Blood Oxygen at Altitude: What to Expect
Why your SpO2 reads lower at altitude, why your nights read lower still, and how to track your overnight oxygen across a trip to the mountains.
Fly into a mountain town, put on a pulse oximeter, and you'll probably see a lower number than you get at home. That's expected. It happens to everyone who goes up.
Why does your oxygen read lower up high?
The air still has about 21% oxygen, the same as at sea level. What changes is the pressure. The air is thinner, so each breath carries less oxygen into your lungs, and less reaches your blood.
Your whole reading shifts down. That's different from the short, repeated dips you might look for at home.
How much lower?
It depends on how high you are, how long you've been there, and you. People at the same height can read several points apart.
Our altitude oxygen tool gives a typical figure for the height you enter. You can also browse destinations by elevation, or check blood oxygen on a plane for the flight there. A plane cabin counts too. See blood oxygen on flights.
Why are nights lower still?
Your breathing slows when you fall asleep, so your oxygen sits lower than it did during the day. Your lowest readings of the trip will probably come at night.
Many people also get periodic breathing at altitude. That's a repeating cycle of deep breaths, then shallow ones or a short pause. On your chart it looks like a regular wave in your SpO2. A review of sleep at high altitude named it as the main thing that disturbs sleep up there. The same review found it can get stronger the higher you go and the longer you stay, so don't expect it to fade after a few nights.
Your T90 will jump as well, because much more of the night is spent below 90%.
What changes as you acclimatize?
Over the first days your body adjusts. You breathe more, and your resting SpO2 usually creeps back up.
How do you track a trip?
- Record a few nights at home first. That's your baseline. Here's how many nights to record.
- Record every night at altitude. A daytime spot check misses the lowest part.
- Tag each night with the height. In PulseOxTracker you can add a tag.
- Compare night one with night three or four. Use compare sessions or the free compare nights tool.
- Keep altitude nights out of your home averages. They aren't comparable.
Away from your phone or charger? The Checkme O2 Max can hold several nights between syncs.
When should you stop or go down?
Go by how you feel, not by the number. The Wilderness Medical Society's guidelines recommend going up gradually. They also say that anyone with symptoms of altitude illness should stop going up, and should go down if the symptoms get worse or don't ease.
- Stop and rest. Headache with nausea, dizziness or unusual tiredness.
- Go down and get medical help now. Confusion, an unsteady walk, or breathlessness at rest that won't settle.
Altitude illness can be life-threatening. A reading on your finger doesn't clear you.
If you have a heart or lung condition, ask your doctor before you travel.
Not a medical device. Does not diagnose, treat or prevent any condition. Discuss results and changes with a clinician.
References
- Ainslie PN, Lucas SJE, Burgess KR. Breathing and sleep at high altitude. Respiratory Physiology & Neurobiology. 2013;188(3):233-256. doi:10.1016/j.resp.2013.05.020
- Luks AM, Auerbach PS, Freer L, et al. Wilderness Medical Society Clinical Practice Guidelines for the Prevention and Treatment of Acute Altitude Illness: 2019 Update. Wilderness & Environmental Medicine. 2019;30(4S):S3-S18. doi:10.1016/j.wem.2019.04.006