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Adult Cancers Near High-voltage Overhead Power Lines

Elliott, Paul; Shaddick, Gavin; Douglass, Margaret; de Hoogh, Kees; Briggs, David J.; Toledano, Mireille B.

doi: 10.1097/EDE.0b013e31827e95b9
Cancer
Free
SDC

Background: Extremely low-frequency magnetic fields are designated as possibly carcinogenic in humans, based on an epidemiologic association with childhood leukemia. Evidence for associations with adult cancers is weaker and inconsistent.

Methods: We conducted a case-control study to investigate risks of adult cancers in relation to distance and extremely low-frequency magnetic fields from high-voltage overhead power lines using National Cancer Registry Data in England and Wales, 1974–2008. The study included 7823 leukemia, 6781 brain/central nervous system cancers, 9153 malignant melanoma, 29,202 female breast cancer cases, and 79,507 controls frequency-matched on year and region (three controls per case except for female breast cancer, one control per case) 15–74 years of age living within 1000 m of a high-voltage overhead power line.

Results: There were no clear patterns of excess risk with distance from power lines. After adjustment for confounders (age, sex [except breast cancer], deprivation, rurality), for distances closest to the power lines (0–49 m) compared with distances 600–1000 m, odds ratios (ORs) ranged from 0.82 (95% confidence interval = 0.61–1.11; 66 cases) for malignant melanoma to 1.22 (0.88–1.69) for brain/central nervous system cancer. We observed no meaningful excess risks and no trends of risk with magnetic field strength for the four cancers examined. In adjusted analyses at the highest estimated field strength, ≥1000 nanotesla (nT), compared with <100 nT, ORs ranged from 0.68 (0.39–1.17) for malignant melanoma to 1.08 (0.77–1.51) for female breast cancer.

Conclusion: Our results do not support an epidemiologic association of adult cancers with residential magnetic fields in proximity to high-voltage overhead power lines.

Supplemental Digital Content is available in the text.

From the aSmall 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; and bDepartment of Mathematical Sciences, University of Bath, Bath, United Kingdom.

Submitted 19 March 2012; accepted 16 November 2012; posted 18 January 2013.

P.E. and D.J.B. received support for the study from UK Department of Health, grant number RRX106. The Energy Networks Association also contributed funding through a grant to the Department of Health. A Steering Committee comprising independent experts and representatives of the funders advised on study design and commented on the protocol. The Department of Health, Energy Networks Association, and National Grid were not involved in the writing or interpretation of this report, which is the responsibility of the authors alone.

Supplemental digital content is available through direct URL citations in the HTML and PDF versions of this article (www.epidem.com). This content is not peer-reviewed or copy-edited; it is the sole responsibility of the author.

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: p.elliott@imperial.ac.uk; m.toledano@imperial.ac.uk.

Extremely low-frequency magnetic fields are designated as possibly carcinogenic in humans based on an epidemiologic association with childhood leukemia, though no biologic mechanism has been identified.1 In a national case-control study of childhood cancers in England and Wales, both a significant trend (P < 0.01) in risk of leukemia with distance from power lines and a significant dose-response relationship comparing risk of leukemia for those living <200 m and 200–600 m from a power line with those living >600 m were observed.2 These associations were, however, not fully explained by exposure to magnetic fields because no statistically significant excess risk was observed with increasing magnetic fields.3

Some reports have also implicated exposures to extremely low-frequency magnetic fields in the incidence of adult cancers, including leukemia,1,4–6 brain and central nervous system cancers,7,8 malignant melanoma,9 and female breast cancer.10 However, evidence for an association between magnetic fields and cancer risks in adults is inconsistent and weaker than for childhood leukemia.5 Several studies in adults have included only small number of cases and based their exposure estimates solely on distance from power line, which is a relatively poor proxy for magnetic field strength.11 To address these shortcomings, we carried out a large national case-control study of cancer incidence among adults living near high-voltage overhead power lines in England and Wales. We included the estimation of magnetic fields and distance from power lines. We focused on the high-voltage power lines where residential exposures to magnetic fields from the power lines tend to dominate over other sources.5,12

