Incidence of breast cancer in a Norwegian cohort of women with potential workplace exposure to 50 Hz magnetic fields

Background The risk of breast cancer was investigated in a large dynamic po pulation-based cohort of all 1.1 million economically active women in Norwa y with potential exposure to 50 Hz magnetic fields at the censuses of 1960, 1970, and 1980. Methods The follow-up period for the cohort was 1961-1992. For each woman, date of birth and census information on occupation and socioeconomic status were ascertained. These data were linked to the breast cancer morbidity in formation in the Cancer Registry of Norway. Exposure to magnetic fields was assessed a priori using two different approaches. In the first approach, h ours per week in a potential magnetic field above background level (0.1 mu T) were classified by an expert panel. In the second approach, measured mag netic fields from a separate study of men at work were allocated to the wom en's census job titles. In both approaches, exposure was cumulated over the years of employment (work hours and mu T-years, respectively). Results The Poisson regression analysis showed a risk ratio (RR) of 1.14 (9 5% confidence interval (CI) = 1.10-1.29) in the highest exposure category c ompared to the lowest when using the first approach, and the corresponding RR was 1.08 (95% CI = 1.01-1.16) when using the second approach. For women younger than 50 years, RR was 1.20 (95% CI = 1.11-1.29) and 1.12 (95% CI = 0.98-1.28), respectively. Conclusions The results give some support to the hypothesis that exposure t o 50 Hz magnetic fields may increase the risk of breast cancer. However; si nce no direct information on exposure was available, no firm conclusions ca n be drawn. Am. J. Ind. Med. 36:147-154, 1999. (C) 1999 Wiley-Liss, Inc.