There are 850k people living in La Paz, Bolivia with the equivalent of 13.2% sea level oxygen and they seem to be doing just fine.

kagis

https://www.reddit.com/r/askscience/comments/thj9zj/could_a_human_survive_on_a_planet_with_a_thinner/

So, the answer is yes. The term "partial pressure" is often heard in this topic: as long as at least 0.2 bar of pressure are produced by oxygen alone, then you can add as much (or as little) as you want of a diluent gas on top of it. Of course this diluent gas has to be inert enough to avoid interfering with respiration, e.g. helium or nitrogen.

I don't think that that's authoritative as a lower limit -- the guy is just saying that what matters is the partial pressure of oxygen. Assuming that that is right...

There is an upper limit on the partial pressure of oxygen. Above 1.6 bars oxygen toxicity becomes a problem.

Yup an a scuba diver, 1.6 at rest is the most we use while decompressing. 1.4 is the recommendation while exercising and 1.2 if you're doing a long dive.

The military, as I understand it can push up to 2.0, but they have people that are super fit & they test people for oxygen tolerance too.

These aren't gonna be hard limits, though, just maximums on what you'd subject a person to intentionally.

https://www.checkyourmath.com/convert/pressure/atmospheres_bars.php

There are 1.01325 bars in an atmosphere.

https://en.wikipedia.org/wiki/Death_zone

The concentration of oxygen (O2) in air is 20.9% so the partial pressure of O2 (PO2) at sea level is about 21.2 kPa (6.3 inHg; 3.07 psi).

Humans have survived for 2 years at 5,950 m (19,520 ft) [475 millibars (14.0 inHg; 6.89 psi) of atmospheric pressure], which appears to be near the limit of the permanently tolerable highest altitude.[13]

So "normal" at sea level is 1 atmosphere, with 20.9% of that being oxygen.

If WP is correct, we can get down to 46% of that partial pressure of oxygen at normal atmospheric mix at sea level (though we couldn't be climbing mountains then, would cut into our survivable altitude range). So we could get down to 9.614% atmospheric oxygen and be okay at sea level.

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

Lambertsen concluded in 1987 that 0.5 bar (50 kPa) could be tolerated indefinitely.

So that'd be 50.7% oxygen, or about 2.42 times the partial pressure of oxygen at sea level.

Going off that range -- 9.614% to 50.7% oxygen at sea level -- we could handle all of the oxygen levels on the chart. But two important caveats:

• That's only at sea level. The "dead zone" altitude on mountains and such would drop when the oxygen level is lower than it is in the current atmosphere.

• We probably wouldn't perform as well as we do today towards the extremes. It might be survivable, but "survivable" can be a long way from "biologically optimal".