Air changes per hour is also a unit that defines the number of times the volume of air in a room will be extracted in an hour. So before opting for an exhaust fan, one must figure out the size of the space that one is trying to cover through an exhaust fan. If it's a tiny toilet compartment, then the exhaust fan that you require must have around 5-8 air changes per hour. And as the size of a particular space increases, the requirement for the number of air changes per hour increases as well.  So, for instance, if you require an exhaust fan for a room where the levels of moisture and other pollutants are too high, then the number of air changes per hour might reach up to 20 or 25 as well. If you have a huge conference room, then the exhaust fan that you require must have the number of air changes per hour about 15 to 20. And similarly, it can vary as per the size of the room.