How smartwatch blood oxygen readings work, and where they fall short
A watch estimates blood oxygen by shining red and infrared light into your wrist. The method is sound and the wrist is a hard place to do it.
Kivorent News desk, , 2 min read
What to know
- Blood oxygen saturation, or SpO2, is the share of your hemoglobin that is carrying oxygen.
- Watches estimate it from how blood absorbs red and infrared light.
- Motion, cold hands, a loose band, tattoos and darker skin tones can all reduce accuracy.
Blood oxygen was once a number you saw only in a clinic, from a clip on your fingertip. Many watches and rings now estimate it on their own, mostly overnight.
What is being measured
Red blood cells carry oxygen using a protein called hemoglobin. SpO2 is the percentage of hemoglobin in your arteries that currently has oxygen attached. For most healthy people at sea level it sits in the mid to high 90s. It runs lower at altitude.
The light trick
Hemoglobin changes color depending on whether it is carrying oxygen. Oxygen-rich blood absorbs more infrared light and lets more red light through. Oxygen-poor blood does the reverse.
A sensor shines both colors into your skin and measures each one. The ratio between the two, tracked across the pulse, gives an estimate of saturation.
Fingertip and wrist
A fingertip clip shines light straight through the finger to a detector on the other side. The fingertip is thin and full of blood vessels, which makes for a strong, clean signal.
A watch cannot shine light through your wrist. It measures the small amount that bounces back to detectors next to the lights. That reflected signal is much weaker and easier to disturb.
What degrades a wrist reading
- Movement. The largest source of error, and the reason most devices measure during sleep.
- A loose fit. Gaps let outside light in.
- Cold skin. Less blood flows near the surface.
- Tattoos. Ink under the sensor absorbs light.
- Skin tone. Research on pulse oximeters has found they can overestimate oxygen levels in people with darker skin. That finding has prompted regulators to revisit how the devices are tested.
Reading the result
- Overnight averages are more useful than spot checks. One odd reading is usually a bad measurement.
- Trends matter. A consistent shift from your own normal is more meaningful than a single low point.
- Altitude changes the baseline. Lower numbers in the mountains are expected.
What a watch reading is for
On most watches, blood oxygen is a wellness feature. It has not been reviewed as a medical measurement unless the maker says so specifically. Whether a product can make medical claims is a regulatory question, covered in what the FDA's 2026 wellness rules let your wearable tell you.
A watch reading should not be used to decide whether you need care. If you are short of breath, confused, or your lips or face look bluish, get medical help regardless of what the watch shows. If you have a lung or heart condition and have been told to monitor your oxygen, use the device your clinician recommended.
This article is general information about how technology works. It is not medical advice. Talk to a clinician about your own health.
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