When the epidemiological studies are pooled there is a suggestion that exposure to magnetic fields might double
the risk of childhood leukaemia. By itself this factor of two is unpersuasive,
partly because of the difficulties in choosing suitable controls to compare with the cases
and the likely presence of confounding factors (such as higher road traffic density near power lines)
that might not have been fully eliminated.
More background to epidemiology is given in Library Research Paper 94/119, Overhead Power Lines and Health.