Blind spot in the control? Researchers suspect numerous chemicals in pesticides to increase the risk of breast cancer.

Researchers suspect that hundreds of chemicals, including those used in pesticides, may increase the risk of breast cancer. This applies to both the active ingredients and the inert ingredients in pesticides, according to a new study from the American research institute Silent Spring Institute. Several of these are also found in pesticides used in Denmark.

The incidence of breast cancer is rising worldwide, and this cannot be fully explained by known risk factors such as smoking, overweight, genetics, and women having children at a later age. Better diagnostics do not explain it either. This suggests that there is something else at play.

Here at home, breast cancer is the most common cancer among women, with about 5,200 new cases each year, and according to Johnni Hansen, senior researcher at the Danish Cancer Society, an increase in cases is also observed in Denmark, especially among women under 45, which cannot be explained by the known risk factors.

"Therefore, it is naturally interesting to search for explanations for the large proportion of breast cancers for which we still do not know the cause," he tells Organic Now. He was not part of the research team behind the study.

He finds the study interesting because it uses a new method to identify substances that may be relevant to breast cancer:

"But it is also a scientific weakness to use unproven robust methods. Moreover, it is interesting that the study focuses on the 'total pesticides' and not just on the 'active ingredients'," he says to Organic Now.

Is this enough for women or cancer patients to consider buying organic instead of conventional based on a precautionary principle?

"One can interpret the study as supporting a political precautionary principle to avoid certain chemicals, including pesticides, which can be done by eating organic food instead of conventional food."

A "blind spot"

To understand the new findings, we need to rewind two years. In 2024, Jennifer Kay, who researches the carcinogenic effects of chemicals at Silent Spring Institute, co-published another study that identified 920 different substances, all suspected of increasing the risk of breast cancer.

The suspicion is based on their ability to induce tumors in the mammary glands of animals or disrupt hormone balance in ways that promote breast cancer. For example, when they cause cells to produce more estrogen or progesterone—hormones that play a role in the development of breast cancer.

She and her colleagues set out to find the sources of these potentially harmful substances, and in this context, pesticides appeared.

They discovered that 485 of the 920 substances are either demonstrably or likely to be found in pesticides as active or inert ingredients. Of these, 145 are currently approved as active ingredients in the USA, while 77 are inert ingredients that are or have been approved. Some substances can be both inert and active ingredients.

"Our results highlight a regulatory blind spot regarding 'inert' ingredients," says Jennifer Kay in a press release.

"When we think about pesticide safety, we usually focus on the active ingredient, but people are exposed to the entire mixture, and our results show that many of the other chemicals may also pose a risk," she continues.

Inert ingredients are added for various reasons; to make the active ingredient water-soluble, to help the plant or pest absorb the product faster, or to extend shelf life.

"This not only suggests that the active ingredients in pesticides are not thoroughly tested for their effects on the breast, but also that hundreds of other potentially risky substances in pesticide mixtures slip by unnoticed as supposedly inactive ingredients (inert ingredients)," says Jennifer Kay.

Using AI tool Claude, Organic Now has cross-checked the substances in the American study with the Danish Environmental Protection Agency's list of approved pesticides (as of September 29) and verified the results. At least 30 of the substances are used as active ingredients in pesticides and biocides approved in Denmark. Four of them have caused tumors in the mammary glands in animal tests.

Tumor finding dismissed

The American authorities know the full composition of pesticides because manufacturers must disclose it when a product is approved, but there is variation in how much the authorities weigh different parts of the data.

In 2020, researchers from Silent Spring Institute reviewed over 400 documents from the US Environmental Protection Agency (EPA) describing health effects of registered pesticides. They found 28 pesticides linked to tumors in the mammary glands in animal studies.

"Breast cancer is strongly influenced by sex hormones, which cause cells in the breast to divide and make them more susceptible to tumors. Therefore, it is important that authorities consider this kind of evidence. If they do not, they risk exposing people to pesticides that are carcinogenic," said Ruthann Rudel, chief researcher at Silent Spring Institute, at the time.

