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 1-hour averaging time would be appropriate (e.g., Colorado
Petroleum Association (CPA), Dow, NAM, Petroleum Association of Wyoming
(PAW), Utah Petroleum Association (UPA)). As discussed above, industry
commenters who disagreed with setting a new 1-hour standard generally
based this conclusion on their interpretation of the scientific
evidence and their conclusion that this evidence does not support the
need to revise the current annual standard. These comments, and EPA's
responses, are discussed in more detail above (section II.E) and in the
Response to Comments document.
c. Conclusions on Averaging Time
    In considering the most appropriate averaging time for the
NO2 primary NAAQS, the Administrator notes the available
scientific evidence as assessed in the ISA, the air quality analyses
presented in the REA, the conclusions of the policy assessment chapter
of the REA, CASAC recommendations, and public comments received. These
considerations are described below.
    When considering averaging time, the Administrator notes that the
evidence relating short-term (minutes to hours) NO2
exposures to respiratory morbidity was judged in the ISA to be
``sufficient to infer a likely causal relationship'' (ISA, section
<IP_ADDRESS>) while the evidence relating long-term (weeks to years)
NO2 exposures to adverse health effects was judged 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>). Thus, the Administrator concludes that these judgments most
directly support an averaging time that focuses protection on short-
term exposures to NO2.
    As in past reviews of the NO2 NAAQS, the Administrator
notes that it is instructive to evaluate the potential for a standard
based on annual average NO2 concentrations, as is the
current standard, to provide protection against short-term
NO2 exposures. To this end, the Administrator notes that
Table 10-1 in the REA reported the ratios of short-term to annual
average NO2 concentrations. Ratios of 1-hour daily maximum
concentrations (98th and 99th percentile \11\) to annual average
concentrations across 14 locations ranged from 2.5 to 8.7 while ratios
of 24-hour average concentrations to annual average concentrations
ranged from 1.6 to 3.8 (see Thompson, 2008 for more details). The
policy assessment chapter of the REA concluded that the variability in
these ratios across locations, particularly those for 1-hour
concentrations, suggested that a standard based on annual average
NO2 concentrations would not likely be an effective or
efficient approach to focus protection on short-term NO2
exposures. For example, in an area with a relatively high ratio (e.g.,
8), the current annual standard (53 ppb) would be expected to allow 1-
hour daily maximum NO2 concentrations of about 400 ppb. In
contrast, in an area with a relatively low ratio (e.g., 3), the current
standard would be expected to allow 1-hour daily maximum NO2
concentrations of about 150 ppb. Thus, for purposes of protecting
against the range of 1-hour NO2 exposures, the REA noted
that a standard based on annual average concentrations would likely
require more control than necessary in some areas and less control than
necessary in others, depending on the standard level selected.
---------------------------------------------------------------------------

    \11\ As discussed below, 98th and 99th percentile forms were
evaluated in the REA. A 99th percentile form corresponds
approximately to the 4th highest 1-hour concentration in a year
while a 98th percentile form corresponds approximately to the 7th or
8th highest 1-hour concentration in a year. A 4th highest
concentration form has been used previously in the O3
NAAQS while a 98th percentile form has been used previously in the
PM2.5 NAAQS.
---------------------------------------------------------------------------

    In considering the level of support available for specific short-
term averaging times, the Administrator notes that the policy
assessment chapter of the REA considered evidence from both
experimental and epidemiologic studies. Controlled human exposure
studies and animal toxicological studies provide evidence that
NO2 exposures from less than 1-hour up to 3-hours can result
in respiratory effects such as increased airway responsiveness and
inflammation (ISA, section <IP_ADDRESS>). Specifically, the ISA concluded
that NO2 exposures of 100 ppb for 1-hour (or 200 ppb to 300
ppb for 30-min) can result in small but significant increases in
nonspecific airway responsiveness (ISA, section <IP_ADDRESS>). In contrast,
the epidemiologic literature provides support for short-term averaging
times ranging from approximately 1-hour up to 24-hours (ISA, section
<IP_ADDRESS>). A

