The smartwatch's Blood Oxygen app lets you measure the oxygen level of your blood on-demand. It's been MIA for more than a year thanks to a patent spat, but it's back in action now.

A new sensor could soon allow hospital patients' blood oxygen levels to be checked via their breath. The technology is claimed to be more reliable and less painful than existing traditional methods.

Thanks to new OS updates rolling out now, you can once again use your Apple Watch to measure the oxygen level in your blood.

Like most modern wrist wearables, that ECG sensor uses LED lights to monitor your heart rate and pulse transit time (PTT). The rear PPG passively determines blood oxygen levels as well.

Masimo's device offers continuous real-time oxygen saturation and pulse rate monitoring, and it can be used as a medical device in hospitals, clinics, and at home.

The change doesn’t affect previously sold models with the original version of the feature or units bought outside the U.S.

The smartwatch's Blood Oxygen app lets you measure the oxygen level of your blood on-demand. It's been MIA for more than a year thanks to a patent spat, but it's back in action now.