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 notice 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.
(Note: _g has been substituted for greek letter mu followed by g as some web browsers do not support mu)
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/m3, annual mean, and 50 _g/m3, 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/m3 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/m3. 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 decision must be received by January
29, 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., S.W.,
Washington, DC 20460.
(Comments of Citizens for a Sound Economy Foundation)
(Comments of Competetive Enterprise Institute)
PUBLIC HEARINGS: The EPA will announce in a separate Federal Register
notice the date, time, and address of the public hearing on this proposed
decision.
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.
B.Related Control Requirements
C. Review of Air Quality Criteria and Standards for PM
II.Rationale for Proposed Decisions on Primary
Standards
E.Averaging Time of PM2.5
Standards
H.Conclusions Regarding
the Current PM10 Standards
III.Rationale for Proposed Decision on the
Secondary Standards
A.PM2.5 Computations and Data Handling
Conventions
V.Reference Methods for the Determination
of Particulate Matter as PM10 and PM2.5 in the Atmosphere
VII.Regulatory
and Environmental Impact Analyses
References
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 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 500-1000
_g/m3 of PM and sulfur dioxide (SO2)) 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).
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
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).
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
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):
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.
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
B.Quantitative Risk
Assessment
1.Overview
Risk estimates for PM-associated mortality and morbidity health effects have
been estimated for alternative annual PM2.5 standards of 15 and 20 _g/m3,
alone and in combination with alternative daily standards 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. 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.
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 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.
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.
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.
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.
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 "
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 will be proposed for public comment in
the future.
Table of Contents
The following topics are discussed in today's preamble:
A.Legislative Requirements
1.Averaging Time and Form
2.Levels for Alternative
Averaging Times
A.Visibility Impairment
B.Materials Damage and Soiling Effects
IV.Revisions to Appendix K --
Interpretation of the PM NAAQS
B.PM10 Computations and Data Handling
Conventions
Proposed Regulatory Text
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
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. 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.
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 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).
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. 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).
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) and coarse (PM10-2.5) 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.
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 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
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
From 1987 to present, numerous epidemiological studies using improved statistical
techniques and expanded particle monitoring data have reported statistically
significant 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).
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 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.
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).
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.
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 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):
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.
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)
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. The Criteria Document and Staff Paper
evaluated the studies with respect to each of these issues, as summarized
below:
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.
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/m3 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
Figure 1. Relationship Between Relative Risk per 50 _g/m3 PM10 and Specific
Causes of Mortality and Morbidity in Adults and Children
[--- Unable To Translate Box ---]
assuming causal relationships between mortality and morbidity effects and
PM exposure (U.S. EPA, 1996a, Section 13.4.1.1).
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
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. 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/m3
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).
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.
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.
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. This approach of estimating 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.
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. 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.
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. 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. 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 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.
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. 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.
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.
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. 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.
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.
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.
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.
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.
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 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. 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.
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.
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. 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.
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 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 associated with annual average
concentrations might be the result of repeated seasonally high exposures.
However, different seasons are likely of concern in different parts of the
country, and the current evidence does not provide a satisfactory quantitative
basis for setting a national fine particle standard in terms of a seasonal
averaging time.
In addition, the Administrator recognizes that an annual standard would have
the effect of controlling air quality broadly across the yearly distribution
of 24-hour PM2.5 concentrations, although such a standard would not as
effectively limit peak 24-hour concentrations as would a 24-hour standard.
Thus, as discussed above in Section B above (see especially Figures 2a, 2b,
2c), an annual standard could also provide protection from health effects
associated with short-term exposures to PM2.5.
For these reasons, the Administrator has concluded that a long-term PM2.5
standard with an annual averaging time can serve to control both long- and
short-term ambient PM2.5 concentrations, thus providing protection from health
effects associated with long-term (seasonal to several years) and, to some
degree, short-term exposures to PM2.5.
3.Combined Effect of Annual and 24-Hour Standards
Having concluded that both 24-hour and annual PM2.5 standards are appropriate,
the Administrator considered the potential combined effects of such standards
on PM concentration levels and distributions prior to considering the form
and level of each standard. The existing health effects evidence could, of
course, be used to assess the form and level of each standard independently,
with short-term health effects evidence being used as the basis for a 24-hour
standard and the long-term health effects evidence as the basis for an annual
standard. Some CASAC panel members apparently used this approach as a basis
for their views on appropriate averaging times and standard levels. In
particular, a few members focused only on a 24-hour PM2.5 standard in light
of the relative strength of the short-term exposure studies. On the other
hand, two members focused only on an annual standard, recognizing that strategies
to meet an annual standard would provide protection against effects of both
short- and long-term exposures.
