Myth or fact

Your oxygen number tells you if you have altitude sickness

Short answerNo

By GyroscopeUpdated November 14, 20268 sources

No. The CDC says people with altitude sickness usually show a number that’s normal for the altitude, or only a little lower. Everyone’s number drops up high, sick or well. A headache with nausea, tiredness or dizziness after a climb is what decides it.

Why it sounds right

Altitude sickness is caused by thin air, and the oximeter measures what thin air does to your blood. Connecting the two seems obvious.

The number does tell you the air is thin. What it can’t tell you is how your body is handling it.

Can a fuel gauge tell you about a flat tire?

A car’s fuel gauge is a good instrument. It tells you how much is in the tank, and it’s useless for telling you a tire is going flat. You’d find that out from how the car drives.

Up high, your oximeter is the fuel gauge. It shows less oxygen in your blood, which is true of everyone up there. Altitude sickness shows up in how you feel.

Why can’t the number tell sick from well?

• Everyone’s number falls. Each breath at 10,000 feet carries about 69% of the oxygen it does at sea level, the CDC says. Your number drops whether or not you feel fine.
• The normal is a wide range. Doctors who study altitude say the normal number at a given elevation “should be viewed as a range of values, rather than a specific number”, and that decisions shouldn’t rest on small differences between people or from one check to the next.
• The line gets steep. In the Apple Watch study of 33,080 people, the straight line between elevation and blood oxygen held up to about 8,200 feet and bent above it. Up there, a small change in the air moves the number a lot, for everyone.
• The sensors drift. In 13 healthy lowlanders after a fast climb to 14,957 feet, blood tests averaged 75%, a medical oximeter showed 81% and a Garmin 85%. Device accuracy declines once true saturation falls under 80%, and darker skin pushes numbers higher too. 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’.

What does decide it?

The 2018 Lake Louise score, the check altitude doctors use. After a recent climb and at least 6 hours at the new elevation, rate four symptoms from 0 (none) to 3 (severe, hard to function).

Three points or more, with at least one from the headache, is acute mountain sickness. Symptoms usually start 2 to 12 hours after arrival, the CDC says, and new symptoms after 3 days at the same elevation shouldn’t be put down to it.

It’s common. In 3,158 adult visitors to Colorado resorts from 6,300 to 9,700 feet, a quarter got it, and those living below 3,000 feet were 3.5 times as likely. The trip tool includes the morning check for each day.

Is the number ever useful up high?

Yes, with the right symptoms beside it. Fluid in the lungs at altitude (high-altitude pulmonary edema) starts with chest congestion, a cough, much more breathlessness than usual on effort and falling performance, and there the CDC says oxygen numbers of 50% to 70% are common. The other serious form, high-altitude cerebral edema, brings confusion, clumsiness and drowsiness. Either set of signs means get medical help now, and the number is a detail.

The best case for the myth

“Mountain guides and expedition doctors use oximeters all the time.” Some do, as one input next to how a person looks and feels, and the CDC notes a cheap oximeter can help travelers follow how they’re getting used to altitude. But the CDC’s own rules for when to stop or go down are written around symptoms. Never go higher to sleep with symptoms, and go down if they get worse despite rest.

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