Radon is a naturally occurring radioactive gas, colourless and odourless and one of the breakdown products of uranium. It is present everywhere on the surface of the earth, in larger or smaller quantities depending on the nature and texture of the soil. It is sometimes found dissolved in water. It can accumulate and reach high concentrations in buildings. Studies in recent decades have shown that inhaling radon at high concentrations very greatly increases the risk of lung cancer.
Radon seeps from the ground in Devon and Cornwall in the west of England, because of the high concentrations of uranium in the underlying rocks. Several thousands of houses are reported as having abnormally high radiation levels, some as much as eight times the level permitted for workers in the nuclear industry. Up to 12% of America's 75 million houses may have enough radon to warrant remedial action.
An extensive survey, covering as much as 92,000 homes, found that about 100,000 homes in England – one in 200 – contained levels of radon gas in 1992.
In 1992, the world average level indoors was about 40 becquerels per cubic metre. The UK average was 21, but the homes at risk had 200 or more; some had 2,000 becquerels.