WHO is setting up a global network of scientists and other experts who will research risks associated with radon and the cost-effectiveness of possible measures to prevent it seeping into houses.

Most exposure occurs in houses, where concentrations of the gas -- the world's second leading cause of lung cancer after smoking --depend on the amount of radon-producing uranium in underlying rocks and how easily it can get in, the World Health Organization said.

"Radon accumulates within the house, so people are breathing this radioactive gas, which is tasteless, colorless," said Dr. Mike Repacholi, WHO's radiation and environmental health chief.

Greatest Exposure Occurs in the Home

WHO recommends improving underfloor ventilation and sealing cracks in floors in existing homes in radon-rich areas -- including many parts of northern and eastern Europe.

Houses in particularly affected areas can be fitted with new basement ventilation systems, although this is more expensive.

Radon gas is also present outdoors, but levels are usually very low because of dilution in the air.

High concentrations can be found in caves, mines and water treatment centers, but by far the greatest exposure for people occurs in the home, WHO said.

High levels can also be found in some drinking water sources.

Radon is also more likely to gather in houses where walls and roofs have been insulated against cold weather, as this cuts down airflow.

Risks Associated with Radon

The risk of contracting lung cancer is significantly greater for smokers who live in houses where radon accumulates, Repacholi said.

The health agency said it was setting up a global network of scientists and other experts who will research risks associated with radon and the cost-effectiveness of possible measures to prevent it seeping into houses.

The results of their work will provide guidelines to help national authorities increase public awareness of radon's potential dangers.

People who use radon spas -- which are popular in central and eastern Europe as well as Japan -- may also be exposing themselves to increased risk of lung cancer, Repacholi added.