6560-50-P

ENVIRONMENTAL PROTECTION AGENCY

40 CFR Parts 50 and 58

 [EPA-HQ-OAR-2006-0922; FRL-]

Primary National Ambient Air Quality Standard for Nitrogen Dioxide

AGENCY:  Environmental Protection Agency (EPA).

ACTION:  Final Rule

SUMMARY:  Based on its review of the air quality criteria for oxides of
nitrogen and the primary national ambient air quality standard (NAAQS)
for oxides of nitrogen as measured by nitrogen dioxide (NO2), EPA is
making revisions to the primary NO2 NAAQS in order to provide requisite
protection of public health.  Specifically, EPA is establishing a new
1-hour standard at a level of 100 ppb, based on the 3-year average of
the 98th percentile of the yearly distribution of 1-hour daily maximum
concentrations, to supplement the existing annual standard.  EPA is also
establishing requirements for an NO2 monitoring network that will
include monitors at locations where maximum NO2 concentrations are
expected to occur, including within 50 meters of major roadways, as well
as monitors sited to measure the area-wide NO2 concentrations that occur
more broadly across communities.

DATES: This final rule is effective on [insert date 60 days after date
of publication in the Federal Register].

ADDRESSES EPA has established a docket for this action under Docket ID
No. EPA-HQ-OAR-2006-0922.  All documents in the docket are listed on the
  HYPERLINK "http://www.regulations.gov"  www.regulations.gov  website.
Although listed in the index, some information is not publicly
available, e.g., confidential business information or other information
whose disclosure is restricted by statute.  Certain other material, such
as copyrighted material, will be publicly available only in hard copy
form.  Publicly available docket materials are available either
electronically through   HYPERLINK "http://www.regulations.gov"
www.regulations.gov  or in hard copy at the Air and Radiation Docket and
Information Center, EPA/DC, EPA West, 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<PHONE_NUMBER> and the
telephone number for the Air and Radiation Docket and Information Center
is<PHONE_NUMBER>.

FOR FURTHER INFORMATION CONTACT:  Dr. Scott Jenkins,  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:<PHONE_NUMBER>; fax:
919-541-0237; email:   HYPERLINK<EMAIL_ADDRESS><EMAIL_ADDRESS>.

SUPPLEMENTARY INFORMATION:

Table of Contents

The following topics are discussed in this preamble:

I.	Background

A.	Summary of revisions to the NO2 primary NAAQS

B.	Legislative requirements

C.	Related NO2 control programs

D.	Review of the Air Quality Criteria and Standards for Oxides of
Nitrogen

E.	Summary of proposed revisions to the NO2 primary NAAQS

F.	Organization and approach to final NO2 primary NAAQS decisions

II.	Rationale for Final Decisions on the NO2 Primary Standard

A.	Characterization of NO2 air quality

	1.	Current patterns of NO2 air quality

	2.	NO2 air quality and gradients around roadways

B.	Health effects information

1.	Adverse respiratory effects and short-term exposure to NO2

2.	Other effects with short-term exposure to NO2

	a.	Mortality

	b.	Cardiovascular effects

		3.	Health effects with long-term exposure to NO2

		a.	Respiratory morbidity

		b.	Mortality

c.	Carcinogenic, cardiovascular, and reproductive/developmental effects

4.	NO2-related impacts on public health

C.	Human exposure and health risk characterization

D.	Approach for reviewing the need to retain or revise the current
standard

E.	Adequacy of the current standard

1.	Rationale for proposed decision

2.	Comments on the adequacy of the current standard

a.	Comments on EPA’s interpretation of the epidemiologic evidence

b.	Comments on EPA’s interpretation of the controlled human exposure
evidence

c.	Comments on EPA’s characterization of NO2-associated exposures and
health risks

3.	Conclusions on the adequacy of the current standard

F.	Elements of a new short-term standard

1.	Indicator

	a.	Rationale for proposed decision

	b.	Comments on indicator

	c.	Conclusions regarding indicator

2.	Averaging time

	a.	Rationale for proposed decision

b.	Comments on averaging time

	c.	Conclusions on averaging time

3.	Form

a.	Rationale for proposed decision

	b.	CASAC and public comments on form

	c.	Conclusions on form

4.	Level

			a.	Rationale for proposed decisions on approach and level

b.	Rationale for alternative decisions on approach and level

c.	Comments on approach and level

	i.	CASAC comments on the approach to setting the standard

	ii.	Public comments on the approach to setting the standard

iii.	CASAC comments on standard level

iv.	Public comments on standard level

	d.	Conclusions on approach and standard level

G.	Annual standard

H.	Summary of final decisions on the primary NO2 standard

III.	Ambient Monitoring and Reporting Requirements

A.	Monitoring Methods

1.	Chemiluminescence FRM and Alternative Methods

2.	Allowable FRM and FEMs for comparison to the NAAQS

a.	Proposed changes to FRM and FEMs that may be compared to the NAAQS

b.	Comments

c.	Decisions on allowable FRM and FEMs for comparison to the NAAQS

B.	Network Design

1.	Two-tiered Network Design

a.	Proposed Two-tier Network Design

b.	Comments

c.	Conclusions Regarding the Two-tier Network Design

2.	First Tier (Near-road Monitoring Component) of the NO2 Network Design

a.	Proposed First Tier (Near-road Monitoring Component) of the Network
Design

b.	Comments

c.	Conclusions Regarding the First Tier (Near-road Monitoring Component)
of the Network Design

3.	Second Tier (Area-wide Monitoring Component) of the Network Design

a.	Proposed Second Tier (Area-wide Monitoring Component) of the Network
Design

b.	Comments

c.	Conclusions on the Second Tier (Area-wide Monitoring Component) of
the Network Design

4.	Regional Administrator Authority

a.	Proposed Regional Administrator Authority

b.	Comments

c.	Conclusions on Regional Administrator Authority

5.	Monitoring Network Implementation

a.	Proposed Monitoring Network Implementation Approach

b.	Comments

c.	Conclusions on Monitoring Network Implementation

6.	Near-road Site Selection

a.	Proposed Near-road Site Selection Criterion

b.	Comments

c.	Conclusions on Near-road Site Selection

7.	Near-road Siting Criteria

a.	Proposed Near-road Siting Criteria

b.	Comments

c.	Conclusions on Near-road Siting Criteria

8.	Area-wide Monitor Site Selection and Siting Criteria

a.	Proposed Area-wide Monitor Site Selection and Siting Criteria

b.	Comments

c.	Conclusions on Area-wide Monitor Site Selection and Siting Criteria

9.	Meteorological Measurements

a.	Proposed Meteorological Measurements

b.	Comments

c.	Conclusions on Meteorological Measurements

C.	Data Reporting

1.	Proposed Data Quality Objectives and Measurement Uncertainty

2.	Comments

3.	Conclusions on Data Quality Objectives and Measurement Uncertainty

IV. 	Appendix S--Interpretation of the Primary NAAQS for Oxides of
Nitrogen and Revisions to the Exceptional Events Rule

A.	Interpretation of the primary NAAQS for oxides of nitrogen for the
annual primary standard

1.	Proposed interpretation of the annual standard

2.	Comments on interpretation of the annual standard

	3.	Conclusions on interpretation of the annual standard

B.	Interpretation of the primary NAAQS for oxides of nitrogen 1-hour
primary standard

1.	Proposed interpretation of the 1-hour standard

2.	Comments on interpretation of the 1-hour standard

		         3.         Conclusions on interpretation of the 1-hour
standard

	C.        Exceptional Events Information Submission Schedule

V.	Designation of Areas

A.  	Proposed process

B.	Public comments

C.  	Final designations process

VI.	Clean Air Act Implementation Requirements

A.	Classifications

1.	Proposal

2.	Public comments

3.	Final

B. 	Attainment Dates

1.	Attaining the NAAQS

			a.	Proposal

b.	Final

2.	Consequences of Failing to Attain by the Statutory Attainment Date

a.	Proposal

b.	Final

C.	Section 110(a)(2) NAAQS Infrastructure Requirements

1.	Proposal

2.	Final

D.  	Attainment Planning Requirements

1.	Nonattainment Area SIPs

a.	Proposal

b.	Public comments

c.	Final

2.	New Source Review and Prevention of Significant Deterioration
Requirements

a.	Proposal

b.	Public comments

c.	Final

3.	General Conformity

a.	Proposal

4.	Transportation Conformity

a.	Proposal

b.	Public comments

c.	Final

VII.	Communication of Public Health Information

VIII.	Statutory and Executive Order Reviews

References

I.	Background

A.	Summary of revisions to the NO2 primary NAAQS

Based on its review of the air quality criteria for oxides of nitrogen
and the primary national ambient air quality standard (NAAQS) for oxides
of nitrogen as measured by nitrogen dioxide (NO2), EPA is making
revisions to the primary NO2 NAAQS in order to provide requisite
protection of public health as appropriate under section 109 of the
Clean Air Act (Act or CAA).  Specifically, EPA is supplementing the
existing annual standard for NO2 of 53 parts per billion (ppb) by
establishing a new short-term standard based on the 3-year average of
the 98th percentile of the yearly distribution of 1-hour daily maximum
concentrations.  EPA is setting the level of this new standard at 100
ppb.  EPA is making changes in data handling conventions for NO2 by
adding provisions for this new 1-hour primary standard.  EPA is also
establishing requirements for an NO2 monitoring network.  These new
provisions require monitors at locations where maximum NO2
concentrations are expected to occur, including within 50 meters of
major roadways, as well as monitors sited to measure the area-wide NO2
concentrations that occur more broadly across communities.  EPA is
making conforming changes to the air quality index (AQI).

B.	Legislative Requirements

Two sections of the CAA govern the establishment and revision of the
NAAQS.  Section 108 of the Act directs the Administrator to identify and
list air pollutants that meet certain criteria, including that the air
pollutant “in [her] judgment, cause[s] or contribute[s] to air
pollution which may reasonably be anticipated to endanger public health
and welfare” and “the presence of which in the ambient air results
from numerous or diverse mobile or stationary sources.”  42 U.S.C. 21
7408(a)(1)(A) & (B).  For those air pollutants listed, section 108
requires the Administrator to issue air quality criteria  that
“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 ambient air . . .”  42 U.S.C. 7408(2).

Section 109(a) of the Act directs the Administrator to promulgate
“primary” and “secondary” NAAQS for pollutants for which air
quality criteria have been issued.  42 U.S.C. 7409(1).  Section
109(b)(1) defines a primary standard as one “the attainment and
maintenance of which in the judgment of the Administrator, based on [the
air quality] criteria and allowing an adequate margin of safety, are
requisite to protect the public health.”  42 U.S.C. 7409(b)(1).  A
secondary standard, in turn,  must “specify a level of air quality the
attainment and maintenance of which, in the judgment of the
Administrator, based on [the air quality] criteria, is requisite to
protect the public welfare from any known or anticipated adverse effects
associated with the presence of such pollutant in the ambient air.”
42 U.S.C. 7409(b)(2).

The requirement that primary standards include an adequate margin of
safety is intended to address uncertainties associated with inconclusive
scientific and technical information available at the time of standard
setting.  It is also intended to provide a reasonable degree of
protection against hazards that research has not yet identified. Lead
Industries Association v. EPA, 647 F.2d 1130, 1154 (D.C. Cir 1980),
cert. denied, 449 U.S. 1042 (1980); American Petroleum Institute v.
Costle, 665 F.2d 1176, 1186 (D.C. Cir. 1981), cert. denied, 455 U.S.
1034 (1982).  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 include 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.

In addressing the requirement for a margin of safety, EPA considers such
factors as the nature and severity of the health effects involved, the
size of the at-risk population(s), and the kind and degree of the
uncertainties that must be addressed.  The selection of any particular
approach to providing an adequate margin of safety is a policy choice
left specifically to the Administrator’s judgment. Lead Industries
Association v. EPA, supra, 647 F.2d at 1161-62.

In setting standards that are “requisite” to protect public health
and welfare, as provided in section 109(b), EPA’s task is to establish
standards that are neither more nor less stringent than necessary for
these purposes.  In so doing, EPA may not consider the costs of
implementing the standards.  Whitman v. American Trucking Associations,
531 U.S. 457, 471, 475-76 (2001).

Section 109(d)(1) of the Act requires the Administrator to periodically
undertake a thorough review of the air quality criteria published under
section 108 and the NAAQS and to  revise the criteria and standards as
may be appropriate.   42 U.S.C. 7409(d)(1).  The Act also requires the
Administrator to appoint an independent scientific review committee
composed of seven members, including at least one member of the National
Academy of Sciences, one physician, and one person representing State
air pollution control agencies, to review the air quality criteria and
NAAQS and to “recommend to the Administrator any new … standards and
revisions of existing criteria and standards as may be appropriate under
section 108 and subsection (b) of this section."  42 U.S.C. 7409(d)(2).
This independent review function is performed by the Clean Air
Scientific Advisory Committee (CASAC) of EPA’s Science Advisory Board.

C.	Related NO2 Control Programs

States are primarily responsible for ensuring attainment and maintenance
of ambient air quality standards once EPA has established them.  Under
section 110 of the Act, 42 U.S.C. 7410, and related provisions, States
are to submit, for EPA approval, State implementation plans (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 EPA, also administer the prevention of
significant deterioration program that covers these pollutants.  See 42
U.S.C. 7470-7479.   In addition, Federal programs provide for nationwide
reductions in emissions of these and other air pollutants under Title II
of the Act, 42 U.S.C. 7521–7574, which involves controls for
automobile, truck, bus, motorcycle, nonroad engine and equipment, and
aircraft emissions; the new source performance standards under section
111 of the Act, 42 U.S.C. 7411; and the national emission standards for
hazardous air pollutants under section 112 of the Act, 42 U.S.C. 7412.

