It will probably surprise many readers of this Journal to know that occupational exposures to chemicals are rarely assessed with quantitative data, either for epidemiology or for control of hazardous conditions. Only about 13% of epidemiology studies published in the 1980s, on the aetiology of chronic diseases, used any exposure measurements,1 and even those tended to rely on trivial sample sizes (median n = 4).2 Thus, uncertainty in exposure levels is the largest source of error in defining exposure–response relationships. Likewise, industrial surveys of air contaminants typically obtain only a single air measurement per occupational group.3 Surprisingly, this was not always the case. In the period between 1920 and 1960, engineers and health professionals routinely collected many air samples from a given working population, despite the primitive state of measurement technology.4^–7 The likely motivation for such effort was the tremendous variability of air concentrations observed over time and among locations; thus, many measurements were needed to accurately assess exposures. The resulting databases allowed early investigators to delve into the sources and magnitudes of exposure, relationships between exposures and health outcomes, and strategies for comparing air levels with occupational exposure limits (OELs). Such applications are beautifully illustrated in a classic set of contributions to the British Journal of Industrial Medicine (BJIM), published more than 50 years ago.

I vividly recall discovering these papers in the Berkeley library shortly after joining the faculty of the University of California in 1976. Having been charged with developing a graduate course in exposure assessment, I reviewed the usual reference books of …