Issue Date: April 23, 2007
Tracking The Chemicals In Us
LARGE-SCALE MEASUREMENTS of chemicals in human tissues, or biomonitoring, is only about two decades old. But it has already led to a revolution in how chemical exposures are assessed in humans, to new opportunities in disease detection and prevention, and to a recognition that endocrine disrupters—compounds that mimic hormones or interfere with hormonal action—in the environment may cause a variety of adverse effects.
The benefits and challenges of biomonitoring, particularly of endocrine disrupters, were the subjects of an interdisciplinary conference hosted by the Chemical Heritage Foundation (CHF) in late March in Philadelphia. Attendees discussed the topics from different perspectives: how to detect the adverse effects of environmental chemicals in humans and wildlife; how to remediate endocrine disrupters in the environment; and how current regulations and conventional toxicology are ill-suited to deal with biomonitoring and endocrine disrupters. The conference was organized by Jody Roberts, Gordon Cain Fellow at CHF.
At the meeting, Larry L. Needham, chief of the organic analytical toxicology branch at the Centers for Disease Control & Prevention, presented an overview of what biomonitoring can and cannot do. In particular, he discussed two national biomonitoring surveys that CDC had conducted. These measured environmental chemicals or their metabolites in blood and urine from about 5,000 participants as a part of the National Health & Nutrition Examination Survey (NHANES).
The NHANES biomonitoring program aims to track the sources of chemicals found in humans and gain a broad picture of human exposures, Needham said. It establishes average exposures, tracks averages over time, and sets priorities in efforts to link exposures to disease, he said.
The survey in 1999-2000 (NHANES II) involved measurement of 116 chemicals, including metals, organochlorine pesticides, phthalate metabolites, dioxins, furans, polychlorinated biphenyls, and the nicotine metabolite cotinine. The survey in 2003-04 (NHANES III) included chemicals analyzed during the previous survey and a number of additional substances, such as pyrethroids, some herbicides, and more phthalate metabolites.
What biomonitoring cannot usually do is establish whether a specific substance detected in human tissue is responsible for disease, Needham observed. "The presence of a chemical in blood or urine does not imply that it is causing disease."
Nevertheless, NHANES data have benefited public health in several ways, Needham said. For example, they were used to assess the effectiveness of efforts to reduce exposures to lead and persistent organic pollutants, such as DDT, he explained. When lead was first banned from gasoline in 1976, models predicted that average levels of lead in blood would decline almost imperceptibly over the next four years, he said. But in fact, NHANES II data showed that average lead levels dropped from about 15.6 µg per dL in 1976 to about 9.5 µg per dL in 1980. As a result, the Environmental Protection Agency placed tougher restrictions on lead in gasoline, and by 1991, the average level in U.S. populations had declined to 3.5 µg per dL, he said. "However, lead paint is still a problem in the inner city where many young children have higher-than-average blood lead levels from exposure to lead paint chips and dust."
Biomonitoring was also crucial in assessing the effects of the July 1976 dioxin disaster, when a trichlorophenol plant in Seveso, Italy, exploded and released a kilogram amount of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) to the atmosphere, Needham said. Analysis of serum samples from members of the local population that were most highly exposed to the release showed TCDD levels in blood as high as 56,000 ppt. Some people in the zone closest to the disaster—zone A—developed chloracne, a permanent condition that resembles serious acne.
In addition, before the accident, the sex ratio of births in zone A was normal, about one to one, Needham said. But during the period beginning nine months after the accident until December 1984, 48 females and only 26 males were born in zone A, he explained. After 1984, the sex ratio returned to normal. The close dose-response correlation between TCDD exposure and female births strongly suggests that TCDD skewed the sex ratio in zone A, he explained. "This information would not have been available without biomonitoring."
Even so, biomonitoring is not well-understood by the general public. Often people react with despair when they realize from biomonitoring studies that almost everyone is contaminated with hundreds of chemicals, noted John Peterson Myers, chief executive officer and chief scientist at Environmental Health Services, a nonprofit think tank. "But this science presents us with opportunities to prevent human disease that we didn't have 10 to 15 years ago."
MANY ENVIRONMENTAL chemicals are found at parts-per-billion ranges in human blood, and skeptics say that such levels are not biologically active and therefore couldn't have adverse effects, Myers explained. " 'What is one in a billion?' they ask. It is one pancake in a stack 4,000 miles high."
But common pharmaceuticals often exert their biological effects at parts-per-billion ranges, Myers said, so it is likely that parts-per-billion levels of some environmental chemicals are also biologically active. For example, the target steady-state blood plasma concentration for bioactivity for albuterol, a drug used to prevent wheezing and shortness of breath, is 5 ppb.
"Experimental mice offer a dramatic lesson," Myers continued. He cited as an example the work of Retha Newbold, a toxicologist at the National Institute of Environmental Health Sciences, who exposed one group of mice to 1 ppb of diethylstilbestrol (DES) during gestation. Starting from birth, the exposed mice grew abnormally fast and became grotesquely obese as adults, while the unexposed control mice gained weight normally. Many other estrogenic chemicals cause similar effects in rodents, he said. "Exposure during prenatal development may permanently alter mechanisms involved in weight homeostasis."
