Altitude sickness and acclimatization on the Everest Base Camp trek

The air at the top of this trek holds the same share of oxygen as the air outside your house. There is simply less of it. The higher you go, the less air is packed into the same space, so every breath you take delivers less oxygen into your blood than the identical breath does at home.

Your body notices this immediately and starts changing the way it works. The changes come in stages and the stages run at very different speeds. Within minutes you breathe harder and deeper without deciding to. Over the next day or two your body adjusts the chemistry of your blood so that all that extra breathing does not push everything else out of balance. Over weeks, which is longer than this trek, it builds more of the cells that carry oxygen around.

Acclimatization is the middle one. It is not fitness, it is not willpower and it is not something you can do at home in advance. It is a chemical adjustment that takes days, and the only thing that buys it is time spent at a height before you go higher. That is the whole idea, and every sensible decision on this trek follows from it.

Altitude sickness is what you feel when you have gone above what your body has adjusted to. It is not an infection and it is not bad luck. It is the gap between how high you are and how high your body is ready for, and the way to close that gap is to stop climbing or to go down.

The heights involved are real. You fly into Lukla at 2,860 m (9,383 ft), sleep at Namche Bazaar at 3,440 m (11,286 ft), sleep at Dingboche at 4,410 m (14,469 ft), sleep at Lobuche at 4,910 m (16,109 ft), and walk out to the Everest Base Camp trek’s end point at 5,364 m (17,598 ft). Every one of those heights asks something of you that the one before it did not. The full set of route heights, and why the air at them behaves the way it does, is on the page about how high Everest Base Camp is.

This is the single most important sentence on this website, so here it is in full, with nothing softened. Altitude sickness is caused by climbing higher than your body has adjusted to, and how fit you are does not protect you from it. A marathon runner and a person who walks the dog twice a day are at the same risk on the same schedule. Fitness changes how much you enjoy the walking. It does not change how your body handles thin air.

We are not saying that to be dramatic. We are saying it because the belief that fitness is protection is the belief that gets people into trouble on this route, and because we have watched it happen. The strongest people in a group are often the ones who struggle at height, and there is a plain reason for it. Strong walkers climb faster. They get ahead. They arrive first, they have energy left, and they go for an extra walk above the village in the afternoon. Every one of those things is a bigger height gain in a shorter time, which is exactly the thing that causes the problem.

There is a second reason, and it is about character rather than lungs. People who are proud of being fit are the least likely to admit they feel bad. They have spent years pushing through discomfort and being rewarded for it. On this trek that instinct is backwards, because the discomfort you are pushing through is information.

So if you take one thing from this page, take this. Nothing you do in the gym buys you altitude tolerance. What buys it is a schedule that goes up slowly enough, and the honesty to say when it is not working for you.

None of which means fitness is pointless. Being fit makes a long day on the climb to Lobuche, 4,910 m (16,109 ft), ordinary instead of grim, and being unfit makes every problem harder to deal with. It is simply not the thing that protects you from altitude. How hard the walking actually is, in hours and consecutive days, is on the page about how difficult the Everest Base Camp trek really is, and what to do about it before you leave home is on the training plan for this trek.

Mild altitude sickness feels like a bad hangover that you did not earn. A headache that will not shift. No appetite, even when the food is good and you have walked all day. Feeling sick. Feeling dizzy when you stand up. Being exhausted in a way that a normal hard day does not explain. Lying awake all night in a warm bed. Most people on this route get at least one of those at some point, and most of the time it means nothing more than that your body is working.

Here is the part that matters. Those feelings are not the danger. The danger is what people do with them, which is to keep quiet and keep walking.

We see the same five sentences every season. I am just tired. It is only a headache. I did not sleep well. I do not want to hold everybody up. I paid a lot of money for this. Every one of those is a person deciding, alone, at altitude, that they are fine. And the organ making that decision is the organ the altitude is affecting, which is why people at height are genuinely poor judges of their own condition. Somebody who is not coping usually thinks they are coping. The person walking next to them can often see it first.

The reason we push so hard on this is arithmetic, not drama. Somebody who says on the first afternoon that their head hurts has a whole set of easy options. Stay where you are for a day. Walk down to the last village and come back up. Change the plan slightly. Nobody misses anything. Somebody who says nothing for two days and then cannot walk at Lobuche, 4,910 m (16,109 ft), has almost no easy options left, and it is dark, and it is cold, and the road is a very long way away.

