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Considering the hazardous effects of particulate matter (PM) exposure on students and teachers and the high PM concentration issue in South Korea, air purifiers have recently been installed in most classrooms to improve air quality. However, some on-site challenges, such as operational costs and noise, have been issues with the continuous operation of air purifiers. Therefore, a guideline is needed to dynamically predict the indoor PM concentration based on the changes in outdoor PM concentration and activate the air purifiers only when necessary. This study develops a grey-box model that uses measured data and physical differential equations to perform the given objective and verifies its accuracy using ASTM D5157. Modeling and analysis results have obtained information that can form the basis for developing guidelines to address PM issues in schools: The air purifier should be operated during periods where the predicted values exceed the limit in closed windows and the air purifier is not operating. It was also confirmed that the need for the operation of the air purifier varies between schools and classrooms under the same outdoor PM concentration. Indoor PM concentration increased significantly after students' simultaneous mass movement, necessitating air purifiers' operation before and after the events. The pre-filter of the heater also aided in the removal of coarse PM. Additionally, the limitations and future development directions of the model were discussed. Copyright © 2023 American Association for Aerosol Research [ABSTRACT FROM AUTHOR]

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 PM 2.5 concentrations, creating a trade-off problem between the two risks. This study quantitatively analyzed the impact of ventilation rate on indoor PM 2.5 concentrations and COVID-19 transmission risk in a typical classroom. The results showed that indoor PM 2.5 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. [ABSTRACT FROM AUTHOR]

Exposure to particulate matter (PM) in school environments has been associated with respiratory illnesses among children. Although using air cleaners was reported to reduce PM exposure and improve residents' health in homes, their effects in classrooms are not well understood. We examined how the use of air cleaners in classrooms and school/classroom characteristics affect the levels of indoor PM. Our environmental study included 102 classrooms from 34 elementary schools located on the mainland peninsula and an island in Korea. Indoor and outdoor PM were monitored simultaneously with portable aerosol spectrometers, and indoor gravimetric PM levels were measured with low volume, size-selective samplers during the class hours. Correlations among PM measurements were computed and final multiple regression models for indoor PM were constructed with a model building procedure. Correlation between indoor and outdoor PM 2.5 (PM < 2.5 μm in aerodynamic diameter) was higher (r = 0.78, p < 0.01) than that of PM 10 (PM < 10 μm) (r = 0.49, p < 0.01). School location, classroom occupant density, and ambient PM levels significantly (p-values<0.05) affected classroom PM concentrations. The adjusted PM levels in classrooms using air cleaners were significantly (p-values<0.01) lower by approximately 35% than in classrooms not using them. However, air cleaners appeared to remove PM 2.5 more effectively than PM 10 , perhaps because coarse particles settle more rapidly than fine particles on surfaces, or their resuspension and generation rate by occupants exceeds the removal rate by air cleaners. Our study suggests that routine cleaning to remove surface dust along with the use of air cleaners might be required to effectively reduce occupants' exposure in classrooms. • Indoor PM had proportionally more coarse PM (72%) than did outdoor PM (56%). • Correlation between classroom and outdoor PM 2.5 was higher than for PM 10. • Using air cleaners was significantly associated with a 35% reduction in PM levels. • PM 2.5 was more effectively removed by air cleaners than PM 10 in classrooms. • Combined use of surface cleaning and air cleaner is best to reduce PM exposure. [ABSTRACT FROM AUTHOR]