text:
Main Research Priorities for Airborne Particulate Matter IV: Continuing Research Progress
Research Priorities for Airborne Particulate Matter IV: Continuing Research ProgressCommittee on Research Priorities for Airborne Particulate Matter, National Research Council
In 1997, the U.S. Environmental Protection Agency (EPA) established regulatory standards to address health risks posed by inhaling tiny particles from smoke, vehicle exhaust, and other sources. At the same time, Congress and the EPA began a multimillion dollar research effort to better understand the sources of these airborne particles, the levels of exposure to people, and the ways that these particles cause disease. To provide independent guidance to the EPA, Congress asked the National Research Council to study the relevant issues. The result was a series of four reports on the particulate-matter research program. The first two books offered a conceptual framework for a national research program, identified the 10 most critical research needs, and described the recommended timing and estimated costs of such research. The third volume began the task of assessing initial progress made in implementing the research program. This, the fourth and final volume, gauged research progress made over a 5-year period on each of the 10 research topics. The National Research Council concludes that particulate matter research has led to a better understanding of the health effects caused by tiny airborne particles. However, the EPA, in concert with other agencies, should continue research to reduce further uncertainties and inform long-term decisions.
You can write a book review and share your experiences. Other readers will always be interested in your opinion of the books you've read. Whether you've loved the book or not, if you give your honest and detailed thoughts then people will find new books that are right for them.
Committee on Research Priorities for Airborne Particulate Matter Board on Environmental Studies and Toxicology Division on Earth and Life Studies THE NATIONAL ACADEMIES PRESS 500 Fifth Street, NW Washington, DC 20001 NOTICE: The project that is the subject of this report was approved by the Governing Board of the National Research Council, whose members are drawn from the councils of the National Academy of Sciences, the National Academy of Engineering, and the Institute of Medicine. The members of the committee responsible for the report were chosen for their special competences and with regard for appropriate balance. This project was supported by Contract No. 68-C-98-003 between the National Academy of Sciences and the U.S. Environmental Protection Agency. Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the organizations or agencies that provided support for this project. Library of Congress Control Number 2004112036 International Standard Book Number 0-309-09199-3 (Book) 0-309-53170-5 (PDF) Additional copies of this report are available from: The National Academies Press 500 Fifth Street, NW Box 285 Washington, DC 20055 800-624-6242 202-334-3313 (in the Washington metropolitan area) http://www.nap.edu Copyright 2004 by the National Academy of Sciences. All rights reserved. Printed in the United States of America The National Academy of Sciences is a private, nonprofit, self-perpetuating society of distinguished scholars engaged in scientific and engineering research, dedicated to the furtherance of science and technology and to their use for the general welfare. Upon the authority of the charter granted to it by the Congress in 1863, the Academy has a mandate that requires it to advise the federal government on scientific and technical matters. Dr. Bruce M. Alberts is president of the National Academy of Sciences. The National Academy of Engineering was established in 1964, under the charter of the National Academy of Sciences, as a parallel organization of outstanding engineers. It is autonomous in its administration and in the selection of its members, sharing with the National Academy of Sciences the responsibility for advising the federal government. The National Academy of Engineering also sponsors engineering programs aimed at meeting national needs, encourages education and research, and recognizes the superior achievements of engineers. Dr. Wm. A. Wulf is president of the National Academy of Engineering. The Institute of Medicine was established in 1970 by the National Academy of Sciences to secure the services of eminent members of appropriate professions in the examination of policy matters pertaining to the health of the public. The Institute acts under the responsibility given to the National Academy of Sciences by its congressional charter to be an adviser to the federal government and, upon its own initiative, to identify issues of medical care, research, and education. Dr. Harvey V. Fineberg is president of the Institute of Medicine. The National Research Council was organized by the National Academy of Sciences in 1916 to associate the broad community of science and technology with the Academy's purposes of furthering knowledge and advising the federal government. Functioning in accordance with general policies determined by the Academy, the Council has become the principal operating agency of both the National Academy of Sciences and the National Academy of Engineering in providing services to the government, the public, and the scientific and engineering communities. The Council is administered jointly by both Academies and the Institute of Medicine. Dr. Bruce M. Alberts and Dr. Wm. A. Wulf are chair and vice chair, respectively, of the National Research Council www.national-academies.org COMMITTEE ON RESEARCH PRIORITIES FOR AIRBORNE PARTICULATE MATTER JONATHAN SAMET (Chair), Johns Hopkins University, Baltimore, MD JUDITH CHOW, Desert Research Institute, Reno, NV BART CROES, California Air Resources Board, Sacramento ROBERT FORSTER, University of Pennsylvania, Philadelphia DANIEL GREENBAUM, Health Effects Institute, Boston, MA PHILIP HOPKE, Clarkson University, Potsdam, NY PETROS KOUTRAKIS, Harvard University, Boston, MA DANIEL KREWSKI, University of Ottawa, Ottawa, ON PAUL LIOY, University of Medicine and Dentistry of New Jersey, Piscataway JOE MAUDERLY, Lovelace Respiratory Research Institute, Albuquerque, NM ROGER MCCLELLAN, Albuquerque, NM GÜNTER OBERDÖRSTER, University of Rochester, Rochester, NY REBECCA PARKIN, George Washington University Medical Center, Washington, DC JOYCE PENNER, University of Michigan, Ann Arbor RICHARD SCHLESINGER, Pace University, New York, NY FRANK SPEIZER, Harvard Medical School, Boston, MA MARK UTELL, University of Rochester, Rochester, NY RONALD WHITE, Johns Hopkins University, Baltimore, MD WARREN WHITE, University of California, Davis RONALD WYZGA, Electric Power Research Institute, Palo Alto, CA TERRY YOSIE, American Chemistry Council, Arlington, VA Project Staff RAYMOND A. WASSEL, Project Director EILEEN N. ABT, Senior Staff Officer KULBIR BAKSHI, Senior Staff Officer K. JOHN HOLMES, Senior Staff Officer KARL GUSTAVSON, Staff Officer AMANDA C. STAUDT, Staff Officer RUTH E. CROSSGROVE, Editor RACHEL HOFFMAN, Intern MIRSADA KARALIC-LONCAREVIC, Research Assistant EMILY L. BRADY, Senior Program Assistant SAMMY BARDLEY, Library Assistant v BOARD ON ENVIRONMENTAL STUDIES AND TOXICOLOGY Members JONATHAN M. SAMET (Chair), Johns Hopkins University, Baltimore, MD DAVID ALLEN, University of Texas, Austin THOMAS BURKE, Johns Hopkins University, Baltimore, MD JUDITH C. CHOW, Desert Research Institute, Reno, NV COSTEL D. DENSON, University of Delaware, Newark E. DONALD ELLIOTT, Yale Law School, New Haven, CT CHRISTOPHER B. FIELD, Carnegie Institute of Washington, Stanford, CA WILLIAM H. GLAZE, Oregon Health and Science University, Beaverton SHERRI W. GOODMAN, Center for Naval Analyses, Alexandria, VA DANIEL S. GREENBAUM, Health Effects Institute, Cambridge, MA ROGENE HENDERSON, Lovelace Respiratory Research Institute, Albuquerque, NM CAROL HENRY, American Chemistry Council, Arlington, VA ROBERT HUGGETT, Michigan State University, East Lansing BARRY L. JOHNSON Emory University, Atlanta, GA JAMES H. JOHNSON, Howard University, Washington, DC JUDITH L. MEYER, University of Georgia, Athens PATRICK V. O'BRIEN, Chevron Research and Technology, Richmond, CA DOROTHY E. PATTON, International Life Sciences Institute, Washington, DC STEWARD T.A. PICKETT, Institute of Ecosystem Studies, Millbrook, NY ARMISTEAD G. RUSSELL, Georgia Institute of Technology, Atlanta LOUISE M. RYAN, Harvard University, Boston, MA KIRK SMITH, University of California, Berkeley LISA SPEER, Natural Resources Defense Council, New York, NY G. DAVID TILMAN, University of Minnesota, St. Paul CHRIS G. WHIPPLE, Environ Incorporated, Emeryville, CA LAUREN A. ZEISE, California Environmental Protection Agency, Oakland Senior Staff JAMES J. REISA, Director DAVID J. POLICANSKY, Associate Director RAYMOND A. WASSEL, Senior Program Director for Environmental Sciences and Engineering KULBIR BAKSHI, Program Director for Toxicology ROBERTA M. WEDGE, Program Director for Risk Analysis K. JOHN HOLMES, Senior Staff Officer SUSAN N.J. MARTEL, Senior Staff Officer SUZANNE VAN DRUNICK, Senior Staff Officer EILEEN N. ABT, Senior Staff Officer ELLEN K. MANTUS, Senior Staff Officer RUTH E. CROSSGROVE, Managing Editor vi OTHER REPORTS OF THE BOARD ON ENVIRONMENTAL STUDIES AND TOXICOLOGY Air Quality Management in the United States (2004) Endangered and Threatened Species of the Platte River (2004) Atlantic Salmon in Maine (2004) Endangered and Threatened Fishes in the Klamath