Report

THE EPA’S PARTICULATE MATTER (PM)
HEALTH EFFECTS RESEARCH CENTERS PROGRAM

A Mid-Course (2 1/2 year) Report of Status, Progress, and Plans

Prepared for: Public Review by EPA’s Science Advisory Board (SAB)

February 11 and 12, 2002, Washington, DC

Prepared by: Directors and Senior Associates of the PM Centers

Headquartered at:

UCLA
University of Washington
University of Rochester
New York University
Harvard University

Report Issued: January 8, 2002

[This document has not been formally reviewed by EPA. The views expressed are solely those of the authors.]

TABLE OF CONTENTS
Cover Page	1
Table of Contents	2
Executive Summary	3
Overview Summary	17
Biological Mechanisms for PM Health Effects	23
Acute Health Effects of PM	41
Chronic Health Effects of PM	56
Dosimetry	66
Exposure Assessment	77
Why it is Critical to have a Coordinated PM Centers Program	90
Definition of Terms	99

EXECUTIVE SUMMARY

Numerous epidemiological studies have reported associations between ambient air particulate matter (PM) concentrations and a variety of health outcomes, including increased mortality, hospital admissions, emergency room and clinician visits, medication usage, and lost-time from work and school. The consistency of these findings was remarkable, considering the diverse study populations, meteorological conditions, air quality characteristics, and the variety of study designs.

Questions concerning the validity of epidemiological results focused on the use of ambient PM concentrations as population exposure surrogates and the potential confounding effects of gaseous co-pollutants. Both of these questions have now been effectively addressed, in part, by the PM Centers Program. Concerns over biological plausibility were raised due to the incomplete state of knowledge regarding: 1) appropriate human and animal models which produce biological responses at levels comparable to those reported in the epidemiological studies; 2) techniques to expose subjects to concentrated ambient particles (CAPs), or laboratory generated surrogates that focus on specific particle characteristics such as size and/or chemistry; 3) information concerning the physical and chemical properties of PM that are responsible for the reported adverse health effects and; 4) the potentially additive or synergistic effect of gaseous co-pollutant exposures on acute and chronic effects.

To address these scientific concerns, Congress directed the EPA, in 1996, to substantially increase its level of funding on PM health effects research. It also mandated that a National Research Council (NRC) Committee (i.e., the Committee on Research Priorities for Airborne Particulate Matter) be established to provide scientific oversight for the PM research. In the first of its three reports, the NRC Committee recommended a multi-year research program that included the establishment of academically-based research Centers to create a comprehensive and integrated PM health effects research program. The PM Centers were needed to foster interdisciplinary collaborations within and among institutions. Research that arose through these collaborations was, in turn, intended to help EPA address scientific issues about PM health effects in a timely and effective manner.

The EPA, through its STAR program for investigator-initiated research, issued a request for proposals (RFP) for Centers for PM Health Effects Research in 1998. It received and peer-reviewed twenty-one applications, and awarded five PM Center Grants in the summer of 1999. The successful applicants were (in alphabetical order): 1) a California Consortium, centered at UCLA; Harvard University; New York University; the University of Rochester; and the University of Washington.

The PM Centers, in their first 2½ years, have initiated research directed at the specific critical issues identified by the NRC Committee, and have also initiated collaborative activities including sponsorship of Scientific Workshops to further research in key areas, such as characterizing respiratory and cardiovascular health effects associated with PM exposures, assessing costs and health benefits of air pollution controls, examining the health impacts of gasoline emissions, and developing methods for apportioning PM sources.

Through their individual and collective activities during the initial years of the PM Centers, considerable progress has been made towards understanding ambient air PM health effects and addressing areas of remaining scientific uncertainty. Future research activities at the PM Centers will include both epidemiological and inhalation studies to enhance our understanding of the health effects of PM, with an increasing focus on long-term effects associated with chronic PM exposures. These goals are consistent with EPA’s Multiyear Plan for PM research as defined in its November 2001 presentation to the NRC Oversight Committee.

This report provides a synopsis of the research accomplishments to date, short-term goals (during the two and a half remaining years of Center support) and long-term goals (beyond initial 5 years of Center support) for the PM Health Effects Centers. This report consists of six sections. Sections 1-5 address issues relating to: 1) Biological Mechanisms; 2) Acute Effects; 3) Chronic Effects; 4) Dosimetry; and 5) Exposure Assessment. Section 6 describes the specific attributes of a coordinated PM Centers Program. Progress and plans discussed in each of these five sections is as follows:

Biological Mechanisms

A major rationale for the establishment of the PM Centers was to explore biological plausibility and mechanisms of PM-associated health outcomes. Over the past two years, substantial contributions have been made toward developing hypotheses based on experimental observations – in large part because of the interdisciplinary research environment fostered by the PM Centers’ structure. Several mechanistic pathways have been investigated that may link PM exposure with adverse health effects. The portal of entry for PM air pollution is the lung, and PM interactions with respiratory epithelium likely mediate a wide range of effects, including respiratory, systemic, and cardiovascular effects. Additionally, PM, or its reaction products, may mediate effects via stimulation of airway sensory nerves, or by translocation to non-pulmonary organs and tissues. Thus the potential mechanisms involved are considerably more complex than envisioned even a few years ago.

Mechanistic areas in which particular progress has been made by the PM Centers include: 1) inflammation and immunity; 2) mechanisms for cardiovascular effects; and 3) the role of reactive oxygen species (ROS). The presence or absence of an inflammatory response is an important issue, because inflammation may induce systemic effects, including an acute phase response with increased blood viscosity and coagulability, and possibly an increased risk for myocardial infarction in patients with severe coronary artery disease. Inflammation is a key pathophysiologic feature of chronic respiratory diseases such as asthma and COPD. Chronic, repeated inflammatory challenge of the airways, may result in airway remodeling that leads to irreversible lung disease. Thus, inflammation may be involved in both acute and chronic effects. Recent PM Center studies have shown that inhalation of PM at concentrations only slightly above peak ambient levels cause airway inflammation.

Results from PM Center clinical studies suggest that exposures to either CAPs or ultrafine particles may initiate endothelial and leukocyte activation, with shedding of intracellular adhesion molecule-1 (ICAM-1), a key initial step in leukocyte recruitment. These findings may have implications for cardiovascular and respiratory disease.

Both human and animal studies are being conducted by the PM Centers to identify mechanisms linking ambient PM to cardiovascular effects. For example, in healthy subjects exposed to ultrafine carbon particles, frequency-domain analysis of the continuous electrocardiographic recording indicated that response of the parasympathetic nervous system was blunted during recovery from exercise immediately after exposure. PM exposures were shown to alter cardiac repolarization, possibly induced either through an indirect effect via the autonomic nervous system, or by directly affecting ion channel function in ventricular myocardium through a yet unknown mechanism.

PM Center research has shown that PM generates ROS, which provide pro-inflammatory stimuli to bronchial epithelial cells and macrophages. These cellular targets respond with cytokine and chemokine production, which can enhance the response to allergens. This hypothesis is being tested using in vitro and animal studies. In one study using an allergic mouse model, animals were exposed to high doses of nebulized Diesel Exhaust Particles (DEP). DEP markedly enhanced antibody response and lipid peroxidation, with these effects being abrogated by antioxidant treatment. These findings may explain, in part, the increased number and severity of asthma attacks in urban settings associated with air pollution episodes characterized by high DEP levels.

It is likely there are major, as yet undetermined pathways leading to PM effects, and an important objective will be to identify additional mechanistic links. For example, neural pathways may play a role in mediating cardiovascular effects. A continuing research objective is to further examine the role of PM composition, size, surface area, gaseous co-pollutants and other factors in mediating effects. This objective can be accomplished, in part, by studying the in vitro toxicity of ultrafine, fine and coarse PM and its relation to their chemical composition. Finally, determining the mechanisms underlying individual susceptibility to PM effects is another major objective underway in each of the PM Centers. Host susceptibility factors being investigated include age, gender, underlying disease, infections, and genetic factors.

Acute Effects

Since the establishment of the PM Centers, substantial progress has been made in our understanding of PM health effects. Collectively, the PM Centers have addressed a large number of scientific issues regarding acute PM health effects. It has been suggested that air pollution-related deaths mainly impact already frail or sick individuals whose deaths are being brought forward by only a few days (the harvesting hypothesis). New methodologies have been developed to examine the harvesting hypothesis using moving averages of mortality and exposure data (from 1 to 45 days). Study findings show that the associations remain significant and that the relative risk increased as the averaging periods increased, suggesting that the harvesting hypothesis is false. Moreover, use of analytical methods, developed to combine smoothed exposure-response curves from multiple locations, provide evidence that the PM10-mortality relationships multiple cities in the U.S. are linear down to the lowest observed exposure concentrations, supporting the no-threshold hypothesis.

