Revealed: The secret push to bury a weedkiller’s link to Parkinson’s disease

Internal documents from chemical giant Syngenta reveal tactics to sponsor sympathetic scientific papers and mislead regulators about unfavorable research

The global chemical giant Syngenta has sought to secretly influence scientific research regarding links between its top-selling weedkiller and Parkinson's, internal corporate documents show.

While numerous independent researchers have determined that the weedkiller, paraquat, can cause neurological changes that are hallmarks of Parkinson's, Syngenta has always maintained that the evidence linking paraquat to Parkinson's disease is "fragmentary" and "inconclusive".

But the scientific record they point to as proof of paraquat's safety is the same one that Syngenta officials, scientists and lawyers in the US and the UK have worked over decades to create and at times, covertly manipulate, according to the trove of internal Syngenta files reviewed by the Guardian and the New Lede.

The files reveal an array of tactics, including enlisting a prominent UK scientist and other outside researchers who authored scientific literature that did not disclose any involvement with Syngenta; misleading regulators about the existence of unfavorable research conducted by its own scientists; and engaging lawyers to review and suggest edits for scientific reports in ways that downplayed worrisome findings.

The files also show that Syngenta created what officials called a "Swat team" to be ready to respond to new independent scientific reports that could interfere with Syngenta's "freedom to sell" paraquat. The group, also referred to as "Paraquat Communications Management Team", was to convene "immediately on notification" of the publication of a new study, "triage the situation" and plan a response, including commissioning a "scientific critique".

A key goal was to "create an international scientific consensus against the hypothesis that paraquat is a risk factor for Parkinson's disease," the documents state.

In another example of a company tactic, an outside lawyer hired by Syngenta to work with its scientists was asked to review and suggest edits on internal meeting minutes regarding paraquat safety. The lawyer pushed scientists to alter "problematic language" and scientific conclusions deemed "unhelpful" to the corporate defense of paraquat.

Syngenta's decision to involve lawyers in the editing of its scientific reports and other communications in ways that downplayed concerning findings potentially related to public health is unacceptable, said Wendy Wagner, a law professor at the University of Texas who has served on several National Academies of Science committees. "Clearly the lawyers are involved in order to limit liability," she said.

"It happens regularly in cases where a corporation's internal research puts it at a high risk of expensive lawsuits. Regrettably, this kind of effective legal ghostwriting of scientific reports happens far too often in the chemical industry. Scientifically it doesn't seem acceptable," Wagner said.

When asked to comment about the contents of the documents, a Syngenta spokesperson said: "We care deeply about the health and wellbeing of farmers and are dedicated to providing them safe and effective products. As a responsible company, we have spent millions of dollars on testing our products to make them safe for their intended use."

Syngenta further said there had been more than 1,200 studies of paraquat and none have "established a causal connection between paraquat and Parkinson's disease".

Syngenta spokesperson Saswato Das wrote:

We care deeply about the health and well-being of farmers and are dedicated to providing them safe and effective products. As a responsible company, we have spent millions of dollars on testing our products to make them safe for their intended use.

There have been 1,200+ studies of paraquat and not one – I repeat: not one – peer-reviewed scientific publication has established a causal connection between paraquat and Parkinson's disease. In a 2021 peer-reviewed article, Dr. Douglas Weed, a physician and epidemiologist with over 25 years of experience in epidemiological research with no ties to Syngenta, concluded following a review of the scientific literature, "No author of any published review stated that it has been established that exposure to paraquat causes Parkinson's disease, regardless of methods used and independent of funding source." As such, he determined that a "consensus exists in the scientific community that the available evidence does not warrant a claim that paraquat causes Parkinson's disease."

In addition, the Agricultural Health Study, which is sponsored by the U.S. EPA and several independent public health institutions and has followed 66,110 participants for 30 years, recently issued a report finding no statistically significant link between paraquat and Parkinson's. (Shrestha 2020).

Many scientists disagree with that position, however. Paraquat has been shown in some research to increase the risk of Parkinson's by 150% and is cited in a 2020 book, Ending Parkinson's Disease, by four of the world's leading neurologists as a causal factor for the disease.

The documents revealing Syngenta's efforts to influence science build on other evidence of questionable corporate practices with regard to paraquat. A set of internal documents revealed last year by the Guardian and the New Lede made clear, among other things, that Syngenta had evidence 50 years ago that paraquat could accumulate in the human brain.

Those documents showed that Syngenta was aware decades ago of evidence that exposure to paraquat could impair the central nervous system, triggering tremors and other symptoms in experimental animals similar to those suffered by people with Parkinson's.

