The three stages of altitude sickness are acute mountain sickness (AMS), high altitude pulmonary edema (HAPE), and high altitude cerebral edema (HACE). They range from a bad headache you can push through to life-threatening fluid buildup in the lungs or brain. AMS is extremely common, affecting 25% to 43% of people who travel above 2,500 meters (about 8,200 feet). HAPE and HACE are rarer but far more dangerous.
Stage 1: Acute Mountain Sickness (AMS)
AMS is the mildest and most common form of altitude sickness. It typically develops within 6 to 12 hours of arriving at elevations above 2,500 meters, though some people notice symptoms as low as 2,000 meters. About 25% of visitors sleeping above 2,450 meters (8,000 feet) in Colorado experience it. The hallmark symptom is headache, and a diagnosis technically requires one. Beyond that, you may feel nauseous, lose your appetite, feel unusually fatigued, or experience dizziness and lightheadedness.
The good news: AMS is self-limiting. If you stop ascending and give your body time to adjust, symptoms generally resolve on their own. Most people improve within one to two days at the same altitude. The condition becomes a problem when people ignore early symptoms and keep climbing, which can allow it to progress to stage 2 or stage 3.
Doctors and researchers rate AMS severity on a scale of 0 to 12 using the Lake Louise scoring system, which grades headache, nausea, fatigue, and dizziness from none to incapacitating. A score of 3 to 5 is considered mild, 6 to 9 moderate, and 10 to 12 severe. You don’t need to memorize this scale, but it’s worth knowing that mild AMS (a nagging headache plus some fatigue) and severe AMS (incapacitating symptoms across the board) are very different experiences with different levels of risk.
Stage 2: High Altitude Pulmonary Edema (HAPE)
HAPE is fluid accumulation in the lungs. It happens because low oxygen at altitude causes blood vessels in the lungs to constrict much more than normal, driving up pressure inside those vessels. That pressure can force fluid through the capillary walls and into the air sacs where gas exchange happens. The result is essentially drowning from the inside.
Early HAPE feels like worsening fitness: a dry cough, shortness of breath during exertion, and noticeably reduced exercise performance. As it progresses, you become breathless even at rest. Your lips or fingertips may turn blue, your heart rate climbs, and breathing becomes rapid. In advanced cases, you can hear a gurgling sound in the chest, and coughing may produce pink, frothy sputum. A low-grade fever (generally under 38.5°C or 101.3°F) is also common.
HAPE is relatively rare at moderate altitudes, occurring in roughly 1 in 10,000 skiers in Colorado. But above 4,300 meters (14,000 feet), up to 1 in 100 travelers may develop it. Importantly, HAPE can strike on its own without AMS symptoms coming first. Someone who feels fine apart from a cough and breathlessness can still be developing a serious problem.
The single most effective treatment is descending to lower elevation. Even dropping 500 to 1,000 meters can produce rapid improvement. Supplemental oxygen helps if available. HAPE is a medical emergency, and waiting to see if it gets better on its own is dangerous.
Stage 3: High Altitude Cerebral Edema (HACE)
HACE is the most severe form of altitude sickness. It involves swelling of the brain itself and is considered “end-stage” AMS, meaning it represents the worst possible progression of that initial headache and nausea. It is rare, particularly below 4,300 meters (14,000 feet), but it can be fatal within 24 hours if untreated.
The defining symptoms are confusion and loss of coordination. A person with HACE may stagger as if drunk, unable to walk in a straight line (a sign called truncal ataxia). They become lethargic, disoriented, and may not make sense when they speak. Consciousness can deteriorate rapidly. In some cases, HACE develops as a secondary consequence of the severe oxygen deprivation that accompanies HAPE, meaning both the lungs and brain are affected simultaneously.
HACE demands immediate descent. Unlike AMS, it will not resolve by simply resting at the same altitude. Every hour at elevation with brain swelling increases the risk of permanent neurological damage. Even after successful treatment and descent, some people experience lingering memory problems or cognitive difficulties that take weeks to fully resolve.
Why Some People Are More Vulnerable
The single biggest risk factor is how fast you go up. Trekkers climbing Kilimanjaro, where the standard routes ascend quickly, have higher rates of altitude sickness than those in Nepal’s Everest region, where the trail naturally enforces a slower pace. Current guidelines recommend limiting your sleeping altitude gain to no more than 600 meters per day once you’re above 2,500 meters, and adding an extra rest day for every 600 to 1,200 meters of elevation gained.
If you’ve had altitude sickness before, you’re more likely to get it again at similar elevations. Conversely, recent experience at high altitude offers some protection. One thing that does not reliably protect you is being physically fit. People in excellent shape tend to have a similar risk of AMS as less-trained individuals, partly because fit people often ascend faster and push harder, both of which increase risk. In controlled studies, exercise at altitude significantly reduced oxygen levels in the blood and worsened AMS symptoms. Avoiding strenuous exertion in the first day or two after a rapid ascent is one of the simplest ways to reduce your risk.
How the Three Stages Relate to Each Other
AMS, HAPE, and HACE are not always a neat staircase where one leads to the next. AMS can progress to HACE if ignored, but HAPE can develop independently, without any prior AMS symptoms. HAPE and HACE can also occur together. Think of them as three overlapping conditions that share a root cause (not enough oxygen at altitude) but affect different organ systems in different ways.
The practical takeaway is straightforward. A headache and nausea above 2,500 meters is your body’s early warning system. If symptoms improve with rest and hydration, you’re likely dealing with mild AMS. If they worsen, or if you develop a persistent cough, breathlessness at rest, confusion, or trouble walking, the situation has escalated. Descending even a modest amount is the most reliable treatment at every stage.