Tiny, nasty, invisible gas particles in flatus (farts), in particular hydrogen sulphide, that jump inside your nose and get trapped by the cilia on a patch of special olfactory neurons in the top of your nasal passage setting in motion a series of chemical reactions that tell your brain "Someone shit their pants".
1. I was taking Phil Johnson to the corner store the other day and he ripped one in the car. I swear I could almost taste the poop farticles.
2. That brown residue on the walls of your bathroom could be the result of farticles combined with condensation from steaming hot showers.