3
UPDATING THE RESEARCH PORTFOLIO

The committee intends its portfolio of PM research recommendations to be a dynamic document that will be updated and revised as research results are obtained and changing circumstances warrant. In this chapter, the committee updates and discusses further the ten high-priority research topics presented in its first report (NRC 1998). Most of the recommendations remain substantially unchanged, but research topics 3 and 4 are revised and renamed because of recent developments and further consideration of the current and planned emissions characterization, air-quality model development, and ambient monitoring activities of EPA and other agencies and organizations.

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 potentially susceptible subpopulations and individuals?

Background

As discussed in the committee's first report, currently available information is not sufficient to characterize the relationships of ambient



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Research Priorities for Airborne Particulate Matter: • II •, Evaluating Research Progress and Updating the Portfolio 3 UPDATING THE RESEARCH PORTFOLIO The committee intends its portfolio of PM research recommendations to be a dynamic document that will be updated and revised as research results are obtained and changing circumstances warrant. In this chapter, the committee updates and discusses further the ten high-priority research topics presented in its first report (NRC 1998). Most of the recommendations remain substantially unchanged, but research topics 3 and 4 are revised and renamed because of recent developments and further consideration of the current and planned emissions characterization, air-quality model development, and ambient monitoring activities of EPA and other agencies and organizations. 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 potentially susceptible subpopulations and individuals? Background As discussed in the committee's first report, currently available information is not sufficient to characterize the relationships of ambient

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Research Priorities for Airborne Particulate Matter: • II •, Evaluating Research Progress and Updating the Portfolio air concentrations of PM and gases to actual human exposures that include indoor environments. Most people spend the majority of their time indoors exposed to a mixture of particles that penetrate from outdoors or are generated indoors. Personal exposures to certain air pollutants have consistently been found to differ from estimates based on corresponding outdoor concentrations. The differences are largely due to the variable contributions of outdoor air to indoor environments, the indoor fate of outdoor contaminants, and the substantial contribution of indoor sources and sinks to total personal PM exposures (Lioy et al. 1990). Studies have found that a significant fraction (50–90%) of small indoor particles have outdoor origins (Koutrakis et al. 1992; Clayton et al. 1993; Thomas et al. 1993). Once indoors, particles may deposit on surfaces, or they can be altered through volatilization, as with ammonium nitrate, or through reactions with other pollutants present indoors, as with neutralization of sulfuric acid by ammonia. Particles are further generated by myriad indoor particle sources, including cooking, resuspension, cleaning, tobacco smoking, pets, insects, and molds. The emission rates of most indoor-particle sources, however, have not been adequately quantified. Furthermore, factors that affect the contribution of outdoor particles to indoor concentrations have not been well characterized. In its first report, the committee concluded that information is needed on relationships between particulate matter in outdoor air and personal exposure to particulate matter, especially for subpopulations that may be susceptible to the effects of PM exposures, such as the elderly, individuals with respiratory or cardiovascular disease, and children. Hypothesis-driven exposure studies must be designed to provide fundamental information on actual human breathing-zone exposures to PM and gaseous pollutants (NRC 1991). The recommended studies should be used to determine the exposure metrics that are most suitable for establishing exposure-response relationships. To attain these goals, the following specific research activities were recommended in the committee's first report: Field studies that quantify the contributions to personal exposures to PM and gaseous pollutants attributed to outdoor ambient air and to the penetration of ambient air indoors.

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Research Priorities for Airborne Particulate Matter: • II •, Evaluating Research Progress and Updating the Portfolio Longitudinal panel studies, in which groups of individuals are studied at successive points in time, to examine interpersonal and intrapersonal variability, as well as seasonal and temporal variability in PM exposure. Analyses of information collected from such field and longitudinal studies to determine the contributions of outdoor versus indoor sources for each pollutant, and to examine the degree to which the use of more accurate exposure information would, or would not, alter the findings of epidemiological time-series studies concerning particulate matter and adverse health effects. The sampling of particulate matter should include measurements of PM2.5 and PM10. as well as other relevant descriptors of particulate matter that might be of value in understanding the impact of particulate matter on human health. The design and execution of a typical panel exposure-assessment study will take approximately 3 years for subject recruitment, field measurements, collection of time-activity data, and data analysis. This research should include potentially susceptible populations in various geographical locations. Update At the committee's June 1998 meeting, Dr. Judith Graham of EPA's Office of Research and Development (ORD) presented a particulate-exposure research plan initiated by EPA in response to the committee's first report. That plan includes substantial changes in EPA's allocation of research resources that the committee finds highly commendable. The program will bring about new in-house and extramural research projects and related personnel changes that are highly consistent with the committee's recommendations. Much of the plan has been implemented and has begun to yield results. Overall, these recent efforts by EPA to support human exposure-assessment research for particulate matter are substantive and promising. In budget terms, Congress increased the resources devoted to research in this area from $3.6 million in the President's proposed budget

