PELs   1 of 2  Permissible Exposure Limits
   
Most (not all) OSHA-established PELs are "8h-TWA," i.e., 8-hour time weighted averages (for airborne contamination).
   

For example, the PEL for acetone is 1000 ppm 8h-TWA. An exposure of 2000 ppm for 4 h followed by 0 ppm for 4 h averages to 1000 ppm over the total 8 h.
   
Chloroform's PEL is not an 8h-TWA, but a ceiling.  50 ppm.  Employee exposures are never to exceed this concentration.
   
Look them all up in Z-1, Z-2, and Z-3 tables at OSHA web site.
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