Which hazardous substance measurement is averaged over a standard workweek?

The correct measurement that is averaged over a standard workweek is the Threshold Limit Value - Time Weighted Average (TLV-TWA). TLV-TWA represents the average concentration of a hazardous substance in the air, to which workers can be exposed over the course of a typical 8-hour workday and 40-hour workweek without suffering harmful effects. This averaging takes into account fluctuations in exposure levels throughout the workday, allowing for a more comprehensive assessment of long-term exposure risks in occupational settings.

In contrast, other acutely defined exposure limits serve different purposes. TLV-STEL (Short-Term Exposure Limit) is focused on shorter durations, specifically for exposures that should not be exceeded for any more than 15 minutes at a time, and it involves a different consideration for acute exposure risk. TLV-Ceiling indicates a concentration that must not be exceeded at any time during the work period, providing a strict upper limit that does not incorporate averaging. TLV-Peak addresses instances of maximum intensity exposure that may occur infrequently, emphasizing immediate health effects without regard to the standard workweek context.

Thus, TLV-TWA is the most appropriate measurement for evaluating the average exposure of workers to hazardous substances over a standard workweek.