However, the EPA only recognized the increased risk for nine of the 28 substances and dismissed the connection for the remaining 19.

Hundreds of thousands of tons emitted into the air

In addition to the substances in pesticides, the researchers identified 500 potentially breast-cancer-causing substances in consumer products, 462 in foods, 162 in drinking water, 142 in medicines, and 106 in building materials.

The researchers state in the study that the suspected breast cancer chemicals are so widespread that exposure to them is "omnipresent": In 2020 alone, hundreds of thousands of tons of these substances were emitted into the air.

"When it comes to cancer, we need to look at total exposure. People are exposed throughout their lives to hundreds of chemicals from many sources, and these exposures add up. But authorities do not assess these exposures collectively," says Jennifer Kay.

According to her, another problem is that endocrine-disrupting chemicals can cause damage at low doses but not necessarily at high doses. When authorities test chemicals at high doses, they may overlook risks associated with the small amounts we are exposed to daily, she explains in an interview with The Guardian.

Traditionally, it is assumed that the higher the dose, the greater the effect. Therefore, chemicals are tested at high doses on laboratory animals. The dose at which no effects are observed is determined, and a safety margin is set below that. The logic: if the chemical is harmless at high doses, it is also harmless at low doses.

However, this is not necessarily the case.

The body's hormones work in extremely small amounts. Substances that mimic hormones can therefore have effects at very low doses. At high doses, the effect may disappear or change. This can happen because hormone receptors become saturated or "shut down," or because other toxic effects overshadow the hormone effect. This is called non-monotonic dose-response.

The EU's food safety authority EFSA has previously confirmed that non-monotonic dose-response occurs in certain studies. The phenomenon is especially relevant for effects that act via receptors, i.e., the way hormones and hormone-like substances work. If the effect occurs at low doses, it can influence current risk assessments, according to EFSA's scientific committee.

Co-author Ruthann Rudel, research director at Silent Spring Institute, also advocates minimizing people's pesticide exposure:

"Breast cancer can take years to develop, so monitoring and reducing people's exposure to pesticides from various sources is a crucial part of cancer prevention, and necessary to protect public health," she states in the institute's press release.

If consistently choosing organic is not an option, an alternative is to only replace foods that are sources of the highest pesticide residues. This typically includes fruits, vegetables, and grains, according to surveys from the DTU Food Institute.

Praise and criticism

Michael Antoniou, emeritus professor of molecular genetics and toxicology at King's College London in England, who did not contribute to the study himself, praises it as highly relevant and well-conducted.

"There is now extensive evidence from laboratory studies with cultured cells and—more importantly—in animal models relevant to legislation, including research from my own research group, showing that commercial pesticide mixtures are far more toxic than the active ingredient alone—which proves that inert ingredients are highly toxic. Nevertheless, government regulatory agencies have not yet fully incorporated inert ingredients into their pesticide risk assessments," he tells Science Media Centre.

Oliver Jones, a chemistry professor at RMIT University in Australia, is skeptical and calls the study "alarmist," partly because they have not measured the concentrations to which people are exposed.

"For example, both alcohol and UV rays in sunlight are carcinogenic substances in class 1. This does not mean that a glass of wine or going outside will give you cancer. Similarly, it does not automatically mean that something is harmful just because it is present in the environment. A chemical can be detected without it necessarily causing cancer," he explains and continues.

"It is also wrong to say that risk assessments do not consider the full mixture. Chemists and toxicologists are aware that more than one chemical exists simultaneously in the world. That is partly why they incorporate large safety factors into chemical risk assessments."

The industry organization American Chemistry Council, representing nearly 200 companies in the chemical industry, argues that there are several uncertainties in the new study:

"The authors themselves point out that many of the exposure estimates used in the analysis are based on predictive models built on limited data and assumptions and are subject to significant uncertainty," says a spokesperson to Inside Climate News.

The study is published in the journal Environmental Health Perspectives.

Organic Now has contacted the Danish Environmental Protection Agency with several questions, but the agency has not responded before the deadline.