Concentration-dependent aerosol size alters regional deposition and inhalation dose translation

Inhalation toxicity assessment relies heavily on quantifying both atmospheric concentration (AC) and region-specific internal dose (ID) within the respiratory tract, especially when comparing in vivo toxicity tests with in vitro respiratory cellular toxicity tests. However, for particulate substances, the precise relationship between ID and AC often becomes unclear due to variations in aerosol particle size distribution (PSD). This study aimed to investigate how the PSD of aerosols, generated from non-volatile and water-soluble biocides (benzalkonium chloride, didecyldimethylammonium chloride, polyhexamethylene guanidine phosphate, and paraquat), influences the relationship between AC and region-specific ID in the respiratory tract. Aerosols were generated at various concentrations using an ultrasonic humidifier in a 0.125 m³ acrylic chamber, with PSDs (0.01-10 μm) measured using real-time instruments. Regional deposition rates in the rat respiratory tract were subsequently calculated via the multi-path particle dosimetry (MPPD) model to estimate ID. As solution concentration increased, both AC and the mass median aerodynamic diameter (MMAD) also rose. The ID/AC ratio showed saturating increases in the head (H) and tracheobronchial (TB) regions, but a decreasing trend in the pulmonary (P) region, attributed to a reduction in fine particle fraction. A lognormal distribution-based analysis revealed that the TB region was most sensitive to changes in geometric standard deviation (GSD), whereas the H and P regions were primarily affected by MMAD variations. Crucially, a notable mismatch in ID/AC ratios was identified between the measured PSDs and those assumed under lognormal distribution models, underscoring the vital role of aerosol size characteristics in understanding respiratory deposition efficiency and dose metrics. These findings strongly suggest that a quantitative approach, incorporating measured PSDs, is essential for accurately interpreting and comparing inhalation exposure data across different toxicity evaluation systems. This goes beyond evaluations based solely on external concentrations, highlighting the necessity of detailed aerosol characterization.