Understanding the Difference between O2 and SpO2


Oxygen is vital for human survival, but what exactly is oxygen, and how do oxygen levels differ from blood oxygen saturation? Oxygen (O2) is the gas inhaled through the nose and mouth to the lungs. It then enters the bloodstream, where it is transported to the cells. 

On the other hand, peripheral capillary oxygen saturation (SpO2) is the measurement of the percentage of oxygen-bound hemoglobin in the blood. In a nutshell, oxygen is a gas, and SpO2 is the measurement of that gas in the bloodstream. 


What Is Oxygen (O2)?

Oxygen, with the atomic number 8 and the chemical symbol O, is a naturally occurring element. It is typically found in the form of dioxygen (O2), an odorless, colorless, and tasteless gas composed of two oxygen atoms. The molecular mass for O2 is 32. 

Oxygen is a vital component of the Earth’s atmosphere, making up approximately 21% of it. It is essential for the survival of plants, animals, and humans. 

While many individuals can breathe in oxygen naturally, some may require additional oxygen through oxygen therapy when their oxygen saturation levels fall below normal due to breathing problems or other medical conditions.

There are three primary grades of oxygen:

  • Medical-grade oxygen: This oxygen is used as a medical treatment for lung diseases, such as pneumonia, asthma, and COVID-19. It is made for human use, meaning that the oxygen inside the tank is of higher purity. Medical oxygen is administered as an inhalation gas and is only available on prescription.
  • Industrial-grade oxygen: Oxygen is made for industrial plants for many purposes, such as steelmaking, glass and ceramic manufacture, welding, and cutting. Industrial oxygen is not meant for human use since it may have pollutants leading to health issues.
  • Aviation-grade oxygen: This type is used by pilots in case they fly at high altitudes, which may cause their SpO2 to decrease.


Why Is Oxygen So Important?

Oxygen is crucial for the proper functioning of cells in the human body. The mitochondria use oxygen to produce chemical energy to power cells. Without enough oxygen, the body cannot perform its daily functions. Because tissues need oxygen to survive, which is measured through tissue oxygen consumption, low oxygen levels can lead to organ failure and even death.


What Is SpO2?

SpO2, commonly known as oxygen saturation or O2 Sat, stands for peripheral capillary oxygen saturation. It’s a measure of the percentage of oxygenated hemoglobin in the blood.

Hemoglobin (Hb or Hgb) is a protein found in red blood cells (RBC). It transports O2 from the lungs to the body’s tissues, and it’s what gives blood its red color. Hemoglobin can be oxygenated or deoxygenated.

Each hemoglobin molecule is composed of four heme groups. One hemoglobin molecule transports a maximum of four oxygen molecules. 

After meeting tissue needs, blood returns to the heart with either little or more oxygen, which is measured in mixed venous oxygen saturation (SvO2). 


How Do You Measure Oxygen Saturation?

SpO2 is commonly measured using a pulse oximeter. In more severe cases, arterial blood may be drawn for measurement. Monitoring blood oxygen levels is vital in healthcare as it helps professionals understand and provide appropriate patient care, particularly in hospitals.


Pulse Oximeter

Pulse oximetry is a painless, noninvasive test used to measure the oxygen in the blood using a pulse oximeter (or pulse ox). It’s a painless and noninvasive test. Normal oxygen saturation levels are between 95%–100%.

A pulse oximeter is composed of a light source and a photodetector. It is placed on a finger, earlobe, or toe to measure a person’s SpO2. It works by emitting light (red and infrared) through the skin and measuring the amount of light absorbed by the blood, which is then used to calculate the oxygen saturation level (SpO2) of the blood.

A pulse oximeter uses the difference in light absorption between oxygenated and deoxygenated hemoglobin to calculate the oxygen saturation level of the blood. The results are then displayed on the device, which also shows the heart rate and pulse wave. 

There are various types of pulse oximeters available, such as handheld, wrist-worn, tabletop, and fetal pulse oximeters. Check out our selection of mobile pulse oximeters that all come with a 3 month warranty:

11931 Fingertip Oximeter with a 0.1% low perfusion rate. OEM Replacement: Nonin Onyx II 9550, ONYX VANTAGE 9590

11930 Fingertip Oximeter with a 0.025% low perfusion rate. OEM Replacement: Masimo MightySat

10758 SpO2 Fingertip Oximeter. OEM Replacement: Contec CMS50M, CMS50NA

It’s important to note that a pulse oximeter measures the total amount of hemoglobin bound to oxygen and carbon monoxide (CO), and returns it as oxyhemoglobin. It cannot differentiate between the two. Therefore, it’s essential to consider other factors or test results to determine the cause of low oxygen saturation levels.


Arterial Blood Gas Analysis

The measurement of arterial oxygen saturation (SaO2) through a blood test is known as an ABG analysis. While more accurate than pulse oximetry, ABG analysis is an invasive procedure and may be painful. It’s typically used in more critical medical situations or when more precise oxygen saturation levels are needed.

This test provides a more accurate measurement of oxygen saturation levels, measuring other important parameters, such as:

  • Oxygen content (O2CT)
  • Hemoglobin 
  • Blood pH level
  • Partial pressure of oxygen (PaO2), 
  • Partial pressure of carbon dioxide (PaCO2), 
  • Bicarbonate (HCO3)

In short, ABG analysis provides a more comprehensive understanding of the patient’s respiratory and metabolic status.


