Principle of microporous air permeability. Liquid water has a strong molecular hydrogen bond, so the molecular formula of its associating molecules is about (H2O)8 at room temperature, while water molecules in the gaseous state are smaller, and its molecular formula is H20. According to this principle, a unique porous plastic film has been designed. The pore size of the film can penetrate the small volume of water vapor molecules H20, but not enough to associate water molecules (H2O)8 through the large volume of liquid. This kind of film can be used as packaging film for sanitary products, fruits and vegetables, and can also be used for making breathable raincoats, gloves and easy-to-handle diapers.