The air in every American community will be safe and 

healthy to breathe. In particular, children, the elderly, 

and people with respiratory ailments will be 

protected from health risks of 

breathing polluted air.

—EPA Strategic Plan 2000
The EPA Particulate Matter Research Program

What Have We Learned About PM Since 1997?

EXECUTIVE SUMMARY

EPA 600/S-04/057

July 2004

US Environmental Protection Agency

Offi ce of Research and Development

Washington, DC 20460

W h a t H a v e W e L e a r n e d A b o u t P M S i n c e 1 9 9 7 ?

The U. S. Environmental Protection Agency (EPA, or the Agency) Clean Air Goal aims for every 

American community to have safe and healthy air. However, research has shown that exposure 

to particulate matter (PM) air pollution continues to be linked to increases in respiratory 

health problems, hospitalization for heart or lung disease, and even premature death. The set 

of National Ambient Air Quality Standards (NAAQS) for PM, promulgated by EPA in 1997, was designed 

to respond to this research and move the nation closer to achieving the Clean Air Goal. Currently, EPA 

estimates that its regulations to reduce air pollution will prevent tens of thousands of premature deaths and 

reduce hospitalizations for cardiovascular and respiratory illness by tens of thousands more people each year. 

The monetary benefi ts of reducing mortality alone are estimated to be up to approximately $100 billion 

per year; the benefi ts of reducing illness and minimizing the number of lost workdays and consequences of 

restricted activity are estimated to provide savings of billions more dollars each year. 

EPA views the reduction of PM emissions through its NAAQS as key to accomplishing its Clean Air Goal. 

By expanding its PM Research Program over the last fi ve years, EPA’s Offi ce of Research and Development 

(ORD)1 is successfully strengthening the science base for the NAAQS and facilitating its implementation. 

The PM Research Program

In 1997, EPA revised the NAAQS for PM, with the most 

signifi cant change being a new standard for PM smaller 

than 2.5 μm in aerodynamic diameter (PM2.5 or fi ne PM). 

Developed largely on the basis of epidemiological studies 

that found consistent associations between ambient PM 

concentrations and various adverse health effects, the revised 

standards were shadowed by concerns regarding the true 

public health signifi cance and credibility of the adverse 

effects of ambient PM. In 1998, these concerns led Congress 

to increase the President’s recommended EPA budget for 

the PM Research Program of $27.8 million by $22.4 million 

per year—an increase that was largely sustained for the 

ensuing fi ve years. EPA’s specifi c charge was to accelerate its 

investigation of the role of PM in health effects associated 

with air pollution and to strengthen the science to support 

implementation of scientifi cally defensible regulatory actions. 

1 EPA research programs include both intramural components (ORD laboratories) and extramural components [Science To Achieve Results 

(STAR) Program grants, cooperative agreements, and other mechanisms]. References to ORD work in this document apply to both its 

intramural and extramural work.

1

Smoke plumes from wildfi res. EPA’s PM 

Research Program is working to develop more 

accurate estimates of PM emissions from wildfi re 

events and other sources to help state and local 

agencies develop effective plans to achieve air 

quality standards for PM.
EPA’s Particulate Matter Research Program 

As a result of this intensifi ed research program: 

 Credibility has replaced skepticism concerning the nature and extent of ambient PM’s effects on 

cardiopulmonary disease and mortality.

 Scientists are uncovering the many of the complex interactions involving PM attributes and human 

characteristics, among other factors, that contribute to unwanted health outcomes.

 Clear advances have been made in characterizing air pollution sources and atmospheric processes. This 

information is needed to plan and implement effective emission reductions to reduce PM.

The emergence of this new information was the result of a comprehensive, national research endeavor 

involving the coordinated planning efforts of EPA’s intramural scientists, extramural investigators funded by 

EPA, and EPA partners such as the Health Effects Institute (HEI). Other federal organizations (including 

the National Institutes of Health and the Department of Energy) and others participating in the Air 

Quality Research Subcommittee of the Federal Committee 

on Environment and Natural Resources (CENR) joined in 

a coordinated effort to enhance the research addressing PM. 

This research has been performed within a scientifi c framework 

developed by the National Research Council (NRC) of the 

National Academy of Sciences, an independent committee of 

experts assembled at the request of EPA. The NRC Committee 

on Research Priorities for Airborne Particulate Matter (NRC 

Committee) issued a series of reports that outlined a research 

agenda to address the key scientifi c questions about PM and 

provided periodic assessments of progress. 

The NRC Committee’s fi rst report (Research Priorities for 

Airborne Particulate Matter: I. Immediate Priorities and a LongRange Research Portfolio, March 1998) set the stage for the 

evolution of EPA’s PM Research Program. EPA quickly focused 

the expertise of both intramural and extramural scientists to address the NRC priorities. Intramurally, the 

program adjusted its priorities in keeping with the NRC recommendations and established goals, timelines, and associated internal funding support. The STAR Program, an extramural grant program managed 

by the EPA’s National Center for Environmental Research (NCER), developed Requests for Applications 

(RFAs) for PM research in numerous environmental science, exposure, engineering, and health areas. This 

included the establishment of fi ve PM Research Centers that, together with the EPA intramural program, 

could broadly address the PM issue. A major goal of the program has been to communicate and coordinate 

new fi ndings and research priorities with partners and other Federal agencies. For example, the basic PM 

I

n three reports, the NRC 

Committee identifi ed important 

research topic areas and 

recommended to EPA a multiyear research portfolio and plan 

of the highest priority research 

topics designed to strengthen and 

expand the scientifi c understanding 

of the links between ambient PM 

and adverse health effects. ORD 

has relied upon these priorities in 

setting its PM research agenda.

2
W h a t H a v e W e L e a r n e d A b o u t P M S i n c e 1 9 9 7 ?

Research Program analyses and other analyses conducted independently by the HEI have yielded signifi cant 

contributions that have advanced and refi ned our knowledge of PM. The integration of EPA’s PM Research 

Program with the programs of partners and other Federal agencies and the communication among them have 

been key elements in achieving signifi cant scientifi c and regulatory advances.

This report highlights and summarizes the salient scientifi c 

advances in PM-related health, exposure, and implementation 

research conducted by ORD and EPA-funded researchers since 

1997. The following discussions are organized by the priority 

research needs stated in the three NRC Committee reports 

(Research Priorities for Airborne Particulate Matter I–III ) and in 

the context of the program and regulatory needs of EPA’s Offi ce of Air and Radiation (OAR). 

EPA’s PM research is conducted through intramural and 

extramural research partnerships.

3

Overview of major research areas being addressed by EPA’s PM Research Program.
EPA’s Particulate Matter Research Program 

Air monitoring equipment used in

EPA’s PM Supersites Program.
W h a t H a v e W e L e a r n e d A b o u t P M S i n c e 1 9 9 7 ?

Outdoor Measures

Versus

Actual Human Exposures

What are the quantitative relationships between 

concentrations of particulate 

matter and gaseous copollutants measured at stationary 

outdoor air-monitoring sites 

and the contributions of 

these concentrations to actual 

personal exposures, especially for 

subpopulations and individuals?

5

RESEARCH TOPIC 1
EPA’s Particulate Matter Research Program 

Understanding the relationship between ambient PM2.5 concentrations and personal PM exposure is 

fundamental to assessing the health effects of air pollution. In 1997, it was diffi cult to quantitatively 

link outside measurements of PM at ambient monitoring sites to what individuals were actually 

exposed, especially because people spend about 90% of their time indoors. 

Initially, EPA designed studies that focused on understanding the relationship between PM mass measured at 

ambient monitors and local outdoor, indoor, and personal exposure concentrations. Likewise, the effects that 

human activities, housing characteristics, and other factors had on these relationships were also investigated.

Longitudinal exposure studies were conducted in eight locations around the country representing diverse 

conditions in weather, housing, and pollution sources. Researchers studied potentially susceptible subjects with 

asthma, chronic obstructive lung disease, or cardiovascular disease. 

The exposure studies revealed a number of important relationships between personal PM exposure estimates 

and data from community monitors. Associations between the ambient monitor and personal exposure are 

strongest for fi ne particle sulfate—which forms a substantial 

fraction of PM2.5 in the eastern part of the nation—followed 

by PM2.5 mass, and then PM10 (PM smaller than 10 μm in 

aerodynamic diameter) mass. The strength of the association 

between ambient concentrations and estimates of personal 

exposure demonstrated in these studies has verifi ed that, 

for fi ne PM mass and sulfate, measurements at central 

monitoring sites should serve as an adequate surrogate for 

personal exposure to ambient PM2.5 mass in community-based 

epidemiological studies.

