Altitude sickness: recognizing AMS, HACE, and HAPE in the backcountry

Above about 8,000 feet (2,400 m), there is less oxygen in every breath you take, and your body needs days to adjust. Push higher faster than it can adapt and you get altitude illness. It comes in three forms: acute mountain sickness, which feels like a hangover that will not lift; high altitude cerebral edema, which swells the brain; and high altitude pulmonary edema, which floods the lungs. The first is miserable but survivable. The other two kill, and they kill people who were walking and talking a few hours earlier. This guide covers how to recognize each one, what to do about it, and how to plan an ascent so you never have to find out.

One principle runs through everything below. Altitude illness is caused by going too high too fast, and it is cured by going down. Every drug, every gadget, and every clever trick in this guide is a distant second to descent. If you remember nothing else, remember that a person who is getting worse at altitude needs to lose elevation, and that almost nobody has to die of this, because the warning signs arrive hours before the danger does.

Why thin air makes you sick

The air at altitude holds the same 21 percent oxygen it does at sea level. What changes is pressure. With less atmosphere pressing down, each breath delivers fewer oxygen molecules to your blood. At around 10,000 feet (3,000 m), the oxygen pressure reaching your lungs is roughly 69 percent of the sea level value, and an unacclimatized person’s blood oxygen saturation can drop into the high 80s within hours of arriving.

Your body responds by breathing harder and deeper, which is the single most important adjustment it makes. Over the first three to five days it also shifts blood chemistry and cerebral blood flow to cope. That acute phase is what matters to a hiker. Altitude illness can develop while acclimatization is still in progress, but not after it is complete, which is why the first two nights at a new elevation are the dangerous ones.

Two details follow from this and they explain most of what goes wrong. First, hypoxia is worst during sleep, when your breathing drive falls. That means sleeping altitude is the number that counts, not the highest point you touched during the day. Climbing high and sleeping low is genuinely protective, and a day trip to a summit with an evening return to a lower camp is far less stressful on your body than staying up there. Second, because increased breathing is your main defence, anything that suppresses it works against you. Alcohol, opiate painkillers, and sedatives are all respiratory depressants, and all of them are a bad idea in your first days at altitude.

Who gets it, and how high is high

Risk begins for unacclimatized travellers at a sleeping altitude of roughly 8,000 feet (2,400 m), and occasionally lower. That is not an exotic threshold in North America. Trail Ridge Road in Rocky Mountain National Park tops out at 12,183 feet (3,713 m) and its Alpine Visitor Center sits at 11,799 feet (3,596 m), both reachable by car in an afternoon from Denver. Roughly a quarter of visitors sleeping above 8,000 feet (2,400 m) in Colorado develop acute mountain sickness. Many trailheads across the Rockies, the Sierra, and the Cascades start higher than most people’s homes ever get, and our national park guides list elevations so you can check a destination before you commit to it.

You are also at risk again every time you move up, even after adjusting to a given elevation. A gain of 2,000 to 3,000 feet (600 to 900 m) in sleeping altitude is enough to restart the problem.

What surprises most people is who it happens to. Susceptibility is largely genetic and there is no screening test for it. Fitness and training do not protect you. Strong, young, well-trained hikers get altitude sickness constantly, partly because they can climb fast enough to outrun their own acclimatization. The pacing discipline in our guide to hiking all day matters more at elevation than anywhere else, for exactly that reason. Sex plays little or no role. Children are as susceptible as adults. People over fifty are, if anything, slightly less at risk.

The most useful predictor is your own history. If you have been to a similar altitude at a similar rate of ascent, how you felt then is a reasonable guide to how you will feel now. It is not a guarantee, and a bad experience once does not condemn you forever, since a slower ascent on the next trip often solves it entirely.

Acute mountain sickness

Acute mountain sickness, or AMS, is the common form and the one you are most likely to meet. The diagnosis rests entirely on how the person feels, because there is nothing to find on examination and no field test that confirms it.

The cardinal symptom is headache, and it comes with at least one of: loss of appetite, nausea or vomiting, unusual fatigue or lassitude, and dizziness or light-headedness. The comparison everyone reaches for is an alcohol hangover, and it is a good one. Symptoms usually begin two to twelve hours after arriving at a new elevation, often during or just after the first night.

Timing helps you diagnose it. AMS follows a recent ascent and generally resolves within 12 to 48 hours if you stop climbing. If someone falls ill more than three days after arriving at an altitude they have not exceeded since, it is probably not AMS and you should look for another cause. In young children who cannot yet describe a headache, watch instead for lost appetite, irritability, and pallor.

