The air vent valve has two important indexes: air permeability and protection level. Its working principle is as follows: the air vent valve is carefully manufactured with expanded polytetrafluoroethylene (ePTFE) microporous membrane. The micropore diameter of the ePTFE membrane is between 0.1 to 10μm, while the gas molecule is only about 0.0004μm. The pore diameter of the ePTFE membrane is 250-25000 times larger than that of the gas, so the gas can pass through smoothly. The diameter of drizzle is 400μm, which is 40-4000 times larger than the pore size of the film. In addition, because the surface energy of ePTFE film is very low, with the contact angle is 135.6°and under the action of surface tension of water molecules, water vapor condenses into small water droplets and forms large water droplets on the surface of ePTFE membrane, which can effectively prevent liquid water wetting and capillary penetration, so it has good waterproof and air permeability performance.