Children living close to high voltage overhead power lines at birth may be at an increased risk of leukaemia, finds a large study in this week's BMJ.
But the authors emphasise that these results may be due to chance and further research is needed to find out whether there really is a link.
About one child in 2000 develops leukaemia before the age of 15 years.
Although we don't yet understand its causes, researchers have studied a variety of possible explanations including genetic susceptibility, ionising radiation, unusual patterns of exposure to infection, and electromagnetic fields.
The electric power system produces extremely low frequency electric and magnetic fields, and since 1979 there has been concern that these fields may be associated with cancer. In 2001, the International Agency for Research on Cancer classified extremely low frequency magnetic fields as "possibly carcinogenic" while others, such as the UK Childhood Cancer Study, dispute the risk.
Over 29,000 children with cancer, including 9,700 with leukaemia, were included in the study, the largest of its kind to date. The children were aged 0-14 years and were born in England and Wales between 1962 and 1995. They were compared with a group of control children individually matched for sex, approximate date of birth, and birth registration district. The distance of each child's home address at birth from the nearest high voltage power line was calculated.
Children who lived within 200m of high voltage power lines at birth appeared to have a 70% raised risk of leukaemia compared with those who lived beyond 600m. There was also a slightly increased risk for those living 200-600m from the lines at birth.
To put these risks into perspective, about five of the 400-420 cases of childhood leukaemia that occur annually in England and Wales may be associated with power lines.
No excess risk was found for other childhood cancers.