High levels of radon gas are thought to be present in 91,000 houses in the Republic, the Radiological Protection Institute of Ireland warned yesterday.
The institute has advised householders, particularly those in high-risk areas, to have their homes tested because of the link between radon and lung cancer.
The high levels - levels which exceed the Government's "reference level" for radon in houses - were estimated in The Radon in Dwellings Report, a comprehensive survey commissioned by the institute.
Radon is classified as a class C carcinogen, a feature which suggests a direct link between radon and the induction of lung cancer.
The report found particularly high levels of the gas in the south-east and the west. Counties in the south- east included Carlow, Kilkenny, Waterford, Wexford and Wicklow. In the west they included Clare, Galway, Mayo and Sligo.
The report stresses, however, that high levels of radon can be found in houses in any part of the country and that the vast majority of homes with excessive radon have yet to be detected.
