# National Ambient Air Quality Standards for Particulate Matter: Proposed Decision

> Briefs, arguments, decisions, and more.

URL: https://www.frixlaw.com/law-library/documents/fr%3A96-30897

## Record

- **Collection:** Federal Register
- **Document type:** Proposed Rule
- **Published:** December 13, 1996
- **Citation:** 61 FR 65638

## Text

SUMMARY: In accordance with sections 108 and 109 of the Clean Air Act
(Act), EPA has reviewed the air quality criteria and national ambient
air quality standards (NAAQS) for particulate matter (PM) and for ozone
(O3). Based on these reviews, EPA proposes to change the standards
for both classes of pollutants. This document describes EPA's proposed
changes with respect to the NAAQS for PM. The EPA's proposed actions
with respect to O3 are being proposed elsewhere in today's Federal
Register.
With respect to PM, EPA proposes to revise the current primary
PM10 standards by adding two new primary PM2.5 standards set
at 15 g/m\3\, annual mean, and 50 g/m\3\, 24-hour
average, to provide increased protection against a wide range of PM-
related health effects, including premature mortality and increased
hospital admissions and emergency room visits (primarily in the elderly
and individuals with cardiopulmonary disease); increased respiratory
symptoms and disease (in children and individuals with cardiopulmonary
disease such as asthma); decreased lung function (particularly in
children and individuals with asthma); and alterations in lung tissue
and structure and in respiratory tract defense mechanisms. The proposed
annual PM2.5 standard would be based on the 3-year average of the
annual arithmetic mean PM2.5 concentrations, spatially averaged
across an area. The proposed 24-hour PM2.5 standard would be based
on the 3-year average of the 98th percentile of 24-hour PM2.5
concentrations at each monitor within an area. The EPA also solicits
comment on two alternative approaches for selecting the levels of
PM2.5 standards. The EPA proposes to revise the current 24-hour
primary PM10 standard of 150 g/m\3\ by replacing the 1-
expected-exceedance form with a 98th percentile form, averaged over 3
years at each monitor within an area, and solicits comment on an
alternative proposal to revoke the 24-hour PM10 standard. The EPA
also proposes to retain the current annual primary PM10 standard
of 50 g/m\3\. Further, EPA proposes new data handling
conventions for calculating 98th percentile values and spatial averages
(Appendix K), proposes to revise the reference method for monitoring PM
as PM10 (Appendix J), and proposes a new reference method for
monitoring PM as PM2.5 (Appendix L).
The EPA proposes to revise the current secondary standards by
making them identical to the suite of proposed primary standards. In
the Administrator's judgment, these standards, in conjunction with the
establishment of a regional haze program under section 169A of the Act,
would provide appropriate protection against PM-related public welfare
effects including soiling, material damage, and visibility impairment.

DATES: Written comments on this proposed rule must be received by
February 18, 1997.

ADDRESSES: Submit comments in duplicate if possible on the proposed
action to: Office of Air and Radiation Docket and Information Center
(6102), Attention: Docket No. A-95-54, U.S. Environmental Protection
Agency, 401 M St., SW., Washington, DC 20460.
Public hearings: The EPA will announce in a separate Federal
Register document the date, time, and address of the public hearing on
this proposed rule.

FOR FURTHER INFORMATION CONTACT: Ms. Patricia Koman, MD-15, Air Quality
Strategies and Standards Division, Office of Air Quality Planning and
Standards, U.S. Environmental Protection Agency, Research Triangle
Park, North Carolina 27711, telephone: (919) 541-5170.

SUPPLEMENTARY INFORMATION:

Docket

Docket No. A-95-54 incorporates by reference the docket established
for the air quality criteria document (Docket No. ECAO-CD-92-0671). The
docket may be inspected at the above address between 8:00 a.m. and 5:30
p.m. on weekdays, and a reasonable fee may be charged for copying.

Availability of Related Information

Certain documents are available from the U.S. Department of
Commerce, National Technical Information Service, 5285 Port Royal Road,
Springfield, Virginia 22161. Available documents include: Air Quality
Criteria for Particulate Matter (Criteria Document) (three volumes,
EPA/600/P-95-001aF thru EPA/600/P-95-001cF, April 1996, NTIS # PB-96-
168224, $234.00 paper copy); and Review of the National Ambient Air
Quality Standards for Particulate Matter: Policy Assessment of
Scientific and Technical Information (Staff Paper) (EPA-452/R-96-013,
July 1996, NTIS # PB-97-115406, $47.00 paper copy and $19.50
microfiche). (Add a $3.00 handling charge per order.) A limited number
of copies of other documents generated in connection with this standard
review, such as technical support documents pertaining to air quality,
monitoring, and health risk assessment, can be obtained from: U.S.
Environmental Protection Agency Library (MD-35), Research Triangle
Park, NC 27711, telephone (919) 541-2777. These and other related
documents are also available for inspection and copying in the EPA
docket identified above.
The Staff Paper and human health risk assessment support documents
are now available on the Agency's Office of Air Quality Planning and
Standards' (OAQPS) Technology Transfer Network (TTN) Bulletin Board
System (BBS) in the Clean Air Act Amendments area, under Title I,
Policy/Guidance Documents. To access the bulletin board, a modem and
communications software are necessary. To dial up, set your
communications software to 8 data bits, no parity and one stop bit.
Dial (919) 541-5742 and follow the on-screen instructions to register
for access. After registering, proceed to choice `` Gateway to TTN
Technical Areas'', then choose `` CAAA BBS''. From the main menu,
choose `` Title I: Attain/Maint of NAAQS'', then `` Policy
Guidance Documents.'' To access these documents through the World Wide
Web, click on ``TTN BBSWeb'', then proceed to the Gateway to TTN
Technical areas, as above. If assistance is needed in accessing the
system, call the help desk at (919) 541-5384 in Research Triangle Park,
NC.

Implementation Activities

When revisions to the primary and secondary PM standards are
implemented by the States, the utility, petroleum, mining, iron and
steel, automobile, and chemical industries are likely to be affected,
as well as other manufacturing concerns that emit PM or precursors to
PM. The extent of such effects will depend on implementation policies
and control strategies adopted by the States to assure attainment and
maintenance of revised standards.
The EPA is developing appropriate policies and control strategies
to assist States in the implementation of the proposed revisions to the
PM NAAQS. The resulting implementation strategies

[[Page 65639]]

will be proposed for public comment in the future.

Table of Contents

The following topics are discussed in today's preamble:

I. Background
A. Legislative Requirements
B. Related Control Requirements
C. Review of Air Quality Criteria and Standards for PM
II. Rationale for Proposed Decisions on Primary Standards
A. Health Effects Information
1. Nature of the Effects
2. Sensitive Subpopulations
3. Evaluation of Health Effects Evidence
4. Particulate Matter Fractions of Concern
B. Quantitative Risk Assessment
1. Overview
2. Key Observations
C. Need for Revision of the Current Primary PM Standards
D. Indicators of PM
1. Indicators for the Fine Fraction of PM10
2. Indicators for the Coarse Fraction of PM10
E. Averaging Time of PM2.5 Standards
1. Short-term PM2.5 Standard
2. Long-term PM2.5 Standard
3. Combined Effect of Annual and 24-Hour Standards
F. Form of PM2.5 Standards
1. Annual Standard
2. 24-Hour Standard
G. Levels for the Annual and 24-Hour PM2.5 Standards
H. Conclusions Regarding the Current PM10 Standards
1. Averaging Time and Form
2. Levels for Alternative Averaging Times
I. Proposed Decisions on Primary Standards
III. Rationale for Proposed Decision on the Secondary Standards
A. Visibility Impairment
B. Materials Damage and Soiling Effects
C. Proposed Decision on Secondary Standards
IV. Revisions to Appendix K--Interpretation of the PM NAAQS
A. PM2.5 Computations and Data Handling Conventions
B. PM10 Computations and Data Handling Conventions
V. Reference Methods for the Determination of Particulate Matter as
PM10 and PM2.5 in the Atmosphere
A. Revisions to Appendix J--Reference Method for PM10
B. Appendix L--New Reference Method for PM2.5
VI. Implementation Program
VII. Regulatory and Environmental Impact Analyses References

I. Background

A. Legislative Requirements

Two sections of the Act govern the establishment, review, and
revision of NAAQS. Section 108 (42 U.S.C. 7408) directs the
Administrator to identify pollutants which ``may reasonably be
anticipated to endanger public health and welfare'' and to issue air
quality criteria for them. These air quality criteria are to
``accurately reflect the latest scientific knowledge useful in
indicating the kind and extent of all identifiable effects on public
health or welfare which may be expected from the presence of [a]
pollutant in the ambient air * * * .''
Section 109 (42 U.S.C. 7409) directs the Administrator to propose
and promulgate ``primary'' and ``secondary'' NAAQS for pollutants
identified under section 108. Section 109(b)(1) defines a primary
standard as one ``the attainment and maintenance of which, in the
judgment of the Administrator, based on the criteria and allowing an
adequate margin of safety, [are] requisite to protect the public
health.'' The margin of safety requirement was intended to address
uncertainties associated with inconclusive scientific and technical
information available at the time of standard setting, as well as to
provide a reasonable degree of protection against hazards that research
has not yet identified. Both kinds of uncertainties are components of
the risk associated with pollution at levels below those at which human
health effects can be said to occur with reasonable scientific
certainty. Thus, by selecting primary standards that provide an
adequate margin of safety, the Administrator is seeking not only to
prevent pollution levels that have been demonstrated to be harmful but
also to prevent lower pollutant levels that she finds may pose an
unacceptable risk of harm, even if the risk is not precisely identified
as to nature or degree. The Act does not require the Administrator to
establish a primary NAAQS at a zero-risk level, but rather at a level
that reduces risk sufficiently so as to protect public health with an
adequate margin of safety.
A secondary standard, as defined in section 109(b)(2), must
``specify a level of air quality the attainment and maintenance of
which, in the judgment of the Administrator, based on [the] criteria,
are requisite to protect the public welfare from any known or
anticipated adverse effects associated with the presence of [the]
pollutant in the ambient air.'' Welfare effects as defined in section
302(h) [42 U.S.C. 7602(h)] include, but are not limited to, ``effects
on soils, water, crops, vegetation, manmade materials, animals,
wildlife, weather, visibility and climate, damage to and deterioration
of property, and hazards to transportation, as well as effects on
economic values and on personal comfort and well-being.''
Section 109(d)(1) of the Act requires periodic review and, if
appropriate, revision of existing air quality criteria and NAAQS.
Section 109(d)(2) requires appointment of an independent scientific
review committee to review criteria and standards and recommend new
standards or revisions of existing criteria and standards, as
appropriate. The committee established under section 109(d)(2) is known
as the Clean Air Scientific Advisory Committee (CASAC), a standing
committee of EPA's Science Advisory Board.

B. Related Control Requirements

States are primarily responsible for ensuring attainment and
maintenance of ambient air quality standards once EPA has established
them. Under section 110 of the Act (42 U.S.C. 7410) and related
provisions, States are to submit, for EPA approval, State
implementation plans (SIP's) that provide for the attainment and
maintenance of such standards through control programs directed to
sources of the pollutants involved. The States, in conjunction with
EPA, also administer the prevention of significant deterioration
program (42 U.S.C. 7470-7479) for these pollutants. In addition,
Federal programs provide for nationwide reductions in emissions of
these and other air pollutants through the Federal Motor Vehicle
Control Program under Title II of the Act (42 U.S.C. 7521-7574), which
involves controls for automobile, truck, bus, motorcycle, and aircraft
emissions; the new source performance standards under section 111 (42
U.S.C. 7411); and the national emission standards for hazardous air
pollutants under section 112 (42 U.S.C. 7412).

