Human Studies of Human Health Hazards: Comparison of Epidemiology and Toxicology

The two principal methods of evaluating environmental health risks are observational epidemiologic studies of exposed populations and laboratory studies of the effects of agents on experimental animals. The principal advantages of the laboratory approach are random exposure assignment and precise exposure assessment. Epidemiologic studies have the virtue of studying the species of ultimate interest (humans) under natural exposure conditions. Given these characteristics, a strategy is offered for resolving contradictory results from these approaches. Epidemiologic evidence of a hazard with contradictory laboratory data may indicate confounding of the human data from other risk factors or bias due to systematically erroneous data. Animal studies might have erred in selecting an inappropriate species or exposure conditions. Where animal studies suggest a hazard but human studies do not, the human studies may be in error due to random exposure misclassification. Animal studies may have overstated risks due to high exposure levels or selection of an unusually sensitive species for study. Studies of electromagnetic fields suggest an effect on cancer risk based on epidemiologic data with negative results from the laboratory, possibly reflecting confounding or bias in the human research or poor selection of the species or exposure conditions in the laboratory. Improved coordination across these disciplines would benefit those who must reconcile these lines of research in assessing risks to human populations.