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Research Priorities for Airborne Particulate Matter: • II •, Evaluating Research Progress and Updating the Portfolio for Fiscal Year 1999 to $8.2 million in 1999 appropriations, and EPA has proposed a budget of $7.9 million for Fiscal Year 2000 (see Table 1.4). Studies conducted by EPA scientists in EPA laboratories will characterize microenvironmental particulate exposures to provide information for the development of exposure models. They will characterize and assess human exposures to fine particles to enhance understanding of sources of these particles and their spatial and temporal profiles. Using an experimental model home, EPA scientists will also investigate processes governing the penetration of outdoor particles into indoor environments and their deposition onto indoor surfaces to determine the contribution of outdoor sources to indoor and personal exposures. EPA is establishing cooperative agreements with universities to characterize the exposures of sensitive subpopulations to particulate matter and related gaseous pollutants. Under these cooperative agreements, studies will be conducted in several urban environments that are characterized by different particle composition and meteorological conditions, including New York, Boston, Atlanta, Los Angeles, and Seattle. The main objectives of these studies are (1) to characterize the personal particulate and gaseous exposures of sensitive as well as healthy individuals; (2) to identify factors affecting such exposures and their corresponding personal exposure versus outdoor concentration relationships; (3) to develop models to predict individual exposures and population exposures to fine-particle mass; and (4) to determine the contribution of outdoor and indoor sources to personal particulate exposures. Under one of the cooperative agreements, studies will also determine the correlations between personal particulate and gaseous exposures. In each of the cooperative agreements, a large number of personal, indoor, and outdoor measurements will be made in winter and summer for several susceptible subpopulations, including children, senior citizens, asthmatics, and individuals with chronic obstructive pulmonary disease and myocardial infractions. Together, these studies will provide a powerful data base that can be used to characterize particulate exposures for susceptible populations, to determine inter-and intrapersonal variability in particulate exposures, and to quantify and statistically characterize the measurement error that results from exposures

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Research Priorities for Airborne Particulate Matter: • II •, Evaluating Research Progress and Updating the Portfolio estimated from single outdoor monitoring sites. These findings will be useful for the interpretation of results from epidemiological studies and for the accurate determination of exposure-response relationships. In addition to the mass-based PM2.5 monitoring program being implemented for regulatory compliance assessment, EPA's air regulatory program office is planning to develop two additional particle-monitoring programs: a chemical-speciation monitoring program and a supersites program. The Primary objective of the speciation-monitoring program is to measure particle composition to aid states and EPA to develop and evaluate particle control strategies. The main objective of the supersites program is to develop a better scientific understanding of source-receptor relationships through intensive monitoring of several representative airsheds using state-of-the-art sampling and analytical techniques. In response to the committee's first report and other considerations, EPA convened expert panels to provide design guidance for both of the additional monitoring programs. Both panels included exposure scientists who contributed to the final monitoring design. It is anticipated that these monitoring programs will produce data that may be used by future particle-exposure studies, because the networks will monitor the spatial and temporal variability of particle mass and its components in several locations throughout the United States. The committee also urged the integration of exposure and health-effects studies into the supersites program. The committee believes that stronger interactions between the atmospheric-modeling and health-science communities are needed to ensure that the programs will be of significant value. EPA's National Exposure Research Laboratory is hiring additional exposure-assessment experts who will conduct and oversee related research activities. These experts will be assisted by scientists within EPA's exposure and engineering laboratories who have been assigned to work on particle-exposure projects. Further support will be provided by several postdoctoral fellows who have recently joined EPA to conduct particle-related research. In addition, EPA has begun to improve coordination of its exposure research activities with those conducted by other organizations such as the Health Effects Institute (HEI), Electric Power Research Institute

