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Traveler Summary


Altitude illness occurs when a person ascends more rapidly than the body can adjust ("acclimatize") to the decreased oxygen at a higher altitude. Some people adjust very easily, while others cannot go above even moderate heights without experiencing symptoms. There is no way to know ahead of time whether one will be a good acclimatizer, except based on past experience. If symptoms occurred before, it is likely they may occur again at the same altitude. The ability to acclimatize may be genetic.

Altitude illness falls into 3 categories: acute mountain sickness (AMS), high altitude pulmonary edema (HAPE), and high altitude cerebral edema (HACE).

Symptoms can range from mild to life-threatening, but most symptoms can be prevented or minimized by proper acclimatization and/or preventive medications. Prevention strategies will vary depending on the type of travel planned: travel to typical tourist destinations at relatively moderate heights or trekking in extreme high altitude situations. See Tables 1 and 2 below for information and advice on common tourist destinations and popular trekking destinations.



AMS symptoms include headache (which can be mild to extremely painful), loss of appetite (which can progress to nausea and vomiting), and extreme fatigue.


HACE begins like AMS but the symptoms become more severe, including changes in consciousness and loss of coordination; HACE can progress rapidly to coma and death. HACE can occur alone or in combination with HAPE.


HAPE symptoms include unusual breathlessness upon exertion and, eventually, even while at rest. Cough is usually present (although cough at high altitude is common from other causes). The trekker may feel fullness in the chest. Descent is mandatory as soon as HAPE is suspected, because the symptoms can progress rapidly and death can occur within hours. Unfortunately, exertion considerably worsens HAPE, so exertion by the sick person should be minimized during descent, if at all possible.

Other Conditions

  • Periodic breathing can occur in persons sleeping above 3,000 m (9.800 ft). While sleeping, increasingly deep breaths are followed by a brief (5-30 seconds) period of not breathing. If periodic breathing at altitude is disturbing to the trekker, acetazolamide 125 mg taken before bed can relieve the problem.
  • Peripheral edema (swelling of the face, hands, and feet) can occur. Although harmless by itself, edema indicates poor acclimatization that can lead to other symptoms of altitude illness. As persons with peripheral edema begin to acclimatize, they often experience a profound diuresis (increased urine flow) and relief of symptoms. Trekkers should not ascend if other symptoms develop.
  • High altitude retinopathy (retinal bleeding) is a rare development at high altitude.


Personal Risk Factors

Persons are at low risk if they:

  • Have no prior history of altitude illness and they are ascending to less than 2,800 m (9,200 ft)
  • Allow 2 or more days to arrive at 2,500-3,000 m (8,000-9,800 ft), and keep subsequent increases in elevation to sleep less than 500 m (1,600 ft) per day

Persons are at moderate risk if they:

  • Have a prior history of AMS and are ascending to 2,500-3,000 m (8,000-9,800 ft) in 1 day
  • Have no prior history of AMS and are ascending to more than 2,800 m (9,200 ft) in 1 day
  • Are ascending more than 500 m (1,600 ft) per day (increase in sleeping elevation) at altitudes greater than 3,000 m (9,800 ft)

Persons are at high risk if they:

  • Have a history of AMS and are ascending to 2,800 m (9,200 ft) or higher in 1 day
  • Have a prior history of HAPE or HACE
  • Are ascending higher than 3,500 m (11,400 ft) in 1 day
  • Are ascending more than 500 m (1,600 ft) per day (increase in sleeping elevation) above 3,500 m (11,500 ft)
  • Make very rapid ascents (e.g., less than 7-day ascent of Mount Kilimanjaro)

Risk by Type of Travel

Typical Tourist Destinations

At typical tourist destinations (e.g., at altitudes of around 3,000 m or 9,800 ft), mild symptoms can occur but the severe forms of AMS (HAPE or HACE) rarely occur.

  • Most mountain resorts are located at these lower altitudes (ranging from, 1,200 to 3,000 m; 3,900 to 9,800 ft).
  • Travelers may go higher during daytime activities (e.g., skiing, hiking, sightseeing) but risk is lessened by descending again, to sleep at the resort altitude.

