ArticlesExposure to power-frequency magnetic fields and the risk of 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.
Section snippets
Methods
The UKCCS was a population-based collaborative case-control study covering the whole of England, Wales, and Scotland, based on eight regional centres in England and a centre each in Scotland11, 12 and Wales.11
Results
87% of all eligible cases diagnosed in the UKCCS in England, Scotland, and Wales in the defined periods were included, with at least one of the parents interviewed. The corresponding response rate among controls was 64% (figure), with evidence of under-representation of those living in the most deprived census areas. 2226 case-control pairs were eligible for analysis (58% of interviewed case-control sets, 50% of all eligible cases). The main reason for non-inclusion in the EMF part of the study
Discussion
We found no evidence that magnetic fields associated with the electricity supply increase risk of childhood leukaemia, malignant brain (or other central nervous system) tumours, or any other childhood cancer.
Of the total number of cases eligible for inclusion in the main UKCCS study, only 50% had EMF measurements, as did only 40% of first-choice controls. Although there was some under-representation of individuals living in more deprived areas among controls compared with cases, the association
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