River Basin (2004) Cumulative Environmental Effects of Alaska North Slope Oil and Gas Development (2003) Estimating the Public Health Benefits of Proposed Air Pollution Regulations (2002) Biosolids Applied to Land: Advancing Standards and Practices (2002) The Airliner Cabin Environment and Health of Passengers and Crew (2002) Arsenic in Drinking Water: 2001 Update (2001) Evaluating Vehicle Emissions Inspection and Maintenance Programs (2001) Compensating for Wetland Losses Under the Clean Water Act (2001) A Risk-Management Strategy for PCB-Contaminated Sediments (2001) Acute Exposure Guideline Levels for Selected Airborne Chemicals (4 volumes, 2000-2004) Toxicological Effects of Methylmercury (2000) Strengthening Science at the U.S. Environmental Protection Agency (2000) Scientific Frontiers in Developmental Toxicology and Risk Assessment (2000) Ecological Indicators for the Nation (2000) Waste Incineration and Public Health (1999) Hormonally Active Agents in the Environment (1999) Research Priorities for Airborne Particulate Matter (4 volumes, 1998-2004) The National Research Council's Committee on Toxicology: The First 50 Years (1997) Carcinogens and Anticarcinogens in the Human Diet (1996) Upstream: Salmon and Society in the Pacific Northwest (1996) Science and the Endangered Species Act (1995) Wetlands: Characteristics and Boundaries (1995) Biologic Markers (5 volumes, 1989-1995) Review of EPA's Environmental Monitoring and Assessment Program (3 volumes, 1994-1995) Science and Judgment in Risk Assessment (1994) Pesticides in the Diets of Infants and Children (1993) Dolphins and the Tuna Industry (1992) Science and the National Parks (1992) Human Exposure Assessment for Airborne Pollutants (1991) Rethinking the Ozone Problem in Urban and Regional Air Pollution (1991) Decline of the Sea Turtles (1990) Copies of these reports may be ordered from the National Academies Press (800) 624-6242 or (202) 334-3313 www.nap.edu vii Dedication to the Memory of Professor Glen Cass (1947-2001) We dedicate this report to the memory of our late colleague and fellow committee member, Professor Glen Cass. Glen represented superbly the multidisciplinary research approach that we advocate in this report. Over the past two decades, from his Ph.D. research on sulfates to subsequent work on ozone, nitrates, primary and secondary carbon, visibility reduction, and indoor air quality, Glen and his research group unraveled the sources, atmospheric processes, toxicity, and emission controls needed to curtail the detrimental impacts of particulate matter on public health and welfare. Glen had a unique ability to combine elegant scientific approaches with sound engineering judgment to arrive at practical solutions that have been incorporated into air quality management practices in Los Angeles, the northeastern United States, and Asia. His legacy includes a large body of research and a rigorous, yet practical, approach to training a generation of air pollution scientists, who continue to lay the foundation needed for science-based decisionmaking. As a colleague, a friend, and an inspiration, we all sorely miss him. viii Preface Under the Clean Air Act, particulate matter (PM) is one of the major air pollutants for which National Ambient Air Quality Standards (NAAQS) are to be established on the basis of the scientific evidence on risks to human health. The U.S. Environmental Protection Agency (EPA), other federal and state government agencies, and nongovernment organizations are conducting a major multiyear research effort to improve scientific understanding of airborne PM and its effects on human health. An overall objective is to reduce uncertainties in the scientific evidence used to set the NAAQS for airborne PM in the United States. At the request of Congress and EPA, the National Research Council's Committee on Research Priorities for Airborne Particulate Matter was established in 1998 and given the charge of developing a research agenda for this purpose and then monitoring research progress. Biosketches of the committee members are presented in Appendix A. This report, the committee's fourth, comes 6 years after its first report, which proposed a 13-year research portfolio. This report evaluates research progress since the first report, evaluates possible barriers to continued progress, and makes recommendations for future research directions and research management. The first of the committee's four planned reports, Research Priorities for Airborne Particulate Matter: I. Immediate Priorities and a Long-Range Research Portfolio, was published in 1998. It identified 10 high-priority research topics linked to key policy-related scientific uncertainties and presented a 13-year "research investment portfolio" containing recommended short-term and long-term phasing and estimated costs of research on each topic. Congress, EPA, and the scientific community gave strong support to the committee's recommendations and implemented substantial changes in research efforts in response to them. The committee's second report, Research Priorities for Airborne Particulate Matter: II. Evaluating Research Progress and Updating the Portfolio, published in 1999, described the committee's plans for monitoring the progress of research. In addition, the research recommendations from the committee's first report were updated, and recommendations related to emissions and air quality models were substantially revised. The committee's third report, published in 2001, monitored the progix x Preface ress of the research begun in 1998 or later to address the priority research topics identified by the committee. Although much research had been initiated, not enough time had elapsed by then for many of the projects to be completed and their results reported. The third report should be viewed as a preliminary assessment of research progress. In this final report, the committee faced the challenge of gauging research progress on each of its 10 research topics. It developed an approach for characterizing the degree to which new evidence has reduced uncertainty and then gathered and evaluated the evidence coming from research over 5 years since the first report. The committee was assisted by the many individuals (listed below) who participated in workshops and public sessions of committee meetings held for the purpose of learning about relevant findings. Research progress reflects not only the creativity and efforts of researchers but also the efficiency of research management. This report also provides recommendations for future consideration of research on PM, as this committee's work is now finished. The committee received oral or written presentations or both from the following individuals: John Bachmann, U.S. Environmental Protection Agency; Tina Bahadori, American Chemistry Council; John Bailar, University of Chicago; David V. Bates, University of British Columbia; William Bennett, University of North Carolina at Chapel Hill; Michael Brauer, University of British Columbia; Richard Burnett, Health Canada; Lilian Calderon-Garciduenas, University of North Carolina at Chapel Hill; Aaron Cohen, Health Effects Institute; Daniel Costa, U.S. Environmental Protection Agency; Robin Dennis, U.S. Environmental Protection Agency; Robert Devlin, U.S. Environmental Protection Agency; Douglas Dockery, Harvard School of Public Health; Francesca Dominici, Johns Hopkins Bloomberg School of Public Health; Ed Edney, U.S. Environmental Protection Agency; Mark Frampton, University of Rochester; John Froines, University of California at Los Angeles; Patrick Gaffney, California Air Resources Board; Chris Geron, U.S. Environmental Protection Agency; Frank Gilliland, University of Southern California; Ian Gilmour, U.S. Environmental Protection Agency; John Godleski, Harvard School of Public Health; Judy Graham, while at the U.S. Environmental Protection Agency, currently at the American Chemistry Council; Jack Harkema, Michigan State University; Bruce Harris, U.S. Environmental Protection Agency; James Hogg, University of British Columbia; Patrick Kinney, Columbia University School of Public Health; Michael Kleinman, University of California at Irvine; Jane Koenig, University of Washington; Timothy Larson, University of Washington; Allen Lefohn, A.S.L. & Associates; Chuck Lewis, U.S. Environ- Preface xi mental Protection Agency; Joellen Lewtas, U.S. Environmental Protection Agency; Morton Lippmann, New York University School of Medicine; Phillip Lorang, U.S. Environmental Protection Agency; Robert Mason, National Jewish Medical and Research Center; Andrew Miller, U.S. Environmental Protection Agency; Fred Miller, CIIT Centers for Health Research; David Mobley, U.S. Environmental Protection Agency; D. Warner North, NorthWorks; William Ollison, American Petroleum Institute; Wayne Ott, Stanford University; Pedro Oyola, Chilean National Commission for the Environment; Joseph Paisie, U.S. Environmental Protection Agency; Giovanni Parmigiani, Johns Hopkins University; Robert Phalen, University of California at Irvine; C. Arden Pope III, Brigham Young University; Peter Preuss, U.S. Environmental Protection Agency; Charles Rodes, Research Triangle Institute; Joseph Rodricks, ENVIRON International Corporation; Armistead Russell, Georgia Institute of Technology; Richard Scheffe, U.S. Environmental Protection Agency; Kenneth Schere, U.S. Environmental Protection Agency; John Seitz, while at the U.S. Environmental Protection Agency, currently at Sonnenschein, Nath & Rosenthal, LLP; Linda Sheldon, U.S. Environmental Protection