Altitude affects everything: your breathing, your sleep, your appetite, your ability to think clearly, and — in extreme cases — whether you live or die. The Himalayas, the Andes, the East African Highlands, the Colorado Rockies: millions of travelers visit high-altitude destinations every year, and the vast majority experience some degree of altitude illness without lasting harm. But the small percentage who develop severe high-altitude cerebral edema (HACE) or high-altitude pulmonary edema (HAPE) face genuine life-threatening emergencies, often in locations where evacuation takes hours or days.
The frustrating thing about altitude illness is that it's almost entirely preventable with proper acclimatization. The "climb high, sleep low" rule, proper ascent rates, adequate hydration, and recognition of early symptoms would prevent the vast majority of serious cases. The tragedy is that many of the worst outcomes involve travelers who didn't know these basic rules, or knew them but pushed through symptoms they should have taken seriously.
How Altitude Affects Your Body
At sea level, oxygen saturation in your blood is approximately 95-100%. At 3,500 meters (11,500 feet), it drops to around 85-90%. At 5,500 meters (18,000 feet), it's around 70-75%. Your body responds to this reduced oxygen delivery through a process called acclimatization: breathing rate increases, heart rate increases, and red blood cell production accelerates over days to weeks. But these adaptations take time — and if you ascend faster than your body can adapt, illness results.
Acute Mountain Sickness (AMS) is the mildest and most common form, affecting roughly 50% of travelers ascending above 3,000 meters without proper acclimatization. Symptoms include headache, nausea, fatigue, dizziness, and difficulty sleeping. These symptoms are unpleasant but typically resolve with rest and further acclimatization. They're also easily confused with other conditions: dehydration, exhaustion, alcohol hangover, or heat illness share similar presentations.
High-Altitude Cerebral Edema (HACE) is the severe progression of AMS. Fluid accumulates in the brain, causing confusion, clumsiness, stumbling, altered consciousness, and eventually coma and death. HACE develops over hours to days and requires immediate descent and medical attention. High-Altitude Pulmonary Edema (HAPE) is a separate condition where fluid accumulates in the lungs, causing cough, shortness of breath at rest, and eventually pink frothy sputum and respiratory failure. Both HACE and HAPE are life-threatening medical emergencies that can kill within 24-48 hours.
The Golden Rules of Safe Ascent
Above 3,000 meters, don't increase your sleeping elevation by more than 500 meters per day. This means if you hike to 4,000 meters to spend the day exploring, you should descend to sleep at or below 3,500 meters. "Climb high, sleep low" is the foundation of all altitude safety advice and the rule most consistently violated by time-pressured travelers.
Take rest days. Every third day — or every 1,000 meters of gain — build in a day where you don't ascend further. This allows your body to consolidate its acclimatization gains. Trekking itineraries in Nepal, Peru, and Kilimanjaro that ascend too rapidly for "efficiency" are responsible for most of the serious altitude illness cases I witnessed during years of high-altitude travel.
Ascending from sea level to high altitude in a single day is a recipe for disaster. If you're flying directly to a high-altitude city (La Paz at 3,650m, Cusco at 3,400m, Lhasa at 3,650m), spend two to three days at the city elevation before attempting any further ascent. The additional risk from the rapid altitude change from flying is significant, and the acclimatization time required is longer than for gradual approaches.
Hydration and Nutrition at Altitude
Altitude dramatically increases fluid loss through breathing (you lose more water with every exhaled breath at altitude than at sea level) and through urination (altitude diuresis is a real phenomenon). Dehydration at altitude is almost universal and contributes to both AMS symptoms and the risk of more serious illness. Drink 3-4 liters of water per day at altitude, more if you're active. Your urine should be clear to pale yellow; dark yellow indicates inadequate hydration.
Avoid alcohol at altitude — it's a diuretic, impairs acclimatization, worsens altitude-related sleep disruption, and is associated with increased AMS risk. This advice is particularly unwelcome at altitude bars in Cusco and trekking lodges in Nepal, but the data is unambiguous: alcohol and altitude don't mix well. If you do drink, wait until after you've fully acclimatized (usually after 3-4 days at a given elevation), and reduce your intake to half of what you'd normally drink.
Altitude suppresses appetite while increasing metabolic rate, creating a paradox where you're burning more calories but eating less. For multi-day treks, force yourself to eat adequate calories even without appetite. Carbohydrate-rich foods are easier to digest at altitude than fats and proteins. Bring snacks you actually want to eat; trail mix, chocolate, energy bars, and dried fruit are more appealing than freeze-dried trekking meals when your body is resisting food.
Medication and Prevention
Acetazolamide (Diamox) is the most studied and most effective medication for AMS prevention and treatment. It works by increasing respiratory drive, thereby improving oxygen saturation at altitude. For prevention, 125mg twice daily starting 24 hours before ascent and continuing for 48 hours after reaching altitude is effective for most people. For treatment of established AMS, 250mg twice daily facilitates faster recovery.
Side effects of acetazolamide include increased urination, tingling in fingers and toes, altered taste of carbonated beverages (flat soda tastes bizarre), and occasional drowsiness. Sulfa allergy is a contraindication. It's worth noting that acetazolamide doesn't eliminate the need for proper acclimatization — it's a supplement to, not a replacement for, appropriate ascent rates.
Dexamethasone (a steroid) is reserved for treatment of established moderate-severe AMS and HACE, not prevention. Ibuprofen (600mg three times daily) can reduce altitude headache but doesn't address the underlying acclimatization deficit. Ginkgo biloba, coca leaves, and garlic — traditional remedies promoted in various cultures — have not demonstrated consistent efficacy in controlled studies and should not be relied upon for AMS prevention.
When to Descend
The treatment for altitude illness is always the same: descent. If someone develops symptoms consistent with moderate-severe AMS (worsening headache, vomiting, shortness of breath at rest, confusion), they need to descend immediately. The minimum safe descent is 500-1,000 meters. Symptoms often improve within hours of descent.
HACE and HAPE are medical emergencies requiring immediate descent and, ideally, supplemental oxygen and medical care. If you're on a guided trek, your guides should be trained to recognize these conditions and have evacuation protocols. If you're trekking independently in remote areas, these conditions can be fatal before evacuation is possible. This is why prevention through proper acclimatization is so important — the treatment options at remote altitude are extremely limited.
Pulse oximeters — small devices that clip onto your finger and measure blood oxygen saturation — are increasingly carried by altitude travelers. They're useful for monitoring but shouldn't drive decisions in isolation. A reading of 70% in someone who feels fine may be their normal; a reading of 80% in someone with a splitting headache and confusion is an emergency. Context and symptoms matter more than the number.