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Air Quality Management in the United States (2004)

Chapter: 2 Setting Goals and Standards

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Suggested Citation:"2 Setting Goals and Standards." National Research Council. 2004. Air Quality Management in the United States. Washington, DC: The National Academies Press. doi: 10.17226/10728.
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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:

  1. 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;

  2. to initiate and accelerate a national research and development program to achieve the prevention and control of air pollution;

  3. 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

  4. 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

Suggested Citation:"2 Setting Goals and Standards." National Research Council. 2004. Air Quality Management in the United States. Washington, DC: The National Academies Press. doi: 10.17226/10728.
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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.

Suggested Citation:"2 Setting Goals and Standards." National Research Council. 2004. Air Quality Management in the United States. Washington, DC: The National Academies Press. doi: 10.17226/10728.
×

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.”

Suggested Citation:"2 Setting Goals and Standards." National Research Council. 2004. Air Quality Management in the United States. Washington, DC: The National Academies Press. doi: 10.17226/10728.
×

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.

Suggested Citation:"2 Setting Goals and Standards." National Research Council. 2004. Air Quality Management in the United States. Washington, DC: The National Academies Press. doi: 10.17226/10728.
×

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.

Suggested Citation:"2 Setting Goals and Standards." National Research Council. 2004. Air Quality Management in the United States. Washington, DC: The National Academies Press. doi: 10.17226/10728.
×

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

Suggested Citation:"2 Setting Goals and Standards." National Research Council. 2004. Air Quality Management in the United States. Washington, DC: The National Academies Press. doi: 10.17226/10728.
×

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

Suggested Citation:"2 Setting Goals and Standards." National Research Council. 2004. Air Quality Management in the United States. Washington, DC: The National Academies Press. doi: 10.17226/10728.
×

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.

Suggested Citation:"2 Setting Goals and Standards." National Research Council. 2004. Air Quality Management in the United States. Washington, DC: The National Academies Press. doi: 10.17226/10728.
×

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 [1990]). 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.

Suggested Citation:"2 Setting Goals and Standards." National Research Council. 2004. Air Quality Management in the United States. Washington, DC: The National Academies Press. doi: 10.17226/10728.
×

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.

Suggested Citation:"2 Setting Goals and Standards." National Research Council. 2004. Air Quality Management in the United States. Washington, DC: The National Academies Press. doi: 10.17226/10728.
×

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.

Suggested Citation:"2 Setting Goals and Standards." National Research Council. 2004. Air Quality Management in the United States. Washington, DC: The National Academies Press. doi: 10.17226/10728.
×

sources, which are defined as “any stationary source or group of stationary sources located within a contiguous area and under common control that emits or has the potential to emit … 10 tons per year or more of any hazardous air pollutant or 25 tons per year or more of any combination of hazardous air pollutants”; and (2) a sufficient number of area sources “to ensure that area sources (excluding mobile sources) representing 90 percent of the area source emissions of the 30 (or more) hazardous air pollutants that present the greatest threat to public health in the largest number of urban areas are subject to regulation.” In the case of major sources, 174 different types of sources were identified as emitters of HAPs and targeted for regulation by Congress. In a separate portion of the CAA Amendments of 1990 (Section 202), EPA was instructed to develop and promulgate emission standards for vehicles and fuels “on those categories of emissions that pose the greatest risk to human health … [and] at a minimum, apply to emissions of benzene and formaldehyde.”

In the first regulatory phase of the program described in Section 112 of the 1990 CAA Amendments for major and area sources, EPA was directed to promulgate national technology-based emission standards for HAPs using available control technologies or work practices. In this regard, the CAA defined two types of emission standards for promulgation:

  • Maximum achievable control technologies (MACTs) are emission standards that achieve “the maximum degree of reduction in emissions of the hazardous air pollutants … that the Administrator, taking into consideration the cost of achieving such emission reduction, and any non-air quality health and environmental impacts and energy requirements, determines is achievable.”

  • Generally available control technologies (GACTs) are less stringent emission standards based on the use of more standard technologies and work practices.

Despite the use of the word “technologies” in those definitions, Congress did not intend them to be technologically prescriptive. They were intended to be technology-neutral, performance-oriented standards that set a maximum allowable emission rate, based on the emissions obtained using MACT or GACT, and that allowed the affected industries and facilities to choose any combination of technologies and practices to achieve these performance levels (EPA 2000a).8

Congress mandated that MACT be applied to all major stationary sources of HAPs. For area sources, the EPA administrator was directed to

8  

A more detailed discussion of the implementation of MACT and GACT controls and their effectiveness as a promulgator of technology-neutral standards is presented in Chapter 5.

