# National Ambient Air Quality Standards for Particulate Matter

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URL: https://www.frixlaw.com/law-library/documents/fr%3A97-18577

## Record

- **Collection:** Federal Register
- **Document type:** Rule
- **Published:** July 18, 1997
- **Citation:** 62 FR 38652

## Text

ENVIRONMENTAL PROTECTION AGENCY

40 CFR Part 50

[AD-FRL-5725-2]
RIN 2060-AE66

National Ambient Air Quality Standards for Particulate Matter

AGENCY: Environmental Protection Agency (EPA).
ACTION: Final rule.
SUMMARY: This document describes EPA's decision to revise the national
ambient air quality standards (NAAQS) for particulate matter (PM) based
on its review of the available scientific evidence linking exposures to
ambient PM to adverse health and welfare effects at levels allowed by
the current PM standards. The current primary PM standards are revised
in several respects: Two new PM2.5 standards are added, set
at 15 g/m3 , based on the 3-year average of annual
arithmetic mean PM2.5 concentrations from single or multiple
community-oriented monitors, and 65 g/m 3 , based
on the 3-year average of the 98th percentile of 24-hour
PM2.5 concentrations at each population-oriented monitor
within an area; and the current 24-hour PM10 standard is
revised to be based on the 99th percentile of 24-hour
PM10 concentrations at each monitor within an area. The new
suite of primary standards will 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 current secondary standards are revised by making them
identical in all respects to the new suite of primary standards. The
new secondary standards, in conjunction with a regional haze program,
will provide appropriate protection against PM-related public welfare
effects including soiling, material damage, and visibility impairment.
In conjunction with the new PM2.5 standards, a new reference
method has been specified for monitoring PM as PM2.5 .
EFFECTIVE DATE: This action is effective September 16, 1997.
ADDRESSES: A docket containing information relating to the EPA's review
of the PM primary and secondary standards (Docket No. A-95-54) is
available for public inspection in the Central Docket Section of the
U.S. Environmental Protection Agency, South Conference Center, Rm. 4,
401 M St., SW., Washington, DC. This docket incorporates the docket
established for the air quality Criteria Document (Docket No. ECAO-CD-
92-0671). The docket may be inspected between 8 a.m. and 3 p.m., Monday
through Friday, except legal holidays, and a reasonable fee may be
charged for copying. The information in the docket constitutes the
complete basis for the decision announced in this document. For the
availability of related information, see ``SUPPLEMENTARY INFORMATION.''
FOR FURTHER INFORMATION CONTACT: John H. Haines, MD-15, Air Quality
Strategies and Standards Division, Office of Air Quality Planning and
Standards, U.S. Environmental Protection Agency, Research Triangle
Park, NC 27711; telephone: (919) 541-5533; e-mail:
[email protected].
SUPPLEMENTARY INFORMATION:

Related Final Rules on PM Monitoring

In a separate document published elsewhere in this issue of the
Federal Register, EPA is amending its ambient air quality surveillance
requirements (40 CFR part 58) and its ambient air monitoring reference
and equivalent methods (40 CFR part 53) for PM.

Availability of Related Information

Certain documents are available from the U.S. Department of
Commerce, National Technical Information Service, 5285 Port Royal Road,
Springfield, VA 22161. Available documents include:
(1) 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).
(2) 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: 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 at the address under
``ADDRESSES,'' at the beginning of this document.

Electronic Availability

The Staff Paper and human health risk assessment support documents
are available on the Agency's Office of Air Quality Planning and
Standards' (OAQPS) Technology Transfer Network (TTN) Bulletin Board
System (BBS) in the Clean Air Act Amendments area, under Title I,
Policy/Guidance Documents. To access the bulletin board, a modem and
communications software are necessary. To dial up, set your
communications software to 8 data bits, no parity and one stop bit.
Dial (919) 541-5742 and follow the on-screen instructions to register
for access. After registering, proceed to choice `` Gateway to TTN
Technical Areas'', then choose `` CAAA BBS''. From the main menu,
choose `` Title I: Attain/Maint of NAAQS'', then `` Policy
Guidance Documents.'' To access these documents through the World Wide
Web, click on ``TTN BBSWeb'', then proceed to the Gateway to TTN
Technical areas, as above. If assistance is needed in accessing the
system, call the help desk at (919) 541-5384 in Research Triangle Park,
NC.

Implementation Strategy For Revised Air Quality Standards

On Wednesday, July 16, 1997, President Clinton signed a memorandum
to the Administrator specifying his goals for the implementation of the
O3 and PM standards. Attached to the President's memorandum
is a strategy prepared by an interagency Administration group outlining
the next steps that would be necessary for implementing these
standards. The EPA will prepare guidance and proposed rules consistent
with the President's memorandum. Copies of the Presidential document
are available in paper copy by contacting the U.S. Environmental
Protection Agency Library at the address under ``Availability of
Related Information'' and in electronic form as discussed above in
``Electronic Availability.''
The following topics are discussed in this preamble:
I. Background
A. Legislative Requirements
B. Related Control Requirements
C. Review of Air Quality Criteria and Standards for PM
D. Summary of Proposed Revisions to the PM Standards
II. Rationale for the Primary PM Standards
A. Introduction
B. Need for Revision of the Current Primary PM Standards

[[Page 38653]]

C. Indicators of PM
D. Averaging Time of PM2.5 Standards
E. Form of PM2.5 Standards
F. Levels for the Annual and 24-Hour PM2.5 Standards
G. Conclusions Regarding the Current PM10 Standards
H. Final Decisions on Primary PM Standards
III. Rationale for the Secondary Standards
A. Need for Revision of the Current SecondaryStandards
B. Decision on the Secondary Standards
IV. Other Issues
A. Consideration of Costs
B. Margin of Safety
C. Data Availability
D. 1990 Amendments
V. Revisions to 40 CFR Part 50, Appendix K--Interpretation of the PM
NAAQS
A. PM2.5 Computations and Data Handling Conventions
B. PM10 Computations and Data Handling Conventions
C. Changes that Apply to Both PM2.5 and
PM10 Computations
VI. Reference Methods for the Determination of Particulate Matter as
PM10 and PM2.5 in the Atmosphere
A. Revisions to 40 CFR Part 50, Appendix J--Reference Method for
PM10
B. 40 CFR Part 50, Appendix L--New Reference Method for
PM2.5
VII. Effective Date of the Revised PM Standards and Applicability of
the Existing PM10 Standards
VIII. Regulatory and Environmental Impact Analyses
A. Executive Order 12866
B. Regulatory Flexibility Analysis
C. Impact on Reporting Requirements
D. Unfunded Mandates Reform Act
E. Environmental Justice
F. Submission to Congress and the Comptroller General
IX. Response to Petition for Administrator Browner's Recusal
X. References

I. Background

A. Legislative Requirements

Two sections of the Clean Air Act (Act) govern the establishment,
review, and revision of NAAQS. Section 108 of the Act (42 U.S.C. 7408)
directs the Administrator to identify certain 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 of the Act (42 U.S.C. 7409) directs the Administrator
to propose and promulgate ``primary'' and ``secondary'' NAAQS for
pollutants identified under section 108 of the Act. Section 109(b)(1)
of the Act 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. The selection
of any particular approach to providing an adequate margin of safety is
a policy choice left specifically to the Administrator's judgment. Lead
Industries Ass'n v. EPA, 647 F.2d 1130, 1161-1162 (D.C. Cir.1980).
A secondary standard, as defined in section 109 (b)(2) of the Act,
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) of the Act (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) of the Act 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) of the Act 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, nonroad
engine, and aircraft emissions; the new source performance standards
under section 111 of the Act (42 U.S.C. 7411); and the national
emission standards for hazardous air pollutants under section 112 of
the Act (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 published in the Federal Register (52 FR 24854, July
1, 1987). In that decision, EPA changed the indicator for PM from total
suspended particles (TSP) to

[[Page 38654]]

