! We're not all dead! Great planet hunting skills indeed! Lets land and start planting flags!
That's a pretty rich mix (O2 @ 82.3%), close to what you would choose to breathe if you were mountain climbing, and the planet's air pressure (0.261 ATM) is around what you'd expect at ten thousand meters high, or 1,200m higher than Everest, but at around 60°F.
The best thing about your planet is the high O2 air content (or partial pressure) at 82.3%. Just like any mountain climber, you will need that when the air is thin. With your planet's low air pressure it's hard to push the oxygen into the blood system. That means your lungs and diaphragm have to work harder breathing at sea level on your planet just like at higher altitudes on Earth. That is, if you only had 21% O2.
At low altitudes on Earth the normal healthy adult resting rate is 12 breaths per minute holding about 500mL in the lungs, giving a total of 6 liters per minute. Light work needs about 50 liters per minute and 100 liters per minute is considered hard work. Around 180 liters per minute is maximum effort at 21% O2.
So, at around a quarter of the normal air pressure on your planet (compared to Earth) you would need to hit your maximum breathing effort to get just around 50 liters per minute of ANY air. That means just sitting there minding your own business would exhaust you, that is though, if you only had the same percentage of oxygen as we do on Earth, 21%. Thank goodness your planet has packed the air O2 for us humans!
Here's a few cool calculators to find out if you can breathe at different ATMs with various O2 partial pressures.Air Pressure at Elevation CalculatorAltitude Air Pressure calculator
With the first, calculate what your elevation would be on Earth based on air pressure or ATM. (You have .261 ATM so that's about 10,000m). Then on the next calculator enter your altitude or elevation to find out the percentage of available oxygen at your Elevation. (at 10,000m inspired O2 is about 28% of 21% of partial pressure oxygen, or 5.88% in your blood, meaning death, but at 28% of 82.3% is 23% partial pressure of oxygen, YAY!) It looks like SE calculated 21.4%Effects on Body's Oxygen Supply
This last calculator is really cool!
On this one, enter how hard you are breathing, lets say 15 breaths per minute, enter your elevation (as if you were on Earth), Enter the FiO2, the fraction of inspired (breathed in) oxygen or the same as the partial pressure (82.3%). On Earth including water vapor it would be around 20.7%. But, all the water vapor has fallen out of the sky on your frigid planet though. The air is so dry it would desiccate your nasal and throat passages. You might need a neck contraption that squirts water vapor up your nose now and then, making you look pretty scary to any native life on the planet.
The last calculator shows that on your planet with .261 ATM and 82.3% partial pressure of O2, the pressure of O2 in your blood would be within tolerable ranges even when not breathing hard. Your hemoglobin content would be just a percentage or two less than walking about on Earth smelling the flowers. On your planet your might seriously consider giving up cigarettes because I still think you're going to need to breathe hard for a good gulp of air. Just try not to hyperventilate.
What would be super cool are the Acetylene (C2H2) snow flurries all over the planet. Even with the high oxygen content it would not combust. It would have a very small window to be in liquid form at such low air pressures. It would be a blast trying to set the snow storms on fire!
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