Time Weighted Average (TWA)

Definition - What does Time Weighted Average (TWA) mean?

A time weighted average (TWA) is the average exposure within the workplace to any hazardous contaminant or agent using the baseline of an 8 hour per day or 40 hours per week work schedule. The TWA reflects the maximum average exposure to such hazardous contaminants to which workers may be exposed without experiencing significant adverse health effects over the standardized work period.

The TWA is expressed in units of parts per million (ppm) or mg/m3.


Safeopedia explains Time Weighted Average (TWA)

TWA for the exposure to any substance may be used when the concentration and time of exposure varies. It is also applicable to short term samples such as a 15-minute TWA. TWA considers particular variables, dose rate and also the amount of time.

For example, if a worker is exposed to different doses of a chemical vapor for different amounts of time, we may find TWA to determine the worker's average amount of exposure to that particular chemical.

TWA may be calculated as follows: TWA= (t1c1+t2c2+…+tncn) / (t1+t2+…+tn)

Here, ci = concentration during nth interval, ti = duration of nth interval

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