A database of approximately 20,000 chemical exposures has been constructed in close co-operation between the School of Public Health of the University of North Carolina at Chapel Hill and the Department of Air Pollution of the Wageningen Agricultural University. A special feature of this database is that only multiple measurements of exposure from the same workers were included. This enabled estimation of within- and between-worker variance components of occupational exposure to chemical agents throughout industry. Most of the groups were not uniformly exposed as is generally assumed by occupational hygienists. In fact only 42 out of a total of 165 groups (25%), based on job title and factory, had 95% of individual mean exposures within a two-fold range. On the contrary, about 30% of the groups had 95% of individual mean exposures in a range which was greater than 10-fold. Environmental and production factors were shown to have distinct influences on the within-worker (day-to-day) variability, but not on the between-worker variability. Groups working outdoors and those working without local exhaust ventilation showed more day-to-day variability than groups working indoors and those working with local exhaust ventilation. Groups consisting of mobile workers, those working with an intermittent process and those where the source of contamination was either local or mobile also showed great day-to-day variability. In a multivariate regression model, environment (indoors-outdoors) and type of process (continuous-intermittent) explained 41% of the variability in the within-worker component of variance. Another model, in which only type of process (continuous-intermittent) had a significant effect, explained only 13% of the variability in the between-worker component of variance.