Because of Tibet's high altitude, travelers with a pre-existing problem of heart, lungs or anaemia should consult a doctor before considering a visit. Most other travelers, once they are acclimatized , rarely suffer more than mild discomfort form the altitude.

Acclimatization is the adjustment of the human body to the diminished supply to oxygen at high altitudes. Bone marrow produces quantities of extra red blood cells to take oxygen from the air in amounts needed for good health, a process that may take insufficient flow of oxygen to the brain and other vital organs. It can affect anybody above 3,000 meters( 1000 feet).

Each person has a different tolerance for altitude that has nothing to do with age, sex or fitness. One person will get a headache at 3,400 meters ( 11,000 feet), another not until 5,500 ( 18,000 feet). The symptoms of mountain sickness include headache, nausea and shortness of Breath, singly or together. About half the people arriving in Lhasa suffer at least one symptom in the first two days before recovering.

Usually rest and two aspirins will relieve the discomfort. However, the serious- sometimes fatal- conditions of pulmonary and cerebral oedema also begin with these same symptoms. If a headache does not respond to aspirin or a good night's rest, if a dry cough with frothy sputum develops, or if there are any signs of sever lethargy or poor coordination, go the hospital at once. Better yet, take the next plane to Chengdu. A lower altitude is the surest cure.

Over-exertion contributes to mountain sickness, and dehydration may be a predisposing factor. Some precautions include:
 
   - Stick to a schedule of very mild activity and test for the first two days.
   - Drink plenty of fluids. Four litres ( seven pints) every day are recommended to maintain a clear, copious urine.
   - Don't smoke, or at least keep it to a minimum.
   - Avoid sedatives such as sleeping medicine or tranquilizers. They tend to depress respiration and limit oxygen intake.
   - Diamox ( acetazolamide), a mild prescription diuretic that stimulates oxygen intake, is used by doctors of the Himalayan Rescue Association in Kathmandu for climbers making sudden ascents. One 250 milligram tablet taken on the plane from Chengdu and another at bedtime the first night in Lhasa may help to forestall discomfort for people known to be susceptible to mountain sickness. Consult a doctor. Diamox can cause unpleasant side-effects in some people.

It is not unusual to wake up at night at high altitudes gasping for breath. This complaint, known as' periodic breathing' and caused by a change in the brain's control of breathing while you sleep, is normally quite harmless. Normal breathing can be quickly restored by relaxation, rhythmic deep breathing, and understanding that there is nothing to worry about.