Long-Term Monitoring of Mechanical Ventilation and Window Airing in Classrooms: A Controlled Observational Study

Indoor environmental quality is essential for pupils‘ health, comfort, and academic performance. However, recent studies indicate that indoor air quality (IAQ) in classrooms is often inadequate. This observational study examines the impact of three ventilation concepts on IAQ and thermal comfort under real-life school conditions: manual window airing combined with CO2 traffic lights, decentralized mechanical ventilation, and centralized mechanical ventilation. Eight classrooms in three elementary schools were monitored from October 2023 to April 2024. Continuous long-term measurements covered CO2, PM2.5, VOCs, indoor air temperature, relative humidity and window opening states in the classrooms, and ambient data including PM2.5 at each school. Significant differences were found in all five indoor parameters across the three ventilation concepts. The decentralized ventilation group achieved the lowest CO2 concentrations (18–22% lower), while the window airing group showed the highest PM2.5 levels (mean of 6 µg/m3) and the lowest temperatures (21% of the time below 20 °C). Relative humidity tended to be too low for all concepts, particularly with mechanical ventilation (medians below 40%). Windows in the window airing group were opened approximately twice as long. The findings highlight the benefits of well-operated mechanical ventilation systems and underscore the importance of user awareness and system management.