BACKGROUND

Exposure science is essential for the protection of public health and the environment. However, the challenges and opportunities for exposure science are considerable. The ability to address them will influence advances in human health and ecosystem protection. Exposure science also will play a role in decision-making in other arenas, including consumer-product safety, environmental planning, climate-change mitigation, and energy development. This report provides a roadmap to navigate the future of exposure science to achieve greater integration and maximize its utility in the environmental and occupational health sciences, environmental-systems science, risk assessment, sustainability science, and industrial ecology.

Exposure science addresses the contact of humans and other organisms with chemical, physical, or biologic (CBP) stressors¹ (EPA 2003; EPA 2011b) over space and time and the fate of these stressors within the ecosystem and organisms—including humans. Although methods of assessment will depend on the situation, exposure science has two primary goals: to understand how stressors affect human and ecosystem health and to prevent or reduce contact with harmful stressors or to promote contact with beneficial stressors to improve public and ecosystem health. The impact of environmental stressors on human and ecologic health is enormous.

For example, the World Health Organization estimates that 24-40% of global disease burden (healthy life-years lost) can be attributed to environmental factors (Smith et al. 1999; WHO 2004; Prüss-Üstün and Corvalán 2006). However, it is not possible to be exact in such calculations, partly because what is “environmental” is not defined consistently (see Box 1-1 for use of the term environmental in this report). In a burden-of-disease context, environmental factors play a role in nearly all diseases, even ones that are not caused directly by environmental risk factors, by altering the course of disease initiated by other causes. In addition, if the total burden of disease is simply decomposed into “nature” or “nurture”, it fails to account directly for the possibly large proportion that could be due to the interplay between the two (gene—environment interactions). Improving our understanding of environmental factors and their relationships with disease is critical for preventing illness and death.

BOX 1-1

Definition and Scope of Exposure Science. Exposure science is defined by this committee as the collection and analysis of quantitative and qualitative information needed to understand the nature of contact between receptors (such as people or ecosystems) (more...)

With respect to ecosystems, the 1999 National Research Council report Our Common Journey: A Transition Toward Sustainability (NRC1999) reported that the rising losses of wild nature, species number, species diversity, and ecosystem integrity were associated with exposures to environmental stressors, including those related to urban and agricultural land conversion and climate change.

Figure 1-1 illustrates the relationship of exposure to other key elements along the environmental-health continuum from the source of a stressor to an outcome. This figure has evolved from previous diagrams (for example, Smith 1988a; Lioy 1990; NRC 1998; EPA 2009a). For more than 20 years, this framework has demonstrated the central role of exposure science in environmental health science in that exposure sits midway between the sources of pollution (and other stressors) on the left—elements that typically can be controlled—and adverse health outcomes on the right, which need to be prevented. Exposure is strategically located upstream of dose and yet provides information and metrics that inform source control and health risk.

FIGURE 1-1

The classic environmental-health continuum. Figure 1-2 illustrates the revised version discussed in the present report. Source: Adapted from EPA 2009a.

There are many notable examples of the roles that exposure science can play in protecting public health. Consider how measurements of childhood blood lead concentrations since the 1970s reveal the dramatic efficacy of lead removal from gasoline in reducing exposure to this neurotoxicant in children (Muntner et al. 2005; Jones et al. 2009). Population-scale measurements of cotinine in urine document the reduction of exposure to second-hand tobacco smoke that resulted from control of tobacco-smoking in the workplace and public areas (EPA SAB 1992). Exposure modeling from the U.S. Environmental Protection Agency’s (EPA) National-Scale Air Toxics Assessment program has provided valuable information for communities on their exposure sources, concentrations, and risks and has helped to shed light on exposure disparities and environmental-justice issues (for example, Pastor et al. 2005).

Exposure science has played a critical role in understanding the influence of stressors on ecologic systems. For example, extensive exposure assessments of polycyclic aromatic hydrocarbons (PAHs) have been linked to liver damage in bottom-dwelling fish in Puget Sound, and field studies have demonstrated that containment of PAH sources has led to declines in PAH concentrations and a resulting decline in liver damage in fish (Myers et al. 2003).

