On site you can easily predict with O2 saturation in the blood, small portable testers are available for decades. You see any problems coming a bit in advance,

On site you can easily predict with O2 saturation in the blood, small portable testers are available for decades. You see any problems coming a bit in advance, btw this is also true for general health in normal altitude. Now why the number is as it is in altitude while next guy was more fit down below, but now vomiting furiously is another question, I agree not very clear. Maybe red blood cell pace of production but even that is not a complete picture.