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Research Priorities for Airborne Particulate Matter: • II •, Evaluating Research Progress and Updating the Portfolio (EPRI), and American Petroleum Institute (API). This coordination will help to ensure that cost-effective research is being conducted to address all particulate exposure research needs. The committee believes that the EPA efforts described above will make it possible to address most of the exposure research needs included in Research Topic 1 of the committee's first report. The exposure-assessment research recommendations in the committee's first report placed initial emphasis on needs for short-term measurements of personal exposure to be compared with outdoor exposures to particulate matter in a cross section of locations, using sensitive subgroups and healthy members of the general population. The next set of exposure measurements needed will be concurrent long-term studies of population exposure to particulate matter and copollutants. The design and resources required will be dependent upon the results of the near term research described in the first report. Of particular interest will be the results obtained to define personal exposure to particulate matter and its constituents and to identify biologically active agents and mechanisms of action. 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 ? Background In its first report, the committee recommended a group of studies to begin about the year 2001, as information improves on PM constituents and as specific chemical constituents or particle-size fractions are indicated as plausible causal agents. These studies should quantify exposures to those constituents for the general public and susceptible subpopulations. The process of obtaining information on such exposures will be iterative, with information developed from earlier studies

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Research Priorities for Airborne Particulate Matter: • II •, Evaluating Research Progress and Updating the Portfolio guiding the planning of later studies. Population-based field studies will provide information on the distribution and intensity of exposure of the general population to experimentally defined particle components and size fractions. The studies should be conducted for statistically representative groups of the general population and for potentially susceptible subgroups. The studies could be coupled to health-outcome investigations, but they should be designed to determine the extent to which members of the population contact these biologically important constituents and size fractions of concern in outdoor air, outdoor air that has penetrated indoors, and air pollutants generated indoors. In its first report, the committee recommended that the following specific research tasks be addressed in Research Topic 2, after obtaining and interpreting results of studies and information from Research Topic 1: Measure population exposures to the most biologically important constituents and size fractions of particulate matter. These exposure studies should include members of the general population and potentially susceptible subgroups, using personal-monitoring studies and ambient stationary sites to examine the contributions of outdoor pollutants to total personal exposure. Refine sampling and analysis tools to permit their routine application for the determination of biologically important chemical constituents and size ranges of particulate matter. Some of the recommended population-exposure studies could be initiated soon, but a more targeted set of studies under this research topic should await a better understanding of the physical, chemical, and biological properties of airborne particles associated with the reported mortality and morbidity outcomes. The exposure research should then be conducted expeditiously to inform decisions on source-reduction strategies. Focusing on the specific toxicity-determining attributes of particulate matter will be essential to make population-exposure studies cost-effective.

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Research Priorities for Airborne Particulate Matter: • II •, Evaluating Research Progress and Updating the Portfolio Update Research Topic 2 of the committee's first report called for a more comprehensive approach to particle-exposure assessment, emphasizing the need for characterization of particulate exposures of large populations. Most of the research activities included in this topic are not expected to commence until about 2001, when information from toxicological studies becomes available. However, the committee recommends that it would be beneficial to start small-scale studies immediately to develop new tools and information to design the new generation of exposure research projects. Examples of such studies include the following: Comprehensive intercomparison study of personal samplers—Intercomparison studies of personal samplers are necessary to evaluate the performance of personal sampling devices and to ensure that data sets from different exposure studies are comparable. Temporal-variability studies of personal exposures—Studies examining temporal variability in personal exposures will enable better information to be obtained about the duration and frequency of human exposures. The National Ambient Air Quality Standard (NAAQS) for PM2.5 was based on 24-hour time intervals because epidemiological studies examined the exposure-effect association using 24-hour ambient particle concentrations and health measurements. However, future toxicological investigations and future epidemiological studies may provide a better scientific basis for choosing the appropriate time interval of the exposure. Exposure misclassification studies—As mentioned previously, several exposure studies will examine the relationship between personal exposures and outdoor concentrations. Will these data be sufficient to investigate the effect of exposure misclassification? Do we need to perform additional studies to provide complementary data sets? To make this decision, epidemiologists and exposure scientists will need to work together to develop the analytical framework for this research. Care should be taken to define the