Exposure science has applications in industrial, military, commercial, and global contexts. It is central to tracking chemicals and other agents that are introduced into global commerce at increasing rates, often with little information on their hazard potential (GAO 2005). Increasingly, exposure science is used for homeland security and the protection of deployed soldiers. Rapid detection of potentially harmful radiation or toxic chemicals is essential for protecting troops and the general public (IOM 2000). The ability to detect chemical contaminants in drinking water at low, biologically relevant concentrations quickly can help to identify emerging health threats, and monitoring of harmful algal blooms and airborne pollen can help to identify health-relevant effects of a changing climate.

As described in more detail in Chapter 3, applications of exposure science are critical for toxicology, epidemiology, risk assessment, and risk management. For example, toxicology provides information about how different chemical concentrations may affect public or ecologic health in laboratory studies or computer models, but the value of information is greatly increased when it is combined with comprehensive and reliable exposure information. Similarly, epidemiology requires exposure information to compare outcomes in populations that have different exposures. Collection of better exposure data can also provide more precise information regarding alternative control or regulatory measures and lead to more efficient and cost-effective protection of public and ecologic health.

In addition to its applications to other fields, exposure science data can be used independently to define trends, assess spatial or population variability, provide information on prevention and intervention, identify populations or ecosystems that have disproportionate exposures, and evaluate regulatory effectiveness.²

Exposure science is also poised to play a critical role in improving the ability to understand and address increasingly important human health and ecologic challenges and to support the development of sustainable industrial, agricultural, and energy technologies. Recognizing the need for a prospective examination of exposure science, EPA and the National Institute of Environmental Health Sciences asked the National Research Council to develop a long-range vision for exposure science and a strategy for implementing the vision over the next 20 years (see Appendix B for statement of task). In response to this request, the National Research Council convened the Committee on Human and Environmental Exposure Science in the 21st Century. The committee—which comprised experts in monitoring, modeling, environmental transport and transformation, geographic information science and related technologies, measurement and analytic techniques, risk assessment and risk management, epidemiology, occupational health, risk communication, ethics, informatics, and ecologic services—prepared this report.

DEFINING THE SCOPE OF EXPOSURE SCIENCE

Exposure science—sometimes defined as the study of the contact between receptors (such as humans or ecosystems) and physical, chemical, or biologic stressors—can be thought of most simply as the study of stressors, receptors, and their contact, including the roles of space and time. For example, ecosystems are receptors for such stressors as mercury, which may cascade from the ecosystem to populations to individuals within the ecosystem because of concentration and accumulation in the food web, which leads to exposure of humans and other species. As the stressor (mercury in this case) is absorbed into the bodies of individuals, it may come into contact with other receptors, such as tissues and organs.

As the scientific communities generating and using exposure data have evolved, so have the terms and definitions used to characterize exposures. Some refer to dose (exposure dose, target dose, or external dose), others to exposure (for example, external or internal exposure), and yet others to an amalgam (exposure is external, dose is internal). A consistent language for the field of exposure science is important for communicating within the field and among disciplines and for developing exposure-science metrics for source monitoring and exposure prevention and reduction. The evolution of the field over the past 15 years has included a greater emphasis on the use of internal markers of exposure to assist in defining exposure-response relationships. As such, the conceptual basis of the field includes both external and internal exposures, using external measurement and modeling methods and internal markers as tools for characterizing past or current exposures. Appendix C provides more detailed discussion on the application of this terminology.

To reflect the definition of exposure science and to embrace a broader view of the role that exposure science plays in human-health and ecosystem-health protection, the committee developed the conceptual framework in Figure 1-2.

FIGURE 1-2

Core elements of exposure science.

The conceptual framework identifies and links the core elements of exposure science: sources of stressors, environmental intensity (such as pollutant concentrations³), time—activity and behavior, contact of stressors and receptors, and outcomes of contact. Figure 1-2 shows the role of upstream human and natural factors in determining which stressors are mobilized and transported to key receptors. (Examples of factors include choosing to use natural gas vs diesel buses, or choosing to pay more for gasoline to drive a car vs taking the bus, where the choice influences the source and can also influence behavior.) It indicates the role of behavior of receptors and time in modifying the contact that results from environmental intensities that influence exposure. It brings both external and internal environments within exposure but retains the idea that exposure is measured at some boundary between the source and receptor and that dose is the amount of material that passes or otherwise has influence across the boundary to come into contact with the target system, organ, or cell and produces an outcome. For example, a dose in one tissue, such as the blood, can serve as an exposure of another tissue that the blood perfuses. Figure 1-2 recognizes the feedbacks inherent in exposure science. Consider, for example, how behavior changes in a diseased person or organism and influences exposure. The outcome can also affect the source, as when a person who has an environmentally mediated infectious disease becomes a source of pathogens in water supplies (Eisenberg et al. 2005).

