Elevation sickness, more commonly called altitude sickness, is a group of conditions triggered when you ascend to heights where the air contains less available oxygen than your body is used to. The fundamental problem is straightforward: as altitude increases, barometric pressure drops, and with it the amount of oxygen pushed into your bloodstream with each breath. That oxygen shortfall, called hypoxemia, is the root cause behind every form of altitude illness, from a mild headache at a ski resort to life-threatening fluid buildup in the lungs or brain. What makes the topic more interesting than “go up, feel bad” is that altitude sickness comes in distinct types with very different mechanisms, timelines, and levels of danger.
Why Altitude Makes You Sick
At sea level, the atmosphere presses down with enough force to push oxygen efficiently from your lungs into your blood. As you climb, that pressure falls. At around 3,000 meters (roughly 10,000 feet), there is about 30 percent less effective oxygen per breath than at sea level. Your body notices this almost immediately. Peripheral chemoreceptors in the neck detect the drop in blood oxygen and trigger a cascade of compensatory responses, the most important of which is hyperventilation: you breathe faster and deeper to pull in more air. This hyperventilatory response is the first and most critical step in acclimatization, and it kicks in within hours of arriving at altitude.
The catch is that hyperventilation also blows off carbon dioxide, making your blood more alkaline. That alkalinity actually blunts the drive to keep breathing harder, creating a tug-of-war between the need for oxygen and the body’s discomfort with changing blood chemistry. If acclimatization goes well, the kidneys compensate over days by excreting bicarbonate, restoring a more normal blood pH and allowing ventilation to stay elevated. If the process is too slow or the altitude too high, symptoms develop.
Acute Mountain Sickness
Acute mountain sickness (AMS) is the most common and mildest form of elevation sickness. It typically appears within six to twelve hours of reaching altitudes above about 2,500 meters. The hallmark symptom is headache, and researchers have standardized its diagnosis using the Lake Louise Acute Mountain Sickness Score, a questionnaire system first published in 1991 and most recently revised in 2018. That revision dropped “disturbed sleep” from the scoring criteria after studies showed that poor sleep at altitude is caused by hypoxia itself rather than being closely tied to AMS as a syndrome.
The remaining scored symptoms are headache, gastrointestinal problems like nausea or vomiting, fatigue or weakness, and dizziness or lightheadedness. A score of three or more, with headache present, qualifies as AMS. Most cases resolve on their own within one to three days if you stop ascending and give your body time to catch up. AMS is uncomfortable but not directly dangerous. The danger lies in ignoring it and continuing to climb, because AMS can progress to the two severe forms of altitude illness.
High-Altitude Cerebral Edema
High-altitude cerebral edema (HACE) is what happens when altitude illness reaches the brain. It is a potentially fatal metabolic encephalopathy in which the brain swells with excess fluid. The classic progression is headache giving way to confusion, loss of coordination (ataxia), and eventually stupor or coma. HACE typically develops at altitudes above 4,000 meters, though it can occur lower in susceptible people, and it tends to emerge after AMS symptoms have been present for a day or more.
The swelling is believed to involve vasogenic edema, meaning fluid leaks from blood vessels into brain tissue. MRI research has shed some light on the sequence of events: when healthy volunteers were exposed to simulated high altitude, cerebral oxygen delivery was initially maintained through increased blood flow velocity and arterial dilation, but over roughly 22 hours, brain white matter volume increased measurably. Researchers also observed compression of small, deep cerebral veins, suggesting that restricted venous outflow contributes to the fluid buildup, a mechanism that had not been clearly demonstrated before.
HACE is a medical emergency. Descent is the single most effective treatment. Supplemental oxygen and dexamethasone can buy time, but getting to lower elevation is the priority. Left untreated, HACE can kill within 24 hours.
