Molecular structure of a fart

OK, this has to be the most bizarre conversation I have ever had with anyone (well maybe not THE most bizarre but it was certainly wacky!)

"These particles have been inside someone's bottom and now they are inside your nose!"
EuuuuW! How gross is that? What on earth would even possess someone to think about a fart in those terms? How did they even start thinking about it like that?
Over to you Toni...................please do explain! LMFAO