2019 No https://pubmed.ncbi.nlm.nih.gov/30734572/ Rejc, T Europe Observational Elementary (K-5) Carbon Dioxide (CO2) Bacteria Fungi Air quality Kindergartens Microorganisms Black carbon Carbon dioxide (CO2) he highest mean concentration of black carbon in indoor and outdoor air was obtained in winter. Concentrations of CO2 exceeded 1000 ppm in 89.3% of the indoor air measurements. The reduction of the number of children in individual playrooms and more frequent ventilation are recommended for lowering the concentration of CO2 and the number of microorganisms in the air, especially potential pathogen fungi. The renovation of buildings with installation of effective artificial ventilation is recommended as well.