About Oxygen

The blood oxygen sensor was introduced in 2020 with the Apple Watch Series 6, but were disabled in Jan 2024 in the US due to a patent dispute. Apple Watch Series 6, 7, 8, 9, and 10, and the Apple Watch Ultra and Ultra 2 have oxygen sensing hardware (even if disabled). The Apple Watch SE series do not have blood oxygen sensors.

How the Apple Watch measures your blood oxygen

Oxygenated and deoxygenated blood absorb red and infrared light differently.

Both optical heart rate and oxygen sensors operate using the same principles: they shine light of a specific wavelength into your skin, and measure the light reflected back. Heart rate sensing uses both green light (525 nanometers) and infrared light (850-940 nanometers). Blood oxygen sensing adds a third red sensor (660 nanometers in wavelength).

On the third generation Apple heart rate sensor (used in the Series 6 and above), there are four groups of photodiodes each with infrared, red, and green sensors, as well as additional infrared sensors (of a slightly different wavelength) in the middle. They’re arranged in a pattern as shown below:

To estimate blood oxygen, the Apple Watch software calculates a ratio of absorbed red light to absorbed infrared light, and then combines them with some calibration constants derived from experimental data to estimate the percentage of blood oxygen saturation.

Source: Apple’s developer documentation on SensorKit.

How accurate is Apple Watch’s blood oxygen sensor?

One research study showed the Apple Watch’s blood oxygen sensor is as reliable as a medical grade device. The study showed that the average absolute difference between Apple Watch and a medical-grade oximeter was less than 1%. So is blood oxygen on apple watch accurate? Yes. The researchers concluded that the Apple Watch’s oxygen sensor can “reliably detect states of reduced blood oxygen saturation”.

However, there are caveats. Studies have shown medical-grade pulse oximeters can over-estimate oxygen stauration for people with darker skin pigmentation. In Feb 2024, the FDA issued a draft document on performance evaluation of pulse oximeters taking into consideration skin pigmentation, race, and ethnicity; as of October 2024, new FDA guidelines were expected to be issued but have not been.

Ongoing patient dispute between Apple and Masimo

As of November 2024, the blood oxygen sensor is disabled in the US due to a patent dispute with Masimo. The U.S. International Trade Commission (ITC) ruled that Apple’s blood oxygen sensor infringed on Masimo’s patents, leading to a potential import ban. In order to comply with the ruling and continue selling watches in the United States, Apple disabled the sensor via a firmware update on January 18, 2024. New Apple Watches are still being manufactured with the sensing hardware. If you buy a new Apple Watch in the US, it will have the oxygen sensor but it will be disabled.

Apple Watches sold internationally still have oxygen sensing enabled.

The patents under dispute expire in 2028, and, of course, a negotiated settlement could lead to oxygen sensing being re-enabled prior to 2018.

What low blood oxygen means

Normal oxygen saturation levels are 95% to 100%. Oxygen saturation levels below 90% are considered abnormal.

If your blood oxygen saturation is low, here are some likely causes.

For blood oxygen, trends are more important than individual outliers

First, let’s get an important caveat out of the way: trends matter more than individual points.

Earlier, we reported that Apple Watch’s oxygen sensor is comparable in accuracy to medical device. However, the error bounds for FDA-cleared oximeters are wider than you might expect. To get an FDA clearance, about 95% of oxygen values must fall within 4-6% of the gold-standard value. This means that if your SpO2 value reads 92%, it is “really” between 88% and 96%.

That means that individual outliers aren’t very meaningful. You have to look at the trend.

Sleep apnea and blood oxygen time below 90%

While the Apple Watch’s breathing disturbance feature uses the accelerometer, not blood oxygen, low blood oxygen during sleep can be a sign of sleep apnea. In our analysis of sleep apnea vs Apple Watch health signals, low sleeping blood oxygen was the #1 predictor of sleep apnea. The variance in oxygen readings was the #4 predictor. Low REM sleep, low cardio recovery, and high respiratory rate rounded out the top 5 signals of sleep apnea risk.

Talk with a doctor about sleep apnea testing if you suspect sleep apnea.

Covid infection

A COVID infection can reduce blood oxygen levels due to lung inflammation, fluid buildup, ARDS, and potential long-term damage to lung tissue, as well as increased risk for blood clots.

COPD or asthma

Asthma and COPD flare-ups, often triggered by infections or pollutants, can significantly worsen oxygen levels, necessitating supplemental oxygen or hospitalization.

High altitude

Your blood oxygen may drop if you travel to a place at high altitude. It may take several days to acclimate. Even after acclimation, your blood oxygen level may be lower than the standard range.

Other lung and heart conditions

For example, heart failure, pulmonary edema (fluid accumulation in the lungs), pulminary embolism (a blood clot in the lungs, which is a medical emergency), anemia (sometimes caused by low iron), and other health conditions may cause low blood oxygen. Check with your doctor if you have concerns.