The NRPB provides a lot of information, including access to detailed information of radon distribution in the UK.

The Cornwall Radon Gas Centre gives lots of information and advice to people living in a high-risk area.

Radon formed when atoms of uranium-238 decay (see decay series notes). Radon will also decay, and, if it is inhaled will emit alpha particles that can damage the internal lung surfaces. Therefore the main health concern is an increased lung cancer risk from breathing the gas.

The radon concentration in the average UK home is about 20 becquerels per cubic metre. If the concentration is found to be as high 200 becquerels per cubic metre the Department of Health recommends that action is taken to lower the concentration by venting the gas into the atmosphere. When homes were not centrally heated and draught-proofed this would not have been necessary, especially with the big open chimneys.
Companies have developed equipment to measure and monitor radon levels. Local councils and environmental agencies are concerned about the levels of radon in a locality and raise awareness of local people to the problems involved.
We became aware of the dangers posed by radon gas after World War Two when large uranium mines were opened around the world to feed the nuclear industry. Miners became exposed to high levels of radon and suffered increased rates of lung cancer. This was due to mutations in their DNA from ionising radiation damage.