National Academies Press: OpenBook
« Previous: 2 Current and Historical Uses of Burn Pits in the Military
Suggested Citation:"3 Approach to the Task." Institute of Medicine. 2011. Long-Term Health Consequences of Exposure to Burn Pits in Iraq and Afghanistan. Washington, DC: The National Academies Press. doi: 10.17226/13209.
×

3

Approach to the Task

The organizing principles, themes and structure for this study are derived from two sources. First, the statement of task asks the Institute of Medicine (IOM) to determine “the potential long-term health effects of exposure to burn pits in Iraq and Afghanistan” and to design a future study of long-term health effects in veterans who may have experienced these exposures. Although the statement of task specifies that Joint Base Balad (JBB) in Iraq be considered as an example of a military base with a burn pit, it is clear that the Department of Veterans Affairs (VA’s) interest extends to all military personnel deployed to bases in Iraq and Afghanistan who may have been exposed to burn pit emissions. These dual objectives shaped the committee’s comprehensive search for information on relevant exposures and health outcomes, as well as its highly focused analyses of data and information from military reports specific to JBB and other burn pit locations.

Second, meeting the statement of task objectives entailed an in-depth examination of a substantial body of information on exposure to combustion products and health effects. For example, the health effects literature on dioxin-like compounds and particulate matter (PM) is vast, and they are but two of the numerous and diverse environmental agents likely to be found in burn bit emissions. Similarly, a cursory review of the scientific literature reveals a wide range of health outcomes—cancer, pulmonary disease, cardiovascular disease, neurological effects—in human populations exposed to environmental agents found in combustion products, singly and in mixtures. Comparable studies on laboratory animals often complement and supplement the data from human studies.

Effective use of this wide-ranging literature requires a systematic approach. The “risk assessment paradigm” introduced in the seminal report Risk Assessment in the Federal Government, Managing the Process (NRC 1983) offers a simple, flexible template. Drawing on advances in the theory and practice of risk assessment in the regulatory context, subsequent reports and guidelines have modified and expanded the paradigm to take into account issues specific to, for example, air pollutants (NRC 1994), ecological risk (EPA 1998), and the relationship between risk assessment and decision-making (NRC 2009). This paradigm, now called a conceptual framework, has been updated with more emphasis on problem formulation and improvements in technologic analyses and to provide more guidance on expanding the utility of risk assessments (NRC 2009). As shown in Figure 3-1, this paradigm calls for several analyses:

  • Exposure assessment—including the analysis of exposures, qualitative and quantitative, currently experienced or anticipated under existing or expected conditions;
Suggested Citation:"3 Approach to the Task." Institute of Medicine. 2011. Long-Term Health Consequences of Exposure to Burn Pits in Iraq and Afghanistan. Washington, DC: The National Academies Press. doi: 10.17226/13209.
×
  • Hazard identification—which requires determining whether exposure to a particular environmental agent has the potential to increase the incidence of an adverse health effect and, for particular exposure conditions, which agents can lead to adverse effects, thus requiring follow-up through dose–response or exposure-response analysis; and
  • Exposure–response/dose–response or toxicity assessment—which quantifies the relationship between the exposure or dose in humans and the incidence of adverse effects or health outcomes in human populations or laboratory animals; and Risk characterization—which synthesizes and integrates data and information to develop a quantitative estimate of the incidence of the effect in a given population, along with a discussion of related qualitative considerations, strengths, limitations, and uncertainties.

This report is organized around the assessment process with modifications to address the specific issues raised in the statement of task. As shown in Figure 3-2, the process focuses first on research and data collection related to exposures at JBB, health effects that might result from those exposures, and health outcomes in other human populations exposed to some of the environmental agents found in ambient samples and potentially emitted by the JBB burn pit. Based on this information, assessments are then made on potential exposures and health effects that might occur in relevant populations. Rather than conduct a formal, quantitative risk assessment and risk characterization, the committee prepared a synthesis and summary of key findings and applied that informa-

image

FIGURE 3-1 Elements of the risk assessment paradigm.
SOURCE: Adapted from NRC (1983, 2009).

