Filter medium for air filter, filter pack, air filter unit and method for producing the filter medium for air filter

A method for producing a filter medium for an air filter for collecting dust in air, the method comprising:

• (a) a step of obtaining a first porous film and a second porous film, both of the films primarily containing a fluororesin;
• (b) a step of arranging the first porous film on a side that is upstream of an air stream of the second porous film;
• and
• (c) a step of arranging a pre-collection member on a side that is upstream of the air current of the first porous film so as to collect a portion of the dust in the air current,
• the first porous film and the second porous film each having a pressure loss of 30 Pa or more and 90 Pa or less when air is passed through each porous film at a flow rate of 5.3 cm/s, and having a collection efficiency for NaCl particles having a particle diameter of 0.3 μm of 95% or more and 99.9% or less when air containing the NaCl particles is passed through each of the porous films at a flow rate of 5.3 cm/s,
• the pre-collection member having the pressure loss of 5 Pa or more and less than 55 Pa when air is passed through the member at a flow rate of 5.3 cm/s, the collection efficiency of 15% or more and less than 85% for NaCl particles having a diameter of 0.3 μm when air containing the particles is passed through the member at a flow rate of 5.3 cm/s, and a thickness of more than 0.4 mm, and
• the method further comprising a step of integrating the first porous film, the second porous film, and the pre-collection member by heat lamination.