Altitude Sickness: What Height Does to Bodies and How to Acclimatize

The flight lands in Cusco, Peru, at 11,150 feet, and for the first hour everything seems fine. You wheel your bag through the terminal, admire the mountains, order lunch. Then, somewhere around dinner, a dull band of pressure settles behind your eyes. Your appetite vanishes. Climbing one flight of hotel stairs leaves you breathing like you sprinted for a bus.

None of this means anything went wrong. It means physics did exactly what physics does. Your body just moved from an ocean of air into a shallower one, and it needs time — measured in hours and days, not minutes — to adjust.

The frustrating part is how much folklore surrounds that adjustment. Travelers swear by herbal teas, breathing tricks, or sheer toughness. The evidence tells a simpler, more useful story: the single variable you control is how fast you go up. Here is what height actually does to a body, and how to work with it.

What Is Altitude Sickness, Really?

Altitude sickness — doctors call the common form acute mountain sickness, or AMS — is your body’s protest against a rapid drop in available oxygen. Here’s the detail most people get wrong: the air at 12,000 feet still contains about 21 percent oxygen, the same proportion as at sea level. What changes is barometric pressure. With less atmosphere pressing down from above, each breath packs fewer oxygen molecules into your lungs. At 8,200 feet (2,500 meters), you’re getting roughly 25 percent fewer molecules per breath; at Everest Base Camp, about half.

Your blood oxygen level falls, and a cascade begins. You breathe faster and deeper without noticing. Your heart rate climbs. Blood vessels in the brain dilate to keep oxygen delivery steady — a leading explanation for the signature headache. Fluid shifts subtly in tissues, which is why some travelers notice puffy hands or a tight face on the first morning.

According to the CDC, roughly a quarter of people who sleep above 8,000 feet develop at least mild AMS, and the proportion rises steeply with speed of ascent and sleeping elevation. This is not a disease of the weak or unprepared. It is a normal, predictable physiological response that becomes a problem only when the ascent outpaces the adaptation — or when warning signs get ignored.

What Does Altitude Sickness Feel Like?

The most honest comparison, used by clinicians at Mayo Clinic and the NHS alike: it feels like a bad hangover you didn’t earn. The headache typically arrives first — throbbing, often worse at night and on waking, aggravated by bending over. Around it cluster the supporting symptoms:

• Nausea, sometimes with vomiting, and a flat loss of appetite (dinner looks fine; you just don’t want it)
• Fatigue that’s out of proportion to what you’ve done that day
• Dizziness or lightheadedness, especially when standing up
• Fragmented, restless sleep, often with strange dreams and frequent waking
• Shortness of breath with exertion that would be trivial at home

Timing matters as much as the symptoms themselves. AMS typically begins 6 to 12 hours after arriving at a new altitude — rarely immediately, which is why that first pleasant hour in a mountain town can be misleading. Symptoms usually peak on the first or second night and, if you stay at the same elevation and rest, fade over one to three days.

One distinction worth memorizing: breathlessness during exertion at altitude is expected. Breathlessness at rest is not. That single difference separates ordinary acclimatization from a developing emergency, and we’ll return to it.

What Are the Three Stages of Altitude Sickness?

Search engines love the phrase “three stages,” and there’s a clinically real idea behind it, though doctors describe it as a spectrum of three related illnesses rather than an automatic progression.

Stage one: acute mountain sickness (AMS). The hangover-like syndrome described above — headache plus at least one other symptom such as nausea, fatigue, or dizziness. Common, usually self-limiting, and a signal to stop ascending until it resolves.

Stage two: high-altitude cerebral edema (HACE). Fluid accumulates in the brain. The headache worsens and stops responding to rest; the person becomes confused, irritable, or drowsy, and — the classic sign — starts stumbling as if drunk. Clinicians test this with the tandem gait: heel-to-toe walking in a straight line. HACE is rare, affecting well under 1 percent of trekkers at typical elevations, but it can progress from first symptoms to coma within hours.

