I personally do not own a Watch 5, but I am very much familiar with people voicing the same complaint.
Community post about someone having noticed burn-esque marks on their forearm, and other people reporting the same in the replies ( I can not say how accurate the replies really are. An account literally called "GalaxyWatch5Burns" posted on there for the first time, and the damage shown in their picture specifically is much more severe then the others) :
https://eu.community.samsung.com/t5/wearables/burns-from-galaxy-watch-5/td-p/6308923
Here a review that also mentions skin irritation:
https://eu.community.samsung.com/t5/wearables/from-practical-life-with-the-galaxy-watch5-pro/m-p/599...
There seem to be two major ideas about what is causing it:
The watch not being worn properly, or allergic reactions.
Not being worn porperly: The underside of the watch not being cleaned often enough, which can cause irritation from dirt and dead skin cells accumulating and damaging the healthy skin, causing an inflammatory reaction.
Wearing it too loosely, with just friction from it wandering constanly irritating the skin, or it being worn too tightly. It is important to remember: The watch has an infrared sensor. It is extremely low power, but if it is sitting on the same spot, firing possibly hundreds of times a day to take readings, you can notice damaging effects. Infrared light making contact with the body, basically instantly turns into heat, so just constanly putting pressure on one spot, the skin not being exposed to air a lot, and the sensor firing all the time can be an issue. Ontop of the watch having a 2 core CPU, and a battery in a very small space, just adding more heat directly onto the skin, drying it out, making it flaky. Basically looks like a rash, because it pretty much is from the skin being put under constant stress in that region. (There should be an option to increase the interval between readings. Some people report that increasing it to 10 minutes or more helped resolve some of the problems.)
Allergic reactions: The underside of the watch contains nickel, something a surprising amount of people are either sensitive to, or downright allergic.
A study about nickel allergy in jewelry:
https://pubmed.ncbi.nlm.nih.gov/10863220/
From what I was able to find out, the nickel content is quite low, and far below what it could legally be at the most, but it is something a lot of people react to, and do not know about, or just dont realize that, yes, wearing something with direct, and frequently too tight contact to the skin, for many hours each day for weeks, months or years. That will 100% at some point cause complications.
Important to remember: It is not a normal watch, like the wind up, or battery powered quartz watches.
It is designed, and for its proper function necessary, to have constant proper contact with the skin, and there is actual logic in it, with every part from the display, to the battery itself generating heat, due to unavoidable inefficency. 5G and bluetooth also are not to be ignored, the wifi/BT themselves are pretty much irrelevant, but such a small device processing sensor data, transmitting said data constanly. the wifi/BT modules, even if you cant really feel it, do generate a good amount of heat just by themselves.
I apologize for the long reply, but I have seen so many people basically accuse Samsung of "branding" them, simply because they were not handling their devices responsibly.
I am not accusing you of being that way, but I finally felt the need to make a post, in hopes that more people actually pay attention to how they interact with these kinds of electronics.
