[Federal Register Volume 61, Number 241 (Friday, December 13, 1996)]
[Proposed Rules]
[Pages 65638-65713]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 96-30897]


      

[[Page 65637]]

_______________________________________________________________________

Part II





Environmental Protection Agency





_______________________________________________________________________



40 CFR Part 50



National Ambient Air Quality Standards for Particulate Matter; Proposed 
Rule

  Federal Register / Vol. 61, No. 241 /  Friday, December 13, 1996 /  
Proposed Rules  

[[Page 65638]]



ENVIRONMENTAL PROTECTION AGENCY

40 CFR Part 50

[AD-FRL-5659-5]
RIN 2060-AE66


National Ambient Air Quality Standards for Particulate Matter: 
Proposed Decision

AGENCY: Environmental Protection Agency (EPA).

ACTION: Proposed rule.

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

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

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

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

SUPPLEMENTARY INFORMATION:

Docket

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

Availability of Related Information

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

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

[[Page 65639]]

will be proposed for public comment in the future.

Table of Contents

    The following topics are discussed in today's preamble:

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

I. Background

A. Legislative Requirements

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

B. Related Control Requirements

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

C. Review of Air Quality Criteria and Standards for PM

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

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July 1987 with notice of a final decision to revise the existing 
standards (52 FR 24854, July 1, 1987). In that decision, EPA changed 
the indicator for particles from total suspended particles (TSP) to 
PM10.1 Identical primary and secondary PM10 standards 
were set for two averaging times: (1) 50 g/m3, expected 
annual arithmetic mean, averaged over 3 years, and (2) 150 g/
m3, 24-hour average, with no more than one expected exceedance per 
year.2
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    \1\ PM10 refers to particles with an aerodynamic diameter 
less than or equal to a nominal 10 micrometers.
    \2\A more complete history of the PM NAAQS is presented in 
section II.B of the OAQPS Staff Paper, Review of National Ambient 
Air Quality Standards for Particulate Matter: Assessment of 
Scientific and Technical Information (U.S. EPA, 1996b).
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    The EPA formally initiated the current review of the air quality 
criteria for PM in April 1994 by announcing its intention to develop a 
revised Air Quality Criteria Document for Particulate Matter 
(henceforth, the ``Criteria Document''). Thereafter, the EPA presented 
its plans for review of the criteria and standards for PM under a 
highly accelerated, court-ordered schedule 3 at a public meeting 
of the CASAC in December 1994. Several workshops were held by EPA's 
National Center for Environmental Assessment (NCEA) to discuss 
important new health effects information in November 1994 and January 
1995. External review drafts of the Criteria Document were made 
available for public comment and were reviewed by CASAC at public 
meetings held in August and December 1995 and February 1996. The CASAC 
came to closure in its review of the Criteria Document, advising the 
Administrator in a March 15, 1996 closure letter (Wolff, 1996a) that 
``although our understanding of the health effects of PM is far from 
complete, a revised Criteria Document which incorporates the Panel's 
latest comments will provide an adequate review of the available 
scientific data and relevant studies of PM.'' CASAC and public comments 
from these meetings and from subsequent written comments and the 
closure letter were incorporated as appropriate in the final Criteria 
Document (U.S. EPA, 1996a).
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    \3\ A court order entered in American Lung Association v. 
Browner, CIV-93-643-TUC-ACM (D. Ariz., October 6, 1994), as 
subsequently modified, requires publication of proposed and final 
decisions on the review of the PM NAAQS by November 29, 1996 and 
June 28, 1997, respectively.
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    External review drafts of a staff paper prepared by the Office of 
Air Quality Planning and Standards (OAQPS), Review of the National 
Ambient Air Quality Standards for Particulate Matter: Assessment of 
Scientific and Technical Information (henceforth, the ``Staff Paper'') 
were made available for public comment and were reviewed by CASAC at 
public meetings in December 1995 and May 1996.4 The CASAC came to 
closure in its review of the Staff Paper, advising the Administrator in 
a June 13, 1996 closure letter (Wolff, 1996b) that ``the Staff Paper, 
when revised, will provide an adequate summary of our present 
understanding of the scientific basis for making regulatory decisions 
concerning PM standards.'' CASAC and public comments from these 
meetings, subsequent written comments, and the CASAC closure letter 
were incorporated as appropriate in the final Staff Paper (U.S. EPA, 
1996b).
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    \4\ The Staff Paper evaluates policy implications of the key 
studies and scientific information in the Criteria Document, 
identifies critical elements that EPA staff believes should be 
considered, and presents staff conclusions and recommendations of 
suggested options for the Administrator's consideration.
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    The principal focus of this current review of the air quality 
criteria and standards for PM is on recent epidemiological evidence 
reporting associations between ambient concentrations of PM and a range 
of serious health effects. Particular attention has been given to 
several size-specific classes of particles, including PM10 and the 
principal fractions of PM10, referred to as the fine (PM2.5) 
5 and coarse (PM10-2.5) 6 fractions. As discussed in the 
Criteria Document, fine and coarse fraction particles can be 
differentiated by their sources and formation processes and their 
chemical and physical properties, including behavior in the atmosphere. 
Detailed discussions of atmospheric formation, ambient concentrations, 
and health and welfare effects of PM, as well as quantitative estimates 
of human health risks associated with exposure to PM, can be found in 
the Criteria Document and Staff Paper.
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    \5\ PM2.5 refers to particles with an aerodynamic diameter 
less than or equal to a nominal 2.5 micrometers.
    \6\ PM10-2.5 refers to those particles with an aerodynamic 
diameter less than or equal to a nominal 10 micrometers but greater 
than 2.5 micrometers.
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    This review of the scientific criteria for PM has occurred 
simultaneously with the review of the criteria for ozone (O3). 
These criteria reviews as well as related implementation strategy 
activities to date have brought out important linkages between O3 
and PM. A number of community epidemiological studies have found 
similar health effects to be associated with exposure to O3 and 
PM, including, for example, aggravation of respiratory disease (e.g., 
asthma), increased respiratory symptoms, and increased hospital 
admissions and emergency room visits for respiratory causes. Laboratory 
studies have found potential interactions between O3 and various 
constituents of PM. Other key similarities relating to exposure 
patterns and implementation strategies exist between O3 and PM, 
specifically fine particles. These similarities include: (1) 
Atmospheric residence times of several days, leading to large urban and 
regional-scale transport of the pollutants; (2) similar gaseous 
precursors, including NOX and VOC, which contribute to the 
formation of both O3 and fine particles in the atmosphere; (3) 
similar combustion-related source categories, such as coal and oil-
fired power generation and industrial boilers and mobile sources, which 
emit particles directly as well as gaseous precursors of particles 
(e.g., SOX, NOX, VOC) and O3 (e.g., NOX, VOC); and 
(4) similar atmospheric chemistry driven by the same chemical reactions 
and intermediate chemical species that form both high O3 and fine 
particle levels. High fine particle levels are also associated with 
significant impairment of visibility on a regional scale.
    These similarities provide opportunities for optimizing technical 
analysis tools (i.e., monitoring networks, emission inventories, air 
quality models) and integrated emission reduction strategies to yield 
important co-benefits across various air quality management programs. 
These co-benefits could result in a net reduction of the regulatory 
burden on some source category sectors that would otherwise be impacted 
by separate O3, PM, and visibility protection control strategies.
    In recognition of the multiple linkages and similarities in effects 
and the potential benefits of integrating the Agency's approaches to 
providing for appropriate protection of public health and welfare from 
exposure to O3 and PM, EPA plans to complete the review of the 
NAAQS for both pollutants on the same schedule. Accordingly, today's 
Federal Register contains a separate notice announcing proposed 
revisions to the O3 NAAQS. Linking the O3 and PM review 
schedules provides an important opportunity to materially improve the 
nation's air quality management programs--both in terms of 
communicating a more complete description of the health and welfare 
effects associated with the major components of urban and regional air 
pollution, and by helping the States and local areas to plan jointly to 
address both PM and O3 air pollution at the same time with one 
process, and to

[[Page 65641]]

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

II. Rationale for Proposed Decisions on Primary Standards

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

A. Health Effects Information

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

a. Mortality

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

[[Page 65642]]

1000 g/m 3 of PM 7 and sulfur dioxide (SO 2)) 
was causally related to increased mortality. Further analyses of daily 
mortality over 14 London winters suggested that particles were more 
likely to be responsible for the associations of health effects with 
air pollution than SO2, and that the association continued to the 
lower concentrations of PM measured in London (150 g/m3, 
measured as BS).
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    \7\ Measured as British Smoke (BS), which gauges the darkness of 
PM collected on a filter and is most sensitive to combustion 
generated carbon particles. When calibrated to a mass measurement, 
as in the historical London studies, BS is an indicator of fine mode 
particles.
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    From 1987 to present, numerous epidemiological studies using 
improved statistical techniques and expanded particle monitoring data 
have reported statistically significant 8 positive associations 
between increased daily or several-day average concentrations of PM [as 
measured by a variety of indices, including TSP, PM10, PM2.5, 
sulfate, and BS] and premature mortality in communities across the U.S. 
as well as in Europe and South America. Of 38 analyses and reanalyses 
of these studies (referred to as daily mortality studies) published 
between 1988 and 1996, most found statistically significant 
associations between increases in short-term ambient PM concentrations 
and total non-accidental mortality (U.S. EPA, 1996a, Table 12-2).
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    \8\ Statistically significant results are reported at a 95% 
confidence level.
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    More specifically, the effects estimates for PM10 reported in 
these studies fall within a range of approximately 2 to 8 percent 
increase in the relative risk 9 of mortality for a 50 g/
m3 increase in 24-hour average PM10 concentrations. The 
consistency in these results is notable, particularly since these 
studies examined PM-mortality relationships in 18 different locations 
varying significantly in climate, human activity patterns, aerosol 
composition, and amounts of co-occurring gaseous pollutants [e.g., 
SO2 and ozone(O3)], using a variety of statistical 
techniques. A rough estimate of the incremental relative risk 
attributed to PM concentrations seen in the worst London episode also 
falls within this range (U.S. EPA, 1996b, p. V-13). It is also 
important to note that the magnitude of the relative risks, while 
significant from a public health perspective because the potentially 
exposed population is large, are small compared to those usually found 
in epidemiological studies of occupational and other risk factors.
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    \9\ Many of the recent epidemiological studies report effects 
estimates in terms of a percentage increase in the risk of mortality 
in the study population (as compared to the baseline rate in the 
population as a whole) associated with a specific increase in 
ambient PM concentrations measured by one or more outdoor monitors. 
These effects estimates generally are based on a statistical model 
of the entire study period, which typically spanned multiple years 
or seasons.
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    Some of these daily mortality studies examined PM-mortality 
associations for both total non-accidental mortality and cause-specific 
mortality. In general, such studies have reported higher relative risks 
for respiratory and cardiovascular causes of death than for total 
mortality, as well as higher risks for mortality in the elderly (>65 
years of age) than for mortality in the general population.
ii. Long-Term Exposure Studies
    By the time of the previous review of the PM criteria in 1987, 
numerous epidemiological studies of a cross-sectional design had 
reported statistically significant associations linking higher long-
term (single or multi-year) concentrations of various indices of PM 
with higher mortality rates across numerous U.S. communities. However, 
the usefulness of such studies for quantitative purposes was at that 
time limited by the lack of supporting evidence available from daily 
mortality studies or the toxicological literature, and by unaddressed 
confounders and methodological problems inherent in these cross-
sectional studies.
    More recently, epidemiological studies of a prospective-cohort 
design have been conducted, including in particular the Six City study 
(Dockery et al., 1993) and the American Cancer Society (ACS) study 
(Pope et al., 1995), that lend support to the earlier cross-sectional 
studies of mortality. These two recent studies reflect significant 
methodological advances over the earlier studies, including the use of 
subject-specific information, and provide evidence for an association 
between long-term PM concentrations and mortality. At least some 
fraction of mortality was reported to reflect cumulative PM impacts in 
addition to those associated with short-term concentrations (U.S. EPA, 
1996a, p. 13-34).
    The Six City study, which followed more than 8,000 adults for 14 
years, found that long-term PM concentrations (PM15/10, 
PM2.5, and sulfate) in six U.S. cities were statistically 
significantly associated with increased rates of total mortality and 
cardiopulmonary mortality, even after adjustment for smoking, education 
level, and occupation. Specifically, this study reported increases in 
relative risk of 26% and 37% for total and cardiopulmonary-related 
mortality, respectively, between the cities with the highest and lowest 
PM concentrations. The ACS study was designed to follow up on the 
findings from the Six City study, using a much larger number of 
individuals (more than half a million adults followed for seven years) 
and cities. The ACS investigators reported that, after adjustment for 
other risk factors, multi-year concentrations of PM2.5 (for 47 
U.S. cities) and sulfate (for 151 cities) were found to be 
statistically significantly associated with both total and 
cardiopulmonary mortality. The ACS study reported increases in relative 
risk of 17% and 31% for total and cardiopulmonary mortality, 
respectively.
    Some reviewers have raised concerns regarding the adequacy of the 
adjustment for confounders in these prospective-cohort studies, 
maintaining that other uncontrolled factors may be responsible for the 
observed mortality rates (Lipfert and Wyzga, 1995; Moolgavkar and 
Luebeck, 1996; Moolgavkar, 1994). The Criteria Document indicates, 
however, that it is unlikely that these studies overlooked plausible 
confounders, although the addition of factors not taken into account 
might well alter the magnitude of the association (U.S. EPA, 1996a, p. 
12-180). In particular, the Criteria Document cautions that the 
magnitude of relative risks associated with PM concentrations reported 
in these studies may be overestimated because some of the effects may 
be due to historical PM concentrations that were significantly higher 
than the ones used to estimate population exposures in these studies.
    The Criteria Document concludes that the Six City and ACS studies, 
taken together with the earlier cross-sectional studies, suggest that: 
1) there may be increases in mortality in disease categories that are 
consistent with long-term exposure to PM, and 2) at least some fraction 
of these deaths reflects cumulative PM impacts greater than those 
reported in the daily mortality studies (U.S. EPA, 1996a, p. 13-34).
iii. Degree of Lifespan Shortening
    The degree of lifespan shortening associated with PM exposure in 
these studies is viewed by many as an important consideration in 
evaluating mortality effects in a public health context. The 
epidemiological findings of associations between short- and long-term 
ambient PM concentrations and premature mortality provide some insight 
into this issue. The mortality effects estimates associated with long-
term PM concentrations in the prospective-cohort studies are

[[Page 65643]]

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

b. Aggravation of Respiratory and Cardiovascular Disease

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

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

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

c. Altered Lung Function and Increased Respiratory Symptoms

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

d. Alteration of Lung Tissue and Structure

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

[[Page 65644]]

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

e. Changes in Respiratory Defense Mechanisms

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

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

a. Interpretation of Individual Study Results

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

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

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

[[Page 65645]]

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

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

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

[[Page 65646]]

b. Consistency and Coherence of the Health Effects Evidence

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

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

[[Page 65649]]

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

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

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

B. Quantitative Risk Assessment

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

[[Page 65650]]

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

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

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

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

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

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

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

[[Page 65651]]

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

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

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

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

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

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

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

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

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

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

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

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

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

[[Page 65652]]

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

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

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

BILLING CODE 6560-50-P

[GRAPHIC] [TIFF OMITTED] TP13DE96.048

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

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

[[Page 65653]]

[GRAPHIC] [TIFF OMITTED] TP13DE96.050


BILLING CODE 6560-50-C

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

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

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

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

C. Need for Revision of the Current Primary PM Standards

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

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

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

[[Page 65654]]