Suggested Citation:"3 Approach to the Task." Institute of Medicine. 2011. Long-Term Health Consequences of Exposure to Burn Pits in Iraq and Afghanistan. Washington, DC: The National Academies Press. doi: 10.17226/13209.
×

image

FIGURE 3-2 Approach to the statement of task: The types of information reviewed and the process used to develop this report.

tion to suggest design elements for a future epidemiologic study to address data gaps. The resulting three-stage analytical process is outlined below:

  1. Data collection and review: For this report, the “exposure assessment,” “hazard identification,” and “dose–response” tasks in the 2009 framework are merged as “data collection and review” activities focused on chemicals detected in air monitoring at JBB and on smoke exposures per se. These activities revolve around two closely related but distinct topics: exposure and health effects. The in-depth review begins with identifying and selecting agents and corresponding reliable studies to be given priority for evaluating health outcomes potentially associated with burn pit exposures. This includes establishing criteria for evaluating the quality and relevance of the studies along with their limitations and uncertainties (see Chapters 4, 5, and 6).
  2. Synthesis: Assessing “the potential long-term health effects of exposure to burn pits in Iraq and Afghanistan” requires the identification and evaluation of the key findings from the comprehensive review of environmental monitoring data and health effects information. One focus is on identifying which, if any, of the many environmental agents detected in the air at JBB are present in toxicologically significant quantities, and which, if any, are associated with the burn pit, and determining the reliability of those data. Another focus is the major findings on health effects seen in other similarly exposed populations as discussed in Chapter 6, including the extent to which these populations experienced exposures relevant (with respect to chemical composition and level of exposure) to those at JBB. A related question is how comparable those populations are to military personnel stationed at JBB. This synthesis includes a discussion of the committee’s confidence in the quality and utility of these findings for the VA (see Chapter 7).
  3. New study design: The exposure–response information developed in Chapters 5 and 6, combined with information on the potential exposures of JBB personnel discussed in Chapter 4, provide the basis for evaluating feasibility and design issues specific to an epidemiologic study to assess health outcomes associated with exposure to burn pit emissions at JBB and other burn pit sites (see Chapter 8).
Suggested Citation:"3 Approach to the Task." Institute of Medicine. 2011. Long-Term Health Consequences of Exposure to Burn Pits in Iraq and Afghanistan. Washington, DC: The National Academies Press. doi: 10.17226/13209.
×

Each of the following sections corresponds to one of the three steps in the analytical process. The next section, “Data Collection and Review,” outlines the kinds of data and information needed to evaluate long-term health effects in line with the statement of task and related risk assessment principles, and explains the principles and processes guiding the evaluation and selection of data presented in the remainder of the report. In the section titled “Summary and Synthesis,” the committee summarizes and highlights critical findings from the previous section. In the final section, “New Study Design,” this information is used to propose design elements for a future epidemiologic study of health effects in veterans who may have been exposed to burn pit emissions at JBB.

DATA COLLECTION AND REVIEW

Data collection for this report centered on two general topics: first, information on environmental releases and concentrations of combustion products at JBB and the resulting potential human exposures, and second, the potential for long-term health effects resulting from those exposures. Principles and methods guiding the committee’s assessment of the data are outlined in the following sections.

Environmental Exposure

Exposure assessment requires characterization of the frequency, magnitude, and duration of exposure to an agent of concern in an exposed population (NRC 2010). The committee began assessing possible burn pit emission exposures by reviewing Department of Defense (DoD) air monitoring data collected at JBB and attempting to determine whether the air pollutants detected during the sampling periods could be attributed to the burn pits or might possibly originate from other sources, either natural or anthropogenic (see Chapter 4). Three DoD documents were used to characterize ambient concentrations: the 2008 Screening Health Risk Assessment Burn Pit Exposures, Balad Air Base, Iraq and Addendum Report (Taylor et al. 2008), a second addendum to the report (CHPPM and AFIOH 2009), and a follow-up Screening Health Risk Assessments study in 2010 (USAPHC 2010). The committee asked for and received from DoD raw air-sampling data collected in 2007 and 2009 to help in identifying potential sources of the air pollutants. The raw data was not only useful for determining which chemicals were tested for in the samples, but they were also useful in determining which chemicals were found at concentrations above the analytical detection limit. Those chemicals present above the detection limit were considered to warrant further assessment.

The committee recognized that air quality in a region is affected by many factors, such as temperature, humidity, meteorological events, and atmospheric inputs from both natural sources such as dust storms and from anthropogenic sources such as power plants, agriculture, and industrial facilities. Thus, the committee asked for and received from DoD meteorological information for the JBB area during the air monitoring periods.

