I'm working on a science-fiction story, and a recent scene deals with someone tampering with the O2 levels of a ship's recycled atmosphere. I find myself wondering, what is the ideal oxygen level for longterm health in humans? In other words, what were the ship's O2 levels set to before they were changed?

According to my research Earth's air generally runs around 20.8-21% oxygen, and can be dangerous if it dips below 19.5% or so. The ISS, for example, uses a basic sea-level air mixture of about 21%. On the flip side, many athletes spend time in oxygen-rich environments to improve their training, and I know many scuba divers who use Nitrox tanks with up to 40% oxygen to give them energy and increase their focus, etc. However, I also know that using enriched air like that can lead to long-term issues and oxygen toxicity at certain levels.

So my question is, are there oxygen mixtures more beneficial to longterm human life than the Earth-standard 21%? If you could control the air mixture of a ship, would it be more beneficial to set it at 25%, or 30%, etc?

And just to be clear: I'm looking only for the longterm effects of the oxygen mixture on human health, and not other practical considerations (like chance of combustion, decompression, etc).

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    There are sites on SE that deal with "what if" scenarios, including Worldbuilding.SE. Biology.SE deals with questions about evolution. This site deals more with health issues. As a doc myself, my stance is that since our atmosphere is what it is, and breathing a different %O2 would be impossible for all but a few without great expense, the point is moot. (I would propose, however, that NASA docs are not slackers; if there were a better %O2, the people up there would be breathing it. If you doubt NASA docs, read about Joe Kittinger and other test pilots.) – anongoodnurse Aug 31 '15 at 18:44
  • @anongoodnurse My question isn't about "what if" scenarios, it has nothing to do with evolution, and breathing different oxygen ratios happens all the time, I'm just asking about the long term effects. I'm asking about health, that's why I posted in Health. As for relevance, there are plenty of people who deal with oxygen ratios and air recycling every day: scuba divers, astronauts, submariners, and high-altitude climbers, just to name a few. – Nerrolken Aug 31 '15 at 18:50
  • Please forgive me. I mistook the "I'm working on a science-fiction story..." for "what if", and the fact that we evolved in an atmosphere of 21% O2 making a "better" ratio more "beneficial" to be theoretical. "breathing different oxygen ratios happens all the time" - not long term (hence my comment.) I know (as I suspect you do) that O2 rich air is toxic to lungs. Ventilator studies will bear that out. Maybe you can start there. – anongoodnurse Aug 31 '15 at 18:53
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    @anongoodnurse No problem! The "science-fiction story" bit was just backstory to explain my reason for asking, and when I said "more beneficial" I meant in terms of improving a person's health. For example, nitrox gives scuba divers increased energy, but might have other detrimental effects over time (I'm not sure). I didn't mean better for our species, I'm asking about the health effects of differing levels of O2 for an individual, long-term. But, apology accepted. :) – Nerrolken Aug 31 '15 at 18:58

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