2024 Yes Analysis of indoor air pollution with radon in the kindergartens of two Bulgarian districts https://doi.org/10.1093/rpd/ncad094 Djounova, Jana Europe Exposure Elementary (K-5) Radon Radon Radon levels in 411 rooms of 157 Bulgarian kindergartens ranged 10-1087 Bq/m³; 10% exceeded 300 Bq/m³. Basements reduced radon levels, but renovations increased them. Findings stress the need for radon measurement before building upgrades, especially energy-efficiency measures. 2023