PM Center investigators have examined the relationship between mortality and source-specific PM concentrations. Initial results have shown significant associations between combustion sources and several adverse health outcomes, including mortality. In addition, new models have been developed to examine the potential confounding effect of gaseous co-pollutants. Results from studies suggest that the association between PM10 and daily deaths was not confounded by gaseous air pollutants.

PM Center studies have examined the impact of PM exposures on the elderly, children and individuals with respiratory and cardiac disease. Recently, diabetics have also been identified as an important susceptible population. In several single-city studies, the risk of PM-associated hospital admissions for heart disease for diabetics was double that for the general population.

A large number of controlled exposure studies in humans and animals have been conducted by the PM Centers to investigate the acute biological responses induced by particle exposures. These studies have shown associations between PM2.5 or ultrafine particle exposures and a variety of acute responses, providing direct evidence that PM2.5 is biologically active at current peak exposure levels. Health outcomes that may warrant further examination in epidemiological studies include heart rate, heart rate variability, QTc interval changes, arrhythmias and ischemia, blood ICAM-1 and IL-6, and oxidant stress markers.

During the next several years, we will continue to develop and apply new analytical techniques to address important scientific issues relating to harvesting, confounding, exposure-response relationships and susceptibility. The novel statistical approaches developed will be applied to additional U.S. populations for a variety of health outcomes. In addition, we will use these new methodologies to examine the relationship between acute and sub-acute effects. These investigations will, in turn, be very valuable in our efforts to quantify chronic PM health effects.

Chronic Effects

Little information is available on chronic effects associated with PM exposures. This is due to the complexity and cost of chronic studies. The Harvard Six Cities and American Cancer Society (ACS) Cohort Studies provided most of the justification for the annual PM2.5 NAAQS in 1997. As a results, they generated a number of concerns that were expressed in public comments including: 1) the validity of underlying study findings due to the lack of public access to raw data; 2) the need to consider city-specific characteristics and other alternative explanations for the observed differences in mortality rates; 3) the lack of adequate controls for the individual characteristics such as age, smoking, occupation, obesity, and socio-economic factors; and 4) the appropriateness of ambient PM measurements as surrogates of community personal exposures. A number of these issues have been addressed in the HEI Re-analysis.

Research currently conducted by the PM Centers includes:

Annual Mortality Cohort Studies: Two of the PM Centers have been engaged in continued follow-up of the original Six Cities and ACS cohorts respectively for an additional nine years. For the Six Cities cohort, PM2.5 and sulfate concentrations continued to be associated with decreased survival, increased mortality from cardiovascular and pulmonary causes, and increased lung cancer deaths. For the ACS cohort, PM2.5 and sulfate also continued to be associated with increased cardiovascular and pulmonary mortality, and lung cancer is now also significantly associated with PM2.5.
Effects of Subchronic Exposures: Analyses of the association between daily mortality and hospital admissions with PM concentrations during the preceding weeks to months have highlighted the importance of subchronic exposures. In Boston, for example, an increase in the two-day average PM2.5 of 10 mg/m³ was associated with an increase in mortality by 2.1%, while for the same increase in monthly average, mortality increased by 3.8%. In the ACS cohort study, this same increase in annual average PM2.5 was associated with a 6.8% increase in mortality.
Children’s Lung Growth Cohort Studies: Studies of PM chronic effects on childrens respiratory health have been conducted by two of the PM Centers. Results from the Children’s Health Study (CHS), for example, showed that PM2.5 is significantly associated with slower growth of lung function in children residing in communities with higher than average annual PM2.5 concentrations.

Continuous follow-up of the Six Cities and ACS study participants will assess the impact of specific PM sources on chronic mortality (longevity reduction). This will include the assessment of individual characteristics (i.e., disease state, socio-economic status, smoking, occupational exposures) that affect susceptibility. Furthermore, previously collected data from the 24 Cities and Children’s Health Studies will be pooled and analyzed to assess effects of PM sources (e.g., vehicular versus power plant) and composition (e.g., nitrate versus sulfate) on children’s health. A new epidemiological study of participants in the NHLBI Womens' Health Initiative Observational Study (93,000 women aged 50-79 yrs from 40 centers around the country) is currently underway. In prior epidemiological studies in Erfurt and Augsburg, Germany, acute effects of ultrafine particles on daily mortality and on myocardial infarction were observed. Cohorts that were established in these cities will be used to investigate the potential long-term effects of ultrafine particles.

Dosimetry

A critical link in the evaluation of the relationship between individual exposures to PM and health responses is dosimetry. Dosimetry investigates the amounts and distribution of the deposited PM in the respiratory tract, but also the pathways by which this material is translocated to other sites in both the respiratory tract and to more distal organ systems. Furthermore, knowledge of inter- and intra-species differences is critical to our effort to extrapolate results from experimental studies to the population-at-large.

To address the paucity of data regarding the PM deposition, a pilot PM Center project investigated the potential for retrieval of morphometric data from three-dimensional images of tracheobronchial airways obtained in vivo by x-ray Computerized Tomography (CT). The study also explored the potential for the use of stereolithography (STL) to produce hollow airway casts for experimental verification of particle deposition models. A volumetric rendering of the interior surface of an airway tree phantom was generated, producing a surface representation of the airway tree. These three-dimensional images were then converted to a STL file format required for the rapid prototyping of airway casts. The STL unit uses a computer controlled arm connected to a plastic extrusion device to build volumetric structures layer-by-layer. When these new technologies were applied to a CT scan of a previously used hollow lung airway cast, the replica cast made by STL was virtually identical to the original.

Figure 1. Cross sectional luminal airway surface of original hollow airway cast made from a normal human tracheobronchial tree in mm2 vs. corresponding airway surface in mm2 of a replica produced by stereolithography using data obtained by CT-scanning of the original cast.

PM Center methodology was also developed to determine total lung deposition of ultrafine particles in humans as well as of concentrated ambient particles in dogs. The methods included the use of a breath-by-breath respiratory monitoring system that provided both rate and tidal volume as well as measurement of inhaled and exhaled particle number classified by size. In the human studies, deposition of specific fractions of ambient PM are ongoing using inhaled ultrafine particles. Total respiratory deposition fractions (DF) for particle number and mass were determined. Eleven non-smoking, healthy subjects and eleven non-smoking subjects with mild asthma were exposed for two hours. Results showed that the total respiratory deposition of inhaled ultrafine particles is high in healthy subjects, and is further increased during exercise, with enhanced deposition in asthmatics. It is evident that, under conditions of increased minute ventilation, the deposited amount of ultrafine particles is substantially increased. Asthmatics may be at greater risk of being affected by inhaled ultrafine particles because of increased deposition in central airways.

Ambient particle deposition patterns observed for the canines were similar to that described in the International Commission on Radiological Protection (ICRP) human particle deposition model. Additional studies in rats revealed a material dependent difference in the disposition of ultrafine particles: Significant amounts of ultrafine carbon particles were translocated to the liver, whereas ultrafine metal (iridium as a model of insoluble material) particles were not. This raises a number of important dosimetric questions which are presently addressed by Center investigators, including translocation pathways as well as other extrapulmonary target tissues.

Future research will focus on the following topics: 1) identifying the population receiving the highest local doses in their tissues; 2) defining the appropriate particle doses for use in in vitro mechanistic studies; 3) collecting data on tracheobronchial airway particle deposition patterns and efficiencies using bronchial airway casts based on in vivo CT images of human and animal lungs; 4) measuring heterogeneous deposition patterns of inhaled particles; 5) identifying vulnerable target cells in the tissues and; 6) understanding dose implications of abnormal particle deposition, impaired clearance, and sequestration of particles.