They also showed that Syngenta worked covertly to keep a highly regarded scientist studying causes of Parkinson's from sitting on an advisory panel for the US Environmental Protection Agency (EPA), the chief US regulator for paraquat and other pesticides.

The new documents have emerged at a sensitive time for Syngenta. In less than six months, the Swiss chemical giant faces a first-ever trial in litigation brought by US farmers and others who allege the company's paraquat weedkiller causes Parkinson's.

It was 2003, and Syngenta officials should have been celebrating: the company's self-proclaimed "blockbuster" paraquat herbicide product, sold under the brand name Gramoxone, was considered one of the world's top weedkillers, used by farmers across the globe. Sales of $420m were forecast for steady growth.

But at the same time, multiple independent researchers were increasingly reporting evidence that the herbicide might be a cause of rising levels of Parkinson's, a disease particularly seen in farmers. Roughly 90,000 Americans are diagnosed each year with Parkinson's. Symptoms include tremors, rigidity of the muscles, a loss of coordination, and difficulty speaking.

In the face of the developing research, the new documents show, Syngenta decided that it needed a "coherent strategy across all disciplines focusing on external influencing, that proactively diffuses the potential threats that we face", according to the minutes of a June 2003 company meeting.

To achieve that goal, the company set several objectives, including attempting to "influence future work by external researchers where possible".

A key strategy was the engagement of scientists outside the company who could write papers that supported Syngenta's defense of paraquat.

Similar strategies have been pursued by other chemical companies and in other industries when safety questions arose about profitable products. Monsanto, for example, was found to have ghostwritten scientific studies about a widely used chemical called glyphosate, the active ingredient in Monsanto's Roundup herbicide.

The newly uncovered records show that among the scientists with which Syngenta had a consulting arrangement was the prominent British pathologist Sir Colin Berry, who in 2003 became president of the British Academy of Forensic Sciences.

According to testimony given in a deposition by the top Syngenta scientist Philip Botham, and other records, Berry became a participant in Syngenta's "extended health science team", attending company meetings on paraquat. The company had several similar relationships with outside scientists who authored papers to submit to scientific journals, the records show.

Berry co-authored a paper published in 2010 titled "Paraquat and Parkinson's Disease" in Cell Death & Differentiation, a journal owned by the Nature Portfolio, It concluded that the link between paraquat and Parkinson's was weak and evidence linking the chemical to the disease was "limited" and based on "insufficient" data. Along with Berry, two other external scientists were listed as authors.

The paper's ethics declaration did not disclose that any of the three had a relationship with Syngenta specifically. It only stated that "the researchers have worked with pharmaceutical and chemical companies as external advisors. This work reflects their scientific experience and independent views."

But a memorandum from a lawyer advising Syngenta suggests that the work was not independent. The memo stresses the "importance of proactively publishing research studies that discredit the alleged connection between paraquat and Parkinson's disease" – and cites, in this context, the "continuing (Syngenta-sponsored) work" by Berry and the other two authors of the 2010 paper.

The same memorandum noted that public knowledge of "Syngenta-sponsored" work could have "adverse consequences".

Syngenta cites the study on its "Paraquat information center" website.

When asked about his work for Syngenta, Berry acknowledged an ongoing relationship, but said the 2010 paper was not "sponsored" by the company. He said he currently served as chair of a Syngenta "ethics committee".

Another author of the paper, Pierluigi Nicotera, scientific director and chairman of the executive board of the German Center for Neurodegenerative Diseases, said that his consultant arrangement with Syngenta ended in 2008 and he was not paid to write the 2010 article. He said the paper "reflected the views of the authors based on the available data at the time". He said he did not know why Syngenta would refer to work by him and Berry and the other author as company sponsored.

"As of today, I do remain strongly skeptical about the link between use of paraquat and Parkinson," Nicotera said. "A link between exposure and disease is only suggested by epidemiological studies, which as you know, do not establish a cause effect relationship, but only generic risks."

The third author did not respond to a request for comment.

Though it worked to publicize research that supported paraquat safety, Syngenta kept quiet about a series of in-house animal experiments that analysed paraquat impacts in the brains of mice, according to company records and deposition testimony.

Scientists who study Parkinson's disease have established that symptoms develop when dopamine-producing neurons in a specific area of the brain called the substantia nigra pars compacta (SNpc) are lost or otherwise degenerate. Without sufficient dopamine production, the brain is not capable of transmitting signals between cells to control movement and balance.