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

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

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

D. Indicators of PM

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

[[Page 65655]]

    In specifying a precise size range for a fine particle standard, 
both the staff and CASAC recommend PM2.5 as the indicator of fine 
particles (Wolff, 1996b). The particle diameter reflecting the mass 
minimum between the fine and coarse modes typically lies between 1 and 
3 m, and the scientific data support a sampling cut point to 
delineate fine particles in this range. Because of the potential 
overlap of fine and coarse particle mass in this intermodal region, EPA 
recognizes that any specific sampling cut point would result in only an 
approximation of the actual fine-mode particle mass. Thus, the choice 
of a specific diameter within this size range is largely a policy 
judgment. The staff and CASAC recommendation for a 2.5 m 
sampling cut point is based on considerations of consistency with the 
community health studies, the limited potential for intrusion of coarse 
fraction particles into the fine fraction, and availability of 
monitoring technology.27 PM2.5 encompasses all of the 
potential agents of concern in the fine fraction, including most 
sulfates, acids, fine particle transition metals, organics, and 
ultrafine particles, and includes most of the aggregate surface area 
and particle number in the entire distribution of atmospheric 
particles.
---------------------------------------------------------------------------

    \27\ Some commentors have recommended the use of a smaller 
cutpoint at 1 m (PM1) to further reduce coarse 
particle intrusion. PM1 has not been used in health studies, 
although in most cases collected mass should be similar to those for 
cutpoints of 2.1 or 2.5 m. While this indicator could 
reduce intrusion of coarse particles, it might also omit portions of 
hygroscopic acid sulfates in high humidity environments. PM1 
sampling technologies have been developed; however, PM1 
samplers have not been widely used in the field to date, and there 
are some concerns about loss of certain organic materials relative 
to an instrument with a larger size cut.
---------------------------------------------------------------------------

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

E. Averaging Time of PM2.5 Standards

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

[[Page 65656]]

uncertain, appear to be larger than those associated with short-term 
exposures. Based on the available epidemiology, and consistent with the 
limited relevant toxicological and dosimetric information, the 
Administrator concludes that significant, and potentially independent, 
health consequences are likely associated with long-term PM exposures.
    The Administrator has considered this evidence, which suggests that 
some health endpoints reflect the cumulative effects of PM exposures 
over a number of years. In such cases, an annual standard would provide 
effective protection against persistent long-term (several years) 
exposures to PM. Requiring a much longer averaging time would also 
complicate and unnecessarily delay control strategies and attainment 
decisions.
    The Administrator has also considered the seasonality of emissions 
of fine particles and their precursors in some areas (e.g., wintertime 
smoke from residential wood combustion, summertime regional acid 
sulfate and ozone formation), which suggests that some effects 
associated with annual average concentrations might be the result of 
repeated seasonally high exposures. However, different seasons are 
likely of concern in different parts of the country, and the current 
evidence does not provide a satisfactory quantitative basis for setting 
a national fine particle standard in terms of a seasonal averaging 
time.
    In addition, the Administrator recognizes that an annual standard 
would have the effect of controlling air quality broadly across the 
yearly distribution of 24-hour PM2.5 concentrations, although such 
a standard would not as effectively limit peak 24-hour concentrations 
as would a 24-hour standard. Thus, as discussed above in Section B 
above (see especially Figures 2a, 2b, 2c), an annual standard could 
also provide protection from health effects associated with short-term 
exposures to PM2.5.
    For these reasons, the Administrator has concluded that a long-term 
PM2.5 standard with an annual averaging time can serve to control 
both long- and short-term ambient PM2.5 concentrations, thus 
providing protection from health effects associated with long-term 
(seasonal to several years) and, to some degree, short-term exposures 
to PM2.5.
3. Combined Effect of Annual and 24-Hour Standards
    Having concluded that both 24-hour and annual PM2.5 standards 
are appropriate, the Administrator considered the potential combined 
effects of such standards on PM concentration levels and distributions 
prior to considering the form and level of each standard. The existing 
health effects evidence could, of course, be used to assess the form 
and level of each standard independently, with short-term health 
effects evidence being used as the basis for a 24-hour standard and the 
long-term health effects evidence as the basis for an annual standard. 
Some CASAC panel members apparently used this approach as a basis for 
their views on appropriate averaging times and standard levels. In 
particular, a few members focused only on a 24-hour PM2.5 standard 
in light of the relative strength of the short-term exposure studies. 
On the other hand, two members focused only on an annual standard, 
recognizing that strategies to meet an annual standard would provide 
protection against effects of both short- and long-term exposures.
    The Administrator has focused on a policy approach that considers 
the consistency and coherence, as well as the limitations, of the body 
of evidence as a whole, and recognizes that there are various ways to 
combine two standards to achieve an appropriate degree of public health 
protection. Such an approach to standard setting that integrates the 
body of health effects evidence and air quality analyses, and considers 
the combined effect of the standards, has the potential to result in a 
more effective and efficient suite of standards than an approach that 
only considers short- and long-term evidence, analyses, and standards 
independently.
    In considering the combined effect of such standards, the 
Administrator notes that while an annual standard focuses on annual 
average PM2.5 concentrations, it would also result in fewer and 
lower 24-hour peak concentrations. Alternatively, a 24-hour standard 
which focuses on peak concentrations would also result in lower annual 
average concentrations. Thus, either standard could be viewed as 
providing both short- and long-term protection, with the other standard 
serving as a ``backstop'' in situations where the daily peaks and 
annual averages are not consistently correlated.
    The Administrator believes that the suite of PM2.5 standards 
can be most effectively and efficiently defined by treating the annual 
standard as the generally controlling standard for lowering both short- 
and long-term PM2.5 concentrations. As a supplement to the annual 
standard, the 24-hour standard would serve as a backstop to provide 
additional protection against days with high peak PM2.5 
concentrations, localized ``hot spots,'' and risks arising from 
seasonal emissions that would not be well controlled by a national 
annual standard. In reaching this view, the Administrator took into 
account the factors discussed below.
    (1) Based on one of the key observations from the quantitative risk 
assessment (Section B, Figures 2a, 2b, 2c), the Administrator notes 
that much if not most of the aggregate annual risk associated with 
short-term exposures results from the large number of days during which 
the 24-hour average concentrations are in the low- to mid-range, below 
the peak 24-hour concentrations. As a result, lowering a wide range of 
ambient 24-hour PM2.5 concentrations, as opposed to focusing on 
control of peak 24-hour concentrations, is the most effective and 
efficient way to reduce total population risk. Further, there is no 
evidence suggesting that risks associated with long-term exposures are 
likely to be disproportionately driven by peak 24-hour concentrations. 
Thus, an annual standard that controls an area's attainment status is 
likely to reduce aggregate risks associated with both short- and long-
term exposures with more certainty than a 24-hour standard.
    (2) The consistency and coherence of the health effects data base 
is more directly related to long-term measures of air quality (e.g., 
the annual distributions of 24-hour PM concentrations), rather than to 
24-hour concentrations on individual days. More specifically, judgments 
about the quantitative consistency of the large number of short-term 
exposure studies reporting associations with 24-hour concentrations 
arise from comparing the relative risk results derived from analyzing 
the associations across the entire duration of the studies, which 
typically spanned at least an annual time frame.
    (3) An annual average measure of air quality is more stable over 
time than are 24-hour measures. Thus, a controlling annual standard is 
likely to result in the development of more consistent risk reduction 
strategies over time, since an area's attainment status will be less 
likely to change due solely to year-to-year variations in 
meteorological conditions that affect the formation of fine particles, 
than under a controlling 24-hour standard.
    Under this policy approach, the annual PM2.5 standard would 
serve in most areas as the target for control programs designed to be 
effective in lowering the broad distribution of PM2.5 
concentrations, thus protecting not only

[[Page 65657]]

against long-term effects but also short-term effects as well. In 
combination with such an annual standard, the 24-hour PM2.5 
standard would be set so as to protect against the occurrence of peak 
24-hour concentrations and those that present localized or seasonal 
effects of concern in areas where the highest 24-hour-to-annual mean 
PM2.5 ratios are appreciably above the national average.
    The Administrator recognizes that this policy approach represents a 
new way of thinking about the combined effects of short- and long-term 
standards, and that there are alternative views about this approach. 
Accordingly, the Administrator solicits comment on this policy approach 
for defining the most effective and efficient suite of PM2.5 
standards.

F. Form of PM2.5 Standards

1. Annual Standard
    As discussed in some detail during the last review of the PM NAAQS 
(see 49 FR 10408, March 20, 1984; 52 FR 24634, July 1, 1987), the 
expected annual arithmetic mean (i.e., the annual arithmetic mean 
averaged over 3 years) is a relatively stable measure of air quality 
that reflects the total cumulative dose of PM to which an individual or 
population is exposed. Short-term peaks have an influence on the 
arithmetic mean that is proportional to their frequency, magnitude, and 
duration, and, thus, their contribution to cumulative exposure and 
risk. As a result, the annual arithmetic mean form of an annual 
standard provides protection across a wide range of the air quality 
distribution contributing to exposure and risk, in contrast to other 
forms, such as the geometric mean, that deemphasize the effects of 
short-term peak concentrations. On this basis, the Administrator 
concurs with the Staff Paper recommendation, supported by CASAC, to use 
the 3-year average annual arithmetic mean as the form for an annual 
PM2.5 standard, consistent with the current form of the annual 
PM10 standard.
    The Staff Paper and some CASAC panel members also recommended that 
consideration be given to calculating the PM2.5 annual arithmetic 
mean for an area by averaging the annual arithmetic means derived from 
multiple, primarily population-oriented monitoring sites within a 
monitoring planning area. In considering a calculation method for 
annual arithmetic averages that involves spatial averaging of 
monitoring data, the Administrator specifically took into account the 
following factors: 28
---------------------------------------------------------------------------

    \28\ Spatial averaging of monitoring data is also discussed in 
the notice of a proposed decision on the O3 NAAQS published 
today. Different considerations apply in the two cases principally 
because of differences between (1) the nature of the health effects 
evidence for PM2.5 and O3; (2) the proposed suite and 
annual and 24-hour PM2.5 standards, in contrast to a single 
proposed O3 standard; and (3) the existence of an established, 
extensive O3 monitoring network, in contrast to the absence at 
present of such a network for PM2.5.
---------------------------------------------------------------------------

    (1) Many of the community-based epidemiological studies examined in 
this review used spatial averages, when multiple monitoring sites were 
available, to characterize area-wide PM exposure levels and the 
associated population health risk. Even in those studies that used only 
one monitoring location, the selected site was chosen to represent 
community-wide exposures, not the highest value likely to be 
experienced within the community. Thus, spatial averages are most 
directly related to the epidemiological studies used as the basis for 
the proposed revisions to the PM NAAQS.
    (2) Under the policy approach advanced earlier, the annual 
PM2.5 standard would be intended to reduce aggregate population 
risk from both long- and short-term exposures by lowering the broad 
distribution of PM2.5 concentrations across the community. An 
annual standard based on spatially averaged concentrations would better 
reflect area-wide PM exposure levels than would a standard based on 
concentrations from a single monitor with the highest measured values.
    (3) Under this policy approach, the 24-hour PM2.5 standard 
would be intended to supplement a spatially averaged annual PM2.5 
standard by providing protection against peak 24-hour concentrations, 
localized ``hot spots,'' and risk arising from seasonal emissions that 
would not be as well controlled by an annual standard. Accordingly, the 
24-hour PM2.5 standard should be based on the single population-
oriented monitoring site within the monitoring planning area with the 
highest measured values.
    Based on these considerations, the Administrator believes that the 
form of a PM2.5 annual standard should be expressed as the annual 
arithmetic mean, temporally averaged over 3 years and spatially 
averaged over all designated monitoring sites. Such designations would 
be based on criteria contained in the proposed revision to the 
monitoring siting guidance in 40 CFR Part 58 that accompanies this 
notice. In the Administrator's judgment, an annual PM2.5 standard 
expressed in this form, established in conjunction with a 24-hour 
PM2.5 standard, would provide the most appropriate target for 
reducing area-wide population exposure to fine particle pollution.
    On the other hand, the Administrator is mindful that adoption of 
spatial averaging for an annual PM2.5 standard would add a degree 
of complexity to the monitor siting requirements for a new PM2.5 
monitoring network and the specification of those areas across which 
spatial averaging should be permitted. These issues are addressed more 
fully in the accompanying proposed revisions to 40 CFR Part 58. Of 
particular concern is whether appropriate and effective criteria can be 
developed and implemented for determining areas within which spatial 
averaging would be reflective of the area-wide population risk. The EPA 
recognizes that some monitoring planning areas may have to be 
subdivided into smaller subareas to reflect gradients in particle 
levels (e.g., upwind suburban sites, central city sites, downwind 
sites) as well as topographical barriers or other factors that may 
result in a monitoring planning area having several distinct air 
quality regimes.
    Because of the importance of this issue, the notice of proposed 
revisions to 40 CFR Part 58 specifically requests broad public input on 
the approaches advanced in that notice with respect to the selection of 
sites and designation of areas for spatial averaging. Recognizing the 
complexities that spatial averaging may introduce into risk management 
programs and that unforeseen issues may arise from public comment on 
the 40 CFR Part 58 notice, the Administrator also requests comment on 
the alternative of basing the annual standard for PM2.5 on the 
population-oriented monitor site within the monitoring planning area 
with the highest 3-year average annual mean. Based on comments 
received, the Administrator may choose either of these two approaches 
for specifying the form of the annual PM2.5 standard at the time 
of promulgation of any revisions to the PM standards. Proposed methods 
for using monitored concentrations to make a comparison with a 
spatially averaged annual mean standard, as well as associated 
calculations and other data handling conventions, are presented below 
in the section on proposed revisions to Appendix K.
2. 24-Hour Standard
    The current 24-hour PM10 standard is expressed in a ``1-
expected-exceedance'' form. That is, the standard is formulated on the 
basis of the expected number of days per year (averaged over 3 years) 
on which the level of the standard will be exceeded. The test for 
determining attainment of the current 24-hour

