1
Introduction

COMMITTEE’S TASK

This is the fourth and final report of the National Research Council (NRC) Committee on Research Priorities for Airborne Particulate Matter. The committee was convened in January 1998 at the request of the U.S. Environmental Protection Agency (EPA) pursuant to directions from Congress in EPA’s fiscal year 1998 appropriations report. The congressional request for this independent NRC study arose from a need to address scientific uncertainties surrounding EPA’s July 1997 decision to establish new National Ambient Air Quality Standards (NAAQS) for airborne particulate matter (PM) smaller than about 2.5 micrometers (µm) in aerodynamic diameter (EPA 1997).1 Contemplating the review of the PM NAAQS in 2003 and every 5 years thereafter, as well as EPA’s proposed schedule for regulatory implementation of the new standards, Congress mandated and appropriated substantial funds for EPA to conduct a major research program to reduce scientific uncertainties. It also directed the EPA Administrator to arrange for the NRC to provide independent guidance for planning the research program and monitoring its implementation. Specifically, the committee was charged with assessing research priorities, developing a conceptual research plan, and monitoring research progress made over 5

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PM10 refers to particulate matter collected by a sampling device with a size-selective inlet that has a 50% collection efficiency for particles with an aerodynamic diameter of 10 μm. PM2.5 is similarly defined except with reference to a 2.5μm size cut. Total suspended particles (TSP) refers to the particle mass collected up to a range of 25-50 μm aerodynamic diameter.



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Research Priorities for Airborne Particulate Matter: IV - Continuing Research Progress 1 Introduction COMMITTEE’S TASK This is the fourth and final report of the National Research Council (NRC) Committee on Research Priorities for Airborne Particulate Matter. The committee was convened in January 1998 at the request of the U.S. Environmental Protection Agency (EPA) pursuant to directions from Congress in EPA’s fiscal year 1998 appropriations report. The congressional request for this independent NRC study arose from a need to address scientific uncertainties surrounding EPA’s July 1997 decision to establish new National Ambient Air Quality Standards (NAAQS) for airborne particulate matter (PM) smaller than about 2.5 micrometers (µm) in aerodynamic diameter (EPA 1997).1 Contemplating the review of the PM NAAQS in 2003 and every 5 years thereafter, as well as EPA’s proposed schedule for regulatory implementation of the new standards, Congress mandated and appropriated substantial funds for EPA to conduct a major research program to reduce scientific uncertainties. It also directed the EPA Administrator to arrange for the NRC to provide independent guidance for planning the research program and monitoring its implementation. Specifically, the committee was charged with assessing research priorities, developing a conceptual research plan, and monitoring research progress made over 5 1   PM10 refers to particulate matter collected by a sampling device with a size-selective inlet that has a 50% collection efficiency for particles with an aerodynamic diameter of 10 μm. PM2.5 is similarly defined except with reference to a 2.5μm size cut. Total suspended particles (TSP) refers to the particle mass collected up to a range of 25-50 μm aerodynamic diameter.

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Research Priorities for Airborne Particulate Matter: IV - Continuing Research Progress years (1998-2002) in understanding the relationships among airborne PM, its various sources, and its effects on public health.2 The committee’s formal statement of task is presented in Appendix B. This final report of the committee addresses progress since its first report, published in 1998 (NRC 1998). That report, Research Priorities for Airborne Particulate Matter: I. Immediate Priorities and a Long-Range Research Portfolio, proposed a conceptual framework for a national program of PM research (Figure 1-1); identified 10 high-priority research topics linked to key policy-related scientific uncertainties (Box 1-1); and presented a 13-year integrated “research investment portfolio” containing recommended short- and long-term phasing of research and estimated costs of such research. This fourth report assesses progress on the research agenda set out in the portfolio and offers an enhancement of the portfolio and recommendations for follow-up monitoring, oversight, and evaluation as research on PM extends beyond the time-frame of this committee. This report also draws on the committee’s second and third reports, published in 1999 and 2001, respectively. In its second report, Research Priorities for Airborne Particulate Matter: II. Evaluating Research Progress and Updating the Portfolio, the committee described its initial plans for monitoring the progress of research (NRC 1999). In addition, it reviewed and updated the research recommendations from the committee’s first report and substantially revised two of the recommended research topics related to emissions characterization and air pollution modeling. The third report, Research Priorities for Airborne Particulate Matter. III. Early Research Progress, provided an overview of progress through approximately mid-2000 on the research agenda (NRC 2001). In preparing that report, the committee attempted to catalog the major studies that had been initiated subsequent to its first report, as well as ongoing but relevant studies that were started before the first report. Its review covered research funded by EPA, the Health Effects Institute, and several other agencies that were supporting research on PM. The committee found substantial progress on several research topics, particularly studies directed at assessment of exposure to PM2.5 (research topic 1) and methodological issues (research topic 10). Progress and planning were particularly lacking for research 2   In addition to effects on human health, airborne particles have other important effects, such as reduction of atmospheric visibility, and interference with particle borne nutrients that can affect ecosystem and agricultural productivity. However, the work of the committee has been directed solely on research that elucidates the human health effects of PM.

