Researchers found that potential breast cancer-related chemical exposures may come from far more sources than previously recognized.
A new study from the Silent Spring Institute has identified hundreds of chemicals in pesticide formulations that could potentially increase breast cancer risk. The list includes both active pesticide ingredients and many so-called “inert” ingredients, which generally receive less safety testing.
“Our findings highlight a regulatory blind spot around so-called inert ingredients,” says lead author Dr. Jennifer Kay, a research scientist at Silent Spring. “When we think about pesticide safety, we tend to focus on the active ingredient. But people are exposed to the entire formulation, and our findings show that many of those other chemicals may also pose a risk.”
Hundreds of Chemicals Identified
The study, published in Environmental Health Perspectives, builds on earlier research that identified more than 900 chemicals that may increase breast cancer risk. Those chemicals were flagged because they caused mammary tumors in animals or altered hormones associated with breast cancer.
For the new analysis, researchers compared that list with data from more than a dozen U.S. government databases. They found 485 potential breast cancer-related chemicals in pesticide formulations, including 145 currently registered with the U.S. Environmental Protection Agency (EPA) as active ingredients.
“Not only does this suggest that active ingredients in pesticides are not being thoroughly tested for their effects on the breast, but that hundreds of other potentially risky ingredients in pesticide formulations are flying under the radar as supposedly inert ingredients,” says Kay.
Exposure Extends Beyond Pesticides
The researchers also found chemicals on the list in consumer products, food, drinking water, pharmaceuticals, and construction materials. These findings show where the chemicals occur, but they do not prove that individual products or exposures cause breast cancer in humans.
Federal biomonitoring data show that 84 of the chemicals have been detected in people’s bodies. Organophosphate pesticides (like malathion, which is commonly used on crops) were found in more than 90 percent of the U.S. population, while DDT was detected in more than 40 percent despite being banned decades ago.
Researchers Call for Broader Testing
The study does not show that any single identified chemical will cause breast cancer. Instead, it maps chemicals with experimental or biological evidence relevant to breast cancer and identifies common sources of exposure.
“EPA continues to approve new pesticides without adequate oversight and with little consideration of their potential effects on breast cancer,” says co-author Ruthann Rudel, Director of Research at Silent Spring. “Breast cancer can take years to develop, so tracking and reducing people’s exposure to pesticides from multiple sources is an essential part of cancer prevention and is necessary to protect public health.”
The authors say the findings could help guide additional testing, stronger evaluation of complete pesticide formulations, and future studies of workers and communities with high chemical exposures.
Reference: “Guiding Risk Reduction for Breast Cancer-Relevant Chemicals (BC-Chems): Chemical Production Volumes, Sources, Intake Levels, Emissions, and Regulations” by Jennifer E. Kay, Rashmi Shakti, Jillian T. McEneaney, Claudia Polsky, Megan Schwarzman and Ruthann A. Rudel, 23 September 2026, Environmental Health Perspectives.
DOI: 10.1021/EHP.6c00022
Funding for this project was provided by the National Institute for Environmental Health Sciences of the National Institutes of Health (grant #K01-ES036182), the California Breast Cancer Research Program (grant #23QB-1881), and charitable donations to the Safer Chemicals Program at Silent Spring Institute.
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