As mentioned, some of the air ducts are made entirely of asbestos, although others are often made of metal that is wrapped with some insulation. By nature, asbestos is really great at insulating and it is for this reason why it is commonly used in the attic and air ducts of some of the houses in Los Angeles, and on some other parts in the world.
This product looks like a corrugated cardboard that is often available in off white color. Remember that using the asbestos as insulation is not really harmful, but it is when the product would break down and deteriorate that it could trigger some health problems. So before you and your loved ones would suffer, you better get an asbestos removal
and air duct replacement