Abstract
Estimation of past exposures to potential health hazards is one of the most difficult problems for industrial hygiene research. Although it is very difficult, it is not impossible. There are two conceptual models that can guide the hygienist in this task: the source-receptor model and the task-TWA (time-weighted average) model of full-shift exposures. The source-receptor model is a formalization of the professional decision criteria used by hygienists in their initial assessments of a workplace to identify where hazards might exist and where to sample. Thus, by performing a detailed assessment of the nature of past operations, raw materials, and job activities, it is frequently possible to identify where exposures might have occurred and what agents might have been involved. The task-TWA model is useful for developing historical estimates of full-shift exposures by integrating information on changes in job activities with limited monitoring data. It is particularly useful for extrapolating TWA exposures from short-term task data obtained where exposures might have been high. The statistical distributions of full-shift TWA exposures resulting from various combinations of task distributions were examined and cases where lognormal distributions are produced were identified, as were some cases where they do not. The application of this approach in the assessment of historical exposures of gasoline transport drivers is presented. These two conceptual models can also be used in the identification of sources of error and assessing their relative importance in estimates of past exposures.