2019 Yes https://doi.org/10.2478/aiht-2019-70-3343 Brdarić, Dario Europe Observational Exposure Study Students Elementary (K-5) Ventilation rates Carbon Dioxide (CO2) Carbon dioxide (CO2) Indoor air quality Ventilation Measured CO2 levels in 60 Croatian elementary school classrooms during the heating season; all exceeded WHO limits, showing poor ventilation. Results call for multidisciplinary solutions to improve indoor air quality.