In Germany, radon in residential buildings has been shown to be a major risk factor for lung cancer. It's clear that this is a worldwide health concern.
A research paper in the current edition of Deutsches Ärzteblatt International by Klaus Schmid of the University of Erlangen-Nuremberg and his coauthors discusses the 1900 deaths from lung cancer that they see every year in Germany, purely due to radon within residential buildings.
The authors base their assessment on the results of relevant German studies which include the recently published guidelines of the German Society for Occupational and Environmental Medicine as well as a current publication from the German Commission on Radiological Protection. These indicate that radon within residential buildings makes a major contribution to the radiological exposure of the general population. Schmid and his colleagues refer to measurements taken from residential areas -- those found mildly unhealthy levels of radon in 36% of homes and severely unhealthy levels in more than another 18%.
Exposure within houses is predominantly due to release of radon-containing subsurface air from the soil into the building. Radon can penetrate into houses through leaks in the foundation or in the walls that are in contact with the soil.
Occupational physicians have long known that radon can cause lung cancer, particularly in uranium miners. For individuals without occupational exposure, radon is regarded as the second most important cause of lung cancer after smoking.