Altitude sickness is a group of symptoms triggered when your body doesn’t get enough oxygen at high elevations. It affects roughly 25% of visitors who sleep above 8,000 feet (2,450 meters) in places like the Colorado mountains, and that rate climbs to nearly 50% at destinations above 11,150 feet (3,400 meters) such as Cusco, Peru, or La Paz, Bolivia. Most cases are mild and resolve on their own, but altitude sickness can progress to life-threatening conditions involving fluid buildup in the lungs or brain.
Why Altitude Makes You Sick
The concentration of oxygen in the air is the same at any altitude, about 21%. What changes is air pressure. As you go higher, the air thins out and each breath delivers less oxygen into your bloodstream. At Everest Base Camp (17,400 feet / 5,300 meters), the pressure pushing oxygen into your lungs is roughly half of what it is at sea level. Your body simply can’t absorb oxygen as efficiently under those conditions.
This state, called hypobaric hypoxia, forces your body into a scramble. Your heart rate increases, you breathe faster, and your blood vessels adjust to try to compensate. When these adjustments can’t keep up with the oxygen deficit, you start to feel it. The speed of your ascent matters more than the final altitude because your body needs time to make these physiological shifts. Fly from sea level to a city at 11,000 feet and you’re far more likely to get sick than if you’d hiked there over several days.
The Three Types of Altitude Illness
Acute Mountain Sickness (AMS)
AMS is the most common and mildest form. Symptoms typically appear within 6 to 12 hours of arriving at a higher elevation and center on four complaints: headache, nausea or loss of appetite, fatigue, and dizziness. Headache is the hallmark, present in virtually every case. You might also notice poor sleep, which compounds the fatigue. Most people with AMS feel like they have a bad hangover.
Clinicians use a standardized tool called the Lake Louise Score to gauge severity. It rates headache, gastrointestinal symptoms, fatigue, and dizziness each on a 0 to 3 scale. A score of 3 or higher with headache present qualifies as AMS. Scores of 3 to 5 are considered mild, 6 to 9 moderate, and 10 to 12 severe. You don’t need a formal scoring system to recognize the pattern: if you’ve recently gained altitude and develop a headache plus at least one other symptom from the list, you likely have AMS.
High-Altitude Cerebral Edema (HACE)
HACE is what happens when AMS progresses. Fluid leaks into the brain, causing swelling that disrupts coordination and mental function. The warning signs are distinct: you become unsteady on your feet (a stumbling, drunken gait), confused, or unusually drowsy. Left untreated, HACE can advance to seizures, coma, and death. It’s relatively rare but extremely dangerous, and ignoring worsening AMS symptoms or pushing higher despite feeling sick is the most common path to this point.
High-Altitude Pulmonary Edema (HAPE)
HAPE involves fluid accumulating in the lungs rather than the brain. It develops when low oxygen causes blood vessels in the lungs to constrict unevenly, creating pockets of high pressure that force fluid through capillary walls. The result is progressively worsening breathlessness, a persistent cough (sometimes producing pink or frothy sputum), and an inability to exert yourself without gasping. HAPE can develop alongside AMS or independently, and it’s the most common cause of death from altitude illness.
Who Gets Altitude Sickness
The biggest risk factor is how fast and how high you ascend. Beyond that, individual susceptibility varies in ways that surprise most people. Physical fitness does not protect you. Elite athletes get altitude sickness at the same rates as anyone else. Age doesn’t appear to make a meaningful statistical difference either.
Women are more susceptible than men, with roughly 54% higher odds of developing AMS after adjusting for other factors. A history of previous altitude sickness is one of the strongest predictors, meaning if you’ve had it before, you’re likely to get it again under similar conditions. One counterintuitive finding from research on Catalan travelers: smokers actually had lower rates of AMS than nonsmokers (about 15% vs. 29%), possibly because chronic exposure to low oxygen primes certain adaptive responses. That’s not a recommendation to smoke, but it illustrates how poorly altitude sickness tracks with conventional ideas about “health.”
How to Prevent It
Gradual ascent is the single most effective prevention strategy. Once above 8,000 feet, increasing your sleeping altitude by no more than about 1,000 to 1,500 feet per day gives your body time to adjust. Building in a rest day every 3,000 feet of elevation gain helps further. This is why well-designed trekking itineraries in Nepal or the Andes include “acclimatization days” where you sleep at the same altitude for an extra night.
Even with a careful ascent schedule, altitude illness rates can approach 30% on popular high-altitude treks like the route to Everest Base Camp. Preventive medication is worth considering if your itinerary forces a rapid gain in altitude, which is common when flying directly into high-altitude cities. A prescription medication that works by making your kidneys excrete bicarbonate, prompting deeper and faster breathing, is the standard choice. It’s typically started a day before ascent. Common side effects include tingling in the fingers and toes, increased urination, and an altered taste for carbonated drinks. It’s a sulfa-based drug, so people with sulfa allergies need an alternative.
Staying well hydrated and avoiding alcohol during the first day or two at altitude are sensible steps, though neither is a substitute for a controlled ascent rate.
What to Do If Symptoms Appear
Mild AMS usually resolves within one to three days if you stop ascending and rest at your current altitude. Over-the-counter pain relievers can help with headache, and anti-nausea medication can ease stomach symptoms. The critical rule is simple: do not go higher while you feel sick. Symptoms that are getting worse rather than better after 24 hours are a signal to descend.
For HACE or HAPE, descent is the primary treatment. Going down even 500 to 600 meters (roughly 1,600 to 2,000 feet) often produces dramatic improvement. Supplemental oxygen, when available, helps bridge the gap. In remote settings where immediate descent isn’t possible, portable pressure bags that simulate lower altitude can buy time. Both HACE and HAPE are medical emergencies. The window between early symptoms and serious deterioration can be surprisingly short, sometimes just hours.
Common Destinations and Risk Levels
Understanding which travel destinations carry altitude risk helps you plan ahead. Colorado ski resorts sit between 8,000 and 12,000 feet, high enough to affect about one in four visitors. Cusco, Peru (11,150 feet) and La Paz, Bolivia (11,975 feet) push rates close to 50%, particularly because most travelers arrive by plane with no time to acclimatize. Kilimanjaro, at 19,341 feet at the summit, and Everest Base Camp, at 17,600 feet, carry the highest risk despite multi-day approaches because the final altitudes are simply extreme.
If you’re planning a trip to any of these destinations, the most practical thing you can do is build extra days into your itinerary. Spending a night or two at a moderate elevation before pushing higher dramatically lowers your odds of getting sick and protects you from the more dangerous forms of altitude illness that cut trips short.