Currently there are no areas in the United States that are designated as
nonattainment of the NO2 NAAQS.  With the revisions to the NO2 NAAQS
that result from this review, however, some areas could be classified as
non-attainment.  Certain States will be required to develop SIPs that
identify and implement specific air pollution control measures to reduce
ambient NO2 concentrations to attain and maintain the revised NO2 NAAQS,
most likely by requiring air pollution controls on sources that emit
oxides of nitrogen (NOX).

While NOX is emitted from a wide variety of source types, the top three
categories of sources of NOX emissions are on-road mobile sources,
electricity generating units, and non-road mobile sources.  EPA
anticipates that NOX emissions will decrease substantially over the next
20 years as a result of the ongoing implementation of mobile source
emissions standards.  In particular, Tier 2 NOX emission standards for
light-duty vehicle emissions began phasing into the fleet beginning with
model year 2004, in combination with low-sulfur gasoline fuel standards.
 For heavy-duty engines, new NOX standards are phasing in between the
2007 and 2010 model years, following the introduction of ultra-low
sulfur diesel fuel.  Lower NOX standards for nonroad diesel engines,
locomotives, and certain marine engines are becoming effective
throughout the next decade.  In future decades, these lower- NOX
vehicles and engines will become an increasingly large fraction of
in-use mobile sources, effecting large NOX emission reductions.

D.	Review of the Air Quality Criteria and Standards for Oxides of
Nitrogen

On April 30, 1971, EPA promulgated identical primary and secondary NAAQS
for NO2 under section 109 of the Act.  The standards were set at 0.053
parts per million (ppm) (53 ppb), annual average (36 FR 8186).  EPA
completed reviews of the air quality criteria and NO2 standards in 1985
and 1996 with decisions to retain the standard (50 FR 25532, June 19,
1985; 61 FR 52852, October 8, 1996).

 EPA initiated the current review of the air quality criteria for oxides
of nitrogen and the NO2 primary NAAQS on December 9, 2005 (70 FR 73236)
with a general call for information.  EPA’s draft Integrated Review
Plan for the Primary National Ambient Air Quality Standard for Nitrogen
Dioxide (EPA, 2007a) was made available in February, 2007 for public
comment and was discussed by the CASAC via a publicly accessible
teleconference on May 11, 2007.  As noted in that plan, NOX includes
multiple gaseous (e.g., NO2, NO) and particulate (e.g., nitrate)
species.  Because the health effects associated with particulate species
of NOX have been considered within the context of the health effects of
ambient particles in the Agency’s review of the NAAQS for particulate
matter (PM), the current review of the primary NO2 NAAQS is focused on
the gaseous species of NOX and is not intended to address health effects
directly associated with particulate species.

The first draft of the Integrated Science Assessment for Oxides of
Nitrogen-Health Criteria (ISA) and the Nitrogen Dioxide Health
Assessment Plan: Scope and Methods for Exposure and Risk Assessment
(EPA, 2007b) were reviewed by CASAC at a public meeting held on October
24-25, 2007.  Based on comments received from CASAC and the public, EPA
developed the second draft of the ISA and the first draft of the Risk
and Exposure Assessment to Support the Review of the NO2 Primary
National Ambient Air Quality Standard (Risk and Exposure Assessment
(REA)).  These documents were reviewed by CASAC at a public meeting held
on May 1-2, 2008.  Based on comments received from CASAC and the public
at this meeting, EPA released the final ISA in July of 2008 (EPA,
2008a).  In addition, comments received were considered in developing
the second draft of the REA, which was released for public review and
comment in two parts.  The first part of this document, containing
chapters 1-7, 9 and appendices A and C as well as part of appendix B,
was released in August 2008.  The second part of this document,
containing chapter 8 (describing the Atlanta exposure assessment) and a
completed appendix B, was released in October of 2008.  This document
was the subject of CASAC reviews at public meetings on September 9 and
10, 2008 (for the first part) and on October 22, 2008 (for the second
part).  In preparing the final REA (EPA, 2008b), EPA considered comments
received from the CASAC and the public at those meetings.

In the course of reviewing the second draft REA, CASAC expressed the
view that the document would be incomplete without the addition of a
policy assessment chapter presenting an integration of evidence-based
considerations and risk and exposure assessment results.  CASAC stated
that such a chapter would be “critical for considering options for the
NAAQS for NO2” (Samet, 2008a).  In addition, within the period of
CASAC’s review of the second draft REA, EPA’s Deputy Administrator
indicated in a letter to the chair of CASAC, addressing earlier CASAC
comments on the NAAQS review process (Henderson, 2008), that the risk
and exposure assessment will include “a broader discussion of the
science and how uncertainties may effect decisions on the standard”
and “all analyses and approaches for considering the level of the
standard under review, including risk assessment and weight of evidence
methodologies” (Peacock, 2008, p.3; September 8, 2008).

Accordingly, the final REA included a new policy assessment chapter.
This policy assessment chapter considered the scientific evidence in the
ISA and the exposure and risk characterization results presented in
other chapters of the REA as they relate to the adequacy of the current
NO2 primary NAAQS and potential alternative primary NO2 standards.  In
considering the current and potential alternative standards, the policy
assessment chapter of the final REA focused on the information that is
most pertinent to evaluating the basic elements of national ambient air
quality standards:  indicator, averaging time, form, and level.  These
elements, which together serve to define each standard, must be
considered collectively in evaluating the health protection afforded.
CASAC discussed the final version of the REA, with an emphasis on the
policy assessment chapter, during a public teleconference held on
December 5, 2008.  Following that teleconference, CASAC offered comments
and advice on the NO2 primary NAAQS in a letter to the Administrator
(Samet, 2008b).

The schedule for completion of this review is governed by a judicial
order resolving a lawsuit filed in September 2005, concerning the timing
of the current review.  The order that now governs this review, entered
by the court in August 2007 and amended in December 2008, provides that
the Administrator will sign, for publication, notices of proposed and
final rulemaking concerning the review of the primary NO2 NAAQS no later
than June 26, 2009 and January 22, 2010, respectively.  In accordance
with this schedule, the Administrator signed a notice of proposed
rulemaking on June 26, 2009 (FR 74 34404).  This action presents the
Administrator’s final decisions on the primary NO2 standard.

E.	Summary of proposed revisions to the NO2 primary NAAQS

For the reasons discussed in the preamble of the proposal for the NO2
primary NAAQS (74 FR 34404), EPA proposed to make revisions to the
primary NO2 NAAQS and to make related revisions for NO2 data handling
conventions in order to provide requisite protection of public health.
EPA also proposed to make corresponding changes to the AQI for NO2.
Specifically, EPA proposed to supplement the current annual standard by
establishing a new short-term NO2 standard that would reflect the
maximum allowable NO2 concentration anywhere in an area.  EPA proposed
that this new short-term standard would be based on the 3-year average
of the 99th percentile (or 4th highest) of the yearly distribution of
1-hour daily maximum NO2 concentrations and solicited comment on using
the 3-year average of the 98th percentile (or 7th or 8th highest) of the
yearly distribution of 1-hour daily maximum NO2 concentrations.  EPA
proposed to set the level of this new 1-hour standard within the range
of 80 to 100 ppb and solicited comment on standard levels as low as 65
ppb and as high as 150 ppb.  EPA proposed to specify the level of the
standard to the nearest ppb.  EPA also proposed to establish
requirements for an NO2 monitoring network at locations where maximum
NO2 concentrations are expected to occur, including monitors within 50
meters of major roadways, as well as area-wide monitors sited to measure
the NO2 concentrations that can occur more broadly across communities.
EPA also solicited comment on the alternative approach of setting a
1-hour standard that would reflect the allowable area-wide NO2
concentration.

F.	Organization and approach to final NO2 primary NAAQS decisions

This action presents the Administrator’s final decisions regarding the
need to revise the current NO2 primary NAAQS.  Revisions to the primary
NAAQS for NO2, and the rationale supporting those revisions, are
described below in section II.  Requirements for the NO2 ambient
monitoring network are described in section III.  Related requirements
for data completeness, data handling, data reporting, rounding
conventions, and exceptional events are described in section IV.
Implementation of the revised NO2 primary NAAQS is discussed in sections
V and VI.  Communication of public health information through the AQI is
discussed in section VII and a discussion of statutory and executive
order reviews is provided in section VIII.

	Today's final decisions are based on a thorough review in the ISA of
scientific information on known and potential human health effects
associated with exposure to NO2 in the air.  These final decisions also
take into account:  (1) assessments in the REA of the most
policy-relevant information in the ISA as well as quantitative exposure
and risk analyses based on that information; (2) CASAC Panel advice and
recommendations, as reflected in its letters to the Administrator and
its public discussions of the ISA, the REA, and the notice of proposed
rulemaking; (3) public comments received during the development of ISA
and REA;  and (4) public comments received on the proposed rulemaking.

Some commenters have referred to and discussed individual scientific
analyses on the health effects of NO2 that were not included in the ISA
(EPA, 2008a) (“new studies”).  In considering and responding to
comments for which such “new studies” were cited in support, EPA has
provisionally considered the cited studies in the context of the
findings of the ISA.

As in prior NAAQS reviews, EPA is basing its decision in this review on
studies and related information included in the ISA and staff’s policy
assessment, which have undergone CASAC and public review.  In this NO2
NAAQS review, staff’s policy assessment was presented in the form of a
policy assessment chapter of the REA (EPA, 2008b).  The studies assessed
in the ISA and REA, and the integration of the scientific evidence
presented in them, have undergone extensive critical review by EPA,
CASAC, and the public.  The rigor of that review makes these studies,
and their integrative assessment, the most reliable source of scientific
information on which to base decisions on the NAAQS, decisions that all
parties recognize as of great import.  NAAQS decisions can have profound
impacts on public health and welfare, and NAAQS decisions should be
based on studies that have been rigorously assessed in an integrative
manner not only by EPA but also by the statutorily mandated independent
advisory committee, as well as the public review that accompanies this
process.  EPA’s provisional consideration of “new studies” did not
and could not provide that kind of in-depth critical review.

This decision is consistent with EPA’s practice in prior NAAQS reviews
and its interpretation of the requirements of the CAA.  Since the 1970
amendments, the EPA has taken the view that NAAQS decisions are to be
based on scientific studies and related information that have been
assessed as a part of the pertinent air quality criteria, and has
consistently followed this approach.  This longstanding interpretation
was strengthened by new legislative requirements enacted in 1977, which
added section 109(d)(2) of the Act concerning CASAC review of air
quality criteria.  See 71 FR 61144, 61148 (October 17, 2006) (final
decision on review of PM NAAQS) for a detailed discussion of this issue
and EPA’s past practice.

As discussed in EPA’s 1993 decision not to revise the NAAQS for ozone
(O3), ‘‘new studies” may sometimes be of such significance that it
is appropriate to delay a decision on revision of a NAAQS and to
supplement the pertinent air quality criteria so the studies can be
taken into account (58 FR at 13013–13014, March 9, 1993).  In the
present case, EPA’s provisional consideration of ‘‘new studies”
concludes that, taken in context, the ‘‘new’’ information and
findings do not materially change any of the broad scientific
conclusions regarding the health effects of NO2 made in the air quality
criteria.  For this reason, reopening the air quality criteria review
would not be warranted even if there were time to do so under the court
order governing the schedule for this rulemaking.

Accordingly, EPA is basing the final decisions in this review on the
studies and related information included in the NO2 air quality criteria
that have undergone CASAC and public review.  EPA will consider the
“new studies” for purposes of decision-making in the next periodic
review of the NO2 NAAQS, which will provide the opportunity to fully
assess these studies through a more rigorous review process involving
EPA, CASAC, and the public.  Further discussion of these ‘‘new
studies” can be found below, in section II.E, and in the Response to
Comments document.

II.	Rationale for Final Decisions on the NO2 Primary Standard

This section presents the rationale for the Administrator’s decision
to revise the existing NO2 primary standard by supplementing the current
annual standard with a new 1-hour standard.  In developing this
rationale, EPA has drawn upon an integrative synthesis of the entire
body of evidence on human health effects associated with the presence of
NO2 in the air.  As summarized below in section II.B, this body of
evidence addresses a broad range of health endpoints associated with
exposure to NO2.  In considering this entire body of evidence, EPA
focuses in particular on those health endpoints for which the ISA finds
associations with NO2 to be causal or likely causal.  This rationale
also draws upon the results of quantitative exposure and risk
assessments, summarized below in section II.C.

As discussed below, a substantial amount of new research has been
conducted since the last review of the NO2 NAAQS, with important new
information coming from epidemiologic studies in particular.  The newly
available research studies evaluated in the ISA have undergone intensive
scrutiny through multiple layers of peer review and opportunities for
public review and comment.  While important uncertainties remain in the
qualitative and quantitative characterizations of health effects
attributable to exposure to ambient NO2, the review of this information
has been extensive and deliberate.

