Other pesticides may affect breast cancer risk by acting as “endocrine disruptors,” also called “hormonally active agents.” These chemicals may mimic a known hormone, such as estrogen, or in another way affect the formation or breakdown of hormones that regulate cell division.

Pesticides that mimic the effect of estrogen may be able to support the growth of an existing estrogen-dependent breast tumor. We know that over half of all breast tumors depend on estrogen for growth.

Scientists are only starting to identify which pesticides are endocrine disruptors, including estrogen mimics. The insecticides DDT and methoxychlor are weak estrogen mimics. The 1996 Food Quality and Protection Act mandates that the Environmental Protection Agency develop screening tests to identify which pesticides act as endocrine disruptors.