My father died of complications relating to severe Parkinson's. Genetic testing revealed that his case was environmental. Looking back at his potential exposure history, it appears that Trichloroethylene (TCE) was present in several work environments such as a print shop job and an aerospace job.
Environmental consultants are now discovering that TCE vapor intrusion (contaminant VOCs entering homes and business from groundwater offgassing) is much more prevalent than previously thought. New testing techniques are confirming that indoor TCE exposure risks are dynamic and must be evaluated using temporal monitoring [http://kahlassociates.com/how-automation-is-resolving-vapor-intrusion-risk-challenges-while-upstaging-conventional-methods/].
This means that some people with Parkinson's may have been exposed to TCE in their own homes over many years. There is now a testing service that can monitor TCE exposures in homes and businesses on a 24hr basis. Most studies take from 2 to 5 days to get a realistic understanding of the exposure cycle if it exists.