Stage three (really a parallel emergency): high-altitude pulmonary edema (HAPE). Fluid leaks into the lungs, typically two to four days after arrival above about 8,000 feet. Breathlessness at rest, a wet-sounding cough, chest tightness, and dramatic loss of stamina are the hallmarks. HAPE causes more altitude-related deaths than HACE and can occur without any preceding AMS.

The honest framing: most people who get stage one never see stages two or three — provided they respond to stage one correctly, by halting the ascent.

At What Height Does Altitude Sickness Start?

There’s no single trigger line, but the risk curve has a well-documented shape. Below about 6,500 feet, altitude illness is rare in healthy travelers. Between 6,500 and 8,000 feet, a minority of sensitive individuals notice symptoms. Above 8,000 feet (2,500 meters) — the threshold used by the CDC and most altitude researchers — AMS becomes common enough that every traveler should plan for it.

Zone	Elevation	Oxygen per breath vs. sea level	Familiar examples
High	8,000–11,500 ft (2,500–3,500 m)	~75–65%	Aspen; Bogotá; Lhasa’s lower valleys
Very high	11,500–18,000 ft (3,500–5,500 m)	~65–50%	Cusco; La Paz; Everest Base Camp
Extreme	Above 18,000 ft (5,500 m)	Below ~50%	High Himalayan and Andean summits

Two variables amplify whatever the map says. First, sleeping elevation matters far more than the highest point you touch during the day — your body does its most oxygen-sensitive work overnight. Second, speed: flying directly from sea level to 11,000 feet produces far more illness than reaching the same town over three days by road. Studies of travelers arriving abruptly at very high elevations report AMS rates of 50 percent or more, versus roughly 25 percent with gradual ascent.

In the “extreme” zone, full acclimatization is not possible for anyone; the body deteriorates over weeks no matter how well adapted. Mountaineers visit; nobody truly lives there.

Why Does Thin Air Make You Sick? The Mechanism, Briefly

Blame the pressure gradient your lungs depend on. Oxygen moves from air into blood passively, driven by the difference in partial pressure across the thin membrane of the alveoli. Shrink the pressure, and the driving force shrinks with it — so even with healthy lungs working perfectly, less oxygen crosses over per breath.

Your body notices within minutes. Chemoreceptors in the carotid arteries detect falling oxygen and instruct you to breathe harder — the hypoxic ventilatory response. That helps, but it creates a side effect: breathing off extra carbon dioxide makes the blood more alkaline, which paradoxically tells the brain to slow breathing. The tug-of-war between those two signals explains a lot of early altitude misery, particularly the bizarre sleep patterns covered later.

Meanwhile, low oxygen makes blood vessels behave in opposite ways depending on location. In the brain, vessels dilate, increasing blood volume inside a rigid skull — hello, headache, and in the worst case, HACE. In the lungs, vessels constrict, and they do so unevenly; some regions get blasted with high pressure that can force fluid out of capillaries into air sacs — the mechanism behind HAPE.

Individual susceptibility varies enormously, and researchers still can’t fully explain why. Genetics clearly play a role: populations with millennia of high-altitude ancestry, such as Tibetans and Andean highlanders, show distinct adaptations. For everyone else, the strongest predictor of trouble on this trip is having had trouble on a previous one.

Who Gets Altitude Sickness — and Why Fitness Doesn’t Protect You

Here is the fact that surprises marathoners in mountain clinics every season: cardiovascular fitness has essentially no protective effect against AMS. Studies summarized by the CDC and altitude medicine researchers consistently find that ultrafit athletes get sick at roughly the same rates as sedentary travelers ascending at the same speed. Acclimatization is a biochemical process — ventilation, kidney function, red-cell production — not a strength contest.

Fitness can actually work against you, in one specific way. Strong hikers ascend faster, and rate of ascent is the dominant modifiable risk factor. The person who “pushes through” to a higher camp because they feel strong is the person the rescue team meets later.

