Oxygen & Altitude

Just a paragraph I found on oxygen levels and increasing altitude that may be helpful to some:
First, the science. The air we breath has a mixture of gases including oxygen and nitrogen. The atmosphere contains about 21% oxygen at all altitudes. What changes as we go higher is the air pressure. Gravity pulls on gas molecules in the air. The closer to the Earth’s surface, the more pressure; the higher, less. At the summit of Everest, the pressure was measured in 1981 by a Nova team at 253 mmHG. At sea level it is 760 mmHG. The reason for the lower pressure is that there is less atmosphere pushing down from above. This reduced pressure allows the air molecules to scatter. This means that on Everest the air is not as dense or “thick”. In other words there is the same amount of oxygen but the molecules are scattered thus the term “thin air”. So in each breath there is less oxygen. To be precise about 66% less!