[Federal Register Volume 62, Number 138 (Friday, July 18, 1997)]
[Rules and Regulations]
[Pages 38652-38760]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 97-18577]



[[Page 38651]]

_______________________________________________________________________

Part II





Environmental Protection Agency





_______________________________________________________________________



40 CFR Part 50



National Ambient Air Quality Standards for Particulate Matter; Final 
Rule

  Federal Register / Vol. 62, No. 138 / Friday, July 18, 1997 / Rules 
and Regulations  

[[Page 38652]]


-----------------------------------------------------------------------

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 ``<1> 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
---------------------------------------------------------------------------

    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).
---------------------------------------------------------------------------

    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).
---------------------------------------------------------------------------

    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.
---------------------------------------------------------------------------

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

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

    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.
---------------------------------------------------------------------------

    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.

---------------------------------------------------------------------------

[[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).
---------------------------------------------------------------------------

    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.
---------------------------------------------------------------------------

    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 <4.5 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 
fully independent. EPA believes that it is more meaningful to examine 
comparisons between the fine and coarse fraction components. Such 
comparisons presented in the Staff Paper suggest that fine particles 
are a better surrogate for those components of PM that are linked to 
mortality and morbidity effects at levels below the current standards 
(U.S. EPA, 1996b, p. VII-18). Moreover, a regulatory focus on fine 
particles would likely also result in controls on gaseous precursors of 
fine particles (e.g., SOx, NOx, VOC), which are 
all components of the complex mixture of air pollution that has most 
generally been associated with mortality and morbidity effects. The 
Staff Paper concludes that, in contrast to fine particles, coarse 
fraction particles are more clearly linked with certain morbidity 
effects at levels above those allowed by the current 24-hour standard.
    Public comments received on the proposed indicators were 
overwhelmingly in favor of EPA's proposal to maintain PM10 
as an indicator for PM, whether as an indicator of coarse particles in 
conjunction with a fine PM standard, or as the sole PM indicator. This 
near unanimity shows strong support for retaining general PM standards. 
While a substantial number of commenters supported EPA's proposal to 
add an indicator for fine PM, a number of other commenters objected to 
any standard revisions, including addition of a fine PM indicator. 
Beyond the general points about the basis for any revisions discussed 
in Unit II.B. of this preamble, these commenters argued either that the 
available epidemiological data did not provide a basis for separating 
fine and coarse fraction particles, or that there were not enough fine 
particle studies to support selecting standard levels. Most of these 
commenters also expressed concerns that there were insufficient ambient 
fine particle data by which to evaluate the relative protection 
afforded by new standards.
    EPA notes that issues relating to the basis for separating 
PM10 fractions were addressed in the Criteria Document and/
or Staff Paper assessments, and these perspectives were also available 
for CASAC consideration in developing its recommendations. The proposal 
states that the main basis for separating the fine and coarse fractions 
of PM10 is that, because they are fundamentally different PM 
components with significantly different physico-chemical properties and 
origins (U.S. EPA 1996b, section V.D), separate standards would permit 
more effective and efficient regulation of PM. While the difficulty in 
separating these classes in the epidemiological studies is noted above, 
the preponderance of the available evidence suggests that strategies to 
control fine particles will more effectively reduce population exposure 
to substances associated with health effects in the recent 
epidemiological studies. Although the number of studies using fine PM 
indicators is more limited than for PM10, there are more 
than 20 community studies showing significant associations for a 
consistent set of mortality and morbidity effects. A substantial subset 
of these studies (Tables V-12 to V-13; U.S. EPA, 1996b) provides a 
sufficient quantitative basis for selecting standard levels, without 
the need to rely on estimates based on PM2.5/PM10 
ratios.
    Having considered the public comments on this issue, the 
Administrator concurs with staff and CASAC recommendations to control 
particles of health concern (i.e., PM10) through separate 
standards for fine and coarse fraction particles. The following units 
outline the basis for the Administrator's decision on specific 
indicators for fine and coarse fraction particle standards.
    1. Indicators for the fine fraction of PM10. The 
Administrator continues to conclude that it is appropriate to control 
fine particles as a group, as opposed to singling out particular 
components or classes of fine particles. The more qualitative 
scientific literature, evaluated in Chapter 11 of the Criteria Document 
and summarized in section V.C of the Staff Paper, has reported various 
health effects associated with high concentrations of a number of fine 
particle components (e.g., sulfates, nitrates, organics, transition 
metals), alone or in some cases in combination with gases. Community 
epidemiolgical studies have found significant associations between fine 
particles or PM10 and health effects in various areas across 
the U.S. where such fine particle components correlate significantly 
with particle mass. As noted above in this unit, it is not possible to 
rule out any one of these components as contributing to fine particle 
effects.26 Thus, the Administrator finds that the present 
data more readily support a standard based on the total mass of fine 
particles. EPA will conduct additional research, in cooperation with 
other Federal agencies and in partnership with State and local agencies 
and the private sector, to better identify which species are of concern 
for human health, and the sources and relative magnitude of such 
species.
---------------------------------------------------------------------------

    26 As discussed above, a number of commenters expressed concerns 
that various portions of fine particles might not be responsible for 
any observed effects. One group (PG&E, 1997) recommended that 
nitrates should be excluded from fine PM mass collected on the basis 
of their assessment of available effects literature on particulate 
and gas phase inorganic nitrates. Based on an examination of this 
information as well as the earlier staff assessment, EPA maintains 
its conclusion that the available evidence is not sufficient to 
exclude nitrates or any other class of fine particles that are 
collected by PM monitors comparable to those used in the recent 
epidemiological studies.
---------------------------------------------------------------------------

    In specifying a precise size range for a fine particle standard, 
both the staff and CASAC recommended PM2.5 as the indicator 
of fine particles (Wolff, 1996b). The particle diameter reflecting the 
mass minimum between the fine and coarse modes typically lies between 1 
and 3 m, and the scientific data support a sampling ``cut 
point'' to delineate fine particles somewhere in this range. Because of 
the potential

[[Page 38668]]

overlap of fine and coarse particle mass in this intermodal region, EPA 
recognizes that any specific sampling cut point would result in only an 
approximation of the actual fine-mode particle mass. Thus, the choice 
of a specific diameter within this size range is largely a policy 
judgment. The staff and CASAC recommendations for a 2.5 m 
sampling cut point were based on considerations of consistency with the 
community health studies, the limited potential for intrusion of coarse 
fraction particles into the fine fraction, and availability of 
monitoring technology.27 PM2.5 encompasses all of 
the potential agents of concern in the fine fraction, including most 
sulfates, acids, fine particle transition metals, organics, and 
ultrafine particles, and includes most of the aggregate surface area 
and particle number in the entire distribution of atmospheric 
particles.
---------------------------------------------------------------------------

    27 The National Mining Association (NMA) and related companies 
submitted comments favoring ultimate selection of a smaller cutpoint 
of 1 m (PM1) to further reduce coarse particle 
intrusion. EPA considered this approach in developing the Staff 
Paper and proposal. PM1 has not been used in health 
studies, although in most cases collected mass should be similar to 
those for cutpoints of 2.1 or 2.5 m. While a PM1 
indicator could reduce intrusion of coarse particles, it might also 
omit portions of hygroscopic PM components such as acid sulfates, 
nitrates, and some organic compounds in higher humidity environments 
picked up by PM2.5 measurements. PM1 sampling 
technologies have been developed, but have not been widely used in 
the field to date; there are some concerns about loss of certain 
organic materials in available models relative to an instrument with 
a larger size cut. NMA has also recommended consideration of a 
methodology that could subtract coarse mass from PM2.5 
measurements where undue coarse particle intrusion resulted in fine 
standard violations. EPA will evaluate this recommendation in the 
context of implementation policies.
---------------------------------------------------------------------------

    The Administrator concurs with the staff and CASAC recommendations 
and concludes that PM2.5 is the appropriate indicator for 
fine particle standards. As discussed in Unit VI.B. of this preamble, 
technical details of how PM2.5 is to be measured in the 
ambient air are specified in the Federal Reference Method (40 CFR part 
50, Appendix L).
    2. Indicators for the coarse fraction of PM10. 
  The Criteria Document and Staff Paper conclude that 
epidemiological information, together with dosimetry and toxicological 
information, support the need for a particle indicator that addresses 
the health effects associated with coarse fraction particles within 
PM10 (i.e., PM10-2.5). As noted above, coarse 
fraction particles can deposit in those sensitive regions of the lung 
of most concern. Although the role of coarse fraction particles in much 
of the recent epidemiological results is unclear, limited evidence from 
studies where coarse fraction particles are the dominant fraction of 
PM10 suggest that significant short-term effects related to 
coarse fraction particles include aggravation of asthma and increased 
upper respiratory illness. In addition, qualitative evidence suggests 
that potential chronic effects may be associated with long-term 
exposure to high concentrations of coarse fraction particles.
    In selecting an indicator for coarse fraction particles, the 
Administrator took into account the views of several CASAC panel 
members who suggested using the coarse fraction directly (i.e., 
PM10-2.5) as the indicator. However, the Administrator notes 
that the existing ambient data base for coarse fraction particles is 
smaller than that for fine particles, and that the only studies of 
clear quantitative relevance to effects most likely associated with 
coarse fraction particles have used undifferentiated PM10. 
In fact, it was the consensus of CASAC that it is reasonable to 
consider PM10 itself as a surrogate for coarse fraction 
particles, when used together with PM2.5 standards. The 
monitoring network already in place for PM10 is large. 
Therefore, in conjunction with the decision to have separate standards 
for PM2.5, the Administrator concludes, consistent with 
CASAC recommendations and public comments, that it is appropriate to 
retain PM10 as the indicator for PM standards intended to 
protect against the effects most likely associated with coarse fraction 
particles.

D. Averaging Time of PM2.5 Standards

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

[[Page 38669]]

    The Administrator has considered this evidence, which suggests that 
some health endpoints reflect the cumulative effects of PM exposures 
over a number of years. In such cases, an annual standard would provide 
effective protection against persistent long-term (several years) 
exposures to PM. Requiring a much longer averaging time would also 
complicate and unnecessarily delay control strategies and attainment 
decisions.
    The Administrator has also considered the seasonality of emissions 
of fine particles and their precursors in some areas (e.g., wintertime 
smoke from residential wood combustion, summertime regional acid 
sulfate and ozone formation), which suggests that some effects 
associated with annual average concentrations might be the result of 
repeated seasonally high exposures. However, different seasons are 
likely of concern in different parts of the country, and the current 
evidence does not provide a satisfactory quantitative basis for setting 
a national fine particle standard in terms of a seasonal averaging 
time.
    In addition, the Administrator recognizes that an annual standard 
would have the effect of improving air quality broadly across the 
entire annual distribution of 24-hour PM2.5 concentrations, 
although such a standard would not as effectively limit peak 24-hour 
concentrations as would a 24-hour standard. The risk assessment 
summarized above found that because such 24-hour peaks contribute much 
less to the total health risk over a year than the more numerous low- 
to mid-range PM2.5 levels, an annual standard could also 
provide effective protection from health effects associated with short-
term exposures to PM2.5 as well as those associated with 
long-term exposures (see figure 2; 61 FR 65652-65653, December 13, 
1996).
    3. Combined effect of annual and 24-hour standards. For the reasons 
outlined in Units II.C.1. and 2. of this preamble, the Administrator 
concluded in the proposal that a short-term PM2.5 standard 
with a 24-hour averaging time can serve to control short-term ambient 
PM2.5 concentrations, thus providing protection from health 
effects associated with short-term (from less than 1-day to up to 5-
day) exposures to PM2.5. Further, a long-term 
PM2.5 standard with an annual averaging time can serve to 
control both long- and short-term ambient PM2.5 
concentrations, thus providing protection from health effects 
associated with long-term (seasonal to several years) and, to some 
degree, short-term exposures to PM2.5.
    EPA received comparatively few public comments on these proposed 
averaging times. Those supporting PM2.5 standards also 
strongly supported adopting both annual and 24-hour averaging times. 
Many of those opposing PM2.5 standards, for the reasons 
discussed in Unit II.B. of this preamble, provided contingent comments 
that variously supported both averaging times for PM2.5 
standards in the event the Administrator disagreed with their overall 
recommendations. Other opponents of PM2.5 standards 
disagreed with having two standards on administrative grounds, or 
because some CASAC members did not support both averaging times.
    The relationship between standards for the two averaging times is 
discussed below in this unit. In essence, based on its examination of 
the effects data and air quality relationships, EPA believes that a 
single PM2.5 standard (24-hour or annual) either would not 
provide adequate protection against effects of concern for all 
averaging times, or would be inefficient in the sense that it was more 
stringent than necessary for at least one averaging time. Contrary to 
commenters who focused on minority CASAC opinions, EPA notes that a 
clear majority of CASAC supported both 24-hour and annual 
standards28. After considering public comments on averaging 
time and the rationale outlined above, the Administrator has concluded 
that both 24-hour and annual PM2.5 standards are 
appropriate.
---------------------------------------------------------------------------

    28 Of the 19 panel members who joined in the consensus for 
PM2.5 standards, 17 (90 percent) recommended a 24-hour 
standard and 13 (70 percent) recommended an annual standard (Wolff, 
1996b).
---------------------------------------------------------------------------

    The Administrator next considered the potential combined effects of 
such standards on PM concentration levels and distributions. The 
existing health effects evidence could, of course, be used to assess 
the form and level of each standard independently, with short-term 
exposure health effects evidence being used as the basis for a 24-hour 
standard and the long-term exposure health effects evidence as the 
entire basis for an annual standard. Some CASAC panel members 
apparently used this approach as a basis for their views on appropriate 
averaging times and standard levels. In particular, a few members 
focused only on a 24-hour PM2.5 standard in light of the 
relative strength of the short-term exposure studies. On the other 
hand, two members focused only on an annual standard, recognizing that 
strategies to meet an annual standard would provide protection against 
effects of both short- and long-term exposures.
    As noted above in this unit, attempting to provide protection for 
all of the effects identified in long- and short-term PM exposure 
studies with a single averaging time would result in either inadequate 
protection for some effects, or unnecessarily stringent control for 
others. The Administrator has, instead, emphasized a policy approach 
that considers the consistency and coherence, as well as the 
limitations, of the body of evidence as a whole, and recognizes that 
there are various ways to combine two standards to achieve an 
appropriate degree of public health protection. Such an approach to 
standard setting, which integrates the body of health effects evidence 
and air quality analyses, and considers the combined effect of the 
standards, has the potential to result in a more effective and 
efficient suite of standards than an approach that only considers 
short- and long-term exposure evidence, analyses, and standards 
independently.
    In considering the combined effect of such standards, the 
Administrator notes that while an annual standard would focus control 
programs on annual average PM2.5 concentrations, it would 
also result in fewer and lower 24-hour peak concentrations. 
Alternatively, a 24-hour standard that focuses controls on peak 
concentrations could also result in lower annual average 
concentrations. Thus, either standard could be viewed as providing both 
short- and long-term protection, with the other standard serving to 
address situations where the daily peaks and annual averages are not 
consistently correlated.
    The Administrator proposed that the suite of PM2.5 
standards could most effectively and efficiently be defined by treating 
the annual standard as the generally controlling standard for lowering 
both short- and long-term PM2.5 concentrations. In 
conjunction with the annual standard, the 24-hour standard would serve 
to provide protection against days with high peak PM2.5 
concentrations, localized ``hot spots,'' and risks arising from 
seasonal emissions that would not be well controlled by a national 
annual standard.
    Relatively few public comments were addressed specifically to the 
proposal that the annual standard be directed toward controlling both 
24-hour and annual levels (thereby basing the annual standard on an 
evaluation of both the short- and long-term health effects 
information), with the 24-hour standard being used to address more 
localized short-term peaks. A number of commenters, notably some among 
the groups opposing any revised PM

[[Page 38670]]

standards, appeared to have ignored this fundamental aspect of the 
proposal, judging by their assertions that the sole basis for EPA's 
proposed annual standards was two long-term exposure studies (Dockery 
et al., 1993; Pope et al. 1995). This is incorrect; as the proposal 
states, EPA based the proposed annual standard level on a wider range 
of short- and long-term exposure studies. Other commenters, including 
some environmental groups, reserved comment on this specific issue, but 
expressed concerns that the specific levels for both standards were not 
stringent enough, regardless of which standard is intended to be 
controlling. Issues regarding specific levels are discussed below in 
Unit II.F. of this preamble.
    Some commenters, however, disagreed with the proposition that EPA's 
proposed approach would necessarily provide the most effective and 
efficient standards. In the view of some who opposed PM2.5 
standards, the likelihood that there are thresholds below which no 
effects occur means that a 24-hour standard would be more efficient 
than an annual standard. In this view, the reductions made on days that 
were below the threshold would provide no protection.29 Some 
commenters also noted that while a majority of CASAC members favored 
both annual and 24-hour standards, more recommended 24-hour standards.
---------------------------------------------------------------------------

    29 A related comment criticized the risk assessment conclusion 
that peak 24-hour concentrations contribute much less to the total 
risk over a year as inconsistent with the experience in historic air 
pollution episodes. EPA disagrees. While the historic London 
episodes were quantitatively different from those assumed in the 
risk assessment, the record over 14 London winters indicates a 
continuum of effects down to the lowest levels. It is therefore 
likely that the cumulative increase in mortality calculated for all 
the days in the whole 14-year period would not be dominated by the 
more limited number of episode days.
---------------------------------------------------------------------------

    While the available epidemiological studies provide strong evidence 
suggesting that PM causes or contributes to health effects at levels 
below the current standards, EPA agrees, as stated previously, that 
uncertainties increase markedly at lower concentrations. Nevertheless, 
the level or even existence of population thresholds below which no 
effects occur cannot be reliably determined by an examination of the 
results from the available studies. Analyses have placed some limits, 
however, and EPA has considered hypothetical thresholds in its risk 
assessment. As noted in Unit II.A. of this preamble, even assuming an 
example threshold of 18 g/m3, the risk assessment 
(see Figure 2c; 61 FR 65653, December 13, 1996) finds that most of the 
annual aggregate risk associated with short-term exposures still 
results from the large number of days at lower to mid-range values 
above the mean. Given that neither the Criteria Document nor commenters 
have provided quantitative evidence regarding the likelihood of a 
threshold at levels much higher than the above example, EPA believes 
that the evidence provided in the risk assessment does not support the 
commenters' position. As noted above, EPA believes that most CASAC 
opinions on averaging time reflect panelists' judgments on the relative 
strength of the short-term exposure epidemiological studies, a judgment 
that EPA shares. Although most CASAC panel members did not offer an 
opinion on the use of short-term exposure studies in specifying annual 
standards, two panelists did support this notion. EPA therefore 
believes this approach is neither inconsistent with the underlying 
science nor discordant with the advice of CASAC.
    Another concern was raised by some air pollution control officials 
who otherwise supported revised PM standards. These commenters state 
that, from an implementation perspective, it is often easier to design 
control strategies for single short-term events than for annual 
averages. Aside from whether this is a proper consideration in 
establishing NAAQS, the point in fact highlights one of the important 
strengths of an annual standard in addressing short-term risks 
associated with PM2.5. As noted by the commenters, risk 
management for a short-term standard focuses on a characteristic 
``design value'' episode responsible for peak concentrations. For PM, 
such peak values can be associated with single source contributions. 
Meteorology, relative source contributions, and resulting particle 
composition for that day may or may not be typical for the area or for 
the year. Yet the short-term exposure epidemiological results are 
largely drawn from studies that associated variations in area-wide 
effects with monitor(s) that gauged the variation in daily levels over 
the course of up to 8 years. The strength of the associations in these 
data is demonstrably in the numerous ``typical'' days in the upper to 
middle portion of the annual distribution, not on the peak 
days.30 For these reasons, strategies that focus only on 
reducing peak days are less likely to achieve reduction of the mix and 
sources of urban and regional-scale PM pollution most strongly 
associated with health effects. Although designing control strategies 
to reduce annual levels may be more difficult than for 24-hour 
standards, the available short- and long-term epidemiological data 
suggest it is also likely to result in a greater reduction in area-wide 
population exposure and risk.
---------------------------------------------------------------------------

    30 This point is buttressed by studies that have taken out a 
limited number of higher PM concentration days with little effect on 
the effects estimates or significance of the association (e.g., 
Schwartz et al., 1996; Pope and Dockery, 1992).
---------------------------------------------------------------------------

    The Administrator concludes that the most effective and efficient 
approach to establishing PM2.5 standards is to treat the 
annual standard as the generally controlling standard for lowering both 
short- and long-term PM2.5 concentrations, while the 24-hour 
standard would serve to provide protection against days with high peak 
PM2.5 concentrations, localized ``hot spots,'' and risks 
arising from seasonal emissions that would not be well controlled by a 
national annual standard. In reaching this view, the Administrator took 
into account the public comments and the factors discussed below in 
this unit.
    (1) Based on one of the key observations from the quantitative risk 
assessment summarized above (see Figures 2a,b,c; 61 FR 65652-65653, 
December 13, 1996), the Administrator notes that much if not most of 
the aggregate annual risk associated with short-term exposures results 
from the large number of days during which the 24-hour average 
concentrations are in the low- to mid-range, below the peak 24-hour 
concentrations. As a result, lowering a wide range of ambient 24-hour 
PM2.5 concentrations, as opposed to focusing on control of 
peak 24-hour concentrations, is the most effective and efficient way to 
reduce total population risk. Further, there is no evidence suggesting 
that risks associated with long-term exposures are likely to be 
disproportionately driven by peak 24-hour concentrations. Thus, an 
annual standard that controls an area's attainment status is likely to 
reduce aggregate risks associated with both short- and long-term 
exposures with more certainty than a 24-hour standard.
    (2) The consistency and coherence of the health effects data base 
are, therefore, more directly related to the more frequently occurring 
PM exposures reflected in study period mean measures of air quality 
(e.g., the annual distributions of 24-hour PM concentrations), than to 
the potentially site-specific and/or otherwise infrequent PM exposures 
reflected in a limited number of peak 24-hour concentrations. More 
specifically, judgments about the quantitative consistency of the large 
number of short-term exposure studies

[[Page 38671]]

reporting associations with 24-hour concentrations arise from comparing 
the relative risk results per PM increment as derived from analyzing 
the associations across the entire duration of the studies. These 
studies typically spanned at least an annual time frame and the 
reported associations are most strongly influenced by the large number 
of days toward the middle of the distribution.
    (3) An annual average measure of air quality is more stable over 
time than are 24-hour measures. Thus, a controlling annual standard is 
likely to result in the development of more consistent risk reduction 
strategies over time, since an area's attainment status will be less 
likely to change due solely to year-to-year variations in 
meteorological conditions that affect the formation of fine particles, 
than under a controlling 24-hour standard.
    Under this policy approach, the annual PM2.5 standard 
would serve in most areas as the target for control programs designed 
to be effective in lowering the broad distribution of PM2.5 
concentrations, thus protecting not only against long-term effects but 
also short-term effects as well. In combination with such an annual 
standard, the 24-hour PM2.5 standard would be set so as to 
protect against the occurrence of peak 24-hour concentrations, 
particularly peak concentrations that present localized or seasonal 
exposures of concern in areas where the highest 24-hour-to-annual mean 
PM2.5 ratios are appreciably above the national average.

E. Form of PM2.5 Standards

    1. Annual standard. As discussed in some detail during the last 
review of the PM NAAQS (see 49 FR 10408, March 20, 1984; 52 FR 24634, 
July 1, 1987) and in the December 13, 1996 proposal, the annual 
arithmetic mean form of the current annual PM10 standard 
(i.e., the annual arithmetic mean averaged over 3 years) is a 
relatively stable measure of air quality that reflects the total 
cumulative dose of PM to which an individual or population is exposed. 
Short-term peaks have an influence on the arithmetic mean that is 
proportional to their frequency, magnitude, and duration, and, thus, 
their contribution to cumulative exposure and risk. As a result, the 
annual arithmetic mean form of an annual standard provides protection 
across a wide range of the air quality distribution contributing to 
exposure and risk, in contrast to other forms, such as the geometric 
mean, that de-emphasize the effects of short-term peak concentrations.
    While almost no commenters took specific issue with use of an 
annual arithmetic mean, a number of commenters disagreed with averaging 
over 3 years for both the annual and 24-hour standards because of their 
desire for quick action in the initial implementation of 
PM2.5 controls. The Administrator recognizes the importance 
of promptly implementing appropriate control programs, but she does not 
believe that implementation start-up concerns are an adequate basis for 
adopting a form (e.g., a single year annual average) that would provide 
less stable risk reduction in the long-run. Therefore, the 
Administrator continues to concur with the Staff Paper recommendation, 
supported by CASAC, to use the annual arithmetic mean, averaged over 3 
years, as the form for an annual PM2.5 standard consistent 
with the current form of the annual PM10 standard. 
Nevertheless, EPA intends to address the concerns of those who 
commented that the 3-year form might prevent the public from being 
informed about the air quality status of their communities. As outlined 
in Unit II.H. of this preamble, EPA plans to issue revised Pollutant 
Standard Index criteria for PM2.5, to ensure the public is 
informed promptly about air quality status.
    The Staff Paper and some CASAC panel members also recommended that 
consideration be given to calculating the PM2.5 annual 
arithmetic mean for an area by averaging the annual arithmetic means 
derived from multiple monitoring sites within a monitoring planning 
area. In proposing a calculation method for annual arithmetic averages 
that involves spatial averaging of monitoring data, the Administrator 
reasoned as follows:
    (1) Many of the community-based epidemiological studies examined in 
this review used spatial averages, when multiple monitoring sites were 
available, to characterize area-wide PM exposure levels and the 
associated population health risk. In those studies that used only one 
monitoring location, the selected site was chosen to represent 
community-wide exposures, not the highest value likely to be 
experienced within the community. Thus, spatial averages are most 
directly related to the epidemiological studies used as the basis for 
the proposed revisions to the PM NAAQS.
    (2) As a part of the overall policy approach discussed in Unit 
II.D. of this preamble, the annual PM2.5 standard would be 
intended to reduce aggregate population risk from both long- and short-
term exposures by lowering the broad distribution of PM2.5 
concentrations across the community. An annual standard based on 
spatially averaged concentrations would better reflect area-wide PM 
exposure levels than would a standard based on concentrations from a 
single monitor with the highest measured values.
    (3) Under this policy approach, the 24-hour PM2.5 
standard would be intended to work in conjunction with a spatially 
averaged annual PM2.5 standard by providing protection 
against peak 24-hour concentrations, localized ``hot spots,'' and 
higher PM2.5 concentrations arising from seasonal emissions 
and meteorology that would not be as well controlled by an annual 
standard. Accordingly, the 24-hour PM2.5 standard should be 
based on the single population-oriented monitoring site within the 
monitoring planning area with the highest measured values.
    Based on these considerations, the Administrator proposed that the 
form of an annual PM2.5 standard be expressed as the annual 
arithmetic mean, temporally averaged over 3 years and spatially 
averaged over all designated monitoring sites,31 which, in 
conjunction with a 24-hour PM2.5 standard, was intended to 
provide the most appropriate target for reducing area-wide population 
exposure to fine particle pollution. Recognizing the complexities that 
spatial averaging might introduce into risk management programs, in the 
proposal the Administrator also requested comment on the alternative of 
basing the annual standard for PM2.5 solely on the single 
population-oriented monitor site within the monitoring planning area 
with the highest 3-year average annual mean.
---------------------------------------------------------------------------

    31 The notice of proposed revisions to 40 CFR part 58 recognized 
that a single appropriately sited monitor could suffice for an area 
in place of an average of multiple monitors.
---------------------------------------------------------------------------

    The proposed approach to designating sites that are appropriate for 
spatial averaging was based on criteria and constraints contained in 
the proposed revision to the monitoring siting and network planning 
requirements in 40 CFR part 58. In proposing this approach, the 
Administrator noted concerns regarding the development and 
implementation of appropriate and effective criteria for the selection 
of sites and designations of areas for spatial averaging.
    A number of commenters who otherwise favored setting 
PM2.5 standards objected to the concept of population-
oriented monitors and expressed the view that any monitor regardless of 
where it was sited should be eligible for comparison to the annual 
PM2.5 standard. They further maintained that the proposed 
provisions for spatial averaging would fail to provide adequate health 
protection because

[[Page 38672]]

``clean areas'' and ``dirty areas'' would be averaged together. Some 
commenters expressed concern that the proposed constraints on spatial 
average would not be sufficient to prevent use of such averaging to 
avoid pollution abatement. Others may not have fully understood the 
implications of the specific constraints and siting requirements 
discussed in the proposed revisions to 40 CFR part 58, which were 
intended to ensure that the population-oriented monitors used for the 
annual standard were actually reflective of community-wide exposures 
and that the spatial averages did not include non-representative 
monitored values from either ``clean areas'' or ``dirty 
areas.''32 In order to clarify the intent that the spatially 
averaged annual standard protect those in smaller communities, as well 
as those in larger population centers, the final revisions to 40 CFR 
part 58 adopt the term ``community-oriented'' monitors.
---------------------------------------------------------------------------

    32 The 40 CFR part 58 proposed rule identified the proposed 
criteria for monitors to be averaged; namely, monitors must be 
properly sited to reflect population-orientation, primarily 
influenced by similar sources, and within +/-20 percent of the 
average levels and a specific degree of correlation (or meet a 
``homogeneity'' constraint). Additional criteria include 
demonstrations that the monitors to be averaged are influenced 
primarily by similar sources (e.g., to prevent the placement of 
monitors upwind in unrepresentative locations), EPA oversight of the 
monitoring program which includes regular review and approval of the 
State PM monitoring network design, and other criteria to ensure 
proper monitor siting. The final rule includes the addition of 
provisions that the State PM monitoring network design be available 
for public inspection.
---------------------------------------------------------------------------

    Other commenters, who supported PM2.5 annual standards, 
endorsed the concept of spatial averaging as being more reflective of 
the air quality data used in the underlying health studies and because 
there is general uniformity of fine particle concentrations across an 
area. Opponents of the PM2.5 standards expressed contingent 
support for spatial averaging in concept, again citing the linkage to 
the underlying health studies. Indeed, they advocated the extension of 
spatial averaging to the daily form of the standard, and/or recommended 
less constrained spatial averaging to allow for averaging across entire 
metropolitan areas.
    The Administrator, of course, shares commenters' concerns that the 
form of the standards, in conjunction with other components of the 
standards, must protect public health adequately against risks 
associated with PM. It was for this reason that EPA proposed a policy 
approach providing for greatest overall risk reduction for all citizens 
in the community from exposures to the mix of urban and regional scale 
PM pollution most strongly associated with health effects. In 
specifically considering whether to allow for the use of spatial 
averaging, the Administrator placed great weight on consistency with 
the underlying body of health effects evidence. The Administrator is 
mindful that some community studies relied inherently on exposure and 
effects estimates that reflect comparatively broad spatial scales, as 
highlighted by those commenters desiring to extend permissible 
averaging; however, this type of exposure characterization may not be 
appropriate for all circumstances and might leave some areas without 
adequate protection.33
    For these reasons, the 40 CFR part 58 proposal package contained 
criteria and constraints on spatial averaging. These criteria and 
constraints were intended to ensure that spatial averaging would not 
result in inequities in the level of protection provided by the PM 
standards. The Administrator again recognizes that either a single 
properly sited community-oriented monitor, or an average of more than 
one such monitors, are both appropriate indices of area-wide population 
exposures. Both are consistent with monitoring approaches used in 
community epidemiological studies upon which the standards are based. 
On the other hand, comparing the annual PM2.5 standard to 
the maximum concentrations at a site that is not representative of 
community exposures, as some have suggested, would be inconsistent with 
the Administrator's goal of using the annual standard to reduce urban 
and regional scale exposures and risks. Further, the Administrator 
believes that the criteria and, siting requirements contained in 40 CFR 
part 58, provide adequate safeguards against inappropriate application 
of spatial averaging. Therefore, the Administrator continues to believe 
that an annual PM2.5 standard reflective of area-wide 
exposures, in conjunction with a 24-hour standard designed to provide 
adequate protection against localized peak or seasonal PM2.5 
levels, reflects the most appropriate approach for public health 
against the effects of PM reported in the scientific 
literature.34
---------------------------------------------------------------------------

    33 Daily mortality studies generally use urban or metro-areawide 
effects statistics in conjunction with single or multiple monitors 
that index day-to-day pollution changes across the area. Ito et al. 
(1995) found that spatial averages from multiple PM monitors in 
Chicago were better correlated with daily mortality than were most 
single monitors, but that single monitors were also associated. A 
number of morbidity studies (e.g., Schwartz et al., 1994; Neas et 
al., 1995; Raizenne et al.; 1996) used community scale monitors and 
effects information from a defined group of subjects from the 
community, who were more closely represented by the monitor.
    34 Because the 24-hour standard is designed to address localized 
peaks, it would be inappropriate to extend spatial averaging forms 
to this standard.
---------------------------------------------------------------------------

    The majority of comments from States stressed the need for 
flexibility in specifying network designs and spatial averaging, given 
that the nature and sources of particle pollution vary from one area to 
another. One State agency specifically requested the flexibility to 
choose whether to use a single community-oriented monitor or a spatial 
average of several of such monitors, arguing that it is appropriate to 
provide this flexibility as PM2.5 monitoring networks evolve 
and to address the diversity of local conditions.
    As a result of EPA's evaluation of these comments, the requirements 
of 40 CFR part 50, Appendix K, and 40 CFR part 58 have been revised to 
clarify that the implementing agencies have the flexibility to compare 
the annual PM2.5 standard either to the measured value at a 
single representative community-oriented monitoring site, or to the 
value resulting from an average of community-oriented monitoring sites 
that meet the revised criteria and constraints enumerated in the 40 CFR 
part 58 final rule.
    In the Administrator's view, the final criteria and siting 
requirements contained in 40 CFR part 58 and in the new 40 CFR part 50, 
Appendix N, address the concerns raised by these commenters about the 
protection afforded by the form of the annual standard. Therefore, the 
Administrator continues to believe that the form of a PM2.5 
annual standard should be expressed as an annual arithmetic mean, 
averaged over 3 years, from single or multiple community-oriented 
monitors, in accordance with 40 CFR part 50, Appendix N and 40 CFR part 
58. In her judgment, an annual standard expressed in this manner and 
set at an appropriate level, in conjunction with a 24-hour 
PM2.5 standard, will adequately protect public health.
    2. 24-hour standard. The current 24-hour PM10 standard 
is expressed in a ``1-expected-exceedance'' form. That is, the standard 
is formulated on the basis of the expected number of days per year 
(averaged over 3 years) on which the level of the standard will be 
exceeded. The test for determining attainment of the current 24-hour 
standard is presented in Appendix K to 40 CFR part 50.
    As discussed in the proposal, since promulgation of the current 24-
hour PM10 standard in 1987, a number of concerns have been 
raised about the 1-

[[Page 38673]]

expected-exceedance form. These include, in particular, the year-to-
year stability of the number of exceedances, the stability of the 
attainment status of an area, and the complex data handling conventions 
specified in 40 CFR part 50, Appendix K, including the procedures for 
making adjustments for missing data and less-than-every-day monitoring.
    In light of these concerns, the Staff Paper and several CASAC panel 
members (Wolff, 1996b) recommended that consideration be given to 
adoption of a more stable and robust form for 24-hour standards. In 
considering this recommendation for the proposal, the Administrator 
noted that the use of a concentration-based percentile form would have 
several advantages over the current 1-expected-exceedance form:
    (1) Such a concentration-based form would be more directly related 
to the ambient PM concentrations that are associated with health 
effects. Given that there is a continuum of effects associated with 
exposures to varying levels of PM, the extent to which public health is 
affected by exposure to ambient PM is related to the actual magnitude 
of the concentration, not just whether the concentration is above a 
specified level. With an exceedance-based form, days on which the 
ambient concentration is well above the level of the standard are given 
equal weight to those days on which the concentration is just above the 
standard (i.e., each day is counted as one exceedance), even though the 
public health impact on the 2 days is significantly different. With a 
concentration-based form, days on which higher concentrations occur 
would weigh proportionally more than days with lower concentrations for 
the design value, since the actual concentrations would be used 
directly in determining whether the standard is attained.
    (2) A concentration-based percentile form would also compensate for 
missing data and less-than-every-day monitoring, thereby reducing or 
eliminating the need for complex data handling procedures in the 40 CFR 
part 50, Appendix K test for attainment. As a result, an area's 
attainment status would be based directly on monitoring data rather 
than on a calculated value adjusted for missing data or less-than-
every-day monitoring.
    (3) Further, a concentration-based form, averaged over 3 years, 
would also have greater stability than the expected exceedance form 
and, thus, would facilitate the development of more stable 
implementation programs by the States.
    The proposal discussed various specific percentile values for such 
a form (e.g., 90th to 99th percentiles), taking 
into account two factors. First, the 24-hour PM2.5 standard 
is intended to supplement the annual PM2.5 standard by 
providing additional protection against extremely high peak days, 
localized ``hot spots,'' and risks arising from seasonal emissions. 
Second, given an appropriate level of health protection, the form of 
the 24-hour PM2.5 standard should provide an appropriate 
degree of increased stability relative to the current form. The 
Administrator noted in the proposal that a more stable statistic would 
reduce the impact of a single high exposure event that may be due to 
unusual meteorological conditions alone, and thus would provide a more 
stable basis upon which to design effective control programs.
    With these purposes in mind, the Administrator observed in the 
proposal that while a percentile value such as the 90th or 
95th would provide substantially increased stability when 
compared to a more extreme air quality statistic (e.g., the current 1-
expected-exceedance form), it would likely not serve as an effective 
supplement to the annual standard, because it would allow a large 
number of days with peak PM2.5 concentrations above the 
standard level. For example, in a 365-day data base, the 
90th and 95th percentiles would equal the 
37th and 19th highest 24-hour concentrations, 
respectively. On the other hand, a percentile value selected much 
closer to the tail of the air quality distribution (e.g. a 
99th or greater percentile) would not likely provide 
significantly more health protection or significantly increased 
stability as compared to a 1-exceedance form. In balancing these issues 
in the proposal, the Administrator ultimately proposed a 
98th percentile value form of the standard.
    Some commenters maintained that EPA should retain the current 1-
expected-exceedance form for the 24-hour PM2.5 standard to 
limit the number of days per year that the standard is exceeded. These 
commenters apparently gave little weight to EPA's rationale that a 
concentration-based form is more directly related to ambient PM 
concentrations that are associated with health effects because it takes 
into account the magnitude of PM concentrations, not just whether the 
concentrations are above a specific level. These commenters also 
discounted the other advantages of a concentration-based percentile 
form outlined above in this unit. A number of other commenters 
supported the concentration-based percentile form for the reasons 
outlined in the proposal but, as discussed below in this unit, argued 
for alternative percentile values that were higher or lower than the 
proposed 98th percentile value.
    EPA continues to believe that a concentration-based percentile form 
is more reflective of the health risk posed by elevated PM 
concentrations, because it gives proportionally greater weight to days 
when concentrations are well above the level of the standard than to 
days when the concentrations are just above the standard. This factor, 
coupled with the other advantages outlined above in this unit, leads 
EPA to conclude that a concentration-based percentile form will provide 
for more effective health protection than a 1-expected-exceedance form.
    Some commenters supporting a single exceedance form or a more 
restrictive concentration-based percentile form (e.g. a 99th 
percentile) expressed concern that the proposed 98th 
percentile form could allow too many high concentration excursions, and 
thus fail to provide adequate protection against seasonal emissions 
problems or localized peaks. In particular, some commenters expressed 
concerns that in areas with strongly seasonal emissions, such as 
western areas with winter inversions, over a three year period an area 
could experience several excursions in which levels could reach as high 
as 250 g/m3 and still comply with both the annual 
and daily standards if the remainder of the days had low levels (e.g., 
10 g/m3). Although this combination of events is 
theoretically possible, EPA believes it is unlikely. Moreover, if such 
episodic events did occur, the Act provides for emergency State or 
Federal action to address them.35 In view of the limits on 
truely episodic peak concentrations, EPA believes that an appropriately 
selected 24-hour standard with a concentration-based 98th 
percentile form can provide a stable and adequately protective 
supplement to the annual standard in areas with periodic peak 
concentrations.
---------------------------------------------------------------------------

    35 See sections 303, 110(a)(2)(y); 40 CFR part 51. EPA intends 
to establish a significant harm level for PM2.5 and 
associated guidance so States can develop appropriate emergency 
episode plans. The significant harm and episode criteria will be 
included in forthcoming proposed revisions to 40 CFR part 51 and 40 
CFR part 58 implementation guidance. In the interim, existing 
PM10 emergency episode plans should be triggered by 
events of this magnitude.
---------------------------------------------------------------------------

    Other commenters who were also concerned with monitoring 
requirements associated with spatial averaging in the annual standard, 
argued that a 98th percentile form, coupled with the 
proposed monitoring requirements that would limit

[[Page 38674]]

compliance monitors for the 24-hour standard to population-oriented 
sites, would not protect people residing in or near localized ``hot 
spots'' in some areas.36 The Administrator believes that the 
siting requirements as proposed and finalized in 40 CFR part 58 for 
population-oriented sites will provide adequate safeguards for such 
residential areas.
---------------------------------------------------------------------------

    36 The 40 CFR part 58 monitoring rule proposed to limit sites 
that would be eligible for comparisons to the 24-hour standard to 
population-oriented monitoring sites.
---------------------------------------------------------------------------

    Other commenters, who otherwise opposed setting PM2.5 
standards, recommended that alternative lower percentiles (e.g., 
95th percentiles) be used, if EPA proceeds to set such 
standards. As discussed above in this unit, however, EPA continues to 
hold the view that a 90th to 95th percentile form 
would not provide an adequate limit against periodic peak values in 
areas with low annual values and periodic high seasonal or source-
oriented peaks.
    After carefully assessing the comments received, the Administrator 
is persuaded that the adoption of a 98th percentile form for 
the 24-hour PM2.5 standard measured at each population-
oriented monitoring site in an area would provide an effective 
supplement to the annual PM2.5 standard. This form will 
provide adequate protection against 24-hour peak PM2.5 
levels in locations dominated by single point sources, as well as in 
areas dominated by seasonal emissions. The Administrator also believes 
that a 98th percentile form, with more frequent sampling and 
averaged over 3 years, will provide increased stability and robustness 
as recommended by several members of the CASAC panel. For these 
reasons, the Administrator has decided to adopt the 98th 
percentile form for the final PM2.5 24-hour standard. The 
24-hour PM2.5 standard would be attained when the 3-year 
average of the 98th percentile of 24-hour concentrations at 
each populated oriented monitor within an area is less than or equal to 
the level of the standard. Further details regarding the interpretation 
of the form, as well as associated calculations and other data handling 
conventions are specified in the new 40 CFR part 50, Appendix N.

F. Levels for the Annual and 24-Hour PM2.5 Standards

    As discussed in Unit II.D. of this preamble, the Administrator 
believes that an annual PM2.5 standard can provide the 
requisite reduction in risk associated with both annual and 24-hour 
averaging times in most areas of the United States. Under this 
approach, the 24-hour standard would be intended to provide 
supplemental protection against extreme peak fine particle levels that 
may occur in some localized situations or in areas with distinct 
variations in seasonal fine particle levels. In reaching judgments as 
to appropriate levels to propose for both the annual and 24-hour 
PM2.5 standards, the Administrator has considered the 
combined protection afforded by both the annual and 24-hour standards, 
taking into account the forms discussed in Unit II.E. of this preamble.
    With this approach in mind, the Administrator has considered the 
available health effects evidence and related air quality information 
presented in the Criteria Document and summarized in chapters IV--VII 
of the Staff Paper, which provides the basis for decisions on standard 
levels that would reduce risk sufficiently to protect public health 
with an adequate margin of safety, recognizing that such standards will 
not be risk-free. In so doing, the Administrator has considered both 
the strengths and the limitations of the available evidence and 
information, as well as alternative interpretations of the scientific 
evidence advanced by various CASAC panel members (Wolff, 1996b; 
Lippmann et al., 1996) and public commenters, arising primarily from 
the inherent uncertainties and limitations in the health effects 
studies.
    Beyond those factors, but clearly related to them, a range of views 
have been expressed by CASAC panel members and the public as to the 
appropriate policy response to the available health effects evidence 
and related air quality information. Toward one end of the spectrum, 
the view has been expressed that only a very limited policy response is 
appropriate in light of the many key uncertainties and unanswered 
questions that, taken together, call into question the fundamental 
issue of causality in the reported associations between ambient levels 
of PM2.5 and mortality and other serious health effects. 
Toward the other end, the view has been expressed that the consistency 
and coherence of the epidemiological evidence should be interpreted as 
demonstrating causality in the relationships between PM2.5 
and health endpoints that are clearly adverse, and that uncertainties 
in the underlying health effects information should be treated, 
regardless of their nature, as warranting a maximally precautionary 
policy response. A third view would suggest an alternative policy 
response, taking into account not only the consistency and coherence of 
the health effects evidence, but also the recognition of key 
uncertainties and unanswered questions that increasingly call into 
question the likelihood of PM-related effects as PM2.5 
concentrations decrease below the mean values in areas where effects 
have been observed and/or as such concentrations approach background 
levels.
    Reflecting these divergent views, both of the science itself and of 
how the science should be used in making policy decisions on proposed 
standards, the Administrator considered three alternative approaches to 
selecting appropriate standard levels, as described in the proposal, 
ultimately deciding to propose standards based on a balanced view of 
the strengths and uncertainties of the scientific information that 
reflects the intermediate approach.
    Judging by the public comments received, EPA accurately reflected 
the bases for divergent views. A substantial body of public comments 
supported revising the PM standards by adding PM2.5 
standards with levels at least as stringent as those proposed by the 
Administrator. In general, however, comments on levels for 
PM2.5 standards revealed a strong dichotomy between those 
who recommended even stronger standards than proposed, and those who 
counseled against revising the standards at all. As noted above in this 
unit, many in this latter group made contingent recommendations with 
respect to the levels and other aspects of PM2.5 standards, 
if the Administrator concluded that any revisions were appropriate.
    This latter group of ``contingent'' commenters recommended levels 
well above those proposed by the Administrator. These commenters placed 
great weight on factors outlined in Units II.B. and II.C. of this 
preamble that led them to oppose any revisions to the PM standards, 
including the uncertainties and limitations in the available health 
effects studies considered individually, such as the possible existence 
of effects thresholds and unanswered questions regarding the causal 
agent(s) responsible for the reported health effects. Further, they 
emphasized the limited amount of research currently available that has 
measured PM2.5 directly. A substantial group recommended 
that PM2.5 standards be selected so as to be equivalent or 
close in stringency to the current PM10 standards, and cited 
the opinions of some CASAC PM panel members as support. Some of these 
commenters provided supplemental analyses of air quality data, arguing 
that they demonstrate that ``equivalent'' standards would be at 
PM2.5 levels as

[[Page 38675]]

high as approximately 95 g/m3 24-hour average and 
27 g/m3 annual average.
    Having evaluated these comments, the Administrator rejects both 
their underlying rationale and the specific recommendations for 
PM2.5 standard levels that result in similar or only 
marginally more protection than that afforded by the current 
PM10 standards. Aside from technical problems in the 
commenters' supporting analyses on the issue of defining ``equivalent'' 
standards,37 the Administrator finds this approach 
inconsistent with her conclusions regarding the adequacy of the current 
standards and the need to provide additional protection as articulated 
in Unit II.B. of this preamble. The Administrator believes that, 
despite well recognized uncertainties, the consistency and coherence of 
the epidemiological evidence and the seriousness of the health effects 
require a more protective response than provided by ``equivalence'' or 
a marginal strengthening of the standards. Moreover, EPA believes that 
the standard levels should be based on the most recent assessment of 
the scientific criteria for PM, not on applying uncertain ratios to 
standard decisions based on much more limited evidence in 1987. The 
Administrator also rejects the premise of some38 who suggest 
that adopting a standard that prompts little or no additional control 
would cause no delay in risk reduction as compared to conducting 
monitoring and research now and setting a more stringent standard after 
the next review. These comments do not consider the realities of 
implementing air quality standards, which ensure that such an approach 
would add several years to the risk reduction process. Thus, aside from 
her obligations under the statute,39 the Administrator 
believes that the most prudent and appropriate course is to establish 
appropriately protective standards now that put into motion monitoring 
and strategy development programs, while at the same time pursuing an 
expanded research program to improve implementation and to inform the 
next periodic review of the criteria and standards.
---------------------------------------------------------------------------

    37 Nationwide PM2.5 estimates have been derived from 
the current PM air quality data base, but reflect a significant 
degree of uncertainty due to the highly variable relationship 
between PM2.5 and PM10 air quality values 
across locations and seasons (Fitz-Simons et al., 1996). The 
American Iron and Steel Institute (AISI) submitted a useful data 
base (Cooper Associates, 1997) on PM2.5/PM10 
relationships that examines both these predictions and the issue of 
equivalence. An EPA examination of this material, which found some 
problems with the analysis and with commenters' conclusions that 
appear inconsistent with the Cooper report, is included in the 
Response to Comments.
    38 Some commenters suggest that CASAC and EPA support for 
PM2.5 standards is based on the need to stimulate 
additional monitoring and research. While the Administrator agrees 
that the additional monitoring and research that would accompany 
establishment of equivalent or marginally tighter PM2.5 
standards are very important goals, they do not form an adequate 
rationale for establishing air quality standards.
    39 As stated previously, section 109(d) of the Act requires 
that, after reviewing the existing criteria and standards for PM, 
the Administrator make such revisions in the standards and 
promulgate such new standards as are appropriate under section 
109(b) of the Act.
---------------------------------------------------------------------------

    In sharp contrast to the commenters discussed immediately above, a 
number of other commenters strongly supported standard levels more 
stringent than those proposed by EPA. These commenters supported EPA's 
conclusions regarding the epidemiological studies, but would place much 
less weight on uncertainties related to the concentration-response 
relationships for PM2.5 as a surrogate for PM and the 
relative importance of various PM components. Based on their evaluation 
of the information, and citing the support of some CASAC panel members, 
these commenters variously recommended 24-hour PM2.5 
standards as low as 18 to 20 g/m3 and annual 
standards of 10 to 12 g/m3.40
---------------------------------------------------------------------------

    40 This range of levels for a 24-hour PM2.5 standard 
is close to the lower bound levels recommended by four CASAC panel 
members (20 g/m3); no member supported an annual 
PM2.5 standard as low as 10 to 12 g/
m3.
---------------------------------------------------------------------------

    EPA notes that setting such standards would result in commensurate 
reductions in health risks only if, in fact, there is a continuum of 
health risks down to the lower end of the ranges of air quality 
observed in the key epidemiological studies, and only if the reported 
associations are, in fact, causally related to PM2.5 at the 
lowest concentrations measured. Setting standards at low levels where 
the possibility of effects thresholds is greater, and where there is 
greater potential that other elements in the air pollution mix (or some 
subset of particles within the fine fraction) become more responsible 
for (or modify) the effects being causally attributed to 
PM2.5, might result in regulatory programs that go beyond 
those that are needed to effectively reduce risks to public health. 
While placing substantial weight on the results of the key health 
studies in the higher range of concentrations observed, EPA is 
persuaded that the inherent scientific uncertainties are too great to 
support standards based on the lowest concentrations measured in such 
studies, which approach the maximum range of PM2.5 values 
estimated for short-term background conditions.
    Having considered the comments reflecting the two contrasting views 
summarized above in this unit, the Administrator concludes that the 
approach she set forth in the proposal is the most appropriate for 
selecting levels for annual and 24-hour PM2.5 standards. 
This approach focuses primarily on standard levels designed to limit 
annual PM2.5 concentrations to somewhat below those where 
the body of epidemiological evidence is most consistent and coherent, 
in recognition of both the strengths and the limitations of the full 
range of scientific and technical information on the health effects of 
PM, as well as associated uncertainties, as interpreted by the Criteria 
Document, Staff Paper, and CASAC. The Administrator believes that this 
approach appropriately reflects the weight of the evidence as a whole.
    In identifying PM2.5 standard levels consistent with 
this overall approach, the Administrator has placed greatest weight on 
those epidemiological studies reporting associations between health 
effects and direct measures of fine particles, most notably those 
recent studies conducted in North America (summarized in Tables V-12 
and V-13 of the Staff Paper).41 Key considerations and study 
results upon which this approach is based are presented as follows.
---------------------------------------------------------------------------

    41 Some confusion is apparent in comments regarding the basis on 
which the Administrator selected levels for the proposed 
PM2.5 standards, with some commenters suggesting two or 
at most three studies were used, and others suggesting that EPA 
relied extensively on uncertain conversion factors to estimate 
levels for the standards. These comments are in error. To clarify, 
as stated in the proposal, the Administrator is basing her decision 
to revise the standards on the full range of PM health effects 
studies summarized in the Criteria Document and Staff Paper, but in 
selecting specific levels for PM2.5 standards, is relying 
chiefly on U.S. and Canadian studies, listed in Tables V-12 and V-13 
of the Staff Paper, that measured fine PM levels. To ease 
identification and use of these key studies, the short-term exposure 
studies and key PM air quality statistics are cited in Koman (1996) 
and all long-term exposure studies are cited in this preamble. The 
referenced memorandum (Koman, 1996) has been updated (Koman, 1997) 
to clarify key aspects of the studies cited and relevant air quality 
statistics. In accordance with EPA and CASAC views on the relative 
strength of these studies, greater weight is placed on short-term 
exposure studies than on long-term exposure studies. Where studies 
found statistically significant associations with PM2.5 
components (e.g., sulfates and/or acids, in Thurston et al., 1994; 
Dockery et al., 1996), the corresponding PM2.5 or 
PM2.1 values from the study are cited. No conversions 
were made from the original measurements used in these studies.
---------------------------------------------------------------------------

