There's a strong correlation between pesticide use and cancer risk. Counties with the highest pesticide use tend to have above-average cancer rates.
The link is even more clear when specific pesticides are matched with the cancers they’re known to cause, like glyphosate and non-Hodgkin lymphoma and picloram and early-onset colon cancer.
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PICLORAM
When scientists compared the incidence of early-onset colorectal cancer to pesticide use by county, the strongest link was to picloram. Glyphosate, the main ingredient in Monsanto (now Bayer)’s Roundup herbicide, was a close second.
Scientists also used epigenetic fingerprints to pinpoint exposure risks, again finding that early-onset colorectal cancer is consistently associated with picloram.
The correlation between cancer and counties where picloram is sprayed suggests that exposure is environmental. The data the scientists used to determine pesticide use by county came from the National Water-Quality Assessment, which measures pesticide pollution of surface waters. Picloram is found in drinking water in many parts of the country, so it’s likely that’s how people are being exposed, but the scientists noted that “dietary exposure to residues of picloram is plausible as it has been found in grain and meat byproducts, and the effects of long-term use on human health have not been described so far.”
Picloram is an herbicide sold under the brand name Tordon by Corteva Agriscience, a spinoff of Dow-DuPont. The U.S. purchased picloram from Dow for use as a chemical weapon in its war against Vietnam. It was mixed with 2,4-D to create Agent White. Evidence that Tordon is carcinogenic began to surface as soon as it was used at home.
In addition to picloram, Tordon is contaminated with hexachlorobenzene and ethylene oxide, which are known to cause cancer, as well as birth defects, and other reproductive harms.
GLYPHOSATE
Glyphosate is the most-used weedkiller in the country. It’s the main ingredient in Monsanto (now Bayer)’s flagship herbicide Roundup. Use has skyrocketed since the 1990s when Monsanto started selling genetically modified seeds engineered to produce plants that are impervious to its pesticide.
Cancer clusters match patterns of glyphosate use. In Iowa, 95 percent of the counties with the most glyphosate use have more than their fair share of late-stage non-Hodgkin lymphoma diagnoses and 61 percent are non-Hodgkin lymphoma hotspots.
Monsanto had long been aware of glyphosate’s carcinogenicity, and the EPA was, too. In 1985, the consensus position of eight Environmental Protection Agency scientists was that glyphosate was a Class C carcinogen, but Monsanto convinced the EPA to overrule its experts. Relying on studies funded – and manipulated – by Monsanto, the EPA decided that glyphosate was not likely to be carcinogenic to humans.
Eventually, the truth came out. In 2015, the World Health Organization's International Agency for Research on Cancer (IARC) found considerable evidence that glyphosate caused non-Hodgkin lymphoma.
ATRAZINE
In 2025, the second-most-used weedkiller, atrazine, the main ingredient in Syngenta (ChemChina)'s AAtrex, was also found to be a “probable human carcinogen.”
That the top two most-used herbicides are carcinogens explains the mysterious rise of cancer among young adults in the Corn Belt.
Cancer rates for young adults in their 20s, 30s and 40s are trending up even as overall cancer rates decline—with the highest rates in the states that use the most glyphosate and atrazine.
In the six leading states for corn production—Iowa, Nebraska, Illinois, Minnesota, Indiana, and Kansas—rates are 5 percent higher than in the overall population.
A 2024 analysis of population-level data in the journal Frontiers in Cancer Control and Society found that “the impact of pesticide use on cancer incidence may rival that of smoking."
“Iowa has a super high rate (of cancer) and when you look at all of our modifiable risk factors … tobacco, obesity, too many calories, highly processed foods, lack of physical activity, alcohol consumption, getting vaccinated for HPV, sun exposure, and so on, Iowa doesn’t really stand out dramatically at any of those,” Dr. Richard Deming, medical director at MercyOne Cancer Center in Des Moines, told Investigate Midwest. “But one thing that distinguishes Iowa from other states is our environmental exposure to agricultural chemicals.”
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