Currently available nasal filters are not well-suited for protecting humans against the fine and ultrafine airborne particles. In this research, we designed and evaluated a novel nanofiber nasal filter (NNF) capable of reducing personal exposure not only to large allergenic particles but also to ultrafine particles, thus reducing respiratory health risks. A new hybrid filter (HF) medium for the NNF was fabricated by overlaying a carbon filter substrate with nylon nanofibers produced by electrospinning. After optimizing the filter's production parameters, the HF was produced using the Nylon-6 polymer solution with a concentration of 15 wt%, a substrate based on a MERV 5 carbon filter with a density of 61 kg m(-3), and a nanofiber surface coating density of 0.72 g m(-2) (or 0.54 g m(-2) as a second choice). The new HF was tested with fluorescent polystyrene latex beads sized 0.026-3.1 mu m and at operating flow rates of 7.5-30 L min(-1). The newly developed NNF showed more than a 90% collection efficiency for particles > 1 mu m, representing bacteria and molds, and more than a 50% efficiency for particles < 0.5 mu m, including ultrafine particles-about a 2.3-fold improvement compared to commercially available nasal filters. Thus, this NNF may serve as a useful tool to minimize our exposure to airborne pollutants.