Most attention has been paid to childhood leukaemia. However, two recent papers have looked at studies on childhood brain cancer. The first is a meta-analysis (which takes the published results from relevant studies and combines them) and the second is a pooled analysis (which takes the raw data from relevant studies and combines them).
The meta-analysis concludes:
"In conclusion, our meta-analysis did not find a robust increase in childhood brain cancer risk with various proxies of residential exposure to magnetic fields. A moderate risk increase, however, cannot be excluded with certainty at high exposure levels. "
The pooled analysis concludes:
"These results provide little evidence for an association between ELF-MF exposure and childhood brain tumors."
The key results from the meta-analsysis are
- for exposures above 0.2 µT: relative risk = 1.14 (95% confidence interval 0.78-1.67)
- for exposures above 0.3 or 0.4 µT (a reduced number of studies): relative risk = 1.68 (95% confidence interval 0.83-3.43)
The key results from the pooled analysis are given in this table and shown in the graph, which compares them with the Ahlbom pooled analysis of leukaemia:
exposure <0.1 µT 0.1-0.2 µT 0.2-0.4 µT >0.4 µT relative risk 1 0.95 0.70 1.14 confidence interval reference category 0.65-1.41 0.40-1.22 0.61-2.13
These results do not suggest that there is an association with childhood brain tumours.
Epidemiological studies of childhood leukaemia have found an association, primarily at the higher exposure levels. But we do not know if this is because there actually is a causal effect, or whether it is a result of bias in the studies. If there is bias in the studies, we might expect the same bias to apply to the brain tumour results as to the leukaemia results. So if the association is caused by bias, we would probably expect a similar association for brain tumours as for leukaemia. But of course, if magnetic fields were to cause a wide range of different cancers, this would also lead to associations with both brain tumours and leukaemia.
The results do not suggest there is a risk for brain tumours. But they are fairly imprecise, so equally, they do not actually exclude the possibility that there is an association similar to the one for leukaemia. So we can't definitively tell these possibilities apart at present.
The study abstracts
The study abstracts
Epidemiology. 2008 May;19(3):424-30.
Mezei G, Gadallah M, Kheifets L.
BACKGROUND: We conducted a meta-analysis of studies on magnetic field exposure and childhood brain tumors to evaluate homogeneity in the results, to examine reasons for heterogeneity, and to derive a summary effect estimate. Comparison of results from studies of childhood brain cancer and childhood leukemia may also help to assess the potential for selection bias in childhood leukemia studies. METHODS: We included results from 13 studies. Using an inverse variance-weighted method, summary effect estimates were calculated separately for distance, wire codes, and measured and calculated magnetic fields. Sensitivity analyses were conducted to assess the influence of individual studies, the potential for selection bias, and the possibility of publication bias. RESULTS: With the exception of wire-code studies, results were compatible with homogeneity across studies. The summary odds ratios (95% confidence intervals) were 0.88 (0.57-1.37) for distance <50 m and 1.14 (0.78-1.67) for calculated or measured magnetic fields above 0.2 microT. For measured or calculated exposures above 0.3 or 0.4 microT, the summary odds ratio was 1.68 (0.83-3.43), with no differences by method of exposure assessment. No single study had a substantial effect on the summary estimates. There was no indication of publication bias. CONCLUSIONS: With the exception of high cut-point analyses (0.3/0.4 microT), where the possibility of a moderate risk increase cannot be excluded, no increase in childhood brain cancer risk was evident for any of the exposure metrics.
Am J Epidemiol. 2010 Aug 9. [Epub ahead of print]