What does TWA stand for in occupational exposure contexts?

• A. Time Weighted Allocation
• B. Time Weighted Average
• C. Total Weight Average
• D. Temperature Weighted Average

Answer: (B)

Time Weighted Average captures how an exposure changes over a shift by weighting each exposure level by the time spent at that level, then averaging over the total period. It stands for Time Weighted Average. This approach reflects the real dose a worker receives during a work period, rather than just a single moment or an unweighted mean.

In practice, you multiply each concentration by the time spent at that concentration, sum those values, and divide by the total sampling time (often 8 hours). For example, if exposure is 0.5 mg/m3 for 2 hours and 1.0 mg/m3 for 6 hours, the TWA is (0.5×2 + 1.0×6) / 8 = 0.875 mg/m3. Regulators compare this shift-based average to allowable exposure limits to assess compliance.

Other terms like time-weighted allocation, total weight average, or temperature-weighted average describe different concepts and aren’t used for assessing occupational exposure over a work period.