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Research Priorities for Airborne Particulate Matter: • II •, Evaluating Research Progress and Updating the Portfolio spatial and temporal variability of outdoor, indoor, and personal exposure concentrations. Pilot studies characterizing the physicochemical characteristics of pollutants to which individuals are exposed—Pilot projects investigating the physicochemical characteristics of pollutants to which individuals are exposed will make it possible to identify factors affecting exposures to different particle components and sizes. This is necessary for the design of the future population-exposure studies. Such pilot studies should include development of sampling and analytical methods for biological-particle components and refinement of questionnaires for selection of representative populations. Individual and population-exposure modeling studies—The development of individual-and population-exposure models will allow personal exposures to be characterized for large study populations without the need for expensive personal monitoring. These models will likely use information on human activity patterns and microenvironmental concentrations or characteristics to predict short-and long-term exposures. REFOCUSING RESEARCH TOPICS 3 AND 4 Sound strategies to manage PM risks will require an improved understanding of relationships among the sources of particulate matter, the atmospheric processes that transport and transform it, and the concentrations of particulate matter to which populations are exposed. Computer models based on such understanding will provide state and local air-quality agencies with the tools necessary to develop state implementation plans that control emissions in ways that (1) focus on the most biologically active and relevant components of the PM mixture, and (2) identify cost-effective strategies for reducing the exposure of the general population to those emissions. In preparing this second report, the committee reviewed current and

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Research Priorities for Airborne Particulate Matter: • II •, Evaluating Research Progress and Updating the Portfolio planned emission-source characterization, model development, and ambient monitoring activities of EPA, other federal, state, and local agencies, the research community, and the private sector. The committee received briefings from the CENR Air Quality Subcommittee, NARSTO, and EPA's Office of Air and Radiation (OAR) on plans for speciation monitoring and supersite monitoring. Based on that information, the committee has refocused and renamed topics 3 and 4 of the research portfolio to integrate more closely these implementation-related research activities, including the development and evaluation of emission-source characterization techniques (see revised Research Topic 3), and source-oriented and receptor-oriented air-quality models (see revised Research Topic 4). The attainment of national PM risk-reduction goals will require active collaboration (see Figure 3.1) among the research community, OAR and ORD of EPA, other federal, state, and local agencies, and the private sector. Research efforts will be needed to develop and evaluate source-measurement techniques and source-oriented and receptor-oriented models, as described below in revised topics 3 and 4. However, the ultimate success of these research efforts will depend in large part on the collection of key data on particle and precursor concentrations and composition, regardless of whether those data are collected by research organizations, federal and state regulatory programs, or others. Specifically, these efforts will require Collection of emissions data for major categories of sources, and development of the speciated and size-resolved emission inventories necessary to operate and evaluate particle models. Collection, through compliance, speciation, and supersite monitoring programs, of the meteorological and air-quality data that will be necessary to evaluate the performance of particle models. The paradigm presented in Figure 3.1 calls for a synchronization of activities and extensive collaboration among atmospheric, exposure, and health scientists, as well as among federal, state, and private research and control programs. Efforts by health-effects and exposure-

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Research Priorities for Airborne Particulate Matter: • II •, Evaluating Research Progress and Updating the Portfolio Figure 3.1 A paradigm for scientific and technical activities related to particulate matter.

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Research Priorities for Airborne Particulate Matter: • II •, Evaluating Research Progress and Updating the Portfolio on the effects of particulate matter in susceptible subpopulations. The committee recommends that EPA explore greater collaboration with other federal agencies and private research organizations to leverage resources and maximize ongoing research opportunities. EPA's work must take into account opportunities offered by such major studies as the Women's Health Initiative, the EPRI Veterans' Study, Harvard studies of medical personnel, inner-city asthma studies in various areas, and studies by large health-management organizations (including health maintenance organizations). A strategy containing short-and long-term components of the PM research program is needed to ensure the effective identification of health effects among susceptible subpopulations as well as general population cohorts, and to identify the most appropriate and cost-effective means to address the effects. The com mittee recommends that the strategy include Updating the inventory of large-cohort, health-research studies with relevant co-variables. Identifying what health outcomes need to be addressed. Identifying and prioritizing what susceptible subpopulations still need to be studied. Identifying what research needs, as discussed above and in the committee's first report, will not be addressed by ongoing studies. Identifying potential collaborators with access to cohorts in selected cities (e.g., Los Angeles and Atlanta). RESEARCH TOPIC 9: MECHANISMS OF INJURY What are the underlying mechanisms (local pulmonary and systemic) that can explain the epidemiological findings of mortality/morbidity associated with exposure to ambient particulate matter?

