People in three states living near large livestock feeding operations experience higher rates of cancer, a new analysis found.

The study, published in the Environmental Research journal, focused on California, Iowa and Texas because of the availability of cancer incidence data, the number of feeding operations in those states and the variety of animals on feedlots.

While the results have raised alarm, the study’s authors note that it doesn’t prove that concentrated animal feeding operations, or CAFOs, are causing the cancers. Meanwhile, industry representatives dispute its findings, pointing out that another study shows lower rates of many cancers near the facilities.

Yale University researchers mapped out the CAFOs in operation and overlaid those charts with county-level cancer data. The research accounted for other factors, like the type of facility, management practices and regulatory enforcement. Their findings have prompted concerns from environmental health advocates over how the facilities impact the surrounding air and water—and by extension human health.

“There are many pathways through which CAFOs could impact cancer, including changes to air quality, water quality, noise, and odor,” Nicole Deziel, a Yale University researcher and co-author of the paper, told Inside Climate News in an email on Friday.

While more research is needed, Deziel said the study “raises important public health concerns given the consistency of findings” across all three states.

Cancers of all types were 4 percent higher in highly exposed California counties and 8 percent higher in highly exposed Iowa and Texas counties when compared to those with lower density of CAFOs, the study found. The researchers defined “high exposure” counties as those in the top 25 percent of CAFO density in their states.

They also correlated CAFO density to higher associations of bladder cancer in California, colorectal cancer in Iowa and lung and bronchus cancer in Texas.

The Yale study is “groundbreaking,” said Amanda Claire Starbuck, research director at the nonprofit Food & Water Watch.

“I don’t think this topic of CAFOs and cancer has really been emphasized enough and explored enough,” Starbuck said..

Instead, studies have focused on pollutants from factory farms, including those documented in the air and water, she said, and the impact of manure released from these facilities on communities.

CAFOs are massive industrial agricultural facilities raising large numbers of livestock that are typically confined to tight spaces to maximize production of meat, milk or eggs.

National estimates show agriculture emits more methane, a potent climate pollutant, than the production of oil and gas. In 2024, about 35 percent of agricultural greenhouse gas emissions in the U.S. came from cattle, which is more than any other farm animal, the Congressional Budget Office notes.

Iowa has the highest number of CAFOs nationally, with federal estimates indicating the state could have nearly 4,000 of the facilities. California is ranked fifth, with an estimated 1,000, and Texas is sixth, with about the same number, according to Environmental Protection Agency figures.

Led by Jiyoung Son, the Yale University researchers reviewed cancer data from 2000 to 2021. They cited the array of harmful pollutants animal feeding operations emit as possible drivers of the higher cancer rates: gaseous emissions like ammonia and hydrogen sulfide, particulate matter, volatile organic compounds and bioaerosols containing endotoxins and antibiotic-resistant bacteria.

“Chronic exposure to these pollutants has been linked to inflammation, oxidative stress, and immunosuppression,” which may contribute to cancer development, study authors wrote.

In addition to impacts to the air, CAFOs can be major water pollution sources. The operations produce large amounts of manure that is often distributed as fertilizer, which can contaminate water with nitrate.

More research—including testing people who live near CAFOs—is needed to understand how different cancer types are affected by things like air and water quality from these feeding operations, said Michelle Bell, the senior author on the Yale study.

The study did not test individual people’s exposure levels to contaminants from CAFOs, the Yale researchers said, and does not prove that CAFOs cause cancer. Moreover, some cancers (such as breast cancer in Iowa and Texas) did not correlate with a high number of CAFOs in relatively small geographic regions.

Industry representatives have cast doubt on the report’s findings.

“In addition to not properly accessing factors like actual pollutant measurements, duration of exposure, facility size, management, or emissions, the study misses other critical cancer-related exposures like occupational exposures, health access/screening rates, diet, obesity, alcohol use, and regional baseline cancer risk differences,” Hannah Thompson-Weeman, the president of the Animal Agriculture Alliance, said Friday after reviewing the study.

What’s more, managers of CAFOs must follow “important environmental regulations,” added Thompson-Weeman, whose group represents farmers, ranchers and food companies.

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The Yale research is not as thorough as other analysis on the same industries, said Wendy Brannen, vice president of communications and marketing for the National Pork Producers Council, which represents more than 60,000 pork operations.

“There is rigorous, long-term research available that has followed real farmers for 30 years and found that Iowa farmers are significantly less likely to develop cancers than the general population, not more,” Brannen said in a statement that referenced a study published in March by the Iowa Cancer Registry.

Yale’s county-level comparisons, which the study’s authors described as “exploratory,” does “not refute” Iowa’s long-term data, Brannen said.

The Iowa Cancer Registry’s report found 13 percent fewer cancers among farmers in the state compared to the general population there.

“They had fewer cases than expected of cancers of the colon and rectum, lung, bladder, oral cavity and pharynx, pancreas, esophagus, larynx, liver, and tongue,” Iowa officials wrote in a summary of the findings. “However, they were diagnosed with more cases of prostate cancer and lip cancer than expected.”

The varied findings in the Iowa study show how complex the issue is, said Starbuck of Food & Water Watch.

“You have to take the whole weight of evidence, and I think that’s kind of the problem here,” Starbuck said. “It’s very easy for industry to cherry pick and be like, well, this study is bad. But look at the study over here, therefore, everything’s fine. That’s not the way science works.”

Lori Pesante, director of the Sierra Club’s Kern-Kaweah chapter in California, found the Yale University study concerning but not surprising, and agreed more research is needed to understand the relationship between cancer and large-scale livestock operations.

When she looks over methane satellite data, the largest plumes are often evident over CAFO feedlots.

“We know that major sources of air and water pollution like oil and gas wells and large feedlots are often built in lower-income areas that lead to disproportionate negative health impacts,” Pesante said.

She called for stronger regulations to monitor and prevent pollution linked to the feeding operations. “Reforming agriculture is a major part of solving the climate crisis, but also making communities healthier.”