There is more risk for the traveler who hikes vigorously or flies directly to higher destinations, because these modes do not allow for gradual acclimatization.

  • Examples of destinations that allow access, without hiking, to relatively high altitudes include: La Paz, Bolivia; Lhasa, Tibet; and Cuzco, Peru.
  • See Table 1, below.
High Altitude Trekking

Trekkers are at higher risk of HAPE and HACE.

  • Altitude illness affects 50% or more trekkers on popular high-altitude routes.
  • There are 2-3 trekker deaths a year from complications of altitude sickness in Nepal.
  • The risk of dying from altitude sickness in the Himalaya is also higher, even though the trek allows one to acclimatize gradually; this is because the trekker is at high altitude for longer periods of time.
  • Most trekking itineraries take a "one-size fits all" approach towards the pace of the trek, and thus cannot guarantee that altitude illness will not occur.
  • Trekking agencies feel pressure to offer shorter expeditions for busy people who cannot take long holidays. For example, Mount Kilimanjaro treks that summit in 5 days are offered when even a 7-day ascent offers altitude gains that are more rapid than typical Himalaya treks.
  • See Table 2, below.



Trekkers and tourists should:

  • Ascend gradually.
    • This allows time to acclimatize.
    • Do not ascend directly to altitudes higher than 3,000 m (9,800 ft), if possible.
    • If an abrupt ascent is unavoidable (e.g., flying directly to the destination), acetazolamide might be needed.
  • Return to a lower altitude to sleep.
    • If participating in activities at altitudes higher than 3,000 m (9,800 ft) during the day, return to a lower altitude to sleep. Many mountain resorts are purposely located at lower altitudes (1,200 to 3,000 m; 4,000-10,000 ft).
    • For example: Travelers typically stay in Mammoth Lakes, California (2,400 m; 7,900 ft) or nearby areas and ski at the higher altitudes of the mountain.
  • Ascend no more than 500 m per day (1,600 ft per day).
    • Once the traveler is at an altitude of 3,000 m (10,000 ft), he or she should ascend no higher the 500 m (1,600 ft) each day to sleep.
  • Avoid alcohol and only participate in mild exercise for the first 48 hours.
  • It is important that the traveler learn to recognize symptoms of altitude illness in the event they occur.
    • Deaths from altitude illness almost invariably result because symptoms were ignored or not recognized.
  • Never ascend to sleep at a higher altitude with any symptoms of altitude illness.

In addition to the above, trekkers should:

  • "Climb high, sleep low."
    • Climbers who reach higher altitudes during the day should return to the valleys to sleep.
  • If possible, have a high altitude exposure (higher than 3,000 m; 9,800 ft) for 2 or more nights sometime within 30 days before a trek.
  • Note that in organized trekking groups, there is a great deal of pressure to keep up with the group schedule. If symptoms occur, do not allow group pressure to decide what actions to take.


Acetazolamide (Diamox)
  • For prevention of AMS: Start taking the drug the day before ascending, take each day during ascent, and continue to take for 24-48 hours after arrival at highest altitude.
    • Adult dose: 125 mg every 12 hours
    • Pediatric dose: 2.5 mg/kg/dose every 12 hours
  • Precautions:
    • Persons with multiple drug allergies or a history of a life-threatening reaction to sulfa drugs should have acetazolamide administered in a controlled environment before the trip.
    • Persons with a history of mild sulfa reactions or rashes can take acetazolamide safely.
  • Side effects:
    • Almost always causes numbness of fingers and toes, and occasionally oral
    • Occasionally causes nausea
Dexamethasone (Decadron)
  • For prevention of AMS (in rare situations): 2-4 mg every 8-12 hours
  • Side effects: euphoria; can increase the need for insulin or oral agents in diabetics
Ibuprofen (Advil, Motrin)
  • For prevention of headache: 600 mg every 8 hours, starting a few hours before ascent.
  • Side effects: gastrointestinal irritation and bleeding.
Sildenafil (Viagra)
  • For prevention of HAPE in known susceptible individuals: 50 mg every 8 hours
  • Precautions: Do not take if hypertensive, hypotensive, or coronary artery disease is present.
  • Side effects: flushing, indigestion, headache, insomnia, visual disturbance
Tadalafil (Cialis)
  • For prevention of HAPE in known susceptible individuals: 10 mg every 12 hours
  • Precautions: Do not take if hypertensive or hypotensive, or of arrhythmia or coronary artery disease is present.
  • Side effects: flushing, indigestion, nausea, muscle aches, headache, respiratory tract infection


