Ventilation strategy for simultaneous management of indoor particulate matter and airborne transmission risks – A case study for urban schools in South Korea

During the COVID-19 pandemic, schools in South Korea augmented ventilation rates by opening windows to avoid airborne transmission risks. However, this also elevated indoor PM2.5 concentrations, creating a trade-off problem between the two risks. This study quantitatively analyzed the impact of ventilation rate on indoor PM2.5 concentrations and COVID-19 transmission risk in a typical classroom. The results showed that indoor PM2.5 concentrations asymptotically approached outdoor ones with the augmentation of ventilation. The impact of ventilation augmentation on indoor concentration decreased with the ventilation rate. The time evolution of air transmission risk was obtained from the balance equation of quanta generated by infected people to estimate the infection probability and effective reproduction number. The transmission risk was a complex function of the ventilation rate, quanta generation rate per infected person, and mask filtration efficiency. The combination of information from the analyses of the two risks provides feasible solutions to the problem of resolving the two conflicting risks simultaneously. For example, high ventilation rates are allowable in mild climates with low outdoor PM concentrations. They can ease mask filtration efficiencies and elevate allowable activity levels while wearing high-efficiency masks and limiting students' activity levels must be enforced under cold climates with high outdoor PM concentrations. This study provides important insights into the trade-off between ventilation rate and COVID-19 transmission risk in classrooms. In addition, the results can be used to develop strategies to mitigate both risks and ensure the safety of students and staff. •Airborne transmission risks fell with augmented ventilation during the pandemic.•Augmented ventilation increased the risk from indoor particulate matter.•The trade-off relation between the two risks was quantitatively investigated.•Wearing a proper mask, the two problems could be solved for most student activities, regardless of ventilation conditions.•Reducing student activity and using active ventilation could lower the required mask filtration efficiency.