What is chlorpyrifos

Chlorpyrifos is a widely used organophosphate insecticide that is used for control of several home (e.g., cockroaches, termites, fleas) and agricultural (cattle ticks and crop-destroying insects) pests.[1] Its use has been banned in the European Union, California, New York, and Hawaii, and several other states are considering imposing a ban on use of the pesticide. Its main biological actions are through overstimulation of the nervous system, but chlorpyrifos has also been shown to act as an endocrine disruptor, interfering with both estrogenic and androgenic hormonal pathways. [2],[3] Laboratory studies indicate that exposures to chronic low doses of chlorpyrifos lead to an increased risk of developing mammary tumors.

What evidence links chlorpyrifos exposures to breast cancer?

Wives of pesticide applicators who used chlorpyrifos in their homes had a significantly increased risk of developing breast cancer; the effect was most pronounced for women diagnosed with premenopausal breast cancer.[4]

Adult female rats who were fed low doses of chlorpyrifos for eight weeks developed several reproductive system abnormalities. They also developed abnormalities in their mammary glands that are predictive of increased mammary tumor risk (e.g., higher numbers of terminal end buds [TEB] and ductal branching, larger TEB diameters, higher oxidative stress).[5]

Another set of studies, from a different lab, found similar results: At low doses considered to be nontoxic by federal regulators, exposures to chlorpyrifos of young adult rats led to structural changes in the mammary gland, including increased hyperplasia (high cell proliferation rates), that predict increased risk of later development of malignant mammary tumors.  There were also changes in the expression of both estrogen and progesterone receptors in the mammary ducts of these rats.[6]

In a follow-up study, longer term exposure to low doses of chlorpyrifos led to increased rates of mammary tumors, and a shorter latency to develop tumors in animals who were also treated with low doses of another mammary carcinogen (NMU). NMU is a carcinogen that exerts its cancer-causing effects via the estrogen and prolactin hormonal systems. Epigenetic changes were found in the mammary glands of rats treated with the lowest doses of chlorpyrifos, but not in those treated with higher doses.[7]

As with the results in rats, exposure of cultured human breast cancer cells led to increased cell proliferation. In both hormone receptor positive (MCF-7) and negative (MDA-MB231) cells, chlorpyrifos also led to increased cell migration and changes in molecular targets in areas of the mammary gland that have been implicated in the development of breast cancer.[8]

Who is most likely to be exposed to chlorpyrifos?

Agricultural workers and their families have the highest exposures. Consumers eating fruits, nuts or other crops sprayed with chlorpyrifos will ingest the chemical.

Who is most vulnerable to the health effects of chlorpyrifos?

Anyone exposed to chlorpyrifos is susceptible to the neural and associated effects of the chemical. Research related to breast cancer has only explored the effects of exposures in adult animals. But chlorpyrifos is a demonstrated endocrine disruptor, acting through estrogenic and androgenic pathways suggesting that exposures during critical periods of breast development (e.g., gestation, early childhood, adolescence, pregnancy) may lead to deleterious effects on later risk for developing breast cancer.

What are the top tips to avoid exposure to chlorpyrifos?

• Wash produce well before eating. Where possible, buy USDA certified organic produce, which has not been treated with pesticides or herbicides.
• For both yourselves and your pets, avoid flea and tick collars that contain chlorpyrifos.
• Wear fitted protective clothing including long sleeves, pants, boots, gloves and masks when exposed to pesticides containing toxic chemicals. Clothing made of synthetic material is more resistant to penetration by pesticides.[9],[10]

Updated 2021