Exposure Assessment

The extent to which outdoor measurements accurately reflect PM exposures, has been the subject of considerable scientific debate. Results from early exposure studies suggested that personal PM exposures may differ substantially from outdoor concentrations due to contributions from indoor sources. Towards this end, the PM Centers have investigated the association between outdoor PM concentrations and corresponding personal exposures. Ambient PM2.5 concentrations, for example, were shown to be significant predictors of corresponding personal exposures, over time, for a cohort of healthy senior citizens. Although the strength of these associations varied by individual and season, the results suggest that for certain individuals, ambient PM2.5 concentrations are appropriate surrogates of personal PM2.5 exposures. When the subject-specific data were aggregated and analyzed together (i.e., cross-sectionally), the association between personal exposures and outdoor concentration was weaker, further highlighting the inadequacy of previous cross-sectional analytical methods for assessing true personal-ambient PM2.5 associations for time-series studies.

Relationships of PM10 and gaseous pollutant concentrations measured at various urban and sub-urban sites were examined as a function of the distance between sites. Stronger spatial correlations among sites were found for PM10, NO2 and O3 as compared to those for CO and SO2. PM Center findings are consistent with results from previous studies in showing relatively little within-city spatial variation in ambient PM2.5 concentrations in eastern U.S. locations. However, this may not be the case in other locations in the U.S. PM2.5 mass concentrations in Seattle exhibited modest, yet significant, spatial variability within a radius of 20 km. These differences were associated with both proximity to major highways and the elevation of the monitoring location. In Los Angeles, PM2.5 and PM10concentrations measured at various distances from highways (10-1000 meters) showed little spatial variability. However, particle number and black carbon concentrations decreased rapidly with distance from highways. These findings of traffic-related concentration gradients are currently being used to examine children’s health effects associated with specific PM sizes and components. Of particular importance is the effect of exposure to specific organic PM constituents, in the formation of ROS.

In addition, other exposure-related studies have included investigations of the impact of indoor PM sources on personal exposures; examination of the variability of outdoor PM penetration efficiencies; measurement of personal exposures to specific toxic PM components and; development of novel methods for the measurement of personal exposures to gaseous co-pollutants.

During the next two-to-three years, the PM Centers will: 1) complete exposure studies currently being conducted in various locations with diverse study populations, meteorological and air quality conditions; 2) examine associations between personal exposures to PM and its gaseous co-pollutants in various locations throughout the U.S.; and 3) develop models of chronic PM exposures. Chronic exposure models will be based on advances in short-term exposure models, re-examinations of historical data on air pollution, time-activity information, and indirect measures of PM exposure.

Special Attributes of PM Centers

The central question facing EPA is how to capitalize on its greatly expanded PM research program and maximize the benefits of the resource commitment. In particular, a key issue is determining the most effective approach to incorporate university-based expertise that can assist EPA in meeting its regulatory needs to protect public health. University-based scientists have enormous expertise and skills required to address complex and important scientific issues, such as the health effects of ambient particles.

Research centers, as opposed to individual investigator grants, are most beneficial where there is a well-defined set of scientific questions, such as those developed by the NRC Committee, and where the approaches needed to answer those questions cross over traditional disciplinary boundaries. Identifying the mechanisms responsible for PM health effects requires an interdisciplinary approach, utilizing the skills and background knowledge of chemists, engineers, aerosol scientists, toxicologists, epidemiologists, pulmonologists, cardiologists, immunologists, molecular biologists, statisticians, and experts in exposure and risk assessment. Each of the five PM Centers brings together such diverse disciplines in a common endeavor. In addition to these intra-Center interactions, inter-Center collaborations are also developing. These efforts are addressing knowledge gaps in many areas, such as sources, exposure, dosimetry, acute and chronic health effects, and underlying biological mechanisms.

A major benefit of EPA's PM Centers Program, over that emphasizing individual grants or contracts, is the ability to build a solid, ongoing infrastructure and to create a critical mass of faculty researchers, professional staff, postdoctoral fellows and students. This enhanced research capacity is one of the most exciting elements of the PM Center approach, since it has the greatest potential for achieving long-term contributions and relevant research findings. It also has already been a key element in the development of collaborative, multidisciplinary research because of the overall resources, equipment, support structures, and technology available for members uses. Examples include analytical chemistry, coordinated analyses of electrocardiographic recordings, exposure assessment, and PM delivery systems for controlled exposures. These are areas where enormous technical resources can be applied to a wide range of research projects. In addition, PM Centers are more readily able than individual research grants to develop collaborative workshops, conferences and other means to describe, discuss and develop research agendas. The PM Center infrastructure also provides a base for the public to seek information, answers to questions, and linkages with other investigators.

The PM Centers are well positioned to participate in the important tasks of investigating public health risks associated with exposure to air pollution. In carrying out their individual and joint research agendas, the Centers have focused on science relevant to public health and public policy. Indeed, a major motivation for the expansion of the PM research program and the establishment of the PM Centers was to reduce uncertainty in setting the NAAQS for PM10 and PM2.5. The PM Centers can address the following key policy questions: What properties of PM are responsible for health effects? Are there specific risk factors or effect modifiers for particle effects? To what particles are people exposed? What types of particles pose the greatest health risks? What effects are directly attributable to PM and what health responses may be confounded by other air pollutants?

The PM Centers have met on an annual basis since their inception. At the first two PM Center Directors' meeting, the Directors and their colleagues described research currently planned or underway within their respective PM Centers. This was a period of familiarization and was characterized by informative reports, but, except in some limited areas, little discussion of potential interaction. At the third meeting in Boston (July 2001), there was a sea change in the approach of the PM Centers to their mission. It became apparent that there needed to be greater interaction across PM Centers in an intellectual context as well as greater collaboration in a wide range of research areas. There was a clear need expressed to communicate more effectively on an ongoing basis, and to interact with EPA in a collaborative context to assist in the EPA research program more fully. Since July 2001, the PM Center Directors and selected colleagues have held conference calls on almost a weekly basis to discuss directions in research and collaborations. These discussions have resulted in development of both short and long-term research goals with inter-center collaborations as an important mechanism in achieving these goals.

OVERVIEW REPORT
December 2001
EPA-Supported Centers for Particulate Matter (PM) Health Effects Research

INTRODUCTION

Over the past fifteen years, an ever increasing number of epidemiologic studies have shown significant associations between the mass concentration of ambient air particulate matter (PM) and adverse respiratory and cardiovascular health effects. These effects include PM concentration-related excess rates of daily and annual mortality, hospital admissions, emergency room and clinician visits for respiratory and cardiac diseases, increased usage of medications, and lost time from work and school. By the mid 1990s, the evidence for these associations was sufficiently compelling for EPA to propose revised and more stringent National Ambient Air Quality Standards (NAAQS) for PM whose form and stringency were endorsed by EPA’s Clean Air Scientific Advisory Committee (CASAC), a group of external scientific advisors whose charter was established by the Clean Air Act (CAA) Amendments of 1977.

The revised PM NAAQS, promulgated in July 1997, retained, with minor modification, the previous daily maxima and annual average PM NAAQS for PM whose aerodynamic diameters were less than 10 µm (PM10). It also established new PM NAAQS for particles with aerodynamic diameters below 2.5 µm (PM2.5), since excess mortality was found to be more strongly associated with PM2.5 than with PM10.

EPA (and CASAC) acknowledged that the data base supporting the judgements that PM2.5 and PM10 exposures were likely causal factors for adverse health effects was not fully supported, by or consistent with, available knowledge of the underlying biological mechanisms. Among the likely factors for discrepancies between observations in human populations and corresponding observations in controlled animals and human studies are:

Epidemiological observations of adverse effects had largely been confined to subpopulations who may be especially susceptible because of underlying pre-existing disease or who were very young or very old. In contrast, most human clinical studies had examined effects in healthy individuals of intermediate ages due to practical and ethical considerations. Similarly, most previous animal studies have focused on healthy and younger animals and used exposures to concentrations that were much higher than those encountered in ambient air.
Toxicological studies have been limited since the most active components of PM remain a matter of speculation. Subsequent studies would need to utilize more realistic ambient mixtures e.g., concentrated ambient air particles CAPs or laboratory-generated surrogates that focus on specific particle characteristics such as particle size (e.g., ultrafines) and/or chemistry (e.g., metals, PAHs, quinones). Also, it is possible that responses may require a mixture of PM components and/or the simultaneous or sequential exposure to gaseous pollutants in the ambient air mixture (e.g., SO2, NO2, O3, CO, and volatile organic compounds).
There had been no laboratory-based toxicological studies involving chronic or even subchronic exposures to ambient air PM mixtures at concentrations at the upper end of the range of current U.S. ambient air concentrations.