The location and activities of the military personnel stationed at JBB was also a factor for assessing exposure. The committee noted that not only did the personnel work on the base, but they also lived there and thus were potentially exposed to the burn pit emissions 24 hours a day, 7 days a week, although potentially at different levels during on- and off-work hours. Furthermore, it was assumed that some personnel would have greater exposure to burn pits either because they actually worked in the pits or may have had a job that required them to work near the pit. The committee did not ask the DoD for specific information regarding the number of military personnel that worked at or in close proximity to the burn pit. Because JBB had both Army and Air Force personnel, and the base was used as a transit stop for some personnel, the duration of exposure to the burn pit emissions was also highly variable. The committee assumed that some unknown number of personnel would experience high exposure to burn pit emissions based on their military occupation specialty and their housing location and other personnel might have relatively low exposure.

Potential Long-Term Health Effects

Assessing potential long-term health effects from exposure to environmental agents depends on two closely related but different aspects of exposure. The environmental exposure information developed in Chapter 4 describes

Suggested Citation:"3 Approach to the Task." Institute of Medicine. 2011. Long-Term Health Consequences of Exposure to Burn Pits in Iraq and Afghanistan. Washington, DC: The National Academies Press. doi: 10.17226/13209.
×

the occurrence and concentration of environmental agents in external media, in this case air. This information can provide a starting point for evaluating internal exposure—the presence and behavior of agents in living systems—and the biological response to that exposure. Alternatively, the information may be used directly in exposure-response evaluations without a full evaluation of dose. Ideally, however, assessment of health effects includes data on internal dose—the amount of agent in the tissues of the organism. Many factors determine internal dose, including the concentration of the agent in environmental media, the physical and chemical properties of the agent, and individual metabolic processes of the organism. Information on the majority of these exposure factors for military personnel living and working at JBB is unavailable or incomplete. Thus, the committee conducted its review assuming that deployed personnel were exposed to burn pit emissions primarily through inhalation and they received some unquantified internal dose of the chemicals present in burn pits emissions. The committee recognized that some dermal and ingestion exposure was possible, albeit at relatively minor levels. However, the lack of data on contaminant levels in soil, food, water, and surfaces at JBB precluded further consideration of ingestion and dermal routes of exposure.

Studies on Health Effects

For chemical mixtures such as burn pit emissions, toxicity and other health effects data on the mixture itself are generally scarce or nonexistent. In view of this paucity of data, the committee focused on studies of exposure to individual chemicals, and exposure to similar mixtures in surrogate populations. First, studies of exposure to known individual chemicals in laboratory animals provide information on health effects (or lack thereof) attributable to specific chemicals at relevant exposure levels (Chapter 5). These studies can be important sources of both qualitative (the nature and range of health effects observed such as cancer, neurological effects, and reproductive effects) and quantitative (dose–response or exposure–response relationships) information on health effects in test animals associated with exposure to specific chemicals under controlled laboratory conditions. However, the extent to which animal studies are predictive of health effects (or lack of effects) in humans varies considerably.

The variety of materials burned at JBB produced a complex mixture of chemicals of differing toxic potential, occurring in different amounts and physical forms. The emissions from the burn pits were also mixed with numerous air pollutants from other local and regional sources such as industry and dust storms. Based on the air monitoring data analyzed in Chapter 4, the committee determined the likely adverse health effects that might be associated with those chemicals found above the analytical detection limit or that otherwise were expected to pose the greatest risk to JBB personnel. To identify potential health effects from these exposures, the committee relied on published summaries from diverse sources, including IOM and NRC reports, government reports such as toxicological profiles from the Agency for Toxic Substances and Disease Registry, and established databases such as the U.S. Environmental Protection Agency’s (EPA’s) Integrated Risk Information System (IRIS).1 The committee did not re-examine the underlying data or methodology for these sources; rather, it relied on them as well established sources of health effects information (see Chapter 5).

Epidemiologic Studies

Given the lack of information on health effects that may result from exposure to burn pit emissions per se, the committee sought information on health effects from epidemiologic studies of other (non-Balad) human populations exposed to smoke, combustion products, or other complex chemical mixtures that include at least some of the chemical constituents present in burn pit emissions (see Chapter 6). The committee gave special attention to studies involving other military populations, firefighters, and residents living near municipal waste incinerators. Those studies provide data on demographically similar populations (relatively young and healthy, and predomi-

_____________

1Although scientists throughout the world use the IRIS database for its valuable toxicity and risk information, many IRIS profiles need to be updated to take into account new pollutant-specific data and advances in methodology or to correct inaccurate values (NRC 2008). Although the committee recognizes these concerns and the resulting uncertainties they create, these values, with those from other health agencies, are the best readily available data for assessing pollutants emitted from open burning.

