A Time-Weighted Average (TWA) exposure is typically calculated over what period?

• A. 24 hours
• B. 8 hours
• C. 1 hour
• D. 2 hours

Answer: (B)

Time-Weighted Average reflects the average exposure over a defined time window, weighting each period by how long it lasts. In occupational settings, this window is typically the length of a standard work shift—eight hours. This aligns with many exposure limits that are set as an eight-hour TWA, so calculating over eight hours provides a single value to assess compliance. If exposure varies during the shift, you sum the product of concentration and time for each segment and divide by the total time (eight hours). For example, exposure at 0.5 mg/m3 for 2 hours and 0.2 mg/m3 for 6 hours yields a TWA of (0.5×2 + 0.2×6) / 8 = 0.275 mg/m3. While eight hours is the typical period, some regulations or scenarios may use a different window.