What genuinely predicts risk, per the evidence:

• Your own history. Previous AMS, HACE, or HAPE is the single best predictor of recurrence at similar elevations and ascent rates.
• Speed and sleeping altitude. Flying or driving directly above 9,000 feet multiplies risk.
• Home elevation. Residents of low-lying areas are more susceptible than people living at moderate elevation.
• Age, modestly and in reverse of expectation. Some studies suggest adults over 50 fare slightly better than younger travelers, possibly because they ascend more conservatively.

Children get AMS at rates similar to adults. Sex appears to make little difference for AMS, though HAPE is reported more often in males. And no — there is no blood test or fitness assessment that can tell you in advance how you’ll respond. Your first careful ascent is the test.

How Your Body Acclimatizes, Hour by Hour and Week by Week

Acclimatization is a layered project, and each layer runs on its own clock.

Minutes to hours: breathing rate and depth increase, and the heart beats faster to move the available oxygen around more quickly. Resting heart rate at a new altitude commonly runs 10 to 20 percent above your sea-level baseline for the first days.

Days one through four: the kidneys quietly do the most elegant work. Rapid breathing blows off carbon dioxide and alkalinizes the blood, which suppresses the drive to breathe; the kidneys correct this by excreting bicarbonate, restoring the blood’s acid-base balance and unlocking a deeper, sustained increase in ventilation. This renal recalibration is a major reason most AMS resolves in one to three days at a constant elevation. You’ll notice its side effect: you urinate more. Altitude experts consider brisk urine output a reassuring sign of adaptation.

Days to weeks: low oxygen triggers the kidneys to release erythropoietin, the hormone that instructs bone marrow to produce more red blood cells. New cells start entering circulation within about four or five days, but meaningfully expanded oxygen-carrying capacity takes two weeks or more. This is why expedition itineraries build in long, boring acclimatization blocks — the biology cannot be rushed.

The practical upshot: feeling better on day three doesn’t mean you’re fully adapted. It means the first layers have engaged. Ascending aggressively right then, before the deeper layers are in place, is a classic setup for illness higher up.

The Golden Rules of Ascent: Climb High, Sleep Low

If you take one operational rule from this article, take the mountaineer’s motto: climb high, sleep low. Day hikes to higher elevation stimulate adaptation; sleeping back at a lower camp gives your body a gentler overnight workload. The pattern lets you gain acclimatization without paying the full nightly price.

The numbers, drawn from CDC and wilderness-medicine guidance:

• Once above about 10,000 feet (3,000 meters), increase your sleeping elevation by no more than roughly 1,600 feet (500 meters) per day.
• Build in a rest day — same sleeping elevation two nights running — every three to four days, or for every 3,300 feet (1,000 meters) gained.
• If your itinerary forces a big jump (a flight into a high city, say), spend at least one night at an intermediate elevation when geography allows, and plan your first two high days as genuinely easy ones.

Averages count. If the trail dictates one 800-meter day, balance it with shorter days around it. And treat symptoms as traffic signals: mild AMS means stop ascending until it resolves — usually a day or two — not push on and hope. The NHS phrases the non-negotiable version plainly: never go higher, and especially never sleep higher, while you have symptoms.

There is a reason guides repeat these rules with religious intensity. Nearly every serious altitude emergency in trekking regions traces back to the same plot: symptoms appeared, the schedule felt important, and the group kept climbing.

How Do You Treat Altitude Sickness? The Honest Answer

People search “how to cure altitude sickness,” and the honest answer is that there is no cure to buy — there is a decision to make. Every effective response is a version of restoring oxygen to your body, and the most reliable way to do that is to lose elevation.

For mild AMS, the treatment is a full stop: remain at your current sleeping elevation, rest, hydrate, avoid alcohol, and let acclimatization catch up. Simple over-the-counter pain relief can ease the headache — ask a clinician before your trip what’s appropriate for you. Most mild cases resolve within one to three days. If symptoms fully clear, cautiously resuming the ascent is generally considered reasonable.

For worsening AMS — a headache that intensifies despite rest, vomiting, mounting fatigue — descend. Even 1,000 to 3,000 feet (300 to 1,000 meters) often produces dramatic improvement within hours. Descent is not failure; it is the therapy.