High-Altitude Pulmonary Edema
High-altitude pulmonary edema (HAPE) is the other severe altitude illness, and it is the leading cause of death from altitude sickness. Rather than the brain, it targets the lungs. HAPE usually appears two to four days after arrival at high altitude and is driven by an exaggerated constriction of blood vessels in the lungs in response to low oxygen. This constriction raises pulmonary artery pressure, and when the pressure climbs high enough, fluid is forced out of capillaries and into the air spaces of the lungs.
Research has shown that people susceptible to HAPE have significantly higher pulmonary artery and capillary pressures at altitude compared with non-susceptible individuals. In one study, every participant who developed HAPE had a pulmonary capillary pressure above 19 mmHg, while every susceptible participant who stayed healthy remained below that threshold. The condition also involves what researchers call stress failure of the capillary wall, where the mechanical pressure physically damages the thin barrier between blood vessels and lung tissue, allowing fluid and even red blood cells to leak through.
Early symptoms include a dry cough, unusual breathlessness during exertion, and reduced exercise tolerance. As it progresses, you may hear a gurgling sound when breathing, develop frothy or pink-tinged sputum, and experience severe shortness of breath even at rest. Like HACE, HAPE requires urgent descent. Supplemental oxygen helps, and the calcium channel blocker nifedipine has been shown to significantly reduce pulmonary artery pressure and prevent HAPE in susceptible individuals.
Who Is Most at Risk
Your single biggest risk factor is how fast you go up. A case-control study of trekkers in the Himalayas found that rapid ascenders were roughly six times more likely to develop altitude sickness than those who climbed slowly. A randomized trial on Muztagh Ata (7,546 meters) confirmed that even a few extra days of acclimatization made a dramatic difference: climbers on a slower 19-day protocol had significantly lower symptom scores and were far more likely to reach high camp without AMS than those on a 15-day schedule.
Personal history matters just as much. The Himalayan study found that people with a previous episode of altitude sickness were about ten times more likely to develop it again on a subsequent trip. Other recognized risk factors include your home elevation (people living at sea level are more vulnerable), maximum altitude and sleeping altitude, intensity of physical exertion, age, and pre-existing health conditions. One study also identified a body mass index above 24 as a risk factor for AMS, though the relationship between fitness and susceptibility is more complicated than most people assume. Being in excellent cardiovascular shape does not protect you. In fact, very fit people sometimes ascend faster and push harder, which can increase risk.
The Genetic Component
If you have ever wondered why your hiking partner breezes through while you feel terrible at the same altitude, genetics is part of the answer. Researchers have identified variants in at least 16 genes across multiple biological pathways that show positive associations with AMS susceptibility, suggesting it is a polygenic disorder influenced by many small genetic contributions rather than a single gene. Genomic techniques over the past decade have strengthened the case that susceptibility to altitude illness has a meaningful heritable component, though the exact mechanisms remain unclear.
One specific example: a study of soldiers found that a variant in the EDN1 gene, which codes for endothelin-1 (a potent blood vessel constrictor), was significantly associated with AMS risk. Individuals carrying certain alleles of that gene were more likely to develop symptoms. This kind of finding helps explain the wide individual variation that altitude medicine researchers observe. Two people of the same age, fitness level, and ascent rate can have dramatically different experiences, and at least some of that gap appears to be written into their DNA.
Chronic Mountain Sickness
All the conditions discussed so far are acute, meaning they develop within days of altitude exposure. But there is also a chronic form that affects people who live permanently at high altitude. Chronic mountain sickness (CMS), sometimes called Monge’s disease, is a progressive syndrome caused by lifelong exposure to hypoxia. It is characterized by an excessive increase in red blood cell production: hemoglobin levels rise above 19 g/dL in women and 21 g/dL in men, thickening the blood and worsening oxygen delivery rather than improving it.
CMS patients experience deep hypoxemia, and the condition is frequently accompanied by pulmonary hypertension. In advanced cases, the right side of the heart enlarges and fails from the strain of pumping against high-pressure lung vessels, a progression known as cor pulmonale. Millions of people worldwide live above 2,500 meters, and a significant portion are at risk. Current evidence points to a genetic predisposition, with certain highland populations showing higher rates than others. Treatment options include descent to lower altitude (often impractical for economic reasons), phlebotomy to reduce blood thickness, and medications that target the overproduction of red blood cells.