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Research Priorities for Airborne Particulate Matter: • II •, Evaluating Research Progress and Updating the Portfolio Background As discussed in the committee's first report, this is another critical area of research. The value of epidemiological studies will be greatly enhanced if results obtained in controlled exposure studies can provide plausible explanations of underlying biological mechanisms for health effects. This research is needed to provide an understanding of local pulmonary and systemic responses. Although research in several other areas will also contribute to elucidation of pathophysiological mechanisms of particle-induced injuries (see research topics 5–8), this section focuses on the use of clinical, animal, and in vitro models to evaluate such mechanisms. Clinical studies are controlled experimental exposures of humans to a substance. When possible, such studies with laboratory-generated surrogate particles or concentrated ambient air particles are an approach of choice. The use of human subjects avoids the need to extrapolate from other species. Human-exposure studies use laboratory atmospheric conditions that can be relevant for ambient pollutant concentrations, and they document physiological responses resulting from exposure that can often be linked to health effects. In these studies, highly controlled environments or well-characterized CAPS can be used to identify health responses to individual pollutants and to characterize exposure-response relationships. In addition, a controlled environment provides the opportunity to examine toxicological interactions among pollutants or with other variables, such as exercise. In laboratory-animal studies, animal models are used as surrogates for humans. The necessity and urgency of developing and validating appropriate animal models of susceptible human subpopulations remains a critical area of research. The use of such animal models is essential for characterizing potential adverse effects of inhaled particles, as well as for determining specific mechanisms that seem to be operating only in the susceptible organism. Among the different animal models that mimic human diseases, the usefulness of transgenic animals should be evaluated to investigate specific mechanistic hypotheses. In vitro studies can be used to evaluate some specific mechanistic

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Research Priorities for Airborne Particulate Matter: • II •, Evaluating Research Progress and Updating the Portfolio hypotheses related to health effects from exposure to particles. As reviewed in the committee's first report, the use of relevant target cells, as well as the use of particle doses comparable to exposure scenarios encountered in vivo, is crucial. Although initially higher dose levels may have to be tested, mechanisms that operate at high-particulate doses may be irrelevant for low doses. For example, it is known that high exposure concentrations to low-toxicity particles lead to particle overload, impaired clearance, and specific lesions, such as fibrosis and tumors not seen at lower concentrations. Thus, extrapolation from in vitro data and mechanisms elucidated from the use of high doses may be flawed unless it can be shown that health responses observed following exposure to low doses do indeed follow the same mechanistic pathways. It is also critical that all cell and tissue models be validated; the issue of whether responses observed in these in vitro models reflect those that would occur in vivo is also critical. Update In response to the committee's first report, which recommended increased work in this research area, EPA and Congress increased the resources devoted to this research from $4.3 million in the President's budget for Fiscal Year 1999 to $8.3 million in 1999 appropriations, and EPA has proposed $6.8 million in Fiscal Year 2000 (see Table 1.4). To evaluate particle-induced effects, clinical studies are being conducted or planned in healthy volunteers and individuals with underlying cardiopulmonary diseases, such as asthma, chronic obstructive pulmonary disease, and angina. A variety of techniques to assess airway inflammation will be used, including bronchoalveolar lavage, sputum induction, and measurement of nitric oxide in exhaled air, as well as assessment of systemic effects by measurement of bloodborne biomarkers of effects and exposure. Assessment of cardiac rhythm and the coagulation cascade will also be studied. These studies will provide important information on early physiological responses from exposures to particles and will potentially provide important information on mechanisms of injury related to acute and chronic health effects. Emphasis has increased on using animal models of diseases that are

