Oxygen saturation is a measure of how much oxygen is in your blood. Your organs and tissues need oxygen to work. Oxygen can "hitch a ride" in your red blood cells and travel through the bloodstream to get where it needs to go in your body.
For most healthy adults, a normal oxygen saturation level is between 95% and 100%. An "O2 sat" level below this range requires medical attention because it means your body isn't getting enough oxygen.
This article covers conditions that affect the amount of oxygen in your blood and the complications of having low oxygen saturation. You will also learn how to measure your blood oxygen levels and when to seek treatment.
Table of Contents
How Blood Gets Oxygenated
To understand how blood gets saturated with oxygen, you have to know about the air sacs in the lungs called alveoli. There are millions of these microscopic air sacs in the lungs. They exchange oxygen and carbon dioxide molecules to-and-from the bloodstream.
As oxygen molecules pass through the alveoli, they bind to a substance in the blood called hemoglobin.
Oxygen "hitches a ride" on the hemoglobin as it circulates until it gets dropped off to the body's tissues. Then, hemoglobin picks up carbon dioxide from the tissues and transports it back to the alveoli. Once there, the cycle can begin all over again.
The level of oxygen in your blood depends on several 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. However, some diseases reduce hemoglobin's ability to bind to oxygen.
Each one of your blood cells contains around 270 million molecules of hemoglobin. Any health condition that limits your body's ability to produce red blood cells can result in low hemoglobin levels, which in turn limits the amount of oxygen that can saturate your blood.
Conditions That Affect Oxygen Saturation
Blood disorders, problems with circulation, and lung issues can prevent your body from absorbing or transporting enough oxygen. When this happens, it 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): Any condition that affects your ability to breathe will affect 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: Not having enough 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
How to Measure Your Blood Oxygen Levels
Oxygen saturation is measured in one of two ways: an arterial blood gas test (ABG or Sa02) or pulse oximetry (Sp02).
ABG is usually only done in a hospital, while pulse oximetry can be done in other healthcare settings (like a provider's office) and even at home.
An ABG value refers to the levels of oxygen and carbon dioxide in blood running through your veins.
During an ABG, 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.
Your ABG value can give your healthcare provider a sense of how efficiently the hemoglobin in your blood exchanges oxygen and carbon dioxide.
A pulse oximetry reading reflects the percentage of oxygen found in arterial blood.
Unlike the ABG test, pulse oximetry does not involve a needle (non-invasive). Instead, the test uses a sensor to read wavelengths reflected from the blood. The probe is attached to your finger, earlobe, or another place on the body. A pulse oximeter can give results on a screen in just a few seconds.
People can monitor their oxygen saturation levels using wearable pulse oximetry devices—some smartwatches even have this feature. You can also 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%|
What Causes Decreased Oxygen Saturation?
A drop in oxygen saturation in the blood is called hypoxemia. It can be caused by:
- Less oxygen in the air (for example, when you are flying in an airplane)
- Conditions that affect breathing (such as asthma and COPD)
- Conditions that affect oxygen absorption (such as pneumonia)
- Having too few red blood cells or hemoglobin (anemia)
- Breathing in another substance (such as carbon monoxide or cyanide) that binds more strongly to hemoglobin than oxygen does
How to Raise Blood Oxygen Fast
If your O2 saturation level is low, you need to call your provider. If they're very low, you might need to seek emergency medical care.
You can also take a few steps right away to try to increase your blood oxygen levels:
- Sit up straight rather than laying flat
- Get some fresh air or go inside if you are outdoors and it is very hot/very cold
- Cough to loosen up any mucus (like from allergies or a cold/the flu)
- Check that you are not taking "shallow" breaths; you need to take deep, full breaths to get oxygen into your blood
Complications of Low Oxygen Saturation
Low oxygen saturation in the blood can mean there's less oxygen in the body's tissues, including the organs and muscles. When this happens, it's called hypoxia.
Your cells can adapt to a lack of oxygen when the deficiency is small. However, with larger deficiencies, cell damage and cell death can happen.
Hypoxia usually happens because there is not enough oxygen in the blood (hypoxemia). However, it can also happen when:
- There are not enough red blood cells to carry oxygen to the tissues (e.g., from severe bleeding after a trauma or conditions like sickle cell anemia).
- There is inadequate blood flow (e.g., a stroke occurs when there is low blood flow to a region of the brain; a heart attack occurs when there is low blood flow to the heart muscles).
- The tissues require even more oxygenated blood than can be delivered (e.g., severe infections that cause sepsis may result in hypoxemia and eventually organ failure)
Treating Low Oxygen Saturation
Generally speaking, an oxygen saturation level below 95% is considered abnormal. An O2 sat below 90% is an emergency.
