[Federal Register Volume 79, Number 242 (Wednesday, December 17, 2014)]
[Proposed Rules]
[Pages 75234-75411]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 2014-28674]
[[Page 75233]]
Vol. 79
Wednesday,
No. 242
December 17, 2014
Part II
Environmental Protection Agency
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40 CFR Parts 50, 51, 52, et al.
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National Ambient Air Quality Standards for Ozone; Proposed Rule
Federal Register / Vol. 79 , No. 242 / Wednesday, December 17, 2014 /
Proposed Rules
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ENVIRONMENTAL PROTECTION AGENCY
40 CFR Parts 50, 51, 52, 53, and 58
[EPA-HQ-OAR-2008-0699; FRL-9918-43-OAR]
RIN 2060-AP38
National Ambient Air Quality Standards for Ozone
AGENCY: Environmental Protection Agency.
ACTION: Proposed rule.
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SUMMARY: Based on its review of the air quality criteria for ozone
(O3) and related photochemical oxidants and national ambient
air quality standards (NAAQS) for O3, the Environmental
Protection Agency (EPA) proposes to make revisions to the primary and
secondary NAAQS for O3 to provide requisite protection of
public health and welfare, respectively. The EPA is proposing to revise
the primary standard to a level within the range of 0.065 to 0.070
parts per million (ppm), and to revise the secondary standard to within
the range of 0.065 to 0.070 ppm, which air quality analyses indicate
would provide air quality, in terms of 3-year average W126 index
values, at or below a range of 13-17 ppm-hours. The EPA proposes to
make corresponding revisions in data handling conventions for
O3 and conforming changes to the Air Quality Index (AQI); to
revise regulations for the prevention of significant deterioration
(PSD) program to add a transition provision for certain applications;
and to propose schedules and convey information related to implementing
any revised standards. The EPA is proposing changes to the
O3 monitoring seasons, the Federal Reference Method (FRM)
for monitoring O3 in the ambient air, Federal Equivalent
Method (FEM) procedures for testing, and the Photochemical Assessment
Monitoring Stations (PAMS) network.
Along with proposing exceptional event schedules related to
implementing any revised O3 standards, the EPA is proposing
to apply this same schedule approach to other future revised NAAQS and
to remove obsolete regulatory language for expired exceptional event
deadlines. The EPA is proposing to make minor changes to the procedures
and time periods for evaluating potential FRMs and equivalent methods
(including making the requirements for nitrogen dioxide consistent with
the requirements for O3) and to remove an obsolete
requirement for the annual submission of documentation by manufacturers
of certain particulate matter monitors. For additional information, see
the Executive Summary, section I.A.
DATES: Written comments on this proposed rule must be received by March
17, 2015.
Public Hearings: The EPA intends to hold three public hearings on
this proposed rule in January 2015. These will be announced in a
separate Federal Register notice that provides details, including
specific dates, times, addresses, and contact information for these
hearings.
ADDRESSES: Submit your comments, identified by Docket ID No. EPA-HQ-
OAR-2008-0699, to the EPA by one of the following methods:
Federal eRulemaking Portal: http://www.regulations.gov.
Follow the online instructions for submitting comments.
Email: [email protected]. Include docket ID No. EPA-
HQ-OAR-2008-0699 in the subject line of the message.
Fax: (202) 566-9744.
Mail: Environmental Protection Agency, EPA Docket Center
(EPA/DC), Mailcode 28221T, Attention Docket ID No. OAR-2008-0699, 1200
Pennsylvania Ave. NW., Washington, DC 20460. Please include a total of
two copies.
Hand/Courier Delivery: EPA Docket Center, Room 3334, EPA
WJC West Building, 1301 Constitution Ave. NW., Washington, DC. Such
deliveries are only accepted during the Docket's normal hours of
operation, and special arrangements should be made for deliveries of
boxed information.
Instructions: Direct your comments to Docket ID No. EPA-HQ-OAR-
2008-0699. The EPA's policy is that all comments received will be
included in the public docket without change and may be made available
online at www.regulations.gov, including any personal information
provided, unless the comment includes information claimed to be
Confidential Business Information (CBI) or other information whose
disclosure is restricted by statute. Do not submit information that you
consider to be CBI or otherwise protected through www.regulations.gov
or email. The www.regulations.gov Web site is an ``anonymous access''
system, which means the EPA will not know your identity or contact
information unless you provide it in the body of your comment. If you
send an email comment directly to the EPA without going through
www.regulations.gov your email address will be automatically captured
and included as part of the comment that is placed in the public docket
and made available on the Internet. If you submit an electronic
comment, the EPA recommends that you include your name and other
contact information in the body of your comment and with any disk or
CD-ROM you submit. If the EPA cannot read your comment due to technical
difficulties and cannot contact you for clarification, the EPA may not
be able to consider your comment. Electronic files should avoid the use
of special characters, any form of encryption, and be free of any
defects or viruses. For additional information about EPA's public
docket visit the EPA Docket Center homepage at http://www.epa.gov/epahome/dockets.htm.
Docket: The EPA has established dockets for these actions as
discussed above. All documents in these dockets are listed on the
www.regulations.gov Web site. This includes documents in the rulemaking
docket (Docket ID No. EPA-HQ-OAR-2008-0699) and a separate docket,
established for the Integrated Science Assessment (ISA) (Docket No.
EPA-HQ-ORD-2011-0050) that has have been incorporated by reference into
the rulemaking docket. Although listed in the index, some information
is not publicly available, e.g., CBI or other information whose
disclosure is restricted by statute. Certain other material, such as
copyrighted material, is not placed on the Internet and may be viewed,
with prior arrangement, at the EPA Docket Center. Publicly available
docket materials are available either electronically in
www.regulations.gov or in hard copy at the Air and Radiation Docket and
Information Center, EPA/DC, EPA WJC West Building, Room 3334, 1301
Constitution Ave. NW., Washington, DC. The Public Reading Room is open
from 8:30 a.m. to 4:30 p.m., Monday through Friday, excluding legal
holidays. The telephone number for the Public Reading Room is (202)
566-1744 and the telephone number for the Air and Radiation Docket and
Information Center is (202) 566-1742. For additional information about
EPA's public docket visit the EPA Docket Center homepage at: http://www.epa.gov/epahome/dockets.htm.
FOR FURTHER INFORMATION CONTACT: Ms. Susan Lyon Stone, Health and
Environmental Impacts Division, Office of Air Quality Planning and
Standards, U.S. Environmental Protection Agency, Mail code C504-06,
Research Triangle Park, NC 27711; telephone: (919) 541-1146; fax: (919)
541-0237; email: [email protected].
SUPPLEMENTARY INFORMATION:
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General Information
What should I consider as I prepare my comments for EPA?
1. Submitting CBI. Do not submit this information to the EPA
through www.regulations.gov or email. Clearly mark the part or all of
the information that you claim to be CBI. For CBI information in a disk
or CD ROM that you mail to EPA, mark the outside of the disk or CD ROM
as CBI and then identify electronically within the disk or CD ROM the
specific information that is claimed as CBI. In addition to one
complete version of the comment that includes information claimed as
CBI, a copy of the comment that does not contain the information
claimed as CBI must be submitted for inclusion in the public docket.
Information so marked will not be disclosed except in accordance with
procedures set forth in 40 CFR part 2.
2. Tips for Preparing Your Comments. When submitting comments,
remember to:
Identify the rulemaking by docket number and other
identifying information (subject heading, Federal Register date and
page number).
Follow directions--The agency may ask you to respond to
specific questions or organize comments by referencing a Code of
Federal Regulations (CFR) part or section number.
Explain why you agree or disagree, suggest alternatives,
and substitute language for your requested changes.
Describe any assumptions and provide any technical
information and/or data that you used.
Provide specific examples to illustrate your concerns, and
suggest alternatives.
Explain your views as clearly as possible, avoiding the
use of profanity or personal threats.
Make sure to submit your comments by the comment period
deadline identified.
Availability of Related Information
A number of documents relevant to this rulemaking are available on
EPA Web sites. The ISA for Ozone and Related Photochemical Oxidants is
available on the EPA's National Center for Environmental Assessment
(NCEA) Web site. To obtain this document, go to http://www.epa.gov/ncea, and click on Ozone in the Quick Finder section. This will open a
page with a link to the February 2013 ISA. The 2014 Policy Assessment
(PA), Health and Welfare Risk and Exposure Assessments (HREA and WREA,
respectively), and other related technical documents are available on
EPA's Office of Air Quality Planning and Standards (OAQPS) Technology
Transfer Network (TTN) Web site. The final 2014 PA is available at:
http://www.epa.gov/ttn/naaqs/standards/ozone/s_o3_2008_pa.html, and the
final 2014 Health and Welfare Risk and Exposure Assessments and other
related technical documents are available at: http://www.epa.gov/ttn/naaqs/standards/ozone/s_o3_2008_rea.html. These and other related
documents are also available for inspection and copying in the EPA
docket identified above.
Environmental Justice
Analyses evaluating the potential implications of a revised
O3 NAAQS for environmental justice populations are discussed
in appendix 9A of the Regulatory Impact Analysis (RIA) that accompanies
this notice of proposed rulemaking. The RIA is available on the Web,
through the EPA's Technology Transfer Network Web site at http://www.epa.gov/ttn/naaqs/standards/ozone/s_o3_index.html.
Table of Contents
The following topics are discussed in this preamble:
I. Background
A. Executive Summary
B. Legislative Requirements
C. Related Control Programs To Implement O3 Standards
D. Review of Air Quality Criteria and Standards for
O3
E. Ozone Air Quality
II. Rationale for Proposed Decision on the Primary Standard
A. Approach
B. Health Effects Information
1. Overview of Mechanisms
2. Nature of Effects
3. Adversity of O3 Effects
4. Ozone-Related Impacts on Public Health
C. Human Exposure and Health Risk Assessments
1. Air Quality Adjustment
2. Exposure Assessment
3. Quantitative Health Risk Assessments
D. Conclusions on the Adequacy of the Current Primary Standard
1. Summary of Evidence-Based Considerations in the PA
2. Summary of Exposure- and Risk-Based Considerations in the PA
3. Policy Assessment Conclusions on the Current Standard
4. CASAC Advice
5. Administrator's Proposed Conclusions Concerning the Adequacy
of the Current Standard
E. Conclusions on the Elements of the Primary Standard
1. Indicator
2. Averaging Time
3. Form
4. Level
F. Proposed Decision on the Primary Standard
III. Communication of Public Health Information
IV. Rationale for Proposed Decision on the Secondary Standard
A. Approach
B. Welfare Effects Information
1. Nature of Effects and Biologically Relevant Exposure Metric
2. Potential Impacts on Public Welfare
C. Exposure and Risk Assessment Information
1. Air Quality Analyses
2. Tree Seedling Growth, Productivity, Carbon Storage and
Associated Ecosystem Services
3. Crop Yield
4. Visible Foliar Injury
D. Conclusions on Adequacy of the Current Secondary Standard
1. Evidence- and Exposure/Risk-Based Considerations in the
Policy Assessment
2. CASAC Advice
3. Administrator's Proposed Conclusions on Adequacy of the
Current Standard
E. Consideration of Alternative Secondary Standards
1. Indicator
2. Consideration of a Cumulative, Seasonal Exposure-based
Standard in the Policy Assessment
3. CASAC Advice
4. Air Quality Analyses
5. Administrator's Proposed Conclusions
F. Proposed Decision on the Secondary Standard
V. Appendix U: Interpretation of the Primary and Secondary NAAQS for
O3
A. Background
B. Data Selection Requirements
C. Data Reporting and Data Handling Requirements
D. Considerations for the Possibility of a Distinct Secondary
Standard
E. Exceptional Events Information Submission Schedule
VI. Ambient Monitoring Related to Proposed O3 Standards
A. Background
B. Revisions to the Length of the Required O3
Monitoring Seasons
C. Revisions to the Photochemical Assessment Monitoring Stations
(PAMS)
1. Network Design
2. Speciated VOC Measurements
3. Carbonyl Sampling
4. Nitrogen Oxides Sampling
5. Meteorology Measurements
6. PAMS Season
7. Timing and Other Implementation Issues
D. Addition of a New Federal Reference Method (FRM) for
O3
E. Revisions to the Procedures for Testing Performance
Characteristics and Determining Comparability Between Candidate
Methods and Reference Methods
VII. Implementation of Proposed O3 Standards
A. NAAQS Implementation Plans
1. Background
2. Timing of Rules and Guidance
3. Section 110 State Implementation Plans
4. Nonattainment Area Requirements
B. Implementing a Distinct Secondary O3 NAAQS, if One
is Established
C. Designation of Areas
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D. Prevention of Significant Deterioration and Nonattainment New
Source Review Programs for the Proposed Revised Primary and
Secondary O3 NAAQS
1. Prevention of Significant Deterioration (PSD)
2. Nonattainment New Source Review
E. Transportation and General Conformity Programs
1. What are transportation and general conformity?
2. Why is the EPA discussing transportation and general
conformity in this proposed rulemaking?
3. When would transportation and general conformity apply to
areas designated nonattainment for a revised O3 NAAQS, if
one is established?
4. Will transportation and general conformity apply to a
distinct secondary O3 NAAQS, if one is established?
5. What impact would the implementation of a revised
O3 NAAQS have on a State's transportation and/or general
conformity SIP?
F. How Background O3 Is Addressed in CAA
Implementation Provisions
1. Introduction
2. Exceptional Events Exclusions
3. Rural Transport Areas
4. International Transport
VIII. Statutory and Executive Order Reviews
A. Executive Order 12866: Regulatory Planning and Review and
Executive Order 13563: Improving Regulation and Regulatory Review
B. Paperwork Reduction Act
C. Regulatory Flexibility Act
D. Unfunded Mandates Reform Act
E. Executive Order 13132: Federalism
F. Executive Order 13175: Consultation and Coordination With
Indian Tribal Governments
G. Executive Order 13045: Protection of Children From
Environmental Health & Safety Risks
H. Executive Order 13211: Actions That Significantly Affect
Energy Supply, Distribution, or Use
I. National Technology Transfer and Advancement Act
J. Executive Order 12898: Federal Actions To Address
Environmental Justice in Minority Populations and Low-Income
Populations
References
I. Background
A. Executive Summary
This section summarizes information about the purpose of this
regulatory action (I.A.1), the major provisions of this proposal
(I.A.2), and provisions related to implementation (I.A.3).
1. Purpose of This Regulatory Action
Sections 108 and 109 of the Clean Air Act (CAA) govern the
establishment, review, and revision, as appropriate, of the NAAQS to
protect public health and welfare. The CAA requires the EPA to
periodically review the air quality criteria--the science upon which
the standards are based--and the standards themselves. This rulemaking
is being conducted pursuant to these statutory requirements. The
schedule for completing this review is established by a federal court
order, which requires that the EPA sign a proposal by December 1, 2014,
and make a final determination by October 1, 2015.
The EPA completed its most recent review of the O3 NAAQS
in 2008. As a result of that review, EPA took four principal actions:
(1) Revised the level of the 8-hour primary O3 standard to
0.075 parts per million (ppm); (2) expressed the standard to three
decimal places; (3) revised the 8-hour secondary O3 standard
by making it identical to the revised primary standard; and (4) made
conforming changes to the AQI for O3.
In subsequent litigation, the U.S. Court of Appeals for the
District of Columbia Circuit upheld the EPA's 2008 primary
O3 standard, but remanded the 2008 secondary standard. State
of Mississippi v. EPA, 744 F. 3d 1334 (D.C. Cir. 2013). With respect to
the primary standard, the court held that the EPA reasonably determined
that the existing primary standard, set in 1997, did not protect public
health with an adequate margin of safety and required revision. In
upholding the EPA's revised primary standard, the court dismissed
arguments that the EPA should have adopted a more stringent standard.
The court remanded the secondary standard to the EPA after rejecting
the EPA's explanation for setting the secondary standard identical to
the revised 8-hour primary standard. The court held that because the
EPA had failed to identify a level of air quality requisite to protect
public welfare, the EPA's comparison between the primary and secondary
standards for determining if requisite protection for public welfare
was afforded by the primary standard failed to comply with the CAA.
This proposal reflects the Administrator's proposed conclusions
based on a review of the O3 NAAQS that began in September
2008. In conducting this review, the EPA has carefully evaluated the
currently available scientific literature on the health and welfare
effects of ozone, focusing particularly on the new literature available
since the conclusion of the previous review in 2008. In addition, the
EPA has also addressed the remand of the Agency's 2008 decision on the
secondary standard. Between 2008 and 2014, the EPA prepared draft and
final versions of the Integrated Science Assessment, the Health and
Welfare Risk and Exposure Assessments, and the Policy Assessment.
Multiple drafts of these documents were available for public review and
comment, and as required by the CAA, were peer-reviewed by the Clean
Air Scientific Advisory Committee (CASAC), an independent scientific
advisory committee established by the CAA and charged with providing
advice to the Administrator. The final documents reflect the EPA
staff's consideration of the comments and recommendations made by CASAC
and the public on draft versions of these documents.
2. Summary of Major Provisions
The EPA is proposing that the current primary O3
standard set at a level of 0.075 ppm is not requisite to protect public
health with an adequate margin of safety, and that it should be revised
to provide increased public health protection. Specifically, the EPA is
proposing to retain the indicator (ozone), averaging time (8-hour) and
form (annual fourth-highest daily maximum, averaged over 3 years) of
the existing primary O3 standard and is proposing to revise
the level of that standard to within the range of 0.065 ppm to 0.070
ppm. The EPA is proposing this revision to increase public health
protection, including for ``at-risk'' populations such as children,
older adults, and people with asthma or other lung diseases, against an
array of O3-related adverse health effects. For short-term
O3 exposures, these effects include decreased lung function,
increased respiratory symptoms and pulmonary inflammation, effects that
result in serious indicators of respiratory morbidity such as emergency
department visits and hospital admissions, and all-cause (total
nonaccidental) mortality. For long-term O3 exposures, these
health effects include a variety of respiratory morbidity effects and
respiratory mortality. Recognizing that the CASAC recommended a range
of levels from 0.060 ppm to 0.070 ppm, and that levels as low as 0.060
ppm could potentially be supported, the Administrator solicits comment
on alternative standard levels below 0.065 ppm, and as low as 0.060
ppm. However, the Administrator notes that setting a standard below
0.065 ppm, down to 0.060 ppm, would inappropriately place very little
weight on the uncertainties in the health effects evidence and
exposure/risk information. Given alternative views of the currently
available evidence and information expressed by some commenters, the
EPA is taking comment on both the Administrator's proposed decision to
revise the current primary O3 standard and the option of
retaining that standard.
In addition to proposing changes to the level of the standard, the
EPA is
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proposing conforming changes to the Air Quality Index (AQI) by
proposing to set an AQI value of 100 equal to the level of the 8-hour
primary O3 standard, and proposing adjustments to the AQI
values of 50, 150, 200 and 300.
The EPA also proposes to revise the secondary standard to provide
increased protection against vegetation-related effects on public
welfare. As an initial matter, the Administrator proposes to conclude
that air quality in terms of a three-year average seasonal W126 index
value, based on the three consecutive month period within the
O3 season with the maximum index value, with daily exposures
cumulated for the 12-hour period from 8:00 a.m. to 8:00 p.m., within
the range from 13 ppm-hrs to 17 ppm-hrs would provide the requisite
protection against known or anticipated adverse effects to the public
welfare. The EPA solicits comment on this proposed conclusion. In
considering how to achieve that level of air quality, the Administrator
recognizes that air quality data analyses suggest that air quality in
terms of three-year average W126 index values of a range at or below 13
to 17 ppm-hrs would be provided by a secondary standard level within
the range of 0.065 to 0.070 ppm, and that to the extent areas need to
take action to attain a standard in the range of 0.065 to 0.070 ppm,
those actions would also improve air quality as measured by the W126
metric. Thus, the Administrator proposes to revise the level of the
current secondary standard to within the range of 0.065 to 0.070 ppm.
The EPA solicits comments on this proposed revision of the secondary
standard.
The EPA also solicits comments on the alternative approach of
revising the secondary standard to a W126-based form, averaged over
three years, with a level within the range of 13 ppm-hrs to 17 ppm-hrs.
The EPA additionally solicits comments on such a distinct secondary
standard with a level within the range extending below 13 ppm-hrs down
to 7 ppm-hrs. Further, the EPA solicits comments on retaining the
current secondary standard without revision, along with the alternative
views of the evidence that would support retaining the current
standard.
3. Provisions Related to Implementation
As directed by the CAA, reducing pollution to meet national air
quality standards always has been a shared task, one involving the
federal government, states, tribes and local air agencies. This
partnership has proved effective since the EPA first issued
O3 standards more than three decades ago, and is evidenced
by significantly lower O3 levels throughout the country. To
provide a foundation that helps air agencies build successful
strategies for attaining new O3 standards, the EPA will
continue to move forward with federal regulatory programs, such as the
proposed Clean Power Plan and the final Tier 3 motor vehicle emissions
standards. To facilitate the development of CAA-compliant
implementation plans and strategies to attain new standards, the EPA
intends to issue timely and appropriate implementation guidance and,
where appropriate and consistent with the law, new rulemakings to
streamline regulatory burdens and provide flexibility in
implementation. In addition, given the regional nature of O3
air pollution, the EPA will continue to work with states to address
interstate transport of O3 and O3 precursors.
This notice contains several proposed provisions related to
implementation of the proposed standards. In addition to revisions to
the primary and secondary NAAQS, the EPA is proposing to make
corresponding revisions in data handling conventions for O3;
to revise regulations for the Prevention of Significant Deterioration
(PSD) permitting program to add a provision grandfathering certain
pending permits from certain requirements with respect to the proposed
revisions to the O3 NAAQS; and to convey schedules and
information related to implementing any revised standards.
In conjunction with proposing exceptional event schedules related
to implementing any revised O3 standards, the EPA is also
proposing to extend the new schedule approach to other future revised
NAAQS and to remove obsolete regulatory language associated with
expired exceptional event deadlines for historical standards for both
O3 and other NAAQS pollutants. The EPA is also proposing to
make minor changes to the procedures and time periods for evaluating
potential FRMs and equivalent methods, including making the
requirements for nitrogen dioxide consistent with the requirements for
O3, and removing an obsolete requirement for the annual
submission of documentation by manufacturers of certain particulate
matter monitors.
B. Legislative Requirements
Two sections of the CAA govern the establishment and revision of
the NAAQS. Section 108 (42 U.S.C. 7408) directs the Administrator to
identify and list certain air pollutants and then to issue air quality
criteria for those pollutants. The Administrator is to list those air
pollutants that in her ``judgment, cause or contribute to air pollution
which may reasonably be anticipated to endanger public health or
welfare;'' ``the presence of which in the ambient air results from
numerous or diverse mobile or stationary sources;'' and ``for which . .
. [the Administrator] plans to issue air quality criteria . . . .'' Air
quality criteria are intended 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 . . . .'' 42
U.S.C. 7408(b). Section 109 (42 U.S.C. 7409) directs the Administrator
to propose and promulgate ``primary'' and ``secondary'' NAAQS for
pollutants for which air quality criteria are issued. Section 109(b)(1)
defines a primary standard as one ``the attainment and maintenance of
which in the judgment of the Administrator, based on such criteria and
allowing an adequate margin of safety, are requisite to protect the
public health.'' \1\ 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
such criteria, is requisite to protect the public welfare from any
known or anticipated adverse effects associated with the presence of
[the] pollutant in the ambient air.'' \2\
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\1\ The legislative history of section 109 indicates that a
primary standard is to be set at ``the maximum permissible ambient
air level . . . which will protect the health of any [sensitive]
group of the population,'' and that, for this purpose, ``reference
should be made to a representative sample of persons comprising the
sensitive group rather than to a single person in such a group.'' S.
Rep. No. 91-1196, 91st Cong., 2d Sess. 10 (1970).
\2\ 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, man-made 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.''
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The requirement that primary standards provide an adequate margin
of safety was intended to address uncertainties associated with
inconclusive scientific and technical information available at the time
of standard setting. It was also intended to provide a reasonable
degree of protection against hazards that research has not yet
identified. See State of Mississippi v. EPA, 744 F. 3d 1334, 1353 (D.C.
Cir. 2013) (``By requiring an `adequate margin of safety', Congress was
directing EPA to build a buffer to protect against uncertain and
unknown dangers to human health''); see also Lead Industries
Association v. EPA, 647 F.2d 1130, 1154 (D.C. Cir 1980); American
Petroleum Institute v. Costle,
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665 F.2d 1176, 1186 (D.C. Cir. 1981); American Farm Bureau Federation
v. EPA, 559 F. 3d 512, 533 (D.C. Cir. 2009); Association of Battery
Recyclers v. EPA, 604 F. 3d 613, 617-18 (D.C. Cir. 2010). 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, in 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 may pose an unacceptable risk of harm, even if the risk is not
precisely identified as to nature or degree. The CAA does not require
the Administrator to establish a primary NAAQS at a zero-risk level or
at background concentrations, see Lead Industries v. EPA, 647 F.2d at
1156 n.51; State of Mississippi v. EPA, 744 F. 3d at 1351, but rather
at a level that reduces risk sufficiently so as to protect public
health with an adequate margin of safety.
In addressing the requirement for an adequate margin of safety, the
EPA considers such factors as the nature and severity of the health
effects, the size of sensitive population(s) \3\ at risk, and the kind
and degree of the uncertainties that must be addressed. The selection
of any particular approach for providing an adequate margin of safety
is a policy choice left specifically to the Administrator's judgment.
See Lead Industries Association v. EPA, 647 F.2d at 1161-62; State of
Mississippi, 744 F. 3d at 1353.
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\3\ As used here and similarly throughout this document, the
term ``population'' refers to people having a quality or
characteristic in common, including a specific pre-existing illness
or a specific age or life stage.
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In setting primary and secondary standards that are ``requisite''
to protect public health and welfare, respectively, as provided in
section 109(b), the EPA's task is to establish standards that are
neither more nor less stringent than necessary for these purposes. In
so doing, the EPA may not consider the costs of implementing the
standards. See generally, Whitman v. American Trucking Associations,
531 U.S. 457, 465-472, 475-76 (2001). Likewise, ``[a]ttainability and
technological feasibility are not relevant considerations in the
promulgation of national ambient air quality standards.'' American
Petroleum Institute v. Costle, 665 F. 2d at 1185.
Section 109(d)(1) requires that ``not later than December 31, 1980,
and at 5-year intervals thereafter, the Administrator shall complete a
thorough review of the criteria published under section 108 and the
national ambient air quality standards . . . and shall make such
revisions in such criteria and standards and promulgate such new
standards as may be appropriate . . . .'' Section 109(d)(2) requires
that an independent scientific review committee ``shall complete a
review of the criteria . . . and the national primary and secondary
ambient air quality standards . . . and shall recommend to the
Administrator any new . . . standards and revisions of existing
criteria and standards as may be appropriate . . . .'' Since the early
1980's, the Clean Air Scientific Advisory Committee (CASAC) has
performed this independent review function.\4\
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\4\ Lists of CASAC members and of members of the CASAC Ozone
Review Panel are available at: http://yosemite.epa.gov/sab/sabpeople.nsf/WebCommitteesSubCommittees/Ozone%20Review%20Panel.
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C. Related Control Programs To Implement O3 Standards
States are primarily responsible for ensuring attainment and
maintenance of ambient air quality standards once the EPA has
established them. Under section 110 of the CAA, and related provisions,
states are to submit, for the EPA's approval, state implementation
plans (SIPs) 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 the EPA, also
administer the PSD program (CAA sections 160 to 169). In addition,
federal programs provide for nationwide reductions in emissions of
O3 precursors and other air pollutants through the federal
motor vehicle and motor vehicle fuel control program under title II of
the CAA (sections 202 to 250) which involves controls for emissions
from mobile sources and controls for the fuels used by these sources,
and new source performance standards for stationary sources under
section 111 of the CAA. For some stationary sources, the national
emissions standards for hazardous air pollutants under section 112 of
the CAA may provide ancillary reductions in O3 precursors.
After the EPA establishes a new or revised NAAQS, the CAA directs
the EPA and the states to take steps to ensure that the new or revised
NAAQS is met. One of the first steps, known as the initial area
designations, involves identifying areas of the country that either are
attaining or not attaining the new or revised NAAQS along with the
nearby areas that contribute to the violations. Upon designation of
nonattainment areas, certain states would then be required to develop
SIPs to attain the standards. In developing their attainment plans,
states would first take into account projected emission reductions from
federal and state rules that have been already adopted at the time of
plan submittal. A number of significant emission reduction programs
that will lead to reductions of O3 precursors are in place
today or are expected to be in place by the time any new SIPs will be
due. Examples of such rules include the Nitrogen Oxides
(NOX) SIP Call, Clean Air Interstate Rule (CAIR), and Cross-
State Air Pollution Rule (CSAPR),\5\ regulations controlling onroad and
nonroad engines and fuels, the utility and industrial boilers hazardous
air pollutant rules, and various other programs already adopted by
states to reduce emissions from key emissions sources. States would
then evaluate the level of additional emission reductions needed for
each nonattainment area to attain the O3 standards ``as
expeditiously as practicable,'' and adopt new state regulations as
appropriate. Section VII of this preamble includes additional
discussion of designation and implementation issues associated with any
revised O3 NAAQS.
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\5\ The Cross-State Air Pollution Rule was recently upheld by
the Supreme Court in Environmental Protection Agency v. EME Homer
City Generation, U.S. (2014). The DC Circuit has since lifted the
stay of the rule. Order, Document #1518738, EME Homer City
Generation, L.P. v. EPA, Case #11-1302 (D.C. Cir. Oct. 23, 2014).
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D. Review of Air Quality Criteria and Standards for O3
The EPA first established primary and secondary NAAQS for
photochemical oxidants in 1971 (36 FR 8186, April 30, 1971). The EPA
set both primary and secondary standards at a level of 0.08 parts per
million (ppm), 1-hr average, total photochemical oxidants, not to be
exceeded more than one hour per year. The EPA based the standards on
scientific information contained in the 1970 Air Quality Criteria for
Photochemical Oxidants (U.S. DHEW, 1970). The EPA initiated the first
periodic review of the NAAQS for photochemical oxidants in 1977. Based
on the 1978 Air Quality Criteria for Ozone and Other Photochemical
Oxidants (U.S. EPA, 1978), the EPA published proposed revisions to the
original NAAQS in 1978 (43 FR 16962) and final revisions in 1979 (44 FR
8202). At that time, the EPA revised the level of the primary and
secondary standards from 0.08 to 0.12 ppm and changed the
[[Page 75239]]
indicator from photochemical oxidants to O3, and the form of
the standards from a deterministic (i.e., not to be exceeded more than
one hour per year) to a statistical form. This statistical form defined
attainment of the standards as occurring when the expected number of
days per calendar year with maximum hourly average concentration
greater than 0.12 ppm equaled one or less.
Following the final decision in the 1979 review, the City of
Houston challenged the Administrator's decision arguing that the
standard was arbitrary and capricious because natural O3
concentrations and other physical phenomena in the Houston area made
the standard unattainable in that area. The U.S. Court of Appeals for
the District of Columbia Circuit (D.C. Circuit) rejected this argument,
holding (as noted above) that attainability and technological
feasibility are not relevant considerations in the promulgation of the
NAAQS. The court also noted that the EPA need not tailor the NAAQS to
fit each region or locale, pointing out that Congress was aware of the
difficulty in meeting standards in some locations and had addressed
this difficulty through various compliance related provisions in the
CAA. See API v. Costle, 665 F.2d 1176, 1184-6 (D.C. Cir. 1981).
In 1982, the EPA announced plans to revise the 1978 Air Quality
Criteria document (47 FR 11561), and in 1983, the EPA initiated the
second periodic review of the O3 NAAQS (48 FR 38009). The
EPA subsequently published the 1986 Air Quality Criteria for Ozone and
Other Photochemical Oxidants (U.S. EPA, 1986) and the 1989 Staff Paper
(U.S. EPA, 1989). Following publication of the 1986 Air Quality
Criteria Document (AQCD), a number of scientific abstracts and articles
were published that appeared to be of sufficient importance concerning
potential health and welfare effects of O3 to warrant
preparation of a Supplement (U.S. EPA, 1992). On August 10, 1992, under
the terms of a court order, the EPA published a proposed decision to
retain the existing primary and secondary standards (57 FR 35542). The
notice explained that the proposed decision would complete the EPA's
review of information on health and welfare effects of O3
assembled over a 7-year period and contained in the 1986 AQCD and its
1992 Supplement. The proposal also announced the EPA's intention to
proceed as rapidly as possible with the next review of the air quality
criteria and standards for O3 in light of emerging evidence
of health effects related to 6- to 8-hour O3 exposures. On
March 9, 1993, the EPA concluded the review by affirming its proposed
decision to retain the existing primary and secondary standards (58 FR
13008).
In August 1992, the EPA announced plans to initiate the third
periodic review of the air quality criteria and O3 NAAQS (57
FR 35542). In December 1996, the EPA proposed to replace the then-
existing 1-hour primary and secondary standards with 8-hour average
O3 standards set at a level of 0.08 ppm (equivalent to 0.084
ppm using standard rounding conventions) (61 FR 65716). The EPA also
proposed to establish a new distinct secondary standard using a
biologically based cumulative, seasonal form. The EPA completed this
review on July 18, 1997 (62 FR 38856) by setting the primary standard
at a level of 0.08 ppm, based on the annual fourth-highest daily
maximum 8-hr average concentration, averaged over three years, and
setting the secondary standard identical to the revised primary
standard. In reaching this decision, the EPA identified several reasons
supporting its decision to reject a potential alternate standard set at
0.07 ppm. Most importantly, the EPA pointed out the scientific
uncertainty at lower concentrations and placed significant weight on
the fact that no CASAC panel member supported a standard level set
lower than 0.08 ppm (62 FR 38868). In addition to noting the
uncertainties in the health evidence for exposure concentrations below
0.08 ppm and the advice of CASAC, the EPA noted that a standard set at
a level of 0.07 ppm would be closer to peak background concentrations
that infrequently occur in some areas due to nonanthropogenic sources
of O3 precursors (62 FR 38856, 38868; July 18, 1997).
On May 14, 1999, in response to challenges by industry and others
to the EPA's 1997 decision, the D.C. Circuit remanded the O3
NAAQS to the EPA, finding that section 109 of the CAA, as interpreted
by the EPA, effected an unconstitutional delegation of legislative
authority. American Trucking Assoc. v. EPA, 175 F.3d 1027, 1034-1040
(D.C. Cir. 1999) (``ATA I''). In addition, the court directed that, in
responding to the remand, the EPA should consider the potential
beneficial health effects of O3 pollution in shielding the
public from the effects of solar ultraviolet (UV) radiation, as well as
adverse health effects. Id. at 1051-53. In 1999, the EPA petitioned for
rehearing en banc on several issues related to that decision. The court
granted the request for rehearing in part and denied it in part, but
declined to review its ruling with regard to the potential beneficial
effects of O3 pollution. 195 F.3d 4, 10 (D.C. Cir., 1999)
(``ATA II''). On January 27, 2000, the EPA petitioned the U.S. Supreme
Court for certiorari on the constitutional issue (and two other
issues), but did not request review of the ruling regarding the
potential beneficial health effects of O3. On February 27,
2001, the U.S. Supreme Court unanimously reversed the judgment of the
D.C. Circuit on the constitutional issue. Whitman v. American Trucking
Assoc., 531 U.S. 457, 472-74 (2001) (holding that section 109 of the
CAA does not delegate legislative power to the EPA in contravention of
the Constitution). The Court remanded the case to the D.C. Circuit to
consider challenges to the O3 NAAQS that had not been
addressed by that court's earlier decisions. On March 26, 2002, the
D.C. Circuit issued its final decision on remand, finding the 1997
O3 NAAQS to be ``neither arbitrary nor capricious,'' and so
denying the remaining petitions for review. American Trucking
Associations, Inc. v. EPA, 283 F.3d 355, 379 (D.C. Cir., 2002) (``ATA
III'').
Specifically, in ATA III, the D.C. Circuit upheld the EPA's
decision on the 1997 O3 standard as the product of reasoned
decision-making. With regard to the primary standard, the court made
clear that the most important support for EPA's decision to revise the
standard was the health evidence of insufficient protection afforded by
the then-existing standard (``the record is replete with references to
studies demonstrating the inadequacies of the old one-hour standard''),
as well as extensive information supporting the change to an 8-hour
averaging time. 283 F.3d at 378. The court further upheld the EPA's
decision not to select a more stringent level for the primary standard
noting ``the absence of any human clinical studies at ozone
concentrations below 0.08 [ppm]'' which supported EPA's conclusion that
``the most serious health effects of ozone are `less certain' at low
concentrations, providing an eminently rational reason to set the
primary standard at a somewhat higher level, at least until additional
studies become available.'' Id. (internal citations omitted). The Court
also pointed to the significant weight that the EPA properly placed on
the advice it received from CASAC. Id. at 379. In addition, the court
noted that ``although relative proximity to peak background
O3 concentrations did not, in itself, necessitate a level of
0.08 [ppm], EPA could consider that factor when choosing among the
three alternative levels.'' Id.
Independently of the litigation, the EPA responded to the court's
remand to
[[Page 75240]]
consider the potential beneficial health effects of O3
pollution in shielding the public from effects of UV radiation. The EPA
provisionally determined that the information linking changes in
patterns of ground-level O3 concentrations to changes in
relevant patterns of exposures to UV radiation of concern to public
health was too uncertain, at that time, to warrant any relaxation in
1997 O3 NAAQS. The EPA also expressed the view that any
plausible changes in UV-B radiation exposures from changes in patterns
of ground-level O3 concentrations would likely be very small
from a public health perspective. In view of these findings, the EPA
proposed to leave the 1997 8-hour NAAQS unchanged (66 FR 57268, Nov.
14, 2001). After considering public comment on the proposed decision,
the EPA published its final response to this remand on January 6, 2003,
re-affirming the 8-hour O3 NAAQS set in 1997 (68 FR 614).
The EPA initiated the fourth periodic review of the air quality
criteria and O3 standards in September 2000 with a call for
information (65 FR 57810). The schedule for completion of that review
was ultimately governed by a consent decree resolving a lawsuit filed
in March 2003 by plaintiffs representing national environmental and
public health organizations, who maintained that the EPA was in breach
of a mandatory legal duty to complete review of the O3 NAAQS
within a statutorily mandated deadline. On July 11, 2007, the EPA
proposed to revise the level of the primary standard within a range of
0.075 to 0.070 ppm (72 FR 37818). Documents supporting this proposed
decision included the Air Quality Criteria for Ozone and Other
Photochemical Oxidants (U.S. EPA, 2006a) and the Staff Paper (U.S. EPA,
2007) and related technical support documents. The EPA also proposed
two options for revising the secondary standard: (1) Replace the
current standard with a cumulative, seasonal standard, expressed as an
index of the annual sum of weighted hourly concentrations cumulated
over 12 daylight hours during the consecutive 3-month period within the
O3 season with the maximum index value, set at a level
within the range of 7 to 21 ppm-hrs, or (2) set the secondary standard
identical to the proposed primary standard. The EPA completed the
review with publication of a final decision on March 27, 2008 (73 FR
16436). In that final rule, the EPA revised the NAAQS by lowering the
level of the 8-hour primary O3 standard from 0.08 ppm to
0.075 ppm, not otherwise revising the primary standard, and adopting a
secondary standard identical to the revised primary standard. In May
2008, state, public health, environmental, and industry petitioners
filed suit challenging the EPA's final decision on the 2008
O3 standards. On September 16, 2009, the EPA announced its
intention to reconsider the 2008 O3 standards, and initiated
a rulemaking to do so. At the EPA's request, the Court held the
consolidated cases in abeyance pending the EPA's reconsideration of the
2008 decision.
On January 19, 2010 (75 FR 2938), the EPA issued a notice of
proposed rulemaking to reconsider the 2008 final decision. In that
notice, the EPA proposed that further revisions of the primary and
secondary standards were necessary to provide a requisite level of
protection to public health and welfare. The EPA proposed to decrease
the level of the 2008 8-hour primary standard from 0.075 ppm to a level
within the range of 0.060 to 0.070 ppm, and to change the secondary
standard to a new cumulative, seasonal standard expressed as an annual
index of the sum of weighted hourly concentrations, cumulated over 12
hours per day (8 a.m. to 8 p.m.), during the consecutive 3-month period
within the O3 season with a maximum index value, set at a
level within the range of 7 to 15 ppm-hours. The Agency also solicited
CASAC review of the proposed rule on January 25, 2010 and solicited
additional CASAC advice on January 26, 2011. After considering comments
from CASAC and the public, the EPA prepared a draft final rule, which
was submitted for interagency review pursuant to Executive Order 12866.
On September 2, 2011, consistent with the direction of the President,
the Administrator of the Office of Information and Regulatory Affairs
(OIRA), Office of Management and Budget (OMB), returned the draft final
rule to the EPA for further consideration. In view of this return and
the fact that the Agency's next periodic review of the O3
NAAQS required under CAA section 109 had already begun (as announced on
September 29, 2008), the EPA deferred the decisions involved in the
reconsideration until it completed its statutorily required periodic
review.
In light of EPA's decision to consolidate the reconsideration with
the current review, the D.C. Circuit proceeded with the litigation on
the 2008 final decision. On July 23, 2013, the Court upheld the EPA's
2008 primary O3 standard, but remanded the 2008 secondary
standard to the EPA. State of Mississippi v. EPA, 744 F.3d 1334. With
respect to the primary standard, the court first held that the EPA
reasonably determined that the existing standard was not requisite to
protect public health with an adequate margin of safety, and
consequently required revision. Specifically, the court noted that
there were ``numerous epidemiologic studies linking health effects to
exposure to ozone levels below 0.08 ppm and clinical human exposure
studies finding a causal relationship between health effects and
exposure to ozone levels at and below 0.08 ppm.'' 744 F.3d at 1345. The
court also specifically endorsed the weight of evidence approach
utilized by the EPA in its deliberations. Id. at 1344.
The court went on to reject arguments that the EPA should have
adopted a more stringent primary standard. Dismissing arguments that a
clinical study (as properly interpreted by the EPA) showing effects at
0.06 ppm necessitated a standard level lower than that selected, the
court noted that this was a single, limited study. Id. at 1350. With
respect to the epidemiologic evidence, the court accepted the EPA's
argument that there could be legitimate uncertainty that a causal
relationship between O3 and 8-hour exposures less than 0.075
ppm exists, so that associations at lower levels reported in
epidemiologic studies did not necessitate a more stringent standard.
Id. at 1351-52.\6\
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\6\ The court cautioned, however, that ``perhaps more [clinical]
studies like the Adams studies will yet reveal that the 0.060 ppm
level produces significant adverse decrements that simply cannot be
attributed to normal variation in lung function,'' and further
cautioned that ``agencies may not merely recite the terms
`substantial uncertainty' as a justification for their actions.''
Id. at 1350, 1357 (internal citations omitted).
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The court also rejected arguments that an 8-hour primary standard
of 0.075 ppm failed to provide an adequate margin of safety, noting
that margin of safety considerations involved policy judgments by the
agency, and that by setting a standard ``appreciably below'' the level
of the current standard (0.08 ppm), the agency had made a reasonable
policy choice. Id. Finally, the court rejected arguments that the EPA's
decision was inconsistent with CASAC's scientific recommendations
because CASAC had been insufficiently clear in its recommendations
whether it was providing scientific or policy recommendations, and the
EPA had reasonably addressed CASAC's policy recommendations. Id. at
1357-58.
With respect to the secondary standard, the court held that because
the EPA had failed to identify a level of air quality requisite to
protect public welfare, the EPA's comparison between
[[Page 75241]]
the primary and secondary standards for determining if requisite
protection for public welfare was afforded by the primary standard did
not comply with the CAA. The court thus rejected the EPA's explanation
for setting the secondary standard identical to the revised 8-hour
primary standard, and remanded the secondary standard to the EPA. Id.
at 1360-62.
At the time of the court's decision, the EPA had already completed
significant portions of its next statutorily required periodic review
of the O3 NAAQS. On September 29, 2008, the EPA announced
the initiation of a new periodic review of the air quality criteria for
O3 and related photochemical oxidants and issued a call for
information in the Federal Register (73 FR 56581, Sept. 29, 2008). A
wide range of external experts, as well as the EPA staff, representing
a variety of areas of expertise (e.g., epidemiology, human and animal
toxicology, statistics, risk/exposure analysis, atmospheric science,
ecology, biology, plant science, ecosystem services) participated in a
workshop. This workshop was held on October 28-29, 2008 in Research
Triangle Park, NC. The workshop provided an opportunity for a public
discussion of the key policy-relevant issues around which the EPA would
structure this O3 NAAQS review and the most meaningful new
science that would be available to inform our understanding of these
issues.
Based in part on the workshop discussions, the EPA developed a
draft Integrated Review Plan (IRP) outlining the schedule, process, and
key policy-relevant questions that would guide the evaluation of the
air quality criteria for O3 and the review of the primary
and secondary O3 NAAQS. A draft of the IRP was released for
public review and comment in September 2009. This IRP was the subject
of a consultation with the CASAC on November 13, 2009 (74 FR 54562;
October 22, 2009).\7\ The EPA considered comments received from that
consultation and from the public in finalizing the plan and in
beginning the review of the air quality criteria. The EPA's overall
plan and schedule for this review is presented in the Integrated Review
Plan for the Ozone National Ambient Air Quality Standards.\8\
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\7\ See http://yosemite.epa.gov/sab/sabproduct.nsf/WebProjectsbyTopicCASAC!OpenView for more information on CASAC
activities related to the current O3 NAAQS review.
\8\ EPA 452/R-11-006; April 2011; Available: http://www.epa.gov/ttn/naaqs/standards/ozone/data/2011_04_OzoneIRP.pdf.
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As part of the process of preparing the O3 ISA, the
EPA's NCEA hosted a workshop to review and discuss preliminary drafts
of key sections of the ISA on August 6, 2010 (75 FR 42085, July 20,
2010). The CASAC and the public reviewed the first external review
draft ISA (U.S. EPA, 2011a; 76 FR 10893, February 28, 2011) at a
meeting held in May 19-20, 2011 (76 FR 23809; April 28, 2011). Based on
CASAC and public comments, NCEA prepared a second draft ISA (U.S. EPA,
2011b; 76 FR 60820, September 30, 2011). CASAC and the public reviewed
this draft at a January 9-10, 2012 (76 FR 236, December 8, 2011)
meeting. Based on CASAC and public comments, NCEA prepared a third
draft ISA (U.S. EPA 2012a; 77 FR 36534; June 19, 2012), which was
reviewed at a CASAC meeting in September 2012. The EPA released the
final ISA (EPA/600/R-10/076F) in February 2013.
The EPA presented its plans for conducting the Risk and Exposure
Assessments (REAs) that build on the scientific evidence presented in
the ISA, in two planning documents titled Ozone National Ambient Air
Quality Standards: Scope and Methods Plan for Health Risk and Exposure
Assessment and Ozone National Ambient Air Quality Standards: Scope and
Methods Plan for Welfare Risk and Exposure Assessment (henceforth,
Scope and Methods Plans).\9\ These planning documents outlined the
scope and approaches that staff planned to use in conducting
quantitative assessments, as well as key issues that would be addressed
as part of the assessments. The EPA released these documents for public
comment in April 2011, and consulted with CASAC on May 19-20, 2011 (76
FR 23809; April 28, 2011). In designing and conducting the initial
health risk and welfare risk assessments, the EPA considered CASAC
comments (Samet, 2011) on the Scope and Methods Plans and also
considered public comments. In May 2012, the EPA issued a memo titled
Updates to Information Presented in the Scope and Methods Plans for the
Ozone NAAQS Health and Welfare Risk and Exposure Assessments that
described changes to elements of the scope and methods plans and
provided a brief explanation of each change and the reason for it.
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\9\ EPA-452/P-11-001 and -002; April 2011; Available: http://www.epa.gov/ttn/naaqs/standards/ozone/s_o3_2008_pd.html.
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In July 2012, the EPA made the first drafts of the Health and
Welfare REAs available for CASAC review and public comment (77 FR
42495, July 19, 2012). The first draft PA \10\ was made available for
CASAC review and public comment in August 2012. These documents were
reviewed by the CASAC O3 Panel at a public meeting in
September 2012. The second draft REAs and PA, made available by the EPA
in January 2014 (79 FR 4694, January 29, 2014), were prepared with
consideration of advice from CASAC (Frey and Samet, 2012a, 2012b) and
comments from the public. These drafts were reviewed by the CASAC
O3 Panel at a public meeting on March 25-27, 2014. The CASAC
issued final reports on the second drafts of the HREA on July 1, 2014
(Frey, 2014a), and the WREA on June 18, 2014 (Frey, 2014b),
respectively. The CASAC issued a final report on the second draft PA on
June 26, 2014 (Frey, 2014c). The final versions of the HREA (U.S. EPA
2014a), WREA (U.S. EPA, 2014b), and PA (U.S. EPA, 2014c) were made
available by the EPA in August, 2014. These documents reflect staff's
consideration of the comments and recommendations made by CASAC, as
well as comments made by members of the public, in their review of the
draft versions of these documents.
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\10\ The PA is prepared by the staff in the EPA's Office of Air
Quality Planning and Standards (OAQPS). It presents a staff
evaluation of the policy implications of the key scientific and
technical information in the ISA and REAs for the EPA's
consideration. The PA provides a transparent evaluation, and staff
conclusions, regarding policy considerations related to reaching
judgments about the adequacy of the current standards, and if
revision is considered, what revisions may be appropriate to
consider. The PA is intended to help ``bridge the gap'' between the
agency's scientific assessments presented in the ISA and REAs, and
the judgments required of the EPA Administrator in determining
whether it is appropriate to retain or revise the NAAQS.
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E. Ozone Air Quality
Ozone is formed near the Earth's surface due to chemical
interactions involving solar radiation and precursor pollutants
including volatile organic compounds (VOCs), nitrogen oxides
(NOX), methane (CH4) and carbon monoxide (CO).
The precursor emissions leading to O3 formation can result
from both man-made sources (e.g., motor vehicles and electric power
generation) and natural sources (e.g., vegetation and wildfires).
Occasionally, O3 that is created naturally in the
stratosphere can also contribute to O3 levels near the
surface. Once formed, O3 can be transported by winds before
eventually being removed from the atmosphere via chemical reactions or
deposition to surfaces. In sum, O3 concentrations are
influenced by complex interactions between precursor emissions,
meteorological conditions, and surface characteristics.
In order to continuously assess O3 air pollution levels,
state and local environmental agencies operate O3 monitors
at various locations and
[[Page 75242]]
subsequently submit the data to the EPA. At present, there are
approximately 1,400 monitors across the U.S. reporting hourly
O3 averages during the times of the year when local
O3 pollution can be important. Much of this monitoring is
focused on O3 measurements in urban areas where precursor
emissions tend to be largest, as well as locations directly downwind of
these areas, but there are also over 100 sites in rural areas where
high levels of O3 can periodically exist due to transport
from upwind sources. Based on data from this national network, the EPA
estimates that approximately 133 million Americans live in counties
where O3 concentrations were above the level of the existing
health-based NAAQS of 0.075 ppm at least 4 days in 2012. High
O3 values can occur almost anywhere within the contiguous 48
states, although locations in California, Texas, and the Northeast
Corridor are especially subject to poor O3 air quality. From
a temporal perspective, the highest daily peak O3
concentrations generally tend to occur during the afternoon within the
warmer months due to higher solar radiation and other conducive
meteorological conditions during these times. The exceptions to this
general rule include: (1) Some rural sites where transport of
O3 from upwind areas of regional production can occasionally
result in high nighttime levels of O3, (2) high-elevation
sites periodically influenced by stratospheric intrusions, and (3)
certain locations in the western U.S. where large quantities of
O3 precursors emissions associated with oil and gas
development can be trapped by strong inversions associated with snow
cover during the colder months and efficiently converted to
O3.
One of the challenging aspects of developing plans to reduce
emissions leading to high O3 concentrations is that the
response of O3 to precursor reductions is nonlinear. In
particular, NOX causes both the formation and destruction of
O3. The net impact of NOX emissions on
O3 concentrations depends on the local quantities of
NOX, VOC, and sunlight which interact in a set of complex
chemical reactions. In some areas, such as urban centers where
NOX emissions typically are high, NOX leads to
the net destruction of O3, making O3 levels lower
in the immediate vicinity. This phenomenon is particularly pronounced
under conditions that lead to low O3 concentrations (i.e.
during cool, cloudy weather and at night when photochemical activity is
limited or nonexistent). However, while NOX can initially
destroy O3 near the emission sources, these same
NOX emissions eventually do react to form more O3
downwind. Photochemical model simulations suggest that the additional
expected reductions in NOX emissions will slightly increase
O3 concentrations on days with lower O3
concentrations in areas in close proximity to NOX sources,
while at the same time decreasing the highest O3
concentrations in outlying areas. See generally, U.S. EPA, 2014a
(section 2.2.1).
At present, both the primary and secondary NAAQS use the annual
fourth-highest daily maximum 8-hour concentration, averaged over 3
years, as the form of the standard. An additional air quality metric,
referred to as W126, is often used to assess cumulative impact of
O3 exposure on ecosystems and vegetation. W126 is a seasonal
aggregate of weighted hourly O3 values observed between 8
a.m. and 8 p.m. As O3 precursor emissions have decreased
across the U.S., O3 design values \11\ have concurrently
shown a modest downward trend. Ozone design values decreased by
approximately 9% on average between 2000 and 2012. Air quality model
simulations estimate that peak O3 levels will continue to
improve over the next decade as additional reductions in O3
precursors from power plants, motor vehicles, and other sources are
realized.
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\11\ A design value is a statistic that describes the air
quality status of a given location relative to the level of the
NAAQS.
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In addition to being affected by changing emissions, future
O3 concentrations will also be affected by climate change.
Modeling studies in EPA's Interim Assessment (U.S. EPA, 2009b) and
cited in support of the 2009 Endangerment Finding (74 FR 66,496; Dec.
15, 2009) show that, while the impact is not uniform, climate change
has the potential to cause increases in summertime O3
concentrations over substantial regions of the country, with increases
tending to occur during higher peak pollution episodes in the summer,
if offsetting emissions reductions are not made. Increases in
temperature are expected to be the principal factor in driving any
ozone increases, although increases in stagnation frequency may also
contribute (Jacob and Winner, 2009). These increases in O3
pollution over broad areas of the U.S., including in the largest
metropolitan areas with the worst O3 problems, increase the
risk of morbidity and mortality. Children, people with asthma or other
lung diseases, older adults, and people who are active outdoors,
including outdoor workers, are among the most vulnerable to these
O3-related health effects. If unchecked, climate change has
the potential to offset some of the improvements in O3 air
quality, and therefore some of the improvements in public health, that
are expected from reductions in emissions of O3 precursors.
Another challenging aspect of the O3 issue is the
involvement of sources of O3 and O3 precursors
beyond those from domestic, anthropogenic sources. Modeling analyses
have suggested that nationally the majority of O3
exceedances are predominantly caused by anthropogenic emissions from
within the U.S. However, observational and modeling analyses have
concluded that O3 concentrations in some locations in the
U.S. can be substantially influenced by sources that may not be suited
to domestic control measures. In particular, certain high-elevation
sites in the western U.S. are impacted by a combination of non-local
sources like international transport, stratospheric O3, and
O3 originating from wildfire emissions. Ambient
O3 from these non-local sources is collectively referred to
as background O3. See generally section 2.4 of the Policy
Assessment (U.S. EPA, 2014c). The analyses suggest that, at these
locations, there can be episodic events with substantial background
contributions where O3 concentrations approach or exceed the
level of the current NAAQS (i.e., 75 ppb). These events are relatively
infrequent and the EPA has policies that allow for the exclusion of air
quality monitoring data from design value calculations when they are
substantially affected by certain background influences. Wildfires pose
a direct threat to air quality and public safety--threats that can be
mitigated through management of wildland vegetation. The use of
wildland prescribed fire can influence the occurrence of catastrophic
wildfires which may help manage the contribution of wildfires to
background O3 levels and periodic peak O3 events.
Prescribed fire mimics a natural process necessary to manage and
maintain fire-adapted ecosystems and climate change adaptation, while
reducing risk of uncontrolled emissions from catastrophic wildfires.
Wildfire emissions may make it more challenging to meet the NAAQS.
However, the CAA requires the EPA to set the NAAQS at levels requisite
to protect public health and welfare without regard to the source of
the pollutant. API, 665 F. 2d at 1185-86. The EPA may consider
proximity to background levels as a factor in the decision whether and
how to revise the NAAQS when considering levels within the range of
reasonable values
[[Page 75243]]
supported by the air quality criteria and judgments of the
Administrator. ATA III, 283 F. 3d at 379. It is in the implementation
process that states and the EPA can address how to develop effective
public policy in locations in which background sources contribute
substantially to high O3. Section VII.F provides more detail
about how background O3 can be addressed via CAA
implementation provisions.
II. Rationale for Proposed Decision on the Primary Standard
This section presents the Administrator's rationale for her
proposed decision to revise the existing primary O3 standard
by lowering the level of the standard to within the range of 0.065 to
0.070 ppm. As discussed more fully below, this rationale draws from the
thorough review in the ISA of the available scientific evidence,
published through July 2011, on human health effects associated with
the presence of O3 in the ambient air. This rationale also
takes into account: (1) Analyses of O3 air quality, human
exposures to O3, and O3-associated health risks,
as presented and assessed in the HREA; (2) the EPA staff assessment of
the most policy-relevant scientific evidence and exposure/risk
information in the PA; (3) CASAC advice and recommendations, as
reflected in discussions of drafts of the ISA, REA, and PA at public
meetings, in separate written comments, and in CASAC's letters to the
Administrator; and (4) public input received during the development of
these documents, either in connection with CASAC meetings or
separately.
Section II.A below provides an overview of the approaches used to
consider the scientific evidence and exposure/risk information as it
relates to the primary O3 standard. This includes summaries
of the approach adopted by the Administrator in the 2008 review of the
O3 NAAQS and of the approach adopted in the PA in the
current review. Section II.B summarizes the body of evidence for health
effects attributable to short- or long-term O3 exposures,
with a focus on effects for which the ISA judges that there is a
``causal'' or a ``likely to be causal'' relationship with O3
exposures. Section II.C summarizes the HREA's quantitative estimates of
O3 exposures and health risks, including key results and
uncertainties. Sections II.D and II.E present the Administrator's
proposed conclusions on the adequacy of the current primary
O3 standard and alternative primary standards, respectively.
A. Approach
In the 2008 review of the O3 NAAQS, Administrator
Stephen L. Johnson revised the level of the 8-hour primary
O3 standard from 0.08 ppm \12\ to 0.075 ppm (75 parts per
billion (ppb) \13\). This decision was based on his consideration of
the available scientific evidence and exposure/risk information, the
advice and recommendations of CASAC, and comments from the public. The
Administrator placed primary emphasis on the body of available
scientific evidence, while viewing the results of exposure and risk
assessments as providing supporting information. Specifically, he
judged that a standard set at 75 ppb would be appreciably below the
concentration at which adverse effects had been demonstrated in the
controlled human exposure studies available at that time (i.e., 80
ppb), and would provide a significant increase in protection compared
to the then-current standard. The Administrator further concluded that
the body of evidence did not support setting a lower standard level,
given the increasing uncertainty in the evidence at lower O3
concentrations (U.S. EPA, 2014c, Chapter 1).
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\12\ Due to rounding convention, the 1997 standard level of 0.08
ppm corresponded to 0.084 ppm (84 ppb).
\13\ The level of the O3 standard is specified as
0.075 ppm rather than 75 ppb. However, in the PA we refer to ppb,
which is most often used in the scientific literature and in the
ISA, in order to avoid the confusion that could result from
switching units when discussing the evidence in relation to the
standard level. Similarly, in the preamble to this notice we refer
to ppb.
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In the current review, the EPA's approach to informing decisions on
the primary O3 standard builds upon the general approach
used in the last review and reflects the broader body of scientific
evidence, updated exposure/risk information, and advances in
O3 air quality modeling now available. This approach,
described in detail in the PA (U.S. EPA, 2014c, section 1.3.1), is
based most fundamentally on using the EPA's assessment of the available
scientific evidence and associated quantitative analyses to inform the
Administrator's judgments regarding a primary standard for
O3 that is ``requisite'' (i.e., neither more nor less
stringent than necessary) to protect public health with an adequate
margin of safety. Specifically, it is based on consideration of the
available body of scientific evidence assessed in the ISA (U.S. EPA,
2013a), exposure and risk analyses presented in the HREA (U.S. EPA,
2014a), advice and recommendations from CASAC (Frey, 2014a, c), and
public comments. Based on the application of this approach, the PA
assesses and integrates the evidence and information, and reaches
conclusions for the Administrator's consideration about the range of
policy options that could be supported. The remainder of this section
describes the PA's approach to reviewing the primary O3
standard, and to informing the Administrator's proposed decisions on
the current and alternative standards.
As an initial matter, the PA recognizes that the final decision to
retain or revise the current primary O3 standard is a public
health policy judgment to be made by the Administrator and will draw
upon the available scientific evidence for O3-attributable
health effects and on analyses of population exposures and health
risks, including judgments about the appropriate weight to assign the
range of uncertainties inherent in the evidence and analyses. The PA's
general approach to informing these public health policy judgments
recognizes that the available health effects evidence reflects a
continuum from relatively higher O3 concentrations, at which
scientists generally agree that health effects are likely to occur,
through lower concentrations, at which the likelihood and magnitude of
a response become increasingly uncertain. Therefore, the conclusions in
the PA reflect an interpretation of the available scientific evidence
and exposure/risk information that, in the views of the EPA staff,
neither overstates nor understates the strengths and limitations of
that evidence and information.\14\ This approach is consistent with the
requirements of sections 108 and 109 of the CAA, as well as with how
the EPA and the courts have historically interpreted the CAA.
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\14\ Draft versions of the PA were subject to review by CASAC
and the final PA reflects consideration of the advice received from
CASAC during the review process. CASAC concluded that ``Overall, we
find the Second Draft PA to be adequate for its intended purpose of
providing a strong scientific basis for findings regarding the
inadequacy of current primary and secondary ozone air quality
standards'' (Frey, 2014c, p. v).
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The PA draws upon an integrative synthesis of the entire body of
available scientific evidence for O3-related health effects,
including the evidence newly available in the current review and the
evidence from previous reviews, as presented in the ISA (U.S. EPA,
2013a). Consideration of the scientific evidence is based fundamentally
on information from controlled human exposure and epidemiologic
studies, supplemented by information from animal toxicology studies. In
the PA, such evidence informs the consideration of the health
[[Page 75244]]
endpoints and at-risk populations \15\ on which to focus the current
review, and the consideration of the O3 concentrations at
which various health effects can occur.
---------------------------------------------------------------------------
\15\ In this review, the term ``at-risk population'' is used to
encompass populations or lifestages that have a greater likelihood
of experiencing health effects related to exposure to an air
pollutant due to a variety of factors; other terms used in the
literature include susceptible, vulnerable, and sensitive. These
factors may be intrinsic, such as genetic factors, lifestage, or the
presence of preexisting diseases, or they may be extrinsic, such as
socioeconomic status (SES), activity pattern and exercise level, or
increased pollutant exposures (U.S. EPA 2013, p. lxx, 8-1, 8-2). The
courts and the CAA's legislative history refer to these at-risk
subpopulations as ``susceptible'' or ``sensitive'' populations. See,
e.g., American Lung Ass'n v. EPA, 134 F. 3d 388, 389 (D.C. Cir.
1998) (``NAAQS must protect not only average health individuals, but
also `sensitive citizens'--children, for example, or people with
asthma, emphysema, or other conditions rendering them particularly
vulnerable to air pollution'' (quoting S. Rep. No. 91-1196 at 10).
---------------------------------------------------------------------------
Since the 2008 review of the O3 NAAQS, the EPA has
developed formal frameworks for characterizing the strength of the
scientific evidence with regard to health effects associated with
exposures to O3 in ambient air and factors that may increase
risk in some populations or lifestages. These frameworks provide the
basis for robust, consistent, and transparent processes for evaluating
the scientific evidence, including uncertainties in the evidence, and
for drawing weight-of-evidence conclusions on air pollution-related
health effects and at-risk populations. These frameworks for
characterizing the strength of the scientific evidence are discussed in
detail in the ISA (U.S. EPA, 2013a, Preamble; Chapter 8).
With regard to characterization of health effects, the ISA uses a
five-level hierarchy to classify the overall weight of evidence into
one of the following categories: causal relationship, likely to be a
causal relationship, suggestive of a causal relationship, inadequate to
infer a causal relationship, and not likely to be a causal relationship
(U.S. EPA, 2013a, Preamble Table II). In using the weight-of-evidence
approach to inform judgments about the degree of confidence that
various health effects are likely to be caused by exposure to
O3, confidence increases as the number of studies
consistently reporting a particular health endpoint grows and as other
factors, such as biological plausibility and the strength, consistency,
and coherence of evidence, increase. Conclusions about biological
plausibility and about the consistency and coherence of O3-
related health effects are drawn from the integration of epidemiologic
studies with mechanistic information from controlled human exposure and
animal toxicological studies, as discussed in the ISA (U.S. EPA, 2013a,
EPA Framework for Causal Determination, p. 1viii). The PA places the
greatest weight on the health effects for which the evidence has been
judged in the ISA to support a ``causal'' or a ``likely to be causal''
relationship with O3 exposures.
The PA further considers the evidence base assessed in the ISA with
regard to the types and levels of exposure at which health effects are
indicated. This consideration of the evidence, which directly informs
conclusions regarding the adequacy of current or alternative standards,
differs from consideration of the evidence in the ISA with regard to
overarching determinations of causality. Therefore, studies that inform
determinations of causality may or may not be concluded to be
informative with regard to the adequacy of the current or alternative
standards.\16\
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\16\ For example, the PA judges that health studies evaluating
exposure concentrations near or below the level of the current
standard and epidemiologic studies conducted in locations meeting
the current standard are particularly informative when considering
the adequacy of the public health protection provided by the current
standard (U.S. EPA, 2014c, Chapters 3 and 4).
---------------------------------------------------------------------------
As with health endpoints, the ISA's characterization of the weight
of evidence for potential at-risk populations is based on the
evaluation and synthesis of evidence from across scientific
disciplines. The ISA uses the collective evidence to examine the
coherence of effects across disciplines and to determine the biological
plausibility of reported effects. Based on this approach, the ISA
characterizes the evidence for a number of ``factors'' that have the
potential to place populations at increased risk for O3-
related effects. The categories considered in evaluating the evidence
for these potential at-risk factors are ``adequate evidence,''
``suggestive evidence,'' ``inadequate evidence,'' and ``evidence of no
effect.'' For the ``adequate evidence'' category, the ISA concludes
that this category is appropriate when multiple high-quality studies
show ``there is substantial, consistent evidence within a discipline to
conclude that a factor results in a population or lifestage being at
increased risk of air pollutant-related health effect(s) relative to
some reference population or lifestage'' (U.S. EPA, 2013a, p. 8-2). In
addition, where applicable, the ``adequate evidence'' category reflects
a conclusion that there is coherence in the evidence across
disciplines. The other categories reflect greater uncertainty in the
evidence. In this review, the PA focuses on those factors for which the
ISA judges there is adequate evidence of increased risk (U.S. EPA,
2013a, Table 8-5). At-risk populations are discussed in more detail in
section 3.1.5 of the PA (U.S. EPA, 2014c) and these categories are
discussed in more detail in the ISA (U.S. EPA, 2013a, chapter 8, Table
8-1).
Using the available scientific evidence to inform conclusions on
the current and alternative standards is complicated by the recognition
that a population-level threshold has not been identified below which
it can be concluded with confidence that O3-attributable
effects do not occur (U.S. EPA, 2013a, section 2.5.4.4). In the absence
of a discernible threshold, the PA's general approach to considering
the available O3 health evidence involves characterizing
confidence in the extent to which O3-attributable effects
occur, and the extent to which such effects are adverse, over the
ranges of O3 exposure concentrations evaluated in controlled
human exposure studies and over the distributions of ambient
O3 concentrations in locations where epidemiologic studies
have been conducted. As noted above, the PA recognizes that the
available health effects evidence reflects a continuum from relatively
high O3 concentrations, at which scientists generally agree
that adverse health effects are likely to occur, through lower
concentrations, at which the likelihood and magnitude of a response
become increasingly uncertain. Aspects of the approach used in this
review to evaluate evidence from controlled human exposure and
epidemiologic studies, respectively, are discussed below.
Controlled human exposure studies provide direct evidence of
relationships between pollutant exposures and human health effects
(U.S. EPA, 2013a, p.lx). Controlled human exposure studies provide data
with the highest level of confidence since they provide human effects
data under closely monitored conditions and can provide exposure
response relationships. Such studies are particularly useful in
defining the specific conditions under which pollutant exposures can
result in health impacts, including the exposure concentrations,
durations, and ventilation rates under which effects can occur. As
discussed in the ISA, controlled human exposure studies provide clear
and compelling evidence for an array of human health effects that are
directly attributable to acute exposures to O3 per se (i.e.,
as opposed to O3 and other photochemical oxidants, for which
O3 is an indicator, or other co-occurring pollutants) (U.S.
EPA, 2013a, Chapter 6). Together with animal toxicological studies,
which can provide
[[Page 75245]]
information about more serious health outcomes as well as the effects
of long-term exposures and mode of action, controlled human exposure
studies also help to provide biological plausibility for health effects
observed in epidemiologic studies.
The PA considers the evidence from controlled human exposure
studies in two ways. First, the PA considers the extent to which
controlled human exposure studies provide evidence for health effects
following exposures to different O3 concentrations, down to
the lowest-observed-effects levels in those studies. Second, the PA
uses these studies to help evaluate the extent to which there is
confidence in health effect associations reported in epidemiologic
studies down through lower ambient O3 concentrations, where
the likelihood and magnitude of O3-attributable effects
become increasingly uncertain.
The PA considers the range of O3 exposure concentrations
evaluated in controlled human exposure studies, including
concentrations near or below the level of the current standard. The PA
considers both group mean responses, which provide insight into the
extent to which observed changes are due to O3 exposures
rather than to chance alone, and interindividual variability in
responses, which provides insight into the fraction of the population
that might be affected by such O3 exposures (U.S. EPA,
2013a, section 6.2.1.1). When considering the relative weight to place
on various controlled human exposure studies, the discussion in the PA
focuses on the exposure conditions evaluated (e.g., exercising versus
resting, exposure duration); the nature, magnitude, and likely
adversity of effects over the range of reported O3 exposure
concentrations; the statistical precision of reported effects; and the
consistency of results across studies for a given health endpoint and
exposure concentration. In addition, because controlled human exposure
studies typically involve healthy individuals and do not evaluate the
most sensitive individuals in the population (U.S. EPA, 2013a, Preamble
p. lx), when considering the implications of these studies for
evaluation of the current and alternative standards, the PA also
considers the extent to which reported effects are likely to reflect
the magnitude and/or severity of effects in at-risk groups.
The PA also considers epidemiologic studies of short- and long-term
O3 concentrations in ambient air. Epidemiologic studies
provide information on associations between variability in ambient
O3 concentrations and variability in various health
outcomes, including lung function decrements, respiratory symptoms,
school absences, hospital admissions, emergency department visits, and
premature mortality (U.S. EPA, 2013a, Chapters 6 and 7). Epidemiologic
studies can inform understanding of the effects in the study population
(which may include at-risk groups) of real-world exposures to the range
of O3 concentrations in ambient air, as well as provide
evidence of associations between ambient O3 levels and more
serious acute and chronic health effects that cannot be assessed in
controlled human exposure studies. For these studies, the degree of
uncertainty introduced by confounding variables (e.g., other
pollutants, temperature) and other factors (e.g., effects modifiers
such as averting behavior) affects the level of confidence that the
health effects being investigated are attributable to O3
exposures, alone and in combination with copollutants.
Available epidemiologic studies have generally not indicated a
discernible population threshold below which O3 is no longer
associated with health effects (U.S. EPA, 2013a, section 2.5.4.4).
However, the currently available epidemiologic evidence indicates
decreased confidence in reported concentration-response relationships
for O3 concentrations at the lower ends of ambient
distributions due to the low density of data in this range (U.S. EPA,
2013a, section 2.5.4.4). As discussed more fully in Chapter 1 of the PA
(U.S. EPA, 2014c), the general approach to considering the results of
epidemiologic studies within the context of the current and alternative
standards focuses on characterizing the range of ambient O3
concentrations over which studies indicate the most confidence in
O3-associated health effects, and the concentrations below
which confidence in such health effect associations becomes appreciably
lower.
In placing emphasis on specific epidemiologic studies, as in past
reviews, the discussion in the PA focuses on the epidemiologic studies
conducted in the U.S. and Canada. Such studies reflect air quality and
exposure patterns that are likely more typical of the U.S. population,
since studies conducted outside the U.S. and Canada may well reflect
different demographic and air pollution characteristics.\17\ The PA
also focuses on studies reporting associations with effects judged in
the ISA (U.S. EPA, 2013a) to be robust to confounding by other factors,
including co-occurring air pollutants.
---------------------------------------------------------------------------
\17\ Though the PA recognizes that a broader body of studies,
including international studies, informs the causal determinations
in the ISA.
---------------------------------------------------------------------------
To put staff conclusions about O3-related health effects
into a broader public health context, the PA also considers exposure
and risk estimates from the HREA, which develops and applies models to
estimate human exposures to O3 and O3-related
health risks in urban study areas across the United States (U.S. EPA,
2014a). The HREA estimates exposures of concern, based on interpreting
quantitative exposure estimates within the context of controlled human
exposure study results; lung function risks, based on applying
exposure-response relationships from controlled human exposure studies
to quantitative estimates of exposures; and epidemiologic-based risk
estimates, based on applying concentration-response relationships drawn
from epidemiologic studies to adjusted air quality. Each of these types
of assessments is discussed briefly below.
As in the 2008 review, the HREA estimates exposures at or above
benchmark concentrations of 60, 70, and 80 ppb, reflecting exposure
concentrations of concern based on the available health evidence.\18\
Estimates of exposures of concern, defined as personal exposures while
at moderate or greater exertion to 8-hour average ambient O3
levels, at or above these discrete benchmark concentrations provide
perspective on the public health risks of O3-related health
effects that have been demonstrated in controlled human exposure and
toxicological studies. However, because of a lack of exposure-response
information across a range of exposure concentrations in these studies,
these risks cannot be assessed using a quantitative risk assessment.
Though this analysis is conducted using discrete benchmark
concentrations, information from the broad body of evidence indicates
that health-relevant exposures are more appropriately viewed as a
continuum with greater confidence and certainty about the existence of
health effects at higher O3 exposure concentrations and less
confidence and certainty at lower exposure concentrations. This
approach recognizes that there is no sharp breakpoint within the
exposure-response relationship for exposure concentrations at and above
80 ppb down to 60 ppb.
---------------------------------------------------------------------------
\18\ For example, see 75 FR 2945-2946 (January 19, 2010) and 73
FR 16441-16442 (March 27, 2008) discussing ``exposures of concern.''
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Within the context of this continuum, estimates of exposures of
concern at these discrete benchmark
[[Page 75246]]
concentrations provide some perspective on the public health impacts of
O3-related health effects, such as pulmonary inflammation,
that are plausibly linked to the more serious effects seen in
epidemiologic studies but cannot be evaluated in quantitative risk
assessments. They also help elucidate the extent to which such impacts
may be reduced by meeting the current and alternative standards.
Estimates of the number of people likely to experience exposures of
concern cannot be directly translated into quantitative estimates of
the number of people likely to experience specific health effects due
to individual variability in responsiveness. Only a subset of
individuals can be expected to experience such adverse health effects,
and at-risk populations or lifestages, such as people with asthma or
children, are expected to be affected more by such exposures than
healthy adults.
The HREA also generates quantitative estimates of O3
health risks for air quality adjusted to just meet the current \19\ and
alternative standards. One approach to estimating O3 health
risks is to combine modeled exposure estimates with exposure-response
relationships derived from controlled human exposure studies of
O3-induced health effects. The HREA uses this approach to
estimate the occurrence of O3-induced lung function
decrements in at-risk populations, including school-age children,
school-age children with asthma, adults with asthma, and older adults.
The available exposure-response information does not support this
approach for other endpoints evaluated in controlled human exposure
studies (U.S. EPA, 2014a, section 2.3).
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\19\ For purposes of the exposure and risk estimates with
adjusted air quality, the REA considered any value <76 ppb to be
``just meeting'' the current 75 ppb standard (U.S. EPA, 2014a).
---------------------------------------------------------------------------
The other approach used in this review to estimate O3-
associated health risks is to apply concentration-response
relationships derived from short- and/or long-term epidemiologic
studies to air quality adjusted to just meet current and alternative
standards. The concentration-response relationships drawn from
epidemiologic studies are based on population exposure surrogates, such
as 8-hour concentrations averaged across monitors and over more than
one day (U.S. EPA, 2013a, Chapter 6). The HREA presents epidemiologic-
based risk estimates for O3-associated mortality, hospital
admissions, emergency department visits, and respiratory symptoms (U.S.
EPA, 2014a, section 2.3). These estimates are derived from the full
distributions of ambient O3 concentrations estimated for the
study locations.\20\ In addition, the HREA estimates mortality risks
associated with various portions of distributions of short-term
O3 concentrations (U.S. EPA, 2014a). The PA considers risk
estimates based on the full distributions of ambient O3
concentrations and, when available, estimates of the risk associated
with various portions of those ambient distributions.\21\ In doing so,
the PA takes note of the ISA conclusions regarding confidence in linear
concentration-response relationships over distributions of ambient
concentrations (see above), and of the extent to which health effect
associations at various ambient O3 concentrations are
supported by the evidence from experimental studies for effects
following specific O3 exposures.
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\20\ In previous reviews, including the 2008 review and
reconsideration, such risks were separately estimated for
O3 concentrations characterized as above policy-relevant
background concentrations. Policy-relevant background concentrations
were defined as the distribution of O3 concentrations
attributable to sources other than anthropogenic emissions of
O3 precursor emissions (e.g., VOC, CO, NOX) in
the U.S., Canada, and Mexico. The decision in this review to
estimate total risk across the full range of O3
concentrations reflects consideration of advice from CASAC (Frey and
Samet, 2012b).
\21\ In a series of sensitivity analyses, the HREA also
evaluates a series of threshold models for respiratory mortality
associated with long-term O3 concentrations. The PA
considers these risk estimates based on threshold models, in
addition to HREA core estimates based on the linear model (U.S. EPA,
2014a, sections 3.2.3.2, 4.4.2.3).
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B. Health Effects Information
This section outlines key information contained in the ISA (U.S.
EPA, 2013a, Chapters 4 to 8) and in the PA (U.S. EPA, 2014c, Chapters 3
and 4) on the known or potential effects on public health which may be
expected from the presence of O3 in the ambient air. The
information highlighted here summarizes: (1) New information available
on potential mechanisms for health effects associated with exposure to
O3 (II.B.1); (2) the nature of effects that have been
associated directly with both short- and long-term exposure to
O3 and indirectly with the presence of O3 in
ambient air (II.B.2); (3) considerations related to the adversity of
O3-attributable health effects (II.B.3); and (4)
considerations in characterizing the public health impact of
O3, including the identification of ``at risk'' populations
(II.B.4).
The decision in the 2008 rulemaking emphasized the large number of
epidemiologic studies published since the 1997 review that continued to
report associations with respiratory hospital admissions and emergency
department visits, as well as additional health endpoints, including
the effects of acute (short-term and prolonged) and chronic exposures
to O3 on lung function decrements and enhanced respiratory
symptoms in asthmatic individuals, school absences, and premature
mortality. It also emphasized controlled human exposure studies showing
respiratory effects with prolonged O3 exposures at levels
below 80 ppb, changes in lung host defenses, and increased airway
responsiveness, and animal toxicology studies that provided information
about mechanisms of action.
The ISA (U.S. EPA, 2013a) prepared for this review emphasizes a
large number of new epidemiologic studies published since the last
review on effects associated with both short- and long-term exposures,
including new epidemiologic studies about risk factors. It also
emphasizes important new information from controlled human exposure,
dosimetry and toxicology studies. Highlights of the new evidence
included:
(1) Two controlled human exposure studies new since the 2008
review are now available that examine respiratory effects associated
with prolonged, 6.6-hour, O3 exposures to levels of 72
ppb \22\ and 60 ppb. These studies observed effects in healthy
adults, including lung function decrements combined with respiratory
symptoms at 72 ppb, and lung function decrements and pulmonary
inflammation at 60 ppb. These studies expand on evidence of lung
function decrements with O3 exposure at 60 ppb available
in the last review, and provide new evidence of airway inflammation,
a mechanism by which O3 may cause other more serious
respiratory effects (e.g., asthma exacerbations).
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\22\ As noted below, for the 70 ppb exposure concentration,
Schelegle et al. (2009) reported that the actual mean exposure
concentration was 72 ppb.
---------------------------------------------------------------------------
(2) Recent multicity and single city epidemiologic studies
continue to report associations between short-term O3
exposures and respiratory hospital admissions and respiratory
emergency department visits. Recent multicity studies and a multi-
continent study have reported consistent positive associations
between short-term O3 exposure and total (nonaccidental)
mortality, expanding upon evidence available in the last review.
They also observed associations between O3 exposure and
cardiovascular and respiratory mortality.\23\
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\23\ The consideration of ambient O3 concentrations
in the locations of these epidemiologic studies are discussed in
sections II.D.1.b and II.E.4.a below, for the current standard and
alternative standards, respectively.
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(3) Recent controlled human exposure studies reporting systemic
inflammation and cardiac changes provide support for the expanded
body of epidemiologic evidence for
[[Page 75247]]
cardiovascular mortality, although lack of coherence with
epidemiologic studies of cardiovascular morbidity remains an
important uncertainty.
(4) New epidemiologic studies provide expanded evidence for
respiratory effects associated with long-term or repeated
O3 concentrations (e.g., seasonal average of 1- or 8-hour
daily max concentrations). Recent studies report interactions
between exercise or different genetic variants and both new-onset
asthma in children and increased respiratory symptom effects in
individuals with asthma; additional studies of respiratory morbidity
and mortality support the association between long-term exposure to
O3 and a range of respiratory health effects.
(5) New evidence of risk factors (i.e., people with certain
genetic variants related to antioxidant status or inflammation, and
people with reduced intake of antioxidant nutrients) strengthens our
understanding of the potential modes of action from O3-
induced effects.
1. Overview of Mechanisms
The purpose of this section is to describe the ISA's
characterization of the key events and pathways that contribute to
health effects resulting from both short-term and long-term exposures
to O3. The information in this section draws from section
5.3 of the ISA (U.S. EPA, 2013a). Mode of action refers to a sequence
of key events and processes that result in a given toxic effect.
Elucidation of mechanisms provides a more detailed understanding of
these key events and processes. Experimental evidence elucidating modes
of action and/or mechanisms contributes to our understanding of the
biological plausibility of adverse O3-related health
effects, including respiratory effects and effects outside the
respiratory system (U.S. EPA, 2013a, Chapters 6 and 7).
Figure 3.1 in the PA (U.S. EPA, 2014c) shows the current
understanding of key events in the toxicity pathway of O3,
based on the available evidence. These key events are described briefly
here and in more detail in section 3.1.1 of the PA. The initial key
event is the formation of secondary oxidation products in the
respiratory tract (U.S. EPA, 2013a, section 5.3). This mainly involves
direct reactions with components of the extracellular lining fluid
(ELF). Although the ELF has inherent capacity to quench (based on
individual antioxidant capacity), this capacity can be overwhelmed,
especially with exposure to elevated concentrations of O3.
The resulting secondary oxidation products transmit signals to the
epithelium, pain receptive nerve fibers and, if present, immune cells
(i.e., eosinophils, dendritic cells and mast cells) involved in
allergic responses. Thus, the available evidence indicates that the
effects of O3 are mediated by components of ELF and by the
multiple cell types found in the respiratory tract. Further, oxidative
stress is an implicit part of this initial key event.
It is well understood that secondary oxidation products initiate
numerous responses at the cellular, tissue, and whole organ level of
the respiratory system. These responses include the activation of
neural reflexes leading to lung function decrements, airway
obstruction, and extrapulmonary effects such as slow resting heart
rate; initiation of inflammation; alteration of barrier epithelial
function; sensitization of bronchial smooth muscle; modification of
lung host defenses; and airways remodeling (U.S. EPA, 2013a, section
5.3.10, Figure 5-8). Each of these effects is discussed in more detail
in section 3.1.1 of the PA (U.S. EPA, 2014c).
Persistent inflammation and injury, which are observed in animal
models of chronic and intermittent exposure to O3, are
associated with airways remodeling (see Section 7.2.3 of the ISA, U.S.
EPA 2013). Chronic intermittent exposure to O3 has also been
shown to result in effects on the developing lung and immune system.
Systemic inflammation and vascular oxidative/nitrosative stress are
also key events in the toxicity pathway of O3.
Extrapulmonary effects of O3 occur in numerous organ
systems, including the cardiovascular, central nervous, reproductive,
and hepatic systems (U.S. EPA, 2013a, sections 6.3 to 6.5 and sections
7.3 to 7.5).
Responses to O3 exposure are variable within the
population. Studies have shown a large range of pulmonary function
(i.e., spirometric) responses to O3 among healthy young
adults, while responses within an individual are relatively consistent
over time. Other responses to O3 have also been
characterized by a large degree of interindividual variability. For
example, a 3- to 20-fold difference among subjects in their studies in
airways inflammation (i.e., neutrophilia influx) following
O3 exposure has been reported (Schelegle et al., 1991 and
Devlin et al., 1991, respectively). Reproducibility of an individual's
inflammatory response to O3 exposure in humans, measured as
sputum neutrophilia, was demonstrated by Holz et al (1999). Since
individual inflammatory responses were relatively consistent across
time, it was thought that inflammatory responsiveness reflected an
intrinsic characteristic of the subject (Mudway and Kelly, 2000). While
the basis for the observed interindividual variability in
responsiveness to O3 is not clear, section 5.4.2 of the ISA
discusses mechanisms that may underlie the variability in responses
seen among individuals. Certain functional genetic polymorphisms, pre-
existing conditions or diseases, nutritional status, lifestages, and
co-exposures contribute to altered risk of O3-induced
effects. Experimental evidence for such O3-induced changes
contributes to our understanding of the biological plausibility of
adverse O3-related health effects, including a range of
respiratory effects as well as effects outside the respiratory system
(e.g., cardiovascular effects) (U.S. EPA, 2013a, Chapters 6 and 7).
2. Nature of Effects
The health effects of O3 are described in detail and
assessed in the ISA (U.S. EPA, 2013a). Based on this assessment, the
ISA determined that a ``causal'' relationship exists between short-term
exposure to O3 in ambient air \24\ and effects on the
respiratory system and that a ``likely to be causal'' relationship \25\
exists between long-term exposure to O3 in ambient air and
respiratory effects (U.S. EPA 2013a, pp. 1-6 to 1-7). As stated in the
ISA, ``[c]ollectively, a very large amount of evidence spanning several
decades supports a relationship between exposure to O3 and a
broad range of respiratory effects'' (US. EPA, 2013a, p. 1-6). The ISA
summarizes the longstanding body of evidence for O3
respiratory effects as follows (U.S. EPA, 2013a, p. 1-5):
---------------------------------------------------------------------------
\24\ In determining that a causal relationship exists for
O3 with specific health effects, the EPA has concluded
that ``[e]vidence is sufficient to conclude that there is a causal
relationship with relevant pollutant exposures'' (U.S. EPA, 2013a,
p. lxiv).
\25\ In determining a ``likely to be a causal'' relationship
exists for O3 with specific health effects, the EPA has
concluded that ``[e]vidence is sufficient to conclude that a causal
relationship is likely to exist with relevant pollutant exposures,
but important uncertainties remain'' (U.S. EPA, 2013a, p. lxiv).
The clearest evidence for health effects associated with
exposure to O3 is provided by studies of respiratory
effects. Collectively, a very large amount of evidence spanning
several decades supports a relationship between exposure to
O3 and a broad range of respiratory effects (see Section
6.2.9 and Section 7.2.8). The majority of this evidence is derived
from studies investigating short-term exposures (i.e., hours to
weeks) to O3, although animal toxicological studies and
recent epidemiologic evidence demonstrate that long-term exposure
---------------------------------------------------------------------------
(i.e., months to years) may also harm the respiratory system.
Additionally, the ISA determined that the relationships between short-
term exposures to O3 in ambient air and both total mortality
and cardiovascular effects are likely to be causal, based on expanded
evidence bases in the current review (U.S. EPA, 2013a, pp. 1-7 to 1-
[[Page 75248]]
8). In the ISA, the EPA additionally determined that the currently
available evidence for additional endpoints is ``suggestive'' of causal
relationships between short-term (central nervous system effects) and
long-term exposure (cardiovascular effects, reproductive and
developmental effects, central nervous system effects and total
mortality) to ambient O3.
Consistent with emphasis in past reviews on O3 health
effects for which the evidence is strongest, in this review the EPA
places the greatest emphasis on studies of health effects that have
been judged in the ISA to be caused by, or likely to be caused by,
O3 exposures (U.S. EPA, 2013a, section 2.5.2). This section
discusses the evidence for health effects attributable to O3
exposures, with a focus on respiratory morbidity and mortality effects
attributable to short- and long-term exposures, and cardiovascular
system effects (including mortality) and total mortality attributable
to short-term exposures. This section focuses particularly on
considering the extent to which the scientific evidence available in
the current review has been strengthened since the last review, and the
extent to which important uncertainties and limitations in the evidence
from the last review have been addressed.
a. Respiratory Effects--Short-Term
The 2006 O3 AQCD concluded that there was clear,
consistent evidence of a causal relationship between short-term
O3 exposure and respiratory effects (U.S. EPA, 2006a). This
conclusion was substantiated by evidence from controlled human exposure
and toxicological studies indicating a range of respiratory effects in
response to short-term O3 exposures, including pulmonary
function decrements and increases in respiratory symptoms, lung
inflammation, lung permeability, and airway hyperresponsiveness.
Toxicological studies provided additional evidence for O3-
induced impairment of host defenses. Combined, these findings from
experimental studies provided support for epidemiologic evidence, in
which short-term increases in ambient O3 concentration were
consistently associated with decreases in lung function in populations
with increased outdoor exposures, especially children with asthma and
healthy children; increases in respiratory symptoms and asthma
medication use in children with asthma; and increases in respiratory-
related hospital admissions and asthma-related emergency department
visits (U.S. EPA, 2013a, pp. 6-1 to 6-2).
As discussed in detail in the ISA (U.S. EPA, 2013a, section 6.2.9),
studies evaluated since the completion of the 2006 O3 AQCD
support and expand upon the strong body of evidence that, in the last
review, indicated a causal relationship between short-term
O3 exposures and respiratory health effects. Recent
controlled human exposure studies conducted in young, healthy adults
with moderate exertion have reported forced expiratory volume in 1
second (FEV1) decrements and pulmonary inflammation
following prolonged exposures to O3 concentrations as low as
60 ppb, and respiratory symptoms following exposures to concentrations
as low as 72 ppb (based on group mean responses).\26\ Epidemiologic
studies provide evidence that increases in ambient O3
exposures are associated with lung function decrements, increases in
respiratory symptoms, and pulmonary inflammation in children with
asthma; increases in respiratory-related hospital admissions and
emergency department visits; and increases in respiratory mortality.
Some of these studies report such associations even for O3
concentrations at the low end of the distribution of daily
concentrations. Recent epidemiologic studies report that associations
with respiratory morbidity and mortality are stronger during the warm/
summer months and remain robust after adjustment for copollutants.
Recent toxicological studies reporting O3-induced
inflammation, airway hyperresponsiveness, and impaired lung host
defense continue to support the biological plausibility and modes of
action for the O3-induced respiratory effects observed in
the controlled human exposure and epidemiologic studies. Further
support is provided by recent studies that found O3-
associated increases in indicators of airway inflammation and oxidative
stress in children with asthma (U.S. EPA, 2013a, section 6.2.9).
Together, epidemiologic and experimental studies support a continuum of
respiratory effects associated with O3 exposure that can
result in respiratory-related emergency department visits, hospital
admissions, and/or mortality (U.S. EPA, 2013a, section 6.2.9).
---------------------------------------------------------------------------
\26\ Schelegle et al. (2009) reported a statistically
significant increase in respiratory symptoms in healthy adults at a
target O3 exposure concentration of 70 ppb, averaged over
the study period. For this 70 ppb target exposure concentration,
Schelegle et al. (2009) reported that the actual mean exposure
concentration was 72 ppb.
---------------------------------------------------------------------------
Across respiratory endpoints, evidence indicates antioxidant
capacity may modify the risk of respiratory morbidity associated with
O3 exposure (U.S. EPA, 2013a, section 6.2.9, p. 6-161). The
potentially elevated risk of populations with diminished antioxidant
capacity and the reduced risk of populations with sufficient
antioxidant capacity is supported by epidemiologic studies and from
controlled human exposure studies. Additional evidence characterizes
O3-induced decreases in antioxidant levels as a key event in
the mode of action for downstream effects.
Key aspects of this evidence are discussed below with regard to
lung function decrements; pulmonary inflammation, injury, and oxidative
stress; airway hyperresponsiveness; respiratory symptoms and medication
use; lung host defense; allergic and asthma-related responses; hospital
admissions and emergency department visits; and respiratory
mortality.\27\
---------------------------------------------------------------------------
\27\ CASAC concurred that these were ``the kinds of identifiable
effects on public health that are expected from the presence of
ozone in the ambient air'' (Frey 2014c, p. 3).
---------------------------------------------------------------------------
i. Lung Function Decrements
In the 2008 review, a large number of controlled human exposure
studies\28\ reported O3-induced lung function decrements in
young, healthy adults engaged in intermittent, moderate exertion
following 6.6 hour exposures to O3 concentrations at or
above 80 ppb. Although two studies also reported effects following
exposures to lower concentrations, an important uncertainty in the last
review was the extent to which exposures to O3
concentrations below 80 ppb result in lung function decrements. In
addition, in the last review epidemiologic panel studies had reported
O3-associated lung function decrements in a variety of
different populations (e.g., children, outdoor workers) likely to
experience increased exposures. In the current review, additional
controlled human exposure studies are available that have evaluated
exposures to O3 concentrations of 60 or 72 ppb. The
available evidence from controlled human exposure and panel studies is
[[Page 75249]]
assessed in detail in the ISA (U.S. EPA, section 6.2.1) and is
summarized below.
---------------------------------------------------------------------------
\28\ The controlled human exposure studies emphasized in the PA
utilize only healthy adult subjects. In the near absence of
controlled human exposure data for children, HREA estimates of lung
function decrements are based on the assumption that children
exhibit the same lung function responses following O3
exposures as healthy 18 year olds (U.S. EPA, 2014a, section 6.2.4
and 6.5). This assumption is justified in part by the findings of
McDonnell et al. (1985), who reported that children (8-11 years old)
experienced FEV1 responses similar to those observed in
adults (18-35 years old). Thus, the conclusions about the occurrence
of lung function decrements that follow generally apply to children
as well as to adults.
---------------------------------------------------------------------------
Controlled exposures to O3 concentrations that can be
found in the ambient air can result in a number of lung function
effects, including decreased inspiratory capacity, mild
bronchoconstriction, and rapid, shallow breathing patterns during
exercise. Reflex inhibition of inspiration results in a decrease in
forced vital capacity (FVC) and total lung capacity (TLC) and, in
combination with mild bronchoconstriction, contributes to a decrease in
FEV1 (U.S. EPA, 2013a, section 6.2.1.1). Accumulating
evidence indicates that such effects are mediated by activation of
sensory nerves, resulting in the involuntary truncation of inspiration
and a mild increase in airway obstruction due to bronchoconstriction
(U.S. EPA, 2013a, section 5.3.10).
Data from controlled human exposure studies show that increasing
the duration of O3 exposures and increasing ventilation
rates decreases the O3 exposure concentrations required to
impair lung function. Ozone exposure concentrations well above those
typically found in ambient air are required to impair lung function in
healthy resting adults, while exposure to O3 concentrations
at or below those in the ambient air have been reported to impair lung
function in healthy adults exposed for longer durations while
undergoing intermittent, moderate exertion (U.S. EPA, 2013a, section
6.2.1.1). With repeated O3 exposures over several days,
FEV1 responses become attenuated in both healthy adults and
adults with mild asthma, though this attenuation of response is lost
after about a week without exposure (U.S. EPA, 2013a, section 6.2.1.1;
p. 6-27).
When considering controlled human exposure studies of
O3-induced lung function decrements, the ISA and PA evaluate
both group mean changes in lung function and the interindividual
variability in the magnitude of responses. An advantage of
O3 controlled human exposure studies (i.e., compared to the
epidemiologic panel studies discussed below) is that reported effects
necessarily result from exposures to O3 itself.\29\ To the
extent studies report statistically significant decrements in mean lung
function following O3 exposures after controlling for other
factors, these studies provide greater confidence that measured
decrements are due to the O3 exposure itself, rather than to
chance alone. As discussed below, group mean changes in lung function
are often small, especially following exposures to relatively low
O3 concentrations (e.g., 60 ppb). However, even when group
mean decrements in lung function are small, some individuals could
experience decrements that are ``clinically meaningful'' (Pellegrino et
al., 2005; ATS, 1991) with respect to criteria for spirometric testing,
and/or that could be considered adverse with respect to public health
policy decisions (see section II.B.3, below).
---------------------------------------------------------------------------
\29\ The ISA notes that the use of filtered air responses as a
control for the assessment of responses following O3
exposure in controlled human exposure studies serves to eliminate
alternative explanations other than O3 itself in causing
the measured responses (U.S. EPA, 2013a, section 6.2.1.1).
---------------------------------------------------------------------------
At the time of the last review, a number of controlled human
exposure studies had reported lung function decrements in young,
healthy adults following prolonged (6.6-hour) exposures while at
moderate exertion to O3 concentrations at and above 80 ppb.
In addition, there were two controlled human exposure studies by Adams
(2002, 2006) that examined lung function effects following exposures to
60 ppb O3. The EPA's analysis of the data from the Adams
(2006) study reported a small but statistically significant
O3-induced decrement in group mean FEV1 following
exposures of young, healthy adults to 60 ppb O3 while at
moderate exertion, when compared with filtered air controls (Brown et
al., 2008).\30\ Further examination of the post-exposure
FEV1 data, and mean data for other time points and other
concentrations, indicated that the temporal pattern of the response to
60 ppb O3 was generally consistent with the temporal
patterns of responses to higher O3 concentrations in this
and other studies (75 FR 2950, January 19, 2010). This suggested a
pattern of response following exposures to 60 ppb O3 that
was consistent with a dose-response relationship, rather than random
variability. See also State of Mississippi v. EPA, F. 3d at 1347
(upholding EPA's interpretation of the Adams studies).
---------------------------------------------------------------------------
\30\ Adams (2006) did not find effects on FEV1 at 60
ppb to be statistically significant. In an analysis of the Adams
(2006) data, Brown et al. (2008) showed that even after removal of
potential outliers, the average effect on FEV1 at 60 ppb
was small, but highly statistically significant (p <0.002) using
several common statistical tests.
---------------------------------------------------------------------------
Figure 6-1 in the ISA summarizes the currently available evidence
from multiple controlled human exposure studies evaluating group mean
changes in FEV1 following prolonged O3 exposures
(i.e., 6.6 hours) in young, healthy adults engaged in moderate levels
of physical activity (U.S. EPA, 2013a, section 6.2.1.1). With regard to
the group mean changes reported in these studies, the ISA specifically
notes the following (U.S. EPA, 2013a, section 6.2.1.1, Figure 6-1):
1. Prolonged exposure to 40 ppb O3 results in a small
decrease in group mean FEV1 that is not statistically
different from responses following exposure to filtered air (Adams,
2002; Adams, 2006).
2. Prolonged exposure to an average O3 concentration of
60 ppb results in group mean FEV1 decrements ranging from
1.8% to 3.6% (Adams 2002; Adams, 2006; \31\ Schelegle et al., 2009;
\32\ Kim et al., 2011). Based on data from multiple studies, the
weighted average group mean decrement was 2.7%. In some analyses, these
group mean decrements in lung function were statistically significant
(Brown et al., 2008; Kim et al., 2011), while in other analyses they
were not (Adams, 2006; Schelegle et al., 2009).\33\
---------------------------------------------------------------------------
\31\ Adams (2006); (2002) both provide data for an additional
group of 30 healthy subjects that were exposed via facemask to 60
ppb (square-wave) O3 for 6.6 hours with moderate exercise
(VE = 23 L/min per m\2\ BSA). These subjects are
described on page 133 of Adams (2006) and pages 747 and 761 of Adams
(2002). The FEV1 decrement may be somewhat increased due
to a target VE of 23 L/min per m\2\ BSA relative to other
studies having the target VE of 20 L/min per m\2\ BSA.
The facemask exposure is not expected to affect the FEV1
responses relative to a chamber exposure.
\32\ For the 60 ppb target exposure concentration, Schelegle et
al. (2009) reported that the actual mean exposure concentration was
63 ppb.
\33\ Adams (2006) did not find effects on FEV1 at 60
ppb to be statistically significant. In an analysis of the Adams
(2006) data, Brown et al. (2008) addressed the more fundamental
question of whether there were statistically significant differences
in responses before and after the 6.6 hour exposure period and found
the average effect on FEV1 at 60 ppb to be small, but
highly statistically significant using several common statistical
tests, even after removal of potential outliers. Schelegle et al.
(2009) reported that, compared to filtered air, the largest change
in FEV1 for the 60 ppb protocol occurred after the sixth
(and final) exercise period.
---------------------------------------------------------------------------
3. Prolonged exposure to an average O3 concentration of
72 ppb results in a statistically significant group mean decrement in
FEV1 of about 6% (Schelegle et al., 2009).\34\
---------------------------------------------------------------------------
\34\ As noted above, for the 70 ppb exposure group, Schelegle et
al. (2009) reported that the actual mean exposure concentration was
72 ppb.
---------------------------------------------------------------------------
4. Prolonged square-wave exposure to average O3
concentrations of 80 ppb, 100 ppb, or 120 ppb O3 results in
statistically significant group mean decrements in FEV1
ranging from 6 to 8%, 8 to 14%, and 13 to 16%, respectively (Folinsbee
et al., 1988; Horstman et al., 1990; McDonnell et al., 1991; Adams,
2002; Adams, 2003; Adams, 2006).
As illustrated in Figure 6-1 of the ISA, there is a smooth dose-
response
[[Page 75250]]
curve without evidence of a threshold for exposures between 40 and 120
ppb O3 (U.S. EPA, 2013a, Figure 6-1). When these data are
taken together, the ISA concludes that ``mean FEV1 is
clearly decreased by 6.6-hour exposures to 60 ppb O3 and
higher concentrations in [healthy, young adult] subjects performing
moderate exercise'' (U.S. EPA, 2013a, p. 6-9).
With respect to interindividual variability in lung function, in an
individual with relatively ``normal'' lung function, with recognition
of the technical and biological variability in measurements, within-day
changes in FEV1 of >=5% are clinically meaningful
(Pellegrino et al., 2005; ATS, 1991). The ISA (U.S. EPA, 2013a, section
6.1.) focuses on individuals with >10% decrements in FEV1
for two reasons. A 10% FEV1 decrement is accepted by the
American Thoracic Society (ATS) as an abnormal response and a
reasonable criterion for assessing exercise-induced bronchoconstriction
(Dryden et al., 2010; ATS, 2000). (U.S. EPA, 2013a, section 6.2.1.1).
Also, some individuals in the Schelegle et al. (2009) study experienced
5-10% FEV1 decrements following exposure to filtered air.
In previous NAAQS reviews, the EPA has made judgments regarding the
potential implications for individuals experiencing FEV1
decrements of varying degrees of severity.\35\ For people with lung
disease, the EPA judged that moderate functional decrements (e.g.,
FEV1 decrements >10% but <20%, lasting up to 24 hours) would
likely interfere with normal activity for many individuals, and would
likely result in more frequent use of medication (75 FR 2973, January
19, 2010). In previous reviews CASAC has endorsed these conclusions. In
the context of standard setting, in the last review of the
O3 NAAQS CASAC indicated that it is appropriate to focus on
the lower end of the range of moderate functional responses (e.g.,
FEV1 decrements >=10%) when estimating potentially adverse
lung function decrements in people with lung disease, especially
children with asthma (Henderson, 2006c; transcript of CASAC meeting,
day 8/24/06, page 149). More specifically, CASAC stated that ``[a] 10%
decrement in FEV1 can lead to respiratory symptoms,
especially in individuals with pre-existing pulmonary or cardiac
disease. For example, people with chronic obstructive pulmonary disease
have decreased ventilatory reserve (i.e., decreased baseline
FEV1) such that a >=10% decrement could lead to moderate to
severe respiratory symptoms'' (Samet, 2011). In this review, CASAC
reiterated its support for this conclusion, stating that ``[a]n FEV1
decrement of >=10% is a scientifically relevant surrogate for adverse
health outcomes for people with asthma and lung disease'' (Frey, 2014c
p. 3). Therefore, in considering interindividual variability in
O3-induced lung function decrements in the current review,
the EPA also focuses on the extent to which individuals were reported
to experience FEV1 decrements of 10% or greater.\36\
---------------------------------------------------------------------------
\35\ Such judgments have been made for decrements in
FEV1 as well as for increased airway responsiveness and
symptomatic responses (e.g., cough, chest pain, wheeze). Ranges of
pulmonary responses and their associated potential impacts are
presented in Tables 3-2 and 3-3 of the 2007 Staff Paper (U.S. EPA,
2007).
\36\ The approach to using results from controlled human
exposure studies conducted in healthy adults to provide perspective
on the potential public health impacts of O3-related
respiratory health effects is discussed in section II.A above, and
in sections II.C.2 and II.C.3 below.
---------------------------------------------------------------------------
New studies (Schelegle et al., 2009; Kim et al., 2011) add to the
previously available evidence for interindividual variability in the
responses of healthy adults following exposures to O3.
Following prolonged exposures to 80 ppb O3 while at moderate
exertion, the proportion of healthy adults experiencing FEV1
decrements greater than 10% was 17% by Adams (2006), 26% by McDonnell
(1996), and 29% by Schelegle et al. (2009). Following exposures to 60
ppb O3, that proportion was 20% by Adams (2002), 3% by Adams
(2006), 16% by Schelegle et al. (2009), and 5% by Kim et al. (2011).
Across these studies, the weighted average proportion (i.e., based on
numbers of subjects in each study) of young, healthy adults with >10%
FEV1 decrements is 25% following exposure to 80 ppb
O3 and 10% following exposure to 60 ppb O3, for
6.6 hours at moderate exertion (U.S. EPA, 2013a, page 6-18 and 6-
19).\37\ \38\ The ISA notes that responses within an individual tend to
be reproducible over a period of several months, indicating that
interindividual differences reflect differences in intrinsic
responsiveness. Given this, the ISA concludes that ``[t]hough group
mean decrements are biologically small and generally do not attain
statistical significance, a considerable fraction of exposed
individuals experience clinically meaningful decrements in lung
function'' when exposed for 6.6 hours to 60 ppb O3 during
quasi continuous, moderate exertion (U.S. EPA, 2013a, section 6.2.1.1,
p. 6-20).
---------------------------------------------------------------------------
\37\ The ISA notes that by considering responses uncorrected for
filtered air exposures, during which lung function typically
improves (which would increase the size of the change, pre-and post-
exposure), 10% is an underestimate of the proportion of healthy
individuals that are likely to experience clinically meaningful
changes in lung function following exposure for 6.6 hours to 60 ppb
O3 during intermittent moderate exertion (U.S. EPA, 2012,
section 6.2.1.1).
\38\ Based on the data available at 60 ppb, 1% of subjects
experienced decrements >20% (also uncorrected for filtered air
exposures).
---------------------------------------------------------------------------
This review has marked an advance in the ability to make reliable
quantitative predictions of the potential lung function response to
ozone exposure, and thus to reasonably predict the degree of
interindividual response of lung function to that exposure. McDonnell
et al. (2012) and Schelegle et al. (2012) developed models using data
on O3 exposure concentrations, ventilation rates, duration
of exposures, and lung function responses from a number of controlled
human exposure studies. See section 6.2.1.1 of the ISA (U.S. EPA 2013a,
p. 6-15). The McDonnell et al. (2012) and Schelegle et al. (2012)
studies analyzed large datasets to fit compartmental models that
included the concept of a dose of onset in lung function response or a
response threshold based upon the inhaled O3 dose. The
McDonnell et al. (2012) model was fit to a dataset consisting of the
FEV1 responses of 741 young, healthy adults (18-35 years of
age) from 23 individual controlled exposure studies. Concentrations
across individual studies ranged from 40 ppb to 400 ppb,\39\ activity
level ranged from rest to heavy exercise, duration of exposure was from
2 to 7.6 hours. The extension of the McDonnell et al. (2012) model to
children and older adults is discussed in section 6.2.4 of the HREA
(U.S. EPA, 2014a). Schelegle et al. (2012) also analyzed a large
dataset with substantial overlap to that used by McDonnell et al.
(2012). The Schelegle et al. (2012) model was fit to the
FEV1 responses of 220 young healthy adults (taken from a
dataset of 704 individuals) from 21 individual controlled exposure
studies. The resulting empirical models can estimate the frequency
distribution of individual responses for any exposure scenario as well
as summary measures of the distribution such as the mean or median
response and the proportions of individuals with FEV1
decrements >10%, 15%, and 20%.
---------------------------------------------------------------------------
\39\ Responses to O3 in these studies were adjusted
for responses observed following exposure to filtered air.
---------------------------------------------------------------------------
The predictions of the McDonnell and Schelegle models are
consistent with the observed results from the individual studies of
O3-induced FEV1 decrements. Specifically,
McDonnell et al. (2012) estimated that 9% of healthy exercising adults
would experience FEV1 decrements greater than 10% following
[[Page 75251]]
6.6 hour exposure to 60 ppb O3, and that 22% would
experience such decrements following exposure to 80 ppb O3
(U.S. EPA, 2013a, p. 6-18 and Figure 6-3).\40\ Schelegle et al. (2012)
estimated that, for a prolonged (6.6 hours) O3 exposure with
moderate, quasi-continuous exercise, the average dose of onset for
FEV1 decrement would be reached following 4 to 5 hours of
exposure to 60 ppb, and following 3 to 4 hours of exposure to 80 ppb.
However, 14% of the individuals were estimated to have a dose of onset
that was less than 40% of the average. Those individuals were estimated
to reach their dose of onset following 1 to 2 hours of exposure to 50
to 80 ppb O3 (U.S. EPA, 2013a, p. 6-16), which is consistent
with the threshold FEV1 responses reported by McDonnell et
al. (2012).
---------------------------------------------------------------------------
\40\ Also consistent with the data from published studies (see
above), this model predicts that 1% of people would experience
FEV1 decrements >20% following 6.6 hour exposure to 60
ppb O3.
---------------------------------------------------------------------------
CASAC agreed that these models mark a significant technical advance
over the exposure-response modeling approach used in the last review
(Frey, 2014a), stating that ``the comparison of the MSS [McDonnell-
Stewart-Smith] model results to those obtained with the exposure-
response (E-R) model is of tremendous importance. Typically, the MSS
model gives results about a factor of three higher than the E-R model
for school-aged children, which is expected because the MSS model
includes responses for a wider range of exposure protocols (under
different levels of exertion, lengths of exposure, and patterns of
exposure concentrations) than the E-R model'' (Frey, 2014a, p. 7).
CASAC explicitly found ``the updated and expanded lung finds the MSS
model to be scientifically and biologically defensible.'' (Frey, 2014a,
pp. 2, 8).
As discussed above and in the ISA (U.S EPA, 2013a, Section 5.3.2),
secondary oxidation products formed following O3 exposures
can activate neural reflexes leading to decreased lung function. The
McDonnell and Schelegle models included mathematical approaches to
simulate the potential protective effect of antioxidants in the ELF at
lower ambient O3 concentrations, and include a dose
threshold below which changes in lung function do not occur.
Epidemiologic studies \41\ have consistently linked short-term
increases in ambient O3 concentrations with lung function
decrements in diverse populations and lifestages, including children
attending summer camps, adults exercising or working outdoors, and
groups with pre-existing respiratory diseases such as asthmatic
children (U.S. EPA, 2013a, section 6.2.1.2). Some of these studies
reported O3-associated lung function decrements accompanied
by respiratory symptoms \42\ in asthmatic children (Just et al., 2002;
Mortimer et al., 2002; Ross et al., 2002; Gielen et al., 1997; Romieu
et al., 1997; Thurston et al., 1997; Romieu et al., 1996). In contrast,
studies of children in the general population have reported similar
O3-associated lung function decrements but without
accompanying respiratory symptoms (Ward et al., 2002; Gold et al.,
1999; Linn et al., 1996) (U.S. EPA, 2013a, section 6.2.1.2).
---------------------------------------------------------------------------
\41\ Unless otherwise specified, the epidemiologic studies
discussed in the PA evaluate only adults.
\42\ Reversible loss of lung function in combination with the
presence of symptoms meets the ATS definition of adversity (ATS,
2000).
---------------------------------------------------------------------------
Several epidemiologic panel studies \43\ reported statistically
significant associations with lung function decrements at relatively
low ambient O3 concentrations. For outdoor recreation or
exercise, associations were reported in analyses restricted to 1-hour
average O3 concentrations less than 80 ppb (Spektor et al.,
1988a; Spektor et al., 1988b), 60 ppb (Brunekreef et al., 1994; Spektor
et al., 1988a), and 50 ppb (Brunekreef et al., 1994). Among outdoor
workers, Brauer et al. (1996) found a robust association with daily 1-
hour max O3 concentrations less than 40 ppb. Ulmer et al.
(1997) found a robust association in schoolchildren with 30-minute
maximum O3 concentrations less than 60 ppb. For 8-hour
average O3 concentrations, associations with lung function
decrements in children with asthma were found to persist at
concentrations less than 80 ppb in a U.S. multicity study (Mortimer et
al., 2002) and less than 51 ppb in a study conducted in the Netherlands
(Gielen et al., 1997).
---------------------------------------------------------------------------
\43\ Panel studies include repeated measurements of health
outcomes, such as respiratory symptoms, at the individual level
(U.S. EPA, 2013a, p. 1x).
---------------------------------------------------------------------------
Epidemiologic panel studies investigating the effects of short-term
exposure to O3 provided information on potential confounding
by copollutants such as particulate matter with a median aerodynamic
diameter less than or equal to 2.5 microns (PM2.5),
particulate matter with a median aerodynamic diameter less than or
equal to 10 microns (PM10), nitrogen dioxide
(NO2), or sulfur dioxide (SO2). These studies
varied in how they evaluated confounding. Some studies of subjects
exercising outdoors indicated that ambient concentrations of
copollutants such as NO2, SO2, or acid aerosol
were low, and thus not likely to confound associations observed for
O3 (Hoppe et al., 2003; Brunekreef et al., 1994; Hoek et
al., 1993). In other studies of children with increased outdoor
exposures, O3 was consistently associated with decreases in
lung function, whereas other pollutants such as PM2.5,
sulfate, and acid aerosol individually showed variable associations
across studies (Thurston et al., 1997; Castillejos et al., 1995; Berry
et al., 1991; Avol et al., 1990; Spektor et al., 1988a). Studies that
conducted copollutant modeling generally found O3-associated
lung function decrements to be robust (i.e., most copollutant-adjusted
effect estimates fell within the 95% confidence interval (CI) of the
single-pollutant effect estimates) (U.S. EPA, 2013a, Figure 6-10 and
Table 6-14). Most O3 effect estimates for lung function were
robust to adjustment for temperature, humidity, and copollutants such
as PM2.5, PM10, NO2, or
SO2. Although examined in only a few epidemiologic studies,
O3 also remained associated with decreases in lung function
with adjustment for pollen or acid aerosols (U.S. EPA, 2013a, section
6.2.1.2).
Several epidemiologic studies demonstrated the protective effects
of vitamin E and vitamin C supplementation, and increased dietary
antioxidant intake, on O3-induced lung function decrements
(Romieu et al., 2002) (U.S. EPA, 2013a, Figure 6-7 and Table 6-8).\44\
These results provide support for the new, quantitative models
(McDonnell et al., 2012; Schelegle et al., 2012), discussed above,
which make use of the concept of oxidant stress to estimate the
occurrence of lung function decrements following exposures to
relatively low O3 concentrations.
---------------------------------------------------------------------------
\44\ Evidence from controlled human exposure studies is mixed,
suggesting that supplementation may be ineffective in the absence of
antioxidant deficiency (U.S. EPA, 2013a, p. 5-63).
---------------------------------------------------------------------------
In conclusion, new information from controlled human exposure
studies considerably strengthens the evidence and reduces the
uncertainties, relative to the evidence that was available at the time
of the 2008 review, regarding the presence and magnitude of lung
function decrements in healthy adults following prolonged exposures to
O3 concentrations below 80 ppb. As discussed in Section
6.2.1.1 in the ISA (U.S. EPA, 2013, p. 6-12), there is information
available from four separate studies that evaluated exposures to 60 ppb
O3 (Kim et al., 2011; Schelegle et al., 2009; Adams 2002;
2006). Although not consistently statistically significant, group mean
FEV1 decrements following exposures to 60 ppb O3
are consistent
[[Page 75252]]
among these studies. Moreover, as is illustrated in Figure 6-1 of the
ISA (U.S. EPA, 2013a), the group mean FEV1 responses at 60
ppb fall on a smooth intake dose-response curve for exposures between
40 and 120 ppb O3. Based on the data in these studies, 10%
of young, healthy adults experience clinically meaningful decrements in
lung function when exposed for 6.6 hours to 60 ppb O3 during
intermittent, moderate exertion. One recent study has also reported
statistically significant decrements following exposures to 72 ppb
O3 (Schelegle et al., 2009). Predictions from newly
developed quantitative models are consistent with these experimental
results. Additionally, as discussed in more detail in section II.B.4
below, epidemiologic studies continue to provide evidence of lung
function decrements in people who are active outdoors, including people
engaged in outdoor recreation or exercise, children, and outdoor
workers, at low ambient O3 concentrations. While few new
epidemiologic studies of O3-associated lung function
decrements are available in this review, previously available studies
have reported associations with decrements, including at relatively low
ambient O3 concentrations.
ii. Pulmonary Inflammation, Injury, and Oxidative Stress
Ozone exposures result in increased respiratory tract inflammation
and epithelial permeability. Inflammation is a host response to injury,
and the induction of inflammation is evidence that injury has occurred.
Oxidative stress has been shown to play a key role in initiating and
sustaining O3-induced inflammation. Secondary oxidation
products formed as a result of reactions between O3 and
components of the ELF can increase the expression of molecules (i.e.,
cytokines, chemokines, and adhesion molecules) that can enhance airway
epithelium permeability (U.S. EPA, 2013a, sections 5.3.3 and 5.3.4). As
discussed in detail in the ISA (U.S. EPA, 2013a, section 6.2.3),
O3 exposures can initiate an acute inflammatory response
throughout the respiratory tract that has been reported to persist for
at least 18-24 hours after exposure.
Inflammation induced by exposure of humans to O3 can
have several potential outcomes: (1) Inflammation induced by a single
exposure (or several exposures over the course of a summer) can resolve
entirely; (2) continued acute inflammation can evolve into a chronic
inflammatory state; (3) continued inflammation can alter the structure
and function of other pulmonary tissue, leading to diseases such as
asthma; (4) inflammation can alter the body's host defense response to
inhaled microorganisms, particularly in potentially at-risk populations
or lifestages such as the very young and old; and (5) inflammation can
alter the lung's response to other agents such as allergens or toxins
(U.S. EPA, 2013a, section 6.2.3). Thus, lung injury and the resulting
inflammation provide a mechanism by which O3 may cause other
more serious morbidity effects (e.g., asthma exacerbations).\45\
---------------------------------------------------------------------------
\45\ CASAC also addressed this issue: ``The CASAC believes that
these modest changes in FEV1 are usually associated with
inflammatory changes, such as more neutrophils in the
bronchoalveolar lavage fluid. Such changes may be linked to the
pathogenesis of chronic lung disease'' (Frey, 2014a p. 2).
---------------------------------------------------------------------------
In the last review, controlled human exposure studies reported
O3-induced airway inflammation following exposures at or
above 80 ppb and animal toxicological studies provided evidence for
increases in inflammation and permeability in rabbits at levels as low
as 100 ppb O3. In the current review, the link between
O3 exposures and airway inflammation and injury has been
evaluated in additional controlled human exposure studies, as well as
in recent epidemiologic studies. Controlled human exposure studies have
generally been conducted in young, healthy adults or in adults with
asthma using lavage (proximal airway and bronchoalveolar), bronchial
biopsy, and more recently, induced sputum. These studies have evaluated
one or more indicators of inflammation, including neutrophil \46\ (PMN)
influx, markers of eosinophilic inflammation, increased permeability of
the respiratory epithelium, and/or prevalence of proinflammatory
molecules (U.S. EPA, 2013a, section 6.2.3.1). Epidemiologic studies
have generally evaluated associations between ambient O3 and
markers of inflammation and/or oxidative stress, which plays a key role
in initiating and sustaining inflammation (U.S. EPA, 2013a, section
6.2.3.2).
---------------------------------------------------------------------------
\46\ Referred to as either neutrophils or polymorphonuclear
neutrophils (or PMNs), these are the most abundant type of white
blood cells in mammals. PMNs are recruited to the site of injury
following trauma and are the hallmark of acute inflammation. The
presence of PMNs in the lung has long been accepted as a hallmark of
inflammation and is an important indicator that O3 causes
inflammation in the lungs. Neutrophilic inflammation of tissues
indicates activation of the innate immune system and requires a
complex series of events, that then are normally followed by
processes that clear the evidence of acute inflammation.
---------------------------------------------------------------------------
There is an extensive body of evidence from controlled human
exposure studies indicating that short-term exposures to O3
can cause pulmonary inflammation. A single acute exposure (1-4 hours)
of humans to moderate concentrations of O3 (200-600 ppb)
while exercising at moderate to heavy intensities resulted in a number
of cellular and biochemical changes in the lung, including inflammation
characterized by increased numbers of PMNs, increased permeability of
the epithelial lining of the respiratory tract, cell damage, and
production of proinflammatory molecules (i.e., cytokines and
prostaglandins, U.S. EPA, 2006a). A meta-analysis of 21 controlled
human exposure studies (Mudway and Kelly, 2004) using varied
experimental protocols (80-600 ppb O3 exposures; 1-6.6 hours
exposure duration; light to heavy exercise; bronchoscopy at 0-24 hours
post-O3 exposure) reported that PMN influx in healthy
subjects is linearly associated with total O3 dose.
Several studies, including one published since the last review
(Alexis et al., 2010), have reported O3-induced increases in
PMN influx and permeability following exposures at or above 80 ppb
(Alexis et al., 2010; Peden et al., 1997; Devlin et al., 1991), and
eosinophilic inflammation following exposures at or above 160 ppb
(Scannell et al., 1996; Peden et al., 1997; Hiltermann et al., 1999;
Vagaggini et al., 2002). In addition, one recent controlled human
exposure study has reported O3-induced PMN influx following
exposures of healthy adults to 60 ppb O3 (Kim et al., 2011),
the lowest concentration at which inflammatory responses have been
evaluated in human studies.
As with FEV1 responses to O3, inflammatory
responses to O3 are generally reproducible within
individuals, with some individuals experiencing more severe
O3-induced airway inflammation than indicated by group
averages (Holz et al., 2005; Holz et al., 1999). Unlike O3-
induced decrements in lung function, which are attenuated following
repeated exposures over several days (U.S. EPA, 2013a, section
6.2.1.1), some markers of O3-induced inflammation and tissue
damage remain elevated during repeated exposures, indicating ongoing
damage to the respiratory system (U.S. EPA, 2013a, section 6.2.3.1).
Most controlled human exposure studies have reported that
asthmatics experience larger O3-induced inflammatory
responses than non-asthmatics.\47\ Specifically, asthmatics
[[Page 75253]]
exposed to 200 ppb O3 for 4-6 hours with exercise show
significantly more neutrophils in bronchoalveolar lavage fluid (BALF)
than similarly exposed healthy individuals (Scannell et al., 1996;
Basha et al., 1994). Bosson et al. (2003) reported significantly
greater expression of a variety of pro-inflammatory cytokines in
asthmatics, compared to healthy subjects, following exposure to 200 ppb
O3 for 2 hours. In addition, research available in the last
review, combined with a recent study newly available in this review,
indicates that pretreatment of asthmatics with corticosteroids can
prevent the O3-induced inflammatory response in induced
sputum, though pretreatment did not prevent FEV1 decrements
(Vagaggini et al., 2001; 2007). In contrast, Stenfors et al. (2002) did
not detect a difference in the O3-induced increases in
neutrophil numbers between 15 subjects with mild asthma and 15 healthy
subjects by bronchial wash at the 6 hours postexposure time point,
although the neutrophil increase in the asthmatic group was on top of
an elevated baseline.
---------------------------------------------------------------------------
\47\ When evaluated, these studies have also reported
O3-induced respiratory symptoms in asthmatics.