Agency; Lianne Sheppard, University of Washington; Dean Smith, U.S. Environmental Protection Agency; Paul Solomon, U.S. Environmental Protection Agency; Helen Suh, Harvard School of Public Health; Joseph Tikvart, U.S. Environmental Protection Agency; Paige Tolbert, Emory University; Sverre Vedal, National Jewish Medical and Research Center; James Vickery, U.S. Environmental Protection Agency; Russell Weiner, U.S. Environmental Protection Agency; Ronald Williams, U.S. Environmental Protection Agency; William Wilson, U.S. Environmental Protection Agency; and Denis Zmirou, French Institute of Health and Medical Research. We are grateful for the assistance of the NRC staff in preparing the report. We wish to thank Raymond Wassel, project director, and James Reisa, director of BEST. Scientific and technical information was provided by Eileen Abt, Kulbir Bakshi, K. John Holmes, Karl Gustavson, Amanda Staudt, Mirsada Karalic-Loncarevic, and Rachel Hoffman. Invaluable logistical support was provided by Emily Brady. The report was ably edited by Ruth Crossgrove. Finally, I would like to thank all the members of the committee for their dedicated efforts throughout the development of this report. Jonathan Samet, Chair Committee on Research Priorities for Airborne Particulate Matter Acknowledgment of Review Participants This report has been reviewed in draft form by individuals chosen for their diverse perspectives and technical expertise, in accordance with procedures approved by the National Research Council's Report Review Committee. The purpose of this independent review is to provide candid and critical comments that will assist the institution in making its published report as sound as possible and to ensure that the report meets institutional standards for objectivity, evidence, and responsiveness to the study charge. The review comments and draft manuscript remain confidential to protect the integrity of the deliberative process. We wish to thank the following individuals for their review of this report: Carol Henry, American Chemistry Council; George Hidy, ENVAIR; Morton Lippmann, New York University Medical Center; Ronald Low, University of Medicine and Dentistry of New Jersey; D. Warner North, NorthWorks; Robert Phalen, University of California at Irvine; C. Arden Pope, Brigham Young University; and Armistead Russell, Georgia Institute of Technology. Although the reviewers listed above have provided many constructive comments and suggestions, they were not asked to endorse the conclusions or recommendations, nor did they see the final draft of the report before its release. The review of this report was overseen by Annetta Watson, Oak Ridge National Laboratory. Appointed by the NRC she was responsible for making certain that an independent examination of this report was carried out in accordance with institutional procedures and that all review comments were carefully considered. Responsibility for the final content of this report rests entirely with the authoring committee and the institution. xii Contents SUMMARY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 Committee's Task, 24 Status of the EPA Review of the NAAQS for Particulate Matter, 28 The Need for Research on Particulate Matter, 29 Research on Particulate Matter, 37 The Committee's Approach to Its Task, 45 Organization of the Report, 46 2 COMMITTEE'S APPROACH TO EVALUATION OF RESEARCH PROGRESS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47 Introduction, 47 Background, 48 The Committee's Evaluative Approach, 60 Considerations in Interpreting Research Progress on Particulate Matter, 63 3 SYNTHESIS OF RESEARCH PROGRESS ON PARTICULATE MATTER . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64 Introduction, 64 Research Topic 1: Outdoor Measures Versus Actual Human Exposures, 65 Research Topic 2: Exposures of Susceptible Subpopulations to Toxic Particulate Matter Components, 70 Research Topic 3: Characterization of Emission Sources, 72 Research Topic 4: Air Quality Model Development and Testing, 79 Research Topic 5: Assess Hazardous Particulate Matter Components, 88 Research Topic 6: Dosimetry: Deposition and Fate of Particles in the Respiratory Tract, 95 xiii xiv Contents Research Topic 7: Combined Effects of Particulate Matter and Gaseous Copollutants, 98 Research Topic 8: Susceptible Subpopulations, 102 Research Topic 9: Mechanisms of Injury, 106 Research Topic 10: Analysis and Measurement, 111 Summary, 116 4 LOOKING ACROSS THE RESEARCH TOPICS . . . . . . . . . . . . . . . 118 Introduction, 118 Health Outcomes and Susceptible Subpopulations, 119 Assessing Hazardous Particulate Matter Components, 120 Increasing Emphasis on Exposure-Dose-Response Relationships, 123 Mixtures and Copollutants, 124 NAAQS Implications, 125 5 THE CHALLENGES AHEAD FOR PARTICULATE MATTER RESEARCH . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127 Introduction, 127 Developing a Systematic Program to Assess the Toxicity of Different Components of the Particulate Matter Mixture, 130 Enhancing Air Quality Monitoring for Research, 132 Investigating the Health Effects of Long-Term Exposure to Air Pollution, 134 Improved Toxicological Approaches, 139 From a Particulate Matter Research Program to a Multipollutant Research Program, 141 Integrating Across the Disciplines, 146 Summary and Conclusions, 149 6 THE WAY FORWARD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 150 Introduction, 150 Sustained Research Management, 151 Improved Tools for Science Tracking and Synthesis, 162 Conclusion, 165 7 CONCLUSION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 166 REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 169 TERMS AND ABBREVIATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187 Contents xv APPENDIX A: BIOGRAPHICAL INFORMATION ON THE COMMITTEE ON RESEARCH PRIORITIES FOR AIRBORNE PARTICULATE MATTER . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 191 APPENDIX B: THE COMMITTEE'S STATEMENT OF TASK . . . . . . . . 202 APPENDIX C: DETAILED ASSESSMENT OF PARTICULATE MATTER RESEARCH PROGRESS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 203 Research Priorities for Airborne Particulate Matter: IV. Continuing Research Progress Summary This report is the fourth in a series by the Committee on Research Priorities for Airborne Particulate Matter. The committee was convened by the National Research Council (NRC) in January 1998 at the request of the U.S. Environmental Protection Agency (EPA) following directions from Congress in EPA's fiscal year 1998 appropriations report. The congressional request for this study arose from the need to reduce uncertainties in the scientific evidence considered by EPA in reaching the July 1997 decision to establish new National Ambient Air Quality Standards (NAAQS) for particulate matter (PM) less than 2.5 micrometers (:m) in aerodynamic diameter (PM2.5). Anticipating information needs of the required reviews of the PM NAAQS every 5 years, Congress appropriated substantial funds for a major research program to reduce the scientific uncertainties. It also directed EPA to arrange for this independent NRC study to provide guidance for planning the research program and then monitoring research progress. This report focuses on the progress since early 1998 in reducing scientific uncertainties. THE PARTICULATE MATTER RESEARCH PROGRAM EPA initiated a substantial program that largely followed the 13-year research portfolio set out over 5 years ago by this committee in its first report. The portfolio identifies 10 high-priority research topics linked to key scientific uncertainties relevant to those sources that cause adverse health effects (see Box S-1). Table S-1 shows funding levels budgeted during fiscal years 1998-2003 by Congress for EPA's PM research and related technical work. Much of the research called for by the committee has been carried out by EPA investigators, but many scientists from academia and 3 4 Research Priorities for Airborne Particulate Matter research institutes in the United States and in other countries are also involved. REVIEW OF PROGRESS AND STATUS OF RESEARCH In preparing this final report, the committee matched PM research and resulting contributions to the scientific literature from projects sponsored by EPA and other institutions against the committee's recommended research portfolio. It assessed the extent to which completed and ongoing research is addressing gaps in the evidence on major issues that decisionmakers will consider as they review the scientific evidence relevant to the PM NAAQS. The committee systematically reviewed progress made between 1998 and mid-2002 in PM research on its topics 1-10. Review updates were performed over the next year as this report was written. Brief summaries for each topic follow. Research Topic 1. Outdoor Measures Versus Actual Human Exposures Substantial progress has been made in addressing topic 1, in part because of leadership by EPA in this area. The committee was able to identify a large number of studies initiated after its first report, and for most, the field work is now complete, and results are being published in peerreviewed literature. Advances in personal monitoring made data collection feasible not only for healthy adults but also for children and persons with chronic heart and lung diseases. Monitoring studies of groups of individuals measured at successive points in time supported the conclusion that ambient (outdoor) particle concentrations are a key determinant of variation in personal exposure to particles. Such an association of ambient concentrations with personal PM exposures supports the use of ambient concentrations in time-series analyses and as a relevant metric for public health. Although substantial monitoring data have been collected and analyzed, understanding remains incomplete on the quantitative relationships between outdoor measures of airborne PM and actual personal exposures to PM. Monitoring data should also be used to evaluate existing exposure models and develop new models when necessary. In addition, there is still only sparse information about the exposures of susceptible individuals for example, those with chronic heart and lung diseases to particles and other air pollutants. However, studies on such individuals might best be deferred Summary 5 BOX S-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. 