text: Air does contain oxygen, but is actually something like 70 to 75% nitrogen, as well as other inert gases like carbon dioxide.  People that need oxygen for medical uses carry pure oxygen in those tanks - usually, it's because they can't get enough filtered through their lungs and circulatory system.  This is pretty simplistic, I'm sure somebody will give you a more technical explanation.\n\nI forgot about altitude - good catch on that one!  \n\nAnother interesting thing - deep sea divers don't use pure oxygen in their tanks, I think it's usually a mixture of helium and oxygen (though this may have changed, it's been several years since I learned that).  I think the reason is that when your lungs are functioning properly, pure oxygen can make you "high" or dizzy...hence the "oxygen bar" fad a few years ago.