For the issue at hand the temperature of the phone, even on the outside, is not very relevant. What matters are - whether it feels uncomfortable to humans -

For the issue at hand the temperature of the phone, even on the outside, is not very relevant. What matters are - whether it feels uncomfortable to humans - whether it is unsafe for humans I think both are mostly about the amount of heat the device transfers to human skin per unit of area per second. The temperature of the outside of the phone is _a_ factor in that, but not the only one. Thermal conductivity of the material (compared to metals, wood doesn’t feels hot in summer or cold in winter, for example) and grip strength needed to reliably hold an object play a role, too.