6 Research Priorities for Airborne Particulate Matter TABLE S-1 EPA Funding for PM Research and Related Technical Work (in millions of dollars) Fiscal Year Budgets 1998 1999 2000 2001 2002 2003 PM research 42.0 47.3 53.7 59.0 61.1 58.1 Related technical work 8.2 8.3 8.7 6.3 6.6 8.8 50.2 55.6 62.4 65.3 67.7 66.9 TOTAL until monitoring methods can be enhanced to characterize exposures to specific PM components. Research Topic 2. Exposures of Susceptible Subpopulations to Toxic Particulate Matter Components A small number of studies have included measurements of personal exposures to various particulate constituents, including sulfate, nitrate, ammonium, elemental and organic carbon, and other substances. However, research conducted to date on such exposures has focused on methods development. These efforts will be useful in initial chemical characterizations of exposure of potentially susceptible subpopulations and in directing the design of future exposure studies. However, before implementing research on this topic, the committee's sequence of research calls for substantial advances in identifying biologically relevant PM characteristics. Research Topic 3. Characterization of Emission Sources Although critically important for implementation of new PM NAAQS, a comprehensive and cohesive emissions characterization program (previously recommended by the committee) has not yet been implemented by EPA or other responsible sponsors, including state agencies. Characterizing PM emission sources requires accurate measurements of mass emission rates, composition, and size distributions from a representative sample of an individual source type. In addition, accurate emission rates of reactive precursor gases, such as sulfur dioxide, are needed. Since 1997, the committee has identified several advances, the greatest of which has been improvement in estimating on-road mobile-source emissions (particularly Summary 7 from heavy-duty diesel trucks) of PM mass, ultrafine particles with diameters less than 0.1 :m, ammonia, and semivolatile organic vapors. Additional test methods and testing are needed for the many other sources that contribute major fractions of ambient PM (such as residential wood combustion, wildfires, cooking, and nonroad engines). Both direct and precusor sources of carbon emissions are the most poorly characterized of the emissions contributing to PM. Also, more efforts are needed to understand the uncertainties in emissions inventory estimates. As the committee emphasized in its third report, EPA should develop a comprehensive plan for systematically translating new source-test methods into a completed, comprehensive national emissions inventory based on contemporary source tests of comparable quality. The first step in planning the source-test program would involve the systematic creation of a master list of the representative sources that should be given highest priority. The timeline for this testing must allow for incorporation of revised and updated data into an overall emissions inventory of predetermined quality and completeness before the next round of PM implementation plans are drafted. Research Topic 4. Air Quality Model Development and Testing EPA's ultimate goal should be to have integrated, flexible, and welltested aerosol models available for development of emission-control strategies for ambient PM management. It is still not clear that EPA is making appropriate commitments to develop models for local air quality management. Taking into consideration limited progress on emissions characterization and on source- and receptor-oriented models, the committee remains concerned about the air quality management community's access to fully operational tools for the NAAQS implementation tasks to be undertaken in the coming years. Although instruments needed to monitor air quality are largely in place, much remains to be done if the resulting data are to be used effectively. Access to these data by researchers and other potential users outside of EPA should be made easier.1 Users of the data also face the difficulty of accounting for substantial sampling and analytical uncertainties in the measurement of major particle components—most critically, organic material. 1 The internet portal that once served this function has been replaced since 2001 by a notice to submit a Freedom of Information Act request to EPA to obtain hourly or daily monitoring values. 8 Research Priorities for Airborne Particulate Matter Various models earmarked for regulatory application will be operationally useful only to the extent that data needed to support them are routinely available. Characterizing emissions is a major need for both source- and receptor-oriented modeling. The rigorous evaluation of sourceoriented models will also require large-scale, three-dimensional field studies, because spatially limited measurements are insufficient to challenge a model's mass accounting. Although data sets from already-completed efforts may be adequate in some locations, additional studies are needed in regions of differing climate and emissions. The committee also recognizes the need to improve bridging between geographic scales typically used for PM source-oriented modeling and those needed to conduct exposure assessment. Although EPA alone might not have adequate resources for model evaluation, it can participate in, and coordinate, efforts involving other government agencies and private institutions with substantial field monitoring programs, enhancing such efforts in ways that synergistically increase the value of the resulting data for its own applications. EPA leadership is needed for a coordinated effort to document and compare models and to identify capabilities and limitations of models for decisionmaking purposes. Research Topic 5. Assessment of Hazardous Particulate Matter Components The current NAAQS for PM is both size and mass-based and implicitly assumes that all particles of a given size have the same toxicity per unit mass, irrespective of chemical composition. In the committee's judgment, this mass-based NAAQS greatly oversimplifies complex biological phenomena. Progress on assessment of hazardous PM components is central to the national research portfolio and to any refinement of the current massbased NAAQS for PM. Without progress on topic 5, targeted exposure studies cannot be developed under topic 2, nor can emission inventories and models be refined under topics 3 and 4. Also, new information on hazardous PM components would help to guide efforts to better understand the deposition and fate of relevant particles (topic 6). Research on biological mechanisms (topic 9) and identification of susceptible populations (topic 8) could be enhanced by research accomplishments in topic 5. A better understanding of characteristics that modulate toxicity could lead to targeted control strategies specifically addressing those sources having the most significant adverse effects on public health. The diversity of PM characteristics and the array of possible health 9 Summary effects define a potentially large and complex matrix for investigation; in fact, different features of particles might be relevant to different health outcomes. Since the committee's first report in 1998, even though progress in assessing specific hazardous characteristics of PM has been fragmented, an increasing number of studies have been published that address specific characteristics of particles, including size and chemistry. The resulting evidence provides some new insights concerning toxicity for many of the specific particle characteristics related to size and chemical composition, as discussed in this report. However, there is a large variety of possible relationships between PM characteristics and health outcomes and, to date, those relationships have not been approached systematically. Epidemiological research has been limited by the lack of detailed ambient monitoring data on particle characteristics collected over periods of time and thought to be relevant to exposure and health effects. The research has also been limited by the high correlations among some particle components. Given the complexity of the hypotheses to be tested, large data sets with rich detail on particle characteristics are needed to reduce uncertainties related to topic 5. Also, the sustained research efforts to obtain air quality monitoring data to estimate exposures for study participants and communities for epidemiological studies are only now coming into place. The initial toxicological studies are providing promising findings, pointing to possible roles for the ultrafine fraction of PM, metals, and other components of the PM mixture in air. Despite the large number of research projects directed at this issue, progress since 