## 3 Results and Discussion  
Figure 1b shows a photograph of the synthesized PVA NF membrane peeled off from the nonwoven fabric substrate (here, the electrospun PVA NF-based filter was not dried). Figure 1c shows the surface FE-SEM images of a nonwoven fabric substrate and a synthesized PVA NF-based filter. As shown in the figure, the pore size was effectively reduced by the formation of PVA NFs with an average diameter of \(\sim\)150 nm on a nonwoven substrate with an average diameter of \(\sim\)20 \(\mu\)m. This means that the pore size of the PVA NF-based filter could be adjusted by controlling the electrospinning time; thus, the limit of the PM size collected by the filter can be reduced (see Figure S3) [1; 13]. Figure 2a shows the reflectance spectrum in the visible region (wavelengths from approximately 400 to 800 nm) of the nonwoven fabric substrate before and after PVA NF formation. The inset shows the appearance of the samples. As shown in the graph, the reflectance of the nonwoven fabric substrate significantly increased after the formation of PVA NFs. In fact, the nonwoven fabric substrate appeared brighter after the PVA NF formation. This is because the PVA NFs accumulate on the nonwoven fabric substrate, reducing the pore size and increasing the physical thickness, resulting in a decrease in the amount of light transmitted through the sample [15; 19]. Table 1 shows the measured average fiber diameter, filtration efficiency (\(E\)), pressure drop (\(\Delta P\)), and calculated quality factor (QF) of the nonwoven fabric substrate, PVA NF-based filters, and commercial air filters used in this study. The average fiber diameter of the prepared PVA NF-based filter was just one-hundredth that of the commercial air filter media. This implies that the PVA NF-based filter has a much larger specific surface area than the commercial air filter media, which means that it can have a larger dust-holding capacity [16]. Additionally, the filtration efficiency and pressure drop of the PVA NF-based filter increased with increasing electrospinning time [13]. Here, the QF of the optimized PVA NF-based filter (electrospinning time: 300 s) was 0.010606, which is lower than that of a HEPA filter (0.015394), but higher than that of a cabin air filter (0.010517) and a dust mask (0.009102). This shows that the fabricated PVA NF-based filter has a performance level capable of practical use. Figure S4 demonstrates the use of a PVA NF-based filter to block high-concentration PM pollution. The right glass bottle contained a hazardous level of PM with a concentration higher than 1000 \(\mathrm{\SIUnitSymbolMicro g}\)/m\({}^{3}\), and a PVA NF-based filter with a quality factor of 0.01060 \(\mathrm{Pa}^{-1}\) (filtration efficiency of ~86.81% and pressure drop of ~191 Pa) was placed between the two glass bottles. The PVA NF-based filter successfully blocked the PM moving from the right to left glass bottle. The PM concentration in the left glass bottle was safe (<30 \(\mathrm{\SIUnitSymbolMicro g}\)/m\({}^{3}\)). This shows that the PVA NF-based filter can be used as a dust mask and window screen that can block the inflow of PM [15; 19]. Figure 2: (**a**) Reflectance spectrum of nonwoven fabric and poly(vinyl alcohol) nanofiber (PVA NF)-based filter; (**b**) photograph of the device for the PM removal performance test in polycarbonate chamber; (**c**) comparison of the chamber test results of commercial nonwoven fabric- and PVA NF-based filters. Figure 2b shows the test equipment used in this study for removing the PM particles in the chamber, comprising an air purifier equipped with a filter, and Figure 2c shows the experimental results. As shown in the experimental results, the performance of the air purifier with a PVA NF-based filter was superior to that of the dust mask filter and comparable to that of the commercial HEPA filter. This result shows that the PVA NF-based filter can be used as a semi-HEPA filter for air purifiers [13,16].