  Peripheral blood oxygen saturation Sp02 is a vital measure in healthcare.
Modern off-the-shelf wrist-worn devices, such as the Apple Watch, FitBit, and
Samsung Gear, have an onboard sensor called a pulse oximeter. While pulse
oximeters are capable of measuring both Sp02 and heart rate, current wrist-worn
devices use them only to determine heart rate, as Sp02 measurements collected
from the wrist are believed to be inaccurate. Enabling oxygen saturation
monitoring on wearable devices would make these devices tremendously more
useful for health monitoring and open up new avenues of research. To the best
of our knowledge, we present the first study of the reliability of Sp02 sensing
from the wrist. Using a custom-built wrist-worn pulse oximeter, we find that
existing algorithms designed for fingertip sensing are a poor match for this
setting, and can lead to over 90% of readings being inaccurate and unusable. We
further show that sensor placement and skin tone have a substantial effect on
the measurement error, and must be considered when designing wrist-worn Sp02
sensors and measurement algorithms. Based on our findings, we propose
\codename, an alternative approach for reliable Sp02 sensing. By selectively
pruning data, \codename achieves an order of magnitude reduction in error
compared to existing algorithms, while still providing sufficiently frequent
readings for continuous health monitoring.
