Many times, bathrooms were built without bathroom exhaust fans. This seems odd when the benefits are quite obvious: Remove moisture, when there is bath or shower, freshening air, etc. We have seen where an exhaust fan had been vented into the attic, this is not good. An exhaust fan should be vented to the outside.
That depends on a number of factors.
In residential (house) is there an attic? In commercial, is there space above the bathroom to run ducting? How far is to an outside wall? Is there a light nearby, that electrical could be taken from- and it would work to have the fan and light switched together?
There are many types of bathroom exhaust fans.
Most bathroom exhausts were designed to be installed during the original construction. That is, before the drywall went on the walls and ceiling. However, these are not good to use when the ceiling is already finished. Therefore, look for a bathroom exhaust that has wording such as, “Remodel” or “Old Work” or “Retro-fit” if it is not new construction.
Fans are also sold with ratings as to air volume (Cubic Feet per Minute, CFM) and how loud they are (rated in “scones” with one scone being about as loud as a refrigerator).
Also, some fans come with built in lights.
If you are doing a full bathroom remodel, taking down the ceiling etc, then it is a matter of taste, which fan do you like the best.
If you only want to add an exhaust fan to an existing bathroom with minimum work, including minimum ceiling damage, then using a remodel fan is the way to go. The selection is minimal in this category so you don’t have to worry about which one to choose.
Each bathroom exhaust fan project needs to be evaluated on its own. We have found many of our customers very much like having a new bathroom exhaust fan installed and vented to the outside.