Key Terms

Permissible Exposure Limit (PEL) is the legal standard set by OSHA for the maximum concentration of a chemical or substance in the air. Most OSHA PELs are based on an 8-hour time-weighted average exposure. Approximately 500 PELs have been established.

Time-Weighted Averages (TWAs) is the average employee exposure to a substance over an 8-hour work shift based on chemical measurements. Most chemicals with PELs have a TWA value. Some chemicals have a ceiling or short-term exposure limits in addition to or instead of TWA values.

Ceiling Limit (C) is the maximum allowable exposure level for a chemical. It must never be exceeded.

Short Term Exposure Limit (STEL) is an exposure level for a chemical that must not be exceeded when averaged over a specified short period of time, usually 15 minutes. When a substance has a STEL value, exposure must never exceed the ceiling limit, and the 8-hour average must still remain at or below the TWA.

Action Level (AL) is a term used by OSHA to express a level of toxicant that requires medical surveillance. Exposure at or above action level is termed occupational exposure. Exposure below this level can also be harmful. This level prompts regulatory action, such as employee training, medical monitoring, and recordkeeping. The action level is generally half the PEL.

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