Suggested Citation:"2 Setting Goals and Standards." National Research Council. 2004. Air Quality Management in the United States. Washington, DC: The National Academies Press. doi: 10.17226/10728.
×

select between MACT and GACT as deemed appropriate. For HAPs with a health threshold, EPA could consider the threshold with an ample margin of safety in establishing the emission standard. Congress further mandated a 10-year schedule for the promulgation of MACT standards for major sources, with certain standards being promulgated in the first 2 years, 25% in the first 4 years, an additional 25% promulgated not later than the seventh year, and the remaining 50% not later than the tenth year. As of February 2003, EPA had promulgated 79 MACT standards affecting 123 source categories (T. Clemons, EPA, Washington, DC, personal commun., March 31, 2003). In addition, two standards regulating solid waste were promulgated under Section 129 of the CAA Amendments of 1990. In total, over 100 HAPs fell under these regulations (EPA 2000a). In May 2003, EPA indicated that it expects to finalize all MACT standards by the time that states will be required to set MACT limits on a facility-by-facility basis according to Section 112(j), as amended (EPA 2003b). Clearly, the promulgation of MACT has proved to be a complex task for EPA and has occurred more slowly than mandated by Congress. This delay has given rise to criticism of EPA by some environmental groups (for example, Williams 2003), as well as litigation (Sierra Club v. U.S. Environmental Protection Agency, No. 02-1135, 2002 [DC Circuit]).

In the second regulatory phase of the HAPs program, EPA was instructed to conduct an assessment of and report on the residual risk due to HAPs emitted from the regulated major and area sources discussed above. Then, in the absence of any specific action by Congress for a 2-year period following issuance of the report, the EPA administrator was to promulgate additional emission standards to “provide an ample margin of safety to protect public health or to prevent, taking into consideration costs, energy, safety, and other relevant factors, an adverse environmental effect.” The promulgation of these additional emission standards was to occur no later than 8 years after EPA’s initial promulgation of the technology-based standards. Although no formal standard for acceptable residual risk was mandated in the CAA Amendments of 1990, the act cited an example of such a standard: the reduction of excess cancer risk for the most exposed individuals to less than 1 in 1 million for a lifetime of exposure to a particular HAP.

Because of the difficulties in assessing residual risk, completion of the residual risk analysis and promulgation of additional emission controls mandated in the CAA are still some years off. The agency is proceeding to investigate the residual risk that is likely to remain after attainment of the MACT and GACT standards. In 1999, EPA reported to Congress on its progress in determining residual risk for cancer and non-cancer health effects. The report describes how EPA intends to calculate risk, what standard it will apply (the 1 in 1 million as applied in the 1989 benzene

Suggested Citation:"2 Setting Goals and Standards." National Research Council. 2004. Air Quality Management in the United States. Washington, DC: The National Academies Press. doi: 10.17226/10728.
×

NESHAP), and progress in furthering the understanding of the health effects of HAPs (EPA 1999b). The 1990 CAA Amendments also called for a number of special studies related to assessing risk resulting from exposure to HAPs, including studies on emissions from electric utility-steam generating units and publicly owned treatment works.

In the case of area sources, the EPA administrator was directed to first undertake a research program of monitoring and analysis to identify the area sources of HAPs in representative urban locations. On the basis of the research, the administrator was to then propose a “comprehensive strategy” to control the emissions of urban area sources that in the aggregate accounted for at least 90% of the emissions of the 30 or more HAPs that presented the greatest health risk in the largest number of urban areas. Congress further mandated that one specific goal of the strategy was to be a 75% reduction in the incidence of cancers attributable to all sources of HAPs. The comprehensive strategy was to be completed within 5 years of the passage of the CAA Amendments of 1990 and implemented “as expeditiously as practicable, assuring that all sources are in compliance with all requirements not later than 9 years after the … enactment of the” CAA Amendments of 1990. To meet the requirements, EPA developed the integrated urban air toxics strategy (EPA 2000b) and later issued a notice in the Federal Register (67 Fed. Reg. 43112 [2002]) that focused on 33 HAPs and listed 47 area sources that are or will be subject to standards. In developing the strategy, EPA attempted to address areas of high exposure by characterizing exposure and risk as a function of geography and demography as a means of selecting the highest priority HAPs from among the 188. Subsequently, EPA refined this approach by conducting the National Air Toxics Assessment (NATA), which summarizes the exposure and risk levels for each of the 33 highest priority HAPs (a subset of 32 of the list of 188 plus diesel particulate matter) (EPA 2002e). To better understand the sources and health risks associated with HAPs, the strategy includes activities to expand HAPs monitoring, improve emission inventories and national- and local-scale modeling, investigate health effects and exposures to HAPs in ambient and indoor air, and improve assessment tools.

In Title II of the CAA Amendments of 1990, several actions were included to reduce emissions of mobile-source HAPs. The act required that EPA implement, by January 1, 1995, a new reformulated gasoline (RFG) program for O3 nonattainment areas that for the first time required caps on a number of toxic and volatile constituents of gasoline, especially benzene. In Section 202, the 1990 CAA Amendments also required the mobile-source HAPs program to conduct a motor-vehicle-source HAPs study and to promulgate regulations. Implementation of this requirement is discussed in Chapter 4.

Suggested Citation:"2 Setting Goals and Standards." National Research Council. 2004. Air Quality Management in the United States. Washington, DC: The National Academies Press. doi: 10.17226/10728.
×

GOALS FOR MITIGATING VISIBILITY DEGRADATION

Most visibility impairment is caused by the presence of fine PM suspended in the atmosphere; these particles scatter and absorb light and, in so doing, reduce visibility (see Figure 2-4A and 2-4B). Haze is a common phenomenon in most parts of the United States. Although most severe in the East and California, hazy conditions are encountered episodically in virtually all areas of the country, even those remote from major centers of population and industrial development (see Figure 2-4C). The affected areas include many of the nation’s most beautiful national parks and wilderness areas. Some visibility impairment is natural (for example, from wind-blown dust or wildfires); however, much of it is caused by pollutant emissions of particles and gases, such as SO2, that are converted into particles in the atmosphere.

