by Evy Lewis, Investigate Midwest, Investigate Midwest
October 23, 2024

In 2004, John Platt and his wife purchased 26 acres of untouched woodlands in Florida’s panhandle. As they transformed it into a horse ranch, they toppled trees and stripped the undergrowth with the herbicide paraquat. Through 2012, Platt sprayed the powerful weedkiller for multiple days in a row each year, by hand.

When Platt bought the land, he weighed around 190 pounds, he said. Now, as he battles Parkinson’s disease, he’s dropped to under 150 pounds. His symptoms, he said, are incessant. He has tremors, difficulty recalling words and severe fatigue. He blames the paraquat. 

“We now know that had a significant impact on my life,” Platt said. “We wouldn’t have continued to use it if we had known what impact it was going to have.”

Platt is one of approximately 6,000 people currently suing Syngenta, which sells paraquat under the trade name Gramoxone. They allege the popular weedkiller led to their Parkinson’s disease, a condition that destroys motor functions. Syngenta, one of the largest chemical companies in the world, has disputed the allegations. Over the years, the company has maintained that there is no connection between paraquat and Parkinson’s disease.

However, thousands of pages of records released in litigation and first reported by The Guardian, show the company’s own scientists determined that paraquat had the potential to damage the brain and nervous system as far back as the 1950s. 

Additional documents, also first reported by The Guardian, showed that as evidence of a connection between paraquat exposure and Parkinson’s disease mounted, Syngenta attempted to discredit critical scientists and limit the spread of information that could threaten paraquat sales.

“Due possibly to good publicity on our part, very few people here believe that paraquat causes any sort of problem in the field and we have the support of the official side,” a toxicologist at Syngenta’s predecessor company wrote to a Chevron toxicologist in 1975, in response to early concerns about paraquat’s long-term health impacts.

Court documents show that over the course of more than five decades on the market, Syngenta elected against following up on early research suggesting that paraquat was neurotoxic, lobbied to keep a prominent paraquat researcher off an EPA panel, and kept quiet about the results of its own studies when they appeared unfavorable to the company.

Syngenta is an international company headquartered in Basel, Switzerland, and owned by Sinochem, a Chinese state-owned conglomerate. It is one of the largest agricultural chemical companies in the world, with a higher market share than its primary competitors: Bayer, Corteva, and BASF. Syngenta has more than 30,000 employees, and reported $19.1 billion in sales in 2023.

Syngenta did not respond to multiple requests for comment over the course of several weeks. On its website, it said that science does not support a connection between paraquat exposure and Parkinson’s disease, and that it is a victim of a “Mass Tort Machine” of plaintiffs’ lawyers attempting to “enrich themselves” by securing settlements. 

In 2021, the EPA completed a decade-long review of paraquat’s risks to human health and re-approved it for sale. The EPA’s decision concluded that “the weight of evidence was insufficient” to link paraquat to Parkinson’s disease. After it was challenged in court in May 2022 by a group of nonprofits, the EPA agreed to reconsider its decision, a process that will take until January 2025 to complete. 

State and federal lawmakers have attempted to ban the herbicide. California Assemblymember Laura Friedman introduced a bill this year that, if passed, would require the state to reevaluate paraquat to determine whether it should be prohibited.

Friedman said she wants the EPA to take a more active role in pesticide regulation. 

“We’ve seen our federal agencies being in much more of a reactive mode, waiting for people to get sick, waiting for years and years of evidence of real harm being caused before they take action,” she said. “Other countries don’t operate that way.” 

On the federal level, Sen. Cory Booker introduced a bill in 2023 that would ban the use of paraquat, among other provisions restricting pesticide use. However, it has made no movement through Congress since its introduction.

Paraquat is banned in more than 60 countries, including the U.K., the European Union, China and Brazil. In late 2022, paraquat was removed from the Canadian market. 

It is difficult to assess how much paraquat is used in the U.S. today. The EPA’s decision said it was one of the most widely used herbicides in the country from 2014 to 2018, according to the most recent available data. The amount of paraquat sprayed nationwide increased significantly during that time, according to estimates by the U.S. Geological Survey. 

Paraquat is mainly used on the U.S.’s three primary cash crops: corn, soybeans and cotton. Like dicamba, paraquat gained popularity once weeds resistant to glyphosate, the active ingredient in Roundup, became a common problem for farmers. 