number of epidemiologic studies have detected positive associations
between respiratory morbidity and 1-hour (daily maximum) and/or 24-hour
NO2 concentrations. A few epidemiologic studies have
considered both 1-hour and 24-hour averaging times, allowing
comparisons to be made. The ISA reported that such comparisons in
studies that evaluate asthma emergency department visits failed to
reveal differences between effect estimates based on a 1-hour averaging
time and those based on a 24-hour averaging time (ISA, section
<IP_ADDRESS>). Therefore, the ISA concluded that it is not possible, from
the available epidemiologic evidence, to discern whether effects
observed are attributable to average daily (or multi-day)
concentrations (24-hour average) or high, peak exposures (1-hour
maximum) (ISA, section <IP_ADDRESS>).
    As noted in the policy assessment chapter of the REA, given the
above conclusions, the experimental evidence provides support for an
averaging time of shorter duration than 24 hours (e.g., 1-h) while the
epidemiologic evidence provides support for both 1-hour and 24-hour
averaging times. The Administrator concludes that, 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. However, she also notes that it is important to
consider the ability of a 1-hour averaging time to protect against 24-
hour average NO2 concentrations. To this end, the
Administrator notes that Table 10-2 in the REA presented correlations
between 1-hour daily maximum NO2 concentrations and 24-hour
average NO2 concentrations (98th and 99th percentile) across
14 locations (see Thompson, 2008 for more detail). Typical ratios
ranged from 1.5 to 2.0, though one ratio (Las Vegas) was 3.1. These
ratios were far less variable than those discussed above for annual
average concentrations, suggesting that a standard based on 1-hour
daily maximum NO2 concentrations could also be effective at
protecting against 24-hour NO2 concentrations. The REA
concluded that the scientific evidence, combined with the air quality
correlations described above, support the appropriateness of a standard
based on 1-hour daily maximum NO2 concentrations to protect
against health effects associated with short-term exposures.
    Based on these considerations, the Administrator concludes that a
standard with a 1-hour averaging time can effectively limit short-term
(i.e., 1- to 24-hours) exposures that have been linked to adverse
respiratory effects. This conclusion is based on the observations
summarized above and in more detail in the proposal, particularly that:
(1) The 1-hour averaging time has been directly associated with
respiratory effects in both epidemiologic and experimental studies and
that (2) results from air quality analyses suggest that a 1-hour
standard could also effectively control 24-hour NO2
concentrations. In addition, the Administrator notes the support
provided for a 1-hour averaging time in comments from CASAC, States,
environmental groups, and medical/public health groups. The
Administrator notes that arguments offered by some industry groups
against setting a 1-hour NO2 standard generally focus on
commenters' conclusions regarding uncertainties in the scientific
evidence. As discussed in more detail above (section II.E.2), the
Administrator disagrees with the conclusions of these commenters
regarding the appropriate interpretation of the scientific evidence and
associated uncertainties. Given these considerations, the Administrator
judges that it is appropriate to set a new NO2 standard with
a 1-hour averaging time.
3. Form
    This section discusses considerations related to the form of the 1-
hour NO2 primary NAAQS. Specifically, this section
summarizes the rationale for the Administrator's proposed decision
regarding form (II.F.4.a; see section II.F.3 of the proposal for more
detail), discusses comments related to form (II.F.4.b), and presents
the Administrator's final conclusions regarding form (II.F.4.c).
a. Rationale For Proposed Decision
    When considering alternative forms in the proposal, the
Administrator noted the conclusions in the policy assessment chapter of
the REA. Specifically, she noted the conclusion that the adequacy of
the public health protection provided by the combination of standard
level and form should be the foremost consideration. With regard to
this, she noted that concentration-based forms can better reflect
pollutant-associated health risks than forms based on expected
exceedances. This is the case because concentration-based forms give
proportionally greater weight to years when pollutant concentrations
are well above the level of the standard than to years when the
concentrations are just above the standard, while an expected
exceedance form would give the same weight to years with concentrations
that just exceed the standard as to years when concentrations greatly
exceed the standard. The Administrator also recognized the conclusion
in the policy assessment chapter of the REA that it is desirable from a
public health perspective to have a form that is reasonably stable and
insulated from the impacts of extreme meteorological events. With
regard to this, she noted that a form that calls for averaging
concentrations over three years would provide greater regulatory
stability than a form based on a single year of concentrations.
Therefore, consistent with recent reviews of the O3 and PM
NAAQS, the proposal focused on concentration-based forms averaged over
3 years, as evaluated in the REA.
    In considering specific concentration-based forms, the REA focused
on 98th and 99th percentile concentrations averaged over 3 years. This
focus on the upper percentiles of the distribution is appropriate given
the reliance, in part, on NO2 health evidence from
experimental studies, which provide information on specific exposure
concentrations that are linked to specific health effects. The REA
noted that a 99th percentile form for a 1-hour daily maximum standard
would correspond approximately to the 4th highest daily maximum
concentration in a year (which is the form of the current O3
NAAQS) while a 98th percentile form (which is the form of the current
short-term PM2.5 NAAQS) would correspond approximately to
the 7th or 8th highest daily maximum concentration in a year (REA,
Table 10-4; see Thompson, 2008 for methods).
    Consideration in the REA of an appropriate form for a 1-hour
standard was based on analyses of standard levels that reflected the
allowable area-wide NO2 concentration, not the maximum
allowable concentration. Therefore, in their review of the final REA,
CASAC did not have the opportunity to comment on the appropriateness of
specific forms in conjunction with a standard level that reflects the
maximum allowable NO2 concentration anywhere in an area.
Given this, when considering alternative forms for the 1-hour standard
in the proposal, the Administrator judged that it was appropriate to
consider both forms evaluated in the REA (i.e., 98th and 99th
percentiles). Therefore, she proposed to adopt either a 99th percentile
or a 4th highest form, averaged over 3 years, and she solicited comment
on both 98th percentile and 7th or 8th highest forms.
b. CASAC and Public Comments on Form
    In their letter to the Administrator, CASAC discussed the issue of
form within the context of the proposed

approach of setting a 1-hour standard level that reflects the maximum
allowable NO2 concentration anywhere in an area. CASAC
recommended that, for such a standard, EPA adopt a form based on the 3-
year average of the 98th percentile of the distribution of 1-hour daily
maximum NO2 concentrations. Specifically, they stated the
following in their comments on the proposal (Samet, 2009):

    The 98th percentile is preferred by CASAC for the form, given
the likely instability of measurements at the upper range and the
absence of data from the proposed two-tier approach.