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Research Priorities for Airborne Particulate Matter: • II •, Evaluating Research Progress and Updating the Portfolio thought likely to increase human susceptibility to inhaled particulate matter. There is now a broad recognition that research based solely on the use of young, healthy animals is not likely to lead to an adequate understanding of health risks to susceptible human subpopulations. Several genetic and experimentally induced laboratory models of emphysema, asthma, inflammation, infection, aging, and cardiovascular disease are now being used in PM research. Animals are also being used to explore the influence of age on susceptibility. Because all existing animal models have limitations in their ability to fully mimic human conditions, continued effort is needed to make adequate progress in developing and evaluating animal models of the full range of human conditions of interest. The development of advanced molecular biology techniques makes it possible to employ methods such as in vitro transfection models for evaluating specific mechanisms of particle-cell interactions. Also useful for evaluating specific mechanisms are ex vivo studies in which specific cells of the respiratory tract are isolated after in vivo exposures of animals, and then subsequent in vitro studies are performed. Careful consideration must be given to the extent to which the animal models represent the human conditions being modeled. For example, interspecies differences in dose to critical tissues, organ-level physiology, and genetics must be considered when judging the degree to which the animal models mimic human doses and responses. Few, if any, animal models accurately represent all features of a human disease, so consideration must be given to determine which features of the disease are modeled by the animal-test systems. Understanding mechanisms of toxicity remains a high-priority research area, because the primary goal is to provide biological plausibility for the epidemiological findings related to ambient particulate matter. Although the overall funding planned by EPA in this area approaches the recommendations of the committee's first report, it is not clear to the committee how the funds will be allocated among clinical, animal, and in vitro studies. As discussed under Research Topic 3 in the committee's first report, research is needed to develop advanced analytical methods to monitor responses to toxic components of particulate matter. Such research will involve the use of animal models and human subjects. In its first

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Research Priorities for Airborne Particulate Matter: • II •, Evaluating Research Progress and Updating the Portfolio report, the committee estimated the cost of this research at $1.5 million per year for 3 years beginning in 2001. However, the committee has decided to move that research from Research Topic 3 to Subtopic 9c to consolidate similar activities together. 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? Background In its first report, the committee noted that a persistent source of the uncertainty associated with epidemiological studies on the health effects of particulate matter can be tied to questions about the statistical methods used to analyze the data and to the inherent errors associated with variables in the analyses. Several alternative methods have been used in these analyses, and the influence of those methods on the results and conclusions have not been fully understood. In addition, the observational data used in the studies are subject to measurement error. The influence of any methodological approach on the results of an analysis must be understood. There is also value in determining whether results and conclusions are robust compared with alternative methods. In addition, measurement error can have extensive influence on the results of an analysis. It can bias the estimates of association and dose-response between pollution and health end points; the extent of such bias needs to be assessed and methods need to be found to correct for it.

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Research Priorities for Airborne Particulate Matter: • II •, Evaluating Research Progress and Updating the Portfolio Update The above issues will be amplified in new studies that consider exposures to many pollutants. The inclusion of many pollutants, some of which are correlated with each other, are creating a situation in which it will be difficult to determine the specific pollutants most highly associated with a health outcome. Differences in measurement error compound the difficulties introduced by this situation. As we achieve greater understanding of the biological basis of health effects of particulates, it will be important to factor this understanding into models and statistical analyses. The current generation of models includes very flexible and powerful tools that can incorporate complex relationships between variables. The challenge is to articulate these relationships and then incorporate them in data analyses. In epidemiological studies of the relationship between particles and health, an individual's exposure to particles is estimated most often by measurements taken at an ambient monitor. Rarely are measurements of personal exposure available. The difference between actual exposures and measured ambient-air concentrations can be considered as measurement error. Measurement error for air-pollution exposure has three significant components: instrument error (the accuracy and precision of the monitoring instrument), error resulting from nonrepresentativeness of a monitoring site (reflected by spatial variability of the pollutant measured), and differences between the average personal exposure of a pollutant and the monitored level. Several efforts are under way to characterize the difference between personal exposures and ambient monitored levels of pollution variables (see Research Topic 1). Those efforts are addressing several populations in several regions. Data are especially needed on human subpopulations thought to be susceptible to air pollution. The distribution of differences between personal exposures and monitored ambient levels must be described. As the number of variables increase, this task will become more difficult. Differences in the underlying attributes of measurement error across population subgroups should be characterized. Differences in error distribution across subgroups could affect causal interpretations.