    As previously discussed, the Administrator has concluded that it is 
appropriate to select the level of the annual standard so as to protect 
against the range of effects associated with both short- and long-term 
exposures to PM,

[[Page 38676]]

with the 24-hour standard level selected to provide supplemental 
protection against peak concentrations that might occur over limited 
areas and/or for limited time periods. In selecting the level for the 
annual standard, therefore, the Administrator has considered both 
short- and long-term exposure studies.
    In accordance with EPA staff and CASAC views on the relative 
strengths of the epidemiological studies, the Administrator has placed 
greater emphasis on the short-term exposure studies in selecting the 
level of the annual standard. The approach she took to this issue 
consisted of determining a provisional level based on the short-term 
exposure studies, and then determining whether the long-term exposure 
studies are consistent with that level or, instead, suggest the need 
for a lower level. The effects estimates from the short-term exposure 
studies (in Table V-12 of the Staff Paper) are based on analyses of 
daily PM2.5 concentrations that occurred over the course of 
the study period. While effects may occur over the full range of 
concentrations observed in the studies, consistent with the discussion 
of this issue in Unit II.D. of this preamble, the strongest evidence 
for short-term PM2.5 effects occurs at concentrations near 
the long-term (e.g., annual) average. More specifically, the strength 
of the evidence of effects increases for concentrations that are at or 
above the long-term (e.g., annual) mean levels reported for these 
studies.42 Given the serious nature of the potential 
effects, the Administrator believes it is both prudent and appropriate 
to select a level for an annual standard at or below such 
concentrations. An examination of the long-term means from the combined 
six city analyses of daily mortality (Schwartz et al., 1996a) and 
morbidity (Schwartz et al., 1994), together with those from studies in 
individual cities for which statistically significant PM-effects 
associations are reported (from Table V-12 in the Staff Paper), finds 
mean concentrations ranging from about 16 to about 21 g/
m3 (Koman, 1996; 1997). In addition, the mean concentrations 
in cities where short-term exposure associations are characterized in 
the Criteria Document as nearly statistically significant (U.S. EPA, 
1996a, p. 13-40) range from about 11 g/m3 to 30 
g/m3. Taken together, and placing greatest weight 
on those studies that were clearly statistically significant, this 
evidence suggests that an annual standard level of 15 g/
m3 is appropriate to reduce the risk of effects from short-
term exposure to fine particles.
---------------------------------------------------------------------------

    42 As discussed in the proposal and Appendix E of the Staff 
Paper (U.S. EPA, 1996b, p. E-4), there is generally greatest 
statistical confidence in observed associations for levels at and 
above the mean concentration.
---------------------------------------------------------------------------

    Before reaching a final conclusion, the Administrator also examined 
this level in light of the effects reported in epidemiological studies 
of long-term exposures to fine particles (Table V-13 in the Staff 
Paper), which may reflect the accumulation of daily effects over time 
as well as potential effects uniquely associated with long-term 
exposures. Even though subject to additional uncertainties, the long-
term exposure studies provide important insights with respect to the 
overall protection afforded by an annual standard. These studies were 
examined for general consistency and support for the levels derived 
from the short-term exposure studies, and to determine whether they 
provide evidence that a more stringent level is needed.
    The most direct comparison with the daily fine particle mortality 
studies is provided by two long-term prospective cohort studies 
(Dockery et al., 1993; Pope et al., 1995). The annual mean 
PM2.5 concentration for the multiple cities included in 
these studies (6 and 50 cities, respectively) was 18 g/
m3 (Dockery et al., 1993), and about 21-22 g/
m3 for the larger Pope et al. (1995) study.43 The 
Staff Paper assessment of the concentration-response results from 
Dockery et al. (1993) concluded that the evidence for increased risk 
was more apparent at annual concentrations at or above 15 g/
m3 (Table E-3; U.S. EPA; 1996b).44 EPA notes that 
the estimated mean values for most of the cities in Pope et al. (1995) 
are above 15 g/m3. As noted in the Staff Paper and 
the Criteria Document, the estimated magnitude of effects in both long-
term exposure mortality studies may be related to higher historical 
concentrations than the affected communities experienced during the 
time period of the studies; this consideration suggests that a level of 
15 g/m3 would incorporate a margin of safety. An 
examination of morbidity effects and long-term exposures is provided by 
the recent ``24 city'' studies, which found that reduced lung function 
and increased respiratory symptoms in children followed the gradient in 
annual mean concentrations of fine particles and/or acid-sulfate 
components of fine particles (Raizenne et al., 1996; Dockery et al., 
1996). The results indicate a greater likelihood of effects at annual 
mean PM2.1 levels above about 15 g/m3 
(U.S. EPA, 1996b; Figure V-7). In the judgment of the Administrator, 
these studies are consistent with a standard level of 15 g/
m3. While they provide some suggestion of risks extending to 
lower concentrations, they do not provide a sufficient basis for 
establishing a lower annual standard level.
---------------------------------------------------------------------------

    43 Based on a public comment, EPA found that the mean of 18 
g/m3 in Pope et al. (1995) reported in the 
Criteria Document and elsewhere was actually the mean of median 
values. Based on typical air quality relationships, the conventional 
arithmetic mean would be approximately 21 to 22 g/
m3 (Freas, 1997). The lowest median concentration 
measured in this study (9 g/m3), which was 
relied upon by some commenters as a basis for annual standards of 10 
g/m3, is about 11 to 12 g/m3 
as an arithmetic mean.
    44 Based on public comments and a further evaluation of the 
underlying study, EPA concludes that the comparable assessment of 
the concentration-response function summarized in Table E-3 for Pope 
et al. (1995) is not appropriate, because it was based on a 
supplemental ``ecologic'' comparison for these cities and not on the 
far more reliable prospective-cohort analysis that was the main 
focus of the paper.
---------------------------------------------------------------------------

    Taking the epidemiological studies of both short- and long-term 
exposures together, the Administrator believes the concordance of 
evidence for PM effects and associated levels provides clear support 
for an annual PM2.5 standard level of 15 g/
m3. This level is below the range of annual data most 
strongly associated with both short- and long-term exposure effects, 
and because even small changes in annual means in this concentration 
range can make significant differences in overall risk reduction and 
total population exposures, the Administrator believes it will provide 
an adequate margin of safety against the effects observed in these 
epidemiological studies. Moreover, the means in areas where 
PM2.5 concentrations were statistically significantly 
associated with daily mortality (about 16 to 21 g/
m3) reflect a 7 to 9-year average; thus, the use of a 3-year 
mean will provide additional protection. Although the possibility of 
effects at lower annual concentrations cannot be excluded, the evidence 
for that possibility is highly uncertain and, as previously discussed, 
the likelihood of significant health risk, if any, becomes smaller as 
concentrations approach the lower end of the range of air quality 
observed in the key epidemiological studies and/or background levels.
    The final annual standard will provide substantial protection 
against short-term as well as long-term exposures to particles. 
Nevertheless, for the reasons specified above, a spatially averaged 
annual standard cannot be expected to offer an adequate margin of 
safety against the effects of all potential short-term exposures in 
areas with strong local or seasonal sources. The

[[Page 38677]]

broad-based community studies considered in this review generally could 
not evaluate such peak exposure conditions directly. Given the public 
health purposes of the 24-hour standard, the Administrator believes it 
should be set at a level that generally supplements the control 
afforded by an annual standard and proposed an approach based on 
providing a reasonable degree of protection against the peak levels 
observed or expected in communities where health effects have been 
associated with daily levels of fine particles.
     For the reasons specified in the previous unit, the Administrator 
has decided to use a 98th percentile concentration-based 
form of the standard. As noted in the proposal, the 98th 
percentile 24-hour PM2.5 concentrations in cities with 
statistically significant or nearly significant short-term fine 
particle exposure-effects associations ranged from 34 g/
m3 to as high as 90 g/m3 (Koman, 1996, 
1997). Based on an examination of these results, EPA originally 
proposed a level for the 24-hour standard of 50 g/
m3, and solicited comments on higher and lower alternative 
levels.
    In considering comments on alternative levels for the purpose of 
making a final decision on the 24-hour standard, the Administrator 
recognizes the significant uncertainties in identifying the extent of 
the incremental risk associated with single peak exposures to 
PM2.5 in areas where the annual standard is met. Clearly, 
the risks associated with the 98th percentile air quality 
data used in the selecting the proposed level are from the same study 
cities that experienced long-term levels at varying amounts above that 
selected for the annual standard. It is unclear what risks might have 
been associated with such peak levels had the long-term averages in 
these areas been below that selected for the annual standard. 
Regardless of this uncertainty, it is clear that reducing the annual 
concentrations in such areas to that of the annual standard would 
reduce the risk associated with peak days, whatever the magnitude, as 
well as that associated with the far more numerous days with 
concentrations near the annual average. Given these uncertainties and 
the significant degree of protection afforded by the annual standard, 
the Administrator is persuaded that it is appropriate to adopt a 
different approach for selecting the levels of the 24-hour standard 
than the one proposed.
     In making a final decision on an appropriate level for the 24-hour 
standard, the Administrator considered several key factors: the 
significant protection afforded against short-term exposures by the 
annual PM2.5 standard; the role of the 24-hour standard in 
providing supplemental protection against peak exposures not addressed 
by the annual standard; the air quality and effects information in the 
studies cited above; the uncertainties in the risks associated with 
infrequent and isolated peak exposures in areas that meet the annual 
standard; the range of levels recommended by EPA staff and CASAC panel 
members; and the extensive public comment on the alternative levels 
proposed, which ranged between 20 and 65 g/m3. 
Because of the approach of establishing the annual standard as the 
controlling standard, and, in particular, the decision to set the level 
at the lower end of the annual range, there is no need to consider 
levels in the lower portion of the 24-hour range below the level 
proposed. Therefore, the Administrator focused on evaluating the margin 
of safety associated with levels between 50 and 65 g/
m3.
    As has been discussed in previous units, the extent of total risk 
over the course of a year associated solely with a limited number of 
peak exposures is uncertain, but it is considerably smaller than that 
associated with the entire air quality distribution. Further, the risk 
associated with infrequent peak 24-hour exposures in otherwise clean 
areas is not well enough understood at this time to provide a basis for 
selecting the more restrictive levels in the range of 50 to 65 
g/m3. On the other hand, it is clear that any 
standard level within this range would provide some margin of safety. 
Taking into account the factors outlined above, the Administrator has 
concluded that a 24-hour standard at the level of 65 g/
m3 would provide an effective limit in the role as a 
supplement to the annual standard. This level is at the upper end of 
the range recommended by staff and most CASAC panel members, and below 
the levels suggested by some CASAC panel members and by a number of 
public commenters. Although this level is not risk free, the 
Administrator believes that it would provide an appropriate degree of 
additional protection over that provided by the annual PM2.5 
standard. Accordingly, after weighing these factors in light of the 
scientific uncertainties, the Administrator believes that a 
98th percentile 24-hour PM2.5 standard of 65 
g/m3 would provide an adequate margin of safety 
against infrequent or isolated peak concentrations that could occur in 
areas that attain the annual standard of 15 g/m3.
    In the Administrator's judgment, the factors discussed above 
provide ample reason to believe that both annual and 24-hour 
PM2.5 standards are appropriate to protect public health 
from adverse health effects associated with short- and long-term 
exposures to ambient fine particles. Further, she believes these 
factors provide a clear basis for judging that an annual 
PM2.5 standard set at 15 g/m3, in 
combination with a 24-hour standard set at 65 g/m3, 
will protect public health with an adequate margin of safety.

G. Conclusions Regarding the Current PM10 Standards

    1. Averaging time and form. In conjunction with 
PM2.5 standards, the new function of PM10 
standard(s) is to protect against potential effects associated with 
coarse fraction particles in the size range of 2.5 to 10 m. 
Coarse fraction particles are plausibly associated with certain effects 
from both long- and short-term exposures (EPA 1996a,b). Based on 
qualitative considerations, deposition of coarse fraction particles in 
the respiratory system could be expected to aggravate effects in 
individuals with asthma. The Criteria Document and Staff Paper found 
support for this expectation in limited epidemiological evidence on the 
effects of coarse fraction particles, suggesting that aggravation of 
asthma and respiratory infections and symptoms may be associated with 
daily or episodic increases in PM10 that are dominated by 
coarse fraction particles. The potential build-up of insoluble coarse 
fraction particles in the lung after long-term exposures to high levels 
should also be considered.
    Based on assessments of the available information in the Criteria 
Document and Staff Paper, both the staff and CASAC recommended 
retention of an annual PM10 standard. The staff, with CASAC 
concurrence, recommended retention of the current annual arithmetic 
mean form of the standard, which is the same form being adopted for the 
annual PM2.5 standard. As noted in the staff assessment, the 
current annual PM10 standard offers substantial protection 
against the effects of both long- and short-term exposure to coarse 
fraction particles. Public comment was nearly unanimous in recommending 
retention of this standard. The Administrator therefore has decided to 
continue a long-term PM10 standard as an annual arithmetic 
mean, averaged over 3 years.
    The staff and CASAC also recommended that consideration be

[[Page 38678]]

given to retention of a 24-hour standard to provide additional 
protection against potential effects of short-term exposures to coarse 
fraction particles. The staff, with CASAC concurrence, also recommended 
that if a 24-hour standard is retained, the form of the standard should 
be revised to provide a more robust target for coarse fraction particle 
controls. The Administrator originally proposed a 98th 
percentile form for the 24-hour PM10 standard based 
primarily on the reasons outlined above in this unit regarding the 
proposed form of the 24-hour PM2.5 standard.
    The EPA received few comments supporting elimination of the 24-hour 
PM10 standard. The main exceptions were some industries, 
most notably the mining industry, which as noted above in this unit, 
argued that the available data provide little evidence for coarse 
particle effects at current ambient levels. These groups, who generally 
opposed PM2.5 standards, also argued that the daily 
PM10 standard could be eliminated if PM2.5 
standards were set. Based on the potential aggravation of respiratory 
symptoms from short-term exposure to coarse fraction particles 
discussed in the Criteria Document and by numerous commenters, as well 
as the recommendations of a majority of CASAC panelists who also 
supported PM2.5 standards, the Administrator concludes it is 
appropriate to retain a 24-hour PM10 standard.
    In general, comments received on the form of the 24-hour 
PM10 standard paralleled those on the form of the 
PM2.5 standard. Substantial concerns were expressed by 
environmental groups, some States, and others that the 98th 
percentile would not provide an adequate limit on the number and 
magnitude of 24-hour peak PM10 excursions. While a number of 
these commenters suggested keeping the current 1-expected-exceedance 
form, EPA believes that a concentration- based percentile form offers 
significant advantages, as outlined above in this unit, for both PM 
indicators. Some air pollution control officials, who were concerned 
about the extent to which the 24-hour PM10 standard would be 
relaxed under the proposed form, suggested consideration of a 
99th percentile form with increased monitoring as an 
appropriately protective form. Other commenters, particularly some 
industry groups and some States, strongly supported concentration-based 
percentile forms, with some recommending consideration of the 
95th percentile form.
    The proposal noted that a percentile value selected closer to the 
``tail'' of the air quality distribution (e.g., a 99th or 
greater percentile) would not significantly increase stability as 
compared to the current form. However, an association of 8 State air 
pollution agencies commented that a 99th percentile form 
could provide increased stability if combined with a daily or 1-in-3-
day sampling frequency and with greater data capture. In addition, EPA 
notes that this concentration-based form is inherently more stable than 
the current exceedance-based form.
     Many of these and other commenters were concerned that the 
uncertainties in the available scientific information on the effects of 
coarse particles were a reason to be concerned that, assuming the 
current standard level was kept, a 98th percentile form 
would represent a significant relaxation in protection relative to the 
current standards. Unlike the situation for the new 
PM2.5 standards, in the case of the PM10 
standards, the 24-hour standard has generally been the ``controlling'' 
standard, making changes to the form of the 24-hour standard 
potentially more significant to the overall national level of 
protection afforded. Given the uncertainties in the available 
scientific evidence with respect to the potential health effects of 
short-term exposures to coarse fraction particles, the Administrator is 
persuaded that the somewhat more cautious approach with respect to 
revising the 24-hour PM10 standard recommended by many 
commenters is appropriate. The only approaches available for increasing 
the extent of protection for this standard as compared to that of the 
proposed standard involve modifying the form or reducing the level. For 
reasons discussed in the following section, the Administrator believes 
it is not appropriate to revise the level of the standard. In order to 
provide adequate protection against the potential risk associated with 
multiple short-term peak exposures to coarse fraction particles, the 
Administator accepts commenters' recommendations to decrease the 
frequency of peak values, while still providing for a more stable 
control target than afforded by the current 1-expected-exceedance form. 
Therefore, the Administrator concludes that the 99th 
percentile concentration-based form, averaged over 3 years, and 
combined with more frequent sampling, would be an appropriate form for 
a 24-hour PM10 standard.
    2. Levels for the annual and 24-hour PM10 standards--a. 
Annual PM10 standard. As a result of the more limited 
information for coarse fraction particles, the Administrator's approach 
for selecting a level of the standard is directly related to the 
approach taken in the last review of the PM NAAQS. In that review, 
evidence from limited quantitative studies was used in conjunction with 
support from the qualitative literature in selecting the level of the 
current annual PM10 standard. In the current review, the 
staff assessment of the major quantitative basis for the level of that 
standard (Ware et al., 1986), together with a more recent related study 
(Dockery et al., 1989), recommended the same range of levels of concern 
(40 to 50 g/m3) as in the 1986 staff paper. The 
staff concludes that it is possible, but not certain, that coarse 
fraction particles, in combination with fine particles, may have 
influenced the observed effects at these levels. Based on particle 
deposition considerations, it is possible that cumulative deposition of 
coarse fraction particles could be of concern in children, who are more 
prone to be active outdoors than sensitive adult populations.
    Qualitative evidence of other long-term coarse particle effects, 
most notably from long-term build-up of silica-containing materials, 
supports the need for a long-term standard, but does not provide 
evidence of effects below the range of 40 to 50 g/
m3 (U.S. EPA, 1996a, p. 13-79). The staff concludes that the 
qualitative evidence with respect to biological aerosols also supports 
the need to limit coarse materials, but should not form the major basis 
for a national standard (U.S. EPA, 1996a, p. 13-79). In addition, staff 
notes that the nature and distribution of such materials, which vary 
from endemic fungi (e.g., valley fever) to pollens larger than 10 
m, are not appropriately addressed by traditional air 
pollution control programs.
    Based on its review of the available information, CASAC found ``a 
consensus that retaining an annual PM10 NAAQS at the current 
level is reasonable at this time'' (Wolff, 1996b). With few exceptions, 
public comments supported levels at least as stringent as the current 
annual PM10 standard.45 Taking into account these 
comments and the above considerations, as more fully detailed in the 
Staff Paper and the

[[Page 38679]]

CASAC recommendations, the Administrator has decided to retain the 
current annual PM10 standard of 50 g/m3 
to protect against the known and potential effects of long-term 
exposure to coarse fraction particles.
---------------------------------------------------------------------------

    45 Some commenters, including some environmental groups and the 
State of California (Cal EPA, 1997), suggested that the large number 
of recent studies showing effects at PM10 levels below 
the current standards provides a basis for establishing stricter 
annual and 24-hour PM10 standards, in conjunction with 
PM2.5 standards. As discussed in Units II.B. and C. of 
this preamble, while these studies could be used either to tighten 
the PM10 standards or to add standards that tighten 
control of the fine fraction of PM10, the weight of 
evidence from all of the relevant information more readily supports 
the development of additional protection for the PM2.5 
fraction.
---------------------------------------------------------------------------

    b. 24-hour PM10 standard. As discussed above in this 
unit, EPA staff and CASAC also recommended that consideration be given 
to a 24-hour standard for coarse fraction particles as measured by 
PM10. Unlike the case for the annual standard, however, the 
staff found that the original quantitative basis for the level of the 
current 24-hour PM10 standard (150 g/m3) 
is no longer appropriate. Instead, the staff found that the main 
quantitative basis for a short-term standard is provided by the two 
recent community studies of exposure to fugitive dust (Gordian et al., 
1996; Hefflin et al., 1994). Because these studies reported multiple 
large exceedances of the current 24-hour standard, and because of 
limitations in the studies themselves, the staff concluded that they 
provide no basis to lower the level of the standard below 150 
g/m3. Moreover, staff concluded that none of the 
qualitative literature regarding the potential effects of short-term 
exposure to coarse particles provides a basis for a lower standard 
level. Both EPA staff and CASAC recommended that if a 24-hour 
PM10 standard is retained, the level of the standard should 
be maintained at 150 g/m3, although with a revised 
form. Beyond the comments summarized above recommending elimination of 
the 24-hour standard, no commenters recommended a less stringent level, 
while some others, as summarized above in this unit, recommended more 
stringent levels. Most comments favored the current level.
    Having considered these factors and the public comments, the 
Administrator judges that, retention of a 24-hour PM10 
standard at the level of 150 /m3 with a 
99th percentile form is appropriate and will provide 
adequate protection against the known and potential effects of short-
term coarse fraction particle exposures that have been identified to 
date in the scientific literature.

H. Final Decisions on Primary PM Standards

    For the reasons discussed above in this unit, and taking into 
account the information and assessments presented in the Criteria 
Document and the Staff Paper, the advice and recommendations of CASAC, 
and public comments received on the proposal, the Administrator is 
revising the current PM NAAQS by adding new PM2.5 standards 
and by revising the form of the current 24-hour PM10 
standard. Specifically, the Administrator is making the following 
revisions:
    (1) The suite of PM standards is revised to include an annual 
primary PM2.5 standard and a 24-hour PM2.5 
standard.
    (2) The annual PM2.5 standard is met when the 3-year 
average of the annual arithmetic mean PM2.5 concentrations, 
from single or multiple community-oriented monitors (in accordance with 
EPA's final rule on monitoring siting guidance, 40 CFR part 58, 
published in a separate document elsewhere in this issue of the Federal 
Register) is less than or equal to 15 g/m3, with 
fractional parts of 0.05 or greater rounding up.
    (3) The 24-hour PM2.5 standard is met when the 3-year 
average of the 98th percentile of 24-hour PM2.5 
concentrations at each population-oriented monitor within an area is 
less than or equal to 65 g/m3, with fractional 
parts of 0.5 or greater rounding up.
    (4) The form of the current 24-hour PM10 standard is 
revised to be based on the 3-year average of the 99th 
percentile of 24-hour PM10 concentrations at each monitor 
within an area.
In addition, the Administrator is retaining the current annual 
PM10 standard at the level of 50 g/m3, 
which is met when the 3-year average of the annual arithmetic mean 
PM10 concentrations at each monitor within an area is less 
than or equal to 50 g/m3, with fractional parts of 
0.5 or greater rounding up.
    As discussed below in Units V. and VI. of this preamble, data 
handling conventions and completeness criteria for the revised 
standards are being established (40 CFR part 50, Appendix N). The 
reference method for monitoring PM as PM10 for the revised 
standards has been established (40 CFR part 50, Appendix M). A new 
reference method is being established for monitoring PM as 
PM2.5 (40 CFR part 50, Appendix L). In a separate document 
published elsewhere in this issue of the Federal Register, EPA is 
providing opportunity for public comment on supplemental information 
relating to the new reference method for monitoring PM as 
PM2.5 (40 CFR part 50, Appendix L).
     As indicated previously, EPA plans to propose related revisions to 
the Pollutant Standards Index for PM (40 CFR 58.50) and the significant 
harm level program (40 CFR 51.66) at a later date.

III. Rationale for the Secondary Standards

    The Criteria Document and Staff Paper examined the effects of PM on 
such aspects of public welfare as visibility, materials damage, and 
soiling. The following discussion of the rationale for revising the 
secondary standards for PM focuses on those considerations most 
influential in the Administrator's decision.

A. Need for Revision of the Current Secondary Standards

    1. Visibility impairment. This unit of the document presents the 
Administrator's decision to address the welfare effects of PM on 
visibility by setting secondary standards identical to the suite of 
PM2.5 primary standards, in conjunction with the 
establishment of a regional haze program under section 169A of the 
Act.46 In the Administrator's judgment, this approach is the 
most effective way to address visibility impairment given the regional 
variations in concentrations of non-anthropogenic PM as well as other 
regional factors that affect visibility, such as humidity. By 
augmenting the protection provided by secondary standards set identical 
to the suite of PM2.5 primary standards with a regional haze 
program, the Administrator believes that an appropriate degree of 
visibility protection can be achieved in the various regions of the 
country.
---------------------------------------------------------------------------

    46 Congress adopted section 169A of the Act because of concern 
that the NAAQS and Prevention of Significant Deterioration programs 
might not provide adequate visibility protection nationally, 
particularly for ``areas of great scenic importance.'' See H.R. Rep. 
No. 95-294,at 203-205 (1977).
---------------------------------------------------------------------------

    In coming to this decision, the Administrator took into account 
several factors, including: The pertinent scientific and technical 
information in the Criteria Document and Staff Paper, difficulties 
inherent in attempting to establish national secondary standards to 
address visibility impairment, the degree of visibility improvement 
expected through attainment of secondary standards equivalent to the 
suite of PM2.5 primary standards, the effectiveness of 
addressing the welfare effects of PM on visibility through the 
combination of a regional haze program and secondary standards for 
PM2.5 equivalent to the suite of primary standards, and 
comments received during the public comment period. The Administrator's 
consideration of each of these factors is discussed below in this unit.
    The Administrator first concluded, based on information presented 
and referenced in the Criteria Document and

[[Page 38680]]

Staff Paper, that particulate matter can and does produce adverse 
effects on visibility in various locations, depending on the PM 
concentrations involved and other factors discussed below. It has been 
demonstrated that impairment of visibility is an important effect of PM 
on public welfare, and that it is experienced throughout the United 
States, in multi-state regions, urban areas, and remote mandatory Class 
I Federal areas47 alike. Visibility is an important welfare 
effect because it has direct significance to people's enjoyment of 
daily activities in all parts of the country. Individuals value good 
visibility for the well-being it provides them directly, both where 
they live and work, and in places where they enjoy recreational 
opportunities. Visibility is highly valued in significant natural 
areas, such as national parks and wilderness areas, because of the 
special emphasis given to protecting these lands now and for future 
generations. The Criteria Document cites many studies designed to 
quantify the benefits associated with improvements in visibility.
---------------------------------------------------------------------------

    47 There are 156 mandatory Class I Federal areas protected by 
the visibility provisions in sections 169A and 169B of the Act. 
These areas are defined in section 162 of the Act as those national 
parks exceeding 6,000 acres, wilderness areas and memorial parks 
exceeding 5,000 acres, and all international parks which were in 
existence on August 7, 1977.
---------------------------------------------------------------------------

    The Administrator considered information from the Staff Paper and 
Criteria Document regarding the effect of the composition of 
particulate matter on visibility. Visibility conditions are determined 
by the scattering and absorption of light by particles and gases, from 
both natural and anthropogenic sources. Visibility can be described in 
terms of visual range, light extinction, or deciview48. The 
classes of fine particles principally responsible for visibility 
impairment are sulfates, nitrates, organic matter, elemental carbon 
(soot), and soil dust. Fine particles are more efficient per unit mass 
at scattering light than coarse particles. The scattering efficiency of 
certain classes of fine particles, such as sulfates, nitrates, and some 
organics, increases as relative humidity rises because these particles 
can absorb water and grow to sizes comparable to the wavelength of 
visible light. In addition to limiting the distance that one can see, 
the scattering and absorption of light caused by air pollution can also 
degrade the color, clarity, and contrast of scenes.
---------------------------------------------------------------------------

    48 Visual range can be defined as the maximum distance at which 
one can identify a black object against the horizon sky. It is 
typically described in miles or kilometers. Light extinction is the 
sum of light scattering and absorption by particles and gases in the 
atmosphere. It is typically expressed in terms of inverse megameters 
(Mm-1), with larger values representing poorer 
visibility. The deciview metric describes perceived visual changes 
in a linear fashion over its entire range, analogous to the decibel 
scale for sound. A deciview of 0 represents pristine conditions. 
Under many scenic conditions, a change of 1 deciview is considered 
perceptible by the average person.
---------------------------------------------------------------------------

    The Administrator next considered what would be an appropriate 
level for a secondary standard to address adverse effects of 
particulate matter on visibility. The determination of a single 
national level is complicated by regional differences in visibility 
impairment due to several factors, including background and current 
levels of PM, composition of particulate matter, and average relative 
humidity.
    The Criteria Document and Staff Paper describe estimated background 
levels of PM and natural light extinction. In the United States, 
estimated annual mean background levels of PM2.5 are 
significantly lower in the West than in the East. Based on estimated 
background fine particle and light extinction levels summarized in 
Table VIII-2 of the Staff Paper, naturally occurring visual range in 
the East is approximately 105 to 195 kilometers, whereas in the West it 
is approximately 190 to 270 kilometers. This significant regional 
difference in estimated background conditions results from two main 
factors. First, in the western United States, visibility is more 
sensitive to an additional 1-2 g/m3 of 
PM2.5 in the atmosphere than in the eastern United States. 
Secondly, light scattering is increased for certain particles (e.g., 
sulfates, nitrates, and some organics) due to higher average relative 
humidity in the East.
    The combination of naturally occurring and manmade emissions also 
leads to significant differences in current visibility conditions 
between the eastern United States, 23-39 kilometers average visual 
range, and western United States, 55-150 kilometers average visual 
range. Table VIII-4 of the Staff Paper indicates that the current level 
of annual average light extinction in several western locations, such 
as the Colorado Plateau, is about equal to the level of background 
light extinction, i.e., the level generally regarded as representing 
the absence of anthropogenic emissions in North America, in the East. 
This regional difference is due to higher background particle 
concentrations in the East, a composition of fine particles in the East 
that, in association with higher eastern humidity levels, is more 
efficient at light scattering, and significantly lower concentrations 
of anthropogenic PM in remote western locations as compared with remote 
eastern sites.
    Because of these regional differences, it is the Administrator's 
judgment that a national secondary standard intended to maintain or 
improve visibility conditions on the Colorado Plateau or other parts of 
the West would have to be set at or even below natural background 
levels in the East, which would effectively require elimination of all 
eastern anthropogenic emissions. Conversely, a national secondary 
standard that would achieve an appropriate degree of visibility 
improvement in the East would permit further degradation in the West. 
Due to this regional variability in visibility conditions created by 
differing background fine particle levels, fine particle composition, 
and humidity effects, the Administrator finds that addressing 
visibility solely through setting more stringent national secondary 
standards would not be an appropriate means to protect the public 
welfare from adverse impacts of PM on visibility in all parts of the 
country.49 Aside from the problem of regional variability, 
the Administrator has also determined that the Agency currently lacks 
sufficient information to establish a level for a national secondary 
standard that would represent a threshold above which visibility 
conditions would always be adverse and below which visibility 
conditions would always be acceptable. Because visibility varies not 
only with PM concentration, but also with PM composition and humidity 
levels, attaining even a low concentration of fine particles might or 
might not provide adequate protection, depending on these factors.
---------------------------------------------------------------------------

    49 Congress adopted a visibility protection program in section 
169A of the Act because it recognized the impracticability of 
revising the NAAQS to protect visibility in all areas of the 
country: ``It would be impracticable to require a major city such as 
New York or Los Angeles to meet the same visibility standards as the 
Grand Canyon and Yellowstone Park.'' See H.R. Rep. No. 95-294 at 
205. (1977)
---------------------------------------------------------------------------

    The Administrator next assessed potential visibility 
improvements50 that would result from attainment of the new 
primary standards for PM2.5. The spatially averaged form of 
the annual standard is well suited to the protection of visibility, 
which involves effects of PM throughout an extended viewing distance 
across an urban area. Indeed, as

[[Page 38681]]

the generally controlling standard focused on reducing urban and 
regional scale fine particle levels, most of the visibility protection 
provided by the PM2.5 primary standards would be derived 
from the annual standard. In many cities having annual mean 
PM2.5 concentrations exceeding 17 g/m3, 
improvements in annual average visibility resulting from attainment of 
the new annual PM2.5 primary standard are expected to be 
perceptible (i.e., to exceed 1 deciview). Based on annual mean 
PM2.5 data reported in Table 12-2 of the Criteria Document 
and Table V-12 in the Staff Paper, many cities in the Northeast, 
Midwest, and Southeast, as well as Los Angeles, would be expected to 
see perceptible improvement in visibility if the annual 
PM2.5 primary standard is attained.
---------------------------------------------------------------------------

    50 Estimates of annual average visibility improvements assume 
that, on a percentage basis, the reduction for each fine particle 
component is equal to the % reduction in the mass of fine particles, 
and that the overall light extinction efficiency of the fine 
particle pollutant mix does not change. Further, for the estimates 
presented here, the reductions in fine mass at monitored locations 
are assumed to reflect the spatial average concentrations through 
the viewing distance. (Damberg and Polkowsky, 1996.)
---------------------------------------------------------------------------

    In Washington, DC, for example, where the IMPROVE 
network51 shows annual mean PM2.5 concentrations 
at about 19 g/m3 during 1992-1995, approximate 
annual average visibility would be expected to improve from 21 km 
visual range (29 deciview) to 27 km (27 deciview). Annual average 
visibility in Philadelphia, where annual PM2.5 levels have 
been recently measured at 17 g/m3, would be 
expected to change from about 24 to 27 km, an improvement of about 1 
deciview. In Los Angeles, where recent data shows annual mean 
PM2.5 concentrations at approximately 30 g/
m3, visibility would be expected to improve from about 19 to 
34 km (30 to 24 deciview) if the new annual primary PM2.5 
standard is attained.
---------------------------------------------------------------------------

    51 IMPROVE (Interagency Monitoring of PROtected Visual 
Environments) is a visibility monitoring network managed 
cooperatively by EPA, Federal land management agencies, and State 
representatives. An analysis of IMPROVE data for 1992-1995 is found 
in Sisler et al. (1996).
---------------------------------------------------------------------------

    It is important to note that some urban areas, many in the eastern 
United States, would be expected to have annual mean PM2.5 
concentrations reduced below the primary standard level of 15 
g/m3 when implementation of regional control 
strategies for PM and other air quality programs, such as those 
addressing acid rain and mobile sources, are taken into account 
together. On the other hand, some urban areas with annual 
PM2.5 levels at or below the 15 g/m3 
level would be expected to see little, if any, improvement in annual 
average visibility. This may be particularly true of certain western 
urban areas that are dominated by coarse rather than fine particles.
    The Administrator also considered the potential effect on urban 
visibility if the 24-hour 98th percentile PM2.5 standard of 
65 m3 is attained. In areas with violations caused by 
localized hot spots, the 24-hour standard might have little effect 
other than on visible source emissions. In other areas, for example, 
with seasonally high woodsmoke, a more areawide improvement is 
possible. In such urban areas, attainment of the 24-hour standard would 
be expected to reduce, to some degree, the number and intensity of 
``bad visibility'' days, i.e., the 20% of days having the greatest 
impairment over the course of a year. For example, maximum 24-hour 
PM2.5 concentrations have been recorded in recent years at 
over 140 g/m3 at several California locations. If 
the level and frequency of peak PM concentrations are reduced, 
improvements would be expected in those days where visibility is worst, 
even in urban areas having annual averages below the annual 
PM2.5 primary standard.
    Having concluded that attainment of the annual and 24-hour 
PM2.5 primary standards would lead to visibility 
improvements in many eastern and some western urban areas, the 
Administrator also considered potential improvements to visibility on a 
regional scale. In the rural East, attainment of the PM2.5 
primary standards could result in regional visibility improvement, 
e.g., in certain mandatory Class I Federal areas such as Shenandoah and 
Great Smoky Mountains National Park, if regional control strategies are 
adopted and carried out in order to reduce the impact of long-range 
transport of fine particles such as sulfates. Fine particle emission 
reductions achieved by other air quality programs, such as those to 
reduce acid rain or mobile source emissions, are also expected to 
improve Eastern regional visibility conditions (U.S. EPA, 1993). In the 
West, strategies to attain the primary PM2.5 standards are 
less likely to significantly improve visibility on a regional basis. 
However, areas downwind from large urban areas, such as Southern 
California, would likely see some improvement in annual average 
visibility.
    Based on the foregoing, the Administrator concludes that attainment 
of PM2.5 secondary standards set at the level of the primary 
standards for PM2.5 would be expected to result in 
visibility improvements in the eastern United States at both urban and 
regional scales, but little or no change in the western United States 
except in and near certain urban areas. Additionally, the Administrator 
determined that attainment of secondary standards equivalent to the 
suite of PM2.5 primary standards for particulate matter 
would address some but not all of the effects of particulate matter on 
visibility. The extent to which these effects would be addressed is 
expected to vary regionally.
    The Administrator then considered the potential effectiveness of a 
regional haze program to address the remaining effects of particulate 
matter on visibility (i.e., those that would not be addressed through 
attainment of secondary standards identical to the suite of 
PM2.5 primary standards). A program to address the 
widespread, regionally uniform type of haze caused by a multitude of 
sources is required by sections 169A and 169B of the Act. In 1977, 
Congress established as a national goal ``the prevention of any future, 
and the remedying of any existing, impairment of visibility in 
mandatory Class I Federal areas which impairment results from manmade 
air pollution'', section 169A(a)(1) of the Act. The EPA is required by 
section 169A(a)(4) of the Act to promulgate regulations to ensure that 
``reasonable progress'' is achieved toward meeting the national goal. 
EPA originally deferred establishment of a program to address regional 
haze in 1980 due to the need for greater scientific and technical 
knowledge, but the current Criteria Document and Staff Paper cite 
information supporting the Administrator's conclusion that the 
scientific state of understanding and analytical tools are now adequate 
to develop such a program. Because regional emission reductions are 
needed to make visibility improvements in mandatory Class I Federal 
areas, the structure and requirements of sections 169A and 169B of the 
Act, provide for visibility protection programs that can be more 
responsive to the factors contributing to regional differences in 
visibility than can programs addressing a nationally applicable 
secondary NAAQS. The visibility goal is more protective than a 
secondary NAAQS since the goal addresses any man-made impairment rather 
than just impairment at levels determined to be adverse.
    Thus, an important factor considered in this review is whether a 
regional haze program, in conjunction with secondary standards set 
identical to the suite of PM2.5 primary standards, would 
provide appropriate protection for visibility in non-Class I areas. The 
Administrator continues to believe that the two programs and associated 
control strategies should provide such protection due to the regional 
approaches needed to manage emissions of pollutants that impair 
visibility in many of these areas. Regional strategies implemented to 
attain the NAAQS, meet other air program goals, and make reasonable 
progress toward the national visibility goal in mandatory Class I 
Federal areas are expected to improve

[[Page 38682]]

visibility in many urban and non-Class I areas as well. The following 
recommendation from the 1993 report of the National Research Council, 
Protecting Visibility in National Parks and Wilderness Areas, addresses 
this point:

    Efforts to improve visibility in Class I areas also would 
benefit visibility outside these areas. Because most visibility 
impairment is regional in scale, the same haze that degrades 
visibility within or looking out from a national park also degrades 
visibility outside it. Class I areas cannot be regarded as potential 
islands of clean air in a polluted sea.

    Before making a final decisions on the secondary standards, the 
Administrator also considered a number of public comments that 
addressed this aspect of the proposal. Some commenters suggested 
setting secondary standards for PM2.5 more stringent than 
the proposed primary standards for the purpose of addressing visibility 
impairment and other environmental effects. For the reasons discussed 
above in this unit, however, the Administrator has concluded that this 
may not be an effective and would not be an appropriate means of 
protecting against visibility impairment in all parts of the country. 
Other commenters raised the possibility of establishing a nationally 
applicable secondary standard defined as a ``floor,'' or increment, 
above regionally specific background levels of PM2.5 or 
associated visibility. Although this idea is of interest and may 
warrant further study, the Administrator determined that it was not 
appropriate to pursue such an approach at this time for two principal 
reasons. First, the Agency does not currently have adequate scientific 
information to establish a specific floor or increment level that would 
protect against adverse effects nationally, nor is it clear as a 
conceptual matter whether further information would support selection 
of a single, uniform increment as providing an appropriate degree of 
protection in all areas of the country. Second, there are serious, 
unresolved questions about whether such an approach is consistent with 
the statutory language and purposes of section 109 of the Act.
    Other commenters argued that national secondary standards 
equivalent to the proposed PM2.5 primary standards are not 
necessary or not supported by the Administrator's findings. As noted 
earlier, however, it is clear that coarse and fine particles can cause 
adverse effects on visibility and significant quantitative data exist 
to demonstrate that visibility impairment occurs at small 
concentrations of PM2.5. Substantial efforts have been put 
forth to assess the effects of PM on visibility. For example, the Grand 
Canyon Visibility Transport Commission52 spent several years 
and significant effort studying the effects of pollution on 16 
mandatory Class I Federal areas on the Colorado plateau and has made 
recommendations to the Administrator for actions to improve visibility 
in these areas (GCVTC, 1996). All of the mandatory Class I Federal 
areas studied by the GCVTC with monitoring data have annual mean 
PM2.5 concentrations below 5 g/m3 
(Sisler, 1996) while also documenting anthropogenic visibility 
impairment. The Southern Appalachian Mountain Initiative53 
is currently assessing air pollution impacts on visibility, terrestrial 
resources, and aquatic resources in the southeastern U.S. in order to 
recommend measures to remedy existing and prevent future adverse 
effects on these air quality related values. The IMPROVE network shows 
that all of the mandatory Class I Federal areas in the SAMI region have 
annual mean PM2.5 concentrations for 1992-95 between 11.0-
13.5 g/m3 (Sisler, 1996). The inclusion in section 
169A of the Act of a national visibility goal of no manmade impairment 
also places significant value on reducing PM concentrations and 
resulting visibility impairment to low levels.54 The 
differences between the fine particle levels associated with visibility 
impairment in eastern and western mandatory Class I Federal areas 
provide further impetus to act under the provisions of sections 169A 
and 169B enabling the Administrator to establish a regionally-tailored 
visibility program to address impairment of visibility in mandatory 
Class I Federal areas. For these reasons, the Administrator has 
concluded that a national regional haze program allowing for regional 
approaches to addressing fine particle pollution, combined with a 
nationally applicable level of protection achieved through secondary 
PM2.5 standards set equal to the suite of primary standards, 
would be more effective in addressing regional variations in the 
adverse effects of PM2.5 on visibility than establishing 
national secondary standards for particulate matter that are lower than 
the suite of PM2.5 primary standards. The Administrator 
emphasizes that in order to appropriately address the regional 
differences in adverse effects of particulate matter on visibility, it 
is essential to establish secondary standards for PM2.5 
equivalent to the primary standards and an effective new regional haze 
program. A regional haze program will be particularly important in 
those areas of the country that do not exceed any of the primary 
standards for PM2.5, yet still experience significant 
visibility impairment due to particulate matter. The EPA will propose a 
regional haze regulation in the near future.
---------------------------------------------------------------------------

    52 EPA established the Grand Canyon Visibility Transport 
Commission (GCVTC) in 1991 under section 169B of the Act. Section 
169B(d) requires visibility transport commissions to assess the 
``adverse impacts on visibility from potential or projected growth 
in emissions'' and to recommend to EPA measures to remedy such 
adverse impacts. The Commission issued its final report in June 
1996.
    53 The Southern Appalachian Mountain Initiative is a voluntary 
effort begun in 1993. Participants include eight southeastern 
States, Federal land managers, EPA, and representatives from 
industry and environmental groups. A final report has not been 
issued to date.
    54 Indeed, Congress recognized when it adopted section 169A that 
the ``visibility problem is caused primarily by emission into the 
atmosphere of sulfur dioxide, oxides of nitrogen and particulate 
matter, especially fine particulate matter, from inadequately 
controlled sources.'' H.R. Rep. No. 95-294 at 204 (1977).
---------------------------------------------------------------------------

    In addition to providing a more regionally tailored approach than 
establishing a more stringent national secondary standard, an effective 
regional haze program will also fulfill the Administrator's regulatory 
responsibility under sections 169A and 169B of the Act to address both 
reasonably attributable impairment and regional haze impairment in 
mandatory Class I Federal areas. Indeed, regional haze has been shown 
to be the principal cause of visibility impairment in mandatory Class I 
Federal areas today. Thus, the promulgation of a regional haze program 
in conjunction with secondary standards for PM2.5 equivalent 
to the suite of primary standards will serve as an appropriate approach 
for addressing adverse effects of visibility that vary regionally, and 
it will also establish a comprehensive program for making reasonable 
progress toward the national visibility goal in mandatory Class I 
Federal areas by addressing visibility impairment in the form of both 
source-specific impacts and regional haze. Further, the regional haze 
rulemaking will fulfill the Administrator's responsibilities to address 
the visibility protection recommendations of the Grand Canyon 
Visibility Transport Commission, pursuant to section 169B(e) of the 
Act.
    The Administrator recognizes that people living in certain urban 
areas may place a high value on unique scenic resources in or near 
these areas, and as a result might experience visibility problems 
attributable to sources that would not necessarily be addressed by the 
combined effects of a regional haze program and secondary standards 
identical to the suite of primary standards for PM2.5. 
Commenters from

[[Page 38683]]

certain western cities and States raised this issue. In the 
Administrator's judgment, State or local regulatory approaches, such as 
past action in Colorado to establish a local visibility standard for 
the City of Denver, would be more appropriate and effective in 
addressing these special situations because of the localized and unique 
characteristics of the problems involved. Visibility in an urban area 
located near a mandatory Class I Federal area can also be improved 
through State implementation of the current visibility regulations, by 
which emission limitations can be imposed on a source or group of 
sources found to be contributing to ``reasonably attributable'' 
impairment in the mandatory Class I Federal area. EPA also intends to 
pursue opportunities to obtain information on urban and non-Class I 
area visibility through examination of available fine particle 
monitoring data. Current or planned monitoring networks and 
initiatives, such as monitoring and chemical analysis of 
PM2.5 in urban and background sites, efforts to better 
characterize real-time environmental conditions in major populations 
centers, and new automated airport visibility monitoring networks 
should provide data needed to evaluate trends in these areas. This 
information should help to better characterize the nature and spatial 
extent of urban and non-Class I visibility problems and thus serve to 
inform future decisions on NAAQS revisions or other appropriate 
measures.
    Based on all of the considerations discussed, the Administrator has 
decided to establish secondary standards identical to the suite of 
PM2.5 primary standards, in conjunction with a regional haze 
program under sections 169A and 169B of the Act, as the most 
appropriate and effective means of addressing the welfare effects 
associated with visibility impairment. Together, the two programs and 
associated control strategies should provide appropriate protection 
against the effects of PM on visibility and enable all regions of the 
country to make reasonable progress toward the national visibility 
goal.
    2. Materials damage and soiling effects. Annual and 24-hour 
secondary standards for materials damage and soiling effects of PM were 
established in 1987 at levels equal in all respects to the primary 
standards. As discussed in the Criteria Document and Staff Paper, 
particles affect materials by promoting and accelerating the corrosion 
of metals, by degrading paints, and by deteriorating building materials 
such as concrete and limestone. Soiling is found to reduce the 
aesthetic quality of buildings and objects of historical or social 
interest. Past studies have found that residential properties in highly 
polluted areas typically have lower values than those in less polluted 
areas. Thus, at high enough concentrations, particles become a nuisance 
and result in increased cost and decreased enjoyment of the 
environment.
    In the proposal, EPA proposed to establish secondary standards for 
PM10 and PM2.5 identical to the suite of proposed 
primary standards. Several comments recommended setting secondary 
standards at levels more stringent than the proposed primary standards 
in order to address various welfare effects of PM, including soiling 
and materials damage, acid deposition, and visibility. Some commenters 
specifically suggested changing the form or level of the proposed 24-
hour, 98th percentile PM standards to better protect against elevated 
PM episodes and associated soiling, materials damage, and visibility 
effects.
    After reviewing the extent of relevant studies and other 
information provided since the 1987 review of the PM standards, the 
Administrator concurs with staff and CASAC conclusions that the 
available data do not provide a sufficient basis for establishing a 
separate secondary standard based on soiling or materials damage alone. 
In the Administrator's judgment, however, setting secondary standards 
identical to the suite of PM2.5 and PM10 primary 
standards would provide increased protection against the effects of 
fine particles and retain an appropriate degree of control on coarse 
particles. Accordingly, the Administrator establishes the secondary 
standards for PM2.5 identical to the suite of primary 
standards to protect against materials damage and soiling effects of 
PM.

B. Decision on the Secondary Standards

    The Administrator establishes secondary standards identical to the 
suite of primary standards. In the Administrator's judgment, the 
establishment of these standards, in conjunction with implementation of 
a regional haze program, will provide appropriate protection against 
the welfare effects associated with particle pollution.

IV. Other Issues

    Commenters have raised a number of legal and procedural issues that 
are discussed in this unit. These include:
    (1) Whether EPA must give consideration to costs and similar 
factors in setting NAAQS.
    (2) Whether EPA erred in its selection of a methodology for 
determining the level of a NAAQS that protects public health with an 
adequate margin of safety.
    (3) Whether EPA committed a procedural error by not entering into 
the rulemaking docket underlying data from certain epidemiological 
studies.
    (4) Whether the 1990 amendments to the Act preclude EPA from 
revising the PM NAAQS to establish a new PM2.5 indicator.

Responses to other legal and procedural issues are included in the 
Response-to-Comments Document.

A. Consideration of Costs

    For more than a quarter of a century, EPA has interpreted section 
109 of the Act as precluding consideration of the economic costs or 
technical feasibility of implementing NAAQS in setting them. As 
indicated in the proposal, a number of judicial decisions have 
confirmed this interpretation. Natural Resources Defense Council v. 
Administrator, 902 F.2d 962, 972-973 (D.C. Cir. 1990)(PM 
NAAQS)(``PM10''), vacated, in part, dismissed, 921 F.2d 326 
(D.C. Cir.), certs. dismissed, 498 U.S. 1075, and cert. denied, 498 
U.S. 1082 (1991); Natural Resources Defense Council v. EPA, 824 F.2d 
1146, 1157-1159 (D.C. Cir. 1987)(en banc)(CAA section 112 standards for 
vinyl chloride)(``Vinyl Chloride''); American Petroleum Institute v. 
Costle, 665 F.2d 1176, 1185-1186 (D.C. Cir. 1981)(ozone 
NAAQS)(``Ozone''), cert. denied, 455 U.S. 1034 (1982); Lead Industries 
Ass'n v. EPA, 647 F.2d 1130, 1148-1151 (D.C. Cir.)(lead NAAQS)(Lead 
Industries), cert. denied, 449 U.S. 1042 (1980).
    Some commenters have argued that costs and similar factors should, 
nonetheless, be considered, both in this rulemaking and in the 
rulemaking on proposed revisions to the NAAQS for ozone. Although most 
of the commenters' arguments are inconsistent with the judicial 
decisions cited in this unit, several commenters have argued that those 
decisions are not dispositive. For reasons discussed in this unit and 
in the Response-to-Comments Document, EPA disagrees with these comments 
and maintains its longstanding interpretation of the Act as precluding 
consideration of costs and similar factors in setting NAAQS.
    1. Background. Given the nature of the points raised, a brief 
review of the issue seems useful before addressing the comments. The 
requirement that EPA establish national ambient air quality standards 
for certain pollutants, to be implemented by the States, was enacted in 
1970 as part of a set of comprehensive amendments that established the 
basic framework for

[[Page 38684]]

Federal, State, and local air pollution control. When EPA promulgated 
the original NAAQS in 1971, its first Administrator, William D. 
Ruckelshaus, concluded that costs and similar factors could not be 
considered in that decision.55 This conclusion was not 
challenged in litigation on the original NAAQS. It has been confirmed 
since then, however, by every judicial decision that has considered the 
issue.
---------------------------------------------------------------------------

    55 36 FR 8186, April 30, 1971. EPA has maintained this 
interpretation consistently since then.
---------------------------------------------------------------------------

    As discussed in this unit, EPA's interpretation rests primarily on 
the language, structure, and legislative history of the statutory 
scheme adopted in 1970. It is also supported by the judicial decisions 
cited in this unit, as well as by legislative developments since 1970 
that reaffirm Congress' original approach to the issue.
    Without cataloguing all relevant aspects of the 1970 amendments and 
their legislative history, several basic points should be noted. Under 
section 109(b) of the Act, NAAQS are to be ``based on'' the air quality 
criteria issued under section 108 of the Act. Under section 108(a)(2) 
of the Act, the kind of information EPA is required to include in 
criteria documents is limited to information about health and welfare 
effects ``which may be expected from the presence of [a] pollutant in 
the ambient air * * * .'' There is no mention of the costs or 
difficulty of implementing the NAAQS, nor of ``effects'' that might 
result from implementing the NAAQS (as opposed to effects of pollution 
in the air).56 By contrast, Congress explicitly provided for 
consideration of costs and similar factors in decisions under other 
sections of the Act.57 Moreover, States were permitted to 
consider economic and technological feasibility in developing plans to 
implement the NAAQS to the extent such consideration did not interfere 
with meeting statutory deadlines for attainment of the 
standards.58 Finally, the legislative history indicated that 
Congress had considered the issue and had deliberately chosen to 
mandate NAAQS that would protect health regardless of concerns about 
feasibility.59
---------------------------------------------------------------------------

    56 That consideration of such factors was not intended in NAAQS 
decisions is also supported by section 109(a)(1) of the Act. For 
pollutants for which air quality criteria had been issued prior to 
the 1970 amendments, that provision required EPA to propose NAAQS 
within 30 days after enactment and to take final action 90 days 
later. The criteria issued previously did not include information on 
costs and similar factors, and it would have been difficult if not 
impossible for EPA to supplement them in time to include meaningful 
consideration of such factors in NAAQS proposed 30 days after 
enactment.
    57 See, e.g., sections 110(e)(1), 111(a)(1), 231(b) of the 1970 
Act; see also, e.g., sections 113(d)(4)(C)(ii), 125(a)(3), 
202(a)(3)(C), 317 of the 1977 Act.
    58 Union Electric Co. v. EPA, 427 U.S. 246, 257-58 (1976).
    59 The Senate report on the 1970 amendments stated: ``In the 
Committee discussions, considerable concern was expressed regarding 
the use of the concept of technical feasibility as the basis of 
ambient air standards. The Committee determined that (1) the health 
of people is more important than the question of whether the early 
achievement of ambient air quality standards protective of health is 
technically feasible; and, (2) the growth of pollution load in many 
areas, even with application of available technology, would still be 
deleterious to public health.''
    ``Therefore, the Committee determined that existing sources of 
pollutants either should meet the standard of the law or be closed 
down * * * .''
    S. Rep. No. 91-1196, at 2-3 (1970).
---------------------------------------------------------------------------

    The first judicial decision on the issue came in the Lead 
Industries case. An industry petitioner argued that EPA should have 
considered economic and technological feasibility in allowing a 
``margin of safety'' in setting primary standards for lead. Based on a 
detailed review of the language, structure, and legislative history of 
the statutory scheme, the U.S. Court of Appeals for the District of 
Columbia Circuit concluded that:

    This argument is totally without merit. [The petitioner] is 
unable to point to anything in either the language of the Act or its 
legislative history that offers any support for its claim * * * . To 
the contrary, the statute and its legislative history make clear 
that economic considerations play no part in the promulgation of 
ambient air quality standards under section 109.

