Towards a new radon hazard map of Norway using Trøndelag county as a test area

Indoor radon constitutes a serious health concern to the population of Norway and this study discusses the need for a new and improved radon hazard map of Norway. To this end, we present a procedure for producing a radon hazard map, using Trøndelag county as a test area, based on equivalent-uranium (eU) from 12 airborne gamma-ray spectrometer surveys and 486 whole-rock chemical analyses of bedrock samples covering large tracts of the county. The current radon hazard map for Norway is based on extrapolation of indoor radon measurements using existing bedrock maps. A previously published relationship between eU concentration from airborne surveys and indoor radon measurements in the Oslo Fjord region forms the basis of the new map. Ground natural gamma-radiation measurements along main roads, supplementary XRF analyses from anomalous sites and indoor radon measurements have been used to validate the new radon hazard map. In many areas, the new radon hazard map for Trøndelag county deviates significantly from the current radon hazard map. For example, the current map shows ‘Moderate to low risk’ in areas with anomalous high eU values. The new Coop-project radiometric compilation also reveals other eU anomalies along the coast of southern and western Norway that are not reflected in the current radon awareness map. We conclude that there is a need to produce a new radon hazard map for all of Norway based on data with a dense regional coverage rather than sparse, clustered indoor measurements.