Altitude Sickness

Altitude Sickness (also known as Acute Mountain Sickness) is a physical reaction that your body experiences to the lower amounts of oxygen (and air pressure) at high altitudes. Everyone is different and therefore altitude sickness can impact people in various ways. The most common symptoms are feelings of a headache, heavy head, nausea, shortness of breathing, and feeling tired or exhausted.

Caution: Mountain Sickness can result in death if ignored or left untreated. So, please read this article carefully and be prepared when climbing or hiking above 10,000 ft or 3000m.

Altitude Sickness

Altitude sickness is very dangerous for four reasons: it can come on suddenly and progress quickly, it can be fatal, sufferers are often some distance from medical help and are difficult to evacuate swiftly, and in many cases sufferers are reliant on their health because they’re doing a lot of physical activity in dangerous environments.

Altitude sickness is a great danger for high-altitude mountaineering (above 4000 or 5000 m), a moderate danger for mountain sports (such as skiing at 3000–4000 m, notably in Colorado), and a moderate danger when flying in to a high-altitude city around 3500 m, notably Tibet (Lhasa), Peru (Cusco, especially for the Inca Trail), and Bolivia (La Paz).

For moderate altitudes (such as 3500 m), the main solution is to acclimatize for a night or two at a lower altitude (near 2500 m) and take it easy for the first few days, rather than flying in and immediately going skiing or hiking. For higher altitudes much more care, preparation, and gradual ascent are necessary, and potent treatments are available.

Particularly dangerous are tall, easy mountains, notably Kilimanjaro (5895m) and Aconcagua (6961m), where it’s easy to get dangerously high quickly. Acclimation requires time, and rushing causes altitude sickness.

Rule of thumb: Only book with tour operators that have a detailed plan for on how to deal with people suffering from AMS.

Mountain Sickness Causes

The further you move away from sea level up into higher altitudes, the lower the air pressure is. The body has two main problems with high altitude and the corresponding lower air pressure:

1. Air at lower pressure has less oxygen per lungful. Your body adjusts to this by making more red blood cells to carry oxygen more efficiently. However, the process takes several days, sometimes more than a week, and in the meanwhile, you may be ill.

2. At lower air pressure, water evaporates faster. This can lead to dehydration.

The difficulty your body has maintaining a good oxygen supply and keeping related problems under control is directly related to how high up you are, and also to recent changes in your altitude. The altitude where you sleep is also important because most of the extra red cells needed to improve your oxygen management are manufactured while you sleep.

How High is Too High

High altitude is defined as:

High Altitude: 1500–3500 m (5000–11,500 ft)
Very High Altitude: 3500–5500 m (11,500–18,000 ft)
Extreme Altitude: above 5500 m (above 18,000 ft)

However, in practice altitude sickness rarely occurs below 2500m -2800m (~9000 ft).

A minority of people, about 20%, have some symptoms of altitude sickness if they ascend to about 2500m (8000 feet) above sea level and sleep there. However, most people will acclimatize to 3000m (10,000 feet) with relative ease, perhaps having symptoms after the first night.

Acclimatizing to heights of 3000–5000m (10,000–16,000 feet) is much more difficult, and it is here that it is absolutely necessary to ascend slowly and return to a lower altitude to sleep if you have been traveling around at a higher altitude during the day. Over 50% of people will become ill if they ascend rapidly from sea level to 3500m (11,000 feet) without acclimatization, and everyone will if they ascend rapidly to 5000m (16,000 feet).

Over 50% of people will become ill if they ascend rapidly from sea level to 3500m (11,000 feet) without acclimatization, and everyone will if they ascend rapidly to 5000m (16,000 feet).

It is thought to be impossible to permanently acclimatize to heights above 5500m (18,000 feet). It is possible to spend several weeks sleeping as high as 6000m (20,000 feet) once acclimatized, but a gradual deterioration of physical well-being will still occur.

Death Zone

Regions above 8000m (26,000 feet) are referred to as the death zone: you will deteriorate noticeably while you remain at such high altitudes, some of your body’s major systems will shut down and climbers will only remain there for two or three days. Death rates from altitude sickness above 7000m (23,000 feet) are estimated at 4% of all people who venture that high.

