Childhood cancer in relation to distance from high voltage power lines in England and Wales: a case-control study

¹ Childhood Cancer Research Group, University of Oxford, Oxford OX2 6HJ, ² National Grid Transco plc, London WC2N 5EH

Abstract

Introduction

Methods

Results

Discussion

Conclusions

Disclosures

References

Objective:  To determine whether there is an association between distance of home address at birth from high voltage power lines and the incidence of leukaemia and other cancers in children in England and Wales.

Design:  Case-control study.

Setting:  Cancer registry and National Grid records.

Subjects:  Records of 29 081 children with cancer, including 9700 with leukaemia.  Children were aged 0-14 years and born in England and Wales, 1962-95.  Controls were individually matched for sex, approximate date of birth, and birth registration district. No active participation was required.

Main outcome measures: Distance from home address at birth to the nearest high voltage overhead power line in existence at the time.

Results: Compared with those who lived > 600 m from a line at birth, children who lived within 200 m had a relative risk of leukaemia of 1.69 (95% confidence interval 1.13 to 2.53); those born between 200 and 600 m had a relative risk of 1.23 (1.02 to 1.49).  There was a significant (P < 0.01) trend in risk in relation to the reciprocal of distance from the line.  No excess risk in relation to proximity to lines was found for other childhood cancers.

Conclusions: There is an association between childhood leukaemia and proximity of home address at birth to high voltage power lines, and the apparent risk extends to a greater distance than would have been expected from previous studies.  About 4% of children in England and Wales live within 600 m of high voltage lines at birth.  If the association is causal, about 1% of childhood leukaemia in England and Wales would be attributable to these lines, though this estimate has considerable statistical uncertainty.  There is no accepted biological mechanism to explain the epidemiological results; indeed, the relation may be due to chance or confounding.

Introduction

The electric power system produces extremely low frequency electric and magnetic fields.  Since 1979 there has been concern that these fields may be associated with cancer.¹  Concern has concentrated on magnetic rather than electric fields and on childhood leukaemia in particular.  A pooled analysis of nine studies that met specified quality criteria found that children living in homes with 24 hour average fields of 0.4 µT have twice the risk of leukaemia.²  In 2001 the International Agency for Research on Cancer classified extremely low frequency magnetic fields as "possibly carcinogenic" on the basis of "limited" epidemiological evidence and "inadequate" evidence from animals.

Magnetic fields in homes arise mainly from low voltage distribution wiring, house wiring, and domestic appliances.  Only a small fraction of homes are close to high voltage overhead power lines (transmission lines), but in these homes the power line is likely to be the main source of magnetic field.

We investigated whether proximity of home address at birth to transmission lines in England and Wales is associated with increased risks of childhood cancer.  It is not known which period of life, if any, is relevant to induction of cancer by magnetic fields.  Previous research has considered address at diagnosis or throughout some specified period.  Over half (55%) of cases of childhood leukaemia and 43% of other cancers in childhood occur by the age of 5 years.

Methods

Cases and controls
Children aged 0-14 years with cancer (malignant neoplasms and tumours of the central nervous system and brain) in England, Scotland, and Wales, ascertained through several sources including the National Cancer Registration System and the UK Children's Cancer Study Group, are included in the National Registry of Childhood Tumours at the Childhood Cancer Research Group.

We identified nearly 33 000 cases of childhood cancer in children born in England and Wales, 1962-95, and diagnosed in England, Wales, or Scotland over the same period.  We obtained birth information for just over 31 000 cases, 1700 having been excluded because the child was adopted or the birth record could not be traced.  For each case we selected from birth registers a control matched for sex, date of birth (within six months), and birth registration district.  Registration districts vary greatly in size and are frequently redefined; there are currently about 400.  We attempted to find the postcode and approximate grid reference of the address at birth for all cases and controls, but this was not always possible.  The final dataset comprised 29 081 matched case-control pairs (9700 for leukaemia) that we could map with respect to transmission lines.

Calculation of distance from power lines
We looked at overhead power lines forming the National Grid in England and Wales — that is, all 275 and 400 kV overhead lines (the highest voltages used) plus a small fraction of 132 kV lines, about 7000 km altogether.  We obtained the grid references of all 21 800 pylons concerned from the records of National Grid Transco.  Using the postcode at birth we identified subjects living within 1 km of a transmission line.  For 93% of these addresses we obtained, from the Ordnance Survey product AddressPoint, a 0.1 m grid reference and hence calculated the shortest distance to any of the transmission lines that had existed in the year of birth, re-creating previous locations of lines when necessary and possible.  For calculated distances less than 50 m, we took the average of the nearest and furthest points of the building from the line, using large scale maps.  We aimed to obtain a complete set of accurate distances for all subjects within 600 m of a line, a distance chosen to be well beyond that at which the magnetic field from the line is thought to be important.

Statistical analysis
We used conditional logistic regression on the matched case-control pairs to calculate relative risks and ² values.

