What Are Blood Oxygen Levels?

Medically Reviewed on 12/21/2022
Blood oxygen levels can also be measured using an instrument known as a pulse oximeter.
Blood oxygen levels can also be measured using an instrument known as a pulse oximeter.

Blood oxygen levels (arterial oxygen) indicate oxygen levels present in the blood that flows through the arteries. 

  • Blood oxygen levels can be checked by withdrawing blood from an artery in the wrist, elbow, or groin.
  • You may feel a sharp pain when the needle enters the artery.
  • The blood is put in an ABG machine (blood gas analyzer) that provides blood oxygen level readings in the form of the partial pressure of oxygen (PaO2).
  • In clinical studies, the blood oxygen level measurement feature on the Apple Watch has shown good accuracy when compared to a traditional pulse oximeter, which is a device commonly used to measure blood oxygen levels.

What are normal oxygen levels by age?

Pulse oximetry measures blood oxygen levels using a pulse oximeter, which is a small, mobile medical device. It is most commonly used when doctors want to quickly assess your blood oxygen levels. It is also used to monitor the response of your body to certain medications or supplemental oxygen. 
The procedure is very simple and requires only clipping the pulse oximeter in between any of the fingers for a few seconds. The reading appears on the pulse oximeter screen. Blood oxygen levels are indicated as SpO2, which is the percent saturation of oxygen in the blood.
  • 95%-100%: Normal oxygen levels in a pulse oximeter usually range from 95%-100%. Note: Normal levels may vary if you have lung disorders. Your doctor can tell what levels are normal for you.
  • Below 95%: People with SpO2 level fall below 95% such as 92% should see a healthcare provider as it may be a sign of hypoxemia.
  • Below 80-85%: The brain gets affected when the SpO2 level falls below 80-85%.
  • Below 67%: Cyanosis develops when the SpO2 level drops below 67%.
Blood Oxygen Levels by Age Using a Pulse Oximetry Chart
Conditions By Age SpO2
Normal Adults & Children 95% to 100%
Normal >70 years old about 95%
Brain is affected Adults & Children 80% to 85%
Cyanosis Adults & Children Below 67%

Pulse oximetry may be slightly less accurate than ABG but is easier to perform than ABG tests when results are needed quickly. 

How are blood oxygen levels checked using the ABG test?

An ABG test uses blood drawn from an artery, where the oxygen and carbon dioxide levels can be measured before they enter body tissues.  
The blood is put in an ABG machine (blood gas analyzer) that provides your blood oxygen levels in the form of the partial pressure of oxygen (PaO2).
  • Above 120 mmHg: Hyperoxemia is defined as blood oxygen levels above 120 mmHg. Hyperoxemia is mostly seen in hospitals when patients are exposed to high pressures of supplemental oxygen for prolonged periods (3 to more than 10 hours).
  • 75-100 mmHg: Normal arterial oxygen pressure (PaO2) is approximately 75 to 100 millimeters of mercury (75-100 mmHg).
  • Below 75 mmHg: When the levels drop below 75 mmHg, the condition is called hypoxemia.
  • Below 60 mmHg: Levels below 60 mmHg are considered very low and indicate the need for supplemental oxygen. Supplemental oxygen is provided through an oxygen cylinder that is connected to the nose via a tube, with or without a mask.
Conditions PaO2 Levels in mmHg
Hyperoxemia > 120
Normal arterial oxygen pressure 75-100
Hypoxemia < 75
Critical low (supplemental oxygen is needed) < 60


COPD (chronic obstructive pulmonary disease) is the same as adult-onset asthma. See Answer

What causes blood oxygen levels to become low?

Blood oxygen levels can get low due to any of the problems that include:

What are the signs and symptoms of low blood oxygen levels (hypoxemia)?

Hypoxemia can give rise to many signs and symptoms depending on its severity, which may include:

Not having enough blood oxygen levels results in inadequate oxygen supply to the organs and tissues of the body. Severe hypoxemia can become dangerous. If left untreated for a long time, it can end up affecting the brain or heart.

How is hypoxemia treated?

Treating hypoxemia involves measures that increase the levels of oxygen in the blood, which is achieved by:

  • Treating underlying conditions: Treating the cause of hypoxemia is the most important part of the therapy that gives long-lasting effects. Many times, medications are given through an inhaler that quickly enables you to breathe the medicine into your lungs.
  • Oxygen therapy: You are given supplemental oxygen through an oxygen cylinder (which can be portable). The pressure of delivered oxygen can be controlled.

How can you prevent hypoxemia?

You can maintain your normal blood oxygen levels after you recover from hypoxemia. Lifestyle changes that can help prevent hypoxemia include:

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Medically Reviewed on 12/21/2022
Hypoxemia. Available at: https://www.mayoclinic.org/symptoms/hypoxemia/basics/definition/sym-20050930

Hafen BB, et al. Oxygen Saturation. In: StatPearls [Internet]. Available from: https://www.ncbi.nlm.nih.gov/books/NBK525974/

Cooper JS, et al. Oxygen Toxicity. In: StatPearls [Internet]. Available from: https://www.ncbi.nlm.nih.gov/books/NBK430743/

Starr N, et al. Pulse oximetry in low-resource settings during the COVID-19 pandemic. The Lancet Global Health. 2020 Sep 1;8(9):e1121-2.