The Administrator has focused on a policy approach that considers the consistency
and coherence, as well as the limitations, of the body of evidence as a whole,
and recognizes that there are various ways to combine two standards to achieve
an appropriate degree of public health protection. Such an approach to standard
setting that integrates the body of health effects evidence and air quality
analyses, and considers the combined effect of the standards, has the potential
to result in a more effective and efficient suite of standards than an approach
that only considers short- and long-term evidence, analyses, and standards
independently.
In considering the combined effect of such standards, the Administrator notes
that while an annual standard focuses on annual average PM2.5 concentrations,
it would also result in fewer and lower 24-hour peak concentrations.
Alternatively, a 24-hour standard which focuses on peak concentrations would
also result in lower annual average concentrations. Thus, either standard
could be viewed as providing both short- and long-term protection, with the
other standard serving as a "backstop" in situations where the daily peaks
and annual averages are not consistently correlated.
The Administrator believes that the suite of PM2.5 standards can be most
effectively and efficiently defined by treating the annual standard as the
generally controlling standard for lowering both short- and long-term PM2.5
concentrations. As a supplement to the annual standard, the 24-hour standard
would serve as a backstop to provide additional protection against days with
high peak PM2.5 concentrations, localized "hot spots," and risks arising
from seasonal emissions that would not be well controlled by a national annual
standard. In reaching this view, the Administrator took into account the
factors discussed below.
1)Based on one of the key observations from the quantitative risk assessment
(Section B, Figures 2a, 2b, 2c), the Administrator notes that much if not
most of the aggregate annual risk associated with short-term exposures results
from the large number of days during which the 24-hour average concentrations
are in the low- to mid-range, below the peak 24-hour concentrations. As a
result, lowering a wide range of ambient 24-hour PM2.5 concentrations, as
opposed to focusing on control of peak 24-hour concentrations, is the most
effective and efficient way to reduce total population risk. Further, there
is no evidence suggesting that risks associated with long-term exposures
are likely to be disproportionately driven by peak 24-hour concentrations.
Thus, an annual standard that controls an area's attainment status is likely
to reduce aggregate risks associated with both short- and long-term exposures
with more certainty than a 24-hour standard.
2)The consistency and coherence of the health effects data base is more directly
related to long-term measures of air quality (e.g., the annual distributions
of 24-hour PM concentrations), rather than to 24-hour concentrations on
individual days. More specifically, judgments about the quantitative consistency
of the large number of short-term exposure studies reporting associations
with 24-hour concentrations arise from comparing the relative risk results
derived from analyzing the associations across the entire duration of the
studies, which typically spanned at least an annual time frame.
3)An annual average measure of air quality is more stable over time than
are 24-hour measures. Thus, a controlling annual standard is likely to result
in the development of more consistent risk reduction strategies over time,
since an area's attainment status will be less likely to change due solely
to year-to-year variations in meteorological conditions that affect the formation
of fine particles, than under a controlling 24-hour standard.
Under this policy approach, the annual PM2.5 standard would serve in most
areas as the target for control programs designed to be effective in lowering
the broad distribution of PM2.5 concentrations, thus protecting not only
against long-term effects but also short-term effects as well. In combination
with such an annual standard, the 24-hour PM2.5 standard would be set so
as to protect against the occurrence of peak 24-hour concentrations and those
that present localized or seasonal effects of concern in areas where the
highest 24-hour-to-annual mean PM2.5 ratios are appreciably above the national
average.
The Administrator recognizes that this policy approach represents a new way
of thinking about the combined effects of short- and long-term standards,
and that there are alternative views about this approach. Accordingly, the
Administrator solicits comment on this policy approach for defining the most
effective and efficient suite of PM2.5 standards.