So the rule we ask of every person who walks with us is simple and it is not about medicine. Say it out loud. Say it early. Say it even if you think it is nothing. You are not being difficult and you are not spoiling anything. You are giving your guide the one thing that lets them fix a small problem while it is still small.

One more thing worth knowing before you go. There is no rule that says a person who felt terrible at Namche Bazaar, 3,440 m (11,286 ft), will feel terrible at Dingboche, 4,410 m (14,469 ft), and no rule that says a person who sailed through the first week is safe in the second. It varies between people and it varies in the same person between trips. Somebody who has been to altitude before and was fine has learned something useful about themselves, but they have not bought a guarantee.

Most altitude sickness stays mild. A small number of cases do not, and there are two severe forms that everybody walking this route should be able to recognize by name. We are describing them so that you know what you are looking at and tell somebody immediately. We are deliberately not giving you a list of criteria to score yourself against, because that is a clinician’s job and not ours.

High altitude pulmonary edema, which affects the lungs

Pulmonary means lungs. Edema means fluid where fluid should not be. This is fluid collecting in the lungs at altitude, and the reason it matters is that it makes breathing worse at exactly the height where breathing is already hard. The picture people describe is being far more out of breath than the effort explains, being out of breath even while resting or lying down, a cough that will not stop, and a chest that feels tight or wet. Somebody with it often cannot keep up on ground they walked easily the day before.

It is often noticed by other people before the person themselves says anything, because being breathless at altitude feels normal enough to explain away.

High altitude cerebral edema, which affects the brain

Cerebral means brain. This is swelling in the brain at altitude, and it is the more frightening of the two because it changes the person rather than just how they feel. The signs people notice from the outside are the important ones. Somebody walking like they are drunk, unsteady on flat ground, unable to walk a straight line. Somebody confused, saying things that do not make sense, unable to answer a simple question. Somebody who has become oddly sleepy and hard to rouse, or who has stopped behaving like themselves.

That last one is why we ask you to watch the people you are walking with rather than only yourself. A person whose brain is affected by altitude cannot reliably judge that their brain is affected by altitude. Somebody else has to notice.

One practical note that costs nothing. Learn these two names before you leave home, and learn what they look like from the outside. On the trail you may be the person who notices. A trekker who can say “he is walking like he is drunk and he could not tell me what day it is” has given a guide the single most useful sentence there is, and has given it in time for it to matter.

There is one thing that reliably reverses altitude sickness and it is not complicated, not expensive and not carried in anybody’s bag. It is losing height. Go down and the air gets thicker, every breath starts delivering more oxygen again, and the problem that the height caused starts to unwind.

What surprises people is how little you have to lose. You do not have to go back to Kathmandu and you usually do not have to abandon the trek. Dropping a few hundred meters, which on this route often means walking back down to the village you slept in two nights ago, is enough to make a real difference to how somebody feels. That is why the shape of this valley matters so much: for most of the route, down is a walk rather than a rescue.

The other thing worth understanding is direction of travel. Staying put at the same height is not the same as going down. Resting for a few hours and then carrying on up is not the same as going down. If somebody is getting worse, the only move that changes the cause is losing height, and the longer that decision is put off the fewer choices there are about how it happens.

This is also the reason we talk about it before anybody is ill. Deciding to go down is easy in a warm room in January and hard at four in the afternoon at 4,910 m (16,109 ft) with base camp one day away. Agreeing in advance that it is the normal, expected, no argument answer takes the decision away from the moment when it is hardest to make.

What happens after that decision, how an evacuation off this route actually works, and who is involved is on the page about safety and risk on the Everest Base Camp trek. Whether your policy will pay for it is a different question and it has a real answer, on the page about travel insurance for this trek. Read that one before you book. Most standard policies stop below the height you are going to.

The main defense against altitude sickness on this trek is not equipment and not medicine. It is the shape of the schedule. Our itinerary has two days built into it that exist for one purpose, which is to give your body the time the middle stage of acclimatization needs. One is at Namche Bazaar, 3,440 m (11,286 ft). The other is at Dingboche, 4,410 m (14,469 ft).

They look like days off on a schedule and they are not days off. On both of them you get up, walk several hundred meters uphill, spend some time up there, and then walk back down and sleep in the same bed you slept in the night before. The name for that is climb high, sleep low.