Specifically, Scannell et al. (1996), Basha et al. (1994), and
Vagaggini et al. (2001, 2007) reported increased symptoms in
addition to inflammation.
---------------------------------------------------------------------------
In people with allergic airway disease, including people with
rhinitis and asthma, evidence available in the last review indicated
that proinflammatory mediators also cause accumulation of eosinophils
in the airways (Jorres et al., 1996; Peden et al., 1995 and 1997;
Frampton et al., 1997; Hiltermann et al., 1999; Holz et al., 2002;
Vagaggini et al., 2002). The eosinophil, which increases inflammation
and allergic responses, is the cell most frequently associated with
exacerbations of asthma (72 FR 37846, July 11, 2007).
Studies reporting inflammatory responses and markers of lung injury
have clearly demonstrated that there is important variation in the
responses of exposed subjects (72 FR 37831, July 11, 2007). Some
individuals also appear to be intrinsically more susceptible to
increased inflammatory responses from O3 exposure (Holz et
al., 2005). In healthy adults exposed to each 80 and 100 ppb
O3, Devlin et al. (1991) observed group average increases in
neutrophilic inflammation of 2.1- and 3.8-fold, respectively. However,
there was a 20-fold range in inflammatory responses between individuals
at both concentrations. Relative to an earlier, similar study conducted
at 400 ppb (Koren et al., 1989), Devlin et al. (1991) noted that
although some of the study population showed little or no increase in
inflammatory and cellular injury indicators analyzed after exposures to
lower levels of O3 (i.e., 80 and 100 ppb), others had
changes that were as large as those seen when subjects were exposed to
400 ppb O3. The study authors concluded that, ``while the
population as a whole may have a small inflammatory response to near-
ambient levels of ozone, there may be a significant subpopulation that
is very sensitive to these low levels'' (Devlin et al., 1991).
A number of studies report that O3 exposures increase
epithelial permeability. Increased BALF protein, suggesting
O3-induced changes in epithelial permeability, has been
reported at 1 hour and 18 hours postexposure (Devlin et al., 1997;
Balmes et al., 1996). A meta-analysis of results from 21 publications
(Mudway and Kelly, 2004) for varied experimental protocols (80-600 ppb
O3; 1-6.6 hours duration; light to heavy exercise;
bronchoscopy at 0-24 hours post-O3 exposure; healthy
subjects), showed that increased BALF protein is associated with total
inhaled O3 dose. As noted in the 2009 PM ISA (U.S. EPA,
2009a), it has been postulated that changes in permeability associated
with acute inflammation may provide increased access of inhaled
antigens, particles, and other inhaled substances deposited on lung
surfaces to the smooth muscle, interstitial cells, immune cells
underlying the epithelium, and the blood (U.S. EPA, 2013a, sections
5.3.4, 5.3.5). As has been observed with FEV1 responses,
within individual changes in permeability are correlated with changes
following sequential O3 exposures (Que et al., 2011).
Changes in permeability and AHR apear to be mediated by different
pathways. Animal toxicology studies have provided some support for this
hypothesis (Adamson and Prieditis, 1995; Chen et al., 2006), though
these studies did not specifically evaluate O3 exposures
(U.S. EPA, 2009a).
The limited epidemiologic evidence reviewed in the 2006
O3 AQCD (U.S. EPA, 2006a) reported associations between
short-term increases in ambient O3 concentrations and
airways inflammation in children (1-hour max O3 of
approximately 100 ppb). In the 2006 O3 AQCD (U.S. EPA,
2006a), there was limited evidence for increases in nasal lavage levels
of inflammatory cell counts and molecules released by inflammatory
cells (i.e., eosinophilic cationic protein, and myeloperoxidases).
Since 2006, as a result of the development of less invasive methods,
there has been a large increase in the number of studies assessing
ambient O3-associated changes in airway inflammation and
oxidative stress, the types of biological samples collected, and the
types of indicators. Most of these recent studies have evaluated
biomarkers of inflammation or oxidative stress in exhaled breath, nasal
lavage fluid, or induced sputum (U.S. EPA, 2013a, section 6.2.3.2).
These recent studies form a larger database to establish coherence with
findings from controlled human exposure and animal studies that have
measured the same or related biological markers. Additionally, results
from these studies provide further biological plausibility for the
associations observed between ambient O3 concentrations and
respiratory symptoms and asthma exacerbations.
A number of epidemiologic studies provide evidence that short-term
increases in ambient O3 exposure increase pulmonary
inflammation and oxidative stress in children, including those with
asthma (Sienra-Monge et al., 2004; Barraza-Villarreal et al., 2008;
Romieu et al., 2008; Berhane et al., 2011). Multiple studies examined
and found increases in exhaled nitric oxide (eNO)\48\ (Berhane et al.,
2011; Khatri et al., 2009; Barraza-Villarreal et al., 2008). In some
studies of subjects with asthma, increases in ambient O3
concentration at the same lag were associated with both increases in
pulmonary inflammation and respiratory symptoms (Khatri et al., 2009;
Barraza-Villarreal et al., 2008). Although more limited in number,
epidemiologic studies also found associations with cytokines such as
IL-6 or IL-8 (Barraza-Villarreal et al., 2008; Sienra-Monge et al.,
2004), eosinophils (Khatri et al., 2009), antioxidants (Sienra-Monge et
al., 2004), and indicators of oxidative stress (Romieu et al., 2008)
(U.S. EPA, 2013a, section 6.2.3.2). Because associations with
inflammation were attenuated with higher antioxidant intake in the
study by Sienra-Monge et al. (2004), this study provides additional
evidence that inhaled O3 is likely to be an important source
of reactive oxygen species in airways and/or may increase pulmonary
inflammation via oxidative stress-mediated mechanisms among all age
groups. Limitations in some recent studies have contributed to
inconsistent results in adults (U.S. EPA, 2013a, section 6.2.3.2).
---------------------------------------------------------------------------
\48\ Exhaled NO has been shown to be a useful biomarker for
airway inflammation in large population-based studies (Linn et al.,
2009) (U.S. EPA, 2013a, section 7.2.4).
---------------------------------------------------------------------------
Exposure to ambient O3 on multiple days can result in
larger increases in pulmonary inflammation and oxidative stress, as
discussed in section 6.2.3.2 of the ISA (U.S. EPA, 2013a). In studies
that examined multiple O3 lags,
[[Page 75254]]
multiday averages of 8-hour maximum or 8-hour average concentrations
were associated with larger increases in pulmonary inflammation and
oxidative stress (Berhane et al., 2011; Delfino et al., 2010; Sienra-
Monge et al., 2004), consistent with controlled human exposure (U.S.
EPA, 2013a, section 6.2.3.1) and animal studies (U.S. EPA, 2013a,
section 6.2.3.3) reporting that some markers of pulmonary inflammation
remain elevated with O3 exposures repeated over multiple
days. Evidence from animal toxicological studies also clearly indicates
that O3 exposures result in damage and inflammation in the
lung (U.S. EPA, 2013a, section 5.3). In the few studies that evaluated
the potential for confounding, O3 effect estimates were not
confounded by temperature or humidity, and were robust to adjustment
for PM2.5 or PM10 (Barraza-Villarreal et al.,
2008; Romieu et al., 2008; Sienra-Monge et al., 2004).
In conclusion, a relatively small number of controlled human
exposure studies evaluating O3-induced airway inflammation
have become available since the last review. For purposes of reviewing
the current O3 NAAQS, the most important of these recent
studies reported a statistically significant increase in airway
inflammation in healthy adults at moderate exertion following exposures
to 60 ppb O3, the lowest concentration that has been
evaluated for inflammation. In addition, a number of recent
epidemiologic studies report O3-associated increases in
markers of pulmonary inflammation, particularly in children. Thus,
recent studies continue to support the evidence for airway inflammation
and injury that was available in previous reviews, with new evidence
for such effects following exposures to lower concentrations than had
been evaluated previously.
iii. Airway Hyperresponsiveness
Airway hyperresponsiveness (AHR) refers to a condition in which the
conducting airways undergo enhanced bronchoconstriction in response to
a variety of stimuli. Airway hyperresponsiveness is an important
consequence of exposure to ambient O3 because its presence
reflects a change in airway smooth muscle reactivity, and indicates
that the airways are predisposed to narrowing upon inhalation of a
variety of ambient stimuli including specific triggers (i.e.,
allergens) and nonspecific triggers (e.g., SO2, and cold
air). People with asthma are generally more sensitive to
bronchoconstricting agents than those without asthma, and the use of an
airway challenge to inhaled bronchoconstricting agents is a diagnostic
test in asthma (U.S. EPA, 2013, section 6.2.2). Standards for airway
responsiveness testing have been developed for the clinical laboratory
(ATS, 2000), although variation in the methodology for administering
the bronchoconstricting agent may affect the results (Cockcroft et al.,
2005). There is a wide range of airway responsiveness in people without
asthma, and responsiveness is influenced by a number of factors,
including cigarette smoke, pollutant exposures, respiratory infections,
occupational exposures, and respiratory irritants. Dietary antioxidants
have been reported to attenuate O3-induced bronchial
hyperresponsiveness in people with asthma (Trenga et al., 2001).
Evidence for airway hyperresponsiveness (AHR) following
O3 exposures is derived primarily from controlled human
exposure and toxicological studies (U.S. EPA, 2013a, section 6.2.2).
Airway responsiveness is often quantified by measuring changes in
pulmonary function following the inhalation of an aerosolized allergen
or a nonspecific bronchoconstricting agent (e.g., methacholine), or
following exposure to a bronchoconstricting stimulus such as cold air.
In the last review, controlled human exposure studies of mostly adults
(>=18 years of age) had shown that exposures to O3
concentrations at or above 80 ppb increase airway responsiveness, as
indicated by a reduction in the concentration of specific (e.g.,
ragweed) and non-specific (e.g., methacholine) agents required to
produce a given reduction in lung function (e.g., as measured by
FEV1 or specific airway resistance) (U.S. EPA, 2013a,
section 6.2.2.1). This O3-induced AHR has been reported to
be dose-dependent (Horstman et al., 1990). Animal toxicology studies
have reported O3-induced AHR in a number of species, with
some rat strains exhibiting hyperresponsiveness following 4-hour
exposures to O3 concentrations as low as 50 ppb (Depuydt et
al., 1999). Since the last review, there have been relatively few new
controlled human exposure and animal toxicology studies of
O3 and AHR, and no new studies have evaluated exposures to
O3 concentrations at or below 80 ppb (U.S. EPA, 2013a,
section 6.2.2.1).
Airway hyperresponsiveness is linked with the accumulation and/or
activation of eosinophils in the airways of asthmatics, which is
followed by production of mucus and a late-phase asthmatic response
(section II.B.4.a.ii). In a study of 16 intermittent asthmatics,
Hiltermann et al. (1999) found that there was a significant inverse
correlation between the O3-induced change in the percentage
of eosinophils in induced sputum and the concentration of methacholine
causing a 20% decrease in FEV1. Hiltermann et al. (1999)
concluded that the results point to the role of eosinophils in
O3-induced AHR. Increases in O3-induced
nonspecific airway responsiveness incidence and duration could have
important clinical implications for children and adults with asthma,
such as exacerbations of their disease.
Airway hyperresponsiveness after O3 exposure appears to
resolve more slowly than changes in FEV1 or respiratory
symptoms (Folinsbee and Hazucha, 2000). Studies suggest that
O3-induced AHR usually resolves 18 to 24 hours after
exposure, but may persist in some individuals for longer periods
(Folinsbee and Hazucha, 1989). Furthermore, in studies of repeated
exposure to O3, changes in AHR tend to be somewhat less
susceptible to attenuation with consecutive exposures than changes in
FEV1 (Gong et al., 1997; Folinsbee et al., 1994; Kulle et
al., 1982; Dimeo et al., 1981) (U.S. EPA, 2013a, section 6.2.2). In
animal studies a 3-day continuous exposure resulted in attenuation of
O3-induced AHR (Johnston et al., 2005) while repeated
exposures for 2 hours per day over 10 days did not (Chhabra et al.,
2010), suggesting that attenuation could be lost when repeated
exposures are interspersed with periods of rest (U.S. EPA, 2013a,
section 6.2.2.2).
As mentioned above, in addition to human subjects a number of
species, including nonhuman primates, dogs, cats, rabbits, and rodents,
have been used to examine the effect of O3 exposure on AHR,
(U.S. EPA, 1996, Table 6-14; and U.S. EPA, 2006a, Annex Table AX5-12,
p. AX5-36). A body of animal toxicology studies, including some recent
studies conducted since the last review, provides support for the
O3-induced AHR reported in humans (U.S. EPA, 2013a, section
6.2.2.2). Although most of these studies evaluated O3
concentrations above those typically found in ambient air in cities in
the United States (i.e., most studies evaluated O3
concentrations of 100 ppb or greater), one study reported that a very
low exposure concentration (50 ppb for 4 hours) induced AHR in some rat
strains (Depuydt et al., 1999). Additional recent rodent studies
reported O3-induced AHR following exposures to O3
concentrations from 100 to 500 ppb (Johnston et al., 2005; Chhabra et
al., 2010; Larsen et al., 2010).
[[Page 75255]]
In characterizing the relevance of these exposure concentrations, the
ISA noted that a study using radiolabeled O3 suggests that
even very high O3 exposure concentrations in rodents could
be equivalent to much lower exposure concentrations in humans.
Specifically, a 2000 ppb (2 ppm) O3 exposure concentration
in resting rats was reported to be roughly equivalent to a 400 ppb
exposure concentration in exercising humans (Hatch et al., 1994). Given
this relationship, the ISA noted that animal data obtained in resting
conditions could underestimate the risk of effects for humans (U.S.
EPA, 2013a, section 2.4, p. 2-14).
The 2006 AQCD (U.S. EPA, 2006a, p. 6-34) concluded that spirometric
responses to O3 are independent of inflammatory responses
and markers of epithelial injury (Balmes et al., 1996; Blomberg et al.,
1999; Torres et al., 1997). Significant inflammatory responses to
O3 exposures that did not elicit significant spirometric
responses have been reported (Holz et al., 2005). A recent study (Que
et al., 2011) indicates that AHR also appears to be mediated by a
differing physiologic pathway. These results from controlled human
exposure studies indicate that O3-induced lung function
decrements, inflammatory responses and pulmonary injury (leading to
increased epithelial permeability), and AHR, are mediated by apparently
different physiologic pathways. Except for lung function decrements, we
do not have concentration or exposure response information about the
other, potentially more sensitive,\49\ clinical endpoints (i.e.,
inflammation, increased epithelial permeability, AHR) that would allow
us to quantitatively estimate the size of the population affected and
the magnitude of their responses.
---------------------------------------------------------------------------
\49\ CASAC noted that ``while measures of FEV1 are quantitative
and readily obtainable in humans, they are not the only measures--
and perhaps not the most sensitive measures--of the adverse health
effects induced by ozone exposure.'' (Henderson, 2006).
---------------------------------------------------------------------------
In summary, a strong body of controlled human exposure and animal
toxicological studies, most of which were available in the last review
of the O3 NAAQS, report O3-induced AHR after
either acute or repeated exposures (U.S. EPA, 2013a, section 6.2.2.2).
People with asthma often exhibit increased airway responsiveness at
baseline relative to healthy controls, and they can experience further
increases in responsiveness following exposures to O3.
Studies reporting increased airway responsiveness after O3
exposure contribute to a plausible link between ambient O3
exposures and increased respiratory symptoms in asthmatics, and
increased hospital admissions and emergency department visits for
asthma (U.S. EPA, 2013a, section 6.2.2.2).
iv. Respiratory Symptoms and Medication Use
Respiratory symptoms are associated with adverse outcomes such as
limitations in activity, and are the primary reason for people with
asthma to use quick relief medication and seek medical care. Studies
evaluating the link between O3 exposures and such symptoms
allow a direct characterization of the clinical and public health
significance of ambient O3 exposure. Controlled human
exposure and toxicological studies have described modes of action
through which short-term O3 exposures may increase
respiratory symptoms by demonstrating O3-induced AHR (U.S.
EPA, 2013a, section 6.2.2) and pulmonary inflammation (U.S. EPA, 2013a,
section 6.2.3).
The link between subjective respiratory symptoms and O3
exposures has been evaluated in both controlled human exposure and
epidemiologic studies, and the link with medication use has been
evaluated in epidemiologic studies. In the last review, several
controlled human exposure studies reported respiratory symptoms
following exposures to O3 concentrations at or above 80 ppb.
In addition, one study reported such symptoms following exposures to 60
ppb O3, though the increase was not statistically different
from filtered air controls. Epidemiologic studies reported associations
between ambient O3 and respiratory symptoms and medication
use in a variety of locations and populations, including asthmatic
children living in U.S. cities. In the current review, additional
controlled human exposure studies have evaluated respiratory symptoms
following exposures to O3 concentrations below 80 ppb and
recent epidemiologic studies have evaluated associations with
respiratory symptoms and medication use (U.S. EPA, 2013a, sections
6.2.1, 6.2.4).
In controlled human exposure studies available in the last review
as well as newly available studies, statistically significant increases
in respiratory symptoms have been reported in healthy adult volunteers
engaged in intermittent, moderate exertion following 6.6 hour exposures
to average O3 concentrations of 80 ppb (Adams, 2003; Adams,
2006; Schelegle et al., 2009) and 72 ppb (Schelegle et al., 2009). Such
symptoms have been reported to increase with increasing O3
exposure concentrations, duration of exposure, and activity level
(McDonnell et al., 1999).
Results have been less consistent for lower exposure
concentrations. A recent study by Schelegle et al. (2009) reported a
statistically significant increase in respiratory symptoms in healthy
adults following 6.6 hour exposures to an average O3
concentration of 72 ppb, but not 60 ppb. Kim et al. (2011) also did not
find statistically significant increases in respiratory symptoms
following exposures of healthy adults to 60 ppb O3. Adams
(2006) reported an increase in respiratory symptoms in healthy adults
during a 6.6 hour exposure protocol with an average O3
exposure concentration of 60 ppb. This increase was significantly
different from initial respiratory symptoms, but not from filtered air
controls. The findings for O3-induced respiratory symptoms
in controlled human exposure studies, and the evidence integrated
across disciplines describing underlying modes of action, provide
biological plausibility for epidemiologic associations observed between
short-term increases in ambient O3 concentration and
increases in respiratory symptoms (U.S. EPA, 2013a, section 6.2.4).
In epidemiologic panel studies of respiratory symptoms, data
typically are collected by having subjects (or their parents) record
symptoms and medication use in a diary without direct supervision by
study staff. Several limitations of symptom reports are well
recognized, as described in the ISA (U.S. EPA, 2013a, section 6.2.4).
Nonetheless, symptom diaries remain a convenient tool to collect
individual-level data from a large number of subjects and allow
modeling of associations between daily changes in O3
concentration and daily changes in respiratory morbidity over multiple
weeks or months. Importantly, many of the limitations in these studies
are sources of random measurement error that can bias effect estimates
to the null or increase the uncertainty around effect estimates (U.S.
EPA, 2013a, section 6.2.4). Because respiratory symptoms are associated
with limitations in activity and daily function and are the primary
reason for using medication and seeking medical care, the evidence is
directly coherent with the associations consistently observed between
increases in ambient O3 concentration and increases in
asthma emergency department visits, discussed below (U.S. EPA, 2013a,
section 6.2.4).
Most epidemiologic studies of O3 and respiratory
symptoms and medication use have been conducted in children
[[Page 75256]]
and/or adults with asthma, with fewer studies, and less consistent
results, in non-asthmatic populations (U.S. EPA, 2013a, section 6.2.4).
The 2006 AQCD (U.S. EPA, 2006a, U.S. EPA, 2013a, section 6.2.4)
concluded that the collective body of epidemiologic evidence indicated
that short-term increases in ambient O3 concentrations are
associated with increases in respiratory symptoms in children with
asthma. A large body of single-city and single-region studies of
asthmatic children provides consistent evidence for associations
between short-term increases in ambient O3 concentrations
and increased respiratory symptoms and asthma medication use in
children with asthma (U.S. EPA, 2013a, Figure 6-12, Table 6-20, p. 79).
Methodological differences among studies make comparisons across
recent multicity studies of respiratory symptoms difficult. Because of
fewer person-days of data (Schildcrout et al., 2006) or examination of
19-day averages of ambient O3 concentrations (O'Connor et
al., 2008), the ISA did not give greater weight to results from recent
multicity studies than results from single-city studies (U.S. EPA,
2013a, section 6.2.4.5).\50\ While evidence from the few available U.S.
multicity studies is less consistent (O'Connor et al., 2008;
Schildcrout et al., 2006; Mortimer et al., 2002), the overall body of
epidemiologic evidence with respect to the association betweeen
exposure to O3 and respiratory symptoms in asthmatic
children remains compelling (U.S. EPA, 2013a, section 6.2.4.1).
Findings from a small body of studies indicate that O3 is
also associated with increased respiratory symptoms in adults with
asthma (Khatri et al., 2009; Feo Brito et al., 2007; Ross et al., 2002)
(U.S. EPA, 2013a, section 6.2.4.2).
---------------------------------------------------------------------------
\50\ Though, as discussed below, for other endpoints (e.g.,
hospital admissions, emergency department visits) the ISA focused
primarily on multicity studies.
---------------------------------------------------------------------------
Available evidence indicates that O3-associated
increases in respiratory symptoms are not confounded by temperature,
pollen, or copollutants (primarily PM) (U.S. EPA, 2013a, section
6.2.4.5; Table 6-25; Romieu et al., 1996; Romieu et al., 1997; Thurston
et al., 1997; Gent et al., 2003). However, identifying the independent
effects of O3 in some studies was complicated due to the
high correlations observed between O3 and PM or different
lags and averaging times examined for copollutants. Nonetheless, the
ISA noted that the robustness of associations in some studies of
individuals with asthma, combined with findings from controlled human
exposure studies for the direct effects of O3 exposure,
provide substantial evidence supporting the independent effects of
short-term ambient O3 exposure on respiratory symptoms (U.S.
EPA, 2013a, section 6.2.4.5).
Epidemiologic studies of medication use have reported associations
with 1-hour maximum O3 concentrations and with multiday
average O3 concentrations (Romieu et al., 2006; Just et al.,
2002). Some studies reported O3 associations for both
respiratory symptoms and asthma medication use (Escamilla-Nu[ntilde]ez
et al., 2008; Romieu et al., 2006; Schildcrout et al., 2006; Jalaludin
et al., 2004; Romieu et al., 1997; Thurston et al., 1997) while others
reported associations for either respiratory symptoms or medication use
(Romieu et al., 1996; Rabinovitch et al., 2004; Just et al., 2002;
Ostro et al., 2001).
In summary, both controlled human exposure and epidemiologic
studies have reported respiratory symptoms attributable to short-term
O3 exposures. In the last review, the majority of the
evidence from controlled human exposure studies in young, healthy
adults was for symptoms following exposures to O3
concentrations at or above 80 ppb. Although studies that have become
available since the last review have not reported increased respiratory
symptoms in young, healthy adults following exposures with moderate
exertion to 60 ppb, one recent study did report increased symptoms
following exposure to 72 ppb O3. As was concluded in the
2006 O3 AQCD (U.S. EPA, 2006a; U.S. EPA, 1996), the
collective body of epidemiologic evidence indicates that short-term
increases in ambient O3 concentration are associated with
increases in respiratory symptoms in children with asthma (U.S. EPA,
2013a, section 6.2.4). Recent studies of respiratory symptoms and
medication use, primarily in asthmatic children, add to this evidence.
In a smaller body of studies, increases in ambient O3
concentration were associated with increases in respiratory symptoms in
adults with asthma.
v. Lung Host Defense
The mammalian respiratory tract has a number of closely integrated
defense mechanisms that, when functioning normally, provide protection
from the potential health effects of exposures to a wide variety of
inhaled particles and microbes. These defense mechanisms include
mucociliary clearance, alveolobronchiolar transport mechanism, alveolar
macrophages,\51\ and adaptive immunity \52\ (U.S. EPA, 2013a, section
6.2.5). The previous O3 AQCD (U.S. EPA, 2006a) concluded
that animal toxicological studies provided evidence that acute exposure
to O3 concentrations as low as 100 to 500 ppb can increase
susceptibility to infectious diseases due to modulation of these lung
host defenses. This conclusion was based, in large part, on animal
studies of alveolar macrophage function and mucociliary clearance (U.S.
EPA, 2013a, section 6.2.5).
---------------------------------------------------------------------------
\51\ Phagocytic white blood cells within the alveoli of the
lungs that ingest inhaled particles.
\52\ The adaptive immune system, is also known as the acquired
immune system. Acquired immunity creates immunological memory after
an initial response to a specific pathogen, leading to an enhanced
response to subsequent encounters with that same pathogen.
---------------------------------------------------------------------------
Integrating animal study results with human exposure evidence, the
2006 Criteria Document concluded that available evidence indicates that
short-term O3 exposures have the potential to impair host
defenses in humans, primarily by interfering with alveolar macrophage
function. Any impairment in alveolar macrophage function may lead to
decreased clearance of microorganisms or nonviable particles.
Compromised alveolar macrophage functions in asthmatics may increase
their susceptibility to other O3 effects, the effects of
particles, and respiratory infections (U.S. EPA, 2006a, p. 8-26). These
conclusions were based largely on studies conducted in animals exposed
for several hours up to several weeks to O3 concentrations
from 100 to 250 ppb (Hurst et al., 1970; Driscoll et al., 1987; Cohen
et al., 2002). Consistent with the animal evidence, a controlled human
exposure study available in the last review had reported decrements in
the ability of alveolar macrophages to phagocytize yeast following
exposures of healthy volunteers to O3 concentrations of 80
and 100 ppb for 6.6 hours during moderate exercise (Devlin et al.,
1991).
Alveolobronchiolar transport mechanisms refers to the transport of
particles deposited in the deep lung (alveoli) which may be removed
either up through the respiratory tract (bronchi) by alveolobronchiolar
transport or through the lymphatic system. The pivotal mechanism of
alveolobronchiolar transport involves the movement of alveolar
macrophages with ingested particles to the bottom of the conducting
airways. These airways are lined with ciliated epithelial cells and
cells that produce mucous, which surrounds the macrophages. The
ciliated epithelial cells move the
[[Page 75257]]
mucous packets up the resiratory tract, hence the term ``mucociliary
escalator.'' Although some studies show reduced tracheobronchial
clearance after O3 exposure (U.S. EPA, 2013a, section
6.2.5.1), alveolar clearance of deposited material is accelerated,
presumably due to macrophage influx, which in itself can be damaging.
With regard to adaptive immunity, a limited number of epidemiologic
studies have examined associations between O3 exposure and
hospital admissions or emergency department visits for respiratory
infection, pneumonia, or influenza. Results have been mixed, and in
some cases conflicting (U.S. EPA, 2013a, sections 6.2.7.2 and 6.2.7.3).
With the exception of influenza, it is difficult to ascertain whether
cases of respiratory infection or pneumonia are of viral or bacterial
etiology. A recent study that examined the association between
O3 exposure and respiratory hospital admissions in response
to an increase in influenza intensity observed an increase in
respiratory hospital admissions (Wong et al., 2009), but information
from toxicological studies of O3 and viral infections is
ambiguous.
In summary, relatively few studies conducted since the last review
have evaluated the effects of O3 exposures on lung host
defense. When the available evidence is taken as a whole, the ISA
concludes that acute O3 exposures impair the host defense
capability of animals, primarily by depressing alveolar macrophage
function and perhaps also by decreasing mucociliary clearance of
inhaled particles and microorganisms. Coupled with limited evidence
from controlled human exposure studies, this suggests that humans
exposed to O3 could be predisposed to bacterial infections
in the lower respiratory tract (U.S. EPA, 2013a, section 6.2.5.5).
vi. Allergic and Asthma-Related Responses
Effects resulting from combined exposures to O3 and
allergens have been studied in a variety of animal species, generally
as models of experimental asthma. Pulmonary function and AHR in animal
models of asthma are discussed in detail in Section 6.2.1.3 and Section
6.2.2.2, respectively, in the ISA (U.S. EPA, 2013a). Studies of
allergic and asthma-related responses are discussed in detail in
sections 5.3.6 and 6.2.6 of the ISA (U.S. EPA, 2013a).
Evidence available in the last review indicates that O3
exposure skews immune responses toward an allergic phenotype and could
also make airborne allergens more allergenic. In humans, allergic
rhinoconjunctivitis symptoms are associated with increases in ambient
O3 concentrations (Riediker et al., 2001). Controlled human
exposure studies have observed O3-induced changes indicating
allergic skewing. Airway eosinophils, which are white blood cells that
participate in allergic disease and inflammation, were observed to
increase in volunteers with atopy \53\ and mild asthma (Peden et al.,
1997). In a more recent study, expression of IL-5, a cytokine involved
in eosinophil recruitment and activation, was increased in subjects
with atopy but not in healthy subjects (Hernandez et al., 2010).
Epidemiologic studies describe associations between eosinophils in both
short- (U.S. EPA, 2013a, section 6.2.3.2) and long-term (U.S. EPA,
2013a, section 7.2.5) O3 exposure, as do chronic exposure
studies in non-human primates. Collectively, findings from these
studies suggest that O3 can induce or enhance certain
components of allergic inflammation in individuals with allergy or
allergic asthma.
---------------------------------------------------------------------------
\53\ Atopy is a predisposition toward developing certain
allergic hypersensitivity reactions. A person with atopy typically
presents with one or more of the following: eczema (atopic
dermatitis), allergic rhinitis (hay fever), allergic conjunctivitis,
or allergic asthma.
---------------------------------------------------------------------------
Evidence available in the last review indicates that O3
may also increase AHR to specific allergen triggers (75 FR 2970,
January 19, 2010). Two studies (J[ouml]rres et al., 1996; Holz et al.,
2002) observed increased airway responsiveness to O3
exposure with bronchial allergen challenge in subjects with preexisting
allergic airway disease. Ozone-induced exacerbation of airway
responsiveness persists longer and attenuates more slowly than
O3-induced lung function decrements and respiratory symptom
responses and can have important clinical implications for asthmatics.
Animal toxicology studies indicate that O3 enhances
inflammatory and allergic responses to allergen challenge in sensitized
animals. In addition to exacerbating existing allergic responses,
toxicology studies indicate that O3 can also act as an
adjuvant to produce sensitization in the respiratory tract. Along with
its pro-allergic effects (inducing or enhancing certain components of
allergic inflammation in individuals with allergy or allergic asthma),
O3 could also make airborne allergens more allergenic. When
combined with NO2, O3 has been shown to enhance
nitration of common protein allergens, which may increase their
allergenicity (Franze et al., 2005).
vii. Hospital Admissions and Emergency Department Visits
The 2006 O3 AQCD evaluated numerous studies of
respiratory-related emergency department visits and hospital
admissions. These were primarily time-series studies conducted in the
U.S., Canada, Europe, South America, Australia, and Asia. Based on such
studies, the 2006 O3 AQCD concluded that ``the overall
evidence supports a causal relationship between acute ambient
O3 exposures and increased respiratory morbidity resulting
in increased emergency department visits and [hospital admissions]
during the warm season'' \54\ (U.S. EPA, 2006a). This conclusion was
``strongly supported by the human clinical, animal toxicologic[al], and
epidemiologic evidence for [O3-induced] lung function
decrements, increased respiratory symptoms, airway inflammation, and
airway hyperreactivity'' (U.S. EPA, 2006a).
---------------------------------------------------------------------------
\54\ Epidemiologic associations for O3 are more
robust during the warm season than during cooler months (e.g.,
smaller measurement error, less potential confounding by
copollutants). Rationale for focusing on warm season epidemiologic
studies for O3 can be found at 72 FR 37838-37840.
---------------------------------------------------------------------------
The results of recent studies largely support the conclusions of
the 2006 O3 AQCD (U.S. EPA, 2013a, section 6.2.7). Since the
completion of the 2006 O3 AQCD, relatively fewer studies
conducted in the U.S., Canada, and Europe have evaluated associations
between short-term O3 concentrations and respiratory
hospital admissions and emergency department visits, with a growing
number of studies conducted in Asia. This epidemiologic evidence is
discussed in detail in the ISA (U.S. EPA, 2013a, section 6.2.7).\55\
---------------------------------------------------------------------------
\55\ The consideration of ambient O3 concentrations
in the locations of these epidemiologic studies are discussed in
sections II.D.1.b and II.E.4.a below, for the current standard and
alternative standards, respectively.
---------------------------------------------------------------------------
In considering this body of evidence, the ISA focused primarily on
multicity studies because they examine associations with respiratory-
related hospital admissions and emergency department visits over large
geographic areas using consistent statistical methodologies (U.S. EPA,
2013a, section 6.2.7.1). The ISA also focused on single-city studies
that encompassed a large number of daily hospital admissions or
emergency department visits, included long study-durations, were
conducted in locations not represented by the larger studies, or
examined population-specific characteristics that may impact the risk
of O3-related health effects but were not evaluated in the
larger studies (U.S. EPA, 2013a, section 6.2.7.1). When
[[Page 75258]]
examining the association between short-term O3 exposure and
respiratory health effects that require medical attention, the ISA
distinguishes between hospital admissions and emergency department
visits because it is likely that a small percentage of respiratory
emergency department visits will be admitted to the hospital;
therefore, respiratory emergency department visits may represent
potentially less serious, but more common outcomes (U.S. EPA, 2013a,
section 6.2.7.1).
Several recent multicity studies (e.g., Cakmak et al., 2006; Dales
et al., 2006) and a multi-continent study (Katsouyanni et al., 2009)
report associations between short-term O3 concentrations and
increased respiratory-related hospital admissions and emergency
department visits. These multicity studies are supported by results
from single-city studies also reporting consistent positive
associations using different exposure assignment approaches (i.e.,
average of multiple monitors, single monitor, population-weighted
average) and averaging times (i.e., 1-hour max and 8-hour max) (U.S.
EPA, 2013a, sections 6.2.7.1 to 6.2.7.5). When examining cause-specific
respiratory outcomes, recent studies report positive associations with
hospital admissions and emergency department visits for asthma
(Strickland et al., 2010; Stieb et al., 2009) and chronic obstructive
pulmonary disease (COPD) (Stieb et al., 2009; Medina-Ramon et al.,
2006), with more limited evidence for pneumonia (Medina-Ramon et al.,
2006; Zanobetti and Schwartz, 2006). In seasonal analyses (U.S. EPA,
2013a, Figure 6-19, Table 6-28), stronger associations were reported in
the warm season or summer months, when O3 concentrations are
higher, compared to the cold season, particularly for asthma
(Strickland et al., 2010; Ito et al., 2007) and COPD (Medina-Ramon et
al., 2006). The available evidence indicates that children are at
greatest risk for effects leading to O3-associated hospital
admissions and emergency department visits (Silverman and Ito, 2010;
Mar and Koenig, 2009; Villeneuve et al., 2007).
Although the collective evidence across studies indicates a mostly
consistent positive association between O3 exposure and
respiratory-related hospital admissions and emergency department
visits, the magnitude of these associations may be underestimated to
the extent members of study populations modify their behavior in
response to air quality forecasts, and to the extent such behavior
modification increases exposure misclassification (U.S. EPA, 2013,
Section 4.6.6). Studies examining the potential confounding effects of
copollutants have reported that O3 effect estimates remained
relatively robust upon the inclusion of PM and gaseous pollutants in
two-pollutant models (U.S. EPA, 2013a, Figure 6-20, Table 6-29).
Additional studies that conducted copollutant analyses, but did not
present quantitative results, also support these conclusions
(Strickland et al., 2010; Tolbert et al., 2007; Medina-Ramon et al.,
2006) (U.S. EPA, 2013a, section 6.2.7.5).
In the last review, studies had not evaluated the concentration-
response relationship between short-term O3 exposure and
respiratory-related hospital admissions and emergency department
visits. A preliminary examination of this relationship in studies that
have become available since the last review found no evidence of a
deviation from linearity when examining the association between short-
term O3 exposure and asthma hospital admissions (U.S. EPA,
2013a, page 6-157; Silverman and Ito, 2010). In addition, an
examination of the concentration-response relationship for
O3 exposure and pediatric asthma emergency department visits
found no evidence of a threshold at O3 concentrations as low
as 30 ppb (for daily maximum 8-hour concentrations) (Strickland et al.,
2010). However, in both studies there is uncertainty in the shape of
the concentration-response curve at the lower end of the distribution
of O3 concentrations due to the low density of data in this
range (U.S. EPA, 2013a, page 6-157).
viii. Respiratory Mortality
The controlled human exposure, epidemiologic, and toxicological
studies discussed in section 6.2 of the ISA (U.S. EPA, 2013a) provide
evidence for respiratory morbidity effects, including emergency
department visits and hospital admissions, in response to short-term
O3 exposures. Moreover, evidence from experimental studies
indicates multiple potential pathways of respiratory effects from
short-term O3 exposures, which support the continuum of
respiratory effects that could potentially result in respiratory-
related mortality in adults (U.S. EPA, 2013a, section 6.2.8). The 2006
O3 AQCD found inconsistent evidence for associations between
short-term O3 concentrations and respiratory mortality (U.S.
EPA, 2006a). Although some studies reported a strong positive
association between O3 and respiratory mortality, additional
studies reported small associations or no associations. New
epidemiologic evidence for respiratory mortality is discussed in detail
in section 6.2.8 of the ISA (U.S. EPA, 2013a). The majority of recent
multicity studies have reported positive associations between short-
term O3 exposures and respiratory mortality, particularly
during the summer months (U.S. EPA, 2013a, Figure 6-36).
Specifically, recent multicity studies from the U.S. (Zanobetti and
Schwartz, 2008b), Europe (Samoli et al., 2009), Italy (Stafoggia et
al., 2010), and Asia (Wong et al., 2010), as well as a multi-continent
study (Katsouyanni et al., 2009), reported associations between short-
term O3 concentrations and respiratory mortality (U.S. EPA,
2013a, Figure 6-37, page 6-259). With respect to respiratory mortality,
summer-only analyses were consistently positive and most were
statistically significant. All-year analyses had more mixed results,
but most were positive.
Of the studies evaluated, only the studies by Katsouyanni et al.
(2009) and by Stafoggia et al. (2010) analyzed the potential for
copollutant confounding of the O3-respiratory mortality
relationship. Based on the results of these analyses, the ISA concluded
that O3 respiratory mortality risk estimates appear to be
moderately to substantially sensitive (e.g., increased or attenuated)
to inclusion of PM10. However, in the APHENA study
(Katsouyanni et al., 2009), the mostly every-6th-day sampling schedule
for PM10 in the Canadian and U.S. datasets greatly reduced
their sample size and limits the interpretation of these results (U.S.
EPA, 2013a, section 6.2.8).
In summary, recent epidemiologic studies support and reinforce the
epidemiologic evidence for O3-associated respiratory
hospital admissions and emergency department visits from the last
review. In addition, the evidence for associations with respiratory
mortality has been strengthened since the last review, with the
addition of several large multicity studies. The biological
plausibility of the associations reported in these studies is supported
by the experimental evidence for respiratory effects.
b. Respiratory Effects--Long-Term
Since the last review, the body of evidence indicating the
occurrence of respiratory effects due to long-term O3
exposure has been strengthened. This evidence is discussed in detail in
the ISA (U.S. EPA, 2013a, Chapter 7) and summarized below for new-onset
asthma and asthma prevalence, asthma hospital admissions, pulmonary
structure and function, and respiratory mortality.
[[Page 75259]]
i. New-Onset Asthma and Asthma Prevalence
Asthma is a heterogeneous disease with a high degree of temporal
variability. The on-set, progression, and symptoms can vary within an
individual's lifetime, and the course of asthma may vary markedly in
young children, older children, adolescents, and adults. In the
previous review, longitudinal cohort studies that examined associations
between long-term O3 exposures and the onset of asthma in
adults and children indicated a direct effect of long-term
O3 exposures on asthma risk in adults (McDonnell et al.,
1999, 15-year follow-up; Greer et al., 1993, 10-year follow-up) and
effect modification by O3 in children (McConnell et al.,
2002). Since that review, additional studies have evaluated
associations with new onset asthma, further informing our understanding
of the potential gene-environment interactions, mechanisms, and
biological pathways associated with incident asthma.
In children, the relationship between long-term O3
exposure and new-onset asthma has been extensively studied in the
Children's Health Study (CHS), a long-term study that was initiated in
the early 1990's which has evaluated effects in several cohorts of
children. The CHS was initially designed to examine whether long-term
exposure to ambient pollution was related to chronic respiratory
outcomes in children in 12 communities in southern California. In the
CHS, new-onset asthma was classified as having no prior history of
asthma at study entry with subsequent report of physician-diagnosed
asthma at follow-up, with the date of onset assigned to be the midpoint
of the interval between the interview date when asthma diagnosis was
first reported and the previous interview date. The results of one
study (McConnell et al., 2002) available in the previous review
indicated that within high O3 communities, asthma risk was
3.3 times greater for children who played three or more outdoor sports
as compared with children who played no sports.
For this review, as discussed in section 7.2.1.1 of the ISA (U.S.
EPA, 2013a), recent studies from the CHS provide evidence for gene-
environment interactions in effects on new-onset asthma by indicating
that the lower risks associated with specific genetic variants are
found in children who live in lower O3 communities. These
studies indicate that the risk for new-onset asthma is related in part
to genetic susceptibility, as well as behavioral factors and
environmental exposure. The onset of a chronic disease, such as asthma,
is partially the result of a sequence of biochemical reactions
involving exposures to various environmental agents metabolized by
enzymes related to a number of different genes. Oxidative stress has
been proposed to underlie the mechanistic hypotheses related to
O3 exposure. Genetic variants may impact disease risk
directly, or modify disease risk by affecting internal dose of
pollutants and other environmental agents and/or their reaction
products, or by altering cellular and molecular modes of action.
Understanding the relation between genetic polymorphisms and
environmental exposure can help identify high-risk subgroups in the
population and provide better insight into pathway mechanisms for these
complex diseases.
The CHS analyses (Islam et al., 2008; Islam et al., 2009; Salam et
al., 2009) have found that asthma risk is related to interactions
between O3 and variants in genes for enzymes such as heme-
oxygenase (HO-1), arginases (ARG1 and 2), and glutathione S transferase
P1 (GSTP1). Biological plausibility for these findings is provided by
evidence that these enzymes have antioxidant and/or anti-inflammatory
activity and participate in well-recognized modes of action in asthma
pathogenesis. As O3 is a source of oxidants in the airways,
oxidative stress serves as the link among O3 exposure,
enzyme activity, and asthma. Further, several lines of evidence
demonstrate that secondary oxidation products of O3 initiate
the key modes of action that mediate downstream health effects (U.S.
EPA, 2013a, section 5.3). For example, HO-1 responds rapidly to
oxidants, has anti-inflammatory and antioxidant effects, relaxes airway
smooth muscle, and is induced in the airways during asthma. Cross-
sectional studies by Akinbami et al. (2010) and Hwang et al. (2005)
provide further evidence relating O3 exposures with asthma
prevalence. Gene-environment interactions are discussed in detail in
Section 5.4.2.1 in the ISA (U.S. EPA, 2013a).
ii. Asthma Hospital Admissions
In the 2006 AQCD, studies on O3-related hospital
discharges and emergency department visits for asthma and respiratory
disease mainly looked at short-term (daily) metrics. The short-term
O3 studies presented in section 6.2.7.5 of the ISA (U.S.
EPA, 2013a) and discussed above in section 3.1.2.1 continue to indicate
that there is evidence for increases in both hospital admissions and
emergency department visits in children and adults related to all
respiratory outcomes, including asthma, with stronger associations in
the warm months. New studies, discussed in section 7.2.2 of the ISA
(U.S. EPA, 2013a) also evaluated long-term O3 exposure
metrics, providing a new line of evidence that suggests a positive
exposure-response relationship between the first hospital admission for
asthma and long-term O3 exposure, although the ISA cautions
in attributing the associations in that study to long-term exposures
since there is potential for short-term exposures to contribute to the
observed associations.
Evidence associating long-term O3 exposure to first
asthma hospital admission in a positive concentration-response
relationship is provided in a retrospective cohort study (Lin et al.,
2008b). This study investigated the association between chronic
exposure to O3 and childhood asthma admissions by following
a birth cohort of more than 1.2 million babies born in New York State
(1995-1999) to first asthma admission or until December 31, 2000. Three
annual indicators (all 8-hour maximum from 10:00 a.m. to 6:00 p.m.)
were used to define chronic O3 exposure: (1) Mean
concentration during the follow-up period (41.06 ppb); (2) mean
concentration during the O3 season (50.62 ppb); and (3)
proportion of follow-up days with O3 levels >70 ppb. The
effects of copollutants were controlled, and interaction terms were
used to assess potential effect modifications. A positive association
between chronic exposure to O3 and childhood asthma hospital
admissions was observed, indicating that children exposed to high
O3 levels over time are more likely to develop asthma severe
enough to be admitted to the hospital. The various factors were
examined and differences were found for younger children (1-2 years),
poor neighborhoods, Medicaid/self-paid births, geographic region and
others. As shown in the ISA, Figure 7-3 (U.S. EPA, 2013a, p. 7-16),
positive concentration-response relationships were observed. Asthma
admissions were significantly associated with increased O3
levels for all chronic exposure indicators.
In considering the relationship between long-term pollutant
exposures and chronic disease health endpoints, where chronic
pathologies are found with acute expression of chronic disease,
K[uuml]nzli (2012) hypothesizes that if the associations of pollution
with events are much larger in the long-term studies, it provides some
indirect evidence that air pollution increases the pool of subjects
with chronic disease, and that more acute events are to be
[[Page 75260]]
expected to be seen for higher exposures. The results of Lin et al
(2008a) for first asthma hospital admission, presented in Figure 7-3
(U.S. EPA, 2013a, p. 7-16), show effects estimates that are larger than
those reported in a study of childhood asthma hospital admission in New
York State (Silverman and Ito, 2010), discussed above. The ISA (U.S.
EPA, 2013a, p. 7-16) notes that this provides some support for the
hypothesis that O3 exposure may not only have triggered the
events but also increased the pool of asthmatic children, but cautions
in attributing the associations in the Lin et al. (2008) study to long-
term exposures since there is potential for short-term exposures to
contribute to the observed associations.
iii. Pulmonary Structure and Function
In the 2006 O3 AQCD, few epidemiologic studies had
investigated the effect of chronic O3 exposure on pulmonary
function. The definitive 8-year follow-up analysis of the first cohort
of the CHS (U.S. EPA, 2013a, section 7.2.3.1) provided little evidence
that long-term exposure to ambient O3 was associated with
significant deficits in the growth rate of lung function in children.
The strongest evidence was for medium-term effects of extended
O3 exposures over several summer months on lung function
(FEV1) in children, i.e., reduced lung function growth being
associated with higher ambient O3 levels. Short-term
O3 exposure studies presented in the ISA (U.S. EPA, 2013a,
section 6.2.1.2) provide a cumulative body of epidemiologic evidence
that strongly supports associations between ambient O3
exposure and decrements in lung function among children. A later CHS
study (Islam et al., 2007) included in this review (U.S. EPA, 2013a,
section 7.2.3.1) also reported no substantial differences in the effect
of O3 on lung function. However, in a more recent CHS study,
Breton et al. (2011) hypothesized that genetic variation in genes on
the glutathione metabolic pathway may influence the association between
ambient air pollutant exposures and lung function growth in children,
and found that variation in the GSS locus was associated with
differences in risk of children for lung function growth deficits
associated ambient air pollutants, including O3. A recent
study (Rojas-Martinez et al., 2007) of long-term exposure to
O3, described in section 7.2.3.1 of the ISA (U.S. EPA,
2013a, p. 7-19), observed a relationship with pulmonary function
declines in school-aged children where O3 and other
pollutant levels were higher (90 ppb at high end of the range) than
those in the CHS. Two studies of adult cohorts provide mixed results
where long-term exposures were at the high end of the range.
Long-term studies in animals allow for greater insight into the
potential effects of prolonged exposure to O3 that may not
be easily measured in humans, such as structural changes in the
respiratory tract. Despite uncertainties, epidemiologic studies
observing associations of O3 exposure with functional
changes in humans can attain biological plausibility in conjunction
with long-term toxicological studies, particularly O3-
inhalation studies performed in non-human primates whose respiratory
systems most closely resemble that of the human. An important series of
studies, discussed in section 7.2.3.2 of the ISA (U.S. EPA, 2013a),
have used nonhuman primates to examine the effect of O3
alone, or in combination with an inhaled allergen, house dust mite
antigen (HDMA), on morphology and lung function. Animals exhibit the
hallmarks of allergic asthma defined for humans (NHLBI, 2007). These
studies and others have demonstrated changes in pulmonary function and
airway morphology in adult and infant nonhuman primates repeatedly
exposed to environmentally relevant concentrations of O3
(U.S. EPA, 2013a, section 7.2.3.2).
The initial observations in adult nonhuman primates have been
expanded in a series of experiments using infant rhesus monkeys
repeatedly exposed to 0.5 ppm O3 starting at 1 month of age
(Plopper et al., 2007; Schelegle et al. 2003). The purpose of these
studies was to determine if a cyclic regimen of O3
inhalation would amplify the allergic responses and structural
remodeling associated with allergic sensitization and inhalation in the
infant rhesus monkey; they provide evidence of an O3-induced
change in airway resistance and responsiveness provides biological
plausibility of long-term exposure, or repeated short-term exposures,
to O3 contributing to the effects of asthma in children.
In addition, significant structural changes in the respiratory
tract development, during which conducting airways increase in diameter
and length, have been observed in infant rhesus monkeys after cyclic
exposure to O3 (Fanucchi et al., 2006). These effects are
noteworthy because of their potential contribution to airway
obstruction and AHR which are central features of asthma. A number of
studies in both non-human primates and rodents demonstrate that
O3 exposure can increase collagen synthesis and deposition,
including fibrotic-like changes in the lung (U.S. EPA, 2013a, section
7.2.3.2).
Collectively, evidence from animal studies strongly suggests that
chronic O3 exposure is capable of damaging the distal
airways and proximal alveoli, resulting in lung tissue remodeling and
leading to apparent irreversible changes. Potentially, persistent
inflammation and interstitial remodeling play an important role in the
progression and development of chronic lung disease. Further discussion
of the modes of action that lead to O3-induced morphological
changes can be found in section 5.3.7 of the ISA (U.S. EPA, 2013a).
Discussion of mechanisms involved in lifestage susceptibility and
developmental effects can be found in section 5.4.2.4 of the ISA (U.S.
EPA, 2013a). The findings reported in chronic animal studies offer
insight into potential biological mechanisms for the suggested
association between seasonal O3 exposure and reduced lung
function development in children as observed in epidemiologic studies
(U.S. EPA, 2013a, section 7.2.3.1).
iv. Respiratory Mortality
A limited number of epidemiologic studies have assessed the
relationship between long-term exposure to O3 and mortality
in adults. The 2006 O3 AQCD concluded that an insufficient
amount of evidence existed ``to suggest a causal relationship between
chronic O3 exposure and increased risk for mortality in
humans'' (U.S. EPA, 2006a). Though total and cardio-pulmonary mortality
were considered in these studies, respiratory mortality was not
specifically considered.
In the most recent follow-up analysis of the American Cancer
Society (ACS) cohort (Jerrett et al., 2009), cardiopulmonary deaths
were separately subdivided into respiratory and cardiovascular deaths,
rather than combined as in the Pope et al. (2002) work. Increased
O3 exposure was associated with the risk of death from
respiratory causes, and this effect was robust to the inclusion of
PM2.5. The association between increased O3
concentrations and increased risk of death from respiratory causes was
insensitive to the use of different models and to adjustment for
several ecologic variables considered individually. The authors
reported that when seasonal averages of 1-hour daily maximum
O3 concentrations ranged from 33 to 104 ppb, there was no
statistical deviation from a linear concentration-response relationship
between O3 and respiratory mortality across 96 U.S. cities
(U.S. EPA, 2013a, section 7.7). However, the authors also
[[Page 75261]]
evaluated the degree to which models incorporating thresholds provided
a better fit to the data. Based on these analyses, Jerrett et al.
(2009) reported ``limited evidence'' for an effect threshold at an
O3 concentration of 56 ppb (p=0.06).
Additionally, a recent multicity time series study (Zanobetti and
Schwartz, 2011), which followed (from 1985 to 2006) four cohorts of
Medicare enrollees with chronic conditions that might predispose to
O3-related effects, observed an association between long-
term (warm season) exposure to O3 and elevated risk of
mortality in the cohort that had previously experienced an emergency
hospital admission due to COPD. A key limitation of this study is the
inability to control for PM2.5, because data were not
available in these cities until 1999.
c. Cardiovascular Effects
A relatively small number of studies have examined the potential
effect of short-term O3 exposure on the cardiovascular
system. The 2006 O3 AQCD (U.S. EPA, 2006a, p. 8-77)
concluded that ``O3 directly and/or indirectly contributes
to cardiovascular-related morbidity,'' but added that the body of
evidence was limited. This conclusion was based on a controlled human
exposure study that included hypertensive adult males; a few
epidemiologic studies of physiologic effects, heart rate variability,
arrhythmias, myocardial infarctions, and hospital admissions; and
toxicological studies of heart rate, heart rhythm, and blood pressure.
More recently, the body of scientific evidence available that has
examined the effect of O3 on the cardiovascular system has
expanded. There is an emerging body of animal toxicological evidence
demonstrating that short-term exposure to O3 can lead to
autonomic nervous system alterations (in heart rate and/or heart rate
variability) and suggesting that proinflammatory signals may mediate
cardiovascular effects. Interactions of O3 with respiratory
tract components result in secondary oxidation product formation and
subsequent production of inflammatory mediators, which have the
potential to penetrate the epithelial barrier and to initiate toxic
effects systemically. In addition, animal toxicological studies of
long-term exposure to O3 provide evidence of enhanced
atherosclerosis and ischemia/reperfusion (I/R) injury, corresponding
with development of a systemic oxidative, proinflammatory environment.
Recent experimental and epidemiologic studies have investigated
O3-related cardiovascular events and are summarized in
section 6.3 of the ISA (U.S. EPA, 2013a). Overall, the ISA summarized
the evidence in this review as follows (U.S. EPA, 2013a, p. 6-211).
In conclusion, animal toxicological studies demonstrate
O3-induced cardiovascular effects, and support the strong
body of epidemiologic evidence indicating O3-induced
cardiovascular mortality. Animal toxicological and controlled human
exposure studies provide evidence for biologically plausible
mechanisms underlying these O3-induced cardiovascular
effects. However, a lack of coherence with epidemiologic studies of
cardiovascular morbidity remains an important uncertainty.
Controlled human exposure studies discussed in previous AQCDs have
not demonstrated any consistent extrapulmonary effects. In this review,
evidence from controlled human exposure studies suggests cardiovascular
effects in response to short-term O3 exposure (U.S. EPA,
2013a, section 6.3.1) and provides some coherence with evidence from
animal toxicology studies. Controlled human exposure studies also
support the animal toxicological studies by demonstrating
O3-induced effects on blood biomarkers of systemic
inflammation and oxidative stress, as well as changes in biomarkers
that can indicate the potential for increased clotting following
O3 exposures. Increases and decreases in high frequency
heart rate variability (HRV) have been reported following relatively
low (120 ppb during rest) and high (300 ppb with exercise)
O3 exposures, respectively. These changes in cardiac
function observed in animal and human studies provide preliminary
evidence for O3-induced modulation of the autonomic nervous
system through the activation of neural reflexes in the lung (U.S. EPA
2013a, section 5.3.2).
Overall, the ISA concludes that the available body of epidemiologic
evidence examining the relationship between short-term exposures to
O3 concentrations and cardiovascular morbidity is
inconsistent (U.S. EPA, 2013a, section 6.3.2.9). Across studies,
different definitions (i.e., ICD-9 diagnostic codes) were used for both
all-cause and cause-specific cardiovascular morbidity (U.S. EPA, 2013a,
Tables 6-35 to 6-39), which may contribute to inconsistency in results.
However, within diagnostic categories, no consistent pattern of
association was found with O3. Generally, the epidemiologic
studies used nearest air monitors to assess O3
concentrations, with a few exceptions that used modeling or personal
exposure monitors. The inconsistencies in the associations observed
between short-term O3 and cardiovascular disease (CVD)
morbidities are unlikely to be explained by the different exposure
assignment methods used (U.S. EPA, 2013a, section 4.6). The wide
variety of biomarkers considered and the lack of consistency among
definitions used for specific cardiovascular disease endpoints (e.g.,
arrhythmias, HRV) make comparisons across studies difficult.
Despite the inconsistent evidence for an association between
O3 concentration and CVD morbidity, mortality studies
indicate a consistent positive association between short-term
O3 exposure and cardiovascular mortality in multicity
studies and in a multi-continent study. When examining mortality due to
CVD, epidemiologic studies consistently observe positive associations
with short-term exposure to O3. Additionally, there is some
evidence for an association between long-term exposure to O3
and mortality, although the association between long-term ambient
O3 concentrations and cardiovascular mortality can be
confounded by other pollutants (U.S. EPA, 2013a). The ISA (U.S. EPA
2013a, section 6.3.4) states that taken together, the overall body of
evidence across the animal and human studies is sufficient to conclude
that there is likely to be a causal relationship between relevant
short-term exposures to O3 and cardiovascular system
effects.
d. Total Mortality
The 2006 O3 AQCD concluded that the overall body of
evidence was highly suggestive that short-term exposure to
O3 directly or indirectly contributes to nonaccidental and
cardiopulmonary-related mortality in adults, but additional research
was needed to more fully establish underlying mechanisms by which such
effects occur (U.S. EPA, 2013a, p. 2-18). In building on the 2006
evidence for mortality, the ISA states the following (U.S. EPA, 2013a,
p. 6-261).
The evaluation of new multicity studies that examined the
association between short-term O3 exposures and mortality
found evidence that supports the conclusions of the 2006 AQCD. These
new studies reported consistent positive associations between short-
term O3 exposure and all-cause (nonaccidental) mortality,
with associations persisting or increasing in magnitude during the
warm season, and provide additional support for associations between
O3 exposure and cardiovascular and respiratory mortality.
The 2006 O3 AQCD reviewed a large number of time-series
studies of associations between short-term O3 exposures and
total mortality including single- and multicity studies, and meta-
analyses. In the large U.S. multicity
[[Page 75262]]
studies that examined all-year data, effect estimates corresponding to
single-day lags ranged from a 0.5-1% increase in all-cause
(nonaccidental) total mortality per a 20 ppb (24-hour), 30 ppb (8-hour
maximum), or 40 ppb (1-hour maximum) increase in ambient O3
(U.S. EPA, 2013a, section 6.6.2). Available studies reported some
evidence for heterogeneity in O3 mortality risk estimates
across cities and across studies. Studies that conducted seasonal
analyses reported larger O3 mortality risk estimates during
the warm or summer season. Overall, the 2006 O3 AQCD
identified robust associations between various measures of daily
ambient O3 concentrations and all-cause mortality, which
could not be readily explained by confounding due to time, weather, or
copollutants. With regard to cause-specific mortality, consistent
positive associations were reported between short-term O3
exposure and cardiovascular mortality, with less consistent evidence
for associations with respiratory mortality. The majority of the
evidence for associations between O3 and cause-specific
mortality were from single-city studies, which had small daily
mortality counts and subsequently limited statistical power to detect
associations. The 2006 O3 AQCD concluded that ``the overall
body of evidence is highly suggestive that O3 directly or
indirectly contributes to nonaccidental and cardiopulmonary-related
mortality'' (U.S. EPA, 2013a, section 6.6.1).
Recent studies have strengthened the body of evidence that supports
the association between short-term O3 concentrations and
mortality in adults. This evidence includes a number of studies
reporting associations with nonaccidental as well as cause-specific
mortality. Multi-continent and multicity studies have consistently
reported positive and statistically significant associations between
short-term O3 concentrations and all-cause mortality, with
evidence for larger mortality risk estimates during the warm or summer
months (U.S. EPA, 2013a, Figure 6-27; Table 6-42). Similarly,
evaluations of cause-specific mortality have reported consistently
positive associations with O3, particularly in analyses
restricted to the warm season (U.S. EPA, 2013a, Figure 6-37; Table 6-
53).\56\
---------------------------------------------------------------------------
\56\ Respiratory mortality is discussed in more detail above.
---------------------------------------------------------------------------
In assessing the evidence for O3-related mortality, the
2006 AQCD also noted that multiple uncertainties remained regarding the
relationship between short-term O3 concentrations and
mortality, including the extent of residual confounding by
copollutants; characterization of the factors that modify the
O3-mortality association; the appropriate lag structure for
identifying O3-mortality effects; and the shape of the
O3-mortality concentration-response function and whether a
threshold exists. Many of the studies, published since the last review,
have attempted to address one or more of these uncertainties. The ISA
(U.S. EPA, 2013a, section 6.6.2) discusses the extent to which recent
studies have evaluated these uncertainties in the relationship between
O3 and mortality.
In particular, recent studies have evaluated different statistical
approaches to examine the shape of the O3-mortality
concentration-response relationship and to evaluate whether a threshold
exists for O3-related mortality. In an analysis of the
National Morbidity and Mortality Air Pollution Study (NMMAPS) data,
Bell et al. (2006) evaluated the potential for a threshold in the
O3-mortality relationship. The authors reported positive and
statistically significant associations with mortality in a variety of
restricted analyses, including analyses restricted to days with 24-hour
area-wide average O3 concentrations below 60, 55, 50, 45,
40, 35, and 30 ppb. In these restricted analyses O3 effect
estimates were of similar magnitude, were statistically significant,
and had similar statistical precision. In analyses restricted to days
with 24-hour average O3 concentrations below 25 ppb, the
O3 effect estimate was similar in magnitude to the effect
estimates resulting from analyses with the higher cutoffs, but had
somewhat lower statistical precision, with the estimate approaching
statistical significance (i.e., based on observation of Figure 2 in
Bell et al., 2006). In analyses restricted to days with lower 24-hour
average O3 concentrations (i.e., below 20 and 15 ppb),
effect estimates were similar in magnitude to analyses with higher
cutoffs, but with notably less statistical precision, and were not
statistically significant (i.e., confidence intervals included the
null, indicating no O3-associated mortality, based on
observation of Figure 2 in Bell et al., 2006). Ozone was no longer
positively associated with mortality when the analysis was restricted
to days with 24-hour O3 concentrations below 10 ppb. Given
the relatively small number of days included in these restricted
analyses, especially for cut points of 20 ppb and below,\57\
statistical uncertainty is increased.
---------------------------------------------------------------------------
\57\ For example, Bell et al. (2006) reported that for analyses
restricted to 24-hour O3 concentrations at or below 20
ppb, 73% of days were excluded on average across the 98 communities.
---------------------------------------------------------------------------
Bell et al. (2006) also evaluated the shape of the concentration-
response relationship between O3 and mortality. Although the
results of this analysis suggested the lack of threshold in the
O3-mortality relationship, the ISA noted that it is
difficult to interpret such a curve because: (1) There is uncertainty
around the shape of the concentration-response curve at 24-hour average
O3 concentrations generally below 20 ppb; and (2) the
concentration-response curve does not take into consideration the
heterogeneity in O3-mortality risk estimates across cities
(U.S. EPA, 2013a, section 6.6.2.3).
Several additional studies have used the NMMAPS dataset to evaluate
the concentration-response relationship between short-term
O3 concentrations and mortality. For example, using the same
data as Bell et al. (2006), Smith et al. (2009) conducted a subset
analysis, but instead of restricting the analysis to days with
O3 concentrations below a cutoff, the authors only included
days above a defined cutoff (cutoffs from 15 and 60 ppb). The results
of this analysis were consistent with those reported by Bell et al.
(2006). Specifically, the authors reported consistent positive
associations for all cutoff concentrations up to concentrations where
the total number of days available were so limited that the variability
around the central estimate was increased (i.e., cutoff values at or
above about 50 ppb) (U.S. EPA, 2013a, section 6.6.2.3). In addition,
using NMMAPS data for 1987-1994 for Chicago, Pittsburgh, and El Paso,
Xia and Tong (2006) reported evidence for a threshold around a 24-hour
average O3 concentration of 25 ppb, though the threshold
values estimated in the analysis were sometimes in the range of where
data density was low (U.S. EPA, 2013a, section 6.6.2.3). Stylianou and
Nicolich (2009) examined the existence of thresholds following an
approach similar to Xia and Tong (2006) using data from NMMAPS for nine
major U.S. cities (i.e., Baltimore, Chicago, Dallas/Fort Worth, Los
Angeles, Miami, New York, Philadelphia, Pittsburgh, and Seattle) for
the years 1987-2000. The authors reported that the estimated
O3-mortality risks varied across the nine cities, with the
models exhibiting apparent thresholds in the 10-45 ppb range for
O3 (24-hour average). However, given the city-to-city
variation in risk estimates, combining the city-specific estimates into
an overall estimate complicates the interpretation of the results.
Additional studies in
[[Page 75263]]
Europe, Canada, and Asia did not report the existence of a threshold
(Katsouyanni et al., 2009), with inconsistent and/or inconclusive
results across cities, or a non-linear relationship in the
O3-mortality concentration-response curve (Wong et al.,
2010).
3. Adversity of O3 Effects
In making judgments as to when various O3-related
effects become regarded as adverse to the health of individuals, in
previous NAAQS reviews, the EPA has relied upon the guidelines
published by the American Thoracic Society (ATS) and the advice of
CASAC. In 2000, the ATS published an official statement on ``What
Constitutes an Adverse Health Effect of Air Pollution?'' (ATS, 2000),
which updated and built upon its earlier guidance (ATS, 1985). The
earlier guidance defined adverse respiratory health effects as
``medically significant physiologic changes generally evidenced by one
or more of the following: (1) Interference with the normal activity of
the affected person or persons, (2) episodic respiratory illness, (3)
incapacitating illness, (4) permanent respiratory injury, and/or (5)
progressive respiratory dysfunction,'' while recognizing that
perceptions of ``medical significance'' and ``normal activity'' may
differ among physicians, lung physiologists and experimental subjects
(ATS, 1985). The 2000 ATS guidance builds upon and expands the 1985
definition of adversity in several ways. The guidance concludes that
transient, reversible loss of lung function in combination with
respiratory symptoms should be considered adverse. There is also a more
specific consideration of population risk (ATS, 2000). Exposure to air
pollution that increases the risk of an adverse effect to the entire
population is adverse, even though it may not increase the risk of any
individual to an unacceptable level. For example, a population of
asthmatics could have a distribution of lung function such that no
individual has a level associated with clinically important impairment.
Exposure to air pollution could shift the distribution to lower levels
that still do not bring any individual to a level that is associated
with clinically relevant effects. However, this would be considered to
be adverse because individuals within the population would have
diminished reserve function, and therefore would be at increased risk
to further environmental insult (U.S. EPA, 2013a, p. lxxi; and 75 FR at
35526/2, June 22, 2010).
The ATS also concluded that elevations of biomarkers such as cell
types, cytokines and reactive oxygen species may signal risk for
ongoing injury and more serious effects or may simply represent
transient responses, illustrating the lack of clear boundaries that
separate adverse from nonadverse events. More subtle health outcomes
also may be connected mechanistically to health effects that are
clearly adverse, so that small changes in physiological measures may
not appear clearly adverse when considered alone, but may be part of a
coherent and biologically plausible chain of related health outcomes
that include responses that are clearly adverse, such as mortality
(U.S. EPA, 2014c, section 3.1.2.1).
In this review, the new evidence provides further support for
relationships between O3 exposures and a spectrum of health
effects, including effects that meet the ATS criteria for being adverse
(ATS, 1985 and 2000). The ISA determination that there is a causal
relationship between short-term O3 exposure and a full range
of respiratory effects, including respiratory morbidity (e.g., lung
function decrements, respiratory symptoms, inflammation, hospital
admissions, and emergency department visits) and mortality, provides
support for concluding that short-term O3 exposure is
associated with adverse effects (U.S. EPA, 2013a, section 2.5.2).
Overall, including new evidence of cardiovascular system effects, the
evidence supporting an association between short-term O3
exposures and total (nonaccidental, cardiopulmonary) respiratory
mortality is stronger in this review (U.S. EPA, 2013a, section 2.5.2).
And the judgment of likely causal associations between long-term
measures of O3 exposure and respiratory effects such as new-
onset asthma, prevalence of asthma, asthma symptoms and control, and
asthma hospital admissions provides support for concluding that long-
term O3 exposure is associated with adverse effects ranging
from episodic respiratory illness to permanent respiratory injury or
progressive respiratory decline (U.S. EPA, 2013a, section 7.2.8).
Application of the ATS guidelines to the least serious category of
effects related to ambient O3 exposures, which are also the
most numerous and, therefore, are also potentially important from a
public health perspective, involves judgments about which medical
experts on CASAC panels and public commenters have in the past
expressed diverse views. To help frame such judgments, in past reviews,
the EPA has defined gradations of individual functional responses
(e.g., decrements in FEV1 and airway responsiveness) and
symptomatic responses (e.g., cough, chest pain, wheeze), together with
judgments as to the potential impact on individuals experiencing
varying degrees of severity of these responses. These gradations were
used in the 1997 O3 NAAQS review and slightly revised in the
2008 review (U.S. EPA, 1996, p. 59; 2007, p. 3-72; 72 FR 37849, July
11, 2007). These gradations and impacts are summarized in Tables 3-2
and 3-3 in the 2007 O3 Staff Paper (U.S. EPA, 2007, pp. 3-74
to 3-75).
For active healthy people, including children, moderate levels of
functional responses (e.g., FEV1 decrements of >=10% but
<20%, lasting 4 to 24 hours) and/or moderate symptomatic responses
(e.g., frequent spontaneous cough, marked discomfort on exercise or
deep breath, lasting 4 to 24 hours) would likely interfere with normal
activity for relatively few sensitive individuals (U.S. EPA, 2007, p.
3-72; 72 FR 37849, July 11, 2007); whereas large functional responses
(e.g., FEV1 decrements >=20%, lasting longer than 24 hours)
and/or severe symptomatic responses (e.g., persistent uncontrollable
cough, severe discomfort on exercise or deep breath, lasting longer
than 24 hours) would likely interfere with normal activities for many
sensitive individuals (U.S. EPA, 2007, p. 3-72; 72 FR 37849, July 11,
2007) and, therefore, would be considered adverse under ATS guidelines.
For the purpose of estimating potentially adverse lung function
decrements in active healthy people in the 2008 O3 NAAQS
review, the CASAC panel for that review indicated that a focus on the
mid to upper end of the range of moderate levels of functional
responses is most appropriate (e.g., FEV1 decrements >=15%
but <20%) (Henderson, 2006; U.S. EPA, 2007, p. 3-76). In this review,
CASAC concurred that the ``[e]stimation of FEV1 decrements
of >=15% is appropriate as a scientifically relevant surrogate for
adverse health outcomes in active healthy adults'' (Frey, 2014c, p. 3).
However, for children and adults with lung disease, even moderate
functional (e.g., FEV1 decrements >=10% but <20%, lasting up
to 24 hours) or symptomatic responses (e.g., frequent spontaneous
cough, marked discomfort on exercise or with deep breath, wheeze
accompanied by shortness of breath, lasting up to 24 hours) would
likely interfere with normal activity for many individuals, and would
likely result in additional and more frequent use of
[[Page 75264]]
medication (U.S. EPA, 2007, p. 3-72; 72 FR 37849, July 11, 2007). For
people with lung disease, large functional responses (e.g.,
FEV1 decrements >=20%, lasting longer than 24 hours) and/or
severe symptomatic responses (e.g., persistent uncontrollable cough,
severe discomfort on exercise or deep breath, persistent wheeze
accompanied by shortness of breath, lasting longer than 24 hours) would
likely interfere with normal activity for most individuals and would
increase the likelihood that these individuals would seek medical
treatment (U.S. EPA, 2007, p. 3-72; 72 FR 37849, July 11, 2007). In the
last O3 NAAQS review, for the purpose of estimating
potentially adverse lung function decrements in people with lung
disease the CASAC panel indicated that a focus on the lower end of the
range of moderate levels of functional responses is most appropriate
(e.g., FEV1 decrements >=10%) (Henderson, 2006; U.S. EPA,
2007, p. 3-76). In addition, in their letter advising the Administrator
on the reconsideration of the 2008 final decision, CASAC stated that
``[a] 10% decrement in FEV1 can lead to respiratory symptoms,
especially in individuals with pre-existing pulmonary or cardiac
disease. For example, people with chronic obstructive pulmonary disease
have decreased ventilatory reserve (i.e., decreased baseline
FEV1) such that a >=10% decrement could lead to moderate to
severe respiratory symptoms'' (Samet, 2011). In this review, CASAC
concurred that ``[a]n FEV1 decrement of >=10% is a scientifically
relevant surrogate for adverse health outcomes for people with asthma
and lung disease'' (Frey, 2014c, p. 3).
In judging the extent to which these impacts represent effects that
should be regarded as adverse to the health status of individuals, in
previous NAAQS reviews, the EPA has also considered whether effects
were experienced repeatedly during the course of a year or only on a
single occasion (U.S. EPA, 2007). Although some experts would judge
single occurrences of moderate responses to be a nuisance, especially
for healthy individuals, a more general consensus view of the adversity
of such moderate responses emerges as the frequency of occurrence
increases. Thus it has been judged that repeated occurrences of
moderate responses, even in otherwise healthy individuals, may be
considered to be adverse since they could well set the stage for more
serious illness (61 FR 65723). The CASAC panel in the 1997 NAAQS review
expressed a consensus view that these ``criteria for the determination
of an adverse physiological response were reasonable'' (Wolff, 1995).
In the review completed in 2008, estimates of repeated occurrences
continued to be an important public health policy factor in judging the
adversity of moderate lung function decrements in healthy and asthmatic
people (72 FR 37850, July 11, 2007).
Evidence new to this review indicates that 6.6-hour exposures to 60
ppb O3 during moderate exertion can result in pulmonary
inflammation in healthy adults (based on study mean). As discussed in
the ISA, the initiation of inflammation can be considered as evidence
that injury has occurred. Inflammation induced by a single
O3 exposure can resolve entirely but, as noted in the ISA
(U.S. EPA, 2013a, p. 6-76), ``continued acute inflammation can evolve
into a chronic inflammatory state,'' which would be adverse.
Responses measured in controlled human exposure studies indicate
that the range of effects elicited in humans exposed to ambient
O3 concentrations include: Decreased inspiratory capacity;
mild bronchoconstriction; rapid, shallow breathing pattern during
exercise; and symptoms of cough and pain on deep inspiration (U.S. EPA,
2013a, section 6.2.1.1). Young, healthy adults exposed for 6.6 hours to
O3 concentrations >=60 ppb, while engaged in intermittent
moderate exertion, develop reversible, transient decrements in lung
function. In addition, depending on the exposure concentration and the
duration of exposure, young healthy adults have been shown to
experience symptoms of breathing discomfort and inflammation if minute
ventilation or duration of exposure is increased sufficiently (U.S.
EPA, 2013a, section 6.2.1.1). Among healthy subjects there is
considerable interindividual variability in the magnitude of the
FEV1 responses, but when data were combined across studies
at 60 ppb (U.S. EPA, 2013a, pp. 6-17 to 6-18), 10% of healthy subjects
had >10% FEV1 decrements. Moreover, consistent with the
findings of the ISA (U.S. EPA, 2013a, section 6.2.1.1), CASAC concluded
that ``[a]sthmatic subjects appear to be at least as sensitive, if not
more sensitive, than non-asthmatic subjects in manifesting ozone-
induced pulmonary function decrements'' (Frey, 2014c, p. 4). The
combination of lung function decrements and respiratory symptoms, which
has been considered adverse in previous reviews, has been demonstrated
in healthy adults following prolonged (6.6 hour) exposures, while at
intermittent moderate exertion, to 72 ppb. For these types of effects,
information from controlled human exposure studies, which provides an
indication of the magnitude and thus adversity of effects at different
O3 concentrations, combined with estimates of occurrences in
the population from the HREA, provide information about their
importance from a policy perspective.
4. Ozone-Related Impacts on Public Health
Setting standards to provide appropriate public health protection
requires consideration of the factors that put populations at greater
risk from O3 exposure. In order to estimate the potential
for public health impacts, it is important to consider not only the
adversity of the health effects, but also the populations at greater
risk and potential behaviors that may reduce exposures.
a. Identification of At-Risk Populations and Lifestages
The currently available evidence expands the understanding of
populations that were identified to be at greater risk of
O3-related health effects at the time of the last review
(i.e., people who are active outdoors, people with lung disease,
children and older adults and people with increased responsiveness to
O3) and supports the identification of additional factors
that may lead to increased risk (U.S. EPA, 2006, section 3.6.2; U.S.
EPA, 2013a, Chapter 8). Populations and lifestages may be at greater
risk for O3-related health effects due to factors that
contribute to their susceptibility and/or vulnerability to
O3. The definitions of susceptibility and vulnerability have
been found to vary across studies, but in most instances
``susceptibility'' refers to biological or intrinsic factors (e.g.,
lifestage, sex, preexisting disease/conditions) while ``vulnerability''
refers to non-biological or extrinsic factors (e.g., socioeconomic
status [SES]) (U.S. EPA, 2013a, p. 8-1; U.S. EPA, 2010c, 2009d). In
some cases, the terms ``at-risk'' and ``sensitive'' have been used to
encompass these concepts more generally. In the ISA and PA, ``at-risk''
is the all-encompassing term used to define groups with specific
factors that increase their risk of O3-related health
effects.
There are multiple avenues by which groups may experience increased
risk for O3-induced health effects. A population or
lifestage \58\ may exhibit greater effects than other populations or
lifestages exposed to the same
[[Page 75265]]
concentration or dose, or they may be at greater risk due to increased
exposure to an air pollutant (e.g., time spent outdoors). A group with
intrinsically increased risk would have some factor(s) that increases
risk through a biological mechanism and, in general, would have a
steeper concentration-risk relationship, compared to those not in the
group. Factors that are often considered intrinsic include pre-existing
asthma, genetic background, and lifestage. A group of people could also
have extrinsically increased risk, which would be through an external,
non-biological factor, such as socioeconomic status (SES) and diet.
Some groups are at risk of increased internal dose at a given exposure
concentration, for example, because of breathing patterns. This
category would include people who work or exercise outdoors. Finally,
there are those who might be placed at increased risk for experiencing
greater exposures by being exposed to higher O3
concentrations. This would include, for example, groups of people with
greater exposure to ambient O3 due to less availability or
use of home air conditioners such that they are more likely to be in
locations with open windows on high O3 days. Some groups may
be at increased risk of O3-related health effects through a
combination of factors. For example, children tend to spend more time
outdoors when O3 levels are high, and at higher levels of
activity than adults, which leads to increased exposure and dose, and
they also have biological, or intrinsic, risk factors (e.g., their
lungs are still developing) (U.S. EPA, 2013a, Chapter 8). An at-risk
population or lifestage is more likely to experience adverse health
effects related to O3 exposures and/or, develop more severe
effects from exposure than the general population.
---------------------------------------------------------------------------
\58\ Lifestages, which in this case includes childhood and older
adulthood, are experienced by most people over the course of a
lifetime, unlike other factors associated with at-risk populations.
---------------------------------------------------------------------------
i. People With Specific Genetic Variants
There is adequate evidence for populations with certain genotypes
being more at-risk than others to the effects of O3 exposure
on health (U.S. EPA, 2013a, section 8.1). Controlled human exposure and
epidemiologic studies have reported evidence of O3-related
increases in respiratory symptoms or decreases in lung function with
variants including GSTM1, GSTP1, HMOX1, and NQO1. NQO1 deficient mice
were found to be resistant to O3-induced AHR and
inflammation, providing biological plausibility for results of studies
in humans. Additionally, studies of rodents have identified a number of
other genes that may affect O3-related health outcomes,
including genes related to innate immune signaling and pro- and anti-
inflammatory genes, which have not been investigated in human studies.
ii. People With Asthma
Previous O3 AQCDs identified individuals with asthma as
a population at increased risk of O3-related health effects.
Multiple new epidemiologic studies included in the ISA have evaluated
the potential for increased risk of O3-related health
effects in people with asthma, including: Lung function; symptoms;
medication use; AHR; and airway inflammation (also measured as exhaled
nitric oxide fraction, or FeNO). A study of lifeguards in Texas
reported decreased lung function with short-term O3 exposure
among both individuals with and without asthma; however, the decrease
was greater among those with asthma (Thaller et al., 2008). A Mexican
study of children ages 6-14 detected an association between short-term
O3 exposure and wheeze, cough, and bronchodilator use among
asthmatics but not non-asthmatics, although this may have been the
result of a small non-asthmatic population (Escamilla-Nu[ntilde]ez et
al., 2008). A study of modification by AHR (an obligate condition among
asthmatics) reported greater short-term O3-associated
decreases in lung function in elderly individuals with AHR, especially
among those who were obese (Alexeeff et al., 2007). With respect to
airway inflammation, in one study, a positive association was reported
for airway inflammation among asthmatic children following short-term
O3 exposure, but the observed association was similar in
magnitude to that of non-asthmatics (Barraza-Villarreal et al., 2008).
Similarly, another study of children in California reported an
association between O3 concentration and FeNO that persisted
both among children with and without asthma as well as those with and
without respiratory allergy (Berhane et al., 2011). Finally, Khatri et
al. (2009) found no association between short-term O3
exposure and altered lung function for either asthmatic or non-
asthmatic adults, but did note a decrease in lung function among
individuals with allergies.
New evidence for difference in effects among asthmatics has been
observed in studies that examined the association between O3
exposure and altered lung function by asthma medication use. A study of
children with asthma living in Detroit reported a greater association
between short-term O3 and lung function (i.e.,
FEV1) for corticosteroid users compared with
noncorticosteroid users (Lewis et al., 2005). Conversely, another study
found decreased lung function among noncorticosteroid users compared to
users, although in this study, a large proportion of non-users were
considered to be persistent asthmatics (Hern[aacute]ndez-Cadena et al.,
2009). Lung function was not related to short-term O3
exposure among corticosteroid users and non-users in a study taking
place during the winter months in Canada (Liu et al., 2009).
Additionally, a study of airway inflammation reported a
counterintuitive inverse association with O3 of similar
magnitude for all groups of corticosteroid users and non-users (Qian et
al., 2009).
Controlled human exposure studies that have examined the effects of
O3 on adults with asthma and healthy controls are limited.
Based on studies reviewed in the 1996 and 2006 O3 AQCDs,
subjects with asthma appeared to be more sensitive to acute effects of
O3 in terms of FEV1 and inflammatory responses
than healthy non-asthmatic subjects. For instance, Horstman et al.
(1995) observed that mild-to-moderate asthmatics, on average,
experienced double the O3-induced FEV1 decrement
of healthy subjects (19% versus 10%, respectively, p=0.04). Moreover, a
statistically significant positive correlation between FEV1
responses to O3 exposure and baseline lung function was
observed in individuals with asthma, i.e., responses increased with
severity of disease. Minimal evidence exists suggesting that
individuals with asthma have smaller O3-induced
FEV1 decrements than healthy subjects (3% versus 8%,
respectively) (Mudway et al., 2001). However, the asthmatics in that
study also tended to be older than the healthy subjects, which could
partially explain their lesser response since FEV1 responses
to O3 exposure diminish with age. Individuals with asthma
also had significantly more neutrophils in the BALF (18 hours
postexposure) than similarly exposed healthy individuals (Peden et al.,
1997; Scannell et al., 1996; Basha et al., 1994). Furthermore, a study
examining the effects of O3 on individuals with atopic
asthma and healthy controls reported that greater numbers of
neutrophils, higher levels of cytokines and hyaluronan, and greater
expression of macrophage cell-surface markers were observed in induced
sputum of atopic asthmatics compared with healthy controls (Hernandez
et al., 2010). Differences in O3-induced epithelial cytokine
expression were noted in bronchial biopsy samples from asthmatics and
healthy controls (Bosson et al., 2003). Cell-surface marker and
cytokine expression results, and the
[[Page 75266]]
presence of hyaluronan, are consistent with O3 having
greater effects on innate and adaptive immunity in these asthmatic
individuals. In addition, studies have demonstrated that O3
exposure leads to increased bronchial reactivity to inhaled allergens
in mild allergic asthmatics (Kehrl et al., 1999; Jorres et al., 1996)
and to the influx of eosinophils in individuals with pre-existing
allergic disease (Vagaggini et al., 2002; Peden et al., 1995). Taken
together, these results point to several mechanistic pathways which
could account for the enhanced sensitivity to O3 in subjects
with asthma (U.S. EPA, 2013a, section 5.4.2.2).
As noted in the previous review (72 FR 37846, July 11, 2007)
asthmatics present a differential response profile for cellular,
molecular, and biochemical parameters (U.S. EPA, 2006a, Figure 8-1)
that are altered in response to acute O3 exposure. Ozone-
induced increases in neutrophils, IL-8 and protein were found to be
significantly higher in the BAL fluid from asthmatics compared to
healthy subjects, suggesting mechanisms for the increased sensitivity
of asthmatics (Basha et al., 1994; McBride et al., 1994; Scannell et
al., 1996; Hiltermann et al., 1999; Holz et al., 1999; Bosson et al.,
2003). Neutrophils, or PMNs, are the white blood cell most associated
with inflammation. IL-8 is an inflammatory cytokine with a number of
biological effects, primarily on neutrophils. The major role of this
cytokine is to attract and activate neutrophils. Protein in the airways
is leaked from the circulatory system, and is a marker for increased
cellular permeability.
Bronchial constriction following provocation with O3
and/or allergens presents a two-phase response. The early response is
mediated by release of histamine and leukotrienes that leads to
contraction of smooth muscle cells in the bronchi, narrowing the lumen
and decreasing the airflow. In people with allergic airway disease,
including people with rhinitis and asthma, these mediators also cause
accumulation of eosinophils in the airways (Bascom et al., 1990; Jorres
et al., 1996; Peden et al., 1995 and 1997; Frampton et al., 1997a;
Michelson et al., 1999; Hiltermann et al., 1999; Holz et al., 2002;
Vagaggini et al., 2002). In asthma, the eosinophil, which increases
inflammation and allergic responses, is the cell most frequently
associated with exacerbations of the disease. A study by Bosson et al.
(2003) evaluated the difference in O3-induced bronchial
epithelial cytokine expression between healthy and asthmatic subjects.
After O3 exposure the epithelial expression of IL-5 and GM-
CSF increased significantly in asthmatics, compared to healthy
subjects. Asthma is associated with Th2-related airway response
(allergic response), and IL-5 is an important Th2-related cytokine. The
O3-induced increase in IL-5, and also in GM-CSF, which
affects the growth, activation and survival of eosinophils, may
indicate an effect on the Th2-related airway response and on airway
eosinophils. The authors reported that the O3-induced Th2-
related cytokine responses that were found within the asthmatic group
may indicate a worsening of their asthmatic airway inflammation and
thus suggest a plausible link to epidemiological data indicating
O3-associated increases in bronchial reactivity and hospital
admissions.
The accumulation of eosinophils in the airways of asthmatics is
followed by production of mucus and a late-phase bronchial constriction
and reduced airflow. In a study of 16 intermittent asthmatics,
Hiltermann et al. (1999) found that there was a significant inverse
correlation between the O3-induced change in the percentage
of eosinophils in induced sputum and the change in PC20, the
concentration of methacholine causing a 20% decrease in
FEV1. Characteristic O3-induced inflammatory
airway neutrophilia at one time was considered a leading mechanism of
airway hyperresponsiveness. However, Hiltermann et al. (1999)
determined that the O3-induced change in percentage
neutrophils in sputum was not significantly related to the change in
PC20. These results are consistent with the results of Zhang et al.
(1995), which found neutrophilia in a murine model to be only
coincidentally associated with airway hyperresponsiveness, i.e., there
was no cause and effect relationship (U.S. EPA, 2006a, AX 6-26).
Hiltermann et al. (1999) concluded that the results point to the role
of eosinophils in O3-induced airway hyperresponsiveness.
Increases in O3-induced nonspecific airway responsiveness
incidence and duration could have important clinical implications for
asthmatics.
Toxicological studies provide additional evidence of the biological
basis for the greater effects of O3 among those with asthma
or AHR (U.S. EPA, 2013a, section 8.2.2). In animal toxicological
studies, an asthmatic phenotype is modeled by allergic sensitization of
the respiratory tract. Many of the studies that provide evidence that
O3 exposure is an inducer of AHR and remodeling utilize
these types of animal models. For example, a series of experiments in
infant rhesus monkeys have shown these effects, but only in monkeys
sensitized to house dust mite allergen. Similarly, adverse changes in
pulmonary function were demonstrated in mice exposed to O3;
enhanced inflammatory responses were in rats exposed to O3,
but only in animals sensitized to allergen. In general, it is the
combined effects of O3 and allergic sensitization which
result in measurable effects on pulmonary function. In a pulmonary
fibrosis model, exposure to O3 for 5 days increased
pulmonary inflammation and fibrosis, along with the frequency of
bronchopneumonia in rats. Thus, short-term exposure to O3
may enhance damage in a previously injured lung (U.S. EPA, 2013a,
section 8.2.2).
In the 2006 O3 AQCD, the potential for individuals with
asthma to have greater risk of O3-related health effects was
supported by a number of controlled human exposure studies, evidence
from toxicological studies, and a limited number of epidemiologic
studies. In section 8.2.2, the ISA reports that in the recent
epidemiologic literature some, but not all, studies report greater risk
of health effects among individuals with asthma. Studies examining
effect measure modification of the relationship between short-term
O3 exposure and altered lung function by corticosteroid use
provided limited evidence of O3-related health effects.
However, recent studies of behavioral responses have found that studies
do not take into account individual behavioral adaptations to
forecasted air pollution levels (such as avoidance and reduced time
outdoors), which may underestimate the observed associations in studies
that examined the effect of O3 exposure on respiratory
health (Neidell and Kinney, 2010). This could explain some
inconsistency observed among recent epidemiologic studies. The evidence
from controlled human exposure studies provides support for increased
detriments in FEV1 and greater inflammatory responses to
O3 in individuals with asthma than in healthy individuals
without a history of asthma. The collective evidence for increased risk
of O3-related health effects among individuals with asthma
from controlled human exposure studies is supported by recent
toxicological studies which provide biological plausibility for
heightened risk of asthmatics to respiratory effects due to
O3 exposure. Overall, the ISA finds there is adequate
evidence for asthmatics to be an at-risk population.
iii. Children
Children are considered to be at greater risk from O3
exposure because their respiratory systems undergo lung
[[Page 75267]]
growth until about 18-20 years of age and are therefore thought to be
intrinsically more at risk for O3-induced damage (U.S. EPA,
2006a). It is generally recognized that children spend more time
outdoors than adults, and, therefore, would be expected to have higher
exposure to O3 than adults. Children aged 11 years and older
and adults have higher absolute ventilation rates than younger children
aged 1-11 years. However, younger children have higher ventilation
rates relative to their lung volumes, which tends to increase dose
normalized to lung surface area. In all ages, exercise intensity has a
substantial effect on ventilation rate, high intensity activity results
in nearly double the ventilation rate for moderate activity. For more
information on time spent outdoors and ventilation rate differences by
age group, see section 4.4.1 in the ISA (U.S. EPA, 2013a).
The 1996 O3 AQCD reported clinical evidence that
children, adolescents, and young adults (<18 years of age) appear, on
average, to have nearly equivalent spirometric responses to
O3 exposure, but have greater responses than middle-aged and
older adults (U.S. EPA, 1996). Symptomatic responses (e.g., cough,
shortness of breath, pain on deep inspiration) to O3
exposure, however, appear to increase with age until early adulthood
and then gradually decrease with increasing age (U.S. EPA, 1996).
Complete lung growth and development is not achieved until 18-20 years
of age in women and the early 20s for men; pulmonary function is at its
maximum during this time as well.
Recent epidemiologic studies have examined different age groups and
their risk to O3-related respiratory hospital admissions and
emergency department visits. Evidence for greater risk in children was
reported in several studies. A study in Cyprus of short-term
O3 concentrations and respiratory hospital admissions
detected possible effect measure modification by age with a larger
association among individuals <15 years of age compared with those >15
years of age; the effect was apparent only with a 2-day lag (Middleton
et al., 2008). Similarly, a Canadian study of asthma-related emergency
department visits reported the strongest O3-related
associations among 5- to 14-year olds compared to the other age groups
(ages examined 0-75+) (Villeneuve et al., 2007). Greater O3-
associated risk in asthma-related emergency department visits were also
reported among children (<15 years) as compared to adults (15 to 64
years) in a study from Finland (Halonen et al., 2009). A study of New
York City hospital admissions demonstrated an increase in the
association between O3 exposure and asthma-related hospital
admissions for 6- to 18-year olds compared to those <6 years old and
those >18 years old (Silverman and Ito, 2010). When examining long-term
O3 exposure and asthma-related hospital admissions among
children, associations were determined to be larger among children 1 to
2 years old compared to children 2 to 6 years old (Lin et al., 2008). A
few studies reported positive associations among both children and
adults and no modification of the effect by age.
The evidence reported in epidemiologic studies is supported by
recent toxicological studies which observed O3-induced
health effects in immature animals. Early life exposures of multiple
species of laboratory animals, including infant monkeys, resulted in
changes in conducting airways at the cellular, functional, ultra-
structural, and morphological levels. The studies conducted on infant
monkeys are most relevant for assessing effects in children. Carey et
al. (2007) conducted a study of O3 exposure in infant rhesus
macaques, whose respiratory tract closely resemble that of humans.
Monkeys were exposed either acutely or in episodes designed to mimic
human exposure. All monkeys acutely exposed to O3 had
moderate to marked necrotizing rhinitis, with focal regions of
epithelial exfoliation, numerous infiltrating neutrophils, and some
eosinophils. The distribution, character, and severity of lesions in
episodically exposed infant monkeys were similar to that of acutely
exposed animals. Neither exposure protocol for the infant monkeys
produced mucous cell metaplasia proximal to the lesions, an adaptation
observed in adult monkeys exposed in another study (Harkema et al.,
1987). Functional and cellular changes in conducting airways were
common manifestations of exposure to O3 among both the adult
and infant monkeys (Plopper et al., 2007). In addition, the lung growth
of the distal conducting airways in the infant monkeys was
significantly stunted by O3 and this aberrant development
was persistent 6 months postexposure (Fanucchi et al., 2006).
Age may also affect the inflammatory response to O3
exposure. Toxicological studies reported that the difference in effects
among younger lifestage test animals may be due to age-related changes
in antioxidants levels and sensitivity to oxidative stress. Further
discussion of these studies may be found in section 8.3.1.1 of the ISA
(U.S. EPA, 2013a, p. 8-18).
The previous and recent human clinical and toxicological studies
reported evidence of increased risk from O3 exposure for
younger ages, which provides coherence and biological plausibility for
the findings from epidemiologic studies. Although there was some
inconsistency, generally, the epidemiologic studies reported positive
associations among both children and adults or just among children. The
interpretation of these studies is limited by the lack of consistency
in comparison age groups and outcomes examined. However, overall, the
epidemiologic, controlled human exposure, and toxicological studies
provide adequate evidence that children are potentially at increased
risk of O3-related health effects.
iv. Older adults
The ISA notes that older adults are at greater risk of health
effects associated with O3 exposure through a variety of
intrinsic pathways (U.S. EPA, 2013a, section 8.3.1.2). In addition,
older adults may differ in their exposure and internal dose. Older
adults were outdoors for a slightly longer proportion of the day than
adults aged 18-64 years. For more information on time spent outdoors by
age group, see Section 4.4 in the ISA (U.S. EPA, 2013a). The gradual
decline in physiological processes that occurs with aging may lead to
increased risk of O3-related health effects (U.S. EPA,
2006a). Respiratory symptom responses to O3 exposure appears
to increase with age until early adulthood and then gradually decrease
with increasing age (U.S. EPA, 1996); lung function responses to
O3 exposure also decline from early adulthood (U.S. EPA,
1996). The reductions of these responses with age may put older adults
at increased risk for continued O3 exposure. In addition,
older adults, in general, have a higher prevalence of preexisting
diseases compared to younger age groups and this may also lead to
increased risk of O3-related health effects (U.S. EPA,
2013a, section 8.3.1.2). With the number of older Americans increasing
in upcoming years (estimated to increase from 12.4% of the U.S.
population to 19.7% between 2000 to 2030, which is approximately 35
million and 71.5 million individuals, respectively) this group
represents a large population potentially at risk of O3-
related health effects (SSDAN CensusScope, 2010a; U.S. Census Bureau,
2010).
The majority of recent studies reported greater effects of short-
term O3 exposure and mortality among older adults, which is
consistent with the findings of the 2006 O3 AQCD. A study
(Medina-Ram[oacute]n and Schwartz, 2008)
[[Page 75268]]
conducted in 48 cities across the U.S. reported larger effects among
adults >=65 years old compared to those <65 years. Further
investigation of this study population revealed a trend of
O3-related mortality risk that gets larger with increasing
age starting at age 51 (Zanobetti and Schwartz, 2008a). Another study
conducted in 7 urban centers in Chile reported similar results, with
greater effects in adults >=65 years old (Cakmak et al., 2007). More
recently, a study conducted in the same area reported similar
associations between O3 exposure and mortality in adults
aged <64 years old and 65 to 74 years old, but the risk was increased
among the older age group (Cakmak et al., 2011). A study performed in
China reported greater effects in populations >=45 years old (compared
to 5 to 44 year olds), with statistically significant effects present
only among those >=65 years old (Kan et al., 2008). An Italian study
reported higher risk of all-cause mortality associated with increased
O3 concentrations among individuals >=85 year old as
compared to those 35 to 84 years old (Stafoggia et al., 2010). The Air
Pollution and Health: A European and North American Approach (APHENA)
project examined the association between O3 exposure and
mortality for those <75 and >=75 years of age. In Canada, the
associations for all-cause and cardiovascular mortality were greater
among those >=75 years old. In the U.S., the association for all-cause
mortality was slightly greater for those <75 years of age compared to
those >=75 years old in summer-only analyses. No consistent pattern was
observed for CVD mortality. In Europe, slightly larger associations for
all-cause mortality were observed in those <75 years old in all-year
and summer-only analyses. Larger associations were reported among those
<75years for CVD mortality in all-year analyses, but the reverse was
true for summer-only analyses (Katsouyanni et al., 2009).
With respect to epidemiologic studies of O3 exposure and
hospital admissions, a positive association was reported between short-
term O3 exposure and respiratory hospital admissions for
adults >=65 years old but not for those adults aged 15 to 64 years
(Halonen et al., 2009). In the same study, no association was observed
between O3 concentration and respiratory mortality among
those >=65 years old or those 15 to 64 years old. No modification by
age (40 to 64 year olds versus >64 year olds) was observed in a study
from Brazil examining O3 levels and COPD-related emergency
department visits.
Although some outcomes reported mixed findings regarding an
increase in risk for older adults, recent epidemiologic studies report
consistent positive associations between short-term O3
exposure and mortality in older adults. The evidence from mortality
studies is consistent with the results reported in the 2006
O3 AQCD and is supported by toxicological studies providing
biological plausibility for increased risk of effects in older adults.
Also, older adults may be experiencing increased exposure compared to
younger adults. Overall, the ISA (U.S. EPA, 2013a) concludes adequate
evidence is available indicating that older adults are at increased
risk of O3-related health effects.
v. People With Diets Lower in Vitamins C and E
Diet was not examined as a factor potentially affecting risk in
previous O3 AQCDs, but recent studies have examined
modification of the association between O3 and health
effects by dietary factors. Because O3 mediates some of its
toxic effects through oxidative stress, the antioxidant status of an
individual is an important factor that may contribute to increased risk
of O3-related health effects. Supplementation with vitamins
C and E has been investigated in a number of studies as a means of
inhibiting O3-mediated damage.
Two epidemiologic studies have examined effect modification by diet
and found evidence that certain dietary components are related to the
effect O3 has on respiratory outcomes. In one recent study,
the effects of fruit/vegetable intake and Mediterranean diet were
examined. Increases in these food patterns, which have been noted for
their high vitamins C and E and omega-3 fatty acid content, were
positively related to lung function in asthmatic children living in
Mexico City, and modified by O3 exposure (Romieu et al.,
2009). Another study examined supplementation of the diets of asthmatic
children in Mexico with vitamins C and E (Sienra-Monge et al., 2004).
Associations were detected between short-term O3 exposure
and nasal airway inflammation among children in the placebo group but
not in those receiving the supplementation.
The epidemiologic evidence is supported by controlled human
exposure studies, discussed in section 8.4.1 of the ISA (U.S. EPA,
2013a), that have shown that the first line of defense against
oxidative stress is antioxidants-rich extracellular lining fluid (ELF)
which scavenges free radicals and limit lipid peroxidation. Exposure to
O3 depletes antioxidant levels in nasal ELF probably due to
scrubbing of O3; however, the concentration and the activity
of antioxidant enzymes either in ELF or plasma do not appear to be
related to O3 responsiveness. Controlled studies of dietary
antioxidant supplementation have demonstrated some protective effects
of [alpha]-tocopherol (a form of vitamin E) and ascorbate (vitamin C)
on spirometric measures of lung function after O3 exposure
but not on the intensity of subjective symptoms and inflammatory
responses. Dietary antioxidants have also afforded partial protection
to asthmatics by attenuating postexposure bronchial
hyperresponsiveness. Toxicological studies discussed in section 8.4.1
of the ISA (U.S. EPA, 2013a) provide evidence of biological
plausibility to the epidemiologic and controlled human exposure
studies.
Overall, the ISA (U.S. EPA, 2013a) concludes adequate evidence is
available indicating that individuals with diets lower in vitamins C
and E are at risk for O3-related health effects. The
evidence from epidemiologic studies is supported by controlled human
exposure and toxicological studies.
vi. Outdoor Workers
Studies included in the 2006 O3 AQCD reported that
individuals who participate in outdoor activities or work outside to be
a population at increased risk based on consistently reported
associations between O3 exposure and respiratory health
outcomes in these groups (U.S. EPA, 2006a). Outdoor workers are exposed
to ambient O3 concentrations for a greater period of time
than individuals who spend their days indoors. As discussed in section
4.7 of the ISA (U.S. EPA, 2013a) outdoor workers sampled during the
work shift had a higher ratio of personal exposure to fixed-site
monitor concentrations than health clinic workers who spent most of
their time indoors. Additionally, an increase in dose to the lower
airways is possible during outdoor exercise due to both increases in
the amount of air breathed (i.e., minute ventilation) and a shift from
nasal to oronasal breathing. The association between FEV1
responses to O3 exposure and minute ventilation is discussed
more fully in section 6.2.3.1 of the 2006 O3 AQCD (U.S. EPA,
2006a).
Previous studies have shown that increased exposure to
O3 due to outdoor work leads to increased risk of
O3-related health effects, specifically decrements in lung
function (U.S. EPA, 2006a). The strong evidence from the 2006
O3 AQCD, which demonstrated increased exposure, dose, and
ultimately risk of O3-related health effects in this
population, supports the
[[Page 75269]]
conclusion that there is adequate evidence to indicate that increased
exposure to O3 through outdoor work increases the risk of
O3-related health effects.
In some cases, it is difficult to determine a factor that results
in increased risk of effects. For example, previous assessments have
included controlled human exposure studies in which some healthy
individuals demonstrate greater O3-related health effects
compared to other healthy individuals. Interindividual variability has
been observed for lung function decrements, symptomatic responses,
pulmonary inflammation, AHR, and altered epithelial permeability in
healthy adults exposed to O3, and these results tend to be
reproducible within a given individual over a period of several months
indicating differences in the intrinsic responsiveness. In many cases
the reasons for the variability is not clear. This may be because one
or some of the factors described above have not been evaluated in
studies, or it may be that additional, unidentified factors influence
individual responses to O3 (U.S. EPA, 2013a, section 8.5).
As discussed in chapter 8 of the ISA (U.S. EPA, 2013a), there is a
lack of information regarding the extent to which some factors may
increase risk from O3 exposures. Due to this lack of
information, the ISA concluded that for some factors, such as sex, SES,
and obesity, there is only ``suggestive'' evidence of increased risk,
or that for a number of factors the evidence is inadequate to draw
conclusions about potential increase in risk of effects. Overall, the
factors for which the ISA concludes there is adequate evidence of
increased risk for experiencing O3-related effects were
related to asthma, lifestage (children and older adults), genetic
variability, dietary factors, and working outdoors.
b. Size of At-Risk Populations
One consideration in the assessment of potential public health
impacts is the size of various population groups for which there is
adequate evidence of increased risk for health effects associated with
O3-related air pollution exposure (U.S. EPA, 2014c, section
3.1.5.2). The factors for which the ISA judged the evidence to be
``adequate'' with respect to contributing to increased risk of
O3-related effects among various populations and lifestages
included: asthma; childhood and older adulthood; diets lower in
vitamins C and E; certain genetic variants; and working outdoors (U.S.
EPA, 2013a, section 8.5). No statistics are available to estimate the
size of an at-risk population based on nutritional status or genetic
variability.
With regard to asthma, Table 3-7 in the PA (U.S. EPA, 2014c,
section 3.1.5.2) summarizes information on the prevalence of current
asthma by age in the U.S. adult population in 2010 (Schiller et al.
2012; children--Bloom et al., 2011). Individuals with current asthma
constitute a fairly large proportion of the population, including more
than 25 million people. Asthma prevalence tends to be higher in
children than adults. Within the U.S., approximately 8.2% of adults
have reported currently having asthma (Schiller et al., 2012) and 9.5%
of children have reported currently having asthma (Bloom et al.,
2011).\59\
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\59\ As noted below (II.C.3.a.ii), asthmatics can experience
larger O3-induced respiratory effects than non-asthmatic,
healthy adults. The responsiveness of asthmatics to O3
exposures could depend on factors that have not been well-evaluated
such as asthma severity, the effectiveness of asthma control, or the
prevalence of medication use.
---------------------------------------------------------------------------
With regard to lifestages, based on U.S. census data from 2010
(Howden and Meyer, 2011), about 74 million people, or 24% of the U.S.
population, are under 18 years of age and more than 40 million people,
or about 13% of the U.S. population, are 65 years of age or older.
Hence, a large proportion of the U.S. population (i.e., more than a
third) is included in age groups that are considered likely to be at
increased risk for health effects from ambient O3 exposure.
With regard to outdoor workers, in 2010, approximately 11.7% of the
total number of people (143 million people) employed, or about 16.8
million people, worked outdoors one or more days per week (based on
worker surveys).\60\ Of these, approximately 7.4% of the workforce, or
about 7.8 million people, worked outdoors three or more days per week.
---------------------------------------------------------------------------
\60\ The O*NET program is the nation's primary source of
occupational information. Central to the project is the O*NET
database, containing information on hundreds of standardized and
occupation-specific descriptors. The database, which is available to
the public at no cost, is continually updated by surveying a broad
range of workers from each occupation. http://www.onetcenter.org/overview.html. http://www.onetonline.org/find/descriptor/browse/Work_Context/4.C.2/.
---------------------------------------------------------------------------
The health statistics data illustrate what is known as the
``pyramid'' of effects. At the top of the pyramid, there are
approximately 2.5 million deaths from all causes per year in the U.S.
population, with about 250 thousand respiratory-related deaths (CDC-
WONDER, 2008). For respiratory health diseases, there are nearly 3.3
million hospital discharges per year (HCUP, 2007), 8.7 million
respiratory emergency department visits (HCUP, 2007), 112 million
ambulatory care visits (Woodwell and Cherry, 2004), and an estimated
700 million restricted activity days per year due to respiratory
conditions (Adams et al., 1999). Combining small risk estimates with
relatively large baseline levels of health outcomes can result in quite
large public health impacts. Thus, even a small percentage reduction in
O3 health impacts on cardiopulmonary diseases would reflect
a large number of avoided cases.
c. Impacts of Averting Behavior
The activity pattern of individuals is an important determinant of
their exposure (U.S. EPA, 2013a, section 4.4.1). Variation in
O3 concentrations among various microenvironments means that
the amount of time spent in each location, as well as the level of
activity, will influence an individual's exposure to ambient
O3. Activity patterns vary both among and within
individuals, resulting in corresponding variations in exposure across a
population and over time. Individuals can reduce their exposure to
O3 by altering their behaviors, such as by staying indoors,
being active outdoors when air quality is better, and by reducing their
activity levels or reducing the time being active outdoors on high-
O3 days (U.S. EPA, 2013a, section 4.4.2).
The widely reported Air Quality Index (AQI) conveys advice to the
public, and particularly at-risk populations, on reducing short- or
prolonged-exposures on days when ambient levels of common, criteria air
pollutants (except lead), are elevated (www.airnow.gov). Information
communicated by the AQI is based on the evidence and exposure/risk
information assessed in the review of the NAAQS; it is updated and
revised as necessary during the review of each standard. Proposed
changes to the AQI sub-index for O3, based on evidence and
exposure/risk information assessed in this review, are discussed in
section III below.
The AQI describes the potential for health effects from
O3 (and other individual pollutants) in six color-coded
categories of air-quality, ranging from Good (green), Moderate
(yellow), Unhealthy for Sensitive Groups (orange), Unhealthy (red), and
Very Unhealthy (purple), and Hazardous (maroon). Levels in the
unhealthy ranges (i.e., Unhealthy for Sensitive Groups and above) come
with recommendations about reducing exposure. Forecasted and actual AQI
values for O3 are reported to the public
[[Page 75270]]
during the O3 season. The AQI advisories explicitly state
that children, older adults, people with lung disease, and people who
are active outdoors, may be at greater risk from exposure to
O3. People are advised to reduce exposure depending on the
predicted O3 levels and the likelihood of risk. This advice
includes being active outdoors when air quality is better, and reducing
activity levels or reducing the time being active outdoors on high-
O3 days. Staying indoors to reduce exposure is not
recommended until air quality reaches the Very Unhealthy or Hazardous
categories.
Evidence of individual averting behaviors in response to AQI
advisories has been found in several studies, including activity
pattern and epidemiologic studies, especially for the at-risk
populations, such as children, older adults, and people with asthma,
who are targeted by the advisories. Such effects are less pronounced in
the general population, possibly due to the opportunity cost of
behavior modification. Epidemiologic evidence from a study (Neidell and
Kinney, 2010) conducted in the 1990's in Los Angeles, CA reports
increased asthma hospital admissions among children and older adults
when O3 alert days (1-hour max O3 concentration
>200 ppb) were excluded from the analysis of daily hospital admissions
and O3 concentrations (presumably thereby eliminating
averting behavior based on high O3 forecasts). If averting
behavior reduces exposure to ambient O3, then epidemiologic
studies that do not account for averting behavior may produce effect
estimates that are biased toward the null due to exposure
misclassification (U.S. EPA, 2013, section 4.6.6).
C. Human Exposure and Health Risk Assessments
To put judgments about health effects that are adverse for
individuals into a broader public health context, the EPA has developed
and applied models to estimate human exposures to O3 and
O3-associated health risks. Exposure and risk estimates
based on such models are presented and assessed in the HREA (U.S. EPA,
2014a). In reviewing the draft HREA, CASAC expressed the view that the
document is ``well-written, founded based upon comprehensive analyses
and adequate for its intended purpose'' (Frey, 2014a, p. 1). Analyses
in the HREA inform consideration of the O3 exposures and
health risks that could be allowed by the current standard and
alternative standards, and consideration of the kind and degree of
uncertainties inherent in estimates of O3 exposures and
health risks.
The following sections discuss the air quality adjustment approach
used in the HREA for exposure and health risk estimates (II.C.1); the
approach taken to estimate exposures, key exposure results, and
important uncertainties (II.C.2); and the approaches taken to estimate
O3 health risks, key risk results, and important
uncertainties (II.C.3).
1. Air Quality Adjustment
As discussed above (section I.E), O3 is formed near the
Earth's surface due to chemical interactions involving solar radiation
and precursor pollutants including VOCs, NOX, CH4
and CO. The response of O3 to changes in precursor
concentrations is nonlinear. In particular, NOX causes both
the formation and destruction of O3. The net impact of
NOX emissions on O3 concentrations depends on the
local quantities of NOX, VOC, and sunlight, which interact
in a set of complex chemical reactions. In some areas, such as urban
centers where NOX emissions typically are high,
NOX leads to the net destruction of O3,
decreasing O3 concentrations in the immediate vicinity. This
phenomenon is particularly pronounced under conditions that lead to low
ambient O3 concentrations (i.e. during cool, cloudy weather
and at night when photochemical activity is limited or nonexistent).
However, while NOX can initially destroy O3 near
emission sources, these same NOX emissions eventually react
to form O3 downwind of those sources. Photochemical model
simulations suggest that reductions in NOX emissions will
slightly increase O3 concentrations near NOX
sources on days with lower O3 concentrations, while at the
same time decreasing the highest O3 concentrations in
outlying areas. The atmospheric chemistry that influences ambient
O3 concentrations is discussed in more detail in the ISA
(U.S. EPA, 2013a, Chapter 3) and the PA (U.S. EPA, 2014c, Chapter 2)
(see also Frey, 2014a, pp. 10 and 11).
The HREA uses a photochemical model to estimate sensitivities of
O3 to changes in precursor emissions in order to estimate
ambient O3 concentrations that would just meet the current
and alternative standards (U.S. EPA, 2014a, Chapter 4).\61\ For the 15
urban study areas evaluated in the HREA,\62\ this model-based
adjustment approach estimates hourly O3 concentrations at
each monitor location when modeled U.S. anthropogenic precursor
emissions (i.e., NOX, VOC) \63\ are reduced. The HREA
estimates air quality that just meets the current and alternative
standards for the 2006-2008 and 2008-2010 periods.\64\
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\61\ The HREA uses the Community Multi-scale Air Quality (CMAQ)
photochemical model instrumented with the higher order direct
decoupled method (HDDM) to estimate O3 concentrations
that would occur with the achievement of the current and alternative
O3 standards (U.S. EPA, 2014a, Chapter 4).
\62\ The urban study areas assessed are Atlanta, Baltimore,
Boston, Chicago, Cleveland, Dallas, Denver, Detroit, Houston, Los
Angeles, New York, Philadelphia, Sacramento, St. Louis, and
Washington, DC.
\63\ Exposure and risk analyses for most urban study areas focus
on reducing U.S. anthropogenic NOX emissions alone. The
exceptions are Chicago and Denver. Exposure and risk analyses for
Chicago and Denver are based on reductions in emissions of both
NOX and VOC (U.S. EPA, 2014a, section 4.3.3.1; Appendix
4D).
\64\ These simulations are illustrative and do not reflect any
consideration of specific control programs designed to achieve the
reductions in emissions required to meet the specified standards.
Further, these simulations do not represent predictions of when,
whether, or how areas might meet the specified standards.
---------------------------------------------------------------------------
As discussed in Chapter 4 of the HREA (U.S. EPA, 2014a), this
approach to adjusting air quality models the physical and chemical
atmospheric processes that influence ambient O3
concentrations. Compared to the quadratic rollback approach used in
previous reviews, it provides more realistic estimates of the spatial
and temporal responses of O3 to reductions in precursor
emissions. Because ambient NOX can contribute both to the
formation and destruction of O3 (U.S. EPA, 2014a, Chapter
4), as discussed above, the response of ambient O3
concentrations to reductions in NOX emissions is more
variable than indicated by the quadratic rollback approach. This
improved approach to adjusting O3 air quality is consistent
with recommendations from the National Research Council of the National
Academies (NRC, 2008). In addition, CASAC strongly supported the
improved approach, stating that ``the quadratic rollback approach has
been replaced by a scientifically more valid Higher-order Decoupled
Direct Method (HDDM)'' and that ``[t]he replacement of the quadratic
rollback procedure by the HDDM procedure is important and supported by
the CASAC'' (Frey, 2014a, pp.1 and 3).
Consistent with the O3 chemistry summarized above, in
locations and time periods when NOX is predominantly
contributing to O3 formation (e.g., downwind of important
NOX sources, where the highest O3 concentrations
often occur), model-based adjustment to the current and alternative
standards decreases
[[Page 75271]]
estimated ambient O3 concentrations compared to recent
monitored concentrations (U.S. EPA, 2014a, section 4.3.3.2). In
contrast, in locations and time periods when NOX is
predominantly contributing to O3 titration (e.g., in urban
centers with high concentrations of NOX emissions, where
ambient O3 concentrations are often suppressed and thus
relatively low \65\), model-based adjustment increases ambient
O3 concentrations compared to recent monitored
concentrations (U.S. EPA, 2014a, section 4.3.3.2; Frey, 2014a, p. 10).
---------------------------------------------------------------------------
\65\ Titration is also prominent during time periods when
photochemistry is limited, and ambient O3 concentrations
are relatively low, such as at night and on cool, cloudy days (U.S.
EPA, 2014a, Chapter 4).
---------------------------------------------------------------------------
Within urban study areas, the overall impacts of model-based air
quality adjustment are to reduce the O3 concentrations at
the upper ends of ambient distributions and to increase the
O3 concentrations at the lower ends of those distributions
(U.S. EPA, 2014a, section 4.3.3.2, Figures 4-9 and 4-10).\66\ Seasonal
means of daily O3 concentrations generally exhibit only
modest changes upon model adjustment, reflecting the seasonal balance
between daily decreases in relatively higher concentrations and
increases in relatively lower concentrations (U.S. EPA, 2014a, Figures
4-9 and 4-10). The resulting compression in the seasonal distributions
of ambient O3 concentrations is evident in all of the urban
study areas evaluated, though the degree of compression varies
considerably across areas (U.S. EPA, 2014a, Figures 4-9 and 4-10).
---------------------------------------------------------------------------
\66\ It is important to note that sensitivity analyses in the
HREA indicate that the increases in low O3 concentrations
are smaller when NOX and VOC emissions are reduced than
when only NOX emissions are reduced (U.S. EPA, 2014a,
Appendix 4-D, section 4.7).
---------------------------------------------------------------------------
This compression in the distributions of ambient O3
concentrations has important implications for exposure and risk
estimates in urban study areas. Estimates influenced largely by the
upper ends of the distribution of ambient concentrations (i.e.,
exposures of concern and lung function risk estimates, as discussed in
sections 3.2.2 and 3.2.3.1 of the PA (U.S. EPA, 2014c)) decrease with
adjustment of air quality to the current and alternative standards. In
contrast, seasonal risk estimates influenced by the full distribution
of ambient O3 concentrations (i.e., epidemiology-based risk
estimates, as discussed in section 3.2.3.2 of the PA) either decrease
or increase in response to air quality adjustment, depending on the
balance between the daily decreases in high O3
concentrations and increases in low O3 concentrations.\67\
---------------------------------------------------------------------------
\67\ In addition, because epidemiology-based risk estimates use
``area-wide'' average O3 concentrations, calculated by
averaging concentrations across multiple monitors in urban study
areas (U.S. EPA, 2014c, section 3.2.3.2), risk estimates on a given
day depend on the daily balance between increasing and decreasing
O3 concentrations at the individual monitors that are
averaged together to calculate the ``area-wide'' concentration.
---------------------------------------------------------------------------
In their review of the second draft HREA, CASAC considered this
issue, in particular noting that ``reductions in nitrogen oxides
emissions can lead to less scavenging of ozone and free radicals,
resulting in locally higher levels of ozone'' (Frey, 2014a, p. 10).
CASAC recommended that ``the EPA should identify and discuss whether
and to what extent health risks in the urban core may be affected by
NOX reductions or other possible strategies'' and, in
particular, concluded that it would ``be of interest to learn if there
would be any children or outdoor workers in the more urban areas who
would experience significantly higher exposures to ozone as a result of
possible changes in the ozone NAAQS'' (Frey, 2014a, p. 10). Consistent
with this advice, the exposure and risk implications of the spatial and
temporal patterns of ambient O3 following air quality
adjustment in urban study areas are discussed in the final HREA (U.S.
EPA, 2014a, Chapter 9) and the final PA (U.S. EPA, 2014c, sections
3.2.2, 3.2.3), and are summarized below within the context of the PA's
consideration of exposure estimates (II.D.2.a) and risk estimates
(II.D.2.b and II.D.2.c).
2. Exposure Assessment
This section discusses the HREA assessment of human exposures to
O3. Section II.C.2.a provides an overview of the approach
used in the HREA to assessing exposures and the approach in the PA to
considering exposure estimates, and summarizes key results. Section
II.C.2.b summarizes the important uncertainties in exposure estimates.
a. Overview and Summary of Key Results
The exposure assessment presented in the HREA (U.S. EPA, 2014a,
Chapter 5) provides estimates of the number and percent of people
exposed to various concentrations of ambient O3, while at
specified exertion levels. The HREA estimates exposures in the 15 urban
study areas for four study groups, all school-age children (ages 5 to
18), asthmatic school-age children, asthmatic adults (ages 19 to 95),
and all older adults (ages 65 to 95), reflecting the evidence
indicating that these populations are at increased risk for
O3-attributable effects (U.S. EPA, 2013a, Chapter 8). An
important purpose of these exposure estimates is to provide perspective
on the extent to which air quality adjusted to just meet the current
O3 NAAQS could be associated with exposures to O3
concentrations reported to result in respiratory effects.\68\ Estimates
of such ``exposures of concern'' provide perspective on the potential
public health impacts of O3-related effects, including
effects that cannot currently be evaluated in a quantitative risk
assessment.\69\
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\68\ In addition, the range of modeled personal exposures to
ambient O3 provide an essential input to the portion of
the health risk assessment based on exposure-response functions (for
lung function decrements) from controlled human exposure studies.
The health risk assessment based on exposure-response information is
discussed below (II.C.3).
\69\ In this review, the term ``exposure of concern'' is defined
as a personal exposure, while at moderate or greater exertion, to 8-
hour average ambient O3 concentrations at and above
specific benchmarks. As discussed below, benchmarks represent
exposure concentrations at which O3-induced health
effects are known to occur, or can reasonably be anticipated to
occur, in some individuals.
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In the absence of large scale exposure studies that encompass the
general population, as well as at-risk populations, modeling is the
preferred approach to estimating exposures to O3 (U.S. EPA,
2014a, Chapter 5). The use of exposure modeling also facilitates the
estimation of exposures resulting from ambient O3
concentrations differing from those present during exposure studies. In
the HREA, population exposures to ambient O3 concentrations
are estimated using the current version of the Air Pollutants Exposure
(APEX) model. The APEX model simulates the movement of individuals
through time and space and estimates their exposures to a given
pollutant in indoor, outdoor, and in-vehicle microenvironments (U.S.
EPA, 2014a, section 5.1.3). APEX takes into account important factors
that contribute to total human exposure to ambient O3,
including the temporal and spatial distributions of people and
O3 concentrations throughout an urban area, the variation of
O3 concentrations within various microenvironments, and the
effects of exertion on breathing rate in exposed individuals (U.S. EPA,
2014a, section 5.1.3). To the extent spatial and/or temporal patterns
of ambient O3 concentrations are altered upon model
adjustment, as discussed above, exposure estimates reflect population
exposures to those altered patterns.
The HREA estimates 8-hour exposures at or above benchmark
concentrations of
[[Page 75272]]
60, 70, and 80 ppb for individuals engaged in moderate or greater
exertion (i.e., to approximate conditions in the controlled human
exposure studies on which benchmarks are based). Benchmarks reflect
exposure concentrations at which O3-induced respiratory
effects are known to occur in some healthy adults engaged in moderate,
intermittent exertion, based on evidence from controlled human exposure
studies (U.S. EPA, 2013a, section 6.2; U.S. EPA, 2014c, section
3.1.2.1). The amount of weight to place on the estimates of exposures
at or above specific benchmark concentrations depends in part on the
weight of the scientific evidence concerning health effects associated
with O3 exposures at those benchmark concentrations. It also
depends on judgments about the importance, from a public health
perspective, of the health effects that are known or can reasonably be
inferred to occur as a result of exposures at benchmark concentrations
(U.S. EPA, 2014c, sections 3.1.3, 3.1.5).
As discussed in more detail above (II.B.2), the health evidence
that supports evaluating exposures of concern at or above benchmark
concentrations of 60, 70, and 80 ppb comes from a large body of
controlled human exposure studies reporting a variety of respiratory
effects in healthy adults. The lowest O3 exposure
concentration for which controlled human exposure studies have reported
respiratory effects in healthy adults is 60 ppb (based on changes in
group mean responses), with more evidence supporting this benchmark
concentration in the current review than in the last review. In healthy
adults, 6.6 hour exposures to 60 ppb O3 have been reported
to decrease lung function and to increase airway inflammation.
Exposures of healthy adults to 72 ppb O3 for 6.6 hours have
been reported to result in larger average lung function decrements,
compared to 60 ppb, as well as in increased respiratory symptoms.
Exposures of healthy adults to 80 ppb O3 for 6.6 hours have
been reported to result in larger average lung function decrements than
following exposures to 60 or 72 ppb and, depending on the study, to
increase airway inflammation, increase respiratory symptoms, increase
airways responsiveness, and decrease lung host defense (based on
changes in group means) (U.S. EPA, 2014c, section 3.1.2.1). In
commenting on the evidence for benchmark concentrations, CASAC stated
the following (Frey, 2014c, p. 6):
The 80 ppb-8hr benchmark level represents an exposure level for
which there is substantial clinical evidence demonstrating a range
of ozone-related effects including lung inflammation and airway
responsiveness in healthy individuals. The 70 ppb-8hr benchmark
level reflects the fact that in healthy subjects, decreases in lung
function and respiratory symptoms occur at concentrations as low as
72 ppb and that these effects almost certainly occur in some people,
including asthmatics and others with low lung function who are less
tolerant of such effects, at levels of 70 ppb and below. The 60 ppb-
8hr benchmark level represents the lowest exposure level at which
ozone-related effects have been observed in clinical studies of
healthy individuals. Based on its scientific judgment, the CASAC
finds that the 60 ppb-8hr exposure benchmark is relevant for
consideration with respect to adverse effects on asthmatics.