Suggested Citation:"3 Approach to the Task." Institute of Medicine. 2011. Long-Term Health Consequences of Exposure to Burn Pits in Iraq and Afghanistan. Washington, DC: The National Academies Press. doi: 10.17226/13209.
×

nantly male) exposed to similar pollutants (chemical mixtures produced by burning). In selecting studies for this report, the committee focused on factors such as exposure duration, populations of interest, criteria for identifying key studies, and determining categories of association between exposures and health effects.

Exposure Duration

In general, the available studies involved one of three exposure–response patterns: (1) medium-term exposure (6 months to a year) to higher pollutant levels that might increase the risk of chronic disease later in life (medium-term exposure/long-term health effects); (2) short-term exposure (over a few days) to higher pollutant levels that might increase the risk of chronic disease later in life (acute exposure/long-term health effects); and (3) short-term exposure to higher pollutant levels that might increase the risk of an acute adverse health event immediately following or shortly after exposure (acute exposure/acute health effects). For relevance to veterans of the current conflicts, the committee focused on long-term health outcomes that persist after exposure ceases, rather than acute health effects.

Populations of Interest

Assessing potential future effects of exposure in a particular population such as JBB personnel depends on available information for that population together with observational data on effects in theoretically similar populations exposed under generally comparable conditions. Three occupational populations were of special interest to the committee because of their exposure to complex mixtures of combustion products potentially similar to burn pit emissions: firefighters (wildlands and urban), municipal waste incinerator workers, and military personnel from several earlier deployments (Bosnia, Kuwait, Iraq, Afghanistan). The IOM has evaluated the scientific literature on military personnel deployed to the 1990–1991 Persian Gulf War in previous reports (IOM 2005, 2006, 2010), which this report incorporates by reference where appropriate. Residents living in the vicinity of municipal waste incinerators were also included in the committee’s deliberations as personnel at JBB not only work on the base but live there as well with the potential for 24-hour exposure. Finally, the committee sought, but did not find, epidemiologic information on health effects seen in Iraqi civilians living near bases with burn pits or other sources of combustion products, such as local waste burning operations or wild fires.

Selection of Key Studies

The studies reviewed in Chapter 6 are designated “key” or “supporting” depending on several criteria as described in that chapter. Consistent with previous IOM reports (IOM 2010), to be designated “key” a study had to include information about the putative exposure and specific health outcomes, demonstrate rigorous methods, include methodological details sufficient to allow a thorough assessment, and include an appropriate control or reference group. A “supporting” study typically had methodologic limitations, such as lack of a rigorous or well-defined diagnostic method or lack of an appropriate control group. The committee’s evaluations of key and supporting studies used in reaching its conclusions are described in greater detail in Chapter 6.

Categories of Association

The committee considered the studies identified in Chapter 6 in terms of the strength of the apparent association between the exposures to combustion products and potential health effects. As detailed in Chapter 6, the five categories used by the committee represent different strengths of association and different levels of confidence in the potential relevance and utility of each study. The validity of an observed association is likely to vary with the extent to which common sources of spurious associations can be ruled out as contributing factors. Accordingly, the criteria for each category express a degree of confidence based on the extent to which sources of error were reduced.

Suggested Citation:"3 Approach to the Task." Institute of Medicine. 2011. Long-Term Health Consequences of Exposure to Burn Pits in Iraq and Afghanistan. Washington, DC: The National Academies Press. doi: 10.17226/13209.
×

SUMMARY AND SYNTHESIS

Understanding the potential for long-term health effects associated with human exposure to burn pit emissions depends on the reliability and utility of the available data on exposures at JBB and other burn pit locations, as well as exposure and outcome data from studies on other populations exposed to some of the same combustion products, singly or in mixtures. Each of the three main data sources—data specific to JBB, other military reports, and the peer-reviewed literature—provides useful and relevant information, but each has major limitations.

