What’s Driving Iowa’s Outlier Cancer Rate? It’s Complicated

In Iowa, a state with a population of around 3.2 million, around 6,000 people will die from cancer in a given year.

Each year, the Iowa Cancer Registry is tasked with reporting on the status of the disease within the state. In 2024, their report made national headlines because of Iowa’s outlier status as the state with the fastest growing cancer incidence and the second highest incidence rate behind Kentucky. The state registry chose to focus its report on the carcinogenic effects of alcohol and high rates of binge drinking in the state, with no mention of the millions of gallons of factory farm manure pumped into Iowa waterways each year.

In its 2024 “strategies to significantly reduce the burden of cancer in Iowa” section, the Iowa Cancer Registry did not mention agricultural pollution once.

In Iowa, animals in factory farms (around 55 million chickens, 53.4 million hogs, 11.5 million turkeys, and 3.7 million cattle and cows) produce around 109 billion pounds of manure. Some of that manure is sprayed on crops as fertilizer. Some of it is illegally discharged from manure lagoons into public waterways. Pesticides and other chemicals are also sprayed on crops and can leach into groundwater. These non-behavioral exposures to carcinogens are causing some experts — including those criticized for not doing enough — to call for further research into the intense industry that uses the vast majority of land in the state.

The issue with cancer, like many other diseases, is that causation is difficult to tie to one source, says Peter Thorne, professor of Occupational and Environmental Health at the University of Iowa College of Public Health. If someone was diagnosed with bladder cancer who lived in an area with high levels of arsenic (a known human carcinogen) in their drinking water, but that person was also exposed to chemicals in their water, and pesticides, and might also eat grilled meat, exactly what caused the bladder cancer is difficult to prove. Another carcinogenic exposure pathway in Iowa is radon, a colorless, odorless radioactive gas that is present in Iowa due to prior glacial activity.

“In a case like that, you can’t say this person’s bladder cancer derived from that one exposure,” Thorne tells Sentient. “You don’t have sufficient data on that person’s lifetime exposure to say that, and even if you did, you can’t be sure that in their case, [one thing] caused it.”

The Nitrate Problem

Despite the difficulty to determine causation for most cancers, one carcinogenic substance is the devil lurking beneath the surface in agricultural states like Iowa. Nitrate is an agricultural byproduct: Ammonia in natural and synthetic fertilizer is converted by soil bacteria into highly water-soluble nitrate through a process called nitrification. If over-applied, nitrate leaks into aquifers and can contaminate water systems.

Throughout the last two decades, the link between nitrate exposure and cancer has become more well-defined. According to Brandi Janssen, clinical associate professor in the Department of Occupational and Environmental Health at the University of Iowa College of Public Health, the colorectal cancer linkages to nitrate are “fairly substantial.” Colorectal cancer is the one of most common cancers in Iowa, the Registry reports.

In internal emails reviewed by Sentient, in an email to Senator Charles Grassley ahead of a meeting with his office in D.C., Iowa Cancer Registry director and principal investigator Mary Charlton describes the “knee jerk” reactions people have to the potential carcinogenic risks of agricultural contamination:

“As you can imagine, it has been a tricky road to alert Iowans about our high cancer risks and try to focus on mitigating the known risks for cancer, while navigating the barrage of knee jerk reactions many people have about it being due to chemicals in the water or the pesticides, nitrates, etc,” Charlton wrote in an email on May 13, 2024.

In a request for comment, Charlton clarified to Sentient that she was not dismissing potential agricultural exposures. “Any time we promote established public health messages about any lifestyle factors that have been demonstrated to contribute to cancer, we are perceived by some as trying to evade the role of agricultural exposures.”

She continued: “People are certainly right to be concerned, and we feel strongly that more research needs to be done to better quantify the contribution of agricultural exposures to our cancer incidence while taking into account the other known risk factors at the individual level such as tobacco use, physical inactivity, poor diet, alcohol consumption, ultraviolet light exposure, and infectious agents, to name a few.”

Based on all of the questions her office has received about environmental exposures, Charlton wrote to Sentient that she asked the University of Iowa Environmental Health Sciences Research Center to consider making fact sheets on nitrate and other environmental risk exposures in Iowa, which they have done.

The nitrate fact sheet states that exposure to nitrate is a probable human cancer risk. A 2020 systematic review of the literature on nitrate contamination and water found that, “there is an association between the intake of nitrate from drinking water and a type of cancer in humans.”

The current legal limit of nitrate in public drinking water is 10 mg/L, a standard set to protect infants from methemoglobinemia, also known as blue-baby syndrome. Keeve Nachman, Professor of Environmental Health and Engineering at Johns Hopkins and Associate Director of the Center for a Livable Future, tells Sentient that a lot has changed in nitrate research since the Environmental Protection Agency first established those limits.

“As with many chemicals, it’s important to go through the process of systematically evaluating the evidence and drawing conclusions from a formal analysis. I also think it’s really important that we don’t wait,” he says.

