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Here me out. I think I can explain the sience behind fire users.
So a fire typically needs three things to be created.
Heat, Oxygen and fuel.
If you think about it your core could he considered the heat aspect of the whole thing. The fat on your body is a burnable object that can be considered a fuel, so with enough of it you could be able to burn. Now Oxygen was a little tricky but I remembered that we have to breath that shit and while it comes in it could create that fire.
Typically the more fat the faster the death. But and here me out.
The less fat the longer it burns which shouldn't make since bit reverse psychology and all that.
Now considering the fact that Touya can burn hotter than Endeavor and hold his flames longer, I'm going to say his core has adjusted to higher temperatures than any normal user can handle due to the fact he was always surrounded by a user with fire that had potentially harmed him.
Due to the fact his core is hotter and therefore can burn hotter. But due to the fact he has no fucking meat on his bones he burns faster than Endeavor which results in his skin deteriorating faster than normal.
The addition of his skin graphs as well! In the hospital after his three year coma, that skin has not adjusted to his core and none of it was really his to begin with.
Now we all know he hates the fucking heat (Because if he doesn't then that's just fuckin weird) but I bet he's in it half the time to restore himself and the cold is the only way to keep him from actually burning himself up.
I can't explain the math behind ice quirks. That's too much for me and this was just a little dive hole I went down but due to him having ice resistance (No I'm not including the ice quirk because one I love spoilers but I am not prepared for that yet) he doesn't freaking die by frost bite or shit.
(IDK but I just thought of this earlier and thought to share.)