The new study from UCLA links long-term exposure to the pesticide chlorpyrifos with a higher chance of developing Parkinson’s disease, using decades of human data along with mouse and zebrafish experiments to explore possible biological mechanisms and public health implications.
A team compared hundreds of people diagnosed with Parkinson’s to a similar number without the disease, tracking proximity to chlorpyrifos over many years and looking for patterns. The analysis found a striking association between sustained exposure and increased Parkinson’s risk. Researchers say the risk rose the longer people lived near places where the pesticide was used.
To explore how chlorpyrifos might harm the brain, the investigators ran controlled mouse studies where animals inhaled the chemical under conditions meant to mimic human exposure. Those mice developed movement problems that resembled Parkinson’s symptoms and showed loss of dopamine-producing neurons. The animals also had increased markers of brain inflammation and harmful protein accumulation, which help explain functional decline.
Zebrafish were used to examine damage at the cellular level, and scientists observed brain cell death tied to failures in cells’ cleanup systems. That kind of cellular breakdown lines up with processes implicated in human neurodegenerative diseases. The cross-species findings strengthened the researchers’ confidence that chlorpyrifos might act on conserved biological pathways.
Dr. Jeff Bronstein, director of the Movement Disorders Program at UCLA and a professor of neurology and molecular toxicology, shared a striking observation from the work. “[We were] surprised that the mechanism of toxicity was apparent in both mice and zebrafish,” he said. The team noted that such consistency across models is uncommon and lends weight to the biological story.
The human portion of the study covered roughly 45 years and included more than 800 people with Parkinson’s and a comparable control group. Long-term residential proximity to chlorpyrifos use correlated with more than a 2.5-fold increase in Parkinson’s risk in that dataset. The authors caution the design is observational, so it shows association rather than definitive proof that the pesticide causes the disease.
Researchers acknowledged limits in how exposure was estimated, since they relied on location data rather than direct measures of personal intake, diet, or indoor contamination. They also pointed out that chlorpyrifos might have been applied alongside other chemicals, complicating the task of isolating its specific effects. Animal studies offer biological plausibility, but their results cannot be directly translated into exact human outcomes.
Chlorpyrifos has long been used against pests such as termites, mosquitoes, and crop-destroying worms and appears on a range of agricultural commodities. People can encounter it by breathing contaminated air or by ingesting residues on food or in water. The study’s authors advised practical steps to reduce exposure in daily life.
“People should avoid exposure to CPF and similar pesticides (organophosphates) by not using them in their home, eating organics, and washing fruits and vegetables before eating them,” Bronstein advised. The simple measures focus on cutting household uses and lowering dietary intake where possible. Those are precautionary moves while regulatory and scientific reviews continue.
The regulatory history of chlorpyrifos has been turbulent: a ban on food-crop uses was put in place, then later challenged and partially reversed, and agencies have continued to review the chemical under federal pesticide law. “Chlorpyrifos is subject to registration review, a process required under FIFRA (the Federal Insecticide, Fungicide, and Rodenticide Act) in which registered pesticides are comprehensively evaluated every 15 years against current safety standards and the latest scientific evidence,” noted a federal statement shared with reporters. “EPA is currently developing a revised human health risk assessment for chlorpyrifos as part of that review, and will consider this study alongside any other relevant submissions. Where the science calls for stronger protections or tolerance revocations, EPA will act without hesitation and without delay.”
Some manufacturers have already moved away from producing chlorpyrifos because of sales trends and regulatory uncertainty. One company declared years ago it would cease production; another requested cancellation of its registrations for products containing the chemical. “BASF does not manufacture chlorpyrifos and does not have any pesticide registrations issued by the U.S. EPA for chlorpyrifos-containing products,” the company said in a formal comment.
While the new work does not close the book on causation, it adds a significant set of human and animal data to the conversation about pesticide safety and neurological health. The study’s findings are likely to inform ongoing regulatory reviews and public discussions about how best to balance pest control needs with long-term health protections. Scientists and policy makers will watch for follow-up studies that measure individual exposures more directly and examine interactions with other chemicals.
