Abstract
Due to the ongoing COVID-19 pandemic, different strategies have been employed to reduce the risk of contagion for the occupants of the built environment, in particular in educational buildings. The adoption of higher ventilation rates and personal protection devices, such as facial masks, are among the most common solutions to reduce the infection risk and allow students to keep attending classes. Since several studies, dedicated to developing strategies to limit the COVID-19 airborne transmission, have been focusing on isolated environments under steady-state conditions, the possibility for pathogens to spread to adjacent environments or the effects of dynamic occupancy schedules have been marginally investigated, albeit being of special interest for school buildings. In a previous work, a statistical model able to evaluate the risk of infection was coupled with a TRNSYS building simulation model of three classrooms at the campus of the Free University of Bozen-Bolzano (UNIBZ), Italy. Two out of three classrooms are part of the UNIBZ Living Labs, where the indoor environmental conditions are monitored by a network of sensors. Different scenarios had been investigated, by varying ventilation strategy, number of occupants and mask utilization, and assessing for each case the airborne contagion risk. The study showed a strong correlation between a poorly ventilated environment and high risk of infection, underling also the possibility for the pathogen to spread into the adjacent environments in case of dynamic occupancy or if the classrooms doors are left open. Drawing on the results of the previous research, this work expanded the scope of TRNSYS and TRNFLOW, extending the building model and the airflow network to the whole floor, including classrooms of different size and some offices. In such a way, the internal airflows were assessed dynamically by considering also cross-ventilation effects, improving the accuracy of the calculated infiltration rates and concentrations of COVID-19 pathogen. After a new calibration and validation against the experimental data collected in the UNIBZ Living Labs, a series of transient scenarios were simulated by accounting for the strategies listed before, as well as by considering the effects of three different vaccines – Pfizer, Moderna, AstraZeneca – with a fixed coverage percentage of full/partially/not- vaccinated occupants, new COVID-19 variants, and introducing portable air cleaners. The spread of new COVID-19 variants, which in some cases have proved to be more easily transmitted, made it necessary to re-evaluate and, possibly, adapt the strategies implemented so far, also accounting for different relative vaccination efficacies. High risk of infection was found related to environments with poor ventilation, especially if the Omicron variant was present, even with the additional protection provided by the vaccines. Air purifiers were a viable solution to reduce the risk of infection in those cases where mechanical ventilation and/or natural ventilation was missing also for the Omicron variant, especially if combined with the effects of a mixed ventilation strategy and mask utilization.