The remainder of this section provides background information that
informed the Administrator’s decisions on the primary standard and
discusses the rationale for those decisions.  Section II.A presents a
discussion of NO2 air quality.  Section II.B includes an overview of the
scientific evidence related to health effects associated with NO2
exposure.  This overview includes discussion of the health endpoints and
at-risk populations considered in the ISA.  Section II.C discusses the
approaches taken by EPA to assess exposures and health risks associated
with NO2, including a discussion of key results.  Section II.D
summarizes the approach that was used in the current review of the NO2
NAAQS with regard to consideration of the scientific evidence and
exposure-/risk-based results related to the adequacy of the current
standard and potential alternative standards.  Sections II.E-II.G
discuss the Administrator’s decisions regarding the adequacy of the
current standard, elements of a new 1-hour standard, and retention of
the current annual standard, respectively, taking into consideration
public comments on the proposed decisions.  Section II.H summarizes the
Administrator’s decisions with regard to the NO2 primary NAAQS.

A.	Characterization of NO2 air quality

1.	Current patterns of NO2 air quality

The size of the State and local NO2 monitoring network has remained
relatively stable since the early 1980s, and currently has approximately
400 monitors reporting data to EPA’s Air Quality System (AQS)
database. At present, there are no minimum monitoring requirements for
NO2 in 40 CFR part 58 Appendix D, other than a requirement for EPA
Regional Administrator approval before removing any existing monitors,
and that any ongoing NO2 monitoring must have at least one monitor sited
to measure the maximum concentration of NO2 in that area (though, as
discussed below monitors in the current network do not measure peak
concentrations associated with on-road mobile sources that can occur
near major roadways because the network was not designed for this
purpose).  EPA removed the specific minimum monitoring requirements for
NO2 of two monitoring sites per area with a population of 1,000,000 or
more in the 2006 monitoring rule revisions (71 FR 61236), based on the
fact that there were no NO2 nonattainment areas at that time, coupled
with trends evidence showing an increasing gap between national average
NO2 concentrations and the current annual standard.  Additionally, the
minimum requirements were removed to provide State, local, and tribal
air monitoring agencies flexibility in meeting higher priority
monitoring needs for pollutants such as O3 and PM2.5, or implementing
the new multi-pollutant sites (NCore network) required by the 2006 rule
revisions, by allowing them to discontinue lower priority monitoring.
There are requirements in 40 CFR part 58 Appendix D for NO2 monitoring
as part of the Photochemical Assessment Monitoring Stations (PAMS)
network.  However, of the approximately 400 NO2 monitors currently in
operation, only about 10 percent may be due to the PAMS requirements.

An analysis of the approximately 400 monitors comprising the current NO2
monitoring network (Watkins and Thompson, 2008) indicates that the
current NO2 network has largely remained unchanged in terms of size and
target monitor objective categories since it was introduced in the May
10, 1979 monitoring rule (44 FR 27571).  The review of the current
network found that the assessment of concentrations for general
population exposure and maximum concentrations at neighborhood and
larger scales were the top objectives.  A review of the distribution of
listed spatial scales of representation shows that only approximately 3
monitors are described as microscale, representing an area on the order
of several meters to 100 meters, and approximately 23 monitors are
described as middle scale, which represents an area on the order of 100
to 500 meters. This low percentage of smaller spatially representative
scale sites within the network of approximately 400 monitoring sites
indicates that the majority of monitors have, in fact, been sited to
assess area-wide exposures on the neighborhood, urban, and regional
scales, as would be expected for a network sited to support the current
annual NO2 standard and PAMS objectives.  The current network does not
include monitors placed near major roadways and, therefore, monitors in
the current network do not necessarily measure the maximum
concentrations that can occur on a localized scale near these roadways
(as discussed in the next section).  It should be noted that the network
not only accommodates NAAQS related monitoring but also serves other
monitoring objectives, such as support for photochemistry analysis, O3
modeling and forecasting, and particulate matter precursor tracking.

2.	NO2 air quality and gradients around roadways

	On-road and non-road mobile sources account for approximately 60% of
NOX emissions (ISA, table 2.2-1) and traffic-related exposures can
dominate personal exposures to NO2 (ISA section 2.5.4).  While driving,
personal exposure concentrations in the cabin of a vehicle could be
substantially higher than ambient concentrations measured nearby (ISA,
section 2.5.4).  For example, estimates presented in the REA suggest
that on/near roadway NO2 concentrations could be approximately 80% (REA,
section 7.3.2) higher on average across locations than concentrations
away from roadways and that roadway-associated environments could be
responsible for the majority of 1-hour peak NO2 exposures (REA, Figures
8-17 and 8-18).  Because monitors in the current network are not sited
to measure peak roadway-associated NO2 concentrations, individuals who
spend time on and/or near major roadways could experience NO2
concentrations that are considerably higher than indicated by monitors
in the current area-wide NO2 monitoring network.

Research suggests that the concentrations of on-road mobile source
pollutants such as NOX, carbon monoxide (CO), directly emitted air
toxics, and certain size distributions of particulate matter (PM), such
as ultrafine PM, typically display peak concentrations on or immediately
adjacent to roads (ISA, section 2.5).  This situation typically produces
a gradient in pollutant concentrations, with concentrations decreasing
with increasing distance from the road, and concentrations generally
decreasing to near area-wide ambient levels, or typical upwind urban
background levels, within a few hundred meters downwind. While such a
concentration gradient is present on almost all roads, the
characteristics of the gradient, including the distance from the road
that a mobile source pollutant signature can be differentiated from
background concentrations, are heavily dependent on factors such as
traffic volumes, local topography, roadside features, meteorology, and
photochemical reactivity conditions (Baldauf, et al., 2009; Beckerman et
al., 2008; Clements et al., 2008; Hagler et al., 2009; Janssen et al.,
2001; Rodes and Holland, 1981; Roorda-Knape et al., 1998; Singer et al.,
2004; Zhou and Levy, 2007).

Because NO2 in the ambient air is due largely to the atmospheric
oxidation of NO emitted from combustion sources (ISA, section 2.2.1),
elevated NO2 concentrations can extend farther away from roadways than
the primary pollutants also emitted by on-road mobile sources.  More
specifically, review of the technical literature suggests that NO2
concentrations may return to area-wide or typical urban background
concentrations within distances up to 500 meters of roads, though the
actual distance will vary with topography, roadside features,
meteorology, and photochemical reactivity conditions (Baldauf et al.,
2009; Beckerman et al., 2008; Clements et al., 2008; Gilbert et al.
2003; Rodes and Holland, 1981; Singer et al., 2004; Zhou and Levy,
2007).  Efforts to quantify the extent and slope of the concentration
gradient that may exist from peak near-road concentrations to the
typical urban background concentrations must consider the variability
that exists across locations and for a given location over time.  As a
result, we have identified a range of concentration gradients in the
technical literature which indicate that, on average, peak NO2
concentrations on or immediately adjacent to roads may typically be
between 30 and 100 percent greater than concentrations monitored in the
same area but farther away from the road (ISA, Section 2.5.4; Beckerman
et al., 2008; Gilbert et al., 2003; Rodes and Holland, 1981;
Roorda-Knape et al., 1998; Singer et al., 2004).  This range of
concentration gradients has implications for revising the NO2 primary
standard and for the NO2 monitoring network (discussed in sections
II.F.4 and III).

B.	Health effects information

In the last review of the NO2 NAAQS, the 1993 NOX Air Quality Criteria
Document (1993 AQCD) (EPA, 1993) concluded that there were two key
health effects of greatest concern at ambient or near-ambient
concentrations of NO2 (ISA, section 5.3.1).  The first was increased
airway responsiveness in asthmatic individuals after short-term
exposures.  The second was increased respiratory illness among children
associated with longer-term exposures to NO2.  Evidence also was found
for increased risk of emphysema, but this appeared to be of major
concern only with exposures to NO2 at levels much higher than then
current ambient levels (ISA, section 5.3.1).  Controlled human exposure
and animal toxicological studies provided qualitative evidence for
airway hyperresponsiveness and lung function changes while epidemiologic
studies provided evidence for increased respiratory symptoms with
increased indoor NO2 exposures.  Animal toxicological findings of lung
host defense system changes with NO2 exposure provided a
biologically-plausible basis for the epidemiologic results.
Subpopulations considered potentially more susceptible to the effects of
NO2 exposure included persons with preexisting respiratory disease,
children, and the elderly.  The epidemiologic evidence for respiratory
health effects was limited, and no studies had considered endpoints such
as hospital admissions, emergency department visits, or mortality (ISA,
section 5.3.1).

As summarized below and discussed more fully in section II.B of the
proposal notice, evidence published since the last review generally has
confirmed and extended the conclusions articulated in the 1993 AQCD
(ISA, section 5.3.2).  The epidemiologic evidence has grown
substantially with the addition of field and panel studies, intervention
studies, time-series studies of endpoints such as hospital admissions,
and a substantial number of studies evaluating mortality risk associated
with short-term NO2 exposures.  While not as marked as the growth in the
epidemiologic literature, a number of recent toxicological and
controlled human exposure studies also provide insights into
relationships between NO2 exposure and health effects.  This body of
evidence focuses the current review on NO2-related respiratory effects
at lower ambient and exposure concentrations than considered in the
previous review.

1.	Adverse respiratory effects and short-term exposure to NO2

The ISA concluded that the findings of epidemiologic, controlled human
exposure, and animal toxicological studies provide evidence that is
sufficient to infer a likely causal relationship for respiratory effects
following short-term NO2 exposure (ISA, sections 3.1.7 and <IP_ADDRESS>).
The ISA (section 5.4) concluded that the strongest evidence for an
association between NO2 exposure and adverse human health effects comes
from epidemiologic studies of respiratory symptoms, emergency department
visits, and hospital admissions.  These studies include panel and field
studies, studies that control for the effects of co-occurring
pollutants, and studies conducted in areas where the whole distribution
of ambient 24-hour average NO2 concentrations was below the current
NAAQS level of 53 ppb (annual average).  With regard to this evidence,
the ISA concluded that NO2 epidemiologic studies provide “little
evidence of any effect threshold” (ISA, section <IP_ADDRESS>, p. 5-15).  In
studies that have evaluated concentration-response relationships, they
appear linear within the observed range of data (ISA, section <IP_ADDRESS>).

Overall, the epidemiologic evidence for respiratory effects has been
characterized in the ISA as consistent, in that associations are
reported in studies conducted in numerous locations with a variety of
methodological approaches, and coherent, in that the studies report
associations with respiratory health outcomes that are logically linked
together.  In addition, a number of these associations are statistically
significant, particularly the more precise effect estimates (ISA,
section <IP_ADDRESS>).  These epidemiologic studies are supported by evidence
from toxicological and controlled human exposure studies, particularly
those that evaluated airway hyperresponsiveness in asthmatic individuals
(ISA, section 5.4).  The ISA concluded that together, the epidemiologic
and experimental data sets form a plausible, consistent, and coherent
description of a relationship between NO2 exposures and an array of
adverse respiratory health effects that range from the onset of
respiratory symptoms to hospital admissions.

In considering the uncertainties associated with the epidemiologic
evidence, the ISA (section 5.4) noted that it is difficult to determine
“the extent to which NO2 is independently associated with respiratory
effects or if NO2 is a marker for the effects of another traffic-related
pollutant or mix of pollutants.”  On-road vehicle exhaust emissions
are a widespread source of combustion pollutant mixtures that include
NOX and are an important contributor to NO2 levels in near-road
locations.  Although the presence of other pollutants from vehicle
exhaust emissions complicates efforts to quantify specific NO2-related
health effects, a number of epidemiologic studies have evaluated
associations with NO2 in models that also include co-occurring
pollutants such as PM, O3, CO, and/or SO2.  The evidence summarized in
the ISA indicates that NO2 associations generally remain robust in these
multi-pollutant models and supports a direct effect of short-term NO2
exposure on respiratory morbidity (see ISA Figures 3.1-7, 3.1-10,
3.1-11).  The plausibility and coherence of these effects are also
supported by epidemiologic studies of indoor NO2 as well as experimental
(i.e., toxicological and controlled human exposure) studies that have
evaluated host defense and immune system changes, airway inflammation,
and airway responsiveness (see subsequent sections of this proposal and
the ISA, section <IP_ADDRESS>).  The ISA (section 5.4) concluded that the
robustness of epidemiologic findings to adjustment for co-pollutants,
coupled with data from animal and human experimental studies, support a
determination that the relationship between NO2 and respiratory
morbidity is likely causal, while still recognizing the relationship
between NO2 and other traffic related pollutants.

The epidemiologic and experimental studies encompass a number of
respiratory-related health endpoints, including emergency department
visits and hospitalizations, respiratory symptoms, airway
hyperresponsiveness, airway inflammation, and lung function.  The
findings relevant to these endpoints, which provide the rationale to
support the judgment of a likely causal relationship, are described in
more detail in section II.B.1 of the proposal.

