The freezing point of water seems a hell of a lot more relevant to what humans consider 'cold'...which is why it's the zero. The boiling point of water isn't the zero in Celsius after all.
Also 'cold' as a concept is often represented with symbols related to frozen water such as snow flakes and icicles.
I was in a sauna at +95 Celsius for several minutes the other day. And within the same week I felt -35 Celsius cold on my bare skin.
Both could kill me provided a bit more exposure, but they don't instantly. Meanwhile, +4 Celsius can also cause death by hypothermia pretty easily in the right circumstances.
So, while I like the idea, I think implementation will be hard as there is no clear death number on either end of the spectrum. Not to mention humidity, clothing, exertion, level of hydration, etc...