Mmm... As you go higher the _percent_ of oxygen remains the same while the total pressure drops. As the total pressure drops, so does the partial pressure of every component, including Oxygen. Partial Pressure O2 = Percent O2 * Total Pressure Partial pressure of oxygen is the primary driver of (loss of) consciousness at altitude. By raising the percentage of oxygen, you raise the partial pressure of oxygen, which is what you need to stay conscious and functional. The more oxygen you consume, the higher the partial pressure. Incidentally, consuming compressed air will do absolutely nothing useful. It's still 20.9% oxygen, and you'll go unconscious just as fast breathing it out of a can as you would out of the air. (Tech diver... monitoring partial pressure is what we do. Pure O2 too deep, you die. Bottom mix too shallow, you die. All based on the differences in partial pressure.)