1998 has been modest, reflecting the challenging set of scientific questions to be addressed. The matrix of relationships between particle composition and possible health responses has been only partially explored. Researchers have focused on PM collected from specific emission sources or on PM components of their particular interest, or they have focused on popular hypotheses or suggestive experimental evidence. A more systematic approach will be required to ensure that research encompasses a broader range of potentially important PM characteristics and yields data that can be used to establish the relative toxicity of different components. Research Topic 6. Dosimetry: Deposition and Fate of Particles in the Respiratory Tract Dosimetry provides a critical link between exposure to atmospheric concentrations of particles and the doses of particles reaching critical sites 10 Research Priorities for Airborne Particulate Matter in the respiratory tract, the clearance of particles from those sites, and the movement of particles from the respiratory tract to other organs. The greatest policy-relevant advance in the understanding of PM dosimetry since the committee's first report is the convergence of evidence from studies in multiple laboratories that demonstrate an increase in the portion of inhaled PM2.5 depositing in the respiratory tracts of people having obstructive lung disease (a highly prevalent condition). The available evidence on dosimetry confirms earlier work suggesting that abnormalities of airway structure or intrapulmonary gas distribution are likely to increase the total deposited dose for a given exposure concentration. The greater deposition fraction and the heterogeneity of deposition in abnormal lungs offer one possible mechanism for increased susceptibility of persons with underlying lung disease to inhaled particles—increased lung dose to localized lung regions—in comparison with persons having normal lungs. Differences in fractional and regional deposition associated with aging have not yet been adequately characterized. Clearance has been less well studied than deposition, and the effects of gender, age, and respiratory abnormalities on clearance remain largely uncertain. Translocation of PM or PM constituents from the lung to other organs has been demonstrated, but this phenomenon has not yet been well characterized. Evidence is more limited for specific size fractions of particles, such as ultrafine particles and particular chemical components of particles. Additionally, more information on dosimetry in animal models of human disease is needed to facilitate extrapolation of findings from these models to humans. Research Topic 7. Combined Effects of Particulate Matter and Gaseous Pollutants This research topic addresses the extent to which the effects of PM on health are independent of coinciding effects of other pollutants and the possibility that PM effects vary with concentrations of other pollutants. Research is complicated by the possibility that modification of the effects of PM by other pollutants could vary among health outcomes. The committee's review found little new direct evidence related to topic 7, other than newer observational studies that have continued to demonstrate an independent effect of particles that is robust to statistical adjustment for other pollutants. Assessments of effect modification in epidemiological and toxicological studies have provided little evidence to indicate whether PM effects vary with concentrations of other major pollutants in ambient air. The committee concludes that further research is 11 Summary needed to address topic 7, while acknowledging the challenges in carrying out such studies whether based on epidemiological observations or toxicological experiments. Modification of PM effects by other pollutants, particularly ozone, can be more powerfully explored in planned larger studies, such as extensions of the approach used in the National Morbidity, Mortality, and Air Pollution Study (NMMAPS). More comprehensive characterization of mixtures considered in source-oriented exposure studies would make such studies more informative. A better understanding is needed of why some mixtures and potentially those from different sources have different effects in different diseases. Research Topic 8. Susceptible Subpopulations Research on susceptible populations is needed to ensure that all populations are protected against risks from PM, including those groups who might be most susceptible. There have been several new findings relevant to susceptible subpopulations. Research results show that following PM exposures, • Persons with diabetes might be at increased risk for adverse health effects, including increased mortality. • Persons with asthma or chronic obstructive pulmonary disease (COPD) exhibit greater deposition of inhaled fine and ultrafine PM, resulting in higher doses and associated risks. • Older adults experience adverse changes in cardiac physiology. • Older adults show hematological changes that are relevant to risk for cardiovascular disease (for example, changes in blood coagulation factors). Animal models are yielding consistent findings. Dogs with coronary occlusion and hypertensive rats demonstrate adverse cardiac and vascular effects when exposed to PM. Studies involving rodent models of aging show enhanced susceptibility to PM and an increase in the effect of PM with infection. To obtain evidence needed to understand the impacts of PM on susceptible subpopulations, research should more effectively address different time scales of exposure (from short-term peaks [acute] to long durations [chronic]), characteristics of exposure, cellular and molecular mechanisms, the range of potential adverse health effects, and potential effect modifiers. 12 Research Priorities for Airborne Particulate Matter Some current concerns focus on whether chronic PM exposures relate to the development of disease and of organ dysfunction, the extent to which ultrafine particles induce adverse effects in asthma and COPD patients, and the magnitude of life-shortening from PM exposures. Research Topic 9. Mechanisms of Injury The major potential biological processes suggested to underlie the reported human health effects from ambient PM exposures include oxidative stress and pulmonary or systemic inflammation. Among the postulated physiological consequences of injury by PM are increased airway hyperreactivity; alterations in the cardiovascular system, such as changes in blood viscosity, heart rate, and rhythm; and heart rate variability. Since 1997, research has broadened to address more subtle pulmonary responses relevant to current exposures and also cardiovascular responses to inhalation of PM. Investigators have begun to consider the respiratory tract as a portal of entry for particles producing systemic effects rather than simply as a target organ for PM. Results from epidemiological, clinical and animal studies are converging to indicate that PM exposures, both to PM2.5 and ultrafine particles, have adverse cardiovascular effects. Although research has provided some leads on potential mechanisms underlying these effects, uncertainty remains regarding the role of copollutants. It is becoming more evident from clinical and toxicological studies that ambient fine PM induces respiratory and cardiovascular events that in susceptible, compromised people can explain the morbidity and mortality observed in epidemiological studies. Research has documented that components of hypothesized mechanistic sequences do actually take place, supplying a biological basis for explaining some effects of PM observed in susceptible subpopulations. Despite substantial research since 1997, uncertainties remain concerning potential mechanisms underlying the various adverse effects of PM observed in epidemiological studies. In part, these uncertainties reflect the limitations of extending findings from toxicological studies that by necessity use small sample sizes to human studies in which large populations are studied to detect effects. Efforts to apply toxicological findings include the challenges of extrapolating from in vitro systems and high-dose animal experiments at concentrations sometimes orders of magnitude greater than those received by people from ambient exposures. There is also uncertainty about mechanistic observations based on nonphysiological exposure routes, 13 Summary such as intratracheal instillation. The next step is to more clearly understand mechanisms underlying exposure-dose-response relationships, recognizing that it is likely that most mechanisms will have some element of exposure (dose) dependence. The findings from the clinical, animal, and in vitro experimental work have often not addressed exposure-dose-response relationships, which may provide critical insights into the relevance of the experimental findings for interpreting epidemiological research. In addition, similar physiological, cellular, and molecular responses to PM in different species help to provide a mechanistic underpinning to the epidemiological observations. Another major uncertainty relates to the lack of understanding of the relationships between the mechanisms responsible for acute versus chronic health effects. As the focus shifts to findings from epidemiological studies on chronic health effects, a similar shift will be required of the mechanistic studies. At present, it is unclear how the mechanisms characterized in the acute health-effects studies relate to the mechanisms underlying chronic health effects. Research