<P> Oxygen saturation is the fraction of oxygen - saturated hemoglobin relative to total hemoglobin (unsaturated + saturated) in the blood . The human body requires and regulates a very precise and specific balance of oxygen in the blood . Normal blood oxygen levels in humans are considered 95--100 percent . If the level is below 90 percent, it is considered low resulting in hypoxemia . Blood oxygen levels below 80 percent may compromise organ function, such as the brain and heart, and should be promptly addressed . Continued low oxygen levels may lead to respiratory or cardiac arrest . Oxygen therapy may be used to assist in raising blood oxygen levels . Oxygenation occurs when oxygen molecules (O) enter the tissues of the body . For example, blood is oxygenated in the lungs, where oxygen molecules travel from the air and into the blood . Oxygenation is commonly used to refer to medical oxygen saturation . </P> <P> In medicine, oxygen saturation (SO), commonly referred to as "sats", measures the percentage of hemoglobin binding sites in the bloodstream occupied by oxygen . At low partial pressures of oxygen, most hemoglobin is deoxygenated . At around 90% (the value varies according to the clinical context) oxygen saturation increases according to an oxygen - hemoglobin dissociation curve and approaches 100% at partial oxygen pressures of> 10 kPa . A pulse oximeter relies on the light absorption characteristics of saturated hemoglobin to give an indication of oxygen saturation . </P>

What is the normal oxygen level in the human body