Time Weighted Average Exposure Value (TWAEV)

Last Updated: August 28, 2020

Definition - What does Time Weighted Average Exposure Value (TWAEV) mean?

A time weighted average exposure value (TWAEV) is the value assigned to a hazard exposure calculated from averaging the level of several samples taken over a specified period of time.

A TWAEV may be used to define safe exposure levels for various workplace hazards. Often, a TWAEV will be imposed for exposures to hazardous substances such as chemicals or other airborne contaminants. When workplace safety regulators set permissible exposure limits (PELs), the limit may be based on the average exposure that occurs during an 8-hour work day or a 40-hour work week.

Time weighted average exposure values may also be referred to as time weighted average (TWA). The term may also be hyphenated (time-weighted average exposure value).

WorkplaceTesting explains Time Weighted Average Exposure Value (TWAEV)

Regulations often use a time weighted average exposure value (TWAEV) when exposure levels of a hazard, such as noise or an airborne contaminant, fluctuate throughout the workday. When a TWAEV is used to determine a permissible exposure limit (PEL), a worker may be exposed to levels above an below the actual numeric value of the limit. However, when averaged throughout the day, his or her exposure may not exceed the TWAEV.

For example, an employee may be exposed to intermittent loud noises in the workplace. But if the average noise level throughout the day does not exceed the TWAEV, then the exposure is not considered hazardous. An exception may exist if any single exposure exceeds a maximum or ceiling level.

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