As indicated in their letter, CASAC concluded that the potential
instability in higher percentile NO2 concentrations near
major roads argues for a 98th, rather than a 99th, percentile form.
Several State organizations and agencies (e.g., NESCAUM and agencies in
IN, NC, SD, VA) and industry groups (e.g., AAM, ACC, API, AirQuality
Research and Logistics (AQRL), CPA, Dow, ExxonMobil, IPAMS, PAW, UPA)
also recommended a 98th percentile form in order to provide regulatory
stability. In contrast, a small number of State and local agencies
(e.g., in MO and TX), several environmental organizations (e.g., EDF,
EJ, GASP, NRDC), and medical/public health organizations (e.g., ALA,
ATS) recommended either a 99th percentile form or a more stringent form
(e.g., no exceedance) to further limit the occurrence of NO2
concentrations that exceed the standard level in locations that attain
the standard.
c. Conclusions On Form
    The Administrator recognizes that there is not a clear health basis
for selecting one specific form over another. She also recognizes that
the analyses of different forms in the REA are most directly relevant
to a standard that reflects NO2 concentrations permitted to
occur broadly across a community, rather than the maximum concentration
that can occur anywhere in the area. In contrast, as discussed below
(section II.F.4.c), the Administrator has judged it appropriate to set
a new 1-hour standard that reflects the maximum allowable
NO2 concentration anywhere in an area. In light of this, the
Administrator places particular emphasis on the comments received on
form from CASAC relating to a 1-hour standard level that reflects the
maximum allowable NO2 concentration anywhere in an area. In
particular, the Administrator notes that CASAC recommended a 98th
percentile form averaged over 3 years for such a standard, given the
potential for instability in the higher percentile concentrations
around major roadways.
    In considering this recommendation, the Administrator recognizes
that the public health protection provided by the 1-hour NO2
standard is based on the approach used to set the standard and the
level of the standard (see below), in conjunction with the form of the
standard. Given that the Administrator is setting a standard that
reflects the maximum allowable NO2 concentration anywhere in
an area, rather than a standard that reflects the allowable area-wide
NO2 concentration, she agrees with CASAC that an appropriate
consideration with regard to form is the extent to which specific
statistics could be unstable at locations where maximum NO2
concentrations are expected, such as near major roads. When considering
alternative forms for the standard, the Administrator notes that an
unstable form could result in areas shifting in and out of attainment,
potentially disrupting ongoing air quality planning without achieving
public health goals. Given the limited available information on the
variability in peak NO2 concentrations near important
sources of NO2 such as major roadways, and given the
recommendation from CASAC that the potential for instability in the
99th percentile concentration is cause for supporting a 98th percentile
form, the Administrator judges it appropriate to set the form based on
the 3-year average of the 98th percentile of the annual distribution of
1-hour daily maximum NO2 concentrations.
4. Level
    As discussed below and in more detail in the proposal (section
II.F.4), the Administrator has considered two different approaches to
setting the 1-hour NO2 primary NAAQS. In the proposal, each
of these approaches was linked with a different range of standard
levels. Specifically, the Administrator proposed to set a 1-hour
standard reflecting the maximum allowable NO2 concentration
anywhere in an area and to set the level of such a standard from 80 to
100 ppb. The Administrator also solicited comment on the alternative
approach of setting a standard that reflects the allowable area-wide
NO2 concentration and setting the standard level from 50 to
75 ppb. This section summarizes the rationale for the Administrator's
proposed approach and range of standard levels (II.F.3.a), describes
the alternative approach and range of standard levels (II.F.3.b),
discusses comments related to each approach and range of standard
levels (II.F.3.c), and presents the Administrator's final conclusions
regarding the approach and level (II.F.3.d).
a. Rationale For Proposed Decisions on Approach and Level
    In assessing the most appropriate approach to setting the 1-hour
standard and the most appropriate range of standard levels to propose,
the Administrator considered the broad body of scientific evidence
assessed in the ISA, including epidemiologic and controlled human
exposure studies, as well as the results of exposure/risk analyses
presented in the REA. In light of the body of available evidence and
analyses, as described above, the Administrator concluded in the
proposal that it is necessary to provide increased public health
protection for at-risk individuals against an array of adverse
respiratory health effects linked with short-term (i.e., 30 minutes to
24 hours) exposures to NO2. Such health effects have been
associated with exposure to the distribution of short-term ambient
NO2 concentrations across an area, including higher short-
term (i.e., peak) exposure concentrations, such as those that can occur
on or near major roadways and near other sources of NO2, as
well as the lower short-term exposure concentrations that can occur in
areas not near major roadways or other sources of NO2. The
Administrator's proposed decisions on approach and level, as discussed
in detail in the proposal (section II.F.4), are outlined below.
    In considering a standard-setting approach, the Administrator was
mindful in the proposal that the available evidence and analyses from
the ISA and REA support the public health importance of roadway-
associated NO2 exposures. The exposure assessment described
in the REA estimated that roadway-associated exposures account for the
majority of exposures to peak NO2 concentrations (REA,
Figures 8-17, 8-18). The ISA concluded (section 4.3.6) that
NO2 concentrations in heavy traffic or on freeways ``can be
twice the residential outdoor or residential/arterial road level.'' In
considering the potential variability in the NO2
concentration gradient, the proposal noted that available monitoring
studies suggest that NO2 concentrations could be 30 to 100%
higher than those in the same area but away from the road.\12\
---------------------------------------------------------------------------

    \12\ In addition, the air quality analyses presented in the REA
estimated that on-road NO2 concentrations are about 80%
higher on average than concentrations away from the road (REA,
section 7.3.2) and that NO2 monitors within 20 m of roads
measure NO2 concentrations that are, on average across
locations, 40% higher than concentrations measured by monitors at
least 100 m from the road (REA, compare Tables 7-11 and 7-13).

