In the study presented here, a mathematical approach for the deposition and clearance of rigid and chemically stable fibers in the human respiratory tract (HRT) is described in detail. For the simulation of fiber transport and deposition in lung airways an advanced concept of the aerodynamic diameter is applied to a stochastic lung model with individual particle trajectories computed according to a random walk algorithm. Interception of fibrous material at airway bifurcations is considered by implementation of correction factors obtained from previously published numerical approaches to fiber deposition in short bronchial sequences. Fiber clearance is simulated on the basis of a multicompartment model, within which separate clearance scenarios are assumed for the alveolar, bronchiolar, and bronchial lung region and evacuation of fibrous material commonly takes place via the airway and extrathoracic path to the gastrointestinal tract (GIT) or via the transepithelial path to the lymph nodes and blood vessels. Deposition of fibrous particles in the HRT is controlled by the fiber aspect ratio beta in as much as particles with diameters <0.1 mu m deposit less effectively with increasing beta, while larger particles exhibit a positive correlation between their deposition efficiencies and beta. A change from sitting to light-work breathing conditions causes only insignificant modifications of total fiber deposition in the HRT, whereas alveolar and, above all. tubular deposition of fibrous particles with a diameter >= 0.1 mu m are affected remarkably. For these particles enhancement of the inhalative flow rate results in an increase of the extrathoracic and bronchial deposition fractions. Concerning the clearance of fibers from the HRT, 24-h retention is noticeably influenced by beta and, not less important, by the preferential deposition sites of the simulated particles. The significance of beta with respect to particle size may be regarded as similar to that determined for the deposition scenarios, while breathing conditions do not have a valuable effect on clearance. (C) 2009 Elsevier B.V. All rights reserved.