Iowa farmers are awash in Roundup weed killer.

That was a federal judge’s takeaway after seeing slides that “sort of seared into my brain” that the main chemical in Monsanto Co.’s Roundup is ubiquitous in the Hawkeye state’s cropland.

“I got the impression that everybody takes a shower in glyphosate every day in Iowa,” U.S. District Judge Vince Chhabria said in court, with a hint of hyperbole.

The slides were presented by Beate Ritz, a public health professor at the University of California at Los Angeles who is a witness on behalf of more than 700 farmers, landscapers and gardeners claiming that exposure to glyphosate – through skin contact or inhalation – caused their non-Hodgkin lymphoma.

Chhabria will be the first judge to weigh in on the toxicity of the world’s most popular herbicide, the source of a heated debate among scientists and regulators worldwide for more than 30 years.

He is scheduled to hear final arguments Wednesday about whether Ritz and other witnesses who testified from both sides last week are qualified as experts to present their conclusions to a jury. Chhabria is acting as a gatekeeper to exclude evidence not backed by scientific rigor in an often fact-challenged lawsuit.

Probable Carcinogen

Any key witnesses who are cut from the lineup may profoundly shape the outcome of more than 300 lawsuits collected before Chhabria – all the cases in federal courts that seek to hold Monsanto liable for its failure to warn about the risks of using Roundup.

The lawsuits were filed after glyphosate was declared a probable human carcinogen in an assessment in 2015 by the International Agency for Research on Cancer, an arm of the World Health Organization. In stark contrast, the U.S. Environmental Protection Agency in its most recent evaluation last year said glyphosate is “not likely to be carcinogenic to humans” at current exposure levels.

Monsanto’s dominance of the glyphosate market has declined since its patent covering the chemical expired in 2000 and generic producers stepped in. Nonetheless, the weed killer remains the primary money-maker for its Agriculture Productivity division, which brought in about a quarter of the St. Louis-based company’s total 2017 revenue, or $3.7 billion out of $14.6 billion.

Control Groups

The company contends that lawyers for the plaintiffs have selectively side-stepped data working in its favor and relied instead on “junk science.”

The plaintiffs argue they weighed many different studies, on animals as well as humans, and across disciplines, to conclude glyphosate causes cancer. Ritz emphasized the importance of using control groups – those not exposed to glyphosate or free of cancer – as comparisons.

Chhabria heard from about a dozen witnesses, including toxicologists, statisticians and an oncologist. But he took an especially keen interest in Ritz and Monsanto’s answer to her, Harvard University professor Lorelei Mucci. Both are epidemiologists who study how humans contract disease, a field Monsanto says is the standard by which the cancer claims should be measured. The judge directed lawyers to be ready to focus primarily on Ritz’s opinions and testimony at Wednesday’s hearing.

The company is relying heavily on the National Institute for Health’s Agricultural Health Study, an investigation of 54,000 farmers in Iowa and North Carolina periodically updated over decades. With a 2017 update, the study is the “most powerful scientific evidence that now exists” that glyphosate isn’t linked to cancer, Monsanto says.

Roundup Ready

Ritz used her slides to try to undercut that claim. She showed a map of U.S. use of glyphosate in 1994, two years before Monsanto introduced Roundup Ready seeds – the corn, soybeans, and other crops genetically modified for the primary purpose of making them resistant to Roundup.

She also displayed a 2014 map, which shows that after explosive growth in consumption of Roundup, almost every square mile of Iowa was blanketed in more than 88 pounds of glyphosate in a single year.

“Every farmer must be using glyphosate, and not only using it but using it at a very high level,” Ritz testified. “So in 2014 Iowa, it would not even be possible to estimate any risk from glyphosate anymore.”

Her point: With so much glyphosate in use, it’s impossible to accurately measure exposure levels among Iowa’s farmers, and thus impossible to ascertain glyphosate’s effects on cancer risk.

When Chhabria put that criticism to Harvard’s Mucci, she countered that the farmer study encompassed an ample range of accurate exposure levels. What Ritz termed a flaw, Mucci said, is in fact the “real strength” of the NIH study.

“If something were to be associated with cancer, what you’d expect is a lot more exposure to it would be associated with even stronger risk,” Mucci told the judge. The NIH scientists examined exposure to glyphosate up to 10 times greater than the studies plaintiffs’ relied on, she said.

“But again, we don’t see any association there,” she said. “So it provides some reassurance.”

Mucci also said the plaintiffs haven’t properly accounted for farmers’ exposures to other pesticides, especially when their use is independently associated with cancer. It was a concern Chhabria inquired about repeatedly throughout the week.

“There’s no way to come to a causal conclusion about glyphosate and non-Hodgkin lymphoma,” Mucci concluded.

Chhabria admitted during the hearing that his own math skills “are less than rudimentary,” but he pledged to catch up with the scientific literature. He was joined by a judge from Oakland handling hundreds of similar state court cases against Monsanto. And he invited other judges across the U.S. handling thousands more such state court cases to “make these proceedings part of their record.”

The invitations could make Chhabria’s already sizeable influence even greater. Most mass lawsuits like the case against Monsanto either settle or are resolved by a judge’s ruling before they reach a trial, said Brooke Coleman, a professor at Seattle University Law School.

“Product liability cases often turn on a battle of the experts,” she said. The sheer size of cases like the Monsanto litigation “make one judge’s decision as to experts quite impactful.”

The case is In re: Roundup Products Liability Litigation, MDL 2741, U.S. District Court, Northern District of California (San Francisco).