In response to growing concerns about deteriorating visibility in our nation’s recreational areas, the CAA Amendments of 1977 (Section 169A) established a national goal of preventing and remedying visibility impairment due to anthropogenic pollution in Class I areas, which include most U.S. large parks and wilderness areas. The CAA Amendments of 1990 provided additional emphasis on regional haze issues. In 1999, EPA promulgated a Regional Haze Rule, which established a 65-year program to return 156 national parks and wilderness areas to their natural visibility conditions. To accomplish that, anthropogenic emissions in the United States would have to be reduced until visibility in all Class I areas is not noticeably poorer than that under natural conditions. The EPA rule establishes “reasonable-progress” goals that are based on a uniform rate of visibility improvement between baseline conditions (measured from 2000 to 2004) and natural visibility conditions to be achieved by 2064.9 SIPs outlining plans for achieving visibility goals are due no later than 2008, and these plans are to be updated every 10 years thereafter to ensure that reasonable progress has been achieved.

STANDARDS FOR MITIGATING EFFECTS OF ACID RAIN

“Acid rain” (also known as acid deposition) refers to the wet and dry deposition of acidic compounds to the earth’s surface that can have deleterious effects on ecosystems and materials (see Figure 2-5). In the United States, most of the harmful acidity is in the form of sulfuric and nitric acids, which are produced in the atmosphere from the chemical conversion of SO2 and NOx. SO2 enters the atmosphere primarily via the combustion of sulfur-containing fossil fuel largely in coal-fired power plants. NOx enters

9  

Implementation of the EPA residual haze rule is discussed in Chapter 3.

Suggested Citation:"2 Setting Goals and Standards." National Research Council. 2004. Air Quality Management in the United States. Washington, DC: The National Academies Press. doi: 10.17226/10728.
×

FIGURE 2-4 The impact of haze on visibility. (A) Good air quality contrasted with (B) poor air quality in Big Bend National Park. SOURCE: NPS 2002a,b,c. (C) Standard visual range for the period 1996–1998 in units of kilometers. SOURCE: VIEWS 2003.

Suggested Citation:"2 Setting Goals and Standards." National Research Council. 2004. Air Quality Management in the United States. Washington, DC: The National Academies Press. doi: 10.17226/10728.
×

FIGURE 2-5 Anthropogenic sources and natural sources contribute emissions that result in the deposition of acidic compounds. SOURCE: EPA 1999c.

the atmosphere primarily via high temperature combustion of coal, oil, and gas in power plants and of gasoline and diesel fuels in automobiles and trucks. Because it typically takes days to weeks for atmospheric SO2 and NOx to be converted to acids and deposited to the earth’s surface, acid deposition occurs on a multistate scale hundreds (if not thousands) of miles away from its sources.

Controls on Acid Rain Precursors before the CAA Amendments of 1990

The establishment of NAAQS for SO2 and NO2 as a result of the CAA Amendments of 1970 marked the beginning of a nationwide program to control their emissions. (In addition to the NAAQS, EPA, pursuant to the 1970 CAA Amendments, imposed a new-source performance standard (NSPS) of a maximum of 1.2 pounds (lb) of SO2 per million British thermal units (Btu) generated for all new power plants.) However, the initial response to the establishment of NAAQS for SO2 and NO2 did not necessarily result in a reduction of their emissions from stationary sources. It was generally found that the most cost effective method to limit the contribution of stationary-source emissions of SO2 and NO2 to local nonattainment events was to install tall stacks, which dispersed the emissions and thereby

Suggested Citation:"2 Setting Goals and Standards." National Research Council. 2004. Air Quality Management in the United States. Washington, DC: The National Academies Press. doi: 10.17226/10728.
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eliminated local areas of elevated concentrations in the immediate vicinity of the facility. As a result, 429 tall stacks, many over 500 feet tall, were constructed on coal-fired boilers in the electricity industry during the 1970s (Regens and Rycroft 1988), and, by 1980, the vast majority of urban areas in the United States were in attainment of the NAAQS for SO2 and NO2. However, the use of tall stacks had an unintended consequence: by facilitating the long-distance transport of SO2 and NO2 and their conversion to sulfuric and nitric acids before deposition, the installation of tall stacks exacerbated the acid rain problem.

The CAA Amendments of 1977 required EPA to address new coal utility plants (those built after 1978). EPA promulgated a standard allowing new plants to either (1) remove 90% of potential SO2 emissions (as determined by the sulfur content of the fuel burned) and operate with an emission rate below 1.2 lb of SO2 per million Btu, or (2) remove 70% of potential SO2 emissions and operate with an emission rate less than 0.6 lb of SO2 per 1 million Btu. (The 1977 Amendments also prohibited using tall stacks to comply with emission standards.) The percent-reduction requirement effectively forced all new coal plants to operate with flue gas desulfurization. It also substantially reduced the advantage of using low-sulfur coal as a means of compliance, because a facility using low-sulfur coal would still be required to remove at least 70% of the potential emissions. Because coal in the western United States has lower sulfur, the statute had the effect of imposing a more stringent emission cap on new sources in the West than in the East, perhaps to help prevent significant deterioration of existing air quality in the West.

