One of the biggest source of indoor pollution can sometimes lie unnoticed - the air ducts that move hot or cold air through your home. Filters remove some of the dust, pollen, and mold spores that fly through these ducts, but much more clings to the side as it settles out of the relatively slwo moving air. To safeguard the system, the first line of defense is to filter the air to keep the stuff out of the ductwork in the first place. But older ducts - twently years and up - may need a bigger fix. Professional duct-cleaning services can reach in with superlong vacuum tubes and clean everything out. How do you know if you need this? Check your nose. If you or someone in your household has chronic or mysterious allergies, the cause may like within the ducts.