Altitude sickness is common enough to affect ordinary travel plans, but reported rates vary sharply by altitude, ascent speed, destination and diagnostic definition. The CDC cites acute mountain sickness (AMS) in 25% of visitors sleeping above 2,450 m in Colorado, while rates can approach 50% among travelers who fly rapidly to destinations above 3,400 m without staged acclimatization. The figures below compare distinct populations and should not be pooled as though they measured one universal risk.
Contents
- How altitude and destination change prevalence
- Rates in traveler and trekker cohorts
- Severe illness, HAPE and HACE
- Ascent speed and acclimatization
- Demographic and physiological correlates
- Symptoms, scoring and travel impact
How altitude and destination change prevalence
The CDC’s High-Altitude Travel and Altitude Illness guidance reports that altitude-illness rates can approach 50% when travelers reach destinations above 3,400 m rapidly, without staged acclimatization. At a lower sleeping altitude, the CDC cites AMS in 25% of visitors sleeping above 2,450 m in Colorado. On some standard Nepal Everest Base Camp itineraries, prevalence can approach 30% at higher elevations. These are practical destination estimates, not a single controlled comparison.
Older field research from the Swiss Alps illustrates the steep change across elevations. A 1990 study found AMS in 9% of climbers at 2,850 m, 13% at 3,050 m, 34% at 3,650 m and 53% at 4,559 m. The study year matters: these results describe that 1990 cohort and should not be presented as a current global rate. The source is the Prevalence of acute mountain sickness in the Swiss Alps.
HAPE, or high-altitude pulmonary edema, is less frequent than uncomplicated AMS but becomes more important at greater altitude. The CDC Yellow Book summary gives HAPE incidence as roughly 1 per 10,000 skiers in Colorado and at most 1 per 100 travelers above 4,300 m. Those figures describe different settings and exposure patterns, so the lower Colorado estimate should not be extrapolated to high-altitude trekking.
Rates in traveler and trekker cohorts
City arrival can produce substantial short-term illness. In a 2017 study of 1,274 travelers surveyed at the Cusco airport, 37% met the study’s AMS definition during their first 2 days in the city. Among the travelers who met that definition, 38% had a Lake Louise score above 5, classified in the study as moderate-to-severe. The findings come from Incidence of acute mountain sickness and healthcare related behaviors among travelers to Cusco, Peru.
At a much higher destination, a 2012 Nepalese pilgrim cohort found 34.0% AMS incidence on arrival at Gosainkunda, 4,380 m. Within that cohort, the reported relative risk was 1.57 for females versus males, and participants older than 35 had a relative risk of 1.63 compared with those aged 35 or younger. These are associations within that pilgrim population, not proof that the same relative risks apply to every traveler.
Kilimanjaro data show how occupation and travel style can coincide with different rates. In a prospective cohort running from December 2019 through March 2020, severe altitude-illness symptoms affected 8.6% of recreational hikers and 1.5% of porters and guides. Hospitalization for severe altitude-illness symptoms occurred in 1.1% of recreational hikers. The source is the Incidence and predictors of severe altitude illness symptoms in Mt. Kilimanjaro hikers.
At 4,470 m in a 2011 Nepal study, AMS affected 21% of foreign trekkers and 8% of indigenous porters. Because the groups differed in background and likely exposure history, the percentages are best read as cohort-specific observations rather than a simple traveler-versus-porter biological comparison. See A comparison of the incidence and understanding of altitude illness between porters and trekkers in the Solu Khumbu Region of Nepal.
| Setting and period | Population or altitude | Reported result |
|---|---|---|
| Colorado, CDC guidance, 2025 Yellow Book / 2026 edition | Visitors sleeping above 2,450 m | 25% AMS |
| Cusco, 2017 | 1,274 travelers, first 2 days | 37% met AMS definition |
| Gosainkunda, Nepal, 2012 | Pilgrims arriving at 4,380 m | 34.0% AMS |
| Kilimanjaro, Dec 2019–Mar 2020 | Recreational hikers | 8.6% severe symptoms |
| Nepal, 2011 | Foreign trekkers at 4,470 m | 21% AMS |
| Nepal, 2011 | Indigenous porters at 4,470 m | 8% AMS |
Severe illness, HAPE and HACE
Clinical series can make severe conditions look more common than they are among all travelers because they include people who sought care. In a Kathmandu travel-clinic series of 2,564 altitude-related patients from 2001 through 2018, 50% had AMS, 25% had HAPE, 16% had HACE, and 9% had combined HACE and HAPE. The series recorded 81% of altitude-related travelers as outpatients. It is a clinic distribution, not population prevalence. The source is the Health problems in travellers to Nepal visiting CIWEC clinic in Kathmandu — A GeoSentinel analysis.
