(ii) average indoor occupancy time per person was 7000 h y–1;
(iii) equilibrium factor between radon and its progeny was equal to 0.4 and
(iv) dose conversion factor for radon exposure was 9 nSv (Bq h m23). The annual effective dose due to inhalation corresponding to the concentration of 1 Bq L–1 in tap water was 2.5 mSv y–1. The World Health Organization(11) and EU Council(12) recommended a 0.1 mSv y–1 annual effective dose from drinking water to be the safe limit from these three radioisotopes: 222Rn, 3H and 40K.