TWA = CaTa + CbTb + ... + CnTn
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TWA = Time-Weighted Average
Ca = Concentration of contaminant during the sampling period "a"
Ta = Time of sampling period "a"
Cb = Concentration of contaminant during the sampling period "b"
Tb = Time of sampling period "b"
Cn = Concentration of contaminant during the sampling period "n", where "n" is the last of a series of contaminants
Tn = Time of sampling period "n"
Note: All concentrations are in parts contaminant per million parts of air or milligrams of contaminant per cubic meter of air.
Air Sampling Problem: You are an industrial hygienist that is conducting personal sampling on a worker who is overseeing a production process involving use of toluene (Note: The Occupational Safety and Health Administration has established a Permissible Exposure Limit of 200 parts per million (ppm)). You are able to measure the following exposures:
500 ppm for 1 hour
100 ppm for 4 hours
225 ppm for 3 hours