C. Review of Air Quality Criteria and Standards for PM

Particulate matter is the generic term for a broad class of
chemically and physically diverse substances that exist as discrete
particles (liquid droplets or solids) over a wide range of sizes.
Particles originate from a variety of anthropogenic stationary and
mobile sources as well as from natural sources. Particles may be
emitted directly or formed in the atmosphere by transformations of
gaseous emissions such as sulfur oxides (SOX), nitrogen oxides
(NOX), and volatile organic compounds (VOC). The chemical and
physical properties of PM vary greatly with time, region, meteorology,
and source category, thus complicating the assessment of health and
welfare effects.
The last review of PM air quality criteria and standards was
completed in

[[Page 65640]]

July 1987 with notice of a final decision to revise the existing
standards (52 FR 24854, July 1, 1987). In that decision, EPA changed
the indicator for particles from total suspended particles (TSP) to
PM10.1 Identical primary and secondary PM10 standards
were set for two averaging times: (1) 50 g/m3, expected
annual arithmetic mean, averaged over 3 years, and (2) 150 g/
m3, 24-hour average, with no more than one expected exceedance per
year.2
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\1\ PM10 refers to particles with an aerodynamic diameter
less than or equal to a nominal 10 micrometers.
\2\A more complete history of the PM NAAQS is presented in
section II.B of the OAQPS Staff Paper, Review of National Ambient
Air Quality Standards for Particulate Matter: Assessment of
Scientific and Technical Information (U.S. EPA, 1996b).
---------------------------------------------------------------------------

The EPA formally initiated the current review of the air quality
criteria for PM in April 1994 by announcing its intention to develop a
revised Air Quality Criteria Document for Particulate Matter
(henceforth, the ``Criteria Document''). Thereafter, the EPA presented
its plans for review of the criteria and standards for PM under a
highly accelerated, court-ordered schedule 3 at a public meeting
of the CASAC in December 1994. Several workshops were held by EPA's
National Center for Environmental Assessment (NCEA) to discuss
important new health effects information in November 1994 and January
1995. External review drafts of the Criteria Document were made
available for public comment and were reviewed by CASAC at public
meetings held in August and December 1995 and February 1996. The CASAC
came to closure in its review of the Criteria Document, advising the
Administrator in a March 15, 1996 closure letter (Wolff, 1996a) that
``although our understanding of the health effects of PM is far from
complete, a revised Criteria Document which incorporates the Panel's
latest comments will provide an adequate review of the available
scientific data and relevant studies of PM.'' CASAC and public comments
from these meetings and from subsequent written comments and the
closure letter were incorporated as appropriate in the final Criteria
Document (U.S. EPA, 1996a).
---------------------------------------------------------------------------

\3\ A court order entered in American Lung Association v.
Browner, CIV-93-643-TUC-ACM (D. Ariz., October 6, 1994), as
subsequently modified, requires publication of proposed and final
decisions on the review of the PM NAAQS by November 29, 1996 and
June 28, 1997, respectively.
---------------------------------------------------------------------------

External review drafts of a staff paper prepared by the Office of
Air Quality Planning and Standards (OAQPS), Review of the National
Ambient Air Quality Standards for Particulate Matter: Assessment of
Scientific and Technical Information (henceforth, the ``Staff Paper'')
were made available for public comment and were reviewed by CASAC at
public meetings in December 1995 and May 1996.4 The CASAC came to
closure in its review of the Staff Paper, advising the Administrator in
a June 13, 1996 closure letter (Wolff, 1996b) that ``the Staff Paper,
when revised, will provide an adequate summary of our present
understanding of the scientific basis for making regulatory decisions
concerning PM standards.'' CASAC and public comments from these
meetings, subsequent written comments, and the CASAC closure letter
were incorporated as appropriate in the final Staff Paper (U.S. EPA,
1996b).
---------------------------------------------------------------------------

\4\ The Staff Paper evaluates policy implications of the key
studies and scientific information in the Criteria Document,
identifies critical elements that EPA staff believes should be
considered, and presents staff conclusions and recommendations of
suggested options for the Administrator's consideration.
---------------------------------------------------------------------------

The principal focus of this current review of the air quality
criteria and standards for PM is on recent epidemiological evidence
reporting associations between ambient concentrations of PM and a range
of serious health effects. Particular attention has been given to
several size-specific classes of particles, including PM10 and the
principal fractions of PM10, referred to as the fine (PM2.5)
5 and coarse (PM10-2.5) 6 fractions. As discussed in the
Criteria Document, fine and coarse fraction particles can be
differentiated by their sources and formation processes and their
chemical and physical properties, including behavior in the atmosphere.
Detailed discussions of atmospheric formation, ambient concentrations,
and health and welfare effects of PM, as well as quantitative estimates
of human health risks associated with exposure to PM, can be found in
the Criteria Document and Staff Paper.
---------------------------------------------------------------------------

\5\ PM2.5 refers to particles with an aerodynamic diameter
less than or equal to a nominal 2.5 micrometers.
\6\ PM10-2.5 refers to those particles with an aerodynamic
diameter less than or equal to a nominal 10 micrometers but greater
than 2.5 micrometers.
---------------------------------------------------------------------------

This review of the scientific criteria for PM has occurred
simultaneously with the review of the criteria for ozone (O3).
These criteria reviews as well as related implementation strategy
activities to date have brought out important linkages between O3
and PM. A number of community epidemiological studies have found
similar health effects to be associated with exposure to O3 and
PM, including, for example, aggravation of respiratory disease (e.g.,
asthma), increased respiratory symptoms, and increased hospital
admissions and emergency room visits for respiratory causes. Laboratory
studies have found potential interactions between O3 and various
constituents of PM. Other key similarities relating to exposure
patterns and implementation strategies exist between O3 and PM,
specifically fine particles. These similarities include: (1)
Atmospheric residence times of several days, leading to large urban and
regional-scale transport of the pollutants; (2) similar gaseous
precursors, including NOX and VOC, which contribute to the
formation of both O3 and fine particles in the atmosphere; (3)
similar combustion-related source categories, such as coal and oil-
fired power generation and industrial boilers and mobile sources, which
emit particles directly as well as gaseous precursors of particles
(e.g., SOX, NOX, VOC) and O3 (e.g., NOX, VOC); and
(4) similar atmospheric chemistry driven by the same chemical reactions
and intermediate chemical species that form both high O3 and fine
particle levels. High fine particle levels are also associated with
significant impairment of visibility on a regional scale.
These similarities provide opportunities for optimizing technical
analysis tools (i.e., monitoring networks, emission inventories, air
quality models) and integrated emission reduction strategies to yield
important co-benefits across various air quality management programs.
These co-benefits could result in a net reduction of the regulatory
burden on some source category sectors that would otherwise be impacted
by separate O3, PM, and visibility protection control strategies.
In recognition of the multiple linkages and similarities in effects
and the potential benefits of integrating the Agency's approaches to
providing for appropriate protection of public health and welfare from
exposure to O3 and PM, EPA plans to complete the review of the
NAAQS for both pollutants on the same schedule. Accordingly, today's
Federal Register contains a separate notice announcing proposed
revisions to the O3 NAAQS. Linking the O3 and PM review
schedules provides an important opportunity to materially improve the
nation's air quality management programs--both in terms of
communicating a more complete description of the health and welfare
effects associated with the major components of urban and regional air
pollution, and by helping the States and local areas to plan jointly to
address both PM and O3 air pollution at the same time with one
process, and to

[[Page 65641]]

work together with industry to address common sources of air pollution.
The EPA believes this integrated approach will lead to more effective
and efficient protection of public health and the environment.

II. Rationale for Proposed Decisions on Primary Standards

This notice presents the Administrator's proposed decisions to
establish new annual and 24-hour PM2.5 primary standards and to
revise the form of the current 24-hour PM10 primary NAAQS, based
on a thorough review, in the Criteria Document, of the latest
scientific information on known and potential human health effects
associated with exposure to PM at levels typically found in the ambient
air. These decisions also take into account and are consistent with:
(1) Staff Paper assessments of the most policy-relevant information in
the Criteria Document, upon which staff recommendations for new and
revised primary standards are based; (2) CASAC advice and
recommendations, as reflected in discussions of drafts of the Criteria
Document and Staff Paper at public meetings, in separate written
comments, and in the CASAC's closure letters to the Administrator; and
(3) public comments received during the development of these documents,
either in connection with CASAC meetings or separately.
As discussed more fully below, the rationale for the proposed
revisions of the PM primary NAAQS includes consideration of: (1) Health
effects information, and alternative views on the appropriate
interpretation and use of the information, as the basis for judgments
about the risks to public health presented by population exposures to
ambient PM; (2) insights gained from a quantitative risk assessment
conducted to provide a broader perspective for judgments about
protecting public health from the risks associated with PM exposures;
and (3) specific conclusions regarding the need for revisions to the
current standards and the elements of PM standards (i.e., indicator,
averaging time, form, and level) that, taken together, would be
appropriate to protect public health with an adequate margin of safety.
As with virtually any policy-relevant scientific research, there is
uncertainty in the characterization of health effects attributable to
exposure to ambient PM. As discussed below, however, there is now a
greatly expanded body of health effects information as compared with
that available during the last review of the PM standards. Moreover,
the recent evidence on PM-related health effects has undergone an
unusually high degree of scrutiny and reanalysis over the past several
years, beginning with a series of workshops held early in the review
process to discuss important new information. A number of opportunities
were provided for public comment on successive drafts of the Criteria
Document and Staff Paper, as well as for intensive peer review of these
documents by CASAC at several public meetings attended by many
knowledgeable individuals and representatives of interested
organizations. In addition, there have been a number of important
scientific conferences, symposia, and colloquia on PM issues, sponsored
by the EPA and others, in the U.S. and abroad, during this period.
While significant uncertainties exist, the review of the health effects
information has been thorough and deliberate. In the judgment of the
Administrator, this intensive evaluation of the scientific evidence has
provided an adequate basis for regulatory decision making at this time,
as well as for the comprehensive research plan recently developed by
EPA, and reviewed by CASAC and others, for improving our future
understanding of the relationships between ambient PM exposures and
health effects.

A. Health Effects Information

This section outlines key information contained in the Criteria
Document (Chapters 10-13) and the Staff Paper (Chapter V) on known and
potential health effects associated with airborne PM, alone and in
combination with other pollutants that are routinely present in the
ambient air. The information highlighted here summarizes: (1) The
nature of the effects that have been reported to be associated with
ambient PM; (2) sensitive subpopulations that appear to be at greater
risk to such effects; (3) an integrated evaluation of the health
effects evidence; and (4) the PM fractions of greatest concern to
health.
Since the last review of the PM criteria and standards, the most
significant new evidence on the health effects of PM is the greatly
expanded body of community epidemiological studies. The Criteria
Document stated that these recent studies provide ``evidence that
serious health effects (mortality, exacerbation of chronic disease,
increased hospital admissions, etc.) are associated with exposures to
ambient levels of PM found in contemporary U.S. urban airsheds even at
concentrations below current U.S. PM standards'' (U.S. EPA, 1996a, p.
13-1). Although a variety of responses to constituents of ambient PM
have been hypothesized to contribute to the reported health effects,
the relevant toxicological and controlled human studies published to
date have not identified an accepted mechanism(s) that would explain
how such relatively low concentrations of ambient PM might cause the
health effects reported in the epidemiological literature. The
discussion below notes the key issues raised in assessing community
epidemiological studies, including alternative interpretations of the
evidence, both for individual studies and for the evidence as a whole.
1. Nature of the Effects
As discussed in the Criteria Document and Staff Paper, the key
health effects categories associated with PM include: (1) Premature
mortality; (2) aggravation of respiratory and cardiovascular disease
(as indicated by increased hospital admissions and emergency room
visits, school absences, work loss days, and restricted activity days);
(3) changes in lung function and increased respiratory symptoms; (4)
changes to lung tissues and structure; and (5) altered respiratory
defense mechanisms. Most of these effects have been consistently
associated with ambient PM concentrations, which have been used as a
measure of population exposure, in a number of community
epidemiological studies. Additional information and insights on these
effects are provided by studies of animal toxicology and controlled
human exposures to various constituents of PM conducted at higher-than-
ambient concentrations. Although, as noted above, mechanisms by which
particles cause effects have not been elucidated, there is general
agreement that the cardio-respiratory system is the major target of PM
effects.

a. Mortality

i. Short-Term Exposure Studies
As discussed in the Staff Paper, the most notable evidence on the
health effects of community air pollution containing high
concentrations of PM has come from the dramatic pollution episodes of
Belgium's industrial Meuse Valley, Donora, Pennsylvania, and London,
England. Based on analyses of a series of episodes in London, there was
general acceptance in the last Criteria Document (U.S. EPA, 1982a) and
in critical reviews of PM-associated health effects that London air
pollution at high concentrations (at or above 500-