2.	Other effects with short-term exposure to NO2

a.	Mortality

The ISA concluded that the epidemiologic evidence is suggestive, but not
sufficient, to infer a causal relationship between short-term exposure
to NO2 and all-cause and cardiopulmonary-related mortality (ISA, section
<IP_ADDRESS>).  Results from several large United States and European
multicity studies and a meta-analysis study indicate positive
associations between ambient NO2 concentrations and the risk of
all-cause (nonaccidental) mortality, with effect estimates ranging from
0.5 to 3.6% excess risk in mortality per standardized increment (20 ppb
for 24-hour averaging time, 30 ppb for 1-hour averaging time) (ISA,
section 3.3.1, Figure 3.3-2, section <IP_ADDRESS>).  In general, the ISA
concluded that NO2 effect estimates were robust to adjustment for
co-pollutants.  Both cardiovascular and respiratory mortality have been
associated with increased NO2 concentrations in epidemiologic studies
(ISA, Figure 3.3-3); however, similar associations were observed for
other pollutants, including PM and SO2.  The range of risk estimates for
excess mortality is generally smaller than that for other pollutants
such as PM.  In addition, while NO2 exposure, alone or in conjunction
with other pollutants, may contribute to increased mortality, evaluation
of the specificity of this effect is difficult.  Clinical studies
showing hematologic effects and animal toxicological studies showing
biochemical, lung host defense, permeability, and inflammation changes
with short-term exposures to NO2 provide limited evidence of plausible
pathways by which risks of mortality may be increased, but no coherent
picture is evident at this time (ISA, section <IP_ADDRESS>).

b.	Cardiovascular effects

The ISA concluded that the available evidence on cardiovascular health
effects following short-term exposure to NO2 is inadequate to infer the
presence or absence of a causal relationship at this time (ISA, section
<IP_ADDRESS>).  Evidence from epidemiologic studies of heart rate
variability, repolarization changes, and cardiac rhythm disorders among
heart patients with ischemic cardiac disease are inconsistent (ISA,
section <IP_ADDRESS>).  In most studies, associations with PM were found to
be similar or stronger than associations with NO2.  Generally positive
associations between ambient NO2 concentrations and hospital admissions
or emergency department visits for cardiovascular disease have been
reported in single-pollutant models (ISA, section <IP_ADDRESS>); however,
most of these effect estimate values were diminished in multi-pollutant
models that also contained CO and PM indices (ISA, section <IP_ADDRESS>).
Mechanistic evidence of a role for NO2 in the development of
cardiovascular diseases from studies of biomarkers of inflammation, cell
adhesion, coagulation, and thrombosis is lacking (ISA, section <IP_ADDRESS>).
 Furthermore, the effects of NO2 on various hematological parameters in
animals are inconsistent and, thus, provide little biological
plausibility for effects of NO2 on the cardiovascular system (ISA,
section <IP_ADDRESS>).

3.	Health effects with long-term exposure to NO2

a.	Respiratory morbidity

The ISA concluded that overall, the epidemiologic and experimental
evidence is suggestive, but not sufficient, to infer a causal
relationship between long-term NO2 exposure and respiratory morbidity
(ISA, section <IP_ADDRESS>).  The available database evaluating the
relationship between respiratory illness in children and long-term
exposures to NO2 has increased since the 1996 review of the NO2 NAAQS
(see section II.B.3 of the proposal for a more detailed discussion).  A
number of epidemiologic studies have examined the effects of long-term
exposure to NO2 and reported positive associations with decrements in
lung function and partially irreversible decrements in lung function
growth (ISA, section 3.4.1, Figures 3.4-1 and 3.4-2).  While animal
toxicological studies may provide biological plausibility for the
chronic effects of NO2 that have been observed in epidemiologic studies
(ISA, sections 3.4.5 and <IP_ADDRESS>), the high correlation among
traffic-related pollutants in epidemiologic studies makes it difficult
to accurately estimate independent effects (ISA, section <IP_ADDRESS>).

b.	Mortality

The ISA concluded that the epidemiologic evidence is inadequate to infer
the presence or absence of a causal relationship between long-term
exposure to NO2 and mortality (ISA, section <IP_ADDRESS>).  In the United
States and European cohort studies examining the relationship between
long-term exposure to NO2 and mortality, results have been inconsistent
(ISA, section <IP_ADDRESS>).  Further, when associations were suggested, they
were not specific to NO2 but also implicated PM and other traffic
indicators.  The relatively high correlations reported between NO2 and
PM indices make it difficult to interpret these observed associations at
this time (ISA, section <IP_ADDRESS>).

c.	Carcinogenic, cardiovascular, and reproductive/developmental effects

The ISA concluded that the available epidemiologic and toxicological
evidence is inadequate to infer the presence or absence of a causal
relationship for carcinogenic, cardiovascular, and reproductive and
developmental effects related to long-term NO2 exposure (ISA, section
<IP_ADDRESS>).  Epidemiologic studies conducted in Europe have shown an
association between long-term NO2 exposure and increased incidence of
cancer (ISA, section <IP_ADDRESS>). However, the animal toxicological studies
have provided no clear evidence that NO2 acts as a carcinogen (ISA,
section <IP_ADDRESS>).  The very limited epidemiologic and toxicological
evidence do not suggest that long-term exposure to NO2 has
cardiovascular effects (ISA, section <IP_ADDRESS>).  The epidemiologic
evidence is not consistent for associations between NO2 exposure and
fetal growth retardation; however, some evidence is accumulating for
effects on preterm delivery (ISA, section <IP_ADDRESS>).  Scant animal
evidence supports a weak association between NO2 exposure and adverse
birth outcomes and provides little mechanistic information or biological
plausibility for the epidemiologic findings.

4.	NO2-related impacts on public health

Specific groups within the general population are likely at increased
risk for suffering adverse effects from NO2 exposure.  This could occur
because they are affected by lower levels of NO2 than the general
population or because they experience a larger health impact than the
general population to a given level of exposure (susceptibility) and/or
because they are exposed to higher levels of NO2 than the general
population (vulnerability).  The term susceptibility generally
encompasses innate (e.g., genetic or developmental) and/or acquired
(e.g., age or disease) factors that make individuals more likely to
experience effects with exposure to pollutants.  The severity of health
effects experienced by a susceptible subgroup may be much greater than
that experienced by the population at large.  Factors that may influence
susceptibility to the effects of air pollution include age (e.g.,
infants, children, elderly); gender; race/ethnicity; genetic factors;
and pre-existing disease/condition (e.g., obesity, diabetes, respiratory
disease, asthma, chronic obstructive pulmonary disease (COPD),
cardiovascular disease, airway hyperresponsiveness, respiratory
infection, adverse birth outcome) (ISA, sections 4.3.1, 4.3.5, and
<IP_ADDRESS>).  In addition, certain groups may experience relatively high
exposure to NO2, thus forming a potentially vulnerable population (ISA,
section 4.3.6).  Factors that may influence susceptibility and
vulnerability to air pollution include socioeconomic status (SES),
education level, air conditioning use, proximity to roadways, geographic
location, level of physical activity, and work environment (e.g., indoor
versus outdoor) (ISA, section 4.3.5).  The ISA discussed factors that
can confer susceptibility and/or vulnerability to air pollution with
most of the discussion devoted to factors for which NO2-specific
evidence exists (ISA, section 4.3).  These factors include pre-existing
disease (e.g., asthma), age (i.e., infants, children, older adults),
genetic factors, gender, socioeconomic status, and proximity to roadways
(see section II.B.4 in proposal for more detailed discussion of these
factors).

As discussed in more detail in the proposal (section II.B.4), the
population potentially affected by NO2 is large.  A considerable
fraction of the population resides, works, or attends school near major
roadways, and these individuals are likely to have increased exposure to
NO2 (ISA, section 4.4).  Based on data from the 2003 American Housing
Survey, approximately 36 million individuals live within 300 feet (~90
meters) of a four-lane highway, railroad, or airport (ISA, section 4.4).
 Furthermore, in California, 2.3% of schools, with a total enrollment of
more than 150,000 students were located within approximately 500 feet of
high-traffic roads, with a higher proportion of non-white and
economically disadvantaged students attending those schools (ISA,
section 4.4).  Of this population, asthmatics and members of other
susceptible groups discussed above will have even greater risks of
experiencing health effects related to NO2 exposure.  In the United
States, approximately 10% of adults and 13% of children (approximately
22.2 million people in 2005) have been diagnosed with asthma, and 6% of
adults have been diagnosed with COPD (ISA, section 4.4).  The prevalence
and severity of asthma is higher among certain ethnic or racial groups
such as Puerto Ricans, American Indians, Alaskan Natives, and African
Americans (ISA, section 4.4).  A higher prevalence of asthma among
persons of lower SES and an excess burden of asthma hospitalizations and
mortality in minority and inner-city communities have been observed
(ISA, section 4.4).  In addition, based on United States census data
from 2000, about 72.3 million (26%) of the United States population are
under 18 years of age, 18.3 million (7.4%) are under 5 years of age, and
35 million (12%) are 65 years of age or older.  Therefore, large
portions of the United States population are in age groups that are
likely at-risk for health effects associated with exposure to ambient
NO2.  The size of the potentially at-risk population suggests that
exposure to ambient NO2 could have a significant impact on public health
in the United States.

C.	Human exposure and health risk characterization

To put judgments about NO2-associated health effects into a broader
public health context, EPA has drawn upon the results of the
quantitative exposure and risk assessments.  Judgments reflecting the
nature of the evidence and the overall weight of the evidence are taken
into consideration in these quantitative exposure and risk assessments,
discussed below.  These assessments provide estimates of the likelihood
that asthmatic individuals would experience exposures of potential
concern and estimates of the incidence of NO2-associated respiratory
emergency department visits under varying air quality scenarios (e.g.,
just meeting the current or alternative standards), as well as
characterizations of the kind and degree of uncertainties inherent in
such estimates.  As discussed more fully in section II.C of the
proposal, this section summarizes the approach taken in the REA to
characterize NO2-related exposures and health risks.  Goals of the REA
included estimating short-term exposures and potential human health
risks associated with (1) recent levels of ambient NO2; 2) NO2 levels
adjusted to simulate just meeting the current standard; and 3) NO2
levels adjusted to simulate just meeting potential alternative
standards.

For purposes of the quantitative characterization of NO2 health risks,
the REA determined that it was appropriate to focus on endpoints for
which the ISA concluded that the available evidence is sufficient to
infer either a causal or a likely causal relationship.  This was
generally consistent with judgments made in other recent NAAQS reviews
(e.g., see EPA, 2005).  As noted above in section II.A, the only health
effect category for which the evidence was judged in the ISA to be
sufficient to infer either a causal or a likely causal relationship is
respiratory morbidity following short-term NO2 exposure.  Therefore, for
purposes of characterizing health risks associated with NO2, the REA
focused on respiratory morbidity endpoints that have been associated
with short-term NO2 exposures.

In evaluating the appropriateness of specific endpoints for use in the
NO2 risk characterization, the REA considered both epidemiologic and
controlled human exposure studies.  As described in more detail in the
proposal (section II.C.1), the characterization of NO2-associated health
risks was based on an epidemiology study conducted in Atlanta, Georgia
by Tolbert et al. (2007) and a meta-analysis of controlled human
exposure studies of NO2 and airway responsiveness in asthmatics (ISA,
Table 3.1-3).

As noted above, the purpose of the assessments described in the REA was
to characterize air quality, exposures, and health risks associated with
recent ambient levels of NO2, with NO2 levels that could be associated
with just meeting the current NO2 NAAQS, and with NO2 levels that could
be associated with just meeting potential alternative standards.  To
characterize health risks, the REA employed three approaches.  In the
first approach, for each air quality scenario, NO2 concentrations at
fixed-site monitors and simulated concentrations on/near roadways were
compared to potential health effect benchmark values derived from the
controlled human exposure literature.  In the second approach, modeled
estimates of exposures in asthmatics were compared to potential health
effect benchmarks.  In the third approach, concentration-response
relationships from an epidemiologic study were used in conjunction with
baseline incidence data and recent or simulated ambient concentrations
to estimate health impacts.  An overview of the approaches to
characterizing health risks is provided in the proposal (section II.C.2)
and each approach, along with its limitations and uncertainties (see
proposal, section II.C.3) has been described in more detail in the REA
(chapters 6 through 9).

Chapters 7-9 of the REA estimated exposures and health risks associated
with recent air quality and with air quality, as measured at monitors in
the current area-wide network, which had been adjusted to simulate just
meeting the current and potential alternative standards.  The specific
standard levels evaluated, for an area-wide standard based on the 3-year
average of the 98th and 99th percentile 1-hour daily maximum NO2
concentrations, were 50, 100, 150, and 200 ppb.  In interpreting these
results within the context of the current revisions to the NO2 primary
NAAQS (see below), we note that simulation of different standard levels
was based on adjusting NO2 concentrations at available area-wide
monitors.  Therefore, the standard levels referred to above reflect the
allowable area-wide NO2 concentrations, not the maximum allowable
concentrations.  As a consequence, the maximum concentrations in an area
that just meets one of these standard levels would be expected to be
higher than the standard level.  For example, given that near-road NO2
concentrations can be 30% to 100% higher than area-wide concentrations
(see section II.E.2), an area-wide concentration of 50 ppb could
correspond to near-road concentrations from 65 to 100 ppb.

Key results of the air quality, exposure, and risk analyses were
presented in the policy assessment chapter of the REA and summarized in
the proposal (Table 1 in proposal).  In considering these results, the
policy assessment chapter of the REA concluded that the risks estimated
to be associated with just meeting the current annual standard can be
judged important from a public health perspective.  The results for
specific 1-hour standard levels estimate that limiting the 98th/99th
percentile of the distribution of 1-hour daily maximum NO2
concentrations measured at area-wide monitors to 50 or 100 ppb could
substantially reduce exposures to ambient NO2 and associated health
risks (compared to just meeting the current standard).  In contrast,
limiting these area-wide NO2 concentrations to 150 or 200 ppb is
estimated to result in similar, or in some cases higher, NO2-associated
exposures and health risks than just meeting the current standard.  The
pattern of results was similar for standards just meeting either the
98th or the 99th percentile 1-hour daily maximum area-wide standards
(REA, Chapters 7, 8, and 9).

