145 Testing the performance of a recent radon-hazard evaluation in the municipality of Gran, eastern Norway Mark A. Smethurst 1 , Aud Venke Sundal 2 , Terje Strand 3 and Bernard Bingen 1 1 Geological Survey of Norway (NGU), 7491 Trondheim, Norway. 2 Institute of Geography, University of Edinburgh, Drummond Street, Edinburgh EH8 9XP, UK. 3 Department of Physics, University of Oslo, P.O. Box 1048 Blindern, 0316 Oslo, Norway. E-mail:mark.smethurst@ngu.no The Geological Survey of Norway and Norwegian Radiation Protection Authority have produced a new overview of radon hazard in the most densely populated part of Norway around the capital city, Oslo, and Oslojord. We look closely at the performance of the radon-hazard evaluation within a rectangular area 520 km 2 in size centred on the municipality of Gran. The uranium- rich alum shale and supericial deposits associated with it are the principal sources of radon in the area. Small, isolated granite bodies and local, high-permeability glacioluvial deposits are likely to contribute to elevated radon-hazard levels. The multidisciplinary hazard evaluation is based on direct measurements of radon in indoor air, bedrock geology, drift geology, and radon- daughter (bismuth) mapping using helicopter-borne instruments. Our testing of the hazard evaluation shows that combining signs of radon hazard from each of the data sets in the Gran region produces a liberal hazard map that encloses most of the known areas of severe radon contamination in dwellings, and reveals additional uninhabited areas where similar levels of contamination can be expected if those areas are taken into use for residential purposes without mitigating action. The hazard evaluation in the Gran area is 80.7% eicient in enclosing high indoor radon measurements in the high-hazard zone when this zone occupies 50% of the total geographic area. The probability of this distribution happening by chance is 0.054%. If the hazard prediction bore no relation to the actual distribution of high indoor radon measurements, the eiciency would be around 50%. The radon-hazard evaluation works well over the alum shale and should be used to improve the eiciency of indoor radon mapping programmes and develop strategies for the implementation of radon mitigation measures. Smethurst, M.A., Sundal, A.V., Strand, T. and Bingen, B. (2008) Testing the performance of a recent radon-hazard evaluation in the municipality of Gran, eastern Norway. In Slagstad, T. (ed.) Geology for Society, Geological Survey of Norway Special Publication, 11, pp. 145–154.