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.