Secret files suggest chemical giant feared weedkiller’s link to Parkinson’s disease

Exclusive: Documents seen by Guardian detail effort to refute scientific research into paraquat and derail nomination of key EPA adviser

For decades, Swiss chemical giant Syngenta has manufactured and marketed a widely used weed-killing chemical called paraquat, and for much of that time the company has been dealing with external concerns that long-term exposure to the chemical may be a cause of the incurable brain ailment known as Parkinson's disease.

Syngenta has repeatedly told customers and regulators that scientific research does not prove a connection between its weedkiller and the disease, insisting that the chemical does not readily cross the blood-brain barrier, and does not affect brain cells in ways that cause Parkinson's.