Home Air OEHHA Acute, 8-hour and Chronic Reference Exposure Level (REL)s

Air Toxicology and Epidemiology

All OEHHA Acute, 8-hour and Chronic Reference Exposure Levels (chRELs) as of June 2014

Follow the links below to download documentation on the reference exposure levels.

Footnotes:

[1]REL types: A = acute, 8 = 8-hour, C = chronic. Exposure averaging time for acute RELs is 1 hour. For 8-hour RELs, 

the exposure averaging time is 8 hours, which may be repeated. Chronic RELs are designed to address continuous 

exposures for up to a lifetime: the exposure metric used is the annual average exposure.

[2]Species used in key study for REL development: D = dog; Gb = gerbil; GP = guinea pig: H = human; Ha = hamster; M 

= mouse; Mk = monkey; R = rat; Rb = rabbit

[3]These peer-reviewed chronic REL values were developed under the Toxic Air Contaminant (TAC) Program mandated 

by AB1807.

[4]REL based on benchmark dose (BMC) approach.

[5]REL developed using the revised methodology (OEHHA, 2008)].

OEHHA Acute, 8-hour and Chronic Reference Exposure Level (REL) Summary1

Substance

REL 

type

[1]

Inhalation 

REL

(ug/m3) 

Oral REL

(ug/kg BW-day)

Hazard Index

Target Organs 

Species

[2]

Acetaldehyde (75-07-0)

A 470[5] Eyes; respiratory system 

(sensory irritation) 

H

8 300[4,5] Respiratory system R

C 140[4,5] Respiratory system R

Acrolein (107-02-8)

A 2.5[5] Eyes, respiratory system 

(sensory irritation)

H 

8 0.7[5] Respiratory system R

C 0.35[5] Respiratory system R

Acrylic Acid (79-10-7) A 6,000 Respiratory system; eyes R

Acrylonitrile (107-13-1) C 5[4] Respiratory system R

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Ammonia (7664-41-7)

A 3200[4] Respiratory system; eyes H

C 200 Respiratory system H

Arsenic (7440-38-2) & inorganic arsenic 

compounds (including arsine)

A 0.20[5] Development; 

cardiovascular system; 

nervous system

M

8 0.015[5] Development; 

cardiovascular system; 

nervous system; 

respiratory system; skin

H

C 0.015[5] 0.0035[5] Inhalation & oral: 

Development; 

cardiovascular system; 

nervous system; 

respiratory system; skin

H

Benzene (71-43-2)

A 27 Developmental; Immune 

system; Hematologic 

system

M

8 3 Hematologic system H

C 3 Hematologic system H

Benzyl Chloride(100-44-7) A 240 Respiratory system; eyes M, R

Beryllium& beryllium compounds (7440-41-7)

C 0.007 2.0 Inhalation: Respiratory 

system; immune system 

Oral: Alimentary system 

(Gastrointestinal tract) 

H

Butadiene (106-99-0)

A 660[4,5] Development M

8 9[4,5] Reproductive system M

C 2[4,5] Reproductive system M

Cadmium & cadmium compounds (7440-

43-9)

C 0.02 0.5 Inhalation: Kidney; 

respiratory system

Oral: kidney 

H

Carbon disulfide (75-15-0)

A 6,200 Reproductive/ 

Development; nervous 

system

R

C 800[4] Nervous system; 

reproductive system

H

Carbon monoxide (630-08-0) A 23,000 Cardiovascular system H

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Caprolactam (105-60-2) 

A 50 Eyes (sensory irritation) H

8 7 Respiratory system R

C 2.2 Respiratory system R

Carbon tetrachloride (56-23-5) 

A 1,900 Alimentary system (liver); 

Reproductive/ 

Developmental; nervous 

system

R

C 40 Alimentary and nervous 

systems; development

GP

Chlorinated dibenzo-p dioxins and

dibenzofurans

Unspeciated mixtures treated as 

2,3,7,8-tetrachlorodibenzo-p-dioxin (1746-01-6)

C 0.00004 1 x 10-5 Inhalation and Oral: 

Alimentary (liver) 

reproductive, endocrine, 

respiratory, hematologic 

systems; development

R

Chlorine (7782-50-5) 

A 210 Respiratory system; eyes H

C 0.2[4] Respiratory system R

Chlorine dioxide (10049-04-4) C 0.6 Respiratory system R

Chlorobenzene (108-90-7)

C 1,000 Alimentary system (liver); 

kidney; reproductive 

system

R

Chloroform (67-66-3) 

A 150 Reproductive/ 

Developmental; 

respiratory system; 

nervous system

R

C 300 Alimentary system; 

kidney; development

R

Chloropicrin (76-06-2) 

A 29 Respiratory system; eyes M

C 0.4[4] Respiratory system M

Chromic trioxide (as chromic acid mist)

