A house in Lifford has been found to have the highest levels of radon gas in County Donegal.
The Radiological Protection Institute of Ireland (RPII) said the levels are 17 times above what is considered acceptable.
The radiation dose is equivalent to receiving about 12 chest X-rays per day, or 4,250 per year.
The RPII said it "is concerned" that other homes in the county may also have high levels of radon gas.
Mr David Fenton, senior scientist with the RPII, said people living in homes with high levels of radon gas "are at increased risk of developing lung cancer."
"Radon is the second biggest cause of lung cancer after smoking and is linked to up to 200 lung cancer deaths each year in Ireland," he said.
"The home located near Lifford had an average radon level of 3,400 becquerels of radon per cubic metre (Bq/m3).
"That is 17 times the acceptable level of 200 Bq/m3 and the highest level ever found in any house in the county.
"Radon has no smell, taste or colour.
"Because of this people cannot tell if they are living with high radon levels in their home unless they take the radon test. "
The RPII has measured 1,270 homes throughout Donegal with 60 recording levels in excess of the acceptable level.
The highest level found previously was three times the acceptable level.