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Hydroquinone

PubChem CID
785
Structure
Hydroquinone_small.png
Hydroquinone_3D_Structure.png
Hydroquinone__Crystal_Structure.png
Molecular Formula
Synonyms
  • hydroquinone
  • 123-31-9
  • Benzene-1,4-diol
  • 1,4-benzenediol
  • Quinol
Molecular Weight
110.11 g/mol
Computed by PubChem 2.2 (PubChem release 2021.10.14)
Dates
  • Create:
    2004-09-16
  • Modify:
    2025-01-18
Description
Hydroquinone appears as light colored crystals or solutions. May irritate the skin, eyes and mucous membranes. Mildly toxic by ingestion or skin absorption.
Hydroquinone is a benzenediol comprising benzene core carrying two hydroxy substituents para to each other. It has a role as a cofactor, a carcinogenic agent, an Escherichia coli metabolite, a human xenobiotic metabolite, a skin lightening agent, an antioxidant and a mouse metabolite. It is a benzenediol and a member of hydroquinones.
Hydroquinone is a topical lightening product found in OTC products, and is used to correct skin discoloration associated with disorders of hyperpigmentation including melasma, post-inflammatory hyperpigmention, sunspots, and freckles. It can be used alone, but is more frequently found in combination with other agents such as alpha-hydroxy acids, corticosteroids, retinoids, or sunscreen. Hydroquinone has come under scrutiny due to several complications associated with its use, including dermal irritation, exogenous onchronosis, and carginogenicity. As a result of these concerns, hydroquinone has been banned in the EU and UK.
See also: Phenol (has active moiety); Quinhydrone (is active moiety of); Fluocinolone acetonide; hydroquinone; tretinoin (component of) ... View More ...

1 Structures

1.1 2D Structure

Chemical Structure Depiction
Hydroquinone.png

1.2 3D Conformer

1.3 Crystal Structures

1 of 3
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CCDC Number
Crystal Structure Data
Crystal Structure Depiction
Crystal Structure Depiction

2 Names and Identifiers

2.1 Computed Descriptors

2.1.1 IUPAC Name

benzene-1,4-diol
Computed by Lexichem TK 2.7.0 (PubChem release 2021.10.14)

2.1.2 InChI

InChI=1S/C6H6O2/c7-5-1-2-6(8)4-3-5/h1-4,7-8H
Computed by InChI 1.0.6 (PubChem release 2021.10.14)

2.1.3 InChIKey

QIGBRXMKCJKVMJ-UHFFFAOYSA-N
Computed by InChI 1.0.6 (PubChem release 2021.10.14)

2.1.4 SMILES

C1=CC(=CC=C1O)O
Computed by OEChem 2.3.0 (PubChem release 2024.12.12)

2.2 Molecular Formula

C6H6O2
Computed by PubChem 2.2 (PubChem release 2021.10.14)

C6H6O2

C6H4(OH)2

2.3 Other Identifiers

2.3.1 CAS

123-31-9
3225-30-7

2.3.3 Deprecated CAS

57534-13-1, 8027-02-9
8027-02-9

2.3.4 European Community (EC) Number

2.3.5 UNII

2.3.6 UN Number

2.3.7 ChEBI ID

2.3.8 ChEMBL ID

2.3.9 DrugBank ID

2.3.10 DSSTox Substance ID

2.3.11 HMDB ID

2.3.12 ICSC Number

2.3.13 KEGG ID

2.3.14 Metabolomics Workbench ID

2.3.15 NCI Thesaurus Code

2.3.16 Nikkaji Number

2.3.17 NSC Number

2.3.18 Pharos Ligand ID

2.3.19 RTECS Number

2.3.20 RXCUI

2.3.21 Wikidata

2.3.22 Wikipedia

2.4 Synonyms

2.4.1 MeSH Entry Terms

  • 1,4-benzenediol
  • 1,4-dihydroxybenzene
  • Artra
  • beta-quinol
  • Eldopaque
  • Eldoquin
  • Esoterica
  • Hidroquilaude
  • Hidroquin
  • Hidroquinona Isdin
  • hydroquinone
  • hydroquinone, copper (1+) salt
  • hydroquinone, lead (2+) salt (2:1)
  • hydroquinone, monocopper (2+) salt
  • Licostrata
  • Lustra
  • Melanasa
  • Melanex
  • Melpaque
  • Melquin
  • Neostrata HQ
  • p-benzenediol
  • Phiaquin
  • Solaquin
  • Ultraquin

2.4.2 Depositor-Supplied Synonyms

3 Chemical and Physical Properties

3.1 Computed Properties

Property Name
Molecular Weight
Property Value
110.11 g/mol
Reference
Computed by PubChem 2.2 (PubChem release 2021.10.14)
Property Name
XLogP3
Property Value
0.6
Reference
Computed by XLogP3 3.0 (PubChem release 2021.10.14)
Property Name
Hydrogen Bond Donor Count
Property Value
2
Reference
Computed by Cactvs 3.4.8.18 (PubChem release 2021.10.14)
Property Name
Hydrogen Bond Acceptor Count
Property Value
2
Reference
Computed by Cactvs 3.4.8.18 (PubChem release 2021.10.14)
Property Name
Rotatable Bond Count
Property Value
0
Reference
Computed by Cactvs 3.4.8.18 (PubChem release 2021.10.14)
Property Name
Exact Mass
Property Value
110.036779430 Da
Reference
Computed by PubChem 2.2 (PubChem release 2021.10.14)
Property Name
Monoisotopic Mass
Property Value
110.036779430 Da
Reference
Computed by PubChem 2.2 (PubChem release 2021.10.14)
Property Name
Topological Polar Surface Area
Property Value
40.5 Ų
Reference
Computed by Cactvs 3.4.8.18 (PubChem release 2021.10.14)
Property Name
Heavy Atom Count
Property Value
8
Reference
Computed by PubChem
Property Name
Formal Charge
Property Value
0
Reference
Computed by PubChem
Property Name
Complexity
Property Value
54.9
Reference
Computed by Cactvs 3.4.8.18 (PubChem release 2021.10.14)
Property Name
Isotope Atom Count
Property Value
0
Reference
Computed by PubChem
Property Name
Defined Atom Stereocenter Count
Property Value
0
Reference
Computed by PubChem
Property Name
Undefined Atom Stereocenter Count
Property Value
0
Reference
Computed by PubChem
Property Name
Defined Bond Stereocenter Count
Property Value
0
Reference
Computed by PubChem
Property Name
Undefined Bond Stereocenter Count
Property Value
0
Reference
Computed by PubChem
Property Name
Covalently-Bonded Unit Count
Property Value
1
Reference
Computed by PubChem
Property Name
Compound Is Canonicalized
Property Value
Yes
Reference
Computed by PubChem (release 2021.10.14)

3.2 Experimental Properties

3.2.1 Physical Description

Hydroquinone appears as light colored crystals or solutions. May irritate the skin, eyes and mucous membranes. Mildly toxic by ingestion or skin absorption.
Pellets or Large Crystals; Other Solid; Dry Powder; Liquid
Light-tan, light-gray, or colorless crystals; [NIOSH]
Solid
COLOURLESS CRYSTALS.
Light-tan, light-gray, or colorless crystals.

3.2.2 Color / Form

White crystals
Lewis, R.J. Sr.; Hawley's Condensed Chemical Dictionary 15th Edition. John Wiley & Sons, Inc. New York, NY 2007., p. 669
Monoclinic prisms (sublimation); needles from water; prisms from methanol
Haynes, W.M. (ed.). CRC Handbook of Chemistry and Physics. 94th Edition. CRC Press LLC, Boca Raton: FL 2013-2014, p. 3-304
Light-tan, light-gray, or colorless crystals
NIOSH. NIOSH Pocket Guide to Chemical Hazards. Department of Health & Human Services, Centers for Disease Control & Prevention. National Institute for Occupational Safety & Health. DHHS (NIOSH) Publication No. 2010-168 (2010). Available from: https://www.cdc.gov/niosh/npg

3.2.3 Odor

Odorless
Cavender FL, O'Donohue J; Phenol and Phenolics. Patty's Toxicology. 6th ed. (1999-2014). New York, NY: John Wiley & Sons, Inc. On-line posting date: 17 Aug 2012

3.2.4 Taste

A slightly bitter taste in aqueous solutions
Cavender FL, O'Donohue J; Phenol and Phenolics. Patty's Toxicology. 6th ed. (1999-2014). New York, NY: John Wiley & Sons, Inc. On-line posting date: 17 Aug 2012

3.2.5 Boiling Point

545 to 549 °F at 760 mmHg (EPA, 1998)
U.S. Environmental Protection Agency. 1998. Extremely Hazardous Substances (EHS) Chemical Profiles and Emergency First Aid Guides. Washington, D.C.: U.S. Government Printing Office.
285-287
O'Neil, M.J. (ed.). The Merck Index - An Encyclopedia of Chemicals, Drugs, and Biologicals. Cambridge, UK: Royal Society of Chemistry, 2013., p. 891
285-287 °C
O'Neil, M.J. (ed.). The Merck Index - An Encyclopedia of Chemicals, Drugs, and Biologicals. Cambridge, UK: Royal Society of Chemistry, 2013., p. 891
286.00 to 288.00 °C. @ 760.00 mm Hg
The Good Scents Company Information System
287 °C
545 °F

3.2.6 Melting Point

338 to 340 °F (EPA, 1998)
U.S. Environmental Protection Agency. 1998. Extremely Hazardous Substances (EHS) Chemical Profiles and Emergency First Aid Guides. Washington, D.C.: U.S. Government Printing Office.
170-171
O'Neil, M.J. (ed.). The Merck Index - An Encyclopedia of Chemicals, Drugs, and Biologicals. Cambridge, UK: Royal Society of Chemistry, 2013., p. 891
170-171 °C
O'Neil, M.J. (ed.). The Merck Index - An Encyclopedia of Chemicals, Drugs, and Biologicals. Cambridge, UK: Royal Society of Chemistry, 2013., p. 891
MP: 173 °C; BP 288 °C
Haynes, W.M. (ed.). CRC Handbook of Chemistry and Physics. 94th Edition. CRC Press LLC, Boca Raton: FL 2013-2014, p. 3-304
Heat of fusion at melting point = 2.71X10+7 J/kmol
Daubert, T.E., R.P. Danner. Physical and Thermodynamic Properties of Pure Chemicals Data Compilation. Washington, D.C.: Taylor and Francis, 1989.
172.3 °C
172 °C
338 °F

3.2.7 Flash Point

329 °F (EPA, 1998)
U.S. Environmental Protection Agency. 1998. Extremely Hazardous Substances (EHS) Chemical Profiles and Emergency First Aid Guides. Washington, D.C.: U.S. Government Printing Office.
329 °F
329 °F (CLOSED CUP)
Lewis, R.J. Sr. (ed) Sax's Dangerous Properties of Industrial Materials. 11th Edition. Wiley-Interscience, Wiley & Sons, Inc. Hoboken, NJ. 2004., p. 1992
165 °C (329 °F) (Closed cup)
Sigma-Aldrich; Material Safety Data Sheet for Hydroquinone. Product Number: H17902, Version 4.7 (Revision Date 06/13/2014). Available from, as of October 8, 2014: https://www.sigmaaldrich.com/safety-center.html
329 °F (165 °C) (Closed cup)
National Fire Protection Association; Fire Protection Guide to Hazardous Materials. 14TH Edition, Quincy, MA 2010, p. 325-72
165 °C
329 °F (Molten)

3.2.8 Solubility

10 to 50 mg/mL at 68 °F (NTP, 1992)
National Toxicology Program, Institute of Environmental Health Sciences, National Institutes of Health (NTP). 1992. National Toxicology Program Chemical Repository Database. Research Triangle Park, North Carolina.
6.72g/L at 20 deg C
Yalkowsky, S.H., He, Yan, Jain, P. Handbook of Aqueous Solubility Data Second Edition. CRC Press, Boca Raton, FL 2010, p. 254
In water, 72,000 mg/L at 25 °C
Granger FS, Nelson JM; J Am Chem Soc 43: 1403-7 (1921)
In water, 6.72 g/L at 20 °C. Also water solubility = 3.85 g/L at 0 °C; 5.12 g/L at 10 °C; 5.40 g/L at 15 °C; 8.76 g/L at 30 °C; 11.5 g/L at 40 °C; 25/9 g/L at 60 °C; 46.8 g/L at 80 °C; 66.4 g/L at 100 °C
Yalkowsky, S.H., He, Yan, Jain, P. Handbook of Aqueous Solubility Data Second Edition. CRC Press, Boca Raton, FL 2010, p. 254
7% soluble in water at 25 °C
Cavender FL, O'Donohue J; Phenol and Phenolics. Patty's Toxicology. 6th ed. (1999-2014). New York, NY: John Wiley & Sons, Inc. On-line posting date: 17 Aug 2012
Grams of hydroquinone per 100 g solvent (30 °C); ethanol 46.4; acetone 28.4; water 8.3; benzene 0.06; tetrachloromethane 0.01
Hudnall PM; Hydroquinone. Ullmann's Encyclopedia of Industrial Chemistry. 7th ed. (1999-2014). New York, NY: John Wiley & Sons. Online Posting Date: 15 Jun 2000
For more Solubility (Complete) data for HYDROQUINONE (6 total), please visit the HSDB record page.
72.0 mg/mL at 25 °C
Solubility in water, g/100ml at 15 °C: 5.9
7%

3.2.9 Density

1.332 at 59 °F (EPA, 1998) - Denser than water; will sink
U.S. Environmental Protection Agency. 1998. Extremely Hazardous Substances (EHS) Chemical Profiles and Emergency First Aid Guides. Washington, D.C.: U.S. Government Printing Office.
1.330 g/cu cm at 20 °C
Haynes, W.M. (ed.). CRC Handbook of Chemistry and Physics. 94th Edition. CRC Press LLC, Boca Raton: FL 2013-2014, p. 3-304
Density of "saturated" air = 1.011 (150 °C) (air = 1) ... Crystals remain relatively stable in air if dry ... Reacts with molecular oxygen undergoing autooxidation in aqueous media
Cavender FL, O'Donohue J; Phenol and Phenolics. Patty's Toxicology. 6th ed. (1999-2014). New York, NY: John Wiley & Sons, Inc. On-line posting date: 17 Aug 2012
Relative density (water = 1): 1.3
1.332 at 59 °F
1.33

3.2.10 Vapor Density

3.81 (EPA, 1998) - Heavier than air; will sink (Relative to Air)
U.S. Environmental Protection Agency. 1998. Extremely Hazardous Substances (EHS) Chemical Profiles and Emergency First Aid Guides. Washington, D.C.: U.S. Government Printing Office.
3.81 (Air = 1)
Lewis, R.J. Sr. (ed) Sax's Dangerous Properties of Industrial Materials. 11th Edition. Wiley-Interscience, Wiley & Sons, Inc. Hoboken, NJ. 2004., p. 1992
Relative vapor density (air = 1): 3.8
3.81

3.2.11 Vapor Pressure

4 mmHg at 302 °F (EPA, 1998)
U.S. Environmental Protection Agency. 1998. Extremely Hazardous Substances (EHS) Chemical Profiles and Emergency First Aid Guides. Washington, D.C.: U.S. Government Printing Office.
0.000019 [mmHg]
1.9X10-5 mm Hg at 25 °C
Jones AH; J Chem Eng Data 5: 196-200 (1960)
Vapor pressure, Pa at 20 °C: 0.12
4 mmHg at 302 °F
0.00001 mmHg

3.2.12 LogP

0.59
Hansch, C., Leo, A., D. Hoekman. Exploring QSAR - Hydrophobic, Electronic, and Steric Constants. Washington, DC: American Chemical Society., 1995., p. 20
log Kow = 0.59
Hansch, C., Leo, A., D. Hoekman. Exploring QSAR - Hydrophobic, Electronic, and Steric Constants. Washington, DC: American Chemical Society., 1995., p. 20
0.59
HANSCH,C ET AL. (1995)

3.2.13 Stability / Shelf Life

Its solution becomes brown in air due to oxidation.
O'Neil, M.J. (ed.). The Merck Index - An Encyclopedia of Chemicals, Drugs, and Biologicals. Cambridge, UK: Royal Society of Chemistry, 2013., p. 891
Stable under recommended storage conditions.
Sigma-Aldrich; Material Safety Data Sheet for Hydroquinone. Product Number: H17902, Version 4.7 (Revision Date 06/13/2014). Available from, as of October 8, 2014: https://www.sigmaaldrich.com/safety-center.html

3.2.14 Autoignition Temperature

960 °F (USCG, 1999)
U.S. Coast Guard. 1999. Chemical Hazard Response Information System (CHRIS) - Hazardous Chemical Data. Commandant Instruction 16465.12C. Washington, D.C.: U.S. Government Printing Office.
960 °F (516 °C)
National Fire Protection Association; Fire Protection Guide to Hazardous Materials. 14TH Edition, Quincy, MA 2010, p. 325-72
515 °C

3.2.15 Heat of Combustion

-2.74X10+3 kJ/mol
Hudnall PM; Hydroquinone. Ullmann's Encyclopedia of Industrial Chemistry. 7th ed. (1999-2014). New York, NY: John Wiley & Sons. Online Posting Date: 15 Jun 2000

3.2.16 Ionization Potential

7.95 eV

3.2.17 Ionization Efficiency

Ionization mode
Negative
logIE
-2.45
pH
10.5
Instrument
Agilent 6495
Ion source
JetStream
Additive
ammonia (10nM)
Organic modifier
MeCN (80%)

3.2.18 Refractive Index

Index of refraction = 1.632 at 25 °C
Haynes, W.M. (ed.). CRC Handbook of Chemistry and Physics. 94th Edition. CRC Press LLC, Boca Raton: FL 2013-2014, p. 3-304

3.2.19 Dissociation Constants

pKa = 10.85 at 25 °C
Pearce PJ, Simkins RJJ; Can J Chem 46: 241-8 (1968)
pKa = 9.96
McEvoy, G.K. (ed.). AHFS Drug Information 90. Bethesda, MD: American Society of Hospital Pharmacists, Inc., 1990 (Plus Supplements 1990)., p. 2081

3.2.20 Collision Cross Section

128.79 Ų [M+H]+ [CCS Type: DT; Method: stepped-field]

119.07 Ų [M-H]- [CCS Type: DT; Method: stepped-field]

118.7 Ų [M-H]-

113.9 Ų [M+H]+

S50 | CCSCOMPEND | The Unified Collision Cross Section (CCS) Compendium | DOI:10.5281/zenodo.2658162

3.2.21 Kovats Retention Index

Standard non-polar
1334 , 1327 , 1334
Semi-standard non-polar
1241 , 220.6 , 207.49
Standard polar
2693 , 2693

3.2.22 Other Experimental Properties

MP: 172.3 °C; BP: 286 °C [
ACGIH Documentation of the Threshold Limit Values and Biological Exposure Indices. 7th ed. Cincinnati, OH: American Conference of Governmental Industrial Hygienists, 2014
Solution becomes brown in the air due to oxidation; oxidation is very rapid in presence of alkali
O'Neil, M.J. (ed.). The Merck Index - An Encyclopedia of Chemicals, Drugs, and Biologicals. Cambridge, UK: Royal Society of Chemistry, 2013., p. 891
Liquid molar volume = 0.123 cu m/kmol (determined at triple point)
Daubert, T.E., R.P. Danner. Physical and Thermodynamic Properties of Pure Chemicals Data Compilation. Washington, D.C.: Taylor and Francis, 1989.
Ideal Gas Heat of Formation = -2.6X10+8 J/kmol
Daubert, T.E., R.P. Danner. Physical and Thermodynamic Properties of Pure Chemicals Data Compilation. Washington, D.C.: Taylor and Francis, 1989.
Vapor Pressure = 4.0 mm Hg at 150 °C; 40 mm Hg at 192 °C; 200 mm Hg at 238 °C. Saturation concentration in air = 0.00006 g/cu m at 25 °C
Verschueren, K. Handbook of Environmental Data on Organic Chemicals. Volumes 1-2. 4th ed. John Wiley & Sons. New York, NY. 2001, p. V2: 1291
Enthalpy of sublimation = 24.8; Absolute entropy = 344.17 J/mole-K
Jones AH; J Chem Eng Data 5: 196-200 (1960)
For more Other Experimental Properties (Complete) data for HYDROQUINONE (8 total), please visit the HSDB record page.

3.3 SpringerMaterials Properties

3.4 Chemical Classes

Other Classes -> Phenols
Antioxidant
S120 | DUSTCT2024 | Substances from Second NORMAN Collaborative Dust Trial | DOI:10.5281/zenodo.13835254

3.4.1 Drugs

Pharmaceuticals -> Listed in ZINC15
S55 | ZINC15PHARMA | Pharmaceuticals from ZINC15 | DOI:10.5281/zenodo.3247749
3.4.1.1 Human Drugs
Breast Feeding; Lactation; Milk, Human; Antioxidants; Depigmentors; Dermatologic Agents; Hydroquinones
Human drug -> Active ingredient (HYDROQUINONE)

3.4.2 Cosmetics

Cosmetic ingredients (Hydroquinone) -> CIR (Cosmetic Ingredient Review)
Antioxidant; Reducing; Hair dyeing; Bleaching
S13 | EUCOSMETICS | Combined Inventory of Ingredients Employed in Cosmetic Products (2000) and Revised Inventory (2006) | DOI:10.5281/zenodo.2624118

3.4.3 Pesticides

Environmental transformation -> Pesticide transformation products (metabolite, successor)
S60 | SWISSPEST19 | Swiss Pesticides and Metabolites from Kiefer et al 2019 | DOI:10.5281/zenodo.3544759

4 Spectral Information

4.1 1D NMR Spectra

1 of 2
1D NMR Spectra
NMR: 10350 (Sadtler Research Laboratories Spectral Collection)
2 of 2
1D NMR Spectra

4.1.1 1H NMR Spectra

1 of 4
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Spectra ID
Instrument Type
Bruker
Frequency
600 MHz
Solvent
Water
pH
7.00
Shifts [ppm]:Intensity
6.81:100.00
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Spectra ID
Instrument Type
JEOL
Frequency
300 MHz
Solvent
DMSO-d6
Shifts [ppm]:Intensity
6.58:1000.00, 8.59:500.00
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4.1.2 13C NMR Spectra

1 of 3
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Spectra ID
Instrument Type
NEVA
Frequency
15.08 MHz
Solvent
DMSO-d6
Shifts [ppm]:Intensity
149.73:379.00, 115.67:1000.00
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Source of Sample
Eastman Organic Chemicals, Rochester, New York
Copyright
Copyright © 1980, 1981-2024 John Wiley & Sons, Inc. All Rights Reserved.
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4.2 2D NMR Spectra

4.2.1 1H-13C NMR Spectra

2D NMR Spectra Type
1H-13C HSQC
Spectra ID
Instrument Type
Bruker
Frequency
600 MHz
Solvent
Water
pH
7.00
Shifts [ppm] (F2:F1):Intensity
6.82:119.16:1.00
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4.3 Mass Spectrometry

4.3.1 GC-MS

1 of 13
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Spectra ID
Instrument Type
EI-B
Ionization Mode
positive
Top 5 Peaks

110.0 99.99

53.0 26.10

39.0 16.90

54.0 12.50

82.0 9.30

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Notes
instrument=Unknown
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Spectra ID
Instrument Type
EI-B
Ionization Mode
positive
Top 5 Peaks

110.0 99.99

81.0 48.49

53.0 36.94

55.0 26.51

39.0 18.60

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Notes
instrument=HITACHI M-80A

4.3.2 MS-MS

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Spectra ID
Ionization Mode
Negative
Top 5 Peaks

109.02917 100

76.96878 55.90

60.97355 50.60

108.01952 37.40

53.03926 36.70

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Spectra ID
Ionization Mode
Positive
Top 5 Peaks

62.98964 100

43.01692 69.10

78.98438 62.70

82.96115 33.40

41.48511 21.80

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4.3.3 LC-MS

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Authors
Kevin S. Jewell; Björn Ehlig; Arne Wick
Instrument
TripleTOF 5600 SCIEX
Instrument Type
LC-ESI-QTOF
MS Level
MS2
Ionization Mode
POSITIVE
Ionization
ESI
Collision Energy
10
Fragmentation Mode
CID
Column Name
Zorbax Eclipse Plus C18 2.1 mm x 150 mm, 3.5 um, Agilent
Retention Time
3.674 min
Precursor m/z
111.0441
Precursor Adduct
[M+H]+
Top 5 Peaks

111.0451 999

93.0345 65

65.0416 28

83.9988 13

83.0497 9

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License
dl-de/by-2-0
2 of 8
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Authors
Kevin S. Jewell; Björn Ehlig; Arne Wick
Instrument
TripleTOF 5600 SCIEX
Instrument Type
LC-ESI-QTOF
MS Level
MS2
Ionization Mode
POSITIVE
Ionization
ESI
Collision Energy
20
Fragmentation Mode
CID
Column Name
Zorbax Eclipse Plus C18 2.1 mm x 150 mm, 3.5 um, Agilent
Retention Time
3.674 min
Precursor m/z
111.0441
Precursor Adduct
[M+H]+
Top 5 Peaks

