Question

An employee spends the first 3.5 hours of her shift in the vicinity of a solvent degreasing operation, where the concentration of tetrachloroethylene (TCE) is 0.2 ppm. She then spends 1 hour in the lunch room, where the concentration of TCE is negligible. The employee spends the next 2 hours in the presence of 3.8 ppm TCE. Finally, she spends the last 1.5 hours of her shift in the presence of 2.2 ppm TCE. What is the TWA exposure to TCE?

Answer 1

A TWA is equal to the sum of the portion of each time period multiplied by the levels of the substance or agent during the time period divided by the hours in the workday. “TWA” indicates “time-weighted average.” “t” indicates “duration.” “c” indicates “concentration.

TWA = t1c1+t2c2+t3c3……tncn/ t1+t2+t3+…tn

TWA exposure to TCE is = (3.5X0.2)+(1X0)+(2X3.8)+(1.5X2.2)/3.5+1+2+1.5

         = 0.7+0+7.6+3.3/8

        = 11.6/8

       = 1.45 ppm