Filter medium and filter unit

A filter medium comprising:

• a first polytetrafluoroethylene (PTFE) porous membrane;
• a first air-permeable supporting member having a mass per unit area of 20 to 70 g/m2;
• a second PTFE porous membrane;
• and
• a second air-permeable supporting member having a mass per unit area of 20 to 70 g/m2, wherein
• the filter medium has a laminate structure composed of the first PTFE porous membrane, the first air-permeable supporting member, the second PTFE porous membrane, and the second air-permeable supporting member laminated in this order, and
• the first PTFE porous membrane constitutes an exposed surface of the filter medium,
• a thickness of the first PTFE porous membrane is 2 μm or more and 12 μm or less,
• a thickness of the second PTFE porous membrane is 6 μm or more and 9 μm or less,
• wherein the thickness of the one of the first PTFE porous membrane and the second PTFE porous membrane having the larger thickness is defined as a larger thickness (T1),
• wherein the thickness of the other of the first PTFE porous membrane and the second PTFE porous membrane having the smaller thickness is defined as a smaller thickness (T2), and
• wherein T2 divided by T1 satisfies the following inequity:
• 0.22≤T2/T1≤0.75, and
• a thickness of the filter medium is more than 420 μm and 450 μm or less,
• an average fiber diameter of the first air-permeable supporting member is 10 to 30 μm,
• an average fiber diameter of the second air-permeable supporting member is 10 to 30 μm, and
• a ratio TR1/TR2 of a transmittance TR1 of the first PTFE porous membrane to a transmittance TR2 of the second PTFE porous membrane is 100 or more and 5000000 or less,
• where the transmittance TR1 and the transmittance TR2 are values determined by the following formulae from, respectively, collection efficiency CE1 (unit:
• %) of the first PTFE porous membrane and collection efficiency CE2 (unit:
• %) of the second PTFE porous membrane measured using monodisperse dioctyl sebacate particles having an average particle diameter of 0.1 μm at a permeate flow rate of 5.3 cm/sec:
• transmittance TR1 (%)=100−collection efficiency CE1;
• transmittance TR2 (%)=100−collection efficiency CE2;
• and
• wherein a transmittance TRF of the filter medium is 5% or less,
• where the transmittance TRF is a value determined by the following formula from collection efficiency CEF (unit:
• %) of the filter medium measured using monodisperse dioctyl sebacate particles having an average particle diameter of 0.1 μm at a permeate flow rate of 5.3 cm/sec:
• transmittance TRF (%)=100−collection efficiency CEF.