Deposition of element-specific particulate matter in the respiratory system of a Caucasian-type healthy adult male and female was computed by a stochastic lung deposition model for different reference levels of physical exertion using mass size distributions in the aerodynamic diameter interval of 0.125-16 mum experimentally determined in urban environments. Particles with an aerodynamic diameter smaller than about 0.3 mum are deposited in the whole respiratory system decreasingly with rising physical exertion, while the opposite is observed for particles with an aerodynamic diameter larger than about 0.7 mum (except for the highest physical activity). It is the light exercise that causes the largest extrathoracic (ET) deposition efficiency of the particles in this last diameter range, and, consequently, the smallest tracheobronchial and acinar depositions. The results obtained indicate that ET deposition depends primarily on the size distribution of the inhaled particles, while physical exertion plays a minor role. In contrast, deposition fractions of different aerosol species in the lungs are very similar to each other for a given physical exertion, despite the rather diverse size distributions of some species, but depend significantly on the subject's physical exertion. The differential deposition curves generally exhibit two peaks, one in the tracheobronchial and one in the acinar region. Both differential and regional deposition fractions do not change in a monotonical fashion with physical exertion but display maximum values approximately at the exertion level corresponding to the switching point between the nose-breathing mode and the combined nose- and mouth-breathing mode. Deposition rates (mass doses), however, increase monotonically with physical exertion due to increased ventilation rates, and more particles reach the deeper parts of the lung. (C) 2002 Elsevier Science Ltd. All rights reserved.