## 3 Results and Discussion
### Filtration Efficiency  
Figure 5 shows the filtration efficiency of materials at different face velocities for PM2.5, PM1, and particles in \(\mathrm{d_{p}}\) = 0.3-0.52 \(\mathrm{\upmu m}\) range. The certified masks had the highest efficiency of all particle sizes; P100 and N95 had 100% filtration for all particle channels and tested face velocities. Surgical mask filtration was lower, especially for particles smaller than 0.5 \(\mathrm{\upmu m}\); however, the surgical mask outperformed all other tested materials for homemade masks. The high FE of the certified materials for the ultrafine particles is due to electrostatically charged fibers densely woven by the melt-blown extrusion process. The mechanical filtration mechanisms (e.g., inertial deposition, interception, and diffusion) are combined with electrostatic deposition to filter both large and small particles. Commonly used textiles, such as cotton and cotton blends, had filtration efficiency below 10%. More dense Kona cotton approached 25% FE; however, its pressure drop was 3 to 10 times greater than other textiles and about the same as the surgical mask. Synthetic materials showed increased FE at lower face velocities for smaller particles. This trend indicates that the primary filtration mechanisms for PM1 are diffusion and electrostatic attraction, as synthetic fibers are likely to carry permanent electrostatic charges or be charged by triboelectrification, which improves the deposition of small particles [27; 68; 69]. However, certified multilayer material such as surgical masks has clear advantages due to the removal of moisture in the outer layer and the strong attraction of electrostatically charged fibers. Natural fiber textiles did not show this trend, and in the case of the paper coffee filter, higher velocity resulted in higher filtration for all particle sizes.  
Figure 6 shows the FE for multiple-layered materials. As the aerosol sizes increase, the effects from interception and inertial impaction filtration mechanisms begin to appear, increasing the FE with increasing face velocity. The trend between FE and face velocity seen by the single-layer materials continued as additional layers were added. Additional layers provided a higher FE, as seen by the surgical mask, which required two layers to reach a consistent FE above 95%. These results represent the FE when a mask tightly fits the wearer with no leakage. Masks that are not properly fitted would not be able to provide the same level of FE, making the fit of masks a vital component to ensure maximum protection for the wearer [27; 40; 70].  
Figure 4: Pressure drop for single-layered materials as a function of RH. Bar colors represent the corresponding RH value, and results are plotted as the average pressure drop from three runs, with the error bars representing the standard deviation.  
Figure 5: FE results for single-layered materials as a function of face velocity. Bar colors represent the corresponding face velocity values, with the results plotted at the average FE from three experimental runs with the error bars representing the standard deviation.  
Figure 6: FE results for layered materials as a function of face velocity. Bar colors represent the corresponding face velocity values; the results are plotted at the average FE from three measurements, with the error bars representing the standard deviation.