The research examines the dynamics of radon (Rn) depending on the geological basis of the location and meteorological factors, the building characteristics and the work regime and ventilation patterns in selected educational institutions in Slovenia and Montenegro. Based on the radon risk maps for Slovenia and Montenegro, we selected four educational buildings, namely two in Slovenia (SI-1 and SI-2) in the area of moderate radon risk and two in Montenegro (ME-1 and ME-2) in an area of high radon risk. Building SI-1 has medium air tightness of the building envelope and natural ventilation, while building SI-2 has high air tightness of the building envelope and hybrid ventilation. Buildings ME-1 and ME-2 have low airtightness of the building envelope and natural ventilation. The work regime is the same in all selected buildings. In all four buildings, we carried out several weeks of continuous measurements of the Rn concentration in the air (mostly in the playrooms) during the transition period from the cold to the warm half of the year (February 4−April 25, 2022 in SI-1 and SI-2 and March 18−April 14, 2022 in ME-1 and ME-2). In SI-1 and SI-2 in Slovenia, we additionally continuously measured the concentration of CO2 (indicator of air quality in rooms during occupancy and ventilation efficiency). In ME-1 and ME-2 in Montenegro, we additionally measured average Rn concentrations in selected rooms and in the outside air. We were most interested in the average concentrations of Rn in the playrooms during the operation of the kindergartens (8 hours a day), which we calculated from continuous measurements for the entire period and obtained the following values: (i) Slovenia, building SI-1, playroom I1_SI-1, CRn=109 65 Bq m−3 , building SI-2, playroom I1_SI-2 CRn=79 68 Bq m−3 and playroom I2_SI-2 CRn=69 57 Bq m−3 , (ii) Montenegro, building ME-1, playroom I1_ME-1 CRn=227 207 Bq m−3 , building ME-2 playroom I2_ME-2 CRn=264 191 Bq m−3 and dressing room G1_ME-2 CRn=290 208 Bq m−3 . Rn concentrations are higher during working hours in playrooms ME-1 and ME2 in Montenegro by a factor of about 3 than in playrooms in SI-1 and SI-2 in Slovenia. In ME-1 and ME-2, morning Rn concentrations are usually between 600 and 1000 Bq m−3 , and in SI-1 and S1-2 between 200 and 300 Bq m−3 . In SI-1 and SI-2 in Slovenia, CO2 concentrations have been exceeded several times during working hours. The dynamics of Rn in the air of all four educational buildings is similar, in addition to the geological basis of the area, the Rn concentration in the room is crucially influenced by the airtightness of the building envelope. In ME-1 and ME-2, this is poor, especially at the contact of the foundation slab with the ground (shafts, cracks), so Rn reach soil air easily enters both buildings, where Rn accumulates. In ME-1 and ME-2, satisfactorily low Rn concentrations cannot be ensured by ventilation alone, radon remediation of the buildings would be necessary. In SI-1 and SI-2, the concentration of Rn is satisfactorily low, but it would be advisable to optimize ventilation during working hours (increase the frequency and duration of ventilation) due to the often exceeded CO2 concentrations.
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