I did have a slight bit to add regarding your excellent question about how synergistic effects may be assessed. This is a very new and difficult area in the science of risk assessment, because it's often hard to understand all of the context for these interactive exposures. But our more complex assessments do indeed try to come to grips with this to some degree.
A good example is the assessment we did on smog to understand what kinds of atmospheric contaminants combine to form the essential components of smog, how this happens, and what the right levels of these precursors would be to avoid risk. As well, quite often we look at what a particular substance degrades to in the environment and what the impacts of those degradation products might be. It is an extremely difficult scientific question to be able to understand all of the various components of this, but it is an area of our ongoing research.