. There are no specific guidelines for interpretation of epidemiologic data in risk assessments that go beyond the conventionally applied criteria for causality. For cancer, guidelines for interpreting the strength of evidence have been published, for example, by the International Agency for Research on Cancer (WHO I ARC 1972). However, these guidelines are not rigid criteria and, as with the widely applied criteria for causality, there may be disagreement on the proper classification of epidemiologic evidence for the purpose of hazard identification. For example, the interpretation of negative epidemiologic findings in the hazard identification process is a major source of disagreement.
Nonetheless, epidemiologic data have played a central role in some risk assessments, e.g., lung cancer/environmental tobacco smoke (US EPA 1992a) and lung cancer/indoor radon (US EPA 1992b). In its 1992 risk assessment, the Environmental Protection Agency classified environmental tobacco smoke as a class A carcinogen (i.e., a carcinogen for which definitive human carcinogenicity was available). The report noted that the data from active smokers, in combination with an understanding of mechanisms and dose-response relations, were sufficient for this classification. However, a principal basis for the classification was the result of a meta-analysis of the epidemiologic studies on lung cancer risk in never-smoking women married to smokers. The agency's analysis was also careful to consider potential sources of bias affecting the findings of the epidemiologic studies. In the example of radon, there is convincing evidence on human carcinogenicity from epidemiologic studies of radon-exposed miners (NRC 1988). All of the studies show strongly increased overall risks of lung cancer, approximately increased by three- to sixfold, and radon exposure alone causes lung cancer in laboratory models. Approximately 20 studies have shown excess lung cancer occurrence; the excess cannot be explained by confounding by smoking or other factors and the effect is strong. In assessing the risk of indoor radon, the agency has considered that the findings from epidemiologic studies of underground miners are sufficient for the hazard identification step.
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