Altitude Sickness Explained: AMS vs HACE vs HAPE, and the Rules That Prevent Them (2026)

The single most important thing to understand before any high trek in Nepal is that “altitude sickness” is not one illness — it is three, and two of them can kill you within hours. Most trekkers only ever meet the mild one. But knowing how to tell acute mountain sickness (AMS) apart from high-altitude cerebral edema (HACE) and high-altitude pulmonary edema (HAPE) — and knowing the four rules that prevent all three — is what keeps a bad night at Dingboche from becoming an emergency. Here is the plain-language physiology, and what to actually do.

What altitude actually does to you

At Everest Base Camp (5,364 m) the air holds roughly half the oxygen it does at sea level. Nothing changes in the oxygen itself — the air is thinner, so each breath delivers less. Your body responds by breathing faster and making more red blood cells, a process called acclimatisation that takes days, not hours. Altitude illness is simply what happens when you climb faster than your body can adapt. It is not caused by cold, by being unfit, or by being weak. The one thing that reliably triggers it is ascending too fast.

AMS: the common one

Acute mountain sickness is the mild, common form — and the warning shot. It usually begins 6 to 12 hours after arriving at a new altitude and feels, honestly, like a hangover: a headache, plus some combination of nausea or loss of appetite, fatigue, dizziness and broken sleep. The clinical benchmark, the Lake Louise score, calls it AMS when a headache appears with a total symptom score of 3 or more after a recent climb. AMS itself is not dangerous. What is dangerous is climbing higher while you have it, because AMS is the doorway to the two conditions that are.

HACE: when it reaches the brain

High-altitude cerebral edema is AMS that has progressed to swelling of the brain. The symptom that separates it from an ordinary bad headache is unmistakable once you know it: ataxia — loss of coordination. A trekker with HACE cannot walk a straight heel-to-toe line, seems drunk, becomes confused, drowsy or behaves out of character. This is a medical emergency. HACE can progress from “a bit off” to unconscious in hours. The heel-to-toe walk test is the field check every guide uses; if someone fails it at altitude, they are going down, now.

HAPE: when it reaches the lungs

High-altitude pulmonary edema is fluid collecting in the lungs, and it is the leading cause of altitude death. It can strike without any AMS beforehand. The signs: breathlessness that is out of proportion — gasping at rest, not just on the climb — a dry cough that turns wet and may produce pink, frothy sputum, chest tightness, blue-tinged lips, and a collapse in exercise tolerance where a trekker who walked strongly yesterday cannot keep up today. Like HACE, the treatment is immediate descent.

The four golden rules that prevent all three

The Wilderness Medical Society’s 2024 guidelines come down to four things any trekker can follow:

1. Ascend slowly. Above 3,000 m, do not increase your sleeping altitude by more than about 500 m per night, and take an extra rest day for every 1,000 m gained. This is the single most protective rule there is.

2. Climb high, sleep low. On acclimatisation days, hike up a few hundred metres, then come back down to sleep. It is why a well-built Everest Base Camp itinerary parks you two nights at Namche (3,440 m) and again at Dingboche (4,410 m) rather than pushing straight through.

3. Never ascend with symptoms — and descend if they worsen. Descent is the definitive treatment for every form of altitude illness. Losing 300–1,000 m of elevation can reverse HACE and HAPE that no drug will fix in place.

4. Consider Diamox, but do not rely on it. Acetazolamide (Diamox), typically 125 mg twice a day starting the day before you go high, speeds acclimatisation and is well proven for prevention. It is a helper, not a substitute for a sensible ascent profile. Discuss it with your doctor before you travel.

What the numbers on the Everest trail look like

The Everest Base Camp trek is the clearest example of why pacing matters, because it climbs relentlessly:

Fitness will not save you here. Studies of Everest trekkers have found AMS in anywhere from a quarter to nearly three-quarters of walkers depending on how fast they went — and strong, young, fit people are not spared, because the illness is about the rate of ascent, not the strength of the legs. Trip length is the real safety feature. A 10-day dash to base camp gives your body far less time to adapt than a 14-day itinerary with two proper acclimatisation days built in. If you want the deeper rule-set for the Everest trail specifically, see our guide to the altitude rules that keep Everest trekkers safe.

Cover photo: Robert Kern via Wikimedia Commons (CC BY-SA 3.0).