Sheet of microfiber assembly

An air-permeable sheet of microfiber assembly comprising:

• microfibers, said microfibers being a mixture of water-insoluble but ethanol-soluble acetalized polyvinyl alcohol resin and water-insoluble but ethanol-soluble fluorine resin,
• wherein
• the microfibers comprise a matrix of the polyvinyl alcohol resin and the fluorine resin is blended into the matrix,
• said air-permeable sheet of microfiber assembly is an air-permeable filter having a filtering property for oil mist,
• each microfiber is obtained by electro-spinning an alcohol solution of said acetalized polyvinyl alcohol resin and said fluorine resin, wherein each microfiber has a fiber diameter in a range of 30 nm to 800 nm,
• said air-permeable sheet of microfiber assembly is laminated on an air-permeable nonwoven fabric having air permeability higher than that of said air-permeable sheet of microfiber assembly,
• said air-permeable nonwoven fabric contains thermoplastic synthetic fibers and has a basis weight of 20 to 50 g/m2, and
• individual fibers or a fiber bundle of the thermoplastic synthetic fibers has a flat radial cross-sectional shape to define a smooth surface of the air-permeable nonwoven fabric, the individual fibers being aligned in one direction.