Abstract In this study, the indoor radon ( 222Rn) levels in summer and winter seasons were measured by using a total of 537 CR-39 detectors. The arithmetic mean values (ranges) of radon activity concentrations (RACs) in summer and winter seasons were found to be 71 Bq m −3 (27–313 Bq m−3) and 241 (89–1047 Bq m−3), respectively. In 20 houses, the RAC was higher than 400-Bq m−3 critical value declared by the Turkish Atomic Energy Authority (TAEK). The mean radon concentrations in summer and winter seasons were below the critical value declared by TAEK. According to normality test, the radon distributions in summer and winter seasons were determined as log-normal. The annual mean effective dose equivalent and the mean excess lifetime cancer risk were calculated as 8 mSv y−1 and 26 × 10−3.