PM10.1 Identical primary and secondary
PM10 standards were set for two averaging times: 50
g/m3, expected annual arithmetic mean, averaged
over 3 years, and 150 g/m3, 24-hour average, with
no more than one expected exceedance per year.2
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1 PM10 refers to particles with an aerodynamic
diameter less than or equal to a nominal 10 micrometers. Technical
details further specifying the measurement of PM10 are
contained in 40 CFR part 50, Appendices J and M.
2 A more complete history of the PM NAAQS is presented in
section II.B of the OAQPS Staff Paper, Review of National Ambient
Air Quality Standards for Particulate Matter: Assessment of
Scientific and Technical Information (U.S. EPA, 1996b).
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The EPA initiated this current review of the air quality criteria
and standards 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 schedule3 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).
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3 A court order entered in American Lung Association v. Browner,
CIV-93-643-TUC-ACM (D. Ariz.,October 6, 1994), as subsequently
modified, requires publication of EPA's final decision on the review
of the PM NAAQS by July 19, 1997.
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External review drafts of a Staff Paper prepared by the Office of
Air Quality Planning and Standards (OAQPS), Review of the National
Ambient Air Quality Standards for Particulate Matter: Assessment of
Scientific and Technical Information (henceforth, the ``Staff Paper''),
were made available for public comment and were reviewed by CASAC at
public meetings in December 1995 and May 1996.4 The CASAC
came to closure in its review of the Staff Paper, advising the
Administrator in a June 13, 1996 closure letter (Wolff, 1996b) that
``the Staff Paper, when revised, will provide an adequate summary of
our present understanding of the scientific basis for making regulatory
decisions concerning PM standards.'' CASAC and public comments from
these meetings, subsequent written comments, and the CASAC closure
letter were incorporated as appropriate in the final Staff Paper (U.S.
EPA, 1996b).
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4 The Staff Paper evaluates policy implications of the key
studies and scientific information in the Criteria Document,
identifies critical elements that EPA staff believes should be
considered, and presents staff conclusions and recommendations of
suggested options for the Administrator's consideration.
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On November 27, 1996, EPA announced its proposed decision to revise
the NAAQS for PM (61 FR 65638, December 13, 1996) (hereafter
``proposal'') as well as its proposed decision to revise the NAAQS for
ozone (O3)(61 FR 65716, December 13, 1996). In the proposal,
EPA identified proposed revisions, based on the air quality criteria
for PM, and solicited public comments on alternative primary standards
and on the proposed forms of the standards.
To ensure the broadest possible public input on the PM and
O3 proposals, EPA took extensive and unprecedented steps to
facilitate the public comment process beyond the normal process of
providing an opportunity to request a hearing and receiving written
comments submitted to the rulemaking docket. The EPA established a
national toll-free telephone hotline to facilitate public comments on
the proposed revisions to the PM and O3 NAAQS, and on
related notices dealing with the implementation of revised PM and
O3 standards, as well as a system for the public to submit
comments on the proposals electronically via the Internet. Over 14,000
calls and over 4,000 electronic mail messages were received through
these channels. The public could also access key supporting documents
(including the Criteria Document, Staff Paper, related technical
documents and fact sheets) via the Internet.
The EPA also held several public hearings and meetings across the
country to provide direct opportunities for public comment on the
proposed revisions to the PM and O3 NAAQS and to disseminate
information to the public about the proposed standard revisions. On
January 14 and 15, 1997, EPA held concurrent, 2-day public hearings in
Boston, MA, Chicago, IL, and Salt Lake City, UT. A fourth public
hearing, which focused primarily on PM monitoring issues, was held in
Durham, NC on January 14, 1997. Over 400 citizens and organizations
testified during these public hearings. EPA also held two national
satellite telecasts to answer questions on the standards and
participated in meetings sponsored by the Air and Waste Management
Association on the proposed revisions to the standards at more than 10
locations across the country. Beyond that, several EPA regional offices
held public meetings and workshops and participated in hearings that
States and cities held around the country.
As a result of this intensive effort to solicit public input, over
50,000 written and oral comments were received on the proposed
revisions to the PM NAAQS by the close of the public comment period on
March 12, 1997. Major issues raised in the comments are discussed
throughout the preamble of this final decision. A comprehensive summary
of all significant comments, along with EPA's response to such comments
(hereafter ``Response to Comments''), can be found in the docket for
this rulemaking (Docket No. A-95-54).
The principal focus of this current review of the air quality
criteria and standards for PM is on recent epidemiological evidence
reporting associations between ambient concentrations of PM and a range
of serious health effects. Particular attention has been given to
several size-specific classes of particles, including PM10
and the principal fractions of PM10, referred to as the fine
(PM2.5)5 and coarse
(PM10-2.5)6 fractions. As discussed in the
Criteria Document, fine and coarse fraction particles can be
differentiated by their sources and formation processes and their
chemical and physical properties, including behavior in the atmosphere.
Detailed discussions of atmospheric formation, ambient concentrations,
and health and welfare effects of PM, as well as quantitative estimates
of human health risks associated with exposure to PM, can be found in
the Criteria Document and in the Staff Paper.
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5 PM2.5 refers to particles with an aerodynamic
diameter less than or equal to a nominal 2.5 micrometers, as further
specified in 40 CFR part 50, Appendix L in this document.
6 PM10-2.5 refers to those particles with an
aerodynamic diameter less than or equal to a nominal 10 micrometers
but greater than 2.5 micrometers. In other words, it refers to the
inhalable particles that remain if fine (PM2.5) particles
are removed from a sample of PM10 particles.

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[[Page 38655]]

D. Summary of Proposed Revisions to the PM Standards

For reasons discussed in the proposal, the Administrator proposed
to revise the current primary standards for PM (as indicated by
PM10), by adding two new primary PM2.5 standards
set at 15 g/m3, annual mean, and 50 g/
m3, 24-hour average. 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 population-oriented monitor
within an area. The proposal solicited comment on two alternative
approaches for selecting the levels of PM2.5 standards. The
Administrator also proposed 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, solicited comment
on an alternative proposal to revoke the 24-hour PM10
standard, and proposed to retain the current annual primary
PM10 standard of 50 g/m3. The proposal
also solicited comment on proposed revisions to 40 CFR part 50,
Appendix K to establish new data handling conventions for calculating
98th percentile values and spatial averages, revisions to 40
CFR part 50, Appendix J to modify the reference method for monitoring
PM as PM10, and a proposed new reference method for
monitoring PM as PM2.5 (40 CFR part 50, Appendix L).
With regard to the secondary standards, the Administrator proposed
to revise the current secondary standards by making them identical to
the suite of proposed primary standards, in conjunction with the
establishment of a regional haze program under section 169A of the Act.

II. Rationale for the Primary Standards

A. Introduction

1. Overview. This document presents the Administrator's final
decisions regarding the need to revise the current primary ambient air
quality standards for PM, and, more specifically, regarding the
establishment of new annual and 24-hour PM2.5 primary
standards and revisions to the form of the current 24-hour
PM10 primary NAAQS. These decisions are 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:
(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.
(3) Public comments received during the development of these
documents, either in connection with CASAC meetings or separately.
(4) Extensive public comments received on the proposed decisions
regarding the primary PM standards.
After taking this information and comments into account, and for
the reasons discussed below in this unit, the Administrator concludes
that revisions to the current primary standards to provide increased
public health protection against a variety of health risks are
appropriate. More specifically, the Administrator has determined that
it is appropriate to establish new annual and 24-hour PM2.5
standards, to revise the current 24-hour PM10 standard, and
to retain the current annual PM10 standard. As discussed
more fully below in this unit, the rationale for the final decisions
regarding 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.
(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 in the proposal, 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 needs document 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.
The health effects information and human risk assessment were
summarized in the proposal and are only briefly outlined below in this
unit. Subsequent units provide a more complete discussion of the
Administrator's rationale, in light of key issues raised in public
comments, for concluding that it is appropriate to revise the current
primary standards (Unit II.B. of this preamble) and to revise the
specific elements of the standards including indicator (Unit II.C. of
this preamble); averaging time, form, and level of new PM2.5
standards (Units II.D., II.E., and II.F. of this preamble); and
averaging time, form, and level of revised PM10 standards
(Unit II.G. of this preamble).
2. Summary of the health effects evidence. In brief, 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 standard'' (U.S. EPA, 1996a; p. 13-1). Although a
variety of

[[Page 38656]]

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 any
accepted mechanism(s) that would explain how such relatively low
concentrations of ambient PM might cause the health effects reported in
the epidemiological literature.
Unit II.A. of the proposal further outlines key information
contained in the Criteria Document, Chapters 10-13, and the Staff
Paper, Chapter V, on the 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
there summarizes:
(1) The nature of the effects that have been reported to be
associated with ambient PM, which include premature mortality,
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), changes in
lung function and increased respiratory symptoms, changes to lung
tissues and structure, and altered respiratory defense mechanisms.
(2) Sensitive subpopulations that appear to be at greater risk to
such effects, specifically individuals with respiratory disease and
cardiovascular disease and the elderly (premature mortality and
hospitalization), children (increased respiratory symptoms and
decreased lung function), and asthmatic children and adults
(aggravation of symptoms).
(3) An integrated evaluation of the health effects evidence, with
an emphasis on 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.
(4) The PM fractions of greatest concern to health.

The summary in the proposal will not be repeated here. EPA emphasizes
that the final decisions on these standards take into account the more
comprehensive and detailed discussions of the scientific information on
these issues contained in the Criteria Document and Staff Paper, which
were reviewed by the CASAC and the public.
3. Key insights from the risk assessment. The Staff Paper presents
the results of a quantitative assessment of health risks for two
example cities, including risk estimates for several categories of
health effects associated with: existing PM air quality levels,
projected PM air quality levels that would occur upon attainment of the
current PM10 standards, and projected PM air quality levels
that would occur upon attainment of alternative PM2.5
standards. 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. The risk
assessment is described more fully in the Staff Paper and summarized in
the proposal. Related technical reports and updates7 have
been placed in the docket (Abt Associates, 1996a,b; 1997a,b).
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7 The risk assessment results that appear in the Staff Paper and
are summarized in the proposal have been updated to include analyses
of the particular forms of standard alternatives contained in the
proposal and to correct estimates for one effects category
(mortality from long-term exposure) to reflect the actual statistics
used in the study upon which they were based (Pope et al., 1995).
The corrections, which cumulatively reduce estimates of mortality
associated with long-term exposures by 20 to 35%, have no effect on
risk estimates for mortality associated with short-term exposures or
the estimates for any other effects. Because the key sensitivity
analyses that provide additional insights regarding thresholds,
copollutants, averaging time and related issues involved the short-
term exposure studies, none of these results are affected by changes
to the long-term exposure risk estimates.
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EPA emphasizes that it places greater weight on the overall
conclusions derived from the studies--that PM air pollution is likely
causing or contributing to significant adverse effects at levels below
those permitted by the current standards--than on the specific
concentration-response functions and quantitative risk estimates
derived from them. These quantitative risk estimates include
significant uncertainty and, therefore, 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. Keeping in mind the
important uncertainties inherent in any such analyses, the key insights
from the risk assessment that are most pertinent to the current
decision include:
(1) Fairly wide ranges of estimates of the incidence of PM-related
mortality and morbidity effects and risk reductions associated with
attainment of alternative standards were calculated for the two
locations analyzed when the effects of key uncertainties and
alternative assumptions were considered. Significantly, the combined
analysis for these two cities alone found that the risk remaining after
attaining the current PM10 standards was on the order of
hundreds of premature deaths each year, hundreds to thousands of
respiratory-related hospital admissions, and tens of thousands of
additional respiratory related symptoms in children.
(2) 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.
(3) Over the course of a year, the few 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 aggregated risks associated with the
low to mid-range concentrations.
(4) There is greater uncertainty about both the existence and the
magnitude of estimated excess mortality and other effects associated
with PM exposures as one considers increasingly lower concentrations
approaching background levels.

