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Subsoil – this is the most important and commonest source of radon. Radon is actively sucked from the subsoil through leakages in the substructure of the building. This is due to the under-pressure in the lower parts of buildings generated by the stack effect and wind forces (Fig. 1). Radon can easily penetrate through a concrete structure with waterproofing – it is transported through cracks, through leakages around service pipe entries, and through imperfectly sealed inspection chambers (Fig. 2).

Building materials – Building materials made from sand, gravel, brickmaker’s clay, fly ash and slag or waste materials from ore processing usually contain some amount of uranium. These materials therefore contain some radium, the mother radionuclide of radon. A higher amount of radium results in a higher concentration of radon, and also a higher level of gamma radiation in the building. All building materials nowadays available in the Czech construction market should be safe from the radon point of view.

Water – Water supplied into buildings is a minor source of radon. It can increase the radon concentration in bathrooms and kitchens while water is being used, and afterwards. If these rooms are sufficiently ventilated, increased irradiation due to radon from water is unlikely to occur.

Fig. 1 - Wind forces and the stack effect caused by the teperature difference between indoors and outdoors generate a slight underpressure in the lower parts of buildings.

Fig. 2 - Typical radon entry points.