A.2 Time Weighted Average (TWA) The maximum average concentration to which an unprotected worker maybe exposed over an eight hour working day is called the Time Weighted Average or TWA value. TWA values are calculated by taking the sum of exposure to a particular toxic gas in the current operating session in terms of parts-per-million-hours and dividing by an eight-hour period.
48 Toxic Gas Measurement – Warning, Danger, STEL, and TWA alarms IQ Force™ Gas Detector Reference Manual Appendix A.3 Short Term Exposure Limits (STEL): Toxic substances may have short term exposure limits which are higher than the eight hour TWA. The STEL is the maximum average concentration to which an unprotected worker maybe exposed in any fifteen minute interval during the day. During this time, neither the eight hour TWA nor the ceiling concentration maybe exceeded. Any fifteen minute periods in which the average STEL concentration exceeds the permissible eight hour TWA must be separated from each other by at least one hour. A maximum of four of these periods are allowed per eight hour shift.