Deposition of ultrafine particles in the upper airways: An empirical analysis

Abstract

Deposition efficiencies measured in replicate casts of the upper bronchial airways of the human respiratory tract were compared with values predicted for deposition by diffusion and impaction for particles with mean diameters from 0.04 μm to 0.2 μm. In addition, mass deposition fractions were calculated for polydisperse particles ( σ g =1.3) to compare with predictions for monodisperse aerosols ( σ g =1). Utilizing the experimental flow rates and particle sizes it was found that the deposition efficiency by impaction was much smaller than that by diffusion, except for the case of a 0.2 μm diameter particle at a high inspiratory flow rate. An empirical expression for the diffusional deposition efficiency of particles in the upper airways was obtained based on the experimental data. In the uppermost airways of the lung the deposition predicted using this expression was about twice the value obtained using Ingham's equation for particle deposition by diffusion from parabolic flow. The predictions converged as penetration increased to deeper branching levels. The behavior was analogous with predicted deposition of diffusive particles in developing flows. Detailed knowledge of deposition efficiency for these particles is required for evaluation of the risk from exposure to ambient short-lived radon decay products.