Topic 10. Analysis and Measurement In its previous reports, the committee outlined several methodological issues needing further study. The issues included the sensitivity of findings in epidemiological studies to the statistical models used, the consequences of measurement error in epidemiological studies, and the analytical approaches to addressing the degree of life shortening associated with PM exposures. Since 1997, several new statistical methods have been introduced to analyze the temporal association between air quality measures and health. These models have been used to examine large national databases and to pool evidence across multiple locations. Although the committee's previous reports had found substantial progress related to this topic, recent findings on the sensitivity of time-series results to modeling approaches are an indication that further methodological research is needed. After the committee's third report, a problem was recognized in the application of a widely used statistical package in time-series analyses, resulting in extensive reanalyses of several studies that were central to characterizing the short-term risks of air pollution. These reanalyses yielded somewhat lower estimates of the effect of PM on risk for hospitalization and mortality. Nonetheless, time-series studies are likely to remain important for estimating the health effects of air pollution on populations, and a more complete 14 Research Priorities for Airborne Particulate Matter understanding of the implications of modeling approaches is needed. In adddition, the issue of "harvesting" or mortality displacement2 needs further investigation. It would also be important to understand any relationship between the results of the times-series studies and the cohort studies. A framework was also developed to address measurement error)the difference between the actual exposure and the measured exposures of individuals in a study. The framework implies that measurement error per se will not create the positive associations found between air pollution and health effects. More precise estimates of the magnitudes and statistical distributions of measurement error need to be incorporated into multipollutant models to provide more reliable quantitative estimates of the impact of measurement error and of the relative importance of various pollutants on health impacts. Greater consideration of this issue would give more credence to risk assessments used to support regulatory decisions. LOOKING ACROSS THE RESEARCH TOPICS As the context for PM research has evolved, five cross-cutting issues have emerged. An Increasing Number of Adverse Health Outcomes Associated with PM and the Related Susceptible Subpopulations Research results under the topics of outdoor measures versus actual human exposures, dosimetry, combined effects of PM and gaseous pollutants, susceptible subpopulations, and mechanisms of injury indicate a broadening scope of health concerns since the committee's 1998 report. At that time, emphasis was largely placed on total morbidity and mortality from respiratory causes, such as exacerbation of chronic respiratory diseases, including COPD and asthma, and the respiratory health of children. Subsequently, the list of particle-related health outcomes has broadened and now includes several adverse cardiac outcomes, such as changes in heart rate variability, cardiac arrhythmias, ischemic events, and congestive heart 2 The terms "harvesting" or "mortality displacement" refer to questions of whether deaths from air pollution occur in people who are highly susceptible and near death (and die a few days earlier because of air pollution than they otherwise would have) or whether air pollution leads to the death of people who are not otherwise near death. Summary 15 failure, as well as reproductive outcomes. Health-outcome research has also focused on many susceptible subpopulations, such as those with preexisting cardiopulmonary illnesses, children, and older adults. Although findings on several of these outcomes remain preliminary and inconsistent, interest has grown in investigating these outcomes and exploring new ones. Particle Toxicity in Relation to Different Particle Characteristics and Emission-Source Types Ambient particles contain a large spectrum of individual compounds. Research findings from the Supersites Program and other atmospheric characterization studies have elegantly demonstrated the complexity of ambient particle characteristics. Research to assess hazardous PM components seeks to understand the comparative toxicity of particles in relation to their specific characteristics (for example, size or composition). Information needed to relate particle characteristics to their potential health risk remains largely incomplete, and the committee views this as a critical gap. Identifying toxicity-determining components of PM is the cornerstone for moving from strategies directed at particles generally to those directed at particular sources of more toxic particles. Such information is helpful for the development of effective controls on emission sources. Increasing Emphasis on Exposure-Dose-Response Relationships Emphasis needs to be shifted from research directed primarily at the question of whether particles are causing particular health effects (that is, hazard identification) to characterizing exposure-dose-response relationships (that is, the form of the quantitative relationship between exposure and risk for an outcome). Knowledge of exposure-dose-response relationships is important in establishing the NAAQS for PM and implementing effective control strategies. This quantitative understanding can guide decisionmaking, offering a foundation for estimating the burden of morbidity and mortality caused by particles and comparing the benefits of alternative scenarios of air quality management. The design of research directed toward characterization of exposure-dose-response relationships may differ from that directed toward hazard identification. In particular, information on exposure-dose-response relationships is needed across a range of particle exposures relevant to those received by people at contemporary concentra- 16 Research Priorities for Airborne Particulate Matter tions. The incorporation of approaches that will allow examination of exposure-dose-response relationships for in vitro systems and laboratory animal experiments will be critical in further research evaluating the comparative toxicity (potency) of PM components. Considering PM Health Effects within the Broader Context of Other Pollutants in the Ambient Air For some time, investigators have recognized that surrogates for "dirty air" have been derived from assessing one pollutant at a time, and possible health effects attributed to a single pollutant have often been used in part to make regulatory decisions. However, in recent years, scientists and regulators have been concerned that, except in specific occupational settings, humans are exposed to air pollutant mixtures. In addition, the recognition of a broader range of health impacts as being putatively related to ambient pollution raises the possibility that single pollutants may be acting together to increase risk through interactive mechanisms. Research findings on the combined effects of particles and gaseous copollutants, susceptible populations, mechanisms of injury, and human exposures have implications that extend beyond PM. The finding of interactions between PM and the other five criteria pollutants listed in the Clean Air Act could provide a rationale for more integrated standards reflecting realistic atmospheric mixture exposures to populations at risk and reflecting the potential for overall mixture toxicity. Designing PM Research Programs to Inform Most Effectively the Setting and Implementing of the PM NAAQS Important PM research needs have implications for the setting of the PM NAAQS. Research results on exposure assessment, emissions characterization, development and testing of air quality models, assessment of hazardous PM components, assessment of the combined effects of PM and gaseous pollutants, and understanding the mechanisms of injury may have significant impacts on the four elements of the NAAQS for PM, namely, the pollutant indicator, averaging time, numerical level of the indicator, and the statistical form of the standard. Measurements of source emission rates for PM and precursor gases as well as evaluations of models are needed as states begin the process of developing NAAQS attainment plans. 