Studies that have examined average magnetic fields measured in homes after a diagnosis has been made have all been severely limited by missing data, and no firm conclusions can be drawn from them. The data that have been generated do not support an association between childhood leukemia and magnetic fields, in contrast to the data generated from wire codes.
The factors that explain the association between wire codes and childhood leukemia have not been identified. The original and continued interest in wire codes is their presumed correlation with long-term average magnetic fields in homes. However, epidemiologic studies have generated little evidence that average magnetic fields account for the observed association between wire codes and childhood leukemia. Wire codes are not strong predictors of magnetic-field strengths in homes, although they do distinguish very high fields from outdoor wiring from lesser fields reasonably well. Other explanatory factors, such as neighborhood characteristics, other measurements of exposure to electric and magnetic fields, or air pollution, have received even less support, leaving open the question of what accounts for the observed association.
Epidemiologic evidence of an association between magnetic fields and childhood cancers (other than leukemia), adult cancers, pregnancy outcome, and neurobehavioral disorders is not, in the aggregate, supported.
A number of studies examined health outcomes other than childhood leukemia, and some of them reported positive associations. However, the number of well-designed studies supportive of such an association is not sufficient to conclude that any of the associations are actually present.
Epidemiology can be defined as the study of patterns of health and disease in human populations to understand causes and identify methods of prevention. Interpretation of epidemiologic evidence regarding potential causal relations between exposures and health outcomes is a complex process and relies on a wide range of supporting data. No simple checklist can be used to make judgments about the quality of research and the certainty of the results, although a number of considerations might bear favorably or unfavorably on a causal interpretation. As a prelude to the review of epidemiologic studies addressing possible health effects of electric and magnetic fields, some of the key methods used to interpret epidemiologic data are reviewed. For more thorough consideration of these and related principles, see work by Rothman (1986) and Kelsey et al. (1986).
Compared with laboratory approaches to the study of health and disease (e.g., toxicology), observational epidemiology has a number of strengths and weaknesses. The principal deficiency of an observational approach is the inability to assign exposure randomly. This inability introduces the possibility of confounding the effect of the exposure of interest by other disease determinants. Because exposures