Severity is what drives the decision. Mild AMS is a headache plus one or more mild symptoms, with no abnormal signs. Moderate to severe AMS is the same picture at an intensity that stops the person functioning normally. The honest field test is not a score, it is a question: can this person still look after themselves and enjoy the day? If the answer is no, treat it as moderate or worse.

Treatment for mild AMS is straightforward. Stop ascending. Rest, drink normally, and treat the headache with ibuprofen or acetaminophen. Most people improve within a day or two and can then continue up. Descending even 1,000 feet (300 m) usually produces rapid relief, and that response is itself a useful confirmation that you were dealing with AMS. Anyone with AMS of any severity should stop climbing, and nobody with ongoing symptoms should sleep higher, no matter how mild it feels or how much the schedule wants it.

HACE: when it reaches the brain

High altitude cerebral edema is AMS that has progressed into an encephalopathy. It is rare, particularly below about 14,000 feet (4,300 m), and it is a genuine emergency. Untreated, coma can follow within 12 to 24 hours of the first clear signs.

What separates HACE from AMS is that HACE has signs, not just symptoms. The earliest and most reliable one is ataxia, meaning loss of coordination and balance. Altered mental status follows: confusion, apathy, irritability, drowsiness, and an inability to carry out simple self-care such as putting on a jacket or packing a bag. The whole picture looks a great deal like drunkenness, which is exactly why groups miss it and write the person off as tired or grumpy.

The field test is the heel-to-toe walk. Have the person walk a straight line placing one heel directly against the toes of the other foot, arms out, for ten paces. A person who cannot do it, or who staggers and needs to put a foot out to catch themselves, has ataxia until proven otherwise. Run this test on anyone who has AMS and is not improving, and run it on any member of the party who has gone quiet, clumsy, or oddly passive. Test it on flat ground, and compare against how they walked earlier in the trip.

Suspected HACE means descend immediately, day or night. Give supplemental oxygen if you have it, and dexamethasone if someone in the party carries it and knows how to use it. If descent is genuinely impossible, oxygen or a portable hyperbaric chamber plus dexamethasone can buy the hours you need. Focal problems such as weakness on one side, or a seizure, point away from HACE and towards a stroke, a head injury, or low blood sodium, but the immediate action is the same: get down and get help.

HAPE: when it reaches the lungs

High altitude pulmonary edema is fluid leaking into the lungs. It can occur on its own, without any AMS beforehand, which is one reason it catches people out. It is uncommon, on the order of one in ten thousand Colorado skiers, but the rate climbs to as high as one in a hundred above 14,000 feet (4,300 m). It can kill faster than HACE, and early recognition is the whole game.

The first sign is almost always performance. The person becomes far more breathless on exertion than they were yesterday, than the terrain warrants, or than everyone else in the party on the same slope. They fall to the back and stay there. Add a dry cough, a feeling of chest congestion or gurgling, weakness, and unusual fatigue, and you have the early picture.

Left alone, it progresses over a day or two to breathlessness at rest, obvious respiratory distress, a cough that turns wet and eventually pink and frothy, and a bluish tinge to lips and fingertips. In someone with a poor breathing response, it can instead present mainly as confusion and drowsiness while their oxygen levels are dangerously low, which is easily mistaken for HACE.

Descent is urgent and mandatory. Aim to lose at least 3,300 feet (1,000 m), or enough that symptoms resolve. Give oxygen if available. Critically, exert the patient as little as you possibly can, because effort raises pulmonary artery pressure and makes the edema worse. Carry their pack. Put them on a vehicle, a horse, or a helicopter if any of those exist. This is the opposite of the usual backcountry instinct to have the casualty walk themselves out, and it matters more here than almost anywhere else. If descent is delayed, oxygen or a portable chamber becomes critical, with nifedipine as a fallback when nothing else is available. Descent and oxygen both beat any medication.

What else it might be

Altitude gets blamed for a lot of things it did not do, and it also gets overlooked in favour of gentler explanations. Work through the alternatives before you settle.

