Your chest heaves, lungs burning, but you can’t catch your breath. Your head pounds, vision blurring, but you feel like you’ll faint any second. You’re definitely fit enough to climb a flight of stairs without trouble — is this some sort of nightmare?
Possibly, but if you’re up in the mountains, you’re more likely experiencing altitude sickness. When you travel to a place above about 8,000 feet, your body starts telling you there’s something seriously wrong with the air up there. The “thin” air at high altitudes has considerably less oxygen and pressure. This is because the earth’s gravity holds the oxygen close to the surface — so much so that half of the oxygen in the atmosphere is found below 18,000 feet. For comparison, Mount Everest is about 29,000 feet. The highest inhabited town in the world is La Rinconada, Peru in the Andes Mountains at nearly 17,000 feet.
With so much less oxygen, your body has to breathe more to get the same amount of the essential molecule.