How High-Altitude Acclimatization Works: The Basics

ALTITUDE: QUICK REFERENCE

Page 1 of the Sergio Photo Tours altitude information sheet. Keep this page with you. It is the one you will want if you feel unwell at elevation.

THREE RULES THAT COVER ALMOST EVERYTHING

1. If symptoms appear, go no higher until they have gone away completely.
2. If symptoms get worse, or any severe sign appears, go down. Descent always works.
3. Tell your guide how you feel, early. We would rather change a day than manage an emergency.

Telling the three forms apart

AMS (mild)

• When: Common. Usually 6 to 12 hours after arriving at a new elevation
• Key signs: Headache, plus fatigue, no appetite, nausea, poor sleep
• What it means: Common and usually settles on its own in 1 to 3 days
• What we do: Stop ascending. Rest, fluids, light food. Watch closely

HAPE (lungs, severe)

• When: Uncommon on a paced ascent. When it happens, within 2 to 4 days of a fast gain in sleeping altitude, rarely the 1st night
• Key signs: Breathlessness at rest is the first reliable sign. Then a cough that turns wet or frothy, and a sudden drop in what you can do
• What it means: Fluid in the lungs. The most common cause of death from altitude illness
• What we do: Descend at once with oxygen. Get to a hospital

HACE (brain, severe)

• When: Uncommon. Usually follows untreated AMS. Onset can be rapid
• Key signs: Loss of coordination is the classic sign. Ask the person to walk heel to toe in a straight line. Also confusion and severe headache
• What it means: Swelling of the brain. The most immediately life-threatening form
• What we do: Descend at once with oxygen, accompanied. Get to a hospital

Fluid in the lungs and fluid in the brain can occur together. Either one is treated as an emergency.

TELL YOUR GUIDE IMMEDIATELY IF YOU NOTICE ANY OF THESE

• Breathlessness that does not settle with rest, or that wakes you at night
• A cough that turns wet, or brings up frothy or pink-tinged liquid
• Trouble walking a straight line, stumbling, or clumsiness
• Confusion, disorientation, or behavior that seems out of character
• Blue or gray lips or fingernails
• A severe headache that will not respond to rest, fluids, and ordinary pain relief
• Repeated vomiting

Do not treat a symptom and keep climbing.

Painkillers and oxygen change how you feel, not what is happening to you, and neither makes it safe to go higher.

HEALTH MATTERS: Altitude and Your Body

Pre-departure information for travelers going to high elevation. Reviewed September 2026.

What this document IS, and what it is NOT

This sheet explains what altitude does to the human body, how acclimatization works, what altitude illness looks like, and how we manage it in the field. It exists so that you arrive informed rather than surprised. The quick reference on the previous page is the part to keep with you.

A document like this necessarily spends most of its length on what can go wrong, which can leave a misleading impression. So the next section puts the actual risks in numbers before anything else. Please read that part first.

We are guides and photographers. We are not physicians, nurses, or paramedics. Nothing here is medical advice, a diagnosis, or a recommendation to take or avoid any medication. Where this document names a medication, it does so only so that you have something concrete to discuss with your own doctor. Only your physician can decide what is appropriate for you.

Please see a doctor familiar with travel or altitude medicine before you travel and bring your itinerary and the elevations involved to that appointment. Medical guidance changes over time. If you are reading this well after the review date above, check for newer information.

Independent sources worth reading

We would rather you read the primary sources than take our word for any of this. Two good places to start are the US Centers for Disease Control and Prevention guide to high-altitude travel and the altitude research published by the University of Colorado Anschutz Medical Campus. The clinical content in this document is drawn from the Wilderness Medical Society guidelines named above and from the CDC Yellow Book.

Putting this in perspective, before anything else

Read this section before the rest, because the rest of this document describes things that can go wrong, and a list of things that can go wrong is misleading without knowing how likely any of them is.

Here is the honest shape of it. The mild form of altitude illness is common and most travelers to the high Andes feel some version of it. It is uncomfortable, it passes, and it is not dangerous when handled properly. The two severe forms are genuinely serious, and they are also genuinely rare, and they are overwhelmingly driven by one thing: going up too fast.

That last point is the whole reason our itineraries look the way they do. Rate of ascent is not a minor variable. It is the dominant one, and it is the one an operator controls. The published figures make that plain.

How much difference pacing makes

Figures from published altitude medicine literature. Same mountains, same people, different pace, and the risk moves by a factor of ten or more.