As a result of the aforementioned emission standards, U.S. SO2 emissions peaked in the early 1970s and declined steadily throughout the remainder of the 1970s and early 1980s (see Figure 2-6). By 1985, SO2 emissions nationwide had declined by about 25% from the peak emissions of the 1970s. However, after 1985, the NSPS lost its ability to affect further SO2 emission reductions. The change seems to have been caused by the focus of the statutes on new generating plants, which had two significant effects: (1) it created a significant gap between the SO2 emissions of older plants and those permitted for new plants; and (2) it raised the costs of new plants and thus created an economic incentive for keeping old plants on line beyond their original design lifetimes. As a result, it became increasingly common to keep older plants on line through lifetime extension projects. By 1985, 83% of power-plant SO2 emissions in the United States came from generating plants not meeting the 1971 NSPS, and SO2 emissions in the United States were relatively stable (Ellerman et al. 2000).

In contrast to SO2 emissions, NOx emissions in the United States remained fairly flat over the 1970s and 1980s (see Figure 2-6). The emissions peaked in the late 1970s and decreased somewhat over the next 5 years as

Suggested Citation:"2 Setting Goals and Standards." National Research Council. 2004. Air Quality Management in the United States. Washington, DC: The National Academies Press. doi: 10.17226/10728.
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FIGURE 2-6 Trends in nationwide SO2 and NO2 emissions by year since 1940. In 2003, EPA revised some of its emission estimates for the years 1970 and later (see EPA [2003r] for the revised profiles). SOURCE: EPA 2000c.

Suggested Citation:"2 Setting Goals and Standards." National Research Council. 2004. Air Quality Management in the United States. Washington, DC: The National Academies Press. doi: 10.17226/10728.
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a result of the imposition of NOx emission standards on new motor vehicles (see Chapter 4) and then inched upward for the remainder of the 1980s.

Acid Rain Goals Set by the CAA Amendments of 1990

Acid rain gradually emerged as a serious environmental concern in the late 1970s. A growing body of scientific evidence had accumulated that documented the deleterious impacts of acid rain on ecosystems, aquatic life, and property, particularly in regions where soils are acidic, such as eastern Canada and the northeastern United States. Pressure for remedial action began to build from environmental groups and officials from the northeastern states most affected by acid rain. The Canadian government also began to pressure the United States, claiming that its ecosystems were being damaged from the transport of acid rain precursors from the United States.

In response, Congress created the National Acid Precipitation Assessment Program (NAPAP) in 1980 to study the impacts of acid deposition, recommend if emission controls were needed to mitigate these impacts and, if so, the magnitude of the emission reductions needed (see Box 2-1). Ten years

BOX 2-1
The Role of NAPAP in Shaping the Acid Rain Provision of the CAA Amendments of 1990

The National Acid Precipitation Assessment Program (NAPAP) was established by Congress to study the impacts of acid deposition, recommend if emission controls were needed to mitigate these impacts, and, if so, the magnitude of the emissions reductions needed.

A major endeavor of NAPAP was the development of the regional acid deposition model (RADM) (Chang et al. 1987), a state-of-the-science 3-dimensional grid model capable of simulating the physical and chemical processes leading to the formation and deposition of acidic speciesa (NAPAP, 1991a). Model development started in 1983 and its application was completed in the early 1990s. The model (together with similar tools like the ADOM model developed for Canada) provided important insights into the source-receptor relationships of the acid deposition problem in the United States. However, these modeling exercises (together with the rest of the synthesis of the acid deposition research) came to fruition very close to the time of the completion of the CAA Amendments of 1990 and appear to have played a minor role in the development of the acid rain provisions in the 1990 Amendments. Instead it appears that a complex set of technological, legal, and political considerations played the most critical roles in shaping the emission targets for the acid rain program in the 1990 Amendments. These other considerations included the technological and economic feasibility of reducing emissions as well as the need to reach consensus on a regionally and politically divisive issue.

a  

A discussion of air quality models is presented in Chapter 3.

Suggested Citation:"2 Setting Goals and Standards." National Research Council. 2004. Air Quality Management in the United States. Washington, DC: The National Academies Press. doi: 10.17226/10728.
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later, in Title IV of the CAA Amendments of 1990, Congress enacted specific legislation to mitigate the adverse effects of acid rain as well as to “encourage energy conservation, use of renewable and clean alternative technologies, and pollution prevention as a long-range strategy … for reducing air pollution and other adverse impacts of energy production and use.”

The legislation in Title IV represented a significant departure from the regulatory approaches prescribed by Congress for criteria pollutants and HAPs. Instead of directing the EPA administrator to set standards to protect human health and welfare, Congress imposed their own standards—standards that, as noted in Box 2-1, were influenced to some extent by scientific understanding of the effects of acid rain but also by nontechnical economic and political considerations. The standards prescribed by Congress were aimed at the emissions of SO2 and NOx from electric utilities and were designed to bring about significant reductions in these emissions nationwide. Electric utilities were targeted because they were estimated to contribute two-thirds of all SO2 emissions and one-third of NOx emissions (EPA 1999c).