Non-Drug Treatment

Three options are available for treatment. Descent should always be the first option, whether a tourist or trekker. Oxygen is helpful for both categories of traveler. Pressurization bags are used only for trekkers.

  • Descent invariably improves altitude illness.
  • Descend if symptoms are getting worse while resting at the same altitude.
  • It is not necessary to descend until all symptoms are gone, because symptoms can take up to 48-72 hours to clear.
  • In severe cases, however, descent must continue until clear signs of improvement are recognized or until the person is below the altitude at which symptoms started.
  • Oxygen is available at many tourist locations, often from the front desk of the hotel, and is helpful is treating mild altitude illness.
  • Bottled oxygen is carried by many trekking expeditions. However, it is expensive and heavy to carry, and thus there is usually insufficient oxygen available.
Pressurization Bags
  • Groups on long treks or climbs to very high altitude where rapid descent might not be possible should consider carrying a pressurization bag which can effectively mimic descent. "Gamow" is one brand name.
  • A 1-hour treatment in a portable pressurization bag is usually enough to dramatically improve mild to moderate AMS. In more severe cases, several hours in the bag may be necessary.

Drug Treatment

Discuss with the health care provider what drugs might be used for treatment and when to use them. For side effects and precautions, see drugs listed under Prevention.

Acetazolamide (Diamox)

Diamox is used for treatment of AMS or periodic breathing and sleep apnea at altitude.

Dexamethasone (Decadron)

Dexamethasone is effective in treating mild to moderate AMS and in improving HACE.

Other Treatments

Ibuprofen and aspirin are effective in treating headache associated with high altitude.

Gingko biloba is not recommended as an effective therapy for altitude illness.

Effect of High Altitudes on Preexisting Medical Conditions

In general, the more severely limited one's exercise tolerance is at sea level, the worse he or she will do at altitude. In addition, many high altitude destinations are far from the nearest medical help. Some travelers with preexisting conditions should avoid trekking if medical care is not readily available.

Cardiovascular System

Travelers with sedentary lifestyles should avoid high altitude trekking and any sudden increase in activity at lower altitudes. A gradual training program should be undertaken in order to prepare for a trek.

Trekkers should be able to hike steadily for at least 4 hours over steep terrain.

Persons with angina should avoid trekking.

Persons with congestive heart failure should limit themselves to moderate activity and stay in areas with medical care readily available.

Travelers who are well controlled on anti-hypertensive medications and are going for a short tourist trip to moderate altitude do not need to adjust dosage. Expatriates and long-stay travelers at altitudes above 2,500 m (8,200 ft) may require dose adjustment after arrival. Persons with unstable blood pressure need close monitoring at altitude and access to a medical setting where drugs can be quickly adjusted or blood pressure rapidly stabilized.

Pulmonary System

Persons with chronic obstructive pulmonary disease (COPD) or primary pulmonary hypertension should avoid trekking.

Persons with asthma have generally done well at altitude, possibly due to the greatly decreased presence of allergens at high altitude. Persons with asthma should carry their medications with them at all times.

Neurologic System

Persons with uncontrolled or poorly controlled seizures should avoid high altitude.

Hematological System

Even moderate altitudes, such as those encountered in airplane travel, can trigger a sickle cell crisis in a person with sickle cell trait or sickle cell disease. Typical tourist altitudes will often cause crises, even without physical exertion.

