The new Apple Watch Series 6 includes a sensor that allows the watch to measure blood oxygen levels. The device is a long-awaited addition to Apple’s suite of health tools, but it tracks oxygen levels at the wrist, which can be less accurate than measurements typically gathered at the fingertip.
Most oxygen sensors, including Apple’s, measure the amount of oxygen in your blood using light. These devices are called pulse oximeters, and they typically clip on to your fingertip. A standard version sends both red and infrared light through the finger, where there’s lots of blood close to the surface. A protein in the blood absorbs more infrared light when it has oxygen and more red light when it’s doesn’t. A sensor on the other side of the finger calculates how much of each type of light travels through, providing an oxygen reading.
The Apple Watch Series 6 also has red and infrared lights, but instead of sending that light through a body part, it measures the lights’ reflection. It’s the same strategy used by Garmin and Fitbit, which already have similar blood oxygen features. But the reflective method at the wrist may be less accurate, especially when oxygen levels start to drop, according to some research. There are a few reasons why: outside light sources might be able to skew the reflected light, and compared to a finger, the outside of the wrist doesn’t have as many blood vessels close to the surface of the skin.
The Apple Watch’s blood oxygen sensor isn’t a medical device and won’t be able to diagnose or monitor any medical conditions. The company says the feature is simply there to help users understand their fitness and wellness. But Apple did connect the feature back to the COVID-19 pandemic during the product announcement: “Blood oxygen and pulse oximetry are terms that we’ve heard a lot about during the COVID pandemic,” said Sumbul Ahmad Desai, Apple’s VP of health.
Early on in the pandemic, doctors found that monitoring COVID-19 patients using pulse oximeters could help detect serious problems with their oxygen levels before they started to feel short of breath. The gadgets suddenly became a must-have item and flew off the shelves, creating shortages. Some people turned to devices like Garmin watches as a workaround. Others called for Apple to activate sensors that were built into older versions of the watch and appeared capable of measuring blood oxygen levels.
Blood oxygen monitors in non-medical, wearable devices like the Apple Watch are fairly new, so there hasn’t been much independent evaluation to see how well they actually match up to typical fingertip monitors. Apple isn’t saying that its blood oxygen measure can treat a medical condition, so it doesn’t need to get clearance from the Food and Drug Administration, which would require coughing up some of that reliability data.
Normally, a “wellness”-focused feature that offers some information about your oxygen levels could still be useful information and a good party trick. But there are risks to relying on inaccurate blood oxygen metrics, especially during the COVID-19 pandemic. This disease is weird. Unlike many illnesses, health care workers can’t always trust that a patient’s symptoms will accurately indicate how sick they are. They need help from reliable devices that can help both patients and caregivers get an accurate read on the situation.
Apple and the other smartwatch makers haven’t cleared that bar yet. There’s a good chance they’ll publish some data on their blood oxygen sensors eventually — Apple is partnering with outside researchers to study ways their tech could be used to monitor health conditions from asthma to COVID-19. But until that’s available, it’s still probably a good idea to be a bit skeptical of smartwatch oxygen readings — and maybe get a second opinion.