Figure 1-3 frames an exposure narrative that plays out in space and time, and is intended to elucidate the stressor-receptor linkages at different levels of intergation. As a human (or fish, bird, or other organism) has changing contacts with different habitats, the intensity of a stressor changes, as do the number and duration of contacts. Here, exposure amounts to a multidimensional description of the location, time, and intensity of the target—stressor contacts. The exposure narrative covers relationships between receptors and locations and between locations and stressors; it provides a basis for drawing inferences about receptor—outcome relationships. That often requires recognition that any receptor can be associated with multiple environments (locations) and that locations can be associated with multiple stressors. Exposure science can be applied at any level of biologic organization—ecologic, community, or individual—and, within the individual, at the level of external exposure, internal exposure, or dose.

FIGURE 1-3

An illustration of how exposures can be measured or modeled at different levels of integration in space and time, from source to dose, and among different human, biologic, and geographic systems. That is exposure science can be applied at any level of (more...)

THE PAST MILLENNIA

To appreciate the vision for exposure science in the 21st century (discussed in Chapter 2), it is important to understand its historical context. Exposure science arose from such disciplines as industrial hygiene, radiation protection, and environmental toxicology, in which the importance of assessing exposure has been demonstrated. In one of the earliest efforts to address exposure, the ancient Greek physician Hippocrates (about 400 BC) demonstrated in his treatise Air, Water, and Places that the appearance of disease in human populations is influenced by the quality of air, water, and food; the topography of the land; and general living habits (Wasserstein 1982). In the 1500s, the physician and alchemist Paracelsus framed the widely cited toxicologic concept that “dose makes the poison” (Binswanger and Smith 2000). Ramazzini, in his 1703 treatise Diseases of the Workers, identified workplace exposures to single and multiple agents and the migration of contaminants into the community environment as causing disease (Ramazzini 1703). Percivall Pott first demonstrated the association between cancer and exposure to soot with his studies of scrotal cancer in chimney sweeps (Pott 1775). John Snow’s study of water-use patterns and their relation to disease in London allowed him to link a source of water contamination to cholera (Snow 1885). The avoidance of potentially harmful exposures through the separation of land use between human residences and industrial facilities was proposed in the latter part of the 19th century (Howard 1898).

Use of exposure assessment in radiation health protection can be traced back to roughly 1900 after the discovery of x rays. During the 1920s, Alice Hamilton established the formal study of industrial medicine in the United States. The metrics for and applications of exposure science to radiation protection have grown in sophistication and reliability over the last century (NRC 2006; ICRP 2007; EPA 2011c). Many of the basic principles for measuring, monitoring, and modeling exposures to airborne contaminants, including the earliest use of exposure biology, come from the field of industrial hygiene. The publication of Silent Spring (Carson 1962) and its focus on the transfer and magnification of persistent pollutants through food webs fostered the growth of environmental toxicology and chemistry, which address chemical fate and transport through multiple media and multiple pathways.

By the middle 1980s, exposure evaluations had evolved into an established scientific discipline that moved beyond single routes, single chemicals, and single pathways toward an understanding of “total” exposure. The 1991 National Research Council report Human Exposure Assessment for Airborne Pollutants (NRC 1991a) laid the foundation for further development of the field by defining the core principles of exposure assessment. Between 1980 and 1985, the Total Exposure Assessment Methodology (TEAM) study was conducted to assess personal exposures of 600 residents in seven US cities to chemical exposures by one or more routes of entry into the body and to estimate the exposures and body burdens of urban populations in several cities (EPA 1987). The TEAM studies established a framework for examining total human exposure covering multiple routes of entry into the body (Wallace 1987).

By promoting the concept that it is important to “measure where the people are” (Wallace 1977), the TEAM studies revealed new source categories and control options to reduce or prevent exposures. For example, application of the concept resulted in increasing attention to exposures indoors, where people spend a substantial portion of their lives (Smith 1988a). Globally, it pointed to the importance of indoor pollution in rural areas of developing countries, where a large portion of the world’s breathing is done but relatively little research or monitoring was being conducted (Smith 1988b).