C 0.002 20 Inhalation: Respiratory 

system

Oral: Hematologic system 

H

Chromium (hexavalent) (18540-29-9) & soluble 

hexavalent chromium compounds (except 

chromic trioxide) 

C 0.2[4] 20 Inhalation: Respiratory 

system 

Oral: Hematologic system 

R

Copper and compounds A 100 Respiratory system H

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Cresol mixtures (1319-77-3) C 600 Nervous system R

Dichlorobenzene (1,4-) (106-46-7)

C 800 Nervous and respiratory; 

alimentary systems 

(liver); kidney

R

Dichloroethylene (1,1) (75-35-4) C 70 Alimentary system (liver) GP

Diesel Exhaust C 5[3] Respiratory system R

Diethanolamine (111-42-2) C 3 Respiratory and 

hematologic systems

R

Dimethylformamide (N,N-) (68-12-2) C 80 Alimentary (liver) and 

respiratory systems

H

Dioxane (1,4-) (123-91-1) 

A 3,000 Respiratory system; eyes H

C 3,000 Alimentary system; 

kidney; cardiovascular 

system

R

Epichlorohydrin (106-89-8) 

A 1,300 Respiratory system; eyes H

C 3 Respiratory system; eyes R

Epoxybutane (1,2-) (106-88-7) C 20 Respiratory system; 

cardiovascular system

M

Ethylbenzene(100-41-4)

C 2,000 Alimentary system (liver); 

kidney; endocrine 

system; development

M, R

Ethyl chloride (75-00-3) C 30,000 Development; alimentary 

system (liver)

M

Ethylene dibromide (106-93-4) C 0.8 Reproductive system H

Ethylene dichloride (107-06-2) C 400 Alimentary system (liver) R

Ethylene glycol (107-21-1) C 400 Respiratory system; 

kidney; development

H

Ethylene glycol monobutyl ether (111-76-2) A 14,000 Respiratory system; eyes H

Ethylene glycol monoethyl ether (110-80-5) A 370 Reproductive/ 

Development

R

C 70 Reproductive system; 

hemotologic system

Rb

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Ethylene glycol monoethyl ether acetate

(111-15-9)

A 140 Reproductive/ 

Development; nervous 

system

R

C 300 Development Rb

Ethylene glycol monomethyl ether (109-86-4) 

A 93 Reproductive/ 

Development

R

C 60 Reproductive system Rb

Ethylene glycol monomethyl ether acetate

(110-49-6) 

C 90 Reproductive system Rb

Ethylene oxide (75-21-8) C 30 Nervous system R

Fluorides (except Hydrogen Fluoride - listed 

below separately)

C 13[4] 40 Inhalation: Bone and 

teeth; respiratory system

Oral: Bone and teeth

H

Formaldehyde (50-00-0)

A 55[5] Eyes (Sensory irritation) H 

8 9[5] Respiratory system H

C 9[5] Respiratory system H 

Glutaraldehyde (111-30-8) C 0.08[4] Respiratory system M

Hexane (n-)(110-54-3) C 7000 Nervous system H

Hydrazine (302-01-2) C 0.2 Alimentary system (liver); 

endocrine system 

Ha

Hydrogen chloride (7647-01-0) 

A 2,100 Respiratory system; eyes H

C 9 Respiratory system H

Hydrogen cyanide (74-90-8)

A 340 Nervous system H

C 9 Nervous system; 

endocrine system; 

cardiovascular system

H

Hydrogen fluoride (7664-39-3) A 240 Respiratory system; eyes H

C 14[4] 40 Inhalation: Bone and 

teeth; respiratory system 

(See “fluorides” 

summary)

H

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Oral: Bone and teeth

Hydrogen selenide (7783-07-5) A 5 Respiratory system; eyes GP

Hydrogen sulfide (7783-06-4) 

A 42 Nervous system H

C 10 Respiratory system M

Isophorone (78-59-1) C 2,000 Development; alimentary 

system (liver)

R, M

Isopropanol (67-63-0) 

A 3,200 Eyes; respiratory system H

C 7,000 Kidney; development R, M

Maleic anhydride (108-31-6) C 0.7[4] Respiratory system R,Ha, 

Mk

Manganese (7439-96-5) & manganese 

compounds

8 0.17[4,5] Nervous system H

C 0.09[4,5] Nervous system H

Mercury (7439-97-6) & inorganic mercury 

compounds

A 0.6[5] Nervous system; 

development

R

8 0.06[5] Nervous system; 

development; kidney

H

C 0.03[5] 0.16[5] Inhalation & Oral: 

Nervous system; 

development; kidney

H

Methanol (67-56-1) 

A 28,000 Nervous system H

C 4,000[4] Development M

Methyl bromide (74-83-9) 

A 3900 Nervous system; 

respiratory system; 

Reproductive/ 

development

H

C 5 Respiratory system; 

nervous system; 

development

R

Methyl chloroform (71-55-6) 