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METHODS

Design

We chose a case-control design over other approaches such as Poisson regression of small area data because it enabled analysis of individual-level information, including residential addresses of cases and controls, which is available within the National Cancer Registry to a ~0.1 m accuracy. This high level of geographic resolution is important because magnetic fields fall off rapidly with distance from a power line. Furthermore, population data for England and Wales are collected from census only every 10 years, leading to possible inaccuracies in denominator information in the intercensual years.13 A similar case-control design was used for the national study of childhood cancers and power lines across England and Wales.2,3

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Study Population

We identified eligible cases and controls from the National Cancer Registry held by the Small Area Health Statistics Unit at Imperial College London.14 Case cancers in England and Wales were 15–74 years of age with a diagnosis of leukemia, brain or central nervous system cancers, malignant melanoma, or female breast cancer diagnosed 1974–2008, and living within 1000 m of a high-voltage overhead power line. Diagnostic codes of the case cancers according to the eighth, ninth, and tenth revisions of the International Classification of Disease, together with controls selected from a range of cancers not considered to be associated with electric and magnetic fields, are given in eTable 1 (http://links.lww.com/EDE/A641). Exclusions from the controls were malignant neoplasms of lymphatic and hematopoietic tissues other than leukemias; nonmelanoma skin cancer; cancers of the lip, oral cavity, pharynx, and respiratory cancers, including larynx and lung (because it has been hypothesized that electric fields emitted by high-voltage power lines, and the charged ions they produce, might be associated with these cancers)15; and cancers of ill-defined, secondary, and unspecified sites.

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Distance Calculations

We obtained grid references and construction dates (by year) of the ~21,800 pylons for all the high-voltage (400 and 275 kV) overhead power lines in England and Wales from National Grid, together with the few 132 kV lines (0.1% of the total at this voltage, by length) that form part of the National Grid rather than regional distribution networks. We constructed zones delineating 1000 m of the power lines for each year 1969–2008 and linked these to the cancer database addresses within a Geographical Information System.

We located residential addresses (0·1 m accuracy) of case and control cancers recorded at the time of diagnosis using ADDRESS-POINT16; depending on the year, we successfully located 89% to 96% of the cancer database addresses. On the basis of diagnosis address, we first calculated distances from all operating power lines for the year before diagnosis. This was done to ensure that cases and controls were living near an operational power line on the date of diagnosis because information on the power lines network was available only annually and new power lines could be added to the network at any point during a year. We included all eligible cases of leukemia, brain/central nervous system cancers, and malignant melanoma within 1000 m of a power line. For female breast cancer, we selected a 50% random sample because the large numbers precluded calculation of magnetic fields for the complete set of cases, though we used both the 50% sample and all eligible cases for distance-based analyses. We identified a pool of 164,095 possible control cancers within 1000 m of a power line in the year before diagnosis. We randomly selected control cancers from the pool, with replacement, for the various case cancers; of the total of 79,507 control cancers thus selected, 18,767 (23.6%) were used for more than one case cancer type (see eTable 1, http://links.lww.com/EDE/A641). We included one control per case for female breast cancer and three per case for the other cancers, frequency-matched to cases by year of diagnosis and region (there are 10 regions in England and Wales; regions are administrative areas, the highest tier in the subnational division used by central Government). We then obtained distances of diagnosis address from operating power lines for year of diagnosis and 5 years before diagnosis for the analyses.

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Magnetic Fields

National Grid provided estimates (in 2011) of magnetic fields (blind of case/control status) using their EM2D program,12 which was also used to estimate magnetic fields in the national study of overhead power lines and childhood cancers.3 Magnetic field estimates were provided for year of diagnosis and for 5 years before diagnosis, for all diagnosis addresses (cases/controls) lying within 200 m of an operating high-voltage power line, except those with transposed phasing for which magnetic fields within 100 m of a line were provided.12 Magnetic fields for people living further from a power line were assumed to be <100 nanotesla (nT) in categorical analyses and zero in analyses of magnetic fields considered as a continuous variable. To maintain confidentiality, the list of addresses provided to National Grid was supplemented by an additional 10% addresses within 200 m.

