Ducts may be made of asbestos material or be wrapped with asbestos material. Determining how safe asbestos ducting depends on several factors.
ACM stands for Asbestos Containing Material ( Material containing 1% or more of asbestos)
Asbestos ducts; basically two types
1. Metal ducts wrapped with ACM
2. Ducts that are made of ACM’s
Metal ducts wrapped with ACM
If metal ducts are wrapped with asbestos insulation on the outer surfaces of the metal duct, where it is not exposed to an airstream in the ducts, then there is less opportunity for contamination of the circulating air. From the mid-1950s to the early 1970’s it was common to wrap metal ducts with a cardboard like insulating material that contained some asbestos.
Ducts made of ACM’s
Ducts which are made of ACM are much less common than metal ducts wrapped with ACMs. If a duct is made of ACMs and it is punctured or damaged, then asbestos fibers may be circulating in the air. If this is the case, it should be reviewed and corrected.
Is there a health risk?
The answer to this depends on what type of ducts you have and the condition of the ducts and duct material. Generally, there are no building codes that require a buyer to remove asbestos ducting.
SOURCE: https://buyersask.com/furnaces-acs-fireplaces/ducts/asbestos-ducting/