A 68,000 Nervous system H

C 1000 Nervous system Gb

Methylene chloride (75-09-2) A 14,000 H

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Cardiovascular system; 

Nervous system

C 400 Cardiovascular system; 

nervous system

H

Methylene dianiline (4,4’-) (101-77-9) C 20 Eyes; alimentary system 

(liver)

GP

Methylene diphenyl isocyanate (101-68-8) C 0.7[4] Respiratory system R

Methyl ethyl ketone (78-93-3) A 13,000 Respiratory system; eyes H

Methyl isocyanate (624-83-9) C 1 Respiratory system; 

reproductive system

R

Methyl t-butyl ether (1634-04-4) C 8,000 Kidney; eyes; alimentary 

system (liver)

R

Naphthalene (91-20-3) C 9 Respiratory system H

Nickel & nickel compounds (except nickel 

oxide for chronic inhalation exposures) 

(Inhalation concentrations as μg Ni/m3: oral 

dose as μg Ni/kg-day)

A 0.2[5] Immune system M

8 0.06[5] Respiratory, immune 

systems R

C 0.014[5] 11

[5]

Inhalation: Respiratory 

system; hematologic 

system

Oral: Development

R

Nickel oxide (1313-99-1) (Inhalation 

concentration as μg Ni/m3: oral dose as μg 

Ni/kg-day) 

C 0.02[5] 11[5] Inhalation: Respiratory 

system

Oral: Development

M

R

Nitric acid (7697-37-2) A 86 Respiratory system H

Nitrogen dioxide (10102-44-0) A 470 Respiratory system H

Ozone (10028-15-6) A 180 Respiratory system; eyes H

Perchloroethylene (127-18-4) 

(syn. Tetrachloroethylene)[3]

A 20,000 Nervous system; 

respiratory system; eyes

H

C 35 Kidney; alimentary 

system (liver)

M

Phenol (108-95-2) A 5,800 Respiratory system; eyes H

C 200 Alimentary system; 

cardiovascular system; 

kidney; nervous system

R

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Phosgene (75-44-5) A 4 Respiratory system R

Phosphine (7803-51-2)

C 0.8 Respiratory system; 

alimentary system (liver); 

nervous system; kidney; 

hematologic system

M

Phosphoric acid (7664-38-2) C 7[4] Respiratory system R

Polychlorinated biphenyls (PCBs) 

Individual congeners evaluated using TEF 

methodology, relative to as 

2,3,7,8-tetrachlorodibenzo-p-dioxin (see 

Appendix C in the TSD for Cancer Potency 

Factors – online at: 

http://oehha.ca.gov/air/hot_spots/tsd052909.html

C - - Inhalation & oral: 

Alimentary (liver) 

reproductive, endocrine, 

respiratory, hematologic 

systems; development

R

Phthalic anhydride (85-44-9) C 20 Respiratory system H

Propylene(115-07-1) C 3,000 Respiratory system R

Propylene glycol monomethyl ether (107-

98-2) 

C 7,000 Alimentary system (liver) R

Propylene oxide (75-56-9) 

A 3,100 Respiratory system; eyes; 

reproductive/development

H

C 30 Respiratory system R

Selenium and selenium compounds (other than 

hydrogen selenide) 

C 20 5 Inhalation & oral: 

Alimentary system (liver); 

cardiovascular system; 

nervous system

H

Silica (crystalline, respirable) C 3[4] Respiratory system H

Sodium hydroxide (1310-93-2) A 8 Respiratory system; eyes; 

skin

H

Styrene (100-42-5)

A 21,000 Respiratory system; eyes; 

reproductive/development

H

C 900[4] Nervous system H

Sulfates A 120 Respiratory system H

Sulfur dioxide (7446-09-5) A 660 Respiratory system H

Sulfuric acid (7664-93-9) [& oleum, acute only] A 120 Respiratory system H

C 1 Respiratory system Mk

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Toluene (108-88-3) 

A 37,000 Respiratory, nervous 

systems; eyes 

reproductive/development

H

C 300 Nervous system; 

respiratory system; 

development

R

Toluene diisocyanates (2,4- & 2,6-) C 0.07 Respiratory system H

Trichloroethylene (79-01-6) C 600 Nervous system; eyes H

Triethylamine (121-44-8) 

A 2,800 Nervous system; eyes H

C 200 Eyes R

Vanadium pentoxide(1314-62-1) A 30 Respiratory system; eyes H

Vinyl acetate (108-05-4) C 200 Respiratory system R, M

Vinyl chloride (75-01-4) A 180,000 Nervous system; 

respiratory system; eyes

H

Xylenes: technical mixture (1330-20-7) and 

o-xylene (95-47-6), m-xylene (108-38-3) and 

p-xylene (106-42-3) isomers.

A 22000 Nervous & respiratory 

systems; eyes

H

C 700 Nervous & respiratory 

systems; eyes

H

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