2021 No https://onlinelibrary.wiley.com/doi/10.1111/ina.12802 Szabados, Máté Europe Observational Students Elementary (K-5) Ventilation rates Temperature Carbon Dioxide (CO2) Volatile Organic Compounds (VOCs) Radon Hazard quotient values were below the threshold value of 1 with one exception. In contrast, 31% of the school buildings were characterized by hazard index values higher than 1. The maximum cumulative ratio approach highlighted that the concern for non-carcinogenic health effects was either low or the health risk was driven by more substances. The median excess lifetime cancer risk values exceeded the acceptable value of 1 × 10−6 in the case of radon and formaldehyde. PM2.5 mass concentration values exceeded the 24 h and annual guideline values set by the World Health Organization in 56 and 85% of the cases, respectively. About 80% of the schools could not manage to comply with the recommended concentration value for carbon dioxide (1000 ppm).