111.0436 999

65.0418 767

93.0354 462

83.9994 73

83.0499 62

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License
dl-de/by-2-0

4.3.4 Other MS

1 of 3
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Other MS
MASS: 1529 (NIST/EPA/MSDC Mass Spectral Database, 1990 version)
2 of 3
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Authors
MASS SPECTROSCOPY SOC. OF JAPAN (MSSJ)
Instrument
Unknown
Instrument Type
EI-B
MS Level
MS
Ionization Mode
POSITIVE
Top 5 Peaks

110 999

53 261

39 169

54 125

82 93

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License
CC BY-NC-SA

4.4 UV Spectra

MAX ABSORPTION (WATER): 288 NM (LOG E= 3.36)
Weast, R.C. (ed.). Handbook of Chemistry and Physics. 60th ed. Boca Raton, Florida: CRC Press Inc., 1979., p. C-159
UV: 60 (Sadtler Research Laboratories Spectral Collection)
Lide, D.R., G.W.A. Milne (eds.). Handbook of Data on Organic Compounds. Volume I. 3rd ed. CRC Press, Inc. Boca Raton ,FL. 1994., p. V1: 893

4.4.1 UV-VIS Spectra

Copyright
Copyright © 2008-2024 John Wiley & Sons, Inc. All Rights Reserved.
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4.5 IR Spectra

IR Spectra
IR: 2882 (Coblentz Society Spectral Collection)

4.5.1 FTIR Spectra

1 of 2
Technique
KBr WAFER
Source of Sample
Tennessee Eastman Corporation
Copyright
Copyright © 1980, 1981-2024 John Wiley & Sons, Inc. All Rights Reserved.
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2 of 2
Technique
KBr WAFER
Source of Sample
Eastman Chemcial Products, Inc., Kingsport, Tennessee
Copyright
Copyright © 1980, 1981-2024 John Wiley & Sons, Inc. All Rights Reserved.
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4.5.2 ATR-IR Spectra

1 of 2
Source of Sample
Aldrich
Catalog Number
240125
Copyright
Copyright © 2018-2024 Sigma-Aldrich Co. LLC. - Database Compilation Copyright © 2018-2024 John Wiley & Sons, Inc. All Rights Reserved.
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2 of 2
Source of Sample
Sigma-Aldrich
Catalog Number
H17902
Copyright
Copyright © 2018-2024 Sigma-Aldrich Co. LLC. - Database Compilation Copyright © 2018-2024 John Wiley & Sons, Inc. All Rights Reserved.
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4.5.3 Vapor Phase IR Spectra

1 of 2
Instrument Name
DIGILAB FTS-14
Technique
Vapor Phase
Copyright
Copyright © 1980, 1981-2024 John Wiley & Sons, Inc. All Rights Reserved.
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2 of 2
Technique
Vapor Phase
Source of Spectrum
Sigma-Aldrich Co. LLC.
Source of Sample
Aldrich
Catalog Number
240125
Copyright
Copyright © 2018-2024 Sigma-Aldrich Co. LLC. - Database Compilation Copyright © 2018-2024 John Wiley & Sons, Inc. All Rights Reserved.
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4.6 Raman Spectra

1 of 2
Instrument Name
Bio-Rad FTS 175C with Raman accessory
Technique
FT-Raman
Source of Sample
Tennessee Eastman Corporation
Copyright
Copyright © 1980, 1981-2024 John Wiley & Sons, Inc. All Rights Reserved.
Thumbnail
Thumbnail
2 of 2
Catalog Number
240125
Copyright
Copyright © 2017-2024 Sigma-Aldrich Co. LLC. - Database Compilation Copyright © 2017-2024 John Wiley & Sons, Inc. All Rights Reserved.
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4.7 Other Spectra

Intense mass spectral peaks: 81 m/z, 110 m/z
Pfleger, K., H. Maurer and A. Weber. Mass Spectral and GC Data of Drugs, Poisons and their Metabolites. Parts I and II. Mass Spectra Indexes. Weinheim, Federal Republic of Germany. 1985., p. 135

6 Chemical Vendors

7 Drug and Medication Information

7.1 Drug Indication

Hydroquinone is used as an OTC topical lightening agent for disorders of hyperpigmentation including melasma, post-inflammatory hyperpigmention, sunspots and freckles.

7.2 Drug Classes

Breast Feeding; Lactation; Milk, Human; Antioxidants; Depigmentors; Dermatologic Agents; Hydroquinones

7.3 FDA Approved Drugs

7.4 FDA Orange Book

7.5 FDA National Drug Code Directory

7.6 Drug Labels

Drug and label
Active ingredient and drug

7.7 Clinical Trials

7.7.1 ClinicalTrials.gov

7.7.2 EU Clinical Trials Register

7.7.3 NIPH Clinical Trials Search of Japan

7.8 Therapeutic Uses

For the gradual temporary bleaching of hyperpigmented skin conditions such as chloasma, melasma, freckles, senile lentigines, and other unwanted areas of melanin hyperpigmentation.
Drug Facts and Comparisons 2013. Wolters Kluwer Health St. Louis, MO 2013, p. 3264
TRI-LUMA Cream is a combination of fluocinolone acetonide (a corticosteroid), hydroquinone (a melanin synthesis inhibitor), and tretinoin (a retinoid) that is indicated for the short-term treatment of moderate to severe melasma of the face, in the presence of measures for sun avoidance, including the use of sunscreens. /Included in US product label/
US Natl Inst Health; DailyMed. Current Medication Information for Tri-Luma (fluocinolone acetonide, hydroquinone, and tretinoin) (April 2014). Available from, as of November 12, 2014: https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?id=8729

7.9 Drug Warnings

This medication is for external use only. A mild transient stinging may occur in people with sensitive skin. Do not use on broken or irritated skin. Discontinue use if irritation or rush occurs. Avoid contact with eyes and mucous membranes. In case of contact, rinse thoroughly with water.
Drug Facts and Comparisons 2013. Wolters Kluwer Health St. Louis, MO 2013, p. 3265
To evaluate possible susceptibility to irritation, or sensitivity, each patient should begin with applying the medication to a small portion of unbroken skin at or near the pigmented area (approx 1 sq cm) over a period of several days. If no irritation occurs within 24 hr, begin treatment. Minor redness is not necessarily a contraindication, but treatment should be discontinued if itching, excessive inflammation, or vesicle formation occurs. Use of hydroquinone products in paranasal and infraorbital areas increase the chance of irritation. If no improvement is seen after 2 mo of treatment, use of this product should be discontinued.
Drug Facts and Comparisons 2013. Wolters Kluwer Health St. Louis, MO 2013, p. 3265
Sunscreen use is an essential aspect of hydroquinone therapy since even minimal sunlight exposure stimulates melanocyte activity. The sunscreens in some hydroquinone products provide the necessary sun protection during skin bleaching activity. During the depigmentation maintenance treatment subsequent to the intensive depigmentation therapy, sun exposure of the bleached skin should be avoided to prevent repigmentation.
Drug Facts and Comparisons 2013. Wolters Kluwer Health St. Louis, MO 2013, p. 3265
Concurent use of peroxide may result in transient dark staining of skin ares due to oxidation of hydroquinone. Staining can be removed by discontinuing concurrent use and by normal soap cleansing.
Drug Facts and Comparisons 2013. Wolters Kluwer Health St. Louis, MO 2013, p. 3265
For more Drug Warnings (Complete) data for HYDROQUINONE (9 total), please visit the HSDB record page.

7.10 Reported Fatal Dose

Fatal cases have been reported after ingestion of 5 to 12 g.
Clayton, G. D. and F. E. Clayton (eds.). Patty's Industrial Hygiene and Toxicology: Volume 2A, 2B, 2C: Toxicology. 3rd ed. New York: John Wiley Sons, 1981-1982., p. 2591

8 Food Additives and Ingredients

8.1 Associated Foods

9 Agrochemical Information

9.1 Agrochemical Transformations

Hydroquinone is a known environmental transformation product of Aclonifen.
S60 | SWISSPEST19 | Swiss Pesticides and Metabolites from Kiefer et al 2019 | DOI:10.5281/zenodo.3544759

10 Pharmacology and Biochemistry

10.1 MeSH Pharmacological Classification

Radiation-Protective Agents
Drugs used to protect against ionizing radiation. They are usually of interest for use in radiation therapy but have been considered for other purposes, e.g. military. (See all compounds classified as Radiation-Protective Agents.)
Antioxidants
Naturally occurring or synthetic substances that inhibit or retard oxidation reactions. They counteract the damaging effects of oxidation in animal tissues. (See all compounds classified as Antioxidants.)
Mutagens
Chemical agents that increase the rate of genetic mutation by interfering with the function of nucleic acids. A clastogen is a specific mutagen that causes breaks in chromosomes. (See all compounds classified as Mutagens.)

10.2 FDA Pharmacological Classification

1 of 2
FDA UNII
XV74C1N1AE
Active Moiety
HYDROQUINONE
Pharmacological Classes
Physiologic Effects [PE] - Depigmenting Activity
Pharmacological Classes
Established Pharmacologic Class [EPC] - Melanin Synthesis Inhibitor
Pharmacological Classes
Mechanisms of Action [MoA] - Melanin Synthesis Inhibitors
FDA Pharmacology Summary
Hydroquinone is a Melanin Synthesis Inhibitor. The mechanism of action of hydroquinone is as a Melanin Synthesis Inhibitor. The physiologic effect of hydroquinone is by means of Depigmenting Activity.
2 of 2
Non-Proprietary Name
HYDROQUINONE
Pharmacological Classes
Depigmenting Activity [PE]; Melanin Synthesis Inhibitor [EPC]; Melanin Synthesis Inhibitors [MoA]

10.3 ATC Code

D - Dermatologicals

D11 - Other dermatological preparations

D11A - Other dermatological preparations

D11AX - Other dermatologicals

D11AX11 - Hydroquinone

10.4 Absorption, Distribution and Excretion

A toxicology review of hydroquinone noted several reports indicating relatively rapid absorption of hydroquinone via the oral route, including a study involving rats that ingested 3% hydroquinone in developer solution. In addition, in CD and F344 rats dosed with 350 mg/kg, >90% absorption was measured in blood levels, with peak levels observed within 1 hr.[DHHS/NTP: Nomination Profile Hydroquinone
123-31-9]. Supporting Information for Toxicological Evaluation by the National Toxicology Program. p.10 (2009). Available from, as of November 12, 2014: https://ntp-server.niehs.nih.gov
Following intravenous (iv) administration of radiolabeled hydroquinone, radioactivity (either hydroquinone or a metabolite) was detected within 2 hr in bone marrow and thymus of rats given 1.2-12 mg/kg. Radioactivity was also detected in the liver and bone marrow of these rats up to 24 hr. Whether given in single or repeated oral doses, radioactivity was found in various rat tissues, with the highest concentrations in the liver and kidneys. Following i.v. administration of radiolabeled hydroquinone in dogs, radioactivity was found in the skin, liver, and intestine. When mice were administered 75 mg/kg radiolabeled hydroquinone by intraperitoneal (ip) injection, radioactivity was detected covalently bound to proteins in the liver, kidneys, blood, and bone marrow, with 10-fold higher specific activity in the liver than in the bone marrow...[DHHS/NTP: Nomination Profile Hydroquinone
123-31-9]. Supporting Information for Toxicological Evaluation by the National Toxicology Program. p.12 (2009). Available from, as of November 12, 2014: https://ntp-server.niehs.nih.gov
When 2% [(14)C]-hydroquinone was administered to human forearms (n = 4 males) in an unspecified cream, hydroquinone moved rapidly and continuously into the stratum corneum and radiolabel was detected in plasma samples within 0.5 hr. Over an 8 hr plasma sampling period, hydroquinone levels peaked at 4 hr (0.04 equivalents/ mL). Following application of the 2% cream on the foreheads of 6 male volunteers for 24 hr, the recovery of hydroquinone in urine was 45.3% (SD = 11.2%).[DHHS/NTP: Nomination Profile Hydroquinone
123-31-9]. Supporting Information for Toxicological Evaluation by the National Toxicology Program. p.11 (2009). Available from, as of November 12, 2014: https://ntp-server.niehs.nih.gov
Human absorption of hydroquinone upon topical application is less efficient than with oral administration. When absorption was measured as elimination 10 of hydroquinone via urine following application (2.0% in alcohol) to the foreheads of human volunteers (6 males per preparation) for 24 hr, the average percutaneous absorption reported was 57% (SD = 11%) with peak elimination within 12 hr and complete elimination by 5 days. The addition of a sunscreen (3.0% Escalol 507) significantly decreased the absorption (26%, SD = 14%), and the addition of a penetration enhancer (0.5% Azone) did not significantly increase absorption in the presence or absence of the sunscreen (35%, SD = 17% and 66%, SD = 13%, respectively).[DHHS/NTP: Nomination Profile Hydroquinone
123-31-9]. Supporting Information for Toxicological Evaluation by the National Toxicology Program. p.10-11 (2009). Available from, as of November 12, 2014: https://ntp-server.niehs.nih.gov
For more Absorption, Distribution and Excretion (Complete) data for HYDROQUINONE (17 total), please visit the HSDB record page.

10.5 Metabolism / Metabolites

Hydroquinone is absorbed through the skin and metabolized primarily to sulfate and glucuronide conjugates, which are excreted in the urine.[DHHS/NTP: Nomination Profile Hydroquinone
123-31-9]. Supporting Information for Toxicological Evaluation by the National Toxicology Program. p.2 (2009). Available from, as of November 12, 2014: https://ntp-server.niehs.nih.gov
Absolute recovery of approximately 45 ng (18%) of hydroquinone as microsomal metabolite of benzene was determined.
ROSTON DA, KISSINGER PT; ANAL CHEM 54 (11): 1798 (1982)
/This study/ investigated the metabolism of hydroquinone in naive and hydroquinone pretreated male Sprague-Dawley rats. (14)C hydroquinone was administered by gavage in single doses of 5, 30, or 200 mg/kg to naive rats. Hydroquinone was given repeatedly by gavage to male rats at 200 mg/kg for 4 consecutive days followed by a single dose with 200 mg/kg of (14)C hydroquinone. In separate studies rats were fed 5.6% unlabeled hydroquinone in the diet for 2 days or were dosed by gavage with 311 mg/kg (14)C hydroquinone. The excretion patterns of (14)C hydroquinone and its metabolites were similar for rats dosed singly or repeatedly. Rats given a single dose of 200 mg/kg of (14)C hydroquinone excreted 91.9% of the dose in the urine within 2-4 days; 3.8% was excreted in the feces, about 0.4% was excreted in expired air, and 1.2% remained in the carcass. Radioactivity was widely distributed throughout the tissues with higher concentrations in the liver and kidneys. A decrease in (14)C tissue concentrations occurred from 48 to 96 hr. The only radiolabeled compounds in the urine were hydroquinone (1.1-8.6% of the dose), hydroquinone monosulfate (25-42%), and hydroquinone monoglucuronide (56-66%). Similar findings were observed for rats given hydroquinone in the feed. There were no significant increases from controls for absolute or relative liver weights, liver microsomal protein concentrations, cytochrome b-5, cytochrome P450 or cytochrome c reductase activity in rats dosed repeatedly with 200 mg/kg hydroquinone. Cytochrome P450 values were slightly but significantly decreased in rats dosed repeatedly with hydroquinone compared with controls.
Divincenzo GD et al; Toxicol 33 (1): 9-18 (1984)
The metabolite 2-(S-glutathionyl)hydroquinone is formed when a microsomal incubation mixture containing either benzene or phenol is supplemented with glutathione. This metabolite is derived from the conjugation of benzoquinone, an oxidation product of hydroquinone. However, neither the glutathione conjugate or its mercapturate, N-acetyl-S-(2,5-dihydroxyphenyl)-L-cysteine, have been identified as metabolites resulting from in vivo metabolism of benzene, phenol, or hydroquinone. To determine if a hydroxylated mercapturate is produced in vivo, we treated male Sprague-Dawley rats with either benzene (600 mg/kg), phenol (75 mg/kg), or hydroquinone (75 mg/kg) and collected the urine for 24 hr. HPLC coupled with electrochemical detection confirmed the presence of a metabolite that was chromatographically and electrochemically identical to N-acetyl-S-(2,5-dihydroxyphenyl)-L-cysteine. The metabolite was isolated from the urine samples and treated with diazomethane to form the N-acetyl-S-(2,5-dimethoxyphenyl)-L-cysteine methyl ester derivative. The mass spectra obtained from these samples were identical to that of an authentic sample of the derivative. The results of these experiments indicate that benzene, phenol, and hydroquinone are metabolized in vivo to benzoquinone and excreted as the mercapturate, N-acetyl-S-(2,5-dihydroxyphenyl)-L- cysteine.
Nerland DE, Pierce WM Jr; Drug Metab Dispos 18 (6): 958-61 (1990)
For more Metabolism/Metabolites (Complete) data for HYDROQUINONE (14 total), please visit the HSDB record page.
Hydroquinone is a known human metabolite of phenol.
S73 | METXBIODB | Metabolite Reaction Database from BioTransformer | DOI:10.5281/zenodo.4056560

10.6 Biological Half-Life

/Hydroquinone was administered alone and in combination with phenol at 75 mg/kg each to B6C3F1 mice/. The half-life of hydroquinone was increased from 9 +/- 2 to 15 +/- 3 min /when phenol was administered in combination with hydroquinone/.
Legathe A et al; Toxicol Appl Pharmacol 124 (1): 131-8 (1994)

10.7 Mechanism of Action

Hydroquinone reduces melanin pigment production through inhibition of the tyrosinase enzyme, which is involved in the initial step of the melanin pigment biosynthesis pathway. Hydroquinone takes several months to take effect.
Benzene is a well-established human carcinogen. Benzene metabolites hydroquinone (HQ) and benzoquinone (BQ) are highly reactive molecules capable of producing reactive oxygen species and causing oxidative stress. /This study/ investigated the role of the Nrf2, a key nuclear transcription factor that regulates antioxidant response element (ARE)-containing genes, in defense against HQ- and BQ-induced cytotoxicity in cultured human lung epithelial cells (Beas-2B). When the cells were exposed to HQ or BQ the activity of an ARE reporter was induced in a dose-dependent manner, meanwhile Nrf2 protein levels were elevated and accumulated in the nucleus. Increased expression of well-known Nrf2-dependent proteins including NQO1, GCLM, GSS and HMOX was also observed in the HQ/BQ-treated cells. Moreover, transient overexpression of Nrf2 conferred protection against HQ- and BQ-induced cell death, whereas knockdown of Nrf2 by small interfering RNA resulted in increased apoptosis. /This study/ also found that the increased susceptibility of Nrf2-knockdown cells to HQ and BQ was associated with reduced glutathione levels and loss of inducibility of ARE-driven genes, suggesting that deficiency of Nrf2 impairs cellular redox capacity to counteract oxidative damage. Altogether, these results suggest that Nrf2-ARE pathway is essential for protection against HQ- and BQ-induced toxicity.
Rubio V et al; Toxicol In Vitro. 25(2):521-9. (2011).

10.8 Human Metabolite Information

10.8.1 Tissue Locations

  • Bone Marrow
  • Epidermis
  • Fibroblasts
  • Kidney
  • Liver

10.8.2 Cellular Locations

Cytoplasm

10.8.3 Metabolite Pathways

10.9 Biochemical Reactions

10.10 Transformations

11 Use and Manufacturing

11.1 Uses

Cosmetic Ingredient Review Link
CIR ingredient: Hydroquinone
Hydroquinone is used as a developing agent in black-and-white photography, lithography, and x-ray films. It is also used as an intermediate to produce antioxidants for rubber and food. It is added to a number of industrial monomers to inhibit polymerization during shipping, storage, and processing.
EPA CPDat Chemical and Product Categories
The Chemical and Products Database, a resource for exposure-relevant data on chemicals in consumer products, Scientific Data, volume 5, Article number: 180125 (2018), DOI:10.1038/sdata.2018.125
Sources/Uses
Used as a developer (photographic & lithographic), stabilizer in paints and oils, polymerization inhibitor, and chemical intermediate. [ACGIH] Used as a photo developer, oil and fat antioxidant, chemical intermediate, topical medication, and inhibitor of vinyl acetate and acrylic polymerization; [HSDB]
ACGIH - Documentation of the TLVs and BEIs, 7th Ed. Cincinnati: ACGIH Worldwide, 2020.
Industrial Processes with risk of exposure

Plastic Composites Manufacturing [Category: Industry]

Photographic Processing [Category: Other]

The largest demand for hydroquinone is as a photographic developer, principally for black-and-white film, lithography, photochemical machining, microfilm, and X-ray film. ... The second largest consumer of hydroquinone is the rubber industry, which requires hydroquinone for the production of antioxidants and antiozonants.
Hudnall PM; Hydroquinone. Ullmann's Encyclopedia of Industrial Chemistry 7th ed. (1999-2014). NY, NY: John Wiley & Sons. Online Posting Date: June 15, 2000
... Used extensively in the vinyl monomer industry to inhibit free-radical polymerization during both processing and storage ... Used as stabilizers for unsaturated polyester resins.
Hudnall PM; Hydroquinone. Ullmann's Encyclopedia of Industrial Chemistry 7th ed. (1999-2014). NY, NY: John Wiley & Sons. Online Posting Date: June 15, 2000
Hydroquinone is an aromatic compound that functions in cosmetics as an antioxidant, fragrance, reducing agent, or polymerization inhibitor...
Andersen FA et al; Int J Toxicol. 29(6 Suppl):274S-87. (2010).
Hydroquinone (U.S.P. grade) ... /is/ used in topical formulations as /a/ skin bleaching and depigmenting agent.
Hudnall PM; Hydroquinone. Ullmann's Encyclopedia of Industrial Chemistry 7th ed. (1999-2014). NY, NY: John Wiley & Sons. Online Posting Date: June 15, 2000
For more Uses (Complete) data for HYDROQUINONE (12 total), please visit the HSDB record page.
Hydroquinone has a variety of uses principally associated with its action as a reducing agent which is soluble in water. It is a major component in most photographic developers where, with the compound Metol, it reduces silver halides to elemental silver. In human medicine, hydroquinone is used as a topical application in skin whitening to reduce the color of skin.