The logic is worth saying plainly, because once you see it you will judge every itinerary you are offered by it. Going up during the day is the signal to your body that it needs to adapt. Coming back down to sleep is what lets it do the adapting somewhere it can still rest and eat and recover. You take the stress and then you take it away. The height of your bed is what your body spends the most hours at, which is why the rule is about where you sleep and not about how high you walked.

Take those two days out and something specific happens. The height of your bed rises every single night, from the night you leave Lukla to the highest night of the trek. Your body never once gets a night at a height it has already seen. The adjustment that needs a day or two never gets its day or two. You then arrive at the highest, coldest night of the walk with the least adaptation behind you, which is exactly the wrong way round.

There is a second thing those days buy, and it is not physiological. They are when a guide finds out who is in trouble. A person who is quietly not coping looks identical to a person who is tired, right up until you watch them on a day when nothing is being asked of them. Cut the day and the first honest information anybody gets about that person arrives higher up, where the options are much worse.

Which day each of those falls on, what you walk up to on each of them, and where you sleep every night of the route is on the day by day Everest Base Camp itinerary. We will not sell this trek with either acclimatization day removed. If your dates do not fit them, the honest answer is a different trip rather than a compressed version of this one.

Altitude sickness is the one everybody asks about, and it is not the only thing that makes people miserable on this route. These six are the ones we see most. None of them is altitude sickness, all of them make altitude harder to handle, and several of them are easy to prevent with things you already own.

Dehydration, which is more common than people expect

The air up here is extremely dry and very cold, and you lose a lot of water just by breathing it. You also lose it by sweating under a pack on a climb and by needing the bathroom more than usual at height. Meanwhile the thing that normally tells you to drink, being thirsty, is unreliable when you are cold. So people quietly get behind on water without noticing. It makes you feel headachy, tired and weak, which is unpleasant on its own and also looks a lot like the early signs of altitude sickness, which makes everybody’s job harder. Our guides push water at every stop for exactly that reason. We publish no daily figure in liters, and the reason why is further down this page.

The Khumbu cough, which almost everybody gets a version of

It is named after this valley because this valley produces it. Air that is cold, dry and thin, breathed hard, for days, dries out and irritates your airways. The result is a dry, hacking cough that arrives in the second week for most people and can be bad enough to keep you and your roommate awake. It is not an infection and it is usually not serious, and it goes away after you come down. It is worth knowing about in advance for one reason: a cough at altitude frightens people who have read about the lung condition above, and the two are not the same thing. If you are coughing and also unusually breathless at rest, that is the moment to tell your guide rather than to look it up.

Sunburn, which is far worse up here than the temperature suggests

There is less atmosphere above you to absorb ultraviolet light, and above the treeline there is snow and pale rock throwing it back up at you. People get burned on days when they were cold the whole time, which is why they do not see it coming. The places that catch people out are the ones facing upward: the underside of the nose, the ears, the lips, and the roof of the mouth of anybody walking uphill with their mouth open. Cover up and reapply more often than feels reasonable. What to bring is on the packing list for this trek.

Snow blindness, which is sunburn of the eyes

The same reflected light does the same thing to the surface of your eyes. It is painful, it usually arrives hours after the exposure rather than at the time, and it is entirely preventable by wearing proper sunglasses that block ultraviolet light and cover the sides. Take a spare pair. A lost pair of sunglasses on a bright day above Dingboche, 4,410 m (14,469 ft), is a genuine problem rather than an inconvenience.

Blisters, which ruin more treks than altitude does

This is the least dramatic thing on this page and it is the one that most often changes somebody’s trip. You are walking many consecutive days, a lot of it downhill on stone, in boots that are carrying more weight than usual. New boots are the usual cause. Wet socks are the second. The fix is boring and it works: walk your boots in properly at home before you come, and tell your guide the moment something feels hot rather than waiting until it is a wound. How to build up to consecutive days is on the training plan.

Stomach upsets, which are not altitude but make altitude harder

Stomach trouble is common on any trip like this and it comes from food and water rather than from height. We mention it on a page about altitude for one reason. If you cannot keep food or water down, you are dehydrated and underfed at the exact moment your body needs both to acclimatize, so a stomach problem at height is a bigger deal than the same problem at home. It also overlaps with the early signs of altitude sickness closely enough that the two get confused. Tell your guide what is happening rather than deciding on your own which one it is.

