Domestic exposure to radon gas is responsible for a significant number of lung cancer deaths, research has found.
Radon gas is widespread
The risk appears to be much higher for smokers.
The researchers conclude radon in the home causes approximately 20,000 lung cancer deaths in the European Union each year - about 1,000 in the UK.
The British Medical Journal study, funded by Cancer Research UK and the European Union, is the largest ever of its type.
Radon is a naturally occurring, colourless, odourless, radioactive gas found at varying levels in all houses in the UK and across Europe.
This research combines information from 13 smaller studies across Europe, which involved 7,000 people who had developed lung cancer and 14,000 without the disease.
It found that radon exposure can cause lung cancer in lifelong non-smokers, but the risk is low.
However, for any given level of exposure to radon, smokers have about 25 times the risk of developing lung cancer than those who do not smoke.
Previous studies of radon in homes have not been large enough to assess the risks reliably.
Nor have they been able to examine risks separately in smokers and non-smokers.
Professor Sarah Darby, of the University of Oxford, who led the collaboration, said: "By putting together many different studies we have shown that radon in ordinary homes is causing about 9% of lung cancer deaths each year in Europe, which is 2% of all cancer deaths.
"In the UK, where radon levels are lower than in many European countries, radon in ordinary homes causes about 1,000 deaths each year, which is about 1% of all cancer deaths."
Professor Sir Richard Peto, also at Oxford University, said that on average in Europe the absolute risks of getting lung cancer by age 75 years at usual radon concentrations of 0, 100, and 400 becquerels per cubic metre (Bq/m 3 ) would be about 0.4%, 0.5%, and 0.7%, respectively, for lifelong non-smokers.
However, for cigarette smokers the risks are about 25 times greater - 10%, 12% and 16%.
Radon is formed from the natural disintegration of uranium, which is present in ordinary surface rocks and in soil.
Radon that diffuses into the atmosphere usually disperses rapidly but it can accumulate indoors, especially in small buildings such as houses.
As radon decays it creates particles that can damage the cells lining the airways of the lung.
This damage can lead to cancer and, as the lungs of smokers may have many cells that are already somewhat damaged, the extra risk from radon is much greater for them than it is for non-smokers.
The UK's National Radiological Protection Board (NRPB) has estimated around 100,000 properties in Britain are significantly affected by radon.
Earlier this year the NRPB reported concentrations of radon up to 85 times higher than recognised safety limits in two homes in Cornwall.
High radon levels in existing houses can usually be reduced by changes to the ventilation system, such as improving underfloor air bricks and extracting radon from beneath the building with a fan, although this can cost up to £1,000 to install and £50 per year to run.
When constructing new buildings, however, low concentrations can usually be achieved by enhancing the damp-proof membrane across the full footprint of the building, at an extra cost of only around £100.
The researchers found lung cancer risk to be raised by 16% for every 100 Bq/m 3 of radon present in the home.
Professor Darby said: "We also found that there is a detectable risk even in homes with levels below 200 Bq/m 3, which is the currently recommended 'action level' in the UK.
"Indeed we estimate that about ninety per cent of radon-induced lung cancers occurred in homes with levels of radon below 200 Bq/m 3."