647 F.2d at 1148.
    The Court cited a number of reasons for this conclusion. Id. at 
1148-1150. Among other things, it noted the contrast between section 
109(b) of the Act and other provisions in which Congress had explicitly 
provided for consideration of economic and technological feasibility, 
as well as the requirement that NAAQS be based on air quality criteria 
defined without reference to such factors. Id. at 1148-1149 and n.37. 
The Court also noted that, in developing plans to implement NAAQS, 
States may consider economic and technological feasibility only to the 
extent that this does not interfere with meeting the statutory 
deadlines for attainment of the standards; and that EPA may not 
consider such factors at all in deciding whether to approve State 
implementation plans. Id. at 1149 n.37 (citing Union Electric Co. v. 
EPA, 427 U.S. 246, 257-258, 266 (1976)).60
---------------------------------------------------------------------------

    60 These limitations would, of course, make little sense if such 
factors could be considered in setting the NAAQS themselves.
---------------------------------------------------------------------------

    As to the legislative history of the 1970 amendments, the Court 
observed that:

    [T]he absence of any provision requiring consideration of these 
factors was no accident; it was the result of a deliberate decision 
by Congress to subordinate such concerns to the achievement of 
health goals.

Id. at 1149. Citing several leading Supreme Court decisions, as well as 
the Senate report quoted in this unit, the Court noted that Congress 
had intended a drastic change in approach toward the control of air 
pollution in the 1970 amendments and was well aware that sections 108-
110 of the Act imposed requirements of a ``technology-forcing'' 
character. Id.61
---------------------------------------------------------------------------

    61 Such requirements ``are expressly designed to force regulated 
sources to develop pollution control devices that might at the time 
appear to be economically or technologically infeasible.''' Id. 
(quoting Union Electric Co. v. EPA, 427 U.S. at 257).
---------------------------------------------------------------------------

    The Court also noted that Congress had already acted, in further 
amendments adopted in 1977, to relieve some of the burdens imposed by 
the 1970 amendments. Id. at 1150 n.38. Observing that Congress had, 
however, declined to amend section 109(b) of the Act to provide for 
consideration of costs and similar factors as requested by industrial 
interests, Id. n.39, the Court concluded:

    A policy choice such as this is one which only Congress, not the 
courts and not EPA, can make. Indeed, the debates on the [1970 
amendments] indicate that Congress was quite conscious of this fact 
* * * .

    * * * [I]f there is a problem with the economic or technological 
feasibility of the lead standards, [the petitioner], or any other 
party affected by the standards, must take its case to Congress, the 
only institution with the authority to remedy the problem.

Id. at 1150.
    After the decision in Lead Industries, Supreme Court review was 
sought on the question whether costs and similar factors could be 
considered in setting NAAQS, among other issues. The Supreme Court 
declined to review the decision. Lead Industries Ass'n v. EPA, 449 U.S. 
1042 (1980). The subsequent decisions in Ozone, Vinyl Chloride, and 
PM10, cited in this unit, strongly reaffirmed the 
interpretation adopted in Lead Industries.62 Supreme Court

[[Page 38685]]

review of the Ozone and PM10 decisions was sought but 
denied. American Petroleum Institute v. Gorsuch, 455 U.S. 1034 (1984); 
American Iron and Steel Institute v. EPA, 498 U.S. 1082 (1991).
---------------------------------------------------------------------------

    62 In the PM10 case, for example, the Court 
considered an argument that EPA should have considered potential 
health consequences of unemployment that might result from revision 
of the primary NAAQS for PM:

    ``This claim is entirely without merit. In three previous cases, 
this court has emphatically stated that Sec. 109 does not permit EPA 
to consider such costs in promulgating national ambient air quality 
standards * * * . It is only health effects relating to pollutants 
in the air that EPA may consider * * * . Consideration of costs 
associated with alleged health risks from unemployment would be 
flatly inconsistent with the statute, legislative history and case 
law on this point.''
    902 F.2d at 973 (emphasis in original; citations omitted).
---------------------------------------------------------------------------

    The Lead Industries opinion focused largely, though not 
exclusively, on the 1970 amendments and their legislative history. 
Perhaps as a result, it did not canvass all the factors that, in fact, 
supported its conclusions at the time. For example, when Congress 
enacted major amendments to the Act in 1977, it was clearly aware that 
some areas of the country had experienced difficulty in attempting to 
attain some of the NAAQS.63 It was also aware that there 
might be no health-effects thresholds for the pollutants involved, and 
that significant uncertainties are inherent in setting health-based 
standards under the Act.64 In response, Congress made 
significant changes in the provisions for implementation of the NAAQS, 
including changes intended to ease the burdens of attainment. It also 
amended sections 108 and 109 of the Act in several ways; for example, 
by requiring periodic review and, if appropriate, revision of air 
quality criteria and NAAQS and by establishing a special scientific 
advisory committee (CASAC) to advise EPA on such reviews. Notably, 
Congress recognized that implementation of NAAQS could cause ``adverse 
public health, welfare, social, economic, or energy effects'' and 
charged CASAC with advising EPA on such matters.65 Yet it 
made no changes in section 109(b) or section 108(a)(2) of the Act; that 
is, in the substantive criteria for setting or revising NAAQS. In other 
words, Congress chose to address economic and other difficulties 
associated with attainment of the NAAQS by adjusting the scheme for 
their implementation, rather than by changing the instructions for 
setting them.66
---------------------------------------------------------------------------

    63 See, e.g., H.R. Rep. No. 95-294, at 207-217 (l977).
    64 See, e.g., Id. at 110-112; Id. at 43-51.
    65 Section 109(d)(2)(C)(iv) of the Act. Some commenters have 
argued that this provision requires EPA to consider such effects in 
setting NAAQS. From the language and structure of section 109(d) of 
the Act, however, it is clear that CASAC's responsibility to advise 
on these factors is separate from its responsibility to review and 
recommend revision of air quality criteria and NAAQS, and that the 
advice pertains to the implementation of NAAQS rather than to 
setting them. The legislative history confirms this view, indicating 
that the advice was intended for the benefit of the States and 
Congress. See H.R. Rep. No. 95-294, at 183 (1977).
    66 The 1977 amendments also required EPA to prepare economic 
impact assessments for specified actions but limited the requirement 
to non-health-based standards, excluding decisions under sections 
109 and 112 of the Act. Section 317; H.R. Rep. No. 95-294, at 51-52 
(1977). In this and other respects, Congress continued the approach 
it took in the l970 amendments, making careful choices as to when 
consideration of costs and similar factors would be required and 
giving paramount priority to protection of health. See 123 Cong. 
Rec. H8993 (daily ed. Aug. 4, 1977) (Clean Air Conference Report 
(1977); Statement of Intent; Clarification of Select Provisions), 
reprinted in 3 Senate Committee on Environment and Public Works, 
95th Cong., A Legislative History of the Clean Air Act Amendments of 
1977, at 319 (1978).
---------------------------------------------------------------------------

    Congress enacted major amendments to the Act again in 1990, well 
after the Lead Industries and Ozone decisions that interpreted section 
109 of the Act as precluding consideration of costs in NAAQS 
decisions.67 In doing so, Congress was clearly aware of 
intervening developments such as EPA's decision to revise the PM NAAQS 
in 1987--the result of an elaborate review in which the Administrator 
strongly underscored the scientific uncertainties 
involved68--and the Vinyl Chloride case drawing a sharp 
distinction between sections 109 and 112 of the Act with regard to 
consideration of costs and similar factors.69 Indeed, the 
legislative history of the 1990 amendments reflects Congress' 
understanding that primary NAAQS were to be based on protection of 
health ``without regard to the economic or technical feasibility of 
attainment.''70 Again, however, Congress chose to respond to 
severe, widespread, and persistent problems with attaining the NAAQS by 
adjusting the scheme for their implementation rather than by changing 
the basis for setting them. See, e.g., sections 181-192 of the Act.
---------------------------------------------------------------------------

    67 In the interim, the National Commission on Air Quality had 
also submitted its report to Congress as required by a provision of 
the 1977 amendments. Among other things, the Commission recommended 
that the statutory approach of requiring NAAQS to be set at levels 
necessary to protect public health, without consideration of 
economic factors, be continued without change. National Commission 
on Air Quality, To Breathe Clean Air 55 (1981).
    68 As the Administrator indicated in EPA's proposal to revise 
the PM standards:
    ``[T]hat review has revealed a highly limited data base--
particularly where quantitative studies are concerned--and a wide 
range of views among qualified professionals about the exact 
pollution levels at which health effects are likely to occur. The 
setting of an `adequate margin of safety' below these levels calls 
for a further judgment--in an area for which the scientific data 
base is even more sparse and uncertain * * * .''

    ``* * * [L]ong and expert review of public health issues has to 
date revealed no scientific method of assessing exactly what level 
of standards public health requires. The scientific review indicates 
substantial uncertainties concerning the health risks associated 
with lower levels of particulate matter.'' (49 FR 10408, 10409, 
March 20, l984)
    69 Congress was clearly aware of the 1987 decision to revise the 
PM NAAQS, which among other things involved changing the indicator 
for particulate matter from ``total suspended particulate'' to 
PM10, because it enacted special nonattainment 
provisions, as well as provisions for PSD increments, applicable to 
PM10. Sections 188-190 of the Act; section 166(f) of the 
Act. It was clearly aware of the Vinyl Chloride decision because it 
amended section 112 of the Act in response to that decision, 
essentially creating a new scheme for setting emission standards for 
hazardous pollutants.
    70 H.R. Rep. No. 101-490, pt. 1, at 145 (1990). See also S. Rep. 
No. 101-228, at 5 (1989).
---------------------------------------------------------------------------

    2. Public comments. As noted previously, a number of commenters 
have argued that costs and similar factors should be considered in 
EPA's final decisions on revision of both the particulate and ozone 
NAAQS. Aside from arguments that are simply inconsistent with the 
judicial decisions cited in this unit, some of the commenters argue 
that those decisions are not dispositive for a variety of reasons. One 
commenter submitted a particularly comprehensive version of this 
argument; the following discussion focuses primarily on points raised 
by that commenter, among others.71
---------------------------------------------------------------------------

    71 Additional responses to points raised by this commenter and 
others are included, as appropriate, in the Response-to-Comments 
Document.
---------------------------------------------------------------------------

    As a general matter, the commenter acknowledges that Congress 
intended to preclude consideration of economic costs and similar 
factors in setting NAAQS. The commenter argues, however, that this is 
so only when the scientific basis for NAAQS is ``clear and compelling'' 
or ``unambiguous.'' From that premise, the commenter advances three key 
assertions:
    a. Where non-threshold pollutants are involved and the health 
evidence is ambiguous, section 109 of the Act must be interpreted to 
allow consideration of all relevant factors, including the practical 
consequences of EPA's decisions.
    b. To the extent the judicial decisions cited in this unit are read 
as precluding this, they rest on a faulty analysis that pre-dates and 
cannot survive scrutiny under Chevron, U.S.A. v. Natural Resources 
Defense Council, 467 U.S. 837 (1984).72
---------------------------------------------------------------------------

    72 Several other commenters argue that the cited decisions are 
not dispositive because they held only that EPA is not required to 
consider costs and similar factors in setting NAAQS. As discussed in 
this unit in connection with Chevron, however, the decisions clearly 
concluded that Congress intended to preclude consideration of such 
factors, and that EPA is not free to alter that congressional 
choice. Although these conclusions are technically dicta, nothing in 
the Court's opinions suggests that it would have interpreted section 
109 of the Act differently had EPA claimed authority to consider 
costs and similar factors in NAAQS decisions. Indeed, the tone of 
the opinions argues to the contrary. See, e.g., PM10, 902 
F.2d at 973. Cf. Ethyl Corp. v. EPA, 51 F.3d 1053 (D.C. Cir. 1995).
---------------------------------------------------------------------------

    c. Because EPA has discretion to consider costs and similar factors 
where the health evidence is ambiguous, it must do so in light of 
Executive Order 12866 (58 FR 51735, October 4, 1993), and two recent 
statutes, the Unfunded

[[Page 38686]]

Mandate Reform Act of 1995, 2 U.S.C. 1501-1571 (UMRA), and the Small 
Business Regulatory Enforcement Fairness Act of 1996, Pub. L. 104-121, 
110 Stat. 857 (SBREFA), which in part amended the Regulatory 
Flexibility Act, 5 U.S.C. 601-808.
    EPA believes all three assertions are clearly incorrect. Regarding 
the first point, it should be evident, both from previous NAAQS 
decisions and from the court opinions upholding them, that the 
scientific basis for NAAQS decisions has never pointed clearly and 
unambiguously to a single ``right answer.''73 This is 
inherent in the statutory scheme for the establishment and revision of 
NAAQS, which in effect requires them to be based on the ``latest 
scientific knowledge'' on potential health and welfare effects of the 
pollutant in question. See sections 109(b) and 108(a)(2) of the Act. 
Although advances in science increase our understanding of such 
effects, they also raise new questions. For this reason, the key 
studies for any given decision on revision of a NAAQS are, almost by 
definition, ``at the very `frontiers of scientific 
knowledge.'''73 That is, studies that call into question the 
adequacy of a standard are always those that go beyond previous 
studies--by reporting new kinds of effects, for example, or effects at 
lower concentrations than those at which effects have been reported 
previously.
---------------------------------------------------------------------------

    73 See, e.g., Lead Industries, 647 F.2d at 1146-1147, 1153-1156, 
1160-1161, 1167 n.106. In enacting the 1970 amendments, Congress was 
aware that there were gaps in the scientific information available 
then as a basis for establishing the original NAAQS. See, e.g., S. 
Rep. No. 91-1196, at 9-11 (1970). If anything, Congress had an even 
greater understanding of the point when it enacted the 1977 
amendments without changing the substantive criteria for setting 
NAAQS. See H.R. Rep. No. 95-294, at 43-51, 181-182 (1977).
    74 Lead Industries, 647 F.2d at 1147 (quoting Ethyl Corp. v. 
EPA, 541 F.2d 1, 24-27 (D.C. Cir.) (en banc), cert. denied, 426 U.S. 
941 (1976)).
---------------------------------------------------------------------------

    As with pioneering work in other fields, such studies may have a 
variety of strengths and limitations.875 As a result, the 
validity and implications of such studies may be both uncertain and 
highly controversial. Given the precautionary nature of section 109 of 
the Act,76 however, it is precisely these kinds of studies 
that the Administrator must grapple with when advances in science 
suggest that revision of a NAAQS is appropriate.
---------------------------------------------------------------------------

    75 They may have methodological flaws, for example, but 
nonetheless report effects that are of serious medical significance; 
or they may be of impeccable quality but involve effects of 
uncertain significance. Others may involve results that are striking 
but hard to explain in terms of previous knowledge, or results that 
seem plausible and important but are not yet replicated by other 
studies.
    76 See, e.g., Lead Industries, 647 F.2d at 1155-1156; H.R. Rep. 
No. 94-295, at 43-51 (1977).
---------------------------------------------------------------------------

    As a result, the EPA staff typically recommends for consideration, 
and the Administrator may propose for comment, a range of alternatives 
based on what the commenter would call ``ambiguous'' science. In this 
respect, the current reviews of the NAAQS for ozone and particulate 
matter are not unusual and do not differ, for example, from the review 
that led to adoption of the PM10 NAAQS in 1987.77 
Indeed, the NAAQS that were upheld in the Lead Industries, Ozone, and 
PM10 decisions were all based on highly controversial health 
evidence; the Lead Industries decision took note of congressional 
statements recognizing that there may be no thresholds for criteria 
pollutants; and the Ozone and PM10 decisions noted the 
Administrator's findings that clear thresholds could not be identified 
for ozone and particulate matter, respectively.78 Thus, the 
present decisions on revision of the NAAQS for ozone and particulate 
matter cannot be distinguished from those past decisions in terms of 
the nature of the health evidence or pollutants involved.78
---------------------------------------------------------------------------

    77 As previously discussed, the Administrator strongly 
emphasized the uncertainties involved in that review. As a result of 
the uncertainties, he proposed ``relatively broad'' ranges for 
comment, though he focused on lower levels within the ranges as 
providing greater margins of safety against the health risks 
involved. See 49 FR 10408, 10409, March 20, l984.
    78 See, e.g., Lead Industries, 647 F.2d at 1152-53 and n. 43, 
1159-60; Ozone, 665 F.2d at 1185, 1187; PM10, 902 F.2d at 
969-71, 972.
    79 Indeed, the present decisions on the NAAQS for PM and ozone 
are based on some of the best scientific information the Agency has 
ever been able to rely on in NAAQS decision-making. In particular, 
the science underlying these decisions is much more extensive and of 
much better quality than the science underlying the existing NAAQS 
for PM and ozone.
---------------------------------------------------------------------------

    Regarding the second of the commenter's key assertions, EPA 
determines it is clear that the judicial decisions cited in this unit 
were correctly decided and continue to be good law under Chevron. In 
Chevron, the Supreme Court essentially reaffirmed the principle that 
courts must defer to reasonable agency interpretations of the statutes 
they administer where Congress has delegated authority to them to 
elucidate particular statutory provisions. Where the intent of Congress 
on an issue is clear, however, it must be given effect by the agency 
and the courts. See 467 U.S. at 842-45. Thus, the first question on 
review of an agency's interpretation under Chevron is ``whether 
Congress has directly spoken to the precise question at issue.'' If the 
court determines that it has not, the remaining question for the court 
is ``whether the agency's answer is based on a permissible construction 
of the statute.'' 467 U.S. at 842-843 (footnote omitted). In 
determining whether Congress ``had an intention on the precise question 
at issue,'' a court employs ``traditional tools of statutory 
construction.'' Id. at 843 n.9.80
---------------------------------------------------------------------------

    80 In practice, analysis of this question is sometimes referred 
to as a ``Chevron step one'' analysis.
---------------------------------------------------------------------------

    In essence, the commenter's argument here is that the Lead 
Industries decision did not address whether Congress had ``spoken 
directly'' to the precise issue posed by the commenter; that is, 
whether section 109 of the Act must be interpreted differently for 
NAAQS decisions involving non-threshold pollutants and ``ambiguous'' 
health evidence. The Lead Industries opinion, which pre-dated Chevron, 
did not pose the question in those terms. Its focus, however, was 
clearly on what Congress intended to be the basis for NAAQS decisions, 
in a context the Court understood to involve considerable uncertainty 
and debate about the health evidence, as well as the possibility that 
there was no threshold for health effects of the 
pollutant.81 In short, the health evidence was hardly 
``unambiguous,'' yet the Court interpreted section 109 of this Act as 
precluding consideration of costs and similar factors even in allowing 
a margin of safety. Nothing in the Lead Industries decision or in the 
subsequent cases suggests in any way that section 109 of the Act should 
be interpreted differently based on the nature of the pollutants or 
health evidence involved, and the Court's findings on congressional 
intent admit of no exceptions:

     * * * [T]he statute and its legislative history make clear that 
economic considerations play no part in the promulgation of ambient 
air quality standards under Section 109.

647 F.2d at 1148.
---------------------------------------------------------------------------

    81 See, e.g., 647 F.2d at 1148-51, 1152-53 and n.43, 1160-61.
---------------------------------------------------------------------------

    Alternatively, the commenter argues that the Lead Industries case 
decided the issue incorrectly in light of the principles announced 
subsequently in Chevron. In this context, the commenter essentially 
argues that the Lead Industries decision rested on two factors that are 
no longer probative:
    (1) That there was no indication that Congress meant to allow 
consideration of costs in NAAQS decisions, and
    (2) That Congress specifically provided for such consideration in 
other sections of the Act but not in section 109.

[[Page 38687]]

    On the first point, the commenter argues that EPA is free under 
Chevron to consider costs and similar factors (by reinterpreting 
section 109 of the Act) unless there is evidence that Congress intended 
to restrict its discretion. As to the second point, the commenter 
argues that similar reasoning was rejected in Vinyl Chloride.
    In Vinyl Chloride, however, an en banc decision that post-dated 
Chevron, the Court essentially underscored the point that such issues 
cannot be decided mechanically but must turn, instead, on more 
analytical attention to relevant indicia of congressional intent. See, 
e.g., 824 F.2d at 1157 n.4; Id. at 1157-1163. With reference to NAAQS 
decisions in particular, the Court concluded that there were concrete 
indications of congressional intent to preclude consideration of costs 
and similar factors; for example, the fact that section 108 of the Act 
``enumerate[s] specific factors to consider and pointedly exclude[s] 
feasibility.'' 824 F.2d at 1159. In a later case, moreover, the same 
Court held that EPA could not consider certain factors, in decisions 
under section 211(f)(4) of the Act, for reasons exactly parallel to 
those that the commenter criticizes in Lead Industries. See Ethyl Corp. 
v. EPA, 51 F.3d 1053, 1057-1063 (D.C. Cir. 1995).
     Beyond this, the commenter's characterization of the Lead 
Industries decision ignores or discounts much of the key evidence cited 
by the Court, including the language, structure, and legislative 
history of the statutory scheme established in 1970, for its conclusion 
that Congress intended to preclude consideration of costs and similar 
factors in NAAQS decisions.82 As indicated in this unit, the 
Vinyl Chloride and PM10 cases, both of which post-dated 
Chevron, reached the same conclusion.
---------------------------------------------------------------------------

    82 See 647 F.2d at 1148-51. By contrast, the commenter's 
argument that Congress actually intended EPA to consider such 
factors relies heavily on statements made in subsequent legislative 
history, most of which were made in floor debate, that sought to 
justify controversial amendments to establish a different program 
than the NAAQS and did not involve any proposed changes in section 
109 of the Act or related provisions; and statements in early 
judicial decisions involving programs under other statutory 
provisions. In context, EPA determines these and other statements 
cited by the commenter are consistent with and do not alter the 
conclusion that Congress intended to preclude consideration of costs 
and similar factors under section 109 of the Act.
---------------------------------------------------------------------------

     Moreover, this series of decisions went far beyond mere deference 
to an agency interpretation. As indicated in the Vinyl Chloride case, 
the Lead Industries court found ``clear evidence'' of Congressional 
intent, which was to limit the factors EPA may consider under section 
109 of the Act. 824 F.2d 1159. Consistent with Chevron, these findings 
were based on traditional tools of statutory construction. See Id. at 
1157-1159; Lead Industries, 647 F.2d at 1148-1151. In terms of the 
analytical framework later established by Chevron, these were Chevron 
step one findings, meaning that the statute spoke directly to the issue 
and that the courts, as well as the agency, must give effect to 
Congress' intent as so ascertained. See 467 U.S. at 842-
843.83 Thus, absent a more recent legislative enactment 
overriding that intent, EPA has no discretion to alter its longstanding 
interpretation that consideration of costs and similar factors is 
precluded in NAAQS decisions under section 109 of the Act.84
---------------------------------------------------------------------------

    83 The commenter argues that the post-Chevron cases accepted the 
Lead Industries analysis uncritically rather than re-examining it 
under Chevron. Clearly, this elevates form over substance. It is 
true that neither case referred to Chevron in discussing the point 
at issue. In Vinyl Chloride, however, the Court retraced the steps 
in the Lead Industries analysis in some detail, characterized some 
of the key evidence reviewed in that analysis in terms going beyond 
mere rote repetition (e.g., ``a far clearer statement than anything 
in the present case that Congress considered the alternatives''), 
and used Chevron-like language in discussing the significance of 
that evidence; that is, that it demonstrated congressional intention 
on the point at issue. E.g., 824 F.2d at 1159. Given that the Vinyl 
Chloride case was decided three years after Chevron, that it was an 
en banc decision of the D.C. Circuit involving interpretation of 
statutory language very similar to that in Lead Industries, and that 
the Court cited Chevron twice in analyzing the language and history 
of section 112 of the Act, it seems highly unlikely that the Court 
was unmindful of Chevron principles in concluding that Congress 
intended to preclude consideration of costs under section 109 of the 
Act but not under section 112 of the Act.
    In the PM10 decision, the Court confirmed the sharp 
distinction it had drawn, based on such evidence of congressional 
intent, between sections 109 and 112 of the Act in Vinyl Chloride. 
902 F.2d at 972-973. Although discussion of the point was brief and 
did not mention Chevron, the industry petitioner raising the point 
had cited Chevron in arguing that the Lead Industries interpretation 
was not binding, and that EPA's decision on the PM10 
standards should be reversed on the ground that it rested on a legal 
position that EPA unjustifiably believed was mandated by Congress. 
Reply Brief of the American Iron and Steel Institute at 11 and n.10, 
Natural Resources Defense Council v. Administrator, 902 F.2d 962 
(D.C. Cir. 1990) (Nos. 87-1438 et al.). Thus, Chevron issues were 
properly before the Court and were brought squarely to its 
attention.
    84 See also 52 FR 24854, July 1, 1987.
---------------------------------------------------------------------------

    As to the commenter's third key assertion, Executive Order 12866, 
UMRA sections 202 and 205, and the Regulatory Flexibility Act (RFA), as 
amended by SBREFA, do not conflict with this interpretation or require 
a different result. Basically, the commenter argues that the Executive 
Order, UMRA, and the RFA (as amended by SBREFA) require agencies to use 
cost (or similar factors) as a decisional criterion in making 
regulatory decisions, and that this modifies the Clean Air Act's 
directive that EPA is precluded from considering costs when setting a 
NAAQS. The commenter's argument is flawed on a number of grounds. 
First, UMRA and the RFA (as amended by SBREFA) do not conflict with 
section 109 of the Act because they do not apply to this decision, as 
discussed in Unit VIII. of this preamble. Second, the Executive Order 
and both statutes are quite clear that they do not override the 
substantive provisions in an authorizing statute. Third, the 
commenter's premise that UMRA and the RFA (as amended by SBREFA) 
establish substantive decisional criteria that agencies are required to 
follow is wrong.
     As a matter of law, the Executive Order cannot (and does not 
purport to) override the Clean Air Act. The Executive Order does not 
conflict with section 109 of the Act because the requirement that 
agencies ``select approaches that maximize net benefits'' does not 
apply if a ``statute requires another regulatory approach.'' Executive 
Order 12866, section (1)(a), (58 FR 51735, October 4, 1993). More 
generally, the Executive Order provides that agencies are to adhere to 
its regulatory principles only ``to the extent permitted by law.'' Id., 
section (1)(b).
     UMRA sections 202 and 205 do not apply to this decision, as 
discussed in Unit VIII. of this preamble. Even when they do apply to a 
regulatory action, they do not establish decisional criteria that an 
agency must follow, much less override decisional criteria established 
in the statute authorizing the regulatory action. UMRA does not require 
an agency to select any particular alternative. Rather, an agency can 
select an alternative that is not the least costly, most cost-effective 
or least burdensome if the agency explains why. Section 205(b)(1) of 
UMRA. Such an explanation is not required if the least costly, most 
cost-effective or least burdensome alternative would have been 
``inconsistent with law,'' section 205(b)(2) of UMRA, and the only 
alternatives that an agency should consider are ones that ``achieve[] 
the objectives of the rule,'' section 205(a) of UMRA. The UMRA 
Conference Report confirms that UMRA does not override the authorizing 
statute. ``This section [202] does not require the preparation of any 
estimate or analysis if the agency is prohibited by law from 
considering the estimate or analysis in adopting the rule.'' 141 Cong. 
Rec. H3063 (daily ed. March 13, 1995).
    The RFA (as amended by SBREFA) also does not apply to this 
decision, as

[[Page 38688]]

discussed in Unit VIII. of this preamble. As is the case with UMRA, 
even when the RFA (as amended by SBREFA) does apply to a regulatory 
action, it does not establish decisional criteria that an agency must 
follow, much less override the underlying substantive statute. When the 
RFA was adopted in 1980, Congress made clear that it did not alter the 
substantive standards contained in authorizing statutes: ``The 
requirements of section 603 and 604 of this title [to prepare initial 
and final regulatory flexibility analyses] do not alter in any manner 
standards otherwise applicable by law to agency action.'' Section 606 
of the RFA. The legislative history further explains that section 606 
``succinctly states that this bill does not alter the substantive 
standard contained in underlying statutes which defines the agency's 
mandate.''85 When Congress passed SBREFA in 1996 and amended 
parts of the RFA, it did not amend section 606.
---------------------------------------------------------------------------

    85 126 Cong. Rec. 21452, 21455 (1980) (Description of Major 
Issues and Section-By-Section Analysis of Substitute for S. 299).
---------------------------------------------------------------------------

    Even when a regulatory decision is subject to sections 603 and 604 
of the RFA and an agency is therefore required to analyze alternatives 
that minimize significant economic impacts on small entities, the RFA 
(as amended by SBREFA) does not establish decisional criteria that an 
agency is required to follow. Both section 603 and 604 of the RFA 
provide that the alternatives an agency should consider are to be 
``consistent with the stated objectives of applicable statutes.'' 
Section 603(c) and 604(a)(5) of the RFA. Furthermore, although the RFA 
(as amended by SBREFA) requires agencies to consider alternatives that 
minimize impacts on small entities subject to the rules' requirements 
and to explain their choice of regulatory alternatives, it does not 
require agencies to select such alternatives. For these reasons, the 
RFA (as amended by SBREFA) does not conflict with or override the Clean 
Air Act's preclusion of considering costs and similar factors in 
setting NAAQS.
     3. Conclusion. In summary, EPA determines that the judicial 
decisions cited in this unit are both correct and dispositive on the 
question of considering costs in setting NAAQS, and that the Agency is 
not free to reinterpret the Act on that question.

B. Margin of Safety

     Several commenters questioned the approach used by the 
Administrator in specifying PM standards that protect public health 
with an adequate margin of safety. Rather than the integrative approach 
applied by the Administrator, these commenters maintained that EPA must 
employ a two-step process. One line of argument was that the 
Administrator must first determine a ``safe level'' and then apply a 
margin of safety taking into account costs and societal impacts. It was 
argued that this was the only approach that would enable the 
Administrator to reach a reasoned decision on a standard level that 
protects public health against unacceptable risk of harm, such that any 
remaining risk was ``acceptable.'' In effect, these commenters adopted 
the two-step methodology endorsed by Vinyl Chloride, 824 F.2d 1146, for 
setting hazardous air pollutant standards under section 112 of the Act. 
Another commenter also maintained that the Administrator must apply a 
two-step process but from a different perspective. It was argued that 
EPA should first identify the lowest observed effect level and then 
apply a margin of safety to address uncertainties and to protect the 
most sensitive individuals within the at-risk population(s). This 
commenter also maintained that the use of risk assessment in 
establishing a NAAQS was a departure from past practice, and that this 
departure was not adequately explained.
    In recognition of the complexities facing the Administrator in 
determining a standard that protects public health with an adequate 
margin of safety, the courts have declined to impose any specific 
requirements on the Administrator's methodological approach. Thus, in 
Lead Industries the court held that the selection of any particular 
approach to providing an adequate margin of safety ``is a policy choice 
of the type Congress specifically left to the Administrator's judgment. 
This court must allow him the discretion to determine which approach 
will best fulfill the goals of the Act.'' 647 F.2d at 1161-1162. As a 
result, the Administrator is not limited to any single approach to 
determining an adequate margin of safety and, in the exercise of her 
judgment, may choose an integrative approach, a two-step approach, or 
perhaps some other approach, depending on the particular circumstances 
confronting her in a given NAAQS review.
     With respect to the approaches advanced in comment, the 
PM10 case made clear that the two-step process endorsed in 
Vinyl Chloride was necessary because of the need under section 112 of 
the Act to ``sever determinations that must be based solely on health 
considerations from those that may include economic and technical 
considerations.'' 902 F.2d at 973. Because the Administrator may not 
consider cost and technological feasibility under section 109 of the 
Act, however, the Court concluded that ``the rationale for parsing the 
Administrator's determination into two steps is inapposite.'' Id.
     The claim that EPA must follow a two-step process of first 
identifying the lowest observed effects level and then applying a 
margin of safety has also been rejected by the courts. In Lead 
Industries, the Court specifically held that the Administrator need not 
apply a margin of safety at the end of the analytical process but may 
take into account margin of safety considerations throughout the 
process as long as such considerations are fully explained and 
supported by the record. 647 F.2d 1161-1162. Accord, PM10, 
902 F.2d at 973-974.
     Because such factors as the nature and severity of the health 
effects involved, the size of the sensitive population(s) at risk, the 
types of health information available, and the kind and degree of 
uncertainties that must be addressed will vary from one pollutant to 
another, the most appropriate approach to establishing a NAAQS with an 
adequate margin of safety may be different for each standard under 
review. Thus, no generalized paradigm such as that imbedded in EPA's 
cancer risk policy can substitute for the Administrator's careful and 
reasoned assessment of all relevant health factors in reaching such a 
judgment. As noted in this unit, both Congress and the courts have left 
to the Administrator's discretion the choice of analytical approaches 
and tools, including risk assessments, rather than prescribing a 
particular formula for reaching such determinations.86 
Because of the inherent uncertainties that the Administrator must 
address in margin of safety determinations, they are largely judgmental 
in nature, particularly with respect to non-threshold pollutants, and 
may not be amenable to quantification in terms of what risk is 
``acceptable'' or any other metric. In view of these considerations, 
the task of the Administrator is to select an approach that best takes 
into account the nature of the health effects and other information 
assessed in the air quality criteria for the pollutant in question and 
to apply appropriate and reasoned analysis to ensure that scientific

[[Page 38689]]

uncertainties are taken into account in an appropriate manner.
---------------------------------------------------------------------------

    86 Contrary to one of the comments received, EPA's use of risk 
assessment in this rulemaking is by no means a departure from past 
practice. The EPA first considered and began applying risk 
assessment methods in the late 1970's (44 FR 8210, 8211, February 8, 
1979).
---------------------------------------------------------------------------

     In this instance, the Administrator has clearly articulated the 
factors she has considered, the judgments she has had to make in the 
face of uncertain and incomplete information, and alternative views as 
to how such information should be interpreted, in reaching her decision 
on standard specifications that will protect public health with an 
adequate margin of safety. See Unit II. of this preamble. Her 
conclusions on these matters are fully supported by the record.

C. Data Availability

     Several commenters questioned EPA's ability to rely on studies 
demonstrating an association between PM and excess mortality without 
obtaining and disclosing the raw ``data'' underlying these studies for 
public review and comment. In particular, a number of commenters cited 
Dockery, D.W., et al. 1993 and Pope, C.A. III, et al., 1995, as studies 
upon which EPA relied without obtaining and disclosing the underlying 
raw data. One commenter also cited J. Schwartz et al., 1996 in the same 
context.87 According to the commenters, without the 
underlying data used in these studies, the reliability of these studies 
cannot be assessed accurately. These commenters requested that EPA 
obtain the relevant data and make it available for public review. In 
light of the court-ordered requirement that EPA publish its rule by 
July 19, 1997, the commenters argued that EPA must retain the current 
PM10 NAAQS pending additional review of the raw data and the 
studies at issue. One commenter, the American Petroleum Institute (API) 
requested that EPA remove the studies from the docket, unless the 
underlying data was also included in the docket.88
---------------------------------------------------------------------------

    87 Contrary to this commenter's assertion, both the health and 
air quality data used in the 1996 Schwartz study are available to 
interested parties. EPA's Office of Research and Development 
maintains a copy of the air pollution database used in the Schwartz 
study and it has previously been made available in response to 
Freedom of Information Act requests from interested parties, such as 
the American Iron and Steel Institute (AISI). The Harvard School of 
Public Health has also made this data available to several 
collaborators and to the Health Effects Institute. With regard to 
the health data underlying the Schwartz study, that mortality data 
was compiled by the National Center for Health Statistics (NCHS) and 
can be purchased from the NCHS by interested parties. Thus, there is 
no real data availability concern with regard to the 1996 Schwartz 
study. However, even were this not the case, for the reasons 
discussed more fully in this unit and elsewhere in the preamble, EPA 
believes it would be entitled to rely upon this study and other 
studies, including the Dockery and Pope studies, regardless of the 
availability of the underlying health data.
    88 API's letter stated that ``API petitions EPA to identify all 
studies that rely, in any way, on data not available for public 
review as part of the rulemaking process and remove those studies 
from the record.'' To the extent this letter constitutes a 
``petition'' for EPA action, EPA hereby denies the ``petition'' for 
the reasons stated in this unit and elsewhere in this preamble.
---------------------------------------------------------------------------

     A few commenters argued that section 307(d) of the Act requires 
that EPA obtain the raw data underlying these studies and that a 
failure to do so contradicts the plain language of section 307(d)(3) of 
the Act, which requires EPA to place in the docket any ``factual data 
on which the proposed rule is based.'' Other commenters argued that 
under section 307(d)(8) of the Act, a failure to obtain and disclose 
the underlying raw data used in the studies would constitute an error 
``so serious and related to matters of such central relevance to the 
rule that there is a substantial likelihood that the rule would have 
been significantly changed if such errors had not been made.'' Id. 
According to one commenter, without the raw data and an opportunity for 
an analysis of it, ``EPA has no legal alternative other than to 
conclude that no new air quality standard would be appropriate within 
the meaning of CAA section 109(a)(1)(B).'' Finally, a number of 
commenters have argued that recent caselaw under the Clean Air Act and 
other statutes makes clear that EPA has a legal obligation to obtain 
and disclose the data used in these studies.89
---------------------------------------------------------------------------

    89 One commenter argued that the failure to obtain and disclose 
the underlying data was a violation of the Administrative Procedure 
Act (APA). The NAAQS rulemaking is promulgated under section 307(d) 
of the Act; the APA generally does not apply to such rulemakings. 
See section 307(d)(1) of the Act.
---------------------------------------------------------------------------

     In developing the proposed revisions to the PM NAAQS, the 
Administrator relied on the scientific studies cited in the rulemaking 
record, rather than on the raw data underlying them.90 In 
this case, the raw data consists of responses to health questionnaires 
based on information supplied by individual citizens, or computer 
tabulations of this information, which remains confidential, and air 
quality and monitoring data, most of which is now publicly available. 
EPA does not generally undertake evaluations of raw, unanalyzed 
scientific data as part of its public health standard setting process. 
Only in extreme cases--for example where there are credible allegations 
of fraud, abuse or misconduct--would a review of raw data be warranted. 
It would be impractical and unnecessary for EPA to review underlying 
data for every study upon which it relies as support for every proposed 
rule or standard. If EPA and other governmental agencies could not rely 
on published studies without conducting an independent analysis of the 
enormous volume of raw data underlying them, then much plainly relevant 
scientific information would become unavailable to EPA for use in 
setting standards to protect public health and the environment. In 
addition, such data are often the property of scientific investigators 
and are often not readily available because of the proprietary 
interests of the investigators or because of arrangements made to 
maintain confidentiality regarding personal health status and lifestyle 
information of individuals included in such data. Without provisions of 
confidentiality, the possibility of conducting such studies could be 
severely compromised.91
---------------------------------------------------------------------------

    90 It is important to note that while EPA did use the Dockery 
and Pope studies to confirm its conclusions regarding the health 
effects of fine particulate air pollution and thus as support for 
its decision to revise the PM standard, these studies do not provide 
the sole (or even primary) basis for EPA's decision regarding 
PM2.5, despite the assertions of numerous commenters. The 
proposed standards are based on a consideration of a large body of 
epidemiological studies, a clear majority of which suggest PM is 
strongly linked to mortality and other serious health effects at 
concentrations permitted under the current standards. Although the 
specific levels of the PM2.5 standards are based on a 
more limited number of studies that actually measured fine particles 
and/or components of fine particles, the Dockery and Pope studies 
were not used in initially selecting the annual fine particle 
standard level, which was principally based on examination of other 
daily mortality and respiratory effects studies (Koman, 1996, 1997) 
that found significant associations between fine PM and effects in 
cities with annual average PM2.5 concentrations of about 
16 to 21 g/m3. Only then were the long-term Dockery and 
Pope studies examined and used to help corroborate this result; in 
the opinion of the Administrator, neither study alone (or together) 
provided sufficient evidence to support more stringent levels below 
those identified from the daily studies. Thus, removal of the 
Dockery and Pope studies would not affect the conclusions about the 
significance of the risks and therefore, while these long-term 
studies tend to strengthen the need for fine particle control and 
provide important insights into the nature of PM effects, removal of 
these two studies from consideration would not have changed the 
selected standard level.
    91 Some commenters noted that with regard to the health data 
underlying the 1993 Dockery and 1995 Pope studies, since EPA 
provided partial funding for these studies, EPA has access to this 
data and cannot shield itself from the duty to obtain this data by 
claiming that it is not in its possession. Although a legal argument 
potentially exists that EPA may obtain access to such data, this 
legal argument has not been tested in the courts. More importantly, 
EPA's ability to rely on studies without reviewing the raw data 
should not depend on whether some Agency of the Federal government 
funded the science.
---------------------------------------------------------------------------

    In this case, the merits of the studies considered and used in 
developing the PM2.5 standard have been discussed and 
debated extensively over the past several years, both as part of the 
EPA review of the pertinent science and in a number of other public 
forums. The studies at issue were critically evaluated

[[Page 38690]]

by the Agency's Office of Research and Development (ORD) and by the 
EPA's independent Clean Air Scientific Advisory Committee (CASAC), in a 
multi-year process for assessment of the science at issue. As with 
other studies on which EPA relied, particular attention was given to 
the strengths and limitations of the Dockery, Schwartz and Pope studies 
during this process, which involved numerous opportunities for public 
participation and extensive input from interested parties. The results 
of these studies are not only consistent with each other, but they are 
also consistent with the results of other studies demonstrating 
significant associations between long-term exposure to fine particle 
indicators and mortality. See U.S. EPA, 1996b, p. V-62. The CASAC 
concluded that EPA's assessments of the pertinent science properly 
characterized both the current state of knowledge and the range of 
policy options for revising the standards.
    In fact, many peer reviewed studies have reported associations 
between PM and premature death; the Dockery, Schwartz and Pope studies 
are among the most recent studies to corroborate this association. In 
the early 1990s, several studies were published showing associations at 
levels below the current PM standards. Some critics began raising 
questions about the extent to which the results could be reproduced and 
the unavailability of underlying data. In response, an independent 
group of investigators under the auspices of the Health Effects 
Institute (HEI), a highly respected research organization jointly 
funded by EPA and several motor vehicle manufacturers, undertook a 
reanalysis of several such studies. The original investigators of 
several studies, including studies conducted at Harvard University, 
Brigham Young University, and the San Francisco Bay Area Air Quality 
Management District provided their raw air quality data sets to the HEI 
investigation team for reanalysis. HEI's reanalysis produced numerical 
results from the data sets for all six cities that closely agree with 
and, in general, confirm the results of the original investigators. 
Thus, as noted in Unit II. of this preamble, these reanalyses by 
respected independent scientists confirmed the reliability and 
reproduceability of prior work of the original investigators, including 
work by Dockery et al. (1992), Pope et al. (1992), Schwartz and Dockery 
(1992a), and Schwartz (1993).
    Thus, the 1993 Dockery and 1995 Pope studies build upon previous 
studies done by a number of different researchers and have been subject 
to an extensive peer review process by EPA's ORD, CASAC and HEI. They 
also underwent a peer review process at the time of their publication 
in reputable scientific journals. Given the consistency and coherence 
of the scientific evidence and the scrutiny the studies have received 
in peer review and in the extensive scientific review process described 
in this unit, EPA does not agree that review of the underlying data for 
these studies is also necessary. Considering the various reviews 
described in this unit and the fact that EPA has received no specific 
and substantiated reason, such as plausible allegations of fraud or 
scientific abuse, to doubt the overall validity of their conclusions, 
EPA agrees with CASAC that revision of the standard is appropriate, 
based on these and other studies.
    In spite of EPA and CASAC's conclusion that it is appropriate to 
rely on the Pope, Dockery and other studies to establish a 
PM2.5 NAAQS, EPA also believes in public disclosure and 
supports efforts to seek appropriate release of data underlying the 
studies in question. On January 31, 1997, EPA wrote to the principal 
scientific investigators at the Harvard School of Public Health and at 
Brigham Young University and urged them to make the data associated 
with their studies available to interested parties. Studies conducted 
by these investigators relied on data compiled as part of the Harvard 
Six-Cities Study and data compiled by the American Cancer Society (ACS) 
as part of the Cancer Prevention Study II.
    The studies in question combined health data on individuals with 
air pollution data. The air pollution data are publicly available. The 
health data consist of personal and confidential information, e.g. age, 
sex, weight, eduction level, smoking history, occupational exposures, 
medical history. These data are not publicly available. In compiling 
these data, researchers have promised study participants that private, 
personal information would be kept confidential under signed assurances 
of confidentiality. Data-sharing arrangements with outside parties 
must, therefore, accommodate interests both in making data accessible 
and in protecting the confidentiality of the information contained 
within them.
    Both the Harvard School of Public Health and the American Cancer 
Society have made such arrangements. Both have processes which allow 
ouside scientists, in collaboration with Harvard and ACS researchers, 
to access their databases for the conduct of legitimate scientific 
research. Scientists from all over the world have applied for and have 
been granted such access and numerous studies have been conducted and 
published using the databases.
    Because of increased interest resulting from EPA's rulemaking on PM 
standards and at the request of the Harvard School of Public Health, 
HEI is taking additional steps to provide a forum for outside 
researchers to access health data associated with the Harvard-Six 
Cities Study and perhaps others. HEI has convened an expert panel of 
esteemed scientists to access underlying data and to conduct additional 
reanalyses. This arrangement appears to provide a constructive venue 
for testing legitimate scientific hypotheses while protecting the 
confidentiality of the underlying data.
    Nevertheless, as noted previously, EPA has full confidence in the 
scientific integrity of the Dockery, Schwartz, and Pope studies and 
their suitability for use in the Agency's rulemaking on PM, without 
undertaking a separate or additional review and analysis of the 
underlying raw data. The decision to propose revisions of the current 
PM standards was based on careful assessment of the scientific and 
technical information presented in the PM Criteria Document and Staff 
Paper. The decision was also consistent with the consensus of CASAC 
that ``although an understanding of health effects of PM is far from 
complete, 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.'' The extensive PM 
epidemiological data base provides evidence that serious adverse health 
effects, e.g., mortality, exacerbation of chronic disease, increased 
hospital admissions, respiratory symptoms, and pulmonary function 
decrements, in sensitive subpopulations, e.g., the elderly, individuals 
with cardiopulmonary disease and children, are attributable to PM at 
levels below the current standards. The increase in risk is significant 
from an overall public health perspective because of the large number 
of individuals in sensitive subpopulations that are exposed to ambient 
PM and the significance of the health effects. These considerations, as 
well as others discussed in the proposal and Staff Paper, such as the 
need to consider fine and coarse particles as distinct classes, led 
both the Administrator and CASAC to conclude that revision of the 
current standards is clearly appropriate. This conclusion remains 
unchanged despite the fact that EPA is without the actual raw and

[[Page 38691]]

unanalyzed health data underlying the studies.
     A number of commenters cited section 307(d) of the Act in support 
of their position that EPA is required to obtain and disclose the 
underlying raw data. Under section 307(d)(3) of the Act, EPA is 
required to issue a notice of proposed rulemaking in the Federal 
Register that is accompanied by a ``statement of basis and purpose'' 
that includes ``a summary'' of:
     (A) The factual data on which the proposed rule is based.
     (B) The methodology used in obtaining the data and in analyzing 
the data.
    Thus, it is clear from the language of section 307(d) of the Act 
that where EPA relies on any ``data'' as support in its rulemakings 
under the Clean Air Act, it has an obligation to include such data or 
information in the rulemaking docket that is open to the public. Where 
EPA fails to do so and the error is ``so serious and related to matters 
of such central relevance to the rule that there is a substantial 
likelihood that the rule would have been significantly changed if such 
errors had not been made,'' a reviewing court may overturn the rule.
     In this case, as noted previously, EPA did not rely upon the raw 
health data supporting the Dockery and Pope studies; it relied instead 
upon the studies themselves. These studies may properly be considered 
``data.'' The EPA has never had the raw health data in its possession; 
thus EPA has neither reviewed it nor had an opportunity to place it in 
the docket. The EPA did rely on the studies and these studies are 
included in the docket and are available for public review. Because EPA 
neither reviewed nor relied upon the raw data, there is no obligation 
to obtain it or to make it available.
     Some commenters argued that the language of section 307(d) of the 
Act, which refers to the ``factual data'' and which also discusses the 
``methodology used in obtaining and analyzing the data'' distinguishes 
between raw data and studies. In the view of these and other 
commenters, the plain language of section 307(d) of the Act requires 
that EPA obtain and disclose the raw data used in the Dockery and Pope 
studies. According to these commenters, without such raw ``data,'' EPA 
cannot legally promulgate its rule.
     The EPA disagrees with this narrow interpretation of the word 
``data'' and of section 307(d) of the Act. Data can take many forms, 
including studies, reports, tabulations, graphs and summaries, as well 
as more raw forms, such as questionnaire responses, test results and 
even actual physical specimens. The ``factual data'' called for by 
section 307(d) of the Act may clearly include peer-reviewed scientific 
studies. Nor does section 307(d) of the Act prohibit EPA from relying 
on a study for standard setting without obtaining the raw, underlying 
data supporting a study. Indeed, as noted in the legislative history to 
section 307(d) of the Act,

    * * * [t]he [House Commerce] Committee recognizes that the 
factual support needed for a rule may vary greatly according to the 
subject being addressed and that rules on some subjects, such as 
procedures, may not require any factual basis at all. There is no 
intention to increase the amount of `factual' support now required 
to support `policy judgments where no factual certainties exist or 
where facts alone do not provide the answer,' Industrial Union 
Department, AFL-CIO v. Hodgson, 499 F.2d 467, 476 (D.C. Cir. 1974). 
Nor is there any intent to diminish the Administrator's authority to 
adopt precautionary regulations based on a showing of risk * * * .

