Altitude sickness is a group of medical conditions developing when the body cannot adapt quickly enough to reduced oxygen at elevation — typically above 2,500 m (8,200 ft). Three forms of increasing severity: AMS (Acute Mountain Sickness) mildest and most common, symptoms headache plus nausea/fatigue/dizziness/sleep disturbance, onset 6-24 hours after rapid ascent, affects 25-50% above 3,500 m. HAPE (High-Altitude Pulmonary Edema) life-threatening fluid in lungs, symptoms severe breathlessness at rest, dry cough progressing to pink/frothy sputum, drowning sensation, blue lips, usually above 2,500 m within 2-5 days, affects 0.2-6% of climbers. HACE (High-Altitude Cerebral Edema) life-threatening brain swelling, symptoms severe confusion, inability to walk straight (ataxia), hallucinations, loss of consciousness, usually follows AMS above 4,000 m, affects 0.5-1% of climbers. Why altitude sickness happens: at sea level atmospheric pressure 760 mmHg with oxygen 21%. At 3,500 m pressure drops to ~500 mmHg. Percentage of oxygen remains same but partial pressure decreases significantly. Lower oxygen pressure means less oxygen per breath reaches bloodstream. Body cannot deliver sufficient oxygen — triggering physiological responses and symptoms. Who gets it: anyone regardless of age, fitness, or experience. Individual susceptibility varies enormously. Prior altitude sickness strongly predicts future episodes. Fitness does NOT prevent altitude sickness. Ascent rate is biggest modifiable risk factor. Altitude categories: low sea level to 1,500 m, moderate 1,500-2,500 m, high 2,500-3,500 m (altitude sickness begins), very high 3,500-5,500 m, extreme above 5,500 m.

Altitude sickness symptoms range from mild discomfort (AMS) to life-threatening emergencies (HAPE/HACE). AMS symptoms: headache hallmark (bilateral, worse with exertion), nausea with or without vomiting, loss of appetite, fatigue and weakness, dizziness, sleep disturbances (insomnia, vivid dreams, periodic breathing), irritability, Lake Louise Score ≥3 with headache diagnostic. HAPE symptoms: shortness of breath at rest (not just with exertion), cough initially dry progressing to pink/frothy sputum, drowning sensation, cyanosis (blue lips/fingernails), elevated heart rate, fever possible, crackling sounds in lungs, severe weakness. HACE symptoms: severe progressive headache unresponsive to medications, confusion, disorientation, behavioral changes, ataxia (unable to walk heel-to-toe), slurred speech, hallucinations, loss of consciousness possible, can follow AMS rapidly (hours), frequently coexists with HAPE. Red flag combinations requiring immediate descent: AMS symptoms NOT improving after 24 hours, AMS symptoms WORSENING despite staying, any HAPE symptom (especially breathlessness at rest), any HACE symptom (especially ataxia or confusion), peripheral oxygen saturation below 80% at 4,000+ m. Lake Louise Score for AMS diagnosis: headache 0-3 points, GI 0-3 points, fatigue/weakness 0-3 points, dizziness 0-3 points. Total ≥3 with headache = AMS. 3-5 mild, 6-9 moderate, 10+ severe. Early recognition essential — AMS can rapidly progress to HAPE or HACE. When in doubt, descend.

Prevention centers on controlled ascent rate plus proper acclimatization, with medications as supplementary support. Primary prevention: gradual ascent 300-500 m per day sleeping elevation above 3,000 m, rest day every 1,000 m, climb high sleep low (hike higher during day return to lower elevation for sleeping), arrival acclimatization 2-3 days at moderate altitude before higher ascents, hydration 3-4 liters daily, avoid alcohol first 48 hours, avoid sleeping pills (suppress breathing), maintain carbohydrate-rich diet, avoid smoking. Ascent rate rules: below 3,000 m generally safe rapid ascent, 3,000-4,000 m 300-500 m per day sleeping, above 4,000 m strictly 300-500 m/day rule, every 1,000 m gained spend 2 nights at same elevation, build in active rest days with minor higher hikes. Medication prevention: Acetazolamide (Diamox) gold-standard, 125-250 mg twice daily starting 1-2 days before altitude, continuing first 2 days at target altitude, reduces AMS by ~50%, side effects tingling, frequent urination, altered taste. Dexamethasone 2 mg four times daily or 4 mg twice daily, reserved for high-risk or known susceptibility. Ibuprofen 600 mg three times daily may reduce AMS. Who should consider medication: previous history of AMS/HAPE/HACE, rapid ascent unavoidable (flying to La Paz, Lhasa), known individual susceptibility, essential travel above 3,500 m. Pre-acclimatization strategies: hypoxic tents at home, 3-5 days at moderate altitude before trek, multiple shorter altitude exposures weeks before main trip, cardiovascular fitness training (doesn’t prevent AMS but improves performance). See our acclimatization science guide.

