Red blood cells contain a pigment called haemoglobin. This binds to oxygen molecules to bring oxygen from the lungs and it also brings carbon dioxide back to the lungs. Oxygen saturation (SO₂) is simply the percentage of cells with oxygenated haemoglobin. The saturation level of cells in the arteries is an important baseline and this is called arterial oxygen saturation (SaO₂). Pulse oximeters measure peripheral oxygen saturation (SpO₂), which is a good estimate of SaO₂. When a cell in oxygenated, it absorbs more infrared light than red light, giving it a vibrant red colour; the opposite is the case when it is deoxygenated, making it look darker. Pulse oximeters pass both red and infrared light through a finger, toe or earlobe and, by comparing how much of each is absorbed, calculate the percentages of oxygenated and deoxygenated cells. This makes it a very simple, quick and painless test.
Saturation levels of 95% to 100% are considered normal; below that is low and if it drops below 90% the condition is known as hypoxaemia.
On average, just over 6% of the population have low oxygen saturation levels (below 95%) but this rises to almost 20% in those with a BMI of 35 kg/m² or more, and 15.5% in those with COPD. It is higher in men than women and higher in those over 65 than those under 65.  Pulse oximetry has become an essential tool in monitoring the well-being of individuals travelling on high altitude expeditions, where changes in oxygen saturation can vary widely depending on the individual and on the duration of acclimatisation. Development of pulse oximeter validation
ISO protocols for the validation of pulse oximeters have been developed by the ISO since 1992, the most recent of which is ISO 80601-2-61:2011. This standard is required for FDA premarket 510k submissions.  However, there is almost a total absence of peer-reviewed publications showing the validity of pulse oximeters.