Parabens, estrogen-mimicking chemicals commonly found in an array of personal care products, may be more dangerous at lower doses than previously thought, according to a new study.

The findings, published in Environmental Health Perspectives (2015; doi:10.1093/carcin/bgv039), could have implications for the development of breast cancer and other diseases that are influenced by estrogens. The study also raises questions about current safety-testing methods that may not predict the true potency of parabens and their effects on human health.

Parabens are a class of preservatives widely used in consumer products such as shampoos, cosmetics, body lotions, and sunscreens. The chemicals are considered estrogenic because they activate the same estrogen receptor as the natural hormone estradiol. Studies have linked exposure to estradiol and related estrogens with an increased risk of breast cancer, as well as reproductive problems. As a result, the use of parabens in consumer products increasingly has become a public health concern.

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How much parabens might contribute to breast cancer risk is unclear.

“Although parabens are known to mimic the growth effects of estrogens on breast cancer cells, some consider their effect too weak to cause harm,” said lead investigator Dale Leitman, MD, PhD, a gynecologist and molecular biologist at University of California Berkeley. “But this might not be true when parabens are combined with other agents that regulate cell growth.”

However, existing chemical safety tests, which measure the effects of chemicals on human cells, look at parabens only in isolation and fail to take into account that the agents could interact with other types of signaling molecules in cells to increase breast cancer risk.

“Scientists and regulators are using potency estimates from these kinds of tests and are assuming they are relevant to what goes on in real life. But if you don’t design the right test, you can be off by a lot,” said co-author Ruthann Rudel, MS, a toxicologist at Silent Spring Institute.

To better reflect what goes on in real life, the researchers looked at breast cancer cells expressing 2 types of receptors: estrogen receptors (ERs) and human epidermal growth factor receptor 2 (HER2). Approximately 25% of breast cancers produce an abundance of HER2. HER2-positive tumors tend to grow and spread more aggressively than other types of breast cancer.

The researchers activated the HER2 receptors in breast cancer cells with the growth factor heregulin, which is naturally made in breast cells, while exposing the cells to parabens. Not only did the parabens trigger the estrogen receptors by turning on genes that caused the cells to proliferate, the effect was significant: Exposure to parabens stimulated breast cancer cell growth in the HER2-activated cells at concentrations 100 times lower than in the cells deprived of heregulin.

The study demonstrates that parabens may be more potent at lower doses than previous studies have suggested, which may spur scientists and regulators to rethink the potential impact of parabens on the development of breast cancer, particularly on HER2- and ER-positive breast cells.

“While this study focused on parabens, it’s also possible that the potency of other estrogen mimics have been underestimated by current testing approaches,” said co-author Chris Vulpe, MD, PhD, a toxicologist now at the Center for Environmental and Human Toxicology at the University of Florida College of Veterinary Medicine.