A further complication with both scientists' and the public's understanding of environmental chemicals and their effects is that classic toxicology assumes that the high-dose effects of a chemical are predictive of low-dose effects, Myers said. But with natural hormones and with endocrine disrupters that act as hormones, complex dose-response curves are common. Often, one set of responses appears at very low doses, and another at high doses, he explained.
Among various endocrine disrupters, Paul M. D. Foster, deputy director of the Center for the Evaluation of Risks to Human Reproduction, singled out phthalates.
"Phthalates are interesting compounds because for years, toxicologists have considered them relatively nontoxic," Foster said. In addition to their primary use as plasticizers, phthalates are employed as inert compounds in drugs and pesticides and as solvents in cosmetics, and they are rarely listed as ingredients on the label, he noted.
Medium- to long-length phthalates are known to act as antiandrogens by disturbing androgen signaling in the testis and reproductive tract of rodent fetuses, Foster said. With this mechanism, they produce a suite of male reproductive tract malformations. In rodents, the defects include reduced sperm counts, undescended testicles, hypospadias (abnormalities of the penis), and early-stage testicular cancer.
For example, Foster said, when pregnant rats are treated with 500 mg per kg of body weight per day of dibutyl phthalate (DBP), the male pups have undescended testicles, decreased testosterone levels, and undeveloped germ cells in the testis. Undeveloped germ cells in the human testis cause testicular cancer.
The problems in rodents attributed to phthalates are similar to those that have been increasing in human populations, Foster pointed out. "For humans, there probably is something to worry about from phthalates," he concluded. In maternal urine, the DBP metabolite-monobutyl phthalate (MBP)-is mostly conjugated, meaning it is biologically inactive, he said. "But in amniotic fluid, most of the MBP is unconjugated," he explained. The human fetus could be harmed by DBP exposure because the levels of metabolite found in human amniotic fluid are about one-fifth the levels that cause adverse effects in rodents, he said. This means the margin of safety is only five, a ratio that risk assessors consider much too low.
"Furthermore, because humans are exposed to more than one biologically active phthalate and they act in the same way, an aggregate risk assessment of all the problematic phthalates should be done," Foster added.
FURTHER EXPOUNDING on the effects of endocrine disrupters was Terrence J. Collins, chemistry professor at Carnegie Mellon University. "Endocrine disrupters represent a tectonic-plate shift in how chemicals interact with society," he said.
The effects of endocrine disrupters on health are becoming more and more obvious, Collins said. For example, average levels of testosterone in serum among American males are declining 1% per year, probably as a result of endocrine disrupters in the environment, he said. One such endocrine disrupter is bisphenol A (BPA), which causes mammary gland hyperplasia (abnormal increase in the number of cells), and tumors in rodents and is likely responsible for some breast cancer in humans, he said. "The chemical industry has been built on the underlying premise that any useful chemical commercialized for anything other than drug purposes will not have a profound effect on health. This premise is wrong."
"We still do not have a comprehensive assessment of U.S. exposure to BPA," because only 394 archived NHANES III urine samples have been analyzed for the chemical, CDC's Needham observed. "In those samples, we found conjugates of BPA and benzophenone-3 (a chemical frequently used in personal care products), and there is a lot of debate about how much of the BPA is bound and how much is free—that is, biologically active."
To reduce human exposures to endocrine disrupters, Collins is working on ways to deal with BPA and other contaminants in water and factory effluents. He has been designing catalysts to degrade contaminants for more than 20 years. Most of the catalysts, trade-named TAML activators, are formed entirely from the biochemically common elements—carbon, hydrogen, oxygen, nitrogen, and iron—he said. They are nontoxic, water soluble, effective at very low concentrations, and about 1% of the size of the enzymes they mimic. They perform many different functions, he explained, including cleaning up chlorinated pollutants, BPA, and other endocrine disrupters; decontaminating pesticides; and removing trace pharmaceuticals from water.
Taking a broader look at human exposure to substances in the environment, Carl Cranor, philosophy professor at the University of California, Riverside, observed: "It is arguable that the current moral basis for legally regulating exposure to toxic substances is problematic." First, he explained, the current harm-based, or risk-of-harm-based, legal structure does not work well enough. Firms proposing to manufacture a new chemical, other than a pharmaceutical or pesticide, are required to submit only what they know about the product to EPA, he said, and for many new chemicals, the firms have no toxicity information.
And even if the legal structure could be made to work better with sufficient political will, there would still be a moral concern about the basis for current regulations, Cranor continued. "Because most substances are subject to postmarket regulation, the existing legal structure results in involuntary experiments on citizens. The bodies of the citizenry are invaded and trespassed on by commercial substances, arguably a moral wrong," he said. "If we were to recognize that chemical invasion is a wrong," then we could authorize actions—especially testing—to prevent additional wrongs, he said. "We can gain greater sovereignty over our bodies by requiring no trespass without testing."
Conference organizer Roberts summarized the research opportunities ahead. "Environmental endocrine disrupters present challenges to our current systems of monitoring and regulating synthetic chemicals in the environment," he said. They have potential activity at doses that are orders of magnitude lower than current dose limits for other toxicants, he explained. In many cases, their effects can often be observed years or even decades after the initial exposure, especially if it occurs in the womb, he said.
"Understanding these processes requires new instrumental and analytical tools," and researchers need to "begin thinking about what that new set of tools would look like," he said. "We need better ways to know how chemicals are acting inside our bodies."
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