ABSTRACTA theoretical model was developed to calculate the deposition of inhaled fibrous particles in the human lung. In the derivation of deposition formulae, the simultaneous effects of the velocity shear in the air flow and Brownian rotation on particle orientation were considered. Total deposition of fibers in the lung at mouth breathing was found to be smaller than that of spherical ones with the same mass, and deposition in the lung at nose breathing showed considerably lower deposition than that at mouth breathing. Calculated deposition in the alveolar region of the lung from this model compared favorably with the postmortem data.