B. Need for Revision of the Current Primary PM Standards

1. Introduction. 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 PM standards should be revised and, if so, what revised or new
standards would be appropriate. The concluding section of the
integrative synthesis of health effects information in the Criteria
Document, which CASAC characterized as EPA's ``best ever example of a
true integrative summary of the state of knowledge about the health
effects of airborne PM,'' (Wolff, 1996b) provides the following summary
of the science with respect to this issue:

The evidence for PM-related effects from epidemiological 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 epidemiological
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 epidemiological studies should be interpreted
cautiously, they nonetheless provide ample reason to be concerned
that there are detectable health effects attributable

[[Page 38657]]

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,
which is based on a large number of studies that used PM10
measurements, as well as studies using other indicators of PM, clearly
indicates that revision of the current PM NAAQS is appropriate. Quite
apart from the issue of whether PM10 should be the sole
indicator for the PM NAAQS, the extensive PM epidemiological data base
provides evidence of serious health effects (e.g., mortality,
exacerbation of chronic disease, increased hospital admissions) in
sensitive populations (e.g., the elderly, individuals with
cardiopulmonary disease), as well as significant adverse health effects
(e.g., increased respiratory symptoms, school absences, and lung
function decrements) in children. Moreover, these effects associations
are observed in areas or at times when the levels of the current
PM10 standards are met. Although the increase in relative
risk is small for the most serious outcomes, EPA believes it is
significant from an overall public health perspective, because of the
large number of individuals in sensitive populations that are exposed
to ambient PM, as well as the significance of the health effects
involved (U.S. EPA, 1996a, p. 1-21). The results of the two-city PM
risk assessment reinforce these conclusions regarding the significance
of the public health risk--even under a scenario in which the current
PM10 standards are attained.
While the lack of demonstrated mechanisms that explain the
extensive body of epidemiological findings is an important caution,
which presents difficulties in providing an integrated assessment of PM
health effects research, a number of potential mechanisms have been
hypothesized in the recent literature (U.S. EPA, 1996b; p. V-5 to V-8;
appendix D). Moreover, 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 inhalation of
PM.8 Indeed, as discussed in the Criteria Document and
section V.E of the Staff Paper, the consistency of the results of the
epidemiological studies from a large number of different locations and
the coherent nature of the observed effects9 are suggestive
of a likely causal role of ambient PM in contributing to the reported
effects.
---------------------------------------------------------------------------

8 As discussed more fully below in this unit, epidemiological
studies alone cannot be used to demonstrate mechanisms of action,
but they can provide evidence useful in making inferences with
regard to causal relationships (U.S. EPA, 1996b, p. V-9).
9 As noted in the proposal, the kinds of effects observed in the
epidemiological studies are logically related. For example, the
association of PM with mortality is mainly linked to respiratory and
cardiovascular causes, which is coherent with observed PM
associations with respiratory and cardiovascular hospital admissions
and respiratory symptoms. Further, similar categories of effects are
seen in long- and short-term exposure studies.
---------------------------------------------------------------------------

2. Comments on scientific basis for revision. A majority of the
public comments received on the proposal agreed that, based on the
available scientific information, the current PM10 standards
are not of themselves sufficient to protect public health and it would
be appropriate to revise them. Included in those calling for revisions
to the current standards are many public health professionals,
including numerous medical doctors and academic researchers. For
example, a group of 27 members of the scientific and medical community
recognized as having substantial expertise in conducting research on
the health effects of air pollution stated:

Health studies conducted in the U.S. and around the world have
demonstrated that levels of particulate and ozone air pollution
below the current U.S. National Air Quality Standards exacerbate
serious respiratory disease and contribute to early death. A large
body of scientific and medical evidence clearly indicates that the
current NAAQS are not sufficiently protective of public health.
[Thurston, 1997]

Similar conclusions were reached in a letter signed by more than
1,000 scientists, clinicians, researchers, and other health care
professionals (Dickey, 1997). The cosigners to this letter argued that
tens of thousands of hospital visits and premature deaths could be
prevented with the proposed air quality standard revisions. In fact,
these commenters argued that even stronger standards than those
proposed by EPA are needed to protect the health of the most vulnerable
residents of our communities.
A number of State and local government authorities also submitted
comments in support of adopting new air quality standards for fine
particulate matter. The commenters concurred with conclusions reached
through the EPA's peer review process that the PM standards should be
revised to protect public health. A number of these commenters
suggested that the standards proposed by EPA should be even stronger,
while several other State agencies recommended that EPA adopt
PM2.5 standards, but at less stringent levels. A number of
the comments from states supporting even stronger standards
acknowledged the lack of demonstrated mechanism(s) and other
uncertainties but stressed the strength of the other evidence in urging
EPA to set protective standards.
Many comments were also received from representatives of
environmental or community health organizations that supported the
adoption of air quality standards for PM2.5. These
commenters agreed with EPA's finding that a large body of compelling
evidence demonstrates that exposure to particulate matter pollution, in
general, is associated with premature death, aggravation of heart and
lung diseases, increased respiratory illness and reduced lung function.
They agreed with EPA that these studies present a consistent and
coherent relationship between exposure to PM and both mortality and
various measures of morbidity. However, the majority of these
commenters argued that EPA's proposed standards for PM2.5
were inadequate and recommended adoption of more stringent levels of
the 24-hour and/or annual air quality standards for PM2.5.
Many of these commenters also urged EPA to revise the NAAQS for
PM10 to be more protective of public health. These
commenters based their recommendations on the findings of the studies
that were reviewed in the preparation of the Criteria Document and
Staff Paper. One commenter used results from five of these studies as
the basis for recommending PM2.5 standards of 10 g/
m3 (annual) and 18 g/m3 (24-hour)
(Dockery et al., 1993; Pope et al., 1995; Schwartz et al., 1996;
Schwartz et al., 1994; Thurston et al., 1994). The commenters agreed
with EPA on the significance of these studies' results and the need to
revise the PM standards, while differing with EPA's interpretation of
the findings for purposes of developing the proposed PM standards.
Several commenters made reference to the conclusions of a number of
international scientific panels regarding the health effects of
exposure to airborne particulate matter--the British Expert Panel on
Air Quality Standards, the British Committee on the Medical Effects of
Air Pollutants, the World Health Organization, the Canadian Ministry of
Environment, Lands and Parks, and the Health Council of the Netherlands
-- and argued that all these panels found that PM concentrations
equivalent to the current U.S. standards for PM10 are not
protective of human health and made recommendations for greater
protection. One commenter noted that the findings of the British Health
Panel have resulted in a British

[[Page 38658]]

proposal to adopt a 24-hour PM10 standard of 50 g/
m3, which is one-third the level of the current U.S. NAAQS.
In these comments, some toxicological studies were cited as
providing evidence for toxicity of particulate pollution. These
commenters disagreed with arguments that PM standards cannot be adopted
due to a lack of a sufficient understanding of the biological mechanism
of injury. The commenters argued that there is sufficient evidence that
particulate pollution is associated with adverse health effects to make
it inappropriate to delay the establishment of standards while further
studies are undertaken. This group of commenters was also critical of
arguments against the establishment of additional PM standards based on
the possibility of confounding by other pollutants, and urged that more
attention be paid instead to the possible additive or synergistic
effects of multiple pollutant exposures.
In general, the EPA agrees with these commenters' arguments
regarding the need to revise the PM standards. The scientific studies
cited by these commenters were the same studies used in the development
of the Criteria Document and the Staff Paper, and the EPA agrees that
there is a sufficient body of evidence that the current NAAQS for PM
are not adequately protective of the public health. For reasons
detailed in Unit II.F. of this preamble and in the Response to
Comments, EPA disagrees with aspects of these commenters' views on the
level of protection that is appropriate and supported by the available
scientific information.
Another body of commenters, including almost all commenters
representing businesses and industry associations, many local
governmental groups and private citizens, and some States opposed
revising the standards. Many of these commenters argued that the
available scientific evidence does not provide an adequate basis for
revising the current standards. The central arguments made by these
commenters can be divided into two categories: (1) General comments on
the appropriateness of relying on the epidemiological evidence for
making regulatory decisions, and (2) more specific comments challenging
EPA's appraisal of the consistency and coherence of the available
information, EPA's conclusions regarding causality, and the use of
these studies for risk assessment and decisions on whether to revise
the standards. While EPA has included comprehensive responses to these
comments in the Response to Comments, certain key points are summarized
below in this unit.
a. General comments on the use of epidemiological studies. The
first category of comments was largely derived from ad hoc panels of
occupational and other epidemiological experts, consulting groups, and
individual consultants. Most of these individuals and groups commented
on the use of epidemiology in reaching scientific and policy
conclusions primarily from an occupational or hazard assessment
perspective, in contrast to the perspective of the review of ambient PM
criteria and standards, where the use of community air pollution
epidemiological studies are central. Citing accepted criteria used in
evaluating epidemiological studies to assess the likelihood of
causality (most notably those of Sir Austin Bradford Hill, 1965), these
commenters argued that in the absence of a demonstrated biological
mechanism, the relative risks of effects in the PM epidemiological
studies are too low (less than values variously cited as 1.5 to 2.0) to
reach any conclusions regarding causality or to form the basis for
regulations. In general, the commenters applied these criteria to a
subset of studies evaluated in the Criteria Document, including as few
as two long-term exposure studies (EOP Group) (API, 1997), a group of 9
selected studies (Greenland panel) (API, 1997), those studies cited in
the proposal (AIHC, 1997), or as many as 23 selected short-term
exposure studies examined in a recently published review paper (Gamble
and Lewis, 1996).
Based on a careful review of these comments, EPA notes a number of
limitations in these commenters' evaluations of the epidemiological
studies that they considered, as discussed in detail in the Response to
Comments. In summary, EPA notes that these commenters provided
scientific advice and conclusions that are in substantial disagreement
with the conclusions of the review reflected in the Criteria Document
and Staff Paper. EPA stands behind the scientific conclusions reached
in these documents regarding the appropriate use of the available
community epidemiological studies. These documents were the product of
an extended public process that included conducting public workshops
involving the leading researchers in the field, drafts of the Criteria
Document and Staff Paper providing opportunities for public scrutiny
and comment on, and, not least, receiving the advice of an independent
panel of air pollution experts, including epidemiologists.
EPA clearly specified the key criteria by which it evaluated the
available epidemiological studies in section 12.1.2 of the Criteria
Document, with substantial reliance on those specified by Hill (1965).
In rejecting results with relative risks less than 1.5 to 2 as
meaningful absent demonstrated biological mechanisms, the commenters
fail to note that Hill and other expert groups (U.S. DHEW, 1964) have
emphasized that no one criterion is definitive by itself, nor is it
necessary that all be met in order to support a determination of
causality (U.S. EPA, 1996a, p. 12-3).
With respect to biological plausibility, Hill noted that ``this is
a feature I am convinced we cannot demand. What is biologically
plausible depends upon the biological knowledge of the day'' (Hill,
1965). This statement is clearly pertinent to the toxicological and
mechanistic understanding of the effects of PM and associated air
pollutants, especially at lower concentrations. It is also important to
stress that while the mechanistic evidence published as of the time the
Criteria Document closed does not provide quantitative support for the
epidemiological results, neither can such limited evidence refute these
findings. It is also important to stress that our understanding of
biological mechanisms for PM pollution effects is not sufficient to
explain the effects observed at much higher concentrations in air
pollution episodes, for which causality is generally accepted.
Moreover, the toxicological literature has only recently begun to
examine animal models (or controlled human studies) that might reflect
the sensitive populations in question (the elderly, individuals with
chronic respiratory and cardiovascular disease) or that adequately
reproduce all of the physico-chemical properties of particles in the
ambient atmosphere. In short, the absence of evidence of a particular
mechanism is hardly proof that there are no mechanisms that could
explain the effects observed so consistently in the epidemiological
studies. The absence of biological mechanisms did not deter CASAC from
recommending revisions to the PM standards in 1982, 1986, and again in
1996.
While Hill appropriately emphasized the strength of the association
as important (e.g., size of the relative risk), he also pointed out
that ``We must not be too ready to dismiss a cause-and-effect
hypothesis merely on the ground that the observed association appears
to be slight. There are many occasions in medicine when this in truth
is so'' (Hill, 1965). EPA believes that the effects of air pollution
containing PM is such a

