No, not everything exists in nature. DDT, for instance, is only man-made. Methoxychlor, too. Plutonium, technetium, latex paint are not found in nature.
But if something is found in environmental samples naturally, then it probably means that we have adapted to it, it’s not too poisonous.
http://www.newscientist.com/article/dn7057-perchlorate-found-in-breast-milk-across-us.html
So if we start a program to remove perchlorate from human breast milk to keep our kids safe, how much would it cost and who would do it and what penalties would you assign to a mom who fed her baby contaminated breast milk? Then we move on to cow’s milk and Ben and Jerry’s.
Plutonium, an element, is indeed found in nature. Just not in a pure form.
Some of its ores are concentrated enough naturally to be harmful.