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PHILADELPHIA (CBS/CNN) — You might want to think twice before you reach for those cereal boxes in the morning. A new study finds that 26 of 28 breakfast foods studied tested positive for trace amounts of herbicide found in the weed killer Roundup, which could cause cancer.

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Dozens of common breakfast cereals and snack bars have trace amounts of the controversial herbicide, according to a report released today by an environmental advocacy group.

The Environmental Working Group (EWG) found that 26 of the 28 products it tested had levels of Roundup’s main ingredient, glyphosate, that were “higher than what EWG scientists consider protective of children’s health.” An earlier report found similar results in over thirty oat-based foods.

Manufacturers say their products are safe, but the EWG report argues that the vast majority of foods tested — such as Honey Nut Cheerios and Quaker Simply Granola Oats — have glyphosate levels that might pose a cancer risk with long-term consumption.

None of the foods violated EPA limits on the herbicide, but the EWG uses a far more conservative health benchmark. California’s proposed glyphosate limit, which would be the most restrictive in the country, still allows for glyphosate levels that are over a hundred times higher than the EWG’s threshold.

The environmental group says its lower threshold includes an added buffer for children, as “exposure during early life can have more significant effects on development later in life,” according to Dr. Alexis Temkin, the lead scientist on EWG report.

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But manufacturers dispute that threshold. Quaker said in a statement that the “EWG report artificially creates a ‘safe level’ for glyphosate that is detached from those that have been established by responsible regulatory bodies in an effort to grab headlines.”

General Mills, whose products were also cited in the report, maintained that glyphosate levels in its foods do not pose any health risks. “The extremely low levels of pesticide residue cited in recent news reports is a tiny fraction of the amount the government allows,” the company said in statement to CNN.

“Consumers are regularly bombarded with alarming headlines, but rarely have the time to weigh the information for themselves,” the company said. “We feel this is important context that consumers should be aware of when considering this topic.”

In August, a jury in San Francisco ordered Roundup’s manufacturer, Monsanto, to pay $289 million in damages to a school groundskeeper who argued that the glyphosate-based weed killer caused his cancer. A judge on Monday upheld that decision but slashed Monsanto’s payout to $78 million.

Pharmaceutical giant Bayer recently purchased Monsanto and said in a statement that the company plans to appeal the court’s decision. “Glyphosate-based herbicides have been used safely and successfully for over four decades worldwide,” the company said in a statement to CNN.

“There is an extensive body of research on glyphosate and glyphosate-based herbicides, including more than 800 rigorous registration studies required by EPA, European and other regulators, that confirms that these products are safe when used as directed.”

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The EPA concluded in 2017 that glyphosate “is not likely to be carcinogenic to humans,” but a World Health Organization agency, the International Agency for Research on Cancer (IARC), determined in 2015 that glyphosate is “probably carcinogenic to humans.”

Dr. Chensheng Lu, an associate professor of environmental exposure biology at Harvard’s T.H. Chan School of Public Health, defended the WHO group, calling it “a world renowned and reputable academic and research institute in cancer epidemiology.”

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The EPA, in contrast, “is a regulatory agency, and in many ways a political agency,” he said. “In 2018, I would not hold EPA’s view on glyphosate as a fact.”

The IARC has vigorously defended its finding, but a separate WHO panel assessing pesticide residues determined in 2016 that “glyphosate is unlikely to pose a carcinogenic risk to humans from exposure through the diet,” adding to a dizzying array of contradictory findings.

Puzzling conclusions like those are not uncommon in cancer research, according to Dr. Otis W. Brawley, the American Cancer Society’s chief medical and scientific officer.

“IARC, I think, is very, very reasonable in their assessments,” he said, “but IARC will sometimes make an assessment that is not satisfying to many of us.”

Brawley noted that the other commonly-consumed substances are also classified as potentially carcinogenic by the IARC. Based on limited evidence, for example, the IARC says that “drinking very hot beverages probably causes cancer of the esophagus in humans,” yet hundreds of millions of people drink coffee every day.

“There are some groups that really want to alarm people and advocate for what’s called the precautionary principle,” Brawley said. “The precautionary principle, taken to its extreme, means you literally wouldn’t get up in the morning.”

Brawley said that parents should instead make sure their kids are eating fruits, vegetables, and getting the nutrition they need. More children “are definitely going to be harmed by inappropriate diets,” he said, “than by a small amount of glyphosate in their oatmeal.”

Glyphosate can make its way into processed foods after being used on farms that grow oats. “Most crops grown in fields use some form of pesticides and trace amounts are found in the majority of food we all eat,” said General Mills in a statement.

“We continue to work closely with farmers, our suppliers and conservation organizations to minimize the use of pesticides on the ingredients we use in our foods,” the company said.

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Still, some experts are urging parents to be vigilant. “I think it’s very important for people to realize how widespread exposure to glyphosate is,” said Dr. Sarah Evans, an assistant professor at the Icahn School of Medicine at Mount Sinai.

“There are things in the foods that they purchase that aren’t listed on the label and that they probably don’t want to be giving to their children,” said Evans, who runs Mount Sinai’s Children’s Environmental Health Center.

“I don’t think that people should become hysterical,” she said, “but people need to be really aware of where their food is coming from and what’s getting into their foods.”

And when dealing with children, Harvard’s Lu believes that parents should err on the side of caution. “What is more scary?” he asked. “Choosing cereals between organic and conventional, or being told by your doctor that you or your children have cancer?”

The EWG’s earlier report found glyphosate in 5 of the 16 organic breakfast samples they tested, but none of those levels exceeded the group’s health benchmark. Temkin, who was also lead author on that report, said that parents don’t need to “throw out their half-eaten box of cheerios” just yet.

“We are talking about lifetime cumulative exposure,” she said. “But if you do want to reduce your risk, we do know that organic oats and organic cereals are a better option.”

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