Bacterial and fungal bioaerosols in urban air: A narrative review and conceptual framework for air quality degradation

Urban air pollution has long been assessed through a chemical lens, overlooking the quantitative and functional significance of bacterial and fungal bioaerosols. This narrative review synthesizes recent evidence showing that airborne microbes are ubiquitous in cities, with concentrations ranging from 102 to 105 CFU m−3, and contribute meaningfully to PM₁₀ and PM₂.₅ mass. Critically, bioaerosols are not inert: they remain metabolically active, secrete organic acids, nitrogenous compounds, and redox-active metabolites, and participate in atmospheric chemistry including secondary aerosol formation, cloud processing, and oxidative potential enhancement. Their presence amplifies particulate toxicity via endotoxins, allergens, and biologically active fragments. This work presents a conceptual framework that positions bioaerosols as dynamic participants in urban atmospheric systems. Despite this, regulatory frameworks exclude biological metrics, leading to underestimation of health risks. We argue for integrating bioaerosols into next-generation air quality models and standards.