National Institute for Occupational Safety and Health (NIOSH)-certified N95 filtering facepiece respirators (FFRs) are used for respiratory protection in some workplaces handling engineered nanomaterials. Previous NIOSH research has focused on filtration performance against nanoparticles. This article is the first NIOSH study using human test subjects to compare N95 FFR faceseal leakage (FSL) performance against nanoparticles and "all size" particles. In this study, estimates of FSL were obtained from fit factor (FF) measurements from nine test subjects who participated in previous fit-test studies. These data were analyzed to compare values obtained by: 1) using the PortaCount Plus (8020A, TSI, Inc., MN, USA) alone (measureable particle size range 20 nm to > 1,000 nm, hereby referred to as the "all size particles test"), and 2) using the PortaCount Plus with N95-Companion(TM) accessory (8095, TSI, Inc., Minn.) accessory (negatively charged particles, size range ∼40 to 60 nm, hereby referred to as the "nanoparticles test"). Log-transformed FF values were compared for the "all size particles test" and "nanoparticles test" using one-way analysis of variance tests (significant at P < 0.05). For individual FFR models, geometric mean (GM) FF using the "nanoparticles test" was the same or higher than the GM FFs using "all size particles test." For all three FFR models combined, GM FF using the "nanoparticles test" was significantly higher than the GM FF using "all size particles test" (P < 0.05). These data suggest that FSL for negatively charged ∼40-60 nm nanoparticles is not greater than the FSL for the larger distribution of charged and uncharged 20 to > 1,000 nm particles.