As the nation's fears focus on contaminated beef, one scientist believes he has identified another threat to public health - overhead power lines. Since Channel Four's Dispatches programme reported on new research at Bristol University showing how overhead power lines may be linked to cancer, Professor Denis Henshaw's reputation has burgeoned worldwide. His telephone has been ringing continuously. As many as 5,000 individuals from 28 different countries have burrowed into the Internet site describing his research published in the Journal of Radiation Biology.

The body established to protect the British public - the National Radiological Protection Board - remains adamant, however, that Henshaw's studies do not show the mechanism for any cause and effect link between overhead power lines and cancer. In fact its public relations director, Matt Gaines, sounds mildly apoplectic at the thought. "His research doesn't show he has established anything," he says. "The link with cancer has not been found. He has made a few measurements, but it's a big jump from that to say you've established a convincing mechanism."

The argument has all the makings of a major scientific spat. The NRPB is outraged that a respectable physicist at an important English university should have taken what they regard as a speculative research paper to a campaigning TV programme. Says Gaines: "We saw Professor Henshaw's paper as a way of setting up his case for more funding."

Henshaw is, indeed, hoping for more research funding - though that was not why he talked to Dispatches - and was applying to grant-making bodies as we went to press. He also intends to take more measurements under real power lines outdoors, to back up the thesis derived from his simulated laboratory model. "The epidemiological studies which are showing consistent effects are with childhood leukaemia and childhood brain cancer under power lines," says Henshaw. "We have already shown the effect in our model. But it would be a bit of a bull's eye in the jigsaw to measure it for real, and to measure airborne levels of radioactivity for real we need to do it outdoors under a real power line." (The NRPB is also carrying out experiments under power lines and will publish the results.) The Bristol University research hinges on new evidence which Henshaw says may help to explain the link between exposure to electromagnetic fields associated with electrical wiring and overhead power lines and the incidence of childhood leukaemia and brain cancer. It has also been found that there is a higher incidence of post-natal depression among women living near power lines.

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He and his team have discovered that the mains leads to ordinary domestic electrical appliances, such as microwave ovens and hairdriers, are able to attract the radioactive products of radon present in air within rooms. However, Henshaw does not believe this effect results in health problems because it occurs only very close to the leads. What concerns him much more are results from another piece of research - the evidence that the same harmful concentrations of radon products may be present around overhead power lines. The electromagnetic fields around power lines have the ability to concentrate radon, and may also concentrate other harmful substances into a potent cocktail of carcinogens.

Radon is an odourless, invisible radioactive gas which is all around us. It occurs naturally, is present in all houses in Britain and has been linked to cancer. It decays to produce other elements known as radon decay products which are themselves radioactive, potentially cancer-causing agents.

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The team led by Henshaw at Bristol's physics department used alpha-particle radiation detectors to measure radioactive radon decay products near mains power cables, domestic appliance leads and other commonly found electrical equipment. They found that the wires and cables attached to electrical equipment and plugged into a wall socket attracted radon decay products in the same way as a magnet attracts iron filings.

The radon decay products attach themselves to tiny particles called aerosols - small drops of water and gas - that float in the air we breathe. The aerosols are polarised and attracted by electricity even when they themselves are electrically neutral. If the aerosols are electrically charged, they oscillate at the mains frequency which increases the chance of their sticking to surfaces such as skin and also increases the likelihood of their being inhaled, according to the researchers.

For Henshaw this provides the answer to the medical puzzle of how electromagnetic fields lead to increased risk of cancer. "We've broken the previous conceptual barrier that there's no convincing mechanism by which electromagnetic fields can actually affect cancer," he says. "The problem has been that electromagnetic fields are not strong enough to disrupt the DNA, so the dilemma has been that there is an epidemiological link but no obvious biology. What we've shown is that the electromagnetic fields can concentrate around them a known carcinogen, namely alpha particles and other aerosols in the air. There is a plausible route. We rest our case at that."

While the NRPB does not deny there is some epidemiological evidence for an association between exposure to electromagnetic fields and cancer, it argues the studies carried out to date do not establish that it is a cause. When does an association cease and a cause begin? When a mechanism can be found connecting harmful substances with cancer, according to Jon Miles, a physicist with the National Radiological Protection Board.

