U.S. Department of Labor Occupational Safety and Health Administration

Are employers required to conduct exposure monitoring?
In construction and shipyard work, unless you are able to demonstrate that employee exposures will be below the PELs (a "negative exposure assessment"), you are generally required to conduct daily monitoring for workers in Class I and II regulated areas.

For workers in other operations where exposures are expected to exceed one of the PELs, you must conduct periodic monitoring.

In general industry, you must perform initial monitoring for workers who may be exposed above a PEL or above the excursion limit.

You must conduct subsequent monitoring at reasonable intervals, and in no case at intervals greater than six months for employees exposed above a PEL.