2024 Yes Assessment of CO2 and aerosol (PM2.5, PM10, UFP) concentrations during the reopening of schools in the COVID-19 pandemic: The case of a metropolitan area in Central-Southern Spain https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8003457/ Villanueva, Florentina Europe Observational Epidemiological Elementary (K-5) Middle/Jr High (6-8) High School (9-12) Carbon Dioxide (CO2) Particulate Matter (PM) COVID-19 pandemic Particulate matter (PM) SARS-COV-2 transmission risk mitigation School environment Ventilation conditions In 19 Spanish classrooms, CO₂ levels exceeded safe limits in 26% of rooms, especially in secondary schools. PM2.5 and PM10 also surpassed WHO limits, mainly in preschools. Findings support improving ventilation and filtration to reduce exposure risks. 2021