Friday, June 03, 2005

Powerlines and childhood leukemia

The British Medical Journal (available here for a short time) has just published a piece by the Oxford Childhood Cancer Research Group and National Grid Transco plc (electricity industry) on childhood cancer in relation to distance from high voltage power lines in England and Wales. It is a case - control study, where the records of more than 29,000 children with cancer were compared with the records of children individually matched on approximate date of birth, sex and registration district to examine differences in the distance of the child's home address at birth to the nearest high voltage overhead powerline. Children who lived within 200 meters of a high tension line had a 70% increased risk of leukemia (i.e., a relative risk of 1.7, 95% confidence interval 1.1 to 2.5, referent >600 meters), while those living 200 to 600 meters had an increased of 20% (relative risk 1.2, CI 1.0 to 1.5). There was a significant trend in risk as the residence at birth was closer to the line. Childhood cancers other than leukemia did not show the same relationship with power lines.

This apparently came as a surprise, although it shouldn't have. Other studies have also shown this relationship (although many have not). Perhaps the support of the electricity industry was partially responsible for the attempt to minimize the "surprising" results:
“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.”


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.” (New Scientist)
It is true that at these distances (200 meters) the magnetic field would likely be relatively low but no data were given in the paper on calculated or measured fields (something which could easily have been done). The comparison with the earth's magnetic field is not appropriate, as the earth's is a DC field while the powerline fields are low frequency 50 - 60 Hz AC fields.

The main (scientific) reason researchers have been reluctant to accept the reasonably abundant epidemiological evidence that there is a causal association between powerline frequency magnetic fields and childhood leukemia is the lack of an accepted mechanism. While several mechanisms have been suggested, none is sufficiently convincing to prompt most scientists to accept the evidence that continues to accrue. But it is worth remembering there is as yet no accepted mechanism for asbestos fiber carcinogenesis either, although no one now doubts that asbestos causes cancer.

The authors of the BMJ study fall back on the explanation that this might be a statistical anomaly. Sounds a bit like "whistling past the graveyard" to our ears. But the other thing that occurs to us is this. If the researchers were so sure exposure at 200 meters wouldn't show an effect, why were they looking at it in the first place? This sounds like an attempt to produce a "negative study" that "went wrong." We give the Oxford group credit for reporting it out. We wonder why they agreed to do it in the first place.