It's battery powered and you only feel this numbness while operating it, on the hand you are using it, not the rest of the body. It stops once you turn it off. My girlfriend also commented on the heat, but it doesn't travel a distance other than 2-3cm, so it may be just the heat the chip/battery generates.

It also seems to be a common complaint among buyers: newer generations (like some Bebird devices) advertise a "non-heating chip" on Alibaba.

Just commented in case anyone out there tried one of these devices, they are usually not SAR rated at all and don't seek to meet industry standards, plus Wi-fi is a bigger deal than Bluetooth, otherwise everyone would be complaining about their bluetooth in-ear headsets.