en Radiation Biology Radiation Biology تحقيق بديع Original Research <span style="font-weight: bold">Background: </span>While in the open air the amount of radon gas is very small and does not pose a health risk, in confined spaces, radon can accumulate to relatively high levels and become a health hazard. Exposure to high levels of radon has been associated with an increased risk of lung cancer, depending on the time length of exposure. Radon level in dwelling of Shiraz with 1,200,000 populations has been sampled and analyzed in this study. Our study could be considered the largest radon study in Iran both time and area wise. <span style="font-weight: bold">Materials and Methods:</span> In this study, radon (²²²Rn) concentration in residential dwellings in Shiraz-Iran was sampled and measured during two consecutive six month periods in 2009- 2010. We used Solid State Nuclear Track Detectors (SSNTD), CR-39 polycarbonate films. The survey parameters of radon concentrations were floor types, construction materials and dwelling’s age. <span style="font-weight: bold">Results:</span> Annual average indoor radon concentration for the survey period was 94±52 Bq/m³. The calculated mean annual effective doses in basements and different floors were less than the lowest limit recommended action level of 3 mSv by ICRP. <span style="font-weight: bold">Conclusion:</span> High radon concentrations are measured in basements and old dwellings however, due to rapidly changing housing structures and ventilation practices with no intervention, lower levels of radon concentration has been expected in Shiraz. <span style="font-weight: bold">Iran. J. Radiat. Res., 2011 9(3): 175-182 </span> Indoor radon, CR-39, effective dose, electrochemical etching, radon atlas 175 182 http://ijrr.com/browse.php?a_code=A-10-1-399&slc_lang=en&sid=1 K. Hadad 790031947532846002401 790031947532846002401 No M.R. Hakimdavoud 790031947532846002402 790031947532846002402 No M. Hashemi-Tilehnoee mehdi.hashemi.t@gmail.com 790031947532846002403 790031947532846002403 Yes