A 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

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... at mouth breathing. Calculated deposition in the alveolar region of the lung from this model compared favorably with the postmortem data. INTBODOCTION Deposition of fibrous particles in the human respiratory tract is a very complicated process because of the sequential branching of the airway passages, the change in the orientation of individual passages, and the complex behavior of long fibrous particles in this system. There are only a few theoretical studies made to date wl\ich address this problem. Beeckmans (1972) extended the deposition results of spherical particles to fibrous ones by assuming that the particles were randomly oriented in the airways at all times. Be showed that the deposition result for fibrous particles could not be derived directly from that for spherical ones by means of an equivalent spherical particle diameter because this diameter differs depending on the mechanism of deposition. Further, in Beeckmans' analysis, particle removal from the nasopharyngeal air stream and interception deposition at the bifurcation sites were neglected. These loss mechanisms are important for large and long fibers. Another very extensive deposition model for fibrous particles was developed by Barris and Fraser (1976) , using existing and new formulae for calculating the deposition of fibrous particles in an airway due to different mechanisms. The particles were assumed to be oriented either with their major axes along the flow or randomly in space, depending upon the flow condition and the strength of particle Brownian motion. They also derived a deposition formula in the nasopharynx by considering impaction deposition and interception deposition by nasal