Scientific advances that have made it possible to detect the tiniest traces of chemicals in the human body and the environment are shaping efforts to modernize U.S. chemical policies and regulations.
At issue is the main chemical law, the 1976 Toxic Substances Control Act, or TSCA, which was enacted when relatively little was known about the effects of chemical exposure on human health.
A modern tool that is likely to play a central role in the debate over a TSCA overhaul: biomonitoring, which measures chemicals at minute levels in human tissue and bodily fluids and provides evidence of direct exposure.
The Centers for Disease Control and Prevention, which runs the most comprehensive U.S. biomonitoring program, says the technique can identify people or groups with higher exposure levels than average and track them over time.
"Biomonitoring is far ahead of the science of interpreting what exposures mean for health," said Henry Falk, acting director of the CDC's National Center for Environmental Health, at a recent Senate hearing.
But interpreting test results isn't easy.
"Understanding the connection between our health and our environment, with its mixture of chemicals, diet and lifestyle stressors, is no less complex than understanding the intricacies of the human genome," said Linda Birnbaum, director of the National Institute of Environmental Health Sciences.
Consider, for example, that the time of exposure to a chemical can matter as much as the type of chemical.
Thomas Zoeller, a University of Massachusetts endocrinologist, said animal studies show the effects of chemicals that interfere with hormones differ depending on when that exposure occurs. Exposure matters most when an animal's brain is young and developing. But biomonitoring, he said, usually measures chemicals in adults without any idea of what kinds of exposure occurred during development.
So to prove a direct causation, scientists must link the timing of the chemical measurement to some damage believed to be connected to that specific time, Zoeller said. But that kind of information is hard to get. And teasing out problems caused by chemical exposures remains a challenge, he said.
"If I want to know what might happen -- the negative consequences -- of poor building materials, I would need to talk to a builder," Zoeller said. "We don't have the equivalent of that for brain development. No one knows how the body gets put together. Superimposed on that, we're trying to figure out what's interfering with the building blocks of development. What's that going to do? We're really at a disadvantage by not knowing."
So making links between chemical exposures and consequences requires a formal human study that sets parameters and controls for confounding factors, Zoeller said. But such a study would be unethical.
Some advocacy groups say the sheer quantity of chemicals detected in human bodies should spur government action to reduce toxicity and exposure to chemicals. The latest CDC biomonitoring study of 2,500 people detected 212 chemicals in their bodies.
The Learning and Developmental Disabilities Initiative last week released a biomonitoring study with the purpose of exploring the relationship between toxic chemicals and increasing rates of learning disabilities, such as autism and attention-deficit hyperactivity disorder.
It found what the researchers said were 61 "neurotoxic and endocrine disrupting chemicals" in the 12 people tested.
The study cannot establish that the presence of any chemicals cause developmental or other health disorders, but the report's authors say the detection of so many chemicals of concern should be enough to take strong regulatory action.
"I realize now more than ever why reforming our federal toxics law is absolutely essential to protecting our health and our children's health," said Maureen Swanson with the Learning Disabilities Association of America.
Others argue that biomonitoring data must be interpreted in context and that regulators should not bypass traditional risk-assessment processes that consider both exposure and hazard data.
The National Institute of Environmental Health Sciences is working to develop methods that allow for improved risk assessments. The institute is funding 32 projects focused on testing technologies that can measure environmental exposures, diet, physical activities, psychosocial stress and other factors in disease development, said Birnbaum, the agency's director.
Scientific uncertainties complicate lawmakers' efforts to revamp TSCA.
"Biomonitoring is really a biomarker of exposure, not of effect," said Richard Becker, senior toxicologist for the American Chemistry Council, an industry group. "You have to link the different pieces of information together to understand the significance."
The group has an ally in Sen. James Inhofe (R-Okla.). "You can't start legislating these levels where the science isn't known," Inhofe said. "The most important thing is that we do it with sound science."
Protecting human health
But for endocrinologist Zoeller, the fact that animal studies have shown chemicals such as the controversial plastics additive bisphenol A, or BPA, interfere with hormone signaling at low levels is enough to prompt regulatory action.
"We need to make sure we get effective legislation that regulates chemicals in a way that protects human health and, frankly, that just hasn't happened," Zoeller said. "If we know a chemical interferes with hormone signaling pathways, the argument becomes, 'How much does a human have to take to be affected?'"
He continued, "The underlying assumption is that we should be able to contaminate people up to some level. But I don't think we can continue to do that. We don't know enough to be confident about what those levels would be when a human has a lifetime to collect exposures."
Sen. Frank Lautenberg (D-N.J.), who is expected to introduce a TSCA reform bill soon, is in Zoeller's camp.
"There's no question that chemicals are essential to our modern lives ... but when we use these products, the chemicals in them can end up in our bodies," Lautenberg said. "And when the chemicals used in flame retardants, plastics or rocket fuel show up in our children's bodies, we have a potentially dangerous situation."