For HACE or HAPE, descent is urgent and immediate, day or night, with the sick person exerting as little as possible. Supplemental oxygen and portable hyperbaric bags — inflatable chambers that simulate lower altitude — are used by expeditions and clinics as bridges, not substitutes, for getting down.

Prescription medications exist that can speed acclimatization or treat established illness, and travel-medicine clinicians routinely discuss them with people heading on fast or high itineraries. That conversation belongs in a pre-trip appointment, tailored to your health history — not in a blog’s dosing chart, and not at 14,000 feet.

HACE and HAPE: The Two Emergencies You Must Recognize

Most altitude stories end with a headache and an early night. A small number don’t, and the difference between outcomes is usually recognition speed — often by a companion, because the sick person’s own judgment may be the first casualty.

HACE — brain swelling. Watch for ataxia (a stumbling, drunk-appearing walk), confusion, unusual irritability, slurred speech, or escalating drowsiness. The field test costs nothing: ask the person to walk heel-to-toe along a line. If they can’t, treat it as HACE and begin descent immediately. Untreated, HACE can progress to coma within 12 to 24 hours.

HAPE — lung flooding. The signature is breathlessness at rest, typically appearing two to four days after arrival at altitude. Other signs: a cough that turns wet or produces frothy (sometimes pink-tinged) sputum, chest tightness, audible gurgling with breaths, bluish lips or nail beds, and a collapse in walking pace — the strong friend who suddenly can’t keep up on flat ground. HAPE is the leading cause of death from altitude illness, per CDC data, yet it responds remarkably well to prompt descent and oxygen.

Three field rules cover nearly every scenario, and they’re worth saying aloud to your group before the trip: symptoms at altitude are altitude illness until proven otherwise; never ascend with symptoms; never leave a person with worsening symptoms alone or send them down unaccompanied.

Why Sleep Gets So Strange at Altitude

Ask returning trekkers about their nights and you’ll hear the same reports: vivid dreams, constant waking, and — most unnerving — jolting awake gasping, or watching a tentmate’s breathing simply stop for ten seconds before restarting with a snort.

That stop-start pattern is called periodic breathing, and above about 9,000 feet it’s nearly universal, not a disease. It’s the mechanical result of the tug-of-war described earlier. During sleep, your conscious breathing drive switches off and chemistry takes over: low oxygen says breathe harder, but the resulting drop in carbon dioxide says stop. The brain overshoots in both directions — a burst of deep breaths, then a pause of five to fifteen seconds, then another burst — cycling all night. Each pause can nudge you toward waking, which is why altitude sleep feels shallow even when you’ve logged eight hours.

A few evidence-aligned points help:

• Periodic breathing tends to ease as acclimatization proceeds, though at very high elevations it may persist for weeks.
• Sleeping elevation is the lever: descending even a few hundred meters to sleep often improves the night dramatically.
• Alcohol and sedatives can worsen nighttime oxygen dips and are best avoided in the first days — if sleep is a persistent problem on a high itinerary, that’s a pre-trip conversation with a clinician, not a self-medication project.

One reassurance: poor sleep alone, without headache or other AMS symptoms, is usually acclimatization noise rather than illness. Annoying, yes. Alarming, no.

Hydration, Alcohol, and the Habits That Actually Move the Needle

Altitude advice attracts rituals, so it’s worth sorting what the evidence supports from what merely feels virtuous.

Hydration: useful, oversold. You lose more water at altitude — faster breathing in dry air, increased urination as the kidneys adapt — so drinking enough to keep urine pale is sensible, and dehydration symptoms can mimic and compound AMS. But drinking extra water does not prevent altitude sickness; studies have not shown a protective effect, and forcing fluids can dilute blood sodium. Drink normally and steadily, not heroically.

Alcohol: skip it early. It depresses breathing during sleep — exactly when your oxygen is lowest — and its hangover is clinically indistinguishable from AMS, which muddies decision-making. Most guidance suggests avoiding it for the first 48 hours at a new altitude.

