We looked at underground duct work for a customer that was getting a lot of fine dust blowing out of the registers. We found all the duct work was PVC, but the connectors and elbows were metal that were heavily rusted out allowing the backfill under the slab to enter the duct work and when the furnace would run it allowed the dust to blow up in the house. We cleaned all the ducts, applied spin cast mortar to all the bad connectors and elbows. Lastly we applied duct armor to seal the system.