Filtering medium for air filter, filter pack, air filter unit, and method for producing filtering medium for air filter

A method of producing a filtering medium for an air filter for collecting fine particles in air, the method comprising:

• preparing a first porous membrane by using a fluororesin and a liquid lubricant;
• preparing a second porous membrane to have a smaller average pore diameter than the first porous membrane by using a fluororesin and a liquid lubricant;
• and
• producing the filtering medium by laminating the first porous membrane, the second porous membrane, and a plurality of air permeable supports which support the first porous membrane and the second porous membrane,
• the producing is carried out such that the plurality of air permeable supports are disposed to form at least an outermost layer of the filtering medium,
• the filtering medium having the pressure loss of less than 200 Pa,
• a collecting efficiency of NaCl particles with a particle diameter of 0.3 μm being 99.97% or more when the air containing the NaCl particles is passed through at a flow rate of 5.3 cm/sec, and
• the dust retention amount of polyalphaolefin being 25 g/m2 or more when the air containing the polyalphaolefin particles with a count median diameter of 0.25 μm is continuously passed through at a flow rate of 5.3 cm/sec and the pressure loss is increased by 250 Pa.