The Apple Watch Series 6 has a new sensor built in that can measure the oxygen level in the blood. This is probably the biggest innovation and to could have a real benefit in judging health and reacting to the first signs of conditions such as heart failure, asthma and Coronavirus.

How Apple Watch measures oxygen levels

To make oxygen measurements possible Apple has changed the lay out of sensors on the lower side of the Apple Watch with the Series 6. Now, in addition to the green and infrared LEDs, there are additional red LEDs and some additional photodiodes.

The principle of oxygen measurement is the same or similar to that of pulse measurement: the four LED clusters illuminate the skin and underlying vessels, the photodiodes register the light reflected back and can use algorithms to calculate how much oxygen the red blood cells are currently transporting in the body. This calculation is based on the knowledge that the more oxygen binds to the red blood cells, the redder the blood appears. The protein responsible for oxygen binding, haemoglobin, contains iron-containing complexes that can bind to the oxygen molecules. Upon binding, the colour changes from dark red to light red, this change can be picked up by the photo diodes of the Apple Watch.

An extra app is required on the Apple Watch for the measurement. This app guides the user through the measurement and displays the measured data. This information is also added to the health app under the new tab Blood Oxygen. 

What the data can tell you

According to Apple, normal blood oxygen saturation should be 95 to 99 percent, but in some people this limit is slightly lower. Even during sleep, saturation can drop below the 95 percent limit.

Being able to tell how much oxygen is in the blood has important medical implimentations as it could help to identify heart failure - when the heart is unable to plump blood sufficiently around the body, such as in Peripartum Cardiomyopathy (PPCM). It may also be able to warn of an asthma attack, and it could tell if you are experiencing Coronavirus-related breathing problems.

What you need

The Apple Watch Series 6 is the only Apple Watch capable of blood oxygen measurement.

It will need to be running watchOS 7 and the paired iPhone will need to be running iOS 14.

Apple has indicated that oxygen measurement will only be available in some countries, but it hasn’t yet confirmed which countries will have the app that has the ability to measure blood oxygen. When Apple launched the Apple Watch Series 4 with the capability to take an ECG the feature wasn’t available in the UK initially. The UK had to wait until Marcy 2019 for the feature due to Apple needing to get the app cleared for use in the European Economic Area. Hopefully this won’t be the case with blood oxygen monitoring.

Similar to the ECG app, the oxygen app is only activated for users aged 18 and over. The user must also be over 18 years of age to share Apple Watch data with a family member’s iPhone.

How to measure Blood Oxygen with Apple Watch

Before you can take the first measurement you have to set up the app.

  1. Open the Health app on the iPhone.
  2. Click on the Discover tab.
  3. Select Airways.
  4. Choose Oxygen saturation and activate it.

To ensure that the app works reliably, Apple recommends taking your measurement while sitting.

You hand must not move, the watch should sit firmly on the wrist and not slip.

The measurement takes fifteen seconds, after which it will present a percentage of the oxygen content in the blood.

How to take background measurements

The watch can measure the blood oxygen in the background even without the app open.

This usually happens when the user is not moving.

In order for the Apple Watch to take its measurements while sleeping, the user must activate the sleep plan in the Health app.

The results of the background measurements can be seen in the Health app in the Respiratory System area. Since the red light could be a nuisance in the dark, the app on the watch allows the user to deactivate the background measurement. You can do this in theatre mode, for example.

Why Apple Watch might not get an accurate measurement

If the heartbeat is fast (150 beats per minute or more), the watch cannot measure the oxygen content accurately.

A tattoo on the wrist at the point where the sensors are attached can also prevent accurate measurements. Some types and colours of tattoos can completely block the light from the sensors so that the measurement is not possible.

Especially important for the background measurements: Apple has found that if the arms are held next to the body, or the hands formed into fists, it can falsify the results.

If it is particularly cold, the blood flow properties in the skin change, which can also lead to falsified results.

Read about how to use an Apple Watch to measure heart rate and how Apple Watch can remind you to wash your hands. Here's everything you need to know about Apple Watch Series 6 and our Apple Watch 5 vs Apple Watch 6 comparison. 

If you want to buy an Apple Watch Series 6 you can order one from Apple here.