The OSHA Lab Standard states that exposure monitoring -- measuring of airborne concentration(s) with instrumentation or other suitable devices -- must be performed by the employer "if there is reason to believe that exposure levels for that substance routinely exceed the action level (or in the absence of an action level, the PEL)."

Action levels are commonly (but not always) half of the PEL.

The particulars of exposure monitoring, medical surveillance, reporting and recordkeeping are detailed in the Lab Standard (paragraph (d))