Remodling and found under slab ducts are transite and had flooded at one point in time as there is mud in them - assuming when the slab was poured back in the 60's Whats the danger factor in keeping these - are they very friable as they are known to contain asbestos? should I be concerned about having a duct cleaning company clean them out and using them or should I abandon the system and run return through the attic?