Although pooled analysis of the results shows a strong relationship between exposure and central site 

concentrations, results for individuals show high variability. Much of this variability is due to the varying 

effect of outdoor particles on indoor environments. Building type and ventilation strongly infl uence the 

penetration of ambient PM indoors. Although until recently particles were assumed to readily penetrate into 

buildings, studies have shown that this is often not the case, and that penetration effi ciencies vary by city and 

season. Not surprisingly, the greater the air turnover in a dwelling (via open windows, air leaks, or mechanical 

systems), the better the indoor PM2.5 refl ects the PM2.5 measured by the outdoor monitor. PM10 and ultrafi ne 

(< 0.1 μm) PM (most often associated with recently generated or “fresh” combustion emissions) penetrate less 

well under similar circumstances and penetrate especially poorly into relatively well-sealed dwellings. High 

levels of ultrafi nes in many homes appear to be associated with human activity and specifi c indoor sources 

(e.g., cooking, space heaters, cigarette smoking), which can also contribute to co-pollutant exposure. 

...results from the longitudinal 

exposure studies have verifi ed that 

for fi ne PM mass and sulfate, the 

ambient monitoring site should 

serve as an adequate surrogate 

for personal exposure...

6
W h a t H a v e W e L e a r n e d A b o u t P M S i n c e 1 9 9 7 ?

Because individuals typically spend most of their time indoors, they are exposed to lower levels of ambient 

PM than would be predicted by central site monitors; thus, the strength of the associations between 

ambient PM concentration and adverse health effects may be underestimated. Even with this variability 

in the relationships between ambient PM and personal exposure, recent studies have not shown signifi cant 

differences in personal exposure to ambient PM as a result of health condition.

Gases such as NO2, O3, CO, and SO2 are found along with PM in pollutant mixtures and may complicate the 

assessment of health outcomes associated with PM. Strong correlations were found to exist between ambient 

PM2.5 concentrations and ambient gaseous co-pollutant concentrations (i.e., O3, NO2). However, only weak 

correlations existed between personal PM2.5 exposure and personal exposures to gaseous co-pollutants. These 

data suggest that ambient gas concentrations do not signifi cantly interfere with risk estimates for the effect of 

ambient PM exposure on health outcomes. 

To assimilate these fi ndings into a tool of use to state and regional regulators, EPA has developed the 

Stochastic Human Exposure and Dose Simulation (SHEDS) Model from laboratory and fi eld data collected 

in the exposure studies. It is now being improved to incorporate the effect of emission sources on personal 

exposures and dose. This model will provide a relevant and sophisticated regulatory tool that will relate 

community monitoring or associated atmospheric modeling data to personal PM exposure. 

The research activities associated with this topic have, by design, provided critical data to address fundamental 

questions regarding PM exposure. These data have now conceptually linked regulatory monitoring data with 

the health-outcome fi ndings that were the basis of the initial PM studies. This research has also provided 

a platform from which to develop the predictive models needed to implement the NAAQS. As such, the 

original objectives of this topic area, as identifi ed in the NRC Committee reports, are nearly complete. 

However, to provide more focused exposure estimates for health assessments, to guide model development, 

and to enable assessment of the regional variability associated with exposure to putative toxic components, 

data for particular PM constituents continue to be gathered in specifi c geographic areas. 

7

Data from personal exposure 

monitors are highly correlated 

with measurements of PM2.5

mass concentrations taken 

at ambient monitoring sites. 

This implies that changes in 

community-level ambient PM2.5

mass concentrations are very 

similar to the changes in PM2.5

concentration to which people are 

exposed, even when they spend 

most of their time indoors.
EPA’s Particulate Matter Research Program 
W h a t H a v e W e L e a r n e d A b o u t P M S i n c e 1 9 9 7 ?

What are the exposures to Wbiologic biologically impor y important 

constituents and specific 

characteristics of particulate matter 

that cause responses in potentially 

susceptible subpopulations and the 

general population?

Exposures of Susceptible 

Subpopulations to Toxic 

Particulate Matter Components

9

RESEARCH TOPIC 2
EPA’s Particulate Matter Research Program 

This topic extends the Research Topic 1 research from exposure to PM mass to exposure to 

potentially toxic components of PM. Research in this area supports the hypothesis that health 

outcomes related to PM are determined by its specifi c physicochemical attributes. However, 

because current evidence suggests that health effects are likely to be associated with a signifi cant number 

of the originally hypothesized toxic agents, the focus has shifted from evaluating individual components to 

evaluating the association between PM from specifi c sources and adverse health effects.

First, exposure research on individual PM species has been initiated without waiting for their toxic 

components to be defi nitively identifi ed. Studies are being performed to investigate exposure relationships 

for as many of the hypothesized toxic components as are feasible with current technology. Results of these 

exposure studies can then be used to inform health studies. Second, source apportionment techniques are 

being incorporated into exposure research in order to evaluate the ambientpersonal exposure relationship for PM from various sources as well as for 

individual PM constituents. 

Initial work in this area has focused on developing methods for monitoring 

personal exposure to determining PM components and characteristics; 

identifying specifi c sources of PM; and developing lightweight, multipollutant samplers for susceptible individuals to use. EPA’s support of 

improvements in analytical methods will improve the capability to identify sources of PM found in personal, 

residential, and ambient PM samples. Methods used to better defi ne contributions to exposure from wood 

smoke and diesel and gasoline combustion are also being improved. Such methods have produced preliminary 

results showing that the high amounts of ultrafi ne particles near highways fall dramatically in relation to 

distance from the highway centerline and that ultrafi ne particles do not readily penetrate into buildings.

Many of the samples that were collected as part of the longitudinal PM exposure studies of Research Topic 

1 will be analyzed for the major PM constituents and size fractions and, in some cases, speciated organics. 

Preliminary results have shown that penetration effi ciencies for ultrafi ne particles are very low and that, even 

in the absence of indoor sources, indoor-outdoor correlations are poor. Other studies have shown that outdoor 

concentrations of elemental carbon and several organic species are not homogenous across airsheds and are 

infl uenced by both mobile and stationary sources. For those species that show weak associations between 

central site concentrations and exposures or that are not well distributed across airsheds, data and models 

will be needed to develop better exposure surrogates for epidemiological studies and to conduct high quality 

exposure and risk assessments. Several models that refi ned the source attribution of PM fractions and their 

statistical applications are being expanded to be associated with health outcomes and the possible infl uence of 

the seasons. Research also includes development of new modeling techniques with which to evaluate exposure 

to PM constituents and sources. 

Personal exposures to ambient PM are not 

substantially different for 

healthy and susceptible 

adult populations.

10
W h a t H a v e W e L e a r n e d A b o u t P M S i n c e 1 9 9 7 ?

New fi eld monitoring studies are being initiated to address the many remaining questions about personal 

exposures to PM with specifi c attributes or from specifi c source types and how these relate to what is 

measured at ambient monitoring sites. Based on the information from these studies, monitoring sites such as 

the PM Speciation Trends Network2 sites can provide information about exposure to PM constituents across 

the country. This information will provide the basis for airshed-specifi c health studies which aim to explore 

specifi c associations with these measures and/or their predominant sources in order to improve associated risk 

assessments. Improvements in models directed at the spatial and temporal distributions of constituents and 

their source attribution will permit the development of models designed to predict exposure at community, 

residential, and personal scales. 

...new studies are being planned and initiated to more fully 

understand and model exposures to PM constituents and 

PM from various sources.

2

 The Speciation Trends Network is a national network of approximately 50 ambient monitoring sites that has the capability to measure 

ambient levels of different chemical constituents of PM, including sulfates, nitrates, and elemental and organic carbon.

The penetration of particles into the indoor environment is highly size dependent. Larger particles (>1 μm) are typically 

excluded by their inability to penetrate with the exchange of air. The smallest particles (<0.1 μm) easily adhere to surfaces and 

other items of contact and as a result also do not penetrate effi ciently. Those particles between these sizes appear to penetrate 

most easily and also are highly respirable. While only slight differences in outdoor-indoor penetration exist between day and 

night, there are likely to be seasonal and behavioral differences, such as opened windows. 

