Bioaccessibility and transformation of PM2.5 organics and microbiota in simulated lung fluids: Implications for fluid-specific respiratory risks

Fine particulate matter (PM 2.5 ) contains organic pollutants and microorganisms that interact with pulmonary fluids upon inhalation, potentially leading to lung microbiota dysbiosis and respiratory diseases. However, how lung fluids affect the bioaccessibility and transformation of PM 2.5 remains unclear, impeding precision risk assessment. We extracted and incubated ambient PM 2.5 samples in simulated lung fluids (SLFs) modeling extracellular healthy lung fluid (modified Gamble's solution, MGS) versus intracellular acidic lung fluid (artificial lysosomal fluid, ALF). Organic compounds and microorganisms were analyzed using ultra-high-performance liquid chromatography-mass spectrometry (UHPLC-MS) and high-throughput sequencing, respectively. The results show fluid-dependent bioaccessibility and changes: seven most abundant organic compounds (e.g. malic acid) are exclusively bioaccessible in MGS, whereas four organics (e.g. 4-nitrophenol) are bioaccessible only in ALF. High-risk toxins like nicotine, skatole, and N,N -dimethylacetamide show high bioavailability in both fluids. While ALF extracts minimal microorganisms, MGS exhibits higher release for some pathogens including Corynebacterium , Staphylococcus , and Acinetobacter . After 96-hour cultivation, MGS exhibits greater compositional shifts in both organic compounds (carboxylic acids, amine compounds, and pyridines) and microbiota (ten different genera including Acinetobacter and Pseudomonas ) than ALF, owing to more pronounced microbial activities in neutral pH. In MGS, Acinetobacter and Pseudomonas were positively associated with carboxylic acids, amines, and aromatics, indicating putative microbe–organic interactions that require mechanistic confirmation. These fluid-specific patterns suggest that identical PM 2.5 exposure may yield different health impacts from pollutants and microorganisms depending on an individual's pulmonary microenvironment. This study underscores the importance of incorporating bioaccessibility into PM 2.5 risk assessments.