Treatment depends on severity. Golden rule: when in doubt, descend. AMS treatment (mild): STOP ascending immediately, rest 24-48 hours at current elevation, hydration 3-4 liters daily, acetaminophen or ibuprofen for headache (avoid aspirin), anti-nausea medication if needed, if symptoms improve resume slow ascent, if symptoms worsen descend. AMS treatment (moderate-severe): Acetazolamide 250 mg twice daily (treatment dose higher than prevention), Dexamethasone 4 mg every 6 hours (moderate-severe cases), descend 300-1,000 m (almost always relieves symptoms), supplemental oxygen, Gamow bag if descent impossible. HAPE treatment (life-threatening): IMMEDIATE DESCENT 500-1,000 m essential, supplemental oxygen, Nifedipine 30 mg extended release every 12 hours (reduces pulmonary artery pressure), Sildenafil or tadalafil alternative, Gamow bag during evacuation, Acetazolamide 250 mg twice daily adjunct, Dexamethasone 4 mg every 6 hours if HACE also present. HACE treatment (life-threatening): IMMEDIATE DESCENT life-saving, Dexamethasone 8 mg initial then 4 mg every 6 hours, supplemental oxygen, Gamow bag if descent delayed, evacuation to lower altitude hospital mandatory. Treatment tools: Gamow bag (hyperbaric chamber) simulates descent of 1,500-3,000 m portable device for emergencies. Pulse oximeter monitors SpO2 — below 80% at 4,000 m indicates severe. Oxygen cylinders at major trekking camps. Satellite phones/InReach essential for evacuation. When to call evacuation: any HAPE symptoms not improving with descent, any HACE symptoms, inability to descend, loss of consciousness, cyanosis, ataxia. Recovery timeline: mild AMS 24-72 hours, moderate-severe 24-48 hours after descent, HAPE 1-3 days lung clearance 2-4 weeks, HACE days to weeks.

The Lake Louise Score (LLS) is the standardized medical assessment tool for diagnosing and grading AMS. Developed at the 1991 International Hypoxia Symposium in Lake Louise, Alberta. Self-assessed questionnaire with 4 symptom categories. Each category scored 0-3 points. Total ranges 0-12. AMS diagnosis requires recent ascent above 2,500 m, headache present, total score ≥3. The 4 categories and scoring: Headache 0 none, 1 mild, 2 moderate, 3 severe/incapacitating. GI (nausea/vomiting) 0 good appetite, 1 poor appetite or nausea, 2 moderate nausea or vomiting, 3 severe. Fatigue/weakness 0 none, 1 mild, 2 moderate, 3 severe. Dizziness 0 none, 1 mild, 2 moderate, 3 severe. 2018 revision removed sleep disturbance as standalone category. Score interpretation: 0-2 no AMS (may not yet be acclimatized), 3-5 with headache mild AMS, 6-9 with headache moderate AMS, 10-12 with headache severe AMS. Clinical decision-making: 3-5 mild stop ascending rest and hydrate may resume in 24 hours if resolved, 6-9 moderate descend 300-1,000 m start acetazolamide, 10-12 severe descend 500+ m consider dexamethasone monitor for HAPE/HACE. Any ataxia or confusion overrides score — treat as HACE. When to use: daily self-assessment above 3,500 m, evening evaluation of trekking day, before descending from high camps, when any symptoms appear, group-wide assessments. Limitations: doesn’t assess HAPE or HACE directly, subjective, other conditions can mimic AMS, should complement not replace clinical judgment, pulse oximetry provides objective data alongside LLS.

Several medications prevent and treat altitude sickness. Acetazolamide (Diamox) is most common for prevention, dexamethasone reserved for emergency treatment. All require physician prescription. Acetazolamide (Diamox) primary prevention: brand Diamox generic acetazolamide. Mechanism carbonic anhydrase inhibitor promotes bicarbonate excretion causing mild metabolic acidosis stimulating faster breathing — accelerating natural acclimatization. Prevention dose 125-250 mg twice daily starting 1-2 days before ascent above 2,500 m continuing first 2 days at target altitude. Treatment dose 250 mg twice daily. Reduces AMS incidence by ~50%. Side effects tingling in fingers/toes/face (paresthesia), frequent urination, altered taste (carbonated drinks taste flat), mild nausea. Contraindications sulfa drug allergy, kidney disease, liver disease, pregnancy. Dexamethasone emergency drug: brand Decadron. Potent corticosteroid reduces inflammation stabilizes cerebral edema in HACE. Prevention dose 2 mg every 6 hours or 4 mg twice daily. HACE treatment 8 mg initial then 4 mg every 6 hours. Moderate-severe AMS treatment 4 mg every 6 hours. Dramatic effect — reduces cerebral edema rapidly. Can mask serious illness — user must still descend. Side effects mood changes, GI upset, insomnia, increased urination. CRITICAL WARNING: NEVER continue ascending on dexamethasone — only masks symptoms, must descend after administration. Pulmonary vasodilators for HAPE: Nifedipine 30 mg extended-release every 12 hours reduces pulmonary artery pressure. Sildenafil (Viagra) 50 mg three times daily alternative. Tadalafil (Cialis) 10 mg twice daily alternative. Other: Ibuprofen 600 mg three times daily may prevent AMS headache. Ondansetron 4-8 mg for nausea. Acetaminophen safer for altitude headache than ibuprofen. Avoid aspirin, sleeping pills, strong opioids. All require physician prescription.