11

Particle Diameter (nm)

0.01 0.1 1

Correlation Coefficient (r

2

) 0.0

0.2

0.4

0.6

0.8

1.0 Night

Day

Williams et al., 2000
EPA’s Particulate Matter Research Program 
W h a t H a v e W e L e a r n e d A b o u t P M S i n c e 1 9 9 7 ?

What are the size distribution, chemical 

composition, and mass-emission 

rates of particulate matter emitted 

from the collection of primaryparticle sources in the United States 

and what are the emissions of 

reactive gases that lead to secondary 

particle formation through 

atmospheric chemical reactions?

Characterization of

Emission Sources

13

RESEARCH TOPIC 3
EPA’s Particulate Matter Research Program 

ORD, in collaboration with OAR, has made substantial progress in compiling data on conventional 

(industrial) and less conventional (agricultural and other) sources of PM and PM-precursor 

emissions. Field and laboratory studies have signifi cantly advanced our understanding of the 

chemical and physical characteristics of these source emissions resulting in new and revised “source profi les” 

that can be used to generate more complete and accurate airshed emissions estimates needed to allow air 

quality models to accurately predict ambient PM levels. Researchers are developing the specialized and 

complex measurement techniques needed to determine accurate particle 

sizes and compositions from diverse and often widely dispersed sources 

such as wildfi res and concentrated animal waste facilities which have 

generally been underappreciated in terms of their contributions to ambient 

PM. 

Based on the NRC priorities and guidance from OAR and state and local 

regulatory agencies, EPA focused on fi ve primary areas related to emission 

sources.

(1) Establish standard test methods to measure particle size and 

chemical composition for combustion sources. Scientists developed new 

or modifi ed measurement approaches (hardware and operating procedures) for characterizing PM, including 

a dilution sampling system that collects emissions and allows the behavior of an exhaust plume to be modeled. 

The sampler also allows more relevant profi ling of both organic and inorganic PM constituents than earlier 

versions. This work provided much of the technical foundation for EPA’s new regulatory measurement 

methodology. Researchers also developed new methods to dynamically characterize the diesel exhaust from 

heavy-duty trucks during highway operation. This effort has provided especially valuable data concerning how 

PM emissions and characteristics change with fl uctuations in real-world engine operation as well as on how 

PM evolves in an exhaust plume.

(2) Characterize primary particle size and composition of emissions. To develop or improve the 

emission profi les needed for modeling, ORD collected data to improve mass emission factors as well as PM 

composition and size information for many important emission sources. These sources have, to date, included 

residential wood combustion devices (including wood stoves), a heavy-duty diesel truck tested during highway 

operation, and various industrial boilers and combustors. The effects of fuel type and composition, as well as 

varied operating conditions, were determined for several of the above source types, along with performance 

data for pollution-control devices. Selected portions of this work also addressed questions about the toxicity of 

specifi c emission fractions with the future goal of associating toxicity with sources of ambient PM.

ORD’s research efforts have focused on 

developing measurements 

and methods for the source 

types that are the most 

diffi cult to measure and to 

address areas for which the 

data currently available are 

highly uncertain.

14
W h a t H a v e W e L e a r n e d A b o u t P M S i n c e 1 9 9 7 ?

In cooperation with scientists from EPA Regions, ORD has addressed the effects of open biomass burning, 

which is common in the western U.S. The use of unique tracers or chemical fingerprints is being studied 

to estimate emission rates and assess how specific burning activities may influence local ambient air quality. 

ORD is also characterizing fugitive road dust contributions to the general air mix. Together, these data 

on primary emissions and precursors will update EPA’s database of source chemical profiles and emission 

factors and will improve atmospheric models used to predict ambient concentrations and secondary particle 

formation.

(3) Develop new measurement methods and collect data in order to characterize diffuse sources of 

gas-phase ammonia and organic vapors. Gas-phase ammonia is a critical co-pollutant in the evolution 

of ambient PM2.5, especially in the eastern half of the country. ORD developed a method using a Fourier 

transform infrared (FTIR) laser system to measure ammonia emissions from hog barns and lagoons, which 

are significant sources of ammonia. Recent studies indicate that this same methodology can be used to 

measure other compounds, including methane and low molecular weight organic compounds from diffuse 

sources that usually cannot be measured by traditional stack-testing techniques. 

(4) Translate new source-test procedures and source-test data into comprehensive, national, emission 

inventories. EPA research has substantially contributed to development of comprehensive, national emission 

inventories by OAR and individual states. Directly, EPA provided updated emission factors and improved 

speciation and size-distribution data for use in national emission inventories. Indirectly, EPA provided expert 

consultation and guidance for data collection and analysis. The advances in source-test procedures and data 

were used in national emission inventories in 2002 and are being used currently; the results will reduce the 

uncertainty associated with both mass emission rates and size distribution for many source types.

(5) Evaluation of PM and PM-precursor control technologies. Although the area of control technologies 

was not addressed by the NRC Committee report, ORD has conducted modest efforts to evaluate the 

performance of technologies to control PM and PM precursors to support the implementation of regulatory 

strategies to achieve the air quality standards. ORD has partnered with several other organizations, including 

industry, to leverage resources and expertise toward development of advanced PM control technologies.

Research has been and continues to be conducted to evaluate innovative approaches to improving the 

capture of fine PM from coal-fired power plants. By applying an electric field to a conventional baghouse, 

an electrostatically enhanced fabric filter system has been developed that can be retrofitted to plants 

currently using an electrostatic precipitator (ESP) or fabric filter system providing a cost-effective approach 

15
EPA’s Particulate Matter Research Program 

to incremental PM reductions. Work is also proceeding to address a potential problem associated with the 

installation of existing emissions reduction technologies at coal-fi red power plants. In a limited number of 

cases, installation of SO2 and NOx controls can result in the formation of visible acid aerosol plumes. A 

review of current literature and available data has been conducted, and experiments are being performed to 

identify methods to prevent or eliminate the formation of these plumes.

16

Ambient PM

Derives from

Varied Sources

Cars and Trucks

Natural Sources

Non-Road Vehicles
W h a t H a v e W e L e a r n e d A b o u t P M S i n c e 1 9 9 7 ?

17

Agricultural Burning

Industrial Sources

Forest Fires

Wood Burning Stoves

Heavy Duty Diesel Engines
EPA’s Particulate Matter Research Program 

The CMAQ modeling system contains three types of modeling 

components: a meteorological modeling system for the 

description of atmospheric states and motions, emission models 

for man-made and natural emissions that are injected into the 

atmosphere, and a chemistry-transport modeling system for 

simulation of the chemical transformation and fate.
W h a t H a v e W e L e a r n e d A b o u t P M S i n c e 1 9 9 7 ?

What are the linkages between emission sources 

and ambient concentrations of the 

biologically important components 

of particulate matter?

Air-Quality Model

Development and Testing

19

RESEARCH TOPIC 4
EPA’s Particulate Matter Research Program 

ORD has invested substantial effort in developing modeling approaches to address the links between 

ambient PM and emission sources that could contribute to health risks. Source-based models are 

predictive, forecasting the effects of emissions on primary (directly from the source) and secondary 

(chemically transformed) atmospheric PM. On the other hand, receptor-based models attempt to unravel the 

source attribution at the time of ambient measurements. When used together, these source- and receptororiented models provide regulators with the necessary data to develop, test, and evaluate the effectiveness of 

PM emission reduction strategies for State Implementation Plans (SIPs).

Source-based air quality models for PM were developed from earlier models designed for acid rain and ozone. 

These disparate models were integrated and updated with new information on atmospheric chemistry to 

create the initial Community Multiscale Air Quality (CMAQ) Model, which was released in 1998. The most 

recent version of the CMAQ Model, released in June 2002, incorporated the current state of the science in 

atmospheric processes at that time. The model is to be further evaluated with the PM2.5 data now becoming 

available from air monitoring networks and large fi eld studies. In addition, the CMAQ Model will also be 

improved through the incorporation of newly developed chemical modules that describe the complex PM 

chemistry with a specifi c focus on the formation of secondary organic aerosols. Through model evaluation and 

incorporation of improved atmospheric chemistry, signifi cant improvements in the predictive capability of the 

CMAQ Model are planned by its next release in 2005. This release will provide a valuable tool for modeling 

air quality for states to use in developing their SIPs. 

