Altitude sickness

Page last reviewed: 13/07/2011

Altitude sickness, also known as acute mountain sickness (AMS), is a potentially life-threatening condition. It can happen when a person is exposed to a high altitude without acclimatising (gradually getting used to the higher altitude).

Altitude sickness is caused by a decrease in atmospheric pressure, which makes breathing difficult. Other common symptoms include:

  • Headache.
  • Nausea.
  • Dizziness.
  • Exhaustion.

In severe cases, altitude sickness can cause pulmonary oedema (fluid on the lungs) and cerebral oedema (fluid on the brain). These are very serious and require immediate medical attention.

At what altitude does altitude sickness occur?

In its mildest form, altitude sickness can occur at about 2,500m (8,000 feet) above sea level, which is a common height for many ski resorts. However, the more severe symptoms of altitude sickness occur at altitudes of 3,600m (about 12,000 feet) and above.

How does altitude sickness occur?

As you climb higher up a mountain, the percentage of oxygen in the atmosphere remains constant (21%). However, as the atmospheric pressure decreases, we breathe in less oxygen. This means that your body needs to adapt to breathing in less oxygen.  

Who is affected by altitude sickness?

There are no specific factors, such as age, sex or physical condition, that increase a person's likelihood of getting altitude sickness. Certain people are affected while others are less susceptible to it.

How common is altitude sickness?

Altitude sickness is fairly common among people who spend time at high altitudes, such as mountaineers and skiers. For example, at a height of over 3,000m (10,000 feet), approximately 75% of people will experience mild symptoms (see Symptoms, above).

It is not possible to get altitude sickness in Ireland because the highest mountain, Carrantoohill , is just over 1000 m high .


Sickness: Sickness is when you feel unwell.

Acute: Acute means occurring suddenly or over a short period of time.

Oxygen: Oxygen is an odourless, colourless gas that makes up about 20% of the air we breathe.

Page last reviewed: 13/07/2011

Altitude sickness is common at high altitudes. The onset and severity of symptoms and the altitude at which they occur vary according to:

  • the individual,
  • how fast you get to a high altitude, and
  • the amount of time spent at high altitude.

Symptoms of mild altitude sickness

The symptoms of mild altitude sickness include:

  • headache,
  • nausea,
  • dizziness,
  • tiredness,
  • loss of appetite,
  • upset stomach,
  • feeling unsteady,
  • shortness of breath,
  • increased heart rate,
  • difficulty sleeping, and
  • a feeling of being unwell.

The symptoms of altitude sickness are usually worse at night. If you have mild altitude sickness, you can continue with normal activities. For example, if you are trekking or climbing, you can continue to climb at a steady pace.

Symptoms usually develop between 12 and 24 hours of being at altitude and begin to ease after 48 hours, as the body starts to acclimatise (gets used to the changes in atmospheric pressure).

Symptoms of moderate altitude sickness

The symptoms of moderate altitude sickness include:

  • severe headache,
  • nausea and vomiting,
  • increased shortness of breath, and
  • lack of co-ordination (ataxia).

You will find normal activities difficult and symptoms can only be relieved by taking specialist medications (see Treatment, above) and descending to a lower altitude.

A good way to test whether someone has moderate altitude sickness is to ask them to walk heel to toe in a straight line. If they have problems with their co-ordination and cannot walk in a straight line, they should descend to a lower altitude immediately.

Severe symptoms of altitude sickness

The symptoms of severe altitude sickness include:

  • worsening of the symptoms above,
  • persistent, irritable cough,
  • breathlessness (even when resting),
  • bubbling sound in the chest (caused by fluid in the lungs),
  • coughing up pink frothy liquid (sputum),
  • clumsiness and difficulty walking,
  • irrational behaviour,
  • double vision,
  • convulsions (fits),
  • drowsiness, and
  • confusion (caused by swelling of the brain, or fluid on the brain).

Severe altitude sickness is a medical emergency. Someone with severe altitude sickness should immediately descend by at least 600m (2,000 feet).


