Women heavily exposed to the pesticide DDT during childhood are five times as likely to develop breast cancer, a new scientific study suggests.

For decades, scientists have tried to determine whether there is a connection between breast cancer and DDT, the most widely used insecticide in history.

“There was very broad exposure to this pesticide, and with this study, we have evidence that women exposed when young were the most affected,” said Barbara A. Cohn, director of UC Berkeley’s Child Health and Development Studies, who lead the study of 129 women. “If this finding holds up, those who were young and more highly exposed could be the women in greater risk.”

Women born between 1945 and 1959 were most likely to have been heavily exposed as children to DDT, which was sprayed throughout the United States to kill mosquitoes and other insects. DDT used began in 1945, peaked in 1959 and was banned nationwide in 1972 because it was building up in the environment.

Because the pesticide was ubiquitous, the authors wrote, “the public health significance of DDT exposure in early life maybe large.”

If the early-exposure theory is true, breast cancer rates could rise as the DDT generation ages. Two-thirds of women with invasive breast cancer are 55 or older when they are diagnosed, according to the American Cancer Society.

Several larger, earlier studies found no evidence that DDT caused breast cancer.

However, those studies were based on amounts found in the blood of middle-age and older women, after they had contracted cancer and decades after DDT was banned.

The new study looked for the first time at DDT concentrations in women when they were primarily in their 20s, closer to when their breasts developed and during a time of widespread spraying. The UC Berkeley team measured DDT in blood collected between 1959 and 1967 from 129 women.

Their study, funded by the National Cancer Institute, is published in Environmental Health Perspectives, October 2007.

The women in the top third of DDT concentrations who were exposed before age 14 were 5 times as likely to get breast cancer as the women with the lowest levels, according to the study.

The Berkeley study “is very compelling and important and addresses a question about timing of exposure that many of the existing studies could not address,” said Mary Beth B. Terry, an associate professor of epidemiology at Colombia University’s Mailman School of Public Health. She co-wrote the Long-Island study.

“Their findings in general support their hypothesis that the earlier you were exposed, the stronger the effect,” Terry said. “We think with organo-chlorines and other exposures, the timing may be more important in terms of breast cancer.”

A 5-fold increase in breast cancer – 400% - is considered very high. Most traditional risk factors, such as late menopause, obesity and older age at first pregnancy, increase risk by 50-100%.

DDT is prohibited today in most of the world, though it is used in small volumes in some malaria-plagued African nations.

But virtually everyone on the planet still carries residue because the pesticide persists in the environment and in tissues, breaking down slowly.

Many environmental toxicologists and epidemiologists have in recent years altered their thinking about toxic exposures. They used to focus on lifetime exposure. But now they suspect that chemicals may activate genes or damage DNA in the womb or during early childhood, resulting in diseases decades earlier.

Other evidence suggests that breast cancer can be triggered early in life. In lab animals, prenatal doses of chemicals can trigger cancerous cells in fetal mammary glands. Also, Japanese females who were younger than 20 in 1945 developed the highest breast cancer rates among those exposed to radiation from the atomic bombs.

Source: Los Angeles Times, 30 September 2007