The Permissible Exposure Limit (PEL) is the legal limit that a U.S. employee can be exposed to an airborne chemical or substance. PELs are established by the Occupational Safety and Health Administration (OSHA) to protect workers against health effects caused by exposure to hazardous substances. The limits are defined using two standards. Time-weighted averages (TWAs) specify a worker’s average exposure over a standard 8-hour work day. Although it is permissible for workers to experience higher levels of exposure than the TWA for a part of the work day as long as they are exposed to lesser amounts for the rest of the day to ensure that the daily concentration is less than the TWA, there is an exposure limit that at no time should ever be exceeded. This is the second form in which the PEL can be expressed, known as the Ceiling Value or Short Term Exposure Limit. It addresses time periods of 15-30 minutes. It should be noted that PELs are not guarantees of safety and should be viewed only as helpful guidelines. Long-term health data regarding all substances are not available, some health hazards have not been discovered, and since exposure limits are set for the average worker, exceptions occur depending on the individual.