Time-weighted averages, peaks, and other indices of exposure in occupational epidemiology

Dose surrogates commonly used in occupational epidemiology are exposure intensity, exposure duration, and cumulative exposure. The appropriateness of any of these measures as dose indicators depends on the nature of the induction process for the disease under consideration. Peak exposure intensity is often associated with acute health outcomes, whereas cumulative exposure is generally more relevant for diseases with long induction times, i.e., "chronic" diseases. However, there may be situations where peak exposure is etiologically relevant in chronic disease induction, such as might occur with nonlinear rates of damage during brief intervals of very high exposure. An approach is described for evaluating the effect of peak exposures in which peaks may be defined on a relative basis for each worker, or with respect to an absolute value, such as the permissible occupational exposure limit. The analytic strategy is illustrated with data from a case-control study of silicosis in relation to quantitative estimates of silica exposure. In this example, relative peak exposures and average non-peak exposures appear to be better predictors of silicosis risk than cumulative exposure.