Altitude Sickness

Altitude sickness is common. It is difficult to predict who will get it, but treatment is simple. The following brief guide provides some basic definition. However, we recommend anyone trekking at altitude downloads the Medex booklet, and read it before you set out.
(High Altitude Travel Booklet).

What Causes Altitude Sickness?

Altitude illness is occurs when the amount of oxygen in the air is reduces. This happens because at sea level the concentration of oxygen in the air is around 21% and the barometric pressure is approximately 760 mmHg. As altitude increases, the concentration of oxygen remains the same, but the oxygen molecules per breath are reduced. At about 3,500 meters the barometric pressure is 483 mmHg, so there are roughly 40% fewer oxygen molecules per breath ( To compensate for this the brain increases breathing, but this can only partially counter act the reduction in oxygen.

Different people adjust at different rates to the low oxygen that occurs at altitude. Doctors do not know who will adjust to altitude; there are no specific markers e.g. fitness, age, gender. However, we do know that biggest cause of altitude sickness is going to high too quickly. If the ascent is gradual most people can acclimatize. The general rule is that it takes 2-3 days at each altitude over 2,500 meters to acclimatize. Above 3,000 meters, only increase your altitude by 300 meters per day, and take a rest day for every 900 meters.

Acclimatisation usually starts with the brain making the breathing faster (event at rest). The body then produces more red blood cells to carry oxygen. This followed by the body adapting to become more efficient at carrying the oxygen to the body tissues. Drinking plenty is important because acclimatization associated with dehydration.

Some people take Diamox (Acetazolamide) to aid acclimatisation. This drug causes the body to breathe faster so more oxygen is taken into the lungs and the blood. It is suggested that Diamox is taken 24 hours before ascending to altitude. The Himalayan Rescue Association Medical Clinic recommendation that 125 mg is taken twice a day ( Some people find it causes tingling of the lips and fingers and blurred vision. Check with Dr before taking it as some people can have a reaction. There are other drugs that can help acclimatization e.g. Dexamethasone but these should be only on the advice of a physician.

What is Acute Mountain Sickness (AMS)?

AMS is common, e.g. over 3,000 meters 75% of people will have mild symptoms including headache, dizziness, fatigue, shortness of breath, loss of appetite, nausea, disturbed sleep ( These symptoms usually resolve in 2-4 days. As AMS becomes more severe the headache worsens and is not relived by painkillers. Nausea and vomiting occur and the tiredness is more profound. The only cure for AMS at this stage is to descent. The person should remain low until they have become acclimatized (usually 2-3 days). There are two from of AMS that are life threatening: High Altitude Cerebral Odema (HACE) and High Altitude Pulmonary Odema (HAPE). HACE is caused by fluid in the brain and HAPE by fluid in the lung that prevents oxygen getting into the blood.

Symptoms of HAPE are breathlessness, coughing frothy white, or blood stained fluid, confusion, fatigue and difficult behaviour. HACE causes headache, lack of coordination, confusion/disorientation weakness, loss of memory and difficult behaviour. In both situations immediate decent is required.