In this study, trajectories of inhaled particulate matter (PM) were simulated within a three-dimensional (3-D) computer model of the human upper respiratory tract (URT). The airways were described by computer-reconstructed images of a silicone rubber cast of the human head, throat, and trachea and main bronchi. Computational fluid dynamics simulations of airflow patterns were performed using commercially available software (CFX-F3D) on a Sun Sparc-5 workstation. For each simulation, particles were introduced within a nostril grid and their trajectories calculated. A typical computer run of 400 iterations took 25 h. Particle deposition was designated in nasal (N), laryngeal (L), and tracheobronchial (T) regions, or penetration (P) of the URT. Deposition was calculated as a function of particle size (0.5-5 mum), density (1 and 2 g/cm(3)), and flow rate (9, 17, and 34 l/min). The computations should be addressed on a case-by-case basis as detailed herein; however, it can be stated that in a given N, L, or T region, deposition fractions as determined by the calculation of trajectories increased more with flow rate than particle size. This knowledge of factors affecting particle trajectories and deposition patterns may have important implications for PM risk assessment programs. Published by Elsevier Science Ltd.