Parkinson's disease is one of those conditions many of us know from a distance.
Maybe through a grandparent, a neighbor, a public figure whose hands slowly started to shake. For most people, it feels like something that strikes almost by chance.
A new study from UCLA Health suggests that for some, it may not be entirely random.
Long-term residential exposure to a farm pesticide called chlorpyrifos was linked to more than 2.5 times the risk of developing Parkinson's disease.
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Chlorpyrifos is an insecticide that was used in American homes for decades until household use was banned in 2001.
It is still sprayed on a range of food crops across the United States.
How researchers reached the 2.5-fold figure
The paper, published in Molecular Neurodegeneration in December 2025, drew on data from 1,653 Californians. 829 had Parkinson's disease, and 824 did not.
The team used California's pesticide-use records combined with each person's past home and work addresses.
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That allowed them to estimate how much chlorpyrifos each person was likely exposed to over many years.
People with the heaviest workplace exposure had about 2.7 times the odds of Parkinson's. Those with long-term residential exposure also saw their risk rise.
Living in areas where the chemical was sprayed raised the odds more than 2.5 times compared with people with no such exposure.
What the pesticide may do to brain cells
To test whether the link could be causal, the researchers also ran animal experiments. Mice breathing in chlorpyrifos for 11 weeks started to lose coordination.
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In their brains, dopamine-producing neurons died off. These are the same cells that are gradually lost in people with Parkinson's.
The mice also showed abnormal clumps of alpha-synuclein, a protein that builds up in Parkinson's.
Experiments in zebrafish pointed to the likely mechanism. The pesticide seems to disrupt autophagy, the body's cellular cleanup system that normally clears away damaged proteins.
Senior author Dr. Jeff Bronstein is a professor of neurology at UCLA Health. He told the UCLA newsroom that the matching evidence from animals makes the human link look likely causal, not just an association.
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Where chlorpyrifos is still used today
According to the US Environmental Protection Agency (EPA), chlorpyrifos tolerances on food were reinstated in 2024 after a federal court ruling.
The agency is now proposing to revoke most food uses while keeping it on 11 specific crops in certain states.
Those 11 crops are alfalfa, apple, asparagus, tart cherry, citrus, cotton, peach, soybean, strawberry, sugar beet, and spring and winter wheat.
If finalized, the restriction is expected to cut annual chlorpyrifos use in the US by about 70 percent compared with historical levels.
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The EPA has flagged particular concern over the pesticide's neurological effects at high exposure. The new UCLA findings add Parkinson's disease to that list, especially for people living for years near fields where it is applied.
The pesticide remains legal on apples, strawberries, citrus and the other eight crops in states listed by the EPA until the agency finalizes its proposed revocation.
This article is made and published by Ida-Marie Palm Varbæk, who may have used AI in the preparation.