One might think that their blood might start to boil at 150°C, but I guess it is Gensokyo after all.
Also, nice to know that the third eye also functions as a thermometer.
Humans (maybe also vampires and satori?) have perspiration for a reason. The body is pretty good at controlling its temperature for a good while even in temperatures where water could boil, as long as you take breaks and keep hydrated. Ask any Finnish person and they can tell from experience that you can easily have a sauna with 100+°C temperatures.
That said, 150°C is pretty extreme. The upper limit for hardcore heat lovers tends to be around 120-130°C depending on the sauna. You wouldn't sit that comfortably in such a hot sauna.