Risk factors, causality, and policy initiatives: the case of aluminum and mental impairment.

The question addressed is when can the evidence concerning a health issue be regarded as adequate to implement policy initiatives. The approach is illustrated by comparing evidence about the effects of cigarette smoking with evidence for the aluminum (Al) hypothesis (that Al contributes to mental impairment and especially to Alzheimer's Disease). The criteria for evaluating the evidence are based on the consistency and strength of the association between a putative risk factor and the relevant outcome variable, the likelihood that the relative risk represents a causal relationship, whether possible mechanisms are available, the number of persons affected, and the costs of modifying the risk factor.