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

    The Administrator also considered that millions of people in the
United States live, work, and/or attend school near important sources
of NO2 such as major roadways (ISA, section 4.4), and that
ambient NO2 concentrations in these locations vary depending
on the distance from major roads (i.e., the closer to a major road, the
higher the NO2 concentration) (ISA, section 2.5.4).
Therefore, these populations, which likely include a disproportionate
number of individuals in groups with higher prevalence of asthma and
higher hospitalization rates for asthma (e.g. ethnic or racial
minorities and individuals of low socioeconomic status) (ISA, section
4.4), are likely exposed to NO2 concentrations that are
higher than those occurring away from major roadways.
    Given the above considerations, the Administrator proposed an
approach to setting the 1-hour NO2 primary NAAQS whereby the
standard would reflect the maximum allowable NO2
concentration anywhere in an area. In many locations, this
concentration is likely to occur on or near a major roadway. EPA
proposed to set the level of the standard such that, when available
information regarding the concentration gradient around roads is
considered, appropriate public health protection would be provided by
limiting the higher short-term peak exposure concentrations expected to
occur on and near major roadways, as well as the lower short-term
exposure concentrations expected to occur away from those roadways. The
Administrator concluded that this approach to setting the 1-hour
NO2 NAAQS would be expected to protect public health against
exposure to the distribution of short-term NO2
concentrations across an area and would provide a relatively high
degree of confidence regarding the protection provided against peak
exposures to higher NO2 concentrations, such as those that
can occur around major roadways. The remainder of this section
discusses the proposed range of standard levels.
    In considering the appropriate range of levels to propose for a
standard that reflects the maximum allowable NO2
concentration anywhere in an area, the Administrator considered the
broad body of scientific evidence and exposure/risk information as well
as available information on the relationship between NO2
concentrations near roads and those away from roads. Specifically, she
considered the extent to which a variety of levels would be expected to
protect at-risk individuals against increased airway responsiveness,
respiratory symptoms, and respiratory-related emergency department
visits and hospital admissions.
    After considering the scientific evidence and the exposure/risk
information (see sections II.B, II.C, and II.F.4.a.1 through II.F.4.a.3
in the proposal), as well as the available information on the
NO2 concentration gradient around roadways (section II.A.2
above and in the proposal), the Administrator concluded that the
strongest support is for a standard level at or somewhat below 100 ppb.
The Administrator's rationale in reaching this proposed conclusion is
provided below.
    The Administrator noted that a standard level at or somewhat below
100 ppb in conjunction with the proposed approach would be expected to
limit short-term NO2 exposures to concentrations that have
been reported to increase airway responsiveness in asthmatics (i.e., at
or above 100 ppb). While she acknowledged that exposure to
NO2 concentrations below 100 ppb could potentially increase
airway responsiveness in some asthmatics, the Administrator also noted
uncertainties regarding the magnitude and the clinical significance of
the NO2-induced increase in airway responsiveness, as
discussed in the policy assessment chapter of the REA (section
<IP_ADDRESS>, discussed in section II.F.4.e in the proposal). Given these
uncertainties, the Administrator concluded in the proposal that
controlled human exposure studies provide support for limiting
exposures at or somewhat below 100 ppb NO2.
    The Administrator also noted that a standard level at or somewhat
below 100 ppb in conjunction with the proposed approach would be
expected to maintain peak area-wide NO2 concentrations
considerably below those measured in locations where key U.S.
epidemiologic studies have reported associations with more serious
respiratory effects, as indicated by increased emergency department
visits and hospital admissions. Specifically, the Administrator noted
that 5 key U.S. studies provide evidence for such associations in
locations where the 99th percentile of the distribution of 1-hour daily
maximum NO2 concentrations measured at area-wide monitors
ranged from 93 to 112 ppb (Ito et al., 2007; Jaffe et al., 2003; Peel
et al., 2005; Tolbert et al., 2007; and a study by the New York State
Department of Health, 2006).\13\ The Administrator concluded that these
studies provide support for a 1-hour standard that limits the 99th
percentile of the distribution of 1-hour daily maximum area-wide
NO2 concentrations to below 90 ppb (corresponds to a 98th
percentile concentration of 85 ppb), and that limiting area-wide
concentrations to considerably below 90 ppb would be appropriate in
order to provide an adequate margin of safety. The Administrator noted
that, based on available information about the NO2
concentration gradient around roads, a standard level at or somewhat
below 100 ppb set in conjunction with the proposed approach would be
expected to accomplish this. Specifically, she noted that given
available information regarding NO2 concentration gradients
around roads (see section II.A.2), a standard level at or below 100 ppb
(with either a 99th or 98th percentile form) would be expected to limit
peak area-wide NO2 concentrations to approximately 75 ppb or
below.\14\ Therefore, the Administrator concluded that a standard level
at or somewhat below 100 ppb under the proposed approach would be
expected to maintain peak area-wide NO2 concentrations well
below 90 ppb across locations despite the expected variation in the
NO2 concentration gradient that can exist around roadways in
different locations and over time.
---------------------------------------------------------------------------

    \13\ The 98th percentile concentrations in these study locations
ranged from 85 to 94 ppb.
    \14\ For a standard of 100 ppb, area-wide concentrations would
be expected to range from approximately 50 ppb (assuming near-road
concentrations are 100% higher than area-wide concentrations) to 75
ppb (assuming near-road concentrations are 30% higher than area-wide
concentrations).
---------------------------------------------------------------------------

    The Administrator also noted that a study by Delfino provides mixed
evidence for effects in a location with area-wide 98th and 99th
percentile 1-hour daily maximum NO2 concentrations of 50 and
53 ppb, respectively. In that study, NO2 effect estimates
were positive, but some reported 95% confidence limits for the odds
ratio (OR) that included values less than 1.00. Given the mixed results
of the Delfino study, the Administrator concluded that it may not be
necessary to maintain area-wide NO2 concentrations at or
below 50 ppb to provide protection against the effects reported in
epidemiologic studies.
    In addition to these evidence-based considerations, the
Administrator noted that a standard level at or somewhat below 100 ppb
under the proposed approach would be consistent with the

results of the exposure and risk analyses presented in the REA. As
discussed in section II.C of the proposal, the results of these
analyses provide support for setting a standard that limits 1-hour
area-wide NO2 concentrations to between 50 and 100 ppb. As
described above, a standard level of 100 ppb that reflects the maximum
allowable NO2 concentration would be expected to maintain
area-wide NO2 concentrations at or below approximately 75
ppb. Given all of these considerations, the Administrator concluded in
the proposal that a standard level at or somewhat below 100 ppb (with a
99th percentile form), in conjunction with the proposed approach, would
be requisite to protect public health with an adequate margin of safety
against the array of NO2-associated health effects.
    In addition to the considerations discussed above, which support
setting a standard level at or somewhat below 100 ppb, the
Administrator also considered the extent to which available evidence
could support standard levels below 100 ppb. The Administrator
concluded that the evidence could support setting the standard level
below 100 ppb to the extent the following were emphasized:
     The possibility that an NO2-induced increase in
airway responsiveness could occur in asthmatics following exposures to
concentrations below 100 ppb and/or the possibility that such an
increase could be clinically significant.
     The mixed results reported in the study by Delfino et al.
(2002) of an association between respiratory symptoms and the
relatively low ambient NO2 concentrations measured in the
study area.
    Specifically, she noted that a standard level of 80 ppb (99th
percentile form), in conjunction with the proposed approach, could
limit area-wide NO2 concentrations to 50 ppb \15\ and would
be expected to limit exposure concentrations to below those that have
been reported to increase airway responsiveness in asthmatics. For the
reasons stated above, the Administrator proposed to set the level of a
new 1-hour standard between 80 ppb and 100 ppb.
---------------------------------------------------------------------------

