Steady 3-D airflow and scalar transport of ultrafine particles, d(p) < 0.1 μm, and fuel vapors within the human upper airways are simulated and analyzed for laminar as well as locally turbulent flow conditions. Presently, our respiratory system consists of two major segments of a simplified human cast replica, i.e., a representative oral airway from mouth to trachea (Generation 0) and a symmetric four-generation upper bronchial tree model (G0-G3). The simulation has been validated with experimental data in terms of ultrafine particle deposition efficiencies. The present computational results show the following: (1) At low breathing rates (Q(in) &AP;15 l/min), ambient temperature variations (&UDelta;T-max = 47 &DEG;C) influence the local velocity fields and vapor concentrations, however, the total and segmental deposition fractions of fuel vapor in the upper airway are essentially unaffected. (2) The inlet flow rate has a significant effect on vapor deposition, i.e., the higher the flow rate the lower the deposition fraction. (3) The convective heat transfer coefficient averaged over an individual bifurcation unit can be correlated as Nu = 0.568(RePr)(0.495) (600 < Re < 6000). (4) Two new Sherwood number correlations capture the convective mass transfer for the oral airway and individual bifurcations.The methodology outlined and physical insight provided can be also applied to other intake configurations, such as engine ports and inlets to air-breathing propulsion systems. (C) 2003 Elsevier Ltd. All rights reserved.