Radon was discovered around 1899 to 1900 through the work of Ernest Rutherford, Robert Owens and Friedrich Ernst Dorn, who traced a radioactive gas emanating from radium and thorium; William Ramsay and Robert Whytlaw-Gray isolated and characterised it in 1910. Its name derives from radium, from which it forms by decay. Radon is a colourless, odourless, radioactive noble gas, the heaviest of that family, and the principal natural source of radiation dose to the general public.
Seeping from uranium in rocks and soil, radon can build up in basements and mines, where it is a leading cause of lung cancer after smoking, so that testing and ventilation against the invisible gas are routine safety measures; it was once sealed in tiny tubes for radiotherapy. Despite radon's well-documented cancer risk, a number of European spas, including Bad Gastein in Austria and Jachymov in the Czech Republic, continue to offer radon inhalation and bathing treatments marketed for arthritis and other chronic pain, a practice with roots in early twentieth-century radioactivity enthusiasm and one that mainstream oncology and radiation-safety bodies regard as unsupported by solid evidence and medically unwise, an echo of the same era's radium quackery persisting into contemporary wellness tourism.