“We’ve seen our federal agencies being in much more of a reactive mode, waiting for people to get sick, waiting for years and years of evidence of real harm being caused before they take action … Other countries don’t operate that way.” 

Laura Friedman, California Assemblymember

In June 2021, the thousands of cases that plaintiffs like Platt have filed against Syngenta were combined into one federal proceeding in the District of Southern Illinois, and the case is ongoing. Trial proceedings, originally scheduled for November 2022, have been repeatedly delayed.

Sarah Doles, Platt’s lawyer and co-lead counsel for the plaintiffs, said she worries about the human costs: Her clients are elderly people in ill health. Many plaintiffs, she said, have died without seeing their cases resolved. 

“These clients just really want to tell their story and tell their story while they can,” she said, “because they’re losing the ability to do so.”

1955 – 1979 — ‘A growing problem’

Paraquat’s potential as a weedkiller was discovered in 1955 at Imperial Chemical Industries, or ICI, a British chemical company that would eventually become Syngenta. 

Three years later, before paraquat was ever commercially sold, a scientist in ICI’s medical division wrote to a high-ranking toxicologist that a chemical then called 2,2′ dipyridyl — paraquat — appeared to have “a moderate toxicity mainly by affecting the central nervous system,” or the brain and spinal cord, according to court records. 

In 1965, ICI entered into an agreement with Chevron, the oil and gas giant, to sell paraquat in the U.S. Chevron remained the herbicide’s U.S. distributor for the next two decades.

Paraquat’s acute toxicity was established early. Direct exposure to paraquat, such as through drinking it, can cause serious organ damage and death. Suicides and accidental deaths were reported soon after paraquat’s release. However, the chemical was generally believed to be safe as long as direct exposure was avoided.

The year after paraquat’s U.S. release, a scientific study by three ICI scientists studied the effects of paraquat on rats. The authors wrote that, judging by the symptoms they saw, paraquat appeared to affect the animals’ central nervous systems.

The EPA and Syngenta have said the results of animal studies on paraquat are not relevant to the everyday exposure levels of workers using the chemical, partly because large amounts of paraquat are injected into the test animals in most studies. 

In an EPA update on its paraquat decision released in January, the agency said that injection was not considered a “relevant pathway” to exposure in workers. They would more likely inhale the chemical, lick it off their lips, or get it on their skin, usually in very small quantities.

In 1968, a woman in Japan died after consuming paraquat, and ICI tested tissue samples from her body. Residual paraquat was found in her kidneys, lungs, liver and brain. Ken Fletcher, a doctor at ICI, wrote that the levels found were “rather higher than we would have expected, particularly in the brain, considering the relatively small quantity that was taken.” 

Based on a 1967 study of Malaysian paraquat sprayers, ICI knew that paraquat could get into the blood of workers in the course of working with it, according to court records. Another ICI rodent study in 1973 re-confirmed that, at least in mice, once paraquat was in the body, it could get into the spine and brain. 

But how long paraquat could remain in the brain, and what damage it might do, remained uninvestigated for years.

One of the first instances of public officials raising concerns about paraquat’s long-term safety came in the early 1970s. California state officials questioned the potential chronic health effects from workplace exposure to paraquat. 

In a letter from August 1974, Chevron lead toxicologist Richard Cavalli wrote that he had spoken with a doctor who had identified what he called “paraquat syndrome” in those repeatedly exposed to the herbicide. The syndrome consisted of an array of symptoms ranging from severe headaches to chest tightness. 

The next year, Cavalli wrote to ICI that several people who’d worked with paraquat had alleged permanent central nervous system damage from paraquat, including a man who’d developed a spinal lesion. Such allegations, he wrote, appeared to be “a growing problem in the litigation area.” 

1980 – 1999 — ‘Keenly aware of our dependence on paraquat’

By the 1980s, paraquat’s toxicity was generating headlines. A 1983 article from Science Digest quoted multiple doctors that said paraquat was a serious health threat. The article, which listed several cases of accidental deaths from paraquat in various countries, said many claimed paraquat was “out of control.” 

In one case, a Florida gardener accidentally sprayed himself. Some paraquat got on his shirt and in his mouth. He washed his hands and face and returned to work but was rushed to the hospital five days later. Ultimately, he died after two-and-a-half months of attempts to save his life. A lung transplant didn’t take.

