Altitude sickness has a misleading name. It sounds like a minor seasonal inconvenience, but the same physiology that causes a mild headache at 9,000 feet can kill someone at 14,000 feet if they ignore the warning signs. Mountaineers, hikers, skiers, and increasingly even tourists who fly into high-altitude cities all face the same basic problem: above about 8,000 feet, the partial pressure of oxygen drops, the body has to adapt, and how fast you climb determines whether the adaptation keeps up.

The good news is that altitude sickness is largely preventable with sensible ascent rates, and the mild form (acute mountain sickness, or AMS) resolves with rest or modest descent. Severe forms like high-altitude cerebral edema (HACE) and high-altitude pulmonary edema (HAPE) are medical emergencies that require immediate descent, but they’re also preceded by warning signs that experienced climbers recognize and respond to early.

This article covers what altitude does to the body, where problems typically start, how to recognize AMS, prevention that works, and the signs that warrant descent.

This article is for educational purposes only and does not replace evaluation or treatment by a qualified clinician. Persistent or severe altitude-related symptoms warrant immediate medical care. Last updated: May 30 2026 | By Austin Murphy

Key Takeaways

  • Altitude sickness typically begins above 2,500 meters (8,200 feet) and reflects insufficient time to adapt to lower oxygen1
  • Acute Mountain Sickness (AMS) presents as headache plus one of nausea, fatigue, dizziness, or insomnia within 6-24 hours of arrival at altitude
  • HACE (cerebral edema) and HAPE (pulmonary edema) are medical emergencies requiring immediate descent of at least 1,000 meters
  • Sudden severe headache, confusion, ataxia, gurgling cough, severe shortness of breath at rest, or cyanosis warrant immediate descent and emergency medical evaluation

What Altitude Does to the Body

The percentage of oxygen in the air stays the same at every elevation: roughly 21%. What changes is air pressure. At sea level, atmospheric pressure pushes oxygen into the lungs with enough force that the blood saturates fully. By 8,000 feet (2,440 meters), atmospheric pressure has dropped to about 75% of the sea-level value. Above 14,000 feet (4,270 meters), pressure falls to around 60% of sea level. Higher still at 18,000 feet (5,500 meters), pressure drops to about 50%.

Lower pressure means less oxygen is forced into the lungs with each breath, which means less oxygen reaches the blood and tissues. The body’s first response is to breathe faster and deeper (hyperventilation), which helps. Kidneys excrete more bicarbonate to compensate for the respiratory alkalosis caused by faster breathing. Bone marrow ramps up red blood cell production over days to weeks. These adaptations together let healthy people function reasonably well even at moderate altitudes after acclimatization.

The problem is that some adaptations take days. People who fly directly to high altitude or who climb too fast haven’t had time to adjust. Their tissues end up oxygen-starved, which produces the symptoms collectively known as altitude sickness.

The Elevation Thresholds

Altitude problems typically scale with elevation in three brackets, though individual susceptibility varies considerably.

High altitude: 1,500 to 3,500 meters (5,000 to 11,500 feet). AMS becomes possible above the lower end of this range. Symptoms are usually mild and resolve within 1-3 days as the body adapts. Common destinations in this range: Denver, CO (5,280 ft); Cusco, Peru (11,150 ft); Mexico City (7,350 ft); Aspen, CO (8,000 ft).

Very high altitude: 3,500 to 5,500 meters (11,500 to 18,000 feet). AMS becomes common. HACE and HAPE become real risks, especially with rapid ascent. Common destinations: Everest Base Camp South (17,600 ft); Mount Kilimanjaro summit (19,341 ft, exceeds this range); La Paz, Bolivia (12,000 ft).

Extreme altitude: above 5,500 meters (18,000 feet). The body cannot fully acclimatize at this altitude; deterioration outpaces adaptation. Even acclimatized climbers experience cumulative damage and need supplemental oxygen for extended exposure. Mount Everest summit (29,032 ft) is the most famous example.

Individual susceptibility varies. Some people get AMS at 7,500 feet; others tolerate 14,000 feet without symptoms1. Prior history of AMS is the best predictor of future AMS; fitness level is a poor predictor (fit hikers are no more or less susceptible than average people).

