Monitoring Human Tissues for Toxic Substances (1991)

Contamination of the environment with chemical substances, including man-made chemicals and pollutants, can pose a threat to public health (Buffler et al., 1985; Upton et al., 1989). The concern can be justified by several examples: In 1984, the National Research Council (NRC) reported that more than 60,000 chemicals¹ were in commercial use in the United States (NRC, 1984a), and the number was growing by an estimated 1,000 per year. About 3,400 pesticide ingredients (active or relatively inert), such as solvents, are registered for commercial use, but data adequate for a complete health hazard assessment are available on only an estimated 10%, and toxicity information was lacking (as of 1984) on almost 40% (NRC, 1984a). Comprehensive monitoring data on industrial chemicals other than pesticides are even less abundant. More than 700 organic chemicals, including 40 carcinogens, have been identified in the U.S. drinking water supply (Harris et al., 1987). Numerous industrial chemicals (including trace metals, polycyclic aromatic hydrocarbons, and volatile organic chemicals) have been detected in ambient air (Hunt et al., 1986). Food can also be a source of exposure to industrial chemicals (NRC, 1987; Gunderson, 1988).

Toxic substances in ambient and workplace air, water, and soil, and in the food supply move readily from one medium to another, and data on the presence of individual industries’ chemicals in specific media are sparse. Reliable

monitoring data that include most routes of human exposures are available on only a few chemicals, and as noted in Human Exposure Assessment for Airborne Pollutants (NRC, 1991), monitoring the environment by identifying and measuring concentrations of chemicals in various media rarely can characterize human exposures. However, determination of concentrations of specific chemicals in human tissues (such as blood and adipose tissue) is a major tool for integrating human exposures across media and time.

The Public Health Service established the National Human Monitoring Program (NHMP) in 1967 and transferred it to the Environmental Protection Agency (EPA) when that agency was formed in 1970. The NHMP consists of the National Human Adipose Tissue Survey (NHATS), and various special studies that support other programs that require data on chemical exposures. EPA would also like to introduce the National Blood Network (NBN). Because many chemicals that may be harmful to humans are pesticides or other persistent hydrocarbons, EPA’s Office of Pesticides Programs operated the NHATS to monitor pesticide concentrations in adipose tissue, where they are likely to concentrate. In 1981, EPA’s Office of Toxic Substances (OTS) was given responsibility for the NHATS program and redesigned NHATS to identify chemicals in human tissues, establish baseline data and trends, and identify population groups with unusually high concentrations of toxic chemicals, thus making NHATS more responsive to the needs of OTS. Details of the program’s history are in Chapter 2.

The National Human Monitoring Program, as developed by EPA, has been funded at decreasing levels, and the NHATS is now the only operating component of the NHMP. In late 1987, EPA announced its intention to delete the NHMP from its budget, although funds were committed through fiscal 1990. Congress, however, instructed EPA to retain the program until its utility could be reviewed. OTS therefore asked the National Research Council to review and evaluate the effectiveness and potential applications of the data collected in the NHMP. The NRC Board on Environmental Studies and Toxicology formed the Committee on National Monitoring of Human Tissues to evaluate the NHMP, to provide recommendations regarding its design and utility, and to identify scientific and technical issues that should receive priority attention. The committee, in keeping with its charge, held a workshop at the National Academy of Sciences in Washington, D.C., on January 24–25, 1989. The workshop was organized to enable committee members to hear from persons who had used NHATS data or tissues, were considered

likely to use NHATS data, and who were involved in the operation of specimen and tissue banks. See Appendixes A, B, and C, respectively, for the workshop agenda, participant list, and summary.

There is general agreement about the need to reduce the release of toxic substances into the environment to protect public health and the environment itself. A corollary is the need to anticipate toxic hazards before they cause disease. A recent National Academy of Sciences report concluded that controlling toxic substances in the environment warrants high priority and challenges both the legal and public health systems of the nation (NRC, 1988). To anticipate and control environmental toxicity, new ways to identify and assess human health hazards in the environment in a timely fashion are needed.

The EPA Science Advisory Board (SAB) has recommended a research strategy for preventing or reducing environmental risks, including those resulting from chemical pollution (EPA, 1988). The strategy includes a long-range research program aimed at characterizing the sources, transport, and fate of environmental pollutants; assessment of total environmental exposure with personal monitoring, models, and biologic markers of exposure; and assessment of human health effects of pollutants with biologic markers of disease, extrapolation of animal effects to humans, and epidemiology. The SAB underscored the need for EPA to improve its capability to anticipate environmental problems, and it cited several instances in which chemical contamination problems (including Kepone in the James River, polybrominated biphenyls in feed, and tributyltin in harbors) were not discovered until after substantial health or economic costs were incurred. The SAB stated (p. 12):

A related recommendation of the SAB was that EPA “expand its efforts to understand how and to what extent humans are exposed to pollutants in the real world” (p. 14). Heightened recognition of potential health risks, which has resulted from ever more sensitive chemical measurement coupled with toxicologic characterization of delayed effects of chronic exposures, has led to a search for indicators of biologic responses to exposure that reveal, before disease occurs, the progression of events that lead to disease. Tissue monitoring constitutes a natural link between conventional environmental surveillance and emerging methods of assessing biologic injury that results from exposure to toxic chemicals. For example, detection and measurement of a pollutant chemical in tissue samples would indicate that exposure to it has taken place. A well-designed national program to monitor toxic chemicals in human tissues is directly relevant to identified research needs and is a necessary component of an anticipatory strategy aimed at early identification of and response to health and environmental problems.

