Chemical "soup" clouds connection between toxins and poor health

From Nature:

From plastics to flame retardants, the ubiquitous chemicals of our daily lives have raised public health concerns like never before. Inside the Beltway, however, data-crunching scientists are often no match for industry lobbyists and corporate lawyers. The exception, no doubt, is Linda Birnbaum, the toxicologist who leads, two little-known scientific agencies, the National Institute of Environmental Health Services (NIEHS) and the National Toxicology Program (NTP).

Scientific American sat down with Birnbaum in Washington, D.C., to learn more about environmental health, toxic chemistry and the politics of chemical regulation.

How much of human disease is due to environmental exposures?
The estimates vary, and it depends on how you define environment. People often say it's about 30 percent. I think that's defining environment fairly narrowly, considering only environmental chemical exposures, but your environment includes the food you eat, the drugs you take, the psychosocial stress you're exposed to and so forth. After all, what's the difference between a drug and an environmental chemical? One you intentionally take and the other one you don't. Considering all that, I would say then the environment is much more than 30 percent. We also know—especially from studies of identical versus fraternal twins—that for many different diseases, genetics is not the whole story. Actually, I think it's time to stop asking, "Is this caused by genes or is this caused by the environment?" because in almost all cases, it's going to be both.

Why has it been so difficult to link environmental exposures to specific health consequences?
Nobody is exposed to one chemical at a time, right? I mean we live in a soup of chemicals and we live in a soup of exposures. Here, I'm having a lemonade. Well, it's not only lemon in here. I'm sure there's some sugar. There might be a preservative or something. I don't know what's in this. So think of all those things interacting, but when we test chemicals in the lab we tend to test them one at a time.

More here.