Then, scientists began to suspect a link between paraquat exposure and Parkinson’s disease.

The potential connection was first theorized due to paraquat’s chemical similarity to MPTP. MPTP is a byproduct of synthetic heroin manufacturing known for producing almost instant Parkinson’s symptoms. 

In March 1985, an ICI research manager, according to court records, wrote: “Paraquat is our major product now and will remain one of our major products for many years to come. I am sure that all of us are keenly aware of our dependence on paraquat. Then, it behooves us to do whatever possible to: Extend and defend paraquat markets through innovative research, development and marketing approaches.”

The same year, Canadian neurologist André Barbeau published the first epidemiological evidence of a connection between paraquat and Parkinson’s. He found a high level of correlation between paraquat use and diagnosis of Parkinson’s disease in regions of Quebec. 

Retired Chevron Chairman R. Gwin Follis wrote to Chevron’s then-chairman about Barbeau’s study, warning him about the potential dangers of selling a product linked to a chronic disease:

“Since we don’t want to take any chance of facing an asbestos situation down the road, I am sure your people are following this aspect of the matter most closely,” Follis wrote. “However, I thought I would pass this on to you as I cannot think of anything more horrible for us to bequeath to our successors than an asbestos problem.”

The next year, in 1986, ICI and Chevron ended their distribution partnership. When reached for comment, Chevron, which is now a co-defendant in some of the lawsuits against Syngenta, said that it never manufactured paraquat itself and should not be held liable. “Despite hundreds of studies conducted over the past 60 years, the scientific consensus is that paraquat has not been shown to be a cause of Parkinson’s disease,” the company stated by email.

A year after the split, a neurologist affiliated with the University of Miami reported a case study of a 32-year-old man who had worked with paraquat for 15 years. He had developed very early-onset Parkinson’s disease. 

Over the course of the 1990s, ICI underwent a succession of corporate mergers and demergers, which culminated in the creation of Syngenta as a corporation based in Switzerland in 2000. It is now owned by Chinese conglomerate Sinochem.

2000 – 2009 — ‘Scientific influencing strategy’

In the new millennium, Syngenta started defending paraquat against increasing scientific scrutiny of its potential connection with Parkinson’s. In 2000, it created a “Paraquat Information Center” website, paraquat.com. Around the same time, Syngenta set an ambitious sales goal. By 2010, the company wanted to sell $1 billion worth of paraquat, according to minutes from a 2001 meeting of the company’s Science and Technology Council. 

Of concern to Syngenta was the research of Deborah Cory-Slechta, a researcher at the University of Rochester in New York. In the early 2000s, Cory-Slechta’s studies found that administering paraquat to mice caused cell death in a specific part of the brain named the substantia nigra. The loss of brain cells in that part of the brain causes the hallmark motor symptoms of Parkinson’s. (Cory-Slechta did not return requests for comment.)

In June 2003, at a meeting of Syngenta’s regulatory development team for paraquat, employees laid out a “scientific influencing strategy,” which included publishing in-house research to increase its own credibility and aiming to influence external researchers’ future work, according to meeting minutes presented in court records. 

One rule governing Syngenta’s internal research was to avoid measuring paraquat levels in the brain. The detection of any amount of paraquat in the brain, “no matter how small,” would not “be perceived externally in a positive light,” according to an internal slideshow.

That in-house research, at the time, was led by a scientist named Louise Marks. Marks did not return repeated requests for comment through her new employer, Regulatory Science Associates.

According to court documents, in her first attempt, Marks found no effect of paraquat on the brains of mice, but she realized the methodology she’d used was out of date compared to other scientists’ methods. When she redid her study with a newer method, she found paraquat did, indeed, cause a measurable loss of brain cells. She tried again, with the same results: Paraquat kills cells in the part of the brain where Parkinson’s symptoms develop. 

Shawn Hayley, a professor at Carleton University in Canada who has done similar studies with paraquat in mice, said paraquat kills up to a third of those brain cells in the substantia nigra. 

Syngenta said on its website that it “rejects the claims of a causal link between paraquat and Parkinson’s disease because it is not supported by scientific evidence.” Parkinson’s disease predates the sale of paraquat, and gene mutations are the only known cause of Parkinson’s, Syngenta said. 

