Is There a Connection Between Parkinson's Disease and Pesticides?

Parkinson's disease is a progressive neurological disease that affects movement, balance, thought processes, emotional well-being and more. With Parkinson's disease, levels of dopamine -- a chemical messenger within the nervous system involved with movement and coordination -- are too low.

The condition typically occurs in people over 60, although young-onset Parkinson's disease can happen to adults 50 or younger. Treatment can help manage symptoms, but to date there is no cure.

Parkinson's disease prevention is a major research push, including discovering causes and risk factors that can be mitigated. Certain pesticides and other environmental toxins have been implicated as possible culprits. Experts are grappling with pinning down which pesticides and herbicides might be most toxic to brain cells, how much and what kinds of exposure -- such as occupations like farming or rural living -- increase Parkinson's risk and how to avoid their harmful effects.

[See: Best Foods for Brain Health.]

Pesticides May Impact Brain Health

Rates of Parkinson's disease continue to rise. The global burden of Parkinson's more than doubled from 2.5 million people in 1990 to 6.1 million people by 2016 -- so in just over a single generation -- according to a systematic analysis published in The Lancet Neurology. One factor in the increase is an aging population.

"There are numerous environmental factors, including pesticides and paraquat, as well as other chemicals that have been linked to the disease," says Dr. Ray Dorsey, a professor of neurology at the University of Rochester Medical Center in New York, and a co-author of "Ending Parkinson's Disease: A Prescription for Action," published in March 2020.

Pesticides are substances used to kill, repel or control forms of plant life (like weeds) or animal life (like insects) considered as pests, for instance in industries like farming. The generic term "pesticide" encompasses insecticides, herbicides and fungicides, Dorsey explains. Paraquat, an herbicide, is "the poster child for pesticides that have strong evidence linking them to Parkinson's," he says.

Paraquat is a restricted-use pesticide that can only be applied by licensed professionals wearing gear such as chemical-resistant gloves, air-purifying respirators, chemical aprons and safety glasses.

Chlorpyrifos is another pesticide with brain effects, Dorsey says, including increasing the risk of Parkinson's disease. Production of the pesticide is being phased out by manufacturers, and its use is banned in the European Union and Hawaii, California and New York.

However, "it's widely used on golf courses even today," Dorsey says. "One study suggested that people who lived near a golf course were at higher risk of Parkinson's disease, especially those people who lived downwind of the golf course."

In mid-August, the Environmental Protection Agency announced that it will ban the use of chlorpyrifos on food crops, because of health concerns for farmers and children. "The EPA's decision to ban chlorpyrifos will prevent thousands of Americans from ever developing Parkinson's disease," Dorsey says. "It now needs to revisit its decision to allow continued use of paraquat."

[See: Personal Protective Equipment (PPE): Definition and Examples.]

Emerging Research

Here are some examples from the body of research linking pesticides to Parkinson's disease:

-- Dorsey points to a landmark 2011 study of farmworkers conducted by Dr. Caroline Tanner at University of California--San Francisco and published in the journal Environmental Health Perspectives. "She found that people who were exposed to paraquat had a 150% increased risk of developing Parkinson's disease," he says. Rotenone, an insecticide, was also linked to higher Parkinson's disease risk.

-- Researchers at the University of Guelph in Ontario, Canada, used stem cells of patients who had specific genetic mutations associated with Parkinson's disease, healthy stem cells and gene editing to show that low levels of exposure to the herbicides paraquat and maneb depleted dopamine-producing nerve cells of energy. In people who are more susceptible, the pesticides significantly heighten Parkinson's risk, concluded the study published October 2018 in the Federation of American Societies for Experimental Biology Journal.

-- An editorial co-written by Dorsey summarizes environmental causes fueling increases in Parkinson's including pesticides and other neurotoxic chemicals, published December 2018 in the Journal of Parkinson's Disease.

-- In a study published earlier this year, laboratory animals exposed to paraquat developed features of Parkinson's disease. After inhaling paraquat as an aerosol, male mice had reduced sense of smell for months after the exposure, and the chemical invaded the rodents' brains and other bodily tissues, according to the University of Rochester research findings in the February 2021 issue of Toxicological Sciences.

