Altitude Sickness: What Every Trekker Needs to Know

At 5,000 meters, atmospheric pressure is roughly 53% of sea-level pressure. The percentage of oxygen in the air is exactly the same — 20.9% — but each breath delivers half as many molecules. Your body registers this from the very first hour. The only question is how wisely you allow it to adapt.

This text is not a substitute for a medical consultation before your trip. Think of it as a map of the terrain: what altitude sickness is, what makes it dangerous, how it is prevented, and when you need to stop climbing — before your body makes that decision for you.

What Happens to the Body Above 2,500 Meters

Acute Mountain Sickness (AMS) is a cluster of symptoms that develop in people who ascend to an altitude they are not acclimatized to. The conventional lower threshold is 2,500 m above sea level. At 3,500 m, symptoms occur in about 25% of people; at 4,500 m, roughly 50%; at 5,000 m and above, in the majority of those without prior high-altitude experience.

The physiology is straightforward: as atmospheric pressure drops, the partial pressure of oxygen in the alveoli falls. The body responds — breathing rate increases, heart rate rises, and after several days the production of red blood cells goes up. At the same time, capillary permeability increases, and in some cases fluid begins to leak into lung or brain tissue. This is precisely what turns a "headache and nausea" into HAPE and HACE — conditions where survival is measured in hours.

AMS / HAPE / HACE — Three Stages You Need to Tell Apart

AMS (Acute Mountain Sickness). Develops 6–24 hours after gaining altitude. The canonical diagnostic tool is the Lake Louise score: headache plus at least one additional symptom (nausea, fatigue, dizziness, poor sleep). A mild form is common and no cause for panic. Moderate AMS is reason to stop and not go higher. Severe AMS is reason to descend.

HAPE (High Altitude Pulmonary Edema). Fluid accumulates in lung tissue. Key signs: breathlessness at rest (not just "after exertion"), a dry cough that becomes productive, sometimes pink-tinged sputum, bluish discoloration of the lips. According to the Wilderness Medical Society, untreated mortality reaches 50%. The response is descent of at least 1,000 m and, if available, oxygen therapy.

HACE (High Altitude Cerebral Edema). Fluid accumulates in the brain. The most telling sign is ataxia: the person cannot walk heel-to-toe in a straight line. Confusion, an excruciating headache unresponsive to analgesics, vomiting. This is a critical emergency. Descend immediately — on foot, on a horse, on a yak, on someone's back. Dexamethasone 8 mg orally or intramuscularly as a bridge until descent. Without intervention: coma and death within hours.

Both HAPE and HACE are rare — about 0.5–2% of visitors above 4,500 m, according to expedition registries. But "rare" does not mean "unlikely in my particular case."

Prevention — the Only Strategy That Works

Wilderness and expedition medicine has established a set of practices with a genuine evidence base:

Rate of ascent. Above 3,000 m — no more than 300–500 meters of sleeping altitude per day. "Climb high, sleep low": go higher during the day, sleep lower. Every 1,000 meters above 3,000 m — take a rest day for acclimatization. This is not being overly cautious; it is the median rate at which statistics of trip failures stop climbing.

Hydration. At altitude, breathing is rapid and dry; fluid losses run 3–5 liters per day. Drink until your urine is pale yellow. Alcohol and strong sleep aids — no: they suppress ventilation during the night.

Diamox (acetazolamide). The only drug with confirmed efficacy for AMS prevention (Cochrane review 2017). Standard regimen: 125 mg morning and evening, starting 24 hours before ascending. Prescribed by a doctor, not a guide or a blog. Side effects — frequent urination and tingling in the fingers — are a normal pharmacological response, not an allergy.

A basic medical check-up 4–6 weeks before departure. ECG, full blood count and metabolic panel, discussion of chronic conditions with a GP who is explicitly told the planned altitude. This is not a formality: uncontrolled hypertension, ischemic heart disease, and severe asthma are genuine contraindications for altitudes above 4,500 m.

Fitness helps, but does not protect. Physical condition eases the first days and reduces background fatigue, but it does not predict how your individual body will respond to hypoxia. AMS in elite athletes is routine; easy acclimatization in ordinary people is equally common. The genetics of acclimatization are individual, and the only way to learn yours is to test yourself at 3,000 m before attempting 5,000 m.

When You Need to Descend

The line between "normal background discomfort" and "time to stop" is practical, not philosophical:

• Headache that does not resolve after ibuprofen and an hour of rest, plus at least one Lake Louise symptom — stop, do not go higher.

• Breathlessness at rest (you are lying down and struggling to breathe) — descend.

• Unsteady gait, confusion, vomiting — descend immediately; do not wait until morning.

• Pulse oximeter SpO2 reading below 75% at a sleeping altitude above 4,500 m — grounds for evacuation.

Reaching a pass, base camp, or the high point of your route is not worth a stroke, pulmonary edema, or a helicopter bill in Lhasa. This is not a catchphrase for a travel blog; it is a foundational principle of wilderness medicine, one on which the UIAA Medical Commission and the Wilderness Medical Society agree.

What the Guide Does, and What You Do

A good high-altitude guide conducts daily monitoring: resting saturation, pulse rate, a quick status check over breakfast conversation. The expedition first-aid kit holds dexamethasone, nifedipine (for HAPE), and oxygen for serious cases. The guide makes decisions about pace and who continues to ascend.

But self-monitoring is your responsibility. A guide is watching a group of ten people; only you know your own body. If something feels wrong — say so. Nobody writes in a review "the guide made me descend a day early." Everyone writes "thank you for stopping me in time."

And get insurance that explicitly covers mountain trekking up to the planned altitude of your route, including helicopter evacuation. Standard travel policies do not cover this; read the fine print and get a specialist policy. Evacuation from western Tibet without insurance is a five-figure sum in cash dollars, on the spot.

A Realistic Picture

Nine out of ten trekkers who follow a proper acclimatization schedule on routes like Everest, Annapurna, or Kailash reach the high point without serious trouble. Most failures happen not at the pass itself but in the days before it, and they are almost always linked to the pace of ascent or to ignoring early warning signs.

Altitude is not an adversary. It is an environment with its own rules. Understand and respect those rules, and hypoxia transforms from a threat into a background condition your body adapts to over 7–10 days. Ignore them — and your body will remind you, and the form that reminder takes can be very unforgiving.