Oxygen Levels at Everest Base Camp

Somewhere around the second night in Namche, someone at the table always asks: "So how much oxygen is actually left when I get to Base Camp?"

The short answer is that the air at Everest Base Camp is still about 21% oxygen, the same as in Kathmandu or London. What changes is air pressure. At 5,364 m the atmosphere presses down with roughly half the force it does at sea level. A breath of the same size carries roughly half as many oxygen molecules into your lungs, in a mixture that is still 21% oxygen. Your body responds to that pressure, not to the percentage.

If you've read that Base Camp has "half the oxygen," you've read something close to true but worded badly. This article sorts out the difference, then walks up the trail from Lukla to Kala Patthar to show what thinner air does to your breathing, your pace, your sleep and your pulse oximeter. If you're weighing up the trek itself, the Everest Base Camp Trek page has the full day-by-day plan. 

The air is still 21% oxygen. So what changes?

Air is about 78% nitrogen and 21% oxygen, with a little of everything else. That mix doesn't change as you climb, and it is the same at Base Camp as on the beach.

What changes is how much air is stacked above you. Atmospheric pressure is the weight of that air. Climb, and there is less of it overhead, so the pressure falls. Every litre you breathe in at Base Camp holds fewer molecules than a litre at sea level, and oxygen is still 21% of them.

Doctors use the term partial pressure for this. The partial pressure of oxygen is simply oxygen's share of the total air pressure:

• At sea level the total is about 101 kPa, so oxygen contributes about 21 kPa (roughly 159 mmHg).
• At Base Camp the total is about half that, so oxygen contributes about 11 kPa (roughly 81 mmHg).

Pressure matters because oxygen crosses from the air in your lungs into your blood because of a pressure difference. Lower the pressure and the drive weakens, even though the recipe hasn't changed. The CDC Yellow Book names reduced oxygen partial pressure as the main physiological problem at high altitude.

Think of a tap. The water is the same water, but turn the pressure down to half and you fill a bucket much more slowly. Your lungs are the bucket.

CDC gives a reference point: at around 3,050 m, inspired oxygen pressure is only about 69% of the sea-level value. A standard-atmosphere calculation gives 69% at 3,050 m and about 51% at 5,364 m. At 5,000 m the figure is a little over half. So "about half the sea-level oxygen pressure" is a fair way to describe Base Camp.

The wording matters, though. "Half the oxygen" makes people picture air that is 10% oxygen and 90% something else. That isn't what's happening.

You may also see 47% or 48% quoted for Base Camp. That comes from a slightly different calculation. Air is warmed and humidified inside your airway, and the water vapour takes up some of the pressure. Once you subtract it, the oxygen pressure in the air you've just inhaled is about 47–48% of the same figure at sea level. It's a fair description of what reaches your lungs. It doesn't mean the air is 47% oxygen.

All of these numbers are estimates. Real pressure shifts by a few percent with weather and season, so "about half" is the honest version, not "51.4%." 

Lukla to Kala Patthar: what thinner air means at each stop

Elevations below come from the current Places Nepal itinerary. The pressure column is an approximate standard-atmosphere calculation, rounded to the nearest 5% on purpose.

A few stops are worth more than the table gives them.

Lukla and Phakding. The flight from Kathmandu takes about 35 minutes and lifts you roughly 1,500 m. Your first night is at Phakding (2,610 m), lower than Lukla on purpose. CDC guidance says to avoid going from low altitude straight to a sleeping altitude above about 2,750 m in one day. Phakding stays under that line and Lukla doesn't.

Namche. This is the first stop where the air really starts to matter. The last 600 m of Day 3's climb from Hillary Bridge is where most people first feel their breathing. You then spend two nights here so the body can catch up.

Dingboche. Above Tengboche the trees are gone, the valley opens up, and the pace of the whole group drops without anyone deciding it should. Dingboche is where thin air stops being an abstract idea.

Gorak Shep, Base Camp and Kala Patthar. Gorak Shep at 5,164 m is the highest you sleep. Base Camp is only about 200 m above your bed, and you walk out to it and back. Kala Patthar is higher than Base Camp, at 5,545 m, and you climb it before dawn from Gorak Shep. Base Camp is not the highest point of the trek.

Coming down. The descent to Pheriche (4,252 m) drops you about 900 m below Gorak Shep in a day. Most people notice their breathing settle and their appetite come back.

What thinner air does to your breathing, legs and pace

The chain of events is short:

lower air pressure → lower oxygen pressure in each breath → your body senses it → you breathe deeper and faster.

