I always assumed it was worse than this: when you breath out your lungs still contain a slightly lowered oxygen concentration at atmospheric pressure, so the pa

I always assumed it was worse than this: when you breath out your lungs still contain a slightly lowered oxygen concentration at atmospheric pressure, so the partial pressure of O2 is still quite high. In a vacuum this drops to zero, meaning that your lungs become an oxygen sink. So not only are you unable to get more oxygen, your lungs are actively sucking the remaining oxygen out of your blood. Edit: as was pointed out in this comment a level up on this thread https://news.ycombinator.com/item?id=24495970 https://news.ycombinator.com/item?id=24495970