Blood oxygen (SpO2): what a healthy reading looks like
How oxygen saturation is measured and what the percentage actually represents.
SpO2 stands for peripheral oxygen saturation — the percentage of oxygen-carrying capacity in your blood that's actually occupied by oxygen. For most healthy people at sea level, a normal reading falls between 95% and 100%. Readings in the 90-94% range are generally considered on the low side and worth attention if they persist, and anything below 90% is typically flagged as low oxygen saturation warranting prompt evaluation.
The measurement works by shining light through tissue — traditionally a fingertip — and analyzing how much of it is absorbed. Oxygen-rich blood and oxygen-poor blood absorb light differently, and that difference is what gets translated into a percentage.
A number of everyday factors can nudge a reading temporarily: cold hands, certain nail polishes, altitude, and even how still you're holding a device during measurement. That's why an isolated low reading is worth rechecking under better conditions before treating it as meaningful.
Like most vitals, SpO2 is more informative as a baseline you check in on periodically than as a one-time number — particularly because it tends to stay remarkably stable in healthy people, which makes a real, persistent shift easier to notice.