You will read, on more than one website about this trek, that there is no medical help on this route. That is wrong, and it is the kind of wrong that stops somebody asking for help when they need it. Two of the places on this route exist specifically for the thing this page is about, and both are run by the Himalayan Rescue Association.

The aid post at Pheriche, 4,250 m (13,944 ft). The association has run it since 1973.1 It sits on the main trail to base camp, so most people walking this route pass it, and it is staffed by doctors who see altitude illness every day of the season. It is seasonal rather than year round, so it is open in the trekking seasons and closed between them.

The post at Machhermo, 4,470 m (14,665 ft). A seasonal aid post and porter shelter. Its opening year is not published by the association.1 It sits on the Gokyo side of the region rather than on the classic route to base camp, so you pass it only if your trek crosses over there.

The free altitude talk, and why we send people to it

The Himalayan Rescue Association runs a free altitude illness talk at its aid posts, in its own published words: Daily at 3:00 pm at its aid posts during the trekking season12. It is given by the doctors who staff the post. It is the best hour available to you on this trek and it costs nothing.

We tell our groups to go, and we would tell you to go even if you were walking with somebody else. It is the one place on this route where you can ask an altitude question of a person who is actually qualified to answer it, which is more than this page can offer you. If you are walking independently it matters even more, because you do not have somebody else watching you. It is also something you can act on the same evening you arrive, which is more than most advice about altitude gives you.

One thing worth knowing about how those posts work. They run on donations, and they provide care to people who live in these valleys either free or at reduced cost. Money that comes in from trekkers helps pay for the care that local families get.1 If you attend the talk, consider that when the hat goes round.

These two posts and the talk are on this page because they are part of the answer to what is happening to your body and what to do about it. For the full list of medical help on and around this route, what each place can actually do, and how an evacuation off this trek is arranged, read safety and risk on the Everest Base Camp trek.

Here is what actually happens on a trek we run, described as plainly as we can and with nothing added that we cannot stand behind. A guide who has walked this route many seasons is watching the group all day, and most of what they are watching is not what people expect.

They watch how you eat. Appetite going is one of the earliest things a guide notices, and it is visible at a dinner table before the person mentions anything. They watch how long you take to get ready in the morning, because somebody who is struggling gets slower at small tasks before they get slower on the trail. They watch your breathing on ground they have watched hundreds of people walk, so they know what that stretch normally does to a person. They watch whether you are talking. A group that goes quiet and stays quiet is telling a guide something. They watch how you move on uneven ground at the end of a day.

And they ask. Every evening, directly, person by person, rather than asking the group whether everybody is fine, because asking a group is how you get a room full of people saying yes.

We turn people around. That is the part worth being blunt about, because it is the part that costs us money. If a guide judges that somebody should not go higher, the answer is that they do not go higher, and if they need to lose height then somebody goes down with them. Not on their own, not waiting for the rest of the group, not talked into one more day to see how it goes. It is not a negotiation and it is not a vote, and we tell people that before the trek rather than in the moment.

We would rather you heard the commercial reality of that from us. Turning a client around is the expensive outcome for us. It costs staff time, it costs goodwill, and it means somebody goes home without the photograph they came for. We do it anyway, and any operator who will not say clearly what their turnaround policy is has answered your question.

What we will not dress up: this is judgment built on seasons of watching people on this specific trail, not a clinical assessment. Our guides are mountain professionals. They are not doctors, and on this page we make no claim about medical qualifications for any individual guide. When somebody needs a clinician, the job is to get them to one. The aid posts are described above, and the full list of what is on this route is on safety and risk on this trek.

How we run a departure more generally, including the group sizes and the staff ratios behind all of this, is on how we run the trek.

Almost every page you will read about altitude on this trek carries numbers. A maximum height gain per day. A daily water target in liters. A reading from a device that supposedly tells you whether you are all right. A drug name. This page carries none of them, and we would rather tell you why than let you assume we forgot.

No height gain rule in meters per day. Our own group publishes one. It is in our corpus, it is the sort of number that looks authoritative in a table, and it is recorded in our fact store as a claim that no authority has confirmed. So it does not ship. What we will say instead is the thing the number is a proxy for, which is that the schedule goes up slowly and includes the two acclimatization days, and that is described above without a figure attached to it.

