I was born at 3,440 meters. I have spent my entire life in the thin air of the Khumbu. Which means I am a bad person to ask about altitude sickness in the way that a fish is a bad person to ask about water.
But I have watched it happen to thousands of trekkers. I understand it the way a mechanic understands an engine — not from the inside, but from watching it fail.
The Chemistry
At sea level, your blood is saturated with oxygen to about 98–99%. Your body and your hemoglobin have an agreement: the lungs pick up oxygen, the blood carries it, the muscles use it, everyone is happy.
At 4,000 meters, the atmospheric pressure has dropped enough that your oxygen saturation typically falls to 85–90%. Your body notices immediately. The response is involuntary: breathing rate increases, heart rate increases, blood thickens slightly as plasma volume drops.
These are adaptations. They are helpful. But they come with a cost: headache, fatigue, disturbed sleep, nausea. What people call acute mountain sickness is simply the body’s adaptation process — uncomfortable but, in mild form, not dangerous.
What Goes Wrong
The danger zone is when adaptation cannot keep pace with ascent rate.
High Altitude Pulmonary Edema (HAPE) and High Altitude Cerebral Edema (HACE) are the serious conditions. In HAPE, fluid leaks into the lungs — the trekker becomes breathless at rest, develops a wet cough, appears cyanotic. In HACE, fluid accumulates in the brain — headache becomes severe, coordination fails, rational thought becomes difficult.
Both are emergencies. Both require immediate descent.
In eighteen years of guiding, I have initiated three emergency descents. All three clients survived without lasting effect. The reason is simple: we acted the moment the signs appeared, not after waiting to see if they resolved.
Acclimatization Is Not Optional
The rule is simple: climb high, sleep low.
From Lukla to Base Camp is a vertical gain of approximately 2,500 meters over 10–12 days. That pace, with built-in acclimatization days at Namche and Dingboche, allows most healthy adults to adapt safely.
The trekkers who get into trouble are almost always the ones who pushed on when they should have rested. The ones who felt fine at lunch and pushed to the next teahouse rather than stopping. The ones who confused determination with good judgment.
The mountain does not reward hurry.
Signs to Know
Mild AMS: Headache, fatigue, loss of appetite, slight nausea. Monitor. Do not ascend. Rest. Hydrate.
Moderate AMS: Persistent headache unresponsive to ibuprofen, vomiting, increasing weakness, difficulty sleeping even at rest. Descend at least 300–500 meters. Do not proceed higher until fully resolved.
Severe AMS / HAPE / HACE: Any of the following — breathlessness at rest, ataxia (stumbling gait), confusion, altered mental status. This is an emergency. Descend immediately. Use supplemental oxygen if available. Administer dexamethasone. Evacuate.
The Pulse Oximeter
Every OVERLAND expedition carries one. It costs thirty dollars and weighs nothing. Clip it to your finger and in ten seconds you know your oxygen saturation and heart rate.
Below 70% saturation is dangerous territory regardless of symptoms. Below 60% at altitude is a medical emergency.
It will not make the decision for you. That is still your guide’s job. But it gives you data instead of guesswork.
A Final Note
I have seen people turn around at Namche who had trained for two years. I have seen people reach Base Camp who seemed, at the start, like the least likely candidates.
Altitude does not care about your fitness level, your VO2 max, or how many marathons you have run. It asks a different question of your biology — one you cannot entirely predict the answer to in advance.
What you can control is your pace, your vigilance, and your willingness to listen to the people who have been here before.
Rajan Thapa is OVERLAND’s lead Himalayan guide. He was born in Namche Bazaar and has summited Island Peak fourteen times.