Altitude sickness is a group of illnesses triggered by reduced oxygen at high elevations, typically above 2,500 meters (about 8,200 feet). It ranges from a bad headache and nausea to life-threatening fluid buildup in the lungs or brain. Between 25% and 43% of people who climb to elevations of 2,500 to 4,300 meters experience some form of it, and that number jumps to over 60% above 6,000 meters.
Why High Altitude Makes You Sick
The air at high altitude contains the same percentage of oxygen as the air at sea level. The difference is pressure. As you go higher, atmospheric pressure drops, so each breath delivers less oxygen into your bloodstream. This is called hypobaric hypoxia. For most people, measurable drops in blood oxygen begin between 2,000 and 3,000 meters.
Your body tries to compensate immediately. You breathe faster, your heart rate increases, and your blood thickens slightly as your body concentrates its red blood cells. Over days and weeks, your bone marrow produces more red blood cells to carry oxygen more efficiently. This gradual adjustment is called acclimatization, and it’s the reason Sherpas and people living in Andean highlands function normally at elevations that would flatten most lowlanders.
Altitude sickness happens when you ascend faster than your body can acclimatize. The risk is directly tied to how fast you climb and how high you go. People who already have an infection or inflammatory illness, even something as minor as a respiratory cold or a bout of traveler’s diarrhea, are more susceptible and can develop symptoms at lower elevations or with slower ascents.
Three Forms of Altitude Sickness
Altitude illness exists on a spectrum, from mild and common to rare and fatal. The three recognized forms overlap, and one can progress into another.
Acute Mountain Sickness (AMS)
AMS is the most common form and the one most travelers experience. The defining symptom is headache, accompanied by at least one other problem: nausea, vomiting, fatigue, weakness, or dizziness. It typically develops within 6 to 12 hours of reaching a new altitude. Researchers and expedition medics use the Lake Louise scoring system to gauge severity, rating headache, gastrointestinal symptoms, fatigue, and dizziness on a 0 to 3 scale. A score of 3 or more, with headache present, confirms AMS. Scores of 3 to 5 are considered mild, 6 to 9 moderate, and 10 to 12 severe.
Mild AMS is unpleasant but not dangerous. It usually resolves on its own within one to two days if you stop ascending and let your body catch up. Pushing higher with symptoms is where the real risk begins.
High Altitude Pulmonary Edema (HAPE)
HAPE occurs when fluid leaks into the lungs. At high altitude, reduced oxygen causes blood vessels in the lungs to constrict unevenly, creating pockets of extremely high pressure that can rupture tiny capillaries. The early stage is essentially a mechanical failure: fluid is forced out of damaged capillaries and into the air sacs. If it persists, inflammation sets in and recovery takes much longer. Early symptoms include a dry cough, breathlessness that worsens even at rest, and extreme fatigue. As it progresses, you may hear a gurgling sound with each breath and cough up pink or frothy sputum.
HAPE can develop independently of AMS and sometimes appears without much warning headache. It is the most common cause of death from altitude illness.
High Altitude Cerebral Edema (HACE)
HACE is swelling of the brain caused by oxygen deprivation. It represents the most severe end of the altitude sickness spectrum and is considered the terminal stage of AMS. Low oxygen causes blood vessels in the brain to dilate and leak fluid into surrounding tissue. The hallmark signs are confusion and ataxia, a loss of coordination that makes walking in a straight line difficult. Ataxia is often the earliest specific warning sign. Without treatment, HACE can progress to coma and death from brain herniation within 12 to 24 hours. About 13% to 20% of people diagnosed with HAPE also develop HACE simultaneously.
Who Gets Altitude Sickness
Physical fitness does not protect you. Elite athletes get altitude sickness at the same rates as sedentary travelers. The primary risk factors are speed of ascent, maximum elevation reached, and individual physiology that is largely genetic. Some people are inherently more prone to the uneven blood vessel constriction in the lungs that triggers HAPE, and there is currently no reliable way to predict who will be affected before they’re exposed to altitude.
Previous experience at altitude is a partial guide. If you’ve had altitude sickness before at a given elevation, you’re likely to get it again under similar conditions. But a trouble-free trip doesn’t guarantee the next one will go the same way.
How to Prevent It
The most effective prevention is controlled ascent. Above 2,500 meters, the standard guideline is to increase your sleeping elevation by no more than about 300 to 500 meters per day, with a rest day (no additional altitude gain) built in every three to four days. Your daytime elevation can be higher, as long as you descend to sleep. This gives your body time to adjust overnight.
A prescription medication called acetazolamide can help speed acclimatization. It works by making your blood slightly more acidic, which stimulates deeper and faster breathing, boosting oxygen intake. The CDC recommends 125 mg twice daily, starting the day before ascent and continuing for the first two days at altitude or longer if you keep climbing. The most common side effects are tingling in the fingers and toes and a strange alteration in taste: carbonated drinks taste flat or metallic because the drug interferes with how your tongue processes carbon dioxide.
Treatment When Symptoms Appear
For mild AMS, the treatment is simple: stop ascending. Stay at your current elevation, rest, hydrate, and take standard pain relievers for the headache. Most people feel better within a day or two and can then resume climbing at a slower pace.
For moderate to severe AMS, HAPE, or HACE, descent is the primary treatment and often the only reliable one. Going lower increases the oxygen pressure in each breath, which directly reverses the cascade of problems. Even a descent of 300 to 1,000 meters can produce dramatic improvement, particularly for HAPE caught early, where the fluid buildup is mechanical rather than inflammatory. The longer you wait, the more inflammation develops, and the slower recovery becomes even after descending.
On remote expeditions where descent isn’t immediately possible, portable hyperbaric chambers (often called Gamow bags) serve as a temporary bridge. A person is zipped inside and the bag is pumped up to simulate a descent of roughly 1,500 to 2,500 meters. This buys time but is not a substitute for actually getting lower. Supplemental oxygen works on the same principle, directly increasing the amount of oxygen reaching the blood.
What Normal Oxygen Levels Look Like at Altitude
At sea level, a healthy person’s blood oxygen saturation sits between 95% and 100%. Values below 90% are considered abnormally low by medical standards. At altitude, everyone’s levels drop. A reading of 88% to 92% at 3,500 meters might be perfectly normal for someone who is acclimatizing well and feeling fine. Pulse oximeters, which are small and inexpensive, have become common gear on treks, but interpreting the numbers requires context. A low reading alone does not mean you have altitude sickness, and a relatively normal reading does not mean you’re immune to it. Symptoms matter more than the number on the screen.
The practical takeaway: if you’re above 2,500 meters and develop a headache plus nausea, dizziness, or unusual fatigue, treat it as altitude sickness until proven otherwise. If you or someone in your group becomes confused or can’t walk a straight line, begin descending immediately.