Quantifying Olfactory and Alveolar Deposition of Ultrafine Particles Using Multiscale Modeling: Implications for Brain Exposure

High Resolution Image Download MS PowerPoint Slide Atmospheric particle exposure is commonly characterized using mass-based metrics that inadequately capture particle surface area, respiratory deposition, and direct brain delivery pathways. Ultrafine particles (UFPs) contribute marginally to PM 2.5 mass yet account for a disproportionate fraction of deposited surface area in both the lungs and the nasal olfactory region, a critical gateway for direct particle translocation to the brain. Using multiscale atmospheric modeling from continental to street level, evaluated against NO 2, particulate mass, chemical composition, and size-resolved particle observations, we quantify particle deposition through lung-deposited surface area (LDSA) and olfactory deposition, explicitly accounting for hygroscopic growth. UFPs account for more than one-third of alveolar surface deposition in urban environments, and particles smaller than 400 nm dominate alveolar deposition despite their marginal contribution to PM 2.5 mass. Decoupling between PM 2.5 and deposited surface area persists across cities, source contributions, and population groups, with children experiencing approximately 3-fold higher alveolar doses than adults. Residential wood heating dominates winter PM 2.5 and alveolar deposition, whereas traffic controls particle number and olfactory deposition. Critically, under the tested translocation assumptions, olfactory deposition of UFPs is estimated to exceed blood-borne translocation to the brain by several hundred-fold, supporting the relevance of neuronal pathways for brain-relevant particle delivery and consistent with recent observations of black carbon in the human olfactory bulb.