Several thousand are thought to live in homes affected by naturally occurring radon, the odourless, radioactive gas that seeps up from the ground. The first direct evidence of its effects, published today, shows that it increases the risk of lung cancer by up to half in people exposed to high levels over many years.
The findings, by a team of scientists at Oxford University including Sir Richard Doll, the cancer epidemiologist, will increase pressure on ministers to tighten building regulations to ensure that all new houses built in affected areas have sealed floors to keep out the gas.
One radiation expert said: "It is a small risk but it is definitely there. When you see young children getting a higher dose than workers at Sellafield [the nuclear reprocessing plant] it is clear something should be done."
Around 50,000 people are estimated to live in houses with concentrations of radon gas above 200 becquerels per cubic metre. This is 10 times the average for the whole country.
The worst affected areas are Devon and Cornwall but highlevels are also found in homes in Derbyshire, Northamptonshire, Somerset and in parts of Wales and Scotland.
The researchers, from the Imperial Cancer Research Fund's epidemiology unit at Oxford, say one in 20 cases of lung cancer is attributable to radon.
Previous warnings about the risks have been based on evidence from miners who were exposed to very high levels but there was less certainty about the effect of lower levels present in people's homes. The current study, published in the British Journal of Cancer, involved measuring the level of radon in homes occupied over the previous 35 years by people diagnosed with lung cancer.
The results showed that a level of 200 becquerels was associated with a 20 per cent increased risk and a level of 400 becquerels with a 40 per cent increased risk. The risk continued to rise with rising concentrations. Among adults who smoke, the risk was multiplied.
Sir Richard said radon contributed to 1,800 deaths a year from lung cancer compared with 33,000 caused by smoking. However, if no one smoked there would be 4,000 deaths from lung cancer, of which 200 would be caused by radon.
He said: "Most radon-induced cancers are produced in conjunction with cigarette smoking. At all levels of radon found in UK homes, cigarette smoking remains the major cause."
Dr Michael Clark, of the National Radiological Protection Board, said that people living in affected homes could reduce the risk by digging a sump under the house and pumping out the gas to disperse in the air rather than seeping into the home. New houses should have sealed floors to keep out the gas.
"Our advice is that people living in homes with a level over 200 becquerels should do something about it. Grants are available to do the building work in affected areas and people who fear their house is affected can get a meter to measure it from us."
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