2021 Yes http://www.psychiatria-danubina.com/UserDocsImages/pdf/dnb_vol33_noSuppl%203/dnb_vol33_noSuppl%203_S318.pdf Kulo, Aidą Europe Observational Epidemiology Study Modeling Elementary (K-5) Temperature Humidity Carbon Dioxide (CO2) Indoor air quality Classrooms Green building certification Health COVID-19 During October, except in one school, PM2,5 concentrations were below the upper recommended value (25μg/m³), but started rising in all schools in the heating season. The highest concentrations of PM2,5 were registered in two schools with closed or shortly opened windows. CO2 concentrations were mostly in the recommended range (up to 1000ppm) except in the school with constantly closed windows and in three schools in February when concentrations were higher. Except in one, the same school, and in January, both temperature and relative humidity were out of the recommended range (24,0–27,0°C in non-heating; 20,0–24,0°C in heating season; and 45–55%), with temperature mainly above and relative humidity mainly below it in three schools. The largest deviation in temperature and relative humidity were registered in urban schools. Registered differences may be explained by different internal and external characteristics.