Comparative analyses of the studies of magnetic fields and cancer in electric utility workers: studies from France, Canada, and the United States

Objectives-To summarise and to facilitate comparison of three major studies of electric utility workers that examined the relation between quantitativ e measurements of occupational exposure to magnetic fields and risk of brai n cancer and leukaemia. These studies have been interpreted as providing co nflicting evidence. Methods-A common analytical approach was applied to data from the five coho rts included in the three studies based on original data from four of the c ohorts, and published data from one additional cohort. A nested case-contro l design with conditional logistic regression was used to estimate the rela tive risk/10 microtesla-years (mu T-years) for each of the contributing coh orts and for the combined data. The homogeneity of these estimates among th e studies was also evaluated. Results-Apparent inconsistencies in the findings of these studies can be ex plained by statistical variation. Overall, the studies suggest a small incr ease in risk of both brain cancer and leukaemia. Different methodological c hoices had little impact on the results. Based on a combined analysis of da ta from all five studies, the relative risk/10 mu T-years was 1.12 (95% con fidence interval (95% CI) 0.98 to 1.28) for brain cancer, and 1.09 (95% CI 0.98 to 1.21) for leukaemia. Conclusions-The combined estimates seem to provide the best summary measure s of the data from all studies. However, fluctuations in risks among studie s may reflect real differences, and the exposure measurements in different studies may not be entirely comparable.