Children living near overhead power lines may have an increased risk of leukemia but the association may not be causal, UK researchers say.
The confusing message, which comes from the largest study to date - of over 29,000 children with cancer - is that since "there is no biological mechanism to explain the higher risk", the results, "although statistically significant, may be due to chance".
The study - a collaboration between the Childhood Cancer Research Group at the University of Oxford and National Grid owners, Transco - looked at cancer data in England and Wales between 1962 and 1995, for children aged up to 15 years old.
They were able to map how far each child lived from a high voltage overhead power line. Comparing the children who had cancer with a control group of 29,000 children without cancer but who lived in comparable districts, found that children whose birth address was within 200 metres of an overhead power line had a 70% increased risk of leukemia. Children living 200 to 600 m away from power lines had a 20% increased risk.
"To put these results in perspective, our study shows that about five of the 400 cases of childhood leukemia every year may be linked to power lines - which is about 1% of cases," says Gerald Draper at Oxford University, who led the study. "The condition is very rare and people living near power lines should have no cause for concern."
However, the results are controversial, coming just one month after the major UK Childhood Cancer Study report, which declared that there was no risk to children living these distances away from power lines.
"Statistical artefact"
Although a link between childhood cancer and power lines has been suggested by previous studies, it has only been associated with high exposure - those living within about 60 m of an overhead power line. At the distances Draper looked at, the electromagnetic field created by the power lines should be too low to have any health effects, he says. They were much lower, for example, than those constantly experienced due to the Earth's magnetic field.
"We don't think it is possible that a magnetic field of these low magnitudes could have a causative effect on childhood leukemia," Draper says.
The increase in leukemia risk for those living at distances greater than 60 m was "difficult to interpret, but is most unlikely to be due to any residual electromagnetic field, or other exposures related to the power line", says David Grant, scientific director of Leukemia Research. "It cannot be excluded that it is a statistical artefact."
But given the statistical significance of their results, the researchers had considered other theories. One established link with the disease is low exposure to infection soon after birth - an effect seen most commonly in babies born to higher income, middle-class families, where early social mixing between infants is rarer. Draper's group looked at the population characteristics in areas immediately surrounding power lines.
They found that some were built in areas of low income housing and others in high-income areas. Looking at social status data alone, there was a 10% increase in leukemia for those in middle-class families, but these results were found to be independent of power line location data.
Corona ions
Another theory that the researchers tested, first mooted controversially in 1999 by Dennis Henshaw at Bristol University, UK, concerned "corona ion" effects. Henshaw proposed that the air immediately surrounding a high voltage power line or pylon becomes ionised by the electric field.
Corona ions combine with pollutants in the air, giving rise to charged airborne particles, which may be blown some distance away before being inhaled. Henshaw believes that once breathed in, the particles remain in the lungs, causing cancer.
The researchers attempted to test this theory with Henshaw, and found no discernable difference between leukemia cases upwind or downwind of the power cables. However, they admit that the testing procedure, although the best available at the time, was poor. They are preparing to conduct a more conclusive test, with Henshaw's newly devised equipment. He believes the excess number of children found to have leukemia due to power lines - five per year - "may be the tip of the iceberg".
This paper forms part of Draper's larger study, which is also looking at electromagnetic doses at different distances from power lines.
Journal reference: British Medical Journal (vol 330, p 1290)
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