Exposure to power-frequency magnetic fields and the risk of childhood cancer

Summary

Background

Previous studies have suggested an association between exposure to power-frequency electromagnetic fields (EMF) and the development of childhood malignant disease, especially leukaemia and tumours of the central nervous system. We investigated the relation between all childhood cancer and exposure to power-frequency magnetic fields.

Methods

The UK Childhood Cancer Study was a population case-control study covering the whole of England, Wales, and Scotland. All children with a confirmed malignant disorder were potentially eligible. For each case, we matched two controls on date of birth and sex, randomly chosen from the list of the Family Health Services Authority in England and Wales or Health Board in Scotland. In the main study, 3838 cases and 7629 controls were interviewed. The EMF part of the study included only one control per case, and household EMF measurements and school measurements where relevant were taken on 2226 matched pairs. These measurements, adjusted for historical line load and appliance fields, were used to estimate average exposure in the year before the date of diagnosis, or an equivalent date for controls. Analyses were by conditional logistic regression, incorporating a census-derived deprivation index used as a measure of socioeconomic status.

Findings

For children with mean exposures of more than 0·2 μT compared with children with mean exposures of less than 0·1 μT, the adjusted odds ratios were 0·92 (95% CI 0·47–1·79) for acute lymphoblastic leukaemia, 0·90 (0·49–1·63) for all leukaemia, 0·46 (0·11–1·86) for central-nervous-system tumours, 0·97 (0·46–2·05) for other malignant disease, and 0·87 (0·56–1·35) for all malignant disease combined. Higher exposures (>0·4 μT) were recorded for only 17 (<0·4%) individuals (eight cases, nine controls).

Interpretation

This study provides no evidence that exposure to magnetic fields associated with the electricity supply in the UK increases risks for childhood leukaemia, cancers of the central nervous system, or any other childhood cancer.

Introduction

The causes of childhood malignant disorders are poorly understood. Ionising radiation, some cancer chemotherapy agents, viruses, and genetic factors are each the cause of a small proportion of cases, but for most cases the cause is uncertain. Several hypotheses have been proposed, including parental cigarette smoking and, for childhood leukaemia, population mixing and abnormal responses to infection.

One hypothesis has related the risk of development of childhood cancer, especially leukaemia and brain tumours, to increased exposure to electromagnetic fields (EMF), specifically the magnetic component associated with the distribution and use of electricity. The first report¹ of a link between childhood leukaemia and exposure to magnetic fields at a frequency of 60 Hz was followed by many studies of power-frequency fields (50/60 Hz), with conflicting results.2, 3, 4 An overview of the early studies based on calculated fields suggested that risk increased smoothly with increasing time-weighted mean exposure to more than 0·2 μT, with a relative risk of about 1·8 associated with exposure of 0·6 μT or more.⁵ For exposures lower than 0·2 μT, the relative risk seemed to be constant. Several subsequent studies measured magnetic-field exposure rather than inferring or calculating exposure from the proximity and load of neighbouring power lines.6, 7, 8, 9, 10 The results are inconclusive in that they are consistent with no increase in risk at exposures higher than 0·2 μT, but they cannot exclude the possibility of a moderate excess risk at high exposure.

We present the results of the EMF part of the UK Childhood Cancer Study (UKCCS), a nationwide case-control study of childhood cancer done across the whole of the UK, except for Northern Ireland.¹¹ The UKCCS was set up in the early 1990s, when sufficient evidence had accrued to suggest that the roles of several exposures required investigation.

In the full study, five main hypotheses were tested: childhood cancer might be caused by in-utero or postnatal exposure of the child to ionising radiation; specific types may be caused by in-utero or postnatal exposure of the child to certain chemicals; childhood cancer might be caused by exposure of parental germ cells to ionising radiation or certain chemicals before conception of the child; specific types, especially leukaemia and cancers of the central nervous system, might be caused by postnatal exposure to extremely low-frequency EMF; leukaemia and lymphomas may arise as rare and abnormal responses to infection. Results on the non-EMF hypotheses will be reported elsewhere.

We measured the 50 Hz magnetic fields (and harmonics <800 Hz) generated by the distribution and use of electricity in homes and, when relevant, schools to obtain estimates of individual exposure. Each measurement consisted of a series of readings, summarised as the arithmetic mean. We tested the hypothesis that a mean exposure of more than 0·2 μT in the year before diagnosis would increase risk of childhood leukaemia, specifically acute lymphoblastic leukaemia, and cancers of the central nervous system, compared with a mean exposure of less than 0·1 μT in the year before diagnosis. A further hypothesis was that risk of the same diagnoses would increase smoothly with increasing mean exposure in the year preceding diagnosis. Residential electric fields were measured in a subset of the study, which will be reported elsewhere.