What the data actually reveals
A single resting figure says very little. A continuous series says a great deal: sleep and wake times, exercise, illness, alcohol, and the periods when you were under pressure. Line that series up against a calendar and it becomes a record of which meetings and which people your body reacted to.
None of that is inferred maliciously. It falls out of the data by itself.
On-device is a real architecture, not a slogan
On iPhone the data already lives in a local, encrypted store — Apple Health. An app with read permission can chart it without a server existing anywhere in the design. There is no technical need for a round trip.
Apps upload anyway for reasons that are mostly product decisions: a web dashboard, sync across devices, coaching features, or an account system that assumes a backend. Each is a legitimate feature. Each also moves a copy of your health record onto infrastructure you do not control.
Worth knowing: Apple's rules prohibit using HealthKit data for advertising or sale to data brokers, and require a privacy policy. That is a meaningful floor. It is not the same as the data never leaving your phone.
How to check any health app in two minutes
Look for a forced account. If it demands an email and password before showing you anything, there is a server, and your data is probably on it. Apps that read Health and stay local rarely need a login at all.
Read the App Store privacy label. Check the "Data Linked to You" section specifically. Health & Fitness appearing there means it leaves the device tied to your identity.
Try airplane mode. If charts render and history browses with no connection, the processing is local. If it spins or shows an error, it was fetching from somewhere.
The honest trade-off
Local-only has real costs. No web dashboard. No syncing your history to an iPad. No account recovery — if the phone is lost and you have no backup, the app's own state goes with it, though anything held in Apple Health follows your encrypted device backup. And no server-side analysis, so features that would need heavy computation stay modest.
For a lot of people that is a fair exchange. For someone who wants their cardiologist to see a shared dashboard, it is not, and that is a reasonable thing to want.
Where 70 BPM sits
70 BPM's App Store listing states it directly: on-device by design — processing stays on your iPhone, and it does not upload your heart rate to a cloud. It reads samples from Apple Health, and you choose which calendars are included, with holiday and subscribed calendars off by default. It is free, and iPhone-only.
So the airplane-mode check works on it, and the constraint is one you can verify yourself rather than take on faith.