As a result of the remaining scientific uncertainties, Congress directed the EPA, in 1996, to substantially increase its level of funding on particulate matter health effects research. It also mandated that a National Research Council (NRC) Committee (i.e., the Committee on Research Priorities for Airborne Particulate Matter) be established to provide scientific oversight for the PM research. In the first of its three reports, the NRC’s Committee on Research Priorities for Airborne Particulate Matter recommended a multi-year research program that included the establishment of academically-based research Centers to create a comprehensive and integrated particle health effects research program. The PM Centers were intended to foster interdisciplinary collaborations within and among institutions, with extensive experience in air pollution health effects research. Research that arose through these collaborations were, in turn, intended to help EPA address scientific issues about PM health effects in a timely and effective manner.

A California Consortium, centered at UCLA,
Harvard University,
New York University,
The University of Rochester, and
The University of Washington.
EPA, in its RFP, specified that:

1. Each Center have an external Scientific Advisory Committee to ensure that its research program addresses important knowledge gaps and is scientifically sound. Each of these Committees includes members from academic institutions, industrial companies, public interest groups, and some EPA scientists.

2. The Center Directors have an annual meeting with EPA scientists and administrators for the exchange of information, progress, and future plans, and for coordination of collaborative research.

3. Each Center have an outreach program to communicate with the public on current knowledge of air pollution health effects and of research progress being made to address knowledge gaps.

The Centers have met these requirements and have also initiated collaborative activities including sponsorship of Scientific Workshops to further research in key areas, such as characterizing cardiac health effects associated with PM exposures, assessing costs and health benefits of air pollution controls, examining the health impacts of gasoline emissions in California, and apportioning PM sources and their associated health effects.

Through their individual and collective activities during the initial years of the PM Health Effects Research Centers, considerable progress has been made towards understanding ambient air pollution health effects and addressing areas of remaining scientific uncertainty. In the future, the PM Centers will continue to conduct both epidemiological and toxicological studies to enhance our understanding of the health effects associated with ambient PM exposures. Of particular interest will be investigations of chronic PM health effects. This research objective is consistent with EPA’s Multiyear Plan for PM research as defined in its November 2001 presentation to the NRC Oversight Committee.

This report provides a synopsis of the Centers’ research accomplishments to date. Many of the research studies presented received major support from PM Center funds; while for others there was additional support from other sources, such as NIEHS, HEI, with the availability of PM Center core services and interactions with other PM Center investigators providing important supplemental support. This report consists of six sections. Sections 1-5 address issues relating to: 1) Biological Mechanisms; 2) Acute Effects; 3) Chronic Effects; 4) Dosimetry; and 5) Exposure Assessment. Section 6 summarizes the ability of the PM Centers program to harness the skills and resources of the PM Centers’ investigators for collaborative research addressing EPA’s PM NAAQS development. Each of the five key area sections begins with background summarizing the challenge faced by the PM Centers as they began their work on the Centers program, in terms of what was known and what goals were established to address key knowledge gaps. These background summaries are followed by an outline of progress that has been and is being made by PM Center investigators in resolving knowledge gaps. The report also outlines short-term goals (during the 2 1/2 remaining years of Center support) and long-term goals (beyond initial five years of Center support) for the PM Health Effects Centers.

The PM Centers’ approach is highly interactive, combining research efforts on exposure/source characterization, epidemiological and toxicological effects and biological mechanisms, summarized in Figure 1. Continuous inter-disciplinary dialogue among the PM Center scientists assures that the total breadth of PM research is covered and that new research ideas have a sound scientific basis. This is accomplished, in part, through crucial feedback and comments from the different scientific disciplines represented in the Centers (e.g. Exposure assessors provided data on specific PM constituents to toxicologists; toxicologists providing information on the relative toxicity of PM constituents to exposure assessors and epidemiologists; or in vitro toxicologists working in concert with epidemiologists, clinicians and animal toxicologists to elucidate a PM-specific response mechanism.) This kind of collegial interaction and sharing of research is an advantage of the PM Centers over individual research grants. The five research topics listed in the center section of Figure 1 are the areas jointly addressed by the five Centers to obtain results useful for EPA’s regulatory needs. Examples of our progress in these areas are provided in this document.

Figure 1. PM Center Integration of Research Disciplines.

BIOLOGICAL MECHANISMS FOR PM HEALTH EFFECTS

Background

The justifications for the 1997 PM10 and PM2.5 NAAQS were primarily based on a large and coherent epidemiological data base of significant associations between ambient air PM concentrations and excess mortality and morbidity. Although the 1996 PM Criteria Document provided some support for biological plausibility of causal links between PM and health effects, evidence from controlled human and animal exposure studies was still largely unavailable. Based on this information gap, a major rationale for the establishment of the PM Centers was to explore biological plausibility and mechanisms of PM-associated health outcomes. Over the past two years, substantial contributions have been made toward developing hypotheses based on experimental observations – in large part because of the research environment fostered by the PM Centers’ structure. The following section provides examples of how the PM Centers program has contributed to the development and testing of plausible mechanistic hypotheses for the health effects of PM.

Recently, several mechanistic pathways have been investigated that may link PM exposure with adverse health effects. Figure 1 highlights the complexity and interdependency of some of these pathways (Utell and Frampton, 2000). The portal of entry for PM air pollution is the lung, and PM interactions with respiratory epithelium likely mediate a wide range of effects, as indicated by the central oval in Figure 1. These include respiratory as well as systemic and cardiovascular effects. However, PM, or its reaction products, may stimulate airway sensory nerves, leading to changes in lung function and in autonomic tone, which influences cardiac function. Ultrafine particles, by virtue of their extremely small size, may enter pulmonary capillary blood and be rapidly transported to extrapulmonary tissues, such as liver, bone marrow, and heart, with either direct or indirect effects on organ function.

Progress Made

This section discusses mechanistic areas in which particular progress has been made by the PM Centers such as: 1) inflammation and immunity; 2) mechanisms for cardiovascular effects; and 3) the role of reactive oxygen species (ROS).

Inflammation and Immunity

Airway injury and inflammation is a well known consequence of toxic inhalation exposures. Previous studies involving animal models have shown that instillation or inhalation of particles, such as diesel exhaust particles (DEP), can cause inflammation and epithelial injury at high doses and concentrations. However, there was little evidence, prior to the PM Centers Program, that exposure to ambient concentrations of PM caused significant airway inflammation. The presence or absence of an inflammatory response is an important issue, because inflammation may induce systemic effects, including an acute phase response with increased blood viscosity and coagulability, and possibly an increased risk for myocardial infarction in patients with severe coronary artery disease. In chronic respiratory diseases, such as asthma and COPD, inflammation is a key pathophysiologic feature. Chronic, repeated inflammatory challenge of the airways, may result in airway remodeling that leads to irreversible lung disease. Thus, inflammation may be involved in both acute and chronic effects.

Listed below are several key findings from PM Center research about the role of inflammation and immunity in mediating the health effects of PM.

PM Exposure and Systemic Markers of Inflammation in Humans: The PM Centers in Southern California and Rochester have collaborated in human clinical studies, using identical crossover exposure protocols, subject recruitment criteria, and outcome measures. The California studies used concentrated ambient air particles (CAPs) at approximately 200 µg/m³, and Rochester used laboratory-generated carbonaceous ultrafine particles at two concentrations, 10 and 25 µg/m³. Preliminary findings from both of these studies provide evidence for effects on systemic markers of inflammation and leukocyte recruitment (Frampton et al., 2001; Daigle et al., submitted; Boscia et al., 2000).

In both Centers, subjects were exposed for two hours with intermittent exercise. Before and at intervals after exposure, symptom ratings, lung function testing, and phlebotomy were performed. Sputum induction was performed approximately 24 hours after exposure. In both Centers, blood was analyzed for markers of systemic inflammation, acute phase response, and blood coagulability. One such marker is soluble intercellular adhesion molecule-1 (sICAM-1), a transmembrane protein that is expressed on leukocytes and endothelial cells. sICAM-1 plays an important role in monocyte recruitment to atherosclerotic lesions and inflamed airways, where it is shed into plasma during leukocyte adhesion.

