indicative of associations but also to show some notable inconsistencies. Additional epidemiologic studies might resolve these concerns.

Various researchers have reported anecdotally that the most highly exposed individuals often seem to drive the results of an epidemiologic study. That is, a relatively small number of subjects typically have notably higher exposures than others in the study, and these higher exposures are associated with an increased risk of cancer. The obvious inference from that finding is that conducting another study focusing on the individuals with higher exposures would be most informative. If the risk factors for exposure to electric and magnetic fields operate the same as most chemical risk factors, then individuals with the highest exposures would have the highest risks, and studies of them would have the greatest statistical power and would best address issues of dose-response relationships. Therefore, it might be particularly fruitful to consider the study of children in very-high-current-configuration homes.

Two contradictions remain: Why is no association with childhood leukemia found with spot measurements of magnetic-field strength after cancer diagnosis, even though the association is found with all the other exposure metrics, and why is no consistent dose-response relationship found in these data? With respect to the spot measurements of magnetic fields, if the results of the other exposure metrics are not spurious, two obvious potential explanations exist. First, the surrogate exposure metrics (wire codes, distances from power lines, and calculated fields) might be indicators for the true risk factor and the true risk factor is not related to magnetic-field strength. A number of such plausible risk factors have been investigated, but none has explained the observed association. Second, other surrogate magnetic-field exposure metrics might be more biologically relevant measurements of magnetic-field exposure than spot measurements. These other surrogate metrics might be more representative of long-term integrated averages of magnetic-field strength or of some other aspect of magnetic-field exposure that is related to the cause of the disease (e.g., peak field strength, field variability, frequency of transients above a given field strength, or time above a specific threshold value). What would best advance understanding of this issue are studies that address the inconsistencies, not just more studies similar to those that have been done. The committee's sample-size analyses show that a study would be unlikely to change markedly the existing pattern of results unless hundreds if not thousands of leukemia cases were included in the study.

Another research approach would be to consider different exposure regimes, an approach that might enable investigators to untangle the conflicting results. Therefore, a study of children with high exposure from sources other than outdoor distribution lines might be fruitful. People with homes near high-voltage transmission lines have high exposures and are different from most of the populations studied so far in the United States. Although such studies have been conducted in the Nordic countries, there have been very few cases because of the small populations involved. Much more high-frequency variation (i.e., more on and



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