A worker is exposed to a chemical with a Safety Data Sheet (SDS) showing a Permissible Exposure Limit (PEL) of 50 ppm for an 8-hour time-weighted average. Air monitoring shows exposure levels of 75 ppm for 4 hours and 25 ppm for 4 hours. What is the 8-hour time-weighted average exposure?
Correct Answer
D) 50 ppm
The 8-hour TWA is calculated as: ((75 ppm × 4 hours) + (25 ppm × 4 hours)) ÷ 8 hours = (300 + 100) ÷ 8 = 50 ppm. This equals the PEL, so additional controls should be considered.
Why This Is the Correct Answer
The 8-hour Time-Weighted Average (TWA) is calculated by multiplying each exposure concentration by its duration, adding these products together, and dividing by the total time period. In this case: (75 ppm × 4 hours) + (25 ppm × 4 hours) = 300 + 100 = 400 ppm-hours. Dividing by 8 hours gives 50 ppm. This calculation method accounts for varying exposure levels throughout the workday to determine the average exposure.
Why the Other Options Are Wrong
Option A: 55 ppm
55 ppm is incorrect and doesn't follow the proper TWA calculation formula. This may result from incorrectly averaging the concentrations or making arithmetic errors in the time-weighted calculation.
Option C: 45 ppm
45 ppm is incorrect because it appears to be a simple arithmetic average of the two concentrations (75 + 25) ÷ 2 = 50, then possibly reduced. This ignores the time-weighting component of the TWA calculation.
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