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Research Priorities for Airborne Particulate Matter: IV - Continuing Research Progress FIGURE 1-1 A general framework for integrating particulate matter research. This figure is not intended to represent a framework for research management. Such a framework would include pathways for the flow of information. Sources: Modified from NRC 198

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Research Priorities for Airborne Particulate Matter: IV - Continuing Research Progress BOX 1-1 Research Priorities and Questions Recommended by This NRC Committee Research Topic 1. Outdoor Measures Versus Actual Human Exposures What are the quantitative relationships between concentrations of particulate matter and gaseous copollutants measured at stationary outdoor air monitoring sites and the contributions of these concentrations to actual personal exposures, especially for subpopulations and individuals? Research Topic 2. Exposures of Susceptible Subpopulations to Toxic Particulate Matter Components What are the exposures to biologically important constituents and specific characteristics of particulate matter that cause responses in potentially susceptible subpopulations and the general population? Research Topic 3. Characterization of Emission Sources What are the size distribution, chemical composition, and mass emission rates of particulate matter emitted from the collection of primary-particle sources in the United States, and what are the emissions of reactive gases that lead to secondary formation through atmospheric chemical reactions? Research Topic 4. Air Quality Model Development and Testing What are the linkages between emission sources and ambient concentrations of the biologically important components of particulate matter? Research Topic 5. Assessment of Hazardous Particulate Matter Components What is the role of physiochemical characteristics of particulate matter in eliciting adverse health effects? Research Topic 6. Dosimetry: Deposition and Fate of Particles in the Respiratory Tract What are the deposition patterns and fate of particles in the respiratory tract of individuals belonging to presumed susceptible subpopulations? Research Topic 7. Combined Effects of Particulate Matter and Gaseous Pollutants How can the effects of particulate matter be disentangled from the effects of other pollutants? How can the effects of long-term exposure to particulate matter and other pollutants be better understood? Research Topic 8. Susceptible Subpopulations What subpopulations are at increased risk of adverse health outcomes from particulate matter? Research Topic 9. Mechanisms of Injury What are the underlying mechanisms (local pulmonary and systemic) that can explain the epidemiological findings of mortality and morbidity associated with exposure to ambient particulate matter? Research Topic 10. Analysis and Measurement To what extent does the choice of statistical methods in the analysis of data from epidemiological studies influence estimates of health risks from exposures to particulate matter? Can existing methods be improved? What is the effect of measurement error and misclassification on estimates of the association between air pollution and health? Sources: NRC 1998, 1999, 2001.

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Research Priorities for Airborne Particulate Matter: IV - Continuing Research Progress topics 3 and 4 characterization of emission sources and air quality model development and testing, respectively. Evidence was still insufficient for the committee to predict the program’s likely effectiveness in reducing uncertainty. Overall, the committee found that the pace of implementing new research projects was slower than the original timelines envisioned in its first report, likely reflecting the practical constraints on planning, funding, and implementing major scientific research projects. Nonetheless, the committee concluded that much research was in progress in accordance with its portfolio and that EPA was responsive in following the committee’s research agenda. Because research results were coming more slowly than originally expected, the committee observed that managers of the research program would probably need to adjust the timing of future research activities as they followed the sequence of the portfolio. For this fourth report, the committee builds on the framework developed in the prior three reports. Its approach to identifying completed, initiated, or planned research and to evaluating progress is described in the second chapter. This approach incorporates the three criteria used for developing the original research agenda (scientific value, decisionmaking value, and feasibility and timing) and the three set forth in the second report for tracking implementation and progress (multidisciplinary interaction, integration and planning, and accessibility of information). The committee searched widely and comprehensively, but not exhaustively, to identify research in progress and research findings for this report, using various methods, such as workshops, literature reviews, research publications considered for EPA’s PM “criteria document” (EPA 2002, 2003a), and the PM research inventory database, which was developed by the Health Effects Institute in collaboration with EPA. STATUS OF THE EPA REVIEW OF THE NAAQS FOR PARTICULATE MATTER As this report was being prepared and reviewed, EPA was continuing its process of reviewing the scientific evidence on PM. This lengthy process is based on detailed consideration of the scientific evidence, which is compiled in a criteria document, an encyclopedic document prepared by EPA with assistance from the larger scientific community. EPA reviews the peer-reviewed publications that are relevant to the NAAQS, focusing on those published since the last review. The “staff paper,” prepared by EPA’s Office of Air Quality Planning and Standards, translates the scientific advances into potential policy options, including possible revisions to the