A recent report by the non-profit advocacy group Food & Water Watch details the extent of nitrate pollution in Iowa and the effect this pollution has on the health outcomes of its residents. As Sentient has previously reported, Iowa is a state where illegal manure discharges are the norm, and attempts to increase regulation enforcement have fallen short. These discharges are impairing waterways and contributing to what some are calling a water quality crisis.

On February 27, President Donald Trump announced that the Environmental Protection Agency would be cutting spending by 65 percent. Mary Grant, Water Program Director at Food & Water Watch wrote in a statement that, “Such a cut would have a devastating impact on the critical clean water and environmental programs that communities rely on each year to fix broken drinking water systems, stop sewage spills and clean up toxic sites.”

Nitrate contamination, Thorne says, “is one of the major exposures that people point to as perhaps contributing to the high rate of cancer that we see in Iowa.”

Other Agricultural Risks

Other agricultural contaminants are contributing to Iowa’s worsening water, and health, crisis. Pesticides, which encompass herbicides and other chemicals, are another likely culprit.

The term “pesticides” encompasses a barrage of chemicals, Janssen explains.

“We like stories that are straightforward. One thing causes another, and unfortunately, that’s just not the way it works in cancer. It’s not the way it works in environmental health. We have multiple exposures that can cause multiple different types of health outcomes,” she says. “When people comment on, say, pesticides causing cancer in a particular setting, it’s like, well, what kind of pesticide and what kind of cancer are you talking about?”

For Elise Pohl, a researcher on concentrated animal feeding operations and a community health consultant at the Iowa Department of Health and Human Services, thinking about exposure in a place with a lot of environmental toxins complicates the picture.

“It just kind of makes you think about the environment and ecosystem that you live in and what you’re exposed to,” she says. “Not just the behaviors that you have, or the risk factors that one may have personally, whether they’re obese or they drink alcohol or smoke cigarettes, but also this external environment whether it’s in their homes or outside their homes.”

A new law advancing in the Iowa legislature would ban cancer victims’ ability to sue pesticide manufacturers; the law was written by Bayer (which acquired Monsanto in 2018).

The Future of the State

In January, Iowa Governor Kim Reynolds, whose husband is currently in remission for lung cancer, vowed to allocate $1 million toward cancer research.

“Every case of cancer is a tragedy. And I’m concerned by the data showing that these tragedies disproportionately affect Iowans. Our state has ranked second for new cancer cases two years running, and we’re one of just two states with rising rates,” she said in her 2025 Condition of the State address. “That’s the ‘what’ of this problem; the ‘why’ and the ‘how’ are where things get tricky. According to the Iowa Cancer Registry, we’re in the top-five states for binge-drinking.”

In an email to Sentient, Charlton wrote that the Iowa Cancer Registry “will be partnering with investigators at the National Cancer Institute who direct the Agricultural Health Study – which involves over 50,000 pesticide applicators in Iowa — to get their assistance in summarizing their findings on agricultural exposures and cancer in a future report.”

Additionally, Charlton wrote that she has been working with the University of Iowa Holden Comprehensive Cancer Center to create a panel of epidemiology experts from across the country, “including people with expertise in environmental risk factors, to get their recommendations on studies and analyses that should be undertaken to better understand the driving factors behind our high cancer rates in Iowa.”

Looming above all of this is the constantly changing federal funding landscape, which could impact cancer research. Richard Deming, medical director of the MercyOne Richard Deming Cancer Center in Des Moines, Iowa said at a press conference in Iowa City that he has already seen the cuts impact work in the state.

“We have over 60 clinical trials that are open at our cancer center for enrolling patients. Just two weeks ago, one of them was closed. It happened to be a clinical trial that was looking at special needs of the LGBTQ community, and we were just told it’s closed,” he said. “Just two weeks ago, one of our workers who had been with us for about a year and a half received a pink slip because she was still under the probationary period.”

Mark Burkard, professor of internal medicine in the University of Iowa Carver College of Medicine and Director of Holden Comprehensive Cancer Center, is especially worried about how universities will “train the next generation” without continued funding or philanthropy to fill in the gaps.

“Those graduate students who would have been conducting the research now and over the next few years would be the research leaders who are investing in that career 10 years from now. So I’m very concerned about how this is going to play out,” he said.

The 2025 Iowa Cancer Registry report, released on February 25, focused on cancer survivorship.

“It’s a great day to be alive,” Deming said at the press conference. He announced that there will be about 171,000 cancer survivors in Iowa this year — the mortality rate is decreasing as quality of care increases, he said.

“Forty-five percent of cancers in the United States are caused by modified risk factors, things that we have control over,” Deming said. “The type of food we eat, physical activity, alcohol consumption, whether or not you get an HPV vaccination, radon, environmental chemicals, all of these things contribute to the 45 percent of all cancers that are caused by modifiable risk factors.”

At the press conference in Iowa City, Deming also said that more research needs to be done on “Iowa’s especially high incidence of cancer,” by looking at environmental factors like radon and chemicals.

One “philosophical issue” Deming is especially interested in is helping cancer survivors with is their mental wellbeing:

“How do you find joy in life knowing that it’s going to end from cancer?”