Control measures revealed by a total-exposure framework include measures to increase the time that doors are closed between a house and its garage in the United States and thereby reduce human exposure to tailpipe emissions in the home, even though this has no effect on vehicle emissions or ambient concentrations in the garage. The exposure control included a simple spring on the door to allow it to stay open a shorter time.

Two major advances that helped to establish the credibility of exposure science as a discipline were the formation of the International Society of Exposure Analysis in 1989 (now the International Society of Exposure Science) and the publication of the Journal of Exposure Science and Environmental Epidemiology in 1990.

A number of important milestones followed. In 1992, EPA published its Guidelines for Exposure Assessment, which served as a companion to its toxicology and risk-assessment guidelines. That was followed in 1993 by the initiation of the National Human Exposure Assessment Survey (NHEXAS), which evaluated human exposure to multiple chemicals on a community and regional scale (EPA 2009b). NHEXAS monitored chemicals in blood and urine; incorporated environmental sampling of air, water, soil, and dust; and conducted personal monitoring of air, food, and beverages (NRC 1991b; EPA 2009b). It brought attention to the role of the proximity of emissions as opposed to the magnitude of emissions in determining overall exposure—low-level emissions near human receptors, such as those from indoor environments, need to receive at least as much attention as outdoor stack emissions (Sexton et al. 1995). In 1997, EPA’s Exposure Factors Handbook was published that presents data and evaluation of allometric and behavioral factors that affect exposures. It became an international resource for risk assessors who use these factors to estimate exposures for various pathways.⁴

Over the last 20 years, exposure science has evolved as a theoretical and practical science to include the development of mathematical models and other tools for examining how individuals and populations come into contact with environmental stressors of concern. For example, the discovery that airborne lead from gasoline combustion is deposited on soil, is tracked into homes, and enters children via hand-to-mouth activities greatly expanded the focus on multipathway exposure assessments and the development of exposure models that are validated through biomonitoring. Ott and others introduced time—activity models that were applied to air pollutants (Ott 1995). In the 1990s, exposure models addressed multimedia and multipathway exposures, tracking pollutants from multiple sources through air, water, soil, food, and indoor environments (McKone and Daniels 1991).

Borrowing from the concept of “dose commitment”⁵ in radiation protection, researchers elaborated the concept of “exposure efficiency” in the 1980s and 1990s (for example, Smith 1993). Early in the 21st century, the term intake fraction was adopted to describe that concept (Bennett et al. 2002). It is defined as the amount of material crossing the body’s barriers per unit emitted and thus is dimensionless. For air pollution, population intake fraction is the amount inhaled by the population divided by the amount emitted per unit activity or time. It directly connects the source and environmental-intensity boxes in Figure 1-2 with the exposure box, effectively incorporating the pathways in between without needing to specify them. A striking characteristic of intake fraction is that it varies by orders of magnitude among standard source categories—for example, in the case of air pollution, from 10^-6 for such remote sources as power plants to 10^-4 for urban outdoor sources, roughly 5 x 10^-3 for such indoor sources as unvented stoves, and 1.0 for active smoking. Not only does “dose make the poison”, therefore, but because proximity makes the dose, ultimately “place makes the poison” (NRC 2003). However, the biologically-relevant time and intensity of contact with an agent for each route of exposure needs to be considered (Lioy 1999).

OPPORTUNITIES AND CHALLENGES: THE NEW MILLENNIUM

Since 2000, a number of activities have benefited from advances in exposure science, and new challenges and opportunities have emerged. The Children’s Health Act of 2000 authorized the establishment of the National Children’s Study, a large-scale multiyear prospective study of children’s health and exposures intended to identify and characterize environmental influences (including physical, chemical, biologic, and psychosocial) on children from birth to adulthood. The study is under way, after the completion of the Vanguard Center pilot programs and the incorporation of new tools and approaches to streamline data collection at the household level and to capitalize on existing data for constructing community exposure baselines (IOM 2008; Trasande et al. 2011).

The increasing collection and evaluation of biomarkers of exposure and effect also is providing growing opportunities for exposure science. The Centers for Disease Control and Prevention’s National Health and Nutrition Examination Survey (NHANES) published the first National Human Exposure Report in 2001, which used a subset of its subjects to assess the US population’s exposure to environmental chemicals on the basis of biomonitoring data. The reports have been updated with publications released in 2003, 2005, and 2009, and annual reports are expected. The NHANES data provide a unique and growing potential for evaluating source—exposure and exposure—disease relationships in a national population-based representative sample. California has started its own biomonitoring program (OEHHA 2007), and other states and cities are working on biomonitoring efforts (CDC 2010). The emerging biomonitoring data sets will allow improved tracking of exposures over time, space, and across populations for an increasingly larger number of chemicals. This information will be essential for evaluating the efficacy of exposure reduction policies, and for prioritizing and assessing chemical risks.

