485 chemicals in US pesticide products may raise breast cancer risk, study finds

Hundreds of chemicals used in US pesticide products may increase the risk of breast cancer, according to a new peer-reviewed study that highlights what researchers call a regulatory blind spot around “inert” ingredients.

The paper, published online on 23 September in Environmental Health Perspectives, comes from a team at the Silent Spring Institute led by research scientist Jennifer Kay. The researchers matched their list of 920 breast cancer-relevant chemicals — substances that induce mammary tumours in rodents or activate oestrogen or progesterone signalling in vitro — against more than a dozen US government databases on production volumes, uses, environmental releases and levels in people’s bodies.

They found 485 of the chemicals in pesticide formulations, but only about one-third — 145 — are registered with the US Environmental Protection Agency as active ingredients. The rest are present as so-called inert ingredients, which do not have to be listed on labels and are rarely tested for breast effects.

“Our findings highlight a regulatory blind spot around so-called inert ingredients,” Kay said. “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.”

Exposure to the chemicals is ubiquitous, the study found: billions of pounds were emitted to air in 2020, tens of millions of pounds were released to the environment through industrial activities in 2024, and 143 were produced at one million pounds or more in 2019. The same chemicals also turned up in consumer products (500), food (462), drinking water (162), pharmaceuticals (142) and construction materials (106).

The researchers note an earlier Silent Spring study found the EPA had dismissed evidence linking 19 pesticides to mammary tumours, and they call for health-protective policies such as regulating drinking water contaminants and avoiding use in common products.

The study does not prove the chemicals cause breast cancer — presence in the environment or in blood does not establish causation, and individual susceptibility matters — but the authors say the findings provide quantitative evidence to prioritise chemicals for further health study and exposure reduction.