Notes: The variation of indoor radon concentration was from 11 Bq/m3 to 1314 Bq/m3. High concentrations of radon are found in central and south-eastern Bulgaria. Population-weighted radon concentration was calculated to be 108 Bq/m3 using the statistical data. Results were analyzed ad published in: Ivanova, K., Stojanovska, Z., Kunovska, B., Chobanova, N., Badulin, V. and Benderev, A. (2019). Analysis of the spatial variation of indoor radon concentrations (national survey in Bulgaria). Environmental Science and Pollution Research, 26(7), pp.6971–6979. The value of arithmetic mean is from National Radon Survey. This value will be reassessed each 5 years according the new obtain data from the surveys under the implementation of the National action plan.  