From the Small Area Health Statistics Unit, MRC-HPA Centre for Environment and Health, School of Public Health, Imperial College London, St Mary’s Campus, London, United Kingdom.
Editors’ note: Related articles appear on pages 184 and 191.
Correspondence: Paul Elliott and Mireille Toledano, Small Area Health Statistics Unit, MRC-HPA Centre for Environment and Health, School of Public Health, Imperial College London, St Mary’s Campus, London W2 1PG, United Kingdom. E-mail: email@example.com; firstname.lastname@example.org
Schüz1 questions both the rationale of our study2 and, more generally, the rationale of undertaking research in response to public concern. The National Institute of Environmental Health Sciences defines environmental public health as “the science of conducting and translating research into action to address environmental exposures and health risks of concern to the public.”3 The International Agency for Research on Cancer (IARC) has classified extremely-low-frequency electromagnetic fields as possibly carcinogenic to man (2B) based on an epidemiologic association with leukemia in children, with no known mechanism. IARC regarded the data as inadequate to make judgments for other cancers.4
Our national study was designed to help fill this gap in scientific knowledge and included scores of people with estimated magnetic field exposures ≥1000 nT, substantially increasing the exposure range above that reported in previous studies. At these levels, overhead power lines are the dominant source of exposure to magnetic fields.5 We acknowledge that this type of study, like many epidemiologic studies, is unlikely on its own to establish causal links and, like the studies of magnetic fields and childhood cancer on which the IARC classification is based, does not address a potential mechanism. What such analyses do provide is evidence—based on data rather than speculation—with which to both evaluate the public health question and address legitimate public and policy concerns.6
In the absence of address data, we used as controls a range of cancers not considered to be associated with electric and magnetic fields. We do not agree that this was a “risky gamble.”1 Before selection of cases and controls, we undertook a comprehensive review of the evidence to date, including primary review of the evidence on magnetic fields and adult cancers for the World Health Organization report on extremely-low-frequency fields.4,7 We consider there to be a discernible difference between the strength of evidence available for the cancers recorded for our cases compared with our controls. In addition, we carried out sensitivity analyses using various control pools and found no material differences in risk estimates for any of the cancers under investigation.
We do not think it was simply luck that we did not report a false-positive effect from distance to power lines acting as a proxy for true risk factors such as lifestyle. We were, in fact, one of the first groups to stress the importance of proper adjustment for area-level deprivation in small-area studies (as a proxy for lifestyle factors, such as smoking)8—precisely to deal with the issue that Schüz1 alludes to.9–11 We paid specific attention to possible confounding by deprivation because we were aware that areas near power lines are more affluent than the national average and that the four cancers included in our study are all associated with higher social class.
1. Schüz J. Power lines and cancer in adults: settling a long-standing debate? Epidemiology. 2013;24:191–192
2. 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
4. . IARC Monographs on the Evaluation of Carcinogenic Risks to Humans. Non-ionizing Radiation, Part 1: Static and Extremely Low-Frequency (ELF) Electric and Magnetic Fields. 2002;Vol. 80 Lyon, France IARC Press
5. Swanson J. Methods used to calculate exposures in two epidemiological studies of power lines in the UK. J Radiol Prot. 2008;28:45–59
6. Elliott P, Wartenberg D. Spatial epidemiology: current approaches and future challenges. Environ Health Perspect. 2004;112:998–1006
7. World Health Organization. Extremely Low Frequency Fields. Environmental Health Criteria. 2007;Vol. 238 Geneva World Health Organization
8. Kleinschmidt I, Hills M, Elliott P. Smoking behaviour can be predicted by neighbourhood deprivation measures. J Epidemiol Community Health. 1995;49(suppl 2):S72–S77
9. Jolley DJ, Jarman B, Elliott PElliott P, Cuzick J, English D, Stern R. Socio economic confounding. In: Geographical and Environmental Epidemiology: Methods for Small-area Studies. 1992 Oxford, UK: Oxford University Press:115–124
10. Elliott P, Hills M, Beresford J, et al. Incidence of cancers of the larynx and lung near incinerators of waste solvents and oils in Great Britain. Lancet. 1992;339:854–858
11. Dolk H, Mertens B, Kleinschmidt I, Walls P, Shaddick G, Elliott P. A standardisation approach to the control of socioeconomic confounding in small area studies of environment and health. J Epidemiol Community Health. 1995;49(suppl 2):S9–S14