Filter A: MOLECULAR SIEVE AND ACTIVATED CARBONThe right choice when you need to get breathable air. Molecular sieve removes moisture in the air, and activated carbon removes odors and any oil residue. Filter D: ACTIVATED CARBONActivated carbon is used to remove odors and oil residues. Activated carbon is sensitive to moisture, so to obtain breathable air, it is necessary to use activated carbon together with molecular sieve to remove moisture. CAUTION: The activated carbon-only cartridge should be used when there is no need to obtain breathable air.