The researchers had 24 healthy participants wear an Apple Watch Series 6—which is outfitted with the same blood oxygen reader as the newer Apple Watches—on their left wrist while placing the medical-grade reader on their left middle finger. The participants wore breathers that slightly reduced the oxygen they took in over a few different phases, and the researchers recorded the blood oxygen levels recorded by each device at 30-second intervals. The Apple Watch is very close to achieving accuracy levels of a pulse oximeter. The blood oxygen level (which is called SpO2 in the study) bias across all data points was 0 percent. Here’s the rest: “The bias for SpO2 less than 90% was 1.2%. The differences in individual measurements between the smartwatch and oximeter within 6% SpO2 can be expected for SpO2 readings 90%-100% and up to 8% for SpO2 readings less than 90%.”