H.R. Rep. No. 95-294, at 323 (1977) (footnote omitted). As this 
legislative history makes clear, the language in section 307(d) of the 
Act is not intended to require EPA to change the amount of ``factual 
support'' that EPA must assemble in order to promulgate a rule and EPA 
may adopt ``precautionary'' regulations ``where no factual certainties 
exist.'' Given this clarification in the legislative history, it is 
evident that EPA is entitled under section 307(d) of the Act to rely on 
studies rather than raw data in developing its Clean Air Act rules, 
despite the arguably ambiguous use of the term ``data.''92
---------------------------------------------------------------------------

    92 EPA also does not agree that because the language of section 
307(d) of the Act mentions ``factual data'' as well as ``the 
methodology used in obtaining and analyzing the data,'' EPA cannot 
rely on a study alone. In this case, the study is the ``factual 
data'' and EPA's methodology used in obtaining and analyzing the 
``factual data'' is the method that EPA used to review and rely upon 
the studies. This methodology is discussed extensively in the staff 
paper and summarized in some detail elsewhere in this preamble. In 
fact, as is clear from the overall structure of section 307(d) of 
the Act, as well as the legislative history cited in this unit, 
section 307(d) of the Act merely requires that EPA summarize and 
disclose the information and methodology that it relied upon in 
developing its rule. It leaves unchanged the ``level'' of support 
that an agency must bring to bear in drafting a proposed rule.
---------------------------------------------------------------------------

    Moreover, EPA has relied on studies in the past (including studies 
using the undisclosed Six Cities data) without obtaining or disclosing 
the underlying raw data, and EPA's reliance on such studies to set 
Clean Air Act standards has been upheld in court. In NRDC v. EPA, 902 
F. 2d 962 (D.C. Cir. 1990), the D.C. Circuit declined to delay its 
review of the PM10 NAAQS rulemaking due to concerns raised 
by the American Iron and Steel Institute about the integrity of the Six 
Cities data base. 902 F.2d at 974. In that case, EPA had relied upon an 
earlier Dockery study based on the Six Cities data base. Although the 
National Institutes of Health (NIH) undertook a review of the 
allegations regarding the Six Cities database, the court nevertheless 
upheld EPA's reliance on that Dockery study without waiting for the 
results of the NIH review. NIH eventually concluded that the 
allegations were without merit. According to the court in the NRDC 
case:

    AISI claims that the EPA relied too much on the Six Cities 
Study, which is comprised of the Dockery study and the Ware study * 
* * . We do not agree that the Administrator's selection of the 
twenty-four hour standard lacks the necessary reasoned analysis and 
supportive evidence * * * . After carefully reviewing the record, we 
find EPA's selection of the twenty four hour standard reasonable in 
light of the divergent results in the studies and the agency's 
mandate to provide an adequate margin of safety. Studies contained 
in the record provided evidence of adverse health effects at levels 
below 250 g/m3.

902 F.2d at 969 (footnotes omitted; emphasis in original). The court 
also stated that:
    In setting a standard under section 109 of the Act, the 
Administrator must ``take into account all the relevant studies 
revealed in the record`` and ``make an informed judgment based on 
available evidence.'' American Petroleum Institute v. Costle, 665 
F.2d at 1187. The record shows that the Administrator did so. The 
Administrator relied on studies which showed adverse effects at and 
below the 250 g/m3 level. AISI essentially asks this court 
to give different weight to the studies than did the Administrator. 
We must decline. It is simply not the court's role to ``second-guess 
the scientific judgments of the EPA. * * * [T]he Administrator did 
not act arbitrarily in drawing conclusions from the uncertain and 
conflicting data. The Administrator may reasonably apply his 
expertise to draw conclusions from ``imperfect data,'' Ethyl Corp., 
541 F.2d at 28, as he did here.

Id. at 971.
    As this language makes plain, the term ``data'' may include a study 
relied upon by EPA. It should be equally plain that EPA may properly 
rely on such a study in setting a standard despite the fact that such 
``data'' may be ``imperfect,'' ``conflicting,'' and ``uncertain.'' 
There are numerous other cases in which EPA has relied on studies in 
setting standards under the Clean Air Act. See, e.g., Engine 
Manufacturers Association v. EPA, 88 F. 3d 1075, 1099 (D.C. Cir. 
1996)(upholding EPA's use of the 1993 Dockery study for setting mobile 
source standards); API v. Costle, 665 F.2d 1176, 1185 (D.C. Cir. 
1981)(Administrator's conclusion that normal body functions

[[Page 38692]]

are disrupted by ozone is ``supported by the studies'').
     A number of commenters cited Endangered Species Committee v. 
Babbitt, 852 F. Supp. 32 (D.D.C. 1994) (hereafter ``Gnatcatcher'') in 
support of the proposition that EPA must obtain and disclose the raw 
data underlying the Dockery and Pope studies. Relying on cases such as 
Connecticut Light and Power Co. v. NRC, 673 F.2d 525 (D.C. Cir. 1982), 
Portland Cement v. Ruckelshaus, 486 F.2d 375 (D.C. Cir. 1973), and 
United States v. Nova Scotia Food Processing Corp, 568 F.2d 240 (2nd 
Cir. 1977), these commenters suggest that ``a body of legal decisions 
is emerging whereby federal courts are increasingly dubious of final 
regulatory decisions that are being made absent public scrutiny of the 
data underlying and purportedly supporting such decisions.'' According 
to these commenters, based on Gnatcatcher and other cases, failure by 
EPA to obtain and place in the docket the raw unanalyzed data used in 
the Dockery and Pope studies constitutes serious procedural error under 
the Clean Air Act.
     Under Connecticut Light and Power, agencies must make available 
technical studies and data that have been relied upon during the 
rulemaking process in order for the public to have an adequate 
opportunity for notice and comment. There is no question that EPA has 
done this with regard to the Dockery and Pope studies, which are 
included in the rulemaking docket. The Portland Cement case makes clear 
that where an agency actually relies on factual data it cannot 
``promulgate rules on the basis of inadequate data, or on data that, 
[to a] critical degree, is known only to the agency.'' 486 F.2d at 393. 
See also, Nova Scotia, 568 F.2d 240, at 251 (where all of the research 
was collected by the agency, and none of it was disclosed ``as the 
material upon which the proposed rule would be fashioned,'' error 
resulted); CMA v. EPA, 870 F.2d 177, 200 (5th Cir. 1989) (``fairness 
requires that the agency afford interested parties an opportunity to 
challenge the underlying factual data relied on by the agency'').
     However, in this case, EPA did not rely on, nor did it ever have 
or review, the underlying data used in the Dockery and Pope studies. 
Instead, it relied upon the studies themselves. Thus, the cases cited 
in this unit are inapposite. They stand only for the proposition that 
where an agency actually reviews and relies on ``data,'' which may be 
raw data, a study or a variety of other forms of information, it must 
make these data available. They do not and cannot stand for the 
proposition that an agency may not rely on a study alone and must 
always obtain the raw and unanalyzed data underlying a study. Indeed, 
as one D.C. Circuit case noted: ``Portland Cement and Nova Scotia 
simply cannot be twisted so as to require notices of proposed or 
interim rules to contain elaborate reproductions of underlying 
studies.'' Petry v. Block, 737 F.2d 1193, 1198 (D.C. Cir. 1984). 
Requiring EPA to obtain, analyze and disclose the data underlying the 
Pope and Dockery studies, which EPA neither reviewed nor relied upon, 
would be to require EPA to attempt such an ``elaborate reproduction.'' 
Such a step is not required under the law and would make it extremely 
difficult, if not impossible, for EPA to regulate in complex, technical 
areas ``at the frontiers of science.'' Baltimore Gas and Electric Co. 
v. NRC, 462 U.S. 87 (1983).
     The district court's decision in the Gnatcatcher case is similarly 
inapposite. That case concerned a scientific study regarding the range 
of the California Gnatcatcher, a small insectivorous songbird. As the 
Gnatcatcher opinion itself notes, ``courts have generally allowed 
agencies to rely on scientific reports.'' Gnatcatcher, 852 F.Supp. at 
37. Thus, the question at issue in Gnatcatcher was whether specific 
circumstances exist in which an agency may not be entitled to rely on 
studies alone. In the Gnatcatcher case, a single author had published 
two directly contradictory studies on the same issue, while relying on 
the same data. In light of this clear contradiction, commenters in that 
rulemaking argued that without the underlying data it was impossible to 
determine whether the conclusions in either study were correct. The 
district court noted that:

    The Secretary had before him a report by an author who, two 
years before had analyzed the same data and come to an opposite 
conclusion. It is the disputed nature of this report that 
distinguishes this from other cases where a scientific report alone 
has been considered sufficient for ESA purposes.

Id. Thus, according to the court: ``While courts have generally allowed 
agencies to rely on scientific reports * * * this is not sufficient in 
this case because the report itself is under serious question.'' Id.
     The EPA's current reliance on the Dockery and Pope studies bears 
no resemblance to the circumstances present in the Gnatcatcher 
decision. As noted previously, these studies have been subject to 
extensive peer review and scrutiny, and neither researcher has 
published a contradictory study on the same issue, much less using the 
same data base. The EPA is not aware of, nor have any of the commenters 
raised any particular issues relating to either gross error, fraud or 
scientific abuse arising from the data. Indeed, as noted previously, 
the prior work of these particular researchers has been subject to 
extensive independent scrutiny and reanalysis, which has confirmed, 
rather than called into question, the underlying validity of their 
conclusions and the integrity of their research methods. Reading 
Gnatcatcher to suggest that EPA cannot rely on such a study, where the 
study and its methods have been subject to extensive peer review, would 
place the district court's rationale in Gnatcatcher in conflict with 
applicable D.C. Circuit precedent that makes evident the right of 
agencies to rely on studies alone. See, e.g., Engine Manufacturers 
Association v. EPA, 88 F.3d 1075, 1099 (D.C. Cir 1996); API v. Costle, 
665 F.2d 1176, 1185 (D.C. Cir. 1981), ``studies discussed in the 
Criteria Document constitute a rational basis for the finding that 
adverse health effects occur at ozone levels of 0.15-0.25 ppm for 
sensitive individuals''; see also, NRDC v. Thomas, 805 F.2d 410, 418 
(D.C. Cir. 1986)(EPA use of a summary of confidential data that was not 
disclosed provides ``a reasoned explanation for moving from a 4.0 to 
5.0 long term NOx standard'').
     In addition, to require EPA to obtain and analyze the data prior 
to revising the standard would also contradict the ``common sense 
notion that Congress, in providing for notice and comment under the 
APA, could not have intended to subject the agencies--and the public on 
whose behalf they regulate--to [a] sort of interminable back and 
forth.'' International Fabricare Institute v. EPA, 972 F.2d 384, 399 
(D.C. Cir. 1992). In the view of some commenters, EPA has no choice but 
to either postpone its decision for a year or more awaiting a review of 
data or choose to retain the current standard. Yet were EPA to adopt 
such an approach, these commenters would undoubtedly insist that EPA be 
required to include an analysis of the data in the docket; further 
questions would likely be raised regarding the re-analysis and once 
again EPA might find itself unable to promulgate its rule pending 
review of further hypothetical questions. This type of unending inquiry 
is not required under the law. As the D.C. Circuit has noted:

    * * * [D]isagreement among the experts is inevitable when the 
issues involved are at the ``very frontiers of scientific 
knowledge,'' and such disagreement does not prevent us from finding 
that the Administrator's decisions are adequately supported by the 
evidence in the record * * * . It is not our function to resolve 
disagreement among the experts or to judge

[[Page 38693]]

the merits of competing expert views * * * . Cf. Hercules, Inc. v. 
EPA, 598 F.2d 91,115 (D.C. Cir. 1978) (``[c]hoice among scientific 
test data is precisely the type of judgment that must be made by 
EPA, not this court'').

Lead Industries Association v. EPA, 647 F.2d 1130, 1160 (D.C. Cir. 
1980).
     Neither Gnatcatcher, nor any other case can fairly be read to 
suggest that EPA has an obligation to respond to all possible questions 
that might be raised regarding its scientific conclusions or that where 
EPA relies on a study, it must engage in a multi-phased and possibly 
unending re-examination of the data supporting such a study until all 
commenters are satisfied in full with the details of the underlying 
science. Even assuming that EPA could obtain the confidential Six 
Cities data through litigation, a substantial delay of many months, if 
not years, would likely result, in order for both EPA and industry to 
reanalyze the data. In the meantime, some tens of thousands of 
premature deaths could result. Neither the Clean Air Act nor relevant 
case law requires or permits such a result.
     Indeed, the suggestion that EPA cannot and should not rely upon 
the Pope, Dockery, and Schwartz studies, unless and until interested 
parties have had an opportunity to examine and reanalyze the underlying 
raw data, is extraordinary. Given the precautionary nature of section 
109 of the Act and the need to allow an adequate margin of safety, see 
Lead Industries, 647 F.2d at 1154, 1155, there are limits on EPA's 
discretion to disregard even studies that are clearly flawed, if they 
are nonetheless ``useful'' in indicating the kind and extent of health 
effects that may result from the presence of a pollutant in the ambient 
air. See sections 109(b)(1) and 108(a)(2) of the Act.
     A few commenters cited Kennecott v. EPA, 684 F.2d 1007 (D.C. Cir. 
1982) and argued that under sections 307(d)(8) and 307(d)(9)(D) of the 
Act, a failure by EPA to obtain and include in the docket the data 
underlying the Pope and Dockery studies would constitute an ``error'' 
that is ``so serious and related to matters of such central relevance 
to the rule that there is a substantial likelihood that the rule would 
have been significantly changed if such error[] had not been 
made.''93 EPA disagrees. Peer reviewed studies conducted by 
outside parties were not at issue in Kennecott. Kennecott involved a 
dispute over financial analyses that EPA itself had previously 
conducted and used in earlier rulemakings. The court determined that 
the financial analyses at issue must have provided at least part of the 
factual basis for EPA's rule, and EPA referenced these analyses in the 
preamble to the final rule without placing them in the docket until one 
week before promulgation. The factual circumstances in Kennecott are 
substantially different than the current situation and thus, Kennecott 
cannot fairly be read to establish the applicable legal standard with 
regard to EPA's reliance on peer reviewed studies for use in setting 
the NAAQS.
---------------------------------------------------------------------------

    93 One commenter argued that EPA's failure to place the ``data'' 
in the docket was not an ``error'' but a ``refusal to comply with 
the clear language of the law that should be reviewed by the courts 
under section 307(d)(9)(C), rather than 307(d)(9)(D).'' As noted 
previously, EPA does not agree with this interpretation of section 
307(d)(3) of the Act. Under applicable caselaw, the term ``data'' 
may include information in many forms, including studies that EPA 
has placed in the docket. See Endangered Species Committee v. 
Babbitt, 852 F. Supp. 32, 37 (D.D.C., 1994) (``data can come in many 
forms: it can be a scientific report, it can be graphs and 
tabulations * * * it can be raw numbers'').
---------------------------------------------------------------------------

    In this case, EPA--well before proposal--has placed the information 
that it relied upon in the docket. This information is in the form of 
studies. These studies have been subject to extensive scrutiny and peer 
review. To date no specific allegation has been made that the studies 
are clearly in error or that the data underlying them are the subject 
of fraud, scientific misconduct, or gross error going to the basic 
validity of the studies.94 Instead, various commenters have 
merely stated their view that were the raw data behind these studies 
available, they would be able to better verify and assess the results 
reached in the studies.
---------------------------------------------------------------------------

    94 A number of commenters did argue these studies do not form a 
sufficient basis for EPA's decision to revise the NAAQS and that 
attempts to replicate these studies have not been universally 
successful. These same commenters also listed a number of 
hypothetical questions and issues that might be resolved through a 
review of the underlying data and suggested that before EPA may 
properly rely on these studies to revise the NAAQS, a variety of 
confounders (such as smoking) should also be ruled out by reviewing 
the data. As set forth more fully in Unit II. of this preamble, 
neither EPA nor CASAC agrees that any of these factors precludes 
reliance on the studies in question.
---------------------------------------------------------------------------

     As one commenter noted, ``In the absence of data on which EPA's 
proposal is based, [key scientific] issues remain shrouded in 
uncertainty and skepticism. The disclosure of the data would allow for 
robust scientific analysis and discussion of these issues.'' A 
similarly hypothetical concern is raised by another commenter who 
stated that ``seeing the data would clarify substantial questions of 
methodology'' and ``had the Harvard data been available, a far broader 
evaluation of the defects of the Harvard Studies would have been 
possible with the same expenditure of time and money.'' Yet, despite 
having spent ``in the neighborhood of a million dollars to duplicate 
and reanalyze the Harvard data set'' this commenter was unable to 
allege any particular defect in the methodology or results of these 
studies and noted instead that ``the track record to date suggests that 
the claimed associations to PM2.5 and health effects would 
not have held up under such a broader evaluation.''
     EPA is not required to await the results of such an inquiry before 
proceeding to regulate to protect human health and the environment. The 
concerns raised by the commenters regarding these studies remain 
hypothetical; the comments themselves raise no allegations of fraud, 
scientific misconduct or gross error that calls into question the 
fundamental validity of the studies. Given this fact, EPA does not 
agree with the commenters that reliance on these studies and/or a 
failure to place the underlying data in the docket constitutes an 
error, much less an error that is ``so serious and related to matters 
of such central relevance that there is a substantial likelihood that 
the rule would have been significantly changed.'' EPA is entitled to 
rely upon these studies and it has satisfied its obligation to provide 
the ``factual data'' upon which the proposed rule is based by placing 
these studies in the docket.
    In fact, the concerns raised by the commenters ultimately boil down 
to a disagreement with EPA over the level of scientific certainty 
necessary to adopt the NAAQS revisions. In setting standards under the 
Clean Air Act, EPA is not required to resolve all scientific issues to 
the complete satisfaction of every interested party. As noted by the 
D.C. Circuit in Lead Industries Association v. EPA, 647 F.2d 1130, 1160 
(D.C. Cir. 1980):

     To be sure, the Administrator's conclusions were not 
unchallenged; both LIA and the Administrator are able to point to an 
impressive array of experts supporting each of their respective 
positions. However, disagreement among the experts is inevitable 
when the issues involved are at the ``very frontiers of scientific 
knowledge,'' and such disagreement does not preclude us from finding 
that the Administrator's decisions are adequately supported by the 
evidence in the record. It may be that LIA expects this court to 
conclude that LIA's experts are right, and the experts whose 
testimony supports EPA are wrong. If so, LIA has seriously 
misconceived our role * * * . It is not our function to resolve 
disagreement among the experts or to judge the merits of competing 
expert views * * * . Cf. Hercules, Inc., v. EPA, 598 F.2d 91, 115 
(D.C. Cir. 1978) (``[c]hoice among scientific test data is precisely 
the type of judgment that must be made by EPA, not this court'').


[[Page 38694]]


647 F.2d at 1160 (footnotes omitted).
     The EPA's rationale for proposing to add a fine particle standard 
was detailed in the preamble to the proposed rule, most notably at 61 
FR 65654-65662, December 13, 1996. This decision is based on the 
extensive review of the science and policy issues contained in the PM 
Criteria Document and Staff Paper; the CASAC concluded, after extensive 
review, that both of these documents were appropriate for use in 
decision making on standards. These documents contain a full discussion 
of both what is known about PM and the information gaps and 
uncertainties. Considering the full weight of the scientific evidence, 
including the uncertainties, the CASAC recommended that the 
Administrator adopt fine particle standards and a number of panel 
members based their support for a PM2.5 standard on the 
following reasoning:

    [T]here is strong consistency and coherence of information 
indicating that high concentrations of urban air pollution adversely 
affect human health, there are already NAAQS that deal with all of 
the major components of that pollution except PM2.5, and 
there are strong reasons to believe that PM2.5 is at 
least as important as PM10-2.5 in producing adverse 
health effects.

Wolff, 1996.
     Given the consistency and coherence of the evidence that premature 
mortality and sickness occur in large numbers of Americans at 
concentrations permitted by the current standards, it would be 
irresponsible to delay action that would put more appropriate air 
quality goals into place based on the most recent scientific 
information. After a review of the comments submitted, the Agency's 
conclusion that it is appropriate to rely on the existing studies 
remains unchanged.

D. 1990 Amendments

     Contrary to the view expressed in some public comments, the 
provisions of subpart 4 of Part D of Title I of the Act, enacted in 
1990, do not preclude EPA from adopting PM2.5 as an 
additional indicator for PM and establishing standards for 
PM2.5. The provisions of subpart 4 of Part D of Title I of 
the Act simply do not limit EPA's clear authority under section 109 of 
the Act to revise the PM standards.
     The basic contention is that because the provisions of subpart 4 
of Part D of Title I of the Act refer to PM10, they prohibit 
EPA from regulating any other type of PM, for example, by revising the 
existing NAAQS for PM by adopting an ambient air quality standard for 
PM2.5. These provisions, however, do not lead to such a 
conclusion. Moreover, this view ignores provisions indicating that 
Congress believed that EPA could revise any existing NAAQS or adopt a 
new NAAQS.
     At the outset, it should be noted that Congress expressly 
authorized EPA to revise any ambient air quality standard and to adopt 
a new NAAQS in section 109 of the Act. That section, which requires EPA 
to review and revise, as appropriate, each NAAQS every five years, 
contains no language expressly or implicitly prohibiting EPA from 
revising a NAAQS or adopting a new NAAQS. If Congress had intended to 
preclude EPA from reviewing and revising a NAAQS or adopting a new 
NAAQS, which are part of EPA's fundamental functions, Congress would 
have specifically done so. Clearly, Congress knew how to preclude EPA 
from exercising otherwise existing regulatory authority and did so in 
other instances. See section 202(b)(1)(C) of the Act (expressly 
precluding EPA from modifying certain motor vehicle standards prior to 
model year 2004); section 112(b)(2) of the Act (preventing EPA from 
adding to the list of hazardous air pollutants any air pollutants that 
are listed under section 108(a) of the Act unless they meet the 
specific exceptions of section 112(b)(2) of the Act); section 
249(e)(3), (f) and section 250(b) (limiting EPA's authority regarding 
certain clean-fuel vehicle programs). No such language was included 
either in section 109 of the Act or elsewhere in the Act and no such 
implication may properly be based on the provisions of subpart 4 of 
Part D of Title I of the Act.
    Second, other provisions of the Act expressly contemplate EPA's 
ability to promulgate a new or revised NAAQS, and provide no indication 
that such ability is limited to standards other than those whose 
implementation is the subject of subparts 2, 3 and 4 of Part D of Title 
I of the Act. For example, section 110(a)(2)(H)(i) of the Act provides 
that SIPs are to provide for revisions ``from time to time as may be 
necessary to take account of revisions of such national primary or 
secondary ambient air quality standard * * * .'' Section 107(d)(1)(A) 
of the Act provides a process for designating areas as attainment, 
nonattainment, or unclassifiable ``after promulgation of a new or 
revised standard for any pollutant under section 109 * * * .'' Section 
172(e) of the Act addresses modifications of national primary ambient 
air quality standards. Finally, section 172(a)(1) of the Act expressly 
contemplates that EPA may revise a standard in effect at the time of 
enactment of the 1990 Clean Air Act Amendments. Section 172(a)(1)(A) of 
the Act provides EPA with authority to classify nonattainment areas on 
or after the designation of an area as nonattainment with respect to 
``any revised standard, including a revision of any standard in effect 
on the date of the enactment of the Clean Air Act Amendments of 1990.'' 
Plainly, Congress had no intention of prohibiting EPA from revising any 
of the ambient standards in effect at the time of the enactment of the 
1990 amendments.
     Third, the provisions of subpart 4 of Part D of Title I of the Act 
do not support the contention that they somehow preclude EPA from 
exercising its authority to adopt a revised PM NAAQS based on a metric 
other than PM10. The fact that Congress laid out an 
implementation program for the PM standard existing at the time of the 
1990 amendments in no way suggests that Congress intended to preclude 
EPA from exercising the authority it provided EPA to revise the NAAQS 
when the health data on which EPA bases such decisions warranted a 
change in the standard.
     The fact that Congress drafted subpart 4 of Part D of Title I of 
the Act in 1990 to specify the implementation regime for the PM 
standard then in effect, a PM10 standard, in terms that 
explicitly refer to PM10 in no way suggests that Congress 
meant to preclude EPA from adopting a PM standard based on another 
metric if scientific information supported such a change. Obviously, 
PM10 was the standard in existence in 1990 and Congress 
drafted subpart 4 of Part D of Title I of the Act, the purpose of which 
was to delineate an implementation regime for that standard, in terms 
of that standard. There is simply no language in subpart 4 of Part D of 
Title I of the Act that limits EPA's ability to establish a different 
PM standard if such a standard were warranted under section 109 of the 
Act or indicates any implicit intent on the part of Congress to limit 
EPA's authority under section 109 of the Act in such a way. Subpart 4 
of Part D of Title I of the Act simply does not speak to the question 
of whether EPA may establish a PM standard based on a different metric. 
In addition, section 107(d)(4) of the Act, the only provision outside 
of subpart 4 of Part D of Title I of the Act invoked as a basis for the 
view that the Act prohibits EPA from adopting a PM2.5 
standard, does not support that view. That provision simply preserved 
pre-existing designations for ``total suspended particulates,'' the PM 
metric utilized prior to PM10, for certain purposes. It 
provides no suggestion that Congress intended to prohibit EPA from 
adopting

[[Page 38695]]

a metric other than PM10. Indeed, if anything, it indicates 
that Congress was fully aware that EPA had previously changed the PM 
metric used in the PM NAAQS and confirms the view that Congress would 
have explicitly barred EPA from changing the metric had it intended to 
do so.
     Finally, for the reasons stated in this unit, EPA's analysis of 
its ability to implement a PM2.5 standard under the 
provisions of subpart 1 of Part D of Title I does not support the view 
that Congress prohibited EPA from promulgating such a standard. 
Congress clearly specified an approach to the implementation of the 
PM10 standard in the provisions of subpart 4 of Part D of 
Title I of the Act. The EPA believes that the clear and express linkage 
of that approach to the PM10 standard indicates that a 
different PM standard should be implemented under the general 
principles of subpart 1 of Part D of Title I of the Act. That Congress 
directed specifically how EPA and the States should implement the 
PM10 standard does not carry with it the implication that 
Congress intended to prohibit EPA from exercising its otherwise clear 
and express authority to adopt a PM standard based on a different 
metric in order to carry out one of its fundamental missions, the 
establishment of ambient air quality standards to protect public health 
with an adequate margin of safety. It is entirely reasonable and 
logical for Congress to, on the one hand, specify an implementation 
regime for the PM standard in effect at the time of enactment of the 
1990 amendments, but, on the other hand, leave EPA free to exercise the 
authority provided it by Congress in section 109 of the Act to adopt a 
new or revised standard when EPA determined that such a standard was 
needed to protect public health with an adequate margin of safety. 
Congress explicitly required EPA to review and revise as appropriate 
the NAAQS every five years. If Congress did not intend for EPA to 
revise the NAAQS when warranted, it would not have required EPA to 
review and revise them. If Congress had intended to prohibit EPA from 
exercising such a fundamental authority it would have clearly 
specified, as it did in other instances, that EPA could not do so.

V. Revisions to 40 CFR Part 50, Appendix K--Intrepretation of the 
PM NAAQS

    Because the revocation of the existing PM10 standards 
will become effective at a later date (as discussed in Unit VII. of 
this preamble), EPA is retaining 40 CFR part 50, Appendix K, although 
it is being published today in revised format to conform with the 
format of the other appendices in this part. A new Appendix N to 40 CFR 
part 50 explains the computations necessary for determining when the 
primary and secondary PM2.5 and PM10 standards 
being adopted today are met. The discussion in this unit sometimes 
refers to the contents of the new Appendix N as revisions to Appendix 
K, so as to highlight how the new Appendix N differs from the current 
Appendix K.
    Key elements of the new 40 CFR part 50, Appendix N, particularly as 
they differ from those of Appendix K, are outlined in this unit.

A. PM2.5 Computations and Data Handling Conventions

    As discussed in Unit II.E. of this preamble, the form of the annual 
PM2.5 standard is a spatially averaged annual mean averaged 
over 3 years, and the form of the 24-hour PM2.5 standard is 
a 98th percentile concentration averaged over 3 years.
    With regard to the annual PM2.5 standard, the EPA 
proposed a form expressed as the annual arithmetic mean, averaged over 
3 years and spatially averaged over all designated monitoring sites to 
represent population exposures. As discussed in Unit II.E.1. of this 
preamble, the form of the annual PM2.5 standard has been 
clarified to make explicit that implementing agencies have the 
flexibility to base comparison of the standard level with measured 
values from either a single community-oriented site or an average of 
measured values from such monitors within the constraints enumerated in 
40 CFR part 58. The new Appendix N of 40 CFR part 50 reflects this 
clarification. The spatial average, if used, is to be carried out using 
data from monitoring sites designated in a State PM Monitoring Network 
Description in accordance with the provisions of 40 CFR part 58.
    Also, the EPA proposed that, for spatial averaging, the 
requirements for 3 years of data for comparison with the standard be 
fulfilled by the spatial averaging network as a whole, not by 
individual monitors within the network. The EPA received comments 
regarding the application of the 75 percent data completeness 
requirement to spatial averaging. The commenters stated that the 
inclusion or exclusion of a site not meeting the data completeness 
requirements from a spatial average, based on the level of the single 
site average, would bias the spatial average for that year. The EPA has 
responded to the comment by demonstrating in Example 1 in 40 CFR part 
50, Appendix N the application of the data completeness criterion that 
is consistent with a spatially averaged network. Specifically, the 
application of the data completeness requirement has been altered in 
the example if a particular site has quarters in a year that do not 
meet the minimum data completeness requirement. Instead of comparing a 
site's annual average to the level of the standard to decide whether or 
not to keep the site in the calculations, the annual average for all 
the sites (the spatial average) is compared to the level of the 
standard. If the spatial average is above the level of the standard, 
the site is kept in the calculations. If it is below, the site is 
omitted from the calculations.
    The EPA also proposed that averaging over calendar quarters be 
retained for the annual average form of the standard. Although several 
commenters stated that the step of calculating quarterly averages to 
obtain the annual average was unnecessary, the EPA maintains that 
quarterly averages are important to ensure representative sampling in 
areas with extreme seasonal variation.
    Regarding the 75 percent data completeness requirement, the 
proposal stated that a given year meets data completeness requirements 
when at least 75 percent of the scheduled sampling days for each 
quarter have valid data, and high values measured in incomplete 
quarters shall not be ignored but shall be included if their value 
causes the annual calculation to be above the level of the standard. 
Some commenters felt that this treatment was unfair in that measured 
data below the standard in incomplete quarters are not retained. In 
addition, the commenters felt that this could create a bias where a 
single sample could inflate an annual average to a level above the 
standard. The EPA agrees and has incorporated in 40 CFR part 50, 
Appendix N the following provisions.
    (1) A statement has been added that less than complete data may be 
used in certain cases subject to the approval of the appropriate 
Regional Administrator in accordance with EPA guidance for dealing with 
less than complete data. This statement was considered necessary for 
those situations where measured data and air quality analyses would 
indicate that the area met or did not meet the standard although it did 
not exactly meet the data completeness requirements.
    (2) A provision has been added that a minimal amount of data is 
needed before the requirement to retain high values in an incomplete 
quarter comes into effect for the annual standards. Sites with at least 
11 samples but less than 75 percent data completeness in a quarter will 
have to include high values

[[Page 38696]]

if they result in calculated values which are above the level of the 
standard. This provision is based upon the change in sampling frequency 
set forth in the revisions to 40 CFR part 58 which effectively doubles 
the minimum sampling frequency from 1-in-6 day sampling to 1-in-3 day 
sampling. The data completeness requirement for the annual form of the 
standard under the original 1-in-6 day sampling schedule is equivalent 
to a minimum of 37.5 percent under the new sampling schedule of 1-in-3 
days. This is equivalent to a minimum of 11 samples in each quarter. 
Therefore, a minimum of 11 samples in a quarter should be sufficient 
for an annual average above the level of the standard to be used under 
the new sampling schedule.
    (3) In sharp contrast, this minimum requirement was considered 
unnecessary for the 24-hour form of the standard when the 
98th percentile is above the level of the standard. That is, 
for a site with a 98th percentile above the level of the 
standard that does not meet the 75 percent data completeness 
requirement, the 98th percentile would be equivalent to the 
maximum or second maximum daily concentration in that year. While 
adding more data samples up to the minimum data completeness 
requirement of 75 percent could help to ensure that the second maximum 
value (rather than the maximum value) corresponds to the 
98th percentile, this difference is not considered 
significant enough to require some minimal number of data samples when 
dealing with the form of the 24-hour standard.
    With regard to the 24-hour PM2.5 standard, the proposed 
revision to 40 CFR part 50, Appendix K defined the 98th 
percentile as the daily value out of a year of monitoring data below 
which 98 percent of all values in the group fall. The calculation of 
the percentile form has been revised to reflect general comments that 
the form of the standard and its calculation should be simplified. The 
EPA maintains that the revised calculation is consistent with the 
definition of the percentile being that number below which a certain 
percent of the data fall.
    Regarding the expression of the annual standard to the nearest 0.1 
g/m3 and the 24-hour standard to the nearest 1 
g/m3, virtually no commenters disagreed with the 
EPA's proposed approach. The few that did, however, took issue with the 
overall stringency of the standards, not the rationale discussed in the 
proposal. The EPA maintains its position that instrument sensitivity 
and the number of measured values used in calculating the values to be 
compared to the standard, as discussed at length in the proposal, point 
to keeping the expressions of the standards stated in this unit.

B. PM10 Computations and Data Handling Conventions

    As discussed in Unit II.G. of this preamble, the EPA proposed 
retaining the current annual arithmetic mean, averaged over 3 years, as 
the form of the annual PM10 standard, and changing the form 
of the 24-hour PM10 standard to a 98th percentile 
value form, averaged over 3 years. As discussed in Unit II.G. of this 
preamble, the form of the daily PM10 standard has been 
revised to a 99th percentile instead of the 98th 
percentile, and the related calculations have been revised accordingly. 
The same revision described above in Unit V.A. of this preamble to 
simplify the formula used to calculate the percentile form of the 24-
hour PM2.5 standard also applies to the PM10 
99th percentile calculation.
    The revisions made to the annual and 24-hour PM2.5 
standards regarding the 75 percent data completeness requirement also 
apply to the annual and 24-hour PM10 standards. Appendix N 
of 40 CFR part 50 reflects this change.
    As with the PM2.5 standards, the EPA maintains its 
position that instrument sensitivity and the number of measured values 
used in calculating the values to be compared to the standard, as 
discussed in detail in the proposal, point to keeping the expressions 
of the standards to the nearest 1 g/m3 for the 
annual standard and to the nearest 10 g/m3 for the 
24-hour standard.

C. Changes That Apply to Both PM2.5 and PM10 
Computations

    In the proposal, the EPA stated that revisions to 40 CFR part 50, 
Appendix K would not address the treatment of exceptional events data, 
which are considered part of the standards implementation process. 
Since several commenters mentioned the handling of these events in 
conjunction with the proposed revisions to Appendix K, the EPA has 
addressed this concern in Appendix N of 40 CFR part 50, which states 
that whether to exclude, retain, or make adjustments to data affected 
by uncontrollable or natural events is subject to the approval of the 
appropriate Regional Administrator.
    Comments were also received expressing the desire of some areas to 
conduct seasonal sampling, reducing the frequency of monitoring during 
a period of expected low concentrations to save resources. The proposed 
revision to 40 CFR part 50, Appendix K did not prohibit this course of 
action, and referred matters of sampling frequency to 40 CFR 58.13. For 
clarification, 40 CFR part 50, Appendix N adds that exceptions to 
specified sampling frequencies, such as a reduced frequency during a 
season of expected low concentrations, shall be subject to the approval 
of the appropriate Regional Administrator.

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

    Because the revocation of the existing PM10 standards 
will become effective at a later date (as discussed in Unit VII. of 
this preamble), EPA is retaining Appendix J in its current form. A new 
Appendix M to 40 CFR part 50 establishes the reference method for 
measuring PM10 in the ambient air for the revised 
PM10 standards. The discussion in this unit sometimes refers 
to the contents of the new Appendix M as revisions to Appendix J, so as 
to highlight how the new Appendix M differs from the current Appendix 
J. As discussed below, the only revision to the Reference Method for 
PM10 relates to the calculation of the volume of air 
sampled.
    During the course of this standards review, EPA has received a 
number of comments regarding the appropriateness of the current 
practice of adjusting measured PM10 concentrations to 
reflect standard conditions of temperature and pressure (25 deg. C and 
760 mm Hg, respectively), as required by 40 CFR part 50, Appendix J. 
The practice was originally adopted to provide a standard basis for 
comparing all pollutants measured in terms of mass per unit volume 
(e.g., g/m3). As EPA has reviewed the ambient 
standards for gaseous pollutants, however, technical changes have been 
made to express them on a pollutant volume/air volume basis (i.e., ppm) 
that is insensitive to differences in altitude and temperature. Such an 
approach is not applicable to particulate pollutants. The question 
arises whether continuing the past practice of making temperature and 
pressure adjustments for PM is appropriate or necessary.
    Information in the Criteria Document on the health and welfare 
effects of PM provides no clear basis for making such adjustments. 
Recent health effects studies have been conducted in cool and warm 
climates, and in cities at high altitude, e.g., Denver, as well as near 
sea level, e.g., Philadelphia (U.S. EPA, 1996a). These studies provide 
no evidence that risk associated with PM exposures is affected by 
variations in

[[Page 38697]]

altitude. Accordingly, any effect that would be accounted for by 
temperature and pressure adjustments would be below the detection 
limits of epidemiological studies. While extremes of altitude might be 
expected to increase the delivered dose of PM in those not acclimatized 
to such locations, the dosimetric studies summarized in the Criteria 
Document provide no clear support for any quantitative adjustment to 
standard conditions. With respect to welfare effects, visibility is 
directly related to the actual mass of fine particles in the 
atmosphere. Adjustment of PM concentrations collected at higher 
altitudes to standard conditions would therefore lead to an 
overstatement of the effect of PM on visibility in such locations. 
Similarly, there is no evidence in the Criteria Document suggesting 
that effects on materials damage and soiling are dependent on altitude.
    Based on this assessment, EPA proposed to delete the requirement to 
adjust PM10 concentrations to standard conditions of 
temperature and pressure from 40 CFR part 50, Appendix J for the 
revised standards and to make corresponding revisions in 40 CFR 50.3. 
Comments received on this issue were divided. A number of commentors 
supported EPA's proposal for the reasons set forth above. A few States 
opposed the change because the lack of adjustment for very cold 
temperature in areas near sea level could make the standard more 
stringent. Some commentors were concerned that the proposed change 
would relax protection afforded for areas at high altitude. A few 
commentors expressed concern that ``sojourners'' who visit high 
altitude area would have higher ventilation rates and receive reduced 
protection as compared to local residents whose ventilation patterns 
were more adapted to these conditions.
    The EPA does not believe that the localized comparisons regarding 
increased or decreased stringency of standards relative to the proposed 
change are an appropriate rationale for keeping the current adjustment 
for temperature and pressure. The issue is whether the available 
scientific evidence on the health and welfare effects of PM provides a 
basis for continuing with the traditional adjustments. The comments 
with respect to sojourners at altitude are relevant, but this issue was 
considered in reaching the proposed decision. Furthermore, commentors 
provided neither laboratory nor epidemiologic evidence that would 
support their theoretical concerns regarding increased annual or 24-
hour PM effects at altitudes typical of mountainous urban areas in the 
United States.
    Based on its assessment of the available evidence and public 
comments, EPA concludes that a continuation of the practice of 
adjusting PM10 concentrations to standard conditions of 
temperature and pressure is not warranted or appropriate. Accordingly, 
this requirement is not included in 40 CFR part 50, Appendix M and 
corresponding revisions are made in 40 CFR 50.3. In addition, EPA is 
also incorporating the proposed minor modifications to 40 CFR part 50, 
Appendix J in Appendix M.

B. 40 CFR Part 50, Appendix L - New Reference Method for 
PM2.5

    1. Introduction. A new reference method for the measurement of fine 
particles (as PM2.5) in the ambient air has been developed 
for the primary purpose of determining attainment of the new 
PM2.5 standards. The method is described in the new 40 CFR 
part 50, Appendix L, and joins the other reference methods (or 
measurement principles) specified for other criteria pollutants in 
other appendices to 40 CFR part 50.
    In developing the proposed new reference method for 
PM2.5, EPA staff consulted with a number of individuals and 
groups in the monitoring community, including instrument manufacturers, 
academics, consultants, and experts in State and local agencies. The 
approach and key specifications were submitted to the CASAC Technical 
Subcommittee for Fine Particle Monitoring, which held a public meeting 
to discuss the proposed new reference method for PM2.5 and 
related monitoring issues on March 1, 1996. Comments on the proposed 
method were provided orally and in writing by interested parties. The 
Technical Subcommittee indicated their overall satisfaction with the 
method in a letter (Price, 1996) forwarded by CASAC to the 
Administrator.
    On December 13, 1996, EPA proposed the new 40 CFR part 50, Appendix 
L at 61 FR 65676 for public comment. The proposal described in detail 
the approach taken and the design specifications and performance 
requirements for the new PM2.5 sampler. On January 14, 1997, 
EPA held a public hearing on the proposed new 40 CFR part 50, Appendix 
L and associated 40 CFR parts 53 and 58 requirements.
    2. Basic reference method approach. In addition to the primary 
purpose of the new PM2.5 reference method (determining 
attainment of the standards), EPA considered a variety of possible 
secondary goals and objectives that the PM2.5 reference 
method might also fulfill. Subsequently, various alternative 
PM2.5 measurement techniques were evaluated. From this 
analysis, EPA proposed to base its PM2.5 reference method on 
a conventional type sampler that collects 24-hour integrated 
PM2.5 samples on a 47 mm Teflon filter that is subsequently 
moisture and temperature conditioned and analyzed gravimetrically. The 
sampler is a low volume sampler that operates at a flow rate of 1 cubic 
meter per hour, for a total sample volume of 24 m3 for the 
specified 24-hour sample collection period. The sampler is easy to 
operate, operates over a wide range of ambient conditions, produces a 
measurement that is comparable to large sets of previously collected PM 
data in existing databases, and provides a physical sample that can be 
further analyzed for chemical composition.
    3. Public comments and responses--a. Sampler design. The EPA 
received many general comments concerning the proposed sampler design. 
Commenters suggested the use of a different indicator, use of a 
different size cut, inclusion of additional constituents (e.g., acid 
aerosols, carbon, metals, and semi-volatiles), and/or use of a multi-
filter method. Early in the development process, design decisions were 
based on public input and the advice of CASAC on these and other basic 
design issues. Other factors affecting the basic design of the method 
were the need for historical continuity, high measurement precision, 
and simplicity of operation, all in response to current national 
monitoring objectives and available resources. In selecting the basic 
measurement approach, substantial weight was given to maintaining 
comparability to PM2.5 samplers, such as the ``dichotomous 
sampler,'' that were widely used to obtain the data upon which the new 
standards are based. Given this objective, EPA concludes that the 
conventional PM measurement approach is appropriate and will provide 
PM2.5 measurements that are comparable to the air quality 
data used in the health studies that provide the basis for the 
PM2.5 standards.
    Although the sampler is conventional in configuration, its design 
is much more sophisticated than that of previous PM samplers. This more 
sophisticated sampler, together with improved manufacturing and 
operational quality assurance, is necessary to achieve the more 
stringent data quality objectives established for PM2.5 
monitoring data. To meet precision requirements, the critical 
mechanical components of the inlet, particle size separator, downtube, 
and upper portion of the filter holder

[[Page 38698]]

are specified by design. All other aspects of the sampler are specified 
by performance-based specifications.
    Several commenters felt that the portions of the sampler that were 
specified by design would stifle further improvements and innovations. 
Although the EPA specifies methods by performance whenever possible, 
for the PM2.5 reference method, development of adequate 
performance specifications for inlet aspiration and particle size 
discrimination would have been a very difficult, costly, lengthy, and 
problematic process. Moreover, manufacturer testing of proposed inlet 
and particle size discrimination devices against such performance 
specifications would require elaborate specialized facilities and would 
be extremely costly. For these reasons, the EPA believes that 
specification of these critical components by design is a prudent and 
very cost-effective way to ensure good inter-manufacturer and intra-
manufacturer precision of the PM2.5 measurements. Therefore, 
these components are specified by design, and other aspects of the 
sampler are specified by performance, as proposed. Innovations and 
improved samplers or measurement methods are encouraged and provided 
for as Class II and III equivalent methods (see 40 CFR part 53).
    b. Inlet and impactor design. Several commenters addressed the 
inlet design, noting that the inlet could allow entrance of 
precipitation and possibly insects. In fact, the inlet selected for the 
sampler has been used effectively for many years to obtain many of the 
PM2.5 measurements that formed the basis of the 
epidemiological studies. While EPA acknowledges that there have been 
some reports of intrusion of precipitation, the Agency believes the 
problem is relatively minor. Nevertheless, a modification of the inlet 
has been developed to further reduce the possibility of precipitation 
(and possibly small insects) reaching the sample filter to damage the 
PM2.5 sample. Extensive wind tunnel tests have shown no 
significant compromise in the PM2.5 aspiration performance 
of the modified inlet.
    In addition, a new provision has been added, in 40 CFR part 50, 
Appendix L, section 7.3.8, to require that the sampling air entrance of 
the inlet be at a height of 2  0.2 meters above the 
supporting surface to help ensure homogeneous air samples when 
collocated samplers of different types are operated simultaneously.
    Other commenters addressed the sharpness of the size cut and how it 
is obtained, e.g., whether more than two stages should be used and what 
size cut should be used for each stage. These aspects were carefully 
considered in selecting the sampler configuration. The selection by EPA 
of the previously used PM10 inlet established the size cut 
for the first stage, and the second stage was designed to be simple, 
reliable, and low in cost for user agencies. In EPA's estimation, the 
advantages of this configuration outweigh any modest advantage that 
might have been gained by designing a new inlet/separation 
configuration that would further refine the cut points at each of two 
(or more) stages.
    A few commenters questioned whether the inlet was wind speed 
dependent at high wind speeds. The selected inlet has been shown to 
perform well up to 24 km/hr with 10 m aerosols and is expected 
to perform well at higher speeds with 2.5 m aerosols. The EPA 
again determined that the advantages of using the selected inlet 
outweighed the possible minor improvement in wind-speed characteristics 
that might have been obtained by a newly-designed inlet.
    Some commenters felt that other types of particle discrimination 
techniques such as cyclones and virtual impactors, should be allowed. 
Again, these alternatives were evaluated previously and the specified 
inlet and impactor were determined to best meet the various objectives 
of the sampler. However, EPA has provided for considerations of other 
particle size selection techniques or devices for approval if 
incorporated into candidate equivalent methods for PM2.5.
    Several commenters addressed the impactor design, noting that the 
impactor should be changed to sharpen the size-cut characteristic, to 
address concerns regarding possible contamination and/or performance 
loss due to impactor oil, and to improve ease of access to service. To 
address the first concern, the initial prototype impactor has been 
modified slightly to sharpen its size-cut. The current impactor is 
designed to lower cost and to optimize cut sharpness, loading capacity, 
manufacturing simplicity, manufacturing quality control, 
serviceability, and reliability. A report containing the penetration 
efficiency of the impactor is available in Docket No. A-95-54. With 
regard to impactor oil concerns, the impactor oil selected has a very 
low vapor pressure, and testing has indicated no contamination of the 
sample filters with impactor oil. The EPA believes that the impactor 
design is as accessible as possible, given the design objectives. Some 
flexibility may be allowed for manufacturers to develop improved 
closure devices or other external modifications. Proper maintenance 
will, of course, be very important and will be stressed in the 
associated operator instruction manuals and in other training and 
guidance materials. The EPA has been performing field and laboratory 
tests that will provide detailed guidance for all necessary preventive 
maintenance. Proper installation procedures for the oil and the 
impactor filter, as well as all other maintenance requirements, will be 
available in the quality assurance procedures and guidance contained in 
the new section 2.12 of Appendix L to be added to EPA's Quality 
Assurance Handbook for Air Pollution Measurement Systems (EPA/600/R-94/
038b).
    c. Anodized aluminum surface. All internal surfaces exposed to 
sample air prior to the filter are required to be anodized aluminum as 
stated in 40 CFR part 50, Appendix L, section 7.3.7. A few commenters 
expressed concern that the anodized aluminum surfaces in high volume 
PM10 samplers have shown substantial pitting, particularly 
in the venturi flow control device. The anodized aluminum surfaces are 
required in the PM2.5 sampler to maintain comparability to 
previously used samplers. The EPA believes that the much lower flow 
rate in the PM2.5 sampler will greatly reduce the pitting 
tendency, and the active flow control in the PM2.5 sampler 
is not dependent on the physical dimensions of a critical orifice as it 
is in a venturi flow control device.
    d. Filter for PM2.5 sample collection. The proposed 
reference method called for the sample to be collected on a 47 mm 
Teflon filter. Many of the comments received on the measurement method 
concerned the proposed filter medium and its performance. Commenters 
expressed concerns with the use of Teflon filters and with the 
selection of a single-filter method. Several commenters recommended 
that alternative filter media be allowed, in most cases to support 
speciation and/or to allow the capture of all PM components. Other 
comments noted potential advantages of other media in operating 
characteristics or chemistry requirements. Operational concerns 
expressed about Teflon filters included tearing, possible loss of 
integrity, and high cost. Other concerns were that Teflon is generally 
not conducive to carbon analysis, and that Teflon filters may not hold 
deposited PM. Many commenters recommended use of a multi-filter sampler 
to support chemical speciation in addition to compliance determination.

