Altitude sickness: the mountain illness that respects no fitness
Last updated September 3, 2026.
Altitude sickness (acute mountain sickness) hits when you ascend faster than your body can adapt to lower oxygen, typically above 2,500 meters: headache, nausea, dizziness, and crushing fatigue that feel like a bad hangover. Fitness is no protection (young fit trekkers get it constantly), and the rules that prevent it are about patience, not conditioning. Ignored, it can progress to fluid in the lungs or brain, which kills.
What does it feel like?
Within 6-12 hours of arriving at altitude: a headache as the defining symptom, plus nausea or lost appetite, dizziness, poor sleep, and unusual exhaustion. It genuinely resembles a hangover. The two dangerous progressions have distinct signatures: HACE (brain swelling) brings staggering, confusion, and bizarre behavior, like drunkenness without alcohol; HAPE (lung fluid) brings breathlessness at rest, a cough (sometimes pink froth), and chest tightness. Both are fatal without descent.
Why does it happen?
Altitude thins the air's oxygen, and the body adapts over days (deeper breathing, more red cells) but only if given the time. Ascend faster than the adaptation and the brain swells slightly and the symptoms arrive. The drivers: sleeping altitude gained too fast (the key variable), flying straight into high cities, hard exertion on arrival days, alcohol and sedatives (which blunt the breathing response), and previous altitude sickness, which is the strongest predictor of the next episode.
What actually prevents and treats it?
- Ascend slowly: above 3,000 meters, gain no more than 300-500 meters of sleeping altitude per day, with a rest day every 3-4 days; climb high, sleep low where the route allows.
- The headache rule: at altitude, treat every headache as altitude sickness until proven otherwise: no further ascent with symptoms, full stop.
- Go easy on arrival: no hard exertion, no alcohol, and light food for the first 24-48 hours at a new height.
- Medication: acetazolamide, started before ascent, speeds acclimatization and is standard for rapid or high itineraries (a prescription conversation before travel).
- The definitive treatment is descent: symptoms that worsen despite rest, or any confusion, staggering, or breathlessness at rest: descend immediately, at least 500-1,000 meters, day or night, whatever the plan was.
When is it an emergency?
At altitude, these are descend-or-die signs: breathlessness at rest (not just on exertion), a cough producing pink or frothy sputum, chest tightness, staggering or inability to walk a straight line, confusion, drowsiness, or any bizarre behavior, and symptoms worsening despite a day of rest at the same altitude. Descent is the treatment; oxygen and drugs (dexamethasone, nifedipine) are bridges to descent, not substitutes. Never ascend to sleep higher with any symptoms, and never leave a symptomatic person alone. Pymander's escalation routing is built and tested specifically for this class of decision; see the safety architecture working paper.
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Common questions
I am very fit. Why am I the one who got sick?
Because fitness and acclimatization are separate systems: cardiovascular fitness changes how hard you can work, not how fast your blood and brain adapt to thin air, and fit people arguably raise their risk by ascending fast and pushing hard on arrival days. The predictors that do matter: rate of ascent above all, sleeping altitude gained per day, previous altitude illness (the strongest single one), and individual susceptibility, which is partly genetic and stubbornly unpredictable. The mountains do not care about your VO2 max; patience is the only proven shield.
What are the golden rules of safe ascent?
The ones the mountain medicine consensus repeats: above 3,000 meters, increase sleeping altitude by no more than 300-500 meters per day, build in a rest day every 3-4 days, climb high and sleep low when the terrain allows, and treat any headache at altitude as altitude sickness until proven otherwise, meaning no ascent with symptoms, and descent if they worsen. Arrival days at new heights are for light activity only. Every one of these rules is written over a pile of case reports of people who broke them.
Does drinking water prevent altitude sickness?
Hydration helps you feel and function better (altitude breathing is drying, and dehydration mimics and worsens the headache and fatigue), but water does not prevent altitude sickness itself: the cause is ascent rate, and overhydrating does not acclimatize you any faster. The honest advice: drink to thirst, keep urine pale, and refuse to let liters of water substitute for a sane itinerary. The bottle that actually prevents altitude sickness is acetazolamide, and that is a prescription decision.
What is acetazolamide and should I take it?
Acetazolamide (Diamox) speeds acclimatization by nudging the blood's chemistry so you breathe deeper, especially during sleep, cutting the incidence and severity of altitude sickness meaningfully in trials. It is prescription-only and worth discussing with a travel clinic or GP before fast or high itineraries (flying into high cities, trekking above 3,500-4,000 meters, or a personal history of altitude sickness). Side effects to expect: tingling fingers, and carbonated drinks tasting flat. It is a helper for tight schedules, not permission to ignore the ascent rules or to ascend with symptoms.
How do I know when it has become dangerous?
Learn the two lethal progressions and their signatures. HACE (brain swelling): staggering or inability to walk heel-to-toe in a straight line (the field test), confusion, drowsiness, bizarre or drunk-like behavior, vomiting. HAPE (lung fluid): breathlessness at rest, not just walking, a cough that worsens (pink or frothy sputum is late and grave), chest tightness, extreme weakness, blue lips. Either one is a descend-now emergency: at least 500-1,000 meters down, immediately, night or storm notwithstanding. Waiting for morning has killed people who could have walked down in an hour.
When is it safe to go back up after feeling ill?
The rule is full resolution at rest: you can resume ascent when you have been completely symptom-free at your current altitude (no headache, no nausea, normal energy, normal sleep) for a day, and you then resume slowly, honoring the 300-500 meter sleeping-altitude rule. Symptoms that persist at the same altitude beyond 24-48 hours of rest are an argument to descend, not to wait longer. Anyone who had confusion, ataxia, or breathlessness at rest descends and does not reascend that trip: those are the signatures of the forms that kill, and the mountain will still be there next season.