Time Weighted Average Concentration (TWA) 1
Time Weighted Average Concentration (TWA) 1 : (a) This is a regulatory value defining the concentration of a substance to which a person is exposed in ambient air, averaged over a period, usually 8 hours. For a person exposed to 0.1 mg m_3 for 6 hours and 0.2 mg m_3 for 2 hours, the 8 hour TWA is (0.1 x 6 + 0.2 x 2) / 8 which equals 0.125 mg m_3. [Duffus, 2000: Univ. Edinburgh Med School On-line Chemical Safety Glossary]; (b) The average value of a parameter (such as concentration of an agent in air) that varies over time. [REAP, 1995: Residential Exposure Assessment Project]; (c) The average time, over a given work period (e.g., 8 hour work day), of a person's exposure to a chemical or an agent. The average is determined by sampling for the contaminant throughout the time period. Represented as TLV_TWA. [University of Kentucky, 1999: Glossary of Safety Terms]
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