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 nm at a fixed face velocity of 9.5 cm/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 h of use, and the surgical mask also performed well after 8 h. 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.

已经评估了三种市售口罩（卫生型、外科型和 FFP2）的过滤性能，其气溶胶粒径范围为 10-150 nm，固定面速度为 9.5 cm/s。在这项研究中改变了两个参数：口罩的使用时间和粒子的充电状态。面膜老化是通过佩戴规定的时间来进行的。已经检查了蒸发冷凝产生的 NaCl 颗粒的四种不同充电状态：带正电的颗粒、带负电的颗粒、不带电的颗粒以及前三种类型的混合物（后者称为“混合气溶胶”）。气溶胶充电是用低活度放射源进行的，因此大多数带电粒子在所有情况下都带有任一符号的单一电荷。气溶胶的荷电状态对三种口罩的过滤效率都有相当大的影响。对于带正电和带负电的粒子，获得了最高的过滤效率，但极性似乎起了作用：一些口罩可以更有效地捕获给定极性的粒子，尽管与观察到的相比，带正电和带负电的粒子的捕获效率差异微不足道带电粒子和不带电粒子之间。不带电粒子的过滤效率最低，混合气溶胶导致带电粒子和不带电粒子的过滤效率之间。口罩的使用时间对其过滤性能也有很大影响：虽然卫生口罩从一开始就效果不佳，但经过检测的两个 FFP2 口罩中的一个即使在使用 24 小时后也显示出良好的性能，而外科口罩面膜在 8 小时后也表现良好。已经估计和讨论了机械（扩散和/或拦截）和静电颗粒沉积的相对重要性，作为颗粒直径和老化时间的函数。对于小颗粒和较短的老化时间，静电机制通常占主导地位。还测量了面罩上的压降：FFP2 和外科面罩的压降最大。口罩的使用时间不影响压降。由于部分进入的气溶胶绕过过滤介质，面罩与佩戴者面部之间缺乏良好的贴合会大大降低面罩的实际过滤效率。在切断面罩样品的一小部分表面后，进行了一些额外的效率测量。