To the Editor:
Elliott et al1 recently published in EPIDEMIOLOGY a large and well-conducted case-control study investigating epidemiologic associations between exposure to magnetic fields from residential high-voltage overhead power lines and risk of leukemia, cancers of the brain and nervous system, malignant melanoma, and female breast cancer.
Elliott and colleagues paid particular attention to confounding by deprivation2 and argued that their results do not provide any evidence of exposure-response associations between proximity to power lines or estimated magnetic field exposure and increased cancer risk. They concluded that this “helps to settle a long-standing debate on the safety of residential exposures to extremely low-frequency magnetic fields from high-voltage overhead power lines and adult cancers.”1(p.189)
However, measurement data from the United Kingdom indicate that the magnetic field from 275 to 400 kV transmission lines typically falls to 0.2 µT around 70–80 m, whereas for 132 kV transmission lines the magnetic field falls to 0.2 µT within only 30–50 m.3 This is similarly illustrated in Figure 1 in the article by Elliott et al,1 which further indicates that magnetic field strengths do not exceed normal domestic background levels at distances of >150–200 m. Thus, it is no surprise that exposure-response associations with distances up to 1,000 m were not observed because any increased risks would have been observable within only about 80 m of power lines.
In this light, it is interesting to observe that the point estimates for adult leukemia, in particular, and also cancers of the brain and central nervous system, for which the strongest prior hypotheses of an association exist, indicate moderate increased risk, although without reaching statistical significance. Although this was a very large study (including up to 58,000 cases and 56,000 controls, depending on the outcome), with extensive follow-up, the wide confidence intervals indicate that the study was nonetheless underpowered to investigate exposure-response associations in the appropriate proximity range. This precludes a firm conclusion that “a long-standing debate” has been settled.
An important result of this study is that only 2.5% of the cases and controls lived at addresses with estimated annual average magnetic field exposure over 0.1 µT.1 This is confirmed by independently measured data, which similarly indicate that only about 2% of the UK population has exposure above 0.2 µT.3 Data from other countries also suggest that very few people live close enough to overhead power lines to have exposure levels above background level.3,4 Thus, rather than settling the debate about causality, this study provides assurance that, even if the odds ratio point estimates indicating a 20% increase in leukemia and brain/central nervous system cancers are correct and not attributable to residual confounding,5 the actual public health impact would be small.
Frank de Vocht
Centre for Occupational and Environmental Health
Centre for Epidemiology Institute of Population Health
Manchester Academic Health Sciences Centre
The University of Manchester
Manchester, United Kingdom
1. Elliott P, Shaddick G, Douglass M, de Hoogh K, Briggs DJ, Toledano MB. Adult cancers near high-voltage overhead power lines. Epidemiology. 2013;24:184–190
2. Elliott P, Toledano MB. Rejoinder: adult cancers and magnetic fields from overhead power lines: epidemiologic investigation, not speculation. Epidemiology. 2013;24:193–194
3. Maslanyj MP, Mee TJ, Renew DC, et al. Investigation of the sources of residential power frequency magnetic field exposure in the UK Childhood Cancer Study. J Radiol Prot. 2007;27:41–58
4. Auger N, Joseph D, Goneau M, Daniel M. The relationship between residential proximity to extremely low frequency power transmission lines and adverse birth outcomes. J Epidemiol Community Health. 2011;65:83–85
5. Schüz J. Commentary: power lines and cancer in adults: settling a long-standing debate? Epidemiology. 2013;24:191–192