Previously, we demonstrated that electrospraying, driven by high-intensity electric fields, can be used to generate stable antimicrobial nanoparticles continuously from ethanolic extracts of natural plant products. In this study, we evaluated the performance of antimicrobial air filters made by these electrosprayed natural-product nanoparticles. As a natural antimicrobial suspension, an ethanolic extract of Sophora flavescens Alt. was sprayed in steady cone-jet mode using our specially designed electrospray system (Jung et al., 2011b). Natural-product nanoparticles with a bimodal size distribution were electrosprayed and deposited continuously onto the surfaces of air filters at various concentrations. The physical (filtration efficiency, pressure drop, and fiber morphology) and biological (antimicrobial efficacy against airborne Staphylococcus epidermidis) characteristics were evaluated. Consequently, although the filter pressure drop increased with the amount of nanoparticles on the filter, the antimicrobial activity was enhanced. Compared with the conventional nebulization process, the filters generated by electrospraying S. flavescens natural-product nanoparticles resulted in more effective removal of S. epidermidis bioaerosols. These results will facilitate the implementation of this new technology to control air quality and protect against hazardous airborne microorganisms. (C) 2012 Elsevier Ltd. All rights reserved.