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Air Quality Management in the United States 2 Setting Goals and Standards INTRODUCTION The introduction to the Clean Air Act (CAA) (42 USC § 7401) lists four overarching goals or purposes for the legislation: to protect and enhance the quality of the Nation’s air resources so as to promote the public health and welfare and the productive capacity of its population; to initiate and accelerate a national research and development program to achieve the prevention and control of air pollution; to provide technical and financial assistance to State and local governments in connection with the development and execution of their air pollution prevention and control programs; and to encourage and assist the development and operation of regional air pollution prevention and control programs. In the subsequent sections or titles to the act and its amendments, these overarching goals are further delineated into a variety of more tangible air quality goals and standards, such as those listed in Chapter 1, that are to be achieved through the implementation of rules, regulations, and practices designed to control and limit pollutant emissions. In this chapter, the committee focuses on these air quality goals and standards. We begin with a descriptive discussion of the various standards and the combined responsibilities of Congress and the administrator of the U.S. Environmental Protection Agency (EPA) in setting them. That discussion is followed by a
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Air Quality Management in the United States critical analysis of specific aspects of the standard-setting procedure, especially those aspects relating to the scientific basis for the standards and the procedures used to set them. OVERVIEW OF AIR QUALITY STANDARDS The CAA sets standards in a number of ways: The setting of National Ambient Air Quality Standards (NAAQS) for six principal pollutants (known as criteria pollutants). The setting of emission standards for a variety of stationary and mobile sources for substances that are the criteria pollutants, their precursors, or hazardous air pollutants (HAPs). Promulgate additional emission standards for HAPs that continue to pose a significant residual risk following the implementation of the first round of emission standards. The setting of fuel and product reformulation standards (for example, reformulated gasoline and requirements for chlorinated fluorocarbons). The setting of reduced caps for emissions of certain pollutants from certain industries (for example, the sulfur dioxide [SO2] cap-and-trade program). The CAA also contains many provisions for attaining and maintaining these standards. In this chapter, the committee focuses on, and critiques, the process by which many of these standards are set. Subsequent chapters discuss how they are implemented. The CAA begins by addressing two major categories of pollutants for which standards are set differently: criteria pollutants and HAPs. The principal difference between the two arises from the specification in the CAA that the presence of criteria pollutants “in the ambient air results from numerous or diverse mobile or stationary sources.” No such requirement is stated for HAPs.1 Thus, presumably, criteria pollutants are more ubiquitous, pose a risk to a larger fraction of the general population, and have more widespread impacts on ecosystems and natural resources than HAPs. Criteria pollutants and HAPs are managed through fundamentally different regulatory frameworks. Criteria pollutants are regulated primarily through the setting of ambient-air-concentration and time standards, known as the NAAQS, and taking action to attain these standards. HAPs are regulated through the promulgation of standards that limit the release or emissions of such compounds (as opposed to their ambient concentrations), followed in 1 The only specific limitation currently placed on HAPs in the CAA is that they cannot be criteria pollutants.
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Air Quality Management in the United States the cases of major stationary sources and area sources by assessment of residual risk. The responsibility for setting the standards for both types of pollutants is assigned to the EPA administrator. In addition to the programs to control criteria pollutants and HAPs, the CAA includes provisions to control emissions from mobile sources, protect areas with good air quality, reduce the effects of acid deposition (or acid rain), safeguard stratospheric ozone (O3), and reduce visibility impairment resulting from regional haze. However, the tenor of these provisions is substantially different from those relating to the programs for criteria pollutants and HAPs. Although the CAA directs the EPA administrator to set air quality and emission standards for criteria pollutants and HAPs, it is more explicit about setting standards for the other provisions, with Congress itself often setting the standards. For example, the CAA now includes specific standards for evaporative and exhaust emissions from light-duty and heavy-duty on-road vehicles and engines. Controls are also required for a wide range of nonroad engines (such as lawnmowers, construction equipment, and locomotives), and programs are mandated for clean fuel and inspection and maintenance of light-duty vehicles. Similarly, in the case of acid rain mitigation, the CAA Amendments of 1990 contain language that establishes a specific nationwide cap for SO2 emissions and standards for nitrogen oxides (NOx) emissions from electric utilities. THE STANDARD-SETTING PROCESSES Criteria Pollutants Criteria pollutants were first defined in the 1970 Amendments to the CAA, which directed the administrator of EPA to identify those widespread ambient air pollutants that are reasonably expected to present a danger to public health or welfare.2 On the basis of air quality criteria3—that is, the current state of scientific knowledge on the effects of these pollutants on health and welfare—the administrator is directed to develop and promulgate primary and secondary NAAQS for each criteria air pollutant. In addition to specifying a maximum ambient concentration for each pollutant, promulgation of a standard must also include descriptions of the moni- 2 Within the framework of the CAA, “welfare” refers to the viability of agriculture and ecosystems (such as forests and wildlands), the protection of materials (such as monuments and buildings), and the maintenance of visibility (EPA 2002d). 3 Air quality criteria are defined in Section 108 of the CAA as a summary of the “latest scientific knowledge useful in indicating the kind and extent of all identifiable effects on public health or welfare which may be expected from the presence of such pollutant in the ambient air.”
