Over the years, many scientific studies have been conducted to clarify whether living near high-voltage power lines is somehow associated with childhood cancer. To summarise the findings of the studies, they pointed to an increased incidence of leukaemia in children exposed to fields stronger than an average of 0.3-0.4 µT (microtesla) for longer periods of time. Experimental research and theoretical science have not been able to confirm the existence of an association. Researchers can therefore not say with certainty that magnetic fields were the cause of the statistical results.

Danish study
In 1993, the Danish Cancer Society carried out a statistical demographic study (epidemiological study) on magnetic fields and cancer in children living close to high-voltage power lines. The study neither confirmed nor dismissed the existence of a link between magnetic fields and childhood cancer.

The researchers responsible for the Danish study assessed that the possible increased risk - if there turned out to be a causal link - would be tantamount to one extra case of childhood leukaemia every five years. In Denmark, about 45 children are diagnosed with leukaemia each year. Only little is known about what causes leukaemia, and the remaining cases must therefore be attributed to other causes. If there is an excess risk in the magnitude of one cancer case every five years or so, statistical studies must cover a very long period to include so many cancer cases that statistical coincidences can be ruled out.

After the publication of the Danish and Swedish childhood cancer studies, the Danish health authorities evaluated the study findings. The Danish National Board of Health came to the conclusion that there was no basis for defining exposure limits but instead recommended a precautionary approach.

The World Health Organization's (WHO) most recent evaluation of magnetic fields and health risks from 2007 is based on the Danish study and studies from a long line of other countries.

Danish studies of high quality
Danish studies are generally of a high quality as all cancer cases have been registered in a national cancer register since 1943 and we have had a civil registration system since 1968. Many countries do not have similar systems, and this could give rise to uncertainty.

Other sources of uncertainty about findings in Denmark and other countries could be that:

• it might be difficult to obtain a sufficiently precise idea of the strength of the fields to which a person has been exposed in a given period of time that may go back 20 years
• relatively few children have lived so close to power lines that they have been subjected to what researchers call 'high exposure' 
• the diseases are fairly rare

In its most recent evaluation WHO therefore recommends that the electricity industry and authorities should continue to keep abreast with research and fund research projects that could potentially reduce the scientific uncertainty.

Updating and expansion of the study
The Danish Cancer Society is presently expanding the Danish study to cover the most recent years up to 2004. More data and years may increase the statistical certainty of the study and hopefully provide an extra piece of the puzzle. It is unlikely, however, that one single study will be able to substantially change the existing evaluations prepared by, for example, WHO, which build on a large volume of research. The study may instead provide us with new and more reliable information about the situation in Denmark.