How do you calculate an 8-hour TWA from multiple exposure levels during the day?

• A. TWA = (C1×t1 + C2×t2) / 8
• B. TWA = (C1×t1 + C2×t2 + … + Cn×tn) / 9
• C. TWA = max(Ci)
• D. TWA = (C1×t1 + C2×t2 + … + Cn×tn) / 8

Answer: (D)

TWA over an 8-hour workday is the average exposure that a worker experiences, accounting for how concentration changes over time. To get it, you sum the amount of exposure at each interval by multiplying the concentration in that interval by how long it lasts, then divide that total by the full 8 hours. In formula terms: TWA equals the sum of Ci times ti for all exposure periods i, divided by 8 hours. If there are breaks with no exposure, those times contribute zero to the numerator but still count toward the 8-hour denominator.

This general approach works for any number of exposure periods during the day. The alternatives fail because they either divide by the wrong total time, pick only the maximum concentration, or omit some exposure intervals, all of which misrepresent the true average over the 8 hours.