IT has been branded the silent killer. Odourless, colourless, it seeps into homes leaving no tell-tale signs. And now a new study has blamed it for one in every 20 deaths from lung cancer.

The substance just described is the radioactive gas radon, which occurs naturally in most rocks and soil in this country.

In most cases the levels are totally harmless, but in some areas it can become trapped in porous rock under the ground, which is when it can cause a risk to humans.

A team of Oxford scientists has just discovered that breathing in concentrations of the gas in homes is responsible for 1,800 lung cancer deaths a year.

Although the findings were based on a study of lung cancer patients in Devon and Cornwall they are also likely to be significant for parts of north Oxfordshire, which is known to have levels of the gas above what is considered to be acceptable.

The message from Sir Richard Doll, an honorary consultant for the Imperial Cancer Research Fund, who is based at the Radcliffe Infirmary, Oxford, and was involved in the research, is not to panic.

He said: "In a non-smoker the chances of developing lung cancer are around five per thousand - and if radon is above the action level it increases the risk by 20 or 30 per cent. It's only a slightly increased risk." But he said that for smokers, who have a one in ten chance of developing lung cancer anyway, the presence of radon would clearly pose more of a risk.

He said: "If you have got a one in ten chance of developing lung cancer, another 20 per cent risk on top of that is going to be material.

"The real message is if you don't want to get lung cancer do not smoke. It's much more important for people to stop smoking than to have their houses treated for radon."

Overall the team of scientists discovered that the presence of radon caused a far smaller risk to the public than smoking, which accounts for 33,000 of the 37,000 lung cancer deaths in Britain each year.

A spokesman for the National Radiological Protection Board, which is based near Didcot, added: "In north Oxfordshire we've found a few hundred homes above action levels but we would not want to unduly panic people. "We tested 2,412 homes in Cherwell area and 385 were found to be above action levels. But we have to put it in perspective. It's a lifetime effect and people do not need to panic, even if their home has been proved to have above action levels.

"Smoking does multiply the risk and if they do smoke we would advise them to do something about both their smoking habit and the radon levels in their home."

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