F. Form of PM2.5
Standards
1. Annual Standard
As discussed in some detail during the last review of the PM NAAQS (see 49
FR 10408, March 20, 1984; 52 FR 24634, July 1, 1987), the expected annual
arithmetic mean (i.e., the annual arithmetic mean averaged over 3 years)
is a relatively stable measure of air quality that reflects the total cumulative
dose of PM to which an individual or population is exposed. Short-term peaks
have an influence on the arithmetic mean that is proportional to their frequency,
magnitude, and duration, and, thus, their contribution to cumulative exposure
and risk. As a result, the annual arithmetic mean form of an annual standard
provides protection across a wide range of the air quality distribution
contributing to exposure and risk, in contrast to other forms, such as the
geometric mean, that deemphasize the effects of short-term peak concentrations.
On this basis, the Administrator concurs with the Staff Paper recommendation,
supported by CASAC, to use the 3-year average annual arithmetic mean as the
form for an annual PM2.5 standard, consistent with the current form of the
annual PM10 standard.
The Staff Paper and some CASAC panel members also recommended that consideration
be given to calculating the PM2.5 annual arithmetic mean for an area by averaging
the annual arithmetic means derived from multiple, primarily population-oriented
monitoring sites within a monitoring planning area. In considering a calculation
method for annual arithmetic averages that involves spatial averaging of
monitoring data, the Administrator specifically took into account the following
factors:
1)Many of the community-based epidemiological studies examined in this review
used spatial averages, when multiple monitoring sites were available, to
characterize area-wide PM exposure levels and the associated population health
risk. Even in those studies that used only one monitoring location, the selected
site was chosen to represent community-wide exposures, not the highest value
likely to be experienced within the community. Thus, spatial averages are
most directly related to the epidemiological studies used as the basis for
the proposed revisions to the PM NAAQS.
2)Under the policy approach advanced earlier, the annual PM2.5 standard would
be intended to reduce aggregate population risk from both long- and short-term
exposures by lowering the broad distribution of PM2.5 concentrations across
the community. An annual standard based on spatially averaged concentrations
would better reflect area-wide PM exposure levels than would a standard based
on concentrations from a single monitor with the highest measured values.
3)Under this policy approach, the 24-hour PM2.5 standard would be intended
to supplement a spatially averaged annual PM2.5 standard by providing protection
against peak 24-hour concentrations, localized "hot spots," and risk arising
from seasonal emissions that would not be as well controlled by an annual
standard. Accordingly, the 24-hour PM2.5 standard should be based on the
single population-oriented monitoring site within the monitoring planning
area with the highest measured values.
Based on these considerations, the Administrator believes that the form of
a PM2.5 annual standard should be expressed as the annual arithmetic mean,
temporally averaged over 3 years and spatially averaged over all designated
monitoring sites. Such designations would be based on criteria contained
in the proposed revision to the monitoring siting guidance in 40 CFR Part
58 that accompanies this notice. In the Administrator's judgment, an annual
PM2.5 standard expressed in this form, established in conjunction with a
24-hour PM2.5 standard, would provide the most appropriate target for reducing
area-wide population exposure to fine particle pollution.
On the other hand, the Administrator is mindful that adoption of spatial
averaging for an annual PM2.5 standard would add a degree of complexity to
the monitor siting requirements for a new PM2.5 monitoring network and the
specification of those areas across which spatial averaging should be permitted.
These issues are addressed more fully in the accompanying proposed revisions
to 40 CFR Part 58. Of particular concern is whether appropriate and effective
criteria can be developed and implemented for determining areas within which
spatial averaging would be reflective of the area-wide population risk. The
EPA recognizes that some monitoring planning areas may have to be subdivided
into smaller subareas to reflect gradients in particle levels (e.g., upwind
suburban sites, central city sites, downwind sites) as well as topographical
barriers or other factors that may result in a monitoring planning area having
several distinct air quality regimes.
Because of the importance of this issue, the notice of proposed revisions
to 40 CFR Part 58 specifically requests broad public input on the approaches
advanced in that notice with respect to the selection of sites and designation
of areas for spatial averaging. Recognizing the complexities that spatial
averaging may introduce into risk management programs and that unforeseen
issues may arise from public comment on the 40 CFR Part 58 notice, the
Administrator also requests comment on the alternative of basing the annual
standard for PM2.5 on the population-oriented monitor site within the monitoring
planning area with the highest 3-year average annual mean. Based on comments
received, the Administrator may choose either of these two approaches for
specifying the form of the annual PM2.5 standard at the time of promulgation
of any revisions to the PM standards. Proposed methods for using monitored
concentrations to make a comparison with a spatially averaged annual mean
standard, as well as associated calculations and other data handling conventions,
are presented below in the section on proposed revisions to Appendix K.
