A TWA is a time-weighted average and relates to a worker’s exposure to harmful substances. The Occupational Safety and Health Administration (OSHA) sets their permissible exposure limits (PEL’s) as TWA’s. In short, a worker’s exposure level is averaged over a certain time period—usually a typical eight-hour shift during a 40-hour workweek.
The TWA allows for natural irregularities in a worker’s day. As the worker moves throughout a facility and engages in different activities, their exposure levels will rise and fall. Sometimes their exposure will even briefly exceed the legal PEL. OSHA sets a number for that, as well, or the “acceptable maximum peak above the acceptable ceiling concentration for an 8-hr shift.” OSHA lists a concentration and a maximum duration for this figure. But the key number to focus on is the averaged PEL. Averaging the worker’s exposure over time creates a more accurate measurement.
Source: definition of TWA in federal code is here
Source: OSHA’s annotated Z-2 table