I'm not a toxicity risk type person. There have been studies on dispersants. I sent, just yesterday, some notes I put together outlining at least some of the references for studies that have been done.
The issue with dispersants is they're really a short-term solution. You don't want to be applying dispersants over a long period of time. They're meant more for just trying to break up the sheen and increase the biodegradation so they have a larger surface area of oil to deal with. Any time you apply a dispersant, it's because I would say you are trying to weigh the fact that oil is both physically and chemically toxic and dispersants are more on the chemical toxicity side.
On their impact on the environment, that would be so hard, especially on the open ocean, because they're dispersed. So I would say probably the only way—and again biologists are going to jump all over me—would be to actually go out and do environmental effects monitoring, where you take samples of fish, or things maybe that don't move in the area, like crustaceans or clams or oysters, and test for levels of the dispersants in them.
I don't know if that really answers your question.