Taken together, the two severe forms occur in roughly 1 to 2% of people going to high altitude across all profiles, including the fast and reckless ones. Fluid in the lungs is about twice as common as fluid in the brain. On a slow, staged ascent with a ceiling around 4,300 m, which is what our Andean itineraries are, the figures sit at the very bottom of that range.

Our own record

In thirty years of guiding these routes we have had one case of HAPE, fluid in the lungs. It happened on a fast push to a high camp above 5,000 m, by a client who went higher and faster than we advised him to. We evacuated him to La Paz inside 24 hours and he recovered. On our standard paced itineraries, at the elevations where our photography tours actually take place, we have not had a case.

We are telling you that as a record, not as a promise. Individual susceptibility to altitude illness is real and not fully explained, and there are documented cases in people who did everything right on a well-paced trip. That is precisely why the recognition material in this document exists. The point of knowing the signs is not that we expect you to need them. It is that the rare case is entirely survivable when it is caught early, and dangerous when it is not.

The short version of this whole document: the risk on a properly paced trip is low, the reason it is low is the pacing, and the way it stays low is that you tell us early if something feels wrong.

The territory: altitude bands

These are the conventional definitions used in altitude medicine. Risk rises with each band, which is why the labels are worth knowing before the rest of this makes sense.

• High altitude — 8,000 to 12,000 ft (2,440 to 3,660 m)
• Very high altitude — 12,000 to 18,000 ft (3,660 to 5,500 m)
• Extreme altitude — Above 18,000 ft (Above 5,500 m)

Acute mountain sickness becomes common above roughly 8,000 ft (2,440 m), which is why that figure marks the start of the high-altitude band. Milder effects are well documented lower down. Plenty of people flying from sea level into Denver at 5,280 ft notice a headache, broken sleep, and breathlessness on stairs. Serious altitude illness below about 6,000 ft (1,830 m) is rare, but rapid unacclimatized ascent above that level can produce symptoms in susceptible people.

Most of our photography takes place in the high and very high bands, with sleeping elevations generally between 10,000 and 14,000 ft, all of which is itinerary dependent. Our photography itineraries do not go above 5,000 m (16,400 ft), and where we work near that ceiling we drive to it rather than climb to it, as at Sud Lipez or Chacaltaya, both in Bolivia. Anything higher is sold as an alpine climb, not a photography tour.

A note on the numbers you will see quoted elsewhere

If you read about altitude illness online you will find 2,500 m (8,200 ft) quoted repeatedly as the altitude above which altitude illness occurs. That figure is a floor, not a forecast. It marks where the body begins to be affected at all, not where trouble becomes likely. Cusco, La Paz, Quito, Bogota, Potosi and the Salar de Uyuni are all above it, and several million people visit them every year without incident. Every destination in this document is above 2,500 m. What determines whether you have a problem is not crossing that line. It is how fast you cross it, how high you sleep afterward, and how long you stay before going higher.

The science: what thin air actually does

The proportion of oxygen in the atmosphere is essentially constant at about 21%, at sea level and on the summit of Everest alike. What changes is barometric pressure. As you climb, the air becomes less dense, so each breath contains fewer oxygen molecules. Lower pressure also means a smaller pressure gradient driving oxygen from your lungs into your bloodstream. The result is that the oxygen content of your blood falls, a state called hypoxia.

Your body responds immediately. Breathing becomes deeper and faster, at rest and during sleep. Heart rate rises to circulate the available oxygen more quickly. These changes raise your blood oxygen but do not restore it to sea-level values, and the same physical work still requires the same oxygen. That mismatch is what you feel as breathlessness on a slope, quicker fatigue, and a restless first night.

High-altitude air also holds very little moisture, so you lose water through breathing alone, considerably faster than you would at home. Dehydration will not by itself cause altitude illness, but it makes you feel worse and it makes symptoms harder to interpret.

In some people, and not predictably, low pressure and hypoxia lead to fluid leaking out of small blood vessels into the lungs or the brain. Those are the two severe forms of altitude illness. They are problems of fluid leakage, not of blood clotting.

How the body acclimatizes

Given time, the body adapts. This process is called acclimatization and it cannot be hurried.

Breathing becomes deeper and faster and stays that way. This is the fastest and most important adaptation, and it begins within hours.

The kidneys excrete bicarbonate, which offsets the change in blood chemistry caused by heavy breathing and allows the breathing response to keep increasing.

Over days to weeks, the kidneys release erythropoietin, or EPO, which prompts the bone marrow to produce more red blood cells. This raises oxygen-carrying capacity and makes the blood more viscous.