[[Page 65658]]

standard is presented in Appendix K to 40 CFR Part 50.
    Since promulgation of the current 24-hour PM10 standard in 
1987, a number of concerns have been raised about the 1-expected-
exceedance form. These include, in particular, the year-to-year 
stability of the number of exceedances, the stability of the attainment 
status of an area, and the complex data handling conventions specified 
in Appendix K, including the procedures for making adjustments for 
missing data and less-than-every-day monitoring.
    In light of these concerns, the Staff Paper and several CASAC panel 
members (Wolff, 1996b) recommended that consideration be given to 
adoption of a more stable and robust form for 24-hour PM standards. In 
considering this recommendation, the Administrator noted that the use 
of a concentration-based percentile form would have several advantages 
over the current 1-expected-exceedance form:
    (1) Such a concentration-based form is more directly related to the 
ambient PM concentrations that are associated with health effects. 
Given that there is a continuum of effects associated with exposures to 
varying levels of PM, the extent to which public health is affected by 
exposure to ambient PM is related to the actual magnitude of the PM 
concentration, not just whether the concentration is above a specified 
level. With an exceedance-based form, days on which the ambient PM 
concentration is well above the level of the standard are given equal 
weight to those days on which the PM concentration is just above the 
standard (i.e., each day is counted as one exceedance), even though the 
public health impact on the two days is significantly different. With a 
concentration-based form, days on which higher PM concentrations occur 
would weigh proportionally more than days with lower PM concentrations 
for the design value, since the actual concentrations are used directly 
in determining whether the standard is attained.
    (2) More specifically, a concentration-based percentile form would 
also compensate for missing data and less-than-every-day monitoring, 
thereby reducing or eliminating the need for complex data handling 
procedures in the Appendix K test for attainment. As a result, an 
area's attainment status would be based directly on monitoring data 
rather than on a calculated value adjusted for missing data or less-
than-every-day monitoring.
    (3) Further, a concentration-based form, averaged over 3 years, 
also has greater stability than the expected exceedance form and, thus, 
would facilitate the development of more stable implementation programs 
by the States.
    In light of these advantages, and taking into account the CASAC 
recommendation as well as concerns regarding adjustments for missing 
data and less-than-every-day monitoring, the Administrator believes 
that adoption of a concentration percentile form for the 24-hour 
PM2.5 standard would be appropriate.
    Having reached this view, the Administrator considered various 
specific percentile values for such a form. In doing so, she took into 
account two factors. First, the 24-hour PM2.5 standard is intended 
to supplement the annual PM2.5 standard by providing a ``back 
stop'' to provide additional protection against extremely high peak 
days, localized ``hot spots,'' and risks arising from seasonal 
emissions. Second, the form of the 24-hour PM2.5 standard should 
provide an appropriate degree of increased stability relative to the 
current form. A more stable statistic would reduce the impact of a 
single high exposure event that may be due to unusual meteorological 
conditions alone, and thus would provide a more stable basis upon which 
to design effective control programs.
    With these purposes in mind, the Administrator observed that while 
a percentile value such as the 90th or 95th would provide substantially 
increased stability when compared to a more extreme air quality 
statistic (e.g., the current 1-expected-exceedance form), it would 
likely not serve as an effective ``back stop,'' because it would allow 
a large number of days with peak PM2.5 concentrations above the 
standard level. For example, in a 365 day data base, the 90th and 95th 
percentiles would equal the 37th and 19th highest 24-hour 
concentrations, respectively. On the other hand, a percentile value 
selected much closer to the tail of the air quality distribution (e.g., 
a 99th or greater percentile) would not likely provide significantly 
more health protection nor significantly increased stability as 
compared to the current form. In balancing these issues, the 
Administrator believes that a 98th percentile value form of a standard, 
set at an appropriate level, would achieve the desired outcomes of both 
a 24-hour standard that would serve as an effective supplement to the 
PM2.5 annual standard and a more stable form. Proposed methods for 
using monitored concentrations to make a comparison with a 
concentration percentile form of a 24-hour standard, averaged over 3 
years, as well as associated calculations and other data handling 
conventions, are presented below in the section on proposed revisions 
to Appendix K.

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

    As discussed in Section E above, 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 U.S. 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 above in Section F.
    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 and in Section A above, 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 can appropriately be 
interpreted as

[[Page 65659]]

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 intermediate 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 has considered three alternative 
approaches to selecting appropriate standard levels, as described 
below.
    (1) One approach would place great weight on 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, and on the limited amount of research 
currently available that has measured PM2.5 directly. This 
approach would recognize PM2.5 as a component of air pollution 
that should be addressed through a NAAQS, since serious health effects 
have been linked to the complex mix of urban air pollution containing 
PM (or some subset of particles within the fine fraction for which 
PM2.5 appears to be a reasonable surrogate). Beyond that 
recognition, however, this approach would reflect the judgment that 
significant new regulatory programs directed toward fine particle 
concentrations well below those permitted under the current PM10 
standards may be premature until additional research has addressed the 
key uncertainties and unanswered questions especially with regard to 
plausible physiological mechanisms for effects at such low exposure 
levels.
    Such an approach would be based on the judgment that the current 
scientific evidence has not demonstrated adverse public health effects 
from fine particle concentrations well below those corresponding to the 
current standard and that it would be difficult to target regulatory 
programs toward the specific pollutants that may be responsible for the 
health effects of concern in the absence of an understanding of the 
mechanism(s) by which these effects are produced. Although there is 
currently significant uncertainty regarding nationwide ambient 
concentrations of PM2.5,29 since little actual monitoring 
data are available, the Administrator believes that such an approach 
could be reflected by setting a standard near the upper end of the 
range recommended in the Staff Paper; i.e., an annual standard level up 
to 20 g/m\3\ in combination with a 24-hour standard of up to 
65 g/m\3\.30
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    \29\ Nationwide PM2.5 estimates have been derived from the 
nationwide PM10 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).
    \30\ In presenting their opinions on the appropriate policy 
choice for PM2.5 standards, several CASAC panel members 
supported levels consistent with this approach. In addition, three 
CASAC members expressed a preference for standards that would be 
equivalent in stringency to the current PM10 standards; with 
the suggestion that standard levels of 25 to 30 g/m3, 
annual average, and 75 g/m3, 24-hour 
average (presumably for the same 1-expected-exceedence form used for 
comparison of options in the Staff Paper), would approximate 
equivalence (Wolff, 1996b). As CASAC recognized, the wide 
variability in PM2.5/PM10 ratios in time and location 
precludes defining uniform PM2.5 standards that would provide 
close to ``equivalent'' protection to the current standard in all or 
even most areas. However, based on estimated PM2.5 data for 
1993-95, the combination of 20 g/m3, annual spatially 
averaged mean, and 65 g/m3, 24-hour, 98th percentile, 
standards is likely to be less stringent than the current standards 
in terms of the numbers of counties predicted not to meet that 
alternative.
---------------------------------------------------------------------------

    A policy decision to set PM2.5 standards at these levels would 
recognize that, while the scientific evidence demonstrating adverse 
effects from fine particles specifically is not conclusive, fine 
particles should nonetheless be regulated separately through PM2.5 
standards, to provide public health protection with an adequate margin 
of safety, as specified in the Act. Such standards would result in the 
establishment of new regulatory programs to reduce potential health 
risks in areas where current levels are high enough to warrant serious 
concern. Such standards would also result in the establishment of a new 
monitoring network to better characterize fine particle levels and 
composition in major population areas throughout the U.S. This would in 
turn facilitate further research into health effects associated with 
ambient PM2.5 levels, which would likely lead to a better 
understanding in the future of the key uncertainties and unanswered 
questions that currently exist, especially with regard to mechanisms 
and the identification of components of urban air pollution, and 
specifically of fine particles, on which to focus future regulatory 
efforts.
    (2) In sharp contrast, a second approach would place great weight 
on the consistency and coherence of the entire body of epidemiological 
evidence, the seriousness of the associated health effects (e.g., 
premature mortality and increased hospital admissions), and the 
magnitude of the incidence of such effects that can be estimated from 
plausible assumptions in an analysis of the quantitative effects 
evidence. While recognizing that uncertainties and unanswered questions 
remain, this approach would suggest policy decisions that would result 
in major new regulatory programs directed at fine particles even as 
additional research is ongoing.
    Such an approach could be viewed as a maximally precautionary 
response, reflecting judgments that the likely effects are as serious 
and potentially adverse to large numbers of sensitive individuals as 
the reported evidence might suggest, and that uncertainties in the 
evidence should be treated, regardless of their nature, as warranting 
greater protection. Such an approach would be predicated on 
interpreting the epidemiological evidence as sufficient to have made a 
compelling case for causality in relationships between PM2.5 and 
health effects at the lower concentrations observed in these studies. 
Based on uncertain estimates of PM2.5 air quality, such an 
approach could be reflected by an annual standard level at the lower 
end of the range recommended in the Staff Paper, i.e., an annual 
standard level down to about 12 g/m3, in combination with 
a 24-hour standard set within the lower part of the range recommended 
in the Staff Paper, from 20 g/m3, at which the 24-hour 
standard might primarily control, up to about 50 g/m3, 
where the annual standard might primarily control.31
---------------------------------------------------------------------------

    \31\  This range of levels for a 24-hour PM2.5 standard is 
consistent with the levels recommended by four CASAC panel members, 
although no members supported an annual PM2.5 standard as low 
as 12 g/m3.
---------------------------------------------------------------------------

    A policy decision to set PM2.5 standards at these levels would 
not only result in a new monitoring network and facilitate additional 
health effects research, but would likely result in major reductions in 
PM2.5 levels throughout the U.S., with associated reductions in 
risks to public health. Commensurate reductions in health risks would 
result only if, in fact, there is a continuum of health risk down to 
the lower end of the ranges of air quality observed in the key 
epidemiological studies, and if the reported associations

[[Page 65660]]

are, in fact, causally related to PM2.5. By setting standards at 
levels where the possibility of effects thresholds are more likely and 
there is greater potential that other elements in the air pollution mix 
(or some subset of particles within the fine fraction) are at least in 
part responsible for or modifying the effects being causally attributed 
to PM2.5, such standards might result in regulatory programs that 
go beyond those that are needed to effectively reduce risks to public 
health. The policy goal of such an approach would be to focus maximal 
regulatory efforts on controlling potential risks to public health, 
with a large margin of safety that takes into account the uncertainties 
and limitations in the available evidence or treating them as 
warranting increased protection in all cases.
    In assessing these two sharply contrasting alternative approaches, 
the Administrator is mindful that the proponents of each, both within 
the scientific community and in the public at large, can advance 
reasoned and potentially persuasive arguments in support of their 
preferred policy approaches. In considering the bases for these two 
contrasting views, however, the Administrator was drawn to consider a 
third approach representing an intermediate policy response, as 
discussed below.
    (3) The third approach would focus 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. Such an approach would recognize 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 such an approach would appropriately 
reflect 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 to V-14 
of the Staff Paper). Key considerations and study results upon which 
this approach is based are presented below.
    As previously discussed, the Administrator is proposing 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, 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 of an annual standard, 
therefore, the Administrator has considered epidemiological studies of 
both short- and long-term exposures to fine particles.
    The effects estimates from the daily 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 year(s) studied. While effects may 
occur over the full range of concentrations observed in the studies, 
the strongest evidence for daily PM2.5 effects is associated with 
annual concentrations at or above the mean levels reported for these 
studies.32 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 annual means from the combined Six City analysis of 
daily mortality and respiratory symptoms (Schwartz et al., 1996a), 
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 21 g/m\3\. In addition, the mean concentrations in 
cities where short-term exposure associations characterized in the 
Criteria Document as nearly statistically significant (U.S. EPA, 1996a, 
p. 13-40) range from about 11 g/m\3\ to 30 g/m\3\. 
Taken together, this evidence suggests that an annual standard level of 
about 15 g/m\3\ may be appropriate to reduce the risk of 
short-term effects of fine particles.
---------------------------------------------------------------------------

    \32\ As discussed in Appendix E of the Staff Paper (U.S. EPA, 
1996b, p. E-4), there is generally the greatest statistical 
confidence in the association at and above the mean concentration.
---------------------------------------------------------------------------

    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 studies provide important 
insights with respect to the overall protection afforded by an annual 
standard. The most direct comparison with the daily fine particle 
mortality studies is provided by two long-term cohort studies (Dockery 
et al., 1993; Pope et al., 1995). The annual mean PM2.5 
concentration for the multiple cities included in both of these studies 
(6 and 47 cities, respectively) was 18 g/m\3\ each study (U.S. 
EPA, 1996b, p. E-10). The Staff Paper assessment of the concentration-
response results from these studies concluded that the evidence for 
increased risk was more apparent at annual concentrations at or above 
15 g/m\3\ (Table E-3 in the Staff Paper). As noted in the 
Staff Paper and the Criteria Document, however, the estimated magnitude 
of effects may be related to somewhat higher historical concentrations 
than the affected communities experienced during the time period of the 
studies; this consideration suggests that a level of 15 g/m\3\ 
would incorporate a margin of safety.
    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 about 15 g/m\3\. 
This level is below the range of annual data most strongly associated 
with both short- and long-term 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 would provide an adequate margin of 
safety. Moreover, the means in areas where PM2.5 concentrations 
were statistically significantly associated with daily mortality (about 
16 to 21 g/m\3\) reflect an 8-year average; thus, the proposed 
use of a 3-year average mean would 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.
    For the reasons specified above, however, an annual, spatially 
averaged standard cannot be expected to offer fully effective and 
efficient protection against all potential short-term effects in areas 
with strong local or seasonal sources. The 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 provided by an annual 
standard and reasonably reflects the peak levels observed in

[[Page 65661]]

communities where health effects have been associated with daily levels 
of fine particles.
    An examination of air quality in cities where short-term exposure 
associations are characterized in the Criteria Document as 
statistically significant or nearly so (U.S. EPA, 1996a, p. 13-40) 
shows that the 98th percentile 24-hour average PM2.5 
concentrations ranged from approximately 35 g/m\3\ to 90 
g/m\3\ (Koman, 1996), with the majority of cities ranging from 
above 40 to above 50 g/m\3\. Based on this examination of 
relevant air quality information, the Administrator believes that a 
98th percentile 24-hour PM2.5 standard of 50 g/m\3\ (at 
the monitoring site within the monitoring planning area with the 
highest 3-year average) would provide an appropriate supplement or 
``backstop'' to a spatially averaged annual mean standard of 15 
g/m\3\.
    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 standard set at 15 g/m\3\, in 
combination with a 24-hour standard set at 50 g/m\3\, would 
protect public health with an adequate margin of safety.
    The Administrator is mindful, however, that in assessing these 
factors a series of judgments had to be made with respect to both the 
interpretation of the underlying scientific evidence and the treatment 
of inherent uncertainties and limitations in the available information 
in making policy choices. Accordingly, the Administrator solicits broad 
public comment, not only on her proposed decision to establish new 
PM2.5 standards of 15 g/m\3\, annual average, and 50 
g/m\3\, 24-hour average, but also on the two alternative 
approaches described above. Based on the comments received and the 
accompanying rationale, the Administrator may choose at the time of 
final promulgation to adopt other standards within the range of these 
alternative approaches in lieu of the standards she is proposing today.

H. Conclusions Regarding the Current PM10 Standards

1. Averaging Time and Form
    In conjunction with the proposed PM2.5 standards, the new 
function of PM10 standard(s) would be to protect against potential 
effects associated with coarse fraction particles in the size range of 
2.5 to 10 m. As noted above, coarse fraction particles are 
plausibly associated with certain effects from both long- and short-
term exposures. 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 is dominated by coarse fraction particles. The potential 
buildup 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 expected annual mean 
form of the standard, which is the same form being proposed for the 
annual PM2.5 standard. As noted in the staff assessment, the 
current annual PM10 standard offers substantial protection against 
both long- and short-term effects of coarse fraction particles.
    The staff and CASAC also recommended that consideration be 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 practical coarse particle 
controls. For the reasons outlined above regarding the form of the 24-
hour PM2.5 standard, the Administrator believes the 98th 
percentile concentration based form would also be an appropriate form 
for a 24-hour PM10 standard.
2. Levels for Alternative Averaging Times

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. 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), now finds the same range of 
levels of concern (40-50 g/m \3\) as was found in the previous 
standard review. The staff finds 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 subpopulations.
    Qualitative evidence of other long-term coarse particle effects, 
most notably from long-term buildup of silica-containing materials, 
supports the need for a long-term standard, but does not provide 
evidence of effects below the range of 40-50 g/m\3\ (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, 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). Taking into account the 
above considerations, as more fully detailed in the Staff Paper and the 
CASAC recommendations, the Administrator proposes to retain the current 
annual PM10 standard of 50 g/m\3\ to protect against the 
long- and short-term effects of coarse fraction particles.

b. 24-Hour PM10 Standard

    As discussed above, EPA staff and CASAC also recommended that 
consideration should 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/m\3\) is no longer appropriate. Instead, the staff found the 
main quantitative basis for a short-term standard is provided by the 
two community studies of exposure to fugitive dust referenced above. 
Because these studies reported multiple large

[[Page 65662]]

exceedences of the current 24-hour standard, and because of limitations 
in the studies themselves, they provide no basis to lower the level of 
the standard below 150 g/m\3\. Moreover, none of the 
qualitative literature regarding the potential short-term effects of 
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/m\3\, although with a revised form.
    In the judgment of the Administrator, retention of a 24-hour 
PM10 standard at the level of 150 g/m\3\ with a 98th 
percentile form would provide adequate protection against the short-
term effects of coarse particles that have been identified to date in 
the scientific literature. However, analyses of the available air 
quality relationships show that such a standard might not add greatly 
to the protection afforded by the current PM10 annual standard 
(Fitz-Simons et al., 1996). As noted in the Staff Paper and by some 
CASAC panel members, it is possible that the current annual standard 
might provide adequate protection against both long- and short-term 
effects of coarse particles, especially when viewed in conjunction with 
the overall proposal to add new annual and 24-hour PM2.5 
standards. Therefore, the Administrator also solicits comment on the 
alternative of retaining the current annual PM10 standard and 
revoking the current 24-hour PM10 standard.

