OSHA enforces hundreds of permissible exposure limits (PELs) for toxic air contaminants found in U.S. workplaces. These PELs set enforceable limits on the magnitude and duration of employee exposure to each contaminant. The amount of exposure permitted by a given PEL depends on the toxicity and other characteristics of the particular substance. Two different types of measurement are used for PEL determination. The concentration of gases and liquids in the air is measured in parts per million (ppm). Solids and liquids in the form of mists, dusts, or fumes, are measured in milligrams per cubic meter (mg/m3).
Exposure limits called Threshold Limit Values (TLVs) were developed by the American Conference of Governmental Industral Hygienists (ACGIH). TLVs represent the level of chemicals in the ambient air that most workers can be exposed to on a daily basis without harmful effects.
The air contaminant limits were adopted by OSHA in 1971 from existing national consensus standards issued by the American Conference of Governmental Industrial Hygienists and the American National Standards Institute (ANSI). Consequently, these PELs, which have not been updated since 1971, reflect the results of research conducted in the 1950s and 1960s.
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