The Facts

The higher you go above sea level, the less oxygen there is to breathe. When you reach elevations above 2,000 metres (6,500 feet), your body doesn't always adjust quickly enough to the decrease in oxygen. This can lead to altitude sickness, which is actually a group of potentially life-threatening ailments. Acute mountain sickness (AMS) is the most common type. Other forms of altitude sickness attack the lungs and brain.

Anyone can develop altitude sickness. Not everyone gets it; the effects of altitude vary from one person to the next. Some adjust to the decrease in oxygen more easily than others. But the higher and faster a person climbs, the more likely altitude sickness will develop. About 20% of people who ascend above 2,500 metres (8,000 feet) in a day and about 40% of people who ascend above 10,000 feet (3,000 m) in a day will experience symptoms. Children are the most susceptible. Surprisingly, the risk is twice as high for people under the age of 60 years.


All forms of altitude sickness are caused by low levels of oxygen at very high altitudes. These lower levels result in hypoxia, a shortage of oxygen in the body's tissues. The effects of hypoxia may be mild or even unnoticeable. Altitude sickness is most likely to occur with a rapid increase in elevation, as well as by the cold experienced at high altitudes.

People can adjust to the effects of hypoxia at high altitudes, but only up to a point. At elevations up to 3,000 metres (10,000 feet), most people have no problems after a few days. But no one can survive permanently above 5,100 metres (17,000 feet). At the elevations reached by mountain climbers, bottled oxygen often becomes necessary.

Risk factors for altitude sickness include:

  • abuse of narcotics
  • some chronic conditions (e.g., diabetes, heart disease, COPD, being overweight)
  • fatigue or overwork
  • heavy drinking
  • previous experiences of altitude sickness
  • rapidly increasing altitude too quickly
  • living near sea level
  • sleeping at too high of an altitude

Children are especially vulnerable, and can experience high altitude pulmonary edema (HAPE) and high altitude cerebral edema (HACE) below 2,500 metres (8,000 feet). Limited evidence seems to suggest men may be affected by HAPE more than women , but acute mountain sickness (AMS) and HACE affect men and women equally. If someone has had altitude sickness before, they have a 3 times greater risk. If someone usually lives below 3,000 metres, they are at more than 3 times the risk of those who reside at higher altitudes. However, if someone lives at a high altitude, an increased risk can develop even after a brief stay at lower elevations. Being in better physical condition does not protect you against altitude sickness.

Symptoms and Complications

The most common form of altitude sickness is acute mountain sickness (AMS), which usually begins at elevations as low as 2,500 metres (8,000 feet). If AMS develops at any elevation, it causes tiredness and nausea. It can also cause swelling of the face and hands, difficulty sleeping, dizziness, and severe headaches. Symptoms will generally develop 6 to 10 hours after ascending to a high altitude. Exercise tends to make symptoms worse; however, some experts believe moderate exercise on the day of arrival will help the body acclimatize more quickly. AMS is rarely fatal. If someone with AMS remains at a high altitude, it will often disappear after a few days when the body adjusts to the higher elevation.

However, AMS can be a warning sign of high altitude cerebral edema (HACE). HACE causes swelling in the brain that can result in coma and death within hours. Early signs of HACE include severe headache, confusion, drowsiness, and hallucinations, as well as staggering or stumbling. A person with HACE must descend to a lower altitude immediately to prevent serious consequences.

High-altitude pulmonary edema (HAPE) develops 1 to 3 days after a rapid ascent above 2,500 metres (8,000 feet). When most people climb this high, fluid begins to accumulate in their lungs. With HAPE, the fluid builds up more quickly than it can drain. People become so breathless they remain unable to breathe normally even after long rests. Their skin turns blue, and they may cough up frothy, pink sputum and eventually fall into a coma. If not treated immediately and moved to a lower elevation, people with HAPE can die from the illness. Both HACE and HAPE can develop without AMS occurring first.

Altitude sickness can be complicated by any chronic illness. It can also be complicated by other high-altitude health problems such as frostbite, blood clots in the legs and lungs, dehydration, and swollen feet and ankles.

Making the Diagnosis

Symptoms can only be experienced at high altitudes. Because the symptoms can be caused by flu or hypothermia, diagnosis is made after ruling out other possible illnesses. HAPE can be confirmed with a chest X-ray.

Treatment and Prevention

AMS symptoms usually disappear after 2 days of rest at the same altitude. A medication called acetazolamide* can be taken to help someone adjust. This medication can prevent or lessen symptoms. In severe cases, another medication called dexamethasone can be taken with acetazolamide to further lessen symptoms. Diabetics should take extra caution when using dexamethasone, as it can cause blood sugar levels to rise. Once symptoms subside, someone with AMS should be able to ascend to higher altitudes. If symptoms don't go away, descending 300 metres (1,000 feet) usually helps. Painkillers (e.g., acetaminophen, ibuprofen) are effective for the headache. Slowing down breathing may help relieve headache and dizziness.

Therapy for HAPE or HACE always requires immediate but slow descent to a lower altitude. Treatment consists mainly of bed rest and doses of pure oxygen. Other medications for HACE (e.g., dexamethasone to reduce swelling) and HAPE may be used. When promptly treated, people usually recover from HAPE within 2 days.

The best prevention is to always allow the body to acclimatize to altitudes higher than those the body is used to:

  • Start slowly – begin at an altitude of less than 2,500 metres (8,000 feet).
  • Allow time to adjust – rest a day after arriving.
  • Take it easy – high altitudes make normal activities more difficult. Slow down if you are out of breath or tired.
  • Limit ascent – don't climb more than 900 metres (3,000 feet) per day. If at 3,000 metres (10,000 feet) or above, don't climb more than 500 metres (1,600 feet) per day.
  • Sleep at a lower altitude. At high altitudes the blood oxygen level may fall even lower when sleeping. If above 3,400 metres (11,000 feet) during the day, spend nights at 2,700 metres (9,000 feet) or lower. "Climb high... sleep low."
  • Every third day, spend at least 2 nights at the same elevation.
  • It's also important to keep well hydrated. Drink plenty of fluids, and avoid alcohol, cigarettes and medications which can aggravate symptoms. Do not take sleeping pills as they may worsen the condition by slowing down your breathing thereby causing more hypoxia.

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