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After returning home, she refocused her research on environmental history, the study of the shared history of people and the land. Her first book, published in 1995, explored the root causes of the failing health of forests in the western United States. She followed with an examination of riparian zones, showing how scientific and cultural ideas about nature triggered often-contentious disagreements about how to manage these areas.

Her interest in environmental pollutants was sparked by conversations with one of her graduate students, a native of Wisconsin named Maria. Growing up on the shores of the Fox River, Maria spent her summers swimming in the Green Bay waters where the Fox River empties. Friday feasts of local fish were a family tradition. Only years later did she realize the river was choked with PCBs, released over decades by paper mills lining its banks. The Fox River became a Superfund site, and Maria became an environmental scientist. She became keenly aware of the dangers of PCBs, which can collect in the body, causing cancer and disturbing hormonal activity.

By 2000, Maria confronted a difficult choice. Pregnant with her first child, she worried about whether to breastfeed her baby, knowing that the PCBs she’d accumulated during her childhood could flow into her baby with breast milk. At the same time, how could she not breastfeed her baby, considering all the benefits it provided?

Maria’s dilemma haunted Langston. It also left her curious. What in our history could explain why such painful decisions were necessary, and how might our past end up shaping the future?

“Part of what interests me is that we eat fish in the here-and-now, but fish have the traces, the legacies, of the past five decades of industrialization,” says Langston. “And our children and grandchildren will continue to bear those legacies.”

In her research for Toxic Bodies, Langston went back to the days just after World War II, when advances in the manufacture of synthetic chemicals spawned an array of new industries. In the decades since, synthetic fertilizers, pesticides and pharmaceuticals have flooded the U.S. consumer market, bringing with them scores of benefits. The products have boosted yields of the nation’s most important food crops, kept pests at bay and ushered in an age of better living through chemistry. But we know now that many of these wonder chemicals have a dark side: Their use can exact a devastating toll on the environment and the health of people and animals. And as Langston argues, we often continue to feel the impact of chemicals even decades after they were used.

The focus of her book is diethylstilbestrol, or DES, a hormone-mimicking chemical approved by the then fledgling Food and Drug Administration in 1941. A potent form of synthetic estrogen, DES was shown in early tests to cause cancer and disrupt sexual development in laboratory animals. Nevertheless, the FDA first sanctioned it as a hormone replacement for women during menopause and later as a treatment for pregnant women to prevent miscarriage. DES found further use in the livestock industry, which deployed it to increase meat in chickens, turkeys and cattle without increasing feed. Millions of women were prescribed DES, and millions more were exposed to residues of the chemical through meat and polluted runoff from farms Yet the FDA didn’t fully ban the chemical until the early 1970s.

Why the agency approved DES and then failed to restrict it for so long is central both to Langston’s book and to the situation we face with many other contaminants. Langston explains that since the 1920s, debate has raged over whether chemicals should be regulated based on their potential to cause harm or evidence of actual harm. In many instances through history, the latter argument won out: Regulators agreed to approve use of chemicals where the effects on humans were unknown or unclear.

And there’s the rub. Demonstrating that chemicals will harm us is tough because such lab tests can’t be carried out on people. Typically, the best evidence of a chemical’s effects come from studies on lab animals, but scientists are far from unanimous about how well those studies predict what might happen in human populations. Even extrapolating lab studies to wild animals is tricky. For one, environmental levels of toxins are typically much lower than the doses employed in toxicology tests, says Bill Karasov, a colleague of Langston’s in forest and wildlife ecology who has studied the effects of contaminants on fish-eating birds, including loons and bald eagles. Animals also vary tremendously in their vulnerability to different toxins. Some species may be worse than others at clearing a chemical once they consume it, for example, or they may harbor especially sensitive target sites in the body.