EM2D takes into account the distance from the power line, tower design, clearance above ground, phase arrangement, and historical load data—estimated for the winter peak and scaled to give an annual average.12 For consistency, modeled estimates of magnetic field exposures were obtained at an elevation of 1 m above ground level.12 On the basis of the configuration of power lines for the year of diagnosis, there were 9586 case and control cancer locations within 200 m of a power line; a directly calculated magnetic field estimate of ≥100 nT was obtained using EM2D for 1845 (19.2%) of these, with an estimate of <100 nT (zero in continuous analyses) assigned for 7,576 (79.0%). Data were insufficient to provide an estimate for the remaining 165 (1.7%) case/control addresses (eg, because there were no available load data or the load data were illegible for the relevant power line); this number was slightly higher (254, 2.7%) for the power lines configuration 5 years before diagnosis. Data were insufficient to provide a magnetic field estimate for similar proportions of cases and controls for both year of diagnosis (1.9% [77] cases, 1.6% [88] controls) and 5 years before diagnosis (2.6% [102], 2.7% [152]).

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Information on Potential Confounders

In addition to stratifying by year of diagnosis/region, we included age, sex (except female breast cancer), and small area (enumeration district) measures of rurality and deprivation (because individual-level information on deprivation and smoking are unavailable in the National Cancer Registry) as potential confounders. We used CORINE 1990/2000 land-cover data (codes 1–11, urban; codes >11, rural)17 as a measure of rurality and Carstairs score to denote deprivation18 because this was available for all 35 years of the study. We used Carstairs 1981 and 1991 at the enumeration district level for cancers diagnosed 1974–1985 and 1986–1995, respectively, and thereafter Carstairs 2001, based on census output areas—on average, there are approximately 12,000 enumeration districts or output areas per region. Negative Carstairs scores indicate areas that are more affluent and positive scores, more deprived, than the average across England and Wales.18

Information on migratory patterns and social class of individuals is unavailable in the National Cancer Registry. We examined migration patterns for women, with breast cancer/control cancers, who lived within 1000 m of a power lines and were included in the Office for National Statistics’ Longitudinal Study—a 1% sample of the UK population with data on individuals linked across decennial censuses.19 We also examined individual measures of social class by distance. We used data for 1991 (n = 14,044 women 15–74 years of age) and 2001 (n = 14,451) because residential postcodes are unavailable in the Longitudinal Study for earlier periods.

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Statistical Methods

We used logistic regression to estimate cancer risks by categories of distance and magnetic field. Category breaks were based on those used in the national childhood study,2,3 with an additional magnetic field break between 200 and 400 nT and for the highest exposures (≥1000 nT); this was possible in our study because there are many more cancers in adults than in children. Models were adjusted for age, sex (except female breast cancer), deprivation, and rurality. Because deprivation and rurality are area-level (enumeration district) measures, this leads to a multilevel data structure; we acknowledge the possibility of within-area dependence by including area-level random effects in our models. P values for tests for linear trend across categories were based on median distance/magnetic field in each category. Additional analyses treated distance and magnetic field as continuous measures. For distance, we used (1) inverse rank and (2) reciprocal of distance. For magnetic fields, we tested for nonlinearity by including a quadratic term. We carried out a sensitivity analysis and reran all the models excluding the rarest cancers from the control pool. We excluded the rarest 5% (3,870) of control cancers comprising cancers of other respiratory organs, pituitary, adrenal and other endocrine glands, male genital, other digestive, eye, bone, gallbladder, small intestine, heart, and thyroid. All analyses were carried out in the statistical package R.20 The study received ethical approval from the London MREC Committee.

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RESULTS

Within 1000 m of a power line, 7823 leukemia cases, 6781 brain/central nervous system cancers, 9153 malignant melanoma cases, and 58,404 breast cancer cases were included (Table). Case cancers tended to be younger, slightly more affluent (based on Carstairs score) and, for malignant melanoma and breast cancer, less urban than control cancers. There was a higher proportion of women among malignant melanoma cases than among controls. Mean magnetic field estimates among the controls ranged from 7.4 to 10.6 nT (Table), but much higher values, ranging up to 8592 nT, were calculated very close to a power line (Fig. 1). Cases of female breast cancer lived on average closer to a power line than controls (Table).

TABLE. De

TABLE. De

FIGURE 1

FIGURE 1

Results by distance for year of diagnosis are shown in Figure 2 and eTable 2 (http://links.lww.com/EDE/A641). With adjustment for confounders, odds ratios (ORs) for leukemia, brain/central nervous system cancer, and malignant melanoma closest to the power lines (0–49 m) compared with distances 600–1000 m ranged from 0.82 (95% confidence interval = 0.61–1.11, 66 cases) for malignant melanoma to 1.22 (0.88–1.69) for brain/central nervous system cancer. For female breast cancer, after adjustment for confounders, ORs at 0–49 m compared with 600–100 m were 1.01 (0.85–1.21) and 1.07 (0.93–1.24) for the 50% and 100% sample, respectively (Fig. 2 and eTable 2, http://links.lww.com/EDE/A641).

