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The study presents the analysis of the results for indoor radon concentration in the kindergartens of two Bulgarian districts. The study presents the analysis of indoor radon concentration results in kindergartens of two Bulgarian districts. Measurements were performed with a passive method for the period from February to May 2015, in 411 children's rooms situated in 157 kindergartens. The measured radon concentrations in the children's rooms were between 10 and 1087 Bq per m. The evaluation of the results showed that in 10% of the kindergarten rooms, the radon concentration is above the national reference level of 300 Bq per m. The influence of the presence of a basement and the implementation of building renovation on the radon concentration were examined. The presence of a basement is a prerequisite for reducing the concentration of radon in the building. It has been proven that renovating the building increases the radon concentration in it. The analysis confirms the necessity of measuring the indoor radon concentration before the renovation and repair of the building, especially the implementation of energy efficiency measures. [ABSTRACT FROM AUTHOR]