Altitude sickness can begin as low as 2,500 m (8,200 ft) though most cases develop between 3,000 m and 5,500 m. Exact threshold varies between individuals. Altitude thresholds: below 1,500 m low altitude no risk, 1,500-2,500 m moderate altitude low risk most healthy individuals unaffected, 2,500-3,500 m high altitude AMS begins (10-25% affected with rapid ascent), 3,500-5,500 m very high altitude significantly elevated risk (30-50% AMS HAPE/HACE possible), above 5,500 m extreme altitude no permanent human habitation progressive deterioration. Common destinations: Low/moderate (generally safe) Tour du Mont Blanc max 2,665 m low risk, Torres del Paine W Circuit max ~1,000 m no altitude issues. High altitude (AMS possible) Rocky Mountain Park Colorado up to 3,600 m some AMS, Machu Picchu 2,430 m mild effects, Atlas Mountains Toubkal 4,167 m moderate risk. Very high (significant AMS risk) Everest Base Camp 5,550 m Kala Patthar AMS common, Kilimanjaro summit 5,895 m HAPE/HACE possible with rapid ascent, Aconcagua 6,961 m high AMS risk, Manaslu Circuit 5,106 m max, K2 Base Camp 5,000 m. Extreme (acclimatization essential) 8,000 m peaks death zone oxygen typical, Everest 8,849 m, K2 8,611 m. Individual variation factors: genetics (susceptibility varies 10x between individuals), previous altitude experience, baseline fitness (doesn’t predict AMS), age (teens and young adults often MORE susceptible), prior AMS history strongly predicts future episodes, pre-existing cardiopulmonary conditions. Ascent rate vs absolute altitude: rapid ascent BIGGER risk than absolute altitude. Flying from sea level to 3,500 m (La Paz, Lhasa) causes more AMS than gradual ascent to 4,500 m. Key thresholds: 2,500 m AMS possible, 3,500 m Lake Louise assessments, 4,000 m mandatory acclimatization days, 4,500 m HAPE/HACE screening, 5,000 m extended acclimatization essential, 5,500 m short duration only for most climbers.

No — cardiovascular fitness does NOT prevent altitude sickness. Persistent myth in mountaineering. Fit individuals are just as susceptible as unfit individuals. Why fitness doesn’t prevent altitude sickness: altitude sickness results from inability to acclimatize to reduced oxygen — physiological response unrelated to cardiovascular conditioning. Fitness improves oxygen utilization at current capability but doesn’t increase oxygen uptake beyond what body can extract from low-oxygen environment. Ability to acclimatize primarily genetic and not trainable through fitness. Some studies suggest fitter individuals may push harder and ascend faster — potentially INCREASING AMS risk. What fitness DOES help: endurance for long trekking days, recovery between trekking days, carrying pack weight, overall trip enjoyment and performance, cardiovascular health baseline reducing other risks, mental resilience during challenging conditions. What actually prevents altitude sickness: gradual ascent (300-500 m per day sleeping elevation), proper acclimatization schedule, hydration, appropriate medications (Diamox), individual susceptibility (genetic), recognition and response to early symptoms, climb high sleep low protocols. Common misconceptions: ‘I run marathons so altitude won’t affect me’ Wrong marathon runners get AMS. ‘Young people don’t get altitude sickness’ Actually younger people may be MORE susceptible. ‘I’m acclimatized from prior trips’ Acclimatization doesn’t persist more than 1-2 weeks after return to sea level. ‘Fit climbers can skip acclimatization days’ Common and dangerous mistake. Historical evidence: professional mountaineers still get altitude sickness. World-class climbers have died from HAPE/HACE. Elite Sherpa guides experience altitude effects. Military special forces affected at altitude. What trainable factors matter: previous altitude exposure (1-2 weeks benefit), hypoxic training, psychological preparation, skill at self-assessment, practiced response protocols. Don’t rely on fitness to skip acclimatization. Follow standard ascent rates regardless of fitness. Take Diamox if predisposed. Be prepared to descend even at peak fitness. See our training program guide.