An employee’s time-weighted average exposure concentration (“TWAEC”) is calculated by taking the sum of the products of the analytically- determined concentration for each sampling period, including overtime (see Appendix B), and the duration of the corresponding sampling period and dividing this sum by the total sampling time as shown in the equation below:
C1 T1 + C2 T2 + ... + Cn Tn
T1 + T2 + ... + Tn
Cn = concentration for sample n in mg/m3,
Tn =sampling time for sample n in minutes.
Because sampling time rarely equals eight hours exactly, a decision regarding exposures during any unsampled periods must be made before comparing the result to eight-hour TWA standards (see Appendix B).