If you’ve been hunting for a new smartwatch, you’ve probably come across the terms SpO2 sensors, pulse ox, or blood oxygen levels. SpO2 sensors measure your blood oxygen saturation — or, put more simply, the amount of oxygen you have in your blood. In fact, these sensors and metrics are included in most modern smartwatches and fitness trackers. The only issue is that not every wearables maker uses these sensors in the same way.

Some smartwatches measure your SpO2 passively as you sleep while others will let you take a direct measurement. But don’t worry, we’ll get into what SpO2 sensors are, how they work, and their limitations. We’ll also dive into how to set up SpO2 measurements on some of the more popular wearables that support this metric.

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How SpO2 sensors work

You might be familiar with pulse oximeters, which are clips you wear on your finger that measure your blood oxygen saturation. These devices became more well-known during the early days of the pandemic, as low oxygen levels were a common symptom for many people with COVID-19. This led to pulse oximeters becoming a must-have item.

SpO2 sensors in smartwatches work similarly to the photoplethysmography (PPG) sensors used to measure heart rate. PPG — or optical heart rate — sensors work by shining a green light into your skin to determine your heart rate based on the light that’s reflected back. The difference is that, instead of green light, smartwatch SpO2 sensors shine red and infrared light into your skin. Then, based on the way light reflects back, the devices use an algorithm to estimate your blood oxygen levels.

A “good” score is generally defined as anything above 95 percent, but results above 90 percent are also considered normal. Also, each wearable maker defines what a “good” or “normal” measurement is, so be sure to thoroughly read any included explainers.

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Limits of wearable SpO2 sensors

There are lots of reasons why wearable makers have begun adding these sensors to their devices. Most have added it as a factor in determining your sleep quality. Others, like Fitbit and Withings, have added SpO2 sensors as a potential way to identify conditions like sleep apnea.

But while fingertip pulse oximeters are a Class II medical device, smartwatch SpO2 features are meant for general wellness only. As such, they generally do not require FDA clearance. (The Withings ScanWatch, however, has received FDA clearance for its SpO2 feature to detect breathing disturbances during sleep.) Blood oxygen measurements taken from the wrist are also usually less accurate than those taken from the fingertip. That’s because while fingertip pulse oximeters shine light through your entire finger, wrist-based sensors use the less reliable method of measuring the reflection of light.

In short, you should not use wearable SpO2 readings for any medical purpose whatsoever. You also shouldn’t put too much stock in these readings. The best they can do is give you a sense of what your baseline is. Even then, you should expect wonky readings from time to time.

Now that we’re aware of the limitations, here’s how to use SpO2 settings and spot checks on various wearable platforms.

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Apple Watch

Apple introduced SpO2 sensors in its Series 6 Watch. That means if you want to take SpO2 readings, you’ll need either a Series 6 or Series 7. The Apple Watch SE does not include SpO2 sensors.

On the Apple Watch, you can take 15-second spot checks of your blood oxygen levels. If you wear the device to sleep, it’ll also take background readings while you snooze.

To set up Blood Oxygen on your Apple Watch:

• Open the Health app on your iPhone.
• Tap the Browse tab.
• Scroll down to Respiratory.
• Tap Blood Oxygen.
• Tap Enable.

To take spot readings:

• Open the Blood Oxygen app on the Apple Watch.
• Tap Start.
• Wait for 15 seconds.
• Tap Done.

Keep in mind that spot check readings can be finicky. Try to keep very still while taking measurements, resting your arm on a table or your lap. Also, make sure your strap is comfortably snug and that the sensor array is making good contact with your skin.

To enable or disable background readings:

• Open the Settings app on the Apple Watch.
• Scroll down to Blood Oxygen.
• Toggle on or off Blood Oxygen Measurements for periodic daytime readings.
• Toggle on or off In Sleep Focus for nighttime readings.
• If you want to suppress readings during the movies or other events, you can toggle on or off the In Theater Mode setting.

