Blood oxygen level is an important medical tool used to determine how effectively you are taking oxygen into your body. If you're in good health, normal blood oxygen levels almost always fall within a very predictable range. Values outside of the expected levels can occur with a variety of medical conditions, most often involving the lungs or heart. Medical professionals use two primary measurements for determining your blood oxygen level: oxygen saturation and arterial oxygen. The normal values differ for these measurements.
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Blood Oxygen Saturation
When you take a breath, oxygen enters your lungs and attaches to a carrier protein called hemoglobin inside your red blood cells. Those same red blood cells release carbon dioxide into your lungs, which leaves your body when you exhale. Newly oxygenated red blood cells carry oxygen to your body.
A pulse oximeter uses light to measure the oxygen saturation level, the percentage of hemoglobin in your red blood cells carrying oxygen. Normal oxygen saturation levels fall between 95 and 99 percent.
Arterial Blood Oxygen
While a pulse oximeter is the most common device used to determine your blood oxygen level, it is sometimes necessary to analyze blood taken directly from an artery to assess your oxygen level. This measurement is called an arterial blood oxygen. A normal arterial blood oxygen level typically falls between 75 and 100 mmHg. Your blood carbon dioxide level and pH -- a measure of acidity or alkalinity -- are usually measured along with the arterial blood oxygen level.
Pulse oximetry offers an easy, noninvasive way of determining the oxygen saturation level. However, there are limitations with this measurement that could lead to erroneous results. Black, brown or blue fingernail polish, for example, can cause falsely decreased oxygen saturation levels. Fluorescent lighting, movement and certain abnormalities of the hemoglobin molecules, such as sickle cell disease, can also interfere with the test results. There are fewer sources of interference with the arterial blood oxygen test, which is one reason it is often performed if the oxygen saturation level is abnormally low.
A low blood oxygen level occurs with many medical conditions. Examples of lung-related causes include severe pneumonia, a severe asthma attack, a collapsed lung, a blood clot in the lungs, multiple rib fractures and chronic obstructive pulmonary disease (COPD). A low oxygen level can also occur due to congestive heart failure, with which fluid accumulates in the lungs. Severe anemia, smoke inhalation, shock, and sedative drug overdose are other possible causes, among others.
Reviewed and revised by: Tina M. St. John, M.D.