Thoron Daughter Concentrations in UK Homes

Studies of radon (222Rn) and radon daughter concentrations in UK homes have been carried out by the National Radiological Protection Board over the past decade and are continuing. During an early study in areas of the UK where above-average indoor radon concentrations were to be expected because of geological or other factors, radon daughter and thoron (220Rn) daughter concentrations in room air were measured in some homes. These areas were generally of low population density. More recently, one of the authors has carried out a similar measurement programme in homes in major centres of population. The range of indoor radon gas levels, and hence of radon daughter concentrations, spans more than three orders of magnitude. Elevated indoor radon levels are predominantly due to the ingress of soil gas which may contain radon concentrations exceeding 200 kBq.m-3. Thoron in room air comes mainly from the building materials, with a minor contribution from soil gas, and the variability in indoor thoron daughter concentrations is somewhat smaller. It is confirmed that the average annual effective dose equivalent received by a member of the UK population from the inhalation of thoron daughters in indoor air is 100 µSv.