Altitude sickness is extremely common. At moderate elevations around 3,000 to 4,000 meters (10,000 to 13,000 feet), roughly 10% of trekkers develop symptoms. Above 4,000 meters, that number jumps to about half. Your risk depends heavily on how high you go, how fast you get there, and your individual biology.
Prevalence by Elevation
The relationship between altitude and sickness is steep and predictable. A study of 150 trekkers in the Everest region found 0% prevalence between 2,500 and 3,000 meters (8,200 to 9,800 feet), 10% at 3,000 to 4,000 meters, and 51% at 4,000 to 4,500 meters (13,000 to 14,800 feet). That pattern holds across studies: the higher you sleep, the more likely you are to feel it.
Symptoms typically appear within several hours to three days after arriving at a new elevation, generally above 2,500 meters (8,200 feet). The hallmark symptom is headache, often accompanied by nausea, fatigue, loss of appetite, and dizziness. Most cases are mild and resolve on their own once the body adjusts, but they can make a trip miserable in the meantime.
How Fast You Climb Matters Most
Elevation alone doesn’t tell the whole story. How quickly you gain sleeping altitude is the single biggest controllable risk factor. The CDC and the Wilderness Medical Society break travelers into three risk categories based on ascent speed once above 3,000 meters (9,800 feet):
- Low risk: Gaining no more than 500 meters (1,650 feet) of sleeping altitude per day.
- Moderate risk: Gaining more than 500 meters per day, but building in an extra acclimatization day for every 1,000 meters (3,300 feet) gained.
- High risk: Gaining more than 500 meters per day with no extra acclimatization days.
The practical takeaway: once you’re above 3,000 meters, keep each night’s sleeping elevation no more than 500 meters higher than the last. For every additional 1,000 meters of altitude gained, spend an extra night at the same elevation before continuing up. People who fly directly into high-altitude cities, like Cusco (3,400 meters) or La Paz (3,640 meters), skip all of this gradual adjustment and are especially vulnerable.
Who Gets It More Often
Individual susceptibility varies in ways that aren’t entirely predictable, but a few patterns are clear. A pilot study published in the European Respiratory Journal found that premenopausal women had higher rates of altitude sickness at 3,100 and 3,600 meters compared to men at the same elevations. The reasons aren’t fully understood, but hormonal differences likely play a role.
Physical fitness, surprisingly, does not protect you. Fit hikers sometimes ascend faster and push harder, which can actually increase risk. A history of altitude sickness on a previous trip is one of the strongest predictors that you’ll experience it again. Living at low elevation is another consistent risk factor, simply because your body has no baseline adaptation to thin air.
Altitude Sickness in Children
Diagnosing altitude sickness in kids is tricky. A study of children aged 11 to 12 trekking to 3,886 meters in Taiwan found an overall incidence of 40.6%, which is higher than typical adult rates at similar elevations. But the researchers flagged an important caveat: many symptoms that look like altitude sickness in children, such as nausea, fatigue, and irritability, can also stem from travel fatigue, disrupted routines, homesickness, or simply being uncomfortable sleeping in a crowded mountain hut in a sleeping bag for the first time.
One Colorado study found that 28% of children aged 9 to 14 developed altitude-like symptoms at 2,835 meters, but 21% of children at sea level had similar complaints. That overlap makes it genuinely difficult to separate altitude effects from the normal discomforts of travel in young kids. Sleep disruption, in particular, was reported by nearly 75% of children in the Taiwan study, driven partly by cold, crowding, and missing their parents at night.
When Altitude Sickness Turns Dangerous
The vast majority of altitude sickness cases are uncomfortable but not dangerous. The two serious complications are fluid buildup in the lungs (HAPE) and swelling of the brain (HACE). HAPE incidence ranges from less than 0.01% to as high as 15% depending on the altitude and ascent speed. Among trekkers and mountaineers in the Himalayas and Alps who climb faster than 600 meters per day, the rate is around 4%. HACE is rarer still. Both conditions are medical emergencies, but in children and the general trekking population, reports of either are uncommon and usually involve individual cases rather than widespread occurrence.
The warning signs for these complications are distinct from ordinary altitude sickness. Severe breathlessness at rest, a persistent wet cough, confusion, loss of coordination, or an inability to walk in a straight line all signal that something more serious is happening. Descending even a few hundred meters often produces rapid improvement.
Reducing Your Risk
Gradual ascent is the most effective prevention strategy, and it costs nothing. Beyond pacing your climb, a preventive medication can cut your risk significantly. The Wilderness Medical Society recommends it for anyone at moderate or high risk, and it works by helping your kidneys adjust your blood chemistry to compensate for thinner air. Ibuprofen is a weaker alternative for people who can’t take the standard medication.
A few popular remedies don’t work. Ginkgo biloba, acetaminophen, and inhaled steroid sprays have all been studied and are specifically not recommended for prevention. Hypoxic tents, which simulate altitude at home, also lack reliable evidence for preventing symptoms or improving summit success.
If you do develop mild symptoms, the most reliable treatment is simply stopping your ascent and staying at the same elevation until you feel better. Most people acclimatize within one to three days. Continuing to climb while symptomatic is the single most common mistake that turns a manageable headache into a serious problem.