SAN FRANCISCO (CN) – Kicking off a week of testimony on the alleged toxicity of the world’s most popular weed killer, a scientist on Monday attacked the validity of a study that found Monsanto’s herbicide Roundup is unlikely to cause cancer.

Dr. Beate Ritz, an epidemiologist from UCLA, was the first of 10 scientists set to testify this week about whether glyphosate, the active ingredient in Monsanto’s Roundup herbicide, has caused farmers and landscapers to develop non-Hodgkin’s Lymphoma.

The science has been heavily disputed. In 2015, the World Health Organization’s International Agency for Research on Cancer concluded that glyphosate is “probably carcinogenic to humans.” Two years later, the United States Environmental Protection Agency contradicted that finding with a draft risk assessment finding glyphosate “is not likely to be carcinogenic to humans.”

The fate of more than 300 lawsuits filed against Monsanto and consolidated in a multidistrict case in San Francisco hinges on whether plaintiffs’ experts are allowed to testify before a jury on studies that show a link between the herbicide and cancer.

The suits were filed by farmers and landscapers who say exposure to Roundup’s active ingredient caused them to develop cancer of the lymph nodes, organs crucial for a properly functioning immune system.

U.S. District Judge Vince Chhabria will decide what expert testimony a jury should be allowed to hear after a week of testimony from scientists on both sides of the issue.

Ritz, the first scientist to testify this week, warned that a study finding no significant link between glyphosate and non-Hodgkin’s Lymphoma can’t be trusted due to several design flaws.

The Agricultural Health Study, funded by the U.S. government and published in November 2017, surveyed 54,251 people that used pesticides, asking about their rates of exposure to glyphosate and other pesticides and assessing the impact on their health over two decades.

Ritz said the study only asked participants about their exposure to pesticides in two questionnaires over a 12-year period between 1993 and 2005, estimating the exposure for most years. The study also relied on the participants’ flawed memories and likely misclassified certain participants as exposed or unexposed due to those errors, she said.

“I have to downgrade the importance of the AHS study for time exposure,” Ritz said. “I can’t take this study seriously if it shows no effect because all the effects are drowned in the noise of exposure misclassification.”

Additionally, because glyphosate became so ubiquitous by 2014, the study could no longer credibly estimate its impact on health, she said.

“As soon as exposure becomes ubiquitous, it’s hard to determine what it does,” Ritz said. “It’s just like cellphone use. Once everyone is using cellphones, we can no longer estimate brain cancer risk from cellphones.”

After analyzing several other case studies that investigated potential links between glyphosate and cancer, Ritz said she concluded “that to reasonable degree of scientific certainty, glyphosate-containing products do cause Non-Hodgkin’s Lymphoma.”

On cross examination, Monsanto attorney Joe Hollingsworth challenged Ritz’s assessment, asking if it was true that the AHS study includes “more exposed Non-Hodgkin’s Lymphoma cases” than all the controlled case studies combined that support her conclusion.

“When we have everyone getting exposed, having the most exposed cases is not necessarily a good thing,” Ritz replied.

The expert testimony is expected to continue through Friday. Dr. Dennis Weisenburger, a pathologist from the City of Hope Medical Center, is expected to take the stand next.

The weeklong scientific debate comes on the heels of a state court’s decision last week to block California from making Monsanto place cancer warning labels on its Roundup products.

Glyphosate/Roundup is the most widely used agrichemical in history. Monsanto introduced it in 1974, and its use exploded after Monsanto introduced “Roundup-ready” seeds — engineered to resist glyphosate — in 1996. Since then, Monsanto has gained billions more dollars from selling its patented Roundup-ready seeds than Roundup itself.

According to publicly available information, Monsanto earns $1.9 billion a year from Roundup, and $10.2 billion from “seeds and genomics,” most of that category being Roundup-ready seeds. Monsanto controls 80 percent of the U.S. market for genetically modified seed corn, and 93 percent of the market for genetically modified soybeans. More than 2.6 billion pounds of glyphosate was spread on U.S. farmlands and yards between 1992 and 2012, according to the U.S. Geological Survey. Alarms were raised last fall when scientists reported that the amount of glyphosate in human urine has increased by 500 percent since Roundup-ready seeds were introduced.

In an email, Monsanto’s vice president of global strategy Scott Partridge said science backs up Monsanto’s belief that there’s no cause for alarm when using Roundup. “More than 800 scientific studies, the U.S. EPA, the National Institutes of Health and every regulatory agency around the world have concluded that glyphosate is safe for use. No regulatory agency in the world has concluded that glyphosate is carcinogenic. Glyphosate-based herbicides have a 40-year history of safe use and have been transformative for environmentally sustainable agricultural practices. We look forward to presenting this scientific evidence to the court this week and helping the court understand that there is no credible evidence to support the plaintiffs’ claims,” Partridge said.

Monsanto, based in St. Louis, employs 24,100 workers and earned nearly $14.5 billion in annual revenue as of May 2017, according to Forbes.