Altitude sickness affects most people to some extent when they visit elevations over 8,000 feet, and for some the effects may be severe. Indeed, altitude sickness has been fatal as low as 9,000 feet, but for most only mild symptoms develop at 10,000 to 14,000 feet. Although dehydration is often to blame for many headaches experienced at altitude, the cause of mountain sickness is lack of oxygen. For example, each breath at 12,000 feet elevation provides the body with only 60% of the oxygen it normally receives at sea level, and for climbers reaching the top of Mt. Everest, at more than 29,000 feet above sea level, each breath provides only 25% of the oxygen.

Because less oxygen reaches the muscles and brain at altitude, the heart and lungs compensate by working harder. Also, for reasons not entirely understood, the lower air pressure at altitude allows fluid to leak from capillaries and collect in the brain and lungs. The result may be swelling of the brain, known as cerebral edema, or swelling of the heart, known as pulmonary edema. Both can be fatal, particularly at elevations over 14,000 feet. Common altitude sickness is basically a mild form of cerebral edema that results in a headache.

Altitude sickness usually develops during the first 24 hours at altitude, but may be delayed for up to three weeks. Mild symptoms include headache, laziness, dizziness, difficulty sleeping, and loss of appetite. Symptoms can become more severe without warning and may even be fatal. Severe symptoms include difficulty breathing, a dry, irritating cough (which may progress to the production of pink, frothy sputum), severe headache, loss of coordination and balance, confusion, irrational behaviour, vomiting, drowsiness, and unconsciousness.

The cure for altitude sickness is either acclimitization, or descent to lower elevations. Mild symptoms may be treated by resting at the same altitude until recovery, usually a day or two. Ibuprofin (advil) can be taken for headaches. Severe symptoms require immediate descent. Descending even as little as 1500 feet can help.

There are several things you can do to prevent altitude sickness.

• Ascend slowly with frequent rest days, spending two to three nights for each rise of 3,000 feet elevation. When you reach high elevaations by walking, acclimatization takes place gradually, and you are less likely to be affected than if you fly to high altitude.
• Drink lots of water. If the truth were known, most altitude headaches at elevations between 8,000 and 14,000 feet can be prevented by simply drinking extra water and taking care not to become dehydrated. Mountain air is dry and cold, and moisture is easily lost during normal breathing. Also, evaporation of sweat can go unnoticed and remove even more water from the body. There is no such thing as drinking too much water, so do yourself a favor and force down as much as you can.
• Sleep at lower altitude than the highest elevation reached during the day. Although it is often not practical, take care once you get above 9,000 feet not to increase the sleeping altitude by more than 1,000 feet each day.
• Eat easy to digest, high-energy foods to get more energy.
• Do not drink alcohol, as it increases the risk of dehydration.
• Do not take sleeping pills to sleep at night.