Leaks in the supply ductwork cause expensive conditioned air to be dumped into the attic, crawlspace or garage instead of into the house.  Leaky return ducts pull in dust from dirty attics, basements, or crawlspaces, then pass the polluted air into your furnace or air handler and distribute it into your supply ducts and into the living area of your business or home.  Leaky supply ducts suck in outside air and dust through the walls of your house and into the home or business.  Leaky ducts do create dust in your home.