17 Summary CHALLLENGES FOR THE YEARS AHEAD In 1998, the committee recognized that meeting its research agenda would require a substantial investment as well as development of new research approaches to address complex scientific questions. In reviewing work carried out since its first report, the committee has identified seven remaining scientific challenges that should be of highest priority in further work to complete the PM research agenda. These challenges are central to the PM research agenda; meeting them may require the development of new research approaches and research management strategies. Completing PM Emissions Inventory and Air Quality Models Necessary for NAAQS Implementation Although the committee's charge is to provide independent guidance for planning and monitoring a long-term PM research program, it recognizes that this research program will also provide the tools needed for implementation of current and possible future PM NAAQS. In particular, improved emissions characterization and air quality model testing and development are critical for state implementation plans (SIPs) that detail steps that states and local air districts will use to attain the PM2.5 NAAQS. Although some positive steps have been taken by EPA, rapidly approaching SIP deadlines require faster progress to ensure that these plans are based on the best available information. Research to improve emission inventories and air quality models will also support more targeted emission-control strategies as progress is made in characterizing hazardous PM components. The specificity of the PM NAAQS on the solid phase of ambient pollutants poses significant scientific challenges for source-test methods. The partitioning of many substances between gas and particle phases is sensitive to environmental parameters, such as temperature and humidity, which typically differ greatly between effluent and ambient conditions. Emission-test methods have historically paid limited attention to such distinctions, thereby yielding particulate emission rates of uncertain relevance to ambient conditions. In its second report, the committee stressed the need for developing new dilution-based source-test methods that simulate ambient conditions and for systematically applying these to a wide array of sources. Implementation of PM NAAQS will require comprehensive national and local emission inventories of PM mass emission rates, composition, and size distributions, along with emission rates of reactive 18 Research Priorities for Airborne Particulate Matter precursor gases (sulfur dioxide, oxides of nitrogen, ammonia, and volatile organic compounds). These inventories will in turn require measurements relevant to ambient conditions as a representative sample of an individual source type. Understanding the relationships between emissions and ambient concentrations is also central to regulatory decisionmaking. The committee is concerned that implementation of emission controls to attain PM standards may rely on untested air quality models. There appears to be a tendency to view model development as a research activity that can be decoupled from the regulatory support functions of ambient monitoring and emissions tracking. However, emissions, ambient concentrations, and source-receptor relationships are closely interrelated in certain situations (as when a known source dominates ambient loadings); in such circumstances, there is an opportunity for cross-calibration and verification. The three activities of emissions tracking, air quality modeling, and ambient monitoring need to be viewed as elements of an integrated process. The committee recommends that the EPA PM program better acknowledge these interrelationships to promote iterative improvements in emission inventories, monitoring networks, and air quality models. Developing a Systematic Program to Assess Toxicity of PM Mixture Components This pivotal topic carries implications not only for research directions but also for PM-control strategies. Answering the key questions concerning hazardous components of PM will require a carefully coordinated, longterm multidisciplinary research effort that goes well beyond the work now under way. Although substantial relevant research has already been carried out, the committee's review showed a diffuse assemblage of evidence with little convergence. Also, the use of disparate approaches to assess hazardous PM components by investigators in the diverse research communities has been a barrier to interpreting the resulting evidence. The research strategy for addressing this topic should be reconsidered, moving from solely investigator-initiated approaches toward more carefully structured ones. A more systematic evaluation of PM by characteristics, components, and health outcomes should be combined with investigatorinitiated research that may follow resultant leads or focus more deeply on specific hypotheses. There is also a need for coherent and converging evidence from exposure and atmospheric characterization and from toxicological and epidemiological research that addresses specific components 19 Summary and health outcomes in parallel. Such integrative research would facilitate judging the extent to which toxicological observations, particularly those made at higher doses, can be generalized. This type of orchestrated programmatic research might be conducted by EPA, through the resources of the EPA PM research centers, or through other research mechanisms. Barriers to implementing integrated research include scientific and administrative challenges. There is also a need to foster cross-disciplinary communication and collaboration. The costs for the needed research may substantially exceed those originally estimated for this topic by the committee, but these costs should not be a basis for deferring needed research, given the importance of the air pollution control topic. Enhancing Air Quality Monitoring for Research Meeting the key PM research priorities will require a shift in the current air monitoring paradigm, from primarily assessing compliance with the NAAQS toward serving multiple purposes, such as air quality forecasting, episode alerts, exposure characterization in populations at high risk, health studies, atmospheric process studies, evaluating emission-source zones of influence, and evaluating long-term effectiveness of control strategies. This shift implies less use of federal reference methods at urban locations and greater use of in situ continuous monitors and compoundspecific integrated samples at locations representing background, boundary, transport corridor, regional, urban, and neighborhood spatial distributions. Such an enhanced network should use continuous measures of appropriate indicators with real-time access, attempt to represent less-uniform micro- and middle-scale exposures, and encourage the completion of development of continuous monitors for indicators other than mass concentrations. A carefully designed network serving multiple purposes could be accomplished using the same resources currently dedicated to compliance monitoring that would have much greater public-health and knowledgecreation benefit than the current filter-based network. Investigating the Health Effects of Long-Term Exposure to Air Pollution Long-term epidemiological studies are likely to remain central in assessing the public-health burden caused by air pollution. For quantitative risk assessment and cost-benefit analysis, estimates of the disease burden associated with exposure to particles are needed, and research approaches 20 Research Priorities for Airborne Particulate Matter should continue to be developed on the basis of existing and new cohorts. Mechanisms are needed for enrollment and tracking of cohorts, including their exposures and health outcomes, that would provide an ongoing characterization of any impact of long-term exposure to air pollution on health. It is not yet clear what role chronic exposure studies (that is, encompassing most or all of the life span) of animals could play in predicting human health effects or in enhancing the understanding of the mechanisms and exposure-dose-response relationships of long-term exposure to PM. A likely application of toxicological research to understanding the consequences of long-term human exposures to PM lies in the use of studies incorporating repeated exposures ranging from several days to a few months. Intermediate-term studies could help characterize long-term study needs and designs. Improved Toxicological Approaches The committee has previously recognized the need for complementary epidemiological and toxicological evidence. Toxicological approaches have been limited by the difficulty of replicating real-world inhalation exposures to PM in terms of chemistry, by the relatively high doses often used in animal studies, by the common use of instillation rather than inhalation of the particles, and by the inability to readily replicate human diseases associated with increased susceptibility. Using toxicological approaches to assess hazardous PM components has also proved to be especially challenging. Separating the potential effects of particle size from those of particle chemistry is difficult, because particles of different size might have different chemical characteristics and different rates or routes of clearance, which affect responses. Studies of appropriate design are needed, as are wellcharacterized particle samples for experimental exposures. From a Particulate Matter Research Program to a Multipollutant Research Program Another challenge to completing the committee's research agenda lies in the scientifically artificial separation of research on PM from research on air pollution generally. This separation follows the regulatory approach of setting ambient standards for six individual criteria pollutants and corresponding emission standards on precursors without adequate recognition of 21 Summary their interrelationships in determining their risk to health. Given the need to develop the evidence base for the NAAQS, research has too often been driven by a schedule reflecting the regulatory cycle of NAAQS review and by the scope of research being restricted to single pollutants rather than the air pollution mixture. Although the PM research agenda has appropriately considered other pollutants, it has of necessity been directed at PM. Researchers have considered some other air contaminants, largely other criteria pollutants, as "copollutants" for their potential impact on PM effects. This research tests whether PM effects