system
Given the following post, respond as if you're commenting on Reddit:
human
[Biology] Since air is only about 25% oxygen, does it really matter for humans what the rest of it is, as long as it's not toxic?

Pretty much, do humans need the remainder of the air we breathe to be nitrogen, or would any inert gas do? For example, astronauts on the ISS or Felix Baumgartner have to breathe artificial atmosphere comprised of the same gases we breathe on Earth, but could they still breathe a mixture of, for example, xenon and oxygen, or is there something special about having the nitrogen as a major ingredient?

EDIT: Quick note, although in the title, I said air is "about 25% oxygen", I've had a few people correcting me down below. I was aware that the figure was a little smaller than that, but thank you for the correction because the detail *is* important. The actual proportion is more like 21%.

P.S. I'm glad this was interesting enough to reach the front. Your comments are very informative! :)
gpt
Helium-Oxygen is sometimes used in divers tubes, because it performs better at deep pressure ( ~~helium is less likely to form bubbles in your blood vessels when you resurface than nitrogen is~~ *EDIT:* So people tell me that it's actually because nitrogen is narcotic at high pressures). 

Xenon cannot be used as it is not sufficiently inert. It may be a noble gas, but it can still influence your brain. It is in fact quite a powerful anaesthetic. It's what we would use to keep people asleep during surgery if it was not so damn expensive.

It is possible for human to "breathe" fluorocarbon liquids as they are sufficiently inert and carry enough oxygen. The problem is that human lungs generally cannot circulate the liquid very well, so you'd have to use a pump for it.