In the Southern California CAPs studies, sICAM-1 increased progressively in both healthy and asthmatic subjects (p=0.045), with the largest increase observed at 21 hours after exposure. In the Rochester ultrafine particle studies, there was no significant change in sICAM-1 in the plasma of healthy subjects, but blood monocyte expression of ICAM-1 decreased immediately following particle exposure in a concentration-response fashion compared with air exposure (See Figure 2), and the number of circulating monocytes decreased at 21 hours after exposure. These studies suggest that exposures to either CAPs or ultrafine particles may initiate endothelial and leukocyte activation, with shedding of surface ICAM-1, a key initial step in leukocyte recruitment. These findings may have implications for cardiovascular and respiratory disease. In a major cardiac epidemiological study, plasma sICAM-1 levels were predictive of future coronary events (Ridker et al., 1998).

Figure 2. Blood monocyte expression of intercellular adhesion molecule-1 (ICAM-1, CD54) after air and UFP exposure (10 and 25 µg/m3). Data are mean ± SE change from pre-exposure baseline.

PM Exposure and Inflammation in Animals: Studies in normal dogs exposed to Boston CAPs by inhalation showed increases in pulmonary inflammation by broncho-alveolar lavage and in circulating blood neutrophils associated with increases in specific ambient particle components. In these experiments, mean exposure doses were 203.4 and 360.8 µg/m³ in the lavage and blood studies, respectively (Clarke et al., 2000). These PM Center studies show that ambient particle components have significant pulmonary and systemic inflammatory potential.

Effects of Aging: Determining the mechanisms involved in increased susceptibility to PM comprises another goal of PM Center research which is being explored at many levels. The role of aging is being examined, using animal and human exposure studies as well as in vitro models. Recently, in vitro models have provided important new insights into the role of aging at the cellular level in determining PM responses. Studies were conducted examining cytokine production by alveolar macrophages from aged rats and mice (>20 months old), after in vitro exposure to lipopolysaccharide (LPS) and PM. Macrophages isolated from aged animals were incubated with endotoxin, with laboratory-generated mixed carbon/iron ultrafine particles, or with both LPS and ultrafine particles. Baseline production of cytokines was elevated 30-50% in aged cells compared to cells from young (8-10 weeks) animals. The response to LPS was enhanced at every dose in aged cells. The response to ultrafine particles containing iron was enhanced 2-3 fold in aged cells compared to young cells. Most significantly, in the aged animals co-administration of ultrafine particles and LPS led to synergistic effects at the lowest ultrafine particle dose, while in young cells this was observed only at the highest PM dose (Finkelstein et al., submitted). These findings suggest that there is a cellular basis for age-related increased susceptibility that may relate to increased susceptibility to oxidative stress. Alternatively, the results may be showing a lower threshold due to impaired protective mechanisms (e.g., antioxidant defenses). Investigations of these age-related differences are currently focusing on signal transduction mechanisms.

Effects on Infection/Pneumonia: Epidemiological studies have demonstrated that infection, specifically pneumonia, contributes substantially to the increased morbidity and mortality among elderly individuals following exposure to PM (Shwartz, 1994), suggesting that inhaled PM can act as an immunosuppressive factor that undermines normal host pulmonary immune responses. A combination of particle concentrator technology and animal infectivity models is being used to investigate this hypothesis. A single 5 hour inhalation exposure of bacterially-infected rats to New York City (NYC) CAPs, at concentrations ranging from 65 to 150 µg/m³, altered both pulmonary and systemic immunity, and exacerbated the infection process in a time-dependent manner (Zelikoff et al., 1999). Streptococcus pneumoniae-infected rats exposed to PM demonstrated increased burdens of pulmonary bacteria, numbers of circulating white blood cells, extent of pneumococcal-associated lung lesions, and incidence of bacteremia, compared to air-exposed, infected control rats. Conversely, this same PM exposure resulted in decreased levels of lavageable polymorphonuclear neutrophils (PMN), bronchus-associated lymphoid tissue, and proinflammatory cytokines (i.e., tumor necrosis factor-a, interleukin-1, and interluekin-6) in infected rats. Subsequent studies implicated the iron content of NYC PM in mediating these effects; many of the findings were reproduced with nose-only exposure to soluble iron, but not with soluble forms of other metals (manganese, copper, or nickel). These findings suggest that PM exposure, and specifically the soluble iron component, may affect the host immune response during pulmonary infection, and may help to explain epidemiological observations. In addition to the effects of iron, PM Center investigators have shown that Diesel engine exhaust particles (DEP) and CAPs induce apoptosis in macrophages by an oxidative stress mechanism that is dependent on the organic chemicals in the PM. Macrophage apoptosis will lead to decreased phagocytic defenses in the lung.

Cardiovascular Effects

Determining the mechanisms linking ambient PM to cardiovascular effects is one of the key challenges of the PM Centers. There is growing clinical and epidemiological evidence that ambient air pollution can precipitate acute cardiac events, such as angina pectoris, cardiac arrhythmias, and myocardial infarction, with the majority of excess PM-related deaths attributable to cardiovascular disease. The PM Centers approach to this issue is multifaceted and multidisciplinary. There are ongoing panel studies of susceptible subjects involving cardiovascular monitoring at three PM Centers, animal exposure studies at four PM Centers, and human clinical studies at three PM Centers. For example, three PM Centers share a cardiac monitoring and analysis protocol for human clinical studies.

A major step forward in this area was made with the convening of a workshop “Cardiovascular Effects of Air Pollution: Potential Mechanisms and Methods of Testing”, which met in Rochester in March, 2001. The Workshop featured presentations by PM Center and other investigators, along with clinical and research cardiologists. Each of the five EPA-supported PM Centers participated in the Workshop along with representatives from EPA. New hypotheses and research directions were developed, and practical issues of cardiac monitoring methodology currently in use at each PM Center were reviewed and optimized. This Workshop grew directly from one of the annual PM Center Directors meetings, where the need for inter-center collaboration on this issue was identified. The interchange of ideas served as an important turning point in our thinking about the mechanisms involved in cardiac effects, and new collaborative efforts were initiated. This would not have happened without the PM Center structure.

Key observations in both human and animal studies of cardiovascular effects have been made since the PM Center Program was initiated. A few examples follow.

Human Studies: Investigation of cardiovascular effects of PM has required multidisciplinary collaboration. For human exposure studies, analysis of cardiac monitoring includes a detailed analysis of heart rate variability (HRV) and repolarization intervals before, during, and for a period of 48 hours after exposure. In one study, healthy subjects were exposed by mouthpiece for two hours with intermittent exercise on three separate occasions to air or ultrafine carbon particles at 10 and 25 µg/m³. Frequency-domain analysis of the continuously recorded electrocardiogram indicated that response of the parasympathetic nervous system was blunted during recovery from exercise immediately after exposure to ultrafine particles, compared with air. This diminished vagal response was not observed 3.5 hours later. Monitoring also indicated that exposure to ultrafine particles altered cardiac repolarization, as indicated by the corrected QT interval (QTc) on the cardiogram. Figure 3 shows the change in QTc from the pre-exposure baseline. The increase in QTc following exercise during air exposure was blunted with PM exposure, and this persisted to at least 21 hours after exposure. This change in repolarization was not explained by changes in heart rate (Frampton, 2001; Zareba et al., 2001). It is plausible that ultrafine particle exposure imposes an effect on repolarization, either through an indirect effect via the autonomic nervous system, or by directly affecting ion channel function in ventricular myocardium through a yet unknown mechanism. This observation is important, because changes in the QT interval have been implicated in susceptibility to cardiac arrhythmias in patients with heart disease.

These human clinical studies are complemented by a major panel study involving patients with pre-existing coronary artery disease in Erfurt, Germany. Analysis of the electrocardiogram (EKG) recordings and blood parameters are underway, including detailed analyses of heart rate variability and repolarization, and acute phase proteins using methodology identical to the human clinical studies. These clinical/epidemiological/toxicological collaborations are an example of how the PM Centers Program has fostered research among diverse disciplines and locales.

Figure 3. Cardiac repolarization duration using Bazett's correction for heart rate (QTc interval), after air and UFP exposure (10 and 25 µg/m3). Data are mean ± SE change from pre-exposure baseline.

Animal Studies: Rats and mice are being instrumented for continuous cardiac and blood pressure monitoring. Algorithms have been developed for analysis of heart rate and blood pressure variability using continuous 24-hour recordings in up to 8 rodents simultaneously (Couderc et al., submitted). Presently, a crossover study with aging, spontaneously hypertensive rats exposed to carbonaceous ultrafine particles is ongoing.

