Beyond Infection-Indoor Airborne Pathogens as Contributors to Respiratory Inflammation and Immune Dysregulation: A Narrative Review.

Indoor-air research has largely examined infection, microbial ecology, immune effects and antimicrobial resistance separately, leaving the pathway from indoor biological sources to chronic respiratory outcomes insufficiently integrated. This narrative review synthesises evidence on indoor airborne pathogens and non-viable microbial components as health-relevant biological exposures beyond acute infection. Literature published between 2000 and February 2026 was reviewed from PubMed, Scopus and Web of Science, supplemented by guidance from WHO, ECDC, US EPA and ASHRAE. Viable microorganisms and non-viable components, including endotoxin, β-(1→3)-glucans, microbial DNA and extracellular vesicles, engage epithelial pattern-recognition pathways and promote inflammatory signalling. Findings included 6.5% higher TNF-α and 5% higher IL-8 per log-unit increase in fungal-spore exposure among sawmill workers; uncontrolled asthma in 45% of moisture- or mould-exposed versus 33% of non-exposed children; airborne resistance-gene and mobile-element loads of 0.55-479.44 copies/m³ in hospital departments; and a 32.8% reduction in viral diversity, but no significant reduction in high viral exposure, following classroom HEPA filtration. These findings support biological plausibility but reveal a fragmented evidence base dominated by observational studies, heterogeneous sampling and limited longitudinal exposure-response data. Indoor bioaerosols should be considered continuous exposures within the exposome, requiring research and regulation across microbiology, environmental engineering, medicine and public health.