A critical review of time-weighted average as an index of exposure and dose, and of its key elements.

Time-weighted average (TWA) is widely used in research and practice, in occupational health, as an index of exposure and dose. Its key element, CT, where C is concentration of contaminant and T is duration of contamination, is recognizable as Haber's rule. Neither TWA nor similar measurements have been scientifically validated, and it does not seem appropriately named. In the late 1940s and early 1950s, cautions were expressed about the scientific validity of TWA, but the specific scientific study of it did not begin until 1981. In the interim, the cautions appear to have been largely ignored. In 1985, TWA cannot be said to be scientifically valid. Uses of it often confuse dose and exposure, and take insufficient account of time as a factor. Validation of TWA may well be too complicated a scientific problem for occupational epidemiology. TWA need not be rejected for regulatory or all practical purposes, but its key element ought to be better understood, and all applications which presume its scientific validity need to be reviewed.