    \15\ This conclusion assumes that near-road NO2
concentrations are 65% higher than area-wide concentrations,
reflecting the mid-point in the range of 30 to 100%. Based on
available information suggesting that near-road concentrations can
be 30 to 100% higher than area-wide concentrations, a standard level
of 80 ppb could limit area-wide concentrations to between 40 and 60
ppb.
---------------------------------------------------------------------------

b. Rationale for the Alternative Approach and Range of Levels
    As described above, the Administrator proposed to set a 1-hour
NO2 NAAQS reflecting the maximum allowable NO2
concentration anywhere in an area and to set the level of such a
standard from 80 to 100 ppb. However, prior to the proposal, the
approach of setting a 1-hour NO2 NAAQS that reflects the
maximum allowable NO2 concentration anywhere in an area had
not been discussed by EPA in the REA or considered by CASAC. Rather,
the potential alternative standards discussed in the REA, and reviewed
by CASAC, reflected allowable area-wide NO2 concentrations
(i.e., concentrations that occur broadly across communities).
    Given this, the Administrator noted in the proposal that comments
received on the approach to setting the 1-hour standard (i.e., from
CASAC and from members of the public) could provide important new
information for consideration. Therefore, the Administrator also
solicited comment on the alternative approach of setting a 1-hour
NO2 primary NAAQS that would reflect the allowable area-wide
NO2 concentration, analogous to the standards evaluated in
the REA, and with a level set within the range of 50 to 75 ppb. In
discussing this alternative approach with a standard level from 50 to
75 ppb, the Administrator noted the following in the proposal:
     Such a standard would be expected to maintain area-wide
NO2 concentrations below peak 1-hour area-wide
concentrations measured in locations where key U.S. epidemiologic
studies have reported associations with respiratory-related emergency
department visits and hospital admissions.
     Standard levels from the lower end of the range would be
expected to limit roadway-associated exposures to NO2
concentrations that have been reported in controlled human exposure
studies to increase airway responsiveness in asthmatics. Specifically,
a standard level of 50 ppb under this approach could limit near-road
concentrations to between approximately 65 and 100 ppb, depending on
the relationship between near-road NO2 concentrations and
area-wide concentrations.
     This alternative approach would provide relatively more
confidence regarding the degree to which a specific standard level
would limit area-wide NO2 concentrations and less confidence
regarding the degree to which a specific standard level would limit the
peak NO2 concentrations likely to occur near major roadways.
c. Comments on Approach and Level
    In the proposal, each approach to setting the 1-hour standard, and
each range of standard levels, was linked to different requirements for
the design of the NO2 monitoring network. Specifically, in
conjunction with the proposed approach (i.e., standard reflects the
maximum allowable NO2 concentration anywhere in an area and
the level is set within the range of 80 to 100 ppb), the Administrator
proposed to establish a 2-tiered monitoring network that would include
monitors sited to measure the maximum NO2 concentrations
anywhere in an area, including near major roadways, and monitors sited
to measure maximum area-wide NO2 concentrations. In
conjunction with the alternative approach (i.e., standard reflects the
allowable area-wide NO2 concentration and the level is set
within the range of 50 to 75 ppb), the Administrator solicited comment
on a monitoring network that would only include area-wide
NO2 monitors. Because of these linkages in the proposal,
most commenters combined their comments on the approach to setting a 1-
hour standard and on the standard level with their comments on the
monitoring requirements. In this section, we discuss comments from
CASAC and public commenters on the approach to setting a 1-hour
standard and on the standard level. Comments on the monitoring network
are also discussed in this section to the extent they indicate a
preference for either the proposed or alternative approach to setting
the 1-hour standard. More specific comments on monitor placement and
network design are discussed below in section III.B.2 and in the
Response to Comments document. EPA responses to technical comments on
the scientific evidence and the exposure/response information are
discussed above in section II.E.2 and in the Response to Comments
document. The Administrator's response to commenters' views on the
approach to setting the 1-hour standard and on the standard level is
embodied in the discussed in section II.F.4.d.
i. CASAC Comments on the Approach to Setting the Standard
    A majority of CASAC and CASAC Panel members \16\ favored the
proposed approach of setting a 1-hour standard that reflects the
maximum allowable

NO2 concentration anywhere in an area and linking such a
standard with a 2-tiered monitoring network that would include both
near-road and area-wide monitors, though CASAC did not reach consensus
on this approach. Specifically, in their letter to the Administrator
(Samet, 2009), CASAC stated the following:
---------------------------------------------------------------------------

    \16\ CASAC members were also part of the CASAC Panel for the
NO2 NAAQS review (i.e., the Oxides of Nitrogen Primary
National Ambient Air Quality Standards Panel). Therefore, references
to the CASAC Panel include both CASAC members and Panel members.

    There was a split view on the two approaches among both CASAC
and CASAC panel members with a majority of each favoring the
Agency's proposed two-tiered monitoring network because they thought
this approach would be more effective in limiting near-roadway
exposures that may reach levels in the range at which some
individuals with asthma may be adversely affected. Other members
acknowledged the need for research and development of near-road
monitoring data for criteria pollutants in general but favored
retention of EPA's current area-wide monitoring for NO2
regulatory purposes, due to the lack of epidemiological data based
on near-roadway exposure measurements and issues related to
---------------------------------------------------------------------------
implementing a near-road monitoring system for NO2.

Thus, the recommendation of the majority of CASAC Panel members was
based on their conclusion that the proposed approach would be more
effective than the alternative at limiting near-roadway exposures to
NO2 concentrations that could adversely affect asthmatics.
In addition, these CASAC Panel members noted important uncertainties
with the alternative approach. Specifically, they stated the following
(Samet, 2009):

    Panel members also supported the proposed two-tiered approach
because basing regulations on area-wide monitoring alone was
problematic. Such an approach would require EPA to embed
uncertainties and assumptions about the relationship between area-
wide and road-side monitoring into the area-wide standard.