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Air Quality Management in the United States toring and statistical methods that are to be used to determine whether an area is in compliance with the standard.4 Primary standards are intended to protect public health “with an adequate margin of safety” for the most sensitive population subgroups. Secondary standards are intended to protect against adverse public welfare effects. Although the CAA specifies a date when a given primary standard is to be achieved and provides EPA with authority to enforce state and tribal compliance, no such timetable and enforcement authority are provided for secondary standards (see Chapter 3). In 1971, NAAQS were established for the first time for six criteria pollutants: carbon monoxide (CO), nitrogen dioxide (NO2), SO2, total suspended particulate matter (TSP), hydrocarbons (HCs), and photochemical oxidants. Lead (Pb) was added to the list in 1976, photochemical oxidants were replaced by O3 in 1979, and HCs were removed in 1983. The definition of suspended particles as a criteria pollutant has also changed. TSP was revised in 1987 to include only particles with an equivalent aerodynamic particle diameter of less than or equal to 10 micrometers (μm) (PM10) and was further revised in 1997 to include a separate standard for particles with an equivalent aerodynamic particle diameter of less than or equal to 2.5 μm (PM2.5). The current standards for criteria pollutants are provided in Table 2-1. Although efforts to meet the NAAQS have not always successfully resulted in attainment of the standards, they appear to have been responsible for sizable reductions in pollutant emissions across the nation (see Chapter 6 for further discussion). The Procedure for Setting NAAQS The CAA instructs the EPA administrator to specify primary and secondary NAAQS and to conduct a review of the air quality criteria and NAAQS for each pollutant at least every 5 years. The review process is a complex one that includes input and comment from independent scientific bodies as well as the general public (see Figure 2-1). EPA’s Office of Research and Development (ORD) prepares a detailed summary, called a criteria document, for each criteria air pollutant. The criteria document is based on the existing body of scientific and technical information and typically includes chapters on emission sources, air concentrations, exposure, dosimetry, and health and welfare effects, as well as a concluding synthesis chapter. The research findings summarized include results from studies supported by EPA, other federal agencies, industry, and 4 A discussion of the methods used to establish nonattainment of NAAQS and the subsequent actions a state or local authority must undertake to bring about attainment is presented in Chapter 3.
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Air Quality Management in the United States TABLE 2-1 NAAQS in Effect as of January 2003a Primary Standard (Health-Based) Secondary Standard (Welfare-Based) Pollutant Type of Average Standard Level Concentration Type of Average Standard Level Concentration PM10 Annual arithmetic mean 50 μg/m3 Same as primary standard 24-hr average not to be exceeded more than once per year on average over 3 yr 150 μg/m3 Same as primary standard PM2.5 Spatial and annual arithmetic mean in area 15 μg/m3 Same as primary standard 98th percentile of the 24-hr average 65 μg/m3 Same as primary standard O3b Maximum daily 1-hr average to be exceeded no more than once per year averaged over 3 consecutive years 0.12 ppm Same as primary standard 3-yr average of the annual fourth highest daily 8-hr average 0.08 ppm Same as primary standard NO2 Annual arithmetic mean 0.053 ppm Same as primary standard SO2 Annual arithmetic mean 0.03 ppm 3 hr 0.50 ppm 24-hr average 0.14 ppm CO 8 hr (not to be exceeded more than once per year) 9 ppm No secondary standard 1 hr (not to be exceeded more than once per year) 35 ppm No secondary standard Lead Maximum quarterly average 1.5 μg/m3 Same as primary standard aA more detailed discussion of how an area is determined to be in attainment or nonattainment of a NAAQS is presented in Chapter 3. bEPA is phasing out the 1-hr, 0.12-ppm standards (primary and secondary) and putting in place the 8-hr, 0.08-ppm standards. However, the 0.12-ppm standards will not be revoked in a given area until that area has achieved 3 consecutive years of air quality data meeting the 1-hr standard. Abbreviations: μg/m3, micrograms per cubic meter; ppm, parts per million (by volume); hr, hour; yr, year. SOURCE: Adapted from EPA 2001a.
