By not cleaning the house, buying a filter that you can read a newpaper through. Not changing it as often as you should. Burning candles. Dryer vent leaking in the house. A lot of cooking with out a power vented hood over the stove. Not having it serviced at least once a year. Make sure there is no leaks in the duct work. And don't forget if you have any kind of animals (Dogs or cats) or birds in the house. Everything get dirty if it is not taken care of. Even a HVAC Pro.