    A minority of CASAC Panel members expressed support for the
alternative approach of setting a 1-hour standard that reflects the
allowable area-wide NO2 concentration. These CASAC Panel
members concluded that there would be important uncertainties
associated with the proposed approach. Specifically, they noted that
the key U.S. NO2 epidemiologic studies relied upon area-wide
NO2 concentrations. In their view, the use of area-wide
concentrations in these studies introduces uncertainty into the
selection of a standard level for a standard that reflects the maximum
allowable NO2 concentration anywhere in an area and that is
linked with a requirement to place monitors near major roads. As a
result of this uncertainty, CASAC Panel members who favored the
alternative approach noted that ``it would be better to set the
standard on the same area-wide monitoring basis as employed in the
epidemiologic studies upon which it [the standard] now relies'' (Samet,
2009). These CASAC Panel members also strongly supported obtaining
monitoring data near major roads, while recognizing uncertainties
associated with identifying appropriate monitoring sites near roads
(see section III.B.2 and the Response to Comments document for more
discussion of CASAC's monitoring comments).
ii. Public Comments on the Approach to Setting the Standard
    Consistent with the views expressed by the majority of CASAC
members, a number of commenters concluded that the most appropriate
approach would be to set a 1-hour standard that reflects the maximum
allowable NO2 concentration anywhere in an area and to
couple that standard with a requirement that monitors be placed in
locations where maximum concentrations are expected, including near
major roads. This view was expressed by some State and local agencies
(e.g., in CA, IA, NY, TX, WA, WI), by a number of environmental
organizations (e.g., CAC, EDF, EJ, GASP, NRDC), by the ALA, and
individual commenters. Several additional medical and public health
organizations (ACCP, AMA, ATS, NADRC, NACPR) did not explicitly express
a recommendation regarding the approach though these organizations did
recommend that, in setting a 1-hour standard, particular attention
should be paid to NOX concentrations around major roadways.
In support of their recommendation to adopt the proposed approach and
to focus monitoring around major roads, these commenters generally
concluded that a primary consideration should be the extent to which
the NO2 NAAQS protects at-risk populations that live and/or
attend school near important sources of NO2 such as major
roads. As such, these comments supported the rationale in the proposal
for setting a 1-hour standard that reflects the maximum allowable
NO2 concentration anywhere in an area.
    A number of State commenters expressed the view that area-wide
monitors should be used for attainment/non-attainment determinations
(e.g., NACAA, NESCAUM and agencies in IL, IN, MI, MS, NC, NM, SC). One
State commenter (NESCAUM) agreed with EPA concerns about near-road
exposures but concluded that it is premature to establish a large near-
road monitoring network at this time due to uncertainty regarding the
relationship between near-road and area-wide NO2
concentrations and the variability in that relationship. NESCAUM
recommended that EPA work with States to establish a targeted
monitoring program in select urban areas to gather data that would
inform future modifications to the monitoring network, but that ``[t]he
existing area-wide monitoring network should be used to identify
initial nonattainment areas.'' Other State commenters also concluded
that the most appropriate approach would be to base non-attainment
determinations only on area-wide monitors. Based on their monitoring
comments, many of these commenters appeared to support setting a 1-hour
standard that reflects the allowable area-wide NO2
concentration. State concerns with the proposed approach often included
uncertainties associated with identifying and accessing appropriate
monitor sites near major roads, as well as concerns related to
implementation and cost to States (as discussed further in the Response
to Comments document, the Administrator may not consider cost of
implementation in decisions on a NAAQS).
    One commenter (AAM) concluded that the focus of the proposed
approach on NO2 concentrations around major roadways is not
justified because the REA and the proposal overstate the extent to
which NO2 concentrations near roads are higher than
NO2 concentrations farther away from the road. This
conclusion is based on an analysis of 42 existing NO2
monitors in 6 locations. Comparing NO2 concentrations
measured by these monitors, some of which are closer to roads and
others of which are farther from roads, AAM concluded that ``roadside
monitors are not measuring high NO2 concentrations.''
    We agree that there is uncertainty associated with estimates of
roadway-associated NO2 concentrations (see REA, sections
7.4.6 and <IP_ADDRESS> for detailed discussion of these uncertainties) and
in identifying locations where maximum concentrations are expected to
occur. However, we note that the Administrator's conclusions regarding
the relationship between NO2 concentrations near roads and
those away from roads rely on multiple lines of scientific evidence and
information. Specifically, the Administrator relied in the proposal on
the following in drawing conclusions regarding the distribution of
NO2 concentrations across areas:
     Monitoring studies discussed in the ISA and REA that were
designed to characterize the NO2 concentration gradient
around roads, which indicated that NO2 concentrations near
roads can

be approximately 30 to 100% higher than concentrations away from the
road in the same area.
     Air quality and exposure analyses presented in the REA
which estimate that, on average across locations, NO2
concentrations on roads could be 80% higher than those away from roads
and that roadway-associated exposures account for the majority of
exposures to NO2 concentrations at or above 100 ppb.
    In contrast, the existing NO2 monitoring network, which
was the basis for the analysis submitted by AAM, was not designed to
characterize the spatial gradients in NO2 concentrations
surrounding roadways. Rather, concentrations of NO2 measured
by existing monitors are likely to reflect contributions from a
combination of mobile and stationary sources, with one or the other
dominating depending on the proximity of these sources to the monitors.
Therefore, we conclude that the analysis submitted by AAM, which does
not consider other relevant lines of evidence and information, does not
appropriately characterize the relationship between NO2
concentrations near roads and those away from roads. (See the Response
to Comments document for a more detailed discussion of AAM comments.)
    In addition, we note that, although the Administrator concluded in
the proposal that maximum NO2 concentrations in many areas
are likely to occur around major roads, she also recognized that
maximum concentrations can occur elsewhere in an area. For this reason,
she proposed to set a 1-hour NO2 standard that reflects the
maximum allowable NO2 concentration anywhere in an area,
regardless of where that maximum concentration occurs.\17\ Therefore,
the proposed approach to setting the standard would be expected to
limit the maximum NO2 concentrations anywhere in an area
even if in some areas, as is contended by AAM, those maximum
NO2 concentrations do not occur near roads.
---------------------------------------------------------------------------

    \17\ To measure maximum concentrations, the Administrator
proposed monitoring provisions that would require monitors within 50
meters of major roads and to allow the Regional Administrator to
require additional monitors in situations where maximum
concentrations would be expected to occur in locations other than
near major roads (e.g., due to the influence of multiple smaller
roads and/or stationary sources).
---------------------------------------------------------------------------

iii. CASAC Comments on Standard Level
    In commenting on the proposal, CASAC discussed both the proposed
range of standard levels (i.e., 80-100 ppb) and the alternative range
of standard levels (i.e., 50-75 ppb). CASAC did express the consensus
conclusion that if the Agency finalizes a 1-hour standard in accordance
with the proposed approach (i.e., standard level reflects the maximum
allowable NO2 concentration anywhere in an area), then it is
appropriate to consider the proposed range of standard levels from 80
to 100 ppb. Specifically, the CASAC letter to the Administrator on the
proposal (Samet, 2009) stated the following with regard to the proposed
approach:

    [T]he level of the one-hour NO2 standard should be
within the range of 80-100 ppb and not above 100 ppb. In its letter
of December 2, 2008, CASAC strongly voiced a consensus view that the
upper end of the range should not exceed 100 ppb, based on evidence
of risk at that concentration. The lower limit of 80 ppb was viewed
as reasonable by CASAC; selection of a value lower than 80 ppb would
represent a policy judgment based on uncertainty and the degree of
public health protection sought, given the limited health-based
evidence at concentrations below 100 ppb.

