A new peer-reviewed study has identified at least 485 chemicals in pesticide products sold in the United States that are linked to breast cancer, sharpening concerns over how Washington regulates substances found in food, homes and gardens.

The paper, published in Environmental Health Perspectives, was authored by researchers at the Silent Spring Institute, a US non-profit. It examines active pesticide ingredients as well as hundreds of so-called inert compounds that rarely face a full safety review.

Only about 145 of the chemicals are registered with the US Environmental Protection Agency (EPA) as active ingredients. The rest are largely inert additives, used as surfactants or penetrating agents that help active compounds spread and absorb. The EPA has approved roughly 4,000 such ingredients, including PFAS, known as "forever chemicals."

Jenny Kay, a co-author at the institute, said the label is misleading, since these substances are called inert only because they do not target pests, not because they are harmless to the body. "It is a regulatory blind spot," she told The Guardian.

Exposure extends well beyond farms

The chemicals appear in crop treatments but also in children's lice remedies, home lawn fertilizers, and flea and tick products.

Data from the National Health and Nutrition Examination Survey show 84 of the compounds are already present in the US population, though researchers say the true number is higher because the survey does not track most of the substances the institute flagged.

The institute's wider project has also found about 500 breast cancer-linked chemicals in consumer goods and 462 in drinking water.

Regulators assess ingredients in isolation

The EPA reviews each ingredient on its own rather than the full formulation or combined exposure to several products. In 2023, it rejected a legal petition demanding stricter review of inert ingredients.

Kay added that the agency has dismissed the risks of some hormone-disrupting chemicals because they can cause harm at low doses but not at higher ones, a pattern known as non-monotonic dose response.

Industry influence is also a concern. The heads of the EPA's pesticide and chemical safety offices are former industry lobbyists, raising concerns about industry pressure and the revolving door between the agency and the companies it oversees.

Breast cancer is the most commonly diagnosed cancer in the US after skin cancers. Incidence has climbed about 1 percent a year, according to the American Cancer Society, with a slightly faster rise of 1.4 percent among women under 50.

The institute urged California to expand Proposition 65, which requires warnings for chemicals known to cause cancer or reproductive harm. Sixteen of the pesticides identified are not on the list, and the authors say adding them could discourage companies from using them.

A recent report also found that about 25 percent of US water intake, and some 675,000 wells, draw on water contaminated with atrazine, a hormone-disrupting chemical linked to breast cancer. Water filters using activated carbon or reverse osmosis are generally the most effective, though none are perfect.

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