Last month's article in the British Medical Journal on the dangers posed by radon gas in buildings jogged my awareness of this important hazard (Gray, Read et al. 2009).  Quoting Wikipedia on radon "Radon is the invisible, radioactive mono-atomic gas that results from radioactive decay of some forms of uranium that may be found in rock formations beneath buildings or in certain building materials themselves. There are relatively simple tests for radon gas, but these tests are not commonly done, even in areas of known systematic hazards." and "According to the United States Environmental Protection Agency, radon is reportedly the second most frequent cause of lung cancer, after cigarette smoking; and radon-induced lung cancer the 6th leading cause of cancer death overall." and "Some ... areas, including Cornwall and Aberdeenshire in the United Kingdom have high enough natural radiation levels that nuclear licensed sites cannot be built there - the sites would already exceed legal radiation limits before they opened, and the natural topsoil and rock would all have to be disposed of as low-level nuclear waste."