D.	Approach for reviewing the need to retain or revise the current
standard

EPA notes that the final decision on retaining or revising the current
primary NO2 standard is a public health policy judgment to be made by
the Administrator.  This judgment has been informed by a recognition
that the available health effects evidence reflects a continuum
consisting of ambient levels of NO2 at which scientists generally agree
that health effects are likely to occur, through lower levels at which
the likelihood and magnitude of the response become increasingly
uncertain.  The Administrator’s final decisions draw upon scientific
information and analyses related to health effects, population
exposures, and risks; judgments about the appropriate response to the
range of uncertainties that are inherent in the scientific evidence and
analyses; and comments received from CASAC and the public.

To evaluate whether the current primary NO2 standard is requisite or
whether consideration of revisions is appropriate, EPA has used an
approach in this review that was described in the policy assessment
chapter of the REA.  This approach builds upon those used in reviews of
other criteria pollutants, including the most recent reviews of the Pb,
O3, and PM NAAQS (EPA, 2007d; EPA, 2007e; EPA, 2005), and reflects the
body of evidence and information that is currently available.  As in
other recent reviews, EPA's considerations included the implications of
placing more or less weight or emphasis on different aspects of the
scientific evidence and the exposure/risk-based information, recognizing
that the weight to be given to various elements of the evidence and
exposure/risk information is part of the public health policy judgments
that the Administrator will make in reaching decisions on the standard.

A series of general questions framed this approach to considering the
scientific evidence and exposure-/risk-based information.  First,
EPA’s consideration of the scientific evidence and exposure/risk
information with regard to the adequacy of the current standard has been
framed by the following questions:

To what extent does evidence that has become available since the last
review reinforce or call into question evidence for NO2-associated
effects that were identified in the last review?

To what extent has evidence for different health effects and/or
sensitive populations become available since the last review?

To what extent have uncertainties identified in the last review been
reduced and/or have new uncertainties emerged?

To what extent does evidence and exposure-/risk-based information that
has become available since the last review reinforce or call into
question any of the basic elements of the current standard?

To the extent that the available evidence and exposure-/risk-based
information suggests it may be appropriate to consider revision of the
current standard, EPA considers that evidence and information with
regard to its support for consideration of a standard that is either
more or less protective than the current standard.  This evaluation has
been framed by the following questions:

Is there evidence that associations, especially causal or likely causal
associations, extend to ambient NO2 concentrations as low as, or lower
than, the concentrations that have previously been associated with
health effects?  If so, what are the important uncertainties associated
with that evidence?

Are exposures above benchmark levels and/or health risks estimated to
occur in areas that meet the current standard?  If so, are the estimated
exposures and health risks important from a public health perspective?
What are the important uncertainties associated with the estimated
risks?

To the extent that there is support for consideration of a revised
standard, EPA then considers the specific elements of the standard
(indicator, averaging time, form, and level) within the context of the
currently available information.  In so doing, the Agency has addressed
the following questions:

Does the evidence provide support for considering a different indicator
for gaseous NOX?

Does the evidence provide support for considering different averaging
times?

What ranges of levels and forms of alternative standards are supported
by the evidence, and what are the associated uncertainties and
limitations?

To what extent do specific averaging times, levels, and forms of
alternative standards reduce the estimated exposures above benchmark
levels and risks attributable to NO2, and what are the uncertainties
associated with the estimated exposure and risk reductions?

The questions outlined above have been addressed in the REA, the
proposal, and in this final rulemaking.  The following sections present
the rationale for proposed decisions, discussion of public comments, and
the Administrator’s conclusions on the adequacy of the current
standard and potential alternative standards in terms of indicator,
averaging time, form, and level.

E.	Adequacy of the current standard

	This section discusses considerations related to the decision as to
whether the current NO2 primary NAAQS is requisite to protect public
health with an adequate margin of safety.  Specifically, section II.E.1
provides an overview of the rationale supporting the Administrator’s
conclusion in the proposal that the current standard alone does not
provide adequate public health protection; section II.E.2 discusses
comments received on the adequacy of the current standard; and section
II.E.3 discusses the Administrator’s final decision on whether the
current NO2 primary NAAQS is requisite to protect public health with an
adequate margin of safety.

1.	Rationale for proposed decision

In reaching a conclusion regarding the adequacy of the current NO2 NAAQS
in the proposal (section II.E.5), the Administrator considered the
scientific evidence assessed in the ISA and the conclusions of the ISA,
the exposure and risk information presented in the REA and the
conclusions of the policy assessment chapter of the REA, and the views
expressed by CASAC.  These considerations are discussed in detail in the
proposal (II.E.) and are summarized in this section.  In the proposal,
the Administrator noted the following in considering the adequacy of the
current standard:

The ISA concluded that the results of epidemiologic and experimental
studies form a plausible and coherent data set that supports a
relationship between NO2 exposures and respiratory endpoints, including
respiratory symptoms and respiratory-related hospital admissions and
emergency department visits, at ambient concentrations that are present
in areas that meet the current NO2 NAAQS (ISA, section 5.4).

The policy assessment chapter of the REA concluded that risks estimated
to be associated with air quality adjusted upward to simulate just
meeting the current standard can reasonably be judged important from a
public health perspective (REA, section 10.3.3).

The policy assessment chapter of the REA concluded that exposure- and
risk-based results reinforce the scientific evidence in supporting the
conclusion that consideration should be given to revising the current
NO2 NAAQS so as to provide increased public health protection,
especially for at-risk groups, from NO2-related adverse health effects
associated with short-term, and potential long-term, exposures (REA,
section 10.3.3).

CASAC agreed that the current annual standard alone is not sufficient to
protect public health against the types of exposures that could lead to
these health effects.  Specifically, in their letter to the
Administrator on the final REA, they stated that “CASAC concurs with
EPA’s judgment that the current NAAQS does not protect the public’s
health and that it should be revised” (Samet, 2008b).

Based on these considerations (discussed in more detail in the proposal,
section II.E), the Administrator concluded in the proposal that the
current NO2 primary NAAQS is not requisite to protect public health with
an adequate margin of safety against adverse respiratory effects
associated with short-term exposures.  In considering approaches to
revising the current standard, the Administrator concluded that it is
appropriate to consider setting a new short-term standard in addition to
retaining the current annual standard.  The Administrator noted that
such a short-term standard could provide increased public health
protection, especially for members of at-risk groups, from effects
described in both epidemiologic and controlled human exposure studies to
be associated with short-term exposures to NO2.

2.	Comments on the adequacy of the current standard

This section discusses comments received from CASAC and public
commenters on the proposal that either supported or opposed the
Administrator’s proposed decision to revise the current NO2 primary
NAAQS.  Comments on the adequacy of the current standard that focused on
the scientific and/or the exposure/risk basis for the Administrator’s
proposed conclusions are discussed in sections II.E.2.a-II.E.2.c.
Comments on the epidemiologic evidence are considered in section
II.E.2.a.  Comments on the controlled human exposure evidence are
considered in section II.E.2.b.  Comments on human exposure and health
risk assessments are considered in section II.E.2.c.  To the extent
these comments on the evidence and information are also used to justify
commenters’ conclusions on decisions related to indicator, averaging
time, level, or form, they are noted in the appropriate sections below
(II.F.1-II.F.4).

In their comments on the proposal (Samet, 2009), CASAC reiterated their
support for the need to revise the current annual NO2 NAAQS in order to
increase public health protection.  As noted above, in its letter to the
Administrator on the final REA (Samet, 2008b) CASAC stated that it
“concurs with EPA’s judgment that the current NAAQS does not protect
the public’s health and that it should be revised.”  In supporting
adoption of a more stringent NAAQS for NO2, CASAC considered the
assessment of the scientific evidence presented in the ISA, the results
of assessments presented in the REA, and the conclusions of the policy
assessment chapter of the REA.  As such, CASAC’s rationale for
revising the current standard was consistent with the Administrator’s
rationale as discussed in the proposal.

Many public commenters agreed with CASAC that, based on the available
information, the current NO2 standard is not requisite to protect public
health with an adequate margin of safety and that revisions to the
standard are appropriate.  Among those calling for revisions to the
standard were environmental groups (e.g., Clean Air Council (CAC), Earth
Justice (EJ), Environmental Defense Fund (EDF), Natural Resources
Defense Council (NRDC), Group Against Smog and Pollution (GASP));
medical/public health organizations (e.g., American Lung Association
(ALA), American Medical Association (AMA), American Thoracic Society
(ATS), National Association for the Medical Direction of Respiratory
Care (NAMDRC), National Association of Cardiovascular and Pulmonary
Rehabilitation (NACPR), American College of Chest Physicians (ACCP)); a
large number of State agencies and organizations (e.g., National
Association of Clean Air Agencies (NACAA), Northeast States for
Coordinated Air Use Management (NESCAUM), and state or local agencies in
CA, IA, IL, MI, MO, NC, NM, NY, TX, VA, WI); Tribes (e.g., National
Tribal Air Association (NTAA), Fond du Lac Band of Lake Superior
Chippewa (Fond du Lac)), and a number of individual commenters.  These
commenters concluded that the current NO2 standard needs to be revised
and that a more stringent standard is needed to protect the health of
sensitive population groups.  In supporting the need to adopt a more
stringent NAAQS for NO2, these commenters often referenced the
conclusions of CASAC and relied on the evidence and information
presented in the proposal.  As such, similar to CASAC, the rationale
offered by these commenters was consistent with that presented in the
proposal to support the Administrator’s proposed decision to revise
the current NO2 NAAQS.

Some industry commenters (e.g., Alliance of Automobile Manufacturers
(AAM), American Petroleum Institute (API), Interstate Natural Gas
Association of America (INGAA), Utility Air Regulatory Group (UARG)) and
one State commenter (IN Department of Environmental Management)
expressed support for retaining the current annual standard alone.  In
supporting this view, these commenters generally concluded that the
current standard is requisite to protect public health with an adequate
margin of safety and that the available evidence is not sufficient to
support revision of the standard.  For example, UARG stated that “EPA
has failed to demonstrate that the present NO2 NAAQS is no longer at the
level requisite to protect public health with an adequate margin of
safety.”  In addition, INGAA stated that

“…EPA should be compelled to retain the current standard and defer a
decision on a new short-term standard until the science is more clearly
defined.”

	In support of their views, these commenters provided specific comments
on the epidemiologic and controlled human exposure evidence as discussed
below.  In responding to these specific comments, we note that the
Administrator relied in the proposal on the evidence, information and
judgments contained in the ISA and the REA (including the policy
assessment chapter) as well as on the advice of CASAC.  In considering
the evidence, information, and judgments of the ISA and the REA, the
Agency notes that these documents have been reviewed extensively by
CASAC and have been discussed by CASAC at multiple public meetings (see
section I.D).  In their letter to the Administrator regarding the second
draft ISA (Henderson, 2008b), CASAC noted the following:

Panel members concur with the primary conclusions reached in the ISA
with regard to health risks that are associated with NO2 exposure. In
particular, the Panel agrees with the conclusion that the current
scientific evidence is “sufficient to infer a likely causal
relationship between short term NO2 exposure and adverse effects on the
respiratory system.” The strongest evidence in support of this
conclusion comes from epidemiology studies that show generally positive
associations between NO2 and respiratory symptoms, hospitalizations or
emergency department visits, as summarized in Figure 5.3.1.”

Similarly, in their letter to the Administrator on the final REA (Samet,
2008b), CASAC noted the following:

Overall, CASAC found this version of the REA satisfactory in its
approach to moving from the scientific foundation developed in the
Integrated Science Assessment (ISA) to setting out evidence-based
options for the NAAQS. The REA provides the needed bridge from the
evidence presented in the ISA to a characterization of the exposures and
the associated risks with different profiles of exposure. It draws on
toxicological and epidemiological evidence and addresses risk to an
identified susceptible population, people with asthmatic conditions. EPA
has also systematically described uncertainties associated with the risk
assessments. We commend EPA for developing a succinct and thoughtfully
developed synthesis in Chapter 10. This summary chapter represents a
long-needed and transparent model for linking a substantial body of
scientific evidence to the four elements of the NAAQS.

Therefore, in discussing comments on the interpretation of the
scientific evidence and exposure/risk information, we note that CASAC
has endorsed the approaches and conclusions of the ISA and the REA.
These approaches and conclusions are discussed below in more detail,
within the context of specific public comments.

a.	Comments on EPA’s interpretation of the epidemiologic evidence

	Several industry groups (e.g., API, National Mining Association (NMA),
American Chemistry Council (ACC), AAM, Annapolis Center for
Science-Based Public Policy (ACSBPP), Engine Manufacturers Association
(EMA), ExxonMobil (Exxon), National Association of Manufacturers (NAM))
commented that, given the presence of numerous co-pollutants in the air,
epidemiologic studies do not support the contention that NO2 itself is
causing health effects.

