People living in parts of Co Louth are among those exposed to amounts of radon that exceeds safe limits.
Geologists from Trinity College Dublin have created a map based on new analysis which divides the country into three risk categories - high, medium and low. This is based on the probability of having an indoor radon concentration level above the reference level of 200 becquerels per cubic metre.
The map shows that the probability of living in a home with a concentration above this is calculated to be 19 per cent in high risk areas (around 265,000 people), 8 per cent in medium risk areas (160,000) and 3 per cent in low risk areas (35,000).
High radon areas are classed as those where more than twenty per cent of the homes are estimated to be above the reference level for safe exposure.
Homes, schools and businesses in a large part of the Cooley peninsula fall into this high risk category.
Exposure to radon, a naturally occurring radioactive gas, increases the likelihood of developing lung cancer, especially among smokers, and causes around 250 deaths a year here - more than road casualties.
Householders concerned about radon levels in their homes can apply for a kit to test the radon levels and get information on what steps to take to reduce indoor levels from the Environmental Protection Agency's dedicated website at www.radon.ie