I definitely think it's not only possible, but likely, that chemical or combination of chemical exposure is at least partially responsible for causing PD. I also agree that odds were more stacked against you in the face of genetic mutation.
I would surmise that such exposure would cause PD early on, only giving the appearance of taking years to develop, since symptoms generally appear after significant neuronal loss.
In my case, I suspect that working with fenthion in the first 3 years as a veterinary professional is a factor. I always wore latex gloves when applying it, but the fumes were quite strong. I have worked with a number of insecticides over the years, as have my colleagues, so why am I the only one I know to develop YOPD? I've been tested for genetic mutations through at least 2 different parties with none detected, although I realize there could still be some sort of genetic predisposition. Maybe it's just a perfect storm of multiple factors that are particular only to me. Like you said, how was I supposed to know?
Thanks to all of you who cited links on this intriguing subject. I plan to read them soon. I'm still searching for any articles that make a connection to fenthion exposure. I also wonder about the multiple Rabies vaccines I received around the same time frame. Due to an outbreak in my area that lasted a few years, plus an exposure at the hospital where I was employed, I received multiple boosters over a period of 5-7 years, 4 of which were within a 6 month period. One veterinarian I worked with suggested I check a titer before getting another booster; she indicated there was a possibility of neurological side effects with too many vaccines. My titer was ridiculously high, 1:7000; adequate immunity is achieved at 1:5. Some 15 years later, it was still well beyond adequate, something like 2800. I'll probably get another titer in the next couple of years, just out of sheer curiosity.