I. Proposed Decisions on Primary Standards

    For the reasons discussed above, 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 to date, the Administrator proposes to amend the current suite 
of PM10 standards by adding new PM2.5 standards and by 
revising the form of the current 24-hour PM10 standard. 
Specifically, the Administrator proposes to add two new primary 
PM2.5 standards set at 15 g/m\3\, annual mean, and 50 
g/m\3\, 24-hour average. The proposed new annual PM2.5 
standard would be met when the 3-year average of the annual arithmetic 
mean PM2.5 concentrations, spatially averaged across an area, is 
less than or equal to 15 g/m\3\, with fractional parts of 0.05 
or greater rounding up. The Administrator solicits comment on the 
alternative of using the 3-year average of the annual arithmetic mean 
PM2.5 concentrations at each monitor within an area rather than a 
spatially averaged value. The proposed new 24-hour PM2.5 standard 
would be met when the 3-year average of the 98th percentile of 24-hour 
PM2.5 concentrations at each monitor within an area is less than 
or equal to 50 g/m\3\, with fractional parts of 0.5 or greater 
rounding up. Data handling conventions are specified in proposed 
revisions to Appendix K, as discussed in Section IV below, and a 
reference method for monitoring PM as PM2.5 is specified in a 
proposed new Appendix L, as discussed in Section V below.
    In recognition of alternative views as to the appropriate policy 
response, the Administrator also solicits comments on two alternative 
sets of new annual and 24-hour PM2.5 standards: (1) An annual 
standard set at a level up to 20 g/m\3\, in combination with a 
24-hour standard set at a level up to 65 g/m\3\; and (2) an 
annual standard set at a level as low as 12 g/m\3\, in 
combination with a 24-hour standard set at a level within the range of 
20 to 50 g/m\3\.
    The Administrator also proposes to retain the current annual 
PM10 standard at the level of 50 g/m\3\, which would be 
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/m\3\, with fractional parts of 0.5 or greater rounding 
up. Further, the Administrator proposes to retain the current 24-hour 
PM10 standard at the level of 150 g/m\3\, but to revise 
the form such that the standard would be met when the 3-year average of 
the 98th percentile of the monitored concentrations at the highest 
monitor in an area is less than or equal to 150 g/m\3\, 
rounding to the nearest 10 g/m\3\. Data handling conventions 
are specified in proposed revisions to Appendix K, as discussed in 
Section IV below, and revisions to the reference method for monitoring 
PM as PM10 (Appendix J) are proposed as discussed in Section V 
below. The Administrator also solicits comment on the alternative of 
revoking the current 24-hour PM10 standard.

III. Rationale for Proposed Decision on 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 the proposed 
secondary standards focuses on those considerations most influential in 
the Administrator's proposed decision.

A. Visibility Impairment

    This section of the notice presents the Administrator's proposed 
decision to address the effects of PM on visibility by setting 
secondary standards identical to the suite of proposed primary 
standards, in conjunction with the establishment of a regional haze 
program under section 169A of the Act.33 In the Administrator's 
judgment, this approach is the most effective way to address visibility 
impairment given the sharp regional variations in concentrations of 
non-anthropogenic PM as well as other factors (e.g., humidity) that 
affect visibility. By augmenting the protection provided by secondary 
standards set identical to the proposed suite of 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.
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    \33\ Congress adopted section 169A of the Act because of concern 
that the NAAQS and Prevention of Significant Deterioration programs 
may not provide adequate visibility protection nationally, 
particularly for ``areas of great scenic importance.'' See H.R. Rep. 
No. 294, 95th Congress, 1st Session, 203-205 (1977).
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    In coming to this proposed decision, the Administrator took into 
account several factors, including: (1) Staff assessments of the most 
policy-relevant information in the Criteria Document and Staff Paper; 
(2) the degree of visibility improvement expected through attainment of 
the recommended primary standards; (3) the regional variation of 
naturally occurring levels of PM and visual range; (4) difficulties 
inherent in attempting to address visibility impairment by setting 
national secondary standards; and (5) EPA's authority to develop a 
national regional haze program under section 169A of the Act that can 
allow for regionally-specific approaches to protecting visibility. The 
Administrator's consideration of each of these factors is discussed 
below.
    The Administrator first concluded, based on information presented 
in the Criteria Document and Staff Paper, that impairment of visibility 
is an important effect of PM on public welfare, and that it is 
experienced throughout the U.S., in multi-state regions, urban areas, 
and remote class I Federal areas 34 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

[[Page 65663]]

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.
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    \34\ 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 6000 acres, wilderness areas and memorial parks 
exceeding 5000 acres, and all international parks which were in 
existence on August 7, 1977.
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    Visibility conditions are determined by the scattering and 
absorption of light by particles and gases, from both natural and 
anthropogenic sources. Visibility is often described in terms of visual 
range, light extinction, or deciviews.35 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.
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    \35\ 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 also considered the information in the Criteria 
Document and Staff Paper describing estimated background levels of PM 
and natural light extinction. In the United States, estimated annual 
average background levels of PM2.5 are lower in the West than in 
the East. Because visibility in a pristine environment is very 
sensitive to an additional 1 or 2 g/m3 of PM2.5 in 
the atmosphere, estimated light extinction due to natural background 
levels of PM2.5 varies fairly significantly between the East and 
the West. Based on estimated background 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. 
Increased light scattering of certain particles due to higher average 
relative humidity in the East is an important factor leading to this 
regional difference.
    The Administrator also assessed potential visibility improvements 
36 on urban and regional scales that would result from attainment 
of the proposed primary standards for PM2.5 are attained. In many 
cities having annual average PM2.5 concentrations exceeding 17 
g/m3, improvements in annual average visibility resulting 
from attainment of the proposed primary standards are expected to be 
perceptible (i.e., to exceed 1 deciview). Based on annual average 
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.
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    \36\ Estimates of annual average visibility improvements assume 
(1) that the % reduction for each fine particle constituent is equal 
to the % reduction in the mass of fine particles, and (2) the 
overall light extinction efficiency of the fine particle pollutant 
mix does not change. (Damberg and Polkowsky, 1996)
---------------------------------------------------------------------------

    In Washington, D.C., for example, where the IMPROVE network 37 
shows average PM2.5 levels 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 average 
PM2.5 levels at approximately 30 g/m3, visibility 
would be expected to improve from about 19 to 34 km (30 to 24 deciview) 
if the proposed annual standard is attained.
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    \37\ 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 would be expected to 
have annual average PM2.5 concentrations reduced below the 
proposed 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 when the proposed 24-hour PM2.5 standard of 50 
g/m3 is attained. In some 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, 
and at over 70 g/m3 in Philadelphia. If the level and 
frequency of peak PM concentrations are reduced, improvements would be 
expected in those days where visibility is worst. Some of these 
improvements in peak concentrations may even be experienced in urban 
areas having annual averages below the annual standard.
    Having concluded that attainment of the proposed annual and 24-hour 
PM2.5 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 proposed PM2.5 standards could result in 
regional visibility improvement (e.g., in certain mandatory Federal 
Class I areas such as Shenandoah and Great Smoky Mountains National 
Parks) 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. It is important to recognize that 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 recommended 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 secondary standards set at the level of the proposed primary 
standards for PM2.5 would be expected to result in visibility 
improvements in the eastern U.S. at both urban and regional scales, but 
little or no change in the western U.S. except in and near selected 
urban areas.
    The Administrator also considered whether establishment of a more 
stringent national secondary standard or standards would be effective 
and efficient in providing increased visibility protection in the 
western U.S. Table VIII-4 of the Staff Paper indicates

[[Page 65664]]

that the current level of annual average light extinction (resulting 
from both anthropogenic and background sources of PM) in several 
western locations, such as the Colorado Plateau, is about equal to the 
level of background light extinction (i.e., the level representing 
nonanthropogenic sources only) in the East. This regional difference is 
due to higher background particle concentrations in the East, the 
greater light scattering associated with higher humidity levels in the 
East, 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 national secondary standards intended to maintain or 
improve visibility conditions on the Colorado Plateau would have to be 
set at or even below natural background levels in the East, the 
attainment of which would effectively require elimination of all 
eastern anthropogenic emissions. Conversely, national secondary 
standards 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 and the effect of humidity on 
these background levels, the Administrator concludes that proposing 
more stringent national secondary standards would not be an effective 
or appropriate means to protect the public welfare from adverse impacts 
of PM on visibility in all parts of the country.
    The Administrator then considered the potential effectiveness of a 
regional haze program in addressing regional differences in visibility 
impairment and thereby supplementing the protection that would be 
achieved by setting the secondary standards identical to the suite of 
proposed primary standards. A program to address this 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, manmade impairment of visibility in 
mandatory Class I areas.'' EPA is required by section 169A(b)(2) of the 
Act to ensure that ``reasonable progress'' is achieved toward meeting 
the national goal. The structure and requirements of sections 169A and 
169B, to be implemented by the States, make it clear that visibility 
protection programs can be specific to each affected region, in 
contrast with the national applicability of a secondary NAAQS. The EPA 
is currently engaged in efforts to develop a regional haze program, and 
will have the benefit of the June 1996 recommendations from the Grand 
Canyon Visibility Transport Commission as well as recommendations from 
the Federal Advisory Committee Act (FACA) Subcommittee on Ozone, 
Particulate Matter, and Regional Haze Implementation Programs which are 
expected by the end of the year.
    An important factor considered in this review is whether a regional 
haze program, in conjunction with secondary standards set identical to 
the suite of proposed primary standards for PM, would provide 
appropriate protection for visibility in non-Class I areas. Based on 
the following recommendation from the 1993 report of the National 
Research Council, Protecting Visibility in National Parks and 
Wilderness Areas, the Administrator believes such protection would be 
provided:

    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.

    The Administrator recognizes, however, that people living in 
certain urban areas may place a high value on unique scenic resources 
in or near these areas, yet could have visibility problems attributable 
to local sources that would not necessarily be addressed by the 
combined effects of a regional haze program and secondary standards 
identical to the proposed suite of primary standards for PM. This may 
be particularly true of certain cities located near scenic vistas in 
the West. In the Administrator's judgment, State or local regulatory 
approaches, such as recent action by 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 Class I 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 Class I area.
    Based on the above considerations, the Administrator proposes to 
set secondary standards identical to the proposed suite of 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 allow all regions of the country to make 
reasonable progress toward the national visibility goal.

B. Materials Damage and Soiling Effects

    Annual and 24-hour secondary standards for PM10 effects on 
materials damage and soiling 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.
    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 
secondary standard based on soiling or materials damage alone. In the 
Administrator's judgment, however, setting secondary standards 
identical to the suite of proposed PM2.5 and PM10 primary 
standards, as discussed above, would provide increased protection 
against the effects of fine particles and retain an appropriate degree 
of control on coarse particles. Accordingly, the Administrator proposes 
to set the secondary standards identical to the suite of proposed 
primary standards to protect against materials damage and soiling 
effects of PM.

C. Proposed Decision on the Secondary Standards

    The Administrator proposes to set secondary standards identical to 
the suite of proposed primary standards, in conjunction with 
establishment of a regional haze program. In her judgment, such an 
approach would provide appropriate protection against the welfare 
effects associated with particle pollution.

[[Page 65665]]

    If at the time of final promulgation the most stringent approach to 
setting the PM2.5 primary standards were to be adopted, the 
Administrator would propose to set the secondary standards identical to 
the final suite of primary standards. However, even if the levels of 
the PM2.5 standards were to be set as low as 12 g/m3 
and 25 g/m3, respectively, for the annual and 24-hour 
PM2.5 standards, the Administrator would still foresee the need 
for a regional haze program to supplement the visibility protection 
afforded by such standards. If, on the other hand, the levels of the 
PM2.5 primary standards were to be set at up to 20 g/
m3, annual average, and up to 65 g/m3, 24-hour 
average, the Administrator would find it necessary to re-examine 
whether a separate lower secondary standard would have to be 
established to protect against the welfare effects associated with 
particle pollution. Based on the above discussion, the Administrator 
would consider setting separate secondary standards for PM2.5 at 
15 g/m3, annual average, and 50 g/m3, 24-
hour average, with PM10 standards set identical to the final 
primary PM10 standards. In her judgment, such a suite of secondary 
standards, in conjunction with the establishment of a regional haze 
program, would appropriately protect public welfare from the effects of 
particle pollution.

IV. Revisions to Appendix K--Interpretation of the PM NAAQS

    The EPA is proposing to revise Appendix K to 40 CFR part 50 to 
reflect the proposed forms for the annual and 24-hour standards for 
PM2.5 and PM10. The proposed revisions to Appendix K explain 
the computations necessary for determining when the proposed primary 
and secondary standards are met. More specifically, the proposed 
revisions address data reporting, handling, and rounding conventions, 
with example calculations. The proposed revisions do not address the 
treatment of exceptional events data. Policies for addressing 
exceptional and natural events are part of the standards implementation 
process.
    Key elements of the proposed revisions to Appendix K are outlined 
below.

