text: if the air we breath is oxygen, then why do some people carry an oxygen tank?   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.  i forgot about altitude - good catch on that one!    another 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.