Altitude sickness

What causes it?

The body's muscles and organs need an adequate supply of oxygen to function properly. As altitude increases, the percentage of oxygen in the air remains constant but the pressure decreases and the air becomes thinner, meaning we breathe in fewer oxygen molecules with each breath.

This leaves the body short of its oxygen requirements. Low air pressure also causes fluid to leak out of the capillaries and into the tissues of the lungs, making them congested, and in the brain, causing it to swell. Together these effects cause altitude sickness

Who's affected?

It's well known that mountaineers may be affected by altitude sickness, but anyone at high altitudes can experience symptoms. This includes people who fly to high-altitude destinations and those who go on walking and trekking trips. Some people are more susceptible than others, for reasons that are not yet fully understood.

What are the symptoms?

How severely someone is affected by altitude sickness depends on how high they go and how quickly they ascend, as well as their individual susceptibility. Severe altitude sickness usually only occurs when someone climbs too high too fast, often in what otherwise seem to be very fit individuals (as less fit people would take it more slowly). It's unusual for altitude sickness to occur below 2,400m (8,000ft) but many ski peaks, for example, are above this level.

Symptoms of mild altitude sickness may include:

• Tiredness
• Headache
• Nausea
• Unsteadiness
• Mild shortness of breath

For most people, symptoms start after about six hours of being at high altitude. As long as the person remains at the same altitude, the symptoms will usually disappear within one or two days.

Vomiting, chest pains and shortness of breath are signs that someone is affected more severely. These symptoms may take a day or two to appear.

In very severe cases of altitude sickness, the person may cough up frothy sputum. This is a sign of a condition called high altitude pulmonary oedema (HAPO) where fluid collects in the lungs, interfering further with the supply of oxygen to the body. Meanwhile clumsiness and difficulty walking can occur if the brain swells in a condition called high altitude cerebral oedema (HACO).

If such severe cases of altitude sickness aren't rapidly treated, fits, confusion, irrational behaviour and coma may follow. Severe altitude sickness can be fatal.

What's the treatment?

It's important not to ignore altitude sickness. If symptoms are mild, rest, fluids, a light diet and painkillers will enable the body to acclimatise. No further ascent should be attempted until all the symptoms have disappeared.

It usually takes one to three days for the body to get used to any given altitude. During this time, breathing becomes deeper, the number of red blood cells in the blood available to carry oxygen increases, the lungs become better perfused with blood, and oxygen is released more efficiently in the tissues.

When symptoms are more severe, it is essential to descend as soon as possible to a lower altitude. If this fails to resolve symptoms, hospital treatment is needed. Any swelling of the brain will be treated with oxygen, rest and drugs.

Most people who are treated correctly for altitude sickness make a full recovery, usually within a few days. However, when the condition is more severe, treatment over a longer period may be necessary.

Can it be prevented?

Problems with altitude sickness can usually be avoided if care is taken to prepare properly. This mostly means doing things to help the body to adapt to the lower oxygen level; a process called acclimatisation. Climbers, in particular, are all too aware of the importance of:

• Good physical fitness
• Staged ascents that allow time to acclimatise
• Drinking plenty of liquid – staying hydrated at altitude is very important
• Being aware of the possibility and the symptoms of altitude sickness
• Not ignoring symptoms of altitude sickness if they occur
• Avoid tobacco, alcohol and other depressant drugs which reduce the respiratory drive