11.1.1 Use Classification

Hazardous Air Pollutants (HAPs)
Human Drugs -> FDA Approved Drug Products with Therapeutic Equivalence Evaluations (Orange Book) -> Active Ingredients
Hazard Classes and Categories ->
Environmental transformation -> Pesticide transformation products (metabolite, successor)
S60 | SWISSPEST19 | Swiss Pesticides and Metabolites from Kiefer et al 2019 | DOI:10.5281/zenodo.3544759
Cosmetics -> Antioxidant; Reducing; Hair dyeing; Bleaching
S13 | EUCOSMETICS | Combined Inventory of Ingredients Employed in Cosmetic Products (2000) and Revised Inventory (2006) | DOI:10.5281/zenodo.2624118

11.1.2 Industry Uses

  • Antioxidant
  • Intermediates
  • Not Known or Reasonably Ascertainable
  • Intermediate
  • Photosensitive chemicals
  • Process regulators
  • Processing aids, not otherwise listed

11.1.3 Consumer Uses

  • Other (specify)
  • Not Known or Reasonably Ascertainable
  • Photosensitive chemicals

11.1.4 Household Products

Household & Commercial/Institutional Products

Information on 49 consumer products that contain Hydroquinone in the following categories is provided:

• Auto Products

• Inside the Home

• Personal Care

11.2 Methods of Manufacturing

p-Diisopropylbenzene (p-DIPB) is produced by Friedel-Crafts alkylation of benzene with propene. Purified p-DIPB is subsequently converted to the dihydroperoxide (DHP) by air oxidation under slightly alkaline conditions at 80 - 90 °C. The DHP is separated from the reaction mixture either by extraction or by crystallization, and is then cleaved to hydroquinone and acetone by an acid-catalyzed Hock rearrangement; the DHP solution is treated with sulfuric acid catalyst (0.2 - 1.0%) at 60 - 80 °C. The hydroquinone is crystallized and isolated. The overall yield of hydroquinone (based on p-DIPB) is ca. 80%.
Hudnall PM; Hydroquinone. Ullmann's Encyclopedia of Industrial Chemistry 7th ed. (1999-2014). NY, NY: John Wiley & Sons. Online Posting Date: June 15, 2000
The catalyzed hydroxylation of phenol at ca. 80 °C with 70% aqueous hydrogen peroxide produces a mixture of hydroquinone and catechol. The catalyst may be a strong mineral acid, iron(II), or a cobalt(II) salt. Depending on catalyst selection, the ratio of catechol to hydroquinone can be varied from 3:1 to 0.1:1; in practice, the ratio is typically 1.5:1, i.e., catechol is the major product. The reaction proceeds by an ionic mechanism in which hydrogen peroxide is polarized by the strong acid catalyst and phenol is subsequently hydroxylated the resulting isomers are separated by a series of extractions and solvent-stripping operations. The ratio of the products hydroquinone and catechol may be influenced by the presence of superacids or shape-selective zeolites. Thus, use of a vanadium-modified Nafion perfluorosulfonate polymer for the oxidation of phenol gave a hydroquinone-catechol ratio of 12.5:1. The use of a shape-selective zeolite in the hydroxylation of phenol has yielded hydroquinone with 99% selectivity.
Hudnall PM; Hydroquinone. Ullmann's Encyclopedia of Industrial Chemistry 7th ed. (1999-2014). NY, NY: John Wiley & Sons. Online Posting Date: June 15, 2000
Oxidation of aniline is the oldest process used for the production of hydroquinone. Aniline is oxidized with manganese dioxide (15-20% excess) in aqueous sulfuric acid at 0 - 5 °C to produce p-benzoquinone. This intermediate is removed from the reaction mixture by steam stripping and collected. A byproduct, manganese sulfate, may be retrieved from the depleted reaction mixture and sold for agricultural applications. Hydroquinone is obtained from the intermediate p-benzoquinone by reduction with iron at 55 - 65 °C or by catalytic hydrogenation. The product (usually technical grade) is crystallized, isolated from the typically aqueous stream by centrifugation, and dried in a vacuum dryer. The overall yield of hydroquinone from aniline is ca. 85%.
Hudnall PM; Hydroquinone. Ullmann's Encyclopedia of Industrial Chemistry 7th ed. (1999-2014). NY, NY: John Wiley & Sons. Online Posting Date: June 15, 2000
Prepared by oxidation of aniline;... by reduction of quinone; ... by Elbs persulfate oxidation of phenol; ... from acetylene + carbon monoxide.
O'Neil, M.J. (ed.). The Merck Index - An Encyclopedia of Chemicals, Drugs, and Biologicals. Cambridge, UK: Royal Society of Chemistry, 2013., p. 891

11.3 Formulations / Preparations

... Available in USA in photographic & technical grades; specifications for technical grade are: appearance, light-tan to light gray crystals; 98.5% min active ingredient; 1% max water; 0.07% max ash; mp of 169 Dec C min; & miscible with water ... .
IARC. Monographs on the Evaluation of the Carcinogenic Risk of Chemicals to Humans. Geneva: World Health Organization, International Agency for Research on Cancer, 1972-PRESENT. (Multivolume work). Available at: https://monographs.iarc.fr/ENG/Classification/index.php, p. V15 158 (1977)
In Japan, commercially available hydroquinone has 99% min active ingredient; 0.05% max ignition residue; 30 mg/kg max lead; 30 mg/kg max iron; & mp of 169-174 °C.
IARC. Monographs on the Evaluation of the Carcinogenic Risk of Chemicals to Humans. Geneva: World Health Organization, International Agency for Research on Cancer, 1972-PRESENT. (Multivolume work). Available at: https://monographs.iarc.fr/ENG/Classification/index.php, p. V15 158 (1977)
Skin bleaching stick containing hydroquinone.
Calvo LC et al; U.S. Patent No. 4466955 08/21/84 (Monteil, Germaine, Cosmetiques Corp)
Formulations /Hydroquinone -Topical/
Drug Facts and Comparisons 2013. Wolters Kluwer Health St. Louis, MO 2013, p. 3264
Formulation /Pigment Agent Combinations/
Drug Facts and Comparisons 2013. Wolters Kluwer Health St. Louis, MO 2013, p. 3265

11.4 Consumption Patterns

Photographic chemicals, 46%; rubber antioxidants, 31%; other antioxidants, 4%; polymerization inhibitors, 11%; other, 8% (1984)
CHEMICAL PRODUCTS SYNOPSIS: HYDROQUINONE, 1984

11.5 U.S. Production

Aggregated Product Volume

2019: 10,000,000 - <50,000,000 lb

2018: 20,000,000 lb - <100,000,000 lb

2017: 10,000,000 - <50,000,000 lb

2016: 50,000,000 - <100,000,000 lb

(1972) 5.35X10+9 GRAMS
SRI
(1975) 7.53X10+9 GRAMS
SRI
1,4-Benzenediol is listed as a High Production Volume (HPV) chemical (65FR81686). Chemicals listed as HPV were produced in or imported into the U.S. in >1 million pounds in 1990 and/or 1994. The HPV list is based on the 1990 Inventory Update Rule. (IUR) (40 CFR part 710 subpart B; 51FR21438).
EPA/Office of Pollution Prevention and Toxics; High Production Volume (HPV) Challenge Program. 1,4-Benzenediol (123-31-9). Available from, as of October 1, 2014: https://www.epa.gov/hpv/pubs/general/opptsrch.htm
Production volumes for non-confidential chemicals reported under the Inventory Update Rule.
Year
1986
Production Range (pounds)
>10 million - 50 million
Year
1990
Production Range (pounds)
>10 million - 50 million
Year
1994
Production Range (pounds)
>50 million - 100 million
Year
1998
Production Range (pounds)
>10 million - 50 million
Year
2002
Production Range (pounds)
>50 million - 100 million
US EPA; Non-confidential Production Volume Information Submitted by Companies for Chemicals Under the 1986-2002 Inventory Update Rule (IUR). 1,4-Benzen (123-31-9). Available from, as of February 24, 2009: https://www.epa.gov/oppt/iur/tools/data/2002-vol.html
For more U.S. Production (Complete) data for HYDROQUINONE (6 total), please visit the HSDB record page.

11.6 U.S. Imports

(1975) 8.01X10+6 GRAMS (PRINCPL CUSTMS DISTS)
SRI
(1983) 1.97X10+11 g
USITC. IMPORTS OF BENZENOID CHEM & PROD 1983 p.19

11.7 General Manufacturing Information

Industry Processing Sectors
  • Other (requires additional information)
  • All Other Basic Organic Chemical Manufacturing
  • Photographic Film, Paper, Plate, and Chemical Manufacturing
  • Plastics Material and Resin Manufacturing
  • Rubber Product Manufacturing
EPA TSCA Commercial Activity Status
1,4-Benzenediol: ACTIVE
The alkylation of benzene and cumene to produce hydroquinone produces acetone as a by-product on a slightly more than pound for pound basis with hydroquinone.
CHEMICAL PRODUCTS SYNOPSIS: HYDROQUINONE, 1984
Hydroxylation of phenol with 70% hydrogen peroxide with acid catalyst yields catechol as a by-product in the ratio of 1.5:1 (catechol to hydroquinone)
CHEMICAL PRODUCTS SYNOPSIS: HYDROQUINONE, 1984
The oxidation of aniline with manganese dioxide produces more than four pounds of manganese and ammonium sulfates per pound of hydroquinone produced
CHEMICAL PRODUCTS SYNOPSIS: HYDROQUINONE, 1984

11.8 Sampling Procedures

NIOSH Method 5004. Analyte: Hydroquinone. Matrix: Air. Sampler: Filter (0.8-um cellulose ester membrane). Flow Rate: 1 to 3 l/min: Sample Size: 30-90 l. Shipment: Ship sample solutions. Sample Stability: At least 7 days at 25 C.
U.S. Department of Health and Human Services, Public Health Service. Centers for Disease Control, National Institute for Occupational Safety and Health. NIOSH Manual of Analytical Methods, 3rd ed. Volumes 1 and 2 with 1985 supplement, and revisions. Washington, DC: U.S. Government Printing Office, February 1984., p. 5004-1

12 Identification

12.1 Analytic Laboratory Methods

Method: NIOSH 5004, Issue 2; Procedure: high performance liquid chromatography with ultraviolet detection; Analyte: hydroquinone; Matrix: air; Detection Limit: 0.01 mg per sample.
CDC; NIOSH Manual of Analytical Methods, 4th ed. Hydroquinone (123-31-9). Available from, as of October 2, 2014: https://www.cdc.gov/niosh/docs/2003-154/
Method: OSHA PV2094; Procedure: gas chromatography with a flame ionization detector; Analyte: hydroquinone; Matrix: air; Detection Limit: 1 ug.
U.S. Department of Labor/Occupational Safety and Health Administration's Index of Sampling and Analytical Methods. Hydroquinone (123-31-9). Available from, as of October 2, 2014: https://www.osha.gov/dts/sltc/methods/toc.html
Method: EPA-RCA 8270D; Procedure: gas chromatography/mass spectrometry; Analyte: hydroquinone; Matrix: solid waste matrices, soils, air sampling media and water; Detection Limit: not provided.
National Environmental Methods Index; Analytical, Test and Sampling Methods. Hydroquinone (123-31-9). Available from, as of October 2, 2014: https://www.nemi.gov
Analyte: hydroquinone; matrix: pharmaceutical preparation (cosmetic cream); procedure: high-performance liquid chromatography with ultraviolet detection at 294 nm; limit of detection: 40 ng
Herpol-Borremans M, Masse MO; Int J Cosmet Sci 8 (5): 203-14 (1986)
For more Analytic Laboratory Methods (Complete) data for HYDROQUINONE (15 total), please visit the HSDB record page.

12.2 Clinical Laboratory Methods

Analyte: hydroquinone; matrix: urine (human); procedure: reversed-phase high-performance liquid chromatography with electrochemical array detection; limit of quantitation: 12.5 ng
Wittig J et al; J Chromatography 761 (1): 125-32 (2001)
Analyte: hydroquinone; matrix: urine; procedure: high-performance liquid chromatography with ultraviolet detection at 270 nm; limit of detection: 60 ug/mL
Schad H et al; J Chromatogr 593: 147-151 (1992). As cited in: Lunn G; HPLC and CE Methods for Pharmaceutical Analysis. CD-ROM. New York, NY: John Wiley & Sons (2000)

12.3 NIOSH Analytical Methods

13 Safety and Hazards

13.1 Hazards Identification

13.1.1 GHS Classification

1 of 7
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Note
Pictograms displayed are for > 99.9% (2463 of 2464) of reports that indicate hazard statements. This chemical does not meet GHS hazard criteria for < 0.1% (1 of 2464) of reports.
Pictogram(s)
Corrosive
Irritant
Health Hazard
Environmental Hazard
Signal
Danger
GHS Hazard Statements

H302+H312 (17.9%): Harmful if swallowed or in contact with skin [Warning Acute toxicity, oral; acute toxicity, dermal]

H302 (99.7%): Harmful if swallowed [Warning Acute toxicity, oral]

H312 (17.9%): Harmful in contact with skin [Warning Acute toxicity, dermal]

H317 (> 99.9%): May cause an allergic skin reaction [Warning Sensitization, Skin]

H318 (99.9%): Causes serious eye damage [Danger Serious eye damage/eye irritation]

H341 (99.9%): Suspected of causing genetic defects [Warning Germ cell mutagenicity]

H351 (> 99.9%): Suspected of causing cancer [Warning Carcinogenicity]

H400 (> 99.9%): Very toxic to aquatic life [Warning Hazardous to the aquatic environment, acute hazard]

H410 (20.7%): Very toxic to aquatic life with long lasting effects [Warning Hazardous to the aquatic environment, long-term hazard]

Precautionary Statement Codes

P203, P261, P264, P264+P265, P270, P272, P273, P280, P301+P317, P302+P352, P305+P354+P338, P317, P318, P321, P330, P333+P317, P362+P364, P391, P405, and P501

(The corresponding statement to each P-code can be found at the GHS Classification page.)

ECHA C&L Notifications Summary

Aggregated GHS information provided per 2464 reports by companies from 36 notifications to the ECHA C&L Inventory. Each notification may be associated with multiple companies.

Reported as not meeting GHS hazard criteria per 1 of 2464 reports by companies. For more detailed information, please visit ECHA C&L website.

There are 35 notifications provided by 2463 of 2464 reports by companies with hazard statement code(s).

Information may vary between notifications depending on impurities, additives, and other factors. The percentage value in parenthesis indicates the notified classification ratio from companies that provide hazard codes. Only hazard codes with percentage values above 10% are shown.

13.1.2 Hazard Classes and Categories

Acute Tox. 4 (99.7%)

Acute Tox. 4 (17.9%)

Skin Sens. 1 (> 99.9%)

Eye Dam. 1 (99.9%)

Muta. 2 (99.9%)

Carc. 2 (> 99.9%)

Aquatic Acute 1 (> 99.9%)

Aquatic Chronic 1 (20.7%)

Acute toxicity - category 4

Carcinogenicity - category 2

Germ cell mutagenicity - category 2

Eye damage - category 1

Skin sensitisation - category 1

Hazardous to the aquatic environment (acute) - category 1

13.1.3 NFPA Hazard Classification

1 of 2
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NFPA 704 Diamond
2-1-0
NFPA Health Rating
2 - Materials that, under emergency conditions, can cause temporary incapacitation or residual injury.
NFPA Fire Rating
1 - Materials that must be preheated before ignition can occur. Materials require considerable preheating, under all ambient temperature conditions, before ignition and combustion can occur.
NFPA Instability Rating
0 - Materials that in themselves are normally stable, even under fire conditions.

13.1.4 Health Hazards

This material is very toxic; the probable oral lethal dose for humans is 50-500 mg/kg, or between 1 teaspoon and 1 ounce for a 150 lb. person. It is irritating but not corrosive. Fatal human doses have ranged from 5-12 grams, but 300-500 mg have been ingested daily for 3-5 months without ill effects. Death is apparently initiated by respiratory failure or anoxia. (EPA, 1998)
U.S. Environmental Protection Agency. 1998. Extremely Hazardous Substances (EHS) Chemical Profiles and Emergency First Aid Guides. Washington, D.C.: U.S. Government Printing Office.

13.1.5 Fire Hazards

Dust cloud may explode if ignited in an enclosed area. It can react with oxidizing materials and is rapidly oxidized in the presence of alkaline materials. Oxidizes in air. (EPA, 1998)
U.S. Environmental Protection Agency. 1998. Extremely Hazardous Substances (EHS) Chemical Profiles and Emergency First Aid Guides. Washington, D.C.: U.S. Government Printing Office.
Combustible. Finely dispersed particles form explosive mixtures in air.

13.1.6 Hazards Summary

Hydroquinone is used as a developing agent in photography and as an antioxidant in rubber and food. Tinnitus (ringing in the ears), dizziness, headache, nausea, vomiting, dyspnea, erosion of the gastric mucosa, edema of internal organs, cyanosis, convulsions, delirium, and collapse may result from the ingestion of a large amount of hydroquinone in humans. Hydroquinone is also a skin irritant in humans. Chronic (long-term) occupational exposure to hydroquinone dust can result in eye irritation, corneal effects, and impaired vision. No information is available on the reproductive, developmental, or carcinogenic effects of hydroquinone in humans. There was some evidence of carcinogenic activity in orally-exposed rodents. Increased skin tumor incidence has been reported in mice treated dermally. EPA has not classified hydroquinone for carcinogenicity.
Workers exposed to high concentrations developed corneal ulcers. [ACGIH] Animals exposed to lethal doses have convulsions and methemoglobinemia. [HSDB] Can cause leukoderma (depigmentation of skin); [LaDou, p. 328] Allergic contact dermatitis in photographers; [Marks] Allergic contact dermatitis reported in developers of photographs, x-rays, and microfilms; also reported in dental technicians; [Kanerva, p. 1802] Severe eye irritant; [ICSC] Induces genetic damage in germ cells of humans or animals (3A); [MAK]
ACGIH - Documentation of the TLVs and BEIs, 7th Ed. Cincinnati: ACGIH Worldwide, 2020.
LaDou - LaDou J, Harrison R (eds). Current Occupational & Environmental Medicine, 5th Ed. New York: McGraw-Hill, 2014., p. 328
Marks - Marks JG, DeLeo VA. Contact and Occupational Dermatology, 2nd Ed. St. Louis: Mosby, 1997.
Kanerva - Rustemeyer L, Elsner P, John SM, Maibach HI (eds). Kanerva's Occupational Dermatology, 2nd Ed. Berlin: Springer-Verlag, 2012., p. 1802

13.1.7 Fire Potential

Fire hazard: slight, when exposed to heat or flame; can react with oxidizing materials.
Lewis, R.J. Sr. (ed) Sax's Dangerous Properties of Industrial Materials. 11th Edition. Wiley-Interscience, Wiley & Sons, Inc. Hoboken, NJ. 2004., p. 1993

13.1.8 Skin, Eye, and Respiratory Irritations

Exposure of the eyes to ...hydroquinone dust can cause ocular damage consisting of irritation, light sensitivity, lacrimation, injury to corneal epithelium, and corneal ulceration.
Sullivan, J.B., Krieger G.R. (eds). Clinical Environmental Health and Toxic Exposures. Second edition. Lippincott Williams and Wilkins, Philadelphia, Pennsylvania 1999., p. 1261

13.2 Safety and Hazard Properties

13.2.1 Flammable Limits

Flammability
Combustible Solid; dust cloud may explode if ignited in an enclosed area.

13.2.2 Critical Temperature & Pressure

Critical temperature: 549 °C; critical pressure: 7.45 MPa
Hudnall PM; Hydroquinone. Ullmann's Encyclopedia of Industrial Chemistry. 7th ed. (1999-2014). New York, NY: John Wiley & Sons. Online Posting Date: 15 Jun 2000

13.2.3 Physical Dangers

Dust explosion possible if in powder or granular form, mixed with air.

13.2.4 Explosive Limits and Potential

Potentially explosive reaction with oxygen at 90 °C/ 100 bar...Slight explosion hazard when exposed to heat.
Lewis, R.J. Sr. (ed) Sax's Dangerous Properties of Industrial Materials. 11th Edition. Wiley-Interscience, Wiley & Sons, Inc. Hoboken, NJ. 2004., p. 1993
Dust cloud may explode if ignited in an enclosed area.
U.S. Coast Guard, Department of Transportation. CHRIS - Hazardous Chemical Data. Volume II. Washington, D.C.: U.S. Government Printing Office, 1984-5.

13.2.5 OSHA Standards

Permissible Exposure Limit: Table Z-1 8-hr Time Weighted Avg: 2 mg/cu m.
29 CFR 1910.1000 (USDOL); U.S. National Archives and Records Administration's Electronic Code of Federal Regulations. Available from, as of September 8, 2014: https://www.ecfr.gov

13.2.6 NIOSH Recommendations

Recommended Exposure Limit: 15 Min Ceiling Value: 2 mg/cu m.
NIOSH. NIOSH Pocket Guide to Chemical Hazards. Department of Health & Human Services, Centers for Disease Control & Prevention. National Institute for Occupational Safety & Health. DHHS (NIOSH) Publication No. 2010-168 (2010). Available from: https://www.cdc.gov/niosh/npg

13.3 First Aid Measures

Inhalation First Aid
Fresh air, rest. Artificial respiration may be needed. Refer for medical attention.
Skin First Aid
Remove contaminated clothes. Rinse and then wash skin with water and soap.
Eye First Aid
First rinse with plenty of water for several minutes (remove contact lenses if easily possible), then refer for medical attention.
Ingestion First Aid
Rinse mouth. Induce vomiting (ONLY IN CONSCIOUS PERSONS!). Refer for medical attention .

13.3.1 First Aid

Signs and Symptoms of Acute Hydroquinone Exposure: Signs and symptoms of acute exposure to hydroquinone may be severe and include dyspnea (shortness of breath), a sense of suffocation, increased respiratory rate, and respiratory failure. Pallor (paleness of the skin), cyanosis (blue tint to skin and mucous membranes), and cardiovascular collapse may occur. Neurologic effects include headache, tinnitus (ringing in the ears), dizziness, delirium, muscle twitching, tremor, and convulsions. Nausea, vomiting, and the production of green to brown-green urine may also occur. Hydroquinone may be irritating and corrosive to the skin, eyes, and mucous membranes. Jaundice (yellow tint to skin) may be noticed.

Emergency Life-Support Procedures: Acute exposure to hydroquinone may require decontamination and life support for the victims. Emergency personnel should wear protective clothing appropriate to the type and degree of contamination. Air-purifying or supplied-air respiratory equipment should also be worn, as necessary. Rescue vehicles should carry supplies such as plastic sheeting and disposable plastic bags to assist in preventing spread of contamination.

Inhalation Exposure:

1. Move victims to fresh air. Emergency personnel should avoid self-exposure to hydroquinone.

2. Evaluate vital signs including pulse and respiratory rate, and note any trauma. If no pulse is detected, provide CPR. If not breathing, provide artificial respiration. If breathing is labored, administer oxygen or other respiratory support.

3. Obtain authorization and/or further instructions from the local hospital for administration of an antidote or performance of other invasive procedures.

4. Transport to a health care facility.

Dermal/Eye Exposure:

1. Remove victims from exposure. Emergency personnel should avoid self- exposure to hydroquinone.

2. Evaluate vital signs including pulse and respiratory rate, and note any trauma. If no pulse is detected, provide CPR. If not breathing, provide artificial respiration. If breathing is labored, administer oxygen or other respiratory support.

3. Remove contaminated clothing as soon as possible.

4. If eye exposure has occurred, eyes must be flushed with lukewarm water for at least 15 minutes.

5. Wash exposed skin areas twice with soap and water.

6. Obtain authorization and/or further instructions from the local hospital for administration of an antidote or performance of other invasive procedures.

7. Transport to a health care facility.

Ingestion Exposure:

1. Evaluate vital signs including pulse and respiratory rate, and note any trauma. If no pulse is detected, provide CPR. If not breathing, provide artificial respiration. If breathing is labored, administer oxygen.

2. IMMEDIATELY give the victims water or milk: children up to 1 year old, 125 mL (4 oz or 1/2 cup); children 1 to 12 years old, 200 mL (6 oz or 3/4 cup); adults, 250 mL (8 oz or 1 cup). Water or milk should be given only if victims are conscious and alert.

3. Obtain authorization and/or further instructions from the local hospital for administration of an antidote or performance of other invasive procedures.

4. Vomiting may be induced with syrup of Ipecac. If elapsed time since ingestion of hydroquinone is unknown or suspected to be greater than 30 minutes, do not induce vomiting and proceed to Step

5.Ipecac should not be administered to children under 6 months of age.Warning: Ingestion of hydroquinone may result in sudden onset of seizures or loss of consciousness. Syrup of Ipecac should be administered only if victims are alert, have an active gag-reflex, and show no signs of impending seizure or coma. If ANY uncertainty exists, proceed to Step

5.The following dosages of Ipecac are recommended: children up to 1 year old, 10 mL (1/3 oz); children 1 to 12 years old, 15 mL (1/2 oz); adults, 30 mL (1 oz). Ambulate (walk) the victims and give large quantities of water. If vomiting has not occurred after 15 minutes, Ipecac may be readministered. Continue to ambulate and give water to the victims. If vomiting has not occurred within 15 minutes after second administration of Ipecac, administer activated charcoal.

5. Activated charcoal may be administered if victims are conscious and alert. Use 15 to 30 g (1/2 to 1 oz) for children, 50 to 100 g (1-3/4 to 3-1/2 oz) for adults, with 125 to 250 mL (1/2 to 1 cup) of water.

6. Promote excretion by administering a saline cathartic or sorbitol to conscious and alert victims. Children require 15 to 30 g (1/2 to 1 oz) of cathartic; 50 to 100 g (1-3/4 to 3-1/2 oz) is recommended for adults.

7. Transport to a health care facility. (EPA, 1998)

U.S. Environmental Protection Agency. 1998. Extremely Hazardous Substances (EHS) Chemical Profiles and Emergency First Aid Guides. Washington, D.C.: U.S. Government Printing Office.

(See general first aid procedures)

Eye: Irrigate immediately - If this chemical contacts the eyes, immediately wash (irrigate) the eyes with large amounts of water, occasionally lifting the lower and upper lids. Get medical attention immediately.

Skin: Water flush - If this chemical contacts the skin, flush the contaminated skin with water. Where there is evidence of skin irritation, get medical attention.

Breathing: Respiratory support

Swallow: Medical attention immediately - If this chemical has been swallowed, get medical attention immediately.

13.4 Fire Fighting

Wear self-contained (positive pressure if available) breathing apparatus and full protective clothing.

For small fires use dry chemical, carbon dioxide, water spray or foam. Move container from fire area if you can do so without risk. This compound is a slight fire or explosion hazard. (EPA, 1998)

U.S. Environmental Protection Agency. 1998. Extremely Hazardous Substances (EHS) Chemical Profiles and Emergency First Aid Guides. Washington, D.C.: U.S. Government Printing Office.
Use water spray, powder, foam, carbon dioxide.

13.4.1 Fire Fighting Procedures

To fight fire, use water, carbon dioxide, dry chem ... .
Lewis, R.J. Sr. (ed) Sax's Dangerous Properties of Industrial Materials. 11th Edition. Wiley-Interscience, Wiley & Sons, Inc. Hoboken, NJ. 2004., p. 1993
If material on fire or involved in fire: Extinguish fire using agent suitable for type of surrounding fire (Material itself does not burn or burns with difficulty). Use water in flooding quantities as fog. Apply water from as far a distance as possible. Keep run-off water out of sewers and water sources.
Association of American Railroads; Bureau of Explosives. Emergency Handling of Hazardous Materials in Surface Transportation. Association of American Railroads, Pueblo, CO. 2005, p. 485
Suitable extinguishing media: Use water spray, alcohol-resistant foam, dry chemical or carbon dioxide.
Sigma-Aldrich; Material Safety Data Sheet for Hydroquinone. Product Number: H17902, Version 4.7 (Revision Date 06/13/2014). Available from, as of October 8, 2014: https://www.sigmaaldrich.com/safety-center.html
Advice for firefighters: Wear self contained breathing apparatus for fire fighting if necessary.
Sigma-Aldrich; Material Safety Data Sheet for Hydroquinone. Product Number: H17902, Version 4.7 (Revision Date 06/13/2014). Available from, as of October 8, 2014: https://www.sigmaaldrich.com/safety-center.html

13.4.2 Firefighting Hazards

Special hazards arising from the substance or mixture: Carbon oxides
Sigma-Aldrich; Material Safety Data Sheet for Hydroquinone. Product Number: H17902, Version 4.7 (Revision Date 06/13/2014). Available from, as of October 8, 2014: https://www.sigmaaldrich.com/safety-center.html

13.5 Accidental Release Measures

13.5.1 Isolation and Evacuation

Excerpt from ERG Guide 171 [Substances (Low to Moderate Hazard)]:

IMMEDIATE PRECAUTIONARY MEASURE: Isolate spill or leak area in all directions for at least 50 meters (150 feet) for liquids and at least 25 meters (75 feet) for solids.