Tissue monitoring has valuable attributes if used as one component of an effort to manage environmental quality and to protect public health:

• Tissue samples reflect exposures accumulated over time.

• Tissue samples reflect exposures by all routes, including some that are difficult or impossible to assess by environmental measurement (such as handto-mouth ingestion in young children).

• Pollutants in tissue samples have undergone the modifying effects of physiology (rates of uptake, distribution, bioconversion, elimination, and storage) and biologic availability.

• Some agents are more concentrated, and so more readily detectable, in tissue samples than in the environment.

• Tissue samples offer the opportunity to correlate, within a given person, the tissue concentration of toxicants with other tissue-based biologic markers or indicators of effect that might be predictive of injury or disease.

All those characteristics, taken together, make tissue monitoring as an assessment tool an important adjunct to environmental monitoring that is uniquely valuable in indicating both exposures and doses that lead to potentially harmful effects.

During the early years of the NHATS, few planning documents explicitly stated EPA’s view of the relationship between national human tissue monitoring and the agency’s responsibilities under various legislative mandates. In its 1976 report to Congress (Environmental Research Outlook for FY 1976 Through 1980), the Office of Research and Development did not explicitly refer to human exposure and assessment or to monitoring of biologic samples, although the need for a national plan for monitoring pesticides was identified under the Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA). EPA’s mission in recognition, evaluation, and control of environmental chemical hazards to human health corresponds broadly to the goals of NHMP in surveillance of trends in human exposures to chemical hazards in the United States, but no detailed plan for tying the NHMP to specific EPA regulatory programs or research objectives appears to have been developed. Some specific NHMP results have been widely used and cited by several EPA programs. Environmental Progress and Challenges: EPA’s Update (EPA, 1988) cites NHMP results in monitoring exposures to polychlorinated biphenyls, pesticides, and agents of more recent concern, such as dioxins, in human biologic samples.

More recent planning documents indicate that the NHMP or a successor program could be readily integrated into new research and program objectives. The role of population-based human tissue monitoring in the overall context of EPA’s mission and current goals lies in providing a link between environmental concentrations of pollutants in the various exposure media and biologic consequences that might be predicted or observed. The necessity for measurement of exposures in a manner that leads to realistic projections of dose is explicitly recognized in the EPA planning document Protecting the Environment: A Research Strategy for the 1990s (EPA, 1990). In citing emerging environmental management issues including the need for multimedia management of toxic chemicals and the apparent disparity between relative health risk and regulatory priorities, EPA has announced new initiatives aimed at broadening the research base for agency planning and developing new programs and at transcending the previous compartmentalized “end-of-the-pipe” approach to environmental management. The new emphasis on human health risk as a unifying focus in regulatory programs for air quality, drinking water, waste management, toxic chemicals, pesticides, ecologic protection, etc., would specifically indicate the ongoing need and value of a national human monitoring program. Of the top five priorities identified by EPA for new research programs, third in importance is “development of a national data base on the

extent of human exposure to pollution in the U.S.” (p. 2). Many of the individual subjects for research identified in the report are related directly to aspects of the present NHMP program or to proposed enhancements: development of biologic markers of exposure, development of techniques for the assessment of human exposure, and reliance on indicators of internal concentrations or quantities of pollutants (doses) rather than external quantities (environmental concentrations), for risk assessment purposes.

In summary, the relevance of national human tissue monitoring to EPA’s mission has always been identifiable. Recent strategic planning to identify EPA’s priorities for the next decade give human tissue monitoring a more central role than it had in the past and would require that some aspects of the NHMP be created, if they did not already exist.

An ideal national human monitoring program should:

• Measure concentrations of known chemical contaminants in human tissues and identify new or previously unrecognized hazards related to chemical substances, especially those attributable to human activities.

• Establish trends in body burdens of toxicants that result from changes in manufacture, use, and disposal patterns, and thus monitor the results of programs intended to control specific chemical hazards.

• Provide biologic samples and data for the evaluation of relationships between environmental exposure and toxic effects for purposes of risk assessment.

• Identify population groups (by age, geographic location, etc.) that might be at risk because of high body burdens.

• Provide data for comparison with results of complementary environmental monitoring programs (e.g., NOAA’s Mussel Watch Program and National Institute of Standards and Technology (NIST)’s Environmental Specimen Banking Program).

• Provide human tissues essential for research on related matters, such as determination of body burdens; distribution of chemicals among body compartments; identification of biologic markers; and procurement, storage and analysis of human tissues.

• Allow assessment of past exposure to newly identified toxicants.

The products of such a monitoring program—reports of tissue concentra-

tions of contaminants representing groups of the U.S. population—will stimulate the formulation of questions regarding patterns of exposure and other exposure-related issues. Those questions might not be answerable with existing information, but they can focus regulatory or scientific attention on new problems. Estimates of exposures based on tissue concentrations of contaminants (discussed in Chapter 3) can contribute to quantitative risk assessment of detected agents or of nondetected agents with known analytic detection limits.

Most of this report expands on that recommendation.