I assume a lot of heat related deaths are mostly due to heat stroke and cardiac stress caused by it. People can easily live in high heat conditions (see deserts

I assume a lot of heat related deaths are mostly due to heat stroke and cardiac stress caused by it. People can easily live in high heat conditions (see deserts, for example), but if they're not used to it, it quickly becomes problematic. You can see the effects any time a heat wave hits a usually mild climate. Cold can kill, too, but it usually hits the very poor and those caught in a bad situation ; heat stroke, on the other hand, can be pretty subtle. I can imagine someone who thinks of 20°C as hot is rather vulnerable to it.