[[Page 38699]]

    To address some of these general concerns about the performance of 
the specified filter material, some minor refinements to the filter 
specifications concerning the filter diameter and the filter support 
ring have been made to ensure proper performance of the filter in the 
specified filter holder. Additional clarifications have been made to 
the maximum moisture pickup and the filter weight stability 
requirements. Although Teflon may preclude certain chemical analyses 
(e.g., elemental and organic carbon), the EPA believes that Teflon 
filter material is the best overall choice to meet the objectives of 
compliance monitoring and to provide good measurement precision. Other 
filter media are likely to provide reduced gravimetric precision and 
preclude more types of subsequent chemical analysis. Additional or 
alternative samplers or filter types can be considered as candidate 
equivalent methods under 40 CFR part 53 and can be used for non-
compliance monitoring, where necessary.
    Compliance monitoring based on mass concentration of 
PM2.5 is the primary objective of the reference method. 
Multi-filter capability would have substantially increased the cost and 
complexity of the sampler. However, multi-filter samplers can be 
considered as candidate equivalent methods. In addition, multi-filter 
samplers can be used as special purpose monitors (SPMs) to perform 
characterization studies, develop control strategies, and conduct other 
special studies as has been done previously for PM10.
    In response to numerous comments received on 40 CFR part 50, 
Appendix L and on the provisions of 40 CFR part 58 regarding the need 
for chemical speciation, the EPA is assigning a high priority to a 
chemical speciation trends network through section 105 of the Act grant 
allocation program and will issue guidance describing the monitoring 
methods and scenarios under which speciation should be performed. The 
program will incorporate additional PM2.5 samplers that 
allow for the simultaneous collection of aerosols on multiple filter 
media.
    The associated requirement for archiving filters has been removed 
from 40 CFR part 50, Appendix L, section 10.17 and relocated to 40 CFR 
part 58, Appendix A. This change has been made because this is a 
supplemental monitoring requirement and not an integral part of the 
reference method for determining compliance with the PM2.5 
NAAQS.
    Provisions of 40 CFR part 50, Appendix L have been clarified to 
apply not only to a single-sample sampler, but also to a sequential-
sample sampler, provided that all specifications are met and no 
deviations, modifications, or exceptions are made to the inlet, 
downtube, impactor, or the upper portion of the filter holder. Samplers 
that have minor changes or modifications in these components, have 
changes that alter the aerosol's flow path, or contain other 
significant deviations will be required to meet the requirements of 
Class I equivalent methods, in the amendments to 40 CFR part 53. 
Further, a provision has been added to require that sequential sample 
filters stored in a sequential sampler be adequately covered and 
protected from contamination during storage periods in the sampler.
    A few commenters expressed concern about who must carry out filter 
tests to determine if they meet the filter specifications. In response, 
the filter specifications have been clarified to indicate that filter 
manufacturers should generally carry out most or all of the filter 
performance tests in order to certify that their filters meet the 
filter specifications for the PM2.5 reference method. In 
addition, EPA conducts acceptance tests on filters procured for NAMS/
SLAMS networks prior to distribution to State and local agencies.
    Some commenters requested additional information on the requirement 
that an ID number be attached to each filter. Preliminary information 
indicates that it is not practical at this time for either filter 
manufacturers or users to print an ID number directly on the filter. 
However, EPA is continuing to pursue this goal. In the meantime, 
alternative means, such as attaching an appropriate ID number to the 
filter's storage container, will be necessary. Additional details and 
possible alternative filter identification methods will be provided in 
new section 2.12 of the Quality Assurance Handbook for Air Pollution 
Measurement Systems.
    e. Filter handling/weighing/conditioning requirements. Many 
commenters felt that the filter handling requirements for collected 
PM2.5 samples were too burdensome. However, handling of the 
exposed filter between retrieval from the sampler and commencement of 
the conditioning period is expected to be one of the most significant 
sources of PM2.5 measurement variability. Thus, EPA 
concludes that specific requirements for this activity are necessary, 
and this position was supported by several commenters.
    Some commenters felt that the samples should be kept cold until 
analysis to prevent volatile losses. In response to this concern, the 
restriction on the maximum temperature exposure for collected samples 
has been reduced from 32 to 25 deg. C, and a recommendation has been 
added for sampler operators to keep the samples as cool as practical 
between retrieval from the sampler and delivery to the conditioning 
environment. Further, the length of time permitted between retrieval of 
the filter and post-collection weighing is increased from 10 to 30 
days, provided that the sample is maintained at 4 deg. C or less 
between retrieval and the start of the conditioning period. The new 
section 2.12 of the Quality Assurance Handbook for Air Pollution 
Measurement Systems will provide guidance and techniques for keeping 
samples cool during this period and may suggest devices to document 
maximum temperature exposure of the sample.
    Commenters also requested additional specifications and guidance 
for field blanks. The EPA will provide additional clarification and 
detailed procedures and guidance regarding field blanks in the new 
section 2.12 of the Quality Assurance Handbook for Air Pollution 
Measurement Systems.
    Other commenters felt that the filter weighing requirements were 
too restrictive. Because filter weighing is one of the most significant 
sources of PM2.5 measurement variability, specific 
requirements and restrictions are deemed necessary. However, in 
response to some of the concerns expressed, the proposed requirement 
that both pre- and post-weighings be carried out by the same analyst 
has been reduced to a non-mandatory recommendation. Detailed 
recommendations and guidance on filter weighing, based on information 
obtained in current field tests, will be provided in the new section 
2.12 of the Quality Assurance Handbook for Air Pollution Measurement 
Systems.
    Several commenters questioned the filter conditioning requirements, 
with some requesting a lower humidity range. Since humidity can 
profoundly affect the weight of the PM2.5 on the filter, EPA 
maintains that filter conditioning requirements need to be tight to 
control measurement variability and to ensure satisfactory precision. 
But in response to at least one of the concerns, the filter 
conditioning humidity requirement has been changed to allow 
conditioning at a relative humidity within 5 RH percent of 
the mean ambient humidity during sampling (down to a minimum of 20 RH 
percent) for samples collected at average ambient humidities lower than 
30

[[Page 38700]]

percent. The EPA will provide further details on filter conditioning 
controls in the new section 2.12 of the Quality Assurance Handbook for 
Air Pollution Measurement Systems.
    f. Sampler performance requirements. Several commenters addressed 
sampler performance requirements, including sampler flow control 
specifications, filter temperature control, sampler performance under 
extreme conditions, and data reporting. In response to concerns that 
various sampler flow control specifications are too tight, EPA contends 
that good flow control is necessary to maintain uniform sampling, to 
ensure correct particle size discrimination, and to control measurement 
variability. Sampler manufacturers have been able to meet the specified 
flow control requirements, and field studies to date confirm that 
prototype samplers are able to meet these flow control requirements.
    In response to comments about the ambient temperature plus 3 deg. C 
filter temperature control requirement, EPA believes that fairly tight 
control of the sample filter temperature is necessary to minimize 
losses of semi-volatile components over a wide temperature range, and 
tight temperature control has been strongly recommended by the CASAC. 
Monitoring of the filter temperature difference from ambient 
temperature is necessary to verify that the sampler filter temperature 
control is functioning properly. Testing to date indicates that the 
proposed 3 deg. C (above ambient temperature) limit is somewhat 
difficult to meet; however, a 5 deg. C limit can be reasonably met. 
Therefore, the filter temperature control requirement has been relaxed 
slightly from 3 deg. C to not more than 5 deg. C above the concurrent 
ambient temperature. Ambient and filter temperature sensors will 
require periodic calibration or verification of accuracy. In response 
to a frequent comment, the method has been clarified to indicate that 
exceedance of the filter temperature difference limit would not 
necessarily invalidate the sample.
    In response to concerns about the performance of the sampler under 
extreme weather conditions (e.g., high or low temperatures, low 
pressures, high winds, high or low humidity, fog, dust storms), the EPA 
has established sampler specifications that are intended to cover 
reasonably normal environmental conditions at about 95 percent of 
expected monitoring sites. Qualification test requirements in 40 CFR 
part 53 address most, if not all, of these operational requirements. 
Specification of the sampler performance for sites with extreme 
environmental conditions would substantially raise the cost of the 
sampler for other users, most of whom do not require the extra 
capability. Users requiring operation of samplers under extreme 
conditions are encouraged to develop supplemental specifications for 
modified samplers to cover those specific conditions. Sampler 
manufacturers have indicated a commitment to respond to the need for 
modified samplers for such extreme conditions.
    Although concerns were expressed that the amount of data required 
to be reported from each sampler is excessive, EPA stresses that only a 
portion of the data collected by the sampler needs to be reported to 
AIRS. These limited data reporting requirements (i.e., ambient and 
filter temperature, barometric pressure, sample volume, variation in 
sample run flow rate) are important to establish or verify the 
reliability and confidence of the PM2.5 measurements and to 
aid in utilization of those data. The substantial amount of additional 
data generated by the sampler are of use to the site operator to 
provide confirmation of a given sample's validity, and to aid in 
troubleshooting should outlier measurements appear in the monitoring 
data. A variety of current electronic devices and systems may be used 
to acquire and handle the data, and these devices can easily 
accommodate the amount of data required to be reported, as well as the 
additional, optional data. Printers, modem connections, and alternative 
data output connections or devices are not precluded.
    4. Additional changes. Additional clarifying changes have also been 
made throughout 40 CFR part 50, Appendix L, based on comments received 
or recently obtained field test information. In 40 CFR part 50, 
Appendix L, section 3.1, the lower concentration of the method has been 
revised from 1 to 2 g/m3, based on the results of 
field blanks associated with available field test data. In 40 CFR part 
50, Appendix L, section 3.3, the sample period specification has been 
augmented to clarify that a measured PM2.5 concentration for 
a sample period less than 23 hours that is greater than the NAAQS 
level(s) is to be considered a valid measurement for comparison to the 
NAAQS, even though not valid for other purposes. Sections 4 (Accuracy) 
and 5 (Precision) have been revised to properly reflect associated 
changes to the data quality and method performance assessment 
requirements set forth in 40 CFR part 58, Appendix A.
    A provision has been added in 40 CFR part 50, Appendix L, section 
7.4.17 to require sampler manufacturers to make available computer 
software to input sampler output data and translate the data into a 
standard spreadsheet format (since no specific format is specified for 
output of the sample data acquired by the sampler).
    The requirements for the sampler to display current flow rate, 
temperature, filter temperature, and barometric pressure readings have 
been changed to require updating of these readings at least every 30 
seconds. This change is based on operational experience of prototype 
samplers in 40 CFR part 50, Appendix L, section 7.4.5.1, and will make 
it easier for the operators to perform status checks and calibrations. 
In 40 CFR part 50, Appendix L, section 7.4.8.1, the requirements for 
the ambient temperature sensor have been changed to specify an external 
sensor with a passive sun shield, to provide better uniformity in the 
ambient temperature measurements among different types of reference 
method samplers. The reference method has also been clarified to 
indicate that PM2.5 samples for which the sampler reported 
an out-of-specification (FLAG) occurrence during or after the sample 
period are not necessarily invalid, and that such samples should be 
reviewed by a quality assurance officer (40 CFR part 50, Appendix L, 
section 10.12). Finally, a new reference has been added in section 13 
of the Act to provide applicable standards for meteorological 
measurements and measurement systems.
    5. Decision on 40 CFR part 50, Appendix L. After fully considering 
the public comments on the proposed new reference method for 
PM2.5, EPA has concluded that the proposed design and 
performance specifications for the reference sampler, with the 
modifications discussed in this unit, will achieve the design 
objectives set forth in the proposal and outlined above. Therefore, EPA 
is adopting the sampler and other method requirements specified in 40 
CFR part 50, Appendix L as the reference method for measuring 
PM2.5 in the ambient air.
    Since proposal, a series of field tests have been performed using 
prototype samplers manufactured in accordance with the proposed design 
and performance specifications. The results of these field tests 
confirm that the prototype samplers perform in accordance with design 
expectations. Operational experience gained through these field tests 
did, however, identify the need for minor modifications as discussed 
above in this unit. In addition, EPA made other modifications to the 
proposed design and performance specification in response to public

[[Page 38701]]

comment as discussed above. As part of this process, EPA performed 
laboratory tests to ensure that the modifications achieved the intended 
objective.
    While the results of these field tests and laboratory tests were 
largely confirmatory in nature and did not indicate a need to alter the 
basic design and performance specifications, they did identify areas 
that needed further refinement. Given that these tests were performed, 
by necessity, during and after the close of the public comment period 
and because the results were not available for placement in the docket 
until late in the rulemaking process, EPA is announcing, in a separate 
Federal Register notice being signed today, a supplemental comment 
period for the limited purpose of taking comments on these field and 
laboratory test results.

VII. Effective Date of the Revised PM Standards and Applicability 
of the Current PM10 Standards

    In summary, the primary and secondary NAAQS for PM have been 
revised by establishing annual and 24-hour PM2.5 standards; 
and by changing the form of the existing 24-hour PM10 
standards. The existing PM10 annual standards have been 
retained. Section 50.3 (reference conditions) of 40 CFR part 50 has 
been revised to remove the adjustment of measured PM10 
concentrations to standard conditions of temperature and pressure with 
respect to the revised PM standards. (Although EPA is retaining the 
current annual PM10 standards, the revision of 40 CFR 50.3 
potentially may affect the effective stringency of the annual 
standards.) A new Appendix M has been added to 40 CFR part 50 that 
reflects the revision of 40 CFR 50.3. A new Appendix N to 40 CFR part 
50 has been added to reflect the forms of the PM2.5 and 
revised PM10 standards. Finally, a new Appendix L to 40 CFR 
part 50 has been added that specifies the reference method for 
measuring PM2.5 in the ambient air.
    The revised PM NAAQS, the revisions to 40 CFR 50.3, and the new 
Appendices M, N, and L to 40 CFR part 50 will become effective 
September 16, 1997. Inherent in the establishment of this revised set 
of PM standards and related provisions is the revocation of the current 
set of PM10 standards and associated provisions. To provide 
for an effective transition from the existing PM standards to the 
revised PM standards --in light of the need to establish 
PM2.5 monitoring networks, designate areas, and develop 
control strategies for PM2.5--the Administrator has 
determined that the effective date of the revocation of the current set 
of PM10 standards and associated provisions should be 
delayed so that the existing standards and associated provisions will 
continue to apply for an interim period. The duration of the interim 
period would depend on whether the area in question has attained the 
current PM10 standards, as described below in this unit.
    First, section 172(e) of the Act provides that, if the 
Administrator relaxes a national primary ambient air quality standard, 
she shall, within 12 months after the relaxation, promulgate 
requirements applicable to all areas that have not attained that 
standard as of the date of the relaxation. Those requirements shall 
provide for controls that are not less stringent than the controls 
applicable to areas designated nonattainment before such relaxation. 
Although the set of revised PM standards, viewed as a whole, is more 
stringent than the set of current PM standards, it appears that the 
shift from the current PM10 standards to the revised 
PM10 standards, viewed in and of itself, represents a 
relaxation of the current PM10 standards. As a result, 
section 172(e) of the Act requires EPA to issue a rule within 12 months 
to apply implementation requirements no less stringent than the 
currently applicable requirements for those areas that have not yet 
attained the current PM10 standard(s) by today's 
promulgation. However, the Act does not specifically provide how to 
ensure that States with current PM10 problems should 
maintain the necessary public health protection in the interim between 
promulgation of a relaxed standard and issuance of a rule under section 
l72(e) of the Act. For that reason, EPA believes that it is both 
necessary and appropriate to defer the effective date of the revocation 
of the current PM10 standards, for areas that have not 
attained those standards, until EPA issues the rule called for by 
section 172(e) of the Act.
    Second, since it will take many years for States to identify PM 
problems under the revised standards and to develop effective means for 
addressing those problems, EPA believes it is necessary for even those 
areas that have already attained the current PM10 standards 
(and hence are not subject to the terms of section 172(e) of the Act) 
to continue their current PM10 implementation efforts for 
the purpose of protecting public health in the transition to 
implementation of the revised standards.
    In order to deal with both of these categories of areas--those that 
are not attaining the current PM10 standards and those that 
are in attainment of the current PM10 standards--EPA is 
taking a two-pronged approach towards deferral of the effective date of 
the revocation of the current PM10 standards. For those 
areas that are not attaining the current PM10 standards at 
the time of the promulgation of the revised PM10 standards, 
the current standards will continue to apply until EPA has completed 
its rulemaking under section 172(e) of the Act to prevent backsliding 
in those areas. This will assure that no backsliding can occur in the 
interim period between the promulgation of the revised standards and 
the completion of the rulemaking under section 172(e) of the Act. For 
those areas that are attaining the current PM10 standards at 
the time of promulgation of the revised PM10 standards, the 
existing PM10 standards will continue to apply until the 
areas have an approved SIP that includes any control measures that had 
been adopted and implemented at the State level to meet the current 
PM10 NAAQS and have an approved section 110 SIP for purposes 
of implementing the revised PM standards. If an area has already 
received approval of a PM10 SIP embodying all of the 
measures that had been adopted and implemented at the State level, no 
further Part D submission or approval would be necessary. If an area 
has already submitted such measures, EPA would need to take action to 
approve them. Finally, if an area has not yet submitted such measures 
to EPA for inclusion in the SIP, the area would need to submit them and 
EPA would need to approve them. This submission and approval would 
serve to satisfy both the area's remaining subpart D obligations and, 
in part, its new obligations under section 110(a)(1) of the Act 
regarding the implementation of the revised PM NAAQS. EPA emphasizes 
that it is not requiring an approval of a modeled attainment 
demonstration for the current PM10 NAAQS, only an approval 
of the control measures that had in fact been adopted and implemented 
and that, therefore, were responsible for the area's attainment of the 
current PM10 standards.
    The existing definition of reference conditions and 40 CFR part 50, 
Appendices J and K will remain in force as long as the current 
PM10 standards apply to an area. Additional policies and 
guidance for assuring an effective transition will be set forth in 
future EPA guidance, policies, and/or rules.

VIII. Regulatory and Environmental Impact Analyses

    As discussed in Unit IV of this preamble, the Clean Air Act and 
judicial

[[Page 38702]]

decisions make clear that the economic and technological feasibility of 
attaining ambient standards are not to be considered in setting NAAQS, 
although such factors may be considered in the development of State 
plans to implement the standards. Accordingly, although, as described 
below, a Regulatory Impact Analysis (RIA) has been prepared, neither 
the RIA nor the associated contractor reports have been considered in 
issuing this final rule.

A. Executive Order 12866

    Under Executive Order 12866, 58 FR 51735 (October 4, 1993), the 
Agency must determine whether a regulatory action is ``significant'' 
and, therefore, subject to Office of Management and Budget (OMB) review 
and other requirements of the Executive Order. The order defines 
``significant regulatory action'' as any regulatory action that is 
likely to result in a rule that may:
    (1) Have an annual effect on the economy of $100 million or more or 
adversely affect in a material way the economy, a sector of the 
economy, productivity, competition, jobs, the environment, public 
health or safety, or State, local, or tribal governments or 
communities.
    (2) Create a serious inconsistency or otherwise interfere with an 
action taken or planned by another Agency.
    (3) Materially alter the budgetary impact of entitlements, grants, 
user fees, or loan programs or the rights and obligations of recipients 
thereof.
    (4) Raise novel legal or policy issues arising out of legal 
mandates, the President's priorities, or the principles set forth in 
the Executive Order.
    In view of its important policy implications, this action has been 
judged to be a ``significant regulatory action'' within the meaning of 
the Executive Order. As a result, under section 6 of the Executive 
Order, EPA has prepared an RIA, entitled ``Regulatory Impact Analysis 
for Particulate Matter and Ozone National Ambient Air Quality Standards 
and Proposed Regional Haze Rule (July 1997).'' This RIA assesses the 
costs, economic impacts, and benefits associated with potential State 
implementation strategies for attaining the PM and O3 NAAQS 
and the proposed Regional Haze Rule. Changes made in response to OMB 
suggestions or recommendations will be documented in the public docket 
and made available for public inspection at EPA's Air and Radiation 
Docket Information Center (Docket No. A-95-58). The RIA will be 
publicly available in hard 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.''

B. Regulatory Flexibility Analysis

    The Regulatory Flexibility Act (RFA), 5 U.S.C. 601 et seq., 
provides that, whenever an agency is required to publish a general 
notice of rulemaking for a proposal, the agency must prepare an initial 
regulatory flexibility analysis for the proposal unless the head of the 
agency certifies that the rule will not, if promulgated, have a 
significant economic impact on a substantial number of small entities 
(section 605(b)). The EPA certified the proposed NAAQS rule based on 
its conclusion that the rule would not establish requirements 
applicable to small entities and therefore would not have a significant 
economic impact on small entities within the meaning of the RFA. See 61 
FR 65638, 65668 (PM proposal) and 61 FR 65716, 65764 (ozone proposal), 
both published December 13, 1996. Accordingly, the Agency did not 
prepare an initial regulatory flexibility analysis for the proposal, 
but it did conduct a more general analysis of the potential impact on 
small entities of possible State strategies for implementing any new or 
revised NAAQS.
    At the heart of EPA's certification of the proposed NAAQS rule was 
the Agency's interpretation of the word ``impact'' as used in the RFA. 
Is the ``impact'' to be analyzed under the RFA a rule's impact on the 
small entities that will be subject to the rule's requirements, or the 
rule's impact on small entities in general, whether or not they will be 
subject to the rule? In the case of NAAQS rules, the question arises 
because of the congressionally designed mixture of Federal and State 
responsibilities in setting and implementing the NAAQS.
    As EPA explained in the proposal, NAAQS rules establish air quality 
standards that States are primarily responsible for meeting. Under 
section 110 and Part D of Title I of the Act, every State develops a 
State Implementation Plan (SIP) containing the control measures that 
will achieve a newly promulgated NAAQS. States have broad discretion in 
the choice of control measures. As the U.S. Supreme Court noted in 
Train v. NRDC, 421 U.S. 60 (1975), 95 S. Ct. 1470:

    [P]rimary [NAAQS] deal with the quality of outdoor air and are 
fixed on a nationwide basis at a level which the agency determines 
will protect the public health. It is the attainment and maintenance 
of these standards which section 110(a)(2)(A) requires that State 
plans provide. In complying with this requirement, a State's plan 
must include ``emission limitations'' which are regulations of the 
composition of substances emitted into the ambient air from such 
sources as power plants, service stations and the like. They are the 
specific rules to which operators of pollution sources are subject 
and which, if enforced, should result in ambient air which meets the 
national standards.
    The Agency is plainly charged by the Act with the responsibility 
for setting the national ambient air standards. Just as plainly, it 
is relegated to a secondary role in the process of determining and 
enforcing the specific, source-by-source emission limitations which 
are necessary if the national standards are to be met. Under 
110(a)(2), the Agency is required to approve a State plan which 
provides for the timely attainment and maintenance of the ambient 
air standards, and which also satisfies that sections other general 
requirements. The Act gives the agency no authority to question the 
wisdom of a state's choices of emission limitations if they are part 
of a plan which satisfies the standards of 110(a)(2) and the Agency 
may devise and promulgate a plan of its own only if the State fails 
to submit an implementation plan which satisfies those standards. 
Section 110(c).

421 U.S. 60 at 78-79 (emphasis in original). In short, NAAQS rules 
themselves do not establish any control requirements applicable to 
small entities. State rules implementing the NAAQS may establish such 
requirements and the extent to which they do depends primarily on each 
State's strategy for meeting the NAAQS.95
---------------------------------------------------------------------------

    95 It is worth noting that Federal rules that apply nationally 
also play a role in reducing emissions governed by NAAQS. For 
instance, EPA rules under Title II of the Act require reductions in 
ozone-forming emissions from on and off-road vehicles and the fuels 
that power them. When EPA issues such rules, it conducts the 
analysis required under the RFA. For example, EPA performed 
regulatory flexibility analyses for the reformulated gasoline rule 
issued under section 211(k) of the Act. See 59 FR 7716, February 16, 
1994.
---------------------------------------------------------------------------

    To determine the proper interpretation of impact under the RFA, EPA 
considered the RFA's stated purpose, its requirements for regulatory 
flexibility analyses, its legislative history, the amendments made by 
the Small Business Regulatory Enforcement Fairness Act of 1996 (SBREFA) 
(Pub. L. 104-121), and caselaw. The EPA concluded that all of these 
traditional tools of statutory construction point in one direction--
that an agency is required to assess the impact of a rule on the small 
entities that will be subject to the rule's requirements, because the 
purpose of a regulatory flexibility analysis is to consider ways of 
easing or even waiving a rule's requirements as they will apply to 
small entities, consistent with the statute authorizing

[[Page 38703]]

the rule. That purpose cannot be served in the case of the rules like 
the NAAQS that do not have requirements that apply to small entities.
    More specifically, EPA noted that its interpretation of ``impact'' 
flows from the express purpose of the RFA itself. As the RFA's 
``Findings and Purposes'' section (Pub. L. 96-354, section 2) makes 
clear, Congress enacted the RFA in 1980 out of concern that agencies 
were writing one-size-fits-all regulations that in fact did not fit the 
size and resources of small entities. Congress noted that it is 
generally easier for big businesses to comply with regulations, and 
that small businesses are therefore at a competitive disadvantage in 
complying with uniform rules. Congress also noted that small entities' 
relative contribution to the problem a rule is supposed to solve may 
not warrant applying the same requirements to large and small entities 
alike. In the RFA itself, Congress therefore stated:

    It is the purpose of this Act to establish as a principle of 
regulatory issuance that agencies shall endeavor, consistent with 
the objectives of the rule and of applicable statutes, to fit 
regulatory and informational requirements to the scale of the 
businesses, organizations, and governmental jurisdictions subject to 
regulation.

(Pub. L. 96-354, section 2(b))
    The EPA further noted that the RFA sections governing initial and 
final regulatory flexibility analyses reflect this statement of 
purpose. Sections 603 and 604 of the RFA require that initial and final 
regulatory flexibility analyses identify the types and estimate the 
numbers of small entities ``to which the proposed will apply'' 
(sections 603(b)(3) and 604(a)(3) of the RFA). Similarly, they require 
a description of the ``projected reporting, recordkeeping, and other 
compliance requirements of the proposal, including an estimate of the 
classes of small entities which will be subject to the requirement'' 
(sections 603(b)(4) and 604(a)(4)). At the core of the analyses is the 
requirement that agencies identify and consider ``significant 
regulatory alternatives'' that would ``accomplish the stated objectives 
of applicable statutes and which minimize any significant economic 
impact of the proposal on small entities'' (sections 603(c) and 
604(a)(5)). Among the types of alternatives agencies are to consider 
are the establishment of different ``compliance or reporting 
requirements or timetables'' for small entities and the exemption of 
small entities ``from coverage of the rule, or any part'' of the rule 
(section 603(c)(1) and (4) of the RFA). The RFA thus makes clear that 
regulatory flexibility analyses are to focus on how to minimize rule 
requirements on small entities.
    As EPA further explained, since regulatory flexibility analyses are 
not required for a rule that will not have a ``significant economic 
impact on a substantial number of small entities'', it makes sense to 
interpret ``impact'' in light of the requirements for such analyses. 
Regulatory flexibility analyses, as described in this unit, are to 
consider how a rule will apply to small entities and how its 
requirements may be minimized with respect to small entities. In this 
context, ``impact'' is appropriately interpreted to mean the impact of 
a rule on the small entities subject to the rule's requirements.
    The Agency cited two Federal court cases in support of its 
interpretation. In Mid-Tex Elec. Co-op v. FERC, 773 F.2d 327, 342 (D.C. 
Cir. 1985), petitioners claimed that the RFA required an agency to 
analyze the effects of a rule on small entities that were not regulated 
by the rule but might be indirectly impacted by it. Petitioners noted 
that the Small Business Administration (SBA) also interpreted the RFA 
to require analysis of a rule's impact on small entities not regulated 
by the rule, and argued that the court should defer to the SBA's 
position in light of its compliance monitoring role under the RFA. 
After reviewing the RFA's ``Findings and Purposes'' section, its 
legislative history, and its requirements for regulatory flexibility 
analyses, the Mid-Tex court rejected petitioners' interpretation. As 
the court explained:

    The problem Congress stated it discerned was the high cost to 
small entities of compliance with uniform regulations, and the 
remedy Congress fashioned--careful consideration of those costs in 
regulatory flexibility analyses--is accordingly limited to small 
entities subject to the proposed regulation * * *. [W]e conclude 
that an agency may properly certify that no regulatory flexibility 
analysis is necessary when it determines that the rule will not have 
a significant economic impact on a substantial number of small 
entities that are subject to the requirements of the rule.

Id. at 342. Notably, Congress let this interpretation stand when it 
recently amended the RFA in enacting SBREFA.
    The EPA also cited a recent case affirming the Mid-Tex court's 
interpretation. In United Distribution Companies v. FERC, 88 F.3d 1105, 
1170 (D.C. Cir. 1996), the court noted that the Mid-Tex court:

    * * * conducted an extensive analysis of RFA provisions 
governing when a regulatory flexibility analysis is required and 
concluded that no analysis is necessary when an agency determines 
``that the rule will not have a significant economic impact on a 
substantial number of small entities that are subject to the 
requirements of the rule''.
Id., citing and quoting Mid-Tex (emphasis added by United Distribution 
court). The Agency went on to explain that given the Federal/State 
partnership for attaining healthy air, the proposed NAAQS, if adopted, 
would not establish any requirements applicable to small entities. 
Instead, any new or revised standard would establish levels of air 
quality that States would be primarily responsible for achieving by 
adopting plans containing specific control measures for that purpose. 
The proposed NAAQS rule was thus not susceptible to regulatory 
flexibility analysis as prescribed by the amended RFA. Since it would 
establish no requirements applicable to small entities, it afforded no 
opportunity for EPA to fashion for small entities less burdensome 
compliance or reporting requirements or timetables, or exemptions from 
all or part of the rule. For these reasons, EPA certified that the 
proposal ``will not, if promulgated, have a significant economic impact 
on a substantial number of small entities,'' within the meaning of the 
RFA. Because EPA was not required to prepare an initial regulatory 
flexibility analysis for the rule, it was also not required to convene 
a Small Business Advocacy Review Panel for the rule under section 
609(b) of the RFA, as added by SBREFA.
    Notwithstanding its certification of the proposal, EPA recognized 
that the proposed NAAQS, if adopted, would begin a process of State 
implementation that could eventually lead to small entities having to 
comply with new or different control measures, depending on the 
implementation plans developed by the States. EPA also recognized that 
the Act does not allow EPA to dictate or second-guess how States should 
exercise their discretion in regulating to attain any new or revised 
NAAQS. Under those circumstances, EPA concluded that the best way to 
take account of small entity concerns regarding any new or revised 
NAAQS was to work with small entity representatives and States to 
provide information and guidance on how States could address small 
entity concerns when they write their implementation plans.
    In line with this approach, as part of RIA it prepared for the 
proposed NAAQS, EPA analyzed how hypothetical State plans for 
implementing the proposal might affect small entities. The analysis was 
necessarily speculative and limited, since it depended on projections 
about what States might do several years in the future and did not take 
into account

[[Page 38704]]

any new strategies that might be developed and recommended by the FACA 
subcommittee formed to help devise potential strategies for 
implementing a new or revised NAAQS (see discussion of RIA and FACA 
process in this document). Nevertheless, the analysis provided as much 
information on potential small entity impacts as was reasonably 
available at the time of the proposal.
    The Agency also took steps to ensure that small entities' voices 
were heard in the NAAQS rulemaking itself. With Jere Glover, Chief 
Counsel for Advocacy of the SBA, EPA convened outreach meetings modeled 
on the SBREFA panel process to solicit and convey small entities' 
concerns with the proposed NAAQS. Two meetings were held as part of 
that process, on January 7 and February 28, 1997, with a total 
attendance of 41 representatives of small businesses, small 
governments, and small nonprofit organizations. Both meetings were 
attended by representatives of SBA and OMB, as well as of EPA. The key 
concerns raised by small entities at those meetings related to the 
scientific foundation of the proposed NAAQS and the potential cost of 
implementing it, the same concerns raised by other industry commenters 
on the proposal. The Agency produced a report on the meetings to ensure 
that small entity concerns were part of the rulemaking record when EPA 
made its final decision on the proposal.
    In light of States' pivotal role in NAAQS implementation, EPA also 
undertook a number of additional activities to assist and encourage the 
States to be sensitive to small entity impacts as they implement any 
new or revised NAAQS. With the SBA, EPA began an interagency panel 
process to collect advice and recommendations from small entity 
representatives on how States could lessen any impacts on small 
entities. The EPA plans to issue materials in two phases to help States 
develop their implementation plans. In view of States' discretion in 
implementing the NAAQS, these materials will mostly take the form of 
guidance, which is not subject to the RFA's requirement for initial 
regulatory flexibility analysis. (Under section 603 of the RFA, that 
requirement applies only to binding rules that are required to undergo 
notice and comment rulemaking procedures.) But regardless of the form 
such materials take, EPA is employing panel procedures to ensure that 
small entities have an opportunity to raise any concerns prior to the 
materials being issued in draft form.
    To supplement the input the Agency receives from the ongoing FACA 
process (described previously in this document), EPA also added more 
small entity representatives to the Subcommittee on implementation of 
any new or revised NAAQS. These representatives have formed a small 
entity caucus to develop and bring to the Subcommittee a focused 
approach to small entity issues. These new Subcommittee members are 
also part of the group in the aforementioned panel process. By means of 
these various processes, EPA hopes to promote the consideration of 
small entity concerns and advice throughout the NAAQS implementation 
process.
    In response to the proposal, a number of commenters questioned 
EPA's decision to certify that the proposed NAAQS will not have a 
significant impact on a substantial number of small entities. Some 
commenters disagreed with EPA's view that the proposed NAAQS would not 
establish regulatory requirements applicable to small entities. These 
commenters argued that a number of control requirements applicable to 
small entities would automatically result from promulgation of the 
proposed NAAQS, such as new reasonable further progress, SIP and 
Federal Implementation Plan (FIP) requirements. Other commenters stated 
that it is possible for EPA to assess the impacts of the NAAQS revision 
on small entities and that, to a limited extent, EPA has already done 
so. Further, a number of commenters argued that EPA has a legal 
obligation under the RFA, as amended by SBREFA, to choose a NAAQS 
alternative that minimizes the impact on small entities. Some 
commenters questioned EPA's interpretations of the Mid-Tex and United 
Distribution cases. In addition, other commenters stated that EPA's 
position regarding the NAAQS and the RFA is inconsistent with its past 
practice and the legislative history of the RFA. Finally, a few 
commenters noted that the panel process EPA conducted for the proposed 
NAAQS did not satisfy the requirements of SBREFA.
    EPA disagrees that promulgation of the NAAQS will automatically 
result in control requirements applicable to small entities that EPA 
can and must analyze under the RFA. As noted previously in this unit, a 
NAAQS rule only establishes a standard of air quality that other 
provisions of the Act call on States (or in case of State inaction, the 
Federal government) to achieve by adopting implementation plans 
containing specific control measures for that purpose. Following 
promulgation of a new or revised NAAQS, section 110 of the Act requires 
States and EPA to engage in a designation process to determine what 
areas within each State's borders are attaining or not attaining the 
NAAQS. Under section 110 and Parts C and D of Title I of the Act, 
States then conduct a planning process to develop and adopt their SIPS. 
Depending on an area's designation for the particular NAAQS, these and 
other Title I provisions of the Act require a State's SIP to contain 
certain control programs in addition to the control measures that the 
State decides are also needed to attain and maintain the NAAQS.
    The fact that the Act requires SIPs to contain certain control 
programs under certain circumstances does not mean that EPA either can 
or must conduct a regulatory flexibility analysis of a rule 
establishing a NAAQS. Just from the standpoint of feasibility, EPA 
cannot know which areas will be subject to what mandatory SIP programs 
until after the designation process is completed. Beyond that, any 
mandatory SIP programs are still implemented by the States, and States 
have considerable discretion in how they implement them. For instance, 
the reasonable further progress requirement under section 172 of the 
Act leaves States broad discretion to determine the rate of progress 
and the control measures to achieve that progress.96 As a 
result, EPA cannot be certain where and how any mandatory programs will 
be implemented with respect to small (or large) entities. Much less can 
EPA know about how States will exercise their discretion to develop 
additional controls needed to attain and maintain the NAAQS.
---------------------------------------------------------------------------

    96The SIP requirements of subpart 4 of Part D of Title I of the 
Act apply to SIPs for areas designated as not attaining NAAQS for 
PM10. Those requirements will not apply to SIPs to 
implement the PM2.5 NAAQS. Further, to the extent SIPs 
for areas in nonattainment with the applicable PM10 NAAQS 
remain subject to subpart 4 requirements, there will be no 
incremental change in the impact on sources regulated by the States' 
SIPs pursuant to those requirements as a result of this 
promulgation.
---------------------------------------------------------------------------

    Even if EPA could know exactly how any mandatory SIP programs would 
apply to small entities, the purpose of the RFA is not served by 
attempting a regulatory flexibility analysis of State implementation of 
those programs. As explained previously in this unit, the RFA and the 
caselaw interpreting it clearly establish that the purpose of the RFA 
is to promote Federal agency efforts to tailor a rule's requirements to 
the scale of the small entities that will be subject to it. That 
purpose cannot be served in the case of a NAAQS rule since the rule 
does not establish requirements applicable to small

[[Page 38705]]

entities. In promulgating a NAAQS, the only choice before EPA concerns 
the level of the standard, not its implementation. While mandatory SIP 
programs may ultimately follow from promulgation of the NAAQS, there is 
nothing EPA can do in setting the NAAQS to tailor those programs as 
they apply to small entities. Whether and how the programs will apply 
in particular nonattainment areas is beyond the scope of the NAAQS 
rulemaking and, indeed, beyond EPA's reach in any rulemaking to the 
extent the applicability and terms of the programs are prescribed by 
statute.97 Moreover, any mandatory SIP programs are 
supplemented by discretionary State controls that EPA has no power to 
tailor under the RFA or the Act (see Train v. NRDC, quoted previously 
in this unit).
---------------------------------------------------------------------------

    97 If and when the Agency issues any rules addressing State 
implementation of any statutorily required actions, EPA would 
analyze and address the impact of those rules on small entities as 
appropriate under the RFA.
---------------------------------------------------------------------------

    The commenters' suggestions for minimizing the potential impact of 
the NAAQS rule on small entities run afoul of both the RFA and the Act. 
Some suggested that EPA set a less stringent standard (or no standard 
at all in the case of PM2.5) to reduce the chance that small 
entities would become subject to new or tighter SIP requirements. 
Others suggested that EPA require States to exempt small entities from 
new or tighter SIP requirements. However, as explained previously in 
this document, the RFA neither requires nor authorizes EPA to set a 
less stringent NAAQS than the applicable Clean Air Act provisions allow 
in order to reduce potential small entity impacts. Indeed, the RFA 
provides that any means of providing regulatory flexibility to small 
entities be consistent with the statute authorizing the rule. Moreover, 
even if EPA set a less stringent standard, States could still exercise 
their discretion to obtain any needed emission reductions from small 
entities. As the Supreme Court in Train v. NRDC made clear, EPA has no 
authority to forbid States from obtaining reductions from any 
particular category of stationary sources, including small entities. 
See also, Virginia v. EPA, No. 108 F.3d 1397, 1408 (D.C. Cir. 1997), 
quoting Union Electric v. EPA, 427 U.S. 246, 269 (1976) (``section 110 
left to the states the power to determine which sources would be 
burdened by regulations and to what extent'').
    EPA's approval of SIPs for the new or revised NAAQS also will not 
establish new requirements, but will instead simply approve 
requirements that a State is already imposing. And again, EPA does not 
have authority to disapprove a State's plan except to the extent that 
the plan fails to demonstrate attainment and maintenance of the NAAQS 
as required by Title I of the Clean Air Act. In cases where EPA 
promulgates a FIP, EPA might establish control requirements applicable 
to small entities, and in such a circumstance, EPA would conduct the 
analyses required by the RFA.
    Some commenters argued that under the RFA as amended by SBREFA, EPA 
now has an obligation to choose the alternative that minimizes the 
impact on small entities when setting the NAAQS. As indicated 
previously in this unit, EPA disagrees with the commenters' argument 
for the reasons stated in this document's discussion of the Agency's 
authority to consider costs and other factors not related to public 
health in setting and revising primary NAAQS. In a nutshell, both the 
text and legislative history of the RFA make clear that the RFA does 
not override the substantive provisions of the statute authorizing the 
rule, but only requires agencies to identify and consider ways of 
minimizing the economic impact on small entities subject to the rule in 
a manner consistent with the authorizing statute.
    Some commenters disagreed with EPA's interpretation of the Mid-Tex 
and United Distribution cases. In particular, these commenters noted 
that in those cases the relevant regulatory agency, Federal Energy 
Regulatory Commission (FERC), wholly lacked jurisdiction to regulate 
the small entities at issue. According to these commenters, EPA does 
have the ability and jurisdiction to regulate small entities in the 
case of the NAAQS, and therefore EPA's reliance on Mid-Tex and United 
Distribution is misplaced.
    The commenters' attempt to distinguish the FERC cases from the 
NAAQS rulemaking wholly overlooks the courts' reasoning, which in fact 
fully supports EPA's certification of the proposed NAAQS. As described 
previously in this unit, the Mid-Tex court exhaustively reviewed the 
relevant sections of the RFA and its legislative history. Its analysis 
revealed that Congress passed the RFA out of concern with one-size-
fits-all regulations and fashioned a remedy limited to regulations that 
apply to small entities. This principle is fully applicable to the 
NAAQS, which creates no rule requirements that apply to small entities.
    The fact that FERC had no regulatory authority over the small 
entities indirectly affected by its rules played no essential role in 
the court's rationale. FERC could (and apparently did in the Mid-Tex 
rulemaking) estimate the potential indirect impact of its rules on 
small entities. Presumably, FERC could have also mitigated any indirect 
impact by changing some aspect of the rule (or else the small entities 
would have had no incentive to sue the agency). The court nevertheless 
found it unnecessary for FERC to do either, based on its reading of the 
RFA as limited to analysis of a rule's impact on the small entities 
subject to the rule's requirements. In reaching its decision, the court 
noted that requiring agencies to ``consider every indirect effect that 
any regulation might have on small businesses * * * is a very broad and 
ambitious agenda, * * * that Congress is unlikely to have embarked on * 
* * without airing the matter.'' Mid-Tex, 773 F.d. at 343.
    The commenters also overstate EPA's regulatory authority over small 
entities with respect to the regulation of criteria pollutants. Various 
provisions of the Clean Air Act authorize EPA to regulate various types 
of sources at the Federal level to accomplish specified goals. However, 
EPA's authority to more generally regulate sources, including small 
entities, in the manner of SIPs is limited to instances of State 
default of SIP responsibilities. When that occurs, EPA may issue a FIP 
containing specific control measures, and to the extent a proposed FIP 
would establish control measures applicable to small entities, EPA 
would analyze the small entity impact of those measures as required by 
the RFA. In 1994, for example, EPA prepared an initial regulatory 
flexibility analysis when it proposed a FIP for Los Angeles. See 59 FR 
23264 (May 5, 1994).
    As noted previously in this unit, Congress let the Mid-Tex 
interpretation stand when it recently amended the RFA in enacting 
SBREFA. If it had disagreed with the court's decision, it would have 
revised the relevant statutory provisions or otherwise indicated its 
disagreement when it enacted SBREFA. Instead, Congress actually 
reinforced the Mid-Tex court's interpretation of the RFA in enacting 
section 212(a) of SBREFA. That section requires that an agency issue a 
``small entity compliance guide'' for ``each rule * * * for which an 
agency is required to prepare a final regulatory flexibility analysis 
under section 604'' of the RFA. The guide is ``to assist small entities 
in complying with the rule'' by ``explain[ing] the actions a small 
entity is required to take to comply'' with the rule (section 212(a) of 
SBREFA). Obviously, it makes no sense to prepare a small entity 
compliance guide for a rule that does not apply to small entities. Thus 
SBREFA stands as further confirmation that Congress intended the

[[Page 38706]]

RFA to address only rules that establish requirements small entities 
must meet. Since SBREFA's passage, the United Distribution court has 
affirmed the Mid-Tex court's interpretation.
    Some commenters noted that EPA's informal panel process did not 
comply with the requirements of SBREFA. The EPA did not convene a 
SBREFA panel because such a panel is not required for rules like the 
NAAQS that do not apply to small entities. Under the RFA as amended by 
SBREFA, since the Agency certified the proposal, it was not required to 
convene a panel for it. Nevertheless, EPA conducted the voluntary panel 
process described previously in this unit, as well as other voluntary 
small business outreach efforts. The process could not comply with the 
analytical requirements of the RFA for the reasons given in this unit. 
However, it could and did ensure that EPA heard directly from small 
entities about the NAAQS proposals.
    A few commenters stated that EPA's view of the NAAQS and the RFA is 
inconsistent with EPA's past positions regarding the RFA and NAAQS 
revisions. Some commenters also cited the RIA for the proposed NAAQS 
and noted that this analysis demonstrates EPA's ability to estimate the 
impact of the NAAQS on small entities, thereby undercutting EPA's 
argument that it is not able to perform a regulatory flexibility 
analysis when setting the NAAQS.
    Past Federal Register documents make clear that the nature of the 
NAAQS makes a regulatory flexibility analysis inapplicable to NAAQS 
rulemakings. For instance, in 1984, EPA stated that a ``NAAQS for 
NOx by itself has no direct impact on small entities. 
However, it forces each State to design and implement control 
strategies for areas not in attainment.'' See 49 FR 6866, 6876 
(February 23, 1984); see also, 50 FR 37484, 37499 (September 13, 1985); 
50 FR 25532, 25542 (June 19, 1985) (NAAQS for NO2 do not 
impact small entities directly). EPA stated again in 1987 that the 
NAAQS ``themselves do not contain emission limits or other pollution 
controls. Rather, such controls are contained in state implementation 
plans.'' See 52 FR 24634, 24654 (July 1, 1987).
    EPA has typically performed an analysis to assess, to the extent 
practicable, the potential impact of retaining or revising the NAAQS on 
small entities, depending on possible State strategies for implementing 
the NAAQS. These analyses have provided as much insight into the 
potential small entity impacts of implementing revised NAAQS as could 
be provided at the NAAQS rulemaking stage. In some instances, these 
preliminary analyses were described as ``regulatory flexibility 
analys[es]'' or as analyses ``pursuant to this [Regulatory Flexibility] 
Act.'' See, e.g., 52 FR 24634, 24654 (July 1, 1987); 50 FR 37484, 37499 
(September 13, 1985).
    However, these analyses were based on hypothetical State control 
strategies, and EPA made the point on various occasions that any 
conclusions to be drawn from such analyses were speculative, given that 
the NAAQS themselves do not impose requirements on small entities. 
Although these past analyses reflected the Agency's best efforts to 
evaluate potential impacts, they were not regulatory flexibility 
analyses containing the necessary elements required by the RFA. These 
analyses, for example, did not describe the reporting, recordkeeping, 
and other compliance requirements of the proposed NAAQS rules that 
would apply to small entities, since the NAAQS rules did not apply to 
small entities. Nor did they determine how the proposed NAAQS rules 
could be eased or waived for small entities. Such an analysis is not 
possible in the case of the NAAQS. To the extent EPA labeled these 
analyses regulatory flexibility analyses in the past, that label was 
inappropriate. EPA's current practice is to describe such an analysis 
more accurately as a general analysis of the potential cost impacts on 
small entities. See, e.g., 61 FR 65638, 65669, 65747 (December 13, 
1996) (current O3 and PM NAAQS proposals).98 
EPA's analytical approach to small entity impacts of the NAAQS has thus 
remained consistent over time.
---------------------------------------------------------------------------

    98 As commenters pointed out, the RIA for the proposed PM NAAQS 
does state that ``[t]he screening analysis * * * provides enough 
information for an initial regulatory flexibility analysis (RFA) if 
such an analysis were to be done.'' That statement was mistaken and 
was not made in the RIA for the proposed ozone NAAQS. While both 
RIAs attempted to gauge the potential impact on small entities of 
State implementation of the proposed NAAQS, neither could or did 
identify any specific control or information requirements contained 
in the NAAQS rule that would apply to small entities. Indeed, both 
RIAs made clear that the impact being analyzed was that of potential 
State measures to attain the NAAQS, and that such an analysis was 
inherently speculative and uncertain. Thus, the RIAs actually 
confirm EPA's statement in the preambles for the proposed NAAQS that 
conducting a complete regulatory flexibility analysis is not 
feasible for rules setting or revising a NAAQS.
---------------------------------------------------------------------------

    One commenter noted that the legislative history of the RFA 
suggests that the RFA was intended to apply to the NAAQS. As noted 
previously in this unit, EPA's reading of both the RFA and SBREFA, 
based on the language of the statute as amended and its legislative 
histories and applicable caselaw, is that the RFA requirements at issue 
do not apply to the NAAQS. The legislative history cited by the 
commenter does not change this conclusion.
    In fact, the statement by Senator Culver on which the commenter 
relies does not indicate that the NAAQS should be subject to regulatory 
flexibility analyses. Rather, Senator Culver uses the NAAQS as an 
example of the type of standard that agencies would not change as a 
result of the RFA. According to Senator Culver, section 606 of the RFA 
``succinctly states that this bill does not alter the substantive 
standard contained in underlying statutes which defines the agency's 
mandate.'' 126 Cong. Rec. S 21455 (August 6, 1980) daily ed. After 
citing section 109 of the Act, Senator Culver goes on to describe EPA's 
bubble policy (which addresses the limits on emissions from a 
particular facility) as the type of flexible regulation that agencies 
should consider, once EPA has set a NAAQS. ``The important point for 
purposes of this discussion is that the `bubble concept,' a type of 
flexible regulation, in no manner altered the basic statutory 
substantive standard of the EPA * * *. No regulatory flexibility 
analysis alters the substantive standard otherwise applicable by law to 
agency action.'' Id. Thus, contrary to the suggestion of the commenter, 
Senator Culver's statement actually confirms that the time to consider 
regulatory flexibility is when regulations applicable to sources are 
being established, not when a NAAQS itself is being set.
    Under section 604 of the RFA, whenever an agency promulgates a 
final rule under section 553 of the Administrative Procedure Act, after 
being required by that section or any other law to publish a general 
notice of proposed rulemaking (NPRM), the agency is required to prepare 
a final regulatory flexibility analysis. RFA section 605(b) provides, 
however, that section 603 (re initial regulatory flexibility analyses) 
and section 604 do not apply if the agency certifies that the rule will 
not have a significant economic impact on a substantial number of small 
entities and publishes such certification at the time of publication of 
the NPRM or at the time of the final rule.
    As noted above, EPA certified this final rule at the time of the 
NPRM. After considering the public comments on the certification, EPA 
continues to believe that this final rule will not have a significant 
economic impact on a substantial number of small entities for the 
reasons explained above and that it

[[Page 38707]]

therefore appropriately certified the rule. Further, as required by the 
Clean Air Act, EPA is promulgating this final rule under section 307(d) 
of the Clean Air Act. For all the foregoing reasons, EPA has not 
prepared a final regulatory flexibility analysis for the rule. The 
Agency has nonetheless analyzed in the final RIA for the rule the 
potential impact on small entities of hypothetical State plans for 
implementing the NAAQS. The Agency also plans to issue guidance to the 
States on reducing the potential impact on small entities of 
implementing the NAAQS.

 C. Impact on Reporting Requirements

    There are no reporting requirements directly associated with the 
finalization of ambient air quality standards under section 109 of the 
Act (42 U.S.C. 7400). There are, however, reporting requirements 
associated with related sections of the Act, particularly sections 107, 
110, 160, and 317 (42 U.S.C. 7407, 7410, 7460, and 7617).
    In EPA's final revisions to the air quality surveillance 
requirements (40 CFR part 58) for PM, the associated RIA addresses the 
Paperwork Reduction Act requirements through an Information Collection 
Request.

D. Unfunded Mandates Reform Act

    Title II of the Unfunded Mandates Reform Act of 1995 (UMRA), Pub. 
L. 104-4, establishes requirements for Federal agencies to assess the 
effects of certain regulatory actions on State, local, and tribal 
governments and the private sector. Under section 202 of UMRA, EPA 
generally must prepare a written statement, including a cost-benefit 
analysis, for proposed and final rules with ``Federal mandates'' that 
may result in expenditures by State, local, and tribal governments, in 
the aggregate, or by the private sector, of $100 million or more in any 
1 year. This requirement does not apply if EPA is prohibited by law 
from considering section 202 of UMRA estimates and analyses in adopting 
the rule in question. Before promulgating a final rule for which a 
written statement is needed, section 205 of UMRA generally requires EPA 
to identify and consider a reasonable number of regulatory alternatives 
and adopt the least costly, most cost-effective, or least burdensome 
alternative that achieves the objectives of the rule. These 
requirements do not apply when they are inconsistent with applicable 
law. Moreover, section 205 of UMRA allows EPA to adopt an alternative 
other than the least costly, most cost-effective, or least burdensome 
alternative if the Administrator publishes with the final rule an 
explanation of why that alternative was not adopted. Before EPA 
establishes any regulatory requirements that may significantly or 
uniquely affect small governments, including tribal governments, it 
must have developed under section 203 of UMRA a small government agency 
plan. The plan must provide for notifying potentially affected small 
governments, enabling officials of affected small governments to have 
meaningful and timely input in the development of EPA regulatory 
proposals with significant Federal intergovernmental mandates, and 
informing, educating, and advising small governments on compliance with 
the regulatory requirements. Section 204 of UMRA requires each agency 
to develop ``an effective process to permit elected officers of state, 
local and tribal governments * * * to provide meaningful and timely 
input'' in the development of regulatory proposals containing a 
significant Federal intergovernmental mandate.99
---------------------------------------------------------------------------

    99 As noted in unit VIII.B., a NAAQS rule only establishes a 
standard of air quality that other provisions of the Act call on 
States (or in the case of State inaction, the Federal government) to 
achieve by adopting implementation plans containing specific control 
measures for the purpose. Thus, it is questionable whether the NAAQS 
itself imposes an enforceable duty and thus whether it is a 
significant Federal mandate within the meaning of UMRA. EPA need not 
and does not reach this issue in this document. For the reasons 
given in this unit, even if the NAAQS were determined to be a 
significant Federal mandate, EPA does not have any obligations under 
sections 202 and 205 of UMRA, and EPA has met any obligations it 
would have under section 204 of UMRA.
---------------------------------------------------------------------------

    The EPA has determined that the provisions of sections 202 and 205 
of UMRA do not apply to this decision.``Unless otherwise prohibited by 
law,'' EPA is to prepare a written statement under section 202 of UMRA 
that is to contain assessments and estimates of the costs and benefits 
of a rule containing a Federal mandate. Congress clarified that 
``unless otherwise prohibited by law'' referred to whether an agency 
was prohibited from considering the information in the rulemaking 
process, not to whether an agency was prohibited from collecting the 
information. The Conference Report on UMRA states, ``This section [202] 
does not require the preparation of any estimate or analysis if the 
agency is prohibited by law from considering the estimate or analysis 
in adopting the rule.'' 141 Cong. Rec. H3063 (daily ed. March 13, 
1995). Because the Clean Air Act prohibits EPA, when setting the NAAQS, 
from considering the types of estimates and assessments described in 
section 202 of UMRA, UMRA does not require EPA to prepare a written 
statement under section 202.100 The requirements in section 
205 of UMRA do not apply because those requirements only apply to rules 
``for which a written statement is required under section 202 * * *.''
---------------------------------------------------------------------------

    100In addition to the estimates and assessments described in 
section 202 of UMRA, written statements are also to include an 
identification of the Federal law under which the rule is 
promulgated (section 202(a)(1) of UMRA) and a description of 
outreach efforts under section 204 of UMRA (section 202(a)(5) of 
UMRA). Although these requirements do not apply here because a 
written statement is not required under section 202 of UMRA, this 
preamble identifies the Federal law under which this rule is being 
promulgated and a written statement describing EPA's outreach 
efforts with State, local, and tribal governments will be placed in 
the docket.
---------------------------------------------------------------------------

    The EPA has determined that the provisions of section 203 of UMRA 
do not apply to this decision. Section 203 of UMRA only requires the 
development of a small government agency plan for requirements with 
which small governments might have to comply. Since setting the NAAQS 
does not establish requirements with which small governments might have 
to comply, section 203 of UMRA does not apply. The EPA acknowledges, 
however, that any corresponding revisions to associated SIP 
requirements and air quality surveillance requirements, 40 CFR parts 51 
and 58, respectively, might result in such effects.Accordingly, EPA did 
address unfunded mandates when it proposed revisions to 40 CFR part 58, 
and will do so, as appropriate, when it proposes any revision to 40 CFR 
part 51.
    With regard to the outreach described in section 204 of UMRA, EPA 
did follow a process for providing elected officials with an 
opportunity for meaningful and timely input into the proposed NAAQS 
revisions, although EPA did not describe this process in the proposal. 
The EPA conducted a series of pre-proposal outreach meetings with State 
and local officials and their representatives that permitted these 
officials to provide meaningful and timely input on issues related to 
the NAAQS and the monitoring issues associated with them. Beginning in 
January, 1996, EPA briefed State and local air pollution control 
officials at national meetings with State and Territorial Air Pollution 
Program Administrators (STAPPA) / Association of Local Air Pollution 
Control Officials (ALAPCO) in Washington, DC, North Carolina, Chicago, 
and Nevada. The EPA also held briefings for the Washington, DC 
representatives of several State and local organizations, including 
National Conference of State Legislators, U.S. Conference of Mayors,

[[Page 38708]]

National Governors Association, National League of Cities, and STAPPA/
ALAPCO. EPA also held separate briefings and discussions with State and 
local officials at meetings set up by the National Governors 
Association, the U.S. Conference of Mayors and the Council of State 
Governments. The EPA also conducted in-depth briefings at each EPA 
regional office and regional staff also had several meetings and 
discussions with their State counterparts about the standards. The 
efforts described in this paragraph of this preamble, which provided 
elected officials with opportunity for meaningful and timely input into 
the proposed NAAQS revisions, met any requirements imposed by section 
204 of UMRA. The docket will contain a written statement describing 
these outreach efforts, including a summary of the comments and 
concerns presented by State, local, and tribal governments and a 
summary of EPA's evaluation of those comments and concerns.
    Several commenters disagreed with EPA that sections 202, 203, and 
205 of UMRA do not apply to this decision. These commenters argued that 
EPA is not prohibited from considering costs in setting NAAQS under the 
Clean Air Act and applicable judicial decisions. Some commenters also 
expressed the view that there is no conflict between UMRA and the Clean 
Air Act with regard to the NAAQS. These commenters argued that UMRA and 
the NAAQS can be harmonized by reading UMRA as an information gathering 
statute and that EPA should therefore perform the analyses required by 
UMRA, regardless of whether costs may be considered. Finally, at least 
one commenter argued that in past NAAQS reviews, EPA did not dispute 
its UMRA obligations.
    As discussed more fully in Unit IV. of this preamble, EPA is 
prohibited from considering cost in setting the NAAQS. Given that fact 
(as noted in Unit IV. of this preamble), sections 202 and 205 of UMRA 
do not apply.101 As the Conference Report clarifies, UMRA 
itself states that the section 202 estimates and analyses are not 
required in cases such as the NAAQS, where an agency is prohibited by 
law from considering section 202 estimates and analyses. Reading UMRA 
in the manner suggested by the commenters would effectively read this 
provision out of UMRA; UMRA contains an exception for rules like the 
NAAQS, it must be given effect.
---------------------------------------------------------------------------

    101 One commenter argued that in reviewing the SO2 
NAAQS, EPA determined that it need not revise the S02 
NAAQS, but could instead pursue an alternative regulatory program 
under other authority. This commenter argued that EPA has similar 
flexibility in reviewing the PM and Ozone NAAQS, and thus UMRA 
requires EPA to identify the least burdensome alternative (such as 
retaining the current NAAQS) as part of that process. As discussed 
more fully in Unit IV. of this preamble, EPA does not agree that it 
has flexibility to choose such an alternative; nor does EPA agree 
with the commenter's characterization of the action it took in 
deciding not to revise the SO2 NAAQS. In fact, in 
deciding not to revise the SO2 NAAQS, EPA determined, for 
reasons independent of section 303 of the Clean Air Act that a NAAQS 
revision was not warranted. See 61 FR 25566, 25575 (May 22, 1996).
---------------------------------------------------------------------------

    With regard to EPA's position regarding UMRA in previous NAAQS 
review exercises, EPA simply made plain in those situations that 
because it did not plan on revising the NAAQS, it determined, without 
further review, that sections 202, 203, and 205 of UMRA did not apply. 
EPA thus stated that:

    Because the Administrator has decided not to revise the existing 
primary NAAQS for SO2, this action will not impose any 
new expenditures on governments or on the private sector, or 
establish any new regulatory requirements affecting small 
governments. Accordingly, EPA has determined that the provisions of 
sections 202, 203 and 205 do not apply to this final decision.