A. PM2.5 Computations and Data Handling Conventions

    As discussed in section II.F above, EPA is proposing a spatially 
averaged annual mean as the form of the annual PM2.5 and a 98th 
percentile concentration form of the 24-hour PM2.5 standard. The 
proposed Appendix K explains the data handling conventions and 
computations for the annual and 24-hour forms of the PM2.5 
standards in sections 2.1 and 2.2, respectively; data rounding 
conventions in section 2.3; monitoring considerations in section 2.4; 
and formulas for calculating the annual and 24-hour forms in sections 
2.5 and 2.6, respectively.
    With regard to the annual PM2.5 standard, EPA is proposing to 
spatially average the annual mean values in areas designated to 
represent population exposures. The spatial average is to be carried 
out using data from monitoring sites designated in a State monitoring 
plan in accordance with the proposed revisions to 40 CFR Part 58. Also, 
EPA is proposing that 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 also proposes that intermediate averaging over calendar quarters be 
retained for the annual average form of the standard. Quarterly 
averages may be important to ensure representative sampling in areas 
with extreme seasonal variation; however, this extra calculation has 
little effect on the calculated 3-year average value (SAI, 1996, pp. 6-
9). Thus, EPA solicits comments on whether or not the calculation of 
quarterly means as an intermediate step in deriving the annual mean 
should be retained.
    With regard to the 24-hour PM2.5 standard, the proposed 
Appendix K defines 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.
    State and local agencies are expected to report daily PM2.5 
concentrations to the nearest 0.1 g/m3 for concentrations 
less than 100 g/m3 and to the nearest 1 g/
m3 for higher values. The incremental sensitivity of proposed 
PM2.5 monitors is better than that for PM10, and PM2.5 
measurements can be reported to 3 significant digits.
    In addition to instrument sensitivity, the number of measured 
values used to calculate an averaged value affects the precision of the 
value to be compared with the level of the standard. In calculating a 
3-year average of annual means, many values (typically 144 values to as 
many as 1095 values) are used to calculate the annual mean, whereas 
only 3 values are averaged to calculate the 24-hour standard. As a 
result, the annual and 24-hour standards are expressed with different 
degrees of precision and, thus, different rounding conventions are 
appropriate. Specifically, when calculating a 3-year average of annual 
mean values, the second decimal place shall be rounded (0.05 to be 
rounded up) to fall within the 15% precision goal for the 
PM2.5 measurements. When calculating the 3-year average of the 
98th percentile values, only two significant digits are retained at 
levels near the standard, with the non-significant first decimal place 
rounded (0.5 g/m3 to be rounded up to the next highest 1 
g/m3).
    To determine whether the proposed standards are met, the calculated 
value of the 3-year average of the annual means and the 3-year average 
of the 98th percentile values would be compared to the level of the 
relevant standard. The proposed annual standard of 15.0 g/
m3 is expressed to the nearest 0.1 g/m3, while the 
24-hour standard of 50 g/m3 is expressed to the nearest 1 
g/m3, reflective of the quantitative uncertainties in the 
health effects evidence upon which these standards are based. More 
specifically, these uncertainties include the measurement uncertainty 
inherent in the ambient PM2.5 concentrations used in 
epidemiological studies upon which consideration of the levels of the 
standards have been based. Because the measurement precision is 
expressed as a percentage of the measured value (15%), the 
magnitude of the target concentration affects the appropriate number of 
significant digits for the purpose of comparison to the standard. The 
EPA believes that expressing the proposed annual standard to the 
nearest 0.1 g/m3 and the 24-hour standard to the nearest 
1 g/m3 is consistent with the quality assurance goal for 
PM2.5 measurements, as stated in the proposed Appendix A of 40 CFR 
Part 58, to be within 15%.

B. PM10 Computations and Data Handling Conventions

    As discussed in section II.H above, the EPA is proposing to retain 
the annual mean as the form of the annual PM10 standard, and to 
revise the form of the 24-hour PM10 standard to a 98th percentile 
form. The 98th percentile for the 24-hour PM10 standard would be 
calculated in the same manner as described in section A above for the 
PM2.5 standard. The proposed Appendix K explains the data handling 
conventions and computations for the annual and 24-hour forms of the 
PM10 standards in sections 3.1 and 3.2, respectively; rounding 
conventions in section 3.3; monitoring considerations in section 3.4; 
and formulas for calculating the annual and 24-hour forms in sections 
3.5 and 3.6, respectively.
    State and local agencies report daily PM10 concentrations to 
the nearest 1 g/

[[Page 65666]]

m3 since the typical incremental sensitivity of currently 
PM10 monitors is 1 g/m3. As with the PM2.5 
standards, the number of measured values used to calculate an averaged 
value affects the precision of the value to be compared with the level 
of the standard. As a result, the annual and 24-hour standards are 
expressed with different degrees of precision and different rounding 
conventions. Specifically, when calculating the annual mean 
concentration (i.e., typically with 144 values or greater), the non-
significant first decimal place shall be rounded (with 0.5 rounded up) 
to preserve the number of significant digits in the reported data. When 
calculating the 3-year average of the annual 98th percentile values 
(i.e., 3 values are averaged), only two significant digits are retained 
at levels near the standard, with the non-significant units digit 
rounded (5 g/m3 to be rounded up to the next highest 10 
g/m3).
    To determine whether the proposed standards are met, the calculated 
value of the 3-year average of the annual means and the 3-year average 
of the annual 98th percentile values would be compared to the levels of 
the respective standards. The proposed annual standard of 50 
g/m3 is expressed to the nearest 1 g/m3, 
while the 24-hour standard of 150 g/m3 is expressed to 
the nearest 10 g/m3, reflective of the quantitative 
uncertainties in the health effects evidence upon which these standards 
are based. More specifically, these uncertainties include the 
measurement uncertainty inherent in the ambient PM10 
concentrations used in epidemiological studies upon which consideration 
of the levels of the standards have been based. Because the measurement 
precision is expressed as a percentage of the measured values 
(15%), the magnitude of the target concentration affects 
the number of significant digits for the purpose of comparison to the 
standard. The EPA believes that expressing the proposed annual standard 
to the nearest 1 g/m3 and the 24-hour standard to the 
nearest 10 g/m3 is consistent with the quality assurance 
guidelines that indicate that the precision for PM10 measurements 
shall be within 15%.

V. Reference Methods for the Determination of Particulate Matter as 
PM2.5 and PM10 in the Atmosphere

A. Revisions to Appendix J--Reference Method for PM10

    During the course of this 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 Appendix J to Part 50. 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 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 concludes that a continuation of the 
practice of adjusting PM10 concentrations to standard conditions 
of temperature and pressure is not warranted or appropriate. 
Accordingly, EPA proposes to delete this requirement from Appendix J 
and to make corresponding revisions in 40 CFR Part 50.3. In addition, 
EPA proposes to make minor modifications to update Appendix J.

B. Appendix L--New Reference Method for PM2.5

    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 
proposed method is described in a new Appendix L to part 50, and would 
join the other reference methods (or measurement principles) specified 
for other criteria pollutants in other appendices to part 50.
    In developing a 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 
FRM 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 FRM approach in a letter (Price, 1996) forwarded 
by CASAC to the Administrator.
1. Approach
    In addition to the primary purpose of the new PM2.5 reference 
method (determining attainment of the standards), the EPA considered a 
variety of possible secondary goals and objectives that this 
measurement method might also fulfill. Subsequently, various 
alternative PM2.5 measurement techniques were evaluated. From this 
analysis, the EPA determined that the new reference method should be 
based on a conventional type ambient air sampler that collects 24-hour 
integrated PM2.5 samples on a filter that is subsequently moisture 
and temperature equilibrated and analyzed gravimetrically.
    This type of sampler is relatively inexpensive and easy to use by 
monitoring agency personnel, operates over a wide range of ambient 
conditions, produces a measurement that is comparable to large sets of 
previously collected PM data in existing data bases, and provides a 
physical sample that can be further analyzed for chemical composition. 
The proposed PM2.5 sampler is a low volume sampler operating at 1 
cubic meter per hour, for a total sample volume of 24 m3 for the 
specified 24-hour sample collection period. The sample is collected on 
a 47 mm Teflon filter.

[[Page 65667]]

2. PM Concentrations Based on Actual Air Volume
    In accordance with the proposed change to the PM10 reference 
method in Appendix J, ambient concentrations measured with the new 
reference method would be expressed as micrograms of PM mass per actual 
cubic meter of air sampled (g/m3), rather than mass per 
cubic meter of air adjusted to standard temperature and pressure (25 
deg.C and 760 mm Hg, respectively). This convention would provide PM 
concentration measurements that are more representative of the actual 
mass of PM2.5 present in conditions of cold temperatures and for 
monitoring sites at high altitude.
3. Sampler
    Although the sampler is conventional in configuration, its design 
is more sophisticated than previous samplers used for collection of PM 
samples. 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 filter 
holder are proposed to be specified by design, in the form of 
manufacturing drawings. Performance specifications for these components 
would be quite extensive, and the performance tests that would be 
required are difficult and require very costly test facilities. All 
other aspects of the sampler would be described by performance-based 
specifications. Sample air flow rate would have to be carefully 
controlled and accurately measured. Ambient temperature and barometric 
pressure sensors would be required for accurate measurement of actual 
volumetric sample flow rate and to provide archival documentation of 
these conditions associated with the PM2.5 measurements. Loss of 
semi-volatile components of PM2.5 would be reduced by temperature 
control of the sample filter. The allowable rise of the temperature of 
the filter above ambient temperature is proposed to be limited to 3 
degrees C above ambient temperature during sampling as well as after 
sample collection while the sample is retained in the sampler awaiting 
retrieval.
    The sampler would be required to have a variety of other timing, 
control, and diagnostic functions and to report any abnormal 
operational conditions to the sampler operator. Flow rate, sample 
volume, sample time, and other sample, site, and diagnostic information 
would also be downloadable to a portable data retrieval device through 
an electronic port connection for fast and accurate documentation of 
the sample parameters and site conditions. A built-in sampler leak-
check capability would allow frequent checking of this potentially 
important source of measurement error. Filters would be mounted in 
filter cassettes to facilitate protected installation and retrieval 
from the sampler, and sampler manufacturers would be free to develop 
innovative filter holder opening/closing mechanisms to make filter 
changing fast and reliable.

VI. Implementation Program

    Recognizing that potential adoption of new or revised NAAQS for PM 
and O3, as well as potential new regulations for regional haze, 
could have profound implications for existing State implementation 
programs, EPA established a subcommittee under the Clean Air Act 
Advisory Committee (CAAAC) in 1995 to consider how such actions might 
be implemented. The Subcommittee, comprised of some 58 members 
representing environmental organizations, State and local air pollution 
control agencies, Federal agencies, academia, industry, and other 
public interests, was asked to provide advice and recommendations to 
EPA on developing new, integrated approaches for implementing potential 
new NAAQS for PM and O3, as well as a potential new regional haze 
reduction program. The Subcommittee, through several work groups made 
up of Subcommittee members and other designees recommended by the 
Subcommittee, is examining key aspects of the existing implementation 
programs for PM and O3, to provide for more effective 
implementation of the potential new NAAQS, as well as to provide new 
approaches to better integrate broad regional and national control 
strategies with more localized efforts.
    Upon completion of its work, the Subcommittee will present its 
findings and recommendations to the CAAAC. These recommendations will 
then assist EPA's development of appropriate policies and regulations 
for implementing the potential new PM and O3 NAAQS and regional 
haze regulations in the most efficient and environmentally effective 
manner. These policies and regulations will then be published in the 
Federal Register for further input from the public.
    As discussed in the advance notice of proposed rulemaking, EPA also 
intends to release an interim implementation policy that would take 
effect at the time the new or revised NAAQS for PM and O3 are 
promulgated. The interim implementation policy is intended to provide 
for an effective transition from the existing implementation 
requirements and control strategies for PM and O3 to new ones that 
are under development. Among other things, the policy will address such 
issues as the continuation of existing control requirements during the 
transition period, continued classification of areas, substitution of 
progress requirements, as well as the timing of the applicability of 
certain provisions of new source review requirements.

VII. Regulatory and Environmental Impact Analyses

    The EPA has judged this proposal to be a significant action, and 
has prepared a draft Regulatory Impact Analysis (RIA) for it as 
discussed below. Neither the draft RIA nor the associated contractor 
reports have been considered in issuing this proposal. Judicial 
decisions make clear that the economic and technological feasibility of 
attaining ambient standards are not to be considered in setting them, 
although such factors may be considered to a degree in the development 
of State plans to implement the standards.
    As discussed above, EPA has established a Subcommittee of the CAAAC 
to examine the existing implementation programs for PM and O3, and 
provide advice and recommendations to assist EPA in developing new, 
integrated approaches for implementing potential new or revised NAAQS 
for PM and O3, as well as a potential new regional haze reduction 
program. Because the work of the Subcommittee is still in progress, the 
draft RIA and associated regulatory flexibility assessment that 
accompany this notice do not reflect its advice and recommendations or 
any resulting implementation strategies for PM. The EPA anticipates 
that such strategies will be more efficient and environmentally 
effective than the ones analyzed. While the draft RIA and flexibility 
assessment should be useful in generally informing the public about 
potential costs and benefits associated with implementation of the 
proposed revisions, they do not reflect any new implementation 
requirements or policies that may be proposed after consideration of 
the Subcommittee's advice and recommendations. As EPA develops and 
elaborates such requirements or policies, it will continue to consult 
with the Subcommittee and will prepare further regulatory analyses as 
appropriate.

[[Page 65668]]

A. Executive Order 12866

    Under Executive Order 12866, 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 
one 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 or recipients 
thereof; or
    (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 proposal has 
been judged to be a ``significant regulatory action'' within the 
meaning of the Executive Order, and EPA has submitted it to OMB for 
review. 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-95-54).
    The EPA has prepared and entered into the docket a draft RIA 
entitled ``Regulatory Impact Analysis for Proposed Particulate Matter 
National Ambient Air Quality Standard (November 1996).'' This draft RIA 
assesses the costs, economic impacts, and benefits associated with the 
implementation of the current and several alternative NAAQS for PM as 
discussed above. As discussed in the draft RIA, there are an unusually 
large number of limitations and uncertainties associated with the 
analyses and resulting cost impacts and benefit estimates. Below are 
the estimated costs and benefits associated with partial attainment of 
the alternative levels in 2007. Because judicial decisions make clear 
that cost can not be considered in setting NAAQS, the results of the 
draft RIA have not been considered in developing this proposal.

 Comparison of Annual Benefits and Costs of PM2.5 Alternatives in 2007 a
                            (Billions 1990$)                            
------------------------------------------------------------------------
                             Monetized annual                           
   PM2.5 alternative       benefits of partial        Annual costs of   
   (g/m\3\)          attainmentb c         partial attainment  
------------------------------------------------------------------------
*20/65.................              22-44                        2     
15/50d.................             58-119                        6     
12.5/50................             94-192                      14      
------------------------------------------------------------------------
* Does not include the reductions in costs and benefits associated with 
  revised PM10 studies. This alternative requires less reductions than  
  current PM10 standards.                                               
a All estimates are measured incremental to the baseline PM10           
  alternative (PM10 g/m\3\ annual/150 g/m\3\ daily, 1 
  expected exceedance per year).                                        
b Lower and upper end of benefit range reflects benefits of including   
  the short-term and long-term mortality risk reduction measure,        
  respectively.                                                         
c Partial attainment benefits based upon post-control air quality as    
  defined in the control cost analysis.                                 
d Proposed PM2.5 alternative.                                           

    As discussed in the RIA itself, there are a large number of 
limitations and uncertainties inherent in estimating these national 
costs and benefits over extended periods of time. Results are limited 
by the inability to monetize certain health or welfare benefits for 
comparison with projections of control costs that are usually more 
complete, but are sometimes overstated due to an inability to forecast 
advances in pollution prevention and control. The approaches used for 
the RIA did not attempt to take advantage of flexibilities and savings 
possible in consideration of combined air quality management programs 
for PM and O3. Further, they were limited by availability of 
emissions, air quality monitoring, and related information. Indeed, the 
suite of control measures available to be considered in the cost 
analysis was not sufficient to achieve full attainment in 2007. It is 
for this reason we have only presented the costs and benefits for this 
``partial attainment'' scenario. In the partial attainment scenario, 
there would be 57 residual nonattainment counties representing 29 
million people in 2007 for the proposed level. One implication of this 
scenario is that more time will be needed to attain the standards in 
the areas remaining in nonattainment. Moreover, based on past 
experience, improvements in technologies and creative implementation 
programs are likely to result in more effective programs than can now 
be forecasted. The EPA is planning to improve and expand its analysis 
of the integrated costs and benefits of attaining both the PM and ozone 
standards in association with developing implementation guidance.

