At 30-50,000 feet cruising altitude (higher than Mt. Everest) the atmosphere is perilously thin.
This is great for the plane since it dramatically decreases wind resistance, but it’s not so great for the people inside who like to breathe.
Such low atmospheric pressures cause hyperventilation and possible loss of consciousness with prolonged exposure, especially if you’re from a low-altitude region and not acclimated to it.
The plane isn’t perfectly airtight (nor would you want it to be with 200 people breathing inside) so the cabin pressure needs to be continually vented and boosted.
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