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).