WHAT IS IT?  The legal limit of air contaminant-- absolute ceiling or 8-hour time-weighted average -- to which an employer may expose employees.  Set by OSHA.

E.g. PEL of chloroform is a Ceiling, 50 ppm.

E.g. the PEL of acetone is an 8-h TWA of 1000 ppm.

See Table Z-1