Transport of 0.5-5 mum diameter particles was simulated within a symmetric two-dimensional (213) six-generation structure representative of the human acinus. Aerosol boluses were introduced at the beginning of a breathing cycle.(2-s inspiration, 2-s expiration). Airflow corresponded to a flow rate at the mouth of 500 ml/s. Deposition increased from 1% to 100% for 0.5-5 mum diameter particles. Deposition agreed with the upper limit of alveolar deposition predicted by the one-dimensional (ID) ICRP66 model but only after accounting for multibreath. The 2D model showed that, for each particle size and structure orientation, there was a large heterogeneity in deposition among ducts. Differences of more than one order of magnitude were found between deposition in a single duct of a given generation and the average deposition in that generation. The presence of "hot spots" may have important implications in human health as several studies suggest a strong correlation between airborne pollutants and the onset of pulmonary or cardiovascular diseases. (C) 2002 Elsevier Science Ltd. All rights reserved.