SAN FRANCISCO (CN) - Returning to the witness stand, a scientist on Wednesday defended her opinion that Monsanto's popular weed killer Roundup causes cancer.
UCLA epidemiologist Dr. Beate Ritz was invited back to court after she and other scientists spent a week testifying in March about whether glyphosate, the active ingredient in the world's most popular weed killer, causes Non-Hodgkin's Lymphoma in humans.
Ritz' testimony could advance more than 300 lawsuits filed against Monsanto and consolidated in a multidistrict case in San Francisco. 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 must decide whether there is adequate scientific evidence to support the plaintiffs' claims against Monsanto.
Dismissing arguments that she failed to consider exposure to other pesticides in her analysis, Ritz told Chhabria on Wednesday that "over-adjusting" the numbers can bias the results.
"Even though we have a knee-jerk reaction of put everything in the model, that's the wrong approach," Ritz said.
Illustrating her point, Ritz cited a 1999 study by Dr. Lennart Hardell and Dr. Mikael Eriksson of Sweden, known as the Hardell study. She presented a chart showing that when adjusted for exposure to pesticides, the link between glyphosate and cancer appeared much stronger than when unadjusted.
Because the case study lacks adequate data, adjusting for other pesticides "exploded the model," causing a phenomenon known as "sparse data bias," Ritz said.
"You can't just throw everything in the model and expect to know the truth," she said.
Ritz was responding to criticism from Monsanto and Chhabria, who last month dismissed some of her opinions as "junk science" because she failed to account for exposure to carcinogenic pesticides in her analysis.
Ritz, who based her conclusion on a review of more than a dozen case studies, told the judge that she did review adjusted numbers, but she did not always rely on those sometimes skewed and biased results.
"If it came across like I didn't look at those, then that's not right," Ritz said.
Chhabria posed a hypothetical. He asked Ritz if she were to only focus on numbers that were adjusted for other pesticide use, would she reach the same conclusion that glyphosate causes cancer.
"I did put a plot together with all the adjusted numbers ... and still believe what I said is correct," Ritz replied.
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. 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.
The alleged connection between glyphosate and non-Hodgkin’s lymphoma, along with the science behind it, has been heavily disputed.
In 2015, the World Health Organization’s International Agency for Research on Cancer (IRAC) concluded that glyphosate is “probably carcinogenic to humans.” But other studies, including a 2017 review by the U.S. Environmental Protection Agency, reached the opposite conclusion.
Another scientist, Dr. Christopher Portier, is expected to give additional testimony on behalf of the plaintiffs on Friday.
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