A pulse oximeter shines two lights through your fingertip or earlobe: one red light and one infrared light. Blood containing lots of oxygen absorbs more infrared light and lets more red light pass through it. Blood without enough oxygen absorbs more red light and lets more infrared light pass through it. If your blood cells do not have enough oxygen, they will appear bluer.
Current UK guidelines recommend that people with a resting stable oxygen saturation of 92% or less should be referred for a blood gas assessment to see if oxygen therapy is appropriate.