The World Health Organisation on Tuesday publicly launched a global campaign to minimise the risks from radioactive radon gas, a natural occurrence believed to be the second most important cause of lung cancer after smoking.
The odourless and colourless gas emanates from the soil or bedrock in some areas where there are relatively high concentrations of decaying natural uranium, and concentrations can build up inside homes.
The UN health agency estimated that radon gas causes six to 15 percent of all cases of lung cancer, leading to tens of thousands of deaths annually.
"Most of the radon induced lung cancer cases occur among smokers," it said in a statement, underlining that smokers were 25 times more at risk of being harmed by the gas.
"Radon poses an easily reducible health risk to populations all over the world, but has not up to now received widespread attention," said Mike Repacholi, coordinator of the WHO's radiation and environmental health unit.
The agency plans to produce guidelines based on research over the next three years by scientists and regulators.
"Radon is all around us. Radon in our homes is the main source of exposure to ionising radiation, and accounts for 50 percent of the public's exposure to naturally occurring sources of radiation in many countries," he added.
Outdoor concentrations of the gas are very low but it can build up in some mines or caves, or indoors notably in basements, entering houses through cracks in floors, porous building materials or drains.
Indoor radon pollution can be reduced through careful design and construction of new homes, according to health experts.
Many European countries including Britain, France, Ireland, Switzerland, Sweden and Italy keep track of areas known to have relatively high concentrations of radon.
Twenty countries are taking part in the WHO's International Radon Project.
