A molecular sieve is a material with small pores of uniform size. These pore diameters are small enough to allow small molecules to pass through, however large molecules cannot be absorbed. Many molecular sieves are used as desiccants (drying agents). Commonly used in the filtration of air or gas, a substance is passed through a cartridge filter which, dependent on the application, is filled with molecular sieves and/or activated carbon, in order to remove unwanted particulates.