1)
1)
This followed the WHO standard.
2)
Right. That's しん.
You can't say どう決めたか as a direct quote, i.e., in real conversation. The form must be どう決めたんですか/どう決めましたか(polite), どう決めたの(casual) or like that. If the form is correct, you can use か and "?" in the same time, of course.
3)
被曝/限度/に/なる
他
他 - goo辞書
That's simply "から + の".