State of the Evidence: What is the Connection between Chemicals
& Breast Cancer?
edited by Nancy Evans
© 2003 by The Breast Cancer Fund and Breast Cancer Action
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Executive Summary
B reast cancer rates
have been climbing steadily in the United States and other
industrialized countries since the 1940's. In 2002 in the United
States alone, breast cancer struck an estimated 205,000 women and
killed nearly 40,000. Billions of dollars have been spent in an
effort to stem this unrelenting tide; yet as many as 50 percent of
breast cancer cases remain unexplained by the characteristics and
risk factors associated with the disease.
Ionizing radiation is the best established environmental cause of
human breast cancer. Yet powerful scientific evidence points to some
of the 85,000 synthetic chemicals in use today as responsible for
many of the unexplained cases of cancer in general and potentially
also of breast cancer. While scientists have not yet developed an
ideal method for linking chemical exposures to breast cancer,
several types of research -- experimental, body burden and
ecological studies -- provide strong evidence of the connection
between chemicals and breast cancer.
Because the types of evidence vary, the strength of the evidence
linking chemicals and breast cancer also varies. The strongest
evidence linking chemicals to breast cancer is based on the fact
that lifetime exposure to naturally produced (endogenous) estrogens
increases the risk of breast cancer. New evidence strengthens the
association between compounds that mimic these natural estrogens,
such as hormone replacement therapy and oral contraceptives, and
increased risk of breast cancer. The National Toxicology Program now
lists steroidal estrogens as known human carcinogens.1 Other
compounds with estrogenic activity such as drugs like
diethylstilbestrol (DES), plastic additives like bisphenol-A (BPA),
polyvinyl chloride (PVC) (found in many consumer products), fuels
such as benzene, and some pesticides like dieldrin have all been
found to increase the risk of breast cancer. Synthetic chemicals
strongly linked to breast cancer through experimental evidence
include: organic solvents (used in many manufacturing processes,
including the manufacture of computer components), polycyclic
aromatic hydrocarbons (PAHs) (produced from combustion of fuels,
including gasoline, diesel, and heating oil, cigarettes and other
tobacco products, or by grilling meats and fish at high temperature)
and 1,3 butadiene (a by-product of internal combustion engines and
certain industrial processes).
There are also chemicals for which the evidence indicates a
probable but less direct link to breast cancer. These chemicals
include dioxin (created when plastics or other materials containing
chlorine are burned), the pesticide DDT
(dichloro-diphenyl-trichloroethane) and its metabolite and
environmental breakdown product, DDE, and PCBs (polychlorinated
biphenyls), previously used in the manufacture of electrical
equipment and other industrial and consumer products.
Finally, there is evidence of chemicals that affect how the body
functions in ways that suggest a possible link between these
substances and breast cancer. These chemicals include the
insecticide heptachlor, the herbicide atrazine, sunscreens, and
phthalates, compounds used to make plastic soft and flexible.
We clearly have major gaps in our current knowledge about the
links between breast cancer and the environment. Therefore, we need
to focus our research efforts in areas that are most likely to
provide useful information for framing public policies related to
chemical exposures and our health. The types of research most likely
to produce useful evidence will be those examining: (1) workplace
exposures, (2) household exposures (3) evidence of human
contamination through biomonitoring and (4) carefully conducted
studies in areas where breast cancer incidence is greatly increasing
and the use of chemicals has exploded.
We must pursue research that will lead to more definitive
answers, including new recommendations from the first International
Summit on Breast Cancer and the Environment sponsored by the U.S.
Centers for Disease Control and Prevention, which was convened May
22-25, 2002. At the same time, the existing evidence linking
chemicals to breast cancer demands that we act now as a society to
begin removing many of these substances from our environment.
Considerable resources are spent encouraging women to make changes
in their personal lives in an effort to reduce their risk of breast
cancer. But breast cancer is not just a personal tragedy; it is a
public health crisis that requires political will to change the
status quo.
This crisis must be addressed by beginning now to implement the
precautionary principle as a matter of public policy. Under this
principle, evidence of harm, rather than definitive proof of harm,
is the trigger for policy action. In addition, the precautionary
principle mandates that the burden of proof with regard to chemicals
rests with the manufacturers to demonstrate that the substances are
safe, rather than with the public to show that they are harmful.
Finally, the precautionary principle rests on the democratic
principle that government officials are obligated to serve the
public's interest in human health and environmental protection.
We ignore at our peril the increasing evidence that chemicals are
contributing to the rising tide of breast cancer. The obligation to
understand this evidence, and begin to address it through the
implementation of public policies that put health first, rests with
all of us. It is in our power to change the course we are on. Now is
the time.
