The Apple Watch Series 6 has a sensor built in that can measure the oxygen level in the blood. This ability could have a real benefit in judging health and reacting to the first signs of conditions such as heart failure, asthma and managing Coronavirus symptoms or even identifying COVID-19.

The NHS guide to staying safe while recovering from Coronavirus at home indicates that: "If you use a pulse oximeter, your blood oxygen level is 94% or 93% or continues to be lower than your usual reading where your normal oxygen saturation is below 95%" you could call NHS 111.

In addition the recommedation is that if "your blood oxygen levels are 92% of less [having retaken your reading first] you should go to A&E or call 999 within an hour of the measurement.

With these recommendations it is no surprise that people are currently searching for blood oxygen meters, and of course the Apple Watch 6 has one included. In this article we will look at how to use the Apple Watch 6 to measure your blood oxygen, how accurate it is, and when you should be concerned about a reading.

The Apple Watch 6's oxygen measuring abilities could be a good reason to purchase or borrow one if you are experiencing Coronavirus-related breathing problems.

How Apple Watch 6 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 blood 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 and monitoring of blood oxygen levels. The Blood Oxygen 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. 

Is the Apple Watch 6 O2 measurement accurate?

We have a separate article where we give a pulmonologist's assessment of the Apple Watch.

He notes that the Apple Watch 6 and comparable trackers are not medically tested, so it is currently not possible to say how accurate the measurements are. He adds that if you record a low reading the first thing you should do is assess how you are feeling in that moment. If no further complaints occur, you can possibly check with your doctor and use the doctor's own device to determine whether the warning is correct.

However, if you are already suffering from Coronavirus symptoms we would suggest calling 111 or 999 or visiting A&E if you are concerned about a reading.

What is a safe oxygen saturation level?

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.

However, as per the NHS guide, a continuous blood oxygen reading under 94% or 93% (if your level is usually 95%) is a reason to call NHS 111. While a reading under 92% is a reason to call 999 or go to casulty.

What low blood oxygen levels can tell you

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.

However, right now the blood oxygen monitoring capabilities of the Apple Watch 6 for those infected with COVID-19 are proving invaluable.

A person suffering with Coronavirus symptoms at home can monitor their blood oxygen leavels with the Apple Watch 6 and react quickly should their blood oxygen level fall below a particular point.

How to measure oxygen levels with Apple Watch

What you need:

  • The Apple Watch Series 6 - the only Apple Watch capable of blood oxygen measurement.
  • The watch needs to be running watchOS 7 and the paired iPhone will need to be running iOS 14.
  • You will also need to set up the Blood Oxygen app on the iPhone.
  • Oxygen measurement is only available in some countries - full list here.
  • Similar to the ECG app, the oxygen app is only activated for users aged 18 and over.

How to measure blood oxygen

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

  1. Open the Health app on the iPhone.
  2. Tap on Browse.
  3. Go to Respiratory > Blood Oxygen > Set up Blood Oxygen.
  4. Now on the Watch open the Blood Oxygen app.
  5. Tap Start. 
  6. Wait while the measurement takes place - it takes around 15 seconds and you will need to keep your arm still.
  7. Tap Done.

To ensure that the app works reliably, Apple recommends taking your measurement while sitting, keeping your arm completly still, and making sure the watch sits firmly on the wrist and does 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 ability to take measurements in Sleep Mode in the Health app.

  1. Go to Settings on the Apple Watch.
  2. Tap Blood Oxygen
  3. Turn off In Sleep Mode and In Theatre Mode

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.

Interestingly, a study has shown that the Apple Watch can detect COVID-19 before you know you have it.

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.