In 1990 the National Radiological Protection Board (NRPB) set an advisory limit for radon concentrations in homes of 200 bequerel per cubic metre. One becquerel of activity corresponds to one radioactive decay per second.
Levels of radioactivity of 200 Bq m-3 or higher have been detected in about 0.4% of houses in the UK. To set this figure in context, the average lung capacity of a human adult is about 5 dm3. One cubic metre of air with a radioactivity of 200 Bq m-3 would have 200 radioactive decays per second. A 5 dm3 sample of this air (1/200 of a cubic metre) would undergo one radioactive decay per second.
Levels of radioactivity due to radon-222 of 8000 Bq m-3 have been reported in areas where the bedrock contains high levels of uranium-238. This corresponds to 40 radioactive decays per second in the air contained within the lungs of a typical adult.