If your home is significantly above sea level, you gain a definite leg up on ascending to higher elevations, but that doesn’t make you immune to altitude problems; it just pushes the threshold for their onset higher. Most otherwise healthy people who live at elevations of 1500m (5000 feet) to 2500m (8000 feet), an elevation range containing quite a few major cities, experience little trouble going to 3000m (10,000 feet) or a bit higher, but even they will be at risk of altitude problems at 5000m (16,000 feet).

Can It Happen To You

Past performance at altitude is the main predictor of future performance but is a guide, not a guarantee. Beware that even if you have not suffered acute mountain sickness (AMS) in the past at altitude, you may still suffer it in future, even at lower altitudes.

Altitude sickness tends to affect men more than women, especially men between the ages of 16 and 25. It’s not clear if there’s some unknown biological reason for this, or if it’s just the demographic most likely to attempt too much, too soon. It is important to remember that just because you are young and healthy, and haven’t experienced altitude sickness in the past doesn’t mean you are immune to it on future climbs. Physical fitness is not necessarily a good indicator, and neither is the strength or good health. You may react badly to altitude despite being fit, young and healthy. In fact, the fit, young and healthy have a hidden risk: their general physical capacity leads them to believe that they should handle altitude just fine, which is not always true.

Physical fitness is not necessarily a good indicator, and neither is the strength or good health. You may react badly to altitude despite being fit, young and healthy. In fact, the fit, young and healthy have a hidden risk: their general physical capacity leads them to believe that they should handle altitude just fine, which is not always true.

Bad health, on the other hand, is a risk factor: particularly cardiac or respiratory problems. Healthy hearts and lungs have a hard enough time getting oxygen to your tissues at high altitudes. Naturally, if you have physical problems that make exertion difficult for you, you have reason to think carefully about exertion at high altitude, where it is much harder!

Scuba diving increases the risk of decompression sickness. If you have recently been diving and have not fully gotten rid of the nitrogen in your blood, you should not ascend to a higher altitude (or travel in a plane). See scuba diving for recommendations on how long to wait.

Genetics play a role: some populations, notably Sherpas, have significant genetic adaptations to altitude, and at least eight genetic polymorphisms have been identified as contributing to individual variation: AMS is an environmentally mediated polygenetic disorder. However, for most people, there is currently no screening, genetic or otherwise, available to assess risk for altitude sickness. Thus prior history is used instead.

How To Acclimatize

Decreased blood oxygen saturation, which increases over time
Increased heart rate
Increased ventilation
Increased urination
Shortness of breath during exertion
Changed breathing pattern at night
Awakening frequently at night

It is therefore important to drink water to handle the increased urination and to avoid anything that decreases breathing. Notably, alcohol both dehydrates and decreases breathing (it is a depressant), so it should be avoided or consumed moderately. If you are not urinating more than usual, you are either dehydrated or not acclimatizing well: try drinking more. The unusual night breathing can be scary and can disturb companions (like snoring), but is normal.

Increased Ventilation

You will naturally breathe faster at higher altitudes to compensate for the lower air pressure. It’s possible you won’t notice this: a similar effect happens during air travel. This is called “hypoxic ventilatory response” (HVR); it is frequently misnamed “hyperventilation”.

Altitude Diuresis

Increased urine output is a response to hypoxia: increased breathing reduces CO2 in the blood, resulting in more output of bicarbonate, which increases urination. This will make you urinate a lot at altitude. If you aren’t urinating much more than you usually would, then you might actually be dehydrated, or not acclimatizing.

Periodic Breathing

Because of the disruption to oxygen and carbon dioxide levels in your blood due to the body chemistry changes and hyperventilation that occurs at altitude, your body’s “when to breathe” chemical signals become confused. While you’re awake you will remember to breathe, but when you sleep it is common to have interrupted breathing: holding your breath for up to fifteen seconds and then breathing very rapidly when you start breathing again.

