‘Pooled’ analysis
The findings on childhood leukaemia are understood best not by looking at individual studies but by gathering all the data together.
In 2000, an international group, led by Professor Anders Ahlbom from Sweden, took all the recent separate epidemiological studies of childhood leukaemia and magnetic fields and pooled the results, so that they could perform one single re-analysis of all the available data. They found that, statistically, there was little or no suggestion of an increased risk at the levels of magnetic fields to which the overwhelming majority of children are exposed. The study did, however, find that in the category of homes with a field, averaged over 24 hours, of greater than 0.4 microteslas (which affects fewer than half a percent of children in the UK) there is a statistical suggestion of increased risk. Some of these homes are near power lines, but many are not. They concluded:
“The explanation for the elevated risk is unknown, but selection bias may have accounted for some of the increase.” Full abstract.
More on selection bias
In view of the importance of this study, we have reproduced the main results table and we also discuss alternative presentations and interpretations of the results.
see also
The Childhood Cancer Research Group Study
This study published in 2005 (also known as the “Draper” study) is a collaboration between the CCRG at the University of Oxford and National Grid. It looks at whether children who get cancer were born near power lines or not. It found:
“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.”
The strange thing about this result is that it seems to extend much further from power lines – up to 600 m – than magnetic fields do. The authors say:
“…our results do not seem to be compatible with the existing data on the relation between magnetic fields and risk.”
and
“We have no satisfactory explanation for our results in terms of causation by magnetic fields or association with other factors.”
more detail on the CCRG study.
The United Kingdom Childhood Cancer Study
The biggest of all the epidemiological studies of EMFs and childhood cancer until the Draper study was conducted during the 1990s, called the United Kingdom Childhood Cancer Study (UKCCS). It looked at a number of suggested causes of childhood cancer including EMFs. Its particularly large study population – over 2000 cases of cancer in total, three times larger than the next largest study before it – makes it very powerful. It was conducted by an eminent group of scientists, led by Sir Richard Doll, who first identified the link between smoking and lung cancer.
The UKCCS was designed to look at every case of childhood leukaemia occurring in the UK over roughly a four-year period. It is hard to envisage a study better able to give comprehensive answers about childhood cancer in the UK, and if a link with EMFs exists, this study was expected to find it.
In December 1999, the UKCCS published its report on exposure to magnetic fields and concluded:
“This study provides no evidence that exposure to magnetic fields associated with the electricity supply in the UK increases risk for childhood leukaemia, cancers of the nervous system, or any other childhood cancer.”
Nonetheless, even though the UKCCS on its own provides "no evidence" of a risk, the pooled analysis, which included the UKCCS, still found a statistical association.
more detail on the UKCCS including further publications on electric fields and proximity to power lines.