CASAC also recommended that this level be employed with a 98th
percentile form, in order to promote the stability of the standard (see
above for discussion of form).
iv. Public Comments on Standard Level
    A number of State and local agencies and organizations expressed
support for setting the level of the 1-hour NO2 standard
within the proposed range of 80 to 100 ppb. While some State and local
agencies (e.g., in CA, IA, MI, NY, TX) made this recommendation in
conjunction with a recommendation to focus monitoring near major roads
and other important sources of NO2, a number of State
commenters (e.g., NACAA, NESCAUM and agencies in IL, NC, NM, TX, VA)
recommended a standard level from 80 to 100 ppb in conjunction with a
recommendation that only area-wide monitors be deployed for purposes of
determining attainment with the standard. Based on these monitoring
comments, these State commenters appear to favor an approach where a
standard level from 80 to 100 ppb would reflect the allowable area-wide
NO2 concentration. As discussed above (and in more detail in
section III.B.2 and the Response to Comments document), State
commenters often based these recommendations on uncertainties
associated with designing an appropriate national near-road monitoring
network.
    A number of environmental organizations (e.g., CAC, EDF, EJ, GASP,
NRDC) and medical/public health organizations (e.g., ACCP, ALA, AMA,
ATS, NACPR, NAMDRC) supported setting a standard level below 80 ppb for
a standard that reflects the maximum allowable NO2
concentration anywhere in an area. Several of these groups recommended
a standard level of 50 ppb. This recommendation was typically based on
the commenters' interpretation of the epidemiologic and controlled
human exposure evidence, as described below.
    Some of these commenters noted that the 98th percentile area-wide
NO2 concentration was below 80 ppb in the location of a
single key U.S. epidemiologic study (i.e., 50 ppb in study by Delfino).
Given this, commenters concluded that the standard level should be set
at 50 ppb. Their comments on the monitoring network generally favored a
requirement to place monitors near major roads and, therefore, these
commenters appeared to favor a standard level as low as 50 ppb and to
recommend that such a standard level reflect the maximum allowable
NO2 concentration anywhere in an area. In their comments,
the ALA, EDF, EJ, and NRDC stated the following:

    Considering the Delfino study alone on EPA's terms, that is,
focusing on the 98th percentile of the 1-hour daily maximum
concentrations, EPA reports a concentration of 50 ppb where asthma
symptoms were observed. Based primarily on this study, EPA concluded
in the REA that it was appropriate to set the lower end of the range
at 50 ppb, which corresponded to the lowest-observed effects level
of airway hyperresponsiveness in asthmatics. To provide the
strongest public health protection, we therefore urge the level of
the standard be set at 50 ppb.

In some cases, the same commenters also appeared to recommend setting a
standard level below 50 ppb because mean area-wide NO2
concentrations reported in locations of key U.S. epidemiologic studies
are below this concentration. Specifically, with regard to the key U.S.
epidemiologic studies, these commenters (e.g., ALA, EDF, EJ, NRDC)
stated the following:

    These studies clearly identify adverse health effects such as
emergency room visits and hospital admissions for respiratory causes
at concentrations currently occurring in the United States. Mean
concentrations for all but two of these studies are about or below
50 ppb, suggesting that the standard must be set below this level to
allow for a margin of safety.

The Administrator's consideration of the Delfino study as it relates to
a decision on standard level is discussed below (section II.F.4.d).
Regarding the recommendation to set the level below 50 ppb based on
mean area-wide NO2 concentrations in epidemiologic study

locations, we note that the Administrator proposed to set a standard
that reflects the maximum allowable NO2 concentration
anywhere in an area and to set the form of that standard at the upper
end of the distribution of 1-hour daily maximum NO2
concentrations.\18\ As described in the proposal, such a standard, with
a level from the proposed range of 80 to 100 ppb, would be expected to
maintain peak area-wide NO2 concentrations below the peak
area-wide concentrations measured in locations where key U.S.
epidemiologic studies have reported associations with respiratory-
related emergency department visits and hospital admissions. Because
reducing NOX emissions to meet a 98th percentile
NO2 standard should lower the distribution of NO2
concentrations, including the mean, a standard that limits the 98th
percentile of the distribution of 1-hour daily maximum concentrations
would also be expected to limit mean concentrations. Therefore,
although we acknowledge that the relationship between peak and mean
NO2 concentrations will likely vary across locations and
over time, if peak area-wide NO2 concentrations are
maintained below those in key epidemiologic study locations, mean area-
wide NO2 concentrations would also be expected to be
maintained below the mean area-wide concentrations in those locations
(see ISA, figure 2.4-13 for information on the relationship between
peak and mean NO2 concentrations).
---------------------------------------------------------------------------

    \18\ As discussed above, the Administrator has selected the 98th
percentile as the form for the new 1-hour NO2 standard.
---------------------------------------------------------------------------