Highland Populations and Evolutionary Adaptation
Not everyone who lives at altitude develops CMS, and the reason is evolution. Three major human populations have lived at high altitude for thousands of years and have each evolved distinct solutions to the problem of chronic oxygen scarcity. Andean populations (Aymara and Quechua), Tibetans and Sherpa, and Ethiopian highlanders (Amhara and Oromo) all handle the oxygen cascade differently.
Andeans tend to have higher hemoglobin concentrations, essentially making more red blood cells to carry whatever oxygen is available. Tibetans, by contrast, maintain relatively normal hemoglobin levels but show enhanced blood flow and more efficient oxygen extraction at the tissue level. Ethiopian highlanders appear to use yet another strategy, maintaining oxygen saturation levels closer to what you would see in lowlanders despite living at comparable altitudes. These three independent evolutionary experiments demonstrate that there is no single “correct” adaptation to altitude. Each population found a different physiological path to the same destination: surviving and thriving with less oxygen.
How Sleep Changes at Altitude
Almost everyone who goes to altitude sleeps poorly, and the mechanism is more specific than just feeling uncomfortable. Above about 3,000 meters, nearly all healthy people develop periodic breathing during sleep, a pattern of alternating deep breaths and pauses (central apneas) that fragments the night into a roller coaster of brief arousals. The cause is the heightened sensitivity of chemoreceptors to carbon dioxide changes: each cycle of hyperventilation drives COâ‚‚ low enough to temporarily suppress the breathing drive, leading to a pause, which lets COâ‚‚ rise again, triggering another burst of deep breaths.
Sleep architecture shifts measurably. Light sleep stages increase, while deep slow-wave sleep and REM sleep both decrease. The result is that even if you spend eight or nine hours in bed, you wake feeling unrefreshed. This fragmentation is a major contributor to daytime fatigue and impaired performance at altitude, and it likely feeds into the broader symptom picture of AMS even though the 2018 Lake Louise revision removed sleep disturbance from the formal scoring. Periodic breathing is almost universal among newcomers to altitude but is far less common in long-term high-altitude residents, consistent with the idea that full acclimatization eventually dampens the exaggerated chemoreceptor sensitivity that drives the cycle.
Prevention Through Acclimatization
The most reliable way to prevent altitude sickness is to ascend gradually. The general guideline is to increase your sleeping altitude by no more than about 300 to 500 meters per day once above 2,500 meters, with a rest day every three or four days of ascent. Research supports the practical value of even modest pre-exposure: six days at 2,200 meters substantially reduces AMS and improves physical performance after a rapid jump to 4,300 meters. Evidence also suggests that spending five or more days above 3,000 meters within the preceding two months significantly lowers AMS risk on a subsequent rapid ascent to 4,500 meters.
For athletes and mountaineers, structured pre-acclimatization programs that alternate living at moderate altitude with training at various elevations have proven effective. The principle is that the body retains some acclimatization memory for weeks after returning to low altitude, so a staged approach can compress the timeline. Even intermittent daily exposures of a few hours to altitudes above 4,000 meters can trigger ventilatory acclimatization if repeated over several days.
Medications for Prevention and Treatment
When gradual ascent is not practical, medications can help. Acetazolamide (sold as Diamox) is the best-studied preventive drug for AMS. It works through multiple mechanisms: it causes the kidneys to excrete bicarbonate, producing a mild metabolic acidosis that counteracts the alkalosis from hyperventilation and allows chemoreceptors to respond more fully to low oxygen. It also improves ventilation through tissue-level effects and, perhaps most practically, improves sleep quality by stabilizing breathing patterns at night. Pre-treatment with acetazolamide starting the day before ascent has been shown to improve tissue oxygenation at altitude, though the full benefit may take until the second day of exposure.
