Can't use argon instead of something exotic like SF6?  And why can't space people sound like chipmunks on other planets?  Would be cheaper than to find and transport these gases at celestial volumes.

Oxygen alone will be enough, without any other gases. It must just have a correct pressure.

As long as oxygen pressure doesn't exceed about 150 mm Hg you are on the safe side, that's why I have wrote about correct pressure.

As for CO₂ you are absolutly right. Although I don't remember exact number, too high pressure of CO₂ makes it impossible to remove it from blood. Fast breathing is not induced by lack of oxygen oxygen intake, but by the CO₂ excess. This CO₂ excess lowers pH of blood and that's what makes us pant. Thus if there is enough oxygen, but there is a high level of CO₂ you pant as crazy and you are not able to do anything.

Wow! That was a better response than I had hoped for, thank you all for helping me out.

Dude: As for introducing SF6, I was worried about having helium as the sole buffer gas because I had heard that large amounts (would be 70%) could cause brain damage. Thusly I wanted to lower it, but the only other gases that sounded plausible were argon and nitrogen. Honestly, SF6 sounded cooler, so I started with that. Other possibilities that I found are xenon, neon, water vapor, and krypton. Are any of these better for a sole buffer gas?

gregpawin: Would argon be safe in such large amounts? I don't mind people sounding like chipmunks from the helium(kinda cool actually) but thought that I was being clever

billnotgatez: Thanks for the links, and exactly! At high levels (70% +) most gases become detrimental, so what doesn't Acclimation or gene modification would be the easiest solution to avoid all of this, but I'm looking for something more... pat, know what I mean?

Borek: This question leads away from chemistry and into other scientific areas (sorry ), but, is having a sea level atmospheric pressure (far) below 300 milibars okay (breathable)? I was under the impression that humans needed a certain level of pressure to help with the breathing process (as demonstrated by breathing difficulties at high altitudes). What are acceptable sea level pressures?

Thanks again, and I'm sorry if I missed anyone in this! All of your input helps a lot!

The problem with having helium at any appreciable concentration is that helium is VERY not dense so a good majority of it will move on out into space unless it is confined.  Even if it is confined, it will likely rise to the top of whatever container it's in making the helium concentration at the bottom noticeably less than at the top.  With SF6 you have the opposite problem.  SF6 is one of the densest gases known and will collect in low lying areas.  It will also collect in one's lungs and will not go out.  After some time, it will take up all the space in people's lungs and suffocate them to death.  Chemically it's not toxic at all, but physiologically it's a very good asphyxiant.  

Okay, so SF6 is out I don't think mandatory hourly hand stands would be a good part of the founding constitution!

So, if I made it so that helium was continually pumped into the atmosphere, would that work? I am hesitant about using a straight oxygen atmosphere due to the flammability (the same reason it is no longer used by NASA and other space programs). Any suggestions?

Why are you not using nitrogen like on earth?

I think she just want's it to be different