The Effect of Post-COVID Ventilation Measures on Indoor Air Quality in Primary Schools

The recent COVID pandemic has made people acutely aware of the importance of indoor air quality (IAQ) and building ventilation systems, particularly in densely occupied places like offices and schools. As a result, governments and other public entities are investing increasing amounts of money in installing, maintaining, and upgrading school ventilation systems. However, little is known about the effect of school building ventilation systems on \textit{actual} indoor air quality, and their interaction with occupant behavior. This paper exploits exogenous closing and opening events of schools during the COVID pandemic, combined with policy measures focusing on maximizing ventilation rates inside classrooms, and assessing the effectiveness of building ventilation systems in primary schools. We use a unique sensor network, consisting of measurement devices installed in 252 classrooms across 27 Dutch primary schools, continuously monitoring IAQ indicators such as CO$_2$ levels and fine particle concentrations. Using high-frequency data for the school years from 2018 to 2022, we compare the performance differences between natural and mechanical ventilation. Using a fixed-effect identification strategy, our results show significant, but not very large advantages of mechanical ventilation regarding CO$_2$ and fine particle levels. However, the post-COVID ventilation measures after school reopenings had stronger effects in naturally ventilated classrooms, suggesting behavioral changes at the classroom level. We also investigate the longer-term effects of these new ventilation measures and show some evidence of decay in effectiveness, as well as a strong seasonal effect, particularly in naturally ventilated classrooms, which seems the result of a changing trade-off between ventilation and thermal comfort.