Exposure Level Terminology

There are a few different thresholds that describe how dangerous an atmospheric hazard is. It is useful to know the difference, since they apply to different situations.

Short-Term Exposure Limit (STEL) is the amount of a substance you can be exposed to over a short period of time, usually fifteen minutes, as long as you do not also exceed the time weighted average exposure.

Time-Weighted Average (TWA) takes the average level of exposure to a chemical over a typical eight-hour work day.

Permissible Exposure Level (PEL) is OSHA designation for the highest level of exposure that can be considered safe. This includes both the STEL and the TWA exposure to a substance. This is usually measured in parts per million (ppm).

For example, the PEL for carbon monoxide is 50 ppm in an eight-hour time-weighted average (TWA). The ceiling limit for carbon monoxide is 200 ppm. Even if the exposure level does not exceed the short-term exposure limit, it can still exceed the TWA exposure for the day. OSHA also identifies a level that is immediately dangerous to life or health (IDLH). The IDLH threshold limit for carbon monoxide is 1200 ppm.

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