How Pulse Oximeters Work.

The pulse oximeter measures blood oxygen saturation (SpO2%) of arterial blood and heart rate. LEDs inside the pulse oximeter emit two small beams of light: red and infrared. The light waves pass through the blood stream and are read by the photo-diode opposite the LEDS. The pulse oximeter measures the difference in light absorption between oxygenated and deoxygenated blood. Oxygenated blood absorbs more infrared light, while deoxygenated blood absorbs more red light. Around thirty measurements are taken each second. And a complex computation is executed by the pulse oximeter to give a SpO2 reading.

A normal SpO2 level is 95%-99% for anyone who does not have an preexisting conditions that would warrant lower readings. Such conditions could be COPD or lung disease. Individuals with these conditions can have readings of 88%-94%. For everyone else, consistent readings of 94% and lower warrant a consult with your doctor. And any consistent readings below 90% may warrant an emergency room visit. Low SpO2 readings could indicate hypoxia; a shortage of oxygen in the tissues. Left unchecked could results in organ damage or failure. And even death.

Source: World Health Organization

Wikipedia

American Lung Association