Hundreds of chemicals used in pesticides linked to increased breast cancer risk

Majority of chemicals considered “inert” or inactive ingredients that evade safety testing

A new study by Silent Spring Institute has identified hundreds of chemicals commonly used in pesticide products that could increase the risk of breast cancer. The list includes active pesticide ingredients as well as a significant number of so-called “inert” or inactive ingredients, which typically do not undergo the same level of 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.”

The findings, published in Environmental Health Perspectives, are part of a larger systematic analysis—the first of its kind—to identify where potential breast cancer-causing chemicals (“BC-Chems”) show up in everyday consumer products, the environment, and the economy, as well as the major sources of exposure.

Breast cancer is the most commonly diagnosed cancer in the United States and worldwide, with rates continuing to rise. Rates are rising the fastest among women under 40, many of whom are diagnosed with breast cancer before routine screening is recommended, suggesting that increased screening cannot explain the steep rise in cases. Growing evidence points to environmental exposures as an important contributing factor.

In 2024, Kay and her colleagues published a landmark study identifying more than 900 chemicals that may increase breast cancer risk based on their ability to cause mammary tumors in animals or disrupt hormones in ways that promote breast cancer. More than half of the chemicals on the list cause cells to produce more estrogen or progesterone, hormones known to play a role in breast cancer development.

In the new study, the Silent Spring team tried to pinpoint the major sources of exposure to BC-Chems. To do that, the researchers analyzed data from more than a dozen U.S. government databases on chemical production volumes, uses, environmental releases as well as levels of the chemicals in people’s bodies. These included the National Emissions Inventory, the Toxics Release Inventory, and the National Health and Nutrition Examination Survey (NHANES).

The analysis highlighted pesticide products as an important source of exposure to BC-Chems. Of the 485 chemicals found in pesticide formulations, about one-third (145 chemicals) are 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.

An earlier Silent Spring study found EPA dismissed evidence linking 19 pesticides with mammary tumors, underscoring the need to strengthen EPA’s regulatory approach to better protect against chemicals that contribute to breast cancer.

People can be exposed to pesticides in multiple ways, including through food and drinking water, as well as through products used around the home and garden, such as insect repellents, household pest-control products, lice and athlete’s foot treatments, and sanitizers. Pesticides are also used in many building materials.

In addition to pesticides, the researchers identified several other important sources of exposure to BC-Chems, including consumer products (500 chemicals), food (462), drinking water (162), pharmaceuticals (142), and construction materials (106).

“When it comes to cancer, we need to think about the totality of our exposures,” says Kay. “People are exposed to hundreds of chemicals from multiple sources over the course of their lives, and these exposures add up. But regulators don’t consider these exposures together.”

Data from the National Emissions Inventory revealed that more than 2 billion pounds of BC-Chems were released into the air in 2020. According to NHANES, 84 BC-Chems have been detected in people’s bodies. However, the researchers say this number is an underestimate because the federal government does not monitor most BC-Chems in the general population.

Organophosphate pesticides (like malathion, which is commonly used on crops) were found in more than 90 percent of the U.S. population. DDT, which was banned in the U.S. decades ago, was detected in more than 40 percent of the population, highlighting the persistence of some chemicals in the environment and in the body.

The researchers hope their findings will inform policies to strengthen how chemicals are regulated in the U.S. and ensure that pesticide formulations—including both active and inactive ingredients—are properly evaluated for their potential to cause breast cancer.

“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.”

One potential policy response is to expand the list of chemicals under California’s Proposition 65, which requires warnings for chemicals known to cause cancer or reproductive harm. For instance, 16 of the pesticides identified in the study are not listed under Prop 65. Adding these chemicals could provide one mechanism for discouraging companies from using them in products.

Rudel says the data could also be used to guide future studies that focus on highly exposed workers or people living in communities with high exposures to BC-Chems from air pollution or pesticide drift. This is particularly important as the workforce changes and more women enter occupations that expose them to chemicals that could increase their risk of breast cancer.

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.