The medical name for the underlying problem is hypoxia, meaning tissues aren't getting as much oxygen as they'd like. Low oxygen specifically in the blood is hypoxemia. Breathing harder is your body's first response. Your heart rate usually rises too.

The result is that the same walking pace costs more effort than it did lower down. A gentle slope that felt easy at Lukla can leave you stopping for breath at Lobuche. Recovery takes longer, and you may find you can't talk and walk at the same time. This is the normal response of a healthy body, and it eases when you go down. It isn't damage.

CDC also notes that even fully acclimatized people can't match their low-altitude maximum effort at height. Slowing down at altitude isn't a sign you're unfit.

The same 5 km is not the same 5 km

Distance tells you very little at altitude. Day 2 covers about 7.5 km from Lukla to Phakding in around 3 hours, mostly downhill at 2,600 to 2,800 m. Day 9 covers about 12 km round trip in 7 to 9 hours, with 454 m of climbing, glacier moraine underfoot, cold, and air at roughly half sea-level pressure.

Oxygen isn't the only reason Day 9 is slower. Slope, terrain and temperature all add to the workload. But it's the factor you can't see, and it's why our Everest Base Camp trek map and the elevation and acclimatization breakdown put elevation next to distance. Kilometres alone hide most of the effort.

What a pulse oximeter can and can't tell you

A pulse oximeter clips onto a fingertip and estimates oxygen saturation (SpO₂), the percentage of your blood's oxygen-carrying capacity that is currently loaded with oxygen. It's useful, and easy to over-read.

Three things to know:

• Readings fall with altitude in healthy people. CDC notes that acute exposure at around 3,050 m can bring saturation down into the high 80s to low 90s. A number that would raise eyebrows at sea level can be ordinary at height. Base Camp readings are lower again.
• The spread between people is wide. Two healthy trekkers in the same room can read quite differently. Comparing your number with your tent-mate's tells you very little.
• One number cannot diagnose altitude sickness. CDC says AMS has no characteristic physical finding and is diagnosed from symptoms and recent ascent. Oximeters (CDC notes they cost under US$30) can help gauge acclimatization, but a single reading is a snapshot, not a verdict.

Read a number alongside the altitude, the trend over days, and how you feel. That's the same way our guides use theirs. Every group carries an oximeter, and readings are taken at rest stops.

Cold fingers and movement can throw a reading off, so guides take them at rest with warm hands. Please don't use a reading to diagnose yourself or to argue with your guide. Tell your guide how you feel, whatever the number says. 

Why acclimatization takes days, and where the itinerary builds it in

At altitude your body starts adjusting:

• Breathing increases. This is the biggest early change.
• Circulation shifts, including blood flow to the brain.
• Blood chemistry rebalances. Breathing harder blows off carbon dioxide, which makes the blood more alkaline. The kidneys respond by excreting bicarbonate, and that lets breathing rise further.

CDC describes the acute phase as the first 3 to 5 days after arriving at a new height, and says some adaptation keeps going for weeks to months. Increased red cell production isn't part of the acute phase, despite what many trek articles say. The early rise in haemoglobin concentration comes mostly from a drop in plasma volume. The takeaway is that days matter more than fitness or willpower.

The standard 14-day Everest Base Camp trek builds this in:

• Day 3 to 4: you reach Namche (3,440 m), then spend a full day going up to Hotel Everest View (3,880 m) and coming back to sleep.
• Day 5 to 6: you reach Tengboche (3,867 m), then Dingboche (4,410 m).
• Day 7: you climb Nangkartsang Hill (5,091 m), your first time above 5,000 m, and come back to sleep at Dingboche.
• Day 8 to 9: you sleep at Lobuche (4,910 m), then Gorak Shep (5,164 m).

Acclimatization does not make altitude illness impossible. CDC says the EBC route pushes the limits of many people's ability to adapt, with altitude illness reaching around 30% at higher elevations even on standard schedules. Its advice is that extra days make the trip safer and more enjoyable.

The arithmetic bears this out. Our sleeping altitude climbs about 2,550 m between Phakding and Gorak Shep, and the 14-day plan gives you two extra nights along the way. That's right at the edge of the guideline that CDC and the Wilderness Medical Society use. Some trekkers want more margin, and our 12, 14 and 16 day comparison covers when a longer plan makes sense. We run both fixed-departure group treks and private customized trips, and a private trip can add acclimatization days. The trip page has current dates and options. 

Sleeping high is different from visiting high

Sleeping at altitude is physiologically different from visiting and returning. CDC notes that oxygen in the blood is at its lowest during sleep, and that day trips to a higher point followed by a return to a lower bed are much less stressful than sleeping at the higher point.

