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.
2. The biggest single risk is of lung cancer. One case in 20 may be caused by radon in the home. Estimates of risk are based on lifetime exposure. If you are born and die in the same house, there is a 3 per cent chance that you will die of lung cancer.