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Comment by patall | Hacker News Reader

My impression was that it's mostly about particle pressure and 100% O_2 at 0.2 atmospheres would also be possible. ISS keeps with the normal atmospheric pressure and 80% nitrogen but from what one reads on the internet, that is more for compatibility reasons and not because we humans cannot sustain a pure oxygen, low pressure environment, once you go all in (and stay there for prolonged times).

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100% oxygen at turned the crew of Apollo 1 into ash. It makes stuff like concrete and aluminum as flammable as wood.

That was 100% oxygen at 100% atmospheric pressure.

The lunar module was 100% oxygen at 5 PSI (atmospheric pressure is about 14 PSI.

I’m not a chemist but I think it’s not the percentage, but the number of atoms that counts.

https://history.nasa.gov/alsj/CSM08_LM_&_SLA_Overview_pp61-6...

The Apollo engineers knew this and it factored into their choice. For whatever reason not enough consideration was given to full up ground tests where the same atmosphere would be used, but at 1atm.

100 O2 at regular or reduced pressure?

A 100% O2 atmosphere would produce oxygen toxicity in the human body. Plus, it would create a huge fire hazard.

Now, it's possible that we could be ok with a gas mix that's different from the traditional 21% oxygen 78% nitrogen we have here. But we don't like to experiment on humans. A different gax mix would need a lot of long term testing on animals before we'd ever consider using it.

I believe you may be incorrect, the fraction is not the problem [1]. Apollo also ran with pure O2, even after an early accident under higher pressure [2].

> In such applications as extra-vehicular activity, high-fraction oxygen is non-toxic, even at breathing mixture fractions approaching 100%, because the oxygen partial pressure is not allowed to chronically exceed 0.3 bar (4.4 psi).

1. https://en.m.wikipedia.org/wiki/Oxygen_toxicity

2. https://www.popsci.com/why-did-nasa-still-use-pure-oxygen-af...

Is there any data about long term effects?

That's why I am writing 'at 0.2 atmospheres'! Under that condition, you do get neither any poisoning (which the internet claims appears at 1.4 atmospheres pure oxygen), nor a big fire hazard. You create other problems due to the lower air pressure like lower boiling temperature and more body heat retention but those might be engineered away... On SkyLab, astronauts supposedly experienced 75% oxygen, 25% nitrogen for upto 84 days (again at lower pressure). That's not really long term, but not short either.

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