FIGURE 2

FIGURE 2

Results of the magnetic field analyses for year of diagnosis are shown in Figure 3 and eTable 3 (http://links.lww.com/EDE/A641). In comparison with the reference group (<100 nT), we observed no meaningful excess risks or trends of risk with magnetic field strength for any of the four cancers examined. In adjusted analyses at the highest estimated field strength (≥1000 nT) compared with <100 nT, the OR ranged from 0.68 (0.39–1.17) for malignant melanoma to 1.08 (0.77–1.51) for female breast cancer. Quadratic terms included in the regression analyses did not improve the fit of the model.

FIGURE 3

FIGURE 3

Pearson correlation coefficients between distance/magnetic field from power lines in year of diagnosis and 5 years before diagnosis were r = 1.00 and r = 0.85, respectively. Trends in risk by distance and magnetic field at diagnosis address 5years before diagnosis were similar to those for year of diagnosis (see eTables 2 and 3, http://links.lww.com/EDE/A641). Excluding rare cancers from the control group as a sensitivity analysis had minimal effect on the results for year of diagnosis and 5 years before diagnosis (data not shown). We also examined data for female breast cancer cases younger than and older than 50 years of age. With adjustment for confounders, there were no convincing trends in risk at either age, and there were no differences in trends between the two age groups (data not shown).

In the Longitudinal Study, 68% of women moved into or out of areas within 100 m of power lines from 1991 to 2001. There were no meaningful differences in migration patterns between those with breast cancer and other cancers, and no difference in social class (based on individual occupation) for women living ≤100 m and from >100 to ≤1000 m of a power line (P = 0·66; see eTable 4, http://links.lww.com/EDE/A641).

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DISCUSSION

This national study, with 35 years of observation, is by far the largest to date to investigate the risk of adult cancers near high-voltage overhead power lines.4,6,10,21,22 Because of the size of the population and the focus on the high-voltage overhead power lines (400 and 275 kV), we included scores of people with estimated 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.12 After adjustment for deprivation and other confounders, we found no excess risks or trends for leukemia, brain/central nervous system cancers, malignant melanoma, or female breast cancer in relation to distance or magnetic fields from high-voltage overhead power lines in England and Wales.

In addition to its size, important strengths of our study include the use of a geographical approach to maximize exposure variation23 and estimation of magnetic field exposures at the home address of individual cases and controls using a validated modeling procedure,12 carried out blind to case/control status; we thus identified homes near power lines with high and, in some cases, extremely high, magnetic field exposures. The use of magnetic field estimates in our study is a major strength, given that distance from power lines has been shown to be a relatively poor proxy of residential magnetic field exposure—indeed, studies using distance alone have been considered “uninterpretable.”11 Because magnetic field exposures fall off rapidly with distance, a particular strength was the availability in the cancer registry of individual address locations with high accuracy (~0.1 m), which we used in the estimation of magnetic fields. This provided an important advantage of our case-control design over a small area approach. Because areas near power lines are more affluent than the average for England and Wales, and the four cancers included here are all associated with higher social class,24–27 we needed to pay specific attention to possible confounding by deprivation. This affected, in particular, the distance-based analyses; adjustment for deprivation and other confounders resulted in lower ORs for these cancers with distance from a power line. Adjustment for deprivation should also address possible local variations in the uptake of screening programs for breast cancer (among cases) and cervical and colon cancers (among controls) because evidence to date suggests variation in uptake largely reflecting socioeconomic status.28–30