Recognizing Acute Mountain Sickness (AMS)

AMS is the mild form of altitude sickness. It typically develops within 6-24 hours of arrival at altitude and is defined by headache plus at least one of the following symptoms1:

  • Nausea or vomiting
  • Fatigue or weakness
  • Dizziness or lightheadedness
  • Difficulty sleeping (insomnia, often with periodic breathing patterns)

The headache is usually bifrontal, dull, and worsens with exertion or bending over. It often improves with rest, hydration, and ibuprofen. Without further ascent, symptoms typically resolve within 24-72 hours as the body adapts. Continuing to climb while symptomatic significantly increases the risk of progression to HACE or HAPE.

Recognizing the Dangerous Progressions: HACE and HAPE

Two serious forms of altitude illness can develop from untreated or progressing AMS, or directly without AMS. Both require immediate descent.

High-Altitude Cerebral Edema (HACE)

HACE is brain swelling from prolonged hypoxia. It usually appears at 12,000 feet or above and represents an end-stage progression of AMS. Recognition signs include:

  • Severe headache that doesn’t improve with rest or analgesics
  • Confusion, lethargy, or altered mental status
  • Ataxia (loss of coordination; classic test is walking heel-to-toe in a straight line)
  • Hallucinations
  • Vomiting that doesn’t respond to oral antiemetics
  • Eventual loss of consciousness

HACE progresses to coma and death within hours if not addressed. Immediate descent of at least 1,000 meters (3,300 feet) is the primary treatment. Supplemental oxygen and the medication dexamethasone (administered by a clinician or trained team) are adjunct treatments while arranging descent.

High-Altitude Pulmonary Edema (HAPE)

HAPE is fluid accumulation in the lungs from altitude-related vascular changes. It can develop independent of AMS, usually 2-4 days after arrival at altitude. Recognition signs include:

  • Severe shortness of breath at rest (not just on exertion)
  • Cough that becomes wet or productive, often pink-tinged or frothy
  • Gurgling sounds when breathing (can be heard without a stethoscope in advanced cases)
  • Cyanosis (blue tint to lips, fingernails)
  • Fever, sometimes mild
  • Extreme weakness and lethargy
  • Eventual loss of consciousness

HAPE also progresses to death within hours if untreated. Immediate descent is again primary treatment. Supplemental oxygen and the medication nifedipine (administered by a clinician) are adjuncts.

Prevention That Actually Works

Most altitude illness is preventable with sensible ascent rates. The Wilderness Medical Society guidelines emphasize the following1:

Ascend gradually above 3,000 meters (10,000 feet). Don’t increase sleeping altitude by more than 500 meters (1,640 feet) per day. Take a rest day every 1,000 meters (3,300 feet) of cumulative ascent. “Climb high, sleep low” is the standard formula: you can hike higher during the day, but return to a lower altitude for sleeping.

Acclimatize at intermediate altitude before going higher. Spending 2-3 nights at 7,000-9,000 feet before continuing higher reduces AMS incidence substantially compared to ascending directly from sea level to 12,000 feet.

Stay hydrated, but not over-hydrated. Altitude increases insensible fluid losses through breathing and dry air. Drink to thirst plus a little more, but excessive water intake during prolonged exertion can cause exercise-associated hyponatremia, which can mimic some AMS symptoms in presentation. Sports drinks or electrolyte mixes for longer days.

Acetazolamide for prevention. Diamox (acetazolamide), a prescription diuretic, accelerates acclimatization by mimicking the kidney’s natural response to altitude. Starting it 24 hours before ascent and continuing for 2-3 days at altitude reduces AMS incidence in susceptible individuals. Discuss with a clinician before traveling; not appropriate for everyone (sulfa allergy is a common contraindication).

Avoid alcohol and sleeping pills for the first 48 hours at altitude. Both suppress respiration and worsen the hypoxia that drives altitude sickness.

Don’t fly directly to very high altitude if possible. If you must, plan to do less the first 2-3 days and watch for AMS symptoms carefully.

Treatment of AMS

If AMS develops, the response options ordered from least to most aggressive:

  1. Stop ascending. Don’t go higher until symptoms resolve. Continuing upward is the most common cause of progression to HACE or HAPE.
  2. Rest at current altitude. Most cases resolve within 24-72 hours with rest, hydration, and mild analgesics for headache (ibuprofen 400-600 mg, acetaminophen 500-1000 mg, both within standard adult dosing).
  3. Descend if symptoms worsen. Even 300-500 meters of descent often produces rapid improvement. Don’t wait for symptoms to escalate to HACE or HAPE before descending.
  4. Supplemental oxygen if available. Common in commercial trekking environments; can reverse mild AMS quickly.
  5. Acetazolamide if not already on it. Can be started after onset of AMS to accelerate adaptation, though prevention is more effective than treatment.

