Altitude sickness is a condition resulting from a lack of oxygen, caused by low oxygen pressure at high altitudes. Well over half of those who travel from sea level to an altitude of 3,500m (11,500 feet) will experience symptoms of altitude sickness or acute mountain sickness.

Almost all of those who ascend rapidly to 5,000m (16,500 feet) or higher will have some symptoms. The key word is 'rapidly'. Slow, graded ascent will reduce the chance of suffering.

The symptoms of altitude sickness tend to develop over two or three days after exposure to altitude and include:

• Loss of appetite.
• Headache.
• Tiredness.
• Nausea.
• Vomiting.
• Dizziness.
• Sleep disturbances.
• A feeling that the heart is beating harder and faster.

If remedial action is not taken, the condition may progress to one of two life-threatening variants:

• Fluid can accumulate in the lungs causing breathlessness at rest. This is known as high-altitude pulmonary oedema. The incidence ranges from 0.0001% at 8,800 feet to more than 2% above 13,100 feet.
• Fluid can also accumulate in the brain and this is known as high-altitude cerebral oedema. It is characterised by confusion, drowsiness and coma.

Both these states may produce a sensation of extreme faintness accompanied by difficulty in breathing, dizziness, headaches and vomiting. They are classed as medical emergencies.

Although the proportion of oxygen in the air at higher altitudes remains the same, the pressure is lower and the amount of oxygen available to the body falls. To compensate for this, a climber will tend to breathe harder, but this in turn drives off carbon dioxide and the blood becomes more alkaline, which acts as a brake on the ability to increase breathing.

Over a period of days, the body will start to compensate for this by increasing the amount of blood available - this is when you become 'acclimatised'.

The best way to prevent altitude sickness is to ascend slowly. Above 3,000m (10,000 feet) it is advisable to sleep at an altitude no greater than 300m (1,000 feet) above the previous day's altitude. Ideally, you should descend to sleep: 'climb high, sleep low'. It is also sensible to have a rest day every three days (or every 3,000 vertical feet).

If it is your first trek, try to avoid choosing a route that goes as high as 13,000 feet. The prevalence of altitude sickness in the Swiss Alps ranges from 9% at 10,000 feet to 53% at 15,000 feet; in the Mount Everest region of Nepal, 50% of trekkers reaching 13,000 feet over five or more days develop altitude sickness.

Your doctor may prescribe a drug called acetazolamide (Diamox), which can help prevent altitude sickness. It is usually given at a dose of 250mg twice daily, starting three days before ascent to 3,500m (11,500 feet) and continuing for a further two days at altitude.

Common side-effects include nausea and tingling in the hands and feet, so the drug is normally reserved for those who are particularly prone to altitude sickness or who are planning a more rapid ascent than is generally advisable.

If you do get symptoms of altitude sickness it is important that you go no higher until they have resolved. If symptoms get worse you should descend immediately. Anyone who suffers with undue breathlessness at rest when at altitude or appears inappropriately drowsy or confused must be taken down.

Oxygen and portable hyperbaric chambers (pressure bags that provide rapid pressurisation) can often produce dramatic improvement and buy enough time to save a life.

Gradual adjustment by stages and treatment with diuretics (drugs to make you pass urine, thereby reducing the amount of fluid in the body) may sometimes be beneficial. Recovery follows rapidly on return to a lower altitude.