One of the most important metrics on a Fitbit tracker is the Estimated Oxygen Variation—and now that every tracker is fitted with an SpO2 sensor, everyone can take advantage. Blood oxygen may be used ...

Want more charts like these? See our charts on the secrets of the jobless recovery, the richest 1 percent of Americans, and how the superwealthy beat the IRS. How Rich Are the Superrich? A huge share ...