A PEL is usually given as a time-weighted average (TWA), although some are Short Term Exposure Limits (STEL) or Ceiling Limits. A TWA is the average exposure over a specified period of time, usually a nominal eight hours. This means that, for limited periods, a worker may be exposed to concentrations higher than the PEL, so long as the average concentration over eight hours remains lower.
A Short Term Exposure Limit is one that addresses the average exposure over the a 15-30 minute period of maximum exposure during a single work shift.
A Ceiling Limit is one that may not be exceeded, and is applied to irritants and other materials that have immediate effects.