Effect modification in epidemiologic studies of low-level neurotoxicant exposures and health outcomes

Little attention has been invested in exploring the possibility that the na ture or magnitude of a neurotoxicant's health impact on children depends on host characteristics (e.g., sex, age) or contextual factors (e.g., socioec onomic status, other chemical exposures). Such effect modification is a pro perty of a true association, and should be distinguished from confounding. In epidemiologic studies of children, most efforts to identify effect modif ication have been unsystematic, pursued as part of data analysis rather tha n of study design. As a result, most samples have insufficient statistical power to characterize effect modification with adequate precision. This may contribute to an inconsistency in results across studies. Failure to, asse ss effect modification adequately may also lead to invalid inferences. If t he magnitude of an association between a neurotoxicant exposure and a parti cular end point varies across strata of a third factor, an estimate that su mmarizes the association across strata of this factor will be inappropriate , overestimating the association in a stratum in which the association is a bsent, and underestimating it in a stratum in which it is present. Until su ch dependencies are identified, our understanding of the mechanism(s) of a compound's neurotoxicity will remain incomplete, as will the knowledge base required to formulate public policy that adequately protects the most sens itive subgroups of the population (C) 1999 Elsevier Science Inc. All rights reserved.