[[Page 38659]]

case. Unlike the ``textbook'' examples of unlikely significant
associations provided by some commenters (e.g., ice cream consumption
correlated with heat stroke), the abundant epidemiological literature
on combustion particles documents numerous occasions in which single
short-term episodes of high air pollution produced unequivocally
elevated relative risks. For the week of the well documented 1952
London air pollution episode, for example, the relative risk of
mortality for all causes was 2.6, while the relative risk for
bronchitis mortality was as high as 9.3 (Ministry of Health, 1954).
Hospital admissions also increased by more than a factor of two.
British epidemiologists in the 1950s concluded that increased mortality
was likely when PM (as mass calibrated British Smoke m in
aerodynamic diameter) exceeded 500 g/m3 (Martin and
Bradley, 1960). This is only about a factor of 3 higher than that
allowed by the current PM standard. Unlike the ``textbook'' and other
unlikely statistical associations noted by some commenters, where the
only evidence is for low relative risk, clear and convincing links
between high-level PM concentrations and mortality and morbidity
buttress the findings of similar associations at much lower PM
concentrations as suggested in the more recent epidemiological
literature.
These commenters also appear to ignore several epidemiological
studies conducted at low PM concentrations in U.S. and European cities,
including both short- and long-term exposures to PM air pollution, that
find statistically significant relative risks of respiratory symptom
categories in children in the range of 1.5 to 5 (Schwartz et al., 1994;
Pope and Dockery, 1992; Braun-Fahrlander et al., 1992; Dockery et al.,
1989; Dockery et al., 1996). Concentrations in these studies extend
from moderately above to well below those permitted by the current
PM10 standards. While, as noted in the proposal, most of the
recent epidemiological studies of mortality and hospital admissions
report comparatively small relative risks, the findings of relative
risks well in excess of the 1.5 to 2 criterion noted by commenters for
earlier studies of high PM episodes, as well as the relative risks of
1.5 to 5 reported in more recent studies of less serious, but still
important effects categories, lend credibility to EPA's interpretation
of the results.
In addition to basing their conclusions primarily on their own
assessment of a limited set of studies, this group of commenters
reached different conclusions about the consistency of the observed
associations because of their assumptions that all model building
strategies by all authors are equally valid. Even the most thorough of
these treatments (Gamble and Lewis, 1996) shared this flaw,
particularly in the discussion of the series of Philadelphia mortality
studies and in the discussion of modeling approaches. The authors'
treatment of modeling and confounding issues was further limited
because they did not include the most recent Philadelphia results
(Samet et al., 1996a,b) sponsored by the Health Effects Institute (HEI,
1997). One of the important functions of the Criteria Document is to
evaluate the strengths and limitations of various studies. As discussed
more fully below in this unit, the Criteria Document found that some of
the studies cited by commenters as suggesting a lack of consistency had
important limitations. In general, these commenters' analyses suffered
by ignoring the much more thorough critical review of these studies and
issues contained in the Criteria Document, notably that in section 12.6
on alternative modeling approaches.
EPA also rejects the notion advanced by these commenters that
epidemiological studies must use personal exposure monitoring to be
considered for regulatory purposes. In particular, commenters ignore
the significant strengths of the time-series studies and prospective
cohort studies relied on by EPA as compared to cross-sectional
epidemiological studies. Time-series studies, such as the daily
mortality studies, look at changes in response rate in relation to
changes in weather and air pollution over time intervals of a few days.
This controls for other factors such as smoking and socioeconomic
status, which are little changed during such short intervals.
Prospective cohort studies (e.g., Pope et al., 1995; Raizenne et al.,
1996), on the other hand, look at changes in health status in a
selected cohort of individuals, which allows direct adjustment for
smoking status, socioeconomic status, and other subject-specific
factors. The commenters also ignore the Criteria Document conclusions
on how properly conducted monitoring can provide an adequate index of
population exposure to ambient air pollution in such studies that, as
detailed below, is more relevant to establishing ambient air quality
standards (U.S. EPA 1996a, chapter 7). Although personal monitoring may
be practical for some occupational and epidemiological studies, and has
been employed in some past studies of air pollution, it is not
realistic to require personal monitors in air pollution studies of
daily mortality, which require urban scale population data over a
period of years. Furthermore, the use of community monitoring-based
epidemiological studies as a basis for establishing standards and
guidelines has a long history in air pollution, including the British
authorities' response to the London episodes and the establishment of
the original U.S. NAAQS in 1971. Rejecting the use of the vast array of
such studies on this basis alone would also go against the advice of
the independent scientific experts on every CASAC panel that has
addressed the subject of PM pollution through the years, each of which
has recommended general PM standards based primarily on the results of
community epidemiological studies (Friedlander, 1982; Lippmann, 1986;
Wolff, 1996b). As noted above in this unit, EPA has included a more
detailed discussion of its responses to these comments in the Response
to Comments.
b. Specific comments on epidemiologic studies. The second group of
commenters noted above made more specific challenges to EPA's
assessment of the epidemiological studies. These comments, although
overlapping some of those made by the first group, were generally made
by commenters who have taken a more active role in the review of the
Criteria Document and Staff Paper. These commenters asserted that the
epidemiological evidence on PM is not as consistent and coherent as EPA
has claimed, and, in particular, charged that EPA ignored or downplayed
a number of studies that the commenters argue contradict the evidence
the Agency cited as supporting the consistency and coherence of PM
effects. The studies, all of which commenters contend do a better job
of addressing one or more key issues, such as confounding pollutants,
weather, exposure misclassification, and model specification, than
earlier studies, include several that were available during preparation
of the Criteria Document, and a number that appeared after the Criteria
Document and Staff Paper were completed. Because the status of the
later studies differ from that of the earlier ones for purposes of
decisions under section 109 of the Act, the two categories are
discussed separately below in this unit. Additional responses to
comments relating to both sets of studies have been included in the
Response to Comments. In addition to the inclusion of specific studies,
commenters also raised other issues regarding the limitations of the

[[Page 38660]]

epidemiological information and the use of these studies in EPA's two-
city risk assessment. Both of these topics are also discussed below in
this unit.
(i) Studies available for inclusion in the criteria review. With
some exceptions, most of the above commenters cited somewhat similar
lists of ``negative'' studies that they argue EPA ignored or downplayed
in arriving at conclusions on consistency and coherence. Of the most
commonly cited studies, the following were available for inclusion in
the Criteria Document: daily mortality studies by Styer et al. (1995),
Lyon et al. (1995), Li and Roth (1995), Moolgavkar (1995a,b), Wyzga and
Lipfert (1995), Lipfert and Wyzga (1995), and Samet et al. (1995,
1996a,b); the long-term exposure mortality study by Abbey et al.
(1991); and the re-examination of the Six-City mortality results
(Dockery et al., 1993) by Lipfert (1995).
The written record of EPA's evaluations of these studies
effectively refutes the claim that the Agency ignored any of these
studies and supports the treatment the Agency accorded to each of them.
All of the studies available to EPA at the time of CASAC closure on the
PM Criteria Document (March 1996) were examined for inclusion in the
Criteria Document and Staff Paper, which form the basis for the PM
proposal. ``Negative''10 studies were evaluated in detail
along with ``positive'' studies when they were found to have no
critical methodological deficiencies, or to point out strengths and
limitations. Studies that had more serious problems were generally
discussed in less detail, whether positive or negative, than studies
with fewer or small deficiencies. The EPA assessments were evaluated by
peer reviewers, by CASAC, and by the public.
---------------------------------------------------------------------------

10 The term ``negative'' studies, as used in these comments,
should not be construed to mean those in which there is a negative
effects estimate (either significant or non-significant) for the
nominal cause. As used by these commenters, the term also includes
statistically non-significant positive effect estimates. In other
words, the commenters define ``positive'' studies as including only
those in which the effect estimate is both positive and
statistically significant.
---------------------------------------------------------------------------

Most of the short-term exposure studies cited above in this unit
are reanalyses and extensions of PM/mortality studies that had been
published by other investigators. In general, the Criteria Document
concluded that the most comprehensive and thorough reanalyses were
those in the series conducted for the HEI, which reanalyzed data sets
used in studies from six urban areas in Phase I.A (Samet et al.,
1995)11, with extended analyses for Philadelphia in Phase
I.B (Samet et al., 1996a,b). The most important finding in the HEI
Phase I.A reanalyses of the six areas is ``the confirmation of the
numerical results of the earlier analyses of all six data sets'' (HEI,
1995)12. After replicating the original investigators'
analyses, Samet et al. (1995) also found similar results analyzing the
data using an improved statistical model. The HEI Oversight Committee
found

[I]t is reasonable to conclude that, in these six data sets,
daily mortality from all causes combined, and from cardiovascular
and respiratory causes in particular, increases as levels of
particulate air pollution indexes increase. [HEI, 1995]

It is important to note that these reanalyses by respected
independent scientists confirm the reliability and reproducibility of
the work of the original investigators, particularly in view of the
concerns some commenters have expressed about EPA's reliance on a
number of PM studies published by these authors.
---------------------------------------------------------------------------

