data (Wartenberg and Savitz 1993) or estimated field strengths (Feychting and Ahlbom 1993) have been examined in detail, higher cut-point scores tend to show the largest relative risks. Overall, the data from published studies support an argument for an increased risk with higher exposure level; however, the anomaly between measured magnetic fields and wire codes in different cities severely weakens this interpretation.
There is little evidence that known or suspected childhood-cancer risk factors introduced confounding into earlier studies. Efforts to adjust for those risk factors have had little impact on the results, which is not surprising in light of their having no strong association with childhood cancer or with wire codes. Approaching the question of confounders from both ends—risk factors for childhood cancer or correlates with wire codes—few contenders, other than magnetic fields, explain the association. Childhood-cancer correlates that remain under contention are viral exposure or some other phenomenon associated with housing density, which, in turn, is associated with wire codes. There is, for example, epidemiologic evidence from British experience during wartime evacuation of children to rural areas that the development of childhood leukemia has an infective component (Kinlen and John 1994). The British experience was that a higher incidence of childhood leukemia was associated with children living in densely populated cities relative to those living in the countryside. In addition, introducing children from the city to the country tended to lead to an increase in leukemia in the region where they were relocated.
We might also ask what exposures, other than magnetic-field exposures, might be indicated by the wire code that are the true causes of childhood cancer? An intriguing suggestion that currents on plumbing lines result in exposure to copper ions was offered (Kavet 1991) but was not supported. Application of herbicides near power-line poles or leakage of polychlorinated biphenyls (PCB) from transformers near homes in the high-exposure group might be a candidate for an increased risk. Again, a chain of tenuous assumptions is required: high wire code corresponds to overall closer proximity to poles and transformers, herbicides are commonly sprayed and transformers commonly leak, children spend time in the vicinity of the herbicides and PCB, and herbicides and PCB cause cancer in children. Each link in these various assumptions has questionable credibility, and cumulatively the proposition has so little support that it is of limited interest. Air pollutants associated with living on busy streets (where major power lines are often placed) constitute a more credible candidate, yet the studies that have evaluated that exposure have not found it to explain the association (Wertheimer and Leeper 1979; Savitz et al. 1988; London et al. 1991), and there is very little independent evidence that air pollutants cause childhood cancer.
Whatever other exposures or characteristics might be associated with wire