“…During a study by WHO in 36 countries, the reference level of radon for the existing buildings was between 200–400 Bq/m 3 ( 6 ). The concentrations of radon gas in the indoor air of residential buildings in Finland ( 12 ), Cyprus ( 13 ), Russia ( 14 ), Germany ( 15 ), Romania ( 16 ), Spain ( 16 ), Vietnam ( 17 ), India ( 18 ), Turkey ( 19 ), Saudi Arabia ( 20 ), Babolsar ( 21 ), Gonabad ( 21 ), Yazd ( 22 ), and Tehran (Ekbatan Housing Complex) ( 23 ) were found to be 155, 6.2, 102.8, 52, 400, 2650, 366, 7–100, 46 ± 26, 9–98, 130, 35 ± 83, 79 ± 3, 137.36 ± 149.5, and 18.4 Bq/m 3 , respectively. Based on extensive studies at Ramsar region in Iran, the annual effective dose for an individual was reported as 2.48–71.74 mSv/y, which is much more than the dose received from natural sources ( 23 ).…”