A prime example of the benefits of improved methods of exposure assessment is their use in environmental epidemiology, in which more accurate estimates of the health effects of important stressors have been achieved by reducing exposure misclassification, for example, in air pollution (Jerrett et al. 2005) and ionizing radiation (NRC 2006). There are many opportunities for continued improvements in this arena.

Integration of Human and Ecologic Exposure Science

There has been a gap been between the application of exposure science to human health and its application to ecosystem health, which is due in part to the lack of recognition of the connection between human and ecosystem health—in reality, they are inextricably linked. The connection between human health and ecosystem health is explored in the context of ecosystem services; as seen in Figure 1-4, human welfare depends on ecosystem health.

FIGURE 1-4

Connections between ecosystem services and human well-being. The framework of ecosystem services makes explicit the linkages between human and ecologic health. The strength of the linkages and the potential for mediation differ in different ecosystems (more...)

A better integration of ecologic and human exposure science is critical because ecologic conditions strongly mediate exposures and their consequences for humans and ecosystems. Not only do ecosystems contain multiple stressors that can act synergistically but organisms’ environments are seldom optimal and may heighten their sensitivity to stressors. As illustrated by the examples in Box 1-2, degradation of ecosystems due to human activities increases exposure to or consequences of chemical and biologic stressors in both humans and ecosystems. Elucidating relationships between exposure and key abiotic and biotic ecologic factors is necessary if we are to understand risk.

BOX 1-2

Illustrations Demonstrating How the Degradation of Ecosystems Due to Human Activities Increases Exposures to Chemical and Biologic Stressors. Rising temperatures. Whether caused by shifts in climate or land uses (for example, deforestation, reduced vegetative (more...)

ROADMAP

The present report builds on the concepts presented in the National Research Council reports Toxicity Testing in the 21st Century and Science and Decisions: Advancing Risk Assessment to develop a framework for bringing exposure science to a point where it fully complements toxicology and risk assessment and can be used to protect human health and the environment better. The committee also addresses a set of emerging needs, such as the need to provide rapid assessment protocols and technologies to respond to natural and human-caused disasters and the needs for community participation and environmental justice. The report describes new technologies and opportunities to make exposure science even more effective in its traditional roles of evaluating environmental control measures, improving understanding of the link between environmental stressors and disease, and designing more cost-effective ways to reduce and prevent health risks. Finally, where possible, the committee offers ideas for integrating the applications of exposure science to human health and ecosystem health.

In Chapter 2, the committee presents a vision for exposure science. Chapter 3 describes the opportunities and challenges for applying exposure science to toxicology, epidemiology, and risk assessment and how exposure science can play a more effective role in other fields, such as environmental regulation, urban planning, ecosystem management, and disaster management. Chapter 4 addresses emerging demands for exposure-science information. Chapter 5 identifies scientific and technologic advances that are shaping the field and that support the committee’s vision. Chapter 6 discusses promoting and sustaining public trust in exposure science including the management of personal and environmental exposure data. Chapter 7 describes the path forward for exposure science in the 21st century.⁶

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Footnotes

1

The Environmental Protection Agency defines a stressor as “any physical, chemical, or biological entity that can induce an adverse response” (EPA 2011a).

2

In 2011, the International Society of Exposure Science and the Journal of Exposure Science and Environmental Epidemiology published a compendium of digests (Graham 2011) that illustrate situations in which application of exposure science resulted in substantial health or policy benefits and situations in which lack of exposure information resulted in adverse consequences.

3

Intensity is the preferred term because some stressors, such as temperature excesses, cannot be easily measured as concentrations.

4

A 2011 version has been released (EPA 2011b).

5

Dose commitment is the dose that will accumulate in an individual or population over a given period (for example, 50 years) from releases of radioactivity from a given source.

6

Given its terms of reference, the committee addresses primarily exposure-science issues related to the United States and other developed countries. In addition, the committee does not focus on uses of exposure principles in regulation and policy.