Altitude Illness

Altitude illness is a state of unease a wilderness explorer feels when traveling at high elevations. Traditionally, 10,000 feet is regarded as the height where altitude symptoms—a sluggish sensation often accompanied by a headache—kick in for most people, but they can afflict others as low as 6,000 feet. Acute cases of this illness can be debilitating, even fatal.

more info on this at REI’s Backcountry Health Topics