This can be very alarming when you wake up knowing that you weren’t breathing or were short of breath; or when you notice someone else has stopped breathing. But it is a normal response to altitude and happens to almost everyone. Acclimatisation only improves it a little.

Altitude Related Illnesses

As well as the less dangerous physiological effects, altitude makes you susceptible to actual illnesses, several of them very dangerous. While not all the effects of altitude can be avoided, you should take sensible steps to avoid actual illness, and take it very seriously if it does occur.

High altitude headache is the most common symptom, and first warning sign, affecting about 80% of people who ascend. In itself, this headache is not dangerous, but other symptoms should be monitored. If other symptoms arise, or a headache does not resolve with a liter of fluids, mild analgesics, and a day or two of acclimatization, you are likely suffering mild AMS and are at increased risk of more serious illness.


You need to increase your fluid intake at high altitudes. The loss of appetite, a precursor to nausea, can lead you into a dehydration headache. Unfortunately, it is easy to mistake dehydration headaches for acute mountain sickness (AMS) headaches (below) and vice versa. If a headache does not improve after drinking a liter of fluids it should be considered as an AMS effect.

A dehydration headache can also be recognized by comparing pulse rates: if your pulse rate goes up more than 20% when standing up after lying down for five minutes, you need more fluids.

Acute Mountain Sickness

Acute mountain sickness (AMS) is the most common unhealthy response to altitude: it’s a collection of signs that your body is becoming ill and has not adapted successfully to a higher altitude.

For your own safety, assume any illness at altitude is AMS – denial of AMS is very frequent and dangerous. The most common reasons that people fail to descend as soon as they should are bad assumptions. They assume that having AMS is a sign of weakness; that their level of fitness means they can’t have AMS; or mistake their symptoms for the flu or another illness. Too-aggressive ascent schedules are another cause: if not enough time is budgeted, acknowledging AMS and slowing down may prevent one from successfully summiting a peak, though worsening AMS may well force this anyway.

Too-aggressive ascent schedules are another cause: if not enough time is budgeted, acknowledging AMS and slowing down may prevent one from successfully summiting a peak, though worsening AMS may well force this anyway.

Assume AMS first: it happens to healthy strong people, and if it turns out you are indeed sick with something else, descending to a lower altitude will make it easier for your body to heal anyway.

In particular, if you’ve recently ascended, and you have a headache and any other symptom, you have AMS. The other signs of AMS vary for different people, but include:

loss of appetite
nausea or vomiting
difficulty walking (called gait ataxia)
rattling breath
feeling generally extremely ill

The last three signs, in particular, are signs that you are becoming quite ill, but you should not wait for the onset of these symptoms before acknowledging you have AMS: they’re fairly reliable indicators of the onset of more serious problems, namely high altitude cerebral edema (HACE) or high altitude pulmonary edema (HAPE).

You and your party should keep an eye on each other for signs of AMS, and if you have AMS, for signs of it worsening. Very sick people can become confused and not realize how ill they are. Loss of appetite is a particularly good sign. Anyone who has been walking or climbing at altitude for a day should be very hungry for a good meal in the evening.

If you have symptoms of AMS, do not ascend further. Consider descending, or wait a few days to acclimatize and for symptoms to resolve before ascending further.

High Altitude Cerebral Edema

High altitude cerebral edema (HACE) is the end-stage of AMS (conversely AMS can be thought of as the mild form of HACE). When you have HACE, your brain swells and stops working properly. If you have signs of HACE or HAPE, descend immediately. Your life may depend on it.

HACE symptoms include a number of signs of mental functions failing: confusion, fatigue and weird behavior. But the most reliable one is gait ataxia, and you can test it by walking heel to toe along a straight line on the ground. Healthy people can pass this test easily, anyone who has difficulty balancing while they do it is showing signs of HACE.

HACE is extremely serious, and you may only have a few hours to help someone with HACE. The main treatment is decent, but a person experiencing these symptoms will need significant help.

A 2008 medical study “Why Climbers Die On Mount Everest” shows HACE as the leading cause of death.

