People from California farm communities that had long-term exposure to the insecticide chlorpyrifos were more than twice as likely to later develop Parkinson’s disease compared to residents without exposure, according to a new study.
The study, published in the Molecular Neurodegeneration journal, also found several markers for Parkinson’s disease in exposed mice and zebrafish, cell death, movement problems and brain inflammation.
It is the latest evidence that the insecticide may damage people’s brains and may play a role in causing Parkinson’s disease, an incurable, debilitating brain disease affecting more than one million Americans each year. Parkinson’s symptoms include tremors, shaking in the arms and legs, muscle stiffness, a loss of balance and coordination and difficulty speaking.
“We’ve had a suspicion that chlorpyrifos is associated with increased risk of Parkinson’s, but it’s always hard to prove causality,” said Dr. Jeff Bronstein, a neurologist at the University of California, Los Angeles (UCLA) Health system and the study’s senior author. “[This] confirms the increased risk. I’d like to see the use of chlorpyrifos and similar organophosphate pesticides banned because of toxicity.”
Environmental and health advocates have long pushed the US Environmental Protection Agency (EPA) to fully ban chlorpyrifos over concerns that it harms brains, particularly those of young children. The agency has gone back and forth on tightening regulations over the past few presidential administrations.
“We’ve had a suspicion that chlorpyrifos is associated with increased risk of Parkinson’s, but it’s always hard to prove causality. [This] confirms the increased risk.” – Dr. Jeff Bronstein, UCLA Health
The chemical was banned from residential use in the year 2000, but is still allowed for use on nearly a dozen US crops.
In the new study led by UCLA Health, researchers used data from about 1,600 people who lived in one of three California farming counties: Kern, Fresno and Tulare. Roughly half of the subjects were diagnosed with Parkinson’s disease. The researchers estimated people’s exposure to chlorpyrifos over the previous three decades at their home or workplace using past home and work addresses and the state’s pesticide use reports. Those with the highest long-term exposure to chlorpyrifos were more than 2.5 times more likely to end up with Parkinson’s disease.
Bronstein said looking at decades of exposures strengthens the study as Parkinson’s disease starts decades before it’s diagnosed. “We also controlled for other potential pollution exposures – so we really isolated the effects of chlorpyrifos,” he said.
The researchers also exposed mice to chlorpyrifos and observed brain inflammation and abnormal accumulation of a type of protein that will often clump in the brains of people with Parkinson’s disease. The mice were exposed via inhalation, which is the dominant exposure route for people in the California counties that were studied, Bronstein said.
He and colleagues also used zebrafish to understand the pathways by which people are getting Parkinson’s, Bronstein said, and found chlorpyrifos disrupted autophagy, the cellular process that clears damaged proteins. This is important, he said, because knowing which pathways are causing someone’s Parkinson’s allows doctors to potentially slow or stop the progression.
“In the case of chlorpyrifos that could mean medicines that stimulate autophagy,” Bronstein said. “When we did [stimulated autophagy] in fish, it made them resistant to the effects from chlorpyrifos.”
Pesticides and Parkinson’s
The findings are the latest linking chlorpyrifos to brain problems. Last year scientists pinned chlorpyrifos to impaired brain development and motor function in exposed children, which built on several previous studies that linked the pesticide to harming developing brains. The new research led by UCLA Health, however, is one of just a few studies to examine chlorpyrifos’ potential role in causing Parkinson’s disease, with previous animal studies finding the pesticide induces cell death, and causes altered expression or loss of neurons — all markers of Parkinson’s disease.
“Chlorpyrifos is linked to just about anything that can go wrong in the brain at this point,” said Nathan Donley, the environmental health science director at the Center for Biological Diversity. “There’s really just no question anymore that it’s an enormous public health threat. It’s no surprise that a known neurotoxin is linked to neurodegenerative diseases.”
The causes of Parkinson’s disease remain unclear but the new research comes as the spotlight falls on a different pesticide — the weed killer paraquat — over its potential role in triggering the disease. The New Lede in 2022 obtained and published internal corporate documents showing the chemical giant Syngenta, which manufactures and markets paraquat, was aware of research linking paraquat to brain disease decades ago, but hid internal evidence from the EPA and worked instead to try to suppress information about the potential for paraquat as a risk factor for Parkinson’s disease.
“Chlorpyrifos is linked to just about anything that can go wrong in the brain at this point.” – Nathan Donley, Center for Biological Diversity
The company now faces thousands of lawsuits from people who allege paraquat exposure caused them to develop Parkinson’s.
Meanwhile Parkinson’s disease cases in the US continue to rise. A 2022 study from the Parkinson’s Foundation found that about 90,000 people in the US are diagnosed with Parkinson’s each year — a 50% increase over previous estimates.
Ban scrapped
Environmental and health advocates have long pushed the EPA to ban chlorpyrifos, which the European Food Safety Authority prohibited in 2020.
In 2021, the EPA announced a nationwide ban, calling it overdue and necessary to protect children, farmworkers and the public. The ban applied during the 2022 growing season but was challenged by chemical companies and growers, who argued the decision was unlawful. In 2023, the U.S. Court of Appeals for the Eighth Circuit overturned the ban.
The EPA in 2024 proposed a narrower rule allowing chlorpyrifos to remain in use on 11 crops, including alfalfa, apple, asparagus, cherry, citrus, cotton, peach, soybean, strawberry, sugar beets and wheat. The agency says those crops account for about 55% of total chlorpyrifos use.
Chlorpyrifos was the 11th most frequently found pesticide in food samples in a recent Food and Drug Administration (FDA) pesticide residue monitoring report, and a 2023 US Department of Agriculture pesticide residue report found traces of the chemical in baby food made with pears, as well as in samples of blackberries, celery and tomatoes.
The pesticide is currently under registration review by the EPA with an expected decision sometime this year, but Donley does not have hope the Trump administration will take action.
“I don’t see the appetite for stricter regulations in this administration,” he said.
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