I'm working on a far-future SF story taking place on a terraformed Mars and would like a fact check. Current estimates (see here and here) are that the CO2 on Mars could be released in gas form to warm the planet and raise the atmospheric pressure to about 30% of earth, which is not breathable but is sufficient pressure to not be lethal (about Mt. Everest level).
Assuming the oxygen content can be increased (through some technological means separating CO2 into carbon and oxygen), would a higher proportion of oxygen at low pressure allow a human to breath it? It would certainly be thin, but athletes who climb mountains find their bodies increasing red blood cell count to more effectively capture oxygen. Is Everest-level pressure just too thin even if the percentage of oxygen were higher than Earths (about 21% of air)?
I'm assuming that things like the solar wind stripping away atmosphere due to lack of a magnetic field have been taken care of. The limiting factors are Martian gravity and air pressure and content. I'm also assuming no extra mass (gas or otherwise) has been imported from elsewhere and we are working solely with what Mars has to offer.
I'm aware of this question about giving Mars a survivable atmosphere, but the accepted answer claims that atmospheric pressure would be limited by gravity without explaining why Titan, which has gravity 14% of Earth, is able to sustain a mostly nitrogen atmosphere at higher pressure than Earth.