11 Data sets were those used in the original studies by Dockery
et al. (1992) for St. Louis and Eastern Tennessee; Pope et al.
(1992) for Utah Valley; Schwartz and Dockery (1992a) for
Philadelphia; Schwartz (1993) for Birmingham; and a portion of the
Santa Clara data from Fairley (1990). The data set from the
Moolgavkar et al. (1995a) Philadelphia reanalysis was also included
(Samet et al., 1995).
12 The HEI Board of Directors appointed an eight member
Oversight Committee consisting of leading scientists in several
disciplines relevant to air pollution epidemiology to oversee key
aspects of the project and to prepare HEI's assessment of the
results.
---------------------------------------------------------------------------

The Phase I.A HEI results for Philadelphia also found that it was
difficult to separate the effects of PM from those of co-occurring
SO2, in agreement with the Moolgavkar et al.(1995a)
analysis. Subsequent HEI work, and several of the other so-called
``negative'' studies cited above in this unit, further examined this
issue in terms of confounding or effects modification by one or more
co-occurring gaseous pollutants or weather. Contrary to commenters'
claims, this issue and these studies received considerable attention in
the Criteria Document and Staff Paper, and the overall implications and
conclusions from these assessments were summarized in the proposal. In
particular, the so-called ``negative'' and other findings of
Moolgalvkar et al. (1995a,b) in their Philadelphia and Steubenville
studies were discussed in great detail in section 12.6 of the PM
Critera Document and compared to those of the original investigators
(Schwartz and Dockery, 1992a,b) and other investigators (Li and Roth,
1995; Wyzga and Lipfert, 1995). Further analytical studies of the
Philadelphia data set were carried out by HEI (Samet et al., 1996a,b)
and have largely resolved many of the uncertainties in the earlier
analyses; in EPA's opinion, these studies supersede the results of the
original investigators (Schwartz and Dockery, 1992a) and the several
earlier reanalyses, including Moolgavkar (1995a), Moolgavkar and
Luebeck (1996), Li and Roth (1995), Wyzga and Lipfert (1995), and Samet
et al. (1995). Even though TSP is not the best PM indicator for health
effects, since it includes a substantial fraction of non-thoracic
particles, the extended Criteria Document assessment (U.S. EPA, 1996a,
pp. 12-291 to -299; 12-327) of the Phase I.B HEI analyses in
Philadelphia (Samet et al., 1996a,b) serves to support the following
findings:
(1) The mortality effects estimates for TSP do not depend heavily
on statistical methods when appropriate models are used.
(2) Estimated PM effects are not highly sensitive to appropriate
methods for adjusting for time trends and for weather.
(3) Air pollution has significant health effects above and beyond
those of weather.
(4) Copollutants such as ozone, CO, and NO2 may be
important predictors of mortality, but their effects can be
substantially separated from those of TSP and SO2.
(5) The health effects of TSP in Philadelphia cannot be completely
separated from SO2, which is itself a precursor of fine
particles, based solely on the epidemiological analyses in this single
city.
The most recent HEI Oversight Committee comments on these studies
(HEI, 1997), which were submitted to the docket by HEI, state that:

Although individual air pollutants (TSP, SO2, and
ozone) are associated with increased daily mortality in these data,
the limitations of the Philadelphia data make it impossible to
establish that particulate air pollution alone is responsible for
the widely observed associations between increased mortality and air
pollution in that city. All we can conclude is that it appears to
play a role. [HEI, 1997; p.38.]

While recognizing the limitations in the conclusions that can be made
based on studies in a single city, the Oversight Committee endorses the
approach taken by EPA in evaluating a broader set of epidemiological
studies:

Consistent and repeated observations in locales with different
air pollution profiles can provide the most convincing
epidemiological evidence to support

[[Page 38661]]

generalizing the findings from these models. This has been the
approach reported by the EPA in its recent Criteria Document and
Staff Paper. [HEI, 1997; p. 38.]

As noted in the proposal, based on this approach, EPA's assessment
of numerous mortality studies concludes that when studies are evaluated
on an individual basis, 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); and when a collection of studies from multiple areas
with differing concentrations of PM and co-pollutants are examined
together, the association with PM10 remains reasonably
consistent across a wide range of concentrations of these potentially
influential pollutants (U.S. EPA, 1996a; p. 12-33; U.S. EPA, 1996b; p.
V-55).
In addition to relying on the most comprehensive and best analyses
in evaluating the reanalysis in Philadelphia and other areas, the
Criteria Document gave less weight to both so-called ``negative'' and
``positive'' studies with methodogical limitations. In particular, EPA
agreed with the epidemiological experts on CASAC (Lippmann et al.,
1996; Samet, 1995) that the Li and Roth (1995) study approach of using
a ``panoply'' of different modeling strategies to produce seemingly
conflicting findings provides little useful insight and is superseded
by the HEI report. The attempt by Lipfert and Wyzga (1995) to address
relative effects of different pollutants was considered inconclusive
(Lippmann et al., 1996) and flawed by the use of a metric (elasticity)
that ignores the absolute concentrations of the pollutants being
compared (see Response to Comments).
Further, the Steubenville studies and reanalyses (Schwartz and
Dockery, 1992b; Moolgavkar, 1995b) were discussed in detail to examine
methodologies, and the differences in relative risks between the two
were regarded as small (U.S. EPA, 1996a, p. 12-280 to 283). Both
studies used TSP as the PM indicator variable, and they are augmented
by the more recent findings of Schwartz et al. (1996) that examine
PM10 and its components. The mixed results by Lyon et al.
(1995) in Utah Valley are compromised by loss of information related to
the methodology (U.S. EPA, 1996a, p. 12-58). As noted above, subsequent
reanalyses of the Utah Valley study by HEI (Samet et al., 1995) as well
as by Pope and Kalkstein (1996) confirmed the original findings of Pope
et al. (1992) using different model specifications. The Salt Lake City
study by Styer et al. (1995) was mentioned in the PM Criteria Document,
but received little discussion because aspects of the methodological
approach limited its statistical power to detect effects. The analysis
of Chicago mortality data in the same paper shared these problems,
particularly for seasonal analyses; in this larger city, they
nonetheless found significant associations on an annual basis between
PM10 and mortality that are consistent with other studies.
In short, the record shows that EPA did not ignore these short-term
exposure studies cited by commenters; moreover, EPA's assessment of
these studies is consistent with the views of four researchers on the
CASAC panel who have extensive involvement in conducting population
studies of air pollution (Lippmann et al., 1996).13
---------------------------------------------------------------------------

13 Their March 20, 1996 letter to the Administrator concludes
that the HEI analysis of Philadelphia supersedes earlier analyses,
specifically Moolgavkar et al. (1995a), Lipfert and Wyzga (1995),
and Li and Roth (1995), and points out the limitations of Styer et
al. (1995).
---------------------------------------------------------------------------

Similarly, EPA believes that appropriate treatment and weight were
given to studies of long-term exposure and mortality. EPA concluded
that the lack of associations in the Abbey et al. (1991) prospective
cohort study were not inconsistent with two other such studies because
the use of days of peak TSP levels as the PM indicator (instead of
PM10 or PM2.5) is inappropriate for California
cohorts exposed to both urban smog and fugitive dust episodes, and the
overall sample size may have been too small to detect significant
effects (U.S. EPA, 1996b; pp. V-17 to -18). The inadequacy of Lipfert's
(1995) application of state-wide average sedentary lifestyle data to
adjust mortality for the six cities studied by Dockery et al. (1993),
in which superior subject-specific body mass index data had already
been considered, was also noted and addressed in the Staff Paper (U.S.
EPA, 1996b; p. V-16). Again, EPA did not ignore these studies; the
rationale for giving them less weight was clearly articulated in the
documents reviewed by CASAC and judged appropriate for use in standard
setting.
While the proposal presents only a summary discussion of key
Criteria Document and Staff Paper findings, EPA believes that
discussion is fully consistent with the state of the science.
Furthermore, the proposal highlights the nature of alternative
viewpoints on the epidemiology in a quotation from the Criteria
Document (61 FR 65644, December 13, 1996) and cites explicitly the
views of most of the authors noted above in this unit (Moolgavkar et
al., 1995b; Moolgavkar and Luebeck, 1996; Li and Roth, 1995; Samet et
al., 1996; Wyzga and Lipfert, 1995). The proposal also summarizes EPA
conclusions based on all of the literature as assessed in the Criteria
Document and Staff Paper with respect to issues raised in these and
other studies, including potential confounding by independent risk
factors such as weather and other pollutants, choice of statistical
models, use of outdoor monitors, and exposure misclassification.
More specifically, in the proposal EPA has not ignored the view
advanced by some that the results of individual studies of multiple
pollutants, such as the HEI Philadelphia studies, are more suggestive
of an ``air pollution'' effect than an effect of PM alone. Indeed, the
proposal notes that 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 (61 FR 65645, December 13, 1996). Based on the large body of
evidence at hand, however, EPA cannot accept the suggestion that such
multi-pollutant studies are in any way ``negative'' with respect to
EPA's conclusions that PM, alone or in combination with other
pollutants, is associated with adverse effects at levels below those
allowed by the current standards. This conclusion is based not only on
the consistency of PM effects across areas with widely varying
concentrations of potentially confounding copollutants, but also on the
extended analyses of the Philadelphia studies in the Criteria Document
and Staff Paper.
Because commenters have tended to ignore the latter analyses, it is
appropriate to summarize them here briefly. As noted above in this
unit, the Criteria Document assessment of the Philadelphia studies
finds that PM can reasonably be distinguished from potential effects of
all pollutants except SO2. The Staff Paper builds on this
analysis through an integrated assessment that draws on information
from atmospheric chemistry, human exposure studies, and respiratory
tract penetration results to provide insight as to which of these two
pollutants is more likely to be responsible for mortality in the
elderly and individuals with cardiopulmonary disease (U.S. EPA 1996b;
pp. V-46 to -50). That assessment notes that the inhalable
(PM10), including the fine (PM2.5), components of
TSP are more likely than SO2 to penetrate and remain indoors
where the sensitive population resides most of the time.14
In addition, these PM

[[Page 38662]]

components, especially PM2.5, penetrate far more effectively
to the airways and gas exchange regions of the lung than does
SO2. Furthermore, in Philadelphia, it is possible that
SO2 is a surrogate for fine particulate acid sulfates. For
these reasons, even though statistical analyses of the Philadelphia
data set cannot fully distinguish between these two highly correlated
pollutants, EPA believes that the weight of the available evidence from
an integrated assessment more strongly supports the notion that PM is
playing an important direct role in the observed mortality effects
associations in Philadelphia. Moreover, as noted above in this unit, in
some other locations with significant PM-mortality associations,
ambient SO2 levels are too low to confound PM.
---------------------------------------------------------------------------

