Apple's heart rate monitor is actually a plethysmograph—it looks and acts like a pulse oximeter, but Apple isn’t claiming it can measure your blood oxygen level. Why? Beats us.
A pulse oximeter measures the difference between levels of the red pigment hemoglobin that carry oxygen in your blood. Oxygenated hemoglobin and nonoxygenated hemoglobin are different colors. The pulse oximeter uses two light-emitting diodes. One sends out invisible infrared light and the other sends out red light. By measuring the differences of light absorbed by each type of hemoglobin, the oxy/deoxyhemoglobin ratio can be calculated.
Typically, healthy individuals at sea level have oxygen saturation values between 96% and 99%. If that dropped to 65% you would experience mental impairment and a level below 55% would likely cause a loss of consciousness.
It's unclear why Apple has yet to enable this functionality in the Apple Watch. FDA regulations may be an issue or the company may be planning to surprise with the feature in a software update.
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