A major study has found that living too close to overhead power lines appears to increase the risk of childhood leukaemia.
The study looked at the distance of pylons from houses
BBC News website looks at the evidence.
Q: What did the study look at?
The study, published in the British Medical Journal, was carried out by Dr Gerald Draper and colleagues from the Childhood Cancer Research Group at Oxford University and Dr John Swanson, a scientific adviser at National Grid Transco.
It looked at more than 29,000 children with cancer, including 9,700 with leukaemia, born between 1962 and 1995, and a control group of healthy youngsters in England and Wales.
The researchers measured the distance from children's home addresses at birth from the nearest high voltage power line.
Q: What did they find?
They found that 64 children with leukaemia lived within 200 metres of the line, while 258 lived between 200-600 metres away.
Overall, youngsters living within 200 metres of the lines were about 70% more likely to develop leukaemia, and those living between 200 and 600 metres away about 20% more likely to develop leukaemia than those who lived beyond 600 metres from high voltage pylons.
Q: Does this mean that pylons cause leukaemia?
The researchers say no. Although they found a definite trend between the distance that children lived at birth from high voltage power lines and the risk of leukaemia, they could find no obvious explanation for this.
Some researchers have suggested that magnetic fields produced by pylons might be to blame.
The authors of the BMJ study did not measure magnetic fields.
However, the raised risk that they found was present at more than 200 metres from a line. At this distance magnetic fields from lines are at or below background levels.
The Health Protection Agency says this suggests that at least some of the increased leukaemia risk may be associated with factors other than electromagnetic fields.
However, the UK charity Children with Leukaemia, said there was now a clear case for immediate government action.
"Planning controls must be introduced to stop houses and schools being built close to high voltage overhead power lines," its chairman, Eddie O'Gorman, said.
Q: What are the other possible causes?
The study authors said their findings might be chance.
Alternatively, the link it might be down to factors other than the pylons themselves, such as the type of people who live near pylons or the general environment where pylons are located, which the researchers said they now plan to investigate.
Cancer Research UK said the triggers that cause childhood leukaemia are most likely a random course of events over which a parent has no control.
There is some evidence that there may be a genetic element to leukaemia.
Other researchers have suggested that, in some children, it could be down to abnormal immune system development as an infant caused by a lack of exposure to infections early in the child's life.
When the child then catches an infection later in life, they cannot fight it in the normal way and this triggers leukaemia, they believe.
Others say it is due to immune system damage by chemicals, infections or radiation before the baby is born.
Q: Is it safe for people live near to high voltage power lines?
No one knows for sure.
Both Cancer Research UK and Leukaemia Research said there was no reason why anyone should be advised to move house on the basis of these new results.
Dr John Swanson, one of the study authors and scientific adviser at National Grid Transco, said more research was needed.
"We have got to think about these questions of 'should people carry on living near power lines or not?'" he said.
He added that a stakeholder group, including the electricity industry, government, patient groups and cancer charities, had been set up to investigate.
He and his co-workers estimated that if living in close proximity to power lines at birth did increase the chance of leukaemia, it would account for only 1% of all childhood leukaemias.