Depending on what the insulation type is and it’s age, this flexible ductwork could create health problems. The ductwork insulation material may be glass fiber or another product such as asbestos containing material. The only way to tell for sure is to have a certified public or industrial health lab test a sample to determine if it is glass fiber, asbestos, or another product. Before taking a sample of the material yourself, to avoid generating a hazard, contact the a test lab you will be sending the sample to. The problem with non-rigid insulation within the duct system is that without quality air filteration the duct may act to “catch” or capture particles that move through the air ventilation system and build up over time. This can create potential health hazards. Mold growth might also occur. If the ductwork is rigid sheet metal, the outside insulation materials may be removed if an asbestos hazard is present. Asbestos removal should be done by a qualified/certified asbestos removal professional. At one period, some ductwork was also constructed using rigid asbestos board insulation. This may present a health hazard, especially if it is disintegrating. An asbestos specialist in the state health department may be able to provide additional guidance.