Altitude is the one part of a Himalayan trek that does not care how fit you are. A marathon runner and someone who trains twice a week face the same physiology above 3,000 m, and the runner is at slightly greater risk, because they can climb faster than their body can adjust. Understanding what is actually happening is the difference between a trek that works and one that ends early.
What thin air actually does
The proportion of oxygen in the air does not change with altitude. It is 21% at sea level and 21% on the summit of Everest. What changes is pressure. At Namche Bazaar, 3,440 m, each breath contains about two-thirds the oxygen of the same breath in Kathmandu. At Everest Base Camp, 5,364 m, it is about half.
Your body responds within hours. You breathe faster and deeper. Your heart rate rises. Over days, your kidneys excrete bicarbonate to let your blood chemistry tolerate that faster breathing, and your bone marrow begins producing more red blood cells — a process that takes weeks, not days, and is not what gets you up the valley. The acclimatisation that matters on a two-week trek is the fast part: breathing and blood chemistry.
That process needs time, and time is the only input. There is no fitness level, no supplement and no breathing technique that substitutes for it.
The rule that governs every itinerary
Above 3,000 m, gain no more than 300 to 500 m of sleeping altitude per day, and take a rest day every 3 to 4 days or every 1,000 m, whichever comes first.
The word doing the work is sleeping. What you do during the day matters far less than the altitude at which you spend the night, which is why every well-built itinerary follows a climb-high, sleep-low profile: walk up to a viewpoint several hundred metres above the village, then come back down to sleep. The climb stimulates the adjustment; the descent lets you recover while it happens.
This is why the acclimatisation days in an itinerary are not rest days in the ordinary sense. On the Everest Base Camp trail, the day at Namche usually means walking up towards the Everest View Hotel or Khumjung and returning. The day at Dingboche means climbing Nangkartshang. You are not resting. You are giving your body a reason to adapt and then a chance to do it.
Recognising altitude sickness
Acute mountain sickness — AMS — is common, uncomfortable and usually manageable. Its cardinal symptom is headache, plus at least one of: nausea or loss of appetite, fatigue beyond what the day's walking explains, dizziness, or poor sleep.
The honest difficulty is that this describes a bad night's sleep at any altitude. The distinction that matters is trend. AMS that improves with a rest day and fluids is ordinary. AMS that worsens, or does not improve after a full day at the same altitude, is a reason to descend.
Two conditions are not manageable and are emergencies.
HACE, high-altitude cerebral oedema, is fluid on the brain. The sign guides watch for is ataxia — loss of coordination. Someone who cannot walk a straight line heel-to-toe, who is confused, unusually drowsy or behaving out of character, is a medical emergency regardless of what they say about how they feel.
HAPE, high-altitude pulmonary oedema, is fluid in the lungs. Breathlessness at rest, a persistent dry cough that turns wet, a crackling sound when breathing, and a blue tinge to the lips or fingernails. HAPE can develop in someone with no headache at all, which is why it catches people out.
For both, the treatment is the same and it is not optional: descend, immediately, and keep descending. Five hundred metres often produces dramatic improvement. Oxygen and drugs buy time; they do not replace descent.
What a pulse oximeter tells you, and what it does not
Guides carry pulse oximeters and check the group morning and evening. The device clips on a finger and reports blood oxygen saturation. At Namche, a typical reading might be 88 to 92%. Higher up, low 80s is not unusual.
What matters is not the absolute number but its behaviour. A reading that drops sharply overnight, or one that sits well below the rest of the group at the same altitude, is worth attention. A single low reading in a cold-fingered trekker who has just walked in from outside means very little.
An oximeter is one input alongside how someone looks, what they are eating and how they slept. It has never overruled a guide's judgement and it should not overrule yours about your own body.
Diamox, and the honest version
Acetazolamide — Diamox — speeds the blood-chemistry half of acclimatisation. It is genuinely effective as prevention, typically at 125 mg twice daily starting a day before you go high.
Three things worth knowing. It is a diuretic, so it increases how much you drink and how often you get up at night. It commonly causes tingling in the fingers and toes, and makes carbonated drinks taste strange. And it is a sulfonamide, so it is not for everyone.
Get it from a doctor before you travel, not from a pharmacy in Thamel on the strength of a stranger's recommendation. And treat it as what it is — a help with adjustment, never a licence to climb faster than the schedule.
Practical things that genuinely help
Drink more than you want to. Three to four litres a day. Dehydration mimics AMS closely enough to be mistaken for it, and thin dry air takes more water out of you through breathing alone than most people expect.
Eat, even when you do not feel like it. Appetite drops at altitude exactly when your body needs more fuel. Dal bhat exists for a reason.
Walk slowly. Slower than feels natural. If you cannot hold a conversation, you are going too fast — and the fitter you are, the more deliberately you will have to hold yourself back.
Skip the alcohol until you are heading down. It disrupts sleep and breathing at exactly the moment you need both working properly.
What to say, and when
Tell your guide the moment something feels wrong. Not at dinner, not tomorrow morning.
The most dangerous pattern on any trail is a trekker who does not want to slow the group down. Groups are not slowed by someone who says something early. They are slowed — or ended — by an evacuation that could have been a rest day. Every guide we send out would rather lose an afternoon than a trekker, and no pass and no base camp is worth pushing an unwell person through.