“…The measurement of short lived 222 Rn decay product concentrations in the air has become a routine procedure for controlling the radiation exposure by inhalation. In general, the concentration of 222 Rn and its decay products in indoor air is on average 2-10 times higher than in free atmosphere (McCregor et al, 1980;O'Riordon et al, 1983;Swedjemark and Mjones, 1984;Hogeweg et al, 1984;Langroo et al, 1991;Stelios and George, 1993;Thiessen, 1994;Malanca et al, 1995;Rolle and Lettner, 1996;Yu et al, 1999;Magalhaes et al, 2003;Anastasiou et al, 2003;Papaefthymiou et al, 2003). This is due to low rates of air exchange and the dynamic collection into closed space, with additional contributions from 222 Rn sources such as building materials.…”