The Environmental Protection Agency says that scientific research doesn’t support a link between this powerful herbicide and Parkinson’s disease. Will thousands of lawsuits find otherwise?
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The Environmental Protection Agency says that scientific research doesn’t support a link between this powerful herbicide and Parkinson’s disease. Will thousands of lawsuits find otherwise?
February 12, 2025
The Environmental Protection Agency in Washington, D.C. (Photo credit: skyhobo/Getty Images)
April 15, 2025 Update: Syngenta, manufacturer of the weedkiller paraquat, which has been linked to Parkinson’s disease, has entered into an agreement to potentially settle thousands of lawsuits against the company.
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Farmers use the herbicide paraquat, often sold under the brand name Gramoxone, to clear fields before planting. One of the most popular herbicides in the U.S., paraquat is cheap and effective, able to rapidly kill grasses and perennial weeds, but a growing body of research has connected it to Parkinson’s disease, thyroid cancer, and harm to birds, amphibians, reptiles, and plants.
One of the most popular herbicides in the U.S., paraquat is cheap and effective, able to rapidly kill grasses and perennial weeds, but a growing body of research has connected it to Parkinson’s disease, thyroid cancer, and harm to birds, amphibians, reptiles, and plants.
The mounting research, however, has so far failed to convince federal and state regulators to ban its use in the U.S. It is illegal in more than 70 countries, including the European Union, as well as the U.K. and China, where it is manufactured.
Parkinson’s disease is the world’s fastest-growing neurodegenerative disease. Epidemiological studies linking it to paraquat are at the root of roughly 7,000 lawsuits filed in federal court against Syngenta, the company that manufactures paraquat.
At the same time, environmental groups are engaged in a 2021 lawsuit against the EPA for re-approving paraquat. As these two separate lines of legal action move forward, it remains to be seen how the scientific research will be considered. Studies linking paraquat to Parkinson’s disease haven’t moved regulators to outlaw the pesticide, but lawsuits seeking damages for the neurological disorder will almost certainly depend on them.
Paraquat was first registered for use in the U.S. in 1964. The registration process, which involves a risk assessment of the pesticide’s potential human health and environmental impacts, with data from the manufacturer, must be done every 15 years to ensure that no new science proves the pesticide poses “unreasonable” risks to people or the environment.
In 2021, to mitigate risks as part of the re-registration process, the EPA implemented some interim paraquat safety measures—including limits on aerial spraying and a respirator requirement. However, the agency allowed paraquat use to continue, claiming it hadn’t found a clear link between paraquat exposure and adverse health outcomes such as Parkinson’s disease and cancer.
Last month, the Environmental Protection Agency moved to abandon its newly established mitigation measures for paraquat applications, which included limits on aerial spraying and a respirator requirement.(Photo credit: mladenbalinovac/Getty Images)
Environmental groups sued, saying the protections were inadequate. “EPA is so focused on the limitations of individual studies, they didn’t meaningfully consider the broad consistency of evidence across multiple lines of evidence,” says Jonathan Kalmuss-Katz, senior attorney for toxic exposure and health at Earthjustice, which sued the EPA over its 2021 interim decision.
On January 17, EPA took two actions. The federal agency abandoned their 2021 interim protections and required the manufacturer, Syngenta, to conduct tests to better understand how the pesticide volatilizes—becomes airborne as a gas—and drifts beyond the application site. And now, with chemical industry insiders recently appointed to steer the EPA’s regulation of chemicals, the future of restrictions on paraquat is in question.
On February 10, Kalmuss-Katz filed a legal response to the Ninth Circuit Court of Appeals, requesting the court deny both of EPA’s January 17 actions. Abandoning interim safety measures, the motion stated, “Not only prolong risks but potentially make them more severe by weakening the safeguards that EPA itself found were necessary to address paraquat’s unreasonable adverse effects.” As well, the motion pointed out that the EPA did not state how they would meet the October 2026 re-registration deadline while waiting years for volatilization studies.
“It’s the tired old industry tactic of delay, delay, delay,” said Nathan Donley, environmental health science director at the Center for Biological Diversity, one of several advocacy groups that sued the EPA over its 2021 interim decision.