61 FR 25566, 25577, May 22, 1996; see also 61 FR 52852, 52856, October 
8, 1996 (Same statement for NO2 NAAQS). As this statement 
makes clear, EPA only determined that sections 202, 203, and 205 of 
UMRA did not apply to the NAAQS when EPA fails to revise the standard. 
Having made that determination, EPA had no reason to catalog additional 
bases for finding UMRA inapplicable. Nothing in that statement was 
intended to preclude EPA, or precludes EPA, from concluding for other 
reasons (such as those discussed in this unit) that UMRA also does not 
apply when EPA in fact revises an applicable NAAQS.

E. Environmental Justice

    Executive Order 12848 (58 FR 7629, February 11, 1994) requires that 
each Federal agency make achieving environmental justice part of its 
mission by identifying and addressing, as appropriate, 
disproportionately high and adverse human health or environmental 
effects of its programs, policies, and activities on minorities and 
low-income populations. These requirements have been addressed to the 
extent practicable in the RIA cited in this unit.

F. Submission to Congress and the Comptroller General

    Under 5 U.S.C. 801(a)(1)(A), as added by the Small Business 
Regulatory Enforcement Fairness Act of 1996 (SBREFA), EPA submitted a 
report containing this rule and other required information to the U.S. 
Senate, the U.S. House of Representatives, and the Comptroller General 
of the United States prior to publication of the rule in this issue of 
the Federal Register. This rule is a ``major rule'' for purposes of 
SBREFA.

IX. Response to Petition for Administrator Browner's Rescusal

    On March 13, 1997, the Washington Legal Foundation (WLF), filed a 
petition with EPA asking that I, Carol Browner, disqualify myself in 
rulemaking regarding the NAAQS for PM and ozone. The petition claims 
that my public statements indicate a ``clear and convincing showing'' 
that I had ``already decided to revise the NAAQS for PM and ozone'' and 
that I therefore ``could not give meaningful consideration`` to 
comments adverse to the proposed rule. On May 12, 1997, EPA's General 
Counsel, Jonathan Z. Cannon, sent a letter to WLF regarding the 
petition. This letter and the WLF petition were then placed in the 
dockets for the proposed ozone and PM standards pending ``consideration 
and final response in connection with the Agency's final actions.''
    Contrary to WLF's assertions, I have maintained an open mind 
throughout these proceedings, and have based today's decisions on the 
rulemaking record--including consideration of comments opposed to the 
proposal. The law does not require the Administrator of EPA to 
disqualify herself merely for expressing views on a proposed 
regulation; in fact, it is part of my responsibility to engage in the 
public debate on the proposals. Moreover, the assertions in WLF's 
petition do not accurately represent my views. The petition takes 
quotes out of context and repeatedly misinterprets my statements. For 
example, WLF quotes a statement that I made at the Children's 
Environmental Health Network Research Conference as an indication that 
I had ``prejudged the issue.'' However, my statement that ``I will not 
be swayed'' did not refer to adopting the NAAQS as proposed. Instead, 
as is clear from reviewing the entire speech, I was addressing my 
broader concern about children's health and the range of EPA standards 
affecting children's health. I also appeared at several congressional 
hearings and testified before members of Congress, some of whom were 
strongly opposed to the proposals. At those hearings, I explained the 
basis for the proposals and put forward the reasons why I concluded the 
proposals were appropriate, given the information before me at the 
time. At the same time, I made clear that I took very seriously

[[Page 38709]]

my obligation to keep an open mind, and to consider fully and fairly 
all significant comments that the Agency received. For these reasons 
and others, as set forth in Mr. Cannon's May 12, 1997 response to WLF, 
which I adopt in full, I have decided not to recuse myself from any 
aspect of considering revisions to the NAAQS for ozone and PM. 
Accordingly, I am hereby denying WLF's petition.

X. References

    (1) Abbey, D.E.; Mills, P.K.; Petersen, F.F.; Beeson, W.L. 
(1991) Long-term ambient concentrations of total suspended 
particulates and oxidants as related to incidence of chronic disease 
in California Seventh-Day Adventists. Environmental Health 
Perspectives 94:43-50.
    (2) Abt Associates (1996a) Proposed Methodology for PM Risk 
Analyses in Selected Cities (Draft). Prepared by Abt Associates, 
Inc., Hampden Square, Suite 500, 4800 Montgomery Lane, Bethesda, MD 
20814-5341. February 12, 1996.
    (3) Abt Associates (1996b) A Particulate Matter Risk Analysis 
for Philadelphia and Los Angeles. Prepared by Abt Associates for US 
EPA, OAQPS, Hampden Square, Suite 500, 4800 Montgomery Lane, 
Bethesda, MD 20814-5341. July 3, 1996 (Updated November 1996).
    (4) Abt Associates (1997a) Revision of Mortality Incidence 
Estimates Based on Pope et al. (1995) in the Abt Particulate Matter 
Risk Assessment Report. Prepared by Abt Associates, Inc., Hampden 
Square, Suite 500, 4800 Montgomery Lane, Bethesda, MD 29814-5341. 
June 5, 1997.
    (5) Abt Associates (1997b) Revision of Mortality Incidence 
Estimates Based on Pope et al. (1995) in the Abt Particulate Matter 
Risk Assessment Report. Prepared by Abt Associates, Inc., Hampden 
Square, Suite 500, 4800 Montgomery Lane, Bethesda, MD 29814-5341. 
June 6, 1997.
    (6) American Industrial Health Council. (1997) Comments on EPA's 
Proposed Rule for National Ambient Air Quality Standards for 
Particulate Matter. Docket No. A-95-54, IV-D-2340. March 3, 1997.
    (7) American Petroleum Institute. (1997) Comments of the 
American Petroleum Institute on the Proposed National Ambient Air 
Quality Standards for Particulate Matter. Docket No. A-95-54, IV-D-
2247. March 12, 1997.
    (8) Borja-Aburto, V.H.; Loomis, D.P.; Bangdiwala, S.I.; Shy, 
C.M.; Rascon-Pacheco, R.A. (1997) Ozone, suspended particulates, and 
daily mortality in Mexico City. American Journal of Epidemiology 
145:258-268.
    (9) Braun-Fahrlander, C.; Ackermann-Liebrich, U.; Schwartz, J.; 
Gnehm, H.P.; Rutishauser, M.; Wanner, H.U. (1992) Air pollution and 
respiratory symptoms in preschool children. American Review of 
Respiratory Disease 145:42-47.
    (10) Brown, K. (1997) Predictability of personal exposure to 
airborne particulate matter from ambient concentrations in four 
communities. In: Comments of the American Petroleum Institute on the 
proposed national ambient air quality standards for particulate 
matter (Appendix F). Docket No. A-95-54, IV-D-2247.
    (11) California Environmental Protection Agency. (1997) Comments 
on the proposed national ambient air quality standards. Docket No. 
A-95-54, IV-D-2251. March 11, 1997.
    (12) Chestnut, L.G.; Rowe, R.D. (1990) Preservation values for 
visibility in the national parks. Washington, DC: U.S. Environmental 
Protection Agency.
    (13) Chestnut, L.G.; Dennis, R.L.; Latimer, D.A. (1994) Economic 
benefits of improvements in visibility: acid rain provisions of the 
1990 Clean Air Act Amendments. Proceedings of the aerosols and 
atmospheric optics: radiative balance and visual air quality. Air & 
Waste Management International Specialty Conference, pp. 791-802.
    (14) Cooper, J.A.; Tawney, C.W. (1996) Comments on Office of Air 
Quality Planning and Standards recommended PM2.5 
concentrations equivalent to current PM10 standards. In: 
Comments of the American Iron and Steel Institute. Docket No. A-95-
54, IV-D-2242. March 12, 1997.
    (15) Costa, D.L.; Dreher, K.L. (1997) Bioavailable transition 
metals in particulate matter mediate cardiopulmonary injury in 
healthy and compromised animal models. Environmental Health 
Perspectives (In press)
    (16) Damberg, R.; Polkowsky, B. (1996) Methodology for 
Estimating Visibility Improvements Due to Reductions in Fine 
Particle Concentrations. Memorandum to the docket for the PM 
National Ambient Air Quality Standards review. September 1996.
    (17) Davis, J.M.; Sacks, J.; Saltzman, N.; Smith, R.L.; Styer, 
P. (1996) Airborne particulate matter and daily mortality in 
Birmingham, Alabama. Technical Report #55, National Institute of 
Statistical Sciences, P.O. Box 14162, Research Triangle Park, NC 
27709.
    (18) Delfino RJ, Murphy-Moulton AM, Burnett RT, Brook JR, 
Becklake MR. (1997) Effects of air pollution on emergency room 
visits for respiratory illnesses in Montreal, Quebec. American 
Journal of Respiratory and Critical Care Medicine 155:568-576
    (19) Dickey, J.H. (1997) Letter to President Bill Clinton. 
Docket No. A-95-54, IV-D-2301. March 12, 1997.
    (20) Dockery, D.W.; Cunningham, J.; Damokosh, A.I.; Neas, L.M.; 
Spengler, J.D.; Koutrakis, P.; Ware, J.H.; Raizenne, M.; Speizer, 
F.E. (1996) Health effects of acid aerosols on North American 
children: respiratory symptoms. Environmental Health Perspectives 
104:500-505.
    (21) Dockery, D.W.; Pope, C.A., III; Wu, W.; Spengler, J.D.; 
Ware, J.H.; Fay, M.E.; Ferris, B.G., Jr.; Speizer, F.E. (1993) An 
association between air pollution and mortality in six U.S. cities. 
New England Journal of Medicine 329:1753-1759.
    (22) Dockery, D.W.; Schwartz, J.; Spengler, J.D. (1992) Air 
pollution and daily mortality: associations with particulates and 
acid aerosols. Environmental Research 59:362-373.
    (23) Dockery, D.W.; Speizer, F.E.; Stram, D.O.; Ware, J.H.; 
Spengler, J.D.; Ferris, B.G., Jr. (1989) Effects of inhalable 
particles on respiratory health of children. American Review of 
Respiratory Disease 139:587-594
    (24) Fairley, D. (1990) The relationship of daily mortality to 
suspended particulates in Santa Clara County, 1980-86. Environmental 
Health Perspectives 89:159-168.
    (25) Fitz-Simons, T.; Mintz, D.; Wayland, M. (1996) Proposed 
methodology for predicting PM2.5 from PM10 
values to assess the impact of alternative forms and levels of the 
PM National Ambient Air Quality Standards. Document transmitted to 
members of the Clean Air Act Scientific Advisory Committee on June 
26, 1996.
    (26) Freas, W.P. (1997) Memorandum to the files: Ratios of 
Particulate Matter Annual Arithmetic Mean to Annual Geometric Mean 
Concentrations. Docket No. A-95-54, IV-B-2. May 29, 1997.
    (27) Friedlander, S.K. (1982) Letter from Sheldon K. 
Friedlander, Chair, Clean Air Science Advisory Committee to 
Administrator Anne. M. Gorsuch. Clean Air Scientific Advisory 
Committee Review and Closure of the Office of Air Quality Planning 
and Standards Staff Paper for Particulate Matter. January 29, 1982.
    (28) Gamble, J.F.; Lewis, R.J. (1996) Health and respirable 
particulate (PM10) air pollution: a causal or statistical 
association? Environmental Health Perspectives 104:838-850.
    (29) Grand Canyon Visibility Transport Commission (1996) 
Recommendations for improving Western vistas. Report of the Grand 
Canyon Visibility Transport Commission to the U.S. Environmental 
Protection Agency. June 1996.
    (30) Godleski, J.J.; Sioutas, C.; Katler, M.; Catalano, P.; 
Koutrakis, P. (1996) Death from inhalation of concentrated ambient 
air particles in animal models of pulmonary disease. Proceedings of 
the Second Annual Colloquium on Particulate Air Pollution and Human 
Health. Lee, J.; Phalen, R.; eds. 4:136-143.
    (31) Gordian, M.E.; Ozkaynak, H,; Xue, J.; Morris, S.S.; 
Spengler, J.D. (1996) Particulate air pollution and respiratory 
disease in Anchorage, Alaska. Environmental Health Perspectives 
104:290-297.
    (32) Hefflin, B.J.; Jalaludin, B.; McClure, E.; Cobb, N.; 
Johnson, C.A.; Jecha, L.; Etzel, R.A. (1994) Surveillance for dust 
storms and respiratory diseases in Washington State, 1991. Archives 
of Environmental Health 49:170-174.
    (33) Health Effects Institute. (1997) Particulate Air Pollution 
and Daily Mortality: Analyses of the Effects of Weather and Multiple 
Air Pollutants (The Phase I.B Report of the Particle Epidemiology 
Evaluation Project). Health Effects Institute, Cambridge, MA.
    (34) Health Effects Institute. (1995) Particulate Air Pollution 
and Daily Mortality: Replication and Valication of Selected Studies 
(The Phase I.A Report of the Particle Epidemiology Evaluation 
Project). Health Effects Institute, Cambridge, MA.
    (35) Hill, A.B. (1965) The environment and disease: associations 
or causation? Proceedings of the Royal Society of Medicine 58:295-
300.
    (36) Ito, K.; Kinney, P.L.; Thurston, G.D. (1995) Variations in 
PM10 concentrations within two metropolitan areas and 
their implications for health effects analyses. Inhalation 
Toxicology 7:735-745.

[[Page 38710]]

    (37) Katsouyanni, K.; Touloumi, G.; Spix, C.; Schwartz, J.; 
Balducci, F.; Medina, S.; Rossi, G.; Wojtyniak, B.; Sunyer, J.; 
Bacharova, L.; Schouten, J.P.; Ponka, A.; Anderson, H.R. (1997) 
Short-term effects of ambient sulphur dioxide and particulate matter 
on mortality in 12 European cities: results from the APHEA project. 
British Medical Journal (In press)
    (38) Killingsworth, C.R.; Alessandrini, F.; Krishna Murthy, 
G.G.; Catalano, P.J.; Paulauskis, J.D.; Godleski, J.J. (1997) 
Inflammation, chemokine expression, and death in monocrotaline-
treated rats following fuel oil fly ash inhalation. Inhalation 
Toxicology (In press)
    (39) Koman, P.D. (1996) Memorandum to the files regarding 
PM2.5 Air Quality Values from PM Health Studies. 
September 30, 1996.
    (40) Koman, P.D. (1997) Supplemental Information Regarding 
PM2.5 Air Quality Values from Key Short-term Fine 
Particle Exposure Studies. June 30, 1997.
    (41) Li, Y.; Roth, H.D. (1995) Daily mortality analysis by using 
different regression models in Philadelphia County, 1973-1990. 
Inhalation Toxicology 7:45-58.
    (42) Lipfert, F.W., Wyzga, R.E. (1997) Air pollution and 
mortality: the implications of uncertainties in regression modeling 
and exposure measurement. Journal of Air & Waste Management 47:517-
523.
    (43) Lipfert, F.W.; Urch, R.B. (1997) Personal exposures of 
asthmatics to air pollution in Toronto and implications for 
epidemiological studies. Submitted as attachment to comments on the 
proposed National Ambient Air Quality Standard for particulate 
matter by R.E. Wyzga, Docket No. A-95-54, IV-D-2672. March 11, 1997.
    (44) Lipfert, F.W. (1995) Estimating air pollution-mortality 
risks from cross-sectional studies: prospective vs. ecologic study 
designs. In: Particulate matter: health and regulatory issues: 
proceedings of an international specialty conference; April; 
Pittsburgh, PA. Pittsburgh, PA: Air & Waste Management Association; 
p. 78-102. (Air & Waste Management Association Publication VIP-49)
    (45) Lipfert, F.W.; Wyzga, R.E. (1995) Air pollution and 
mortality: issues and uncertainties. Journal of Air & Waste 
Management Association 45:949-966.
    (46) Lippmann, M.; Shy, C.; Stolwijk, J.; Speizer, F. (1996) 
Letter to Administrator Carol M. Browner. Supplement to the Closure 
Letter from the Clean Air Scientific Advisory Committee. March 20, 
1996.
    (47) Lippman, M. (1986) Letter from Morton Lippmann, Chair, 
Clean Air Act Scientific Advisory Committee, to Administrator Lee M. 
Thomas.
    (48) Lipsett, M.; Hurley, S.; Ostro, B. (1997) Air Pollution and 
emergency room visits for asthma in Santa Clara County, California. 
Environmental Health Perspectives 105(2):216-222
    (49) Lyon, J.L.; Mori, M.; Gao, R. (1995) Is there a causal 
association between excess mortality and exposure to PM10 
air pollution? Additional analyses by location, year, season and 
cause of death. Inhalation Toxicology 7:603-614.
    (50) Martin, A.E.; Bradley, W.H. (1960) Mortality, fog and 
atmospheric pollution: an investigation during the winter of 1958-
1959. Monthly Bulletin of the Minister of Health, Public Health 
Laboratory Service (GB) 19:56-73.
    (51) Ministry of Health - London- Her Majesty's Stationary 
Office (1954) Reports on Public Health and Medical Subjects No. 95. 
Mortality and Morbidity During the London Fog of December 1952.
    (52) Moolgavkar, S.H.; Luebeck, E.G.; Anderson, E.L. (1997) Air 
pollution and hospital admissions for respiratory causes in 
Minneapolis-St. Paul and Birmingham. Epidemiology 8:364-370.
    (53) Moolgavkar, S.H.; Luebeck, E.G. (1996) Particulate air 
pollution and mortality: a critical review of the evidence. 
Epidemiology 7:420-428.
    (54) Moolgavkar, S.H.; Luebeck, E.G.; Hall, T.A.; Anderson, E.L. 
(1995a) Air pollution and daily mortality in Philadelphia. 
Epidemiology 6:476-484.
    (55) Moolgavkar, S.H.; Luebeck, E.G.; Hall, T.A.; Anderson, E.L. 
(1995b) Particulate air pollution, sulfur dioxide, and daily 
mortality: a reanalysis of the Steubenville data. Inhalation 
Toxicology 7:35-44.
    (56) Neas L.M.; Dockery D.W.; Koutrakis P.; Tollerud, D.J.; 
Speizer, F.E. (1995) The association of ambient air pollution with 
twice daily peak expiratory flow rate measurements in children. 
American Journal of Epidemiology 141:111-122.
    (57) National Mining Association. (1997) Comments of the 
National Mining Association on the proposed revisions to the 
national ambient air quality standards for particulate matter. 
Docket No. A-95-54, IV-D-2158. March 12, 1997.
    (58) Ozkaynak, H.; Spengler, J.D. (1996) The role of outdoor 
particulate matter in assessing total human exposure. In: Particles 
in Our Air: Concentrations and Health Effects. Harvard University 
Press. Wilson, R; Spengler, J.D.; eds.
    (59) Peters, A.; Wichmann, H-E.; Tuch, T.; Heinrich, J.; Heyder, 
J. (1997) Respiratory effects are associated with the number of 
ultrafine particles. American Journal of Respiratory Critical Care 
Medicine 155:(In press).
    (60) Pacific Gas and Electric Company (1997) Comments on the 
proposed PM2.5 fine particulate standard. Docket No. A-
95-54, IV-D-2183. March 11, 1997.
    (61) Pope, C.A., III, Kalkstein, J.S. (1996) Synoptic weather 
modeling and estimates of the exposure-response relationship between 
daily mortality and particulate air pollution. Environmental Health 
Perspectives 104:414-420.
    (62) Pope, C.A., III; Thun, M.J.; Namboori, M.M.; Dockery, D.W.; 
Evans, J.S.; Speizer, F.E.; Heath, D.W., Jr. (1995) Particulate air 
pollution as a predictor of mortality in a prospective study of U.S. 
adults. American Journal of Respiratory and Critical Care Medicine 
151:669-674.
    (63) Pope, C.A., III; Schwartz, J.; Ransom, M.R. (1992) Daily 
mortality and PM10 pollution in Utah Valley. Archives 
Environmental Health 47:211-217.
    (64) Pope, C.A., III; Dockery, D.W. (1992) Acute health effects 
of PM10 pollution on symptomatic and asymptomatic 
children. American Review of Respiratory Disease 145:1123-1128.
    (65) Raizenne, M.; Neas, L.M.; Damokosh, A.I.; Dockery, D.W.; 
Spengler, J.D.; Koutrakis, P.; Ware, J.H.; Speizer, F.E. (1996) 
Health effects of acid aerosols on North American children: 
pulmonary function. Environmental Health Perspectives 104:506-514.
    (66) Roth, H.D.; Li, Y. (1997) Analysis of the association 
between air pollutants with mortality and hospital admissions in 
Birmingham, Alabama: 1986-1990. (Submitted for publication)
    (67) Roth, H.D. et al. (1997) Analysis of the association 
between total particulate matter and daily mortality in the Czech 
Republic: 1986-1994. Submitted as an attachment to comments of R.E. 
Wyzga, Docket No. A-95-54, IV-D-2672. March 11, 1997.
    (68) Sacks, J.; Karr, A.F.; Smith, R.L.; Davis, J.M. (1997) 
Comment on Scientific Input to Decision-Making on Airborne 
Particulate Standards. Docket No. A-95-54, IV-D-14,298. March 12, 
1997.
    (69) Samet, J.M.; Zeger, S.L.; Kelsall, J.E.; Xu, J. (1996a) Air 
pollution and mortality in Philadelphia 1973-1988: Report to the 
Health Effects Institute on Phase I.B of the Particle Epidemiology 
Evaluation Project. Health Effects Institute, Cambridge, MA.
    (70) Samet, J.M.; Zeger, S.L.; Kelsall, J.E.; Xu, J.; Kalkstein, 
L.S. (1996b) Weather, air pollution, and mortality in Philadelphia, 
1973-1980: Report to the Health Effects Institute on Phase I.B of 
the Particle Epidemiology Evaluation Project. Health Effects 
Institute, Cambridge, MA.
    (71) Samet, J.M.; Zeger, S.L.; Berthane, K. (1995) The 
association of mortality and particulate air pollution. In: 
Particulate Air Pollution and Daily Mortality: Replication and 
Valication of Selected Studies (The Phase I.A Report of the Particle 
Epidemiology Evaluation Project) pp. 3-104. Health Effects 
Institute, Cambridge, MA.
    (72) Scarlett, J.F.; Abbott, K.J.; Peacock, J.L.; Strachan, 
D.P.; Anderson, H.R. (1996) Acute effects of summer air pollution on 
respiratory function in primary school children in southern England. 
Thorax 51:1109-1114.
    (73) Schulze, W.D.; Brookshire, D.S.; Walther, E.G.; MacFarland, 
K. K.; Thayer, M.A.; Whitworth, R.L.; Ben-David, S.; Malm, W.; 
Molenar, Jr. (1983) The economic benefits of preserving visibility 
in the national parklands of the southwest. Natural Resources 
Journal 23:149-173.
    (74) Schwartz, J.; Dockery, D.W.; Neas, L.M. (1996) Is daily 
mortality associated specifically with fine particles? Journal of 
Air & Waste Management Association 46:927-939.
    (75) Schwartz, J.; Dockery, D.W.; Neas, L.M.; Wypij, D.; Ware, 
J.H.; Spengler, J.D.; Koutrakis, P.; Speizer, F.E.; Ferris, Jr., 
B.G. (1994) Acute effects of summer air pollution on respiratory 
symptom reporting in children. American Journal of Respiratory and 
Critical Care Medicine 150:1234-1242.
    (76) Schwartz, J. (1994) Air pollution and hospital admissions 
for the elderly in Birmingham, Alabama. American Journal of 
Epidemiology 139:589-598.
    (77) Schwartz, J. (1993) Air pollution and daily mortality in 
Birmingham, Alabama. American Journal of Epidemiology 137:1136-1147.

[[Page 38711]]

    (78) Schwartz, J; Dockery, D.W. (1992a) Increased mortality in 
Philadelphia associated with daily air pollution concentrations. 
American Review of Respiratory Disease 145:600-604.
    (79) Schwartz, J.; Dockery, D.W. (1992b) Particulate air 
pollution and daily mortality in Steubenville, Ohio. American 
Journal of Epidemiology 135:12-19
    (80) Sisler, J.; Malm, W.; Molenar, J.; Gebhardt, K. (1996) 
Spatial and Seasonal Patterns and Long Term Variability of the 
Chemical Composition of the Haze in the U.S.: An Analysis of Data 
from the IMPROVE Network. Fort Collins, CO: Cooperative Institute 
for Research in the Atmosphere, Colorado State University. July 
1996.
    (81) Styer, P.; McMillan, N.; Gao, F.; Davis, J.; Sacks, J. 
(1995) Effect of outdoor airborne particulate matter on daily death 
counts. Environmental Health Perspectives 103:490-497.
    (82) Swiss EKL. (1996) Report No. 270. Swiss Federal Commission 
of Air Hygiene (EKL). Docket No. A-95-54, IV-I-59.
    (83) Thurston, G.D. (1997) Letter to President William J. 
Clinton. Docket No. A-95-54, IV-F-97. January 10, 1997.
    (84) Thurston, G.D.; Ito, K.; Hayes, C.G.; Bates, D.V.; 
Lippmann, M. (1994) Respiratory hospital admissions and summertime 
haze air pollution in Toronto, Ontario: consideration of the role of 
acid aerosols. Environmental Research 55:271-290.
    (85) Utility Air Regulatory Group. (1997) Comments on the 
proposed air quality standards. Docket No. A-95-54, IV-D-2250. March 
14, 1997
    (86) United Kingdom Department of Environment. (1997) The United 
Kingdom National Air Quality Strategy. United Kingdom of the 
Environment. Scottish Office. Docket No. A-95-54, IV-I-58. March 
1997.
    (87) U.S. Department of Health, Education and Welfare (1964) 
Smoking and health: report of the Advisory Committee to the Surgeon 
General of the Public Health Service. Washington, DC: Public Health 
Service; p. 60.
    (88) U.S. Environmental Protection Agency (1996a) Air Quality 
Criteria for Particulate Matter. Research Triangle Park, NC: 
National Center for Environmental Assessment. Office of Research and 
Development. April 12, 1996.
    (89) U.S. Environmental Protection Agency (1996b) Review of the 
National Ambient Air Quality Standards for Particulate Matter: 
Policy Assessment of Scientific and Technical Information--Office of 
Air Quality Planning and Standards Staff Paper. Office of Air 
Quality Planning and Standards. Office of Air and Radiation. July 
1996.
    (90) U.S. Environmental Protection Agency (1996c) Transcript of 
the Clean Air Scientific Advisory Committee's Review of the 
Particulate Matter Staff Paper Meetings held on May 6-7, 1996 in 
Chapel Hill, NC.
    (91) U.S. Environmental Protection Agency (1993) Office of Air 
Quality Planning and Standards Effects of the 1990 Clean Air Act 
Amendments on Visibility in Class I Areas: An EPA Report to 
Congress. Research Triangle Park, NC.
    (92) Valdberg, P.A. (1997) Causality has not been demonstrated 
between outdoor levels of particulate matter (PM) and daily 
mortality and morbidity. In: Comments of the Engine Manufacturers 
Association on the proposed revisions to NAAQS for particulate 
matter and ozone. Docket No. A-95-54, IV-D-2328. March 11, 1997.
    (93) Ware, J.H.; Ferris, B.G., Jr.; Dockery, D.W.; Spengler, 
J.D.; Stram, D.O.; Speizer, F.E. (1986) Effects of ambient sulfur 
oxides and suspended particles on respiratory health of children. 
American Review of Respiratory Disease 133:834-842.
    (94) World Health Organization. (1997) Update and Revision of 
the WHO Air Quality Guidelines for Europe. European Centre for 
Environment and Health. (In press). Docket No. A-95-54, IV-I-60.
    (95) Wolff, G.T. (1996a) Letter from George T. Wolff, Chair, 
Clean Air Scientific Advisory Committee, to Administrator Carol M. 
Browner. Closure letter on draft Air Quality Criteria for 
Particulate Matter. March 15, 1996.
    (96) Wolff, G.T. (1996b) Letter from George T. Wolff, Chair, 
Clean Air Scientific Advisory Committee, to Administrator Carol M. 
Browner. Clsoure letter on draft OAQPS Staff Paper (Review of the 
National Ambient Air Quality Standards for Particulate Matter: 
policy Assessment of Scientific and Technical Information). June 13, 
1996.
    (97) Woodruff, T.J.; Grillo, J.; Schoendorf, K.C.; 1997. The 
relationship between selected causes of postneonatal infant 
mortality and particulate air pollution in the United States. 
Environmental Health Perspectives 105:(In press)
    (98) Wordley, J.; Walters, S.; Ayres, J.R. (1997) Short-term 
variations in hospital admissions and mortality and particulate air 
pollution. Occupational and Environmental Medicine 54:108-116.
    (99) Wyzga, R.E.; Lipfert, F.W. (1995) Temperature-pollution 
interactions with daily mortality in Philadelphia. In: Particulate 
matter: health and regulatory issues: proceedings of an 
international specialty conference; April; Pittsburgh, PA. Air & 
Waste Management Association, Pittsburgh, PA (Air & Waste Management 
Association Publication VIP-49)

List of Subjects in 40 CFR Part 50

    Environmental protection, Air pollution control, Carbon monoxide, 
Lead, Nitrogen dioxide, Ozone, Particulate matter, Sulfur oxides.

    Dated: July 16, 1997.

Carol M. Browner,
Administrator.

    Therefore, 40 CFR Chapter I is amended as follows:

PART 50--NATIONAL PRIMARY AND SECONDARY AMBIENT AIR QUALITY 
STANDARDS

    1. The authority citation for part 50 continues to read as follows:

    Authority: Secs. 109 and 301(a), Clean Air Act, as amended (42 
U.S.C. 7409, 7601(a)).

    2. Section 50.3 is revised to read as follows:


Sec. 50.3   Reference conditions.

    All measurements of air quality that are expressed as mass per unit 
volume (e.g., micrograms per cubic meter) other than for the 
particulate matter (PM10 and PM2.5) standards 
contained in Sec. 50.7 shall be corrected to a reference temperature of 
25  deg.C and a reference pressure of 760 millimeters of mercury 
(1,013.2 millibars). Measurements of PM10 and 
PM2.5 for purposes of comparison to the standards contained 
in Sec. 50.7 shall be reported based on actual ambient air volume 
measured at the actual ambient temperature and pressure at the 
monitoring site during the measurement period.
    3. Section 50.6 is amended by revising the section heading and 
adding paragraph (d) to read as follows:


Sec. 50.6   National primary and secondary ambient air quality 
standards for PM10.

*      *      *      *      *
    (d) The PM10 standards set forth in this section will no 
longer apply to an area not attaining these standards as of September 
16, 1997, once EPA takes final action to promulgate a rule pursuant to 
section 172(e) of the Clean Air Act, as amended (42 U.S.C. 7472(e)) 
applicable to the area. The PM10 standards set forth in this 
section will no longer apply to an area attaining these standards as of 
September 16, 1997, once EPA approves a State Implementation Plan (SIP) 
applicable to the area containing all PM10 control measures 
adopted and implemented by the state prior to September 16, 1997, and a 
section 110 SIP implementing the PM standards published on July 18, 
1997. SIP approvals are codified in 40 CFR part 52.
    4. Section 50.7 is added to read as follows:


Sec. 50.7   National primary and secondary ambient air quality 
standards for particulate matter.

    (a) The national primary and secondary ambient air quality 
standards for particulate matter are:
    (1) 15.0 micrograms per cubic meter (g/m3) 
annual arithmetic mean concentration, and 65 g/m3 
24-hour average concentration measured in the ambient air as 
PM2.5 (particles with an aerodynamic diameter less than or 
equal to a nominal 2.5 micrometers) by either:
    (i) A reference method based on Appendix L of this part and 
designated in accordance with part 53 of this chapter; or
    (ii) An equivalent method designated in accordance with part 53 of 
this chapter.
    (2) 50 micrograms per cubic meter (g/m3) annual 
arithmetic mean

[[Page 38712]]

concentration, and 150 g/m3 24-hour average 
concentration measured in the ambient air as PM10 (particles 
with an aerodynamic diameter less than or equal to a nominal 10 
micrometers) by either:
    (i) A reference method based on Appendix M of this part and 
designated in accordance with part 53 of this chapter; or
    (ii) An equivalent method designated in accordance with part 53 of 
this chapter.
    (b) The annual primary and secondary PM2.5 standards are 
met when the annual arithmetic mean concentration, as determined in 
accordance with Appendix N of this part, is less than or equal to 15.0 
micrograms per cubic meter.
    (c) The 24-hour primary and secondary PM2.5 standards 
are met when the 98th percentile 24-hour concentration, as 
determined in accordance with Appendix N of this part, is less than or 
equal to 65 micrograms per cubic meter.
    (d) The annual primary and secondary PM10 standards are 
met when the annual arithmetic mean concentration, as determined in 
accordance with Appendix N of this part, is less than or equal to 50 
micrograms per cubic meter.
    (e) The 24-hour primary and secondary PM10 standards are 
met when the 99th percentile 24-hour concentration, as 
determined in accordance with Appendix N of this part, is less than or 
equal to 150 micrograms per cubic meter.
    5. Appendix K is revised (for conformity with the format of the 
other appendices in this part) to read as follows:

Appendix K to Part 50--Interpretation of the National Ambient Air 
Quality Standards for Particulate Matter

1.0  General.
    (a) This appendix explains the computations necessary for 
analyzing particulate matter data to determine attainment of the 24-
hour and annual standards specified in 40 CFR 50.6. For the primary 
and secondary standards, particulate matter is measured in the 
ambient air as PM10 (particles with an aerodynamic 
diameter less than or equal to a nominal 10 micrometers) by a 
reference method based on appendix J of this part and designated in 
accordance with part 53 of this chapter, or by an equivalent method 
designated in accordance with part 53 of this chapter. The required 
frequency of measurements is specified in part 58 of this chapter.
    (b) The terms used in this appendix are defined as follows:
    Average refers to an arithmetic mean. All particulate matter 
standards are expressed in terms of expected annual values: Expected 
number of exceedances per year for the 24-hour standards and 
expected annual arithmetic mean for the annual standards.
    Daily value for PM10 refers to the 24-hour average 
concentration of PM10 calculated or measured from 
midnight to midnight (local time).
    Exceedance means a daily value that is above the level of the 
24-hour standard after rounding to the nearest 10 g/m\3\ 
(i.e., values ending in 5 or greater are to be rounded up).
    Expected annual value is the number approached when the annual 
values from an increasing number of years are averaged, in the 
absence of long-term trends in emissions or meteorological 
conditions.
    Year refers to a calendar year.
    (c) Although the discussion in this appendix focuses on 
monitored data, the same principles apply to modeling data, subject 
to EPA modeling guidelines.
2.0  Attainment Determinations.
    2.1  24-Hour Primary and Secondary Standards.
    (a) Under 40 CFR 50.6(a) the 24-hour primary and secondary 
standards are attained when the expected number of exceedances per 
year at each monitoring site is less than or equal to one. In the 
simplest case, the number of expected exceedances at a site is 
determined by recording the number of exceedances in each calendar 
year and then averaging them over the past 3 calendar years. 
Situations in which 3 years of data are not available and possible 
adjustments for unusual events or trends are discussed in sections 
2.3 and 2.4 of this appendix. Further, when data for a year are 
incomplete, it is necessary to compute an estimated number of 
exceedances for that year by adjusting the observed number of 
exceedances. This procedure, performed by calendar quarter, is 
described in section 3.0 of this appendix. The expected number of 
exceedances is then estimated by averaging the individual annual 
estimates for the past 3 years.
    (b) The comparison with the allowable expected exceedance rate 
of one per year is made in terms of a number rounded to the nearest 
tenth (fractional values equal to or greater than 0.05 are to be 
rounded up; e.g., an exceedance rate of 1.05 would be rounded to 
1.1, which is the lowest rate for nonattainment).
    2.2  Annual Primary and Secondary Standards. Under 40 CFR 
50.6(b), the annual primary and secondary standards are attained 
when the expected annual arithmetic mean 
PM10 concentration is less than or equal to the level of 
the standard. In the simplest case, the expected annual arithmetic 
mean is determined by averaging the annual arithmetic mean 
PM10 concentrations for the past 3 calendar years. 
Because of the potential for incomplete data and the possible 
seasonality in PM10 concentrations, the annual mean shall 
be calculated by averaging the four quarterly means of 
PM10 concentrations within the calendar year. The 
equations for calculating the annual arithmetic mean are given in 
section 4.0 of this appendix. Situations in which 3 years of data 
are not available and possible adjustments for unusual events or 
trends are discussed in sections 2.3 and 2.4 of this appendix. The 
expected annual arithmetic mean is rounded to the nearest 1 
g/m\3\ before comparison with the annual standards 
(fractional values equal to or greater than 0.5 are to be rounded 
up).
    2.3  Data Requirements.
    (a) 40 CFR 58.13 specifies the required minimum frequency of 
sampling for PM10. For the purposes of making comparisons 
with the particulate matter standards, all data produced by National 
Air Monitoring Stations (NAMS), State and Local Air Monitoring 
Stations (SLAMS) and other sites submitted to EPA in accordance with 
the Part 58 requirements must be used, and a minimum of 75 percent 
of the scheduled PM10 samples per quarter are required.
    (b) To demonstrate attainment of either the annual or 24-hour 
standards at a monitoring site, the monitor must provide sufficient 
data to perform the required calculations of sections 3.0 and 4.0 of 
this appendix. The amount of data required varies with the sampling 
frequency, data capture rate and the number of years of record. In 
all cases, 3 years of representative monitoring data that meet the 
75 percent criterion of the previous paragraph should be utilized, 
if available, and would suffice. More than 3 years may be 
considered, if all additional representative years of data meeting 
the 75 percent criterion are utilized. Data not meeting these 
criteria may also suffice to show attainment; however, such 
exceptions will have to be approved by the appropriate Regional 
Administrator in accordance with EPA guidance.
    (c) There are less stringent data requirements for showing that 
a monitor has failed an attainment test and thus has recorded a 
violation of the particulate matter standards. Although it is 
generally necessary to meet the minimum 75 percent data capture 
requirement per quarter to use the computational equations described 
in sections 3.0 and 4.0 of this appendix, this criterion does not 
apply when less data is sufficient to unambiguously establish 
nonattainment. The following examples illustrate how nonattainment 
can be demonstrated when a site fails to meet the completeness 
criteria. Nonattainment of the 24-hour primary standards can be 
established by the observed annual number of exceedances (e.g., four 
observed exceedances in a single year), or by the estimated number 
of exceedances derived from the observed number of exceedances and 
the required number of scheduled samples (e.g., two observed 
exceedances with every other day sampling). Nonattainment of the 
annual standards can be demonstrated on the basis of quarterly mean 
concentrations developed from observed data combined with one-half 
the minimum detectable concentration substituted for missing values. 
In both cases, expected annual values must exceed the levels allowed 
by the standards.
    2.4  Adjustment for Exceptional Events and Trends.
    (a) An exceptional event is an uncontrollable event caused by 
natural sources of particulate matter or an event that is not 
expected to recur at a given location. Inclusion of such a value in 
the computation of exceedances or averages could result in 
inappropriate estimates of their respective expected annual values. 
To reduce the effect of unusual events, more than 3 years of

[[Page 38713]]

representative data may be used. Alternatively, other techniques, 
such as the use of statistical models or the use of historical data 
could be considered so that the event may be discounted or weighted 
according to the likelihood that it will recur. The use of such 
techniques is subject to the approval of the appropriate Regional 
Administrator in accordance with EPA guidance.
    (b) In cases where long-term trends in emissions and air quality 
are evident, mathematical techniques should be applied to account 
for the trends to ensure that the expected annual values are not 
inappropriately biased by unrepresentative data. In the simplest 
case, if 3 years of data are available under stable emission 
conditions, this data should be used. In the event of a trend or 
shift in emission patterns, either the most recent representative 
year(s) could be used or statistical techniques or models could be 
used in conjunction with previous years of data to adjust for 
trends. The use of less than 3 years of data, and any adjustments 
are subject to the approval of the appropriate Regional 
Administrator in accordance with EPA guidance.
3.0  Computational Equations for the 24-hour Standards.
    3.1  Estimating Exceedances for a Year.
    (a) If PM10 sampling is scheduled less frequently 
than every day, or if some scheduled samples are missed, a 
PM10 value will not be available for each day of the 
year. To account for the possible effect of incomplete data, an 
adjustment must be made to the data collected at each monitoring 
location to estimate the number of exceedances in a calendar year. 
In this adjustment, the assumption is made that the fraction of 
missing values that would have exceeded the standard level is 
identical to the fraction of measured values above this level. This 
computation is to be made for all sites that are scheduled to 
monitor throughout the entire year and meet the minimum data 
requirements of section 2.3 of this appendix. Because of possible 
seasonal imbalance, this adjustment shall be applied on a quarterly 
basis. The estimate of the expected number of exceedances for the 
quarter is equal to the observed number of exceedances plus an 
increment associated with the missing data. The following equation 
must be used for these computations:

Equation 1
[GRAPHIC] [TIFF OMITTED] TR18JY97.180

where:

eq=the estimated number of exceedances for calendar 
quarter q;

vq=the observed number of exceedances for calendar 
quarter q;

Nq=the number of days in calendar quarter q;

nq=the number of days in calendar quarter q with 
PM10 data; and

q=the index for calendar quarter, q=1, 2, 3 or 4.

    (b) The estimated number of exceedances for a calendar quarter 
must be rounded to the nearest hundredth (fractional values equal to 
or greater than 0.005 must be rounded up).
    (c) The estimated number of exceedances for the year, e, is the 
sum of the estimates for each calendar quarter.

Equation 2
[GRAPHIC] [TIFF OMITTED] TR18JY97.181

    (d) The estimated number of exceedances for a single year must 
be rounded to one decimal place (fractional values equal to or 
greater than 0.05 are to be rounded up). The expected number of 
exceedances is then estimated by averaging the individual annual 
estimates for the most recent 3 or more representative years of 
data. The expected number of exceedances must be rounded to one 
decimal place (fractional values equal to or greater than 0.05 are 
to be rounded up).
    (e) The adjustment for incomplete data will not be necessary for 
monitoring or modeling data which constitutes a complete record, 
i.e., 365 days per year.
    (f) To reduce the potential for overestimating the number of 
expected exceedances, the correction for missing data will not be 
required for a calendar quarter in which the first observed 
exceedance has occurred if:
    (1) There was only one exceedance in the calendar quarter;
    (2) Everyday sampling is subsequently initiated and maintained 
for 4 calendar quarters in accordance with 40 CFR 58.13; and
    (3) Data capture of 75 percent is achieved during the required 
period of everyday sampling. In addition, if the first exceedance is 
observed in a calendar quarter in which the monitor is already 
sampling every day, no adjustment for missing data will be made to 
the first exceedance if a 75 percent data capture rate was achieved 
in the quarter in which it was observed.

Example 1

    a. During a particular calendar quarter, 39 out of a possible 92 
samples were recorded, with one observed exceedance of the 24-hour 
standard. Using Equation 1, the estimated number of exceedances for 
the quarter is:

eq=1 x 92/39=2.359 or 2.36.

    b. If the estimated exceedances for the other 3 calendar 
quarters in the year were 2.30, 0.0 and 0.0, then, using Equation 2, 
the estimated number of exceedances for the year is 
2.36+2.30+0.0+0.0 which equals 4.66 or 4.7. If no exceedances were 
observed for the 2 previous years, then the expected number of 
exceedances is estimated by: (1/3) x (4.7+0+0)=1.57 or 1.6. Since 
1.6 exceeds the allowable number of expected exceedances, this 
monitoring site would fail the attainment test.

Example 2

    In this example, everyday sampling was initiated following the 
first observed exceedance as required by 40 CFR 58.13. Accordingly, 
the first observed exceedance would not be adjusted for incomplete 
sampling. During the next three quarters, 1.2 exceedances were 
estimated. In this case, the estimated exceedances for the year 
would be 1.0+1.2+0.0+0.0 which equals 2.2. If, as before, no 
exceedances were observed for the two previous years, then the 
estimated exceedances for the 3-year period would then be (1/
3) x (2.2+0.0+0.0)=0.7, and the monitoring site would not fail the 
attainment test.
    3.2 Adjustments for Non-Scheduled Sampling Days.
    (a) If a systematic sampling schedule is used and sampling is 
performed on days in addition to the days specified by the 
systematic sampling schedule, e.g., during episodes of high 
pollution, then an adjustment must be made in the eqution for the 
estimation of exceedances. Such an adjustment is needed to eliminate 
the bias in the estimate of the quarterly and annual number of 
exceedances that would occur if the chance of an exceedance is 
different for scheduled than for non-scheduled days, as would be the 
case with episode sampling.
    (b) The required adjustment treats the systematic sampling 
schedule as a stratified sampling plan. If the period from one 
scheduled sample until the day preceding the next scheduled sample 
is defined as a sampling stratum, then there is one stratum for each 
scheduled sampling day. An average number of observed exceedances is 
computed for each of these sampling strata. With nonscheduled 
sampling days, the estimated number of exceedances is defined as:

Equation 3
[GRAPHIC] [TIFF OMITTED] TR18JY97.182

where:

eq=the estimated number of exceedances for the quarter;

Nq=the number of days in the quarter;

mq=the number of strata with samples during the quarter;

vj=the number of observed exceedances in stratum j; and

kj=the number of actual samples in stratum j.

    (c) Note that if only one sample value is recorded in each 
stratum, then Equation 3 reduces to Equation 1.

Example 3

    A monitoring site samples according to a systematic sampling 
schedule of one sample every 6 days, for a total of 15 scheduled 
samples in a quarter out of a total of 92 possible samples. During 
one 6-day period, potential episode levels of PM10 were 
suspected, so 5 additional samples were taken. One of the regular 
scheduled samples was missed, so a total of 19 samples in 14 
sampling strata were measured. The one 6-day sampling stratum with 6 
samples recorded 2 exceedances. The remainder of the quarter with 
one sample per stratum recorded zero exceedances. Using Equation 3, 
the estimated number of exceedances for the quarter is:

eq=(92/14) x (2/6+0+. . .+0)=2.19.


[[Page 38714]]


4.0 Computational Equations for Annual Standards.
    4.1 Calculation of the Annual Arithmetic Mean. (a) An annual 
arithmetic mean value for PM10 is determined by averaging 
the quarterly means for the 4 calendar quarters of the year. The 
following equation is to be used for calculation of the mean for a 
calendar quarter:

Equation 4
[GRAPHIC] [TIFF OMITTED] TR18JY97.183

where:
xq= the quarterly mean concentration for quarter q, q=1, 
2, 3, or 4,

nq= the number of samples in the quarter, and

xi= the ith concentration value recorded in the quarter.

    (b) The quarterly mean, expressed in g/m\3\, must be 
rounded to the nearest tenth (fractional values of 0.05 should be 
rounded up).
    (c) The annual mean is calculated by using the following 
equation:

Equation 5
[GRAPHIC] [TIFF OMITTED] TR18JY97.184

where:

x=the annual mean; and

xq=the mean for calendar quarter q.

    (d) The average of quarterly means must be rounded to the 
nearest tenth (fractional values of 0.05 should be rounded up).
    (e) The use of quarterly averages to compute the annual average 
will not be necessary for monitoring or modeling data which results 
in a complete record, i.e., 365 days per year.
    (f) The expected annual mean is estimated as the average of 
three or more annual means. This multi-year estimate, expressed in 
g/m\3\, shall be rounded to the nearest integer for 
comparison with the annual standard (fractional values of 0.5 should 
be rounded up).

Example 4

    Using Equation 4, the quarterly means are calculated for each 
calendar quarter. If the quarterly means are 52.4, 75.3, 82.1, and 
63.2 g/m \3\, then the annual mean is:

x = (1/4) x (52.4+75.3+82.1+63.2)= 68.25 or 68.3.

    4.2  Adjustments for Non-scheduled Sampling Days. (a) An 
adjustment in the calculation of the annual mean is needed if 
sampling is performed on days in addition to the days specified by 
the systematic sampling schedule. For the same reasons given in the 
discussion of estimated exceedances, under section 3.2 of this 
appendix, the quarterly averages would be calculated by using the 
following equation:

Equation 6
[GRAPHIC] [TIFF OMITTED] TR18JY97.185

where:

xq=the quarterly mean concentration for quarter q, q=1, 
2, 3, or 4;

xij=the ith concentration value recorded in stratum j;

kj=the number of actual samples in stratum j; and

mq=the number of strata with data in the quarter.

    (b) If one sample value is recorded in each stratum, Equation 6 
reduces to a simple arithmetic average of the observed values as 
described by Equation 4.