[[Page 65642]]

1000 g/m 3 of PM 7 and sulfur dioxide (SO 2))
was causally related to increased mortality. Further analyses of daily
mortality over 14 London winters suggested that particles were more
likely to be responsible for the associations of health effects with
air pollution than SO2, and that the association continued to the
lower concentrations of PM measured in London (150 g/m3,
measured as BS).
---------------------------------------------------------------------------

\7\ Measured as British Smoke (BS), which gauges the darkness of
PM collected on a filter and is most sensitive to combustion
generated carbon particles. When calibrated to a mass measurement,
as in the historical London studies, BS is an indicator of fine mode
particles.
---------------------------------------------------------------------------

From 1987 to present, numerous epidemiological studies using
improved statistical techniques and expanded particle monitoring data
have reported statistically significant 8 positive associations
between increased daily or several-day average concentrations of PM [as
measured by a variety of indices, including TSP, PM10, PM2.5,
sulfate, and BS] and premature mortality in communities across the U.S.
as well as in Europe and South America. Of 38 analyses and reanalyses
of these studies (referred to as daily mortality studies) published
between 1988 and 1996, most found statistically significant
associations between increases in short-term ambient PM concentrations
and total non-accidental mortality (U.S. EPA, 1996a, Table 12-2).
---------------------------------------------------------------------------

\8\ Statistically significant results are reported at a 95%
confidence level.
---------------------------------------------------------------------------

More specifically, the effects estimates for PM10 reported in
these studies fall within a range of approximately 2 to 8 percent
increase in the relative risk 9 of mortality for a 50 g/
m3 increase in 24-hour average PM10 concentrations. The
consistency in these results is notable, particularly since these
studies examined PM-mortality relationships in 18 different locations
varying significantly in climate, human activity patterns, aerosol
composition, and amounts of co-occurring gaseous pollutants [e.g.,
SO2 and ozone(O3)], using a variety of statistical
techniques. A rough estimate of the incremental relative risk
attributed to PM concentrations seen in the worst London episode also
falls within this range (U.S. EPA, 1996b, p. V-13). It is also
important to note that the magnitude of the relative risks, while
significant from a public health perspective because the potentially
exposed population is large, are small compared to those usually found
in epidemiological studies of occupational and other risk factors.
---------------------------------------------------------------------------

\9\ Many of the recent epidemiological studies report effects
estimates in terms of a percentage increase in the risk of mortality
in the study population (as compared to the baseline rate in the
population as a whole) associated with a specific increase in
ambient PM concentrations measured by one or more outdoor monitors.
These effects estimates generally are based on a statistical model
of the entire study period, which typically spanned multiple years
or seasons.
---------------------------------------------------------------------------

Some of these daily mortality studies examined PM-mortality
associations for both total non-accidental mortality and cause-specific
mortality. In general, such studies have reported higher relative risks
for respiratory and cardiovascular causes of death than for total
mortality, as well as higher risks for mortality in the elderly (>65
years of age) than for mortality in the general population.
ii. Long-Term Exposure Studies
By the time of the previous review of the PM criteria in 1987,
numerous epidemiological studies of a cross-sectional design had
reported statistically significant associations linking higher long-
term (single or multi-year) concentrations of various indices of PM
with higher mortality rates across numerous U.S. communities. However,
the usefulness of such studies for quantitative purposes was at that
time limited by the lack of supporting evidence available from daily
mortality studies or the toxicological literature, and by unaddressed
confounders and methodological problems inherent in these cross-
sectional studies.
More recently, epidemiological studies of a prospective-cohort
design have been conducted, including in particular the Six City study
(Dockery et al., 1993) and the American Cancer Society (ACS) study
(Pope et al., 1995), that lend support to the earlier cross-sectional
studies of mortality. These two recent studies reflect significant
methodological advances over the earlier studies, including the use of
subject-specific information, and provide evidence for an association
between long-term PM concentrations and mortality. At least some
fraction of mortality was reported to reflect cumulative PM impacts in
addition to those associated with short-term concentrations (U.S. EPA,
1996a, p. 13-34).
The Six City study, which followed more than 8,000 adults for 14
years, found that long-term PM concentrations (PM15/10,
PM2.5, and sulfate) in six U.S. cities were statistically
significantly associated with increased rates of total mortality and
cardiopulmonary mortality, even after adjustment for smoking, education
level, and occupation. Specifically, this study reported increases in
relative risk of 26% and 37% for total and cardiopulmonary-related
mortality, respectively, between the cities with the highest and lowest
PM concentrations. The ACS study was designed to follow up on the
findings from the Six City study, using a much larger number of
individuals (more than half a million adults followed for seven years)
and cities. The ACS investigators reported that, after adjustment for
other risk factors, multi-year concentrations of PM2.5 (for 47
U.S. cities) and sulfate (for 151 cities) were found to be
statistically significantly associated with both total and
cardiopulmonary mortality. The ACS study reported increases in relative
risk of 17% and 31% for total and cardiopulmonary mortality,
respectively.
Some reviewers have raised concerns regarding the adequacy of the
adjustment for confounders in these prospective-cohort studies,
maintaining that other uncontrolled factors may be responsible for the
observed mortality rates (Lipfert and Wyzga, 1995; Moolgavkar and
Luebeck, 1996; Moolgavkar, 1994). The Criteria Document indicates,
however, that it is unlikely that these studies overlooked plausible
confounders, although the addition of factors not taken into account
might well alter the magnitude of the association (U.S. EPA, 1996a, p.
12-180). In particular, the Criteria Document cautions that the
magnitude of relative risks associated with PM concentrations reported
in these studies may be overestimated because some of the effects may
be due to historical PM concentrations that were significantly higher
than the ones used to estimate population exposures in these studies.
The Criteria Document concludes that the Six City and ACS studies,
taken together with the earlier cross-sectional studies, suggest that:
1) there may be increases in mortality in disease categories that are
consistent with long-term exposure to PM, and 2) at least some fraction
of these deaths reflects cumulative PM impacts greater than those
reported in the daily mortality studies (U.S. EPA, 1996a, p. 13-34).
iii. Degree of Lifespan Shortening
The degree of lifespan shortening associated with PM exposure in
these studies is viewed by many as an important consideration in
evaluating mortality effects in a public health context. The
epidemiological findings of associations between short- and long-term
ambient PM concentrations and premature mortality provide some insight
into this issue. The mortality effects estimates associated with long-
term PM concentrations in the prospective-cohort studies are

[[Page 65643]]

considerably larger (Six City study) to somewhat larger (ACS study)
than those from the daily mortality studies, suggesting that a
substantial portion of the deaths associated with long-term PM exposure
may be independent of the deaths associated with short-term exposure
(U.S. EPA, 1996a, p. 13-44). The Criteria Document suggests that the
extent of lifespan shortening implied by the long-term exposure studies
could be on the order of years (U.S. EPA, 1996a, p. 13-45).
As discussed in the Staff Paper, attempts to quantitatively
evaluate the extent of lifespan shortening in the daily mortality
studies to date provide no more than suggestive results, with the
investigators recognizing that more research is needed in this area
(U.S. EPA, 1996b, p. V-19-20). The limited analyses available suggest
that at least some portion of the daily mortality associated with PM
may occur in individuals who would have died within days in the absence
of PM exposure (U.S. EPA, 1996b, p. V-19-20). Researchers in this area
also note that it is possible that the reported deaths might be
substantially premature if a person becomes seriously ill but would
have otherwise recovered without the extra stress of PM exposure (U.S.
EPA, 1996b, p. V-19-20).
Quantification of the degree of lifespan shortening inherent in the
long- and short-term exposure mortality studies is difficult and
requires assumptions about life expectancies given other risk factors
besides PM exposure, including the ages at which PM-attributable deaths
occur and the general levels of medical care available to sensitive
subpopulations in an area. Because of these uncertainties, it is not
possible to develop with confidence quantitative estimates of the
extent of life-shortening accompanying the increased mortality rates
that have been associated with exposures to PM (U.S. EPA, 1996a, p. 13-
45).

b. Aggravation of Respiratory and Cardiovascular Disease

Given the statistically significant positive associations between
ambient PM concentrations and mortality outlined above, it is
reasonable to expect that community epidemiological studies should also
find increased PM-morbidity associations. As noted in the Criteria
Document, this is indeed the case. Twelve of the 13 epidemiological
studies of hospital admissions in North America (U.S. EPA, 1996a, Table
13-3) report statistically significant positive associations between
short-term concentrations of PM and hospital admissions for
respiratory-related and cardiac diseases. More specifically, these
studies report increases from 6 to 25 percent in the relative risk of
hospital admissions for respiratory disease, pneumonia, and chronic
obstructive pulmonary disease (COPD), for a 50 g/m3
increase in 24-hour average PM10 concentrations. A smaller, but
statistically significant, increase in relative risk of 2 percent was
reported in one study of hospital admissions for ischemic heart
disease.10
---------------------------------------------------------------------------

\10\ Ischemic heart disease is a general term for heart diseases
in which there is an insufficient blood supply to the heart muscle.
---------------------------------------------------------------------------

Indirect measures of morbidity, including school absences,
restricted activity days, and work loss days have also been used as
indicators of acute respiratory conditions in community studies of PM.
For example, the statistically significant association reported between
short-term PM concentrations and school absences is consistent with an
effect from PM exposure, because respiratory conditions are the most
frequent cause of school absences (U.S. EPA, 1996a, Chapter 12). Recent
studies have also reported statistically significant associations
between short-term PM concentrations and both (1) respiratory-related
restricted activity days and (2) work loss days (U.S. EPA, 1996b, p. V-
22).

c. Altered Lung Function and Increased Respiratory Symptoms

Community epidemiological studies of ambient PM concentrations and
laboratory studies of human and animal exposures to high concentrations
of PM components show that PM exposure can be associated with altered
lung function and increased respiratory symptoms. A number of
epidemiological studies in the U.S. (U.S. EPA, 1996a, Tables 13-3 and
13-4) show associations between short-term PM concentrations and
increased upper and lower respiratory symptoms and cough, as well as
decreases in pulmonary function [e.g., forced expiratory capacity for
one second (FEV1) and peak expiratory flow rate (PEFR)]. Taken
together, these studies suggest that sensitive individuals, such as
children (especially those with asthma or pre-existing respiratory
symptoms), may have increased or aggravated symptoms associated with PM
exposure, with or without reduced lung function.
Results from respiratory symptom studies of long-term PM
concentrations (U.S. EPA, 1996a, Table 13-5) are consistent with and
supportive of the associations reported for short-term PM
concentrations. Studies conducted in multiple U.S. communities in
recent years have reported that increased symptoms of respiratory
ailments in children, including bronchitis, are associated with
increasing annual PM concentrations across the communities (U.S. EPA,
1996a, p. 12-372). Recent evidence for an association between long-term
exposure to PM and decreased lung function in children and adults is
suggestive, but more limited (U.S. EPA, 1996a, p. 12-202).
The increased risk for respiratory symptoms and related respiratory
morbidity reported in the epidemiological studies is important not only
because of the immediate and near-term symptoms produced, but also
because of the longer-term potential for increases in the development
of chronic lung disease. Specifically, recurrent childhood respiratory
illness has been suggested to be a risk factor for later susceptibility
to lung damage (U.S. EPA, 1996b, p. V-27).

d. Alteration of Lung Tissue and Structure

Community epidemiological studies have generally not been used to
evaluate the extent to which exposure to PM directly alters lung
tissues and cellular components, although some autopsy studies have
found limited qualitative evidence of such effects from community air
pollution (U.S. EPA, 1996b, p. V-27). Evidence of morphological (i.e.,
structural) damage from PM exposure has come primarily from animal and
occupational studies of high concentrations of acid aerosols and other
PM components, including coarse particle dusts. While morphological
alterations have been extensively studied for exposures to acid
aerosols, such studies have been conducted at concentrations well above
current ambient levels. Long-term exposure of animals to somewhat lower
concentrations of acid mixtures have been shown to induce morphological
changes, which may be relevant to clinical small airway disease. Recent
work in animals using lower concentrations, approaching ambient levels,
of ammonium sulfate and nitrate suggest morphometric changes that could
lead to a decrease in compliance or a ``stiffening'' of the lung (U.S.
EPA, 1996b, p. V-27-29).
Occupational exposure to crystalline silica, which is a component
of coarse dust, has been associated with a specific form of pulmonary
inflammation and fibrosis (silicosis) (U.S. EPA, 1996a, p. 11-127).
Based on analyses of the silica content of resuspended crustal material
collected from several U.S. cities as part of the last review, staff
concluded that