The brain controls thought, memory and emotion. It sends messages to the body controlling movement, speech and senses.
Drowsiness is when someone feels extremely tired and uncontrollably near to falling asleep.
Blood supplies oxygen to the body and removes carbon dioxide. It is pumped around the body by the heart.
Double vision
Double vision is when you see two images of a single object instead of one.
Loss of appetite
Loss of appetite is when you do not feel hungry or want to eat.
Inflammation is the body's response to infection, irritation or injury. It causes redness, swelling, pain and sometimes a feeling of heat in the affected area.
Lungs are a pair of organs in the chest that control breathing. They remove carbon dioxide from the blood and replace it with oxygen.

Page last reviewed: 13/07/2011

Altitude sickness is caused by going too high, too quickly. The body needs time to adapt to the low air pressure and the decrease in oxygen levels at high altitudes. This is known as acclimatisation.

It usually takes between one and three days for the body to get used to a change in altitude. If you do not spend enough time acclimatising to an altitude before ascending to a higher one, you are more likely to develop altitude sickness.

Altitude sickness is potentially very serious and can be life-threatening. It is your body's response to a reduction in the supply of oxygen to your muscles and brain.

The reduction in oxygen makes your heart and lungs work harder, causing your pulse and breathing rates to increase. In response, red blood cells are made in greater numbers in order to carry more oxygen around your body.

The body's efforts to cope with the decreased amount of oxygen causes changes to your:

  • blood acidity level,
  • lung pressure,
  • electrolytes (salts), and
  • fluid and salt balance.


Oxygen: Oxygen is an odourless, colourless gas that makes up about 20% of the air we breathe.

Brain: The brain controls thought, memory and emotion. It sends messages to the body controlling movement, speech and senses.

Heart: The heart is a muscular organ that pumps blood around the body.

Lungs: Lungs are a pair of organs in the chest that control breathing. They remove carbon dioxide from the blood and replace it with oxygen.

Red blood cells:Red blood cells transport oxygen around the body and remove carbon dioxide.

Page last reviewed: 13/07/2011

Changes in altitude have specific effects on the body, causing symptoms such as headache, nausea and shortness of breath. Usually, the first signs of altitude sickness are:

  • a headache,
  • light-headedness,
  • weakness,
  • difficulty sleeping, and
  • an upset stomach.

If you experience these symptoms, either stop ascending (going up) or descend (go down) to a lower altitude until your symptoms disappear completely.

The formal diagnosis of altitude sickness is based on a headache with one or more other symptoms, if there is no other likely cause and you have ascended to a height of at least 2,500m (8,000 feet).


Page last reviewed: 13/07/2011

As altitude sickness is potentially life-threatening, Do not ignore its signs and symptoms. In the past, people have died from altitude sickness by not recognising the signs or ignoring them.

If you have very mild altitude sickness, continue to ascend but make sure you do so gradually and at a steady rate. Tell the people you are travelling with how you feel, even if your symptoms are mild.

If your symptoms are slightly more severe, stay at your current altitude to let your body adjust.

  • Do not exercise.
  • Drink plenty of fluid (but not alcohol).
  • Do not smoke.
  • Rest until you feel better.

Descending to a lower altitude

If you have altitude sickness, the best treatment is to descend to a lower altitude.

Going down by 300m (1,000 feet) will improve the symptoms of moderate altitude sickness and spending 24 hours at this lower altitude will significantly improve symptoms.

Remain at this altitude until your symptoms have completely disappeared. After two to three days, your body will have become acclimatised and your symptoms should have disappeared. At this point, you can start going up again.

If you have severe altitude sickness, descend immediately by 600m (2,000 feet). You must descend as soon as possible because death can occur if altitude sickness is not treated quickly. If your symptoms do not significantly improve at the lower altitude, continue to descend until they do.

Pure (100%) oxygen can be given to improve severe breathing problems caused by altitude sickness.



Painkillers, such as paracetamol or ibuprofen, can be used to treat headaches that are caused by altitude sickness.


Acetazolamide (Diamox) can help reduce the severity of the symptoms of altitude sickness.

One of the effects of altitude sickness is that it changes the chemical balance of your blood. Acetazolamide corrects the chemical balance of your blood, and it speeds up your breathing rate.