Because the scientific evidence suggested that SO2 emissions made the largest contribution to acid rain (NAPAP 1991a), the most aggressive control program in Title IV was aimed at SO2 emissions. Specifically, SO2 emissions from electric utilities nationwide were capped at an amount that would require a decrease in total emissions by 2010 of 10 million tons (or about 50%) relative to emission levels of 1980. Instead of imposing a technological or emissions-based standard on power-generating facilities, Congress specified that the emission reductions were to be achieved through a market-based mechanism; specifically in this case, a cap-and-trade program. A smaller and more traditional program for reducing NOx emissions was also enacted. This program involved a two-phase strategy to reduce NOx emissions from coal-fired electric utility plants by over 400,000 tons per year between 1996 and 1999 (Phase I) and by approximately 1.17 million tons per year beginning in year 2000 (Phase II). To accomplish these reductions, Congress imposed emission standards on power plants; standards varied depending upon the type of facility. Language was also included to allow a state or group of states to petition the EPA administrator to use a cap-and-trade program instead of emission standards to meet Congress’s NOx reductions goals. A more detailed discussion of the SO2 and NOx acid-rain emission-control programs and their implementation can be found in Chapter 5.

Environmental Justice as an Air Quality Goal

Environmental justice has been defined by EPA as the “fair treatment for people of all races, cultures, and incomes, regarding development of

Suggested Citation:"2 Setting Goals and Standards." National Research Council. 2004. Air Quality Management in the United States. Washington, DC: The National Academies Press. doi: 10.17226/10728.
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environmental laws, regulations and policies” (EPA 2003c). In practice, environmental justice issues have been concerned with the adverse health and economic effects of environmental hazards when disproportionately suffered by minority and low-income communities. Historically, Title VI of the Civil Rights Act of 1964 has been the primary instrument available to these communities to redress and ameliorate environmental injustices. Because this act specifically forbids funding recipients, such as state agencies, from using criteria or administrative methods that have the effect of subjecting individuals to discrimination on the grounds of race, color, or national origin, relief can be obtained in principle by alleging discriminatory environmental and health effects that have resulted from environmental permits issued by state agencies that receive federal funds.

The CAA and its amendments make no direct or specific reference to environmental justice. However, there are clearly environmental justice issues that can arise in the implementation of its many provisions. For example, trading programs, if not carefully designed (see Chapter 5), could result in hot spots10 that might disproportionately affect minority and other low-income communities (Solomon and Lee 2000 and references therein). In addition, the highest ambient air pollution concentrations are most often found in densely populated urban centers, where the highest proportion of minority and other low-income populations are also found, resulting in a disproportionate burden of effects (NRC 2003b). Finally, recent health studies have suggested that people with lower socioeconomic status are more likely to suffer premature mortality from exposure to air pollution than higher-income populations (Krewski et al. 2000a,b). A number of studies have been conducted that show, with varying degrees of uncertainty, that a correlation exists between race, income level, and a disproportionate exposure to environmental toxicants (Brown 1995; Goldman 1994; Perlin et al. 2001; Sexton et al. 1993; Zimmerman 1993; Gwynn and Thurston 2001).

In response to the aforementioned concerns, EPA established the Office of Environmental Justice in 1992 to integrate environmental justice into EPA’s policies, programs, and activities. In February 1994, the President of the United States issued Executive Order 12898 on environmental justice. That order was designed to focus federal attention on the environmental

10  

Hot spots are locales where pollutant concentrations are substantially higher than concentrations indicated by ambient outdoor monitors located in adjacent or surrounding areas. Hot spots can occur in indoor areas (for example, public buildings, schools, homes, and factories), inside vehicles (for example, cars, buses, and airplanes), and outdoor microenvironments (for example, a busy intersection, a tunnel, a depressed roadway canyon, toll plazas, truck terminals, airport aprons, or nearby one or many stationary sources). The pollutant concentrations within hot spots can vary over time depending on various factors including the emission rates, activity levels of contributing sources, and meteorological conditions.

Suggested Citation:"2 Setting Goals and Standards." National Research Council. 2004. Air Quality Management in the United States. Washington, DC: The National Academies Press. doi: 10.17226/10728.
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and health conditions of minority communities and low-income communities. It also called on federal agencies to make environmental justice a part of their missions and to develop an environmental justice strategy. An Interagency Working Group on Environmental Justice (IWG) was established to implement the order, and the EPA administrator was designated to serve as the convener of the IWG. Thus, the assurance of environmental justice has become a general goal of the nation’s AQM system. It remains to be seen how this goal will ultimately affect environmental policy and to what extent improved scientific tools and enhanced monitoring can be used to aid in this effort.

THE SCIENTIFIC BASIS FOR SETTING STANDARDS

The CAA directs the EPA administrator to set primary NAAQS to protect human health with “an adequate margin of safety,” and to set secondary NAAQS to “protect the public welfare from any known or anticipated adverse effects associated with” a criteria pollutant. It similarly directs the EPA administrator to set emission standards for HAPs on the basis of an assessment of the residual risk they pose following implementation of MACT and GACT. To do that, the administrator must have reliable and quantitative information on how human health and welfare outcomes are affected by varying concentrations of air pollutants. This information is typically expressed in terms of a dose-response relationship, which relates an undesirable health or welfare outcome to the concentration of a pollutant and the level of exposure to a pollutant over some specified period of time. Schematic illustrations of such dose-response relationships are presented in Figure 2-7. In this section, we discuss the scientific basis for establishing these relationships in the cases of human health and ecosystem effects.

