Are the pesticides around us evaluated comprehensively enough? This is the question raised by a new study published on September 23, 2026 in the journal Environmental Health Perspectives. Researchers at the Silent Spring Institute have identified 485 chemicals used in pesticide formulations that are associated with characteristics that may contribute to breast cancer risk.
Be careful, however: the study does not show that these substances cause breast cancer in humans. But it highlights a potential risk in the light of new toxicological and environmental data.
485 substances found in pesticide formulations
The researchers cross-referenced data from numerous American databases. And were interested in a category of substances called “BC-Chems”, for “breast cancer-relevant chemicals”. That is to say, substances with characteristics likely to promote the development of breast cancer. In the studies reviewed, some caused mammary tumors in animals. While others could notably disrupt the hormones involved in the development of certain breast cancers.
However, among the 485 substances identified, only 145 are currently registered as active ingredients by the American Environmental Protection Agency (EPA). The others (340, the majority) are notably “co-formulants”, sometimes described as “inert”. And rarely evaluated.
These substances can have different functions: improving the effectiveness of the product, facilitating its application or even contributing to its conservation. But they are less visible than the active ingredient on which regulatory assessments generally focus.
This is precisely what worries Jennifer Kay, researcher at the Silent Spring Institute and lead author of the study.
“Our findings highlight a regulatory gap regarding so-called “inert” ingredients. When we are interested in the safety of pesticides, we tend to focus on the active substance. However, populations are exposed to the formulation as a whole, and our results show that many of these other chemicals can also present a risk..
In other words, according to the researchers, evaluating only the active ingredient may not reflect actual exposure, since the environment and users are in contact with the entire formulation.
“Commercial formulations are much more toxic”
This concern is shared by Professor Michael Antoniou, Emeritus Professor of Molecular Genetics and Toxicology at King’s College London. He says many studies in labs and animal models show a problem in the way we approach the subject.
“THE“commercial formulations of pesticides (…) are much more toxic than the active ingredient alone, which demonstrates the high toxicity of co-formulants”.
For the toxicologist, the problem is particularly due to the fact that long-term regulatory toxicity studies mainly focus on active ingredients, while populations are exposed to commercial formulations as a whole. A “cocktail” effect of sorts.
He too believes that the results of Kay and his colleagues reinforce the need for a risk assessment that takes more into account co-formulants.
Pesticides in our food, our water and certain everyday products
Moreover, the word “pesticide” does not only apply to the agricultural field, or to the workers who handle these products. The researchers identified various very “common” potential sources of exposure:
Food, drinking water, household and gardening products, insect repellents, anti-lice treatments, products against athlete’s foot and even disinfectants are among the products at risk.
Some substances used in pesticides have also been identified in construction materials.
This multiplicity of sources is important, because exposure to a chemical substance does not necessarily occur through a single route.
A link with breast cancer possible, but not yet demonstrated
For Jennifer Kay, these results raise a question that deserves further investigation. Could the presence of these numerous substances in our environment contribute, at least in part, to the evolution of the incidence of breast cancer?
But the researchers do not claim to have demonstrated this link.
As Michael Antoniou points out, biomonitoring studies and epidemiological studies make it possible to identify associations. But cannot alone establish that a given exposure causes disease.
For his part, Professor Oliver Jones, professor of chemistry at RMIT University, considers the study “rather alarmist”. He emphasizes in particular that the authors compiled existing data from cellular and animal models and estimated potential exposures. But did not directly measure the concentrations of these substances in the humans studied.
“The fact that a phenomenon can occur under certain circumstances (for example, high concentrations in an animal model) does not mean that it occurs in daily life.”
The researcher also points out that a substance detected in the environment is not automatically dangerous. The dose, duration and conditions of exposure are decisive in assessing a health risk.
What the study actually shows
The new publication therefore does not allow us to assert that pesticides are responsible for breast cancer. Rather, it highlights a set of substances to monitor and multiple exposure routes. While raising the question of the evaluation of complete formulations.
For Jennifer Kay and her colleagues, the main issue lies there. No longer limit the evaluation of a pesticide to its active ingredient alone when the population is exposed to the entire product.