Watch the air thin as you climb.
Drag the aircraft up the column and watch air pressure, density, oxygen, and blood oxygen (SpO₂) fall — they drop exponentially, not linearly. Then check your field with the free density altitude calculator.
The dots are illustrative; the real number density (from n/V = P/kBT) is below — and it's staggering.
2.55 × 1025 molecules/m³ · 100% of sea level
The S-curve. Above the cliff you're safe; past it, saturation falls off a ledge.
This view tops out at 65,000 ft — the Armstrong limit, where unprotected body fluids boil at body temperature. For perspective, the Kármán line (the conventional edge of space) sits near 330,000 ft / 100 km — roughly 5× higher than the top of this chart.
Five, stacked by temperature: the troposphere (to ~7 miles), stratosphere (to ~31 miles), mesosphere (to ~53 miles), thermosphere (to ~370 miles), and the exosphere. The Kármán line — the edge of space — sits at about 62 miles.
No — the mix stays remarkably constant (~78% nitrogen, ~21% oxygen) up to ~62 miles. What changes is the pressure, not the recipe: at 18,000 ft the air is still ~21% oxygen, but there are roughly half as many molecules per breath — which is why partial pressure, not percentage, drives hypoxia.
As pressure drops, the partial pressure of oxygen drops with it, so each breath delivers less oxygen to your blood. Under FAR 91.211, U.S. pilots need oxygen above 12,500 ft cabin altitude (after 30 min), continuously above 14,000 ft, and must provide it to passengers above 15,000 ft.
For an unacclimatized person, SpO₂ falls from ~97% at sea level to ~87% at 10,000 ft, ~72% at 18,000 ft, and ~50% at 25,000 ft. The drop accelerates because the oxygen–hemoglobin curve falls off a cliff below ~60 mmHg PaO₂.