https://en.wikipedia.org/wiki/Hyperoxia
This was a screenshot I took months ago while watching a Geology Hub upload on YT. It was a lightbulb moment for my understanding of mass extinction events, (the largest was 250ma). I’ve referenced this multiple times, so thought I might share. Perhaps you find it as interesting as I do.
The low oxygen is is also a lie of this chart. There’s a city of two million people that is 2,900 meters above sea level. At 2,900 meters, the oxygen levels are down to around 14.5%.
Now if you were suddenly just zapped there, you’re going to feel like shit, have a headache, and be tired. But it wouldn’t kill you, and after a couple months you would be completely fine living there forever. The current human body knows how to deal with some shit. Your blood would thicken a bit, your hemoglobin would increase, you’d grow additional capillaries and vessels, and your red blood cell count would increase. These changes take place anywhere from a day to a couple months, depending partially on how low the concentration is.
While my early mention of 14.5% at 2,900 meters is an example I gave, I used it because it was a large city with lots of people. Smaller town/villages/monasteries exist higher up, with oxygen concentrations well below 14%.
why do you know all this?
I knew enough to see the graph here in the link and know it was bullshit because I’ve read stories and taken geography classes to know that people live at very high elevations all their lives and I remembered that those elevations have much lower oxygen concentrations (think all the sherpas that escort mt everest climbers up without needing any oxygen). So then I did just a little google fu to find a big city at a high elevation and cross referenced that with a chart showing oxygen concentrations at certain elevations.