Time Weighted Average is defined as

• A. The concentration weighted over a 24-hour period.
• B. The average concentration over an 8-hour work day.
• C. The maximum concentration during a 40-hour week.
• D. The peak exposure during a single shift.

Answer: (B)

Time Weighted Average represents the average exposure a worker experiences when concentrations vary over time, calculated over a standard work period. In industrial hygiene, that standard period is an eight-hour work shift. By integrating the concentration over the shift and dividing by the shift length, you get the mean exposure for the day. This approach captures fluctuations: high exposures for short times and lower exposures for longer times combine to give an overall average that is used to compare to occupational exposure limits.

That’s why the correct description is the average concentration over an eight-hour work day. The other ideas describe peak or instantaneous values, or averages over different timeframes (like a full 24 hours or a longer workweek), which do not reflect how TWA is defined and used in workplace exposure assessments.