Thoron exposure in Dutch dwellings - An overview.

In the Netherlands considerable attention has been given to the exposure from thoron progeny in dwellings. For this purpose a nationwide survey on the thoron exhalation and thoron progeny concentration has been completed in 2015. Furthermore, extensive laboratory studies have been performed to measure activity concentrations and thoron exhalation rates from regular Dutch building materials. The purpose of this study is to demonstrate if the findings from both field experiments and laboratory results are consistent. For this reason measured properties of building materials and surface barriers, in-situ measurements on air ventilation and thoron(progeny) in dwellings as well as advanced computational modelling on indoor air and aerosol behaviour have been used. The results demonstrate that median and mean thoron progeny concentrations of 0.53 and 0.64 Bq·m-3 found in the survey are comparable with the mean concentration of 0.57 Bq·m-3 obtained from laboratory testing and calculation. Furthermore, upper thoron progeny concentrations from the survey and the calculations are with respectively 13 and 14 Bq·m-3 also in good agreement. Such elevated concentrations lead to an effective doses of around 4 mSv per year. The study also includes worst-case scenarios on the application of surface materials high on 232 Th, and the expected reduction in thoron progeny when using mainstream mitigation measures.