Toxicologists Reveal Popular Weed Killer May Harm Teenage Brains

The widespread use of weed killer could be seriously damaging our teenagers' brains, scientists warn.

As of 2021, 1.7 million metric tons of herbicides were used annually around the world, according to data from the United Nations Food and Agriculture Organisation, with 457,390 tons used in the United States alone. These chemicals can linger on our fruit and vegetables and seep into our waterways, meaning that many of us are ingesting them in small quantities on a regular basis.

"There is considerable use of herbicides and insecticides in agricultural industries in both developed and developing nations around the world, raising exposure potential for children and adults, especially if they live in agricultural areas, but we don't know how it impacts each stage of life," Briana Chronister, a doctoral candidate at the University of California, San Diego, said in a statement.

Together with researchers from UC San Diego and Emory University, Chronister collected data from 519 adolescents aged between 11 and 17 in Pedro Moncayo, an agricultural county in Ecuador. By analyzing their urine, the team found that 98 percent of these individuals contained traces of glyphosate—known commercially as Roundup—the most widely used herbicide in the United States. Sixty-six percent of the participants' urine also contained the broadleaf herbicide 2,4-D.

"The herbicides 2,4-D and glyphosate are heavily used in agriculture and forestry, on lawns and gardens, and for weed control in industrial areas," senior author Jose Ricardo Suarez, an associate professor in the Herbert Wertheim School of Public Health, told Newsweek.

"There are multiple pathways that glyphosate and 2,4-D can enter people's bodies, either through inhalation, consumption (eating or drinking sprayed produce/grains), or absorption through the skin, and their exposure can be due to direct and indirect exposure pathways.

"Direct exposure primarily stems from use of these herbicides, by purchasing them and applying them to lawns or crops at home or occupationally (e.g farming), while indirect pathways include exposures without intending to use them, such as exposures through pesticide drift from spray sites to homes nearby, eating produce that had been sprayed with pesticides, or consuming water contaminated with pesticides," he said.

After analyzing the participants' urine, the team assessed their cognitive performance in five key areas: attention and inhibitory control; memory and learning; language; visuospatial processing; and social perception.

From this assessment, the team found that individuals with higher levels of 2,4-D in their urine were more likely to perform poorly in their assessment for attention, memory and learning, and language. Meanwhile, those who had higher concentrations of glyphosate were more likely to perform poorly in social-perception tests. The results were published in the journal Environmental Health Perspectives on October 11.

The team also tested the effects of high levels of the insect-repellent DEET in the participants' urine but did not find any significant associations.

"There have been no studies that specifically look at the effects of 2,4-D with the brain in humans, thus our knowledge stems from animal and in vitro studies," Suarez said.

"[In these studies] it has been found to induce neuronal and glial cell death in rodents by increasing reactive oxygen species and free radicals. These neurotoxic effects have also been linked to changes in neurobehavior, like object recognition and exploration behaviors, in rodents."

For glyphosate, the effects may be due to its effects on chloride ions in our brain cells.

"Reduced emotional recognition abilities have also been associated with autism spectrum disorder, and there is a theory that elevated chloride [levels] in neurons and neocortical tissue may serve a role," Suarez said. "Recent evidence suggests that glyphosate can be a glycine mimetic, which may be causing similar patterns of over-concentrated chloride within neurons that are leading to lower facial affect recognition that we saw in this study."

Children and adolescents are more susceptible to the effects of toxic chemical exposure, which makes them more vulnerable to these neuronal changes.

"As children and adolescents are developing, alterations to the normal progression of their development through herbicide exposure may lead to lifetime changes," Suarez said. "Thus, it is important to identify ways to limit their exposure to toxic chemicals like 2,4-D and glyphosate."

However, Suarez also said that additional research is needed to identify how both of these herbicides interact with the teenage brain.

Of course, the study was performed on a relatively small group of individuals in an agricultural community, which is not the environment that most American teenagers grow up in. These links may also be due to some other confounding factors, like spending more time on the farm than in formal education. However, these results are still striking and important to consider for young people in farming communities (and non-farming communities) around the world.

"Many chronic diseases and mental health disorders in adolescents and young adults have increased over the last two decades worldwide, and exposure to neurotoxic contaminants in the environment could explain a part of this increase," Suarez said.

"Hundreds of new chemicals are released into the market each year, and more than 80,000 chemicals are registered for use today," he added. "Sadly, very little is known about the safety and long-term effects on humans of most of these chemicals. Additional research is needed to truly understand the impact."

So what can we do to limit our exposure to these chemicals?

"Buying organic produce is a way that households can limit their exposure to herbicides in their diet," Suarez said. "Three out of four non-organic fresh produce sold in the U.S. contains residues of potentially harmful pesticides. Edible crops that are sprayed the most with 2,4-D and glyphosate include corn, soybeans, grains, and rice, but it has been also found in almonds, apples, cherries, and apricots, to name a few.

"These herbicides can also be used to dry crops such as wheat, oats, corn, lentils, beans, and potatoes, to help speed up harvesting. You can look for products that feature the Detox Project's 'Glyphosate Residue Free' label."

Update 10/11/23, 7:48 a.m. ET: This article was updated with comment from Jose Ricardo Suarez.