The 8-hour time-weighted average refers to average exposure over what period?

• A. 24 hours
• B. 8 hours
• C. 4 hours
• D. 12 hours

Answer: (B)

Time-weighted average expresses the average concentration over a defined period, weighting each portion by how long it lasts so longer exposures influence the result more. In most workplaces, the standard reference period is eight hours—the length of a typical work shift. So the eight-hour time-weighted average is the average exposure over a full workday, even if concentrations fluctuate during those hours. Other periods describe different summaries and aren’t the conventional TWA for a standard shift. Eight hours.