Pressure in the pulmonary arteries rises, recruiting areas of the lung that are underused at sea level.

Levels of 2,3-DPG in the red blood cells shift, which helps hemoglobin release oxygen into the tissues.

Who gets altitude illness

This is the part travelers most often get wrong, in both directions.

What does not predict it

Athletic fitness does not protect you. Trained runners get altitude illness at the same rate as everyone else. Age by itself is a poor predictor. Sex is not a meaningful predictor. Having been to altitude before without trouble guarantees nothing about the next trip, and it does not excuse you from acclimatizing again. What does raise the risk is hard exertion in the first days, whatever shape you are in.

What does predict it

Rate of ascent. How fast you gain sleeping elevation is the largest factor you control.

The altitude you sleep at. Sleeping elevation matters far more than the highest point you touch during the day.

Prior history of altitude illness. This is the most useful individual warning sign there is. If it has happened to you before, tell your doctor and tell us.

Exertion in the first days, and how well acclimatized you were before arriving.

Weight and smoking

Excess weight is a different question from fitness, and it is a documented risk factor for high-altitude illness. The mechanism is sleep. Altitude already lowers your blood oxygen overnight, and excess weight and sleep-disordered breathing lower it further, so the two compound. It also shapes the days in ways that have nothing to do with medicine: climbing in and out of a raised 4x4 several times a day, stairs in hotels without elevators at 3,600 m, and half an hour walking on uneven ground at 4,000 m. Smoking works against every one of those.

None of this means you need to be an athlete. Our vehicles reach the viewpoints and you rarely walk with a pack for more than 30 to 45 minutes at a stretch. It means arriving fitter than you are now is worth real money to you. Walk, and walk uphill. Take the stairs. Stairmaster, bike, hike. Best of all, hike with your photo pack on, because that is the load you will actually carry. None of it prevents altitude illness. All of it decides how much of the trip you enjoy.

In thirty years I have watched older travelers handle altitude better than young athletic ones more often than not. They pace themselves. That is my observation rather than a study, but it has been a consistent one.

Beyond those, individual susceptibility varies widely and is not well explained. Some people feel something every time they go high and others almost never do. That variation is real, it is not a measure of toughness, and there is nothing to be gained by comparing yourself to the rest of the group.

How we build trips around it

The dominant cause of altitude illness is ascending too high, too fast. Published guidance addresses this directly, and these are the rules our itineraries are built on.

1. Above about 9,800 ft (3,000 m), limit increases in sleeping elevation to roughly 1,600 ft (500 m) per day and take a rest or easy day every third or fourth day. Where the geography allows, we plan more conservatively than this. It is also itinerary dependent, which affects where and how high we go.
2. Avoid going straight from sea level to a sleeping elevation above 9,800 ft (3,000 m) where an intermediate night is possible. Where it is not, as with flights into La Paz, Cusco, Quito or Bogota, the first 24 to 72 hours are rest as needed and light activity only. Driving is fine, easy walks are fine, exertion is not.
3. Climb high, sleep low. Spending part of the day higher aids acclimatization, provided you come back down to sleep. We do this with vehicles and easy walks, then return to a lower sleeping altitude.
4. Do not go up until the signs come down. If symptoms appear, stay at that elevation until they have resolved completely.
5. If symptoms worsen, or any severe sign appears, descend. Descent is the one intervention that always works.

The same rule, six countries

One rule, six countries, and six different ways the ground makes you obey it. The physiology is identical everywhere. What changes is the geography, and the geography decides the mechanics. Listed north to south.

Colombia: two staircases with a reset between them

From Bogota at 2,640 m we work up to paramo days near 4,000 m in Sumapaz and Chingaza, then deliberately drop back to Bogota, then further down to coffee country at about 2,000 m. That descent is not lost time. It consolidates the first block of exposure and it puts cloud forest and hummingbirds between two cold weeks of paramo. Then the second staircase: Termales del Ruiz at 3,500 m for two nights, then Los Nevados and the El Cisne visitor center at 4,050 m.

Nobody sleeps above 4,050 m on the whole trip. Los Nevados sits on an active volcano, so access changes with alert levels and closures, and we confirm routes against current park conditions before every departure.

Ecuador: up and down every day

The Avenue of the Volcanoes lets you gain altitude in daylight and give it back at night. Two nights in Quito at 2,850 m doing nothing strenuous. Then the cable car to about 4,000 m and Rucu Pichincha at 4,696 m, with the night back down at 2,850 m. South the next day to sleep at 3,400 m. A full day between 3,800 and 4,200 m in Cotopaxi National Park. Then the drive to 4,500 m and the short walk up to the Jose Rivas refuge at 4,864 m, and down again to sleep.