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Research Priorities for Airborne Particulate Matter: • II •, Evaluating Research Progress and Updating the Portfolio For most of the criteria pollutants, data are available to characterize spatial variability—another source of measurement error. However, for many particulate subspecies that we are only beginning to measure, these data are not available. For other species, some efforts to characterize the variability are under way. Errors associated with instrument accuracy and precision also need to be characterized. This could be problematic for some older measurements that are based upon measurement instruments and technologies no longer used. Research is under way to characterize the consequences of measurement error. Some of these efforts have focused upon measurement errors with very specific properties. The measurement error is assumed to be random (independent of the true measurement), and the errors are assumed to be independent and identically distributed with a symmetric distribution. Recent research efforts have applied statistical methods to consider multiple pollutants in the same analysis and have examined the sensitivity of model results to assumptions made in times-series models. In addition, two workshops have been held to address these issues. HEI held a workshop on measurement error, and EPA convened a workshop to discuss broad methodological issues. In addition, methods are being developed and applied to estimate the extent to which the timing of death may be advanced by pollution exposure (''mortality displacement" and "harvesting"). Methods (metaanalysis and hierarchical analysis) are also being studied to combine the results from several similar studies and to determine when such methods are appropriate. Several methodological issues need to be addressed further. Systematic investigations of alternative methods and models would provide greater insights on the robustness of results. Alternative valid methods should be systematically applied to a few data sets to indicate the potential sensitivity of results to alternative methods. Data must be collected to ensure that we can adequately characterize the nature and distribution of significant errors for independent variables used in statistical models and that data on the errors are used to make adjustments. Studies with detailed environmental data (e.g., the supersites program) present additional methodological challenges. Finally, as

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Research Priorities for Airborne Particulate Matter: • II •, Evaluating Research Progress and Updating the Portfolio biological understanding improves on the relationships between pollution and health, models must incorporate this understanding. THE UPDATED RESEARCH PORTFOLIO As discussed earlier, the committee is continuing to evaluate PM research needs and accordingly to update its research investment portfolio (see Table 3.1). Specifically, the research activities and resource estimates for research topics 2, 3, 4, and 9 have been revised since the first report. Over the 11-year span of the updated portfolio, these revisions increase the total estimated cost of the research from $357.1 million to $369.9 million, increasing the average from $32.5 million to $33.6 million per year. Research Topic 2 has been divided into two subtopics to distinguish between exposure methods development (Subtopic 2a) and exposure studies (Subtopic 2b). In the first report, Research Topic 3 contained three subtopics that addressed the development of advanced mathematical, modeling, and monitoring tools to represent source-receptor relationships more accurately. Research Topic 4 addressed the research efforts needed to apply these methods and models to link biologically important components of particulate matter to their sources and to efficient air-quality management. In updating and refining its research portfolio, the committee has reconfigured the activities in research topics 3 and 4 and expanded some of the resource estimates to cover the implementation-related research and data collection that are not expected to be conducted by regulatory program efforts. Research Topic 3 has no subtopics in the revised portfolio and is focused on research and development for methods to characterize emission sources. These activities are estimated to cost $2.5 million per year for 4 years. However, that amount does not include testing of the most important source types because that activity is viewed as part of the ongoing source surveillance program needed for implementation of the regulatory program. Such testing is represented in Table 3.2.

