provided by: 
Excess build-up of mold and mildew in your bathroom can be detrimental to your health and lead to the deterioration of a room's structural supports and walls. Good ventilation from a bathroom fan can prevent these problems.
Bathroom ceiling fans are called intermittent ventilation. They are used to capture moisture or pollutants at the source and to exhaust them before they can spread to other areas of the house. Areas often requiring ventilation include bathrooms, kitchens, utility rooms, exercise rooms, workshops, garages and home offices. Bathroom ceiling fan come in a variety of configurations, including lights, nightlights and built-in heater combinations.
Q. What size fan does my bathroom need?
A. For bathrooms up to 100 square feet in area, it is recommended that an exhaust fan provide 1 cfm per square foot at approximately eight air changes per hour. For proper ventilation, the fan should be left on for around 20 minutes after usage.
For an 8x5 room with an 8 ceiling = 40 sq ft, you will need a fan rated at 40cfm. For larger bathrooms install a 150cfm fan so that the air can be pulled through the entire room and exhausted at a central location. Or, you can install multi fans; one over the toilet, in the shower, and over the tub. This second method is very effective and provides ventilation where and when its needed, but both methods work well.
Q. Where should I install the fan?
A. Generally, the exhaust point should be located over or near the shower or tub.
Q. If the fan grill is dripping water or the mirrors stay steamed up, is the fan operating properly?
A. Try leaving the fan on longer to carry out more moisture, or check duct work. Poor duct design or damage may prevent the fan from carrying the moisture. You can insulate the ducts and check the roof jack which may be allowing rain to come into the duct. Another cause can be condensation from warm, humid air in the house striking the cold duct surface.
Q. What is a sone level?
A. Sound levels are measured in sones. The higher the sone level, the noisier the fan. Purchase as quiet a fan as you can afford. The way a fan is installed can also affect its noise level. A low-sone fan attached to a duct with twists and turns, or is kinked or too small, can be just as noisy as the noisiest model.
Visit Faucet.com for a wide variety of quality bathroom fans.
article at HomeRenovationGuide.com