Notes: Spatial variability of CRn has been confirmed. CRn are significantly different between districts (KW, p < 0.0001). The GM in the district varies from 57 Bq/m3 (2.24) for Varna district to 184 Bq/m3 (1.69) for Yambol district. The GM of CRn in 12 districts is higher in comparison to the overall GM of CRn. Results were analyzed ad published in: Ivanova, K. and Stojanovska, Z. (2019). Modelling of the temporal indoor radon variation in Bulgaria. Radiation and Environmental Biophysics, 58(3), pp.337–344. The value of geometric 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.  