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Research Priorities for Airborne Particulate Matter: • II •, Evaluating Research Progress and Updating the Portfolio TABLE 3.1 The Committee's Updated Research Investment Portfolio for Fiscal Year 2000–2010: Timing and Estimated Costs* ($ million/year in 1998 dollars) of Recommended Research on Particulate Matter   2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 SOURCE/CONCENTRATION/EXPOSURE 1. Outdoor vs. human exposure 3.0                     2. Exposure to toxic PM components 2a. Methods 1.0                     2b. Studies   4.0 4.0 4.0 4.0 4.0           3. Emission sources 2.5 2.5 2.5 2.5               4. Models 4a. Source oriented** 4.5 4.5 4.5 4.5 4.5 4.5 4.5         4b. Receptor oriented 1.0 1.0 1.0                 EXPOSURE/DOSE-RESPONSE 5. Assessment of hazardous PM components 5a. Toxicological and clinical studies 8.0 8.0 8.0                 5b. Epidemiology 1.0 6.0 6.0 6.0 6.0 6.0 6.0 6.0 6.0 6.0 6.0 6. Dosimetry 1.5 1.5                   7. Effects of PM and copollutants 7a. Copollutants (toxicology) 4.0 4.0 4.0 4.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 7b. Copollutants/long term (epidemiology) 6.0 6.0 6.0 6.0 6.0 6.0 6.0 6.0 6.0 3.0 3.0 8. Susceptible subpopulations 3.0 3.0 3.0 3.0 3.0 3.0           9. Toxicity mechanisms 9a. Animal models 3.0 3.0 3.0 3.0               9b. In vitro studies 3.0 3.0 3.0 3.0               9c. Human clinical 3.5 3.5 3.5 3.5               ANALYSIS AND MEASUREMENT 10a. Statistical analysis 1.0 1.0 1.0 1.0               10b. Measurement error 1.5 3.0 2.0 2.0               SUBTOTALS ($ MILLION PER YEAR) 47.5 54.0 51.5 42.5 28.5 28.5 21.5 17.0 17.0 14.0 14.0 RESEARCH MANAGEMENT*** (ESTIMATED AT 10%) 4.8 5.4 5.2 4.3 2.9 2.9 2.2 1.7 1.7 1.4 1.4 TOTAL$ ($ MILLION PER YEAR) 52.3 59.4 56.7 46.8 31.4 31.4 23.7 18.7 18.7 15.4 15.4 * The committee's rough but informed collective-judgment cost estimates for the highest-priority research activities recommended in this report. See Chapter 3 of this report and Chapter 4 of NRC, 1998 for explanations. These estimates should not be interpreted as a recommended total particulate-matter research budget for EPA or the nation, for reasons explained in NRC 1998. ** These estimates are in addition to costs for EPA's supersite program and expansion of the nationwide speciation network, as well as likely expenditures by states, local agencies, and industries for source-emissions inventories and field-measurement campaigns in support of model evaluation studies (see Table 3.2). *** Research management includes research planning, budgeting, oversight, review, and dissemination, cumulatively estimated by the committee at 10% of project costs.

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Research Priorities for Airborne Particulate Matter: • II •, Evaluating Research Progress and Updating the Portfolio TABLE 3.2 The Committee's Technical Support Estimates: Timing and Estimated Costsa ($ million/year in 1998 dollars) of additional technical work needed for implementation of emissions control programs for airborne particles   2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 ACTIVITY 1. Source testing by regulatory programs     5.0 5.0 5.0 5.0 5.0         2. Compilation of interim PM emission inventory 1.0 1.0 1.0 1.0               3. Compilation of PM emission inventory based on results of new source information             1.0 1.0 1.0     4. Field studies in support of air quality model evaluation and testing*   20.0 20.0 20.0 20.0 20.0           TOTALS ($ MILLION PER YEAR) 1.0 21.0 26.0 26.0 25.0 25.0 6.0 1.0 1.0     * Technical support expenditures by all public and private sponsoring organizations.

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Research Priorities for Airborne Particulate Matter: • II •, Evaluating Research Progress and Updating the Portfolio In this report, Research Topic 4 has been divided into Subtopic 4a, which addresses the development and evaluation of source-oriented models, and Subtopic 4b, which addresses receptor-oriented models. Because EPA monitoring is not expected to provide adequate data for model evaluation, the committee has increased its resource estimate for source-oriented models from the original estimate in its first report. Subtopic 4b shows the same amount of resources for receptor-oriented model development that was presented in Research Topic 3 of the first report. Research Topic 9 retains the three subtopics from the first report. However, Subtopic 9c ("Human Clinical") was expanded by reallocating, from Research Topic 3 of the first report, $1.5 million per year for 3 years beginning in 2001 for the development of advanced analytic methods for monitoring biological responses to toxic components of particulate matter. It is important to recognize that many parts of the research effort will continue to depend heavily upon data developed in technical programs maintained by the government in areas that fall outside the scope of the research activities recommended by the committee. Examples of such activities are testing of emissions sources, compilation of emissions inventories, and much of the collection of ambient data to support testing and evaluation of air-quality models. These technical programs may be carried out by government regulatory or research programs at the federal, state, or local level, or by nongovernmental organizations (see Table 3.2).