Indoor air quality (IAQ) and ventilation efficiency in after-school educational centers in Athens during the post-COVID-19 era

Abstract Aim Indoor air quality (IAQ) plays an important role in classroom ventilation, student comfort, and the potential transmission of airborne infections. This study aimed to evaluate IAQ conditions and identify factors influencing ventilation efficiency in school classrooms in Athens. Subject and methods A cross-sectional environmental monitoring study was conducted over a 3-month period in 114 classrooms across schools in Athens. IAQ parameters measured included carbon dioxide (CO 2 ), carbon monoxide (CO), oxygen (O 2 ), nitric oxide (NO), nitrogen dioxide (NO 2 ), temperature, and relative humidity. CO 2 concentration was used as an alternative indicator of ventilation adequacy and classroom occupancy. Measurements were collected using portable environmental sensors during regular classroom activities. Results The mean CO 2 concentration was 836 ppm, with peak values reaching 3300 ppm, indicating periods of inadequate ventilation, particularly in naturally ventilated classrooms. The average indoor temperature was 18.3 °C and relative humidity (RH) was 61.1%. Concentrations of NO and NO 2 remained consistently low across classrooms. A strong positive association was observed between CO 2 levels and classroom occupancy ( p < 0.001), while no significant association was found with ventilation type or classroom floor level. Conclusion Although most IAQ parameters remained within acceptable limits, the occurrence of elevated CO 2 peaks suggests occasional ventilation deficiencies. Improving ventilation management and monitoring, particularly during periods of high occupancy and extended classroom use such as after-school activities, may help reduce airborne transmission risks and enhance indoor environmental quality.