At a Glance

Chlorpyrifos is a pesticide and insecticide widely found on produce and home pest control formulas that is an endocrine disruptor and breast carcinogen. Exposure to chronic low doses of chlorpyrifos leads to an increased risk of mammary tumors in lab studies.

Banned in the EU, California, New York, and Hawaii, several other U.S. states are considering a ban on chlorpyrifos.

To avoid consuming this pesticide, choose organic produce when possible and wash produce thoroughly. Also, wear synthetic protective clothing if you are exposed to this chemical in an agricultural or home setting.

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

[1] Agency for Toxic Substances and Disease Registry (ATSDR). Toxicological Profile for Chlorpyrifos. Atlanta, GA: U.S. Department of Health and Human Services, Public Health Service, 1997.

[2] Ventura, Clara et al. “The Organophosphorous Chlorpyrifos as a Breast Cancer Risk Factor,” in: Chlorpyrifos: Toxicological Properties, Uses and Effects on Human Health and the Environment, ed. Caitlin Mayes (Nova Science Publishers, 2015),.15-26.

[3] Kass, Laura et al. “Relationship between agrochemical compounds and mammary gland development and breast cancer.” Molecular and Cellular Endocrinology 508 (2020): 110789. doi:10.1016/j.mce.2020.110789.

[4] Engel, Lawrence S et al. “Insecticide Use and Breast Cancer Risk among Farmers’ Wives in the Agricultural Health Study.” Environmental Health Perspectives 125,9 097002 (2017). doi:10.1289/EHP1295.

[5] Nishi, Kumari, and Swarndeep Singh Hundal. “Chlorpyrifos induced toxicity in reproductive organs of female Wistar rats.” Food and Chemical Toxicology: an international journal published for the British Industrial Biological Research Association 62 (2013): 732-8. doi:10.1016/j.fct.2013.10.006.

[6] Ventura, Clara et al. “Pesticide chlorpyrifos acts as an endocrine disruptor in adult rats causing changes in mammary gland and hormonal balance.” The Journal of Steroid Biochemistry and Molecular Biology 156 (2016): 1-9. doi:10.1016/j.jsbmb.2015.10.010.

[7] Ventura, Clara et al. “Effects of the pesticide chlorpyrifos on breast cancer: Implication of epigenetic mechanisms.” Journal of Steroid Biochemistry and Molecular Biology 186 (2019): 96-104. doi:10.1016/j.jsbmb.2018.09.021.

[8] Lasagna, M. et al. “Chlorpyrifos Subthreshold Exposure Induces Epithelial-Mesenchymal Transition in Breast Cancer Cells.” Ecotoxicology and Environmental Safety 205 (2020): 111312. doi:10.1016/j.ecoenv.2020.111312.

[9] Damalas, Christos A, and Spyridon D Koutroubas. “Farmers’ Exposure to Pesticides: Toxicity Types and Ways of Prevention.” Toxics 4,1 1 (2016). doi:10.3390/toxics4010001.

[10] Alexander, Bruce H et al. “Biomonitoring of 2,4-Dichlorophenoxyacetic Acid Exposure and Dose in Farm Families.” Environmental Health Perspectives 115, 3 (2007): 370-6. doi:10.1289/ehp.8869.


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