SPILL: Increase the immediate precautionary measure distance, in the downwind direction, as necessary.

FIRE: If tank, rail tank car or highway tank is involved in a fire, ISOLATE for 800 meters (1/2 mile) in all directions; also, consider initial evacuation for 800 meters (1/2 mile) in all directions. (ERG, 2024)

13.5.2 Spillage Disposal

Personal protection: particulate filter respirator adapted to the airborne concentration of the substance. Do NOT let this chemical enter the environment. Sweep spilled substance into covered sealable containers. Carefully collect remainder. Then store and dispose of according to local regulations.

13.5.3 Cleanup Methods

Environmental Considerations: Land spill: Dig a pit, pond, lagoon, holding area to contain liquid or solid material. Cover solids with a plastic sheet to prevent dissolving in rain or fire fighting water. Dike surface flow using soil, sand bags, foamed polyurethane, or foamed concrete. Absorb bulk liquid with fly ash, cement powder, or commercial sorbents. /SRP: If time permits, pits, ponds, lagoons, soak holes, or holding areas should be sealed with an impermeable flexible membrane liner./
Association of American Railroads; Bureau of Explosives. Emergency Handling of Hazardous Materials in Surface Transportation. Association of American Railroads, Pueblo, CO. 2005, p. 501
Environmental Considerations: Water spill: Use natural barriers or oil spill control booms to limit spill travel. Use natural deep water pockets, excavated lagoons, or sand bag barriers to trap material at bottom. Remove trapped material with suction hoses.
Association of American Railroads; Bureau of Explosives. Emergency Handling of Hazardous Materials in Surface Transportation. Association of American Railroads, Pueblo, CO. 2005, p. 501
Environmental Considerations: Air spill: Apply water spray or mist to knock down vapors.
Association of American Railroads; Bureau of Explosives. Emergency Handling of Hazardous Materials in Surface Transportation. Association of American Railroads, Pueblo, CO. 2005, p. 501
Accidental Release Measures. Personal precautions, protective equipment and emergency procedures: Use personal protective equipment. Avoid dust formation. Avoid breathing vapours, mist or gas. Ensure adequate ventilation. Evacuate personnel to safe areas. Avoid breathing dust. Environmental precautions: Prevent further leakage or spillage if safe to do so. Do not let product enter drains. Discharge into the environment must be avoided. Methods and materials for containment and cleaning up: Pick up and arrange disposal without creating dust. Sweep up and shovel. Keep in suitable, closed containers for disposal.
Sigma-Aldrich; Material Safety Data Sheet for Hydroquinone. Product Number: H17902, Version 4.7 (Revision Date 06/13/2014). Available from, as of October 8, 2014: https://www.sigmaaldrich.com/safety-center.html

13.5.4 Disposal Methods

SRP: The most favorable course of action is to use an alternative chemical product with less inherent propensity for occupational harm/injury/toxicity or environmental contamination. Recycle any unused portion of the material for its approved use or return it to the manufacturer or supplier. Ultimate disposal of the chemical must consider: the material's impact on air quality; potential migration in soil or water; effects on animal and plant life; and conformance with environmental and public health regulations.
The following wastewater treatment technologies have been investigated for Hydroquinone: Concentration process: Reverse osmosis.
USEPA; Management of Hazardous Waste Leachate, EPA Contract No.68-03-2766 p.E-84 (1982)
The following wastewater treatment technologies have been investigated for Hydroquinone: Concentration process: Activated carbon.
USEPA; Management of Hazardous Waste Leachate, EPA Contract No.68-03-2766 p.F-146 (1982)
Hydroquinone is a waste chemical stream constituent which may be subjected to ultimate disposal by controlled incineration.
USEPA; Engineering Handbook for Hazardous Waste Incineration p.2-7 (1981) EPA 68-03-3025
For more Disposal Methods (Complete) data for HYDROQUINONE (6 total), please visit the HSDB record page.

13.5.5 Preventive Measures

Work clothing that becomes wet or significantly contaminated should be removed and replaced.
NIOSH. NIOSH Pocket Guide to Chemical Hazards. Department of Health & Human Services, Centers for Disease Control & Prevention. National Institute for Occupational Safety & Health. DHHS (NIOSH) Publication No. 2010-168 (2010). Available from: https://www.cdc.gov/niosh/npg
Workers whose clothing may have become contaminated should change into uncontaminated clothing before leaving the work premises.
NIOSH. NIOSH Pocket Guide to Chemical Hazards. Department of Health & Human Services, Centers for Disease Control & Prevention. National Institute for Occupational Safety & Health. DHHS (NIOSH) Publication No. 2010-168 (2010). Available from: https://www.cdc.gov/niosh/npg
The worker should immediately wash the skin when it becomes contaminated.
NIOSH. NIOSH Pocket Guide to Chemical Hazards. Department of Health & Human Services, Centers for Disease Control & Prevention. National Institute for Occupational Safety & Health. DHHS (NIOSH) Publication No. 2010-168 (2010). Available from: https://www.cdc.gov/niosh/npg
SRP: The scientific literature for the use of contact lenses by industrial workers is inconsistent. The benefits or detrimental effects of wearing contact lenses depend not only upon the substance, but also on factors including the form of the substance, characteristics and duration of the exposure, the uses of other eye protection equipment, and the hygiene of the lenses. However, there may be individual substances whose irritating or corrosive properties are such that the wearing of contact lenses would be harmful to the eye. In those specific cases, contact lenses should not be worn. In any event, the usual eye protection equipment should be worn even when contact lenses are in place.
For more Preventive Measures (Complete) data for HYDROQUINONE (11 total), please visit the HSDB record page.

13.6 Handling and Storage

13.6.1 Nonfire Spill Response

Excerpt from ERG Guide 171 [Substances (Low to Moderate Hazard)]:

Do not touch or walk through spilled material. Stop leak if you can do it without risk. Prevent dust cloud. For Asbestos, avoid inhalation of dust. Cover spill with plastic sheet or tarp to minimize spreading. Do not clean up or dispose of, except under supervision of a specialist.

SMALL DRY SPILL: With clean shovel, place material into clean, dry container and cover loosely; move containers from spill area.

SMALL SPILL: Pick up with sand or other non-combustible absorbent material and place into containers for later disposal.

LARGE SPILL: Dike far ahead of liquid spill for later disposal. Cover powder spill with plastic sheet or tarp to minimize spreading. Prevent entry into waterways, sewers, basements or confined areas. (ERG, 2024)

13.6.2 Safe Storage

Separated from strong bases and food and feedstuffs.

13.6.3 Storage Conditions

Keep well closed and protected from light.
O'Neil, M.J. (ed.). The Merck Index - An Encyclopedia of Chemicals, Drugs, and Biologicals. Cambridge, UK: Royal Society of Chemistry, 2013., p. 891
Plug tight and store in a place away from direct sunlight.
ITII. Toxic and Hazardous Industrial Chemicals Safety Manual. Tokyo, Japan: The International Technical Information Institute, 1988., p. 281
Conditions for safe storage, including any incompatibilities: Keep container tightly closed in a dry and well-ventilated place. Air and light sensitive.
Sigma-Aldrich; Material Safety Data Sheet for Hydroquinone. Product Number: H17902, Version 4.7 (Revision Date 06/13/2014). Available from, as of October 8, 2014: https://www.sigmaaldrich.com/safety-center.html

13.7 Exposure Control and Personal Protection

13.7.2 Permissible Exposure Limit (PEL)

2.0 [mg/m3]
PEL-TWA (8-Hour Time Weighted Average)
2 mg/m³
TWA 2 mg/m3

13.7.3 Immediately Dangerous to Life or Health (IDLH)

50 mg/m3 (NIOSH, 2024)

50.0 [mg/m3]

Excerpts from Documentation for IDLHs: Human data: It has been reported that 5 to 12 grams is the lethal oral dose [Zeidman and Deutel 1945]. [Note: An oral dose of 5 to 12 grams is equivalent to a worker being exposed to 3,333 to 8,000 mg/m3 for 30 minutes, assuming a breathing rate of 50 liters per minute and 100% absorption.]

50 mg/cu m.
NIOSH. NIOSH Pocket Guide to Chemical Hazards. Department of Health & Human Services, Centers for Disease Control & Prevention. National Institute for Occupational Safety & Health. DHHS (NIOSH) Publication No. 2010-168 (2010). Available from: https://www.cdc.gov/niosh/npg
50 mg/m³

50 mg/m3

See: 123319

13.7.4 Threshold Limit Values (TLV)

1.0 [mg/m3]
8 hr Time Weighted Avg (TWA): 1 mg/cu m, dermal sensitization.
American Conference of Governmental Industrial Hygienists. Threshold Limit Values for Chemical Substances and Physical Agents and Biological Exposure Indices. ACGIH, Cincinnati, OH 2014, p. 35
Excursion Limit Recommendation: Excursions in worker exposure levels may exceed 3 times the TLV-TWA for no more than a total of 30 minutes during a work day, and under no circumstances should they exceed 5 times the TLV-TWA, provided that the TLV-TWA is not exceeded.
American Conference of Governmental Industrial Hygienists. Threshold Limit Values for Chemical Substances and Physical Agents and Biological Exposure Indices. ACGIH, Cincinnati, OH 2014, p. 5
A3; Confirmed animal carcinogen with unknown relevance to humans.
American Conference of Governmental Industrial Hygienists. Threshold Limit Values for Chemical Substances and Physical Agents and Biological Exposure Indices. ACGIH, Cincinnati, OH 2014, p. 35
1 mg/m
TLV-TWA (Time Weighted Average)
1 mg/m³ [2007]

13.7.5 Occupational Exposure Limits (OEL)

MAK (Maximale Arbeitsplatz Konzentration)
skin absorption (H); sensitization of skin (SH); carcinogen category: 2; germ cell mutagen group: 3A

13.7.6 Inhalation Risk

A harmful contamination of the air will not or will only very slowly be reached on evaporation of this substance at 20 °C.

13.7.7 Effects of Short Term Exposure

The substance is severely irritating to the eyes. The substance is irritating to the skin and respiratory tract.

13.7.8 Effects of Long Term Exposure

Repeated or prolonged contact with skin may cause dermatitis. Repeated or prolonged contact may cause skin sensitization. The substance may have effects on the eyes and skin. This may result in discolouration of the conjunctiva and cornea and skin depigmentation. This substance is possibly carcinogenic to humans.

13.7.9 Personal Protective Equipment (PPE)

Excerpt from NIOSH Pocket Guide for Hydroquinone:

Skin: PREVENT SKIN CONTACT - Wear appropriate personal protective clothing to prevent skin contact.

Eyes: PREVENT EYE CONTACT - Wear appropriate eye protection to prevent eye contact.

Wash skin: WHEN CONTAMINATED - The worker should immediately wash the skin when it becomes contaminated.

Remove: WHEN WET OR CONTAMINATED - Work clothing that becomes wet or significantly contaminated should be removed and replaced.

Change: DAILY - Workers whose clothing may have become contaminated should change into uncontaminated clothing before leaving the work premises.

Provide: EYEWASH (>7%) - Eyewash fountains should be provided in areas where there is any possibility that workers could be exposed to the substances; this is irrespective of the recommendation involving the wearing of eye protection. (>7%) (NIOSH, 2024)

Wear appropriate personal protective clothing to prevent skin contact.
NIOSH. NIOSH Pocket Guide to Chemical Hazards. Department of Health & Human Services, Centers for Disease Control & Prevention. National Institute for Occupational Safety & Health. DHHS (NIOSH) Publication No. 2010-168 (2010). Available from: https://www.cdc.gov/niosh/npg
Wear appropriate eye protection to prevent eye contact.
NIOSH. NIOSH Pocket Guide to Chemical Hazards. Department of Health & Human Services, Centers for Disease Control & Prevention. National Institute for Occupational Safety & Health. DHHS (NIOSH) Publication No. 2010-168 (2010). Available from: https://www.cdc.gov/niosh/npg
Eyewash fountains should be provided in areas where there is any possbility that workers could be exposed to the substance; this is irrespective of the recommendation involving the wearing of eye protection. (>7%)
NIOSH. NIOSH Pocket Guide to Chemical Hazards. Department of Health & Human Services, Centers for Disease Control & Prevention. National Institute for Occupational Safety & Health. DHHS (NIOSH) Publication No. 2010-168 (2010). Available from: https://www.cdc.gov/niosh/npg
Respirator Recommendations: Up to 50 mg/cu m
Assigned Protection Factor (APF)
APF = 25
Respirator Recommendation
Any powered air-purifying respirator with a high-efficiency particulate filter. Substance causes eye irritation or damage; eye protection needed.
Assigned Protection Factor (APF)
APF = 50
Respirator Recommendation
Any air-purifying, full-facepiece respirator with an N100, R100, or P100 filter.
Assigned Protection Factor (APF)
APF = 50
Respirator Recommendation
Any supplied-air respirator that has a tight-fitting facepiece and is operated in a continuous-flow mode. Substance causes eye irritation or damage; eye protection needed.
Assigned Protection Factor (APF)
APF = 50
Respirator Recommendation
Any self-contained breathing apparatus with a full facepiece.
Assigned Protection Factor (APF)
APF = 50
Respirator Recommendation
Any supplied-air respirator with a full facepiece.
NIOSH. NIOSH Pocket Guide to Chemical Hazards. Department of Health & Human Services, Centers for Disease Control & Prevention. National Institute for Occupational Safety & Health. DHHS (NIOSH) Publication No. 2010-168 (2010). Available from: https://www.cdc.gov/niosh/npg
For more Personal Protective Equipment (PPE) (Complete) data for HYDROQUINONE (11 total), please visit the HSDB record page.

(See personal protection and sanitation codes)

Skin: Prevent skin contact - Wear appropriate personal protective clothing to prevent skin contact.

Eyes: Prevent eye contact - Wear appropriate eye protection to prevent eye contact.

Wash skin: When contaminated

Remove: When wet or contaminated

Change: Daily - Workers whose clothing may have become contaminated should change into uncontaminated clothing before leaving the work premises.

Provide: Eyewash (>7%)

13.7.10 Respirator Recommendations

NIOSH/OSHA

Up to 50 mg/m3 :

(APF = 25) Any powered, air-purifying respirator with a high-efficiency particulate filter./

(APF = 50) Any air-purifying, full-facepiece respirator with an N100, R100, or P100 filter.

Click here for information on selection of N, R, or P filters.

(APF = 50) Any supplied-air respirator that has a tight-fitting facepiece and is operated in a continuous-flow mode

(APF = 50) Any self-contained breathing apparatus with a full facepiece

(APF = 50) Any supplied-air respirator with a full facepiece

Emergency or planned entry into unknown concentrations or IDLH conditions:

(APF = 10,000) Any self-contained breathing apparatus that has a full facepiece and is operated in a pressure-demand or other positive-pressure mode

(APF = 10,000) Any supplied-air respirator that has a full facepiece and is operated in a pressure-demand or other positive-pressure mode in combination with an auxiliary self-contained positive-pressure breathing apparatus

Escape:

(APF = 50) Any air-purifying, full-facepiece respirator with an N100, R100, or P100 filter.

Click here for information on selection of N, R, or P filters.

Any appropriate escape-type, self-contained breathing apparatus

Important additional information about respirator selection

13.7.11 Preventions

Fire Prevention
NO open flames. Closed system, dust explosion-proof electrical equipment and lighting. Prevent deposition of dust.
Exposure Prevention
PREVENT DISPERSION OF DUST! AVOID ALL CONTACT!
Inhalation Prevention
Use local exhaust or breathing protection.
Skin Prevention
Protective gloves. Protective clothing.
Eye Prevention
Wear safety goggles.
Ingestion Prevention
Do not eat, drink, or smoke during work. Wash hands before eating.

13.8 Stability and Reactivity

13.8.1 Air and Water Reactions

Darkens on exposure to air and light. Miscible in water. Solutions become brown in air due to oxidation. Oxidation is very rapid in the presence of alkali.

13.8.2 Reactive Group

Phenols and Cresols

Acids, Weak

13.8.3 Reactivity Alerts

13.8.3.1 CSL Reaction Information
1 of 2
CSL No
Reactants/Reagents
ACROLEIN + HYDROQUINONE
Warning Message
Warning - This reagent combination was observed internally to build significant pressure, resulting in the failure of a sealed tube apparatus. Please consider using a solvent such as Xylenes or running the reaction at lower temperature.
GHS Category
Gas Under Pressure
Functional Group
ALDEHYDE
Reference Source
User-Reported
Modified Date
7/8/18
Create Date
2/13/17
2 of 2
CSL No
Reactants/Reagents
HYDROQUINONE + METHYL ACRYLATE
Warning Message
Warning - This reagent combination was observed internally to build significant pressure, resulting in the failure of a sealed tube apparatus. Please consider using a solvent such as Xylenes or running the reaction at lower temperature.
GHS Category
Gas Under Pressure
Reference Source
User-Reported
Modified Date
7/8/18
Create Date
2/13/17

13.8.4 Reactivity Profile

HYDROQUINONE is a slight explosion hazard when exposed to heat. Incompatible with strong oxidizing agents. Also incompatible with bases. It reacts with oxygen and sodium hydroxide. Reacts with ferric salts (NTP, 1992). Hot and/or concentrated NaOH can cause hydroquinone to decompose exothermically at elevated temperature. (NFPA Pub. 491M, 1975, 385)
National Toxicology Program, Institute of Environmental Health Sciences, National Institutes of Health (NTP). 1992. National Toxicology Program Chemical Repository Database. Research Triangle Park, North Carolina.

13.8.5 Hazardous Reactivities and Incompatibilities

Strong oxidizers, alkalis.
NIOSH. NIOSH Pocket Guide to Chemical Hazards. Department of Health & Human Services, Centers for Disease Control & Prevention. National Institute for Occupational Safety & Health. DHHS (NIOSH) Publication No. 2010-168 (2010). Available from: https://www.cdc.gov/niosh/npg
Accidental mixing of the hot crude hydroquinone with concn sodium hydroxide solution led to extensive exothermic decomposition.
Bretherick, L. Handbook of Reactive Chemical Hazards. 4th ed. Boston, MA: Butterworth-Heinemann Ltd., 1990, p. 623
Violent reaction with NaOH.
Lewis, R.J. Sr. (ed) Sax's Dangerous Properties of Industrial Materials. 11th Edition. Wiley-Interscience, Wiley & Sons, Inc. Hoboken, NJ. 2004., p. 1993
Incompatible materials: Strong bases, strong oxidizing agents
Sigma-Aldrich; Material Safety Data Sheet for Hydroquinone. Product Number: H17902, Version 4.7 (Revision Date 06/13/2014). Available from, as of October 8, 2014: https://www.sigmaaldrich.com/safety-center.html
Crude hydroquinone was pumped into a sodium hydroxide storage tank by mistake. The hyrdoquinone liquor at 85 °C decomposed rapidly in the presence of the sodium hydroxide resulting in overflow of the tank and evolution of a considerable amount of heat.
National Fire Protection Association; Fire Protection Guide to Hazardous Materials. 14TH Edition, Quincy, MA 2010, p. 491-180

13.9 Transport Information

13.9.1 DOT Emergency Guidelines

/GUIDE 153: SUBSTANCES - TOXIC and/or CORROSIVE (Combustible)/ Fire or Explosion: Combustible material: may burn but does not ignite readily. When heated, vapors may form explosive mixtures with air: indoors, outdoors and sewers explosion hazards. Those substances designated with a (P) may polymerize explosively when heated or involved in a fire. Contact with metals may evolve flammable hydrogen gas. Containers may explode when heated. Runoff may pollute waterways. Substance may be transported in a molten form.
U.S. Department of Transportation. 2012 Emergency Response Guidebook. Washington, D.C. 2012
/GUIDE 153: SUBSTANCES - TOXIC and/or CORROSIVE (Combustible)/ Health: TOXIC; inhalation, ingestion or skin contact with material may cause severe injury or death. Contact with molten substance may cause severe burns to skin and eyes. Avoid any skin contact. Effects of contact or inhalation may be delayed. Fire may produce irritating, corrosive and/or toxic gases. Runoff from fire control or dilution water may be corrosive and/or toxic and cause pollution.
U.S. Department of Transportation. 2012 Emergency Response Guidebook. Washington, D.C. 2012
/GUIDE 153: SUBSTANCES - TOXIC and/or CORROSIVE (Combustible)/ Public Safety: CALL Emergency Response Telephone Number on Shipping Paper first. If Shipping Paper not available or no answer, refer to appropriate telephone number listed on the inside back cover. As an immediate precautionary measure, isolate spill or leak area in all directions for at least 50 meters (150 feet) for liquids and at least 25 meters (75 feet) for solids. Keep unauthorized personnel away. Stay upwind. Keep out of low areas. Ventilate enclosed areas.
U.S. Department of Transportation. 2012 Emergency Response Guidebook. Washington, D.C. 2012
/GUIDE 153: SUBSTANCES - TOXIC and/or CORROSIVE (Combustible)/ Protective Clothing: Wear positive pressure self-contained breathing apparatus (SCBA). Wear chemical protective clothing that is specifically recommended by the manufacturer. It may provide little or no thermal protection. Structural firefighters' protective clothing provides limited protection in fire situations ONLY; it is not effective in spill situations where direct contact with the substance is possible.
U.S. Department of Transportation. 2012 Emergency Response Guidebook. Washington, D.C. 2012
For more DOT Emergency Guidelines (Complete) data for HYDROQUINONE (8 total), please visit the HSDB record page.

13.9.2 DOT ID and Guide

13.9.3 Shipping Name / Number DOT/UN/NA/IMO

UN 2662; Hydroquinone

13.9.4 Shipment Methods and Regulations

No person may /transport,/ offer or accept a hazardous material for transportation in commerce unless that person is registered in conformance ... and the hazardous material is properly classed, described, packaged, marked, labeled, and in condition for shipment as required or authorized by ... /the hazardous materials regulations (49 CFR 171-177)./
49 CFR 171.2 (USDOT); U.S. National Archives and Records Administration's Electronic Code of Federal Regulations. Available from, as of October 9, 2014: https://www.ecfr.gov

13.9.5 DOT Label

Class 9 Poison

13.9.6 Packaging and Labelling

Do not transport with food and feedstuffs.

