best ways to
acclimatize before
an expedition
YOU’VE DONE EVERYTHING RIGHT: trained super hard, done days of research, and arrived at the nearest town to your objective. You’re psyched and strong. You came from sea level but “feel great” at 10,000 feet. You feel impatient so while the rest of the team stays in town, you speed up to basecamp in a single push. You nurse a slight headache, but inside you’re secretly proud of gaining 15,000 feet of elevation in only six days.

The next day you scope out the peak and stash gear at 16,500 feet. A nagging headache makes dinner unappealing. You spend the night tossing and turning, and the next day you admit to the others (who just arrived) that you’re “not feeling great.” On day three, you lie in your sleeping bag as the team does a light hike. You try carrying a load the next day, but nausea and dizziness force you back. On day five, you’re having trouble standing straight and your cook carries you down to safety. A few days later, the team prepares for their summit push. They will be summiting right as you are arriving back at basecamp.

The above is a lesson in patience—and acclimatization. This process takes time. Fitness, age, and experience make no difference. Hazards from not acclimatizing properly are illness or death. This process of physiological adjustment to the low oxygen content at high altitude differs from person to person. For you, the climber, acclimatization means matching how fast you go up with how fast your body adjusts. There are no hard-and-fast rules with acclimatization, but here are some tips on how to best approach the “climb high, sleep low” guiding principle of acclimatizing. Remember, this information is no substitute for professional care and training. A Wilderness First Aid class is a great way to learn about the hazards of Altitude Sickness.

REI Wilderness First-Aid ›


Altitude sickness is the general term for what happens when you aren’t acclimatized. Symptoms can range from a mild headache to incapacitation and death. Altitude sickness comes in three forms: Acute Mountain Sickness (AMS), High-Altitude Pulmonary Edema (HAPE), and High-Altitude Cerebral Edema (HACE). Any of these three illnesses can strike a highly trained athlete, a veteran mountaineer, or a couch potato with little regard for their lifestyle. Altitude sickness can occur while stepping on the summit of K2 at 28,251 feet or stepping off a plane in Denver at 5,430 feet. No one has immunity from it, people who have never gotten it before can get it unexpectedly.

Altitude sickness is an equal opportunity oppressor. Expedition skier Luke Smithwick says, “No one should take it personally. Some of the best climbers on earth, people who plan to do all fourteen 8,000-meter peaks with no oxygen can take weeks to acclimatize.” It can strike in a few hours. The only cure is to decrease elevation. There are several factors that affect acclimatization—fitness, other illness, level of hydration, what altitude you live at, how much you push yourself physically, and new studies indicate that genetics might play a role in it.

  • Acute Mountain Sickness (AMS) is the mildest and most common version, commonly described as similar to a bad hangover.
  • High-Altitude Pulmonary Edema (HAPE) is the second most common. This is a serious condition in which the lungs fill with fluid, and it is potentially fatal.
  • High-Altitude Cerebral Edema (HACE) is the rarest and most serious condition in which the brain swells and the victim loses coordination. This condition is fatal without treatment.

As with other aspects of expedition climbing, consult professionals prior to departure. Your travel doctor will provide information, guidelines for treatment, and necessary medications. Once again, experience is the best teacher. Err on the side of safety.


There are several ways climbers successfully “pre-acclimatize.” One is to visit and sleep above 10,000 feet prior to your expedition. This is only possible in certain regions, and costs time and money. An alternative process involves sleeping at progressively higher altitudes in your home through the use of a hypobaric chamber set up around your bed. This is an expensive strategy generally used by those who have more money than time. See Hypoxico

A word of caution. Expert big mountain climber and guide Garrett Madison says, “This works over time to build more red blood cells, but the tent can’t mimic the pressure at altitude. I think it’s best to take it easy even if using the tent to cut out part of an expedition. However, it does work. I regularly have climbers who sign up for my ‘Everest Express’ program, where they climb the peak with me in less than a month, rather than the more traditional two-month program.”

