In a micro-gravity environment, like the ISS, there is no natural convection. Air is very still if we do not initiate forced convection. This can be a problem for sleeping astronauts because the CO2 they expire just sits in-front of their face, waiting to be sucked back into their nose and mouth. read more
In the micro-gravity of space, this convection of air is negligent. Unless artificially circulated, air just tends to sit there. This is actually a very real hazard to sleeping astronauts — without fans constantly blowing the air around, a person could suffocate on the CO2 they exhale while asleep. read more