Asbestos is dangerous only if it is in what is called a friable (spelling?) condition. Friable is defined as a physical (as opposed to chemical) condition wherein microscopic particles are released and become airborn. This is the only time asbestos becomes dangerous to humans. And it is dangerous only in the lungs.

Microscopic particles are released only from asbestos material which is deteriorating. The material that was the biggest offender was sprayed-on insulation over heating ducts in offices and especially schools in big cities. For a long time the material was perfectly safe; however, unknown to suppliers and contractors, the material would decompose and powder over long periods of time, and that is when it became dangerous.

The ducts you described are entirely safe, and probably would have remained so indefinitely. They were not deteriorating or powdering, thus not releasing any friable material into the air.

My brother in law is a scientist for decades with the EPA in Washington; he was one of the original researchers for the Love Canal debacle which started the whole "environmental polluion" movement. When we purchased our home, the heating ducts in the basement had asbestos paper wrapped around them; I did a lot of investigating and discovered that they posed no danger to us. Just to be safe I sprayed them with white spray paint, and that is called encapsulation.

And what the Ranch-hand posts is called common-sense, with a good shot of intellect.
Adrian