13.9.7 EC Classification

Symbol: Xn, N; R: 22-40-41-43-50-68; S: (2)-26-36/37/39-61

13.9.8 UN Classification

UN Hazard Class: 6.1; UN Pack Group: III

13.10 Regulatory Information

The Australian Inventory of Industrial Chemicals
Chemical: 1,4-Benzenediol
California Safe Cosmetics Program (CSCP) Reportable Ingredient

Hazard Traits - Carcinogenicity

Authoritative List - CA TACs

Report - if used as a fragrance or flavor ingredient

REACH Registered Substance
New Zealand EPA Inventory of Chemical Status
Hydroquinone: HSNO Approval: HSR003003 Approved with controls

13.10.1 Atmospheric Standards

This action promulgates standards of performance for equipment leaks of Volatile Organic Compounds (VOC) in the Synthetic Organic Chemical Manufacturing Industry (SOCMI). The intended effect of these standards is to require all newly constructed, modified, and reconstructed SOCMI process units to use the best demonstrated system of continuous emission reduction for equipment leaks of VOC, considering costs, non air quality health and environmental impact and energy requirements. Hydroquinone is produced, as an intermediate or final product, by process units covered under this subpart.
40 CFR 60.489 (USEPA); U.S. National Archives and Records Administration's Electronic Code of Federal Regulations. Available from, as of September 8, 2014: https://www.ecfr.gov
Listed as a hazardous air pollutant (HAP) generally known or suspected to cause serious health problems. The Clean Air Act, as amended in 1990, directs EPA to set standards requiring major sources to sharply reduce routine emissions of toxic pollutants. EPA is required to establish and phase in specific performance based standards for all air emission sources that emit one or more of the listed pollutants. Hydroquinone is included on this list.
Clean Air Act as amended in 1990, Sect. 112 (b) (1) Public Law 101-549 Nov. 15, 1990

13.10.2 State Drinking Water Guidelines

(FL) FLORIDA 280 ug/L
USEPA/Office of Water; Federal-State Toxicology and Risk Analysis Committee (FSTRAC). Summary of State and Federal Drinking Water Standards and Guidelines (11/93) To Present

13.10.3 CERCLA Reportable Quantities

Persons in charge of vessels or facilities are required to notify the National Response Center (NRC) immediately, when there is a release of this designated hazardous substance, in an amount equal to or greater than its reportable quantity of 100 lb or 45.4 kg. The toll free number of the NRC is (800) 424-8802. The rule for determining when notification is required is stated in 40 CFR 302.4 (section IV. D.3.b).
40 CFR 302.4 (USEPA); U.S. National Archives and Records Administration's Electronic Code of Federal Regulations. Available from, as of September 8, 2014: https://www.ecfr.gov
Releases of CERCLA hazardous substances are subject to the release reporting requirement of CERCLA section 103, codified at 40 CFR part 302, in addition to the requirements of 40 CFR part 355. Hydroquinone is an extremely hazardous substance (EHS) subject to reporting requirements when stored in amounts in excess of its threshold planning quantity (TPQ) of 500 or 10,000 lbs. Extremely hazardous substances that are solids are subject to either of two threshold planning quantities ... The lower quantity applies only if the solid exists in powdered form and has a particle size less than 100 microns; or is handled in solution or in molten form; or meets the criteria for a National Fire Protection Association (NFPA) rating of 2, 3 or 4 for reactivity. If the solid does not meet any of these criteria, it is subject to the upper ... threshold planning quantity ... .
40 CFR 355 (USEPA); U.S. National Archives and Records Administration's Electronic Code of Federal Regulations. Available from, as of September 8, 2014: https://www.ecfr.gov

13.10.4 TSCA Requirements

Pursuant to section 8(d) of TSCA, EPA promulgated a model Health and Safety Data Reporting Rule. The section 8(d) model rule requires manufacturers, importers, and processors of listed chemical substances and mixtures to submit to EPA copies and lists of unpublished health and safety studies. Hydroquinone is included on this list. Effective date 10/04/84; Sunset date: 10/04/94.
40 CFR 716.120 (USEPA); U.S. National Archives and Records Administration's Electronic Code of Federal Regulations. Available from, as of September 8, 2014: https://www.ecfr.gov

13.10.5 FDA Requirements

Hydroquinone is an indirect food additive for use only as a component of adhesives.
21 CFR 175.105 (USFDA); U.S. National Archives and Records Administration's Electronic Code of Federal Regulations. Available from, as of September 8, 2014: https://www.ecfr.gov

13.11 Other Safety Information

Chemical Assessment

IMAP assessments - 1,4-Benzenediol: Human health tier II assessment

Evaluation - Hydroquinone and p-benzoquinone

13.11.1 History and Incidents

Five hundred forty-four (544) crewmen aboard a large usa navy vessel developed gastrointestinal disease characterized by acute onset of nausea, vomiting, abdominal cramps, & diarrhea which was found to be due to hydroquinone contamination of the chilled water system by automatic photo developing machines on the ship.
HOOPER RR ET AL; MORB MORTAL WKLY REP 27 (28): 237 (1978)
14 cases of medically-diagnosed hydroquinone-associated ochronosis from skin lightening cream use have been reported in the United States between the years 1976 and 1992. The incidence of ochronosis among U.S. users of skin lightening creams is significantly different from that of South African users. The reasons for this difference are complex but they appear to be principally related to the use pattern, which is heavier among South Africans, and the differing compositions of the available skin lighteners. During the time when large numbers of cases of ochronosis were being reported in South Africa, the products used contained 5-8% hydroquinone plus phenol and resorcinol. The lower concentrations of hydroquinone (1.5-2%) used in skin lighteners in the United States has only rarely been associated with ochronosis.
European Commission, ESIS; IUCLID Dataset, hydroquinone (123-31-9) p 128 (2000 CD-ROM edition). Available from, as of March, 4 2009: https://esis.jrc.ec.europa.eu/

13.11.2 Special Reports

J Am Coll Toxicol 5 (3): 123-65 (1986). Final Report on the Safety Assessment of Hydroquinone and Pyrocatechol. A review of physiochemical properties and cosmetic application as couplers in oxidase hair dyes of hydroquinone and pyrocatechol.
Santodonato J et al; Monograph on Human Exposure to Chemicals in the Workplace: Hydroquinone Center for Chemical Hazard Assessment, Syracuse Research Corporation, Syracuse, New York, Report No. SRC TR 84-1042, 32 pp (1985). This report summarized and evaluated toxicologic information relevant to an occupational hazard assessment of hydroquinone including chemical and physical properties, production, and use, extent of occupational exposure, pharmacokinetics, animal carcinogenicity and mutagenicity, and epidemiological studies.
DHHS/NTP; Toxicology and Carcinogenesis Studies of Hydroquinone (Gavage Studies) p.4-5 Technical Report No. 366 (1989) NIH Pub #90-2812 U.S. Department of Health and Human Services, National Toxicology Program, National Institute of Environmental Health Sciences, Research Triangle Park, NC 27709[Available from, as of April 4, 2003: http://ntp.niehs.nih.gov/ntp/htdocs/LT_rpts/tr366.pdf]
Organization for Economic Cooperation and Development; Screening Information Data Set for Hydroquinone (123-31-9) (June 2002).[Available from, as of March 9, 2009: http://www.chem.unep.ch/irptc/sids/OECDSIDS/sidspub.html]
European Commission, ESIS; IUCLID Dataset, hydroquinone (123-31-9) (2000 CD-ROM edition).[Available from, as of March 4, 2009: http://esis.jrc.ec.europa.eu/]

14 Toxicity

14.1 Toxicological Information

14.1.1 Toxicity Summary

IDENTIFICATION AND USE: Hydroquinone (HQ) is an aromatic compound in the form of light tan to gray crystals. It is a high-volume commodity chemical used as a reducing agent, antioxidant, polymerization inhibitor, chemical stabilizer, chemical intermediate, and photographic reducer and developer. It is also used in skin lighteners, in cosmetics, hair dye, glue, and a medication to treat dyschromias. HUMAN EXPOSURE: A great deal of research has been conducted with HQ because it is a metabolite of benzene. In workers engaged in the manufacture, HQ dust oxidizes to brown benzoquinone. This material causes pigmentation of the eye and, in some cases, permanent corneal damage. There are reported cases of keratitis and discoloration of the conjunctiva among men exposed to concentrations ranging from 10 to 30 mg of vapor or dust of HQ per cubic meter of air. Ingestion of 1 g by an adult has caused dizziness, sense of suffocation, increased rate of respiration, vomiting, pallor, muscular twitching, headache, dyspnea, cyanosis and collapse and eventually death due to respiratory failure. Upon ingestion urine is green or brownish-green in color and continues to darken on standing. Five hundred forty-four (544) crewmen aboard a large USA Navy vessel developed GI disease characterized by acute onset of nausea, vomiting, abdominal cramps, and diarrhea which was found to be due to hydroquinone contamination of the chilled water system by automatic photo developing machines on the ship. HQ is only weakly positive in in vivo chromosomal assays when expected human exposure routes are used. Chromosomal effects are increased significantly when parenteral or in vitro assays are used. Hydroquinone impairs several leukocyte cell functions, which alter the immune response. It evokes pro-inflammatory properties in endothelial cells that are triggered by the enhancement of NF-kappaB nuclear translocation-dependent gene transcription. Parenteral administration of HQ is associated with changes in several hematopoietic and immunologic endpoints. This toxicity is more severe when combined with parenteral administration of phenol. It is likely that oxidation of HQ within the bone marrow compartment to the semiquinone or p-benzoquinone (BQ), followed by covalent macromolecular binding, and is critical to these effects. Bone marrow and hematologic effects are generally not characteristic of HQ exposures in animal studies employing routes of exposure other than parenteral. Enhanced Ras signaling increases both hydroquinone-mediated growth inhibition in yeast and genotoxicity in mammalian hematopoietic suggesting that HQ toxicity is modulated by Ras signaling and individuals with abnormal Ras signaling could be more vulnerable to developing myeloid diseases after exposure. Hydroquinone also increases proliferation of CFU-GM progenitor cells in mice with Nf1 null bone marrow relative to WT, the same cell type associated with benzene-associated leukemia. It is confirmed animal carcinogen with unknown relevance to humans. ANIMAL STUDIES: In cancer bioassays, HQ has reproducibly produced renal adenomas in male rats. The mechanism of tumorigenesis is unclear but probably involves a species-, strain-, and sex-specific interaction between renal tubule toxicity and an interaction with the chronic progressive nephropathy that is characteristic of aged male rats. Mouse liver tumors (adenomas) and mononuclear cell leukemia (female rat) have also been reported following HQ exposure, but their significance is uncertain. Various tumor initiation/promotion assays with HQ have shown generally negative results. In two-year studies in rats of each sex given hydroquinone in deionized water by gavage, nearly all male rats and most female rats in all vehicle control and dosed groups had nephropathy. The severity of this disease was judged to be greater in high dose male rats. The data on the effect of HQ on development are conflicting, with several studies reporting minimal to no treatment -related effects on duration of gestation, mean litter size, fetal viability, or lactation index in rats fed diets containing HQ. However, one study reports that female rats fed 0.5 g of hydroquinone in their diet during pregnancy had higher rates of fetal resorption than controls (100% versus 41% of the dams), and a greater number of the total implantations were resorbed (27% versus 11%). HQ was not mutagenic in Salmonella typhimurium strains TA98, TA100, TA1535, or TA1537 with or without exogenous metabolic activation. It induced trifluorothymidine resistance in mouse L5178Y/TK lymphoma cells in the presence or absence of metabolic activation, and induced sister chromatid exchanges in Chinese hamster ovary cells both with or without exogenous metabolic activation and caused chromosomal aberrations in the presence of activation. HQ also induced aneuploidy in yeast by delaying the cell cycle at the G2/M transition.

14.1.2 EPA IRIS Information

Substance
Toxicity Summary
EPA IRIS Summary PDF (Update: Oct-01-1990 )

14.1.3 EPA Provisional Peer-Reviewed Toxicity Values

Chemical Substance
Reference Dose (RfD), Chronic
4 x 10^-2 mg/kg-day
Reference Dose (RfD), Subchronic
4 x 10^-1 mg/kg-day
PPRTV Assessment
Weight-Of-Evidence (WOE)
Likely to be carcinogenic to humans
Last Revision
2009

14.1.4 RAIS Toxicity Values

Oral Chronic Reference Dose (RfDoc) (mg/kg-day)
0.04
Oral Chronic Reference Dose Reference
PPRTV Current
Oral Subchronic Chronic Reference Dose (RfDos) (mg/kg-day)
0.4
Oral Subchronic Chronic Reference Dose Reference
PPRTV Current
Oral Slope Factor (CSFo)(mg/kg-day)^-1
0.06
Oral Slope Factor Reference
PPRTV Current

14.1.5 NIOSH Toxicity Data

14.1.6 Evidence for Carcinogenicity

Evaluation: There is inadequate evidence in humans for the carcinogenicity of hydroquinone. There is limited evidence in experimental animals for the carcinogenicity of hydroquinone. Overall evaluation: Hydroquinone is not classifiable as to its carcinogenicity to humans (Group 3).
IARC. Monographs on the Evaluation of the Carcinogenic Risk of Chemicals to Humans. Geneva: World Health Organization, International Agency for Research on Cancer, 1972-PRESENT. (Multivolume work). Available at: https://monographs.iarc.fr/ENG/Classification/index.php, p. V71 711 (1999)
A3; Confirmed animal carcinogen with unknown relevance to humans.
American Conference of Governmental Industrial Hygienists. Threshold Limit Values for Chemical Substances and Physical Agents and Biological Exposure Indices. ACGIH, Cincinnati, OH 2014, p. 35

14.1.7 Carcinogen Classification

1 of 3
IARC Carcinogenic Agent
Hydroquinone
IARC Carcinogenic Classes
Group 3: Not classifiable as to its carcinogenicity to humans
IARC Monographs

Volume 15: (1977) Some Fumigants, the Herbicides 2,4-D and 2,4,5-T, Chlorinated Dibenzodioxins and Miscellaneous Industrial Chemicals

Volume Sup 7: Overall Evaluations of Carcinogenicity: An Updating of IARC Monographs Volumes 1 to 42, 1987; 440 pages; ISBN 92-832-1411-0 (out of print)

Volume 71: (1999) Re-evaluation of Some Organic Chemicals, Hydrazine and Hydrogen Peroxide (Part 1, Part 2, Part 3)

2 of 3
Substance
NTP Technical Report
TR-366: Toxicology and Carcinogenesis Studies of Hydroquinone (CASRN 123-31-9) in F344/N Rats and B6C3F1 Mice (Gavage Studies) (1989 )
Peer Review Date
Conclusion for Male Rat
Some Evidence Some Evidence
Conclusion for Female Rat
Some Evidence Some Evidence
Conclusion for Male Mice
No Evidence No Evidence
Conclusion for Female Mice
Some Evidence Some Evidence
Summary

Under the conditions of these 2-year gavage studies, there was some evidence of carcinogenic activity of hydroquinone for male F344/N rats, as shown by marked increases in tubular cell adenomas of the kidney. There was some evidence of carcinogenic activity of hydroquinone for female F344/N rats, as shown by increases in mononuclear cell leukemia. There was no evidence of carcinogenic activity of hydroquinone for male B6C3F1 mice administered 50 or 100 mg/kg in water by gavage. There was some evidence of carcinogenic activity of hydroquinone for female B6C3F1 mice, as shown by increases in hepatocellular neoplasms, mainly adenomas.

Administration of hydroquinone was associated with thyroid follicular cell hyperplasia in both male and female mice and anisokaryosis, multinucleated hepatocytes, and basophilic foci of the liver in male mice.

3 of 3
Carcinogen Classification
3, not classifiable as to its carcinogenicity to humans. (L135)

14.1.8 Effects During Pregnancy and Lactation

◉ Summary of Use during Lactation

Topical hydroquinone has not been studied during breastfeeding. Although hydroquinone is not contraindicated during breastfeeding, some experts feel that long-term use of hydroquinone is difficult to justify in a nursing mother. If hydroquinone is used, ensure that the infant's skin does not come into direct contact with the areas of maternal skin that have been treated and the infant does not ingest the product from the mother's skin.

◉ Effects in Breastfed Infants

Relevant published information was not found as of the revision date.

◉ Effects on Lactation and Breastmilk

Relevant published information was not found as of the revision date.

14.1.9 Exposure Routes

The substance can be absorbed into the body by inhalation, through the skin and by ingestion.
inhalation, ingestion, skin and/or eye contact

14.1.10 Symptoms

Inhalation Exposure
Cough. Laboured breathing.
Skin Exposure
Redness.
Eye Exposure
Redness. Pain. Blurred vision.
Ingestion Exposure
Dizziness. Headache. Nausea. Shortness of breath. Convulsions. Vomiting. Ringing in the ears.
irritation eyes: conjunctivitis; keratitis (inflammation of the cornea); central nervous system excitement; colored urine, nausea, dizziness, suffocation, rapid breath; muscle twitching, delirium; collapse; skin irritation, sensitization, dermatitis

14.1.11 Target Organs

Eyes, skin, respiratory system, central nervous system

14.1.12 Adverse Effects

Methemoglobinemia - The presence of increased methemoglobin in the blood; the compound is classified as secondary toxic effect

Skin Sensitizer - An agent that can induce an allergic reaction in the skin.

ACGIH Carcinogen - Confirmed Animal.

14.1.13 Acute Effects

14.1.14 Interactions

Possible interactions between hydroquinone and phenol, two known benzene metabolites, in inducing micronuclei in mouse bone marrow cells were investigated. Hydroquinone and phenol administered alone gave weak and negative results, at the doses tested. However, simultaneous administration of both compounds caused a considerable increase in the induction of micronuclei as well as an increase in bone marrow toxicity. Using 3 different statistical methods, it was shown that the observed joint effect was significantly higher than additive interaction, and was close to multiplicative interaction. These findings bring further support to the hypothesis that the toxic and genotoxic effects of benzene are produced by several metabolites acting synergistically.
Barale R et al; Mutat Res 244 (1): 15-20 (1990)
Phenol, when administered together with (14)C hydroquinone, significantly stimulated the covalent binding of (14)C hydroquinone oxidation products to blood (p< 0.001) and bone marrow (p< 0.05) macromolecules, but had no significant effect on covalent binding of (14)C hydroquinone oxidation products to liver and kidney macromolecules (p> 0.05). Catechol, on the other hand, had no effect on the binding of (14)C hydroquinone oxidation products in either bone marrow, kidney or liver (p> 0.05). When hydroquinone was administered together with (14)C phenol, a stimulation of the covalent binding of phenol oxidation products to bone marrow macromolecules also occurred (p< 0.05).
Subrahmanyam VV et al; Toxicol 62 (1): 107-16 (1990)
Modifying effects of catechol resorcinol and hydroquinone on second stage hepato- and renal carcinogenesis was investigated in rats pretreated with N-ethyl-N-hydroxyethyl-nitrosamine (EHEN). Groups of twenty 6-week-old Wistar/Crj male rats were treated with 0.1% N-ethyl-N-hydroxyethyl-nitrosamine in the drinking water for 3 weeks. Starting 1 week after the termination of N-ethyl-N-hydroxyethyl-N-nitrosamine treatment they were given a diet containing 0.8% catechol, 0.8% resorcinol or 0.8% hydroquinone or basal diet for 36 weeks. Further groups of 15 rats were each treated with the same doses of phenolic compounds or basal diet alone without N-ethyl-N-hydroxyethyl-N-nitrosamine pretreatment. ... At the end of week 40 histopathological assessment revealed significant reduction of the numbers per rat of hepatocellular adenonas and hepatocellular carcinomas by resorcinol whereas hydroquinone significantly enhanced the numbers per rat of renal microadenomas and renal cell tumors. On the other hand the number of alpha 2u-globulin positive tubules in the animals treated with hydroquinone was significantly lower than controls without any alteration in bromodeoxyuridine incorporation. Lipid peroxidation as evaluated by thiobarbituric acid reactive substance was at control levels in the kidneys of rats treated with hydroquinone throughout the experiment. The results showed that the known renal carcinogen hydroquinone potently enhances the second stage of N-ethyl-N-hydroxyethyl-N-nitrosamine-induced renal carcinogenesis, while its isomer resorcinol inhibited hepatocarcinogenesis.
Okazaki S et al; Teratog Carcinog Mutagen 13 (3): 127-37 (1993)
The effects of diethylnitrosamine on hydroquinone induced hepatic enzyme altered foci were studied. Male Sprague-Dawley-rats were injected with 40 mg/kg diethylnitrosamine followed 24 to 48 hr later by a single dose by gavage of hydroquinone. The presence of foci positive for glutathione-S-transferase-P activity was determined after 1 week by immunohistochemistry. Hepatocytes isolated from rats that had been treated with diethylnitrosamine, hydroquinone, and phenobarbital were immunostained for gamma-glutamyl-transpeptidase activity. For studies of enzyme induction rats were treated daily with hydroquinone for 3 days and the activities of several oxidative enzymes measured. Treatment with diethylnitrosamine andhydroquinone resulted in a dose dependent increase in the area ofenyme altered foci and a dose independent increase in the number of enzyme altered foci. Diethylnitrosamine and hydroquinone treatment also resulted in a decrease in the number of glutathione-S-transferase-P positive cells. The toxicity of hydroquinone was potentiated in-vitro by pretreatment with diethylnitrosamine. A glutathione and dose dependent increase in the percentage of gamma-glutamyl-transpeptidase positive cells as a function of time was seen. Hydroquinone demonstrated only minor effects in the induction of oxidative enzymes. These results support a synergistic effect of hydroquinone and diethylnitrosamine on the reduction of enzyme altered foci precursor cells but they do not support selective damage of enzyme altered foci cells by hydroquinone.
Stenius U et al; Cancer Letters 68 (2-3): 149-57 (1993)
For more Interactions (Complete) data for HYDROQUINONE (10 total), please visit the HSDB record page.

14.1.15 Antidote and Emergency Treatment

/SRP:/ Immediate first aid: Ensure that adequate decontamination has been carried out. If patient is not breathing, start artificial respiration, preferably with a demand-valve resuscitator, bag-valve-mask device, or pocket mask, as trained. Perform CPR as necessary. Immediately flush contaminated eyes with gently flowing water. Do not induce vomiting. If vomiting occurs, lean patient forward or place on left side (head-down position, if possible) to maintain an open airway and prevent aspiration. Keep patient quiet and maintain normal body temperature. Obtain medical attention. /Aniline and related compounds/
Currance, P.L. Clements, B., Bronstein, A.C. (Eds).; Emergency Care For Hazardous Materials Exposure. 3rd revised edition, Elsevier Mosby, St. Louis, MO 2007, p. 238
/SRP:/ Basic treatment: Establish a patent airway (oropharyngeal or nasopharyngeal airway, if needed). Suction if necessary. Watch for signs of respiratory insufficiency and assist ventilations if necessary. Administer oxygen by nonrebreather mask at 10 to 15 L/min. Monitor for shock and treat if necessary ... . Anticipate seizures and treat if necessary ... . For eye contamination, flush eyes immediately with water. Irrigate each eye continuously with 0.9% saline (NS) during transport ... . Do not use emetics. For ingestion, rinse mouth and administer 5 mL/kg up to 200 mL of water for dilution if the patent can swallow, has a strong gag reflex, and does not drool. Administer activated charcoal ... . /Aniline and related compounds/
Currance, P.L. Clements, B., Bronstein, A.C. (Eds).; Emergency Care For Hazardous Materials Exposure. 3rd revised edition, Elsevier Mosby, St. Louis, MO 2007, p. 239
/SRP:/ Advanced treatment: Consider orotracheal or nasotracheal intubation for airway control in the patient who is unconscious, has severe pulmonary edema, or is in severe respiratory distress. Monitor cardiac rhythm and treat arrhythmias as necessary ... . Start IV administration of D5W /SRP: "To keep open", minimal flow rate/. Use 0.9% saline (NS) or lactated Ringer's (LR) if signs of hypovolemia are present. For hypotension with signs of hypovolemia, administer fluid cautiously. Consider vasopressors if hypotensive with a normal fluid volume. Watch for signs of fluid overload ... . Administer 1% solution methylene blue if patient is symptomatic with severe hypoxia, cyanosis, and cardiac compromise not responding to oxygen. ... . Treat seizures with diazepam (Valium) or lorazepam (Altivan) ... . Use proparacaine hydrochloride to assist eye irrigation ... . /Aniline and related compounds/
Currance, P.L. Clements, B., Bronstein, A.C. (Eds).; Emergency Care For Hazardous Materials Exposure. 3rd revised edition, Elsevier Mosby, St. Louis, MO 2007, p. 239
/SRP:/ Immediate first aid: Ensure that adequate decontamination has been carried out. If patient is not breathing, start artificial respiration, preferably with a demand-valve resuscitator, bag-valve-mask device, or pocket mask, as trained. Perform CPR as necessary. Immediately flush contaminated eyes with gently flowing water. Do not induce vomiting. If vomiting occurs, lean patient forward or place on left side (head-down position, if possible) to maintain an open airway and prevent aspiration. Keep patient quiet and maintain normal body temperature. Obtain medical attention. /Phenols and related compounds/
Currance, P.L. Clements, B., Bronstein, A.C. (Eds).; Emergency Care For Hazardous Materials Exposure. 3rd revised edition, Elsevier Mosby, St. Louis, MO 2007, p. 276-7
For more Antidote and Emergency Treatment (Complete) data for HYDROQUINONE (6 total), please visit the HSDB record page.

14.1.16 Medical Surveillance

Periodic examinations of exposed personnel annually including studies of liver and kidney function. /Protect/ from exposure those individuals with diseases of kidneys, liver, and skin.
ITII. Toxic and Hazardous Industrial Chemicals Safety Manual. Tokyo, Japan: The International Technical Information Institute, 1988., p. 281

14.1.17 Human Toxicity Excerpts

/HUMAN EXPOSURE STUDIES/ When human subjects ingested 300-500 mg hydroquinone in 3 divided doses with meals daily for 3-5 months, no abnormal results were noted in percent hemoglobin, hematocrit, red blood cell count, differential white blood cell count, sedimentation rate, platelet count, coagulation time, and icteric index in blood samples and no abnormal levels were noted in urinary albumin, reducing sugars, white and red cell counts, casts, and urobilinogen (Carlson and Brewer, 1953). The authors suggested that the lack of toxic response may have been due to the division of dose throughout the day at meals, which may have decreased peak blood levels.[DHHS/NTP: Nomination Profile Hydroquinone
123-31-9]. Supporting Information for Toxicological Evaluation by the National Toxicology Program. p.18 (2009). Available from, as of November 12, 2014: https://ntp-server.niehs.nih.gov
/SIGNS AND SYMPTOMS/ Ingestion may cause tinnitus, nausea, dizziness, sense of suffocation, increased respiration rate, vomiting, pallor, muscle twitching, headache, dyspnea, cyanosis, delirium, and collapse.
O'Neil, M.J. (ed.). The Merck Index - An Encyclopedia of Chemicals, Drugs, and Biologicals. Cambridge, UK: Royal Society of Chemistry, 2013., p. 891
/SIGNS AND SYMPTOMS/ The initial stimulatory effects of acutely toxic doses of HQ on the CNS are well recognized and are similar to the effects of other phenolics. ... Clinical signs of salivation, tremors, and hyperexcitability at lower dose levels and convulsions followed by CNS depression and respiration at lethal levels hae been commonly reported. The onset of clinical signs is typically a short period of time following oral or parenteral dosing...Recovery at sublethal dose levels is typically rapid and complete.
Bingham, E.; Cohrssen, B.; Powell, C.H.; Patty's Toxicology Volumes 1-9 5th ed. John Wiley & Sons. New York, N.Y. (2001)., p. V4 417-8
/SIGNS AND SYMPTOMS/ Reports of the effects of hydroquinone ingestion have listed the following clinical manifestations: vomiting, abdominal pain, tachycardia, seizures, tremors, dyspnea, cyanosis, coma, loss of reflexes, and death.
Sullivan, J.B., Krieger G.R. (eds). Clinical Environmental Health and Toxic Exposures. Second edition. Lippincott Williams and Wilkins, Philadelphia, Pennsylvania 1999., p. 1261
For more Human Toxicity Excerpts (Complete) data for HYDROQUINONE (35 total), please visit the HSDB record page.

14.1.18 Non-Human Toxicity Excerpts

/LABORATORY ANIMALS: Acute Exposure/ Uptake and turnover of the sc administration hydroquinone was quite rapid in most tissues and organs of Carassius auratus. It showed no specific affinity for melanosomes, although it induced cytopathologic alteration in these pigment cells. Only the melanosome-containing pigment cells present at the time of treatment were destroyed.
CHAVIN W; VIRCHOWS ARCH B 9 (4): 343 (1971)
/LABORATORY ANIMALS: Acute Exposure/ Oral LD50 values for rats, mice, guinea pigs, cats, and dogs range from 70 to 550 g/kg of hydroquinone, with the cat having the greatest sensitivity. Hyperexcitability, tremors, convulsions, salivation, and emesis were observed in cats within 90 minutes of administration of lethal doses, and death occurred after several hours.
American Conference of Governmental Industrial Hygienists. Documentation of the TLV's and BEI's with Other World Wide Occupational Exposure Values. CD-ROM Cincinnati, OH 45240-1634 2007.
/LABORATORY ANIMALS: Acute Exposure/ Aqueous solutions of hydroquinone (HQ) were given by gavage to male and female Sprague-Dawley rats ... . The acute dermal toxicity of HQ in rabbits was also determined. For the acute oral toxicity study, groups of five male and five female rats were administered single oral doses of 375, 345, 315, or 285 mg/kg. At all dose levels, animals exhibited minor to moderate tremors and minor convulsions within the first hour after dosing. The acute oral LD50 value for both sexes combined was >375 mg/kg. Dermal application of 2000 mg/kg HQ to rabbits under an occlusive wrap for 24 hr did not result in neurobehavioral effects or mortality.
Topping DC et al: Fd Chem Toxicol 45 (1): 70-8 (2007)
/LABORATORY ANIMALS: Acute Exposure/ ... The effect of hydroquinone and /its metabolite/ 2,3,5-(tris-glutathion-S-yl)hydroquinone, on site-selective cytotoxicity and cell proliferation in rat kidney /is described/. Male Fischer 344 rats (160-200 g) were treated with hydroquinone (1.8 mmol/kg or 4.5 mmol/kg, po) or 2,3,5-(tris-glutathion-S-yl)hydroquinone (7.5 umol/kg; 1.2-1.5 uol/rat, iv), and blood urea nitrogen (BUN), urinary gamma-glutamyl transpeptidase (gamma-GT), alkaline phosphatase (ALP), glutathione-S-transferase (GST) and glucose were measured as indices of nephrotoxicity. Hydroquinone (1.8 mmol/kg, po) is nephrotoxic in some rats, but not others, but cell proliferation (BrDU incorporation) in proximal tubular cells of the S3M region correlates with the degree of toxicity in individual rats. At 4.5 mmol/kg, hydroquinone causes significant increases in the urinary excretion of gamma-GT, ALP and GST. Pretreatment of rats with acivicin prevents hydroquinone-mediated nephrotoxicity, indicating that toxicity is dependent on the formation of metabolites that require processing by gamma-GT. Consistent with this view, 2,3,5-(tris-glutathion-S-yl)hydroquinone, a metabolite of hydroquinone, causes increases in BUN, urinary gamma-GT and ALP, all of which are maximal 12 hr after administration of 2,3,5-(tris-glutathion-S-yl)hydroquinone. In contrast, the maximal excretion of GST and glucose occurs after 24 h. By 72 h, BUN and glucose concentrations return to control levels, while gamma-GT, ALP and GST remain slightly elevated. Examination of kidney slices by light microscopy revealed the presence of tubular necrosis in the S3M segment of the proximal tubule, extending into the medullary rays. Cell proliferation rates in this region were 2.4, 6.9, 15.3 and 14.3% after 12, 24, 48 and 72 hr, respectively, compared to 0.8-2.4% in vehicle controls. Together with the metabolic data, the results indicate a role for hydroquinone-thioether metabolites in hydroquinone toxicity and carcinogenicity.
Peters MM et al; Carcinogenesis 18 (12): 2393-401 (1997)
For more Non-Human Toxicity Excerpts (Complete) data for HYDROQUINONE (80 total), please visit the HSDB record page.

14.1.19 Non-Human Toxicity Values

LD50 Rat oral 320 mg/kg
O'Neil, M.J. (ed.). The Merck Index - An Encyclopedia of Chemicals, Drugs, and Biologicals. Cambridge, UK: Royal Society of Chemistry, 2013., p. 891
LD50 Rat intraperitoneal 170 mg/kg
Lewis, R.J. Sr. (ed) Sax's Dangerous Properties of Industrial Materials. 11th Edition. Wiley-Interscience, Wiley & Sons, Inc. Hoboken, NJ. 2004., p. 1992
LD50 Rat intravenous 115 mg/kg
Lewis, R.J. Sr. (ed) Sax's Dangerous Properties of Industrial Materials. 11th Edition. Wiley-Interscience, Wiley & Sons, Inc. Hoboken, NJ. 2004., p. 1992
LD50 Mouse oral 245 mg/kg
Lewis, R.J. Sr. (ed) Sax's Dangerous Properties of Industrial Materials. 11th Edition. Wiley-Interscience, Wiley & Sons, Inc. Hoboken, NJ. 2004., p. 1992
For more Non-Human Toxicity Values (Complete) data for HYDROQUINONE (13 total), please visit the HSDB record page.

14.1.20 Ongoing Test Status

EPA has released the first beta version (version 0.5) of the Interactive Chemical Safety for Sustainability (iCSS) Dashboard. The beta version of the iCSS Dashboard provides an interactive tool to explore rapid, automated (or in vitro high-throughput) chemical screening data generated by the Toxicity Forecaster (ToxCast) project and the federal Toxicity Testing in the 21st century (Tox21) collaboration. /The title compound was tested by ToxCast and/or Tox21 assays; See the data in Chemical Explorer/[USEPA; ICSS Dashboard Application; Available from, as of June 27, 2014: http://actor.epa.gov/dashboard/]
The following link will take the user to the National Toxicology Program (NTP) Test Agent Search Results page, which tabulates all of the "Standard Toxicology & Carcinogenesis Studies", "Developmental Studies", and "Genetic Toxicity Studies" performed with this chemical. Clicking on the "Testing Status" link will take the user to the status (i.e., in review, in progress, in preparation, on test, completed, etc.) and results of all the studies that the NTP has done on this chemical.[Available from, as of October 1, 2014: http://ntp-apps.niehs.nih.gov/ntp_tox/index.cfm?fuseaction=ntpsearch.searchresults&searchterm=123-31-9]

14.1.21 National Toxicology Program Studies

Fourteen-Day: Fourteen-day gavage studies were conducted by administering hydroquinone in corn oil to rats at doses ranging from 63 to 1,000 mg/kg body weight and to mice at doses ranging from 31 to 500 mg/kg. All rats receiving 1,000 mg/kg and 1/5 male and 4/5 female rats receiving 500 mg/kg died before the end of the 14 days. Compound-related clinical signs in rats included tremors lasting up to 30 minutes after each dosing at 500 and 1,000 mg/kg. In the 14-day gavage studies with mice, 4/5 male mice and 5/5 female mice receiving 500 mg/kg and 3/5 males receiving 250 mg/kg died before the end of the studies. Tremors followed by convulsions were seen at 250 and 500 mg/kg.
DHHS/NTP; Toxicology and Carcinogenesis Studies of Hydroquinone (CAS No. 123-31-9) in F344/N Rats and B6C3F1 Mice (Gavage Studies) (1989) Technical Rpt Series No. 366. Available from, as of November 12, 2014: https://ntp-server.niehs.nih.gov/
13-week studies: In the 13-week studies, doses for rats and mice ranged from 25 to 400 mg/kg. All rats receiving 400 mg/kg and 3/10 female rats receiving 200 mg/kg died before the end of the studies. The mean body weight at necropsy of male rats administered 100 or 200 mg/kg was about 8%-9% lower than that of vehicle controls. Mean body weights of vehicle control and dosed female rats at necropsy were similar. Tremors and convulsions were observed after dosing in most rats receiving 400 mg/kg and in several female rats receiving 200 mg/kg. Inflammation and/or epithelial hyperplasia (acanthosis) of the forestomach were seen in 4/10 male rats and 1/10 female rats receiving 200 mg/kg. Toxic nephropathy, characterized by tubular cell degeneration in the renal cortex, was seen in 7/10 male and 6/10 female rats receiving 200 mg/kg and in 1/10 females receiving 100 mg/kg. In the 13-week studies in mice, 8/10 males and 8/10 females receiving 400 mg/kg and 2/10 male mice receiving 200 mg/kg died early. Mean body weights of dosed and vehicle control mice at necropsy were similar. Liver weight to body weight ratios for dosed male mice were significantly greater than for vehicle controls. Ulceration, inflammation, or epithelial hyperplasia of the forestomach was found in 3/10 male and 2/10 female mice receiving 400 mg/kg and 1/10 females receiving 200 mg/kg.
DHHS/NTP; Toxicology and Carcinogenesis Studies of Hydroquinone (CAS No. 123-31-9) in F344/N Rats and B6C3F1 Mice (Gavage Studies) (1989) Technical Rpt Series No. 366. Available from, as of November 12, 2014: https://ntp-server.niehs.nih.gov/
2-year studies: Based on these collective results, 2-year studies were conducted by administering 0, 25, or 50 mg/kg hydroquinone in deionized water by gavage to groups of 65 rats of each sex, 5 days per week. Groups of 65 mice of each sex were administered 0, 50, or 100 mg/kg on the same schedule. Ten rats and 10 mice from each group were killed after 15 months for an interim evaluation. Body Weights, Organ Weights, and Survival in the Two-Year Studies: Mean body weights of high dose male rats were 5%-13% lower than those of vehicle controls after week 73, and those of low dose male rats were 5%-9% lower than those of vehicle controls after week 89. Mean body weights of dosed female rats were similar to those of vehicle controls throughout the study. The relative kidney and liver weights for high dose male rats were higher than those for vehicle controls. Mean body weights of high dose male mice were 5%-8% lower than those of vehicle controls after week 93, and those of high dose female mice were 5%-14% lower after week 20. Relative liver weights were increased for dosed male and high dose female mice. No significant differences in survival were observed between any groups of rats or mice of either sex after 2 years (male rats: vehicle control, 27/55; low dose, 18/55; high dose, 18/55; female rats: 40/55; 27/55; 32/55; male mice: 33/55; 37/54; 36/55; female mice: 37/55; 39/55; 36/55). Nonneoplastic and Neoplastic Effects in the Two-Year Studies: Nearly all male rats and most female rats in all vehicle control and dosed groups had nephropathy. The severity of this disease was judged to be greater in high dose male rats. Hyperplasia of the renal pelvic transitional epithelium and renal cortical cysts, changes observed with advanced renal disease, were increased in male rats. Renal tubular hyperplasia was seen in 2 high dose male rats, and renal tubular adenomas were seen in 4/55 low dose and 8/55 high dose male rats; none was seen in vehicle controls. Mononuclear cell leukemia in female rats occurred with a positive trend, and the incidences in the dosed groups were greater than that in the vehicle controls (vehicle control, 9/55; low dose, 15/55; high dose, 22/55). The historical incidence of leukemia in water gavage vehicle control female F344/N rats is 25% +/- 15% and in untreated controls is 19% +/- 7%. Compound-related lesions observed in the liver of high dose male mice included anisokaryosis (0/55; 2/54; 12/55), syncytial alteration (5/55; 3/54; 25/55), and basophilic foci (2/55; 5/54; 11/55). The incidences of hepatocellular adenomas were increased in dosed male mice (9/55; 21/54; 20/55), but these increases were offset by decreases in the incidences of hepatocellular carcinomas (13/55; 11/54; 7/55). The incidences of hepatocellular neoplasms, primarily adenomas, were increased in dosed female mice (3/55; 16/55; 13/55). Follicular cell hyperplasia of the thyroid gland was increased in dosed mice (male: 5/55; 15/53; 19/54; female: 13/55; 47/55; 45/55). Follicular cell adenomas were seen in 2/55 vehicle control, 1/53 low dose, and 2/54 high dose male mice and in 3/55 vehicle control, 5/55 low dose, and 6/55 high dose female mice, a follicular cell carcinoma was seen in a seventh high dose female mouse. The highest observed incidence of follicular cell adenomas or carcinomas(combined) in historical water gavage vehicle control female B6C3F1 mice is 3/48 (6%).
DHHS/NTP; Toxicology and Carcinogenesis Studies of Hydroquinone (CAS No. 123-31-9) in F344/N Rats and B6C3F1 Mice (Gavage Studies) (1989) Technical Rpt Series No. 366. Available from, as of November 12, 2014: https://ntp-server.niehs.nih.gov/
Genetic Toxicology: Hydroquinone was not mutagenic in S. typhimurium strains TA98, TA100, TA1535, or TA1537 with or without exogenous metabolic activation. It induced trifluorothymidine (Tft) resistance in mouse L5178Y/TK lymphoma cells in the presence or absence of metabolic activation. An equivocal response was obtained in tests for induction of sex-linked recessive lethal mutations in Drosophila administered hydroquinone by feeding. Hydroquinone induced sister chromatid exchanges (SCEs) in CHO cells both with or without exogenous metabolic activation and caused chromosomal aberrations in the presence of activation.
DHHS/NTP; Toxicology and Carcinogenesis Studies of Hydroquinone (CAS No. 123-31-9) in F344/N Rats and B6C3F1 Mice (Gavage Studies) (1989) Technical Rpt Series No. 366. Available from, as of November 12, 2014: https://ntp-server.niehs.nih.gov/

14.1.22 TSCA Test Submissions

Teratogenicity was evaluated in pregnant female Crl: COBS CD(SD)BR rats (30/group) orally exposed by gavage to hydroquinone at dose levels of 0, 30, 100 or 300 mg/kg on gestation days (GD) 6-15. Surviving rats were sacrificed on GD 20. Significant differences were observed between treated and control animals in the following: decreased combined and female mean fetal body weight (high-dose group). No significant differences were observed between treated and control animals in the following: maternal mortality, body weight and weight gain, histologic examinations of livers and kidneys of high-dose group animals (other groups not examined), liver and kidney weights, pregnancy rates, number of litters with resorptions, corpora lutea, implantation sites, viable fetuses, resorptions/dam, pre- and post-implantation losses, mean gravid uterine weights, fetal ratio, male mean fetal body weight, and external, and internal soft tissue and skeletal examinations of the fetuses.
Eastman Kodak Co., Toxicological Sciences Section; Hydroquinone: A Developmental Toxicity Study in Rats. (1985), EPA Document No. FYI-AX-1285-0468, Fiche No. OTS0000468-0
The mutagenicity of hydroquinone was evaluated in Salmonella tester strains TA98, TA100, TA1535 and TA1537 (Ames Test), both in presence and absence of added metabolic activation by Aroclor-induced rat liver S9 fraction. Hydroquinone was tested at concentrations up 1000 ug/plate using the plate incorporation technique. Toxicity of the treatments was not reported. Hydroquinone did not cause a positive response in any tester strain with or without metabolic activation.
Goodyear Health, Safety & Environmental Laboratory; Mutagenicity Evaluation of Hydroquinone, (1979), EPA Document No. 878210375, Fiche No. OTS0205939
The mutagenicity of hydroquinone was evaluated in Salmonella tester strains TA98, TA100, TA1535 and TA1537 (Ames Test), both in the presence and absence of added metabolic activation by Aroclor-induced rat liver S9 fraction. Hydroquinone, diluted with DMSO, was tested up to concentrations up to 1000 ug/plate without metabolic activation and up to 320 ug/plate with metabolic activation using the plate incorporation technique. Toxicity was observed at 1000ug/plate. Hydroquinone did not cause a positive response in any tester strain with or without metabolic activation.
Goodyear Health, Safety and Environmental Laboratory; Salmonella typhimurium/Microsome Bioassay, (1982), EPA Document No. 878210376, Fiche No. OTS0205939
The mutagenic potential of hydroquinone was evaluated in the germ cells (Sex-Linked Recessive Lethal Assay) of Drosophila melanogaster males. Based on preliminary toxicity determinations, hydroquinone, was fed to 100 males at a nominal concentration of 1000ug/mL. After treatment, each male was mated with three virgin females and offspring were analyzed. The treatment did not cause an statistically significant increase in the mutation frequency relative to the negative controls (sucrose/distilled water).
Goodyear Fiber and Polymer Products Research Division; Evaluation of Hydroquinone Using the Drosophila melanogaster/Sex-Linked Recessive Lethal Test, (1981), EPA Document No. 878210377, Fiche No. OTS0205939
For more TSCA Test Submissions (Complete) data for HYDROQUINONE (8 total), please visit the HSDB record page.

14.1.23 Populations at Special Risk

Glutathione S transferase (GST) gene polymorphism examined among north Indians and correlated with hydroquinone (HQ) genotoxicity to help in clinical prediction of susceptibility of HQ toxicity. Lymphocytes of individuals with/without GSTM1, GSTT1, and GSTP1 (ile/ile or val/val) were exposed to HQ (20, 40, or 80 uM) and examined chromosomal aberrations (CA) or cytokinesis-block micronucleus assays. Among north Indians the frequencies of GSTM1 (null), GSTT1 (null), and both null were found to be 41.1, 21.9, and 12.7%, whereas frequencies of GSTP1 with (ile/ile) or (ile/val), or (val/val) were 52, 42.1, or 5.9%, respectively. Individuals with null GSTM1, GSTT1, and GSTP1 (val/val) showed inhibition of mitotic index (MI) and significant (p < 0.01) induction of CA as compared to individuals with GSTM1, GSTT1, and GSTP1 (ile/ile). Micronucleus formation was found to be significant (p < 0.05 or 0.01) in both the genotypes. Results indicate that GSTM1, GSTT1 (null), and GSTP1 (val/val) are sensitive to HQ genotoxicity.
Kumar M et al; Toxicol Mech Methods. 19(1):59-65. (2009).
...Together /these/ findings show that hydroquinone toxicity is modulated by Ras signaling. Individuals with abnormal Ras signaling could be more vulnerable to developing myeloid diseases after exposure to benzene. /This study notes/ that hydroquinone is used cosmetically as a skin-bleaching agent, including by individuals with cafe-au-lait spots (which may be present in individuals with neurofibromatosis who have a mutation in NF1), which could be unadvisable given /these/ findings.
North M et al; BMC Cancer. 14:6. (2014).

14.2 Ecological Information

14.2.1 Ecotoxicity Values

EC50; Species: Chlorococcales (Green algae); Conditions: freshwater, static; Concentration: 17000 ug/L for 24 hr; Effect: physiology; assimilation efficiency /formulation/
Krebs F; Deutsche Gewasserkundliche Mitteilungen 35 (5/6): 161-70 (1991) Available from, as of January 20, 2009
EC50; Species: Dunaliella tertiolecta (Green algae); Conditions: freshwater, static, 24 °C, pH 6.8, salinity 2.348%, ; Concentration: 29250 ug/L for 24 hr; Effect: population, decreased photosynthesis /formulation/
McFeters GA; Water Res 17 (12): 1757-62 (1983) Available from, as of January 20, 2009
EC50; Species: Selenastrum capricormutum (algae); Concentration: 0.335 mg/L for 72 hr; Effect: growth /Conditions of bioassay not specified in source examined/
Organization for Economic Cooperation and Development; Screening Information Data Set for Hydroquinone (123-31-9) p.17 (June 2002). Available from, as of March 9, 2009: https://www.chem.unep.ch/irptc/sids/OECDSIDS/sidspub.html
EC50; Species: Daphnia magna (Water flea) age 6-24 hr; Conditions: freshwater, static, 20 °C, pH > or =7.0; Concentration: 320 ug/L for 24 hr (95% confidence interval: 270-380 ug/L); Effect: intoxication, immobilization /formulation/
Kuhn, R et al; Water Res 23 (4): 495-9 (1989) Available from, as of January 20, 2009
For more Ecotoxicity Values (Complete) data for HYDROQUINONE (22 total), please visit the HSDB record page.

14.2.2 Ecotoxicity Excerpts

/AQUATIC SPECIES/ Sea bass (Dicentrarchus labrax) were injected intraperitoneally once (single dose) or three times (fractionated dose) with phenol or OH-phenols (hydroquinone, resorcinol, and pyrocatechol). On the basis of the lethal doses, OH-phenols were more toxic than phenol, and pyrocatechol was the most powerful compound. Hematological, metabolic and antioxidant blood parameters were measured 3 days after the end of the treatment. Metabolic variations as specific effects on erythrocytes were revealed and differences between single and fractionated doses were observed. OH-phenols-treated fish showed disorders in the metabolic toxicity indicators as hypoglycemia, low blood urea nitrogen level (BUN) and decrease of alkaline phosphatase activity (ALP).
Roche H, Boge G; Comp Biochem Physiol C Toxicol Pharmacol 125 (3): 345-53 (2000)

14.2.3 US EPA Regional Screening Levels for Chemical Contaminants

Resident Soil (mg/kg)
9.00e+00
Industrial Soil (mg/kg)
3.80e+01
Tapwater (ug/L)
1.30e+00
MCL (ug/L)
5.00e+01
Risk-based SSL (mg/kg)
8.70e-04
Oral Slope Factor (mg/kg-day)-1
6.00e-02
Chronic Oral Reference Dose (mg/kg-day)
4.00e-02
Volatile
Volatile
Mutagen
Mutagen
Fraction of Contaminant Absorbed in Gastrointestinal Tract
1
Fraction of Contaminant Absorbed Dermally from Soil
0.1

14.2.4 US EPA Regional Removal Management Levels for Chemical Contaminants

Resident Soil (mg/kg)
9.00e+02
Industrial Soil (mg/kg)
3.80e+03
Tapwater (ug/L)
1.30e+02
MCL (ug/L)
5.00e+01
Oral Slope Factor (mg/kg-day)-1
6.00e-02
Chronic Oral Reference Dose (mg/kg-day)
4.00e-02
Volatile
Volatile
Mutagen
Mutagen
Fraction of Contaminant Absorbed in Gastrointestinal Tract
1
Fraction of Contaminant Absorbed Dermally from Soil
0.1

14.2.5 ICSC Environmental Data

The substance is very toxic to aquatic organisms.

