Impacts of inhaled exposures on the upper respiratory tract microbiome: a systematic review

Abstract

Background: Inhaled exposures can substantially affect human health. The upper respiratory tract (URT) microbiome forms a critical first point of interaction with inhaled agents (e.g. air pollutants and chemicals), yet its response to most inhaled exposures remains poorly characterised beyond the well-studied effects of tobacco smoking. Methods: We systematically reviewed research articles from 2005 to 2024 investigating the effects of inhaled exposures on the human URT microbiome, using sequencing-based approaches. Database searches in PubMed, Scopus, and EMBASE yielded 5263 unique publications. Following screening using ASReview, 66 studies met inclusion criteria, covering four exposure domains: urban outdoor, rural outdoor, household indoor, and occupational settings. Results: Inhaled exposures were consistently associated with alterations in the URT microbiome, often differing by anatomical niche (e.g. nasal, nasopharynx, oral, oropharynx). Outdoor air pollution and urbanisation were linked to reduced microbial diversity and depletion of commensals, whereas green space and agricultural exposures were associated with higher diversity, enrichment of health-associated taxa, and introduction of animal- and soil-associated microbes. Findings for other exposures (e.g. indoor pollutants, pesticides) were more heterogeneous. Conclusions: Overall, the URT microbiome remains understudied as a mediator of respiratory health effects related to inhaled exposures, while methodological heterogeneity complicates comparability across studies. Future research should prioritise benchmarked protocols, longitudinal designs, and functional analyses (e.g. metagenomics) to clarify how inhaled exposures alter microbial activity, resilience, ecological interactions, and host outcomes. This synthesis highlights the need for integrated environmental health approaches and for assessing the long-term consequences of inhaled exposures.