OK I was a bit naughty with the headline. The headline in the normally reliable Guardian Unlimited Life was “Power cables linked to cancer”, but I think mine describes the results of the study equally well. First, the sensationalist story:
Children living near high-voltage power lines are substantially more likely to develop leukaemia, researchers from Oxford University and the national electricity grid report today in the British Medical Journal.
Those living within 200 metres of the overhead cables were 70% more likely to develop the disease than similar children living more than 600 metres away.
Wow! 70% more likely to develop the disease! Predictably the chairman of Children with Leukaemia weighed in with the not really hysterical:
There is now a clear case for immediate government action. Planning controls must be introduced to stop houses and schools being built close to high-voltage overhead power lines.
“Immediate government action”? Really? Of course, it helps if you read the actual study, which concludes:
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.
By my calculation that puts the normal risk of getting leukemia at 0.0042%, rising to a whopping 0.007% if the child lives within 199m of power lines. That’s a pretty small risk not really conveyed by the “70% increase” headline. (Although I do agree that if you’re one of the five extra cases a year it’s 100% for you.)
The figures can be found in this table under the “leukaemia” column that shows 64 cases of leukemia in the 0-199 meter range compared with 39 in the control group. If you look under the CNS (central nervous system)/brain tumors column you will see the equivalent figures are 33 cases against 45 for the control – a reduction of 17% (hence my headline). Take just the figure for 0-49 meters and the headline could just as easily have been “Power cables linked to 56% reduction in central nervous system and brain tumors” – although that would have been equally misleading. There is no causal link for either statement and so it seems absurd to be calling for “immediate government action”. As we know, correlation is not causation. That doesn’t mean there is definitely no link but it should give us pause.
If there is a link, could it be due to the magnetic fields created by the power lines? The researchers measured magnetic fields near the power lines, but noted that although the increased risk seemed to extend to at least 200 meters, at that distance the magnetic fields from the power lines were less than fields from other sources (appliances, house wiring etc), in the homes. Also, the increased incidence of the disease was noted right up to the 600 meter limit – much more than previous studies had found and way beyond any electromagnetic field from the lines. The study doesn’t support the view that increased risk is due to electromagnetic fields.
It may not be the power lines, but something about areas where power lines are located, or something about the people who live in these areas. It’s hard to know what, since the researchers did control for socioeconomic status, and found that had no effect on the relative risks.
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.
There is no accepted biological mechanism to explain the results. The results could be due to chance (for example, if the controls were not representative) or confounding. Even if the link is causal, only about 1% of childhood leukemia in England and Wales would be attributable to the power lines and even this figure has considerable statistical uncertainty.
The study does arrive at a result that can’t be explained, and so it is reasonable to call for more work to be done. Nevertheless, the headline and article seem a little scare-mongering.