Appendix B - Calculating Exposure Concentrations

Calculating Exposure Concentrations

In order to calculate the concentration of particulate, the sampled volume of air must be determined. The volume is determined by taking the average of the volumetric flowrates from the pump pre- and post-calibration and multiplying it by the time sampled. Corrections to the volume for any difference in air temperature or pressure between the area where calibration is performed and the area where air is sampled should be made using the ideal gas laws:



= T2
x T2
x V1


P1 and T1 are the conditions during calibration in units of mmHg and K, and P2 and T2  are the sampling conditions. V1 is the calculated sample volume, and V2 is the corrected volume.

From the laboratory analysis, which reports the mass of the contaminant collected, the concentration is calculated by dividing the mass of contaminant by the volume of air sampled:




massn (mg)

volumen (m3)


Calculating the Time-Weighted Average Exposure Concentration (TWAEC)

An employee's TWAEC is calculated by taking the sum of the products of the analytically-determined concentration (see above) for each sampling period and the duration of the corresponding sampling period and dividing this sum by the total sampling time as shown below:

C1 T1 + C2 T2 + ... + Cn Tn

T1 + T2 + ... + Tn


Cn = concentration for sample n in mg/m3, and
Tn = sampling time for sample n in minutes

It usually happens that the total sampling time is less than eight hours. If the TWAEC is to be compared with an eight-hour TWA standard such as the TLV, the calculated TWAEC must be adjusted to an eight-hour basis. This can be done by adding one or more CuiTui products to the numerator of the above equation and increasing the denominator to 480. As the added CuiTui product(s) refers to periods during which sampling was not conducted, estimates of Cui must be made. If the person conducting the sampling decides that no exposure occurred during an unsampled period, the Cui for that period would be set to zero.