With SIP development beginning in 2004 and 2005, ORD has endeavored to provide user-friendly versions 

of available receptor-based models to state and local regulators. Two PC-based models have been developed: 

the Chemical Mass Balance (EPA CMB8.2) and Unmix (EPA 

Unmix2.3). CMB fully apportions exposure to the spectrum 

of sources and is highly dependent on the quality of the 

constituent-linked source-profi le database. Unmix internally 

generates external source profi les from ambient data and will 

probably be a useful tool for handling the vast amount of data 

being generated by the national Speciation Trends Network. In 

addition to developing receptor modeling tools, EPA has also 

applied receptor-based models to identify the relative contributions of gasoline vehicles and diesel vehicles to 

ambient fi ne particle levels. 

The Unmix model...[provided the fi rst] quantitative estimation of 

the separate contributions of diesel 

and gasoline engines to ambient 

levels of PM2.5

20
W h a t H a v e W e L e a r n e d A b o u t P M S i n c e 1 9 9 7 ?

21

The CMAQ Model is able to predict changes in particulate sulfate levels by simulating how sulfur dioxide, the 

major precursor to particulate sulfate, is transported and transformed in the atmosphere. This comparison of 

measured sulfate levels (in black) with CMAQ Model results (in orange) shows the model is able to predict changes 

in particulate sulfate concentrations as meteorology and upwind emissions of sulfur dioxide change over the course 

of a year. The ability to identify how particle concentrations change as emissions change is crucial to developing 

effective air quality management strategies.

Monthly Average SO4 for Ohio River Valley (2001)

0.00

2.00

4.00

6.00

8.00

10.00

12.00

Ja

n

Fe

b

Mar

Apr

May

Ju

n

Jul

Au

g

Sept

Oct

No

v

De

c

[S

O4 ] μg/

m 3

Observations

(CASTNET)

CMAQ
EPA’s Particulate Matter Research Program 
W h a t H a v e W e L e a r n e d A b o u t P M S i n c e 1 9 9 7 ?

What is the role of physicochemical 

characteristics of particulate matter 

in eliciting adverse health effects?

Assessment of Hazardous 

Particulate Matter Components

23

RESEARCH TOPIC 5
EPA’s Particulate Matter Research Program 

S

everal hypotheses regarding the role of specifi c hazardous PM attributes3 have emerged from ORD 

research since 1997. There now seems to be little doubt that there are adverse, PM-associated effects 

on human health and that several potential “active” attributes of the PM mixture may be involved.

The “causal” attributes of PM can be classifi ed as either physical, chemical, or biological. EPA has conducted 

toxicological studies to build on previous epidemiological evidence that PM-related effects on cardiovascular 

or respiratory disease mortality and morbidity are strongly associated with exposure to smaller particles 

(effects of PM2.5 > PM10 > total suspended particulate, or TSP). Some evidence also indicates that the smallest 

particles, the ultrafi nes, may have even greater potency. Further, 

in some instances, surface area may be a better dose metric 

than mass in evoking lung injury although not all ultrafi ne 

particles behave in a similar manner when comparably tested. 

Most epidemiological studies have found minimal effects on 

mortality risk for larger, coarser particles (PM10-2.5), which are 

often of crustal origin. Yet time-series epidemiological studies 

of various morbidity end points, including respiratory symptom 

measurements and cause-specifi c hospitalization reports, have 

recently reported associations with both the fi ne and coarse PM 

fractions. It remains to be determined if health end points are differentially responsive to these size modes.

The chemical composition of PM has received considerable scrutiny in toxicological studies; inorganic 

constituents have generated the most data to date. Sulfate and nitrate anions derived from combustion 

Since 1997, empirical toxicological studies have 

provided important, albeit still 

limited, evidence indicting specifi c 

PM attributes as being primarily 

responsible for the cardiopulmonary 

effects linked to ambient PM.

24

1986 1987 1988

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Cu

Zn

Fe

Pb

St

As

Ni

Mn

V

Metal (mg) present in filter extracts

Utah Valley extract

(12 TSP filters/year)

Rats

1986 1987 1988

0.0

1.0

2.0

3.0

4.0 *

*

Humans

1986 1987 1988

0.0

1.0

2.0

3.0

4.0

*

*

Saline

Extract

PMNs (107) PMNs (105)

Dye et al., 2001

Ghio et al., 2001

A one-year closing of a steel-mill in Utah Valley provided a natural experiment to compare the effects

of ambient PM in the Valley when the plan was and was not in operation. The composition of the PM

suggested that metals played a role in its health impact.

3 “Hazardous” in this context does not refer to “hazardous air pollutants” as defi ned in the Clean Air Act, but is intended to convey the hypothesis 

that specifi c attributes of PM are responsible for the adverse health effects associated with exposure to ambient PM.” 
W h a t H a v e W e L e a r n e d A b o u t P M S i n c e 1 9 9 7 ?

emissions or atmospheric processes usually combine with other constituents in PM, especially the water-soluble 

materials. Although the intrinsic, independent toxicities of many sulfates and nitrates appear to be rather low, 

it is hypothesized that they may infl uence the toxicity or bioavailability of other PM components. Little is 

actually known about the cardiovascular effects associated with acidic aerosols or the possibility that they might 

mediate some of the reported PM effects, and this issue is now being explored in EPA-funded programs. 

EPA toxicological studies have found that inhalation of certain metals results in infl ammation in the lung and 

cardiac arrhythmias. While these studies were conducted with doses or concentrations of PM higher than 

typical ambient conditions, they demonstrate the potential for similar effects to occur in humans. Perhaps 

the most striking evidence for the importance of metals is from studies of PM-associated metals extracted 

from ambient fi lters in the Utah Valley at the site of a steel mill that was temporarily closed because of a labor 

dispute. Laboratory tests and human and animal exposure studies using material from particles collected 

when the plant was open and closed demonstrated similar patterns and types of effects. These EPA-supported 

studies corroborate the results of a separate study that found a decrease in hospital admissions for similar 

causes in the local population while the plant was closed. Extracts of the Utah Valley particles were tested in 

humans, animals, and in vitro cell cultures to compare the effects associated with particles collected during 

the periods when the plant was open and closed. Despite the 

relatively high doses used in these test systems, the experimental 

studies showed a pattern of response that was consistent with the 

reported epidemiological fi ndings. Moreover, the experimental 

studies supported the hypothesis of a primary role for metals.

EPA is investigating the toxicity of other chemical attributes of 

PM. Organic constituents are of particular concern due in large 

part to the contribution of various industrial sources as well as 

diesel and other mobile sources to the fi ne PM fraction. While 

not as overtly toxic as the some of the metallic compounds, some organic compounds appear to be able to 

generate oxidants that may cause delayed or subtle effects not readily measured by conventional methods.

Finally, EPA has begun to investigate the relationship between health outcomes and source-specifi c PM. 

If health effects can be linked to particular sources of air pollution, this information will prove useful for 

targeting control strategies. Field, epidemiological, and toxicological studies are emerging that can associate 

responses with sources. These studies are designed to help determine the PM components, characteristics, 

and sources most responsible for health effects. This information is required to ensure that control strategies 

are designed and focused on the PM sources that most strongly affect public health and to provide a sound 

scientifi c basis for the development of future PM standards.

I

n a more focused analysis of 

the data from six U.S. cities, 

signifi cant associations were found 

between mortality and two key 

sources of pollution—traffi c and 

coal combustion—with the largest 

specifi c effect for the traffi c factor.

25
EPA’s Particulate Matter Research Program 
W h a t H a v e W e L e a r n e d A b o u t P M S i n c e 1 9 9 7 ?

What are the deposition patterns and fate of 

particles in the respiratory tract of 

individuals belonging to presumed 

susceptible subpopulations?

Dosimetry: Deposition and

Fate of Particles in the 

Respiratory Tract

27

RESEARCH TOPIC 6
EPA’s Particulate Matter Research Program 

Because the most relevant exposure measure of PM for health-risk assessment is the actual dose of 

particles in the lung, EPA developed a new, non-invasive method to measure the deposition of 

inhaled particles in different regions of the human lung. Studies using this method have significantly 

advanced our understanding of how different size classes of PM are deposited and the influence of age, 

gender, and respiratory diseases on the distribution of particle deposition within the lung. 

