Some years back, I took FAA’s high altitude course (hyperbaric chamber). During the classroom portion, the instructor told us that the partial pressure of oxygen in the atmosphere at approx fl330 equals the pressure required for O2 molecules to pass through the thin walls of your alveoli (sp?), hence no transfer above this altitude without pressure breathing equipment. Keep this in mind the next time you’re a passenger up nice and high in cruise. The mask that is provided for you is just an ornament to satisfy the regulation.-CY
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*deep inhale* I LOVE the smell of jet-a in the morning..