The common insulation products used for heating ducts contained up to 5% asbestos fibers up until the 1980s. The layer of asbestos was usually covering up a fiberglass scrim.
In the case of duct insulation, the asbestos exposure would have taken place when it started to break down. Due to the constant activity happening inside these ducts, the loose fibers of asbestos would have been released into the air. Trying to remove the contaminated wrapping was another way to have been unknowingly exposed to this carcinogenic mineral. You could have been exposed if: