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A new study continues the debate on whether exposure to chemicals in the environment
increases the risk of breast cancer. To date, evidence has been inconclusive, with some
studies showing a link between chemicals and breast cancer and others showing no
association. This latest study, based on the analysis of blood samples from cancer
patients and healthy women, shows that women with breast cancer were more likely to have
residue from the banned pesticides DDT and HCB in their blood. While the study does not
prove that the chemicals caused breast cancer, it does raise questions about environmental
risk factors for breast cancer.
People are typically exposed to low concentrations of environmental chemicals through
diet, water, etc. Researchers believe that certain environmental pollutants or pesticides
may increase breast cancer risk because many of these compounds appear to affect the
body’s metabolism of the hormone, estrogen. In fact, several experimental animal
studies have shown that exposure to chemicals increases the risk of estrogen-sensitive
cancers (i.e., cancers that depend on estrogen to grow). Though many of the suspected
pollutants have been banned for 20 years or longer, they may persist in the environment
because they degrade slowly.
HCB (hexachlorobenzene) and DDT (dichlorodiphenyltrichloroethane) are both pesticides
that were banned in the United States in 1965 and 1972 respectively. Given the
contradictory results of several studies on DDT and HCB, Dr. C. Charlier and colleagues
from the Sart Tilman University Hospital in Belgium decided to study blood samples of
breast cancer patients and healthy women to determine if DDT and HCB levels differed. They
analyzed blood from 159 women with breast cancer and 250 healthy women for traces of the
pesticides.
The results showed that levels of total DDT and HCB were significantly higher in breast
cancer patients than in the healthy women. Thus, Dr. Charlier and his colleagues conclude
that their "results add to the growing evidence that certain persistent pollutants
may occur in higher concentrations in blood samples from breast cancer patients than
[healthy women]."
However, in the study, pesticide levels did not coincide with estrogen sensitive breast
cancers, as previous animal studies suggested they might. Approximately 80% of breast
cancers are estrogen-sensitive; that is, they depend on the hormone estrogen to grow. The
remaining 20% of breast cancers do not depend on estrogen. Women with both
estrogen-sensitive and non-estrogen sensitive breast cancers had pesticide residue levels
in their blood in the study.
While these results do seem to point to pesticides as a risk factor for breast cancer,
the study does not show that the chemicals actually caused cancer. They were simply
present in the blood of cancer patients of the majority of breast cancer patients but only
2.5% of the healthy women. Further research is needed to explore the relationship between
pesticides and breast cancer, including a probe into how women are being exposed to these
banned chemicals.
Researchers are also investigating several other factors that may play a role in how
greatly pollutants could influence a woman’s risk of breast cancer. For example, some
research suggests that women with high body mass indexes may be at greater risk for breast
cancer because the total body load of chemicals may be higher in these individuals. (Body
mass index (BMI) measures a person’s total body fat and is derived by multiplying a
person's weight in pounds by 703 and then dividing it twice by the height in inches.
According to federal guidelines a BMI of 25 to 29.9 is considered overweight and a BMI of
30 or greater is considered obese).
Also, researchers are investigating long-term risk of exposure to pollutants in utero
(via the placenta) and through breast milk. The goal is to determine whether exposure to
these pollutants during fetal and infant life may lead to an increased risk for cancer 20
or 30 years later in life. The prenatal period is thought to be the riskiest period in
terms of exposure to harmful agents. Therefore, exposure to pollutants may be more harmful
if it occurs during the prenatal period than if it occurs during adulthood.
A number of small studies over the past few years have also shown a possible increased
incidence of breast cancer in women who use dry cleaning services or professional lawn
services. However, several experts doubt the scientific validity of these studies whose
data are often contradicted in larger studies.
A number of risk factors for breast cancer have already
been identified. However, most of these risk factors are biological. It is estimated that
80% of women who develop breast cancer have no known risk factors for the disease,
suggesting that environmental components may play a role in determining who is likely to
develop the disease.
Some known risk factors for breast cancer include:
- Age (over 50)
- Personal or family history of breast cancer
- Genes (such as mutations of the BRCA1 or BRCA2 genes)
- Early onset of menstruation (before age 12)
- Late menopause (after age 50)
- Delayed childbirth or never having children
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