However, Hayley said only about a tenth of all Parkinson’s cases can be attributed to genetic factors alone. The vast majority are likely caused by a more difficult-to-trace combination of age, genetic vulnerability and exposure to environmental factors, such as toxins, he said.

While a direct causal relationship has not been and likely cannot be definitively proven in humans, as intentionally exposing humans to paraquat to see if they develop Parkinson’s would be unethical, Hayley said the studies on rodents do show a relationship. 

“Let’s put it this way, if mice and rats were not relevant for the human condition, then all of our biomedical science would be bullshit,” Hayley said. “That would be catastrophic. Literally billions and billions of dollars (of research) around the world are done on mice and rats every year. You have to make that jump.”

Syngenta did not publish or report to the EPA Marks’ studies showing a loss of brain cells. In October 2004 at a Society for Neuroscience conference, Marks presented the results of her initial study, which found no change in the number of brain cells. 

When reached by email, a spokesman for the EPA wrote that under the Federal Insecticide, Fungicide, and Rodenticide Act, companies have “a general obligation to submit additional information regarding the risks or benefits of a product and information which EPA might believe raises concerns about the continued registration of a product.” 

The spokesman said that companies are specifically required to notify the EPA of the results of a study on the toxicity of a pesticide if, “relative to all previously submitted studies, they show an adverse effect.” Syngenta’s corporate witness said in deposition that the company did not have to submit the Marks study results to the EPA because they were not the first to find those results. 

An internal Syngenta presentation from 2005 acknowledged Marks’ studies had confirmed brain cell loss in mice. The presentation listed important targets to “influence” regarding public perception of paraquat’s safety, including Cory-Slechta’s research group and a then-upcoming large government-funded study in the United Kingdom. 

In the U.S., Syngenta saw a problem. Cory-Slechta, who had been vocal about her concerns regarding paraquat’s potential neurotoxicity, had recently been nominated for the FIFRA Scientific Advisory Panel. The panel is made up of seven scientists who advise the EPA on health and safety matters related to pesticides. The members of the panel are not directly involved in policy-making. 

“Their advice is invaluable to the EPA as it strives to protect the American people from risks posed by pesticides,” the EPA spokesman wrote in an email.

Syngenta wanted to keep Cory-Slechta off the panel. It compiled comments critical of her work, including that she “appears single-minded in believing that some pesticides are a primary risk factor for Parkinsons” and that her conclusions were “in reality speculation.” Syngenta passed the comments to the national trade organization for pesticide manufacturers, CropLife America, to repeat to the EPA. 

In the email sent to CropLife, Greg Watson, a member of Syngenta’s regulatory division, wrote, “I would ask that you handle our comments with care & in such a way that they cannot be attributed to Syngenta.” 

Ultimately, Cory-Slechta was not named to the panel. The EPA spokesperson wrote in the email response that while the agency considers public comments when selecting candidates, it also considers many other factors, such as the scientist’s area of expertise and professional qualifications, and that individual comments are not considered in isolation. 

Syngenta said it rejects any claim it acted inappropriately regarding Cory-Slechta’s nomination. Syngenta did not respond to a request for comment from Watson. CropLife America did not respond to requests for comment.

In 2007, according to court records, Syngenta head of regulatory science Lewis Smith attended a neurotoxicology conference where he heard neurologist Caroline Tanner present data from an ongoing study. It involved more than 80,000 participants, one of the largest of its kind. Many were farmworkers who were exposed to paraquat for years. 

Tanner’s latest data indicated exposure to the herbicide increased the risk of Parkinson’s disease. Smith worried about the broad agreement among the scientists that environmental factors, and particularly pesticides, played a primary role in the disease’s development.

Following the conference, Smith wrote in an email to other high-ranking Syngenta employees: “Unless we are able to generate new data on the mechanism of toxicity of paraquat in the brain (…) we shall not halt or far less reverse the perception that paraquat contributes to some extent to the incidence of Parkinson’s disease.” 

In 2008, Syngenta internally re-evaluated paraquat’s safety. In its report, it listed several “major sources of uncertainty,” including the question of how long paraquat remained in the brain and the possibility the brain cell death caused by paraquat exposure could progress even without further exposure.  