Several lawsuits claim that the larger body of research shows that long-term exposure to paraquat causes Parkinson's disease. In its 2020 financial statement, Syngenta, a company that produces paraquat, briefly summarizes a series of pending Parkinson's-related litigation against the company and other paraquat manufacturers. "The Syngenta AG group strongly believes that the claims are without merit and is vigorously defending against the actions," the report section concludes.

Research on farmers who use paraquat suggests that those who protect themselves appropriately might not have increased Parkinson's risk, says Dr. Rebecca Gilbert, a neurologist and vice president and chief scientific officer of the American Parkinson Disease Association. "Your risk is in your hands to a certain degree," she says. "So, if you have to be exposed to toxic chemicals, use the right equipment -- whatever that might be for that particular exposure. There are steps you can take to mitigate your risk."

Population vs. Individual Risk

With a complicated disease like Parkinson's disease, Gilbert says, "the risk is going to be an accumulation of all the genetic risk factors you carry and all the environmental risk factors you've been exposed to. So for any one individual, it's nearly impossible to tease out exactly why you got the disease."

However, that changes when you examine Parkinson's disease risk on a larger scale. "In a population, you can start making more generalizations," Gilbert says. "You can look at a farming state and see that their rates are higher than a state that doesn't have as much interaction with pesticides."

Statewide Parkinson's disease registries can make information collection and environmental connections possible. The California registry has a direct link to electronic medical records that highlight if a patient has Parkinson's, and pesticide exposure in each county is recorded as well, enabling good studies on the relationship in that state, Gilbert explains. Nebraska has a similar registry.

Gilbert emphasizes the importance of expanding registries in more states: "That's how we know who has Parkinson's out there," she says. Population studies are needed to be able to pinpoint the true risk of paraquat exposure in individuals, she adds. "It's not going to make it 100% or even close," she says. "It's going to increase your risk a small amount."

Nonoccupational Exposure

If you're not an agricultural worker, it's possible you could still be indirectly exposed to neurotoxic pesticides.

"Pesticides have been most studied among farmworkers, people who live in rural areas and people who drink well water," Dorsey says. "So, 40 million Americans drink their water not from city water but from private wells, and those private wells aren't regulated by the Safe Drinking Water Act."

Well water isn't regularly tested for chemicals or other hazards, Dorsey says. "People who have wells tend to live in rural areas," he notes. "So, even if they're not farming, the pesticides that are used on nearby farms can wash off those farms as seepage in soil and that can contaminate the underlying groundwater ."

On a personal level, you can take steps that might reduce your exposure to neurotoxic pesticides:

-- Use a carbon water filter at home.

-- If your water supply comes from a well, have the well water tested.

-- Wash fresh fruits and vegetables thoroughly before eating or serving.

-- Consider choosing organic produce if it's available.

What does organic mean? "Produce can be called organic if it's certified to have grown on soil that had no prohibited substances applied for three years prior to harvest," according to the Department of Agriculture website. "Prohibited substances include most synthetic fertilizers and pesticides."

However, it's more important to reap the known health benefits of fruits and vegetables than to avoid them for an uncertain reduction of Parkinson's disease risk, Gilbert says: "We definitely know that eating lots of fruits and vegetables is a very healthy way to help your brain."

[See: Ways to Boost Your Immune System.]

EPA Stance

Paraquat is still permitted in the U.S. In October 2020. The Environmental Protection Agency extended its continued use as a pesticide in an interim decision.

"EPA reviewed a robust set of literature on paraquat exposure which included a range of health outcomes, including Parkinson's disease, lung function and respiratory effects and cancer," the regulatory agency posted. "Based on this review, EPA concluded that there is insufficient evidence to link registered paraquat products to any of the health outcomes investigated, including Parkinson's disease, when used according to the label."

Dorsey strongly disagrees with that decision. "Paraquat is so toxic that over 30 countries including China have banned it, but the United States has not," he says. "The EPA reauthorized its use despite over 100,000 signatures from the Parkinson's community asking that it be banned."

Legislation to ban harmful pesticides must be revisited, Dorsey says: "We can prevent at least a large portion of Parkinson's disease -- the world's fastest-growing brain disease that affects 1.2 million Americans -- by changing our habits and getting rid of these chemicals that are nerve toxins."