Inhalation Study Links Paraquat to Parkinson's Brain Damage

Mice exposed to airborne paraquat particles for 60 days lost dopamine neurons and showed impaired motor function, according to preliminary research published Nov. 3 that adds new weight to the evidence linking the herbicide to Parkinson's disease.

Scientists at the University of Rochester and the University of Georgia conducted the study, funded in part by the National Institutes of Environmental Health Sciences. It is a preprint — a paper posted online before peer review — but its design distinguishes it from prior animal work. Earlier studies dosed animals through diet or injection. This one exposed mice through inhalation, the route closest to how applicators encounter the chemical in the field.

"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.

What does the inhalation route change?

Parkinson's is a progressive, incurable neurodegenerative disorder marked by the loss of dopamine-producing neurons in the substantia nigra, a region of the brain. It ranks second among neurodegenerative diseases, behind Alzheimer's.

Cory-Slechta argues the exposure route matters for regulation. "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," Cory-Slechta said.

The study found greater impacts in male mice than in females — a pattern that mirrors the human disease, where men receive diagnoses at higher rates than women.

Dutch neurologist Bastiaan Bloem, director of the Radboudumc Center of Expertise for Parkinson & Movement Disorders at Radboud University Medical Center, called the findings "hugely important" if they survive peer review. Current regulatory decisions rest largely on dietary-exposure experiments, he noted, yet inhalation bypasses a key protective mechanism.

"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. He added that currently used pesticides should be retested with inhalation experiments and, if not proven safe, removed from the market.

Michael Okun, director of the Norman Fixel Institute for Neurological Diseases and medical advisor to the Parkinson's Foundation, framed the policy contradiction plainly: "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."

Where do regulators and manufacturers stand?

The EPA has discounted hundreds of studies on a possible paraquat–Parkinson's link, including nearly 200 animal studies, citing various deficiencies. Roughly 70 countries ban the herbicide, including China, where most of the world's paraquat is manufactured. The United States still permits its use, including aerial spraying.

Last year, the EPA released an updated review of post-application inhalation exposures from volatilization — the process by which a pesticide converts to vapor and drifts beyond the treated field. The agency said it would incorporate new data from former manufacturer Syngenta and other producers.

The EPA also committed to an Endangered Species Act assessment and an endocrine disruption assessment. Neither has begun, an agency spokesperson confirmed this week, citing a court challenge and "higher priority pesticides."

A paraquat-focused roundtable, first promised for summer, then November, is now planned for the first week of December, though the agency has not set a specific date. Its stated purpose is to convene scientists, farmworkers and stakeholders to "discuss the documented safety challenges" associated with the chemical.

Corporate records obtained by The New Lede and the Guardian in 2022 and 2023 showed Syngenta and its predecessors spent decades working to downplay science linking what the company called its "blockbuster" weed killer to Parkinson's. Reporting in September showed Syngenta did not share certain internal health-effects data with the EPA despite federal reporting requirements. Syngenta announced it would cease paraquat production earlier this year and faces thousands of Parkinson's-related lawsuits.

Vermont has banned paraquat outright, and California regulators announced that manufacturers voluntarily agreed to stop selling it after the state identified health concerns in new research.

Cristobal Lagunas, organizing lead for the Pesticide Action & Agroecology Network, said growers' symptoms are not isolated cases: "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."

With the EPA's December roundtable approaching and litigation mounting, growers who rely on paraquat should watch whether inhalation-based evidence finally shifts the agency's registration review.