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Computational fluid dynamics (CFD) simulations of airflow and particle deposition in geometries representing the human tracheobronchial tree were conducted. Two geometries were used in this work: (1) based on the Weibel A model, and (2) based on a CT scan of a cadaver lung cast. Flow conditions used included both steady state inhalation and exhalation conditions as well as time dependent breathing cycles. Particle trajectories were calculated in each of these models by solving the equations of motion of the particle for the deterministic portion of particle displacement, and adding a stochastic Brownian term at each step. The trapping of particles on the wall surfaces was monitored, and the locations of trapping in each generation were recorded. The results indicate that for simple steady state inhalation conditions, there is not much difference between the Weibel A model and the more realistic CT-scan based model. However, under time dependent flow conditions, as well as under exhalation conditions, there are dramatic differences in the predicted deposition between the two models. The results of this study suggest that under steady state inhalation conditions, a lung geometry based on the Weibel A model may be sufficient to predict deposition accurately, while under any other breathing conditions, a more realistic geometry may be required.