Smartwatches can measure everything from heart rate to sleep quality, but one health metric has become particularly relevant over the past two years: blood oxygen saturation. Two of the world's biggest smartwatch makers, Apple and Samsung, added blood oxygen monitoring to their wearables in 2020. The COVID-19 pandemic also made measuring vitals from home more desirable.
But the arrival of blood oxygen monitoring in smartwatches also raised questions about how useful this information is without the context of a medical professional. In CNET's review of the Apple Watch Series 6, Vanessa Hand Orellana said she wished the Apple Watch could provide more guidance to accompany blood oxygen readings. (When her levels dropped to 92% overnight, she didn't know whether to be concerned.) Most smartwatches also aren't cleared by the US Food and Drug Administration for blood oxygen measurements and can't be used for medical purposes, making it difficult to understand how these metrics should be interpreted.
Roughly two years later, are blood oxygen readings any more useful than they were in 2020? The answer isn't that simple. Medical experts say measuring blood oxygen throughout the day and under different conditions could unlock insights you won't get with a traditional pulse oximeter. Plus, having more access to health data from home is also usually a good thing.
But these sensors still have shortcomings that can limit usefulness. And smartwatch makers are still figuring out the best ways to incorporate blood oxygen measurements into broader features that give users a complete picture of their overall health.
"We know the science behind them is still not as accurate as the ones that are hospital grade with regard to the way the oxygen is determined," said Dr. Albert Rizzo, chief medical officer at the American Lung Association. "But having said that, they've become useful from the standpoint of patients, or not even people who are medically ill, even well persons, to be able to track another vital sign."
Blood oxygen monitoring's breakthrough
To understand whether measuring blood oxygen levels from your smartwatch is useful, it's crucial to know what this metric means and how it's implemented in today's wearables first. Your blood oxygen level, also known as SpO2, refers to how much oxygen your red blood cells carry. It's considered an important indicator of respiratory health since it signals how well your body is able to absorb oxygen.
Blood oxygen saturation is typically measured through a pulse oximeter that clips onto your finger. Smartwatches like the Apple Watch measure this by shining a light through your wrist and measuring the light reflected.
If your current smartwatch or fitness tracker can't measure your blood oxygen levels, chances are the next one you buy will. The technology has become a staple in today's wearables and can be found in products from Apple, Samsung, Fitbit, Garmin and Withings, among others.
The Apple Watch Series 6 and Series 7 both measure blood oxygen levels, as do the Samsung Galaxy Watch 3 and Watch 4. Fitbit devices such as its Sense, Ionic and Versa smartwatches and the Charge 4, Charge 5 and Luxe fitness bands can also measure blood oxygen levels overnight during sleep.
But most companies haven't received FDA clearance for their blood oxygen measuring technology. Withings is the exception; the blood oxygen monitor in its ScanWatch and ScanWatch Horizon are both FDA-cleared. Maxime Dumont, Withings' product manager for smartwatches, says the FDA clearance should make its data more trustworthy to doctors.
"We will never replace a doctor, and we are not intended to make any diagnosis with a watch," he said. "But the watch results are reliable for a physician."
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Even though it was possible to take blood oxygen readings from a smartwatch before 2020, the technology had a breakout moment two years ago. As the pandemic overwhelmed hospitals and the healthcare system, there's been more interest in researching how wearables can monitor bodily changes at home.
Devices from Apple, Fitbit, Garmin and Oura have all been used in research examining whether wearable devices can predict disease early by measuring changes in bodily signals like heart rate and temperature. A 2021 study published in Scientific Reports from researchers at the University of Sao Paolo and Centro Universitário FMABC also found the Apple Watch Series 6 to be reliable at gathering SpO2 and heart rate data in patients with lung disease in a controlled environment.
"The massive number of people that the health system had to deal with made it a little easy for health systems to experiment with these non clinical-grade oxygen sensors," said Dr. Nauman Mushtaq, medical director of cardiology at Northwestern Medicine Central DuPage and Delnor hospitals.