In considering estimates of O3 exposures of concern at
or above benchmarks of 60, 70, and 80 ppb, the PA focuses on modeled
exposures for school-age children (ages 5-18), including asthmatic
school-age children, which are key at-risk populations identified in
the ISA (U.S. EPA, 2014c, section 3.1.5). The percentages of children
estimated to experience exposures of concern are considerably larger
than the percentages estimated for adult populations (i.e.,
approximately 3-fold larger across urban study areas) (U.S. EPA, 2014a,
section 5.3.2 and Figures 5-5 to 5-8). The larger exposure estimates
for children are due primarily to the larger percentage of children
estimated to spend an extended period of time being physically active
outdoors when O3 concentrations are elevated (U.S. EPA,
2014a, sections 5.3.2 and 5.4.1).
Although exposure estimates differ between children and adults, the
patterns of results across the urban study areas and years are similar
among all of the populations evaluated (U.S. EPA, 2014a, Figures 5-5 to
5-8). Therefore, while the PA highlights estimates in children,
including asthmatic school-age children, it also notes that the
patterns of exposures estimated for children represent the patterns
estimated for adult asthmatics and older adults.
Table 1 below summarizes key results from the exposure assessment.
Table 1 presents estimates of the percentages and numbers of all
school-aged children estimated to experience exposures of concern when
air quality was adjusted to just meet the current and alternative 8-
hour O3 standards. The percentage of all school-age children
in the 15 urban study areas estimated to experience exposures of
concern declines when comparing just meeting the current standard to
just meeting alternative 8-hour O3 standards. Substantial
variability is evident across years and urban study areas, as indicated
by the ranges of averaged estimates and estimates for worst-case years
and study areas. As discussed below, the interindividual variability in
responsiveness following exposures of concern means that only a subset
of individuals who are exposed at and above a given benchmark
concentration would actually be expected to experience respiratory
effects.
Table 1--Summary of Estimated Exposures of Concern in All School-age Children for the Current and Alternative O3
Standards in Urban Study Areas
----------------------------------------------------------------------------------------------------------------
% Children--
Standard level Average % Average number of children worst year and
Benchmark concentration (ppb) children exposed [average number of worst area
exposed \70\ asthmatic children] \71\
----------------------------------------------------------------------------------------------------------------
One or more exposures of concern per season
----------------------------------------------------------------------------------------------------------------
>=80 ppb......................... 75 0-0.3 27,000 [3,000].............. 1.1
70 0-0.1 3,700 [300]................. 0.2
65 0 300 [0]..................... 0
60 0 100 \72\ [0]................ 0
>=70 ppb......................... 75 0.6-3.3 362,000 [40,000]............ 8.1
70 0.1-1.2 94,000 [10,000]............. 3.2
65 0-0.2 14,000 [2,000].............. 0.5
60 0 1,400 [200]................. 0.1
>=60 ppb......................... 75 9.5-17 2,316,000 [246,000]......... 25.8
70 3.3-10.2 1,176,000 [126,000]......... 18.9
65 0-4.2 392,000 [42,000]............ 9.5
[[Page 75273]]
60 0-1.2 70,000 [8,000].............. 2.2
----------------------------------------------------------------------------------------------------------------
Two or more exposures of concern per season
----------------------------------------------------------------------------------------------------------------
>=80 ppb......................... 75 0 600 [100]................... 0.1
70 0 0 [0]....................... 0
65 0 0 [0]....................... 0
60 0 0 [0]....................... 0
>=70 ppb......................... 75 0.1-0.6 46,000 [5,000].............. 2.2
70 0-0.1 5,400 [600]................. 0.4
65 0 300 [100]................... 0
60 0 0 [0]....................... 0
>=60 ppb......................... 75 3.1-7.6 865,000 [93,000]............ 14.4
70 0.5-3.5 320,000 [35,000]............ 9.2
65 0-0.8 67,000 [7,500].............. 2.8
60 0-0.2 5,100 [700]................. 0.3
----------------------------------------------------------------------------------------------------------------
b. Key Uncertainties
---------------------------------------------------------------------------
\70\ Estimates for each urban case study area were averaged for
the years evaluated in the HREA (2006 to 2010). Ranges reflect the
ranges across urban study areas. Estimates smaller than 0.05% were
rounded downward to zero (from U.S. EPA, 2014a, Tables 5-11 and 5-
12).
\71\ Numbers of children exposed in each urban case study area
were averaged over the years 2006 to 2010. These averages were then
summed across urban study areas. Numbers were rounded to nearest
thousand unless otherwise indicated. Estimates smaller than 50 were
rounded downward to zero (from U.S. EPA, 2014a, Appendix 5F Table
5F-5). See below for discussion of uncertainties in exposure
estimates.
\72\ As discussed in section 4.3.3 of the HREA, the model-based
air quality adjustment approach used to estimate risks associated
with the current and alternative standards was unable to estimate
the distribution of ambient O3 concentrations in New York
City upon just meeting an alternative standard with a level of 60
ppb. Therefore, for the 60 ppb standard level the numbers of
children and asthmatic children reflect all of the urban study areas
except New York.
---------------------------------------------------------------------------
In considering exposure estimates within the context of the current
and alternative O3 standards, the PA also notes important
uncertainties in these estimates. For example, due to variability in
responsiveness, only a subset of individuals who experience exposures
at or above a benchmark concentration can be expected to experience
health effects.\73\ Given the lack of sufficient exposure-response
information for most of the health effects that informed benchmark
concentrations, estimates of the number of people likely to experience
exposures at or above benchmark concentrations generally cannot be
translated into quantitative estimates of the number of people likely
to experience specific health effects.\74\ The PA views health-relevant
exposures as a continuum with greater confidence and less uncertainty
about the existence of adverse health effects at higher O3
exposure concentrations, and less confidence and greater uncertainty as
one considers lower exposure concentrations. This view draws from the
overall body of available health evidence, which indicates that as
exposure concentrations increase, the incidence, magnitude, and
severity of effects increases.
---------------------------------------------------------------------------
\73\ As noted below (II.C.3.a.ii), in the case of asthmatics,
responsiveness to O3 could depend on factors that have
not been well-evaluated, such as asthma severity, the effectiveness
of asthma control, or the prevalence of medication use.
\74\ The exception to this is lung function decrements, as
discussed below (and in U.S. EPA, 2014c, section 3.2.3.1).
---------------------------------------------------------------------------
Though the PA indicates less confidence in the likelihood of
adverse health effects as O3 exposure concentrations
decrease, it also notes that the controlled human exposure studies that
provided the basis for health benchmark concentrations have not
evaluated at-risk populations. Compared to the healthy individuals
included in controlled human exposure studies, members of at-risk
populations (e.g., asthmatics, children) could be more likely to
experience adverse effects, could experience larger and/or more serious
effects, and/or could experience effects following exposures to lower
O3 concentrations. The CASAC expressed similar views in
their advice to the Administrator (Frey, 2014a, pp. 7 and 14). In
considering estimated exposures of concern (U.S. EPA, 2014c, section
3.4), the PA notes that concerns about the potential for adverse health
effects, including effects in at-risk populations must be balanced
against the increasing uncertainty regarding the likelihood of such
effects following exposures to lower O3 concentrations.
Uncertainties associated with the APEX exposure modeling also have
the potential to be important (U.S. EPA, 2014a, section 5.5.2, Table 5-
6). For example, the HREA concludes that exposures of concern could be
underestimated for some individuals who are frequently and routinely
active outdoors during the warm season (U.S. EPA, 2014a, section
5.5.2). This could include outdoor workers and children who are
frequently active outdoors. The HREA specifically notes that long-term
diary profiles (i.e., monthly, annual) do not exist for such
populations, limiting the extent to which APEX outputs reflect people
who follow similar daily routines resulting in high exposures, over
extended periods of time.
In order to evaluate one dimension of the potential implications of
this uncertainty for exposure estimates, the HREA reports the results
of limited exposure model sensitivity analyses using subsets of
activity diaries specifically selected to reflect groups spending a
larger proportion of time being active outdoors during the
O3 season. When diaries were selected to mimic activity
patterns performed by outdoor workers, the percent of modeled
individuals estimated to experience exposures of concern was higher
than the other adult populations evaluated. The percentages of outdoor
workers estimated to experience exposures of concern were generally
similar to the percentages estimated for children (i.e., using the full
database of diary profiles) in the worst-case urban study area and year
(i.e., urban study area and year with the largest percent of children
estimated to experience exposures of concern) (U.S. EPA, 2014a, section
5.4.3.2, Figure 5-14). In
[[Page 75274]]
addition, when diaries were restricted to children who did not report
any time spent inside a school or performing paid work (i.e., to mimic
children spending large portions of their time outdoors during the
summer), the number experiencing exposures of concern increased by
approximately 30% (U.S. EPA, 2014a, section 5.4.3.1). Though these
sensitivity analyses are limited to single urban study areas, and
though there is uncertainty associated with diary selection approaches
to mimic highly exposed populations, they suggest the possibility that
some at-risk groups could experience more frequent exposures of concern
than indicated by estimates made using the full database of activity
diary profiles.
In further considering activity diaries, the HREA also notes that
growing evidence indicates that people can change their behavior in
response to high O3 concentrations, reducing the time spent
being active outdoors (U.S. EPA, 2014a, section 5.4.3.3). Commonly
termed ``averting behaviors,'' these altered activity patterns could
reduce personal exposure concentrations. Therefore, the HREA also
performed limited sensitivity analyses to evaluate the potential
implications of averting behavior for estimated exposures of concern.
These analyses suggest that averting behavior could reduce the
percentages of children estimated to experience exposures of concern at
or above the 60 or 70 ppb benchmark concentrations by approximately 10
to 30%, with larger reductions possible for the 80 ppb benchmark (U.S.
EPA, 2014a, Figure 5-15). As discussed above for other sensitivity
analyses, these analyses are limited to a single urban case study area
and are subject to uncertainties associated with assumptions about the
prevalence and duration of averting behaviors. However, the results
suggest that exposures of concern could be overestimated, particularly
in children (Neidell, 2009; U.S. EPA, 2013, Figures 4-7 and 4-8), if
the possibility for averting behavior is not incorporated into
estimates.
3. Quantitative Health Risk Assessments
For some health endpoints, there is sufficient scientific evidence
and information available to support the development of quantitative
estimates of O3-related health risks. In the last review of
the O3 NAAQS, the quantitative health risk assessment
estimated O3-related lung function decrements, respiratory
symptoms, respiratory-related hospital admissions, and nonaccidental
and cardiorespiratory-related mortality (U.S. EPA, 2007). In those
analyses, both controlled human exposure and epidemiologic studies were
used for the quantitative assessment of O3-related human
health risks.
In the current review, for short-term O3 concentrations,
the HREA estimates lung function decrements; respiratory symptoms in
asthmatics; hospital admissions and emergency department visits for
respiratory causes; and all-cause mortality (U.S. EPA, 2014a). For
long-term O3 concentrations, the HREA estimates respiratory
mortality (U.S. EPA, 2014a).\75\ Estimates of O3-induced
lung function decrements are based on exposure modeling, combined with
exposure-response relationships from controlled human exposure studies
(U.S. EPA, 2014a, Chapter 6). Estimates of O3-associated
respiratory symptoms, hospital admissions and emergency department
visits, and mortality are based on concentration-response relationships
from epidemiologic studies (U.S. EPA, 2014a, Chapter 7). As with the
exposure assessment discussed above, O3-associated health
risks are estimated for recent air quality and for ambient
concentrations adjusted to just meet the current and alternative
O3 standards, based on 2006-2010 air quality and adjusted
precursor emissions. The following sections discuss the lung function
risk assessment (II.C.3.a) and the epidemiology-based morbidity and
mortality risk assessments (II.C.3.b) from the HREA, including
important sources of uncertainty in these estimates.
---------------------------------------------------------------------------
\75\ Estimates of O3-associated respiratory mortality
are based on the study by Jerrett et al. (2009). This study used
seasonal averages of 1-hour daily maximum O3
concentrations to estimate long-term concentrations.
---------------------------------------------------------------------------
a. Lung Function Risk Assessment
Section II.C.3.a.i provides an overview of the approach used in the
HREA to assessing lung function risks, an overview of the approach in
the PA to considering lung function risk estimates, and a summary of
key results. Section II.C.3.a.ii presents a summary of key
uncertainties in lung function risk estimates.
i. Overview and Summary of Key Results
In the current review, the HREA estimates risks of lung function
decrements in school-aged children (ages 5 to 18), asthmatic school-
aged children, and the general adult population for the 15 urban study
areas. The results presented in the HREA are based on an updated dose-
threshold model that estimates FEV1 responses for
individuals following short-term exposures to O3 (McDonnell
et al., 2012), reflecting methodological improvements since the last
review (II.B.2.a.i, above; U.S. EPA, 2014a, section 6.2.4). The impact
of the dose threshold is that O3-induced FEV1
decrements result primarily from exposures on days with average ambient
O3 concentrations above about 40 ppb (U.S. EPA, 2014a,
section 6.3.1, Figure 6-9).\76\
---------------------------------------------------------------------------
\76\ Analysis of this issue in the HREA is based on risk
estimates in Los Angeles for 2006 unadjusted air quality. The HREA
shows that more than 90% of daily instances of FEV1
decrements >=10% occur when 8-hr average ambient concentrations are
above 40 ppb for this modeled scenario. The HREA notes that the
distribution of responses will be different for different study
areas, years, and air quality scenarios (U.S. EPA, 2014c, Chapter
6).
---------------------------------------------------------------------------
The HREA estimates risks of moderate to large lung function
decrements, defined as FEV1 decrements >=10%, 15%, or 20%.
In evaluating these lung function risk estimates within the context of
considering the current and alternative O3 standards, the PA
focuses on the percent of children estimated to experience one or more
and two or more decrements >=10, 15, and 20%, noting that the
percentage of asthmatic children estimated to experience such
decrements is virtually indistinguishable from the percentage estimated
for all children.\77\ Compared to children, a smaller percentage of
adults were estimated to experience O3-induced
FEV1 decrements (U.S. EPA, 2014a, section 6.3.1, Table 6-4).
As for exposures of concern (see above), the patterns of results across
urban study areas and over the years evaluated are similar in children
and adults. Therefore, while the PA highlights estimates in children,
it notes that these results are also representative of the patterns
estimated for adult populations.
---------------------------------------------------------------------------
\77\ Though see below for discussion of uncertainty in lung
function responses of children and asthmatics.
---------------------------------------------------------------------------
Table 2 below summarizes key results from the lung function risk
assessment. Table 2 presents estimates of the percentages of school-
aged children estimated to experience O3-induced
FEV1 decrements >=10, 15, or 20% when air quality was
adjusted to just meet the current and alternative 8-hour O3
standards. Table 2 also presents the numbers of children, including
children with asthma, estimated to experience such decrements. As shown
in these tables, the percentage of school-age children in the 15 urban
study areas estimated to experience O3-induced
FEV1 decrements declines when comparing just meeting the
current standard to just meeting alternative
[[Page 75275]]
8-hour O3 standards. Substantial variability is evident
across years and urban study areas, as indicated by the ranges of
averaged estimates and estimates for worst-case years and locations.
Table 2--Summary of Estimated O3-Induced Lung Function Decrements for the Current and Potential Alternative O3
Standards in Urban Case Study Areas
----------------------------------------------------------------------------------------------------------------
Number of children (5 to 18 % Children
Lung function decrement Alternative Average % years) [number of asthmatic worst year and
standard level children \78\ children] \79\ area
----------------------------------------------------------------------------------------------------------------
One or more decrements per season
----------------------------------------------------------------------------------------------------------------
>=10%............................. 75 14-19 3,007,000 [312,000]......... 22
70 11-17 2,527,000 [261,000]......... 20
65 3-15 1,896,000 [191,000]......... 18
60 5-11 1,404,000 [139,000] \80\.... 13
>=15%............................. 75 3-5 766,000 [80,000]............ 7
70 2-4 562,000 [58,000]............ 5
65 0-3 356,000 [36,000]............ 4
60 1-2 225,000 [22,000]............ 3
>=20%............................. 75 1-2 285,000 [30,000]............ 2.8
70 1-2 189,000 [20,000]............ 2.1
65 0-1 106,000 [11,000]............ 1.4
60 0-1 57,000 [6,000].............. 0.9
----------------------------------------------------------------------------------------------------------------
Two or more decrements per season
----------------------------------------------------------------------------------------------------------------
>=10%............................. 75 7.5-12 1,730,000 [179,000]......... 14
70 5.5-11 1,414,000 [145,000]......... 13
65 1.3-8.8 1,023,000 [102,000]......... 11
60 2.1-6.4 741,000 [73,000]............ 7.3
>=15%............................. 75 1.7-2.9 391,000 [40,000]............ 3.8
70 0.9-2.4 276,000 [28,000]............ 3.1
65 0.1-1.8 168,000 [17,000]............ 2.3
60 0.2-1.0 101,000 [10,000]............ 1.4
>=20%............................. 75 0.5-1.1 128,000 [13,000]............ 1.5
70 0.3-0.8 81,000 [8,000].............. 1.1
65 0-0.5 43,000 [4,000].............. 0.8
60 0-0.2 21,000 [2,000].............. 0.4
----------------------------------------------------------------------------------------------------------------
ii. Key Uncertainties
---------------------------------------------------------------------------
\78\ Estimates in each urban case study area were averaged for
the years evaluated in the HREA (2006 to 2010). Ranges reflect the
ranges across urban study areas.
\79\ Numbers of children estimated to experience decrements in
each study urban case study area were averaged over 2006 to 2010.
These averages were then summed across urban study areas. Numbers
are rounded to nearest thousand unless otherwise indicated.
\80\ As discussed in section 4.3.3 of the HREA, the model-based
air quality adjustment approach used to estimate risks associated
with the current and alternative standards was unable to estimate
the distribution of ambient O3 concentrations in New York
City upon just meeting an alternative standard with a level of 60
ppb. Therefore, for the 60 ppb standard level the numbers of
children and asthmatic children reflect all of the urban study areas
except New York.
---------------------------------------------------------------------------
As for exposures of concern discussed above, the PA also considers
important uncertainties in estimates of lung function risk. In addition
to the uncertainties noted for exposure estimates, the HREA identifies
several key uncertainties associated with estimates of O3-
induced lung function decrements. An uncertainty with particular
potential to impact consideration of risk estimates stems from the lack
of exposure-response information in children. In the near absence of
controlled human exposure data for children, risk estimates are based
on the assumption that children exhibit the same lung function response
following O3 exposures as healthy 18 year olds (i.e., the
youngest age for which controlled human exposure data is available)
(U.S. EPA, 2014a, section 6.5.3). This assumption is justified in part
by the findings of McDonnell et al. (1985), who reported that children
(8-11 years old) experienced FEV1 responses similar to those
observed in adults (18-35 years old). In addition, as discussed in the
ISA (U.S. EPA, 2013a, section 6.2.1), summer camp studies of school-
aged children reported O3-induced lung function decrements
similar in magnitude to those observed in controlled human exposure
studies using adults. In extending the risk model to children, the HREA
fixes the age term in the model at its highest value, the value for age
18. This approach could result in either over- or underestimates of
O3-induced lung function decrements in children, depending
on how children compare to the adults used in controlled human exposure
studies (U.S. EPA, 2014a, section 6.5.3).
A related source of uncertainty is that the risk assessment
estimates O3-induced decrements in asthmatics using the
exposure-response relationship developed from data collected from
healthy individuals. Although the evidence has been mixed (U.S. EPA,
2013a, section 6.2.1.1), several studies have reported larger
O3-induced lung function decrements in asthmatics than in
non-asthmatics (Kreit et al., 1989; Horstman et al., 1995; Jorres et
al., 1996; Alexis et al., 2000). On this issue, CASAC noted that
``[a]sthmatic subjects appear to be at least as sensitive, if not more
sensitive, than non-asthmatic subjects in manifesting ozone-induced
pulmonary function decrements'' (Frey, 2014c, p. 4). To the extent
asthmatics experience larger O3-induced lung function
decrements than the healthy adults used to develop exposure-response
relationships, the HREA could underestimate the impacts of
O3 exposures on lung function in asthmatics, including
asthmatic children. The implications of this uncertainty for risk
estimates remain unknown at this time (U.S. EPA, 2014a,
[[Page 75276]]
section 6.5.4), and could depend on a variety of factors that have not
been well-evaluated, including the severity of asthma and the
prevalence of medication use. However, the available evidence shows
responses to O3 increase with severity of asthma (Horstman
et al., 1995) and corticosteroid usage does not prevent O3
effects on lung function decrements or respiratory symptoms in people
with asthma (Vagaggini et al., 2001, 2007).
b. Mortality and Morbidity Risk Assessments
As discussed above (II.B.2), epidemiologic studies provide evidence
for the most serious O3-associated public health outcomes
(e.g., mortality, hospital admissions, emergency department visits).
Section II.C.3.b.i below provides an overview of the approach used in
the HREA to assessing mortality and morbidity risks based on
information from epidemiologic studies, discusses the approach in the
PA to considering epidemiology-based risk estimates, and presents a
summary of key results. Section II.C.3.b.ii summarizes key
uncertainties in epidemiology-base risk estimates.
i. Overview and Summary of Key Results
Risk estimates based on epidemiologic studies can provide
perspective on the most serious O3-associated public health
outcomes (e.g., mortality, hospital admissions, emergency department
visits) in populations that often include at-risk groups. The HREA
estimates O3-associated risks in 12 urban study areas \81\
using concentration-response relationships drawn from epidemiologic
studies. These concentration-response relationships are based on
``area-wide'' average O3 concentrations.\82\ The HREA
estimates risks for the years 2007 and 2009 in order to provide
estimates of risk for a year with generally higher O3
concentrations (2007) and a year with generally lower O3
concentrations (2009) (U.S. EPA, 2014a, section 7.1.1).
---------------------------------------------------------------------------
\81\ The 12 urban areas evaluated are Atlanta, Baltimore,
Boston, Cleveland, Denver, Detroit, Houston, Los Angeles, New York,
Philadelphia, Sacramento, and St. Louis.
\82\ In the epidemiologic studies that provide the health basis
for HREA risk assessments, concentration-response relationships are
based on daytime O3 concentrations, averaged across
multiple monitors within study areas. These daily averages are used
as surrogates for the spatial and temporal patterns of exposures in
study populations. Consistent with this approach, the HREA
epidemiologic-based risk estimates also utilize daytime
O3 concentrations, averaged across monitors, as
surrogates for population exposures. In this notice, we refer to
these averaged concentrations as ``area-wide'' O3
concentrations. Area-wide concentrations are discussed in more
detail in section 3.1.4 of the PA (U.S. EPA, 2014c).
---------------------------------------------------------------------------
As in the last review of the O3 NAAQS (U.S. EPA, 2007,
pp. 2-48 to 2-54), the PA recognizes that ambient O3
concentrations, and therefore O3-associated health risks,
result from precursor emissions from various types of sources. Based on
the air quality modeling discussed in chapter 2 of the PA (U.S. EPA,
2014c), approximately 30 to 60% of average daytime O3 during
the warm season (i.e., daily maximum 8-hour concentrations averaged
from April to October) is attributable to precursor emissions from U.S.
anthropogenic sources (U.S. EPA, 2014c, section 2.4.4). The remainder
is attributable to precursor emissions from international anthropogenic
sources and natural sources. Because the HREA characterizes health
risks from all O3, regardless of source, risk estimates
reflect emissions from U.S. anthropogenic, international anthropogenic,
and natural sources.
Compared to the weight given to HREA estimates of exposures of
concern and lung function risks, and the weight given to the evidence
(U.S. EPA, 2014c, section 4.4.1), the PA places relatively less weight
on epidemiologic-based risk estimates. In doing so, the PA notes that
the overall conclusions from the HREA likewise reflect less confidence
in estimates of epidemiologic-based risks than in estimates of
exposures and lung function risks. The determination to attach less
weight to the epidemiologic-based estimates reflects the uncertainties
associated with mortality and morbidity risk estimates, including the
heterogeneity in effect estimates between epidemiologic study areas,
the potential for epidemiologic-based exposure measurement error, and
uncertainty in the interpretation of the shape of concentration-
response functions at lower O3 concentrations (discussed
below). The PA also notes the HREA conclusion that lower confidence
should be placed in the results of the assessment of respiratory
mortality risks associated with long-term O3 exposures,
primarily because that analysis is based on only one study (even though
that study is well-designed) and because of the uncertainty in that
study about the existence and level of a potential threshold in the
concentration-response function (U.S. EPA, 2014a, section 9.6).
In considering the epidemiology-based risk estimates, the PA
focuses on mortality risks associated with short-term O3
concentrations. In doing so, in addition to noting uncertainty in
estimates of respiratory mortality associated with long-term
O3, the PA notes that the patterns of estimated respiratory
morbidity risks across urban study areas, over years, and for different
standards are similar to the patterns of total mortality risk.
The PA considers estimates of total risk (i.e., based on the full
distributions of ambient O3 concentrations) and estimates of
risk associated with O3 concentrations in the upper portions
of ambient distributions. A focus on estimates of total risks would
place greater weight on the possibility that concentration-response
relationships are linear over the entire distribution of ambient
O3 concentrations, and thus on the potential for morbidity
and mortality to be affected by changes in relatively low O3
concentrations. A focus on risks associated with O3
concentrations in the upper portions of the ambient distribution would
place greater weight on the uncertainty associated with the shapes of
concentration-response curves for O3 concentrations in the
lower portions of the distribution. Given that both types of risk
estimates could reasonably inform a decision on standard level,
depending on the weight placed on uncertainties in the occurrence and
the estimation of O3-attributable effects at relatively low
O3 concentrations, the PA considers both types of estimates.
Key results for O3-associated mortality risk are summarized
in Table 3 below. Table 3 presents estimates of the number of
O3-associated deaths in urban study areas, for air quality
adjusted to just meet the current and alternative standards.
[[Page 75277]]
Table 3--Estimates of O3-Associated Deaths Attributable to the Full Distribution of 8-Hour Area-Wide O3
Concentrations and to Concentrations at or Above 20, 40, or 60 ppb O3
[Deaths summed across urban case study areas] \83\
----------------------------------------------------------------------------------------------------------------
Number of O3-associated deaths summed across urban case study areas
-----------------------------------------------------------------------------------------------------------------
Standard level Total O3 20+ ppb 40+ ppb 60+ ppb
----------------------------------------------------------------------------------------------------------------
2007
----------------------------------------------------------------------------------------------------------------
75 ppb.......................................... 7,500 7,500 5,400 500
70 ppb.......................................... 7,200 7,200 4,900 240
65 ppb.......................................... 6,500 6,500 2,800 90
60 ppb \84\..................................... 6,400 6,400 2,300 10
----------------------------------------------------------------------------------------------------------------
2009
----------------------------------------------------------------------------------------------------------------
75 ppb.......................................... 7,000 7,000 4,700 270
70 ppb.......................................... 6,900 6,900 4,300 80
65 ppb.......................................... 6,400 6,400 2,600 40
60 ppb.......................................... 6,300 6,300 2,100 10
----------------------------------------------------------------------------------------------------------------
ii. Key Uncertainties
---------------------------------------------------------------------------
\83\ Table 3 is based on the information in Figures 7-2 and 7-3
in the HREA (U.S. EPA, 2014a). Estimates of the numbers of
O3-associated deaths are based on concentration-response
relationships for total mortality associated with short-term
O3 from the study by Smith et al. (2009). Estimates of
the numbers O3-associated deaths are rounded to the
nearest hundred, unless otherwise indicated.
\84\ As discussed in section 4.3.3 of the HREA, the model-based
air quality adjustment approach used to estimate risks associated
with the current and alternative standards was unable to estimate
the distribution of ambient O3 concentrations in New York
City upon just meeting an alternative standard with a level of 60
ppb. Therefore, the total number of deaths indicated for the 60 ppb
standard level reflect the 60 ppb estimates for all urban study
areas except New York City. For New York City, the estimated number
of O3-associated deaths for the 60 ppb standard level was
assumed to be equal to the number for the 65 ppb level.
---------------------------------------------------------------------------
Compared to estimates of O3 exposures of concern and
estimates of O3-induced lung function decrements (discussed
above), the HREA conclusions reflect lower confidence in epidemiologic-
based risk estimates (U.S. EPA, 2014a, section 9.6). In particular, the
HREA highlights the heterogeneity in effect estimates between
locations, the potential for exposure measurement errors, and
uncertainty in the interpretation of the shape of concentration-
response functions at lower O3 concentrations (U.S. EPA,
2014a, section 9.6). The HREA also concludes that lower confidence
should be placed in the results of the assessment of respiratory
mortality risks associated with long-term O3, primarily
because that analysis is based on only one study, though that study is
well-designed, and because of the uncertainty in that study about the
existence and identification of a potential threshold in the
concentration-response function (U.S. EPA, 2014a, section 9.6).\85\
\86\ This section further discusses some of the key uncertainties in
epidemiologic-based risk estimates, as summarized in the PA (U.S. EPA,
2014c, section 3.2.3.2), with a focus on uncertainties that can have
particularly important implications for the Administrator's
consideration of epidemiology-based risk estimates.
---------------------------------------------------------------------------
\85\ The CASAC also concluded that ``[i]n light of the potential
nonlinearity of the C-R function for long-term exposure reflecting a
threshold of the mortality response, the estimated number of
premature deaths avoidable for long-term exposure reductions for
several levels need to be viewed with caution'' (Frey, 2014a, p. 3).
\86\ There is also uncertainty about the extent to which
mortality estimates based on the long-term metric used in the study
by Jerrett et al. (2009) (i.e., seasonal average of 1-hour daily
maximum concentrations) reflects associations with long-term average
O3 versus repeated occurrences of elevated short-term
concentrations.
---------------------------------------------------------------------------
The PA notes that reducing NOX emissions generally
reduces O3-associated mortality and morbidity risk estimates
in locations and time periods with relatively high ambient
O3 concentrations and increases risk estimates in locations
and time periods with relatively low concentrations (II.C.1, above).
When evaluating uncertainties in epidemiologic risk estimates, it is
important to consider (1) The extent to which the O3
response to reductions in NOX emissions appropriately
represents the trends observed in ambient O3 following
actual reductions in NOX emissions; (2) the extent to which
estimated changes in risks in urban study areas are representative of
the changes that would be experienced broadly across the U.S.
population; and (3) the extent to which the O3 response to
reductions in precursor emissions could differ with emissions reduction
strategies that are different from those used in HREA to generate risk
estimates.
To evaluate the first issue, the HREA conducted a national analysis
evaluating trends in monitored ambient O3 concentrations
during a time period when the U.S. experienced large-scale reductions
in NOX emissions (i.e., 2001 to 2010). Analyses of trends in
monitored O3 indicate that over such a time period, the
upper end of the distribution of monitored O3 concentrations
(i.e., indicated by the 95th percentile) generally decreased in urban
and non-urban locations across the U.S. (U.S. EPA, 2014a, Figure 8-29).
During this same time period, median O3 concentrations
decreased in suburban and rural locations, and in some urban locations.
However, median concentrations increased in some large urban centers
(U.S. EPA, 2014a, Figure 8-28). As discussed in the REA, and above
(II.C.1), these increases in median concentrations likely reflect the
increases in relatively low O3 concentrations that can occur
near important sources of NOX upon reductions in
NOX emissions (U.S. EPA, 2014a, section 8.2.3.1). These
patterns of monitored O3 during a period when the U.S.
experienced large reductions in NOX emissions are
qualitatively consistent with the modeled responses of O3 to
reductions in NOX emissions.
To evaluate the second issue, the HREA conducted national air
quality modeling analyses. These analyses estimated the proportion of
the U.S. population living in locations where seasonal averages of
daily O3 concentrations are estimated to decrease in
response to reductions in NOX emissions, and the proportion
living in locations where such seasonal averages are estimated to
increase. Given the close relationship between changes in
[[Page 75278]]
seasonal averages of daily O3 concentrations and changes in
seasonal mortality and morbidity risk estimates, this analysis informs
consideration of the extent to which the risk results in urban study
areas represent the U.S. population as a whole. This representativeness
analysis indicates that the majority of the U.S. population lives in
locations where reducing NOX emissions would be expected to
result in decreases in warm season averages of daily maximum 8-hour
ambient O3 concentrations. Because the HREA urban study
areas tend to underrepresent the populations living in such areas
(e.g., suburban, smaller urban, and rural areas), risk estimates for
the urban study areas are likely to understate the average reductions
in O3-associated mortality and morbidity risks that would be
experienced across the U.S. population as a whole upon reducing
NOX emissions (U.S. EPA, 2014a, section 8.2.3.2).
To evaluate the third issue, the HREA assessed the O3
air quality response to reducing both NOX and VOC emissions
(i.e., in addition to assessing reductions in NOX emissions
alone) for a subset of seven urban study areas. As discussed in the PA
(U.S. EPA, 2014c, section 3.2.1), in most of the urban study areas the
inclusion of VOC emissions reductions did not alter the NOX
emissions reductions required to meet the current or alternative
standards.\87\ However, the addition of VOC reductions generally
resulted in larger decreases in mid-range O3 concentrations
(25th to 75th percentiles) (U.S. EPA, 2014a, Appendix 4D, section
4.7).\88\ In addition, in all seven of the urban study areas evaluated,
the increases in low O3 concentrations were smaller for the
NOX/VOC scenarios than the NOX alone scenarios
(U.S. EPA, 2014a, Appendix 4D, section 4.7). This was most apparent for
Denver, Houston, Los Angeles, New York, and Philadelphia. Given the
impacts on total risk estimates of increases in low O3
concentrations, these results suggest that in some locations optimized
emissions reduction strategies could result in larger reductions in
O3-associated mortality and morbidity than indicated by HREA
estimates.
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\87\ The exceptions are Chicago and Denver, for which the HREA
risk estimates are based on reductions in both NOX and
VOC (U.S. EPA, 2014a, section 4.3.3.1). Emissions of NOX
and VOC were reduced by equal percentages, a scenario not likely to
reflect the optimal combination for reducing risks.
\88\ This was the case for all of the urban study areas
evaluated, with the exception of New York (U.S. EPA, 2014a, Appendix
4-D, section 4.7).
---------------------------------------------------------------------------
Section 7.4 of the HREA also highlights some additional
uncertainties associated with epidemiologic-based risk estimates (U.S.
EPA, 2014a). This section of the HREA identifies and discusses sources
of uncertainty and presents a qualitative evaluation of key parameters
that can introduce uncertainty into risk estimates (U.S. EPA, 2014a,
Table 7-4). For several of these parameters, the HREA also presents
quantitative sensitivity analyses (U.S. EPA, 2014a, sections 7.4.2 and
7.5.3). Of the uncertainties discussed in Chapter 7 of the HREA, those
related to the application of concentration-response functions from
epidemiologic studies can have particularly important implications for
consideration of epidemiology-based risk estimates, as discussed below.
An important uncertainty is the shape of concentration-response
functions at low ambient O3 concentrations (U.S. EPA, 2014a,
Table 7-4).\89\ Consistent with the ISA conclusion that there is no
discernible population threshold in O3-associated health
effects, the HREA estimates epidemiology-based mortality and morbidity
risks for entire distributions of ambient O3 concentrations,
based on the assumption that concentration-response relationships
remain linear over those distributions. In addition, in recognition of
the ISA conclusion that certainty in the shape of O3
concentration-response functions decreases at low ambient
concentrations, the HREA also estimates total mortality associated with
various ambient O3 concentrations. The PA considers both
types of risk estimates, recognizing greater public health concern for
adverse O3-attributable effects at higher ambient
O3 concentrations (which drive higher exposure
concentrations, section 3.2.2 of the PA (U.S. EPA, 2014c)), as compared
to lower concentrations.
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\89\ A related uncertainty is the existence, or not, of a
threshold. The HREA addresses this issue for long-term O3
by evaluating risks in models that include potential thresholds
(II.D.2.c).
---------------------------------------------------------------------------
A related uncertainty is that associated with the public health
importance of the increases in relatively low O3
concentrations following air quality adjustment. This uncertainty
relates to the assumption that the concentration response function for
O3 is linear, such that that total risk estimates are
equally influenced by decreasing high concentrations and increasing low
concentrations, when the increases and decreases are of equal
magnitude. Even on days with increases in relatively low area-wide
average concentrations, resulting in increases in estimated risks, some
portions of the urban study areas could experience decreases in high
O3 concentrations. To the extent adverse O3-
attributable effects are more strongly supported for higher ambient
concentrations (which are consistently reduced upon air quality
adjustment), the impacts on risk estimates of increasing low
O3 concentrations reflect an important source of
uncertainty.
The HREA also notes important uncertainties associated with using a
concentration-response relationship developed for a particular
population in a particular location to estimate health risks in
different populations and locations (U.S. EPA, 2014a, Table 7-4). As
discussed above, concentration-response relationships derived from
epidemiologic studies reflect the spatial and temporal patterns of
population exposures during the study. The HREA applies concentration-
response relationships from epidemiologic studies to adjusted air
quality in study areas that are different from, and often larger in
spatial extent than, the areas used to generate the relationships. This
approach ensures the inclusion of the actual nonattainment monitors
that often determine the magnitude of emissions reductions for the air
quality adjustments throughout the urban study areas. This approach
also allows the HREA to estimate patterns of health risks more broadly
across a larger area, including a broader range of air quality
concentrations and a larger population. The HREA notes that it is not
possible to quantify the impacts of this uncertainty on risk estimates
in most urban case study locations, though the HREA notes that
mortality effect estimates for different portions of the New York City
core based statistical area (CBSA) vary by a factor of almost 10 (U.S.
EPA, 2014a, section 7.5.3).
An additional, related uncertainty is that associated with applying
concentration-response functions from epidemiologic studies to adjusted
air quality. Concentration-response functions from the O3
epidemiologic studies used in the HREA are based on associations
between day to day variation in ``area-wide'' O3
concentrations (i.e., averaged across multiple monitors) and variation
in health effects. Epidemiologic studies use these area-wide
O3 concentrations, which reflect the particular spatial and
temporal patterns of ambient O3 present in study locations,
as surrogates for the pattern of O3 exposures experienced by
study populations. To the extent adjusting O3 concentrations
to just meet the current standard results in important alterations in
the spatial and/
[[Page 75279]]
or temporal patterns of ambient O3, there is uncertainty in
the appropriateness of applying concentration-response functions from
epidemiologic studies (which necessarily reflect a different air
quality distribution than the modelled distribution) to estimate health
risks associated with adjusted O3 air quality. In
particular, this uncertainty could be important to the extent that (1)
factors associated with space modify the effects of O3 on
health or (2) spatial mobility is a key driver of individual-level
exposures. Although the impact of this uncertainty on risk estimates
cannot be quantified (U.S. EPA, 2014a, Table 7-4), it has the potential
to become more important as model adjustment results in larger changes
in spatial and temporal patterns of ambient O3
concentrations across urban study areas.
The use of a national concentration-response function to estimate
respiratory mortality associated with long-term O3 is a
source of uncertainty. Risk estimates generated in sensitivity analyses
using region-specific effect estimates differ substantially from the
core estimates based on a single national-level effect estimate (U.S.
EPA, 2014a; Table 7-14). Furthermore, the risk estimates generated
using the regional effect estimates display considerable variability
across urban study areas (U.S. EPA, 2014a; Table 7-14), reflecting the
substantial variability in the underlying effect estimates (see Jerrett
et al., 2009, Table 4). While the results of the HREA sensitivity
analyses evaluating this uncertainty point to the potential for
regional heterogeneity in the long-term risk estimates, the relatively
large confidence intervals associated with regional effect estimates
resulted in the HREA conclusion that staff does not have confidence in
the regionally based risk estimates themselves.
Finally, the HREA does not quantify any reductions in risk that
could be associated with reductions in the ambient concentrations of
pollutants other than O3, resulting from control of
NOX. For example, as discussed in chapter 2 of the PA (U.S.
EPA, 2014c), NOX emissions contribute to ambient
NO2, and NOX and VOCs can contribute to secondary
formation of PM2.5 constituents, including ammonium sulfate
(NH4SO4), ammonium nitrate
(NH4NO3), and organic carbon (OC). Therefore, at
some times and in some locations, control strategies that would reduce
NOX emissions (i.e., to meet an O3 standard)
could reduce ambient concentrations of NO2 and
PM2.5, resulting in health benefits beyond those directly
associated with reducing ambient O3 concentrations. In
issuing its advice, CASAC likewise noted the potential reductions in
criteria pollutants other than ozone as a result of NOx reductions, and
the resulting potential public health benefits (Frey, 2014a, pp. 10 and
11).
D. Conclusions on the Adequacy of the Current Primary Standard
The initial issue to be addressed in the current review of the
primary O3 standard is whether, in view of the advances in
scientific knowledge and additional information, the existing standard
should be revised. In evaluating whether it is appropriate to retain or
revise the current standard, the Administrator's considerations build
upon those in the 2008 review, including consideration of the broader
body of scientific evidence and exposure and health risk information
now available, as summarized above (II.A to II.C).
In developing conclusions on the adequacy of the current primary
O3 standard, the Administrator takes into account both
evidence-based and quantitative exposure- and risk-based
considerations. Evidence-based considerations include the assessment of
evidence from controlled human exposure, animal toxicological, and
epidemiologic studies for a variety of health endpoints. The
Administrator focuses on health endpoints for which the evidence is
strong enough to support a ``causal'' or a ``likely to be causal''
relationship, based on the ISA's integrative synthesis of the entire
body of evidence. The Administrator's consideration of quantitative
exposure and risk information draws from the results of the exposure
and risk assessments presented in the HREA.
The Administrator's consideration of the evidence and exposure/risk
information is informed by the considerations and conclusions presented
in the PA (U.S. EPA, 2014c). The purpose of the PA is to help ``bridge
the gap'' between the scientific and technical information assessed in
the ISA and HREA, and the policy decisions that are required of the
Administrator (U.S. EPA, 2014c, Chapter 1). The PA's evidence-based and
exposure-/risk-based considerations and conclusions are summarized
below in sections II.D.1 to II.D.3. CASAC advice to the Administrator
and public commenter views are summarized in section II.D.4. Section
II.D.5 presents the Administrator's proposed conclusions concerning the
adequacy of the public health protection provided by the current
standard, and her proposed decision to revise that standard.
1. Summary of Evidence-Based Considerations in the PA
In considering the available scientific evidence, the PA evaluates
the O3 concentrations in health effects studies (U.S. EPA,
2014c, section 3.1.4). Specifically, the PA characterizes the extent to
which effects have been reported for the O3 exposure
concentrations evaluated in controlled human exposure studies and over
the distributions of ambient O3 concentrations in locations
where epidemiologic studies have been conducted. These considerations,
as they relate to the adequacy of the current standard, are presented
in detail in section 3.1.4 of the PA (U.S. EPA, 2014c) and are
summarized briefly below for controlled human exposure and
epidemiologic panel studies (II.D.1.a), epidemiologic studies of short-
term O3 exposures (II.D.1.b), and epidemiologic studies of
long-term O3 exposures (II.D.1.c). Section II.D.1.d
summarizes the PA conclusions based on consideration of the scientific
evidence.
a. Concentrations in Controlled Human Exposure and Panel Studies
The evidence from controlled human exposure studies and panel
studies is assessed in section 6.2 of the ISA (U.S. EPA, 2013a) and is
summarized in section 3.1.2 of the PA (U.S. EPA, 2014c). As discussed
above (II.B), controlled human exposure studies have generally been
conducted with young, healthy adults, and have evaluated exposure
durations less than 8 hours. Panel studies have evaluated a wider range
of study populations, including children, and have generally evaluated
associations with O3 concentrations averaged over several
hours (U.S. EPA, 2013a, section 6.2.1.2).\90\
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\90\ The PA focuses on panel studies that used on-site
monitoring, and that are highlighted in the ISA for the extent to
which monitored ambient O3 concentrations reflect
exposure concentrations in their study populations (U.S. EPA, 2013a,
section 6.2.1.2).
---------------------------------------------------------------------------
As summarized above (II.B), a large number of controlled human
exposure studies have reported lung function decrements, respiratory
symptoms, airway inflammation, AHR, and/or impaired lung host defense
in young, healthy adults engaged in moderate, intermittent exertion,
following 6.6-hour O3 exposures. These studies have
consistently reported such effects following exposures to O3
concentrations of 80 ppb or greater. In addition to lung function
decrements, available studies have also evaluated respiratory symptoms
or airway
[[Page 75280]]
inflammation following exposures to O3 concentrations below
75 ppb. Table 3-1 in the PA highlights the group mean results of
individual controlled human exposure studies that have evaluated
exposures of healthy adults to O3 concentrations below 75
ppb (U.S. EPA, 2014c). The studies included in Table 3-1 of the PA
indicate a combination of lung function decrements and respiratory
symptoms following 6.6 hour exposures to O3 concentrations
as low as 72 ppb, and lung function decrements and airway inflammation
following 6.6 hour exposures to O3 concentrations as low as
60 ppb (based on group means).
The PA also notes consistent results in some panel studies of
O3-associated lung function decrements. In particular, the
PA notes that epidemiologic panel studies in children and adults
consistently indicate O3-associated lung function decrements
when on-site monitored concentrations were below 75 ppb, although the
evidence becomes less consistent at lower O3 concentrations
(U.S. EPA, 2014c, section 3.1.4.1).\91\
---------------------------------------------------------------------------
\91\ As indicated in the PA (U.S. EPA, 2014c, Table 3-2), key
O3 panel studies evaluated averaging periods ranging from
10 minutes to 12 hours.
---------------------------------------------------------------------------
Thus, controlled human exposure studies and panel studies have
reported respiratory effects in adults and children following exposures
to O3 concentrations below 75 ppb (albeit over shorter
averaging periods than the 8 hour averaging time of the current
O3 standard). The PA notes that such impairments in
respiratory function have the potential to be adverse, based on ATS
guidelines for adversity and based on advice from CASAC (Frey, 2014c,
pp. 5 and 6) (U.S. EPA, 2014c, section 3.1.3). In addition, the PA
notes that if they become serious enough, these respiratory effects
could lead to the types of clearly adverse effects commonly reported in
O3 epidemiologic studies (e.g., respiratory emergency
department visits, hospital admissions). Therefore, the PA concludes
that the respiratory effects experienced following exposures to
O3 concentrations lower than 75 ppb could be adverse in some
individuals, particularly if experienced by members of at-risk
populations (e.g., people with asthma, children).\92\
---------------------------------------------------------------------------
\92\ These effects were reported in healthy individuals.
Consistent with CASAC advice (Samet, 2011; Frey, 2014a, p. 14; Frey,
2014c, p. 7), it is a reasonable inference that the effects would be
greater in magnitude and potential severity for at-risk groups. See
National Environmental Development Ass'n Clean Air Project v. EPA,
686 F. 3d 803, 811 (D.C. Cir. (2012) (making this point).
---------------------------------------------------------------------------
b. Concentrations in Epidemiologic Studies--Short-Term
The PA also considers distributions of ambient O3
concentrations in locations where epidemiologic studies have evaluated
O3-associated hospital admissions, emergency department
visits, and/or mortality (U.S. EPA, 2014c, section 3.1.4.2). When
considering epidemiologic studies within the context of the current
standard, the PA emphasizes those studies conducted in the U.S. and
Canada. Such studies reflect air quality and exposure patterns that are
likely more typical of the U.S. population than the air quality and
exposure patterns reflected in studies conducted outside the U.S. and
Canada (U.S. EPA, 2014c, section 1.3.1.2).\93\ The PA also emphasizes
studies reporting associations with effects judged in the ISA to be
robust to confounding by other factors, including co-occurring air
pollutants. In addition to these factors, the PA considers the
statistical precision of study results, the extent to which studies
report associations in at-risk populations, and the extent to which the
biological plausibility of associations at various ambient
O3 concentrations is supported by controlled human exposure
and/or animal toxicological studies. These considerations help inform
the range of ambient O3 concentrations over which the
evidence indicates the most confidence in O3-associated
health effects, and the range of concentrations over which confidence
in such associations is appreciably lower.
---------------------------------------------------------------------------
\93\ Nonetheless, the PA recognizes the importance of all
studies, including international studies, in the ISA's assessment of
the weight of the evidence that informs causality determinations.
---------------------------------------------------------------------------
This section summarizes the PA conclusions regarding the extent to
which health effect associations have been reported for ambient
O3 concentrations likely to have met the current
O3 standard. Section II.D.1.b.i summarizes PA analyses and
conclusions based on analyses evaluating the extent to which
epidemiologic studies have reported health effect associations in
locations that would likely have met the current O3
standard. Section II.D.1.b.ii summarizes PA conclusions based on
analyses evaluating the O3 air quality in locations where
epidemiologic studies have characterized confidence intervals around
cut point analyses or concentration-response functions. Section
II.D.1.b.iii summarizes the important uncertainties in these analyses.
i. Associations in Locations Likely Meeting Current Standard
The PA considers the extent to which U.S. and Canadian
epidemiologic studies have reported associations with mortality or
morbidity in locations that would likely have met the current
O3 standard during the study period (U.S. EPA, 2014c,
section 3.14.2). Addressing this issue can provide important insights
into the extent to which O3-health effect associations are
present for distributions of ambient O3 concentrations that
would be allowed by the current standard. To the extent associations
are reported in study areas that would have met the current standard,
those associations indicate that the current standard could allow the
types of clearly adverse O3-associated effects reported in
epidemiologic studies (e.g., mortality, hospital admissions, emergency
department visits).\94\ In considering these analyses, the PA also
notes that the lack of such associations in locations meeting the
current standard indicates increased uncertainty in the extent to which
O3-associated health effects would persist upon reducing
O3 precursor emissions in order to meet that standard.
---------------------------------------------------------------------------
\94\ See ATA III, 283 F.3d at 370 (EPA justified in revising
NAAQS when health effect associations are observed in epidemiologic
studies at levels allowed by the NAAQS); State of Mississippi v.
EPA, 744 F. 3d at 1345 (same).
---------------------------------------------------------------------------
The PA identifies U.S. and Canadian studies of respiratory hospital
admissions, respiratory emergency department visits, and mortality
(total, respiratory, cardiovascular) from the ISA (U.S. EPA, 2013a,
Tables 6-28, 6-42, and 6-53, and section 6.2.8; U.S. EPA, 2014c,
Appendix 3D). Analysis of study area air quality indicates that the
large majority of epidemiologic study areas evaluated would have
violated the current standard during study periods (U.S. EPA, 2014c,
Appendix 3D). However, the PA notes that a single-city study conducted
in Seattle, a location that would have met the current standard over
the entire study period, reported positive and statistically
significant associations with respiratory emergency department visits
in children and adults (Mar and Koenig, 2009). The PA also notes four
Canadian multicity studies that reported positive and statistically
significant associations with respiratory morbidity or mortality, and
for which the majority of study cities would have met the current
standard over the entire study periods (Cakmak et
[[Page 75281]]
al., 2006; Dales et al., 2006; Katsouyanni et al., 2009; Stieb et al.,
2009).\95\
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\95\ In addition, a study by Vedal et al. (2003) was included in
the 2006 AQCD (U.S. EPA, 2006a). This study reported positive and
statistically significant associations with mortality in Vancouver
during a time period when the study area would have met the current
standard (U.S. EPA, 2007). This study was not assessed in the ISA in
the current review (U.S. EPA, 2013a).
---------------------------------------------------------------------------
The PA concludes that the single-city study by Mar and Koenig
(2009) indicates the presence of associations with mortality and
morbidity for an ambient distribution of O3 that would have
met the current standard (U.S. EPA, 2014c, section 3.1.4.2). The PA
notes that interpretation of the air quality concentrations in the
multicity study locations evaluated in this review is complicated by
uncertainties in the extent to which multicity effect estimates can be
attributed to ambient O3 in the majority of locations, which
would have met the current standard, versus O3 in the
smaller number of locations that would have violated the standard.
While acknowledging this uncertainty in interpreting air quality in
multicity studies, the PA notes that multicity effect estimates in the
four studies cited above are largely influenced by locations meeting
the current standard (i.e., given that most study areas would have met
this standard). Therefore, the PA concludes that Canadian multicity
studies, in addition to the single-city study in Seattle, suggest
confidence in the presence of associations with mortality and morbidity
for ambient distributions of O3 that would have met the
current standard (U.S. EPA, 2014c, section 3.1.4.2).
ii. Air Quality Associated With Cut Point Analyses and Concentration-
Response Functions
The PA also considers the extent to which additional epidemiologic
studies of mortality or morbidity, specifically those conducted in
locations that would have violated the current standard, can inform
consideration of adequacy of the current standard (U.S. EPA, 2014c,
section 3.1.4.2). In doing so, the PA notes that health effect
associations reported in epidemiologic studies are influenced by the
full distributions of ambient O3 concentrations, including
concentrations below the level of the current standard. The PA focuses
on studies that have explicitly characterized O3 health
effect associations, including confidence in those associations, for
various portions of distributions of ambient O3
concentrations.
The U.S. multicity study by Bell et al. (2006) reported health
effect associations for air quality subsets restricted to ambient
O3 concentrations below one or more predetermined cut
points. In these analyses, effect estimates were based only on the
subsets of days contributing to averaged O3 concentrations
below cut points ranging from 5 to 60 ppb (Bell et al., 2006, Figure
2).\96\ The PA notes that such ``cut point'' analyses can provide
information on the magnitude and statistical precision of effect
estimates for defined distributions of ambient concentrations, which
may in some cases include distributions that would meet the current
standard (U.S. EPA, 2014c, section 3.1.4.2). The cut points below which
confidence intervals become notably wider depend in large part on data
density and, therefore, cut point analyses provide insight into the
ambient concentrations below which the available air quality
information becomes too sparse to support conclusions about the nature
of concentration-response relationships with a high degree of
confidence (U.S. EPA, 2014c, section 3.1.4.2).
---------------------------------------------------------------------------
\96\ In the published study, 2-day rolling averages of 24-hour
average O3 concentrations were calculated in each study
location (based on averaging across monitors in study locations with
multiple monitors).
---------------------------------------------------------------------------
The PA considers the extent to which the cut-point analyses
reported by Bell et al. (2006) indicate health effect associations for
distributions of ambient O3 concentrations that would likely
have met the current standard. The PA particularly focuses on the
lowest cut-point for which the association between O3 and
mortality was reported to be statistically significant (i.e., 30 ppb,
based on visual inspection of Figure 2 in the published study). Based
on the O3 air quality concentrations that met the criteria
for inclusion in the 30 ppb cut point analysis, 95% of study areas had
3-year averages of annual 4th highest daily maximum 8-hour
O3 concentration at or below 75 ppb over the entire study
period (U.S. EPA, 2014c, section 3.1.4.2, Table 3-6). Though there are
important uncertainties in this analysis, as discussed below, the PA
concludes that these results suggest that the large majority of air
quality distributions that provided the basis for the positive and
statistically significant association with mortality at the 30 ppb cut
point would likely have met the current O3 standard.
The PA also analyzes air quality for studies that have reported
confidence intervals around concentration-response functions over
distributions of ambient O3 concentrations (U.S. EPA, 2014c,
section 3.1.4.2). Confidence intervals around concentration-response
functions can provide insights into the range of ambient concentrations
over which the study indicates the most confidence in the reported
health effect associations (i.e., where confidence intervals are
narrowest), and into the range of ambient concentrations below which
the study indicates that uncertainty in the nature of such associations
becomes notably greater (i.e., where confidence intervals become
markedly wider). As with cut point analyses, the concentrations below
which confidence intervals become markedly wider are intrinsically
related to data density, and do not necessarily indicate the absence of
an association.
The PA focuses on two U.S. single-city studies that have reported
confidence intervals around concentration-response functions (Silverman
and Ito, 2010; Strickland et al., 2010). Based on the published
analyses, the PA identifies the ranges of ambient O3
concentrations over which these studies indicate the highest degree of
confidence in the reported linear concentration-response functions
(U.S. EPA, 2014c, section 3.1.4.2). For the lower ends of these ranges,
air quality analyses in the PA indicate that over 99% of days had
maximum 8-hour O3 concentrations (i.e., from highest
monitors in study locations) at or below 75 ppb. For comparison, the
annual 4th highest daily maximum 8-hour O3 concentration
generally corresponds to the 98th or 99th percentile of the seasonal
distribution, depending on the length of the O3 season.
The PA concludes that these analyses of air quality data from the
study locations evaluated by Silverman and Ito (2010) and Strickland et
al. (2010) indicate a relatively high degree of confidence in reported
statistical associations with respiratory health outcomes on days when
virtually all monitored 8-hour O3 concentrations were 75 ppb
or below (U.S. EPA, 2014c, section 3.1.4.2). Though these analyses do
not identify true design values, the presence of O3-
associated respiratory effects on such days provides insight into the
types of health effects that could occur in locations with maximum
ambient O3 concentrations at or below the level of the
current standard.
iii. Important Uncertainties
In considering the above evidence within the context of developing
overall conclusions on the current and potential alternative standards,
the PA also takes into account important uncertainties in these
analyses of air quality in locations of epidemiologic study areas.
These uncertainties are summarized in this
[[Page 75282]]
section. The PA's consideration of the evidence, including the
associated uncertainties, in reaching conclusions on the current and
potential alternative standards is summarized in sections II.D.3
(current standard) and II.E.4.b (potential alternative standards)
below.
The PA notes that while multicity studies generally have greater
statistical power and geographic coverage than single-city studies,
there is often greater uncertainty in conclusions about the extent to
which multicity effect estimates reflect associations with air quality
meeting the current standard (U.S. EPA, 2014c, section 1.3.1.2.1). This
is particularly the case for the multicity studies evaluated in this
review with some study locations meeting the current standard and
others violating that standard. Specifically for the four Canadian
multicity studies discussed above, the PA notes that interpretation of
air quality information is complicated by uncertainties in the extent
to which multicity effect estimates can be attributed to ambient
O3 in the majority of locations, which would have met the
current standard, versus O3 in the smaller number of
locations that would have violated the standard.
The PA also notes important uncertainties in multicity studies that
evaluate the potential for thresholds to exist, as was done in the
study by Bell et al. (2006). Specifically, the ISA highlights the
regional heterogeneity in O3 health effect associations as a
factor that could obscure the presence of thresholds, should they
exist, in multicity studies (U.S. EPA, 2013a, sections 2.5.4.4 and
2.5.4.5). The ISA notes that community characteristics (e.g., activity
patterns, housing type, age distribution, prevalence of air
conditioning) could be important contributors to reported regional
heterogeneity (U.S. EPA, 2013a, section 2.5.4.5). Given this
heterogeneity, the ISA concludes that ``a national or combined analysis
may not be appropriate to identify whether a threshold exists in the
O3-mortality [concentration-response] relationship'' (U.S.
EPA, 2013a, p. 2-33). This represents an important source of
uncertainty when characterizing confidence in reported concentration-
response relationships over distributions of ambient O3
concentrations, based on multicity studies. The PA notes that this
uncertainty becomes increasingly important when interpreting
concentration-response relationships at lower ambient O3
concentrations, particularly those concentrations corresponding to
portions of distributions where data density decreases notably (U.S.
EPA, 2014c, section 3.1.4.2).
Another important uncertainty, related specifically to the PA
analysis of cut points by Bell et al. (2006), is that EPA staff was
unable to obtain the air quality data used to generate the cut-point
analyses in the published study (U.S. EPA, 2014c, section 3.1.4.2).
Therefore, the analyses in the PA identified 2-day averages of 24-hour
O3 concentrations in study locations using the air quality
data available in AQS, combined with the published description of study
area definitions. An important uncertainty in this approach is the
extent to which the PA appropriately recreated the cut-point analyses
in the published study (U.S. EPA, 2014c, section 3.1.4.2).
An uncertainty that applies to epidemiologic studies in general is
the extent to which reported health effects are caused by exposures to
O3 itself, as opposed to other factors such as co-occurring
pollutants or pollutant mixtures. The PA notes that this uncertainty
becomes an increasingly important consideration as health effect
associations are evaluated at lower ambient O3
concentrations. In particular, there is increasing uncertainty as to
whether the observed associations remain plausibly related to exposures
to ambient O3, rather than to the broader mix of air
pollutants present in the ambient air. In considering the potential
importance of this uncertainty at the relatively low ambient
O3 concentrations that are the focus of the PA analyses, the
PA notes that Silverman and Ito (2010) and Strickland (2010) reported
O3 health effect associations in co-pollutant models,\97\
providing support for associations with O3 itself (U.S. EPA,
2014c, section 3.1.4.2). The PA also concludes that air quality
analyses indicate coherence with the results of experimental studies
(i.e., in which the study design dictates that exposures to
O3 itself are responsible for reported effects), and are
consistent with the occurrence of O3-attributable
respiratory hospital admissions and emergency department visits, even
when virtually all monitored concentrations were below the level of the
current standard (U.S. EPA, 2014c, section 3.1.4.2, Tables 3-4, 3-5).
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\97\ In addition, Bell et al. (2006) reported that, based on a
previous study (Bell et al., 2004), associations with mortality were
robust to the inclusion of PM10 in the model.
---------------------------------------------------------------------------
c. Concentrations in Epidemiologic Studies--Long-Term
The PA also considers the extent to which epidemiologic studies
employing longer-term ambient O3 concentration metrics
inform our understanding of the air quality conditions associated with
O3-attributable health effects, and specifically inform
consideration of the extent to which such effects could occur under air
quality conditions meeting the current standard (U.S. EPA, 2014c,
section 3.1.4.3). Unlike for the studies of short-term O3
discussed above, the available U.S. and Canadian epidemiologic studies
evaluating long-term ambient O3 concentration metrics have
not been conducted in locations likely to have met the current 8-hour
O3 standard during the study period, and have not reported
concentration-response functions that indicate confidence in health
effect associations at O3 concentrations meeting the current
standard (U.S. EPA, 2014c, section 3.1.4.3). Therefore, although these
studies contribute to understanding of health effects associated with
long-term or repeated exposures to ambient O3, consideration
of study area air quality does not inform consideration of the extent
to which those health effects may be occurring in locations that meet
the current standard.
d. PA Conclusions Based on Consideration of the Evidence
As discussed above (II.D.1.a to II.D.1.c), in considering the
available scientific evidence, including associated uncertainties, as
it relates to the degree of public health protection provided by the
current primary O3 standard, the PA evaluates the extent to
which health effects have been reported for the O3 exposure
concentrations evaluated in controlled human exposure studies and over
the distributions of ambient O3 concentrations in locations
where epidemiologic studies have been conducted. The PA concludes that
(1) the evidence from controlled human exposure studies provides strong
support for the occurrence of adverse respiratory effects following
exposures to O3 concentrations below the level of the
current standard and that (2) epidemiologic studies provide support for
the occurrence of adverse respiratory effects and mortality under air
quality conditions that would likely meet the current standard. In
further considering the public health protection provided by the
current standard, the PA next considers the results of exposure and
health risk assessments.
[[Page 75283]]
2. Summary of Exposure- and Risk-Based Considerations in the PA
In order to further inform judgments about the potential public
health implications of the current O3 NAAQS, the PA
considers the exposure and risk assessments presented in the HREA (U.S.
EPA, 2014c, section 3.2). Overviews of these exposure and risk
assessments, including summaries of key results and uncertainties, are
provided in section II.C above. This section summarizes key
observations from the PA related to the adequacy of the current
O3 NAAQS, based on consideration of the HREA exposure
assessment (II.D.2.a), lung function risk assessment (II.D.2.b), and
mortality/morbidity risk assessments (II.D.2.c).
a. Exposure Assessment--Key Observations
As discussed above (II.C.2), the exposure assessment provides
estimates of the number and percent of people who would experience
exposures of concern at or above benchmark concentrations of 60, 70,
and 80 ppb. Benchmarks reflect exposure concentrations at which
O3-induced respiratory effects are known to occur in some
healthy adults engaged in moderate, intermittent exertion, based on
evidence from controlled human exposure studies (U.S. EPA, 2014c,
section 3.1.2.1; U.S. EPA, 2013a, section 6.2).
The PA focuses on exposure estimates in children. Compared to
recent (i.e., unadjusted) air quality, the PA notes that adjusting air
quality to just meet the current O3 NAAQS consistently
reduces the estimated occurrence of exposures of concern in children
(U.S. EPA, 2014a, Appendix 5F). When averaged over the years evaluated
in the HREA, reductions of up to about 70% were estimated. These
reductions in estimated exposures of concern, relative to unadjusted
air quality, reflect the consistent reductions in the highest ambient
O3 concentrations upon model adjustment to just meet the
current standard (U.S. EPA, 2014c, section 3.2.1; U.S. EPA, 2014a,
Chapter 4). Such reductions in estimated exposures of concern are
evident throughout urban study areas, including in urban cores and in
surrounding areas (U.S. EPA, 2014a, Appendix 9A).
Based on Figures 3-7 to 3-10 in the PA (U.S. EPA, 2014c), and the
associated details described in the HREA (U.S. EPA, 2014a, Chapter 5),
the PA further highlights key observations with regard to exposures of
concern in children that are estimated to be allowed by the current
standard. These key observations are summarized below for exposures of
concern >=60, 70, and 80 ppb.
For exposures of concern at or above 60 ppb, the PA highlights the
following key observations for air quality adjusted to just meet the
current standard:
(1) On average over the years 2006 to 2010, the current standard is
estimated to allow approximately 10 to 18% of children in urban study
areas to experience one or more exposures of concern at or above 60
ppb. Summing across urban study areas, these percentages correspond to
almost 2.5 million children experiencing approximately 4 million
exposures of concern at or above 60 ppb during a single O3
season. Of these children, almost 250,000 are asthmatics.\98\
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\98\ As discussed above (II.C.2.b), due to variability in
responsiveness, only a subset of individuals who experience
exposures at or above a benchmark concentration can be expected to
experience adverse health effects.
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(2) On average over the years 2006 to 2010, the current standard is
estimated to allow approximately 3 to 8% of children in urban study
areas to experience two or more exposures of concern to O3
concentrations at or above 60 ppb. Summing across the urban study
areas, these percentages correspond to almost 900,000 children
(including almost 90,000 asthmatic children) estimated to experience at
least two O3 exposure concentrations at or above 60 ppb
during a single O3 season.
(3) In the worst-case years (i.e., those with the largest exposure
estimates), the current standard is estimated to allow approximately 10
to 25% of children to experience one or more exposures of concern at or
above 60 ppb, and approximately 4 to 14% to experience two or more
exposures of concern at or above 60 ppb.
For exposures of concern at or above 70 ppb, the PA highlights the
following key observations for air quality adjusted to just meet the
current standard:
(1) On average over the years 2006 to 2010, the current standard is
estimated to allow up to approximately 3% of children in urban study
areas to experience one or more exposures of concern at or above 70
ppb. Summing across urban study areas, almost 400,000 children
(including almost 40,000 asthmatic children) are estimated to
experience O3 exposure concentrations at or above 70 ppb
during a single O3 season.\99\
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\99\ As discussed above (II.C.2.b), due to variability in
responsiveness, only a subset of individuals who experience
exposures at or above a benchmark concentration can be expected to
experience adverse health effects.
---------------------------------------------------------------------------
(2) On average over the years 2006 to 2010, the current standard is
estimated to allow less than 1% of children in urban study areas to
experience two or more exposures of concern to O3
concentrations at or above 70 ppb.
(3) In the worst-case years, the current standard is estimated to
allow approximately 1 to 8% of children to experience one or more
exposures of concern at or above 70 ppb, and up to approximately 2% to
experience two or more exposures of concern, at or above 70 ppb.
For exposures of concern at or above 80 ppb, the PA highlights the
observation that the current standard is estimated to allow about 1% or
fewer children in urban study areas to experience exposures of concern
at or above 80 ppb, even in years with the highest exposure estimates.
b. Lung Function Risk Assessment--Key Observations
As discussed above (II.C.3.a), the HREA estimates risks of moderate
to large lung function decrements (i.e., FEV1 decrements
>=10%, 15%, or 20%) in school-aged children (ages 5 to 18), asthmatic
school-aged children, and the general adult population for 15 urban
study areas. As for exposures of concern, the PA focuses on lung
function risk estimates in children (including children with asthma).
Compared to risks associated with recent air quality, risk
estimates for air quality just meeting the current standard are
consistently smaller across urban study areas (U.S. EPA, 2014a,
Appendix 6B). When averaged over the years evaluated in the HREA, risk
reductions of up to about 40% were estimated compared to recent air
quality. These reductions reflect the consistent decreases in
relatively high ambient O3 concentrations upon adjustment to
just meet the current standard (U.S. EPA, 2014a, Chapter 4). Such
reductions in estimated lung function risks are evident throughout
urban study areas, including in urban cores and in surrounding areas
(U.S. EPA, 2014, Appendix 9A).
Based on Figures 3-11 to 3-14 in the PA (U.S. EPA, 2014c), and the
associated details described in the HREA (U.S. EPA, 2014a, chapter 6),
the PA highlights key observations with regard to lung function risks
estimated in children for air quality adjusted to just meet the current
standard. These key observations are presented below for
FEV1 decrements >=10, 15, and 20%.
With regard to decrements >=10%, the PA highlights the following
key observations for air quality adjusted to just meet the current
standard:
[[Page 75284]]
(1) On average over the years 2006 to 2010, the current standard is
estimated to allow approximately 14 to 19% of children in urban study
areas to experience one or more lung function decrements >=10%. Summing
across urban study areas, this corresponds to approximately 3 million
children experiencing 15 million O3-induced lung function
decrements >=10% during a single O3 season. Of these
children, about 300,000 are asthmatics.
(2) On average over the years 2006 to 2010, the current standard is
estimated to allow approximately 7 to 12% of children in urban study
areas to experience two or more O3-induced lung function
decrements >=10%. Summing across the urban study areas, this
corresponds to almost 2 million children (including almost 200,000
asthmatic children) estimated to experience two or more O3-
induced lung function decrements greater than 10% during a single
O3 season.
(3) In the worst-case years, the current standard is estimated to
allow approximately 17 to 23% of children in urban study areas to
experience one or more lung function decrements >=10%, and
approximately 10 to 14% to experience two or more O3-induced
lung function decrements >=10%.
With regard to decrements >=15%, the PA highlights the following
key observations for air quality adjusted to just meet the current
standard:
(1) On average over the years 2006 to 2010, the current standard is
estimated to allow approximately 3 to 5% of children in urban study
areas to experience one or more lung function decrements >=15%. Summing
across urban study areas, this corresponds to approximately 800,000
children (including approximately 80,000 asthmatic children) estimated
to experience at least one O3-induced lung function
decrement >=15% during a single O3 season.
(2) On average over the years 2006 to 2010, the current standard is
estimated to allow approximately 2 to 3% of children in urban study
areas to experience two or more O3-induced lung function
decrements >=15%.
(3) In the worst-case years, the current standard is estimated to
allow approximately 4 to 6% of children in urban study areas to
experience one or more lung function decrements >=15%, and
approximately 2 to 4% to experience two or more O3-induced
lung function decrements >=15%.
With regard to decrements >=20%, the PA highlights the following
key observations for air quality adjusted to just meet the current
standard:
(1) On average over the years 2006 to 2010, the current standard is
estimated to allow approximately 1 to 2% of children in urban study
areas to experience one or more lung function decrements >=20%. Summing
across urban study areas, this corresponds to approximately 300,000
children (including approximately 30,000 asthmatic children) estimated
to experience at least one O3-induced lung function
decrement >=20% during a single O3 season.
(2) On average over the years 2006 to 2010, the current standard is
estimated to allow less than 1% of children in urban study areas to
experience two or more O3-induced lung function decrements
>=20%.
(3) In the worst-case years, the current standard is estimated to
allow approximately 2 to 3% of children to experience one or more lung
function decrements >=20%, and less than 2% to experience two or more
O3-induced lung function decrements >=20%.
c. Mortality and Morbidity Risk Assessments--Key Observations
As discussed above (II.C.3.b), risk estimates based on
epidemiologic studies can provide perspective on the most serious
O3-associated public health outcomes (e.g., mortality,
hospital admissions, emergency department visits) in populations that
often include at-risk groups. The HREA estimates such O3-
associated risks in 12 urban study areas \100\ using concentration-
response relationships drawn from epidemiologic studies. These
concentration-response relationships are based on ``area-wide'' average
O3 concentrations.\101\ The HREA estimates risks for the
years 2007 and 2009 in order to provide estimates of risk for a year
with generally higher O3 concentrations (2007) and a year
with generally lower O3 concentrations (2009) (U.S. EPA,
2014a, section 7.1.1).
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\100\ The 12 urban areas evaluated are Atlanta, Baltimore,
Boston, Cleveland, Denver, Detroit, Houston, Los Angeles, New York,
Philadelphia, Sacramento, and St. Louis. Morbidity endpoints were
evaluated in subsets of these areas, based on availability of
appropriate studies (U.S. EPA, 2014a, Chapter 7).
\101\ In the epidemiologic studies that provide the health basis
for HREA risk assessments, concentration-response relationships are
based on daytime O3 concentrations, averaged across
multiple monitors within study areas. These daily averages are used
as surrogates for the spatial and temporal patterns of exposures in
study populations. Consistent with this approach, the HREA
epidemiologic-based risk estimates also utilize daytime
O3 concentrations, averaged across monitors, as
surrogates for population exposures. In this notice, these averaged
concentrations are referred to as ``area-wide'' O3
concentrations. Area-wide concentrations are discussed in more
detail in section 3.1.4 of the PA (U.S. EPA, 2014c).
---------------------------------------------------------------------------
In considering these estimates, the PA notes that HREA conclusions
reflect somewhat lower confidence in epidemiologic-based risk estimates
than in estimates of O3 exposures of concern and
O3-induced lung function decrements (U.S. EPA, 2014a,
section 9.6). In particular, the HREA highlights the unexplained
heterogeneity in effect estimates between locations, the potential for
exposure measurement errors, and uncertainty in the interpretation of
the shape of concentration-response functions at lower O3
concentrations (U.S. EPA, 2014a, section 9.6). The HREA also concludes
that lower confidence should be placed in the results of the assessment
of respiratory mortality risks associated with long-term O3
exposures, primarily because that analysis is based on only one study,
though that study is well-designed, and because of the uncertainty in
that study about the existence and identification of a potential
threshold in the concentration-response function (U.S. EPA, 2014a,
section 9.6). These and other uncertainties are considered in the PA in
reaching conclusions on the current and alternative standards (U.S.
EPA, 2014c, sections 3.4, 4.6).
Key observations from the PA are summarized below for mortality and
morbidity risks associated with air quality adjusted to simulate just
meeting the current O3 NAAQS. These include key observations
for estimates of total (nonaccidental) mortality associated with short-
term O3 concentrations, respiratory morbidity associated
with short-term O3 concentrations, and respiratory mortality
associated with long-term O3 concentrations (U.S. EPA,
2014c, section 3.2.3.2).
With regard to total mortality or morbidity associated with short-
term O3, the PA notes the following for air quality adjusted
to just meet the current standard:
(1) When air quality was adjusted to the current standard for
the 2007 model year (the year with generally ``higher''
O3-associated risks), 10 of 12 urban study areas
exhibited either decreases or virtually no change in estimates of
the number of O3-associated deaths (U.S. EPA, 2014a,
Appendix 7B). Increases were estimated in two of the urban
[[Page 75285]]
study areas (Houston, Los Angeles) \102\ (U.S. EPA, 2014a, Appendix
7B).\103\
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\102\ As discussed above (II.C.1), in locations and time periods
when NOX is predominantly contributing to O3
formation (e.g., downwind of important NOX sources, where
the highest O3 concentrations often occur), model-based
adjustment to the current and alternative standards decreases
estimated ambient O3 concentrations compared to recent
monitored concentrations (U.S. EPA, 2014a, section 4.3.3.2). In
contrast, in locations and time periods when NOX is
predominantly contributing to O3 titration (e.g., in
urban centers with high concentrations of NOX emissions,
where ambient O3 concentrations are often suppressed and
thus relatively low), model-based adjustment increases ambient
O3 concentrations compared to recent monitored
concentrations (U.S. EPA, 2014a, section 4.3.3.2). Changes in
epidemiology-based risk estimates depend on the balance between the
daily decreases in high O3 concentrations and increases
in low O3 concentrations following the model-based air
quality adjustment. Commenting on this issue, CASAC noted that
``controls designed to reduce the peak levels of ozone (e.g., the
4th highest annual MDA8) may not be effective at reducing lower
levels of ozone on more typical days and may actually increase ozone
levels on days where ozone concentrations are low'' (Frey 2014a, p.
2). CASAC further noted that risk results ``suggest that the ozone-
related health risks in the urban cores can increase for some of the
cities as ozone NAAQS alternatives become more stringent. This is
because reductions in nitrogen oxides emissions can lead to less
scavenging of ozone and free radicals, resulting in locally higher
levels of ozone'' (Frey 2014c, p. 10).
\103\ For the 2009 adjusted year (i.e., the year with generally
lower O3 concentrations), changes in risk were generally
smaller than in 2007 (i.e., most changes about 2% or smaller).
Increases were estimated for Houston, Los Angeles, and New York
City.
---------------------------------------------------------------------------
(2) In focusing on total risk, the current standard is estimated
to allow thousands of O3-associated deaths per year in
the urban study areas. In focusing on the risks associated with the
upper portions of distributions of ambient concentrations (area-wide
concentrations >=40, 60 ppb), the current standard is estimated to
allow hundreds to thousands of O3-associated deaths per
year in the urban study areas.
(3) The current standard is estimated to allow tens to thousands
of O3-associated morbidity events per year (i.e.,
respiratory-related hospital admissions, emergency department
visits, and asthma exacerbations).
With regard to respiratory mortality associated with long-term
O3, the PA notes the following for air quality adjusted to
just meet the current standard:
(1) Based on a linear concentration-response function, the
current standard is estimated to allow thousands of O3-
associated respiratory deaths per year in the urban study areas.
(2) Based on threshold models, HREA sensitivity analyses
indicate that the number of respiratory deaths associated with long-
term O3 concentrations could potentially be considerably
lower (i.e., by more than 75% if a threshold exists at 40 ppb, and
by about 98% if a threshold exists at 56 ppb) (U.S. EPA, 2014a,
Figure 7-9).\104\
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\104\ Risk estimates for respiratory mortality associated with
long-term O3 exposures are based on the study by Jerrett
et al. (2009) (U.S. EPA, 2014a, Chapter 7). As discussed above
(II.B.2.b.iv) and in the PA (U.S. EPA, 2014c, section 3.1.4.3),
Jerrett et al. (2009) reported that when seasonal averages of 1-hour
daily maximum O3 concentrations ranged from 33 to 104
ppb, there was no statistical deviation from a linear concentration-
response relationship between O3 and respiratory
mortality across 96 U.S. cities (U.S. EPA, 2013a, section 7.7).
However, the authors reported ``limited evidence'' for an effect
threshold at an O3 concentration of 56 ppb (p=0.06). In
communications with EPA staff (Sasser, 2014), the study authors
indicated that it is not clear whether a threshold model is a better
predictor of respiratory mortality than the linear model, and that
``considerable caution should be exercised in accepting any specific
threshold.''
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3. Policy Assessment Conclusions on the Current Standard
As an initial matter, the PA concludes that reducing precursor
emissions to achieve O3 concentrations that meet the current
standard will provide important improvements in public health
protection. This initial conclusion is based on (1) the strong body of
scientific evidence indicating a wide range of adverse health outcomes
attributable to exposures to O3 concentrations commonly
found in the ambient air and (2) estimates indicating decreased
occurrences of O3 exposures of concern and decreased health
risks upon meeting the current standard, compared to recent air
quality.
In particular, the PA concludes that strong support for this
initial conclusion is provided by controlled human exposure studies of
respiratory effects, and by quantitative estimates of exposures of
concern and lung function decrements based on information in these
studies. Analyses in the HREA estimate that the percentages of children
(i.e., all children and children with asthma) in urban study areas
experiencing exposures of concern, or experiencing abnormal and
potentially adverse lung function decrements, are consistently lower
for air quality that just meets the current O3 standard than
for recent air quality. The HREA estimates such reductions consistently
across the urban study areas evaluated and throughout various portions
of individual urban study areas, including in urban cores and the
portions of urban study areas surrounding urban cores. These reductions
in exposures of concern and O3-induced lung function
decrements reflect the consistent decreases in the highest
O3 concentrations following reductions in precursor
emissions to meet the current standard. Thus, populations in both urban
and non-urban areas would be expected to experience important
reductions in O3 exposures and O3-induced lung
function risks upon meeting the current standard.\105\
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\105\ As discussed above (II.C.1), CASAC recommended that the
EPA evaluate how health risks in urban centers, as well as outside
urban centers, change upon reducing NOX emissions, given
the varying impacts of NOX emissions reductions on
ambient O3 concentrations.
---------------------------------------------------------------------------
The PA further concludes that support for this initial conclusion
is also provided by estimates of O3-associated mortality and
morbidity based on application of concentration-response relationships
from epidemiologic studies to air quality adjusted to just meet the
current standard. These estimates, which are based on the assumption
that concentration-response relationships are linear over entire
distributions of ambient O3 concentrations, are associated
with uncertainties that complicate their interpretation (II.C.3).
However, risk estimates for effects associated with short- and long-
term O3 exposures, combined with the HREA's national
analysis of O3 responsiveness to reductions in precursor
emissions and the consistent reductions estimated for the highest
ambient O3 concentrations, suggest that O3-
associated mortality and morbidity would be expected to decrease
nationwide following reductions in precursor emissions to meet the
current O3 standard.
Reductions in O3 precursor emissions (i.e.,
NOX) could also increase public health protection by
reducing the ambient concentrations of pollutants other than
O3. For example, in their advice on the second draft HREA
CASAC acknowledged the potential for ambient NO2
concentrations to be affected by changes in NOX emissions
(Frey, 2014a, p. 10). Consistent with this, the PA notes that
NOX emissions contribute to ambient NO2, and that
NOX and VOCs can contribute to secondary formation of
PM2.5 constituents, including ammonium sulfate
(NH4SO4), ammonium nitrate
(NH4NO3), and organic carbon (OC). Therefore, at
some times and in some locations, control strategies that would reduce
NOX emissions (i.e., to meet an O3 standard)
could reduce ambient concentrations of NO2 and
PM2.5, resulting in health benefits beyond those directly
associated with reducing ambient O3 concentrations.
After reaching the initial conclusion that meeting the current
primary O3 standard will provide important improvements in
public health protection, and that it is not appropriate to consider a
standard that is less protective than the current standard, the PA
considers the adequacy of the public health protection that is provided
by the
[[Page 75286]]
current standard. In considering the available scientific evidence,
exposure/risk information, advice from CASAC (II.D.4, below), and input
from the public, the PA reaches the conclusion that the available
evidence and information clearly call into question the adequacy of
public health protection provided by the current primary standard. In
reaching this conclusion, the PA notes that evidence from controlled
human exposure studies provides strong support for the occurrence of
adverse respiratory effects following exposures to O3
concentrations below the level of the current standard. Epidemiologic
studies provide support for the occurrence of adverse respiratory
effects and mortality under air quality conditions that would likely
meet the current standard. In addition, based on the analyses in the
HREA, the PA concludes that the exposures and risks projected to remain
upon meeting the current standard are indicative of risks that can
reasonably be judged to be important from a public health perspective.
Thus, the PA concludes that the evidence and information provide strong
support for giving consideration to revising the current primary
standard in order to provide increased public health protection against
an array of adverse health effects that range from decreased lung
function and respiratory symptoms to more serious indicators of
morbidity (e.g., including emergency department visits and hospital
admissions), and mortality. In consideration of all of the above, the
PA draws the conclusion that it is appropriate for the Administrator to
consider revision of the current primary O3 standard to
provide increased public health protection.
4. CASAC Advice
Following the 2008 decision to revise the primary O3
standard by setting the level at 0.075 ppm (75 ppb), CASAC strongly
questioned whether the standard met the requirements of the CAA. In
September 2009, the EPA announced its intention to reconsider the 2008
standards, issuing a notice of proposed rulemaking in January 2010 (75
FR 2938). Soon after, the EPA solicited CASAC review of that proposed
rule and in January 2011, solicited additional advice. This proposal
was based on the scientific and technical record from the 2008
rulemaking, including public comments and CASAC advice and
recommendations. As further described above (I.C), in the fall of 2011,
the EPA did not revise the standard as part of the reconsideration
process but decided to defer decisions on revisions to the
O3 standards to the next periodic review, which was already
underway. Accordingly, in this section we describe CASAC's advice
related to the 2008 final decision and the subsequent reconsideration,
as well as its advice on this current review of the O3 NAAQS
that was initiated in September 2008.
In April 2008, the members of the CASAC Ozone Review Panel sent a
letter to EPA stating ``[I]n our most-recent letters to you on this
subject--dated October 2006 and March 2007--the CASAC unanimously
recommended selection of an 8-hour average Ozone NAAQS within the range
of 0.060 to 0.070 parts per million [60 to 70 ppb] for the primary
(human health-based) Ozone NAAQS'' (Henderson, 2008). The letter
continued:
The CASAC now wishes to convey, by means of this letter, its
additional, unsolicited advice with regard to the primary and
secondary Ozone NAAQS. In doing so, the participating members of the
CASAC Ozone Review Panel are unanimous in strongly urging you or
your successor as EPA Administrator to ensure that these
recommendations be considered during the next review cycle for the
Ozone NAAQS that will begin next year . . . numerous medical
organizations and public health groups have also expressed their
support of these CASAC recommendations' . . . [The CASAC did] not
endorse the new primary ozone standard as being sufficiently
protective of public health. The CASAC--as the EPA's statutorily-
established science advisory committee for advising you on the
national ambient air quality standards--unanimously recommended
decreasing the primary standard to within the range of 0.060-0.070
ppm [60 to 70 ppb]. It is the Committee's consensus scientific
opinion that your decision to set the primary ozone standard above
this range fails to satisfy the explicit stipulations of the Clean
Air Act that you ensure an adequate margin of safety for all
individuals, including sensitive populations.
In response to the EPA's solicitation of advice on the EPA's
proposed rulemaking as part of the reconsideration, CASAC conveyed
support (Samet, 2010).
CASAC fully supports EPA's proposed range of 0.060-0.070 parts per
million (ppm) for the 8-hour primary ozone standard. CASAC considers
this range to be justified by the scientific evidence as presented
in the Air Quality Criteria for Ozone and Related Photochemical
Oxidants (March 2006) and Review of the National Ambient Air Quality
Standards for Ozone: Policy Assessment of Scientific and Technical
Information, OAQPS Staff Paper (July 2007). As stated in our letters
of October 24, 2006, March 26, 2007 and April 7, 2008 to former
Administrator Stephen L. Johnson, CASAC unanimously recommended
selection of an 8-hour average ozone NAAQS within the range proposed
by EPA (0.060 to 0.070 ppm). In proposing this range, EPA has
recognized the large body of data and risk analyses demonstrating
that retention of the current standard would leave large numbers of
individuals at risk for respiratory effects and/or other significant
health impacts including asthma exacerbations, emergency room
visits, hospital admissions and mortality.
In response to EPA's request for additional advice on the
reconsideration in 2011, CASAC reaffirmed their conclusion that ``the
evidence from controlled human and epidemiological studies strongly
supports the selection of a new primary ozone standard within the 60-70
ppb range for an 8-hour averaging time'' (Samet, 2011, p ii). As
requested by the EPA, CASAC's advice and recommendations were based on
the scientific and technical record from the 2008 rulemaking. In
considering the record for the 2008 rulemaking, CASAC stated the
following to summarize the basis for their conclusions (Samet, 2011,
pp. ii to iii).
(1) The evidence available on dose-response for effects of
O3 shows associations extending to levels within the range
of concentrations currently experienced in the United States.
(2) There is scientific certainty that 6.6-hour exposures with
exercise of young, healthy, non-smoking adult volunteers to
concentrations >=80 ppb cause clinically relevant decrements of lung
function.
(3) Some healthy individuals have been shown to have clinically
relevant responses, even at 60 ppb.
(4) Since the majority of clinical studies involve young, healthy
adult populations, less is known about health effects in such
potentially ozone sensitive populations as the elderly, children and
those with cardiopulmonary disease. For these susceptible groups,
decrements in lung function may be greater than in healthy volunteers
and are likely to have a greater clinical significance.
(5) Children and adults with asthma are at increased risk of acute
exacerbations on or shortly after days when elevated O3
concentrations occur, even when exposures do not exceed the NAAQS
concentration of 75 ppb.
(6) Large segments of the population fall into what the EPA terms a
``sensitive population group,'' i.e., those at increased risk because
they are more intrinsically susceptible (children, the elderly, and
individuals with chronic lung disease) and those who are more
vulnerable due to increased exposure because they work outside or live
in areas that are more polluted than the mean levels in their
communities.
With respect to evidence from epidemiologic studies, CASAC stated
``while epidemiological studies are
[[Page 75287]]
inherently more uncertain as exposures and risk estimates decrease (due
to the greater potential for biases to dominate small effect
estimates), specific evidence in the literature does not suggest that
our confidence on the specific attribution of the estimated effects of
ozone on health outcomes differs over the proposed range of 60-70 ppb''
(Samet, 2011, p. 10).
Following its review of the second draft PA in the current review,
which considers an updated scientific and technical record since the
2008 rulemaking, CASAC concluded that ``there is clear scientific
support for the need to revise the standard'' (Frey, 2014c, p. ii). In
particular, CASAC noted the following (Frey, 2014c, p. 5):
[T]he scientific evidence provides strong support for the occurrence
of a range of adverse respiratory effects and mortality under air
quality conditions that would meet the current standard. Therefore,
CASAC unanimously recommends that the Administrator revise the
current primary ozone standard to protect public health.\106\
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\106\ CASAC provided similar advice in their letter to the
Administrator on the REA, stating that ``The CASAC finds that the
current primary NAAQS for ozone is not protective of human health
and needs to be revised'' (Frey, 2014a, p. 15).
In supporting these conclusions, CASAC judged that the strongest
evidence comes from controlled human exposure studies of respiratory
effects. The Committee specifically noted that ``the combination of
decrements in FEV1 together with the statistically
significant alterations in symptoms in human subjects exposed to 72 ppb
ozone meets the American Thoracic Society's definition of an adverse
health effect'' (Frey, 2014c, p. 5). CASAC further judged that ``if
subjects had been exposed to ozone using the 8-hour averaging period
used in the standard, adverse effects could have occurred at lower
concentration'' and that ``the level at which adverse effects might be
observed would likely be lower for more sensitive subgroups, such as
those with asthma'' (Frey, 2014c, p. 5).
With regard to lung function risk estimates based on information
from controlled human exposure studies, CASAC concluded that
``estimation of FEV1 decrements of >=15% is appropriate as a
scientifically relevant surrogate for adverse health outcomes in active
healthy adults, whereas an FEV1 decrement of >=10% is a scientifically
relevant surrogate for adverse health outcomes for people with asthma
and lung disease'' (Frey, 2014c, p. 3). The Committee further concluded
that ``[a]sthmatic subjects appear to be at least as sensitive, if not
more sensitive, than non-asthmatic subjects in manifesting
O3-induced pulmonary function decrements'' (Frey, 2014c, p.
4). In considering estimates of the occurrence of these decrements in
urban study areas, CASAC specifically noted that the current standard
is estimated to allow 11 to 22% of school age children to experience at
least one day with an FEV1 decrement >=10% (Frey, 2014c, p.
7).
Although CASAC judged that controlled human exposure studies of
respiratory effects provide the strongest evidence supporting their
conclusion on the current standard, the Committee judged that there is
also ``sufficient scientific evidence based on epidemiologic studies
for mortality and morbidity associated with short-term exposure to
ozone at the level of the current standard'' (Frey, 2014c, p. 5). In
support of the biological plausibility of the associations reported in
these epidemiologic studies, CASAC noted that ``[r]ecent animal
toxicological studies support identification of modes of action and,
therefore, the biological plausibility associated with the
epidemiological findings'' (Frey, 2014c, p. 5).
Consistent with the advice of CASAC, several public commenters
supported revising the primary O3 standard to provide
increased public health protection. In considering the available
evidence as a basis for their views, these commenters generally noted
that the health evidence is stronger in the current review than in past
reviews, with new evidence for effects attributable to short- and long-
term exposures, and new evidence for effects at lower O3
exposure concentrations.
Other public commenters opposed considering revised standards.
These commenters discussed a variety of reasons for their views. A
number of commenters expressed the view that the EPA should not lower
the level of the standard because a lower level would be closer to
background O3 concentrations. In addition, several
commenters challenged the interpretation of the evidence presented in
the ISA. With respect to the risk assessment, several commenters
expressed the view that the EPA should only estimate risks above
O3 background concentrations, or above threshold
concentrations. Some commenters also expressed the view that, based on
the mortality and morbidity risk estimates in the HREA, there is little
to no difference between the risks estimated for the current
O3 standard and the risks estimated for revised standards
with lower levels. These commenters concluded that the HREA and PA have
not shown that the public health improvements likely to be achieved by
a revised O3 standard would be greater than the improvements
likely to be achieved by the current standard.
5. Administrator's Proposed Conclusions Concerning the Adequacy of the
Current Standard
This section discusses the Administrator's proposed conclusions
related to the adequacy of the public health protection provided by the
current primary O3 standard, resulting in her proposed
decision to revise that standard. These proposed conclusions, and her
proposed decision, are based on the Administrator's consideration of
the available scientific evidence, exposure/risk information, the
comments and advice of CASAC, and public input received thus far, as
summarized below.
As an initial matter, the Administrator concludes that reducing
precursor emissions to achieve O3 concentrations that meet
the current primary O3 standard will provide important
improvements in public health protection, compared to recent air
quality. In reaching this initial conclusion, she notes the discussion
in section 3.4 of the PA (U.S. EPA, 2014c), summarized above (II.D.3).
In particular, the Administrator notes that this initial conclusion is
supported by (1) the strong body of scientific evidence indicating a
wide range of adverse health outcomes attributable to exposures to
O3 at concentrations commonly found in the ambient air and
(2) estimates indicating decreased occurrences of O3
exposures of concern and decreased O3-associated health
risks upon meeting the current standard, compared to recent air
quality. Thus, she concludes that it would not be appropriate in this
review to consider a standard that is less protective than the current
standard.\107\
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\107\ While not analyzed quantitatively, consistent with CASAC
advice (Frey, 2014a, p. 10), the Administrator notes that reductions
in O3 precursor emissions (e.g., NOX; VOC) to
achieve O3 concentrations that meet the current standard
could also increase public health protection by reducing the ambient
concentrations of pollutants other than O3 (i.e.,
PM2.5, NO2).
---------------------------------------------------------------------------
After reaching the initial conclusion that meeting the current
primary O3 standard will provide important improvements in
public health protection, and that it is not appropriate to consider a
standard that is less protective than the current standard, the
Administrator next considers the adequacy of the public health
protection that is provided by the current standard. In doing so, the
Administrator first notes that studies evaluated since the completion
of the 2006 O3 AQCD
[[Page 75288]]
support and expand upon the strong body of evidence that, in the last
review, indicated a causal relationship between short-term
O3 exposures and respiratory health effects. This is the
strongest causality finding possible under the ISA's hierarchical
system for classifying weight of evidence for causation. Together,
experimental and epidemiologic studies support conclusions regarding a
continuum of O3 respiratory effects ranging from small
reversible changes in pulmonary function, and pulmonary inflammation,
to more serious effects that can result in respiratory-related
emergency department visits, hospital admissions, and premature
mortality. Recent animal toxicology studies support descriptions of
modes of action for these respiratory effects and augment support for
biological plausibility for the role of O3 in reported
effects. With regard to mode of action, evidence indicates that
antioxidant capacity may modify the risk of respiratory morbidity
associated with O3 exposure, and that the inherent capacity
to quench (based on individual antioxidant capacity) can be
overwhelmed, especially with exposure to elevated concentrations of
O3. In addition, based on the consistency of findings across
studies and evidence for the coherence of results from different
scientific disciplines, evidence indicates that certain populations are
at increased risk of experiencing O3-related effects,
including the most severe effects. These include populations and
lifestages identified in previous reviews (i.e., people with asthma,
children, older adults, outdoor workers) and populations identified
since the last review (i.e., people with certain genotypes related to
antioxidant and/or anti-inflammatory status; people with reduced intake
of certain antioxidant nutrients, such as Vitamins C and E).
The Administrator further notes that evidence for adverse
respiratory health effects attributable to long-term, or repeated
short-term, O3 exposures is much stronger than in previous
reviews, and the ISA concludes that there is ``likely to be'' a causal
relationship between such O3 exposures and adverse
respiratory health effects (the second strongest causality finding).
Uncertainties related to the extrapolation of data generated by rodent
toxicology studies to the understanding of health effects in humans
have been reduced by studies in non-human primates and by recent
epidemiologic studies. The evidence available in this review includes
new epidemiologic studies using a variety of designs and analysis
methods, conducted by different research groups in different locations,
evaluating the relationships between long-term O3 exposures
and measures of respiratory morbidity and mortality. New evidence
supports associations between long-term O3 exposures and the
development of asthma in children, with several studies reporting
interactions between genetic variants and such O3 exposures.
Studies also report associations between long-term O3
exposures and asthma prevalence, asthma severity and control,
respiratory symptoms among asthmatics, and respiratory mortality.
In considering the O3 exposure concentrations reported
to elicit respiratory effects, the Administrator agrees with the
conclusions of the PA and with the advice of CASAC (Frey, 2014c) that
controlled human exposure studies provide the most certain evidence
indicating the occurrence of health effects in humans following
exposures to specific O3 concentrations. In particular, as
discussed further in section II.E.4.d below, she notes that the effects
reported in controlled human exposure studies are due solely to
O3 exposures, and interpretation of study results is not
complicated by the presence of co-occurring pollutants or pollutant
mixtures (as is the case in epidemiologic studies). Therefore, she
places the most weight on information from these controlled human
exposure studies.
In considering the evidence from controlled human exposure studies,
the Administrator first notes that these studies have reported a
variety of respiratory effects in healthy adults following exposures to
O3 concentrations of 60, 72,\108\ or 80 ppb, and higher. The
largest respiratory effects, and the broadest range of effects, have
been studied and reported following exposures of healthy adults to 80
ppb O3 or higher, with most exposure studies conducted at
these higher concentrations. She further notes that recent evidence
includes controlled human exposure studies reporting the combination of
lung function decrements and respiratory symptoms in healthy adults
engaged in intermittent, moderate exertion following 6.6 hour exposures
to concentrations as low as 72 ppb, and lung function decrements and
pulmonary inflammation following exposures to O3
concentrations as low as 60 ppb. As discussed below, compared to the
evidence available in the last review, these studies have strengthened
support for the occurrence of abnormal and adverse respiratory effects
attributable to short-term exposures to O3 concentrations
below the level of the current standard.\109\ The Administrator
concludes that such exposures to O3 concentrations below the
level of the current standard are potentially important from a public
health perspective, given the following:
---------------------------------------------------------------------------
\108\ As noted above, for the 70 ppb target exposure
concentration, Schelegle et al. (2009) reported that the actual mean
exposure concentration was 72 ppb.
\109\ Cf. State of Misisssippi. 744 F.3d 1350 (``Perhaps more
studies like the Adams studies will yet reveal that the 0.060 ppm
level produces significant adverse decrements that simply cannot be
attributed to normal variation in lung function.'').
---------------------------------------------------------------------------
(1) The combination of lung function decrements and respiratory
symptoms reported to occur in healthy adults following exposures to 72
ppb O3 or higher, while at moderate exertion, meet ATS
criteria for an adverse response. In specifically considering the 72
ppb exposure concentration, CASAC noted that ``the combination of
decrements in FEV1 together with the statistically
significant alterations in symptoms in human subjects exposed to 72 ppb
ozone meets the American Thoracic Society's definition of an adverse
health effect'' (Frey, 2014c, p. 5).
(2) With regard to 60 ppb O3, CASAC agreed that ``a
level of 60 ppb corresponds to the lowest exposure concentration
demonstrated to result in lung function decrements large enough to be
judged an abnormal response by ATS and that could be adverse in
individuals with lung disease'' (Frey, 2014c, p. 7). CASAC further
noted that ``a level of 60 ppb also corresponds to the lowest exposure
concentration at which pulmonary inflammation has been reported''
(Frey, 2014c, p. 7).
(3) The controlled human exposure studies reporting these
respiratory effects were conducted in healthy adults, while at-risk
groups (e.g., children, people with asthma) could experience larger
and/or more serious effects. In their advice to the Administrator,
CASAC concurred with this reasoning (Frey, 2014a, p. 14; Frey, 2014c,
p. 5).
(4) These respiratory effects are coherent with the serious health
outcomes that have been reported in epidemiologic studies evaluating
exposure to O3 (e.g., respiratory-related hospital
admissions, emergency department visits, and mortality).
As noted above, the Administrator's proposed conclusions regarding
the adequacy of the current primary O3 standard place a
large amount of weight on the results of controlled human exposure
studies. In particular, given the combination of lung function
[[Page 75289]]
decrements and respiratory symptoms following 6.6 hour exposures to
O3 concentrations as low as 72 ppb, and given CASAC advice
regarding effects at 72 ppb along with ATS adversity criteria, she
concludes that the evidence in this review supports the occurrence of
adverse respiratory effects following exposures to O3
concentrations lower than the level of the current standard.\110\ As
discussed below, the Administrator further considers information from
the broader body of controlled human exposure studies within the
context of quantitative estimates of exposures of concern and
O3-induced FEV1 decrements.
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\110\ The use of evidence from controlled human exposure studies
conducted in healthy adults to characterize the potential for
adverse effects, including in at-risk groups such as children and
asthmatics, is discussed in sections II.C.2 and II.C.3.a, above.
CASAC advice on this issue is discussed in sections II.D.4 and
II.E.4.c.
---------------------------------------------------------------------------
In addition to controlled human exposure studies, the Administrator
also considers what the available epidemiologic evidence indicates with
regard to the adequacy of the public health protection provided by the
current primary O3 standard.\111\ She notes that recent
epidemiologic studies provide support, beyond that available in the
last review, for associations between short-term O3
exposures and a wide range of adverse respiratory outcomes (including
respiratory-related hospital admissions, emergency department visits,
and mortality) and with total mortality. Associations with morbidity
and mortality are stronger during the warm or summer months, and remain
robust after adjustment for copollutants.
---------------------------------------------------------------------------
\111\ As noted above, she places less weight on information from
epidemiologic studies than on information from controlled human
exposure studies.
---------------------------------------------------------------------------
In considering information from epidemiologic studies within the
context of her conclusions on the adequacy of the current standard, the
Administrator considers the extent to which available studies support
the occurrence of O3 health effect associations with air
quality likely to be allowed by the current standard. In doing so, she
places the most weight on air quality analyses in locations of single-
city studies of short-term O3, as discussed in more detail
in section II.E.4.d below.\112\ In particular, she notes that a U.S.
single-city study reported associations with respiratory emergency
department visits in children and adults in a location that would
likely have met the current O3 standard over the entire
study period (Mar and Koenig, 2009). In addition, even in some single-
city study locations where the current standard was likely not met
(i.e., Silverman and Ito, 2010; Strickland et al., 2010), the
Administrator notes PA analyses indicating that reported concentration-
response functions and available air quality data support the
occurrence of O3-health effect associations on subsets of
days with ambient O3 concentrations below the level of the
current standard (II.D.1). Compared to single-city studies, the
Administrator notes additional uncertainty in interpreting the
relationships between air quality in individual study cities and health
effects based on multicity analyses (discussed further in sections
II.D.1 and II.E.4.d). While such uncertainties limit the extent to
which the Administrator bases her conclusions on air quality in
locations of multicity epidemiologic studies, she does note that
O3 associations with respiratory morbidity or mortality have
been reported in several multicity studies when the majority of study
locations (though not all study locations) would likely have met the
current O3 standard. When taken together, the Administrator
reaches the conclusion that single-city epidemiologic studies and
associated air quality information support the occurrence of
O3-associated hospital admissions and emergency department
visits for ambient O3 concentrations likely to have met the
current standard, and that air quality analyses in locations of
multicity studies provide some support for this conclusion for a
broader range of effects (i.e., including mortality).
---------------------------------------------------------------------------
\112\ As discussed in section II.E.4.d of this preamble, this
judgment applies specifically to epidemiologic studies of short-term
O3 concentrations where multicity effect estimates are
presented, based on combining the effect estimates from multiple
individual cities, and where individual city effect estimates are
not presented (as is the case for key multicity studies analyzed in
the PA). Because these reported multicity effect estimates do not
allow health effect associations to be disaggregated by individual
city, it is not possible to assign the health effect association to
the air quality in any one study location, or to the air quality in
a subset of locations. In contrast, for epidemiologic studies of
long-term concentrations, where multicity effect estimates are based
on comparisons across cities, different judgments have been made
with regard to the utility of multicity studies (see, e.g. 78 FR
3086 at 3103/2) (January 15, 2013) (and see discussion below of
study by Jerrett et al., (2009)).
---------------------------------------------------------------------------
Beyond her consideration of the scientific evidence, the
Administrator also considers the results of the HREA exposure and risk
analyses in reaching initial conclusions regarding the adequacy of the
current primary O3 standard. In doing so, as noted above,
she focuses primarily on exposure and risk estimates based on
information from controlled human exposure studies (i.e., exposures of
concern and O3-induced lung function decrements). She places
relatively less weight on epidemiologic-based risk estimates, noting
that the overall conclusions from the HREA likewise reflect less
confidence in estimates of epidemiologic-based risks than in estimates
of exposures and lung function risks (U.S. EPA, 2014, section 9.6).
Consistent with the conclusions in the PA, her determination to attach
less weight to the epidemiologic-based risk estimates reflects her
consideration of key uncertainties, including the heterogeneity in
effect estimates between locations, the potential for exposure
measurement errors, and uncertainty in the interpretation of the shape
of concentration-response functions for O3 concentrations in
the lower portions of ambient distributions (U.S. EPA, 2014, section
9.6) (II.D.2). In particular, she concludes that lower confidence
should be placed in the results of the assessment of respiratory
mortality risks associated with long-term O3 exposures,
primarily because that analysis is based on only one study (even though
that study is well-designed) and because of the uncertainty in that
study about the existence and level of a potential threshold in the
concentration-response function (U.S. EPA, 2014a, section 9.6)
(II.D.2).\113\
---------------------------------------------------------------------------
\113\ CASAC also called into question the extent to which it is
appropriate to place confidence in risk estimates for respiratory
mortality (Frey, 2014a, p. 11).
---------------------------------------------------------------------------
With regard to estimates of exposures of concern, the Administrator
considers the extent to which the current standard provides protection
against exposures to O3 concentrations at or above 60, 70,
and 80 ppb, noting CASAC advice that 60 ppb ``is an appropriate
exposure of concern for asthmatic children'' (Frey, 2014c, p. 8). She
further notes that while single exposures of concern could be adverse
for some people, particularly for the higher benchmark concentrations
(70, 80 ppb) where there is stronger evidence for the occurrence of
adverse effects (discussed further in II.E.4.d, below), she becomes
increasingly concerned about the potential for adverse responses as the
frequency of occurrences increases.\114\ In particular,
[[Page 75290]]
she notes that repeated occurrences of the types of effects shown to
occur following exposures of concern can have potentially adverse
outcomes. For example, repeated occurrences of airway inflammation
could potentially result in the induction of a chronic inflammatory
state; altered pulmonary structure and function, leading to diseases
such as asthma; altered lung host defense response to inhaled
microorganisms; and altered lung response to other agents such as
allergens or toxins (U.S. EPA, 2013a, section 6.2.3). Thus, the
Administrator notes that the types of lung injury shown to occur
following exposures to O3 concentrations from 60 to 80 ppb,
particularly if experienced repeatedly, provide a mode of action by
which O3 may cause other more serious effects (e.g., asthma
exacerbations). Therefore, the Administrator places the most weight on
estimates of two or more exposures of concern (i.e., as a surrogate for
the occurrence of repeated exposures), though she also considers
estimates of one or more, particularly for the 70 and 80 ppb
benchmarks.
---------------------------------------------------------------------------
\114\ Not all people who experience an exposure of concern will
experience an adverse effect (even members of at-risk populations).
For most of the endpoints evaluated in controlled human exposure
studies (with the exception of O3-induced FEV1
decrements, as discussed below), the number of those experiencing
exposures of concern who will experience adverse effects cannot be
reliably quantified.
---------------------------------------------------------------------------
Consistent with CASAC advice (Frey, 2014c), the Administrator
focuses on children in these analyses of O3 exposures,
noting that estimates for all children and asthmatic children are
virtually indistinguishable (in terms of the percent estimated to
experience exposures of concern). Though she focuses on children, she
also recognizes that exposures to O3 concentrations at or
above 60 or 70 ppb could be of concern for adults. As discussed in the
HREA and PA (and II.C.2.a, above), the patterns of exposure estimates
across urban study areas, across years, and across air quality
scenarios are similar in adults with asthma, older adults, all
children, and children with asthma, though smaller percentages of adult
populations are estimated to experience exposures of concern than
children and children with asthma. Thus, the Administrator recognizes
that the exposure patterns for children across years, urban study
areas, and air quality scenarios are indicative of the exposure
patterns in a broader group of at-risk populations that also includes
asthmatic adults and older adults.
As illustrated in Table 1 (above), the Administrator notes that if
the 15 urban study areas evaluated in the HREA were to just meet the
current O3 standard, fewer than 1% of children in those
areas would be estimated to experience two or more exposures of concern
at or above 70 ppb, though approximately 3 to 8% of children, including
approximately 3 to 8% of asthmatic children, would be estimated to
experience two or more exposures of concern to O3
concentrations at or above 60 ppb \115\ (based on estimates averaged
over the years of analysis). To provide some perspective on these
percentages, the Administrator notes that they correspond to almost
900,000 children in urban study areas, including about 90,000 asthmatic
children, estimated to experience two or more exposures of concern at
or above 60 ppb. Nationally, if the current standard were to be just
met the number of children experiencing such exposures would be larger.
In the worst-case year and location (i.e., year and location with the
largest exposure estimates), the Administrator notes that over 2% of
children are estimated to experience two or more exposures of concern
at or above 70 ppb and over 14% are estimated to experience two or more
exposures of concern at or above 60 ppb.
---------------------------------------------------------------------------
\115\ Almost no children in those areas would be estimated to
experience two or more exposures of concern at or above 80 ppb.
---------------------------------------------------------------------------
Although, as discussed above and in section II.E.4.d, the
Administrator is less concerned about single occurrences of exposures
of concern, she notes that even single occurrences can cause adverse
effects in some people, particularly for the 70 and 80 ppb benchmarks.
Therefore, she also considers estimates of one or more exposures of
concern. As illustrated in Table 1 (above), if the 15 urban study areas
evaluated in the HREA were to just meet the current O3
standard, fewer than 1% of children in those areas would be estimated
to experience one or more exposures of concern at or above 80 ppb
(based on estimates averaged over the years of analysis). However,
approximately 1 to 3% of children, including 1 to 3% of asthmatic
children, would be estimated to experience one or more exposures of
concern to O3 concentrations at or above 70 ppb and
approximately 10 to 17% would be estimated to experience one or more
exposures of concern to O3 concentrations at or above 60
ppb. In the worst-case year and location, the Administrator notes that
over 1% of children are estimated to experience one or more exposures
of concern at or above 80 ppb, over 8% are estimated to experience one
or more exposures of concern at or above 70 ppb, and about 26% are
estimated to experience one or more exposures of concern at or above 60
ppb.
In addition to estimated exposures of concern, the Administrator
also considers HREA estimates of the occurrence of O3-
induced lung function decrements. In doing so, she particularly notes
CASAC advice that ``estimation of FEV1 decrements of >=15%
is appropriate as a scientifically relevant surrogate for adverse
health outcomes in active healthy adults, whereas an FEV1
decrement of >=10% is a scientifically relevant surrogate for adverse
health outcomes for people with asthma and lung disease'' (Frey, 2014c,
p. 3). The Administrator notes that while single occurrences of
O3-induced lung function decrements could be adverse for
some people, as discussed above (II.B.3), a more general consensus view
of the potential adversity of such decrements emerges as the frequency
of occurrences increases. Therefore, the Administrator focuses
primarily on the estimates of two or more O3-induced lung
function decrements.
When averaged over the years evaluated in the HREA, the
Administrator notes that the current standard is estimated to allow
about 1 to 3% of children in the 15 urban study areas (corresponding to
almost 400,000 children) to experience two or more O3-
induced lung function decrements >=15%, and to allow about 8 to 12% of
children (corresponding to about 180,000 asthmatic children \116\) to
experience two or more O3-induced lung function decrements
>=10%. Nationally, larger numbers of children would be expected to
experience such O3-induced decrements if the current
standard were to be just met. The current standard is also estimated to
allow about 3 to 5% of children in the urban study areas to experience
one or more decrements >=15% and about 14 to 19% of children to
experience one or more decrements >=10%. In the worst-case year and
location, the current standard is estimated to allow 4% of children in
the urban study areas to experience two or more decrements >=15% (and
7% to experience one or more such decrements) and 14% of children to
experience two or more decrements >=10% (and 22% to experience one or
more such decrements).
---------------------------------------------------------------------------
\116\ As noted above, CASAC concluded that ``an FEV1 decrement
of >=10% is a scientifically relevant surrogate for adverse health
outcomes for people with asthma and lung disease'' (Frey, 2014c, p.
3) and that such decrements ``could be adverse for people with lung
disease'' (Frey, 2014c, p. 7).
---------------------------------------------------------------------------
In further considering the HREA results, the Administrator
considers the epidemiology-based risk estimates. As discussed above,
compared to the weight given to HREA estimates of exposures of concern
and lung function risks, she places relatively less weight on
epidemiology-based risk estimates. In giving some consideration to
these
[[Page 75291]]
risk estimates, the Administrator notes estimates of total risks (i.e.,
based on the full distributions of ambient O3
concentrations) and risks associated with O3 concentrations
in the upper portions of ambient distributions. The Administrator notes
that estimates of total risks are based on the assumption that
concentration-response relationships remain linear over the entire
distributions of ambient O3 concentrations. With regard to
total risks, she notes that the HREA estimates thousands of
O3-associated hospital admissions, emergency department
visits, and deaths per year for air quality conditions associated with
just meeting the current standard in the 12 urban study areas (II.C.3).
However, the Administrator also notes the increasing uncertainty
associated with the shapes of concentration-response curves for
O3 concentrations in the lower portions of ambient
distributions. She particularly notes that there is less certainty in
the shape of concentration-response functions for area-wide
O3 concentrations at the lower ends of warm season
distributions (i.e., below about 20 to 40 ppb depending on the
O3 metric, health endpoint, and study population) (U.S. EPA,
2013a, section 2.5.4.4). The Administrator further notes the evidence
from controlled human exposure studies, which provide the strongest
support for O3-induced effects following exposures to
O3 concentrations corresponding to the upper portions of
typical ambient distributions (i.e., 60 ppb and above). Therefore, the
Administrator judges it appropriate to focus on risks associated with
O3 concentrations in the upper portions of ambient
distributions. Even when considering only area-wide O3
concentrations from the upper portions of seasonal distributions, the
Administrator notes that the current standard is estimated to allow
hundreds to thousands of O3-associated deaths per year in
urban study areas (II.C.3).
Although the Administrator notes the HREA conclusions indicating
somewhat less confidence in estimates of O3-associated
mortality and morbidity risks, compared to estimates of exposures of
concern and risk of lung function decrements, she concludes that the
general magnitude of mortality and morbidity risk estimates suggests
the potential for a substantial number of O3-associated
deaths and adverse respiratory events to occur nationally, even when
the current standard is met. She especially notes that this is the case
based on the risks associated with the upper ends of distributions of
ambient O3 concentrations, where she has the greatest
confidence in O3-attributable effects.
In addition to the evidence and exposure/risk information discussed
above, the Administrator also takes note of the CASAC advice in the
current review and in the 2010 proposed reconsideration of the 2008
decision establishing the current standard. As discussed in more detail
above, the current CASAC ``finds that the current NAAQS for ozone is
not protective of human health'' and ``unanimously recommends that the
Administrator revise the current primary ozone standard to protect
public health'' (Frey, 2014c, p. 5). The prior CASAC O3
Panel likewise recommended revision of the current standard to one with
a lower level. This earlier recommendation was based entirely on the
evidence and information in the record for the 2008 standard decision,
which, as discussed above, has been substantially strengthened in the
current review (Samet, 2011; Samet, 2012).
In consideration of all of the above, the Administrator proposes
that the current primary O3 standard is not adequate to
protect public health, and that it should be revised to provide
increased public health protection. This proposed decision is based on
the Administrator's initial conclusions that the available evidence and
exposure and risk information clearly call into question the adequacy
of public health protection provided by the current primary standard
and, therefore, that the current standard is not requisite to protect
public health with an adequate margin of safety. With regard to the
evidence, she specifically notes that (1) controlled human exposure
studies provide support for the occurrence of adverse respiratory
effects following exposures to O3 concentrations below the
level of the current standard (i.e., as low as 72 ppb), and that (2)
single-city epidemiologic studies provide support for the occurrence of
adverse respiratory effects under air quality conditions that would
likely meet the current standard, with multicity studies providing some
support for this conclusion for a broader range of effects (i.e.,
including mortality). Courts have repeatedly held that this type of
evidence justifies an Administrator's conclusion that it is
``appropriate'' (within the meaning of section 109 (d)(1) of the CAA)
to revise a primary NAAQS to provide further protection of public
health.\117\ In addition, based on the analyses in the HREA, the
Administrator initially concludes that the exposures and risks
projected to remain upon meeting the current standard can reasonably be
judged to be important from a public health perspective. Thus, she
reaches the proposed conclusion that the evidence and information,
together with CASAC advice based on their consideration of that
evidence and information, provide strong support for revising the
current primary standard in order to increase public health protection
against an array of adverse effects that range from decreased lung
function and respiratory symptoms to more serious indicators of
morbidity (e.g., including emergency department visits and hospital
admissions), and mortality.
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\117\ See e.g. State of Mississippi, 744 F. 3d at 1345; American
Farm Bureau, 559 F. 3d at 525-26.
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The Administrator solicits comment on her proposed decision to
revise the current primary O3 NAAQS, including on her
considerations and proposed conclusions based on the scientific
evidence, exposure/risk information, and CASAC advice. In doing so, she
recognizes that some have expressed alternative approaches to viewing
the evidence and information, including alternative approaches to
viewing, evaluating, and weighing important uncertainties. In some
cases, these alternative approaches have led some public commenters to
recommend retaining the current standard. Given these alternative
views, in addition to proposing to revise the current primary
O3 standard, the Administrator solicits comment on the
option of retaining that standard. In doing so, she also solicits
comment on the potential approaches to viewing the scientific evidence
and exposure/risk information that could support a conclusion that the
current standard is requisite to protect public health with an adequate
margin of safety.
E. Conclusions on the Elements of the Primary Standard
Having reached the proposed conclusion that the currently available
scientific evidence and exposure/risk information call into question
the adequacy of the current O3 standard, the Administrator
next considers the range of alternative standards supported by that
evidence and information. Consistent with her consideration of the
adequacy of the current standard, the Administrator's proposed
conclusions on alternative standards are informed by the available
scientific evidence assessed in the ISA, exposure/risk information
presented and assessed in the HREA, the evidence-based and exposure-/
risk-based considerations and conclusions in the PA, CASAC advice, and
input from members of the public. The sections below discuss the
evidence
[[Page 75292]]
and exposure/risk information, CASAC advice and public input, and the
Administrator's proposed conclusions, for the major elements of the
NAAQS: indicator (II.E.1), averaging time (II.E.2), form (II.E.3), and
level (II.E.4).
1. Indicator
In the last review, the EPA focused on O3 as the most
appropriate indicator for a standard meant to provide protection
against ambient photochemical oxidants. In this review, while the
complex atmospheric chemistry in which O3 plays a key role
has been highlighted, no alternatives to O3 have been
advanced as being a more appropriate indicator for ambient
photochemical oxidants. More specifically, the ISA noted that
O3 is the only photochemical oxidant (other than
NO2) that is routinely monitored and for which a
comprehensive database exists (U.S. EPA, 2013a, section 3.6). Data for
other photochemical oxidants (e.g., PAN, H2O2,
etc.) typically have been obtained only as part of special field
studies. Consequently, no data on nationwide patterns of occurrence are
available for these other oxidants; nor are extensive data available on
the relationships of concentrations and patterns of these oxidants to
those of O3 (U.S. EPA, 2013a, section 3.6). In its review of
the second draft PA, CASAC stated ``The indicator of ozone is
appropriate based on its causal or likely causal associations with
multiple adverse health outcomes and its representation of a class of
pollutants known as photochemical oxidants'' (Frey, 2014c, p. ii).
In addition, the PA notes that meeting an O3 standard
can be expected to provide some degree of protection against potential
health effects that may be independently associated with other
photochemical oxidants, even though such effects are not discernible
from currently available studies indexed by O3 alone (U.S.
EPA, 2014c, section 4.1). That is, since the precursor emissions that
lead to the formation of O3 generally also lead to the
formation of other photochemical oxidants, measures leading to
reductions in population exposures to O3 can generally be
expected to lead to reductions in population exposures to other
photochemical oxidants. In considering this information, and CASAC's
advice, the Administrator reaches the proposed conclusion that
O3 remains the most appropriate indicator for a standard
meant to provide protection against photochemical oxidants.\118\
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\118\ The DC Circuit upheld the use of O3 as the
indicator for photochemical oxidants based on these same
considerations. American Petroleum Inst. v. Costle, 665 F. 2d 1176,
1186 (D.C. Cir. 1981).
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2. Averaging Time
The EPA established the current 8-hour averaging time \119\ for the
primary O3 NAAQS in 1997 (62 FR 38856). The decision on
averaging time in that review was based on numerous controlled human
exposure and epidemiologic studies reporting associations between 6 to
8 hour O3 concentrations and adverse respiratory effects (62
FR 38861). It was also noted that a standard with a max 8-hour
averaging time is likely to provide substantial protection against
respiratory effects associated with 1-hour peak O3
concentrations. Similar conclusions were reached in the last
O3 NAAQS review and thus, the 8-hour averaging time was
retained in 2008.
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\119\ This 8-hour averaging time reflects daily max 8-hour
average O3 concentrations.
---------------------------------------------------------------------------
In reaching a proposed conclusion on averaging time in the current
review, the Administrator considers the extent to which the available
evidence continues to support the appropriateness of a standard with an
8-hour averaging time. Specifically, the Administrator considers the
extent to which the available information indicates that a standard
with the current 8-hour averaging time provides appropriate protection
against short- and long-term O3 exposures.
a. Short-Term
As an initial consideration with respect to the most appropriate
averaging time for the O3 NAAQS, the Administrator notes
that the strongest evidence for O3-associated health effects
is for respiratory effects following short-term exposures. More
specifically, the Administrator notes the ISA conclusion that the
evidence is ``sufficient to infer a causal relationship'' between
short-term O3 exposures and respiratory effects. The ISA
also judges that for short-term O3 exposures, the evidence
indicates ``likely to be causal'' relationships with both
cardiovascular effects and mortality (U.S. EPA, 2013a, section 2.5.2).
Therefore, as in past reviews, the strength of the available scientific
evidence provides strong support for a standard that protects the
public health against short-term exposures to O3.
In first considering the level of support available for specific
short-term averaging times, the Administrator notes the evidence
available from controlled human exposure studies. As discussed in more
detail in chapter 3 of the PA, substantial health effects evidence from
controlled human exposure studies demonstrates that a wide range of
respiratory effects (e.g., pulmonary function decrements, increases in
respiratory symptoms, lung inflammation, lung permeability, decreased
lung host defense, and AHR) occur in healthy adults following 6.6 hour
exposures to O3 (U.S. EPA, 2013a, section 6.2.1.1). Compared
to studies evaluating shorter exposure durations (e.g., 1-hour),
studies evaluating 6.6 hour exposures in healthy adults have reported
respiratory effects at lower O3 exposure concentrations and
at more moderate levels of exertion.
The Administrator also notes the strength of evidence from
epidemiologic studies that have evaluated a wide variety of populations
(e.g., including at-risk lifestages and populations, such as children
and people with asthma, respectively). A number of different averaging
times are used in O3 epidemiologic studies, with the most
common being the max 1-hour concentration within a 24-hour period (1-
hour max), the max 8-hour average concentration within a 24-hour period
(8-hr max), and the 24-hour average. These studies are summarized above
and assessed in detail in chapter 6 of the ISA (U.S. EPA, 2013a).
Limited evidence from time-series and panel epidemiologic studies
comparing risk estimates across averaging times does not indicate that
one exposure metric is more consistently or strongly associated with
respiratory health effects or mortality, though the ISA notes some
evidence for ``smaller O3 risk estimates when using a 24-
hour average exposure metric'' (U.S. EPA, 2013a, section 2.5.4.2; p. 2-
31). For single- and multi-day average O3 concentrations,
lung function decrements were associated with 1-hour max, 8-hour max,
and 24-hour average ambient O3 concentrations, with no
strong difference in the consistency or magnitude of association among
the averaging times (U.S. EPA, 2013a, p. 6-71). Similarly, in studies
of short-term exposure to O3 and mortality, Smith et al.
