Don't stick your head down in it then inhale!
I noticed this morning the tape holding temp probe onto the side of the carboy was coming off. So I reached down to slap a new piece of tape on and nearly passed out from the fumes! Burned all the way into my chest!
Duh! CO2 is heavier than air so the thing was filled with CO2 and whatever other crap the yeasties were belching up.