14 In response to comments on this rulemaking, some papers
submitted by industry commenters make statements that are in
substantial agreement with these staff conclusions with respect to
the likelihood of SO2 penetrating to indoor environments
and the lesser likelihood of affecting sensitive populations indoors
(Lipfert and Wyzga, 1997; Lipfert and Urch, 1997).
---------------------------------------------------------------------------

(ii) Recent studies available after completion of criteria review.
As noted above in this unit, other studies cited by some commenters as
so-called ``negative'' evidence ignored by EPA were published or
otherwise made available only after completion of the PM Criteria
Document. EPA agrees that it did not rely on these studies, based on
its long-standing practice of basing NAAQS decisions on studies and
related information included in the pertinent air quality criteria and
available for CASAC review.15 Although EPA has not relied on
such studies in this review and decision process, the Agency
nevertheless has conducted a provisional examination of these and other
recent studies to assess their general consistency with the much larger
body of literature evaluated in the Criteria Document.16 EPA
has placed its examination of recent studies in the rulemaking docket.
---------------------------------------------------------------------------

15 Since the 1970 amendments, the EPA has taken the view that
NAAQS decisions are to be based on scientific studies that have been
assessed in air quality criteria [see e.g., 36 FR 8186 (April 30,
1971) (EPA based original NAAQS for six pollutants on scientific
studies discussed in the air quality criteria and limited
consideration of comments to those concerning validity of scientific
basis); 38 FR 25678, 25679-25680 (September 14, 1973) (EPA revised
air quality criteria for sulfur oxides to provide basis for
reevaluation of secondary NAAQS)]. This longstanding interpretation
was strengthened by new legislative requirements enacted in 1977
(section 109(d)(2) of the Act; section 8(c) of the Environmental
Research, Development, and Demonstration Authorization Act of 1978)
for CASAC review of air quality criteria and reaffirmed in EPA's
decision not to revise the ozone standards in 1993. 58 FR 13008,
13013-13014 (March 9, 1993). Some of the commenters now criticizing
EPA for not considering the most recent PM studies strongly
supported the Agency's interpretation in the 1993 decision (UARG,
1992).
16 As discussed in EPA's 1993 decision not to revise the NAAQS
for ozone, new studies may sometimes be of such significance that it
is appropriate to delay a decision on revision of NAAQS and to
supplement the pertinent air quality criteria so the new studies can
be taken into account. 58 FR at 13014, March 9, 1993. In the present
case, EPA's provisional examination of recent studies suggests that
reopening the air quality criteria review would not be warranted
even if there were time to do so under the court order governing the
schedule for this rulemaking. Accordingly, EPA believes that the
appropriate course of action is to consider the newly published
studies during the next periodic review cycle.
---------------------------------------------------------------------------

Among the most frequently cited new studies relied on by commenters
were Davis et al. (1996), Moolgavkar et al. (1997), and Roth and Li
(1997). Davis et al. (1996) conducted a reanalysis of the Birmingham
mortality data set originally investigated in Schwartz (1993). At the
time of the close of the public comment period, the paper based on this
manuscript had not been accepted for publication in a peer reviewed
journal (Sacks, 1997). Commenters nevertheless highlight the authors'
claim that ``when humidity is included among the meteorological
variables (it is excluded in the analysis by Schwartz [1993]), we find
that the PM10 effect is not statistically significant.''
EPA's review found important factual errors in this study. Contrary to
Davis et al., Schwartz did include humidity in his 1993 study, and his
finding of a hot-and-humid-day effect was reported there. In addition,
the PM-related variables used by Davis et al. in their manuscript were
not, as the authors claimed, the same as those in Schwartz (1993).
Davis et al. also used a different humidity indicator, specific
humidity. Reanalysis by one of the co-authors (R. Smith, personal
communication, February 8, 1997) showed that when Schwartz's PM metric
was used, the estimated PM10 effect was of about the same
magnitude, and statistically significant at the 0.05 level, even using
the characterization of humidity effect proposed by Davis et al. It
therefore appears that the Davis et al. PM10 result was, in
fact, consistent with that of Schwartz, and robust against a very
different weather model specification.
Based on its examination of both the content and the publication
status of this study, EPA believes the heavy reliance and attention
given to it are misguided. In contrast to commenters' assertions, this
study does not contradict EPA's conclusions with respect to consistency
of the epidemiological evidence and confounding by weather variables;
indeed, the consideration of the corrected results would actually
support EPA's conclusions. EPA believes this example reinforces the
importance of relying on peer reviewed studies and also conducting the
kind of critical examination of such studies that takes place in the
criteria and standards review process.
Several commenters note that Roth and Li (1997) also reexamined the
Birmingham mortality data, as well as hospital admissions data from
Schwartz (1994), and produced a number of negative and inconsistent
results that depend on temperature effects and choice of statistical
model. Preliminary findings from this study were presented by Roth at
the May 1996 CASAC meeting. CASAC epidemiologists and statisticians at
the meeting pointed out a number of shortcomings, both in the
analytical strategy and in details of the models being
evaluated.17 As discussed in more detail in the Response to
Comments, the materials from Roth and Li (1997) recently provided to
EPA as attachments to public comments show that the deficiencies
pointed out at the May 1996 CASAC meeting have not been adequately
addressed. EPA concludes that this study does not support commenters'
claims.
---------------------------------------------------------------------------

17 For example, commenting on the Roth examination of
alternative model specifications, Dr. Stolwijk noted ``If you select
out of his [Roth's] matrix the things that other people have done,
he comes to a different conclusion than when he takes his whole
matrix * * *. [Y]ou are going to get a random effect that shows that
there is no effect. He [Roth] did this, I think, on purpose in this
case. Most epidemiologists, I think, have been trained to limit
their observations to something that they can state or would have
stated before they started and observe that and base their
conclusions on it'' [U.S. EPA 1996(c); May 17, 1996 Transcript,
pages 45-46].
---------------------------------------------------------------------------

The paper recently accepted for publication by Moolgavkar et al.
(1997) examines hospital admissions and air pollution in Minneapolis
and Birmingham and comes to different conclusions than earlier
investigators with respect to the role of PM10. While the
paper is a useful addition to the literature, the authors clearly do
not attempt to replicate the original studies, making the kind of
direct comparisons suggested by commenters difficult. The paper finds
an air pollution effect in one city that implicates ozone but is unable
to separate effects of PM from a group of other pollutants. EPA's
provisional examination of this study raises some questions about the
methodology, which might usefully be supplemented to further separate
pollutants as was done by Samet et al. (1996a,b) in Philadelphia, and
about the authors' interpretation of the results in both cities. In any
event, EPA does not believe this study negates the PM associations with
hospital admissions

[[Page 38663]]

reported in a number of other studies cited in the Criteria Document.
Another recent paper by Lipfert and Wyzga (1997) provides analyses
suggesting that differential measurement error might account for some
or all of the observation by Schwartz et al. (1996) that daily
mortality is more strongly associated with fine (PM2.5) than
with coarse (PM10-2.5) PM. EPA staff and CASAC accounted for
this possibility, however, and it was factored into both the Staff
Paper and CASAC recommendations.18
---------------------------------------------------------------------------

18 CASAC panelists recommended a discussion of this issue in the
Staff Paper. The Staff Paper notes: ``While greater measurement
error for the coarse fraction could depress a potential coarse
particle effect, this would not explain the results in Topeka
relative to other cities. Even considering relative measurement
error, these results provide no clear evidence implicating coarse
particles in the reported effects.'' (U.S. EPA, 1996b p. V-64).
EPA's provisional examination of the Lipfert and Wyzga (1997) paper
in the Response to Comments, finds that it is implausible that most
of the effect attributed to PM2.5 could in fact be due to
PM10-2.5, since differential measurement error cannot
make a weaker effect appear stronger than a stronger one, except
under extremely unusual circumstances.
---------------------------------------------------------------------------

Some commenters have highlighted selected individual papers or
summaries from the APHEA19 project conducted in Europe, and
from Roth (1996), calling attention particularly to negative results
found in heavily polluted regions of Eastern Europe. EPA notes that a
number of the recent APHEA and other studies in Western Europe have
shown significant associations between mortality and air pollution
including PM, and that a meta-analysis of 12 Western and Central-
eastern European studies ``is supportive of a causal association
between PM and SO2 exposure and all-cause mortality''
(Katsouyanni et al., 1997). The Eastern and Western European studies
used differing measurement methods for PM, including PM10,
gravimetric ``suspended particles,'' and the British Smoke
method.20 The differences in aerometry and the substantial
differences in location and strength of primary PM emissions sources in
central and eastern Europe as compared to western Europe or the U.S.
might well explain the different results in these unique areas.
Consequently, integration of these results would involve comprehensive
examination of the various PM instruments used, monitor siting in
relation to sources, mass calibration procedures and other aspects of
these studies.21 EPA notes that a number of European
authorities, who are familiar with this recent literature, have
proceeded with recommendations to strengthen their health guidelines,
risk assessments, or regulations for PM.22
---------------------------------------------------------------------------

19 The APHEA (Air Pollution and Health: a European Approach)
project was supported by the European Union Environment 1991-1994
Programme to investigate the possible short-term health effects of
exposure to low or moderate levels of ambient air pollutants. Eleven
European research groups carried out studies in 15 cities
(Amsterdam, Athens, Barcelona, Bratislava, Cracow, Helsinki, Koln,
Lodz, London, Lyon, Milan, Paris, Poznan, Rotterdam and Wroclaw) in
which air pollutant concentration data had been collected for at
least 5 years. Initial findings of studies on mortality and hospital
admissions were published in a series of papers in Supplement 1 to
the Journal of Epidemiology and Community Health in 1996 and a meta-
analysis of the mortality data from 12 cities is currently in press
(Katsouyanni et al., 1997).
20 The Roth et al. (1997) study in Prague used a measurement
termed ``suspended particles'' that appears to be close to TSP. The
relation of this indicator to PM10 or PM2.5 in
this city is not reported. Moreover, this study uses a variant of
the problematic methodology in the Roth analyses cited above.
21 These concerns are consistent with EPA's treatment of a
number of European and South American studies that are included in
the Criteria Document and contributed to the evaluation of the
epidemiology in Chapter 12. Because of differences in aerometry
methods and characteristic source classes between North America and
other regions of the world, however, the integrative assessment
chapter reported results only from studies conducted in the U.S. and
Canada (cf. Tables 13-3 to 13-5) in reaching quantitative
conclusions for effects estimates.
22 See, for example, the United Kingdom Air Quality Strategy,
1997; Swiss Federal Commission of Air Hygiene, 1996; World Health
Organization Revised Air Quality Guidelines for Europe, In Press).
---------------------------------------------------------------------------

