Radon is a naturally-occurring radioactive gas that seeps from rocks and soil beneath our feet. In the open it blows away harmlessly, but the gas can become trapped in pockets under buildings. It accumulates there, and enters buildings through small cracks in foundations or sub-floors.
The cancer risk posed by radon arises when the gas decays into radioactive particles. These are suspended in the air, and if breathed can become lodged in lung tissue.
There the particles deliver a radiation dose that increases the risk of lung cancer.
The higher the radon level the greater the chance that its breakdown particles will be breathed into the lungs.
Long-term exposure to radon causes an estimated 150 to 200 lung cancer cases in Ireland every year. Smokers face even greater risks.
Some areas, including parts of Kilkenny, Louth and Galway, have high radon levels.
About a third of Ireland is classified as a high radon area by the RPII, which recommends that homeowners in all high radon areas test their homes for radon.
All new homes built after July 1998 should include measures to reduce radon levels.
Radon can also be reduced in homes built before that date by fitting radon-reducing systems, although no measures should be taken until a radon test shows that remedial action is necessary.
Information including maps that show Ireland's radon levels, how to organise a radon test and what action homeowners should take after testing is available from the institute's website www.rpii.ie