Sleeping altitude rises about 550 m across those six days while the daytime highs run close to 5,000 m. That gap is the whole method.

Peru: Cusco is the arrival, not the base

The standard mistake in Peru is Lima at sea level, a flight to Cusco at 3,399 m, and Rainbow Mountain above 5,000 m inside about 48 hours. We go the other way: down into the Sacred Valley at around 2,800 m for the first nights, then back up to Cusco and higher once the body has caught up.

For Cordillera Blanca and Huayhuash work the platform is Huaraz at 3,052 m, used as a fixed base with day exposures to Llanganuco at 3,850 m and Portachuelo at 4,767 m before any trek begins.

Bolivia: the arrival descent

Bolivia is the clearest example, so it is worth showing night by night rather than describing. You land on the El Alto rim at 4,061 m and we take you straight down to sleep at 3,100 m, the lowest point of the city. La Paz spans roughly 900 m from its lowest neighborhoods to its highest, which no other city in the world does, and that vertical range is a tool we use on night one.

Sleeping elevation by night on a typical Bolivia photography itinerary. Elevations are approximate and itinerary dependent.

Three things in that line are deliberate.

The arrival descent. A drop of roughly 1,000 m on night one, then two flat nights in La Paz with no gain at all.

The plateaus. Two nights at Lake Titicaca at 3,812 m, then two or three at the Salar de Uyuni at 3,600 m, which is actually slightly lower. Those flat stretches are where the adapting happens.

The ceiling comes last. Laguna Colorada in the Sud Lipez at 4,200 m, then Sajama at 4,330 m, both in the second week rather than the first.

Net gain in sleeping elevation across that itinerary is about 1,230 m over roughly nine nights, with a descent on arrival and two plateaus in the middle. Averaged out that is well under the published guideline, and the ceiling stays at 4,330 m rather than the 5,000 m and above where the severe forms become meaningfully more common. Chacaltaya at 5,395 m (17,700 ft) and the Sud Lipez lagoons are places we drive to rather than climb, so the daytime highs run well above anywhere anyone sleeps.

Chile: one hotel, four altitudes

From San Pedro de Atacama at 2,408 m you never change rooms. Day trips to the Puritama springs at 3,475 m, the Miscanti and Miniques lagoons above 4,100 m, and El Tatio at 4,300 m, with the same bed every night. Repeated high exposure, one sleeping elevation.

The northern route is the harder problem and the more dramatic one. Arica sits at sea level and Lake Chungara at about 4,500 m, and you cannot drive between them in a day. Putre at 3,500 m does the work in between, which is what both CONAF and Chile's tourism authority recommend. Either way, Chile is often the approach rather than the destination: by the time you cross into Bolivia, the acclimatization is already done.

Argentina: the road does the work

Argentina inverts the usual problem. No airport in the country is high enough to hurt you, so you gain elevation by driving over days rather than by landing in it. Salta at 1,152 m, up the Quebrada de Humahuaca to Tilcara at 2,465 m, then the climb to Hornocal at 4,350 m for sunset over the fourteen colors, with the night back down in Tilcara.

The Cuesta de Lipan follows, gaining nearly 2,000 m in about 35 km to a pass at 4,170 m before dropping to Salinas Grandes at 3,450 m and on to San Antonio de los Cobres at 3,775 m, the first night near 3,800 m. Two weeks later and 800 km south, the Ruta de los Seismiles works the same way from the other end: day drives from Cortaderas up to the Balcon del Pissis and the high lagoons, then back down to the same bed. Both ends of the Puna, and you never sleep at the high point.

What the six have in common

Every one of them is the same rule applied to different ground: gain in the day, give it back at night, and let the sleeping altitude rise slowly behind the daytime highs. None of that is accident. It is the shape we build every trip to.

The high border crossings

Several of our itineraries cross from one country into another by road, and those crossings are where you will stand at the highest elevations of the whole trip. Every one of them is a day-only elevation. You drive up, you complete the formalities, you drive down. Most take an hour or two. Hito Cajon between Bolivia and Chile is the exception, where the process can run several hours. None of them is a sleeping elevation, and sleeping elevation is what drives altitude illness, so crossing a high pass and descending to sleep is acclimatization working in your favor rather than a risk.

Listed north to south, the way you would meet them traveling down the Andes.

Pass elevations are approximate. Summit signs, GPS readings and survey figures frequently disagree, sometimes by a hundred meters or more. Other crossings exist, and we confirm access and conditions with our ground teams before any route goes into an itinerary.

