Jack McCall was a fixture at the local farmers market, where he sold avocados and other fruits he grew on his 20-acre ranch in Cambria, on California’s Central Coast.

The U.S. postal worker and Little League coach was “very environmentally friendly,” said Teri McCall, his wife of 41 years. He avoided chemicals, using only his tractor-mower to root out the thistle and other weeds that continually sprouted on the flat areas of the ranch.

But he did make one exception to that rule — a fateful one, his wife now believes. For more than three decades, on the hilly parts of the ranch where he grew the avocados, and around newly planted fruit trees, Jack donned a backpack sprayer and doused weeds with the widely sold herbicide Roundup.

“He believed Roundup was safe,” Teri McCall said, noting that St. Louis-based Monsanto Co. has regularly touted its flagship product as harmless to people and pets.

In 2012, the McCalls’ 6-year-old dog, Duke, who regularly accompanied Jack around the farm, fell ill with swollen lymph nodes in his neck and died shortly afterward of lymphoma — a type of blood cancer. Three years later, Jack discovered swollen lymph nodes in his own neck, Teri said. The diagnosis: a rare form of non-Hodgkin lymphoma, which killed him on Dec. 26, 2015.

“I thought, ‘That’s kind of a coincidence that they both got lumps in their neck,’” Teri recalled. “Then I thought about all the time Duke spent sticking his nose in grass that had been sprayed with Roundup.”

In March 2016, McCall filed a wrongful death lawsuit against Monsanto, alleging that the company concealed the cancer risk posed by a chemical called “glyphosate,” the active ingredient in Roundup, which she now blames for the deaths of her husband and their dog.

Hundreds of similar lawsuits are pending in federal and state courthouses around the United States.

Monsanto vigorously contests them.

“To be clear: The underlying science behind glyphosate is not at question,” said Scott Partridge, the company’s vice president of global strategy. “Monsanto’s glyphosate-based herbicides have a long history of safe use and have been studied in real-world application, including the largest study ever of the actual use of pesticides by farmers.”

Monsanto’s Partridge contended that “cherry-picking isolated documents out of context is an attempt by the plaintiffs’ attorneys in pending litigation to distract from the science, which is not on their side.”

The use of glyphosate has grown exponentially in the past two decades. The chemical has found its way into the food chain — and into people’s bodies. A study published this week in the medical journal JAMA showed that the number of Southern California adults who tested positive for glyphosate in their urine rose dramatically from 1993 to 2016, as did the amount of the chemical in those who excreted it.

In July, California added glyphosate to its list of cancer-causing chemicals under the Safe Drinking Water and Toxic Enforcement Act of 1986. The act, also known as Proposition 65, requires businesses to warn consumers if their products or facilities contain potentially unsafe amounts of any toxic substances known to cause cancer, birth defects or other reproductive harm.

California is the first state in the U.S. to “take regulatory action to protect our residents from this chemical,” said Olga Naidenko, senior science adviser for the Environmental Working Group, a nonprofit research and advocacy organization. The move is “a huge step and has global implications.”

The state’s Office of Environmental Health Hazard Assessment, which is responsible for listing chemicals under Proposition 65, has proposed a threshold of 1.1 milligrams of glyphosate a day for an adult weighing 70 kilograms, or 154 pounds. That’s about 122 times more stringent than the federal Environmental Protection Agency’s safety guideline.

The state agency is studying more than 1,300 written public comments, along with oral testimony from a June hearing, to decide whether it should implement or revise its proposed limit.

The Prop. 65 listing requires warning labels beginning next July.

Other companies, including Dow AgroSciences and DuPont, also sell products containing glyphosate, since Monsanto’s patent expired in 2000.

California’s decision to list the chemical was triggered by a 2015 study from the World Health Organization that described the chemical as “probably carcinogenic to humans” and cited “convincing evidence that glyphosate also can cause cancer in laboratory animals.”

The organization’s International Agency for Research on Cancer found a “positive association” between exposure to glyphosate and malignancy in humans, though it added that other explanations could not be excluded. In particular, the international agency found a possible link to non-Hodgkin lymphoma, the type of cancer that killed Jack McCall.

Monsanto sued in state Superior Court to overrule the California listing but lost in March, and it has appealed that decision. Its bid to temporarily halt the cancer listing pending trial was rejected by a state appellate court and the California Supreme Court. The company says that labeling glyphosate a cancer risk is unjustified.

It argues that the International Agency for Research on Cancer erred by neglecting to consider data suggesting no link between glyphosate and non-Hodgkin lymphoma. That research was in an unpublished part of the multiyear and multifaceted Agricultural Health Study, which assesses the effects of pesticide exposure on farmers. The international cancer agency, an independent panel of scientists, said it weighs only published, peer-reviewed studies.

Other studies also have failed to establish a convincing link between glyphosate and cancer. Earlier this year, the European Union’s chemical safety regulator determined there was not sufficient evidence to classify glyphosate as a carcinogen, though it did say the compound could cause eye damage and long-term harm to aquatic life.

But the international cancer agency, which said it examined about 1,000 studies, determined there was enough information to support its finding of a link between glyphosate and cancer.

Advocates for farmers say California’s plan to require warning labels for glyphosate-based products is wrong-headed. At a June hearing, Cynthia Cory, environmental affairs director for the nonprofit California Farm Bureau Federation, told the board of the health hazard assessment agency that the herbicide is an important tool for farmers. It ultimately benefits the environment, she said, because “it allows us to reduce our tractor passes, which means you have cleaner air.”

Dr. Michelle Perro, a pediatrician who treats children for glyphosate exposure, offered the board a different viewpoint. “What I am seeing is sicker kids,” she said.

Research suggests that Roundup and other glyphosate-based herbicides may be linked not only to cancer but to a variety of other health problems. Recent studies link the compound to DNA and chromosomal damage in human cells, kidney failure, chronic kidney disease, intestinal disorders, Celiac disease and autism.

About 250 million pounds of glyphosate were sprayed on U.S. crops in 2014, a ninefold increase in just under two decades, according to a study in the journal Environmental Sciences Europe. Two-thirds of all the glyphosate used in the U.S. during the 40 years from 1974 to 2014 was sprayed in the last decade.

And you don’t need to live next to farm fields to be exposed to it, said Dr. Paul Winchester, a clinical professor of neonatology at Indiana University School of Medicine and medical director of the neonatal unit at Franciscan St. Francis Health in Indianapolis. “It turns out it’s in almost every [non-organic] food.”

That concerns him in light of a study that suggests chromosomal damage caused by pesticides has the potential to embed in DNA and get passed down to future generations.

Teri McCall said she applauds California’s decision to list glyphosate as a carcinogen and hopes it will help protect others from the kind of loss she’s suffered.

Since her husband’s death, “it’s kind of like my life of living color has gone to black-and-white,” she said. “My life with Jack was just so full of joy and laughter and fun, and this has just left a huge void. … Every day is just a series of efforts to escape the loss and there’s just no escaping it.”

This story was produced by Kaiser Health News, which publishes California Healthline, a service of the California Health Care Foundation.