While we're on the topic of possible cause, here's another new possible cause that blew me away ..

*edit*
Carbon monoxide can cause PD

This is especially relevant to me as my ex and I spent numerous years renovating a brownstone. During the process of tuckpointing (brick/mortor replacement) the workers blocked the chimney by endlessly shoving mortor into it. The chimney was clogged for 2 years of winters and we had condensation on the inside of all our windows and didn't realize the cause. The heating bills were low though so we thought we were doing great till we figured out what was going on. It was a big house (over 4,000 sf and high ceilings). I have always wondered about the long term implications of such a mistake.

It's funny, all the different ways we can get here.