The majority of dust in your ducted system is usually found in the air return duct. This is because air is constantly sucked into the return air grill and passes along the return duct to the heater. You may notice the return grill gets dusty over time, well the build up of dust does not stop there.  The entire length of the return duct and the heater interior collect dust also. Over the years the amount of dust just builds and builds. Billions of dustmites, bacteria and germs live in this dust and then get blown around your home causing itchy eyes and headaches and aggravating coughs, colds, and respiratory problems such as asthma.  It is very important that the entire length of this tube is thoroughly cleaned (not just a few feet from each end) so that all the dust, dustmites, germs, pollens etc are removed from your system.