While EPA has recognized that multiple factors can contribute to the
etiology of respiratory disease and that more than one air pollutant
could independently impact respiratory health, we continue to judge, as
discussed in the ISA, that the available evidence supports the
conclusion that there is an independent effect of NO2 on respiratory
morbidity.  In reaching this judgment, we recognize that a major
methodological issue affecting NO2 epidemiologic studies concerns the
evaluation of the extent to which other air pollutants may confound or
modify NO2-related effect estimates.  The use of multipollutant
regression models is the most common approach for controlling potential
confounding by copollutants in epidemiologic studies.  The issues
related to confounding and the evidence of potential confounding by
copollutants has been thoroughly reviewed in the ISA (see Figures 3.1-10
and 3.1-11) and in previous assessments (e.g., EPA, 2004).  NO2 risk
estimates for respiratory morbidity endpoints, in general, were not
sensitive to the inclusion of copollutants, including particulate and
gaseous pollutants.  As observed in Figures 3.1-10 and 3.1-11 in the
ISA, relative risks for hospital admissions or emergency department
visits are generally unchanged, nor is their interpretation modified,
upon inclusion of PM or gaseous co-pollutants in the models.  Similarly,
associations between short-term NO2 exposure and asthma symptoms are
generally robust to adjustment for copollutants in multipollutant
models, as shown in Figures 3.1-5 and 3.1-7 of the ISA.  These results,
in conjunction with the results of a randomized intervention study
evaluating respiratory effects of indoor exposure to NO2 (ISA, section
<IP_ADDRESS>), led to the conclusion that the effect of NO2 on respiratory
health outcomes is robust and independent of the effects of other
ambient copollutants.

In addition, experimental studies conducted in animals and humans
provide support for the plausibility of the associations reported in
epidemiologic studies.  These controlled human exposure and animal
toxicological studies have reported effects of NO2 on immune system
function, lung host defense, airway inflammation, and airway
responsiveness (ISA, section 5.4).  These experimental study results
support an independent contribution of NO2 to the respiratory health
effects reported in epidemiologic studies (ISA Section 5.4).

In considering the entire body of evidence, including epidemiologic and
experimental studies, the ISA (section 5.4, p. 5-16) concluded the
following:

Although this [presence of co-pollutants] complicates the efforts to
disentangle specific NO2-related health effects, the evidence summarized
in this assessment indicates that NO2 associations generally remain
robust in multi-pollutant models and supports a direct effect of
short-term NO2 exposure on respiratory morbidity at ambient
concentrations below the current NAAQS. The robustness of epidemiologic
findings to adjustment for co-pollutants, coupled with data from animal
and human experimental studies, support a determination that the
relationship between NO2 and respiratory morbidity is likely causal,
while still recognizing the relationship between NO2 and other
traffic-related pollutants.

Comments on specific epidemiologic studies are discussed below.

The National Association of Manufacturers (NAM) commented that the final
REA relied on an epidemiologic study (Delfino et al. 2002) not
critically reviewed in the final ISA.  Contrary to NAM’s contention,
the study by Delfino et al. (2002) was critically reviewed by EPA staff
and pertinent information was extracted from the study.  The respiratory
health effects of NO2 on asthma reported in this study are included in
Figure 5.3-1, Table 5.4-1, and Annex Table AX6.3-2 of the ISA.  While
NAM comments on the narrative discussion of this study in the final ISA,
their contention that EPA scientists did not critically analyze the
study while preparing the final ISA is incorrect.  The inclusion of the
study in the figures and tables in this ISA, as well as inclusion in the
2004 PM AQCD, indicate critical analysis of the study that was
implemented throughout the review process.  The narrative discussion in
the ISA focused on multicity studies (specifically those by Schwartz et
al. 1994, Mortimer et al. 2002 and Schildcrout et al. 2006), which
provide substantial epidemiologic evidence for the respiratory health
effects of NO2 on asthma among children.

Additional comments from NAM contend that EPA’s interpretation of
three individual epidemiologic studies (e.g. Krewski et al. 2000;
Schildcrout et al. 2006; Mortimer et al. 2002) is inconsistent across
different NAAQS reviews.  The NAM comments on all three studies are
discussed below.

NAM stated the following regarding the study by Krewski et al:

In the Final ISA, EPA cites the Krewski, et al. (2000) study as evidence
of a significant association between NO2 exposure and mortality.
Although EPA acknowledges that exposure to NO2 was “highly
correlated” with other pollutants, including PM2.5 and SO2, EPA does
not consider the analysis of the respective contributions of single
pollutants in the same study that EPA included in its prior Staff Paper
for Particulate Matter. In that document, EPA stated: “In
single-pollutant models, none of the gaseous co-pollutants was
significantly associated with mortality except SO2.” If EPA has not
altered its scientific views concerning this study as expressed in the
PM Staff Paper, it is entirely inappropriate for EPA to suggest that the
Krewski, et al. (2000) study provides any evidence of an association
between NO2 exposure and mortality.

In these comments, NAM fails to recognize that the Krewski et al. (2000)
report contains a reanalysis of two cohort studies, the Harvard Six
Cities and the American Cancer Society (ACS) studies.  NAM correctly
quotes from the PM Staff Paper (2005) regarding the results of single
pollutant models on associations with mortality.  However, this
statement in the PM Staff Paper is referring to the results of the ACS
Study.  In contrast, the characterization in the NOX ISA of the study by
Krewski et al. (2000), referenced by NAM in their comments, refers to
the reanalysis of the Harvard Six Cities Study.  As stated in the NOX
ISA (p. 3-74):

Krewski et al. (2000) conducted a sensitivity analysis of the Harvard
Six Cities study and examined associations between gaseous pollutants
(i.e., O3, NO2, SO2, CO) and mortality. NO2 showed risk estimates
similar to those for PM2.5 per “low to high” range increment with
total (1.15 [95% CI: 1.04, 1.27] per 10-ppb increase), cardiopulmonary
(1.17 [95% CI: 1.02, 1.34]), and lung cancer (1.09 [95% CI: 0.76, 1.57])
deaths; however, in this dataset NO2 was highly correlated with PM2.5
(r=0.78), SO4 2– (r=0.78), and SO2 (r=0.84).

In the subsequent paragraph, the NOX ISA presents results from the
Krewski et al. (2000) reanalysis of the ACS study, observing that “NO2
showed no associations with mortality outcomes” (ISA, p.3-74).  This
is consistent with the quote drawn from the PM Staff Paper.  Thus, there
is no inconsistency in the interpretation of the results of the study by
Krewski et al. (2000) in the PM Staff Paper (2005) and the NOX ISA
(2008).  NAM is confusing the conclusions on the results of the
reanalysis of the Harvard Six Cities Study in the NOX ISA with
conclusions regarding the reanalysis of the ACS study in the PM Staff
Paper.

NAM also commented that EPA has relied on a study by Schildcrout et al.
(2006) in the NOX ISA but declined to rely on the same study for the
previous review of the O3 NAAQS.  NAM made the following comment
regarding the study by Schildcrout et al:

Another example of how EPA has reached different scientific conclusions
in the Final ISA than in prior NAAQS documents is provided by the
Schildcrout, et al. (2006) study. In the Final ISA, EPA includes an
extensive discussion of this study of asthmatic children and the
relationship purportedly found in this study between NO2 and various
respiratory symptoms. In contrast, as part of the NAAQS review for
ozone, EPA expressly declined to rely on this same study because of
specific limitations in the study design. Among the limitations EPA
cites were the fact that the Schildcrout, et al. (2006) study included
“children in which the severity of their asthma was not clearly
identified,” and the use of a study population that was “not
comparable to other large multi-city studies.” EPA must explain why it
chose to discount the value of the Schildcrout, et al. (2006) study when
evaluating the effects of ozone, but has relied on it extensively in the
Final ISA for NO2.

The study by Schildcrout et al. (2006) appeared in the peer-review
literature too late to be considered in the 2006 O3 AQCD; however, this
study was included in the O3 Provisional Assessment.  The purpose of the
Provisional Assessment was to determine if new literature materially
changed any of the broad scientific conclusions regarding the health
effects of O3 exposure as stated in the 2006 O3 AQCD.  EPA concluded
that, taken in context, the “new” information and findings did not
materially change any of the broad scientific conclusions regarding the
health effects of O3 exposure made in the O3 AQCD.  Therefore, NAM’s
contention that EPA “declined” to rely on the Schildcrout study for
the O3 review because of limitations in study design is not correct.

The observations NAM draws from the O3 Provisional Assessment regarding
severity of asthma and the study population do not indicate limitations
that resulted in EPA “discounting” the study results.  Rather, these
observations were intended to put the study in perspective for purposes
of interpreting the results within the context of the larger body of O3
health effects evidence.  These observations were drawn from comments
submitted by Dr. Schildcrout regarding the interpretation of the results
of his study in the decision to revise the ozone standards (see docket
ID EPA-HQ-OAR-2005-0172-6991).  The results of this study will be fully
considered in the upcoming review of the ozone NAAQS.

Finally, NAM contends that EPA reached differing scientific conclusions
on the use of self-reported peak expiratory flow (PEF) depending on
regulatory context, particularly in the large multi-city trial by
Mortimer et al.   ADDIN
LC_ITEM<references><reference><heroid>30281</heroid><citation>Mortimer
KM; Neas LM; Dockery DW; Redline S; Tager IB. (2002). The effect of air
pollution on inner-city children with asthma. Eur Respir J, 19:
699-705.</citation><shortcitation>Mortimer et
al.</shortcitation><year>2002</year><link>http://cfpub.epa.gov/ncea/hero
/index.cfm?action=search.view&reference_id=30281</link><format>Year<
/format><prefix></prefix><suffix></suffix></reference></references>
(2002) .  EPA consistently examines clinical measurements of lung
function, which include PEF, forced expiratory flow in 1 second (FEV1),
forced vital capacity (FVC), maximal midexpiratory flow (MMEF), maximal
expiratory flow at 50% (MEF50), maximal expiratory flow at 25% (MEF25),
and forced expiratory flow at 25 to 75% of FVC (FEF25-75).  Evidence for
all of these clinical measurements is considered before drawing a
conclusion related to the association of lung function with a criteria
pollutant.  In different reviews, there may be more evidence from one of
these clinical measurements than another.  In the previous NAAQS review
of ozone, EPA identified statistically significant associations between
increased ozone levels and morning PEF, which remained significant even
when concentrations exceeding 0.08 ppm were excluded from the analysis
(Mortimer et al. 2002).  EPA used this as evidence, along with evidence
of other clinical measurements of lung function, in drawing conclusions
on the relationship between ozone and lung function.  Similarly, in the
final NOX ISA, EPA weighed the all of the evidence pertinent to lung
function, including studies that produced no statistically significant
results for PEF, and the summary section (<IP_ADDRESS>) of the NO2 ISA
states:

In summary, epidemiologic studies using data from supervised lung
function measurements (spirometry or peak flow meters) report small
decrements in lung function   ADDIN
LC_ITEM<references><reference><heroid>46184</heroid><citation>Hoek G;
Brunekreef B. (1995). Effect of photochemical air pollution on acute
respiratory symptoms in children. Am J Respir Crit Care Med, 151:
27-32.</citation><shortcitation>Hoek and
Brunekreef</shortcitation><year>1995</year><link>http://cfpub.epa.gov/nc
ea/hero/index.cfm?action=search.view&reference_id=46184</link><forma
t>Author</format><prefix></prefix><suffix></suffix></reference><referenc
e><heroid>82508</heroid><citation>Linn WS; Shamoo DA; Anderson KR; Peng
R-C; Avol EL; Hackney JD; Gong H Jr. (1996). Short-term air pollution
exposures and responses in Los Angeles area schoolchildren. J Expo Sci
Environ Epidemiol, 6: 449-472.</citation><shortcitation>Linn et
al.</shortcitation><year>1996</year><link>http://cfpub.epa.gov/ncea/hero
/index.cfm?action=search.view&reference_id=82508</link><format>Autho
r</format><prefix></prefix><suffix></suffix></reference><reference><hero
id>90771</heroid><citation>Moshammer H; Hutter H-P; Hauck H; Neuberger
M. (2006). Low levels of air pollution induce changes of lung function
in a panel of schoolchildren. Eur Respir J, 27:
1138-1143.</citation><shortcitation>Moshammer et
al.</shortcitation><year>2006</year><link>http://cfpub.epa.gov/ncea/hero
/index.cfm?action=search.view&reference_id=90771</link><format>Autho
r</format><prefix></prefix><suffix></suffix></reference><reference><hero
id>42026</heroid><citation>Peacock JL; Symonds P; Jackson P; Bremner SA;
Scarlett JF; Strachan DP; Anderson HR. (2003). Acute effects of winter
air pollution on respiratory function in schoolchildren in southern
England. Occup Environ Med, 60: 82-89.</citation><shortcitation>Peacock
et
al.</shortcitation><year>2003</year><link>http://cfpub.epa.gov/ncea/hero
/index.cfm?action=search.view&reference_id=42026</link><format>Autho
r</format><prefix></prefix><suffix></suffix></reference><reference><hero
id>14576</heroid><citation>Schindler C; Kunzli N; Bongard J-P;
Leuenberger P; Karrer W; Rapp R; Monn C; Ackermann-Liebrich U; Swiss
Study on Air Pollution and Lung Diseases in Adults Investigators.
(2001). Short-term variation in air pollution and in average lung
function among never-smokers. Am J Respir Crit Care Med, 163:
356-361.</citation><shortcitation>Schindler et
al.</shortcitation><year>2001</year><link>http://cfpub.epa.gov/ncea/hero
/index.cfm?action=search.view&reference_id=14576</link><format>Autho
r</format><prefix></prefix><suffix></suffix></reference></references>
(Hoek and Brunekreef, 1995; Linn et al., 1996; Moshammer et al., 2006;
Peacock et al., 2003; Schindler et al., 2001) .  No significant
associations were reported in any studies using unsupervised,
self-administered peak flow [PEF] measurements with portable devices.

