Section: 16 | Threshold Limits for Airborne Contaminants |
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The recommended form of citation is:
John R. Rumble, ed., CRC Handbook of Chemistry and Physics, 103rd Edition (Internet Version 2022), CRC Press/Taylor & Francis, Boca Raton, FL.
If a specific table is cited, use the format: "Physical Constants of Organic Compounds," in CRC Handbook of Chemistry and Physics, 103rd Edition (Internet Version 2022), John R. Rumble, ed., CRC Press/Taylor & Francis, Boca Raton, FL.


Several organizations recommend limits of exposure to airborne contaminants in the workplace. These include the United States National Institute for Occupational Safety and Health (NIOSH)-recommended exposure limits (RELs), which are intended to limit exposure to hazardous substances in workplace air to protect workers, and the non-governmental organization, American Conference of Governmental Industrial Hygienists (ACGIH). Threshold Limit Values (TLVs®) are the airborne concentrations of chemical substances under which it is believed that nearly all workers may be repeatedly exposed, day after day, over a working lifetime, without adverse effects.

In contrast to the ACGIH- and NIOSH-recommended limits of exposure, the United States Occupational Safety and Health Administration (OSHA) has established legal occupational exposure limits known as permissible exposure limits (PELs) which are based on a consideration of factors in addition to the adverse effects associated with airborne contaminants. When establishing PELs, OSHA takes into account technical feasibility and economic considerations for employers in addition to the adverse effects associated with the airborne contaminant. While these PELs are legal exposure limits, OSHA recognizes that many of its PELs are outdated and inadequate for ensuring protection of worker health. Most of OSHA’s PELs were issued shortly after adoption of the Occupational Safety and Health (OSH) Act in 1970 and have not been updated since that time. OSHA has stated that alternate occupational exposure limits such as NIOSH RELs and ACGIH TLVs may serve to better protect workers.

The following table gives threshold limit values for substances that may be encountered in the atmosphere of a chemical laboratory or industrial facility. The table includes NIOSH RELs and ACGIH TLVs, but not OSHA PELs given OSHA’s caution about PELs.

NIOSH RELs and the ACGIH TLVs are given in two forms:

All values refer to the concentration in air at 25 °C and normal atmospheric pressure. Data for gases are given in parts per million by volume (ppm). Values for liquids refer to mists or aerosols, and those for solids to dusts or fumes; both are stated in mass concentration units (mg/m3). In some cases, a “Ceiling value” is provided which indicates a ceiling limit that should not be exceeded even for very brief periods because of acute toxic effects of the substance. The notation “levels as low as possible” in the Comments column indicates such a high degree of hazard that no safe limit can be recommended.  Expressions such as "inhalable fraction and vapor" and "inhalable particulate matter" that appear after a TLV indicate that the TLV value has been adjusted to reflect the influence of particle size on the respiratory hazard. See Refs. 1-3 for details.

Substances are listed alphabetically by systematic name. Synonyms are also provided. The Comments provide further information on the physical form of the substance and the basis to which the limit is referred. The Formula column gives the molecular formula in the Hill convention for organic compounds and the customary line formula for inorganic compounds. The ACGIH and NIOSH TWA, STEL and Ceiling limit are provided if available for a substance.

Note that the entries in this table are for substances that have undesirable physiological effects on humans. Many other substances should be avoided in the workplace because of explosion or asphyxia hazards.


  1. 2018 TLVs and BEIs, American Conference of Governmental Industrial Hygienists, 1330 Kemper Meadow Drive, Cincinnati, OH 45240-1634, 2018 <>.
  2. NIOSH Pocket Guide to Chemical Hazards, U.S. Department of Health and Human Services, National Institute for Occupational Health and Safety, 2016 <>.
  3. OSHA Annotated PELs, TABLE Z-1, U.S. Department of Labor, Occupational Safety and Health Administration, 2017 <>.

Threshold Limits for Airborne Contaminants

NameCommentsFormulaCAS Reg. No.Mol. wt.ACGIH Time-weighted averageACGIH Short-term exposure limitACGIH CeilingNIOSH Time-weighted averageNIOSH Short-term exposure limitNIOSH CeilingBasis
Continued on next page...
Abate[Temephos]C16H20O6P2S33383-96-8466.4691 mg/m310 mg/m3 (total); 5 mg/m3 (resp)Cholinesterase inhibitor
Acetaldehyde[Ethanal]CH3CHO75-07-044.05225 ppmEye and upper respiratory tract irritation
AcetamideCH3CONH260-35-559.0671 ppm (inhalable fraction and vapor)Liver cancer and damage
Acetic acid[Ethanoic acid]CH3COOH64-19-760.05210 ppm15 ppm10 ppm15 ppmUpper respiratory tract and eye irritation; pulmonary function
Acetic anhydride[Acetyl acetate]C4H6O3108-24-7102.0891 ppm3 ppm5 ppmEye and upper respiratory tract irritation
Acetone[2-Propanone](CH3)2CO67-64-158.079250 ppm500 ppm250 ppmEye and upper respiratory tract irritation; central nervous system impairment
Acetone cyanohydrinas CNC4H7NO75-86-585.1055 mg/m34 mg/m3 (1 ppm) [15-minute]Upper respiratory tract irritation; headache; hypoxia/cyanosis
Acetonitrile[Methyl cyanide]CH3CN75-05-841.05220 ppm20 ppmLower respiratory tract irritation
Acetophenone[Methyl phenyl ketone]C6H5C=OCH398-86-2120.14910 ppmUpper respiratory tract irritation; central nervous system impairment; pregnancy loss
2-(Acetyloxy)benzoic acid[Acetylsalicylic acid (Aspirin)]C9H8O450-78-2180.1585 mg/m35 mg/m3Skin and eye irritation
Acrolein[2-Propenal]CH2=CHCHO107-02-856.0630.1 ppm0.1 ppm0.3 ppmEye and upper respiratory tract irritation; pulmonary edema; pulmonary emphysema
Acrylamide[2-Propenamide]C3H5NO79-06-171.0780.03 mg/m3 (inhalable fraction and vapor)0.03 mg/m3Central nervous system impairment
Acrylic acid[2-Propenoic acid]C3H4O279-10-772.0632 ppm2 ppmUpper respiratory tract irritation
Acrylonitrile[Propenenitrile]CH2=CHCN107-13-153.0632 ppm1 ppm10 ppm [15-minute]Upper respiratory tract irritation
Alachlor[Acetamide, 2-chloro-N-(2,6-diethylphenyl)-N-(methoxymethyl)-]C14H20ClNO215972-60-8269.7681 mg/m3 (inhalable fraction and vapor)Hemosiderosis (liver, spleen, kidney)
AldrinC12H8Cl6309-00-2364.9100.05 mg/m3 (inhalable fraction and vapor)0.25 mg/m3Central nervous system impairment; liver and kidney damage
Allyl alcohol[2-Propen-1-ol]C3H6O107-18-658.0790.5 ppm2 ppm4 ppmEye and upper respiratory tract irritation
Allyl glycidyl ether[AGE]C6H10O2106-92-3114.1421 ppm5 ppm10 ppmUpper respiratory tract irritation; eye and skin irritation; dermatitis
Allyl propyl disulfideC6H12S22179-59-1148.2890.5 ppm2 ppm3 ppmUpper respiratory tract and eye irritation

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