Understanding Oxygen Saturation

Oxygen saturation, or "O2 sats," indicates that amount of oxygen traveling through your body with your red blood cells. Normal oxygen saturation is usually between 95% and 100% for most healthy adults.

Any level below this is concerning and in need of immediate medical attention, since it means your organs, tissues, and cells aren't getting the oxygen they need to function properly.

This article covers several conditions that affect the amount of oxygen in your blood and the complications that can result from having low oxygen saturation. It will also touch on how to measure blood oxygen, as well as treatments for low levels.

Verywell / Laura Porter

How Blood Becomes Oxygenated

Understanding how blood is saturated with oxygen begins with the alveoli, or air sacs. There are millions of these microscopic air sacs in the lungs. They serve an important function: to exchange oxygen and carbon dioxide molecules to and from the bloodstream.

When oxygen molecules pass through the alveoli, they bind to hemoglobin—a substance in the blood.

As the hemoglobin circulates, oxygen essentially hitches a ride and gets dropped off to the body's tissues. With this, hemoglobin picks up carbon dioxide from the tissues and transports it back to the alveoli so the cycle can begin all over again.

The level of oxygen in your blood depends on several key factors:

  • How much oxygen you breathe in
  • How well the alveoli swap carbon dioxide for oxygen
  • How much hemoglobin is concentrated in red blood cells
  • How well hemoglobin attracts oxygen

Most of the time, hemoglobin contains enough oxygen to meet the body's needs. But some diseases reduce its ability to bind to oxygen.

Each one of your blood cells contains around 270 million molecules of hemoglobin. But any condition that limits your body's ability to produce red blood cells can result in low hemoglobin levels, which limits the amount of oxygen that can saturate your blood.

Conditions Affecting Oxygen Saturation

Blood disorders, problems with circulation, and lung issues may prevent your body from absorbing or transporting enough oxygen. In turn, that can lower your blood's oxygen saturation level.

Examples of conditions that can affect your oxygen saturation include:

  • Respiratory infections (e.g., a cold, the flu, COVID-19), as they can affect your breathing and, therefore, your oxygen intake
  • Chronic obstructive pulmonary disease (COPD): a group of chronic lung diseases that make it difficult to breathe
  • Asthma: a chronic lung disease that causes airways to narrow
  • Pneumothorax: a partial or total collapse of the lung
  • Anemia: a lack of healthy red blood cells
  • Heart disease: a group of conditions that affect the heart's function
  • Pulmonary embolism: when a blood clot causes blockage in an artery of the lung
  • Congenital heart defects: a structural heart condition that is present at birth

Measuring Your Levels

Oxygen saturation is usually measured one of two ways: arterial blood gas test (ABG or Sa02) and pulse oximetry (Sp02).

ABG is usually only done in a hospital setting, while pulse oximetry is done in a variety of healthcare settings, including your healthcare provider's office.


An ABG value refers to the levels of oxygen and carbon dioxide in blood running through your veins.

A nurse or lab technician draws blood from an artery, such as the radial artery in the wrist or the femoral artery in the groin. The sample is immediately analyzed by a machine or in a lab.

The ABG value can give your healthcare provider a sense of how efficiently the hemoglobin exchanges oxygen and carbon dioxide.

Pulse Oximetry

Similarly, a pulse oximetry reading reflects the percentage of oxygen found in arterial blood.

Unlike the ABG test, pulse oximetry is non-invasive. The test uses a sensor to read wavelengths reflected from the blood. This probe is simply attached to your finger, earlobe, or another place on the body. Results appear on a screen within a few seconds.

People can monitor their oxygen saturation levels using wearable pulse oximetry devices; some watches even have this feature. You can buy a pulse oximetry device at your local pharmacy or online.

Oxygen Saturation Levels
Reading ABG Level O Sat Result
Below Normal < 80 mm Hg  < 95%
Normal > 80 mm Hg 95% to 100%

Decreased Oxygen Saturation

A drop in oxygen saturation in the blood is referred to as hypoxemia. Potential causes of hypoxemia include:

  • Less oxygen in the air you breathe—for example, while flying in an airplane
  • Conditions that affect breathing, such as asthma and COPD
  • Conditions that affect oxygen absorption, such as pneumonia
  • Anemia, which causes a lower concentration of hemoglobin
  • Breathing in another substance, such as carbon monoxide or cyanide, that binds more strongly to hemoglobin than oxygen does

Complications of Low Oxygen Saturation

Low oxygen saturation in the blood can affect oxygen concentration in the body's tissues, including the organs and muscles. This condition is called hypoxia.

Your cells can adapt to a lack of oxygen when the deficiency is small. However, with larger deficiencies, cell damage can occur followed by cell death.

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

  • There are not enough red blood cells to carry oxygen to the tissues. Possible causes include severe bleeding due to trauma or sickle cell anemia.
  • There is inadequate blood flow. For example, a stroke occurs when there is low blood flow to a region of the brain, and a heart attack occurs when there is low blood flow to the heart muscles. Both result in cell and tissue death.
  • The tissues require even more oxygenated blood than can be delivered. Severe infections that lead to sepsis may result in hypoxemia and eventually organ failure.


Generally speaking, an oxygen saturation level below 95% is considered abnormal and anything below 90% is an emergency.

When this happens, oxygen therapy is needed—sometimes urgently. The brain is the most susceptible organ to hypoxia, and brain cells can begin to die within five minutes of oxygen deprivation. Should hypoxia last longer, coma, seizures, and brain death can occur.

It is very important to determine the cause of low oxygen saturation in order to correct the problem. With chronic conditions, such as COPD and asthma, the root cause is usually low air exchange in the lungs and alveoli. In addition to oxygen therapy, steroids or bronchodilators (rescue inhalers) may be needed to open the airways.

In circulatory conditions like heart disease, inadequate blood flow can reduce oxygen delivery. Medications that improve heart function, such as beta-blockers for heart failure or prescriptions to treat heart arrhythmias, can help improve oxygenation.

With anemia, blood supply to the tissues is reduced because there are not enough healthy red blood cells with hemoglobin to carry oxygen. Sometimes a red blood cell transfusion is necessary to increase the level of healthy red blood cells.


For people with chronic conditions that affect their lungs, blood, or circulation, regularly tracking oxygen saturation is important. Keep in mind that an O2 sat level below 95% is generally abnormal and calls for urgent medical attention. Unless treated promptly, a drop in oxygen saturation will lead to cell and tissue death.

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