Urbanization and indoor-outdoor bioaerosol dynamics shape the airborne microbiome

Bioaerosols are airborne particles of biological origin which include bacteria, fungi, pollen, viruses and microbial fragments, forming dynamic airborne microbial communities. They can be inhaled by humans while breathing. While some are infectious or pro-inflammatory, most are harmless or may even contribute to immune development.

Over recent decades, research has shown how human activities shapes these airborne communities. Studies in occupational settings or high-risk workplaces, such as farms and wastewater treatment facilities, have found high microbial loads in these environments, which means that workers may be exposed to pathogens. Despite this extensive body of research, fewer studies have addressed how bioaerosol composition changes across urbanization gradients and in everyday environments (e.g., schools ,offices), which account for a significant portion of our daily exposure.

Canada provides a strong comparative context, as its territory is spread across different in ecozones and land use, which are key drivers of airborne communities. Here, we compared the bacterial and fungal air microbiomes of Scarborough (ON) and Chicoutimi (QC) and assessed how outdoor communities impact indoor air bioaerosol composition.

Amplicon sequencing revealed significant differences in the composition of bacterial and fungal communities across both cities, but only bacterial abundance differed (using qPCR). Bacterial abundance and taxonomic richness were higher in Scarborough than Chicoutimi, potentially due to greater wind speeds, which correlated with these metrics. No such correlation was observed for fungal diversity. Microbial assemblages were different across cities, likely due to distinct bioaerosol emission sources. Compared to outdoor air, indoor air exhibited a higher bacterial abundance, but lower diversity, likely due to limited indoor air circulation. Additionally, higher occupancy levels contributed to an increased bacterial load, as humans actively shed bacteria into the environment. By characterizing the links between indoor and outdoor airborne microbiome, this study provides insight into bioaerosol exposure in our daily lives.