Crowded Public Spaces as Hotspots of Airborne Microbial Risk: A Population-Weighted Risk Assessment in Urban Environments

Airborne pathogens and antimicrobial resistance (AMR) pose growing health risks in cities, where enclosed spaces, inadequate ventilation, and high population density enhance their persistence and dissemination. However, the microbial burden and risk associated with high-occupancy public spaces remain poorly quantified. Here, we compared bioaerosol characteristics across university cafeterias and a subway station, dry- and mixed-waste collection facilities (WCFs), and an urban air monitoring site by using culture-based, molecular, source-tracking, and risk-assessment approaches. The results showed that Crowded Public Spaces (CPSs) harbored culturable bacterial and AMR burdens comparable to those in WCFs, both far exceeding levels at the urban air monitoring site. Human-associated sources contributed to ∼50% of airborne bacteria, and multidrug-resistant isolates (∼60%), high-risk β-lactam ARGs, and clinically relevant pathogens were further enriched in CPSs. We further applied a population-weighted infection burden (PWIB) metric that integrates infection risk with pedestrian volume and dwell time. Although contamination levels in CPSs were similar to those in conventional microbial hotspots, CPSs contributed more to the city-scale infection burden once population exposure was taken into account. These findings reveal that urban airborne microbial risk is shaped not only by contamination intensity but also by human occupancy and exposure patterns. This study highlights the value of incorporating human activity into microbial risk assessment in high-density urban environments.