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

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.

But a cache of internal corporate documents dating back to the 1950s reviewed by the Guardian suggests that the public narrative put forward by Syngenta and the corporate entities that preceded it has at times contradicted the company's own research and knowledge.

And though the documents reviewed do not show that Syngenta's scientists and executives accepted and believed that paraquat can cause Parkinson's, they do show a corporate focus on strategies to protect product sales, refute external scientific research and influence regulators.

In one defensive tactic, the documents indicate that the company worked behind the scenes to try to keep a highly regarded scientist from sitting on an advisory panel for the US Environmental Protection Agency (EPA). The agency is the chief US regulator for paraquat and other pesticides. Company officials wanted to make sure the efforts could not be traced back to Syngenta, the documents show.

That prediction of legal consequences has come to pass. Thousands of people who allege they developed Parkinson's because of long-term chronic effects of paraquat exposure are now suing Syngenta. Along with Syngenta, they are also suing Chevron USA, the successor to a company that distributed paraquat in the US until 1986. Both companies deny any liability and maintain that scientific evidence does not support a causal link between paraquat and Parkinson's disease.

"Recent thorough reviews performed by the most advanced and science-based regulatory authorities, including the United States and Australia, continue to support the view that paraquat is safe," Syngenta said in a statement to the Guardian.