(2009) and Darrow et al. (2011) have reported high correlations between
risk estimates calculated using 24-hour average, 8-hour max, and 1-hour
max averaging times (U.S. EPA, 2013a, p. 6-253). Thus, the
Administrator notes that the epidemiologic evidence alone does not
provide a strong basis for distinguishing between the appropriateness
of 1-hour, 8-hour, and 24-hour averaging times.
Considering the health information discussed above, the
Administrator concludes that an 8-hour averaging time remains
appropriate for addressing health effects associated with short-term
exposures to ambient O3. An 8-hour
[[Page 75293]]
averaging time is similar to the exposure periods evaluated in
controlled human exposure studies, including recent studies that
provide evidence for respiratory effects following exposures to
O3 concentrations below the level of the current standard.
In addition, epidemiologic studies provide evidence for health effect
associations with 8-hour O3 concentrations, as well as with
1-hour and 24-hour concentrations. As in previous reviews, the
Administrator notes that a standard with an 8-hour averaging time
(combined with an appropriate standard form and level) would also be
expected to provide substantial protection against health effects
attributable to 1-hour and 24-hour exposures (e.g., 62 FR 38861, July
18, 1997). This conclusion is consistent with the advice received from
CASAC that ``the current 8-hour averaging time is justified by the
combined evidence from epidemiologic and clinical studies'' (Frey,
2014c, p. 6).
b. Long-Term
The ISA concludes that the evidence for long-term O3
exposures indicates that there is ``likely to be a causal
relationship'' with respiratory effects (U.S. EPA, 2013a, chapter 7).
Thus, in this review the Administrator also considers the extent to
which currently available evidence and exposure/risk information
suggests that a standard with an 8-hour averaging time can provide
protection against respiratory effects associated with longer term
exposures to ambient O3.
In considering this issue in the last review of the O3
NAAQS, the Staff Paper noted that ``because long-term air quality
patterns would be improved in areas coming into attainment with an 8-hr
standard, the potential risk of health effects associated with long-
term exposures would be reduced in any area meeting an 8-hr standard''
(U.S. EPA, 2007, p. 6-57). In the current review, the PA further
evaluates this issue, with a focus on the long-term O3
metrics reported to be associated with mortality or morbidity in recent
epidemiologic studies. As discussed in section 3.1.3 of the PA (U.S.
EPA, 2014c, section 4.2), much of the recent evidence for such
associations is based on studies that defined long-term O3
in terms of seasonal averages of daily maximum 1-hour or 8-hour
concentrations.
As an initial consideration, the Administrator notes the risk
results from the HREA for respiratory mortality associated with long-
term O3 concentrations. These HREA analyses indicate that as
air quality is adjusted to just meet the current 8-hour standard, most
urban study areas are estimated to experience reductions in respiratory
mortality associated with long-term O3 concentrations based
on the seasonal averages of 1-hour daily maximum O3
concentrations evaluated in the study by Jerrett et al. (2009) (U.S.
EPA, 2014a, chapter 7).\120\ As air quality is adjusted to meet lower
alternative standard levels, for standards based on 3-year averages of
the annual fourth-highest daily maximum 8-hour O3
concentrations, respiratory mortality risks are estimated to be reduced
further in urban study areas. This analysis indicates that an
O3 standard with an 8-hour averaging time, when coupled with
an appropriate form and level, can reduce respiratory mortality
reported to be associated with long-term O3 concentrations.
---------------------------------------------------------------------------
\120\ Though the Administrator also notes important
uncertainties associated with these risk estimates, as discussed
above (II.C.3.b).
---------------------------------------------------------------------------
In further considering the study by Jerrett et al. (2009), the
Administrator notes the PA comparison of long-term O3
concentrations following model adjustment in urban study areas (i.e.,
adjusted to meet the current and alternative 8-hour standards) to the
concentrations present in study cities that provided the basis for the
positive and statistically significant association with respiratory
mortality. As indicated in Table 4-3 of the PA (U.S. EPA, 2014c,
section 4.2), this comparison suggests that a standard with an 8-hour
averaging time can decrease seasonal averages of 1-hour daily maximum
O3 concentrations, and can maintain those O3
concentrations below the seasonal average concentration where the study
indicates the most confidence in the reported concentration-response
relationship with respiratory mortality (U.S. EPA, 2014c, sections 4.2
and 4.4.1).
The Administrator also notes that the HREA conducted analyses
evaluating the impacts of reducing regional NOX emissions on
the seasonal averages of daily maximum 8-hour O3
concentrations. Seasonal averages of 8-hour daily max O3
concentrations reflect long-term metrics that have been reported to be
associated with respiratory morbidity effects in several recent
O3 epidemiologic studies (e.g., Islam et al., 2008; Lin et
al., 2008; Salam et al., 2009). The HREA analyses indicate that the
large majority of the U.S. population lives in locations where reducing
NOX emissions would be expected to result in decreases in
seasonal averages of daily max 8-hour ambient O3
concentrations (U.S. EPA, 2014a, chapter 8). Thus, consistent with the
respiratory mortality risk estimates noted above, these analyses
suggest that reductions in O3 precursor emissions in order
to meet a standard with an 8-hour averaging time would also be expected
to reduce the long-term O3 concentrations that have been
reported in recent epidemiologic studies to be associated with
respiratory morbidity.
c. Administrator's Proposed Conclusion on Averaging Time
Taken together, the Administrator notes that the analyses
summarized above indicate that a standard with an 8-hour averaging
time, coupled with the current 4th high form and an appropriate level,
would be expected to provide appropriate protection against the short-
and long-term O3 concentrations that have been reported to
be associated with respiratory morbidity and mortality. The CASAC
agreed with this conclusion, stating that ``[t]he current 8-hour
averaging time is justified by the combined evidence from epidemiologic
and clinical studies'' and that ``[t]he 8-hour averaging window also
provides protection against the adverse impacts of long-term ozone
exposures, which were found to be ``likely causal'' for respiratory
effects and premature mortality'' (Frey, 2014c, p. 6). Therefore,
considering the available evidence and exposure risk information, and
CASAC's advice, the Administrator proposes to retain the current 8-hour
averaging time, and not to set an additional standard with a different
averaging time.
3. Form
The ``form'' of a standard defines the air quality statistic that
is to be compared to the level of the standard in determining whether
an area attains that standard. The foremost consideration in selecting
a form is the adequacy of the public health protection provided by the
combination of the form and the other elements of the standard. In this
review, the Administrator considers the extent to which the available
evidence and/or information continue to support the appropriateness of
a standard with the current form, defined by the 3-year average of
annual 4th-highest 8-hour daily maximum O3 concentrations.
The EPA established the current form of the primary O3
NAAQS in 1997 (62 FR 38856). Prior to that time, the standard had a
``1-expected-exceedance'' form.\121\ An advantage of the current
concentration-based form recognized in the 1997 review is that
[[Page 75294]]
such a form better reflects the continuum of health effects associated
with increasing ambient O3 concentrations. Unlike an
expected exceedance form, a concentration-based form gives
proportionally more weight to years when 8-hour O3
concentrations are well above the level of the standard than years when
8-hour O3 concentrations are just above the level of the
standard.\122\ It was judged appropriate to give more weight to higher
O3 concentrations, given that available health evidence
indicated a continuum of effects associated with exposures to varying
concentrations of O3, and given that the extent to which
public health is affected by exposure to ambient O3 is
related to the actual magnitude of the O3 concentration, not
just whether the concentration is above a specified level.
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\121\ For a standard with a 1-expected-exceedance form to be met
at an air quality monitoring site, the fourth-highest air quality
value in 3 years, given adjustments for missing data, must be less
than or equal to the level of the standard.
\122\ As discussed (61 FR 65731), this is because with an
exceedance-based form, days on which the ambient O3
concentration is well above the level of the standard are given
equal weight to those days on which the O3 concentration
is just above the standard (i.e., each day is counted as one
exceedance), even though the public health impact of such days would
be very different. With a concentration-based form, days on which
higher O3 concentrations occur would weigh proportionally
more than days with lower O3 concentrations since the
actual concentrations are used directly to calculate whether the
standard is met or violated.
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During the 1997 review, the EPA considered a range of alternative
``concentration-based'' forms, including the second-, third-, fourth-
and fifth-highest daily maximum 8-hour concentrations in an
O3 season. The fourth-highest daily maximum was selected,
recognizing that a less restrictive form (e.g., fifth highest) would
allow a larger percentage of sites to experience O3 peaks
above the level of the standard, and would allow more days on which the
level of the standard may be exceeded when the site attains the
standard (62 FR 38856). Consideration was also given to setting a
standard with a form that would provide a margin of safety against
possible but uncertain chronic effects, and would provide greater
stability to ongoing control programs.\123\ A more restrictive form was
not selected, recognizing that the differences in the degree of
protection afforded by the alternatives were not well enough understood
to use any such differences as a basis for choosing the most
restrictive forms (62 FR 38856).
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\123\ See American Trucking Assn's v. EPA, 283 F. 3d at 374-75
(less stable implementation programs may be less effective, and
therefore the EPA can consider programmatic stability in determining
the form of a NAAQS).
---------------------------------------------------------------------------
In the 2008 review, the EPA additionally considered the potential
value of a percentile-based form. In doing so, the EPA recognized that
such a statistic is useful for comparing datasets of varying length
because it samples approximately the same place in the distribution of
air quality values, whether the dataset is several months or several
years long. However, the EPA concluded that a percentile-based
statistic would not be effective in ensuring the same degree of public
health protection across the country. Specifically, a percentile-based
form would allow more days with higher air quality values in locations
with longer O3 seasons relative to places with shorter
O3 seasons. Thus, in the 2008 review, the EPA concluded that
a form based on the nth-highest maximum O3 concentration
would more effectively ensure that people who live in areas with
different length O3 seasons receive the same degree of
public health protection.
Based on analyses of forms specified in terms of an nth-highest
concentration (n ranged from 3 to 5), advice from CASAC, and public
comment,\124\ the Administrator concluded that a 4th-highest daily
maximum should be retained (73 FR 16465, March 27, 2008). In reaching
this decision, the Administrator recognized that ``there is not a clear
health-based threshold for selecting a particular nth-highest daily
maximum form of the standard'' and that ``the adequacy of the public
health protection provided by the combination of the level and form is
a foremost consideration'' (73 FR 16475, March 27, 2008). Based on
this, the Administrator judged that the existing form (4th-highest
daily maximum 8-hour average concentration) should be retained,
recognizing the increase in public health protection provided by
combining this form with a lower standard level (i.e., 75 ppb).
---------------------------------------------------------------------------
\124\ In the 2008 review, one group of commenters expressed the
view that the standard was not adequate and supported a more health-
protective form (e.g., a second- or third-highest daily max form).
Another group of commenters expressed the view that the standard was
adequate and did not provide any views on alternative forms that
would be appropriate should the Administrator consider revisions to
the standard. The Administrator considered the protection afforded
by the combination of level and form in revising the standard in
2008 to 75 ppb, as a 3-year average of the annual fourth-highest
daily max 8-hour concentrations (73 FR 16475, March 27, 2008).
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The Administrator also recognized that it is important to have a
form that provides stability with regard to implementation of the
standard. In the case of O3, for example, he noted the
importance of a form insulated from the impacts of the meteorological
events that are conducive to O3 formation. Such events could
have the effect of reducing public health protection, to the extent
they result in frequent shifts in and out of attainment due to
meteorological conditions. The Administrator noted that such frequent
shifting could disrupt an area's ongoing implementation plans and
associated control programs (73 FR 16474, March 27, 2008). In his final
decision, the Administrator judged that a 4th high form ``provides a
stable target for implementing programs to improve air quality'' (73 FR
16475, March 27, 2008).
In the current review, the Administrator considers the extent to
which newly available information provides support for the current
form. In so doing, she takes note of the conclusions of prior reviews
summarized above. She recognizes the value of an nth-high statistic
over that of an expected exceedance or percentile-based form in the
case of the O3 standard, for the reasons summarized above.
The Administrator additionally takes note of the importance of
stability in implementation to achieving the level of protection
specified by the NAAQS. Specifically, she notes that to the extent
areas engaged in implementing the O3 NAAQS frequently shift
from meeting the standard to violating the standard, it is possible
that ongoing implementation plans and associated control programs could
be disrupted, thereby reducing public health protection.
In light of this, while giving foremost consideration to the
adequacy of public health protection provided by the combination of all
elements of the standard, including the form, the Administrator
considers particularly findings from prior reviews with regard to the
use of the nth-high metric. As noted above, the 4th-highest daily
maximum was selected in recognition of the public health protection
provided by this form, when coupled with an appropriate averaging time
and level, and recognizing that such a form can provide stability for
implementation programs. The Administrator concludes that the currently
available evidence and information do not call into question these
conclusions from previous reviews. In reaching this conclusion, the
Administrator notes that CASAC concurred that the O3
standard should be based on the fourth highest, daily maximum 8-hour
average value (averaged over 3 years), stating that this form
``provides health protection while allowing for atypical meteorological
conditions that can lead to abnormally high ambient ozone
concentrations which, in turn, provides programmatic stability'' (Frey,
2014c, p. 6). Thus, a standard with the current 4th high form, coupled
with a level lower than 75 ppb
[[Page 75295]]
as discussed below, would be expected to increase public health
protection relative to the current standard while continuing to provide
stability for implementation programs. Therefore, the Administrator
proposes to retain the current 4th-highest daily maximum form for an
O3 standard with an 8-hour averaging time and a revised
level, as discussed below.
4. Level
The Administrator next considers the extent to which alternative
levels below 75 ppb could provide greater protection than the current
primary standard against short- and long- term exposures to
O3 in ambient air, for a standard based on the 3-year
average of the annual 4th highest daily maximum 8-hour O3
concentration. In doing so, she particularly notes the evidence-based
and exposure-/risk-based considerations in the PA, which take into
account the experimental and epidemiologic evidence as assessed in the
ISA; quantitative estimates of O3 exposures and health risks
in at-risk populations provided by the HREA; uncertainties and
limitations associated with this evidence and information; CASAC
advice; and public input (U.S. EPA, 2014c, sections 4.4 and 4.5).
Section II.E.4.a below summarizes the PA's approach to considering the
scientific evidence and the exposure/risk information related to level
of the primary standard. Section II.E.4.b presents the PA's conclusions
on alternative primary O3 standard levels. Section II.E.4.c
summarizes CASAC advice on the level of the primary standard, and
public input received thus far. Section II.E.4.d presents the
Administrator's proposed conclusions on primary O3 standard
levels.
a. PA Approach to Considering the Evidence and Information Related to
Alternative Levels of the Primary Standard
The PA's approach to reaching conclusions on alternative standard
levels focuses on the evidence from controlled human exposure and
epidemiologic studies, as assessed in the ISA (U.S. EPA, 2013a), and
the exposure and health risk analyses presented in the HREA (U.S. EPA,
2014a). This approach is discussed in detail in Chapters 1 and 4 of the
PA (U.S. EPA, 2014c, sections 1.3, 4.6), and is summarized below.
As an initial matter, the PA notes that controlled human exposure
studies provide the most certain evidence indicating the occurrence of
health effects in humans following exposures to specific O3
concentrations. Consistent with this, CASAC concluded that ``the
scientific evidence supporting the finding that the current standard is
inadequate to protect public health is strongest based on the
controlled human exposure studies of respiratory effects'' (Frey,
2014c, p. 5). As discussed above and in section 3.1.2.1 of the PA (U.S.
EPA, 2014c), controlled human exposure studies have reported a variety
of respiratory effects in healthy adults following exposures to
O3 concentrations of 60, 72,\125\ or 80 ppb, and higher.
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\125\ As noted above, for the 70 ppb exposure concentration
Schelegle et al. (2009) reported that the actual 6.6-hour mean
exposure concentration was 72 ppb.
---------------------------------------------------------------------------
Given the evidence for respiratory effects from controlled human
exposure studies, the PA considers the extent to which standards with
revised levels would be estimated to protect at-risk populations
against exposures of concern to O3 concentrations at or
above the health benchmark concentrations of 60, 70, and 80 ppb (i.e.,
based on HREA estimates of one or more and two or more exposures of
concern). In doing so, the PA notes the CASAC conclusion that (Frey,
2014c, p. 6):
The 80 ppb-8hr benchmark level represents an exposure level for
which there is substantial clinical evidence demonstrating a range
of ozone-related effects including lung inflammation and airway
responsiveness in healthy individuals. The 70 ppb-8hr benchmark
level reflects the fact that in healthy subjects, decreases in lung
function and respiratory symptoms occur at concentrations as low as
72 ppb and that these effects almost certainly occur in some people,
including asthmatics and others with low lung function who are less
tolerant of such effects, at levels of 70 ppb and below. The 60 ppb-
8hr benchmark level represents the lowest exposure level at which
ozone-related effects have been observed in clinical studies of
healthy individuals.
The PA also notes that, due to individual variability in
responsiveness, only a subset of people who experience exposures at or
above the three benchmark concentrations can be expected to experience
associated health effects, and that available data are not sufficient
to quantify that subset of people for most of the endpoints that have
been evaluated in controlled human exposure studies (i.e., with the
exception of FEV1 decrements). The PA views the health
effects evidence as a continuum with greater confidence and less
uncertainty about the occurrence of adverse health effects at higher
O3 exposure concentrations, and less confidence and greater
uncertainty as one considers lower exposure concentrations (U.S. EPA,
2014c, section 3.2.2, p. 3-101).
While there is greater uncertainty regarding the occurrence of
adverse health effects at lower concentrations, the PA also notes that
the controlled human exposure studies that provided the basis for
benchmark concentrations have not evaluated responses in populations at
the greatest risk from exposures to O3 (e.g., children,
people with asthma). Compared to the healthy people included in most
controlled human exposure studies, members of at-risk populations and
lifestages are at greater risk of experiencing adverse effects. Thus,
the effects reported in healthy adults at each of the benchmark
concentrations may underestimate effects in these at-risk groups. In
considering the health evidence within the context of drawing
conclusions on alternative standard levels, the PA balances concerns
about the potential for adverse health effects, especially in at-risk
populations, with the increasing uncertainty regarding the likelihood
of such effects following exposures to lower O3
concentrations.
With respect to the lung function decrements that have been
evaluated in controlled human exposure studies, the PA considers the
extent to which standards with revised levels would be estimated to
protect healthy and at-risk populations against O3-induced
lung function decrements large enough to be adverse in some people
(based on quantitative risk estimates in the HREA). As discussed in
section 3.1.3 of the PA (U.S. EPA, 2014c) and section II.B.3 above,
although some experts would judge single occurrences of moderate
responses to be a nuisance, especially for healthy individuals, a more
general consensus view of the adversity of moderate lung function
decrements emerges as the frequency of occurrence increases. Repeated
occurrences of moderate responses, even in otherwise healthy
individuals, may be considered to be adverse, since they could well set
the stage for more serious illness (73 FR 16448). In reaching
conclusions on alternative standard levels, the PA considers the extent
to which standards with revised levels would be estimated to protect
healthy and at-risk populations against one or more, and two or more,
moderate (i.e., FEV1 decrements >=10% and >=15%) and large
(i.e., FEV1 decrements >=20%) lung function decrements.
In evaluating the epidemiologic evidence within the context of
drawing conclusions on alternative standard levels, the PA considers
the extent to which available studies have reported associations with
emergency
[[Page 75296]]
department visits, hospital admissions, and/or mortality in locations
that would likely have met alternative standards with levels below 75
ppb. In evaluating the epidemiologic evidence in this way, the PA
considers both multicity and single-city studies, recognizing the
strengths and limitations of each. In particular, while single-city
studies are more limited than multicity studies in terms of statistical
power and geographic coverage, conclusions linking air quality in a
specific area with health effect associations in that same area can be
made with greater certainty for single-city studies (i.e., compared to
multicity studies reporting only multicity effect estimates).
The PA also considers the epidemiologic evidence within the context
of epidemiology-based risk estimates. Compared to the weight given to
HREA estimates of exposures of concern and lung function risks, and the
weight given to the evidence, the PA places relatively less weight on
epidemiologic-based risk estimates. In doing so, the PA notes that the
overall conclusions from the HREA likewise reflect less confidence in
estimates of epidemiologic-based risks than in estimates of exposures
and lung function risks. The determination to attach less weight to the
epidemiologic-based estimates reflects the uncertainties associated
with mortality and morbidity risk estimates, including the
heterogeneity in effect estimates between locations, the potential for
exposure measurement errors, and uncertainty in the interpretation of
the shape of concentration-response functions at lower O3
concentrations (U.S. EPA, 2014a, section 9.6). The HREA also concludes
that lower confidence should be placed in the results of the assessment
of respiratory mortality risks associated with long-term O3
exposures, primarily because that analysis is based on only one study
(even though that study is well-designed) and because of the
uncertainty in that study about the existence and level of a potential
threshold in the concentration-response function (U.S. EPA, 2014a,
section 9.6).
In considering the epidemiology-based risk estimates, the PA
focuses on the extent to which potential alternative O3
standards with levels below 75 ppb are estimated to reduce the risk of
O3-associated mortality.\126\ As discussed for the current
standard (II.D.2.c), the PA considers estimates of total risk (i.e.,
based on the full distributions of ambient O3
concentrations) and estimates of risk associated with O3
concentrations in the upper portions of ambient distributions.
---------------------------------------------------------------------------
\126\ Differences in estimated respiratory morbidity risks
between alternative standard levels are similar to the differences
estimated for total mortality associated with short-term
O3 concentrations.
---------------------------------------------------------------------------
b. PA Conclusions on Alternative O3 Standard Levels
Using the approach discussed above to consider the scientific
evidence and exposure/risk information, CASAC advice (II.E.4.c, below),
and public comments, the PA reaches the conclusion that it is
appropriate for the Administrator to consider alternative primary
O3 standard levels from 70 to 60 ppb. The basis for this
conclusion is discussed in detail in sections 4.4.1 and 4.4.2 of the PA
(U.S. EPA, 2014c), and is summarized below.
With regard to controlled human exposure studies, the PA considers
the lowest O3 exposure concentrations at which various
effects have been evaluated and statistically significant effects
reported. The PA also considers the potential for reported effects to
be adverse, including in at-risk populations and lifestages. As
discussed in section 3.1.2.1 of the PA (U.S. EPA, 2014c), controlled
human exposure studies provide evidence of respiratory symptoms
combined with lung function decrements (an adverse response based on
ATS criteria) in healthy adults following 6.6 hour exposures to
O3 concentrations as low as 72 ppb, and evidence of
potentially adverse lung function decrements and airway inflammation
following 6.6 hour exposures to O3 concentrations as low as
60 ppb.
Although some studies show that respiratory symptoms also develop
during exposures to 60 ppb O3, the increase in symptoms has
not been reported to reach statistical significance by the end of the
6.6 hour exposure period (Adams, 2006; Schelegle et al., 2009). Thus,
while significant increases in respiratory symptoms combined with lung
function decrements have not been reported following exposures to 60
ppb O3, this combination of effects is likely to occur to
some degree in healthy adults with 6.6-hour exposures to concentrations
below 72 ppb, and also are more likely to occur with longer (i.e., 8-
hour) exposures.\127\ In addition, pulmonary inflammation, particularly
if experienced repeatedly, provides a mechanism by which O3
may cause other more serious respiratory morbidity effects (e.g.,
asthma exacerbations) and possibly extrapulmonary effects. As discussed
in section 3.1.2.1 of the PA (U.S. EPA, 2014c), the physiological
effects reported in controlled human exposure studies down to 60 ppb
O3 have been linked to aggravation of asthma and increased
susceptibility to respiratory infection, potentially leading to
increased medication use, increased school and work absences, increased
visits to doctors' offices and emergency departments, and increased
hospital admissions.
---------------------------------------------------------------------------
\127\ In addition, CASAC observed that, ``adverse health effects
in young healthy adults occur with exposures to 72 ppb of ozone for
6.6 hours'' and that ``[i]t is the judgment of CASAC that if
subjects had been exposed to ozone using the 8-hour averaging period
used in the standard, adverse effects could have occurred at [a]
lower concentration. Further, in our judgment, the level at which
adverse effects might be observed would likely be lower for more
sensitive subgroups, such as those with asthma'' (Frey, 2014c, p.
5).
---------------------------------------------------------------------------
With regard to the lowest exposure concentration shown to cause
respiratory effects (i.e., 60 ppb),\128\ the PA notes that most
controlled human exposure studies have not evaluated O3
concentrations below 60 ppb. Therefore, 60 ppb does not necessarily
reflect an exposure concentration below which effects such as lung
function decrements and airway inflammation no longer occur. This is
particularly the case given that controlled human exposure studies were
conducted in healthy adults, while people with asthma, including
asthmatic children, are likely to be more sensitive to O3-
induced respiratory effects.
---------------------------------------------------------------------------
\128\ As discussed above (II.B.2), prolonged 6.6 exposure to 40
ppb O3 has been shown to result in a small decrease in
group mean FEV1 that is not statistically different from
responses following exposure to filtered air (Adams, 2002; Adams,
2006).
---------------------------------------------------------------------------
With regard to other O3-induced effects, the PA notes
that AHR and impaired lung host defense capabilities have been reported
in healthy adults engaged in moderate exertion following exposures to
O3 concentrations as low as 80 ppb, the lowest concentration
evaluated for these effects. As discussed in section 3.1.2.1 of the PA
(U.S. EPA, 2014c), these physiological effects have been linked to
aggravation of asthma and increased susceptibility to respiratory
infection, potentially leading to increased medication use, increased
school and work absences, increased visits to doctors' offices and
emergency departments, and increased hospital admissions. These are all
indicators of adverse O3-related morbidity effects, which
are consistent with, and provide plausibility for, the adverse
morbidity effects and mortality effects observed in epidemiologic
studies.
Based on consideration of the above evidence, the PA concludes that
available controlled human exposure studies support considering
alternative O3 standard levels from 70 to 60 ppb in
[[Page 75297]]
the current review. In reaching this conclusion, the PA notes that 70
ppb is just below the O3 exposure concentration reported to
result in lung function decrements and respiratory symptoms in healthy
adults (i.e., 72 ppb), a combination of effects that meet ATS criteria
for an adverse response. In addition, while 70 ppb is well below the 80
ppb exposure concentration shown to cause potentially adverse
respiratory effects such as AHR and impaired host-defense capabilities,
these effects have not been evaluated at exposure concentrations below
80 ppb and there is no reason to believe that 80 ppb represents a
threshold for such effects. In addition, potentially adverse lung
function decrements and pulmonary inflammation have been demonstrated
to occur in healthy adults at 60 ppb. Thus, 60 ppb is a short-term
exposure concentration that may be reasonably concluded to elicit
adverse effects in at-risk groups.
The PA further notes that the range of alternative levels from 70
to 60 ppb is supported by evidence from epidemiologic studies and by
exposure and risk estimates from the HREA. This evidence and exposure/
risk information indicate that a level from anywhere in the range of 70
to 60 ppb would be expected to result in important public health
improvements over the current standard. In particular, compared to the
current standard a revised standard with a level from 70 to 60 ppb
would be expected to (1) more effectively maintain short- and long-term
O3 concentrations below those present in the epidemiologic
studies that reported significant O3 health effect
associations in locations likely to have met the current standard; (2)
reduce the occurrence of exposures of concern to O3
concentrations that result in respiratory effects in healthy adults (at
or above 60, 70, and 80 ppb); (3) reduce the occurrence of moderate-to-
large O3-induced lung function decrements; and (4) reduce
the risk of O3-associated mortality and morbidity,
particularly the risk associated with the upper portions of the
distributions of ambient O3 concentrations. The PA also
notes that the range of levels from 70 to 60 ppb corresponds to the
range of levels recommended for consideration by CASAC, based on the
available evidence and information (Frey, 2014a; Frey, 2014c).
In reaching a conclusion on whether it is appropriate to consider
alternative standard levels below 60 ppb, the PA notes the following:
(1) While controlled human exposure studies provide evidence for
O3-induced respiratory effects following exposures to
O3 concentrations as low as 60 ppb, they do not provide
evidence for adverse effects following exposures to lower
concentrations. On this issue, CASAC concurred that 60 ppb
O3 is an appropriate and justifiable scientifically based
lower bound for a revised primary standard, based upon findings of
``adverse effects, including clinically significant lung function
decrements and airway inflammation, after exposures to 60 ppb ozone
in healthy adults with moderate exertion (Adams, 2006; Schelegle et
al., 2009; Brown et al., 2008; Kim et al., 2011), with limited
evidence of adverse effects below 60 ppb'' (Frey, 2014c, p. 7).
(2) Based on the HREA results, meeting an O3 standard
with a level of 60 ppb would be expected to almost eliminate
exposures of concern to O3 concentrations at or above 60
ppb. To the extent lower exposure concentrations may result in
adverse health effects in some people, a standard level of 60 ppb
would be expected to also reduce exposures to O3
concentrations below 60 ppb.
(3) U.S. and Canadian epidemiologic studies have not reported
O3 health effect associations based primarily on study
locations likely to have met a standard with a level of 60 ppb.
(4) In all of the urban study areas evaluated, a standard with a
level of 60 ppb would be expected to maintain long-term
O3 concentrations below those where a key study indicates
the most confidence in a linear concentration-response relationship
with respiratory mortality.
Given all of the above considerations the PA concludes that,
compared to standards with levels from 70 to 60 ppb, the extent to
which standards with levels below 60 ppb could result in further public
health improvements becomes notably less certain. Therefore, the PA
concludes that it is not appropriate in this review to consider
standard levels below 60 ppb.
The following sections summarize the PA's consideration of the
scientific evidence and exposure/risk information specifically related
to potential alternative O3 standards with levels from the
upper (70 ppb) (II.E.4.c.i), middle (65 ppb) (II.E.4.c.ii), and lower
(60 ppb) (II.E.4.c.iii) portions of the range of 70 to 60 ppb. Key
exposure/risk information considered in the PA is summarized in Tables
4 and 5, below (from U.S. EPA, 2014c, Tables 4-4 and 4-5).
---------------------------------------------------------------------------
\129\ All alternative standard levels evaluated in the HREA were
effective at limiting exposures of concern at or above 80 ppb (U.S.
EPA, 2014c, Figures 4-1 to 4-4). Therefore, Table 4 focuses on
exposures of concern at or above the 70 and 60 ppb benchmark
concentrations.
\130\ Estimates for each urban case study area were averaged for
the years evaluated in the HREA (2006 to 2010). Ranges reflect the
ranges across urban study areas.
\131\ Numbers of children exposed in each urban case study area
were averaged over the years 2006 to 2010. These averages were then
summed across urban study areas. Numbers are rounded to nearest
thousand unless otherwise indicated.
\132\ As noted in section II.C.3.a.ii, the responsiveness of
asthmatics to O3 exposures could depend on factors that
have not been well-evaluated such as asthma severity, the
effectiveness of asthma control, or the prevalence of medication
use.
\133\ Percent reductions in each urban study area were
calculated and averaged across areas.
\134\ Estimates smaller than 0.05% were rounded to zero.
\135\ As discussed in section 4.3.3 of the HREA (U.S. EPA,
2014a), the model-based air quality adjustment approach used to
estimate risks associated with the current and alternative standards
was unable to estimate the distribution of ambient O3
concentrations in New York City upon just meeting an alternative
standard with a level of 60 ppb. Therefore, for the 60 ppb standard
level the numbers of children and asthmatic children reflect all of
the urban study areas except New York.
Table 4--Summary of Estimated Exposures of Concern for Potential Alternative O3 Standard Levels of 70, 65, 60
ppb in Urban Case Study Areas \129\
----------------------------------------------------------------------------------------------------------------
Number of children
Alternative Average % (5 to 18 years) Average % % Children--
Benchmark level standard level children [number of asthmatic reduction from worst year and
(ppb) exposed \130\ children] \131\ current worst area
\132\ standard \133\
----------------------------------------------------------------------------------------------------------------
One or more exposures of concern per season
----------------------------------------------------------------------------------------------------------------
>=70 ppb................. 70 0.1-1.2 94,000 [10,000]..... 73 3.2
65 0-0.2 14,000 [2,000]...... 95 0.5
60 \134\ 0 1,400 [200] \135\... 100 0.1
>=60 ppb................. 70 3.3-10.2 1,176,000 [126,000]. 46 18.9
65 0-4.2 392,000 [42,000].... 80 9.5
[[Page 75298]]
60 0-1.2 70,000 [8,000]...... 96 2.2
----------------------------------------------------------------------------------------------------------------
Two or more exposures of concern per season
----------------------------------------------------------------------------------------------------------------
>=70 ppb................. 70 0-0.1 5,400 [600]......... 95 0.4
65 0 300 [100]........... 100 0
60 0 0 [0]............... 100 0
>=60 ppb................. 70 0.5-3.5 320,000 [35,000].... 61 9.2
65 0-0.8 67,000 [7,500]...... 92 2.8
60 0-0.2 5,100 [700]......... 100 0.3
----------------------------------------------------------------------------------------------------------------
---------------------------------------------------------------------------
\136\ Estimates in each urban case study area were averaged for
the years evaluated in the HREA (2006 to 2010). Ranges reflect the
ranges across urban study areas.
\137\ Numbers of children estimated to experience decrements in
each urban case study area were averaged over 2006 to 2010. These
averages were then summed across urban study areas. Numbers are
rounded to nearest thousand unless otherwise indicated.
\138\ As noted in section II.C.3.a.ii, the responsiveness of
asthmatics to O3 exposures could depend on factors that
have not been well-evaluated such as asthma severity, the
effectiveness of asthma control, or the prevalence of medication
use.
\139\ As discussed in section 4.3.3 of the HREA (U.S. EPA,
2014a), the model-based air quality adjustment approach used to
estimate risks associated with the current and alternative standards
was unable to estimate the distribution of ambient O3
concentrations in New York City upon just meeting an alternative
standard with a level of 60 ppb. Therefore, for the 60 ppb standard
level the numbers of children and asthmatic children reflect all of
the urban study areas except New York.
Table 5--Summary of Estimated Lung Function Decrements for Potential Alternative O3 Standard Levels of 70, 65,
and 60 ppb in Urban Case Study Areas
----------------------------------------------------------------------------------------------------------------
Number of children
(5 to 18 years) Average % % Children
Lung function decrement Alternative Average % [number of asthmatic reduction from worst year and
standard level children \136\ children] \137\ current area
\138\ standard
----------------------------------------------------------------------------------------------------------------
One or more decrements per season
----------------------------------------------------------------------------------------------------------------
>=10%..................... 70 11-17 2,527,000 [261,000]. 15 20
65 3-15 1,896,000 [191,000]. 31 18
60 5-11 1,404,000 [139,000] 45 13
\139\.
>=15%..................... 70 2-4 562,000 [58,000].... 26 5
65 0-3 356,000 [36,000].... 50 4
60 1-2 225,000 [22,000].... 67 3
>=20%..................... 70 1-2 189,000 [20,000].... 32 2.1
65 0-1 106,000 [11,000].... 59 1.4
60 0-1 57,000 [6,000]...... 77 0.7
----------------------------------------------------------------------------------------------------------------
Two or more decrements per season
----------------------------------------------------------------------------------------------------------------
>=10%..................... 70 5.5-11 1,414,000 [145,000]. 17 13
65 1.3-8.8 1,023,000 [102,000]. 37 11
60 2.1-6.4 741,000 [73,000].... 51 7.3
>=15%..................... 70 0.9-2.4 276,000 [28,000].... 29 3.1
65 0.1-1.8 168,000 [17,000].... 54 2.3
60 0.2-1.0 101,000 [10,000].... 71 1.4
>=20%..................... 70 0.3-0.8 81,000 [8,000]...... 34 1.1
65 0-0.5 43,000 [4,000]...... 66 0.8
60 0-0.2 21,000 [2,000]...... 83 0.4
----------------------------------------------------------------------------------------------------------------
i. PA Consideration of an O3 Standard Level of 70 ppb
The PA notes that a level of 70 ppb is below the lowest
O3 exposure concentration that has been reported to elicit a
range of respiratory effects that includes AHR and decreased lung host
defense, in addition to lung function decrements, airway inflammation,
and respiratory symptoms (i.e., 80 ppb). A level of 70 ppb is also
below the lowest exposure concentration at which the combined
occurrence of respiratory symptoms and lung function decrements have
been reported (i.e., 72 ppb), a combination judged adverse by the ATS
(U.S. EPA, 2014c, section 3.1.3). A level of 70 ppb is above the lowest
exposure concentration demonstrated to result in lung function
decrements large enough to be judged an abnormal response by ATS and
above the lowest exposure concentration demonstrated to result in
pulmonary inflammation (i.e., 60 ppb).
Compared to the current standard, the HREA estimates that a revised
O3
[[Page 75299]]
standard with a level of 70 ppb would reduce exposures of concern to
O3 concentrations of 60, 70, and 80 ppb in urban study
areas, with such a standard level estimated to be most effective at
limiting exposures at or above the higher health benchmark
concentrations and at limiting multiple occurrences of such exposures.
On average over the years 2006 to 2010, for a standard with a level of
70 ppb, up to about 1% of children (i.e., ages 5 to 18) are estimated
to experience exposures of concern at or above 70 ppb (73% reduction,
compared to current standard), and far less than 1% are estimated to
experience two or more such exposures (95% reduction, compared to
current standard). In the worst-case location and year (i.e., location
and year with the largest exposure estimate), about 3% of children are
estimated to experience one or more exposures of concern at or above 70
ppb, and less than 1% are estimated to experience two or more. Far less
than 1% of children are estimated to experience exposures of concern at
or above the 80 ppb benchmark concentration, even in the worst-case
year (Table 4, above).\140\
---------------------------------------------------------------------------
\140\ As noted above, due to interindividual variability,
children (or adults) exposed at these levels will not necessarily
experience health effects; the information available for some health
effects is not sufficient to quantify the numbers of children in the
urban study areas who might experience these effects.
---------------------------------------------------------------------------
As noted above, CASAC advised the EPA that 60 ppb is an appropriate
exposure of concern with respect to adverse effects on people with
asthma, including children (Frey, 2014c, pp. 6 and 8). For an
O3 standard with a level of 70 ppb, about 3 to 10% of
children, including asthmatic children, are estimated to experience one
or more exposures of concern at or above 60 ppb in a single
O3 season. Compared to the current standard, this reflects
about a 46% reduction, on average across the urban study areas. About
1% to 4% of children are estimated to experience two or more exposures
of concern at or above 60 ppb (approximately 60% reduction, compared to
current standard). In the worst-case location and year, for a standard
set at 70 ppb, about 19% of children are estimated to experience one or
more exposures of concern at or above 60 ppb, and 9% are estimated to
experience two or more such exposures (Table 4, above).
Compared to the current standard, the HREA estimates that a revised
O3 standard with a level of 70 ppb would also reduce
O3-induced lung function decrements in children. A level of
70 ppb is estimated to be most effective at limiting the occurrences of
moderate and large lung function decrements (i.e., FEV1
decrements >=15% and >=20%, respectively), and at limiting multiple
occurrences of O3-induced decrements. On average over the
years 2006 to 2010, for a standard with a level of 70 ppb, about 2 to
4% of children in the urban study areas are estimated to experience one
or more moderate O3-induced lung function decrements (i.e.,
FEV1 decrement >=15%), which would be of concern for healthy
people, and about 1 to 2.5% of children are estimated to experience two
or more such decrements (approximately 30% reduction, compared to the
current standard). In the worst-case location and year, up to 5% of
children are estimated to experience one or more O3-induced
lung function decrements >=15%, and up to 3% are estimated to
experience two or more such decrements. For a standard set at 70 ppb,
about 2% or fewer children are estimated to experience large
O3-induced lung function decrements (i.e., FEV1
decrement >=20%), and about 1% or fewer children are estimated to
experience two or more such decrements, even in the worst-case years
and locations (Table 5, above).
On average over the years 2006 to 2010, for an O3
standard set at 70 ppb, about 11 to 17% of children in the urban study
areas are estimated to experience one or more moderate O3-
induced lung function decrements (i.e., FEV1 decrement
>=10%), which could be adverse for people with lung disease. This
reflects an average reduction of about 15%, compared to the current
standard. About 6 to 11% of children are estimated to experience two or
more such decrements (17% reduction, compared to current standard). In
the worst-case location and year, for a standard set at 70 ppb, about
20% of children in the urban study areas are estimated to experience
one or more O3-induced lung function decrements >=10%, and
13% are estimated to experience two or more such decrements (Table 5,
above).
Compared to the current standard, a revised standard with a level
of 70 ppb would also more effectively maintain short-term ambient
O3 concentrations below those present in the epidemiologic
studies that reported significant O3 health effect
associations in locations likely to have met the current standard. In
particular, the single-city study by Mar and Koenig (2009) reported
positive and statistically significant associations with respiratory
emergency department visits in children and adults in a location that
likely would have met the current O3 standard over the
entire study period but violated a revised standard with a level of 70
ppb or below. None of the single-city studies evaluated in section
4.4.1 of the PA (U.S. EPA, 2014c) provide evidence for O3
health effect associations in locations meeting a standard with a level
of 70 ppb or below. While this analysis does not provide information on
the extent to which the reported O3-associated emergency
department visits would persist upon meeting an O3 standard
with a level of 70 ppb, or on the extent to which standard levels below
70 ppb could further reduce the incidence of such emergency department
visits,\141\ it suggests that a revised O3 standard with a
level at or below 70 ppb would require reductions in the ambient
O3 concentrations that provided the basis for the health
effect associations reported by Mar and Koenig (2009).
---------------------------------------------------------------------------
\141\ Put another way, one cannot infer from this analysis the
extent to which effects would occur at O3 concentrations
below those observed in the study.
---------------------------------------------------------------------------
As discussed above, compared to single-city studies, there is
greater uncertainty in linking air quality concentrations from
individual study cities to multicity effect estimates. With regard to
the multicity studies in this review, the PA notes that Dales et al.
(2006) reported significant associations with respiratory hospital
admissions based on air quality in 11 Canadian cities, most of which
would likely have met the current standard over the entire study
period, but violated a revised standard with a level of 70 ppb or below
over at least part of that period (Table 4-1). This analysis suggests
that although the current standard would allow the ambient
O3 concentrations in most of the study locations that
provided the basis for the association with hospital admissions, a
revised O3 standard with a level at or below 70 ppb would
require reductions in those ambient O3 concentrations. As
with the study by Mar and Koenig (2009), this analysis does not provide
information on the extent to which the reported O3-
associated hospital admissions would persist upon meeting an
O3 standard with a level of 70 ppb, or on the extent to
which standard levels below 70 ppb could further reduce the incidence
of such hospital admissions.\142\
---------------------------------------------------------------------------
\142\ In addition, for the other multicity studies identified in
Table 4-1 of the PA (Cakmak et al., 2006; Stieb et al., 2009;
Katsouyanni et al., 2009), and for the study by Bell et al. (2006)
(for the 30 ppb cut point) (Table 4-2 of the PA), the majority of
study locations would likely have met a standard with a level of 70
ppb (U.S. EPA, 2014c).
---------------------------------------------------------------------------
With regard to long-term O3 concentrations, the PA
evaluates the long-term O3 metrics reported to be associated
with mortality or morbidity in recent epidemiologic studies (e.g.,
[[Page 75300]]
seasonal averages of 1-hour or 8-hour daily max concentrations).
Compared to the current standard, a revised standard with a level of 70
ppb would be expected to reduce the risk of respiratory mortality
associated with long-term O3 concentrations, based on
information from the study by Jerrett et al. (2009), though the PA
notes the HREA conclusion, discussed above, that lower confidence
should be placed in respiratory mortality risk estimates based on this
study (U.S. EPA, 2014a, section 9.6). In addition, a standard with a
level of 70 ppb would be expected to more effectively maintain long-
term O3 concentrations below those where the study by
Jerrett et al. (2009) indicates the most confidence in the reported
association with respiratory mortality.\143\ Specifically, air quality
analyses indicate this to be the case in 9 out of the 12 urban study
areas for a level of 70 ppb, compared to 6 out of 12 areas for the
current standard. Finally, a revised standard with a level of 70 ppb
would be expected to reduce long-term O3 concentrations
based on the types of metrics that have been reported in recent
epidemiologic studies to be associated with respiratory morbidity
(i.e., seasonal averages of daily maximum 8-hour concentrations).
---------------------------------------------------------------------------
\143\ As discussed in section 3.1.4.3 of the PA (U.S. EPA,
2014c), the study by Jerrett et al. (2009) suggests notably
decreased confidence in the reported linear concentration-response
function for long-term O3 concentrations in the first
quartile (i.e., at or below about 53 ppb), given the widening in
confidence intervals for lower concentrations; the fact that most
study cities contributing to the linear function had O3
concentrations in the highest three quartiles, accounting for
approximately 72% of the respiratory deaths in the cohort (based on
Table 2 in the published study); and the limited evidence presented
in the published study for a threshold at or near 56 ppb.
---------------------------------------------------------------------------
In further considering the potential implications of epidemiology
studies for alternative standard levels, the PA notes estimates of
total mortality associated with short-term O3
concentrations.\144\ As discussed above, the PA considers estimates of
total risk (i.e., based on the full distributions of ambient
O3 concentrations) and estimates of risk associated with
O3 concentrations in the upper portions of ambient
distributions. With regard to total risk the PA notes that, when summed
across urban study areas, a standard with a level of 70 ppb is
estimated to reduce the number of deaths associated with short-term
O3 concentrations by about 4% (2007) and 2% (2009), compared
to the current standard.\145\ Based on a national modeling analysis,
the majority of the U.S. population would be expected to experience
reductions in such risks upon reducing precursor emissions.
---------------------------------------------------------------------------
\144\ As discussed above, compared to the weight given to the
evidence and to HREA estimates of exposures of concern and lung
function risks, the PA places relatively less weight on
epidemiologic-based risk estimates.
\145\ A standard with a level of 70 ppb is also estimated to
reduce respiratory mortality associated with long-term O3
concentrations in urban study areas. However, given uncertainties
associated with these risk estimates, as discussed above, the PA
gives them limited weight.
---------------------------------------------------------------------------
Compared to the total risk estimates noted above, an O3
standard with a level of 70 ppb is estimated to be more effective at
reducing the number of deaths associated with short-term O3
concentrations at the upper ends of ambient distributions.
Specifically, for area-wide O3 concentrations at or above 40
ppb, a standard with a level of 70 ppb is estimated to reduce the
number of deaths associated with short-term O3
concentrations by about 10% compared to the current standard. In
addition, for area-wide concentrations at or above 60 ppb, a standard
with a level of 70 ppb is estimated to reduce O3-associated
deaths by about 50% to 70% (U.S. EPA, 2014c, Figure 4-13).
The PA noted that in providing the advice that 70 ppb is an
appropriate upper bound for consideration, CASAC advised that a level
of 70 ppb would provide little margin of safety for protection of
public health, particularly for sensitive subpopulations (Frey, 2014c,
p. 8). In particular, CASAC stated that:
At 70 ppb, there is substantial scientific certainty of a variety of
adverse effects, including decrease in lung function, increase in
respiratory symptoms, and increase in airway inflammation. Although
a level of 70 ppb is more protective of public health than the
current standard, it may not meet the statutory requirement to
protect public health with an adequate margin of safety (Frey,
2014c, p. 8).\146\
---------------------------------------------------------------------------
\146\ Also see Frey (2014c, p. ii).
However, the committee also acknowledged that ``the choice of a level
within the range recommended based on scientific evidence [i.e., 70 to
60 ppb] is a policy judgment under the statutory mandate of the Clean
Air Act'' (Frey, 2014c, pp. ii and 8).
In summary, compared to the current standard, the PA concludes that
a revised O3 standard with a level of 70 ppb would be
expected to (1) reduce the occurrence of exposures of concern to
O3 concentrations that result in respiratory effects in
healthy adults (at or above 60 and 70 ppb) by about 45 to 95%, almost
eliminating the occurrence of multiple exposures at or above 70 ppb;
(2) reduce the occurrence of moderate-to-large O3-induced
lung function decrements (FEV1 decrements >=10, 15, 20%) by
about 15 to 35%, most effectively limiting the occurrence of multiple
decrements and decrements >=15, 20%; (3) more effectively maintain
short- and long-term O3 concentrations below those present
in the epidemiologic studies that reported significant O3
health effect associations in locations likely to have met the current
standard; \147\ and (4) reduce the risk of O3-associated
mortality and morbidity, particularly the risk associated with the
upper portions of the distributions of ambient O3
concentrations.
---------------------------------------------------------------------------
\147\ Epidemiologic studies also provide some evidence for
O3 health effect associations in locations likely to have
met a standard with a level of 70 ppb, as discussed below for lower
standard levels.
---------------------------------------------------------------------------
ii. PA Consideration of an O3 Standard Level of 65 ppb
The PA also considers a standard with a level of 65 ppb. A level of
65 ppb is well below 80 ppb, an O3 exposure concentration
that has been reported to elicit a range of respiratory effects that
includes airway hyperresponsiveness and decreased lung host defense, in
addition to lung function decrements, airway inflammation, and
respiratory symptoms. A standard level of 65 ppb is also below the
lowest exposure concentration at which the combined occurrence of
respiratory symptoms and lung function decrements has been reported
(i.e., 72 ppb), a combination judged adverse by the ATS (U.S. EPA,
2014c, section 3.1.3). A level of 65 ppb is above the lowest exposure
concentration demonstrated to result in lung function decrements large
enough to be judged an abnormal response by ATS, where statistically
significant changes in group mean responses would be judged to be
adverse by ATS, and which the CASAC has indicated could be adverse in
people with lung disease (i.e., 60 ppb). A level of 65 ppb is also
above the lowest exposure concentration at which pulmonary inflammation
has been reported in healthy adults (i.e., 60 ppb).
Compared to the current standard and a revised standard with a
level of 70 ppb, the HREA estimates that a standard with a level of 65
ppb would reduce exposures of concern to the range of O3
benchmark concentrations analyzed (i.e., 60, 70, and 80 ppb). The HREA
estimates that meeting a standard with a level of 65 ppb would
eliminate exposures of concern at or above 80 ppb in the urban study
areas. Such a standard is estimated to allow far less than 1% of
children in the urban study areas to experience one or more exposures
of concern at or above the 70
[[Page 75301]]
ppb benchmark level, even in the worst-case years and locations, and is
estimated to eliminate the occurrence of two or more exposures at or
above 70 ppb (Table 4, above).
In addition, for a standard with a level of 65 ppb, between 0 and
about 4% of children (including asthmatic children) in urban study
areas are estimated to experience exposures of concern at or above 60
ppb, which CASAC has indicated is an appropriate exposure of concern
for people with asthma, including children. This reflects an 80%
reduction (on average across areas), relative to the current standard.
Less than 1% of children are estimated to experience two or more
exposures of concern at or above 60 ppb (> 90% reduction, compared to
current standard). In the worst-case location and year, about 10% of
children are estimated to experience one or more exposures of concern
at or above 60 ppb, with about 3% estimated to experience two or more
such exposures (Table 4, above).
Compared to the current standard and a revised standard with a
level of 70 ppb, the HREA estimates that a standard with a level of 65
ppb would also further reduce the occurrence of O3-induced
lung function decrements. For a level of 65 ppb, about 4% of children,
or less, are estimated to experience moderate O3-induced
FEV1 decrements >=15% (50% reduction, compared to current
standard), even considering the worst-case location and year. About 2%
of children, or less, are estimated to experience two or more such
decrements. Only about 1% of children, or less, are estimated to
experience large O3-induced lung function decrements (i.e.,
FEV1 decrement >=20%), even in the worst-case year and
location.
In addition, for a standard with a level of 65 ppb, about 3 to 15%
of children are estimated to experience one or more moderate
O3-induced lung function decrements (i.e., FEV1
decrement >=10%), which CASAC has indicated could be adverse for people
with lung disease. This reflects an average reduction of about 30%,
relative to the current standard. About 1 to 9% of children in the
urban study areas are estimated to experience two or more such
decrements (37% reduction, compared to current standard). In the worst-
case location and year, for a standard set at 65 ppb, up to about 18%
of these children are estimated to experience one or more moderate
O3-induced lung function decrements >=10%, and up to 11% are
estimated to experience two or more such decrements.
With regard to O3 epidemiologic studies, the PA notes
that a revised standard with a level of 65 ppb would be expected to
maintain short-term ambient O3 concentrations below those
present in some of the study locations that provided the basis for
reported O3 health effect associations and that were likely
to have met a revised standard with a level of 70 ppb. In particular,
Katsouyanni et al. (2009) reported statistically significant
associations with mortality based on air quality in 12 Canadian cities,
most of which would likely have met a standard with a level of 70 ppb
over the entire study period but violated a revised standard with a
level of 65 ppb or below over at least part of that period (U.S. EPA,
2014c, Table 4-1). This analysis suggests that although the current
standard or a standard with a level of 70 ppb would allow the ambient
O3 concentrations in most of the study locations that
provided the basis for the association with mortality in this study, a
revised O3 standard with a level at or below 65 ppb would
require reductions in those ambient O3 concentrations. As
discussed above for a level of 70 ppb, this analysis does not provide
information on the extent to which O3-associated mortality
would persist upon meeting an O3 standard with a level of 65
ppb, or on the extent to which standard levels below 65 ppb could
further reduce the incidence of this mortality.\148\
---------------------------------------------------------------------------
\148\ For the other multicity studies identified in Table 4-1 of
the PA (Cakmak et al., 2006; Stieb et al., 2009; Katsouyanni et al.,
2009 (for hospital admissions)), and for the study by Bell et al.
(2006) (for the 30 ppb cut point) (Table 4-2 of the PA), the
majority of study locations would have met a standard with a level
of 65 ppb (U.S. EPA, 2014c).
---------------------------------------------------------------------------
With regard to long-term O3 concentrations, as for 70
ppb (above) the PA evaluates the long-term O3 metrics
reported to be associated with mortality or morbidity in recent
epidemiologic studies (e.g., seasonal averages of 1-hour or 8-hour
daily max concentrations). Compared to the current standard or a
revised O3 standard with a level of 70 ppb, a revised
standard with a level of 65 ppb would be expected to further reduce the
risk of respiratory mortality associated with long-term O3
concentrations, based on information from the study by Jerrett et al.
(2009).\149\ In addition, a standard with a level of 65 ppb would be
expected to more effectively maintain long-term O3
concentrations below those where the study by Jerrett et al. (2009)
indicates the most confidence in the reported association with
respiratory mortality. Specifically, air quality analyses indicate this
to be the case in 10 out of the 12 urban study areas for a level of 65
ppb, compared to 6 out of 12 areas for the current standard and 9 out
of 12 for a standard with a level of 70 ppb (U.S. EPA, 2014c, Table 4-
3). Finally, a revised standard with a level of 65 ppb would be
expected to further reduce long-term O3 concentrations based
on the types of metrics that have been reported in recent epidemiologic
studies to be associated with respiratory morbidity (i.e., seasonal
averages of daily maximum 8-hour concentrations).
---------------------------------------------------------------------------
\149\ Though as discussed above, the PA notes the lower
confidence placed in these risk results (U.S. EPA, 2014a, section
9.6).
---------------------------------------------------------------------------
In further considering the potential implications of epidemiology
studies for alternative standard levels, the PA notes estimates of
total mortality associated with short-term O3.\150\ As
discussed above, the PA considers estimates of total risk (i.e., based
on the full distributions of ambient O3 concentrations) and
estimates of risk associated with O3 concentrations in the
upper portions of ambient distributions. With regard to total risk the
PA notes that, when summed across urban study areas, a standard with a
level of 65 ppb is estimated to reduce the number of deaths associated
with short-term O3 exposures by about 13% (2007) and 9%
(2009), compared to the current standard.\151\ For area-wide
concentrations at or above 40 ppb, a standard level of 65 ppb is
estimated to reduce O3-associated deaths by almost 50%
compared to the current standard, when summed across urban study areas.
For area-wide concentrations at or above 60 ppb, a standard level of 65
ppb is estimated to reduce O3-associated deaths by more than
80% (U.S. EPA, 2014c, Figure 4-13).
---------------------------------------------------------------------------
\150\ As discussed above, compared to the weight given to the
evidence and to HREA estimates of exposures of concern and lung
function risks, the PA places relatively less weight on
epidemiologic-based risk estimates.
\151\ A standard with a level of 65 ppb is also estimated to
reduce respiratory mortality associated with long-term O3
concentrations in urban study areas. However, given uncertainties
associated with these risk estimates, as discussed above, we give
them limited weight.
---------------------------------------------------------------------------
In summarizing CASAC's advice regarding a standard with a level of
65, the PA noted CASAC's conclusion that an alternative standard with a
level of 65 ppb would further reduce, though not eliminate, the
frequency of lung function decrements >=15% and would lead to lower
frequency of short-term premature mortality (i.e., compared to a
standard with a level of 70 ppb) (Frey, 2014c, p. 8).
In summary, compared to a standard with a level of 70 ppb, the PA
concludes that a revised standard with a level of
[[Page 75302]]
65 ppb would be expected to further reduce O3 exposures and
health risks. In particular, a standard with a level of 65 ppb is
estimated to (1) reduce the occurrence of exposures of concern by about
80 to 100%, compared to the current standard, decreasing exposures at
or above 60 ppb and almost eliminating exposures at or above 70 and 80
ppb; (2) reduce the occurrence of FEV1 decrements >=10, 15,
and 20% by about 30 to 65%, compared to the current standard; (3) more
effectively maintain short- and long-term O3 concentrations
below those present in the epidemiologic studies that reported
significant O3 health effect associations in locations
likely to have met the current standard; \152\ and (4) further reduce
the risk of O3-associated mortality and morbidity,
particularly the risk associated with the upper portion of the
distribution of ambient O3 concentrations.
---------------------------------------------------------------------------
\152\ Though epidemiologic studies also provide evidence for
O3 health effect associations in locations likely to have
met a standard with a level of 65 ppb, as discussed below for a
level of 60 ppb.
---------------------------------------------------------------------------
iii. PA Consideration of an O3 Standard Level of 60 ppb
The PA also considers a standard with a level of 60 ppb. A level of
60 ppb is well below the O3 exposure concentration that has
been reported to elicit a wide range of potentially adverse respiratory
effects in healthy adults (i.e., 80 ppb). A level of 60 ppb is also
below the lowest concentration where the combined occurrence of
respiratory symptoms and lung function decrements was observed, a
combination judged adverse by the ATS (i.e., 72 ppb). A level of 60 ppb
corresponds to the lowest exposure concentration demonstrated to result
in lung function decrements that are large enough to be judged an
abnormal response by ATS, that meet ATS criteria for adversity based on
a downward shift in the distribution of FEV1, and that the CASAC
indicated could be adverse in people with lung disease. A level of 60
ppb also corresponds to the lowest exposure concentration at which
pulmonary inflammation has been reported in a single controlled human
exposure study.
Based on the HREA analyses of O3 exposures of concern, a
standard with a level of 60 ppb is estimated to eliminate exposures of
concern at or above the 70 and 80 ppb benchmark concentrations and to
be more effective than the higher standard levels at limiting exposures
of concern at or above 60 ppb. On average over the years 2006 to 2010,
for a standard with a level of 60 ppb, between 0 and about 1% of
children, including asthmatic children, in urban study areas are
estimated to experience exposures of concern at or above 60 ppb, which
CASAC indicated is an appropriate exposure of concern for asthmatic
children. This reflects a 96% reduction (on average across areas),
compared to the current standard. Virtually no children are estimated
to experience two or more exposures of concern at or above 60 ppb. In
the worst-case location and year, about 2% of children are estimated to
experience exposures of concern at or above 60 ppb, with far less than
1% estimated to experience two or more such exposures (Table 4, above).
Based on the HREA analyses of O3-induced lung function
decrements, a standard with a level of 60 ppb would be expected to be
more effective than a level of 65 or 70 ppb at limiting the occurrence
of O3-induced lung function decrements. For a standard with
a level of 60 ppb, about 2% of children, or less, in the urban study
areas are estimated to experience one or more moderate O3-
induced FEV1 decrements >=15% (almost 70% reduction,
compared to current standard), and about 1% or less are estimated to
experience two or more such decrements (3% in the location and year
with the largest estimates). About 1% of children, or less, are
estimated to experience large O3-induced lung function
decrements (i.e., FEV1 decrement >=20%), even in the worst-
case locations and year (Table 5, above).
In addition, for a standard with a level of 60 ppb, about 5 to 11%
of children in the urban study areas are estimated to experience one or
more moderate O3-induced lung function decrements that CASAC
indicated could be adverse for people with lung disease (i.e.,
FEV1 decrements >=10%). This reflects an average reduction
of about 45%, compared to the current standard. About 2 to 6% of
children in these areas are estimated to experience two or more such
decrements (51% reduction, compared to current standard). In the worst-
case location and year, for a standard set at 60 ppb, up to about 13%
of children are estimated to experience one or more moderate
O3-induced FEV1 decrements >=10%, and 7% are
estimated to experience two or more such decrements (Table 5, above).
With regard to O3 epidemiologic studies, the PA notes
that a revised standard with a level of 60 ppb would be expected to
maintain short-term ambient O3 concentrations below those
present in some of the study locations that provided the basis for
reported O3 health effect associations and that were likely
to have met a revised standard with a level of 70 or 65 ppb.
Specifically, in all of the U.S. and Canadian epidemiologic studies
evaluated, the majority of study cities had ambient O3
concentrations that would likely have violated a standard with a level
of 60 ppb. Thus, none of the U.S. and Canadian epidemiologic studies
analyzed provide evidence for O3 health effect associations
when the majority of study locations would likely have met a standard
with a level of 60 ppb (U.S. EPA, 2014c, Tables 4-1 and 4-2). As
discussed above, while this analysis does not provide information on
the extent to which the O3-associated morbidity or mortality
would persist upon meeting an O3 standard with a level of 60
ppb, it suggests that a revised O3 standard with a level of
60 ppb would require reductions in the ambient O3
concentrations that provided the basis for those health effect
associations.
With regard to long-term O3 concentrations, compared to
the current standard or a revised O3 standard with a level
of 65 or 70 ppb, a revised standard with a level of 60 ppb would be
expected to further reduce the risk of respiratory mortality associated
with long-term O3 concentrations, based on information from
the study by Jerrett et al. (2009).\153\ In addition, a standard with a
level of 60 ppb would be expected to more effectively maintain long-
term O3 concentrations below those where the study by
Jerrett et al. (2009) indicates the most confidence in the reported
association with respiratory mortality. Specifically, air quality
analyses indicate this to be the case in all of the urban study areas
evaluated at a level of 60 ppb, compared to 6 out of 12 areas for the
current standard, 9 out of 12 for a standard with a level of 70 ppb,
and 10 out of 12 for a standard with a level of 65 ppb (U.S. EPA,
2014c, Table 4-3). Finally, a revised standard with a level of 60 ppb
would be expected to further reduce long-term O3
concentrations based on the types of metrics that have been reported in
recent epidemiologic studies to be associated with respiratory
morbidity (i.e., seasonal averages of daily maximum 8-hour
concentrations).
---------------------------------------------------------------------------
\153\ Though as discussed above, the PA notes the lower
confidence we place in these risk results (U.S. EPA, 2014a, section
9.6).
---------------------------------------------------------------------------
In further considering the potential implications of epidemiology
studies for alternative standard levels, the PA notes estimates of
total mortality associated with short-term O3
concentrations.\154\
[[Page 75303]]
As discussed above, the PA considers estimates of total risk (i.e.,
based on the full distributions of ambient O3
concentrations) and estimates of risk associated with O3
concentrations in the upper portions of ambient distributions. With
regard to total risk the PA notes that, when summed across urban study
areas, a standard with a level of 60 ppb is estimated to reduce the
number of deaths associated with short-term O3 exposures by
about 15% (2007) and 11% (2009), compared to the current standard (U.S.
EPA, 2014c, Figure 4-13).\155\ For area-wide concentrations at or above
40 ppb, a standard with a level set at 60 ppb is estimated to reduce
O3-associated deaths by almost 60% compared to the current
standard. For area-wide concentrations at or above 60 ppb, a standard
level of 60 ppb is estimated to reduce O3-associated deaths
by over 95% compared to the current standard.
---------------------------------------------------------------------------
\154\ As discussed above, compared to the weight given to the
evidence and to HREA estimates of exposures of concern and lung
function risks, we place relatively less weight on epidemiologic-
based risk estimates.
\155\ A standard with a level of 60 ppb is also estimated to
reduce respiratory mortality associated with long-term O3
concentrations in urban study areas. However, given uncertainties
associated with these risk estimates, as discussed above, the PA
gives them limited weight.
---------------------------------------------------------------------------
In summary, compared to a standard with a level of 65 or 70 ppb,
the PA concludes that a revised standard with a level of 60 ppb would
be expected to further reduce O3 exposures and health risks.
In particular, a standard with a level of 60 ppb is estimated to (1)
reduce the occurrence of exposures of concern by about 95 to 100%,
compared to the current standard, almost eliminating exposures at or
above 60 ppb; (2) reduce the occurrence of FEV1 decrements
>=10, 15, and 20% by about 45 to 85%, compared to the current standard;
(3) more effectively maintain short- and long-term O3
concentrations below those present in the epidemiologic studies that
reported significant O3 health effect associations in
locations likely to have met the current standard; \156\ and (4)
further reduce the risk of O3-associated mortality and
morbidity, particularly the risk associated with the upper portion of
the distribution of ambient O3 concentrations.
---------------------------------------------------------------------------
\156\ As discussed above, these studies do not provide
information on the extent to which O3 health effect
associations would persist following reductions in ambient
O3 concentrations in order to meet a standard with a
level of 60 ppb.
---------------------------------------------------------------------------
c. CASAC Advice
The PA recognizes that decisions regarding the weight to place on
various types of evidence, exposure/risk information, and associated
uncertainties reflect public health policy judgments that are
ultimately left to the Administrator. To help inform those judgments
with regard to the range of alternative primary O3 standard
levels appropriate for consideration, CASAC has provided advice to the
Administrator based on their reviews of draft versions of the
O3 ISA, HREA, and PA. This section summarizes the advice
provided by CASAC regarding alternative standard levels, as well as the
views expressed at the CASAC meetings by public commenters. This
section includes CASAC advice from the reconsideration of the 2008
final decision on the level of the standard, as well as CASAC advice
received during the current review as it pertains to alternative
standards.
Consistent with its advice in 2008, CASAC reiterated during the
reconsideration its support for an 8-hour primary O3
standard with a level ranging from 60 to 70 ppb, combined with the
current indicator, averaging time, and form. Specifically, in response
to the EPA's solicitation of CASAC advice during the reconsideration,
the CASAC letter (Samet, 2010) to the Administrator stated:
CASAC fully supports EPA's proposed range of 0.060-0.070 parts per
million (ppm) for the 8-hour primary ozone standard. CASAC considers
this range to be justified by the scientific evidence as presented
in the Air Quality Criteria for Ozone and Related Photochemical
Oxidants (March 2006) and Review of the National Ambient Air Quality
Standards for Ozone: Policy Assessment of Scientific and Technical
Information, OAQPS Staff Paper (July 2007).
Similarly, in response to the EPA's request for additional advice
on the reconsideration in 2011, CASAC reaffirmed its conclusion that
``the evidence from controlled human and epidemiologic studies strongly
supports the selection of a new primary ozone standard within the 60-70
ppb range for an 8-hour averaging time'' (Samet, 2011). CASAC further
concluded that this range ``would provide little margin of safety at
its upper end'' (Samet, 2011, p. 2).
In the current review of the Second Draft PA, CASAC concurred with
staff's conclusions that it is appropriate to consider retaining the
current indicator (O3), averaging time (8-hour average) and
form (3-year average of the 4th highest maximum daily 8-hour average.
With regard to level, CASAC stated the following (Frey, 2014c, pp. ii
to iii):
The CASAC further concludes that there is adequate scientific
evidence to recommend a range of levels for a revised primary ozone
standard from 70 ppb to 60 ppb. The CASAC reached this conclusion
based on the scientific evidence from clinical studies,
epidemiologic studies, and animal toxicology studies, as summarized
in the Integrated Science Assessment (ISA), the findings from the
exposure and risk assessments as summarized in the HREA, and the
interpretation of the implications of these sources of information
as given in the Second Draft PA.
The CASAC acknowledges that the choice of a level within the
range recommended based on scientific evidence [i.e., 70 to 60 ppb]
is a policy judgment under the statutory mandate of the Clean Air
Act. The CASAC advises that, based on the scientific evidence, a
level of 70 ppb provides little margin of safety for the protection
of public health, particularly for sensitive subpopulations.
Thus, our policy advice is to set the level of the standard
lower than 70 ppb within a range down to 60 ppb, taking into account
your judgment regarding the desired margin of safety to protect
public health, and taking into account that lower levels will
provide incrementally greater margins of safety.
The public commenters who expressed the view that the current
primary O3 standard is not adequate (II.D.3) also submitted
comments that supported revising the level of the primary O3
standard. Several of these commenters expressed the view that the level
should be revised to the lower end of the range of 70 to 60 ppb, or in
some cases to a level below 60 ppb. These commenters often placed a
large amount of emphasis on evidence from controlled human exposure
studies for respiratory effects following exposures to 60 ppb
O3.
In addition, as discussed above (II.D.3), some public commenters
expressed the view that revision of the current standard is not
necessary. Consistent with their view that it would not be appropriate
to revise the current standard, these commenters did not provide any
provisional views on alternative levels below 75 ppb that would be
appropriate for consideration.
d. Administrator's Proposed Conclusions on Level
This section discusses the Administrator's proposed conclusions on
the level of the primary O3 standard. In conjunction with
her proposed decisions to retain the current indicator, averaging time,
and form (II.E.1 to II.E.3, above), the Administrator proposes to
revise the level of the primary O3 standard to within the
range of 65 to 70 ppb. In doing so, she is mindful that the selection
of a primary O3 standard that is requisite to protect public
health with an adequate margin of safety requires judgments based on an
interpretation of the scientific evidence and exposure/risk information
that neither overstates nor understates the strengths and limitations
of that evidence and information, nor the appropriate
[[Page 75304]]
inferences to be drawn therefrom.\157\ The rationale supporting the
Administrator's proposed conclusions on alternative standard levels is
discussed below.
---------------------------------------------------------------------------
\157\ As discussed above (I.B), in addressing the requirement
for an adequate margin of safety the EPA considers such factors as
the nature and severity of the health effects, the size of sensitive
population(s) at risk, and the kind and degree of the uncertainties
that must be addressed. The selection of any particular approach for
providing an adequate margin of safety is a policy choice left
specifically to the Administrator's judgment. See Lead Industries
Association v. EPA, 647 F. 2d at 1161-62; State of Mississippi, 744
F. 3d at 1353.
---------------------------------------------------------------------------
The Administrator's proposed conclusions on alternative standard
levels build upon her proposed conclusion that the overall body of
scientific evidence and exposure/risk information call into question
the adequacy of public health protection afforded by the current
primary O3 standard, particularly for at-risk populations
and lifestages (II.D.5). These proposed conclusions are based on
consideration of the scientific evidence assessed in the ISA (U.S. EPA,
2013a); the results of the exposure and risk assessments in the HREA
(U.S. EPA, 2014a); the evidence-based and exposure-/risk-based
considerations and conclusions in the PA (U.S. EPA, 2014c); CASAC
advice and recommendations, as reflected in CASAC's letters to the
Administrator and in public discussions of drafts of the ISA, HREA, and
PA; and public input received during the development of these
documents.
In reaching proposed conclusions on alternative levels for the
primary O3 standard, the Administrator considers the extent
to which various alternatives would be expected to protect the public,
including at-risk populations, against the wide range of adverse health
effects that have been linked with short- or long-term O3
exposures. At-risk populations include people with asthma; children and
older adults; people who are active outdoors, including outdoor
workers; people with certain genetic variants; and people with reduced
intake of certain nutrients.
As was the case for her consideration of the adequacy of the
current primary O3 standard (II.D.5), the Administrator
places the greatest weight on the results of controlled human exposure
studies and on exposure and risk analyses based on information from
these studies. In doing so, she notes that controlled human exposure
studies provide the most certain evidence indicating the occurrence of
health effects in humans following exposures to specific O3
concentrations. The effects reported in these studies are due solely to
O3 exposures, and interpretation of study results is not
complicated by the presence of co-occurring pollutants or pollutant
mixtures (as is the case in epidemiologic studies). She further notes
the CASAC judgment that ``the scientific evidence supporting the
finding that the current standard is inadequate to protect public
health is strongest based on the controlled human exposure studies of
respiratory effects'' (Frey, 2014c, p. 5). Consistent with this
emphasis, the HREA conclusions reflect relatively greater confidence in
the results of the exposure and risk analyses based on information from
controlled human exposure studies (i.e., exposures of concern and risk
of lung function decrements) than the results of epidemiology-based
risk analyses, given the greater uncertainties in the epidemiology-
based risk estimates (U.S. EPA, 2014a, section 9.6). For all of these
reasons, the Administrator has the most confidence in using the
information from controlled human exposure studies to reach proposed
conclusions on alternative standard levels.
In considering the evidence from controlled human exposure studies,
the Administrator first notes that these studies have reported a
variety of respiratory effects in healthy adults following exposures to
O3 concentrations of 60,\158\ 72,\159\ or 80 ppb, and
higher. The largest respiratory effects, and the broadest range of
effects, have been studied and reported following exposures of healthy
adults to 80 ppb O3 or higher, with most exposure studies
conducted at these higher concentrations. Exposures of healthy adults
to O3 concentrations of 80 ppb or higher have been reported
to decrease lung function, increase airway inflammation, increase
respiratory symptoms, result in airway hyperresponsiveness, and
decrease lung host defenses (II.B.2).
---------------------------------------------------------------------------
\158\ As discussed above (II.B.2), exposures to 60 ppb
O3 have been evaluated in studies by Adams (2002, 2006),
Schelegle et al. (2009), and Kim et al. (2011). In the study by
Schelegle, for the 60 ppb target exposure concentration, study
authors reported that the actual mean exposure concentration was 63
ppb.
\159\ As noted above, for the 70 ppb target exposure
concentration, Schelegle et al. (2009) reported that the actual mean
exposure concentration was 72 ppb.
---------------------------------------------------------------------------
The Administrator notes that O3 exposure concentrations
as low as 72 ppb have been shown to both decrease lung function and
increase respiratory symptoms (Schelegle et al., 2009), a combination
that meets the ATS criteria for an adverse response. In considering
effects at 72 ppb, CASAC likewise noted that ``the combination of
decrements in FEV1 together with the statistically
significant alterations in symptoms in human subjects exposed to 72 ppb
ozone meets the American Thoracic Society's definition of an adverse
health effect'' (Frey, 2014c, p. 5).
With regard to lower exposure concentrations, the Administrator
notes that the combination of statistically significant increases in
respiratory symptoms and decrements in lung function has not been
reported. More specifically, she notes that respiratory symptoms have
been evaluated following 6.6-hour exposures to average O3
concentrations of 60 ppb (Adams, 2006; Kim et al., 2011) and 63 ppb
(Schelegle et al., 2009) and that none of these studies reported a
statistically significant increase in respiratory symptoms, compared to
filtered air controls.\160\
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\160\ However, following exposures to 60 ppb O3,
several studies have observed decreases in lung function and one
study (Kim et al., 2011) observed an increase in airway inflammation
(II.B.2).
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Based on this evidence, the Administrator reaches the initial
conclusion that the results of controlled human exposure studies
strongly support setting the level of a revised O3 standard
no higher than 70 ppb. In reaching this initial conclusion, the
Administrator places a large amount of weight on the importance of
setting the level of the standard well below 80 ppb, the O3
exposure concentration shown in healthy adults to result in the
broadest range of respiratory effects, and below 72 ppb, the lowest
O3 exposure concentration shown in healthy adults to result
in the adverse combination of respiratory symptoms and lung function
decrements.
In further considering the potential public health implications of
a standard with a level of 70 ppb, the Administrator also considers the
extent to which such a standard would be expected to limit population
exposures to the broader range of O3 concentrations reported
in controlled human exposure studies to cause respiratory effects.
Given the range of effects reported following exposures to 80 ppb
O3, and the evidence for the adverse combination of lung
function decrements and respiratory symptoms in healthy adults
following exposures as low as 72 ppb, the Administrator concludes that
the evidence in this review supports the occurrence of adverse
respiratory effects for exposures to O3 concentrations at or
above 72 ppb.
The Administrator has decreasing confidence that adverse effects
will occur following exposures to O3 concentrations below 72
ppb. In particular, compared to O3 exposure
[[Page 75305]]
concentrations at or above 72 ppb, she has less confidence that adverse
effects will occur following exposures to O3 concentrations
as low as 60 ppb. In reaching this conclusion, she notes that, as
discussed above, statistically significant increases in respiratory
symptoms, combined with lung function decrements, have not been
reported following exposures to 60 or 63 ppb O3, though
several studies have evaluated the potential for such effects.
Although she has decreasing confidence in the occurrence of adverse
effects following exposures to O3 concentrations below 72
ppb, the Administrator notes the CASAC judgment that the adverse
combination of lung function decrements and respiratory symptoms
``almost certainly occur in some people'' following exposures to lower
concentrations (Frey, 2014c, p. 6). In particular, when commenting on
the extent to which the study by Schelegle et al. (2009) suggests the
potential for adverse effects following O3 exposures below
72 ppb, CASAC judged that:
[I]f subjects had been exposed to ozone using the 8-hour averaging
period used in the standard [i.e., rather than the 6.6 hour
exposures evaluated in the study], adverse effects could have
occurred at lower concentration. Further, in our judgment, the level
at which adverse effects might be observed would likely be lower for
more sensitive subgroups, such as those with asthma [i.e., compared
to the healthy adults evaluated in the study] (Frey, 2014c, p. 5).
Though CASAC did not provide advice as to how far below 72 ppb adverse
effects would likely occur, the Administrator agrees that such effects
could occur following exposures at least somewhat below 72 ppb.
Based on the evidence and CASAC advice noted above, when
considering the extent to which a standard with a level of 70 ppb would
be expected to limit population exposures to the broader range of
O3 concentrations shown to cause respiratory effects, the
Administrator considers the extent to which such a standard would be
expected to limit the occurrence of O3 exposures of concern
at or above 60, 70, and 80 ppb.\161\ In doing so, she notes that an
O3 standard established at a particular level can provide
protection against a range of exposure concentrations, including
concentrations below the standard level. This is because the degree of
protection provided by any NAAQS is due to the combination of all of
the elements of the standard (i.e., indicator, averaging time, form,
level). In the case of the 4th maximum form of the O3 NAAQS,
which the Administrator is proposing to retain in the current review
(II.E.3), the large majority of days in areas that meet the standard
will have 8-hour O3 concentrations below the level of the
standard.
---------------------------------------------------------------------------
\161\ As with her consideration of the current standard
(II.D.5), the Administrator focuses on estimated exposures of
concern in children, including asthmatic children, noting the HREA
analyses indicating that exposures of concern occur in a larger
percentage of children than adults (given that a larger percentage
of children are estimated to spend an extended period of time being
physically active outdoors when O3 concentrations are
elevated) (II.C.2). To the extent alternative standards provide an
appropriate degree of protection for children, she judges that those
standards will also protect adult populations (including at-risk
adult populations).
---------------------------------------------------------------------------
In considering exposures of concern at or above 60, 70, and 80 ppb,
the Administrator judges that the evidence supporting the occurrence of
adverse respiratory effects is strongest for exposures at or above the
70 and 80 ppb benchmarks. While the Administrator has less confidence
that adverse effects will occur following exposures to O3
concentrations as low as 60 ppb, she notes the possibility for adverse
effects following such exposures given that (1) CASAC has indicated the
moderate lung function decrements (i.e., FEV1 decrements
>=10%) that occur in some healthy adults following exposures to 60 ppb
O3, which are large enough to be judged an abnormal response
by ATS, could be adverse to people with lung disease (II.B.3), and that
(2) airway inflammation has been reported following exposures as low as
60 ppb O3. She also takes note of CASAC advice that the
occurrence of exposures of concern at or above 60 ppb is an appropriate
consideration for people (including children) with asthma (Frey, 2014c,
p. 6).
Due to interindividual variability in responsiveness, the
Administrator further notes that not every occurrence of an exposure of
concern will result in an adverse effect.\162\ Repeated occurrences of
some of the effects demonstrated following exposures of concern could
increase the likelihood of adversity. For example, as discussed in the
ISA (U.S. EPA, 2013a, Section 6.2.3), repeated occurrences of airway
inflammation could lead to the induction of a chronic inflammatory
state; altered pulmonary structure and function, leading to diseases
such as asthma; altered lung host defense response to inhaled
microorganisms, particularly in potentially at-risk populations such as
the very young and old; and altered lung response to other agents such
as allergens or toxins. The Administrator notes that the types of lung
injury that can occur following exposures of concern, particularly if
experienced repeatedly, provide a plausible mode of action by which
O3 may cause other more serious effects. Therefore, the
Administrator is most concerned about protecting at-risk populations
against repeated occurrences of exposures of concern.
---------------------------------------------------------------------------
\162\ For most of the effects demonstrated in controlled human
exposure studies (e.g., airway inflammation, AHR, decreased lung
host defense, respiratory symptoms) the available data are not
sufficient to quantify the number of people who would experience
adverse effects due to O3 exposures.
---------------------------------------------------------------------------
Based on the above considerations, the Administrator focuses on the
extent to which a revised standard would be expected to protect
populations from experiencing two or more O3 exposures of
concern (i.e., as a surrogate for repeated exposures). While she
emphasizes the importance of limiting two or more exposures and
reducing their occurrence, compared to the current standard, she
balances this emphasis by noting that (1) not all exposures of concern
will result in adverse effects; (2) she has less confidence in the
occurrence of adverse effects at the 60 ppb benchmark than at the 70 or
80 ppb benchmarks; and (3) the NAAQS are not meant to be zero-risk
standards.\163\ Therefore, in using estimates of exposures of concern
to inform her decisions on alternative standard levels, the
Administrator judges that it would not be appropriate to set a standard
intended to eliminate all exposures of concern for all benchmarks,
particularly the 60 ppb benchmark. Her consideration of specific
estimates of exposures of concern is discussed below.
---------------------------------------------------------------------------
\163\ State of Mississippi, 744 F. 3d at 1343.
---------------------------------------------------------------------------
As illustrated in Table 1 (above), the Administrator notes that, in
urban study areas, a revised standard with a level of 70 ppb would be
expected to eliminate the occurrence of two or more exposures of
concern to O3 concentrations at and above 80 ppb and to
virtually eliminate the occurrence of two or more exposures of concern
to O3 concentrations at and above 70 ppb, even in the worst-
case urban study area and year. For the 70 ppb benchmark, this reflects
about a 95% reduction in the occurrence of two or more exposures of
concern, compared to the current standard (Table 4).
Though the Administrator acknowledges greater uncertainty with
regard to the occurrence of adverse effects following exposures of
concern at or above 60 ppb, she notes that a revised standard with a
level of 70 ppb would also be expected to protect the large majority of
children in the urban study areas (i.e., about 96% to more
[[Page 75306]]
than 99% of children in individual urban study areas) from experiencing
two or more exposures of concern at or above 60 ppb. Compared to the
current standard, this represents a reduction of more than 60% in the
occurrence of two or more exposures of concern (Tables 1 and 4).
Based on the above information, the Administrator concludes that a
revised O3 standard with a level of 70 ppb would be expected
to virtually eliminate the occurrence of two or more O3
exposures of concern for the 70 and 80 ppb benchmarks, and to
substantially reduce the occurrence of two or more O3
exposures of concern for the 60 ppb benchmark, compared to the current
standard.
Although the Administrator is less concerned about single
occurrences of exposures of concern, she acknowledges that even single
exposures to O3 concentrations at or above benchmark
concentrations (particularly for the 70 and 80 ppb benchmarks) could
potentially result in adverse effects. To the extent this may be the
case, the Administrator notes that a standard with a level of 70 ppb
would also be expected to (1) virtually eliminate all occurrences of
exposures of concern at or above 80 ppb, even in the worst-case year
and location and (2) achieve important reductions, compared to the
current standard, in the occurrence of one or more exposures of concern
at or above 70 and 60 ppb (i.e., about a 70% reduction for the 70 ppb
benchmark and almost a 50% reduction for the 60 ppb benchmark) (Tables
1 and 4).
In further evaluating the potential public health impacts of a
standard with a level of 70 ppb, the Administrator also considers the
HREA estimates of O3-induced lung function decrements. To
inform her consideration of these decrements, the Administrator takes
note of CASAC advice that ``estimation of FEV1 decrements of
>=15% is appropriate as a scientifically relevant surrogate for adverse
health outcomes in active healthy adults, whereas an FEV1
decrement of >=10% is a scientifically relevant surrogate for adverse
health outcomes for people with asthma and lung disease'' (Frey, 2014c,
p. 3). Consistent with this advice, she considers estimates of the
occurrence of O3-induced FEV1 decrements >=10 and
15% as surrogates for the occurrence of adverse health outcomes.
While these surrogates provide perspective on the potential for the
occurrence of adverse respiratory effects following O3
exposures, the Administrator agrees with the conclusion in past reviews
that a more general consensus view of the adversity of moderate
responses emerges as the frequency of occurrence increases (61 FR
65722-3) (Dec. 13, 1996). Specifically, she concludes that not every
estimated occurrence of an O3-induced FEV1
decrement will be adverse and that repeated occurrences of moderate
responses, even in otherwise healthy individuals, may be considered to
be adverse since they could set the stage for more serious illness.
Therefore, the Administrator becomes increasingly concerned about the
potential for adversity as the frequency of occurrences increases and,
as a result, she focuses primarily on estimates of two or more
O3-induced FEV1 decrements (i.e., as a surrogate
for repeated exposures).
Given the above considerations, the Administrator does not believe
it would be appropriate to set a standard that is intended to eliminate
all O3-induced FEV1 decrements. She notes that
this is consistent with CASAC advice, which did not include a
recommendation to set the standard level low enough to eliminate all
O3-induced FEV1 decrements >=10 or 15% (Frey,
2014c). Rather, the Administrator considers the extent to which a
standard with a level of 70 ppb would be expected to protect the
population from experiencing O3-induced FEV1
decrements >=10 and 15%, including the extent to which such a standard
would be expected to achieve reductions in the occurrence of
O3-induced FEV1 decrements, relative to the
current standard.\164\
---------------------------------------------------------------------------
\164\ The Administrator additionally notes that, unlike
exposures of concern, the variability in lung function risk
estimates across urban study areas is often greater than the
differences in risk estimates between various standard levels (Table
2, above). Given this, and the resulting considerable overlap
between the ranges of lung function risk estimates for different
standard levels, although the Administrator has confidence in the
lung function risk estimates themselves, she views them as providing
a more limited basis than exposures of concern for distinguishing
between the degree of public health protection provided by
alternative standard levels.
---------------------------------------------------------------------------
The Administrator notes that a revised O3 standard with
a level of 70 ppb is estimated to protect about 98 to 99% of children
in urban study areas from experiencing two or more O3-
induced FEV1 decrements >=15%, and about 89 to 94% from
experiencing two or more decrements >=10%.\165\ Compared to the current
standard, these estimates represent decreases in the occurrence of two
or more O3-induced decrements of about 29 and 17%,
respectively (Tables 2 and 5). Although the Administrator is less
concerned about the public health implications of single O3-
induced lung function decrements, she also gives some consideration to
estimates of one or more O3-induced FEV1
decrements. In particular, she notes that a revised standard with a
level of 70 ppb is estimated to reduce the occurrence of one or more
O3-induced decrements, compared to the current standard, by
about 26% (for decrements >=15%) and 15% (for decrements >=10%) (Tables
2 and 5).
---------------------------------------------------------------------------
\165\ In the worst-case year and location, a standard with a
level of 70 ppb is estimated to protect about 97% of children in
urban study areas from experiencing two or more O3-
induced FEV1 decrements >=15%, and about 87% from
experiencing two or more decrements >=10%.
---------------------------------------------------------------------------
Given all of the above information, the Administrator concludes
that a revised standard with a level of 70 ppb would be expected to
provide substantial protection against O3 exposures of
concern (for benchmark concentrations of 60, 70, 80 ppb) and
O3-induced lung function decrements, and would be expected
to result in important reductions in the occurrence of such exposures
and decrements, compared to the current standard. This is particularly
the case for estimates of two or more occurrences of exposures of
concern and lung function decrements.
In next considering the additional protection that would be
expected from standard levels below 70 ppb, the Administrator evaluates
the extent to which a standard with a level of 65 ppb would be expected
to further limit O3 exposures of concern and O3-
induced lung function decrements.
In addition to eliminating almost all exposures of concern to
O3 concentrations at or above 80 and 70 ppb, even in the
worst-case years and locations, the Administrator notes that a revised
standard with a level of 65 ppb would be expected to protect more than
99% of children in urban study areas (and 100% of children in some
urban study areas) from experiencing two or more exposures of concern
at or above 60 ppb. Compared to the current standard, this represents
about a 95% reduction in the occurrence of two or more exposures of
concern for the 60 ppb benchmark (Tables 1 and 4). In addition, the
Administrator notes that a revised standard with a level of 65 ppb is
estimated to reduce the occurrence of one or more exposures of concern
for the 60 ppb benchmark by about 80%, compared to the current standard
(Tables 1 and 4).
With regard to O3-induced lung function decrements, the
Administrator notes that an O3 standard with a level of 65
ppb is estimated to protect about 98% to more than 99% of children from
experiencing two or more O3-induced FEV1
decrements >=15%, even considering the worst-case year and location,
and about 91 to 99% from
[[Page 75307]]
experiencing two or more decrements >=10% (89% in worst-case year and
location). These estimates reflect reductions, compared to the current
standard, of about 54 and 37%, respectively. A revised standard with a
level of 65 ppb is also estimated to reduce the occurrence of one or
more lung function decrements >=15 and 10%, compared to the current
standard, by about 50 and 31%, respectively.
Taken together, the Administrator initially concludes that the
evidence from controlled human exposure studies, and the information
from quantitative analyses that draw upon these studies (i.e.,
exposures of concern, O3-induced FEV1
decrements), provide strong support for standard levels from 65 to 70
ppb. In particular, she bases this conclusion on the fact that such
standard levels would be well below the O3 exposure
concentration shown to result in the widest range of respiratory
effects (i.e., 80 ppb), and below the lowest O3 exposure
concentration shown to result in the adverse combination of lung
function decrements and respiratory symptoms (i.e., 72 ppb). A standard
with a level from 65 to 70 ppb would also be expected to result in
important reductions, compared to the current standard, in the
occurrence of O3 exposures of concern for all of the
benchmarks evaluated (i.e., 60, 70, and 80 ppb) and in the risk of
O3-induced lung function decrements >=10 and 15%.
In further considering the evidence and exposure/risk information,
the Administrator considers the extent to which the epidemiologic
evidence, and the quantitative risk estimates based on information from
epidemiologic studies, also provide support for standard levels from 65
to 70 ppb. In doing so, as in her consideration of the adequacy of the
current O3 standard, the Administrator focuses on
epidemiologic studies of respiratory-related hospital admissions,
emergency department visits, and mortality. These considerations are
discussed below.
The Administrator first considers the extent to which available
epidemiologic studies have reported associations between short-term
O3 concentrations and emergency department visits, hospital
admissions, and/or mortality in locations that would likely have met
alternative standards with levels from 65 to 70 ppb (U.S. EPA, 2014c,
section 4.4.1). In evaluating the epidemiologic evidence in this way,
the Administrator places the most weight on single-city studies of
short-term O3 concentrations, recognizing that there were no
multicity studies for which air quality data indicated that all cities
included in the analyses would likely have met alternative standard
levels. In particular, she notes that while single-city studies are
more limited than multicity studies in terms of statistical power and
geographic coverage, conclusions linking air quality in a given city
with health effect associations in that same city can be made with
greater certainty for single-city studies of short-term O3,
compared to health effect associations aggregated across multiple
cities in multicity studies. In particular, the Administrator notes
considerable uncertainty in linking multicity effect estimates
(aggregated across multiple cities) for short-term O3 with
the air quality for subsets of study locations (rather than all
locations) likely to have met an alternative standard.\166\
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\166\ In recognizing that multicity studies are often emphasized
over single-city studies for purposes of making weight of evidence
judgments (U.S. EPA, 2013a), the Administrator's judgment in this
case applies specifically to interpreting air quality analyses for
epidemiologic studies of short-term O3 concentrations
where multicity effect estimates are aggregated across cities, and
where individual city effect estimates are not presented (as is the
case for the key O3 studies analyzed in the PA, with the
exception of the study by Stieb et al. (2009) where none of the
city-specific effect estimates for asthma emergency department
visits were statistically significant). Because reported multicity
effect estimates do not allow health effect associations to be
disaggregated by individual city, it is not possible to assign the
multicity health effect association to the air quality in any one
study location, or to the air quality in a particular subset of
locations. In contrast, for epidemiologic studies of long-term
concentrations, where multicity effect estimates are based on
comparisons across cities, different judgments have been made by EPA
with regard to the utility of multicity studies (see, e.g. 78 FR
3086 at 3103/2, January 15, 2013) (and see discussion below of study
by Jerrett et al., 2009).
---------------------------------------------------------------------------
Given the above, the Administrator notes analyses in the PA (U.S.
EPA, 2014c, section 4.4.1) indicating that a revised standard with a
level of 65 or 70 ppb would be expected to maintain short-term ambient
O3 concentrations below those present in the locations of
all of the single-city studies analyzed. As discussed in the PA (U.S.
EPA, 2014c, section 4.4.1), this includes several single-city studies
conducted in locations that would have violated the current standard,
and the single-city study by Mar and Koenig (2009) that reported
positive and statistically significant associations with respiratory
emergency department visits with children and adults in a location that
likely would have met the current standard over the entire study period
but that would likely not have met a revised standard with a level of
70 ppb or below. Thus, the Administrator notes that, while the current
standard would allow the ambient O3 concentrations that
provided the basis for the health effect associations reported by Mar
and Koenig (2009), a revised O3 standard with a level at or
below 70 ppb would require reductions in those ambient O3
concentrations. While the Administrator acknowledges uncertainty in the
extent to which the reported O3-associated emergency
department visits could be further reduced by standard levels below 65
or 70 ppb, she concludes that this analysis indicates that a revised
standard with a level at least as low as 70 ppb would result in
improvements in public health, beyond the protection provided by the
current standard, in the locations of the single-city epidemiologic
studies that reported significant health effect associations.
As discussed above, the Administrator notes the greater uncertainty
in interpreting air quality in locations of multicity epidemiologic
studies of short-term O3 for the purpose of evaluating
alternative standard levels (II.D.1 and U.S. EPA, 2014c, section
4.4.1). Therefore, she places less weight on these studies than on the
single-city studies noted above. Despite this uncertainty, she notes
that PA analyses suggest that standard levels of 65 or 70 ppb would
require additional reductions, beyond those required by the current
standard, in ambient O3 concentrations in several of the
epidemiologic study locations that provided the basis for statistically
significant O3 health effect associations. For example, she
notes that Dales et al. (2006) reported significant associations with
respiratory hospital admissions based on air quality in 11 Canadian
cities, most of which would likely have met the current standard over
the entire study period (i.e., seven cities) but would have violated a
standard with a level of 70 ppb or below over at least part of that
period (U.S. EPA, 2014c, Table 4-1). She further notes that Katsouyanni
et al. (2009) reported statistically significant associations with
mortality based on air quality in 12 Canadian cities, most of which
would likely have met the current standard (i.e., eight study cities)
and a standard with a level of 70 ppb (i.e., seven study cities) over
the entire study period, but would have violated a standard with a
level of 65 ppb over at least part of that period (U.S. EPA, 2014c,
Table 4-1). While most of the other multicity epidemiologic studies
evaluated also suggest that a level from 65 to 70 ppb would result in
public health improvements, compared to the current standard, the
Administrator acknowledges that several multicity epidemiologic studies
reported O3 health effect associations when the majority of
study cities would likely
[[Page 75308]]
have met standards with levels from 65 to 70 ppb. However, given the
important uncertainties in interpreting the air quality in these
multicity studies, the Administrator places limited weight on them
overall, relative to the single-city studies noted above (and relative
to the information based on controlled human exposure studies).
With regard to long-term O3 concentrations, the
Administrator considers the long-term O3 metrics reported to
be associated with mortality or morbidity in recent epidemiologic
studies (e.g., seasonal averages of 1-hour or 8-hour daily max
concentrations). Compared to the current standard, she notes that
analyses in the PA (U.S. EPA, 2014c, section 4.4.1) suggest a revised
standard with a level of 65 or 70 ppb would more effectively maintain
long-term O3 concentrations below those where the multicity
study by Jerrett et al. (2009) indicates the most confidence in the
reported association with respiratory mortality (II.B.2, II.D.1). Based
on additional information from the study by Jerrett et al. (2009), the
Administrator also notes HREA analyses indicating that a revised
standard with a level of 65 or 70 ppb would be expected to reduce the
risk of respiratory mortality associated with long-term O3
concentrations (though she also notes important uncertainties with
these risk estimates, as described below). Finally, she notes analyses
in the HREA suggesting that a revised standard with a level of 65 or 70
ppb would be expected to reduce long-term O3 concentrations,
defined in terms of O3 metrics similar to the long-term
metrics that have been reported in recent epidemiologic studies to be
associated with respiratory morbidity (i.e., seasonal averages of daily
maximum 8-hour concentrations). Given the above evidence and
information, the Administrator concludes that a revised 8-hour standard
with a level from 70 to 65 ppb could increase public health protection,
compared to the current standard, against effects associated with long-
term O3 exposures.
In further evaluating information from epidemiologic studies, the
Administrator also considers the HREA's epidemiology-based risk
estimates of morbidity and mortality associated with short-term
O3 (U.S. EPA, 2014a). Compared to the weight given to the
evidence from controlled human exposure studies, and to HREA estimates
of exposures of concern and lung function risks, she places relatively
less weight on epidemiology-based risk estimates. In doing so, she
notes that the overall conclusions from the HREA likewise reflect
relatively less confidence in estimates of epidemiology-based risks
than in estimates of exposures of concern and lung function risks. As
discussed above (II.C.3.b), this is based on the greater uncertainties
associated with mortality and morbidity risk estimates, including the
heterogeneity in effect estimates between locations, the potential for
exposure measurement errors, and uncertainty in the interpretation of
the shape of concentration-response functions at lower O3
concentrations. The Administrator further notes the HREA conclusion
that lower confidence should be placed in the results of the assessment
of respiratory mortality risks associated with long-term O3
exposures, primarily because that analysis is based on only one study
(even though that study is well-designed) and because of the
uncertainty in that study regarding the existence and identification of
a potential threshold in the concentration-response function (U.S. EPA,
2014a, section 9.6).
In considering epidemiology-based risk estimates, the Administrator
focuses on the extent to which potential alternative O3
standards are estimated to reduce the risk of mortality associated with
short-term exposures to O3, noting the similar patterns of
risk across urban study areas and air quality scenarios for respiratory
morbidity endpoints (II.C.3). Given the uncertainties in epidemiology-
based risk estimates, the Administrator focuses on the general
magnitudes of risk changes estimated for standard levels of 65 and 70
ppb, compared to the current standard, rather than placing a large
amount of weight on the absolute estimates of O3-associated
deaths. In doing so, she notes the CASAC conclusion that ``[a]lthough
the estimates for short-term exposure impacts are subject to
uncertainty, the data supports a conclusion that there are meaningful
reductions in mean premature mortality associated with ozone levels
lower than the current standard'' (Frey, 2014a, p. 10). She further
notes that, as discussed above (II.C.3.b), the HREA risk estimates for
urban study areas are likely to understate the average reductions in
O3-associated mortality and morbidity risks that would be
experienced across the U.S. population as a whole upon meeting
standards with lower levels.
The Administrator's primary focus is on risks associated with
O3 concentrations in the upper portions of ambient
distributions, given the greater uncertainty associated with the shapes
of concentration-response curves for O3 concentrations in
the lower portions of ambient distributions.\167\ The Administrator
further notes that experimental studies provide the strongest evidence
for O3-induced effects following exposures to O3
concentrations corresponding to the upper portions of typical ambient
distributions. In particular, as discussed above, she notes controlled
human exposure studies showing respiratory effects following exposures
to O3 concentrations at or above 60 ppb (II.B).
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\167\ The ISA concludes that there is less certainty in the
shape of concentration-response functions for area-wide
O3 concentrations at the lower ends of warm season
distributions (i.e., below about 20 to 40 ppb) (U.S. EPA, 2013a,
section 2.5.4.4).
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In considering risks associated with O3 concentrations
in the upper portions of ambient distributions, the Administrator
focuses on area-wide O3 concentrations at or above 40 ppb
and 60 ppb. For area-wide O3 concentrations at or above 40
ppb, the Administrator notes that revised standards with levels of 70
or 65 ppb are estimated to reduce the number of premature deaths
associated with short-term O3 concentrations by about 10%
and almost 50%, respectively, compared to the current standard.\168\ In
addition, for area-wide concentrations at or above 60 ppb, revised
standards are estimated to reduce O3-associated premature
deaths by about 50% to 70% for a standard level of 70 ppb, and by more
than 80% for a standard level of 65 ppb.\169\ Risk reductions are
smaller when total risks are considered (II.C.3.b).
---------------------------------------------------------------------------
\168\ For area-wide O3 concentrations at or above 40
ppb, reductions in estimated premature deaths are disproportionately
larger with the 65 ppb standard level than with the 70 ppb standard
level. This results from the larger air quality adjustments required
to meet the 65 ppb level. Across urban study areas, the additional
reductions required to meet 65 ppb result in many fewer days with
area-wide O3 concentrations at or above 40 ppb and,
therefore, many fewer O3-associated deaths for area-wide
concentrations at or above 40 ppb (U.S. EPA, 2014a, Figures 7-2 and
7-3).
\169\ Though only a relatively small number of days in urban
study areas had area-wide O3 concentrations at or above
60 ppb.
---------------------------------------------------------------------------
Given all of the above evidence, exposure/risk information, and
advice from CASAC, the Administrator proposes to revise the level of
the current primary O3 standard to within the range of 65 to
70 ppb. She concludes that a standard with a level from within this
range could reasonably be judged to be requisite to protect public
health with an adequate margin of safety, based on her consideration of
the evidence and information discussed above. In reaching this
conclusion, she particularly notes that a level from anywhere within
this range would be below the lowest O3 exposure
concentration shown to result in the
[[Page 75309]]
adverse combination of respiratory symptoms and lung function
decrements (i.e., 72 ppb), would be expected to maintain ambient
O3 concentrations below those in locations where single-city
studies assessed in the ISA have reported statistically significant
O3 health effect associations, and would be expected to
result in important reductions in O3 exposures and health
risks, compared to the current standard.
The Administrator notes that the determination of what constitutes
an adequate margin of safety is expressly left to the judgment of the
EPA Administrator. She further notes that in evaluating how particular
standards address the requirement to provide an adequate margin of
safety, the Administrator must consider such factors as the nature and
severity of the health effects, the size of sensitive population(s) at
risk, and the kind and degree of the uncertainties that must be
addressed (I.B, above). Consistent with past practice and long-standing
judicial precedent, she takes the need for an adequate margin of safety
into account as an integral part of her decision-making on the
appropriate level, averaging time, form, and indicator of the
standard.\170\
---------------------------------------------------------------------------
\170\ See, e.g., NRDC v. EPA, 902 F. 2d 962, 973-74 (D.C. Cir.
1990).
---------------------------------------------------------------------------
The Administrator notes that the NAAQS are not designed to be zero-
risk or background standards, and that the sizeable risk reductions
that are estimated in the HREA to be associated with standard levels of
65 or 70 ppb represent substantial improvements in public health for
important segments of the population, including at-risk groups such as
children and people with asthma. Although any rationale supporting a
decision to set a specific level within the range of 65 to 70 ppb would
discuss the full body of evidence and information, the Administrator
notes that certain aspects of this evidence and information could be
particularly important in distinguishing between the appropriateness of
a level closer to 65 ppb versus a level closer to 70 ppb.\171\
---------------------------------------------------------------------------
\171\ Although this discussion refers to supporting rationale
for a level of 65 ppb or 70 ppb, the Administrator is proposing the
entire range between 65 and 70 ppb. The Administrator notes that
although neither the PA nor CASAC reached conclusions or provided
advice on a standard set at a specific level between 65 ppb and 70
ppb, there is nothing in either the evidence, exposure/risk
information, or CASAC advice that would preclude such a standard
level.
---------------------------------------------------------------------------
For example, a level at or near 65 ppb could be judged requisite to
protect public health with an adequate margin of safety to the extent
the Administrator places greater weight on the importance of: (1)
Eliminating almost all exposures of concern (even single occurrences)
at or above 70 and 80 ppb, even in worst-case years and locations; (2)
almost eliminating the occurrence of two or more exposures of concern
at or above 60 ppb; (3) achieving additional reductions in
O3-induced FEV1 decrements, beyond those achieved
with a level of 70 ppb (4) maintaining ambient concentrations below
those in locations of single-city studies and more effectively doing so
for multicity studies (i.e., more effectively than 70 ppb); and (5)
achieving substantial reductions, compared to a standard with a level
of 70 ppb, in mortality associated with the upper portion of the
distribution of ambient O3 concentrations, despite
uncertainties in risk estimates.
In contrast, a level at or near 70 ppb could be judged requisite to
protect public health with an adequate margin of safety to the extent
the Administrator places a greater amount of weight (i.e., greater than
for 65 ppb) on the importance of: (1) Almost eliminating the occurrence
of two or more exposures of concern at or above 70 and 80 ppb, even in
the worst-case year and location; (2) substantially reducing, but not
eliminating, the occurrence of two or more exposures of concern at or
above 60 ppb, noting conclusions regarding increasing uncertainty in
adverse effects for the 60 ppb benchmark; (3) reducing, but not
eliminating, the occurrence of one or more exposures of concern, noting
that not all exposures of concern result in adverse effects; (4)
maintaining ambient O3 concentrations below those in
locations of single-city epidemiologic studies, and uncertainties in
analyses of air quality in multicity study locations; and (5)
recognizing uncertainties in epidemiology-based risk estimates.
In considering CASAC advice on the range of standard levels, the
Administrator first notes CASAC's conclusion that there is adequate
scientific evidence to consider a range of levels for a primary
standard that includes an upper end at 70 ppb. For the reasons
discussed above, she agrees with this advice. She also notes that while
CASAC concluded that a standard with a level of 70 ppb ``may not meet
the statutory requirement to protect public health with an adequate
margin of safety'' (Frey, 2014c, p. 8), it further acknowledged that
``the choice of a level within the range recommended based on
scientific evidence is a policy judgment under the statutory mandate of
the Clean Air Act'' (Frey, 2014c, p. ii). While she agrees with CASAC
that it is appropriate to consider levels below 70 ppb, as reflected in
her range of proposed levels from 65 to 70 ppb, for the reasons
discussed above she also concludes that a standard level as high as 70
ppb, which CASAC concluded could be supported by the scientific
evidence, could reasonably be judged to be requisite to protect public
health with an adequate margin of safety.
The Administrator has also considered the appropriateness of
standard levels below 65 ppb. In doing so, she notes the conclusions of
the PA and the advice of CASAC that it would be appropriate for her to
consider standard levels as low as 60 ppb. In particular, she notes
that a decision to set the primary O3 standard level at 60
ppb would place a large amount of weight on the potential public health
importance of virtually eliminating even single occurrences of
exposures of concern at and above 60 ppb, though controlled human
exposure studies have not reported the adverse combination of
respiratory symptoms and decrements in lung function following
exposures to 60 ppb O3; on the potential public health
importance of further reducing the occurrence of O3-induced
lung function decrements >=10 and 15%; on analyses of ambient
O3 concentrations in locations of multicity epidemiologic
studies, despite uncertainties in linking multicity effect estimates
for short-term O3 with air quality in individual study
cities; and on epidemiology-based risk estimates, despite the important
uncertainties in those estimates. However, as discussed more fully
above, given the uncertainties associated with the adversity of
exposures to 60 ppb O3, particularly single occurrence of
such exposures; uncertainties associated with air quality analyses in
locations of multicity epidemiologic studies; and uncertainties in
epidemiology-based risk estimates, particularly uncertainties in the
shape of the concentration-response functions at lower O3
concentrations and uncertainties associated with the heterogeneity in
O3 effect estimates across locations, the Administrator does
not agree that it is appropriate to place significant weight on these
factors or to use them to support the appropriateness of standard
levels below 65 ppb. Compared to O3 standard levels from 65
to 70 ppb, the Administrator concludes that the extent to which
standard levels below 65 ppb could result in further public health
improvements becomes notably less certain. Therefore, she concludes
that it
[[Page 75310]]
is not appropriate to propose standard levels below 65 ppb.\172\
---------------------------------------------------------------------------
\172\ Although, as discussed below, she solicits comment on
standard levels as low as 60 ppb.
---------------------------------------------------------------------------
The Administrator acknowledges that her proposed range of 65 to 70
ppb does not include the lower portion of the range supported by CASAC.
In reaching the conclusion that this is appropriate, she focuses on
CASAC's rationale for levels as low as 60 ppb. In particular, she notes
the following CASAC advice (Frey, 2014c, p. 7):
The CASAC concurs that 60 ppb is an appropriate and justifiable
scientifically based lower bound for a revised primary standard.
This is based upon findings of adverse effects, including clinically
significant lung function decrements and airway inflammation, after
exposures to 60 ppb ozone in healthy adults with moderate exertion
(Adams 2006; Schelegle et al., 2009; Brown et al., 2008; Kim et al.,
2011), with limited evidence of adverse effects below 60 ppb.
In considering this advice, the Administrator notes that CASAC
focused on the importance of limiting exposures to O3
concentrations as low as 60 ppb. As discussed above, the Administrator
agrees with this advice. In particular, she notes that standards within
the proposed range of 65 to 70 ppb would be expected to substantially
limit the occurrence of exposures of concern to O3
concentrations at or above 60 ppb, particularly the occurrence of two
or more exposures.\173\ When she further considers that not all
exposures of concern lead to adverse effects, and that the NAAQS are
not meant to be zero-risk or background standards, the Administrator
judges that alternative standard levels below 65 ppb are not needed to
further reduce such exposures. Therefore, the Administrator's initial
conclusion is that standard levels below 65 ppb would be more than
requisite to protect public health with an adequate margin of safety.
---------------------------------------------------------------------------
\173\ In fact, as noted above (Table 4), a standard with a level
of 70 ppb would be expected to limit multiple occurrences of
exposures of concern at or above the 60 ppb benchmark to as low as
0.5% in urban case study areas (and as low as 0% for a standard with
a level of 65 ppb).
---------------------------------------------------------------------------
In reaching this initial conclusion, the Administrator acknowledges
that alternative approaches to viewing the available scientific
evidence and exposure/risk information, and to viewing the
uncertainties inherent in that evidence and information, could lead one
to reach a different conclusion. In particular, as noted above, she
recognizes that levels as low as 60 ppb could potentially be supported,
to the extent substantial weight is placed on the public health
importance of estimates of one or more occurrences of exposures of
concern at or above 60 ppb and O3-induced lung function
decrements >=10%; analyses of ambient O3 concentrations in
locations of multicity epidemiologic studies; and epidemiology-based
estimates of total risk. This approach would also place a large amount
of weight on the possibility that at-risk groups would experience
adverse effects at lower levels than the benchmarks derived from
clinical studies conducted using healthy adult subjects, despite the
fact that these studies have not reported a statistically significant
increase in respiratory symptoms, combined with lung function
decrements, following exposures to 60 ppb.\174\ Such an approach to
viewing the evidence and exposure/risk information would place very
little weight on the uncertainties in these estimates and analyses. In
some cases, elements of this approach have been supported by public
commenters, leading some commenters to recommend setting the level of
the primary O3 standard at least as low as 60 ppb. In
recognition of such an alternative approach to viewing the evidence and
information, in addition to proposing to set the level of the
O3 standard from 65 to 70 ppb, the Administrator solicits
comment on alternative standard levels below 65 ppb, and as low as 60
ppb. In doing so, the Administrator reiterates that the CAA does not
require the establishment of a primary NAAQS at a zero-risk level or at
background concentration levels, but rather at a level that reduces
risk sufficiently so as to protect public health with an adequate
margin of safety (I.A).
---------------------------------------------------------------------------
\174\ More specifically, as discussed above, respiratory
symptoms have been evaluated following 6.6-hour exposures to average
O3 concentrations of 60 ppb (Adams, 2006; Kim et al.,
2011) and 63 ppb (Schelegle et al., 2009). None of these studies
reported a statistically significant increase in respiratory
symptoms, compared to filtered air controls.
---------------------------------------------------------------------------
F. Proposed Decision on the Primary Standard
For the reasons discussed above, and taking into account
information and assessments presented in the 2013 ISA, 2014 HREA and
integration of this information and assessments into staff conclusions
in the 2014 PA, the advice and recommendations of CASAC, and public
comments received during the development of these documents, the
Administrator proposes to retain the current indicator, averaging time
and form of the primary O3 standard, and to set a new level
for the 8-hour primary O3 standard. Specifically, the
Administrator proposes to set the level of the 8-hour primary
O3 standard to within the range of 65 to 70 ppb. The
proposed 8-hour primary standard would be met at an ambient air
monitoring site when the 3-year average of the annual fourth-highest
daily maximum 8-hour average O3 concentration is less than
or equal to the level of the revised standard that is promulgated.
Thus, the Administrator proposes to set a standard with a level within
this range. For the reasons discussed above, the Administrator also
solicits comment on setting the level of the primary O3
standard below 65 ppb, and as low as 60 ppb.
III. Communication of Public Health Information
Information on the public health implications of ambient
concentrations of criteria pollutants is currently made available
primarily through EPA's Air Quality Index (AQI) program. The AQI has
been in use since its inception in 1999 (64 FR 42530). It provides
accurate, timely, and easily understandable information about daily
levels of pollution (40 CFR 58.50). It is designed to tell individual
members of the public how clean or unhealthy their air is, whether
health effects might be a concern, and, if so, measures individuals can
take to reduce their exposure to air pollution. The AQI focuses on
health effects individuals may experience within a few hours or days
after breathing unhealthy air. The AQI establishes a nationally uniform
system of indexing pollution concentrations for O3, carbon
monoxide, nitrogen dioxide, particulate matter and sulfur dioxide. The
AQI converts pollutant concentrations in a community's air to a number
on a scale from 0 to 500. Reported AQI values enable the public to know
whether air pollution concentrations in a particular location are
characterized as good (0-50), moderate (51-100), unhealthy for
sensitive groups (101-150), unhealthy (151-200), very unhealthy (201-
300), or hazardous (301-500). The AQI index value of 100 typically
corresponds to the level of the short-term NAAQS for each pollutant.
For the O3 NAAQS, an 8-hour average concentration of 75 ppb
corresponds to an AQI value of 100. An AQI value greater than 100 means
that a pollutant is in one of the unhealthy categories (i.e., unhealthy
for sensitive groups, unhealthy, very unhealthy, or hazardous) on a
given day; an AQI value at or below 100 means that a pollutant
concentration is in one of the satisfactory categories (i.e., moderate
or good). An additional consideration in
[[Page 75311]]
selecting breakpoints is for each category to span at least a 15 ppb
range to allow for more accurate forecasting. Decisions about the
pollutant concentrations at which to set the various AQI breakpoints,
that delineate the various AQI categories, draw directly from the
underlying health information that supports the NAAQS review.
The Agency recognizes the importance of revising the AQI in a
timely manner to be consistent with any revisions to the NAAQS.
Therefore EPA is proposing conforming changes to the AQI, in connection
with the Agency's proposed decision on revisions to the O3
NAAQS if revisions to the primary standard are promulgated. These
conforming changes would include setting the 100 level of the AQI at
the same level as the revised primary O3 NAAQS and also
making adjustments based on health information from this NAAQS review
to AQI breakpoints at the lower end of each range (i.e., AQI values of
50, 150, 200 and 300). The EPA does not propose to change the level at
the top of the index (i.e., AQI value of 500) that typically is set
equal to the Significant Harm Level (40 CFR 51.16), which would apply
to state contingency plans.
The EPA is proposing to revise the AQI for O3 by setting
an AQI value of 100 equal to the level of the revised O3
standard (65-70 ppb). The EPA is also proposing to revise the following
breakpoints: An AQI value of 50 to within a range from 49-54 ppb; an
AQI value of 150 to 85 ppb; an AQI value of 200 to 105 ppb, and an AQI
value of 300 to 200 ppb. All these levels are averaged over 8 hours.
The EPA is proposing to set an AQI value of 50, the breakpoint between
the good and moderate categories, at 15 ppb below the value of the
proposed standard, i.e. to within a range from 49 to 54 ppb. The EPA is
taking comment on what level within this range to select, recognizing
that there is no health message for either at-risk or healthy
populations in the good category. Thus, the level selected should be
below the lowest concentration (i.e., 60 ppb) that has been shown in
controlled human exposure studies of healthy adults \175\ to cause
moderate lung function decrements (i.e., FEV1 decrements
>=10%, which could be adverse to people with lung disease), large lung
function decrements (i.e., FEV1 decrements >=20%) in a small
proportion of people, and airway inflammation.\176\ The EPA is
proposing to set an AQI value of 150, the breakpoint between the
unhealthy for sensitive groups and unhealthy categories, at 85 ppb. At
this level, controlled human exposure studies of healthy adults
indicate that up to 25% of exposed people are likely to have moderate
lung function decrements (i.e., 25% have FEV1 decrements
>=10%; 12% have FEV1 decrements >=15%) and up to 7% are
likely to have large lung function decrements (i.e., FEV1
decrements >=20%) (McDonnell et al., 2012; Figure 7). Large lung
function decrements would likely interfere with normal activity for
many healthy people. For people with lung disease, large lung function
decrements would likely interfere with normal activity for most people
and would increase the likelihood that they would seek medical
treatment (72 FR 37850, July 11, 2007). The EPA is proposing to set an
AQI value of 200, the breakpoint between the unhealthy and very
unhealthy categories, at 105 ppb. At this level, controlled human
exposure studies of healthy adults indicate that up to 38% of exposed
people are likely to have moderate lung function decrements (i.e., 38%
have FEV1 decrements >=10%; 22% have FEV1
decrements >=15%) and up to 13% are likely to have large lung function
decrements (i.e., FEV1 decrements >=20%). The EPA is
proposing to set an AQI value of 300, the breakpoint between the very
unhealthy and hazardous categories, at 200 ppb. At this level,
controlled human exposure studies of healthy adults indicate that up to
25% of exposed individuals are likely to have large lung function
decrements (i.e., FEV1 decrements >=20%), which would
interfere with daily activities for many of them. Large lung function
decrements would interfere with daily activities for most people with
lung disease, and likely cause them to seek medical attention.
---------------------------------------------------------------------------
\175\ Effects would likely be greater in people with asthma.
\176\ Exposures to 50 ppb have not been evaluated
experimentally, but are estimated to potentially affect only a small
proportion of healthy adults and with only a half to a third of the
moderate to large lung function decrements observed at 60 ppb
(McDonnell et al., 2012; Figure 7).
Table 6--Proposed AQI Breakpoints
----------------------------------------------------------------------------------------------------------------
Existing
breakpoints Proposed breakpoints (ppb, 8-
AQI category Index values (ppb, 8-hour hour average)
average)
----------------------------------------------------------------------------------------------------------------
Good.......................................... 0-50 0-59 0-(49 to 54).
Moderate...................................... 51-100 60-75 (50 to 55)-(65 to 70).
Unhealthy for Sensitive Groups................ 101-150 76-95 (66 to 71)-85.
Unhealthy..................................... 151-200 96-115 86-105.
Very Unhealthy................................ 201-300 116-374 106-200.
Hazardous..................................... 301-400 375- 201-.
401-500
----------------------------------------------------------------------------------------------------------------
EPA believes that the proposed breakpoints reflect an appropriate
balance between reflecting the health evidence that is the basis for
the proposed primary O3 standard and providing category
ranges that are large enough to be forecasted accurately, so that the
new AQI for O3 can be implemented more easily in the public
forum for which the AQI ultimately exists. However, the EPA recognizes
that some have expressed alternative approaches to viewing the evidence
and information and solicits comment on these proposed revisions to the
AQI.
With respect to reporting requirements (40 CFR part 58, Sec.
58.50), EPA proposes to revise 40 CFR part 58, Sec. 58.50 (c) to
require the AQI reporting requirements to be based on the latest
available census figures, rather than the most recent decennial U.S.
census. This change is consistent with our current practice of using
the latest population figures to make monitoring requirements more
responsive to changes in population.
[[Page 75312]]
IV. Rationale for Proposed Decision on the Secondary Standard
This section presents the rationale for the Administrator's
proposed decisions regarding the need to revise the current secondary
O3 NAAQS and the appropriate revisions to the standard,
including her proposed decisions that the current secondary standard is
not requisite to protect public welfare and should be revised to
provide additional public welfare protection. Based on her
consideration of the full body of welfare effects evidence and related
analyses, the Administrator proposes to conclude that ambient
O3 concentrations in terms of a W126 index value, averaged
across three consecutive years, within the range from 13 ppm-hrs to 17
ppm-hrs would provide the requisite protection against known or
anticipated adverse effects to the public welfare. In considering
policy options for achieving that level of air quality, the
Administrator has further considered the full body of information,
including air quality analyses that relate ambient O3
concentrations in terms of a three-year average W-126-based metric and
in terms of the form and averaging time for the current standard. Based
on this consideration, the Administrator proposes to revise the level
of the current secondary standard to within the range of 0.065 to 0.070
ppm to achieve the appropriate air quality.
As discussed more fully below, this proposal is based on a thorough
review, in the ISA, of the latest scientific information on
O3-induced environmental effects. This proposed decision
also takes into account: (1) Staff assessments in the PA of the most
policy-relevant information in the ISA and WREA analyses of air
quality, exposure, and ecological risks and associated ecosystem
services; (2) CASAC advice and recommendations; and, (3) public
comments received during the development of these documents, either in
connection with CASAC meetings or separately.
This proposed decision draws on the ISA's integrative synthesis of
the entire body of evidence, published through July 2011, on
environmental effects associated with the presence of O3 and
related photochemical oxidants in the ambient air. As summarized in
section IV.B below, this body of evidence addresses the range of
environmental responses associated with exposure to ambient levels of
O3 (U.S. EPA, 2013a, ISA chapters 9-10), and includes more
than four hundred new studies that build on the extensive evidence base
from the last review. This rationale also draws upon the results of
quantitative exposure and risk assessments, summarized in section IV.C
below. Section IV.D presents the Administrator's proposed decisions on
the adequacy of the current secondary standard (section IV.D.3) drawing
on both evidence-based and exposure/risk-based considerations in the PA
(section IV.D.1) and advice from CASAC (section IV.D.2). Proposed
conclusions on alternative standards are summarized in section IV.E.
A. Approach
In evaluating whether it is appropriate to retain or revise the
current secondary O3 standard, the Administrator adopts an
approach in this review that builds upon the general approach used in
the 2008 review \177\ and reflects the broader body of scientific
evidence now available, updated exposure/risk information, advances in
O3 air quality modeling, and air monitoring information.
This review of the standard also considers the July 2013 remand of the
secondary standard by the U.S. Court of Appeals for the D.C. Circuit,
such that the proposed decision described herein incorporates the EPA's
response to this remand.
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\177\ The 2008 revision of the O3 secondary standard,
the proposed reconsideration of the 2008 decision, and the 2013
court decision on the 2008 revision of the secondary standard are
summarized in section I.C above.
---------------------------------------------------------------------------
The Administrator's decisions in the 2008 review were based on an
integration of information on welfare effects associated with exposure
to O3, judgments on the adversity and public welfare
significance of key effects, and judgments as to what standard would be
requisite to protect public welfare. These considerations were informed
by air quality and related analyses, quantitative exposure and risk
assessments, and qualitative assessment of impacts that could not be
quantified. As a result of the 2008 review, the Administrator concluded
the then-current secondary standard did not provide the requisite
public welfare protection and it was revised. The current secondary
standard is 75 ppb based on the annual fourth-highest daily maximum 8-
hour average concentration, averaged over three consecutive years,
which is identical to the current primary standard. In 2008, the
Administrator considered the then-available monitoring data with regard
to relationships between the revised primary standard and degree of
protection of public welfare from cumulative seasonal O3
exposures, expressed in terms of the W126 exposure index (described in
section IV.B.1 below), and decided to revise the secondary standard to
be equal to the revised primary standard (73 FR 16499-16500, March 27,
2008). In remanding the 2008 decision on the secondary standard back to
the EPA (described in section I.C above), the U.S. Court of Appeals for
the D.C. Circuit determined that EPA did not specify what level of air
quality was requisite to protect public welfare from adverse public
welfare effects or explain why any such level would be requisite.
Mississippi, 744 F.3d at 272-73.
