Oxygen is carried around in your red blood cells by a molecule called haemoglobin. Oxygen saturation (SaO2) is a measure of how much oxygen the haemoglobin is carrying.
Low SaO2 means that the lungs are struggling to transfer oxygen into the blood. This is more likely to happen when you exert yourself because that is when blood is travelling faster through the lungs.
Assessing your oxygen levels
A pulse oximetry test uses a sensor placed on your fingertip or earlobe to measure your oxygen saturation. For someone who’s healthy, the normal blood oxygen saturation level will be 95–100%. If yours is 92% or less when you are resting, you should be referred for an assessment to see if supplementary oxygen is appropriate.
At the assessment, your oxygen levels may be measured both at rest and when you walk. You may also have a blood gas analysis, where a small sample of blood is taken from your wrist, the inside of your elbow or your earlobe.