Pilot survey of indoor radon in the dwellings of Bulgaria.

The first systematic indoor radon survey in four districts of Bulgaria was performed. Three hundred and seventy-three selected ground floor dwellings were measured using the nuclear track detectors during 6 months, from October 2011 to May 2012. It was found that indoor radon concentration varied between 20 and 3560 Bq m(-3) with median value of 90 Bq m(-3). The fractions of dwellings in four districts: Sofia city, Sofia districts, Plovdiv and Varna above the reference levels of 300 Bq m(-3) were 3, 9, 14 and 5 %, respectively. Each data set does not follow a log-normal distribution at a significance level of 95 %. The results of the analysis of the variance showed statistically significant differences among the indoor radon concentrations for the regions between urban and rural municipalities as well for the building with and without basement. These results may be utilised to set up the methodology for a more systematic survey in Bulgaria.