In addition to reviewing the most recent scientific information as
required by the CAA, this rulemaking responds to the remand and fully
explains the Administrator's proposed conclusions as to the level of
air quality requisite to protect public welfare from known or
anticipated effects. Our general approach in considering the scientific
information available in this review involves consideration of the
integrative synthesis of the entire body of available scientific
evidence in the ISA (U.S. EPA, 2013a), including information on
biologically relevant exposure indices, exposure/risk and air quality
modeling analyses presented in the WREA (U.S. EPA, 2014b), staff
analyses in the PA; advice and recommendations from CASAC (Frey, 2014b,
c), and public comments. We note that in drawing conclusions on the
secondary standard, the final decision to retain or revise the standard
is a public welfare policy judgment to be made by the Administrator.
The Administrator's final decision will draw upon the available
scientific evidence for O3-attributable welfare effects and
on analyses of exposures and public welfare risks based on impacts to
vegetation, ecosystems and their associated services, as well as
judgments about the appropriate weight to place on the range of
uncertainties inherent in the evidence and analyses. Such judgments in
the context of this review include: The weight to place on the evidence
of specific vegetation-related effects estimated to result across a
range of cumulative seasonal concentration-weighted O3
exposures; the weight to give associated uncertainties, including those
related to the variability in occurrence of such effects in areas of
the U.S., especially areas of particular public welfare significance;
and, judgments on the extent to which such effects in such areas may be
considered adverse to public welfare.
As provided in the CAA, section 109(b)(2), the secondary standard
is to ``specify a level of air quality the attainment and maintenance
of which in the judgment of the Administrator . . .
[[Page 75313]]
is requisite to protect the public welfare from any known or
anticipated adverse effects associated with the presence of such air
pollutant in the ambient air.'' Effects on welfare include, but are not
limited to, ``effects on soils, water, crops, vegetation, man-made
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'' (CAA section 302(h)). As recognized in the last review, the
secondary standard is not meant to protect against all known or
anticipated O3-related effects, but rather those that are
judged to be adverse to the public welfare (73 FR 16496, March 27,
2008). Thus, the level of protection from known or anticipated adverse
effects to public welfare that is requisite for the secondary standard
is a public welfare policy judgment to be made by the Administrator. In
the current review, the Administrator's judgment is informed by
conclusions drawn with regard to adversity of effects to public welfare
in decisions on secondary O3 standards in past reviews.
In the 2008 decision, the Administrator concluded that the degree
to which O3 effects on vegetation should be considered to be
adverse to the public welfare depends on the intended use of the
vegetation and the significance of the vegetation to the public
welfare, and also applied this concept beyond the species level to the
ecosystem level (73 FR 16496, March 27, 2008). In so doing, the
Administrator took note of ``a number of actions taken by Congress to
establish public lands that are set aside for specific uses that are
intended to provide benefits to the public welfare, including lands
that are to be protected so as to conserve the scenic value and the
natural vegetation and wildlife within such areas, and to leave them
unimpaired for the enjoyment of future generations'' (73 FR 16496,
March 27, 2008). The notice for the 2008 decision further noted that
[s]uch public lands that are protected areas of national interest
include national parks and forests, wildlife refuges, and wilderness
areas'' (73 FR 16496, March 27, 2008).178 179 The
Administrator additionally recognized that ``States, Tribes and public
interest groups also set aside areas that are intended to provide
similar benefits to the public welfare, for residents on State and
Tribal lands, as well as for visitors to those areas'' (73 FR 16496,
March 27, 2008). The Administrator took note of the ``clear public
interest in and value of maintaining these areas in a condition that
does not impair their intended use and the fact that many of these
lands contain O3-sensitive species'' (73 FR 16496, March 27,
2008). Similarly, in judgments of adversity to public welfare in the
2010 proposed reconsideration, the Administrator proposed to place the
highest priority and significance on vegetation and ecosystem effects
to sensitive species that are known to or are likely to occur in
federally protected areas such as national parks and other Class I
areas,\180\ or on lands set aside by states, tribes and public interest
groups to provide similar benefits to the public welfare (75 FR 3023-
24, January 19, 2010).
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\178\ For example, the National Park Service Organic Act of 1916
established the National Park Service (NPS) and, in describing the
role of the NPS with regard to ``Federal areas known as national
parks, monuments, and reservations'', stated that the ``fundamental
purpose'' for these Federal areas ``is to conserve the scenery and
the natural and historic objects and the wild life therein and to
provide for the enjoyment of the same in such manner and by such
means as will leave them unimpaired for the enjoyment of future
generations.'' 16 U.S.C. 1.
\179\ As a second example, the Wilderness Act of 1964 defines
designated ``wilderness areas'' in part as areas ``protected and
managed so as to preserve [their] natural conditions'' and requires
that these areas ``shall be administered for the use and enjoyment
of the American people in such manner as will leave them unimpaired
for future use and enjoyment as wilderness, and so as to provide for
the protection of these areas, [and] the preservation of their
wilderness character . . .'' 16 U.S.C. 1131 (a).
\180\ As defined by section 162 of the CAA, Class I areas
include all international parks, national wilderness areas which
exceed 5,000 acres in size, national memorial parks which exceed
5,000 acres in size, and national parks which exceed six thousand
acres in size, provided the park or wilderness area was in existence
on August 7, 1977, as well as other areas designated as Class I
consistent with that section of the Act. The current Class I areas
are specified at 40 CFR part 81.
---------------------------------------------------------------------------
In the current review, our consideration of the scientific evidence
for effects on vegetation is based fundamentally on using information
from controlled chamber studies, free air methodologies, and field-
based observational, survey and gradient studies. Such evidence,
discussed below, informs consideration of welfare endpoints and at-risk
species and ecosystems on which to focus the current review, and
consideration of the ambient O3 conditions under which
various welfare effects are known or anticipated to occur. As in past
reviews, we recognize that the available evidence has not provided
identification of a threshold in exposure or ambient O3
concentrations below which it can be concluded with confidence that
O3-attributable effects on vegetation do not occur, when
considering the broad range of O3-sensitive plant species
growing within the U.S and the array of effects. This is due in part to
the fact that research shows that there is variability in sensitivity
between and within species and that numerous factors, i.e., chemical,
physical, biological, and genetic, can influence the direction and
magnitude of the studied effect (U.S. EPA, 2013a, section 9.4.8). In
the absence of evidence for a discernible threshold, the general
approach to considering the available O3 welfare effects
evidence involves characterizing the confidence in conclusions
regarding O3-attributable vegetation effects over the ranges
of cumulative seasonal O3 exposure values evaluated in
chamber studies and in field studies in areas where O3-
sensitive vegetation are known to occur, as well as characterizing the
extent to which these effects can be considered adverse at the plant
level and beyond. With this approach, we consider the evidence for
O3 affecting other ecosystem components (such as soils,
water, and wildlife) and their associated goods and services, through
its effects on vegetation, as well as the associated uncertainties.
Our general approach further recognizes the complexity of judgments
to be made regarding the identification of particular vegetation
effects as welfare effects and regarding the point that known or
anticipated vegetation-related effects become adverse to the public
welfare. For example, in addition to the magnitude of the ambient
concentrations, the species present, their sensitivity to
O3, and their public welfare importance are also essential
considerations in drawing conclusions regarding the significance of
public welfare impact. Taking this into account, we recognize the
existence of a continuum from relatively higher ambient O3
concentrations and conditions, in areas with sensitive species and
public welfare significance, for which there might be general agreement
that effects on public welfare are likely to occur, through lower
concentrations at which the degree to which public welfare might be
expected to be affected becomes increasingly uncertain.
The evidence base for this review, summarized in section IV.B
below, includes quantitative information across a broad array of
vegetation effects (e.g., growth impairment during seedling, sapling
and mature tree growth stages, visible foliar injury, and yield loss in
annual crops) and across a diverse set of exposure methods from
laboratory and field studies. While considering the full breadth of
information available, we place greater weight on U.S. studies due to
the often species-, site-, and climate-
[[Page 75314]]
specific nature of O3-related vegetation responses, and
particularly emphasize those studies that include O3
exposures that fall within the range of those likely to occur in the
ambient air. We additionally recognize differences across different
study types in what information they provide (U.S. EPA, 2013a, section
9.2.6). For example, because conditions can be controlled in laboratory
studies, responses in such studies may be less variable and smaller
differences may be easier to detect. However, the controlled conditions
may limit the range of responses or incompletely reflect pollutant
bioavailability, so they may not reflect responses that would occur in
the natural environment. Alternatively, field data can provide
important information for assessments of multiple stressors or where
site-specific factors significantly influence exposure. They are also
often useful for analyses of larger geographic scales and higher levels
of biological organization. However, depending on the type of field
study, many field study conditions may not be controlled, which can
make variability higher and differences harder to detect. In some field
studies (e.g., gradient studies), the presence of confounding factors
can also make it difficult to attribute observed effects to specific
stressors.
In developing quantitative exposure and risk assessments for this
review, summarized in section IV.C below, we have placed greatest
emphasis on studies that have evaluated plant response over multiple
exposure levels and developed exposure-response (E-R) relationships
that allow the estimation of plant responses over the range of
O3 exposures pertinent to judgments on the current and
potential alternative standards. In considering the information from
these assessments, we focus particularly on the quantitative risks
related to three types of O3 effects on vegetation and
associated ecosystem services: visible foliar injury, biomass loss in
trees, and crop yield loss. These risks were assessed in a range of
analyses primarily involving national-scale air quality scenarios
developed using model adjustments and interpolation methods. We
consider particularly the national scale assessments for these
scenarios, while recognizing the uncertainties with regard to the
conditions they represent.
With regard to the appropriate characterization of exposures
associated with ambient O3 concentrations, as in the 2008
and 1997 reviews, we continue to recognize the relevance of cumulative,
seasonal, concentration-weighted exposures for assessing vegetation
effects. More specifically, in the 2008 review, the EPA concluded and
the CASAC agreed that the W126 cumulative exposure metric was the most
appropriate to use to evaluate both the adequacy of the current
secondary standard and the appropriateness of any potential revisions.
As discussed in section IV.B.1 below, the information available in this
review continues to support the use of such a metric and it is used in
considering potential public welfare impacts in the sections below.
B. Welfare Effects Information
1. Nature of Effects and Biologically Relevant Exposure Metric
This section describes the nature of O3-induced welfare
effects, including the nature of the exposures that drive the
biological and ecological responses (U.S. EPA, 2013a, chapter 9).
Ozone's phytotoxic effects were first identified on grape leaves in
a study published in 1958 (Richards et al., 1958). In the almost fifty
years that have followed, extensive research has been conducted both in
and outside of the U.S. to examine the impacts of O3 on
plants and their associated ecosystems, since ``of the phytotoxic
compounds commonly found in the ambient air, O3 is the most
prevalent, impairing crop production and injuring native vegetation and
ecosystems more than any other air pollutant'' (U.S. EPA, 1989, 1996a).
As was established in prior reviews, O3 can interfere with
carbon gain (photosynthesis) and allocation of carbon within the plant.
As a result of decreased carbohydrate availability, fewer carbohydrates
are available for plant growth, reproduction, and/or yield. For seed-
bearing plants, these reproductive effects will culminate in reduced
seed production or yield (U.S. EPA, 1996a, pp. 5-28 and 5-29). Recent
studies, assessed in the ISA, together with this longstanding and well-
established literature on O3-related vegetation effects,
further contribute to the coherence and consistency of the vegetation
effects evidence. As described in the ISA, a variety of factors in
natural environments can either mitigate or exacerbate predicted
O3-plant interactions and are recognized sources of
uncertainty and variability. These include: (1) Multiple genetically
influenced determinants of O3 sensitivity; (2) changing
sensitivity to O3 across vegetative growth stages; (3) co-
occurring stressors and/or modifying environmental factors (U.S. EPA,
2013a, section 9.4.8).
Among the studies of vegetation effects, the ISA recognizes
controlled chamber studies as the best method for isolating or
characterizing the role of O3 in inducing the observed plant
effects, and in assessing plant response to O3 at the finer
scales (U.S. EPA, 2013a, sections 9.2 and 9.3). Recent controlled
studies have focused on a variety of plant responses to O3
including the underlying mechanisms governing such responses. These
mechanisms include: (1) Reduced carbon dioxide uptake due to stomatal
closure (U.S. EPA, 2013a, section 9.3.2.1); (2) the upregulation of
genes associated with plant defense, signaling, hormone synthesis and
secondary metabolism (U.S. EPA, 2013a, section 9.3.3.2); (3) the down
regulation of genes related to photosynthesis and general metabolism
(U.S. EPA, 2013a, section 9.3.3.2); (4) the loss of carbon assimilation
capacity due to declines in the quantity and activity of key proteins
and enzymes (U.S. EPA, 2013a, section 9.3.5.1); and (5) the negative
impacts on the efficiency of the photosynthetic light reactions (U.S.
EPA, 2013a, section 9.3.5.1). As described in the ISA, these new
studies ``have increased knowledge of the molecular, biochemical and
cellular mechanisms occurring in plants in response to O3'',
adding ``to the understanding of the basic biology of how plants are
affected by oxidative stress . . .'' (U.S. EPA, 2013a, p. 9-11). The
ISA further concludes that controlled studies ``have clearly shown that
exposure to O3 is causally linked to visible foliar injury,
decreased photosynthesis, changes in reproduction, and decreased
growth'' in many species of vegetation (U.S. EPA, 2013a, p. 1-15).
Such effects at the plant scale can also be linked to an array of
effects at larger spatial scales. For example, recent field studies at
larger spatial scales, together with previously available evidence,
support the controlled exposure study results and indicate that
``ambient O3 exposures can affect ecosystem productivity,
crop yield, water cycling, and ecosystem community composition'' (U.S.
EPA, 2013a, p. 1-15; Chapter 9, section 9.4). The current body of
O3 welfare effects evidence confirms the conclusions reached
in the last review on the nature of O3-induced welfare
effects and is summarized in the ISA as follows (U.S. EPA, 2013a, p. 1-
8).
The welfare effects of O3 can be observed across
spatial scales, starting at the subcellular and cellular level, then
the whole plant and finally, ecosystem-level processes. Ozone
effects at small spatial scales, such as the leaf of an individual
plant, can result in effects along a continuum of larger spatial
scales. These effects include altered rates of leaf gas exchange,
growth, and reproduction at the individual plant level, and can
result
[[Page 75315]]
in broad changes in ecosystems, such as productivity, carbon
storage, water cycling, nutrient cycling, and community composition.
Based on its assessment of this extensive body of science, the ISA
determines that, with respect to vegetation and ecosystems, a causal
relationship exists between exposure to O3 in ambient air
and visible foliar injury effects on vegetation, reduced vegetation
growth, reduced productivity in terrestrial ecosystems, reduced yield
and quality of agricultural crops and alteration of below-ground
biogeochemical cycles \181\ (U.S. EPA, 2013a, Table 1-2). In
consideration of the evidence of O3 exposure and alterations
in stomatal performance, ``which may affect plant and stand
transpiration and therefore possibly affecting hydrological cycling,''
the ISA concludes that ``[a]lthough the direction of the response
differed among studies,'' the evidence is sufficient to conclude a
likely causal relationship between O3 exposure and the
alteration of ecosystem water cycling (U.S. EPA, 2013a, section 2.6.3).
The ISA also concludes that the evidence is sufficient to conclude a
likely causal relationship between O3 exposure and the
alteration of community composition of some terrestrial ecosystems
(U.S. EPA, 2013a, section 2.6.5). Related to the effects on vegetation
growth, productivity and, to some extent, below-ground biogeochemical
cycles, the ISA additionally determines that a likely causal
relationship exists between exposures to O3 in ambient air
and reduced carbon sequestration (also termed carbon storage) \182\ in
terrestrial ecosystems (U.S. EPA, 2013a, p. 1-10 and section 2.6.2).
Modeling studies available in this review consistently found negative
impacts of O3 on carbon sequestration, although the severity
of impact was influenced by ``multiple interactions of biological and
environmental factors'' (U.S. EPA, 2013a, p. 2-39).
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\181\ Based on studies focused on O3-associated
alterations in quality and quantity of carbon input to soil,
microbial community composition, and carbon and nutrient cycling,
the ISA concludes that the evidence is sufficient ``to infer that
there is a causal relationship between O3 exposure and
the alteration of below-ground biogeochemical cycles'' (U.S. EPA,
2013a, pp. 2-41 to 2-42).
\182\ The terms sequestration and storage are used somewhat
interchangeably in the ISA and other documents in this review.
---------------------------------------------------------------------------
The ISA notes that ``[t]he suppression of ecosystem [carbon] sinks
results in more [carbon dioxide] accumulation in the atmosphere'' and
that a recent study has suggested that ``the indirect radiative forcing
caused by O3 exposure through lowering the ecosystem
[carbon] sink could have an even greater impact on global warming than
the direct radiative forcing of O3'' (U.S. EPA, 2013a, p. 2-
39). With regard to direct radiative forcing, however, the ISA makes a
stronger causality conclusion that the evidence supports a causal
relationship between changes in tropospheric O3
concentrations and radiative forcing \183\ (U.S. EPA, 2013a, section
2.7.1). There are, however, ``large uncertainties in the magnitude of
the radiative forcing estimate attributed to tropospheric
O3, making the impact of tropospheric O3 on
climate more uncertain than the effect of the longer-lived greenhouse
gases'' (U.S. EPA, 2013a, p. 2-47). In this regard, the ISA observes
that ``radiative forcing does not take into account the climate
feedbacks that could amplify or dampen the actual surface temperature
response,'' that ``[q]uantifying the change in surface temperature
requires a complex climate simulation in which all important feedbacks
and interactions are accounted for'' and that ``[t]he modeled surface
temperature response to a given radiative forcing is highly uncertain
and can vary greatly among models and from region to region within the
same model'' (U.S. EPA, 2013a, p. 2-47). Even with these uncertainties,
the ISA notes that ``global climate models indicate that tropospheric
O3 has contributed to observed changes in global mean and
regional surface temperatures'' and as a result of such evidence
presented in climate modeling studies, concludes that there is likely
to be a causal relationship between changes in tropospheric
O3 concentrations and effects on climate (U.S. EPA, 2013a,
p. 2-47). The ISA additionally notes, however, that ``[i]mportant
uncertainties remain regarding the effect of tropospheric O3
on future climate change'' (U.S. EPA, 2013a, p. 10-31).
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\183\ Radiative forcing by a greenhouse gas or aerosol is a
metric used to quantify the change in balance between radiation
coming into and going out of the atmosphere caused by the presence
of that substance. For example, a reduction in outgoing infrared
radiation has been associated with O3 by satellite data
(U.S. EPA, 2013a, p. 2-47).
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Given the strong evidence base, and findings of causal or likely
causal relationships with O3 in ambient air, including the
quantitative assessments of relationships between O3
exposure and occurrence and magnitude of effects, we give a primary
focus to three main areas of effects. The three main areas, for which
the evidence is summarized in more detail below, are: 1) impacts on
tree growth, productivity and carbon storage (section IV.B.1.b); 2)
crop yield loss (section IV.B.1.c); and 3) visible foliar injury
(section IV.B.1.a). Consideration of these three areas includes, as
appropriate, consideration of evidence of associated effects at larger
scales, including ecosystems, and on associated ecosystem services.
With regard to biologically based indices of exposure pertinent to
O3 effects on vegetation, the ISA states the following (U.S.
EPA, 2013a, p. 2-44).
The main conclusions from the 1996 and 2006 O3 AQCDs
[Air Quality Criteria Documents] regarding indices based on ambient
exposure remain valid. These key conclusions can be restated as
follows: ozone effects in plants are cumulative; higher
O3 concentrations appear to be more important than lower
concentrations in eliciting a response; plant sensitivity to
O3 varies with time of day and plant development stage;
[and] quantifying exposure with indices that cumulate hourly
O3 concentrations and preferentially weight the higher
concentrations improves the explanatory power of exposure/response
models for growth and yield, over using indices based on mean and
peak exposure values.
The long-standing body of available evidence upon which these
conclusions are based provides a wealth of information on aspects of
O3 exposure that are important in influencing plant
response. Specifically, a variety of ``factors with known or suspected
bearing on the exposure-response relationship, including concentration,
time of day, respite time, frequency of peak occurrence, plant
phenology, predisposition, etc.,'' have been identified (U.S. EPA,
2013a, section 9.5.2). In addition, the importance of the duration of
the exposure and the relatively greater importance of higher
concentrations over lower concentrations in determining plant response
to O3 have been consistently well documented (U.S. EPA,
2013a, section 9.5.3). Much of this evidence was assessed in the 1996
AQCD (U.S. EPA, 1996a), while more recent work substantiating this
evidence is assessed in the subsequent 2006 AQCD and 2013 ISA.
Understanding of the biological basis for plant response to
O3 exposure led to the development of a large number of
``mathematical approaches for summarizing ambient air quality
information in biologically meaningful forms for O3
vegetation effects assessment purposes'' (U.S. EPA, 2013a, section
9.5.3), including those that cumulate exposures over some specified
period while weighting higher concentrations more than lower (U.S. EPA,
2013a, section 9.5.2). As with any summary statistic, these exposure
indices retain information on some, but not all, characteristics of the
original observations. The 1996 AQCD contained an extensive review of
the published
[[Page 75316]]
literature on different types of exposure-response metrics, including
comparisons between metrics, from which the 1996 Staff Paper built its
assessment of forms appropriate to consider in the context of the
secondary NAAQS review. The result of these assessments was a decision
by the EPA to focus on cumulative, concentration-weighted indices,
which were recognized as the most appropriate biologically based
metrics to consider in this context, with attention given primarily to
two cumulative, concentration-weighted index forms: SUM06 and
W126.\184\
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\184\ The SUM06 index is a threshold-based approach described as
the sum of all hourly O3 concentrations greater or equal
to 0.06 ppm observed during a specified daily and seasonal time
window (U.S. EPA, 2013a, section 9.5.2). The W126 index is a non-
threshold approach described as the sigmoidally weighted sum of all
hourly O3 concentrations observed during a specified
daily and seasonal time window, where each hourly O3
concentration is given a weight that increases from zero to one with
increasing concentration (U.S. EPA, 2013a, section 9.5.2).
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In both the 1997 and 2008 reviews, the EPA concluded that the risk
to vegetation comes primarily from cumulative exposures to
O3 over a season or seasons \185\ and focused on metrics
intended to characterize such exposures: SUM06 (61 FR 65716, December
13, 1996) and W126 (72 FR 37818, July 11, 2007) in the 1997 and 2008
reviews, respectively. Although in both reviews the policy decision was
made to set the secondary standard to be identical to a revised primary
standard (with an 8-hour averaging time), the Administrator, in both
cases, also concluded, consistent with CASAC advice, that a cumulative,
seasonal index was the most biologically relevant way to relate
exposure to plant growth response (62 FR 38856, July 18, 1997; 73 FR
16436, March 27, 2008; 75 FR 2938, January 19, 2010). This approach for
characterizing O3 exposure concentrations that are
biologically relevant with regard to potential vegetation effects
received strong support from CASAC in the last review and again in this
review, including strong support for use of such a metric as the form
for the secondary standard (Henderson, 2006, 2008; Samet, 2010; Frey,
2014c).
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\185\ In describing the form as ``seasonal'', the EPA is
referring generally to the growing season of O3-sensitive
vegetation, not to the seasons of the year (i.e., spring, summer,
fall, winter).
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An alternative to using ambient exposure durations and
concentrations to predict plant response has been developed in recent
years, primarily in Europe, i.e., flux models. While ``some researchers
have claimed that using flux models can be used {sic{time} to better
predict vegetation responses to O3 than exposure-based
approaches'' because flux models estimate the ambient O3
concentration that actually enters the leaf (i.e., flux or deposition)
(U.S. EPA, 2013a, p. 9-114), it is important to note that ``[f]lux
calculations are data intensive and must be carefully implemented''
(U.S. EPA, 2013a, p. 9-114). Further, the ISA states, ``[t]his uptake-
based approach to quantify the vegetation impact of O3
requires inclusion of those factors that control the diurnal and
seasonal O3 flux to vegetation (e.g., climate patterns,
species and/or vegetation-type factors and site-specific factors)''
(U.S. EPA, 2013a, p. 9-114). In addition to these data requirements,
each species has different amounts of internal detoxification potential
that may protect species to differing degrees. The lack of detailed
species- and site-specific data required for flux modeling in the U.S.
and the lack of understanding of detoxification processes have
continued to make this technique less viable for use in vulnerability
and risk assessments at the national scale in the U.S. (U.S. EPA,
2013a, section 9.5.4).
Therefore, consistent with the ISA conclusions regarding the
appropriateness of considering cumulative exposure indices that
preferentially weight higher concentrations over lower for predicting
O3 effects of concern based on the long-established
conclusions and long-standing supporting evidence described above, and
in light of continued CASAC support, we continue to focus on cumulative
concentration-weighted indices as the most biologically relevant
metrics for consideration of O3 exposures eliciting
vegetation-related effects. Such a metric has an ``explanatory power''
that is improved ``over using indices based on mean and peak exposure
values'' (U.S. EPA, 2013a, section 2.6.6.1, p. 2-44). In this review as
in the last review, we use the W126 cumulative, seasonal metric (U.S.
EPA, 2013a, sections 2.6.6.1 and 9.5.2) for consideration of the
effects evidence and in the exposure and risk analyses in the WREA.
The subsections below summarize key aspects of the welfare effects
information for O3-elicited visible foliar injury (section
IV.B.1.a), effects on forest tree growth, productivity and carbon
storage (section IV.B.1.b) and reductions in crop yield (section
IV.B.1.c), as well as associated effects.
a. Visible Foliar Injury
Visible foliar injury resulting from exposure to O3 has
been well characterized and documented over several decades of research
on many tree, shrub, herbaceous, and crop species (U.S. EPA, 2013a, p.
1-10; U.S. EPA, 2006a, 1996a, 1986, 1978). Additionally, O3-
induced visible foliar injury symptoms on certain plant species, such
as black cherry, yellow-poplar and common milkweed, are considered
diagnostic of exposure to O3 based on the consistent
association established with experimental evidence (U.S. EPA, 2013a, p.
1-10). The significance of O3 injury at the leaf and whole
plant levels depends on an array of factors, and therefore, it is
difficult to quantitatively relate visible foliar injury symptoms to
vegetation effects such as individual tree growth, or effects at
population or ecosystem levels (U.S. EPA, 2013a, p. 9-39). The ISA
notes that visible foliar injury ``is not always a reliable indicator
of other negative effects on vegetation'' (U.S. EPA, 2013a, p. 9-39).
Factors that influence the significance to the leaf and whole plant
include the amount of total leaf area affected, age of plant, size,
developmental stage, and degree of functional redundancy among the
existing leaf area (U.S. EPA, 2013a, section 9.4.2). Visible foliar
injury by itself is an indication of phytotoxicity due to O3
exposure, which occurs only when sensitive plants are exposed to
elevated O3 concentrations in a predisposing environment, a
major aspect of which is the lack of drought conditions during the year
such injury is assessed (U.S. EPA, 2013a, section 9.4.2).
Recent research is consistent with previous conclusions and that
O3-induced visible foliar injury symptoms are well
characterized and considered diagnostic on certain bioindicator plant
species. Diagnostic usage for these plants has been verified
experimentally in exposure-response studies, using exposure
methodologies such as continuous stirred tank reactors, open-top
chambers (OTCs), and free-air carbon dioxide (and ozone) enrichment
(FACE). Although there remains a lack of robust exposure-response
functions that would allow prediction of visible foliar injury severity
and incidence under varying air quality and environmental conditions,
``experimental evidence has clearly established a consistent
association of the presence of visible foliar injury symptoms with
O3 exposure, with greater exposure often resulting in
greater and more prevalent injury'' (U.S. EPA, 2013a, section 9.4.2, p.
9-41). The research newly available in this review includes: 1)
controlled exposure studies conducted to test and verify the
O3
[[Page 75317]]
sensitivity and response of potential new bioindicator plant species;
2) multi-year field surveys in several National Wildlife Refuges (NWR)
documenting the presence of foliar injury in valued areas; and 3)
ongoing data collection and assessment by the U.S. Forest Service's
Forest Health Monitoring Forest Inventory and Analysis (USFS FHM/FIA)
program, including multi-year trend analysis (U.S. EPA, 2013a, section
9.4.2). These recent studies, in combination with the entire body of
available evidence, thus form the basis for the ISA determinations of a
causal relationship between ambient O3 exposure and the
occurrence of O3-induced visible foliar injury on sensitive
vegetation across the U.S. (U.S. EPA, 2013a, p. 9-42).
Recently available evidence confirms the evidence available in
previous reviews that visible foliar injury can occur when sensitive
plants are exposed to elevated O3 concentrations in a
predisposing environment (i.e., adequate soil moisture) (U.S. EPA,
2013a, section 9.4.2). Recent evidence also continues to support
previous findings that indicated the occurrence of visible foliar
injury at cumulative ambient O3 exposures previously
examined.
With regard to evidence from controlled exposure studies, a recent
study, using continuously stirred tank reactor chambers, evaluated the
occurrence of O3 characteristic visible foliar injury
symptoms on 28 species of plants that were suspected of being
O3 sensitive and most of which grow naturally throughout the
northeast and midwest U.S., including in national parks and wilderness
areas (U.S. EPA, 2013a, section 9.4.2.1; Kline et al., 2008). Across
the 28 tested species, the study reported O3-induced
responses in 12, 20, 28 and 28 species at the 30, 60, 90 and 120 ppb
exposure concentrations,\186\ respectively; the plants were exposed for
7 hours per each weekday over 21 to 29 summer days (Kline et al.,
2008).
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\186\ Two of the target exposure levels, 30 and 60 ppb, fall
below the level of the current standard (75 ppb), although the
exposures were average concentrations for 7-hour exposures across
durations shorter than a month. Because the form of the current
standard targets peak concentrations in a season, an area that just
meets the current standard can be expected to have mean
concentrations well below that level due to variability in ambient
O3 concentrations.
---------------------------------------------------------------------------
A string of recently published multi-year field studies provide a
complementary line of field-based evidence by documenting the incidence
of visible foliar injury symptoms on a variety of O3-
sensitive species over multiple years and across a range of cumulative,
seasonal exposure values in several eastern and midwestern NWRs (U.S.
EPA, 2013a, section 9.4.2.1; Davis and Orendovici, 2006; Davis, 2007a,
b; Davis, 2009). Some of these studies also included information
regarding soil moisture stress using the Palmer Drought Severity Index
(PDSI). While environmental conditions and species varied across the
four NWRs, visible foliar injury was documented to a varying degree at
each site.
By far the most extensive field-based dataset of visible foliar
injury incidence is that obtained by the USFS FHM/FIA biomonitoring
network program. A trend analysis of data from the sites located in the
Northeast and North Central U.S. for the 16 year period from 1994
through 2009 (Smith, 2012) describes evidence of visible foliar injury
occurrence in the field as well as some insight into the influence of
changes in air quality and soil moisture on visible foliar injury and
the difficulty inherent in predicting foliar injury response under
different air quality/soil moisture scenarios (Smith, 2012; U.S. EPA,
2013a, section 9.2.4.1). Study results showed that incidence and
severity of foliar injury were dependent on local site conditions for
soil moisture availability and O3 exposure. Overall, there
was a declining trend in the incidence of visible foliar injury as peak
O3 concentrations declined, although the study additionally
indicated that moderate O3 exposures continued to cause
visible foliar injury at sites throughout the region (U.S. EPA, 2013a,
p. 9-40). In a similar assessment of the USFS FHM/FIA data in the West,
six years (2000 to 2005) of biomonitoring data, during a period where a
large proportion of California sites did not meet the current standard,
indicated O3-related visible foliar injury in 25-37% of
biosites in California (Campbell et al., 2007; U.S. EPA, 2013a, section
9.4.2.1).\187\ These recent studies provide additional evidence of
O3-induced visible foliar injury in many areas across the
U.S. and augment the EPA's understanding of O3-related
visible foliar injury and of factors, such as soil moisture, that
influence associations between O3 exposures or
concentrations and visible foliar injury.
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\187\ See: http://www.epa.gov/airtrends/values.html.
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b. Effects on Forest Tree Growth, Productivity and Carbon Storage
Ozone has been shown to affect a number of important U.S. tree
species with respect to growth, productivity, and carbon storage.
Ambient O3 concentrations have long been known to cause
decreases in photosynthetic rates and plant growth. As discussed in the
ISA, research published since the 2006 AQCD substantiates prior
conclusions regarding O3-related effects on forest tree
growth, productivity and carbon storage. The ISA states, ``previous
O3 AQCDs concluded that there is strong evidence that
exposure to O3 decreases photosynthesis and growth in
numerous plant species'' and that ``[s]tudies published since the 2008
review support those conclusions'' (U.S. EPA, 2013a, p. 9-42). The
available studies come from a variety of different study types that
cover an array of different species, effects endpoints, levels of
biological organization and exposure methods and durations. The
O3-induced effects at the plant scale may translate to the
ecosystem scale, with changes in productivity and carbon storage. As
stated in the ISA, ``[s]tudies conducted during the past four decades
have demonstrated unequivocally that O3 alters biomass
allocation and plant reproduction'' (U.S. EPA, 2013a, p. 1-10).
The previously available strong evidence for trees includes robust
E-R functions for seedling relative biomass loss (RBL) \188\ in 11
species developed under the National Health and Environmental Effects
Research Laboratory-Western Ecology Division program. This series of
experiments used OTCs to study seedling growth response for a single
growing season under a variety of O3 exposures (ranging from
near background to well above current ambient concentrations) and
growing conditions (U.S. EPA, 2013a, section 9.6.2; Lee and Hogsett,
1996). The evidence from these studies shows that there is a wide range
in sensitivity across the studied species in the seedling growth stage
over the course of a single growing season, with some species being
extremely sensitive and others being very insensitive over the range of
cumulative O3 exposures studied (U.S. EPA, 2014c, Figure 5-
1). At the other end of the organizational spectrum, field-based
studies of species growing in natural stands have compared observed
plant response across a number of different sites and/or years when
exposed to varying ambient O3 exposure conditions. For
example, a study conducted in forest stands in the southern Appalachian
Mountains during a period when O3 concentrations exceeded
the current standard found that the cumulative
[[Page 75318]]
effects of O3 decreased seasonal stem growth (measured as a
change in circumference) by 30-50 percent for most of the examined tree
species (i.e., tulip poplar, black cherry, red maple, sugar maple) in a
high O3 year in comparison to a low O3 year (U.S.
EPA, 2013a, section 9.4.3.1; McLaughlin et al., 2007a). The study also
reported that high ambient O3 concentrations can increase
whole-tree water use and in turn reduce late-season streamflow
(McLaughlin et al., 2007b; U.S. EPA, 2013a, p. 9-43).
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\188\ These functions for RBL estimate reduction in a year's
growth as a percentage of that expected in the absence of
O3 (U.S. EPA, 2013a, section 9.6.2; U.S. EPA, 2014b,
section 6.2).
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The magnitude of O3 impact on ecosystem productivity and
on forest composition can vary among plant communities based on several
factors including: the type of stand or community in which the
sensitive species occurs (e.g., single species versus mixed canopy),
the role or position of the species in the stand (e.g., dominant, sub-
dominant, canopy, understory), the sensitivity of co-occurring species
and environmental factors (e.g., drought and other factors). For
example, O3 has been found to have little impact on white
fir, but to greatly reduce growth of ponderosa pine in southern
California, and cause decreased net primary production of most forest
types in the Mid-Atlantic region, although only small impacts on
spruce-fir forest (U.S. EPA, 2013a, section 9.4.3.4).
As noted above, long-standing evidence has demonstrated that
O3 alters biomass allocation and plant reproduction (U.S.
EPA, 2013a, section 9.4.3). Several studies published since the 2006
O3 AQCD further demonstrate that O3 can alter
reproductive processes in herbaceous and woody plant species, such as
the timing of flowering and the number of flowers, fruits and seeds
(U.S. EPA, 2013a, section 9.4.3.3). Further, limited evidence in
previous reviews reported that vegetation effects from a single year of
exposure to elevated O3 could be observed in the following
year. For example, growth affected by a reduction in carbohydrate
storage in one year may result in the limitation of growth in the
following year. Such ``carry-over'' effects have been documented in the
growth of some tree seedlings and in roots (U.S. EPA, 2013a, section
9.4.8; Andersen, et al., 1997). In the current review, additional
field-based evidence expands the EPA's understanding of the
consequences of single and multi-year O3 exposures in
subsequent years. A number of studies were conducted at a planted
forest at the Aspen FACE site in Wisconsin. These studies, which
occurred in a field setting (more similar to natural forest stands than
OTC studies), observed tree growth responses when grown in single or
two species stands within 30-m diameter rings and exposed over a period
of ten years to existing ambient conditions and elevated O3
concentrations. Some studies indicate the potential for carry-over
effects, such as those showing that the effects of O3 on
birch seeds (reduced weight, germination, and starch levels) could lead
to a negative impact on species regeneration in subsequent years, and
that the effect of reduced aspen bud size might have been related to
the observed delay in spring leaf development. These effects suggest
that elevated O3 exposures have the potential to alter
carbon metabolism of overwintering buds, which may have subsequent
effects in the following year (Darbah, et al., 2008, 2007; Riikonen et
al., 2008; U.S. EPA, 2013a, section 9.4.3). Other studies found that,
in addition to affecting tree heights, diameters, and main stem volumes
in the aspen community, elevated O3 over a 7-year study
period was reported to increase the rate of conversion from a mixed
aspen-birch community to a community dominated by the more tolerant
birch, leading the authors to conclude that elevated O3 may
alter intra- and inter-species competition within a forest stand (U.S.
EPA, 2013a, section 9.4.3; Kubiske et al., 2006; Kubiske et al., 2007).
These studies confirm earlier FACE results of aspen growth reductions
from a 6-7 year exposure to elevated O3 and of cumulative
biomass impacts associated with changes in annual production in studied
tree communities (U.S. EPA, 2013a, section 9.4.3; King et al., 2005).
In addition to individual studies, recent meta-analyses have
quantitatively analyzed the effect of O3 on trees across
large numbers of studies. In particular, a recent meta-analysis of 55
peer reviewed studies from the past 40 years indicates a negative
relationship between O3 concentrations in the northern
hemisphere during that period and stomatal conductance and
photosynthesis, which decreases growth (U.S. EPA, 2013a, section
9.4.3.1; Wittig et al., 2007). In this analysis, younger trees (less
than 4 years) were affected less by O3 than older trees
(U.S. EPA, 2013a, section 9.4.3.1; Wittig, et al., 2007). A second
meta-analysis that quantitatively compiled 263 peer-reviewed studies
``demonstrates the coherence of O3 effects across numerous
studies and species that used a variety of experimental techniques, and
these results support the conclusion of the previous AQCD that exposure
to O3 decreases plant growth'' (U.S. EPA, 2013a, p. 9-43).
Other meta-analyses have examined the effect of O3 exposure
on root growth and generally found that O3 exposure reduced
carbon allocated to roots (U.S. EPA, 2013a, pp. 9-45 to 9-46).
As noted above, robust E-R functions have been developed for 11
tree species (black cherry, Douglas fir, loblolly pine, ponderosa pine,
quaking aspen, red alder, red maple, sugar maple, tulip poplar,
Virginia pine, white pine) from the extensive evidence base of
O3-induced growth effects that was also available and relied
upon in the previous review. While the species for which robust E-R
functions have been developed represent only a small fraction (0.8
percent) of the total number of native tree species in the contiguous
U.S. (1,497), this small subset includes eastern and western species,
deciduous and coniferous species, and species that grow in a variety of
ecosystems and represent a range of tolerance to O3 (U.S.
EPA, 2013a, section 9.6.2; U.S. EPA, 2014b, section 6.2, Figure 6-2,
Table 6-1). Each of these species were studied in OTCs, with most
species studied multiple times under a wide range of exposure and/or
growing conditions, with separate E-R functions developed for each
combination of species, exposure condition and growing condition
scenario. These species-specific composite E-R functions have been
successfully used to predict the biomass loss response from tree
seedling species over a range of cumulative exposure conditions (U.S.
EPA, 2013a, section 9.6.2). The 11 robust composite E-R functions
available in the last review, as well as the E-R for eastern cottonwood
(derived from a field study in which O3 and climate
conditions were not controlled), are described in the ISA and graphed
in the WREA to illustrate the predicted responses of these species over
a wide range of cumulative exposures (U.S. EPA, 2014b, section 6.2,
Table 6-1 and Figure 6-2; U.S. EPA, 2013a, section 9.6.2). For some of
these species, the E-R function is based on a single study (e.g., red
maple), while for other species there were as many as 11 studies
available (ponderosa pine). In total, the E-R functions developed for
these 12 species (the 11 with robust composite E-R functions plus
eastern cottonwood) reflect 52 tree seedling studies. A stochastic
analysis in WREA, summarized in section IV.C below, indicates the
potential for within species variability to contribute appreciably to
estimates for each species. Consideration of biomass loss estimates in
the PA and in discussions
[[Page 75319]]
below, however, is based on conventional method and focuses on
estimates for the 11 species for which the robust datasets from OTC
experiments are available, in consideration of CASAC advice.\189\
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\189\ The CASAC cautioned EPA against placing too much emphasis
on the eastern cottonwood data, stating that while the cottonwood
data are important results, they are not as strong as those from
other experiments that developed E-R functions based on controlled
O3 exposure in OTCs; they are from a single gradient
study that did not control for O3 and climatic conditions
and they show extreme sensitivity to O3 compared to other
studies (Frey, 2014c, p. 10).
---------------------------------------------------------------------------
c. Crop Yield Loss
The ``detrimental effect of O3 on crop production has
been recognized since the 1960s'' (U.S. EPA, 2013a, p. 1-10, section
9.4.4). On the whole, the newly available evidence supports previous
conclusions that exposure to O3 decreases growth and yield
of crops. The ISA describes average crop yield loss reported across a
number of recently published meta-analyses and identifies several new
exposure studies that support prior findings for a variety of crops of
decreased yield and biomass with increased O3 exposure (U.S.
EPA, 2013a, section 9.4.4.1, Table 9-17). Studies have also ``linked
increasing O3 concentration to decreased photosynthetic
rates and accelerated aging in leaves, which are related to yield'' and
described effects of O3 on crop quality, such as nutritive
quality of grasses, macro- and micronutrient concentrations in fruits
and vegetable crops and cotton fiber quality (U.S. EPA, 2013a, p. 1-10,
section 9.4.4). The findings of the newly available studies do not
change the basic understanding of O3-related crop yield loss
since the last review and little additional information is available in
this review on factors that influence associations between
O3 levels and crop yield loss (U.S. EPA, 2013a, section
9.4.4.).
The new evidence has strengthened support for previously
established E-R functions for 10 crops (barley, field corn, cotton,
kidney bean, lettuce, peanut, potato, grain sorghum, soybean and winter
wheat), reducing two important areas of uncertainty, especially for
soybean. The established E-R functions were developed from OTC-type
experiments (U.S. EPA, 2013a, section 9.6.3; U.S. EPA, 2014b, section
6.2; U.S. EPA, 2014c, Figure 5-4). In this review, the ISA included an
analysis comparing OTC data for soybean from the National Crop Loss
Assessment Network (NCLAN) with field-based data from SoyFACE (Soybean
Free Air Concentration Enrichment) studies (U.S. EPA, 2013a, section
9.6.3.1).\190\ Yield loss in soybean from O3 exposure at the
SoyFACE field experiment was reliably predicted by soybean E-R
functions developed from NCLAN data, demonstrating a robustness of the
E-R functions developed with NCLAN data to predict relative yield loss
from O3 exposure. A second area of uncertainty that was
reduced is that regarding the application of the NCLAN E-R functions,
developed in the 1980s, to more recent cultivars currently growing in
the field. Recent studies, especially those focused on soybean, provide
little evidence that crops are becoming more tolerant of O3
(U.S. EPA, 2006a; U.S. EPA, 2013a). A meta-analysis of 53 studies found
consistent deleterious effects of O3 exposures on soybean
from studies published between 1973 and 2001 (Morgan et al., 2003).
Further, Betzelberger et al. (2010) utilized the SoyFACE facility to
compare the impact of elevated O3 concentrations across 10
soybean cultivars to investigate intraspecific variability of the
O3 response, finding that the E-R functions derived for
these 10 current cultivars were similar to the response functions
derived from the NCLAN studies conducted in the 1980s (Heagle, 1989),
``suggesting there has not been any selection for increased tolerance
to O3 in more recent cultivars'' (U.S. EPA, 2013a, p. 9-59).
Additionally, the ISA comparisons of NCLAN and SoyFACE data referenced
above ``confirm that the response of soybean yield to O3
exposure has not changed in current cultivars'' (U.S. EPA, 2013a, p. 9-
59; section 9.6.3.1). Thus, the evidence available in the current
review has reduced uncertainties in two areas with regard to the use of
E-R functions for soybean crop yield loss.
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\190\ The NCLAN program, which was undertaken in the early to
mid-1980s, assessed multiple U.S. crops, locations, and
O3 exposure levels, using consistent methods, to provide
the largest, most uniform database on the effects of O3
on agricultural crop yields (U.S. EPA 1996a; U.S. EPA, 2006a; U.S.
EPA, 2013a, sections 9.2, 9.4, and 9.6, Frey, 2014c, p. 9). The
SoyFACE experiment was a chamberless (or free-air) field-based
exposure study conducted in Illinois from 2001-2009 (U.S. EPA,
2013a, section 9.2.4).
---------------------------------------------------------------------------
During past O3 NAAQS reviews, there were very few
studies that estimated O3 impacts on crop yields at large
geographical scales (i.e., regional, national or global). Recent
modeling studies of the impact of O3 concentrations
historically found that increased O3 in the past generally
reduced crop yield, but the impacts varied across regions and crop
species, with the largest O3-induced crop yield losses
estimated to have occurred in high-production areas that had been
exposed to elevated O3 concentrations, such as the Midwest
and the Mississippi Valley regions of the U.S. (U.S. EPA, 2013a,
Section 9.4.4.1). Among affected crop species, the estimated yield loss
for wheat and soybean were higher than rice and maize (i.e., field
corn). Additionally, satellite and ground-based O3
measurements have assessed soybean yield loss estimated to result from
O3 over the continuous area of Illinois, Iowa, and
Wisconsin, finding a relationship which correlates well with the
previous results from FACE- and OTC-type experiments (U.S. EPA, 2013a,
section 9.4.4.1).
Thus, consistent with the conclusions of the 1996 and 2006 CDs, the
current ISA concludes that O3 concentrations in ambient air
can reduce the yield of major commodity crops in the U.S. and focuses
on use of crop E-R functions based on OTC experiments to characterize
the quantitative relationship between ambient O3
concentrations and yield loss (U.S. EPA, 2013a, section 9.4.4). In the
PA, as summarized in sections IV.D and IV.E below, relative yield loss
(RYL) is estimated for 10 different crops using the individual E-R
functions described in the WREA \191\ (U.S. EPA, 2014b, section 6.2;
U.S. EPA, 2014c, section 6.3).
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\191\ These functions for RYL estimate reduction in a year's
growth as a percentage of that expected in the absence of
O3 (U.S. EPA, 2013a, section 9.6.2; U.S. EPA, 2014b,
section 6.2).
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2. Potential Impacts on Public Welfare
The magnitude of a public welfare impact or the degree to which it
may be considered adverse is dependent upon the nature and severity of
the specific welfare or ecological effect, the use or service (and
value) of the affected ecosystem and the relevance and significance of
that use \192\ to the public welfare. In the preamble of the 2012 final
notice of rulemaking on the secondary standards for oxides of nitrogen
and sulfur (NOx/SOx), the EPA stated that ``[a]n evaluation of
adversity to the public welfare might consider the likelihood, type,
magnitude, and spatial scale of the effect, as well as the potential
for recovery and any uncertainties relating to these conditions'' (77
FR 20232, April 3,
[[Page 75320]]
2012). The EPA additionally stated that ``[c]onceptually, changes in
ecosystem services may be used to aid in characterizing a known or
anticipated adverse effect to public welfare'' (77 FR 20232, April 3,
2012).\193\
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\192\ Ecosystem services have been defined as ``the benefits
that people obtain from ecosystems'' (U.S. EPA, 2013a, Preamble, p.
1xxii; UNEP, 2003) and thus are an aspect of the use of a type of
vegetation or ecosystem. Similarly, a definition used for the
purposes of EPA benefits assessments states that ecological goods
and services are the ``outputs of ecological functions or processes
that directly or indirectly contribute to social welfare or have the
potential to do so in the future'' and that ``[s]ome outputs may be
bought and sold, but most are not marketed'' (U.S. EPA, 2006b).
\193\ Ecosystem services analyses were one of the tools used in
that review to inform the decisions made with regard to adequacy and
as such, were used in conjunction with other considerations in the
discussion of adversity to public welfare (77 FR 20241).
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Potential public welfare impacts associated with ecosystems and
associated services have a range of dimensions, including spatial,
temporal, and social, and these likely will vary depending on the type
of effect being characterized. For example, ecosystems can cover a
range of spatial scales, and the services they provide can accrue
locally or be distributed more broadly, such as when crops are sold and
eaten locally and/or also sold in regional, national and world markets.
Accordingly, impacts can be localized or more widely distributed.
Further, ecosystem services can be realized over a range of temporal
scales from immediate up to longer term. The size of the societal unit
receiving benefits from ecosystem services can also vary dramatically.
For example, a national park can provide direct recreational services
to the thousands of visitors that come each year, but also provide an
indirect value to the millions who may not visit but receive
satisfaction from knowing it exists and is preserved for the future
(U.S. EPA, 2014b, chapter 5, section 5.5.1).
As recognized in the last review, judgments regarding adverse
effects to the public welfare depend on the intended use for and
significance of the affected vegetation, ecological receptors,
ecosystems and resources to the public welfare (73 FR 16496, March 27,
2008).\194\ For example, a number of different types of locations
provide services of special significance to the public welfare. As
emphasized in previous O3 NAAQS decisions, and summarized in
section IV.A above, Class I areas and other parks have been afforded
special federal protection to preserve services that provide for the
enjoyment of these resources for current and future generations.
Surveys have indicated that Americans rank as very important the
existence of the resource, the option or availability of the resource
and the ability to bequest or pass on to future generations (Cordell et
al., 2008). These and other services provided by Class I areas and
other areas that have been afforded special protection can flow in part
or entirely from the vegetation that grows there. Aesthetic value and
outdoor recreation depend on the perceived scenic beauty of the
environment. Many outdoor recreation activities directly depend on the
scenic value of the area, in particular scenic viewing, wildlife-
watching, hiking, and camping (U.S. EPA, 2014b, chapters 5 and 7).
Further, analyses have reported that the American public values--in
monetary as well as nonmonetary ways--the protection of forests from
air pollution damage. In fact, studies that have assessed willingness-
to-pay for spruce-fir forest protection in the southeastern U.S. from
air pollution and insect damage have found that values held by the
survey respondents for the more abstract services (existence, option
and bequest) were greater than those for recreation or other services
(U.S. EPA, 2014b, Table 5-6; Haefele et al., 1991; Holmes and Kramer,
1995).
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\194\ As noted in section IV.A above, in judgments regarding
public welfare significance in the last review, emphasis was placed
on vegetation and ecosystem effects to sensitive species that are
known to or are likely to occur in federally protected areas such as
national parks and other Class I areas, or on lands set aside by
states, tribes and public interest groups to provide similar
benefits to the public welfare (73 FR 16496, March 27, 2008; 75 FR
3023-24, January 19, 2010).
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There are several potential public welfare impacts related to the
three main categories of O3 effects on vegetation (i.e.,
effects on tree growth, productivity and carbon storage; crop yield
loss; and, visible foliar injury, as described in section IV.B.1 above)
and their associated ecosystem services. At the same time, these three
categories of effects differ with regard to aspects important to
judging their public welfare significance. Judgments regarding crop
yield loss, for example, depend on considerations related to the heavy
management of agriculture in the U.S., while judgments regarding the
other categories of effects generally relate to considerations
regarding forested areas. For example, while both tree growth-related
effects and visible foliar injury have the potential to be significant
to the public welfare through impacts in Class I and other protected
areas, they differ in how they might be significant and with regard to
the clarity of the data which describes the relationship between the
effect and the services potentially affected.
With regard to effects on tree growth, reduced growth is associated
with effects on an array of ecosystem services including reduced
productivity, altered forest and forest community (plant, insect and
microbe) composition, reduced carbon storage and altered water cycling
(U.S. EPA, 2013a, Figure 9-1, sections 9.4.1.1 and 9.4.1.2; U.S. EPA,
2014b, section 6.1). For example, forest or forest community
composition can be affected through O3 effects on growth and
reproductive success of sensitive species in the community, with the
extent of compositional changes dependent on factors such as
competitive interactions (U.S. EPA, 2013a, sections 9.4.3 and 9.4.3.1).
Depending on the type and location of the affected ecosystem, services
benefitting the public in other ways can be affected as well. For
example, other services valued by people that can be affected by
reduced tree growth, productivity and carbon storage include aesthetic
value, food, fiber, timber, other forest products, habitat,
recreational opportunities, climate and water regulation, erosion
control, air pollution removal, hydrologic and fire regime
stabilization (U.S. EPA 2013a, sections 9.4.1.1 and 9.4.1.2; U.S. EPA,
2014b, section 6.1, Figure 6-1, section 6.4, Table 6-13). Further,
impacts on some of these services (e.g., forest or forest community
composition) may be considered of greater public welfare significance
when occurring in Class I or other protected areas.
Consideration of the magnitude of tree seedling growth effects that
might cause or contribute to adverse effects for trees, forests,
forested ecosystems or the public welfare is complicated by aspects of,
or limitations in, the available information. For example, the evidence
on tree seedling growth effects, deriving from the E-R functions for 11
species, provides no clear threshold or breakpoint in the response to
O3 exposure. Additionally, there are no established
relationships between magnitude of tree seedling growth reduction and
forest ecosystem impacts and, as noted in section IV.B.1.b above, other
factors can influence the degree to which O3-induced growth
effects in a sensitive species affect forest and forest community
composition and other ecosystem service flows from forested ecosystems.
These include: 1) the type of stand or community in which the sensitive
species is found (i.e., single species versus mixed canopy); 2) the
role or position the species has in the stand (i.e., dominant, sub-
dominant, canopy, understory); 3) the O3 sensitivity of the
other co-occurring species (O3 sensitive or tolerant); and
4) environmental factors, such as soil moisture and others. The lack of
such established relationships complicates judgments as to the extent
to which different amounts of tree seedling growth would be significant
to the public welfare and thus an important consideration in the level
of protection for the secondary standard.
During the 1997 review of the secondary standard, views related to
[[Page 75321]]
this issue were provided by a 1996 workshop of 16 then-leading
scientists in the context of discussing their views for a secondary
O3 standard (Heck and Cowling, 1997). In their consideration
of tree growth effects as an indicator for forest ecosystems and crop
yield reduction as an indicator of agricultural systems, the workshop
participants identified annual percentages, of RBL for forest tree
seedlings and RYL for agricultural crops, considered important to their
judgments on the standard. With regard to forest ecosystems and
seedling growth effects as an indicator, the participants selected a
range of 1-2% RBL per year ``to avoid cumulative effects of yearly
reductions of 2%.'' With regard to crops, they indicated an interest in
protecting against crop yield reductions of 5% RYL yet noted
uncertainties surrounding such a percentage which led them to
identifying 10% RYL for the crop yield endpoint (Heck and Cowling,
1997). The workshop report provides no explicit rationale for the
percentages identified (2% RBL and 5% or 10% RYL); nor does it describe
their connection to ecosystem impacts of a specific magnitude or type
and judgments on significance of the effects for public welfare, e.g.,
taking into consideration the intended use and significance of the
affected vegetation (Heck and Cowling, 1997). In recognition of the
complexity of assessing the adversity of tree growth effects and
effects on crop yield in the broader context of public welfare, the
EPA's consideration of those effects in both the 1997 and 2008 reviews
extended beyond the consideration of various benchmark responses for
the studied species, and with regard to crops, additionally took note
of their extensive management (62 FR 38856, July 18, 1997; 73 FR 16436,
March 27, 2008).
While, as noted above, public welfare benefits of forested lands
can be particular to the type of area in which the forest occurs, some
of the potential public welfare benefits associated with forest
ecosystems are not location dependent. A potentially extremely valuable
ecosystem service provided by forested lands and for which the ISA
concludes a likely causal relationship with O3 in ambient
air is carbon storage, a regulating service that is ``of paramount
importance for human society'' (U.S. EPA, 2013a, section 2.6.2.1 and p.
9-37). The service of carbon storage is potentially important to the
public welfare no matter in what location the sensitive trees are
growing, or what their intended current or future use. In other words,
the benefit exists as long as the tree is growing, regardless of what
additional functions and services it provides.
Another example of locations potentially vulnerable to
O3-related impacts but not necessarily identified for such
protection might be forested lands, both public and private, where
trees are grown for timber production, particularly where they are
dominated by a single timber species stand that is sensitive to
O3, such as ponderosa pine. Further, forests in urbanized
areas provide a number of services that are important to the public in
those areas, including air pollution removal, cooling of the heat
island effect, and beautification (U.S. EPA, 2014b, section 6.6.2 and
Appendix 6D; Akbari, 2002).\195\ The presence of O3-
sensitive trees in such areas may place them at risk from elevated
O3 exposures, contributing to potential impacts on important
services provided by urban forests (U.S. EPA, 2014b, sections 6.6.2 and
6.7). There are many other tree species, such as species used in the
USFS biomonitoring network, and various ornamental and agricultural
species (i.e., Christmas trees, fruit and nut trees) that provide
ecosystem services that may be judged important to the public welfare
but whose vulnerability to impacts from O3 on tree growth,
productivity and carbon storage has not been quantitatively
characterized (U.S. EPA, 2014b, Chapter 6; Abt Associates, 1995).
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\195\ For example, see http://www.fs.fed.us/research/urban/environmental-justice.php.
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As noted above, in addition to tree growth-related effects,
O3-induced visible foliar injury also has the potential to
be significant to the public welfare through impacts in Class I and
other similarly protected areas. Visible foliar injury is a visible
bioindicator of O3 exposure in species sensitive to this
effect, with the injury affecting the physical appearance of the plant.
Accordingly visible foliar injury surveys are used by federal land
managers as tools in assessing potential air quality impacts in Class I
areas. These surveys may focus on plant species that have been
identified as potentially sensitive air quality related values (AQRVs)
due to their sensitivity to O3-induced foliar injury (USFS,
NPS, FWS, 2010). An AQRV is defined by the National Park Services as a
``resource, as identified by the FLM for one or more Federal areas that
may be adversely affected by a change in air quality'' and the resource
``may include visibility or a specific scenic, cultural, physical,
biological, ecological, or recreational resource identified by the FLM
for a particular area'' (USFS, NPS, USFWS, 2010).\196\ No criteria have
been established, however, regarding a level or prevalence of visible
foliar injury considered to be adverse to the affected vegetation, and,
as noted in section IV.B.1.a above, there is not a clear relationship
between visible foliar injury and other effects, such as reduced growth
and productivity. Thus, key considerations with regard to public
welfare significance of this endpoint have related to qualitative
consideration of the plant's aesthetic value in protected forested
areas. Depending on the extent and severity, O3-induced
visible foliar injury might be expected to have the potential to impact
the public welfare in scenic and/or recreational areas during the
growing season, particularly in areas with special protection, such as
Class I areas.
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\196\ The identification, monitoring and assessment of AQRVs
with regard to an adverse effect is an approach used for assessing
the potential for air pollution impacts from pending permit actions
in Class I areas (USFS, NPS, USFWS, 2010). An adverse impact is
recognized by the National Park Service as one that results in
diminishment of the Class I areas's national significance or the
impairment of the ecosystem structure or functioning, as well as
impairment of the quality of the visitor experience (USFS, NPS,
USFWS, 2010). Federal land managers (FLMs) make such adverse impact
determinations on a case-by-case basis, using technical and other
information which they provide for consideration by permitting
authorities. The National Park Services has developed is a document
describing an overview of approaches related to assessing projects
under the National Environmental Policy Act and other planning
initiatives affecting the National Park System (http://www.nature.nps.gov/air/Pubs/pdf/AQGuidance_2011-01-14.pdf).
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The ecosystem services most likely to be affected by O3-
induced visible foliar injury (some of which are also recognized above
for tree growth-related effects) are cultural services, including
aesthetic value and outdoor recreation. In addition, several tribes
have indicated that many of the species identified as O3-
sensitive (including bioindicator species) are culturally significant
(U.S. EPA, 2014c, Table 5-1). The geographic extent of protected areas
that may be vulnerable to such public welfare effects of O3
is potentially appreciable. Sixty six species that occur on U.S.
National Park Service (NPS) and U.S. Fish and Wildlife Service lands
\197\ have been identified as sensitive to O3-induced
visible foliar injury and some also have particular cultural importance
to some tribes (U.S. EPA, 2014c, Table 5-1 and Appendix 5-A; U.S. EPA,
2014b, section 6.4.2). Not all species are equally sensitive to
O3, however, and quantitative relationships between
O3 exposure and other important effects, such as seedling
growth reduction, are
[[Page 75322]]
only available for a subset of the 66, as described in section IV.B.1.
above. A diverse array of ecosystem services has been identified for
these twelve species (U.S. EPA, 2014c, Table 5-1). Two of the species
in this group that are relatively more sensitive with regard to effects
on growth are the ponderosa pine and quaking aspen (U.S. EPA, 2014b,
section 6.2), the ranges for which overlap with many lands that are
protected or preserved for enjoyment of current and future generations
(consistent with the discussion above on Class I and other protected
areas), including such lands located in the west and southwest regions
of the U.S. where ambient O3 concentrations and associated
cumulative seasonal exposures can be highest (U.S. EPA, 2014c, Appendix
2B).\198\
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\197\ See http://www2.nature.nps.gov/air/Pubs/pdf/flag/NPSozonesensppFLAG06.pdf.
\198\ Basal area for resident species in national forests and
parks are available in files accessible at: http://www.fs.fed.us/foresthealth/technology/nidrm2012.shtml.
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With regard to agriculture-related effects, the EPA has recognized
other complexities, stating that the degree to which O3
impacts on vegetation that could occur in areas and on species that are
already heavily managed to obtain a particular output (such as
commodity crops or commercial timber production) would impair the
intended use at a level that might be judged adverse to the public
welfare has been less clear (73 FR 16497, March 27, 2008; 75 FR 3024;
January 19, 2010). We note that while having sufficient crop yields is
of high public welfare value, important commodity crops are typically
heavily managed to produce optimum yields. In light of all of the
inputs that go into achieving these yields, such as fertilizer,
herbicides, pesticides, and irrigation, it is difficult to determine at
what point O3-induced yield loss creates an adverse impact
for the producer in the way of requiring increased inputs in order to
maintain the desired yields. Moreover, based on the economic theory of
supply and demand, increases in crop yields would be expected to result
in lower prices for affected crops and their associated goods, which
would primarily benefit consumers. Given these competing impacts on
producers and consumers, it is unclear how to consider these effects in
terms of potential adversity to the public welfare (U.S. EPA, 2014c,
sections 5.3.2 and 5.7).
When agricultural impacts or vegetation effects in other areas are
contrasted with the emphasis on forest ecosystem effects in Class I and
similarly protected areas, it can be seen that the Administrator has in
past reviews judged the significance to the public welfare of
O3-induced effects on sensitive vegetation growing within
the U.S. to differ depending on the nature of the effect, the intended
use of the sensitive plants or ecosystems, and the types of
environments in which the sensitive vegetation and ecosystems are
located, with greater significance ascribed to areas identified for
specific uses and benefits to the public welfare, such as Class I
areas, than to areas for which such uses have not been established. In
summary, several considerations are recognized as important to
judgments on the public welfare significance of the array of effects of
different O3 exposure conditions on vegetation. While there
are complexities associated with the consideration of the magnitude of
key vegetation effects that might be concluded to be adverse to
ecosystems and associated services, there are numerous locations where
O3-sensitive tree species are present that may be vulnerable
to impacts from O3 on tree growth, productivity and carbon
storage and their associated ecosystems and services. It is not
possible to generalize across all studied species regarding which
cumulative exposures are of greatest concern, however, as this can vary
by situation due to differences in exposed species sensitivity, the
importance of the observed or predicted O3-induced effect,
the role that the species plays in the ecosystem, the intended use of
the affected species and its associated ecosystem and services, the
presence of other co-occurring predisposing or mitigating factors, and
associated uncertainties and limitations. These factors contribute to
the complexity of the Administrator's judgments regarding the adversity
of known and anticipated effects to the public welfare.
C. Exposure and Risk Assessment Information
The WREA characterized ambient O3 exposure and its
relationship to tree biomass loss, crop yield loss, and visible foliar
injury and the associated ecosystem services \199\ in national-scale
and case study analyses. The WREA also qualitatively assessed impacts
to some ecosystem services, including impacts on the hydrologic cycle,
pollination regulation, and fire regulation; commercial non-timber
forest products and insect damage; and aesthetic and non-use values. In
the quantitative analyses, the WREA characterized effects associated
with exposures to O3 in ambient air using the W126 metric.
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\199\ In its review of drafts of the WREA and PA, the CASAC
conveyed support for analyses and considerations of ecosystem
services that may be affected by O3 exposures (Frey,
2014b, 2014c).
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The following sections summarize the analyses and adjustment
approach used to develop the O3 concentrations used as
inputs to the vegetation risk analyses for tree biomass and crop yield
loss, and the analyses, including key results and uncertainties, for
tree seedling growth, productivity, carbon storage and associated
ecosystem services (section IV.C.2); crop yield loss (section IV.C.3);
and visible foliar injury (section IV.C.4).
1. Air Quality Analyses
The WREA evaluated O3 exposure and risks for several
national-scale air quality scenarios: recent conditions (2006 to
2008),\200\ the current secondary standard, and W126 index values of 15
ppm-hrs, 11 ppm-hrs, and 7 ppm-hrs, using three-year averages (U.S.
EPA, 2014b, chapter 4). For each of these scenarios, three-year average
W126 index values were estimated at each 12 km by 12 km grid cell in a
national-scale spatial surface. Additionally, some analyses were based
on single-year surfaces.\201\ The method for creating the five
scenarios generally involved two steps (summarized in Table 5-4 of the
PA). The first is derivation of the average W126 index value (across
the three years) at each monitor location. This value is based on
unadjusted O3 concentrations from monitoring data for recent
conditions and adjusted concentrations for the four other scenarios.
Concentrations were adjusted based on model predicted relationships
between O3 and U.S.-wide emissions reductions in oxides of
nitrogen (NOx). The adjusted air quality does not represent an
optimized control scenario that just meets the current standard (or
target W126 index values for other scenarios), but rather characterizes
one potential distribution of air quality across a region when all
monitor locations meet the standard (U.S. EPA, 2014b, section 4.3.4.2).
The development of adjusted concentrations was done for each of nine
regions independently (see U.S. EPA, 2014b, section 4.3.4.1). In the
second step, national-scale spatial surfaces (W126 index values for
each 12 km x 12 km
[[Page 75323]]
grid cell used in the air quality model) were created using the
monitor-location values and the Voronoi Neighbor Averaging (VNA)
spatial interpolation technique (details on the VNA technique are
presented in U.S. EPA, 2014b, Appendix 4A).
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\200\ Certain visible foliar injury analyses assessed recent
conditions from 2006 to 2010 on an annual basis.
\201\ An analysis using data from USFS FHM/FIA O3
biomonitoring sampling sites (``biosites'') and a screening-level
assessment in 214 national parks were done using national-scale
spatial surfaces of unadjusted O3 concentrations (in
terms of the W126 index) created for each year from 2006 through
2010 using the VNA interpolation technique (U.S. EPA, 2014b, section
4.3.2, Appendix 4A).
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In the dataset used to create the recent conditions scenario, the
three-year average W126 index values at the monitor locations (before
application of the VNA technique) ranged from below 5 ppm-hrs to 48.6
ppm-hrs (U.S. EPA, 2014b, Figure 4-4 and Table 4-3). In the nine
modeling regions, the maximum three-year average W126 index values at
monitor locations ranged from 48.6 ppm-hrs in the West region down to
6.6 ppm-hrs in the Northwest region.\202\ After adjustment of the
monitor location concentrations to just meet the current standard in
each region (using relationships described above), the region-specific
maximum three-year average W126 values ranged from 18.9 ppm-hrs in the
West region to 2.6 ppm-hrs in the Northeast region (U.S. EPA, 2014b,
Table 4-3). With the next step, creation of the national surface of air
quality values at grid cell centroids, the highest values were reduced,
such that all the three-year average W126 index values were below 15
ppm-hrs across the national surface with the exception of a very small
area of the Southwest region (near Phoenix) where average W126 index
values were just above 15 ppm-hrs. Thus, it can be seen that
application of the VNA interpolation method to estimate W126 index
values at the centroid of every 12 x 12 km \2\ grid cell rather than
only at each monitor location results in a lowering of the highest
values.
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\202\ The regions referenced here and also with regard to
monitoring data analyses described in section IV.D.4 below are NOAA
climate regions, as shown in Figure 2B-1 of the PA.
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Because the W126 estimates generated for the different air quality
scenarios assessed are inputs to the vegetation risk analyses for tree
biomass and crop yield loss, and also used in the foliar injury
analyses, any uncertainties in the air quality analyses are propagated
into the those analyses (U.S. EPA, 2014b, section 8.5). The WREA
identified sources of uncertainty for the W126 estimates for each air
quality scenario and qualitatively characterized the magnitude of
uncertainty and potential for directional bias (U.S. EPA, 2014b, Table
4-5). As discussed in Chapter 4 and 8 of the WREA, an important large
uncertainty in the analyses is the assumed response of the W126
concentrations to emissions reductions needed to meet the existing
standard (U.S. EPA, 2014b, section 8.5.1). Any approach to
characterizing O3 air quality over broad geographic areas
based on concentrations at monitor locations will convey inherent
uncertainty. The model-based adjustments are based on U.S.-wide
emissions reductions in NOx and characterize only one potential
distribution of air quality across a region when all monitor locations
meet the standard (U.S. EPA, 2014b, section 4.3.4.2).\203\
Additionally, the surface is created from the three-year average at the
monitor locations, rather than creating a surface for each year and
then averaging across years at each grid cell; the potential impact of
this on the resultant estimates is considered in the WREA (U.S. EPA,
2014b, Appendix 4A).
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\203\ The WREA analyses used U.S.-wide NOx emissions reductions
to simulate air quality that independently in each region would just
meet the existing standard and the three W126 scenarios. The NOx
emissions reductions were determined such that the highest monitor
within each region would just meet the target level. In this way,
the adjustment results in broad regional reductions in O3
and includes reductions in O3 at some monitors that were
already meeting or below the target level. Thus, the adjustments
performed to develop a scenario meeting a target level at the
highest monitor in each region did result in substantial reduction
below the target level in some areas of the region. This result at
the monitors already well below the target indicates an uncertainty
with regard to air quality expected from specific control strategies
that might be implemented to meet a particular target level.
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An additional uncertainty related to the W126 index value estimates
for each air quality scenario comes from the creation of a national
W126 surface using the VNA technique to interpolate recent air quality
measurements of O3. In general, spatial interpolation
techniques perform better in areas where the O3 monitoring
network is denser. Therefore, the W126 index values estimated in the
rural areas in the West, Northwest, Southwest, and West North Central
with few or no monitors (U.S. EPA, 2014b, Figure 2-1) are more
uncertain than those estimated for areas with denser monitoring.
Further, this interpolation method generally underpredicts higher 12-
hour W126 exposures. Due to the important influence of higher exposures
in determining risks to plants, the potential for the VNA interpolation
approach to underpredict higher W126 exposures could result in an
underestimation of risks to vegetation in some areas. Underestimation
of the highest W126 index values for the current standard scenario is
an additional impact of the interpolation method that is important to
consider.
2. Tree Seedling Growth, Productivity, Carbon Storage and Associated
Ecosystem Services
For the WREA assessments related to tree growth, productivity,
carbon storage and associated ecosystem services, the sections below
provide an overview of the analyses along with the key results (section
IV.C.2.a) and summarize the key uncertainties (section IV.C.2.b).
a. Overview and Summary of Key Results
The assessments to estimate the exposures and risks for tree
seedling growth, productivity, and carbon storage reflect a range of
spatial scales ranging from the county scale up to the national park,
urban area, and national scales. For the air quality scenarios
described above, the WREA applied the species-specific E-R functions to
develop estimates of O3-associated RBL, productivity, carbon
storage and associated ecosystem services (U.S. EPA, 2014b, Chapter 6).
Some analyses also apply the median across species E-R functions.
The WREA examined multiple approaches for characterizing the median
tree response to O3 exposure based on the 11 robust E-R
functions for tree seedlings from the OTC research and the E-R function
for eastern cottonwood (U.S. EPA, 2014b, section 6.2.1.2 and Figure 6-
5). For some species, only one study was available (e.g., red maple),
and for other species there were as many as 11 studies available (e.g.,
ponderosa pine). To illustrate the effect of within-species variability
associated with the E-R data available on estimates for a median
response across the 12 species, the WREA performed a stochastic
sampling analysis involving multiple iterations of random selection of
E-R functions from the studies available for each of the 12 species.
This analysis produced median values at each cumulative exposure level
that were higher than medians derived by two conventional,
deterministic methods (U.S. EPA, 2014b, section 6.2.1.2 and Figure 6-
5).\204\ For example, the median seasonal W126 index value for which a
two percent biomass loss is estimated in seedlings for the studied
species ranges from approximately 7 ppm-hrs using the conventional
methods up to 14 ppm-hrs when derived by the stochastic method.
Although the stochastic method provides some illustration of the effect
of within-species variability, we focus on the conventional approach
that gives equal weight to each studied species,
[[Page 75324]]
calculating the median response based on the composite E-R functions,
consistent with CASAC advice (Frey, 2014b).
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\204\ These methods were calculating a median using the
composite functions and calculating a median using all tree seedling
studies available.
---------------------------------------------------------------------------
The WREA estimates indicate substantial heterogeneity in plant
responses to O3, both within species, between species, and
across regions of the U.S. The tree species known to be O3-
sensitive are different in the eastern and western U.S. and the eastern
U.S. has far more such species. Ozone exposure and risk is somewhat
easier to assess in the eastern U.S. because of the availability of
more data and the greater number of species to analyze. In addition,
there are more O3 monitors in the eastern U.S. but fewer
national parks (U.S. EPA, 2014b, chapter 8). In consideration of CASAC
advice, the WREA derived RBL and weighted RBL (wRBL) estimates
separately with and without the eastern cottonwood. The results
summarized here are for the analyses that exclude cottonwood.\205\ The
WREA reported RBL estimates relative to a benchmark of 2% RBL for tree
seedlings, as well as relative to other percent RBL values. The 2% RBL
benchmark was considered based on CASAC advice that stated that ``focus
on a 2% loss level for trees . . . is appropriate.'' (Frey, 2014b, p.
6). The main WREA analyses for effects related to tree growth,
productivity and carbon storage are summarized below, with the key
findings for each.
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\205\ The CASAC advised that the eastern cottonwood response
data ``receive too much emphasis'' in a draft version of the PA,
explaining that these ``results are from a gradient study that did
not control for ozone and climatic conditions and show extreme
sensitivity to ozone compared to other studies'' and that
``[a]lthough they are important results, they are not as strong as
those from other experiments that developed E-R functions based on
controlled ozone exposure'' (Frey, 2014b, p. 10).
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Relative biomass loss nationally was estimated for each of the 12
studied species from the composite E-R functions for each species
described above and information on the distribution of those species
across the U.S. (U.S. EPA, 2014b, section 6.2.1.3 and Appendix 6A). As
one example of a tree species near the median of the studied species,
relative biomass loss estimates (reduced growth) for ponderosa pine in
the current standard air quality scenario are below two percent for
most areas where this species is found but estimates of RBL for this
species in some areas of the southwest fall above two percent biomass
loss (U.S. EPA, 2014b, Figure 6-8). Maximum estimates of RBL for all
areas where ponderosa pine is found decrease to just over three percent
and just over two percent for the 15 and 7 ppm-hrs scenarios,
respectively (U.S. EPA, 2014b, Table 6-6).
To provide an indication of ecosystem-level impacts, weighted
estimates of RBL (wRBL) were also developed for each grid cell
nationwide. This is estimated from the species-specific E-R functions
and a weighting approach based on information on prevalence of the
studied species across the U.S. (i.e., the proportion of the total
basal area modeled by USFS across all species for which data were
available). An overall wRBL value for each grid cell is generated by
summing the wRBL values for each studied tree species found within that
grid cell. The wRBL is intended to be an indication of the potential
magnitude of the ecological effect that could occur in some ecosystems.
In general, the higher the wRBL is in a given ecosystem, the larger the
potential ecological effect. (U.S. EPA, 2014b, section 6.8, Table 6-
25).
For the national-scale analysis, the WREA presents the percent of
total basal area with wRBL greater than 2%. The estimates for the
weighted biomass loss analysis reflecting the 11 tree species with
robust E-R functions are as follows (U.S. EPA, 2014b, Table 6-25):
For the current standard scenario, the percent of total
basal area that exceeds a two percent wRBL is 0.2 percent.
For the W126 scenarios of 15, 11 and 7 ppm-hrs, the
percent of total basal area that exceeds a two percent wRBL is 0.2
percent, 0.1 percent, and less than 0.1 percent respectively (U.S. EPA,
2014b, Table 6-25).
In the wRBL analysis for Class I areas, the number of Class I areas
with wRBL greater than 2% is estimated for the grid cells located in
the 145 of the 156 Class I areas for which data were available (U.S.
EPA, 2014b, Table 6-26).
For the current standard scenario, two of the 145 assessed
Class I areas have weighted RBL values above two percent (U.S. EPA,
2014b, Table 6-26).
For the W126 scenarios of 15, 11 and 7 ppm-hrs, there are
two, two and one Class I area with wRBL above two percent,
respectively.
In the county analysis, the WREA estimated the number of U.S.
counties in which any of the studied tree species is estimated to
experience more than two percent RBL, the number of species affected,
and the number of counties for which the median of the species-specific
functions exceeds two percent RBL. In addition to the estimates based
on all 12 studied species and also the 11 species with the exclusion of
eastern cottonwood (in response to CASAC advice), additional estimates
were developed without black cherry to show contribution of that
sensitive species to the multi-species estimates (U.S. EPA, 2014b,
Table 6-7).
In the current standard scenarios, 66% of the 3,109
assessed counties are estimated to have at least one of the 11 species
(excluding cottonwood) with an RBL greater than two percent, with three
counties having three species exceeding two percent. The median RBL
(across the species present) is above two percent in 239 counties. The
maximum number of species in any one county with an RBL greater than
two percent is three (excluding cottonwood). (U.S. EPA, 2014b, Table 6-
7).
For the 15, 11 and 7 ppm-hrs scenarios, the proportion of
3,109 counties with one or more species with an RBL above two percent
decreases to 61 percent, 59 percent, and 58 percent, respectively. For
the 7 ppm-hrs scenario, the median RBL is above two percent in six
percent of the counties (U.S. EPA, 2014b, Table 6-7).
The county RBL estimates are appreciably influenced by
black cherry, a very sensitive species that is widespread in the
Eastern U.S. For 1,805 of the 1,929 counties estimated to have at least
one species with an RBL greater than two percent when air quality is
meeting the current standard, only black cherry exceeds this level of
RBL. If black cherry is excluded, the median RBL for the 10 remaining
species decreases. For the median RBL values, 203 of the 239 counties
estimated to have a median RBL above two percent when air quality is
meeting the current standard are because of the presence of black
cherry (U.S. EPA, 2014b, Table 6-7).
Additionally, the WREA estimated relative yield loss in timber
production and associated changes in consumer and producer/farmer
economic surplus using E-R functions for tree seedlings to calculate
relative yield loss (equivalent to relative biomass loss) across full
tree lifespans and through modeling of the resulting market-based
welfare effects. Because the forestry and agriculture sectors are
related and trade-offs occur between the sectors, the WREA calculated
the resulting market-based welfare effects of O3 exposure in
the forestry and agriculture sectors on consumer and producer
surplus.\206\
[[Page 75325]]
Because demand for most forestry and agricultural commodities is not
highly responsive to changes in price, producer surplus (i.e., producer
gains) often declines. These declines can be more than offset by
changes in consumer surplus gains from lower prices, but, in some
cases, lower prices reduce producer gains more than can be offset by
consumer surplus (U.S. EPA, 2014b, Appendix 6B, Table and B-9).
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\206\ The WREA used the Forest and Agricultural Sector
Optimization Model with Greenhouse Gases (FASOMGHG). FASOMGHG is a
national-scale model that provides a complete representation of the
impacts of meeting alternative standards on the U.S. forest and
agricultural sectors. FASOMGHG simulates the allocation of land over
time to competing activities in both the forest and agricultural
sectors. FASOMGHG results include multi-period, multi-commodity
results over 60 to 100 years in 5-year time intervals when running
the combined forest-agriculture version of the model.
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In the current standard scenario, estimates of the
relative yield loss for timber production are below one percent other
than in the Southwest, Southeast, Central, and South regions (U.S. EPA,
2014b, section 6.3, Table 6-9) (see U.S. EPA, 2014b, Table 6-8 for
clarification on region names). The highest yield loss occurs in upland
hardwood forests in the South Central and Southeast regions at over
three percent per year and in Corn Belt hardwoods at just over two
percent loss per year (U.S. EPA, 2014b, section 6.3, Table 6-9).
For the 15 and 11 ppm-hrs scenarios, relative yield loss
estimates for timber production are above one percent in parts of the
Southeast, Central, and South regions and above two percent in parts of
the Southeast and Central U.S.
For the 7 ppm-hrs scenario, relative yield loss estimates
for timber production are above one percent in the Southeast and South
regions (U.S. EPA, 2014b, section 6.3, Table 6-9).
The WREA also estimated impacts on tree growth and two ecosystem
services provided by urban trees: removal of air pollutants and carbon
storage. The estimates of the tons of carbon monoxide, nitrogen
dioxide, ozone and sulfur dioxide removed are for a 25-year period in
five urban case study areas: Baltimore, Syracuse, the Chicago region,
Atlanta, and the urban areas of Tennessee (U.S. EPA, 2014b, section
6.7).\207\
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\207\ The WREA used the i-Tree model for the urban case studies.
i-Tree is a peer-reviewed suite of software tools provided by USFS.
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Estimates for all five urban case study areas indicate
increased pollutant removal of O3, nitrogen dioxide, carbon
monoxide, and sulfur dioxide in the current standard scenario (U.S.
EPA, 2014b, sections 6.7). The results for the 15 ppm-hrs scenario were
very similar to those for meeting the current standard. For the 11 and
7 ppm-hrs scenarios, all five case study areas indicate smaller
additional increases in air pollutant removal beyond moving from
current conditions to the current standard (U.S. EPA, 2014b, sections
6.7).
The WREA estimated carbon storage related to O3-induced
biomass loss in forests and agricultural crops nationally and also in
forests in five urban areas using the FASOMGHG and i-Tree models noted
above (U.S. EPA, 2014b, section 6.6). Ozone effects on tree growth
affects the climate regulation service provided by ecosystems by
reducing carbon sequestration and storage (U.S. EPA, 2013a, section
9.4.3.4; U.S. EPA, 2014b, chapter 6, section 6.6). Because
O3 exposure affects photosynthesis and CO2 uptake
by trees, forests sequester less carbon and thus more carbon stays in
the atmosphere. In the model used to calculate national-level impacts
to forests and agriculture from O3-related biomass loss,
carbon sequestration reflects carbon in standing (live and dead) trees,
forest soils, the forest understory vegetation, forest floor including
litter and large woody debris, and wood products both in use and in
landfills (U.S. EPA, 2014b, chapter 6, Appendix 6B, section 2.7.1).
Over 30 years for the national-scale analysis, carbon
storage in the forestry sector estimated for the current standard
scenario is just over 89,000 million metric tons of CO2
equivalents (MMtCO2e); this is 11,840 more
MMtCO2e storage associated with the reduced O3-
related growth impact from meeting the current standard as compared
with recent conditions.\208\ The estimates of carbon storage in the
agricultural sector are much smaller (i.e., 8,469 MMtCO2e
for the current standard scenario which is 606 MMtCO2e more
than the recent conditions scenario) (U.S. EPA, 2014b, section 6.6.1
and Appendix 6B). The forestry sector carbon storage estimated for each
of the three W126 scenarios is just slightly greater than that
estimated for the current standard. As a percentage of the current
standard estimate, the three scenario estimates are less than 0.1% (13
MMtCO2e), just under 1% (593 MMtCO2e) and under
2% (1,600 MMtCO2e) for the 15, 11 and 7 ppm-hrs scenarios,
respectively (U.S. EPA, 2014b, Tables 6-19 and B-10).
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\208\ One MMtCO2e is equivalent to 208,000 passenger
vehicles or the electricity to run 138,000 homes for 1 year as
calculated by the EPA Greenhouse Gas Equivalencies Calculator
(updated September 2013 and available at http://www.epa.gov/cleanenergy/energy-resources/calculator.html).
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Estimates of the effects of avoided O3-related
biomass loss on carbon sequestration in forests in the five urban area
case studies indicate the potential for an increase in carbon
sequestration of somewhat more than one MMtCO2e for the
current standard scenario compared to the recent conditions estimate
(U.S. EPA, 2014b, section 6.6.2 and Appendix 6D). The additional
increases in O3-related carbon sequestration estimated
across the five case studies for the three W126 scenarios are
relatively small (U.S. EPA, 2014b, section 6.6.2 and Appendix 6D).
Although not discussed in detail here, the WREA also describes
qualitative assessments for some ecosystem services that may be
affected by O3 effects on tree growth and productivity, such
as commercial non-timber forest products and recreation (U.S. EPA,
2014b, section 6.4), aesthetic and non-use values (U.S. EPA, 2014b,
section 6.4), increased susceptibility to insect attack and fire damage
(U.S. EPA, 2014b, sections 5.3 and 5.4, respectively). Other ecological
effects that are causally or likely causally associated with
O3 exposure, such as effects on terrestrial productivity,
the water cycle, the biogeochemical cycle, and community composition
(U.S. EPA, 2013a, Table 9-19), were not quantitatively addressed in the
WREA due to a lack of sufficient quantitative information.
b. Key Uncertainties
The WREA identified several key limitations and uncertainties in
the biomass loss assessments for trees, which may have a large impact
on both overall confidence and confidence in individual analyses. Key
uncertainties that affect the assessment of impacts on ecosystem
services at the national and case-study scales, as well as across
species, U.S. geographic regions and future years, include those
associated with the interpolated and adjusted O3
concentrations used to estimate W126 exposures in the air quality
scenarios, the available seedling E-R functions, combining effects
across sensitive species, the effects of compounding over time, and
modeling impacts of biomass loss on timber harvesting and urban air
pollutant removal.
With regard to the robust seedling E-R functions, the WREA provided
some characterization of the variability of individual study results
and the impact of that on estimates of W126 index values that might
elicit different percentages of biomass loss in tree seedlings (U.S.
EPA, 2014b, section 6.2.1.2). Even though the evidence shows that there
are additional species affected by O3-related biomass loss,
the WREA only has E-R functions available to quantify this loss for 12
tree species. This limited information only allows a partial
characterization of the O3-related biomass loss impacts in
trees associated
[[Page 75326]]
with recent O3 index values and with just meeting the
existing and potential alternative secondary standards. In addition,
there are uncertainties inherent in these E-R functions, including the
extrapolation of relative biomass loss rates from tree seedlings to
adult trees and information regarding within-species variability. The
overall confidence in the E-R function varies by species based on the
number of studies available for that species. Some species have low
within-species variability (e.g., many agricultural crops) and high
seedling/adult comparability (e.g., aspen), while other species do not
(e.g., black cherry). The uncertainties in the E-R functions for
biomass loss and in the air quality analyses are propagated into the
analysis of the impact of biomass loss on ecosystem services, including
provisioning and regulating services (U.S. EPA, 2014b, Table 6-27). The
WREA characterizes the direction of potential influence of E-R function
uncertainty as unknown, yet its magnitude as high, concluding that
further studies are needed to determine how accurately the assessed
species reflect the larger suite of O3-sensitive tree
species in the U.S. (U.S. EPA, 2014b, Table 6-27).
Another uncertainty associated with interpretation of the WREA
biomass loss-related estimates concerns the potential for
underestimation of compounding of growth effects across multiple years
of varying concentrations. Though tree biomass loss impacts were
estimated using air quality scenarios of three-year average W126 index
values, the WREA also conducted an analysis to compare the impact of
using a variable compounding rate based on yearly variations in W126
exposures to that of using a W126 index value averaged across three
years. The WREA compared the compounded values for an example species
occurring in the eastern U.S. and another example species occurring in
the western U.S. In both examples, one species (tulip polar and
ponderosa pine, respectively) and one climate region where that species
occurred (Southeast and Southwest regions, respectively) were chosen
and air quality values associated with just meeting the existing
standard of 75 ppb were used. Within each region, the WREA analysis
used both the W126 index value at each monitor in the region for each
year and the three-year average W126 index value using the method
described in Chapter 4 of the WREA. The results show that the use of
the three-year average W126 index value may underestimate RBL values
slightly (U.S. EPA, 2014b, section 6.2.1.4 and Figure 6-14). In both
regions, the three-year average W126 index value is sometimes above and
sometimes below the individual year W126 index value.
The WREA recognizes uncertainty regarding the extent to which the
subset of studied tree species encompass the O3 sensitive
species in the U.S. and the extent to which it represents U.S.
vegetation as a whole (U.S. EPA, 2013a, pp. 9-123 to 9-125; U.S. EPA,
2014b, Table 6-27). There are also uncertainties associated with
estimating the national scale ecosystem-level impacts using wRBL. For
example the wRBL estimates are likely biased low as there may be other
unstudied O3-sensitive tree species in some areas that are
also being affected at those levels, although this analysis does not
take into account the effects of competition, which could further
affect forest biomass loss.
Uncertainties are recognized in the national-scale analyses of
timber production, agricultural harvesting, and carbon sequestration,
for which the WREA used the FASOMGHG model. These uncertainties include
those associated with the functions for carbon sequestration, the
assumptions made regarding proxy species where there are insufficient
data, and the non-W126 E-R functions for three crops. The FASOMGHG
model does not include agriculture and forestry on public lands,
changes in exports due to O3 into international trade
projections, or forest adaptation. Despite the inherent limitations and
uncertainties, the WREA concludes that the FASOMGHG model reflects
reasonable and appropriate assumptions for a national-scale assessment
of changes in the agricultural and forestry sectors due to changes in
vegetation biomass associated with O3 exposure (U.S. EPA,
2014b, sections 6.3, 6.5, 6.6, and 8.5.2, and Table 6-27).
In the case study analyses of five urban areas, the WREA used the
i-Tree model, which includes an urban tree inventory for each area and
species-specific pollution removal and carbon sequestration functions.
However, i-Tree does not account for the potential additional VOC
emissions from tree growth, which could contribute to O3
formation. Uncertainties are also recognized with regard to the base
inventory of city trees, the functions used for air pollutant removal
and for carbon storage (U.S. EPA, 2014b, sections 6.6.2 and 6.7, and
Table 6-27). Despite the inherent limitations and uncertainties, the
WREA concludes that the i-Tree model reflects reasonable and
appropriate assumptions for a case study assessment of pollution
removal and carbon sequestration for changes in biomass associated with
O3 exposure (U.S. EPA, 2014b, sections 6.6.2, 6.7, and
8.5.2).
3. Crop Yield
Section IV.C.3.a below provides an overview of the assessments
performed in the WREA to estimate the exposures and risks for crop
yield, as well as the key results. Section IV.C.3.b summarizes the key
uncertainties.
a. Overview and Summary of Key Results
The WREA conducted two analyses to estimate O3 impacts
related to crop yield, including annual yield losses estimated for 10
commodity crops grown in the U.S. with E-R functions and how these
losses affect producer and consumer economic surpluses (U.S. EPA,
2014b, sections 6.2, 6.5). Summary estimates for crop yield loss
related effects in the WREA are presented relative to a 5% yield loss
benchmark based on consideration of CASAC's recommendation to consider
a benchmark of 5% for median crop yield loss and to consider 5% yield
loss for individual crop species. In addition, other benchmarks levels
are considered in the WREA (e.g. 10% and 20%).
The WREA derived estimates of crop RYL estimates nationally and in
a county-specific analysis. Crop-specific estimates of O3-
related RYL nationally were derived for each of the air quality
scenarios from the 10 E-R functions for crops described above combined
with information regarding crop distribution (U.S. EPA, 2014b, section
6.5). The WREA also reported crop RYL results at the county-level, as
well as the number of crop-producing counties with greater than five
percent RYL (U.S. EPA, 2014b, section 6.5.1, Appendix 6B).
The largest reduction in O3-induced crop yield
loss and yield changes occurs when moving from the recent conditions
scenario to the current standard scenario (U.S. EPA, 2014b, section
6.5). Among the major commercial crops, winter wheat and soybeans are
more sensitive to ambient O3 levels than other crops.
In the current standard scenario, no counties have RYL
estimates at or above 5% (U.S. EPA, 2014b, section 6.5).\209\
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\209\ In the air quality scenario for the current standard, a
monitor that already met the current standard but was located within
the same region as another monitor that was above the current
standard would have had its concentration adjusted downward. This is
due to the fact that concentrations were adjusted independently for
each region, applying reductions to all monitors within the region,
such that all monitors located within a region meet the standard
(U.S. EPA, 2014b, section 4.3.4.2).
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[[Page 75327]]
The WREA also estimated O3-related crop impacts on
producer and consumer surplus.\210\ These are national-scale estimates
of the effects of yield loss on agricultural harvesting, which supply
provisioning services of food and fiber for each of the air quality
scenarios. Overall effect on agricultural yields and producer and
consumer surplus depends on (1) the ability of producers/farmers to
substitute other crops that are less O3 sensitive, and (2)
the responsiveness, or elasticity, of demand and supply (U.S. EPA,
2014b, section 6.5).
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\210\ Welfare economics focuses on the optimal allocation of
resources and goods and how those allocations affect total social
welfare. Total welfare is also referred to as economic surplus,
which is the overall benefit a society, composed of consumers and
producers, receives when a good or service is bought or sold, given
a quantity provided and a market price. Economic surplus is divided
into two parts: Consumer and producer surplus (U.S. EPA, 2014b, p.
ES-6).
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Estimates of consumer surplus, or consumer gains, were
generally higher in the current standard scenario in the agricultural
sector because higher productivity under lower O3
concentrations increased total yields and reduced market prices (U.S.
EPA, 2014b, Tables 6-17 and 6-18). Combined gains in producer and
consumer surplus for forestry and agriculture were essentially
unchanged for the 15 ppm-hrs scenario, but annualized gains increased
by $21 million beyond the current standard scenario for the 11 ppm-hrs
scenario and $231 million for the 7 ppm-hrs scenario. In some cases,
lower prices reduce producer gains more than can be offset by higher
yields (U.S. EPA, 2014b, Table 6-18).
Because demand for most agricultural commodities is not
highly responsive to changes in price, producer surplus or producer
gains often declined. For agricultural welfare, annualized combined
consumer and producer surplus gains were estimated to be $2.6 trillion
in 2010 for the current standard scenario (U.S. EPA, 2014b, Table 6-
17).
b. Key Uncertainties
The WREA discusses multiple areas of uncertainty associated with
the crop yield loss estimates, including those associated with the
model-based adjustment methodology as well as those associated with the
projection of yield loss using the FASOMGHG model at the estimated
O3 concentrations (U.S. EPA, 2014b, Table 6-27, section
8.5). Because the W126 estimates generated in the air quality analyses
are inputs to the vegetation risk analyses for crop yield loss, any
uncertainties in the air quality analyses are propagated into the those
analyses (U.S. EPA, 2014b, Table 6-27, section 8.5). Therefore, the air
quality scenarios in the crop yield analyses have the same
uncertainties and limitations as in the biomass loss analyses
(summarized above), including those associated with the model-based
adjustment methodology (U.S. EPA, 2014b, section 8.5).
4. Visible Foliar Injury
Section IV.C.4.a below provides an overview of the assessment in
the WREA of O3-related visible foliar injury and associated
ecosystem services impacts, as well as the key results. Section
IV.C.4.b summarizes the key uncertainties.
a. Overview and Summary of Key Results
The WREA presents a number of analyses of O3-related
visible foliar injury and associated ecosystem services impacts (U.S.
EPA, 2014b, Chapter 7). An initial analysis using USFS FHM/FIA biosite
data included the development of benchmark criteria reflecting
different prevalences of visible foliar injury at different W126
exposures and soil moisture conditions. These criteria were then used
in a screening-level characterization of the potential risk of foliar
injury incidence in 214 national parks and a case study assessment of
three national parks, which also provides limited characterization of
the associated ecosystem services.
In the biosite data analysis, the WREA used the biomonitoring site
data from the USFS FHM/FIA Network (USFS, 2011),\211\ associated soil
moisture data during the sample years, and national surfaces of ambient
air O3 concentrations based on spatial interpolation of
monitoring data from 2006 to 2010 to calculate the proportion of
biosites with any visible foliar injury. The proportion of biosites
metric is derived by first ordering the data (across biosites and
sample years) by W126 index value estimated for that biosite and year.
Then for each W126 index value, the proportion of biosites is
calculated with any foliar injury for all observations at or below that
W126 index value. (U.S. EPA, 2014b, section 7.2). This analysis
indicates that the proportion of biosites showing the presence of any
foliar injury increases rapidly from zero to about 20 percent at
relatively low W126 index values. Specifically: (1) the proportion of
biosites exhibiting foliar injury rises rapidly with increasing W126
index values below approximately 10 ppm-hrs (W126 <10.46 ppm-hrs), and
(2) there is relatively little change in this proportion with
increasing W126 index values above approximately 10 ppm-hrs (W126
>10.46 ppm-hrs). The data for biosites during normal moisture years are
very similar to the dataset as a whole, with an overall proportion of
close to 18 percent for presence of any foliar injury. Among the
biosites with a relatively wet season, the proportion of biosites
showing injury is much higher and the relationship with annual W126
index value is much steeper. Much lower proportions of biosites show
injury with relatively dry seasons (U.S. EPA, 2014b, section 7.2.3,
Figures 7-10), consistent with the ISA finding that many studies have
shown that dry periods tend to decrease the incidence and severity of
O3-induced visible foliar injury (U.S. EPA, 2013a, section
9.4.2). While these analyses indicate the potential for foliar injury
to occur under conditions that meet the current standard, the extent of
foliar injury that might be expected under such conditions is unclear
from these analyses.
---------------------------------------------------------------------------
\211\ Data were not available for several western states
(Montana, Idaho, Wyoming, Nevada, Utah, Colorado, Arizona, New
Mexico, Oklahoma, and portions of Texas).
---------------------------------------------------------------------------
The national-scale screening-level assessment in 214 parks employed
benchmark criteria developed from the above analysis.212 213
For example, annual O3 concentrations corresponding to a
W126 index value of 10.46 ppm-hrs represents the O3 exposure
concentration where the slope of exposure-response relationship changes
for FHM biosites, with the percentage of biosites showing injury
remaining relatively constant for higher W126 index values. The WREA
refers to this as the ``base scenario'' benchmark. The
[[Page 75328]]
W126 benchmarks across this and the other four scenarios range from
3.05 ppm-hrs (foliar injury observed at five percent of biosites,
normal moisture) up to 24.61 ppm-hrs (foliar injury observed at 10
percent of biosites, dry). For the scenario of 10 percent biosites with
injury, W126 index values were approximately 4, 6, and 25 ppm-hrs for
wet, normal and dry years, respectively. The national-scale screening-
level assessment applied these benchmarks to 42 parks with
O3 monitors and a total of 214 parks with O3
exposure estimated from the interpolated national O3
surfaces for individual years from 2006 to 2010 (U.S. EPA, 2014b,
Appendix 7A and section 7.3).
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\212\ The parks assessed in the WREA include lands managed by
the NPS in the continental U.S., which includes National Parks,
Monuments, Seashores, Scenic Rivers, Historic Parks, Battlefields,
Reservations, Recreation Areas, Memorials, Parkways, Military Parks,
Preserves, and Scenic Trails.
\213\ This analysis considered the approach in Kohut (2007),
which assessed the risk of O3-induced visible foliar
injury on O3 bioindicators (i.e., O3-sensitive
vegetation) in 244 parks managed by the NPS. Consistent with advice
from CASAC (Frey and Samet, 2012a), however, the WREA modified the
approach used by Kohut (2007) to apply the W126 metric alone. The
WREA applied different foliar injury benchmarks in this assessment
after further investigation into the benchmarks applied in Kohut
(2007), which were derived from biomass loss rather than visible
foliar injury. Kohut cited a threshold of 5.9 ppm-hrs for highly
sensitive species from Lefohn (1997), which was based on the lowest
W126 estimate corresponding to a 10 percent growth loss for black
cherry. For soil moisture, Kohut (2007) qualitatively assessed
whether there appeared to be an inverse relationship between soil
moisture and high O3 exposure.
---------------------------------------------------------------------------
Based on NPS lists, 95 percent of the 214 parks in this
screening-level assessment contain at least one vegetation species
sensitive to O3-induced foliar injury (U.S. NPS, 2003,
2006).
In the current standard scenario, none of the 214 parks
had O3 concentrations estimated to exceed the annual
benchmark of a W126 index value above 10.46 ppm-hrs (U.S. EPA, 2014b,
section 7.3.3.3).
The case study analyses focused on Great Smoky Mountains National
Park (GRSM), Rocky Mountain National Park (ROMO), and Sequoia and Kings
Canyon National Parks (SEKI). Information on visitation patterns,
recreational activities and visitor expenditures was considered. For
example, visitor spending in 2011 exceeded $800 million, $170 million
and $97 million dollars in GRSM, ROMO and SEKI, respectively. In each
park, the percent cover of species sensitive to foliar injury was
estimated and the overlap between recreation areas within the park and
elevated W126 concentrations was described. (U.S. EPA, 2014b, section
7.4).
In the current standard scenario, the three-year average
W126 index values were at or below 7 ppm-hrs in all areas of two of the
three parks (GRSM and SEKI). Three-year average W126 index values were
below 7 ppm-hrs in a little more than half of the area of the third
park (ROMO) and between 7 and 11 ppm-hrs in the remainder of the park
(U.S. EPA, 2014b, section 7.4).
For the 15, 11 and 7 ppm-hrs scenarios, all areas of the
three specific national parks evaluated (GRSM, SEKI, and ROMO) had
three-year average W126 index values at or below 7 ppm-hrs, well below
the 10.46 ppm-hrs benchmark. However, the extent of foliar injury that
might be expected under these scenarios is unclear from these analyses.
Although not discussed in detail here, the WREA also describes
qualitative assessments for some of the ecosystem services most likely
to be affected by O3-induced foliar injury such as cultural
services, including aesthetic value and outdoor recreation. Aesthetic
value and outdoor recreation depend on the perceived scenic beauty of
the environment. Many outdoor recreation activities directly depend on
the scenic value of the area, in particular scenic viewing, wildlife-
watching, hiking, and camping. These activities and services are of
significant importance to public welfare as they are enjoyed by
millions of Americans every year and generate millions of dollars in
economic value (U.S. EPA, 2014b, Chapters 5 and 7). Although data are
not available to explicitly quantify O3 effects on ecosystem
services, the WREA includes several qualitative analyses.
b. Key Uncertainties
Uncertainties associated with these analyses are discussed in the
WREA, sections 7.5 and 8.5.3, and in WREA Table 7-24, and also
summarized in the PA (e.g., U.S. EPA, 2014c, section 6.3). As discussed
in the WREA (section 8.5.3), evaluating soil moisture is more
subjective than evaluating O3 exposure because of its high
spatial and temporal variability within the O3 season, and
there is considerable subjectivity in the categorization of relative
drought. The WREA generally concludes that the spatial and temporal
resolution for the soil moisture data is likely to underestimate the
potential for foliar injury to occur in some areas. In addition, there
is lack of a clear threshold for drought below which visible foliar
injury would not occur. In general, low soil moisture reduces the
potential for foliar injury, but injury could still occur, and the
degree of drought necessary to reduce potential injury is not clear.
Studies in the ISA provide additional information regarding the role of
soil moisture in influencing visible foliar injury response, (U.S. EPA,
2013a, section 9.4.2). These studies confirm that adequate soil
moisture creates an environment conducive to greater visible foliar
injury in the presence of O3 than drier conditions. As
stated in the ISA, ``[a] major modifying factor for O3-
induced visible foliar injury is the amount of soil moisture available
to a plant during the year that the visible foliar injury is being
assessed . . . because lack of soil moisture generally decreases
stomatal conductance of plants and, therefore, limits the amount of
O3 entering the leaf that can cause injury'' (U.S. EPA,
2013a, p. 9-39). As a result, ``many studies have shown that dry
periods in local areas tend to decrease the incidence and severity of
O3-induced visible foliar injury; therefore, the incidence
of visible foliar injury is not always higher in years and areas with
higher O3, especially with co-occurring drought (Smith,
2012; Smith et al., 2003)'' (U.S. EPA, 2013a, p. 9-39). This ``. . .
partial `protection' against the effects of O3 afforded by
drought has been observed in field experiments (Low et al., 2006) and
modeled in computer simulations (Broadmeadow and Jackson, 2000)'' (U.S.
EPA, 2013a, p. 9-87). In considering the extent of any protective role
of drought conditions, however, the ISA also notes that other studies
have shown that ``drought may exacerbate the effects of O3
on plants (Pollastrini et al., 2010; Grulke et al., 2003)'' and that
``[t]here is also some evidence that O3 can predispose
plants to drought stress (Maier-Maercker, 1998)''. Accordingly, the ISA
concludes that ``the nature of the response is largely species-specific
and will depend to some extent upon the sequence in which the stressors
occur'' (U.S. EPA, 2013a, p. 9-87).
Due to the absence of biosite injury data in the Southwest region
and limited biosite data in the West and West North Central regions,
the W126 benchmarks for foliar injury that the WREA developed and
applied in the national park screening assessment may not be applicable
to these regions. The WREA applied the benchmarks from the national-
scale analysis to a screening-level assessment of 214 national parks
and case studies of three national parks. Therefore, uncertainties in
the foliar injury benchmarks are propagated into these analyses.
Other uncertainties associated with these analyses include
uncertainty associated with our understanding of the number and
sensitivity of O3 sensitive species, uncertainties
associated with spatial assignment of foliar injury biosite data to 12
km x 12 km grid cells, and uncertainties associated with O3
exposure data of vegetation and recreational areas within parks (U.S.
EPA, 2014b, Table 7-22).
There are also important uncertainties in the estimated
O3 concentrations for the different air quality scenarios
evaluated (U.S. EPA, 2014b, section 8.5), as discussed earlier in this
section. These uncertainties only apply to the national park case
studies because these are the only foliar injury analyses that rely on
the air quality scenarios, but any uncertainties in the air quality
analyses are propagated into those analyses. The WREA identifies
additional uncertainties that are associated with
[[Page 75329]]
the national park case studies. Specifically, there is uncertainty
inherent in survey estimates of participation rates, visitor spending/
economic impacts, and willingness-to-pay. These surveys potentially
double-count impacts based on the allocation of expenditures across
activities but also potentially exclude other activities with economic
value. In general, the national level surveys apply standard
approaches, which minimize potential bias. Other sources of uncertainty
are associated with the mapping, including park boundaries, vegetation
species cover, and park amenities, such as scenic overlooks and trails.
In general, the WREA concludes that there is high confidence in the
park mapping (U.S. EPA, 2014b, Table 7-24).
D. Conclusions on Adequacy of the Current Secondary Standard
The initial issue to be addressed in the current review of the
secondary O3 standard is whether, in view of the currently
available scientific evidence, exposure and risk information and air
quality analyses, discussed in the PA, the existing standard should be
revised. In drawing conclusions on adequacy of the current
O3 secondary standard, the Administrator has taken into
account both evidence-based and quantitative exposure- and risk-based
considerations, and advice from CASAC. Evidence-based considerations
draw upon the EPA's assessment and integrated synthesis of the
scientific evidence from experimental and field studies evaluating
welfare effects related to O3 exposure, with a focus on
policy-relevant considerations, as discussed in the PA. Air quality
analyses inform these considerations with regard to cumulative,
seasonal exposures occurring in areas of the U.S. that meet the current
standard. Exposure- and risk-based considerations draw upon EPA
assessments of risk of key welfare effects, including O3
effects on forest growth, productivity, carbon storage, crop yield and
visible foliar injury, expected to occur in model-based scenarios for
the current standard, with appropriate consideration of associated
uncertainties.
The following sections describe consideration of the evidence and
the exposure/risk information in the PA and advice received from CASAC,
as well as the comments received from various parties, and the
Administrator's proposed conclusions regarding the adequacy of the
current secondary standard.
1. Evidence- and Exposure/Risk-Based Considerations in the Policy
Assessment
Staff assessments in the PA focus on the policy-relevant aspects of
the assessment and integrative synthesis of the currently available
welfare effects evidence in the ISA, analyses of air quality
relationships with exposure metrics of interest, the exposure and risk
assessments in the WREA, comments and advice of CASAC and public
comment on drafts of the PA, ISA and WREA. The PA describes evidence-
and exposure/risk-based considerations and presents staff conclusions
for the Administrator to consider in reaching her proposed decision on
the current standard. The focus of the initial PA conclusions is
consideration of the question: Does the currently available scientific
evidence and exposure/risk information, as reflected in the ISA and
WREA, support or call into question the adequacy and/or appropriateness
of the protection afforded by the current secondary O3
standard?
The PA's general approach to informing judgments by the
Administrator recognizes that the available welfare effects evidence
demonstrates a range of O3 sensitivity across studied plant
species and documents an array of O3-induced effects that
extend from lower to higher levels of biological organization. These
effects range from those affecting cell processes and individual plant
leaves to effects on the physiology of whole plants, as well as the
range from species effects and effects on plant communities to effects
on related ecosystem processes and services. Given this evidence, the
PA notes that it is not possible to generalize across all studied
species regarding which cumulative exposures are of greatest concern,
as this can vary by situation due to differences in exposed species
sensitivity, the importance of the observed or predicted O3-
induced effect, the role that the species plays in the ecosystem, the
intended use of the affected species and its associated ecosystem and
services, the presence of other co-occurring predisposing or mitigating
factors, and associated uncertainties and limitations. Therefore, in
developing conclusions in the PA, staff takes note of the complexity of
judgments to be made by the Administrator regarding the adversity of
known and anticipated effects to the public welfare and are mindful
that the Administrator's ultimate judgments on the secondary standard
will most appropriately reflect an interpretation of the available
scientific evidence and exposure/risk information that neither
overstates nor understates the strengths and limitations of that
evidence and information (U.S. EPA, 2014c, section 5.7).
In considering the estimates of exposures and risks for air quality
scenarios assessed in the WREA, the PA: (1) Evaluates the weight of the
scientific evidence concerning vegetation effects associated with those
O3 exposures; (2) considers the importance, from a public
welfare perspective, of the O3-induced effects on sensitive
vegetation and associated ecosystem services that are known or
anticipated to occur as a result of exposures in the assessed air
quality scenarios; and, (3) recognizes that predictions of effects
associated with any given O3 exposure may be mitigated or
exacerbated by actual conditions in the environment (i.e., co-occurring
modifying environmental and genetic factors). When considering WREA
analyses that involve discrete exposure levels or varying levels of
severity of effects, the PA's approach recognizes that the available
welfare effects evidence demonstrates a wide range in O3
sensitivities across studied plant species. The PA additionally
considers the uncertainties associated with this information.
As an initial matter, the PA recognizes that the CAA does not
require that a secondary standard be protective of all effects
associated with a pollutant in the ambient air, but rather those
considered adverse to the public welfare (as described in section
IV.B.2 above). In considering the extent to which it may be appropriate
to consider particular welfare effects adverse, the PA applies a
paradigm used in past reviews. As discussed in section IV.B.2 above,
this paradigm recognizes that the significance to the public welfare of
O3-induced effects on sensitive vegetation growing within
the U.S. can vary depending on the nature of the effect, the intended
use of the sensitive plants or ecosystems, and the types of
environments in which the sensitive vegetation and ecosystems are
located. Accordingly, any given O3-related effect on
vegetation and ecosystems (e.g., biomass loss, crop yield loss, visible
foliar injury) may be judged to have a different degree of impact on
the public welfare depending, for example, on whether that effect
occurs in a Class I area, a city park, or commercial cropland. In the
last review, the Administrator took note of actions taken by Congress
to establish public lands that are set aside for specific uses that are
intended to provide benefits to the public welfare, including lands
that are to be protected so as to conserve the scenic value and the
natural vegetation
[[Page 75330]]
and wildlife within such areas for the enjoyment of future generations
(73 FR 16497, March 27, 2008). Such public lands that are protected
areas of national interest include national parks and forests, wildlife
refuges, and wilderness areas (73 FR 16497, March 27, 2008). The PA
notes that effects occurring in such areas would likely have the
highest potential for being classified as adverse to the public
welfare, given the expectation of preserving these areas to ensure
their intended use is met (U.S. EPA, 2014c, section 5.1). In
considering uses of vegetation and forested lands, the paradigm also
includes consideration of impacts to ecosystem goods and services. In
summary, the paradigm considered in the PA, consistent with the
discussion in section IV.B.2 above, integrates the concepts of: (1)
Variability in public welfare significance given intended use and value
of the affected entity, such as individual species; (2) relevance of
associated ecosystem services to public welfare; and (3) variability in
spatial, temporal, and social distribution of ecosystem services
associated with known and anticipated welfare effects. Further, the PA
recognizes that there is no bright-line rule delineating the set of
conditions or scales at which known or anticipated effects become
adverse to public welfare.
With respect to the scientific evidence, the PA takes note of the
longstanding evidence base that demonstrates O3-induced
effects that occur across a range of biological and ecological scales
of organization, as described in the ISA and summarized in section
IV.B.1 above (U.S. EPA, 2013a, p. 1-8). Many of the recent studies
evaluated in this review have focused on and further increased our
understanding of the molecular, biochemical and physiological
mechanisms that explain how plants are affected by O3 in the
absence of other stressors (U.S. EPA, 2013a, section 9.3). These recent
studies, in combination with the extensive and long-standing evidence,
have further strengthened the coherence and consistency of the entire
body of research since the last review. Consistent with conclusions in
the 2006 AQCD, the ISA determined that a causal relationship exists
between O3 exposure and visible foliar injury on sensitive
vegetation, reduced plant growth, reduced productivity in terrestrial
ecosystems, reduced yield and quality of agricultural crops and
alteration of below-ground biogeochemical cycles (U.S. EPA, 2013a,
Table 1-2 and section 2.6). The relationship between O3
exposures and reduced carbon sequestration in terrestrial ecosystems,
alteration of terrestrial ecosystem water cycling and alteration of
terrestrial community composition was concluded to be likely causal
(U.S. EPA, 2013a, Table 1-2).
The PA recognizes that consistent with conclusions drawn in the
last review, the currently available evidence base also strongly
supports that effects on vegetation are attributable to cumulative
seasonal O3 exposures. Moreover, on the basis of the entire
body of evidence in this regard, the ISA concludes that ``quantifying
exposure with indices that cumulate hourly O3 concentrations
and preferentially weight the higher concentrations improves the
explanatory power of exposure/response models for growth and yield,
over using indices based on mean and peak exposure values'' (U.S. EPA,
2013a, p. 2-44). Accordingly, as in other recent reviews, the evidence
continues to provide a strong basis for concluding that it is
appropriate to judge impacts of O3 on vegetation, related
effects and services, and the level of public welfare protection
achieved, using a cumulative, seasonal exposure metric, such as the
W126-based metric. In this review, as in the last review, the CASAC
concurs with this conclusion (Frey, 2014c, p. iii). Thus, based on the
consistent and well-established evidence described above, the PA
concludes that the most appropriate and biologically relevant way to
relate O3 exposure to plant growth, and to determine what
would be adequate protection for public welfare effects attributable to
the presence of O3 in the ambient air is to characterize
exposures in terms of a cumulative seasonal form, and in particular the
W126 metric.
In considering the current standard with regard to protection from
the array of O3-related effects recognized in this review,
the PA first considers effects related to forest tree growth,
productivity and carbon storage, effects for which the ISA concludes
the evidence supports a causal or likely causal relationship with
exposures to O3 in ambient air (U.S. EPA, 2014c, sections
5.2 and 5.7). In so doing, the PA notes that while changes in biomass
affect individual tree species, the overall effect on forest ecosystem
productivity depends on the composition of forest stands and the
relative sensitivity of trees within those stands. In considering the
evidence for these effects and the extent to which they might be
expected to occur under conditions that meet the current secondary
standard, the PA focused particularly on RBL estimates for the 11
species for which robust E-R functions have been developed. The PA
recognized that recent studies, such as multiple-year exposures of
aspen and birch, have provided additional evidence on tree biomass or
growth effects associated with multiple year exposures in the field,
including the potential for cumulative and carry-over effects. For
example, findings from these studies indicate that effects of
O3 on birch seeds (reduced weight, germination, and starch
levels) could lead to a negative impact on species regeneration in
subsequent years and may have the potential to alter carbon metabolism
of overwintering buds, potentially affecting growth in the following
year. Other studies have reported that multiple-year exposures reduced
tree size parameters in an aspen community, and increased the rate of
conversion from a mixed aspen-birch community to a community dominated
by the more tolerant birch, such that elevated O3 may alter
intra- and inter-species competition within a forest stand (U.S. EPA,
2013a, section 9.4.3; U.S. EPA, 2014c, section 5.2). In giving
particular attention to tree seedling biomass loss estimates, the PA
notes that CASAC ``concurs that biomass loss in trees is a relevant
surrogate for damage to tree growth that affects ecosystem services
such as habitat provision for wildlife, carbon storage, provision of
food and fiber, and pollution removal'' (Frey, 2014c, p. 10).
In evaluating the current evidence and exposure/risk information
associated with tree growth, productivity and carbon storage, with
regard to the adequacy of public welfare protection afforded by the
current standard, the PA considers the evidence of vegetation and
welfare impacts in areas of the U.S. likely to have met the current
standard. With regard to O3 effects on tree growth,
productivity and carbon storage and associated ecosystems and services,
the PA focuses on relative biomass loss estimates based on the OTC-
based E-R functions, noting that analyses newly performed in this
review have reduced the uncertainty associated with using OTC E-R
functions to predict tree growth effects in the field (U.S. EPA, 2014c,
section 5.2.1; U.S. EPA, 2013a, section 9.6.3.2).
In focusing on RBL estimates, the PA recognized that comparison to
an array of benchmarks would be informative to considerations of
significance to public welfare. Included in this array were RBL values
of 2% and 6% given emphasis by CASAC (Frey, 2014c). In considering the
RBL estimates for different O3 conditions associated with
the current standard, the PA focused first on the median of the
species-specific (composite) E-R functions. In so doing,
[[Page 75331]]
the PA takes note of CASAC's comments that a 6% median RBL is
``unacceptably high'', and that the 2% median RBL is an important
benchmark to consider (Frey, 2014c).\214\ Based on the summary of RBL
estimates in the PA, the PA notes that the median species RBL estimate
is at or below 2% for W126 exposure index values less than or equal to
7 ppm-hrs (U.S. EPA, 2014c, Table 6-1 and Appendix 5C). The median
species RBL is at or above 6% for W126 index values of 19 ppm-hrs and
higher.
---------------------------------------------------------------------------
\214\ The CASAC provided several comments related to 2% RBL for
tree seedlings both with regard to its use in summarizing WREA
results and with regard to consideration of the potential
significance of vegetation effects, as summarized in sections IV.D.2
and IV.E.3. In identifying 2% as an important benchmark, CASAC
referenced the 1996 workshop sponsored by the Southern Oxidants
Study group at which, as noted in section IV.B.2 above, participants
identified annual percentages of tree seedling growth reduction and
crop yield loss they considered important to their judgments on a
secondary standard. The workshop report provides no explicit
rationale for the percentages identified or specification with
regard to number or proportion of species for which such percentages
should be met (Heck and Cowling, 1997).
---------------------------------------------------------------------------
In recognition of the significance of welfare effects in Class I
areas, the PA gives appreciable weight to consideration of the
occurrence of O3 concentrations associated with the
potential for RBL estimates above benchmarks of interest in Class I
areas that meet the current standard. Based on air quality data for the
period from 1998 to 2012, the PA focused consideration on 22 Class I
areas, in which during one or more three-year periods the air quality
met the current standard and the three-year average W126 index value
was at or above 15 ppm-hrs (see Table 7 below, drawn from U.S. EPA,
2014c, Table 5-2). Across these 22 Class I areas, the highest single-
year W126 index values for these three-year periods ranged from 17.4 to
29.0 ppm-hrs. In 20 of the areas, distributed across eight states (AZ,
CA, CO, KY, NM, SD, UT, WY) and four regions (West, Southwest, West
North Central and Central), this range was 19.1 to 29.0 ppm-hrs,
exposure values for which the corresponding median species RBL
estimates equal or exceed 6%, which CASAC has termed ``unacceptably
high''. Recognizing that in any given year, other environmental factors
can influence the extent to which O3 may have the impact
predicted by the E-R functions, the PA looked beyond single year
occurrences of such magnitudes of W126 index values. For example,
focusing on the highest three-year periods that include these highest
annual values for 21 areas, the PA notes that in 10 areas (across five
states in the West and Southwest regions), the three-year average W126
values (for the highest three-year period that includes these annual
values) are at or above 19 ppm-hrs, ranging up to 22.5 ppm-hrs (for
which the median species RBL estimate is above 7%). This indicates that
the W126 value above 19 ppm-hrs is not simply a single year in a period
of lower years, but that in these cases there were sustained higher
values that contributed to a three-year W126 also above 19 ppm-hrs. In
terms of the highest three-year values observed (regardless of single-
year values), the PA additionally notes that the highest three-year
average W126 index value (during periods meeting the current standard)
was at or above 19 (ranging up to 22.5 ppm-hrs) in 11 areas,
distributed among five states in the West and Southwest regions (U.S.
EPA, 2014c, Table 5-2, Appendix 5B).
Table 7--O3 Concentrations in Class I Areas During Period From 1998 to 2012 that Met the Current Standard and Where Three-year Average W126 Index Value
was at or Above 15 ppm-hrs
--------------------------------------------------------------------------------------------------------------------------------------------------------
3-year Average W126 (ppm-
Class I Area State/county Design value hrs)* (# >=19 ppm-hrs, Annual W126 (ppm-hrs)* Number of 3-
(ppb)* range) (# >=19 ppm-hrs, range) year periods
--------------------------------------------------------------------------------------------------------------------------------------------------------
Bandelier Wilderness Area QA, DF, PP. NM/Sandoval.................. 70-74 15.8-20.8 (2, 20.0-20.8) 12.1-25.3 (4, 19.2-25.3) 8
Bridger Wilderness Area QA, DF....... WY/Sublette.................. 69-72 15.1-17.4............... 9.9-19.2 (1, 19.2)...... 5
Canyonlands National Park QA, DF, PP. UT/San Juan.................. 69-73 15.0-20.5 (2, 19.8-20.5) 9.9-24.8 (5, 19.3-24.8). 9
Carlsbad Caverns National Park PP.... NM/Eddy...................... 69 15.0-15.3............... 8.6-26.7 (1, 26.7)...... 3
Chiricahua National Monument DF, PP.. AZ/Cochise................... 69-73 15.7-18.0............... 13.2-21.6 (2, 19.3-21.6) 7
Grand Canyon National Park QA, DF, PP AZ/Coconino.................. 68-74 15.3-22.2 (7, 19.2-22.2) 11.3-26.7 (7, 19.8-26.7) 12
John Muir Wilderness Area QA, DF, PP. CA/Inyo...................... 71-72 16.5-18.6............... 10.1-25.8 (2, 23.9-25.8) 3
Lassen Volcanic National Park DF, PP. CA/Shasta.................... 75 15.3.................... 13.6-18.7............... 1
Mammoth Cave National Park BC, C, LP, KY/Edmonson.................. 74 15.9.................... 12.5-22.5 (1, 22.5)..... 1
RM, SM, VP, YP.
Mesa Verde National Park DF.......... CO/Montezuma................. 67-73 15.5-21.0 (2, 19.0-21.0) 10.7-23.6 (4, 19.7-23.6) 10
Mokelumne Wilderness Area DF, PP..... CA/Amador.................... 74 17.6.................... 14.8-22.6 (1, 22.6)..... 1
Petrified Forest National Park....... AZ/Navajo.................... 70 15.7.................... 12.9-19.2 (1, 19.2)..... 1
Pinnacles National Monument.......... CA/San Benito................ 74 15.1.................... 13.1-17.4............... 1
Rocky Mountain National Park QA, DF, CO/Boulder................... 73-75 15.1-19.3 (1, 19.3)..... 9.5-25.1 (5, 20.7-25.1). 6
PP.
CO/Larimer................... 74 15.0-18.3............... 8.1-25.8 (3, 19.1-25.8). 3
Saguaro National Park DF, PP......... AZ/Pima...................... 69-74 15.4-18.9............... 11.0-23.1 (3, 20.0-23.1) 6
Sierra Ancha Wilderness Area DF, PP.. AZ/Gila...................... 72-75 17.9-22.4 (3, 20.2-22.4) 14.8-27.5 (4, 20.3-27.5) 4
[[Page 75332]]
Superstition Wilderness Area PP......
