Long-term chlorpyrifos exposure linked to higher Parkinson's disease risk

Long-term residential exposure to the pesticide chlorpyrifos was linked to more than a 2.5-fold higher risk of Parkinson’s disease in a UCLA study that focused on people living and working near treated areas. The finding puts farmworkers, pesticide applicators and rural families closest to the center of the exposure question: who is still coming into contact with a chemical that regulators have tried, and often failed, to keep off the market?

The UCLA team used data from 829 people with Parkinson’s disease and 824 controls in the Parkinson’s Environment and Genes study, then estimated exposure with California pesticide-use reports and participants’ home and work addresses. In plain terms, the result means the exposed group was not just slightly more likely to get sick. It was more than twice as likely, a scale of risk that would matter to clinicians, regulators and families living near agricultural spraying. UCLA Health said the association was tied to long-term residential exposure, not a single incident.

The paper appeared in Molecular Neurodegeneration and was accepted on November 29, 2025, before publication on December 11, 2025. Jeff M. Bronstein and Beate Ritz were among the authors, along with Will Houston, Kazi Md Mahmudul Hasan, Lisa M. Barnhill, Kimberly C. Paul, Edward A. Burton and Clive Svendsen. UCLA’s release said nearly one million Americans live with Parkinson’s disease, a progressive disorder that causes tremor, stiffness and slowed movement.

The study went beyond an epidemiological signal. In mice exposed to aerosolized chlorpyrifos for 11 weeks, UCLA said the animals developed movement problems, lost dopamine-producing neurons, showed brain inflammation and accumulated alpha-synuclein. Zebrafish experiments pointed to disrupted autophagy as a possible pathway of damage. Taken together, the human, rodent and zebrafish work strengthened the biological case that chronic exposure may do more than accompany disease risk, even as the human data still describe an association rather than a single proven cause.

The policy backdrop is unsettled. The U.S. Environmental Protection Agency revoked all tolerances for chlorpyrifos in August 2021, effectively ending its use on food and animal feed, but the Eighth U.S. Circuit Court of Appeals vacated that rule in November 2023. EPA says it is preparing a new proposed rule and has identified 11 major crop uses, including alfalfa, apples, asparagus, tart cherries, citrus, cotton, peaches, soybeans, strawberries, sugar beets and spring and winter wheat, as accounting for about 55% of average annual pounds applied between 2014 and 2018. The European Union did not renew chlorpyrifos approval in January 2020.

UCLA also said it will lead a $9 million, three-year project with Aligning Science Across Parkinson’s and The Michael J. Fox Foundation, alongside Cedars-Sinai and the University of Münster, to study how pesticides and air pollution affect Parkinson’s risk and progression. The chlorpyrifos result now sits inside that larger effort to trace how environmental exposure, genetics and disease intersect.