One thing that concerns me about Proposition 65 is that, from the reply they sent, it says hazardous materials in "harmful concentrations". How do they determine what exactly is a "harmful amount"? From what I've read, for example, there is no "safe" amount of mercury, and it varies from person to person. I guess that the amalgam fillings are generally considered to not give off more than is considered "safe", at least here in the UK.

This isn't about the alginate, just something that occurred to me now while re-reading this thread.