PUBLIC fears that overhead power lines pose a cancer risk have been stirred up again following research published by scientists at Bristol University this week. The work suggests that the amount of radon - a naturally occurring radioactive gas - found in the home can be influenced by the presence of electric fields.

Electric and magnetic fields are created by all types of house wiring, by domestic appliances such as washing machines and hairdryers, and also by overhead electricity transmission lines.

For decades researchers have attempted to identify what dangers, if any, are posed by living in these fields. Most of the focus has been on whether electromagnetic fields, and more particularly magnetic fields, can induce changes or damage within cells that might ultimately lead to cancers.

Despite decades of scientific work and an international investment worth millions of pounds, no definitive answer to this question has been found. Those strongly opposed to new power lines can cite research which appears to demonstrate a risk or a possible cancer cause, but this is readily countered by electricity suppliers who can present a vast body of work rebutting any claims.

The Bristol research is noteworthy in that its authors have made a different kind of connection linking electric fields and cancer. They suggest that electric fields can cause accumulations of the radioactive particles which come from radon, its "daughter particles.

Radon is a gas which occurs naturally in the Earth's surface, coming from the breakdown A radium. It seeps out of the ground and can accumulate in open, still spaces under buildings and in rooms. A radiation hazard arises when radon in turn breaks down, giving off a radioactive emission as it forms daughter products.

Because radon is a gas it can be breathed in. If the radon breaks down while in the lungs it can cause tissue damage that can lead to lung cancers.

The Radiological Protection Institute of Ireland is particularly concerned about radon and is currently involved in a nationwide study of radon in Irish homes, according to Dr Ann McGarty, a principal scientific officer with the institute.

"We do find high radon levels in areas where we don't expect it. That is the reason we need to do the national survey. The principal risk is lung cancer," she explained. The survey will be finished in 1998.

There are about 1,500 lung cancer cases in the Republic each year and 15 to 20 per cent of these can be linked to radon, she said. A person has a one in 50 chance of contracting lung cancer by 70, assuming an exposure to, high levels of the gas over an entire lifetime.

Even though radon represents a genuine radiation hazard, one of the Bristol researchers, Prof Denis Henshaw, believes his work represents a "good news story" and not a reason to be fearful. Radon daughters as they form first connect themselves to minute water droplets and dust and then are transported towards the electric fields given off by household wiring and appliances.

He believes that, although a person might expect to encounter increased radon daughter levels wherever electric fields occur, over time these fields will mop up the particles like a magnet attracting iron filings, thereby cleaning the air. The work indicates that changes in radon levels indoors caused by electric fields will "be too small to be important", he said.

He maintains, however, that an unexpected radon risk occurs outdoors, directly below high tension electricity lines. The strong electric fields created by the lines will attract and hold particles. "I would suggest that outdoor exposure [to radon under power lines] is the greatest concern," he added, although he has yet to begin tests to prove his theory.

Asked to comment on the Bristol research, the RPII said: "We believe the suggestion that there could be such an effect is a new one, and at this stage we will study the newly announced findings."

Its British equivalent, the National Radiological Protection Board, countered the research, saying that it viewed radon and electromagnetic fields as separate health issues".

It added: "The authors of the paper go on to speculate that there may be some mechanism by which electric fields cause radiation doses from the inhalation of radon daughters to be increased, but offer no credible explanation why this should occur.

"The theory is implausible: the weight of evidence would suggest the presence of electric fields will, if anything, slightly reduce human exposure to radon daughters."

"I think a cautious approach is necessary," said one of Ireland's experts in the area of radon, Dr James McLaughlin of UCD. The earliest research on the movement of radon daughters under the influence of electric fields dates back to 1901, he said, and he has carried out extensive research on the use of electric fields to mop up radon particles. "You actually substantially reduce the level of radon in a room," he said.

It is too soon to deliver a verdict on the power line controversy, he warned. "The jury is still our and the best thing to do is not to put [overhead power] lines near houses."

Mr John Royds, a consultant specialising in electric and magnetic fields given off by power lines, argues against any assumption that power lines pose no health risk. "I am not saying you should turn the lights off," he said, but he points to international research which suggests there is a risk.

For its part, the ESB applies international safety standards when installing equipment and power lines. "They are based on the known biological effects of electromagnetic radiation," said Mr Brian Cronly, an ESB senior project manager specialising in electromagnetic fields. "We do surveys for new lines and we try to locate them as far as possible from inhabited areas.

There have been up to 12,000 similar studies on power line fields over the years, he said, and this is yet another that awaits further analysis before it can be taken as correct.