Evaporative air cooling works by drawing in hot, dry air, and fanning out cooler, more moist air. After a while, though, your room will need ventilation or a cross-breeze to circulate the more humid air out the room and restart the process with fresh dry air. It is recommended to keep a window and/or door open for adequate cross ventilation. In larger rooms, air from outdoors is not required, but there must be some type of airflow to keep the cooling action moving.