In this section, the committee summarizes and highlights key findings on materials burned at JBB and other military burn pit locations, health effects data on the combustion products detected at JBB, and studies on health effects in non-Balad populations potentially exposed to similar chemicals. The committee comments on its confidence in these findings and on their usefulness in providing the VA with information for medical follow-up and future studies. The committee also considers the possible impact of co-exposure to combustion products from local and regional air pollution sources other than the JBB burn pit.

NEW STUDY DESIGN

The statement of task directs the committee to “examine the feasibility and design issues for an epidemiologic study of veterans exposed at the Balad burn pit.” Two recent IOM reports on other military populations offer useful models. First, a study on depleted uranium in military and veteran populations (IOM 2008) identifies key features of a well-designed epidemiology study of potential health effects associated with environmental exposure.

Second, the review of the DoD’s Enhanced Particulate Matter Surveillance Program (NRC 2010) identifies exposure assessment as a central element in the study design for that proposed study because “if data are not available to characterize exposure, any study of association between health outcomes and exposure will not provide valid results.” Any study of JBB exposures is necessarily incomplete because of the limited exposure information available. Specifically, the military collected environmental samples at fixed locations, but the relation to individual exposures may be difficult to ascertain, be subject to large uncertainties, or both. Since the JBB burn pit was closed in 2009, no additional opportunities are available to collect environmental or personal monitoring data on burn pit emissions at JBB, although it is still possible to conduct air monitoring, including personal monitoring, at other military sites with operational burn pits.

Epidemiologic studies conducted at JBB and other bases with burn pits have substantial acknowledged limitations (for example, lack of adequate follow-up for the diagnosis of conditions with long latency such as chronic bronchitis, emphysema, systemic lupus erythematosus, and cancer). Taking these considerations and the key findings in this report into account, the committee closes with a discussion of design issues for a future epidemiologic study of the JBB population and outlines a proposed approach for that study.

REFERENCES

CHPPM (U.S. Army Center for Health Promotion and Preventive Medicine) and AFIOH (U.S. Air Force Institute for Operational Health). 2009. Addendum 2. Screening health risk assessment burn pit exposures, Balad Air Base, Iraq, May 2008. USACHPPM Report No. 47-MA-08PV-08/AFIOH Report No. IOH-RS-BR-TR-2008-0001. Aberdeen Proving Ground, MD: U.S. Army Center for Health Promotion and Preventive Medicine. August.

EPA (U.S. Environmental Protection Agency). 1998. Guidelines for ecological risk assessment. EPA/630/R-95/002F. Washington, DC: Risk Assessment Forum.

IOM (Institute of Medicine). 2005. Gulf War and health: Volume 3. Fuels, combustion products, and propellants. Washington, DC: The National Academies Press.

IOM. 2006. Gulf War and health: Volume 4. Health effects of serving in the Gulf War. Washington, DC: The National Academies Press.

IOM. 2008. Epidemiological studies of veterans exposed to depleted uranium: Feasability and design issues. Washington, DC: The National Academies Press.

IOM. 2010. Gulf War and health: Volume 8. Update of health effects of serving in the Gulf War. Washington, DC: The National Academies Press.

Suggested Citation:"3 Approach to the Task." Institute of Medicine. 2011. Long-Term Health Consequences of Exposure to Burn Pits in Iraq and Afghanistan. Washington, DC: The National Academies Press. doi: 10.17226/13209.
×

NRC (National Research Council). 1983. Risk assessment in the federal government: Managing the process. Washington, DC: National Academy Press.

NRC. 1994. Science and judgment in risk assessment. Washington, DC: National Academy Press.

NRC. 2009. Science and decisions: Advancing risk assessment. Washington, DC: The National Academies Press.

NRC. 2010. Review of the Department of Defense Enhanced Particulate Matter Surveillance Program report. Washington, DC: National Academies Press.

Taylor, G., V. Rush, A. Deck, and J. A. Vietas. 2008. Screening health risk assessment burn pit exposures, Balad Air Base, Iraq, and addendum report. IOH-RS-BR-TR-2008-0001/USACHPPM 47-MA-08PV-08. Brooks City-Base, TX: Air Force Institute for Operational Health and U.S. Army Center for Health Promotion and Preventative Medicine.

USAPHC (U.S. Army Public Health Command). 2009. Screening health risk assessments, Joint Base Balad, Iraq, 11 May–19 June 2009. With author information redacted. Aberdeen Proving Ground, MD: U.S. Army Public Health Command. July.