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Research Priorities for Airborne Particulate Matter: IV - Continuing Research Progress four elements of the NAAQS: (1) the pollutant indicator (such as PM2.5), (2) the concentration of the indicator in the air, (3) the time over which measurements are made or averaged, and (4) the statistical form of the standard used to determine the allowable number of exceedances. The staff paper references only those papers cited in the criteria document that specifically affect the setting of the four elements of the NAAQS. The criteria document and the staff paper are reviewed by EPA’s Clean Air Scientific Advisory Committee. The proposed schedule has been slowed by the magnitude of the task of preparing and reviewing the criteria document and by the identification of a substantive statistical issue affecting the results of the time-series epidemiological studies,3 necessitating reanalysis of major studies considered by EPA (HEI 2003). The current schedule is to finalize the criteria document in 2004, release a first-draft staff paper for review in 2004, and decide whether to revise the PM NAAQS within a year of the release of the final version of the staff paper. The next complete review would follow in 5 years. Given the pace of the NAAQS review, the committee was unable to track the influence of new research on the recommendations of the staff paper. THE NEED FOR RESEARCH ON PARTICULATE MATTER When Congress asked the NRC to establish this committee, it recognized the continuing scientific uncertainties in the evidence that led to the 1997 revision of the NAAQS for PM and the need for gains in scientific understanding necessary for implementing the NAAQS. It anticipated the reduction of uncertainty by additional research and saw a clear need for greater certainty, given the effort and cost of implementing the NAAQS revisions. This committee’s research framework was intended to be responsive to the call from Congress, and its research portfolio was structured around key sources of uncertainty. Moreover, the research findings were expected to facilitate the development of the implementation program and enhance its effectiveness in reducing PM emissions. To reduce the uncertainties about airborne PM, research evidence is needed that fills in data gaps related to the scope of PM health effects and to the underlying processes by which PM exposure causes health effects. 3   Time-series epidemiological studies evaluate associations between changes in health effects and changes in exposure indicators (for example, ambient PM concentrations) preceding or simultaneous with the observed outcome.

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Research Priorities for Airborne Particulate Matter: IV - Continuing Research Progress Research evidence is also needed that addresses the links from emission sources to human exposures, particularly for those particles found to be injurious to health. The needed research is inherently multidisciplinary, involving engineers, atmospheric scientists, exposure assessors, toxicologists and other basic scientists, epidemiologists, and clinicians. Research topics 1 and 2 are largely the domain of exposure assessors but often with input from atmospheric scientists and epidemiologists. Topics 3 and 4 are of concern to atmospheric scientists and engineers. Topics 5, 7, and 8 need parallel investigation by epidemiologists and toxicologists, and topic 9 involves laboratory-based and clinical toxicological approaches, along with parallel population approaches by epidemiologists. Topics 6 and 10 call for specific disciplinary expertise. Dialogue is needed among these investigative communities, if the overall goal of reducing uncertainty within an integrated paradigm, like that proposed by the committee, is to be achieved. Challenges in carrying out this research agenda are evident. Research evidence on PM in support of setting the NAAQS is almost inevitably subject to uncertainties, reflecting the dissonance between the complex nature of exposures of the population to multiple air pollutants and the setting of a pollutant-specific NAAQS. Air pollution in both urban and nonurban settings is a complex mixture of gaseous and solid pollutants, having both natural and anthropogenic sources. Many sources of PM also emit gases so that concentrations of PM and other criteria pollutants4 are often correlated to a substantial degree. Motor vehicles, for example, emit oxides of nitrogen and volatile organic compounds, which contribute to secondary particle formation and which also have adverse health effects. Particles are diverse in physical and chemical characteristics, depending on their sources and atmospheric conditions. Beyond the science needed to better inform the decisions on the PM NAAQS, the committee’s research portfolio was also designed to improve the scientific basis for the large number of decisions that will be needed to implement the PM NAAQS. Improvements that need to be achieved include better measurement and characterization of both ambient PM and PM 4   The Clean Air Act requires EPA to set NAAQS for certain pollutants known to be hazardous to human health and the public welfare (for example, damage to forests and degradation of atmospheric visibility). EPA has identified and set standards to protect human health and public welfare for six pollutants: ozone, carbon monoxide, particulate matter (PM10 and PM2.5), sulfur dioxide, lead, and nitrogen dioxide. The term criteria pollutants derives from the requirement that EPA must describe the characteristics and potential health and welfare effects of these pollutants. It is on the basis of such criteria that NAAQS are set or revised.

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Research Priorities for Airborne Particulate Matter: IV - Continuing Research Progress from specific sources; models that can accurately identify the sources of particles to inform the development of national, state, and local plans for reductions; and enhanced toxicological and epidemiological study on the effects of the different sources and components of the PM mixture to assist in setting future reduction priorities. These gains in knowledge are needed for the implementation of any NAAQS; in the case of PM, they are critical because of PM’s unique nature among criteria pollutants as a complex mixture of particle sizes, characteristics, and constituents. Although some work is under way in these areas, much remains to be done as the nation moves toward implementation of the PM NAAQS. Characterizing Particulate Matter In attempting to link sources to effects to support the development of NAAQS (Figure 1-1), PM is generally considered within a variety of particle-size categories. These size categories have been considered to have biological significance, although characteristics of particles other than size are likely to be relevant to determining their toxicity. Particle composition and the potential for particles to impact onto surfaces can vary within and across particle-size categories. The types of particles within specific size categories may also be affected by the concentrations of various gaseous pollutants in the air. Further, the size and shape of inhaled particles influence where and how much mass will be deposited in various regions of the respiratory system. The size categories most commonly distinguished in health-related research are PM: mixtures of any or all sizes. PM0.1: particles less than about 0.1 µm in aerodynamic diameter, called ultrafines. PM2.5: particles less than about 2.5 µm in aerodynamic diameter, called fine particles. PM10: particles less than about 10 µm in aerodynamic diameter. PM10-2.5: particles between 2.5 and 10 µm in aerodynamic diameter, called coarse particles. PM10+: particles greater than 10 µm in aerodynamic diameter. Assessing the health effects of PM is complicated by the diversity and richness of particles typically present in ambient air. Compared with a gaseous pollutant, whose effects at a given time and place are uniquely determined by its concentration, PM is only loosely defined. In concept, PM is whatever material is carried by suspended liquid and solid particles