Example 5

    a. During one calendar quarter, 9 observations were recorded. 
These samples were distributed among 7 sampling strata, with 3 
observations in one stratum. The concentrations of the 3 
observations in the single stratum were 202, 242, and 180 
g/m\3\. The remaining 6 observed concentrations were 55, 
68, 73, 92, 120, and 155 g/m\3\. Applying the weighting 
factors specified in Equation 6, the quarterly mean is:

xq = (1/7)  x  [(1/3)  x  (202 + 242 + 180) + 155 + 68 + 
73 + 92 + 120 + 155] = 110.1

    b. Although 24-hour measurements are rounded to the nearest 10 
g/m\3\ for determinations of exceedances of the 24-hour 
standard, note that these values are rounded to the nearest 1 
g/m\3\ for the calculation of means.
    6. Appendix L is added to read as follows:

Appendix L to Part 50--Reference Method For the Determination of 
Fine Particulate Matter as PM2.5 in the Atmosphere

1.0 Applicability.
    1.1 This method provides for the measurement of the mass 
concentration of fine particulate matter having an aerodynamic 
diameter less than or equal to a nominal 2.5 micrometers 
(PM2.5) in ambient air over a 24-hour period for purposes 
of determining whether the primary and secondary national ambient 
air quality standards for fine particulate matter specified in 
Sec. 50.6 of this part are met. The measurement process is 
considered to be nondestructive, and the PM2.5 sample 
obtained can be subjected to subsequent physical or chemical 
analyses. Quality assessment procedures are provided in part 58, 
Appendix A of this chapter, and quality assurance guidance are 
provided in references 1, 2, and 3 in section 13.0 of this appendix.
    1.2 This method will be considered a reference method for 
purposes of part 58 of this chapter only if:
    (a) The associated sampler meets the requirements specified in 
this appendix and the applicable requirements in part 53 of this 
chapter, and
    (b) The method and associated sampler have been designated as a 
reference method in accordance with part 53 of this chapter.
    1.3 PM2.5 samplers that meet nearly all 
specifications set forth in this method but have minor deviations 
and/or modifications of the reference method sampler will be 
designated as ``Class I'' equivalent methods for PM2.5 in 
accordance with part 53 of this chapter.
2.0 Principle.
    2.1 An electrically powered air sampler draws ambient air at a 
constant volumetric flow rate into a specially shaped inlet and 
through an inertial particle size separator (impactor) where the 
suspended particulate matter in the PM2.5 size range is 
separated for collection on a polytetrafluoroethylene (PTFE) filter 
over the specified sampling period. The air sampler and other 
aspects of this reference method are specified either explicitly in 
this appendix or generally with reference to other applicable 
regulations or quality assurance guidance.
    2.2 Each filter is weighed (after moisture and temperature 
conditioning) before and after sample collection to determine the 
net gain due to collected PM2.5. The total volume of air 
sampled is determined by the sampler from the measured flow rate at 
actual ambient temperature and pressure and the sampling time. The 
mass concentration of PM2.5 in the ambient air is 
computed as the total mass of collected particles in the 
PM2.5 size range divided by the actual volume of air 
sampled, and is expressed in micrograms per cubic meter of air 
(g/m3).
3.0 PM2.5 Measurement Range.
    3.1 Lower concentration limit. The lower detection limit of the 
mass concentration measurement range is estimated to be 
approximately 2 g/am3, based on noted mass 
changes in field blanks in conjunction with the 24 m3 
nominal total air sample volume specified for the 24-hour sample.
    3.2 Upper concentration limit. The upper limit of the mass 
concentration range is determined by the filter mass loading beyond 
which the sampler can no longer maintain the operating flow rate 
within specified limits due to increased pressure drop across the 
loaded filter. This upper limit cannot be specified precisely 
because it is a complex function of the ambient particle size 
distribution and type, humidity, the individual filter used, the 
capacity of the sampler flow rate control system, and perhaps other 
factors. Nevertheless, all samplers are estimated to be capable of 
measuring 24-hour PM2.5 mass concentrations of at least 
200 g/m3 while maintaining the operating flow 
rate within the specified limits.
    3.3 Sample period. The required sample period for 
PM2.5 concentration measurements by this method shall be 
1,380 to 1500 minutes (23 to 25 hours). However, when a sample 
period is less than 1,380 minutes, the measured concentration (as 
determined by the collected PM2.5 mass divided by the 
actual sampled air volume), multiplied by the actual number of 
minutes in the sample period and divided by 1,440, may be used as if 
it were a valid concentration measurement for the specific purpose 
of determining a violation of the NAAQS. This value assumes

[[Page 38715]]

that the PM2.5 concentration is zero for the remaining 
portion of the sample period and therefore represents the minimum 
concentration that could have been measured for the full 24-hour 
sample period. Accordingly, if the value thus calculated is high 
enough to be an exceedance, such an exceedance would be a valid 
exceedance for the sample period. When reported to AIRS, this data 
value should receive a special code to identify it as not to be 
commingled with normal concentration measurements or used for other 
purposes.
4.0 Accuracy.
    4.1 Because the size and volatility of the particles making up 
ambient particulate matter vary over a wide range and the mass 
concentration of particles varies with particle size, it is 
difficult to define the accuracy of PM2.5 measurements in 
an absolute sense. The accuracy of PM2.5 measurements is 
therefore defined in a relative sense, referenced to measurements 
provided by this reference method. Accordingly, accuracy shall be 
defined as the degree of agreement between a subject field 
PM2.5 sampler and a collocated PM2.5 reference 
method audit sampler operating simultaneously at the monitoring site 
location of the subject sampler and includes both random (precision) 
and systematic (bias) errors. The requirements for this field 
sampler audit procedure are set forth in part 58, Appendix A of this 
chapter.
    4.2 Measurement system bias. Results of collocated measurements 
where the duplicate sampler is a reference method sampler are used 
to assess a portion of the measurement system bias according to the 
schedule and procedure specified in part 58, Appendix A of this 
chapter.
    4.3 Audits with reference method samplers to determine system 
accuracy and bias. According to the schedule and procedure specified 
in part 58, Appendix A of this chapter, a reference method sampler 
is required to be located at each of selected PM2.5 SLAMS 
sites as a duplicate sampler. The results from the primary sampler 
and the duplicate reference method sampler are used to calculate 
accuracy of the primary sampler on a quarterly basis, bias of the 
primary sampler on an annual basis, and bias of a single reporting 
organization on an annual basis. Reference 2 in section 13.0 of this 
appendix provides additional information and guidance on these 
reference method audits.
    4.4 Flow rate accuracy and bias. Part 58, Appendix A of this 
chapter requires that the flow rate accuracy and bias of individual 
PM2.5 samplers used in SLAMS monitoring networks be 
assessed periodically via audits of each sampler's operational flow 
rate. In addition, part 58, Appendix A of this chapter requires that 
flow rate bias for each reference and equivalent method operated by 
each reporting organization be assessed quarterly and annually. 
Reference 2 in section 13.0 of this appendix provides additional 
information and guidance on flow rate accuracy audits and 
calculations for accuracy and bias.
5.0 Precision. A data quality objective of 10 percent coefficient of 
variation or better has been established for the operational 
precision of PM2.5 monitoring data.
    5.1 Tests to establish initial operational precision for each 
reference method sampler are specified as a part of the requirements 
for designation as a reference method under Sec. 53.58 of this 
chapter.
    5.2 Measurement System Precision. Collocated sampler results, 
where the duplicate sampler is not a reference method sampler but is 
a sampler of the same designated method as the primary sampler, are 
used to assess measurement system precision according to the 
schedule and procedure specified in part 58, Appendix A of this 
chapter. Part 58, Appendix A of this chapter requires that these 
collocated sampler measurements be used to calculate quarterly and 
annual precision estimates for each primary sampler and for each 
designated method employed by each reporting organization. Reference 
2 in section 13.0 of this appendix provides additional information 
and guidance on this requirement.
6.0 Filter for PM2.5 Sample Collection. Any filter 
manufacturer or vendor who sells or offers to sell filters 
specifically identified for use with this PM2.5 reference 
method shall certify that the required number of filters from each 
lot of filters offered for sale as such have been tested as 
specified in this section 6.0 and meet all of the following design 
and performance specifications.
    6.1 Size. Circular, 46.2 mm diameter 0.25 mm.
    6.2 Medium. Polytetrafluoroethylene (PTFE Teflon), with integral 
support ring.
    6.3 Support ring. Polymethylpentene (PMP) or equivalent inert 
material, 0.38 0.04 mm thick, outer diameter 46.2 mm 
0.25 mm, and width of 3.68 mm ( 0.00, -0.51 
mm).
    6.4 Pore size. 2 m as measured by ASTM F 316-94.
    6.5 Filter thickness. 30 to 50 m.
    6.6 Maximum pressure drop (clean filter). 30 cm H2O 
column @ 16.67 L/min clean air flow.
    6.7 Maximum moisture pickup. Not more than 10 g weight 
increase after 24-hour exposure to air of 40 percent relative 
humidity, relative to weight after 24-hour exposure to air of 35 
percent relative humidity.
    6.8 Collection efficiency. Greater than 99.7 percent, as 
measured by the DOP test (ASTM D 2986-91) with 0.3 m 
particles at the sampler's operating face velocity.
    6.9 Filter weight stability. Filter weight loss shall be less 
than 20 g, as measured in each of the following two tests 
specified in sections 6.9.1 and 6.9.2 of this appendix. The 
following conditions apply to both of these tests: Filter weight 
loss shall be the average difference between the initial and the 
final filter weights of a random sample of test filters selected 
from each lot prior to sale. The number of filters tested shall be 
not less than 0.1 percent of the filters of each manufacturing lot, 
or 10 filters, whichever is greater. The filters shall be weighed 
under laboratory conditions and shall have had no air sample passed 
through them, i.e., filter blanks. Each test procedure must include 
initial conditioning and weighing, the test, and final conditioning 
and weighing. Conditioning and weighing shall be in accordance with 
sections 8.0 through 8.2 of this appendix and general guidance 
provided in reference 2 of section 13.0 of this appendix.
    6.9.1 Test for loose, surface particle contamination. After the 
initial weighing, install each test filter, in turn, in a filter 
cassette (Figures L-27, L-28, and L-29 of this appendix) and drop 
the cassette from a height of 25 cm to a flat hard surface, such as 
a particle-free wood bench. Repeat two times, for a total of three 
drop tests for each test filter. Remove the test filter from the 
cassette and weigh the filter. The average change in weight must be 
less than 20 g.
    6.9.2 Test for temperature stability. After weighing each 
filter, place the test filters in a drying oven set at 40  deg.C 
2  deg.C for not less than 48 hours. Remove, condition, 
and reweigh each test filter. The average change in weight must be 
less than 20 g.
    6.10 Alkalinity. Less than 25 microequivalents/gram of filter, 
as measured by the guidance given in reference 2 in section 13.0 of 
this appendix.
    6.11 Supplemental requirements. Although not required for 
determination of PM2.5 mass concentration under this 
reference method, additional specifications for the filter must be 
developed by users who intend to subject PM2.5 filter 
samples to subsequent chemical analysis. These supplemental 
specifications include background chemical contamination of the 
filter and any other filter parameters that may be required by the 
method of chemical analysis. All such supplemental filter 
specifications must be compatible with and secondary to the primary 
filter specifications given in this section 6.0 of this appendix.
7.0 PM2.5 Sampler.
    7.1 Configuration. The sampler shall consist of a sample air 
inlet, downtube, particle size separator (impactor), filter holder 
assembly, air pump and flow rate control system, flow rate 
measurement device, ambient and filter temperature monitoring 
system, barometric pressure measurement system, timer, outdoor 
environmental enclosure, and suitable mechanical, electrical, or 
electronic control capability to meet or exceed the design and 
functional performance as specified in this section 7.0 of this 
appendix. The performance specifications require that the sampler:
    (a) Provide automatic control of sample volumetric flow rate and 
other operational parameters.
    (b) Monitor these operational parameters as well as ambient 
temperature and pressure.
    (c) Provide this information to the sampler operator at the end 
of each sample period in digital form, as specified in Table L-1 of 
section 7.4.19 of this appendix.
    7.2 Nature of specifications. The PM2.5 sampler is 
specified by a combination of design and performance requirements. 
The sample inlet, downtube, particle size discriminator, filter 
cassette, and the internal configuration of the filter holder 
assembly are specified explicitly by design figures and associated 
mechanical dimensions, tolerances, materials, surface finishes, 
assembly instructions, and other necessary specifications. All other 
aspects of the

[[Page 38716]]

sampler are specified by required operational function and 
performance, and the design of these other aspects (including the 
design of the lower portion of the filter holder assembly) is 
optional, subject to acceptable operational performance. Test 
procedures to demonstrate compliance with both the design and 
performance requirements are set forth in subpart E of part 53 of 
this chapter.
    7.3 Design specifications. Except as indicated in this section 
7.3 of this appendix, these components must be manufactured or 
reproduced exactly as specified, in an ISO 9001-registered facility, 
with registration initially approved and subsequently maintained 
during the period of manufacture. See Sec. 53.1(t) of this chapter 
for the definition of an ISO-registered facility. Minor 
modifications or variances to one or more components that clearly 
would not affect the aerodynamic performance of the inlet, downtube, 
impactor, or filter cassette will be considered for specific 
approval. Any such proposed modifications shall be described and 
submitted to the EPA for specific individual acceptability either as 
part of a reference or equivalent method application under part 53 
of this chapter or in writing in advance of such an intended 
application under part 53 of this chapter.
    7.3.1 Sample inlet assembly. The sample inlet assembly, 
consisting of the inlet, downtube, and impactor shall be configured 
and assembled as indicated in Figure L-1 of this appendix and shall 
meet all associated requirements. A portion of this assembly shall 
also be subject to the maximum overall sampler leak rate 
specification under section 7.4.6 of this appendix.
    7.3.2 Inlet. The sample inlet shall be fabricated as indicated 
in Figures L-2 through L-18 of this appendix and shall meet all 
associated requirements.
    7.3.3 Downtube. The downtube shall be fabricated as indicated in 
Figure L-19 of this appendix and shall meet all associated 
requirements.
    7.3.4 Impactor.
    7.3.4.1 The impactor (particle size separator) shall be 
fabricated as indicated in Figures L-20 through L-24 of this 
appendix and shall meet all associated requirements. Following the 
manufacture and finishing of each upper impactor housing (Figure L-
21 of this appendix), the dimension of the impaction jet must be 
verified by the manufacturer using Class ZZ go/no-go plug gauges 
that are traceable to NIST.
    7.3.4.2 Impactor filter specifications:
    (a) Size. Circular, 35 to 37 mm diameter.
    (b) Medium. Borosilicate glass fiber, without binder.
    (c) Pore size. 1 to 1.5 micrometer, as measured by ASTM F 316-
80.
    (d) Thickness. 300 to 500 micrometers.
    7.3.4.3 Impactor oil specifications:
    (a) Composition. Tetramethyltetraphenyltrisiloxane, single-
compound diffusion oil.
    (b) Vapor pressure. Maximum 2 x 10-8 mm Hg at 25 
deg.C.
    (c) Viscosity. 36 to 40 centistokes at 25  deg.C.
    (d) Density. 1.06 to 1.07 g/cm3 at 25  deg.C.
    (e) Quantity. 1 mL 0.1 mL.
    7.3.5 Filter holder assembly. The sampler shall have a sample 
filter holder assembly to adapt and seal to the down tube and to 
hold and seal the specified filter, under section 6.0 of this 
appendix, in the sample air stream in a horizontal position below 
the downtube such that the sample air passes downward through the 
filter at a uniform face velocity. The upper portion of this 
assembly shall be fabricated as indicated in Figures L-25 and L-26 
of this appendix and shall accept and seal with the filter cassette, 
which shall be fabricated as indicated in Figures L-27 through L-29 
of this appendix.
    (a) The lower portion of the filter holder assembly shall be of 
a design and construction that:
    (1) Mates with the upper portion of the assembly to complete the 
filter holder assembly,
    (2) Completes both the external air seal and the internal filter 
cassette seal such that all seals are reliable over repeated filter 
changings, and
    (3) Facilitates repeated changing of the filter cassette by the 
sampler operator.
    (b) Leak-test performance requirements for the filter holder 
assembly are included in section 7.4.6 of this appendix.
    (c) If additional or multiple filters are stored in the sampler 
as part of an automatic sequential sample capability, all such 
filters, unless they are currently and directly installed in a 
sampling channel or sampling configuration (either active or 
inactive), shall be covered or (preferably) sealed in such a way as 
to:
    (1) Preclude significant exposure of the filter to possible 
contamination or accumulation of dust, insects, or other material 
that may be present in the ambient air, sampler, or sampler 
ventilation air during storage periods either before or after 
sampling; and
    (2) To minimize loss of volatile or semi-volatile PM sample 
components during storage of the filter following the sample period.
    7.3.6 Flow rate measurement adapter. A flow rate measurement 
adapter as specified in Figure L-30 of this appendix shall be 
furnished with each sampler.
    7.3.7 Surface finish. All internal surfaces exposed to sample 
air prior to the filter shall be treated electrolytically in a 
sulfuric acid bath to produce a clear, uniform anodized surface 
finish of not less than 1000 mg/ft2 (1.08 mg/
cm2) in accordance with military standard specification 
(mil. spec.) 8625F, Type II, Class 1 in reference 4 of section 13.0 
of this appendix. This anodic surface coating shall not be dyed or 
pigmented. Following anodization, the surfaces shall be sealed by 
immersion in boiling deionized water for not less than 15 minutes. 
Section 53.51(d)(2) of this chapter should also be consulted.
    7.3.8 Sampling height. The sampler shall be equipped with legs, 
a stand, or other means to maintain the sampler in a stable, upright 
position and such that the center of the sample air entrance to the 
inlet, during sample collection, is maintained in a horizontal plane 
and is 2.0 0.2 meters above the floor or other 
horizontal supporting surface. Suitable bolt holes, brackets, tie-
downs, or other means should be provided to facilitate mechanically 
securing the sample to the supporting surface to prevent toppling of 
the sampler due to wind.
    7.4 Performance specifications.
    7.4.1 Sample flow rate. Proper operation of the impactor 
requires that specific air velocities be maintained through the 
device. Therefore, the design sample air flow rate through the inlet 
shall be 16.67 L/min (1.000 m3/hour) measured as actual 
volumetric flow rate at the temperature and pressure of the sample 
air entering the inlet.
    7.4.2 Sample air flow rate control system. The sampler shall 
have a sample air flow rate control system which shall be capable of 
providing a sample air volumetric flow rate within the specified 
range, under section 7.4.1 of this appendix, for the specified 
filter, under section 6.0 of this appendix, at any atmospheric 
conditions specified, under section 7.4.7 of this appendix, at a 
filter pressure drop equal to that of a clean filter plus up to 75 
cm water column (55 mm Hg), and over the specified range of supply 
line voltage, under section 7.4.15.1 of this appendix. This flow 
control system shall allow for operator adjustment of the 
operational flow rate of the sampler over a range of at least 
15 percent of the flow rate specified in section 7.4.1 
of this appendix.
    7.4.3 Sample flow rate regulation. The sample flow rate shall be 
regulated such that for the specified filter, under section 6.0 of 
this appendix, at any atmospheric conditions specified, under 
section 7.4.7 of this appendix, at a filter pressure drop equal to 
that of a clean filter plus up to 75 cm water column (55 mm Hg), and 
over the specified range of supply line voltage, under section 
7.4.15.1 of this appendix, the flow rate is regulated as follows:
    7.4.3.1 The volumetric flow rate, measured or averaged over 
intervals of not more than 5 minutes over a 24-hour period, shall 
not vary more than 5 percent from the specified 16.67 L/
min flow rate over the entire sample period.
    7.4.3.2 The coefficient of variation (sample standard deviation 
divided by the mean) of the flow rate, measured over a 24-hour 
period, shall not be greater than 2 percent.
    7.4.3.3 The amplitude of short-term flow rate pulsations, such 
as may originate from some types of vacuum pumps, shall be 
attenuated such that they do not cause significant flow measurement 
error or affect the collection of particles on the particle 
collection filter.
    7.4.4 Flow rate cut off. The sampler's sample air flow rate 
control system shall terminate sample collection and stop all sample 
flow for the remainder of the sample period in the event that the 
sample flow rate deviates by more than 10 percent from the sampler 
design flow rate specified in section 7.4.1 of this appendix for 
more than 60 seconds. However, this sampler cut-off provision shall 
not apply during periods when the sampler is inoperative due to a 
temporary power interruption, and the elapsed time of the 
inoperative period shall not be included in the total sample time 
measured and reported by the sampler, under section 7.4.13 of this 
appendix.
    7.4.5 Flow rate measurement.
    7.4.5.1 The sampler shall provide a means to measure and 
indicate the instantaneous sample air flow rate, which shall be 
measured as volumetric flow rate at the

[[Page 38717]]

temperature and pressure of the sample air entering the inlet, with 
an accuracy of 2 percent. The measured flow rate shall 
be available for display to the sampler operator at any time in 
either sampling or standby modes, and the measurement shall be 
updated at least every 30 seconds. The sampler shall also provide a 
simple means by which the sampler operator can manually start the 
sample flow temporarily during non-sampling modes of operation, for 
the purpose of checking the sample flow rate or the flow rate 
measurement system.
    7.4.5.2 During each sample period, the sampler's flow rate 
measurement system shall automatically monitor the sample volumetric 
flow rate, obtaining flow rate measurements at intervals of not 
greater than 30 seconds.
    (a) Using these interval flow rate measurements, the sampler 
shall determine or calculate the following flow-related parameters, 
scaled in the specified engineering units:
    (1) The instantaneous or interval-average flow rate, in L/min.
    (2) The value of the average sample flow rate for the sample 
period, in L/min.
    (3) The value of the coefficient of variation (sample standard 
deviation divided by the average) of the sample flow rate for the 
sample period, in percent.
    (4) The occurrence of any time interval during the sample period 
in which the measured sample flow rate exceeds a range of 
5 percent of the average flow rate for the sample period 
for more than 5 minutes, in which case a warning flag indicator 
shall be set.
    (5) The value of the integrated total sample volume for the 
sample period, in m3.
    (b) Determination or calculation of these values shall properly 
exclude periods when the sampler is inoperative due to temporary 
interruption of electrical power, under section 7.4.13 of this 
appendix, or flow rate cut off, under section 7.4.4 of this 
appendix.
    (c) These parameters shall be accessible to the sampler operator 
as specified in Table L-1 of section 7.4.19 of this appendix. In 
addition, it is strongly encouraged that the flow rate for each 5-
minute interval during the sample period be available to the 
operator following the end of the sample period.
    7.4.6 Leak test capability.
    7.4.6.1 External leakage. The sampler shall include an external 
air leak-test capability consisting of components, accessory 
hardware, operator interface controls, a written procedure in the 
associated Operation/Instruction Manual, under section 7.4.18 of 
this appendix, and all other necessary functional capability to 
permit and facilitate the sampler operator to conveniently carry out 
a leak test of the sampler at a field monitoring site without 
additional equipment. The sampler components to be subjected to this 
leak test include all components and their interconnections in which 
external air leakage would or could cause an error in the sampler's 
measurement of the total volume of sample air that passes through 
the sample filter.
    (a) The suggested technique for the operator to use for this 
leak test is as follows:
    (1) Remove the sampler inlet and installs the flow rate 
measurement adapter supplied with the sampler, under section 7.3.6 
of this appendix.
    (2) Close the valve on the flow rate measurement adapter and use 
the sampler air pump to draw a partial vacuum in the sampler, 
including (at least) the impactor, filter holder assembly (filter in 
place), flow measurement device, and interconnections between these 
devices, of at least 55 mm Hg (75 cm water column), measured at a 
location downstream of the filter holder assembly.
    (3) Plug the flow system downstream of these components to 
isolate the components under vacuum from the pump, such as with a 
built-in valve.
    (4) Stop the pump.
    (5) Measure the trapped vacuum in the sampler with a built-in 
pressure measuring device.
    (6) (i) Measure the vacuum in the sampler with the built-in 
pressure measuring device again at a later time at least 10 minutes 
after the first pressure measurement.
    (ii) Caution: Following completion of the test, the adaptor 
valve should be opened slowly to limit the flow rate of air into the 
sampler. Excessive air flow rate may blow oil out of the impactor.
    (7) Upon completion of the test, open the adaptor valve, remove 
the adaptor and plugs, and restore the sampler to the normal 
operating configuration.
    (b) The associated leak test procedure shall require that for 
successful passage of this test, the difference between the two 
pressure measurements shall not be greater than the number of mm of 
Hg specified for the sampler by the manufacturer, based on the 
actual internal volume of the sampler, that indicates a leak of less 
than 80 mL/min.
    (c) Variations of the suggested technique or an alternative 
external leak test technique may be required for samplers whose 
design or configuration would make the suggested technique 
impossible or impractical. The specific proposed external leak test 
procedure, or particularly an alternative leak test technique, 
proposed for a particular candidate sampler may be described and 
submitted to the EPA for specific individual acceptability either as 
part of a reference or equivalent method application under part 53 
of this chapter or in writing in advance of such an intended 
application under part 53 of this chapter.
    7.4.6.2 Internal, filter bypass leakage. The sampler shall 
include an internal, filter bypass leak-check capability consisting 
of components, accessory hardware, operator interface controls, a 
written procedure in the Operation/Instruction Manual, and all other 
necessary functional capability to permit and facilitate the sampler 
operator to conveniently carry out a test for internal filter bypass 
leakage in the sampler at a field monitoring site without additional 
equipment. The purpose of the test is to determine that any portion 
of the sample flow rate that leaks past the sample filter without 
passing through the filter is insignificant relative to the design 
flow rate for the sampler.
    (a) The suggested technique for the operator to use for this 
leak test is as follows:
    (1) Carry out an external leak test as provided under section 
7.4.6.1 of this appendix which indicates successful passage of the 
prescribed external leak test.
    (2) Install a flow-impervious membrane material in the filter 
cassette, either with or without a filter, as appropriate, which 
effectively prevents air flow through the filter.
    (3) Use the sampler air pump to draw a partial vacuum in the 
sampler, downstream of the filter holder assembly, of at least 55 mm 
Hg (75 cm water column).
    (4) Plug the flow system downstream of the filter holder to 
isolate the components under vacuum from the pump, such as with a 
built-in valve.
    (5) Stop the pump.
    (6) Measure the trapped vacuum in the sampler with a built-in 
pressure measuring device.
    (7) Measure the vacuum in the sampler with the built-in pressure 
measuring device again at a later time at least 10 minutes after the 
first pressure measurement.
    (8) Remove the flow plug and membrane and restore the sampler to 
the normal operating configuration.
    (b) The associated leak test procedure shall require that for 
successful passage of this test, the difference between the two 
pressure measurements shall not be greater than the number of mm of 
Hg specified for the sampler by the manufacturer, based on the 
actual internal volume of the portion of the sampler under vacuum, 
that indicates a leak of less than 80 mL/min.
    (c) Variations of the suggested technique or an alternative 
internal, filter bypass leak test technique may be required for 
samplers whose design or configuration would make the suggested 
technique impossible or impractical. The specific proposed internal 
leak test procedure, or particularly an alternative internal leak 
test technique proposed for a particular candidate sampler may be 
described and submitted to the EPA for specific individual 
acceptability either as part of a reference or equivalent method 
application under part 53 of this chapter or in writing in advance 
of such intended application under part 53 of this chapter.
    7.4.7 Range of operational conditions. The sampler is required 
to operate properly and meet all requirements specified in this 
appendix over the following operational ranges.
    7.4.7.1 Ambient temperature. -30 to +45  deg.C (Note: Although 
for practical reasons, the temperature range over which samplers are 
required to be tested under part 53 of this chapter is -20 to +40 
deg.C, the sampler shall be designed to operate properly over this 
wider temperature range.).
    7.4.7.2 Ambient relative humidity. 0 to 100 percent.
    7.4.7.3 Barometric pressure range. 600 to 800 mm Hg.
    7.4.8 Ambient temperature sensor. The sampler shall have 
capability to measure the temperature of the ambient air surrounding 
the sampler over the range of -30 to +45  deg.C, with a resolution 
of 0.1  deg.C and accuracy of 2.0  deg.C, referenced as 
described in reference 3 in section 13.0 of this appendix, with and 
without maximum solar insolation.

[[Page 38718]]

    7.4.8.1 The ambient temperature sensor shall be mounted external 
to the sampler enclosure and shall have a passive, naturally 
ventilated sun shield. The sensor shall be located such that the 
entire sun shield is at least 5 cm above the horizontal plane of the 
sampler case or enclosure (disregarding the inlet and downtube) and 
external to the vertical plane of the nearest side or protuberance 
of the sampler case or enclosure. The maximum temperature 
measurement error of the ambient temperature measurement system 
shall be less than 1.6  deg.C at 1 m/s wind speed and 1000 W/m2 
solar radiation intensity.
    7.4.8.2 The ambient temperature sensor shall be of such a design 
and mounted in such a way as to facilitate its convenient 
dismounting and immersion in a liquid for calibration and comparison 
to the filter temperature sensor, under section 7.4.11 of this 
appendix.
    7.4.8.3 This ambient temperature measurement shall be updated at 
least every 30 seconds during both sampling and standby (non-
sampling) modes of operation. A visual indication of the current 
(most recent) value of the ambient temperature measurement, updated 
at least every 30 seconds, shall be available to the sampler 
operator during both sampling and standby (non-sampling) modes of 
operation, as specified in Table L-1 of section 7.4.19 of this 
appendix.
    7.4.8.4 This ambient temperature measurement shall be used for 
the purpose of monitoring filter temperature deviation from ambient 
temperature, as required by section 7.4.11 of this appendix, and may 
be used for purposes of effecting filter temperature control, under 
section 7.4.10 of this appendix, or computation of volumetric flow 
rate, under sections 7.4.1 to 7.4.5 of this appendix, if 
appropriate.
    7.4.8.5 Following the end of each sample period, the sampler 
shall report the maximum, minimum, and average temperature for the 
sample period, as specified in Table L-1 of section 7.4.19 of this 
appendix.
    7.4.9 Ambient barometric sensor. The sampler shall have 
capability to measure the barometric pressure of the air surrounding 
the sampler over a range of 600 to 800 mm Hg referenced as described 
in reference 3 in section 13.0 of this appendix; also see part 53, 
subpart E of this chapter. This barometric pressure measurement 
shall have a resolution of 5 mm Hg and an accuracy of 10 
mm Hg and shall be updated at least every 30 seconds. A visual 
indication of the value of the current (most recent) barometric 
pressure measurement, updated at least every 30 seconds, shall be 
available to the sampler operator during both sampling and standby 
(non-sampling) modes of operation, as specified in Table L-1 of 
section 7.4.19 of this appendix. This barometric pressure 
measurement may be used for purposes of computation of volumetric 
flow rate, under sections 7.4.1 to 7.4.5 of this appendix, if 
appropriate. Following the end of a sample period, the sampler shall 
report the maximum, minimum, and mean barometric pressures for the 
sample period, as specified in Table L-1 of section 7.4.19 of this 
appendix.
    7.4.10 Filter temperature control (sampling and post-sampling). 
The sampler shall provide a means to limit the temperature rise of 
the sample filter (all sample filters for sequential samplers), from 
insolation and other sources, to no more 5  deg.C above the 
temperature of the ambient air surrounding the sampler, during both 
sampling and post-sampling periods of operation. The post-sampling 
period is the non-sampling period between the end of the active 
sampling period and the time of retrieval of the sample filter by 
the sampler operator.
    7.4.11 Filter temperature sensor(s).
    7.4.11.1 The sampler shall have the capability to monitor the 
temperature of the sample filter (all sample filters for sequential 
samplers) over the range of -30 to +45  deg.C during both sampling 
and non-sampling periods. While the exact location of this 
temperature sensor is not explicitly specified, the filter 
temperature measurement system must demonstrate agreement, within 1 
deg.C, with a test temperature sensor located within 1 cm of the 
center of the filter downstream of the filter during both sampling 
and non-sampling modes, as specified in the filter temperature 
measurement test described in part 53, subpart E of this chapter. 
This filter temperature measurement shall have a resolution of 0.1 
deg.C and accuracy of 1.0  deg.C, referenced as 
described in reference 3 in section 13.0 of this appendix. This 
temperature sensor shall be of such a design and mounted in such a 
way as to facilitate its reasonably convenient dismounting and 
immersion in a liquid for calibration and comparison to the ambient 
temperature sensor under section 7.4.8 of this appendix.
    7.4.11.2 The filter temperature measurement shall be updated at 
least every 30 seconds during both sampling and standby (non-
sampling) modes of operation. A visual indication of the current 
(most recent) value of the filter temperature measurement, updated 
at least every 30 seconds, shall be available to the sampler 
operator during both sampling and standby (non-sampling) modes of 
operation, as specified in Table L-1 of section 7.4.19 of this 
appendix.
    7.4.11.3 For sequential samplers, the temperature of each filter 
shall be measured individually unless it can be shown, as specified 
in the filter temperature measurement test described in Sec. 53.57 
of this chapter, that the temperature of each filter can be 
represented by fewer temperature sensors.
    7.4.11.4 The sampler shall also provide a warning flag indicator 
following any occurrence in which the filter temperature (any filter 
temperature for sequential samplers) exceeds the ambient temperature 
by more than 5  deg.C for more than 30 consecutive minutes during 
either the sampling or post-sampling periods of operation, as 
specified in Table L-1 of section 7.4.19 of this appendix, under 
section 10.12 of this appendix, regarding sample validity when a 
warning flag occurs. It is further recommended (not required) that 
the sampler be capable of recording the maximum differential between 
the measured filter temperature and the ambient temperature and its 
time and date of occurrence during both sampling and post-sampling 
(non-sampling) modes of operation and providing for those data to be 
accessible to the sampler operator following the end of the sample 
period, as suggested in Table L-1 of section 7.4.19 of this 
appendix.
    7.4.12 Clock/timer system.
    (a) The sampler shall have a programmable real-time clock 
timing/control system that:
    (1) Is capable of maintaining local time and date, including 
year, month, day-of-month, hour, minute, and second to an accuracy 
of 1.0 minute per month.
    (2) Provides a visual indication of the current system time, 
including year, month, day-of-month, hour, and minute, updated at 
least each minute, for operator verification.
    (3) Provides appropriate operator controls for setting the 
correct local time and date.
    (4) Is capable of starting the sample collection period and 
sample air flow at a specific, operator-settable time and date, and 
stopping the sample air flow and terminating the sampler collection 
period 24 hours (1440 minutes) later, or at a specific, operator-
settable time and date.
    (b) These start and stop times shall be readily settable by the 
sampler operator to within 1.0 minute. The system shall 
provide a visual indication of the current start and stop time 
settings, readable to 1.0 minute, for verification by 
the operator, and the start and stop times shall also be available 
via the data output port, as specified in Table L-1 of section 
7.4.19 of this appendix. Upon execution of a programmed sample 
period start, the sampler shall automatically reset all sample 
period information and warning flag indications pertaining to a 
previous sample period. Refer also to section 7.4.15.4 of this 
appendix regarding retention of current date and time and programmed 
start and stop times during a temporary electrical power 
interruption.
    7.4.13 Sample time determination. The sampler shall be capable 
of determining the elapsed sample collection time for each 
PM2.5 sample, accurate to within 1.0 minute, 
measured as the time between the start of the sampling period, under 
section 7.4.12 of this appendix and the termination of the sample 
period, under section 7.4.12 of this appendix or section 7.4.4 of 
this appendix. This elapsed sample time shall not include periods 
when the sampler is inoperative due to a temporary interruption of 
electrical power, under section 7.4.15.4 of this appendix. In the 
event that the elapsed sample time determined for the sample period 
is not within the range specified for the required sample period in 
section 3.3 of this appendix, the sampler shall set a warning flag 
indicator. The date and time of the start of the sample period, the 
value of the elapsed sample time for the sample period, and the flag 
indicator status shall be available to the sampler operator 
following the end of the sample period, as specified in Table L-1 of 
section 7.4.19 of this appendix.
    7.4.14 Outdoor environmental enclosure. The sampler shall have 
an outdoor enclosure (or enclosures) suitable to protect the filter 
and other non-weatherproof components of the sampler from 
precipitation, wind, dust, extremes of temperature and humidity; to 
help maintain temperature control of the

[[Page 38719]]

filter (or filters, for sequential samplers); and to provide 
reasonable security for sampler components and settings.
    7.4.15 Electrical power supply.
    7.4.15.1 The sampler shall be operable and function as specified 
herein when operated on an electrical power supply voltage of 105 to 
125 volts AC (RMS) at a frequency of 59 to 61 Hz. Optional operation 
as specified at additional power supply voltages and/or frequencies 
shall not be precluded by this requirement.
    7.4.15.2 The design and construction of the sampler shall comply 
with all applicable National Electrical Code and Underwriters 
Laboratories electrical safety requirements.
    7.4.15.3 The design of all electrical and electronic controls 
shall be such as to provide reasonable resistance to interference or 
malfunction from ordinary or typical levels of stray electromagnetic 
fields (EMF) as may be found at various monitoring sites and from 
typical levels of electrical transients or electronic noise as may 
often or occasionally be present on various electrical power lines.
    7.4.15.4 In the event of temporary loss of electrical supply 
power to the sampler, the sampler shall not be required to sample or 
provide other specified functions during such loss of power, except 
that the internal clock/timer system shall maintain its local time 
and date setting within 1 minute per week, and the 
sampler shall retain all other time and programmable settings and 
all data required to be available to the sampler operator following 
each sample period for at least 7 days without electrical supply 
power. When electrical power is absent at the operator-set time for 
starting a sample period or is interrupted during a sample period, 
the sampler shall automatically start or resume sampling when 
electrical power is restored, if such restoration of power occurs 
before the operator-set stop time for the sample period.
    7.4.15.5 The sampler shall have the capability to record and 
retain a record of the year, month, day-of-month, hour, and minute 
of the start of each power interruption of more than 1 minute 
duration, up to 10 such power interruptions per sample period. (More 
than 10 such power interruptions shall invalidate the sample, except 
where an exceedance is measured, under section 3.3 of this 
appendix.) The sampler shall provide for these power interruption 
data to be available to the sampler operator following the end of 
the sample period, as specified in Table L-1 of section 7.4.19 of 
this appendix.
    7.4.16 Control devices and operator interface. The sampler shall 
have mechanical, electrical, or electronic controls, control 
devices, electrical or electronic circuits as necessary to provide 
the timing, flow rate measurement and control, temperature control, 
data storage and computation, operator interface, and other 
functions specified. Operator-accessible controls, data displays, 
and interface devices shall be designed to be simple, 
straightforward, reliable, and easy to learn, read, and operate 
under field conditions. The sampler shall have provision for 
operator input and storage of up to 64 characters of numeric (or 
alphanumeric) data for purposes of site, sampler, and sample 
identification. This information shall be available to the sampler 
operator for verification and change and for output via the data 
output port along with other data following the end of a sample 
period, as specified in Table L-1 of section 7.4.19 of this 
appendix. All data required to be available to the operator 
following a sample collection period or obtained during standby mode 
in a post-sampling period shall be retained by the sampler until 
reset, either manually by the operator or automatically by the 
sampler upon initiation of a new sample collection period.
    7.4.17 Data output port requirement. The sampler shall have a 
standard RS-232C data output connection through which digital data 
may be exported to an external data storage or transmission device. 
All information which is required to be available at the end of each 
sample period shall be accessible through this data output 
connection. The information that shall be accessible though this 
output port is summarized in Table L-1 of section 7.4.19 of this 
appendix. Since no specific format for the output data is provided, 
the sampler manufacturer or vendor shall make available to sampler 
purchasers appropriate computer software capable of receiving 
exported sampler data and correctly translating the data into a 
standard spreadsheet format and optionally any other formats as may 
be useful to sampler users. This requirement shall not preclude the 
sampler from offering other types of output connections in addition 
to the required RS-232C port.
    7.4.18 Operation/instruction manual. The sampler shall include 
an associated comprehensive operation or instruction manual, as 
required by part 53 of this chapter, which includes detailed 
operating instructions on the setup, operation, calibration, and 
maintenance of the sampler. This manual shall provide complete and 
detailed descriptions of the operational and calibration procedures 
prescribed for field use of the sampler and all instruments utilized 
as part of this reference method. The manual shall include adequate 
warning of potential safety hazards that may result from normal use 
or malfunction of the method and a description of necessary safety 
precautions. The manual shall also include a clear description of 
all procedures pertaining to installation, operation, periodic and 
corrective maintenance, and troubleshooting, and shall include parts 
identification diagrams.
    7.4.19 Data reporting requirements. The various information that 
the sampler is required to provide and how it is to be provided is 
summarized in the following Table L-1.

                                            Table L-1.--Summary of Information To Be Provided By the Sampler                                            
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                                             Availability                                        Format                 
                                    Appendix L section -------------------------------------------------------------------------------------------------
    Information to be provided          reference                        End of        Visual                                                           
                                                          Anytime1       period2      display3    Data output4    Digital reading5          Units       
--------------------------------------------------------------------------------------------------------------------------------------------------------
Flow rate, 30-second maximum       7.4.5.1............         ............                   *   XX.X...............  L/min              
 interval.                                                                                                                                              
Flow rate, average for the sample  7.4.5.2............            *                    *          XX.X...............  L/min              
 period.                                                                                                                                                
Flow rate, CV, for sample period.  7.4.5.2............            *                    *     XX.X...............  %                  
Flow rate, 5-min. average out of   7.4.5.2............                         On/Off.............  ...................
 spec. (FLAG6).                                                                                                                                         
Sample volume, total.............  7.4.5.2............            *                   XX.X...............  m3                 
Temperature, ambient, 30-second    7.4.8..............         ............         ............  XX.X...............   deg.C             
 interval.                                                                                                                                              
Temperature, ambient, min., max.,  7.4.8..............            *                   XX.X...............   deg.C             
 average for the sample period.                                                                                                                         
Baro pressure, ambient, 30-second  7.4.9..............         ............         ............  XXX................  mm Hg              
 interval.                                                                                                                                              

[[Page 38720]]

                                                                                                                                                        
Baro pressure, ambient, min.,      7.4.9..............            *                   XXX................  mm Hg              
 max., average for the sample                                                                                                                           
 period.                                                                                                                                                
Filter temperature, 30-second      7.4.11.............         ............         ............  XX.X...............   deg.C             
 interval.                                                                                                                                              
Filter temperature differential,   7.4.11.............            *                   On/Off.............  ...................
 30-second interval, out of spec.                                                                                                                       
 (FLAG6).                                                                                                                                               
Filter temperature, maximum        7.4.11.............            *             *             *             *   X.X, YY/MM/DD HH:mm   deg.C, Yr./Mon./  
 differential from ambient, date,                                                                                                     Day Hrs. min      
 time of occurrence.                                                                                                                                    
Date and time....................  7.4.12.............         ............         ............  YY/MM/DD HH:mm.....  Yr./Mon./Day Hrs.  
                                                                                                                                      min               
Sample start and stop time         7.4.12.............                              YY/MM/DD HH:mm.....  Yr./Mon./Day Hrs.  
 settings.                                                                                                                            min               
Sample period start time.........  7.4.12.............  ............                  YYYY/MM/DD HH:mm...  Yr./Mon./Day Hrs.  
                                                                                                                                      min               
Elapsed sample time..............  7.4.13.............            *                   HH:mm..............  Hrs. min           
Elapsed sample time, out of spec.  7.4.13.............  ............                  On/Off.............  ...................
 (FLAG6).                                                                                                                                               
Power interruptions >1 min.,       7.4.15.5...........            *                    *          1HH:mm, 2HH:mm, etc  Hrs. min           
 start time of first 10.                                                                                         ....                                   
User-entered information, such as  7.4.16.............                         As entered.........  ...................
 sampler and site identification.                                                                                                                       
--------------------------------------------------------------------------------------------------------------------------------------------------------
 Provision of this information is required.                                                                                                      
 Provision of this information is optional. If information related to the entire sample period is optionally provided prior to the end of the sample    
  period, the value provided should be the value calculated for the portion of the sampler period completed up to the time the information is provided. 
 Indicates that this information is also required to be provided to the AIRS data bank; see Sec.  Sec.  58.26 and 58.35 of this chapter.           
                                                                                                                                                        
1 Information is required to be available to the operator at any time the sampler is operating, whether sampling or not.                                
2 Information relates to the entire sampler period and must be provided following the end of the sample period until reset manually by the operator or  
  automatically by the sampler upon the start of a new sample period.                                                                                   
3 Information shall be available to the operator visually.                                                                                              
4 Information is to be available as digital data at the sampler's data output port specified in section 7.4.16 of this appendix following the end of the
  sample period until reset manually by the operator or automatically by the sampler upon the start of a new sample period.                             
5 Digital readings, both visual and data output, shall have not less than the number of significant digits and resolution specified.                    
6 Flag warnings may be displayed to the operator by a single-flag indicator or each flag may be displayed individually. Only a set (on) flag warning    
  must be indicated; an off (unset) flag may be indicated by the absence of a flag warning. Sampler users should refer to section 10.12 of this appendix
  regarding the validity of samples for which the sampler provided an associated flag warning.                                                          

8.0 Filter Weighing. See reference 2 in section 13.0 of this 
appendix, for additional, more detailed guidance.
    8.1 Analytical balance. The analytical balance used to weigh 
filters must be suitable for weighing the type and size of filters 
specified, under section 6.0 of this appendix, and have a 
readability of 1 g. The balance shall be 
calibrated as specified by the manufacturer at installation and 
recalibrated immediately prior to each weighing session. See 
reference 2 in section 13.0 of this appendix for additional 
guidance.
    8.2 Filter conditioning. All sample filters used shall be 
conditioned immediately before both the pre- and post-sampling 
weighings as specified below. See reference 2 in section 13.0 of 
this appendix for additional guidance.
    8.2.1 Mean temperature. 20 - 23  deg.C.
    8.2.2 Temperature control. 2  deg.C over 24 hours.
    8.2.3 Mean humidity. Generally, 30-40 percent relative humidity; 
however, where it can be shown that the mean ambient relative 
humidity during sampling is less than 30 percent, conditioning is 
permissible at a mean relative humidity within 5 
relative humidity percent of the mean ambient relative humidity 
during sampling, but not less than 20 percent.
    8.2.4 Humidity control. 5 relative humidity percent 
over 24 hours.
    8.2.5 Conditioning time. Not less than 24 hours.
    8.3 Weighing procedure.
    8.3.1 New filters should be placed in the conditioning 
environment immediately upon arrival and stored there until the pre-
sampling weighing. See reference 2 in section 13.0 of this appendix 
for additional guidance.
    8.3.2 The analytical balance shall be located in the same 
controlled environment in which the filters are conditioned. The 
filters shall be weighed immediately following the conditioning 
period without intermediate or transient exposure to other 
conditions or environments.
    8.3.3 Filters must be conditioned at the same conditions 
(humidity within 5 relative humidity percent) before 
both the pre- and post-sampling weighings.
    8.3.4 Both the pre- and post-sampling weighings should be 
carried out on the same analytical balance, using an effective 
technique to neutralize static charges on the filter, under 
reference 2 in section 13.0 of this appendix. If possible, both 
weighings should be carried out by the same analyst.
    8.3.5 The pre-sampling (tare) weighing shall be within 30 days 
of the sampling period.
    8.3.6 The post-sampling conditioning and weighing shall be 
completed within 240 hours (10 days) after the end of the sample 
period, unless the filter sample is maintained at 4  deg.C or less 
during the entire time between retrieval from the sampler and the 
start of the conditioning, in which case the period shall

[[Page 38721]]

not exceed 30 days. Reference 2 in section 13.0 of this appendix has 
additional guidance on transport of cooled filters.
    8.3.7 Filter blanks.
    8.3.7.1 New field blank filters shall be weighed along with the 
pre-sampling (tare) weighing of each lot of PM2.5 
filters. These blank filters shall be transported to the sampling 
site, installed in the sampler, retrieved from the sampler without 
sampling, and reweighed as a quality control check.
    8.3.7.2 New laboratory blank filters shall be weighed along with 
the pre-sampling (tare) weighing of each set of PM2.5 
filters. These laboratory blank filters should remain in the 
laboratory in protective containers during the field sampling and 
should be reweighed as a quality control check.
    8.3.8 Additional guidance for proper filter weighing and related 
quality assurance activities is provided in reference 2 in section 
13.0 of this appendix.
9.0 Calibration. Reference 2 in section 13.0 of this appendix 
contains additional guidance.
    9.1 General requirements.
    9.1.1 Multipoint calibration and single-point verification of 
the sampler's flow rate measurement device must be performed 
periodically to establish and maintain traceability of subsequent 
flow measurements to a flow rate standard.
    9.1.2 An authoritative flow rate standard shall be used for 
calibrating or verifying the sampler's flow rate measurement device 
with an accuracy of 2 percent. The flow rate standard 
shall be a separate, stand-alone device designed to connect to the 
flow rate measurement adapter, Figure L-30 of this appendix. This 
flow rate standard must have its own certification and be traceable 
to a National Institute of Standards and Technology (NIST) primary 
standard for volume or flow rate. If adjustments to the sampler's 
flow rate measurement system calibration are to be made in 
conjunction with an audit of the sampler's flow measurement system, 
such adjustments shall be made following the audit. Reference 2 in 
section 13.0 of this appendix contains additional guidance.
    9.1.3 The sampler's flow rate measurement device shall be re-
calibrated after electromechanical maintenance or transport of the 
sampler.
    9.2 Flow rate calibration/verification procedure.
    9.2.1 PM2.5 samplers may employ various types of flow 
control and flow measurement devices. The specific procedure used 
for calibration or verification of the flow rate measurement device 
will vary depending on the type of flow rate controller and flow 
rate measurement employed. Calibration shall be in terms of actual 
ambient volumetric flow rates (Qa), measured at the 
sampler's inlet downtube. The generic procedure given here serves to 
illustrate the general steps involved in the calibration of a 
PM2.5 sampler. The sampler operation/instruction manual 
required under section 7.4.18 of this appendix and the Quality 
Assurance Handbook in reference 2 in section 13.0 of this appendix 
provide more specific and detailed guidance for calibration.
    9.2.2 The flow rate standard used for flow rate calibration 
shall have its own certification and be traceable to a NIST primary 
standard for volume or flow rate. A calibration relationship for the 
flow rate standard, e.g., an equation, curve, or family of curves 
relating actual flow rate (Qa) to the flow rate indicator 
reading, shall be established that is accurate to within 2 percent 
over the expected range of ambient temperatures and pressures at 
which the flow rate standard may be used. The flow rate standard 
must be re-calibrated or re-verified at least annually.
    9.2.3 The sampler flow rate measurement device shall be 
calibrated or verified by removing the sampler inlet and connecting 
the flow rate standard to the sampler's downtube in accordance with 
the operation/instruction manual, such that the flow rate standard 
accurately measures the sampler's flow rate. The sampler operator 
shall first carry out a sampler leak check and confirm that the 
sampler passes the leak test and then verify that no leaks exist 
between the flow rate standard and the sampler.
    9.2.4 The calibration relationship between the flow rate (in 
actual L/min) indicated by the flow rate standard and by the 
sampler's flow rate measurement device shall be established or 
verified in accordance with the sampler operation/instruction 
manual. Temperature and pressure corrections to the flow rate 
indicated by the flow rate standard may be required for certain 
types of flow rate standards. Calibration of the sampler's flow rate 
measurement device shall consist of at least three separate flow 
rate measurements (multipoint calibration) evenly spaced within the 
range of -10 percent to +10 percent of the sampler's operational 
flow rate, section 7.4.1 of this appendix. Verification of the 
sampler's flow rate shall consist of one flow rate measurement at 
the sampler's operational flow rate. The sampler operation/
instruction manual and reference 2 in section 13.0 of this appendix 
provide additional guidance.
    9.2.5 If during a flow rate verification the reading of the 
sampler's flow rate indicator or measurement device differs by 
2 percent or more from the flow rate measured by the 
flow rate standard, a new multipoint calibration shall be performed 
and the flow rate verification must then be repeated.
    9.2.6 Following the calibration or verification, the flow rate 
standard shall be removed from the sampler and the sampler inlet 
shall be reinstalled. Then the sampler's normal operating flow rate 
(in L/min) shall be determined with a clean filter in place. If the 
flow rate indicated by the sampler differs by 2 percent 
or more from the required sampler flow rate, the sampler flow rate 
must be adjusted to the required flow rate, under section 7.4.1 of 
this appendix.
    9.3 Periodic calibration or verification of the calibration of 
the sampler's ambient temperature, filter temperature, and 
barometric pressure measurement systems is also required. Reference 
3 of section 13.0 of this appendix contains additional guidance.