Despite this, the evaluation concluded the margins of safety were adequate. The herbicide was, effectively, safe when used as instructed. 

2010 – 2012 — ‘Potential for future legal activity’

In 2010, paraquat was the second most-sold herbicide in the world after glyphosate, and represented $400 million in annual sales for Syngenta. By this point, Syngenta faced competition from competitors selling generic paraquat formulations for lower prices, and it was focused on protecting its brand identity and market share.

Syngenta organized its own epidemiology study of former workers at four shuttered paraquat production plants in Widnes, England, with the goal of determining whether a disproportionate number had died of Parkinson’s. The study, published 2011, only examined the listed causes on workers’ death certificates. Neither living workers with Parkinson’s nor deceased workers who may have had Parkinson’s but died of other causes were counted in the study, according to testimony given by one of the study’s authors. The study found no statistically significant increase in deaths from Parkinson’s.

A medical journal specializing in the study of workplace hazards and human health rejected the Widnes study because it did not examine living subjects. An update from 2021, which Syngenta cites on its webpage devoted to paraquat, also used only death certificates.

Syngenta considered alerting the Widnes workers that it was conducting a study, but elected not to. Philip Botham, Syngenta’s head of product safety, wrote in an email that “in spite of the positive health messages in the publication, this action could precipitate concern and the potential for future legal activity.”

One question that has been raised in litigation when it comes to paraquat’s neurotoxicity is how long paraquat remains in the human brain once it gets in. Paraquat typically gets into farmworkers’ bodies only in tiny amounts, but if paraquat that gets to the brain isn’t processed out quickly and instead accumulates there, then those many small exposures could build up and cause damage.

In 2011, Syngenta finished its analysis of a collection of brain tissue samples from spider monkeys exposed to paraquat. The samples were taken two, four and eight weeks after exposure. The amount of paraquat detected did not decline over time. 

This indicated the time paraquat remains in monkey brains without breaking down or passing out of the brain is at least six weeks — twice as long as in mice — and potentially much longer. In 2022, Syngenta’s corporate witness said that the company still does not know how long paraquat remains in primate brains. Studies on monkeys are generally, though not always, considered to be more accurate than other animal studies when it comes to predicting health outcomes in human beings. 

Syngenta considered reporting this finding to the EPA, but decided the findings of paraquat remaining in monkey brains “do not represent an adverse effect or a precursor to an adverse event,” and so did not meet the requirements to submit to the EPA, according to court documents.

The same year, Tanner published the results of her long-term research into paraquat and Parkinson’s. It found agricultural workers who had sprayed or been around paraquat had a 250% higher chance of developing Parkinson’s disease. (Tanner did not return requests for comment.)

Syngenta’s response to the Tanner study was immediate. It posted on paraquat.com that the study’s results were potentially flawed because it did not clarify whether its results were based on incidence — the number of new cases diagnosed — or prevalence — the total number of cases in a population at a given time.  

Syngenta acquired the underlying data from the Tanner study from the National Institutes of Health via a Freedom of Information Act request and hired a consulting firm to re-analyze it with incidence specifically in mind. The firm confirmed the data did contain numbers for both incidence and prevalence of Parkinson’s. Both were increased by 250%.

Present day — ‘One of the luckier ones’

Platt still owns and lives on the ranch he once sprayed with paraquat, although his ability to maintain the property has deteriorated due to his illness. His wife now has to do more of the upkeep. At one point, the Platts had hoped to sell the property to move into a smaller and more handicap-accessible home, but the plans fell through. Platt had to retire early from his job as a professor at the University of West Florida due to his disease. 

“I understand that I am one of the luckier ones, because I have the benefit of an education, and I could do a job that wasn’t impacted by the paraquat,” Platt said.

Platt said he wants programs to better support farmworkers, as well as to provide therapy for those with Parkinson’s disease. And he wants paraquat taken off the market. 

“There’s a lot of anger, in the way I feel,” Platt said. “I think that it’s sad that we have a country that has so much, and takes so much away.” 

Editor’s note: This story has been updated with a reference and link to The Guardian’s previous reporting on documents related to Syngenta’s weedkilling product and its link to various health concerns. An earlier version of the story did not intend to claim any documents were being first reported by Investigate Midwest.

This article first appeared on Investigate Midwest and is republished here under a Creative Commons license.