Understanding Oxygen Saturation, Partial Pressure, and Oxygen Content

As previously discussed, oxygen saturation (O2 sat) is the measurement of the percentage of oxygen-bound hemoglobin in the blood. Partial pressure of oxygen (PaO2) can also be measured when an arterial blood sample is taken, providing insight into the pressure of oxygen in the arteries.

PaO2 is typically measured with a blood gas test. A normal arterial PaO2 is between 80 and 100 mmHg, while the ideal reading would be 104 mmHg. When PaO2 dips below the acceptable range, it can indicate hypoxemia:

  • Mild hypoxemia: 60–79 mmHg
  • Moderate hypoxemia: 40–59 mmHg
  • Severe hypoxemia: less than 40 mmHg

Unlike SpO2, where you can use pulse oximetry, the only way to check PaO2 is through an ABG.

Partial pressure of oxygen and arterial oxygen saturation does measure the number of oxygen molecules in the blood. It indicates “how much” O2 is in the blood since PaO2 represents unbound oxygen molecules dissolved in plasma, not those bound to hemoglobin. 

Similarly, oxygen saturation cannot tell how much oxygen is in the blood. That’s where O2 content comes in.

Arterial oxygen content (CaO2) is the measure of the amount of oxygen present in a specific volume of blood (including unbound oxygen and dissolved oxygen in arterial blood), measured in milliliters per deciliter (mL/dL). 

It is a more accurate measure of the overall oxygen in the blood than SaO2 or PaO2 alone, which gives a more comprehensive understanding of the patient’s oxygenation status.

There’s no better way to describe the relationship between oxygen saturation and partial pressure than to use the oxygen-hemoglobin dissociation curve. 

“Oxyhemoglobin dissociation curve of fetal and adult hemoglobin” by Bernhard Schwaberger et al., licensed under CC BY 4.0

This graphical representation of fetal and adult hemoglobin illustrates how oxygen binds to hemoglobin across a range of oxygen pressures. Particularly, there is about 15g of hemoglobin in 100 ml of blood, which has the oxygen-carrying capacity of 20.1 ml. It is essential information for medical professionals treating respiratory and cardiac patients. 

The tissue oxygen supply can be impacted by conditions that cause the curve to shift, with the greatest effects at low oxygen partial pressures. Oxygen affinity to hemoglobin increases under conditions that cause the oxygen-hemoglobin dissociation curve to shift to the left (red line), such as high temperatures, high hydrogen ion concentrations, high CO2 levels, and high intraerythrocytic 2,3-DPG


What Happens When SpO2 Gets Too Low?

When blood oxygen saturation levels fall too low, it can lead to a condition called hypoxemia. The severity of hypoxemia can range from mild (91%–94%), moderate (86%–90%), or severe (below 85%). If left untreated, hypoxemia can be fatal. 

It’s crucial to seek medical attention immediately if you fall below normal oxygen levels. A healthcare professional can evaluate the situation and develop a treatment plan to restore your oxygen levels to a healthy range.


Are O2 Saturation Readings Always Accurate?

The accuracy of oxygen saturation readings depends on the method used for measurement. ABG analysis is considered to be more accurate than pulse oximetry. 

Pulse oximetry readings can be affected by various factors, including movement, nail polish, skin pigmentation, temperature, light, and perfusion. However, most pulse oximeters have an error rate of ± 2%. While not as accurate as ABG, they can still be a useful tool for tracking one’s health, even in a home setting.


What Affects Blood Oxygen Saturation

The following can affect SpO2 readings:

  • High altitude
  • Some strong medications
  • Smoking
  • Irregular blood pressure
  • Stress and anxiety
  • Activity such as intense workouts
  • Some medical conditions, especially lung and lung diseases.


What Are the Signs of Low Oxygen in the Blood?

Your blood oxygen saturation levels may be low if you experience the following:

  • Shortness of breath
  • Chest pain
  • Rapid breathing
  • Fast heart rate
  • Dizziness or lightheadedness
  • Bluish lips or skin


How to Increase Oxygen Saturation

In case your oxygen saturation levels take a dip, you can try the following tips to increase the levels:

  • Breathe deeply and slowly. Deep breaths allow your lungs to inflate fully and absorb as much oxygen as possible. You can also practice other breathing techniques, such as pursed lip breathing.
  • Exercise regularly. Exercising helps increase your overall fitness and respiratory system health.
  • Avoid high altitudes. If you find that high altitudes are having a negative impact on your oxygen levels, it is best to avoid staying in these areas for prolonged periods of time or taking frequent breaks to return to lower altitudes. This can help to alleviate any symptoms related to low oxygen levels.
  • Eat healthy foods. Certain foods such as garlic, lemons, spinach, and berries may have beneficial effects on blood oxygen levels. Incorporating these foods into your diet could potentially help to improve oxygenation. 
  • Quit smoking. Smoking can increase the levels of carbon monoxide in the blood, which competes with oxygen for binding sites on hemoglobin, thus reducing the overall oxygen levels in the body. When a person quits smoking, the levels of CO in the blood decrease, making it easier for oxygen to reach the organs and tissues. This can result in an improvement in overall oxygenation and overall health.
  • Drink lots of water. Your lungs need to be hydrated to eliminate carbon dioxide and increase oxygen in the blood. 


Ensure to visit a health care provider immediately if you notice your SpO2 readings are low. Doing so can prevent severe health complications that might come with a lack of enough oxygen in your body.