SOURCES: Charles Benbrook, Ph.D., research professor, Washington State University Center for Sustaining Agriculture and Natural Resources; Jennifer Sass, Ph.D., senior scientist, Natural Resources Defense Council, New York City; Kenneth Portier, Ph.D., vice president, Statistics and Evaluation Center, American Cancer Society; Aug. 19, 2015, statement, Dow AgroSciences, Indianapolis; Aug. 20, 2015, New England Journal of Medicine
WEDNESDAY, Aug. 19, 2015 (HealthDay News) -- Farm fields are becoming soaked with increasing amounts of suspected cancer-causing herbicides, thanks to the spread of genetically modified crops that are immune to these chemicals, two researchers contend.
They make their argument in a Perspective piece in the Aug. 20 issue of the New England Journal of Medicine.
Farmers' use of glyphosate -- a weedkiller most commonly known as Roundup -- has increased by a factor of more than 250 in the United States, climbing from 0.4 million kilograms in 1974 to 113 million kilograms in 2014, the researchers said.
This increase is due to crops such as corn and soybeans that have been genetically altered to be "Roundup-Ready," so they can't be affected by these herbicides, said one of the researchers, Charles Benbrook. He is a research professor at the Washington State University Center for Sustaining Agriculture and Natural Resources.
The herbicides leave the genetically modified crops unscathed, while still controlling weed growth on farm fields, according to Benbrook and colleague Dr. Philip Landrigan, who's with the department of preventive medicine at the Icahn School of Medicine at Mount Sinai in New York City.
Cancer researchers have recently raised concerns about the safety of these farm chemicals, which had long been considered benign to humans because they attack plants and not animals, said Jennifer Sass, a senior scientist with the Natural Resources Defense Council in New York City.
The International Agency for Research on Cancer (IARC) earlier this year classified glyphosate as a "probable human carcinogen," following studies that linked the chemical to non-Hodgkin's lymphoma, the Perspective authors noted. The IARC also classified a second widely used herbicide, 2,4-dichlorophenoxyacetic acid (2,4-D), as a "possible human carcinogen."
Both of these herbicides have been combined in a new product from Dow AgroSciences called Enlist Duo, which received U.S. Environmental Protection Agency approval in 2014.
Benbrook and Sass are concerned that heavy use of these two herbicides could pose a risk to the health of people who work on or live near farm fields.
"If you keep hitting somebody over and over, even if each punch isn't great, you're going to do as much or more damage as you would with one hard punch," Sass said. "That's what these herbicides are doing."
Farmers are using larger quantities of herbicide on their fields because weeds are becoming resistant to the chemicals, much in the same way genetically modified crops are resistant, Benbrook said.
The advent of genetically modified corn and soybeans meant that farmers could just spray an appropriate herbicide across their entire field, coating both crops and weeds, he said. The crops would ignore the herbicide, but the weeds would succumb.
But weeds that are resistant to glyphosate have emerged on nearly 100 million acres in 36 states, requiring fields to be treated with multiple herbicides that include 2,4-D, the authors said in their paper.
"As the resistant weeds have become a bigger problem, farmers have responded by spraying more," Benbrook said, increasing both the quantity of weedkiller they use and the number of times they spray their fields.
Those chemicals have begun to find their way into the water supply surrounding large farm operations, Benbrook said. Recent studies have found traces of glyphosate in surface water, runoff from rainfall, and even urine from humans.
In a company statement, Dow AgroSciences said the safety of glyphosate and 2,4-D has been shown through "extensive data."
"These are two of the most thoroughly studied herbicides in the world, and farmers have used them safely and effectively for decades," the company said.
Glyphosate's potential risk to humans is mitigated by a few factors, said Kenneth Portier, vice president of the American Cancer Society's Statistics and Evaluation Center.
Humans do not process glyphosate and 2,4-D in the same way that they absorb harmful pesticides like DDT, he said.
"With glyphosate, most of it is excreted in fecal material, which means it's not bioprocessed," Portier said. "Your stomach and intestines don't take it up and put it in the bloodstream."
Also, researchers have not been able to rule out other chemicals as potential sources of the cancer risk that has been ascribed to glyphosate, he said.
"They see increased risk in these agricultural workers, but there's the understanding that they are exposed to more than just glyphosate," Portier said. "They're exposed to this whole mix of chemicals, so it's hard to tease out what's the effect of this one chemical."
That said, the American Cancer Society does take seriously the potential threat posed by glyphosate and 2,4-D, Portier said, and would like manufacturers to come up with alternative products.
"We don't like to see man-made carcinogens freely circulating in the environment," he said. "We would prefer to use chemicals in agricultural production that are not human carcinogens, even probable carcinogens."
Given the cancer concerns regarding glyphosate and 2,4-D, Benbrook and Landrigan ask the U.S. Environmental Protection Agency to delay implementation of its decision to permit use of Enlist Duo.
Dow AgroSciences disagrees.
"Contrary to the claims of the authors, regulatory approvals for Enlist Duo herbicide are based on detailed government health and environmental assessments, including review of recent, published state-of-the-art studies," the company said in a statement. "The U.S. Environmental Protection Agency spent nearly four years on its review and consideration of public input, repeatedly extending the comment period."
The EPA review took into account the cancer concerns raised by the IARC, "and found no basis for concern relative to potential carcinogenicity," Dow AgroSciences said.
"It is important to note that IARC only focuses on whether a substance theoretically could cause cancer, not whether it will cause cancer in real-world circumstances," the company said. "No national regulatory body in the world considers these herbicides a carcinogen."
For more on glyphosate, visit the International Agency for Research on Cancer.