Quantifying crowding-ventilation effect on respiratory infections.

Crowded and poorly ventilated indoor spaces drive outbreaks of respiratory infections such as coronavirus disease 2019 (COVID-19), often leading to superspreading events. While overcrowding heightens risk, its interplay with space size and ventilation remains underexplored. We introduce a framework to assess this risk, using effective clean flow rate, which combines clean airflow (outdoor air, filtration, ultraviolet germicidal irradiation) with volume-time air clearance (VTAC), i.e., dilution from indoor air volume per person over time, and intake fraction time, which quantifies aerosol exposure risk independent of infectious quanta generation. Analysis of real outbreak data from jails, earthquake shelters, and dormitories shows modest spaciousness (16-32 m3/person) increases effective clean flow by 8.7 to 17.6 L/s per person in low-ventilation settings (0.5 L/s per person, 1-hour exposure), reducing exposure. Current standards, such as American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) 62.1-2022, fall short in crowded spaces, heightening infection risks. Our model predicts infection patterns, showing VTAC is a critical yet overlooked factor in limiting long-range airborne transmission. These insights call for updated ventilation and spaciousness standards to strengthen public health strategies.