A mathematical description of inhaled particle behavior suitable for analysis of factors affecting deposition in the human upper respiratory tract (nasopharyngeal and oropharyngeal compartments), larynx, and ciliated airways is presented. When upper respiratory tract and larynx filtering efficiencies are incorporated in the model, theoretical calculations of aerosol deposition agree with tracheobronchial (TB) data reported in the open literature. The model predicts the sigmoidal shape of TB deposition patterns measured in human subjects, the slopes of the quasilinear segments of these curves, and the positions of their two asymptotes. The model could therefore be used to evaluate the health effects of toxic airborne particles.