If someone's oxygen saturation is dangerously low, they will need oxygen therapy—sometimes urgently.
The brain is the most susceptible organ to hypoxia. Brain cells can begin to die within five minutes of oxygen deprivation. If hypoxia lasts longer, it can lead to coma, seizures, and brain death.
It is very important to find out the cause of low oxygen saturation so the problem can be fixed.
For example, with chronic conditions such as COPD and asthma, the cause of hypoxia is usually low air exchange in the lungs and alveoli. In addition to oxygen therapy, steroids or rescue inhalers (bronchodilators) might be needed to open the airways.
In circulatory conditions like heart disease, inadequate blood flow reduces oxygen delivery. In this case, medications that improve heart function, such as beta-blockers for heart failure or prescription medications to treat heart arrhythmias, can help improve oxygenation.
With anemia, the 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 a person's level of healthy red blood cells.
Oxygen saturation is the measure of how much oxygen is traveling through your body in your red blood cells. Normal oxygen saturation for healthy adults is usually between 95% and 100%.
If you have a chronic health condition that affects your lungs, blood, or circulation, regularly tracking your oxygen saturation is important. An O2 sat level below 95% is not normal and requires immediate medical attention.
Frequently Asked Questions
Which finger is best for an oximeter?
Most oximeters work best when used on the middle finger. Some studies have shown that the right middle finger works best. In some people, the right thumb may also work well.
How long does it take for oxygen levels to return to normal after COVID?
COVID-19 can damage your lungs and affect your breathing. A lot of people who have COVID experience low 02 sats—sometimes dangerously low.
We are still learning about how COVID affects the lungs. It can take months for the lungs to heal after COVID. Some people develop lasting breathing problems after they have the infection.
What is a normal O2 sat for a child?
Normal oxygen saturation levels are the same for kids as they are in adults; between 95% and 100%.
Is 93 SpO2 normal while sleeping?
Oxygen levels above 90% are normal when you're asleep. If your O2 sats are lower than that while you're sleeping, it could be a sign that you have a disorder like sleep apnea.
Thanks for your feedback!
What are your concerns?
Hafen BB, Sharma S. Oxygen Saturation. [Updated 2021 Aug 12]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2022 Jan-.
Minnesota Department of Health. Oxygen Saturation, Pulse Oximeters, and COVID-19.
U.S. Food and Drug Administration. Pulse oximeter accuracy and limitations: FDA safety communication.
McGill - Office for Science and Society. Under the microscope: Blood.
Collins JA, Rudenski A, Gibson J, Howard L, O'Driscoll R. Relating oxygen partial pressure, saturation and content: the haemoglobin-oxygen dissociation curve. Breathe. 2015;11(3):194–201. doi:10.1183/20734735.001415
Sarkar M, Niranjan N, Banyal PK. Mechanisms of hypoxemia. Lung India. 2017 Feb;34(1):47-60. doi:10.4103/0970-2113.197116
Doyle GR, McCutcheon, JA. 5.6: Management of Hypoxia. Clinical Procedures for Safer Patient Care; Chapter 5: Oxygen Therapy.
McKenna H, Murray A, Martin D. Human adaptation to hypoxia in critical illness. J Appl Physiol. 2020 Sep;129(4):656-663. doi:10.1152/japplphysiol.00818.2019
Caboot J, Allen J. Hypoxemia in sickle cell disease: Significance and management. Paediatr Respir Rev. 2014 Mar;15(1)17-23. doi:10.1016/j.prrv.2013.12.004
Ferdinand P, Roffe C. Hypoxia after stroke: a review of experimental and clinical evidence. Exp Trans Stroke Med. 2016 Dec;8(9):1-8. doi:10.1186/s13231-016-0023-0
Pavez N, Kattan E, Vera M, et al. Hypoxia-related parameters during septic shock resuscitation: Pathophysiological determinants and potential clinical implications. ATM. 2020 Jun;8(12):784. doi:10.21037/atm-20-2048
Yale Medicine. Should you really have a pulse oximeter at home?.
National Institute of Neurological Disorders and Stroke. Cerebral hypoxia information page.
Cortés Buelvas A. Anemia and transfusion of red blood cells. Colomb Med. 2013 Dec;44(4):236-242.
Basaranoglu G, Bakan M, Umutoglu T, Zengin SU, Idin K, Salihoglu Z. Comparison of SpO2 values from different fingers of the hands. Springerplus. 2015;4:561. Published 2015 Sep 29. doi:10.1186/s40064-015-1360-5
Johns Hopkins Medicine. COVID-19 Lung Damage.
Children's Health. Pediatric Oxygen Titrations.
University of Iowa Health Care. Pulse oximetry basic principles and interpretation.