Radon exhalation rates of building materials: experimental, analytical protocol and classification criteria

The strict correlation between indoor radon exposure and potential health hazard to occupants is well known. The indoor radon concentrations mainly depend on radon exhalation from surrounding soil, but also on exhalation from building materials and radon in domestic water supply. The radon emanating from building materials achieves a larger relevance in some areas of the world, where rocks enriched in radon precursors, are used in construction industry, either as cut-stone or in a granular form to prepare cements. The parameter that better expresses the indoor accumulation of radon released by geological materials is the radon exhalation rate. With a view to this, it is very important to study factors that influence the phenomenon and to standardise the experimental procedure to measure radon exhalation rates. An experimental set-up to measure simultaneously 222Rn and 220Rn release from building material is presented. The method makes use of a continuous monitor equipped with a solid-state alpha detector, in-line connected to a small accumulation chamber. Parameters controlling exhalation rates are discussed: temperature, air mixing, humidity and particle size. Guidelines for a standard experimental protocol are advanced and a tentative classification of building materials is proposed on the basis of radon exhalation rates required to reach legal indoor radon action levels.