• Dehydration and exhaustion produce headache and fatigue at any elevation. They are real and common, and they often coexist with AMS rather than replacing it. Fix them and reassess.
• Carbon monoxide poisoning from cooking inside a tent or a closed hut mimics AMS almost exactly, headache and nausea included. If several people in one shelter feel ill at once, get everyone into fresh air first and think about altitude second.
• Hyponatremia, low blood sodium from drinking large volumes of plain water without salt, causes headache, nausea, and confusion. It is one reason forcing fluids is not the answer to altitude illness.
• Hypothermia also produces stumbling, confusion, and poor judgment. Cold and altitude travel together, so consider both. Our guide to recognizing and treating hypothermia covers the staging in detail, cold injury to fingers and toes is a companion risk on the same trips, and staying warm through a cold night covers the high camp itself.
• A caffeine withdrawal headache is a genuinely common confounder in people who quit coffee for the trip. If you drink it daily at home, keep drinking it on the mountain.
• Viral illness usually brings fever, chills, a runny nose, or muscle aches. AMS brings none of those.

When you cannot tell, treat it as altitude illness. The cost of being wrong in that direction is a lost day. The cost of being wrong in the other direction can be a life. Altitude belongs to the same family as heat illness and hypothermia: environmental conditions that make an otherwise healthy person sick, all of which the backcountry first aid basics primer sets in order of priority.

The three rules that prevent deaths

The published guidance on this reduces to three sentences, and almost every altitude fatality involves breaking one of them.

1. Know the early symptoms and be willing to admit you have them. The illness announces itself as a hangover. Say so out loud to your party rather than hoping it passes.
2. Never sleep higher while you have any symptoms of altitude illness, however minor they seem. This is the rule that summit fever breaks.
3. Descend if symptoms get worse despite rest and treatment at the same elevation. Not tomorrow morning. Then.

The reason these work is that altitude illness moves slowly and predictably compared with most wilderness emergencies. Hours pass between the first headache and real danger. Nobody should die of this unless weather or terrain has trapped them somewhere they cannot descend from, which is a planning failure more than a medical one. High country generates its own weather and closes escape routes quickly, so read our guide to staying ahead of backcountry storms alongside this one.

Agree the rules with your group before the trip, when nobody has anything invested in the summit yet. Decide who calls it, and agree that the call is not up for negotiation at 13,000 feet (4,000 m). Our guide to building a pre-trip safety plan covers turnaround rules and how to set them so they hold.

Descent: how far, and how to move someone

Descent is the treatment. How far depends on what you are treating.

• AMS: 1,000 feet (300 m) is usually enough to produce clear improvement, more if it does not.
• HACE: descend until the person is clearly better, and keep going towards definitive care. Do not stop at the first sign of improvement.
• HAPE: at least 3,300 feet (1,000 m), or until symptoms resolve.

Three practical points make the difference between a descent that works and one that goes wrong.

Go at night if you have to. The instinct to wait for dawn has killed people. A HACE or HAPE patient who is deteriorating cannot wait eight hours. Descending in the dark is a real hazard and needs headlamps, spare batteries, and care on the ground, but it is the lesser risk. Our guide to navigating in fog, whiteout, and darkness covers moving safely when you cannot see, and crossing difficult terrain covers the footing.

Do the work for them. Take the sick person’s pack, and if you are now carrying two, our guide to fitting and packing a backpack covers getting that weight onto your hips where it belongs. Rope up or short-rope them if the ground demands it and you know how. For HAPE especially, every avoided step helps. Never let an ataxic person walk exposed ground unassisted, because HACE takes balance before it takes consciousness.

Never send them down alone. Someone who is confused, unsteady, or breathless will make bad decisions and may not arrive. Two people go down together, minimum. If that splits the party awkwardly, the party goes down together.

Call for help early rather than late. A satellite messenger such as the Garmin inReach Mini 2 lets you describe symptoms and get advice while you descend, and a subscription-free beacon like the ACR ResQLink 400 summons a rescue outright. High peaks are exactly where helicopter evacuation is hardest and slowest, so the earlier the request goes out, the better. Our guide to signaling for rescue covers what to do once you have called.

Planning an ascent that works

Nearly all of this is preventable with a schedule. The published ascent guidance is specific and worth following literally.

• Do not go from low elevation to sleeping above 9,000 feet (2,700 m) in a single day. Break the trip.
• Once above roughly 9,800 feet (3,000 m), increase your sleeping altitude by no more than about 1,600 feet (500 m) per day.
• Take a rest day every three to four days, meaning a night at the same sleeping altitude as the night before, and add an extra acclimatization night for every 3,300 feet (1,000 m) of sleeping altitude gained.
• Sleep low, climb high. Going up during the day and coming back down to sleep is a proven way to acclimatize without the overnight cost.

