Magnetic Field Exposure Associated With Childhood Leukemia

The study, reported in two separate papers in the current issues of the International Journal of Cancer (IJC) and the journal Cancer Causes and Control (CCC), shows that children with higher exposures to magnetic fields in residences are two to four times more likely to develop leukemia compared to children who are less exposed. The study comprehensively measured magnetic field exposures inside and outside the children's homes. Wire code was also assigned to each residence. For some children, EMF exposures were measured by a personal monitor.

The authors report in IJC that overall, wire code -- a surrogate indicator of magnetic field exposure based on the physical characteristics of the line and proximity of the residence to power lines -- was not associated with an increased risk of developing leukemia. However, leukemia risk was elevated for children diagnosed at less than six years of age in relation to magnetic field exposures in residences occupied during the first two years of life.

A subset of the total study population wore a personal monitoring device which measured EMF during usual activities in the home over 48 hours. As reported in CCC, exposure to magnetic fields was associated with a two-fold increase in risk of developing leukemia.

When other factors such as residential mobility, power consumption, child's medical history and other environmental exposures were taken into account, children exposed to higher levels of magnetic fields were 4.5 times more likely to develop leukemia compared to less exposed children. Risks were higher for children diagnosed at less than six years of age and for those with acute lymphoblastic leukemia -- the most common type of leukemia in children.

"As the methods of assessing exposure were refined, we found that the association between magnetic fields and the risk of developing childhood leukemia became stronger, particularly in children diagnosed at a younger age," says lead author Dr. Lois Green, epidemiologist in the department of public health sciences at U of T and at Ontario Power Generation. "But this study does not establish that magnetic fields cause cancer. To date, laboratory research has not shown a plausible biologic mechanism supporting a cause and effect relationship."

Researchers compared 201 children living in the Greater Toronto Area (GTA) who were diagnosed with leukemia at 0 to 14 years of age between 1985 and 1993 at the Hospital for Sick Children with 406 control children.

Where possible, magnetic field exposures were measured at all the GTA residences occupied by the child during the period of inquiry -- the only study to date to consider several different exposure time periods. A detailed questionnaire was also administered to gather information about other factors such as family history, which might be related to leukemia risk.

"Acute lymphoblastic leukemia is most commonly seen in children two to six years of age. The association we saw with this age group is interesting because the range of exposures to possible risk factors is smaller and shorter," says Dr. Mark Greenberg, professor of pediatrics at U of T and pediatric oncologist at the Hospital for Sick Children. "We don't know what it means however, because there is no good biologic explanation for how such exposure might work."

Another consideration, Green adds, is the possibility that lifetime exposures to magnetic fields may have been measured more accurately for younger children because the interval between diagnosis and measurement was short, thus reducing possible misclassification of exposure.

Funding was provided in part by the Ontario Hydro Services Company (formerly part of Ontario Hydro) and the Canadian Electrical Association.