What happens to your Apple Health data when an app reads it?

What is Apple Health, technically?

A store on the device. The Health app shows its contents; HealthKit is the interface that other apps use to read from it and write to it (Apple, HealthKit). The store is encrypted on the phone and, if you sync it to iCloud, end-to-end encrypted in transit and at rest. Nothing in the store is visible to an app until you say so.

What does an app have to ask for?

Permission, per category and per direction. Reading sleep is separate from reading heart rate, and reading is separate from writing. When an app requests access you see a sheet listing each category with its own switch. You can grant some and refuse others, and the app is not told which read permissions you refused: it simply sees no data for that category. The sheet is the moment to be deliberate. An app that asks for forty categories to do one job is telling you something.

What may an app do with what it read?

Apple’s rules for apps that use HealthKit are among the strictest on the platform. Under the App Store Review Guidelines, section 5.1.3, apps may not use Health data for advertising or data mining, may not sell it, may not share it with third parties for anything other than improving health or medical research with your consent, and may not store it in iCloud (Apple, Review Guidelines 5.1.3). Within those rules, what happens is governed by the app’s own privacy policy.

Which leaves a wide range:

The App Store privacy label summarises this: Health & Fitness data listed under “Data Linked to You” means it leaves the device tied to your identity (Apple, app privacy details).

Can an app change what is in Apple Health?

Only with a separate write permission. Reading your sleep and writing a symptom into Health are different grants. An app that writes can only write the categories you allowed, and everything it writes is labelled with its name in the Health app, where you can see it, filter it out and delete it.

How do you check and change access?

1. Open Settings → Privacy & Security → Health, or the Health app → your profile → Apps and Services.
2. Tap the app. You see every category it asked for, read and write, each with a switch.
3. Turn off anything you did not mean to grant. The change is immediate. The app keeps whatever it already read, which is why the policy matters.
4. In the Health app, any record an app wrote can be found by source and deleted.

What does Longnote do with Apple Health?

Longnote reads only the categories you approve (sleep, activity, workouts, heart measurements, cycle data and the symptom types Health offers), shows them beside your own entries, and processes them on the device. There is no Longnote server, so nothing read from Health is sent to us, and entries read from Health are never included in Longnote’s backups. Longnote writes to Health only if you turn on Save symptoms to Health, and then only the symptoms you log on trackers Health has a symptom type for: which symptom, when, and how severe. One connected detail does leave the phone: if a workout has a recorded route, the app can fetch Apple Maps tiles for the area, to draw a small map beside it. That has its own switch under Settings → Privacy & data. The support article on Apple Health walks through each setting.