AC duct in a home is not likely to be asbestos. Only testing will tell you however. It is most likely fiberglass of some sort. Dust can be a problem, but there are ways to deal with that. First, dust is mostly made of human skin, fibers from clothes, insulation, and dirt. Normal house level dust is not a danger. Unless there is an additional source, don’t worry. However, if you are seeing dust, you should clean it up. If there is an AC duct over your bed, either move your bed or at hardware stores, you can get filter screens to go over the outlet to reduce the dust level. Change twice per year to keep them working properly (or more often if they get dirty). If you really are concerned with the duct, see if you can find a piece of it (full depth) and take it to a local asbestos lab. For around $25, they will confirm the content.