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As COVID-19 pandemic has caused more than 24 million confirmed cases globally (as of August 28 th , 2020), it is critical to slow down the spreading of SARS-CoV-2 to protect the healthcare system from overload. Wearing a respirator or a mask has been proven as an effective method to protect both the wearer and others, but commercially available respirators and masks should be reserved for healthcare workers under a currently desperate shortage. The use of alternative materials becomes an option for the general public to make the do-it-yourself (DIY) masks, with their efficacy seldom reported. In this study, we tested commercial respirators and masks, furnace filters, vacuum cleaner filters, and common household materials. We evaluated the materials' fractional filtration efficiency and breathing resistance, which are primary factors affecting respiratory protection. To compare the efficiency-resistance tradeoff, the figure of merit of each tested common material was also calculated. Filter media with electrostatic charges (electret) is recommended due to its high efficiency with low flow resistance; multiple-layer household fabrics and sterilization wraps are acceptable materials; a coffee filter is inadvisable due to its low efficiency. The outcome of this study can not only offer guidance for the general public under the current pandemic but also suggest the appropriate alternative respiratory protection materials under heavy air pollution episodes.

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We developed two types of high performance nanofiber filters by increasing the uniformity of the fiber layer's deposition on the substrate media and by fabricating nanofiber with a branched morphology, in which additional fibers were spun to radiate from the main fibers. We subsequently compared them with conventional and beaded nanofiber filters in terms of particle removal efficiency, filter pressure drop, and particle loading capacity. First, the uniformity of the nanofiber's deposition, which was visually evaluated using optical and electron microscopes, was increased by removing the substrate surface charge during electrospinning. The uniform nanofiber filters demonstrated a higher filtration efficiency with a lower pressure drop than their conventionally electrospun counterparts, which exhibited irregular fiber layers. Second, branched nanofiber was fabricated by adjusting the viscosity of the polymer solution. The performance of the nanofiber filters was dramatically improved by the decrease in air flow resistance, which resulted from (1) a longer distance streamwise between the fibers and (2) a smaller diameter for the fibers. The enhanced efficiency was also due to (3) the additional fibers branching out from the main fibers.