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 proposed rule, the agency must prepare 
regulatory flexibility analyses for the proposed and final rule unless 
the head of the agency certifies that it will not have a significant 
economic impact on a substantial number of small entities. In judging 
what kinds of economic impacts are relevant for this determination, it 
is appropriate to consider the purposes and requirements of the RFA. 
Mid-Tex Electrical Co-op v. FERC, 773 F.2d 327, 341-42 (D.C. Cir. 
1985).
    Review of the findings and purposes section of the RFA makes clear 
that Congress enacted the RFA to address the economic impact of rules 
on small entities subject to the rule's requirements. Pub. L. 96-354, 
section 2 (1980); see also 126 Cong. Rec. 21,452, 21,453 (1980). In 
explaining the need for the RFA, Congress generally expressed concern 
about the problematic consequences of applying regulations uniformly to 
large and small entities. Specifically, Congress stated that ``laws and 
regulations designed for application to large scale entities have been 
applied uniformly to small [entities] even though the problems that 
gave rise to government action may not have been caused by those small 
entities,'' that ``uniform Federal regulatory and reporting 
requirements have in numerous instances imposed unnecessary and 
disproportionately burdensome demands . . . upon small [entities] with 
limited resources,'' that ``the failure to recognize differences in the 
scale and resources of regulated entities has in numerous instances 
adversely affected competition in the marketplace,'' and that ``the 
practice of treating all regulated [entities] as equivalent may lead to 
inefficient use of regulatory agency resources.'' Id. To address these 
concerns, Congress enacted the RFA ``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 [entity] subject to 
regulation'' (emphasis added). Id.
    The statutory requirements for regulatory flexibility analyses 
confirm that the economic impact to be analyzed is the impact of the 
rule on small entities that will have to comply with the rule's 
requirements. In both initial

[[Page 65669]]

and final regulatory flexibility analyses, for example, the agency 
issuing the rule is required to describe and (where feasible) estimate 
the number of small entities ``to which the proposed rule will apply''; 
describe the reporting, recordkeeping and other ``compliance 
requirements'' of the proposed rule; and estimate the classes of small 
entities that ``will be subject to the requirement.'' See RFA sections 
603 and 604. The agency must also discuss and address significant 
regulatory alternatives that are consistent with the applicable 
statutes and would minimize any significant economic impact on small 
entities. Among the possible alternatives listed by the RFA are the 
establishment of differing compliance and reporting requirements that 
take into account the resources available to small entities and partial 
or total exemptions from the rule for small entities. See RFA section 
603(c). The RFA's requirements for regulatory flexibility analyses thus 
establish that the focus of such analyses are the regulatory 
requirements small entities will be required to meet as a result of the 
rule and ways to tailor those requirements to reduce the burden on 
small entities. Mid-Tex Electrical Co-op, 773 F.2d at 342 (``[I]t is 
clear that Congress envisioned that the relevant `economic impact' was 
the impact of compliance with the proposed rule on regulated small 
entities'').
    The scope of regulatory flexibility analyses in turn informs the 
scope of the analysis necessary to support a certification that a rule 
will not have ``a significant economic impact on a substantial number 
of small entities.'' Thus, ``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. (emphasis added); see also United Distribution Companies v. 
FERC, 88 F.3d 1105, 1170 (D.C. Cir. 1996).
    In view of the RFA's purposes and the requirements it establishes 
for regulatory flexibility analyses, EPA believes that today's proposal 
to revise the PM NAAQS will not have a significant economic impact on 
small entities within the meaning of the RFA. The proposed rule, if 
promulgated, will not establish requirements applicable to small 
entities. Instead, it will establish a standard of air quality that 
other Clean Air Act provisions will call on states (or in case of state 
default, the federal government) to achieve by adopting implementation 
plans containing specific control measures for that purpose. In other 
words, state (or federal) regulations implementing the NAAQS might 
establish requirements applicable to small entities, but the NAAQS 
itself would not.38
---------------------------------------------------------------------------

    \38\ Because the proposed rule would not establish requirements 
applicable to small entities, EPA cannot in fact perform the 
analyses contemplated by the RFA.
---------------------------------------------------------------------------

    For these reasons, the Administrator certifies that this proposed 
rule will not have a significant economic impact on a substantial 
number of small entities.
    While the statutory requirements for regulatory flexibility 
analyses are thus inapplicable to NAAQS standard-setting, EPA is 
nonetheless interested in assessing to the extent possible the 
potential impact on small entities of implementing a revised PM NAAQS. 
EPA has accordingly conducted a more general analysis of the potential 
cost impacts on small entities of control measures that states might 
adopt to attain and maintain a revised NAAQS, and has included that 
analysis in the RIA cited above.
    That analysis examines industry-wide cost and economic impacts for 
those sectors likely to be affected when the proposed revisions to the 
PM NAAQS are implemented by States. As part of the draft RIA, the EPA 
has analyzed various industries for the existence of small entities to 
ascertain whether small entities within a given industry category are 
likely to be differentially affected when compared to the industry 
category as a whole. This information will serve to inform potentially 
affected small entities, thus enabling them to participate more 
effectively in EPA's review and potential revision of existing 
implementation requirements and policies and in development of any 
necessary State implementation plan revisions. As indicated previously, 
EPA will prepare further analyses as appropriate as it develops new 
implementation requirements or policies.
    EPA's finding that today's proposal will not have a significant 
economic impact on small entities also entails that the new small-
entity provisions in Section 244 of the Small Business Regulatory 
Enforcement Fairness Act (SBREFA) do not apply. Nevertheless, EPA 
intends to fulfill the spirit of SBREFA on a voluntary basis. To 
accomplish this, following the proposal of new air quality standards 
for O3 and PM, EPA intends to work with the Small Business 
Administration (SBA) to hold two separate panel exercises to collect 
comments, advice and recommendations from representatives of small 
businesses, small governments, and other small organizations. The first 
panel, soliciting comments on the new standards themselves, will be 
held shortly after proposal. The second panel, covering implementation 
of the standards, will be held a few months later. Both panel exercises 
will be carried out using a panel process modeled on the ``Small 
Business Advocacy Review Panel'' provisions in Section 244 of SBREFA. 
We are also adding a number of small-entity representatives to our 
Federal advisory committee focusing on NAAQS implementation; we expect 
the small-entity advice from this committee will help the 
aforementioned implementation panel accomplish its purpose.

C. Impact on Reporting Requirements

    There are no reporting requirements directly associated with an 
ambient air quality standard proposed 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 proposed 
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 their regulatory actions on State, local, and tribal 
governments and the private sector. Under section 202 of the 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 to State, local, and tribal governments, in 
the aggregate, or to the private sector, of $100 million or more in any 
one year. This requirement does not apply if EPA is prohibited by law 
from considering section 202 estimates and analyses in adopting the 
rule in question. Before promulgating an EPA rule for which a written 
statement is needed, section 205 of the 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 allows EPA to adopt an alternative other 
than the least costly, most cost-effective,

[[Page 65670]]

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 the 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.
    As indicated previously, EPA cannot consider in setting a NAAQS the 
economic or technological feasibility of attaining ambient air quality 
standards, although such factors may be considered to a degree in the 
development of State plans to implement the standards. Moreover, the 
proposed revisions to the PM NAAQS, if adopted, will not in themselves 
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 of the UMRA do not apply to this proposed decision. The 
EPA acknowledges, however, that any corresponding revisions to 
associated State implementation plan requirements and air quality 
surveillance requirements, 40 CFR part 51 and 40 CFR part 58, 
respectively, might result in such effects. Accordingly, EPA has 
addressed unfunded mandates in the notice that announces the proposed 
revisions to 40 CFR part 58, and will, as appropriate, when it proposes 
any revisions to 40 CFR part 51.

E. Environmental Justice

    Executive Order 12848 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 draft RIA cited 
above.

List of Subjects in 40 CFR Part 50

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

    Dated: November 27, 1996.
Carol M. Browner,
Administrator.

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Fitz-Simons, T.; Mintz, D.; Wayland, M. (1996) Proposed methodology 
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Hoek, G.; Brunekreef, B. (1993) Acute effects of a winter air 
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air pollution as a predictor of mortality in a prospective study of 
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Dated July 9, 1996.

[[Page 65671]]

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Schwartz, J. (1994f) PM10, ozone, and hospital admissions for 
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    For the reasons set forth in the preamble, Part 50 of Chapter I of 
Title 40 of the Code of Federal Regulations is proposed to be 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, 7801(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 particulate 
matter (PM10 and PM2.5) 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 
shall be reported based on actual air volume measured at the actual 
temperature and pressure at the monitoring site during the measurement 
period.
    3. Section 50.6 is revised to read as follows:


Sec. 50.6  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 50 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:

[[Page 65672]]

    (i) A reference method based on Appendix L 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 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:
    (i) A reference method based on Appendix J 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 K to 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 K to this part, is less than or equal to 50 
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 K 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 98th percentile 24-hour concentration, as determined in 
accordance with Appendix K of this part, is less than or equal to 150 
micrograms per cubic meter.
    4. Appendix J is amended as follows:
    a. Section 2.2 is revised.
    b. The last sentence of Section 3.1 is revised.
    c. The first sentence of Section 7.3 is revised.
    d. The last sentence of Section 8.1.2 is removed.
    e. Section 8.2.1 is revised.
    f. The first sentence of Section 8.2.2 is revised.
    g. Section 11.1 is revised.
    h. Section 11.2 is revised.
    i. Section 11.3 is removed.

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

* * * * *
    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).
* * * * *
    3.1  * * * Nevertheless, all samplers should be capable of 
measuring 24-hour PM10 mass concentrations of at least 300 
g/m3 while maintaining the operating flow rate within 
the specified limits.
* * * * *
    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 Standards and Technology 
(NIST).
* * * * *
    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. The general procedure given here serves 
to illustrate the steps involved in the calibration. Consult the 
sampler manufacturer's instruction manual and Reference 2 for 
specific guidance on calibration. Reference 14 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.
* * * * *
    11.1  Calculate the total volume of air sampled as:

V=Qa x t

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  Calculate the PM10 concentration as:

PM10=(Wf - Wi)  x  106/V

Where:

PM10=mass concentration of PM10, g/m3,
Wf, Wi=final and initial weights of filter collecting 
PM10 particles, g, and
106 = conversion of g to g.

    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 [S(Wf - Wi)] is used to 
calculate the PM10 mass concentration.
* * * * *
    5. Appendix K is revised in its entirety to read as follows:

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

1.0  General

     This appendix explains the data handling conventions and 
computations necessary for determining whether the annual and 24-
hour primary and secondary national ambient air quality standards 
for particulate matter specified in part 50.6 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 J of this part for PM10 and 
on appendix L 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 
reporting, 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.
    Several terms used throughout this appendix are defined here. A 
``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. The term ``98th percentile'' means the 
daily value out of a year of monitoring data below which 98% of all 
values in the group fall. The terms ``average'' and ``mean'' refer 
to an arithmetic mean. All particulate matter standards are 
expressed in terms of 3-year averages of annual values: the 3-year 
average of the annual means for the annual standards, and the 3-year 
average of the 98th percentile values for each year for the 24-hour 
standards. The term ``year'' refers to a calendar year. ``Designated 
monitors'' are those monitoring sites designated in a State 
monitoring plan for spatial averaging in areas designated for 
spatial averaging in accordance with part 58 of this chapter.

2.0  Comparisons With the PM2.5 Standards

2.1  Annual PM2.5 Standard

    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 three consecutive years.
    The steps can be summarized as follows:
    (a) Average 24-hour measurements to obtain quarterly means at 
each monitor,
    (b) Average quarterly means to obtain annual means at each 
monitor,
    (c) Average across designated monitoring sites to obtain an 
annual spatial mean for an area, and
    (d) Average 3 years of annual spatial means to obtain a 3-year 
average of spatially averaged annual means.
    For the annual PM2.5 standard, a year meets data 
completeness requirements when at

[[Page 65673]]

least 75 percent of the scheduled sampling days for each quarter 
have valid data. Three years of spatial averages are required to 
demonstrate that the standard has been met. Sites with less than 3 
years of data shall be included in spatial averages for those years 
that data completeness requirements are met. The formulas for 
calculating the 3-year average annual mean of the PM2.5 
standard are given in Section 2.5.
    Although 3 complete years of data are required to demonstrate 
that the standard has been met, years with high concentrations shall 
not be ignored just because they have less than complete data. Thus, 
in computing annual spatially averaged means, sites with less than 
75 percent data completeness for each quarter in a year shall be 
included in the computation if the resulting annual mean 
concentration is greater than the level of the standard.

2.2  24-Hour PM2.5 Standard

    The 24-hour PM2.5 standard for is met when the 3-year 
average of the 98th percentile values at each monitoring site is 
less than or equal to 50 g/m\3\. This comparison shall be 
based on three consecutive, complete years of air quality data. A 
year meets data completeness criteria when at least 75 percent of 
the scheduled sampling days have valid data for each quarter. The 
formula for calculating the 3-year average of the annual 98th 
percentile values is given in Section 2.6.
    Although three complete years of data are required to 
demonstrate that the standard has been met, years with high 
concentrations shall not be ignored just because they have less than 
complete data. Thus, in computing the 3-year average 98th percentile 
value, years with less than 75 percent data completeness shall be 
included in the computation if the annual 98th percentile value is 
greater than the level of the standard.

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. 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/m\3\ 
(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/m\3\ 
(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

    Part 58.13 of this chapter specifies the required minimum 
frequency of sampling for PM2.5. Part 58 also specifies which 
monitors shall be used in making comparisons with the particulate 
matter standards.
    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, their concentrations shall be averaged before an 
area-wide spatial average is calculated, and such monitors will then 
be considered as one monitor.

2.5 Formulas 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.
[GRAPHIC] [TIFF OMITTED] TP13DE96.002

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 formula is then to be used for calculation of 
the annual mean:
[GRAPHIC] [TIFF OMITTED] TP13DE96.003

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) 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:
[GRAPHIC] [TIFF OMITTED] TP13DE96.004

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.

    In the event that an area designated for spatial averaging has 
one or more sites at the same location producing data for the same 
time periods, the sites are averaged together before using formula 
[3] by:
[GRAPHIC] [TIFF OMITTED] TP13DE96.005

Where:

xy,s*=the annual mean for year y for the sites at the same 
location (which will now be considered one site),
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 formula:
[GRAPHIC] [TIFF OMITTED] TP13DE96.006

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.

    In an area designated for spatial averaging, four designated 
monitors recorded data in at least 1 year of a particular 3-year 
period. Using formulas [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 are also 
shown.

--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                                                       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\)....          13.3          17.4           9.8  ............         15.35
                                                 % data completeness..............          90            63            40             0    ............
Year 3.........................................  Annual mean (g/m\3\)....          12.9          16.7          12.3          20.1         15.50
                                                 % data completeness..............          90            80            85            50    ............
3-year mean....................................  .................................  ............  ............  ............  ............         14.52
--------------------------------------------------------------------------------------------------------------------------------------------------------

    The data from these sites are averaged in the order described in 
section 2.1. Note that the annual mean from site #3 in year 2 does 
not enter in the spatial mean since the data completeness criteria 
are not met. However, the annual means from site #2 in year 2 and 
from site #4 in year 3 are included, even though the data 
completeness criteria are not met, since they are above the level of 
the standard. The 3-year mean is rounded to 14.5 g/m\3\, 
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.

    In an area designated for spatial monitoring, six designated 
monitors, with

[[Page 65674]]

two monitors at the same location (#5 and #6), recorded data in a 
particular 3-year period.
    Using formulas [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.

