The joint PHE-GSNI-BGS digital Radon Potential Dataset for Northern Ireland provides the current definitive map of radon Affected Areas in Northern Ireland. The Radon Potential map for Northern Ireland shows the estimated percentage of homes in an area exceeding the radon Action Level. This is the basic information to assigning the level of protection required for new buildings and extensions, as described in the Building Research Establishment guidance BR-413 Radon: Guidance on protective measures for new dwellings in Northern Ireland (2004). The Radon Potential map for Northern Ireland is based on PHE indoor radon measurements and 1:10 000 or 1: 250 000 scale digital geology information provided by the Geological Survey of Northern Ireland (GSNI). The indoor radon data is used with the agreement of the Northern Ireland Environment Agency and PHE. Confidentiality of measurement locations is maintained through data management practices. Access to the data is restricted. Radon is a natural radioactive gas, which enters buildings from the ground. Exposure to high concentrations increases the risk of lung cancer. Public Health England (PHE) recommends that radon levels should be reduced in homes where the annual average exceed 200 becquerels per cubic metre (200 Bq m-3), the Action Level. PHE defines radon Affected Areas as those with 1% chance or more of a house having a radon concentration at or above the Action Level. Further information on radon can be obtained from www.ukradon.org