10.0 PM2.5 Measurement Procedure The detailed procedure 
for obtaining valid PM2.5 measurements with each specific 
sampler designated as part of a reference method for 
PM2.5 under part 53 of this chapter shall be provided in 
the sampler-specific operation or instruction manual required by 
section 7.4.18 of this appendix. Supplemental guidance is provided 
in section 2.12 of the Quality Assurance Handbook listed in 
reference 2 in section 13.0 of this appendix. The generic procedure 
given here serves to illustrate the general steps involved in the 
PM2.5 sample collection and measurement, using a 
PM2.5 reference method sampler.
    10.1 The sampler shall be set up, calibrated, and operated in 
accordance with the specific, detailed guidance provided in the 
specific sampler's operation or instruction manual and in accordance 
with a specific quality assurance program developed and established 
by the user, based on applicable supplementary guidance provided in 
reference 2 in section 13.0 of this appendix.
    10.2 Each new sample filter shall be inspected for correct type 
and size and for pinholes, particles, and other imperfections. 
Unacceptable filters should be discarded. A unique identification 
number shall be assigned to each filter, and an information record 
shall be established for each filter. If the filter identification 
number is not or cannot be marked directly on the filter, 
alternative means, such as a number-identified storage container, 
must be established to maintain positive filter identification.
    10.3 Each filter shall be conditioned in the conditioning 
environment in accordance with the requirements specified in section 
8.2 of this appendix.
    10.4 Following conditioning, each filter shall be weighed in 
accordance with the requirements specified in section 8.0 of this 
appendix and the presampling weight recorded with the filter 
identification number.
    10.5 A numbered and preweighed filter shall be installed in the 
sampler following the instructions provided in the sampler operation 
or instruction manual.
    10.6 The sampler shall be checked and prepared for sample 
collection in accordance with instructions provided in the sampler 
operation or instruction manual and with the specific quality 
assurance program established for the sampler by the user.
    10.7 The sampler's timer shall be set to start the sample 
collection at the beginning of the desired sample period and stop 
the sample collection 24 hours later.
    10.8 Information related to the sample collection (site location 
or identification number, sample date, filter identification number, 
and sampler model and serial number) shall be recorded and, if 
appropriate, entered into the sampler.
    10.9 The sampler shall be allowed to collect the 
PM2.5 sample during the set 24-hour time period.
    10.10 Within 96 hours of the end of the sample collection 
period, the filter, while still contained in the filter cassette, 
shall be carefully removed from the sampler, following the procedure 
provided in the sampler operation or instruction manual and the 
quality assurance program, and placed in a protective container. 
This protective container shall be made of metal and contain no 
loose material that could be transferred to the filter. The 
protective container shall hold

[[Page 38722]]

the filter cassette securely such that the cover shall not come in 
contact with the filter's surfaces. Reference 2 in section 13.0 of 
this appendix contains additional information.
    10.11 The total sample volume in actual m3 for the 
sampling period and the elapsed sample time shall be obtained from 
the sampler and recorded in accordance with the instructions 
provided in the sampler operation or instruction manual. All sampler 
warning flag indications and other information required by the local 
quality assurance program shall also be recorded.
    10.12 All factors related to the validity or representativeness 
of the sample, such as sampler tampering or malfunctions, unusual 
meteorological conditions, construction activity, fires or dust 
storms, etc. shall be recorded as required by the local quality 
assurance program. The occurrence of a flag warning during a sample 
period shall not necessarily indicate an invalid sample but rather 
shall indicate the need for specific review of the QC data by a 
quality assurance officer to determine sample validity.
    10.13 After retrieval from the sampler, the exposed filter 
containing the PM2.5 sample should be transported to the 
filter conditioning environment as soon as possible ideally to 
arrive at the conditioning environment within 24 hours for 
conditioning and subsequent weighing. During the period between 
filter retrieval from the sampler and the start of the conditioning, 
the filter shall be maintained as cool as practical and continuously 
protected from exposure to temperatures over 25  deg.C. See section 
8.3.6 of this appendix regarding time limits for completing the 
post-sampling weighing. See reference 2 in section 13.0 of this 
appendix for additional guidance on transporting filter samplers to 
the conditioning and weighing laboratory.
    10.14. The exposed filter containing the PM2.5 sample 
shall be re-conditioned in the conditioning environment in 
accordance with the requirements specified in section 8.2 of this 
appendix.
    10.15. The filter shall be reweighed immediately after 
conditioning in accordance with the requirements specified in 
section 8.0 of this appendix, and the postsampling weight shall be 
recorded with the filter identification number.
    10.16 The PM2.5 concentration shall be calculated as 
specified in section 12.0 of this appendix.
11.0 Sampler Maintenance
    The sampler shall be maintained as described by the sampler's 
manufacturer in the sampler-specific operation or instruction manual 
required under section 7.4.18 of this appendix and in accordance 
with the specific quality assurance program developed and 
established by the user based on applicable supplementary guidance 
provided in reference 2 in section 13.0 of this appendix.
12.0 Calculations
    12.1 (a) The PM2.5 concentration is calculated as:
    PM2.5 = (Wf - Wi)/Va
    where:
    PM2.5 = mass concentration of PM2.5, 
g/m3;
    Wf, Wi = final and initial weights, 
respectively, of the filter used to collect the PM2.5 
particle sample, g;
    Va = total air volume sampled in actual volume units, 
as provided by the sampler, m3.
    (b) Note: Total sample time must be between 1,380 and 1,500 
minutes (23 and 25 hrs) for a fully valid PM2.5 sample; 
however, see also section 3.3 of this appendix.
13.0 References.
    1. Quality Assurance Handbook for Air Pollution Measurement 
Systems, Volume I, Principles. EPA/600/R-94/038a, April 1994. 
Available from CERI, ORD Publications, U.S. Environmental Protection 
Agency, 26 West Martin Luther King Drive, Cincinnati, Ohio 45268.
    2. Copies of section 2.12 of the Quality Assurance Handbook for 
Air Pollution Measurement Systems, Volume II, Ambient Air Specific 
Methods, EPA/600/R-94/038b, are available from Department E (MD-
77B), U.S. EPA, Research Triangle Park, NC 27711.
    3. Quality Assurance Handbook for Air Pollution Measurement 
Systems, Volume IV: Meteorological Measurements, (Revised Edition) 
EPA/600/R-94/038d, March, 1995. Available from CERI, ORD 
Publications, U.S. Environmental Protection Agency, 26 West Martin 
Luther King Drive, Cincinnati, Ohio 45268.
    4. Military standard specification (mil. spec.) 8625F, Type II, 
Class 1 as listed in Department of Defense Index of Specifications 
and Standards (DODISS), available from DODSSP-Customer Service, 
Standardization Documents Order Desk, 700 Robbins Avenue, Building 
4D, Philadelphia, PA 1911-5094.
14.0 Figures L-1 through L-30 to Appendix L.

[[Page 38723]]

[GRAPHIC] [TIFF OMITTED] TR18JY97.022



[[Page 38724]]

[GRAPHIC] [TIFF OMITTED] TR18JY97.023



[[Page 38725]]

[GRAPHIC] [TIFF OMITTED] TR18JY97.024



[[Page 38726]]

[GRAPHIC] [TIFF OMITTED] TR18JY97.025



[[Page 38727]]

[GRAPHIC] [TIFF OMITTED] TR18JY97.026



[[Page 38728]]

[GRAPHIC] [TIFF OMITTED] TR18JY97.027



[[Page 38729]]

[GRAPHIC] [TIFF OMITTED] TR18JY97.028



[[Page 38730]]

[GRAPHIC] [TIFF OMITTED] TR18JY97.029



[[Page 38731]]

[GRAPHIC] [TIFF OMITTED] TR18JY97.030



[[Page 38732]]

[GRAPHIC] [TIFF OMITTED] TR18JY97.031



[[Page 38733]]

[GRAPHIC] [TIFF OMITTED] TR18JY97.032



[[Page 38734]]

[GRAPHIC] [TIFF OMITTED] TR18JY97.033



[[Page 38735]]

[GRAPHIC] [TIFF OMITTED] TR18JY97.034



[[Page 38736]]

[GRAPHIC] [TIFF OMITTED] TR18JY97.035



[[Page 38737]]

[GRAPHIC] [TIFF OMITTED] TR18JY97.036



[[Page 38738]]

[GRAPHIC] [TIFF OMITTED] TR18JY97.037



[[Page 38739]]

[GRAPHIC] [TIFF OMITTED] TR18JY97.038



[[Page 38740]]

[GRAPHIC] [TIFF OMITTED] TR18JY97.039



[[Page 38741]]

[GRAPHIC] [TIFF OMITTED] TR18JY97.040



[[Page 38742]]

[GRAPHIC] [TIFF OMITTED] TR18JY97.041



[[Page 38743]]

[GRAPHIC] [TIFF OMITTED] TR18JY97.042



[[Page 38744]]

[GRAPHIC] [TIFF OMITTED] TR18JY97.043



[[Page 38745]]

[GRAPHIC] [TIFF OMITTED] TR18JY97.044



[[Page 38746]]

[GRAPHIC] [TIFF OMITTED] TR18JY97.045



[[Page 38747]]

[GRAPHIC] [TIFF OMITTED] TR18JY97.046



[[Page 38748]]

[GRAPHIC] [TIFF OMITTED] TR18JY97.047



[[Page 38749]]

[GRAPHIC] [TIFF OMITTED] TR18JY97.048



[[Page 38750]]

[GRAPHIC] [TIFF OMITTED] TR18JY97.049



[[Page 38751]]

[GRAPHIC] [TIFF OMITTED] TR18JY97.050



[[Page 38752]]

[GRAPHIC] [TIFF OMITTED] TR18JY97.051



[[Page 38753]]

    7. Appendix M is added to read as follows:

Appendix M to Part 50--Reference Method for the Determination of 
Particulate Matter as PM10 in the Atmosphere

1.0  Applicability.
    1.1  This method provides for the measurement of the mass 
concentration of particulate matter with an aerodynamic diameter 
less than or equal to a nominal 10 micrometers (PM1O) in 
ambient air over a 24-hour period for purposes of determining 
attainment and maintenance of the primary and secondary national 
ambient air quality standards for particulate matter specified in 
Sec. 50.6 of this chapter. The measurement process is 
nondestructive, and the PM10 sample can be subjected to 
subsequent physical or chemical analyses. Quality assurance 
procedures and guidance are provided in part 58, Appendices A and B 
of this chapter and in references 1 and 2 of section 12.0 of this 
appendix.
2.0  Principle.
    2.1  An air sampler draws ambient air at a constant flow rate 
into a specially shaped inlet where the suspended particulate matter 
is inertially separated into one or more size fractions within the 
PM10 size range. Each size fraction in the 
PM1O size range is then collected on a separate filter 
over the specified sampling period. The particle size discrimination 
characteristics (sampling effectiveness and 50 percent cutpoint) of 
the sampler inlet are prescribed as performance specifications in 
part 53 of this chapter.
    2.2 Each filter is weighed (after moisture equilibration) before 
and after use to determine the net weight (mass) gain due to 
collected PM10. The total volume of air sampled, measured 
at the actual ambient temperature and pressure, is determined from 
the measured flow rate and the sampling time. The mass concentration 
of PM10 in the ambient air is computed as the total mass 
of collected particles in the PM10 size range divided by 
the volume of air sampled, and is expressed in micrograms per actual 
cubic meter (g/m3).
    2.3  A method based on this principle will be considered a 
reference method only if the associated sampler meets the 
requirements specified in this appendix and the requirements in part 
53 of this chapter, and the method has been designated as a 
reference method in accordance with part 53 of this chapter.
3.0  Range.
    3.1  The lower limit of the mass concentration range is 
determined by the repeatability of filter tare weights, assuming the 
nominal air sample volume for the sampler. For samplers having an 
automatic filter-changing mechanism, there may be no upper limit. 
For samplers that do not have an automatic filter-changing 
mechanism, the upper limit is determined by the filter mass loading 
beyond which the sampler no longer maintains the operating flow rate 
within specified limits due to increased pressure drop across the 
loaded filter. This upper limit cannot be specified precisely 
because it is a complex function of the ambient particle size 
distribution and type, humidity, filter type, and perhaps other 
factors. Nevertheless, all samplers should be capable of measuring 
24-hour PM10 mass concentrations of at least 300 
g/m\3\ while maintaining the operating flow rate within the 
specified limits.
4.0  Precision.
    4.1  The precision of PM10 samplers must be 5 
g/m\3\ for PM10 concentrations below 80 
g/m\3\ and 7 percent for PM10 concentrations 
above 80 g/m\3\, as required by part 53 of this chapter, 
which prescribes a test procedure that determines the variation in 
the PM10 concentration measurements of identical samplers 
under typical sampling conditions. Continual assessment of precision 
via collocated samplers is required by part 58 of this chapter for 
PM10 samplers used in certain monitoring networks.
5.0  Accuracy.
    5.1  Because the size of the particles making up ambient 
particulate matter varies over a wide range and the concentration of 
particles varies with particle size, it is difficult to define the 
absolute accuracy of PM10 samplers. Part 53 of this 
chapter provides a specification for the sampling effectiveness of 
PM10 samplers. This specification requires that the 
expected mass concentration calculated for a candidate 
PM10 sampler, when sampling a specified particle size 
distribution, be within 10 percent of that calculated 
for an ideal sampler whose sampling effectiveness is explicitly 
specified. Also, the particle size for 50 percent sampling 
effectiveness is required to be 100.5 micrometers. Other 
specifications related to accuracy apply to flow measurement and 
calibration, filter media, analytical (weighing) procedures, and 
artifact. The flow rate accuracy of PM10 samplers used in 
certain monitoring networks is required by part 58 of this chapter 
to be assessed periodically via flow rate audits.
6.0  Potential Sources of Error.
    6.1  Volatile Particles. Volatile particles collected on filters 
are often lost during shipment and/or storage of the filters prior 
to the post-sampling weighing \3\. Although shipment or storage of 
loaded filters is sometimes unavoidable, filters should be reweighed 
as soon as practical to minimize these losses.
    6.2  Artifacts. Positive errors in PM10 concentration 
measurements may result from retention of gaseous species on filters 
4, 5. Such errors include the retention of sulfur dioxide 
and nitric acid. Retention of sulfur dioxide on filters, followed by 
oxidation to sulfate, is referred to as artifact sulfate formation, 
a phenomenon which increases with increasing filter alkalinity \6\. 
Little or no artifact sulfate formation should occur using filters 
that meet the alkalinity specification in section 7.2.4 of this 
appendix, Artifact nitrate formation, resulting primarily from 
retention of nitric acid, occurs to varying degrees on many filter 
types, including glass fiber, cellulose ester, and many quartz fiber 
filters 5, 7, 8, 9, 10. Loss of true atmospheric 
particulate nitrate during or following sampling may also occur due 
to dissociation or chemical reaction. This phenomenon has been 
observed on Teflon filters \8\ and inferred for quartz 
fiber filters 11, 12. The magnitude of nitrate artifact 
errors in PM10 mass concentration measurements will vary 
with location and ambient temperature; however, for most sampling 
locations, these errors are expected to be small.
    6.3  Humidity. The effects of ambient humidity on the sample are 
unavoidable. The filter equilibration procedure in section 9.0 of 
this appendix is designed to minimize the effects of moisture on the 
filter medium.
    6.4  Filter Handling. Careful handling of filters between 
presampling and postsampling weighings is necessary to avoid errors 
due to damaged filters or loss of collected particles from the 
filters. Use of a filter cartridge or cassette may reduce the 
magnitude of these errors. Filters must also meet the integrity 
specification in section 7.2.3 of this appendix.
    6.5  Flow Rate Variation. Variations in the sampler's operating 
flow rate may alter the particle size discrimination characteristics 
of the sampler inlet. The magnitude of this error will depend on the 
sensitivity of the inlet to variations in flow rate and on the 
particle distribution in the atmosphere during the sampling period. 
The use of a flow control device, under section 7.1.3 of this 
appendix, is required to minimize this error.
    6.6  Air Volume Determination. Errors in the air volume 
determination may result from errors in the flow rate and/or 
sampling time measurements. The flow control device serves to 
minimize errors in the flow rate determination, and an elapsed time 
meter, under section 7.1.5 of this appendix, is required to minimize 
the error in the sampling time measurement.
7.0  Apparatus.
    7.1  PM10 Sampler.
    7.1.1  The sampler shall be designed to:
    (a) Draw the air sample into the sampler inlet and through the 
particle collection filter at a uniform face velocity.
    (b) Hold and seal the filter in a horizontal position so that 
sample air is drawn downward through the filter.
    (c) Allow the filter to be installed and removed conveniently.
    (d) Protect the filter and sampler from precipitation and 
prevent insects and other debris from being sampled.
    (e) Minimize air leaks that would cause error in the measurement 
of the air volume passing through the filter.
    (f) Discharge exhaust air at a sufficient distance from the 
sampler inlet to minimize the sampling of exhaust air.
    (g) Minimize the collection of dust from the supporting surface.
    7.1.2  The sampler shall have a sample air inlet system that, 
when operated within a specified flow rate range, provides particle 
size discrimination characteristics meeting all of the applicable 
performance specifications prescribed in part 53 of this chapter. 
The sampler inlet shall show no significant wind direction 
dependence. The latter requirement can generally be satisfied by an 
inlet shape that is circularly symmetrical about a vertical axis.
    7.1.3  The sampler shall have a flow control device capable of 
maintaining the sampler's operating flow rate within the flow rate 
limits specified for the sampler inlet over normal variations in 
line voltage and filter pressure drop.

[[Page 38754]]

    7.1.4  The sampler shall provide a means to measure the total 
flow rate during the sampling period. A continuous flow recorder is 
recommended but not required. The flow measurement device shall be 
accurate to 2 percent.
    7.1.5  A timing/control device capable of starting and stopping 
the sampler shall be used to obtain a sample collection period of 24 
1 hr (1,440 60 min). An elapsed time meter, 
accurate to within 15 minutes, shall be used to measure 
sampling time. This meter is optional for samplers with continuous 
flow recorders if the sampling time measurement obtained by means of 
the recorder meets the 15 minute accuracy specification.
    7.1.6  The sampler shall have an associated operation or 
instruction manual as required by part 53 of this chapter which 
includes detailed instructions on the calibration, operation, and 
maintenance of the sampler.
    7.2  Filters.
    7.2.1  Filter Medium. No commercially available filter medium is 
ideal in all respects for all samplers. The user's goals in sampling 
determine the relative importance of various filter characteristics, 
e.g., cost, ease of handling, physical and chemical characteristics, 
etc., and, consequently, determine the choice among acceptable 
filters. Furthermore, certain types of filters may not be suitable 
for use with some samplers, particularly under heavy loading 
conditions (high mass concentrations), because of high or rapid 
increase in the filter flow resistance that would exceed the 
capability of the sampler's flow control device. However, samplers 
equipped with automatic filter-changing mechanisms may allow use of 
these types of filters. The specifications given below are minimum 
requirements to ensure acceptability of the filter medium for 
measurement of PM10 mass concentrations. Other filter 
evaluation criteria should be considered to meet individual sampling 
and analysis objectives.
    7.2.2  Collection Efficiency. 99 percent, as measured 
by the DOP test (ASTM-2986) with 0.3 m particles at the 
sampler's operating face velocity.
    7.2.3  Integrity. 5 g/m\3\ (assuming 
sampler's nominal 24-hour air sample volume). Integrity is measured 
as the PM10 concentration equivalent corresponding to the 
average difference between the initial and the final weights of a 
random sample of test filters that are weighed and handled under 
actual or simulated sampling conditions, but have no air sample 
passed through them, i.e., filter blanks. As a minimum, the test 
procedure must include initial equilibration and weighing, 
installation on an inoperative sampler, removal from the sampler, 
and final equilibration and weighing.
    7.2.4  Alkalinity. <25 microequivalents/gram of filter, as 
measured by the procedure given in reference 13 of section 12.0 of 
this appendix following at least two months storage in a clean 
environment (free from contamination by acidic gases) at room 
temperature and humidity.
    7.3  Flow Rate Transfer Standard. The flow rate transfer 
standard must be suitable for the sampler's operating flow rate and 
must be calibrated against a primary flow or volume standard that is 
traceable to the National Institute of Standard and Technology 
(NIST). The flow rate transfer standard must be capable of measuring 
the sampler's operating flow rate with an accuracy of 2 
percent.
    7.4  Filter Conditioning Environment.
    7.4.1  Temperature range. 15 to 30 C.
    7.4.2  Temperature control. 3 C.
    7.4.3  Humidity range. 20% to 45% RH.
    7.4.4  Humidity control. 5% RH.
    7.5  Analytical Balance. The analytical balance must be suitable 
for weighing the type and size of filters required by the sampler. 
The range and sensitivity required will depend on the filter tare 
weights and mass loadings. Typically, an analytical balance with a 
sensitivity of 0.1 mg is required for high volume samplers (flow 
rates >0.5 m\3\/min). Lower volume samplers (flow rates <0.5 m\3\/
min) will require a more sensitive balance.
8.0  Calibration.
    8.1  General Requirements.
    8.1.1  Calibration of the sampler's flow measurement device is 
required to establish traceability of subsequent flow measurements 
to a primary standard. A flow rate transfer standard calibrated 
against a primary flow or volume standard shall be used to calibrate 
or verify the accuracy of the sampler's flow measurement device.
    8.1.2 Particle size discrimination by inertial separation 
requires that specific air velocities be maintained in the sampler's 
air inlet system. Therefore, the flow rate through the sampler's 
inlet must be maintained throughout the sampling period within the 
design flow rate range specified by the manufacturer. Design flow 
rates are specified as actual volumetric flow rates, measured at 
existing conditions of temperature and pressure (Qa).
    8.2  Flow Rate Calibration Procedure.
    8.2.1 PM10 samplers employ various types of flow 
control and flow measurement devices. The specific procedure used 
for flow rate calibration or verification will vary depending on the 
type of flow controller and flow rate indicator employed. 
Calibration is in terms of actual volumetric flow rates 
(Qa) to meet the requirements of section 8.1 of this 
appendix. The general procedure given here serves to illustrate the 
steps involved in the calibration. Consult the sampler 
manufacturer's instruction manual and reference 2 of section 12.0 of 
this appendix for specific guidance on calibration. Reference 14 of 
section 12.0 of this appendix provides additional information on 
various other measures of flow rate and their interrelationships.
    8.2.2  Calibrate the flow rate transfer standard against a 
primary flow or volume standard traceable to NIST. Establish a 
calibration relationship, e.g., an equation or family of curves, 
such that traceability to the primary standard is accurate to within 
2 percent over the expected range of ambient conditions, i.e., 
temperatures and pressures, under which the transfer standard will 
be used. Recalibrate the transfer standard periodically.
    8.2.3  Following the sampler manufacturer's instruction manual, 
remove the sampler inlet and connect the flow rate transfer standard 
to the sampler such that the transfer standard accurately measures 
the sampler's flow rate. Make sure there are no leaks between the 
transfer standard and the sampler.
    8.2.4  Choose a minimum of three flow rates (actual m\3\/min), 
spaced over the acceptable flow rate range specified for the inlet, 
under section 7.1.2 of the appendix, that can be obtained by 
suitable adjustment of the sampler flow rate. In accordance with the 
sampler manufacturer's instruction manual, obtain or verify the 
calibration relationship between the flow rate (actual m\3\/min) as 
indicated by the transfer standard and the sampler's flow indicator 
response. Record the ambient temperature and barometric pressure. 
Temperature and pressure corrections to subsequent flow indicator 
readings may be required for certain types of flow measurement 
devices. When such corrections are necessary, correction on an 
individual or daily basis is preferable. However, seasonal average 
temperature and average barometric pressure for the sampling site 
may be incorporated into the sampler calibration to avoid daily 
corrections. Consult the sampler manufacturer's instruction manual 
and reference 2 in section 12.0 of this appendix for additional 
guidance.
    8.2.5  Following calibration, verify that the sampler is 
operating at its design flow rate (actual m\3\/min) with a clean 
filter in place.
    8.2.6  Replace the sampler inlet.
9.0  Procedure.
    9.1  The sampler shall be operated in accordance with the 
specific guidance provided in the sampler manufacturer's instruction 
manual and in reference 2 in section 12.0 of this appendix. The 
general procedure given here assumes that the sampler's flow rate 
calibration is based on flow rates at ambient conditions 
(Qa) and serves to illustrate the steps involved in the 
operation of a PM10 sampler.
    9.2  Inspect each filter for pinholes, particles, and other 
imperfections. Establish a filter information record and assign an 
identification number to each filter.
    9.3  Equilibrate each filter in the conditioning environment 
(see 7.4) for at least 24 hours.
    9.4  Following equilibration, weigh each filter and record the 
presampling weight with the filter identification number.
    9.5  Install a preweighed filter in the sampler following the 
instructions provided in the sampler manufacturer's instruction 
manual.
    9.6   (a) Turn on the sampler and allow it to establish run-
temperature conditions. Record the flow indicator reading and, if 
needed, the ambient temperature and barometric pressure. Determine 
the sampler flow rate (actual m\3\/min) in accordance with the 
instructions provided in the sampler manufacturer's instruction 
manual.
    (b) Note: No onsite temperature or pressure measurements are 
necessary if the sampler's flow indicator does not require 
temperature or pressure corrections or if seasonal average 
temperature and average barometric pressure for the sampling site 
are incorporated into

[[Page 38755]]

the sampler calibration, under section 8.2.4 of this appendix. If 
individual or daily temperature and pressure corrections are 
required, ambient temperature and barometric pressure can be 
obtained by on-site measurements or from a nearby weather station. 
Barometric pressure readings obtained from airports must be station 
pressure, not corrected to sea level, and may need to be corrected 
for differences in elevation between the sampling site and the 
airport.
    9.7  If the flow rate is outside the acceptable range specified 
by the manufacturer, check for leaks, and if necessary, adjust the 
flow rate to the specified setpoint. Stop the sampler.
    9.8  Set the timer to start and stop the sampler at appropriate 
times. Set the elapsed time meter to zero or record the initial 
meter reading.
    9.9  Record the sample information (site location or 
identification number, sample date, filter identification number, 
and sampler model and serial number).
    9.10  Sample for 241 hours.
    9.11  Determine and record the average flow rate (Qa) 
in actual m\3\/min for the sampling period in accordance with the 
instructions provided in the sampler manufacturer's instruction 
manual. Record the elapsed time meter final reading and, if needed, 
the average ambient temperature and barometric pressure for the 
sampling period, in note following section 9.6 of this appendix.
    9.12  Carefully remove the filter from the sampler, following 
the sampler manufacturer's instruction manual. Touch only the outer 
edges of the filter.
    9.13  Place the filter in a protective holder or container, 
e.g., petri dish, glassine envelope, or manila folder.
    9.14  Record any factors such as meteorological conditions, 
construction activity, fires or dust storms, etc., that might be 
pertinent to the measurement on the filter information record.
    9.15  Transport the exposed sample filter to the filter 
conditioning environment as soon as possible for equilibration and 
subsequent weighing.
    9.16  Equilibrate the exposed filter in the conditioning 
environment for at least 24 hours under the same temperature and 
humidity conditions used for presampling filter equilibration (see 
section 9.3 of this appendix).
    9.17  Immediately after equilibration, reweigh the filter and 
record the postsampling weight with the filter identification 
number.
10.0  Sampler Maintenance.
    10.1  The PM10 sampler shall be maintained in strict 
accordance with the maintenance procedures specified in the sampler 
manufacturer's instruction manual.
11.0  Calculations.

    11.1 Calculate the total volume of air sampled as:

V = Qat

where:

V = total air sampled, at ambient temperature and 
pressure,m3;

Qa = average sample flow rate at ambient temperature and 
pressure, m3/min; and

t = sampling time, min.

    11.2   (a) Calculate the PM10 concentration as:

PM10 = (Wf-Wi) x 10\6\/V

where:

PM10 = mass concentration of PM10, g/
m\3\;

Wf, Wi = final and initial weights of filter 
collecting PM1O particles, g; and

10\6\ = conversion of g to g.

    (b) Note: If more than one size fraction in the 
PM10 size range is collected by the sampler, the sum of 
the net weight gain by each collection filter 
[(Wf-Wi)] is used to calculate the 
PM10 mass concentration.
12.0  References.
    1. Quality Assurance Handbook for Air Pollution Measurement 
Systems, Volume I, Principles. EPA-600/9-76-005, March 1976. 
Available from CERI, ORD Publications, U.S. Environmental Protection 
Agency, 26 West St. Clair Street, Cincinnati, OH 45268.
    2. Quality Assurance Handbook for Air Pollution Measurement 
Systems, Volume II, Ambient Air Specific Methods. EPA-600/4-77-027a, 
May 1977. Available from CERI, ORD Publications, U.S. Environmental 
Protection Agency, 26 West St. Clair Street, Cincinnati, OH 45268.
    3. Clement, R.E., and F.W. Karasek. Sample Composition Changes 
in Sampling and Analysis of Organic Compounds in Aerosols. Int. J. 
Environ. Analyt. Chem., 7:109, 1979.
    4. Lee, R.E., Jr., and J. Wagman. A Sampling Anomaly in the 
Determination of Atmospheric Sulfate Concentration. Amer. Ind. Hyg. 
Assoc. J., 27:266, 1966.
    5. Appel, B.R., S.M. Wall, Y. Tokiwa, and M. Haik. Interference 
Effects in Sampling Particulate Nitrate in Ambient Air. Atmos. 
Environ., 13:319, 1979.
    6. Coutant, R.W. Effect of Environmental Variables on Collection 
of Atmospheric Sulfate. Environ. Sci. Technol., 11:873, 1977.
    7. Spicer, C.W., and P. Schumacher. Interference in Sampling 
Atmospheric Particulate Nitrate. Atmos. Environ., 11:873, 1977.
    8. Appel, B.R., Y. Tokiwa, and M. Haik. Sampling of Nitrates in 
Ambient Air. Atmos. Environ., 15:283, 1981.
    9. Spicer, C.W., and P.M. Schumacher. Particulate Nitrate: 
Laboratory and Field Studies of Major Sampling Interferences. Atmos. 
Environ., 13:543, 1979.
    10. Appel, B.R. Letter to Larry Purdue, U.S. EPA, Environmental 
Monitoring and Support Laboratory. March 18, 1982, Docket No. A-82-
37, II-I-1.
    11. Pierson, W.R., W.W. Brachaczek, T.J. Korniski, T.J. Truex, 
and J.W. Butler. Artifact Formation of Sulfate, Nitrate, and 
Hydrogen Ion on Backup Filters: Allegheny Mountain Experiment. J. 
Air Pollut. Control Assoc., 30:30, 1980.
    12. Dunwoody, C.L. Rapid Nitrate Loss From PM10 
Filters. J. Air Pollut. Control Assoc., 36:817, 1986.
    13. Harrell, R.M. Measuring the Alkalinity of Hi-Vol Air 
Filters. EMSL/RTP-SOP-QAD-534, October 1985. Available from the U.S. 
Environmental Protection Agency, EMSL/QAD, Research Triangle Park, 
NC 27711.
    14. Smith, F., P.S. Wohlschlegel, R.S.C. Rogers, and D.J. 
Mulligan. Investigation of Flow Rate Calibration Procedures 
Associated With the High Volume Method for Determination of 
Suspended Particulates. EPA-600/4-78-047, U.S. Environmental 
Protection Agency, Research Triangle Park, NC 27711, 1978.
    8. Appendix N is added to read as follows:

Appendix N to Part 50--Interpretation of the National Ambient Air 
Quality Standards for Particulate Matter

1.0 General.
    (a) This appendix explains the data handling conventions and 
computations necessary for determining when the annual and 24-hour 
primary and secondary national ambient air quality standards for PM 
specified in Sec. 50.7 of this chapter are met. Particulate matter 
is measured in the ambient air as PM10 and 
PM2.5 (particles with an aerodynamic diameter less than 
or equal to a nominal 10 and 2.5 micrometers, respectively) by a 
reference method based on Appendix M of this part for 
PM10 and on Appendix L of this part for PM2.5, 
as applicable, and designated in accordance with part 53 of this 
chapter, or by an equivalent method designated in accordance with 
part 53 of this chapter. Data handling and computation procedures to 
be used in making comparisons between reported PM10 and 
PM2.5 concentrations and the levels of the PM standards 
are specified in the following sections.
    (b) Data resulting from uncontrollable or natural events, for 
example structural fires or high winds, may require special 
consideration. In some cases, it may be appropriate to exclude these 
data because they could result in inappropriate values to compare 
with the levels of the PM standards. In other cases, it may be more 
appropriate to retain the data for comparison with the level of the 
PM standards and then allow the EPA to formulate the appropriate 
regulatory response. Whether to exclude, retain, or make adjustments 
to the data affected by uncontrollable or natural events is subject 
to the approval of the appropriate Regional Administrator.
    (c) The terms used in this appendix are defined as follows:
    Average and mean refer to an arithmetic mean.
     Daily value for PM refers to the 24-hour average concentration 
of PM calculated or measured from midnight to midnight (local time) 
for PM10 or PM2.5.
    Designated monitors are those monitoring sites designated in a 
State PM Monitoring Network Description for spatial averaging in 
areas opting for spatial averaging in accordance with part 58 of 
this chapter.
    98th percentile (used for PM2.5) means the 
daily value out of a year of monitoring data below which 98 percent 
of all values in the group fall.

[[Page 38756]]

    99th percentile (used for PM10) means the 
daily value out of a year of monitoring data below which 99 percent 
of all values in the group fall.
    Year refers to a calendar year.
    (d) Sections 2.1 and 2.5 of this appendix contain data handling 
instructions for the option of using a spatially averaged network of 
monitors for the annual standard. If spatial averaging is not 
considered for an area, then the spatial average is equivalent to 
the annual average of a single site and is treated accordingly in 
subsequent calculations. For example, paragraph (a)(3) of section 
2.1 of this appendix could be eliminated since the spatial average 
would be equivalent to the annual average.
2.0 Comparisons with the PM2.5 Standards.
    2.1 Annual PM2.5 Standard.
    (a) The annual PM2.5 standard is met when the 3-year 
average of the spatially averaged annual means is less than or equal 
to 15.0 g/m3. The 3-year average of the 
spatially averaged annual means is determined by averaging quarterly 
means at each monitor to obtain the annual mean PM2.5 
concentrations at each monitor, then averaging across all designated 
monitors, and finally averaging for 3 consecutive years. The steps 
can be summarized as follows:
    (1) Average 24-hour measurements to obtain quarterly means at 
each monitor.
    (2) Average quarterly means to obtain annual means at each 
monitor.
    (3) Average across designated monitoring sites to obtain an 
annual spatial mean for an area (this can be one site in which case 
the spatial mean is equal to the annual mean).
    (4) Average 3 years of annual spatial means to obtain a 3-year 
average of spatially averaged annual means.
    (b) In the case of spatial averaging, 3 years of spatial 
averages are required to demonstrate that the standard has been met. 
Designated sites with less than 3 years of data shall be included in 
spatial averages for those years that data completeness requirements 
are met. For the annual PM2.5 standard, a year meets data 
completeness requirements when at least 75 percent of the scheduled 
sampling days for each quarter have valid data. However, years with 
high concentrations and more than a minimal amount of data (at least 
11 samples in each quarter) shall not be ignored just because they 
are comprised of quarters with less than complete data. Thus, in 
computing annual spatially averaged means, years containing quarters 
with at least 11 samples but less than 75 percent data completeness 
shall be included in the computation if the resulting spatially 
averaged annual mean concentration (rounded according to the 
conventions of section 2.3 of this appendix) is greater than the 
level of the standard.
    (c) Situations may arise in which there are compelling reasons 
to retain years containing quarters which do not meet the data 
completeness requirement of 75 percent or the minimum number of 11 
samples. The use of less than complete data is subject to the 
approval of the appropriate Regional Administrator.
    (d) The equations for calculating the 3-year average annual mean 
of the PM2.5 standard are given in section 2.5 of this 
appendix.
    2.2 24-Hour PM2.5 Standard.
    (a) The 24-hour PM2.5 standard is met when the 3-year 
average of the 98th percentile values at each monitoring 
site is less than or equal to 65 g/m3. This 
comparison shall be based on 3 consecutive, complete years of air 
quality data. A year meets data completeness requirements when at 
least 75 percent of the scheduled sampling days for each quarter 
have valid data. However, years with high concentrations shall not 
be ignored just because they are comprised of quarters with less 
than complete data. Thus, in computing the 3-year average 
98th percentile value, years containing quarters with 
less than 75 percent data completeness shall be included in the 
computation if the annual 98th percentile value (rounded 
according to the conventions of section 2.3 of this appendix) is 
greater than the level of the standard.
    (b) Situations may arise in which there are compelling reasons 
to retain years containing quarters which do not meet the data 
completeness requirement. The use of less than complete data is 
subject to the approval of the appropriate Regional Administrator.
    (c) The equations for calculating the 3-year average of the 
annual 98th percentile values is given in section 2.6 of 
this appendix.
    2.3 Rounding Conventions. For the purposes of comparing 
calculated values to the applicable level of the standard, it is 
necessary to round the final results of the calculations described 
in sections 2.5 and 2.6 of this appendix. For the annual 
PM2.5 standard, the 3-year average of the spatially 
averaged annual means shall be rounded to the nearest 0.1 
g/m3 (decimals 0.05 and greater are rounded up 
to the next 0.1, and any decimal lower than 0.05 is rounded down to 
the nearest 0.1). For the 24-hour PM2.5 standard, the 3-
year average of the annual 98th percentile values shall 
be rounded to the nearest 1 g/m3 (decimals 0.5 
and greater are rounded up to nearest whole number, and any decimal 
lower than 0.5 is rounded down to the nearest whole number).
    2.4 Monitoring Considerations.
    (a) Section 58.13 of this chapter specifies the required minimum 
frequency of sampling for PM2.5. Exceptions to the 
specified sampling frequencies, such as a reduced frequency during a 
season of expected low concentrations, are subject to the approval 
of the appropriate Regional Administrator. Section 58.14 of 40 CFR 
part 58 and section 2.8 of Appendix D of 40 CFR part 58, specify 
which monitors are eligible for making comparisons with the PM 
standards. In determining a spatial mean using two or more 
monitoring sites operating in a given year, the annual mean for an 
individual site may be included in the spatial mean if and only if 
the mean for that site meets the criterion specified in Sec. 2.8 of 
Appendix D of 40 CFR part 58. In the event data from an otherwise 
eligible site is excluded from being averaged with data from other 
sites on the basis of this criterion, then the 3-year mean from that 
site shall be compared directly to the annual standard.
    (b) For the annual PM2.5 standard, when designated 
monitors are located at the same site and are reporting 
PM2.5 values for the same time periods, and when spatial 
averaging has been chosen, their concentrations shall be averaged 
before an area-wide spatial average is calculated. Such monitors 
will then be considered as one monitor.
    2.5 Equations for the Annual PM2.5 Standard.
    (a) An annual mean value for PM2.5 is determined by 
first averaging the daily values of a calendar quarter:

Equation 1 
[GRAPHIC] [TIFF OMITTED] TR18JY97.000

where:

xq,y,s = the mean for quarter q of year y for site s;

nq = the number of monitored values in the quarter; and

xi,q,y,s = the ith value in quarter q for year 
y for site s.

    (b) The following equation is then to be used for calculation of 
the annual mean:

Equation 2 
[GRAPHIC] [TIFF OMITTED] TR18JY97.001

where:

xy,s = the annual mean concentration for year y (y = 1, 
2, or 3) and for site s; and

xq,y,s = the mean for quarter q of year y for site s.

    (c) (1) The spatially averaged annual mean for year y is 
computed by first calculating the annual mean for each site 
designated to be included in a spatial average, xy,s, and 
then computing the average of these values across sites:

Equation 3 
[GRAPHIC] [TIFF OMITTED] TR18JY97.002

where:

xy = the spatially averaged mean for year y;

xy,s = the annual mean for year y and site s; and

ns = the number of sites designated to be averaged.

    (2) In the event that an area designated for spatial averaging 
has two or more sites at the same location producing data for the 
same time periods, the sites are averaged together before using 
Equation 3 by:

Equation 4 
[GRAPHIC] [TIFF OMITTED] TR18JY97.003

where:

xy,s* = the annual mean for year y for the sites at the 
same location (which will now be considered one site);


[[Page 38757]]


nc = the number of sites at the same location designated 
to be included in the spatial average; and

xy,s = the annual mean for year y and site s.

    (d) The 3-year average of the spatially averaged annual means is 
calculated by using the following equation:

Equation 5 
[GRAPHIC] [TIFF OMITTED] TR18JY97.004

where:

x = the 3-year average of the spatially averaged annual means; and

xy = the spatially averaged annual mean for year y.

Example 1--Area Designated for Spatial Averaging That Meets the 
Primary Annual PM2.5 Standard.

    a. In an area designated for spatial averaging, four designated 
monitors recorded data in at least 1 year of a particular 3-year 
period. Using Equations 1 and 2, the annual means for 
PM2.5 at each site are calculated for each year. The 
following table can be created from the results. Data completeness 
percentages for the quarter with the fewest number of samples are 
also shown.

                                                        Table 1.--Results from Equations 1 and 2                                                        
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                                                       Site #1       Site #2       Site #3       Site #4    Spatial mean
--------------------------------------------------------------------------------------------------------------------------------------------------------
Year 1.........................................  Annual mean (g/m\3\)....          12.7  ............  ............  ............         12.7 
                                                 % data completeness..............          80             0             0             0    ............
Year 2.........................................  Annual mean (g/m\3\)....          12.6          17.5          15.2  ............         15.05
                                                 % data completeness..............          90            63            38             0    ............
Year 3.........................................  Annual mean (g/m\3\)....          12.5          18.5          14.1          16.9         15.50
                                                 % data completeness..............          90            80            85            50    ............
3-year mean....................................  .................................  ............  ............  ............  ............         14.42
--------------------------------------------------------------------------------------------------------------------------------------------------------

    b. The data from these sites are averaged in the order described 
in section 2.1 of this appendix. Note that the annual mean from site 
#3 in year 2 and the annual mean from site #4 in year 3 do not meet 
the 75 percent data completeness criteria. Assuming the 38 percent 
data completeness represents a quarter with fewer than 11 samples, 
site #3 in year 2 does not meet the minimum data completeness 
requirement of 11 samples in each quarter. The site is therefore 
excluded from the calculation of the spatial mean for year 2. 
However, since the spatial mean for year 3 is above the level of the 
standard and the minimum data requirement of 11 samples in each 
quarter has been met, the annual mean from site #4 in year 3 is 
included in the calculation of the spatial mean for year 3 and in 
the calculation of the 3-year average. The 3-year average is rounded 
to 14.4 g/m3, indicating that this area meets 
the annual PM2.5 standard.

Example 2--Area With Two Monitors at the Same Location That Meets 
the Primary Annual PM2.5 Standard.

    a. In an area designated for spatial averaging, six designated 
monitors, with two monitors at the same location (#5 and #6), 
recorded data in a particular 3-year period. Using Equations 1 and 
2, the annual means for PM2.5 are calculated for each 
year. The following table can be created from the results.

                                                        Table 2.--Results From Equations 1 and 2                                                        
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                                                                                                  Average of    Spatial 
           Annual mean (g/m\3\)              Site #1      Site #2      Site #3      Site #4      Site #5      Site #6     #5 and #6     mean   
--------------------------------------------------------------------------------------------------------------------------------------------------------
Year 1............................................         12.9          9.9         12.6         11.1         14.5         14.6       14.55       12.21
Year 2............................................         14.5         13.3         12.2         10.9         16.1         16.0       16.05       13.39
Year 3............................................         14.4         12.4         11.5          9.7         12.3         12.1       12.20       12.04
3-Year mean.......................................  ...........  ...........  ...........  ...........  ...........  ...........  ..........       12.55
--------------------------------------------------------------------------------------------------------------------------------------------------------

    b. The annual means for sites #5 and #6 are averaged together 
using Equation 4 before the spatial average is calculated using 
Equation 3 since they are in the same location. The 3-year mean is 
rounded to 12.6 g/m3, indicating that this area 
meets the annual PM2.5 standard.

Example 3--Area With a Single Monitor That Meets the Primary Annual 
PM2.5 Standard.

    a. Given data from a single monitor in an area, the calculations 
are as follows. Using Equations 1 and 2, the annual means for 
PM2.5 are calculated for each year. If the annual means 
are 10.28, 17.38, and 12.25 g/m3, then the 3-
year mean is: 
[GRAPHIC] [TIFF OMITTED] TR18JY97.005

    b. This value is rounded to 13.3, indicating that this area 
meets the annual PM2.5 standard.
    2.6 Equations for the 24-Hour PM2.5 Standard.
    (a) When the data for a particular site and year meet the data 
completeness requirements in section 2.2 of this appendix, 
calculation of the 98th percentile is accomplished by the 
following steps. All the daily values from a particular site and 
year comprise a series of values (x1, x2, 
x3, ..., xn), that can be sorted into a series 
where each number is equal to or larger than the preceding number 
(x[1], x[2], x[3], ..., 
x[n]). In this case, x[1] is the smallest 
number and x[n] is the largest value. The 98th 
percentile is found from the sorted series of daily values which is 
ordered from the lowest to the highest number. Compute (0.98)  x  
(n) as the number ``i.d'', where ``i'' is the integer part of the 
result and ``d'' is the decimal part of the result. The 
98th percentile value for year y, P0.98, y, is 
given by Equation 6:

Equation 6 
[GRAPHIC] [TIFF OMITTED] TR18JY97.006

where:
P0.98,y = 98th percentile for year y;

x[i+1] = the (i+1)th number in the ordered 
series of numbers; and

i = the integer part of the product of 0.98 and n.


[[Page 38758]]


    (b) The 3-year average 98th percentile is then 
calculated by averaging the annual 98th percentiles:

Equation 7 
[GRAPHIC] [TIFF OMITTED] TR18JY97.007

    (c) The 3-year average 98th percentile is rounded 
according to the conventions in section 2.3 of this appendix before 
a comparison with the standard is made.

Example 4--Ambient Monitoring Site With Every-Day Sampling That 
Meets the Primary 24-Hour PM2.5 Standard.

    a. In each year of a particular 3 year period, varying numbers 
of daily PM2.5 values (e.g., 281, 304, and 296) out of a 
possible 365 values were recorded at a particular site with the 
following ranked values (in g/m3):

                                  Table 3.--Ordered Monitoring Data For 3 Years                                 
----------------------------------------------------------------------------------------------------------------
               Year 1                                Year 2                                Year 3               
----------------------------------------------------------------------------------------------------------------
      j rank            Xj value            j rank            Xj value            j rank            Xj value    
----------------------------------------------------------------------------------------------------------------
275..............            57.9                296               54.3                290               66.0   
276..............            59.0                297               57.1                291               68.4   
277..............            62.2                298               63.0                292               69.8   
----------------------------------------------------------------------------------------------------------------

    b. Using Equation 6, the 98th percentile values for 
each year are calculated as follows: 
[GRAPHIC] [TIFF OMITTED] TR18JY97.008


 [GRAPHIC] [TIFF OMITTED] TR18JY97.009


 [GRAPHIC] [TIFF OMITTED] TR18JY97.010

    c. 1. Using Equation 7, the 3-year average 98th 
percentile is calculated as follows: 
[GRAPHIC] [TIFF OMITTED] TR18JY97.011

    2. Therefore, this site meets the 24-hour PM2.5 
standard.
3.0 Comparisons with the PM10 Standards.
    3.1 Annual PM10 Standard.
    (a) The annual PM10 standard is met when the 3-year 
average of the annual mean PM10 concentrations at each 
monitoring site is less than or equal to 50 g/
m3. The 3-year average of the annual means is determined 
by averaging quarterly means to obtain annual mean PM10 
concentrations for 3 consecutive, complete years at each monitoring 
site. The steps can be summarized as follows:
    (1) Average 24-hour measurements to obtain a quarterly mean.
    (2) Average quarterly means to obtain an annual mean.
    (3) Average annual means to obtain a 3-year mean.
    (b) For the annual PM10 standard, a year meets data 
completeness requirements when at least 75 percent of the scheduled 
sampling days for each quarter have valid data. However, years with 
high concentrations and more than a minimal amount of data (at least 
11 samples in each quarter) shall not be ignored just because they 
are comprised of quarters with less than complete data. Thus, in 
computing the 3-year average annual mean concentration, years 
containing quarters with at least 11 samples but less than 75 
percent data completeness shall be included in the computation if 
the annual mean concentration (rounded according to the conventions 
of section 2.3 of this appendix) is greater than the level of the 
standard.
    (c) Situations may arise in which there are compelling reasons 
to retain years containing quarters which do not meet the data 
completeness requirement of 75 percent or the minimum number of 11 
samples. The use of less than complete data is subject to the 
approval of the appropriate Regional Administrator.
    (d) The equations for calculating the 3-year average annual mean 
of the PM10 standard are given in section 3.5 of this 
appendix.
    3.2 24-Hour PM10 Standard.
    (a) The 24-hour PM10 standard is met when the 3-year 
average of the annual 99th percentile values at each 
monitoring site is less than or equal to 150 g/
m3. This comparison shall be based on 3 consecutive, 
complete years of air quality data. A year meets data completeness 
requirements when at least 75 percent of the scheduled sampling days 
for each quarter have valid data. However, years with high 
concentrations shall not be ignored just because they are comprised 
of quarters with less than complete data. Thus, in computing the 3-
year average of the annual 99th percentile values, years 
containing quarters with less than 75 percent data completeness 
shall be included in the computation if the annual 99th 
percentile value (rounded according to the conventions of section 
2.3 of this appendix) is greater than the level of the standard.
    (b) Situations may arise in which there are compelling reasons 
to retain years containing quarters which do not meet the data 
completeness requirement. The use of less than complete data is 
subject to the approval of the appropriate Regional Administrator.
    (c) The equation for calculating the 3-year average of the 
annual 99th percentile values is given in section 2.6 of 
this appendix.
    3.3 Rounding Conventions. For the annual PM10 
standard, the 3-year average of the annual PM10 means 
shall be rounded to the nearest 1 g/m3 (decimals 
0.5 and greater are

[[Page 38759]]

rounded up to the next whole number, and any decimal less than 0.5 
is rounded down to the nearest whole number). For the 24-hour 
PM10 standard, the 3-year average of the annual 
99th percentile values of PM10 shall be 
rounded to the nearest 10 g/m3 (155 g/
m3 and greater would be rounded to 160 g/
m3 and 154 g/m3 and less would be 
rounded to 150 g/m3).
    3.4 Monitoring Considerations. Section 58.13 of this chapter 
specifies the required minimum frequency of sampling for 
PM10. Exceptions to the specified sampling frequencies, 
such as a reduced frequency during a season of expected low 
concentrations, are subject to the approval of the appropriate 
Regional Administrator. For making comparisons with the 
PM10 NAAQS, all sites meeting applicable requirements in 
part 58 of this chapter would be used.
    3.5 Equations for the Annual PM10 Standard.
    (a) An annual arithmetic mean value for PM10 is 
determined by first averaging the 24-hour values of a calendar 
quarter using the following equation:

Equation 8 
[GRAPHIC] [TIFF OMITTED] TR18JY97.012

where:
xq,y = the mean for quarter q of year y;

nq = the number of monitored values in the quarter; and

xi,q,y = the ith value in quarter q for year 
y.

    (b) The following equation is then to be used for calculation of 
the annual mean:

Equation 9 
[GRAPHIC] [TIFF OMITTED] TR18JY97.013

where:

xy = the annual mean concentration for year y, (y=1, 2, 
or 3); and

xq,y = the mean for a quarter q of year y.

    (c) The 3-year average of the annual means is calculated by 
using the following equation:

Equation 10 
[GRAPHIC] [TIFF OMITTED] TR18JY97.014

where:

x = the 3-year average of the annual means; and

xy = the annual mean for calendar year y.

Example 5--Ambient Monitoring Site That Does Not Meet the Annual 
PM10 Standard.

    a. Given data from a PM10 monitor and using Equations 
8 and 9, the annual means for PM10 are calculated for 
each year. If the annual means are 52.42, 82.17, and 63.23 
g/m3, then the 3-year average annual mean is: 
[GRAPHIC] [TIFF OMITTED] TR18JY97.015

    b. Therefore, this site does not meet the annual PM10 
standard.
    3.6 Equation for the 24-Hour PM10 Standard.
    (a) When the data for a particular site and year meet the data 
completeness requirements in section 3.2 of this appendix, 
calculation of the 99th percentile is accomplished by the 
following steps. All the daily values from a particular site and 
year comprise a series of values (x1, x2, 
x3, ..., xn) that can be sorted into a series 
where each number is equal to or larger than the preceding number 
(x[1], x[2], x[3], ..., 
x[n]). In this case, x[1] is the smallest 
number and x[n] is the largest value. The 99th percentile 
is found from the sorted series of daily values which is ordered 
from the lowest to the highest number. Compute (0.99)  x  (n) as the 
number ``i.d'', where ``i'' is the integer part of the result and 
``d'' is the decimal part of the result. The 99th 
percentile value for year y, P0.99,y, is given by 
Equation 11:

Equation 11 
[GRAPHIC] [TIFF OMITTED] TR18JY97.016

where:

P0.99,y = the 99th percentile for year y;

x[i+1] = the (i+1)th number in the ordered 
series of numbers; and

i = the integer part of the product of 0.99 and n.

    (b) The 3-year average 99th percentile value is then 
calculated by averaging the annual 99th percentiles:

Equation 12 
[GRAPHIC] [TIFF OMITTED] TR18JY97.017

    (c) The 3-year average 99th percentile is rounded 
according to the conventions in section 3.3 of this appendix before 
a comparison with the standard is made.

Example 6--Ambient Monitoring Site With Sampling Every Sixth Day 
That Meets the Primary 24-Hour PM10 Standard.

    a. In each year of a particular 3 year period, varying numbers 
of PM10 daily values (e.g., 110, 98, and 100) out of a 
possible 121 daily values were recorded at a particular site with 
the following ranked values (in g/m3):

                                  Table 4.--Ordered Monitoring Data For 3 Years                                 
----------------------------------------------------------------------------------------------------------------
               Year 1                                Year 2                                Year 3               
----------------------------------------------------------------------------------------------------------------
      j rank            Xj value            j rank            Xj value            j rank            Xj value    
----------------------------------------------------------------------------------------------------------------
108..............             120                 96                143                 98                140   
109..............             128                 97                148                 99                144   
110..............             130                 98                150                100                147   
----------------------------------------------------------------------------------------------------------------

    b. Using Equation 11, the 99th percentile values for 
each year are calculated as follows: 
[GRAPHIC] [TIFF OMITTED] TR18JY97.018


 [GRAPHIC] [TIFF OMITTED] TR18JY97.019


 
[[Page 38760]]

[GRAPHIC] [TIFF OMITTED] TR18JY97.020


    c. 1. Using Equation 12, the 3-year average 99th 
percentile is calculated as follows: 
[GRAPHIC] [TIFF OMITTED] TR18JY97.021

    2. Therefore, this site meets the 24-hour PM10 
standard.

[FR Doc. 97-18577 Filed 7-17-97; 8:45 am]
BILLING CODE 6560-50-F