KANSAS CITY — A long-standing debate over the safety of the chemical glyphosate was reignited last week after a study revealed residue from the herbicide was found in 43 of 45 samples of breakfast foods made with conventionally grown oats.

The Environmental Working Group commissioned testing of more than a dozen brands of oat-based foods using a “no significant risk level” for glyphosate of 1.1 milligrams per day for an average adult. That level — suggested by California’s Office of Environmental Health Hazard Assessment in conjunction with listing glyphosate as a carcinogen on the state’s Proposition 65 list — represents a one-in-100,000 increased cancer risk over the lifetime of an adult. It is more than 60 times lower than the Environmental Protection Agency’s established safety level, but far higher than the one-in-1-million lifetime increased risk level set for many cancer-causing drinking water contaminants.

The E.W.G. said an additional 10-fold margin of safety was supported by the federal Food Quality Protection Act due to the increased susceptibility of children and fetuses to carcinogens. The group also

said the one-in-1-million increased cancer risk can be reached by ingesting 0.01 mg of glyphosate per day, the amount contained in a single 60-gram serving of food with glyphosate levels of 160 parts per billion.

Three fourths of the 43 positive samples contained glyphosate at levels above the 160 p.p.b. threshold for protection of children’s health with an adequate margin of safety. Sixteen of the samples were made with organically grown oats, and five of those contained glyphosate levels between 10 and 30 p.p.b., well below the E.W.G.’s health threshold.

The highest levels seen in the E.W.G.’s study were two samples of Quaker Old Fashioned Oats, which registered at 1,000 p.p.b. Three samples of General Mills’ Cheerios brand turned out glyphosate levels from 470 to 530 p.p.b. Twelve samples didn’t reach the E.W.G.’s benchmark, and two contained no traces of glyphosate.

In response, Quaker and General Mills each said their products are safe and cited E.P.A. and European Food Safety Authority safety rules and standards governing producers, suppliers, millers and manufacturers.

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

“While our products comply with all safety and regulatory requirements, we are happy to be part of the discussion and are interested in collaborating with industry peers, regulators and other interested parties on glyphosate,” Quaker officials said.

The impetus for the study was a Food and Drug Administration scientist’s presentation at the 2016 North American Chemical Residue Workshop, which showed glyphosate had been detected in several oat-based food products since testing was implemented in 2016 in response to consumer concerns and activist outcries.

After a Freedom of Information Act request, the F.D.A. released documentation revealing “a fair amount” of glyphosate residue was found in several processed foods. The F.D.A. said last week “the results are being evaluated, and those results will be included when the agency releases the 2017 pesticide report,” a process it noted could stretch to two years.

Monsanto, which merged with Bayer A.G. this year, manufactures glyphosate under the trade name Roundup. It has been in use in the United States for around 40 years and is the most popular herbicide in the country, with more than 250 million lbs of it sprayed annually on the 250 agricultural crops for which it is approved as a herbicide and, less frequently as a desiccant. In November, a five-year extension of glyphosate use was approved in the European Union.

Glyphosate often is used in conjunction with "Roundup Ready" corn and soybeans genetically engineered to thrive despite contact with the herbicide, while killing weeds that choke field crops and combines during harvests. In recent years, glyphosate has sometimes been applied to genetically natural wheat, barley, beans and oats just prior to harvest in order to kill the crop and speed the drying process for an earlier harvest.

The glyphosate debate has drawn allegations of faulty research, both for and against the use of the chemical. In 2015, glyphosate was called a “probable human carcinogen” by the World Health Organization’s International Agency for Research on Cancer. That study has been challenged, with critics alleging scientific lapses and unsubstantiated findings.

The I.A.R.C. ruling has proven to be a controversial outlier in research on the pesticide.  A study funded by the National Cancer Institute and the National Institute of Environmental Health Sciences, the Agricultural Health Study, found no firm link between exposure to glyphosate and cancer.

Other contrary conclusions were drawn by several other international organizations, including the European Food Safety Authority and the Joint Food and Agriculture Organization/World Health Organization Meeting on Pesticide Residues.  And in December, the E.P.A. concluded “glyphosate is not likely to be carcinogenic to humans,” noting the body “found no other meaningful risks to human health when the product is used according to the pesticide label.”

Still, based on the I.A.R.C. determination, California added glyphosate to its Safe Drinking Water and Toxic Enforcement Act of 1986 (Proposition 65) list last year and began enforcing labeling in July. Any consumer-facing product containing carcinogens must display the warning “known to the state of California to cause cancer, birth defects or other reproductive harm” via a label, sign or newspaper notice. The state Supreme Court on Aug. 17 declined to hear Monsanto’s appeal to remove glyphosate from the list.

Last week, Bayer was ordered by a judge to pay $289 million to a former school groundskeeper whose attorneys convinced a California jury that the company’s failure to warn of the potential risks of repeated use of pesticides was responsible for the man’s terminal case of non-Hodgkin's lymphoma.

Immediately following the verdict, Bayer’s stock lost more than $14 billion in value, a more-than-10% drop. The company said the jury’s verdict remains subject to post-trial motions in the trial court and to an appeal through which courts ultimately will find that Monsanto and glyphosate were not responsible for the groundskeeper’s illness. The company said that case is the first to trial of about 5,200 lawsuits by farm and grounds crew workers alleging their cancers were caused by years of exposure to Roundup.

The F.D.A., which regulates all food under E.P.A. guidelines, said it has not found any violations of E.P.A. glyphosate standards, but will consider E.W.G.’s latest findings on oat-based foods.