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Estrogen is a hormone that is important for the normal growth and development of female reproductive organs, including the breasts. Since the 1930s, evidence demonstrates that increased estrogen is linked to increased cancer risk, including breast cancer. While estrogen is not a mutagen, it acts as a carcinogen due to its role in stimulating cell growth and division in organ tissues that are particularly sensitive to its effects — the reproductive organs, the heart, and bones. Estrogen-stimulated cell division increases the risk of replicating cellular DNA mutations, making mutations permanent, which leads to carcinogenesis.(7) The International Agency for Research on Cancer (IARC), an agency of the World Health Organization, lists estrogens and their component hormones as known human carcinogens.(10) In addition, the National Toxicology Program (NTP) includes hormone replacement therapy (HRT) and oral contraceptives on their list of known human carcinogens.(11)   However, it must be noted that there is debate regarding whether the use of birth control pills increases a woman’s risk for breast cancer; “This may depend on  the level of estrogen present in the birth control pill, the length of use, and the time in a woman's life when the pills were used.” (7)

A woman’s lifetime exposure to hormones (especially estrogen) increases the likelihood of her developing breast cancer.(12) This lifetime exposure, starting from the fetal period, is influenced by relatively natural-occurring events, such as the age of the first menstrual period (which in turn may be influenced by environmental factors), number of pregnancies, breast feeding behavior, and age at menopause. Additional sources of estrogen or estrogen-like chemicals in a woman’s body may increase this lifetime estrogen load. Women may be exposed to excess estrogen through the use of birth control pills, hormone replacement therapy, and exposure to some synthetic, manufactured chemicals present in our environment known as endocrine disruptors (EDs). To understand how and why these chemical pollutants, or EDs, put us at risk, it is necessary to understand how hormones work. Like a key fitting into a lock, hormones (including estrogens) act by fitting into specific protein receptors in cells; this estrogen/receptor complex activates biochemical reactions determining whether certain genes are expressed. Endocrine disruptors, as the term implies, can interfere with this process and disrupt normal hormonal activity in different ways.(12) It is thought these chemicals that mimic estrogen may affect the growth of estrogen-dependent breast tumors, which account for half of all breast tumors.(13)

As a result of scientific controversy and, as some claim, industry pressure, neither IARC nor NTP categorize EDs as carcinogens in humans.(4)  The source of the controversy is the fact that designing experiments to assess the toxic effects of environmental chemicals can be quite complicated. However, evidence from wildlife studies, lab studies, and human epidemiological studies indicate that EDs in our environment play a role in the increased rates of breast cancer we are experiencing.(4) The Silent Spring Institute in Massachusetts reviewed scientific evidence from national and international regulatory agencies and compiled a list of 216 substances in the environment that act as potential mammary carcinogens in animals.(14) The list includes —

• Industrial chemicals
• Chlorinated solvents
• Products of combustion
• Pesticides
• Dyes
• Drinking water disinfection byproducts
• Pharmaceuticals
• Hormones (including; estriol, estrone, conjugated estrogens, progesterone, and testosterone, among others listed)
• Radiation
• Natural products (i.e. bracken fern, carboxymethylnitrosourea, methyleugenol, ochratoxin A wood dust,  methanol extract)
• Research chemicals 

Many of the listed chemicals are EDs, though not all EDs have been tested for their carcinogenicity. Other non-ED chemicals that put women at risk for breast cancer will be discussed later.

We are all exposed to these chemicals in a variety of ways; they can transfer between the environmental media of air, water, and land, and cross geographical boundaries to travel long distances. They end up in the air we breathe, the food we eat, the water we drink, and even some of the fragrances and body products we use. Exposures occur in our communities, homes, and places of work. Many of these chemicals bio-accumulate in the human body and are stored for extended periods of time in fat cells, including those of the breast. Additionally, it is important to note that we are not being exposed to just one discreet chemical at a time, but to a mixture of chemicals, and the synergistic or additive effect of these chemicals is unknown as toxicological studies typically look at one chemical at a time.

breast cancer, chemicals, endocrine distruptors, carcinogen

Last updated 390 days ago by Katie Huffling