childhood leukemia and residential proximity to particular configurations of external electric wires has captured the interest and concern of the public, scientists, and government officials. The committee carefully considered the reports of weak positive associations and found them consistent and not explainable by other known factors. This chapter considers the use of those data for estimating the nationwide childhood cancer risk from residential exposure to magnetic fields.

The conclusions in this chapter about the possible risks of exposure to electric and magnetic fields relate mainly to cancer, as is typical in risk assessments. At the end of the chapter, the committee comments on what the data might say about any possible association between exposure to magnetic fields and other health-effect end points, such as adverse effects on reproduction or on learning ability.

RISK ASSESSMENT

Risk assessment is a method designed to evaluate human-health risk (NRC 1983; Wartenberg and Chess 1992). Together with information about costs, benefits, and sociopolitical concerns, risk assessments are used to formulate policies to reduce risks, a process called risk management. The basic tenet of risk assessment is that data on health effects detected in small populations exposed to high concentrations of suspected hazardous agents, usually chemicals, can be extrapolated to predict health effects in large populations exposed to lower concentrations of the same agent. Thus, if workers in a factory are exposed to a high concentration of the solvent toluene and no one develops cancer after follow-up testing for a sufficient number of years, toluene can be inferred to be less carcinogenic than another substance known to produce cancer in workers. Furthermore, if community residents are exposed to toluene at a low concentration, it can be inferred that they will not incur a substantial increased risk of cancer either.

In concept, risk assessment is meant to be an objective approach to risk evaluation for making informed public policy. Although components of the method allow for some flexibility, permitting risk assessors to accommodate a range of circumstances, many federal agencies have explicit guidelines for carrying out risk assessment. Thus, some federal officials believe that little room is left for subjective judgments if agency guidelines are followed. In practice, however, many staunch supporters of risk assessment acknowledge the imprecision of the technique, recognizing that it entails a considerable measure of subjective judgment.

Risk assessment is a four-stage process (NRC 1983). The goals of the first stage, known as hazard identification, are to catalogue situations in which an agent can pose a risk to human health and to predict all possible adverse health effects. It is meant to address a hazardous agent regardless of the amount present



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