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Fitbit

Fitbit was one of the first to incorporate SpO2 sensors in 2017 with the Fitbit Ionic. However, it wasn’t until 2020 that it began actively incorporating the metric. Every current Fitbit device except for the Inspire 2 is capable of tracking your blood oxygen levels.

Unlike the Apple Watch, you cannot take a SpO2 spot check reading on Fitbit devices. The metric is much more passive. You don’t have to do anything to enable the metric, but here’s how you can view your SpO2 baseline.

To view your nightly SpO2 metric:

• Tap the Today tab in the Fitbit app.
• Tap the Sleep tile.
• Tap a day’s sleep log to find the Estimated Oxygen Variation graph.
• For Fitbit Premium users, tap the Restoration tile in a sleep log to see the EOV graph.

To view your SpO2 trends on the Fitbit Charge 4, Charge 5, Luxe, Sense, Versa 2, and Versa 3:

• You must have a Fitbit Premium subscription.
• Make sure your device is up to date.
• Wear your device for a full 24 hours.
• The next morning, sync your device.
• Tap the Today tab.
• Tap the Health Metrics tile.
• Scroll to Oxygen Saturation (SpO2).

You can also install the SpO2 clockface to view the information on your wrist.

• Tap the Today tab.
• Tap your profile picture.
• Tap the picture of the device you want to install the clockface on.
• Tap Gallery.
• Navigate to the Clocks tab.
• Search for the SpO2 clockface. You can tap View All to see the complete list.
• Select the SpO2 clockface.
• Tap Install.

Keep in mind that it may take an hour or two after your daily sync for the clockface to display your SpO2 data on the wrist.

If you have the Sense or Versa 3, you can also enable background SpO2 readings via the SpO2 Tracker app. To install:

• Follow the first four steps in the last section.
• Search for the SpO2 Tracker app.
• Tap Install.

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Garmin

Garmin devices refer to SpO2 as Pulse Ox. There are various settings for how frequently your tracker or smartwatch samples blood oxygen levels. You can also view your blood oxygen levels via the Pulse Ox widget on supported devices.

To take a spot check reading:

• Navigate to your device’s menu.
• Tap the Pulse Ox widget.
• Sit still as it calculates your blood oxygen percentage.

To enable or disable blood oxygen tracking:

• Open the Garmin Connect app.
• On Android, select the three-bar icon in the top left.
• On iOS, tap More on the bottom right.
• Tap Garmin Devices.
• Select Activity Tracking.
• Tap Pulse Ox.
• Toggle on or off the Sleep Tracking or All-Day Tracking settings.

To view sleep Pulse Ox metrics:

• Follow the first three steps in the last section.
• Tap Health Stats.
• Select Sleep.
• You’ll see three tabs: Stages, Pulse Ox, and Respiration.
• Tap Pulse Ox.

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Samsung Galaxy Watch 4 and Watch 4 Classic

Like the Apple Watch, the Samsung Galaxy Watch 4 and Watch 4 Classic allow you to take spot check readings. For passive sleep measurements, you’ll have to manually turn on that setting, as it’s turned off by default.

To add the Blood Oxygen tile:

• From the main screen, swipe left to access your tiles.
• If you do not already have the Blood Oxygen tile, swipe all the way to the left.
• Tap Add Tile.
• Scroll until you find Blood Oxygen.
• Tap the Blood Oxygen tile to add it to your watch.

To take spot check readings:

• Swipe left on your watch until you reach the Blood Oxygen tile.
• Tap Measure.
• Scroll through the onscreen instructions.
• Tap Ok.
• Stay still while it takes your reading.

To enable blood oxygen measurements during sleep:

• Open the Samsung Health app on the watch.
• Scroll down to Settings.
• Tap Measurements.
• Tap Blood Oxygen During Sleep.
• Toggle on Measure Constantly.

As you can see, SpO2 features on smartwatches are still in their infancy. In the future, we may see more advanced applications of these sensors and will update this guide accordingly. In the meantime, it’s worth remembering that SpO2 features are currently aimed at providing extra context as to your baselines and how your body recovers from exercise — and not much more.