It's because for a lot of diseases, the causes are not clear. So if there were a list of everything someone in a particular trade was exposed to, then maybe they could be given the benefit of the doubt that having been exposed to something on that list.... Those chemicals evolve constantly. When I joined, I was using chemicals that are no longer used because they are just too dangerous—things like a fuel filled with benzene. Now the fuel is not as toxic as it was. I was exposed to that, but I have no proof.
I don't know. It would make sense that we have something we can go by to explain our diseases. So many veterans are affected by weird diseases that it's hard not to make a link, and yet we can't prove it.