As many as 460,000 people in Ireland may be exposed to radon levels that are deemed to be unsafe, new research has found.

Radon is a naturally occurring radioactive gas present in all rocks and soils. When it surfaces in the open air, it is quickly diluted to harmless concentrations. However when it enters an enclosed space, such as a house, it can sometimes build up to high concentrations, leading to an potentially dangerous health risk.

Globally, radon is the second highest cause of lung cancer, coming after smoking. The gas is linked to around 250 cancer deaths in Ireland every year.

A research team led by geologists from Trinity College Dublin (TCD) has produced a new ‘risk map' using indoor radon concentration measurements and relevant geological information.

They found that including more geological data, such as bedrock and glacial geology, provided a more detailed picture of the risks posed by radon.

According to this map, around 10% of Ireland's population is exposed to radon levels that exceed the references safe level - that is around 460,000 people who may currently be at risk.

This new analysis 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% in high risk areas (around 265,000 people), 8% in medium risk areas (160,000) and 3% in low risk areas (35,000).

This map now needs to be validated using new annually available indoor radon data.

"EU member states need to translate European radiation protection legislation into national law, and this requires an accurate definition of radon-prone areas. Our research provides one example of how national-scale radon risk maps can be produced, which is especially relevant to countries developing their national radon programmes," explained Quentin Crowley, assistant professor in isotopes and the environment at TCD's School of Natural Sciences.

The researchers emphasised that according to the map, even some homes in the low risk category ‘will have elevated radon levels'.

"No model, no matter how sophisticated, can substitute for having indoor radon levels tested. For this reason we advise all householders to test their homes for radon and, if high levels are found, to have their houses fixed. Further information is available on radon.ie," commented Barbara Rafferty of the Environmental Protection Agency.

Details of this research have been published in the journal, Science of the Total Environment.