Health Effects Studies

The effects data used to set health-related standards and goals (such as the primary NAAQS) are generated typically from two types of studies: (1) experimental or toxicological, and (2) observational or epidemiological. Experimental or toxicological studies involve either direct measurements of the effects of pollutants on health outcomes of human or animal subjects (see Figure 2-8 and Figure 2-9) or in vitro experiments in which the effects of pollutants on specific human or animal cells are examined.

In general, observational or epidemiological studies examine statistical relationships between the actual exposure of a population to a pollutant and some measure of adverse health effects in the population (for example, emergency room visits for asthma attacks or morbidity) over that same time

Suggested Citation:"2 Setting Goals and Standards." National Research Council. 2004. Air Quality Management in the United States. Washington, DC: The National Academies Press. doi: 10.17226/10728.
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FIGURE 2-7 Schematic illustrating dose-response relationships between pollutant exposure and (A) human health effects and (B) crop or vegetation effects. In A, two types of dose-response relationships are illustrated: the upper one with no threshold for an adverse effect and the lower one with a threshold. In B, the different lines are to indicate that the response to pollutants typically varies substantially among plant species or among varieties within a given species. Dose-response relationships for health effects are usually plotted with risk increasing with increasing dose, but dose-response relationships for welfare effects are often plotted in terms of a diminishing return as a function of exposure.

Suggested Citation:"2 Setting Goals and Standards." National Research Council. 2004. Air Quality Management in the United States. Washington, DC: The National Academies Press. doi: 10.17226/10728.
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FIGURE 2-8 Exercising volunteer being exposed to ultrafine particles and monitored for health response. SOURCE: HEI 2000. Reprinted with permission; copyright 2000, Health Effects Institute.

Suggested Citation:"2 Setting Goals and Standards." National Research Council. 2004. Air Quality Management in the United States. Washington, DC: The National Academies Press. doi: 10.17226/10728.
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FIGURE 2-9 Evidence of health impact of ozone on human respiratory system based on an experimental study involving human subjects. (a) A healthy lung airway, and (b) the constricted opening of a lung airway inflamed from exposure to ozone. SOURCE: EPA 1999d.

period. Population exposure is in turn estimated from observations of pollutant concentrations and, in some cases, information on activity patterns within the population. To estimate the public health impact at any given level of exposure, relationships are derived from these studies.

In addition to observations of the concentration of the pollutant of interest, epidemiological studies require data on many other parameters that can affect health—such as meteorological condition and the concentrations of other pollutants—so that the influence of these potentially confounding influences on health outcomes can be accounted for. As noted in Chapter 1, identifying and quantifying the health impact of a specific pollutant is a challenging task and typically requires the use of large sample sizes and sophisticated statistical methods.

Each approach has its advantages and disadvantages. A major advantage of the toxicological approach is that, because the experiments can be carried out under controlled conditions, a much stronger cause-and-effect link exists between the administered exposure and the observed health effect. For this reason, experimental studies are often considered more convincing than those based on epidemiological data. Moreover, laboratory experiments provide an opportunity through ancillary observations to elucidate the physiological mechanism that results in the adverse health effect. However, the experimental approach has drawbacks. In addition to ethical questions that might be raised, there are a number of potential

Suggested Citation:"2 Setting Goals and Standards." National Research Council. 2004. Air Quality Management in the United States. Washington, DC: The National Academies Press. doi: 10.17226/10728.
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technical difficulties. For example, experimental conditions might not replicate the actual conditions in which population exposures occur (for example, meteorological conditions, mixtures of pollutants, or human activities). Further, animal experimental studies might not be relevant to humans, and human experimental studies typically have been unable, for ethical reasons, to include the frailest subgroups of the population that are suspected to be the most sensitive to air pollution effects. In contrast, epidemiological studies apply to the specific conditions and activities that exist at the time of exposure. Moreover, epidemiological studies are able to focus on specific segments of the population in terms of either physical condition (for example, elderly or people with asthma) or social condition (for example, the economically disadvantaged persons or those living near hot spots). Epidemiological studies have more difficulty characterizing the exposures of individual members of the population and distinguishing the effects of air pollution from a variety of other environmental, societal, and economic factors that can also affect health (for example, smoking behavior or socioeconomic status). Because epidemiological studies rely on ambient monitoring data, it must be assumed that the monitored pollutant concentrations actually reflect population exposure to the pollutant or pollutants. That assumption is not necessarily accurate and may explain the apparent inconsistencies in the findings of observational studies where one pollutant appears to have a stronger association with adverse health effects in one setting, but a different pollutant, or set of pollutants, is more strongly associated with effects in another setting (Samet et al. 2000).

In an attempt to address the limitations of the experimental and observational approaches, a hybrid approach involving the use of personal-exposure monitors has received increased attention in recent years. A personal-exposure monitor is essentially a miniaturized and automated air quality monitor that a person can carry during the course of his or her daily activities (see Figure 2-10). In this approach, selected members of a population are given personal exposure monitors to develop a quantitative record of their actual exposure to air pollutants and, at the same time, are carefully monitored for signs of any related adverse health effects. Because the approach includes clinical measurements of affected individuals, it (like the more standard experimental approach) has the potential to elucidate the specific physiological mechanisms that result in adverse health effects from air pollution. On the other hand, it does not have the disadvantage of the standard experimental approach of not being able to replicate the actual conditions under which the exposure occurs. In that regard, the use of personal-exposure monitors is even superior to the observational or epidemiological method, which must infer personal exposure from ambient measurements and personal-activity data. However, a major disadvantage is that only a small segment of the population can be studied at any one time.

