Altitude Sickness: Prevention, Symptoms, and Treatment (2026)
Altitude sickness derails more mountain trips than any other factor, and most cases are preventable. The principles are well-established: ascend slowly, sleep low, recognize symptoms early, and know when to descend. This guide gives you the knowledge to apply all four.
In This Guide
What Causes Altitude Sickness
At high altitude, the partial pressure of oxygen in the air is lower. There are the same proportion of oxygen molecules as at sea level — about 21% — but fewer molecules per breath because the air is less dense. Your lungs take in less oxygen per breath, blood oxygen saturation drops, and the body initiates a cascade of adaptations to compensate: deeper and faster breathing, increased heart rate, and eventually increased red blood cell production.
Altitude sickness occurs when the body cannot adapt fast enough to the reduced oxygen availability. The rate of ascent is the primary controllable factor — giving the body more time to adjust at each elevation band before moving higher is the core prevention strategy.
AMS vs HACE vs HAPE
AMS — Acute Mountain Sickness
The most common form. Headache is the defining symptom, plus at least one of: nausea, fatigue, dizziness, or poor sleep. Usually appears 2-12 hours after arrival at a new elevation. Typically resolves in 1-2 days with rest at the same altitude.
HACE — High-Altitude Cerebral Edema
Severe progression of AMS. Brain swelling causes confusion, loss of coordination (ataxia — test by walking heel-to-toe), severe headache not relieved by ibuprofen, and eventually loss of consciousness. Medical emergency requiring immediate descent.
HAPE — High-Altitude Pulmonary Edema
Fluid accumulation in the lungs. Persistent dry cough (later productive or frothy), breathlessness at rest or with minimal exertion, blue lips. Can be rapidly fatal. Medical emergency requiring immediate descent and supplemental oxygen if available.
Acclimatization Schedule
The standard guideline above 8,000 feet: ascend no more than 1,000-1,500 feet (300-500m) of sleeping elevation per day, and take a rest day every 3,000 feet of elevation gain. The phrase is "climb high, sleep low" — it is fine to hike to higher elevations during the day as long as you return to sleep at a lower camp.
For a trip to Kilimanjaro (19,341 ft), this means at least 6-8 days of ascent. For Colorado 14ers starting from 10,000 ft, spending 2-3 days at 8,000-10,000 ft before attempting the summit significantly reduces AMS risk. Arriving at your destination altitude and immediately hiking to the summit is the scenario most likely to cause serious problems.
Prevention Strategies
- Ascend slowly: Follow the 1,000 ft/day sleeping elevation guideline above 8,000 ft.
- Hydrate well: Dehydration worsens AMS symptoms. Drink more than you think you need, especially in dry mountain air.
- Avoid alcohol and sedatives: Both suppress breathing and worsen oxygen desaturation during sleep.
- Pre-acclimatize: If time allows, spend 1-2 nights at moderate altitude (6,000-8,000 ft) before ascending to your target elevation.
- Consider Diamox: Pharmacological prevention is effective and appropriate for rapid ascent scenarios where slow acclimatization is not possible.
Medications (Diamox)
Acetazolamide (Diamox) is the most studied and effective medication for AMS prevention. It works by inhibiting carbonic anhydrase, which causes the kidneys to excrete bicarbonate — effectively making the blood more acidic and stimulating faster breathing. More breaths per minute means more oxygen.
Typical prevention dose: 125-250mg twice daily, starting 24-48 hours before ascent. Side effects include frequent urination, tingling in hands and feet, and occasionally altered taste of carbonated beverages. It is not appropriate for everyone (sulfa allergy, kidney disease, pregnancy) — talk to a physician before your trip. Dexamethasone is the alternative for AMS treatment when descent is not immediately possible; it treats symptoms but does not aid acclimatization.
Symptoms and Self-Assessment
The Lake Louise Scoring System is a simple field assessment for AMS. Score yourself on headache (0-3), nausea/vomiting (0-3), fatigue (0-3), dizziness (0-3), and sleep difficulty (0-3). A score of 3 or above with headache indicates AMS. A score of 5 or above indicates significant AMS requiring action.