Suggested Citation:"3 Approach to the Task." Institute of Medicine. 2011. Long-Term Health Consequences of Exposure to Burn Pits in Iraq and Afghanistan. Washington, DC: The National Academies Press. doi: 10.17226/13209.
×
Page 23
Suggested Citation:"3 Approach to the Task." Institute of Medicine. 2011. Long-Term Health Consequences of Exposure to Burn Pits in Iraq and Afghanistan. Washington, DC: The National Academies Press. doi: 10.17226/13209.
×
Page 24
Suggested Citation:"3 Approach to the Task." Institute of Medicine. 2011. Long-Term Health Consequences of Exposure to Burn Pits in Iraq and Afghanistan. Washington, DC: The National Academies Press. doi: 10.17226/13209.
×
Page 25
Suggested Citation:"3 Approach to the Task." Institute of Medicine. 2011. Long-Term Health Consequences of Exposure to Burn Pits in Iraq and Afghanistan. Washington, DC: The National Academies Press. doi: 10.17226/13209.
×
Page 26
Suggested Citation:"3 Approach to the Task." Institute of Medicine. 2011. Long-Term Health Consequences of Exposure to Burn Pits in Iraq and Afghanistan. Washington, DC: The National Academies Press. doi: 10.17226/13209.
×
Page 27
Suggested Citation:"3 Approach to the Task." Institute of Medicine. 2011. Long-Term Health Consequences of Exposure to Burn Pits in Iraq and Afghanistan. Washington, DC: The National Academies Press. doi: 10.17226/13209.
×
Page 28
Suggested Citation:"3 Approach to the Task." Institute of Medicine. 2011. Long-Term Health Consequences of Exposure to Burn Pits in Iraq and Afghanistan. Washington, DC: The National Academies Press. doi: 10.17226/13209.
×
Page 29
Suggested Citation:"3 Approach to the Task." Institute of Medicine. 2011. Long-Term Health Consequences of Exposure to Burn Pits in Iraq and Afghanistan. Washington, DC: The National Academies Press. doi: 10.17226/13209.
×
Page 30
Next: 4 Evaluation of Air Monitoring Data and Determinants of Exposure »
Long-Term Health Consequences of Exposure to Burn Pits in Iraq and Afghanistan Get This Book
×
Buy Paperback | $45.00 Buy Ebook | $35.99
MyNAP members save 10% online.
Login or Register to save!
Download Free PDF

Many veterans returning from the conflicts in Iraq and Afghanistan have health problems they believe are related to their exposure to the smoke from the burning of waste in open-air "burn pits" on military bases. Particular controversy surrounds the burn pit used to dispose of solid waste at Joint Base Balad in Iraq, which burned up to 200 tons of waste per day in 2007. The Department of Veterans Affairs asked the IOM to form a committee to determine the long-term health effects from exposure to these burn pits. Insufficient evidence prevented the IOM committee from developing firm conclusions. This report, therefore, recommends that, along with more efficient data-gathering methods, a study be conducted that would evaluate the health status of service members from their time of deployment over many years to determine their incidence of chronic diseases.

  1. ×

    Welcome to OpenBook!

    You're looking at OpenBook, NAP.edu's online reading room since 1999. Based on feedback from you, our users, we've made some improvements that make it easier than ever to read thousands of publications on our website.

    Do you want to take a quick tour of the OpenBook's features?

    No Thanks Take a Tour »
  2. ×

    Show this book's table of contents, where you can jump to any chapter by name.

    « Back Next »
  3. ×

    ...or use these buttons to go back to the previous chapter or skip to the next one.

    « Back Next »
  4. ×

    Jump up to the previous page or down to the next one. Also, you can type in a page number and press Enter to go directly to that page in the book.

    « Back Next »
  5. ×

    Switch between the Original Pages, where you can read the report as it appeared in print, and Text Pages for the web version, where you can highlight and search the text.

    « Back Next »
  6. ×

    To search the entire text of this book, type in your search term here and press Enter.

    « Back Next »
  7. ×

    Share a link to this book page on your preferred social network or via email.

    « Back Next »
  8. ×

    View our suggested citation for this chapter.

    « Back Next »
  9. ×

    Ready to take your reading offline? Click here to buy this book in print or download it as a free PDF, if available.

    « Back Next »
Stay Connected!