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Research Priorities for Airborne Particulate Matter: IV - Continuing Research Progress of diverse size, shape, and composition. Some of the PM is in dynamic chemical equilibrium with the surrounding mixture of gases. As illustrated in Figures 1-2 and 1-3, characterizations of individual particles cover a heterogeneous collection of material. In setting NAAQS in recent decades, EPA has attempted to distinguish more-hazardous from less-hazardous PM by reference to aerodynamic behavior as an indicator of particle size. Thus, PM10 and PM2.5 are defined in terms of filter samples collected with the use of size-selective inlets passing 50% of particles (approximated by spheres of 1 g/cm3 density) of 10 μm or 2.5 μm diameters. As Figure 1-4 indicates, observed particle diameters range over several orders of magnitude, with those below about 0.05 μm typically dominating particle number in urban air, those below about 0.5 μm dominating particle surface area, and those at about 10 μm dominating particle volume (or mass). The existing distinctions at 10 μm and 2.5 μm have dosimetric significance and hint at generic categories of origin, because combustion-generated particles tend to be smaller than 2.5 μm and mechanically generated dust grains tend to be larger. Figure 1-2 shows clearly, however, that particles of similar sizes can have widely differing origins and composition. More refined categorizations can be based on chemical analyses of PM sampled by various methods, with or without aerodynamic sorting by size. Analytical strategies range from a focus on major mass fractions, often broadly defined, to targeted studies of biologically relevant species or properties or to comprehensive efforts to resolve trace elements and compounds indicative of specific emissions. Trade-offs are of course inevitable among the many possible dimensions of resolution—in time, space, particle size, and physical state and in elemental and molecular composition (McMurry 2000). The descriptive frameworks that emerge directly from aerodynamic classification and chemical analysis are not necessarily the ones best suited to assessments of health effects or emissions management strategies. Generic types of emissions, such as diesel exhaust particles (DEP) or residual oil fly ash (ROFA), may consist of chemically complex particles over a broad range of sizes whose statistical distributions in the aggregate are, nonetheless, reasonably uniform and unvarying. Therefore, the degrees of freedom actually exhibited by ambient PM for testing hypotheses might be significantly fewer than the number of chemical components and size classes required for its detailed description. A given sample of PM can be described by a variety of different schemes of characterization, some better suited to different purposes—for example, source characterization or assessment of risks to health.

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Research Priorities for Airborne Particulate Matter: IV - Continuing Research Progress FIGURE 1-2 Electron micrographs of atmospheric particles. (a) Internal mixture of sulfate and soot; arrow points to a soot aggregate. The surrounding halo is ammonium sulfate crystals formed as the sulfate dehydrated in the microscope’s vacuum. (b) Sea salt. (c) Branching soot aggregate typical of some combustion processes. (d) Internal mixture of terrestrial silicate with sea salt and anhydrite (calcium sulfate [CaSO4]) likely formed by reaction of sulfur dioxide with carbonate particles. Source: Buseck and Posfai 1999. The multiplicity of legitimate descriptive frameworks for PM is easily overlooked in interdisciplinary conversations, particularly once specific indices, such as PM2.5, have been singled out in NAAQS and other regulatory settings. The complexity of the particle mixture nevertheless requires explicit acknowledgment if the needed integration of research is to be achieved. Research Approaches Many exposure assessment studies have now been carried out to measure personal exposures of people to particles and to understand the relationship between personal exposures and concentrations of particles recorded at centrally sited monitors. These studies have proved feasible,

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Research Priorities for Airborne Particulate Matter: IV - Continuing Research Progress FIGURE 1-3 Positive and negative ion mass spectra obtained by aerosol time-of-flightmass spectrometry (ATOFMS) for typical individual particlesin the San Joaquin Valley of California. (a) Carbonaceous particle containing sulfates and nitrates of presumably secondary origin. (b) Elemental carbon particle. (c) Sodium-containing particle likely formed by reactionof sea salt withgas-phase nitrogen oxides and sulfuric acid. (d) Calcium-rich dust particle. (See also Figure 6 in Whiteakeret al. [2002].) Source: Whiteaker et al. 2002. Reprinted with permission;copyright 2002, American Chemical Society.