For HACE or HAPE, immediate descent is non-negotiable. Don’t wait for daylight, don’t wait for weather, don’t wait for porters: descent of at least 1,000 meters is needed, and every hour of delay increases mortality risk.

Common Misconceptions

“Fitness protects against altitude sickness.” It doesn’t. Olympic athletes have died from HAPE on relatively modest peaks. Genetic factors and prior history of altitude problems predict susceptibility better than fitness level.

“Altitude pills (over-the-counter supplements) prevent altitude sickness.” No over-the-counter supplement has reliable evidence for AMS prevention1. Acetazolamide (prescription) is the only widely-supported pharmacologic preventive. Some evidence supports ginkgo biloba for AMS prevention but evidence is mixed.

“Drinking lots of coca tea prevents altitude sickness.” Coca tea is a traditional practice in the Andes but isn’t a substitute for proper acclimatization. Some report subjective benefit; rigorous evidence is limited.

“Oxygen canisters solve altitude problems.” Brief supplemental oxygen helps acutely, but the canister-based products sold commercially provide only minutes of additional oxygen. They don’t substitute for descent in serious cases.

“You can tell altitude sickness is coming because you’ll feel short of breath.” Everyone feels short of breath at altitude during exertion; that’s normal. Altitude sickness presents as headache plus other symptoms, not just shortness of breath unless it’s HAPE (and HAPE shortness of breath is at rest, not just during exertion).

When to See a Doctor or Descend Immediately

Several signs warrant immediate descent and emergency medical attention:

  • Severe headache that doesn’t improve with rest or analgesics
  • Confusion, lethargy, or any altered mental status
  • Loss of coordination, particularly inability to walk heel-to-toe in a straight line
  • Severe shortness of breath at rest (not just during exertion)
  • Cough producing pink-tinged or frothy sputum
  • Gurgling sounds when breathing
  • Cyanosis (blue tint to lips, fingernails, or tongue)
  • Persistent vomiting that prevents oral fluid intake
  • Chest pain or palpitations beyond exertional response
  • Loss of consciousness or seizure
  • Fever combined with altitude symptoms
  • Worsening symptoms despite 24+ hours of rest at current altitude

The general principle: any progression of AMS symptoms warrants descent1. Waiting to see if mild symptoms get worse is the most common reason climbers progress to HACE or HAPE. Descend early, descend often, live to climb another day. These adjustments support healthy ascent and adaptation; they do not treat established HACE or HAPE, which require descent and emergency medical care.

Frequently Asked Questions

Can children get altitude sickness?

Yes, at similar rates to adults. Children may have more difficulty communicating their symptoms, so caregivers should watch for behavior changes (irritability, decreased activity, refusal to eat) as well as the standard symptom list. Pediatric altitude illness response follows the same descent principles as adult illness.

How long does it take to acclimatize?

Initial adjustments (faster breathing, increased heart rate) happen within hours. Bicarbonate excretion takes 1-3 days. Red blood cell increase takes 1-3 weeks. “Fully acclimatized” depends on the altitude reached; for moderate altitudes (8,000-10,000 feet), 2-3 days is usually sufficient.

Does altitude sickness happen on airplane flights?

Commercial aircraft cabins are pressurized to approximately 6,000-8,000 feet, which is at the lower threshold for AMS in some sensitive individuals. Mild altitude effects (mild headache, fatigue, dehydration) are reported by some passengers, but full AMS during a flight is uncommon.

Should I take acetazolamide as a precaution?

This is a medical decision to make with your clinician. Acetazolamide is generally well-tolerated for short courses but has side effects (tingling in extremities, altered taste, frequent urination, and others). Discuss based on your destination altitude, ascent rate, and any prior altitude history.

How do I know I’m acclimatized?

Subjective signs: no headache, normal appetite, normal sleep, energy levels back to baseline. Objective measures (pulse oximetry, resting heart rate) can confirm but aren’t necessary for healthy travelers. If you’ve been at altitude 2-3 days without symptoms and tolerated mild exertion well, you’re likely adapted enough to continue higher carefully.

Sources

  1. Luks AM, Auerbach PS, Freer L, et al. Wilderness Medical Society Clinical Practice Guidelines for the Prevention, Diagnosis, and Treatment of Acute Altitude Illness. Wilderness Environ Med. 2024;35(1):2-30. https://www.wemjournal.org/