The single highest-value move for most North American trips is an intermediate night. A sea level visitor heading to a Colorado trailhead above 9,000 feet (2,700 m) should spend a night around Denver’s 5,280 feet (1,610 m) first. Two or three nights at 8,000 to 9,000 feet (2,400 to 2,700 m) before going higher is markedly protective. If you have been above 9,000 feet (2,700 m) for two or more nights within the fortnight before the trip, that helps too, and the closer to departure the better.

For the first 48 hours at a new elevation: no alcohol, only mild exercise, and keep your normal caffeine habit. Drink to thirst rather than forcing fluids, and treat what you find up there properly, since high alpine streams are no safer than any other (see purifying water in the backcountry). Expect to sleep badly, because periodic breathing, where your breath repeatedly fades and then catches up with a gasp, is close to universal above 9,000 feet (2,700 m) and disturbs almost everyone’s first nights.

Expect your appetite to disappear as well, which is a normal altitude effect and a nuisance on a long trip. Plan food you will actually eat when you feel queasy, which usually means simple carbohydrates and things you can nibble. Our guide to what food to bring covers building a menu that survives contact with a bad day.

Build the schedule into the plan rather than hoping to improvise it. The Trip Planner is a good place to lay out the nights, and it is worth checking your route’s high camps against the numbers above before you book anything.

Medications, honestly

Drugs help, and they are secondary to a sensible schedule. Most of these are prescription medicines in North America, so this is a conversation to have with a doctor who understands high altitude travel, well before you leave. Do not improvise dosing from an article, including this one.

Acetazolamide is the mainstay for preventing AMS. It works by making your blood slightly more acidic, which drives you to breathe more, and it compresses the ventilatory acclimatization that normally takes three to five days into roughly one. The usual prevention dose is 125 mg every 12 hours, started the day before you go up and continued for the first couple of days at altitude or longer if you keep climbing. A common treatment dose is 250 mg every 12 hours. It is worth considering whenever a rapid ascent is unavoidable, and it is strongly encouraged for high-risk itineraries such as flying straight into a high airport. Expect tingling in the fingers and toes, more trips to the toilet, and carbonated drinks tasting strange. It is a sulfonamide but not an antibiotic one, and people with sulfa antibiotic allergies can generally take it, though a history of anaphylaxis to any drug calls for caution.

Dexamethasone is a steroid used mainly for treatment of moderate to severe AMS and of HACE, usually alongside descent. It is more reliable than acetazolamide for treating serious illness. Importantly, it masks symptoms without helping you acclimatize, so anyone who takes it should not continue upward until they are well without it. It is the drug expedition parties carry for the HACE emergency.

Ibuprofen at 600 mg every 8 hours treats high altitude headache and does reduce AMS, though less effectively than acetazolamide. It is over the counter, cheap, and well tolerated, which makes it a reasonable option for someone who cannot or will not take the prescription drugs.

Nifedipine and tadalafil lower pulmonary artery pressure and are used for HAPE, essentially only by people with a known history of it, or as a treatment fallback when descent and oxygen are both unavailable.

Two cautions worth stating plainly. Any medication that masks symptoms can tempt a party upward when they should be going down, and none of these replaces the ascent schedule. And never use alcohol, opiates, or sedating sleep aids to get to sleep at altitude, because they suppress the breathing you are depending on.

What does not work

A large industry sells altitude remedies, and the evidence for most of it is absent.

• Canned oxygen. The small handheld cans hold a few litres at most. They may briefly ease a headache and they cannot treat altitude illness. Real supplemental oxygen means a cylinder delivering a steady flow for hours.
• Oxygen bars and other brief oxygen exposures do nothing for prevention.
• Coca leaves, coca tea, and coca products. Culturally embedded in the Andes, but not shown to prevent altitude illness.
• Forced or over-hydration. Drinking large amounts of water does not prevent altitude illness and can cause low blood sodium, which produces symptoms that look like the thing you were trying to avoid. Drink to thirst.
• Powders, patches, and supplements marketed for altitude. No demonstrated benefit.
• Being fit. Worth repeating, because it is the most persistent myth in the mountains. Training does not protect you, and it may encourage you to ascend faster than you should.

Preacclimatization in a hypoxic tent is a partial exception. It can work, but only with long daily exposures over weeks, which puts it out of reach for most trips.

Pulse oximeters and their limits

A fingertip pulse oximeter is small, inexpensive, and genuinely interesting to have at altitude. It is also easy to misuse, and it should never be the thing that makes your decision.