AZ/Maricopa.................. 75 22.4 (1, 22.4).......... 14.5-28.6 (2, 27.4-28.6) 1
AZ/Pinal..................... 73-75 18.7-22.5 (2, 20.8-22.5) 14.8-29.0 (3, 22.6-29.0) 3
Weminuche Wilderness Area QA, DF, PP. CO/La Plata.................. 70-74 15.0-19.1 (1, 19.1)..... 10.9-21.0 (2, 20.8-21.0) 5
West Elk Wilderness Area QA, DF...... CO/Gunnison.................. 68-73 15.6-20.1 (1, 20.1)..... 12.9-23.9 (3, 21.1-23.9) 8
Wind Cave National Park QA, PP....... SD/Custer.................... 70 15.4.................... 12.2-20.6 (1, 20.6)..... 1
Yosemite National Park QA, DF, PP.... CA/Tuolumne.................. 73-74 20.7-20.8 (2, 20.7-20.8) 19.7-22.1 (4, 19.7-22.1) 2
Zion National Park QA, DF, PP........ UT/Washington................ 70-73 17.8-21.1 (2, 20.3-21.1) 14.9-24.2 (5, 19.3-24.2) 4
--------------------------------------------------------------------------------------------------------------------------------------------------------
* Based on data from http://www.epa.gov/ttn/airs/airsaqs/detaildata/downloadaqsdata.htm. W126 values are truncated after first decimal place.
Superscript letters refer to species present for which E-R functions have been developed. QA=Quaking Aspen, BC=Black Cherry, C=Cottonwood, DF=Douglas
Fir, LP=Loblolly Pine, PP=Ponderosa Pine, RM=Red Maple, SM=Sugar Maple, VP=Virginia Pine, YP=Yellow (Tulip) Poplar. Sources include USDA-NRCS
(2014,http://plants.usda.gov), USDA-FS (2014, http://www.fs.fed.us/foresthealth/technology/nidrm2012.shtml) UM-CFCWI (2014, http://www.wilderness.net/printFactSheet.cfm?WID=583) and Phillips and Comus (2000).
In considering the data analysis for 22 Class I areas described
above, the PA additionally considers the species-specific RBL estimates
for quaking aspen and ponderosa pine, two tree species that are found
in many of these 22 areas and have a sensitivity to O3
exposure that places them near the middle of the group for which E-R
functions have been established (U.S. EPA, 2014c, sections 5.2 and
5.7). In the Class I areas where ponderosa pine is present, the highest
single year W126 index values ranged from 18.7 to 29.0 ppm-hrs and the
highest three-year average W126 values in which these single year
values were represented ranged from 15 to 22.5 ppm-hrs, with these
three-year values above 19 ppm-hrs in eight areas across five states.
The ponderosa pine RBL estimates for 29 and 22.5 ppm-hrs are
approximately 12% and 9%, respectively (U.S. EPA, 2014c, Appendix 5C).
In Class I areas where quaking aspen is present, the highest single
year W126 index values ranged from 19.2 to 26.7 ppm-hrs and the highest
three-year average W126 values in which these single year values were
represented ranged from 15.0 to 22.2 ppm-hrs, with these three-year
values above 19 ppm-hrs in eight areas across five states. The quaking
aspen RBL estimates for 26.7 and 22.2 ppm-hrs are approximately 16% and
13%, respectively (U.S. EPA, 2014c, Appendix 5C).
The PA describes the above observations, particularly in light of
advice from CASAC, summarized in section IV.D.2 below, as evidence of
the occurrence in Class I areas during periods where the current
standard is met of cumulative seasonal O3 exposures of a
magnitude for which the tree growth impacts indicated by the estimated
median species RBL might reasonably be concluded to be important to
public welfare (U.S. EPA, 2014c, sections 5.2.1 and 5.7).
In considering the WREA analyses of effects on tree growth and
associated ecosystem services in the air quality scenario for the
current standard, the PA first takes note of the potential for the
interpolation method used in creating the national surface of
O3 concentrations for the air quality scenarios to
underestimate the higher W126 values such that W126-based exposures
would be expected to be somewhat higher than those included in each
scenario (U.S. EPA, 2014b, pp. 5-31 to 5-32). While recognizing this,
the PA considers results of the WREA analyses for the current standard
scenario and the 11 species of trees, for which robust E-R functions
are available. These results indicate that O3 can impact
growth of these species across the U.S., as well as an array of
associated ecosystem services provided by forests, including timber
production, carbon storage and air pollution removal (U.S. EPA, 2014b,
sections 6.2-6.8; U.S. EPA, 2014c, section 5.2).
With regard to WREA analyses of ecosystem services, the PA notes
that the national-scale analysis of O3 impacts on carbon
storage indicates appreciably more storage in the air quality scenario
for the current standard (approximately 11,000 MMtCO2e, over
30 years) compared to the scenario for recent, higher O3
conditions (U.S. EPA, 2014b, Appendix 6B, Table B-10). The PA
additionally considers the WREA estimates of tree growth and ecosystem
services provided by urban trees over a 25-year period for five urban
areas based on case-study scale analyses that quantified the effects of
biomass loss on carbon storage and pollution removal (U.S. EPA, 2014b,
sections 6.6.2 and 6.7; U.S. EPA, 2014c, sections 5.2 and 5.7). The
urban areas included in this analysis represent diverse geography in
the Northeast, Southeast, and Central regions, although they do not
include an urban area in the western U.S. Estimates of the effects of
O3-related biomass loss on carbon sequestration indicate the
potential for an increase of somewhat more than a MMtCO2e
for the current standard scenario as compared to the recent conditions
scenario (U.S. EPA, 2014b, section 6.6.2 and Appendix 6D; U.S. EPA,
2014c, sections 5.2 and 5.7). The PA also notes the WREA estimates of
increased pollution removal in the current standard scenario as
compared to the scenario for recent conditions (U.S. EPA, 2014b,
section 6.6.2; U.S. EPA, 2014c, section 5.2.2).
In considering the significance of these WREA analyses of risks for
the associated ecosystem services for timber production, air pollution
removal, and carbon sequestration, the PA takes note of the large
uncertainties associated with these analyses (see U.S. EPA, 2014b,
Table 6-27), and the potential for these findings to underestimate the
response at the national scale. While noting the potential usefulness
of considering predicted and anticipated impacts to these services in
assessing the extent to which the current information supports or calls
into question the adequacy of the protection afforded by the current
standard, the PA also notes that staff places limited
[[Page 75333]]
weight on the absolute magnitude of the risk results for these
ecosystem service endpoints due to the identification of significant
associated uncertainties (U.S. EPA, 2014c, sections 5.2 and 5.7).
In reaching conclusions regarding support for the adequacy of the
current secondary standard provided by the currently available
information on O3-induced effects on trees and associated
services, the PA takes note of: (1) the robust evidence supporting the
causal relationship between cumulative O3 exposures and
effects on tree growth and productivity, and information from model
simulations supporting the determination of a likely causal
relationships for carbon storage in terrestrial ecosystems (U.S. EPA,
2013a, sections 2.6.2.1 and 9.4.3); (2) the tree seedling E-R functions
evidence, which has been strengthened and demonstrates variability in
sensitivity to O3 across species; (3) estimates of median
species RBL at or above 6% associated with W126-based exposure levels
in several areas when O3 concentrations were at or below the
current standard; (4) growth effects estimates associated with exposure
concentrations in several Class I areas based on O3
concentrations from 1998-2012 that were at or below the current
standard; (5) evidence that impacts from single year exposures can
carry over to the subsequent year and/or cumulate over multiple years
with repeated annual exposures; (6) evidence from recent mechanistic
studies and field based studies that support earlier findings from OTC
studies; and (7) WREA analyses indicating that O3-induced
biomass loss can impact ecosystem services provided by forests,
including timber production, carbon storage, and air pollution removal,
even when air quality is adjusted to just meet the current standard.
Given the above, and noting CASAC views (described in section IV.D.2
below), the PA concludes that the current evidence and exposure/risk
information call into question the adequacy of public welfare
protection afforded by the current standard from the known and
anticipated adverse effects associated with O3-induced
impacts on tree growth, productivity and carbon storage, including the
associated ecosystem services assessed in this review. Therefore, the
PA concludes that it is appropriate to consider revision of the
secondary standard to provide increased protection.
With respect to crops, the PA takes note of the extensive and long-
standing evidence on the detrimental effect of O3 on crop
production, which continues to be confirmed by newly available evidence
(U.S. EPA, 2013a, section 9.4.4; U.S. EPA, 2014c, sections 5.3 and
5.7). The PA additionally notes that recent studies have highlighted
the effects of O3 on crop quality, such as through decreases
in the nutritive quality of grasses, and in the macro- and micro-
nutrient concentrations in fruits and vegetable crops (U.S. EPA, 2013a,
section 9.4.4; U.S. EPA, 2014c, section 5.3). Further, the PA notes
that there has been little published evidence that crops are becoming
more tolerant of O3, taking note particularly of the ISA
analyses of data from cultivars used in NCLAN studies, and yield data
for modern cultivars from SoyFACE which confirm that the average
response of soybean yield to O3 exposure has not changed in
current cultivars (U.S. EPA, 2006a; U.S. EPA, 2013a, section 9.6.3;
U.S. EPA, 2014c, section 5.3). In consideration of the currently
available evidence for O3 effects on crops, the PA concludes
that the recently available evidence, as assessed in the ISA, continues
to support the conclusions of the 1996 and 2006 CDs that ambient
O3 concentrations can reduce the yield of major commodity
crops in the U.S, and that the currently available evidence continues
to support the use of the E-R functions developed for 10 crops from OTC
experiment data. Further, the PA recognizes that important
uncertainties have been reduced regarding the exposure-response
functions for crop yield loss, especially for soybean, the second-most
planted field crop in the U.S.,\215\ with the ISA generally reporting
consistent results across exposure techniques and across crop varieties
(U.S. EPA, 2013a, section 9.6.3.2).
---------------------------------------------------------------------------
\215\ See http://www.ers.usda.gov/topics/crops/soybeans-oil-crops/background.aspx
---------------------------------------------------------------------------
With regard to consideration of the quantitative impacts of
O3 on crop yield, the PA considers RYL estimates for
O3 conditions associated with the current standard. As in
the case of the PA considerations of RBL estimates for tree seedlings,
the PA recognized CASAC comments, which described greater than 5% RYL
for the median crop species as ``unacceptably high'' and 5% RYL for the
median crop species as adverse, while noting the opportunities to alter
management of annual crops (Frey, 2014c, pp. iii and 14). The PA notes
that staff analyses of recent monitoring data (2009-2011) indicate that
O3 concentrations in multiple agricultural areas in the U.S.
that meet the current standard correspond to W126 index levels above 12
ppm-hrs, a value for which soybean RYL estimates are greater than 5%.
In particular, the PA notes that while the design values for two
counties in the Midwest met the current standard in 2009-2011, both had
a maximum annual W126 of 19 ppm-hrs (in 2011) for which the soybean
annual RYL estimate, based on the E-R function, is 9%.\216\
---------------------------------------------------------------------------
\216\ The monitoring data reflect observations in locations that
meet the current standard. The WREA analysis that assessed crop
yield loss used a model-developed air quality scenario to reflect
air quality associated with the current standard (as described in
section IV.C.1 above). In so doing, adjustments are made to create
air quality that meets the standard and when the highest monitor in
an area is adjusted downward to meet the standard, concentrations at
nearby monitors that already meet the standard are also reduced.
---------------------------------------------------------------------------
In considering the evidence and exposure/risk-based information for
effects on crops, the PA notes the CASAC comments regarding the use of
crop yields as a surrogate for consideration of public welfare impacts,
in which it noted that ``[c]rops provide food and fiber services to
humans'' and that ``[e]valuation of market-based welfare effects of
O3 exposure in forestry and agricultural sectors is an
appropriate approach to take into account damage that is adverse to
public welfare'' (Frey, 2014c, p. 10; U.S. EPA, 2014c, section 5.7).
The PA additionally notes, however, as recognized in section IV.B.2
above that the determination of the point at which O3-
induced crop yield loss becomes adverse to the public welfare is still
unclear, given that crops are heavily managed with additional inputs
that have their own associated markets and that benefits can be
unevenly distributed between producers and consumers. The PA further
notes that to the extent protection is provided by the current standard
with regard to impacts on trees, protection may also be provided for
commodity crops (U.S. EPA, 2014c, sections 5.3 and 5.7).
In reaching conclusions regarding support provided for the adequacy
of the current secondary standard by the currently available
information on O3-related crop effects, the PA notes: (1)
the support for a causal relationship between cumulative O3
exposures and effects on crop yields and quality (U.S. EPA, 2013a,
section 9.4.4); (2) the evidence supporting E-R functions for 10 crops,
which has been strengthened in this review and which demonstrates
variability in sensitivity to O3 across species; (3)
evidence from recent mechanistic studies and field based studies
supporting earlier findings from OTC studies; (4) evidence that crops,
and in particular soybean, have not become more tolerant of
O3 (U.S. EPA, 2013a, section 9.6.3, 9.4.4.1); and, (5) WREA
analysis results indicating that O3-induced crop yield loss
can impact producer and consumer surpluses and
[[Page 75334]]
the interaction between agriculture and timber production.
With regard to visible foliar injury, the PA recognizes the long-
standing evidence that has established that O3 causes
diagnostic visible injury symptoms on studied bioindicator species and
that soil moisture is a major confounding effect that can decrease the
incidence and severity of visible foliar injury under dry conditions
and vice versa (U.S. EPA, 2014c, sections 5.4 and 5.7). As at the time
of the last review, the most extensive dataset regarding visible foliar
injury incidence across the U.S. is that collected by the U.S. Forest
Service (USFS) Forest Health Monitoring/Forest Inventory and Analysis
(FHM/FIA) Program, which has documented incidence of visible foliar
injury in both the eastern and western U.S. Evidence available in the
current review includes studies using controlled exposures as well as
multi-year field surveys. In addition to supporting prior conclusions,
the newly available studies also address some uncertainties identified
in the last review, such as the influence of soil moisture on visible
injury development (U.S. EPA, 2013a, section 9.4.2). As stated in the
ISA, ``many studies have shown that dry periods in local areas tend to
decrease the incidence and severity of O3-induced visible
foliar injury; therefore, the incidence of visible foliar injury is not
always higher in years and areas with higher O3, especially
with co-occurring drought'' (U.S. EPA, 2013a, p. 9-39). The ISA
additionally concludes, however, that ``the nature of the response is
largely species-specific and will depend to some extent upon the
sequence in which the stressors occur'' (U.S. EPA, 2013a, p. 9-87). As
recognized in the PA, this area of uncertainty complicates
characterization of the potential for visible foliar injury and its
severity or extent of occurrence for any given air quality conditions
and thus complicates identification of air quality conditions that
might be expected to provide a specific level of protection from this
effect (U.S. EPA, 2014c, sections 5.4 and 5.7).
Information available in this review indicates the occurrence of
visible foliar injury in some Class I areas during times when
O3 concentrations met or would be expected to meet the
current standard (U.S. EPA, 2014c, sections 5.4.1 and 5.7). In noting
this occurrence in Class I areas, the PA notes it has particular public
welfare significance in light of direction from Congress that these
areas merit a high level of protection (U.S. EPA, 2014c, sections 5.1,
5.4.1 and 5.7). The PA also notes that visible foliar injury surveys
are used by the federal land managers to assess potential O3
impacts in Class I areas (USFS, NPS, FWS, 2010). Given this focus on
visible foliar injury, the PA concludes that such O3-induced
impacts have the potential to impact the public welfare in scenic and/
or recreational areas on an annual basis. Visible foliar injury is
associated with important cultural and recreational ecosystem services
to the public, such as scenic viewing, wildlife-watching, hiking, and
camping, that are of significance to the public welfare and enjoyed by
millions of Americans every year, generating millions of dollars in
economic value (U.S. EPA, 2014b, section 7.1). In addition, several
tribes have indicated that many of the O3-sensitive species
(including bioindicator species) are culturally significant (U.S. EPA,
2014c, Table 5-1). With respect to agricultural species, such visible
effects of O3 exposure can affect the market value of
certain crops and ornamentals for which leaves are the product, such as
spinach (U.S. EPA, 2006a, p. AX-9-189). The PA additionally notes CASAC
comments that ``visible foliar injury can impact public welfare by
damaging or impairing the intended use or service of a resource'',
including through ``visible damage to ornamental or leafy crops that
affects their economic value, yield, or usability; visible damage to
plants with special cultural significance; and visible damage to
species occurring in natural settings valued for scenic beauty or
recreational appeal'' (Frey, 2014c, p. 10).
With regard to the exposure and risk-based information, the PA
takes note of the WREA analyses of the nationwide dataset (2006-2010)
for USFS/FHM biosites, including the observation that the proportion of
biosites with injury varies with soil moisture conditions and
O3 W126 index values (U.S. EPA, 2014b, Chapter 7, Figure 7-
10; U.S. EPA, 2014c, section 5.4.2). These analyses indicate that the
proportion of biosites showing visible foliar injury incidence
increases steeply with W126 index values up to approximately 10 ppm-
hrs, with little difference in incidence across higher W126 index
levels. The screening-level assessment of national parks indicated that
risk of visible foliar injury is likely to be lower in most national
parks after simulating just meeting the current standard, although
visible foliar injury would likely continue to occur at lower
O3 exposures, including some sensitive species growing in
National Parks and other Class I areas that may provide important
cultural ecosystem services to the public. The PA also notes the WREA
recognition that many of the outdoor recreational activities which
directly depend on the scenic value of the area are of significant
importance to public welfare as they are enjoyed by millions of
Americans every year and generate millions of dollars in economic value
(U.S. EPA, 2014b, Chapter 5, Chapter 7).
In reaching conclusions regarding support for the adequacy of the
current secondary standard provided by the currently available
information on O3-induced visible foliar injury, the PA took
note of: (1) The evidence for many species of native plants, including
trees, that have been observed to have visible foliar injury symptoms
in both OTC and field settings, some of which have also been identified
as bioindicators of O3 exposure by the USFS; (2) the finding
that visible foliar injury incidence can occur at very low cumulative
exposures, but due to confounding by soil moisture and other factors,
it is difficult to predictively relate a given O3 exposure
to plant response; (3) information indicating the occurrence of visible
foliar injury in some Class I areas under air quality conditions
expected to meet the current standard; and, (4) WREA analyses, based on
USFS biosite data, indicating a relationship of the proportion of
biosites showing visible foliar injury incidence with W126 index values
below approximately 10 ppm-hrs (U.S. EPA, 2014c, section 5.7).
The PA additionally recognizes a lack of guidance for federal land
managers regarding what spatial scale or degree of severity of visible
foliar injury is considered sufficient to trigger protective action for
O3 sensitive AQRVs. Further, there does not appear to be any
consensus in the literature in this regard, and CASAC, while
identifying benchmarks to consider for percent biomass loss and yield
loss for tree seedlings and commodity crops, respectively, did not
provide a similar recommendation for this endpoint. Likewise, as in
previous reviews, the ISA notes the difficulty in relating visible
foliar injury symptoms to other vegetation effects such as individual
plant growth, stand growth, or ecosystem characteristics (U.S. EPA,
2013a, section 9.4.2, p. 9-39) and further notes that the full body of
evidence indicates that there is wide variability in this endpoint,
such that although evidence shows visible foliar injury can occur under
very low cumulative O3 concentrations, ``. . .the degree and
extent of visible foliar injury development varies from year to year
and site to site . . ., even among co-members of a population exposed
to similar O3 levels, due to the influence
[[Page 75335]]
of co-occurring environmental and genetic factors'' (U.S. EPA, 2013a,
section 9.4.2, p. 9-38).
Given the above, and taking note of CASAC views, the PA recognizes
visible foliar injury as an important O3 effect which,
depending on severity and spatial extent, may reasonably be concluded
to be of public welfare significance, especially when occurring in
nationally protected areas. While noting the uncertainties associated
with describing the potential for visible foliar injury and its
severity or extent of occurrence for any given air quality conditions,
the PA notes the occurrence of O3-induced visible foliar
injury in areas, including federally protected Class I areas that meet
the current standard, and suggests it may be appropriate to consider
revising the standard to achieve greater protection, while recognizing
that the degree to which O3-induced visible foliar injury
would be judged important and potentially adverse to public welfare is
uncertain (U.S. EPA, 2014c, section 5.7).
With regard to other welfare effects, for which the ISA determined
a causal or likely causal relationships with O3 in ambient
air, such as alteration of ecosystem water cycling and changes in
climate, the PA concludes there are limitations in the available
information which affect our ability to consider potential impacts of
air quality conditions associated with the current standard.
In reaching conclusions on options for the Administrator's
consideration, the PA indicates that the final decision to retain or
revise the current secondary O3 standard is a public welfare
policy judgment to be made by the Administrator, based on her judgment
as to what level of air quality would be requisite (i.e., neither more
nor less stringent than necessary) to protect the public welfare from
any known or anticipated adverse effects. This final decision will draw
upon the available scientific evidence for O3-attributable
welfare effects and on quantitative analyses of vegetation and
ecosystem exposures and associated risks to vegetation, ecosystems and
their associated services, and judgments about the appropriate weight
to place on the range of uncertainties inherent in the evidence and
analyses. In making this decision, the Administrator will need to weigh
the importance of these effects and their associated ecosystem services
in the overall context of public welfare protection.
Based on the considerations described in the PA and summarized
here, the PA concludes that the currently available evidence and
exposure/risk information call into question the adequacy of the public
welfare protection provided by the current standard and provides
support for considering potential alternative standards to achieve
increased public welfare protection, especially for sensitive
vegetation and ecosystems in federally protected Class I and similarly
protected areas. In this conclusion, staff gives particular weight to
the evidence indicating the occurrence in Class I areas that meet the
current standard of cumulative seasonal O3 exposures
associated with estimates of tree growth impacts of a magnitude that
may reasonably be considered important to public welfare.
2. CASAC Advice
Beyond the evidence- and exposure/risk-based considerations in the
PA discussed above, the EPA's consideration of the adequacy of the
current secondary standard also takes into account the advice and
recommendations of CASAC.
In its advice offered in the current review, based on the updated
scientific and technical record since the 2008 rulemaking, the CASAC
stated that they ``support the conclusion in the Second Draft PA that
the current secondary standard is not adequate to protect against
current and anticipated welfare effects of ozone on vegetation'' (Frey,
2014c, p. iii) and that the PA ``clearly demonstrates that ozone-
induced injury may occur in areas that meet the current standard''
(Frey, 2014c, p. 12). The Panel further stated ``[w]e support EPA's
continued emphasis on Class I and other protected areas'' (Frey, 2014c,
p. 9). Additionally, the CASAC indicated support for the concept of
ecosystem services ``as part of the scope of characterizing damage that
is adverse to public welfare'' and ``concurs that trees are important
from a public welfare perspective because they provide valued services
to humans, including aesthetic value, food, fiber, timber, other forest
products, habitat, recreational opportunities, climate regulation,
erosion control, air pollution removal, and hydrologic and fire regime
stabilization'' (Frey, 2014c, p. 9). Similar to comments from CASAC in
the last review, including comments on the proposed reconsideration,
the current CASAC also endorsed the PA discussions and conclusions on
biologically relevant exposure metrics and the focus on the W126 index
accumulated over a 12-hour period (8am-8pm) over the three-month
summation period of a year resulting in the maximum value (Frey, 2014c,
p. iii).
In addition, CASAC stated that ``relative biomass loss for tree
species, crop yield loss, and visible foliar injury are appropriate
surrogates for a wide range of damage that is adverse to public
welfare'' (Frey, 2014c, p. 10). With respect to relative biomass loss
for tree species, CASAC states that it is appropriate to ``include
levels that aim for not greater than 2% RBL for the median tree
species'' and that a median tree species RBL of 6% is ``unacceptably
high.'' With respect to crop yield loss, CASAC points to a benchmark of
5%, stating that a crop RYL for median species over 5% is
``unacceptably high'' (Frey, 2014c, p. 13).
3. Administrator's Proposed Conclusions on Adequacy of the Current
Standard
In considering the adequacy of the current secondary O3
standard, the Administrator has considered the assessment of the
current evidence in the ISA, findings of the WREA, including associated
limitations and uncertainties, considerations and staff conclusions and
associated rationales presented in the PA, views expressed by CASAC,
and public comments. In taking into account the information discussed
above with regard to the nature of O3-related effects on
vegetation, the Administrator has taken particular note of: the PA
analysis of the magnitude of tree seedling growth effects (biomass
loss) estimated for different cumulative, seasonal, concentration-
weighted exposures in terms of the W126 metric; the monitoring analysis
in the PA of W126 exposures occurring in locations where the current
standard is met, including those locations in Class I areas, and
associated estimates of tree seedling growth effects; the analyses in
the WREA illustrating the geographic distribution of tree species for
which E-R functions are available and relative differences estimated
for O3-related growth impacts across areas of the U.S. for
the air quality scenarios, taking into account the identified potential
for the WREA's scenario for the current standard to underestimate the
highest W126-based O3 values that would be expected to
occur.
As an initial matter, the Administrator recognizes the
appropriateness and usefulness of the W126 metric, as described in
sections IV.B.1 and IV.D.1 above, in evaluating O3 exposures
of potential concern for vegetation effects. In so doing, the
Administrator additionally notes support conveyed by CASAC for such a
use for this metric.
With regard to considering the adequacy of public welfare
protection provided by the current secondary standard, the
Administrator focuses first on welfare effects related to reduced
[[Page 75336]]
native plant growth and productivity in terrestrial ecosystems, taking
note of the ISA conclusion of a causal relationship between
O3 in the ambient air and these effects. In considering the
assessment of the information available in this review with regard to
O3 effects on vegetation growth and productivity, the
Administrator takes note of the evidence from OTC studies of the
effects of O3 exposure on tree seedling growth that support
robust E-R functions for 11 tree seedling species, and the
characterization of growth effects across these species for different
cumulative seasonal concentration-weighted exposures using the W126
metric. Reductions in growth of sensitive species, as recognized in
section IV.B above, have the potential to result in effects on
ecosystem productivity, as well as, on forest and forest community
composition. The Administrator takes particular note of the evidence,
described in section IV.D.1 above, of the occurrence in Class I areas
during periods where the current standard is met of cumulative seasonal
O3 exposures for which median species RBL estimates are of a
magnitude that CASAC has termed ``unacceptably high.'' In so doing, the
Administrator also takes note of a number of actions taken by Congress
to establish public lands that are set aside for specific uses intended
to provide benefits to the public welfare, including lands that are to
be protected so as to conserve the scenic value and the natural
vegetation and wildlife within such areas for the enjoyment of future
generations. Such public lands that are protected areas of national
interest include national parks and forests, wildlife refuges, and
wilderness areas (many of which have been designated Class I
areas).\217\
---------------------------------------------------------------------------
\217\ As noted in section IV.A above, Congress has established
areas such as national parks and wilderness areas with specific
purposes including the preservation of the areas for future
generations, and has identified many of those areas as Class I
areas.
---------------------------------------------------------------------------
While recognizing the variability in the various environmental
factors that can influence the occurrence and severity of the effect of
ambient O3 concentrations on vegetation in different
locations, the Administrator concludes that the information referenced
above including the currently available, extensive evidence base and
also factors affecting the significance of impacts to public welfare,
as well as WREA estimates regarding the potential for occurrence of
impacts important to public welfare, provides an appropriate basis to
inform a conclusion as to whether the current standards provide
adequate protection against O3-related vegetation effects on
public welfare. With regard to the results of the monitoring analysis,
the Administrator takes note of the PA conclusions that the impacts on
tree growth (and the potential for associated ecosystem effects)
estimated for W126 values found to occur in Class I areas when meeting
the current standard are reasonably concluded to be important from a
public welfare standpoint in terms of both the magnitude of the
vegetation effects and the significance to public welfare of such
effects in such areas, calling into question the adequacy of the
current secondary standard.
The Administrator also recognizes the causal relationships between
O3 in the ambient air and visible foliar injury, reduced
yield and quality of agricultural crops and alteration of below-ground
biogeochemical cycles associated with effects on growth and
productivity. As to visible foliar injury, the Administrator takes note
of the complexities and limitations in the evidence base regarding
characterizing air quality conditions with respect to the magnitude and
extent of risk for visible foliar injury. She additionally recognizes
the challenges of associated judgments with regard to adversity of such
effects to public welfare. In taking note of the conclusions with
regard to crops, she recognizes the complexity of considering adverse
O3 impacts to public welfare due to the heavy management
common for achieving optimum yields and market factors that influence
associated services and additionally takes note of the PA conclusions
that placing emphasis on the protection afforded to trees inherently
also recognizes a level of protection afforded for crops.
Based on her consideration of the conclusions in the PA, and with
particular weight given to PA findings pertaining to tree growth-
related effects, as well as with consideration of CASAC's conclusion
that the current standard is not adequate, the Administrator proposes
to conclude that the current standard is not requisite to protect
public welfare from known or anticipated effects and that revision is
needed to provide increased public welfare protection, especially for
sensitive vegetation and ecosystems in federally protected Class I
areas and in other areas providing similar public welfare benefits. The
Administrator further concludes that the scientific evidence and
quantitative analyses on tree growth-related effects provide strong
support for consideration of alternative standards that would provide
increased public welfare protection beyond that afforded by the current
O3 secondary standard. She further notes that a revised
standard would provide increased protection for other growth-related
effects, including for carbon storage and for areas for which it is
more difficult to determine public welfare significance, as recognized
in section IV.B.2 above, as well other welfare effects of
O3, including visible foliar injury and crop yield loss.
In giving particular focus to tree growth-related effects of
O3 on public welfare, the Administrator additionally
recognizes that there are alternative approaches to viewing the
evidence and information, including alternative approaches to viewing,
evaluating, and weighing important uncertainties. In some cases, these
alternative approaches have been expressed by public commenters,
leading some public commenters to recommend retaining the current
standard. Given these alternative views, in addition to proposing to
revise the current secondary standard, the Administrator also solicits
comment on the option of retaining the standard without revision.
E. Consideration of Alternative Secondary Standards
Given her proposed conclusion that the current secondary standard
is inadequate, the Administrator has then considered what revisions to
the standard may be appropriate, focusing on revisions to the key
standard elements of indicator, form, averaging time, and level. On the
basis of the strength and coherence of the vegetation effects evidence
indicating a cumulative, seasonal, concentration-weighted metric as the
most appropriate approach for judging potential impacts of and
protection from O3 in ambient air, the Administrator judges
that it is appropriate to consider revisions to the secondary standard
that reflect this understanding and to use such a metric in identifying
an appropriate level of protection and considering the protection
afforded by potential alternative standards. The Administrator also
judges that the current averaging time and form may also provide
protection to vegetation when set at an appropriate level. Therefore,
the Administrator considered whether revision to the level of the
current secondary standard might provide sufficient protection to also
achieve the level of air quality that is determined requisite to
protect the public welfare.
The sections below address the indicator for the secondary standard
(section IV.E.1), consideration of a cumulative, seasonal exposure-
based
[[Page 75337]]
standard in the PA (section IV.E.2), CASAC advice and public input
(section IV.E.3), analyses of air quality in the PA and subsequent to
the PA (section IV.E.4) and the Administrator's proposed conclusions
regarding an alternative secondary standard (section IV.E.5)
1. Indicator
In the last review of the air quality for O3 and other
photochemical oxidants and of the O3 standard, as in other
prior reviews, the EPA focused on a standard for O3 as the
most appropriate surrogate for ambient photochemical oxidants. Ozone is
a long-established surrogate for ambient photochemical oxidants, among
which it is by far the most widely studied with regard to effects on
welfare and specifically on vegetation. The information available in
this review adds to the understanding of the atmospheric chemistry for
photochemical oxidants and O3 in particular (as described in
the ISA, sections 3.2 and 3.6, and summarized in section 2.2 in the
PA). The 1996 Staff Paper noted that the database on vegetation effects
is generally considered to raise concern at levels found in the ambient
air for O3 and, therefore, control of ambient O3
levels has previously been concluded to provide the best means of
controlling other photochemical oxidants of potential welfare concern
(U.S. EPA, 1996b, p. 277). In the current review, while the complex
atmospheric chemistry in which O3 plays a key role has been
highlighted, no alternatives to O3 have been advanced as
being a more appropriate surrogate for ambient photochemical oxidants.
Ozone continues to be the only photochemical oxidant (other than
nitrogen dioxide) that is routinely monitored and for which a
comprehensive database exists (U.S. EPA, 2013a, section 3.6).
Thus, the Administrator concludes that ozone is the appropriate
indicator and proposes to continue to use O3 as indicator
for a secondary standard that is intended to address effects associated
with exposure to O3, alone and in combination with related
photochemical oxidants. In so doing, the Administrator recognizes that
measures leading to reductions in ecosystem exposures to O3
will also reduce exposures to other photochemical oxidants.
2. Consideration of a Cumulative, Seasonal Exposure-based Standard in
the Policy Assessment
In recognition of the extensive evidence supporting a cumulative,
seasonal exposure index as a biologically relevant metric for assessing
potential for O3 effects on vegetation, discussed in
sections IV.B.1 above, as well as advice from CASAC in the current and
last O3 NAAQS reviews, summarized in sections IV.D.3 above
and IV.E.3 below, the PA focused its consideration of alternative
standards on a revised secondary standard based on a cumulative,
seasonal, concentration-weighted form. The PA considered the currently
available information that has been critically analyzed and
characterized in the ISA, the risk and exposure information presented
in the WREA, and CASAC advice and public comment with regard to support
for consideration of options for alternative standards that might be
expected to provide increased protection from ambient O3
exposures over the current standard.
a. Form and Averaging Time
In considering potential forms for a revised secondary standard,
the PA considers the characterization of the evidence in the ISA,
summarized in section IV.B.1 above, including the ISA conclusion that
exposure indices that cumulate and differentially weight the higher
hourly average concentrations over a season and also include the mid-
level values, such as the W126 index, offer the most scientifically
defensible approach for characterizing vegetation response to ambient
O3 and comparing study findings, as well as for defining
indices for vegetation protection (U.S. EPA, 2013a, section 2.6.6.1).
The PA additionally considers CASAC advice in the current review, as
well as that from the last review, all of which provided support for
such a form. Thus, in considering alternative forms of a revised
standard, the PA concludes that it is reasonable and appropriate to
consider a cumulative, concentration-weighted form to provide
protection against cumulative, seasonal exposures to O3 that
are known or anticipated to harm sensitive vegetation or ecosystems.
The PA recognizes that such a metric is specifically designed to focus
on the kind of O3 exposures that have been shown to cause
harm to vegetation and states that it would have a distinct advantage
over the form of the current standard in characterizing air quality
conditions potentially of concern for vegetation and in more directly
demonstrating that the desired degree of protection against those
conditions was being achieved (U.S. EPA, 2014c, sections 6.2 and 6.6).
With regard to the appropriate index for a cumulative seasonal
form, the PA considers the evidence and background for a number of
different cumulative concentration weighted indices that have been
developed and evaluated in the scientific literature and in past NAAQS
reviews in terms of their ability to predict vegetation response and
their usefulness in the NAAQS context (U.S. EPA, 2006a, pp. 9-11 to 9-
15 and pp. AX9-159 to AX9-187; U.S. EPA, 2007, pp. 7-15 to 7-16). While
these various forms have different strengths and limitations, the PA
notes the ISA conclusion that the W126 index, described in section
IV.B.1 above, has some important advantages over other non-sigmoidally
weighted cumulative indices, including its lack of a cut-off in its
weighting scheme which allows for cumulation of lower O3
concentrations (U.S. EPA, 2013a, section 9.5; U.S. EPA, 2014c, sections
6.2 and 6.6). Additionally, the W126 metric adds increasing weight to
hourly concentrations from about 40 ppb to about 100 ppb, which is an
important feature because ``as hourly concentrations become higher,
they become increasingly likely to overwhelm plant defenses and are
known to be more detrimental to vegetation'' (U.S. EPA, 2013a, p. 9-
104). The PA additionally takes note of CASAC advice in the current and
last review that concurred with a focus on the W126 form (Frey, 2014c,
p. iii; Henderson, 2006; Samet, 2010). Based on the considerations
summarized here, the PA concludes that the W126 index is the most
appropriate cumulative seasonal form to consider in the context of the
secondary O3 NAAQS review.
The PA next considers the exposure periods--diurnal and seasonal--
over which the W126 index would be summed in any given year. The
currently available information continues to provide support for a
definition of the diurnal period of interest as the 12-hour period from
8:00 a.m. to 8:00 p.m., which the EPA identified in past reviews as
appropriately capturing the diurnal window with most relevance to the
photosynthetic process (U.S. EPA, 2013a, section 9.5.3; 72 FR 37900,
July 11, 2007). The CASAC has generally supported this 12-hour daylight
period as well (Frey, 2014c; Henderson, 2006, 2007). Based on these
considerations, the PA concludes that the 12-hour daylight window (8:00
a.m. to 8:00 p.m.) represents the portion of the diurnal exposure
period that is most relevant to predicting or inducing plant effects
related to photosynthesis and growth and thus is an appropriate diurnal
period to use in conjunction with a W126 cumulative metric (U.S. EPA,
2014c, sections 6.2 and 6.6). With regard
[[Page 75338]]
to a seasonal period of interest, the current evidence base continues
to provide support for a seasonal period with a minimum duration of
three months, as described more fully in the ISA and considered in the
PA (U.S. EPA, 2013a, section 9.5.3; U.S. EPA, 2014c, sections 6.2 and
6.6). The CASAC has also indicated support for such a three month
period (Frey, 2014c; Samet, 2010; Henderson, 2006). The PA thus
concludes that it is appropriate to identify the seasonal W126 index
value as that derived from the consecutive 3-month period within the
O3 season with the highest W126 index value.
The PA additionally considers the period of time over which a
cumulative seasonal W126-based standard should be evaluated,
considering the support for both a single year form and a form averaged
over three years (U.S. EPA, 2014c, pp. 6-29 through 6-33). The PA
considers the evidence of effects associated with single year and
multiple year exposures as well as their potential public welfare
significance. The PA also considers comments from CASAC, including
their comment in the current review that ``[t]he CASAC does not
recommend the use of a three-year averaging period'' and that they
``favor a single-year period for determining the highest three-month
summation which will provide more protection for annual crops and for
the anticipated cumulative effects on perennial species'' (Frey, 2014c,
p. iii).
The PA considered O3-induced effects that can occur with
a single year's exposure, including visible foliar injury, growth
reduction in annual and perennial species and yield loss in annual
crops (U.S. EPA, 2014c, section 6.3). While recognizing that there are
a number of O3-induced effects that have the potential for
public welfare significance within the annual timeframe, the PA also
notes the uncertainties associated with these effects that complicate
consideration of the level of appropriate protection on an annual basis
for such effects in order to protect the public welfare from known or
anticipated adverse effects, and thus recognizes the possibility that a
multiple-year form could be considered to provide a more consistent
target level of protection for certain effects (U.S. EPA, 2014c, pp. 6-
29 to 6-31). With regard to visible foliar injury, the ISA notes that
``the degree and extent of visible foliar injury development varies
from year to year and site to site . . . even among co-members of a
population exposed to similar O3 levels, due to the
influence of co-occurring environmental and genetic factors'' (U.S.
EPA, 2013a, p. 9-38; U.S. EPA, 2014c, p. 6-30). Additionally, the PA
takes note of the difficulty and complexity shown by the WREA analyses
with regard to identifying W126 index values that would provide
consistent protection on an annual basis given likely fluctuations in
annual O3 and soil moisture conditions (U.S. EPA, 2014c, p.
6-30).
The PA additionally notes evidence of some O3 effects on
perennial species that may result from a single season's elevated
O3 exposures, such as reduced bud size or starch content,
which may have the potential for some ``carry over'' of effects on
plant growth or reproduction in the subsequent season. Another effect
where such potential for ``carry over'' has been noted with elevated
O3 exposure is reduction in below-ground carbohydrate
reserves which can impair growth in subsequent seasons (U.S. EPA,
2014c, pp. 6-30 to 6-31; U.S. EPA, 2013a, pp. 9-43 to 9-44 and p. 9-
86). The PA notes that the occurrence of such annual effects of
elevated O3 exposures over multiple years may contribute to
a potential to be compounded, increasing the potential for effects at
larger scales (e.g., population, ecosystem). In the PA, staff notes
that multiple consecutive years of critical O3 exposures
might be expected to result in larger impacts on forested areas than
intermittent occurrences of such exposures due to the potential for
compounding or carry-over effects on tree growth (U.S. EPA, 2014c, pp.
6-29 to 6-31).
In light of the above summarized considerations for potential
compounding of carry-over effects, the PA concludes that the public
welfare significance of the effects that can occur as a result of
three-year O3 exposures are potentially greater than those
associated with a single year of such exposure. Thus, to the extent
that the focus for public welfare protection to be afforded by the
secondary O3 standard is on long-term effects that occur in
sensitive tree species in natural forested ecosystems, including
federally protected areas such as Class I areas or on lands set aside
by States, Tribes and public interest groups to provide similar
benefits to the public welfare, the PA concludes that a standard with a
form that evaluates the cumulative seasonal index across multiple years
might be considered to provide a more appropriate match to the nature
of O3-related effects on vegetation upon which the secondary
O3 standard is focused. In considering such forms, the PA
focuses on one that averages the W126 index values across three years
(U.S. EPA, 2014c, section 6.2).
With regard to single-year and three-year forms, the PA considers a
WREA analysis that examined the extent to which cumulative RBL across a
three-year period might be underestimated when each year's RBL is
derived from the three-year average W126 index value versus each
single-year W126 index value for each of three years (in which no other
influence on plant growth is presumed to change). This analysis
indicates that use of the three-year average may lead to an
underestimation, although of relatively small magnitude (U.S. EPA,
2014b, section 6.2.1.4). The PA notes that this limited analysis does
not account for moisture levels and other environmental factors that
could affect plant growth and that vary from year to year. When
considering an appropriate level for a form that averages W126 index
values across three years, the PA also recognizes the importance of
considering the extent to which the cumulative effect of different
average W126 exposures across the three-year period would be judged
adverse (U.S. EPA, 2014c, p. 6-31).
Although single-year W126 index values were not separately analyzed
in the PA analysis of recent monitoring data, the data indicate
appreciable variation in cumulative, seasonal O3
concentrations among monitor locations meeting different levels of a
standard of the current form (U.S. EPA 2014c, section 6. Appendix
2B).Therefore, a standard with an annual form would have the cumulative
seasonal index values be at or lower than the level of the standard in
all years and, noting the inter-annual observed variability in seasonal
W126 index values, could be appreciably below the standard level in
some years. For a standard with a form that averages the cumulative
seasonal index values across three consecutive years, the annual
seasonal index value could be above the level in some years, but would
have to be below it in others within the same three-year period, thus
restricting the air quality for a given area to have no more than two
years out of three with a W126 index value above the standard level,
and depending on magnitude of each year's index, potentially having no
more than one.
In its consideration of one year as compared to three year forms,
the PA also considers implications with regard to stability of air
quality programs that implement the NAAQS (U.S. EPA, 2014c, pp. 6-31 to
6-32). The PA notes that a standard based on a single year W126 index
would be expected to have a less stability relative to a standard based
on a form that averages seasonal indices across three consecutive
years, given the potential for large year-to-year variability in annual
W126 index values in areas across the country. Thus, a
[[Page 75339]]
three-year evaluation period can contribute to greater public welfare
protection by limiting year-to-year disruptions in ongoing control
programs that would occur if an area was frequently shifting in and out
of attainment due to extreme year-to-year variations in meteorological
conditions. This greater stability in air quality management programs
would thus facilitate achievement of the protection intended by a
standard. Such considerations of stability often receive particular
weight in NAAQS reviews, such as those resulting in selection of the
form for the current O3 primary and secondary standards (62
FR 38856, July 18, 1997), as well as the primary standards for nitrogen
dioxide (75 FR 6474, February 9, 2010) and sulfur dioxide (75 FR 35520,
June 22, 2010). See also ATA III, 283 F. 3d at 374-75 (recognizing
programmatic stability as a legitimate consideration in the NAAQS
standard-setting process).
Thus, to the extent that emphasis continues to be placed on
protecting against effects associated with multi-year exposures and
maintaining more year-to-year stability of public welfare protection,
the PA concludes that it is appropriate to consider a secondary
standard form that is an average of the seasonal W126 index values
across three consecutive years. The PA concludes that such a form might
be appropriate for a standard intended to achieve the desired level of
protection from longer-term effects, including those associated with
potential compounding, and that such a form might be concluded to
contribute to greater stability in air quality management programs, and
thus, greater effectiveness in achieving the desired level of public
welfare protection, than that might result from a single year form
(U.S. EPA, 2014c, section 6.6).
The PA additionally recognized that to the extent the Administrator
finds it useful to consider the public welfare protection that might be
afforded by a revised primary standard, this is appropriately judged by
evaluating the impact of attainment of such a revised primary standard
on O3 exposures in terms of the cumulative seasonal W126-
based exposure index.
b. Level
In considering an appropriate range of levels to consider for a
W126-based standard, the PA notes that, due to the variability in the
importance of the associated ecosystem services provided by different
species at different exposures and in different locations, as well as
differences in associated uncertainties and limitations, both the
species present and their public welfare significance, in addition to
the magnitude of the ambient concentrations, are essential
considerations in drawing conclusions regarding the significance or
magnitude of public welfare impact. Therefore, in development of the PA
conclusions, staff took note of the complexity of judgments to be made
by the Administrator regarding the adversity of known and anticipated
effects to the public welfare and recognized that the Administrator's
ultimate judgments on the secondary standard will most appropriately
reflect an interpretation of the available scientific evidence and
exposure/risk information that neither overstates nor understates the
strengths and limitations of that evidence and information.
As described in section IV.D.1 above, the PA employed a paradigm,
which has evolved over the course of the O3 and other
secondary NAAQS reviews, to assist in putting the available science and
exposure/risk information into the public welfare context (U.S. EPA,
2014c, section 5.1). This paradigm recognizes that the significance to
the public welfare of O3-induced effects on sensitive
vegetation growing within the U.S. can vary depending on the nature of
the effect, the intended use of the sensitive plants or ecosystems, and
the types of environments in which the sensitive vegetation and
ecosystems are located. Accordingly, any given O3-related
effect on vegetation and ecosystems (e.g., biomass loss, crop yield
loss, visible foliar injury) may be judged to have a different degree
of impact on or significance to the public welfare depending, for
example, on whether that effect occurs in a Class I area, a city park,
or commercial cropland. This approach also includes consideration of
impacts to ecosystem goods and services, which are an important
category of public welfare effects with an obvious relationship to
consideration of intended use (73 FR 16492, March 27, 2008).
In considering potential levels for an alternative standard based
on the W126 metric, the PA focused primarily on impacts on tree growth,
crop yield loss, and visible foliar injury, as well as impacts on the
associated ecosystem services, while taking note of the uncertainties
and limitations associated with several key aspects of this
information. In addition to uncertainties related to the WREA air
quality scenarios and assessments summarized in section IV.C above, the
PA also recognized uncertainties associated with the evidence
underlying the tree seedling and crop E-R functions (U.S. EPA, 2014c,
section 6.3). These include uncertainties regarding intra-species
variability due to the different numbers of studies that exist for
different species so that the weight of evidence is not the same for
each species. Those species with more than one study show variability
in response and E-R functions. The potential variability in less well-
studied species is, however, unknown (U.S. EPA, 2013a, pp. 9-123 to 9-
125; U.S. EPA, 2014b, section 6.2.1.2, and Table 6-27). The PA also
recognizes uncertainty regarding the extent to which tree seedling E-R
functions can be used to represent mature trees since seedling
sensitivity has been shown in some cases to not reflect mature tree
O3 sensitivity in the same species and uncertainty in the
relationship of O3 effects on tree seedlings (e.g., relative
biomass loss) in one or a few growing seasons to effects that might be
expected to accrue over the life of the trees extending into adulthood
(U.S. EPA, 2013a, section 9.6 and pp. 9-52 to 9-53; U.S. EPA, 2014b,
sections 6.2.1.1, 6.2.1.4 and Tables 6-5 and 6-27).
With respect to tree growth, the PA gave primary consideration to
relative biomass loss estimates derived from the E-R functions,
described in section IV.B.1.b above and in the PA, while also
considering WREA risk/exposure estimates related to this effect (U.S.
EPA, 2014c, section 6.4). The PA takes note of the different index
value estimates presented in Table 6-1 of PA (Table 8 below) with
regard to the number of studied species below different response
benchmarks, as well as with regard to the median response. The PA
additionally considers the WREA estimates regarding: (1) percent of
assessed geographic area exceeding 2% weighted relative biomass (U.S.
EPA, 2014c, Table 6-2); (2) number of assessed Class I areas with tree
seedling weighted relative biomass loss estimates below 2% (U.S. EPA,
2014c, Table 6-3); and (3) the percent median biomass loss across
counties for different air quality scenarios (U.S. EPA, 2014c, Table 5-
5). The PA further notes other WREA estimates for effects on ecosystem
services related to public welfare, such as carbon sequestration and
air pollutant removal. With respect to crop yield loss, the PA notes
the summary of RYL estimates for individual crop species and for the
median across species (Table 8), and the WREA risk/exposure estimates
(U.S. EPA, 2014b, Section 6.3). The PA also notes information available
on visible foliar damage to species occurring in areas preserved for
their natural character,
[[Page 75340]]
such as federal Class I areas, and the analyses in the WREA evaluating
biosite data and several benchmarks of injury (U.S. EPA, 2014b, section
5.4.2).
Table 8--Tree Seedling Biomass Loss and Crop Yield Loss Estimated for O3 Exposure Over a Season
----------------------------------------------------------------------------------------------------------------
Tree seedling RBL \a\ Crop RYL \c\
W126 value for exposure --------------------------------------------------------------------------------------
period Median value Individual species Median value Individual species
----------------------------------------------------------------------------------------------------------------
21 ppm-hrs............... Median species w. <= 2% loss: 3/11 Median species w. <=5% loss: 4/10
6.8% loss \b\. species. 7.7% loss \d\. species.
<=5% loss: 5/11 >5, <10% loss: 3/10
species. species.
<=10% loss: 7/11 >10, <20% loss: 3/
species. 10 species.
<=15% loss: 10/11
species.
>40% loss: 1/11
species.
19 ppm-hrs............... Median species w. <=2% loss: 3/11 Median species w. <=5% loss: 5/10
6.0% loss \b\. species. 6.4% loss \d\. species.
<5% loss: 5/11 >5, <10% loss: 3/10
species. species.
<=10% loss: 7/11 >10, <20% loss: 2/
species. 10 species.
<=15% loss: 10/11
species.
>30% loss: 1/11
species.
17 ppm-hrs............... Median species w. <=2% loss: 5/11 Median species w. <=5% loss: 5/10
5.3% loss \b\. species. 5.1% loss \d\. species.
<5% loss: 5/11 >5, <10% loss: 3/10
species. species.
<=10% loss: 9/11 >10, <20% loss: 2/
species. 10 species.
15% loss: 10/11
species.
>30% loss: 1/11
species.
15 ppm-hrs............... Median species w. <=2% loss: 5/11 Median species w. <=5% loss: 6/10
4.5% loss \b\. species. <=5% loss \d\. species.
<=5% loss: 6/11 >5, <10% loss: 4/10
species. species.
<=10% loss: 10/11
species.
>30% loss: 1/11
species.
13 ppm-hrs............... Median species w. <=2% loss: 5/11 Median species w. <=5% loss: 6/10
3.8% loss \b\. species. <=5% loss \d\. species.
<5% loss: 7/11 >5, <10% loss: 4/10
species. species.
<10% loss: 10/11
species.
>20% loss: 1/11
species.
11 ppm-hrs............... Median species w. <=2% loss: 5/11 Median species w. <=5% loss: 9/10
3.1% loss \b\. species. <=5% loss \d\. species.
<=5% loss: 8/11 >5, <10% loss: 1/10
species. species.
<=10% loss: 10/11
species.
>20% loss: 1/11
species.
9 ppm-hrs................ Median species w. <=2% loss: 5/11 Median species w. <=5% loss: all
2.4% loss \b\. species. <=5% loss \d\. species.
<=5% loss: 10/11
species.
>20% loss: 1/11
species.
7 ppm-hrs................ Median species w. <=2% loss: 7/11 Median species w. <=5% loss: all
<=2% loss \b\. species. <=5% loss \d\. species.
<=5% loss: 10/11
species.
>15% loss: 1/11
species.
----------------------------------------------------------------------------------------------------------------
\a\ Estimates are based on the 11 E-R functions for tree seedlings described in WREA, Appendix 6F and discussed
in the PA, section 5.2.1, with the exclusion of cottonwood in consideration of CASAC comments on differences
of that study from the other controlled E-R studies (Frey, 2014b, 2014c).
\b\ This is the median of the composite E-R functions for 11 tree species from the WREA, Appendix 6F (discussed
in the PA, section 5.2.1).
\c\ Estimates here are based on the 10 E-R functions for crops (from the PA, Appendix 6F and section 5.3.1).
\d\ This median value is the median of the composite E-R functions for 10 crops from WREA, Appendix 6F (also
discussed in the PA, section 5.3.1).
Given the wide variation in sensitivity of studied tree species to
O3-induced relative biomass loss, the PA focused
consideration on both median species values and individual species
responses and RBL estimates for a given range of W126 index values. In
this consideration, the PA took note of CASAC's advice regarding RBL
levels, specifically their emphasis on a benchmark of median relative
tree biomass loss at or below 2% and their view that a 6% median
relative biomass loss is ``unacceptably high.'' The median tree species
RBL estimate is at or below 2% only at the lowest W126 level assessed,
7 ppm-hrs. At incrementally higher W126 index levels, the median RBL is
also incrementally higher, so that at W126 index values of 9, 11, 13,
15, 17 and 19 ppm-hrs, the median RBL increases to 2.4%, 3.1%, 3.8%,
4.5%, 5.3% and 6.0%, respectively. Thus, the median species biomass
loss is below 6%, the level characterized by the CASAC as unacceptably
high, across the W126 range of 7 to 17 ppm-hrs, for which it varies
from approximately 2% to approximately 5%. Given this finding, the PA
discussion of a range of levels appropriate to consider focuses on this
range. In focusing on this range, the PA considers the full array of
CASAC advice with regard to interpretation of the evidence and
exposure/risk information on vegetation-related effects of
O3, as well as the role of the Administrator's judgments in
identifying the level of air quality that is requisite to protect
public welfare from adverse effects, as noted in section IV.A
above.\218\
---------------------------------------------------------------------------
\218\ In the context of the O3 standard, such
judgments include: The weight to give the evidence of specific
vegetation-related effects estimated to result across a range of
cumulative seasonal concentration-weighted O3 exposures;
the weight to give associated uncertainties, including those related
to the variability in occurrence of such effects in specific areas
of U.S., such as those of particular public welfare significance;
and, judgments on the extent to which such effects in such areas may
be considered adverse to public welfare.
---------------------------------------------------------------------------
The PA recognizes that public welfare judgments may reasonably be
informed by a range of biomass loss benchmarks, in contexts of
considering both median RBL estimates and RBL estimates for individual
species. Accordingly, in considering individual tree species estimates,
the PA notes the value of additionally characterizing the RBL
[[Page 75341]]
estimates in comparison to higher loss levels such as 10% or 15%.
For every W126 value over the full range from 7 to 17 ppm-hrs, the
RBLs for each of five species is less than 2% (Table 8), which is the
lower benchmark that CASAC identified for tree species. Accordingly,
the PA focused attention on the remaining six more sensitive studied
species (i.e., eastern white pine, aspen, tulip poplar, ponderosa pine,
red alder, and black cherry) to evaluate the protection against tree
seedling biomass loss at different W126 levels within the range from 17
to 7 ppm-hrs. At a W126 index value of 17 ppm-hrs, one of these six
species (red alder) has a RBL estimate below 6%, while at the W126
index value of 7 ppm-hrs, five of these six species have RBLs below 6%
(eastern white pine, aspen, tulip poplar, ponderosa pine, red alder).
Taken together with the more tolerant species, the proportion of the
studied tree species with RBLs below 6% are 6 of 11, 7 of 11, 8 of 11,
and 10 of 11 at W126 index values of 17, 15, 13, and 11 ppm-hrs,
respectively.
With regard to other, higher, RBL benchmark levels and estimates
for all 11 species, the PA notes that 9 of 11 studied tree species have
a predicted RBL below 10% at the W126 level of 17 ppm-hrs, while 10 of
11 species have a predicted RBL below 10% for W126 levels of 15 to 7
ppm-hrs. In addition, 10 of 11 studied tree species have a predicted
RBL below 15% for W126 levels of 17 to 7 ppm-hrs. The PA notes that the
RBL estimates for black cherry, the most sensitive of the 11 species,
remain above 15% for W126 index values across the range from 17 to 7
ppm-hrs, making unclear the extent to which black cherry estimates
might inform consideration of different W126 exposures within this
range (U.S. EPA, 2014c, section 6.6 and Table 6-1; U.S. EPA, 2014b,
section 6.2 and Appendix 6A).
While recognizing the limitations and uncertainties associated with
the WREA air quality scenarios with regard to their representation of
conditions just meeting different three-year average W126 index values
(as summarized in section IV.C.1 above), including the potential
underestimation of the highest O3 concentrations, the PA
additionally considers several WREA RBL analyses (U.S. EPA, 2014c,
section 6.3). In the WREA characterization of the number of counties
where the median RBLs were greater than 2%, 7% of the counties have
median RBLs greater than 2% in the 15 and 11 ppm-hrs W126 scenarios, as
compared to 8% for the current standard (U.S. EPA, 2014c, Table 5-5;
U.S. EPA, 2014b, Table 6-7). The percentage is 6% in the 7 ppm-hrs W126
scenario. Of the 221 counties (7% of counties) estimated to have a
median RBL above 2% for the 15 ppm-hrs scenario, 203 of those counties
have a RBL greater than 2% because of the presence of black cherry
(U.S. EPA, 2014c, section 6.3).
In considering the potential magnitude of the ecosystem impact of
O3-related biomass effects on tree growth, the PA
additionally focused on the WREA estimates of weighted RBL for the W126
air quality scenarios (U.S. EPA, 2014b, section 6.8). For the current
standard and the three W126 scenarios, the percent of total assessed
area having weighted RBL greater than 2% was 0.2%, 0.2%, 0.1% and
<0.1%, respectively (U.S. EPA, 2014c, Table 6-2; U.S. EPA 2014b, Table
6-25). In giving particular attention to estimates for Class I areas,
the PA notes that for all four scenarios, the WREA estimates indicate
weighted RBL greater than 2% in one or two of the 145 assessed
nationally protected Class I areas (U.S. EPA, 2014c, sections 6.3 and
6.6).
In considering potential impacts on ecosystem services related to
reductions in O3 effects on tree growth, the PA particularly
recognizes that impacts on climate regulation can reasonably be
concluded to be potentially significant from a public welfare
perspective. In additionally recognizing that carbon sequestration has
been identified as a potentially important tool for managing
anthropogenic impacts on climate, the PA considers the WREA estimates
of potential increases in forestry carbon storage for ambient
O3 reductions in the three W126 air quality scenarios (U.S.
EPA, 2014c, sections 6.3 and 6.6; U.S. EPA, 2014b, section 6.6.1). The
WREA estimates additional forestry carbon storage potential of 13, 593
and 1,600 MMtCO2e (over 30 years) for the W126 scenarios of
15, 11 and 7 ppm-hrs, respectively, as compared to the current standard
(U.S. EPA, 2014b, Table 6-18). Compared to the absolute estimate for
the current standard scenario (approximately 89,000 MMtCO2e,
over 30 years), these amounts represent additional storage of less than
0.1%, just under 1% and under 2% for the 15, 11 and 7 ppm-hrs
scenarios, respectively (U.S. EPA, 2014b, section 6.6.1 and Appendix
6B).
The PA additionally considers the WREA estimates for five urban
areas of how reduced growth of O3-sensitive trees in urban
forests may affect air pollutant removal (U.S. EPA, 2014b, sections
6.6.2 and 6.7 and Appendix 6D). As with the national estimates,
estimates for all five case study areas indicate generally small
differences between the current standard and the three W126 scenarios
(U.S. EPA, 2014c, Table 6-5). The PA additionally notes significant
uncertainties and limitations associated with WREA estimates related to
carbon sequestration and air pollution removal (U.S. EPA, 2014b, Table
6-27; U.S. EPA, 2014c, sections 6.3 and 6.6), some of which are
summarized in section IV.C.2.b above. The PA recognizes that, as with
consideration of other pertinent evidence and exposure/risk
information, the Administrator's consideration of WREA estimates for
these ecosystem services will involve judgments regarding the
appropriate weight to place on such uncertainties as well as the
potential impacts to the public welfare of the estimates.
The PA additionally considers the biomass effects of O3
on crops estimated for different W126 index values across the range
identified above. For this consideration, the PA focuses on the 10
crops for which robust E-R functions have been established, as
described in section IV.B.1 above: Barley, lettuce, field corn, grain
sorghum, peanut, winter wheat, field cotton, soybean, potato and kidney
bean (U.S. EPA, 2013a; U.S. EPA, 2014b, section 6.5 and Figure 6-3). In
evaluating this information, the PA takes note of CASAC's comment
regarding significance of 5% for median crop relative yield loss (RYL).
The PA finds that the median crop RYL is at or below 5% for all W126
index values from 7 to 17 ppm-hrs and observes that this finding makes
it unclear to what extent this information informs consideration of
levels within this range. The RBL estimates for half of the ten
individual species are below 5% RYL at 17 ppm-hrs. The number of the
ten individual crops with RYL below 5% is six for W126 values of 15 and
13 ppm-hrs, nine for a W126 value of 11 ppm-hrs and ten for W126 levels
of 9 and 7 ppm-hrs. Recognizing that different crops are likely to have
different values or importance to public welfare, the PA also considers
the RYL estimates across the W126 range for individual species.
In considering these RYL estimates, the PA recognizes that they do
not reflect the influence of the heavy management of agricultural crops
that is common in the U.S. and so cannot be easily interpreted with
regard to potential public welfare significance. In light of the median
RYL estimates of approximately 5% or lower for W126 index values at and
below 17 ppm-hrs, the PA gives less emphasis to consideration of crop
RYL, while noting that this information indicates that a secondary
standard revised to provide
[[Page 75342]]
additional protection for vegetation with attention to tree growth,
would be expected to also provide additional protection to crops over
that provided by the current standard (U.S. EPA, 2014c, section 6.6).
The PA also considers the evidence and exposure/risk information
with regard to visible foliar injury and the extent to which that might
inform consideration of potential alternative secondary standards
appropriate for the Administrator to consider. Specifically, the PA
notes the findings of the WREA analyses of the nationwide USFS/FHM
biosite dataset (2006-2010) that while soil moisture conditions
influence the proportion of biosites with O3-related visible
foliar injury, as described in section IV.B.1.a above, the proportion
of such sites increases appreciably with increasing W126 index values
up to approximately 10 ppm-hrs, while relatively little or no change in
incidence of injury is seen with O3 exposures at higher W126
index values (U.S. EPA 2014b, Chapter 7, Figure 7-10). The PA
additionally notes that visible foliar injury has been identified by
the federal land managers as a diagnostic tool for informing
conclusions regarding potential O3 impacts on potentially
sensitive AQRVs (USFS, NPS, FWS, 2010), which the PA concludes
indicates that such O3-induced impacts might be considered
to have the potential to impact the public welfare in scenic and/or
recreational areas during years they occur.
The PA was unable, however, to identify any guidance for federal
land managers regarding at what spatial scale or what degree of
severity visible foliar injury might be sufficient to trigger
protective action based on this potential impact on AQRVs. The PA
states that there does not appear to be consensus in the literature
regarding severity of visible foliar injury and risks to plant
functions or services, additionally noting that CASAC, while
identifying percent biomass loss and yield loss benchmarks for tree
seedlings and commodity crops, respectively, did not provide any
benchmark or criteria for consideration of O3 impacts
related to this endpoint. Further, as in previous reviews, the ISA
concludes visible foliar injury is not always a reliable indicator of
other negative effects on vegetation, making it difficult to relate
visible foliar injury symptoms to other vegetation effects such as
individual plant growth, stand growth, or ecosystem characteristics
(U.S. EPA, 2013a, section 9.4.2, p. 9-39). Additionally, although
evidence shows visible foliar injury can occur under very low
cumulative O3 exposures, ``. . . the degree and extent of
visible foliar injury development varies from year to year and site to
site . . ., even among co-members of a population exposed to similar
O3 levels, due to the influence of co-occurring
environmental and genetic factors'' (U.S. EPA 2013a, section 9.4.2, p.
9-38). Thus, while the PA recognizes visible foliar injury as an
important O3 effect which, depending on severity and spatial
extent may reasonably be concluded to be of public welfare
significance, most particularly in nationally protected areas such as
Class I areas, it additionally recognizes the appreciable variability
in this endpoint, which poses challenges to giving it primary emphasis
in identifying potential alternative standard levels.
On the basis of all the considerations described above, including
the evidence and exposure/risk analyses, and advice from CASAC, the PA
concludes that a range of W126 index values appropriate for the
Administrator to consider in identifying a secondary standard that
might be expected to provide the requisite protection to the public
welfare from any known or anticipated adverse effects, extends from 7
to 17 ppm-hrs. The PA notes, however, the role of judgments by the
Administrator in such decisions, as recognized above. In selecting this
range, the PA primarily considers the evidence- and exposure/risk-based
information for cumulative seasonal O3 exposures represented
by W126 index values (including those represented by the WREA average
W126 scenarios) associated with biomass loss in studied tree species,
both in and outside areas that have been afforded special protections.
The PA recognizes that tree biomass loss can be an indicator of more
significant ecosystem-wide effects which might reasonably be concluded
to be significant to public welfare. For example, when biomass loss
occurs over multiple years at a sufficient magnitude, it is linked to
some level of effects on an array of ecosystem-level processes, such as
nutrient and water cycles, changes in above and below ground
communities, carbon storage and air pollution removal, that benefit the
public welfare (U.S. EPA, 2014c, Figure 5-1). In focusing on tree
biomass effects, the PA gave emphasis to CASAC's judgment that a 6%
median RBL is unacceptably high, and that the 2% median RBL is an
important benchmark to consider. The PA notes that for the lower W126
value of 7 ppm-hrs that the median tree species biomass loss is at or
below 2% and that for the upper value of 17 ppm-hrs that the median
tree biomass loss is below 6%.\219\
---------------------------------------------------------------------------
\219\ We note that a W126 index value of 19 ppm-hrs is estimated
to result in a median RBL value of 6%, as shown in Table 2 above.
---------------------------------------------------------------------------
In considering the stability and potential for associated greater
public welfare protection offered by a three-year form, as well as
based on the recognition that in any given year in the environment,
other environmental factors can influence the extent to which
O3 may have the impact predicted by the E-R functions on
which much of the range discussion above focuses, the PA gave careful
consideration to the support for consideration of potential alternative
W126 based standards with levels in the range identified above (17 ppm-
hrs to 7 ppm-hrs) with a three-year average form.
Thus, the PA concludes that in staff's view, the evidence- and
exposure/risk-based information relevant to tree biomass loss and the
associated ecosystem services important to the public welfare support
consideration of a W126-based secondary standard with index values
within the range of 7 to 17 ppm-hrs, and a form averaged over three
years. In reaching this conclusion, the PA gave particular
consideration to the importance of considering the lasting or carry-
over effects that can derive from single year exposures of perennial
plants, recognizing the importance of considering the available
evidence and exposure/risk based information related to such effects,
as well as associated uncertainties. The PA additionally recognized
that there is limited information to discern differences in the level
of protection afforded for cumulative growth-related effects by
potential alternative W126-based standards of a single year form as
compared to a three-year average form. Lastly, the PA recognizes the
role of policy judgments required of the Administrator with regard to
the public welfare significance of identified effects, the appropriate
weight to assign the range of uncertainties inherent in the evidence
and analyses, and, ultimately, in identifying the requisite protection
for the secondary O3 standard. Examples of areas where the
Administrator's judgments would be expected include those stemming from
consideration of the effects associated with longer-term conditions and
the role that year-to-year exposure variability may play in associated
public welfare impacts, as well as the objectives for consideration of
tree species biomass loss estimates in relationship to identified
benchmarks (e.g., 2% or greater).
[[Page 75343]]
The PA also concludes that, to the extent the Administrator finds
it useful to consider the public welfare protection that might be
afforded by a revised primary standard, this is appropriately judged
through the use of a cumulative seasonal W126-based exposure metric, a
metric considered appropriate for evaluating impacts on vegetation. For
example, comparison of the air quality conditions (expressed in terms
of W126 exposures) expected to result from a revised primary standard
to the W126-based exposures concluded to provide requisite public
welfare protection would thus inform a judgment of whether a secondary
standard set identical to a revised primary standard would be expected
to achieve the appropriate level of air quality. The PA notes that such
a comparison would be in terms of a metric considered appropriate for
evaluating impacts on vegetation which inform conclusions on public
welfare impacts. The PA further concludes that the drawing of
conclusions with regard to the public welfare protection afforded by
such a standard should entail consideration of the air quality
conditions likely to be achieved in terms of the cumulative seasonal
W126-based metric described above.
Accordingly, the PA describes several analyses of air quality data
that might inform such consideration (U.S. EPA, 2014c, section 6.4),
and notes the importance of taking into account associated
uncertainties, including those associated with the limited monitor
coverage in many rural areas, such as those in the West and Southwest
regions and at high elevation sites. Additional such analyses, based on
more recent O3 monitoring data, have been developed since
the completion of the PA. All of these analyses are summarized in
section IV.E.4 below. In reaching conclusions on appropriate policy
options for a revised secondary standard the Administrator has
considered the findings of these analyses, as described in section
IV.E.5 below.
3. CASAC Advice
Beyond the evidence- and exposure/risk-based considerations in the
PA discussed above, the EPA's consideration of a revised secondary
standard also takes into account the advice and recommendations of
CASAC. The EPA also considered public comments received to date, some
of which urged the consideration of a secondary standard with a
cumulative seasonal form using the W126 metric and a level within the
range of 7 to 15 ppm-hrs or in the low end of this range,\220\ while
others have urged retaining the existing form and averaging time due to
their view of a lack of new information to support a distinct secondary
standard.
---------------------------------------------------------------------------
\220\ Public comment received thus far in this review are in the
docket EPA-HQ-OAR-2008-0699, accessible at www.regulations.gov
---------------------------------------------------------------------------
In advice offered on a revised secondary standard in the current
review, similar to advice in the last review, including advice offered
on the 2010 proposed reconsideration, the CASAC recommended ``retaining
the current indicator (ozone) but establishing a revised form of the
secondary standard to be the biologically relevant W126 index
accumulated over a 12-hour period (8 a.m.-8 p.m.) over the 3-month
summation period of a single year resulting in the maximum value of
W126'' (Frey, 2014c, p. iii). With regard to the level, the CASAC
recommended that ``that the level associated with this form be within
the range of 7 ppm-hrs to 15 ppm-hrs to protect against current and
anticipated welfare effects of ozone'' and that ``CASAC does not
support a level higher than 15 ppm-hrs'' (Frey, 2014c, p. iii). The
CASAC additionally stated that ``[i]n reaching its scientific judgment
regarding the indicator, form, summation time, and range of levels for
a revised secondary standard, the CASAC has focused on the scientific
evidence for the identification of the kind and extent of adverse
effects on public welfare,'' while also acknowledging ``that the choice
of a level within the range recommended based on scientific evidence is
a policy judgment under the statutory mandate of the Clean Air Act''
(Frey, 2014c, p. iii).
In providing advice on a range for the secondary standard, the
CASAC noted a W126 index value for which the median tree species RBL
estimate was 6 percent, and the median crop species RBL estimate was
over 5 percent, stating that ``[t]hese levels are unacceptably high''
(Frey, 2014c, p. iii).\221\ In addition, regarding consideration of
relative biomass loss benchmarks for tree seedlings, the CASAC stated
that ``[a] 2% biomass loss is an appropriate scientifically based value
to consider as a benchmark of adverse impact for long-lived perennial
species such as trees, because effects are cumulative over multiple
years'' (Frey, 2014c, p. 14). In so stating, the CASAC referenced
findings for biomass loss in aspen exposed to elevated O3
over seven years, citing Wittig et al., 2009.\222\ The CASAC
additionally pointed to the report of the 1996 workshop sponsored by
the Southern Oxidants Study group (Heck and Cowling, 1997, noted in
section IV.B.2 above) which described a general consideration of 1-2%
per year growth reduction in making judgments the group identified as
appropriate for the endpoint of growth effects in trees, without
providing an explicit rationale for the identified percentages (Frey,
2014c, p. 14). The CASAC also commented that ``it is appropriate to
identify a range of levels of alternative W126-based standards that
includes levels that aim for not greater than 2% RBL for the median
tree species'' (Frey, 2014c, p. 14). The CASAC noted that the ``level
of 7 ppm-hrs is the only level analyzed for which the relative biomass
loss for the median tree species is less than or equal to 2 percent''
indicating that 7 ppm was appropriate lower bound for the recommended
range (Frey, 2014c, p. 14).
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\221\ The CASAC made this comment while focusing on Table 6-1 in
the second draft PA and the entry for 17 ppm-hrs. That table was
revised for inclusion in the final PA in consideration of CASAC
comments on the E-R function for eastern cottonwood, such that the
RBL estimates for 17 ppm-hrs in the final table (see Table 2 above)
are below the values CASAC viewed as ``unacceptably high''.
\222\ The way in which the statement pointing to the aspen
seven-year biomass loss value from Wittig et al (2009) relates to
CASAC's view with regard to 2%, however, is unclear as the original
source for this finding (cited in Wittig et al., 2009) indicates
yearly relative biomass loss values during this seven year exposure
that are each well above 2%, and, in fact, are all above 20% (King,
et al., 2005).
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With regard to consideration of effects on crops, the CASAC, as
noted above, described median species RYL over 5% yield loss as
``unacceptably high.'' The CASAC further noted that ``[c]rop loss
appears to be less sensitive than these other indicators, largely
because of the CASAC judgment that a 5% yield loss represents an
adverse impact, and in part due to more opportunities to alter
management of annual crops'' (Frey, 2014c, p. 14).
The CASAC acknowledged that ``the choice of a level within the
range recommended based on scientific evidence is a policy judgment
under the statutory mandate of the Clean Air Act'', while further
providing its own policy recommendations, including the following
(Frey, 2014c, p. iii).
[T]he CASAC advises that a level of 15 ppm-hrs for the highest
3-month sum in a single year is requisite to protect crop yield
loss, but that lower levels provide additional protection against
crop yield loss. Furthermore, there are specific economically
significant crops, such as soybeans, that may not be protected at 15
ppm-hrs but would be protected at lower levels. A level below 10
ppm-hrs is required to reduce foliar injury. A level of 7 ppm-hrs is
protective of relative biomass loss for trees and offers additional
[[Page 75344]]
protection against crop yield loss and foliar injury. Therefore, 7
ppm-hrs is protective of ecosystem services. Thus, lower levels
within the recommended range offer a greater degree of protection of
more endpoints than do higher levels within the range.
Additionally, in regard to consideration of form, the CASAC noted
that ``[i]f, as a policy matter, the Administrator prefers to base the
secondary standard on a three-year averaging period for the purpose of
program stability, then the level of the standard should be revised
downward such that the level for the highest three-month summation in
any given year of the three-year period would not exceed the
scientifically recommended range of 7 ppm-hrs to 15 ppm-hrs'' (Frey,
2014c, pp. iii and iv). In related manner, the CASAC noted that a
three-year average W126 level of 13 ppm-hrs may be appropriate
depending on consideration of year-to-year variability and such policy
considerations (Frey, 2014c, p. iv).
Lastly, in comments recognizing uncertainties associated with the
evidence and exposure and risk analyses, the CASAC stated that ``there
is sufficient scientific evidence, and sufficient confidence in the
available research results, to support the advice we have given above
for this review cycle of the primary and secondary standards'' (Frey,
2014c, p. iv).
4. Air Quality Analyses
As described in section II.D. above, the PA concludes with regard
to the primary standard that it is appropriate for the Administrator to
consider revision of the level to within the range of 60 to 70 ppb. In
consideration of this conclusion for the primary standard, although the
PA also concludes it is appropriate to consider a revised secondary
standard with a cumulative, seasonal, concentration-weighted form, the
PA recognized that, it may be practical to consider the extent to which
a revised secondary standard in the form of the current secondary
standard might be expected to also reduce and provide protection from
cumulative seasonal exposures of concern, noting that, for example, if
a clear and robust relationship was found to exist between 8-hour daily
peak O3 concentrations and cumulative, seasonal exposures,
the averaging time and form of the current standard might be concluded
to have the potential to be effective as a surrogate (U.S. EPA, 2014c,
section 6.4).
Therefore, the PA evaluated what the available information
indicated with regard to control of cumulative O3 exposures
that might be afforded by alternative secondary standards with the
averaging time and form of the current standard (a three-year average
of 4th highest 8-hour average concentrations). The available
information addressing this point includes a ``focus study'' in the
ISA, and several air quality analyses described in the PA, chapters 2,
5 and 6 and Appendix 2b.\223\ Additionally, a similar air quality
analysis performed with more recent monitoring data is now available
and is also described here.
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\223\ This information and analyses were included in the second
draft PA (U.S. EPA, 2014j), reviewed by CASAC in early 2014, and
drafts of the ISA, reviewed by CASAC earlier in the review.
---------------------------------------------------------------------------
The focus study described in the ISA examined the diel variability
in O3 concentrations in six rural areas between 2007 and
2009 (U.S. EPA, 2013a, pp. 3-131 to 3-133). The ISA reported that
``[t]here was considerable variability in the diel patterns observed in
the six rural focus areas'' with the three mountainous eastern sites
exhibiting a ``generally flat profile with little hourly variability in
the median concentration and the upper percentiles,'' while the three
western rural areas demonstrated a ``clear diel pattern to the hourly
O3 data with a peak in concentration in the afternoon
similar to those seen in the urban areas,'' which was especially
obvious at the San Bernardino National Forest site, 90 km east of Los
Angeles at an elevation of 1,384 meters (U.S. EPA, 2013a, p. 3-132).
Thus, while the western sites that are influenced by upwind urban
plumes may have increased cumulative seasonal values coincident with
increased daily 8-hour peak O3 concentrations, this analysis
indicates that, in sites without such an urban influence (the eastern
sites in this analysis), such a relationship does not occur (U.S. EPA,
2013a, section 3.6.3.2). Thus, the lack of such a relationship
indicates that in some locations, O3 air quality patterns
can lead to elevated cumulative, seasonal O3 exposures
without the occurrence of elevated daily maximum 8-hour average
O3 concentrations (U.S. EPA, 2013a, section 3.6.3.2).
Further, staff notes that the prevalence and geographic extent of such
locations is unclear, since as in the last review, there continue to be
relatively fewer monitors in the western U.S., including in high
elevation remote sites. In considering the findings of this analysis,
the PA additionally recognized, however, that the cumulative seasonal
values for the eastern rural sites, where cumulative seasonal
O3 concentrations appear to be relatively less related to
daily maximum 8-hour concentrations, are lower in general than those of
the western, urban-influenced sites.
In addition to the focus study described in the ISA (U.S. EPA,
2013a, section 3.6.3.2), the PA considers additional analyses of air
quality monitoring data. For example, Chapter 2 of the PA characterized
recent monitoring data of O3 air quality in rural areas.
While approximately 80 percent of the O3 monitoring network
is urban focused, about 120 rural monitors are divided among CASTNET,
NCore, and portable O3 monitors (POMs) sites (U.S. EPA,
2014c, Chapter 2, pp. 2-2 to 2-3, Figure 2.1). Specifically, as stated
in Chapter 2 of the PA, ``[a]lthough rural monitoring sites tend to be
less directly affected by anthropogenic pollution sources than urban
sites, rural sites can be affected by transport of O3 or
O3 precursors from upwind urban areas and by local
anthropogenic sources such as motor vehicles, power generation, biomass
combustion, or oil and gas operations'' (U.S. EPA, 2013a, section
3.6.2.2). In addition, O3 tends to persist longer in rural
than in urban areas due to lower rates of chemical scavenging in non-
urban environments. At higher elevations, increased O3
concentrations can also result from stratospheric intrusions (U.S. EPA,
2013a, sections 3.4, 3.6.2.2). As a result, O3
concentrations measured in some rural sites can be higher than those
measured in nearby urban areas (U.S. EPA, 2013a, section 3.6.2.2) and
the ISA concludes that ``cumulative exposures for humans and vegetation
in rural areas can be substantial, and often higher than cumulative
exposures in urban areas'' (U.S. EPA, 2013a, p. 3-120). These known
differences between urban and rural sites suggest that there is the
potential for an inconsistent relationship between 8-hour daily peak
O3 concentrations and cumulative, seasonal exposures in
those areas. However, the PA also notes that reductions in NOx
emissions that occur in urban areas to attain primary standards would
also have the effect of reducing downwind, rural concentrations over
the season (U.S. EPA, 2014c, section 6.4).
In addition, as was done in both the 1997 and 2008 reviews, the PA
analyzed relationships between O3 levels in terms of the
current averaging time and form and a W126 cumulative form, based on
recent air quality data. One analysis in the PA describes the W126
index values and current standard design values at each monitor for two
periods: 2001-2003 and 2009-2011 (e.g., U.S. EPA, 2014c, Appendix 2B,
Figures 2B-2 and 2B-3). This shows that between the two periods, during
which broad scale O3
[[Page 75345]]
precursor emission reductions occurred, O3 concentrations in
terms of both metrics were reduced. There is a fairly strong, positive
degree of correlation between the two metrics (U.S. EPA, 2014c,
Appendix 2B).\224\ Focusing only on the latter dataset (2009-2011), it
can be seen that at monitors just meeting the current standard (three-
year average fourth-highest daily maximum 8-hour average concentration
equal to 0.075 ppm), W126 index values (in this case three-year
averages) varied from less than 3 ppm-hrs to approximately 20 ppm-hrs
(U.S. EPA, 2014c, Appendix 2B, Figure 2B-3b). At sites with a three-
year average fourth-highest daily maximum 8-hour average concentration
at or below a potential alternative primary standard level of 70 ppb,
three-year W126 index values were above 17 ppm-hrs at no monitors,
above 15 ppm-hrs at one monitor, and above 13 ppm-hrs at 8 monitors in
the West and Southwest NOAA climate regions. At sites with a three-year
average fourth-highest daily maximum 8-hour average concentration at or
below a potential alternative primary standard level of 65 ppb, three-
year W126 index values were above 11 ppm-hrs at no monitors, above 9
ppm-hrs at three monitors, and above 7 ppm-hrs at 9 monitors
(distributed across five regions). The majority of these nine
monitoring sites are located in the West and Southwest regions and
include the states of Arizona, California, Colorado, Nevada, New
Mexico, and Utah. At sites with a three-year average fourth-highest
daily maximum 8-hour average concentration at or below a potential
alternative primary standard level of 60 ppb, three-year W126 index
values were at or below 7 ppm-hrs at all monitors (U.S. EPA, 2014c,
Figure 2B-3b).
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\224\ Appendix 2B in the PA additionally observes that the
program implemented for reducing precursor emissions, especially
NOx, appears to have been an effective strategy for lowering both
design values and W126 index values.
---------------------------------------------------------------------------
Another analysis in Chapter 2 of the PA presents the data for sets
of recent three-year periods back to 2006-2008 and indicates that among
the counties with O3 concentrations that met the current
standard, the number of counties with three-year W126 index values
above 15 ppm-hrs ranges from fewer than 10 to 24 (U.S. EPA, 2014c,
Appendix 2B, Figure 2B-9). In general during this longer period, W126
index values above 15 ppm-hrs and meeting the current standard were
predominantly in Southwest region. As the first analysis in Appendix 2B
of the PA (for the 2001-2003 and 2009-2011 periods) indicates, monitors
in the West and Southwest tend to have higher W126 index values
relative to their design values than do monitors in other regions. This
pattern is noteworthy because the Southwest region has a less dense
monitoring network than regions in the eastern U.S. (see U.S. EPA,
2014c, Figure 2-1), so that the extent to which this pattern occurs
throughout these regions is uncertain.
An additional air quality analysis was performed for this review
that is documented in a technical memorandum (Wells, 2014). This
analysis examines the relationships between O3 levels in
terms of the form and averaging time for the current standard (the
``4th max'' metric) and a three-year average, W126-based metric. The
first part of the analyses focus on the air quality values for the most
recent three-year period, 2011-2013. Based on this information, it can
be seen that at monitors just meeting the current standard (three-year
average fourth-highest daily maximum 8-hour average concentration equal
to 0.075 ppm), W126 index values (in this case three-year averages)
varied from less than 3 ppm-hrs to up to 23 ppm-hrs (Figure 5a). At
sites with a three-year average fourth-highest daily maximum 8-hour
average concentration at or below a level of 70 ppb (566 monitors
distributed across all regions of the U.S.), three-year W126 index
values were above 17 ppm-hrs at no monitors, above 15 ppm-hrs at 4
monitors, and above 13 ppm-hrs at 16 monitors (1% of the monitors in
full dataset and less than 3% in this group). These 16 monitors are
located in the Southwest (15 monitors) and West North Central NOAA
climate regions and include the states of Arizona, Colorado, New
Mexico, Utah and Wyoming. At sites with a three-year average fourth-
highest daily maximum 8-hour average concentration at or below a level
of 65 ppb (220 monitors distributed across all regions of the
U.S.\225\), three-year W126 index values were above 11 ppm-hrs at no
monitors, above 7 ppm-hrs at 15 monitors. These 15 monitoring sites are
predominantly located in the West North Central and Southwest regions.
At all sites with a three-year average fourth-highest daily maximum 8-
hour average concentration at or below a level of 60 ppb, three-year
W126 index values were at or below 7 ppm-hrs (Wells, 2014, Figure 5b).
---------------------------------------------------------------------------
\225\ This memo utilizes the same regional specifications as are
used in the PA and WREA (e.g., U.S. EPA, 2014c, Appendix 2B, Figure
2B-1).
---------------------------------------------------------------------------
Further analysis in the technical memorandum focused on a
comparison of monitors with a three-year average fourth-highest daily
maximum 8-hour average concentration at or below a level of 70 ppb and
a three-year W126 index values above 13 ppm-hrs for sets of three-year
periods between 2001-2003 and 2011-2013 (Wells, 2014, Figure 8). This
analysis found that the number of sites meeting 70 ppb while exceeding
13 ppm-hrs has remained relatively constant over the past decade, with
these sites consistently being limited to a small number in the West
and Southwest. In addition, the number of sites meeting both 70 ppb and
13 ppm-hrs has increased over time, while the number of sites exceeding
both 70 ppb and 13 ppm-hrs has decreased by a similar amount.
The second part of the analysis in the technical memorandum focused
on trends in the relationships between O3 levels in terms of
the 4th high metric and a three-year average W126 metric, starting with
the 2001-2003 period and ending with the 2011-2013 period. Based on
analysis of 729 monitors, trends in both the 4th high metric and the
three-year average, W126 metric showed decreasing values between 2001-
2003 and 2011-2013. In addition, the amount of year-to-year variability
in the two metrics tended to decrease over time with decreasing
O3 concentrations, especially for the W126 metric. Most
sites in the eastern U.S. and California saw large, widespread
decreases in both the 4th high metric and the three-year average W126
metrics over the past decade as a result of regional NOX
control programs. In the inter-mountain west, where control programs
have been more localized, the decreases observed in the 4th high metric
and three-year average W126 metrics were typically much smaller in
magnitude, with a small number of sites showing significant increases.
As part of this analysis, regional comparisons were included on the
relative changes in the relationships between O3 levels in
terms of the 4th high metric and a three-year average W126 metric
between the periods of 2001-2003 and 2011-2013. Figure 12 in the
technical memorandum shows that a positive, linear relationship
persists within each region between the changes in 4th high and three-
year average W126 metrics. Nationally, the three-year average W126
metric decreased by approximately 0.7 ppm-hrs per unit ppb decrease in
the 4th high metric. In addition, the Southwest and West regions, which
have the greatest potential for sites to measure elevated cumulative,
seasonal O3 exposures without the occurrence of elevated
daily maximum 8-hour average O3 concentrations, exhibited
the greatest
[[Page 75346]]
response in W126 value change per unit change in 4th high metric
(Wells, 2014, Table 6).
The technical memorandum concludes that the 4th high metric and a
three-year average W126 metric are highly correlated, as are the
relative changes in these two metrics over the past decade. In this
way, the technical memorandum concludes that that future control
programs designed to help meet a revised primary O3 standard
based on the three-year average of the 4th highest daily maximum 8-hour
concentration are expected to also result in decreases in the values of
a three-year average W126 metric.
The above information suggests that depending on the level for a
standard of the current averaging time and form, the current form and
averaging time of the secondary standard can be expected to achieve
control of cumulative seasonal O3 exposures, providing air
quality that may meet specific three-year average W126 index values. As
discussed above, we recognize limitations in the dataset and associated
analyses, including those related to monitor coverage, which may
contribute uncertainties to conclusions related to the relationships
described. With respect to monitor coverage, the current O3
monitoring network is urban focused, with fewer monitors in some parts
of the country, particular rural areas of the southwestern and western
U.S. Because of this, there are potential uncertainties in the extent
to which the monitoring information discussed above represents air
quality patterns and relationships that would occur in areas without
monitors. There is some information suggesting that there is a
potential for inconsistencies in the relationship between W126 measures
of seasonal O3 concentrations and the fourth highest peak
O3 concentrations assessed by the current standard averaging
time and form, but the available data suggest that air quality in areas
meeting a standard of the current form and averaging time with a level
in the range of 65 to 70 ppb would also meet a three-year W126 index
value falling in the range of 13 to 17 ppm-hrs, and that to the extent
areas need to take action to attain a primary standard in the range of
65 to 70 ppb, those actions would also improve air quality as measured
by the W126 metric.\226\ To the extent to which the monitoring data can
be expected to describe future relationships in air quality, we
acknowledge potential uncertainties in specifying future air quality
but note that these uncertainties are limited by the fact that the data
analysis includes over a decade of O3 measurements, with
similar patterns and trends observed in air quality over this period of
time.
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\226\ EPA notes that areas can be expected to have air quality
at least as good as that specified by the primary standard, so to
the extent there are inconsistencies between fourth highest peak
concentrations and W126 values such that some areas meeting a
standard of 0.065 to 0.070 ppm might be well below the range of 13
to 17 ppm-hours, those inconsistencies are less relevant to
consideration of the appropriate form and level for the secondary
standard.
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5. Administrator's Proposed Conclusions
In considering what revisions to the secondary standard are
appropriate, the Administrator has drawn on the ISA conclusions
regarding the weight of the evidence for a range of welfare effects
associated with O3 in ambient air, and associated areas of
uncertainty; quantitative risk and exposure analyses in the WREA for
different adjusted air quality scenarios and associated limitations and
uncertainties; staff evaluations of the evidence, exposure/risk
information and air quality information in the PA; additional air
quality analyses of relationships between air quality metrics based on
form and averaging time of the current standards and a cumulative
seasonal exposure index; and CASAC advice; and, public comments
received thus far in the review.
As described in section IV.E.1 above, the Administrator concludes
it is appropriate to continue to use O3 as the indicator for
a secondary standard intended to address adverse effects to public
welfare associated with exposure to O3 alone and in
combination with related photochemical oxidants. In this review, no
alternatives to O3 have been advanced as being a more
appropriate surrogate for ambient photochemical oxidants. Thus, as is
the case for the primary standard (discussed above in section II.E.1),
the Administrator proposes to continue to use O3 as the
indicator for a standard that is intended to address effects associated
with exposure to O3 alone and in combination with related
photochemical oxidants. In so doing, the Administrator recognizes that
measures leading to reductions in ecosystem exposures to O3
would also be expected to reduce exposures to other photochemical
oxidants.
The Administrator has next considered the array of information with
regard to identifying policy options for a revised secondary standard
for O3 that in her judgment would provide appropriate
protection for public welfare effects associated with O3 in
ambient air. This information includes ISA conclusions, WREA analysis
findings, staff considerations and conclusions in the PA and CASAC
advice, as well as the Administrator's conclusions in the last review,
with regard to a biologically relevant exposure metric for
O3 vegetation-related effects. The information also includes
PA conclusions and CASAC advice with regard to key aspects of the
definition of such a metric, as summarized in section IV.E.2 and IV.E.3
above. Additionally, the Administrator has considered findings of staff
evaluations in the PA with regard to potential impacts on vegetation
and forested ecosystems associated with a range of values for such a
metric and identified uncertainties and limitations of such
information, as summarized in section IV.E.2 above. Additionally
important to her deliberations here are findings of air quality
analyses of relationships between the W126-based exposure metric and
levels of a standard of the same form and averaging time as the current
standards, as described in section IV.E.4 above. Based on consideration
of this array of information, as described below, the Administrator has
drawn conclusions with regard to policy options for a revised secondary
standard. In drawing conclusions on such options, she recognizes that
the Act does not require that NAAQS be set at zero-risk or background
levels, but rather at levels that reduce risk sufficiently to protect
public welfare from adverse effects.
As an initial matter, the Administrator recognizes the longstanding
evidence, described in the ISA, of O3 effects on vegetation
and associated terrestrial ecosystems. Further, in reaching a proposed
conclusion on the appropriate form and averaging time for a revised
secondary standard that would provide increased protection against
vegetation-related effects on public welfare, the Administrator takes
note of the conclusions drawn in the ISA, the PA and by CASAC in this
review that the scientific evidence continues to demonstrate the
cumulative nature of O3-induced plant effects and the need
to give greater weight to higher concentrations, as summarized in
sections IV.B.1, IV.D.1, IV.D.2, IV.E.2.a and IV.E.3 above. Based on
these considerations, the Administrator concurs with the CASAC that a
cumulative, seasonal, concentration-weighted exposure-based form and
averaging time provides the most direct link between O3 in
ambient air and O3-related effects on vegetation. The
Administrator further concludes that in judging the extent of public
welfare protection that might be afforded by a revised standard, it is
appropriate to use
[[Page 75347]]
a cumulative, seasonal concentration-weighted metric.
In identifying a cumulative, seasonal, concentration-weighted
metric for use in judging public welfare protection, the Administrator
gives weight to the PA conclusions regarding consideration of a revised
secondary standard in terms of the cumulative, seasonal, concentration-
weighted form, the W126 index. As described in section IV.B.1 above,
the ISA has recognized the strength of the W126 index in its weighting
of potentially damaging O3 concentrations that contributes
to the advantages it offers over other weighted cumulative indices. The
Administrator notes the PA conclusions regarding the W126 metric,
specifically use of the three consecutive month period within the
O3 season with the maximum index value as the seasonal
period over which to cumulate hourly O3 exposures and the
cumulation of daily exposures for the 12-hour period from 8:00 a.m. to
8:00 p.m. The Administrator additionally takes note of CASAC support
for consideration of the W126 index defined in this way and concludes
it is appropriate to use the cumulative seasonal W126-based metric
derived in this way.
In further considering the PA conclusions regarding a revised
secondary standard in terms of the W126 index, the Administrator takes
note of considerations in the PA of a three-year or single-year
evaluation period. Such considerations include the variability in
ambient air O3 concentrations from year to year, as well as
variability and uncertainties related to environmental factors that
influence the occurrence and magnitude of O3-related
effects. The Administrator additionally notes the PA observation of
greater significance for effects associated with multiple-year
exposures. Based on these and related considerations described in the
PA (and summarized in section IV.E.2 above), the Administrator, in
identifying a metric for use in judging public welfare protection
afforded, agrees with the PA conclusion that it is appropriate to
consider a form that averages W126 index values across three
consecutive years, and to do so in conjunction with identification of
levels for such a form that might be judged to provide the appropriate
degree of public welfare protection from O3 effects across
multiple years. In so doing, the Administrator takes note of the ISA
conclusions regarding the role of environmental factors in variability
associated with effects of ambient air O3 and the year-to-
year variability commonly observed in such environmental factors.
Further, the Administrator also recognizes uncertainties associated
with determining the degree of vegetation impacts for annual effects
that would be adverse to public welfare. Even in the case of annual
crops, the assessment of public welfare significance is unclear for the
reasons discussed below related to agricultural practices. The
considerations identified here lead the Administrator to conclude it is
appropriate to use an index averaged across three years.
In reaching this conclusion regarding a three-year average metric,
the Administrator has considered CASAC comments that it favors a W126-
based secondary standard with a single year form and that its
recommended range of levels relates to such a form. The Administrator
concurs with CASAC that it is important to consider impacts associated
with a single year that may be of a magnitude concluded to represent an
adverse effect on public welfare. The Administrator further concludes
that such an occurrence can be addressed through use of a three-year
average metric, chosen with consideration of the relevant factors. As
noted above, the Administrator gives consideration to the
variabilities, as well as the uncertainties, associated with single
year and multiple year impacts. Based on all of these considerations,
the Administrator recognizes greater confidence in judgments related to
public welfare impacts based on a three-year average metric.
Thus, based on all of the above, the Administrator proposes, for
purposes of judging the extent of public welfare protection that might
be afforded by a revised standard and whether it meets the appropriate
level of protection, to use the average W126 index value across three
years, with each year's value identified as that for the three-month
period yielding the highest seasonal value and with daily O3
exposures within a three-month period cumulated for the 12-hour period
from 8:00 a.m. to 8:00 p.m.
In reaching a conclusion on the appropriate range of W126 index
values that describe the O3 conditions expected to provide
the requisite protection of public welfare, the Administrator has given
careful consideration to the following: (1) The nature and degree of
effects of O3 to the public welfare, including what
constitutes an adverse effect; (2) the strengths and limitations of the
evidence that is available regarding known or anticipated adverse
effects from cumulative, seasonal exposures, and its usefulness in
informing selection of a proposed range; and (3) CASAC's views
regarding a range of W126 levels appropriate to consider, as well as on
the strength of the evidence and its adequacy to inform a range of
levels. In this consideration, the Administrator recognizes that the
choice of a range of W126 index values (and the form of the W126 index)
that might be expected to provide protection of the public welfare from
any known or anticipated adverse effects requires judgments about the
interpretation of the evidence and other information, such as the
quantitative analyses of air quality monitoring, exposure and risk,
that neither overstates nor understates the strengths and limitations
of the evidence and information nor the appropriate inferences to be
drawn as to risks to public welfare. The CAA does not require that a
secondary standard be protective of all effects associated with a
pollutant in the ambient air but rather those considered adverse to the
public welfare (as described in section IV.B.2 above). The
Administrator additionally recognizes that there is not a bright line
clearly directing the choice of a range of W126 index values and that
the choice of what is appropriate is a public welfare policy judgment
entrusted to the Administrator.
In determining the range of three-year average W126 index values
that might be expected to provide the appropriate level of public
welfare protection, the Administrator first considers the nature and
degree of effects of O3 on the public welfare. The
Administrator recognizes that the significance to the public welfare of
O3-induced effects on sensitive vegetation growing within
the U.S. can vary, depending on the nature of the effect, the intended
use of the sensitive plants or ecosystems, and the types of
environments in which the sensitive vegetation and ecosystems are
located. Any given O3-related effect on vegetation and
ecosystems (e.g., biomass loss, visible foliar injury), therefore, may
be judged to have a different degree of impact on the public depending,
for example, on whether that effect occurs in a Class I area, or a
residential or commercial setting. The Administrator notes that such a
distinction is supported by CASAC advice in this review. In her
judgment, like those of the Administrator in the last review, it is
appropriate that this variation in the significance of O3-
related vegetation effects should be taken into consideration in making
judgments with regard to the level of ambient O3
concentrations that is requisite to protect the public welfare from any
known or anticipated adverse effects. As a result, the Administrator
concludes that of those known and anticipated O3-related
vegetation and ecosystem effects
[[Page 75348]]
identified and discussed in this notice, particular significance should
be ascribed to those that occur on sensitive species that are known to
or are likely to occur in federally protected areas such as Class I
areas \227\ or on lands set aside by States, Tribes and public interest
groups to provide similar benefits to the public welfare, for residents
on those lands, as well as visitors to those areas.
---------------------------------------------------------------------------
\227\ For example, the Wilderness Act of 1964 defines designated
``wilderness areas'' in part as areas ``protected and managed so as
to preserve [their] natural conditions'' and requires that these
areas ``shall be administered for the use and enjoyment of the
American people in such manner as will leave them unimpaired for
future use and enjoyment as wilderness, and so as to provide for the
protection of these areas [and] the preservation of their wilderness
character.''16 U.S.C. 1131(a).
---------------------------------------------------------------------------
Likewise, the Administrator also notes that the same known or
anticipated O3-induced effects occurring in other areas may
call for less protection. For example, the maintenance of adequate
agricultural crop yields is extremely important to the public welfare
and is currently achieved through the application of intensive
management practices. With respect to commercial production of
commodities, the Administrator notes that judgments about the extent to
which O3-related effects on commercially managed vegetation
are adverse from a public welfare perspective are particularly
difficult to reach, given that the extensive management of such
vegetation (which, as CASAC noted, may reduce yield variability) may
also to some degree mitigate potential O3-related effects.
The management practices used on these lands are highly variable and
are designed to achieve optimal yields, taking into consideration
various environmental conditions. In addition, changes in yield of
commercial crops and timber may affect producers and consumers
differently, further complicating the question of assessing overall
public welfare impacts. Thus, the Administrator concludes that
agricultural crops do not have same need for additional protection from
the NAAQS as forested ecosystems and, while research on agricultural
crop species remains useful in illuminating mechanisms of action and
physiological processes, information from this sector on O3-
induced effects is considered less useful in informing judgments on
what level(s) would be sufficient but not more than necessary to
protect the public welfare. The CASAC identified a crop RYL benchmark
of 5% for the median species and indicated they found higher
percentages unacceptably high. Although the Administrator has not drawn
a conclusion with regard to this specific benchmark, the Administrator
finds the public welfare impacts associated with crop yield loss to be
a less important consideration in this review for the reasons discussed
here, including the extensive management of crop yields and the
dynamics of agricultural markets, and thus is not focusing on crop
yield loss in selecting a revised standard. She notes, however, the PA
finding that median species crop RYL estimates for W126 index values in
the PA identified range (17 to 7 ppm-hrs) fall below the 5% benchmark
emphasized by CASAC for this endpoint. The Administrator also notes
that a standard revised to increase protection for forested ecosystems
would also be expected to provide some increased protection for
agricultural crops.
The Administrator also recognizes that O3-related
effects on sensitive vegetation can occur in other areas that have not
been afforded special federal protections, ranging from effects on
vegetation growing in managed city parks and residential or commercial
settings, such as ornamentals used in urban/suburban landscaping or
vegetation grown in land use categories that are heavily managed for
commercial production of commodities such as timber. For vegetation
used for residential or commercial ornamental purposes, the
Administrator believes that there is not adequate information at this
time to establish a secondary standard based specifically on impairment
of these categories of vegetation, but notes that a secondary standard
revised to provide protection for sensitive natural vegetation and
ecosystems would likely also provide some degree of protection for such
vegetation.
Based on the above, the Administrator finds that the type of
information most useful in informing the selection of an appropriate
range of protective levels is appropriately focused on information
regarding exposures and responses of sensitive trees and other native
species known or anticipated to occur in protected areas such as Class
I areas or on lands set aside by States, Tribes and public interest
groups to provide similar benefits to the public welfare, for residents
on those lands, as well as visitors to those areas.
With regard to the available evidence, the Administrator finds the
coherence and strength of the weight of evidence from the large body of
available literature compelling. This evidence addresses a broad array
of O3-induced effects on a variety of tree species across a
range of growth stages (i.e., seedlings, saplings and mature trees)
using diverse field-based (e.g., free air, gradient and ambient) and
OTC exposure methods. The Administrator gives particular attention to
the effects related to native tree growth and productivity, recognizing
their relationship to a range of ecosystem services, including forest
and forest community composition.
With regard to selection of the values for use with the W126 index
for the purpose of identifying a range of O3 conditions
expected to provide the appropriate level of protection from vegetation
effects of particular concern, the Administrator, as an initial matter,
takes note of the PA conclusion that, with regard to a target level of
protection for a revised standard, it is appropriate to give
consideration to a range of levels from 17 ppm-hrs to 7 ppm-hrs,
expressed in terms of the W126 index averaged across three consecutive
years. As summarized in section IV.E.2.b above, this PA conclusion
draws heavily on considerations related to estimates of tree seedling
growth impacts (in terms of relative biomass loss) associated with a
range of W126-based index values developed from the robust E-R
functions for 11 tree species. This conclusion also gives weight to
CASAC comments as to an unacceptably high magnitude of relative biomass
loss (6%) for the median species and a magnitude of median relative
biomass loss on which to focus considerations (2%). The Administrator
takes particular note of the CASAC view of a median species RBL of 6%
as unacceptably high.
In considering the basis for the range of W126 index levels
identified by the PA, for which 17 ppm-hrs is the upper end, the
Administrator considers the CASAC advice, including their view that a
6% median tree seedling species RBL is unacceptably high, their
consideration of Table 6-1 in the second draft PA which indicated such
a RBL estimate for a W126 index value of 17 ppm-hrs, and their
consequent lack of support for levels higher than 15 ppm-hrs (Frey,
2014c, p. iii; U.S. EPA 2014j, Table 6-1). As noted in section IV.E.3
above, revisions to this table in the final PA, made in consideration
of CASAC comments have resulted in changes to the median species RBL
estimates such that the median species RBL estimate for a W126 index
value of 17 ppm-hrs in this table in the final PA (5.3%) is nearly
identical to the median species estimate for 15 ppm-hrs (the value
corresponding to the upper end of the CASAC-identified range) in the
second draft PA (5.2%) (U.S. EPA, 2014c, Table 6-1; U.S. EPA, 2014j,
Table 6-1).
[[Page 75349]]
The Administrator additionally takes note of the PA observations
that the number and proportion of individual species with RBL estimates
at or below 2%, a benchmark given emphasis by CASAC, do not vary across
W126 index values from 17 ppm-hrs down to 9 ppm-hrs (as seen in Table 8
above), providing little distinction with regard to the significance of
growth impacts for exposures across this large portion of the PA range.
The Administrator also notes the CASAC recommendation regarding a
lowering of the level with consideration of a three-year average index;
however, the Administrator's judgments on a three-year average index,
as described above, focus on confidence in conclusions that might be
drawn with regard to single as compared to multiple year impacts. For
example, the Administrator, while recognizing the strength of the
evidence with regard to quantitative characterization of O3
effects on growth of tree seedlings and crops, in addition to noting
the additional difficulties for assessing welfare impacts of crops,
takes note of the uncertainty associated with drawing conclusions with
regard to the extent to which small percent reductions in annual growth
contribute to adverse effects on public welfare and the role of annual
variability in environmental factors that affect plant responses to
O3. Moreover, as explained above, the Administrator
concludes that concerns related to the possibility of a singly
unusually damaging year can be addressed through use of a three-year
average metric, chosen with consideration of the relevant factors.
Accordingly, she judges it appropriate to include 17 ppm-hrs, without
adjustment, in the range of three-year average W126 index values
appropriate to consider in determining what secondary standard will
provide air quality associated with the appropriate level of public
welfare protection. She thus judges it appropriate to focus on a range
for three-year average W126 levels with 17 ppm-hrs at the upper end. In
so doing, she additionally notes CASAC's recognition that, within a
scientifically appropriate range, the choice of levels is a public
policy judgment by the Administrator.
In turning to consideration of the low end for the W126 index
range, the Administrator considers the full range of W126 levels
identified in the PA with regard to the evidence and exposure/risk-
based information, and associated uncertainties, identified in the PA,
as well as CASAC advice. The Administrator notes the CASAC policy view
regarding protection provided for trees and associated ecosystem
services from a W126 index value of 7 ppm-hrs, which is based on the
W126 index value for which the median species estimate falls below 2%
RBL. The Administrator recognizes, however, as noted above, the greater
uncertainty associated with the extent to which estimates of benefits
in terms of ecosystem services and reduced effects on vegetation at
lower O3 exposures might be judged significant to the public
welfare.
The Administrator additionally notes the results of the EPA's
quantitative exposure and risk assessments for the air quality
scenarios for W126 levels at and below 11 ppm-hrs, including the
relatively small additional benefits and increased uncertainty with the
ecosystem services estimates in these lower W126 scenarios. With regard
to the PA evaluation of RBL estimates, the Administrator, while noting
the PA observations of similarity in the number of species with less
than 2% RBL across the W126 range from 17 to 9 ppm-hrs, as stated
above, additionally notes PA observations of a similar number of
studied species with RBL estimates below 5% for W126 index values of 13
and 11 ppm-hrs. Thus, to the extent that weight is given to the
importance of 5% RBL for individual species, both W126 index values are
observed to provide RBL estimates below this benchmark.
With regard to considerations of O3 effects beyond
biomass loss in tree seedlings, the Administrator takes note of the
lack of new quantitative E-R relationships for larger trees growing in
the field that would help inform consideration of a standard level
within the lower part of PA range. Thus, the Administrator recognizes
that important uncertainties remain in interpreting the quantitative
O3-related growth effects for tree seedlings assessed in OTC
studies for the purpose of characterizing long-term growth effects, and
other more subtle but important effects on sensitive tree species,
natural forests, and forested ecosystems in the broader context of
protection of public welfare. Additionally, while the Administrator
notes that there is evidence that O3-related visible foliar
injury can occur at such lower levels (below a W126 index value of 13
ppm-hrs), she recognizes, as summarized in sections IV.C.3.c and IV.D.1
above, the significant challenges in judging the extent to which such
effects should be considered adverse to public welfare, in light of the
variability and the lack of clear quantitative relationship with other
effects on vegetation, as well as the lack of established criteria or
objectives that might inform consideration of potential public welfare
impacts related to this vegetation effect.
Thus, in the Administrator's judgment, focus on a three-year
average W126 index value below 13 ppm-hrs would not give sufficient
attention to the important uncertainties and limitations inherent in
the currently available scientific evidence and in the quantitative
assessments conducted for the current review. Taking into account the
uncertainties that remain in interpreting the evidence, the
Administrator observes that the likelihood of obtaining benefits to
public welfare decreases with a standard set below a level of 13 ppm-
hrs, while the likelihood of requiring reductions in ambient
concentrations that go beyond those that are needed to reduce adverse
impacts to public welfare increases.
Based on the above considerations and based on the entire body of
evidence and information currently available, the Administrator
identifies the range of three-year average W126 index values extending
from 13 to 17 ppm-hrs as appropriate to consider in identifying the
ambient O3 concentrations that would provide the appropriate
level of public welfare protection. In so doing, the Administrator
notes CASAC recognition that a three-year average W126 level of 13 ppm-
hrs may be appropriate depending on consideration of year-to-year
variability and policy considerations. Thus, based on the discussion
above, and with consideration of CASAC advice on these issues, the
Administrator proposes that ambient O3 concentrations
resulting in cumulative seasonal O3 exposures of a level
within the range from 13 ppm-hrs to 17 ppm-hrs, in terms of a W126
index averaged across three consecutive years, would provide the
requisite protection against known or anticipated adverse effects to
the public welfare. The EPA solicits comments on levels within this
range.
The Administrator next turns to consideration of policy options for
a revised secondary standard that would provide this level of
protection. The Administrator takes note of staff conclusions that it
is appropriate to consider a revised secondary standard in terms of the
cumulative, seasonal, concentration-weighted form, the W126 index.
Further, she gives extensive consideration to CASAC advice to set such
a secondary standard. Such a standard, as mentioned above, would be
directly linked to O3 exposures to which vegetation are most
responsive and thus might be expected to provide some
[[Page 75350]]
confidence that such exposures of concern would be controlled.
In considering different policy options for a revised secondary
standard, the Administrator finds it useful to consider what can be
concluded from the available information with regard to relationships
between air quality characterized in terms of the current form and
averaging time and also in terms of the W126 metric. She has considered
particularly what such analyses and relationships indicate with regard
to the extent to which W126-based O3 concentrations may be
controlled by a revised secondary standard set identical to a revised
primary standard (in the range identified in section II.F above). In so
doing, she considers the air quality analyses in the PA and also the
analyses of more recent air quality data summarized in the EPA
technical memo (described in section IV.E.4 above), focusing
particularly on analyses examining the W126-based O3
exposure achieved in locations found to meet potential alternative
standards within the range of primary standards proposed in section
II.F above.
Findings from these analyses of recent O3 measurements
and trends in the relationship between the current standard and the
W126 metrics were substantially similar for the various time periods
examined over the past decade. There is some information suggesting
that there is a potential for inconsistencies in the relationship
between W126 measures of seasonal O3 concentrations and the
fourth highest peak O3 concentrations assessed by the
current standard averaging time and form, but the available data
suggest that air quality in areas meeting a primary standard in the
range of 65 to 70 ppb would also meet a three-year W126 index value
falling in the range of 13 to 17 ppm-hrs, and that to the extent areas
need to take action to attain a standard in the range of 0.065 to 0.070
ppm, those actions would also improve air quality as measured by the
W126 metric. The Administrator also recognizes that the relatively
lesser density of monitors in rural areas, including in areas of the
West and Southwest NOAA climatic regions currently meeting the current
standard where O3 W126 index values are generally higher,
makes uncertain the degree to which a revised level for the current
standard would provide the appropriate degree of protection for
vegetation-related effects on public welfare in these areas. The
Administrator takes note of the PA finding, referenced in section
IV.D.3 above, that reductions in NOX emissions that occur in
urban areas to attain primary standards would also have the effect of
reducing downwind, rural concentrations over the season. Thus, while
the potential for underprotection may exist, depending on the specific
levels chosen, the extent of such areas and of such a risk is not
clear.
Based on the most recent period of monitoring data, the
Administrator notes that in all areas in which the O3
concentrations would have met a primary standard with a revised level
of 70 ppb (which includes over 500 monitors distributed across all
regions of U.S), the three-year average W126 index values are at or
below 17 ppm-hrs. In the same areas, only 16 monitors (or less than 3%
of all monitors in this group, all but one of which is located in the
Southwest region) had three-year average W126 index values above 13
ppm-hrs. She further notes that in all areas in which the O3
concentrations would have met a primary standard with a revised level
of 65 ppb (which includes 220 monitors distributed across all regions
of U.S), the three-year average W126 index values are at or below 13
ppm-hrs.
In considering these findings regarding cumulative seasonal
O3 exposures in areas that would have met a primary standard
with a revised level within the proposed range, the Administrator also
takes note of the high correlation observed between the design value
for the current secondary (and primary) standard and values for the
three-year average, W126 metric, as well as the high correlation in the
relative changes in these two metrics based on air quality analyses of
O3 measurements from over the past decade. This finding
supports a conclusion that the air quality analyses indicate that
future control programs designed to reduce O3 concentrations
to help meet a revised primary O3 standard that retains the
current form and averaging time (three-year average of the 4th highest
daily maximum 8-hour concentration) would also be expected to result in
reductions in three-year average, W126 index values. Further, she notes
the conclusion from the air quality analysis that the Southwest and
West regions, which have the greatest potential for sites to measure
elevated cumulative, seasonal O3 exposures without the
occurrence of elevated daily maximum 8-hour average O3
concentrations, exhibited the greatest response in W126 index value
change per unit change in metric based on the current standard form and
averaging time. While recognizing the limitations of such analyses in
projections of future air quality patterns, the Administrator also
notes that the time period over which the analyses focused involved
emissions control programs to achieve O3 reductions such
that their findings would be expected to be informative of further
similar control activities, such as those to meet a revised standard
with a lower level, in the future.
Based on the findings from these analyses, the Administrator finds
it appropriate to consider the policy option of retaining the form and
averaging time of the current secondary standard and revising the level
to within the range of 65 to 70 ppb. In such consideration, the
Administrator first notes her proposed conclusion that the requisite
protection from known or anticipated adverse effects to public welfare
may be achieved by cumulative, seasonal, concentration-weighted
O3 concentrations characterized in terms of a W126 index
value that falls within the range from 13 to 17 ppm-hrs. Her final
decision on the W126 index value in this range that affords the
requisite protection will be based on a series of judgments, as
described above. Given the focus on tree seedling growth effects in
identifying this range, such judgments will include the weight to give
the evidence of specific vegetation-related effects estimated to result
from W126 index values within this range, including the objectives for
consideration of tree species biomass loss estimates in relationship to
identified benchmarks (e.g., median species RBL of 2% and greater), the
weight to give associated uncertainties, including those related to the
variability in occurrence of such effects in forested areas, the
associated ecosystem services including those of particular public
welfare significance, and judgments on the extent to which such effects
in forested areas may be considered adverse to public welfare. This
final decision will also take into account judgments with regard to the
weight to give the evidence and quantitative analyses, and associated
uncertainties, related to other effects of O3 (summarized in
sections IV.C, IV.D.1 and IV.E.2 above), particularly including those
for which the ISA concludes causal or likely causal relationships with
O3 exposures. As noted above, a standard that provides the
appropriate level of protection for growth effects would also be
expected to provide additional protection for other effects including
visible foliar injury, crops and carbon storage.
The Administrator notes that based on the above analyses, the
proposed range of levels for a revised primary standard provide air
quality, in terms of three-year average W126 index values, of a
[[Page 75351]]
range at or below the range which the Administrator has identified for
consideration with regard to the requisite public welfare protection.
Thus, depending on final judgments on revisions to the primary standard
and the requisite protection for the secondary standard, a revised
secondary standard identical to the revised primary standard may
provide sufficient protection for public welfare. Therefore, the
Administrator proposes to retain the current averaging time and form of
the secondary standard and revise the level to within the range of 65
to 70 ppb.
In reaching such a conclusion, the Administrator recognizes that
such a strengthening of the secondary standard would be expected to
provide significant additional protection for public welfare, including
effects related to vegetation and associated ecosystem services (and
others discussed above), over that afforded by the current secondary
standard.
Thus, based on her consideration of the full range of information
as described above, the Administrator judges that ambient O3
concentrations in terms of a three-year average W126 index value within
the range extending from 13 ppm-hrs to 17 ppm-hrs would provide
requisite public welfare protection. She further judges that it would
be appropriate to achieve that level of air quality by retaining the
existing averaging time and form, and revising the level to within the
range of 65 to 70 ppb. In recognition of CASAC's recommendation and the
PA conclusion with regard to a distinct secondary standard, the
Administrator additionally solicits comment on the policy option of
revising the form and averaging time for the secondary standard to a
W126 index value, averaged across three years, with each year's value
identified as that for the three-month period yielding the highest
seasonal value and with daily O3 exposures within a three-
month period cumulated for the 12-hour period from 8:00 a.m. to 8:00
p.m., and a level within the range from 13 ppm-hrs to 17 ppm-hrs.
F. Proposed Decision on the Secondary Standard
The Administrator proposes to revise the level of the current
secondary standard within the range of 0.065 to 0.070 ppm. The EPA
solicits comments on this proposed revision of the secondary standard.
Further, the EPA solicits comments on the proposed conclusion that air
quality in terms of a W126 index value, averaged across three
consecutive years, within the range of 13 ppm-hrs to 17 ppm-hrs would
provide requisite protection against known or anticipated adverse
effects to the public welfare. Additionally, the EPA solicits comments
on alternative values for a three-year average W126 index for such a
purpose within the range extending below 13 ppm-hrs down to 7 ppm-hrs.
The EPA also solicits comments on the alternative approach of
revising the secondary standard to a cumulative, seasonal,
concentration-weighted form, the W126 index based on the three
consecutive month period within the O3 season with the
maximum index value, with daily exposures cumulated for the 12-hour
period from 8:00am to 8:00pm and with a form that averages seasonal
W126 values across three consecutive years and a level within the range
of 13 to 17 ppm-hrs. The EPA additionally solicits comments on such a
distinct secondary standard with a level within the range extending
below 13 ppm-hrs down to 7 ppm-hrs. Further, the EPA solicits comments
on retaining the current secondary standard without revision, along
with the alternative views of the evidence that would support retaining
the current standard.
V. Appendix U: Interpretation of the Primary and Secondary NAAQS for
O3
A. Background
The EPA is proposing to create Appendix U to 40 CFR part 50 to
reflect the proposed revisions to the primary and secondary NAAQS for
O3 discussed in previous sections of this preamble. The
proposed Appendix U explains the computations necessary for determining
when the proposed primary and secondary O3 NAAQS are met at
an ambient air quality monitoring site, similar to Appendix P to 40 CFR
part 50 which deals with interpretation of the O3 NAAQS
promulgated in 2008. Specifically, the proposed Appendix U addresses
data selection requirements (section V.B), data reporting and data
handling requirements (section V.C), and data completeness
requirements. The EPA is proposing to maintain the data completeness
requirements from the previous O3 NAAQS.
Given that the EPA is soliciting public comment on a distinct
secondary standard based on the W126 metric, section V.D of this
preamble contains a discussion of additional data handling requirements
that would be adopted in Appendix U in the event that the Administrator
decides to set a distinct secondary standard based on public comments
received.
The proposed Appendix U also provides specific requirements for the
handling of data affected by exceptional events in accordance with 40
CFR 50.14. Section V.E of this preamble addresses O3-
specific deadlines related to the flagging and submission of
demonstrations for exceptional event data for the proposed
O3 NAAQS.
B. Data Selection Requirements
The EPA is proposing to clarify which data are to be used in
comparisons with the NAAQS. First, the EPA proposes to maintain the
existing regulatory requirements that only O3 data collected
by a federal reference method specified in Appendix D to 40 CFR part
50, or an equivalent method designated in accordance with 40 CFR part
53, and meeting all applicable monitoring requirements listed in 40 CFR
part 58, are eligible for comparison to the proposed O3
NAAQS.
Second, the EPA is proposing in Appendix U that O3
design values are to be calculated on a site-level basis. Past practice
has been to calculate a design value for each individual O3
monitor. However, this practice could be viewed as inconsistent with
the stated purpose of the previous O3 data handling
appendix, which is to determine ``whether the national 8-hour primary
and secondary ambient air quality standards for ozone (O3)
specified in Sec. 50.15 are met at an ambient O3 air
quality monitoring site.'' (40 CFR part 50, Appendix P, section 1
(emphasis added)). Given the level of consistency in the measurement
data obtained across the various federal reference and equivalent
O3 monitoring instruments currently in operation (U.S. EPA,
2013a, section 3.5.2.1), the EPA believes that it would be appropriate
to combine data across O3 monitors operating at the same
site. Therefore, the EPA is proposing an analytic approach for
combining data collected from multiple O3 monitors at a site
in order to obtain a single set of hourly O3 concentration
data for each site.
The proposed approach allows the monitoring agencies to designate
one monitor as the ``primary monitor'' for each site. In the absence of
a primary monitor designation, the primary monitor would default to the
monitor with the most complete hourly dataset in each year. Once a
primary monitor has been determined for the site, missing hourly
O3 concentrations for the primary monitor would be
substituted from any other monitors at the site. In the event of three
or more monitors operating at the same site, missing hourly
O3 concentrations for the primary monitor would be
substituted with hourly values averaged across the other monitors. The
EPA notes that at the time of this proposal, there were approximately
20 sites operating two
[[Page 75352]]
monitors simultaneously, and no O3 sites operating three or
more monitors simultaneously. This proposed approach for combining data
across monitors at a site is consistent with the existing approach
described in Appendix N to Part 50 for the PM2.5 NAAQS. The
EPA invites public comment on the scientific validity of combining data
across O3 monitors, and the merits of the proposed approach
for combining data across multiple O3 monitors at a site.
Third, the EPA proposes to maintain the existing practice of
combining data from nearby monitoring sites in order to determine a
valid design value, known as a ``site combination''. Site combinations
typically involve situations where sites have been replaced or
relocated a short distance away, and the monitoring agency wishes to
combine the data from the two sites in order to maintain a continuous
data record. The EPA regional offices have approved over 100 site
combinations for O3 since the promulgation of the 1997
O3 NAAQS. The EPA has maintained records of approved site
combinations, but these records are not easily accessible by the
public.
The EPA proposes to replace the current procedure for approving
O3 site combinations with a more formal procedure in
Appendix U, which would allow states to submit site combination
requests to the appropriate Regional Administrator. Site combinations
may be approved by the Regional Administrator, after he or she has
determined that the measured air quality concentrations do not differ
substantially between the two sites. In order to make this
determination, the Regional Administrator may request additional
information from the states, including detailed information on the
locations and distance between the two sites, levels of ambient
concentrations measured at the two sites, and local emissions or
meteorology data.
In order to improve transparency, the EPA will make records of all
approved site combinations available in their Air Quality System (AQS)
database, and will update design value calculations in AQS so that
approved site combinations are implemented. The EPA invites public
comment on the merits of the proposed process for approving site
combinations in order to obtain valid design values for the
O3 NAAQS.
C. Data Reporting and Data Handling Requirements
The EPA is proposing to maintain the requirement that hourly
O3 concentration data be reported in parts per million (ppm)
to three decimal places. Any decimal digits reported beyond three
decimal digits will be truncated, consistent with past practice (40 CFR
part 50, Appendix P, section 2.1) and the typical measurement
uncertainty associated with most O3 monitoring instruments.
The proposed Appendix U clarifies that hourly O3
concentrations are to be reported in Local Standard Time (LST),
consistent with how the values are currently stored in AQS.