[[Page 65644]]

the risk of silicosis at levels permitted by the current annual
PM10 NAAQS was low. The 1982 Staff Paper (U.S. EPA, 1982b)
summarized qualitative evidence for morphometric changes associated
with long-term exposure to crustal dusts, as suggested by autopsy
studies of humans and animals exposed to various crustal dusts near or
slightly above current ambient levels in the Southwest; however, no
inferences regarding quantitative exposures of concern can be drawn
from these studies.

e. Changes in Respiratory Defense Mechanisms

Responses to air pollutants often depend upon their interaction
with respiratory tract defense mechanisms that can detoxify or
physically remove inhaled material (e.g., antigenic stimulation of the
immune system and mucocilliary clearance). Either depression or over-
activation of such defense systems may be involved in the development
of lung diseases (U.S. EPA, 1996a, p. 11-55). Acid aerosols
(H2SO4) have been shown to alter mucocilliary clearance in
healthy human subjects at levels as low as 100 g/m3; such
effects are also reported in animals (U.S. EPA, 1996a, pp. 11-60-61).
Persistent impairment of clearance may lead to the inception or
progression of acute or chronic respiratory disease, and may be a
plausible link between acid aerosol exposure and respiratory disease.
Alveolar macrophages play a role in resistance to bacterial
infection, the induction and expression of immune reactions, and the
production of a number of biologically active chemicals that are
involved in respiratory defense mechanisms (U.S. EPA, 1996a, pp. 11-56-
66). Various exposures to PM constituents (e.g., acid aerosols,
sulfates, and road dust) at concentrations that range from near to well
above ambient levels have been shown to affect such macrophage
functions in experimental animals (U.S. EPA, 1996b, pp. V-29-31).
2. Sensitive Subpopulations
The recent epidemiological information summarized in the Criteria
Document provides evidence that several subpopulations are apparently
more sensitive (i.e., more susceptible than the general population) to
the effects of community air pollution containing PM. As discussed
above, the observed effects in these subpopulations range from the
decreases in pulmonary function reported in children to increased
mortality reported in the elderly and in individuals with
cardiopulmonary disease. Such subpopulations may experience effects at
lower levels of PM than the general population, and the severity of
effects may be greater.
Based on a qualitative assessment of the epidemiological evidence
of effects associated with PM for subpopulations that appear to be at
greatest risk with respect to particular health endpoints (U.S. EPA,
1996a, Tables 13-6, 13-7), the Staff Paper draws the following
conclusions with respect to sensitive subpopulations (U.S. EPA, 1996b,
pp. V-31-36):

(1) Individuals with respiratory disease (e.g., COPD, acute
bronchitis) and cardiovascular disease (e.g., ischemic heart
disease) are at greater risk of premature mortality and
hospitalization due to exposure to ambient PM.
(2) Individuals with infectious respiratory disease (e.g.,
pneumonia) are at greater risk of premature mortality and morbidity
(e.g., hospitalization, aggravation of respiratory symptoms) due to
exposure to ambient PM. Also, exposure to PM may increase
individuals susceptibility to respiratory infections.
(3) Elderly individuals are also at greater risk of premature
mortality and hospitalization for cardiopulmonary causes due to
exposure to ambient PM.
(4) Children are at greater risk of increased respiratory
symptoms and decreased lung function due to exposure to ambient PM.
(5) Asthmatic children and adults are at risk of exacerbation of
symptoms associated with asthma, and increased need for medical
attention, due to exposure to PM.
3. Evaluation of Health Effects Evidence
As discussed above, a range of serious health effects in sensitive
subpopulations has been associated with ambient PM concentrations in a
large number of community epidemiological studies. Questions as to
whether the reported associations represent causal relationships can be
addressed by consideration of the adequacy and strength of the
individual studies; the consistency of the associations, as evidenced
by repeated observations by different investigators, in different
places, circumstances, and time; the coherence of the associations
(i.e., the logical or systematic interrelationships between different
types of health effects); and the biological plausibility of the
reported associations. Because of limitations in the available evidence
from controlled laboratory studies of PM components, it is generally
recognized that an understanding of biological mechanisms that could
explain the reported associations has not yet emerged. Thus, the
following discussion focuses on the epidemiological evidence as a basis
for assessing the weight of evidence for inferences about the causality
of the relationships between health effects and exposures to ambient PM
concentrations. In particular, issues associated with interpreting
individual study results are presented, followed by a discussion of the
consistency and coherence of the health effects evidence as a whole.

a. Interpretation of Individual Study Results

While it is widely accepted that serious effects are causally
related to the high concentrations of air pollution observed in the
historical episodes, there is less consensus as to the most appropriate
interpretation of the more recent studies finding associations of such
effects with ambient PM concentrations below the levels of the current
NAAQS (e.g., Schwartz, 1994b; Dockery et al., 1995; Moolgolvkar et al.,
1995b; Moolgolvkar and Luebeck, 1996; Li and Roth, 1995; Samet et al.,
1996; Wyzga and Lipfert, 1995b):

In this regard, several viewpoints currently exist on how best
to interpret the epidemiology data: one sees PM exposure indicators
as surrogate measures of complex ambient air pollution mixtures and
reported PM-related effects represent those of the overall mixture;
another holds that reported PM-related effects are attributable to
PM components (per se) of the air pollution mixture and reflect
independent PM effects; or PM can be viewed both as a surrogate
indicator as well as a specific cause of health effects. In any
case, reduction of PM exposure would lead to reductions in the
frequency and severity of the PM-associated health effects. (U.S.
EPA, 1996a, p. 13-31)

Such alternative interpretations as to the causality underlying the
reported PM-effects associations result from a number of specific
issues that have been raised regarding the adequacy and strength of
individual studies.
Of particular concern is the possibility that independent risk
factors, related to both ambient PM concentrations and the reported
effects, could potentially confound or modify the apparent PM-effects
associations. Possible independent risk factors include weather-related
variables and other pollutants present in the ambient air (e.g.,
SO2, CO, O3, NO2), which have been addressed to varying
degrees in most of the epidemiological studies. Other concerns are
related to the influence of the choice of statistical models used by
investigators and to the uncertainties introduced by the imprecision in
measurements of ambient air pollutants, as well as the use of such
measurements as surrogates for population exposures.11 The
Criteria

[[Page 65645]]

Document and Staff Paper evaluated the studies with respect to each of
these issues, as summarized below:
---------------------------------------------------------------------------

\11\ In subsequent discussions, the term ``exposure
misclassification'' is used to refer to combined uncertainties
introduced by the related issues of errors in measurement of
pollution and in the use of outdoor measurements to index population
exposures.
---------------------------------------------------------------------------

(1) Many recent studies, including a reanalysis by the Health
Effects Institute (HEI) (Samet et al., 1996), have considered the
influence of weather on the results reported in studies of short-term
exposures, because fluctuations in weather are associated with both
changes in PM and other pollutant levels and the reported health
effects. The Criteria Document concludes that the PM effects estimates
are relatively insensitive to the different methods of weather
adjustment used in these studies, that the role of weather-related
variables has been addressed adequately, and that it is highly unlikely
that weather can explain a substantially greater portion of the health
effects attributed to PM than has already been accounted for in the
models (U.S. EPA, 1996a, p. 13-54).
(2) A number of recent reanalyses of daily mortality studies have
examined the influence of other pollutants that commonly occur in the
ambient air together with PM. Most attention has been focused on
Philadelphia, where extensive data are available on TSP, NO2,
O3, CO, and SO2. In fact, reanalyses of the Philadelphia data
have led HEI investigators to conclude that a single pollutant cannot
be readily identified as the best predictor of air pollution-related
mortality in Philadelphia based on analyses of Philadelphia data alone
(Samet et al., 1996). Based on such single-city analyses, some have
argued that estimated PM effects may be overstated or potentially non-
existent due to confounding by other pollutants that might actually be
responsible for the effects. While it is reasonable to expect that
other pollutants may play a role in modifying the magnitude of the
estimated effects of PM on mortality, either through pollutant
interactions or independent effects, the extent of any such co-
pollutant modification is less clear. The Criteria Document notes that
some mortality and morbidity studies have found little change in the PM
relative risk estimates after inclusion of other co-pollutants in the
model, and, in analyses where the PM relative risk estimates were
reduced, the PM effects estimates typically remained statistically
significant. Accordingly, the Criteria Document concludes that the PM-
effects associations are valid and, in a number of studies, not
seriously confounded by co-pollutants (U.S. EPA, 1996a, p. 13-57).
(3) Many investigators have examined how the choice of statistical
models or the ways in which they were specified may have influenced
reported PM-effects associations. In reviewing this issue, the Criteria
Document finds that, while model specification is important and can
influence PM-effects estimates, appropriate modeling strategies have
been adopted by most investigators (U.S. EPA, 1996a, section 13.4.2.2).
The Criteria Document concludes that, ``the largely consistent specific
results, indicative of significant positive associations of ambient PM
exposures and human mortality/morbidity effects, are not model
specific, nor are they artifactually derived due to misspecification of
any specific model. The robustness of the results of different modeling
strategies and approaches increases our confidence in their validity''
(U.S. EPA, 1996a, p. 13-54).
(4) A difficulty noted by many reviewers in interpreting the
epidemiological studies, particularly for quantitative purposes, is the
uncertainty and possible bias introduced by the use of outdoor monitors
to estimate a population-level index of exposure. Even in studies where
outdoor PM levels near population centers are well represented by
monitors, the extent to which fluctuations in outdoor concentrations
are found to affect indoor concentrations and personal exposure to PM
of outdoor origin remains an issue of importance. This issue is
particularly salient since some of the sensitive subpopulations in the
daily mortality and hospital admissions studies can be expected to
spend more time indoors than the general population. Some commentors
have expressed concerns regarding the lack of correlation shown in some
studies that made cross-sectional comparisons of outdoor PM with indoor
or personal exposures to PM (which includes PM from the indoor and
personal environment). The Criteria Document found, however, that on a
longitudinal basis (e.g., day-to-day), personal exposure to PM10
can be well correlated with outdoor measurements, and that the effects
reported in the short-term epidemiological studies are not due to
indoor-generated particles (U.S. EPA, 1996a, p. 1-10). Specifically,
the Criteria Document concluded that ``the measurements of daily
variations of ambient PM concentrations, as used in the time-series
epidemiological studies of Chapter 12, have a plausible linkage to the
daily variations of human exposures to PM from ambient sources, for the
populations represented by the ambient monitoring stations'' (U.S. EPA,
1996a, p. 1-10).
The strength of the correspondence between outdoor concentrations
and personal exposure levels on a day-to-day basis serves to reduce,
but not eliminate, the potential error introduced by using outside
monitors as a surrogate for personal exposure. Some commentors have
suggested the net effect of misclassifying total exposure to PM might
bias reported relationships between outdoor PM and mortality (or
morbidity) effects towards a linear, non-threshold relationship, when
in fact a threshold model of response may be more appropriate. While
such a threshold has not been demonstrated in studies to date, the
potential influence of exposure misclassification serves to increase
the uncertainty in the reported concentration-response relationships,
particularly for the lower range of concentrations.
(5) A closely related issue, namely errors in the measurement of
the concentrations of air pollutants, can also introduce uncertainty
and bias in effects estimates reported in epidemiological studies of PM
and co-pollutants. While questions about the magnitude of measurement
error and its effect on the PM-health effects associations have not
been resolved, some aspects of this issue have been examined in two
recent studies (Schwartz and Morris, 1995; Schwartz et al., 1996).
These results suggest that the influence of measurement error for
individual variables is to bias the PM-effects estimates downward
(i.e., to underestimate effects). These analyses, however, do not
assess the potential effect of exposure misclassification on effects
estimates for different components of PM, or for other co-pollutants.
In such multiple pollutant analyses, measurement error or, more
generally, exposure misclassification can theoretically bias effects
estimates of PM or co-pollutants in either direction, introducing
further uncertainties in the estimated concentration-response
relationships for all pollutants (U.S. EPA, 1996b, pp. V-39-43). A
comprehensive, formal treatment of the potential influences of exposure
misclassification is, therefore, an important research need. As noted
below, however, the available evidence on the consistency of the PM
effects relationships in multiple urban locations with widely varying
indoor/outdoor conditions and a variety of monitoring approaches makes
it less likely that the observed findings are an artifact of errors in
measurement of pollution or of exposure.