Surprisingly, the smallest and largest 

of the potentially respirable PM, the 

ultrafine and coarse (PM10-PM2.5) PM 

fractions, are deposited in a healthy 

human respiratory tract in the same 

areas. It has long been understood that 

fine particles, including the ultrafine 

fraction, penetrate more deeply into 

the respiratory tract than coarse 

PM. However, it appears that the 

deposition patterns (as opposed to 

the penetration patterns) of ultrafine 

particles tend to be more similar to 

patterns of coarse particle deposition 

than to the patterns of fine particles. 

Clearly, the absolute dose deposited 

is different for different size fractions 

and depends largely on the size 

distributions of exposure aerosols. For 

urban aerosols with a dominant fine particle mode, mass deposition is similar for fine and coarse particles. 

While the number of ultrafine particles deposited may be much larger than the number of fine and coarse 

particles, the deposited mass of ultrafine particles tends to be negligible in most cases.

This dosimetry information is important for understanding how a complex mixture of ambient PM may cause 

adverse health effects and why some people are more susceptible to adverse effects than others. Preliminary 

data suggest that some PM may, as a function of size and composition, migrate from the lung to other organs 

and tissues. Although these data need further confirmation, their potential implications are significant, 

especially in light of the cardiac effects attributed to PM. 

Fraction of inhaled small particles deposited in the lungs of human 

subjects. Those with various forms of airway and lung disease have a 

higher rate of deposition than normal healthy subjects.

28

0

0.05

0.1

0.15

0.2

0.25

0.3

Normal

Airway Dis.

COPD

Diameter = 1 :m

Smoker

Asthma

Kim, C. S. and T. C. Kang (1997)
W h a t H a v e W e L e a r n e d A b o u t P M S i n c e 1 9 9 7 ?

PM deposition in healthy adults of different ages and in men and women was also studied. EPA researchers 

found that the respiratory dose in young and older adults is similar. Therefore, dose itself may not be 

responsible for differences in susceptibility to the health effects of PM by age. Similarly, overall respiratory 

dose is comparable in men and women but; in general, women tend to receive greater doses in their upper 

airway regions than men, possibly because their airways are somewhat narrower. This is consistent with some 

data that suggest women are more sensitive to irritant gases. 

The respiratory dose of inhaled PM is unevenly distributed within the lung, and the actual dose in local 

airway regions can be many times greater than the average lung dose. People with impaired lungs may 

experience doses to airway “hot spots” that exceed those in a similar location of a healthy lung by a factor of 

8 or 10. Therefore, exposure relationships alone may be inadequate to address responsiveness in potentially 

susceptible groups without some assessment of the dosimetry.

Generally, PM dosimetry in adult, healthy, human males has 

been well described by EPA researchers. Models have been 

developed to describe particle deposition for uniform breathing 

(both during rest or exercise) over a wide range of PM size. This 

idealized dosimetry has long been the standard for estimating 

lung doses of PM exposure. There are also analogous models 

for the laboratory rodents that are typically used in toxicological 

studies. These models are important for understanding 

exposure-dose relationships in toxicological studies conducted 

to address questions of mechanism, susceptibility, causality, and composition-specifi c effects on the lung. In 

contrast, dosimetry in impaired lungs is not well defi ned in animal models; consequently, its quantitative use 

in extrapolation to humans is limited. 

The efforts in basic human dosimetry studies have waned in response to program priorities. ORD is now 

assessing the effect of cardiopulmonary diseases on the distribution of PM of various sizes in the various 

compartments of the respiratory system and how these distributions are mimicked in the animal models of 

these diseases. 

The respiratory dose of inhaled PM is distributed unevenly 

within the lung, and the actual dose 

at local airway regions can be many 

times greater than the overall lung 

dose.

29
EPA’s Particulate Matter Research Program 
W h a t H a v e W e L e a r n e d A b o u t P M S i n c e 1 9 9 7 ?

How can the effects of particulate matter be 

disentangled from the effects of 

other pollutants? How can the 

effects of long-term exposure 

to particulate matter and other 

pollutants be better understood?

Combined Effects of Particulate 

Matter and Gaseous Pollutants

31

RESEARCH TOPIC 7
EPA’s Particulate Matter Research Program 

Much of what has been learned about the combined effects of PM and other air pollutants has 

emerged from the work done by extramural STAR Program grantees, PM Research Centers, 

and HEI. To date, these ORD-sponsored epidemiological and panel studies have found either 

no infl uences or only minor infl uences from co-pollutant exposure on the estimates of health risk from PM. 

The effects of PM appear to be independent from those of other criteria air pollutants.

Long-term health effects from PM that cause shortening 

of life, accelerated dysfunction, or exacerbated disease 

remain a major concern. At the time the PM regulations 

were set in 1997, two national-scale studies of long-term 

exposure to PM had been published in the peer reviewed 

literature. The Harvard Six Cities Adult Cohort Study and 

the American Cancer Society Cohort Study both reported 

signifi cant associations between risk of premature mortality 

and long-term exposure to PM. To verify these results, HEI 

sponsored an extensive re-analysis effort which confi rmed 

the fi ndings of the original studies. Additionally, the ACS 

investigators extended the original study, doubling the 

follow-up time from 8 to 16 years; the results replicated the 

fi ndings of increased cardiopulmonary risk and documented a 

signifi cant association with mortality from lung cancer. Early 

methodological criticisms of these studies have been largely resolved, supporting the concerns about adverse 

effects of long-term exposure to PM.

Other studies in children exposed to a mixed oxidant-PM atmosphere suggest retardation of lung growth that 

is not fully reversible. Ongoing work at the U.S.-Mexico border suggests that asthma and economic status are 

important factors in the overall response. Work funded by the STAR Program is beginning to use established 

cohorts with the extensive medical histories or unique lifestyles that make them appropriate subjects for 

understanding the effects of long-term exposure to ambient PM. 

This topic area, perhaps more than any other research priority 

identifi ed by the NRC, is in its relative infancy due to the 

complexity, expense, and duration of longitudinal epidemiological 

studies of PM health effects and the lack of historical fi neparticle monitoring data. Efforts are underway to address 

the effects of chronic exposure to ambient PM including new 

initiatives to study the health effects of long-term exposure 

Pope et al. (2002) extended the original Adult Cohort Study by 

8 years and replicated the fi ndings 

of increased cardiopulmonary 

mortality risk, but in addition, 

reported a signifi cant association 

with mortality from lung cancer.

32
W h a t H a v e W e L e a r n e d A b o u t P M S i n c e 1 9 9 7 ?

initiated under STAR Program grants. Other initiatives to study chronic health effects in controlled animal 

studies have begun focusing on issues associated with the potential influence of the seasons and on identifying 

relevant biomarkers for long-term human studies. It is anticipated that the National Monitoring Network 

will be a key resource for many of the planned epidemiological studies and that laboratory animal studies will 

likewise attempt to use this rich database to assess specific causality and susceptibility hypotheses that link to 

potential chronic health outcomes. 

33
EPA’s Particulate Matter Research Program 

A researcher checks the performance of a personal exposure 

monitor. Exposure research is helping EPA to understand 

when and how people are exposed to ambient PM, even 

when they spend significant amounts of time indoors.
W h a t H a v e W e L e a r n e d A b o u t P M S i n c e 1 9 9 7 ?

What subpopulations are at an increased risk of 

adverse health outcomes from 

particulate matter?

Susceptible

Subpopulations

35

RESEARCH TOPIC 8
EPA’s Particulate Matter Research Program 

Epidemiological studies in the 1990s indicated that risks of adverse effects from PM are higher in the 

elderly, children, and people with cardiovascular or respiratory disease. Children, infants, diabetics, 

and those with hypertension may also be at increased risk. EPA research, including clinical studies 

using human volunteers and novel animal models of disease, has begun to examine the biochemical and 

physiological mechanisms of PM-associated risks. For example, a number of studies have investigated the 

underlying biology of lung disease to understand why disease induces heightened sensitivity to PM. These 

studies used animal models of lung disease to show that animals with existing pulmonary infl ammation may 

be intrinsically more responsive to PM. Similarly, a growing body of work has demonstrated PM-induced 

alterations in cardiac physiology in both humans and animals and has reinforced the hypothesis that PM 

exacerbates cardiovascular disease conditions. Studies in healthy elderly adults have demonstrated that 

exposure to concentrated ambient PM causes subtle changes in autonomic control of cardiac function and 

clotting factors. Although small, these changes are considered clinically signifi cant based on other studies of 

cardiac disease progression. 

