Radon is a naturally occurring radioactive gas, which is produced by the radioactive decay of radium which, in turn, is derived from the radioactive decay of uranium.
Uranium is found in small quantities in all soils and rocks, although the amount varies from place to place. Radon released from rocks and soils is quickly diluted in the atmosphere. Concentrations in the open air are normally very low and do not present a hazard.
Radon that enters buildings may reach high concentrations in some circumstances. The construction method and degree of ventilation will influence radon levels in individual buildings. A person's exposure to radon will also vary according to how particular buildings and spaces are used.
Inhalation of the radioactive decay products of radon gas increases the chance of developing lung cancer. Radon is the second largest cause of lung cancer- the first is smoking. People who are exposed to high levels of radon are more likely to get lung cancer (much more so if they are smokers as well).
In order to limit the risk to individuals, the Public Health England* recommends that radon levels should be reduced in homes where the average is more than 200 becquerels per cubic metre (200 Bq m-3). This recommendation has been endorsed by the Government.
The Public Health England defines radon Affected Areas as those with one per cent probability or more of a home having radon above the Action Level.
Guidance on measuring radon can be obtained from the Public Health England.
Download publications about radon from DEFRA at Radioactivity — radon
Acknowledgement: Some of the text in our radon pages is derived from the DEFRA publications at the link above.