The guidelines are given in terms of cumulative exposure over a period of time or in terms of maximal concentrations. The Threshold Limit Value (TLV) is the time-weighted average concentration of a normal 8-hour workday or 40-hour work week. The Short-Term Exposure Limit (STEL) is defined as a 15-minute time-weighted average exposure that should not be exceeded at any time during a workday, even if the 8-hour time-weighted average is within the TLV. Exposures at the STEL should not be repeated more than four times per day, with at least 60 minutes between successive exposures at the STEL. The ceiling limit is the concentration that should never be exceeded.

Finally, it should be noted that the guidelines are established for individual chemicals, without consideration of complex mixtures that may contain these chemicals. The behavior of the chemicals in a complex mixture over time is likely to be complicated. In summary, the direct comparisons of these guidelines with ambient levels measured in natural or experimental conditions should be made with caution. In some cases, the comparison may be inappropriate.


American Conference of Governmental Industrial Hygienists (ACGIH). Documentation of the Threshold Limit Values and Biological Exposure Indices, fifth ed. Cincinnati, Ohio: ACGIH, 1986. 743 pp.

National Institute for Occupational Safety and Health (NIOSH). NIOSH/OSHA Pocket Guide to Chemical Hazards. DHEW Publ. No. 85–14. Cincinnati, Ohio: National Institute for Occupational Safety and Health, 1985. 241 pp.

Swedish Board of Occupational Safety and Health (Arbetarksyddsstyrelsens). Hygieniska Gransvarden. Stockholm, Sweden: Liber Distribution, 1984. 60 pp.

The National Academies of Sciences, Engineering, and Medicine
500 Fifth St. N.W. | Washington, D.C. 20001

Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement