DECODING THE ENVIRONMENT: Despite leads linking many chemicals to chronic diseases, some researchers argue that it is time to take a broader look at every possible internal and external exposures to detect more specific correlations Image: ISTOCKPHOTO/AZGEK
Anxious about BPA? Petrified of pesticides? Plenty of scientific literature shows that concerns about certain chemicals' potential to up the risk for chronic disease are justified. And although genetics can predispose a person to many ills, more than half of disease risks—and possibly as much as 90 percent—likely stem from environmental factors, according to recent epidemiological research.
Hard data—of the quality now gleaned from genetic studies—however, has been lacking in the environmental field. And if there is to be any hope of untangling the complex web of risks behind chronic diseases, many scientists argue, researchers need to develop an "exposome," a highly detailed map of environmental exposures that might occur throughout a lifetime, which can be mapped onto the etiology (the study of causes) of major illnesses, including cancer, diabetes and heart disease.
Environmental factors have long been relegated to questionnaires in epidemiological research, often requiring subjects to estimate a lifetime of exposure in a single question. Even for studies that have focused on environmental correlations, researchers "just ask people what their exposures are," Steve Rappaport, a professor of environmental health at the University of California, Berkeley, says. "How can you imagine you're going to get any resolution like that?"
Although broad-picture trends can help researchers draw connections among exposures and disease, with those loose associations, "you can't really tell what's going on" at a biological level, he notes.
Furthermore, chemicals do not just enter the body and persist in an isolated state. Once inside, they can interact with a wide range of cells inside the body, and often themselves undergo changes.
And chemical exposures of interest to researchers such as Rappaport do not only come from the world beyond our skin, but from natural processes occurring within our bodies as well. From biological processes that produce oxidative stress or inflammation, our bodies face a constantly changing internal environment. "We really need to think about the environment as what's going on inside the body and accept things that come from every source," says Rappaport, who lays out evidence in support of moving forward with exposome research in an essay published online October 21 in Science.
"We've never looked at the whole environment we have inside our bodies in a way that allows us to discover these things," Rappaport says.
But how does one profile a seemingly infinite set of external—and internal—factors?
Atul Butte, an assistant professor at Stanford University School of Medicine who has also worked in the field of environmental links to disease, admits that it is no small task. "You're looking at an unbounded set of variables here," he says. "But that doesn't mean we shouldn't try to start measuring them."
The toxins within
Many environmental risk studies have turned their sights on the everyday world people inhabit. Children and mothers-to-be have toted around air quality monitors; researchers have sampled drinking water for a litany of compounds. But Rappaport and his co-author, Martyn Smith, also a professor at Berkeley's School of Public Health, argued in their new essay that exposures should be assessed from within the body, such as via blood samples.
"People do think of chemical exposures are coming in from outside the body," Rappaport says. But, he argues, "if people are always thinking about air pollution and water pollution, we're not going to get very far." In fact, he adds, "there are so many natural processes that produce chemicals that are toxic—and are going on right inside the body."
And when compared with headline-grabbing pollutants, such as bisphenol A, phthalates or benzene, toxic exposures coming from within the body are much more common. "The blood concentrations of these things are really high compared to the concentrations you get from exogenous chemicals," Rappaport notes.
New findings about the human microbiome, for example, have shown that even more that we could have—or might have liked to—imagine. "We're full of bacteria that are generating waste products," Rappaport says. And from an exposure standpoint, "there's just no reason to think that we can ignore something like that."