Posted by Bill Couzens

Key Words Brest Cancer, Davis, Devra Davis, Devra Lee Davis

Breast Cancer and Work: The Case for Prevention

by Devra Lee Davis, PhD, MPH, Director, Center for Environmental Oncology of UPCI

Patterns of breast cancer are unexplained. (1) While rates of breast cancer are higher in white women than in black women, the chances of dyinfrom breast cancer are generally greater for blacks than for whites. One in five cases of breast cancer occurs in white women under the age of 40, while one in four cases occurs in black women this same age. The reasons why breast cancer is more lethal in blacks than whites and more common in young black women remain a topic of intense research interest. Only one in ten cases of breast cancer occurs in a woman who has inherited the risk of the disease from either her mother or father. Identical twins begin life from a single egg, but only one may develop cancer. This means that most cases come about as a result of things that happen in the course of a lifetime of good and bad exposures. Thus, most cases of breast cancer do not result from inherited defects, but arise as a result of complex interactions that take place over the course of a lifetime between genes and the environments in which women live and work.

A substantial body of scientific evidence indicates that exposures to common chemicals and radiation, alone and in combination, are important causes of breast cancer. The challenge in understanding the causes of breast cancer is considerable as the disease can arise decades after critical exposures take place. The disease may result from either hormonally active materials or those that directly damage DNA. (2) Pre-menopausal breast cancer and post-menopausal breast cancer are likely to have different causes. The National Institute of Environmental Health Sciences has a major program of research on several different classes of environmental hazards that have been found to increase the risk of breast cancer, including some hormones and endocrine-disrupting compounds, synthetic-solvents, heavy metals, pesticides, fossil fuel combustion by-products, and both ionizing and nonionizing radiation. (1 , 3)

The State of the Evidence report of the Breast Cancer Fund evaluated more than 600 peer-reviewed studies on breast cancer and concluded that there are major opportunities to reduce the risk of the disease through lowering exposures in many different workplaces. (4) The fundamental challenge to researchers and policy makers remains this: we seldom can identify specific distinct causes of breast cancer through public health research. Yet, every proven cause of cancer in humans has also been shown to cause cancer in animals when adequately studied. Policymakers have agreed that in order to prevent cancer in humans, we should rely on experimental findings. Insisting on proof that a given compound or profession increases the risk of breast cancer in groups of women who have been well studied over time before acting to control or restrict suspected exposures effectively means waiting for sufficient numbers of illness or deaths to amass before acting to prevent future harm.

We have made remarkable progress in finding and treating breast cancer today. Recently, deaths from breast cancer and new cases of the disease have dropped in older women. Some scientists believe that the reduction in use of Hormone Replacement Therapy accounts in large part for some of these recent declines. (5) Other known hormonally-active compounds that are tied with increased breast cancer include the pharmaceutical hormone diethylstilbestrol (DES) which was given to women to prevent miscarriage. Both mothers and their daughters who were exposed to DES have an increased risk of breast cancer.

The fact that specific hormones have been linked with increased risk of breast cancer strongly suggests that compounds that act like hormones—termed endocrine disruptors or hormone mimics—may similarly affect breast cancer risk. The arena of prevention of the disease remains ripe for major advances. Numerous studies show that workplace factors can significantly increase the risk of the disease. Developing appropriate policies to reduce such exposures should play an important role in reducing the burden of breast cancer in the future.

In each of the professions listed in the blue box on page four, observations of increased risks of breast cancer have been generated based on detailed exposure studies or an estimate of typical exposures associated with certain jobs that are consistently found to be associated with a greater chance that breast cancer will develop in thousands of workers. The types of studies conducted include case comparison analyses that contrast the risk of breast cancer in individuals with well characterized exposures to some toxicants compared to other individuals without such exposures. Other studies of workplace risks have been developed by following large numbers of workers with well characterized common exposures over time and comparing their overall patterns of disease to patterns that occur in other groups of workers that generally are not exposed to the same agents.

Modern life is a mixture. As a result, the ability to draw conclusions about the causes of breast cancer or any other chronic illness from observations of workplace conditions remains one of the most challenging parts of public health research. Efforts to prevent breast cancer necessarily must rely on experimental studies conducted in animals or in cell cultures. With support from the Susan G. Komen for the Cure Foundation, The Silent Spring Institute evaluated studies on more than 200 different compounds found to cause breast tumors in animals in the journal Cancer in 2007 (6) and concluded:

Many environmental pollutants that cause mammary gland tumors in animals are also hormonally active and mimic estrogen, which is a breast cancer risk factor. Human studies of breast cancer risk factors have focused chiefly on those associated with childbearing, behavior and medical histories. Efforts to study the environmental links to breast cancer in humans have been hampered by limited funding and major difficulties in conducting such research. Breast cancer can take decades to form. Relevant exposures to hormonally mimicking compounds can also occur over long periods of time. In instances of pre-menopausal breast cancer, pre-natal conditions and exposures can also be important determinants. Despite the serious limits in epidemiological research in this area, studies of workers have consistently found elevated risks for women who work with a number of chemicals, including polychlorinated biphenyls (PCBs)—banned chemicals previously used in electrical equipment and other products. Women who work with organic solvents, or common air pollutants and dioxins also have developed greater rates of breast cancer than those without such exposures.

Several major reviews of the field agree that human studies of environmental risks for breast cancer should take advantage of new tools such as the ability to identify molecular markers of exposure and indicators of genetic susceptibility. In the absence of better studies in humans, a prudent public policy position has been crafted by the World Health Organization, International Agency for Research on Cancer. Studies that a given compound induces cancer in experimental animals should be interpreted as indicating that this same compound constitutes a risk to human health.

The Silent Spring Institute combined information from national and international sources and identified 216 different chemicals that should be regarded as causes of breast cancer in humans, because they are proven to cause mammary tumors in animals. The Women’s Health Trial has shown that the increased risk of breast cancer tied with hormone replacement therapy declined soon after this therapy was discontinued. This strongly implies that changing patterns of exposure to other hormonally active compounds may also have major benefits to reducing breast cancer risk. (7)

“While it’s disturbing to learn that so many chemicals may be linked to breast cancer,” says Dr. Julia Brody, executive director of Silent Spring Institute, “we must remember that we have a great opportunity to save thousands of lives by identifying those links, limiting exposure, and finding safer alternatives. It’s critical that we integrate this information into policies that govern chemical exposures.”

1. Breast Cancer and the Environment Research Centers of the National Institute of Environmental Health Sciences,
2. California Breast Cancer Research Program,
3. Cornell University. Breast Cancer and Environmental Risk Factors,
4. Breast Cancer Fund:,
5. Vassar College, Environmental Risks and Breast Cancer,
6. Silent Spring Institute:
7. Chlebowski et al. Breast Cancer after Use of Estrogen plus Progestin in Postmenopausal Women. NEJM, (2009) 360: 573-87.