A pooled analysis of magnetic fields and childhood leukaemia

Previous studies have suggested an association between exposure to 50-60 Hz magnetic fields (EMF) and childhood leukaemia. We conducted a pooled analy sis based on individual records from nine studies, including the most recen t ones. Studies with 24/48-hour magnetic field measurements or calculated m agnetic fields were included. We specified which data analyses we planned t o do and how to do them before we commenced the work. The use of individual records allowed us to use the same exposure definitions, and the large num bers of subjects enabled more precise estimation of risks at high exposure levels. For the 3203 children with leukaemia and 10 338 control children wi th estimated residential magnetic field exposures levels < 0.4 mu T we obse rved risk estimates near the no effect level, while for the 44 children wit h leukaemia and 62 control children with estimated residential magnetic fie ld exposures greater than or equal to 0.4 mu T the estimated summary relati ve risk was 2.00 (1.27-3.13), P value = 0.002). Adjustment for potential co nfounding variables did not appreciably change the results. For North Ameri can subjects whose residences were in the highest wire code category, the e stimated summary relative risk was 1.24 (0.82-1.87). Thus, we found no evid ence in the combined data for the existence of the so-called wire-code para dox. In summary, the 99.2% of children residing in homes with exposure leve ls < 0.4 mu T had estimates compatible with no increased risk, while the 0. 8% of children with exposures greater than or equal to 0.4 mu T had a relat ive risk estimate of approximately 2, which is unlikely to be due to random variability. The explanation for the elevated risk is unknown, but selecti on bias may have accounted for some of the increase. (C) 2000 Cancer Resear ch Campaign.