The term biologic effect is intended to be a neutral term; it implies no judgment about whether an effect is good or bad. Some biologic effects of electric and magnetic fields have already been found to be beneficial; for example, the ability of fields to stimulate tissue and bone growth has been known and used for a number of years to speed the healing of fractures and burns. Other effects might be harmful.

Health Effects

The committee has focused its attention on three kinds of adverse health effects that have become the chief concerns of the public and of health officials. These are cancer, primarily childhood leukemia; reproductive and developmental effects, primarily abnormalities and premature pregnancy termination; and neurobiologic effects, primarily learning disabilities and behavioral modifications. Each of those effects has been reported in epidemiologic studies to be associated with exposure to some indirect estimates of the strength of power-frequency electric and magnetic fields. Childhood leukemia has attracted the most attention because of studies conducted in Denver, Los Angeles, and the Nordic countries that reported an increased risk of the disease in association with various indicators of exposure to electric or magnetic fields.

Exposure Assessment in Epidemiologic Studies

Determining the amounts and types of environmental agents to which an individual is exposed is often difficult. For example, persons exposed to environmental (secondhand) tobacco smoke might also be exposed to byproducts of tobacco as past or present smokers. That example is similar to the case of exposure to power-frequency electric and magnetic fields where there are multiple opportunities for exposure and almost no way to reconstruct the history of the exposure sources associated with any eventual adverse effect.

Multiple sources of possible adverse health effects are also difficult to separate in epidemiologic studies. For example, studies intending to determine if lead in the residential environment is a hazard can be altered by the fact that study subjects might be exposed to lead not only in their homes but also from outside air or in their workplaces. An individual who is exposed to one agent thought to be associated with an adverse effect might also be exposed to other agents that could contribute to the risk of the disease. Lung cancer might be assumed to be caused by an individual's smoking, even though the individual was exposed instead or in addition to another potential causative agent, such as radon or asbestos. Determining exposure to power-frequency electric and magnetic fields can be confounded similarly, and thus it is difficult to associate accurately the purported exposure with health effects in individuals.

Several dissimilar methods have been used to assess exposure to electric

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