Dark-skinned patients born outside the U.S., especially in developing countries, may never have been tested for sickle trait as children. High altitude is contraindicated in this population.

People with low red cell counts could experience difficulty adjusting to high altitude, because their oxygen-carrying capacity would already be low.

Patients with polycythemia could have a risk of blood clots.

Endocrine System

Persons with stable diabetes can travel safely to high altitude if they are comfortable with self-monitoring and pay very close attention to their glucose balance. Glucose meters may lose calibration at high altitude. In addition, it may be difficult to keep insulin supplies at close hand and unfrozen during a long, cold, backcountry journey.

High altitude can be associated with severe ketoacidosis and has led to deaths. Risk factors include illness such as gastroenteritis, respiratory infection, and altitude illness.

Other Considerations


Pregnant women should not take part in high altitude trekking because medical care (in the event of early labor or other complications) is not readily available.

Pregnant women should avoid altitudes above 3,000-3,600 m (9,800-11,800 ft), due to the possibility of lower oxygen levels.

Infants and Children

Children are at risk for AMS just as adults are, but symptoms may be more difficult to assess in young children, especially those who are not yet talking. For prevention, acetazolamide may be advised.

HAPE and HACE are not well reported in traveling children due to the infrequency of children traveling to high altitude, but HAPE may be more likely to occur when the child also has a viral illness. For treatment of AMS and HACE in children, dexamethasone might be advised.

Oral Contraceptives

Women taking oral contraceptives who will not spend much time higher than 4,200 m (13,800 ft) can continue to take oral contraceptives.

Women climbing at extremely high altitude on expeditions (above 6,000 m; 19,700 ft) should consider discontinuing the oral contraceptives.

Table 1: Typical Tourist Destinations
Destination Peak altitude Comments
Cusco, Peru 3,400 m (11,200 ft) For travelers flying from Lima to Cusco, acetazolamide is recommended.

Alternatives to sleeping in Cusco after arriving on a flight are 1) descend to Ollantaytambo (2,800 m; 9,200 ft) for the first 2 nights, or 2) go to Arequipa (2,300 m; 7,500 ft) for a few days before land transportation to Cusco.
Mammoth Mountain, California, U.S. 3,400 m (11,200 ft) Travelers typically stay in Mammoth Lakes (2,400 m; 7,900 ft) or nearby areas and ski at the higher altitudes of the mountain.
Mont Blanc, France and Italy 4,810 m (15,800 ft) Travelers typically stay in Chamonix (1,035 m; 3,400 ft) or other villages in the valley (up to 1,462 m; 4,800 ft) and go to higher altitudes during the day.
Quito, Ecuador 2,800 m (9,200 ft) Some travelers fly into Quito and may benefit from acetazolamide while others may carry the medication to be used in response to altitude symptoms.
La Paz, Bolivia 3,800 m (12,500 ft) For travelers flying into La Paz, acetazolamide is recommended for prevention.
Lhasa, Tibet 3,700 m (12,100 ft) For travelers flying into Lhasa, acetazolamide is recommended for prevention.

Table 2: Popular Trekking Destinations
Destination Peak altitude Comments
Annapurna Circuit, Nepal 5,416 m (17,800 ft) Most trekkers arrive to Pokhara (up to 1,740 m; 5,700 ft) and can acclimatize gradually during the trek. Because some routes reach significantly higher elevations, acetazolamide is beneficial for prevention.
Everest Base Camp, Nepal 5,364 m (17,600 ft) Routes to the peak vary in their rates of ascent.

Generally, start acetazolamide as the climber ascends; continue until descent to the starting point. Dexamethasone and nifedipine are usually prescribed for emergency treatment.
Kilimanjaro, Tanzania 5,895 m (19,300 ft) Routes to the peak vary in rate of ascent; descent typically takes only 1-2 days.

Generally, start acetazolamide as the climber ascends; continue until descent to the starting point. Dexamethasone and nifedipine are usually prescribed for emergency treatment.