Breathing more rapidly enables you to take on board more oxygen, which helps to minimise symptoms such as headache, nausea, and dizziness. This can be particularly helpful at night.

As acetazolamide is an unlicensed, prescription-only medication, you should consult your GP before taking it. There are a number of minor side effects associated with acetazolamide including:

  • paraesthesia  (numbness or a tingling sensation of the face, fingers or toes),
  • polyuria (excessive urination), and
  • in rare cases, blurred vision.


Dexamethasone can be very useful for treating severe cases of altitude sickness, such as high altitude cerebral oedema (HACO) (see Complications, above).

Dexamethasone is a strong steroid which decreases inflammation (swelling) of the brain. It is usually taken in 4mg doses three times a day. Symptoms usually improve within about six hours.

Dexamethasone is particularly useful for 'buying time' until it is safe to make a descent. For example, it can be used during the night to relieve symptoms, when making a descent is not possible or may be dangerous.

Dexamethasone is effective in treating people with altitude sickness. If you are going on a climbing or trekking expedition, it is a good idea to take it with you for use in emergencies.

However, dexamethasone can cause side effects, such as:

  • stomach upset,
  • euphoria (a state of excitement or elation), or
  • depression.

As with acetazolamide, you should consult your GP before taking dexamethasone.


Nifedipine is often used to treat high blood pressure (hypertension). It can be useful in treating cases of altitude sickness, particularly high altitude pulmonary oedema (HAPO) (see Complications, above).

Nifedipine decreases the narrowing of the pulmonary artery caused by HAPO, helping to reduce chest tightness and ease breathing.

The recommended dosage of nifedipine is 20mg taken at six to eight-hour intervals. Nifedipine can cause a sudden drop in blood pressure, so if you take it, do not get up too quickly from a lying or sitting position.

Portable hyperbaric chambers

Portable hyperbaric chambers (Gamow or Certec bags) are often used to treat altitude sickness. You are placed inside the chamber and it is pumped full of air, increasing the amount of oxygen inside. The effect of the treatment is equivalent to descending by 2,000m (6,500 feet). Treatment should be continued for at least two hours after which time your symptoms should have improved significantly, allowing you to descend more safely. 

Page last reviewed: 13/07/2011

Severe altitude sickness can lead to two very serious conditions that are known as:

  • high altitude cerebral oedema (HACO), and
  • high altitude pulmonary oedema (HAPO).

HACO and HAPO are rare conditions, but they can occur if a person ascends to a very high altitude too quickly and stays there.

Note: High altitude cerebral oedema (HACO) is sometimes referred to as high altitude cerebral edema (HACE), and high altitude pulmonary oedema (HAPO) as high altitude pulmonary edema (HAPE). HACE and HAPE are alternative, American spellings. This article uses HACO and HAPO.

High altitude cerebral oedema

High altitude cerebral oedema (HACO) occurs when a lack of oxygen causes fluid to leak through the capillary walls and into the brain, causing the brain tissue to swell. Capillaries are tiny blood vessels that surround major organs, such as the brain, heart and lungs.

The symptoms of HACO include:

  • headache,
  • weakness,
  • disorientation,
  • loss of co-ordination,
  • memory loss,
  • hallucinations and psychotic behaviour (an inability to distinguish between reality and imagination),
  • loss of consciousness, and
  • coma.

HACO can occur if a person stays at a very high altitude for a week or longer. If it is not treated immediately, it is likely that HACO will lead to death. To prevent this, an immediate descent of at least 600m (2,000 feet) is necessary.

The steroid medication dexamethasone can be used to treat HACO. However, someone with the condition should be transferred to hospital as soon as possible to receive follow-up treatment.

High altitude pulmonary oedema

High altitude pulmonary oedema (HAPO) occurs when fluid builds up in the lungs. The fluid prevents oxygen from being transferred from the lungs into the bloodstream. As the condition gets worse, the amount of oxygen in the blood decreases, which causes the following symptoms:

  • cyanosis (a blue tinge to the skin),
  • severe breathing difficulties (even when resting),
  • tightness in the chest (caused by the pulmonary artery narrowing),
  • a persistent cough, bringing up white frothy liquid (sputum),
  • extreme tiredness and weakness,
  • confusion and disorientation,
  • irrational behaviour, and
  • eventually death.