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Research Priorities for Airborne Particulate Matter: IV - Continuing Research Progress but some of the groups considered most susceptible—persons with advanced chronic heart and lung diseases—are not yet well studied. Additionally, the extent to which findings from any particular location can be generalized is uncertain, and many studies to date have focused primarily on total particle mass, rather than more detailed particle characteristics, such as their chemical composition. Epidemiological studies take advantage of naturally occurring variation in exposure, across groups or over time, to estimate the effect of PM on one or more health outcome indicators. In an effort to provide evidence relevant to the NAAQS for PM, epidemiologists design studies that have the potential to estimate the effect of PM without contamination (confounding) by the effects of other pollutants. This approach implicitly assumes that inhaled particles have effects on health that are independent of other pollutants, an underlying assumption in having a NAAQS for PM. Alternatively, the effect assigned to PM may reflect the total effect of the air pollution mixture or some other factor that varies with PM, and PM is serving as a surrogate index. Even with careful design and analysis, there is the possibility of some residual confounding of the effect of PM by other pollutants or other factors. Some epidemiological studies take advantage of historical data on air quality and community health. Other studies use air quality and health data collected prospectively to address specific hypotheses. Controlled human exposure studies offer the opportunity to study small numbers of human subjects under carefully controlled exposure conditions and gain valuable insights into both the relative deposition of inhaled particles and the resulting health effects. Individuals studied can range from healthy people to individuals with cardiac or respiratory diseases of varying degrees of severity. In all cases, the specific protocols defining the subjects, the exposure conditions, and the evaluation procedures must be reviewed and approved by institutional review boards providing oversight for human experimentation. The exposure atmospheres studied vary, ranging from well-defined, single-component aerosols (such as black carbon5 or sulfuric acid) to atmospheres produced by recently developed particle concentrators, which concentrate the particles present in ambient air. The concentrations of particles studied are limited by ethical considerations and by concern for the range of concentrations, from the experimental setting to typical ambient concentration, over which findings need to be extrapolated. Toxicological studies with laboratory animals provide the opportunity 5   “Black carbon” is a general term that is often used interchangeably with “elemental carbon” or “soot.”

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Research Priorities for Airborne Particulate Matter: IV - Continuing Research Progress to develop information under defined experimental conditions that will complement the data from epidemiological studies or controlled exposures of human subjects. The results of such studies introduce the issue of the validity of extrapolations from laboratory species to people. Nonetheless, by careful selection of the animal species and strain, including genetically modified animals, and appropriate experimental manipulation, certain aspects of both normal and diseased states in people can be reproduced in the laboratory animals. As a corollary to animal studies, in vitro exposure studies can be performed. As in the controlled human exposure studies, a range of exposure atmospheres can be studied. The goal of the studies is not necessarily to recreate the complex mixtures of pollutants to which people are exposed in their daily lives. Rather, as in studies of diluted emissions from a specific source, such as a diesel engine, the intent is to attempt to derive some understanding about the influence of emissions from that source on health outcomes. In other cases, one or a few types of atmospheres, including specific forms of PM, such as black carbon or residual oil fly ash, have been studied. Ultimately, through the study of particles with varied characteristics, including particle size and chemical composition, insight can be gained into the extent that responses are generic or specific to particle size and composition. To enhance the potential for observing effects and thereby increase the efficiency of the research, the exposure concentrations used in toxicological studies are typically greater than those to which people are usually exposed. Particle concentrations have been used that allow the study of increased concentrations of ambient PM. Some toxicological studies have been conducted in the field at sites near particular sources, such as freeways. Such field studies come with the challenge of ensuring that the experiments are not complicated by confounding factors, such as the occurrence of infectious diseases or not readily controlled factors not usually encountered in the laboratory. RESEARCH ON PARTICULATE MATTER In response to the need for further research on PM, a substantial national program was initiated, largely following the 13-year research portfolio set out over 5 years ago by this committee. The committee’s research investment portfolio for fiscal years 2000-2010 is shown in Table 1-1, and its estimates for technical support are presented in Table 1-2. This is the same portfolio shown in the committee’s second report (NRC 1999). Available time and resources did not allow the committee to provide a detailed review and update of the research portfolio. Much of the research

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Research Priorities for Airborne Particulate Matter: IV - Continuing Research Progress has been carried out by EPA investigators, but many scientists from academia and research institutes have also been involved. In addition, a number of federal agencies formed the PM Research Coordination Working Group under the auspices of the Air Quality Research Subcommittee of the Committee on Environment and Natural Resources (CENR 2002). The working group formed an overall federal PM research strategy, and several agencies developed agency-specific strategies to guide their internal and external programs. Responding to the same need for additional evidence, other organizations in the United States have developed research programs, and the European Union and other countries have also supported research on PM. Although not readily estimated, the total funding is substantial. EPA, for example, has funded a total of $368 million on PM research and related technical work for fiscal years 1998-2003, including $66.7 million for fiscal year 2003. Table 1-3 summarizes the levels of resources allocated to the 10 categories of research recommendations by this committee. The table shows the amounts of funding allocated to intramural and extramural research funding for each category. In addition, other organizations have provided funding for PM research. For example, the Electric Power Research Institute (EPRI) provided $30.1 million for the years 1998-2002, and the Health Effects Institute provided $20.9 million for that same period.6 The research has followed the committee’s agenda through a variety of mechanisms. At EPA, research on PM was organized and placed under a manager to ensure some cohesion and exchange among investigators. EPA also engaged the extramural community of researchers through targeted cooperative agreements and requests for proposals. The committee’s first report supported the funding of PM research centers that would support interdisciplinary research groups. Five centers were funded for a 5-year period in 2000. EPA has funded the Supersite program to provide intensive monitoring and characterization of particles; the data collection phase at most sites is coming to an end. The total extent of research on PM and the resulting publications cannot be readily tracked, largely because of the research scope and the impossibility of specifying criteria to separate relevant from nonrelevant research. Together, the Health Effects Institute and EPA have maintained a database of research projects, but maintaining complete and up-to-date information has proved difficult. The approaches used in this report provide an indication of the substantial scope of the research now in progress. 6   Approximately half of the funding was provided by EPA and is reported in its accounting.

