The anti-scuba

Out in space, including any outdoor activity in a low-grav planet or moon, astronauts must wear suits. They can breathe near-pure oxygen at low pressure, or nitrogen/oxygen at 100 kPa like we do down here. Low-pressure is for the nimble suits, the ones which do the high-precision work. On the move between higher to lower pressures those astronauts get Da Bends, when the nitrogen naturally locked up in our system boils out to the bloodstream. Rapture-of-the-deep is what happens when the nitrogen dissolves back in. Space is, thus, the anti-scuba.

Better if our astronauts didn't have gas in their bodies in the first place. That was the idea behind Apollo One, to fill the cabin with pure oxygen. Ooops.

To replace nitrogen we need such nontoxic, nonflammable, room-temperature gas as doesn't dissolve in ... well, in us. That is: in lipids, per the Meyer[-Overton] correlation. Our mix needs be the exact opposite of chloroform!

I have heard tell of deep-sea divers breathing helium/oxygen. Helium of course does not dissolve. It is also expensive and makes your voice squeaky. But then I started thinking about other noble gases, or any other insoluble gas. Argon is cheap. Is there an argon/oxygen mix out there?

Turns out: there is. There's research in using argon / nitrogen / oxygen at 40:40:20, for Mars. Mars' atmo is 1.9:1.9:96.2, where that last is mostly carbon dioxide. Scrub that out by... leaving the mix overnight and scraping the dry-ice off. For heavens' sake the atmo at the poles may be at that mix already. Mind you, nobody on Mars is breathing over 100 kPa...

I was actually surprised that scuba divers don't use "Argox" already but, turns out, divers warn us that this is more narcotic than the 0.9:78:21 we get naturally. It does dissolve in oil. Argox has been mooted as a decompression-gas up top to fight off the bends; a narcotic might actually help, in that case. Nobody's studied this much on humans.

Apparently hydrogen is 55% soluble as nitrogen, which I readily believe, given how much unbound hydrogen isn't in our hydrosphere. But good luck finding a volunteer for a hydrogen/oxygen mix in a spaceship. Apollo One meets Hindenberg!

Neon is comparable to helium here and, indeed, is sometimes used in place of nitrogen. It is more expensive than helium however.

If Argox dissolves more readily than nitrogen, it might bubble out more slowly and spread out the Grecian Bends effect. We still want as little of it as possible so I'm unsure it's a good fit for the ship's cabin.

The best idea is to have a high-oxygen chamber-suite next to the outer airlock. They sit there doing mostly non-electronical chores whilst their nitrogen boils off; literally in detox. We might not have space for that in a short-duration ship fresh off Earth, but we can assuredly get one in a larger space station, or in a Mars colony. If you're Kim Stanley Robinson or (inshallah) Elon Musk, your home is indeed an apartment in the colony, you can let the outdoorsmen deal with this little problem.

Speaking of. Bob Zubrin in How To Live On Mars didn't talk much about shifting between 100 kPa argox and low-pressure oxygen when he sent his libertarian pioneers off roaming the surface. I don't recall that Andy Weir harped on this, either - but to his credit, Weir did illustrate what happens when some damnfool botanist allows his surface hab[itat] to skew high in oxygen. Both models are close to the Great Outdoors with easy access to the suit. So - yeah, not going Apollo One should be given more weight than it is.