While heating, ventilating, and air-conditioning (HVAC) systems can provide healthy and comfortable indoor environments, virtually any part of an HVAC system can support active microbial growth if sufficient nutrients are present. In this study, we introduced a methodology to enhance the one-pass antibacterial performance of air ions against aerosolized bacteria in a ventilation duct flow. Staphylococcus epidermidis (S. epidermidis) was aerosolized, mixed with the duct flow, and exposed to air ions generated by carbon fiber ionizers installed inside the duct. The S. epidermidis was then sampled at the exit of the duct and incubated to evaluate their cultivability as functions of the ion exposure time, ion concentration, and ion polarity. When the ionizers produced bipolar air ions for 2 s, a high antibacterial efficiency of 85% was obtained when four ionizers were positioned both at the top and bottom walls of the duct in a configuration in which there were three changes in ion polarity (one positive ionizer, one negative ionizer, one positive ionizer, and one negative ionizer in series along the flow direction). When the ion exposure time was decreased to 0.2 s, an antibacterial efficiency of 50% was realized by applying a configuration with seven changes in ion polarity. By using a scanning electron microscope (SEM), cell contraction of S. epidermidis caused by the bipolar ion treatment was observed. (C) 2013 Elsevier Ltd. All rights reserved.