https://techcrunch.com/2020/09/15/apple-watch-series-6-will-measure-blood-oxygen-levels/

Apple’s new Series 6 watch has some exciting new health features. Thanks to a new health sensor, the Apple Watch Series 6 is able to measure your blood oxygen levels in 15 seconds, the company announced today.

The sensor uses green, red and infrared LEDs, in addition to the four photodiodes on the Apple Watch’s back crystal, to measure the light reflected back from your blood. Using an algorithm built into the Blood Oxygen app, the watch then measures your blood oxygen level between 70% to 100%.  The watch will also record background measurements while you sleep.