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Research Priorities for Airborne Particulate Matter: IV - Continuing Research Progress TABLE 1-1 The Committee’s 1999 Research Investment Portfolio for Fiscal Year 2000-2010: Timing and Estimated Costsa,b ($ million/year in 1998 dollars) of Recommended Research on Particulate Matter   2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 SOURCE, CONCENTRATION EXPOSURE   Outdoor vs. human exposure 3.0     Exposure to toxic PM components   Methods 1.0     Studies   4.0 4.0 4.0 4.0 4.0     Emission sources 2.5 2.5 2.5 2.5     4. Models   Source orientedc 4.5 4.5 4.5 4.5 4.5 4.5 4.5     Receptor oriented 1.0 1.0 1.0   EXPOSURE–DOSE-RESPONSE RELATIONSHIP   Assessment of hazardous PM components   Toxicological and clinical studies 8.0 8.0 8.0     Epidemiology 1.0 6.0 6.0 6.0 6.0 6.0 6.0 6.0 6.0 6.0 6.0   Dosimetry 1.5 1.5     Effects of PM and copollutants   Copollutants (toxicology) 4.0 4.0 4.0 4.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0   Copollutants/long term (epidemiology) 6.0 6.0 6.0 6.0 6.0 6.0 6.0 6.0 6.0 3.0 3.0   Susceptible subpopulations 3.0 3.0 3.0 3.0 3.0 3.0     Toxicity mechanisms   Animal models 3.0 3.0 3.0 3.0     In vitro studies 3.0 3.0 3.0 3.0     Haman clinical 3.5 3.5 3.5 3.5  

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Research Priorities for Airborne Particulate Matter: IV - Continuing Research Progress   2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 ANALYSIS AND MEASUREMENT   Statistical analysis 1.0 1.0 1.0 1.0     Measurement error 1.5 3.0 2.0 2.0   SUBTOTALS ($ million per year) 47.5 54.0 51.5 42.5 28.5 28.5 21.5 17.0 17.0 14.0 14.0 RESEARCH MANAGEMENTd (estimated at 10%) 4.8 5.4 5.2 4.3 2.9 2.9 2.2 1.7 1.7 1.4 1.4 TOTALS ($ million per year) 52.3 59.4 56.7 46.8 31.4 31.4 23.7 18.7 18.7 15.4 15.4 aThe committee’s rough but informed collective-judgment cost estimates for the highest-priority research activities recommended in this report. See Chapter 3 of NRC (2001) and Chapter 4 of NRC (1998) for explanations. These estimates should not be interpreted as a recommended total particulate-matter research budget for EPA or the nation, for reasons explained in NRC (1998) bThe committee provided these cost estimates as initial guidance to the development of this research investment portfolio. Available time and resources did not allow the committee to revise and up date these figures since its second report (NRC 1999). Instead, the committee focused its efforts on assessing theactual scientific progress. cThese estimates are in addition to costs for EPA’s Supersites Program and expansion of the nationwide speciation network, as well as likely expenditures by states, local agencies, and industries for source-emissions inventories and field-measurement campaigns in support of model evaluation studies (see Table 1-2). dResearch management includes research planning, budgeting, oversight, review, and dissemination, cumulatively estimated by the committee at 10% of project costs. Source: NRC 1999.

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Research Priorities for Airborne Particulate Matter: IV - Continuing Research Progress TABLE 1-2 The Committee’s 1999 Technical Support Estimates: Timing and Estimated Costsa,b ($ million/year in 1998 dollars) of Additional Technical Work Needed for Implementation of Emission-Control Programs for Airborne Particles   2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 ACTIVITY   Source testing by regulatory programs     5.0 5.0 5.0 5.0 5.0           Compilation of interim PM emissioninventory 1.0 1.0 1.0 1.0     Compilation of PM emission inventory based on results of new source information   1.0 1.0 1.0       4.Field studies in support of air quality model evaluation and testing   20.0 20.0 20.0 20.0 20.0   TOTALS ($ MILLION PER YEAR) 1.0 21.0 26.0 26.0 25.0 25.0 6.0 1.0 1.0     aTechnical-support expenditures by all public and private sponsoring organizations. bThe committee provided these cost estimates as initial guidance to the development of this research investment portfolio. Available time and resources did not allow the committee to revise and update these figures since its second report (NRC 1999). Instead,thecommittee focused its efforts on assessing theactual scientific progress. Source: NRC 1999.

