Altitude sickness is a potentially life-threatening condition that can happen when you're high up on a mountain. It's caused by climbing up too quickly.

The decrease in atmospheric pressure makes breathing difficult, because you are not able to take in as much oxygen. You may also experience:

• headache
• nausea
• dizziness
• exhaustion

In severe cases, altitude sickness can cause fluid to build up either on the lungs or on the brain. These are very serious conditions and require immediate medical attention.

See Altitude sickness - symptoms and complications for more information.

When you may be at risk

Altitude sickness is fairly common among those who spend time at high altitudes, such as mountaineers and skiers.

In its mildest form, altitude sickness can occur at heights over 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 tend to occur at altitudes of 3,600m (about 12,000 feet) and above.

It is not possible to get altitude sickness in the UK because the highest mountain, Ben Nevis in Scotland, is only 1,344m (4,406 feet) high.

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.

Preventing altitude sickness

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

For example, once you are above 3,000m (10,000 feet), try not to increase the altitude at which you sleep by more than 300m a night.

See Altitude sickness - prevention for more advice.

What to do if you get ill

If you have mild symptoms of altitude sickness, continue to ascend but do so gradually and at a steady rate.

If your symptoms are more severe, stay at your current altitude to let your body adjust. If your symptoms do not get better (or they get worse), descend to a lower altitude. See Altitude sickness - prevention for more advice.

While one person may be severely unwell at a high altitude, another person at the same altitude may not be affected at all.

This is because the experience of altitude sickness and the altitude at which it occurs depends on:

• how sensitive the individual is to changes in atmospheric pressure
• how quickly they reached the high altitude
• the amount of time they spend 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
• generally feeling unwell

The symptoms 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 (get 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
• lack of co-ordination

You will find normal activities difficult and symptoms can only be relieved by taking specialist medications and descending to a lower altitude. See treatment of altitude sickness for more information.

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 co-ordination problems and cannot walk in a straight line, they should descend to a lower altitude immediately.

Symptoms of severe altitude sickness

The symptoms of severe altitude sickness include:

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

Altitude sickness results from climbing to a high altitude too quickly.

Your body needs time to adapt to the low air pressure and decrease in the amount of oxygen you're able to breathe in. This process 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 a given altitude before ascending to a higher one, you are at risk of developing the more severe symptoms of 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. It 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, and fluid and salt balance.

If you have very mild altitude sickness, continue to climb 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. Also:

• 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 more severe symptoms of altitude sickness, or mild symptoms that do not go away over 24-48 hours, the best thing to do is 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 should 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 as much as possible and by at least 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.

Bottled oxygen can be given to improve severe breathing problems caused by altitude sickness.

Medication

Painkillers

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

Acetazolamide

Acetazolamide (Diamox) can help reduce the severity of the symptoms, and can also be used to help prevent altitude sickness occurring.

One of the effects of altitude sickness is that it changes the chemical balance of your blood. Acetazolamide helps correct this chemical imbalance.

Breathing more rapidly also 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:

• numbness or tingling of the face, fingers or toes
• excessive urination
• blurred vision (although this is rare)

Dexamethasone

Dexamethasone can be very useful for treating severe complications of altitude sickness, such as high altitude cerebral oedema.

Dexamethasone is a strong steroid that reduces swelling of the brain. It is usually taken in 4mg doses three times a day. Symptoms usually start to 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)
• depression

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

Nifedipine

Nifedipine is often used to treat high blood pressure. It can be useful in treating complications of altitude sickness, such as high altitude pulmonary oedema.

Nifedipine decreases the narrowing of the artery supplying blood to the lungs, 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.

Severe altitude sickness can lead to two very serious conditions: cerebral oedema and pulmonary oedema.

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

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 an inability to distinguish between reality and imagination
• loss of consciousness
• coma

HACO can occur if a person ascends rapidly to high altitude, or 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. Oxygen should be given if available. However, if possible, someone with the condition should be transferred to hospital as soon as possible to receive follow-up treatment.

In practice, this may involve transfer by emergency helicopter, although in many parts of the world no formal rescue services exist, and it should also be remembered that at extreme altitudes (for example on the upper slopes of 8,000m peaks), helicopters may not be able to operate.

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

Eventually, it will lead to 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 can be used to treat HAPO. 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.