The EPA did not respond to multiple requests for an interview or to emailed questions about its paraquat decision, sent before publication.
The EPA’s decision last month to abandon protections and order more testing cited a recent preliminary report published in California, one of the nation’s largest users of paraquat. The report, which EPA said it had not had time to evaluate, was released by the state’s Department of Pesticide Regulation (DPR), and is the initial stage of the state’s own comprehensive evaluation of recent science to determine whether to continue allowing the use of paraquat in California.
“EPA is so focused on the limitations of individual studies, they didn’t meaningfully consider the broad consistency of evidence across multiple lines of evidence.”
The report, which followed an initial review of 4,000 public comments and 150 scientific studies, highlighted research indicating paraquat can cause thyroid cancer and have ecological impacts on wildlife. However, DPR stated that, so far, its “review of existing human health studies, including epidemiological studies, does not indicate a causal association between paraquat exposure and Parkinson’s Disease.”
When asked for comment, DPR Deputy Director of Communications and Outreach Leia Bailey wrote in an email to Civil Eats, “DPR considered epidemiological studies along with all other toxicology studies to determine areas of risk that can lead to restrictions on pesticide use.”
She further noted that epidemiological studies span several decades. “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,” she wrote.
In other words, the epidemiological studies span decades of paraquat exposure before 2021’s more stringent application rules for the herbicide, which lowered exposure—and presumably the risk.
California DPR’s review continues, with a final decision due by 2029.
Over 1 million people suffer from Parkinson’s disease in the U.S., and each year, 90,000 new cases are diagnosed. Parkinson’s disease can develop when the nerve cells that produce the chemical dopamine die; dopamine controls critical body functions such as memory, mood, and movement.
In recent years, it’s become clear that Parkinson’s is largely caused by interactions between genetics and environmental factors, including exposure to metals, solvents, or pesticides. According to a recent study that offered free genetic testing for Parkinson’s disease to more than 10,000 participants with the disease, only 13 percent were predisposed genetically.
Said another way, 87 percent of the Parkinson’s cases analyzed had no known genetic risk factor. As a result, researchers are investigating how genes and environmental exposures interact to create hot spots of Parkinson’s disease.
California’s Central Valley, which produces one-quarter of the nation’s food, is one such hotspot. “I call California’s Central Valley my personal lab,” says study co-author Beate Ritz, a Parkinson’s disease researcher at University of California at Los Angeles (UCLA) and a retained expert consultant in lawsuits against Syngenta. “The levels of Parkinson’s disease there are outrageous.”
In 2021, California dedicated $8.4 million to fund and expand the California Parkinson’s Disease Registry, one of 14 state registries. Registries provide data sources that help researchers identify high-risk groups, determine accurate prevalence rates, and improve links between the disease and risk factors such as pesticide exposure.
In addition, a number of Parkinson’s studies have focused on California, because it has the most rigorous reporting of pesticide usage in the country; in 2023, California farmers bought over 92,000 pounds of paraquat. Between 2017 and 2021, over 5.3 million pounds of paraquat were sprayed in California, according to the Environmental Working Group. These data makes it possible to begin identifying correlations and better understand how pesticides impact the health of agricultural communities.
“I call California’s Central Valley my personal lab. The levels of Parkinson’s disease there are outrageous.”
It’s not just farmworkers or those who apply pesticides who face risks. Ritz and co-author Kimberly Paul, also a Parkinson’s disease researcher at UCLA, published a 2024 epidemiological study that shows that people who live or work within 500 meters of fields on which paraquat has been applied for at least two decades had double the risk of Parkinson’s disease compared to the rest of the region’s population.
A separate Ritz and Paul study also identified 10 additional pesticides that kill neurons and result in Parkinson’s disease. And a new paper by Brittany Krzyzanowski, a researcher at the Barrow Neurological Institute in Phoenix, Arizona, found that people who live near sunflower, winter wheat, and alfalfa fields—where paraquat is just one of dozens of pesticides used on these crops—have increased risk of Parkinson’s disease.
One of the biggest oversights, many scientists agree, is that the EPA only considers one pesticide at a time. Farming communities are often exposed to multiple pesticides at once, so the EPA’s assessments don’t reflect real-world exposures, says Paul.
