What altitude actually does to you
The air at 5,000 m has the same 21% oxygen as the air at sea level. What changes is pressure — there's simply less of everything in each breath. At Everest Base Camp you're taking in roughly half the oxygen you would at home, and at Kala Patthar a little under that.
Your body responds by breathing faster, raising your heart rate, and — over days, not hours — producing more red blood cells. That last part is the one that matters, and it's the part you cannot rush. Acclimatisation is a waiting game, and the schedule is set by your bone marrow, not your fitness.
Being fit does not protect you from altitude sickness. Strong walkers are arguably at higher risk, because they climb faster than their acclimatisation can keep up with. The trail rewards patience over power.
How acclimatisation days work
An acclimatisation day is not a rest day. You'll usually walk several hundred metres higher than your lodge, spend an hour or two up there, then come back down to sleep. The principle is climb high, sleep low — you expose your body to thinner air while it's alert, then recover at a height it already tolerates.
On a classic Everest Base Camp itinerary there are two of these built in:
- Namche Bazaar, 3,440 m — typically a walk up to the Everest View Hotel at 3,880 m, or to Khumjung village, then back down.
- Dingboche, 4,410 m — usually Nangkartshang Peak at around 5,083 m, which is higher than Base Camp itself and does most of the preparatory work.
Skipping either to save time is the most common cause of a trip ending early. The days feel unproductive. They're the reason the last week works.
The rule of thumb for ascent
Above 3,000 m, the widely used guideline is to raise your sleeping altitude by no more than 300–500 m per night, with a full acclimatisation day every 3–4 days or roughly every 1,000 m gained.
You can walk considerably higher than that during the day — what your body reacts to is the altitude you sleep at. That's the whole logic behind climb-high-sleep-low.
| Stage | Sleeping altitude | Gain on the night before |
|---|---|---|
| Lukla | 2,860 m | — |
| Namche Bazaar | 3,440 m | +830 m, then a rest day |
| Tengboche | 3,867 m | +427 m |
| Dingboche | 4,410 m | +543 m, then a rest day |
| Lobuche | 4,940 m | +530 m |
| Gorak Shep | 5,164 m | +224 m |
Symptoms, and when to worry
Mild acute mountain sickness (AMS) is common and not in itself dangerous. Most people feel some of it above 3,500 m.
Normal, and usually fine: headache, poor sleep, odd vivid dreams, breathlessness on exertion, reduced appetite, mild nausea. The standard response is to stop ascending, drink, rest, and see whether it settles overnight.
Stop and descend: a headache that painkillers don't touch, vomiting, breathlessness at rest, a persistent wet cough, confusion, or loss of coordination — if someone can't walk a straight line heel-to-toe, that is not something to sleep on.
Descent. Not tea, not rest, not oxygen, not paracetamol. Those help symptoms; going down fixes the cause. Losing 500–1,000 m usually produces dramatic improvement within hours, and no reputable guide will ever make you feel awkward about asking for it.
What your guide should be doing
On any guided trek worth paying for, someone is monitoring the group daily rather than waiting for a person to speak up. That means:
- Morning and evening pulse oximeter readings, recorded so trends are visible — a single number means little, a falling trend means a lot.
- Watching appetite and sleep, which drop before people admit to feeling unwell.
- Willingness to change the itinerary. A guide who treats the schedule as fixed is the wrong guide.
- An evacuation plan, insurance details, and knowledge of where the nearest help is — in the Khumbu that usually means the Himalayan Rescue Association posts at Pheriche or Manang.
Ask about this before you book. The answer tells you a great deal about an operator.
Practical things that help
- Walk slower than feels natural. If you can't hold a conversation, you're going too fast. Sherpa guides walk at a pace that feels almost comical on day one and exactly right on day eight.
- Drink more than you want to. Three to four litres a day. Dry air and heavy breathing dehydrate you far faster than at home, and dehydration mimics AMS symptoms.
- Eat, even without appetite. Appetite disappears at altitude, but the body needs carbohydrate. Dal bhat exists for a reason and comes with refills.
- Skip alcohol and sleeping pills above 3,000 m — both suppress breathing at exactly the wrong time.
- Discuss acetazolamide (Diamox) with a doctor before you travel. It genuinely helps some people acclimatise, has real side effects, and is a medical decision — not something to sort out in a lodge at 4,400 m.
This is general background from operating experience, not a substitute for a doctor. Speak to a travel-health professional before you go, especially about medication or if you have any heart or lung condition.