This evaluation of the evidence is consistent with the way that the
evidence from multiple clinical measures of lung function was used in
making similar statements during the NAAQS review for ozone.

b.	Comments on EPA’s interpretation of the controlled human exposure
evidence

A number of industry groups (e.g., AAM, ACC, API, Dow Chemical Company
(Dow), EMA, NAM, UARG) disagreed with EPA’s reliance on a
meta-analysis of controlled human exposure studies of airway
responsiveness in asthmatics.  Based on this meta-analysis (ISA, Table
3.1-3 for results), the ISA concluded that “small but significant
increases in nonspecific airway hyperresponsiveness were observed…at
0.1 ppm NO2 for 60-min exposures in asthmatics” (ISA, p. 5-11).
Industry groups raised a number of objections to this analysis and the
way in which it has been used in the current review.

Several of these industry groups concluded that, in relying on this
analysis, EPA has inappropriately relied on a new unpublished
meta-analysis that has not been peer-reviewed, was not reviewed by
CASAC, and was not conducted in a transparent manner.  For example, as
part of a Request for Correction submitted under EPA’s Information
Quality Guidelines, NAM stated that “EPA’s substantial reliance on
an unpublished assessment described as a “meta-analysis” of the
relation between NO2 exposure and changes in airway responsiveness
violates EPA Guidelines requiring “transparency about data and
methods.”

EPA disagrees with this characterization of the updated meta-analysis
included in the final ISA.  As described in the ISA (p. 3-16), this
meta-analysis is based on an earlier analysis by Folinsbee (1992) that
has been subject to peer-review, that was published in a scientific
journal (Toxicol Ind Health. 8:1-11, 1992), and that was reviewed by
CASAC as part of the previous review of the NO2 NAAQS (EPA, 1993, Table
15-10).  The updates to this earlier analysis did not include
substantive changes to the approach.  As discussed in the final ISA (p.
3-16), the changes made to the analysis were to remove the results of
one allergen study and add results from a non-specific responsiveness
study, which focused the meta-analysis on non-specific airway
responsiveness, and to discuss results for an additional exposure
concentration (i.e., 100 ppb).  The information needed to reproduce this
meta-analysis is provided in the ISA (Tables 3.1-2 and 3.1-3, including
footnotes).

While the ISA meta-analysis reports findings on airway responsiveness in
asthmatics following exposure to 100 ppb NO2, a concentration not
specifically discussed in the findings of the original report by
Folinsbee (1992), this does not constitute a substantive change to that
original analysis.  For exposures at rest, four of the studies included
in the analysis by Folinsbee evaluated the effects of exposure to 100
ppb NO2.  In that original meta-analysis, these studies were grouped
with another study that evaluated exposures to 140 ppb NO2.  When
analyzed together, exposures to NO2 concentrations of 100 ppb and 140
ppb (grouped together in the manuscript and described as less than 0.2
ppm) increased airway responsiveness in 65% of resting asthmatics (p <
0.01).  Therefore, reporting results at 100 ppb NO2 in the ISA
meta-analysis reflects a change in the way the data are presented and
does not reflect a substantive change to the study.  This change in
presentation allows specific consideration of the potential for
exposures to 100 ppb NO2 to increase airway responsiveness, rather than
grouping results at 100 ppb with results at other exposure
concentrations.

In addition, the updated meta-analysis was considered by CASAC during
their review of the REA (REA, Table 4-5 reports the results of the
updated meta-analysis), which based part of the assessment of
NO2-associated health risks on the results of the meta-analysis.  In
their letter to the Administrator on the final REA (Samet, 2008b), CASAC
stated that “[t]he evidence reviewed in the REA indicates that adverse
health effects have been documented in clinical studies of persons with
asthma at 100 ppb” and that “CASAC firmly recommends that the upper
end of the range [of standard levels] not exceed 100 ppb, given the
findings of the REA.”  In addition, in their comments on the proposal,
CASAC reiterated this advice in their statement that “the level of the
one-hour NO2 standard should be within the range of 80-100 ppb and not
above 100 ppb.”  These statements indicate that CASAC did specifically
consider the results of the updated meta-analysis and that they used
those results to inform their recommendations on the range of standard
levels supported by the scientific evidence.

In summary, we note the following:

The original meta-analysis was published in a peer-reviewed journal and
was reviewed by CASAC in the previous review of the NO2 NAAQS

The updated meta-analysis does not include substantive changes to the
methodology of this original analysis

The changes that were made are clearly described in the ISA

CASAC specifically reviewed and considered the ISA meta-analysis in
making recommendations regarding the range of standard levels supported
by the science

Many of these same industry groups also referred in their comments to a
recent meta-analysis of controlled human exposure studies evaluating the
airway response in asthmatics following NO2 exposure (Goodman et al.,
2009).  These groups generally recommended that EPA rely on this
meta-analysis and on the authors’ conclusions with regard to NO2 and
airway responsiveness.  Specific comments based on the manuscript by
Goodman et al., as well as EPA’s responses, are discussed below in
more detail.

Industry commenters generally claimed that the meta-analysis by Goodman
et al. supports the conclusion that no adverse effects occur following
exposures up to 600 ppb NO2.  However, Table 4 of the Goodman study
reports that 64% (95% Confidence Interval: 58%, 71%) of resting
asthmatics exposed to NO2 experienced an increase in airway
responsiveness.  Furthermore, Figure 2a of this manuscript reports that
for exposures <0.2 ppm, the fraction affected is 0.61 (95% CI: 0.52,
0.70) while for exposures of 0.2 ppm to <0.3 ppm, the fraction affected
is 0.66 (95% CI: 0.59, 0.74).  These findings are consistent with those
reported in the meta-analysis by Folinsbee and in the updated
meta-analysis that was included in the final ISA.

Also based on the meta-analysis by Goodman et al. (2009), several
industry commenters concluded that NO2-induced airway
hyperresponsiveness is not adverse and, therefore, should not be
considered in setting standards.  The basis for this comment appears to
be the conclusions reached by Goodman et al. that there is no
dose-response relationship for NO2 and that the magnitude of any NO2
effect on airway responsiveness is too small to be considered adverse.

Due to differences in study protocols in the NO2-airway response
literature (ISA, section 3.1.3), EPA disagrees with the approach taken
in the Goodman study to use existing data to attempt to evaluate the
presence of a dose-response relationship and to determine the magnitude
of the NO2 response.  Examples of differences in the study protocols
include the NO2 exposure method (i.e., mouthpiece versus chamber),
subject activity level (i.e., rest versus exercise) during NO2 exposure,
choice of airway challenge agent, and physiological endpoint used to
quantify airway responses.  Goodman et al. (2009) also recognized
heterogeneity among studies as a limitation in their analyses.

As a result of these differences, EPA judged it appropriate in the ISA
meta-analysis to assess only the fraction of asthmatics experiencing
increased or decreased airway responsiveness following NO2 exposure.  We
have acknowledged in the REA, the proposal, and in this final rulemaking
that there is uncertainty with regard to the magnitude and the
clinical-significance of NO2-induced increases in airway responsiveness
(see sections II.C.3 and II.F.4.a in the proposed rulemaking as well as
II.F.3 in this final rulemaking).  The REA stated the following (p.
302):

[O]ne of the important uncertainties associated with these [NO2-induced
airway hyperresponsiveness] results is that, because the meta-analysis
evaluated only the direction of the change in airway responsiveness, it
is not possible to discern the magnitude of the change from these data.
This limitation makes it particularly difficult to quantify the public
health implications of these results.

While we acknowledge this uncertainty, EPA disagrees with the conclusion
that the NO2-induced increase in airway responsiveness in asthmatics
exposed to NO2 concentrations up to 600 ppb is not adverse and should
not be considered in setting standards.  Specifically, we note that the
ISA concluded that “[t]ransient increases in airway responsiveness
following NO2 exposure have the potential to increase symptoms and
worsen asthma control” (ISA, section 5.4).  The uncertainty over the
adversity of the response reported in controlled human exposure studies
does not mean that the NO2-induced increase in airway responsiveness is
not adverse.  Rather, it means that there is a risk of adversity,
especially for asthmatics with more than mild asthma, but that this risk
cannot be fully characterized based on existing studies.  The studies of
NO2 and airway responsiveness included in the meta-analysis have
generally evaluated mild asthmatics, rather than more severely affected
asthmatics who could be more susceptible to the NO2-induced increase in
airway responsiveness (ISA, section <IP_ADDRESS>).  Given that this is the
case, and given the large percentage of asthmatics that experienced an
NO2-induced increase in airway responsiveness in the studies and the
large size of the asthmatic population in the United States, the REA
concluded that it is appropriate to consider NO2-induced airway
hyperresponsiveness in characterizing NO2-associated health risks (REA,
section 10.3.2).  As noted above, CASAC endorsed this conclusion in
their letters to the Administrator on the final REA and on the proposal
(Samet, 2008b; Samet, 2009).

c.	Comments on EPA’s characterization of NO2-associated exposures and
health risks

Several commenters discussed the analyses of NO2-associated exposures
and health risks presented in the REA.  As in past reviews (EPA 2005,
2007d, 2007e), EPA has estimated allowable risks associated with the
current standard and potential alternative standards to inform judgments
on the public health risks that could exist under different standard
options.  Some industry commenters (e.g., API, NMA) concluded that the
Administrator should consider modeled exposures and risks associated
with actual NO2 air quality rather than with NO2 concentrations adjusted
to simulate just meeting the current annual standard or potential
alternative 1-hour standards.  These commenters pointed out that such
simulations require large adjustments to air quality and are highly
uncertain and that NAAQS are intended to address actual, rather than
highly improbable, risks to health.

We disagree with these commenters that exposure- and risk-related
considerations in the NAAQS review should rely only on unadjusted air
quality.  In considering whether the current standard is requisite to
protect public health with an adequate margin of safety, air quality
adjustments allow estimates of NO2-related exposures and health risks
that could exist in areas that just meet that standard.  That is, these
adjustments allow consideration of exposures and risks that would be
permissible under the current standard.  Therefore, such adjustments are
clearly useful to inform a decision on the issue before EPA (i.e., the
adequacy of the level of public health protection associated with
allowable NO2 air quality under the standard).  Similarly, air quality
adjustments to simulate different potential alternative standards
provide information on exposures and risks that would be permissible
under these alternatives.   As noted above, in their letter to the
Administrator on the final REA (Samet, 2008b), CASAC concluded that
“The REA provides the needed bridge from the evidence presented in the
ISA to a characterization of the exposures and the associated risks with
different profiles of exposure.”

We agree that there are uncertainties inherent in air quality
adjustments.  These uncertainties are discussed thoroughly in the REA
(sections 7.4, 8.12, 9.6, and <IP_ADDRESS>) and in the proposed rule
(section II.C.3).  For example, the policy assessment chapter of the REA
(section <IP_ADDRESS>) noted the following regarding adjustment of NO2
concentrations:

In order to simulate just meeting the current annual standard and many
of the alternative 1-h standards analyzed, an upward adjustment of
recent ambient NO2 concentrations was required.  We note that this
adjustment does not reflect a judgment that levels of NO2 are likely to
increase under the current standard or any of the potential alternative
standards under consideration.  Rather, these adjustments reflect the
fact that the current standard, as well as some of the alternatives
under consideration, could allow for such increases in ambient NO2
concentrations.  In adjusting air quality to simulate just meeting these
standards, we have assumed that the overall shape of the distribution of
NO2 concentrations would not change.  While we believe this is a
reasonable assumption in the absence of evidence supporting a different
distribution and we note that available analyses support this approach
(Rizzo, 2008), we recognize this as an important uncertainty.  It may be
an especially important uncertainty for those scenarios where
considerable upward adjustment is required to simulate just meeting one
or more of the standards.

These air quality adjustments are not meant to imply an expectation that
NO2 concentrations will increase broadly across the United States or in
any given area (REA, section <IP_ADDRESS>).  Rather, as noted above, they
are meant to estimate NO2-related exposures and health risks that would
be permitted under the current and potential alternative standards.
Such estimates can inform decisions on whether the current standard, or
particular potential alternative standards, provide the requisite
protection of public health.

3.	Conclusions regarding the adequacy of the current standard

In considering the adequacy of the current standard, the Administrator
has considered the scientific evidence assessed in the ISA, the exposure
and risk results presented in the REA, the conclusions of the policy
assessment chapter of the REA, and comments from CASAC and the public.
These considerations are described below.

In considering the scientific evidence as it relates to the adequacy of
the current standard, the Administrator notes that the epidemiologic
evidence has grown substantially since the last review with the addition
of field and panel studies, intervention studies, and time-series
studies of effects such as emergency department visits and hospital
admissions associated with short-term NO2 exposures.  No epidemiologic
studies were available in 1993 assessing relationships between NO2 and
outcomes such as hospital admissions or emergency department visits.  In
contrast, dozens of epidemiologic studies on such outcomes, conducted at
recent and current ambient NO2 concentrations, are now included in this
evaluation (ISA, chapter 3).

