Scientists have studied whether paraquat exposure is linked to Parkinson’s disease for more than a decade. Some studies report a link and others do not. EPA says it has not found a clear link from labeled uses, while other researchers see stronger evidence. Here is what the main studies found, and why no single study settles the question.
Two ideas to keep straight
Association is not causation. An association means two things show up together more often than chance would predict. That does not prove one caused the other. Other factors can explain a link, and a study can find a link by chance.
Human studies and lab studies answer different questions. Studies of people look at real exposures, but they rely on records and memories that can be wrong. Lab studies in cells or animals can show how a chemical might harm nerve cells, but animals get controlled doses that may not match what people face.
The 2011 study
In 2011, Dr. Caroline Tanner and colleagues published a study in Environmental Health Perspectives. It drew on the Agricultural Health Study, a large federal study of farmers and pesticide applicators. Movement-disorder specialists confirmed each Parkinson’s diagnosis. The study compared 110 people with Parkinson’s disease to 358 people without it. (Tanner et al., 2011)
People who had used paraquat had about 2.5 times the odds of Parkinson’s disease. The authors linked this to oxidative stress, a kind of cell damage that paraquat is known to cause. This is a case-control study. It looks back at past exposure, so it can show an association but cannot prove cause and effect.
Later research with different findings
In 2020, researchers published an update from the same Agricultural Health Study. It followed about 38,000 applicators and 28,000 of their spouses for more than 20 years. (Shrestha et al., 2020) EPA says this update reported no association between paraquat exposure and Parkinson’s disease, and did not repeat the 2011 finding. (EPA)
Reviews that pool many studies have reached mixed results. A 2019 review of observational studies found a positive association. It also said the evidence was not strong enough to assume a clear cause-and-effect link, and it called for better studies. (Vaccari et al., 2019)
A 2024 study in California’s Central Valley estimated exposure from where people lived and worked, using state pesticide-use records. It found that living or working near paraquat spraying was associated with Parkinson’s disease. (Paul et al., 2024) The journal notes that two authors have been retained as expert consultants for plaintiffs in paraquat litigation.
What the agencies say
EPA reviewed hundreds of studies. It says studies on paraquat and Parkinson’s disease “range in quality and provide conflicting results.” Its conclusion is that it “has not found a clear link between paraquat exposure from labeled uses” and Parkinson’s disease. (EPA) EPA’s review focuses on exposure when the product is used as its label directs.
The National Institute of Environmental Health Sciences, part of the National Institutes of Health, describes paraquat as one of several pesticides its research has linked to Parkinson’s disease. It also says the exact cause of Parkinson’s disease is unknown and likely involves both genes and environment. (NIEHS)
Federal agencies, in short, have not spoken with one voice, and the research has not settled the question.
What this means for one person
Even a strong finding about a large group cannot show what caused one person’s illness. In court, a claim needs evidence about the product, how much exposure there was and when, the diagnosis, and other possible causes. In the federal paraquat litigation, the court in 2024 excluded the plaintiffs’ expert on whether paraquat can cause Parkinson’s disease, and then dismissed the first four cases set for trial. (court order)
Our paraquat page explains what a review looks at. This article is general information, not medical or legal advice. Talk to a doctor about symptoms or treatment, and do not change prescribed treatment based on this article.
This article is general information about how these cases work. It is not legal advice, it does not create an attorney-client relationship, and no two claims are alike. If something here sounds like your situation, the useful next step is a conversation about the facts.
