A common pesticide was tied to more than 2.5 times the risk of Parkinson’s in a UCLA study

Long-term residential exposure to chlorpyrifos, a widely used agricultural insecticide, was associated with more than 2.5 times the risk of Parkinson’s disease in a UCLA Health study of 1,653 people. The team, led by neurologist Jeff Bronstein, paired that human comparison with mouse and zebrafish experiments meant to show how the chemical could injure the brain cells that Parkinson’s destroys.

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The human result is an association, and the authors say so plainly. The animal work gives the signal a plausible biological route, which is what makes the paper more than another correlation.

Chlorpyrifos exposure mapped from California spray records

The human arm drew on UCLA’s long-running Parkinson’s Environment and Genes study, which enrolled 829 people with Parkinson’s and 824 people without it. Rather than asking participants to recall what they had been exposed to, the researchers combined California’s pesticide application records with each person’s residential and workplace address history, producing an estimate of chlorpyrifos exposure over time. The design is described in the UCLA Health release of January 7, 2026, and the full paper appeared in Molecular Neurodegeneration.

People with prolonged residential exposure had more than 2.5 times the risk of Parkinson’s compared with people who had none. The release does not give an odds ratio or confidence interval for that figure, so the exact range around it is something only the paper can supply. Because the comparison rests on where people lived and worked rather than on measured blood or tissue levels, it can show that the two things travel together without proving that one produced the other.

Parkinson’s itself has no single cause. The National Institute of Neurological Disorders and Stroke describes risk as arising from genes and environment together, with inherited variants in genes such as GBA1 and LRRK2 on one side and pesticide exposure from farming or landscaping on the other. A compound-specific result like this one is useful because it names a single chemical with a regulatory history instead of a vague class of products, and because California’s records let the team trace that chemical to particular addresses and particular years.

Autophagy failure and alpha-synuclein in exposed mice and zebrafish

To test causation, the team exposed mice to aerosolized chlorpyrifos for 11 weeks, a schedule meant to mimic breathing the chemical in the environment. The animals developed movement difficulties, lost dopamine-producing neurons, showed inflammation in the brain and built up abnormal clumps of the protein alpha-synuclein. Each of those changes is a recognizable feature of the human disease.

Zebrafish carried the mechanism work. Their results pointed to disrupted autophagy, the cell’s housekeeping process for clearing damaged proteins and debris, so that damaged material piled up inside neurons. When the researchers restored the cleanup process or removed synuclein, the neurons were protected. The authors stress that this protective effect has been shown only in laboratory models and that its effect in people is unknown. The National Institute of Neurological Disorders and Stroke notes that by the time symptoms appear, most patients have already lost roughly 60 to 80 percent or more of their dopamine-producing cells, which is why a way to protect those neurons early would matter.

Bronstein, the senior author, said the study establishes chlorpyrifos as a specific environmental risk factor for Parkinson’s disease, and that the animal findings make the association likely causal. That second claim leans mostly on the mice and fish, since the human data alone cannot settle it.

Where chlorpyrifos is still allowed in the United States

American residential use of the insecticide ended in 2001, and agricultural uses were restricted from 2021, but it has not disappeared. According to the Environmental Protection Agency, the agency revoked all food tolerances in August 2021, the Eighth Circuit vacated that rule on November 2, 2023, and the tolerances were reinstated when the court’s mandate issued that December.

The registrant later asked to cancel food and feed uses except on 11 crops, and as of July 1, 2025, no one may legally use chlorpyrifos on the cancelled crops. The remaining uses cover alfalfa, apple, asparagus, tart cherry, citrus, cotton, peach, soybean, strawberry, sugar beet and wheat. In December 2024 the agency proposed revoking every tolerance outside those crops, and its page shows no final rule yet.

The paper lists 17 authors, among them Kazi Md. Mahmudul Hasan, Lisa M. Barnhill, Beate Ritz, Julian Whitelegge and Edward Burton alongside Bronstein, which reflects how many kinds of work the project needed: environmental epidemiology, animal exposure chambers and zebrafish experiments. None of the summaries reviewed names a funder.

That leaves a practical gap between the science and the rules. The study’s exposure estimates came from California spray records over many years, so they describe a period of heavier use than the 11-crop limit now allows. Whether current agricultural exposure carries the same risk, and whether the autophagy pathway Bronstein’s group identified can be turned into a treatment, are questions the ScienceDaily summary of the work leaves open.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

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