--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                                                                                                  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............................................         14.2         11.5          8.7         10.9         16.9         14.5       15.70       12.21
Year 2............................................         16.4         13.3         10.3         12.3         15.5         13.8       14.65       13.39
Year 3............................................         12.9         12.4          9.5         11.2         15.1         13.3       14.20       12.04
3-Year mean.......................................  ...........  ...........  ...........  ...........  ...........  ...........  ..........       12.55
--------------------------------------------------------------------------------------------------------------------------------------------------------

    The annual means for sites #5 and #6 are averaged together using 
formula [4] before the spatial average is calculated using formula 
[3] since they are in the same location. The 3-year mean is rounded 
to 12.6 g/m\3\, 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.

    Given data from a single monitor in an area designated for 
spatial averaging, the calculations are as follows. Using formulas 
[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/
m\3\, then the 3-year mean is:

X=(\1/3\) x (10.28+17.38+12.25)=13.303 g/m\3\.

    This value is rounded to 13.3, indicating that this area meets 
the annual PM2.5 standard.

2.6  Formulas for the 24-Hour PM2.5 Standard

    When the data for a particular site and year meet the data 
completeness requirements in section 2.2, 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 formula [6]:
[GRAPHIC] [TIFF OMITTED] TP13DE96.007

where:

P0.98,y=98th percentile for year y,
X[j]=the jth number in the ordered series of numbers,
``i''=the integer part of the product of 0.98 and n (the number of 
values in the series), and
``d''=the decimal part of the product of 0.98 and n.

    The 3-year average 98th percentile is then calculated by 
averaging the annual 98th percentiles:
[GRAPHIC] [TIFF OMITTED] TP13DE96.008

    The 3-year average 98th percentile is rounded according to the 
conventions in section 2.3 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.

    In each year of a particular 3 year period, varying numbers of 
daily PM2.5 values (e.g., 278, 300, and 293) out of a possible 
365 values were recorded at a particular site with the following 
ranked values (in g/m \3\):

----------------------------------------------------------------------------------------------------------------
               Year 1                                Year 2                                Year 3               
----------------------------------------------------------------------------------------------------------------
      j rank            Xj value            j rank            Xj value            j rank            Xj value    
----------------------------------------------------------------------------------------------------------------
* * *............           * * *              * * *              * * *              * * *              * * *   
272..............            44.1                293               41.4                287               50.3   
273..............            45.0                294               43.5                288               52.1   
274..............            47.4                295               48.0                289               53.2   
* * *............           * * *              * * *              * * *              * * *              * * *   
----------------------------------------------------------------------------------------------------------------

    Using formula [6], the 98th percentile values for each year are 
calculated as follows:
[GRAPHIC] [TIFF OMITTED] TP13DE96.009

    Using formula [7], the 3-year average 98th percentile is 
calculated as follows:

[GRAPHIC] [TIFF OMITTED] TP13DE96.010


    Therefore, this site meets the 24-hour PM2.5 standard.

[[Page 65675]]

3.0  Comparisons with the PM10 Standards

3.1  Annual PM10 Standard

    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/m \3\. 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:
    (a) Average 24-hour measurements to obtain a quarterly mean,
    (b) Average quarterly means to obtain an annual mean, and
    (c) Average annual means to obtain a 3-year mean.
    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. The formulas for 
calculating the 3-year average annual mean of the PM10 standard 
are given in Section 3.5.
    Although 3 complete years of data are required to demonstrate 
that the standard has been met, years with high concentrations shall 
not be ignored just because they have less than complete data. Thus, 
in computing the 3-year average annual mean concentration, years 
with less than 75 percent data completeness shall be included in the 
computation if the annual mean concentration is greater than the 
level of the standard.

3.2  24-Hour PM10 Standard

    The 24-hour PM10 standard is met when the 3-year average of 
the annual 98th percentile values at each monitoring site is 
less than or equal to 150 g/m \3\. This comparison shall be 
based on 3 consecutive, complete years of air quality data. A year 
meets data completeness criteria when at least 75 percent of the 
scheduled sampling days have valid data each quarter. The formula 
for calculating the 3-year average of the annual 98th 
percentile values is given in Section 3.6.
    Although 3 complete years of data are required to demonstrate 
that the standard has been met, years with high concentrations shall 
not be ignored just because they have less than complete data. Thus, 
in computing the 3-year average of the annual 98th percentile 
values, years with less than 75 percent data completeness shall be 
included in the computation if the annual 98th percentile value 
is greater than the level of the standard.

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/
m \3\ (decimals 0.5 and greater are 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 98th percentile values of PM10 shall 
be rounded to the nearest 10 g/m \3\ (155 g/m \3\ 
and greater would be rounded to 160 g/m \3\ and 154 
g/m \3\ and less would be rounded to 150 g/m \3\).

3.4  Monitoring Considerations

    Part 58.13 of this chapter specifies the required minimum 
frequency of sampling for PM10. For making comparisons with the 
PM10 NAAQS, all sites meeting applicable requirements in part 
58 of this chapter would be used.

3.5  Formulas 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 formula:
[GRAPHIC] [TIFF OMITTED] TP13DE96.011

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 formula is then to be used for calculation of 
the annual mean:
[GRAPHIC] [TIFF OMITTED] TP13DE96.012

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 formula:
[GRAPHIC] [TIFF OMITTED] TP13DE96.013

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.

    Given data from a PM10 monitor and using formulas [9] and 
[10], the annual means for PM10 are calculated for each year. 
If the annual means are 52.42, 82.17, and 63.23
g/m \3\, then the 3-year average annual mean is

x=(\1/3\)  (52.42 + 82.17 + 63.23)=65.94 which is rounded to 
66 g/m\3\. Therefore, this site does not meet the annual 
PM10 standard.

3.6  Formula for the 24-Hour PM10 Standard

    When the data for a particular site and year meet the data 
completeness requirements in section 3.2, 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 formula [12]:
[GRAPHIC] [TIFF OMITTED] TP13DE96.014

Where:

P0.098,y=the 98th percentile for year y,
X[j]=the jth number in the ordered series of numbers,
``i''=the integer part of the product of 0.98 and n (the number of 
observations in the series), and
``d''=the decimal part of the product of 0.98 and n.

    The 3-year average 98th percentile value is then calculated by 
averaging the annual 98th percentiles:
[GRAPHIC] [TIFF OMITTED] TP13DE96.015

    The 3-year average 98th percentile is rounded according to the 
conventions in section 3.3 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.

    In each year of a particular three year period, varying numbers 
of PM10 daily values (e.g., 55, 49, and 50) out of a possible 
61 daily values were recorded at a particular site with the 
following ranked values (in g/m\3\):

[[Page 65676]]



----------------------------------------------------------------------------------------------------------------
               Year 1                                Year 2                                Year 3               
----------------------------------------------------------------------------------------------------------------
      j rank            Xj value            j rank            Xj value            j rank            Xj value    
----------------------------------------------------------------------------------------------------------------
* * *............           * * *              * * *              * * *              * * *              * * *   
53...............             120                 47                143                 48                140   
54...............             128                 48                148                 49                144   
55...............             130                 49                150                 50                147   
* * *............           * * *              * * *              * * *              * * *              * * *   
----------------------------------------------------------------------------------------------------------------

    Using formula [12], the 98th percentile values for each year are 
calculated as follows:
[GRAPHIC] [TIFF OMITTED] TP13DE96.016

    Using formula [3], the 3-year average 98th percentile is 
calculated as follows:
[GRAPHIC] [TIFF OMITTED] TP13DE96.017

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

     6. Appendix L is added to read as follows:

Appendix L--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 chapter are met. The measurement process is considered to be 
nondestructive, and physical or chemical analyses. Quality 
assessment procedures are provided in part 58, Appendices A and B, 
of this chapter and quality assurance procedures and guidance are 
provided in References 1 and 2.
    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,
    (b) the method and associated sampler have been designated as a 
reference method in accordance with part 53 of this Chapter, and
    (c) the national operating performance of the associated 
sampler, as determined in accordance with part 58, Appendix A, 
section 6 of this chapter, continue to meet the specifications set 
forth in part 58, Appendix A, section 6.3.3 of this chapter.
    1.3  PM2.5 samplers that meet all specifications set forth 
in this method but have minor deviations and/or modifications of the 
reference method sampler necessary to obtain sequential operation 
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 
equilibration) before and after sample collection to determine the 
net weight (mass) 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 actual cubic meter of air (g/m 
\3\).

3.0  PM2.5 Measurement Range

    3.1  Lower concentration limit. The lower limit of the mass 
concentration range should be 1 g/m3 or less and is 
determined primarily by the repeatability (precision) of filter 
blanks, based on 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 should 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 1380 to 1500 
minutes (23 to 25 hours). However, when a sample period is less than 
1380 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 1440, may be used as a valid concentration measurement 
for purposes of determining violations of the NAAQS. This number 
represents the minimum concentration that would have been measured 
for the full 24-hour sample period. When reported to AIRS, this data 
value should receive a special code.

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 samplers 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 for other (equivalent) 
methods for PM2.5 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. This field 
sampler audit procedure is set forth in section 6 of part 58, 
Appendix A of this chapter.
    4.2.1  Test of concordance (reproducibility). Annual assessment 
of reproducibility for each designated reference

[[Page 65677]]

method sampler is required under the provisions of Appendix A of 
Part 58 of this chapter. This assessment is based on the concordance 
correlation, using 6 measurements per year at regular intervals of 
each reference method sampler operated in a SLAMS network to a 
collocated audit reference sampler. The assessment audits may be 
performed by either the reporting agency itself or by a third party 
and must meet criteria specified in Appendix A of part 58 of this 
Chapter. A test procedure is described in section 6.1 of part 58, 
Appendix A that determines the bias in the primary sampler as 
compared to the reference method sampler under actual network 
operational sampling conditions. The lower 95 percent probability 
limit of the concordance correlation for PM2.5 samplers, as 
determined by this procedure, must be equal to or greater than 0.94 
for each designated reference method sampler to retain its 
designation.
    4.2.2  Annual assessment of the bias of each designated 
reference method sampler is required under the provisions of part 58 
of this chapter. This assessment is based on comparisons made six 
times per year at regular intervals of each reference method sampler 
operated in a SLAMS network to a collocated audit sampler. The 
assessment audits may be performed by either the reporting agency 
itself or by a third party and must meet criteria specified in 
Appendix A of part 58 of this chapter. A screening test procedure is 
described in section 6.2 of part 58, Appendix A that examines for 
bias between the primary sampler and the reference method sampler 
under actual network operational sampling conditions. The test uses 
a simple counting procedure and leads to a conclusion of bias only 
when the evidence is quite strong (p=0.01).
    4.3  In addition, part 58, Appendix A of this chapter requires 
that the flow rate accuracy of PM2.5 samplers used in SLAMS 
monitoring networks be assessed periodically via audits of the 
sampler's operational flow rate.

5.0  Precision.

    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 part 53 of this chapter 
(Sec. 53.56).
    5.2  Annual assessments of routine operational precision are 
also required.
    5.2.1  Annual assessment of the pooled operational precision of 
each designated reference method sampler is required under the 
provisions of part 58 of this chapter. This assessment is based on 
comparisons made six times per year at regular intervals of each 
reference method sampler operated in a SLAMS network to a collocated 
audit sampler. The assessment audits may be performed by either the 
reporting agency itself or by a third party and must meet criteria 
specified in Appendix A of part 58 of this Chapter. A test procedure 
is described in section 6.1 of part 58, Appendix A that determines 
the variation in the PM2.5 concentration measurements of 
reference method samplers under the actual network operational 
sampling conditions. The pooled operational precision of PM2.5 
samplers, as determined by this procedure, must meet the 
specification in section 6 of Appendix A, part 58 for each 
designated reference method sampler to retain its designation.
    5.2.2  A screening test for bias and excessive imprecision is 
required under the provisions of part 58 of this chapter. This 
assessment is based on comparisons made six times per year at 
regular intervals of each reference method sampler operated in a 
SLAMS network to a collocated audit sampler. The assessment audit 
may be performed by either the reporting agency itself or by a third 
party and must meet criteria specified in Appendix A section 6.2 of 
part 58 of this Chapter. A screening test procedure is described in 
section 6 of part 58, Appendix A that examines for excessive 
imprecision (>15%) in one or both of the samplers. The test uses a 
simple counting procedure and leads to a conclusion of excessive 
imprecision only when evidence is quite strong (p-0.01) under the 
actual network operational sampling conditions.

6.0  Filter for PM2.5 Sample Collection

    6.1  Size: Circular, 47 mm diameter.
    6.2  Medium: Polytetrafluoroethylene (PTFE) with integral 0.38 
0.04 mm thick polymethylpentene (PMP) or equivalent 
support ring.
    6.3  Pore size: 2 m as measured by ASTM F 316-80
    6.4  Thickness: 20-60 m
    6.5  Maximum pressure drop: 30 cm H2O column @ 16.67 L/min 
clean air flow.
    6.6  Maximum moisture pickup: 0.0% weight increase after 24-hour 
exposure at 48% relative humidity at 23  deg.C.
    6.7  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.
    6.8  Filter weight stability. Filter weight loss  20 
g, measured as specified in the following two tests. Filter 
weight loss shall be the average difference between the initial and 
the final weights of a random sample of test filters selected from 
each lot prior to shipment. The number of filters tested shall be 
not less than 0.1% 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 equilibration and weighing, the test, and final 
equilibration and weighing. Equilibration and weighing shall be in 
accordance with section 8 and guidance provided in Reference 2.
    6.8.1  Test for surface particle contamination. Install each 
test filter in a filter cassette (Drawing numbers L-25, L-26) and 
drop the cassette from a height of 25 cm to a flat hard surface, 
such as a particle-free wood bench. Repeat three times. Remove the 
test filter from the cassette and weigh the filter. The average 
change in weight must be less than 20 g.
    6.8.2  Test of temperature stability. Place randomly selected 
test filters in a drying oven set at 40 deg.C 2  deg.C 
for not less than 48 hours. Remove, equilibrate, and reweigh each 
test filter. The average change in weight must be less than 20 
g.
    6.9  Alkalinity. Less than 25 microequivalents/gram of filter, 
as measured by the procedure given in Reference 2.
    6.10  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.

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, timer, outdoor environmental enclosure, and suitable 
mechanical, electrical, or electronic control capability to provide 
the design and functional performance as specified in this section 
7. The performance specifications require that the sampler:
    (a) provide automatic control of sample flow rate and other 
operational parameters,
    (b) monitor these operational parameters as well as ambient 
temperature and pressure, and
    (c) provide this information to the sampler operator at the end 
of each sample period in digital form, either visually or as 
electronic data available for output through a data output port 
connection.
    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, and the 
internal configuration of the filter holder assembly are specified 
explicitly by design drawings and associated mechanical dimensions, 
tolerances, materials, surface finishes, assembly instructions, and 
other necessary specifications. All other aspects of the 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. These components must be 
manufactured or reproduced exactly as specified in an ISO 9001-
registered facility, with registration initially approved and 
subsequently maintained.
    7.3.1  Sample inlet assembly. The sample inlet assembly, 
consisting of the inlet, downtube, and impactor shall be assembled 
as indicated in drawing No. L-1 and shall meet all associated 
requirements. A portion of this assembly shall also be subject to 
the maximum overall sampler leak rate specification (see section 
7.4.6).