Two of them deserve real respect, for a reason that has nothing to do with the pass itself. Tambo Quemado and Hito Cajon are both higher than La Paz, and both are usually crossed while climbing up from the Chilean coast or from the Atacama. The pass is not the issue. The day is. If you have gained serious elevation getting there, the first night on the Bolivian side is a rest night rather than a celebration.

Entering Chile: what you cannot bring, and why the wait is worth it

This applies at every Chilean entry point, road or air. Chile is an island in agricultural terms, protected by desert, ocean and the Andes, and it defends that seriously. Nothing that can grow and nothing that came from an animal comes in with you. No fresh fruit or vegetables, no seeds or nuts, no meat, no cheese or dairy, no honey, no plants, no soil on your boots. The apple somebody picked up at breakfast in Uyuni counts. Fines are real and they are not a warning.

The remedy is simple. Eat it or bin it before you reach the border, knock the mud off your boots, and declare anything you are unsure about. Declaring costs you nothing at all. Failing to declare is what gets you fined.

At Hito Cajon in particular, expect a thorough job. Dogs, bags out of the vehicle, everything scanned, questions asked. It can take a few hours and it happens at 4,480 m, so keep a warm layer and water within reach rather than buried in the roof box. This is a serious counter-narcotics and biosecurity operation rather than an inconvenience, and we would rather travel through a border that works than one that does not.

The three forms of altitude illness, in detail

The quick reference on page one is the field version. This section is the full picture. Altitude illness is conventionally divided into one mild form and two severe ones. They are related, and the mild form can progress to the severe forms if it is ignored.

1. Acute Mountain Sickness (AMS)

AMS is the mild and by far the most common form, known throughout the Andes as soroche. It typically appears within 6 to 12 hours of arriving at a new elevation, though it can start sooner or as late as a day or two later. In most people it resolves within one to three days at the same elevation, provided they stop climbing, rest and hydrate. It is not usually dangerous by itself. It matters because it is the stage at which the severe forms can still be prevented.

How common is it? On rapid ascents in the Alps, measured rates were about 9% at 2,850 m, 13% at 3,050 m, 34% at 3,650 m, and 53% at 4,559 m. Those figures are for going up fast. On a staged ascent they fall considerably, but you should still expect that some people in any group at 3,500 m and above will feel something. Feeling it is normal. It is not a sign that anything has gone wrong, and it is not a reason to be alarmed. It is a reason to stop climbing for a day.

Signs and symptoms

Headache is the cardinal symptom. Alongside it: fatigue, weakness, loss of appetite, nausea, occasional vomiting, dizziness and poor sleep. Sleep is often broken by an irregular breathing pattern that alternates between rapid breaths and pauses, which is normal at altitude and not itself a danger sign. Mild swelling of the hands, face or feet can occur. Children may be unable to describe what they feel, so they are watched closely.

What to do

Go no higher until symptoms have resolved completely.

Rest, hydrate and eat lightly. Most cases settle on their own, and each person needs their own time.

Descend if symptoms do not improve, or if they worsen at any point. Even a few thousand feet of descent usually produces rapid relief.

Acetaminophen or ibuprofen are effective for the headache. Published guidance notes that they treat the headache without necessarily improving the other symptoms.

Supplemental oxygen relieves symptoms and we carry it on every high-altitude trip. Our Amazon itineraries are at low elevation and do not carry it, because there is no altitude to treat. Oxygen treats the symptom rather than the cause, and it is never a reason to stay high or to continue ascending.

2. High Altitude Pulmonary Edema (HAPE)

HAPE is fluid accumulating in the lungs. Two things need saying together about it, and neither one means much without the other.

It is uncommon. On a paced ascent to the elevations we work at, published rates are under 0.2%, and we have never had a case on a standard itinerary. It is also the most common cause of death from altitude illness, with an untreated mortality that has been reported as high as 50%. Both of those are true at once. The resolution is that HAPE caught early and treated with descent and oxygen is fully reversible, and HAPE ignored is not. Recognition is the entire game.

When it does occur, it is almost always after a fast gain in sleeping altitude, typically appearing within two to four days of that gain, and rarely on the first night. It is uncommon after a prolonged stay at one elevation, which is why our plateaus matter. It can develop in someone who had only mild AMS beforehand, or none at all, so it is not something you can rule out by feeling fine yesterday.

Signs and symptoms, in the order they usually appear

Breathlessness that does not settle with rest, and out of proportion to the exertion. This is the earliest reliable sign.

