Definition
The percentage of hemoglobin in the blood that is currently carrying oxygen, compared to the maximum amount it could carry. In aviation, it is most commonly measured at the fingertip with a pulse oximeter and reported as SpO2. Healthy values at sea level are typically 95 to 100 percent. As altitude increases, the lower partial pressure of oxygen in the lungs reduces saturation, and values below roughly 90 percent indicate the onset of hypoxia.
Plain English
How full your blood is of oxygen, shown as a percentage. The higher the number, the more oxygen your body is getting.
Context Anchor
Seen when using a pulse oximeter before or during flight, especially at higher altitudes or when using supplemental oxygen.
Derivation
From Latin saturare, meaning 'to fill.' The blood's oxygen carriers (hemoglobin) can only hold a fixed amount of oxygen, so 'saturation' describes how full they are. 100 percent means completely filled.
Why Pilots Care
Low readings warn of hypoxia risk that can impair judgment and lead to unconsciousness.
Analogy
Think of hemoglobin like seats on a shuttle bus and oxygen like passengers. Oxygen saturation is the percentage of seats that are filled.
Grounding Statement
At altitude, a pilot may feel normal while a pulse oximeter shows oxygen saturation dropping, which is a cue to take the oxygen situation seriously.
Intuition Check
Do not read “saturation” as meaning the air is full of oxygen. Here it means the oxygen-carrying part of the blood is filled with oxygen.
Example Sentence 1
Cruising at 12,500 feet without supplemental oxygen, the pilot checked her pulse oximeter and saw her oxygen saturation had dropped to 88 percent, so she descended.
Example Sentence 2
A drop in oxygen saturation below 90 percent prompted immediate use of supplemental oxygen.