In response to the study by Dr David Lloyd and colleagues published in the British Journal of Cancer today, Professor Denis Henshaw of Bristol University has questioned whether the study provides evidence that powerlines do not cause childhood leukaemia, given the already acknowledged link between childhood leukaemia risk and exposure to magnetic fields, such as those found near high voltage powerlines.

Two major pooled analyses of international studies published in 2000, by Professor Anders Ahlbom of the Karolinska Institute in Stockholm and Professor Sander Greenland of the University College of Los Angeles have clearly demonstrated that there is a doubling of childhood leukaemia risk associated with magnetic field exposures above 0.3 or 0.4 microtesla, well below that found under high voltage powerlines.

Many lines of investigation have been followed in searching for mechanisms by which magnetic fields may cause an increase in childhood leukaemia risk. One such line concerns the ability of magnetic fields to disrupt both the production and action in the body of the natural anti-cancer agent melatonin. At magnetic field levels as low as 0.2 microtesla, again well below that typically seen under high voltage powerlines, there are at least nine studies in human population groups showing that such fields reduce the night-time production of melatonin in the pineal gland.

The study carried out by Dr Lloyd and colleagues did not involve melatonin. Therefore, while publication of this study is to be welcomed, failure to observe an effect of magnetic fields on these cells may be unsurprising. In particular, the study does not provide evidence that exposure to magnetic fields near powerlines does not cause childhood leukaemia.

Studies on breast cancer cells in the laboratory have shown that while melatonin can inhibit the growth of these cells, this inhibiting action can be suppressed by magnetic fields, again at levels lower than that typically seen under powerlines. No such studies have been done on blood cells. However, it is known that melatonin is highly protective of radiation damage to blood cells of the type looked at in Dr Lloyd's study and the type that could lead to leukaemia. If magnetic fields, which disrupt the production of melatonin in the pineal gland, also inhibit its protective action that has been shown for breast cancer cells, then clearly this would provide a potential mechanism by which magnetic fields such as seen near powerlines increase the risk of childhood leukaemia.

Information on childhood leukaemia and magnetic fields and the statements by various international agencies may be found at www.electric-fields.bris.ac.uk

Dr David Lloyd's paper Possible associations between ELF electromagnetic fields, DNA damage response processes and childhood leukaemia was published in the British Journal of Cancer on 11 June 2003