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Research Priorities for Airborne Particulate Matter: IV - Continuing Research Progress TABLE 1-3 EPA Intramural and Extramural PM-Research Enacted Budgets for FY 1998-2001 ($ million/year in actual dollars)a NRC Committee-Recommended Research Topic Recipientb FY 1998 FY 1999 FY 2000 FY 2001 FY 2002 FY 2003   Outdoor vs. human exposure Total $6.3 $8.2 $8.1 $5.3 $1.6 $1.3 Intramural $4.1 $8.2 $7.6 $4.8 $1.3 $0.5 Extramural $2.2 $0.0 $0.5 $0.5 $0.3 $0.8   Exposure to toxic PM components Total $0.5 $0.0 $0.6 $0.6 $1.7 $3.9 Intramural $0.0 $0.0 $0.0 $0.0 $1.4 $3.2 Extramural $0.5 $0.0 $0.6 $0.6 $0.3 $0.7   Emission sources Total $5.5 $7.0 $4.7 $4.5 $5.2 $6.1 Intramural $3.6 $5.6 $4.2 $4.0 $4.3 $5.1 Extramural $1.9 $1.4 $0.5 $0.5 $0.9 $1.0   Air-quality models Total $0.5 $0.4 $6.6 $7.2 $7.4 $9.2 Intramural $0.0 $0.4 $6.0 $6.7 $5.5 $7.0 Extramural $0.5 $0.0 $0.6 $0.6 $1.9 $2.2   Assessment of hazardous PMcomponents Total $7.9 $7.9 $8.1 $6.7 $16.1 $11.1 Intramural $4.1 $3.3 $4.8 $4.5 $12.1 $8.0 Extramural $3.8 $4.6 $3.2 $2.2 $4.1 $3.0   Dosimetry Total $1.5 $0.6 $1.3 $1.1 $0.5 $0.4 Intramural $1.0 $0.6 $0.8 $0.6 $0.4 $0.0 Extramural $0.4 $0.0 $0.5 $0.5 $0.2 $0.4   Effects of PM and copollutants Total $2.3 $7.4 $6.4 $11.7 $7.2 $6.5 Intramural $0.0 $2.6 $2.3 $4.7 $3.4 $4.1 Extramural $2.3 $4.9 $4.1 $7.0 $3.8 $2.5

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Research Priorities for Airborne Particulate Matter: IV - Continuing Research Progress   Susceptible subpopulations Total $8.4 $2.7 $2.9 $2.7 $4.7 $4.8 Intramural $3.9 $2.4 $1.9 $1.7 $1.7 $2.2 Extramural $4.6 $0.3 $1.0 $1.0 $3.0 $2.6   Toxicity mechanisms Total $5.6 $8.3 $8.2 $8.4 $7.1 $6.5 Intramural $2.5 $2.7 $3.0 $3.6 $2.4 $2.5 Extramural $3.1 $5.7 $5.2 $4.8 $4.7 $4.1   Analysis and measurement Total $1.6 $1.2 $1.0 $1.0 $0.8 $0.6 Intramural $1.1 $1.2 $0.5 $0.5 $0.2 $0.1 Extramural $0.5 $0.0 $0.5 $0.5 $0.6 $0.5 SUBTOTAL   $40.1 $43.8 $47.8 $49.3 $52.4 $50.5 INTRAMURAL   $20.3 $27.0 $31.1 $31.1 $32.7 $32.7 EXTRAMURAL   $19.8 $16.9 $16.7 $18.2 $19.8 $17.8 Management expensesc   $1.9 $3.6 $5.9 $1.5 $1.7 $1.7 Working capital and operating expenses   —d —e —e $8.2 $7.0 $5.9 TOTAL FOR NRC RESEARCH TOPICS   $42.0 $47.3 $53.7 $59.0 $61.1 $58.1 Implementation-Related Activityf Technical support Total $2.9 $3.4 $3.2 $1.7 $3.0 $5.4 Intramural $2.9 $3.4 $3.2 $1.7 $2.4 $4.7 Extramural $0.0 $0.0 $0.0 $0.0 $0.6 $0.6 Supersites       $2.9 $2.0 $1.0 0 Emissions characteristics, factors and controls Total $4.0 $3.5 $1.2 $1.0 $1.3 $1.4 Intramural $3.7 $3.2 $1.2 $1.0 $1.3 $1.4 Extramural $0.4 $0.4 $0.0 $0.0 $0.0 $0.0

