At what altitude can you not breath at?

If mountain climbing to an altitude of over 20k feet can cause brain damage, then can holding your breath for 40-60 seconds also damage your brain?

• http://www.scientificamerican.com/article.cfm?id=brain-cells-into-thin-air Speaking of minor brain damage that most people can't detect

• Answer:

  No, an unprepared person could not hold their breath long enough to do brain damage. Even divers who can hold their breath for longer, hyperventilate first. Mountain climbers are exposed to constant low oxygen conditions while doing physical labor so the effect of lack of oxygen becomes quickly acute and they cannot just take a deep breath as someone just holding their breath could.