[en] Elevated indoor radon concentrations are a more extensive problem in Norway than in many other countries. It has been estimated that indoor radon causes approximately 300 deaths from lung cancer each year in Norway. On average, avoiding lung cancer increases life expectancy by 14 to 18 years. Radon is a radioactive noble gas formed continually is a decay product from uranium. Uranium is a natural constituent existing in varying concentrations in bedrock, minerals and soils. For this reason, both the soil air and groundwater contain radon. Radon in buildings normally originates from the soil air in the underlying ground. Indoor air pressure is often low, so that radon-containing air from the surrounding ground gets sucked in through cracks in the building foundations. Elevated indoor radon concentrations can be due to household water drawn from groundwater wells, and radon gas can also be emitted from building materials such as certain types of stone or concrete containing high levels of natural radioactivity. Norway, Sweden and Finland are among the the countries in the world with the highest average indoor radon concentrations. Geological conditions and the cool climate pose a big challenge, but the radon problem can be solved in a cost-effective way. Radon is the most common cause of lung cancer after active smoking. At a radon concentration of 100 Bq/m3, which is not far from the estimated average for Norwegian housing, the risks of dying of radon-induced lung cancer before the age of 75 are 0.1 % for non-smokers and 2 % for smokers, respectively. Many buildings in Norway have radon levels that exceed this. The most important health impact of radon exposure is the increased risk of lung cancer. This increase in risk is assumed to be linear in relation to radon concentration (i.e., the risk is 10 times higher at 1000 Bq/m3 compared to 100 Bq/m3). The risk also increases linearly with exposure time, i.e. there is a tenfold greater risk of contracting lung cancer caused by radon if you have lived in a house for 30 years, compared to if you lived in the same house (with the same radon levels) for 3 years. The above relationships are especially important when considering which mitigation measures should be implemented. All reduction of radon concentration and exposure time will have a favourable impact on health. Taking the population as a whole, the health benefit will be appreciable, even if the exposure reduction that is achieved from the measures is modest - so long as the reduction applies to a large number of buildings. The risk of radon induced lung cancer increases with exposure and is proportional to the indoor radon concentration and the exposure time. Moreover, there is no lowest threshold radon concentration where no risk occurs. The risk is greatest for smokers and ex-smokers, though never-smokers can also develop lung cancer as a result of radon exposure. In recent years, three important large-scale international pooled analyses of available reliable data have confirmed earlier risk estimates. (Author)