Normal saturation falls predictably as you go up, so a reading that would alarm you at home may be entirely expected at 12,000 feet (3,700 m). There is no single correct number for a given elevation, only a range. Saturation also swings quickly at altitude in response to small changes, and consumer devices lose accuracy badly once readings fall below about 80 percent. Cold fingers, bright ambient light, and a poorly fitting probe all produce garbage readings, and cold fingers are the normal state of affairs on a mountain.

Where it earns its place is in comparison. A person whose saturation sits ten or more points below everyone else in the party at the same camp, and who is breathless, is a real HAPE concern. Readings in the 50 to 70 percent range in someone who looks unwell are meaningful. What you should not do is treat a two-point drop as evidence of anything, or a reassuring number as permission to keep climbing someone who feels terrible. How the person looks and functions outranks the display, every time.

Kids, pregnancy, and pre-existing conditions

Children are as susceptible as adults and much worse at telling you about it. In a preverbal child, altitude illness shows up as fussiness, refusing food, unusual paleness, and poor sleep. Ascend more slowly with kids than you would alone, and take a low threshold for turning around. Drug doses for children are weight-based and dexamethasone is not used for prevention in kids, so this needs a paediatrician’s input rather than a rule of thumb.

In pregnancy, there is no evidence of harm from brief high altitude travel, but the cautious and common recommendation is to avoid sleeping above 10,000 feet (3,000 m). The larger concern is the remoteness: a pregnancy complication in high mountain terrain is a serious problem regardless of oxygen.

Several conditions warrant a conversation with a doctor familiar with altitude before you go, even when they are well controlled: coronary artery disease, any chronic lung disease, obstructive sleep apnea, sickle cell trait, and diabetes. Sleep apnea in particular tends to worsen at altitude. Diabetics should know that altitude illness can trigger ketoacidosis and that not all glucometers read accurately up high. People who have had radial keratotomy can develop sudden farsightedness at very high elevation, which is a genuine hazard if you are relying on your eyes to descend.

What to carry

Nothing on this list substitutes for the schedule, and each item earns its weight when something goes wrong.

• Ibuprofen or acetaminophen, enough for several days, in a compact waterproof first-aid kit. For groups and longer trips, a larger kit gives you room for altitude medications alongside everything else.
• Any prescribed altitude medications, obtained and discussed with a doctor before departure, and carried by someone who knows the doses.
• A two-way satellite messenger or a personal locator beacon. High country has poor cell coverage and hard evacuations, so this is not optional above the treeline.
• A reliable headlamp with spare batteries, because the descent you most need to make is the one that starts at night.
• Trekking poles, which do more for a wobbly, tired person on a long descent than any other piece of gear.
• Warm layers beyond what the forecast suggests. Altitude illness and cold compound each other, and a descent at 2am is colder than anything you planned for. See what to wear in the bush.
• Simple, appealing food you can still face when nauseated, plus sun protection, since ultraviolet exposure climbs sharply with elevation.

Plan your ascent with the Trip Planner

Key takeaways

Altitude illness is caused by climbing faster than your body can adjust, and the number that matters is where you sleep, not where you stood. Risk starts around 8,000 feet (2,400 m), fitness does not protect you, and children are as vulnerable as adults. Learn the three pictures: AMS is a headache plus nausea, fatigue, or dizziness that feels like a hangover; HACE adds stumbling, confusion, and an inability to walk heel to toe, and it is a brain emergency; HAPE is breathlessness far out of proportion to the effort, with a cough, and it can kill fastest of all. Follow the three rules without negotiation: know the symptoms, never sleep higher with symptoms, and descend when things worsen despite rest. Descent is the treatment, at night if necessary, with the patient carrying nothing and never travelling alone. Plan an intermediate night, cap your sleeping gain at about 1,600 feet (500 m) a day above 9,800 feet (3,000 m), and build in rest days. Talk to a doctor about acetazolamide well before you leave, ignore the canned oxygen and the supplements, and call for help early rather than late, because the mountain gives you hours of warning and it only takes one of them to start going down.

Verified against the Wilderness Medical Society Clinical Practice Guidelines for the Prevention, Diagnosis, and Treatment of Acute Altitude Illness (2024 Update) and the CDC Yellow Book, 2026 edition, chapter on High-Altitude Travel and Altitude Illness. This is general education, not a substitute for medical training or professional care, and altitude medications require a prescription and a physician’s advice.

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