SpO2

SpO2 is the peripheral oxygen saturation, the percentage of haemoglobin in your blood that is carrying oxygen. Watches and bands show an estimate of it using small light sensors on the underside of the case.

How it works

The sensor shines red and infrared light into the skin and measures how much returns to a detector beside it. Blood rich in oxygen and blood low in oxygen absorb these two colours differently, so the device works out a percentage from the difference. This method is called reflective pulse oximetry. A fingertip clinical oximeter works differently, because light passes through the finger. On a wearable, the result is usually shown as a single number after the wearer keeps still for a short time.

What the numbers mean

• The reading is a percentage. Healthy people at rest usually read in the mid-90s or higher.
• Height above sea level can lower a healthy reading.
• Some devices measure only on request, and others also measure during sleep.

What it does not promise

A wearable SpO2 reading is an estimate, not a diagnosis. Movement, a loose strap, cold hands, tattoos, skin tone and wrist position can all affect it. Most consumer wearables are not medical devices, and a reading cannot replace a clinical test or a doctor's advice.