Air filter medium, air filter pack, and air filter unit

1. An air filter medium has a first main surface and a second main surface, the air filter medium comprising:

a first porous PTFE membrane;

a second porous PTFE membrane; and

the first porous PTFE membrane and the second porous PTFE membrane are arranged so that an air flow moving from the first main surface to the second main surface passes through the first porous PTFE membrane and subsequently through the second porous PTFE membrane,

a contact angle of the first porous PTFE membrane with water is in the range of 155° to 157°, and

a contact angle of the second porous PTFE membrane with water is in the range of 148° to 153°.

2. The air filter medium according to claim 1 , wherein when poly-α-olefin in the form of polydisperse particles with an average particle diameter of 0.15 μm is passed through the air filter medium at a concentration of 20 to 40 g/m 3 and a linear flow velocity of 5.3 cm/sec to measure change in pressure drop, an amount of poly-a-olefin collected by the air filter medium is 20 mg/m 2 /Pa or more at a time point where the pressure drop reaches 500 Pa.

3. The air filter medium according to claim 1 , wherein when NaCl in the form of polydisperse particles with an average particle diameter of 0.5 μm is passed through the air filter medium at a concentration of 1 to 3 g/m 3 and a linear flow velocity of 5.3 cm/sec to measure change in pressure drop, an amount of NaCl collected by the air filter medium is 8 mg/m 2 /Pa or more at a time point where the pressure drop reaches 500 Pa.

4. The air filter medium according to claim 1 , wherein the pressure drop across the first porous PTFE membrane is 10 to 45 Pa.

5. An air filter pack comprising the air filter medium according to claim 1 , the air filter medium being pleated.

6. An air filter unit comprising the air filter pack according to claim 5 and a frame supporting the air filter pack.