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From Wikipedia, the free encyclopedia. The permissible exposure limit (PEL or OSHA PEL) is a legal limit in the United States for exposure of an employee to a chemical substance or physical agent such as loud noise. Permissible exposure limits are established by the Occupational Safety and Health Administration (OSHA).
TWA - Time Weighted Average. 1  This is the level of CO (in PPM) that a person has been exposed to on average over some period of time (usually 8 hours). 2  PEL - Permissible Exposure Limit. This is the term OSHA uses for the limits it publishes. For CO, it happens that all of OSHA's PELs are actually TWA limits. Ceiling - An upper limit on the concentration of CO at any moment. For example, a ceiling value of 200ppm means that the level of CO should never rise above 200ppm. STEL - Short Term Exposure Limit. This is a TWA defined over a short time period of 15 minutes.