The guide

What happens to your blood oxygen at altitude?

By GyroscopeUpdated November 14, 202612 sources

Each breath carries less oxygen the higher you go, about 82% of the sea-level amount in Denver and 69% at 10,000 feet. Your number holds up well to about 8,000 feet, then falls fast. Pace the elevation you sleep at, and let symptoms decide, because the number can’t tell you if you’re getting sick.

Why would a normal number fool you up high?

People bring an oximeter on a trek for the same reason they’d bring a thermometer, expecting it to say when something’s wrong. Up high that logic breaks. Everyone’s number drops, sick or well, and the CDC says people with altitude sickness usually show a number that’s within the normal range for the altitude, or a little lower.

Doctors who study altitude add that the normal number at a given elevation should be thought of as a range, and that decisions shouldn’t rest on small differences between people or from one check to the next. A fit friend at 88 and a sick friend at 86 can’t be told apart by the numbers.

Why is there less oxygen up high?

The air is the same mix all the way up. Oxygen makes up about 21% of it at every altitude. What changes is the pressure. The higher you go, the less air is pressing down from above, so each breath holds fewer molecules of everything, oxygen included.

A simple way to picture it

Think of a bag of trail mix where one handful in five is peanuts. On the mountain, the mix in the bag is the same, but your scoop gets smaller. You still get one peanut in five, just fewer of them in each scoop.

How much oxygen is in each breath?

The CDC gives the same 69% for 10,000 feet. Its risk bands for the elevation you sleep at are low under 9,000 feet, medium from 9,000 to 11,150, and high above that. Oxygen by elevation works it out for any town or height.

What does your number do as you climb?

Your hemoglobin holds onto oxygen well while there’s plenty around, then lets go fast once the air gets thin. That’s why the number barely moves at first.

In the Apple Watch study of 33,080 people, the night average fell about 1.2 points for every kilometre (3,300 feet) higher people lived, and by day about 0.8. The straight line held to about 8,200 feet and bent above it.

Above the bands, there’s one real example. In a study published in 2021, 13 healthy lowlanders made a fast climb from about 3,700 feet to 14,957 feet (4,559 m). Their blood tests averaged 75%, a medical finger oximeter showed 81% and a Garmin Fenix 5X Plus 85%. The Garmin was 7 points too high on average, and the researchers said it “cannot be recommended” for oxygen at high altitude.

If your skin is darker, any sensor up high can show higher than your true oxygen, the same way it can at sea level. In hospital patients whose finger oximeter showed 92 to 96, true oxygen was under 88 for 11.7% of Black patients’ measurements and 3.6% of White patients’.

Who gets altitude sickness?

In 1993 a team led by Honigman surveyed 3,158 adult visitors at Colorado resorts from 6,300 to 9,700 feet. A quarter got acute mountain sickness, and about two in three of those got it within the first 12 hours. Visitors who lived below 3,000 feet were 3.5 times as likely to get it, and people who’d had it before 2.8 times as likely.

The surprise was age. Visitors under 60 were about twice as likely to get it as older visitors. More than half of the people who got sick cut back on their activities.

How do you know if you’re getting it?

Symptoms usually start 2 to 12 hours after you arrive. The doctors who study it use a simple morning score, the 2018 Lake Louise score. Rate each of four symptoms from 0 (none) to 3 (severe, hard to function).

• Headache
• Stomach, from poor appetite or nausea up to vomiting
• Tiredness or weakness
• Dizziness or light-headedness

Three points or more, with at least one from the headache, after a recent climb and at least 6 hours at the new elevation, is acute mountain sickness. Sleep used to be on the list and was dropped in 2018, because poor sleep up high comes from the thin air itself. The CDC adds that symptoms that start after 3 days at the same elevation shouldn’t be put down to altitude sickness.

Two rarer forms of altitude illness are emergencies. One affects the brain (high-altitude cerebral edema) and brings confusion, clumsiness and drowsiness. The other is fluid in the lungs (high-altitude pulmonary edema), which starts with chest congestion, a cough, much more breathlessness than usual on effort and falling performance, and there oxygen numbers of 50% to 70% are common. Any of those signs up high means get medical help now.

How fast should you go up?

The CDC’s travel guide paces a trip by the elevation you sleep at.

1. From under 4,000 feet, don’t sleep at 9,000 feet or higher the first night.
2. Above 9,800 feet, sleep no more than 1,650 feet higher than the night before.
3. Above 9,800 feet, add an extra night with no gain for every 3,300 feet you climb.
4. Never go higher to sleep with symptoms. Go down if they get worse despite rest. Going down 1,000 feet or more usually helps quickly.

Two common trips break the first rule. Flying from the coast to a ski town at about 9,600 feet puts your first night over the 9,000-foot line, and one night in Denver on the way fixes it. Flying into Cusco, at about 11,150 feet, puts you there on night one, and our trip tool suggests sleeping lower first, near Machu Picchu, then going up. It flags every night that breaks a rule and finds the one change that clears the most.

The CDC also names a prescription medicine, acetazolamide, for medium and high risk trips. Ask your doctor whether it fits yours.

Who should talk to a doctor before going high?

The CDC says people with heart disease, chronic lung disease or sleep apnea should talk to a doctor who knows altitude medicine before they go. It advises people with severe sleep apnea to avoid high-altitude travel unless they have supplemental oxygen.

What if you live up high?

Then your usual number is lower, and that’s your normal. MedlinePlus notes numbers can be lower at higher elevations. People living around Denver often sit a point or two under sea-level numbers. A new awake number at or under 92 is still a reason to call your doctor.