Some Apple Watch Series 12 and Ultra 4 owners have experienced unexpected skin irritation with the new models, according to reports on the MacRumors forums and Reddit . Affected users complain of a burning sensation and redness under the sensor area. The Apple Watch Series 12 and Ultra 4 measure heart rate every five seconds, and some people have wondered if that's the cause of the skin irritation. Along with more frequent heart rate readings, the Series 12 and Ultra 4 have a larger ECG electrode surface, photodiodes in a ring-shaped arrangement, and larger green LEDs. It is unknown if any of these design changes are a problem, but wrist irritation from the Apple Watch is a complaint that dates back to the original model. Apple says reactions can be caused by wearing a band too tight or too loose, or by irritants like soap and sweat under the watch. Some Apple Watch models contain nickel, but it is a low amount below EU limits, and Apple says it's unlikely to be a problem. The Apple Watch case and certain Apple Watch bands have trace amounts of acrylates and methacrylates from adhesives, but Apple says it designs the watch and bands to keep those components away from skin. Apple recommends making sure a band fits well and keeping the Apple Watch, bands, and skin clean and dry. Apple adds that cleaning may be required after workouts or exposure to liquids like sweat, soap, sunscreen, and lotion. Some users have seen the issue clear up after an adjustment period wearing the watch, and cleaning the back of the watch and the band has also helped in some cases.