Exertion: keep it light at first. Hard efforts in the first day or two are associated with higher AMS rates. The old guide’s pace — slow enough to hold a conversation — is a genuinely evidence-friendly habit.

Eating: modest effect, real comfort. Appetite often drops at altitude; regular carbohydrate-leaning meals help maintain energy, and there’s some physiological rationale that carbohydrates require slightly less oxygen to metabolize.

Herbal remedies: unproven. Coca tea in the Andes, ginkgo, assorted supplements — none has consistent trial evidence for preventing AMS. Enjoy the tea as culture, not as medicine, and treat any supplement claim with the skepticism it has earned.

Traveling High With Kids, Older Adults, or a Health Condition

Altitude doesn’t only belong to mountaineers. Families visit high national parks; grandparents join treks; ski towns sit at sleeping elevations of 8,000 to 9,500 feet. A few groups deserve tailored planning.

Children develop AMS at roughly adult rates, but young children can’t report a headache — they show it as fussiness, poor feeding, unusual sleepiness, or vomiting. Pediatric and travel-medicine guidance urges extra caution with infants and slower itineraries for families, with a low threshold for descending if a child seems off.

Older adults generally tolerate moderate altitude well, and some data suggest slightly lower AMS rates over 50. The bigger issue is that altitude stresses the heart and lungs, which matters more if those systems already carry a diagnosis.

People with heart or lung conditions — coronary artery disease, heart failure, COPD, sleep apnea, pulmonary hypertension — should have a specific pre-travel conversation with their clinician, because reduced oxygen increases cardiac workload and can worsen underlying disease. Sickle cell disease deserves special mention: altitude can trigger serious complications, and medical advice before any high travel is essential.

Pregnancy: short visits to moderate altitude are generally considered acceptable for uncomplicated pregnancies, per mainstream guidance, but this too warrants an individualized medical conversation rather than a rule of thumb.

The common thread: altitude rarely says no outright. It says plan — slower ascents, conservative sleeping elevations, and a clinician who knows both your itinerary and your chart.

When to See a Doctor

Most altitude sickness never needs a clinic. Knowing the exceptions is the whole game.

Seek emergency care — and descend without waiting for it — if anyone in your group has:

• Confusion, disorientation, unusual drowsiness, or behavior that seems “not them”
• A stumbling, uncoordinated walk (the failed heel-to-toe test)
• Shortness of breath at rest, or breathlessness that doesn’t settle within minutes of stopping
• A wet, gurgling, or frothy cough; chest tightness; blue-tinged lips or fingertips
• A severe headache that keeps worsening despite rest and stopping the ascent
• Repeated vomiting or inability to keep fluids down

Arrange a same-trip medical visit if mild symptoms haven’t improved after two to three days at a constant elevation, if they return each time you attempt to ascend, or if you have a heart, lung, or blood condition and feel worse than expected.

Book a pre-trip appointment if you’re planning to sleep above roughly 9,000 feet, your itinerary forces a fast ascent (a direct flight to a high city, a compressed trek), you’ve had altitude illness before, or you manage a chronic condition. A travel-medicine clinician can review your route day by day and discuss preventive options suited to your health history.

One last note for groups: designate someone to watch for symptoms in others. HACE, in particular, erodes exactly the judgment a sick person needs to save themselves. On mountains, the most valuable medical instrument is often an honest friend.

Frequently asked questions

How do you cure altitude sickness?

There is no cure in the medicine-cabinet sense — descent is the definitive treatment, because it restores the oxygen your body is missing. Mild cases usually resolve on their own within one to three days if you stop ascending, rest, and stay hydrated. Worsening symptoms call for going down 1,000 to 3,000 feet, which often brings dramatic relief within hours. Severe symptoms — confusion, stumbling, breathlessness at rest — require immediate descent plus emergency medical care.

What does altitude sickness feel like?