PM Center investigators are evaluating both inflammation and cardiovascular effects in animal models. The hypothesis being tested is that inhaled PM causes release of inflammatory mediators from cells in the lung that then become blood borne and target the cardiovascular system. The research plan uses transgenic mouse strains with specific cardiovascular genetic alterations in order to create susceptibility models. Initial studies have been using a mouse model of atherosclerosis, the apolipoprotein E-deficient mouse generated via a targeted disruption of the mouse apo-E gene. The deficiency of apo-E leads to a spontaneous hypercholesterolemia. The nonhypertensive animals form atherosclerotic lesions throughout the vasculature which resemble, in part, human atherosclerotic lesions at 3 to 5 months of age. Animals are instrumented and monitored for blood pressure and heart rate using radiotelemetry. Individual mice were dosed with 125 µg of Washington, DC urban dust in 50 µl saline by oropharyngeal aspiration into the lungs. Heart rate was reduced after exposure to PM in normal and in apoE-/- mice. ApoE-/- mice showed a trend to increased blood pressure and increased variability of blood pressure after PM exposure that did not differ significantly from that of the normal mice (Luchtel et al., 2002). Additional experiments are being performed as the control and apoE-/- mice age, and studies involving more realistic exposures are being planned.

In another PM Center study, 18 month old male Fischer 344 rats with implanted EKG transmitters were used to determine the effects of PM on the frequency of spontaneous arrhythmias. Since old rats were found to have many spontaneous arrhythmias, a standardized definition for each type of arrhythmia was developed and a procedure for quantifying the frequency of spontaneous arrhythmias was established. Rats were exposed to New York City CAPs or filtered air for 4 hours. The rats were exposed twice with a crossover design so each rat could serve as its own control. The CAP concentration was 160 µg/m³ and 200 µg/m³ for the first and second exposures, respectively. EKG tracings demonstrated a significant increase in the frequency of supraventricular arrhythmias following exposure to CAPs compared to filtered air-exposed control animals. The same rats were also exposed twice to 1 ppm SO2 and twice to air in a repeated crossover design. No significant change in the frequency of any category of spontaneous arrhythmia following exposure to SO2 (or filtered air) was observed (Nadziejko et al., 2002).

The effects of PM on myocardial ischemia has also been the focus of PM Center research. The extent to which inhaled PM exacerbates ischemia in a clinically relevant model of coronary artery occlusion in conscious dogs has been quantified. For the ischemia studies, dogs undergo thoracotomy for implantation of a vascular occluder around the left anterior descending coronary artery and tracheostomy to facilitate particulate exposure. After recovery, pairs of dogs are exposed by inhalation for 6 hr/day on 4 consecutive days. Immediately following each exposure, each dog undergoes a 5-min preconditioning coronary artery occlusion followed 20 min later by the 5-min experimental study coronary artery occlusion. Peak ST-segment elevation, heart rate, and arrhythmia incidence during occlusion are determined from continuous EKG recordings. Exposure to CAPs increased peak ST-segment elevation during a 5-min coronary artery occlusion (Figure 4). Neither heart rate nor arrhythmia incidence was affected by CAPs exposure. The finding that PM exacerbates acute myocardial ischemia suggests that vascular responses and pathophysiological events leading to changes in vessels may be key mechanisms by which PM may trigger acute cardiac events (Wellenius et al., submitted). PM Center research has also studied the effects of combinations of CAPs and CO in the rat model of myocardial infarction. In other studies of the vascular response to inhaled particles, a variety of cell and molecular biologic methods have been used.

Figure 4. Changes in peak ST-segment elevation in canine acute myocardial ischemia studies following CAPs or sham.

Reactive Oxygen Species (ROS)

It is likely that more than one biological mechanism is involved in the health effects of PM. However, a major finding has been that PM generate ROS, which provide pro-inflammatory stimuli to bronchial epithelial cells and macrophages. These cellular targets respond with cytokine and chemokine production, which can enhance the response to allergens. PM may therefore act as an adjuvant that strengthens the response of the immune system to environmental allergens. Hallmarks of allergic inflammation include increased immunoglobin E (IgE) production, eosinophilic bronchial inflammation, airway hyperresponsiveness, and an increase of NO in exhaled air.

Figure 5. Thiol antioxidant interferes in the adjuvant effects of DEP during ovalbumin sensitization in a murine allergic inflammation model.

This hypothesis is being tested using in vitro and animal studies (Ning et al., 2002; Whitekus et al., 2002). In one study using an allergic mouse model, animals were exposed at high doses to nebulized DEP (2000 mg/m³)for 1 hour, followed by nebulized antigen (ovalbumin, OVA) for 20 minutes, daily for 10 days. A control group received saline instead of DEP followed by OVA. To determine the role of reactive oxygen species, the same exposure groups were pretreated every day with intraperitoneal N-acetyl cysteine (NAC) (320 mg/kg). Control animals received saline intraperitoneally. Two days after the last exposure, blood was obtained and assayed for IgE and IgG1, and the lungs were assayed for carbonyl proteins and lipid peroxides. DEP markedly enhanced the antibody response (Figure 5) and lipid peroxidation, and these effects were abrogated by antioxidant treatment. Follow-up studies using more realistic exposure levels of DEP and/or CAPs are being planned. Furthermore, human nasal challenge studies confirmed the role of DEP as an adjuvant in already established allergic responses, as well as in exposure to neo-allergens. Taken together, these findings may explain the increased number and severity of asthma attacks in an urban setting after a surge in PM levels, and may implicate DEP as a factor in asthma exacerbations.

Reactive oxygen species associated with exposure to PM may play a role in cardiovascular effects. 9,10-Phenanthroquinone (9,10-PQ) is a potent inhibitor of neuronal form of nitric oxide synthase (NOS). 9,10-PQ also inhibits the endothelial form of NOS which plays a critical role in vascular tone, thereby causing the suppression of NO-dependent vasorelaxation of aorta and significant increase in blood pressure in rats. Therefore, quinones and other compounds producing ROS, e.g., nitro-PAHs may contribute to diseases related to vascular dysfunction caused by exposure to urban air particles. In addition to the production of ROS, quinones, PAHs, nitro-PAHs, and related compounds may also undergo electrophilic addition to macromolecules producing complementary toxicity.

The key role of the PM Centers in facilitating this line of investigation on ROS has been the collaboration between scientists with expertise in particle physics (e.g., particle concentrators), animal asthma models, the cellular biology of oxidative stress and inflammation, and inhalation toxicology. For example, the collective expertise in the Southern California PM center facilitated the use of CAPs to replace DEP for mechanistic in vitro and in vivo studies. Asthma animal models are now being used to compare the pro-oxidative and pro-inflammatory effects of CAPs collected on California freeways and various source-receptor sites. In addition, human panel studies and CAPs exposure studies now typically include the role of oxidative stress in airway inflammation (e.g., assays for NO and CO content in the expired air, and measures of cytokines in induced sputum, blood, and breath condensate).

Another important development by the PM Centers is collaboration between organic/analytical chemists, particle engineers and biologists in exploring how chemicals constituents of CAPs contribute to ROS generation and inflammation. An important observation has been that organic components present in the organic carbon fraction generate ROS through their ability to undergo redox cycling. The in vitro reactions correlate well with the ability of organic PM components to generate oxidative stress in epithelial cells and macrophages. Preliminary evidence indicates that polycyclic aromatic hydrocarbons and their oxidized derivatives (quinones and ketones) play a key role in ROS generation at the cellular level. The in vitro toxicity studies predict a hierarchical or stratified oxidative stress response, in which the biological effects range from: (a) protective (e.g., expression of anti-oxidant enzymes) — (b) pro-inflammatory (e.g., production of cytokines and chemokines) — (c) cytotoxic (e.g., cellular apoptosis and necrosis), depending on the level of oxidative stress. The ability to relate the inherent redox-cycling and oxidative stress capabilities of a PM sample to specific biological effects allows a more rational interpretation of the in vivo toxicity data being generated in the community, freeway and source/receptor studies.

Short-Term PM Research Goals

Objectives of the PM Centers during the remaining funding period will include a greater utilization of inter-Center collaborations. In addition to the collaborative efforts described in this section, new interactive Center initiatives will elucidate mechanisms of PM effects. These may also include sharing of PM collected from various sources at different PM Centers for use in animal and in vitro studies, sharing of exposure technology, and developing common laboratory protocols.

It is likely there are major, as yet undetermined pathways leading to PM effects, and an important objective will be to identify additional mechanistic links. For example, neural pathways may play a role in mediating cardiovascular effects. If studies that are currently underway confirm that there are major PM effects on autonomic regulation of cardiovascular function, we will need to further define the mechanisms by which those effects are initiated, including the cells and cell mediators responsible. Research on these mechanisms will provide vital clues about individual susceptibility and potential approaches to the prevention of adverse health effects.

