What is it about kryptonite that renders Superman vulnerable? In our classic comic books it was radioactive in a way that was generally only harmful to kryptonians. But why would their home planet be toxic to them? It never really made sense. I think Zack Snyder addressed this issue in a clever way in Man of Steel. The atmosphere of Krypton did not hurt Cal-El, it simply rendered him normal. My theory: Superman is not allergic to his home planet, he is allergic to Earth. When the Kryptonians were first exposed to Earth, it made them sick. In fighting through that sickness, their powers developed. So the powers are in fact an immune system response to the toxic atmosphere of Earth. In effect, Kryptonite serves as an antidote rather than a poison. A cosmic antihistamine that soothes the allergic response and allows them to return to normal. It's an interesting idea to think that Superman is only super because Earth makes him sick. This could lead to some interesting ways for it to be used against him. Will Batman neutralize him with a booster shot?