Radon is a dense gas, occurring in small amounts in spring water, streams, and the air, being formed from the natural radioactive decay of radium. Only three isotopes occur in nature; the longest half-life for which is 3.82 days (Rn-222).
It is estimated that there are 100,000 homes in Britain where Radon is believed to give off hazardous levels of radioactivity. High levels are to be found in the south west of England. The UK radon website allows users to obtain an estimate of the probability that an individual property in England and Wales is above the Action Level for radon - the search is by postcode.