The itinerary uses this on both acclimatization days:

• Hotel Everest View (3,880 m): you go up from Namche (3,440 m) and come back to sleep.
• Nangkartsang Hill (5,091 m): you climb 681 m from Dingboche and return to sleep at 4,410 m.

Kala Patthar works the same way. You climb to 5,545 m at dawn, but your bed the night before was at 5,164 m, and by nightfall you're at Pheriche (4,252 m). That's why the highest sleeping altitude on the trek is Gorak Shep and not Kala Patthar.

Why sleep goes sideways above 4,000 m

Sleep disturbance is the most common complaint at altitude, and it's usually not a sign of anything wrong. CDC says some degree of periodic breathing becomes nearly universal above about 2,700 m. Your breathing speeds up, slows, and briefly pauses in cycles. Sleep stages shift and you wake more often. It's normal for someone to wake with a start, feeling short of breath, after one of those pauses.

Cold and unheated bedrooms don't help. At the highest stops the dining room is the only room with a stove.

Broken sleep isn't altitude sickness on its own. Mention it to your guide if it comes with headache, nausea or vomiting, or breathlessness at rest. CDC says alcohol and opioids should not be used as sleep aids at altitude, and we don't recommend sedatives. Sleep often improves with acclimatization, though not always. Descent in the following days usually helps.

Low oxygen and altitude sickness are related, not identical

Thin air is why altitude is stressful, but altitude illness isn't diagnosed just because oxygen is low. Symptoms, the ascent profile and each person's response all matter. Two trekkers at the same altitude can feel completely different. The full breakdown of AMS, HAPE and HACE is in our altitude sickness guide, so this article doesn't repeat it.

What matters here is knowing when to say something. Tell your guide if you notice:

• a headache that's getting worse
• nausea or vomiting
• fatigue that's unusual for the day
• dizziness
• confusion or unusual behaviour
• trouble walking normally
• breathlessness while resting

If symptoms are getting worse, do not keep climbing just because you've reached a planned stopping point. Tell your guide. For severe altitude illness, descent and medical care can be urgent. Your travel insurance should cover emergency evacuation at altitude (it isn't included in the trek price, so check yours).

Do you need bottled oxygen on an ordinary EBC trek?

Routine supplemental oxygen is not normally part of a standard Everest Base Camp trek. A well-planned trip relies on gradual ascent, acclimatization, pacing, watching symptoms, and descending when it's medically necessary. That approach is why oxygen bottles aren't needed for most people.

That doesn't mean oxygen is never used. Emergency oxygen is part of medical and rescue care. CDC says low-flow oxygen can ease an AMS headache within about half an hour, though it's rarely available in the field, and that supplemental oxygen is used alongside descent for severe illness. The small handheld cans sold in some places hold at most about 5 litres, which is too little for lasting relief.

Emergency oxygen is a medical tool, not a substitute for acclimatization. Please don't rely on portable oxygen to keep climbing while unwell. Carrying your own cylinder isn't standard trekking kit. If a doctor has given you a specific medical plan that includes oxygen, tell us before you book so we can plan around it.

Where Diamox fits

Acetazolamide (Diamox) doesn't change the air. It helps your body acclimatize by increasing breathing, which raises blood oxygen, especially during sleep. It doesn't remove the need for a slow ascent and it isn't right for everyone. Whether to take it is a decision for you and your own doctor, made before you fly. Our dedicated Diamox article covers it in detail.

Why guides slow the group down above 4,000 m

Above 4,000 m you'll notice three things: the pace drops, rests get more frequent, and evenings get colder and less predictable. From Dingboche through Lobuche and Gorak Shep to Base Camp, the walking days aren't long by distance, but they feel long. This is where you'll hear the "Khumbu shuffle," a very slow, steady rhythm that our guides set deliberately.

A slower pace isn't the guide being cautious for the sake of it. It:

• reduces unnecessary exertion at a time when every step costs more
• helps you manage the thinner air
• keeps the group together
• gives the guide more chances to notice a change in how someone looks or walks

There isn't one "medically optimal" speed, and we don't claim there is. The right pace is one where you can still breathe comfortably and hold a short conversation.

Being fit doesn't make you immune to low oxygen

Fitness helps with the physical workload: carrying a daypack, recovering after a long day, having reserves if you need to walk down. It doesn't protect you from altitude illness. CDC states that training and physical fitness don't affect risk, and altitude response varies between individuals in ways nobody can predict. Age doesn't work the way people expect either. CDC notes that people over 50 have slightly lower risk, while children are as susceptible as adults. Our guide to Everest Base Camp for seniors covers pacing and the medical conversation for older trekkers.

