Corrected entry: Just after Han realizes that they are, in fact, inside a critter instead of a cave on an asteroid, you see Leia and Han stumbling back to the Millennium Falcon. Pay special attention to Han; as he makes his way to the hatch you see the end of his hose come detached and flap around as he is running. So much for the oxygen he was breathing.
Rebecca Bennett
15th Nov 2006
Correction: First off, the masks were just a precaution (there's nothing that would keep the toxic fumes from coming into the Falcon after they land, there's no air-lock on the ship). Secondly, what's to say that he couldn't just hold his breath? I've been scuba diving and the same thing has happened, I just held my breath until I got it back in my mouth.
Nick Bylsma