It’s been a big year for oxygen. For many people, the ability to breathe has become a primary concern in a world gripped by a virus that wreaks havoc on the respiratory system. And of course, if you’re on the West coast, wildfire smoke makes it tougher to fill your lungs.
In response, a number of tech companies have ramped up efforts to put features that detect blood oxygen levels in their devices. Samsung’s Galaxy Watch 3 shipped this summer with a blood oxygen sensor. In September, Apple announced that its Watch Series 6 would also have the ability to monitor blood oxygen levels right from your wrist. Garmin and Fitbit have both sold products with similar pulse oximetry features for even longer.
So what is blood oxygen saturation?
Oxygen is absorbed by a protein in your blood called hemoglobin. When you breathe, your lungs load up blood cells with oxygen, then the pumping of your heart circulates the oxygen-rich blood through the rest of your body. Fresh, oxygen-rich blood keeps everything from your brain to the tips of your toes functioning and healthy. A pulse oximeter measures the amount of oxygen being carried by blood cells through your system and reports it as a percentage. That percentage is your oxygen saturation level (also called SpO2). Normal oxygen levels are between 95 and 100 percent. A rating lower than 95 can indicate problems with your body’s circulation, but your normal baseline might vary. A person’s SpO2 can also be lower because of preexisting conditions, the type of device taking the measurement, or even the amount of light in the room. (More on that in a moment.)
I feel fine. Why do I need to monitor my blood oxygen level?
If you’re asking that question, there’s a good chance that you don’t need to worry about it. SpO2 sensors are often used by climbers, free divers, marathoners, and enthusiasts who enjoy the kind of exercise that has the potential to make their body’s oxygen saturation levels dip. Think of them as oxygen superusers. The rest of us don’t really need to check our Sp02 as often.
“Do you need to have an SpO2 monitor on your watch? No, you don’t,” says Plinio Morita, who researches wearables and medical technology at the University of Waterloo in Ontario, Canada. “The only people who need an SpO2 monitor on their watch are people who are either in that superuser category or people who are sick.”
In medical situations, blood oxygen monitoring can be critical. SpO2 is an important metric for monitoring patients afflicted with respiratory illnesses like sleep apnea, emphysema, COPD, or Covid-19. Since poor circulation can lead to an inability to breathe regularly, routinely monitoring a patient with a pulse oximeter can help doctors catch potentially dangerous SpO2 levels early.
What about Covid-19?
Even in the early days of the pandemic, demand for pulse oximeters exploded as people were eager to monitor themselves or loved ones for Covid-19 any way they could. As we’ve seen in recent days, blood oxygen levels are one of the most important indicators of how well someone who has Covid-19 is coping with the virus. Oxygen levels can dip without the patient noticing, allowing pneumonia to develop undetected. Medical institutions have issued wearables to Covid patients that let them monitor their own blood oxygen levels from home.
It’s important to note that an abnormal SpO2 rating alone isn’t enough to diagnose Covid, or any other illness. Conversely, if a device isn’t taking readings properly, a seemingly normal SpO2 rating might belie other underlying problems and give wearers a false sense of security. There are a whole host of signs and symptoms that can indicate that someone has contracted the virus. You should always consult with a doctor before relying on any information gleaned from a device to make critical decisions about your health.
Look, I just skipped past all that explainery stuff. How do I actually determine my blood oxygen level?
Oximeters take measurements differently depending on where they are placed on the body. Wrist-mounted devices, like the new Fitbit and Apple Watch, measure light that is reflected back into the sensor. Flip your watch face down. See the array of LEDs on the back? That’s the sensor. Oximeters that attach to the finger—the last time you were at the doctor, a nurse might have taped one to your pointer—takes a reading by measuring the light that goes all the way through the fingertip. Cells are darker if they’re oxygen-deprived, and by measuring the color of your blood cells, the sensor can give a percentage of oxygen saturation.
If you have one of these devices, follow the directions they give you as closely as possible. (Here’s how to take a reading on the Apple Watch, Fitbit devices, Garmin watches, and the Samsung Galaxy Watch.) Even then, know that you aren’t always going to get a perfect reading.
How accurate are these things?
Even if you wear a sensor in the right spot, its readings can be affected by a whole host of factors. That’s especially true for devices that aren’t the standard fingertip readers.
“On reflective devices, the performance can be a little worse,” says John Feiner, a respiratory physiology researcher at UC San Francisco. “Anything that shines through the fingertip usually is a little better, but all of these devices can be affected by profusion, cold, blood flow. There are many things that can affect this.”
These possible inaccuracies are why companies like Apple, Fitbit, and Garmin are very careful to emphasize that the readings you get from their wearables are not meant to be used for any kind of medical diagnosis.
And since we’re talking about measuring color and light, even skin color can possibly affect oximeter readings. If you are in a position where you’d still like to monitor your SpO2 levels, there are some ways to potentially make the readings more consistent.
“Personally, one thing that I would do is to put the watch on and put a wristband on top of it—something black to cover it and keep it enclosed,” Morita says. “I would monitor myself only in controlled conditions. So I would go into my bedroom, turn off all the lights, and measure my SpO2, let's say, once an hour.” If you’re regularly monitoring your levels for the sake of your everyday health, it’s best to do so when your breathing levels are normal, and not after a period of physical exertion. “I would not use the information from the watch coming back after a run, or after cooking or walking around the house, because all of that activity is adding noise,” Morita says.
Is there a better way?
Just because wrist-mounted oximeters might lack precision, it doesn’t mean they’re entirely useless. Feiner, who has studied pulse oximetry in the medical field for over 20 years, says that the availability of these features in consumer devices shows how far the technology has come.
“Let's say you've got an agitated patient in the intensive care unit, and you're trying to get a reading and they’re moving,” Feiner says. “You need to know if you can trust that this is even in the ballpark. Many years ago, we would’ve gotten nothing. Now these devices actually perform pretty well. It’s pretty amazing.”
Both Feiner and Morita say their real concern with pulse oximetry in consumer devices is less about accuracy and more about how users will interpret the data that is presented to them.
“The biggest problem with all the wearable market that's out there is not the technology, the sensors, or the data they're collecting,” Morita says. “It’s always how that data is being presented to the user.”
A quick snapshot that tells you your SpO2 levels right at that moment isn’t very useful without context. Ultimately, if you’re concerned about your oxygen levels, your best bet is still consulting with a doctor who can track that data over time, and then interpret the results in meaningful ways.