A dry cough that becomes wet, sometimes producing frothy or pink-tinged sputum.

Marked fatigue and a sharp drop in exercise tolerance. Someone who kept up yesterday cannot keep up today.

Chest tightness, congestion, rapid heart rate and rapid breathing at rest.

Crackling or bubbling sounds in the chest.

Cyanosis, a blue or gray tint to the lips and fingernails, indicating low blood oxygen.

Confusion or altered behavior, which suggests the brain is affected as well. This is a late and serious development.

What to do

Descend immediately. Descent is the definitive treatment. Published guidance recommends a descent of at least 1,000 m (about 3,300 ft) where feasible, together with supplemental oxygen, targeting a blood oxygen saturation above 90%. Minimize the patient's exertion during descent. Get to a hospital. Where descent is impossible or delayed and oxygen is unavailable, nifedipine may be used as an adjunct under medical direction.

A correction worth stating plainly: acetazolamide has no established role in the treatment of HAPE, neither do diuretics or beta-agonists. The treatment is descent, oxygen and medical care as soon as possible.

3. High Altitude Cerebral Edema (HACE)

HACE is swelling of the brain caused by fluid leakage. It is the rarest of the three forms, roughly half as common as HAPE, and it is uncommon below 3,500 m with most cases occurring above 4,000 m. It is also the most immediately life-threatening, which is why it gets space here out of all proportion to how often anyone sees it.

It is almost always the progression of untreated AMS, and it can occur together with HAPE. Onset can be rapid. That combination, rare but fast and severe, is the reason we do not simply wait out AMS at elevation and hope.

Signs and symptoms

Loss of coordination, most reliably detected by asking the person to walk heel to toe in a straight line. Inability to do so is the classic sign and is treated as HACE until proven otherwise.

Severe headache unresponsive to rest, fluids and ordinary pain relief.

Confusion, disorientation, memory loss, irrational or out-of-character behavior, hallucinations.

Declining level of consciousness, drowsiness progressing toward stupor and coma.

Persistent vomiting.

What to do

Descend immediately, with oxygen if available, accompanied by others. Do not let the person descend alone, and do not wait for daylight or better conditions if descent is possible. Published guidance identifies dexamethasone as the primary drug treatment alongside descent and oxygen, with acetazolamide as an adjunct. Any use of these is a medical decision. Evacuation to a hospital is necessary, and follow-up care is required.

The pulse oximeter and oxygen, what the numbers actually tell you

Your guides carry a finger pulse oximeter and we carry supplemental oxygen. Both are emergency equipment. Equipment in remote fieldwork gets damaged, lost and sometimes stolen, so I will tell you we carry these things rather than promise that a particular item will be always available.

You will see readings that would alarm your doctor at home, and they are usually normal for where you are standing. It is worth understanding what the device is and is not good for, because travelers routinely read too much into it.

At sea level a healthy reading is around 96 to 99%. Blood oxygen saturation falls as you go up, for everyone, and it recovers partially over days as you acclimatize. Readings in the 80s are ordinary at the elevations we sleep at. There is no single number that means you are ill and no single number that means you are fine, because the normal range at any given altitude is wide and varies a great deal between individuals.

Oxygen is for emergencies, not for getting through a trip. It relieves symptoms and buys time while we descend. If someone needs oxygen to keep going, that is not a trip to continue; it is a trip to come off. We will get that person to a hospital as soon as possible.

What it does not tell you

It does not diagnose mild altitude illness. In acute mountain sickness, oximeter readings are usually within the normal range for that altitude or only slightly below it. Someone with a bad headache can read perfectly well, and someone who feels excellent can read low. So how you feel is the information that matters, and it is why we keep asking you rather than just measuring you.

What it is genuinely good for

It is useful for spotting fluid in the lungs, and the useful signal is comparison rather than an absolute value. In HAPE, readings are commonly at least 10 points below those of healthy people standing at the same altitude, and can fall far lower. That is why your guide takes readings from the whole group and records a baseline for each person early in the trip. One person sitting well below everyone else, or well below their own earlier readings, is the pattern worth acting on.

Two practical notes. Cold fingers, nail polish and movement all produce false low readings, so a single odd number gets repeated on a warm hand before anyone reacts to it. And consumer oximeters lose accuracy below roughly 80%, so very low readings are treated as a reason to act rather than as a precise measurement.

Before you travel

See your own doctor, well in advance. Bring your itinerary and the elevations. Discuss your medical history, every medication you take, and whether any altitude medication is appropriate for you.

