Cannabis 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 enviroment. The best way to do this is with a fan of some kind. If the grow room is large enough then an regualar 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.
If a large number of plants are to be kept a dehumidifier may be needed. If humidity levels are too high then the chances of mold will dramaticly increase. A dehumidifier will cost a grower about $100 so it isn,t really practical for the closet grower.