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Air Quality Management in the United States FIGURE 2-1 Flow diagram illustrating the process by which the EPA administrator reviews and sets a new NAAQS. CASAC refers to the Clean Air Scientific Advisory Committee. Diamonds are used to denote official actions by the administrator. SOURCE: Greenbaum et al. 2001. Reprinted with permission from the American Journal of Epidemiology; copyright 2001, Oxford Press. private funding organizations. Investigators from both inside and outside EPA collaborate in writing the document. Next, EPA’s Office of Air Quality Planning and Standards (OAQPS) in the Office of Air and Radiation prepares a document, called the staff paper, which recommends and provides the justification for policy options presented to the EPA administrator, who makes a determination on whether to retain an existing standard or propose a new one. In preparing the staff paper, EPA uses the information included in the criteria document and also conducts analyses of population exposure to characterize risk and to recommend standards. Both the criteria document and the staff paper are made available for public comment. The two documents, along with the public comments, are reviewed by the Clean Air Scientific Advisory Committee (CASAC), which is a committee composed of independent experts from outside EPA and is organizationally situated within the EPA Science Advisory Board (SAB). CASAC makes recommendations to EPA staff on revisions to both the criteria document and the staff paper, resulting in one or more rounds of revision and review. When satisfied, CASAC informs the EPA administrator that the document fully and fairly represents the
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Air Quality Management in the United States current state of the science. Because of the effort involved in their preparation and the rigorous review process involved, criteria documents and staff papers have traditionally served as comprehensive reviews of the current understanding of air pollution health effects at the time of their publication and, as such, have stimulated new focused research. On the basis of the criteria document and staff paper, the EPA administrator publishes proposals for new or revised NAAQS in the Federal Register, and a new round of public comment ensues. The administrator then makes final decisions on the NAAQS, taking into account both public input and advice from CASAC. The primary and secondary standards, including the levels and the forms of the standards, together with their justification, are published in the Federal Register as part of the standard promulgation process. The Supreme Court has determined that the CAA requires that the setting of primary NAAQS is to be done without consideration of the economic consequences (Whitman v. American Trucking Association, 531 U.S. 457, 2001). However, there are two points in the process where costs are assessed: (1) under Executive Orders dating back to the Carter administration and the Small Business Regulatory Enforcement Fairness Act, major federal regulations are subject to a regulatory impact analysis by the Office of Management and Budget, and (2) the Congressional Review Act provides Congress up to 60 days following promulgation of any rule to conduct hearings and review the rule (although no congressional action is required for the NAAQS to take effect). According to the CAA, each of the NAAQS for each of the criteria pollutants must be reviewed every 5 years. However, the complexity of the process and the sheer volume of new research results on some criteria pollutants have made it necessary to extend the periods between reviews (see Figure 2-2). As a result, EPA has been sued by some stakeholders and at times required by the courts to complete an overdue review on a court-ordered schedule. Protection of Ecosystems and the Establishment of Secondary NAAQS Although the CAA empowers EPA to set independent primary and secondary standards for each criteria pollutant, and criteria documents prepared by EPA have included reviews of the available data on impacts to ecosystems, visibility, human-made structures, and other aspects of public welfare, SO2 is the only criteria pollutant for which there is a unique secondary standard (Table 2-1). The promulgation of common standards for protecting public health and welfare could simply reflect a judgment on EPA’s part that humans and ecosystems have similar sensitivity to air pollutants or that humans are more sensitive; thus, a single standard can adequately protect both. It is more likely, however, that the correspondence
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Air Quality Management in the United States FIGURE 2-2 Timeline illustrating historical sequence of the periodic NAAQS reviews and final decisions carried out by EPA since the passage of the 1970 CAA Amendments. For particulate matter, standards were set in 1971 for total suspended particles (TSP). In 1987, the standards were revised to more stringent PM10 standards. In 1997, those standards were revised by adding more stringent PM2.5 standards. For ozone, photochemical oxidant standards were set in 1971. They were revised to less stringent 1-hr ozone standards in 1979. Then, in 1997, the standards were revised to more stringent 8-hr standards. For sulfur dioxide, the decision to reaffirm the primary standards was remanded in 1996. EPA’s response to the remand is ongoing.
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Air Quality Management in the United States between the two standards reflects a historical and reasonable tendency in EPA to set priorities to protect human health over aspects of public welfare and to focus on urban rather than nonurban pollution. The inability to enforce standards to protect public welfare by the requirement for attainment by a specific date might also play a part. Whatever the reason that led EPA to use identical primary and secondary NAAQS in the past, it is becoming increasingly evident that a new approach will be needed in the future. There is growing evidence that the current forms of the NAAQS are not providing adequate protection to sensitive ecosystems and crops (see Figure 2-3) (Driscoll et al. 2001a; Mauzerall and Wang 2001). Moreover, new research documenting the economic importance of the functions and services supplied to society by ecosystems (Daily 1997; Ecological Society of America [ESA] 1997a) suggests that air pollution damage to ecosystems exacts economic as well as environmental costs to the nation. At the same time, air quality management (AQM) and science are experiencing a shift in focus to problems related to multistate (and by extension rural) air quality problems.5 Thus, the nation’s AQM system may now be in a better position to tackle the problem of air pollution damage to sensitive ecosystems and crops. In the CAA Amendments of 1990, Congress instructed EPA to undertake a comprehensive review of the need for and use of standards to protect public welfare (42 USC § 7409 ). However, such a study was never undertaken. In Chapter 7, the committee advances more specific recommendations for improving the scientific basis for setting secondary standards by strengthening the nation’s ability to monitor ecosystems and their exposure and response to air pollution. National Emission Standards Mandated by Congress to Help Attain NAAQS The general procedure for attainment of NAAQS is specified in the CAA. The process includes the monitoring of ambient air pollution concentrations, the designation of nonattainment areas on the basis of the data from this monitoring, and the development of state implementation plans (SIPs) to achieve the emission reductions necessary to bring areas into attainment.6 However, Congress recognized that the states could not be expected to achieve NAAQS for criteria pollutants on their own without potential substantial economic disruptions. In particular, it was recognized that economies of scale could be realized through the promulgation of more 5 Later in the report, a number of regional approaches to address these types of problems are discussed. 6 A detailed discussion of the SIP process and its implementation is provided in Chapter 3.