Suggested Citation:"2 Setting Goals and Standards." National Research Council. 2004. Air Quality Management in the United States. Washington, DC: The National Academies Press. doi: 10.17226/10728.
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FIGURE 2-10 Volunteer wearing a personal exposure monitor to measure actual exposures to PM and gases during daily activities. SOURCE: HEI 2000. Reprinted with permission; copyright 2000, Health Effects Institute.

The monitors also can seldom be used by themselves to determine the ambient contribution to personal exposure.

Studies of Air Pollution Effects on Ecosystems

Standards and goals (such as the secondary NAAQS) designed to protect ecosystems are primarily based on four types of studies: (1) laboratory chamber studies, in which specific types of soil and vegetation are exposed to pollutants in greenhouses or carefully controlled (but artificial) environmental chambers (see Figure 2-11); (2) field studies, in which effects of air pollutants on ecosystems and the biotic components of these are monitored; (3) field studies, where a portion (for example, forest plot and stream reach)

Suggested Citation:"2 Setting Goals and Standards." National Research Council. 2004. Air Quality Management in the United States. Washington, DC: The National Academies Press. doi: 10.17226/10728.
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FIGURE 2-11 Four-chamber greenhouse-based exposure system constructed to study effects of elevated CO2 on plants. SOURCE: Photograph courtesy of Alberta Research Council Inc., Vegreville, Alberta, Canada.

of or entire (for example, lake and watershed) in situ ecosystems have been exposed to varying levels of acidity to demonstrate the effects of acidity in the otherwise natural environment (Hall et al. 1980; Schlinder et al. 1985; Hedin et al. 1990; Driscoll et al. 1996; Norton et al. 1994); and (4) hybrid studies, in which chamber experiments are conducted in the field using filters to scrub pollutants, such as O3 so that exposure concentrations are less than those at ambient conditions, or pollutant concentrations are increased above ambient concentrations (see Figure 2-12). The first two approaches suffer from many of the same shortcomings discussed above for toxicological and epidemiological studies of effects on humans. Controlled experiments have the advantage of helping to establish cause-and-effect relationships. However, it may be difficult to relate these experiments to field conditions. Field measurements have the advantage of being representative of field conditions, but biotic response under field conditions is the integrated response of all stresses an organism is exposed to, air pollution being only one. Under field conditions, it is difficult to establish a cause-and-effect relationship. A significant shortcoming of these approaches is that they are limited to studying the effects experienced by a small number of species of organisms over a relatively short period of time. Although often adequate for agricultural crops, which are usually grown as isolated cultivars over one growing season, these approaches are not adequate for

Suggested Citation:"2 Setting Goals and Standards." National Research Council. 2004. Air Quality Management in the United States. Washington, DC: The National Academies Press. doi: 10.17226/10728.
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FIGURE 2-12 Studies in open-top field chambers have shown the response of plants to ambient levels of O3. Plants grown in chambers receiving air filtered with activated charcoal to reduce O3 concentrations, do not develop symptoms that occur on plants grown in nonfiltered air at ambient O3 concentrations. SOURCE: USDA-ARS 1998.

unmanaged ecosystems filled with a multitude of species, some of which have life cycles that span decades. These approaches also fail to address the indirect effects of air pollution on, for example, soils and aquatic ecosystems. As a result, the effects of pollutants on specific agricultural crops are generally better defined than the long-term effects of pollutants on unmanaged ecosystems, such as forests, grasslands, wetlands, lakes, and estuaries. One potential technique that might be useful for examining long-term effects of air pollutants on intact ecosystems has thus far been applied to studies of the long-term ecological effects of increased atmospheric carbon dioxide (CO2) and is referred to as free air CO2 enrichment (FACE) studies. In a FACE facility, scientists can increase the concentration of a trace gas, such as CO2, in a controlled way in the air surrounding an intact ecosystem and measure plant and soil responses to the altered conditions over years to decades in accordance with the long life span of trees (see Figure 2-13 and Delucia et al. 1999). The FACE systems are not without fault but offer an alternative approach to enable long-term experimental exposure to air pollutants and evaluation of effects on whole ecosystems and their components under otherwise natural environmental conditions.

Suggested Citation:"2 Setting Goals and Standards." National Research Council. 2004. Air Quality Management in the United States. Washington, DC: The National Academies Press. doi: 10.17226/10728.
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FIGURE 2-13 Free air CO2 Experiment (FACE) is used to elucidate forest ecosystem responses to elevated CO2. A similar type of experimental approach could be used to better understand the long-term ecological effects of elevated pollutant concentrations. SOURCE: DOE 2001.

