Influence of aerosol electrical charging state and time of use on the filtration performance of some commercial face masks for 10–150 nm particles

Abstract

Abstract The filtration performance of three types of commercially available face masks (hygienic, surgical, and FFP2) has been evaluated for aerosol particle size in the range 10–150\xa0nm at a fixed face velocity of 9.5\xa0cm/s. Two parameters have been varied in this study: the time of use of the mask, and the electrical charging state of the particles. Mask aging has been carried out by wearing it for a prescribed period of time. Four different charging states of NaCl particles generated by evaporation-condensation have been examined: positively charged particles, negatively charged particles, uncharged particles, and a mixture of the three former types (the latter referred to as “mixed aerosol”). Aerosol charging was carried out with a low activity radioactive source so that most of the charged particles carried a single charge of either sign in all instances. The charging state of the aerosol exerts a considerable effect on filtration efficiency for the three types of masks. Highest filtration efficiencies are attained for positively and negatively charged particles, but polarity seems to play a role: some masks capture more efficiently particles of a given polarity, although differences in the capture efficiencies of positively and negatively charged particles are insignificant in comparison with those observed between charged and uncharged particles. Uncharged particles give the lowest efficiency and the mixed aerosol leads to filtration efficiencies between those of charged and uncharged particles. The time of use of the mask has also a great influence on its filtration performance: while hygienic masks give poor results from the very beginning, one of the two FFP2 masks assayed has shown a good performance even after 24\xa0h of use, and the surgical mask also performed well after 8\xa0h. The relative importance of mechanical (diffusion and or interception) and electrostatic particle deposition, as a function of particle diameter and aging time, has been estimated and discussed. The electrostatic mechanism generally dominates for small particles and short aging times. Pressure drop across the mask has also been measured: the largest pressure drop was observed for the FFP2 and the surgical masks. The time of use of the masks did not affect the pressure drop. Lack of a good fit between the mask and the face of the wearer drastically reduces the actual filtration efficiency of the mask because part of the incoming aerosol bypasses the filtering medium. A few additional efficiency measurements were done after cutting off a small surface area of the mask sample.