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Air Quality Management in the United States FIGURE 2-3 Foliar injury to cotton induced by chronic exposure to ozone. Chronic exposures consist of relatively low concentrations (for example, less than 40 parts per billion), with periodic, random, intermittent episodes or relatively high ozone concentrations (for example, greater than 80 parts per billion) throughout the plant growth season. Symptoms of chronic injury include premature senescence and purple pigmentation in the interveinal areas. Photograph courtesy of P.J. Temple (retired, U.S. Department of Agriculture Forest Service). SOURCE: Krupa et al. 1998. uniform national regulations for certain key sources of criteria pollutants. To that end, the CAA includes a number of programs to reduce criteria pollutant emissions from stationary and mobile sources. As noted earlier, these national controls have been implemented in large part by setting specific emission standards within the act itself or, barring that, by providing specific instructions to the EPA administrator. Detailed discussions of the emission-control programs mandated in the CAA for mobile and stationary sources are presented in Chapters 4 and 5, respectively.
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Air Quality Management in the United States Hazardous Air Pollutants The CAA Amendments of 1970 required EPA to identify and list all air pollutants (not already identified as criteria pollutants) that “may reasonably be anticipated to result in an increase in mortality or an increase in serious irreversible or incapacitating reversible illness.” For each pollutant identified, EPA was to then promulgate national emission standards for hazardous air pollutants (NESHAPs) at levels that would ensure the protection of public health with “an ample margin of safety” and to prevent any significant and adverse environmental effects, which may reasonably be anticipated, on wildlife, aquatic life, or other natural resources. During the 1970s and 1980s, EPA began developing risk assessment methods necessary to establish the scientific basis for regulating HAPs (EPA 2000a). Despite advances in risk assessment methods gained through this work, the chemical-by-chemical regulatory approach based solely on risk proved difficult. Legal, scientific, and policy debates ensued over the risk assessment methods and assumptions, how much health risk data are needed to justify regulation, analyses of the costs to industry and benefits to human health and the environment, and decisions about “how safe is safe” (EPA 2000a). In the 20 years following enactment of the 1970 legislation, EPA identified only eight pollutants as HAPs and regulated sources of seven of them (asbestos, benzene, beryllium, inorganic arsenic, mercury, radionuclides, and vinyl chloride) (NRC 1994). With the CAA Amendments of 1990, Congress mandated a new approach. To expedite control of HAPs without explicit consideration of their inherent toxicity and potential risk, Congress provided a list of 189 compounds to be controlled by EPA as HAPs (see Appendix C). The EPA administrator was given the responsibility to review and amend the list of regulated HAPs periodically as dictated by new scientific information. However, since passage of the CAA Amendments of 1990, one compound has been deleted from the list (caprolactam), the scope of chemicals covered by glycol ethers was reduced, and no compound has been added to the list.7 Current Standard-Setting Procedure for HAPs In contrast to criteria pollutants for which ambient concentration standards are used, the control of HAPs is based on an initial promulgation of emission standards and a subsequent assessment of risk that remains after implementation of these standards (the CAA defines this remaining risk as the “residual risk”). The standards are to be imposed on (1) all major 7 On May 30, 2003, EPA proposed to remove the compound methyl ethyl ketone (MEK) from the HAPs list, and on November 21, 2003, EPA proposed to remove ethylene glycol monobutyl ether from the list.
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Air Quality Management in the United States recent research. A more systematic approach is needed to ensure that the criteria document includes all potentially relevant findings in the scientific review, identifies in an objective fashion the most valuable studies for assessing effects and setting standards, and does not overly emphasize findings of some studies without adequate consideration of their limitations. Accounting for Lack of Thresholds for Health Effects of Some Criteria Pollutants Several recent epidemiological studies have introduced a new complication into the health-based standard-setting process. These studies suggest that there is no threshold concentration for O3 (EPA/SAB, 1995), Pb (Canfield et al. 2003; Selevan et al. 2003), and PM (EPA 2002f) below which no observable health effects occur in the population (for example, see Figure 2-14). Although the validity of these findings still needs to be confirmed by additional research (EPA 2002f), the possibility that concentration thresholds may not exist for some pollutants raises serious questions about the technical feasibility of setting primary NAAQS that are consistent with the language in the CAA. In it, the EPA administrator is required to set primary NAAQS to protect public health with “an adequate margin of safety.” Implicit in this FIGURE 2-14 Concentration-response estimation from the reanalysis of the Pope/ American Cancer Society Study on cardiopulmonary disease mortality (excluding Boise, Idaho). Each dot represents the risk of mortality for one of the cities studied as compared with the risk for a city at the mean PM concentration. (The risk is shown on the y axis as standardized residual where larger positive values indicate greater risk.) Note that the relative risk continues to decline even when annual average levels of fine particles (PM2.5) are reduced to below the current NAAQS. SOURCE: Krewski et al. 2000a. Reprinted with permission; copyright 2000, Health Effects Institute.
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Air Quality Management in the United States instruction is the assumption that a NAAQS can be formulated by specifying a particular concentration below which the public health is protected from an adverse health effect of a pollutant. If a threshold does not exist, however, there might be no concentration below which the most susceptible members of the population are protected, raising the challenge for the administrator of how to arrive at an “adequate” margin of safety. A variety of alternative approaches that would better reflect the observed dose-response relationships could be considered for pollutants without thresholds. One example would be the use of a “cumulative” form of the standard in which risk increases with concentrations accumulated over a period of time, and the standard would be set at some allowable level of risk. However, this approach would present problems. It would require incorporation of the concept of acceptable risk into the NAAQS-setting procedure, a controversial issue in its own right. It would also require accurate quantitative estimates of the nature and extent of adverse health affects in sensitive populations, and the models upon which such estimates might be based are currently clouded in considerable scientific uncertainty. Static List of Hazardous Air Pollutants At the national level, toxic air pollutants are controlled when they are listed as hazardous air pollutants (HAPs), but the list of chemicals that fall under this regulatory regime has remained virtually unchanged since it was first developed in the CAA Amendments of 1990. That is the case despite the statute’s directing the administrator to review the list periodically, to add a substance when it “is known to cause or may reasonably be anticipated to cause any adverse effects to human health or adverse environmental effects,” and to delete a substance from the list when it “may not reasonably be anticipated to cause any adverse effects to human health or adverse environmental effects.” At this time, no formal process seems to have been established at EPA for routinely reviewing the list of HAPs. In fact, EPA has encountered resistance—in the form of extensive regulatory debates, conflicting scientific evidence, lawsuits, and political pressure—when trying to add substances or remove them from the list. (One example is the recent debate over the potential removal of methanol from the list.) The static nature of the HAPs list is problematic. In the United States, it is estimated that approximately 300 new chemicals are introduced into the environment each year by industry,11 and yet not a single new air toxic has 11 This information was obtained from the Notice of Commencement Database maintained by the inventory section in EPA’s Office of Pollution Prevention and Toxics.
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Air Quality Management in the United States been added to the list in more than a decade. The Toxic Substances Control Act (P.L. 94-469) is intended to require chemical testing and, if necessary, impose controls on new chemicals that are manufactured or imported. However, implementation of this act has been difficult, and an analysis by EPA showed that for even those chemicals produced or imported at over 1 million pounds per year, only 7% had a full set of basic toxicity information (EPA 1998a). There are two consequences of potential concern: (1) it is possible that the public health and welfare is not being adequately protected from all HAPs; indeed, without a formal procedure for reviewing the list of HAPs and assessing the risks associated with unregulated HAPs, it is difficult to assess the level of danger associated with this possibility; and (2) by leaving some harmful toxicants off the list of HAPs, the overall risks and costs to society of exposure to HAPs are probably underestimated. For example, although it is difficult to quantify the precise cancer risk from exposure to diesel exhaust (HEI 1999), some efforts to include diesel exhaust in estimates of cancer risk from exposure to HAPs have resulted in substantial increases in the overall estimates of risk (SCAQMD 2000). Another problem with the current HAPs framework is that chemicals that do not meet the evidentiary threshold for regulatory action and that have only suggestive evidence of adverse health or environmental effects are not addressed by the federal regulatory system, and the resources needed to better quantify the risks of these chemicals are often not available. As an example, consider polybrominated diphenylethers (PBDEs). These compounds, widely used as flame retardants, have been found in dust emanating from old furniture and manufacturing facilities (Hermanson et al. 2003) and are now accumulating in the environment (Hale et al. 2001a,b; Ikinomou et al. 2002). PBDEs are structurally similar to polychlorinated biphenyls, which are regulated by EPA, and could elicit similar toxic effects (Hooper and McDonald 2000; Eriksson et al. 2001; McDonald 2002). Moreover, the concentrations of PBDEs in human tissue in the United States are on the rise (Mazdai et al. 2003; Schecter et al. 2003). However, because the toxicology of PBDEs is not well-documented, these compounds are unregulated air pollutants at the federal level in the United States. In Chapter 7, suggestions are put forward for addressing many of the problems related to HAPs described above. Need for a Coordinated Strategic Program to Assess Ecosystem Effects The nation has not had a continued strategic approach for conducting the research needed to characterize and understand the effects of air pollution on ecosystems, and that has made it extremely challenging to establish appropriate standards to protect ecosystems. Indeed, for the most part,
