How do you calculate a time-weighted average exposure from multiple short-term samples collected during an 8-hour shift?

• A. Convert each sample to an 8-hour equivalent by multiplying concentration by the fraction of shift time, sum all (C_i × t_i), then divide by 8 hours.
• B. Average the concentrations.
• C. Take the maximum concentration.
• D. Sum concentrations and divide by the number of samples.

Answer: (A)

Time-weighted average exposure is the average level of exposure over the full 8-hour shift, accounting for how long you’re exposed at each concentration. To get this from short-term samples, each measured concentration must be scaled to its 8-hour impact and then all these impacts are summed and divided by the total shift time.

Multiply each concentration by the amount of time it applied during the shift (in hours) to get its exposure contribution, then sum these contributions and divide by 8 hours. In formula form, sum(C_i × t_i) for all samples and divide by 8. Equivalently, convert each to an 8-hour equivalent by multiplying by the fraction of the shift (t_i/8) and sum these fractions.

This captures how exposure varies throughout the day; simply averaging concentrations ignores how long each concentration persisted, and taking the maximum overlooks all other periods of exposure. Similarly, summing concentrations and dividing by the number of samples ignores time durations entirely. For example, a short high value for 1 hour and a longer lower value for 7 hours will yield a lower TWA than the peak alone but higher than just the long-duration low value, reflecting the overall 8-hour exposure.