Aside from the recent literature cited by these commenters, there
are a number of other recent epidemiological studies that, if
considered in today's decision, would tend to support EPA's conclusions
about the effects of PM at lower concentrations, assuming their results
were accepted following a full review in the criteria and CASAC
process. For example, in addition to the APHEA studies, several other
recent epidemiologic studies have reported significant positive
associations between PM and health effects (Lipsett et al., 1997;
Peters et al., 1997; Borja-Aburto et al., 1997; Delfino et al., 1997;
Scarlett et al., 1996; Woodruff et al., 1997; Wordley et al., 1977). In
addition, a number of recent toxicologic papers have been accepted or
appear in proceedings (Costa and Dreher, 1997; Killingsworth et al.,
1997; Godleski et al., 1997) that involve exposure to concentrated
ambient fine particles or PM constituents and appear to provide
supportive evidence as to the plausibility of the effects that have
been reported epidemiologically. If considered in this decision, these
studies would also provide biological support for the epidemiological
observation that certain susceptible groups (notably those with
cardiopulmonary disease) are most likely to be affected by PM, again
assuming the results were sustained in the full criteria and CASAC
review process.
In summary, EPA has conducted a provisional assessment of the more
recent scientific literature. Based on this provisional assessment, EPA
disagrees with commenters' assertion that full consideration of
selected new studies in this decision would materially change the
Criteria Document and Staff Paper conclusions on the consistency and
coherence of the PM data, or on the need to revise the current
standards.
(iii) Other specific comments on the epidemiological studies. Aside
from their assertion that EPA ignored or downplayed particular studies,
this second group of commenters raise additional objections, based on
the statistical modeling strategies used and the potential importance
of personal exposure misclassification, to EPA's conclusions regarding
the consistency of the epidemiological evidence. EPA conclusions on
these topics were summarized in the proposal and supported by extensive
treatments in the Criteria Document and Staff Paper. With respect to
the first issue, commenters argued that sufficient flexibility exists
in the analyses of large data sets that it may be possible to obtain
almost any result desired through choice of statistical method.
Analytical choices include the specific statistical model; methods used
to adjust for seasonal variation and the trends in the data; treatment
of other variables (e.g., other pollutants, weather, and day of week);
``lag'' structure; and study population.
A more detailed discussion of this issue, which expands on the
assessment summarized in the Criteria Document, is included in the
Response to Comments. In summary, EPA must reject commenters'
contention that legitimate alternative analyses can obtain ``almost any
result.'' As outlined above in this unit, EPA's detailed reviews of
individual studies have shown that not all methods are equally valid or
legitimate. Moreover, strong arguments can be made that the methods and
analytical strategies in the studies EPA relied upon are more
appropriate approaches than those cited by commenters (e.g., Li and
Roth, 1995; Lipfert and Wyzga, 1995; Davis et al., 1996; Roth and Li,
1997). While not all studies have addressed each of the above issues in
this unit equally well, the most comprehensive analyses of these issues
(e.g., Samet et al., 1995, 1996a,b; Pope and Kalkstein, 1996), as well
as the EPA analyses comparing study results for each issue (U.S. EPA,
1996a, pp. 12-261 to 12-305) found that the authors of studies on which
EPA

[[Page 38664]]

chiefly relied made appropriate modeling choices. The Criteria Document
concludes that: ``[T]he 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
artifactualy 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].'' While it is true, as evidenced in Li and Roth (1995), that
PM-effects data can be randomly manipulated to produce apparently
conflicting results, commenters have provided no evidence that
different plausible model specifications could lead to markedly
different conclusions.
Some commenters have expressed concerns about the reliability of
the epidemiological results because some studies showed a lack of
correlation in cross-sectional comparisons between outdoor PM measured
at central locations and indoor or personal exposures to PM (which
includes PM from the outdoor, indoor and personal
environments).23 EPA acknowledged and responded to this
issue in chapter 7 of the Criteria Document and the proposal (61 FR
65645, December 13, 1996). The major premise underlying commenters'
arguments on this issue is incorrect.24 The question is not
whether central monitoring site measurements contain a signal
reflecting actual exposures to total PM from both outdoor and indoor
sources at the individual level; the relevant question is whether
central monitoring site measurements contain a signal reflecting actual
exposures to ambient PM for the subject population, including both
ambient PM, while individuals are outdoors, and ambient PM that has
infiltrated indoors, while individuals are indoors. The PM standards
are intended to protect the public from exposure to ambient PM, not PM
generated by indoor or personal sources. There is ample evidence, as
discussed in chapter 7 of the Criteria Document, that personal exposure
to ambient PM, while outdoors and while in indoor micro-environments,
does correlate on a day-to-day basis with concentrations measured at
properly sited central monitors (U.S. EPA, 1996a, p. 1-10). EPA has,
therefore, concluded that it is reasonable to presume that a reduction
in ambient PM concentrations will reduce personal exposure to ambient
PM, and that this will protect the public from adverse health outcomes
associated with personal exposure to ambient PM.
---------------------------------------------------------------------------

23 Paradoxically, some commenters have argued (e.g., Valdberg,
1997) that the PM results are confounded because the weather and
other factors that cause daily variations in outdoor pollution will
cause similar daily variations in indoor generated air pollution.
For this to be true, outdoor ambient pollution concentrations would
have to be correlated with personal exposure to indoor generated air
pollution such as that from smoking, cleaning, and cooking. This
argument is logically inconsistent with the other comments on the
lack of any such correlation with personal exposure, and these
commenters have offered no scientific evidence to support their
claim. In response, EPA has performed and included in the Response
to Comments a numerical analysis of the relevant information from
the PTEAM exposure study that finds no evidence for such a
correspondence in the actual data.
24 As documented in Chapter 7 of the Criteria Document, time-
series community studies observe the effects of varying levels of
ambient air pollution; therefore the effects of indoor-generated air
pollution would be independent of and in addition to the effects
found in these epidemiological studies. Commenters apparently
believe EPA is claiming such studies are detecting the effects of
daily variations in total PM personal exposure from indoor and
outdoor sources. This misunderstanding is evidenced, for example, by
Wyzga and Lipfert's (1995) treatment of the difference between
ambient monitors and actual personal exposures as ``exposure
errors'' and Brown's comment for API that ``if (ambient) PM is
causally related to mortality/morbidity, then it is personal PM
exposure that must be reduced to have an effect.'' On the contrary,
it is personal exposure to ambient PM that must be reduced to
address the risk identified in community air pollution studies. Any
lack of significant correlation between outdoor PM concentrations
and personal exposure to total PM from all sources is irrelevant,
except to the extent it may decrease the power of time-series
studies to detect the effects of ambient pollution.
---------------------------------------------------------------------------

Commenters have also restated theoretically based concerns on a
related issue, namely errors in the measurement of the concentrations
of air pollutants, that was summarized in the proposal. In multiple
pollutant analyses, measurement error or, more generally, exposure
misclassification, could 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 to V-43). Relevant insights on this issue in
material appended to public comments (Ozkaynak and Spengler, 1996) have
prompted an expanded statistical analysis of the conditions under which
such errors could inflate the magnitude of the effects estimates or the
significance of PM relative to gaseous pollutants, as has been
suggested by Lipfert and Wyzga (1995). This analysis, which is
summarized in the Response to Comments, finds that the conditions under
which measurement error could inflate the effects estimates or
significance of PM relative to other pollutants are restricted to a
limited set of statistical relationships. Commenters have not provided
evidence that suggest such conditions are likely to occur with respect
to the measurement of ambient PM in relation to those for gaseous co-
pollutants commonly used in epidemiological studies.25
Therefore, it appears unlikely that measurement and exposure errors for
PM and other pollutants have inflated the estimated effects of PM, even
in multivariate analyses. More importantly, 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
associations of PM with serious health effects at levels allowed under
the current NAAQS are an artifact of errors in measurement of pollution
or of exposure (U.S. EPA 1996b, pp. V-39 to V-43).
---------------------------------------------------------------------------

25 The EPA analysis finds that in order for measurement errors
in one pollutant variable to significantly bias the estimated effect
of another pollutant, three conditions are necessary: (1) The
measurement error in the poorly measured pollutant must be very
large, roughly at least the same size as the population variability
in that pollutant; (2) the poorly measured pollutant must be highly
correlated with the other pollutant, either positively or
negatively; and (3) the measurement errors for the two pollutants
must be highly negatively correlated (Response to Comments, Appendix
D). This important factor was not considered in Lipfert and Wyzga
(1995) or by commenters.
---------------------------------------------------------------------------

(iv) Comments on the PM risk assessment. As noted in the proposal,
uncertainties about measurement errors, exposure misclassification, and
the relative effects of copollutants are more important to the
quantitative estimates of risk associated with PM than to the existence
of valid PM-effects associations at levels found in recent studies. A
number of commenters argued that EPA's risk assessment is flawed and
incomplete. Chief among the reasons they advanced is that the
assessment is based on the same epidemiological studies these
commenters argued are inadequate for the reasons summarized and
responded to above. Specific comments also addressed the extent to
which the risk assessment might overstate risk estimates because it
assumes a linear no-threshold relationship and the use of studies that
might inflate PM risk due to inadequate consideration of co-pollutants
and other potential confounders. The full risk assessment acknowledges
these issues and uncertainties, however, and it illustrates the
potential influence of such uncertainties in sensitivity analyses (U.S.
EPA 1996b; chapter 6, appendix F; Abt Associates, 1996a,b; 1997a,b).
For example, Figure 2c in the proposal (61 FR 65653, December 13, 1996)