Our study has limitations. In the absence of population address data, we used as controls a range of cancers not considered to be associated with electric and magnetic fields, as was done in a Taiwanese study.4 The use of cancer controls has the advantage that, as with the case cancers, ascertainment levels are high,31 they are known to have been living near a power line at the time of diagnosis, and magnetic field exposures can be estimated to the same degree of accuracy as for cases; however, the use of cancer controls might introduce bias (toward the null) if magnetic fields are positively associated with the control cancers. We had no direct information on the migration of cases or controls, and so were unable to allow for cumulative exposures or latency, except for providing estimates at diagnosis address 5 years before diagnosis, which were highly correlated with estimates for year of diagnosis. Data from the Longitudinal Study indicate that over a 10-year period, there was substantial in- and out-migration among the population living within 100 m of power lines. To the extent that migration affected case and control cancers similarly, this would tend to dilute any risk, if exposed people moved out of the area and unexposed moved in. Furthermore, calculation of magnetic fields was based on historical load data with inherent inaccuracies12; these were unlikely to be differential with respect to case or control location, and so this may have led to underestimation of any risk. We were only able to take into account the magnetic fields from overhead power lines and had no measurements of actual exposures within the home. However, long-term average background fields in homes32 are at least an order of magnitude below our highest category of magnetic field (≥1000 nT).

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Comparison With Other Studies

The International Agency for Research on Cancer (IARC) designated extremely low-frequency magnetic fields as possibly carcinogenic in humans, based on an epidemiologic association with childhood leukemia.1 Results of the national study of childhood cancers and overhead power lines in England and Wales were consistent with the IARC evaluation, showing a statistically significant dose-response association between childhood leukemia and proximity of home address at birth to high-voltage power lines: relative risk = 1.69 (95% confidence interval = 1.13–2.53) and 1.23 (1.02–1.49) for those living <200 m and 200–600 m from a power line compared with those who lived >600 m from a line, respectively.2 In the same national study, the risk of childhood leukemia was 2.00 (0.18–22.04) in the highest field category (≥400 nT) compared with the lowest (<100 nT) and 1.14 (0.57–2.32) with each 200 nT higher magnetic field. No statistically significant associations were thus observed with magnetic field exposures. However, annual average magnetic fields were fairly low in the study; only 2.5% of cases and controls lived at addresses where the field was estimated to be ≥100 nT giving wide confidence intervals around the risk estimates.3 The study authors concluded that exposure to magnetic fields in year of birth was unlikely to fully explain the observed association between childhood leukemia and distance from power lines.3

The national childhood cancer study used a case-control approach similar to that adopted here for adults, with use of the National Grid EM2D program to estimate magnetic fields based on power line characteristics and residential addresses of cases and controls. Evidence in adults, including for leukemia and brain cancer, was considered by the IARC review to be inconsistent and weaker than for childhood leukemia, whereas evidence for breast cancer, in particular, was inadequate to make causal judgments.1 Subsequently, the World Health Organization5 concluded that the evidence did not support an epidemiologic association between extremely low-frequency magnetic fields and female breast cancer, and a recent meta-analysis reached a similar conclusion.33 National, population-based studies in Finland,22 Sweden,21 and Norway10,34 reported conflicting results for risks of adult leukemia and breast cancer associated with residential exposures to magnetic fields from high-voltage power lines, though compared with our study, numbers were small, particularly for those with the highest exposures (categorized as >200 nT in those studies).

In summary, our results do not support an epidemiologic association of adult cancers with proximity to residential magnetic fields from high-voltage overhead power lines. Unless new biologic hypotheses emerge, our findings should help 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.

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ACKNOWLEDGMENTS

We thank Sara Morris, Norman Cobley, Catherine Keshishian, Nina Iszatt, and Peter Hambly (Imperial College London) for their help with data acquisition and analysis early on in the study. We are grateful to John Swanson (National Grid) who provided data on the locations of power lines and carried out the magnetic fields analyses in EM2D, blinded to case-control status. We thank the cancer registries in England and Wales and the Office for National Statistics for providing data on cancer incidence. The permission of the Office for National Statistics to use the Longitudinal Study is gratefully acknowledged (clearance no. 30104), as is the help provided by staff of the Centre for Longitudinal Study Information and User Support (CeLSIUS). CeLSIUS is supported by the ESRC Census of Population Programme (Award Ref: RES-348-25-0004). Census output is Crown copyright and is reproduced with the permission of the Controller of HMSO and the Queen’s Printer for Scotland. The Small Area Health Statistics Unit is funded by the Health Protection Agency as part of the MRC-HPA Centre for Environment and Health at Imperial College London. P.E. acknowledges support from the National Institute for Health Research (NIHR) Biomedical Research Centre based at Imperial College Healthcare NHS Trust and Imperial College London. P.E. is a NIHR Senior Investigator.

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