The risk to health of radioactive radon gas in people's homes may have been overestimated, say researchers in the US.

Radon is an invisible, odourless gas given off by the decay of underground rocks. It seeps into buildings and if not dispersed can cause lung cancer.

It is a particular problem in parts of Britain abundant in granite, including much of the west of England, Northamptonshire, Derbyshire and Aberdeenshire.

Radon is thought to be responsible for about 2,500 deaths annually from lung cancer, with smokers at particular risk.

DNA damage

In the US, radon is blamed for the deaths of about 15,000 lung cancer sufferers a year.

But a study, reported in the Proceedings of the National Academy of Sciences (PNAS), suggests these figures may be overestimates.

Radon and its decay products emit a type of radiation called alpha particles, which can cause malignant changes in cells lining the airways by damaging their DNA.

Advice is available on reducing the risk from radon

Until now, estimates of the risks of domestic radon exposure have been made by extrapolating from epidemiological data on uranium miners.

The miners are exposed to much higher radon levels than are present in most homes.

And their airway cells may have been traversed, or "hit", by several alpha particles during a short period.

But the study says the cells of people exposed at normal domestic levels are unlikely to be hit by more than one alpha particle in a lifetime.

Now researchers at Columbia University have developed a technique to assess the cancer-causing effects of single alpha particles, by using a "microbeam" machine that delivers a predefined, exact number of particles to individual cells.

They say a single alpha-particle hit on a cell may not significantly raise the likelihood that cancerous changes will take place in that cell.

And they say that almost all of the risk comes from cells that receive multiple alpha-particle hits.

Smoking invariably worsens the risk

The US study is in marked contrast with a study conducted last year in the west of England.

It was funded by the Imperial Cancer Research Fund, the National Radiological Protection Board, several government departments and the European Commission.

The researchers were concerned that the estimates of radon damage in the population at large were derived indirectly from the experience of miners.

So they sought direct evidence from 982 lung cancer patients and compared the radon concentrations in their homes with those of 3,185 controls.

One of the investigators said: "This study provides, for the first time, direct evidence on the risks associated with residential radon in the UK.