Because asbestos was easy to attain and had a lot of properties that were useful in many industries it was widely used during a large part of the 20th century. Asbestos is strong, fire-resistant, and an ideal insulator so it was used in a wide variety of insulation, including HVAC insulation ducts found in floors and above ceilings in large buildings.
The Consumer Product Safety Commission and the Environmental Protection Agency restricted the use of asbestos products in construction materials beginning in 1977. Strict laws governed the use of asbestos because the hazards related to the ingestion of asbestos became widely known to the public. Constant and repeated exposure can cause lung cancer, not to mention even more serious diseases such as asbestosis and mesothelioma. In mesothelioma, the asbestos affects the inner lining of the chest and abdomen.
Insulation ducts are not necessarily hazardous just because they contain asbestos. It only becomes dangerous if and when the fibers get into the air. If the asbestos fibers in your insulation ducts are intact and in good shape there is nothing to worry about. But once the asbestos-containing material is disturbed they can get into the air and be inhaled.