No filter is 100 percent effective at filtering out contaminants from your duct system. As a result, every time your system turns on, the dust, pet dander, mites and other contaminants that settle in the ducts gets blown back into your home and into the air that you and your family breathe. In addition, the process of heating or cooling your home creates the kind of temperature and humidity changes in your ducts that enable the growth of unhealthy mold, fungus, and bacteria. This results in a poor indoor air quality that can lead to increased symptoms for allergy sufferers.