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Health & Wellness

What is Blood Oxygen, What Are Normal Levels & How to Measure It

September 26, 2021

We discuss what blood oxygen is, what levels are normal, how to measure it, and causes and symptoms of low blood oxygen levels.


Your blood oxygen level refers to the amount of oxygen you have circulating in your blood. It is most commonly measured through a process known as “pulse oximetry.” Low blood oxygen levels can be associated with a variety of different health issues.

Below we’ll take a deeper dive into the basics of blood oxygen levels and pulse oximetry.


What Does Blood Oxygen Level Show?

The human body needs to constantly be supplied with oxygen in order to function. We breathe it in through the nose and mouth, then our lungs transfer it into the bloodstream where it is then distributed throughout the body. Oxygen is carried in your blood by red blood cells. As it travels around the body, oxygen helps provide energy, replace worn out cells, and support internal organs and systems.

Your blood oxygen level shows how much oxygen is being circulated by your red blood cells, and it is very closely regulated by your body. Keeping it within a specific range is necessary to ensure that all the cells in your body are getting enough oxygen. Maintaining the proper balance of oxygen saturation in your blood is extremely important for your health.

The level of oxygen in your blood indicates how efficiently your body is dispersing oxygen from your lungs to your cells. Low levels can be a sign that there is something wrong with your lungs or blood circulation.

If you’ve tested positive for COVID-19 and are experiencing symptoms, low blood oxygen levels may be a red flag that you’re in need of medical attention.

Learn More: Knowing Your Baseline – Case Studies in Respiratory Rate in Time of COVID-19


What is a Normal Blood Oxygen Level?

There are two ways to measure your blood oxygen level. One is an arterial blood gas test (ABG test), which is uncomfortable and invasive and requires a doctor to draw blood from one of your arteries (arteries carry oxygenated blood and deliver it to the body, then veins return it to the heart).

The other is with pulse oximetry, which does it quickly and painlessly by shining light through capillary veins near the surface of your skin. Pulse oximetry measures the percentage of oxygen in your blood (oxygen saturation level) and is known as SpO2.

A normal blood oxygen level is between 95% and 100%.

This number is likely to vary for people with lung diseases and other particular medical issues. An SpO2 below 90% is considered low, and referred to as hypoxemia.

Learn More: How to Increase Blood Oxygen Level


Causes and Symptoms of Low Blood Oxygen Levels

Here are several potential symptoms of low blood oxygen levels:

  • Heavy breathing or shortness of breath
  • Chest pains
  • Elevated heart rate and blood pressure
  • Headaches and dizziness
  • Restless feeling
  • Confusion or loss of coordination
  • Impaired vision
  • Cyanosis

Low levels of blood oxygen can be caused by changes in your environment, most notably high-altitude conditions where there is less oxygen in the air.

From a medical standpoint, a low SpO2 may be caused by anemia (insufficient red blood cells) or congenital heart diseases where the heart is unable to pump enough oxygenated blood. Additionally, lung ailments and things that decrease your body’s ability to take in oxygen often lead to low blood oxygen levels as well, including:

  • Asthma
  • Bronchitis
  • Sleep apnea
  • Pneumonia
  • COVID-19 or other viral infections
  • Emphysema (when the lungs’ air sacs are damaged, usually from smoking)
  • Pulmonary edema (fluid buildup that causes lungs to swell) or fibrosis (lung tissue becomes scarred)


How WHOOP Monitors Blood Oxygen

While the most familiar pulse oximeters are those that are placed on your finger, the same technology is utilized in many other devices, including the WHOOP 4.0, to monitor blood oxygen levels from the wrist.

The WHOOP 4.0 does this by sending two wavelengths of light (red and infrared) through your skin where it is absorbed and reflected by blood vessels. The red and infrared light each get absorbed differently by hemoglobin (the red blood cells that can bind with oxygen), and their absorption changes if the hemoglobin is oxygenated or not. By comparing the relative amount of light that gets reflected back from each of the wavelengths, we are able to track what percentage of red blood cells are oxygenated.

So, for example, if 96% of them are oxygenated and 4% are not, then your SpO2 is 96%.

The WHOOP 4.0 measures your blood oxygen level while you’re sleeping at night (which allows for the most consistent and reliable readings), then reports it to you each morning via the Health Monitor, which tracks key vitals including live heart rate, resting heart rate, heart rate variability, respiratory rate, and skin temperature.

whoop health monitor shows spo2

The WHOOP 4.0 Health Monitor alerts you if blood oxygen levels or other key physiological metrics stray too far from their normal baseline.

The products and services of WHOOP are not medical devices, are not intended to diagnose COVID-19, the flu or any other disease, and should not be used as a substitute for professional medical advice, diagnosis or treatment. All content available through the products and services of WHOOP is for general informational purposes only.


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