High altitude pulmonary edema

High altitude pulmonary edema (HAPE) is another severe altitude illness. It sometimes occurs in conjunction with AMS or HACE, but sometimes on its own — it’s thought to have different causes. When you have HAPE, your lungs fill with fluid.

Signs include extreme fatigue; breathlessness (when not due to interrupted breathing — give yourself 30 seconds to recover upon waking); a cough, especially if it is wet and has blood in it; rattling or gurgling breath; chest congestion; very fast heart rate; very fast breathing; and blue extremities. A fever is sometimes present. It most commonly sets in at night.

It is an extremely serious illness and as such it should be treated as a critical emergency. Rapid descent is very important.

Cheyne Stokes Breathing

Above 3000m (10,000 feet), some people experience a periodic breathing during sleep known as Cheyne-Stokes respiration. The pattern begins with a few shallow breaths and increases to deep sighing respirations then falls off rapidly. Respirations may cease entirely for a few seconds and then the shallow breaths begin again. During the period when breathing stops the person often becomes restless and may wake with a sudden feeling of suffocation. This can disturb sleeping patterns, exhausting the climber.

How To Prevent AMS

Gradually Acclimatize To Altitude

Acclimatization is the process of getting your body to adapt to the lower oxygen levels by ascending slowly into higher altitudes, spending some time at each one to adapt. It is essential to budget sufficient time, and have a realistic ascent profile: ascend gradually, and leave extra days in case it is necessary to spend extra time acclimatizing.

Over-aggressive schedules, such as 6-day ascent/descent of Kilimanjaro, pose a very high risk of AMS, and a significant risk that you will not successfully complete the expedition due to not having enough time to acclimatize, instead of being forced to turn back.

The most important factor is to increase your sleeping elevation (the altitude where you’re spending the night) slowly. If you’re on a hiking or climbing holiday, a typical strategy is to spend a day (or initially part of a day) at a higher altitude and return to a lower altitude to sleep: “climb high, sleep low”.

This is particularly used on summit day for a high summit (such as Kilimanjaro), or over a high pass (such as the Inca trail). This also works for people doing winter sports at high altitudes: ski at the top of the resort and sleep at the bottom.

Acetazolamide (ACZ)

The drug Acetazolamide (ACZ) jump-starts and speeds acclimatization, via the same biological pathway, and is effective in preventing AMS and reducing its severity. However, it is no substitute for a reasonable ascent schedule.

Avoid Rapid Ascents

Rapid ascents are the opposite of acclimatization; you make a rapid ascent when you’re gaining altitude faster than recommended. This may mean climbing and camping higher than recommended, but you can also make an even more rapid ascent by driving to a high altitude location, and flying from low altitude to high altitude is an even more rapid ascent.

For example, flying from sea level to Lhasa, Tibet, which is 3700m (12,000 feet) high, is distinctly unwise. Consider spending a week or so at an intermediate altitude. If you are going to travel around Tibet — where some inhabited areas are over 5000m (16,000 feet) and some mountains over 8000m (26,000 feet), do not set out until you are thoroughly acclimatized in Lhasa.

The same is true if you travel to the Andes; destinations such as Cusco, La Paz or the Inca Trail are located well over 3000m above sea level.

Where possible, avoid ascents more rapid than recommended above, particularly any sudden ascent to 3000m (10,000 feet) or higher. Even if you are taking Acetazolamide (below) a rapid ascent makes it more likely you’ll get AMS and makes AMS progress to serious illness faster, so you will have less time to respond and descend.

Consider road or rail travel rather than flying directly to somewhere with a very high altitude — but remember that the surface option often involves a lot higher altitudes: the Manali-Leh road (in India), for example, will take you from below 2000m (7000 feet) to 5000m (16,000 feet) in less than a day. Or fly in stages, stopping somewhere at moderate altitude in between.

If you must fly to any destination about 3000m (10,000 feet) at least spend a few days at some intermediate destination en route. If flying to a more moderate altitude above 2500m (8000 feet), you will still want to spend several nights at that altitude before setting off into higher country.

