Key takeaways
- Altitude sickness comes from ascending too fast for the body to adjust to thinner air, usually above ~8,000 ft (2,500 m).
- Mild AMS feels like a hangover: headache, nausea, fatigue, poor sleep.
- HAPE (lungs) and HACE (brain) are life-threatening — descend immediately if they appear.
- Prevention is gradual ascent (acclimatization): climb high, sleep low, and don't ascend with symptoms.
This is general educational information, not medical advice. Seek qualified medical guidance for altitude illness, and descend and get professional help in an emergency.
What causes it
At altitude the air holds less oxygen, and ascending faster than your body can adapt triggers altitude sickness. It typically begins above about 8,000 ft (2,500 m) and depends on how high you go and how fast — not on fitness. The body’s adjustment process is called acclimatization.
Types
- AMS (acute mountain sickness) — the common, mild form: headache, nausea, fatigue, poor sleep.
- HAPE — fluid in the lungs; breathlessness at rest, cough, weakness. Life-threatening.
- HACE — fluid in the brain; confusion, loss of coordination. Life-threatening.
A trekker who flies into a 3,500 m town takes an extra rest day, day-hikes higher and returns to sleep low, and treats a persistent headache as a reason to wait rather than climb — textbook acclimatization.
Prevention & treatment
Prevent it by ascending gradually, climbing high and sleeping low, hydrating, and never going higher with symptoms. For severe HAPE or HACE the treatment is immediate descent, supported where available by supplemental oxygen. See how the two severe forms differ in HAPE vs HACE.
The bottom line
Altitude sickness ranges from a miserable-but-mild headache to the rapidly fatal HAPE and HACE, and the difference can be hours. The same simple discipline prevents and treats it: ascend slowly to acclimatize, never push higher with symptoms, and descend at the first sign of severe illness. When in doubt, go down.
Frequently asked questions
What are the symptoms of altitude sickness?
Mild acute mountain sickness (AMS) resembles a hangover: headache plus nausea, fatigue, dizziness, or poor sleep, usually starting 6–12 hours after arriving at altitude. Warning signs of severe illness include breathlessness at rest and a wet cough (HAPE) or confusion and loss of coordination (HACE).
How do you prevent altitude sickness?
Ascend gradually so your body can acclimatize: above ~3,000 m, increase your sleeping elevation slowly, 'climb high and sleep low', take rest days, stay hydrated, and never ascend further with symptoms. The drug acetazolamide can aid acclimatization under medical advice.
What do you do if altitude sickness gets severe?
Descend immediately. HAPE (fluid in the lungs) and HACE (fluid in the brain) are life-threatening and the definitive treatment is going down — even a few hundred metres can be enough. Supplemental oxygen and medications can help, but they do not replace descent.
Sources
- Altitude illness — Wilderness Medical Society
- High-altitude travel — CDC
- Mountain medicine — UIAA