Refinements in magnetic field exposure assignment for a case-cohort study of electrical utility workers

This study examined the effect of refinements in exposure assignment on ann ual and career exposure to 60 Hz magnetic fields, using all deaths from bra in cancer (145) and leukemia (164) and a random sample of 800 workers from a cohort of 138 905 men. Reassessment of 1060 job titles in the measurement database generated 20 subcategories in addition to 28 occupational categor ies used in the original cohort mortality study, Furthermore, previously mi sclassified jobs were corrected. The complete work history of each sub-coho rt member was re-examined. Original and refined average annual exposures we re 0.086 and 0.088 mu T, respectively. The average career cumulative exposu res were 1.40 and 1.44 mu T-years, respectively. Spearman correlation coeff icients between the original and refined methods across the companies were 0.81 for annual exposure and 0.93 for career cumulative exposure, 23% of th e workers were assigned to another exposure ranking after refinement, but 8 5% of these moved to an adjacent group, suggesting that the differences in exposure ranking are small. The results of this study indicate that refinem ents have modest influence on the average annual and career exposures, Howe ver, the refinements may only change a very rough exposure assessment into one that is slightly less crude, The proportion of workers assigned to anot her exposure ranking indicated that nondifferential exposure misclassificat ion in the original cohort mortality study may have occurred. Implications of these changes for the risk estimates of brain cancer and leukemia cases will to be examined, (C) 1999 British Occupational Hygiene Society. Publish ed by Elsevier Science Ltd. All rights reserved.