Here are the recommended maximum increases in your sleeping elevation which will stop most people from proceeding to AMS:

Go no higher than 2400m-2800m (8000 – 9000 feet) the first night.
Increase your sleeping elevation by 300m-450m (1000 – 1500 feet) per night after 3000m (10,000 feet).
After every 1000m (3000 feet) of hiking, you should spend a second night at the same altitude. This will be every fourth night if you have been ascending at the maximum pace recommended above.

Keep Hydrated

Remember to drink adequately – around one liter extra of fluids per day. During acclimatization, drink a lot of non-alcoholic drinks, due to increased urination. Pushing large volumes of water does not protect against AMS, and can give the same symptoms (headaches, nausea, vomiting and more) as severe AMS from electrolyte imbalance.

Check blood oxygen and pulse

You can use a pulse oximeter to measure your blood oxygen saturation and pulse rate, which can help you detect problems before you get symptoms. These are inexpensive and easily available, with good accuracy. However, interpreting the numbers is complicated: normal readings vary between individuals and change with altitude.

As a rule of thumb, within a group, individuals with lower SpO2 (either resting or after exercise) at a given altitude are more likely to develop AMS at higher altitudes, though precise cutoff values are difficult to give.

Smoking and Alcohol

Refrain from smoking (due to impact on lungs) and alcohol (due to dehydration) when you arrive by plane in a high altitude area from lower altitudes.


As soon as the symptoms of AMS appear, your first priority is recovering. You must not ascend any further until the symptoms have disappeared – “don’t go up until the symptoms go down”. This may take up to 48 hours – if it takes longer, descend! You could also descend on the onset of symptoms; this will make them disappear much faster, probably within hours, and even minor descents (100 m) can help significantly.

If you are getting sicker or showing signs of HACE or HAPE, you must descend to a lower altitude as quickly as possible. If it is night time, do not wait for the morning if you have a choice at all. You should descend at least as far as you were the last night you had no AMS symptoms. You may need to seek hospital care.

People with HACE and HAPE are frequently confused or exhausted and are likely to need help with the descent. Help them down!

Supplemental oxygen can alleviate symptoms of AMS, HACE, and HAPE, but is no substitute for descent.

Caffeine, through either caffeinated beverages, and coca leaves (primarily and legally available in the Andes) widen the blood vessels and thereby help oxygen transport in the body. Though, if you are not used to caffeine, be aware of adverse effects like fastened heart-beat. Chewed coca leaves and coca tea have a milder onset and are thus easier on the body but might get you into trouble when facing a drug-check back at home. In the Andes, cocaine is also widely available (though technically illegal), but most people from the west are not used to high-grade cocaine even when they consider themselves “cocaine experts” – it is therefore very unwise to use cocaine to prevent AMS!

Headache medication

One can relieve symptoms of AMS via usual means: treat headaches with headache medication (non-opiate analgesics, like aspirin, acetaminophen (Tylenol), NSAIDs, etc.) and treat nausea and vomiting with anti-nausea drugs (antiemetics, like Zofran).

Coca leaves

Coca leaves, available primarily in Andean regions of Peru and Bolivia (in coca tea, chewing, or in candies), is a mild stimulant and alleviate symptoms to some degree, particularly headaches (like the caffeine in coffee or tea) though they do not speed acclimatization. Some people find that vegetarian or starchy food helps them somewhat.

Antacids may help with nausea, but do not help with acclimatization. There is the occasional misconception that antacids have some impact on acclimatization, presumably due to confusion between blood acidity (which is related to acclimatization) and stomach acidity (which is not).

If symptoms get worse while staying at the same altitude, you are in danger: descend immediately.

Oxygen and hyperbaric chambers

Supplemental oxygen (2 L/minute) will relieve AMS headaches quickly, and resolve AMS over hours; it is also lifesaving in cases of HAPE, and important for HACE. Oxygen typically isn’t available in the field, but is available at hospitals, and at some airports, such as Cusco, for arriving passengers. An alternative field treatment is a hyperbaric chamber (high atmospheric pressure tent), which increases the amount of oxygen available in the air.