But hasn't Henshaw found that mechanism? No, says Miles, who has been exchanging letters with the physics professor. "We're not contesting his experimental results. What we're saying is that the effects of the electromagnetic fields on the radon decay products is to reduce exposure of people to them." That is because of a well-known concept called plateout, whereby the radon particles deposit themselves on surfaces such as floors, walls and clothing. Therefore, says Mr Miles, fewer particles will remain in the air to be breathed in.

In reply, Henshaw says the NRPB has got it wrong. Its physics is totally incorrect, he adds. It has not understood what he and his team measured. Plateout is not the issue anyway. There is a drift, a migration of radon decay products towards the electric field source. Theoretical physics in the aerosol field actually predicts these effects, says Henshaw. "All my physics colleagues understand this."

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Which raises the question of why there is this misunderstanding. Henshaw points out the board is a regulatory organisation. The board has no specialists in this area, "yet it thinks it can shoot from the hip without any supporting numbers, calculations, arguments, anything", he says.

In its response to the research, the board says Henshaw offers no credible explanation why electric fields cause radiation doses to be increased. Henshaw says it is very simple. There is higher activity in the air around the source. If there is higher activity, you are going to breathe it in. It is as simple as that, he says. Once in your mouth, in your saliva, radon passes all the way round the body, including to the foetus, says Henshaw.

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And so the argument rages. Scientists who want to pursue the various strands in all their minutiae should read next month's International Journal of Radiation Biology, which contains an NRPB response and Henshaw's reply. Meanwhile the public is all at sea. Who are they to believe? The board points to an inquiry being chaired by Sir Richard Doll, the veteran cancer researcher, who is examining causes of childhood leukaemia. But not everyone in the field is satisfied that his inquiry will be comprehensive enough or report quickly enough.

Other countries, including Sweden, Denmark and the United States, appear less willing than the United Kingdom to accept the risks associated with overhead power lines. The Swedish government, for example, has acted to stop the building of homes very close to power lines. Its policy is one of "cautious avoidance". In Britain homes continue to be built under power lines and the National Radiological Protection Board continues to argue that no biological mechanism has been found to show how electromagnetic fields can cause cancer.

OUR ELECTROMAGNETIC KILLING FIELDS?

How much electromagnetic exposureis safe? This question is beingexamined in Bristol by Alan Preece, a medical physicist at the Bristol Oncology Centre and by Jean Golding at the Institute of Child Health. Funded by the US Department of Energy, the research is part of a much bigger study of morethan 14,000 families in Avon, known as "Children of the 1990s", that hopesto discover why some childrengrow up healthy and reach their full development potential , and others do not, by tracking mothers and children born between April 1991 andDecember 1992.

So far, however, the electromagnetic exposure research has covered only50 homes selected at random. It involved taking spot measurements in every room, leaving electromagneticfield meters in the bedroom, kitchenand living room and persuading the mother to wear a meter and to keep a diary of her activities. It appears that a significant proportion of exposure from electromagnetic fields comes from pavements, supermarkets, shops and theatres. Checkout machinery in shops gives off high exposure, for example,but electricity substations do not.

The research also investigatedhow you measure exposure. Initially people were asked how often theyused the cooker, a hairdrier, how they heated their homes, and whether theyused an electric blanket. "We have shown this isn't a very good way of measuring exposure," says Preece."Too many variables come in."The only reliable method is to put an electromagnetic field meter into the home. Appliances which expose people the most to electromagnetism are cookers, microwave ovens, dishwashers,bedside clocks and central heating pumps, the researchers think.

They are seeking funding forfurther research to see why peoplecollect more exposure outdoorsthan indoors. In particular they want to examine the effects of living close to power lines. None of the families inthe study of 50 families lived close to power lines. But several hundred ofthe more than 14,000 families in the "Children of the 1990s" study do.When the research began, theintention was that it would last until the children were seven. At the momentthey are at the pre-school stage.But now there is pressure, particularly from educationists, for the research, which includes screening the children for genes linked to learning difficulties, to continue.

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