Pulse oximetry measures the amount of oxygen in the blood, through a finger clip test and infrared light. Oxygenated blood absorbs infrared light and allows red light to pass through. Deoxygenated blood absorbs red light and allows infrared light to pass through. A range of 96% to 100% oxygen in the blood is generally considered normal. Anything below 90% could quickly lead to lifethreatening complications. Knowing this is very useful for patients with respiratory and/or cardiac problems, especially those with Chronic Obstructive Pulmonary Disease (COPD).