sures. I really wanted to look more closely at this window.” Although she started out looking for cancer markers, she was soon curious about other pressing health conditions in the Harlem community. That Perera was even looking at environmental causes of illness was both unusual and unfashionable. For the past two decades, cancer researchers and other molecular biologists have spent much of their time riveted by the genome, believing it would unlock the secrets of disease. But cellular life isn’t determined simply by the blueprints of DNA. It’s now understood that cells are designed to interact nimbly with the outside world and that genes get turned on or off—and are sometimes mutated altogether—by environmental exposures from diet, inhalation, even transmission through the skin. To truly understand human health and disease, scientists need to look at both the genome and—to use a term coined in 2005 by Christopher Wild, a cancer epidemiologist—the “exposome.” Perera decided to do just that. Today, as director of the Columbia center, she oversees what has become one of the most respected epidemiological data troves in the country. (She is also a trustee of NRDC.) Starting in late 1998, Perera and her colleagues recruited more than 700 pregnant women from hospitals in Harlem, Washington Heights, and the South Bronx for what’s known as the Mothers and Children Study. Now, 15 years later, the team has retained three-quarters of its original participants, and the first babies are entering their teenage years. Such “prospective” studies, which follow a group over time and measure their health outcomes, are considered the gold standard in scientific research because they don’t rely on retrospective memory or old, imperfect medical records. Blood and urine samples have been banked since the mothers’ pregnancies and deliveries, as have samples from the children, and researchers can go back to these as they ponder new questions. It’s now known that many chemicals can cross the placenta, once believed to be a sacrosanct barrier. This is disquieting, because the vast majority of these chemicals have never been tested for human health effects. Furthermore, the medical community agrees that many diseases and conditions, including obesity, cancer, and autism, are modulated by both genes and fetal exposures. In September, the American College of Obstetricians and Gynecologists and the American Society for Reproductive Medicine issued a joint statement saying, “The scientific evidence over the last 15 years shows that exposure to toxic environmental agents before conception and during pregnancy can have significant and long-lasting effects on reproductive health.” (The chemical industry, however, is seeding doubt. The American Chemistry Council’s chief medical officer responded that ACOG’s evidence was based on “a limited number of flawed studies.”) It makes sense to developmental biologists that fetal exposures matter; this is when the cells in the body and brain are on the biggest adventure of their lives, differentiating and replicating like a one-way train. Once it’s left the station, it doesn’t go back. Children, of course, are harder to study than lab animals, because they are exposed to so many different conditions that can confound the results. That’s why large, prospective epidemiological studies are so critical. Perera knows both what’s in these babies’ bloodstreams and what happens to them as they age. The larger the study, the stronger the statistical power. Even so, epidemiological findings are necessarily couched in terms of “associations” rather than causal 3 2 onearth
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links. So epidemiologists often look to better-controlled animal studies as a guide, as well as to molecular lab investigations of human blood and tissue samples. With enough replication and diverse strategies, a picture begins to emerge. The center’s early studies found significant associations between pollutants measured in the mothers and difficult birth outcomes, including low birth weight and small head circumference. The main culprits were PAH and chlorpyrifos, a then-common organophosphate pesticide used indoors to kill roaches and bedbugs. (Later the team would look at flame retardants.) Perera also documented that these chemicals damaged cellular DNA. Chlorpyrifos was found in the umbilical cords of virtually every mother in the early samples. Since 2001 it has been phased out of residential use, but exterminators are still caught using it. It also remains a common agricultural pesticide and so ends up as a residue in food. Perera and her colleagues knew that reduced head circumference had been linked to lower IQ scores, and animal experiments had shown that chlorpyrifos killed developing brain cells and induced behavioral changes in rats. In some well-known experiments, for example, rats given low doses of the pesticide while in the womb or shortly after birth later had frederica perera trouble learning their way around a maze. began by lookAlthough Perera and ing for cancer her team didn’t know how these chemicals markers, but she might be changing the brain, they were deterwas soon curimined to find out all ous about other they could about the children’s growth and pressing health development. The children (now teenagers), conditions in who are mostly Dominithe harlem can and African American, would be regularly community. tested on everything from reading ability and motor skills to psychological yardsticks like aggression, risk-taking, and depression. By the time they were 3 years old, the children with the highest exposures to the pesticide tested as much as six points lower on motor skills and three points lower on mental development and were significantly more likely than those with lower exposures to suffer from attention and hyperactivity problems. These findings were published in 2006 in Pediatrics. Other studies showed that children most exposed to PAH were nearly three times as likely to show cognitive developmental delays. By the time they were 7, the children most exposed to chlorpyrifos were showing deficits in working memory, a key component of IQ. Their working memory declined by 2.8 percent and their full-scale IQ by 1.4 percent, after adjusting for variables like tobacco smoke and maternal intelligence. The center’s findings on the cognitive impacts of PAH and pesticides bolster what other researchers have found in Boston, Cincinnati, and California. Perera teamed up with a similar cohort study in Poland that found that higher prenatal exposure corresponded to an average