en Radiation Biology Radiation Biology تحقيق بديع Original Research <span style="font-weight: bold">Background: </span>The exposure of human being to ionize radiation from natural sources is a continuing, inescapable feature of life on earth. Direct measurement of absorbed dose rates in air has been carried out in many countries of the world during the last few decades. Such investigations can be useful for assessment of public dose rates, the performance of epidemiological studies, and keeping reference-data records to ascertain possible changes in the environmental radioactivity duo to nuclear, industrial, and other human activities. <span style="font-weight: bold">Materials and Methods:</span> The measurements of the outdoor and indoor- environmental exposures including cosmic and terrestrial components were accomplished by a portable Geiger Muller detector (RDS -110). The measurements were made during daylight from September to October 2009, in five areas within nine big cities of Lorestan province. In each area, one building was randomly selected for indoor and outdoor measurements. Measurements were made for each region and an average value was used to calculate the exposure rate from gamma background radiation. <span style="font-weight: bold">Results:</span> The results showed the Maximum and minimum outdoor dose rates as 166±44 and 65±8 nSvh^-1 in Borujerd and Pol-e- dokhtar, respectively. The average of outdoor dose rates was determined 113±26 nSvh^-1. Also the maximum and minimum values of indoor dose were 157±52 and 74±14 nSvh^-1 in Borujerd and Pol-e-dokhtar, respectively. The average indoor dose rates were determined as 119±27 nSvh^-1. <span style="font-weight: bold">Conclusion:</span> The average annual effective dose for gamma background radiation in Lorestan province has been 0.72 mSv, with the range of 0.3– 0.6 mSv which was more than the global value (0.48 mSv). A poor correlation coefficient between was observed altitude and absorbed dose rates. Iran. J. Radiat. Res., 2011 9(2): 89-93 Terrestrial radiation, average annual effective dose, cosmic rays, Lorestan province 89 93 http://ijrr.com/browse.php?a_code=A-10-1-388&slc_lang=en&sid=1 M. Gholami gholami@resident.mui.ac.ir 790031947532846002249 790031947532846002249 Yes S. Mirzaei 790031947532846002250 790031947532846002250 No A. Jomehzadeh 790031947532846002251 790031947532846002251 No