Few three-letter acronyms have invited as much scientific controversy as DDT — stepping in for the cumbersome dichlorodiphenyltrichloroethane.

The once-heralded pesticide was first used to great, inexpensive, effect in curbing disease-carrying insect populations in World War II, with peak worldwide use occurring after its release to the public in 1945. Health organizations, governments, and farmers alike used DDT to protect their crops and fight back against the scourge of malaria, spread by various types of mosquitos.

Though DDT did prove incredibly useful in both those tasks, concerns over its incidental, lasting effects on wildlife, especially birds, and in humans led to the United States eventually banning its use wholesale in the 1970s. Many, but not all, nations would follow suit, leaving only a few scattered countries such as India that regularly rely on it for its antimalarial qualities today.

But while the detrimental effects of DDT on the environment are widely accepted, particularly when it’s used as an agricultural pesticide, the smoking gun for human harm has been rather elusive. "Interestingly, none of the...pesticides (i.e. DDT/DDE) considered could be conclusively linked to an increased risk of breast cancer," explained a 2014 report from the Institute of Medicine, noting that further research would be needed to determine any such risk.

It’s research that may have just arrived, as scientists in the Journal of Clinical Endocrinology & Metabolism claim that they were able to find a definitive and significant link between breast cancer and DDT exposure. The wrinkle to their study is that this effect was seen when women were exposed to the chemical while they rested in their mothers' wombs.

"We observed a sizable, statistically significant association between in utero DDT exposure and risk of breast cancer in young women and a possible association with more aggressive tumors," the authors concluded. "These findings are the first ever reported for a prospective observation of a large pregnancy cohort."

Rather ingenuously, the authors went back and examined the blood samples of pregnant women enrolled in the Child Health and Development Studies (CHDS), a longitudinal study of Oakland, California residents. As the women were pregnant from 1959 to 1967, this enabled the current authors to examine rates of DDT exposure when it was still prevalent in America. Out of the some-twenty thousand pregnancies recorded by the CHDS, the authors focused on the 9,300 daughters that arrived as a result.

For more than 50 years, both mothers and daughters were regularly followed up with, and by 2012, 137 cases of breast cancer had occurred among the daughters. The researchers then attempted to see if high levels of DDT in the mothers’ blood serum could be correlated to the risk of their daughters developing breast cancer. One particular component of commercial DDT, o,p’-DDT, was strongly associated with breast cancer. "Independent of a maternal history of breast cancer, elevated maternal serum o,p’-DDT significantly predicted a nearly 4-fold increase in the daughter’s risk of breast cancer," they concluded.

In certain aggressive cancers affiliated with the presence of human epidermal growth factor receptor 2 (HER2) in the tumor, o,p’-DDT was even more strongly linked, validating past experiments showing that it could activate the production of HER2 in breast cancer cell lines. "These results suggest a strong effect of in utero o,p’-DDT on breast cancer stage, and HER2 status in this population,"

Their findings, while not showing a causative link, are built on a base of evidence showing that chemicals similar to DDT, which are classified as estrogen disturbers for their ability to interfere with the normal production of the hormone, are tied to later breast cancer risk. Past research by the authors has also shown a relationship between DDT exposure and cancer risk, but only for women before the age of 14, possibly explaining why studies of fully-grown adults have failed to find a conclusive association. And as Medical Daily has reported, DDT exposure has also recently been linked to obesity, diabetes and high cholesterol in mice populations.

Though DDT use has been widely curtailed, the authors feel their results are still very relevant. "[T]he women exposed most heavily while in utero during the 1960’s are currently reaching the age of heightened breast cancer risk,” they wrote. “In addition, DDT remains a global environmental contaminant due to its environmental persistence and semivolatility."

Far from unnecessarily drudging up the past, they’re hopeful that their findings can promote an informed discussion about whether continued DDT use is worth the human cost, though they acknowledge the need to more concretely demonstrate its possible effects. "Experimental studies are essential to confirm findings and discover causal mechanisms.” they wrote. ”If confirmed, these findings could lead to discovery of biomarkers and interventions for DDT-associated breast cancer."

Source: Cohn B, Merrill M, Krigbaum N, et al. DDT Exposure in Utero and Breast Cancer. Journal of Clinical Endocrinology & Metabolism. 2015.