Results

Table 1 shows the distribution of distances from the nearest line for cases, subdivided into leukaemia, central nervous system/brain, and "other," and for matched controls.  Most (97%) of these distances were 600 m.  The relative risk is an estimate of the incidence compared with that at distances 600 m.  For leukaemia, at each distance category < 600 m the relative risks are greater than 1.0; there is some evidence that the risk varies according to distance from the line, though there is no smooth trend. For the other diagnoses, our data suggest no increased risk.

Table 1 Distance of address at birth from nearest National Grid line for cases and controls in each diagnostic group, and estimated relative (RR)

In general, emanations from a line source are expected to reduce in strength as the reciprocal of distance, but the magnetic field from a power line generally falls as the inverse square of distance, or sometimes the inverse cube.³  For each diagnostic group, we tested whether the risk is some function of distance (d) from the nearest line (table 2), using three models: that the risk depends on the rank of the distance band, the reciprocal of the distance (1/d), or the inverse square (1/d²).  There were no significant results for central nervous system/brain tumours or for "other tumours."  For leukaemia, the results of two of the trend analyses were significant (P < 0.01); these analyses suggest the risk might depend either on the rank of the distance category or on the reciprocal of distance.  The latter seems more plausible.

Table 2 Tests of hypotheses relating trends in relative risks to alternative measures of proximity to nearest line (based on the eight distance categories^* in table 1).  Figures are ² for trend (with 1 df) and P value

We therefore retabulated the results for leukaemia at intervals corresponding to roughly equal intervals of 1/d (table 3).  This change in the grouping of the data does not change the pattern of relative risk estimates shown in table 1 or the significance of the test for trend with 1/d.  For simplicity we also analysed risk of leukaemia in bands 0-199 m and 200-599 m.  The risks relative to 600 m were 1.69 and 1.23; the trend with 1/d was significant (P < 0.01).

Table 3 Relative risk (RR) estimates for leukaemia using revised distance categories (see text)

We examined the possibility that the relation between distance and risk of leukaemia is a consequence of a relation between distance and socioeconomic status.  We used the Carstairs deprivation index to allocate a measure of socioeconomic status to the census ward in which each child was living at birth.⁴  The results in table 4 confirm the previously reported association between affluence and risk of childhood leukaemia (P for trend < 0.01).⁵  Adjustment for socioeconomic status had no effect on the relative risks for distance (table 3).

Table 4 Relative risks for categories of socioeconomic status

Power lines produce small air ions through a process known as "corona."  Fews et al suggest that this could lead to health effects when winds blow the ions away from the line.⁶  We have made an initial test of this hypothesis using a simple model suggested by Preece et al (personal communication), assuming the prevailing wind is from the south west.  The case-control ratio was no greater downwind than upwind of power lines, so, using this admittedly oversimplified approach, we have no evidence to support this hypothesis.

Discussion

To date this is the largest study of childhood cancer and power lines, with roughly twice the number of children living close to power lines than in the next largest study.⁷  We found that the relative risk of leukaemia was 1.69 (95% confidence interval 1.13 to 2.53) for children whose home address at birth was within 200 m of a high voltage power line compared with those more than 600 m from the nearest line.  For 200-600 m the relative risk was 1.23 (1.02 to 1.49).  The finding that the increased leukaemia risk apparently extends so far from the line is surprising in view of the very low level of magnetic field that could be produced by power lines at these distances.

Possible explanations for findings
There is no obvious source of bias in the choice of cases or controls.  The study is based on records of childhood cancer in England and Wales over most of the period that the National Grid has existed.  Registration for childhood cancer is nearly complete, and it seems improbable that the likelihood of registration is related to proximity of birth address to transmission lines.  Controls were selected from registers compiled through the legally required process of birth registration.  No participation by cases or controls was required. We calculated distances without knowing case-control status, and we were able to include 88% of the eligible cases, each with a matched control.

Populations near power lines may have different characteristics from the rest of the population.  In our control data there is a slight tendency in urban areas for greater affluence (measured by the Carstairs index) closer to lines, though in rural areas there is no clear trend.  There is known to be a positive association between affluence and rates of childhood leukaemia.  However, adjustment for socioeconomic status of the census ward of birth address did not explain our finding.  Population mixing has been associated with childhood leukaemia,⁸ but in our cases individual mobility, measured by changes of postcode between birth and diagnosis, was no more common for those whose home at birth was closer to the lines.  Other characteristics of the population (for instance parity, which has sometimes been found to be associated with childhood leukaemia⁹) may vary with proximity to power lines, but we do not have the data to determine whether these explain our result.

The results are highly significant but could nevertheless be due to chance—for example, if the leukaemia controls are not sufficiently representative of the relevant population.  Some support for this explanation can be derived from the different distance distributions observed for the leukaemia and non-leukaemia controls in table 1. Comparison of the leukaemia cases with the latter still suggests that there is an increased risk for leukaemia but it is much lower than that found using the matched controls.  We emphasise, however, that the use of the matched controls is the most appropriate approach.

