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. The UC Berkeley research, based on a small number of Bay Area women, tested a theory that the person’s age during exposure was critical, and provided the first evidence of a substantial effect on breast cancer.
“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 led the study of 129 women. “If this finding holds up, those who were young and more highly exposed could be the women at greatest risk.”
Women born between 1945 and 1965 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 use began in 1945, peaked in 1959 and was banned nationwide in 1972 because it was building up in the environment.
“This does speak to a generation of us, the baby boomer generation,” said Peggy Reynolds, an epidemiologist at the Northern California Cancer Center and consulting professor at Stanford University School of Medicine. She was not involved in the study.
“There’s nothing we can do now about the exposures we may have had back then,” Reynolds said. “But it’s prudent to say that we should be mindful of the fact that we may have higher risks by virtue of those environmental exposures back then.”
Because the pesticide was ubiquitous, the authors wrote, “the public health significance of DDT exposure in early life may be 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.
“A single study doesn’t necessarily translate into truth, if you will,” Reynolds said. “But a study like this -- which has such dramatic and provocative findings, and is consistent with what we have suspected about early life exposures -- does call for careful examination of the results.”
Several larger, earlier studies found no evidence that DDT caused breast cancer. The largest, a 2002 study involving more than 3,000 women in Long Island, N.Y., concluded that the breast cancer rate did not rise with increasing DDT levels in their blood. To some, that seemed to put the question to rest.
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 who had just given birth in Kaiser Permanente hospitals in the Oakland area.
Their study, funded by the National Cancer Institute, will be published Monday in the October edition of the journal Environmental Health Perspectives.
The women in the top third of DDT concentrations who were exposed before age 14 were five times as likely to get breast cancer as the women with the lowest levels, according to the study. No relationship between cancer and the insecticide was found in the women born before 1931, who would have been older during any exposure.
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 Columbia 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 organochlorines and other exposures, the timing may be more important in terms of breast cancer.”
Scientists said the study was particularly important because the blood was drawn when DDT was still heavily used, so it offered a snapshot of women with levels an order of magnitude higher than today.
“It really turns back the clock in a very unique way,” said Steven Stellman, a professor of clinical epidemiology at Columbia University who has studied DDT and breast cancer.
A fivefold 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% to 100%.
However, because relatively few women were involved, the study is prone to statistical weakness, which may mean the result is partly attributable to chance, Stellman said.
Terry agreed: “Certainly if you have a larger study, the estimates you get are more stable. No one study can be definitive. It would be good to try to replicate the finding in another population of girls who were highly exposed.”
But it is rare to find blood stored for 40 years, so replication would be difficult.
Exposure to DDT for the Bay Area women was probably no more extensive than elsewhere in the country at the time. Most of the 129 women did not live on farms, so they would have been exposed through food or urban spraying.
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 later.
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.
The new study does not indicate the age of greatest vulnerability to exposure. Breast development is most critical in the womb and at puberty.
Whether or not DDT promotes breast cancer, there are many other risk factors, including alcohol consumption, hormone therapy and age at menstruation.
The known risk factors are believed responsible for up to half of cases.
“We truly believe it’s not one exposure that’s going to determine whether you get breast cancer or don’t get breast cancer,” Reynolds said.
“While it’s true that our generation may be more at risk from those exposures, there are a whole lot of other things involved too.”