Altitude sickness is the most common response of the human body to hypoxia at high altitude. It affects nearly one in four climbers who ascend rapidly above 2,500 metres, often catching physically fit individuals off guard, and can evolve into a medical emergency if ignored. Yet it is almost always preventable.
What is Acute Mountain Sickness (AMS)
Acute Mountain Sickness (AMS) is a syndrome caused by rapid exposure to high altitude without adequate acclimatization. The underlying mechanism is straightforward: as you gain altitude, atmospheric pressure drops, and with it the partial pressure of oxygen. The lungs receive less "push" to transfer oxygen to the blood, and the brain — the tissue most sensitive to hypoxia — responds with a recognizable set of symptoms.
AMS is not a sign of physical weakness or poor preparation. Individual susceptibility depends largely on genetics, and even elite climbers with decades of experience can develop it.
At Oxymeter you can calculate in real time the percentage of oxygen available at your current altitude and assess your physiological risk level before any ascent.
Symptoms: how to recognize altitude sickness
AMS symptoms typically appear 6–12 hours after arriving at a new altitude, often during the night or in the morning. The cardinal symptom — a necessary condition for diagnosis — is headache: a dull, throbbing pain that tends to worsen when lying down and does not fully respond to common analgesics.
Additional symptoms, in variable combination:
- Nausea or vomiting (more frequent in moderate-severe forms)
- Marked fatigue — exhaustion disproportionate to the effort
- Dizziness and instability
- Insomnia with frequent awakenings, often caused by periodic Cheyne-Stokes breathing
- Anorexia — loss of appetite, common in the first hours
The Lake Louise Score
Field diagnosis uses the Lake Louise Score (2018 version), a scoring system that assigns 0 to 3 points to each main symptom. A total score ≥ 3 with headache present indicates AMS; scores ≥ 5 identify moderate-severe forms requiring immediate action.
Risk factors
Not everyone is equally vulnerable. The main risk factors are:
- Personal history of AMS: the most reliable predictor. Those who have suffered altitude sickness at the same elevation are highly likely to experience it again
- Rate of ascent: the most controllable factor. Exceeding 300–500 m of sleeping altitude gain per day above 2,500 m significantly increases risk
- Starting altitude: those already at elevation can ascend faster than those arriving from sea level
- Young age: counterintuitively, children and young adults are more susceptible than older individuals
- Physical exertion in the first 24 hours: intense activity in the hours following arrival at a new altitude increases oxygen consumption and worsens hypoxia
Physical fitness, contrary to popular belief, does not protect against altitude sickness. A well-trained athlete consumes oxygen faster during exertion, but this does not improve the adaptive response to chronic hypoxia.
Prevention: the rules that work
1. Gradual ascent
The golden rule: do not exceed 300–500 metres of sleeping altitude gain per day above 2,500 m. For every 1,000 m of elevation gained, include a rest day at the same altitude.
The "climb high, sleep low" principle — ascending to higher elevations during the day and returning to sleep lower — is one of the most effective methods to stimulate acclimatization without increasing overnight risk.
You can verify oxygen availability at your target altitude with the Oxymeter calculator before planning your ascent.
2. Adequate hydration
At altitude, the body loses fluids more rapidly (hyperventilation + dry air + increased urine output). Drinking 3–4 litres of water per day is the minimum above 3,000 m. Urine colour is the best indicator: pale yellow = optimal hydration.
3. Avoid alcohol and sedatives
Alcohol and sleeping pills suppress the hypoxic ventilatory response — the reflex that drives faster breathing in response to falling oxygen. Avoid them in the first 48 hours at any new altitude.
4. Prophylactic acetazolamide
Acetazolamide (Diamox), taken on medical prescription, significantly reduces the risk of AMS. The mechanism: it stimulates renal bicarbonate excretion, lowering blood pH and stimulating ventilation. Standard prophylactic dose: 125–250 mg twice daily, starting 24 hours before ascent.
Medical note. Acetazolamide requires a medical prescription. Consult a doctor before any high-altitude expedition.
Treatment: what to do when symptoms appear
Mild forms (Lake Louise 3–4)
- Stay at the same altitude — do not ascend further until symptoms resolve
- Rest for the first 12–24 hours
- Analgesics for headache (ibuprofen 400 mg or paracetamol 1000 mg)
- Plenty of fluids
- Monitor progression: if symptoms improve within 24 hours, a slow resumption of ascent may be considered
Moderate-severe forms (Lake Louise ≥ 5)
- Descend immediately by at least 500–1,000 m — this is the single most effective intervention
- Acetazolamide 250 mg twice daily accelerates recovery
- Supplemental oxygen (2–4 l/min) if available
- Portable hyperbaric chamber (Gamow bag) as a bridge to descent
The warning sign you must not ignore
If the headache worsens despite rest, or if confusion, difficulty walking in a straight line, or breathlessness at rest appear, we are facing evolution toward HAPE (pulmonary oedema) or HACE (cerebral oedema). In both cases: immediate descent + emergency services (112 in Europe, 911 in North America).
Read more: HACE and HAPE — life-threatening altitude emergencies | All guides on altitude health
Frequently Asked Questions
Is altitude sickness dangerous?
In mild forms, altitude sickness is unpleasant but not dangerous, and resolves spontaneously with rest at the same altitude. It becomes potentially fatal only if ignored and allowed to progress to HAPE (High Altitude Pulmonary Edema) or HACE (High Altitude Cerebral Edema). The key is recognizing symptoms early and not ascending further until they resolve.
Can I prevent altitude sickness with medication?
Acetazolamide (Diamox), taken on medical prescription 24 hours before ascent, significantly reduces the risk. It is not a substitute for gradual ascent: even with pharmacological prophylaxis, respecting acclimatization schedules remains essential.
Who is most at risk for altitude sickness?
Those who have previously experienced AMS, those who ascend too quickly, and — counterintuitively — young adults. Physical fitness does not protect: susceptibility depends primarily on individual genetics.
