VO₂ max, blood oxygen, and breathing rate: what your Apple Watch measures and MeteoHealth shows

Open Apple Health after a night of sleep or a morning run and three numbers tend to show up together: blood oxygen, breathing rate, and VO₂ max. They look like a single dashboard, but they come from three different places on your wrist, produced in three different ways. This is informational material, not medical advice — none of it is a diagnosis. And it's worth being precise about where these numbers actually come from: your Apple Watch is what measures or estimates them. MeteoHealth only reads what's already in Apple Health and shows it next to the day's weather — it never measures or estimates any of these itself.

Blood oxygen (SpO₂): a sensor reading, for wellness only

The Blood Oxygen app works with light. Apple describes it plainly: "Blood oxygen measurements use a bright red light that shines against your wrist" (Apple Support), and what comes back is "the percentage of oxygen your red blood cells carry from your lungs to the rest of the body" (Apple Support). That's a real optical measurement, not a guess — the sensor is genuinely reading something physical off your wrist.

What it isn't is a clinical instrument. Apple states this directly, without hedging: "Blood Oxygen app measurements are not intended for medical use" (Apple Support). It's a wellness feature, not a medical device, and it isn't built to flag any condition. The honest way to use the number is the same as most of what your watch collects: watch it over time, as your own baseline, rather than reading any single reading as a verdict.

Breathing rate: estimated while you sleep

Unlike SpO₂'s direct optical reading, breathing (respiratory) rate is a step removed — Apple Watch estimates it overnight from patterns of body movement, as part of the Vitals feature that also tracks heart rate, wrist temperature, and sleep duration while you're asleep (Apple Support). The Vitals app doesn't hand you a single "good" number to hit. Instead it "estimates certain overnight health metrics and establishes a typical range for each" (Apple Support) — a range that's personal to you, built from your own recent nights.

That framing is worth taking seriously rather than skimming past. There's no universal cutoff where a breathing-rate number becomes something to worry about; there's only whether tonight's estimate sits inside or outside the range your own history has established. Watching your own trend, night over night, tells you more than comparing any single figure to somebody else's number.

VO₂ max: an estimate, not a lab test

Of the three, VO₂ max sits furthest from a raw sensor reading. The real, direct measurement is a cardiopulmonary exercise test (CPET) — riding a bike or running on a treadmill while wearing a face mask that measures the oxygen in every breath you take in and out. Apple Watch does not do this. It generates what Apple itself calls "an estimate of VO2 max using Apple Watch" (Apple, 2021), inferred from your heart-rate response during ordinary outdoor walks, runs, or hikes on relatively flat ground — Apple specifies a grade under 5 percent — given a solid GPS and heart-rate signal and enough exertion.

Apple's own validation puts a number on how close that estimate lands: against a reference on a validation set of 221 people, the mean difference was 1.4 ± 4.7 ml/kg/min (Apple, 2021) — close on average, with real spread around that average. An independent study on the Apple Watch Series 7, with 19 participants, found something less flattering: lab-measured VO₂ max averaged 45.88 ml/kg/min against the watch's 41.37, a mean underestimate of about 4.5 ml/kg/min, a mean absolute percentage error of 15.79%, and poor overall reliability (ICC 0.47) that only reached moderate (0.60) for participants who were already quite fit. The authors put it directly: "Similar to other smartwatches, the Apple Watch also overestimates or underestimates the VO2max in individuals with poor or excellent fitness levels, respectively" (Caserman et al., 2024). Apple's own paper adds further caveats: chronotropic incompetence can push the estimate too high, while dehydration, caffeine, extreme heat, or a recent change in altitude can push it too low (Apple, 2021). None of that makes the number worthless — it makes it a fitness estimate worth tracking as a trend, not a lab result and not a medical verdict on your heart.

What the watch does, and what the app does

Laid out plainly, the chain is short. Apple Watch measures blood oxygen with its optical sensor; it estimates breathing rate overnight from movement, and it estimates VO₂ max from your heart-rate response to outdoor exertion. Whatever it produces — measured or estimated — gets written to Apple Health. MeteoHealth reads that data on-device and shows it next to the day's weather and your own symptom log, so you can see how the numbers move together. It doesn't touch the sensors, doesn't run its own estimate of any of these three metrics, and doesn't diagnose anything from them.

Where this fits in MeteoHealth

Inside MeteoHealth, blood oxygen, breathing rate, and VO₂ max sit alongside your Apple Health overnight vitals, your local weather, and whatever you've logged about how you feel. The point is pattern-spotting, not scoring: does your breathing rate drift outside its usual range on damp, low-pressure nights? Does a run in the heat correlate with a lower VO₂ max estimate that week? Those are comparisons you can make for yourself, on-device, with nothing leaving your phone to make them. Each number stays exactly what the watch made it — a measurement or an estimate — and MeteoHealth never promotes it to something more certain than that.

Bottom line

Three numbers, three different levels of certainty: blood oxygen is a genuine optical measurement, meant for wellness and not for medical use; breathing rate is an overnight estimate best read against your own typical range; VO₂ max is a heart-rate-based estimate with a real, documented margin of error, useful as a trend and nothing like a lab test. Your Apple Watch is what produces all three. MeteoHealth simply reads them from Apple Health and puts them next to the weather — never measuring, never estimating, never diagnosing, only showing you your own pattern.