Corona discharge based unipolar ionizers are commonly used for reducing aerosol particle concentrations in indoor environments. The ions (usually negative) emitted from the ionizer help in charging the airborne particles, which are then removed from air space by electro-migration effects to the walls of the room due to the space charge induced electric fields. A system of equations has been formulated by considering various processes responsible for aerosol removal in the presence of the ionizers. These pertain to the space charge generated electric fields, particle charging, ion and particle removal rates under the combined action of diffusion, gravitational sedimentation, external ventilation and electric fields. The study includes steady-state situations corresponding to a continuous injection of particles as well as time dependent depletion of an initially injected aerosol. Model computations have been made to examine the dependence of the particle depletion characteristics and concentration reduction factors (CRFs) on particle sizes, concentrations and external ventilation rates. The ionizer induced concentration reduction factor is found to increase at higher particle concentrations. However, it is considerably reduced in the presence of even a small amount of external ventilation. The model predictions are found to compare favourably with the results of some of the experimental studies reported by earlier investigators as well as that carried out as a part of the present study. (C) 2004 Elsevier Ltd. All rights reserved.