Multilevel composite structures give an insight into highly integrated and efficient filtration media for various applications, including personal, industrial, and environmental protection, because of their fascinating features of controllable pore size, gradually varied pore structure, and high porosity. In view of this novel design strategy, Zhang et al. reported the fabrication of highly integrated multilevel PSU/PAN/PA-6 hybrid fibrous membranes to capture airborne particles from polluted air via the sequential electrospinning process, as shown in Fig. 12.13. Each layer of the integrated PSU/PAN/PA-6 composite membranes possessed the particular diameters and pore sizes for removing particles of different diameters. The PSU microfiber layer, with a fiber diameter of w1 mm and pore size of w2.2 mm, was employed to filter particles with a diameter of >2 mm; the PAN nanofiber layer, with fiber diameter of w200 nm and pore size of w0.55 mm, was used to filter >0.5-mm particles; while the PA-6 nanonet could remove w300-nm particles because of their unique 2D Steiner tree structures with extremely small pore size of 0.27 mm. These orderly assembled layers, with varying diameters, different ranges of pore size, and high porosity, enabled the composite membrane filters to work efficiently and avoid blockage of the pore structures by gradually screening particles with certain sizes. Benefiting from the gradient structure, PSU/PAN/PA-6 membranes can remove 300-nm NaCl aerosol particles with a high efficiency of 99.992%, the low air resistance of 118 Pa, and high-quality factor value, using physical sieving. Furthermore, these composite membranes successfully got rid of the impact of electret failure and high humidity and displayed robust mechanical (tensile strength of 5.6 MPa) and hydrophobic properties (water contact angle w130 degrees), which made them promising candidates to be used in a broad range of applications for filtration and separation devices.