The Syngenta scientist Louise Marks did a series of mouse studies between 2003 and 2007 that confirmed the same type of brain impacts from paraquat exposure that outside researchers had found. She concluded that paraquat injections in the laboratory mice resulted in a "statistically significant" loss of dopamine levels in the substantia nigra pars compacta.

Syngenta did not publish the Marks research, nor share the results with the EPA. Instead, the documents show that when Syngenta met with EPA officials in February 2013 to update the agency on its internal research on the potential for paraquat to cause Parkinson's disease, there was no mention of the adverse findings of the Marks studies. Instead, Syngenta told the EPA that internal studies showed high doses of paraquat did not reduce the dopamine-producing neurons, directly contrary to Marks's conclusions.

In a follow-up "Paraquat Research Program Update" presentation to EPA officials in February 2017, Syngenta held to that position. The presentation stated that a series of Syngenta animal studies found no "statistically significant effect of [paraquat] on dopaminergic neuronal cell numbers". Again, the company did not mention the study findings by Marks to the EPA, according to deposition testimony from the Syngenta executive Montague Dixon, who acts as the company's main liaison to the EPA.

The presentation to the EPA concluded that paraquat had "no effect" in the brain and that a "causal relationship between paraquat and Parkinson's was "not supported".

Sign up to Down to Earth

The planet's most important stories. Get all the week's environment news - the good, the bad and the essential

after newsletter promotion

When asked in the deposition if the information presented to the EPA was "a lie", Dixon said that Syngenta was not hiding the results of the Marks studies from the EPA, but was instead choosing to focus on other studies. The presentation to the EPA was "not geared to the Dr Marks studies", Dixon said in the deposition.

It was not until 2019 that the company told the EPA about the Marks research – and only after being pressured to do so by an attorney who was by then suing the company on behalf of people with Parkinson's disease.

While Syngenta determined which studies to share with the EPA, company officials were also on alert for outside research related to paraquat and Parkinson's. Part of that involved the internal unit Syngenta referred to as its "Swat team".

The work of the Syngenta Swat team included not just scientists but representatives from the company's legal department and corporate affairs, and involved a variety of potential tactics for responding to independent scientific papers, the records show. In a 2011 email, designated "CONFIDENTIAL AND PRIVILEGED COMMUNICATION", flagged an epidemiology study analysing risk factors for causes of Parkinson's by non-Syngenta scientists to be addressed by the Swat team for a response.

Suggested actions included production of a company "position statement" or a "broader critical review of the approach" used by the outside researchers in their paper.

It was early 2008 when Syngenta scientists gathered in Atlanta, Georgia, to discuss the latest research looking at paraquat and Parkinson's disease. A corporate defense lawyer named Jeffrey Wolff attended the meeting.

Though the meeting was ostensibly called as a "Scientific Review", Wolff spent 30 minutes advising the scientists on how they should be taking notes and managing their communications in ways that might allow the company to later keep the work from public view by claiming "attorney client privilege" in the event of litigation, according to deposition testimony of a top Syngenta scientist, and internal documents.

Wolff "was giving us guidance on how to communicate", the scientist Philip Botham said in his deposition.

"Action notes" from that meeting stated "Study work should be labelled Work Product Doctrine Material Confidential, and carry the Attorney Client Privilege statement."

Wolff then became more deeply involved, records show. The lawyer was asked to comment on a paraquat science strategy document detailing a plan for certain paraquat studies to be carried out, and sent back comments "directed at improving it in the event it falls into the hands of adversaries".

In July 2008, an in-house Syngenta lawyer emailed Wolff for his "review and comment" on notes and minutes of internal meetings related to a risk assessment of paraquat exposure. The in-house lawyers told Wolff that there were "a number of statements in the paper which taken out of context would potentially be unhelpful".

For example, Syngenta scientists had written that, in laboratory tests with paraquat, "The one consistent finding from the body of animal studies is the loss of dopaminergic neurones in the substantia nigra pars compacta (of male mice.) This finding is judged to be real, to be related to treatment and to be adverse in nature. In the absence of evidence to the contrary, it is prudent to assume that this finding is potentially qualitatively relevant to man."

Wolff wrote back suggesting the removal of the words "and to be adverse in nature", questioning the phrasing of the relevance to humans, and other changes, agreeing with the in-house attorney that the statement overall was "unhelpful".

Among other instances, in 2009, records show that Wolff worked with an in-house company lawyer to edit a presentation by a company scientist for Syngenta's leadership team titled "Paraquat and Parkinson's Disease".

Wolff expressed concerns about "blunt statements" and the "sensitive nature of the subject", and advised that only a single electronic copy be presented because it was "not in Syngenta's interest for multiple copies of this document to be in circulation".

