There's a new wearable tech metric in town – and it's all about blood oxygen. It might sound complicated and a little pointless, but pulse ox can reveal conditions like sleep apnea – and help athletes recover.
Pulse ox is now on the spec sheet of pretty much everything wearable and fitness tracker out there.
The Apple Watch Series 7, Fitbit Charge 5, Huawei Band 6 all track it – and in fact, it's now harder to find a wearable that's NOT keeping tabs on blood oxygen levels.
So why is putting a pulse oximeter inside of a wearable a big deal?
We explore what it is, how it works and what it's going to bring to the wearable party.
What is a pulse oximeter/SpO2 sensor?
When we talk about pulse oximeters or pulse oximetry, we are delving into the realm of medical tech and talking about a device that's able to measure oxygen levels or oxygen saturation in the blood.
That tech used to take form of a clip-on device that you place on your finger, a toe or even on your ear lobe.
Optical SpO2 sensors use red and infrared light sensors to detect your oxygen levels, sensing changes in those levels by looking at the color of your blood.
It measures the volume of oxygen based on the way the light passes through your finger and delivers the data to the device's screen, which will tell you the percentage of oxygen in your blood.
Why track blood oxygen?
The Fitbit Estimated Oxygen Variation score
An oxygen saturation percentage greater than 95% is considered to be a normal reading. If you see a score of 92% or less, then it could be time to further investigate, and find out whether it's related to an as yet undetected health issue.
John Hopkins Medicine explains how measuring oxygen levels through pulse oximetry can offer insights into a range of health related issues.
It can be used to check whether someone needs assistance with their breathing via a ventilator, measure a person's ability to handle intensive physical activities, and it can also check whether you're experiencing breathing issues when sleeping.
COVID-19 has put the focus on blood oxygen on the map, but actually, there are many less extreme reasons to keep an eye on blood oxygen levels.
Athletes or people spending time at altitude will want to monitor blood oxygen levels.
However, it's sleep apnea that really makes SpO2 a worthwhile sensor. It's a disorder which if left untreated or undetected could lead to an increase in the risk of high blood pressure, obesity and can even cause a heart attack. And that's where many health watches and wearables can come in useful.
It's estimated that 22 million Americans suffer with sleep apnea, but the majority aren't even aware they have the disorder.
It can also be a valuable piece of health data for people suffering from a range of conditions including asthma, pneumonia, heart failure and lung cancer.
The origins of the pulse oximeter
The first oxygen saturation meter is said to be from as far back as the 1930s, when the exploration of light transmission through skin and the information it could provide really began.
It wasn't until the 1960s and 70s when we began to see the pulse oximeter devices shape into the ones that are now used in hospitals, and which can be purchased to carry out those measurements from your home. Hewlett Packard was the first company to make an ear oximeter, which was largely used inside of clinical sleep labs due to its hulking size.
But it was Japanese bioengineer Takuo Aoyagi, in the early 1970s, who first developed a noninvasive way of using the light transmitted through the ear and went on to develop a pulse oximeter.
From then up until today, the size of the tech has become smaller and – crucially – cheaper to build, so more people were able to get their hands on it.
Pulse oximeter and wearables you can buy right now
Pulse oximeters are starting to find their way into some big name wearables and that data is being used in very different ways.
Arguably it started with the Withings Pulse Ox fitness tracker, which measured blood oxygen levels when you placed your finger on the sensor on the back of the device. But things have changed since then, and now the process of taking those measurements happen much more easily from the wrist.
SpO2 was finally introduced on the Apple Watch Series 6, which can now take spot readings and analyse blood oxygen during sleep.
You can use the Blood Oxygen app to take a 15 second SpO2 test, which is logged in the Apple Health app.
However, if you wear the Series 6/7 to bed then it will take spot readings while you sleep. This means it can be used as an early warning system of sleep apnea, and something you can use to start a conversation with your doctor.
The sensor isn't available on the Apple Watch SE, however, so you need the flagship Apple Watch to track it.
Fitbit has been doing SpO2 for a while and it's available on all Fitbit Versa smartwatches, as well as Charge 5 and the Fitbit Sense health watch.
Unlike other brands the focus of the SpO2 isn't on spot checks, and is used to enrich sleep data if you subscribe to Fitbit Premium. You get the Estimated Oxygen Variation graph which shows your blood ox throughout sleep, using a traffic light system to show any (possibly) apneic events.
And now things go further too. Breathing rate is a new metric for Fitbit Premium subscribers, which offers extra data. As part of the Fitbit COVID-19 study, a change of breathing rate was shown to pre-empt symptoms, so it can be a useful metric to track.
What's more, the company has launched a Fitbit Versa SpO2 watch face, which puts the information front-and-center.
Read our guide to choosing the best Fitbit for your needs.
Garmin has introduced pulse ox sensors into a large part of its range, with the Fenix 7, Fenix 6X, Fenix 5X, Vivoactive 4 and Forerunner 245/645/945 all packing an SpO2 sensor.
In this instance the on-screen widget will offer a blood oxygen percentage, coupled with data on your altitude, to show the levels of oxygen in the blood.
This is particularly useful for anyone that's into hiking, alpine sports and going on big expeditions. With elevation data you can view how oximeter readings are changing relative to your elevation.
However, many wearables use pulse ox for a different reason: sleep apnea. This is a condition that 8/10 suffers do not know they have, where blood oxygen levels dip during sleep.
The Garmin Vivosmart 4 able to log blood oxygen levels during sleep, and it's reported in your sleep data. However, the device stops short of alerting you to sleep apnea in particular, and you need to look at the data yourself.
What's more, on many devices the pulse ox is turned off by default as it impacts battery life. As such, Garmin offers you the chance to spot check pulse ox, but you can turn on night time tracking and continuous monitoring, if you're willing to charge your device much more often.
The Withings ScanWatch is now out in Europe, and uses an SpO2 sensor to monitor blood oxygen.
The company hasn't got clearance for a full sleep apnea detection, but has a graph called Breathing Disturbances in the sleep metrics, which is rated low, medium or high.
You can also take spot readings too, by cycling through the watch menu on the device itself.
This is presented as a true blood oxygen percentage score in Withings Health Mate.
Read our Withings ScanWatch review.
Huawei Watch GT 3
The Huawei Watch GT 3 has included an SpO2 sensor, and you'll find it on the GT2e and GT2 Pro as well.
However, you can only perform spot readings, so it won't take a reading during sleep.
This does make it a lot less useful as a health monitoring device, although you can check readings if you spend time at altitude.
Check out our Huawei Watch GT 3 review.
Blood oxygen is a major part of Whoop 4.0, which has added a the Health Monitor with the latest version. The Whoop Health Monitor is designed as a quick-look traffic light system to show whether you're good to go, or there's something under-the-hood that's not normal.
Whoop spends two weeks establishing baselines, so any readings are compared to your average, adding vital personal context.
The readings are taken during sleep – although there's no warning system for sleep apnea specifically. However, you will be warned if there are dips in nightly blood oxygen.
How we test