No daily water figure in liters. Also published by our own group, also unconfirmed by any medical authority, and it is on our open verification list waiting for one. Drinking properly matters and we say so. We are not going to put a number on how much water your body needs at altitude, because that is a clinical question and we are not the people to answer it.

No oxygen saturation figure and no threshold. You will see numbers presented as the line between fine and not fine. We publish none, we hold none as verified, and we make no claim on this page about what our guides carry to measure anything. A number a trekker misreads is worse than no number, in both directions: it talks people out of saying something when they feel wrong, and it frightens people who are fine.

No drug name and no dose. Medication for altitude exists and your doctor can talk to you about it. We are a trekking company. Publishing drug guidance would be us practicing medicine on strangers we have never met, some of whom have conditions we know nothing about, and no amount of care in the wording would make that right. Ask your own doctor, before you travel, and tell them how high you are going and how long you will be up there.

No checklist to score yourself with. The scoring systems used for altitude illness are clinical tools, and handing one to somebody with a headache at 4,410 m (14,469 ft) invites them to score themselves as fine and go back to bed. The instruction we give instead is the one on this page: tell your guide, early, and let somebody who is not inside your head make the call.

If any of those figures becomes something an authority has published and we can check, it will appear here with its source and the date we read it, and the review date at the foot of this page will move. How we decide what is verified and how to ask us to correct something is on our editorial and corrections policy.

Some people on this trek do not reach base camp. That is the honest sentence and we would rather you read it here, months in advance, than hear it for the first time from a guide at four in the afternoon at 4,910 m (16,109 ft).

Most of the time going down is not dramatic. You walk, with a member of staff with you, to a village lower down. You spend a night or two there. Quite often people feel much better within a day and the conversation becomes what to do with the rest of the trip. Sometimes people rejoin the group lower down on the way out. Sometimes they stay put somewhere comfortable and the group comes back through. Sometimes it is the end of the walking for that person and they go out to Lukla, 2,860 m (9,383 ft), and on to Kathmandu.

A smaller number of situations are not a walk. When somebody cannot safely walk down, a helicopter evacuation is called, and that is a real thing that happens on this route rather than a horror story. On our treks your own insurer decides and sends the helicopter under your policy, and our guide makes the call to your insurer from the trail. That is the single best reason to read your insurance policy properly before you leave, because the thing that determines how smoothly that goes is usually paperwork rather than mountains. What cover this trek needs, and the height limit that most standard policies quietly stop at, is on travel insurance for the Everest Base Camp trek. What an evacuation involves is on safety and risk on this trek.

The part people find hardest is not physical. It is the feeling of having come a very long way and spent a lot of money and stopped short. We understand that completely and we are not going to pretend it is fine. What we will tell you is what we have watched happen over many seasons. The people who go down early and come back the following year finish. The people who push on with something building finish their trip in a helicopter, or worse, and they do not go back.

There is one thing that makes this easier to live with, and it is worth doing before you go. Decide now, while you are calm, that you will accept a guide who tells you to go down. Tell whoever you are traveling with that you have decided that. Then it is not a decision you have to make while you feel terrible and disappointed, and it is not a thing anybody has to argue you into.

We are telling you this rather than leaving you to assume otherwise. Everything above is written by guides, from what we see on this trail and from sources we name. No doctor has read it yet. Until one has, treat it as the experience of an operator who walks this route, not as clinical information.

A page about how altitude affects a human body should be checked by somebody qualified in altitude medicine, and that review is the reason this page carries no drug names, no doses, no thresholds and no diagnostic criteria. Those are the parts that need a clinician, so those are the parts we left out rather than guessed at.

When that review has happened we will say so here, we will name the reviewer and their credential, we will date the review, and we will say what they changed. If they tell us something on this page is wrong, we will correct it and record the correction in the open log on our editorial and corrections policy rather than editing it quietly.

Until then, the advice at the top of this page stands and it is the most important thing on it. Talk to your own doctor before you book, especially if you have a heart condition, a lung condition, a pregnancy, or any chronic illness or regular medication. Go to the free altitude talk when you are on the route. And tell your guide early when something feels wrong.

Every number above is listed here with the body that publishes it and the date we last read it. If a figure has changed since, write to us and we will correct the page and say that we did.