Marijuana like all other plants puts out waste through the stomata on it's leaves. Outdoors wind, rain and sun are present to evaporate these toxins from the leaf surface. Indoors the grower must create an environment. The best way to do this is with a fan of some kind. If the grow room is large enough then an regular cooling fan can be placed inside and left on all the time. If you are running a small closet operation then just opening the door twice a day to look at them will create enough air movement for healthy growth. A fan controlled by a thermostat will also work well. These can be found at most electronics stores. JUST SOME INFO I FOUND FOR YOU, HOPE IT HELPS!!!