Radon is a radioactive gas that emanates from rocks and soils and tends to concentrate in enclosed spaces, such as underground mines or houses. Soil gas infiltration is recognized as the most important source of residential radon. Other sources, including building materials and water extracted from wells, are less important in most circumstances. Radon is a major contributor to the ionizing radiation dose received by the general population.
Studies on indoor radon and lung cancer in Europe, North America, and Asia provide strong evidence that radon causes a substantial number of lung cancer cases in the general population. Estimates of the proportion of lung cancers attributable to radon range from 3% to 14%, depending on the average radon concentration in the country concerned and the calculation methods. The analyses indicate that the lung cancer risk increases proportionally with increased radon exposure. As many people are exposed to low and moderate radon concentrations, the majority of lung cancers related to radon are caused by these exposure levels rather than by higher concentrations. Radon is the second-leading cause of lung cancer after smoking. Most of the radon-induced lung cancer cases occur among smokers due to a strong combined effect of smoking and radon.
A national reference level for radon represents the maximum accepted radon concentration in a residential dwelling and is an important component of a national program. For homes with radon concentrations above these levels, remedial actions may be recommended or required. When setting a reference level, various national factors such as the distribution of radon, the number of existing homes with high radon concentrations, the arithmetic mean indoor radon level, and the prevalence of smoking should be taken into consideration.