SpO2 stands for "Blood Oxygen Saturation Level" and is a measure of the amount of oxygen present in the hemoglobin. SpO2 sensors, also called pulse oximeters, measure blood oxygen levels or oxygen saturation in your blood. In smartwatches, SpO2 sensors work similarly to the photoplethysmography (PPG) sensors used to measure heart rate. Smartwatch SpO2 sensors shine red and infrared light into your skin, and based on the way light reflects back, the devices use an algorithm to estimate your blood oxygen levels.

It is important to note that while fingertip pulse oximeters are a Class II medical device, smartwatch SpO2 features are meant for general wellness only and are not as accurate as those taken from the fingertip. However, SpO2 monitoring has become a standard feature in many smartwatches and can be beneficial when tracking your sleep or when youre at a high altitude.

The following smartwatches have SpO2 sensors:

• Apple Watch Series 6 and 7
• Garmin Forerunner 245, Vivoactive 4, Fenix 6, Forerunner 945
• Fitbit Versa 3, Sense, Ionic (2017), and most devices released from 2020 to the present

To measure SpO2 levels on an Apple Watch, you can take 15-second blood oxygen level spot-checks, and it will monitor your blood oxygen levels while you sleep. Other smartwatches use different methods to measure blood oxygen, but they can take better measurements when youre still and have watch bands that fit snuggly.