If cannabis and/or users were to all get their trachea and bronchial tubes scraped, a black substance would be found. It just takes time for you to become use to the burning sensation. When the tar builds up enough (another theory) on the surface of one's trachea and bronchial tubes, you won't feel pain anymore, because the skin is covered too much for you too feel anything. Unless one was to use a water pipe or vaporizer.