Animal studies based on the heirarchical or stratified oxidative stress model are being planned. The application of the oxidative stress model will also consider the possible identification of susceptible human subjects with weakened oxidative stress defenses. This could involve polymorphisms of the heme oxygenase 1 gene, which is a very sensitive antioxidant defense mechanism that protects cells against redox-cycling DEP chemicals and contributes to CO and NO production during in vivo DEP exposure. The elucidation of susceptible individuals who can be studied with rational endpoints will enhance epidemiological studies and will also help to monitor the impact of regulatory measures to reduce adverse health effects.

A continuing research objective is to further examine the role of PM composition, size, surface area, gaseous co-pollutants and other factors in mediating effects. This involves a variety of experimental approaches, from detailed morphologic and chemical analysis of ambient air PM to in vivo and in vitro exposure studies using both CAPs and laboratory-generated PM of carefully defined composition. A related longer term goal is to determine the role of complex mixtures in eliciting health effects using factor analysis to identify the sources of PM and associated gaseous air pollutants.

Determining the mechanisms underlying individual susceptibility to PM effects is another major objective underway in each of the PM Centers. Host susceptibility factors being investigated include age, gender, underlying disease, infections, and genetic factors.

A key goal of PM Centers research is to develop biologic markers of specific mechanistic pathways that can be used to link findings from animal, human, and epidemiological studies. There are many examples that are currently being investigated, including plasma sICAM-1 and monocyte ICAM-1 as indicators of enhanced leukocyte endothelial interaction, and measurement of changes in heart rate variability as a measure of cardiovascular autonomic effects.

Long-Term PM Research Goals

One major long-term objective of the PM Centers is to determine the mechanisms involved in the chronic health effects of ambient PM exposure. Epidemiological studies have indicated that exposure to PM2.5 leads to a shortening of life span, and this finding was a major impetus in the establishment of the annual average PM2.5 NAAQS. Does PM2.5 exposure exacerbate underlying disease, or contribute to the genesis of disease (or both)? Some of the key findings currently being investigated in the PM Centers, and summarized here, have implications for chronic (long-term) effects.

For example, one hypothesis is that recurrent inflammatory or allergic challenges to the airway leads to airway remodeling, a key feature in the development of irreversible airways obstruction in asthma and COPD. Secondly, there is growing evidence that atherosclerotic cardiovascular disease is an inflammatory process. PM Centers are testing the hypothesis that immune or inflammatory effects of PM exposure may promote or accelerate atherosclerosis. Thirdly, diabetes is associated with severe, accelerated atherosclerotic vascular disease and increased susceptibility to infection, and a recent study identified diabetics as particularly susceptible to the health effects of PM. Determining the mechanisms involved in this susceptibility of diabetics may help to shed new light on PM effects mechanisms in general.

It is known that ambient air PM, particularly DEP, contain carcinogens, and there is recent epidemiological evidence that mortality due to cancer is increased in relation to PM exposure. Investigating the PM components and mechanisms for carcinogenesis will be an important long-term goal of the PM Centers.

Finally, a major long term goal of the PM Centers is to contribute new scientific data and risk assessment tools and information that will assist in refining air quality standards for PM, and to evaluate the public health benefits of reductions in PM exposure. This will involve more comparisons of PM potency across animal, clinical, and in vitro endpoints coupled with ever-increasing specificity as to aerosol characteristics, leading to better indicators of biological mechanism and links of particular sources of PM to specific health effects.

References

Boscia, J.A., Chalupa, D., Utell, M.J., Zareba, W., Konecki, J.A., Morrow, P.E., Gibb, F.R., Oberdörster, G., Azadniv, M., Frasier, L.M., Speers, D.M., and Frampton, M.W. Airway and cardiovascular effects of inhaled ultrafine carbon particles in resting, healthy, nonsmoking adults. Am. J. Respir. Crit. Care Med. 161:A239 (2000).

Clarke, R.W., Coull, B.A., Reinisch, U., Catalano, P.J., Killingsworth, C.R., Koutrakis, P., Kavouras, I., Lawrence, J., Lovett, E.G., Wolfson, J.M., Verrier, R.L., and Godleski, J.J. Inhaled concentrated ambient particles are associated with hematologic and bronchoalveolar lavage changes in canines. Environ. Health Perspect. 108(12):1179-1187 (2000).

Couderc, J-P., Elder, A.C.P., Cox, C., Zareba, W., and Oberdörster, G. Power spectrum analysis and time-domain analysis of heart rate variability in short-term ECGs recorded using telemetry in unrestrained rats. Ann. Biomed. Eng. (Submitted).

Couderc, J-P., Elder, A.C.P., Cox, C., Zareba, W., and Oberdörster, G. Limitation of power spectrum and time-domain analysis of heart rate variability in short-term ECG recorded using telemetry in unrestrained rats. Computers in Cardiology, IEEE Computer Society Press, Vol. 28, 2001 (In press).

Daigle, C.C., Speers, D.M., Chalupa, D., Stewart, J.C., Frasier, L.M., Azadniv, M., Phipps, R.P., Utell, M.J., and Frampton, M.W. Ultrafine particle exposure alters expression of CD40 ligand (CD154) in healthy subjects and subjects with asthma. (Submittted).

Finkelstein, J.N., Reed, C., Johnston, C., and Oberdörster, G. Age alters macrophage responses to endotoxin and particle-induced cytokine gene expression. Abstract 2001.

Frampton, M.W. Systemic and cardiovascular effects of airway injury and inflammation: Ultrafine particle exposure in humans. Environ. Health Perspect. 109(Suppl. 4):529-532 (2001).

Frampton, M.W., Azadniv, M., Chalupa, D., Morrow, P.E., Gibb, F.R., Oberdörster, G., Boscia, J., Speers, D.M., and Utell, M.J. Blood leukocyte expression of LFA-1 and ICAM-1 after inhalation of ultrafine carbon particles. Am. J. Respir. Crit. Care Med. 163:A264 (2001).

Li, N., Kim, S., Wang, M., Froines, J., Sioutas, C., and Nel, A.E. Use of a stratified oxidative strerss model to study the biological effects of ambient concentrated and diesel exhaust particulate matter. Inhal. Toxicol. (In press, 2002).

Luchtel, D., Fu, C., and Ghatpande, P. A mouse model to study toxicity of particulate matter (PM). ATS abstract—submitted (2002).

Nadziejko, C., Fang, K., Gordon, T., and Chen, L.C. Quantitative analysis of arrhythmias, Presented at the “Workshop on Cardiovascular Effects Associated with Air Pollution”, Rochester, NY, March 7-8, 2001.

Ridker, P.M., Hennekens, C.H., Roitman-Johnson, B., Stampfer, M.J., and Allen, J. Plasma concentration of soluble intercellular adhesion Molecule 1 and risks of future myocardial infarction in apparently healthy men. Lancet 351:88-92 (1998).

Schwartz, J. Total suspended particulate matter and daily mortality in Cincinnati. Environ. Health Perspect. 102:186-189 (1994).

Utell, M.J., and Frampton, M.W. Acute health effects of ambient air pollution: The ultrafine particle hypothesis. J. Aerosol Med. 13:355-359 (2000).

Wellenius, G.A., Lovett, E.G., Verrier, R.L., Coull, B.A., Catalano, P., Koutrakis, P., Reinisch, U., and Godleski, J.J. Exposure to concentrated air particles potentiates ischemic electrocardiographic changes during acute myocardial ischemia. Submitted to Circulation.

Whitekus, M.J., Li, N., Zhang, M., Wang, M., Horwitz, M., Nelson, S.K., Brechun, N., Diaz-Sanchez, D., and Nel, A.E. Thiol antioxidants inhibit the adjuvant effects of aerosolized diesel exhaust particles in a murine model for ovalbumin sensitization. J. Immunol. (In press, 2002).

Zareba, W., Couderc, J.P., Nomura, A., Frampton, M., Utell, M.J., Peters, A., and Oberdörster, G. Cardiac effects of air pollution: What to measure in ECG? Society of Toxicology, 2001.

