A chemical used in agriculture for decades may be doing more than protecting crops from insects. Scientists have uncovered evidence that long-term exposure to a widely used pesticide could be associated with a substantially increased risk of Parkinson’s disease, a progressive neurological condition that affects movement, coordination and everyday life.
The chemical is called chlorpyrifos. Researchers at the University of California, Los Angeles (UCLA), found that people with prolonged residential exposure had more than 2.5 times the risk of developing Parkinson’s disease compared with people without such exposure.
But the research goes beyond identifying a statistical association. Laboratory experiments also revealed how the pesticide may interfere with the brain’s natural ability to remove damaged proteins, potentially contributing to the destruction of neurons responsible for controlling movement.
The findings raise an important question: could exposure to certain agricultural chemicals be contributing to neurological diseases that emerge decades later?
The pesticide at the centre of the investigation
Chlorpyrifos is an organophosphate insecticide that has been used extensively in agriculture to control insects affecting crops.
For years, it was also used in some residential pest-control products. However, concerns about its potential effects on human health led to restrictions in several countries.
In the United States, most residential uses were phased out or banned in 2001, while agricultural uses faced further regulatory restrictions in 2021. Some agricultural applications have continued under permitted uses.
The chemical deserves attention because exposure is not necessarily limited to people who directly handle pesticides. Depending on application practices and environmental conditions, people living or working near treated agricultural areas may also encounter pesticide drift.
The UCLA investigation focused on whether prolonged exposure to chlorpyrifos could be associated with Parkinson’s disease.
The research was published in the peer-reviewed journal Molecular Neurodegeneration, under the title The pesticide chlorpyrifos increases the risk of Parkinson’s disease.
What did the UCLA researchers discover?
The research team examined information from 829 people diagnosed with Parkinson’s disease and 824 individuals without the condition.
All participants were part of UCLA’s long-running Parkinson’s Environment and Genes study.
Rather than relying exclusively on participants remembering which chemicals they had encountered, researchers combined residential and workplace address histories with California’s official pesticide application records.
This allowed them to estimate long-term exposure based on where people lived and worked in relation to areas where chlorpyrifos had been applied.
The results were striking.
People with prolonged residential exposure to chlorpyrifos had more than 2.5 times the risk of developing Parkinson’s disease compared with those without the estimated exposure.
This does not mean that everyone exposed to chlorpyrifos will develop Parkinson’s. Nor does it establish that the pesticide alone caused the disease.
It does, however, identify a potentially important environmental risk factor that deserves further investigation.
How does chlorpyrifos damage the brain?
To understand the findings, it helps to look at dopamine, a chemical messenger essential for controlling movement.
Dopamine-producing neurons are concentrated in a region of the brain called the substantia nigra. In Parkinson’s disease, these specialised cells progressively deteriorate, reducing dopamine availability.
This contributes to characteristic symptoms such as:
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Tremors, particularly when the affected limb is at rest.
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Muscle stiffness and rigidity.
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Slowness of movement.
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Problems with balance and coordination.
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Changes in walking and posture.
The UCLA researchers investigated whether chlorpyrifos could contribute to this deterioration.
In animal experiments, mice inhaled aerosolised chlorpyrifos over 11 weeks. Researchers observed movement difficulties, loss of dopamine-producing neurons, brain inflammation and abnormal accumulation of alpha-synuclein, a protein strongly associated with Parkinson’s disease.
The team also investigated the effects in zebrafish, identifying disruption of a cellular process called autophagy.
The brain’s cleaning system may hold the answer
Autophagy is one of the body’s essential cellular maintenance systems.
Think of it as an internal waste-disposal and recycling service. Cells use this process to identify damaged proteins and unwanted components, break them down and recycle useful materials.
When autophagy works properly, potentially harmful protein accumulations can be removed before they interfere with normal cellular functions.
However, the UCLA experiments suggested that chlorpyrifos interferes with this protective mechanism.
When the cellular cleaning system becomes impaired, abnormal proteins such as alpha-synuclein may accumulate. Over time, this could place additional stress on neurons and contribute to their deterioration.
One particularly interesting finding was that restoring autophagy or removing synuclein protected neurons from pesticide-related damage in experimental models.
This could eventually help scientists identify ways to protect vulnerable brain cells, although no such protective treatment has yet been established for people exposed to chlorpyrifos.
What does this mean for agricultural workers and nearby communities?
The research has implications beyond laboratory science.
Agricultural workers, pesticide applicators and people living near frequently treated farmland may experience repeated environmental exposure to agricultural chemicals.
The concern is particularly relevant because Parkinson’s disease can develop gradually, sometimes becoming clinically apparent years after the biological changes begin.
Nevertheless, the study does not establish an individual’s personal risk from a particular exposure. It also does not demonstrate that chlorpyrifos exposure is responsible for every case of Parkinson’s disease.
The US Environmental Protection Agency continues to review chlorpyrifos and its permitted uses. Its current regulatory framework includes restrictions on which crops can be treated and where certain applications are allowed. Read the EPA’s latest chlorpyrifos guidance.
What happens next?
The findings provide researchers with two important directions for future investigation.
First, scientists need to determine whether other commonly used pesticides interfere with autophagy in similar ways.
Second, they need to investigate whether treatments that restore cellular waste-removal mechanisms could help protect dopamine-producing neurons.
UCLA is also leading further research into how pesticides, air pollution and genetic susceptibility interact in Parkinson’s disease. Read about UCLA’s wider Parkinson’s research programme.
For now, the chlorpyrifos study is an important warning about the possible long-term neurological consequences of environmental chemical exposure.
It does not prove that pesticides cause Parkinson’s in every exposed person. But it provides a stronger biological explanation for a connection scientists have been investigating for decades.
The most important discovery may ultimately be what happens inside the brain: a chemical exposure that interferes with the very cleaning system neurons depend on to survive.
