This graph is the famous oxyhemoglobin dissociation curve. If standing on the top of Everest ( without using bottled O2) the horizontal PaO2 would be 25-30 range, incompatible with life. But on the vertical axis you see that with a high-altitude left-shifted curve, ones hemoglobin saturation % ("O2 sat") is a just barely life-possible 60%. I can tell you that it really doesn't feel good unless it is 75, and even that would be low enough to put you in the ICU back home.
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I wish that I would have taken my O2 sat at the summit but quite frankly - I did not care!!!!
