Chemical exposure, decoding the limits

When talking about exposure to hazardous chemicals, the safety industry has developed some acronyms that make it difficult to understand what the actual exposure limitations are.

ppm: parts per million

OSHA: Occupational Safety and Health Administration http://www.osha.gov/

NIOSH: National Institute for Occupational Safety and Health http://www.cdc.gov/niosh/

ACGIH: American Conference of Governmental Industrial Hygenists http://www.acgih.org/home.htm

TWA: Time Weighted Average. The averaged exposure to a chemical over certain period of time (usually 8 hours for OSHA).

PEL: Permissible Exposure Limit. Usually provided in either ppm or in a concentration such as mg/M³ (micrograms per cubic meter) over an 8 hour TWA.

STEL: Short Term Exposure Limit. Provided in either ppm or in a concentration such as mg/M³ (micrograms per cubic meter) over a short term time period such as 15 minutes. This is also a TWA (time weighted average).

REL: Recommended Exposure Limit. This is purely a NIOSH term that takes into account OSHA data as well as “real life”, and sets a limit at which health and life may be affected for exposures beyond the REL.

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