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Air Quality Management in the United States EPA has opted not to set unique secondary NAAQS for criteria pollutants. However, as noted earlier, there is growing evidence that tighter standards to protect sensitive ecosystems in the United States are needed, and an enhanced program of research on air pollution impacts on ecosystems is needed. The development of a quantitative understanding of how air pollutants affect ecosystems is going to require the design and implementation of a substantially enhanced research strategy—one that monitors pollutants, as well as ecosystem structure and function, in a comprehensive and holistic way. A more detailed discussion of the monitoring networks that are in place and that would be needed to support such a research strategy is provided in Chapter 6. Need for Alternative Forms of Air Quality Standards to Protect Ecosystems The CAA currently directs the administrator to protect ecosystems from criteria pollutants through the promulgation and enforcement of ambient-concentration-based standards (that is, the secondary NAAQS). However, concentration-based standards are inappropriate for some resources at risk from air pollutants, including soils, groundwaters, surface waters, and coastal ecosystems. For such resources, a deposition-based standard would be more appropriate. One approach for establishing such a deposition-based standard is through the use of so-called “critical loads.” As described in Box 2-2, this approach has been adopted to protect ecosystems from acid rain by the European Union with some success. Limitations of Establishing Standards for One Pollutant at a Time The CAA directs the EPA administrator to establish air quality standards for individual criteria pollutants and HAPs in isolation from other pollutants. That approach has contributed to the development of an AQM system in the United States that tends to focus on only one pollutant at a time, probably introduces inefficiencies into the pollution control program, and might even give rise to unintended consequences. Many air pollutants have common sources, and multipollutant strategies that take advantage of these common sources probably can achieve economies of scale that control strategies that target one pollutant at a time cannot. Moreover, pollutants can also be connected by similar precursors or chemical reactions once in the atmosphere. Thus, control strategies that target one pollutant may affect others, perhaps in unintended ways. Because the standard-setting procedure for criteria pollutants and HAPs has also largely focused on individual pollutant effects, most health
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Air Quality Management in the United States BOX 2-2 Critical Loads: Europe’s Approach to Setting Acid Rain Goals Critical loads came to the forefront in air quality policy development as part of the United Nations Economic Commission for Europe (UNECE) Convention on Long-Range Transboundary Air Pollution (LRTAP). LRTAP was signed in 1979 and entered into force in 1983; critical loads were adopted in 1988 as part of the protocol development process. Critical loads are scientifically determined estimates of the maximum long-term exposure to pollution that an ecosystem can withstand without significant harmful effects (Grennfelt and Nilsson 1988). Defining a critical load requires the integration of data on an ecosystem’s soil type, land use, geology, rainfall, and other characteristics to classify its sensitivity to deposition of a pollutant or combination of pollutants. Initial efforts in Europe focused on sulfur deposition but have been expanded to include nitrogen and more recently heavy metals and persistent organic pollutants (POPs). Three approaches have been used to calculate critical loads: (1) empirical, (2) mass-balance based, and (3) dynamic modeling. The empirical approach uses empirical relationships developed from experimental studies and field observations of soil and water chemistry across pollution gradients. The mass-balance approach involves calculations to determine an ecosystem’s ability to neutralize inputs of acidity. These calculations assume that ecosystems are at steady-state conditions and that soil chemical pools do not evolve in response to changes in atmospheric deposition. Dynamic models take the mass-balance approach a step further by assessing the time-dependent response to changes in deposition (Jefferies 1997; Jenkins et al. 1998). Although the mass-balance approach is limited by the steady-state assumption, it requires far less information than dynamic models and has been widely embraced in Europe for critical-load calculations. After the critical load is assessed for an area, it is possible to determine the exceedance level, which is the difference between the critical load and the actual deposition. The exceedance level indicates the emission reductions necessary to protect the ecosystem. Much like critical load values, exceedance levels can be calculated by different approaches, ultimately, shaping environmental policy. One method is to determine the land area of ecosystem types that exceeds the critical load. A more refined method, the average accumulated exceedance, produces a weighted average of the exceedance by the amount of area the ecosystems cover in the grid. In Europe, a 5-percentile critical load map was adopted in the second sulfur protocol in which a deposition level is considered to be less than the critical load if 95% of the ecosystems in the grid will not be harmed. Once areas of excess deposition are identified, optimal emission-reduction strategies that take cost into consideration can be negotiated and implemented. The Convention on LRTAP determined that the cost to meet the 5-percentile goal was not economically feasible. Thus, policy makers identified “target loads” that accounted for economic and political considerations as an intermediate step to reducing emission levels to below critical load levels. The UNECE chose to set an interim load that would reduce the 1980 exceedance level by 60% by 2010. This objective is then revisited every 5 to 6 years to determine if additional measures need to be enacted.
