These materials are comprised of fiberglass overlain by a thin coating of a rubber compound called neoprene. It serves both as a filler and gives rubber its characteristic black color. The deterioration can be much greater in areas prone to higher ozone concentrations associated with air pollution. When you place your hand on the duct liner surface, it is likely that it will be soiled by a fine black powder. As such small black particles may be blown through your duct system and enter the air space of your home.
Particle deposition occurs along thermal gradients.
It may also occur on surfaces that are electrostatically charged. Such thermal gradients do occur near electrical outlets on exterior walls. Neoprene contains chlorine. If a dust sample is collected and has a high chlorine content, it is likely to be contain neoprene particles. Candle soot particles on the other hand are too small to be seen individually under such magnification.