Among 350 Everest-trek participants studied in 2017–2018, 9% met the study’s HAPE-risk symptom/sign combination and 5% showed signs consistent with HACE. The wording matters: these were study-defined symptom and sign combinations, not necessarily confirmed clinical diagnoses. The same sample produced different AMS estimates depending on the scoring method: 72% reported at least one AMS symptom, 25% had manifest AMS under the study’s symptom-oriented score, and 12.3% met AMS classification under the 1993 Lake Louise scoring system. Results are from Cardiovascular Risk Profiles and Pre-Existing Health Conditions of Trekkers in the Solu-Khumbu Region, Nepal.
A small 2004 Mt. Kailash pilgrim study found high-altitude illness in 35.13% of 37 pilgrims. In that group, 24.32% developed AMS and 10.81% developed HAPE. The sample was only 37 people, so the percentages can shift substantially with a small change in case counts. The source is Incidence of High Altitude Illness.
Ascent speed and acclimatization
The CDC identifies 2–3 nights at about 2,450–2,750 m before going higher as markedly protective against AMS. Above 3,000 m, its guidance recommends limiting sleeping-altitude gain to 500 m per night and adding one extra acclimatization night for each 1,000 m of sleeping-altitude gain. These are ascent-planning recommendations, not guarantees that illness will not occur.
A 2001 double-blind controlled trial in Nepali porters reported AMS in 16.1% of lowland-born porters and 0% of highland-born porters. Among lowland-born porters, AMS affected 4.94% of those who stayed at or below 2,500 m during the pretrial week, compared with 11.11% of those who stayed above 2,500 m. The source is Prevention of Acute Mountain Sickness by Acetazolamide in Nepali Porters.
The same trial reported lower AMS percentages with slower ascent and rest days. Lowland-born porters ascending 300 m or less per day had AMS in 4% of cases, compared with 12% among those ascending more than 300 m per day. AMS affected 4% of porters who took at least 1 rest day and 12% of those who took no rest days. These comparisons describe trial subgroups and should not be treated as a universal dose-response calculation.
A Mustang district hospital model, covering June 2018 through June 2019, estimated twice the moderate-to-severe AMS risk for a 2,978 m one-day ascent compared with gaining the same altitude over 3 days. The result is an estimate from hospital data and modeling, not a direct randomized travel experiment. See Ascent rate and the Lake Louise scoring system.
Demographic and physiological correlates
A 2021 ascent study followed 332 lowland-dwelling volunteers ascending to 5,300 m. Overall AMS was reported in 73.5%; 23.2% had mild AMS and 50.3% had moderate-to-severe AMS. Participants without a previous ascent above 5,000 m were more likely to develop moderate-to-severe AMS. The study is Physiological responses during ascent to high altitude and the incidence of acute mountain sickness.
Oxygen-saturation differences in that study were small in absolute terms but associated with severity. At 3,500 m, participants with moderate-to-severe AMS had resting oxygen saturation of 88.5%, versus 89.6% among those without it. End-exercise saturation was 82.2% for participants with mild AMS versus 83.8% in unaffected participants. For moderate-to-severe AMS, end-exercise saturation was 81.5%, versus 83.8% in unaffected participants.
The Nepalese pilgrim findings add demographic context: females had a reported AMS relative risk of 1.57 versus males, and people older than 35 had a relative risk of 1.63 versus those aged 35 or younger. Such measures describe relative differences in that study and do not replace an individual medical assessment.
Symptoms, scoring and travel impact
The CDC describes typical AMS onset as 2–12 hours after arrival or ascent to a higher elevation. When travelers do not ascend farther, AMS generally resolves within 12–48 hours; the usual course is developing and resolving over 1–3 days. These timing statements come from the CDC’s High-Altitude Travel and Altitude Illness.
In the Cusco study, 20% of travelers with AMS said it affected their travel plans, 10% reported having to stay in bed, and 6% reported missing a tour. Treatment behavior also varied: 69% used coca-leaf tea, 24% took acetazolamide, and 19% consulted a physician. These percentages are among travelers with AMS in that 2017 sample, not among all visitors to Cusco.
Scoring thresholds can materially change the reported rate. In the Everest-trek sample, 72% reported at least one symptom, but 25% met the study’s symptom-oriented definition and 12.3% were classified as having AMS under the 1993 Lake Louise system. In the Mt. Kailash study, AMS scores were 3 in 5.4% of all 37 pilgrims, 4–6 in 13.5%, and 7–9 in 5.4%.
The practical lesson is to read altitude sickness statistics with their altitude, itinerary, population, measurement period and case definition attached. The figures above come from different destinations and designs; clinic and hospital series are not population prevalence estimates, and Lake Louise thresholds and symptom definitions differ across studies. Percentages should therefore be compared cautiously rather than pooled.