Zelikoff, J.T., Nadziejko, C., Fang, K., Gordon, T., Premdass, C., and Cohen, M.D. Short-term, low-dose inhalation of ambient particulate matter exacerbates ongoing pneumococcal infections in Streptococcus pneumoniae-infected rats. In: Proceedings of the Third Colloquium on Particulate Air Pollution and Human Health (R. Phalen and Y. Bell, Eds.), pp. 8-94–8-101.

ACUTE HEALTH EFFECTS OF PM

Background

The acute health effects of PM exposures have been extensively examined by a large number of epidemiological studies conducted worldwide. These studies have consistently shown significant associations between daily average ambient PM concentrations and corresponding cardiopulmonary mortality, morbidity, and functional impairments. When EPA promulgated its 24-hour PM2.5 NAAQS in 1997, it relied primarily on this large body of epidemiological data relating PM exposures to daily deaths and hospital admissions. However, critics of the revised PM NAAQS raised a number of key issues in their challenges to the credibility of, or need for, a new 24-hour PM2.5 NAAQS such as: 1) the associations represented deaths of frail individuals whose deaths were brought forward by only a few days or weeks (harvesting), and thus had little public health significance; 2) particles originating from different sources have varying toxicities, therefore, the relative health impacts of particle sources should be assessed prior to regulating their emissions; 3) the associations were potentially confounded by season, weather, and other gaseous pollutants; 4) the associations were implausible because ambient PM2.5 concentrations were not appropriate surrogates of personal PM2.5 exposures; 5) the associations were implausible because there was limited support from controlled human and animal studies; 6) the identification of populations susceptible to PM2.5 health effects is necessary prior to promulgating a new PM2.5 standard and; 7) the exposure-response curves, showing no thresholds, were unlikely from a biological standpoint.

Many of these issues have been addressed in recent epidemiological and toxicological investigations examining the acute health effects associated with short-term PM exposures. Collaborations between epidemiological and toxicological communities have led to the development of common study hypotheses and common health endpoints, such as lung function, arterial oxygen saturation, heart rate, heart rate variability, blood pressure, tissue biomarkers of effects, exhaled nitric oxide, cardiac dysrhythmias, and respiratory symptoms. As a result, our understanding of ambient PM acute health effects has been advanced substantially. For example, results from recent controlled laboratory studies have shown that short-term exposures to CAPs, as compared to artificially generated particles, result in acute health effects that were comparable to those reported in the epidemiological literature. These toxicological findings provided some evidence about biological plausibility and mechanisms that will be of paramount importance to our efforts to examine the validity of the observed epidemiological study findings.

Progress Made

Since the establishment of the PM Centers, substantial progress has been made in our understanding of PM health effects. Collectively, the PM Centers have addressed a large number of scientific issues regarding acute PM health effects. These investigations are broadly categorized as either observational or controlled studies, and are summarized below.

Observational Studies

Harvesting: It has been suggested that air pollution-related deaths mainly impact already frail or sick individuals whose deaths are being brought forward by only a few days (the harvesting hypothesis). If this were true, then PM-related mortality would have little or no importance on total mortality rates. Several studies examining the harvesting hypothesis were conducted by the PM Centers. Using a series of moving averages of mortality and exposure data (7 day, 15 day, etc.), Schwartz (2000a) examined whether the strength of the associations changed over the various averaging periods. Based on the harvesting hypothesis one would expect that estimated PM-mortality associations would become weaker as the averaging periods increased. However, the results from this analysis showed that the association between PM10 and mortality remained significant, and the estimated relative risks were, in fact, higher for the longer averaging periods. Similar results were found for hospital admissions data (Schwartz 2001). An alternate analytical approach was used in a 10-city meta-analysis study, where averaging periods were increased incrementally from 1 to 45 days. Results from this analysis showed that the PM effect increased by a factor of 2.5, again suggesting significant shortening of life (Zanobetti et al., in press). Similar analyses are currently being conducted to examine cause-specific mortality.

Text Box:
Figure 1. Percent increase in daily deaths in six U.S. cities as a function of community average PM2.5. The shaded area represents the 95% confidence interval.

Figure 1. Percent increase in daily deaths in six U.S. cities as a function of community average PM2.5. The shaded area represents the 95% confidence interval.

Threshold/Exposure-Response: Analytical methods were developed to combine smoothed exposure-response curves from multiple locations to examine whether a threshold in the PM10 and daily death relationship exists. Results from multiple cities in the U.S. and Spain suggest that the PM10-mortality relationships are linear down to the lowest observed exposure concentrations, supporting the no-threshold hypothesis (Schwartz and Zanobetti, 2000; Schwartz et al., 2001). Similar results were found in subsequent studies of PM2.5 and mortality in six U.S. cities (Figure 1) and of PM10 and hospital admissions (Zanobetti and Schwartz, 2001a). In the PM10-hospital admissions study, the methodology was modified to examine sources of heterogeneity in the exposure-response relationship by calculating a random slope for each city. In the near future, we plan to apply the random slope model to mortality data and to examine other responses to PM exposures, such as electrocardiogram (EKG) changes.

Particle-Specific Associations: PM Center investigators have examined the relationship between mortality and source-specific PM concentrations. Laden et al. (2000), for example, used source-apportionment techniques to group elemental PM concentrations from six U.S. cities into a small numbers of categories, or “factors”. These factors were attributed to different PM sources, such as vehicular emissions, oil combustion, soil, etc. For each factor, a daily score was calculated. Subsequently, excess daily mortality was regressed against the daily scores using multiple regression analyses. Significant associations were found between mortality and the traffic and coal combustion factors, with the largest effect size for the traffic factor. No significant associations were observed for the oil and soil factors. Mar (2000) applied factor analysis to PM2.5, PM10, PM10-2.5, sulfates, non-soil PM2.5, organic carbon, elemental carbon, total carbon, gaseous co-pollutants and cardiac mortality data in Phoenix, AZ. Results from this study showed that combustion-related pollutants and sulfates were associated with cardiovascular mortality.

Figure 2. Association of Motor Vehicle PM10 and CVD Hospital Admissions. The size of the symbol reflects the size of the population group studied.

Source-specific effects were also examined by Janssen et al. (2001). The investigators used source emission and home characteristics to explain observed variability in the city-specific PM10-hospital admissions coefficients. As shown in Figure 2, the PM10 coefficient for CVD-related hospital admissions increased with the fraction of PM10 emissions from traffic-related sources, suggesting higher relative risks from PM10 for cities with a greater number of traffic-related PM10 sources.

Susceptibility: Several studies have shown higher PM-associated health risks for susceptible subpopulations, such as the elderly or those with existing cardiopulmonary diseases. PM Center investigators have attempted to identify important susceptible subpopulations. Results from these studies have shown that socio-economic factors and race do not affect susceptibility to PM-associated mortality (Zanobetti and Schwartz, 2000); however, females were shown to be at greater risk.

The elderly have long been thought to be particularly susceptible to PM pollution. Consequently, several panel studies have focused on examining the impact of PM exposures on the elderly. In Seattle, the relationship between PM exposures and cardio-pulmonary health effects was examined for three panels of elderly subjects (healthy, with CVD and with COPD). Results from this study showed that during paced breathing exercises, a 10 mg/m³ increase in outdoor PM2.5 (lagged by 1 hour) was associated with a 25% (-45%, 01%) decrease in the median of high frequency heart rate variability (HF HRV) in subjects with CVD (Sullivan et al., submitted). Decreases in HF HRV were also observed for 4- and 24-hour lagged periods. No effects were observed either for healthy subjects or those with COPD.

Results from these studies have not been entirely consistent, however, as a recent study showed no significant association between PM2.5 exposure (0-2 day lag) and coached FEV1 for subjects with COPD or CVD. In contrast, a decrement in FEV1of 215ml for a 10 mg/m³ increase in local outdoor PM2.5 exposure was shown in the same study for normal subjects without reported COPD or CVD (Trenga et al., submitted).

Other PM Center-sponsored studies examining whether individuals with pre-existing disease are at increased risk from PM exposures are currently underway. Data collection and analyses for a study in Erfurt, in eastern Germany, of patients with pre-existing CVD or COPD is currently being conducted to examine whether ultrafine particles exacerbate cardiovascular diseases through different mechanisms than PM2.5 (Wichmann and Peters, 2000). Analyses of 683 EKG recordings and blood parameter analyses are ongoing. Fieldwork for the study on COPD patients started in October 2001 and will be completed in spring 2002.

Children are also hypothesized to be particularly susceptible to PM exposures. Koenig et al. (2002) conducted a panel s

Report

Site Navigation

Research Project Search