[[Page 65646]]

b. Consistency and Coherence of the Health Effects Evidence

As discussed above, the individual epidemiological studies indicate
that health effects are likely associated with PM, even after taking
into account issues regarding the adequacy and strength of these
studies. However, because individual studies are inherently limited as
a basis for addressing questions of causality, the consistency and
coherence of the evidence across the studies have also been considered
in the Criteria Document (U.S. EPA, 1996a, section 13.4.2.5) and Staff
Paper (U.S. EPA, 1996b, pp. V-54-58), as summarized below.
Of the more than 80 community epidemiological studies that
evaluated associations between short-term concentrations of various PM
indicators and mortality and morbidity endpoints (U.S. EPA, 1996a,
Tables 12-2, 12-8 to 13), more than 60 such studies reported positive,
statistically significant associations. These studies have been
conducted by a number of different investigators, in a number of
geographic locations throughout the world (with different climates and
co-pollutants), using a variety of statistical techniques, and with
varying temporal relationships. Despite these differences, the finding
of statistically significant associations is relatively consistent
across the studies (U.S. EPA, 1996a, Table 12-2).
More specifically, in looking across those studies that evaluated
associations between short-term PM10 concentrations and mortality
and morbidity endpoints, various aspects of consistency and coherence
can be observed. These observations are discussed below in reference to
Figure 1 (adapted from Figure V-2 in the Staff Paper). Figure 1
displays the estimated relative risk for a 50 g/m\3\ increase
in measured 24-hour PM10 levels, derived from studies that the
Criteria Document concluded permit quantitative comparisons across
various cause-specific mortality and morbidity endpoints (i.e.,
respiratory hospital admissions, COPD or ischemic heart disease
hospital admissions, and cough and lower and upper respiratory
symptoms) (U.S. EPA, 1996b, Tables V-4, V-6; U.S. EPA, 1996a, Section
12.3.2.2).
Figure 1 illustrates that the effects estimates for each health
endpoint are relatively consistent across the studies. Some variation
would be expected, however, due to the differences among the study
areas in the concentrations and relative composition of PM and other
air pollutants, and in the demographic and socioeconomic
characteristics of the study populations, including the distributions
of sensitive subpopulations, as well as a result of random error. Thus,
the Criteria Document concludes that the relatively small ranges of
variability in the effects estimates observed in these studies are
consistent with expectations based on assuming causal relationships
between mortality and morbidity effects and PM exposure (U.S. EPA,
1996a, Section 13.4.1.1).

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As noted above, it is reasonable to expect that co-pollutants
present in the study areas might modify the apparent effects of PM by
atmospheric interactions (e.g., through dissolution/adsorption or
aerosol formation reactions) or by independent and/or interactive
effects on sensitive subpopulations (e.g., respiratory function changes
from exposures to O3 or SO2). Moreover, the possibility of
exposure misclassification for primary gaseous pollutants (e.g., CO,
SO2) could diminish their apparent significance relative to PM. If
such PM effects modification was occurring to an appreciable degree,
the associations with PM would be expected to be consistently high in
areas with high co-pollutant concentrations, and consistently low in
areas with low co-pollutant concentrations. On the contrary, in an
examination of reported PM10-mortality associations as a function
of the varying levels of co-pollutants in study areas, consistent
effects estimates were observed across wide ranges of co-pollutant
concentrations (U.S. EPA, 1996b, Figures V-3a, V-3b). While it is
possible that different pollutants may serve to confound or otherwise
influence particles in different areas, it seems unlikely that this
would lead to such similar associations and consistent relative risk
estimates as have been reported for PM in a large number of studies.
In addition to the consistency observed in the PM associations for
each health endpoint, these studies also exhibit coherence in the kinds
of health effects that have been associated with PM exposure. For
example, the association of PM with mortality is mainly linked to
respiratory and cardiovascular causes, which is coherent with the
observed PM associations with respiratory- and cardiovascular-related
hospital admissions.
Coherence is also observed across studies of both short- and long-
term exposures to PM. For example, the existence of statistically
significant PM-mortality associations from long-term as well as short-
term exposures reinforces the likelihood that PM is a causal factor for
premature mortality relative to that which might be reasonably inferred
from either type of study alone. Furthermore, the fact that mortality
has been associated with both short- and long-term exposures is
important with respect to the credibility of ambient PM as a cause of
mortality involving significant life-years lost. If there was no
evidence of excess mortality from studies of long-term exposures, it
might be inferred based on the short-term studies that reported daily
mortality was due solely to lifespan shortening of only days or weeks
in individuals already near death.
This qualitative coherence is further supported by the quantitative
coherence across several health endpoints. For example, if the
relationships were causal, PM-related hospitalization would be expected
to occur substantially more frequently than PM-related mortality (even
though many deaths attributed to air pollution probably do not occur in
hospitals). The Criteria Document notes that is indeed the case (U.S.
EPA, 1996a, p. 13-64 and Table 13-8). Based on the relative risk
estimates from the short-term exposure studies, expected increases in
respiratory- and cardiovascular-related hospital admission rates
associated with PM are substantially larger than the expected increases
in mortality rates for the same causes.
The coherence in the epidemiological evidence is strengthened by
those studies in which different health effects are associated with
ambient PM concentrations in the same study population. Specifically,
studies of Detroit, Birmingham, Philadelphia, and Utah Valley all find
that ambient PM concentrations in each of these cities are associated
with increases in a variety of respiratory- and cardiovascular-related
health effects in the elderly and adult subpopulations in these cities
(U.S. EPA, 1996a, p. 13-66).
As summarized above, there is evidence that PM exposure is
associated with increased risk for health effects ranging in severity
from asymptomatic pulmonary function decrements, to respiratory and
cardiopulmonary illness requiring hospitalization, to excess mortality
from respiratory and cardiovascular causes (U.S. EPA, 1996a, p. 13-67).
The consistency and coherence of the epidemiological evidence greatly
adds to the strength and plausibility of the reported associations. The
Criteria Document concludes that the overall coherence of the health
effects evidence suggests (a likely causal role of ambient PM in
contributing to the reported effects) (U.S. EPA, 1996a, p. 13-1).
4. Particulate Matter Fractions of Concern
The previous criteria and standards review included an integrated
examination of available literature on the potential mechanisms,
consequences, and observed responses to particle deposition in the
major regions of the respiratory tract (U.S. EPA, 1982b). The review
concluded with general agreement that particles that deposit in the
thoracic region (tracheobronchial and alveolar regions) (i.e.,
particles smaller than 10 m diameter), were of greatest
concern for public health. Thus, the PM NAAQS were revised as a result
of the last review from TSP to PM10 standards. Particle dosimetry
and mechanistic considerations developed in the current review continue
to support the view that, for particles that typically occur in the
ambient air, those that are capable of penetrating to the thoracic
regions of the respiratory tract are of greatest concern to health
(U.S. EPA, 1996b, Section V).
Section V.F of the Staff Paper summarizes the evidence regarding
the health effects associated with the fine (PM2.5) and coarse
(PM10-2.5) fractions of PM10. Both fine and coarse fraction
particles can deposit in the thoracic regions of the respiratory tract.
However, based on atmospheric chemistry, exposure, and mechanistic
considerations, the Criteria Document concludes it would be most
appropriate to ``consider fine and coarse mode particles as separate
subclasses of pollutants'' (U.S. EPA, 1996a, p. 13-94), and to measure
them separately as a basis for planning effective control strategies.
Given the significant physical and chemical differences between the
two subclasses of PM10 (U.S. EPA, 1996b, pp. V-69-78), it is
reasonable to expect that differences may exist between fine and coarse
fraction particles in both the nature of potential effects and the
relative concentrations required to produce such effects. The Criteria
Document highlights a number of specific components of PM that could be
of concern to health, including components typically within the fine
fraction (e.g., acid aerosols including sulfates, certain transition
metals, diesel particles, and ultrafine particles), and other
components typically within the coarse fraction (e.g., silica,
resuspended dust, and bioaerosols). While components of both fractions
can produce health effects, in general the fine fraction appears to
contain more of the reactive substances potentially linked to the kinds
of effects observed in the epidemiological studies. The fine fraction
also contains by far the largest number of particles and a much larger
aggregate surface area than the coarse fraction. The greater surface
area of the fine fraction increases the potential for surface
absorption of other potentially toxic components of PM (e.g., metals,
acids, organic materials), and dissolution or absorption of pollutant

[[Page 65649]]

gases and their subsequent deposition in the thoracic region.
The Staff Paper presents the available quantitative and qualitative
information on the effects of fine particles and its constituents (U.S.
EPA, 1996b, pp. V-60-63). Because of the number of pertinent studies
published since the last review, far more quantitative epidemiological
data exist today for relating fine particles to mortality, morbidity,
and lung function changes in sensitive subpopulations, in terms of both
short- and long-term ambient concentrations, than was the case for
PM10 at the conclusion of the last review.\12\ Like the more
numerous PM10 studies, the fine particle studies (e.g., studies
using PM2.5, sulfates) generally find statistically significant
positive associations between fine particle concentrations and
mortality and morbidity endpoints, with more than 20 studies conducted
in a number of geographic locations throughout the world, including the
U.S., Canada, and Europe. More specifically, daily mortality effects
estimates reported for PM2.5 fall within the range of
approximately 3 to 6 percent increases in relative risk for a 25
g/m\3\ increase in 24-hour average PM2.5 concentrations,
for those cities with statistically significant positive associations
(U.S. EPA, 1996b, Table V-12). This collection of studies shows
qualitative coherence in the types of health effects associated with
fine particle exposure including mortality, morbidity, symptoms, and
changes in lung function (U.S. EPA, 1996b, Tables V-11 to V-13).
---------------------------------------------------------------------------

\12\ The 1986 Staff Paper cited PM studies conducted in
essentially 3 locations as a basis for the 24-hour standard, and 4
studies involving a total of 10 cities as a basis for the annual
standard; none measured PM10 directly (EPA, 1986b).
---------------------------------------------------------------------------

By contrast, the current review finds much less direct
epidemiological or toxicological evidence regarding the potential
effects of coarse fraction particles at typical ambient concentrations.
As discussed in the Staff Paper, community epidemiological studies
directly comparing the effects of fine and coarse fraction particles
provide evidence that reported PM associations with mortality and
decreased lung function in children are more likely associated with
fine fraction particles (U.S. EPA, 1996b, pp. V-63-67). On the other
hand, both past and current reviews of occupational and toxicological
literature have found ample qualitative reasons for concern about
higher-than-ambient concentrations of coarse fraction particles. At
such elevated levels, coarse fraction particles are linked to short-
term effects such as aggravation of asthma and increased upper
respiratory illness, which are consistent with enhanced deposition of
coarse fraction particles in the tracheobronchial region (U.S. EPA,
1996a, p. 13-51). Children may be particularly sensitive to such an
effect, since they typically spend more time in outdoor activities,
such that they may encounter higher exposures and doses of coarse
fraction particles than other potentially sensitive populations.
In addition, long-term deposition of insoluble coarse fraction
particles in the alveolar region may have the potential for enhanced
toxicity, in part because clearance from this region of the lung is
significantly slower than from the tracheobronchial region. Limited
qualitative support for this concern is found in autopsy studies of
animals and humans exposed to various ambient crustal dusts at or
slightly above ambient levels typical in the Southwest.
Unlike the case for fine particles, the clearest community
epidemiological evidence regarding coarse fraction particles finds such
effects only in areas with numerous marked exceedances of the current
PM10 standard (U.S. EPA, 1996a, p. 13-51). In this regard, it
appears that the weight of the available evidence allowing direct
comparisons between the two size fractions of PM10 suggests that
ambient coarse fraction particles are either less potent or a poorer
surrogate for community effects of air pollution than are fine fraction
particles.

