Altitude and acclimatisation
What altitude illness is, why a slow climb matters, and why going down is the first answer when someone is unwell.
This is not a substitute for training, a qualified guide or medical advice. Climbing at altitude can kill. Learn from qualified instructors and climb with licensed Nepali guides.
What altitude illness is
Altitude illness is a family of conditions caused by climbing to a height where the air holds less usable oxygen than the body is ready for. The mildest and most common is acute mountain sickness (AMS). Its main symptom is a headache, often with poor appetite, dizziness, tiredness or nausea.12
The Himalayan Rescue Association says AMS usually shows up at around 8,000 ft (about 2,440 m) or higher. It stresses that AMS is preventable and that early warning signs should not be ignored.2
The serious forms: HACE and HAPE
High-altitude cerebral edema (HACE) is swelling of the brain. The CDC Yellow Book describes it as the end stage of AMS, with confusion, drowsiness and unsteady walking. Without descent it can progress to coma within about 12 to 24 hours of the onset of unsteadiness.1
High-altitude pulmonary edema (HAPE) is fluid in the lungs. It can appear alone or alongside AMS or HACE, and early signs include a cough and getting breathless on effort. The Yellow Book notes it can be more rapidly fatal than HACE.12
Why acclimatisation matters
Acclimatisation is the body's gradual adjustment to lower oxygen. The CDC Yellow Book says the main acute adjustment takes place over the first 3 to 5 days at altitude, and that altitude illness can develop before it is complete.1
The Himalayan Rescue Association describes the idea as a kind of truce that only works if you ascend gradually. Its advice includes the saying 'climb high, sleep low' and taking rest days every second or third day.2
Going up slowly, going down when ill
The Yellow Book gives rules of thumb that it attributes to Wilderness Medical Society recommendations: above 3,000 m, raise your sleeping altitude by no more than about 500 m a day, and add a rest day for every 1,000 m gained. It adds that these figures may be too fast for some people and too slow for others, so they are a guide to the idea and not a schedule to follow on your own.1
The central rule is simple. Do not go higher while you have symptoms, and go down if they get worse. Both the Yellow Book and the Himalayan Rescue Association call descent the key treatment, and the Yellow Book says descent and oxygen work much better than medication.12
Know your limits
This page is an introduction, not medical advice. Altitude illness can turn serious quickly, and the right plan depends on your health, your route and your team. Talk to a travel doctor before a trip and rely on an experienced guide on the mountain.12
Sources
- High-Altitude Travel and Altitude Illness (CDC Yellow Book), Centers for Disease Control and Prevention. Retrieved 4 November 2026.
- Health problems (altitude illness guidance), Himalayan Rescue Association. Retrieved 4 November 2026.
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