Most people describe it as a hangover they didn’t earn: a throbbing headache (often worse at night and when bending over), nausea, loss of appetite, unusual fatigue, dizziness, and broken sleep. Symptoms typically start 6 to 12 hours after arriving at a new elevation, not immediately. Breathlessness during exertion is normal at altitude; breathlessness while resting is a warning sign of something more serious and should prompt descent and medical attention.

What are the three stages of altitude sickness?

Doctors describe a spectrum: acute mountain sickness (AMS), the common hangover-like illness; high-altitude cerebral edema (HACE), a rare brain swelling marked by confusion and a stumbling walk; and high-altitude pulmonary edema (HAPE), fluid in the lungs causing breathlessness at rest and a wet cough. AMS doesn’t automatically progress to the severe forms — most cases never do — but ignoring AMS and continuing to ascend is the classic path to both emergencies.

What are the main symptoms of altitude sickness?

The core symptoms are headache plus at least one of the following: nausea or vomiting, loss of appetite, fatigue, dizziness or lightheadedness, and difficulty sleeping. They usually appear within 6 to 12 hours of reaching elevations above 8,000 feet and peak on the first or second night. Red-flag symptoms — confusion, an uncoordinated walk, breathlessness at rest, or a frothy cough — indicate the severe forms, HACE or HAPE, and are medical emergencies.

How long does altitude sickness last?

Mild acute mountain sickness typically resolves within one to three days, provided you stay at the same elevation and rest while your body acclimatizes. If symptoms persist beyond two to three days at a constant altitude, worsen despite rest, or return every time you try to go higher, that’s a signal to descend and to seek medical advice. Severe forms don’t wait — HACE can progress to coma within 12 to 24 hours without descent.

Can you prevent altitude sickness?

You can dramatically reduce the risk, mainly by ascending slowly. Above 10,000 feet, limit sleeping-elevation gains to about 1,600 feet per day, take a rest day every three to four days, and spend a night at intermediate elevation before big jumps. Avoid alcohol and heavy exertion for the first 48 hours. Preventive prescription options exist for fast or high itineraries — discuss them with a travel-medicine clinician before you go, since suitability depends on your health history.

Does being fit prevent altitude sickness?

No. Studies consistently show that cardiovascular fitness offers essentially no protection against acute mountain sickness — elite athletes get sick at similar rates to sedentary travelers ascending at the same pace. Acclimatization depends on breathing adjustments, kidney chemistry, and red blood cell production, none of which training accelerates. Fitness can even backfire by tempting strong hikers to climb faster, and ascent speed is the single biggest modifiable risk factor.

Does drinking lots of water prevent altitude sickness?

No — this is one of the most persistent altitude myths. Staying normally hydrated matters, because you lose extra water at altitude through faster breathing in dry air and increased urination, and dehydration can mimic and worsen symptoms. But research has not shown that drinking extra water prevents acute mountain sickness, and overdrinking can dangerously dilute blood sodium. Aim for pale-yellow urine and steady intake, not heroic quantities.

Can children get altitude sickness?

Yes, at rates roughly similar to adults. The challenge is recognition: young children can’t describe a headache, so altitude sickness may show up as fussiness, poor feeding, unusual sleepiness, or vomiting. Travel-medicine guidance recommends slower ascents for families, extra caution with infants, and a low threshold for descending if a child seems unwell at altitude. Any confusion, breathing difficulty, or marked lethargy in a child warrants immediate descent and medical care.

Is it safe to fly directly to a high-altitude city?

Millions do it every year, but abrupt arrival above 9,000–11,000 feet substantially raises the odds of acute mountain sickness — studies report rates of 50 percent or more with rapid ascent versus about 25 percent with gradual approaches. Reduce the risk by scheduling nothing strenuous for the first two days, avoiding alcohol initially, and sleeping at the lowest elevation your plans allow. Travelers with heart, lung, or blood conditions should consult a clinician before booking.

References

• MedlinePlus: Acute Mountain Sickness
• NHS: Altitude Sickness
• Cleveland Clinic: Altitude Sickness

This article is for general information only and is not a substitute for professional medical advice. Please consult a qualified doctor about your individual situation.