Hypoxia and altitude

How high can you go on air alone?

Altitude zones

Read from the ground up: each band needs more help to keep oxygen reaching the body.

• Total space equivalent complete vacuum Needs spacecraft life support.
• Partial space equivalent sealed cabin or pressure suit The Armstrong line, where body fluids boil, is at 63,000 ft.
• Oxygen under pressure 100% O2, positive pressure Forces oxygen into the lungs against the low outside pressure.
• 100% oxygen pure oxygen needed Keeps sea-level oxygen pressure in the lungs.
• Oxygen and air mix diluter-demand Supplementary oxygen mixed with air.
• Physiological zone air is enough The body works normally on air.

Memory hook: Air, mix, pure, pressure, suit: one more step every band.

Takeaway: Air up to about 10,000 ft, then an oxygen mix, 100% oxygen, oxygen under pressure, and above 50,000 ft a sealed cabin or suit.

Why is there less oxygen if it is still 21%?

Thin air

Each column is the air pressing on you. Its height is the total pressure.

Takeaway: The mix never changes, but the total pressure falls, so oxygen's partial pressure falls with it. That is Dalton's law, and it explains altitude hypoxia.