The EPA is proposing to maintain the existing procedures for
calculating moving 8-hour averages from the hourly O3 data
(40 CFR part 50, Appendix P, section 2.1), with one minor exception. In
instances where fewer than six hourly O3 concentrations are
available during an 8-hour period (i.e. less than 75% completeness),
the EPA is proposing to substitute zero (i.e. 0.000 ppm) instead of one
half of the O3 monitoring instrument's minimum detectable
limit (MDL) for the missing concentration values to determine if the
resulting 8-hour average is greater than the level of the NAAQS. The
purpose of this ``data substitution test'' is to identify any 8-hour
periods that do not meet the requirements for a valid 8-hour average,
but have reported concentrations that are so high that the NAAQS is
exceeded even when substituting low values for the missing
concentrations. The EPA believes that a constant substitution value of
zero is preferable to 1/2 MDL, which may vary across O3
monitoring instruments. The MDL value for most O3 monitoring
instruments is 0.005 ppm, and the 1/2 MDL value is 0.002 ppm (with
truncation); thus, in practice, the difference is slight. The EPA notes
that a value of zero micrograms per cubic meter (ug/m\3\) is used in
data substitution tests for 24-hour average PM2.5
concentrations, as specified in Appendix N to 40 CFR part 50. The EPA
invites public comment on the merits of using zero instead of 1/2 MDL
for the 8-hour average data substitution test.
The EPA is proposing new procedures for determining daily maximum
8-hour average O3 concentrations. Past practice allows for
daily maximum 8-hour average O3 concentrations from two
consecutive days to have some hours in common (40 CFR part 50, Appendix
P, section 2.1). One implication of this is that an O3 site
may be counted as having exceeded the NAAQS on two distinct days based
on two 8-hour periods having up to 7 hours in common. Theoretically,
this could result in an annual fourth-highest value greater than the
NAAQS based on high overnight O3 concentrations occurring
only twice during the year.
The EPA performed an analysis based on ambient O3
concentration data from 2004 to 2013 (Wells, 2014b), which showed that
at least one instance of overlapping daily maximum 8-hour averages
occurred at 99.5% of O3 sites during that time period.
Overlapping daily maximum 8-hour averages were infrequent at most
sites, but in some cases, these values occurred quite regularly (up to
60 times per year). Overlapping daily maximum 8-hour averages
contributed to additional exceedances of the proposed O3
NAAQS at 14% of sites for a level of 0.070 ppm, and at 23% of sites for
a level of 0.065 ppm. In addition, 8% of sites had overlapping daily
maximum 8-hour averages which contributed to a higher annual fourth-
highest daily maximum value in one or more years. Finally, the analysis
showed that O3 sites located in non-urban areas affected by
long-range transport, especially those sites at higher elevations, were
most likely to have additional exceedances of the proposed
O3 NAAQS due to the occurrence of overlapping daily maximum
8-hour averages.
Based on this analysis, the EPA initially concludes that
overlapping daily maximum 8-hour averages are more likely to contribute
to additional exceedances of the O3 NAAQS as the level of
the standard is lowered. Therefore, the EPA is proposing a new
procedure for determining daily maximum 8-hour average O3
concentrations for the proposed NAAQS that is based on 17 consecutive
8-hour periods in each day, beginning with the 8-hour period from 7:00
a.m. to 3:00 p.m., and ending with the 8-hour period from 11:00 p.m. to
7:00 a.m. Given that 8-hour averages are stored in the beginning hour
of each period, this corresponds to the 8-hour averages from 7:00 a.m.
to 11:00 p.m.
The rationale for the proposed approach is twofold. First, it
avoids any possibility of ``double counting'' exceedances of the NAAQS
based on 8-hour periods with one or more hours in common, while
continuing to make use of all of the hourly concentration data, and
keeping the calculations simple and straightforward. Second, it is more
consistent with the physical processes involved in the formation and
transport of ground-level O3. Specifically, the chemical
reactions involved in the formation of new ground-level O3
require sunlight. Therefore, it is appropriate to begin the
``O3 day'' at sunrise, which for simplicity is assumed to be
7:00 a.m. LST. Similarly, any daily maximum 8-hour averages occurring
after sunset are assumed to be caused by transport of O3
molecules which
[[Page 75353]]
originated before sunset. Therefore, it is appropriate to end the
``O3 day'' with the 8-hour period beginning at 11:00 p.m.
and ending at 7:00 a.m.
In order to accommodate the above proposed approach to the hours
considered in an ``O3 day'', the EPA is also proposing to
modify the requirement for determining whether a daily maximum 8-hour
average O3 concentration is valid for assessing compliance
with the NAAQS (40 CFR part 50, Appendix P, section 2.1). The proposed
Appendix U requires valid 8-hour averages for 13 of the 17 8-hour
periods in a day in order to determine a valid daily maximum value. The
requirement of 13 valid 8-hour averages was chosen because 13/17 is the
smallest ratio greater than 75%, which is consistent with the long
standing requirement of 75% data completeness for daily and annual
NAAQS-related statistics. In addition, the EPA is proposing to maintain
the existing provision allowing daily maximum 8-hour averages greater
than the level of the NAAQS to be considered valid (40 CFR part 50,
Appendix P, section 2.1). The EPA invites public comment on the merits
of the proposed procedure for determining daily maximum 8-hour average
O3 concentrations, and the merits of the proposed daily
validity criteria.
Finally, the EPA has included additional language in the proposed
Appendix U codifying existing data handling procedures for the previous
O3 NAAQS. First, the proposed Appendix U maintains the
provision that hourly O3 concentrations approved under 40
CFR 50.14 as having been affected by exceptional events are to be
counted as missing or unavailable when calculating 8-hour averages, and
that these concentrations are to be included when determining whether
the daily validity criteria have been met for a given day. Effectively,
this means that it is possible for an 8-hour period affected by
exceptional events to lack sufficient data to determine an 8-hour
average, yet the 8-hour period may still be counted toward meeting the
daily validity criteria. Second, the proposed Appendix U maintains the
existing practice of including monitored days outside of the
O3 monitoring season when determining the annual fourth-
highest daily maximum value. Finally, the proposed Appendix U maintains
the existing practice of using only daily maximum 8-hour average values
for days where the daily validity criteria have been met when
determining the annual fourth-highest daily maximum value.
D. Considerations for the Possibility of a Distinct Secondary Standard
Given that the EPA is soliciting public comment on setting a
distinct secondary O3 NAAQS based on the W126 index, the EPA
is including a discussion on the data handling requirements for a
distinct secondary standard. In the event that the Administrator
decides to set a distinct secondary O3 standard based on the
W126 index, the EPA will adopt data handling requirements for the
secondary standard similar to those proposed during the reconsideration
of the 2008 O3 NAAQS in 2010 (see 75 FR 3049-3052, January
19, 2010).
Two changes would need to be made to the data handling provisions
for the secondary standard proposed in 2010 in order to provide
consistency with what the EPA is proposing for the primary standard in
Appendix U. First, the secondary standard design value (i.e. the 3-year
average of the annual W126 index) would be truncated after the decimal
point, instead of being rounded to the nearest whole number. Second,
paragraph 4(c)(ii) would be modified to read:
``If one or more months during the ozone monitoring seasons of
three consecutive years has less than 75% data completeness, the three
years shall nevertheless be used in the computation of a valid design
value for the site, if, after adjusting the monthly W126 index values
for the months with less than 75% data completeness by a factor of 4/3,
the resulting design value is greater than the level of the standard.''
E. Exceptional Events Information Submission Schedule
States \228\ are responsible for identifying air quality data that
they believe warrant special consideration, including data affected by
exceptional events. States identify such data by flagging (making a
notation in a designated field in the electronic data record) specific
values in the AQS database. States flag the data and submit supporting
documentation showing that the data have been affected by exceptional
events if they wish the EPA to consider excluding the data in
regulatory decisions, including determining whether or not an area is
attaining the proposed revised O3 NAAQS, if a different
standard is finalized.
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\228\ References to ``state'' are meant to include state, local
and tribal agencies responsible for preparing and submitting
exceptional event documentation as identified in the Exceptional
Events Rule (72 FR 13560, March 22, 2007).
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All states and areas of Indian country that include areas that
could exceed or contribute to an exceedance of any revised
O3 NAAQS in a nearby area and could therefore be designated
as nonattainment have the potential to be affected by this rulemaking.
Therefore, this action applies to all states; to local air quality
agencies to which a state has delegated relevant responsibilities for
air quality management including air quality monitoring and data
analysis; and to tribal air quality agencies, where appropriate.
The ``Treatment of Data Influenced by Exceptional Events; Final
Rule'' (72 FR 13560, March 22, 2007), known as the Exceptional Events
Rule and codified at 40 CFR 50.1, 50.14 and 51.930, contains generic
deadlines for a state to submit to the EPA specified information about
exceptional events and associated air pollutant concentration data.
Under this generic flagging schedule in 40 CFR 50.14(c)(2)(iii), a
state must initially notify the EPA that data have been affected by an
event by July 1 of the calendar year following the year in which the
event occurred. This is done by flagging the data in AQS and providing
an initial event description. According to the generic demonstration
schedule in 40 CFR 50.14(c)(3)(i), the state must also, after notice
and opportunity for public comment, submit a demonstration to justify
any claim within 3 years after the quarter in which the data were
collected. This section of the regulation also states that if the EPA
must make a regulatory decision based on the data, the state must
submit all information to the EPA no later than 1 year before the
decision is to be made.
These generic deadlines in the Exceptional Events Rule apply to
data influencing redesignation efforts or other regulatory decisions
made by the EPA after the EPA promulgates initial area designations for
a new or revised NAAQS. However, these same generic deadlines in the
Exceptional Events Rule may not work well with the timing of the
initial area designation process and schedule under a new or revised
NAAQS. Until the EPA promulgates the level and form of the NAAQS, a
state does not know whether the criteria for excluding data (which are
tied to the level and form of the NAAQS) were met for a given event. In
some cases, the generic deadlines, especially the deadlines for
flagging some relevant data, may have already passed by the time the
EPA promulgates the new or revised NAAQS. This scheduling constraint
could result in the EPA's being unable to consider whether an
exceptional event has affected the data relied on for initial area
designations and further result in an area being designated
nonattainment based on data
[[Page 75354]]
that might have been excluded as having been influenced by an
exceptional event if the EPA had been able to consider it during the
designation process. For this reason, the EPA has historically
undertaken rulemaking as part of the NAAQS promulgation process to
adjust the generic deadlines in sections 50.14(c)(2)(iii) and
50.14(c)(3)(i) of the Exceptional Events Rule to accommodate the
initial area designation process and schedule under a new or revised
NAAQS.
The Exceptional Events Rule at section 50.14(c)(2)(vi) indicates
``when EPA sets a NAAQS for a new pollutant or revises the NAAQS for an
existing pollutant, it may revise or set a new schedule for flagging
exceptional event data, providing initial data descriptions and
providing detailed data documentation in AQS for the initial
designations of areas for those NAAQS.'' The EPA intends to issue its
final action promulgating a revised O3 NAAQS or determine
that it is not necessary to do so in October 2015.
The CAA provides requirements regarding the schedule for initial
area designations. Section 107(d)(1) of the CAA states that, ``By such
date as the Administrator may reasonably require, but not later than 1
year after promulgation of a new or revised national ambient air
quality standard for any pollutant under section 109, the Governor of
each state shall . . . submit to the Administrator a list of all areas
(or portions thereof) in the State, designating . . . '' those areas as
nonattainment, attainment, or unclassifiable.\229\ No later than 120
days prior to promulgating designations, the EPA is required to notify
states of any intended modifications to their designation
recommendations as the EPA may deem necessary. Section 107(d)(1)(B)(i)
further provides, ``Upon promulgation or revision of a NAAQS, the
Administrator shall promulgate the designations of all areas (or
portions thereof) . . . as expeditiously as practicable, but in no case
later than 2 years from the date of promulgation. Such period may be
extended for up to one year in the event the Administrator has
insufficient information to promulgate the designations.'' As described
in more detail in section VII.C of this proposal, the EPA intends to
complete designations for any revised O3 NAAQS promulgated
in 2015 following the standard 2-year process. The EPA is required by
Court Order to take final action for this O3 NAAQS review no
later than October 1, 2015. The EPA does not intend to establish a date
earlier than the 1 year submission period provided in CAA section
107(d)(4); thus, state Governors (and tribes, if they choose) would be
required to submit their initial designation recommendations for any
revised NAAQS no later than 1 year after promulgation (i.e., by October
1, 2016, if the EPA promulgates a revised NAAQS on October 1, 2015).
State Governors (and tribes, if they choose) would likely use air
quality data from the years 2013 to 2015 as the basis for their
recommendations. The EPA would notify states and tribes of intended
modifications to their recommendations no later than June 2017 and the
EPA would promulgate initial designations for any revised NAAQS in
October 2017. We anticipate that the EPA's notification of intended
modifications and the final designations would be based on air quality
data from the years 2014 to 2016, because air quality data from 2016 is
required to be certified by the state no later than May 1, 2017, and
thus would be available for consideration for purposes of initial area
designations by October 2017.
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\229\ While the CAA says ``designating'' with respect to the
Governor's letter, in the full context of the CAA section it is
clear that the Governor actually makes a recommendation.
---------------------------------------------------------------------------
As indicated above, and as explained in additional detail in
section VII.C of this preamble, section 107(d)(1)(B) of the CAA allows
the Administrator to extend the designations schedule for up to 1 year
in the event the Administrator has insufficient information to
promulgate the designations for a newly promulgated NAAQS. If the EPA
were to determine that it is necessary to extend the schedule for
designating areas for a revised O3 NAAQS (promulgation in
October 2015) from 2017 to 2018, then it is possible that air quality
data from 2017 could be considered for designations. This could raise
concerns about whether influences from exceptional events in 2017 could
be investigated and submitted by the state and reviewed by the EPA in
sufficient time for consideration during the designation process.
For purposes of initial designations, where the EPA considers the
most recent air quality monitoring data in a relatively quick
timeframe, the EPA is proposing revisions to the flagging and data
submission schedule in 40 CFR 50.14 applicable to the initial area
designations process. The proposed exceptional events schedule is based
on following a standard 2 year designation process. However, because
the CAA also provides for a 3-year process in the event the
Administrator has insufficient information to promulgate the
designations for a newly promulgated NAAQS within 2 years and provides
for the promulgation of designations as ``expeditiously as
practicable,'' which could include accelerating the designations
schedule ahead of the 2-year schedule, the proposed exceptional event
schedule also includes provisions for both an accelerated designations
process and a 3-year process. If the EPA were to pursue a designations
schedule other than a 2- or 3-year process, the EPA would notify the
state Governors of the intended date for final designations through
notification letters, guidance and/or Federal Register notices.
These proposed revised exceptional event scheduling provisions
would, if promulgated, apply to submission of information supporting
claimed exceptional events affecting pollutant data for initial area
designations under any new or revised NAAQS, including any revised
O3 NAAQS promulgated in October 2015. The general data
flagging deadlines in the Exceptional Events Rule at 40 CFR
50.14(c)(2)(iii) and the general schedule for submission of
demonstrations at 40 CFR 50.14(c)(3)(i) would continue to apply to
regulatory decisions other than those related to the initial area
designations process under a new or revised NAAQS. The EPA believes
these proposed revisions to the exceptional events scheduling
provisions will provide adequate time for states to determine whether
data have been influenced by an exceptional event, to notify the EPA by
flagging the relevant data and providing an initial description in AQS,
and to submit documentation to support claims for exceptional events.
Therefore, using the authority provided in CAA section 319(b)(2),
the EPA proposes to modify the schedule for data flagging and
submission of demonstrations for exceptional events data considered for
initial area designations by replacing the deadlines and information in
Table 1 in 40 CFR 50.14 with the deadlines and information presented in
Table 9. The EPA is also providing Table 10 to illustrate how the
proposed schedule might apply to the designations process for any
revised O3 NAAQS promulgated in October 2015 or to
designations processes for future new or revised NAAQS. The EPA invites
comment on these proposed changes, shown in Table 9, to the exceptional
event data flagging and documentation submission deadlines for future
new or revised
[[Page 75355]]
NAAQS, including any revised O3 NAAQS promulgated in 2015.
Table 9--Proposed Schedule for Exceptional Event Flagging and
Documentation Submission for Data To Be Used in Initial Area
Designations
------------------------------------------------------------------------
Exceptional event
Exceptional event/regulatory action deadline schedule
\d\
------------------------------------------------------------------------
Exceptional event data flagging and initial If state and tribal
description deadline for data years 1, 2 and 3 initial designation
\a\. recommendations for
the new/revised
NAAQS are due
August through
January, then the
flagging and
initial description
deadline will be
the July 1 prior to
the recommendation
deadline. If state
and tribal
recommendations for
the new/revised
NAAQS are due
February through
July, then the
flagging and
initial description
deadline will be
the January 1 prior
to the
recommendation
deadline.
Exceptional event demonstration submittal deadline No later than the
for data years 1, 2 and 3 \a\. date that state and
tribal
recommendations are
due to EPA.
Exceptional event data flagging, initial By the last day of
description, and exceptional event demonstration the month that is 1
submittal deadline for data year 4 \b\ and year and 7 months
potential data year 5 \c\. after promulgation
of a new or revised
NAAQS, unless
either option a or
b applies.
a. If the EPA
follows a 3-year
designation
schedule, the
deadline is 2 years
and 7 months after
promulgation of a
new or revised
NAAQS.
b. If the EPA
notifies the state/
tribe via Federal
Register notice,
letter or guidance
that it intends to
complete the
initial area
designations
process according
to a schedule other
than a 2-year or 3-
year timeline, the
deadline is 5
months prior to the
date specified for
final designations
decisions in such
EPA notification.
------------------------------------------------------------------------
\a\ Where data years 1, 2, and 3 are those years expected to be
considered in state and tribal recommendations.
\b\ Where data year 4 is the additional year of data that the EPA may
consider when it makes final area designations for the new/revised
NAAQS under the standard designations schedule.
\c\ Where data year 5 is the additional year of data that the EPA may
consider when it makes final area designations for the new/revised
NAAQS under an extended designations schedule.
\d\ The date by which air agencies must certify their ambient air
quality monitoring data in AQS is annually on May 1 of the year
following the year of data collection. The EPA cannot require air
agencies to certify data prior to this date. In some cases, however,
air agencies may choose to certify a prior year's data in advance of
May 1 of the following year, particularly if the EPA has indicated its
intent to promulgate final designations in the months of May, June,
July or August. Exceptional event flagging, initial description, and
demonstration deadlines for ``early certified'' data will follow the
deadlines for ``year 4'' and ``year 5'' data.
[[Page 75356]]
[GRAPHIC] [TIFF OMITTED] TP17DE14.000
[[Page 75357]]
[GRAPHIC] [TIFF OMITTED] TP17DE14.001
Additionally, in conjunction with proposing exceptional event
schedules related to implementing any revised O3 standards,
the EPA is also proposing to remove obsolete regulatory language
associated with exceptional event
[[Page 75358]]
schedules for historical standards. The EPA expects to propose
additional revisions to the Exceptional Events Rule in a future notice
and comment rulemaking effort and will solicit public comment on other,
non-schedule related, aspects of the Exceptional Events Rule at that
time.
VI. Ambient Monitoring Related to Proposed O3 Standards
A. Background
The EPA is proposing to: Revise the state-by-state O3
monitoring seasons; revise the PAMS monitoring requirements; revise the
FRM for measuring O3; and revise the FEM testing
requirements. The EPA is also proposing to make additional minor
changes to the FEM testing requirements for NO2 and
particulate matter in part 53 as discussed below.
The EPA is proposing to extend the length of the required
O3 monitoring season in some states to be appropriate for
the O3 NAAQS revision finalized in 2008, as well as a final
revised O3 standard, if a revision is finalized in 2015.
The EPA is proposing to make changes to the PAMS monitoring
requirements in 40 CFR part 58, Appendix D section 5. Section VI.C of
this preamble provides background on the current PAMS monitoring
requirements, recent efforts to re-evaluate the current PAMS
requirements, and a summary of the proposed PAMS requirement revisions.
The EPA is proposing to revise the FRM to establish a new,
additional technique for measuring O3 in the ambient air.
This new technique is based on nitric oxide-chemiluminescence (NO-CL)
methodology. Because of the similarity of this new chemiluminescence
technique to the existing ethylene-chemiluminescence (ET-CL)
methodology, the EPA proposes that it be incorporated into the existing
O3 FRM, using the same calibration procedure. Appendix D of
40 CFR part 50 would be revised to include both the original ET-CL as
well as the new NO-CL methodology. A minor change is proposed to the
existing O3 FRM calibration procedure, which would be
applicable to both of the chemiluminescence FRM methodologies. The
proposed change in section 4.5.2.3 of the calibration procedure in
appendix D provides for more flexibility in the range of the linearity
test.
The only substantial changes proposed to the requirements of 40 CFR
part 53 are in Tables B-1 and B-3 of subpart B. Table B-1 has been
updated in recent years with regard to FRM and FEM methods for
SO2 (74 FR 64877, December 8, 2009) and CO (76 FR 54294,
August 31, 2011) to be more consistent with current analyzer
performance capabilities. Similar changes to Table B-1 are proposed
here for methods for O3. Modest changes to Table B-3 would
add new interferent test concentrations specifically for NO-CL
analyzers, adding a test for NO2. Also, the table would
clarify that the existing test concentrations apply to ET-CL
O3 analyzers.
In addition, the EPA is making minor additional changes to Part 53
including: conforming changes to the FEM testing requirements in Table
B-1 and Figure B-5 for NO2; extending the period of time for
the Administrator to take action on a request for modification of a FRM
or FEM from 30 days to 90 days; and removing an obsolete provision for
manufacturers to submit Product Manufacturing Checklists for certain PM
monitors.
B. Revisions to the Length of the Required O3 Monitoring
Seasons
Unlike the ambient monitoring requirements for other criteria
pollutants that mandate year-round monitoring, O3 monitoring
is only required during the seasons of the year that are conducive to
O3 formation. These seasons vary in length from place to
place as the conditions conducive to the formation of O3
(i.e., seasonally-dependent factors such as ambient temperature,
strength of solar insolation, and length of day) differ by
location.\230\ In some locations, conditions conducive to O3
formation are limited to the summer months of the year. For example, in
states with colder climates such as Montana and South Dakota, the
currently required O3 monitoring season is four months long.
However, in other states with warmer climates such as California,
Nevada, and Arizona, the currently required O3 monitoring
season is year-round.\231\
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\230\ See 40 CFR part 58 Appendix D, section 4.1, Table D-3 for
a table of required O3 seasons.
\231\ Certain states, such as California and Arizona, have
approved shorter seasons for a subset of O3 sites, based
on Regional Administrator review and approval (see 40 CFR part 58,
Appendix D, section 4.1(i) for the waiver authority).
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Based on the O3 NAAQS revision that was finalized in
2008, as well as the proposed NAAQS revisions discussed in this
rulemaking, the EPA has determined that lengthening the O3
monitoring seasons may be appropriate. Ambient O3
concentrations could approach or exceed the level of the 2008 NAAQS, as
well as the proposed NAAQS, more frequently and during more months of
the year. The EPA has done an analysis to address the issue of whether
extensions of currently required monitoring seasons are appropriate
(Rice, 2014). In this analysis, we determined the number of days where
one or more monitors had a daily maximum 8-hour O3 average
equal to or above 0.060 ppm in the months outside the currently-
required state O3 monitoring season using data from monitors
that collected O3 data year-round in 2010-2013.\232\ We find
that this level, taking into consideration reasonable uncertainty,
serves as an appropriate indicator of ambient conditions that may be
conducive to the formation of O3 concentrations that
approach or exceed the 2008 NAAQS or the proposed 8-hour average range
of 0.065 to 0.070 ppm. Although we refer to these days as ``exceedance
days'' in the analysis, this 0.060 ppm threshold is simply a
conservative benchmark that is below the levels proposed for the
revised NAAQS. Proposals for revising each state's required monitoring
season are based on the observed ``exceedance days'' where the 8-hour
average daily maximum was >=0.060 ppm in and surrounding the state. The
EPA considered a number of factors including out-of-season ``exceedance
days'' either before or after the current O3 monitoring
season, the pattern of ``exceedance days'' in the out-of-season months,
and regional consistency. We note that seasonal O3 patterns
vary year-to-year due primarily to highly variable meteorological
conditions conducive to the formation of early or late season elevated
O3 concentrations in some years and not others. The EPA
believes it is important that O3 monitors operate during all
periods when there is a reasonable possibility of ambient levels
approaching the level of the proposed NAAQS.
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\232\ Approximately 800 O3 monitors are currently
operated year-round, representing greater than 50% of the total
O3 monitoring network of about 1500 monitors. They
include monitors that are mandated to operate year-round due to the
required O3 season and other monitors that are
voluntarily operated year-round by states and other organizations
including EPA-operated monitors at Clean Air Status and Trends
Network (CASTNET) sites.
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The EPA reviewed the year-round, O3 data for 2010
through 2013. A year-round monitor was identified as ``year-round'' if
it had at least 20 daily observations in all 12 months, for at least 1
year of the 4 year period. During the 2010-2013 data period, all states
operated a portion of their monitoring network outside of their
required O3 monitoring season and reported the data to the
EPA Air Quality System (AQS).
[[Page 75359]]
The EPA's analysis found the frequency of observed ``exceedance days''
of daily maximum 8-hour average O3 readings of >=0.060 ppm
to be quite high in several states across the country in months outside
of the currently required monitoring season. A total of 43 states
experienced at least one ``exceedance day'' outside of their current
O3 season; 21 states had ``exceedance days'' only before the
required monitoring season; 4 states had ``exceedance days'' only after
the required monitoring season; and 18 states had ``exceedance days''
both before and after the required monitoring season. In some cases,
the frequency of ``exceedance days'' before the current O3
season was high, with four states (South Dakota, Colorado, Wyoming, and
Utah) experiencing between 31 and 230 out-of-season ``exceedance days''
from 2010 to 2013 at monitors operating year-round.
Basing O3 monitoring season requirements on the goal of
ensuring monitoring when ambient O3 levels approach or
exceed the level of the proposed NAAQS supports established monitoring
network objectives described in Appendix D of Part 58, including the
requirement to provide air pollution data to the general public in a
timely manner \233\ and to support comparisons of an area's air
pollution levels against the NAAQS. The EPA believes that frequency of
``exceedance days'' in which daily maximum of 8-hour O3
levels are observed to be greater than or equal to a threshold level of
0.060 ppm in months outside the currently required O3
monitoring season supports the proposed lengthening of the
O3 monitoring season requirements for certain states.
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\233\ Public reporting requirements are detailed in 40 CFR part
58 Appendix G, Uniform Air Quality Index (AQI) and Daily Reporting.
---------------------------------------------------------------------------
The operation of O3 monitors during periods of time when
ambient levels approach or exceed the level of the proposed NAAQS
ensures that persons unusually sensitive to O3 are alerted
to potential levels of health concern allowing them to take
precautionary measures. The majority of O3 monitors in the
U.S. report to AIRNOW,\234\ as well as to state-operated Web sites and
automated phone reporting systems. These programs support many
objectives including real-time air quality reporting to the public,
O3 forecasting programs, and the verification of real-time
air quality forecast models.
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\234\ See http://airnow.gov/.
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The specific proposed changes to the required state O3
monitoring seasons are detailed in the proposed changes to Table D-3 of
40 CFR part 58, Appendix D (O3 Monitoring Season by State).
Although 43 states had at least one exceedance day outside the current
monitoring season, changes are proposed for only 33 of those states.
These proposed changes would entail an increase of 1 month for 23
states (Connecticut, Delaware, District of Columbia, Idaho, Illinois,
Iowa, Kansas, Maryland, Massachusetts, Minnesota, Missouri, Nebraska,
New Hampshire, New Jersey, New York, North Carolina, Ohio,
Pennsylvania, Rhode Island, South Carolina, Northern Texas, Virginia,
and West Virginia), an increase of one and one half months for
Wisconsin, an increase of two months for four states (Indiana,
Michigan, Montana, and North Dakota), an increase of four months for
Florida and South Dakota, an increase of five months for Colorado, and
an increase of seven months for Utah. For Wyoming, we are proposing to
add three months at the beginning of the season and remove one month at
the end of the season, resulting in a net increase of two months. Ozone
season requirements are currently split by Air Quality Control Region
(AQCR) in Louisiana and Texas. Included in the state-by-state
accounting is the proposal to lengthen the required season in the
northern part of Texas (AQCR 022, 210, 211, 212, 215, 217, and 218) by
one month. Southern Texas O3 monitors in AQCRs 106, 153,
213, 214, and 216 would remain on a year-round schedule. In some states
with limited available data and few exceedance days outside the current
season, proposed changes were made by considering regional consistency
and using supporting information from the surrounding states; these
changes were all minor, involving the proposed addition of 1 month to
the current required season in Iowa, Missouri, and West Virginia.
The EPA solicits comment on the proposed changes to the required
O3 monitoring seasons. We note that EPA Regional
Administrators have previously approved certain deviations from the
required O3 monitoring seasons through rulemakings (64 FR
3028, January 20, 1999; 67 FR 57332, September 10, 2002; and 69 FR
52836, August 30, 2004). The current ambient monitoring rule, in
paragraph 4.1(i) of 40 CFR part 58 Appendix D (71 FR 61319, October 17,
2006), allows the EPA Regional Administrators to approve changes to the
O3 monitoring season without rulemaking. The EPA is
retaining the rule language allowing such deviations from the required
O3 monitoring seasons in the proposed revision to paragraph
4.1(i) of 40 CFR part 58, Appendix D. The proposed changes to
O3 monitoring season requirements, if finalized, will revoke
previous Regional Administrator-granted waiver approvals. As
appropriate, monitoring agencies could seek new waivers. Post-final
rule requests submitted along with relevant supporting information by
states for monitoring season waivers from the revised requirements will
be reviewed by Regional Administrators using, at a minimum, occurrences
of the moderate AQI level, the frequency of out-of-season O3
NAAQS exceedances, and regional consistency. Any deviations based on
the Regional Administrator's waiver of requirements must be described
in the state's annual monitoring network plan and updated in the AQS.
Current regulations permit O3 monitors located at NCore
multi-pollutant stations to be counted toward meeting minimum network
monitoring requirements. The NCore network requirements were
promulgated in the October 17, 2006 (71 FR 61317) revisions to ambient
monitoring regulations in order to build a long-term, nationwide
network that supports multiple objectives including air quality trends
analyses, model evaluation, ecosystem studies, and assessment of
transport between urban and rural areas. In the 2006 rulemaking, the
EPA did not propose a different O3 monitoring season for
NCore stations.
NCore stations are required to operate a full suite of gaseous and
particulate matter monitors as well as basic meteorology to support the
objectives. Given the potential value of NCore data to support year-
round scientific studies, the EPA believes that it is appropriate to
require O3 monitors at NCore stations to be operated year-
round. Accordingly, the EPA proposes that the required monitoring
season for NCore stations be January through December regardless of the
length of the required O3 monitoring season for the
remainder of the SLAMS (State and Local Air Monitoring Stations)
monitors within a state.
The EPA has estimated the cost of the proposed changes to the
O3 seasons. The results are detailed in the EPA ICR #2313.03
and summarized in Section VIII.B., ``Paperwork Reduction Act''. The
estimated cost is $1,668,433 which is about 7% of the total average
annual cost of $24,115,182 for the national O3 monitoring
network. This estimate is based on the current requirements in 40 CFR
part 58 and the proposed requirements in this rule. We note however,
that greater than 50% of the monitors are currently operated year-round
due to existing requirements, as
[[Page 75360]]
well as other monitors that are voluntarily operated year-round by the
states. Taking into consideration the number of year-round
O3 monitors that are operated due to existing requirements,
as well as on a discretionary basis by states, the incremental cost of
these proposed changes is reduced from $1,668,433 to approximately
$230,000, which is less than 1% of the total average annual cost of the
national O3 monitoring network.
Considering the timing of this proposal and the final rulemaking
(court ordered deadline of October 1, 2015) and associated burden on
state/local monitoring agencies, we propose that implementation of the
revised O3 seasons become effective at SLAMS (including
NCore sites) on January 1, 2017. The EPA is proposing to add paragraph
58.13 (g) of 40 CFR part 58 to require that monitors operating under
the requirements of section 4.1 of 40 CFR part 58, Appendix D operate
on the applicable required O3 monitoring seasons effective
January 1, 2017 as listed in Table D-3 of appendix D to this part. We
solicit comment on whether the revised seasons could be implemented
beginning January 1, 2016 for all monitors or for a subset of monitors,
such as those currently operating year-round or on a schedule that
corresponds to the proposed O3 season. If we determine,
based on any such comments that implementation could occur earlier in
such cases, we could proceed to final action requiring earlier
implementation.
C. Revisions to the Photochemical Assessment Monitoring Stations (PAMS)
Section 182 (c)(1) of the CAA required the EPA to promulgate rules
for enhanced monitoring of O3, oxides of nitrogen, and VOCs
for nonattainment areas classified as serious (or above) to obtain more
comprehensive and representative data on O3 air pollution.
In addition, Section 185B of the CAA required the EPA to work with the
National Academy of Sciences (NAS) to conduct a study on the role of
O3 precursors in tropospheric O3 formation and
control. In 1992, the NAS issued the report entitled, ``Rethinking the
Ozone Problem in Urban and Regional Air Pollution'', (NAS, 1991).
In response to the CAA requirements and the recommendations of the
NAS report, on February 12, 1993 (58 FR 8452), the EPA revised the
ambient air quality surveillance regulations to require PAMS in each
O3 nonattainment area classified as serious, severe, or
extreme (``PAMS areas'').\235\ As noted in EPA's Technical Assistance
Document (TAD) for Sampling and Analysis of Ozone Precursors (U.S. EPA,
1998), the objectives of the PAMS program are to: (1) Provide a
speciated ambient air database which is both representative and useful
in evaluating control strategies and understanding the mechanisms of
pollutant transport by ascertaining ambient profiles and distinguishing
among various individual VOCs; (2) provide local, current
meteorological and ambient data to serve as initial and boundary
condition information for photochemical grid models; (3) provide a
representative, speciated ambient air database which is characteristic
of source emission impacts to be used in analyzing emissions inventory
issues and corroborating progress toward attainment; (4) provide
ambient data measurements which would allow later preparation of
unadjusted and adjusted pollutant trends reports; (5) provide
additional measurements of selected criteria pollutants for attainment/
nonattainment decisions and to construct NAAQS maintenance plans; and
(6) provide additional measurements of selected criteria and non-
criteria pollutants to be used for evaluating population exposure to
air toxics as well as criteria pollutants.
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\235\ Additional information on the O3 designation
process can be obtained at EPA's O3 designations Web page
at http://www.epa.gov/groundlevelozone/designations/.
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The original PAMS requirements called for two to five sites per
area depending on the area's population. Four types of PAMS sites were
identified including upwind (Type 1), maximum precursor emission rate
(Type 2), maximum O3 (Type 3), and extreme downwind (Type 4)
sites. Each PAMS site was required to measure O3, NO,
NO2, speciated VOCs, selected carbonyl compounds, and
selected meteorological parameters. In addition, upper air
meteorological monitoring was required at one site in each PAMS area.
In the October 17, 2006 monitoring rule (71 FR 61267), the EPA
revised the PAMS requirements to only require two PAMS sites per PAMS
area.\236\ The intent of the revision was to ``allow PAMS monitoring to
be more customized to local data needs rather than meeting so many
specific requirements common to all subject O3 nonattainment
areas; the PAMS changes would also give states the flexibility to
reduce the overall size of their PAMS programs--within limits--and to
use the associated resources for other types of monitoring they
consider more useful.'' In addition to reducing the number of required
sites per PAMS area, the 2006 revisions also limited the requirement
for carbonyl measurements (specifically formaldehyde, acetaldehyde, and
acetone) to areas classified as serious or above for the 8-hour
O3 standard. This change was made in recognition of carbonyl
sampling issues which were believed to cause significant uncertainty in
the measured concentrations.
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\236\ One Type 2 site and either a Type 1 or a Type 3 site are
currently required.
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Twenty-two areas were classified as serious or above O3
nonattainment at the time the PAMS requirements were promulgated in
1993. On July 18, 1997 (62 FR 38856), the EPA revised the averaging
time of the O3 NAAQS from a 1-hour averaging period to an 8-
hour averaging period. On June 15, 2005 (70 FR 44470), the EPA revoked
the 1-hour standard in most areas of the country; however, PAMS
requirements were identified as requirements that had to be retained in
the anti-backsliding provisions\237\ included in that action.
Therefore, PAMS requirements continue to be applicable to areas that
were classified as serious or above nonattainment for the 1-hour
O3 standard as of June 15, 2004. Currently, 25 areas are
subject to the PAMS requirements with a total of 75 sites. As will be
discussed in detail later, the current PAMS sites are concentrated in
the North East and California with relatively limited coverage in the
rest of the country (Cavender, 2014).
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\237\ Refer to 40 CFR part 51.905
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As discussed above, the first PAMS sites began operation in 1994,
and have been in operation for over 20 years. Many changes have
occurred during that time that have changed the O3 problem
in the U.S. as well as our understanding of it. The O3
standard has been revised multiple times since the PAMS program was
first implemented. On July 18, 1997, the EPA revised the O3
NAAQS to a level of 0.08 ppm, with a form based on the 3-year average
of the annual fourth-highest daily maximum 8-hour average O3
concentration. On March 28, 2008 (73 FR 16436), the EPA revised the
O3 standard to a level of 0.075 ppm, with a form based on
the 3-year average of the annual fourth-highest daily maximum 8-hour
average O3 concentration. These changes in the level and
form of the O3 NAAQS, along with notable decreases in
O3 levels in most parts of the U.S., have changed the
landscape of the O3 problem in the U.S. At the time of the
first round of designations for the 8-hour standard
[[Page 75361]]
(June 15, 2005), only five areas were classified as serious or above
for the 8-hour standard as compared to 22 areas that were classified as
serious or above for the 1-hour standard.\238\ While the number of
serious and above areas decreased, the number of nonattainment areas
remained nearly the same. In addition, much of the equipment used at
PAMS sites is old and in need of replacement. New technologies have
been developed since the inception of the PAMS program that should be
considered for use in the network. For these reasons, the EPA
determined that it would be appropriate to re-evaluate the PAMS program
and associated requirements in light of current O3 issues.
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\238\ PAMS requirements continue to apply to all areas
classified as serious or above as of June 15, 2005 due to anti-
backsliding provisions of 40 CFR 51.905.
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In 2011 (U.S. EPA, 2011c), the EPA initiated an effort to re-
evaluate the PAMS requirements in light of changes in the needs of PAMS
data users and the improvements in monitoring technology. The EPA
consulted with CASAC's, Air Monitoring and Methods Subcommittee (AMMS)
to seek advice on potential revisions to the technical and regulatory
aspects of the PAMS program; including changes to required measurements
and associated network design requirements. The EPA also requested
advice on appropriate technology, sampling frequency, and overall
program objectives in the context of the most recently revised
O3 NAAQS and changes to atmospheric chemistry that have
occurred over the past 10-15 years in the significantly impacted areas.
The CASAC AMMS met on May 16 and May 17, 2011, and provided a report
with their advice on the PAMS program on September 28, 2011 (U.S. EPA,
2011c). In addition, the EPA met multiple times with the National
Association of Clean Air Agencies (NACAA) Monitoring Steering Committee
(MSC) to seek advice on the PAMS program. The MSC includes monitoring
experts from various state and local agencies actively engaged in
ambient air monitoring and many members of the MSC have direct
experience with running PAMS sites. As discussed in more detail in the
appropriate sections below, the EPA took into consideration advice from
the CASAC AMMS and the MSC in proposing changes to the PAMS
requirements.
Based on the findings of the PAMS evaluation and the consultations
with the CASAC AMMS and NACAA MSC, the EPA is proposing to revise
several aspects of the PAMS monitoring requirements including changes
in 1) network design, 2) VOC sampling, 3) carbonyl sampling, 4)
nitrogen oxides sampling, and 5) upper air meteorology measurements.
The following paragraphs describe the changes being proposed including
the rationale for the proposed changes. Timing and other implementation
issues associated with these proposed changes are discussed at the end
of this section.
1. Network Design
As discussed above, the current PAMS network design calls for two
sites (a Type 2, and a Type 1 or Type 3) per PAMS area. In their report
(U.S EPA, 2011c), the CASAC AMMS found ``that the existing uniform
national network design model for PAMS is outdated and too resource
intensive,'' and recommended ``that greater flexibility for network
design and implementation of the PAMS program be transferred to state
and local monitoring agencies to allow monitoring, research and data
analysis to be better tailored to the specific needs of each
O3 problem area.'' While stating that the current PAMS
objectives were appropriate, the AMMS report also stated that
``objectives may need to be revised to include both a national and
regional focus because national objectives may be different from
regional objectives.'' The NACAA MSC also advised the EPA that the
existing PAMS requirements were too prescriptive and may hinder state
efforts to collect other types of data that were more useful in
understanding their local O3 problems.
The EPA agrees with CASAC that the PAMS objectives include both
local and national objectives, and believes that the current PAMS
network design is no longer suited for meeting either sets of
objectives. As part of the PAMS evaluation, it was determined that at
the national level the primary use of the PAMS data has been to
evaluate photochemical model performance. Due to the locations of the
current PAMS areas and the current network design, existing PAMS sites
are clustered along the northeast and west coasts leading to
significant redundancy in these areas and very limited coverage
throughout the remainder of the country (Cavender, 2014). The resulting
uneven spatial coverage greatly limits the value of the PAMS data for
evaluation of model performance. CASAC (U.S. EPA, 2011c) noted the
spatial coverage issue and advised that EPA should consider requiring
PAMS measurements in areas in addition to ``areas classified as serious
and above for the O3 NAAQS to improve spatial coverage.''
The EPA also agrees with CASAC and the NACAA that the PAMS requirements
should be revised to provide monitoring agencies greater flexibility in
meeting local objectives.
The EPA is proposing changes to the network design requirements
that we believe will better serve both national and local objectives.
The EPA is proposing a two part network design. The first part of the
design includes a network of fixed sites (required PAMS sites) intended
to support O3 model development and the tracking of trends
of important O3 precursor concentrations. The second part of
the network design includes monitoring agency directed Enhanced
Monitoring Plans which allow monitoring agencies the needed flexibility
to implement additional monitoring capabilities to suit the needs of
their area.
The EPA considered a number of options to revise the fixed site
portion of the network design (Cavender, 2014). An initial option
considered was to require all NCore sites to make PAMS measurements
regardless of O3 attainment status. This option would take
advantage of the existing NCore infrastructure and would result in a
relatively wide geographic distribution of sites. However, it was noted
that this option would place some PAMS measurements in areas with
relatively low O3 levels and would also result in a network
of approximately 80 required sites, which would strain existing
resources with a somewhat larger network than the current situation,
and could make it difficult to also implement the desired state-
directed Enhanced Monitoring Plans. The second option considered was to
require only NCore sites in O3 nonattainment areas to
collect PAMS measurements. This option would provide the benefits
discussed above for collecting PAMS measurements at existing NCore
sites. This option would also reduce the total number of sites required
and focus efforts in areas with higher, non-attaining, levels of
O3. The final option considered would add a population limit
in addition to the consideration of O3 attainment status at
NCore sites. An illustration of this example would be a PAMS
requirement that applied only to NCore sites in O3
nonattainment areas with a population greater than a given threshold,
for example, Core Based Statistical Areas with 1,000,000 people or
more. This approach would continue the current practice of focusing
PAMS resources in areas of elevated O3 readings with an
additional consideration that measurements in these larger population
areas would be
[[Page 75362]]
sufficient to characterize O3 formation on a national basis.
After considering the above options as well as the comments of
CASAC and NACAA, the EPA believes that an approach focused primarily on
the use of the existing NCore sites in O3 nonattainment
areas provides an appropriate balance to the consideration of
O3 levels as well as population, noting that a majority of
NCore sites are already located in the larger urban areas of each
state. Accordingly, the EPA is proposing to require PAMS measurements
at any existing NCore site in an O3 nonattainment area
(either based on the 2008 O3 NAAQS or the 2015 O3
NAAQS if finalized) in lieu of the current PAMS network design
requirements.\239\ The NCore network is a multi-pollutant monitoring
network consisting of 80 sites (63 urban, 17 rural) and is intended to
support multiple air quality objectives including the development and
model evaluation of photochemical models (including both
PM2.5 and O3 models), and the tracking of
regional precursor trends. NCore sites are sited in typical
neighborhood scale locations which are more suitable than source
impacted locations for evaluation of grid models typical of current
photochemical models and tracking of trends in pre-cursor
concentrations. The EPA believes NCore sites are well suited for
O3 model development and evaluation.
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\239\ Although enhanced monitoring for O3, oxides of
nitrogen, and VOCs is specifically required for areas classified at
least serious for the O3 NAAQS by section 182(c)(1) of
the CAA, the EPA has concluded that requiring enhanced monitoring
for all O3 nonattainment areas is appropriate for the
purposes of monitoring ambient air quality and better understanding
O3 pollution.
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The proposal to require PAMS measurements at existing NCore sites
in O3 nonattainment areas would replace the existing PAMS
network design.\240\ This change would keep roughly the same number of
required PAMS sites while improving spatial coverage (Cavender, 2014).
Based on the range of the O3 NAAQS being proposed today and
current O3 design value estimates (based on 2011-2013 air
quality data), the number of required sites is estimated to be between
48 and 65, which compares to 50 currently required sites, and 75
currently operating sites. Potential redundancy in the existing network
would be reduced while important network coverage in the Southeast and
Midwest would be added. The improved spatial coverage will also improve
the EPA's ability to track trends in precursor concentrations
regionally. The EPA notes that in limited situations, an O3
nonattainment area may not have an NCore site and in those cases, the
area would only be subject to the requirement for an Enhanced
Monitoring Plan as discussed in more detail below. The EPA believes
that the network coverage provided by existing NCore sites in
O3 nonattainment areas would be adequate for the national
PAMS objectives discussed above, and that requiring PAMS sites, in
addition to Enhanced Monitoring Plans, in those O3
nonattainment areas without NCore sites would not substantially improve
the network coverage.
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\240\ While the EPA is proposing to replace the multi-site
design, monitoring agencies would be encouraged to identify the type
of PAMS site the NCore site represents. In most cases, NCore sites
would likely be classified as either a Type 2 or Type 3 site. In
limited situations, rural NCore sites might be subject to these
proposed requirements, in which case, these sites would likely be
either Type 1 or Type 4 sites.
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The EPA notes that the proposed network design change would provide
significant cost efficiencies. By adding PAMS measurements to existing
NCore sites, the PAMS network would be taking advantage of existing
infrastructure and measurements currently being collected at NCore
sites. NCore sites already have the larger, climate-controlled shelters
that are necessary to operate the automated gas chromatographs (``auto-
GCs'') used to collect speciated VOCs. In addition, existing NCore
sites currently collect data on many of the required PAMS measurements
including O3, CO, total reactive nitrogen (NOy),
and meteorological measurements including wind speed and direction,
temperature, and relative humidity.
While the EPA believes these proposed changes will result in fixed
network cost savings for the overall network on a national basis,
individual monitoring agencies may see either an increase or a decrease
in burden as a result of these proposed changes. Monitoring agencies in
O3 nonattainment areas who are not currently affected by the
existing PAMS requirements would be required to add PAMS measurements
to their existing NCore sites, while several monitoring agencies with
existing PAMS sites would not be required to continue PAMS monitoring
if these proposed requirements are promulgated.\241\ As discussed later
in this preamble, the EPA is proposing a staggered compliance schedule
for the proposed PAMS requirements in recognition of the need for
capital investment and staff training at these sites.
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\241\ Monitoring agencies would be able to seek approval to shut
down non-required PAMS sites at their discretion pursuant to the
requirements detailed in 40 CFR 58.14.
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The EPA recognizes that in limited situations, existing NCore sites
may not be the most appropriate locations for making PAMS measurements.
For example, an existing PAMS site in an O3 nonattainment
area may be sited at a different location than the existing NCore site.
In this case, it may be appropriate to continue monitoring at the
existing PAMS site to support ongoing research and to maintain trends
information. To account for these situations, the EPA is also proposing
to provide the EPA Regional Administrator the authority to approve an
alternative location for a required PAMS sites where appropriate.
The EPA seeks comment on the network design revision proposed
above, the requirement for PAMS measurements at NCore sites in
O3 nonattainment areas, and the removal of current multi-
site PAMS network design requirements. The EPA also solicits comment on
whether, instead of requiring PAMS measurements at all NCore sites in
nonattainment areas, we should instead adopt one of the other options
discussed above, for example, using both attainment status and
population thresholds, that may result in a fixed PAMS monitoring
network that is either smaller or larger than what will result from the
proposed requirement.
The second part of the proposed PAMS network design includes
monitoring agency directed enhanced O3 monitoring activities
intended to provide data needed to understand an area's specific
O3 issues. To implement this part of the PAMS network
design, the EPA is proposing to add a requirement for states with
O3 nonattainment areas to develop an ``Enhanced Monitoring
Plan.'' These Enhanced Monitoring Plans, which are to be submitted as
part of their required Annual Monitoring Network Plan (40 CFR 58.10),
would be reviewed and approved by the EPA Regional Administrator as
part of the annual plan review process. The purpose of the Enhanced
Monitoring Plan is to improve monitoring for ambient concentrations of
O3, NOX/NOy, VOCs, and meteorology.
The goal of the Enhanced Monitoring Plan is to allow monitoring
agencies flexibility in determining and collecting the data they need
to understand their O3 problems, consistent with this
purpose and the advice obtained from the CASAC AMMS and the NACAA MSC.
Types of activities that might be included in the Enhanced Monitoring
Plan include (but are not limited to) additional PAMS
[[Page 75363]]
sites (e.g., upwind or downwind sites), additional O3 and
NOX monitoring, ozonesondes or other aloft measurements,
rural measurements, mobile PAMS sites, additional meteorological
measurements, and episodic or intensive studies. The savings from a
smaller less costly fixed network of required PAMS sites would be
available for re-investment in the development and implementation of
the proposed Enhanced Monitoring Plans.
2. Speciated VOC Measurements
Measurement of speciated VOCs important to O3 formation
is a key aspect of the PAMS program. Currently, the existing PAMS
requirements allow for a number of options in measuring speciated VOCs
at PAMS sites which include 1) hourly measurements using an auto-GC, 2)
eight 3-hour samples daily using canisters, or 3) one morning and one
afternoon sample with a 3-hour or less averaging time daily using
canisters plus continuous total non-methane hydrocarbon (TNMHC)
measurements.
The EPA believes that the options provided for VOC measurements
limit the comparative value of the data being collected, and is
proposing to require instead that all required PAMS sites measure and
report hourly speciated VOCs using an auto GC. More complete and
consistent speciated VOC data nationally would better help meet certain
objectives of the PAMS program described above (e.g., a speciated
ambient air database useful in evaluating control strategies, analyzing
emissions inventory issues, corroborating progress toward attainment,
and evaluating population exposure to air toxics). Furthermore, as
noted by the CASAC AMMS, hourly VOC data are ``particularly useful in
evaluating air quality models and performing diagnostic emission
attribution studies. These data can be provided on a near real-time
basis and presented along with other precursor species (e.g., oxides of
nitrogen and carbon monoxide) collected over similar averaging times.''
Longer time-averaged data are of significantly lower value for model
evaluation.\242\ In addition, creating consistent monitoring
requirements across the network will provide better data for analyzing
regional trends and spatial patterns.
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\242\ Data of longer than a 1-hour average are often not used in
model evaluations due to the complexity of trying to accommodate
non-hourly averaged data.
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At the time the original PAMS requirements were promulgated, the
canister options were included because the EPA recognized that the
technologies necessary to measure hourly average speciated VOCs
concentrations were relatively new and may not have been suitable for
broad network use. At that time, gas chromatographs designed for
laboratory use were equipped with auto-samplers designed to ``trap''
the VOC compounds from a gas sample, and then ``purge'' the compounds
onto the GC column. The EPA did not believe that auto-GCs were
universally appropriate due to the technical skill and effort necessary
at that time to properly operate an auto-GC.
While the basic principles of auto-GC technology have not changed,
the hardware and software of modern auto-GCs are greatly improved over
that available at the time of the original PAMS requirements. Based on
advice from the CASAC AMMS, the EPA has initiated an evaluation of
current auto-GCs potentially suitable for use in the PAMS network.
Based on the preliminary results, the EPA believes that typical NCore
site operators, with appropriate training, will have the skill
necessary to operate a modern auto-GC successfully. Considering the
advances in auto-GC technology, the added value obtained from hourly
data, and the proposed move of PAMS measurements to NCore sites in
O3 nonattainment areas, the EPA is proposing to require
hourly speciated VOC sampling at all PAMS sites. The EPA notes that
this proposed requirement would effectively prevent the use of
canisters to collect speciated VOCs at the required PAMS sites.
However, canister sampling may continue to be an appropriate method for
collecting speciated VOCs at other locations as part of the proposed
Enhanced Monitoring Plans.
While the EPA believes that the proposed transition to hourly
speciated VOC sampling is the appropriate strategy to take advantage of
improved technology and to broaden the utility of collected data, we
are also mindful of the additional rigidity that the proposed mandatory
use of auto-GCs may have for monitoring agencies, especially those that
have experience with and have established effective and reliable
canister sampling programs. Therefore, the EPA is requesting comment on
the proposed requirement for hourly VOC sampling as well as the range
of alternatives that might be appropriate in lieu of a strict
requirement. Such alternatives could range from a more formal process
where monitoring agencies could request a Regional Administrator-
granted waiver from the hourly VOC requirements through the Annual
Monitoring Network Plan process to collect canister-based speciated VOC
data, to a more flexible set of alternatives where canister sampling
could be retained based on each monitoring agency's evaluation of
programmatic needs as well as their own logistical and technical
capabilities.
3. Carbonyl Sampling
Carbonyls include a number of compounds important to O3
formation that cannot currently be measured using the auto-GCs or
canisters used at PAMS sites to measure speciated VOCs.\243\ The
current method for measuring carbonyls in the PAMS program is
Compendium Method TO-11A (U.S. EPA, 1999). In this method, carbonyl
compounds are adsorbed and converted into stable hydrazones using
dinitrophenylhydrazine (DNPH) cartridges. These cartridges are then
analyzed for the individual carbonyl compounds using liquid
chromatography (LC) techniques. Three carbonyls (formaldehyde,
acetaldehyde, and acetone) are currently required to be measured in the
PAMS program.
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\243\ Carbonyls compounds including formaldehyde and
acetaldehyde are difficult to analyze by GC with Flame Ionization
Detectors (FID). Both of these compounds in their free state, do not
respond well to FID detectors. GC analysis is difficult due to the
chemical composition of these compounds, increased polarity and
their inherently low boiling points.
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In 2006, the EPA revised the PAMS requirements such that carbonyl
sampling was only required in areas classified as serious or above
nonattainment for O3 under the 8-hour O3 standard
which effectively reduced the applicability of carbonyl sampling to a
few areas in California. This change was made in recognition that there
were a number of issues with Method TO-11A that raised concerns with
the uncertainty in the carbonyl data being collected. These issues
include interferences (humidity and O3) and breakthrough
(i.e., overloading of the DNPH cartridge) at high concentrations. While
solutions for these issues have been investigated, these improvements
have not been incorporated into Method TO-11A.
A recent evaluation of the importance of VOCs and carbonyls to
O3 formation determined that carbonyls, especially
formaldehyde, are very important to O3 formation (Cavender,
2013). CASAC AMMS (U.S. EPA, 2011c) also noted the importance of
carbonyls stating that ``There are many compelling scientific reasons
to measure carbonyls. They are a very important part of O3
chemistry almost everywhere.'' Due to the importance of carbonyls to
[[Page 75364]]
understanding O3 chemistry, the EPA believes the need for
carbonyl data outweighs the concerns over the uncertainty in the data.
Therefore, the EPA is proposing to require all required PAMS sites to
measure formaldehyde, acetaldehyde, and acetone. In addition, EPA is
investigating alternatives to further reduce uncertainties in carbonyl
data as described below.
To improve the carbonyl data that would be collected at required
PAMS sites (and National Air Toxics Trends Station, or NATTS sites
which are also currently measuring carbonyls), the EPA has undertaken
an effort to improve carbonyl sampling and analysis methods to reduce
the uncertainty in carbonyl data. This effort will lead to improvements
to the current Method TO-11A by incorporating solutions to sampling and
analysis issues that have been identified since Method TO-11A was
finalized in 1999, such as the inclusion of an O3 scrubber
in the sampling system to reduce the interference from oxidants such as
O3. Also as part of this effort, the EPA is investigating
alternative cartridge materials that have been identified in the
literature as a replacement for DNPH that may have better collection
efficiency with fewer interferences.
4. Nitrogen Oxides Sampling
It is well known that NO and NO2 play important roles in
O3 formation (U.S. EPA, 2011a, Section 3.2.2). Under the
current network design, Type 2 PAMS sites are required to measure
NOX (which by definition is the sum of NO and
NO2), and Types 1, 3, and 4 sites are required to measure
NOy which by definition includes NO, NO2, and
other oxidized nitrogen compounds (NOz). NCore sites are
also currently required to measure NOy but are not required
to measure NO2.
In conventional NOX analyzers, NO2 is
determined as the difference between the measured NO and NOX
concentrations. However, due to the non-selective reduction of oxidized
nitrogen compounds by the molybedenum converter used in conventional
NOX monitors, the NO2 measurement made by
conventional NOX monitors can be biased high due to the
varying presence of NOz compounds that may be reported as
NO2.\244\ The unknown bias from the NOz compounds
is undesirable when attempting to understand O3 chemistry.
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\244\ Nitrogen compounds that would likely be reported (along
with NO2) as NO2 with a conventional
NOX monitor include peroxyacetyl nitrate (PAN),
peroxypropionyl nitrate (PPN), peroxymethacryloyl nitrate (MPAN),
and nitric acid (HNO3), and as well as other nitrogen
compounds not listed here.
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Improvements in reactive nitrogen measurements have been made since
the original PAMS requirements were promulgated that allow for improved
NO2 measurements. Selective photolytic converters have been
developed that are not significantly biased by NOz compounds
(Ryerson et al., 2000). Monitors using photolytic converters are
commercially available and have been approved as FEMs for the
measurement of NO2. In addition, methods that directly read
NO2 have been developed that allow for very accurate
readings of NO2 without some of the issues inherent to the
``difference method'' used in converter based NOX analyzers.
However, these direct reading NO2 analyzers generally do not
provide an NO estimate, and would need to be paired with a converter-
based NOX monitor or NOy monitor in order to also
measure NO.
As discussed above, the EPA is proposing to change the PAMS network
design such that PAMS measurements would be required at existing NCore
sites in O3 nonattainment areas. NCore sites currently are
required to measure NO and NOy. NCore sites are not
currently required to measure NO2. Due to the importance of
accurate NO2 data to the understanding of O3
formation, the EPA is proposing to require NO2 measurements
at required PAMS sites. Since existing NCore sites currently measure
NOy, either a direct reading NO2 analyzer or a
photolytic-converter NOX analyzer should be used to meet the
proposed requirement. The EPA believes conventional NOX
analyzers would not be appropriate for making PAMS measurements due to
the uncertainty caused by interferences caused by NOz
compounds.
5. Meteorology Measurements
Monitoring agencies are currently required to collect surface
meteorology at all PAMS sites. As noted in EPA's TAD (U.S. EPA, 1998)
for the PAMS program, the PAMS requirements do not provide specific
surface meteorological parameters to be monitored. As part of the
implementation efforts for the original PAMS program, a list of
recommended parameters was developed and incorporated into the TAD
which includes wind direction, wind speed, temperature, humidity,
atmospheric pressure, precipitation, solar radiation, and ultraviolet
(UV) radiation. Currently, NCore sites are required to measure the
above parameters with the exceptions of atmospheric pressure,
precipitation, solar radiation, and UV radiation. In recognition of the
importance of these additional measurements for O3, the EPA
is proposing to specify that required PAMS sites are required to
collect wind direction, wind speed, temperature, humidity, atmospheric
pressure, precipitation, solar radiation, and UV radiation. This
proposed revision will provide clarity and consistency to the
collection of surface meteorological parameters important to the
understanding of O3 formation. If PAMS measurements are
moved to NCore sites in O3 nonattainment areas, as is being
proposed, the net impact of this proposed revision to the surface
meteorological requirements for PAMS sites is to add the requirement
for the monitoring of atmospheric pressure, precipitation, solar
radiation, and UV radiation at affected NCore sites.
The existing PAMS requirements also require the collection of upper
air meteorological measurements at one site in each PAMS area. The term
``upper air meteorological'' is not well defined in the existing PAMS
requirements. As part of the implementation efforts for the original
PAMS program ``mixing height'' was added to the PAMS TAD as a
recommended meteorological parameter to be monitored.
Most monitoring agencies installed radar profilers to meet the
requirement to collect upper air meteorology. Radar profilers provide
data on wind and speed at multiple heights in the atmosphere. Radio
acoustic sounding system (RASS) profilers are often included with radar
profilers to obtain atmospheric temperature at multiple heights in the
atmosphere and to estimate mixing height. The EPA recognizes that the
upper air data on wind speed and wind direction from radar profilers
can be very useful in O3 modeling. However, many of the
current PAMS radar profilers are old and in need of replacement or
expensive maintenance. In addition, the cost to install and operate
radar profilers at all NCore sites would be prohibitive. Therefore, the
EPA is not proposing to require upper air wind speed and direction as
required meteorological parameters to be monitored at PAMS sites. Where
monitoring agencies find the radar profiler data valuable, continued
operation of existing radar profilers or the installation of new radar
profilers would be appropriate to consider as part of the state's
Enhanced Monitoring Plan.
As discussed above, mixing height is one upper air meteorological
measurement that has historically been measured at PAMS sites. A number
of methods can be used to measure mixing height in addition to radar
profiler
[[Page 75365]]
technology discussed above. Recent developments in ceilometer
technology allow for the measurement of mixing height by changes in
particulate concentrations at the top of the boundary layer (Eresmaa et
al., 2006). Ceilometers provide the potential for continuous mixing
height data at a fraction of the cost of radar profilers. Due to the
importance of mixing height measurements for O3 modeling,
the EPA is proposing to require monitoring agencies to measure mixing
height at PAMS sites. The EPA is aware of a large network of
ceilometers operated by the National Oceanic and Atmospheric
Administration (NOAA) as part of the Automated Surface Observing System
(ASOS). The EPA has been in discussions with NOAA regarding the
potential for these systems to provide the needed mixing height data,
however, the ASOS ceilometers are not currently equipped to provide
mixing height data. Nonetheless, the EPA will continue to work with
NOAA to determine if the ASOS ceilometers can be upgraded to meet the
need for mixing height data, and is including proposed regulatory
language that will allow states a waiver to use nearby mixing height
data from ASOS or other sources to meet the requirement to collect
mixing height data at required PAMS sites.
6. PAMS Season
Currently, PAMS measurements are required to be taken during the
months of June, July, and August. This 3-month period is referred to as
the ``PAMS Season''. As part of the PAMS re-evaluation, the EPA
considered changes to the PAMS season. The 3-month PAMS season was
originally selected to represent the most active period for
O3 formation. However, the EPA notes that in many areas the
highest O3 concentrations are observed outside of the PAMS
season.\245\ As an example, the highest O3 concentrations in
the Mountain-West often occur during the winter months. Data collected
during the current PAMS season would have limited value in
understanding winter O3 episodes.
---------------------------------------------------------------------------
\245\ The current O3 monitoring season by state in 40
CFR part 58, appendix D, requires monitoring seasons from 4 to 12
months. As noted in section VI.B. of this preamble, the EPA proposes
to lengthen the seasons further for 33 states.
---------------------------------------------------------------------------
The CASAC AMMS (U.S. EPA, 2011c) noted in their report to the EPA
that ``it would be desirable to extend the PAMS monitoring season
beyond the current June, July, August sampling period,'' but that ``the
monitoring season should not be mandated and rigid; it should be
flexible and adopted and coordinated on a regional airshed basis (i.e.,
within the same O3 region).'' The EPA agrees with CASAC on
the need for flexibility in determining when PAMS measurements should
be taken to meet local monitoring needs but also agrees with CASAC that
the flexibility ``should not conflict with national goals for the PAMS
program.'' A significant benefit of the standard PAMS season is that it
ensures data availability from all PAMS sites for national- or
regional-scale modeling efforts.
While the EPA agrees with the potential benefit of extending the
availability of PAMS measurements outside of the current season, we
also considered the burden of requiring monitoring agencies to operate
additional PAMS measurements (e.g., hourly speciated VOC) for periods
that in some cases, might be much longer than the current 3-month
season, for example, if the PAMS season was extended to match each
state's required O3 monitoring season. Being mindful of the
potential burden associated with a lengthening of the PAMS season as
well as the potential benefits of the additional data, the EPA is
proposing to maintain the current 3-month PAMS monitoring season for
required PAMS sites rather than extending the PAMS season to other
periods where elevated O3 may be expected. The EPA believes
that the 3-month PAMS season will provide a consistent data set of
O3 and O3 precursor measurements for addressing
the national PAMS objectives. Monitoring agencies are encouraged to
consider collecting PAMS measurements in additional periods beyond the
required PAMS season as part of the proposed Enhanced Monitoring Plan.
The monitoring agencies should consider factors such as the periods of
expected O3 exceedances and regional consistency when
determining potential expansion of the specific monitoring periods
beyond the required PAMS season.
7. Timing and Other Implementation Issues
The EPA recognizes that the proposed changes to the PAMS
requirements will require resources and a reasonable implementation
schedule if they are promulgated. The proposed network design changes
would require monitoring agencies to start collection of PAMS
measurements at many NCore sites that are not currently collecting PAMS
measurements. These affected monitoring agencies would need to make
capital investments (primarily for the installation of auto-GCs,
NO2 monitors, and ceilometers). Monitoring agencies will
also need time to develop the expertise, by training existing staff or
otherwise, to successfully collect PAMS measurements. The EPA believes
that the current national funding level of the PAMS program is
sufficient to support these proposed changes, especially in light of
the staggered deployment schedule described below. The current grant
guidance includes the maintenance of a PAMS capital equipment reserve
that could be used to assist monitoring agencies with the purchase of
needed equipment. We also recognize that the proposed revisions would
result in a potential shifting of PAMS resources, and we would work
with the regional offices, affected states, and monitoring
organizations such as the NACAA and the Association of Air Pollution
Control Agencies (AAPCA) to facilitate any shifts in funding during the
implementation phase of the program.
For these reasons, the EPA is proposing a staggered deployment
schedule for the proposed changes to the PAMS requirements (including
both the monitoring at required PAMS sites and the Enhanced Monitoring
Plans). For areas currently designated as nonattainment for
O3 based on the 2008 NAAQS, the EPA is proposing to require
monitoring agencies to incorporate the proposed PAMS requirements into
their next annual monitoring network plan following promulgation of
these proposed changes (due July 1, 2016, based on current schedules)
and to comply with these proposed PAMS requirements by the following
PAMS season (June 1, 2017, based on current schedules). For new areas
designated as O3 nonattainment based on the initial round of
designations following the promulgation of a revised O3
standard, the EPA is proposing to require monitoring agencies to
incorporate the proposed PAMS requirements into their next annual
monitoring network plan following designations (due July 1, 2018, based
on current schedules) and to comply with new PAMS requirements by the
following PAMS season (June 1, 2019, based on current schedules).
Finally, the EPA is proposing that areas designated as O3
nonattainment following the initial round of designations be allowed 2
years after designation to comply with the proposed PAMS requirements.
The EPA believes that the proposed compliance schedule will allow
monitoring agencies adequate time to implement the proposed PAMS
requirements. The EPA solicits comments on whether the proposed
implementation schedule is practicable, or whether additional time
would be
[[Page 75366]]
warranted for installation of new PAMS sites, the development of
Enhanced Monitoring Plans, or other specific new PAMS requirements.
D. Addition of a New Federal Reference Method (FRM) for O3
To be used in a determination of compliance with the O3
NAAQS, O3 monitoring data must be obtained using either a
FRM or a FEM, as defined in 40 CFR parts 50 and 53. Nearly all the
monitoring methods for O3 currently used by state and local
monitoring agencies are FEM continuous analyzers utilizing a
measurement principle based on quantitative measurement of the
absorption of UV light by O3. This type of O3
analyzer was introduced into the monitoring networks in the 1980s and
has since become the most predominant type of method used because of
its all-optoelectronic design and ease of installation and use. The
existing O3 FRM utilizes a measurement principle based on
quantitative measurement of the chemiluminescence from the reaction of
O3 with ethylene. Ozone analyzers based on this FRM
principle are no longer used for routine O3 field monitoring
and are no longer commercially available. The current list of all
approved FRMs and FEMs capable of providing ambient O3 data
for use in NAAQS attainment decisions may be found on the EPA's Web
site and in the docket for this action (U.S. EPA, 2014g).
The EPA proposes to revise the FRM to establish a new technique for
measuring O3 in the ambient air. This new technique would be
a new type of analyzer based on Nitric Oxide-chemiluminescence (NO-CL)
methodology. Because of the similarity of this new chemiluminescence
technique to the existing ethylene-chemiluminescence (ET-CL)
methodology, the EPA proposes that it be incorporated into the existing
O3 FRM, using the same calibration procedure. Appendix D of
40 CFR part 50 would be revised to include both the original ET-CL as
well as the new NO-CL methodology. A minor change is proposed to the
existing O3 FRM calibration procedure, which would be
applicable to both chemiluminescence FRM methodologies. The proposed
change in section 4.5.2.3 of the calibration procedure in Part 50
provides for more flexibility in the range of the linearity test.
FRMs, as set forth in several appendices to 40 CFR part 50, serve
two primary purposes. The first is to provide a specified, definitive
methodology for routinely measuring concentrations of various ambient
air pollutants for comparison to the NAAQS in Part 50, for quality
assurance assessment of monitoring data, and for other air monitoring
objectives. The second is to provide a standard of comparison for
determining equivalence to the specified reference method of
alternative and perhaps more practical pollutant measurement methods
(equivalent methods, or FEMs) that can be used in lieu of the FRM for
routine monitoring.
Some of the FRMs contained in appendices to Part 50 (such as the
original SO2 FRM and the lead FRM) are manual methods that
are completely specified in a step-by-step manner. Others (such as the
O3 FRM) are in the form of a measurement principle along
with an associated calibration procedure that must be implemented in a
commercially-produced FRM analyzer model. Such FRM-type analyzers must
be tested and shown to meet explicit performance and other
qualification requirements that are set forth in 40 CFR part 53
(Ambient Air Monitoring Reference and Equivalent Methods). Each
analyzer model is then considered to be an FRM only upon specific
designation as an FRM by the EPA under the provisions of 40 CFR 53.2
(General requirements for a reference method determination).
As pollutant measurement technology advances and changes, the
reference methods in part 50 are assessed by the EPA to determine if
improved or more suitable measurement technology is available to better
meet current FRM needs as well as potential future FRM requirements.
New technology can either be presented to the EPA for evaluation by an
FEM applicant under 40 CFR 53.16 (Supersession of reference methods),
or (as in this case) the EPA can originate the process as provided in
40 CFR 53.7 (Testing of methods at the initiative of the
Administrator).
The current FRM for measuring O3 in the ambient air was
promulgated on April 30, 1971 (36 FR 8186), in conjunction with the
EPA's establishment (originally as 42 CFR part 410) of the first
national ambient air quality standards for six criteria pollutants
(including O3), as now set forth in 40 CFR part 50. On
February 8, 1979 (44 FR 8224), the original O3 FRM
calibration procedure was changed from a wet-chemical standard to a UV
photometric calibration procedure. Minor updates to technical
references were made on July 18, 1997 (62 FR 38895). This FRM is
specified as a measurement principle and calibration procedure in
Appendix D of Part 50. The measurement principle of the FRM is based on
the quantitative measurement of chemiluminescent light intensity
emitted by the chemical reaction of O3 in an air sample with
ethylene gas mixed in a measurement cell. This ET-CL measurement is
calibrated by the specified calibration procedure, which is based on
photometric assay of O3 calibration concentrations in a
dynamic flowing system, using measurement of the absorption of UV light
by the O3 calibration concentrations at a nominal wavelength
of 254 nm.
At the time of the FRM's original promulgation, analyzers based on
the ET-CL FRM were widely used for field monitoring of O3.
Laboratory testing prior to, during, and following analyzer development
indicated that interferences to which the method was susceptible were
few and relatively minor in magnitude. Further, subsequent field
experience with the FRM analyzers showed them to be stable, accurate,
and reliable. Operation of these FRM analyzers requires a supply of
ethylene gas, provided by an attendant high-pressure compressed gas
cylinder. Installation of this high-pressure cylinder of flammable and
potentially explosive gas proved problematic at many field-monitoring
sites due to fire codes or other safety restrictions. Further, the
ethylene gas cylinder required periodic replacement--a considerable
cost and operational inconvenience.
Following the development of FEM O3 analyzers based on
UV absorption, use of these newer UV FEM analyzers eventually
supplanted the ET-CL FRM analyzers because the UV analyzers required no
gas supply or other reagents and were much easier to install and
operate. Currently, nearly all compliance monitoring in the U.S. is
carried out with UV absorption type FEM analyzers (Long, 2014). This
transition from ET-CL FRM analyzers to UV absorption analyzers in U.S.
(as well as world-wide) monitoring networks has become so extensive
that analyzer manufacturers no longer manufacture the ET-CL FRM
analyzers. The last new O3 FRM analyzer was designated by
EPA in 1979. As a result, no FRM O3 analyzers are
commercially available to serve as reference standards for testing and
designation of new O3 FEM analyzers, for O3
compliance monitoring, and for quality assurance of field monitors. FRM
units manufactured years ago are becoming increasingly difficult to
maintain in operational condition due to aging of components and lack
of replacement parts (several of the original FRM analyzer
manufacturers no longer exist).
Until the last few years, relatively few measurement techniques
have been
[[Page 75367]]
successfully implemented in a continuous ambient O3 analyzer
model that has achieved designation by the EPA as either an FRM or FEM
(U.S. EPA, 2014g). These include the ET-CL technique, the UV absorption
technique and differential optical absorption spectroscopy (DOAS, an
open-path method represented by two FEM instrument models from
different manufacturers). A relatively new technology is nitric oxide
(NO)-O3 chemiluminescence, which is represented by two FEM
instrument models from a single manufacturer. An even newer technology
is a ``scrubberless'' UV absorption technique that is represented by a
single analyzer model for which FEM designation was recently achieved.
As noted above, the ET-CL technique is technically advantageous as
an FRM, but its ethylene supply requirement and the lack of
commercially available analyzers severely limit its ability to fulfill
the needs for an O3 FRM. DOAS analyzers are not suitable for
some FRM purposes because of their open-path nature.
Commercial availability of conventional UV-absorption O3
analyzers is excellent, and their widespread use makes the measurement
technique desirable for consideration as an FRM. However, the technique
is susceptible to potential measurement interference from mercury, some
volatile aromatic hydrocarbons, water, and other compounds that
sometimes occur in ambient air (Spicer et al., 2010). These
interferences are substantially reduced by the use of scrubbers (as
discussed below) in UV FEM analyzers, such that the technique can be
used extensively for compliance monitoring. Although the interferences
are substantially reduced by the use of scrubbers, the potential for
interferences prevents the technique from consideration as an FRM.
It is important to make a distinction between use of the UV-
absorption measurement technique for assay of O3
concentrations, as described in the FRM calibration procedure of Part
50, Appendix D, and use of the UV absorption technique for measurement
of O3 in ambient air. For assay of calibration
concentrations, the technique is used in a system with clean, zero air
(air that must be free of contaminants which would cause a detectable
response from the O3 analyzer) such that potential ambient-
air-borne interferences are not an issue. Under these clean-air
conditions, the UV assay technique is very accurate and highly
reproducible, so much so that the National Institute of Standards and
Technology (NIST) utilizes it for its O3 Standard Reference
Photometer.
In contrast, use of the UV-absorption technique to measure
O3 in ambient air is much more difficult because of the need
to deal with UV-absorbing (and hence potential interfering) species
present in ambient air. Ambient UV O3 monitors typically
suppress interferences by using an ``O3 scrubber'' that
attempts to remove O3 from ambient air without removing
potentially interfering species, to create a zero-O3
reference air that still contains any potentially interfering species.
In a differential measurement process that compares the UV absorption
measurement of O3 in the ambient air sample with that in
this zero-O3 reference air, the net effect of interferences
is minimized by cancellation. FEM analyzers using such O3
scrubbers are able to meet the FEM interference test requirements of 40
CFR part 53 and provide adequate O3 monitoring data at most
typical O3 monitoring sites.
On October 7, 2011, the EPA designated two NO-CL O3
analyzers as FEMs (76 FR 62402). These analyzers use a variation of the
current FRM measurement principle, based on measurement of the
chemiluminescence produced by the chemical reaction of O3
with NO rather than with ethylene. As explained below, the EPA believes
that this variation has performance suitable for an O3 FRM
and offers a substantial implementation advantage over the existing
FRM.
The NO-CL measurement technique for O3 is quite similar
to the existing ET-CL FRM technique, in that both are based on the
measurement of the intensity of the chemiluminescence resulting from a
chemical reaction of a reactant with the O3 in the ambient
air sample. The principle difference is that the reactant is NO rather
than ethylene. As a potential variation of the FRM measurement
principle, the measurement would be calibrated with the same
calibration procedure specified in the FRM.
The performance of NO-CL analyzers has been shown to be very
similar to the performance of ET-CL FRM analyzers, providing stable,
accurate, highly reproducible measurements of ambient O3
with minimal potential interferences (U.S. EPA, 2014h). As with ET-CL,
some minor interference from variable humidity in ambient air can be
minimized with a sample air dryer. The analyzers require a supply of NO
gas, typically from a high-pressure compressed gas cylinder. However,
unlike ethylene, NO is neither flammable nor explosive, so use of the
method in field applications is eased considerably relative to use of
ET-CL analyzers. Nitric oxide gas is toxic, but it is possible to use a
cylinder of much less toxic, non-combustible nitrous oxide
(N2O) gas with a photolytic N2O-to-NO converter
to supply NO gas for the instrument as needed. There will be no
requirement for states to switch to NO-CL analyzers; therefore, UV-
absorption FEM analyzers can still be used for routine O3
monitoring. As noted previously, the EPA has designated two NO-CL FEM
analyzers (from the same manufacturer), both of which would qualify for
re-designation as FRMs if the NO-CL technique is finalized as an FRM.
NO-CL analyzers would then be available for those applications where an
FRM analyzer is needed.
Because of the similarity of the NO-CL technique to the existing
ET-CL technique, the EPA is proposing to amend the ET-CL FRM by adding
the NO-CL technique as a variation to the existing FRM measurement
principle specified in Appendix D of Part 50. The specified calibration
procedure would be applicable to both FRM ET-CL and NO-CL measurement
techniques. Since the existing ET-CL FRM measurement principle remains
a technically adequate FRM, and the proposed new NO-CL FRM is
technically adequate, it is prudent to retain the existing FRM
measurement principle. The designation of all currently designated
O3 FEMs is based on comparison to the ET-CL FRM, so
retention of the ET-CL FRM allows those FEM designations to be
retained.
Adding the proposed NO-CL measurement technique to the current
O3 FRM would allow at least two commercially available FRM
analyzer models (currently FEMs) to be re-designated as FRMs to fulfill
FRM analyzer needs. Some older FRM analyzers based on the existing ET-
CL measurement principle may still be in operable condition, and there
is no technical reason to cancel their designation by withdrawing the
original ET-CL FRM technique. Additionally, retaining the existing ET-
CL FRM technique allows for the possibility of an instrument
manufacturer offering an ET-CL FRM analyzer in the future.
The second of the newly introduced O3 measurement
techniques is known as the scrubberless UV absorption (UV-SL)
technique. It utilizes the UV-absorption measurement technique that is
widely used in O3 monitoring networks. The new UV-SL
technique specifies removal of O3 from the sample air for
the zero reference by a gas-phase reaction with NO rather than via a
conventional chemical scrubber. The NO reacts with
[[Page 75368]]
the O3 much faster than with other potential interfering
compounds and is very effective at removing the O3 without
affecting other compounds that may be present in the ambient air
sample. The differential UV measurement can effectively eliminate
interferences to an insignificant level. Other potential interference
arising from changes in water vapor concentration can be minimized with
a sample air dryer.
The UV-SL technique appears to have characteristics that are
advantageous for meeting the requirements of a new O3 FRM.
Analyzers implementing this technique require a supply of NO (such as a
high-pressure gas cylinder). As noted previously in connection with the
NO-CL technique, NO is neither flammable nor explosive, so use of the
method in field applications is eased considerably relative to use of
ET-CL analyzers. Use of N2O gas, also supplied in compressed
gas cylinders but less toxic than NO, is also possible with a
photolytic N2O to NO converter. One commercially available
UV-SL analyzer was approved as an FEM on June 18, 2014 (79 FR 34734).
The performance of the analyzer, as reported by the manufacturer \246\
and some initial field and laboratory studies performed by the EPA
(U.S. EPA, 2014h), suggests that the analyzer may meet existing, as
well as the proposed, requirements for an O3 FRM.
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\246\ 2B FEM test data via http://www.twobtech.com/model_211.htm.
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The CASAC AMMS provided a peer review of the proposed FRM and
changes to the Part 53 requirements on April 3, 2014. The CASAC AMMS
recommended that the EPA consider the UV-SL as a FRM. The EPA is
independently conducting further laboratory and field tests of the UV-
SL analyzer to verify its performance. Although this new UV-SL
methodology shows substantial promise for future consideration as a new
O3 FRM, there is currently insufficient documented test and
performance information available on the method to propose it as a new
FRM at this time. The EPA is continuing to study the method and assess
its potential suitability as a new O3 FRM, and the EPA
solicits comment on its potential and suitability as an FRM.
The EPA is not proposing to supersede (replace) the existing
O3 FRM measurement principle under the provisions of 40 CFR
53.16. Rather, for the reasons in the preamble and having conducted the
necessary tests, the EPA is proposing, consistent with 40 CFR 53.7, to
revise the existing O3 FRM to widen the scope of its ET-CL
measurement principle to include the NO-CL measurement technique as
well.
Following promulgation of the proposed revised O3 FRM
measurement principle, any new candidate O3 FRM analyzers
would be required to use either the ET-CL or NO-CL measurement
principle, and would also be subject to the O3 FRM
performance requirements proposed in 40 CFR part 53. The FRM
calibration procedure specified in Appendix D would apply to both
O3 FRM measurement techniques.
A substantial number of laboratory tests have confirmed the
excellent performance of the NO-CL analyzers as well as very close
agreement with both ET-CL and UV analyzers in collocated field tests.
Therefore, the EPA believes the proposed FRM measurement principle that
incorporates the NO-CL methodology is the best approach to improve the
availability of FRM analyzers for O3. No other currently
known approach or alternative methodology appears to be more
appropriate for a new FRM. Adding the NO-CL technology to the existing
O3 FRM is also endorsed by the EPA's CASAC AMMS. The EPA
solicits comment on the proposal to retain the existing O3
FRM measurement principle and amend it to include the NO-CL variation
as well. Comments are also solicited on the nature and adequacy of the
proposed revised FRM.
The generic description of the FRM measurement principle for the
existing ET-CL FRM in Appendix D would be amended to include the NO-CL
variation (see the proposed rule text for Appendix D). As noted
previously, the new NO-CL technique would also use the same calibration
procedure in Appendix D and would be similarly coupled with the
explicit O3 FRM analyzer performance requirements specified
in subpart B of 40 CFR part 53. In addition to the incorporation of the
NO-CL methodology, numerous minor clarifications, wording changes,
additional details, and a more refined numbering system are being
proposed for Appendix D. Accordingly, the entire Appendix D is proposed
to be revised as identified in the proposed regulatory text.
Because the new NO-CL technique is proposed to be added to the
existing FRM measurement principle, while the existing ET-CL FRM
principle would be retained and remain in effect, all existing
designated FEM analyzer models will continue their designated status.
Thus, this action would cause no negative consequences on monitoring
agencies, and no disruption of, or required change to, their
O3 monitoring programs. Comparative testing has been carried
out at several field monitoring sites under a variety of ambient
conditions, and the results confirm that the proposed new NO-CL FRM
measurement technique provides ambient O3 measurements that
compare and correlate excellently with measurements using the existing
ET-CL measurement principle, with no significant bias, offset, or
discrepancy (U.S. EPA, 2014h).
E. Revisions to the Procedures for Testing Performance Characteristics
and Determining Comparability Between Candidate Methods and Reference
Methods
The only substantial changes proposed to the requirements of Part
53 are in Tables B-1 and B-3 of Subpart B. Table B-1 has been updated
in recent years with regard to FRM and FEM methods for SO2
(74 FR 64877, December 8, 2009) and CO (76 FR 54294, August 31, 2011).
Similar update changes to Table B-1 are proposed here for
O3. Modest changes proposed for Table B-3 would add new
interferent test concentrations specifically for NO-CL analyzers,
adding a test for NO2. The table would also clarify that the
existing test concentrations apply to ET-CL O3 analyzers.
Figure B-5 is revised to correct a minor inconsistency in the
``Calculations'' column for the two ``Precision'' rows to change ``%
URL'' to ``% Standard Deviation.''
Several changes to the performance requirements given in Table B-1
are proposed for O3. The performance requirements for
``standard range'' instruments would be updated to be more consistent
with current O3 analyzer performance capabilities. The noise
requirement limit would be reduced from 0.005 to 0.001 ppm for
O3 analyzers, the lower detectable limit would be reduced
from 0.010 to 0.003 ppm, and the maximum interference equivalent limits
would be reduced from 0.02 to 0.005 ppm for each potential interfering
agent (interferent). The performance limit requirement for the total of
all interferents is proposed to be withdrawn for O3 methods.
This withdrawal is appropriate because O3 analyzer test
performance, as reported in recent FEM applications, has shown that the
limits established for individual interferents are sufficiently
effective to define adequate analyzer performance, and the separate
limit for the total of all interferences is unnecessary.
Maximum zero drift for O3 analyzers would be reduced
from 0.02 to 0.004 ppm. The existing limit for span drift at 20% of the
upper range limit (URL) is proposed to be withdrawn. Analyzer
performance test results have clearly
[[Page 75369]]
shown that the existing 80% URL limits are fully adequate and better
specify span drift performance and that the 20% URL span drift limits
are ineffective and unnecessary. The span drift limit applicable to
O3 analyzers is proposed to be reduced from 5.0%
to 3.0%. Lag time limits would be reduced from 20 to 2
minutes, and rise and fall time limits would be similarly reduced from
15 to 2 minutes.
For precision, the EPA proposes to change the form of the precision
limit specifications (at both 20% and 80% of URL) for O3
analyzers from ppm to percent (of the URL). This change would make the
limits responsive to higher and lower measurement ranges, as
appropriate, and is consistent with the same change previously made in
the corresponding precision requirements for SO2 and CO
analyzers. Both limits would be set at 2% for O3 analyzers,
which is equivalent to, and, therefore effectively unchanged, from the
existing limits of 0.01 ppm (for a URL of 0.5 ppm). Although the
changes to Part 53 proposed here are generally restricted to methods
for O3, this change in form for the precision limits is
proposed to be extended to methods for NO2 as well, to
simplify Table B-1 and make it consistent for all pollutants covered by
the Table. The precision limits that would be applicable to methods for
NO2 are proposed to be changed to 4% and 6% of the URL (for
20% and 80% of the URL, respectively). These values are exactly
equivalent to the existing limits of 0.020 ppm and 0.030 ppm,
respectively, for the specified URL of 0.5 ppm. Therefore, these
precision limits for NO2 remain effectively unchanged, but
specified as a percent rather than an absolute concentration. A new
footnote is proposed for Table B-1 to clarify that these revised
precision limits are given as ``standard deviation expressed as percent
of the URL.'' Therefore, Figure B-5 will be revised to correct a minor
inconsistency in the ``Calculation'' column for the two ``Precision''
rows to change the ``% URL'' to ``% Standard Deviation.''
The EPA has reviewed the documented performance of currently
designated FRM and FEM methods for O3 (that are still in
commercial production or in service in monitoring networks) and has
verified that all would meet the proposed new performance requirements
for O3 methods (Long, 2014). Therefore, adoption of the
proposed new performance requirements in Table B-1 would not require
the withdrawal or cancelation of the FRM or FEM designation of any such
O3 analyzers.
Finally, to meet a need for analyzers with more sensitive
measurement ranges for monitoring in relatively clean areas, new,
``lower range'' performance limit requirements are proposed for
O3 analyzers. These lower range limits are set forth in a
new ``lower range'' column in Table B-1 and would be optional. But
where a lower measurement range is included in the FRM or FEM
designation, these proposed new requirements would provide more
stringent performance for analyzers commensurate with greater accuracy
for low-level measurements in lower-level concentration ranges.
The EPA believes that these proposed changes in the performance
requirements of Tables B-1 and B-3 are appropriate, based on analyzer
performance data available from analyzer manuals and recent FRM and FEM
applications. The EPA solicits comment as to whether the proposed
changes are reasonable, appropriate, beneficial, and achievable without
undue burden.
The EPA is proposing minor changes to the general provisions in
subpart A of Part 53 to ease the administrative burden associated with
processing and reviewing modification requests to existing FRMs and
FEMs. This change in 40 CFR 53.14(c) will extend the length of time for
the Administrator to take action on a request for modification of a
reference or equivalent method from 30 days to 90 days. Section
53.14(c) would read: ``Within 90 calendar days after receiving a report
under paragraph (a) of this section, the Administrator will take one or
more of the following actions:'' The EPA is also proposing to remove
the obsolete provision that manufacturers who offered PM2.5
or PM10-2.5 samplers or analyzers for sale as part of a FRM
or FEM may continue to do so only so long as updates of the Product
Manufacturing Checklist are submitted annually. This change is
accomplished through the removal of section (i) from 40 CFR 53.9 and
Figure E-2 from subpart E of Part 53.
VII. Implementation of Proposed O3 Standards
The proposed revisions to the primary and secondary O3
NAAQS discussed in sections II.E and IV.G of this preamble, if
finalized, would trigger a process under which states \247\ make
recommendations to the Administrator regarding area designations, and
the EPA promulgates the final area designations. States would also be
required to review, modify, and supplement their existing SIPs. The
proposed O3 NAAQS revisions would also affect the
transportation conformity and general conformity processes. The revised
O3 NAAQS and the subsequent designations process could
affect which preconstruction permitting requirements apply to
O3 in some areas and the nature of those requirements in
others.
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\247\ This and all subsequent references to ``state'' are meant
to include state, local, and tribal agencies responsible for the
implementation of an O3 control program.
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The EPA has regulations in place addressing the requirements for
SIPs and several provisions in these existing rules cover O3
(40 CFR part 51). States likewise have provisions in their SIPs to
address air quality for O3 and to implement the existing
O3 NAAQS. The EPA has also provided general guidance on the
development of SIPs for all pollutants and administration of
construction permitting programs, as well as specific guidance on
implementing the O3 NAAQS in some contexts under the CAA and
the EPA regulations.
When the EPA proposes to revise a NAAQS for a particular criteria
pollutant, it considers the extent to which existing EPA regulations
and guidance are sufficient to implement the standard and whether any
revisions or updates to those regulation and guidance would be helpful
or appropriate in facilitating the implementation of the revised
standard by states. The CAA does not require that the EPA promulgate
new implementing regulations every time that a NAAQS is revised.
Likewise, the CAA does not require the issuance of additional
implementing regulations or guidance by the EPA before a revised NAAQS
becomes effective. Existing EPA regulations may be sufficient in many
cases to enable the EPA and the states to begin the process of
implementing a revised NAAQS. However, where the nature of revisions to
a NAAQS indicate that additional EPA regulations or guidance (or
revisions to existing regulations or guidance) may be helpful to
implement unique aspects of the revised standard, the EPA endeavors to
provide those regulations and guidance in a timely way to facilitate
preparation of SIPs plans. It is important to note, however, that the
existing EPA regulations in 40 CFR part 51 applicable to SIPs generally
and to particular pollutants continue to apply even without such
updates. Accordingly, the discussion below provides the EPA's current
thoughts about the extent to which revisions to existing regulations
and additional guidance might be helpful or appropriate to aid in the
[[Page 75370]]
implementation of a revised O3 NAAQS, should one be
finalized through this rulemaking.
This section provides background information for understanding the
possible implications of the proposed NAAQS changes in some areas, and
describes the EPA's plans for providing revised rules or additional
guidance on some subjects in a timely manner to assist states with
their implementation efforts under the requirements of the CAA. This
section also describes existing EPA interpretations of CAA requirements
and other EPA guidance relevant to implementation of revised
O3 NAAQS. Relevant CAA provisions that provide potential
flexibility with regard to meeting implementation timelines are also
discussed.
This section contains a discussion of how existing requirements to
reduce the impact on O3 concentrations from the stationary
source construction in permit programs under the CAA may be affected by
the proposed revisions of the O3 NAAQS. These are the PSD
and Nonattainment New Source Review (NNSR) programs. To facilitate the
timely implementation of the PSD requirements, the EPA proposes as part
of this rulemaking to add a grandfathering provision to its regulations
that would apply to certain PSD permit applications that are pending on
the effective date of the revised O3 NAAQS. If the proposed
NAAQS revisions are finalized, this grandfathering provision could be
finalized at the same time as the revised NAAQS (see section VII.D of
this preamble).
The EPA intends to propose additional regulations and issue
additional guidance, as necessary, related to the implementation
requirements for any revised O3 NAAQS resulting from this
proposal. The EPA intends to take these actions on a schedule that
provides timely assistance to air agencies. Accordingly, in this
section, the EPA solicits comment on several issues that the agency
anticipates addressing in future guidance or regulatory actions to
assist with implementation of the revised O3 NAAQS. Because
these issues are not relevant to the establishment of the NAAQS, and
the CAA does not require that the EPA provide implementation rules or
guidance for each revised NAAQS, the EPA does not expect to respond,
nor is the agency required to respond, to these comments in the final
action on this proposal. However the EPA expects these comments will be
helpful as future guidance and regulations are developed.
A. NAAQS Implementation Plans
1. Background
As directed by the CAA, reducing pollution to meet national air
quality standards always has been a shared task, one involving the
federal government, states, tribes and local air quality management
agencies. The EPA develops regulations and strategies to reduce
pollution on a broad scale, while states and tribes are responsible for
implementation planning and any additional emission reduction measures
necessary to bring areas into attainment. The agency supports
implementation planning with technical resources and guidance, while
states and local agencies use their knowledge of local needs and
opportunities in designing emission reduction strategies that will work
best for their industries and communities.
This partnership has proved effective since the EPA first issued
O3 standards more than three decades ago. For example, 101
areas were designated as nonattainment for the 1-hour O3
standards issued in 1979. As of the end of 2013, air quality in 98 of
those areas meets the 1-hour standards. The EPA strengthened the
O3 standards in 1997, shifting to an 8-hour standard to
improve public health protection, particularly for children, the
elderly, and other sensitive individuals, against effects such as
reduced lung function and respiratory symptoms, hospital and emergency
room visits for asthma, and possible irreversible damage to the lungs.
The 1997 standards drew significant public attention when they were
proposed, with numerous parties voicing concerns about states' ability
to comply. However, after close collaboration between the EPA, states,
tribes and local governments to reduce O3-forming
pollutants, significant progress has been made. Air quality in 90% of
the original 113 areas designated as nonattainment for the 1997
O3 NAAQS now meets the 1997 standards. The EPA designated 46
areas as nonattainment for the 2008 O3 NAAQS in 2012. We
expect these areas to make similar progress in achieving clean air.
The majority of man-made NOX and VOC emissions that
contribute to O3 formation in the U.S. come from the
following sectors: On-road and nonroad mobile sources, industrial
processes (including solvents), consumer and commercial products, and
the electric power industry. In 2011, the most recent year for which
the National Emissions Inventory (NEI) is available, onroad and nonroad
mobile sources accounted for about 60% of annual NOX
emissions; and the electric power industry accounted for about nearly
15%. With respect to VOC, industrial processes (including solvents)
accounted for about 57% of manmade VOC emissions; and mobile sources
accounted for about 39%. Emissions from natural sources, such as trees,
also comprise around 70% of total VOC emissions nationally, with a
higher proportion during the O3 season and in areas with
more vegetative cover. See section VII.F of this preamble for more
detail on background O3.
Since 2000, the EPA has issued numerous emissions and fuels
standards for on-road and nonroad mobile sources, as well as emissions
standards for many types of stationary sources. Benefits from new
engine standards increase each year as older, more-polluting vehicles
and engines are replaced with newer, cleaner models. Benefits from fuel
programs generally begin as soon as a new fuel is available. The
ongoing emission reductions from federal programs such as these will
provide for substantial emissions reductions well into the future, and
will complement state and local efforts to attain any revised
O3 NAAQS.
Over the past 15 years, the EPA has established new emissions
standards under title II of the CAA, 42 U.S.C. 7521-7574, for numerous
classes of automobile, truck, bus, motorcycle, earth mover, aircraft,
and locomotive engines, and for the fuels used to power these engines.
The EPA also established new standards for the smaller engines used in
small watercraft, and lawn and garden equipment. In March 2008, the EPA
promulgated new standards for locomotive and for marine diesel engines
and in April 2010 the EPA promulgated new standards for Category 3 (C3)
engines installed on U.S. ocean-going vessels and to marine diesel
fuels produced and distributed in the U.S. In September 2011, the EPA
and the National Highway Transportation Safety Administration
established greenhouse gas and fuel efficiency standards for new 2014-
2018 model year medium and heavy-duty engines and vehicles. In addition
to improving fuel efficiency and reducing greenhouse gas emissions this
rule reduces emissions of NOX from the subject vehicles. In
March 2014, the EPA promulgated Tier 3 standards for tailpipe and
evaporative emissions from passenger cars, light-duty trucks, medium-
duty passenger vehicles, and some heavy-duty vehicles. The associated
gasoline sulfur standard will enable more stringent vehicle emissions
standards and will make existing emissions control systems more
effective. Compared to current standards, the VOC and NOX
tailpipe
[[Page 75371]]
standards for light-duty vehicles represent approximately an 80%
reduction from today's fleet average, and the heavy-duty tailpipe
standards represent about a 60% reduction in VOC and NOX.
The emission reductions from all of these mobile source programs
are significant and will continue to be realized throughout the
implementation period for any revised O3 NAAQS. The EPA
projects that between 2011 and 2025, onroad and nonroad mobile
NOX will decline by more than 60% and onroad and nonroad
mobile VOC will decline by more than 50%.\248\
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\248\ ``Regulatory Impact Analysis for the Proposed Revisions to
the National Ambient Air Quality Standards for Ground-Level Ozone,''
(December 2014) at http://www.epa.gov/ttnecas1/ria.html.
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The reduction of VOC emissions from industrial processes has been
achieved either directly or indirectly through implementation of
control technology standards, including maximum achievable control
technology (MACT), reasonably available control technology (RACT), and
best available control technology (BACT) standards; or is anticipated
due to proposed or upcoming proposals based on generally available
control technology or best available controls under provisions related
to consumer and commercial products. These standards have resulted in
VOC emission reductions of almost a million tons per year accumulated
starting in 1997 from a variety of sources including combustion
sources, coating categories, and chemical manufacturing. The EPA also
finalized emission standards and fuel requirements for new stationary
engines. In the area of consumer and commercial products, the EPA
finalized new national VOC emission standards for aerosol coatings in
2008 and will review and revise, as necessary, existing rules for
household and institutional consumer products, architectural and
industrial maintenance coatings, and automobile refinish coatings.
Additionally, in O3 nonattainment areas, we anticipate
reductions of an additional 10,000 tons per year as states adopt rules
implementing control techniques recommendations issued in 2008 for four
additional categories of consumer and commercial products, such as
surface coatings and adhesives used in industrial manufacturing
operations. These emission reductions primarily result from solvent
controls and typically occur where and when the solvent is used, such
as during manufacturing processes.
As noted above, the power industry is responsible for a nearly 15%
of NOX emissions across the U.S. Power industry emission
sources include large electric generating units (EGU) and some large
industrial boilers and turbines. The EPA's Clean Air Interstate Rule
(CAIR), issued on March 10, 2005 (70 FR 25612; May 12, 2005), was
designed to permanently reduce power industry emissions of
NOX in the eastern U.S. The first phase of the cap was to
begin in 2009, and a lower second phase cap was to begin in 2015. The
EPA had projected that by 2015, the CAIR and other programs would
reduce NOX emissions during the O3 season by
about 50% and annual NOX emissions by about 60% from 2003
levels in the Eastern U.S. However, on July 11, 2008, and December 23,
2008, the U.S. Court of Appeals for the DC Circuit (DC Circuit) issued
decisions on petitions for review of the CAIR. In its July 11 opinion,
the court found CAIR unlawful and decided to vacate CAIR and its
associated Federal implementation plans (FIPs) in their entirety. State
of North Carolina v. EPA, 531 F. 3d 896. On December 23, 2008, however,
the court granted EPA's petition for rehearing to the extent that it
remanded without vacatur for EPA to conduct further proceedings
consistent with the Court's prior opinion. Under this decision, CAIR
will remain in place only until replaced by EPA with a rule that is
consistent with the Court's July 11 opinion.
The EPA issued the final CSAPR on July 6, 2011 (76 FR 48208; August
8, 2011), to replace CAIR. CSAPR requires states to significantly
improve air quality by reducing power plant emissions that contribute
to O3 and/or fine particle pollution in other states. CSAPR
requires a total of 28 states to reduce annual SO2
emissions, annual NOX emissions and/or O3 season
NOX emissions to assist downwind states in attaining the
1997 O3 and fine particle and 2006 PM2.5 NAAQS.
On December 30, 2011, the D.C. Circuit issued an order staying CSAPR
and ordering the EPA to continue implementing CAIR. Subsequently, on
August 21, 2012, the D.C. Circuit issued an opinion vacating CSAPR. EME
Homer City Generation LP v. EPA, 696 F. 3d 7. In its decision the Court
again instructed the EPA to continue administering CAIR. The U.S. and
other parties appealed the D.C. Circuit decision to the U.S. Supreme
Court and on April 29, 2014, the U.S. Supreme Court issued an opinion
reversing the judgment of the D.C. Circuit, upholding the EPA's
interpretation of the CAA ``good neighbor'' provision (CAA section 110
(a)(2)(d)(ii)), and remanding the case back to the D.C. Circuit for
further proceedings consistent with the Supreme Court opinion. EME
Homer City Generation LP v. EPA, 134 S. Ct. 1584. On June 26, 2014, the
U.S. Government filed a motion with the D.C. Circuit to lift the stay
of the CSAPR. The D.C. Circuit has since lifted the stay of the rule.
Order, Document #1518738, EME Homer City Generation, L.P. v. EPA, Case
#11-1302 (D.C. Cir. Oct. 23, 2014).
The EPA proposed the Clean Power Plan for existing power plants on
June 2, 2014 (79 FR 34830; June 18, 2014). In this action the EPA
proposed state-specific rate-based goals for CO2 emissions
from the power sector, as well as guidelines for states to follow in
developing plans to achieve the state-specific goals. This rule, as
proposed, would continue progress already underway to reduce
CO2 emissions from existing fossil fuel-fired power plants
in the U.S. Actions taken to comply with the proposed guidelines would
reduce emissions of CO2 and other air pollutants, including
SO2, NOX and directly emitted PM2.5,
from the electric power industry. The EPA estimates that the Clean
Power Plan, as proposed, would reduce precursors for both O3
and particulate matter leading to decreases in the concentrations of
those pollutants of approximately 25% in 2030.
It should also be noted, in general, that new EGUs are subject to
NOX limits under New Source Performance Standards (NSPS)
under CAA section 111, as well as either PSD or NNSR requirements. The
EPA's regulations for commercial, industrial and solid waste
incinerators set standards for NOX and several air toxics
for all commercial incinerators, as required under Section 129 of the
Act. Air toxics rules for industrial boilers will yield co-benefit
NOX reductions as a result of tune-ups and energy efficiency
measures, especially from boilers that burn coal. And several new
source performance standards and air toxics standards are expected to
make further cuts to NOX and VOC emissions from new and
existing sources of pollution. These include upcoming review and
revisions for gas turbines and municipal waste combustors, along with
proposed requirements for the petroleum refining industry. The NSPS and
air toxics standards that have recently taken effect for stationary
engines will also make cuts to NOX and VOC emissions. The
EPA also anticipates reductions in O3 precursors to result
from implementation of the Mercury and Air Toxics Standard rule, as
well as from measures to address Regional Haze best
[[Page 75372]]
available retrofit technology (BART) determinations.
While the EPA uses its regulatory opportunities to reduce
NOX and VOCs, the agency also is pursuing non-regulatory
efforts as we strive toward cleaner air. Energy Star, a joint program
of the EPA and the U.S. Department of Energy, protects the environment
and saves money through energy efficient products and practices.
Improving energy efficiency in homes, buildings and industry helps
reduce all emissions from the power sector--including NOX--
while reducing compliance costs for electricity providers. As part of
its new Advance Program, the EPA is working collaboratively with state,
local, and tribal governments that want to take steps to reduce air
pollution in O3 and particulate matter attainment areas.
Although these areas are not currently subject to nonattainment
planning requirements, Advance Program participants are interested in
undertaking their own planning efforts with the goal of keeping their
air healthy and creating an improved buffer against future air quality
violations. Participating areas are implementing a mix of voluntary and
mandatory measures relating to mobile, area, and point sources as well
as energy efficiency measures, and they are also pursuing education and
awareness programs to improve their communities' understanding of air
quality issues.
The EPA recognizes that a number of areas of the country have been
working to reduce O3 precursors for many years and now may
need to turn to newer, more innovative approaches for reducing
emissions as they develop their implementation plans. These approaches,
such as smart growth policies and renewable energy portfolios, hold
great promise for improved air quality and health, and the EPA is
working with air quality agencies and stakeholders to identify ways to
include these types of programs in implementation plans. For example,
the EPA developed a roadmap for giving SIP credit to energy efficiency/
renewable energy projects.\249\ Recognition of innovative programs will
allow states and tribes to pursue effective strategies that address
some of the more challenging issues affecting air quality, such as land
use planning, ever increasing motor vehicle use, and planning for long-
term energy needs.
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\249\ ``The Roadmap for Incorporating Energy Efficiency/
Renewable Energy Policies and Programs into State and Tribal
Implementation Plans,'' (July 2012) at http://epa.gov/airquality/eere/.
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With respect to agricultural sources, the U.S. Department of
Agriculture (USDA) has approved conservation systems and activities
that reduce agricultural emissions of NOX and VOC. The EPA
recognizes that USDA has been working with the agricultural community
to develop site-specific conservation systems and activities to control
emissions of O3 precursors. The EPA will continue to work
with USDA on these activities with efforts to identify and/or improve
the control efficiencies, prioritize the adoption of these conservation
systems and activities, and ensure that appropriate criteria are used
for identifying the most effective application of conservation systems
and activities.
The EPA will work together with USDA and with states to identify
appropriate measures to meet the primary and secondary standards,
including site-specific conservation systems and activities. Based on
prior experience identifying conservation measures and practices to
meet the PM NAAQS requirements, the EPA will use a similar process to
identify measures that could meet the O3 requirements. The
EPA anticipates that certain USDA approved conservation systems and
activities that reduce agricultural emissions of NOX and VOC
may be able to satisfy the requirements for applicable sources to
implement reasonably available control measures for purposes of
attaining the primary and secondary O3 NAAQS.
The agency also is active in work to reduce the international
transport of O3 and other pollutants that can contribute to
``background'' O3 levels in the U.S. Under the Convention on
Long-Range Transboundary Air Pollution (LRTAP) of the United Nations
Economic Commission for Europe, the U.S. has been a party to the
Protocol to Abate Acidification, Eutrophication, and Ground-level Ozone
(known as the Gothenburg Protocol) since 2005. The U.S. is also active
in the LRTAP Task Force for Hemispheric Transport of Air Pollution,
which in 2010 produced a comprehensive assessment of the
intercontinental transport of air pollution (including O3)
in the northern hemisphere.
The U.S. has worked bilaterally with Canada under the US-Canada Air
Quality Agreement to adopt an Ozone Annex to address transboundary
O3 impacts. The EPA also continues to work with rapidly
growing countries such as China on air quality management activities
and the development of analytical tools to help these countries address
significant air quality problems, including the emissions of
O3-forming pollutants. This work includes supporting China's
efforts to rapidly deploy power plant pollution controls that can
achieve NOX reductions of at least 80 to 90%.
We know that developing the implementation plans that outline the
steps a nonattainment area will take to meet an air quality standard
requires a significant amount of work on the part of state, tribal or
local air agencies. The EPA routinely looks for ways to reduce this
workload, including assisting with air quality modeling by providing
inputs such as emissions, meteorological and boundary conditions; and
sharing national-scale model results that states can leverage in their
development of their attainment demonstrations. At the same time, we
work with air agencies to provide implementation flexibility to the
extent allowed by law.
2. Timing of Rules and Guidance
In public comment periods associated with several recent
rulemakings, the EPA received comments from a variety of states and
organizations asking for rules and guidance associated with a revised
NAAQS to be issued in a timely manner. Although issuance of such rules
and guidance is not a part of the NAAQS review process, National Ass'n
of Manufacturers v. EPA, 750 F. 3d 921, 926-27 (D.C. Cir. 2014), toward
that end the EPA intends to produce appropriate revisions to necessary
implementation rules and provided additional guidance in time frames
that would be more useful to states when developing their
implementation plans than has been the case with some previous rules
and guidance.
Certain requirements under the PSD preconstruction permit review
program apply immediately to a revised NAAQS upon the effective date of
that NAAQS, unless the EPA has established a grandfathering provision
through rulemaking. To ensure a smooth transition to a revised
O3 NAAQS, the EPA is proposing a grandfathering provision
similar to the one finalized in the 2012 PM2.5 NAAQS Rule.
See section VII.D of this preamble for more details on the PSD program.
Promulgation of the NAAQS starts a clock for the EPA to designate
areas as either attainment or nonattainment. State recommendations for
area designations are due to the EPA within 12 months of promulgation
of the NAAQS. In an effort to allow states to make more informed
recommendations, the EPA intends to issue guidance concerning the
designations process within 4 months of promulgation of the NAAQS, or
approximately 8 months before state recommendations are due. The EPA
has issued designation
[[Page 75373]]
guidance for several NAAQS in recent years. While generally the EPA
considers information related to the same factors in making designation
decisions, the guidance is tailored to the particular NAAQS. The EPA
anticipates that the guidance for a revised NAAQS resulting from this
proposal would be similar to the designation guidance for the 2008
O3 NAAQS. The EPA generally completes area designations 2
years after promulgation of a NAAQS. See section VII.C of this preamble
for additional details on designations.
Clean Air Act section 110 requires SIPs to be submitted within 3
years of promulgation of a revised NAAQS. These SIPs are referred to as
``infrastructure SIPs.'' The EPA issued general guidance on submitting
infrastructure SIPs on September 13, 2013.\250\ It should be noted that
this guidance did not address certain state planning and emissions
control requirements related to interstate pollution transport. Should
this guidance need to be modified for this prospective O3
NAAQS, the EPA intends to issue that updated guidance no later than 1
year after promulgation of a revised O3 NAAQS. See section
VII.B.3 of this preamble for additional information on infrastructure
SIPs.
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\250\ See memorandum from Stephen D. Page to Regional Air
Directors, ``Guidance on Infrastructure State Implementation Plan
(SIP) Elements under Clean Air Act Sections 110(a)(1) and
110(a)(2)'' September 13, 2013.
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The EPA intends to propose any appropriate rules for assisting with
implementing any revised O3 NAAQS resulting from this
proposal within 1 year after a revised NAAQS is established. The rules
that EPA is considering, as with implementation of previous NAAQS,
would address nonattainment area classification methodologies, SIP due
dates, attainment dates, and required implementation programs such as
NNSR and conformity. At that same time the EPA intends to address any
modifications needed as a result of this revised NAAQS to guidance
pertaining to developing nonattainment area emissions inventories and
attainment demonstrations, and demonstrating conformity. The EPA
anticipates finalizing these items by the time areas are designated
nonattainment. Finalizing rules and guidance by this time would provide
air agencies with the information to develop any CAA-required SIPs
associated with nonattainment designations. In an area designated as
nonattainment, new major sources and major modifications at existing
sources are required to comply with NNSR requirements including the
application of ``lowest achievable emission rate'' (LAER) and emissions
offsets at ratios prescribed by the CAA. See section VII.B.4 of this
preamble for additional information on nonattainment SIPs.
3. Section 110 State Implementation Plans
The CAA section 110 specifies the general requirements for SIPs.
Within 3 years after the promulgation of revised NAAQS (or such shorter
period as the Administrator may prescribe \251\) each state must adopt
and submit ``infrastructure'' SIPs to the EPA to address the
requirements of section 110(a)(1) and (2), as applicable. These
``infrastructure SIPs'' establish the basic state programs to
implement, maintain, and enforce revised NAAQS and provide assurances
of state resources and authorities. States are to develop and maintain
an air quality management infrastructure that includes enforceable
emission limitations, a permitting program, an ambient monitoring
program, an enforcement program, air quality modeling capabilities, and
adequate personnel, resources, and legal authority. Section 110(b) of
the CAA provides that the EPA may extend the deadline for the
``infrastructure'' SIP submission for a revised secondary standard by
up to 18 months beyond the initial 3 years. If both the primary NAAQS
and a distinct secondary NAAQS are finalized, the EPA currently
believes it would be more efficient for states and the EPA if each
affected state submits a single section 110 infrastructure SIP that
addresses both standards at the same time (i.e., within 3 years of
promulgation of the O3 NAAQS), because the EPA does not at
present discern any need for there to be any significant substantive
difference in the infrastructure SIPs for the two standards. However,
the EPA also recognizes that states may prefer the flexibility to
submit the secondary NAAQS infrastructure SIP at a later date. The EPA
solicits comment on these infrastructure SIP submittal timing
considerations, and specifically on challenges that would justify
needing 18 additional months to complete the submission of an
infrastructure SIP for the secondary standard.
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\251\ While the CAA allows the EPA to set a shorter time for
submission of these SIPs, the EPA does not currently intend to do
so.
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It is the responsibility of each state to review its air quality
management program's infrastructure SIP provisions in light of each
revised NAAQS. Most states have revised and updated their
infrastructure SIPs in recent years to address requirements associated
with recently revised NAAQS. It may be the case that for a number of
infrastructure elements, the state may believe it has adequate state
regulations already adopted and approved into the SIP to address a
particular requirement with respect to the revised O3 NAAQS.
For such portions of the state's infrastructure SIP submittal, the
state may provide a ``certification'' specifying that certain existing
provisions in the SIP are adequate. Although the term ``certification''
does not appear in the CAA as a type of infrastructure SIP submittal,
the EPA sometimes uses the term in the context of infrastructure SIPs,
by policy and convention, to refer to a state's SIP submission. If a
state determines that its existing EPA-approved SIP provisions are
adequate in light of the revised O3 NAAQS with respect to a
given infrastructure SIP element (or sub-element), then the state may
make a ``certification'' that the existing SIP contains provisions that
address those requirements of the specific CAA section 110(a)(2)
infrastructure elements. In the case of a certification, the submittal
does not have to include another copy of the relevant provision (e.g.,
rule or statute) itself. Rather, the submittal may provide citations to
the already SIP-approved state statutes, regulations, or non-regulatory
measures, as appropriate, which meet the relevant CAA requirement. Like
any other SIP submittal, such certification can be made only after the
state has provided reasonable notice and opportunity for public
hearing. This ``reasonable notice and opportunity for public hearing''
requirement for infrastructure SIP submittals appears at section
110(a), and it comports with the more general SIP requirement at
section 110(l) of the CAA. Under the EPA's regulations at 40 CFR part
51, if a public hearing is held, an infrastructure SIP submittal must
include documentation by the state that the public hearing was held in
accordance with the EPA's procedural requirements for public hearings.
See 40 CFR part 51, Appendix V, paragraph 2.1(g), and 40 CFR 51.102.
4. Nonattainment Area Requirements
Part D of the CAA describes the various program requirements that
apply to states with nonattainment areas for different NAAQS. Section
182 (found in subpart 2 of Part D) includes the SIP requirements that
govern the O3 program, and supplements the more general
nonattainment area requirements in sections 172 and 173. Under CAA
section 182, states generally are required to submit attainment
[[Page 75374]]
demonstration SIPs within 3 or 4 years of the effective date of area
designations by the EPA, depending on the classification of the area.
These plans need to show how the nonattainment area will attain the
primary O3 standard ``as expeditiously as practicable,'' but
no later than within the relevant time frame from the effective date of
designations associated with the classification of the area.
Section 181(a)(1) of the CAA establishes classification categories
for areas designated nonattainment for the primary O3 NAAQS.
These categories range from ``Marginal,'' the lowest O3
classification with the fewest requirements associated with it, to
``Extreme,'' the highest classification with the most required
programs. Areas with worse O3 problems are given more time
to attain the NAAQS and more associated emission control requirements.
Pursuant to previous O3 NAAQS reviews, the EPA set the
secondary NAAQS equal to the primary NAAQS. Thus, previous
implementation programs for O3 standards did not include a
separate classification threshold methodology for the secondary NAAQS.
For this NAAQS review, which may result in a secondary standard
different in form and level compared to the primary standard, the EPA
is considering methodologies for establishing the air-quality based
thresholds for assigning the section 181 classifications to areas out
of attainment with a secondary O3 NAAQS. Any such methods
would be proposed for comment if the EPA finalizes a distinct secondary
NAAQS.
There are two main EPA rulemakings relating to implementation of
the 2008 O3 NAAQS. In May 2012, the EPA issued the final
Classifications Rule (77 FR 30160; May 21, 2012). The Classifications
Rule detailed the classifications approach, established attainment
deadlines and revoked the 1997 O3 NAAQS for purposes of
transportation conformity. In June 2013, the EPA proposed a SIP
Requirements Rule (78 FR 34178; June 6, 2013) to provide rules and
guidance to states regarding development of SIPs to attain the 2008
O3 NAAQS. The EPA believes that the overall framework and
policy approach of the proposed SIP Requirements Rule for the 2008
O3 NAAQS provides an effective and appropriate template for
the general approach states would follow in planning for attainment of
a revised primary O3 standard. The EPA intends to develop
and propose a new SIP Requirements Rule that will address, to the
extent necessary, any new implementation requirements that would result
from any revised O3 NAAQS. The EPA intends to propose this
implementation rule within 1 year after the revised O3 NAAQS
are promulgated, and finalize the implementation rule by no later than
the time the area designations process is finalized (approximately 2
years after promulgation of the O3 NAAQS).
In general, when developing an attainment plan, the state begins
with the evaluation of the air quality improvements the nonattainment
area can expect in the future due to ``on the books'' existing federal,
state, and local emission reduction measures. The state then must
conduct a further assessment of relevant NOX and VOC
emission sources in the nonattainment area, and the additional
reasonably available control measures (RACM) and reasonably available
control technology (RACT) that can be implemented by these sources, in
determining how soon the area can attain the standard. Under section
172(c)(1) of the CAA as interpreted by the EPA, attainment
demonstrations must include a RACM analysis showing that no additional
reasonably available measures could be adopted and implemented such
that the SIP could specify an attainment date that is 1 or more years
earlier.
The evaluation of these potential emissions reductions and
associated air quality improvement is commonly performed with
sophisticated air quality modeling tools. Given that O3
concentrations are affected both by regionally-transported
O3 and O3 precursor emissions and emissions of
precursors from local sources in the nonattainment area (e.g.,
industrial sources, EGUs, and on-road mobile sources), the EPA
recommends the use of regional grid-based models (such as CMAQ and
CAMx) to develop O3 attainment strategies. Although, as
described above, the EPA projects significant improvements in
O3 concentrations regionally resulting from a number of
ongoing emission reduction programs already in place (e.g., mobile
source engine and fuel standards and regulations for power plants) and
from a number of recently promulgated rules such as the Cross State Air
Pollution Rule (76 FR 48208; August 8, 2011), the Mercury and Air
Toxics Standards rule (77 FR 9304; February 16, 2012) and the Tier 3
rule (79 FR 23414; April 28, 2014) that will result in VOC and
NOX reductions from many geographically dispersed sources,
local reductions of direct O3 precursors can also result in
important health benefits.
States must also ensure that a nonattainment area will make
``reasonable further progress'' (RFP) in accordance with subpart 2 of
the CAA from the time of the nonattainment designation to its
attainment date. The amount of RFP required is based on the
classification of the nonattainment area. Under the approach outlined
in the proposed SIP Requirements Rule for the 2008 O3 NAAQS,
areas designated nonattainment and classified as Moderate would
generally be required to reduce emissions by 15% over the first six
years after the effective date of designations. Areas classified higher
than Moderate would be required to produce additional emission
reductions after this 6-year period for an area that average 3%
reductions per year. All RFP and attainment plans must also include
contingency measures which would apply without significant delay in the
event the area fails to attain by its attainment date or meet RFP
milestones.
The EPA expects that the same general approach for determining
attainment of the previous 1997 and 2008 8-hour O3 primary
standards by the attainment deadline would be followed for determining
attainment with any revised primary O3 standard. Attainment
would be evaluated based on the 3 most recent years of certified,
complete, and quality-assured air quality data in the nonattainment
area. Areas are able to obtain up to two 1-year attainment date
extensions provided under CAA section 181 under certain circumstances.
Under previous 8-hour O3 NAAQS rules, an area whose design
value based on the most recent 3 years of data exceeds the standard
could receive a 1-year attainment date extension if the air quality
concentration for the third year alone does not exceed the level of the
standard. Similarly, an area that has received a 1-year extension could
receive a second 1-year extension if the average of the area's air
quality concentration in the ``extension year'' and the previous year
does not exceed the level of the standard.
B. Implementing a Distinct Secondary O3 NAAQS, if One Is Established
In each of the previous O3 NAAQS reviews the secondary
standard was set equal to the primary standard. As discussed in section
IV of this preamble, the EPA is proposing to retain the current
averaging time and form of the secondary standard and to revise the
level. The EPA is also soliciting comment on the alternative approach
of revising the secondary standard to a cumulative, seasonal,
concentration-weighted form based on the W126 index.
If the EPA were to establish a distinct secondary standard, there
would be
[[Page 75375]]
unique implementation issues to consider. These could include issues
related to, but not limited to, PSD implementation, nonattainment area
classification thresholds, attainment planning, and conformity
demonstrations. These issues would be addressed in future
implementation rules and guidance, as necessary. The EPA solicits
comments on the specific kinds of implementation-related issues (with
examples, where possible) that air agencies and affected sources would
face if a separate and distinct secondary standard is established.
C. Designation of Areas
After the EPA establishes or revises a NAAQS, the CAA directs the
EPA and the states to take steps to ensure that the new or revised
NAAQS is met. One of the first steps, known as the initial area
designations, involves identifying areas of the country that either do
not meet the new or revised NAAQS along with the nearby areas that
contribute to the violations.
Section 107(d)(1) of the CAA provides that, ``By such date as the
Administrator may reasonably require, but not later than 1 year after
promulgation of a new or revised national ambient air quality standard
for any pollutant under section 109, the Governor of each state shall .
. . submit to the Administrator a list of all areas (or portions
thereof) in the state'' that designates those areas as nonattainment,
attainment, or unclassifiable. The EPA must then promulgate the area
designations according to a specified process, including procedures to
be followed if the EPA intends to modify a recommendation. The CAA
defines an area as nonattainment if it is violating the NAAQS or if it
is contributing to a violation in a nearby area.
Section 107(d)(1)(B)(i) further provides, ``Upon promulgation or
revision of a national ambient air quality standard, the Administrator
shall promulgate the designations of all areas (or portions thereof) .
. . as expeditiously as practicable, but in no case later than 2 years
from the date of promulgation of the new or revised national ambient
air quality standard. Such period may be extended for up to one year in
the event the Administrator has insufficient information to promulgate
the designations.'' In certain contexts, with respect to the NAAQS, the
term ``promulgation'' has been interpreted by the courts to be
signature and widespread dissemination of a final NAAQS rule.\252\ By
no later than 120 days prior to promulgating area designations, the EPA
is required to notify states of any intended modifications to their
recommendations that the EPA may deem necessary. States then have an
opportunity to demonstrate why any proposed modification is
inappropriate. Whether or not a state provides a recommendation, the
EPA must timely promulgate the designation that the agency deems
appropriate.
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\252\ American Petroleum Institute v. Costle, 609 F.2d 20 (D.C.
Cir. 1979).
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While section 107 of the CAA specifically addresses states, the EPA
intends to follow the same process for tribes to the extent
practicable, pursuant to CAA section 301(d) regarding tribal authority
and the Tribal Authority Rule (63 FR 7254, February 12, 1998). To
provide clarity and consistency in doing so, the EPA issued a 2011
guidance memorandum on working with tribes during the designation
process.\253\
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\253\ Page, S. (2011). Guidance to Regions for Working with
Tribes during the National Ambient Air Quality Standards (NAAQS)
Designations Process, Memorandum from Stephen D. Page, Director, EPA
Office of Air Quality Planning and Standards to Regional Air
Directors, Regions I-X, December 20, 2011. Available: http://www.epa.gov/ttn/oarpg/t1/memoranda/20120117naaqsguidance.pdf.
