Effects of seasonality and air pollution on the nasal microbiota in healthy Italian adults

Air pollution is a major environmental risk factor for respiratory health, yet its interaction with seasonality in shaping the upper airway microbiota remains poorly understood. We conducted a longitudinal repeated-measures study to investigate whether seasonality modulates the effects of indoor and outdoor air pollution on the nasal microbiota of healthy adults. Twenty-six participants were sampled weekly for three weeks in winter and three weeks in summer . Microbial composition was characterized using 16S rRNA gene sequencing (124 samples) and whole-genome shotgun sequencing (141 samples). Weekly exposure to indoor total suspended particles (TSP) and outdoor pollutants (particulate matter, black carbon, benzene, and carbon monoxide) was assessed using environmental monitoring data. The nasal microbiota was stable within seasons but differed significantly between seasons, with winter enrichment of Moraxella species, particularly among women with children. Across seasons, higher pollutant levels were negatively associated with relative abundance of commensal taxa, particularly Corynebacterium species. In addition, this study identified significant season-pollutant interactions. For example, in summer, commensal bacteria (e.g., Staphylococcus epidermidis and Cutibacterium granulosum) were found to be negatively associated with particulate matter exposure. Among host factors, sex explained the largest proportion of variance in microbial diversity, while household characteristics contributed additional compositional variability. These findings indicate that the respiratory microbiome varies across seasons and is associated with air pollution, suggesting that both seasonality and environmental exposures can contribute to differences in respiratory microbial communities.