36

Susceptible Subpopulations

Effect of PM10 (per 100 μg/m3)

Among People with Each Predisposing Condition

Individual Covariate Increase 95%

Pattern in Risk Confi dence Interval

None of the fi ve conditions

Myocardial Infarction

Diabetes

Congestive Heart Failure

COPD

Conduction Disorder

7.6%

23.7%

15.1%

12.9%

5.8%

6.8%

(-2.9, 19.1)

(-0.6, 53.8)

(-1.6, 34.1)

(-1.0, 28.8)

(-7.9, 21.5)

(-5.6, 20.8)

The risk of mortality associated with exposure to elevated levels of ambient PM increases, in some cases substantially, 

for people with pre-existing health problems. Researchers are studying why certain groups may be more sensitive to the 

effects of PM than others, which can help in the development of public health responses to air pollution events.
W h a t H a v e W e L e a r n e d A b o u t P M S i n c e 1 9 9 7 ?

Epidemiological data are emerging on certain types of 

medical conditions that potentially predispose individuals to 

an increased risk of PM-associated mortality. Recent studies 

found that those with pre-existing cardiopulmonary disease or 

with diabetes were at increased risk for PM-related effects. 

Researchers have begun to investigate the effects of PM 

exposures on physiological development in children. The 

Southern California PM Research Center is supporting 

assessment research on PM and traffi c exposure for the 

ongoing California Children’s Health Study, which has reported an increased risk of impaired lung 

development in children living in areas with higher levels of air pollution. Other studies are examining PM 

effects in young animals to identify possible effects of prenatal and postnatal exposures.

Thus, studies to date suggest that certain subpopulations are indeed more acutely responsive to PM, 

perhaps due to differences in lung deposition or to other biologic aspects of the cardiopulmonary system 

or disease. Emphasis on the identifi cation and characterization of susceptible groups will continue, as will 

determining whether prolonged PM exposure moves one into a more susceptible state by impairing defenses 

or diminishing function. The role of genetic predisposition is largely unknown, but is potentially of great 

importance. Another key topic that has not yet been adequately addressed is individual risk and the elements 

that defi ne or underlie that risk.

The results supported previous fi ndings identifying those 

with pre-existing cardiopulmonary 

conditions at increased risk for 

ambient PM effects and implicated 

another possible risk factor—

diabetes.

...there is suffi cient evidence to conclude that 

certain groups are likely to be more sensitive 

or responsive to PM than others.

37
EPA’s Particulate Matter Research Program 
W h a t H a v e W e L e a r n e d A b o u t P M S i n c e 1 9 9 7 ?

What are the underlying mechanisms (local 

pulmonary and systemic) that 

can explain the epidemiological 

fi ndings of mortality/morbidity 

associated with exposure to ambient 

particulate matter?

Mechanisms

of Injury

39

RESEARCH TOPIC 9
EPA’s Particulate Matter Research Program 

To understand the underlying biological mechanisms that explain how PM might cause adverse 

health effects, ORD has investigated several hypotheses. The mechanisms of PM toxicity are 

potentially quite complex and may involve interactions and/or interdependencies between several 

organ systems or tissues such as the lung, heart, vascular system, and autonomic nervous system. The primary 

portal of entry for PM air pollution is clearly the lung, and PM interactions with the lining of the lung may 

lead to immunologic and other responses that cause a wide range of pulmonary effects. These effects include 

lung injury, infl ammation, and changes in resistance to infection or sensitivity to allergens. Exposure to PM or 

its reaction products may also alter respiratory rate and tidal volumes. Soluble components of PM may diffuse 

into the circulatory system and may be distributed systemically 

or may perhaps activate cells within the lung to secrete mediators 

that likewise can move throughout the body. Not surprisingly 

then, PM appears to exert a number of systemic effects, 

particularly on the cardiovascular system. 

The growing number of reports associating PM with cardiac 

death, morbidity, or altered cardiac function has spurred 

new thinking about how these problems could occur. One route under investigation by EPA is via neural 

mechanisms involving the autonomic nervous system. PM may act via direct pulmonary irritant refl exes in the 

airways or through refl exes activated during pulmonary infl ammation. This effect could affect cardiac function 

via nervous system networks that operate to protect the lung or maintain homeostasis. Infl ammatory changes 

in the heart or stimulation of bone marrow cells could modulate or impair normal function in an already 

diseased or stressed heart. Other studies report altered blood viscosity or circulation of cells, both of which 

are consistent with an increased risk of cardiac events. Because PM is a complex mixture of many different 

components, different components may stimulate different biochemical pathways or interact in other ways to 

alter the dose required to evoke a response. Thus, exposure to PM, depending on its chemical and physical 

makeup, may result in the activation of one or more pathways. 

The presence or absence of an infl ammatory response 

is an important issue because 

infl ammation may induce systemic 

effects.

Chapel Hill (NC) CAPs [concentrated ambient particles] cause mild pulmonary 

infl ammation...

Los Angeles CAPs [and] carbonaceous ultrafi ne particles provide evidence of systemic 

markers of infl ammation.

PM has also been shown to induce changes in conductance and repolarization of the heart.

Some studies are reporting PM induced increases in several clotting and coagulation 

factors and vascular infl ammatory cells...that might trigger cardiovascular events.

40
W h a t H a v e W e L e a r n e d A b o u t P M S i n c e 1 9 9 7 ?

Five years ago, a description of PM health effects would typically include a caveat that the underlying 

biological mechanisms were unknown. Recent developments that led to multiple novel hypotheses indicate 

a rapid growth in our understanding of how PM can cause adverse effects. ORD, through its intramural and 

extramural programs, will be focusing specifi cally on early and primary events to separate the effects of PM 

from those of other stressors. Among various susceptible groups, there may be a common thread that explains 

what contributes to susceptibility to PM and why health effects occur.

41

Potential PM Effects on the Cardiovascular System

Pulmonary Refl exes

Autonomic Nervous

System

Conduction/Repolarization

Heart Rate Cardiac Rhythm

Cardiac

Disruption

Pulmonary Infl ammation

Systemic Infl ammation

Endothelial Cell

Dysfunction

Platelet

Activation

Clotting

Factors

Plaque Rupture

Thrombosis Viscosity

Ambient PM

Sudden

Cardiac

Death

There are numerous pathways by which exposure to ambient PM can lead to cardiac events, including sudden cardiac 

death. EPA’s research is examining these biological pathways to better understand how PM affects health.
EPA’s Particulate Matter Research Program 
W h a t H a v e W e L e a r n e d A b o u t P M S i n c e 1 9 9 7 ?

To what extent does the choice of statistical methods 

in the analysis of data from 

epidemiological studies infl uence 

estimates of health risks from 

exposures to particulate matter? 

Can existing methods be improved? 

What is the effect of measurement 

error and misclassifi cation on 

estimates of the association 

between air pollution and health?

Analysis

and

Measurement

43

RESEARCH TOPIC 10
EPA’s Particulate Matter Research Program 

S

ince the early 1990s, more than 100 epidemiological studies have reported increases in mortality and 

morbidity associated with exposure to ambient PM. Most of these studies relied on novel statistical 

models or approaches and involved monitoring data not designed a priori for research use. To address 

these concerns and in response to recommendations in the fi rst NRC Committee report, EPA and the 

University of Washington held a Workshop on Particulate Methodology in 1998. Since the workshop, many 

statistical concerns have been addressed by more detailed temporal and spatial segregation of important 

variables, independent re-analysis using alternate models, or a combination of databases for comparative 

or more rigorous evaluations. Basic issues such as the adherence to dose-response principles and testing for 

potential interactions with co-pollutants have also been addressed. Generally, investigators have concluded 

that the health effects of ambient PM are not the result of a statistical or methodological anomaly, but 

constitute a real and national problem. 

44

General

Population

Risk

Pool Death

T1

T2 T3

Is There Harvesting in the Association of Airborne Particles

with Daily Deaths?

