Performance evaluation of middle-layer micrometer and nanometer fibers on quality factor of five-ply commercial respirators and surgical masks

In the recent COVID-19 pandemic, N95 Filtering Facepiece Respirators (FFRs), surgical masks, and other protective face coverings played a key role in limiting disease transmission. FFRs and surgical masks were generally composed of three- and five-layered designs with differing middle layer compositions. The middle layer, also known as the filtering layer of a three-layer respirator, is responsible for overall Particle Filtration Efficiency (PFE) and has a reasonable Quality Factor (QF). Despite this, limited studies have explored how middle-layer composition affects the QF of a five-layer respirator. Therefore, in the present study, the performance of three five-layer respirators (N95) and a surgical mask was evaluated for QF. Three five-layer respirators and a surgical mask were tested for PFE and pressure drop at face velocities from 5 to 25 cm sec^-1. Subsequently, the respirator and individual layer QFs were determined by the Single Fiber Filtration Efficiency (SFFE) model and layer analysis. The PFE of the tested respirators was distinct due to different filtration mechanisms acting on particles with diameters of 100 and 300 nm. Efficiency due to fiber charging was dominant at 100 nm, whereas interception was dominant at particle diameters of 300 nm. However, variations in pressure drop were drastic at higher face velocities, resulting in significant variations in QF. Solidity and fiber diameter were lower in the middle layers than in the outer and inner layers. The QF calculated for individual and composite layers (referring to layers of nano- and micro-fibers) showed that the middle layer improved QF compared to the combined inner and outer layers. In addition, charged micrometer-sized fibers improved QF for particle sizes < 100 nm due to electrophoretic forces, while the presence of nanofibers improved QF for particle sizes > 100 nm due to the early onset of interception and impaction filtration mechanisms. The most Penetrating Particle Size (MPPS) analysis concluded that experimentally determined MPPS was mainly dependent on the middle layer, which was an indication that MPPS was influenced by fiber diameter and the presence of charged fibers.