http://viartis.net/parkinsons.disease/toxic.causes.htm

Paraquat is a quaternary ammonium herbicide. Other members of this class include diquat, cyperquat, diethamquat, difenzoquat and morfamquat. Pesticides are known to be associated with an increased rate of Parkinson's Disease. Paraquat structurally resembles MPTP and its metabolite MPP+. MPTP and MPP+ are neurotoxic chemicals, that induce Parkinson's Disease in exposed humans. Paraquat might therefore might, as do MPTP and MPP+ inhibit tyrosine hydroxylation, which is essential for the formation of dopamine.