As an initial consideration with regard to the adequacy of the current
standard, the Administrator notes that the evidence relating long-term
(weeks to years) NO2 exposures at current ambient concentrations to
adverse health effects was judged in the ISA to be either “suggestive
but not sufficient to infer a causal relationship” (respiratory
morbidity) or “inadequate to infer the presence or absence of a causal
relationship” (mortality, cancer, cardiovascular effects,
reproductive/developmental effects) (ISA, sections <IP_ADDRESS>-<IP_ADDRESS>).  In
contrast, the evidence relating short-term (minutes to hours) NO2
exposures to respiratory morbidity was judged to be “sufficient to
infer a likely causal relationship” (ISA, section <IP_ADDRESS>).  This
conclusion was supported primarily by a large body of recent
epidemiologic studies that evaluated associations of short-term NO2
concentrations with respiratory symptoms, emergency department visits,
and hospital admissions.  Given these conclusions from the ISA, the
Administrator judges that, at a minimum, consideration of the adequacy
of the current annual standard should take into account the extent to
which that standard provides protection against respiratory effects
associated with short-term NO2 exposures.

In considering the NO2 epidemiologic studies as they relate to the
adequacy of the current standard, the Administrator notes that annual
average NO2 concentrations were below the level of the current annual
NO2 NAAQS in many of the locations where positive, and often
statistically significant, associations with respiratory morbidity
endpoints have been reported (ISA, section 5.4).  As discussed
previously, the ISA characterized that evidence for respiratory effects
as consistent and coherent.  The evidence is consistent in that
associations are reported in studies conducted in numerous locations and
with a variety of methodological approaches (ISA, section <IP_ADDRESS>).  It
is coherent in the sense that the studies report associations with
respiratory health outcomes that are logically linked together (ISA,
section <IP_ADDRESS>).  The ISA noted that when the epidemiologic literature
is considered as a whole, there are generally positive associations
between NO2 and respiratory symptoms, hospital admissions, and emergency
department visits.  A number of these associations are statistically
significant, particularly the more precise effect estimates (ISA,
section <IP_ADDRESS>).

As discussed in the proposal (II.E.1) and above, the Administrator
acknowledges that the interpretation of these NO2 epidemiologic studies
is complicated by the fact that on-road vehicle exhaust emissions are a
nearly ubiquitous source of combustion pollutant mixtures that include
NO2.  She notes that, in order to provide some perspective on the
uncertainty related to the presence of co-pollutants the ISA evaluated
epidemiologic studies that employed multi-pollutant models,
epidemiologic studies of indoor NO2 exposure, and experimental studies.
Specifically, the ISA noted that a number of NO2 epidemiologic studies
have attempted to disentangle the effects of NO2 from those of
co-occurring pollutants by employing multi-pollutant models.  When
evaluated as a whole, NO2 effect estimates in these models generally
remained robust when co-pollutants were included.  Therefore, despite
uncertainties associated with separating the effects of NO2 from those
of co-occurring pollutants, the ISA (section 5.4, p. 5-16) concluded
that “the evidence summarized in this assessment indicates that NO2
associations generally remain robust in multi-pollutant models and
supports a direct effect of short-term NO2 exposure on respiratory
morbidity at ambient concentrations below the current NAAQS.”  With
regard to indoor studies, the ISA noted that these studies can test
hypotheses related to NO2 specifically (ISA, section <IP_ADDRESS>).  Although
confounding by indoor combustion sources is a concern, indoor studies
are not confounded by the same mix of co-pollutants present in the
ambient air or by the contribution of NO2 to the formation of secondary
particles or O3 (ISA, section <IP_ADDRESS>).  The ISA noted that the findings
of indoor NO2 studies are consistent with those of studies using ambient
concentrations from central site monitors and concluded that indoor
studies provide evidence of coherence for respiratory effects (ISA,
section <IP_ADDRESS>).  With regard to experimental studies, the REA noted
that they have the advantage of providing information on health effects
that are specifically associated with exposure to NO2 in the absence of
co-pollutants.  The ISA concluded that the NO2 epidemiologic literature
is supported by 1) evidence from controlled human exposure studies of
airway hyperresponsiveness in asthmatics, 2) controlled human exposure
and animal toxicological studies of impaired host-defense systems and
increased risk of susceptibility to viral and bacterial infection, and
3) controlled human exposure and animal toxicological studies of airway
inflammation (ISA, section <IP_ADDRESS> and 5.4).  Given the above
consideration of the evidence, particularly the epidemiologic studies
reporting NO2-associated health effects in locations that meet the
current standard, the Administrator agrees with the conclusion in the
policy assessment chapter of the REA that the scientific evidence calls
into question the adequacy of the current standard to protect public
health.

In addition to the evidence-based considerations described above, the
Administrator has considered the extent to which exposure- and
risk-based information can inform decisions regarding the adequacy of
the current annual NO2 standard.  While she acknowledges the
uncertainties associated with adjusting air quality in these analyses,
she judges that such analyses are appropriate for consideration in this
review of the NO2 primary NAAQS.  In reaching this conclusion she notes
the considerations discussed above, particularly the endorsement by
CASAC of the REA and its characterization of NO2-associated exposures
and health risks.

In considering the exposure- and risk-based information with regard to
the adequacy of the current annual NO2 standard to protect the public
health, the Administrator notes the conclusion in the policy assessment
chapter of the REA that risks estimated to be associated with air
quality adjusted upward to simulate just meeting the current standard
can reasonably be concluded to be important from a public health
perspective.  In particular, a large percentage (8-9%) of
respiratory-related ED visits in Atlanta could be associated with
short-term NO2 exposures, most asthmatics in Atlanta could be exposed on
multiple days per year to NO2 concentrations at or above 300 ppb, and
most locations evaluated could experience on-/near-road NO2
concentrations above 100 ppb on more than half of the days in a given
year.  Therefore, after considering the results of the exposure and risk
analyses presented in the REA the Administrator agrees with the
conclusion of the policy assessment chapter of the REA that exposure-
and risk-based results reinforce the scientific evidence in supporting
the conclusion that consideration should be given to revising the
current standard so as to provide increased public health protection,
especially for at-risk groups, from NO2-related adverse health effects
associated with short-term, and potential long-term, exposures.

In reaching a conclusion on the adequacy of the current standard, the
Administrator has also considered advice received from CASAC.  In their
comments on the final REA, CASAC agreed that the primary concern in this
review is to protect against health effects that have been associated
with short-term NO2 exposures.  CASAC also agreed that the current
annual standard is not sufficient to protect public health against the
types of exposures that could lead to these health effects.  As noted in
their letter to the EPA Administrator, “CASAC concurs with EPA’s
judgment that the current NAAQS does not protect the public’s health
and that it should be revised” (Samet, 2008b).

Based on the considerations discussed above, the Administrator concludes
that the current NO2 primary NAAQS alone is not requisite to protect
public health with an adequate margin of safety.  Accordingly, she
concludes that the NO2 primary standard should be revised in order to
provide increased public health protection against respiratory effects
associated with short-term exposures, particularly for susceptible
populations such as asthmatics, children, and older adults.  In
considering approaches to revising the current standard, the
Administrator concludes that it is appropriate to consider setting a new
short-term standard (see below).  The Administrator notes that such a
short-term standard could provide increased public health protection,
especially for members of at-risk groups, from effects described in both
epidemiologic and controlled human exposure studies to be associated
with short-term exposures to NO2.

F.	Elements of a new short-term standard

In considering a revised NO2 primary NAAQS, the Administrator notes the
need to protect at-risk individuals from short-term exposures to NO2 air
quality that could cause the types of respiratory morbidity effects
reported in epidemiologic studies and the need to protect at-risk
individuals from short-term exposure to NO2 concentrations reported in
controlled human exposure studies to increase airway responsiveness in
asthmatics.  The Administrator’s considerations with regard to her
decisions are discussed in the following sections in terms of indicator
(II.F.1), averaging time (II.F.2), level (II.F.3), and form (II.F.4).

1.	Indicator

a.	Rationale for proposed decision

In past reviews, EPA has focused on NO2 as the most appropriate
indicator for ambient NOX.  In making a decision in the current review
on the most appropriate indicator, the Administrator considered the
conclusions of the ISA and the policy assessment chapter of the REA as
well as the view expressed by CASAC.  The policy assessment chapter of
the REA noted that, while the presence of NOX species other than NO2 has
been recognized, no alternative to NO2 has been advanced as being a more
appropriate surrogate.  Controlled human exposure studies and animal
toxicology studies assessed in the ISA provide specific evidence for
health effects following exposure to NO2.  Epidemiologic studies also
typically report levels of NO2 though the degree to which monitored NO2
reflects actual NO2 levels, as opposed to NO2 plus other gaseous NOX,
can vary (REA, section 2.2.3).   In addition, because emissions that
lead to the formation of NO2 generally also lead to the formation of
other NOX oxidation products, measures leading to reductions in
population exposures to NO2 can generally be expected to lead to
reductions in population exposures to other gaseous NOX.  Therefore, an
NO2 standard can also be expected to provide some degree of protection
against potential health effects that may be independently associated
with other gaseous NOX even though such effects are not discernable from
currently available studies indexed by NO2 alone.  Given these key
points, the policy assessment chapter of the REA concluded that the
evidence supports retaining NO2 as the indicator.  Consistent with this
conclusion, the CASAC Panel stated in its letter to the EPA
Administrator that it “concurs with retention of NO2 as the
indicator” (Samet, 2008b).  In light of the above considerations, the
Administrator proposed to retain NO2 as the indicator in the current
review.

b.	Comments on indicator

A relatively small number of comments directly addressed the issue of
the indicator for the standard (CASAC, Dow, API, AAM, and the Missouri
Department of Natural Resources Air Pollution Control Program (MODNR)).
All of these commenters endorsed the proposal to continue to use NO2 as
the indicator for ambient NOX.

 c.	Conclusions on indicator

	Based on the available information discussed above, and consistent with
the views of CASAC and other commenters, the Administrator concludes
that it is appropriate to continue to use NO2 as the indicator for a
standard that is intended to address effects associated with exposure to
NO2, alone or in combination with other gaseous NOX.  In so doing, the
Administrator recognizes that measures leading to reductions in
population exposures to NO2 will also reduce exposures to other nitrogen
oxides.

2.	Averaging time

	This section discusses considerations related to the averaging time of
the NO2 primary NAAQS.  Specifically, this section summarizes the
rationale for the Administrator’s proposed decision regarding
averaging time (II.F.2.a; see section II.F.2 of the proposal for more
detail), discusses comments related to averaging time (II.F.2.b), and
presents the Administrator’s final conclusions regarding averaging
time (II.F.2.c).

a.	Rationale for proposed decision

In considering the most appropriate averaging time for the NO2 primary
NAAQS, the Administrator noted in the proposal the conclusions and
judgments made in the ISA about available scientific evidence, air
quality correlations discussed in the REA, conclusions of the policy
assessment chapter of the REA, and CASAC recommendations (section II.F.2
in the proposal).  Specifically, she noted the following:

Experimental studies in humans and animals have reported respiratory
effects following NO2 exposures lasting from less than 1-hour up to
several hours.  Epidemiologic studies have reported associations between
respiratory effects and both 1-hour and 24-hour NO2 concentrations.
Therefore, the experimental evidence provides support for an averaging
time of shorter duration than 24 hours (e.g., 1-hour) while the
epidemiologic evidence provides support for both 1-hour and 24-hour
averaging times.  At a minimum, this suggests that a primary concern
with regard to averaging time is the level of protection provided
against 1-hour NO2 concentrations.

Air quality correlations presented in the policy assessment chapter of
the REA illustrated the relatively high degree of variability in the
ratios of annual average to short-term NO2 concentrations (REA, Table
10-2).  This variability suggests that a standard based on annual
average NO2 concentrations would not likely be an effective or efficient
approach to focus protection on short-term exposures.

These air quality correlations (REA, Table 10-1) suggested that a
standard based on 1-hour daily maximum NO2 concentrations could also be
effective at protecting against 24-hour NO2 concentrations.

The policy assessment chapter of the REA concluded that the scientific
evidence, combined with the air quality correlations, support the
appropriateness of a standard based on 1-hour daily maximum NO2
concentrations to protect against health effects associated with
short-term exposures.

CASAC concurred “with having a short-term NAAQS primary standard for
oxides of nitrogen and using the one-hour maximum NO2 value” (Samet,
2008b).

Based on these considerations, the Administrator proposed to set a new
standard based on 1-hour daily maximum NO2 concentrations.

b.	Comments on averaging time

As discussed above, CASAC endorsed the establishment of a new standard
with a 1-hour averaging time.  CASAC stated the following in their
comments on the proposal (Samet, 2009):

In reviewing the REA, CASAC supported a short-term standard for NO2 and
in reviewing the proposal, CASAC supports the proposed one-hour
averaging time in EPA's proposed rule.

The supporting rationale offered by CASAC in support of a new 1-hour
standard was generally the same as that put forward in the final REA and
the proposal.  Specifically, that rationale considered the available
scientific evidence, which supports a link between 1-hour NO2
concentrations and adverse respiratory effects, and air quality
information presented in the REA, which suggests that a 1-hour standard
can protect against effects linked to short-term NO2 exposures while an
annual standard would not be an effective or efficient approach to
protecting against these effects.

A large number of public commenters also endorsed the establishment of a
new standard with a 1-hour averaging time.  These included a number of
State agencies and organizations (e.g., NACAA, NESCAUM and agencies in
CA, IL, NM, TX, VA); environmental, medical, and public health
organizations (e.g., ACCP, ALA, AMA, ATS, CAC, EDF, EJ, GASP, NACPR,
NAMDRC, NRDC); and most individual commenters.  The supporting
rationales offered by these commenters often acknowledged the
recommendations of CASAC and the Administrator’s rationale as
discussed in the proposal.

Though many industry commenters recommended not revising the current
annual standard (as discussed above in section II.E.2), several of these
groups did conclude that if a short-term standard were to be set, a