fart dust is microscopic and/or visible bits of feces expelled from the rectum. Feces accumulates on the sphincter after wiping the buttocks because of the wrinkles of the sphincter. Feces dries and when a person farts/expells gas the sphincter opens slightly and the wrinkles open and allow the dried feces to fall and be blown out. Dr. Chuck of The Virginia Fecal Institute performed many experiments to prove the existence of fart dust despite many disbelievers but proved without a shadow of a doubt that it in fact existed. With the help of a "volunteer", he had the test subject fart on a clean sheet of printer paper and the fart dust materialized. Fart dust does not occur with peoples who use baby wipes or bidets for cleansing their buttocks after bathroom use.
Joe awoke hacking up fart dust after Harold farted in his open mouth.