Blood oxygen saturation

The share of the hemoglobin in your blood that is currently carrying oxygen, estimated by shining light through a fingertip.

It is the quickest indication that your lungs are getting oxygen into the blood, and unlike most numbers here it is read for the value on the day rather than for the trend.

Readings usually sit between 95 and 100 percent at sea level. Cold hands, poor circulation, nail polish and a loose fit all drag a fingertip reading down with nothing actually wrong, so repeat a low one on a warm hand before believing it. A watch estimates this less reliably than a dedicated oximeter.

Worth mentioning readings that stay low across several warm-handed attempts, and any that come with breathlessness.