What is a time-weighted average (TWA) exposure and how is it calculated for an 8-hour shift?

What is a time-weighted average (TWA) exposure and how is it calculated for an 8-hour shift?

• A. TWA is the instantaneous concentration at a moment.
• B. TWA is the average exposure concentration over the work shift, calculated as the sum of (Ci × ti) divided by the shift duration (8 hours).
• C. TWA is the maximum concentration reached during the shift.
• D. TWA is the average of peak exposures during the shift.

Answer: (B)

The time-weighted average exposure measures the overall level of exposure a worker experiences over a full shift by weighting each concentration by how long it lasts. For an 8-hour shift, you multiply each concentration during a period by the duration of that period, sum those products, and then divide by the total shift time. In formula form, TWA = Σ(Ci × ti) / 8 hours. This approach captures fluctuations in concentration, giving a single representative value for the entire shift rather than a single moment or just the highest peak. It isn’t the instantaneous concentration at a moment, nor the maximum reached, nor the average of peaks. For example, if the concentration is 2 ppm for 3 hours and 5 ppm for 5 hours, the TWA would be (2×3 + 5×5) / 8 = 31/8 = 3.875 ppm.

The time-weighted average exposure measures the overall level of exposure a worker experiences over a full shift by weighting each concentration by how long it lasts. For an 8-hour shift, you multiply each concentration during a period by the duration of that period, sum those products, and then divide by the total shift time. In formula form, TWA = Σ(Ci × ti) / 8 hours. This approach captures fluctuations in concentration, giving a single representative value for the entire shift rather than a single moment or just the highest peak. It isn’t the instantaneous concentration at a moment, nor the maximum reached, nor the average of peaks. For example, if the concentration is 2 ppm for 3 hours and 5 ppm for 5 hours, the TWA would be (2×3 + 5×5) / 8 = 31/8 = 3.875 ppm.