The symptoms of confusion and irrational behaviour are due to a lack of oxygen to the brain. To prevent death, someone with HAPO should descend immediately by at least 600m (2,000 feet).

Nifedipine is a medication that is sometimes used to treat HACO. However, as with HACO, someone with the symptoms of HAPO should be transferred to hospital as soon as possible for follow-up treatment.

The HAPE database

If you or someone you know has had HACO or HAPO, you can register with the International HAPE Database The database has been set up to assist with research into these two very serious health conditions.

Page last reviewed: 13/07/2011

Before you set off on a climbing or trekking expedition, be aware of the risks, symptoms and treatments of altitude sickness.

If you are travelling to a remote part of the world, it may be difficult to get medical care of the same standard as in Ireland


Proper acclimatisation is the best way to prevent altitude sickness. Ascending slowly will give your body time to adapt to the change in altitude.

Before booking your trip, make sure that there is enough time in the itinerary for acclimatisation so you do not put yourself at unnecessary risk of altitude sickness.

Sleep at a lower altitude

Once you are above 3,000m (10,000 feet), do not increase the altitude at which you sleep by more than 300m a night. You can go up higher during the day, but each night go back down to a camp that is no more than 300m higher than the previous night's camp.

If this is not possible, some travel companies include a day's rest in the itinerary, with a walk down the mountain to help you acclimatise.

Some holiday companies offer to let you climb a mountain in a short time, such as climbing Mont Blanc in a week. Climbing at this rate can lead to altitude sickness. It would be better to do it at the end of a two-week holiday after you have acclimatised by climbing a few lower peaks first.

What to pack

  • Sunglasses.
  • Sun protection cream.
  • Lip balm.
  • Water purification system with iodine tablets.
  • Method of communication (mobile phones now work in some high-altitudes areas, including Kilimanjaro, but for other mountains, you may need a satellite phone).
  • Plastic whistle (six blasts and a pause is the internationally recognised mountaineers’ distress signal).


As well as being used to treat altitude sickness, acetazolamide (Diamox) and dexamethasone can also be used to prevent the onset of symptoms.


For preventing altitude sickness, the recommended dose of acetazolamide is 125-250mg twice a day. Begin taking the medication one to two days before you start to ascend. Continue taking it while you are ascending and for three days after you have reached your highest altitude.

See Treatment, above, for more information about acetazolamide.


The recommended dose of dexamethasone is 4mg twice a day. It is usually taken for about two days, starting when you begin your ascent, to prevent the symptoms of altitude sickness developing.

See Treatment, above, for more information about dexamethasone.

Other preventative advice

As well as acclimatising properly and taking prescription medication, follow the advice below:

  • If you start to develop the symptoms of moderate altitude sickness (see Symptoms, above), stay at your current altitude until your symptoms improve.
  • If your symptoms get worse, immediately descend from your current altitude.
  • As different people acclimatise at different rates, make sure that everyone you are travelling with has fully acclimatised before you go any higher.
  • Keep well hydrated by drinking plenty of fluids (a minimum of four to six litres a day).
  • While at altitude, eat a high-calorie diet.
  • Do not smoke, drink alcohol or use medication such as tranquillisers and sleeping pills while you are at altitude. They could make any symptoms of altitude sickness worse. Speak to your GP if you are unsure.



Vomiting is when you bring up the contents of your stomach through your mouth.


Dose is a measured quantity of a medicine to be taken at any one time.


Numbness is a lack of sensation in a part of the body.


Inflammation is the body's response to infection, irritation or injury. It causes redness, swelling, pain and sometimes a feeling of heat in the affected area.


The brain controls thought, memory and emotion. It sends messages to the body controlling movement, speech and senses.


The sac-like organ of the digestive system. It helps digest food by churning it and mixing it with acids to break it down into smaller pieces.


Depression is when you have feelings of extreme sadness, despair or inadequacy that last for a long time.

Content provided by NHS Choices and adapted for Ireland by the Health A-Z.

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