Parkinson’s disease has long been associated with environmental risk factors, but pinpointing specific culprits has proven difficult. Now, a growing body of evidence is zeroing in on one chemical: chlorpyrifos, a widely used pesticide.
In a large human study, researchers analyzed data from more than 1,600 individuals, comparing those diagnosed with Parkinson’s disease to those without it. By reconstructing long-term exposure using detailed pesticide application records and participants’ residential and workplace histories, the team identified a striking pattern. People with sustained exposure to chlorpyrifos were more than 2.5 times as likely to develop Parkinson’s disease.
The study results were published in Molecular Neurodegeneration. Senior author Jeff Bronstein, professor of Neurology at UCLA Health, said, “This study establishes chlorpyrifos as a specific environmental risk factor for Parkinson’s disease, not just pesticides as a general class. By showing the biological mechanism in animal models, we’ve demonstrated that this association is likely causal.”
Recreating Exposure in the Lab
To move beyond correlation, researchers turned to controlled laboratory experiments. In one set of studies, mice were exposed to aerosolized chlorpyrifos for 11 weeks, mimicking how humans might inhale the chemical in agricultural settings.
The results mirrored key features of Parkinson’s disease. The exposed mice developed movement impairments and showed a significant loss of dopamine-producing neurons—the same cells that degenerate in human patients. Researchers also observed increased inflammation in brain tissue and the accumulation of alpha-synuclein, a protein known to form clumps in the brains of people with Parkinson’s disease.
These findings provide biological plausibility to the human data, demonstrating that chlorpyrifos is not only associated with disease risk but can directly trigger the kinds of neurological damage seen in Parkinson’s disease.
A Breakdown in the Brain’s Cleanup System
Further experiments in zebrafish revealed a critical underlying mechanism. Chlorpyrifos disrupted autophagy, the cellular process that removes damaged proteins from cells.
When this system fails, harmful proteins accumulate, placing stress on neurons and increasing vulnerability to degeneration. In the zebrafish models, impaired autophagy led to the buildup of alpha-synuclein and subsequent loss of dopamine-producing cells.
Crucially, when researchers restored autophagic function—or removed the problematic protein—the neurons were protected. This suggests that the pesticide’s neurotoxic effects may hinge on its ability to interfere with the brain’s natural maintenance systems.
“The discovery that autophagy dysfunction drives the neurotoxicity also points us toward potential therapeutic strategies to protect vulnerable brain cells,” Bronstein said.
A Widespread Chemical With Lasting Impact
Chlorpyrifos has been used for decades in agriculture. Although the European Food Safety Authority banned its use in 2020, and residential use was prohibited in the US in 2001, the chemical is still legally used on certain crops in the US and is widely applied globally.
Because Parkinson’s disease develops slowly over many years, past exposure may continue to influence risk long after contact with the pesticide has ended. Millions of people may have been exposed to chlorpyrifos during periods when its use was more widespread.
The findings also raise broader concerns about similar chemicals. If one pesticide can disrupt neuronal health by impairing autophagy, others may act through comparable pathways.
Toward Prevention and Protection
Identifying chlorpyrifos as a specific risk factor opens new possibilities for both prevention and treatment. Individuals with known exposure histories could benefit from closer neurological monitoring, allowing earlier detection of disease progression.
At the same time, the discovery of autophagy disruption as a central mechanism offers a promising therapeutic target. Drugs designed to enhance the brain’s protein-clearing systems could help counteract environmental damage and protect vulnerable neurons.
The research underscores a critical shift in understanding Parkinson’s disease—not just as a condition driven by genetics, but as one shaped by long-term environmental interactions.
As scientists continue to map these connections, the path forward becomes more clear. By identifying harmful exposures and intervening earlier, it may be possible to reduce risk and slow disease progression. At the same time, understanding how these exposures damage the brain could guide the development of therapies that preserve brain function.
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