Results of model analysis suggest that air pollution has a sustained impact on health. It increases the fl ow of people 

(T1) into the Risk Pool and reduces ‘‘recovery’’ (T2) further increasing the size of the Risk Pool as well as increasing 

the those who succumb and move into T3. (Schwartz 2001)
W h a t H a v e W e L e a r n e d A b o u t P M S i n c e 1 9 9 7 ?

The developments in statistical methods have allowed researchers to evaluate specifi c questions concerning 

the nature of the mortality risks observed. For example, the question of whether deaths from air pollution 

observed in times-series studies occur only in very ill people who are days away from death, even if not 

exposed, has been addressed. These analyses indicate that individuals for whom death is imminent comprise 

only a small portion of the deaths associated with PM exposure and reported in the time-series studies. 

In 2002, statisticians raised the concern that risk estimates for acute PM health outcomes had been 

overestimated because researchers had relied on a pre-programmed default assumption in a commonly 

used statistical model. To address this issue, ORD convened a workshop in November 2002 to consider the 

technical issues and to discuss how to overcome them. The studies deemed most relevant to the reassessment 

of the PM NAAQS process were subsequently re-analyzed using alternative statistical techniques. HEI 

conducted a peer review of the re-analyses and issued a report on its fi ndings in May 2003. The re-analyses 

indicate that the relationship between PM exposures and adverse health outcomes remains statistically 

signifi cant. The revised effects estimates were lower in most cases, although in some revised studies no 

decrease was noted in the effects estimates. The HEI peer-review panel concluded that the fundamental 

understanding of the association of adverse health effects with PM mass was not altered by the re-analyses. 

In keeping with its basic health and implementation research efforts, ORD is attempting to associate 

outcomes with emission sources and with principal components of PM. Improvements in monitoring 

methods, exposure estimates, and data models/analyses will help refi ne risk assessments and will guide 

regulatory activities appropriately. 

[The ORD workshop]...led to a substantial clarifi cation of the 

potential effects of problems in the use of general additive 

models...

...it appears that the existence of a statistical link between 

exposure to ambient PM and health outcomes remains valid...

45
EPA’s Particulate Matter Research Program 

Instrument for measuring PM concentrations.
W h a t H a v e W e L e a r n e d A b o u t P M S i n c e 1 9 9 7 ?

Atmospheric Measurements and Methods4

Technical

Support

47

RESEARCH TOPIC 11

4

 Projects and methods that support issues are addressed under Research Topics 3 and 4.
EPA’s Particulate Matter Research Program 

S

ince 1997, EPA has made significant advances in the development and field application of methods to 

measure PM. These efforts are broad-based, range from state-of-the-art methods development and 

validation to monitoring network design and PM characterization, and have relied upon coordinated 

activities across the Agency and with the broader scientific community to achieve the program’s advances. 

With the establishment of the PM2.5 NAAQS, the refinement and standardization of a Federal Reference 

Method for monitoring PM2.5 became paramount. The PM2.5 methodology has now been validated in the 

field and used in the National Monitoring Network to collect the three years of data necessary for use in 

compliance determinations. With the anticipated need to specifically sample coarse PM, prototype samplers 

are entering their final testing phases. Ultimately, the goal for all these monitoring devices—which, at present, 

use filters and usually provide 24-hour measurements—is to achieve continuous, real-time measurement that 

can reveal rapid cycles and peaks not previously apparent. 

In an effort to define new methods to advance monitoring technology and atmospheric science, EPA 

established the PM Supersites Program. These seven sites distributed throughout the nation were 

competitively selected to intensively characterize PM and associated co-pollutants and to provide 

opportunities to test the latest technologies in aerometric analyses. Early on, EPA made great efforts to 

coordinate with members of the private sector (the Electric Power Research Institute, or EPRI) and other 

Federal agencies, either directly or through CENR and NARSTO.5 The five eastern sites were organized into 

the cooperative Eastern Supersites Program that designed intensive, seasonal studies over a 13-month period 

from 2001 to 2002. These data comprise the most extensive database on PM collected to date. Additionally, 

many new technologies were tested and validated, including approaches to making continuous measurements, 

single particle analyses, and others. The data from these studies are available on the web for public, scientific, 

and programmatic use.

PM measurement techniques have advanced, but much remains to be done to bring these techniques into 

wider application. Continuous and semi-continuous methods for PM mass (fine, coarse, and PM10) and 

its major components are being developed. Likewise, with the emphasis on speciation of PM constituents, 

methods for measuring the chemical composition and physical properties of single particles are also emerging 

to complement more standard methods. In cooperation with the National Institute of Standards and 

Technology (NIST), EPA has been collecting samples to be used as Standard Reference Materials.

5 Formerly an acronym for the North American Strategy for Tropospheric Ozone, the term NARSTO is now commonly used to signify this trinational, public-private partnership for dealing with multiple features of tropospheric pollution, including ozone and suspended particulate matter.

48
W h a t H a v e W e L e a r n e d A b o u t P M S i n c e 1 9 9 7 ?

Conclusions

49
EPA’s Particulate Matter Research Program 

Five Years of Progress

As the preceding pages have illustrated, the first five years of EPA’s expanded research program have led to a 

wealth of new information and significant advances in PM science. EPA established a sound strategy, involved 

many partners, and successfully conducted high-priority research. Highlights of key accomplishments include 

the following:

Verification of PM-associated health effects: Studies showing that exposure to ambient PM can adversely 

affect human health have been replicated many times in a number of locations throughout the U.S. and the 

world. Generally, exposure to PM is associated with morbidity and mortality independent of the effects of 

other gaseous pollutants in the atmosphere. Recent studies have also underscored that the elderly with preexisting cardiopulmonary disease appear to be most at risk. In addition, other groups such as the very young, 

asthmatics, and diabetics may also be susceptible to the effects of PM. Even more striking are the findings 

that suggest that extended exposure to PM can lead to chronic disease and/or shortened life span.

Confirmation of exposure measures: In 1997, the relationship between outside PM measurements and the 

concentrations to which people were actually exposed was not known. Results from exposure studies have 

verified that, for fine PM mass and sulfate, the central-site monitor should serve as an adequate surrogate 

for exposure in community-based epidemiological studies. While the strength of the correlation may vary by 

location, housing characteristics, and season, these data support the assumption used in health studies that 

measurements of outdoor PM2.5 concentrations at central monitoring sites provide a valid representation of 

personal exposure to ambient PM2.5. Exposure studies have also found that gaseous co-pollutants such as 

ozone and nitrogen dioxide measured outdoors can serve as appropriate surrogates for personal PM2.5 and are 

not likely to interfere significantly with estimates of PM health risks.

 

Advances in dosimetry: Prior to the advent of the expanded PM Research Program, little was known about 

the deposition of fine particles in the impaired lung. Since then, laboratory studies have shown that inhaled 

PM is distributed unevenly in the respiratory tract and that actual dose at local airway regions can be many 

times greater than the average lung dose. These studies determined that as many as 10 times more particles 

are deposited in certain regions of the lungs of people with pulmonary disease. This may indicate that their 

increased susceptibility is due to exposure to a higher dose. 

 

Plausible biological mechanisms: While scientists could not explain the observed effects in epidemiological 

studies from a biological basis five years ago, there are now multiple hypotheses describing the mechanisms by 

which very small concentrations of inhaled PM may produce the cardiovascular and pulmonary changes that 

contribute to increased illness and death. Similarly, laboratory studies and animal models that mimic human 

50
W h a t H a v e W e L e a r n e d A b o u t P M S i n c e 1 9 9 7 ?

disease have stimulated several theories about how the physicochemical properties of PM produce toxicity. 

Somewhat surprisingly, there appears to be no single attribute that makes PM toxic, but size and certain 

chemical components (e.g., metals) appear to be involved. This is supported by both laboratory and field 

evidence.

Development of predictive models and evaluative tools to achieve reduction in PM: EPA is improving 

and developing tools to simulate and measure atmospheric processes and concentrations and is working to 

identify and characterize sources of PM. It is developing several models to address the links between ambient 

PM and emission sources. Such models are needed to estimate how much source-designated PM will reach 

populations likely to be affected. EPA’s emission-inventory data and accurate air quality models are being 

applied to regions all around the country. The PM Research Program is also developing the specialized 

measurement techniques needed to determine the detailed particle sizes and compositions of PM from diverse 

and often widely dispersed unconventional sources such as wildfires.

