Largely untested pesticide chemicals could raise breast cancer risk

Clinical takeaway: Patients asking how to lower environmental breast cancer risk can reasonably limit home pesticide use. Exposure is also present through food, drinking water, and OTC products, such as lice and athlete's foot treatments. 

Breast cancer incidence is rising fastest in women too young for routine mammography. Between 2010 and 2019, rates among females aged 15 to 29 climbed 1.85% per year, more than double the pace of any older age group. Screening can't explain a rise in patients who aren't screened, and genetics, obesity, and fertility patterns don't fully account for it either. That points toward the environment, where physicians have had little concrete to offer. 

In 2024, researchers cataloged 920 chemicals that induce mammary tumors in rodents or activate estrogen or progesterone signaling, a list built to flag potential breast carcinogens before human evidence accumulates. A hazard list alone doesn't say where those chemicals reach people. The new analysis fills that in by pairing the hazard list with federal records of where the chemicals are produced, sold, and detected. Pesticide products stand out: much of their contents has drawn little examination for effects on the breast. 

Of the 485 breast cancer-relevant chemicals identified in pesticide formulations, 145 are registered with the US Environmental Protection Agency (EPA) as active ingredients, which means the majority enter products through other routes, with less toxicity review attached. Among them are 77 chemicals the EPA has approved as inert ingredients, more than half with endocrine or genotoxic activity. The active side has its own gaps. Of the 1,060 pesticide active ingredients now registered, 99 (9%) caused mammary tumors in rodents or showed higher-confidence endocrine disruption, a group that includes malathion and atrazine. 

The analysis counted 500 breast cancer-relevant chemicals in consumer products, 462 in food, and 162 in drinking water. Biomonitoring data suggest that many of these chemicals are reaching the general population: metabolites of organophosphate pesticides turn up in the vast majority of the US population, and DDT, banned since the 1970s, was still detectable in more than 40% of people when the National Health and Nutrition Examination Survey (NHANES) last measured organochlorines in 2003 to 2004. 

The researchers mapped their 2024 list of 920 breast cancer-relevant chemicals against federal databases covering chemical production, uses, environmental releases, and NHANES biomonitoring. Chemicals made the list by inducing mammary tumors in rodents or activating estrogen or progesterone pathways in EPA screening assays. The analysis includes no human outcome data. 

Sixteen pesticides that caused rodent mammary tumors, including ametryn and chloropicrin, are absent from California's Proposition 65 list, and adding them would trigger warning requirements that have historically pushed manufacturers away from listed chemicals. At the federal level, the researchers point to TSCA testing orders for chemicals with endocrine and genotoxic activity but no carcinogenicity data, and to the EPA's Unregulated Contaminant Monitoring Rule as a route for tracking the 140 breast cancer-relevant chemicals detected or expected in drinking water but not covered by national standards. 

"Our findings highlight a regulatory blind spot around so-called inert ingredients. When we think about pesticide safety, we tend to focus on the active ingredient," says lead author Jennifer Kay, a research scientist at Silent Spring Institute. "But people are exposed to the entire formulation, and our findings show that many of those other chemicals may also pose a risk." 

Source: Kay JE, et al. (2026 Sep 23) Environ Health Perspect. Guiding risk reduction for breast cancer-relevant chemicals (BC-Chems): Chemical production volumes, sources, intake levels, emissions, and regulations