Experts are urging businesses and households to get their premises tested for radon gas.
The warning from the Irish Radiological Protection Institute follows the discovery of hugely elevated concentrations of the deadly gas at an office building in Co Cork.
The concentration of radon in the offices of the Corkman newspaper in Mallow was found to be 60 times the acceptable level, the highest ever detected in Ireland.
Radon, which can cause lung cancer, naturally seeps up from the ground in many parts of Ireland and frequently gets trapped in buildings.
It is a colourless, odourless, tasteless gas which occurs naturally from the decay of radioactive uranium in rocks and soils.
The RPII says 100,000 homes and workplaces in Ireland may have unsafe levels of the gas.
The institute said levels found would be equivalent to receiving 39 chest X-rays every day, or nearly 10,000 in a year.
RPII's Chief Executive, Dr Ann McGarry, said over 100,000 homes and offices are estimated to be at risk and while the Corkman took the appropriate action, many others were being irresponsible for not testing for radon