A new analysis links high use of the weed killer glyphosate to elevated rates of non-Hodgkin lymphoma (NHL), particularly in the Midwest, reinforcing years of research linking cancer to the weed killer made popular by Monsanto.

The analysis by Food & Water Watch (FWW), a nonprofit public health advocacy group, looked at counties that spray the highest amounts of glyphosate herbicides in the country, focusing on those in the top 20% of glyphosate use on commodity crops.

The group then overlapped that data with NHL incidence rates. Among the findings, the group said 60% of counties that had high glyphosate use had NHL rates above the national average.

The greatest overlap between glyphosate application and NHL was in the Midwest, a key US farming region.

A map of the hotspots shows clusters of NHL rates particularly high in many parts of Iowa, the nation’s top corn-growing state and among the top five states for growing soybeans. Both crops have both been genetically altered to tolerate being sprayed with glyphosate.

Iowa has the second-highest rate of cancer in the nation and is only one three states where cancer is rising, according to the National Institutes of Health. The FWW analysis found that 82% of high glyphosate spray areas had elevated NHL incidence rates.

FWW said it based its analysis on data from the US Department of Agriculture’s (USDA) Agricultural Chemical Use Program and the National Cancer Institute’s State Cancer Profiles database. The map displays only those counties that are in the top 20% for total glyphosate applied per square mile, and that also have county-level data for NHL incidence, FWW said.

“It is compelling that many of the counties that use the most glyphosate are also ‘hot spots’ for non-Hodgkin lymphoma — a cancer associated with glyphosate exposure,” said FWW Research Director Amanda Starbuck.

Sarah Green, executive director for the Iowa Environmental Council, which has an initiative underway to explore the relationship between environmental risk factors and cancer rates in Iowa, said the new analysis was not surprising.

“We know cancer rates in Iowa have reached crisis levels, and growing research shows that Iowans face outsized risks from exposure to agricultural chemicals, including glyphosate, atrazine, acetochlor, and nitrates from fertilizers and manure application,” she said. “What’s especially notable is that Iowans aren’t exposed to just one risk factor, we’re often exposed to several at the same time.”

Green cautioned that correlation does not equal causation, and said there is a need for more comprehensive research into the health impacts of environmental exposures.

Glyphosate was introduced by Monsanto in the 1970s and has long been a widely used herbicide in agriculture, as well as in an array of commercial and residential uses. Commonly known as the Roundup branded product, use of the chemical skyrocketed after Monsanto introduced genetically altered crops designed to tolerate glyphosate, making it easy for farmers to kill weeds in their fields without harming the genetically engineered crops.

As use rose, so did scientific scrutiny of the potential environmental and human health risks of the chemical.

In 2015, the International Agency for Research on Cancer, part of the World Health Organization, classified the pesticide as probably carcinogenic to humans, noting ties to NHL.

In 2019, researchers published a study finding that people with high exposures to glyphosate herbicides had a 41% increased risk of developing NHL, calling the evidence “a compelling link” between the chemical and the cancer.

Several scientists are gathering later this month for a symposium to review glyphosate research with respect to potential health impacts.

University of Washington professor Lianne Sheppard, who is organizing the symposium and is an author of the 2019 study, said evidence continues to mount about the potential health hazards of glyphosate.

“Since our study was published, evidence has accumulated that glyphosate-based herbicides cause DNA damage and affect DNA repair mechanisms, both of which can result in cancer, and that blood cancers such as NHL may be the most impacted,” she said.

Tens of thousands of people around the US have sued Monsanto and the German company Bayer, which bought Monsanto in 2018, alleging exposure to the company’s glyphosate herbicides caused them to develop NHL.

Monsanto, and subsequently Bayer, maintain the products do not cause cancer. The Environmental Protection Agency holds the position that glyphosate is “not likely to be carcinogenic to humans.”

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