[[Page 65678]]

    7.3.2  Inlet. The sample inlet shall be fabricated as indicated 
in drawing Nos. L-2 through L-18 and shall meet all associated 
requirements.
    7.3.3  Downtube. The downtube shall be fabricated as indicated 
in drawing No. L-19 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 drawing Nos. L-20 through L-24 and shall 
meet all associated requirements.
    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/cm 3 at 25  deg.C
    (e) Quantity: 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 (section 6) 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 drawing Nos. L-25 and L-26 and shall accept and seal 
with the filter cassette, which shall be fabricated as indicated in 
drawing Nos. L-27 through L-29.
    (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 below.
    7.3.6  Flow rate measurement adapter. A flow rate measurement 
adapter as specified in drawing No. L-30 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/ft 2 (1.08 mg/cm 2) in 
accordance with military standard specification (mil. spec.) 8625F, 
Type II, Class 1 (Reference 3). This anodic surface coating shall 
not be dyed or pigmented. Following anodization, the surfaces shall 
be sealed by immersion in boiling deionized water for 15 minutes.
    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 sample air flow rate through the inlet, 
downtube, impactor, and filter shall be 16.67 L/min (1.000 m 3/
hour) 5%, measured as actual volumetric flow rate at the 
temperature and pressure of the sample air entering the impactor.
    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 (section 7.4.1) for the specified filter (section 6), at any 
atmospheric conditions specified (section 7.4.7), 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 (section 7.4.15.1). This flow control system shall allow for 
operator adjustment of the operational flow rate of the sampler over 
a range of at least 10 percent of the flow rate 
specified in section 7.4.1.
    7.4.3  Sample flow rate regulation. The sample flow rate shall 
be regulated such that for the specified filter (section 6), at any 
atmospheric conditions specified (section 7.4.7), 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 (section 7.4.15.1), 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; and
    7.4.3.2  The coefficient of variation (sample standard deviation 
divided by the average) of the flow rate, measured at intervals of 
not more than 5 minutes over a 24-hour period, shall not be greater 
than 4 percent.
    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 nominal 
(or cumulative average) sampler flow rate specified in section 7.4.1 
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 will not be included in the total sample time 
measured and reported by the sampler (see section 7.4.13).
    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 temperature and pressure of 
the sample air entering the impactor, with an accuracy of 
2 percent. 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 or average flow rate measurements at 
intervals of not greater than 5 minutes.
    (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) any time during the sample period in which the sample flow 
rate measured 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; and
    (5) the value of the integrated total sample volume for the 
sample period, in m 3.
    (b) Determination or calculation of these values shall properly 
exclude periods when the sampler is inoperative due to temporary 
interruption of electrical power (see section 7.4.13). These 
parameters shall be accessible to the sampler operator as specified 
in Table L-1, section 7.4.19.
    7.4.6  Leak test capability.
    7.4.6.1  External leakage: The sampler shall include components, 
accessory hardware, operator interface controls, a written procedure 
in the associated Operation/Instruction Manual (section 7.4.18), 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.
    (a) The suggested technique for this leak test is as follows: 
The operator:
    (1) removes the sampler inlet and installs the flow rate 
measurement adapter supplied with the sampler (see section 7.3.6),
    (2) closes the valve on the flow rate measurement adapter and 
uses 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),
    (3) plugs the flow system downstream of these components to 
isolate the components under vacuum from the pump, such as with a 
built-in valve,
    (4) stops the pump,
    (5) measures the trapped vacuum in the sampler with a built-in 
pressure measuring device, and
    (6) measures 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, and
    (7) removes the plugs and restores 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 either:

[[Page 65679]]

    (1) 10 mm Hg or
    (2) an alternative 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) The specific proposed external leak test procedure, or 
particularly a proposed alternative leak test technique such as may 
be required for samplers whose design or configuration would make 
the suggested technique impractical, may be described and submitted 
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 application.
    7.4.6.2  Internal (filter bypass) leakage: The sampler shall 
include 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.
    (a) The suggested technique for this leak test is as follows: 
The operator:
    (1) Carries out an external leak test as provided under the 
paragraph 7.4.6.1 which indicates successful passage of the 
prescribed external leak test,
    (2) Installs 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 holder,
    (3) Uses 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) Plugs 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) Stops the pump,
    (6) Measures the trapped vacuum in the sampler with a built-in 
pressure measuring device,
    (7) Measures 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, and
    (8) removes the membrane and plugs and restores 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 either 10 mm Hg or 
an alternative 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. The specific proposed internal leak test procedure, or 
particularly a proposed alternative internal leak test technique 
such as may be required for samplers whose design or configuration 
would make the suggested technique impractical, may be described and 
submitted 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 application.
    7.4.7  Range of Operational Conditions. The sampler is required 
to operate properly and meet all requirements specified herein over 
the following operational ranges:
    7.4.7.1  Ambient temperature: -30 to +45 degrees Celsius (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 degrees Celsius, the sampler should 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 -20 to +40 , with a resolution of 0.1 
C and accuracy of 2.00 C (referenced to National 
Weather Service (NWS) requirements; see part 53, subpart E), with or 
without maximum solar insolation. This ambient temperature 
measurement shall be updated at least every 5 minutes during both 
sampling and standby (non-sampling) modes of operation. A visual 
indication of the current (most recent) value of the ambient 
temperature measurement shall be available to the sampler operator 
during both sampling and standby (non-sampling) modes of operation, 
as specified in Table L-1. This ambient temperature measurement 
shall be used for the purpose of monitoring filter temperature 
deviation from ambient temperature, as required by section 7.4.11.4, 
and may be used for purposes of effecting filter temperature control 
(section 7.4.10) or computation of volumetric flow rate (sections 
7.4.1 to 7.4.5). 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.
    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 to National 
Weather Service (NWS) requirements; see part 53, subpart E). (The 
barometric pressure of the air entering the impactor when sampling 
will be assumed to be the same as the barometric pressure of the air 
surrounding the sampler.) 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 5 minutes. A visual indication 
of the value of the current (most recent) barometric pressure 
measurement shall be available to the sampler operator during both 
sampling and standby (non-sampling) modes of operation, as specified 
in Table L-1. This barometric pressure measurement may be used for 
purposes of computation of volumetric flow rate (sections 7.4.1 to 
7.4.5), if appropriate. Following the end of a sample period, the 
sampler shall report the maximum, minimum, and average barometric 
pressures for the sample period, as specified in Table L-1.
    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, from insolation and other sources, to no more 
than 3  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. The sampler shall have the 
capability to monitor the sample filter temperature via a 
temperature sensor located within 1 cm of the center of the filter 
downstream of the filter and to provide a visual indication of the 
filter temperature to the operator, as specified in Table L-1. The 
sampler shall also provide a warning flag indicator following any 
occurrence in which the filter temperature exceeds the ambient 
temperature by more than 3  deg.C for more than 10 consecutive 
minutes during either the sampling or post-sampling periods of 
operation, as specified in Table L-1. 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 
those data to the sampler operator following the end of the sample 
period, as suggested in Table L-1.
    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; and
    (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. Upon execution 
of a programmed sample period start, the sampler shall automatically 
reset all sample period information and warning indications 
pertaining to a previous sample period. Refer also to section 
7.4.15.4 regarding retention of current date and time and programmed 
start and stop times during a temporary electrical power 
interruption.
    7.4.13  Sampling sample time determination. The sampler shall be 
capable of determining the elapsed sample collection time for each 
PM2.5 sample, accurate to

[[Page 65680]]

within 1.0 minute, measured as the time between the 
start of the sampling period (sec. 7.4.12) and the termination of 
the sample period (sec. 7.4.12 or sec. 7.4.4). This elapsed sample 
time shall not include periods when the sampler is inoperative due 
to a temporary interruption of electrical power (section 7.4.15.4). 
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, 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.
    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 filter; 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 see section 3.3.) 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.
    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. 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.
    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.

                                                                                                                                                        
                                            Table L-1.--Summary of Information To Be Provided by the Sampler                                            
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                                       Availability                                           Format                    
                                      Appendix L -------------------------------------------------------------------------------------------------------
     Information to be provided         section                    End of        Visual                                                                 
                                       reference    Anytime 1     period 2      display 3    Data output     Digital reading 5             Units        
--------------------------------------------------------------------------------------------------4-----------------------------------------------------
Flow rate, instantaneous............     7.4.5.1                                            XX.X..................  L/min                 
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. (FLAG 6).                                                                                                                                        
Sample volume, total................     7.4.5.2            *                   XX.X..................  m 3                   
Temperature, ambient, instantaneous        7.4.8                                            XX.X..................   deg.C                
 or 5-minute average.                                                                                                                                   
Temperature, ambient, min., max.,          7.4.8            *                   XX.X..................   deg.C                
 average for the sample period.                                                                                                                         

[[Page 65681]]

                                                                                                                                                        
Baro pressure, ambient,                    7.4.9                                            XXX...................  mm Hg                 
 instantaneous or 5-minute average.                                                                                                                     
Baro pressure, ambient, min, max,          7.4.9            *                   XXX...................  mm Hg                 
 average for the sample period.                                                                                                                         
Filter temperature, instantaneous...      7.4.11                                            XX.X..................   deg.C                
Filter temperature, instantaneous         7.4.11            *                   On/Off................  ......................
 differential out of spec. (FLAG 1).                                                                                                                    
Filter temp, maximum differential         7.4.11            *             *             *             *   X.X, YY/MM/DD HH.mm...   deg.C, Yr/Mon/Day    
 from ambient, date, time of                                                                                                       Hrs.min              
 occurrence.                                                                                                                                            
Date and Time.......................      7.4.12                                            YY/MM/DD HH.mm........  Yr/Mon/Day Hrs.min    
Sample start and stop time settings.      7.4.12                              YY/MM/DD HH.mm........  Yr/Mon/Day Hrs.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................  ......................
 (FLAG\6\).                                                                                                                                             
Power interruptions >1 min, start       7.4.15.5            *                    *          1HH.mm 2HH.mm ........  Hrs.min               
 time of first 10.                                                                                                                                      
User-entered information, such as         7.4.16                         As entered............  ......................
 sampler and site identification.                                                                                                                       
--------------------------------------------------------------------------------------------------------------------------------------------------------
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 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. The occurrence of a flag warning during a sample period     
  shall not necessarily indicate an invalid sample but shall indicate the need for specific review of the QC data by a quality assurance officer to     
  determine sample validity.                                                                                                                            
* 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.  58.26 and Sec.  58.35 of part 58 of this Chapter.

    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 Table L-1.
    8.0  Filter weighing.
    See Reference 2 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 (section 6) 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, but not less often than once per year. See 
Reference 2 for additional guidance.
    8.2  Filter conditioning/equilibration. All filters used are to 
be conditioned or equilibrated immediately before both the pre- and 
post-sampling weighings as specified below. See Reference 2 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: 30-40 percent relative humidity.
    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 for additional guidance.
    8.3.2  The analytical balance shall be located in the same 
environment in which the filters are conditioned or equilibrated, 
such that the filters can be weighed immediately following the 
conditioning period without intermediate or transient exposure to 
nonequilibration conditions.
    8.3.3  Filters must be equilibrated at the same conditions 
before both the pre- and post-sampling weighings.
    8.3.4  Both the pre- and post-sampling weighings should be 
carried out by the same analyst on the same analytical balance, 
using an effective technique to neutralize static charges on the 
filter.

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    8.3.5  The pre-sampling (tare) weighing shall be within 30 days 
of the sampling period.
    8.3.6  The post-sampling equilibration and weighing shall be 
completed within 240 hours (10 days) after the end of the sample 
period.
    8.3.7  New 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.
    8.3.8  Additional guidance for proper filter weighing is 
provided in Reference 2. See also section 10.17 concerning filter 
archiving.

9.0  Calibration

    See Reference 2 for 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 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, drawing L-30. This flow rate standard 
must have its own certification and be traceable to National 
Institute of Standards and Technology (NIST) primary standards for 
volume or flow rate. If adjustments to the sampler's flow 
calibration are to be made in conjunction with an audit of the 
sampler, such adjustments shall be made following the audit. See 
Reference 2 for 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). 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) and the Quality 
Assurance Handbook (Reference 2) 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 National 
Institute of Standards and Technology (NIST) primary standards for 
volume or flow rate. A calibration relationship for the flow rate 
standard (e.g., an equation, curve, or family of curves) 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 in accordance with the 
operation/instruction manual, such that the flow rate standard 
accurately measures the sampler's flow rate. The sampler operator 
shall 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% to +10% of the sampler's operational flow rate (see 
section 7.4.1). 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 
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 
4 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 
sampler flow rate differs by 2 percent or more from the 
required sampler flow rate, the sampler flow rate must be adjusted 
to the required flow rate (see section 7.4.1).

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. Supplemental guidance is provided in section 2.12 of 
the QA Handbook (Reference 2). 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.
    10.2  Each new filter shall be inspected for correct type and 
size and for pinholes, particles, and other imperfections. A filter 
information record shall be established for, and an identification 
number assigned to, each filter.
    10.3  Each filter shall be equilibrated in the conditioning 
environment in accordance with the requirements specified in section 
8.2.
    10.4  Following equilibration, each filter shall be weighed in 
accordance with the requirements specified in section 8 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 PM 2.5 
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. The 
protective container shall hold the filter cassette securely. The 
cover shall not come in contact with the filter's surfaces. The 
protective container shall be made of metal and contain no loose 
material that could be transferred to the filter. (See reference 2 
for additional information.)
    10.11  The total sample volume in actual m 3 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.
    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 within 24 
hours--for equilibration and subsequent weighing. During the period 
between filter retrieval

[[Page 65683]]

from the sampler and the start of the conditioning or equilibration, 
the filter shall not be exposed to temperatures over 32  deg.C.
    10.14 The exposed filter containing the PM2.5 sample shall 
be re-equilibrated in the conditioning environment in accordance 
with the requirements specified in section 8.2.
    10.15 The filter shall be reweighed immediately after 
equilibration in accordance with the requirements specified in 
section 8, 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.
    10.17 Filter archiving. Following the post-sampling weighing or 
other non-destructive analysis, air pollution control agencies shall 
archive all routinely collected PM2.5 filter samples from all 
SLAMS sites, as well as appropriate, associated laboratory and field 
blanks and other quality assurance replicate samples, for a period 
of not less than 1 year after collection. All PM2.5 filters 
from core NAMS sites shall be archived for a period of not less than 
5 years after collection. These archived filters shall be made 
available for supplemental analyses at the request of the EPA or to 
provide information to State and local agencies on the composition 
and trends for PM2.5. Archived filter samples shall be stored 
in clean, dust-proof, covered containers at a temperature of 4 
3  deg.C; see Reference 2 for additional guidance.

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 and in accordance with the specific 
quality assurance program developed and established by the user 
based on applicable supplementary guidance provided in Reference 2.
    12.0 Calculations.
    12.1 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;
V a = total air volume sampled in actual volume units, as 
provided by the sampler, m3.

    Note: Total sample time must be between 1380 and 1500 minutes 
(23 and 25 hrs) for a fully valid PM 2.5 sample; however, see 
also section 3.3.

 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. Quality Assurance Handbook for Air Pollution Measurement 
Systems, Volume II, Ambient Air Specific Methods (Interim Edition), 
section 2.12. EPA/600/R-94/038b, April 1994. Available from CERI, 
ORD Publications, U.S. Environmental Protection Agency, 26 West 
Martin Luther King Drive, Cincinnati, Ohio 45268. [Section 2.12 is 
currently under development and will not be available from the 
previous address until it is published as an addition to EPA/600/R-
94/038b. Prepublication draft copies of section 2.12 will be 
available from Department E (MD-77B), U. S. EPA, Research Triangle 
Park, NC 27711 or from the contact identified at the beginning of 
this proposed rule].
    3. 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

    Figures L-1 through L-30 are included as part of this appendix 
L.

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[FR Doc. 96-30897 Filed 12-12-96; 8:45 am]
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