Time-weighted average (TWA)

To calculate a TWA, each exposure concentration is multiplied by the time it lasted, the results are added, and the total is divided by the averaging time. If a worker is exposed to a chemical for only part of a shift, the unexposed time counts as zero unless the standard says otherwise. OSHA PELs, NIOSH RELs and ACGIH TLVs are most often written as TWAs, but the averaging time can differ, such as a 10-hour period for NIOSH.

The TWA reflects the idea that for many chemicals the total dose over a shift is what matters most. For chemicals with acute effects, the average alone is not enough, so the limit tables add a Short-term exposure limit (STEL) or a Ceiling limit. Be careful not to read a low TWA as proof that a short peak is fine.

Assessing a TWA normally means personal air sampling with a pump and collection media, or a passive badge, worn in the worker's breathing zone for the shift. The lab results are compared with the limit. Extended shifts may need an adjusted limit because there is less recovery time, and that adjustment should come from a qualified hygienist.

You will see TWA values on the SDS, in Parts per million (ppm) or milligrams per cubic meter, attached to the issuing organization such as OSHA or ACGIH.