Key finding: A UCLA study found long-term exposure to chlorpyrifos was associated with more than a 2.5-fold higher risk of Parkinson’s disease in humans. Complementary animal experiments (mice and zebrafish) showed movement problems, dopamine neuron loss, brain inflammation and cellular cleanup failure consistent with Parkinsonian damage. The research is observational and cannot prove causation; exposure estimates were based on location, and animal results may not directly translate to humans. Regulators are reviewing chlorpyrifos and the EPA signaled plans in 2026 to move toward restricting most uses.
UCLA Study Links Common Pesticide Chlorpyrifos to >2.5× Higher Parkinson’s Risk
Parkinson's risk increases with exposure to common chemical, study suggests
New research from the University of California, Los Angeles suggests that long-term exposure to the pesticide chlorpyrifos is associated with a markedly higher risk of Parkinson’s disease. The study combines a large human epidemiological analysis with experiments in mice and zebrafish to explore possible mechanisms linking the chemical to neurodegeneration.
Chlorpyrifos is often used on agricultural products like soybeans, fruit and nut trees, broccoli, cauliflower and other row crops, according to the U.S. Environmental Protection Agency.(iStock)
Human Study: Long Follow-Up and Strong Association
Investigators compared 829 people diagnosed with Parkinson’s disease to 824 matched controls across a 45-year period, estimating participants’ chlorpyrifos exposure based on residential proximity to application sites. In these analyses, prolonged exposure was associated with more than a 2.5-fold increase in Parkinson’s risk.
A researcher commented that the consistency in results between human and animal subjects is "rare."
Animal Experiments Reveal Mechanisms
Laboratory work supported the human findings. Mice exposed to chlorpyrifos by inhalation for 11 weeks developed movement impairments resembling Parkinsonian symptoms, lost dopamine-producing neurons, showed increased brain inflammation, and accumulated proteins harmful to neurons. Zebrafish experiments demonstrated brain cell death and damage linked to dysfunction of the cell’s ‘‘cleanup’’ systems (autophagy/proteostasis), providing mechanistic clues for how the pesticide may harm nervous tissue.
Chlorpyrifos is used to control different kinds of pests, like termites, mosquitoes and roundworms, among crops.
"[We were] surprised that the mechanism of toxicity was apparent in both mice and zebrafish," said Dr. Jeff Bronstein, director of UCLA’s Movement Disorders Program. "We rarely find such consistent results in different animal models."
Limitations and Cautions
The authors note important limitations. This is an observational study and therefore cannot prove causation. Exposure was estimated from participants’ locations and did not directly measure individual diet, indoor exposures or personal behaviors. Animal-model results, while informative about biological effects, cannot be directly translated into human outcomes. The study also acknowledges potential co-exposure to other pesticides, which can complicate attribution of effects to chlorpyrifos alone.
"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," the agency said in a statement sent to Fox News Digital.
Regulatory Context and Practical Advice
Chlorpyrifos is used to control pests (termites, mosquitoes, roundworms) on a variety of crops including soybeans, fruit and nut trees, broccoli and cauliflower, according to the U.S. Environmental Protection Agency (EPA). People can be exposed by inhalation or by consuming contaminated food or water.
Regulatory history has been contentious: the EPA moved to ban chlorpyrifos on food crops in 2021, a federal appeals court overturned that ban in 2023, and in January 2026 the EPA announced plans to move forward with a rule to prohibit most uses while it updates its human health risk assessment under FIFRA.
Dr. Bronstein recommended reducing personal exposure where possible by avoiding use of chlorpyrifos-containing pesticides at home, choosing organic produce when feasible, and thoroughly washing fruits and vegetables before eating.
Industry Responses
Some manufacturers have reduced or stopped production: Corteva announced plans in 2020 to end chlorpyrifos production, and BASF requested cancellation of its U.S. chlorpyrifos registrations in 2022. The study did not evaluate products from these companies.
Bottom line: This multimodal study strengthens evidence that chlorpyrifos exposure is linked to Parkinson’s-like pathology, but further research and regulatory review are needed to determine causality and guide policy.
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