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the particles expelled when you fart, which linger in the air, causing smell.
PARTICLE: ie, atoms ... etc: expelled from bum-hole, thusly named "Arse Particles"

"Upon waking, James lifted his duvet covers and wafted the arse particles around the room. Hannah turned over...and woke up thinking something had died as the particles lingered in her nostrils."
by PARTICLECHICK November 13, 2011