Current models of submicron particle transport and deposition often ignore particle inertia for aerosols smaller than 200 nm. In the absence of inertial effects, a highly efficient Eulerian transport model can be applied that treats the particle phase as a dilute chemical species. However, the effects of inertia have not been fully quantified for aerosols in the fine and ultrafine ranges. The objective of this study is to evaluate conditions for which current chemical species Eulerian and Lagrangian particle transport models can be applied in order to predict submicron particle deposition characteristics on a regional and local basis in upper and central respiratory models. Differences between the chemical species Eulerian and Lagrangian model results have been used to evaluate conditions for which particle inertia becomes important relative to diffusional effects. The deposition characteristics of particles ranging front 5 nm to I pm have been evaluated in a tubular entrance flow geometry, a double bifurcation model of upper respiratory generations G3-G5 and a double bifurcation model of central respiratory generations G7-G9. Considering the regional area-averaged deposition of submicron aerosols, the minimum particle diameters (and Stokes numbers) for which particle inertia became significant were approximately 20 nm (St = 6.1 x 10(-6)) for the tubular entrance flow geometry, 70 nm (St = 5.1 x 10(-5)) for the upper bifurcation model, and 140 run (St = 4.4 x 10(-5)) for the central bifurcation model. Below these critical particle diameters, numerical estimates of regional deposition were shown to be consistent with currently available analytic correlations of diffusional deposition efficiencies. In comparison to regional-averaged values, the effects of particle inertia on localized deposition characteristics were found to be much more dramatic. For the upper airway bifurcation model, inclusion of particle inertia increased the maximum local microdosimetry factor by one order of magnitude for 40 nm particles at an inhalation flow rate of 30 L/min. Results of this study indicate that particle inertia may be more significant in regional and local depositions of fine and ultrafine aerosol than previous considered. Therefore, effective models of particle transport are necessary that can maintain the efficiency of the chemical species Eulerian approach while accounting for local finite particle inertia. (C) 2006 Elsevier Ltd. All rights reserved.