> Even at sea level the exhaled air is about 16% oxygen (down from 21%). Giving the body access to more oxygen per breath won't make the air supply last longer[

> Even at sea level the exhaled air is about 16% oxygen (down from 21%). Giving the body access to more oxygen per breath won't make the air supply last longer[*]. -Incidentally, in the survival suits we wear when helicommuting in the North Sea contain a rebreather device - which is simply a small bag in the suit's lining with a mouthpiece and a valve. The idea is that when getting underwater is imminent, you take a few deep breaths. When you exhale into the bag, you can then draw the spent air back in for another gulp or two of air. The idea being that in a couple of breaths you'll either be in the clear or dead - so a couple of extra breaths is all you realistically need. (Ever the cynic, I suspect part of the reasoning behind it is to keep you busy and un-panicked in your last few seconds alive, but anyway...)