This paper presents measurements of the geometric shape, perimeter, and cross-sectional area of the human oral passage (from oral entrance to midtrachea) and relates them through dimensionless parameters to the depositional mass transfer of ultrafine particles. Studies were performed in two identical replicate oral passage models, one of which was cut orthogonal to the airflow direction into 3 mm elements for measurement, the other used intact for experimental measurements of ultrafine aerosol deposition. Dimensional data were combined with deposition measurements in two sections of the oral passage (the horizontal oral cavity and the vertical laryngeal–tracheal airway) to calculate the dimensionless mass transfer Sherwood number (Sh). Mass transfer theory suggests that Sh should be expressible as a function of the Reynolds numper (Re) and the Schmidt number (Sc). For inhalation and exhalation through the oral cavity (O-C), an empirical relationship was obtained for flow rates from 7.5–30.0 1 min−1:  
Sh=15.3Re0.812Sc0.986
An empirical relationship was likewise obtained for the laryngeal–tracheal (L-T) region over the same range of flow rates:  
Sh=25.9Re0.861Sc1.37
These relationships were compared to heat transfer in the human upper airways through the well-known analogy between heat and mass transfer. The Reynolds number dependence for both the O-C and L-T relationships was in good agreement with that for heat transfer. The mass transfer coefficients were compared to extrathoracic uptake of gases and vapors and showed similar flow rate dependence. For gases and vapors that conform to the zero concentration boundary condition, the empirical relationships are applicable when diffusion coefficients are taken into consideration.
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