For many scientists, numerous studies, both epidemiological and in-vitro cell studies conducted over decades around the world, leave little doubt that paraquat exposure is associated with Parkinson’s disease. Paraquat is, after all, a neurotoxin that can damage the central nervous system and kill lung, liver, and kidney cells when ingested or inhaled. One teaspoon can cause extreme respiratory and gastrointestinal distress—and even organ failure or death. The EPA’s website says: “One small sip can be fatal and there is no antidote.”
“Studies from researchers around the world link paraquat exposure to Parkinson’s disease in humans and laboratory animals,” says Ray Dorsey, a professor of neurology at the University of Rochester Medical Center in New York and co-author of the 2020 book Ending Parkinson’s Disease: A Prescription for Action.
The financial benefits of reducing pesticide-related Parkinson’s disease are striking. Global sales of paraquat are $400 million each year, yet the economic burden of Parkinson’s disease, in terms of health care costs, is $52 billion a year, according to Dorsey. “Even if paraquat was responsible for just 1 percent of Parkinson’s disease cases, the economic value of preventing its use is $500 million per year,” says Dorsey.
Meanwhile, Syngenta has been aware of possible long-term health impacts of paraquat for decades. A trove of Syngenta’s internal documents—dubbed the “paraquat papers” by a 2022 Environmental Working Group Investigation—detailed how as early as 1975, the company feared it could face legal liability for paraquat’s long-term health impacts. “One company scientist called the situation ‘a quite terrible problem’ for which ‘some plan could be made,’” The Guardian reported.
“[Syngenta’s] own researchers demonstrated that paraquat exposure led to features of Parkinson’s disease in three different mammalian species in the 1960s,” Dorsey says. “I am not sure what additional evidence is needed to conclude that paraquat causes Parkinson’s disease.”
The most recent lawsuits could cost Syngenta and co-defendant Chevron USA—the successor to the company that distributed paraquat in the U.S. until 1986—roughly $1 billion to settle cases, predicted one of the plaintiff’s lawyers.
The EPA gives the greatest weight to studies that demonstrate paraquat causes direct harm in lab mice. The agency places lesser value on in-vitro cell studies or epidemiological studies. In the cell studies, paraquat introduced to cells sharply increased the amount of damaged proteins inherent to Parkinson’s disease; it is these damaged proteins that clog brain cells, causing them to deteriorate.
It’s all about how you interpret correlation versus causation, Donley says. “EPA is looking for a level of confidence for causation that is nearly impossible to achieve in public health research,” he adds.
Ideally, the EPA wants studies that test the blood and urine of people who have been exposed to paraquat for years, explains Donley. “Those studies cost millions of dollars—yet federal funding for academic research is plummeting,” he says. Donley has never seen the EPA require the pesticide makers to produce that data.
The question of cause is already proving contentious in the Parkinson’s disease lawsuits; judges have ruled that they won’t allow testimony from an expert on whether paraquat is capable of causing Parkinson’s disease in the lead up to the first trial, scheduled to start in October. As a result, epidemiological studies that demonstrate links to paraquat exposure and cell-based studies revealing how paraquat damages dopamine cells may be even more important.
“In every other country, industry has to bring a chemical to the regulatory agency with proof that it’s safe,” says Kyla Bennett, director of science policy at the Public Employees for Environmental Responsibility. “In the U.S., it’s more like, ‘Here’s the chemical. [EPA] has 90 days to prove it’s dangerous.’ It’s totally backwards.”
For the time being, it will be business-as-usual with paraquat until October 1, 2026, when EPA is legally required to finalize paraquat’s re-registration review—a timeline that doesn’t allow for the four years needed to conduct volatilization studies.
In California, Paul and others plan to use the Parkinson’s disease registry to continue their research using a state-wide database of patient information to better identify risk factors. Unfortunately, she notes, the registries often can’t get data on everyone potentially affected. “One of the most vulnerable populations are migrant workers,” she says. “Many of those will be missed.”
That’s increasingly likely in the future, given the Trump administration’s new restrictive immigration policies, which may make migrant workers less willing to participate in any kind of government study—even at the state level.
Reporting for this piece was supported by the Nova Institute for Health.
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