14.2.6 Environmental Fate / Exposure Summary

Hydroquinone's production and use as a chemical intermediate, glue and hair dye ingredient, and administration as a skin depigmentor may result in its release to the environment through various waste streams. Hydroquinone occurs naturally in several arthropods and assorted species of plants. If released to air, a vapor pressure of 2.4X10-5 mm Hg at 25 °C indicates hydroquinone will exist in both the vapor and particulate phases in the atmosphere. Vapor-phase hydroquinone will be degraded in the atmosphere by reaction with photochemically-produced hydroxyl radicals; the half-life for this reaction in air is estimated to be 17 hrs. Particulate-phase hydroquinone will be removed from the atmosphere by wet and dry deposition. Hydroquinone contains chromophores that absorb at wavelengths >290 nm and, therefore, may be susceptible to direct photolysis by sunlight. If released to soil, hydroquinone is expected to have moderate mobility based upon an estimated Koc of 240. Volatilization from moist soil surfaces is not expected to be an important fate process based upon an estimated Henry's Law constant of 4.7X10-11 atm-cu m/mole. Hydroquinone is not expected to volatilize from dry soil surfaces based upon its vapor pressure. Hydroquinone was found to biodegrade in the coupled units test, the Zahn-Wellens test, the MITI test, the Sturm test, the AFNOR test, the OECD screening test and the closed bottle test, indicating that biodegradation may be an important environmental fate process in soil and water, although its biodegradation is likely to be strongly concentration-dependent. It may be inhibitory at high concentration. If released into water, hydroquinone is expected to adsorb to suspended solids and sediment based upon the estimated Koc. Volatilization from water surfaces is not expected to be an important fate process based upon this compound's estimated Henry's Law constant. An estimated BCF of 3 suggests the potential for bioconcentration in aquatic organisms is low. Hydrolysis is not expected to be an important environmental fate process since this compound lacks functional groups that hydrolyze under environmental conditions (pH 5 to 9). Aqueous hydroquinone showed 75% decomposition after 44 hours of UV photooxidation. Occupational exposure to hydroquinone may occur through inhalation and dermal contact with this compound at workplaces where hydroquinone is produced or used. Monitoring/use data indicate that the general population may be exposed to hydroquinone via inhalation of wood or cigarette smoke, ingestion of certain foods and dermal contact with consumer products containing hydroquinone. (SRC)

14.2.7 Natural Pollution Sources

Hydroquinone occurs naturally in several arthropods and assorted species of plants. It is present in the African plant Noogoora burr (Xanthium pungens) at concentrations high enough to poison pigs and cattle. It is formed as a by-product of metabolism in several bacteria and marine species(1). Hydroquinone occurs naturally, as a conjugate with beta-D-glucopyranoside, in the leaves, bark and fruit of a number of plants, especially the ericaceous shrubs such as the cranberry, cowberry, bearberry and blueberry(2,3). Its presence may be an important factor in fire-blight resistance in the pear, and it may also play an important part in the defense mechanisms of a number of insects, including the bombadier beetle and the earwig(2).
(1) Cavender FL, O'Donohue J; Phenol and Phenolics. Patty's Toxicology. 6th ed. (1999-2014). New York, NY: John Wiley & Sons, Inc. On-line posting date: 17 Aug 2012
(2) Harbison KG, Belly RT; Environ Toxicol Chem 1: 9-15 (1982)
(3) Hudnall PM; Hydroquinone. Ullmann's Encyclopedia of Industrial Chemistry. 7th ed. (1999-2014). New York, NY: John Wiley & Sons. Online Posting Date: 15 Jun 2000

14.2.8 Artificial Pollution Sources

Hydroquinone's production and use as an intermediate for chemical conversion to inhibitors used to stabilize monomers, paints, varnishes, motor oils, fuels, as a rust inhibitor in cooling towers(1), in glue and hair dye formulations(2), and as a depigmentor(3) to gradually lighten age spots, liver spots, freckles or hyperpigmentation that can occur as a result of pregnancy or use of oral contraceptives(4) may result in its release to the environment through various waste streams(SRC). It may also be released to the environment in the effluent coal-gasification condensate water(5) and cigarette smoke(1).
(1) Cavender FL, O'Donohue J; Phenol and Phenolics. Patty's Toxicology. 6th ed. (1999-2014). New York, NY: John Wiley & Sons, Inc. On-line posting date: 17 Aug 2012
(2) US NLM; Household Products Database. US Natl Lib Med, Natl Inst Health, Health Human Serv. Available from, as of Aug 4, 2014: https://hpd.nlm.nih.gov/index.htm
(3) O'Neil MJ, ed; The Merck Index. Whitehouse Station, NJ: Merck and Co., Inc., p. 891 (2012)
(4) US Natl Inst Health; DailyMed. Current Medical Information. Available from, as of 5 Aug 2014: https://dailymed.nlm.nih.gov/dailymed/about.cfm
(5) Mohr DH, King CJ; Environ Sci Technol 19: 929-35 (1985)

14.2.9 Environmental Fate

TERRESTRIAL FATE: Based on a classification scheme(1), an estimated Koc value of 240(SRC), determined from a strucure estimation method(2), indicates that hydroquinone is expected to have moderate mobility in soil(SRC). Volatilization of hydroquinone from moist soil surfaces is not expected to be an important fate process(SRC) given an estimated Henry's Law constant of 4.7X10-11 atm-cu m/mole(SRC), derived from its vapor pressure, 2.4X10-5 mm Hg(3), and water solubility, 7.2X10+4 mg/L(4). Hydroquinone is not expected to volatilize from dry soil surfaces(SRC) based upon its vapor pressure(3). The compound may be removed from soil by oxidation processes(5,6) or by direct photolysis on the surface(7,8). In a survey of standardized tests, hydroquinone was found to biodegrade in the coupled units test, the Zahn-Wellens test, the MITI test, the Sturm test, the AFNOR test, the OECD screening test and the closed bottle test, although its biodegradation is likely to be strongly concentration-dependent. It may be inhibitory at high concentration(9,10).
(1) Swann RL et al; Res Rev 85: 17-28 (1983)
(2) US EPA; Estimation Program Interface (EPI) Suite. Ver. 4.1. Nov, 2012. Available from, as of Aug 4, 2014: https://www.epa.gov/oppt/exposure/pubs/episuitedl.htm
(3) Daubert TE, Danner RP; Physical and Thermodynamic Properties of Pure Chemicals Data Compilation. Washington, DC: Taylor and Francis (1989)
(4) Granger FS, Nelson JM; J Am Chem Soc 43: 1403-7 (1921)
(5) Laha S, Luthy RG; Environ Sci Tech 24: 363-73 (1990)
(6) Morrill LG et al; Toxic Chemicals in the Soil Environment. Vol 2. Defense Tech Info Center Dugway Proving Ground. Utah NTIS AD-A158-215 (1985)
(7) Freitag D et al; Chemosphere 14: 1589-616 (1985)
(8) Freitag D et al; Ecotox Environ Saf 6: 60-81 (1982)
(9) Gerike P, Fischer WK; Ecotox Environ Saf 5: 45-55 (1981)
(10) Gerike P; Berichte 10: 139-43 (1978)
AQUATIC FATE: Based on a classification scheme(1), an estimated Koc value of 240(SRC), determined from a structure estimation method(2), indicates that hydroquinone is expected to adsorb to suspended solids and sediment(SRC). Volatilization from water surfaces is not expected(3) based upon an estimated Henry's Law constant of 4.7X10-11 atm-cu m/mole(SRC), derived from its vapor pressure, 2.4X10-5 mm Hg(4), and water solubility, 7.2X10+4 mg/L(5). According to a classification scheme(6), an estimated BCF of 3(SRC), from its log Kow(7) and a regression-derived equation(2), suggests the potential for bioconcentration in aquatic organisms is low(SRC). In a survey of standardized tests, hydroquinone was found to biodegrade in the coupled units test, the Zahn-Wellens test, the MITI test, the Sturm test, the AFNOR test, the OECD screening test and the closed bottle test, although its biodegradation is likely to be strongly concentration-dependent. It may be inhibitory at high concentration(8,9).
(1) Swann RL et al; Res Rev 85: 17-28 (1983)
(2) US EPA; Estimation Program Interface (EPI) Suite. Ver. 4.1. Nov, 2012. Available from, as of Aug 4, 2014: https://www.epa.gov/oppt/exposure/pubs/episuitedl.htm
(3) Lyman WJ et al; Handbook of Chemical Property Estimation Methods. Washington, DC: Amer Chem Soc pp. 15-1 to 15-29 (1990)
(4) Daubert TE, Danner RP; Physical and Thermodynamic Properties of Pure Chemicals Data Compilation. Washington, DC: Taylor and Francis (1989)
(5) Granger FS, Nelson JM; J Am Chem Soc 43: 1403-7 (1921)
(6) Franke C et al; Chemosphere 29: 1501-14 (1994)
(7) Hansch C et al; Exploring QSAR. Hydrophobic, Electronic, and Steric Constants. ACS Prof Ref Book. Heller SR, consult. ed., Washington, DC: Amer Chem Soc p. 20 (1995)
(8) Gerike P, Fischer WK; Ecotox Environ Saf 5: 45-55 (1981)
(9) Gerike P; Berichte 10: 139-43 (1978)
ATMOSPHERIC FATE: According to a model of gas/particle partitioning of semivolatile organic compounds in the atmosphere(1), hydroquinone, which has a vapor pressure of 2.4X10-5 mm Hg at 25 °C(2), will exist in both the vapor and particulate phases in the ambient atmosphere. Vapor-phase hydroquinone is degraded in the atmosphere by reaction with photochemically-produced hydroxyl radicals(SRC); the half-life for this reaction in air is estimated to be 17 hrs(SRC), calculated from its rate constant of 2.3X10-11 cu cm/molecule-sec at 25 °C(SRC) that was derived using a structure estimation method(3). Particulate-phase hydroquinone may be removed from the air by wet and dry deposition(SRC). The water solubility of hydroquinone, 72,000 mg/L at 25 °C(4), indicates that it may undergo atmospheric removal by wet deposition processes(SRC). It may also be removed from the atmosphere by a gas-phase reaction with nitrate radicals, an important nighttime oxidant(3). Hydroquinone contains chromophores that absorb at wavelengths >290 nm(5) and, therefore, may be susceptible to direct photolysis by sunlight(SRC).
(1) Bidleman TF; Environ Sci Technol 22: 361-367 (1988)
(2) Daubert TE, Danner RP; Physical and Thermodynamic Properties of Pure Chemicals Data Compilation. Washington, DC: Taylor and Francis (1989)
(3) Meylan WM, Howard PH; Chemosphere 26: 2293-99 (1993)
(4) Granger FS, Nelson JM; J Am Chem Soc 43: 1403-7 (1921)
(5) Lyman WJ et al; Handbook of Chemical Property Estimation Methods. Washington, DC: Amer Chem Soc pp. 8-12 (1990)

14.2.10 Environmental Biodegradation

AEROBIC: Hydroquinone at a concentration of 0.05 mg/L underwent 7.5% removal in 5 days when inoculated with an activated sludge seed(1,2). Sewage sludge activated to phenol was found to oxidize hydroquinone(3,4). Pure culture oxidation of hydroquinone produced 1,4-benzoquinone, 2-hydroxy-1,4-benzoquinone and beta-ketoadipic acid(5). In a screening study using a sewage seed, hydroquinone had a 5 day theoretical BOD of 25.3%(6). Hydroquinone at an initial concentration of 200 mg/L COD underwent 54.2% removal (less than 120 hours) using a thickened adapted activated sludge under aerobic conditions(7). Activated sludges adapted to aniline, phenol or m-cresol were found to biodegrade hydroquinone under aerobic conditions(8). It was listed as undergoing rapid biodegradation in a commercial activated sludge unit under aerobic conditions(9).
(1) Freitag D et al; Chemosphere 14: 1589-616 (1985)
(2) Freiag D et al; Ecotox Environ Saf 6: 60-81 (1982)
(3) Barth EF, Bunch RL; Removability, Biodegradation and Treatability of Specific Pollutants. Municipal Environmental Research Lab, USEPA NTIS PB-80-106438 (1979)
(4) Chambers CW et al; J Water Pollut Contr Fed 35: 1517-28 (1963)
(5) Harbison KG, Belly RT; Environ Toxicol Chem 1: 9-15 (1982)
(6) Heukelekian H, Rand MC; J Water Pollut Contr Assoc 29: 1040-53 (1955)
(7) Pitter P; Water Res 10: 231-5 (1976)
(8) Urushigawa Y et al; Kogai Shigen Kenkyusho Iho 14: 45-64 (1984)
(9) Dagon TJ; J Water Pollut Contr Fed 45: 2123-35 (1973)
AEROBIC: Hydroquinone at an initial concentration of 500 mg/L underwent 3%, 4% and 25% theoretical oxidation when inoculated with an activated sewage sludge seed acclimated to phenol(1,2), benzoic acid and catechol(1). Hydroquinone underwent a 5 day theoretical biological oxygen demand (BOD) of 53% when inoculated with a settled raw wastewater seed(3). It was found to biodegrade when inoculated with an activated sludge acclimated to resorcinol, but not to m-nitrobenzenesulfonate sodium salt(4). Hydroquinone underwent a 5 day theoretical BOD of 37% under aerobic conditions(5). Hydroquinone was completely removed by an activated sludge in 15 days(6). An activated sewage sludge was found to oxidize hydroquinone(7). Hydroquinone, present at 100 mg/L, reached 95% of its theoretical BOD in 2 weeks using an activated sludge inoculum at 30 mg/L in the Japanese MITI test(8-11).
(1) McKinney RE et al; Sew Ind Wastes 28: 547-57 (1956)
(2) Ludzack FJ, Ettinger MB; J Water Pollut Control Fed 32: 1173-1200 (1960)
(3) Young RHF et al; J Water Pollut Contr Fed 40: 354-68 (1968)
(4) Deshpande SD et al; Environ Sci Tech 21: 1003-8 (1987)
(5) Dore M et al; Trib Cebedeau 28: 3-11 (1975)
(6) Grossman A, Kusznik W; Environ Prot Engineer 1: 151-62 (1975)
(7) Belly RT, Goodhue CT; Proc Int Biodegrad Symp 3: 1103-7 (1976)
(8) NITE; Chemical Risk Information Platform (CHRIP). Biodegradation and Bioconcentration. Tokyo, Japan: Natl Inst Tech Eval. Available from, as of Aug 4, 2014: https://www.safe.nite.go.jp/english/db.html
(9) Kawasaki M; Ecotox Environ Safety 4: 444-54 (1980)
(10) Kitano M; Biodegradation & Bioaccumulation Test on Chemical Substances OECD Tokyo Meeting Reference Book TSU-No. 3 (1978)
(11) Sasaki K et al; Bull Environ Contam Toxicol 27: 775-82 (1981)
AEROBIC: Hydroquinone applied at a concentration of 500 mg/kg degraded in 1 day on a hard, carbonaceous woody loam at 19 °C(1,2). Bacteria from soil and related environments raised on a wide variety of different phenolic compounds were found to utilize 95% of 300 ppm hydroquinone within 1 to 2 days under aerobic conditions(3). In a survey of standarized tests, hydroquinone was found to biodegrade in the coupled units test, the Zahn-Wellens test, the MITI test, the Sturm test, the AFNOR test, the OECD screening test and the closed bottle test, although its biodegradation is probably strongly concentration dependent and it may be inhibitory at high concentration(4,5).
(1) Medvedev V, Davidov VD; pp. 245-54 in Decomposition of Toxic and Nontoxic Organic Compounds is soil. Overcash MR ed. MI Ann Arbor Sci Publ. Ann Arbor (1981)
(2) Medvedev V, Davidov VD; pp. 175-81 in Decomposition of Toxic and Nontoxic Organic Compounds in soil. Overcash MR ed. MI Ann Arbor Sci Publ. Ann Arbor Overcash MR ed. MI Ann Arbor Sci Publ. Ann Arbor (1981)
(3) Tabak HH et al; J Bacteriol 87: 910 (1964)
(4) Gerike P, Fischer WK; Ecotox Environ Saf 5: 45-55 (1981)
(5) Gerike P; Berichte 10: 139-43 (1978)
ANAEROBIC: In a 500 mg/L fixed-bed reactor, degradation of hydroquinone at concentrations ranging from 0.55 g/L to 2.2 g/L was complete at a flow rate of 170 mL/hr using a mixed microbial seed under anaerobic conditions(1). It was listed as a compound which should be amenable to biological degradation in wastewater treatment by anaerobic biotechnology(2). Hydroquinone was listed as a compound metabolized by enriched methane cultures from a sewage sludge(3). Methanogenic organisms isolated from freshwater sediment and sewage sludge were found to degrade hydroquinone under anaerobic conditions(4).
(1) Szewzyk U, Bernhard S; Appl Microbiol Technol 32: 346-9 (1989)
(2) Chou WL et al; Biotech Bioeng Symp 8: 391-414 (1979)
(3) Szewzyk U et al; FEMS Microbiol Ecol 31: 79-87 (1985)
(4) Speece RE; Environ Sci Technol 17: 416A-27A (1983)

14.2.11 Environmental Abiotic Degradation

The rate constant for the vapor-phase reaction of hydroquinone with photochemically-produced hydroxyl radicals has been estimated as 2.3X10-11 cu cm/molecule-sec at 25 °C(SRC) using a structure estimation method(1). This corresponds to an atmospheric half-life of about 17 hours at an atmospheric concentration of 5X10+5 hydroxyl radicals per cu cm(1). The half-life for oxidation of hydroquinone by alkylperoxy radicals (present in sunlit waters) in organic solvents is 12 min(2) and at 90 to 95 °C under a UV lamp in aqueous media, 25% photooxidation occurred after 10 hours, 50% after 23 hours, and 75% at 44 hours(3). Autoxidation at 25 °C exhibited half-lives of 111 hours at pH 7.0, 41 hours at pH 8.0, and 0.8 hours at pH 9.0(4). Hydroquinone may also undergo a nighttime reaction with nitrate radicals, which is a facile reaction for similarly substituted phenols(1). Laboratory oxidation of hydroquinone with manganese dioxide is thought to be representative of oxidants commonly present in soil. The result of this test was loss of substrate, indicating that this may be a method of abiotic removal of hydroquinone in soil(5). At pH <9, hydroquinone was found to be oxidized to quinone in the presence of smectite, an isomorphically substituted clay mineral, indicating that oxidation may occur in soil(6).
(1) Meylan WM, Howard PH; Chemosphere 26: 2293-99 (1993)
(2) Mill T; Environ Toxicol Chem 1: 135-41
(3) Knoevenagel K et al; Arch Environ Contam Toxicol 4: 324-33 (1976)
(4) Moussavi M; Water Res 13: 1125-1128 (1979)
(5) Laha S, Luthy RG; Environ Sci Tech 24: 363-73 (1990)
(6) Morrill LG et al; Toxic Chemicals in the Soil Environment Vol. 2. Defense Tech Info Center Dugway Proving Ground. Utah NTIS AD-A158-215 (1985)
Hydroquinone is not expected to undergo hydrolysis in the environment due to the lack of functional groups that hydrolyze under environmental conditions(1). Hydroxyquinone contains chromophores that absorb at wavelengths >290 nm(1) and, therefore, is susceptible to direct photolysis by sunlight(SRC). When irradiated at 290 nm, 100 ng/g hydroquinone, adsorbed to silica gel, underwent 57.4% photomineralization after 17 hrs(2,3). Photolysis of hydroquinone in water by natural sunlight produced superoxide anion, and ultimately, hydrogen peroxide(4). Laboratory irradiation of a 1.0 mM aqueous hydroquinone solution for 5 hrs produced >42 uM oxidant (superoxide radical anion, aqueous electrons or hydrogen peroxide)(5,6). Photolysis of hydroquinone in dilute aqueous solutions and in the presence of oxygen produced quinone through semiquinonic radicals intermediates(7,8). Hydroquinone was oxidized to unidentified colored products when oxygen was bubbled through an aqueous solution(9).
(1) Lyman WJ et al; Handbook of Chemical Property Estimation Methods. Washington, DC: Amer Chem Soc pp. 7-4, 7-5, 8-12 (1990)
(2) Freitag D et al; Chemosphere 14: 1589-616 (1985)
(3) Freitag D et al; Ecotox Environ Saf 6: 60-81 (1982)
(4) Choudhry GG, Webster GRB; Res Rev 96: 79-136 (1985)
(5) Draper WM, Crosby DG; Arch Environ Contam Toxicol 12: 121-6 (1983)
(6) Draper WM, Casida JE; J Agric Food Chem 31: 227-31 (1983)
(7) Perbet G et al; J Chimie Physique 76: 89-96 (1979)
(8) Tissot A et al; Chemosphere 14: 1221-30 (1985)
(9) Grossman A, Kusznik W; Environ Prot Engineer 1: 151-62 (1975) (1982)

14.2.12 Environmental Bioconcentration

An estimated BCF of 3 was calculated in fish for hydroquinone(SRC), using a log Kow of 0.59(1) and a regression-derived equation(2). According to a classification scheme(3), this BCF suggests the potential for bioconcentration in aquatic organisms is low(SRC). A bioaccumulation factor of 40 was measured using Golden ide fish (Leuciscus idus melanotus) exposed for 3 days to 0.05 mg/L hydroquinone(4,5). Experimental 24-hour bioaccumulation factors in alga were 40 and 65 for hydroquinone(4-6).
(1) Hansch C et al; Exploring QSAR. Hydrophobic, Electronic, and Steric Constants. ACS Prof Ref Book. Heller SR, consult. ed., Washington, DC: Amer Chem Soc p. 20 (1995)
(2) US EPA; Estimation Program Interface (EPI) Suite. Ver. 4.1. Nov, 2012. Available from, as of Aug 4, 2014: https://www.epa.gov/oppt/exposure/pubs/episuitedl.htm/
(3) Franke C et al; Chemosphere 29: 1501-14 (1994)
(4) Freitag D et al; Chemosphere 14: 1589-616 (1985)
(5) Freitag D et al; Ecotox Environ Saf 6: 60-81 (1982)
(6) Geyer HJ et al; Chemosphere 10: 1307-13 (1981)

14.2.13 Soil Adsorption / Mobility

Using a structure estimation method based on molecular connectivity indices(1), the Koc of hydroquinone is estimated as 240(SRC). According to a classification scheme(2), this estimated Koc value suggests that hydroquinone is expected to have moderate mobility in soil. Hydroquinone can exhibit chemisorption to transition metal-containing particulate matter via reaction with the copper oxide/silca surfaces present(3).
(1) US EPA; Estimation Program Interface (EPI) Suite. Ver. 4.1. Nov, 2012. Available from, as of Aug 4, 2014: https://www.epa.gov/oppt/exposure/pubs/episuitedl.htm
(2) Swann RL et al; Res Rev 85: 17-28 (1983)
(3) Truong H et al; Environ Sci Technol 44: 1933-1939 (2010)

14.2.14 Volatilization from Water / Soil

The Henry's Law constant for hydroquinone is estimated as 4.7X10-11 atm-cu m/mole(SRC) derived from its vapor pressure, 2.4X10-5 mm Hg(1), and water solubility, 7.2X10+4 mg/L(2). This Henry's Law constant indicates that hydroquinone is expected to be essentially nonvolatile from water and moist soil surfaces(3). Hydroquinone is not expected to volatilize from dry soil surfaces(SRC) based upon its vapor pressure(1).
(1) Daubert TE, Danner RP; Physical and Thermodynamic Properties of Pure Chemicals Data Compilation. Washington, DC: Taylor and Francis (1989)
(2) Granger FS, Nelson JM; J Am Chem Soc 43: 1403-7 (1921)
(3) Lyman WJ et al; Handbook of Chemical Property Estimation Methods. Washington, DC: Amer Chem Soc pp. 15-1 to 15-29 (1990)

14.2.15 Effluent Concentrations

Hydroquinone was detected in 3 of 4 samples of effluents of coal-gasification condensate water at a concentration ranging from 1 to 35 mg/L(1). It is typically present in the water of photographic processes(2). Atmospheric emissions in hydroquinone have been associated with the following industries; industrial organics, plastic materials and resins, pharmaceutical preparations, paints and allied products, cyclic crude and intermediates, industrial organics, chemical preparations, petroleum refining, electronic computing equipment, electronic components, and services allied to motion pictures(3). Emission of hydroquinone has been associated with the production of polyvinyl acetate and methyl methacrylate(4). Hydroquinone has been detected in waste ammonia liquor from low temperature carbonization wastes in India at a mean concentration of 33.33 mg/L(5).
(1) Mohr DH, King CJ; Environ Sci Technol 19: 929-35 (1985)
(2) Dagon TJ; J Water Pollut Contr Fed 45: 2123-35 (1973)
(3) PES Inc; Toxic Air Pollutant/Source Crosswalk USEPA-450/4-87-023a (1987)
(4) Carpenter CE et al; Toxic Subst J 10: 323-71 (1990)
(5) Pandey RA et al; J Environ Sci Health 24: 603-32 (1989)

14.2.16 Food Survey Values

Hydroquinone is present in several foods such as wheat products, fruits, and beverages including coffee, teas, beer, and wine(1). Hydroquinone was detected in trace amounts in a mixture of extractable organic substances from New Zealand honeys from unifloral manuka (Leptospermum scoparium) and nodding thistle (Carduus nutans)(2,3).
(1) Cavender FL, O'Donohue J; Phenol and Phenolics. Patty's Toxicology. 6th ed. (1999-2014). New York, NY: John Wiley & Sons, Inc. On-line posting date: 17 Aug 2012
(2) Wilkins AL et al; J Agric Res 32: 3-9 (1993)
(3) Wilkins AL et al; J Agric Food Chem 41: 873-878 (1993)

14.2.17 Plant Concentrations

Hydroquinone occurrence in foods(1).
Genus species
Acacia nilotica
Common Name
Babul
Part
Plant
Genus species
Arbutus unedo
Common Name
Arbutus
Part
Leaf
Genus species
Hyacinthus orientalis
Common Name
Hyacinth
Part
Flower
Genus species
Origanum marjorana
Common Name
Marjoram
Part
Plant
Genus species
Pimpinella anisum
Common Name
Anise
Part
Plant
Genus species
Pyrus communis
Common Name
Pear
Part
Plant
Genus species
Rhododendron sauricum
Common Name
Chinese Alpenrose
Part
Plant
Genus species
Rubus fruticosus
Common Name
Blackberry
Part
Leaf
Genus species
Vaccunium myrtillus
Common Name
Bilberry
Part
Leaf
Genus species
Vaccunium vitis-idaea var minus
Common Name
Cowberry
Part
Leaf
(1) USDA; Dr. Duke's Phytochemical and Ethnobotanical Databases. Plants with a chosen chemical. Hydroquinone. Washington, DC: US Dept Agric, Agric Res Service. Available from, as of Aug 4, 2014: https://www.ars-grin.gov/duke/

14.2.18 Animal Concentrations

Hydroquinone may play an important part in the defense mechanisms of a number of insects, including the bombadier beetle (Carabidae) and the earwig (Forficula auricularia)(1).
(1) Harbison KG, Belly RT; Environ Toxicol Chem 1: 9-15 (1982)

