The pooled analysis by Ahlbom and others is usually interpreted as suggesting a relative risk of two for exposures to magnetic fields of greater than 0.4 µT. A relative risk of two is a doubling of risk.
This means long-term average fields in the child’s bedroom (24 hours or longer) of 0.4 µT, not short-term exposure above this level. More on the difference between fields in homes and personal exposure.
0.4% of children – 1 in 250 - in the UK live in homes with fields this high. This figure comes from the United Kingdom Childhood Cancer Study, the largest source of data that exists in the UK.
A relative risk of two applying to 0.4% of the population equates to roughly 0.4% of cases being attributable to magnetic fields (if, of course, the risk were a real one to start with). About 500 cases of leukaemia occur a year in children in the UK (more on the statistics of cancer). So the risk, if real, corresponds to about 2 extra cases a year in the UK. Of these 2, one would be due to power lines and one due to low voltage sources.
Some things to note about this calculation:
- This is the number of extra cases. Survival for the commonest type of childhood leukaemia, Acute Lymphocytic Leukaemia, is approaching 80% in the UK.
- This calculation takes the epidemiology at face value and assumes it shows a real risk. Of course, the risk may not exist at all (see some of the other things that could account for epidemiological results). Even if it does, the true relative risk would be unlikely to be exactly two. But this calculation is unlikely to be greatly wrong.
- The epidemiological studies compared the children with fields greater than 0.4 µT to a reference group: children with fields less than 0.1 µT. We usually assume there would be no risk in this reference group. If there were a risk, the number of attributable cases would increase. Similarly, we assume there is no risk in the intermediate categories –0.1 – 0.2 µT and 0.2 - 0.4 µT.
The 2005 CCRG study found slightly raised rates of childhood leukaemia (but not any other childhood cancers) round 275 kV and 400 kV power lines in England and Wales. That paper says:
“If the association is causal, about 1% of childhood leukaemia [5 cases per year] in England and Wales would be attributable to these lines, though this estimate has considerable statistical uncertainty.”
This number – five cases per year – is bigger than the calculation from magnetic fields – two per year. This is because the effect the CCRG study found extends further from power lines than the magnetic-field effect would do. It may be evidence that the effect is caused by some other characteristic of the areas power lines pass through.
The 2007 WHO Environmental Health Criteria Monograph considers this issue at a global level and concludes:
"Although a causal relationship between magnetic field exposure and childhood leukaemia has not been established, the possible public health impact has been calculated assuming causality in order to provide a potentially useful input into policy. However, these calculations are highly dependent on the exposure distributions and other assumptions, and are therefore very imprecise. Assuming that the association is causal, the number of cases of childhood leukaemia worldwide that might be attributable to exposure can be estimated to range from 100 to 2400 cases per year. However, this represents 0.2 to 4.9% of the total annual incidence of leukaemia cases, estimated to be 49 000 worldwide in 2000. Thus, in a global context, the impact on public health, if any, would be limited and uncertain
Other possible causes
If magnetic fields don't cause more than a small fraction of childhood leukaemia, if indeed they cause any, what causes the rest? We have a separate page with a discussion of this.
Statistics on cancer generally
This page talks about the numbers of children potentially affected by childhood leukaemia and EMFs. See more information on the statistics of childhood cancer generally.