What is SpO2 and can fitness bracelets be used to measure blood oxygen levels?

Measuring oxygen in blood is not a new feature, but now all the attention will be riveted on it, since Apple has finally added it to its Apple Watch 6. You can treat this company differently, but it creates noise around its products knows how. This means that everyone will start measuring oxygen, completely not understanding what all this means and whether someone really needs it.

What is blood oxygen saturation (SpO2) ? Is it necessary to measure this parameter and can you generally trust the readings of a fitness bracelet or smart watch, which, in fact, are not medical devices?

In this article, we will answer in detail all the questions posed above and see if the fashionable “trick” of modern trackers is another marketing trick to warm up interest in their devices.

What is SpO2 and why do we need oxygen?

The body’s cells need energy to move, synthesize protein, and create other chemicals. This is the foundation of any life.

Imagine your body as a car engine, inside which gasoline is burned and pistons move. In order for a fire to occur, air is required, or rather the oxygen contained in it.

Such “gasoline” (nutrients) + oxygen is also needed by our cells to oxidize and obtain ATP molecules (this is the main source of energy in the body).

But how does oxygen get into cells?

Blood is 40% composed of cells, the remaining 60% is plasma (a light liquid made from water, salts and minerals). The vast majority of cells are erythrocytes , also called red blood cells . They make up 99% of all blood cells (approximately 20-25 billion pieces):

Each such erythrocyte contains more than 250 million hemoglobin molecules . It is this molecule that is capable of binding with oxygen to transfer it to all tissues of the body.

In other words, hemoglobin is a kind of “taxi” for transporting oxygen throughout the body. When blood flows through the lungs, hemoglobin “snatches” oxygen molecules from there and delivers them to the tissues, and from there, on the way back, it takes “waste material” – carbon dioxide.

Now, when the blood flows through the lungs again, the hemoglobin will leave the “working off” there and take a new portion of oxygen. And we, in turn, will take another exhalation and inhalation in order to clear the lungs of carbon dioxide and collect fresh material from the air.

And here a small clarification should be made. Not every hemoglobin molecule is an exemplary driver. Someone may simply not take the “passenger” and ride all the way without oxygen, “light”.

SpO2 just shows the ratio of hemoglobin containing oxygen to the total amount of hemoglobin in the blood.

What is the SpO2 (blood oxygen saturation) rate?

The normal SpO2 for a healthy person is considered to be 96-99% oxyhemoglobin (this is the name of hemoglobin containing oxygen). In other words, virtually all of the hemoglobin must contain oxygen.

By the way, it is oxygen that gives the blood its characteristic color. The more oxygen there is in hemoglobin, the brighter the color will be. Therefore, arterial blood, which carries oxygen to the tissues, has a bright red color, and venous blood (flowing from the tissues) is dark red, since it already contains one third less oxygen.

It is important to note that SpO2 levels may drop slightly during exercise and this is completely normal.

If the saturation falls below 90% , hypoxemia (lack of oxygen in the blood) sets in, which can lead to hypoxia (low oxygen levels in the tissues of the body).

In some diseases, oxygen saturation of hemoglobin is also reduced. For example, an exacerbation of bronchial asthma can lead to a fall in SpO2 below 90%, which requires immediate hospitalization.

How does fitness bracelets measure blood oxygen levels? Can these indicators be trusted?

The answer to the second question will be short – yes, you can trust the performance of a fitness tracker.

The principle of measuring the saturation of hemoglobin with oxygen on a fitness bracelet is no different from the principle of operation of a medical pulse oximeter device and is as follows:

The LED emits light that passes through the tissue, is reflected, and hits the photo detector. A certain amount of this light is absorbed by the blood, and how much depends on the degree of its saturation with oxygen.

If you are interested in learning more about how exactly all this works, why there are not enough green lights on the bracelet and you definitely need an infrared LED, what happens to the light inside the wrist, how a simple light passing through the blood can report the amount of oxygen, read our exciting new material .

A typical sensor consisting of a photodetector and emitters looks like this:

Even many smartphones are able to measure SpO2 and do it quite accurately. For example, most Samsung flagships (Galaxy S and Galaxy Note series) measure blood oxygen levels without any additional devices. The SpO2 sensor is located at the back, in the area of ​​the camera:

To do this, open the Samsung Health app and select SpO2 measurement or stress measurement :

In the latest updates, Samsung Health does not measure SpO2 separately, but does it as part of the stress level measurement.

Now let’s try to answer the main question.

Why measure SpO2? What is the meaning of this feature on fitness bracelets?

There are situations in which the SpO2 measurement is critical to a person’s life. But, unfortunately, fitness bracelets have nothing to do with these situations:

  • SpO2 monitoring is required mainly for anesthesia during surgery. This is one of the main tasks of the anesthesiologist.
  • It can also be important to monitor oxygen levels while recovering from major surgery or previous health problems such as a heart attack.

But in the current implementation, fitness bracelets are not able to replace specialized devices (more on this at the very end of the article). But for an ordinary user, it makes little sense to measure SpO2. Although, if you wish, you can think of several areas for using such an “oxygen sensor”:

1. Professional sports

As mentioned above, the SpO2 level drops with exercise. By tracking this indicator during intense training, you can find out the limits of your body and try to improve your physical performance.

On the other hand, in professional sports, VO2 max is more important , which some fitness trackers and smartwatches are also trying to measure today. But this is already a topic for a completely different conversation.

2. Mountaineering

As you know, the air in the mountains is thinner, that is, it contains less oxygen. And with a certain decrease in oxygen levels, you may even feel better. But if the decline continues, it could have bad consequences.

Therefore, by controlling SpO2 while climbing, you can reduce the risk of problems and feeling unwell. For example, when the percentage of oxygen begins to drop dramatically, you need to reduce the load – change the rate of ascent, take a break, or even revise your plans.

Added on 04/06/2020 : As it turned out , a fitness bracelet with SpO2 measurement function has become an excellent tool for diagnosing coronavirus infection COVID-19!


Again, there are much more convenient tools for each of these tasks.

The main problem with most fitness bracelets and smartwatches at the moment is not measurement accuracy, but the lack of a continuous SpO2 measurement mode.

Today, almost all bracelets take one-time measurements within 15-30 seconds and are not able to signal a drop in the oxygen level in the blood. And this is the main task of pulse oximeters.

What fitness bracelets and watches can measure blood oxygen levels (SpO2)?

SpO2 measurement is not only a software function. The bracelet must be equipped with infrared and red LEDs. If the bracelet does not have a red / infrared LED, it will not be able to measure the oxygen saturation level of the blood. A software update will not fix this problem.

Today the following devices are able to measure SpO2:

  • Apple Watch 6
  • Fitbit Charge 4
  • Garmin Vivosmart 4
  • Garmin Fenix ​​6
  • Honor Band 5
  • Honor Band 5i
  • Huawei Band 4
  • Huawei Band 4 Pro
  • Huawei Watch GT 2
  • Huawei Watch GT 2e
  • Honor MagicWatch 2
  • Samsung Galaxy Watch 3

Also, smartphones from Samsung are able to measure SpO2, starting with Galaxy S6 and ending with Galaxy S10 inclusive + Galaxy Note line up to Galaxy Note 9 inclusive. All subsequent flagships from Samsung lack this sensor.

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Igor Smith/ author of the article
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