Filters, no matter how technologically advanced or expensive, cannot completely get out the pollutants that are in the ducts. Furthermore, these pollutants do not always have to go through where the filter is but can get in through other places. When air gets into the filter, it gets drawn in through the furnace and air conditioner. Therefore, each time you turn on the system, the dust that has settled gets shaken up, hence blowing it into your home. Also, when you switch on the heater or the fan, the temperature changes give mold, fungus, and bacteria perfect breeding ground.