Air filter medium, pleated filter medium, air filter unit, mask filter medium, and method for regenerating air filter medium

A mask comprising:

• a base including an intermediate opening;
• straps provided to the base;
• and
• a pleated filter medium attached to the intermediate opening of the base, wherein
• the pleated filter medium has a shape including a mountain fold portion and a valley fold portion,
• the pleated filter medium, in a position attached to the intermediate opening of the base, comprises an area that is greater than or equal to 22 cm2 and less than or equal to 61 cm2 when viewed in the air flow direction,
• a pleat interval is 2.0 mm or more and 4.5 mm or less,
• the pleated filter medium has an air filter medium,
• the air filter medium comprises a fluororesin porous membrane and a supporting member stacked on top of each other,
• wherein a pressure loss of the fluororesin porous membrane, when air is passed at a flow rate of 5.3 cm/s, is 40 Pa or more and 80 Pa or less, and
• the air filter medium has a total area of 200 cm2 or more and 1300 cm2 or less,
• wherein a permeability ratio (permeability after disinfection treatment/permeability before disinfection treatment), which is a ratio of a permeability of the air filter medium after a disinfection treatment of the fluororesin porous membrane as determined using NaCl particles having a particle size of 0.1 μm to a permeability of the air filter medium before the disinfection treatment of the fluororesin porous membrane as determined using NaCl particles having a particle size of 0.1 μm, is 5.0 or less,
• a pressure loss ratio (pressure loss after disinfection treatment/pressure loss before disinfection treatment), which is a ratio of a pressure loss of the air filter medium after the disinfection treatment of the fluororesin porous membrane as determined when air is passed at a flow rate of 5.3 cm/s to a pressure loss of the air filter medium before the disinfection treatment of the fluororesin porous membrane as determined when air is passed at a flow rate of 5.3 cm/s, is 0.83 or more and 1.15 or less,
• the air filter medium before the disinfection treatment has a PF of 19 or more, the PF being determined from a formula:
• PF={−log ((100−collection efficiency (%))/100)}/(pressure loss (Pa)/1000) using the pressure loss and a collection efficiency determined using NaCl particles having a particle size of 0.1 μm, and
• the air filter medium is reused after the disinfection treatment,
• wherein regardless of whether the disinfection treatment is exposure to an alcohol, exposure to a surfactant, exposure to a chlorine-based bleaching agent, exposure to a hypochlorous acid solution, exposure to a hydrogen peroxide solution, exposure to a temperature environment of 75° C. or higher and 150° C. or lower, irradiation with ultraviolet rays, or exposure to ozone, the permeability ratio of the air filter medium is 5.0 or less and the pressure loss ratio of the air filter medium is 0.83 or more and 1.15 or less.