Looking Ahead

As recognized by the NRC Committee in its original report, five years of expanded PM research are not 

sufficient to answer all of the most pressing questions. The first, very productive portion of this program has 

tackled many issues and laid the groundwork for addressing others. In addition, new research topics such 

as linking sources of PM to ambient concentrations and to health effects have emerged as promising areas 

for further research. Looking ahead, EPA will sustain its commitment to PM research to ensure that future 

reviews of the PM NAAQS are on the strongest scientific footing possible and that tools are available to 

implement the standards efficiently, effectively, and as inexpensively as possible. Highlights of future efforts 

include the following: 

Effects of long-term exposure to PM: The role of long-term PM exposure in the development of chronic 

disease and how this long-term exposure combines with short-term fluctuations in PM levels to trigger acute 

events such as heart attacks is far from understood. In 2003, EPA issued a request for research grant proposals 

to conduct a 10-year longitudinal epidemiological study to examine the health consequences of long-term 

exposure to PM. 

Biological mechanisms explaining susceptibility to PM effects: Now that several plausible mechanistic 

hypotheses have emerged, EPA will investigate their significance. These investigations will likely focus on 

human subpopulations and animal models with cardiopulmonary and associated systemic diseases that appear 

to contribute to PM sensitivity.

51
EPA’s Particulate Matter Research Program 

PM attributes and source apportionment of health effects: We still do not understand enough about the 

physicochemical properties of PM and how they relate to health outcomes. Source attribution of health effects 

is a growing area of interest, and ORD plans to expand exposure, toxicological, and epidemiologic research to 

pursue source-based linkages of hazardous components and effects. Through these efforts, the contributors to 

PM’s adverse health effects can be more appropriately targeted for mitigation. 

Air Quality Criteria Document (AQCD) for PM: By the end of 2004, EPA will complete its comprehensive 

assessment of PM science, the Air Quality Criteria Document for Particulate Matter. This document, which 

is required prior to the regulatory review of the NAAQS, will consider the findings of over 2,000 studies 

published since the last assessment in 1996. With completion of EPA’s Office of Air Quality Planning and 

Standards (OAQPS) Staff Paper that evaluates the policy implications of the key scientific information, EPA 

will issue a decision concerning possible revision of the PM NAAQS based on the science delineated by the 

AQCD.

Human exposure to PM constituents and PM from specific source types: Exposure studies suggest that 

ambient concentrations provide an adequate surrogate for personal exposure to ambient fine particle mass 

and sulfate in community-based epidemiological studies. However, the same may not hold true for other 

components. Research is needed to evaluate exposures and health effects for those components associated 

with specific source types. EPA’s exposure program will provide information to associate actual human 

exposure to ambient levels of PM components and PM from specific sources, which will provide the basis for 

more complete understanding of the relationships between source-specific PM and adverse health effects.

Tools for developing PM control strategies: With more than two years of PM2.5 monitoring data now 

available from the National Monitoring Network, there is a pressing need for accelerated research related 

to the development of the tools and data needed by states and others for the development of approaches 

designed to achieve the PM standards. Such data includes the characterization of emissions and improved 

regional and local atmospheric modeling of PM2.5. The next generation of analytical tools to help EPA 

and the states with NAAQS implementation was released by EPA in 2003. In 2004 and 2005, OAR will 

announce which states and regions are not in compliance with the NAAQS based on PM2.5 monitoring and 

modeling data. This announcement will require the states and regions to develop plans and strategies that will 

enable them to meet the NAAQS. Additional EPA research will focus on likely causes of states and regions 

being unable to attain the standards beyond failure to implement current and pending regulations to reduce 

emissions such as local concentrations of carbonaceous particles.

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Continuation of research partnerships: EPA’s successes in the sciences that have expanded our 

understanding of pollutant emissions, transport, and transformation, as these relate to exposure and 

associated health effects of ambient PM have resulted from the combined work of EPA scientists, EPAfunded researchers, and our federal and academic partners. As the scope of studies increases, it becomes 

increasingly important to build upon these partnerships, such as with HEI, to achieve the goals of the 

program. Epidemiological research to evaluate the effects of PM and co-pollutants across multiple and 

diverse locations as a means to better understand how the emissions from specific source types are linked to 

health effects is one example of a large-scale effort that can benefit from strategic partnerships, and continue 

to provide the science needed to refine future air quality standards.

In addition to the research directions highlighted previously, EPA will rely on the final report of the NRC 

Committee to determine whether any additional changes in research direction are warranted. In this way, 

EPA’s partnership between ORD and OAR remains an essential part of its efforts to achieve the Agency’s 

Clean Air Goal. EPA will continue to integrate its diverse, yet targeted, health and implementation 

research agenda to ensure that air quality standards are scientifically sound and that approaches to achieving 

compliance with these standards are based on information and technical tools that are as accurate and effective 

as possible.

53
EPA’s Particulate Matter Research Program 

54

References

National Research Council (1998). Research Priorities for Airborne Particulate Matter: I. Immediate Priorities and a LongRange Research Portfolio. Washington, DC: National Academies Press. ISBN 0-309-06094-X.

National Research Council (1999). Research Priorities for Airborne Particulate Matter: II. Evaluating Research Progress and 

Updating the Portfolio. Washington, DC: National Academies Press. ISBN 0-309-06638-7.

National Research Council (2001). Research Priorities for Airborne Particulate Matter: III. Early Research Progress.

Washington, DC: National Academies Press. ISBN 0-309-07337-5.

Lippmann, M., M. Fampton, J. Schwarz, D. Dockery, R. Schlesinger, P. Koutrakis, J. Froines, A. E. Nel, J. Finkelstein, J. 

Godleski, J. Kaufman, J. Koening, T. Larson, D. Luchtel, L.-J. S. Liu, G. Oberdorster, A. Peters, J. Sarnat, C. Sioutas, H. 

Suh, J. Sullivan, M. Utell, E. Wichmann and J. T. Zelikoff (2003). “The EPA’s Particulate Matter (PM) Health Effects 

Research Centers Program: A Midcourse Report of Status, Progress, and Plans.” Environ Health Persp 111(8). DOI: 

10.1289/ehp.5750.

Pope, C. A. I., R. T. Burnett, M. J. Thun, E. E. Calle, D. Krewski, K. Ito and G. D. Thurston (2002). “Lung cancer, 

cardiopulmonary mortality and long-term exposure to fine particulate air pollution.” J Am Med Assoc 287: 1132-1141.

Schwartz, J. (2001). “Is there harvesting in the association of airborne particles with daily deaths and hospital 

admissions?” Epidemiology (12)1:55-61.

U.S. Environmental Protection Agency (2003). Air Quality Criteria for Particulate Matter (Fourth External Review 

Draft). EPA 600/P-99/002aD-bD. Research Triangle Park, NC: U.S. EPA, NCEA. NTIS PB2003-106438; PB2003-

106439.

Kim, C.S., and T.C. Kang (1997). “Comparative measurement of lung deposition of inhaled fine particles in normal 

subjects and patients with obstructive airway disease.” Am J Respir Crit Care Med 155(3):899-905.

Ghio, A. J. and R. B. Devlin (2001). “Inflammatory lung injury after bronchial instillation of air pollution particles.” Am 

J Respir Crit Care Med 164(4): 704-708.

Dye, J. A., J. R. Lehmann, J. K. McGee, D. W. Winsett, A. D. Ledbetter, J. I. Everitt, A. J. Ghio and D. L. Costa 

(2001). “Acute pulmonary toxicity of particulate matter (PM) filter extracts in rats: Coherence with epidemiological 

studies in Utah Valley residents.” Environ Health Persp 109(Supplement 3):395-403.

Williams, R., J. Creason, R. Zweidinger, R. Watts, L. Sheldon and C. Shy (2000). “Indoor, outdoor, and personal 

exposure monitoring of particulate air pollution: The Baltimore elderly epidemiology-exposure pilot study.” Atmospheric 

Environment 34:4193-4204.

Williams, R., J. Suggs, J. Creason, C. Rodes, P. Lawless, R. Kwok, R. Zweidinger and L. Sheldon (2000). “The 1998 

Baltimore particulate matter epidemiology-exposure study: Part 1-Comparison of ambient, residential outdoor, indoor 

and apartment particulate matter monitoring.” J. Expos. Anal. Environ. Epid. 10:518-532.
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