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Research Priorities for Airborne Particulate Matter: IV - Continuing Research Progress NRC Committee-Recommended Research Topic Recipientb FY 1998 FY 1999 FY 2000 FY 2001 FY 2002 FY 2003 Criteria document development Total Intramural $1.3 $1.4 $1.4 $1.6 $1.3 $1.8 SUBTOTAL   $8.2 $8.3 $8.7 $6.3 $6.6 $8.6 INTRAMURAL   $7.9 $8.0 $5.8 $4.3 $5.0 $7.9 EXTRAMURAL   $0.4 $0.4 $2.9g $2.0g $1.6g $0.6 GRAND TOTAL   $50.2 $55.6 $62.4 $65.3 $67.7 $66.7 INTRAMURAL   $28.2 $35.0 $36.9 $35.4 $37.7 $40.6 EXTRAMURAL   $20.2 $17.3 $19.6 $20.2 $21.4 $18.4 aSums of intramural and extramural costs may differ from their respective totals shown in the table because of round- off error. bExtramural consists of competitive and noncompetitive awards. It includes the Science to Achieve Results (STAR) Program, PM centers, interagency agreements, cooperative agreements with universities, and supersite funding. The distribution of research efforts of PM centers to the NRC topics is based on input from each center. Intramural includes EPA personnel salaries and expenses, contracts, and cooperative agreements. cManagement expenses includes salaries and expenses for EPA management personnel. dIn FY 1998, working capital and operating expenses were tracked under a different budget element than that for PM. eWorking capital and operating expenses for scientific infrastructure are allocated to EPA laboratories and EPA centers based on program need. Those expenses are included under “Management expenses” for FY 1999 and FY 2000. Expenses for FY 2001- 2003 have been reported explicitly. fNot identified by committee as among highest priorities. gSupersites. Source: Modified from NRC 2001.

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Research Priorities for Airborne Particulate Matter: IV - Continuing Research Progress THE COMMITTEE’S APPROACH TO ITS TASK For this report, the committee focused on the task of tracking research progress from the time of its first report. While assessing research progress to date, the committee recognizes that future research extends beyond the existence of the committee. A recommendation concerning future oversight is presented in Chapter 6. As in prior reports, the committee did not attempt to synthesize the available information to formulate its own recommendations for a PM NAAQS. To complete its task, the committee evaluated progress on a research agenda intended to resolve key scientific uncertainties, but the outcome of the evaluation largely reflects process rather than findings. In this regard, the committee’s work is distinct from that of the Clean Air Scientific Advisory Committee of EPA’s SAB, which critically evaluates the criteria document and staff paper. Other NRC committees have also addressed aspects of the NAAQS process. For example, the NRC Committee on Air Quality Management in the United States prepared a report that provides scientific and technical recommendations for strengthening the nation’s air quality management system, including setting, implementing, and tracking progress of the NAAQS (NRC 2004). In addition, the NRC Committee on Estimating the Health-Risk-Reduction Benefits of Proposed Air Pollution Regulations completed a report that reviews recent EPA analyses and provides recommendations for improvement of the methods used. That committee addressed issues concerned with the structure of the analysis, such as the regulatory options to evaluate, the time frame to use, and the assumptions to make about conditions with and without regulation (NRC 2002). In approaching its task for this report, the present committee used a multipronged approach to gauge research progress. It formed working groups directed at specific research topics, and these groups organized workshops that brought together committee members and researchers to discuss the evolution of the evidence on particular topics. The committee also carried out a comparable process of discussion in developing the evaluation and synthesis approach used in this report.7 In addition, it developed a database of peer-reviewed publications that were collected by EPA in preparing the PM criteria document (EPA 2002, 2003a). The 7   The names listed in the preface of this report include those who participated in the committee’s workshops and discussion to develop an evaluation and synthesis approach.

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Research Priorities for Airborne Particulate Matter: IV - Continuing Research Progress database served as one basis for identifying research results that have been published over the years of the committee’s consideration and for assessing the extent to which important scientific uncertainties have been reduced. The committee specifically did not evaluate the EPA-funded PM Research Centers Program, because some of its members are involved in the centers. However, the EPA Science Advisory Board (SAB) completed an evaluation of the centers in 2002 (EPASAB 2002). The SAB report concluded that the centers program has and will likely continue to produce benefits beyond those that would come from individual investigator-initiated research. It also stated that the research centers offered a number of advantages, including flexibility and adaptability in pursuing PM research, ability to foster a multidisciplinary approach from a study’s inception, and ability to leverage additional resources. The SAB report also said the centers offered the possibility of undertaking research on methods development, validation, and pilot studies that are often difficult to finance by individual investigators. The SAB report concluded that the PM centers program merits continuation beyond its expiration in fiscal year 2004—through a new, fully competitive round of applications—as one part of a diverse PM research portfolio at EPA. The SAB report recommended that an overarching mechanism be developed to provide scientific advice across all the centers. It also recommended enhanced interactions among the centers and ongoing intensive air quality monitoring efforts. In addition, the report recommended a continued focus of the centers’ efforts on the most critical PM research needs identified by this committee and EPA. The SAB report also emphasized the importance of ensuring that the work of the centers does not become isolated from that of other researchers within EPA and in the academic community. ORGANIZATION OF THE REPORT In Chapter 2, the committee discusses its approach for evaluating PM research progress. Chapter 3 provides a synthesis of PM research progress in each of the 10 topics in the research portfolio since 1997. Detailed assessments of progress for each research topic are presented in Appendix C. Chapter 4 provides an integration of progress made across the research topics, and Chapter 5 identifies key, overarching scientific challenges for the years ahead in completing the research portfolio on PM. Chapter 6 provides guidance on key management issues that the committee expects to be relevant for successfully addressing key priorities for PM research in the future, and Chapter 7 provides an overall synthesis and conclusions.