14.2.19 Other Environmental Concentrations

Hydroquinone is present at trace levels in cigarette smoke. An estimated 5X10+4 kg of hydroquinone is generated per year during cigarette smoking(1).
(1) Cavender FL, O'Donohue J; Phenol and Phenolics. Patty's Toxicology. 6th ed. (1999-2014). New York, NY: John Wiley & Sons, Inc. On-line posting date: 17 Aug 2012
Characterization of particulate emissions from fireplace combustion of the most prevalent woods found in the United States revealed the presence of hydroquinone. Smoke samples were collected during a pyrolysis experiment and analyzed by GC/MS. The concentrations were reported in mg/g pyrolysis product/wood weight as follows: 7.609 yellow poplar (Liriodendron tulipifera); 1.621, white ash (Faxinus americana); 1.435, sweetgum (Liquidambar styraciflua); 10.119, mockernut hickory (Carya tomentosa); 0.763, loblolly pine (Pinus taeda); 0.295, slash pine (Pinus elliottii)(1). Combustion of wood in residential fireplaces in the Los Angeles area resulted in hydroquinone emissions of 61.78 mg/kg of pine logs burned and 22.52 mg/kg of oak logs burned; hydroquinone was not detected when burning synthetic logs(2).
(1) Fine PM et al; Environ Sci Technol 36: 1442-1451 (2002)
(2) Rogge WF et al; Environ Sci Technol 32: 13-22 (1998)
Hydroquinone was present at 36.9, 84.2 and 55.3 mg/kg dry wood burned in open, half-closed and closed airflow emissions, respectively, using woodheaters commonly used in central Australia. Higher burn rates (open fair flow) producing much less particle mass per kg wood burned than the low burn rates (closed airflow). Wood used was White gum (Eucalyptus viminalis)(1).
(1) Jordan TB, Seen AJ; Environ Sci Technol 39: 3601-3610 (2005)

14.2.20 Probable Routes of Human Exposure

According to the 2006 TSCA Inventory Update Reporting data, the number of persons reasonably likely to be exposed in the industrial manufacturing, processing, and use of hydroquinone is 1000 or greater; the data may be greatly underestimated(1).
(1) US EPA; Inventory Update Reporting (IUR). Non-confidential 2006 IUR Records by Chemical, including Manufacturing, Processing and Use Information. Washington, DC: U.S. Environmental Protection Agency. Available from, as of Aug 4, 2014: https://cfpub.epa.gov/iursearch/index.cfm
NIOSH (NOES Survey 1981-1983) has statistically estimated that 442,749 workers (144,063 of these were female) were potentially exposed to hydroquinone in the US(1). Occupational exposure to hydroquinone may occur by inhalation or dermal contact in workplaces where hydroquinone is produced or used(SRC). Occupational exposure to hydroquinone may occur through inhalation and dermal contact with this compound at workplaces where hydroquinone is produced or used. Monitoring/use data indicate that the general population may be exposed to hydroquinone via inhalation of wood or cigarette smoke, ingestion of certain foods and dermal contact with consumer products containing hydroquinone(SRC).
(1) NIOSH; NOES. National Occupational Exposure Survey conducted from 1981-1983. Estimated numbers of employees potentially exposed to specific agents by 2-digit standard industrial classification (SIC). Available from, as of Aug 4, 2014: https://www.cdc.gov/noes/

14.2.21 Body Burden

During an environmental benzene exposure assessment for parent-child pairs in Rouen, France, hydroquinone, a urinary metabolite of benzene, was detected in urine samples collected from a group of parents and their children. The parent's urine samples (n = 22) contained a mean hydroquinone concentration of 2.50 mg/L while the children's samples (n = 21) contained a mean concentration of 1.11 mg/L(1).
(1) Kouniali A et al; France Sci Total Environ 308: 73-82 (2003)

15 Associated Disorders and Diseases

Associated Occupational Diseases with Exposure to the Compound
Contact dermatitis, allergic [Category: Skin Disease]
Disease
Uremia
References

PubMed: 2026685, 8087979, 9607216, 17132244, 6520173, 21359215, 15353324, 11865086, 9573551, 10509899, 7482520, 19309105, 24023812, 22626821, 12675874

Merck Manual of Diagnosis and Therapy.

Geigy Scientific Tables, 8th Rev edition, pp. 165-177. Edited by C. Lentner, West Cadwell, N.J.: Medical education Div., Ciba-Geigy Corp., Basel, Switzerland c1981-1992.

Geigy Scientific Tables, 8th Rev edition, pp. 80-82. Edited by C. Lentner, West Cadwell, N.J.: Medical education Div., Ciba-Geigy Corp., Basel, Switzerland c1981-1992.

Geigy Scientific Tables, 8th Rev edition, pp. 130. Edited by C. Lentner, West Cadwell, N.J.: Medical education Div., Ciba-Geigy Corp. Basel, Switzerland c1981-1992.

David F. Putnam Composition and Concentrative Properties of Human Urine. NASA Contractor Report. July 1971

National Health and Nutrition Examination Survey (NHANES Survey) 2013

16 Literature

16.1 Consolidated References

16.2 NLM Curated PubMed Citations

16.3 Springer Nature References

16.4 Thieme References

16.5 Wiley References

16.6 Nature Journal References

16.7 Chemical Co-Occurrences in Literature

16.8 Chemical-Gene Co-Occurrences in Literature

16.9 Chemical-Disease Co-Occurrences in Literature

17 Patents

17.1 Depositor-Supplied Patent Identifiers

17.2 WIPO PATENTSCOPE

17.3 Chemical Co-Occurrences in Patents

17.4 Chemical-Disease Co-Occurrences in Patents

17.5 Chemical-Gene Co-Occurrences in Patents

18 Interactions and Pathways

18.1 Protein Bound 3D Structures

18.1.1 Ligands from Protein Bound 3D Structures

PDBe Ligand Code
PDBe Structure Code
PDBe Conformer

18.2 Chemical-Target Interactions

18.3 Pathways

19 Biological Test Results

19.1 BioAssay Results

20 Taxonomy

The LOTUS Initiative for Open Natural Products Research: frozen dataset union wikidata (with metadata) | DOI:10.5281/zenodo.5794106
S29 | PHYTOTOXINS | Toxic Plant Phytotoxin (TPPT) Database | DOI:10.5281/zenodo.2652993
A metabolome atlas of the aging mouse brain. Nat Commun. 2021 Oct 15;12(1):6021. DOI:10.1038/s41467-021-26310-y. PMID:34654818; PMCID:PMC8519999.
The Metabolome Atlas of the Aging Mouse Brain: https://mouse.atlas.metabolomics.us

21 Classification

21.1 MeSH Tree

21.2 NCI Thesaurus Tree

21.3 ChEBI Ontology

21.4 KEGG: ATC

21.5 KEGG: Target-based Classification of Drugs

21.6 KEGG: Drug Groups

21.7 WHO ATC Classification System

21.8 FDA Pharm Classes

21.9 ChemIDplus

21.10 CAMEO Chemicals

21.11 ChEMBL Target Tree

21.12 UN GHS Classification

21.13 EPA CPDat Classification

21.14 NORMAN Suspect List Exchange Classification

21.15 CCSBase Classification

21.16 EPA DSSTox Classification

21.17 International Agency for Research on Cancer (IARC) Classification

21.18 Consumer Product Information Database Classification

21.19 EPA TSCA and CDR Classification

21.20 LOTUS Tree

21.21 FDA Drug Type and Pharmacologic Classification

21.22 EPA Substance Registry Services Tree

21.23 MolGenie Organic Chemistry Ontology

22 Information Sources

  1. Australian Industrial Chemicals Introduction Scheme (AICIS)
  2. CAMEO Chemicals
    LICENSE
    CAMEO Chemicals and all other CAMEO products are available at no charge to those organizations and individuals (recipients) responsible for the safe handling of chemicals. However, some of the chemical data itself is subject to the copyright restrictions of the companies or organizations that provided the data.
    https://cameochemicals.noaa.gov/help/reference/terms_and_conditions.htm?d_f=false
    CAMEO Chemical Reactivity Classification
    https://cameochemicals.noaa.gov/browse/react
  3. CAS Common Chemistry
    LICENSE
    The data from CAS Common Chemistry is provided under a CC-BY-NC 4.0 license, unless otherwise stated.
    https://creativecommons.org/licenses/by-nc/4.0/
  4. ChemIDplus
    ChemIDplus Chemical Information Classification
    https://pubchem.ncbi.nlm.nih.gov/source/ChemIDplus
  5. DrugBank
    LICENSE
    Creative Common's Attribution-NonCommercial 4.0 International License (http://creativecommons.org/licenses/by-nc/4.0/legalcode)
    https://www.drugbank.ca/legal/terms_of_use
  6. DTP/NCI
    LICENSE
    Unless otherwise indicated, all text within NCI products is free of copyright and may be reused without our permission. Credit the National Cancer Institute as the source.
    https://www.cancer.gov/policies/copyright-reuse
  7. EPA Chemical Data Reporting (CDR)
    LICENSE
    The U.S. Government retains a nonexclusive, royalty-free license to publish or reproduce these documents, or allow others to do so, for U.S. Government purposes. These documents may be freely distributed and used for non-commercial, scientific and educational purposes.
    https://www.epa.gov/web-policies-and-procedures/epa-disclaimers#copyright
  8. EPA Chemicals under the TSCA
    EPA TSCA Classification
    https://www.epa.gov/tsca-inventory
  9. EPA DSSTox
    CompTox Chemicals Dashboard Chemical Lists
    https://comptox.epa.gov/dashboard/chemical-lists/
  10. EPA Integrated Risk Information System (IRIS)
  11. EPA Provisional Peer-Reviewed Toxicity Values (PPRTVs)
  12. European Chemicals Agency (ECHA)
    LICENSE
    Use of the information, documents and data from the ECHA website is subject to the terms and conditions of this Legal Notice, and subject to other binding limitations provided for under applicable law, the information, documents and data made available on the ECHA website may be reproduced, distributed and/or used, totally or in part, for non-commercial purposes provided that ECHA is acknowledged as the source: "Source: European Chemicals Agency, http://echa.europa.eu/". Such acknowledgement must be included in each copy of the material. ECHA permits and encourages organisations and individuals to create links to the ECHA website under the following cumulative conditions: Links can only be made to webpages that provide a link to the Legal Notice page.
    https://echa.europa.eu/web/guest/legal-notice
  13. FDA Global Substance Registration System (GSRS)
    LICENSE
    Unless otherwise noted, the contents of the FDA website (www.fda.gov), both text and graphics, are not copyrighted. They are in the public domain and may be republished, reprinted and otherwise used freely by anyone without the need to obtain permission from FDA. Credit to the U.S. Food and Drug Administration as the source is appreciated but not required.
    https://www.fda.gov/about-fda/about-website/website-policies#linking
  14. Hazardous Substances Data Bank (HSDB)
  15. Human Metabolome Database (HMDB)
    LICENSE
    HMDB is offered to the public as a freely available resource. Use and re-distribution of the data, in whole or in part, for commercial purposes requires explicit permission of the authors and explicit acknowledgment of the source material (HMDB) and the original publication (see the HMDB citing page). We ask that users who download significant portions of the database cite the HMDB paper in any resulting publications.
    http://www.hmdb.ca/citing
  16. ILO-WHO International Chemical Safety Cards (ICSCs)
  17. New Zealand Environmental Protection Authority (EPA)
    LICENSE
    This work is licensed under the Creative Commons Attribution-ShareAlike 4.0 International licence.
    https://www.epa.govt.nz/about-this-site/general-copyright-statement/
  18. NJDOH RTK Hazardous Substance List
  19. Occupational Safety and Health Administration (OSHA)
    LICENSE
    Materials created by the federal government are generally part of the public domain and may be used, reproduced and distributed without permission. Therefore, content on this website which is in the public domain may be used without the prior permission of the U.S. Department of Labor (DOL). Warning: Some content - including both images and text - may be the copyrighted property of others and used by the DOL under a license.
    https://www.dol.gov/general/aboutdol/copyright
    HYDROQUINONE (DIHYDROXYBENZENE)
    https://www.osha.gov/chemicaldata/175
  20. Risk Assessment Information System (RAIS)
    LICENSE
    This work has been sponsored by the U.S. Department of Energy (DOE), Office of Environmental Management, Oak Ridge Operations (ORO) Office through a joint collaboration between United Cleanup Oak Ridge LLC (UCOR), Oak Ridge National Laboratory (ORNL), and The University of Tennessee, Ecology and Evolutionary Biology, The Institute for Environmental Modeling (TIEM). All rights reserved.
    https://rais.ornl.gov/
  21. The National Institute for Occupational Safety and Health (NIOSH)
    LICENSE
    The information provided using CDC Web site is only intended to be general summary information to the public. It is not intended to take the place of either the written law or regulations.
    https://www.cdc.gov/Other/disclaimer.html
  22. California Safe Cosmetics Program (CSCP) Product Database
  23. BindingDB
    LICENSE
    All data curated by BindingDB staff are provided under the Creative Commons Attribution 3.0 License (https://creativecommons.org/licenses/by/3.0/us/).
    https://www.bindingdb.org/rwd/bind/info.jsp
  24. Comparative Toxicogenomics Database (CTD)
    LICENSE
    It is to be used only for research and educational purposes. Any reproduction or use for commercial purpose is prohibited without the prior express written permission of NC State University.
    http://ctdbase.org/about/legal.jsp
  25. Drug Gene Interaction database (DGIdb)
    LICENSE
    The data used in DGIdb is all open access and where possible made available as raw data dumps in the downloads section.
    http://www.dgidb.org/downloads
  26. Therapeutic Target Database (TTD)
  27. Toxin and Toxin Target Database (T3DB)
    LICENSE
    T3DB is offered to the public as a freely available resource. Use and re-distribution of the data, in whole or in part, for commercial purposes requires explicit permission of the authors and explicit acknowledgment of the source material (T3DB) and the original publication.
    http://www.t3db.ca/downloads
  28. Haz-Map, Information on Hazardous Chemicals and Occupational Diseases
    LICENSE
    Copyright (c) 2022 Haz-Map(R). All rights reserved. Unless otherwise indicated, all materials from Haz-Map are copyrighted by Haz-Map(R). No part of these materials, either text or image may be used for any purpose other than for personal use. Therefore, reproduction, modification, storage in a retrieval system or retransmission, in any form or by any means, electronic, mechanical or otherwise, for reasons other than personal use, is strictly prohibited without prior written permission.
    https://haz-map.com/About
  29. ChEBI
  30. E. coli Metabolome Database (ECMDB)
    LICENSE
    ECMDB is offered to the public as a freely available resource.
    https://ecmdb.ca/citations
  31. EPA Air Toxics
  32. FDA Pharm Classes
    LICENSE
    Unless otherwise noted, the contents of the FDA website (www.fda.gov), both text and graphics, are not copyrighted. They are in the public domain and may be republished, reprinted and otherwise used freely by anyone without the need to obtain permission from FDA. Credit to the U.S. Food and Drug Administration as the source is appreciated but not required.
    https://www.fda.gov/about-fda/about-website/website-policies#linking
  33. LOTUS - the natural products occurrence database
    LICENSE
    The code for LOTUS is released under the GNU General Public License v3.0.
    https://lotus.nprod.net/
  34. NCI Thesaurus (NCIt)
    LICENSE
    Unless otherwise indicated, all text within NCI products is free of copyright and may be reused without our permission. Credit the National Cancer Institute as the source.
    https://www.cancer.gov/policies/copyright-reuse
  35. Open Targets
    LICENSE
    Datasets generated by the Open Targets Platform are freely available for download.
    https://platform-docs.opentargets.org/licence
  36. CCSbase
    CCSbase Classification
    https://ccsbase.net/
  37. NORMAN Suspect List Exchange
    LICENSE
    Data: CC-BY 4.0; Code (hosted by ECI, LCSB): Artistic-2.0
    https://creativecommons.org/licenses/by/4.0/
    Hydroquinone
    NORMAN Suspect List Exchange Classification
    https://www.norman-network.com/nds/SLE/
  38. ChEMBL
    LICENSE
    Access to the web interface of ChEMBL is made under the EBI's Terms of Use (http://www.ebi.ac.uk/Information/termsofuse.html). The ChEMBL data is made available on a Creative Commons Attribution-Share Alike 3.0 Unported License (http://creativecommons.org/licenses/by-sa/3.0/).
    http://www.ebi.ac.uk/Information/termsofuse.html
  39. ClinicalTrials.gov
    LICENSE
    The ClinicalTrials.gov data carry an international copyright outside the United States and its Territories or Possessions. Some ClinicalTrials.gov data may be subject to the copyright of third parties; you should consult these entities for any additional terms of use.
    https://clinicaltrials.gov/ct2/about-site/terms-conditions#Use
  40. Consumer Product Information Database (CPID)
    LICENSE
    Copyright (c) 2024 DeLima Associates. All rights reserved. Unless otherwise indicated, all materials from CPID are copyrighted by DeLima Associates. No part of these materials, either text or image may be used for any purpose other than for personal use. Therefore, reproduction, modification, storage in a retrieval system or retransmission, in any form or by any means, electronic, mechanical or otherwise, for reasons other than personal use, is strictly prohibited without prior written permission.
    https://www.whatsinproducts.com/contents/view/1/6
    Consumer Products Category Classification
    https://www.whatsinproducts.com/
  41. Cosmetic Ingredient Review (CIR)
  42. EPA Chemical and Products Database (CPDat)
  43. Crystallography Open Database (COD)
    LICENSE
    All data in the COD and the database itself are dedicated to the public domain and licensed under the CC0 License. Users of the data should acknowledge the original authors of the structural data.
    https://creativecommons.org/publicdomain/zero/1.0/
  44. DailyMed
  45. Drugs and Lactation Database (LactMed)
  46. Drugs@FDA
    LICENSE
    Unless otherwise noted, the contents of the FDA website (www.fda.gov), both text and graphics, are not copyrighted. They are in the public domain and may be republished, reprinted and otherwise used freely by anyone without the need to obtain permission from FDA. Credit to the U.S. Food and Drug Administration as the source is appreciated but not required.
    https://www.fda.gov/about-fda/about-website/website-policies#linking
  47. KNApSAcK Species-Metabolite Database
  48. Natural Product Activity and Species Source (NPASS)
  49. West Coast Metabolomics Center-UC Davis
    Hydroquinone
  50. FDA Orange Book
    LICENSE
    Unless otherwise noted, the contents of the FDA website (www.fda.gov), both text and graphics, are not copyrighted. They are in the public domain and may be republished, reprinted and otherwise used freely by anyone without the need to obtain permission from FDA. Credit to the U.S. Food and Drug Administration as the source is appreciated but not required.
    https://www.fda.gov/about-fda/about-website/website-policies#linking
  51. EPA Regional Screening Levels for Chemical Contaminants at Superfund Sites
  52. EU Clinical Trials Register
  53. Hazardous Chemical Information System (HCIS), Safe Work Australia
  54. NITE-CMC
    Hydroquinone - FY2012 (Revised classification)
    https://www.chem-info.nite.go.jp/chem/english/ghs/12-mhlw-2013e.html
    Hydroquinone - FY2007 (Revised classification)
    https://www.chem-info.nite.go.jp/chem/english/ghs/07-meti-2007e.html
    Hydroquinone - FY2006 (New/original classication)
    https://www.chem-info.nite.go.jp/chem/english/ghs/06-imcg-0173e.html
  55. Regulation (EC) No 1272/2008 of the European Parliament and of the Council
    LICENSE
    The copyright for the editorial content of this source, the summaries of EU legislation and the consolidated texts, which is owned by the EU, is licensed under the Creative Commons Attribution 4.0 International licence.
    https://eur-lex.europa.eu/content/legal-notice/legal-notice.html
    1,4-dihydroxybenzene; hydroquinone; quinol
    https://eur-lex.europa.eu/eli/reg/2008/1272/oj
  56. National Drug Code (NDC) Directory
    LICENSE
    Unless otherwise noted, the contents of the FDA website (www.fda.gov), both text and graphics, are not copyrighted. They are in the public domain and may be republished, reprinted and otherwise used freely by anyone without the need to obtain permission from FDA. Credit to the U.S. Food and Drug Administration as the source is appreciated but not required.
    https://www.fda.gov/about-fda/about-website/website-policies#linking
  57. FooDB
    LICENSE
    FooDB is offered to the public as a freely available resource. Use and re-distribution of the data, in whole or in part, for commercial purposes requires explicit permission of the authors and explicit acknowledgment of the source material (FooDB) and the original publication.
    https://foodb.ca/about
  58. IUPAC Digitized pKa Dataset
  59. NMRShiftDB
  60. MassBank Europe
  61. MassBank of North America (MoNA)
    LICENSE
    The content of the MoNA database is licensed under CC BY 4.0.
    https://mona.fiehnlab.ucdavis.edu/documentation/license
  62. NIST Mass Spectrometry Data Center
    LICENSE
    Data covered by the Standard Reference Data Act of 1968 as amended.
    https://www.nist.gov/srd/public-law
  63. SpectraBase
    HYDROQUINONE*POLYMERIZATION INHIBITOR AND ARRESTER
    https://spectrabase.com/spectrum/LTFxvguZBuD
    HYDROQUINONE*POLYMERIZATION INHIBITOR AND ARRESTER
    https://spectrabase.com/spectrum/8VZAo7LWtUK
  64. International Agency for Research on Cancer (IARC)
    LICENSE
    Materials made available by IARC/WHO enjoy copyright protection under the Berne Convention for the Protection of Literature and Artistic Works, under other international conventions, and under national laws on copyright and neighbouring rights. IARC exercises copyright over its Materials to make sure that they are used in accordance with the Agency's principles. All rights are reserved.
    https://publications.iarc.fr/Terms-Of-Use
    IARC Classification
    https://www.iarc.fr/
  65. NTP Technical Reports
  66. Japan Chemical Substance Dictionary (Nikkaji)
  67. KEGG
    LICENSE
    Academic users may freely use the KEGG website. Non-academic use of KEGG generally requires a commercial license
    https://www.kegg.jp/kegg/legal.html
    Anatomical Therapeutic Chemical (ATC) classification
    http://www.genome.jp/kegg-bin/get_htext?br08303.keg
    Target-based classification of drugs
    http://www.genome.jp/kegg-bin/get_htext?br08310.keg
  68. Kruve Lab, Ionization & Mass Spectrometry, Stockholm University
    4-hydroxyphenol
  69. Metabolomics Workbench
  70. Nature Chemistry
  71. NIOSH Manual of Analytical Methods
    LICENSE
    The information provided using CDC Web site is only intended to be general summary information to the public. It is not intended to take the place of either the written law or regulations.
    https://www.cdc.gov/Other/disclaimer.html
  72. NIPH Clinical Trials Search of Japan
  73. NLM RxNorm Terminology
    LICENSE
    The RxNorm Terminology is created by the National Library of Medicine (NLM) and is in the public domain and may be republished, reprinted and otherwise used freely by anyone without the need to obtain permission from NLM. Credit to the U.S. National Library of Medicine as the source is appreciated but not required. The full RxNorm dataset requires a free license.
    https://www.nlm.nih.gov/research/umls/rxnorm/docs/termsofservice.html
  74. Pharos
    LICENSE
    Data accessed from Pharos and TCRD is publicly available from the primary sources listed above. Please respect their individual licenses regarding proper use and redistribution.
    https://pharos.nih.gov/about
  75. Pistoia Alliance Chemical Safety Library
    ACROLEIN + HYDROQUINONE
    https://safescience.cas.org/
  76. Protein Data Bank in Europe (PDBe)
  77. RCSB Protein Data Bank (RCSB PDB)
    LICENSE
    Data files contained in the PDB archive (ftp://ftp.wwpdb.org) are free of all copyright restrictions and made fully and freely available for both non-commercial and commercial use. Users of the data should attribute the original authors of that structural data.
    https://www.rcsb.org/pages/policies
  78. Rhea - Annotated Reactions Database
    LICENSE
    Rhea has chosen to apply the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/). This means that you are free to copy, distribute, display and make commercial use of the database in all legislations, provided you credit (cite) Rhea.
    https://www.rhea-db.org/help/license-disclaimer
  79. Springer Nature
  80. SpringerMaterials
  81. The Cambridge Structural Database
  82. Thieme Chemistry
    LICENSE
    The Thieme Chemistry contribution within PubChem is provided under a CC-BY-NC-ND 4.0 license, unless otherwise stated.
    https://creativecommons.org/licenses/by-nc-nd/4.0/
  83. WHO Anatomical Therapeutic Chemical (ATC) Classification
    LICENSE
    Use of all or parts of the material requires reference to the WHO Collaborating Centre for Drug Statistics Methodology. Copying and distribution for commercial purposes is not allowed. Changing or manipulating the material is not allowed.
    https://www.whocc.no/copyright_disclaimer/
  84. Wikidata
  85. Wikipedia
  86. Wiley
  87. PubChem
  88. Medical Subject Headings (MeSH)
    LICENSE
    Works produced by the U.S. government are not subject to copyright protection in the United States. Any such works found on National Library of Medicine (NLM) Web sites may be freely used or reproduced without permission in the U.S.
    https://www.nlm.nih.gov/copyright.html
  89. GHS Classification (UNECE)
  90. EPA Substance Registry Services
  91. MolGenie
    MolGenie Organic Chemistry Ontology
    https://github.com/MolGenie/ontology/
  92. PATENTSCOPE (WIPO)
  93. NCBI
CONTENTS