Using a window to exhaust moisture-laden air can cause mold and mildew growth around the top of the window, especially in humid climates. Ventilation fans help remove heat, moisture and excess airborne particles from bathrooms to allow gases to escape naturally and reduce humidity from the air inside your house. Most bathroom fans are installed with a wall switch that activates when you turn on the light in your bathroom. Installing a bathroom vent fan helps prevent mold, mildew and odors from forming in your bathroom or anywhere else inside your house as well.