Generally, any inert gas heavier than air. Maybe for only one breath, in an O2 rich environ. Would the gas be too heavy to exhale? Other than the asphyixiation, is there a poisoning risk? I think there wouldn’t because noble gasses are inert and don’t bond (except one tope of Ar)
What’s the dope?
Well, there’s certainly no poisoning problem. Argon is the third-most abundant component of air, making up nearly 1%. I suspect that any patially-inert atmosphere would be harmless, so long as it contained the requisite 22% oxygen, but I’m not certain of that.
Not an expert or anything, but isn’t everybody breathing in both Xenon and Argon? (In really small amounts)
Aren’t they naturally occurring gasses? So they’d be everywhere. Just in tiny concentrations. And breathing in and out would keep the air in your lungs mixed up, so the heavier gasses wouldn’t “pool” and kill you.
Breathing any gas straight would be a bad idea. (Including straight O[sub]2[/sub]) But sucking down the stray Argon molecule wouldn’t be so bad. (If I remember HS Chem. right, 2 Argon atoms smack together to make 1 stable Argon molecule.)(If I don’t remember right then: But sucking down the stray Argon atom wouldn’t be so bad.)
Argon is one of the Nobel Gases which don’t combine with anything (at least under normal conditions).
I once gleefully got to point this out to a safety twit during a design review. She was worried that our argon atmosphere was going to react with some cooling water. :rolleyes:
To address the OP:
A Diving Web Site
You could safely breathe a 80-20 mix of argon and oxygen indefinitely, as far as I know. Such mixtures are sometimes used by professional divers, as argon is far less absorbable into blood than nitrogen, presenting less risk of decompression sickness.
Breathing a 80-20 mix of xenon and oxygen might prove dangerous in the long term, because xenon is substantially heavier than oxygen; the increased weight of such a mixture could possibly strain the lungs, and you might end up with pockets of a xenon-carbon dioxide mixture in the lungs that you might not be able to readily expel.
It should be safe to take a single deep breath of xenon (which will give you an incredibly deep and gravelly voice on the way back out) as long as your lungs are in good shape. It may take several breaths to clear all of the xenon back out afterwards, and I shouldn’t advise strenuous activity immediately afterwards as your total gaseous exchange capacity will be somewhat compromised while the remaining xenon diffuses back out.
Neither argon nor xenon is inherently toxic. Extended attempts to breathe pure argon or pure xenon will kill you, obviously.
Argon and xenon are both noble gases. Of the six noble gases, helium and neon are not known to form compounds under any conditions, while argon, krypton, xenon, and radon can be forced to form chemical compounds with fluorine or oxygen under extreme conditions not generally to be found in the places where humans routinely travel.
Xenon at STP would be narcotic if it was used to replace the nitrogen in the air (as are all essentially inert gases; “rapture of the deep”). Indeed, I believe that xenon has been used experimentally as an anesthetic (IANADOEPOOT); medical-grade xenon is so expensive, however, that it is not a practical use.
I breathed in AKron for a whole day… smelled like rubber 
Oooh! Oooh! You diddnt ask meee! I asked Mr. N about that and he said that you tecnicly can, you’d just have to beat your chest to get it out. and your voice would go wayyyyyyyyy low. diddnt we discuss this in Chem?
Oh, that’s right. It’s the atoms that end in “gen” (oxygen, hydrogen…) and “ine” (fluorine, iodine…) that forms a stable molecule from two atoms of the same element.
Unless I got that wrong, too. It’s been a long time since HS Chem. Oddly, I know about the Noble Gasses not combining with anything, but I had an attack of Chemistry Ignorance and figured “Hey, they’re just combining with themselves, so that doesn’t count, right?”
[duh]Yeah it counts.[/duh]