That's definitely a danger zone for healthy people but interestingly enough people with things like COPD may have a blood oxygen level in the 80s and while that is indicative of the disease, they may be totally stable and may not even need oxygen [1].
My grandmother's heart was completely fucked, so they'd have to adjust the alarms on the hospital monitors after checking their files when she went in. It's like "OK, well that's the problem... consults notes... Nope, apparently that is normal for her, now lets figure out what's actually wrong". It wasn't keeping regular time and it would sometimes skip, but apparently it was pumping well enough to keep her alive for several years.
Normal in humans is definitely relative and medicine has tended to assume that if we average 1000 humans (in too many cases, 1000 white college age men) that's what human normal is, which is crazy even beyond obvious problems like " people normally have 1.999 legs apparently".
Bodies are generally pretty amazing in that sense. As long as things go out of spec _slowly_, we will often adapt quite well. In the short term, we will tend to balance even fairly extreme changes out through various chemical processes and in the long term people can even develop heritable genetic changes. (E.g., how people acclimatize and have in some cases adapted to living at higher altitudes[0])
Obviously not. I did the experiment with a finger pulse ox and a Garmin device to check. You just hold your breath. My Apple watch was pretty good at it too. It's very uncomfortable and you'll get visual snow but I'm not dead, super or otherwise. Use your hand to clamp over your mouth and shut your nostrils if you want to try.
I had some momentary readings lower than 80 during a sleep study prior to going on CPAP. I didn't snore, or choke, or anything. Just ... didn't breathe. With CPAP, 98% all the time.
