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Extremely low frequency electromagnetic fields (ELFEMF) have been the subject of much controversy. Recent extensive studies of electric utility workers show a minimal increase in relative risk of brain tumors (1.12) and of leukemia (1.09) per 10 μT years of exposure, although both risk had 95% confidence intervals (CI) that spanned 1.0.73
A meta-analysis of all available studies of childhood leukemia, none of which was individually significant, showed a slight but consistent elevation of OR for association of leukemia with residential magnetic field exposure.74 In addition there are two recent pooled analyses. In one, based on nine well-conducted studies, Ahlbom and colleagues75 observed a twofold excess risk for exposure above 0.4 μT. In the other, based on 16 studies, Greenland and colleagues76 observed a relative risk of 1.7 for exposure above 0.3 μT. The two studies are closely consistent.
A study of ELFEMF exposure during pregnancy and postnatally on the incidence of acute lymphoblastic leukemia up to the age of 14 was carried out retrospectively by interview with 640 mothers and 640 mothers of matched control children. The OR for use of an electric blanket or mattress pad during pregnancy was 1.59 and during childhood was 2.75, both with CI above 1.0. Risk rose with increasing hours of television watching, but, paradoxically, there was no relationship to usual distance from the screen. Similar inconsistencies existed in other multiple comparisons. In a companion study, measured 60-Hz magnetic fields and wire category coding showed no effect.
Indeed, those living in the highest wire-code category homes had an OR of 0.88 when compared to the lowest category. The authors from the Division of Cancer Epidemiology and Genetics of the National Cancer Institute caution that these contrary residential data must be considered before ascribing causality to the observed effects of household exposure to electromagnetic fields.77, 78
Using a different approach, childhood cancer patients were compared to controls for measured ELFEMF exposure from ground currents, which are often found in homes with uninterrupted metallic plumbing paths to other houses. The OR for high magnetic exposure was 3.0 (CI 1.3 to 68) in children who had lived in the same houses throughout the study period, suggesting a positive effect.79
The most recent and largest study lends no support to the proposition, however. In the United Kingdom, 3,838 cases of childhood cancer of all kinds were compared with 7,629 randomly selected age and sex matched controls. Interviews were conducted in all, and ELFEMF measurements made at home and often at school for 2,226 matched pairs. For lymphoblastic leukemia, all leukemia, central nervous system tumors and all tumors, there was no evidence of greater mean exposure to electromagnetic fields.80
Studies conducted in the 1980s and early 1990s among workers exposed to ELFEMF show a possible increased risk of leukemia, brain tumors, and male breast cancer. Interpretation of these studies is difficult, mainly because of methodologic limitations and lack of appropriate measurements.81
Up to now, the results of the numerous epidemiologic researches carried out on children resident in houses in the vicinity of electricity power lines and occupationally exposed workers have indicated a potential carcinogenic risk from electricity-generated electromagnetic fields. However the epidemiologic evidence is insufficient to provide qualitative and quantitative data for use as guidelines for development and operative intervention to safeguard public health. For this purpose we need suitable experimental data.
Long-term carcinogenicity bioassays on extremely low frequency magnetic fields (ELFMF) have been conducted in Canada,82 Japan,83 and the United States.84
The studies performed in Canada and Japan cannot be considered adequate to show the carcinogenicity of ELFMF.
The most comprehensive study to date to evaluate the potential carcinogenicity of ELFMF was conducted in the United States by the NTP. The results of the study were reported by Boorman and colleagues.84 In this study, conducted following Good Laboratory Practices (GLP), groups of 100 Fischer 344 rats and 100 B6C3F1 mice of each sex were exposed to one of several magnetic field conditions: 2, 200, or 1,000 μT continuously or 1,000 μT intermittently (1 h on/1 h off), 60-Hz linearly polarized magnetic fields; one group received sham exposure. Exposure began when the animals were 6 to 7 weeks of age and continued for 18.5 h per day for 2 years. After two years of exposure, the animals still alive were sacrificed. The report concluded that there was equivocal evidence for the carcinogenic activity of 60 Hz magnetic fields in Fischer 344 rats on the basis of the increased incidence of thyroid gland C-cell neoplasms in males exposed to 2 or 200 μT. There was no evidence of carcinogenicity in female rats, or in male and female mice.
In a recently published monograph on ELFEMF, the International Agency for Research on Cancer classified ELFEMF as possibly carcinogenic to humans (Group 2B). 81
The avalanche of conflicting data means that this controversial topic will continue to command attention, and clearly the question of causality needs to be solved. For this purpose more adequate experimental studies should be performed, using larger groups of animals exposed for their whole life span from the embryo state on, as are being conducted at the Cancer Research Center laboratory of the European Ramazzini Foundation.