Six of the studies included in the pooled analysis referred to above² contain, or have been extended to include, analyses of proximity to power lines.^{7 10-14}  Of these, one, a previous UK study,¹⁰ with 1582 cases of leukaemia diagnosed during 1992-6 (most of which will be contained within our 9700), found a relative risk of 1.42 (0.85 to 2.37) for acute lymphocytic leukaemia within 400 m for 275 and 400 kV lines; this supports our results.  Studies in Canada¹¹ and Sweden⁷ also found increased risks for childhood leukaemia (Canada: relative risk 1.8 (0.7 to 4.7) for residence within 100 m of transmission lines of 50 kV or more, and 1.3 within 50 m; Sweden: 2.9 (1.0 to 7.3) for residence £ 50 m versus 101-300 m from 220 and 400 kV power lines, with no increase for other childhood cancers).  Studies from Denmark,¹² Norway,¹³ and the United States¹⁴ found relative risks below 1.0 but were based on smaller numbers.  None of these estimates relates to distances as great as ours; some used a reference category that is within the distance where we found an increased risk.

Our study concerned home address at birth, whereas much previous magnetic field epidemiology has concerned address at other times.  Half of the children with leukaemia in this study had the same address at diagnosis as at birth; we have no corresponding information for the control group.

The most obvious explanation of the association with distance from a line is that it is indeed a consequence of exposure to magnetic fields.  For magnetic fields in the home the pooled analysis by Ahlbom et al found a relative risk of 2.00 (1.27 to 3.13) for exposures 0.4 µT versus < 0.1 µT; the risks for fields < 0.4 µT were near the no effect level.²  Another pooled analysis, including additional studies, found a similar result with a threshold of 0.3 µT.¹⁵ For the power lines we investigated, the magnetic field falls to 0.4 µT at an average of about 60 m from the line (based on calculations using one year of recorded loads for a sample of 42 lines).  Our increased risk seems to extend to at least 200 m, and at that distance typical calculated fields from power lines are < 0.1 µT, and often < 0.01 µT—that is, less than the average fields in homes from other sources.  Thus our results do not seem to be compatible with the existing data on the relation between magnetic fields and risk.  The estimated relative risk was more closely related to the reciprocal of the distance from the line than to the square of the reciprocal of the distance.

Conclusion

While few children in England and Wales live close to high voltage power lines at birth, there is a slight tendency for the birth addresses of children with leukaemia to be closer to these lines than those of matched controls.  An association between childhood leukaemia and power lines has been reported in several studies, but it is nevertheless surprising to find the effect extending so far from the lines.  We have no satisfactory explanation for our results in terms of causation by magnetic fields or association with other factors.  Neither the association reported here nor previous findings relating to level of exposure to magnetic fields are supported by convincing laboratory data or any accepted biological mechanism.

Assuming that the higher risk in the vicinity of high voltage lines is indeed a consequence of proximity to the lines we can estimate the attributable annual number of cases of childhood leukaemia in England and Wales.  The annual incidence of childhood leukaemia in England and Wales is about 42 per million; the excess relative risks at distances of 0-199 m and 200-599 m are about 0.69 and 0.23, respectively, giving excess rates of 28 and 10 per million.  (These two estimates allow for the fact that the incidence for England and Wales is itself partly based on cases occurring in the vicinity of power lines.)  We estimate that of the 9.7 million children in the population (2003 estimate), at birth about 80 000 would have lived within 199 m of a line and 320 000 between 200 and 599 m.  Thus, of the 400-420 cases of childhood leukaemia occurring annually, about five would be associated with high voltage power lines, though this estimate is imprecise.  We emphasise again the uncertainty about whether this statistical association represents a causal relation.

Disclosures

We are grateful to colleagues at the Childhood Cancer Research Group and at National Grid Transco for help with this study and to cancer registries and the United Kingdom Children's Cancer Study Group for notifications of cases of childhood cancer.

Contributors:  GD was responsible for overall direction of the study and publication.  GD and JS had the initial idea and designed the study.  TV and MEK collected information on cases and controls and carried out the statistical analysis. JS assessed exposures.  GD and JS are guarantors.

Funding:  This study was undertaken as part of a project funded by the United Kingdom Department of Health Radiation Protection Programme.  The Childhood Cancer Research Group also receives funding from the Department of Health and the Scottish Ministers.  The views expressed here are those of the authors and not necessarily those of the Department of Health and the Scottish Ministers.  National Grid Transco provided staff time but no other funding.

Competing interests:  JS is employed by National Grid Transco and worked on this project with their permission.  A written contract exists between the Childhood Cancer Research Group and National Grid Transco specifying that the Childhood Cancer Research Group has complete control over the conduct, interpretation, and publication of this study; this paper has not been approved by anyone in National Grid Transco other than JS in his capacity as author and does not necessarily represent National Grid Transco's views.

Ethical approval:  The Childhood Cancer Research Group has local ethics committee approval and, through membership of the UK Association of Cancer Registries, has approval from the Patient Information Advisory Group with respect to cancer registration function.

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