Time Weighted Average

By Safeopedia Staff
Last updated: February 14, 2024

What Does Time Weighted Average Mean?

Time-weighted average (TWA) is a method of calculating a worker’s daily exposure to hazardous substances such as dust, fumes, chemicals, gases, or vapors. It is averaged to an 8-hour workday or 40-hour work week, along with the average levels of exposure to the hazardous substance and the time spent exposed to them.

The TWA reflects the maximum average exposure a worker can be subjected to without experiencing significant adverse health effects over the standardized eight-hour work period.

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

Safeopedia Explains Time Weighted Average

Time-weighted average calculations are used in situations where the time of exposure to a hazardous substance or the concentration of that substance varies. It is also applicable to short-term samples, such as a 15-minute TWA. The calculation can also consider particular variables, dose rates, and duration.

For example, if a worker is exposed to different doses of a chemical vapor for different amounts of time, a TWA calculation can help a safety professional determine the average level of exposure.

Time-weighted average values are calculated as the sum of exposure during a workday to a particular hazardous substance in ppm-hours and dividing it by an 8-hour period:

TWA= (t1c1+t2c2+…+tncn) / number of hours in the workday

Where t1 – is a portion of a time period (such as 0.25 hours) and c1 – represents the levels of the substance or agent during that time-period.

Is Exposure Limit Being Exceeded?

The 8-hour TWA is a legal limit that should not be exceeded. To determine whether it has been, the TWA of the employees needs to be worked out and compared to the legally permissible limit. If the exposure limit has been exceeded, proper action must be taken to bring exposure within set limits for a particular hazardous substance.

In many cases, the exposure levels are not the same, such as when exposed to a hazardous substance may only be for a part of the workday. The above formula can be used to break down the exposure sessions and work out the TWA. For longer shifts, such as 10 hours, the division is still done by eight to get the 8-hour average.

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Weighted Average Formula

Time-Weighted Average Exposure Value (TWAEV)

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