cancer association. This search is severely hampered by the scarcity of established or even strongly suspected causes of childhood leukemia. This ignorance should not be misinterpreted as evidence against confounding, which can only be produced by identification, measurement, and adjustment for such causes.

Among the sociodemographic and environmental factors considered previously, none has been identified as a strong confounder on empirical or theoretic grounds. A leading contender is socioeconomic status (SES) or some correlated characteristic, yet neither the association between SES and childhood cancer nor the association between SES and wire code is very strong. Air pollution related to traffic density remains a candidate (Savitz and Feingold 1989), yet empirical evaluation has not shown this to be a source of confounding in one study (Savitz et al. 1988), and at present, little evidence exists to establish air pollution as a risk factor for childhood cancer. Nevertheless, a more accurate indicator of air pollution from motor-vehicle traffic might yield a different conclusion.

At present, confounding remains a possible explanation for the wire code and cancer association. However, past efforts to identify such confounders have failed, and few strong candidates can be postulated at present. Either the evidence of an association of possible confounders with cancer is weak, as in the case of age of home, or evidence against an association is fairly good, as in the case of parental smoking.

Is the Association with Cancer Accounted for by Magnetic Fields?

Review of Exposure-Assessment Methods in Residential Studies

A variety of approaches were used in past epidemiologic studies to assess potential magnetic-field exposure; the approaches were largely based on the description of nearby power lines or on spot measurements of fields in and around the home. To interpret past studies, it is useful to identify what causative factors might be represented by the exposure indicators used in the studies. To the extent that associations are found between possible indicators of exposure and cancer, the challenge is to determine the actual exposure responsible for the association. Most epidemiologic studies examine several exposure metrics, and some studies have been reanalyzed with several metrics, so some inferences concerning the appropriateness of the different indicators might be possible. All indicators used, including wire codes and field measurements, are surrogates for some unknown ''causal exposure" (if one is indeed present) and must be considered with regard to their effectiveness as substitute measurements for that unknown exposure. In all instances, the exposures of interest took place in the past, so the relationship between contemporary and historical indicators of exposures is of interest. Thus, all other things being equal, the more applicable an exposure indicator is to historical periods, the more useful it is.

To evaluate the hypothesis that the causal exposure is some aspect of power-frequency

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