withings scanwatch review spo2 sensor pulse oximeter
Jimmy Westenberg / Android Authority

Your fitness tracker is capable of monitoring all kinds of weird and wonderful things. The problem is keeping track of what, exactly, it all means! SpO2 is just one example of a lesser-understood metric that can provide insight into your health and wellbeing. You’ve probably seen it in your health app’s dashboard, but just what does it mean and what are you supposed to do with that information? We break down exactly what SpO2 measures, what good SpO2 levels look like, and more.

What is SpO2?

Pulse Oximeter
Adam Sinicki / Android Authority

Put simply, SpO2 is a measure of blood oxygen saturation. The more oxygen you have in your blood, the higher your SpO2 levels are going to be. Seeing as our bodies need oxygen to survive, we can broadly state that higher SpO2 levels are a good thing. Low levels of blood-oxygen, conversely, can lead to symptoms known as hypoxemia. A normal reading is anything between 95% and 99%.

Devices such as the Apple Watch Series 6, Fitbit Versa 3, Xiaomi Mi Band 5, and countless others all offer this feature. But just how accurate are they? And what can you do with the information?

See also: What is heart rate variability and why does it matter?

It is more accurate to say that SpO2 is a measure of hemoglobin. For those that have forgotten their high school biology, hemoglobin are “red blood cells” that contain oxygen and carry it around the body. The body replenishes its oxygen stores naturally through regular breathing. Here, oxygen enters through the mouth and nose, then passes through tiny sacs and blood vessels in the lungs (called alveoli and capillaries) into the blood stream.

When we run, lift weights, or otherwise “train,” we increase the demand for oxygen. Thus, our breathing and heart rate increase to meet the increased demand.

How do fitness trackers measure SpO2?

apple watch series 6 review heart rate sensor
Jimmy Westenberg / Android Authority

Fitness trackers and other devices called “pulse oximeters” can measure SpO2 levels. These work in a similar manner to wrist-worn heart rate monitors, by shining light of a particular wavelength (usually infrared) through the skin. A highly sensitive photodetector effectively allows the device to “see” the hemoglobin in your blood vessels.

The main difference between a fitness tracker that measures SpO2 and a pulse oximeter, is that the latter is placed on the fingertip. This is the ideal spot to take a reading and that makes these dedicated devices more accurate.

Also read: Fitbit buyer’s guide: Everything you need to know

Even then, this is not an exact science. Everything from skin tone, to ambient light, to movement can all impact accuracy. Fitness trackers can attempt to counteract the challenge of movement skewing the results by using algorithms that account for data from the accelerometer/gyroscope, but this can only achieve a certain degree of accuracy.

For these reasons, manufacturers are very keen to point out that their devices should not be used for medical diagnosis. Only a handful of wearables are certified as medical devices to offer SpO2 readings, one of which is the Withings ScanWatch.

For those keen to get the most accurate readings possible from their trackers, though, a neat trick is to place a black wristband or a long sleeve over your tracker.

How do you act on SpO2 levels?

garmin venu review pulse oximeter sensor spo2

So, why should you care how much oxygen is in your blood?

Well, for the most part… you probably shouldn’t.

SpO2 data is, broadly, useful for two categories of people:

  • Athletes whose training requires them to carefully monitor SpO2 (divers and climbers that train at high altitudes, for example)
  • People with health conditions affecting their breathing

Perhaps a third category could be:

  • Biohackers and those interested in experimenting with techniques such as the Wim Hof method

Sleep apnea

apple watch series 6 review blood oxygen results spo2
Jimmy Westenberg / Android Authority

SpO2 can be a useful indicator of sleep apnea. Sleep apnea is a condition that causes a person to wake up intermittently in the night due to difficulty breathing. This can occur due to physical blockages that hamper breathing (obstructive apnea) or interference in the signals sent by the brain telling the body to breath (central apnea). The individual wakes up in the night to gasp for air but often isn’t consciously aware that this is happening. The result is that such people often wake up feeling extremely unrested and tired, and may even be dangerous behind a wheel; but they often have no idea what’s causing the problem.

A device that measures SpO2 as well as movement during sleep can potentially alert them to this problem and allow them to take action. Of course, your SpO2-capable wearable needs to be able to track blood oxygen levels at night. Some of today’s wearables only allow for spot checks throughout the day, while others run at night. Usually, SpO2 monitoring throughout the night significantly decreases the wearable’s battery life.

This next step would normally involve visiting a sleep specialist.

See also: Fitbit Sleep Score: How does it work?

SpO2 and COVID-19

xiaomi mi watch review spo2 blood oxygen recording 2
Jimmy Westenberg / Android Authority

There is a little confusion surrounding the link between SpO2 levels and COVID-19. Because some healthcare providers use pulse oximeters to help diagnose the coronavirus, there has been a rush to order such devices.

COVID-19 is a condition affecting the lungs, meaning difficulty breathing can be an indicator of the virus. Moreover, this can be useful as a tool to see how a person is coping with the virus. Wearable SpO2 monitors can provide a useful early warning sign for complications such as pneumonia.

But remember, SpO2 monitors are not perfectly accurate and can give false readings. Moreover, this is only one of many symptoms associated with COVID-19; plenty of other conditions can cause difficulty breathing.

Altitude training

Skipping Mask
Adam Sinicki / Android Authority

When training at altitude, it is more difficult for us to extract the proper amount of oxygen from the air. Oxygen levels at sea-level are typically around 21%, but this can drop to 15% at altitudes of over 3000m.

Training at altitude can force useful adaptations in an athlete, helping them to utilize oxygen even more efficiently. After a few weeks, the body will begin producing more red blood cells, for example. Within days, different adaptations can alter the way those red blood cells hold onto oxygen. Until these changes occur, the athlete will find they feel sluggish as less oxygen is delivered to the muscles. Thus, an SpO2 sensor can help that individual train safely and see the benefits of that training.

Side note: altitude masks do not truly simulate the experience of training at altitude and won’t trigger the same adaptations. That said, they can provide other useful benefits, such as strengthening the intercostal muscles.

Breathing techniques

garmin vivoactive 4 review workout screen breathwork elliptical

Biohackers use a variety of breathing techniques to alter their physiology. The Wim Hof technique is perhaps the best known of these and has rapidly grown in popularity owing to claims that it can strengthen the immune system, increase energy levels, and more. Wim himself is an eccentric and charismatic individual who is known for incredible feats; such as running up Mount Everest in his underpants.

This technique involves taking a series of deep inhalations with very shallow exhalations. After 30 rounds of this, you then take a last deep breath and let it out slowly. Finally, you hold your breath for a further 15-20 seconds.

This process saturates the blood with oxygen (which is where the SpO2 reading can come in handy) and drastically reduces carbon dioxide. This doesn’t have the effect you might imagine but rather simulates hypoxia. Why? By lowering carbon dioxide and lowering the blood’s pH level, you prevent the body from using the stored oxygen. It’s through this same process that hyperventilation can cause someone to pass out. The result is a sympathetic response that elevates the heart rate, releases anti-inflammatory cytokines, and provides a rush of energy.

The method is complex and somewhat controversial, but it serves as a useful demonstration of the complexity of SpO2 and the body in general.

Next: The best running watches you can buy

So, that’s how SpO2 works and what you can do with it. For most of us, it’s a useful feature for monitoring sleep and perhaps getting just a little insight into the way our body handles training. For those interested in hacking their physiology or training at altitude, it may have a few other benefits.