Pulse OximeterAt sea-level, healthy people have an oxygen saturation (SpO2) level in the blood of 98-99%. When one goes to high altitude, the percentage of oxygen in the bloodstream goes down quite significantly—and prolonged exposure to these lower blood oxygen levels is the primary cause of the effects of altitude.

The major cause of altitude illnesses is going too high, too fast. Given time, your body can adapt to the decrease in oxygen molecules at a specific altitude. This process is known as acclimatization and generally takes 1-3 days (per atmosphere) at that altitude. For example, if you hike to 10,000 feet (3,048 meters), and spend several days at that altitude, your body acclimatizes to 10,000 feet (3,048 meters). If you climb to 12,000 feet (3,658 meters), your body has to acclimatize once again.

A number of changes take place in the body to allow it to operate with decreased oxygen:

  • The depth of respiration increases.
  • Pressure in pulmonary arteries is increased, “forcing” blood into areas of the lung not normally used during breathing.
  • The body produces more red blood cells to carry oxygen.
  • The body produces more of a particular enzyme that facilitates the release of oxygen from hemoglobin to the body tissues.

Oxygen at Altitude Graphic

How Long Does it Take to Become *FULLY* Acclimatized?

From sea level, here is the average amount of time it takes to become FULLY acclimated to altitude:

  • 6,000 ft (1829 m) — 3 Weeks
  • 8,000 ft (2438 m) — 4 Weeks
  • 10,000 ft (3048 m) — 5 Weeks
  • 12,000 ft (3658 m) — 6 Weeks
  • 14,000 ft (4267 m) — 7 Weeks


Source: CDC – High Altitude Travel Directory

Prevention First


Slow Ascent

A Latin saying goes, “festina lente“, or “make haste slowly”. Like everything in life, it also applies when going up mountains, especially the higher ones. You must make your way to the mountaintop slowly. Doing otherwise can have serious, even fatal consequences. But high altitude illnesses like Acute Mountain Sickness can be prevented and its serious effects avoided—simply by ascending slowly.

The most important thing is to avoid AMS by acclimatizing properly by a gradual ascent. A recommended rate of ascent is to climb no more that 500 meters a day over an altitude of 3000 meters. In addition, you could also take a rest day every third day. If this is not possible, because of the position of the huts or campsites, you could also do staging, where you remain at an intermediate altitude between 3000 and 4000 meters for an extra day before ascending any further.

Also, always sleep at a lower altitude than the highest point reached that day. Sleeping at least 1,500 to 2,000 feet in elevation higher that the night before has been shown to help safely acclimate the body more than sleeping higher than that altitude gain.

Hydrate! Hydrate! Hydrate!

Although dehydration does not *cause* altitude sickness, it does decrease physical performance and the ability to generate heat. This will contribute to chilling and fatigue, which then could lead to altitude sickness, as well as hypothermia and frostbite. Drink at least 4 liters of water a day. Six liters is not too much. It is actually during sleep that your body uses water to help acclimate (adjust) to the atmospheric pressures of being at higher elevations.

Also, always sleep at a lower altitude than the highest point reached that day. Sleeping at least 1,500 to 2,000 feet in elevation higher that the night before has been shown to help safely acclimate the body more than sleeping higher than that altitude gain.

Hydrate! Hydrate! Hydrate!
Cold Injuries

Cold Injuries

When traveling in cold regions, it is not always enough to put on warm clothing to protect yourself from Hypothermia and Frostbite. There are actually several ways to prevent them like being well-fed, well-hydrated and many more. These are discussed in greater detail in this section..


As with everything, many home and herbal remedies as well as generic treatments claim to prevent various mountain sicknesses. These treatments can make AMS worse or have other dangerous side effects – many herbs are poisonous. Only one drug is currently known to prevent AMS and to be safe for this purpose: acetazolamide (diamox). It causes some minor side effects, such as tingling fingers and a funny taste in the mouth.


Acetazolamide (DIAMOX)

Acetazolamide prevents AMS when taken before ascent; it can also help speed recovery if taken after symptoms have developed. The drug works by acidifying the blood and reducing the respiratory alkalosis associated with high elevations, thus increasing respiration and arterial oxygenation and speeding acclimatization. An effective dose that minimizes the common side effects of increased urination and paresthesias of the fingers and toes is 125 mg every 12 hours, beginning the day before ascent and continuing the first 2 days at elevation, or longer if ascent continues.

Allergic reactions to acetazolamide are uncommon. As a nonantimicrobial sulfonamide, it does not cross-react with antimicrobial sulfonamides. However, it is best avoided by people with history of anaphylaxis to any sulfa. People with history of severe penicillin allergy have occasionally had allergic reactions to acetazolamide. The pediatric dose is 5 mg/kg/day in divided doses, up to 125 mg twice a day.

Acetazolamide is not recommended as a prophylactic medication, except under specific limited conditions outlined below.

  • Treatment of persons with AMS.
  • Treatment of persons bothered by periodic breathing at night.
  • Prophylactically for persons on rapid forced ascents (such as flying into Lhasa, Tibet).
  • Prophylactically for those persons who have repeatedly had AMS in the past.

Most people who have a reasonable ascent schedule will not need it, and in addition to some common minor but unpleasant side effects, it carries the risk of any of the severe side effects that may occur with sulfonamides.

Common side effects include numbness, tingling, or vibrating sensations in hands, feet, and lips, taste alterations, and ringing in the ears. These go away when the medicine is stopped. Since acetazolamide works by forcing a bicarbonate diuresis, you will urinate more on this medication. Uncommon side effects include nausea and headache.

Myths About DIAMOX

Among climbers, there are a lot of myths about acetazolamide. Let’s clarify these!

Dexamethasone (DexPak)

Dexamethasone is effective for preventing and treating AMS and HACE and prevents HAPE as well. Unlike acetazolamide, if the drug is discontinued at elevation before acclimatization, mild rebound can occur. Acetazolamide is preferable to prevent AMS while ascending, with dexamethasone reserved as an adjunct treatment for descent. The adult dose is 4 mg every 6 hours. An increasing trend is to use dexamethasone for “summit day” on high peaks such as Kilimanjaro and Aconcagua, in order to prevent abrupt altitude illness.


Nifedipine (Procardia)

Nifedipine both prevents and ameliorates HAPE. For prevention, it is generally reserved for people who are particularly susceptible to the condition. The adult dose for prevention or treatment is 30 mg of extended release every 12 hours or 20 mg every 8 hours.


Other Medications

Phosphodiesterase-5 inhibitors can also selectively lower pulmonary artery pressure, with less effect on systemic blood pressure. Tadalafil (yes, Cialis), 10 mg twice a day during ascent, can prevent HAPE; it may also have use as a treatment. Ginkgo biloba, 100–120 mg taken twice a day before ascent, reduced AMS in adults in some trials. It was not effective in other trials, though, possibly due to variation in ingredients. Recent studies have shown ibuprofen 600 mg every 8 hours to be noninferior to acetazolamide in preventing AMS, although ibuprofen does not improve acclimatization or reduce periodic breathing. It is, however, over-the-counter, inexpensive, and well tolerated.