Carbon filters are used to control odors and ensure that plants receive enough fresh air. Because indoor gardens often do not have much air circulation, humidity and moisture can build up, creating unpleasant and pungent odors. Carbon filters contain activated carbon, which is extremely porous. These pores filter the air as it passes through, trapping the molecules of contaminants. The filters can be attached directly to fans in the grow space, and can either be placed on the floor of the grow space or hung on the walls.