How useful are these sensors in smartwatches?
While health sensors in smartwatches show promise in research, some experts are unsure how often these sensors are being used in everyday circumstances. "I have had a few patients who have used Apple Watches or similar devices to monitor their blood oxygen levels," said Dr. Ashraf Fawzy, a pulmonologist and critical care physician and Assistant Professor of Medicine at Johns Hopkins University. "But it hasn't been as common as I would have thought."
Regarding regular use, Dr. Mushtaq sees blood oxygen sensors in devices like the Apple Watch as most useful for adding more context regarding overall wellness. In most cases, the average healthy person would experience physical warning signs before experiencing hypoxemia, or low blood oxygen, he said.
"I don't think it, to be honest, does anything that is clinically meaningful for an average person," he said.
That doesn't mean medical experts don't see potential. Smartwatches have a big advantage over traditional pulse oximeters: their position on your wrist all day. Many smartwatches can take background blood oxygen measurements in addition to providing spot checks, meaning they can gather data at different times during the day.
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Fitbit, Samsung, Garmin and Apple devices can monitor blood oxygen levels passively during sleep, unlike a traditional pulse oximeter which is used for taking on-demand measurements. Both Apple and Garmin can also sample blood oxygen levels periodically throughout the day.
But smartwatches are only good at checking SpO2 levels at rest, even when taking scans in the background. (Apple says its background measurements happen when the wearer isn't moving, and Garmin says it takes readings less often if it detects high movement).
Measuring blood oxygen levels during strenuous activities would make these devices more useful since it could help doctors know whether to adjust the amount of oxygen a patient is being prescribed, according to Dr. Fawzy. Dr. Mushtaq also said patients with heart failure or pulmonary hypertension could benefit from seeing whether their blood oxygen levels drop during exercise.
"That can certainly help," said Dr. Fawzy. "Because for some people, their oxygen levels only drop when they're being active and will be normal when they're sitting quietly."
Health metrics are most useful when put in context, whether it be blood oxygen levels or how many steps you've taken. The numbers and charts only matter when you know how to put them to good use.
"Ultimately, consumers aren't buying sensors," Julie Ask, vice president and principal analyst at research firm Forrester, said in a previous interview with CNET. "They're not buying data. Consumers are buying what they hope is help achieving an outcome."
So what kind of context do smartwatches need to provide to make blood oxygen readings more useful? Some companies are trying to answer that question by weaving SpO2 results into other features and in-app wellness reports to better understand your overall health. Samsung, for example, incorporates SpO2 measurements into its sleep coaching feature on the Galaxy Watch 4 to help you make sense of your sleep patterns, according to a Samsung representative. Withings uses blood oxygen levels as one of the metrics it analyzes when determining breathing disturbances, along with heart rate and motion.
Phil McClendon, the manager of Garmin's wellness product management team, couldn't comment on future plans when asked whether SpO2 measurements would be factored into other health insights. But he pointed to Garmin's Health Snapshot as an example of the company's approach to making health data more meaningful.
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Health Snapshot compiles various metrics (including heart rate, blood oxygen, heart rate variability, respiration and stress) to provide a high-level view of your cardiovascular status. McClendon said the feature helps people quantify changes that may be happening in their bodies during abnormal events.
"So maybe they're having a panic attack, and they're like 'I want to record this thing and export the PDF to take to my healthcare provider," he said as an example.
Right now, the biggest benefit of measuring blood oxygen levels from your smartwatch is learning what's considered normal for your body. Even though smartwatches aren't meant for medical diagnosis, it's another signal you can take to your doctor if you're not feeling well or notice a bodily change.
"Whatever device you're using, compare it to your baseline, use it as a trend monitor so that you know that you're off from your baseline," said the American Lung Association's Dr. Rizzo. "It may allow you to change what you're doing or seek help sooner than you otherwise would."