The blood oxygen sensor in the Apple Watch measures the percentage of oxygen your red blood cells carry from your lungs to the rest of your body. It's an important health metric since it can help you better understand your overall wellness. According to a 2022 medical study, the measurement of blood oxygen levels (in an Apple Watch Series 6) was not yet up to the medical standard of pulse oximetry; too many of the measurements remain unsuccessful or incorrect. Studies have not been conducted on the most recent Apple Watch models. *Important Note: Earlier this year, a US federal judge has ruled that Apple cannot sell its Series 9 and Ultra 2 smartwatches if they have the blood oxygen sensor, due to a patent violation. As a result, a ban was placed on the import of all Apple watches with the feature. Apple Watch users in the US are no longer able to access the blood oxygen monitor app or sensors.