A novel method based on 220Rn (thoron) exhalation rate of indoor surfaces for robust estimates of 220Rn concentration and equilibrium factor to compute inhalation dose

The research into Rn-220 (thoron) has generated an increasing interest in recent times due to the realisation of its radiological importance in many indoor environments. Though it is assumed that the contribution of Rn-220, per se, to the inhalation dose is negligible in comparison with that of its decay products, this may not be always true. Correct estimation of inhalation dose due to thoron requires a reliable method to measure the concentration of both 220 Rn and its decay products in indoor air. However, due to its very short half-life (55.6 s) Rn-220 shows large variation in its indoor activity concentration. This makes it difficult to have a robust value of Rn-220 concentration which can be considered representative of a house, thus making the dose estimation unreliable. This issue has been addressed in the present study by developing a novel method that utilises the Rn-220 exhalation rate from indoor surfaces as the basis for estimation of average Rn-220 concentration in indoor air. The Rn-220 concentration estimated in this manner can be converted to decay products concentration using a suitable equilibrium factor and finally the inhalation dose using appropriate dose conversion factors. A wall mounting accumulator setup has been developed for easy in-situ measurement of Rn-220 exhalation from room surfaces. The method has been validated through comprehensive measurements in 25 dwellings in two different regions of India. The developed method is very good for large scale field surveys because of fast and easy applicability. (C) 2020 Elsevier Ltd. All rights reserved.