DDT
CATEGORY: IARC possible, NTP known, Endocrine disruptor
USED IN: Banned in many countries, still used for malaria control in 17 countries, persists in the environment even where it is not used
DDT was the first widely used synthetic pesticide. It is credited both with the eradication of malaria in the United States and Europe and with long-term devastating effects on reproductive success in wildlife and adverse health effects in humans (Beard, 2006). Banned in most countries for agricultural use, DDT is still used for malaria control in many countries especially in sub-Saharan Africa (WHO, 2007). Because of its continued use and its persistence in the environment, DDT is found worldwide. Most animals, including humans, ingest DDT-contaminated foods and retain the chemical and its main metabolite, DDE. Significant concentrations of DDT and DDE are still found in the body fat of humans and animals as well as in human breast milk and placenta (Rogan, 2007; Shen, 2007; Zheng, 1999).
Epidemiological data are mixed regarding the effects of DDT/DDE on breast cancer risk. For example, one study from the Long Island Breast Cancer Study Project did not find an association between DDT/DDE (or PCBs) and breast cancer (Gammon, 2002). Like many such studies, however, this project measured contaminant levels near the time of breast cancer diagnosis, without regard to possible exposures during critical early periods of breast development, and did not consider the effect of chemical mixtures nor assess key metabolites.
A recent study explored women’s estimated DDT levels based on aggregate data from their year of birth as well as blood DDT levels at the time the women gave birth to their first children. Researchers then followed the women over the next two decades, noting cases when women either were diagnosed with invasive or non-invasive breast cancer before the age 50, or had died from breast cancer before the age of 50. Results show that exposure to DDT during childhood and early adolescence (younger than age 14) was associated with a fivefold increase in risk of developing breast cancer before the age of 50. As the authors note, “Many U.S. women heavily exposed to DDT in childhood have not yet reached age 50. The public health significance of DDT exposure in early life may be large” (Cohn, 2007).
Laboratory studies have found the estrogen-like form of DDT enhances the growth of estrogen-positive (ER+) mammary tumors (Robison, 1985; Scribner, 1981). ER+ tumors are the most common type of breast cancer. The percentage of breast tumors in the United States that are ER+ rose from 73 percent in 1973 to 78 percent in 1992. This is the period when women exposed to DDT as young girls in the 1950s were expected to exhibit increased incidence of breast cancer related to DDT exposure (Pujol, 1994). Another study, looking at chemical levels in breast fat tissue, did not find an association of DDT/DDE with ER+ tumors. However, data from this study indicated a significant association of higher concentrations of these compounds in breast tissue with tumors that were more aggressive and that had poorer prognoses (Woolcott, 2001).
