One smartwatch carries two different kinds of health feature, and the label tells you which is which. Apple's ECG app went through FDA review as a medical device: the agency's De Novo decision summary DEN180044 records a De Novo date of August 14, 2018, a new classification under 21 CFR 870.2345, product code QDA, and an indication limited to determining the presence of atrial fibrillation or sinus rhythm on a classifiable waveform. The blood oxygen reading on the same wrist did not. Apple's own specification footnote states that the Blood Oxygen app is for wellness purposes only and not for medical use, and says the same of temperature sensing. The condition that decides which features you actually get is the pairing: the cleared features carry age floors and exclusions written into the clearance, not into the marketing.
The clearance names the limits in the same sentence as the claim
DEN180044 states that the device is not intended to provide a diagnosis, that the ECG data is intended for informational use only, that the user is not intended to interpret or take clinical action on the output without consultation of a qualified healthcare professional, and that the app is not intended for use by people under 22 years old. Its limitations section adds that the device was evaluated only for detection of atrial fibrillation or normal sinus rhythm, is not intended to detect any other type of arrhythmia, and cannot detect heart attacks.
The evidence behind it is on the record too. That same summary describes a study enrolling 602 subjects at 5 investigational sites, with sensitivity and specificity performance goals set at 90 percent and 92 percent against physician-adjudicated 12-lead ECG. Those two numbers, and the 602-subject population they came from, are the entire clinical basis for the feature.
Age floors and exclusions travel with each cleared feature
The manufacturer repeats the boundaries in its published footnotes. Apple's Series 12 technical specifications state that the ECG app is intended for use by people 22 years old or older, that sleep apnea notifications are intended to detect signs of moderate to severe sleep apnea for people 18 years old or older without a diagnosis of sleep apnea, and that hypertension notifications are not intended for people under 22, for people previously diagnosed with hypertension, or for pregnant persons. Those exclusions are part of the feature, not fine print around it.
The FDA's September 2025 notice covers wearable blood pressure features
Cuffless blood pressure is the line the agency drew explicitly. In a safety communication dated September 16, 2025, the FDA told consumers not to use unauthorized blood pressure devices, including software features on wearables such as smartwatches and smart rings that claim to measure blood pressure, regardless of brand, and directed them to check the FDA's 510(k) database to confirm whether a device is authorized. The same distinction runs through smart rings, which the communication names by product type.
Apple's 24-hour battery figure is a written script
Battery claims on a watch are test results, and this one publishes its test. Apple states up to 24 hours of normal use, up to 38 hours in Low Power Mode and up to 10 hours of outdoor workout for the Series 12, with fast charging to 80 percent in about 30 minutes and 15 minutes of charging for up to 12 hours of normal use.
The manufacturer's definition of normal use is spelled out in the same footnote: 300 time checks, 90 notifications, 15 minutes of app use, a 60-minute workout with music playback over Bluetooth, and 6 hours of sleep tracking, over 24 hours, with testing conducted by Apple in July and August 2026 on preproduction units.
As an example, count what the script leaves out. Apple's 24-hour figure allocates 6 hours to sleep tracking, so the remaining 18 hours carry 300 time checks and 90 notifications, which is one glance every 3.6 minutes and one notification every 12 minutes. A day with a second workout adds an hour that the 24-hour script never counted, and the published outdoor workout figure of 10 hours is measured on its own, without an iPhone, during outdoor run sessions. The battery number is not wrong; it is a stated recipe, the same way a headlamp's runtime is quoted at a stated output.
Two more manufacturer figures on that same specification page are worth writing down because they are the ones that decide whether the watch survives the week: water resistance of 50 meters under ISO standard 22810:2010, which Apple describes as shallow-water use such as swimming or snorkeling to a depth of 20 feet (6 meters) and not scuba diving or high-velocity water sports, and an IP6X dust rating. The same page notes that water resistance is not a permanent condition and can diminish over time. Evidence quality follows the same rule on a dash cam: the specification tells you the conditions under which the number holds.
Before you buy a smartwatch
- Which health features are FDA-cleared and which are labeled wellness only, since the specification footnotes say so per feature.
- The age floor and exclusions on each cleared feature: 22 and up for ECG and hypertension notifications, 18 and up for sleep apnea notifications.
- The battery figure with its test script attached, including how many hours of that 24 are sleep tracking.
- The fast-charge figures in minutes, since overnight sleep tracking and overnight charging compete for the same hours.
- The water resistance rating with its standard number, here 50 meters under ISO 22810:2010, and the depth limit stated alongside it.
- The phone requirement: which handset and operating system version the watch pairs with, and which features stop working without it.
- The out-of-warranty battery service price, which is what a sealed lithium-ion cell costs at the end of its life.