The ataxia test (walking heel-to-toe in a straight line) is the best field screen for HACE progression. If someone cannot walk a straight line with heels touching toes, HACE is possible and descent is mandatory. Do not wait to see if this symptom resolves.
When to Descend
Descend immediately for: any signs of HACE or HAPE, AMS that is worsening despite 24 hours of rest, or any symptom that impairs judgment or coordination. Descent of 1,000-2,000 feet (300-600m) usually produces rapid improvement in AMS symptoms. Do not wait until morning. Do not wait for helicopter evacuation if the person can walk. Descent is the treatment.
For mild AMS without progression: rest at current altitude, hydrate, take ibuprofen for headache (avoid acetaminophen, which can mask fever from other causes), and do not ascend until symptoms fully resolve. If symptoms are not improving after 24 hours, descend regardless of schedule.
High-Altitude Gear Essentials
- Pulse oximeter — Measures blood oxygen saturation (SpO2). Readings below 90% at altitude indicate significant hypoxia. Lightweight and inexpensive for the information it provides.
- Ibuprofen tablets — First-line treatment for AMS headache. More effective than acetaminophen for altitude headache specifically.
- Altitude acclimatization tent — For serious mountaineers and expedition athletes who pre-acclimatize at home before high-altitude trips.
Frequently Asked Questions
At what altitude does altitude sickness start?▼
Altitude sickness (AMS) can begin at elevations as low as 6,500-8,000 feet (2,000-2,400m) in susceptible individuals, but more commonly becomes a concern above 8,000 feet. Most healthy people who ascend slowly below 10,000 feet do not experience significant symptoms. Above 14,000 feet, even fit people who ascend too quickly will develop AMS.
Does fitness level affect altitude sickness?▼
No. Physical fitness does not protect against altitude sickness. Elite athletes get AMS at the same rate as sedentary individuals. The primary factors are rate of ascent, sleeping altitude, individual genetic susceptibility, and previous altitude exposure. Being fit means you can move faster — which can actually work against you if it leads to ascending too quickly.
How long does it take to acclimatize to high altitude?▼
Initial acclimatization (reduced acute symptoms) happens in 1-3 days at a given altitude. More complete adaptation takes 1-2 weeks. At extreme altitudes above 18,000 feet, full acclimatization is not possible — the body slowly deteriorates regardless. Most mountain trekkers plan 2-3 acclimatization days at intermediate camps before pushing to higher elevations.
Can I take Diamox (acetazolamide) to prevent altitude sickness?▼
Diamox is the standard pharmacological prevention for AMS and is effective. It works by stimulating breathing, which increases blood oxygen saturation. The typical dose is 125-250mg twice daily, started 1-2 days before ascent. Side effects include increased urination and tingling in the extremities. It is a sulfonamide drug — those with sulfa allergies should not take it. Consult a doctor before using it.
When should I descend for altitude sickness?▼
Descend immediately if you experience any signs of HACE (confusion, loss of coordination, severe headache unrelieved by ibuprofen, vomiting) or HAPE (persistent dry cough, breathlessness at rest, pink or frothy sputum). These are medical emergencies. For mild AMS without progression, rest at current altitude for 24 hours. If symptoms do not improve or worsen, descend at least 1,000-2,000 feet.
Can altitude sickness be fatal?▼
Yes. High Altitude Cerebral Edema (HACE) and High Altitude Pulmonary Edema (HAPE) are rare but life-threatening complications of altitude sickness that can develop rapidly and lead to death if untreated. HACE involves brain swelling and causes confusion, loss of coordination, and unconsciousness. HAPE involves fluid accumulation in the lungs and causes increasing breathlessness at rest, a wet cough, and rapid deterioration. Both conditions are most likely above 13,000 feet (4,000 meters) and in people who ascend too quickly. The treatment for both is immediate descent — even 1,000 feet of descent can be life-saving. Portable hyperbaric chambers (Gamow bags) are used in remote settings. Any symptom that worsens at rest — especially coordination loss or breathlessness at rest — is a medical emergency.
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