To examine the cause of reduced performance, the surface potential of the filter web was measured (Figure 3 and Figure S6). The surface charges of the filter media are not consistently positive or negative; instead, both positive and negative charges can exist simultaneously, compensating the overall charges on the filter surface [45]. Therefore, for such cases, the average surface potential over an area can be less meaningful than the variation of potential. In Figure 3, the surface potential across the horizontal line was measured by line-scanning, and the fluctuation of the voltage values was observed as an important parameter. The surface potential of the untreated filter webs from respirators A and B ranged from −1.6 kV to +4.4 kV, and these surface charges contributed to particle capture either by coulombic attraction or induced polarization [46]. When the electret media was exposed to IPA, EtOH, and detergent solution, the range of surface potential was considerably reduced. In particular, IPA-treated respirator exhibited nearly 0 kV invariably across all areas, clearly indicating the loss of surface potential. The EtOH or detergent-treated respirators showed a slightly larger variation than the IPA-treated ones, indicating that surface charges may not be completely lost by those treatments. The remaining charges would contribute to electrostatic filtration mechanism, as indicated by the higher filtration efficiency of EtOH and detergent-treated respirators compared to that of the IPA-treated ones (Figure 2).