Thousands of households are living unaware with a cancer-causing gas, experts have warned.
Tests have found a home in Castleisland, north Kerry, with one of the highest amounts of naturally-occurring radon ever found in Europe only a few kilometres from the record level detected in 2003.
The Radiological Protection Institute of Ireland (RPII) said that out of 5,000 homes tested in the nine months to June, more than 800 showed high levels.
Radon is the second biggest cause of lung cancer after smoking and is directly linked to about 200 deaths from the disease in Ireland each year.
Dr Ann McGarry, RPII chief executive, said tests show Ireland has a significant problem with the colourless, odourless gas in homes.
"We predict that there are thousands more homes across the country with high levels of radon gas," she said.
"To date, only a very small proportion of these homes have been identified. Exposure to high radon levels causes lung cancer and many people are unknowingly living with very high levels in their homes.
"The only way people will know if it is in their homes is by testing."
Radon is created when uranium in rocks and soil breaks down and it is regarded as a class one carcinogen.
Experts estimate that in Ireland a 70-year lifetime exposure to radon at the acceptable level - 200 becquerels per cubic metre (Bq/m3) - carries a one in 50 chance of contracting fatal lung cancer. That is twice the risk of dying in a road accident.
The RPII said smokers, or reformed smokers, face up to 25 times greater risk from radon than non-smokers.
The highest level of gas found in a home in Ireland was discovered in Castleisland and had an average concentration of 37,000 becquerels per cubic metre (Bq/m3) - 185 times above the acceptable level.
It is only a few kilometres from the highest ever reading of 49,000 Bq/m3 in 2003.
Eighteen homes were identified above 2,000 Bq/m3, 10 times above the acceptable level - three in Sligo and Carlow, two in Tipperary, Galway, Cork, Wexford and Kerry and one in Donegal and Waterford.
Another 93 homes had levels between four and 10 times the acceptable level - 18 in Waterford, 14 in Clare and Mayo, 13 in Galway, 11 in Sligo, five in Cork and Kilkenny, four in Wicklow, three in Kerry, two in Carlow and one in Dublin, Roscommon, Tipperary and Wexford. More than 700 homes had levels up to four times acceptable levels.
Radon is tested in a home over a three-month period.
An interactive map is available on the RPII's website - rpii.ie - for householders to see whether their home or workplace is in a high radon area.