Understanding Oxygen Saturation


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Oxygen saturation—sometimes referred to as O sats, or simply, sats—refers to the extent to which hemoglobin is saturated with oxygen. Hemoglobin is an element in the blood that binds with oxygen to carry it through the bloodstream to the organs, tissues, and cells of the body. Normal oxygen saturation is usually between 96 percent and 98 percent. 

About Oxygen Saturation

Each one of our red blood cells contains around 270 million molecules of hemoglobin. Iron, which is present in hemoglobin, is what oxygen binds to after diffusing from the alveoli in the lungs. Most of the time, hemoglobin is fully saturated with oxygen, but in some disease states, oxygen may have trouble binding to hemoglobin.

Oxygen saturation is dependent upon:

  • Oxygen availability
  • Gas exchange in the lungs: The ability of oxygen to reach the alveoli and diffuse through the walls of the alveoli to reach the red blood cells
  • The concentration of hemoglobin in red blood cells
  • The affinity of hemoglobin for oxygen, in other, words how strongly hemoglobin attracts oxygen

Conditions Affecting Oxygen Saturation

Blood disorders, circulatory problems, and lung issues may negatively affect your blood oxygen saturation level, as they may prevent you from adequately absorbing oxygen, utilizing oxygen, or taking a full inhale or exhale. Certain conditions affecting O sat level may include:

  • Chronic obstructive pulmonary disease (COPD), including emphysema and chronic bronchitis
  • Asthma
  • Collapsed lung (pneumothorax)
  • Anemia
  • Heart disease
  • Pulmonary embolism
  • Congenital heart defects

Measuring Oxygen Saturation

Oxygen saturation is most commonly measured by two metrics:

  • Arterial blood gases: The value obtained from arterial blood gases or ABGs (SaO2) describes the oxygen saturation of arterial blood, and is obtained by drawing blood from an artery such as the radial artery in the wrist or the femoral artery in the groin. ABGs are measured in millimeters of mercury (mm Hg) and can be a clue into how efficiently your body is exchanging oxygen and carbon dioxide.
  • Pulse oximetry: The value obtained from peripheral capillary blood using pulse oximetry (SpO2) often closely reflects the levels that would be found in arterial blood. Pulse oximetry has the advantage of being a non-invasive test, which uses a probe attached to a finger or earlobe or other regions of the body that reads the wavelengths of light reflected from the blood. Not only are pulse oximeters standard for monitoring people in the hospital, but today's wearable technology empowers people to track their own saturation levels.
Oxygen Saturation Levels
Reading ABG Level O Sat Result
Below Normal < 80 mm Hg  < 95%
Normal 80 to 100 mm Hg 95% to 100%
Above Normal > 100 mm Hg > 100%

Decreased Oxygen Saturation

A drop in oxygen saturation levels is referred to as desaturation, or hypoxemia, and can be caused by any changes or damage in the variables above.

  • A change in oxygen availability can be caused by a decreased concentration of oxygen in the inspired air such as at higher altitudes and when flying in an airplane.
  • Problems with gas exchange causing a decrease in oxygen saturation can include anything that reduces the ability of oxygen to travel from the outside air down into the alveoli of our bodies or during the process of oxygen transfer from the alveoli into the capillaries of the blood, such as in asthma or COPD.
  • Decreased oxygen saturation may result from a lower concentration of hemoglobin, such as in iron deficiency anemia.
  • A decreased affinity of hemoglobin for oxygen may occur when there is something else present which binds more strongly to hemoglobin than does oxygen, such as in carbon monoxide poisoning in which the compound carboxyhemoglobin is formed.

If you're a smoker, you may find that you have a falsely high pulse oximetry reading, thanks to the buildup of carbon monoxide in the blood from cigarettes. Because the oximeter can't distinguish between this type of gas and oxygen, it may provide an inaccurate result. An ABG test is a more reliable method in this case.

Hypoxemia and Hypoxia

The terms hypoxemia and hypoxia are often used interchangeably but mean quite different things. Hypoxemia is the term which defines a decreased oxygen concentration in the blood. Hypoxia, in turn, describes the consequences that occur due to hypoxemia—a marked change in the oxygen levels found in tissues and muscles. When cells do not get enough oxygen, they may adapt if the deficiency is small. However, in larger deficiencies, the result is cell damage followed by cell death.

Hypoxia is often caused by hypoxemia, but may also occur when:

  • There is anemia because there are too few red blood cells so even fully oxygenated blood doesn't bring enough oxygen to tissues.
  • There is inadequate blood flow so even fully oxygenated blood does not reach the tissues.
  • The tissues are unable to use oxygenated blood that is delivered.
  • The tissues require even more oxygenated blood than can be delivered, such as in severe infections.


There is no set level at which hypoxia occurs—it may vary person to person. But generally speaking, when oxygen saturation drops below 95 percent, levels are considered abnormal or below normal. In these cases, supplemental oxygen therapy is usually needed, and sometimes emergently. The brain is the most susceptible organ to hypoxia, especially at saturation levels of 80 to 85 percent—this is the point at which cognitive and visual function may be impaired.

It is highly important to determine the cause of low oxygen saturation in order to correct the problem. Treating the underlying cause is then the primary goal of treatment.

In cases of chronic conditions such as COPD and asthma, the root cause is usually an obstruction, resulting in inadequate air exchange in the lungs and alveoli. Treatment involves medication such as steroids or bronchodilators to open the airways and pulmonary rehabilitation, in addition to oxygen therapy.

In circulatory conditions like heart disease and edema, inflammation and swelling may result in inadequate blood flow, which limits oxygen's ability to reach the tissues.

In blood conditions like anemia, blood supply to the tissues is reduced thanks to the blood's low ability to carry oxygen attached to hemoglobin.

A Word From Verywell

Oxygen saturation is a helpful metric in determining how efficiently your body can exchange oxygen for carbon dioxide, which can be helpful in assessing how certain therapies are performing when you have a specific condition, such as COPD. In those with chronic conditions affecting their lungs, blood, and circulation, regularly tracking your O sats levels via pulse oximetry can be a useful tool. However, if you don't have a health issue affecting your oxygen state, consistent monitoring of your O sats level is not something to worry about.

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Article Sources

  • Hafen BB, Sharma S. Oxygen Saturation. [Updated 2019 Jun 2]. In: StatPearls Treasure Island (FL): StatPearls Publishing. 2019 Jan-. https://www.ncbi.nlm.nih.gov/books/NBK525974/