Methods for investigating age differences in the effects of prolonged exposures.

People experience physiological changes with age which may lead to changes in sensitivity to many kinds of exposure. In situations of prolonged, low level exposure, differences in the effects of exposures received at different ages may be difficult to evaluate, since study members receive low level exposures over a range of ages. Such investigations require examination of the effects of different age-patterns of exposure. Three approaches to investigating age-related variability in the effects of prolonged exposures are described: subcohort analyses; the use of smooth weighting functions; and the evaluation of separate effects of cumulative exposures received at different age ranges. Each method can contribute to the overall evaluation of age-specific exposure effects. These methods are illustrated with occupational cohort data for employees of Oak Ridge National Laboratory. The methods presented in this paper should facilitate examination of the effects of aging on sensitivity to prolonged exposures.