A recent California court decision and a new report from the state Department of Pesticide Regulation have cast doubt on whether plaintiffs’ attorneys can prove a causal link between the weed killer paraquat and Parkinson’s disease.
In late November, Contra Costa Superior Court Judge Charles Treat issued a ruling excluding the testimony of a plaintiffs’ witness who argued that a causal relationship existed between paraquat and Parkinson’s disease. Treat is overseeing hundreds of lawsuits brought by agricultural workers, farm neighbors and others who allege that the weed killer’s use in the state contributed to their development of Parkinson’s disease.
Treat rejected an expert witness who has testified on behalf of plaintiffs in such cases – Cornell University biostatistics professor Martin Wells – concluding that Wells’ meta-analysis of paraquat studies was haphazard and inconsistent with scholarly practices.
“The key question is whether Dr. Wells employed scientific methodology in conducting his meta-analysis … that is sufficiently reliable for admission,” Treat said in his Nov. 21 decision. “The court has concerns about Dr. Wells’ methodology and scientific technique.”
Treat went on to raise questions about whether Wells improperly excluded relevant studies involving paraquat and Parkinson’s disease because they didn’t support his conclusions..
“... It is a blot on the scientific methodology he employed where ‘form,’ including transparency and objectivity, is an important aspect of the scientific technique and intellectual rigor expected from experts,” the judge said.
Treat’s analysis of Wells’ methods was also peppered with phrases such as “defects … in logical soundness” and “analytical gaps.”
One of the defendants in the paraquat lawsuits, which include federal cases in multidistrict litigation in Illinois, is Syngenta Crop Protection, which expressed satisfaction that Wells’ expert opinion was excluded.
“This ruling is consistent not only with the federal multidistrict litigation (MDL) court ruling in April 2024, but also with decades of peer-reviewed scientific literature and detailed, science-based reviews by multiple regulatory authorities,” a Syngenta spokesperson told the Southern California Record in an email.
Wells’ meta-analysis “used shifting criteria for the studies included and inconsistent application of inclusion/exclusion criteria apparently to achieve a results-driven meta-analysis to support an association and causation opinion,” the spokesman said, quoting from Treat’s opinion.
As a result of the judge’s decision, plaintiffs’ attorneys will now need to find a new expert witness to make their case against defendants in the paraquat cases, including Chevron Chemical Co., which delivered the weed killer in past decades.
A Dec. 30 preliminary report by the California Department of Pesticide Regulation examined U.S. Environmental Protection Agency studies and other research in the scientific literature about potential paraquat-Parkinson’s links in response to public comments it has received.
“The evidence reviewed herein do not demonstrate a direct causal association with exposure to paraquat and the increased risk of developing Parkinson’s disease,” the report concludes.
It goes on to note that many population-based studies the EPA evaluated go back many decades to the mid-1970s. Paraquat exposure at that time would have been different than what it is today, according to the report.
“It is expected that the legal label restrictions for paraquat use currently in place at the federal and state level would significantly reduce exposures compared to exposures that study subjects recall experiencing in the past,” the study says.