No filter, not even the expensive electrostatic kind, is 100% effective at filtering out contaminants from your duct system. What's more, harmful contaminants also get into your ducts from other places aside from where your filter is located. Once air goes through your filter it's sucked through the machinery that makes up your furnace and air conditioner. As a result , every time your system turns on, its moving parts and vibration shake up the dust that typically settles in your ducts -- and the air that just went through your filter now blows that dust into your home. In addition, the process of heating and cooling your home creates the kind of temperature and humidity changes in your ducts that enable the growth of unhealthy mold, fungus and bacteria.