[[Page 38665]]

illustrates the potential influence of what appears to be the most
significant uncertainty in current information, whether a population
threshold exists below which the effects of PM no longer occur (61 FR
65653, December 13, 1996). EPA notes that a full consideration of the
uncertainties, including the analysis summarized above on measurement
error, suggests that the epidemiological studies might well have
understated the total effects of air pollution; thus, both the
direction and the extent of any bias in the risk estimates are less
clear than commenters suggest.
EPA believes that, even recognizing the large uncertainties, the
key qualitative insights derived from the risk assessment and
summarized in Unit II.A.3. of this preamble remain appropriate. While
not placing great weight on the specific numerical estimates, EPA
believes that the risk analysis confirms the general conclusions drawn
primarily from the epidemiological results themselves, that there is
ample reason to be concerned that exposure to ambient PM at levels
allowed under the current air quality standards presents a serious
public health problem.
3. Key considerations informing the decision. Having carefully
considered the public comments on the above matters, EPA believes the
fundamental scientific conclusions on the effects of PM reached in the
Criteria Document and Staff Paper, and restated in the introduction to
this unit, remain valid. That is, the epidemiological evidence for
ambient PM, alone or in combination with other pollutants, shows
associations with premature mortality, hospital admissions, respiratory
symptoms, and lung function decrements. Despite extensive critical
examination in the criteria and standards review, these findings cannot
be otherwise explained by analytical, data, or other problems inherent
in the conduct of such studies. Although the evidence from
toxicological studies available during the criteria review has not
revealed demonstrated mechanisms that explain the range of effects
reported in epidemiological studies, it does not and cannot refute the
observation of such effects in exposed populations. Moreover, the
effects observed in the recent epidemiological studies at lower PM
concentrations are both coherent with each other and plausible based on
the categories of effects observed at much higher concentrations in
historic air pollution episodes, laboratory studies of PM effects at
high doses, and particle dosimetry studies. The consistency of the
results from a large number of locations and the coherent nature of the
observed results suggest a likely causal role of ambient PM in
contributing to the reported effects (U.S. EPA, 1996a; p. 13-1). Many
of the studies showing PM effects were conducted in areas where the
current PM10 standards are largely met, and both the studies
and EPA's risk assessment suggest that the collective magnitude of the
effects reflects a significant public health problem.
For these reasons, and having considered public comments on this
issue, the Administrator concludes that the review of the criteria and
standards provides strong evidence that the current PM10
standards do not adequately protect public health, and that revision of
the standards is not only appropriate, but necessary.
Aside from that conclusion, the appropriateness of continuing to
rely on the use of PM10 as the sole indicator for revised PM
standards is also relevant here. While the basis for decisions on
specific indicators is discussed more fully in Unit II.C. of this
preamble, this issue is related to the Administrator's decision on the
need to revise the standards. Based on both the staff review (U.S. EPA,
1996b, p. VII-3) and the recommendations of some commenters (e.g.,
California EPA), there are two alternative approaches for providing
additional health protection in revising the standards: Adopt tighter
PM10 standards and/or recognize the fundamental differences
between fine and coarse particles and develop separate standards for
the major components of PM10, including fine particles.
Conceptually, the first approach would give weight to comments that
standards should be based on pollutant indicators for which the most
data have been collected, with less consideration of the evidence that
suggests that the current standards provide adequate protection against
the effects of coarse particles, and that tightening the current
PM10 standards in an attempt to control fine particles would
place unnecessary requirements on coarse particles. Because the
PM10 network is in place, a more stringent PM10
standard would also respond to commenters who have expressed a desire
for more immediate implementation of revised standards. The second
approach is based on the view that, in the long run, more effective and
efficient protection can be provided by separately targeting
appropriate levels of controls to fine and coarse PM.
The Staff Paper examined this issue in detail (U.S. EPA 1996b, pp.
VII-3 to VII-11), and concluded that the available information was
sufficient to develop separate indicators for fine and coarse fractions
of PM10, based on the recent health evidence, the
fundamental differences between fine- and coarse-fraction particles,
and implementation experience with PM10. Further, the staff
concluded that:

[C]onsideration of comparisons between fine and coarse fractions
suggests that fine fraction particles are a better surrogate for
those particle components linked to mortality and morbidity effects
at levels below the current standards. In contrast, coarse fraction
particles are more likely linked with certain effects at levels
above those allowed by the current PM10 standards. In
examining alternative approaches to increasing the protection
afforded by PM10 standards, the staff concludes that
reducing the levels of the current PM10 standards would
not provide the most effective and efficient protection from these
health effects. [U.S. EPA 1996b; p. 7-45]

As discussed in Unit II.C. of this preamble, the Administrator
believes that it is more appropriate to provide additional protection
against the risk posed by PM by adding new standards for the fine
fraction of PM10, as opposed to tightening the current
PM10 standards. Although fewer epidemiological studies have
used PM2.5 and other fine particle indicators (e.g.,
sulfates, acids), there are nonetheless significant indications from
the scientific evidence - drawn from the physicochemical studies of PM,
air quality and exposure information, toxicological studies, and
respiratory tract deposition data - that this approach will provide the
most effective and efficient protection of public health.
Several commenters have argued that the decision on whether to
revise the PM standards should be deferred, particularly with regard to
fine particle standards, pending establishment and operation of a
national monitoring network to characterize fine PM and a research
program to reduce uncertainties in the effects information. These
commenters expressed concerns that establishing fine PM standards now
might result in needless regulation of PM components that may be
unrelated to observed health effects. As discussed more fully in Unit
II.F. of this preamble, such commenters recommended, at most, that if
fine PM standards were established, they should be set at a level
``equivalent'' to the current PM standards.
EPA strongly disagrees that the decision on revising the standards
should be delayed to await the results of new PM monitoring and
research programs. Under section 109(d) of the Act, EPA's obligation
after reviewing the

[[Page 38666]]

existing criteria and standards for PM is to make such revisions in the
standards and to promulgate such new standards as are appropriate under
section 109(b) of the Act. Based on her review of the criteria and
standards for PM, the Administrator has concluded that the current
standards are not adequate to protect public health and that revisions
are appropriate. In the face of the available evidence, a delay in
revising the standards would not only be inconsistent with the statute
but -- even under the optimistic assumption that the same extensive
monitoring and strategy assessment as now contemplated would occur in
the absence of a revised standard -- would add approximately 2 years to
the time when significant health benefits can be realized, resulting in
potentially significant numbers of additional premature deaths and even
larger numbers of children and individuals with air pollution-related
illness and symptoms. On the other hand, establishing standards now
will set into motion the development of implementation programs and
monitoring that can be conducted in parallel with additional scientific
research, without undue delays inherent in waiting for the research.
The question of which pollutant components to regulate has been an
issue since the inception of the first PM standards. Other ambient
pollutants (e.g., NO2 or CO) are uniquely defined as
individual chemicals, whether or not they serve as proxies for a larger
class of substances (e.g., ozone as an index of photochemical
oxidants). Regulating general PM, as opposed to multiple chemical
components of PM, raises the spectre of a host of particulate materials
of varying composition, size, and other physicochemical properties, not
all of which are likely to produce identical effects.
Both EPA's past and present regulatory experience with PM control
programs and its successive reviews of the standards have reaffirmed
the wisdom of retaining standards that control particles as a group,
rather than eliminating such standards and waiting for scientific
research to develop information needed to identify more precise limits
for the literally thousands of particle components. Each such decision
recognized the possibility that potentially less harmful particles
might be included in the mix that was regulated, but concluded that the
need to provide protection against serious health effects nonetheless
required action under section 109 of the Act. The success of this
approach is evident in early U.S. control programs that dramatically
reduced ``smoke'' and ``TSP'' in major cities in the 1960's and 1970's
and in the continued improvement in air quality through the current PM
standards. The major refinements that have been recommended through the
course of reviews of PM standards have been to improve the focus of
control efforts by defining scientifically based size classes (i.e.,
moving from TSP to PM10 and now, PM2.5) that will
permit more effective and efficient regulation of those fractions most
likely to present significant risks to health and the environment.
As discussed in Unit II.C. of this preamble, the current review has
examined the available evidence to determine whether it would tend to
support inclusion or exclusion of any physical or chemical classes of
PM, for example sulfates, nitrates, or ultra-fine particles. That
examination concludes that, while both fine and coarse particles can
produce health effects, the fine fraction appears to contain more of
the reactive substances potentially linked to the kinds of effects
observed in the recent epidemiological studies (U.S. EPA 1996b, section
V.F.). However, the available scientific information does not rule out
any one of these components as contributing to fine particle effects.
Indeed, it is reasonable to anticipate that no single component will
prove to be responsible for all of the effects of PM.
EPA recognizes that whether the standards are set for
PM10 only or also for fine particles, there are
uncertainties with respect to the relative risk presented by various
components of PM. In this regard, the Administrator places greater
weight on the concern that by failing to act now, the PM NAAQS would
not control adequately those components of air pollution that are most
responsible for serious effects, than on the possibility they might
also control some component that is not. EPA believes that moving
simultaneously to establish standards based on the best available
scientific evidence and to conduct an aggressive monitoring and
scientific research program designed to help resolve current
uncertainties is a prudent and responsible approach for addressing both
the risks and the uncertainties inherent in this important public
health issue.
In summary, given the evidence that PM-related health effects
appear likely to 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 their May 1996 public meeting (U.S. EPA,
1996c), 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 advice of CASAC, and taking into account public
comments, the Administrator concludes that it is appropriate at this
time to revise the current PM standards to increase the public health
protection provided against the known and potential effects of PM
identified in the air quality criteria.

C. Indicators of PM

In 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, EPA concludes that 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, EPA
continues to find that the following conclusions reached in the Staff
Paper and 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.

[[Page 38667]]

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.
As noted in Unit II.B. of this preamble, however, the Staff Paper
concludes 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 PM (U.S. EPA, 1996b, pp. VII-4 to
VII-11). Based on the recent health effects evidence and the
fundamental physical and chemical differences between fine and coarse
fraction particles, the Criteria Document and Staff Paper conclude that
fine and coarse fractions of PM10 should be considered
separately (U.S. EPA, 1996a, p. 13-93; 1996b, p. VII-18). 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).
Some commenters have noted that it is often difficult to
distinguish the effects of either fine or coarse fraction particles
from those of PM10; this is to be expected because both
fractions are themselves components of PM10, and hence not

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Source: Frix Law Library, https://www.frixlaw.com/law-library/documents/fr%3A97-18577. Public record. Not legal advice.