Toxic chemicals linked with early breast development, difficulty breastfeeding, high breast density, and breast cancer risk

Scientists call on regulators to update their methods for screening chemicals that could harm the breast

According to leading experts in breast cancer and environmental health, exposures to harmful chemicals in everyday products and the environment can harm the breast in a variety of ways. In a new review, the researchers highlight the latest science linking toxic chemicals not only with breast cancer, but also with early breast development, problems breastfeeding, and increased breast density, and call for major changes in the way chemicals are tested for safety in the United States.

“The breast goes through many different changes over the course of a woman’s life—during puberty, pregnancy, and menopause—that are guided by the body’s hormones,” says co-author Jennifer Kay, a research scientist at Silent Spring Institute. “These changes also make the breast vulnerable to environmental influences.”

Scientists have identified hundreds of chemicals, called endocrine disrupting chemicals (EDCs), that disrupt or interfere with natural hormones. The chemicals are used in common products like hair dyes, food packaging, pesticides, and furniture. They also end up in drinking water and accumulate in household dust. Since EDCs are so ubiquitous, people can be exposed to multiple EDCs every day—the effects of which can add up.

Black women, in particular, have higher exposures to some EDCs. They also have higher rates of girls starting puberty early, are less likely to breastfeed, and have more aggressive forms of breast cancer.

Reporting in the journal Current Environmental Health Reports, the researchers present evidence from both human and animal studies describing the many ways EDCs impact the breast and how these impacts coincide with worrisome health trends:

• Breastfeeding: A significant percentage of women stop breastfeeding earlier than they intend to for reasons such as low milk supply, poor infant weight gain, and difficulty breastfeeding. Studies in animals show that chemicals can disrupt milk production by altering breast development and hormone signals. High blood levels of PFAS chemicals during pregnancy and breastfeeding has been associated with shorter breastfeeding duration, and women with high exposures to DDT are more likely to stop breastfeeding early.
• Breast development: Over the last several decades, girls have been developing breasts at earlier ages, suggesting that environmental factors play a role. Early breast development is associated with other health problems, including increased risk of breast cancer later in life. Exposure to chemicals in the womb can influence the timing of puberty. EDCs such as BPA, parabens, and triclosan have been shown to accelerate breast development.
• Breast density: Dense breast tissue is a well-established risk factor for breast cancer and it can mask tumors during mammogram screenings. Women with dense breasts have a more than four-fold increased risk of breast cancer. Using breast imaging, studies have linked higher breast density with higher blood and urine concentrations of several environmental chemicals, including BPA, polycyclic aromatic hydrocarbons (PAHs), phthalates, and air pollution. Studies in animals have also shown that exposure to BPA and PFAS in the womb leads to more dense mammary tissue.
• Breast cancer: Although cancer incidence rates have been going down over the last few decades, breast cancer remains one of the few cancers that continues to increase in prevalence. It is the most commonly diagnosed cancer, surpassing lung cancer. Rates among women under the age of 40 are also on the rise in the US. Scientists have identified hundreds of chemicals, including pesticides, consumer product ingredients, air pollution and drinking water contaminants that wreak havoc in cells in ways that lead to breast cancer. Chemicals can damage DNA but also mimic hormones and cause breast cells to produce more estrogen and progesterone, which are established risk factors. A number of studies in humans have also found significant links between these types of chemical exposures and breast cancer.  

The review authors point out that current methods used by regulators to screen chemicals for toxic effects do not adequately consider their effects on the breast, especially during vulnerable time periods. “These outdated methods are not capturing critical information and are putting women and children at risk,” says co-author Ruthann Rudel, director of research at Silent Spring Institute. The authors describe shortcomings in current practices, which typically rely on experimental studies in rodents to predict effects in humans. Some of the most concerning gaps are:

1. The mammary gland is either not examined at all, or only a tiny sliver of tissue is examined, so important effects can be missed.
2. In most studies designed to measure changes in development that can result from in utero exposures, the mammary gland is not assessed in juvenile animals, while changes in reproductive organs are. As a result, effects relevant to breast development may not be detected.
3. Mammary tumors observed in animals that are exposed to chemicals at low doses are often dismissed unless there are more of them at higher doses, even though high dose toxicity can lower body weight and lead to fewer tumors. In fact, in many studies, only the high dose animals are examined, so tumors that occur at low doses can be missed entirely.
4. Effects on the ability to lactate are only indirectly assessed by measuring pup body weights. When reduced body weights are observed, they are often attributed to overall toxicity in the mother rather than to a specific effect on her ability to produce milk to feed her young.

The authors make specific recommendations, including:

1. Always collect and analyze the whole mammary tissue, as well as cross-sections that cover a larger area.
2. Collect and evaluate the mammary gland from animals tested at all doses and at each examination time point included in the study.
3. Acknowledge effects at low doses even when they’re not seen at high doses.
4. When there are reductions in pup body weight, follow up with assessments of milk production and nursing duration.

“Cures are important, but because of the high prevalence and severity of breast cancer, we should be doing more to bring down incidence rates, and that includes reducing exposures to chemicals that contribute to the disease,” says Kay.

Funding for the study was provided by the National Institute of Environmental Health Sciences, the FREIA project (supported by the European Union’s Horizon 2020 research and innovation programme), and charitable donations to the Safer Chemicals Program at Silent Spring Institute.