Arrive in reasonable cardiovascular condition. Fitness will not prevent altitude illness, but it makes the days more enjoyable and recovery easier. If you smoke, stopping well before the trip helps.

Tell us about any medical condition, medication, or previous experience of altitude illness that could affect you on the trip. The earlier we know, the more we can do about it.

If your schedule allows, arrive a day or two early and spend that time at moderate elevation. Time is the most effective acclimatizer there is.

While you are at altitude

Hydrate deliberately. Roughly 2 to 4 liters a day. Pale, plentiful urine is the simplest indicator. Dark or scant urine means you are behind, so increase fluid intake, but no alcohol.

Eat lightly and favor carbohydrates. Digestion consumes oxygen. Large, fatty meals late in the day are harder work than they are worth.

Avoid alcohol for the first few days, and avoid sleeping pills, sedatives and tranquilizers at altitude. They suppress the breathing response, which is precisely the adaptation you need, and they can mask early symptoms. Do not stop a prescribed medication without talking to your doctor.

Move at half your normal pace. Walk slowly, stand up slowly, and let the terrain set the rhythm. Light activity during the day is generally better than long daytime sleeping, since breathing slows during sleep. Short naps are fine.

Stay warm and avoid getting sick. Strong sun at altitude does not mean warm air, and an ordinary cold makes everything else harder. Dress in layers for the colder parts of the day, morning and evening.

Tell us how you feel. Early and honestly. Reporting a headache is not complaining. It is what allows us to act while the problem is still small. We prefer proactive prevention over hospital visits.

Coca tea and other traditional remedies

Coca leaf has been used in the Andes for centuries, and coca tea, mate de coca, is part of daily life across much of the region. Most travelers find it eases a mild headache and settles the stomach, and it is a genuine part of the culture you are visiting. We should be straight with you though: there is no good clinical trial evidence that it prevents or treats altitude illness, so treat it as a pleasant comfort rather than as protection. Anise and chamomile teas are similarly traditional for digestion. None of this is a substitute for acclimatizing properly or for descending when you need to.

Where you will actually find it varies more than most travelers assume. Where it is available, we carry it.

Colombia. Confined to particular indigenous communities in the far north. Not an everyday item, and not something a hotel will offer you.

Ecuador. No coca tea culture at all. Do not expect it.

Peru. Standard. Grown legally for domestic use and offered in most hotels.

Bolivia. Standard, the same as Peru. They even put it in toothpaste.

Chile. The far north only, around Arica, Parinacota and San Pedro de Atacama. Not elsewhere in the country.

Argentina. The northwest, around Salta and Jujuy, where coca is grown in the region. Not further south.

Three things about coca worth knowing before you travel.

It can produce a positive result on a standard workplace drug screen for several days afterward. If a drug test matters for your employment or your license, skip it entirely.

Do not take it home. Coca leaf and coca tea are a Schedule II controlled substance in the United States and prohibited on import regardless of where you bought them. US Customs treats it as contraband rather than a souvenir and will seize it. The United Kingdom is the same.

Do not carry it across a South American border either. Legal to drink in La Paz does not make it legal to bring into Chile, which enforces this without exception.

Medications: what to discuss with your doctor

Several drugs are used at altitude, and you will see them mentioned in guidebooks and on forums. It is worth knowing the names so you can have a useful conversation with your own physician before you travel.

Acetazolamide, sold as Diamox, is the one most travelers ask about. It speeds acclimatization rather than masking symptoms, and it is used both to prevent and to treat mild altitude sickness. Dexamethasone, a steroid, is used under medical direction for severe cases. Nifedipine is used for the lung form, usually only by people who have had it before. Ordinary painkillers such as ibuprofen and acetaminophen are used for altitude headache, and ibuprofen has also been studied for prevention. A headache treated with a painkiller is still a headache at altitude, and it does not clear you to go higher.

We do not prescribe, recommend, or advise for or against any of these. We are guides and photographers, not physicians. Whether any of them is appropriate for you depends on your medical history, and that is a conversation for your doctor, not for us. Published dosing guidance exists in the Wilderness Medical Society Clinical Practice Guidelines for the Prevention, Diagnosis, and Treatment of Acute Altitude Illness, and your physician will know it or can look it up. Ask them.

Supplemental oxygen is different. We carry it as emergency equipment, and our guides use it as first aid. It works but is never a substitute for descent.