depend on the presence or concentrations of other pollutants without considering the overall toxicity of the air pollution mixture, as the characteristics of the mixture vary. For the future, there is an opportunity and a critical need to shift from the current single-pollutant focus to a multipollutant focus that begins by acknowledging that realworld exposures involve complex mixtures of hundreds of air contaminants falling into several physical-chemical classes. A multiple air pollutant program can be carried forward in a manner that reflects the need to develop information relevant to the setting of standards and the development of air quality management strategies. It is important that research priorities be carefully established in such a research program. One potential payoff of a multipollutant science-based approach is the high likelihood that the resulting information will aid in understanding the relative importance of various pollutants (and thus sources) and their interactions in causing adverse health effects. This knowledge should have clear benefit in optimizing both the cost-effectiveness and the health benefits of future air quality management strategies. Integrating Across Disciplines The need for complementary evidence on PM from multiple disciplines was recognized early by the committee. It called for interdisciplinary research and proposed the PM research centers as one mechanism for fostering interdisciplinary collaboration. Although greater cross-disciplinary integration has occurred in some PM research topics, to a large extent the coordination of toxicological, epidemiological, exposure, and atmospheric research has received more discussion than actual implementation to date. Expanding multidisciplinary strategies and programs will be essential to implement a multipollutant approach. At a minimum, such efforts should include the following: 22 Research Priorities for Airborne Particulate Matter • Active collaborative research design. • A shift by funding agencies toward giving higher priority to research implemented by truly multidisciplinary teams. • Adequate research funding for projects to allow the active involvement of a full team (including senior investigators from multiple disciplines, if needed). • Fellowships or sabbaticals that will enable scientists to collaborate with groups outside their disciplines. • Redoubled efforts of appropriate professional societies to hold joint workshops and meetings and to publish proceedings. MANAGING SCIENCE TO ADDRESS THE KEY REMAINING QUESTIONS Progress to date on several of the committee's priority research topics is encouraging and demonstrates that key uncertainties can be quickly addressed with targeted research initiatives, as in the example of research on outdoor measures of airborne PM versus actual human exposure (research topic 1). The committee has been impressed by EPA's quick incorporation of the PM research portfolio into its planning and its integrated approach to managing PM research within the agency. It is not surprising that much research remains to be done from the committee's original research agenda, as detailed in this report, and new research findings always invite new questions. Beyond continuing to seek answers to those specific questions, the committee has identified some recent progress to address the seven challenges discussed above (for example, implementation of the nationwide speciation monitoring network), but those seven issues need careful attention as the PM research program continues. By addressing these challenges directly, the pace of scientific gain should be quickened and the quality of research evidence strengthened. These challenges also pose significant challenges to the management of scientific research. To address them effectively, a series of steps should be taken to effectively manage this complex scientific enterprise: First, an even higher level of sustained integration and interaction will be needed to successfully complete the research portfolio. This step must be taken among the scientific disciplines and among the full range of public and private research funding organizations. Second, much stronger tools will be necessary to compile and synthesize Summary 23 the large amounts of new information being developed in this research program. Third, and perhaps most important, sustained and substantially enhanced management of this program by EPA, accompanied by a continuing mechanism for independent review and oversight of the program, will be the only way to ensure that this investment is being soundly made. EPA has taken steps toward better management, but recent frequent transitions in management personnel of that effort and a substantial need for new management systems and administrative mechanisms for supporting research—especially on the topic of assessment of hazardous PM components—suggest that EPA will need to enhance its efforts. Also, it will be important to develop some form of continuing independent oversight to provide continued monitoring and guidance for EPA's and others' efforts. Much has been learned from the research investment since 1998, and evidence gained by the investment is already being used in decisionmaking, which will continue even in the face of uncertainty. However, much is still to be learned. A failure to invest in developing greater understanding of the effects of PM and air pollution, in general, on health would result in not taking full advantage of the substantial research investment to date and in limiting the nation's ability to make evidence-based health policy and air quality regulatory choices in the future. Alternatively, continued enhancement of the air pollution and health research effort will undoubtedly yield substantial benefits for years to come. It is clearly the latter choice that offers the most promise to the nation in its effort to improve air quality and public health. 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 (:m) 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 sizeselective inlet that has a 50% collection efficiency for particles with an aerodynamic diameter of 10 :m. PM2.5 is similarly defined except with reference to a 2.5:m size cut. Total suspended particles (TSP) refers to the particle mass collected up to a range of 25-50 :m aerodynamic diameter. 24 Introduction 25 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. 26 Sources of Airborne Particulate Matter or Gaseous Precursor Emissions Particulate Matter in Ambient (Outdoor) Air Mechanisms determining emissions, chemical transformation (including formation of secondary particles from gaseous precursors), and transport in air Personal Exposure Human time-activity patterns, indoor (or microenvironmental) sources and sinks of particulate matter Dose to Target Tissues Deposition, clearance, retention and disposition of particulate matter presented to an individual Human Health Response Mechanisms of damage and repair 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 1983, 1994; Lioy 1990; and Sexton et al. 1992. Introduction 27 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. 28 Research Priorities for Airborne Particulate Matter 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 Introduction 29 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. 30 Research Priorities for Airborne Particulate Matter 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. 31 Introduction 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 :m in aerodynamic diameter, called ultrafines. • PM2.5: particles less than about 2.5 :m in aerodynamic diameter, called fine particles. • PM10: particles less than about 10 :m in aerodynamic diameter. • PM10-2.5: particles between 2.5 and 10 :m in aerodynamic diameter, called coarse particles. • PM10+: particles greater than 10 :m 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 32 Research Priorities for Airborne Particulate Matter 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 :m or 2.5 :m diameters. As Figure 1-4 indicates, observed particle diameters range over several orders of magnitude, with those below about 0.05 :m typically dominating particle number in urban air, those below about 0.5 :m dominating particle surface area, and those at about 10 :m dominating particle volume (or mass). The existing distinctions at 10 :m and 2.5 :m have dosimetric significance and hint at generic categories of origin, because combustion-generated particles tend to be smaller than 2.5 :m 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. 33 Introduction d 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, 34 FIGURE 1-3 Positive and negative ion mass spectra obtained by aerosol time-of-flight mass spectrometry (ATOFMS) for typical individual particles in 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 reaction of sea salt with gas-phase nitrogen oxides and sulfuric acid. (d) Calcium-rich dust particle. (See also Figure 6 in Whiteaker et al. [2002].) Source: Whiteaker et al. 2002. Reprinted with permission; copyright 2002, American Chemical Society. FIGURE 1-4 Average particle size distribution representative of Los Angeles smog during August-September 1969. The distribution is plotted as a fraction of total particle number, total particle surface, and total particle volume. Dp refers to the diameter of a particle. Source: Adapted from Whitby et al. 1972. 35 36 Research Priorities for Airborne Particulate Matter 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." 37 Introduction 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 inte