A case-control study of childhood leukemia in southern Ontario, Canada, and exposure to magnetic fields in residences

A population-based case-control study was conducted in Ontario, Canada, to assess the relation between the risk of childhood leukemia and residential exposure to magnetic fields. Participating subjects consisted of 201 cases, diagnosed at 0 to 14 years of age during 1985-1993, ascertained from the r ecords at the Hospital for Sick Children (Toronto), and 406 individually ma tched controls. Where possible, point-in-time measurements of magnetic fiel ds were made in all residences occupied by subjects during the period of in quiry in the defined catchment area. Three different classification schemes of wire code were assigned to each residence. Detailed information was col lected by interviewer-administered questionnaires, which enabled risk estim ates to be adjusted for socio-economic characteristics, medical history of parent(s) and child and environmental exposures. Inconsistent elevations in risk were associated with time-weighted averages of magnetic fields both i nside and outside the home for subjects having residential point-in-time me asurements that represented at least 70% of their etiological period. These risks increased in magnitude when analysis was restricted to children unde r 6 years of age at diagnosis or to those with acute lymphoblastic leukemia . For children younger than 6 years at diagnosis, outside perimeter measure ments of the residence, greater than or equal to 0.15 mu T, were associated with increased leukemia risk (OR = 3.45, 95% CI = 1.14-10.45). Evaluation of different exposure times for point-in-time magnetic field measurements a nd wire configuration suggested that exposures earliest in the etiological period were associated with greater risks for children diagnosed at a young er age (OR = 2.50, 95% CI = 1.14-5.49). Our findings did not support an ass ociation between leukemia and proximity to power lines with high current co nfiguration. (C) 1999 Wiley-Liss, Inc.