Indoor air ventilation and recommended capacity assessment at university facilities to reduce airborne risk spread of infectious agents

This study presents an in-situ methodology to assess indoor ventilation performance and estimate the recommended percentage of maximum occupancy (RPMO) for reducing airborne disease transmission risks. Using carbon dioxide (CO2) as a tracer gas, Air Changes per Hour (ACH) were determined in university facilities, including classrooms, meeting rooms, auditoriums, and a concert hall, under natural and mechanical ventilation conditions. Multi-point CO2 monitoring enabled the identification of ventilation patterns and under-ventilated zones. Results demonstrated that natural cross-flow ventilation could produce ACH values up to 180% higher than mechanical systems, resulting in RPMO estimates of 95%, compared to the 30% limit implemented during the COVID-19 pandemic. In contrast, some mechanically ventilated spaces achieved 12% RPMO, despite being used at higher levels. This discrepancy highlights the importance of empirical ventilation assessments to avoid under or overestimating recommended indoor capacities. A model for estimating the recommended number of occupants (NOP) was validated during six public events in a concert hall, demonstrating good agreement between predicted and measured CO2 concentrations. The proposed methodology is simple, scalable, and operationally feasible, providing institutions with a practical tool to define evidence-based occupancy limits and improve ventilation management. The findings highlight the role of ventilation in reducing airborne transmission risks in educational and public buildings.