The aim of this work is to develop a mathematical multi-plume multi-layer transport model of active particle behavior in spaces ventilated by a displacement ventilation (DV) system in order to study cross-infection between occupants in typical internal offices. The developed model incorporates particle deposition on walls and the effect of gravitational settling on particles distribution. The model was validated using published data from the literature revealing that the current simplified model is able to capture the physics of the problem and predict particle concentration and transport at low computational cost. The model results show that as the particle diameter increases, the gravitational settling increases, thereby lowering the stratification in particle concentration created by the DV system and thus increasing the particle concentration at the breathing level of the exposed person. For a flow rate of 60 L/s, this effect remains until reaching a particle diameter above 10 mu m where deposition on the floor opposing the DV principle acts as a removal factor. For the critical inhalable range, as the diameter increases, gravitational settling accumulates particles in the occupied zone, thereby increasing the probability of cross-infection. To overcome the settling effect, higher ventilation air flow rates are recommended to provide good indoor air quality (IAQ). (C) 2014 Elsevier Ltd. All rights reserved.