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

Results in a nutshell

• Children who lived within 200 m of high-voltage power lines 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.
• The results do not seem to be compatible with the existing data on magnetic fields and cancer because they extend too far from the line
• There is no evidence the results are explained by the “corona ion” hypothesis

Background to the study

The study was funded by the UK Department of Health. It was conducted by the Childhood Cancer Research Group (CCRG) at the University of Oxford with input from National Grid Transco. There are four authors, three from CCRG (Gerald Draper, Honorary Senior Research Fellow, Tim Vincent, Research Officer, Mary Kroll, Statistician) and one from NGT (John Swanson, Scientific Advisor).

The paper spells out that the overall direction of and responsibility for the study and publication was with Gerald Draper at CCRG, whereas the initial idea and design of study was with Gerald Draper and John Swanson jointly. The paper also spells out that a written contract exists between CCRG and NGT specifying that CCRG has complete control over the conduct, interpretation and publication of this study; the paper has not been approved by anyone in NGT other than John Swanson in his capacity as author and does not necessarily represent NGT’s views.

Methods

Cases and controls
The study concerns nearly 33,000 cases of childhood cancer born in England and Wales between 1962 and 1995, and diagnosed in England, Wales or Scotland over the same period. Birth information was obtained for just over 31,000 cases, 1,700 having been excluded because the child was adopted or the birth record could not be traced. For each case a “matched control” (a child of the same age and sex, but without leukaemia, for comparison purposes) was selected from birth registers. The final analysis dataset comprised 29,081 matched case-control pairs (9700 for leukaemia) that were mapped with respect to transmission lines. See also a picture explaining the numbers of children involved [link to first slide in ppt file as separate page]

Calculation of distance of birth domicile from power lines
This study concerns overhead power lines forming the National Grid in England and Wales, i.e. all 275 and 400 kV overhead lines (the highest voltages used), plus a small fraction of 132 kV lines, about 7,000 route km altogether (more on these lines). The grid references of all 21,800 pylons concerned were obtained from the records of National Grid Transco. Using the postcode of domicile at birth, subjects within 1 km of a transmission line were identified. For 93% of these a grid reference specific to the address was obtained and hence the shortest distance to any of the transmission lines that had existed in the year of birth was calculated. The aim was 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 could be important.

The paper says it is not known which period in life, if any, is relevant to induction of cancer by magnetic fields. This paper considers whether risk is dependent on the proximity of domicile at birth. Previous authors have considered domicile at diagnosis or throughout some specified period. In practice, for half the leukaemia cases, the two addresses are the same anyway.

Results

The paper uses three diagnostic groups: childhood leukaemia, central nervous system/brain tumours, and “other”. It analyses risks within 600 m compared to a reference category of beyond 600 m. For leukaemia, at each distance category below 600 m the relative risks are greater than one, and there is some evidence that the risk varies according to distance from the line though there is not a smooth trend. For the other diagnoses, the authors say their data suggest that there is no increased risk.

The relative risks for leukaemia are presented for various distances, but the version used in the abstract and conclusions is:

See also the full results table including graphs of the results

The paper tests for three different possible trends, and concludes that a trend of risk with the reciprocal of distance is statistically significant (p=0.01) and plausible.

Adjustment for socio-economic status made no virtually no difference to the results, and an initial test of the “corona ion” hypothesis, using the same simple model previously used by Bristol University, assuming the prevailing wind is from the south-west, produced no evidence to support this hypothesis.

Discussion

The authors state that the finding that the increased leukaemia risk apparently extends so far from the line is surprising in view of the low average level of magnetic field exposure at these distances. They consider various possible explanations:

Bias
They conclude there is no obvious source of bias in the choice of cases or controls, and the analyses were conducted “blind”.

Confounding by some other characteristic of the population that varies near power lines
They give evidence that socio-economic status varies near power lines and suggest that other factors may do too. But adjusting for socio-economic status of the census ward of birth domicile did not explain the finding, and they did not have the data to test for other factors.

Chance
The results are highly statistically significant but could nevertheless be due to chance. The authors note that some of the controls seem to be different to others, and this could be evidence that chance is at work.

Magnetic fields
The authors say that the most obvious explanation of the association with distance from a line is that it is indeed a consequence of exposure to magnetic fields. But they then cite the Ahlbom pooled analysis, suggesting a risk for magnetic fields above 0.4 microteslas (µT), and point out that for the power lines they are investigating, the magnetic field falls to 0.4 µT at an average of approximately 60 m from the line (more on fields from power lines). Their elevated risk appears to extend to at least 200 m, and at that distance, typical calculated fields from power lines are less than 0.1 µT, and often less than 0.01 µT, i.e. less than the average fields in homes from other sources. Thus they say their results “do not seem to be compatible with the existing data on the relation between magnetic fields and risk.”

Comparison with previous results
The authors confine their comparison to just those studies that were included in the Ahlbom pooled analysis. Of these, they note that one, the United Kingdom Childhood Cancer Study (UKCCS), with subjects largely also included in the present study, found a relative risk of 1.42 for acute lymphocytic leukaemia within 400 metres for 275 and 400 kV lines; this, they say, supports their results. Two others studies, in Canada and in Sweden, found elevated risks; results from Denmark, Norway and the United States found relative risks below one but are based on smaller numbers. They point out that no previous study has investigated distances as great as they have, and some used a reference category that is within the distance where they found an elevated risk.

Abstract

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.

Participants
29081 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 by cases or controls 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.