Oxygen saturation refers to the amount of oxygen being carried in the red blood cells, which load up with oxygen in the lungs and then transport it to the rest of the body. People who have medical conditions, such as lung disease, may not have enough oxygen in their red blood cells.

Oxygen saturation can be measured by a device called an oximeter. Oxygen saturation of less than 90% is a sign that the lungs and heart are not working well.