Cancer among Ontario electric utility workers, the evaluation of alternate indices of exposure to 60Hz electric and magnetic fields

grantor: University of TorontoThree research objectives were addressed within this thesis. First, the relationships between indices of 60 Hz electric and magnetic field exposures measured in Ontario electric utility workers were evaluated to identify a series of independent exposure metrics for cancer risk assessment. Second, using these exposure indices, cancer risks were estimated using data from a nested case-control study of Ontario Hydro workers assembled as a component of a previous Tri-Utility Study. Cancers examined included: leukemia, brain cancer, malignant melanoma and Non-Hodgkin's lymphoma. Third, a simulation study was undertaken to investigate the gains in power that could be achieved by performing additional sampling to reduce the variance of the mean exposure estimate assigned to each job category, or alternatively, by increasing the number of cases and controls. The principal components and correlational analyses revealed that aspects of field strength, other than those of a central tendency, should be examined when performing cancer risk assessment in this population. In particular, metrics that capture fluctuations in field strength, and exposures above threshold cut-points warrant consideration. Moreover, the poor correlations observed between electric and magnetic field exposure indicate that risk assessment should be undertaken separately for both field entities. The percentage of time spent above an electric field within the range of 10-39 V/m was found to be associated with an increased risk of leukemia and Non-Hodgkin's lymphoma over and above associations explained by either the geometric or arithmetic mean. Duration of employment was associated with an elevated risk of leukemia and malignant melanoma. Magnetic field exposures were not significantly associated with any cancers examined, however, analyses of brain cancer cases were limited by small sample sizes. Taken together, the results are consistent with the hypothesis that electric fields act as a promoting agent in the process of carcinogenesis of adult leukemia and Non-Hodgkin's lymphoma. The simulation study revealed that greater improvements in power were realised by increasing the number of cases and controls rather than by conducting additional exposure assessments on an equivalent number of workers. No appreciable gains in power were achieved by sampling more than 15 workers in each occupational grouping.Ph.D