I wonder how this info correlates with what one would experience using a Pulse Oximeter.    For example, let assume my normal SpO2 reading is 97-98% breathing without face mask.  I put face mask on, and my reading goes down to 94%.  That reduced number would not be attributable to lung efficiency because my lungs are same shape as when I had no mask.  Therefore the reduced number is due to reduced oxygen level available to lungs that convert the oxygen.     Did you know that the percent of oxygen in air is same at all altitude levels, 21%.  But that 21% at higher altitude is 21% of a smaller number, as you go higher.  The barometric pressure at sea level is 760 mmHg.  At 10,000 feet it's 534 mmHg.  So at 10,000 feet it's equivalent to breathing air with on 15% oxygen at sea level.  I am sure the difference between breathing oxygen at reduce oxygen level due to altitude, and that of doing same because of mask, is that at altitude, you are not re-breathing the CO2.  At altitude you can acclimate to reduce oxygen content.  You probably cannot do the same with reduced oxygen percent while also re-breathing CO2, but that would be interesting to know.  How does acclimation work with reduced oxygen vs reduced oxygen and CO2.