How to measure blood oxygen saturation with Apple Watch


The Apple Watch Series 6 can calculate how much oxygen that is in your blood, which could identify asthma attack, heart failure, or even COVID-19.

The Apple Watch Series 6 has a new great sensor that can measure oxygen levels in the blood. It is surely its main gimcrack and could have a real benefit in monitoring your health and anticipating the first symptoms of conditions such as heart failure, asthma or coronavirus.

Apple Watch Series 6

How does Apple Watch measure oxygen levels?

To make it possible to calculate blood oxygen saturation, Apple had to change the distribution of the sensors at the bottom of the Apple Watch with the new Series 6. Now, in addition to the green and infrared LEDs, there is another red and some extra photodiodes.

The operation of the oxygen calculation is the same (or very similar) to that of the pulse: the group of four LEDs illuminate your skin and blood vessels; photodiodes record the light that is reflected and use algorithms to calculate how much oxygen is being carried by the red blood cells.

This calculation is based on the fact that the more oxygen in the red blood cells, the redder the blood will be. The protein responsible for transporting oxygen, hemoglobin, contains iron capable of binding oxygen molecules. With this union, the color goes from dark red to light red, a change that the Apple Watch’s photodiodes can detect.

Apple Watch needs a new app to do this calculation. This app serves as a guide for the user throughout the calculation and shows the data on the screen. Information is also added in the Health app in a new section called ‘Blood Oxygen’.

What data is offered?

According to Apple, normal blood oxygen saturation is between 95% and 99%, but in some people, it may be lower. When you sleep, your saturation can drop below 95%.

Knowing how much oxygen is in our blood is medically important as it can help identify heart failure (when the heart cannot pump enough blood, as in the case of peripartum cardiomyopathy (PPCM) ). It can also alert you to an asthma attack, or if you are experiencing breathing problems due to COVID.

What do you need?

The Apple Watch Series 6 is the only Apple Watch capable of measuring oxygen in the blood. You will need to have watchOS 7 installed and be linked to an iPhone that also has iOS 14 installed.

As with the ACG app, the oxygen measurement app can only be activated by users over 18 years of age. The user must also be of legal age to share information from their Apple Watch with the iPhone of a family member.

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How to measure blood oxygen with Apple Watch

Before you can start measuring your blood oxygen, you must configure the app:

  1. Open the Health app on your iPhone.
  2. Click on the ‘Browse’ tab.
  3. Now select ‘Breathing’.
  4. Choose ‘Blood Oxygen’ and activate the option.

To make sure the app works properly, Apple recommends using it sitting down. Also, you should not move your hand, as it is important that the watch is straight on your wrist and does not fall.

The calculation takes 15 seconds, after which you will see the percentage of your blood oxygen on the screen.

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How to measure blood oxygen in the background

The watch can measure blood oxygen in the background, even without the app open. These readings typically occur when the user is not moving. But for the Apple Watch to do it while you sleep, you must activate it in the Health app.

The results of the readings in the background can also be seen in the Health app, in the section dedicated to the respiratory system. Since the red light could be a nuisance in the dark, the app on the watch allows you to disable background readings. It may be the case, for example, when you are in the theater.

Why might the Apple Watch not get an accurate calculation?

  • If your heart rate is very high (150 beats per minute or higher), the watch cannot accurately measure oxygen levels.
  • A tattoo on the wrist right where the sensors go can lead to inaccurate results. Certain types of tattoos and colors can block the light from the sensors completely, preventing reading.
  • Apple has realized that if the arms are close to the body, or the hands in the shape of a fist, the results can be altered.
  • If it is very cold, the properties of the blood flow can change, which can also lead to altered results. 

Remember that the only way to measure blood oxygen is by having an Apple Watch Series 6


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