Since the early 1990s, experts have known that the chemical, called 4-nonylphenol, binds to estrogen receptors, said senior study author William Baldwin, an assistant professor of biological sciences at the University of Texas at El Paso.

In the liver, 4-nonylphenol stimulates an enzyme system that then boosts production of estriol, a hormone associated with breast cancer. The chemical also has an affinity for estrogen receptors in breast tissue that trigger growth, he said.

"There have been a couple of studies in animals that showed a proliferation of mammary tissue when exposed to NP-4," Baldwin said. "This is believed to be the first study to show that if we carried out that enhanced proliferation for a longer period of time, it actually does lead to cancer in the lab animals."

Part of the problem is that the chemical, which mimics estrogen, may last longer in the body than natural estrogen, Baldwin theorized, but he added, "I have no evidence of this."

The study appears in the July issue of the Journal of Applied Toxicology.

In the study, Baldwin and his team compared the effects of giving differing doses of the chemical and estrogen to mice. When they followed mice genetically engineered to readily develop breast cancer over 32 weeks, many of those given 4-NP developed breast cancer while those given equivalent doses of estrogen did not.

Since the chemical is found in so many materials, should consumers be worried?

"I don't know if we can say that," Baldwin said. "The doses we used were much greater than what you would be exposed to, probably 100 to 1,000 times greater. [But] there might be more risk than we think."

Much more research is needed, Baldwin said. "In 10 or 15 years, we will figure out if environmental estrogens are a cause or part of the cause for breast cancer," he said.

Baldwin and other experts estimate that established risk factors such as aging, early onset of periods, late menopause, delayed childbearing and genetics explain only about 25 percent to 50 percent of breast cancers, and that environmental exposure plays a big role.

Brenda Salgado is program manager for Breast Cancer Action, a San Francisco-based organization that lobbies for environmental reforms, such as seeking legislation to label all carcinogens in cosmetics.

She said that while it's premature to give women advice based on just this study, they "should be conscious of chemicals that have an estrogenic effect and limit exposure to those chemicals."

Previous studies have found that exposure to these estrogen-like chemicals boost the risk of breast cancer, she said. The organization notes on its Web site that 70 percent of women with breast cancer have no known risk factors, suggesting that environmental forces play a major role.

More information: To learn more about breast cancer, visit the American Cancer Society.