The atmosphere as a physical thing

Up to about 80 km, the air's composition is essentially constant: roughly 78% nitrogen, 21% oxygen, 1% other (mostly argon, with a sliver of CO₂). These are dry-air values. The proportions don't change with altitude, that's a common student trap. What changes is the density: there's progressively less of everything as you climb. This matters because hypoxia at altitude isn't about oxygen disappearing from the air, it's about partial pressure dropping, which is Dalton's Law from physics class.