B. Quantitative Risk Assessment

The Staff Paper presents the results of a quantitative assessment
of health risks for two example cities, including risk estimates for
several categories of health effects associated with: (1) existing PM
air quality levels, (2) projected PM air quality levels that would
occur upon attainment of the current PM10 standards, and (3)
projected PM air quality levels that would occur upon attainment of
alternative PM2.5 standards. As an integral part of this
assessment, qualitative and, where possible, quantitative
characterizations of the uncertainties in the resulting risk estimates
have been developed, as well as information on baseline incidence rates
for the health effects considered. The risk assessment is intended as
an aid to the Administrator in judging which alternative PM NAAQS would
reduce risks sufficiently to protect public health with an adequate
margin of safety, recognizing that such standards will not be risk-
free.
As discussed in Section A above, the Criteria Document concludes
that the overall consistency and coherence of the epidemiological
evidence suggests a likely causal role of ambient PM in contributing to
adverse health effects. An alternative interpretation is that PM may be
serving as an index for the complex mixture of pollutants in urban air.
The manner in which the PM epidemiological evidence is used in this
risk assessment is consistent with either of these alternative
interpretations of the evidence.
Despite the consistency and coherence of the epidemiological
evidence reporting health effects associated with PM, EPA cautions that
quantitative risk estimates derived from these studies include
significant uncertainty, and thus, should not be viewed as demonstrated
health impacts. EPA believes, however, that they do represent
reasonable estimates as to the possible extent of risk for these
effects given the available information.
1. Overview
The following discussion briefly summarizes the scope of the risk
assessment and key components of the risk model. A more detailed
discussion of the risk assessment methodology and results is presented
in the Staff Paper and technical support documents (Abt Associates,
1996a, b).
The risk assessment focused on selected health effects endpoints
discussed above for which adequate quantitative information is
available (U.S. EPA, 1996a, Table VI-2), including increased daily
mortality, increased hospital admissions for respiratory and
cardiopulmonary causes, and increased respiratory symptoms in children.
All concentration-response relationships used in the assessment were
based on findings from human epidemiological studies, and consequently
rely on fixed-site, population-oriented, ambient monitors as a
surrogate for actual PM exposures.
Risk estimates were developed for the urban centers of two example
cities, one eastern (Philadelphia County) and one western (Southeast
Los Angeles County), for which sufficient PM10 and PM2.5 air
quality data were available. Risk estimates were calculated only for
ambient PM levels in excess of estimated annual average background
levels. 13 This approach of estimating

[[Page 65650]]

risks in excess of background was judged to be more relevant to policy
decisions regarding ambient air quality standards than risk estimates
that include effects potentially attributable to uncontrollable
background PM concentrations. For these analyses, an estimate of the
annual average background level was used, rather than a maximum 24-hour
value, since estimated risks were aggregated for each day throughout
the year. Risks have been estimated for a recent year of PM air quality
data in each of the two example cities. Risk estimates were calculated
for Los Angeles County with PM levels adjusted downward to just attain
the current PM10 standards. Finally, risk estimates were also
calculated for both example cities where PM levels were further
adjusted to just attain various alternative PM2.5 standards.
---------------------------------------------------------------------------

\13\ As discussed in Chapter IV of the Staff Paper, annual
average background levels of PM2.5 are estimated to range from
approximately 1-4 g/m\3\ in western areas and 2-5
g/m\3\ in eastern areas, with the maximum 24-hour levels
estimated to reach as high as about 15-20 g/m\3\ over the
course of a year. Background PM is defined in the Staff Paper as the
distribution of PM concentrations that would be observed in the U.S.
in the absence of anthropogenic emissions of PM and precursor
emissions of VOC, NOx, and SOx in North America.
---------------------------------------------------------------------------

As discussed in Chapter 13 of the Criteria Document, the
interpretation of specific concentration-response relationships is the
most problematic issue in conducting risk assessments for PM-associated
health effects at this time, due to (1) the absence of clear evidence
regarding mechanisms of action for the various health effects of
interest; (2) uncertainties about the shape of the concentration-
response relationships; and (3) concern about whether the use of
ambient PM2.5 and ambient PM10 fixed-site monitoring data
adequately reflects the relevant population exposures to PM that are
responsible for the reported health effects. The reported study results
used in this assessment are based on linear concentration-response
models extending only down to the lowest PM concentrations observed
within each study. \14\ Thus, concentration-response relationships were
not extrapolated below the range of the PM concentration air quality
data reported in any given study. Alternatively, the data do not rule
out the possibility of an underlying non-linear, threshold
concentration-response relationship. Although these alternative
interpretations of study results could significantly affect estimated
risks, only very limited information is available to aid in resolving
this issue (U.S. EPA, 1996a, section 13.6.5). Thus, the approach taken
in the PM risk assessment is to address alternative concentration-
response models through sensitivity and integrated uncertainty analyses
to develop ranges of estimated risks, rather than characterizing any
particular set of risk estimates as representing the ``best''
estimates.
---------------------------------------------------------------------------

\14\ See Table VI-2 in the Staff Paper (U.S. EPA, 1996b) for
information about the reported PM mean and range of concentration
levels observed in the various epidemiological studies used in the
risk assessment.
---------------------------------------------------------------------------

Risk estimates for PM-associated health effects in excess of
background PM levels (i.e., excess risk) were initially developed based
on a set of ``base case'' assumptions. These base case assumptions
reflect the use of: (1) Mid-point estimates from the ranges of
estimated annual average background concentrations for the eastern and
western regions of the U.S. to represent typical background levels; (2)
essentially linear concentration-response relationships down to the
lowest PM level observed in each study; and (3) annual distributions of
24-hour PM10 and PM2.5 concentrations that were obtained by
taking a recent year of PM air quality data in each example city and
adjusting all PM concentrations exceeding the estimated background
concentration level by the same percentage to simulate attainment of
alternative standards (referred to as a ``proportional rollback''
approach). While there are many different methods of adjusting PM air
quality distributions to reflect future attainment of alternative
standards, analysis of historical data (Abt, 1996b) support the use of
such a proportional method for adjusting air quality values.
For comparison with alternative standards, it is desirable to
estimate health risks associated with PM air quality that do not
include the effect of concentrations in excess of those allowed by the
current PM10 standards. Since the air quality in one of the two
cities examined, Los Angeles, exceeded the current PM10 standards,
both PM10 and PM2.5 concentrations were proportionally rolled
back (preserving the PM2.5/PM10 ratio) to air quality
concentrations that just attain the current PM10 standards. While
this necessarily introduces additional uncertainty into the risk
estimates, it is required in order to compare risks associated with
attaining the current PM10 standards with risks associated with
attainment of alternative PM2.5 standards.
Sensitivity analyses have been conducted to examine the impact on
the risk estimates of these and other assumptions, by varying each
assumption independently. For example, the impact of using alternative
estimates for background concentrations was examined by replacing the
mid-point estimate with the lower and the upper end of the range of
estimated annual average background levels. In addition, integrated
uncertainty analyses have been conducted specifically for the excess
mortality associated with PM exposures to examine the range of risk
estimates when several key assumptions and uncertainties are considered
simultaneously, rather than one at a time. The key issues examined in
the integrated uncertainty analyses include: (1) Variability in the
underlying concentration-response relationship resulting from combining
the results of PM2.5 mortality studies in six cities to estimate
the relative risks in the two example cities; (2) consideration of
alternative potential threshold concentrations; (3) inclusion of the
range of estimates for PM background levels; and (4) use of alternative
PM air quality adjustment procedures to simulate attainment of
alternative standards based on analysis of historical data.
2. Key Observations
The discussion below highlights the key observations and insights
from the risk assessment, together with important caveats and
limitations.

(1) Fairly wide ranges of estimates of the incidence of PM-
related mortality and morbidity effects were calculated for the two
locations analyzed when the effects of key uncertainties and
alternative assumptions were considered.

This point is illustrated below for mortality estimates using base
case and alternative assumptions, as well as for morbidity estimates
using base case assumptions alone.15 For example, the incidence of
mortality associated with short-term PM2.5 exposures upon
attainment of the current PM10 standards was estimated to range
from approximately 400 to 1,000 deaths per year in Los Angeles County
(with a population of 3.6 million) under base case assumptions, and
from approximately 100 to 1,000 deaths using alternative assumptions
considered in the integrated uncertainty analysis.16 For
Philadelphia County (with a population of 1.6 million), a city with
better air quality than Los Angeles and already well below the current
PM10

[[Page 65651]]

standards, estimated mortality associated with short-term PM2.5
exposures ranged from approximately 200 to 500 deaths per year under
base case assumptions, and from approximately 20 to 500 deaths per year
under alternative assumptions considered in the integrated uncertainty
analyses.17
---------------------------------------------------------------------------

\15\ In the examples presented here the ranges of estimated
incidences are based on the 90 percent credible intervals from the
risk analyses. The 90 percent credible interval represents the range
from the 5th percentile to the 95th percentile of the estimated risk
distribution, and provides a reasonable characterization of the
range of estimated values that results from the various
uncertainties that could be incorporated quantitatively in the risk
analyses.
\16\ Incidence estimates of roughly 400 to 1,000 excess deaths
per year represent roughly 2 to 4 percent of the total mortality
incidence in Los Angeles County.
\17\ Incidence estimates of 200 to 500 excess deaths per year
associated with PM exposures represent roughly 1 to 2.5 percent of
total mortality in Philadelphia County.
---------------------------------------------------------------------------

Morbidity effects associated with exposures to PM2.5 are
estimated using base case assumptions to range from approximately 250
to 1,600 respiratory-related hospital admissions per year and from
23,000 to 58,000 cases of respiratory symptoms in children per year for
Los Angeles.18 For Philadelphia County, morbidity effects
associated with exposures to PM2.5 are estimated using base case
assumptions to range from about 70 to 450 respiratory-related hospital
admissions and from 6,000 to 15,000 cases of respiratory symptoms per
year.19
---------------------------------------------------------------------------

\18\ Incidence estimates of 250 to 1,600 respiratory-related
hospital admissions associated with PM exposures represent roughly
1.5 to 10 percent of total respiratory-related hospital admissions
in Los Angeles County. Incidence estimates of 23,000 to 58,000 cases
of respiratory symptoms represent roughly 15 to 40 percent of total
respiratory symptom cases in Los Angeles County.
\19\ Incidence estimates of 70 to 450 cardiopulmonary-related
hospital admissions associated with PM exposures represent roughly
0.5 to 3.5 percent of total respiratory-related hospital admissions
in Philadelphia County. Incidence estimates of 6,000 to 15,000 cases
of respiratory symptoms associated with PM exposures represent
roughly 10 to 30 percent of total respiratory symptom cases in
Philadelphia County.
---------------------------------------------------------------------------

(2) Risk estimates associated with attainment of alternative
PM2.5 standards described in the Staff Paper show highly
variable reductions in PM-associated risk which are a function of
the particular city and the levels of the standards.

