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Researchers Raise Concerns Over Health Risks Of Popular Herbicide

Kerry Klein
The ground underneath these baby almond trees is bare because of the herbicide paraquat, which research suggests is associated with an increased risk of developing Parkinson’s disease.";

It’s springtime in the valley, which, for many of us, means it’s time to clear the weeds out of our backyards. The same goes for growers, but the landscape of industrial weedkillers is changing. A California judge recently ruled that the main ingredient of the popular herbicide RoundUp must be labeled as a carcinogen. Now, another popular herbicide is facing some scrutiny over its health impacts as well.

Weeds kill crops. Kurt Hembree says that’s because they’re pernicious moochers.

“Direct competition for water and nutrients. Whatever the tree likes, the weeds like,” Hembree says.

Hembree is a weed specialist with the UC Cooperative Extension in Fresno County. He advises growers on how to manage their weeds.

We’re standing in a field near Selma that’s got rows and rows of leafy stalks about 2 feet tall. “This is a young almond orchard,” he says. “They call them baby trees.”

He says one product is especially good at protecting young trees from weeds: Paraquat. “Paraquat’s a contact-type herbicide,” he says. “In other words, it's a material that, if you sprayed it on a plant, it'll disrupt the plant's cells.”

He says it essentially melts away plant tissue, clearing the ground of any living plant other than the tree. “And basically in five or six days, whatever it touches, it spots up and it causes necrosis and death on the tissue,” he says.

Paraquat is popular because it’s cheap and effective. But like many chemicals, it’s dangerous. It’s a highly controlled, lethal substance with strict instructions to keep handlers safe. Nonetheless, an emerging body of research suggests mere exposure to the chemical is correlated with increased risk of Parkinson’s disease. Many countries have outlawed the herbicide, including the European Union, which banned it in 2007, partly because of uncertainty surrounding Parkinson’s. As researchers work to tease out the nature of that correlation, paraquat remains one of the most common herbicides in California.

Credit Kerry Klein / KVPR
UC Cooperative Extension weed specialist and farm advisor Kurt Hembree says growers keep weeds at bay for two primary reasons: 1) to prevent resource competition, and 2) to clear the ground so nuts and fruit can easily be harvested.

To be clear, Hembree says paraquat is explicitly labeled as toxic. “There’s different warning label levels,” he says. “This is danger, which is the highest level you can have.”

Swallowing even a small amount can lead to organ failure and death. It’s occasionally used even for suicides, particularly in developing countries. To prevent accidental ingestion, it’s dyed blue and given a sharp smell. It’s typically sprayed by machine to minimize human contact.

“Something like paraquat, you're going to wear rubber boots, you're going to wear goggles while you're spraying,” Hembree says. “You don't want to get this stuff on your skin or on your mouth or anywhere.”

Paraquat is among the top 10 most common herbicides in California, and the San Joaquin Valley gets over three-quarters of the state total. It’s used on scores of crops, primarily tree nuts, grapes, alfalfa and cotton. And before the drought struck, paraquat use was rising. In 2011, Valley growers used almost 80% more of it than they did in 1990, the earliest year data are available.

Some researchers say those numbers are concerning.

“The number of studies that have linked paraquat use occupationally to increased risk of Parkinson’s should raise some concerns if there’s an increasing environmental presence,” says Dr. Caroline Tanner, a neurologist at UC San Francisco who’s dedicated her career to studying Parkinson’s disease. In 2011, she published a highly cited study based on in-depth surveys with agricultural workers in the Midwest.

“People who mixed or applied this chemical had more than double the risk of developing Parkinson’s disease compared to people really very similar as far as where they lived, even what they did for a living, age and gender,” she says.

Notably, Tanner says, people who said they were careful with personal protective equipment were an exception.

“Those people, when exposed to paraquat, still didn’t have a greater risk of Parkinson’s,” she says.

Tanner is careful to call the Parkinson’s-paraquat relationship a correlation, not causation. But it’s one of many studies that suggests that correlation. And Freya Kamel, an epidemiologist with the National Institutes of Health who co-authored the study with Caroline Tanner, argues demonstrating cause and effect isn’t really necessary.

“We can't say that it causes the disease,” Kamel says, “but I also want to emphasize that you don't need to demonstrate a direct and specific causal relationship in order to suggest that there might be a reason for concern.”

Not all researchers are quite as convinced. Sirisha Nandipati is a neurologist with Kaiser Permanente in Marin County. She authored a review last year that compiled other studies of Parkinson’s disease. Of the eight studies she found that mentioned paraquat, one study demonstrated no connection between the two, and three others showed a correlation only when paraquat was combined with other risk factors.

“So it seems like genetic factors can influence it, even synergistic effects between paraquat and other pesticides can lead to some sort of correlation between paraquat and Parkinson's,” Nandipati says, “but the data is certainly mixed.”

Credit Ezra David Romero / KVPR
In the Valley, paraquat is primarily used as an herbicide for almonds and tree nuts, grapes, alfalfa and cotton.

Part of what’s confounding is that Parkinson’s disease is rare and there’s no statewide registry of the disease. Plus, it’s a long-term disease, and it may reflect environmental exposures from decades ago. In 2014, Tulare County had the highest rate of Parkinson’s-related deaths in the Valley, but it wasn’t a standout across the state. Plus, many other Valley counties apply more paraquat.

Syngenta, the chemical giant that manufactures Gramoxone, whose main ingredient is paraquat, declined an interview but did issue a statement claiming its product is not associated with the disease. It has also funded studies that conclude that.

The EPA and CDC do not mention Parkinson’s disease on their paraquat information pages, but the EPA does acknowledge the emerging research in a regulatory filing from March 2016.

Meanwhile, ongoing research out of UCLA aims to pinpoint the environmental exposures associated with Parkinson’s disease here in California.

Kerry Klein is an award-winning reporter whose coverage of public health, air pollution, drinking water access and wildfires in the San Joaquin Valley has been featured on NPR, KQED, Science Friday and Kaiser Health News. Her work has earned numerous regional Edward R. Murrow and Golden Mike Awards and has been recognized by the Association of Health Care Journalists and Society of Environmental Journalists. Her podcast Escape From Mammoth Pool was named a podcast “listeners couldn’t get enough of in 2021” by the radio aggregator NPR One.
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