  • Take Your Time: “The best way to acclimatize is to do it naturally, by going slow and steady.”
  • Be Patient: “Be conservative in going higher. Climbers get into trouble when they push too hard too fast.”
  • Follow the Rule: “Once on the peak and at altitude, a good rule of thumb is 1,000 feet per day of elevation gain. That could mean you climb up 2,000 feet, then rest for a couple of days before moving up higher.”
  • Be Ready to Stop or Descend: “If you start feeling the symptoms of Acute Mountain Sickness, don’t go higher until those symptoms have resolved. If they are not resolving, go down.”
  • Illness can Escalate: “If you continue up, AMS can turn into High-Altitude Pulmonary Edema, which is unpleasant and life-threatening. Once you have HAPE, it’s hard to help yourself; you need capable and knowledgeable people around (mountain guides) to save your life.”
  • Treatment: “The best treatment is immediate descent, along with supplemental oxygen and dexamethasone.”
More Prevention Tips
With all three forms of altitude illness, the adage holds true: An ounce of prevention is worth a pound of cure.
  • Sleep at progressively higher altitudes following the dictum, “climb high, sleep low.” Sleeping at 15,000 feet after climbing at 17,000 feet can do wonders for acclimatization. A 1,000- to 3,000-foot difference is recommended.
  • Stay hydrated. Drink 2 to 4 liters per day. During activity, sip water every 15 to 20 minutes. Add an electrolyte mix to help absorb water and maintain your sodium levels.
  • Avoid alcohol and strenuous exertion for two to three days after arriving at altitude. If possible, avoid alcohol entirely.
  • Acetazolamide (Diamox) speeds acclimatization and prevents High-Altitude Cerebral Edema as well as AMS. This drug requires a prescription and is readily available through your physician.
The higher one goes, the thinner the air gets and less oxygen is available. Though the percentage of oxygen remains consistent, the decreased atmospheric pressure reduces the number of oxygen molecules available in each lungful. Most of us live in an elevation range between sea level and several thousand feet, so it’s no surprise that the general rule considers elevations above 7,500 feet as problematic. It is estimated that 25 to 30 percent of sea level visitors to Colorado ski resorts around 9,000 feet get AMS. The numbers decrease to 8 to 10 percent of those only visiting Denver at 5,280 feet. Consider the elevation at which you live when factoring in how much time to spend acclimatizing.
Symptoms vary according to specific type of ailment. As we mentioned, these can range from hangover-like symptoms to complete debilitation. The one constant is that descent to a lower elevation is the only guaranteed treatment for all altitude illnesses. As we’ve seen, being a tough girl or guy and “climbing through” an illness is a shortcut to failure and potential disaster. With AMS, one can wait at their current altitude to see in symptoms resolve. HAPE might resolve by staying in place, but it is strongly recommended that one descend. With HACE it is critical that the patient descend immediately.
Acute Mountain Sickness
SYMPTOMS: A steady or throbbing headache is the primary symptom of AMS. The headache ties in with nausea, vomiting, fatigue, lethargy, loss of appetite, and difficulty sleeping.

TREATMENT: If one rests and proceeds no higher, AMS usually resolves itself in two days. The following steps help:

  • If you have AMS, stay at the same altitude and rest, or descend. Descending 1,000 feet is generally adequate. Resting at the same altitude sometimes works, even to the point that you may resume climbing. Sound advice is to descend for a few days, and then resume climbing.
  • Get lots of rest and take ibuprofen or acetaminophen for headache.
  • If prescribed, take acetazolamide (Diamox®) as directed. Acetazolamide speeds up the acclimatization process that in turn helps to alleviate symptoms of AMS. Consult your physician and acquire prescriptions and drugs prior to departure.
  • If prescribed, take dexamethasone (Decadron®) as directed. This prescription steroid reduces brain swelling, which in turn alleviates symptoms in moderate to severe altitude sickness. Consult your physician and acquire prescriptions and drugs prior to departure.
  • If available and descent is not possible, an inflatable pressurized bag that simulates lower altitude (Gamow®) can be used. These are rare on all but the most lavish expeditions (costing upwards of $5,000).
High-Altitude Pulmonary Edema

SYMPTOMS: HAPE shows up as shortness of breath accompanied by a cough. It may or may not initially appear as AMS. This condition may emerge from untreated AMS. It can creep up in mere hours. Symptoms include progressive shortness of breath, unusual fatigue and exhaustion following exertion, and wet coughing, often productive (meaning it produces sputum when you cough).

TREATMENT: HAPE is a very serious and potentially fatal condition that can be treated with supplemental oxygen and rest. While resting at the same altitude sometimes works, descending 1,000 feet or more may be necessary. If available and descent is not possible, a hyperbaric, inflatable pressurized bag (Gamow®) might be necessary.

High-Altitude Cerebral Edema

SYMPTOMS: The telltale symptom of HACE is ataxia, or impaired balance. HACE starts with AMS-like symptoms and progresses through severe headache, confusion, lethargy, lack of coordination, irritability, vomiting, seizures, coma, and eventually death if untreated. This condition might emerge over days, or may strike in hours. The signs are inability to walk a straight line, going heel to toe, or the inability to balance on one foot.

TREATMENT: HACE is a very serious and potentially fatal condition. The victim needs to immediately descend to lower altitude. Descent must be followed by professional medical treatment. A person with HACE must be taken to a lower altitude. Canned oxygen therapy should be started if available, and dexamethasone, a steroid medication in pill form, should be given immediately. If the hyperbaric bag (Gamow®) is available, use it right away.