Risk estimates for PM-associated mortality and morbidity health
effects have been estimated for alternative annual PM2.5 standards
20 of 15 and 20 g/m3, alone and in combination with
alternative daily standards 21 ranging from 25 to 65 g/
m3. For two cases considering only annual PM2.5 standards,
the mean estimates (using base case assumptions) of excess mortality
and morbidity associated with short-term PM2.5 exposures in Los
Angeles County were reduced by roughly 45-50% for attainment of an
annual PM2.5 standard level of 15 g/m3, and by
roughly 20-25% for attainment of an annual standard level of 20
g/m3.22 These estimates of risk reduction are
incremental to the risk reductions associated with attainment of the
current PM10 standards as explained above. Similarly, for an area
already in attainment with the current PM10 standards
(Philadelphia County), mean estimates of excess morbidity and mortality
associated with short-term exposures to PM2.5 were not affected by
an annual standard of 20 g/m3 but were reduced by about
15-20% upon attainment of an annual PM2.5 standard of 15
g/m3.23
---------------------------------------------------------------------------

\20\ The annual standards analyzed were simulated by adjusting
the annual average concentration at the population-oriented monitor
in the study area with the highest measured values to the standard
level under consideration.
\21\ The alternative daily standards analyzed were the 1-
expected-exceedance form of the standard.
\22\ In Los Angeles County, a 45-50% reduction in excess
mortality and morbidity associated with short-term PM2.5
exposures represents decreases of roughly 320 excess deaths, 540
cardiopulmonary-related hospital admissions, and 22,000 cases of
respiratory symptoms; a 20-25% reduction represents decreases of
roughly 150 excess deaths, 250 cardiopulmonary-related hospital
admissions, and 11,000 cases of respiratory symptoms.
\23\ In Philadelphia County, a 15-20% reduction in excess
mortality and morbidity associated with short-term PM2.5
exposures represents decreases of roughly 60 excess deaths, 70
cardiopulmonary-related hospital admissions, and 2,000 cases of
respiratory symptoms.
---------------------------------------------------------------------------

As noted above, risk estimates for PM-associated mortality and
morbidity health effects also have been estimated for alternative 24-
hour PM2.5 standards ranging from 25 to 65 g/m3 (in
combination with an annual standard of 20 g/m3). These
combinations of standards result in cases for which the 24-hour
standard was generally controlling the degree of risk reduction. Mean
estimates of excess mortality and morbidity associated with short-term
PM2.5 exposures in Los Angeles County were reduced by roughly 85%
for a daily standard of 25 g/m3, and by roughly 40-50%
for a daily standard of 65 g/m3, beyond the risks
associated with attainment of the current PM10 standards when base
case assumptions were used.24 Similarly, for Philadelphia County,
the mean estimates of excess mortality and morbidity were reduced by
roughly 70-75% for a daily standard of 25 g/m3, and about
10% for a daily standard of 65 g/m3.25
---------------------------------------------------------------------------

\24\ In Los Angeles County, an 85% reduction in excess
mortality and morbidity associated with short-term PM2.5
exposures represents decreases of roughly 590 excess deaths, 1000
cardiopulmonary-related hospital admissions, and 37,000 cases of
respiratory symptoms; a 40-50% reduction represents decreases of
roughly 280 excess deaths, 480 cardiopulmonary-related hospital
admissions, and 20,000 cases of respiratory symptoms.
\25\ In Philadelphia County, a 70-75% reduction in excess
mortality and morbidity associated with short-term PM2.5
exposures represents decreases of roughly 260 excess deaths, 320
cardiopulmonary-related hospital admissions, and 8,000 cases of
respiratory symptoms; a 10% reduction represents decreases of
roughly 40 excess deaths, 50 cardiopulmonary-related hospital
admissions, and 1,000 cases of respiratory symptoms.

(3) Based on the results from the sensitivity analyses of key
uncertainties and the integrated uncertainty analyses, the single
most important factor influencing the uncertainty associated with
the risk estimates is whether or not a threshold concentration
exists below which PM-associated health risks are not likely to
---------------------------------------------------------------------------
occur.

Alternative assumed threshold concentrations considered in these
analyses result in as much as a 3- to 4-fold difference in estimated
risk associated with PM exposures in Los Angeles County (U.S. EPA,
1996b, Figure VI-8; Abt Associates, 1996b, Exhibits 7.19 and 7.20)
depending on the likelihood imputed to various PM2.5 threshold
concentrations. In an area with PM concentrations well below the
current PM standards (e.g., Philadelphia County), differences in risk
associated with a recent year of PM air quality may be even greater for
alternative threshold assumptions, since these locations would be
expected to have a greater proportion of PM concentrations below
assumed threshold concentrations.

(4) Based on results from the sensitivity analyses of key
uncertainties and/or the integrated uncertainty analyses,
quantitative consideration of the following uncertainties is
estimated to have a much more modest impact on the risk estimates:
(a) Inclusion of individual co-pollutant species when estimating PM
effect sizes (based on reported estimates of effects modification);
(b) the choice of approach to adjusting the slope of the
concentration-response relationship when analyzing alternative
possible threshold concentrations; (c) the value chosen to represent
average background PM concentrations; and (d) the choice of air
quality adjustment approaches for simulating attainment of
alternative PM standards.
(5) Additional sources of uncertainty associated with risk
analyses of alternative PM2.5 standard scenarios which could
not be addressed quantitatively include: (a) Uncertainty in the
pattern of air quality concentration reductions that would be
observed across the distribution of 24-hour PM2.5
concentrations in areas attaining the standards, and (b) uncertainty
concerning the degree to which PM concentration-response
relationships may reflect contributions from other pollutants, or
the particular contribution of certain constituents of PM2.5,
and whether such constituents would be reduced in similar proportion
as the reduction in PM2.5.

To the extent concentrations of other combustion source co-
pollutants are reduced more or less than PM2.5 concentrations in
attaining alternative PM2.5 standards, estimates of health effects
reduced by such standards would be expected to be related to the degree
to which these co-pollutants in fact play a role in producing or
modifying PM-associated effects. Similarly, if specific constituents of
PM2.5 mass have differing potencies in

[[Page 65652]]

producing effects relative to other PM2.5 constituents, estimates
of risk reduced would be expected to vary if these constituent
concentrations are reduced to different degrees by control strategies
designed to attain alternative PM2.5 standards.

(6) The peak 24-hour PM2.5 concentrations appear to
contribute a relatively small amount to the total health risk posed
by the entire air quality distribution as compared to the risks
associated with the low to mid-range concentrations.

Standards with a 24-hour averaging time are traditionally based on
the highest 24-hour values observed in a year, concentrations for which
the risk on an individual day is highest. However, examining a typical
distribution of ambient 24-hour PM2.5 concentrations over the
course of a year in conjunction with PM2.5 concentration-response
relationships, as illustrated in Figures 2a, 2b, and 2c, the peak
PM2.5 concentrations contribute much less to the total health risk
over a year than the low- to mid-range PM2.5 concentrations.
More specifically, Figures 2a, 2b, and 2c illustrate some of the
characteristics of the integration of air quality distributions and
concentration-response relationships as used to predict total risk from
ambient particle exposures across a year. These figures show the
relative contribution of different portions of a typical urban ambient
PM2.5 concentration distribution to mortality risk from short-term
exposures. As shown in Figures 2b and 2c, low- to mid-range
concentrations (e.g., 10-50 g/m3) account for the largest
amount of estimated mortality risk on an annualized basis.
The portion of the air quality distribution that contributes
significantly to total health risk over the course of a year is, of
course, smaller if effects thresholds are assumed or if much higher
levels of estimated background PM2.5 concentrations are used
(Figure 2c). However, even with this assumption, most of the aggregate
risk associated with short-term exposures likely results from the large
number of days during which the 24-hour average concentrations are in
the low- to mid-range, below peak 24-hour concentrations. Even though
higher 24-hour concentrations, including peaks above 70 g/
m3, clearly contribute more mortality per day than low- to mid-
range concentrations, the much larger number of days within the low- to
mid-ranges results in this interval being associated with the largest
proportion of the total risk.

BILLING CODE 6560-50-P

[GRAPHIC] [TIFF OMITTED] TP13DE96.048

Figure 2a. Illustrative Air Quality Distribution of 24-Hour
PM2.5 Concentrations--This figure shows an example of a
frequency distribution of the number of days exceeding various 24-
hour average PM2.5 concentrations over a year.
[GRAPHIC] [TIFF OMITTED] TP13DE96.049

Figure 2b. Estimated Mortality Risks Using A Non-Threshold
Concentration-Response Relationship--This figure illustrates the
proportion of estimated mortality incidence, using a non-threshold
concentration-response relationship, associated with each
concentration range shown above in Figure 2a.

[[Page 65653]]

[GRAPHIC] [TIFF OMITTED] TP13DE96.050

BILLING CODE 6560-50-C

Figure 2c. Estimated Mortality Risks Using An Illustrative
Threshold Concentration-Response Relationship--This figure
illustrates the proportion of estimated mortality incidence, using
an example threshold concentration of 18 g/m3
PM2.5, associated with each concentration range shown above in
Figure 2a.

An annual PM2.5 standard would almost certainly require areas
whose air quality concentrations are above those necessary for
attainment to reduce PM2.5 concentrations across a wide range of
the 24-hour air quality distribution rather than just a few high 24-
hour values, thus resulting in more significant risk reduction than
would a 24-hour standard set so as to control the peak concentrations.
Further, an annual standard would be expected to lead to greater
consistency in the risk reduced in different geographic areas having
similar initial air quality than would a 24-hour standard of similar
impact, in terms of the number of areas affected. Such a 24-hour
standard would focus on reducing the highest 24-hour concentrations
rather than on the entire air quality distribution.

(7) There is greater uncertainty about estimated excess
mortality (and other effects) associated with PM exposures as one
considers increasingly lower concentrations approaching background
levels.

As discussed in Section A above, one of the most important
uncertainties related to estimating excess mortality associated with PM
exposures is the shape of the concentration-response relationship. The
existing epidemiological data reporting excess mortality associated
with PM exposures do not rule out the possibility that there may be a
threshold concentration below which excess mortality associated with PM
exposures does not occur. As one considers progressively higher PM
concentrations it is increasingly unlikely that there is a threshold at
these higher levels. In contrast, as one considers increasingly lower
PM concentrations, there is increasing uncertainty about the shape and
magnitude of the estimated concentration-response relationship over the
lower range of concentrations. This increasing uncertainty is due to
questions about: (1) The possible impact of multiple co-pollutants on
the estimated concentration-response relationships; (2) whether
exposure misclassification associated with the use of ambient monitors
as a measure of population exposure might be masking a non-linear
relationship; and (3) whether a biological threshold may exist below
which excess mortality associated with PM exposures does not occur. In
addition, there is uncertainty about background levels, and thus about
the extent to which effects associated with PM exposures at
concentrations approaching estimated background levels are attributable
to controllable, non-background sources of ambient PM.

C. Need for Revision of the Current Primary PM Standards

The overarching issue in the present review of the primary NAAQS is
whether, in view of the advances in scientific knowledge reflected in
the Criteria Document and Staff Paper, the existing standards should be
revised and, if so, what revised or new standards would be appropriate.
The concluding section of the integrative summary of health effects
information in the Criteria Document provides the following summary of
the science with respect to this issue:

The evidence for PM-related effects from epidemiologic studies
is fairly strong, with most studies showing increases in mortality,
hospital admissions, respiratory symptoms, and pulmonary function
decrements associated with several PM indices. These epidemiologic
findings cannot be wholly attributed to inappropriate or incorrect
statistical methods, misspecification of concentration-effect
models, biases in study design or implementation, measurement errors
in health endpoint, pollution exposure, weather, or other variables,
nor confounding of PM effects with effects of other factors. While
the results of the epidemiology studies should be interpreted
cautiously, they nonetheless provide ample reason to be concerned
that there are detectable health effects attributable to PM at
levels below the current NAAQS (U.S. EPA, 1996a, p. 13-92).

Given the nature of the health effects in question, this finding
clearly suggests that revision of the current NAAQS is appropriate. The
extensive PM epidemiological data base provides evidence of serious
health effects (e.g., mortality, exacerbation of chronic disease,
increased hospital admissions) in sensitive subpopulations (e.g., the
elderly, individuals with cardiopulmonary disease). Although the
increase in relative risk is small for the most serious outcomes (see
Figure 1), it

[[Page 65654]]

is likely significant from an overall public health perspective,
because of the large number of individuals in sensitive subpopulations
that are exposed to ambient PM and the significance of the health
effects (U.S. EPA, 1996a, p. 1-21).
While the lack of demonstrated mechanisms that explain the range of
epidemiological findings is an important caution, which presents
difficulties in providing an integrated assessment of PM health effects
research, qualitative information from laboratory studies of the
effects of particle components at high concentrations and dosimetry
considerations suggest that the kinds of effects observed in community
studies (e.g., respiratory- and cardiovascular-related responses) are
at least plausibly related to PM.26 Indeed, the Criteria Document
and Section V.E of the Staff Paper point to the consistency of the
results of the epidemiological studies from a large number of different
locations and the coherent nature of the observed effects as being
suggestive of a likely causal role of ambient PM in contributing to the
reported effects.
---------------------------------------------------------------------------

\26\ Epidemiological studies alone cannot be used to demonstrate
mechanisms of action, but they can provide evidence useful in making
inferences with regard to causal relationships (U.S. EPA, 1996b, p.
V-9).
---------------------------------------------------------------------------

Given the evidence that such effects may occur at levels below the
current standards, the serious nature and potential magnitude of the
public health risks involved, and the need to consider the fine and
coarse fractions as distinct classes of particles, the Staff Paper and
the CASAC (Wolff, 1996b) concluded that revision of the current
standards is clearly appropriate. Moreover, at the May 1996 public
meeting (U.S. EPA, 1996e), and in separate written comments (including
Lippmann et al., 1996), a majority of CASAC panel members recommended
revisions that would strengthen the health protection provided by the
current PM standards. Based on the rationale and recommendations
contained in the Staff Paper and the CASAC closure letter, the
Administrator concludes that the current PM standards should be
revised.

