The last quarter of a century has taught science some newfangled things about breasts. For one thing, they appear to be showing up earlier in young girls, with possible consequences for breast cancer later on. For another, the way they grow and develop varies from woman to woman, and—if lab animals are any indication—normal exposures to commercial chemicals can alter that process. The growing human breast is also more vulnerable than we thought. Data from atomic-bomb survivors in Japan show that it was adolescents—not grown women—near the explosions who were most likely to develop breast cancer in later years. Since then, there’s been similar data for girls who were exposed to medical X-rays or radiation therapy, as well as research showing that the pesticide DDT, now banned but pervasive in the 1950s and 1960s, is associated with a higher risk of breast cancer in women exposed as girls.

So it may come as a surprise that the federal agencies responsible for public health don’t routinely take childhood exposures into account when testing whether commercial chemicals cause mammary tumors. In fact, in many lab-animal tests, they don’t bother to look at the mammary gland at all. Breast cancer may be the No. 1 killer of middle-aged women in the United States, but as a new set of reports makes clear, the breast is a major blind spot in federal chemical-safety policy. “They just throw the mammary glands in the trash can,” says Ruthann Rudel, research director with the nonprofit Silent Spring Institute and lead author of one of the papers, a review of the latest science on mammary gland development and toxic exposures.

The reports, published last week in the journal Environmental Health Perspectives, grew out of a 2009 workshop on mammary gland risk assessment, which involved scientists from federal and international agencies as well as independent groups. Breast cancer is just one of the areas federal agencies neglect, the reports show, along with health issues surrounding lactation and the timing of breast development in puberty. “Few chemicals coming into the marketplace are evaluated for these effects,” state Rudel and her co-authors.

But blowing off these tests is a big mistake. The mammary gland—the breast’s intricate milk-making structure—is uniquely sensitive to toxic chemicals, says Suzanne Fenton, a reproductive endocrinologist with the National Toxicology Program of the National Institutes of Health, and a co-author of the science review. In both rodents and humans, it starts to develop in the fetus, undergoes a colossal growth spurt at puberty, and doesn’t fully develop until late pregnancy. During these times, its cells appear particularly vulnerable to carcinogens and other organ-altering substances. While lab rats and mice aren’t perfect proxies for humans, their mammary glands undergo similar development patterns under similar hormonal influences, says Fenton.

When it comes to breast health, the reports fault the primary federal body responsible for regulating commercial chemicals, the U.S. Environmental Protection Agency, on several fronts. First, in its cancer tests that do assess mammary glands, lab animals are typically dosed with the chemicals in question as adults, past the sensitive age when toxins may cause critical damage to the organ. Second, in other tests that use younger animals, the mammary glands are not examined at all, meaning the tests could be missing signs of poor functioning and development. This is important because chemicals may affect both breastfeeding ability and the timing of puberty, which in turn influences breast-cancer risk. Finally, the EPA’s testing protocols don’t consistently examine the male mammary gland, which also appears to be sensitive to toxins.

These lapses come on top of the already lame reality that the EPA maintains toxicity data on just 1 percent of the 83,000 chemicals in use in this country. Most chemicals enter the market with no health testing at all. In 2009, the agency finally implemented a much-needed new program that screens chemicals for subtle effects on hormone systems. (Previously, the agency only checked to see whether chemicals caused more obvious problems, like tumors, sickness, or death.) Called the Endocrine Disruptor Screening Program, it has begun requiring manufacturers to test pesticides (and only pesticides, so far). These comprehensive tests look for widespread effects in rodent bodies, including on the brain, internal organs, and genitals—but not on mammary glands. Oops. “There are really no other major gaps [in those tests],” says Fenton.

“I wish I could understand the failure of logic in not testing mammary glands,” says Jeanne Rizzo, director of the Breast Cancer Fund, a California-based nonprofit. But she says she’s heartened that the discussion is even on the table. “Ten years ago, when we said we need to look at the environment and we need to look at prevention, people thought we were nuts.”

It’s becoming increasingly clear, though, that mammary glands belong on a slide and not in the dumpster. Recent studies show that some pesticides, pharmaceuticals, and plastic additives appear to change when and how the mammary gland develops. Even low doses, close to what average Americans are exposed to currently, have been linked to altered development, cell growth, and gene expression in animal mammary glands. The chemicals include the notorious baby-bottle chemical bisphenol A, dioxin (a by-product from burning plastic and a common food contaminant), phthalates (plastic additives), atrazine (a top-selling herbicide in the United States, now banned in Europe), flame retardants, and stain repellants. PFOA, a common chemical used to make Teflon, appears to delay puberty in animal pups and reduce the size of the mammary gland, while chemicals that mimic estrogen may accelerate puberty.

The good news is that the mounting data on the sensitivity of breast cells has caught the attention of scientists within state and federal government. Some are calling for reform. “There’s a growing body of animal evidence and sporadic human evidence that things we’re exposed to across a lifetime can cause breast cancer,” says Marion Kavanaugh-Lynch, director of California’s Breast Cancer Research Program, a governmental grant-making organization that helped fund the 2009 workshop with proceeds from the state’s cigarette tax. “If we can identify these chemicals now, we can more easily avoid them,” she says.

The question is whether, and how fast, regulatory agencies can update their toxicity protocols. New tests have to be proven effective and then standardized across numerous labs. In a victory for breast-cancer advocates, the National Toxicology Program, under pressure from its own scientists, is actively rewriting its testing guidelines to look more carefully at mammary glands. But this is only a partial victory, as the NTP does not have the authority to regulate chemicals (it’s a research and advisory body), and the agency tests only about six to eight substances per year.

The NTP’s Fenton is a big fan of a new testing procedure, the gruesome-sounding “whole mount,” a way of preparing dissected rodent tissue to look for changes both subtle and overt. A rodent’s fourth or fifth mammary glands—those near the stomach—are removed, flattened, stained, and mounted flat to a slide, where they can be carefully examined for changes in cells or structures. Soon, Fenton hopes, every government lab will use it. If approved, NTP’s guidelines would also require that certain tests include doses of chemicals both in-the-womb and around the time of puberty.

The EPA is watching closely to see if and how the NTP’s new tests pass political and scientific muster. So is the Organisation for Economic Cooperation and Development, which guides chemicals testing in Europe. With added data from mammary glands, policymakers could have more reasons to keep unsafe substances off the market. If your breasts won’t benefit, your daughters’ just might.