A note on acetazolamide and sulfa allergy

Acetazolamide is a sulfonamide. If you have a known sulfa allergy, tell your doctor before it is prescribed. Published guidance notes that a true allergic reaction to acetazolamide is very unlikely even in people with a sulfonamide antibiotic allergy, and that a supervised trial dose before travel is one option. That assessment is your physician's to make, not ours. Common side effects include tingling in the fingers, toes and lips, increased urination, and an altered taste that makes carbonated drinks taste strange.

What help actually looks like out here

This is the part most travelers have never had explained to them, and it matters more than anything else in this document. Helicopter rescue exists in all six countries, but you should not assume it is an option. Roughly in order of likelihood, it is Chile, then Colombia, Ecuador, Peru, northern Argentina, and Bolivia last, with Patagonia better served than the Argentine north. Whether one is available on the day you need it depends on weather, daylight, distance, the aircraft being serviceable and someone being willing to fly. I cannot guarantee a helicopter anywhere. When one does happen it is extremely expensive, and the cost is yours, not ours. That is the single clearest reason emergency evacuation insurance is a condition of travel with us.

Mountain rescue services of the kind that exist in the Alps or the Rockies do exist across some of the high Andes but getting a rescue team into an area by helicopter is not the norm and should not be expected. Ambulances are rare outside cities, and the nearest facility may be a basic clinic rather than a hospital. Get ready to assume a lot of hours on bad roads. If we end up on pavement eventually, that is a bonus.

So, the plan is always self-rescue. Your field team will get you down and get you out using the vehicle, the oxygen, the route and the daylight available to them at that moment. That is what our itineraries, our pacing and our equipment are built around, and it is why we would rather change a day early than manage an emergency later.

There are limits, and you should know them before you book. River crossings are a good example: some are nothing to worry about, others are deep with a strong current, and in rain or flood conditions the same crossing can change within hours. It depends on the river and the day, and the guide on the ground decides. We will not cross water we judge unsafe, drive a road we judge unsafe, or split the group in a way that leaves anyone alone, in order to reach a clinic faster. We will not put our staff, our other travelers or our vehicle at risk except in a genuine emergency where the alternative is worse. If the drive is twelve hours rather than eight because that is the road that is open, then it is twelve hours.

None of this is a reason not to come. It is the reason our trips are paced the way they are, why we require evacuation insurance, and why we ask you to tell your guide early rather than push through. Everyone's safety is the goal.

Who decides, and who pays

If altitude illness develops, your guide makes the final call on whether the group or an individual descends. That is not a matter of preference. The guide is the person with the field experience, the local knowledge and the responsibility, and on this one subject their decision stands.

You can also make the call yourself. If you want to descend or leave the trip, we will arrange it, provided it can be done safely: without putting other travelers or staff at risk, without leaving anyone unaccompanied, and with the logistics available at that location. What you need to understand before the trip starts is the financial side. Where a descent or departure happens at your request rather than on medical grounds identified by us, the costs that follow are yours. That includes transport, additional accommodation, changed flights and any unused portion of the trip. Sergio Photo Tours does not absorb evacuation or trip-interruption costs under any circumstance. That is what your insurance is for, and it is why we require it.

Insurance and evacuation

Emergency medical and evacuation insurance is a condition of travel with Sergio Photo Tours. It is not optional and proof is due no later than 60 days before departure. This is the cover that matters most: an evacuation from the Puna or the Altiplano can cost many times the price of the trip, and it is your expense, not ours.

Trip cancellation, interruption and baggage cover is also required under our Terms and Conditions, and it costs very little by comparison. We recommend adding a cancel for any reason rider. Full terms are in our Terms and Conditions and the rest of our pre-departure materials.

Important notice

This document provides general educational information about high-altitude travel. It is not medical advice, diagnosis, or treatment, and it is not a substitute for consultation with a qualified physician. Sergio Photo Tours and its staff are not medical professionals and do not diagnose, prescribe, or recommend any drug, dose, or course of treatment. Where this document names a medication, it does so only to help you have a useful conversation with your own physician, who is the person qualified to advise you.

Always follow the guidance of your physician or another qualified health provider over anything written here.

Altitude illness can affect anyone and can develop or worsen despite gradual ascent, acclimatization days, monitoring, supplemental oxygen, and every other precaution described here. Nothing in this document is a guarantee that you will not experience altitude illness, or a guarantee of any particular outcome, response time, medical result, or availability of medical, communications, or evacuation resources. Conditions in remote mountain terrain change without warning, and the availability of any third-party medical or evacuation service is outside our control.

Medical understanding of altitude illness continues to develop, and published guidance is revised periodically.

The information here reflects guidance available as of the review date and may not be current when you read it.