Altitude and travel

What's normal blood oxygen at altitude?

By GyroscopeUpdated November 14, 20267 sources

Lower, and more spread out. On a watch, a typical night falls about 1.2 points for every 3,300 feet (1 km) you live higher, up to about 8,000 feet, and then it falls faster. Up high, your symptoms tell you more than the number does.

How much less oxygen is in each breath?

Air is about 21% oxygen at every elevation. What changes is how much air there is. The higher you go, the lower the air pressure, so each breath pulls in fewer molecules of everything, oxygen included.

These come from NASA’s standard model of the atmosphere, and the CDC gives the same 69% for 10,000 feet.

What’s a normal number at each elevation?

Up to about 8,200 feet, the 33,080-person Apple Watch study gives a typical range. For a 40-year-old with a BMI of 25, where about 95 in 100 watch wearers sit if the spread follows the usual bell curve:

Above about 8,200 feet (2,500 m), the study’s authors say the straight line stops working, so the tools here stop giving a range. The CDC says that at about 10,000 feet, a newly arrived visitor’s blood oxygen can drop to 88 to 91. In 13 healthy lowlanders who climbed fast to 14,957 feet, a blood test averaged 75.

Why does it fall faster up high?

Picture a hill with a long flat top and then a cliff. Healthy blood near sea level sits on the flat top: a little less air barely moves the number, because nearly every oxygen seat stays filled. Somewhere past 8,000 feet you reach the edge, and each step down the cliff costs a lot more. Doctors who study oxygen at altitude say saturation changes fast with small changes in oxygen up there, and that the normal number at a given elevation is best seen as a range of values.

They add two warnings. Don’t make decisions on small differences, between days or between people, and remember that oximeters get less accurate below a true 80. Cold hands and bright sunlight, both common on a mountain, cause errors too.

Why can’t the number tell you if you’re getting sick?

The CDC says people with altitude sickness usually show a number that’s normal for the altitude, or only a little lower. Altitude sickness shows up as a headache with an upset stomach, tiredness or dizziness, so the morning symptom check decides (the symptoms). Pacing where you sleep lowers the risk (how to prevent it).

If you live up high, MedlinePlus says your levels may run lower. The tool below shows the oxygen in each breath and the typical range for any elevation you pick.