New paraquat research again points to Parkinson’s link

Preliminary new research adds to evidence that exposure to paraquat herbicide can damage the brain in ways that lead to Parkinson’s disease, and comes as US regulators say they are giving fresh scrutiny to the chemical.

Scientists affiliated with the University of Rochester and the University of Georgia found that laboratory mice exposed to air containing paraquat particles for 60 days showed neurotoxic brain effects, including reduced dopamine neurons and impaired motor function, among other impacts.

Parkinson disease is a progressive neurodegenerative disorder characterized in part by the degeneration of dopamine-producing neurons in the substantia nigra part of the brain. It is incurable and is the second-most common neurodegenerative disease after Alzheimer’s disease.

“These findings raise significant concerns as to the contribution of [paraquat] exposures to the continued increase in the incidence of [Parkinson’s disease] in the US,” the study concludes.

The work is a preprint, meaning it has not yet been finalized or peer reviewed. It was made available online Nov. 3. The work was funded in part by the National Institutes of Environmental Health Sciences.

Deborah Cory-Slechta, a University of Rochester Medical School neurotoxicologist and co-author of the new study, said the findings add to an already robust database of evidence showing paraquat exposure is associated with the development of Parkinson’s.

“The human studies are strong. The animal studies are strong … I don’t know how much stronger you can get.” – Neurotoxicologist Deborah Cory-Slechta.

This study stands out because it exposed experimental mice to the chemical primarily through inhalation, while other animal studies traditionally exposed animals by adding paraquat to the animals’ diets, or through injection. The inhalation route is more in line with how farm workers and others would be exposed as they apply paraquat, she said.

“The human studies are strong. The animal studies are strong. We never say [something definitely] ‘causes’ in science, but boy, I don’t know how much stronger you can get,” she said.

The study found greater impacts in the male mice than in female mice, Cory-Slechta noted, a distinction similar in Parkinson’s disease – men are diagnosed at a higher rate than women.

EPA questions

The US Environmental Protection Agency (EPA) has discounted hundreds of studies pertaining to a possible paraquat link to Parkinson’s, including nearly 200 animal studies, citing what the agency says are a variety of deficiencies. Roughly 70 other countries ban paraquat, including China, where most of the world’s paraquat is manufactured. But the US continues to allow its use on farm fields and elsewhere, including through aerial spraying.

Last year, the EPA released an updated review of paraquat pertaining to potential “post-application inhalation exposures” to people that could result from the volatilization of paraquat from treated farm fields. 

Volatilization occurs when a pesticide converts into a vapor and then releases into, and moves through, the atmosphere beyond where it was applied. People who live beyond the area where the pesticide is sprayed could thus be impacted, inhaling the pesticide vapor.

The EPA said at that time that it would be updating its analysis of paraquat to take into consideration new data provided by former paraquat maker Syngenta and data requested from other paraquat manufacturers dealing with volatilization of the chemical. 

The agency also said at that time that it intended to complete an Endangered Species Act (ESA) assessment, along with an endocrine disruption assessment.

But an agency spokesperson confirmed this week that neither of those has been initiated. The EPA cited a court challenge and “higher priority pesticides” as factors for not yet starting those assessments.

An EPA spokesperson said this week that the agency is planning a paraquat-focused “roundtable” the first week of December. The agency announced in June that it would hold such an event during the summer, but failed to do so. It then said the event would be held in November. The stated purpose is to convene scientists, farmworkers and other experts and stakeholders to “discuss the documented safety challenges” associated with paraquat.

When asked this week for a specific date for the roundtable in December, an agency spokesperson said that information was not yet determined.

In 2022 and 2023, The New Lede and the Guardian obtained and revealed a set of internal corporate records showing that key paraquat maker Syngenta and its predecessor corporate entities used an array of secret tactics for decades to downplay and discredit science linking what the company called its “blockbuster” weed killer to Parkinson’s and other chronic health issues. 

In September, The New Lede and the Guardian additionally revealed that internal corporate records, depositions and other records show that Syngenta did not share certain internal data and other information with the EPA regarding paraquat health effects and risks despite federal legal requirements for companies to report information about “unreasonable adverse effects” of their products.

Syngenta announced it was ceasing production of paraquat earlier this year. The company faces thousands of lawsuits brought by people alleging they developed Parkinson’s due to exposure to paraquat.

“Hugely important”

 Dutch neurologist Bastiaan Bloem said if the findings of the new study withstand the scrutiny of external peer review, they are “hugely important.”  Bloem is director of the Radboudumc Center of Expertise for Parkinson & Movement Disorders, Department of Neurology at the Radboud University Medical Center in the Netherlands.

Current regulatory actions are largely based on animal experiments in which the animals have been exposed to pesticides through the diet, he noted, though inhalation would be a common route of exposure for people applying pesticides such as paraquat or in areas near where the pesticides are sprayed.

“There are theoretical concerns that a pesticide might well be toxic after inhalation, even when it is found to be safe after oral application. The reason is that there is significant detoxification by the liver after oral exposure, but not after inhalation,” Bloem said. 

The study indicates that inhalation impacts should be studied not only for new pesticides but “currently used pesticide should be retested using such inhalation experiments and, if not proven to be safe, be taken from the market,” he said. 

“We spend billions of dollars developing treatments for Parkinson’s disease, yet we continue to expose people to pesticides and chemicals that may increase their risk of developing it.” – Michael Okun, medical advisor to the Parkinson’s Foundation.  

Michael Okun, director of the Norman Fixel Institute for Neurological Diseases who also serves as medical advisor to the Parkinson’s Foundation, said the preliminary findings raise serious questions about what repeated exposure to airborne pesticides may be doing to the human brain.

“We spend billions of dollars developing treatments for Parkinson’s disease, yet we continue to expose people to pesticides and chemicals that may increase their risk of developing it,” Okun said. “This study is another reminder that preventing Parkinson’s disease must include reducing potentially harmful environmental exposures, not simply developing better medications after the disease appears.”

Several organizations, as well as state and federal lawmakers around the United States have been calling for paraquat to be banned. Vermont recently banned paraquat, with lawmakers there citing risks that include an association between paraquat and Parkinson’s disease shown in years of scientific research.  And California regulators announced paraquat makers voluntarily agreed to stop selling the chemical after the state identified health concerns in new research.

“Every year, we hear stories of farmers who developed tremors and Parkinson’s-like symptoms after spraying paraquat for years. And unfortunately, these are not rare exceptions,” said Cristobal Lagunas, organizing lead for the Pesticide Action & Agroecology Network.

“They are signs of a system that has let science, regulation, and compassion fall behind corporate profit. We deserve policies that don’t force farmworkers to choose between economic survival and physical suffering.”

Featured image by Getty Images for Unsplash +.