The hybrid approach of integrating these types of studies has the advantage of investigating effects of air pollution under general conditions that closely approximate those in the real world, but also allowing investigators to carefully regulate the pollutant exposures experienced by the ecosystem being studied. This hybrid approach might include long-term measurements of air pollutants and their changes, associated ecosystem response to these changes, and coupled integrated experiments and model applications. An example of a site where this hybrid approach has been implemented is at the Hubbard Brook Experimental Forest in New Hampshire. At Hubbard Brook, long-term measurements of atmospheric deposition have been made (Likens and Bormann 1995). Associated with these measurements are long-term studies of soil, vegetation, and stream response to changes in atmospheric deposition (Likens et al. 1996; Driscoll et al. 2001a). These long-term measurements have been supported by integrated field experiments involving forest plots (Christ et al. 1999), streams (Hall et al. 1980; Hedin et al. 1990), and whole watershed manipulations and

Suggested Citation:"2 Setting Goals and Standards." National Research Council. 2004. Air Quality Management in the United States. Washington, DC: The National Academies Press. doi: 10.17226/10728.
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model development and application (Gbondo-Tugbawa et al. 2001; Gbondo-Tugbawa and Driscoll 2002, 2003).

Scientific Basis for Setting Standards for the Nation’s Air Quality Management

Need for Additional Strategic Planning of Research That Underpins Health-Based Standards

At this time and most likely for the foreseeable future, no one single definitive method exists for establishing the dose-response relationships needed for setting health-based air quality standards. For that reason, standards and goals have been and should continue to be established using a combination of data from toxicological and epidemiological studies. A key question is whether there is an optimal mix of these studies that should be used to determine standards. In the past, the mix used by EPA has varied widely among the different criteria pollutants. Moreover, although EPA has normally identified a set of research priorities to fill key gaps at the end of each NAAQS review, those priorities have not explicitly defined the mix of toxicological and epidemiological data needed to establish pollutant standards and goals; nor have they, until recently, been part of a strategic research plan to develop such a consistent mix. For example, a large body of both animal and human experimental evidence on O3 is available, and as a result, the O3 NAAQS has been set with a great reliance on experimental data. For PM, on the other hand, direct human effects studies are scarce, and the bulk of the scientific evidence used to develop the PM NAAQS in 1997 came from epidemiological studies using mostly PM10 data from atmospheric monitoring. This presents a series of challenges—most notably because PM10 is a complex mixture and because toxicological information on the relative toxicities of its different components is sparse. As a result, in 1997 Congress appropriated funds for a major new PM research initiative, and directed EPA to work with the National Research Council to develop the first multiyear strategic research priorities for NAAQS research. In 1998, that committee identified 10 key priorities (NRC 1998b). Since that time, EPA has been investing substantial funds in implementing them, and the NRC committee is currently completing its evaluation of the program. Although EPA has begun developing similar strategies for other pollutants, no similarly comprehensive effort has been initiated to date.

Beyond the need to improve the science base for setting standards, there is also a need to improve how research results are summarized and synthesized in criteria documents. Criteria documents are designed to be comprehensive and have therefore become a useful but extensive catalogue of all

Suggested Citation:"2 Setting Goals and Standards." National Research Council. 2004. Air Quality Management in the United States. Washington, DC: The National Academies Press. doi: 10.17226/10728.
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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.

Suggested Citation:"2 Setting Goals and Standards." National Research Council. 2004. Air Quality Management in the United States. Washington, DC: The National Academies Press. doi: 10.17226/10728.
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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.

Suggested Citation:"2 Setting Goals and Standards." National Research Council. 2004. Air Quality Management in the United States. Washington, DC: The National Academies Press. doi: 10.17226/10728.
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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,

Suggested Citation:"2 Setting Goals and Standards." National Research Council. 2004. Air Quality Management in the United States. Washington, DC: The National Academies Press. doi: 10.17226/10728.
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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

Suggested Citation:"2 Setting Goals and Standards." National Research Council. 2004. Air Quality Management in the United States. Washington, DC: The National Academies Press. doi: 10.17226/10728.
×

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.

Suggested Citation:"2 Setting Goals and Standards." National Research Council. 2004. Air Quality Management in the United States. Washington, DC: The National Academies Press. doi: 10.17226/10728.
×

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,

Suggested Citation:"2 Setting Goals and Standards." National Research Council. 2004. Air Quality Management in the United States. Washington, DC: The National Academies Press. doi: 10.17226/10728.
×

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.

Suggested Citation:"2 Setting Goals and Standards." National Research Council. 2004. Air Quality Management in the United States. Washington, DC: The National Academies Press. doi: 10.17226/10728.
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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.

Suggested Citation:"2 Setting Goals and Standards." National Research Council. 2004. Air Quality Management in the United States. Washington, DC: The National Academies Press. doi: 10.17226/10728.
×

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

Suggested Citation:"2 Setting Goals and Standards." National Research Council. 2004. Air Quality Management in the United States. Washington, DC: The National Academies Press. doi: 10.17226/10728.
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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.

Suggested Citation:"2 Setting Goals and Standards." National Research Council. 2004. Air Quality Management in the United States. Washington, DC: The National Academies Press. doi: 10.17226/10728.
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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.

Suggested Citation:"2 Setting Goals and Standards." National Research Council. 2004. Air Quality Management in the United States. Washington, DC: The National Academies Press. doi: 10.17226/10728.
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The implementation of air quality regulations should be less bureaucratic -- with more emphasis on results than process -- and should be designed to protect ecosystems as well as people. The report recommends that The Environmental Protection Agency (EPA) use an approach to target groups of pollutants instead of individual ones and that revised or new regulations also should consider how air pollution travels from state to state and across international borders. In addition, improved tracking of emissions is needed to accurately assess what populations are at the highest risk of health problems from pollution and to better measure the progress of pollution-control strategies.

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