Dexamethasone, a corticosteroid, is an effective alternative, particularly at altitudes above 4,000 meters at doses of 8 to 16 mg per day. One study found that dexamethasone reduced AMS symptoms by about 63 percent compared with placebo. However, dexamethasone does not fix the underlying physiological problems: oxygenation, sleep apnea, and fluid shifts were unchanged in treated subjects. For that reason, many experts recommend reserving it for situations where descent is impossible or to help a severely symptomatic person cooperate with evacuation. It is also the go-to emergency treatment for HACE.
For HAPE prevention in known susceptible individuals, nifedipine is the best-established option. In a randomized trial at 4,559 meters, only one of ten subjects on nifedipine developed HAPE, compared with seven of eleven on placebo. The drug works by lowering pulmonary artery pressure, directly counteracting the exaggerated vasoconstriction that drives the condition. Phosphodiesterase inhibitors like sildenafil have also been proposed as potential HAPE preventives based on their ability to reduce pulmonary artery pressure through a different pathway, though the clinical evidence base is less established.
Pre-Existing Lung Disease and Altitude
If you have a chronic lung condition, altitude travel deserves extra thought. The physiological challenges of high altitude are essentially the same challenges that lung disease already imposes: reduced oxygen delivery and increased strain on the heart and pulmonary circulation. People with moderate to severe COPD who travel even to relatively modest altitudes around 3,100 meters can experience significant worsening of breathlessness, exercise limitation, and sleep quality. Severe hypoxemia at altitude in COPD patients may be asymptomatic but still carries real risks, including dangerous spikes in systemic and pulmonary blood pressure, cardiac arrhythmias, and even ischemic events in the heart or brain.
The type and severity of the underlying disease determine how much risk altitude adds. Mild, well-controlled asthma, for example, is generally compatible with altitude travel, whereas conditions that already compromise oxygen transfer at baseline require careful medical evaluation. Supplemental oxygen, adjusted medication, and conservative ascent profiles are all part of the conversation for anyone with significant lung disease planning a high-altitude trip.
Pregnancy at Altitude
Pregnant travelers face a unique set of concerns. The fetus depends entirely on the mother’s oxygen supply, and anything that reduces maternal oxygenation can affect fetal well-being. Altitude exposure triggers physiological responses that help preserve oxygen delivery to both mother and baby, but these compensatory mechanisms have limits, especially during exercise. A survey of obstetrical providers in Colorado identified preterm labor and bleeding complications as the most commonly encountered problems among pregnant high-altitude visitors, with dehydration and strenuous exertion before acclimatization contributing to risk.
Pre-existing conditions that raise the risk of hypertension, preeclampsia, or fetal growth restriction are considered contraindications for altitude travel, especially after 20 weeks of pregnancy. The general recommendation for pregnant women visiting altitude is to respect the acclimatization process, avoid medications when possible, allow at least several days before engaging in physical activity, and stay well hydrated. The evidence base is limited compared with altitude medicine in general, so caution tends to be the default advice.
Smartwatches and Oxygen Monitoring
Consumer pulse oximeters and smartwatches that measure blood oxygen saturation (SpOâ‚‚) have become common accessories on high-altitude treks. Research supports the general principle behind this: blood oxygen levels drop measurably in people developing AMS, and SpOâ‚‚ has been identified as an independent predictor of acute mountain sickness. One study found that each percentage-point increase in SpOâ‚‚ decreased the odds of AMS by about 8.6 percent, meaning that people with lower readings were progressively more likely to be symptomatic.
The practical value is real but comes with caveats. Wrist-worn sensors are less accurate than fingertip medical devices, cold hands and poor circulation at altitude introduce errors, and there is no single SpOâ‚‚ cutoff that reliably separates “you are fine” from “you have AMS.” Individual baseline values vary, so a reading of 85 percent might be unremarkable for one person at 4,000 meters and alarming for another. The more useful approach is tracking your own trend: a steady decline over hours, especially paired with a worsening headache, is a stronger signal than any single number. Treating the watch as a supplement to paying attention to how you feel, rather than a replacement for it, is the sensible middle ground.