With the latest scientific data attributing 14 percent of all lung cancer cases to radon (a naturally occurring radioactive gas), the World Health Organization has slashed its exposure safety limits to a tenth of its current level.
"In view of the latest scientific data, WHO proposes a reference level of 100 becquerels per metric cube to minimize health hazards due to indoor radon exposure," said the UN health agency in a report published this week.
"However, if this level cannot be reached under the prevailing country-specific conditions, the chosen reference level should not exceed 300 becquerels per metric cube," it added.
A previous WHO report published in 1996 had fixed the reference level at 1,000 becquerels per cubic metre.
After smoking, radon is the second biggest cause of lung cancer, killing tens of thousands of people a year, said the WHO.
"Cancers attributable to radon range from 3 to 14 percent, depending on the average radon concentration in the country concerned and the calculation methods," said the WHO.
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.
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.
The WHO also recommended that rooms be aired frequently, especially in winter when higher levels of radon are often detected as windows are kept shut to block out the cold.