Because the air is thinner at high altitude there is less oxygen available so breathing gets deeper and quicker to compensate. This ‘acclimatisation’ helps you cope with the altitude better. Being more short of breath for the same exercise as at sea level is normal.
Other changes occur in the blood, which you will be less aware of, allowing the blood to carry more oxygen to where it is needed.
People often develop a dry cough at altitude. It is not entirely clear why this happens, but whilst irritating, it is not usually serious.
Occasionally more serious problems can occur with breathing.
Fluid may collect in the lungs causing a problem known as ‘High Altitude Pulmonary Oedema’ (HAPE). Symptoms include severe breathlessness at rest and frothy bloodstained spit may be coughed up. People who have had HAPE are likely to get it again, often at the same altitude. This is a serious (potentially life-threatening) condition and should not be ignored.
Walk slowly.
Take plenty of rests.
It's not a competition! Some people adapt better than others.
Do not ignore signs of HAPE. Seek medical help if possible and if in doubt DESCEND!
Exercise regularly, preferably the type planned at altitude; don't be short of breath due to unfitness!
An experienced trekker on a medical research expedition arrived at 5,200m to find her blood oxygen had dropped below what would normally keep her alive. Her lungs were filling with water, she became dizzy and had difficulty breathing at night. Descent was impossible without crossing a high pass. She was given acetazolamide tablets 500mg followed by 250mg three times a day. After 24 hours and a lot of peeing, her blood oxygen had risen to the normal level.