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Air Quality Management in the United States The critical-loads and target-loads approaches adopted in Europe have provided an objective framework for stakeholders to debate how the ecological effects of acid deposition and other deposition-based pollutants (for example, nitrogen, mercury) can be curtailed by air pollution control programs. It has resulted in a process that provides for ecosystem protection and has led to reductions in European emissions while accounting for variations in sensitivity between different ecosystems. Countries have typically implemented measures that provide for reasonable progress toward meeting the targets given cost considerations, but they typically have not adopted requirements stringent enough to meet the targets (Blau and Agren 2001). Due to its grounding in scientific assessments, the critical load process enjoys relatively strong political support from the majority of European countries. Nevertheless, although critical loads may provide a consistent scientific framework for evaluating emissions reductions, the calculated values can be highly uncertain, in part due to reliance on steady-state models. Further, the numerous methods for calculating both critical loads and exceedance levels allow for inconsistency in implementation. Another limitation in the approach is that the formulation is in terms of a single threshold for an entire ecosystem. In actuality, ecosystems vary in composition, and pollutant deposition probably has impacts on some species at even very low levels. effects studies that form the current scientific basis for setting air quality standards have attempted to elucidate the effects of individual pollutants in isolation from others. That approach can pose significant challenges to investigators. In epidemiological studies, for example, identification of independent health effects of pollutants requires that a setting be chosen where only one of the pollutants is present (an unrealistic and somewhat artificial requirement) or, more commonly, that statistical modeling of the data be carried out to tease out individual pollutant effects. The latter approach can be hampered by the strong correlations that sometimes exist between air pollutants; the correlations can make it difficult or even impossible to separate the effects of one pollutant from another. Health effects studies that focus on sources instead of individual pollutants offer one method for moving away from an AQM system focused on single pollutants to one focused on multipollutant controls. One approach that science is beginning to pursue is to move away from studies (and ultimately standards) based on concentrations of individual pollutants to studies of the effects of specific pollution sources. In a source-oriented method, scientists define exposure to a specific source and obtain information about the health effects of the exposure to the mix of pollutants from that source. Techniques are being developed to define pollution sources by using source “markers,” which are indirect source measures (for example,
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Air Quality Management in the United States distances from major roadways, determined by geographic information systems, to reflect motor vehicle pollutants), and factor analysis based on extensive chemical analysis of monitor filters (Laden et al. 2000). There also has been significant progress in developing exposure assessment techniques that determine the intake fraction—that is, that portion of a person’s exposure that results from emissions from a particular source (Bennett et al. 2002). Finally, toxicological efforts are under way to systematically assess and compare the effects of well-characterized sources (Seagrave et al. 2002). These source-oriented approaches have potential advantages in focusing regulatory attention and research on potentially toxic emissions from specific sources and in directing public health initiatives to reduce emissions from specific sources rather than attempt to reduce general ambient concentrations of specific pollutants. Need to Address Health Risk Associated with Exposure in Hot Spots and Indoor Environments There is a growing recognition within the scientific and regulatory communities of the potential importance of pollutant exposure in special microenvironments or hot spots. These environments may include highway toll plazas, truck stops, airport aprons, and areas adjacent to one or many stationary sources or busy roadways, as well as transit within vehicles and indoor environments. Pollution concentrations in hot spots may exhibit strong transient spikes, such as with the ebb and flow of traffic or as a result of off-normal (upset) conditions at stationary sources that might result in short-term emissions greater than those usually represented by typical operating conditions or by annual national averages (for example see NRC 2000c). Because of the small spatial scale and possible transient nature of hot spots, mitigation of air pollution within these microenvironments may not be adequately addressed in the nation’s current AQM system, even though segments of the population may experience significant air pollution exposure within these environments (for example, California school bus study [Fitz et al. 2003]). In Chapter 7 of this report, recommendations are made to better address the problem of hot spots by encouraging states to include measures to mitigate hot-spot pollution in their implementation plans to meet NAAQS and other national goals and by evolving the AQM system toward a more risk-based multipollutant paradigm. Within the general problem of hot spots, the health risk that may be associated with exposure to indoor air pollution is of particular concern, because most Americans spend more time indoors than outdoors (EPA 1987). For most Americans, exposure to the indoor environment dominates over the outdoor environment. The most vulnerable—children and older and infirm adults—generally spend over 90% of their time indoors.
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Air Quality Management in the United States Indoor air contains complex mixtures of contaminants. Some enter from the outside (for example, lead, mercury, and fine PM from contaminated soils, power plants, and other sources), and others arise from the presence and use of commercial, industrial, and household products, such as plastics, paint, solvents, pesticides, and stoves (see Figure 2-15). Biological agents—such as molds, dander, and dust mites—are another type of contaminant that can cause pulmonary reactions. The level of air pollution within any indoor environment depends on the specific types of products in use as well as the building’s characteristics (for example, the ventilation and types of carpets) and the habits of the occupants. In spite of the potential dangers, indoor air pollution remains largely unregulated in the United States, and to the extent that the public is protected, it is accomplished through a patchwork of information and some regulatory actions. In some instances, EPA has been able to exercise authority (for example, over asbestos). However, no federal statutes specifically give EPA authority to regulate indoor residential and commercial sources of air pollutants—perhaps because of a reluctance on the part of FIGURE 2-15 Schematic diagram illustrating the source of human exposure to indoor PM pollution. SOURCE: Rodes 2001. Reprinted with permission; copyright 2001, RTI International, Research Triangle Park, NC.
