I read something once that said, if low levels of mercury really made a difference in anything important, it would be apparent on a large scale particularly in

I read something once that said, if low levels of mercury really made a difference in anything important, it would be apparent on a large scale particularly in Japan, due to seafood consumption. So that's my rationale for not obsessing over it in general. Compare to, say, lead, where you can see that the places that were/are last to have it in gasoline are the most violent places in the world. When you start worrying about an insidious problem affecting a huge number of people, it seems to me the first response should be to ask what natural experiment exists.