Study suggests DDT, breast cancer link

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Exposure in childhood is key, quintupling the risk among women with
high levels of the pesticide, researchers say.

By Marla Cone, Los Angeles Times Staff Writer
September 30, 2007

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.

FOR THE RECORD:
DDT and cancer: An article in Sunday's Section A about the link
between DDT and breast cancer identified the researchers as coming
from UC Berkeley's Child Health and Development Studies. The project
was part of the university until 1986 but is now administered by the
nonprofit Public Health Institute in Berkeley. —


"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."