To the Editor: When discussing causes of childhood leukaemia, Ziegler et al stated, “Exposure to electromagnetic fields has been ruled out as playing any significant role”. 1 They cited one large study2 in support of this statement, but overlooked two independent pooled analyses that showed the opposite. Greenland et al analysed 12 studies involving 2656 patients and 7084 controls, 3 and Ahlbom et al analysed nine studies involving 3247 patients and 10 400 controls.4 Each analysis found an association with a doubling of risk of childhood leukaemia at levels of household exposure at and over 0.4 microtesla (4 milligauss). Confounders and sources of bias to explain these findings have been sought without success. Consequently, in 2002, the International Agency for Research on Cancer classified 50 and 60 Hz magnetic fields as a “possible carcinogen” (Group 2b)5 even though the mechanism of an effect is not clear.
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