Particulate suspensions inhaled by humans are typically dilute and hence interactions between particles can be neglected. In such cases conventional Euler-Lagrange or Euler-Euler methods are suitable to simulate micron- or nano-particle transport and deposition in human respiratory systems. However, when challenging conditions, such as large pressure differentials, high velocity gradients and/or intense particle collisions, exist, alternative approaches for numerical analysis are required to capture fluid-particle, particle-particle, and particle-wall interactions. In the present study, the dense discrete phase model (DDPM) in conjunction with the discrete element method (DEM) have been employed to simulate micron-particle transport, interaction and deposition dynamics in different triple bifurcations (i.e., G3-G6, G6-G9, and G9-G12), using ANSYS Fluent 14.0 enhanced by user-defined functions (UDFs). In light of the relatively high computational cost when employing DDPM-DEM for such simulations throughout the human respiratory system, it may be necessary to combine different computational fluid-particle dynamics (CF-PD) models based on the local intensity of particle-particle interactions. Thus, the validity and necessity of the DDPM-DEM approach for different lung airway generations were numerically investigated, considering new parametric criteria for the use of most suitable numerical models. Specifically, the relative intensities of three major particle deposition mechanisms (i.e., inertial impaction, secondary-flow effect, and particle-particle-interaction impact) in idealized lung-airway segments were investigated. As a result, a new criterion for CF-PD model combination in terms of a relationship between inlet-particle stacking-volume fraction, phi, and percentage-of-fate changing particles, Delta beta(p), is proposed. Visualizations of the fluid-particle dynamics in bifurcating airways have been provided as well. Results of this study pave the way for accurate and cost-effective CF-PD simulations of lung-aerosol dynamics, aiming at the improvement of respiratory dose estimation for health risk assessment in case of toxic particles and for treatment options in case of therapeutic particles. (C) 2014 Elsevier Ltd. All rights reserved.