D. Indicators of PM

In formulating alternative approaches to establishing adequately
protective, effective, and efficient PM standards, it is necessary to
specify the fraction of particles found in the ambient air that should
be used as the indicator(s) for the standards. In this regard, the most
recent assessment of scientific information in the Criteria Document,
summarized in Chapters IV and V of the Staff Paper, continues to
support past staff and CASAC recommendations regarding the selection of
size-specific indicators for PM standards. More specifically, the Staff
Paper finds that the following conclusions reached in the 1987 review
remain valid:
(1) Health risks posed by inhaled particles are influenced both by
the penetration and deposition of particles in the various regions of
the respiratory tract and by the biological responses to these
deposited materials.
(2) The risks of adverse health effects associated with deposition
of ambient fine and coarse fraction particles in the thoracic
(tracheobronchial and alveolar) regions of the respiratory tract are
markedly greater than for deposition in the extrathoracic (head)
region. Maximum particle penetration to the thoracic region occurs
during oronasal or mouth breathing.
(3) The risks of adverse health effects from extrathoracic
deposition of general ambient PM are sufficiently low that particles
which deposit only in that region can safely be excluded from the
standard indicator.
(4) The size-specific indicator(s) should represent those particles
capable of penetrating to the thoracic region, including both the
tracheobronchial and alveolar regions.
These conclusions, together with information on the dosimetry of
particles in humans, were the basis for the promulgation in 1987 of a
new size-specific indicator for the PM NAAQS, PM10, that includes
particles with an aerodynamic diameter smaller than or equal to a
nominal 10 m. The recent information on human particle
dosimetry contained in the Criteria Document provides no basis for
changing 10 m as the appropriate cut point for particles
capable of penetrating to the thoracic regions.
The Staff Paper concludes, however, that continued use of PM10
as the sole indicator for the PM standards would not provide the most
effective and efficient protection from the health effects of
particulate matter (U.S. EPA, 1996b, pp. VII-4-11). The recent health
effects evidence and the fundamental physical and chemical differences
between fine and coarse fraction particles have prompted consideration
of separate standards for the fine and coarse fractions of PM10.
In this regard, the Criteria Document concludes that fine and coarse
fractions of PM10 should be considered separately (U.S. EPA,
1996a, p. 13-93). Taking into account such information, CASAC found
sufficient scientific and technical bases to support establishment of
separate standards relating to these two fractions of PM10.
Specifically, CASAC advised the Administrator that ``there is a
consensus that retaining an annual PM10 NAAQS * * * is reasonable
at this time'' and that there is ``also a consensus that a new
PM2.5 NAAQS be established'' (Wolff, 1996b).
While it is difficult to distinguish the effects of either fine or
coarse fraction particles from those of PM10, comparisons between
fine and coarse fraction particles presented in the Staff Paper suggest
that fine particles are a better surrogate for those components of PM
that are linked to mortality and morbidity effects at levels below the
current standards (U.S. EPA, 1996b, P. VII-18). Moreover, a regulatory
focus on fine particles would likely also result in controls on gaseous
precursors of fine particles (e.g., SOX, NOX, VOC), which are
all components of the complex mixture of air pollution that has most
generally been associated with mortality and morbidity effects. The
Staff Paper concludes that, in contrast to fine particles, coarse
fraction particles are more clearly linked with certain morbidity
effects at levels above those allowed by the current 24-hour standard.
The Administrator concurs with staff and CASAC recommendations to
control particles of health concern (i.e., PM10) through separate
standards for fine and coarse fraction particles. The following
sections outline the basis for the Administrator's decision on specific
indicators for fine and coarse particle standards.
1. Indicators for the Fine Fraction of PM10
The Administrator concludes that it is appropriate to control fine
particles as a group, as opposed to singling out particular components
or classes of fine particles. The qualitative literature, evaluated in
Chapter 11 of the Criteria Document and summarized in Section V.C of
the Staff Paper, has reported various health effects associated with
high concentrations of a number of fine particle components (e.g.,
sulfates, nitrates, organics, transition metals), alone or in some
cases in combination with gases. Community studies have found
significant associations between fine particles or PM10 and health
effects in various areas across the U.S. where such fine particle
components correlate significantly with particle mass. As noted above,
it is not possible to rule out any one of these components as
contributing to fine particle effects. Thus, the Administrator finds
that the present data more readily support a standard based on the
total mass of fine particles.

[[Page 65655]]

In specifying a precise size range for a fine particle standard,
both the staff and CASAC recommend PM2.5 as the indicator of fine
particles (Wolff, 1996b). The particle diameter reflecting the mass
minimum between the fine and coarse modes typically lies between 1 and
3 m, and the scientific data support a sampling cut point to
delineate fine particles in this range. Because of the potential
overlap of fine and coarse particle mass in this intermodal region, EPA
recognizes that any specific sampling cut point would result in only an
approximation of the actual fine-mode particle mass. Thus, the choice
of a specific diameter within this size range is largely a policy
judgment. The staff and CASAC recommendation for a 2.5 m
sampling cut point is based on considerations of consistency with the
community health studies, the limited potential for intrusion of coarse
fraction particles into the fine fraction, and availability of
monitoring technology.27 PM2.5 encompasses all of the
potential agents of concern in the fine fraction, including most
sulfates, acids, fine particle transition metals, organics, and
ultrafine particles, and includes most of the aggregate surface area
and particle number in the entire distribution of atmospheric
particles.
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\27\ Some commentors have recommended the use of a smaller
cutpoint at 1 m (PM1) to further reduce coarse
particle intrusion. PM1 has not been used in health studies,
although in most cases collected mass should be similar to those for
cutpoints of 2.1 or 2.5 m. While this indicator could
reduce intrusion of coarse particles, it might also omit portions of
hygroscopic acid sulfates in high humidity environments. PM1
sampling technologies have been developed; however, PM1
samplers have not been widely used in the field to date, and there
are some concerns about loss of certain organic materials relative
to an instrument with a larger size cut.
---------------------------------------------------------------------------

The Administrator concurs with staff and CASAC recommendations, and
concludes that PM2.5 is the appropriate indicator for fine
particle standards. Details of this definition are further specified in
the Federal Reference Method discussed in section V below and proposed
in a new Appendix L.
2. Indicators for the Coarse Fraction of PM10
The Criteria Document and Staff Paper conclude that epidemiological
information, together with dosimetry and toxicological information,
support the need for a particle indicator that addresses the health
effects associated with coarse fraction particles within PM10
(i.e., PM10-2.5). As noted above, coarse fraction particles can
deposit in those sensitive regions of the lung of most concern.
Although the role of coarse fraction particles in much of the recent
epidemiological results is unclear, limited evidence from studies where
coarse fraction particles are the dominant fraction of PM10
suggest that significant short-term effects related to coarse fraction
particles include aggravation of asthma and increased upper respiratory
illness. In addition, qualitative evidence suggests potential chronic
effects associated with long-term exposure to high concentrations of
coarse fraction particles.
In selecting an indicator for coarse fraction particles, the
Administrator took into account the views of several CASAC panel
members who suggested using the coarse fraction directly (i.e.,
PM10-2.5) as the indicator. However, the Administrator notes that
the existing ambient data base for coarse fraction particles is smaller
than that for fine particles, and that the only studies of clear
quantitative relevance to effects most likely associated with coarse
fraction particles have used undifferentiated PM10. In fact, it
was the consensus of CASAC that it is reasonable to consider PM10
itself as a surrogate for coarse fraction particles, when used in
conjunction with PM2.5 standards. The monitoring network already
in place for PM10 is large. Therefore, in conjunction with the
decision to have separate standards for PM2.5, the Administrator
concludes, consistent with CASAC recommendations, that it is
appropriate to retain PM10 as the particle indicator for standards
intended to protect against the effects most likely associated with
coarse fraction particles.

E. Averaging Time of PM2.5 Standards

As discussed above, the Administrator has concluded that PM2.5
is an appropriate indicator for standards intended to provide
protection from effects associated primarily with fine particles. The
recent health effects information includes reported associations with
both short-term (from less than 1 day to up to 5 days) and long-term
(from generally a year to several years) measures of PM. On the basis
of this information, summarized in Chapter V of the Staff Paper, the
Administrator has considered both short- and long-term PM2.5
standards.
1. Short-term PM2.5 Standard
The current 24-hour averaging time is consistent with the majority
of community epidemiological studies, which have reported associations
of health effects with 24-hour concentrations of various PM indicators
such as PM10, fine particles, and TSP. Such health effects,
including premature mortality and increased hospital admissions, have
generally been reported with same-day, previous day, or longer lagged
single-day concentrations, although some studies have reported stronger
associations with multiple-day average concentrations. In any case, the
Administrator recognizes that a 24-hour PM2.5 standard can
effectively protect against episodes lasting several days, since such a
standard would provide protection on each day of a multi-day episode,
while also protecting sensitive individuals who may experience effects
after even a single day of exposure.
Although most reported effects have been associated with daily or
longer measures of PM, evidence also suggests that some effects may be
associated with PM exposures of shorter durations. For example,
controlled human and animal exposures to specific components of fine
particles, such as acid aerosols, suggest that bronchoconstriction can
occur after exposures of minutes to hours. Some epidemiological studies
of exposures to acid aerosols have also found changes in respiratory
symptoms in children using averaging times less than 24 hours. However,
such reported results do not provide a satisfactory quantitative basis
for setting a fine particle standard with an averaging time of less
than 24 hours, nor do current gravimetric mass monitoring devices make
such shorter durations generally practical at present. Further, the
Administrator recognizes that a 24-hour average PM2.5 standard
which leads to reductions in 24-hour average concentrations is likely
to lead as well to reductions in shorter-term average concentrations in
most urban atmospheres, thus providing some degree of protection from
potential effects associated with shorter duration exposures.
For these reasons, the Administrator has concluded that a short-
term PM2.5 standard with a 24-hour averaging time can serve to
control short-term ambient PM2.5 concentrations, thus providing
protection from health effects associated with short-term (from less
than 1-day to up to 5-day) exposures to PM2.5.
2. Long-Term PM2.5 Standard
Community epidemiological studies have reported associations of
annual and multi-year average concentrations of PM10, PM2.5,
sulfates, and TSP with an array of health effects, notably premature
mortality, increased respiratory symptoms and illness (e.g., bronchitis
and cough in children), and reduced lung function. The relative risks
associated with such measures of long-term exposures, although highly

[[Page 65656]]

uncertain, appear to be larger than those associated with short-term
exposures. Based on the available epidemiology, and consistent with the
limited relevant toxicological and dosimetric information, the
Administrator concludes that significant, and potentially independent,
health consequences are likely associated with long-term PM exposures.
The Administrator has considered this evidence, which suggests that
some health endpoints reflect the cumulative effects of PM exposures
over a number of years. In such cases, an annual standard would provide
effective protection against persistent long-term (several years)
exposures to PM. Requiring a much longer averaging time would also
complicate and unnecessarily delay control strategies and attainment
decisions.
The Administrator has also considered the seasonality of emissions
of fine particles and their precursors in some areas (e.g., wintertime
smoke from residential wood combustion, summertime regional acid
sulfate and ozone formation), which suggests that some effects
assoc

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Source: Frix Law Library, https://www.frixlaw.com/law-library/documents/fr%3A96-30897. Public record. Not legal advice.