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As discussed in sections II and IV of this preamble, the EPA is
proposing to revise both the primary and secondary O3 NAAQS,
which currently are identical 8-hour standards that were set at 0.075
ppm in the 2008 NAAQS rule (73 FR 16436; March 27, 2008). If the EPA
revises the primary and secondary O3 NAAQS based on this
proposal, the EPA intends to complete designations for both NAAQS
following the standard 2-year process discussed above. The EPA is
required to sign the final rule for this O3 NAAQS review no
later than October 1, 2015, under a court-ordered deadline. In
accordance with section 107(d)(4) of the CAA, state Governors (and
tribes, if they choose) should submit their initial designation
recommendations for a revised primary and secondary NAAQS no later than
1 year after promulgation of any revised O3 NAAQS (for
example, by October 1, 2016, if the EPA promulgates such NAAQS on
October 1, 2015.) If the EPA intends to modify any state
recommendation, the EPA would notify the appropriate state Governor (or
tribal leader) no later than 120 days prior to making final designation
decisions. A state or tribe that believes the modification is
inappropriate would then have the opportunity to demonstrate to EPA why
it believes its original recommendation (or a revised recommendation)
is more appropriate. The EPA would take any additional input into
account in making the final designation decisions.
Consistent with previous designations, the EPA intends to use area-
specific multi-factor analyses to support area boundary decisions for
any revised primary or secondary O3 standards. Historically,
the EPA has evaluated information related to the following factors for
designations: air quality data, emissions-related data, meteorology,
geography/topography, and jurisdictional boundaries. The EPA solicits
comment related to establishing area designation boundaries for the
proposed revised primary and secondary NAAQS, including any relevant
technical information that should be considered by the EPA and the
extent to which different considerations may be relevant to
establishing boundaries for a distinct secondary NAAQS. As noted
earlier, the EPA intends to issue designation guidance to the states
shortly after the promulgation of any revised O3 NAAQS to
provide information on the designation process and to assist states in
developing their recommendations. The EPA invites preliminary comment
on all aspects of the designation process at this time, which the EPA
will consider in developing that guidance.
D. Prevention of Significant Deterioration and Nonattainment New Source
Review Programs for the Proposed Revised Primary and Secondary O3 NAAQS
The CAA, at parts C and D of title I, contains NSR requirements
that constitute preconstruction review and permitting programs
applicable to new major stationary sources and major modifications of
existing major sources. The preconstruction review of each new major
source and major modification generally applies on a pollutant-specific
basis, and the requirements that apply for each pollutant generally
depend on whether the area is designated as attainment (or
unclassifiable) or nonattainment for that pollutant. For the
O3 NAAQS, in areas designated attainment and unclassifiable,
the PSD requirements under part C apply. In nonattainment areas for
O3, the NNSR requirements under part D apply. Collectively,
those two sets of permit requirements are commonly referred to as the
``major NSR programs.''
Until areas are designated for the proposed revised O3
NAAQS, the NSR provisions applicable under an area's designation for
the 2008 NAAQS (including any applicable anti-backsliding requirements)
would continue to apply. See 40 CFR 51.166(i)(2) and 52.21(i)(2). When
the
[[Page 75376]]
new designations for any revised O3 NAAQS are effective,
they generally will serve to determine whether the PSD or nonattainment
NSR program applies.
1. Prevention of Significant Deterioration (PSD)
The statutory requirements for a PSD permit program set forth under
part C (sections 160 through 169 of the CAA) are addressed by the EPA's
PSD regulations found at 40 CFR 51.166 (minimum requirements for an
approvable SIP) and 40 CFR 52.21 (federal PSD permit program for areas
lacking an EPA-approved PSD program in the applicable SIP and for lands
owned by the federal government and tribal lands). Both sets of
regulations already apply to O3. See 40 CFR 51.166(b)(23),
(49); 40 CFR 52.21(b)(23), (50).. Among other things, in
attainment and unclassifiable areas, the PSD program requires a new
major stationary source or a major modification to an existing major
source to apply BACT for each applicable pollutant and to conduct an
air quality impact analysis to demonstrate that the proposed source or
project will not cause or contribute to a violation of any NAAQS or PSD
increment (see CAA section 165(a)(3)-(4), 40 CFR 51.166(j)-(k), 40 CFR
52.21(j)-(k)). PSD requirements may also include, in appropriate cases,
an analysis of potential adverse impacts on Class I areas (see CAA
sections 162 and 165).\254\ These existing requirements of the PSD
program will remain applicable to O3 and the demonstration
required under 40 CFR 51.166(k) and 52.21(k) will apply to any revised
O3 NAAQS when such NAAQS become effective, except to the
extent that a pending permit application is subject to a grandfathering
provision that the EPA establishes through rulemaking.
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\254\ Congress established certain Class I areas in section
162(a) of the CAA, including international parks, national
wilderness areas, and national parks that meet certain criteria.
Such Class I areas, known as mandatory federal Class I areas, are
afforded special protection under the CAA. In addition, states and
tribal governments may establish Class I areas within their own
political jurisdictions to provide similar special air quality
protection.
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To address ambient O3 impacts of VOC and NOX
precursor emissions from individual stationary sources, Appendix W to
40 CFR part 51 currently directs states to consult with the applicable
EPA Regional Office to determine the appropriate techniques on a case-
by-case basis, which may or may not involve the use of air quality
models, for evaluating whether a PSD source causes or contributes to a
violation of the O3 NAAQS (40 CFR part 51, Appendix W,
section 5.2.1.c). At present, the EPA is evaluating the models and
techniques available to address atmospheric chemistry of O3
formation in assessing such single source impacts, and as part of that
evaluation has conducted discussions of such tools with the regulatory
modeling community. Consistent with its commitment to engage in a
rulemaking process to determine whether updates to Appendix W in 40 CFR
part 51 are warranted,\255\ the EPA is planning to propose a rulemaking
in the spring of 2015 to consider whether to update Appendix W. If the
EPA concludes that it is technically and scientifically appropriate, it
will propose appropriate regulatory updates to Appendix W as part of
that rulemaking and may also make related updates to technical
guidance, as appropriate. In the meantime, in order to demonstrate that
a proposed source or modification does not cause or contribute to a
violation of the applicable O3 NAAQS, PSD permit applicants
would follow the current provisions in Appendix W until any revisions
to them are in effect.
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\255\ See Letter from Gina McCarthy, Assistant Administrator, to
Robert Ukeiley, at 1 (Jan. 4, 2012), available at http://www.epa.gov/scram001/10thmodconf/review_material/Sierra_Club_Petition_OAR-11-002-1093.pdf.
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For PSD, a ``major stationary source'' is one with the potential to
emit 250 tons per year (TPY) or more of any regulated NSR pollutant,
unless the new or modified source is classified under a list of 28
source categories contained in the statutory definition of ``major
emitting facility'' in section 169(1) of the CAA. For those 28 source
categories, a ``major stationary source'' is one with the potential to
emit 100 TPY or more of any regulated NSR pollutant. A ``major
modification'' is a physical change or a change in the method of
operation of an existing major stationary source that results first, in
a significant emissions increase of a regulated NSR pollutant at a
project, and second, in a significant net emissions increase of that
pollutant at the source.\256\ See 40 CFR 51.166(b)(2)(i), 40 CFR
52.21(b)(2)(i).
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\256\ As explained in 40 CFR 52.21(a)(2)(iv)(a) and
51.166(a)(7)(iv)(a), ``[t]he project is not a major modification if
it does not cause a significant emissions increase. If the project
causes a significant emissions increase, then the project is a major
modification only if it also results in a significant net emissions
increase.'' The PSD regulations at 40 CFR 51.166(a)(7) and
52.21(a)(2) also explain in more detail the two-pronged test for
determining whether a proposed project at a facility is a major
modification.
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The EPA's regulations define the term ``regulated NSR pollutant''
to include ``[a]ny pollutant for which a [NAAQS] has been promulgated
and any pollutant identified [in EPA regulations] as a constituent or
precursor to such pollutant'' (40 CFR 51.166(b)(49); 40 CFR
52.21(b)(50)). These regulations identify VOC and NOX as
precursors to O3 in all attainment and unclassifiable areas
(40 CFR 51.166(b)(49)(i)(a); 40 CFR 52.21(b)(50)(i)(a)). Thus, for
O3, the PSD program currently requires the review and
control of emissions of VOC and NOX, as applicable, as
precursors of O3.
As noted above, section 165(a)(3) of the CAA and the implementing
PSD regulations require the owner or operator of a proposed facility
to, among other things, demonstrate that ``emissions from construction
or operation of such facility will not cause, or contribute to, air
pollution in excess of any . . . national ambient air quality standard
in any air control region.'' See also 40 CFR 51.166(k), 40 CFR
52.21(k). The EPA has interpreted this requirement to include any NAAQS
that is in effect as of the date a permit is issued, unless it has
grandfathered permit applications from the requirement to demonstrate
that the proposed facility does not cause or contribute to a violation
of the new or revised NAAQS.\257\ See, e.g., 73 FR 28321, 28324, 28340
(May 16, 2008); 78 FR 3253 (Jan. 15, 2013); Memorandum from Stephen D.
Page, Director, Office of Air Quality Planning & Standards,
``Applicability of the Federal Prevention of Significant Deterioration
Permit Requirements to New and Revised National Ambient Air Quality
Standards'' (April 1, 2010). Consistent with this interpretation, any
revised O3 NAAQS finalized through this rulemaking will need
to be addressed by PSD permit applicants and permitting authorities, in
permits issued on or after the date when the revised NAAQS become
effective, unless the permit application has been grandfathered through
rulemaking, as described below in this proposal.
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\257\ In the past the EPA has asserted the discretion to take
such grandfathering action, under appropriate circumstances, either
by rulemaking or through a case-by-case determination for a specific
permit application. The United States Court of Appeals for the Ninth
Circuit recently vacated a decision by the EPA to issue an
individual PSD permit grandfathering a permit applicant from certain
requirements. See Sierra Club v. EPA, 762 F.3d 971 (9th Cir. 2014).
In light of that decision, the EPA is no longer asserting authority
to grandfather permit applications on a case-by-case basis. However,
in the same opinion the court also stated that it did ``not doubt,
or express any opinion on, the EPA's traditional authority to employ
formal rulemaking to implement grandfathering'' and distinguished
that authority from the permit-specific grandfathering at issue in
the case before it. Id., at 982, n. 7 & 982-983. Thus, the EPA does
not interpret this opinion to limit its authority to grandfather
through rulemaking, but rather believes that the decision offers
support for such authority.
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Because the complex chemistry of O3 formation poses
significant challenges
[[Page 75377]]
for the assessing the impacts of individual stationary sources on
O3 formation, the EPA's judgment has been that it was not
technically sound to designate a specific air quality model that must
be used in the PSD permitting process to make this demonstration for
O3.\258\ The EPA has explained that sources must make the
demonstration required under CAA section 165(a)(3) and the implementing
regulations, that this demonstration necessarily involves an analysis,
and has established a process to determine on a case-by-case basis, in
consultation with the appropriate EPA Regional Office, what analytical
techniques should be used to assess the impact of an individual source
\259\ (40 CFR part 51, Appendix W, Section 5.2.1.c). The EPA has,
however, granted a petition from Sierra Club requesting, among other
things, that it initiate rulemaking to designate air quality models for
O3, and consistent with that petition grant, has been going
through a process to evaluate potential updates to Appendix W.\260\
While that process is underway, individual sources should continue to
follow the existing procedures to determine what method is appropriate
to use to evaluate their impacts on O3 formation.
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\258\ Letter from Gina McCarthy, Assistant Administrator, to
Robert Ukeiley, at 2 (Jan. 4, 2012), available at http://www.epa.gov/scram001/10thmodconf/review_material/Sierra_Club_Petition_OAR-11-002-1093.pdf.
\259\ See, e.g., id.
\260\ See id. at 1 and 3.
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The PSD rules in 40 CFR 51.166(i)(2) and 52.21(i)(2) contain an
exemption for particular pollutants from the PSD requirements if the
owner or operator of the source demonstrates that the area in which the
facility is located is designated as nonattainment for that pollutant
under CAA section 107. Thus, new major sources and modifications will
generally be subject to the PSD program requirements for O3
if they are locating in an area that does not have a current
nonattainment designation under CAA section 107 for O3. As
explained in the recent proposal for the implementation rule for the
2008 O3 NAAQS, references to historical nonattainment
designations for a revoked standard should not be viewed as current
``nonattainment designation[s] under CAA section 107'' within the
meaning of 40 CFR 51.166(i)(2) and 52.21(i)(2) and, therefore, do not
trigger the exemption from PSD requirements that would otherwise result
from those provisions (78 FR 34216, June 6, 2013).
a. PSD Grandfathering Provision
Recognizing that some PSD applications may have already been
submitted and could be in the review process when a revised
O3 NAAQS becomes effective, the EPA is proposing a
transition plan that would enable certain PSD applications to make the
demonstration that the proposed project will not cause or contribute to
a violation of any NAAQS with respect to the O3 NAAQS that
were in effect on the date the reviewing authority determines the
permit application complete or the date the public notice on the draft
permit or preliminary determination is first published (depending on
which grandfathering provision applies), rather than the revised
O3 NAAQS.\261\
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\261\ The proposed grandfathering provision is intended to apply
to pending PSD permit applications that meet one or both of the
specified criteria and that are for sources locating in areas where
PSD continues to apply with respect to O3 at the time the
permit is issued. The proposed grandfathering provision is not
intended to apply to sources locating in areas where NNSR applies at
the time of permit issuance (for example, if the area had been
designated as attainment for O3 when the permit
application was submitted but was subsequently designated as
nonattainment for O3 and that nonattainment designation
would be in effect when the permit would be issued). For such
sources, the permit application must be resubmitted in accordance
with the applicable NNSR requirements.
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The EPA is proposing and taking comment on adding a grandfathering
provision to EPA's regulations at 40 CFR 51.166 and 52.21 that would
apply specifically to two categories of PSD permit applications that
are pending when the EPA issues the revised O3 NAAQS: (1)
Applications for which the reviewing authority has formally determined
that the application is complete on or before the signature date of the
revised NAAQS; and (2) applications for which the reviewing authority
has first published a public notice of a draft permit or preliminary
determination before the effective date of the revised NAAQS. These two
categories are proposed because some states do not do completeness
determinations as part of their permit process.
As explained above, the EPA interprets the CAA and implementing PSD
regulations at 40 CFR 52.21(k)(1) and 51.166(k)(1) to require that PSD
permit applications must include a demonstration that new major sources
and major modifications will not cause or contribute to a violation of
any NAAQS that is in effect as of the date the PSD permit is issued.
Thus, if the EPA revises the O3 NAAQS, any proposed new
source or modification with a PSD permit application pending at the
time the revised O3 NAAQS takes effect would be expected to
conduct an analysis to demonstrate that it does not cause or contribute
to a violation of that NAAQS, absent some type of transition provision
exempting the application from that requirement. This demonstration, as
noted above, should be completed in consultation with the applicable
EPA Regional Office.
Nevertheless, the agency has previously recognized that the CAA
provides discretion for the EPA to grandfather PSD permit applications
from requirements that become applicable while the applications are
pending (45 FR 52683, August 7, 1980; 52 FR 24672, July 1, 1987; 78 FR
3086, January 15, 2013). As discussed in more detail in these
referenced actions, section 165(a)(3) of the CAA requires that a permit
applicant demonstrate that its proposed project will not cause or
contribute to a violation of any NAAQS. At the same time, section
165(c) of the CAA requires that a PSD permit be granted or denied
within 1 year after the permitting authority determines the application
for such permit to be complete. In addition, section 301 of the CAA
authorizes the Administrator ``to prescribe such regulations as are
necessary to carry out his functions under this chapter.'' When read in
combination, these three provisions of the CAA provide the EPA with the
discretion to issue regulations to grandfather pending permit
applications from having to address a revised NAAQS where necessary to
achieve both CAA objectives to protect the NAAQS and to avoid delays in
processing PSD permit applications. Moreover, in a recent opinion the
U.S. Court of Appeals for the Ninth Circuit recognized the EPA's
traditional exercise of grandfathering authority through rulemaking and
indicated that this approach was consistent with statutory requirement
to ``enforce whatever regulations are in effect at the time the agency
makes a final decision'' because it involved identifying ``an operative
date, incident to setting the new substantive standard, and the
grandfathering of pending permit applications was explicitly built into
the new regulations.'' Sierra Club v. EPA, 762 F.3d 971, 983 (9th Cir.
2014).
In the EPA's most recent adoption of a grandfathering provision for
PSD, it adopted a provision for PM2.5 that provides a
reasonable transition for implementing certain new PSD requirements
related to the 2012 PM2.5 NAAQS for pending permit
applications that have met certain criteria. As finalized, the
PM2.5 grandfathering provision included the same two
categories of permit applications that
[[Page 75378]]
today are being proposed for O3. See 40 CFR 51.166(i)(10)
and 52.21(i)(11). In the rulemaking, adding the grandfathering
provision for the PM2.5 NAAQS, the EPA also provided a
detailed rationale and legal basis for including the grandfathering
provision in the PSD program. See 78 FR 3087 at 3253-59 (January 15,
2013); see also 77 FR 39023-24 (June 29, 2012).
When the PM2.5 NAAQS grandfathering provision was
originally proposed, the EPA provided for only one category of pending
PSD applications--applications for which the reviewing authority has
published a public notice on the draft permit prior to the effective
date of the revised PM2.5 NAAQS. A majority of the
commenters supported the adoption of a grandfathering provision but
some responded that a grandfathering milestone based on the submittal
of a complete application would be more appropriate in order to avoid
significant burdens associated with having to withdraw an application.
These commenters pointed out the significant level of effort, resources
and time involved in preparing all of the information necessary for a
complete permit application. They claimed that it would be unfair to
establish grandfathering milestones beyond the complete application
date because the processes and timeframe involved in generating the
draft permit or preliminary determination materials and publishing the
public notice are largely out of the control of the permit applicant
and vary from agency to agency.
Based on this and other pertinent information provided by the
commenters, the EPA concluded in that rulemaking that it should add an
additional grandfathering milestone to avoid substantial additional
burden and delay for permit applications that have reached a stage in
the review process by which significant resources have been expended to
complete PSD analyses and demonstrations that would have to be redone
to address the revised NAAQS. Accordingly, the EPA adopted a
grandfathering provision for the PM2.5 NAAQS in the final
rule that included two milestones for establishing grandfathering
eligibility. The EPA believes that these considerations and this
rationale also apply to pending PSD permit applications that would be
affected by a revised O3 NAAQS. Accordingly, the EPA is
proposing to apply these same two milestones in this proposed
rulemaking for the revised O3 NAAQS.
The proposed grandfathering provision does not apply to any
applicable PSD requirements related to O3 other than the
requirement to demonstrate that the proposed source does not cause or
contribute to a violation of any revised O3 NAAQS. Sources
with projects qualifying under the grandfathering provision will be
required to apply BACT to all applicable pollutants, demonstrate that
the project emissions will not cause or contribute to a violation of
the existing O3 NAAQS, and address any Class I area and
additional O3-related impacts in accordance with the PSD
regulatory requirements.
For the reasons provided both herein and in the prior EPA actions
referenced above, the EPA proposes to amend the federal PSD permitting
regulations at 40 CFR 52.21 to add the described grandfathering
provision for the proposed O3 NAAQS revision. Specifically,
the proposed provision provides that qualifying new sources and
modifications seeking PSD permits under 40 CFR 52.21 shall not be
required to demonstrate that their proposed emissions will not cause or
contribute to a violation of the revised O3 NAAQS, but
instead must demonstrate that their proposed emissions will not cause
or contribute to a violation of the O3 NAAQS in effect on
the date the reviewing authority determines the permit application
complete or the date the public notice on the draft permit or
preliminary determination is first published, depending on which prong
of the grandfathering provision is applicable for that source. See
proposed 40 CFR 52.21(i)(12).
For sources subject to the PSD program under section 52.21, it
should be noted that the EPA intends for a source that satisfies either
milestone in the proposed revisions to section 52.21(i) to be
grandfathered from this requirement if those revisions are finalized.
Accordingly, if a particular source does not qualify under the first
milestone based on a complete application, it may qualify under the
second milestone based on the issuance of a public notice. Conversely,
a source may qualify for grandfathering under the first milestone, even
if it does not satisfy the second. As explained below, states with EPA-
approved PSD programs in their SIPs would have additional flexibility
for implementing the proposed grandfathering provision to the extent
that any alternative approach is at least as stringent as the federal
provision.
The EPA also proposes that states that issue PSD permits under a
SIP-approved PSD permit program should have discretion to
``grandfather'' pending PSD permits in the same manner under these same
circumstances. Therefore, the EPA is proposing to revise its rules at
40 CFR 51.166 to provide a comparable exemption applicable to SIP-
approved PSD programs, although such states are under no obligation to
grandfather. See proposed 40 CFR 51.166(i)(11). The EPA recognizes that
such states interested in grandfathering PSD sources for O3
will not have time to revise their rules and submit them to the EPA for
approval into the SIP, since the need to grandfather sources will occur
immediately upon the effective date of the revised O3 NAAQS.
As explained in an earlier rulemaking, the EPA believes that states
implementing a SIP-approved PSD program have the discretion to allow
grandfathering consistent with the grandfathering provision contained
in the federal rule provisions, even in the absence of an express
grandfathering provision in their state rules, if the particular
state's laws and regulations may be interpreted to provide such
discretion. See 78 FR 3086 at 3258.
Because state SIPs cannot be less stringent than federal
requirements, the states' discretion must be limited to applying
grandfathering consistent with the federal rule provisions for
O3. However, we believe that such consistent application
affords states with ample flexibility for implementing the provision.
Accordingly, a state may elect to apply both milestones or it may elect
to rely solely upon one of the milestones for grandfathering PSD
permits for O3. For example, in states that do not issue a
formal completeness determination, the complete application milestone
would not serve any practical purpose for grandfathering a PSD source,
so the state may choose not to use this milestone. These states may
elect to rely solely upon the public notice milestone, regardless of
whether it issues formal completeness determinations. However, the EPA
anticipates that once a decision is made concerning either the use of
both milestones or only one, states will apply the provision
consistently to all PSD permit applications that would qualify under
the elected milestone(s).
The EPA seeks comments on all aspects of the proposed
grandfathering provisions under either 40 CFR 52.21 or 51.166 as they
would apply to exempt certain pending PSD permit applications from
having to address the revised O3 NAAQS.
b. PSD Screening Tools
The EPA has historically allowed the use of screening tools to help
facilitate the implementation of the NSR program
[[Page 75379]]
by reducing the source's burden and streamlining the permitting process
for circumstances where pollutant emissions or ambient impacts could be
considered de minimis. For example, the EPA has established significant
emission rates or SERs that are used to determine when the NSR
requirements should be applied to a particular new or modified source
with regard to each regulated NSR pollutant. See 40 CFR 51.166(b)(23)
and 52.21(b)(23). For O3, the EPA established a separate SER
in these regulations of 40 tpy for emissions of each O3
precursor--VOC and NOX. For PSD, these SER values for VOC
and NOX are used to determine when the proposed major source
or major modification must complete PSD review for that precursor,
including complying with BACT for that precursor and completing the
appropriate air quality analyses associated with the proposed emissions
increase of that precursor.
Another key screening tool commonly used for PSD is the significant
impact level (SIL). This particular tool is used to determine the
extent to which an ambient impact analysis must be completed for the
applicable pollutant. The EPA has not established a SIL for
O3. The PSD regulations currently state that ``[n]o de
minimis air quality level is provided for ozone. However, any net
emissions increase of 100 tons per year or more of [VOC] or
[NOX] subject to PSD would be required to perform an ambient
impact analysis, including the gathering of ambient air quality data.''
\262\ The EPA intends to consider whether it is appropriate to make any
revisions to the PSD regulations related to the screening tools for
O3 in a separate rulemaking that will specifically address
various implementation issues for O3. However, there are no
such revisions being proposed in today's rulemaking. Until any
rulemaking to amend existing regulations is completed, permitting
decisions should continue to be based on the existing 40 TPY SER for
O3 precursors (NOX and VOC) in existing
regulations. Further decisions regarding the need for an analysis to
assess the impact of an individual source on the O3 NAAQS
and the method of analysis depend on the nature of the source and its
emissions, and, as noted above, should be determined in consultation
with the EPA Regional Office on a case-by-case basis in accordance with
section 5.2.1.c. of Appendix W to 40 CFR part 51.
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\262\ This language is contained in a footnote in the PSD
regulations at 40 CFR 51.166(i)(5)(i) and 52.21(i)(5)(i), and it has
not been revisited by the EPA since the issuance of the 8-hour
O3 NAAQS. These values do not reflect a categorical
conclusion by the EPA that sources emitting less than 100 tpy of
VOCs or NOX will not cause or contribute to a violation
of the current (or any revised) O3 NAAQS, nor does it
reflect a conclusion that such sources should be categorically
excluded from the requirement for an ambient impact analysis.
Instead, the EPA recommends consultation with the appropriate EPA
Regional Office in accordance with section 5.2.1.c of Appendix W
when a review of an application for a new source or modification
involves emissions less than 100 tpy of either O3
precursor. See Letter from Gina McCarthy, Assistant Administrator,
to Robert Ukeiley, at 4 (Jan. 4, 2012), available at http://www.epa.gov/scram001/10thmodconf/review_material/Sierra_Club_Petition_OAR-11-002-1093.pdf.
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c. Other PSD Transition Issues
As explained earlier in this section, the EPA anticipates that the
existing O3 air quality in some areas will no longer be in
attainment of the primary O3 standard when it is revised,
and that these areas will be designated as ``nonattainment'' at a later
date consistent with the designation process set forth for
O3 under the CAA. However, until such nonattainment
designation occurs, proposed new major sources or major modifications
located in any area designated attainment or unclassifiable for the
2008 O3 NAAQS will continue to be required to obtain a PSD
permit.\263\ This raises the question as to how a source can be issued
a PSD permit in light of known existing ambient violations of the
revised NAAQS.
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\263\ Any proposed major stationary source or major modification
for O3 that does not receive its PSD permit by the
effective date of a new nonattainment designation for the area where
the source would locate would then be required to satisfy all of the
applicable NNSR preconstruction permit requirements for
O3.
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Section 165(a)(3)(B) of the CAA requires that a proposed source may
not construct unless it demonstrates that it will not cause or
contribute to a violation of any NAAQS. This statutory requirement is
implemented through a provision contained in the PSD regulations at 40
CFR 51.166(k) and 52.21(k). If a source cannot make this demonstration
or if its initial air quality impact analysis shows that the source's
impact does cause or contribute to a violation, a PSD permit may not be
issued until that adverse impact is mitigated.\264\ The PSD
regulations, however, do not explicitly specify remedial actions that a
prospective source can take to address such a situation. Nevertheless,
the EPA has historically recognized in regulations and through other
actions that sources applying for PSD permits may utilize offsets as
part of the required PSD demonstration under the CAA section
165(a)(3)(B), even though the PSD provisions of the Act do not
expressly reference offsets, in contrast to the NNSR provisions of the
Act.\265\
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\264\ See, e.g., Stephen D. Page, Director, Office of Air
Quality Planning and Standards, to Regional Air Division Directors,
``Guidance Concerning Implementation of the 1-hour SO2
NAAQS for the Prevention of Significant Deterioration Program,''
(August 23, 2010); 44 FR 3278 (January 16, 1979).
\265\ See, e.g., In re Interpower of New York, Inc., 5 E.A.D.
130, 141 (EAB 1994) (describing an EPA Region 2 PSD permit that
relied in part on offsets to demonstrate the source would not cause
or contribute to a violation of the NAAQS). 52 FR 24698 (July 1,
1987); 78 FR 3261-62 (Jan. 15, 2013).
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The EPA has looked to the procedures contained in a separate set of
regulations at 40 CFR 51.165(b) to guide the process by which a source
that is located in an area designated as attainment or unclassifiable
for a NAAQS, but that is determined to cause or contribute to a
violation of that NAAQS in any area, can use offsets to mitigate its
adverse impact on the NAAQS and ultimately meet the PSD demonstration
requirement under CAA section 165(a)(3)(B) and the implementing
regulations.\266\
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\266\ 78 FR 3261 (January 15, 2013); Stephen D. Page, Director,
Office of Air Quality Planning and Standards, to Regional Air
Division Directors, ``Guidance Concerning Implementation of the 1-
hour SO2 NAAQS for the Prevention of Significant
Deterioration Program,'' (August 23, 2010).
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Section 51.165(b) states that plans shall include a preconstruction
review permit program (or its equivalent) to satisfy the requirements
of CAA section 110(a)(2)(D)(i) for major sources and major
modifications, and that the program shall apply to any major stationary
source or major modification locating in an area designated attainment
or unclassifiable for any NAAQS, when that source would cause or
contribute to a NAAQS violation.\267\ Paragraph (b)(3) of that
regulation provides that the required permit program may include a
provision allowing a proposed major source or major modification to
reduce the impact of its emissions on air quality by obtaining
sufficient emissions reductions to, at a minimum, compensate for its
adverse ambient impact where the source or modification would otherwise
cause or contribute to a violation of any NAAQS. Although section
51.165(b) refers explicitly to CAA section 110(a)(2)(D)(i), which now
addresses transport issues, but not CAA section 165(a)(3)(B), the EPA
has previously explained that 51.165(b) may also be interpreted to
apply to the section 165(a)(3)(B) demonstration
[[Page 75380]]
based on the regulatory history (78 FR 3262, n. 256).\268\
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\267\ The definition of ``major stationary source'' and ``major
modification'' in this regulation is based on the respective
definitions in the NNSR regulations at 40 CFR 51.165(a), which are
more inclusive than the respective PSD definitions, but clearly
include major sources covered by the PSD requirements.
\268\ Briefly, in 1980, the EPA had determined that the
statutory requirements under CAA section 165(a)(3)(B), taken
together with the requirements of CAA section 110(a)(2)(D) required
all major sources locating outside a nonattainment area, but causing
or contributing to a NAAQS violation to reduce the impact on air
quality so as to assure attainment and maintenance of the NAAQS. 45
FR 31310 (May 13, 1980). In a footnote, the EPA further indicated
that this offset requirement must apply to sources causing or
contributing to a newly discovered NAAQS violation until the area is
designated nonattainment. See 45 FR 31310 (May 13, 1980). In the
1980 rule, the EPA adopted section 51.18(k), which was later
renumbered section 51.165(b). The EPA revised 51.165(b) to expressly
authorize an offset program to meet the requirements of CAA section
110(a)(2)(D)(i), but this provision may also be interpreted to apply
to section 165(a)(3)(B), consistent with the EPA's reading of
section 51.18(k) in 1980. It is also worth noting that at the time
of these rules, before the 1990 CAA amendments, section 110(a)(2)(D)
required each state to have ``a permit or equivalent program for any
major emitting facility . . . to assure (i) that national air
quality standards are achieved and maintained . . ..''
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Generally, the offset needed to compensate for a proposed source's
adverse impact would be determined by the ability of any particular
emissions reduction to mitigate the source's adverse impact at the
location of the violation. As long as the emissions reduction or offset
can be shown to compensate for the source's adverse impact, there is no
implied requirement that the amount of the emissions reduction be equal
to or greater than the proposed emissions increase. See 44 FR 3278
(January 16, 1979). (``Although full emissions offsets are not
required, such a source must obtain emissions offsets sufficient to
compensate for it air quality impact where the violation occurs.'')
In previous discussions of the use of emissions offsets to help
make the demonstration required under CAA section 165(a)(3)(B), the EPA
has explained that any emissions used for PSD purposes must meet
applicability criteria that are at least as stringent as the offset
criteria set forth in the NNSR requirements for offsets under 40 CFR
51.165(a)(3). See 78 FR 3262. The EPA continues to believe that these
criteria provide the most appropriate criteria for determining the
creditability of PSD offsets.
d. PSD for a Distinct Secondary Standard, if One Is Established
As noted above, the CAA requires that proposed new major stationary
sources and major modifications demonstrate that their emissions
increases will not cause or contribute to a violation of any NAAQS,
which includes the primary and secondary NAAQS. For O3, the
existing primary and secondary NAAQS are defined in the same form and
at the same level. As described earlier in this preamble, the
Administrator is proposing to retain the current averaging time and
form and revise the level of the current secondary standard to within
the range of 70 to 65 ppb. In addition, among other things, the agency
is seeking comment on the alternative approach of revising the
secondary standard to establish a distinct O3 secondary
standard. If the agency were to finalize a secondary standard that
differs from the primary standard, PSD permit applicants would be
required to provide an analysis that specifically addresses the revised
secondary standard and make the necessary showing of compliance with
that standard, as well as any revised primary standard. Moreover, if
such a secondary standard is expressed in a distinctly different form
than the primary standard, the required analysis for making the
compliance demonstration would need to be consistent with that form.
Should the Administrator decide to establish a distinct secondary
NAAQS for O3, the EPA would consider whether the approaches
put forth in any regulatory updates to Appendix W and associated
guidance, as noted in this preamble above, are sufficient for making
the necessary compliance demonstration for that standard for purposes
of PSD. If appropriate, the EPA may consider establishing a surrogacy
policy that would allow a source to make the PSD-required demonstration
of compliance with a distinct secondary O3 NAAQS solely
through a demonstration of compliance with the primary NAAQS.
Therefore, the EPA expects that projects subject to the revised
O3 NAAQS could generally move forward consistent with the
PSD program requirements and NNSR program requirements as subject to
the revised primary and secondary O3 NAAQS. The EPA seeks
comment on this potential approach as well as any other options that
should be considered for showing compliance with any revised primary
and secondary O3 NAAQS.
2. Nonattainment New Source Review
Part D of title I of the CAA includes preconstruction review and
permitting requirements for new major stationary sources and major
modifications when they locate in areas designated nonattainment for a
particular pollutant. As explained in section VII.D.1 of this preamble,
the relevant part D requirements are typically referred to as the NNSR
program. The EPA's regulations for the NNSR programs are contained in
40 CFR 51.165, 52.24 and Part 51, Appendix S. For example, the EPA has
developed minimum program requirements for an NNSR program that is
approvable in a SIP, and those requirements, which include requirements
for O3, are contained in 40 CFR 51.165. In addition, 40 CFR
part 51, Appendix S contains requirements constituting an interim NNSR
program. This program governs NNSR permitting in nonattainment areas
that lack a SIP-approved NNSR permitting program, and applies during
the time between the date of the relevant designation and the date that
the EPA approves into the SIP a NNSR program.\269\ This program is
commonly known as the Emissions Offset Interpretative Rule, and is
applicable to O3 as well.\270\
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\269\ See Appendix S, Part I; 40 CFR 52.24(k).
\270\ As appropriate, certain NNSR requirements under 40 CFR
51.165 or Appendix S can also apply to sources and modifications
located in areas that are designated attainment or unclassifiable in
the Ozone Transport Region. See, e.g., CAA 184(b)(2), 40 CFR
52.24(k).
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As with PSD, the NNSR requirements apply on a pollutant-specific
basis. However, in nonattainment areas, NNSR applies only to
nonattainment pollutants, that is, pollutants for which an area is
designated nonattainment on the date when the permit is issued. As
explained in section VII.D.1 of this preamble, prior to the designation
of areas for any revised O3 NAAQS, applicability of either
PSD or NNSR for O3 to a proposed major new or modified
source will depend on an area's current designations with regard to the
O3 NAAQS. Accordingly, a major stationary source or major
modification proposing to locate in an area currently designated
nonattainment for the 2008 O3 NAAQS must satisfy the NNSR
permit requirements for O3. The EPA is not proposing any new
or revised NNSR requirements in this proposal. As explained in section
VII.A.2 of this preamble, the CAA requires that area designations for
new or revised NAAQS be addressed subsequent to the effective date of
such new or revised NAAQS. The EPA anticipates that the area
designation process for any revised O3 NAAQS will be
completed within 2 years after the revised NAAQS become effective.
Accordingly, any revisions to the existing NNSR requirements for
O3 will be proposed at a later date consistent with the
designation process for any revised O3 NAAQS. The EPA will
also at the same time propose any necessary revisions to the NNSR
requirements under Appendix S so that states will be able to issue NNSR
permits for the revised O3 NAAQS on and after the effective
date of
[[Page 75381]]
designations of new nonattainment areas for O3 until such
time that their own NNSR program is approved as part of their SIP,
where the state does not already have an approved NNSR program
applicable to O3.
This section provides an explanation of some of the key
requirements of the NNSR program as it currently applies to
O3. For NNSR, ``major stationary source'' is generally
defined as a source with the potential to emit at least 100 tpy of the
pollutant for which the area is designated nonattainment. In some
cases, however, the CAA and the NNSR regulations define ``major
stationary source'' for NNSR in terms of a lower rate dependent on the
pollutant. For O3, in addition to the general threshold
level of 100 tpy, lower major source thresholds have been defined for
O3 nonattainment areas based on the stringency of the area's
classification. The NNSR program requires the review and control of
emissions of both VOC and NOX as precursors of
O3, and both are reviewed separately in accordance with the
applicable major source threshold. For example, the threshold for
O3 nonattainment areas classified as Serious is 50 tpy for
both VOC and NOX. See 40 CFR 51.165(a)(1)(iv)(A)(1)(i) and
(a)(1)(iv)(A)(2)(iv), respectively.
As explained earlier in section VII.D.1 of this preamble, a major
modification is a physical change or change in the method of operation
of an existing major stationary source that results in both a
significant emissions increase, and a significant net emissions
increase. To determine whether an emissions increase is significant,
the NNSR rules define significant emissions rates or SERs for each
applicable pollutant. The SER for VOC is 40 tpy, as is the SER for
NOX. See 40 CFR 51.165(a)(1)(x)(A). It should be noted that
there are additional more stringent criteria that must be considered in
determining a major modification in nonattainment areas classified as
Serious, Severe or Extreme for O3. See 40 CFR
51.165(a)(1)(x)(B), (C) and (E).
New major stationary sources and major modifications for
O3 must comply with the LAER as defined in the CAA and NNSR
rules, as well as performing other analyses as required under section
173 of the CAA. In addition, appropriate emissions reductions, known as
emissions offsets, must be secured to offset the proposed emissions
increase of the precursors that trigger NNSR for O3. The
appropriate emissions offset needed for a particular source will depend
upon the classification for the O3 nonattainment area in
which the source or modification will locate. Generally, the ratio of
the total emissions reduction to the emissions increase is at least
1:1; however, more stringent ratios apply to O3
nonattainment areas according to the area classification. See, e.g., 40
CFR 51.165(a)(9) and 40 CFR part 51, Appendix S, IV.G.2.
E. Transportation and General Conformity Programs
1. What are transportation and general conformity?
Conformity is required under CAA section 176(c) to ensure that
federal actions are consistent with (``conform to'') the purpose of the
SIP. Conformity to the purpose of the SIP means that federal activities
will not cause new air quality violations, worsen existing violations,
or delay timely attainment of the relevant NAAQS or interim reductions
and milestones. Conformity applies to areas that are designated
nonattainment, and those nonattainment areas redesignated to attainment
with a CAA section 175A maintenance plan after 1990 (``maintenance
areas'').
The EPA's Transportation Conformity Rule (40 CFR 51.390 and Part
93, subpart A) establishes the criteria and procedures for determining
whether transportation activities conform to the SIP. These activities
include adopting, funding or approving transportation plans,
transportation improvement programs (TIPs) and federally supported
highway and transit projects. For further information on conformity
rulemakings, policy guidance and outreach materials, see the EPA's Web
site at http://www.epa.gov/otaq/stateresources/transconf/index.htm. The
EPA may issue future transportation conformity guidance as needed to
implement a revised O3 NAAQS.
With regard to general conformity, the EPA first promulgated
general conformity regulations in November 1993. (40 CFR part 51,
subpart W, 40 CFR part 93, subpart B) Subsequently the EPA finalized
revisions to the general conformity regulations on April 5, 2010. (75
FR 17254-17279). Besides ensuring that federal actions not covered by
the transportation conformity rule will not interfere with the SIP, the
general conformity program also fosters communications between federal
agencies and state/local air quality agencies, provides for public
notification of and access to federal agency conformity determinations
and allows for air quality review of individual federal actions. More
information on the general conformity program is available at http://www.epa.gov/air/genconform/.
2. Why is the EPA discussing transportation and general conformity in
this proposed rulemaking?
The EPA is discussing transportation and general conformity in this
proposed O3 NAAQS rulemaking in order to provide affected
parties with information on when and how conformity must be implemented
after nonattainment areas are designated for a revised O3
NAAQS. The information presented here is consistent with existing
conformity regulations and statutory provisions that are not addressed
by this O3 NAAQS rulemaking. Affected parties would include
state and local transportation and air quality agencies, metropolitan
planning organizations (MPOs), and federal agencies including the U.S.
Department of Transportation (DOT), the U.S. Department of Defense, the
U.S. Department of Interior, and the U.S. Department of Agriculture.
3. When would transportation and general conformity apply to areas
designated nonattainment for a revised O3 NAAQS, if one is
established?
Transportation and general conformity apply one year after the
effective date of nonattainment designations for a revised
O3 NAAQS. This is because CAA section 176(c)(6) provides a
1-year grace period from the effective date of initial designations for
any revised NAAQS before transportation and general conformity apply in
areas newly designated nonattainment for a specific pollutant and
NAAQS.
4. Will transportation and general conformity apply to a distinct
secondary O3 NAAQS, if one is established?
Section 176(c)(1)(A) of the CAA states that conformity to a SIP
means ``conformity to an implementation plan's purpose of eliminating
or reducing the severity and number of violations of the national
ambient air quality standards and achieving expeditious attainment of
such standards . . .'' In other words, because the CAA refers to the
NAAQS without distinguishing between them, conformity applies to both
the primary and secondary NAAQS for all criteria pollutants. Therefore,
if a distinct secondary O3 NAAQS is established, both
transportation and general conformity will apply in any areas
designated nonattainment for such a NAAQS.
Current transportation and general conformity regulations already
apply to such a secondary NAAQS, and nothing in this proposal affects
those
[[Page 75382]]
transportation and general conformity requirements. The EPA will
consider the need to issue additional guidance concerning the
implementation of transportation and general conformity in areas
designated nonattainment for a distinct secondary O3 NAAQS,
if one is established.
5. What impact would the implementation of a revised O3
NAAQS have on a state's transportation and/or general conformity SIP?
If the EPA revises the O3 NAAQS, but does not make
specific changes to its transportation or general conformity
regulations, then states should not need to revise their transportation
and/or general conformity SIPs. The EPA is not proposing any changes to
its transportation or general conformity regulations. While we are not
proposing any revisions to the general conformity regulations at this
time, we recommend, when areas develop SIPs for a revised O3
NAAQS, that state and local air quality agencies work with federal
agencies with large facilities that are subject to the general
conformity regulations to establish an emissions budget for those
facilities in order to facilitate future conformity determinations
under the conformity regulations. Such a budget could be used by
federal agencies in determining conformity or identifying mitigation
measures if the budget level is included and identified in the SIP.
However, because some federal agencies may not have an established
facility-wide emissions budget in the SIP for the purpose of meeting
general conformity requirements, state, local and tribal agencies are
encouraged to maintain ozone SIP emissions inventories on an annual
basis, at a minimum, to facilitate compliance of federal agencies with
CAA section 176(c). Finally, states with new nonattainment areas may
also need to revise conformity SIPs in order to ensure the state
regulations apply in any newly designated areas if the existing SIP
does include current conformity provisions.
If this is the first time that transportation conformity will apply
in a state, such a state is required by the statute and EPA regulations
to submit a SIP revision that addresses three specific transportation
conformity requirements that address consultation procedures and
written commitments to control or mitigation measures associated with
conformity determinations for transportation plans, TIPs or projects.
(40 CFR 51.390) Additional information and guidance can be found in the
EPA's ``Guidance for Developing Transportation Conformity State
Implementation Plans'' (http://www.epa.gov/otaq/stateresources/transconf/policy/420b09001.pdf).
F. How Background O3 Is Addressed in CAA Implementation Provisions
1. Introduction
The EPA and state, local and tribal air agencies, need to determine
how to most effectively and efficiently use the CAA's various
provisions to provide required public health and welfare protection
from the harmful effects of O3. In most cases, reducing man-
made emissions of NOX and VOCs will reduce O3
formation and provide additional health and welfare protection. The EPA
recognizes, however, that ``background'' O3 levels, which
can be significant in some areas on some days, may present a challenge
to air agencies in preparing clean air plans. That is, O3
and O3-forming pollution from natural and international
sources could prevent ambient levels from reaching attainment levels in
locations where the impacts of such sources are large relative to the
impact of controllable man-made sources of NOX and VOC
emissions within the U.S., especially in locations with few remaining
untapped opportunities for local emission reductions.
Climate change may also influence future O3
concentrations. Modeling studies in EPA's Interim Assessment (U.S. EPA,
2009b) and cited in support of the 2009 Endangerment Finding (74 FR
66,496; Dec. 15, 2009) show that, while the impact is not uniform,
simulated climate change causes increases in summertime O3
concentrations over substantial regions of the country, with increases
tending to occur during higher peak pollution episodes in the summer.
Increases in temperature are expected to be the principal factor in
driving any O3 increases, although increases in stagnation
frequency may also contribute (Jacob and Winner, 2009). These
temperature increases could lead to more prevalent wildfires, the
impacts of which may lessened by various mitigation measures including
taking steps to minimize fuel loading in areas vulnerable to fire.
The term ``background'' O3 is often used to refer to
O3 that originates from natural sources of O3
(e.g., wildfires and stratospheric O3 intrusions) and
O3 precursors, as well as from manmade international
emissions of O3 precursors. Using the term generically,
however, can lead to confusion as to what sources of O3 are
being considered. The PA provides three specific definitions of
background O3: natural background, North American
background, and United States background. Natural background (NB) is
defined as the O3 that would exist in the absence of any
manmade O3 precursor emissions. North American background
(NAB) is defined as that O3 that would exist in the absence
of any manmade O3 precursor emissions from North America.
U.S. background (USB) is defined as that O3 that would exist
in the absence of any manmade emissions inside the U.S. Because
background O3 is difficult to measure, air quality modeling
is conducted to estimate NA, NAB, and USB.
The PA identifies several key findings related to background
O3. First, background O3 can comprise a
considerable fraction of total seasonal mean O3 across the
U.S. Studies have estimated that seasonal mean USB 8-hour O3
values across U.S. locations varied between 25 to 50 ppb in 2007 (U.S.
EPA, 2014c, Figure 211). The largest seasonal average values of
background are modeled to occur at locations in the intermountain
western U.S. and the highest daily USB levels are highest in the spring
and early summer seasons. Second, the modeling indicates that U.S.
anthropogenic emission sources are the dominant contributor to the
majority of modeled O3 exceedances of the NAAQS across the
U.S. This conclusion is based on results that indicate background
contributions are generally similar on high O3 days as on
all other O3 days. As a result, the proportional influence
of background sources tends to be lower on high O3 days.
Third, while the majority of modeled O3 exceedances have
local and regional emissions as their primary cause, there can be
events where O3 levels approach or exceed the concentration
levels being proposed in this notice (i.e., 60-70 ppb) in large part
due to background sources. These cases of high USB levels on high
O3 days typically result from stratospheric intrusions of
O3, wildfire O3 plumes, or long-range transport
of O3 from sources outside the U.S. In most locations in the
U.S., these events are relatively infrequent and the CAA contains
provisions that can be used to help deal with certain events, including
providing varying degrees of regulatory relief for air agencies and
potential regulated entities.
Regulatory relief associated with U.S. background O3 may
include: \271\
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\271\ Note that the relief mechanisms discussed here do not
include the CAA's interstate transport provisions found in sections
110(a)(2)(D) and 126. The interstate transport provisions are
intended to address the cross-state transport of O3 and
O3 precursor emissions from man-made sources within the
continental U.S. rather than background O3 as it is
defined in this section.
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[[Page 75383]]
Relief from designation as a nonattainment area (through
exclusion of data affected by exceptional events)
Relief from the more stringent requirements of higher
nonattainment area classifications (through treatment as a rural
transport area; through exclusion of data affected by exceptional
events; or through international transport provisions)
Relief from adopting more than reasonable controls to
demonstrate attainment (through international transport provisions)
None of these relief mechanisms are completely burden-free, meaning
they all require some level of assessment or demonstration by a state
and/or EPA to legally invoke. In no case does the CAA authorize a
blanket exclusion from the basic application of an air quality
management regime because an area is significantly impacted by
background O3. While any prediction of the exact nature of
future implementation challenges associated with alternative
prospective standards is inherently uncertain, there is no question
that, as the levels of alternative prospective standards are lowered,
background will represent increasingly larger fractions of total
O3 levels and may subsequently complicate efforts to attain
these standards. For a prospective standard of 70 ppb, the EPA does not
believe that background O3 would create significant
implementation-related challenges at locations throughout the U.S. and
prevent attainment of the NAAQS. However, as the levels of prospective
standards are lowered, the areas that would most likely need to use the
relief mechanisms discussed in this section as part of attaining the
lower prospective levels are rural locations in the western U.S.,
consistent with the previously mentioned locations where we have
estimated the largest seasonal average values of background occur. The
remainder of this section discusses these relief mechanisms and the
methods associated with legally invoking them. These relief mechanisms
depend on distinguishing background O3 by the following
types of drivers: routine natural emissions, non-routine natural events
and international emissions. The EPA welcomes comment on any of these
issues related to O3 background and implementation.
2. Exceptional Events Exclusions
A state can request and the EPA can agree to exclude data
associated with event-influenced exceedances or violations of a NAAQS,
including the proposed O3 NAAQS, provided the event meets
the statutory requirements in section 319 of the CAA:
The event ``affects air quality.''
The event ``is not reasonably controllable or
preventable.''
The event is ``caused by human activity that is unlikely
to recur at a particular location or [is] a natural event.'' \272\
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\272\ A natural event is further described in 40 CFR 50.1(k) as
``an event in which human activity plays little or no direct causal
role.''
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The EPA's implementing regulations, the 2007 Exceptional Events
Rule, further specify that states must provide evidence that: \273\
---------------------------------------------------------------------------
\273\ Federal Register (2007). Treatment of Data Influenced by
Exceptional Events; Final Rule. 40 CFR 50 and 51; Federal Register
72:13560.
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``There is a clear causal relationship between the
measurement under consideration and the event that is claimed to have
affected the air quality in the area;''
``The event is associated with a measured concentration in
excess of normal historical fluctuations, including background;'' and
``There would have been no exceedance or violation but for
the event.''
The ISA contains discussions of natural events that may contribute
to O3 or O3 precursors. These include
stratospheric O3 intrusion and wildfire events.\274\ As
indicated above, to satisfy the exceptional event requirements and to
qualify for data exclusion under the Exceptional Events Rule, a state
must develop and submit evidence addressing each of the identified
criteria. The extent to which a stratospheric O3 intrusion
event or a wildfire event contribute to O3 levels can be
uncertain, and in most cases requires detailed investigation and
analysis to adequately determine.
---------------------------------------------------------------------------
\274\ The preamble to the Exceptional Events Rule (72 FR 13560,
March 22, 2007) identifies both stratospheric O3
intrusions and wildfires as natural events that could also qualify
as exceptional events under the CAA and Exceptional Event Rule
criteria. Note that O3 resulting from routine natural
emissions from vegetation, microbes, animals and lightning are not
exceptional events authorized for exclusion under the section 319 of
the CAA.
---------------------------------------------------------------------------
Strong stratospheric O3 intrusion events, most prevalent
at high elevation sites during winter or spring, can be identified
based on measurements of low relative humidity, evidence of deep
atmospheric mixing, and a low ratio of CO to O3 based on
ambient measurements. Accurately determining the extent of weaker
intrusion events remains challenging (U.S. EPA 2013a, p. 3-34).
Although states have submitted only a few exceptional event
demonstrations for stratospheric O3 intrusion, the EPA
recently approved a demonstration from Wyoming for a June 2012
stratospheric O3 event.\275\
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\275\ U.S. EPA (2014) Treatment of Data Influenced by
Exceptional Events: Examples of Reviewed Exceptional Event
Submissions. U.S. Environmental Protection Agency, Research Triangle
Park, NC. Available at http://www.epa.gov/ttn/analysis/exevents.htm.
---------------------------------------------------------------------------
While stratospheric O3 intrusions can increase monitored
ground-level ambient O3 concentrations, wildfire plumes can
either suppress or enhance O3 depending upon a variety of
factors including fuel type, combustion stage, plume chemistry, aerosol
effects, meteorological conditions and distance from the fire (Jaffe
and Wigder, 2012). As such, determining the impact of wildfire
emissions on specific O3 observations is challenging. The
EPA recently approved an exceptional event demonstration for wildfires
affecting 1-hour O3 levels in Sacramento, California in 2008
that successfully used a variety of analytical tools (e.g., regression
modeling, back trajectories, satellite imagery, etc.) to support the
exclusion of O3 data affected by large fires.\276\
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\276\ U.S. EPA (2014) Treatment of Data Influenced by
Exceptional Events: Examples of Reviewed Exceptional Event
Submissions. U.S. Environmental Protection Agency, Research Triangle
Park, NC. Examples of O3-related exceptional event
submissions available at http://www.epa.gov/ttn/analysis/exevents.htm.
---------------------------------------------------------------------------
Because of previously expressed stakeholder feedback regarding
implementation of the Exceptional Events Rule and specific stakeholder
concerns regarding the analyses that can be used to support
O3-related exceptional event demonstrations, the EPA intends
to propose revisions to the Exceptional Events Rule in a future notice
and comment rulemaking effort and will solicit public comment at that
time.
Additionally, the EPA intends to develop guidance to address
implementing the Exceptional Events Rule criteria for wildfires that
could affect ambient O3 concentrations. Wildfire emissions
are a component of background O3 (Jaffe and Wigder, 2012)
and can significantly contribute to periodic high O3 levels
(Emery, 2012). Besides their effect on air quality, wildfires pose a
direct threat to public safety--a threat that can be mitigated through
management of wildland vegetation. Attempts to suppress wildfires have
resulted in unintended consequences, including increased risks to both
humans and ecosystems. Indeed, ``Fire policy that focuses on [wildfire]
suppression only, delays the inevitable, promising more dangerous and
destructive future . . . fires'' (Stephens, S. et al., 2013). The use
of wildland
[[Page 75384]]
prescribed fire can influence the occurrence of catastrophic wildfires
which may help manage the contribution of wildfires to background
O3 levels and periodic peak O3 events.
Additionally prescribed fires can have benefits to those plant and
animal species that depend upon natural fires for propagation, habitat
restoration, and reproduction, as well as myriad ecosystem functions
(e.g., carbon sequestration). As previously indicated, the CAA and the
EPA's implementing regulations allow for the exclusion of air quality
monitoring data from design value calculations when they are
substantially affected by certain background influences. Additionally,
the CAA requires the EPA to set the NAAQS at levels requisite to
protect public health and welfare without regard to the source of the
pollutant. However, EPA understands the importance of prescribed fire
which mimics a natural process necessary to manage and maintain fire-
adapted ecosystems and climate change adaptation, while reducing risk
of uncontrolled emissions from catastrophic wildfires. The EPA is
committed to working with federal land managers, tribes, and states to
effectively manage prescribed fire use to reduce the impact of
wildland-fire related emissions on ozone through policies and
regulations implementing these standards.
3. Rural Transport Areas
Clean Air Act section 182(h) authorizes the EPA Administrator to
determine that an area designated nonattainment can be treated as a
rural transport area. In accordance with the statute, a nonattainment
area may qualify for this determination if it meets the following
criteria:
The area does not contain emissions sources that make a
significant contribution to monitored O3 concentrations in
the area, or in other areas; and
The area does not include and is not adjacent to a
Metropolitan Statistical Area.\277\
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\277\ Note that the EPA interprets the rural transport
provisions of section 182(h) would not apply to an O3
monitor that is located in a relatively rural location, but is
heavily influenced by short-range upwind contributions from a nearby
urbanized area. The EPA will work closely with states to determine
whether a particular monitor violating the NAAQS is considered to be
affiliated with a nearby urban area, or is an isolated rural area
monitor.
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Historically, the EPA has recognized few nonattainment areas under
this statutory provision.\278\ The EPA has not issued separate written
guidance to further elaborate on the interpretation of these CAA
qualification criteria. However, the EPA developed draft guidance in
2005 that explains the kinds of technical analyses that states could
use to establish that transport of O3 and/or O3
precursors into the area is so overwhelming that the contribution of
local emissions to an observed 8-hour O3 concentration above
the level of the NAAQS is relatively minor and determine that emissions
within the area do not make a significant contribution to the
O3 concentrations measured in the area or in other
areas.\279\ While this guidance was not prepared specifically for rural
transport areas, it could be useful to states for developing technical
information to support a request that the EPA treat a specific
O3 nonattainment area as a rural transport area.
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\278\ For the 1979 1-hour O3 standard, Essex County,
New York, and Smyth County, Virginia (White Top Mountain) were
recognized by the EPA as rural transport areas.
\279\ U.S. Environmental Protection Agency (2005). Criteria For
Assessing Whether an Ozone Nonattainment Area is Affected by
Overwhelming Transport [Draft EPA Guidance]. U.S. Environmental
Protection Agency, Research Triangle Park, NC. June 2005. Available
at http://www.epa.gov/scram001/guidance/guide/owt_guidance_07-13-05.pdf.
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An area that qualifies for treatment as a rural transport area is
deemed to have fulfilled all O3-related planning and control
requirements if it meets the CAA's requirements for areas classified
Marginal, which is the lowest classification specified in the CAA.
Therefore, a state would not need to develop an attainment plan or an
attainment demonstration for such an area or adopt the various
mandatory measures required in nonattainment areas classified as
Moderate or above. The only requirements that would apply, regardless
of the level of O3 air quality, would be NNSR (at the
Marginal major source threshold and offset ratio), conformity
requirements associated with a nonattainment designation, and the
emission inventory and source emission statement requirements.
4. International Transport
Clean Air Act section 179B recognizes the possibility that certain
nonattainment areas may be heavily impacted by O3 or
O3 precursor emissions from international sources beyond the
regulatory jurisdiction of the state. The EPA's science review suggests
that the influence of international sources on U.S. O3
levels will be largest in locations that are in the immediate vicinity
of an international border with Canada or Mexico, but other locations
can also potentially be affected when conditions are favorable for
long-range transport (U.S. EPA 2013a, p.3-140). Section 179B allows
states to consider in their attainment demonstrations whether an area
might have met the O3 NAAQS by the attainment date ``but
for'' emissions contributing to the area originating outside the U.S.
If a state is unable to demonstrate attainment in such an area after
adopting all reasonably available control measures (RACM, including
RACT, as required by CAA section 182(b)), the EPA can nonetheless
approve the CAA-required state attainment plan and demonstration using
the authority in section 179B.
When the EPA approves this type of attainment plan, states avoid
potential sanctions and FIPs, and there would be no adverse consequence
for a finding that the area failed to attain the NAAQS by the relevant
attainment date. For example, the area would not be reclassified to the
next highest classification or required to implement a section 185
penalty fee program.
Section 179B authority does not allow the EPA to avoid designating
an area as nonattainment or for the area to be classified with a lower
classification than is indicated by actual air quality. Generally,
monitoring data influenced by international transport may not be
excluded from regulatory determinations, unless the data are influenced
by an excludable exceptional event. Section 179B also does not provide
for any relaxation of mandatory emissions control measures (including
contingency measures) or the prescribed emissions reductions necessary
to achieve RFP.
The EPA's guidance on `but for' demonstrations involving
international emissions indicates that states may want to consider
conducting air quality modeling using O3 episodes that do
not involve international transport of emissions (U.S. EPA 1991)\280\,
running the model with boundary conditions that reflect general U.S.
background concentrations, and analyzing monitoring data if a dense
network has been established. Additional information that may be
helpful at nonattainment areas abutting international borders could
include evaluating changes in O3 with changes in wind
direction at monitors near the border, and comparing emissions on both
sides of the border. States are encouraged to consult with their EPA
Regional Office to establish appropriate
[[Page 75385]]
technical requirements for these analyses.
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\280\ U.S. Environmental Protection Agency (1991). Criteria for
Assessing the Role of Transported Ozone/Precursors in Ozone
Nonattainment Areas. EPA-450/4-91-015. U.S. Environmental Protection
Agency, Office of Air Quality Planning and Standards, Research
Triangle Park, NC. May 1991.
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The EPA has used section 179B authority previously to approve
attainment plans for Mexican border areas in El Paso, TX
(O3, PM10, and CO plans); Nogales, AZ
(PM10 plan); and Imperial Valley, CA (PM10 plan).
The 1-hour O3 attainment plan for El Paso, TX was approved
by EPA as sufficient to demonstrate attainment of the NAAQS by the
Moderate classification deadline of November 15, 1996, taking into
account ``but for'' international emissions sources in Ciudad
Ju[aacute]rez, Mexico (69 FR 32450, June 10, 2004). The state's
demonstration included airshed modeling using only the U.S. emissions
data because emissions data from Ciudad Ju[aacute]rez were not
available.
VIII. Statutory and Executive Order Reviews
Additional information about these statutes and Executive Orders
can be found at http://www2.epa.gov/laws-regulations/laws-and-executive-orders.
A. Executive Order 12866: Regulatory Planning and Review and Executive
Order 13563: Improving Regulation and Regulatory Review
This action is an economically significant regulatory action that
was submitted to the Office of Management and Budget (OMB) for review.
Any changes made in response to OMB recommendations have been
documented in the docket. The EPA prepared an analysis of the potential
costs and benefits associated with this action. This analysis is
contained in the document, Regulatory Impact Analysis for the
O3 NAAQS, November, 2014. A copy of the analysis is
available in the RIA docket (EPA-HQ-OAR-2013-0169) and the analysis is
briefly summarized here. The RIA estimates the costs and monetized
human health and welfare benefits of attaining four alternative
O3 NAAQS nationwide. Specifically, the RIA examines the
alternatives of 60 ppb, 65 ppb, 70 ppb, and 75 ppb. The RIA contains
illustrative analyses that consider a limited number of emissions
control scenarios that states and Regional Planning Organizations might
implement to achieve these alternative O3 NAAQS. However,
the CAA and judicial decisions make clear that the economic and
technical feasibility of attaining ambient standards are not to be
considered in setting or revising NAAQS, although such factors may be
considered in the development of state plans to implement the
standards. Accordingly, although an RIA has been prepared, the results
of the RIA have not been considered in issuing this proposed rule.
B. Paperwork Reduction Act
The information collection requirements in this proposed rule have
been submitted for approval to the Office of Management and Budget
(OMB) under the Paperwork Reduction Act (PRA). The Information
Collection Request (ICR) document prepared by EPA has been assigned EPA
ICR #2313.03. You can find a copy of the ICR in the docket for this
rule, and it is briefly summarized here.
The information collected and reported under 40 CFR part 58 is
needed to determine compliance with the NAAQS, to characterize air
quality and associated health and ecosystems impacts, to develop
emission control strategies, and to measure progress for the air
pollution program. We are proposing to extend the length of the
required O3 monitoring season in 33 states and propose that
the revised O3 monitoring seasons become effective on
January 1, 2017. We are also proposing revisions to the PAMS monitoring
requirements that reduce the number of required PAMS sites while
improving spatial coverage, and proposing to require states with
O3 non-attainment areas to develop an enhanced monitoring
plan as part of the PAMS requirements. For areas currently designated
as nonattainment for O3 based on the 2008 NAAQS, we propose
that these areas comply with the PAMS requirements by June 1, 2017. For
new areas designated based on a revised NAAQS, if finalized, we propose
that those areas comply with the PAMS requirements by January 1, 2019.
In addition, we are proposing to revise the O3 FRM to
establish a new, additional technique for measuring O3 in
the ambient air. We propose that it be incorporated into the existing
O3 FRM, using the same calibration procedure in Appendix D
of 40 CFR part 50. We also propose to make changes to the procedures
for testing performance characteristics and determining comparability
between candidate FEMs and reference methods.
For the purposes of ICR #2313.03, the burden figures represent the
burden estimate based on the requirements contained in the proposed
rule. The burden estimates are for the 3-year period from 2015 through
2017. The implementation of the PAMS changes, if finalized, will occur
beyond the time frame of this ICR with likely implementation dates
between 2017 and 2019. The cost estimates for the PAMS network
(including proposed revisions) will be captured in future routine
updates to the Ambient Air Quality Surveillance ICR that are required
every 3 years by OMB. The proposal for a new FRM in 40 CFR part 50 and
revisions to the O3 FEM procedures for testing performance
characteristics in 40 CFR part 53 does not add any additional
information collection requirements.
The ICR burden estimates are associated with the proposed changes
to the O3 seasons. This information collection is estimated
to involve 158 respondents for a total cost of approximately
$24,115,182 (total capital, labor, and operation and maintenance) plus
a total burden of 339,930 hours for the support of all operational
aspects of the entire O3 monitoring network. The labor costs
associated with these hours are $19,813,692. Also included in the total
are other costs of operations and maintenance of $2,210,132 and
equipment and contract costs of $2,091,358. The actual labor cost
increase to expand the O3 monitoring seasons is $1,668,433.
In addition to the costs at the state, local, and tribal air quality
management agencies, there is a burden to EPA of 41,418 hours and
$2,617,591. Burden is defined at 5 CFR 1320.3(b). State, local, and
tribal entities are eligible for state assistance grants provided by
the Federal government under the CAA which can be used for related
activities. An agency may not conduct or sponsor, and a person is not
required to respond to, a collection of information unless it displays
a currently valid OMB control number. The OMB control numbers for EPA's
regulations in 40 CFR are listed in 40 CFR part 9.
To comment on the Agency's need for this information, the accuracy
of the provided burden estimates, and any suggested methods for
minimizing respondent burden, EPA has established a public docket for
this rule, which includes this ICR, under Docket ID number EPA-HQ-OAR-
2008-0699. Submit any comments related to the ICR to EPA and OMB. Send
comments to the EPA at the Air and Radiation Docket and Information
Center Docket in the EPA Docket Center (EPA/DC), EPA West, Room 3334,
1301 Constitution Ave. NW., Washington, DC. The Docket Center Public
Reading Room is open from 8:30 a.m. to 4:30 p.m., Monday through
Friday, excluding legal holidays. The telephone number for the Reading
Room is (202) 566-1744, and the telephone number for the Air and
Radiation Docket and Information Center Docket is (202) 566-1742. An
electronic version of the public docket is available at
www.regulations.gov. Send comments to OMB at the Office of
[[Page 75386]]
Information and Regulatory Affairs, Office of Management and Budget,
725 17th Street NW., Washington, DC 20503, Attention: Desk Office for
EPA. Since OMB is required to make a decision concerning the ICR
between 30 and 60 days after December 17, 2014, a comment to OMB is
best assured of having its full effect if OMB receives it by December
17, 2014. The final rule will respond to any OMB or public comments on
the information collection requirements contained in this proposal.
C. Regulatory Flexibility Act
I certify that this action will not have a significant economic
impact on a substantial number of small entities under the Regulatory
Flexibility Act (RFA). The reason is that this proposed rule will not
impose any requirements on small entities. Rather, this rule
establishes national standards for allowable concentrations of
O3 in ambient air as required by section 109 of the CAA. See
also American Trucking Associations v. EPA, 175 F. 3d at 1044-45 (NAAQS
do not have significant impacts upon small entities because NAAQS
themselves impose no regulations upon small entities). Similarly, the
proposed revisions to 40 CFR part 58 address the requirements for
states to collect information and report compliance with the NAAQS and
will not impose any requirements on small entities. Similarly, the
addition of a new FRM in 40 CFR part 50 and revisions to the FEM
procedures for testing in 40 CFR part 53 will not impose any
requirements on small entities.
D. Unfunded Mandates Reform Act
This action does not contain any unfunded mandate as described in
the Unfunded Mandates Reform Act (UMRA), 2 U.S.C. 1531--1538, and does
not significantly or uniquely affect small governments. Furthermore, as
indicated previously, in setting a NAAQS the EPA cannot consider 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. See also
American Trucking Associations v. EPA, 175 F. 3d at 1043 (noting that
because the EPA is precluded from considering costs of implementation
in establishing NAAQS, preparation of a Regulatory Impact Analysis
(RIA) pursuant to the Unfunded Mandates Reform Act would not furnish
any information which the court could consider in reviewing the NAAQS).
E. Executive Order 13132: Federalism
This action does not have federalism implications. It will not have
substantial direct effects on the states, on the relationship between
the national government and the states, or on the distribution of power
and responsibilities among the various levels of government.
F. Executive Order 13175: Consultation and Coordination With Indian
Tribal Governments
This action does not have tribal implications, as specified in
Executive Order 13175. It does not have a substantial direct effect on
one or more Indian Tribes as tribes are not obligated to adopt or
implement any NAAQS. In addition, tribes are not obligated to conduct
ambient monitoring for O3 or to adopt the ambient monitoring
requirements of 40 CFR part 58. Thus, Executive Order 13175 does not
apply to this rule.
The EPA specifically solicits comment on this rule from tribal
officials. Prior to finalization of this proposal, the EPA intends to
conduct outreach consistent with the EPA Policy on Consultation and
Coordination with Indian Tribes. Outreach to tribal environmental
professionals will be conducted through participation in the Tribal Air
call, which is sponsored by the National Tribal Air Association. In
addition, the EPA intends to offer formal consultation to the tribes
during the public comment period. If consultation is requested, a
summary of the result of that consultation will be presented in the
notice of final rulemaking and will be available in the docket.
G. Executive Order 13045: Protection of Children From Environmental
Health & Safety Risks
This action is subject to Executive Order 13045 because it is an
economically significant regulatory action as defined by Executive
Order 12866, and the EPA believes that the environmental health risk
addressed by this action may have a disproportionate effect on
children. The rule will establish uniform national ambient air quality
standards for O3; these standards are designed to protect
public health with an adequate margin of safety, as required by CAA
section 109. However, the protection offered by these standards may be
especially important for children because children, especially children
with asthma, along with other at-risk populations\281\ such as all
people with lung disease and people active outdoors, are potentially
susceptible to health effects resulting from O3 exposure.
Because children are considered an at-risk lifestage, we have carefully
evaluated the environmental health effects of exposure to O3
pollution among children. Discussions of the results of the evaluation
of the scientific evidence, policy considerations, and the exposure and
risk assessments pertaining to children are contained in sections II.B
and II.C of this preamble.
---------------------------------------------------------------------------
\281\ As used here and similarly throughout this document, the
term population refers to people having a quality or characteristic
in common, including a specific pre-existing illness or a specific
age or life stage.
---------------------------------------------------------------------------
H. Executive Order 13211: Actions That Significantly Affect Energy
Supply, Distribution, or Use
This action is not a ``significant energy action'' because it is
not likely to have a significant adverse effect on the supply,
distribution, or use of energy. The purpose of this rule is to
establish revised NAAQS for O3, establish an additional FRM,
revise FEM procedures for testing, and revises air quality surveillance
requirements. The rule does not prescribe specific pollution control
strategies by which these ambient standards and monitoring revisions
will be met. Such strategies will be developed by states on a case-by-
case basis, and the EPA cannot predict whether the control options
selected by states will include regulations on energy suppliers,
distributors, or users. Thus, the EPA concludes that this rule is not
likely to have any adverse energy effects and does not constitute a
significant energy action as defined in Executive Order 13211.
I. National Technology Transfer and Advancement Act
This rulemaking involves environmental monitoring and measurement.
Consistent with the Agency's Performance Based Measurement System
(PBMS), the EPA proposes not to require the use of specific, prescribed
analytical methods. Rather, the Agency plans to allow the use of any
method that meets the prescribed performance criteria. Ambient air
concentrations of ozone are currently measured by the Federal reference
method (FRM) in 40 CFR part 50, Appendix D (Measurement Principle and
Calibration Procedure for the Measurement of Ozone in the Atmosphere)
or by Federal equivalent methods (FEM) that meet the requirements of 40
CFR part 53. Procedures are available in part 53 that
[[Page 75387]]
allow for the approval of an FEM for O3 that is similar to
the FRM. Any method that meets the performance criteria for a candidate
equivalent method may be approved for use as an FEM. This approach is
consistent with EPA's PBMS. The PBMS approach is intended to be more
flexible and cost-effective for the regulated community; it is also
intended to encourage innovation in analytical technology and improved
data quality. The EPA is not precluding the use of any method, whether
it constitutes a voluntary consensus standard or not, as long as it
meets the specified performance criteria.
J. Executive Order 12898: Federal Actions To Address Environmental
Justice in Minority Populations and Low-Income Populations
The EPA believes the human health or environmental risk addressed
by this action will not have potential disproportionately high and
adverse human health or environmental effects on minority, low-income
or indigenous populations because it does not affect the level of
protection provided to human health or the environment. This action
proposes the strengthening of the O3 NAAQS. If the proposed
revisions are finalized, the revised O3 NAAQS will increase
public health protection. Analyses evaluating the potential
implications of a revised O3 NAAQS for environmental justice
populations are discussed in appendix 9A of the Regulatory Impact
Analysis (RIA) that accompanies this notice of proposed rulemaking. The
RIA is available on the Web, through the EPA's Technology Transfer
Network Web site at http://www.epa.gov/ttn/naaqs/standards/ozone/s_o3_index.html.
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OAR-2008-0699.
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List of Subjects
40 CFR Part 50
Environmental protection, Air pollution control, Carbon monoxide,
Lead, Nitrogen dioxide, Ozone, Particulate matter, Sulfur oxides.
[[Page 75396]]
40 CFR Part 51
Environmental protection, Administrative practices and procedures,
Air pollution control, Intergovernmental relations.
40 CFR Part 52
Environmental Protection, Administrative practices and procedures,
Air pollution control, Incorporation by reference, Intergovernmental
relations.
40 CFR Part 53
Environmental protection, Administrative practice and procedure,
Air pollution control, Reporting and recordkeeping requirements.
40 CFR Part 58
Environmental protection, Administrative practice and procedure,
Air pollution control, Intergovernmental relations, Reporting and
recordkeeping requirements.
Dated: November 25, 2014.
Gina McCarthy,
Administrator.
For the reasons set forth in the preamble, 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
0
1. The authority citation for part 50 continues to read as follows:
Authority: 42 U.S.C. 7401 et seq.
0
2. Amend Sec. 50.14 by:
0
a. Revising paragraph (c)(2)(iii);
0
b. Removing and reserving paragraphs (c)(2)(iv) and (v);
0
c. Revising paragraph (c)(2)(vi) introductory text and Table 1;
0
d. Revising paragraph (c)(3)(i); and
0
e. Removing and reserving paragraphs (c)(3)(ii) and (iii);
0
3. The revisions read as follows:
Sec. 50.14 Treatment of air quality monitoring data influenced by
exceptional events.
* * * * *
(c) * * *
(2) * * *
(iii) Flags placed on data as being due to an exceptional event
together with an initial description of the event shall be submitted to
EPA not later than July 1st of the calendar year following the year in
which the flagged measurement occurred, except as allowed under
paragraph (c)(2)(vi) of this section.
* * * * *
(vi) Table 1 identifies the data submission process for new or
revised NAAQS, beginning with the 2015 Ozone NAAQS. This process shall
apply to those data that will or may influence the initial designation
of areas for any new or revised NAAQS.
Table 1--Schedule for Exceptional Event Flagging and Documentation
Submission for Data To Be Used in Initial Area Designations
------------------------------------------------------------------------
Exceptional event deadline
Exceptional event/regulatory action schedule \d\
------------------------------------------------------------------------
Exceptional event data flagging and If state and tribal
initial description deadline for data recommendations for the new/
years 1, 2 and 3.\a\ revised NAAQS are due
August through January,
then the flagging and
initial description
deadline will be the July 1
prior to the recommendation
deadline. If state and
tribal recommendations for
the new/revised NAAQS are
due February through July,
then the flagging and
initial description
deadline will be the
January 1 prior to the
recommendation deadline.
Exceptional event demonstration submittal No later than the date that
deadline for data years 1, 2 and 3.\a\ state and tribal
recommendations are due to
EPA.
Exceptional event data flagging, initial By the last day of the month
description, and exceptional event that is 1 year and 7 months
demonstration submittal deadline for data after promulgation of a new
year 4 \b\ and potential data year 5.\c\ or revised NAAQS, unless
either option a or b
applies.
a. If the EPA follows a 3-
year designation schedule,
the deadline is 2 years and
7 months after promulgation
of a new or revised NAAQS.
b. If the EPA notifies the
state/tribe via Federal
Register notice, letter or
guidance that it intends to
complete the initial area
designations process
according to a schedule
other than a 2-year or 3-
year timeline, the deadline
is 5 months prior to the
date specified for final
designations decisions in
such EPA notification.
------------------------------------------------------------------------
\a\ Where data years 1, 2, and 3 are those years expected to be
considered in state and tribal recommendations.
\b\ Where data year 4 is the additional year of data that the EPA may
consider when it makes final area designations for the new/revised
NAAQS under a 2-year designations schedule.
\c\ Where data year 5 is the additional year of data that the EPA may
consider when it makes final area designations for the new/revised
NAAQS under an extended designations schedule.
\d\ The date by which air agencies must certify their ambient air
quality monitoring data in AQS is annually on May 1 of the year
following the year of data collection. The EPA cannot require air
agencies to certify data prior to this date. In some cases, however,
air agencies may choose to certify a prior year's data in advance of
May 1 of the following year, particularly if the EPA has indicated its
intent to promulgate final designations in the months of May, June,
July or August. Exceptional event flagging, initial description, and
demonstration deadlines for ``early certified'' data will follow the
deadlines for ``year 4'' and ``year 5'' data.
(3) Submission of demonstrations. (i) Except as allowed under
paragraph (c)(2)(vi) of this section, a State that has flagged data as
being due to an exceptional event and is requesting exclusion of the
affected measurement data shall, after notice and opportunity for
public comment, submit a demonstration to justify data exclusion to EPA
not later than the lesser of, 3 years following the end of the calendar
quarter in which the flagged concentration was recorded or, 12 months
prior to the date that a regulatory decision must be made by EPA. A
State must submit the public comments it received along with its
demonstration to EPA.
* * * * *
0
3. Section 50.19 is added to read as follows:
Sec. 50.19 National primary and secondary ambient air quality
standards for ozone.
(a) The level of the national 8-hour primary ambient air quality
standard for ozone (O3) is (0.065-0.070) parts per million
(ppm), daily maximum 8-hour average, measured by a reference method
based on appendix D to this part and designated in accordance with part
53 of this chapter or an equivalent method designated in accordance
with part 53 of this chapter.
(b) The 8-hour primary O3 ambient air quality standard
is met at an ambient air quality monitoring site when the 3-year
average of the annual fourth-highest daily maximum 8-hour average
O3
[[Page 75397]]
concentration is less than or equal to (0.065-0.070) ppm, as determined
in accordance with appendix U to this part.
(c) The level of the national secondary ambient air quality
standard for O3 is (0.065-0.070) ppm, daily maximum 8-hour
average, measured by a reference method based on appendix D to this
part and designated in accordance with part 53 of this chapter or an
equivalent method designated in accordance with part 53 of this
chapter.
(d) The 8-hour secondary O3 ambient air quality standard
is met at an ambient air quality monitoring site when the 3-year
average of the annual fourth-highest daily maximum 8-hour average
O3 concentration is less than or equal to (0.065-0.070) ppm,
as determined in accordance with appendix U to this part.
0
4. Revise appendix D to part 50 under subchapter C to read as follows:
Appendix D to Part 50--Reference Measurement Principle and Calibration
Procedure for the Measurement of Ozone in the Atmosphere
(Chemiluminescence Method)
1.0 Applicability.
1.1 This chemiluminescence method provides reference
measurements of the concentration of ozone (O3) in
ambient air for determining compliance with the national primary and
secondary ambient air quality standards for O3 as
specified in 40 CFR part 50. This automated method is applicable to
the measurement of ambient O3 concentrations using
continuous (real-time) sampling and analysis. Additional quality
assurance procedures and guidance are provided in 40 CFR part 58,
appendix A, and in Reference 14.
2.0 Measurement Principle.
2.1 This reference method is based on continuous automated
measurement of the intensity of the characteristic chemiluminescence
released by the gas phase reaction of O3 in sampled air
with either ethylene (C2H4) or nitric oxide
(NO) gas. An ambient air sample stream and a specific flowing
concentration of either C2H4 (ET-CL method) or
NO (NO-CL method) are mixed in a measurement cell, where the
resulting chemiluminescence is quantitatively measured by a
sensitive photo-detector. References 8-11 describe the
chemiluminescence measurement principle.
2.2 The measurement system is calibrated by referencing the
instrumental chemiluminescence measurements to certified
O3 standard concentrations generated in a dynamic flow
system and assayed by photometry to be traceable to a National
Institute of Standards and Technology (NIST) standard reference
photometer for O3 (see Section 4, Calibration Procedure,
below).
2.3 An analyzer implementing this measurement principle is shown
schematically in Figure 1. Designs implementing this measurement
principle must include: An appropriately designed mixing and
measurement cell; a suitable quantitative photometric measurement
system with adequate sensitivity and wave length specificity for
O3; a pump, flow control, and sample conditioning system
for sampling and drying the ambient air and moving it into and
through the measurement cell; a means to supply, meter, and mix a
constant, flowing stream of either C2H4 or NO
gas of fixed concentration with the sample air flow in the
measurement cell; suitable electronic control and measurement
processing capability; and other associated apparatus as may be
necessary. The analyzer must be designed and constructed to provide
accurate, repeatable, and continuous measurements of O3
concentrations in ambient air, with measurement performance that
meets the requirements specified in subpart B of part 53 of this
chapter.
2.4 An analyzer implementing this measurement principle and
calibration procedure will be considered a federal reference method
(FRM) only if it has been designated as a reference method in
accordance with part 53 of this chapter.
2.5 Sampling considerations. The use of a particle filter on the
sample inlet line of a chemiluminescence O3 FRM analyzer
is required to prevent buildup of particulate matter in the
measurement cell and inlet components. This filter must be changed
weekly (or at least often as specified in the manufacturer's
operation/instruction manual), and the sample inlet system used with
the analyzer must be kept clean, to avoid loss of O3 in
the O3 sample air prior to the concentration measurement.
3.0 Interferences.
3.1 Except as described in 3.2 below, the chemiluminescence
measurement system is inherently free of significant interferences
from other pollutant substances that may be present in ambient air.
3.2 A small sensitivity to variations in the humidity of the
sample air is minimized by a sample air dryer. Potential loss of
O3 in the inlet air filter and in the air sample handling
components of the analyzer and associated exterior air sampling
components due to buildup of airborne particulate matter is
minimized by filter replacement and cleaning of the other inlet
components.
4.0 Calibration Procedure.
4.1 Principle. The calibration procedure is based on the
photometric assay of O3 concentrations in a dynamic flow
system. The concentration of O3 in an absorption cell is
determined from a measurement of the amount of 254 nm light absorbed
by the sample. This determination requires knowledge of (1) the
absorption coefficient ([alpha]) of O3 at 254 nm, (2) the
optical path length (l) through the sample, (3) the transmittance of
the sample at a nominal wavelength of 254 nm, and (4) the
temperature (T) and pressure (P) of the sample. The transmittance is
defined as the ratio I/I0, where I is the intensity of
light which passes through the cell and is sensed by the detector
when the cell contains an O3 sample, and I0 is
the intensity of light which passes through the cell and is sensed
by the detector when the cell contains zero air. It is assumed that
all conditions of the system, except for the contents of the
absorption cell, are identical during measurement of I and
I0. The quantities defined above are related by the Beer-
Lambert absorption law,
[GRAPHIC] [TIFF OMITTED] TP17DE14.002
Where:
[alpha] = absorption coefficient of O3 at 254 nm = 308
4 atm - 1 cm - 1 at 0 [deg]C and 760
torr,1, 2, 3, 4, 5, 6, 7
c = O3 concentration in atmospheres, and
l = optical path length in cm.
A stable O3 generator is used to produce
O3 concentrations over the required calibration
concentration range. Each O3 concentration is determined
from the measurement of the transmittance (I/I0) of the
sample at 254 nm with a photometer of path length l and calculated
from the equation,
[GRAPHIC] [TIFF OMITTED] TP17DE14.003
[[Page 75398]]
The calculated O3 concentrations must be corrected
for O3 losses, which may occur in the photometer, and for
the temperature and pressure of the sample.
4.2 Applicability. This procedure is applicable to the
calibration of ambient air O3 analyzers, either directly
or by means of a transfer standard certified by this procedure.
Transfer standards must meet the requirements and specifications set
forth in Reference 12.
4.3 Apparatus. A complete UV calibration system consists of an
O3 generator, an output port or manifold, a photometer,
an appropriate source of zero air, and other components as
necessary. The configuration must provide a stable O3
concentration at the system output and allow the photometer to
accurately assay the output concentration to the precision specified
for the photometer (4.3.1). Figure 2 shows a commonly used
configuration and serves to illustrate the calibration procedure,
which follows. Other configurations may require appropriate
variations in the procedural steps. All connections between
components in the calibration system downstream of the O3
generator must be of glass, Teflon, or other relatively inert
materials. Additional information regarding the assembly of a UV
photometric calibration apparatus is given in Reference 13. For
certification of transfer standards which provide their own source
of O3, the transfer standard may replace the
O3 generator and possibly other components shown in
Figure 2; see Reference 12 for guidance.
4.3.1 UV photometer. The photometer consists of a low-pressure
mercury discharge lamp, (optional) collimation optics, an absorption
cell, a detector, and signal-processing electronics, as illustrated
in Figure 2. It must be capable of measuring the transmittance, I/
I0, at a wavelength of 254 nm with sufficient precision
such that the standard deviation of the concentration measurements
does not exceed the greater of 0.005 ppm or 3% of the concentration.
Because the low-pressure mercury lamp radiates at several
wavelengths, the photometer must incorporate suitable means to
assure that no O3 is generated in the cell by the lamp,
and that at least 99.5% of the radiation sensed by the detector is
254 nm radiation. (This can be readily achieved by prudent selection
of optical filter and detector response characteristics.) The length
of the light path through the absorption cell must be known with an
accuracy of at least 99.5%. In addition, the cell and associated
plumbing must be designed to minimize loss of O3 from
contact with cell walls and gas handling components. See Reference
13 for additional information.
4.3.2 Air flow controllers. Air flow controllers are devices
capable of regulating air flows as necessary to meet the output
stability and photometer precision requirements.
4.3.3 Ozone generator. The ozone generator used must be capable
of generating stable levels of O3 over the required
concentration range.
4.3.4 Output manifold. The output manifold must be constructed
of glass, Teflon, or other relatively inert material, and should be
of sufficient diameter to insure a negligible pressure drop at the
photometer connection and other output ports. The system must have a
vent designed to insure atmospheric pressure in the manifold and to
prevent ambient air from entering the manifold.
4.3.5 Two-way valve. A manual or automatic two-way valve, or
other means is used to switch the photometer flow between zero air
and the O3 concentration.
4.3.6 Temperature indicator. A device to indicate temperature
must be used that is accurate to 1 [deg]C.
4.3.7 Barometer or pressure indicator. A device to indicate
barometric pressure must be used that is accurate to 2
torr.
4.4 Reagents.
4.4.1 Zero air. The zero air must be free of contaminants which
would cause a detectable response from the O3 analyzer,
and it must be free of NO, C2H4, and other
species which react with O3. A procedure for generating
suitable zero air is given in Reference 13. As shown in Figure 2,
the zero air supplied to the photometer cell for the I0
reference measurement must be derived from the same source as the
zero air used for generation of the O3 concentration to
be assayed (I measurement). When using the photometer to certify a
transfer standard having its own source of O3, see
Reference 12 for guidance on meeting this requirement.
4.5 Procedure.
4.5.1 General operation. The calibration photometer must be
dedicated exclusively to use as a calibration standard. It must
always be used with clean, filtered calibration gases, and never
used for ambient air sampling. A number of advantages are realized
by locating the calibration photometer in a clean laboratory where
it can be stationary, protected from the physical shock of
transportation, operated by a responsible analyst, and used as a
common standard for all field calibrations via transfer standards.
4.5.2 Preparation. Proper operation of the photometer is of
critical importance to the accuracy of this procedure. Upon initial
operation of the photometer, the following steps must be carried out
with all quantitative results or indications recorded in a
chronological record, either in tabular form or plotted on a
graphical chart. As the performance and stability record of the
photometer is established, the frequency of these steps may be
reduced to be consistent with the documented stability of the
photometer and the guidance provided in Reference 12.
4.5.2.1 Instruction manual. Carry out all set up and adjustment
procedures or checks as described in the operation or instruction
manual associated with the photometer.
4.5.2.2 System check. Check the photometer system for integrity,
leaks, cleanliness, proper flow rates, etc. Service or replace
filters and zero air scrubbers or other consumable materials, as
necessary.
4.5.2.3 Linearity. Verify that the photometer manufacturer has
adequately established that the linearity error of the photometer is
less than 3%, or test the linearity by dilution as follows: Generate
and assay an O3 concentration near the upper range limit
of the system or appropriate calibration scale for the instrument,
then accurately dilute that concentration with zero air and re-assay
it. Repeat at several different dilution ratios. Compare the assay
of the original concentration with the assay of the diluted
concentration divided by the dilution ratio, as follows
[GRAPHIC] [TIFF OMITTED] TP17DE14.004
Where:
E = linearity error, percent
A1 = assay of the original concentration
A2 = assay of the diluted concentration
R = dilution ratio = flow of original concentration divided by the
total flow
The linearity error must be less than 5%. Since the accuracy of
the measured flow-rates will affect the linearity error as measured
this way, the test is not necessarily conclusive. Additional
information on verifying linearity is contained in Reference 13.
4.5.2.4 Inter-comparison. The photometer must be inter-compared
annually, either directly or via transfer standards, with a NIST
standard reference photometer (SRP) or calibration photometers used
by other agencies or laboratories.
4.5.2.5 Ozone losses. Some portion of the O3 may be
lost upon contact with the photometer cell walls and gas handling
components. The magnitude of this loss must be determined and used
to correct the calculated O3 concentration. This loss
must not exceed 5%. Some guidelines for quantitatively determining
this loss are discussed in Reference 13.
4.5.3 Assay of O3 concentrations. The operator must carry out
the following steps to properly assay O3 concentrations.
4.5.3.1 Allow the photometer system to warm up and stabilize.
4.5.3.2 Verify that the flow rate through the photometer
absorption cell, F, allows the cell to be flushed in a reasonably
short period of time (2 liter/min is a typical flow). The precision
of the measurements is inversely related to the time required for
flushing, since the photometer drift error increases with time.
4.5.3.3 Ensure that the flow rate into the output manifold is at
least 1 liter/min greater than the total flow rate required by the
photometer and any other flow demand connected to the manifold.
[[Page 75399]]
4.5.3.4 Ensure that the flow rate of zero air, Fz, is at least 1
liter/min greater than the flow rate required by the photometer.
4.5.3.5 With zero air flowing in the output manifold, actuate
the two-way valve to allow the photometer to sample first the
manifold zero air, then Fz. The two photometer readings must be
equal (I = I0).
Note: In some commercially available photometers, the operation
of the two-way valve and various other operations in section 4.5.3
may be carried out automatically by the photometer.
4.5.3.6 Adjust the O3 generator to produce an
O3 concentration as needed.
4.5.3.7 Actuate the two-way valve to allow the photometer to
sample zero air until the absorption cell is thoroughly flushed and
record the stable measured value of Io.
4.5.3.8 Actuate the two-way valve to allow the photometer to
sample the O3 concentration until the absorption cell is
thoroughly flushed and record the stable measured value of I.
4.5.3.9 Record the temperature and pressure of the sample in the
photometer absorption cell. (See Reference 13 for guidance.)
4.5.3.10 Calculate the O3 concentration from equation
4. An average of several determinations will provide better
precision.
[GRAPHIC] [TIFF OMITTED] TP17DE14.005
Where:
[O3]OUT = O3 concentration, ppm
[alpha] = absorption coefficient of O3 at 254 nm = 308
atm-1 cm-1 at 0 [deg]C and 760 torr
l = optical path length, cm
T = sample temperature, K
P = sample pressure, torr
L = correction factor for O3 losses from 4.5.2.5 = (1 -
fraction of O3 lost).
Note: Some commercial photometers may automatically evaluate all
or part of equation 4. It is the operator's responsibility to verify
that all of the information required for equation 4 is obtained,
either automatically by the photometer or manually. For
``automatic'' photometers which evaluate the first term of equation
4 based on a linear approximation, a manual correction may be
required, particularly at higher O3 levels. See the
photometer instruction manual and Reference 13 for guidance.
4.5.3.11 Obtain additional O3 concentration standards
as necessary by repeating steps 4.5.3.6 to 4.5.3.10 or by Option 1.
4.5.4 Certification of transfer standards. A transfer standard
is certified by relating the output of the transfer standard to one
or more O3 calibration standards as determined according
to section 4.5.3. The exact procedure varies depending on the nature
and design of the transfer standard. Consult Reference 12 for
guidance.
4.5.5 Calibration of ozone analyzers. Ozone analyzers must be
calibrated as follows, using O3 standards obtained
directly according to section 4.5.3 or by means of a certified
transfer standard.
4.5.5.1 Allow sufficient time for the O3 analyzer and
the photometer or transfer standard to warm-up and stabilize.
4.5.5.2 Allow the O3 analyzer to sample zero air
until a stable response is obtained and then adjust the
O3 analyzer's zero control. Offsetting the analyzer's
zero adjustment to +5% of scale is recommended to facilitate
observing negative zero drift (if any). Record the stable zero air
response as ``Z''.
4.5.5.3 Generate an O3 concentration standard of
approximately 80% of the desired upper range limit (URL) of the
O3 analyzer. Allow the O3 analyzer to sample
this O3 concentration standard until a stable response is
obtained.
4.5.5.4 Adjust the O3 analyzer's span control to
obtain the desired response equivalent to the calculated standard
concentration. Record the O3 concentration and the
corresponding analyzer response. If substantial adjustment of the
span control is necessary, recheck the zero and span adjustments by
repeating steps 4.5.5.2 to 4.5.5.4.
4.5.5.5 Generate additional O3 concentration
standards (a minimum of 5 are recommended) over the calibration
scale of the O3 analyzer by adjusting the O3
source or by Option 1. For each O3 concentration
standard, record the O3 concentration and the
corresponding analyzer response.
4.5.5.6 Plot the O3 analyzer responses (vertical or
Y-axis) versus the corresponding O3 standard
concentrations (horizontal or X-axis). Compute the linear regression
slope and intercept and plot the regression line to verify that no
point deviates from this line by more than 2 percent of the maximum
concentration tested.
4.5.5.7 Option 1: The various O3 concentrations
required in steps 4.5.3.11 and 4.5.5.5 may be obtained by dilution
of the O3 concentration generated in steps 4.5.3.6 and
4.5.5.3. With this option, accurate flow measurements are required.
The dynamic calibration system may be modified as shown in Figure 3
to allow for dilution air to be metered in downstream of the
O3 generator. A mixing chamber between the O3
generator and the output manifold is also required. The flow rate
through the O3 generator (Fo) and the dilution
air flow rate (FD) are measured with a flow or volume standard that
is traceable to a NIST flow or volume calibration standard. Each
O3 concentration generated by dilution is calculated
from:
[GRAPHIC] [TIFF OMITTED] TP17DE14.006
Where:
[O3]'OUT = diluted O3
concentration, ppm
FO = flow rate through the O3 generator, liter/min
FD = diluent air flow rate, liter/min
Note: Additional information on calibration and pollutant
standards is provided in Section 12 of Reference 14.
5.0 Frequency of Calibration.
5.1 The frequency of calibration, as well as the number of
points necessary to establish the calibration curve, and the
frequency of other performance checking will vary by analyzer;
however, the minimum frequency, acceptance criteria, and subsequent
actions are specified in Appendix D of Reference 14: Measurement
Quality Objectives and Validation Templates. The user's quality
control program shall provide guidelines for initial establishment
of these variables and for subsequent alteration as operational
experience is accumulated. Manufacturers of analyzers should include
in their instruction/operation manuals information and guidance as
to these variables and on other matters of operation, calibration,
routine maintenance, and quality control.
6.0 References.
1. E.C.Y. Inn and Y. Tanaka, ``Absorption coefficient of Ozone
in the Ultraviolet and Visible Regions'', J. Opt. Soc. Am., 43, 870
(1953).
2. A.G. Hearn, ``Absorption of Ozone in the Ultraviolet and
Visible Regions of the Spectrum'', Proc. Phys. Soc. (London), 78,
932 (1961).
3. W.B. DeMore and O. Raper, ``Hartley Band Extinction
Coefficients of Ozone in the Gas Phase and in Liquid Nitrogen,
Carbon Monoxide, and Argon'', J. Phys. Chem., 68, 412 (1964).
4. M. Griggs, ``Absorption Coefficients of Ozone in the
Ultraviolet and Visible Regions'', J. Chem. Phys., 49, 857 (1968).
5. K.H. Becker, U. Schurath, and H. Seitz, ``Ozone Olefin
Reactions in the Gas Phase. 1. Rate Constants and Activation
Energies'', Int'l Jour. of Chem. Kinetics, VI, 725 (1974).
6. M.A.A. Clyne and J.A. Coxom, ``Kinetic Studies of Oxy-halogen
Radical Systems'', Proc. Roy. Soc., A303, 207 (1968).
7. J.W. Simons, R.J. Paur, H.A. Webster, and E.J. Bair, ``Ozone
Ultraviolet Photolysis. VI. The Ultraviolet Spectrum'', J. Chem.
Phys., 59, 1203 (1973).
8. Ollison, W.M.; Crow, W.; Spicer, C.W. ``Field testing of new-
technology ambient air
[[Page 75400]]
ozone monitors.'' J. Air Waste Manage. Assoc., 63 (7), 855-863
(2013).
9. Parrish, D.D.; Fehsenfeld, F.C. ``Methods for gas-phase
measurements of ozone, ozone precursors and aerosol precursors.''
Atmos. Environ., 34 (12-14), 1921-1957 (2000).
10. Ridley, B.A.; Grahek, F.E.; Walega, J.G. ``A small, high-
sensitivity, medium-response ozone detector suitable for
measurements from light aircraft.'' J. Atmos. Oceanic Technol., 9
(2), 142-148 (1992).
11. Boylan, P., Helmig, D., and Park, J.H. ``Characterization
and mitigation of water vapor effects in the measurement of ozone by
chemiluminescence with nitric oxide.'' Atmos. Meas. Tech. 7, 1231-
1244 (2014).
12. Transfer Standards for Calibration of Ambient Air Monitoring
Analyzers for Ozone, EPA publication number EPA-454/B-13-004,
October 2013. EPA, Office of Air Quality Planning and Standards,
Research Triangle Park, NC 27711. [Available at www.epa.gov/ttnamti1/files/ambient/qaqc/OzoneTransferStandardGuidance.pdf.]
13. Technical Assistance Document for the Calibration of Ambient
Ozone Monitors, EPA publication number EPA-600/4-79-057, September,
1979. [Available at www.epa.gov/ttnamti1/files/ambient/criteria/4-79-057.pdf.]
14. QA Handbook for Air Pollution Measurement Systems--Volume
II. Ambient Air Quality Monitoring Program. EPA-454/B-13-003, May
2013. [Available at http://www.epa.gov/ttnamti1/files/ambient/pm25/qa/QA-Handbook-Vol-II.pdf.]
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0
5. Appendix U to Part 50 is added to read as follows:
Appendix U to Part 50--Interpretation of the Primary and Secondary
National Ambient Air Quality Standards for Ozone
1. General
(a) This appendix explains the data handling conventions and
computations necessary for determining whether the primary and
secondary national ambient air quality standards (NAAQS) for ozone
(O3) specified in Sec. 50.19 are met at an ambient
O3 air quality monitoring site. Data reporting, data
handling, and computation procedures to be used in making
comparisons between reported O3 concentrations and the
levels of the O3 NAAQS are specified in the following
sections.
(b) Whether to exclude or retain the data affected by
exceptional events is determined by the requirements under
Sec. Sec. 50.1, 50.14 and 51.930.
(c) The terms used in this appendix are defined as follows:
8-hour average refers to the moving average of eight consecutive
hourly O3 concentrations measured at a site, as explained
in section 3 of this appendix.
Annual fourth-highest daily maximum refers to the fourth highest
value measured at a site during a particular year.
Collocated monitors refers to the instance of two or more
O3 monitors operating at the same site.
Daily maximum 8-hour average O3 concentration refers to the
maximum calculated 8-hour average value measured at a site on a
particular day, as explained in section 3 of this appendix.
Design value refers to the metric (i.e., statistic) that is used
to compare ambient O3 concentration data measured at a
site to the NAAQS in order to determine compliance, as explained in
section 4 of this appendix.
Minimum data completeness requirements refer to the amount of
data that a site is required to collect in order to make a valid
determination that the site is meeting the NAAQS.
Monitor refers to a physical instrument used to measure ambient
O3 concentrations.
O3 monitoring season refers to the span of time within a year
when individual states are required to measure ambient O3
concentrations, as listed in Appendix D to part 58 of this chapter.
Site refers to an ambient O3 air quality monitoring
site.
Site data record refers to the set of hourly O3
concentration data collected at a site for use in comparisons with
the NAAQS.
Year refers to calendar year.
2. Selection of Data for use in Comparisons With the Primary and
Secondary Ozone NAAQS
(a) All valid hourly O3 concentration data collected
using a federal reference method specified in Appendix D to this
part, or an equivalent method designated in accordance with part 53
of this chapter, meeting all applicable requirements in part 58 of
this chapter, and submitted to EPA's Air Quality System (AQS)
database, or otherwise available to EPA, shall be used in design
value calculations. Data not meeting these requirements shall not be
used in design value calculations.
(b) All design value calculations shall be implemented on a
site-level basis. If data are reported to EPA from collocated
monitors, those data shall be combined into a single site data
record as follows:
(i) The monitoring agency may designate one monitor as the
primary monitor for the site. If a primary monitor has not been
designated by the monitoring agency, the monitor with the largest
number of hourly O3 concentrations reported for the year
shall be designated as the primary monitor.
(ii) Hourly O3 concentration data from a collocated
monitor shall be substituted into the site data record whenever a
valid hourly O3 concentration is not obtained from the
primary monitor. In the event that hourly O3
concentration data are available for two or more collocated
monitors, the hourly
[[Page 75403]]
concentration data for those monitors shall be averaged and
substituted into the site data record.
(c) In certain circumstances, including but not limited to site
closures or relocations, data from two nearby sites may be combined
into a single site data record for the purpose of calculating a
valid design value. The appropriate Regional Administrator may
approve such combinations after taking into consideration factors
such as distance between sites, spatial and temporal patterns in air
quality, local emissions and meteorology, jurisdictional boundaries,
and terrain features.
3. Data Reporting and Data Handling Conventions
(a) Hourly average O3 concentrations shall be
reported in parts per million (ppm) to the third decimal place, with
additional digits to the right of the third decimal place truncated.
Each hour shall be identified using local standard time (LST).
(b) Moving 8-hour averages shall be computed from the hourly
O3 concentration data for each hour of the year and shall
be stored in the first, or start, hour of the 8-hour period. An 8-
hour average shall be considered valid if at least 6 of the hourly
concentrations for the 8-hour period are available. In the event
that only 6 or 7 hourly concentrations are available, the 8-hour
average shall be computed on the basis of the hours available, using
6 or 7 as the divisor. In addition, in the event that 5 or fewer
hourly concentrations are available, the 8-hour average shall be
considered valid if, after substituting zero for the missing hourly
concentrations, the resulting 8-hour average is greater than the
level of the NAAQS. The 8-hour averages shall be reported to three
decimal places, with additional digits to the right of the third
decimal place truncated. Hourly O3 concentrations that
have been approved under Sec. 50.14 as having been affected by
exceptional events shall be counted as missing or unavailable in the
calculation of 8-hour averages.
(c) The daily maximum 8-hour average O3 concentration
for a given day is the highest of the 17 consecutive 8-hour averages
beginning with the 8-hour period from 7:00 a.m. to 3:00 p.m. and
ending with the 8-hour period from 11:00 p.m. to 7:00 a.m. (i.e.,
the 8-hour averages for 7:00 a.m. to 11:00 p.m.). Daily maximum 8-
hour average O3 concentrations shall be determined for
each day with ambient O3 monitoring data, including days
outside the O3 monitoring season if those data are
available.
(d) A daily maximum 8-hour average O3 concentration
shall be considered valid if valid 8-hour averages are available for
at least 13 of the 17 consecutive 8-hour periods starting from 7:00
a.m. to 11:00 p.m. In addition, in the event that fewer than 13
valid 8-hour averages are available, a daily maximum 8-hour average
O3 concentration shall also be considered valid if it is
greater than the level of the NAAQS. Hourly O3
concentrations that have been approved under Sec. 50.14 as having
been affected by exceptional events shall be included when
determining whether these criteria have been met.
(e) The primary and secondary O3 design value
statistic is the annual fourth-highest daily maximum 8-hour
O3 concentration, averaged over three years, expressed in
parts per million. The fourth-highest daily maximum 8-hour
O3 concentration for each year shall be determined based
only on days meeting the validity criteria in 3(d). The 3-year
average shall be computed using the three most recent, consecutive
years of ambient O3 monitoring data. Design values shall
be reported to three decimal places, with additional digits to the
right of the third decimal place truncated.
4. Comparisons With the Primary and Secondary Ozone NAAQS
(a) The primary and secondary national ambient air quality
standards for O3 are met at an ambient air quality
monitoring site when the 3-year average of the annual fourth-highest
daily maximum 8-hour average O3 concentration (i.e., the
design value) is less than or equal to (0.065-0.070) ppm.
(b) A design value greater than the level of the NAAQS is always
considered to be valid. A design value less than or equal to the
level of the NAAQS must meet minimum data completeness requirements
in order to be considered valid. These requirements are met for a 3-
year period at a site if valid daily maximum 8-hour average
O3 concentrations are available for at least 90% of the
days within the O3 monitoring season, on average, for the
3-year period, with a minimum of at least 75% of the days within the
O3 monitoring season in any one year.
(c) When computing whether the minimum data completeness
requirements have been met, meteorological or ambient data may be
sufficient to demonstrate that meteorological conditions on missing
days were not conducive to concentrations above the level of the
NAAQS. Missing days assumed less than the level of the NAAQS are
counted for the purpose of meeting the minimum data completeness
requirements, subject to the approval of the appropriate Regional
Administrator.
(d) Comparisons with the primary and secondary O3
NAAQS are demonstrated by examples 1 and 2 as follows:
Example 1: Site Meeting the Primary and Secondary O3
NAAQS
----------------------------------------------------------------------------------------------------------------
Percent valid
days within O3 1st highest 2nd highest 3rd highest 4th highest 5th highest
Year monitoring daily max 8- daily max 8- daily max 8- daily max 8- daily max 8-
season hour O3 (ppm) hour O3 (ppm) hour O3 (ppm) hour O3 (ppm) hour O3 (ppm)
----------------------------------------------------------------------------------------------------------------
2014............ 100 0.082 0.080 0.075 0.069 0.068
2015............ 96 0.074 0.073 0.065 0.062 0.060
2016............ 98 0.070 0.069 0.067 0.066 0.060
Average......... 98 .............. .............. .............. 0.065 ..............
----------------------------------------------------------------------------------------------------------------
As shown in Example 1, this site meets the primary and secondary
O3 NAAQS because the 3-year average of the annual fourth-
highest daily maximum 8-hour average O3 concentrations
(i.e., 0.065666 ppm, truncated to 0.065 ppm) is less than or equal
to (0.065-0.070) ppm. The minimum data completeness requirements are
also met because the average percent of days within the
O3 monitoring season with valid ambient monitoring data
is greater than 90%, and no single year has less than 75% data
completeness.
Example 2: Site Failing to Meet the Primary and Secondary O3
O3 NAAQS
----------------------------------------------------------------------------------------------------------------
Percent valid
days within O3 1st highest 2nd highest 3rd highest 4th highest 5th highest
Year monitoring daily max 8- daily max 8- daily max 8- daily max 8- daily max 8-
season hour O3 (ppm) hour O3 (ppm) hour O3 (ppm) hour O3 (ppm) hour O3 (ppm)
----------------------------------------------------------------------------------------------------------------
2014............ 96 0.085 0.080 0.079 0.074 0.072
2015............ 74 0.084 0.083 0.072 0.071 0.068
2016............ 98 0.083 0.081 0.081 0.075 0.074
Average......... 89 .............. .............. .............. 0.073 ..............
----------------------------------------------------------------------------------------------------------------
[[Page 75404]]
As shown in Example 2, this site fails to meet the primary and
secondary O3 NAAQS because the 3-year average of the
annual fourth-highest daily maximum 8-hour average O3
concentrations (i.e., 0.073333 ppm, truncated to 0.073 ppm) is
greater than (0.065-0.070) ppm, even though the annual data
completeness is less than 75% in one year and the 3-year average
data completeness is less than 90%.
PART 51--REQUIREMENTS FOR PREPARATION, ADOPTION, AND SUBMITTAL OF
IMPLEMENATION PLANS
0
6. The authority citation for part 51 continues to read as follows:
Authority: 23 U.S.C. 101; 42 U.S.C. 7401-7671q.
Subpart I--Review of New Sources and Modifications
0
7 Amend Sec. 51.166 by adding paragraph (i)(11) to read as follows:
Sec. 51.166 Prevention of significant deterioration of air quality.
* * * * *
(i) * * *
(11) The plan may provide that the requirements of paragraph (k)(1)
of this section shall not apply to a stationary source or modification
with respect to the national ambient air quality standards for ozone in
effect on [EFFECTIVE DATE OF FINAL RULE] if:
(i) The reviewing authority has determined a permit application
subject to this section to be complete on or before [SIGNATURE DATE OF
FINAL RULE]. Instead, the requirements in paragraph (k)(1) of this
section shall apply with respect to the national ambient air quality
standards for ozone in effect at the time the reviewing authority
determined the permit application to be complete; or
(ii) The reviewing authority has first published before [EFFECTIVE
DATE OF FINAL RULE] a public notice of a preliminary determination or
draft permit for the permit application subject to this section.
Instead, the requirements in paragraph (k)(1) of this section shall
apply with respect to the national ambient air quality standards for
ozone in effect at the time of first publication of a public notice of
the preliminary determination or draft permit.
* * * * *
PART 52--APPROVAL AND PROMULGATION OF IMPLEMENTATION PLANS
0
8. The authority citation for part 52 continues to read as follows:
Authority: 42 U.S.C. 7401 et seq.
0
9. Amend Sec. 52.21 by adding paragraph (i)(12) to read as follows:
Sec. 52.21 Prevention of significant deterioration of air quality.
* * * * *
(i) * * *
(12) The requirements of paragraph (k)(1) of this section shall not
apply to a stationary source or modification with respect to the
national ambient air quality standards for ozone in effect on
[EFFECTIVE DATE OF FINAL RULE] if:
(i) The Administrator has determined a permit application subject
to this section to be complete on or before [SIGNATURE DATE OF FINAL
RULE]. Instead, the requirements in paragraph (k)(1) of this section
shall apply with respect to the national ambient air quality standards
for ozone in effect at the time the Administrator determined the permit
application to be complete; or
(ii) The Administrator has first published before [EFFECTIVE DATE
OF FINAL RULE] a public notice of a preliminary determination or draft
permit subject to this section. Instead, the requirements in paragraph
(k)(1) of this section shall apply with respect to the national ambient
air quality standards for ozone in effect on the date the Administrator
first published a public notice of a preliminary determination or draft
permit.
* * * * *
PART 53--AMBIENT AIR MONITORING REFERENCE AND EQUIVALENT METHODS
0
10. The authority citation for part 53 continues to read as follows:
Authority: Sec. 301(a) of the Clean Air Act (42 U.S.C.
1857g(a)), as amended by sec. 15(c)(2) of Pub. L. 91-604, 84 Stat.
1713, unless otherwise noted.
Subpart A--General Provisions
Sec. 53.9 [Amended]
0
11. Amend Sec. 53.9 by removing paragraph (i).
0
12. Amend Sec. 53.14 by revising paragraph (c) introductory text to
read as follows:
Sec. 53.14 Modification of a reference or equivalent method.
* * * * *
(c) Within 90 calendar days after receiving a report under
paragraph (a) of this section, the Administrator will take one or more
of the following actions:
* * * * *
Subpart B--Procedures for Testing Performance Characteristics of
Automated Methods for SO2, CO, O3, and
NO2
0
13. Amend Sec. 53.23 by revising paragraph (e)(1)(vi) to read as
follows:
Sec. 53.23 Test procedures.
* * * * *
(e) * * *
(1) * * *
(vi) Precision: Variation about the mean of repeated measurements
of the same pollutant concentration, denoted as the standard deviation
expressed as a percentage of the upper range limits.\282\
---------------------------------------------------------------------------
\282\ NO2 precision in Table B-1 is also changed to
percent to agree with the calculation specified in 53.23(e)(10)(vi).
---------------------------------------------------------------------------
* * * * *
0
14. Revise Table B-1 to Subpart B of Part 53 to read as follows:
Table B-1 to Subpart B of Part 53--Performance Limit Specifications for Automated Methods
--------------------------------------------------------------------------------------------------------------------------------------------------------
SO2 O3 CO
------------------------------------------------------------------
Performance parameter Units \1\ Lower Lower Lower NO2 (Std. Definitions and test
Std. range Std. range Std. range range) procedures
range \3\ \2,3\ range \3\ \2,3\ range \3\ \2,3\
--------------------------------------------------------------------------------------------------------------------------------------------------------
1. Range........................ ppm............. 0-0.5 <0.5 0-0.5 <0.5 0-50 <50 0-0.5 Sec. 53.23(a).
2. Noise........................ ppm............. 0.001 0.0005 0.001 0.0005 0.2 0.1 0.005 Sec. 53.23(b).
3. Lower detectable limit....... ppm............. 0.002 0.001 0.003 0.001 0.4 0.2 0.010 Sec. 53.23(c).
4. Interference equivalent
Each interferent............ ppm............. 0.0 minus>0.0 minus>0.0 minus>0.0 minus>1.0 minus>0.5 minus>0.0
05 05 05 05 2
Total, all interferents..... ppm............. ......... ......... ......... ......... ......... ......... 0.04 Sec. 53.23(d).
5. Zero drift, 12 and 24 hour... ppm............. 0.0 minus>0.0 minus>0.0 minus>0.0 minus>0.5 minus>0.3 minus>0.0
04 02 04 02 2
6. Span drift, 24 hour
[[Page 75405]]
20% of upper range limit.... Percent......... ......... ......... ......... ......... ......... ......... 20.
0
80% of upper range limit.... Percent......... 3.0 minus>3.0 minus>3.0 minus>3.0 minus>2.0 minus>2.0 minus>5.0
7. Lag time..................... Minutes......... 2 2 2 2 2.0 2.0 20 Sec. 53.23(e).
8. Rise time.................... Minutes......... 2 2 2 2 2.0 2.0 15 Sec. 53.23(e).
9. Fall time.................... Minutes......... 2 2 2 2 2.0 2.0 15 Sec. 53.23(e).
10. Precision
20% of upper range limit.... ................ ......... ......... ......... ......... ......... ......... ......... Sec. 53.23(e).
Percent \5\..... 2 2 2 2 1.0 1.0 4 Sec. 53.23(e).
................ ......... ......... ......... ......... ......... ......... ......... Sec. 53.23(e).
80% of upper range limit.... Percent \5\..... 2 2 2 2 1.0 1.0 6 Sec. 53.23(e).
--------------------------------------------------------------------------------------------------------------------------------------------------------
\1\ To convert from parts per million (ppm) to [mu]g/m\3\ at 25 [deg]C and 760 mm Hg, multiply by M/0.02447, where M is the molecular weight of the gas.
Percent means percent of the upper measurement range limit.
\2\ Tests for interference equivalent and lag time do not need to be repeated for any lower range provided the test for the standard range shows that
the lower range specification (if applicable) is met for each of these test parameters.
\3\ For candidate analyzers having automatic or adaptive time constants or smoothing filters, describe their functional nature, and describe and conduct
suitable tests to demonstrate their function aspects and verify that performances for calibration, noise, lag, rise, fall times, and precision are
within specifications under all applicable conditions. For candidate analyzers with operator-selectable time constants or smoothing filters, conduct
calibration, noise, lag, rise, fall times, and precision tests at the highest and lowest settings that are to be included in the FRM or FEM
designation.
\4\ For nitric oxide interference for the SO2 UVF method, interference equivalent is 0.0003 ppm for the lower range.
\5\ Standard deviation expressed as percent of the URL.
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0
15. Revise table B-3 to subpart B of part 53 to read as follows:
0
16. Amend appendix A to subpart B of part 53 by revising ``Figure B-5''
to read as follows:
Appendix A to Subpart B of Part 53--Optional Forms for Reporting
Test Results
* * * * *
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[[Page 75410]]
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* * * * *
Subpart C--Procedures for Determining Comparability Between
Candidate Methods and Reference Methods
0
17. Amend Sec. 53.32 by revising paragraph (g)(1)(iii) to read as
follows:
Sec. 53.32 Test procedures for methods for SO2, CO,
O3, and NO2.
* * * * *
(g) * * *
(1) * * *
(iii) The measurements shall be made in the sequence specified in
table C-2 of this subpart.
* * * * *
Figure E-2 to Subpart E of Part 53 [Removed]
0
18. Amend subpart E by removing figure E-2 to subpart E of part 53.
PART 58--AMBIENT AIR QUALITY SURVEILLANCE
0
19. The authority citation for part 58 continues to read as follows:
Authority: 42 U.S.C. 7403, 7405, 7410, 7414, 7601, 7611, 7614,
and 7619.
Subpart B--Monitoring Network
0
20. Amend Sec. 58.10 by adding paragraphs (a)(10) and (11) to read as
follows:
Sec. 58.10 Annual monitoring network plan and periodic network
assessment
(a) * * *
(10) The annual monitoring network plan shall provide for the
required O3 sites to be operating on the first day of the
applicable required O3 monitoring season in effect on
January 1, 2017 as listed in Table D-3 of appendix D of this part.
(11) The annual monitoring network plan shall include the Enhanced
Monitoring Plan (EMP) for areas designated as O3
nonattainment, as required under 40 CFR part 58 Appendix D, section
5(f) beginning with the annual monitoring plans due on July 1, 2016.
* * * * *
0
21. Amend Sec. 58.13 by adding paragraphs (g) and (h) to read as
follows:
Sec. 58.13 Monitoring network completion.
* * * * *
(g) The O3 monitors required under appendix D, section
4.1 of this part must operate on the first day of the applicable
required O3 monitoring season in effect January 1, 2017.
(h) The Photochemical Assessment Monitoring sites required under 40
CFR part 58 Appendix D, section 5(a) must be physically established and
operating under all of the requirements of this part, including the
requirements of appendix A, C, D, and E of this part, no later than
June 1, 2017, or two years following designation as O3
nonattainment.
Subpart F--Air Quality Index Reporting
0
22. Amend Sec. 58.50 by revising paragraph (c) to read as follows:
Sec. 58.50 Index reporting.
* * * * *
(c) The population of a metropolitan statistical area for purposes
of index reporting is the latest available U.S. census population.
Subpart G--Federal Monitoring
0
23. Amend Appendix D to Part 58, under section 4, by revising section
4.1(i) and Table D-3 to Appendix D of part 58 and by revising section 5
to read as follows:
Appendix D to Part 58--Network Design Criteria for Ambient Air Quality
Monitoring
* * * * *
4. Pollutant-Specific Design Criteria for SLAMS Sites
4.1 Ozone (O3) Design Criteria. * * *
(i) Since O3 levels decrease significantly in the
colder parts of the year in many areas, O3 is required to
be monitored at SLAMS monitoring sites only during the ``ozone
season'' as described below in Table D-3 of this appendix. These
ozone seasons are also identified in the AQS files on a state-by-
state basis. Deviations from the O3 monitoring season
must be approved by the EPA Regional Administrator. These requests
will be reviewed by Regional Administrators taking into
consideration, at a minimum, the frequency of out-of-season
O3 NAAQS exceedances, as well as occurrences of the
Moderate air quality index level and regional consistency. Any
deviations based on the Regional Administrator's waiver of
requirements must be described in the annual monitoring network plan
and updated in AQS. Changes to the O3 monitoring season
requirements in Table D-3 revoke any previously approved Regional
Administrator waivers for affected states. Requests for monitoring
season waivers must be accompanied by relevant supporting
information. Information on how to analyze O3 data to
support a change to the O3 season in support of the 8-
hour standard for a specific state can be found in reference 8 to
this appendix. O3 monitors at NCore stations are required
to be operated year-round (January to December).
Table D-3 \1\ to Appendix D of Part 58--Ozone Monitoring Season by State
------------------------------------------------------------------------
State Begin month End month
------------------------------------------------------------------------
Alabama......................... March............. October.
Alaska.......................... April............. October.
Arizona......................... January........... December.
Arkansas........................ March............. November.
California...................... January........... December.
Colorado........................ January........... December.
Connecticut..................... March............. September.
Delaware........................ March............. October.
District of Columbia............ March............. October.
Florida......................... January........... December.
Georgia......................... March............. October.
Hawaii.......................... January........... December.
Idaho........................... April............. September.
Illinois........................ March............. October.
Indiana......................... March............. October.
Iowa............................ March............. October.
Kansas.......................... March............. October.
Kentucky........................ March............. October.
Louisiana (Northern) AQCR March............. October.
019,022.
Louisiana (Southern) AQCR 106... January........... December.
Maine........................... April............. September.
Maryland........................ March............. October.
Massachusetts................... March............. September.
Michigan........................ March............. October.
Minnesota....................... March............. October.
Mississippi..................... March............. October.
Missouri........................ March............. October.
Montana......................... April............. September.
Nebraska........................ March............. October.
Nevada.......................... January........... December.
New Hampshire................... March............. September.
New Jersey...................... March............. October.
New Mexico...................... January........... December.
New York........................ March............. October.
North Carolina.................. March............. October.
North Dakota.................... March............. September.
Ohio............................ March............. October.
Oklahoma........................ March............. November.
Oregon.......................... May............... September.
Pennsylvania.................... March............. October.
Puerto Rico..................... January........... December.
Rhode Island.................... March............. September.
South Carolina.................. March............. October.
South Dakota.................... March............. October.
Tennessee....................... March............. October.
Texas (Northern) AQCR........... March............. November.
022, 210, 211, 212, 215, 217,
218.
Texas (Southern) AQCR........... January........... December.
106, 153, 213, 214, 216.........
Utah............................ January........... December.
Vermont......................... April............. September.
Virginia........................ March............. October.
Washington...................... May............... September.
West Virginia................... March............. October.
Wisconsin....................... March............. October 15.
Wyoming......................... January........... September.
[[Page 75411]]
American Samoa.................. January........... December.
Guam............................ January........... December.
Virgin Islands.................. January........... December.
------------------------------------------------------------------------
\1\ The required O3 monitoring season for NCore stations is January
through December.
* * * * *
5. Network Design for Photochemical Assessment Monitoring Stations
(PAMS) and Enhanced Ozone Monitoring
(a) State and local monitoring agencies are required to collect
and report the following PAMS measurements at each NCore site
required under paragraph 3(a) of this appendix located in an area
designated as nonattainment for O3.
(b) PAMS measurements include:
(1) Hourly averaged speciated volatile organic compounds (VOCs),
(2) 8 3-hour averaged carbonyls daily,
(3) Hourly averaged O3,
(4) Hourly averaged nitrogen oxide (NO), nitrogen dioxide
(NO2), and total reactive nitrogen (NOy),
(5) Hourly averaged 3 meter ambient temperature,
(6) Hourly vector-averaged 10 meter wind direction,
(7) Hourly averaged 10 meter wind speed,
(8) Hourly average atmospheric pressure,
(9) Hourly averaged relative humidity, and
(10) Hourly averaged mixing-height.
(c) The EPA Regional Administrator may grant a waiver to allow
the collection of required PAMS measurements at an alternative
location where the monitoring agency can demonstrate that the
alternative location will provide representative data useful for
regional or national scale modeling and the tracking of trends in
O3 precursors.
(d) The EPA Regional Administrator may also grant a waiver to
allow representative meteorological data from nearby monitoring
stations to be used to meet the requirements to collect temperature,
wind direction, wind speed, atmospheric pressure, relative humidity,
or hourly averaged mixing height where the monitoring agency can
demonstrate the data is collected in a manner consistent with EPA
quality requirements for these measurements.
(e) At a minimum, the monitoring agency shall collect the
required PAMS measurements during the months of June, July, and
August.
(f) States with O3 nonattainment areas are required
to develop and implement an Enhanced Monitoring Plan (EMP) detailing
enhanced O3 and O3 precursor monitoring
activities to be performed which is subject to review and approval
by the EPA Regional Administrator. The EMP will include monitoring
activities deemed important to understanding the O3
problems in the state. Such activities may include, but are not
limited to, the following:
(1) Additional O3 monitors beyond the minimally
required under paragraph 4.1 of this appendix,
(2) Additional NOX or NOy monitors beyond
those required under 4.3 of this appendix,
(3) Additional speciated VOC measurements including data
gathered during different periods other than required under
paragraph 5(e) of this appendix, or locations other than those
required under paragraph 5(a) of this appendix, and
(4) Enhanced upper air measurements of meteorology or pollution
concentrations.
[FR Doc. 2014-28674 Filed 12-16-14; 8:45 am]
BILLING CODE 6560-50-P