Norman Fixel Institute for Neurological Diseases

By Carey Gillam

When US regulators issued a 2019 assessment of the widely used farm chemical paraquat, they determined that even though multiple scientific studies linked the chemical to Parkinson’s disease, that work was outweighed by other studies that did not find such links. Overall, the weight of scientific evidence was “insufficient” to prove paraquat causes the brain disease, officials with the US Environmental Protection Agency (EPA) declared.

The EPA reiterated its assessment in a 2021 report. The agency said after extensive analysis of numerous factors, including concerns about Parkinson’s risk, it determined farmers could safely continue to apply the weed killer across millions of US acres to help in the production of soybeans, corn, cotton and an array of other crops.

The determination has been significant for Syngenta, the longtime maker of paraquat weed killers. The company is facing calls in the US for a paraquat ban, as well as more than 2,000 legal claims brought by farmers and others alleging they developed Parkinson’s disease because of their exposure to paraquat.

Syngenta cites the EPA assessment of the science on paraquat in defense of the pesticide, and says scientific research “does not support” a causal relationship between the chemical and the disease. On a Syngenta-run paraquat information website, the company highlights several studies it says also backs that position, including many conducted by company scientists or by outside scientists who received company funding for their work.

Last week the Guardian and The New Lede reported that internal corporate records show Syngenta had knowledge of science linking paraquat to Parkinson’s decades ago but sought both to refute the evidence with various secret tactics. The documents are available at the Paraquat Papers Media Library.

US farmworker, health and environmental advocacy groups say in contrast to the corporate science, research conducted by independent scientists provides abundant evidence of paraquat’s ability to cause Parkinson’s and other health dangers, and the EPA is improperly discounting that body of research.

More than 50 groups have called for the US to follow the lead of dozens of other countries in banning paraquat. The Michael J. Fox Foundation for Parkinson’s Research submitted a petition to the EPA with 107,000 signatories calling for a ban. The foundation cited a study that found people exposed to paraquat in their teens or as young adults had an increased Parkinson’s risk of 200 to 600 percent, depending on the overall exposure.