Conditioned air can leave the inside of a home through wire and pipe penetrations in the attic. As the air leaves the home it takes some dirt and dust with it. As the insulation filters the air the dirt stays behind. The dirt itself isn't the problem but it is the tell-tail of air escaping the home. Since we pay to condition that air we do not want it escaping into our attic. The other problem is along with that dirt and dust the air has moisture. The humid air is allowed to escape the home and then condensates on the cold roof deck (during cold outside temperatures). This house is only 6 years old and the mold in the attic was very extensive. To prevent these problems it is imperative that all areas that air could leak out of your home and into your attic be sealed with spray foam.