2. 24-hour Standard
The current 24-hour PM10 standard is expressed in a "1-expected-exceedance"
form. That is, the standard is formulated on the basis of the expected number
of days per year (averaged over 3 years) on which the level of the standard
will be exceeded. The test for determining attainment of the current 24-hour
standard is presented in Appendix K to 40 CFR Part 50.
Since promulgation of the current 24-hour PM10 standard in 1987, a number
of concerns have been raised about the 1-expected-exceedance form. These
include, in particular, the year-to-year stability of the number of exceedances,
the stability of the attainment status of an area, and the complex data handling
conventions specified in Appendix K, including the procedures for making
adjustments for missing data and less-than-every-day monitoring.
In light of these concerns, the Staff Paper and several CASAC panel members
(Wolff, 1996b) recommended that consideration be given to adoption of a more
stable and robust form for 24-hour PM standards. In considering this
recommendation, the Administrator noted that the use of a concentration-based
percentile form would have several advantages over the current
1-expected-exceedance form:
1)Such a concentration-based form is more directly related to the ambient
PM concentrations that are associated with health effects. Given that there
is a continuum of effects associated with exposures to varying levels of
PM, the extent to which public health is affected by exposure to ambient
PM is related to the actual magnitude of the PM concentration, not just whether
the concentration is above a specified level. With an exceedance-based form,
days on which the ambient PM concentration is well above the level of the
standard are given equal weight to those days on which the PM concentration
is just above the standard (i.e., each day is counted as one exceedance),
even though the public health impact on the two days is significantly different.
With a concentration-based form, days on which higher PM concentrations occur
would weigh proportionally more than days with lower PM concentrations for
the design value, since the actual concentrations are used directly in
determining whether the standard is attained.
2)More specifically, a concentration-based percentile form would also compensate
for missing data and less-than-every-day monitoring, thereby reducing or
eliminating the need for complex data handling procedures in the Appendix
K test for attainment. As a result, an area's attainment status would be
based directly on monitoring data rather than on a calculated value adjusted
for missing data or less-than-every-day monitoring.
3)Further, a concentration-based form, averaged over 3 years, also has greater
stability than the expected exceedance form and, thus, would facilitate the
development of more stable implementation programs by the States.
In light of these advantages, and taking into account the CASAC recommendation
as well as concerns regarding adjustments for missing data and
less-than-every-day monitoring, the Administrator believes that adoption
of a concentration percentile form for the 24-hour PM2.5 standard would be
appropriate.
Having reached this view, the Administrator considered various specific
percentile values for such a form. In doing so, she took into account two
factors. First, the 24-hour PM2.5 standard is intended to supplement the
annual PM2.5 standard by providing a "back stop" to provide additional protection
against extremely high peak days, localized "hot spots," and risks arising
from seasonal emissions. Second, the form of the 24-hour PM2.5 standard should
provide an appropriate degree of increased stability relative to the current
form. A more stable statistic would reduce the impact of a single high exposure
event that may be due to unusual meteorological conditions alone, and thus
would provide a more stable basis upon which to design effective control
programs.
With these purposes in mind, the Administrator observed that while a percentile
value such as the 90th or 95th would provide substantially increased stability
when compared to a more extreme air quality statistic (e.g., the current
1-expected-exceedance form), it would likely not serve as an effective "back
stop," because it would allow a large number of days with peak PM2.5
concentrations above the standard level. For example, in a 365 day data base,
the 90th and 95th percentiles would equal the 37th and 19th highest 24-hour
concentrations, respectively. On the other hand, a percentile value selected
much closer to the tail of the air quality distribution (e.g., a 99th or
greater percentile) would not likely provide significantly more health protection
nor significantly increased stability as compared to the current form. In
balancing these issues, the Administrator believes that a 98th percentile
value form of a standard, set at an appropriate level, would achieve the
desired outcomes of both a 24-hour standard that would serve as an effective
supplement to the PM2.5 annual standard and a more stable form. Proposed
methods for using monitored concentrations to make a comparison with a
concentration percentile form of a 24-hour standard, averaged over 3 years,