Everyone throws the word "toxins" around now, referring to food based as well as environmental sources. For example, I was telling someone about how I enjoy eating liver, especially because it's one of the most nutritious foods per gram in the world. Their response? "It's also one of the most toxic." All kinds of people recommend a variety of foods because they have less "toxins", from vegans to zero-carb paleos, to everyday standard-American-dieters.

This question asks what people do to avoid them, but I'd like to more about them in general.

So, from a Paleo perspective, what is your take on toxins? Which ones are the worst, and importantly, why should we worry about them? Which ones aren't really a problem? I think a list with some hard science evidence would a great resource for Paleo beginners and the community in general, since it seems to be a very important part of the diet and lifestyle.