iFixit’s teardown of the Apple Watch has revealed that the sophisticated heart-rate monitor used is actually capable of acting as a pulse oximeter, allowing it to calculate the oxygen content of your blood by measuring how much infrared light is absorbed. This data would be useful for health and fitness monitoring, but the functionality is not currently enabled in the watch.

As iFixit notes, there are a couple of possible reasons Apple is not currently allowing to watch to display this data … 

First, it may simply be that the company hasn’t yet achieved the necessary accuracy and reliability from the sensor. If this is indeed the case, we’ll need to wait for a second- or third-generation Apple Watch to take advantage of this capability.

But it may also be that Apple is awaiting FDA approval of this usage of the sensor. If that turns out to be true, then it’s possible that a software update could later enable the feature for existing first-gen Apple Watch owners.

The Food & Drug Administration is on record as stating that it intends light-touch regulation of fitness-orientated wearables, but is likely to apply stricter standards to those features which veer more toward the medical diagnosis end of the scale.