In one key edit, Wolff suggested deleting a statement that read: "The combination of experimental data and epidemiological data provides plausibility to the claim that PQ [paraquat] is implicated in PD [Parkinson's disease]."

Wolff also took issue with a statement that said only a small percentage of Parkinson's cases were genetic, with the "majority resulting from gene-environment or environmental causes". Wolff suggested, instead, that the presentation say "The great majority of PD cases are idiopathic or of unknown cause."

Today it is well-established that the vast majority of Parkinson's cases are not caused by genetics, and that environmental factors, including air pollution and pesticides, play an important role.

In another round of edits to a scientific slide show, Wolff recommended the deletion of a statement that said "We can show loss of cells" in the substantia nigra pars compacta. The statement was "an unhelpful admission verifying unhelpful claims which have been made in the literature" about paraquat. He said the observation could be made verbally.

He additionally asked the scientists to revise a slide that he said "suggests that [paraquat] exposure leads to cell death and direct damage to neuronal cells". The records show revised slides were created.

In 2009, Wolff went a step further, discussing legal involvement in the production of research. He advised the company about using outside legal counsel in preparing for an epidemiology study, which would involve discussions with former workers about their exposure to paraquat at a company plant in Widnes, north-west England.

A company scientist planned to do the interviews. But Wolff wrote in the memo that if the scientist did the interviews "it is highly likely that any information he learns or written interview summaries he prepares would not be protected by either the attorney-client or the work-product privileges".

Interviews performed by a lawyer, on the other hand, could be kept confidential more easily. "The highest level of protection would be provided if the interviews were conducted by outside counsel."

Wolff did not respond to a request for comment.

The involvement of lawyers with the scientists at Syngenta appears similar to highly criticized practices by the tobacco industry in the 1970s and ’80s that downplayed the dangers of smoking, said Thomas McGarity, former EPA legal adviser and co-author of the 2008 book titled Bending Science: How Special Interests Corrupt Public Health Research.

"It looks like the paraquat maker has adopted nearly every strategy we outlined in our book about bending science," McGarity said.

"Science matters. We have to be able to depend on science," he said. "When it is perverted, when it is manipulated, then we get bad results. And one result is that pesticides that cause terrible things like Parkinson's remain on the market."

When he worked at the EPA, pesticide lobbyists were so persistent in trying to influence officials, that agency staffers referred to them as "hall crawlers", McGarity said.

The agency has a history of close relationships with industry, and critics say there is a "revolving door" of employees who move between the two, resulting in lax regulation.

Indeed, the trove of Syngenta documents reveal that its law firm hired a retired top EPA official as an expert witness to help defend the company in the litigation. Jack Housenger, director until February 2017 of the EPA's Office of Pesticide Programs, which is the main regulator of paraquat and other pesticides, agreed to do so for $300 an hour.

Housenger did not respond to a request for comment. In a report that he wrote for Syngenta's defense, he said that the EPA had conducted an "in-depth look" into the association between paraquat and Parkinson's and found there is "insufficient evidence" of a relationship between the weedkiller and the disease.

The collection of records, including the newly obtained internal communications and related documents, are part of a case that was filed in 2017 by a group of people suffering from Parkinson's. Syngenta had to turn the records over to plaintiffs’ lawyers as part of court-ordered discovery. The case was set for trial two years ago, and many of the internal documents would have been made public. Just before the trial was to start, however, Syngenta agreed to pay more than $187m to settle the claims.

The evidentiary files are made up of internal memos, emails, draft scientific reports and other records, along with depositions of key witnesses and scientific experts. Many of the documents cited in this story come from an expert report authored by David Michaels, an occupational health expert and a former top health official in both the Clinton and Obama administrations. It has all been turned over to lawyers who are now representing thousands of additional plaintiffs with Parkinson's disease they blame on paraquat exposure.

The lawsuits rely on Syngenta's internal records to claim the company engaged in "a comprehensive scheme of selective fraudulent research and testing, misleading advertising, and deceptive omissions" with respect to paraquat's link to Parkinson's disease.

The trials are set to start in October in California and Illinois. Many more trials loom as over 3,500 additional people have pending Parkinson's claims against the company.

Syngenta asserts that the allegations lack merit. The company has indicated in court records that its defense will rely in part on the EPA's position that after evaluating "hundreds of studies" the regulatory agency has "not found a clear link" between paraquat exposure and Parkinson's disease.

* This story is co-published with the New Lede, a journalism project of the Environmental Working Group. Carey Gillam is managing editor of the New Lede and the author of two books addressing glyphosate: Whitewash (2017) and The Monsanto Papers (2021)