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Air Quality Management in the United States Congress to regulate any aspect of the inside of people’s homes, public buildings, and offices. Regulatory power for some aspects of indoor air quality in occupational settings is given to the Occupational Safety and Health Administration (OSHA), and for some consumer products, to the Consumer Products Safety Commission (CPSC). However, in the case of OSHA, which is primarily responsible for protecting generally healthy worker populations, indoor air quality standards are generally not as protective as EPA’s ambient (outside) air quality standards. In addition, some states and EPA, in regulating some products for VOC content (for example, surface coatings that are potential sources of VOC precursors to O3 formation), have in effect also reduced exposure to those VOCs in indoor settings where those coatings are applied. For the most part, however, EPA has attempted to reduce indoor air pollution indirectly by identifying indoor air contaminants that pose significant risks (for example, environmental tobacco smoke and radon) and encouraging consumers, corporations, and other organizations to respond to that information or by regulating major sources of outdoor air pollutants that pose a health risk when transported indoors (for example, lead). In some cases, state and local governments have responded by improving their own standards, regulations, or ordinances. However, there is not at this time a coordinated federal program to ensure that the public is protected from unnecessary or avoidable health risks associated with indoor air pollution. A more coordinated approach would seem prudent. Risk Assessment and Priority Setting There is a long-standing recognition of the need for robust quantitative estimates of the risks to human health and welfare associated with exposure to air pollutants. First, Congress expressly directed EPA to use an assessment of residual risk to design the second phase of controls on HAPs. Second, the final report of the National Commission on Risk Assessment and Risk Management, established in the CAA Amendments of 1990, found that risk assessment can be a powerful tool for setting priorities on resources for monitoring and regulating the myriad air pollutants to which humans, ecosystems, and materials are regularly exposed (NCRARM 1997). Moreover, under the present AQM system, setting priorities is done largely by statutory and/or agency fiat. For example, because of the detailed requirements specified for the regulation of criteria pollutants (as discussed in Chapter 3), this subset of pollutants generally receives a substantially larger share of the management effort and resources than do HAPs. Moreover, controls that most effectively reduce concentrations of pollutants in the ambient atmosphere tend to be favored over those that target pollution in hot spots. This emphasis on criteria pollutants in ambient air may or
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Air Quality Management in the United States may not be justified on the basis of actual human health and ecosystem risk. Similarly, it is not clear if all of the currently regulated HAPs pose a greater risk to human health and welfare than many of the untested and unregulated air toxics known to be in the ambient air and at specific hot spots. For the reasons noted above, it would be highly desirable for the nation’s AQM system to have a robust risk-assessment capability that could reliably assess and set priorities on the relative risks posed by all pollutants in the atmosphere—in hot spots and microenvironments, as well as the ambient air. However, although the scientific community has learned a great deal about air pollution in recent decades, and there have been significant advances in the general field of risk assessment (NRC 1994), current knowledge is not yet extensive enough to rank pollutants comprehensively on the basis of risk. There is a lack of sufficient knowledge of the diversity of health and welfare effects associated with different pollutants, and, perhaps more important, with different mixtures of pollutants under environmental conditions. Another major deficiency is our inability to assess pollutant exposures accurately because of a lack of sufficient data on the distribution of pollutants. If these deficiencies are to be addressed, substantial investments over a substantial period of time will be needed for research on air pollution effects research and for more advanced systems to determine the spatial and temporal variability of air pollutants in specific hot spots and in indoor environments as well as the ambient air. SUMMARY Strengths of Goal-Setting Procedures The establishment of the NAAQS has allowed for important and extensive input and feedback from the scientific and technical communities and has catalyzed additional research and understanding of the effects of air pollution. The standards-setting procedure for NAAQS has been responsive to new scientific information and has allowed for adjustments in the standards when scientific understanding so dictated. The establishment of NAAQS has provided targets for regulatory agencies and measures by which to assess improvement in air quality and the effectiveness of the AQM.
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Air Quality Management in the United States Limitations of Goal-Setting Procedures12 The funding of health effects research and the subsequent selection and review of this research in criteria documents and staff papers used to promulgate NAAQS often lack a coherent and standardized strategic plan. The CAA requirements for pollutant-specific air quality standards for criteria pollutants and HAPs have encouraged the evolution of an AQM system with control strategies that largely focus on one pollutant at a time. Changes that foster multipollutant approaches would be advantageous. One method that could be considered would be to assess the health and welfare effects of sources instead of pollutants. The list of HAPs (that is, those specifically regulated by EPA) has been essentially static for a decade and probably does not contain all the air toxics that pose a significant risk to human health and welfare. Current monitoring data and understanding are not sufficient to adequately assess the relative risks to human health and welfare posed by exposure to the myriad pollutants in the environment, as well as to the myriad microenvironments or hot spots in which these exposures may occur. Development of such a capability will be a major challenge and will require a substantial investment in resources for monitoring and effects research over a long period of time. Although progress has been made to improve exposure assessment and to link specific exposures and effects to specific sources, substantial additional work is needed. Indoor air pollution poses a significant health risk to humans and yet is not addressed comprehensively by any agency in the federal government. The current practice of letting the primary standard serve as the secondary standard for most criteria pollutants does not appear to be sufficiently protective of sensitive crops and unmanaged ecosystems. Moreover, concentration-based standards are inappropriate for some resources, such as soils, groundwater, surface water, and coastal ecosystems, that are at risk from the indirect effects that pollutants can foster (for example, eutrophication). A deposition-based standard would be more appropriate in some instances. It is a significant challenge to set ambient or emission standards to protect public health with an adequate margin of safety from harmful exposure to a pollutant if that pollutant does not exhibit a threshold concentration for an adverse health effect. 12 Recommendations that address these limitations are provided in Chapter 7.
Representative terms from entire chapter: