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In vitro measurements of particle filtration were made for nasal geometries of 11 infants aged 3–18 months. The geometries were obtained from computed tomography (CT) scans of seven males and four female infants and replicas were built using rapid prototyping. Particles ranging in aerodynamic diameter from 0.8 to 5.3μm were passed through these replicas with simulated tidal breathing. Filtration was determined from particle counts upstream and downstream of the models using an electrical low pressure impactor (ELPI). Mathematical fits were constructed to predict the measured deposition based on the relevant parameters. The fractional deposition, η, is found to depend on the Reynolds number of the flow (Re), the particle Stokes number (Stk), and an airway dimension D defined as airway volume divided by airway surface area. This dependence is well captured by the formula η=1-(2.164*10⁵/(2.164*10⁵ …