The device is not meant to carry this through clothing like a T-shirt. I wear it directly on the skin. In still comes something that keeps the heat, a vest is suitable. Without cover the heating pad cools down very quickly and it is only mau lau warm. The heating area is too small for me, so you can even take a second heating pad adding to it.
As far as I can tell, the appliance is safe. Also, it will not result in the temperature overheating, Schmorbildung, fire hazard. Other devices cause smoke and or even result in burns to the skin, which probably will not happen with this unit.
I have ordered it to give away. At least what the market offers in this sector, I am satisfied. I give the device still three star. Four or five star, but I'm not really thrilled with the device.