Five things to remember

1. The air does not contain half as much oxygen at Everest Base Camp. It's still about 21% oxygen.
2. Atmospheric pressure is much lower, so oxygen partial pressure is roughly half its sea-level value.
3. Your body has to work harder to get oxygen into your bloodstream, which is why breathing, pace and sleep all change.
4. Acclimatization takes time and is a central part of the EBC itinerary.
5. Feeling unwell matters more than a single pulse oximeter number. Tell your guide.

Sources: CDC Yellow Book 2026, High-Altitude Travel and Altitude Illness; Wilderness Medical Society, Foundations in Wilderness Medicine: Altitude Illness; Luks AM et al., WMS clinical practice guidelines for acute altitude illness, 2024 update, Wilderness & Environmental Medicine 35(1).

FAQ

How much oxygen is there at Everest Base Camp?
The air is still about 21% oxygen. At 5,364 m, air pressure is roughly half its sea-level value, so each breath delivers about half as many oxygen molecules, and the pressure driving oxygen into your blood is about half too.

Does the percentage of oxygen decrease at Everest Base Camp?
No. The percentage stays about 21% at every point you'll walk. It's the pressure that falls.

Why is breathing harder at 5,000 m?
Air pressure at 5,000 m is a little over half its sea-level value, so less oxygen crosses into your blood with each breath. Your body responds by breathing deeper and faster, and your heart rate rises, so the same effort feels harder.

What is the oxygen level at Everest Base Camp in percentage?
The concentration is about 21%. If you mean availability compared with sea level, it's about half, and some sources quote 47–48% for the air you've just inhaled once airway humidity is accounted for. These are approximate figures that vary with weather.

What is oxygen partial pressure?
It's the share of total air pressure that comes from oxygen. At sea level that's about 21 kPa. At Base Camp it's about 11 kPa (approximate).

What happens to oxygen saturation at high altitude?
SpO₂ falls with altitude in healthy people, and the range between individuals is wide. CDC notes that acute exposure at around 3,050 m can bring it into the high 80s to low 90s. Readings improve somewhat with acclimatization but stay below sea-level values.

Is oxygen available at Everest Base Camp?
The air is breathable everywhere on the trek. Emergency oxygen is part of medical and rescue care in the region, but it isn't something to count on being on tap.

Do trekkers need supplemental oxygen?
Not on a standard, well-paced trek. Oxygen is an emergency tool used alongside descent, not routine equipment.

Does Diamox increase oxygen levels?
It doesn't change the oxygen in the air. It helps your body acclimatize by increasing breathing, which raises blood oxygen, especially during sleep. Whether it's appropriate for you is a decision for your doctor.

Does fitness help you handle low oxygen?
Fitness helps with the workload, but CDC says physical fitness doesn't affect altitude illness risk.

Why do you walk slower at high altitude?
The same effort costs more when each breath carries less oxygen pressure. Guides also slow the group to reduce exertion and to watch for changes in how people look and move.

Why is sleeping harder at Everest Base Camp?
Periodic breathing is near-universal above about 2,700 m, sleep stages shift, and you wake more often. Cold, unheated bedrooms add to it. It's usually not a sign of altitude sickness by itself.

Is 50% oxygen available at Everest Base Camp?
Not 50% oxygen in the air. The air is about 21% oxygen. The oxygen pressure is roughly half its sea-level value.

What is the highest point on the standard EBC trek?
Kala Patthar, at about 5,545 m.

Is Kala Patthar higher than Everest Base Camp?
Yes, by roughly 180 m. Base Camp is about 5,364 m, and Kala Patthar is climbed pre-dawn from Gorak Shep.

Can a pulse oximeter tell if you have altitude sickness?
No. AMS is diagnosed from symptoms and recent ascent, and readings fall normally with altitude. An oximeter is one input for your guide, not a diagnosis.

What happens if oxygen saturation gets too low?
There's no single number that works for everyone. A reading well below what's expected for your altitude, falling while you rest, or accompanied by symptoms is a reason to tell your guide. Breathlessness at rest, confusion or trouble walking straight is an emergency and calls for descent.

Does acclimatization increase oxygen levels?
It improves how well your blood is oxygenated compared with your first day up high, though never back to sea-level values. It doesn't change the oxygen in the air.

Does your body make more red blood cells at altitude?
Not in the first few days. CDC says red cell production isn't part of acute acclimatization. The early rise in haemoglobin concentration comes mainly from a drop in plasma volume, and slower changes continue over weeks.