Researchers have identified several ways that pesticides may affect breast cancer risk.

Some pesticides may act as “complete” carcinogens. A complete carcinogen is capable of taking a normal cell through the entire multi-step process to form an invasive tumor. Only a few pesticides have been identified as “complete” carcinogens.

Another way is to act as a tumor promoter. A tumor promoter may give an existing tumor a growth advantage, promoting its further growth and development.

The insecticide DDT has been identified as a tumor promoter in laboratory animals. When given with or after a known cancer-causing chemical (called a carcinogen), a tumor promoter can cause a higher number or earlier appearance of breast tumors than if only the cancer-causing chemical is given.