    As discussed above (section, II.E.2), a number of industry groups
did not support setting a new 1-hour NO2 standard. However,
several of these groups (e.g., AAM, Dow, NAM, NPRA) also concluded
that, if EPA does choose to set a new 1-hour standard, the level of
that standard should be above 100 ppb. As a basis for this
recommendation, these groups emphasized uncertainties in the scientific
evidence. Specifically, as discussed in more detail above (section
II.E.2), these commenters typically concluded that available
epidemiologic studies do not support the conclusion that NO2
causes reported health effects. This was based on their assertion that
the presence of co-pollutants in the ambient air precludes the
identification of a specific NO2 contribution to reported
effects. As a result, these commenters recommended that a 1-hour
standard should be based on the controlled human exposure evidence and
that, in considering that evidence, EPA should rely on the meta-
analysis of NO2 airway responsiveness studies conducted by
Goodman et al., (2009) rather than the meta-analysis included in the
final ISA. As described above, they concluded that in relying on the
ISA meta-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. EPA recognizes
the uncertainties in the scientific evidence that are discussed by
these industry commenters; however, we strongly disagree with their
conclusions regarding the implications of these uncertainties for
decisions on the NO2 NAAQS. These comments, and EPA's
responses, are discussed in detail above (section II.E.2) and in the
Response to Comments document and are summarized briefly below.
    As noted in section II.E.2, we agree that the presence of co-
pollutants in the ambient air complicates the interpretation of
epidemiologic studies; however, our conclusions regarding causality are
based on consideration of the broad body of epidemiologic studies
(including those employing multi-pollutant models) as well as animal
toxicological and controlled human exposure studies. The ISA concluded
that this body of evidence ``supports a direct effect of short-term
NO2 exposure on respiratory morbidity at ambient
concentrations below the current NAAQS level'' (ISA, p. 5-16). In
addition, the ISA (p. 5-15) concluded the following:

    [T]he strongest evidence for an association between
NO2 exposure and adverse human health effects comes from
epidemiologic studies of respiratory symptoms and ED visits and
hospital admissions. These new findings were based on numerous
studies, including panel and field studies, multipollutant studies
that control for the effects of other pollutants, and studies
conducted in areas where the whole distribution of ambient 24-h avg
NO2 concentrations was below the current NAAQS level of
0.053 ppm (53 ppb) (annual average).

Given that epidemiologic studies provide the strongest support for an
association between NO2 and respiratory morbidity, and that
a number of these studies controlled for the presence of other
pollutants with multi-pollutant models (in which NO2 effect
estimates remained robust), we disagree that NO2
epidemiologic studies should not be used to inform a decision on the
level of the 1-hour NO2 standard.
    In addition, we agree that uncertainty exists regarding the extent
to which the NO2-induced increase in airway responsiveness
is adverse (REA, section <IP_ADDRESS>); however, as discussed in detail
above (section II.E.2), we disagree with the conclusion by many
industry commenters that this effect is not adverse in asthmatics
following exposures from 100 to 600 ppb NO2. Specifically,
we do not agree that the approach taken in the study by Goodman et al.
(2009), which was used by many industry commenters to support their
conclusions, was appropriate. The authors of the Goodman study used
data from existing NO2 studies to characterize the dose-
response relationship of NO2 and airway responsiveness and
to calculate the magnitude of the NO2 effect. Given the
protocol differences in existing studies of NO2 and airway
responsiveness, we do not agree that it is appropriate to base such an
analysis on these studies.
    The Administrator's consideration of these uncertainties, within
the context of setting a standard level, is discussed in the next
section.
d. Conclusions on Approach and Standard Level
    Having carefully considered the public comments on the appropriate
approach and level for a 1-hour NO2 standard, as discussed
above, the Administrator believes the fundamental conclusions reached
in the ISA and REA remain valid. In considering the approach, the
Administrator continues to place primary emphasis on the conclusions of
the ISA and the analyses of the REA, both of which focus attention on
the importance of roadways in contributing to peak NO2
exposures, given that roadway-associated exposures can dominate
personal exposures to NO2. In considering the level at which
the 1-hour primary NO2 standard should be set, the
Administrator continues to place primary emphasis on the body of
scientific evidence assessed in the ISA, as summarized above in section
II.B, while viewing the results of exposure and risk analyses,
discussed above in section II.C, as providing information in support of
her decision.
    With regard to her decision on the approach to setting the 1-hour
standard, the Administrator continues to judge it appropriate to
provide increased public health protection for at-risk individuals
against an array of adverse respiratory health effects linked with
short-term exposures to NO2, where such health effects have
been associated with exposure to the distribution of short-term ambient
NO2 concentrations across

an area. In protecting public health against exposure to the
distribution of short-term NO2 concentrations across an
area, the Administrator is placing emphasis on providing a relatively
high degree of confidence regarding the protection provided against
exposures to peak concentrations of NO2, such as those that
can occur around major roadways. Available evidence and information
suggest that roadways account for the majority of exposures to peak
NO2 concentrations and, therefore, are important
contributors to NO2-associated public health risks. In
reaching this conclusion, the Administrator notes the following:
     Mobile sources account for the majority of NOX
emissions (ISA, Table 2.2-1).
     The ISA stated that NO2 concentrations in heavy
traffic or on freeways ``can be twice the residential outdoor or
residential/arterial road level,'' that ``exposure in traffic can
dominate personal exposure to NO2,'' and that
``NO2 levels are strongly associated with distance from
major roads (i.e., the closer to a major road, the higher the
NO2 concentration)'' (ISA, sections 2.5.4, 4.3.6).
     The exposure assessment presented in the REA estimated
that roadway-associated exposures account for the majority of exposures
to peak NO2 concentrations (REA, Figures 8-17, 8-18).
     Monitoring studies suggest that NO2
concentrations near roads can be considerably higher than those in the
same area but away from roads (e.g., by 30-100%, see section II.A.2).
     In their comments on the approach to setting the 1-hour
NO2 standard, the majority of CASAC Panel members emphasized
the importance of setting a standard that limits roadway-associated
exposures to NO2 concentrations that could adversely affect
asthmatics. These CASAC Panel members favored the proposed approach,
including its focus on roads.
    In addition, the Administrator notes that a considerable fraction
of the population resides, works, or attends school near major roadways
or other sources of NO2 and that these populations 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).\19\ 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 (ISA, section 4.4). Of this population, which likely
includes a disproportionate number of individuals in groups with a
higher prevalence of asthma and higher hospitalization rates for asthma
(e.g., ethnic or racial minorities and individuals of low socioeconomic
status) (ISA, section 4.4), asthmatics and members of other susceptible
groups (e.g., children, elderly) will have the greatest risks of
experiencing health effects related to NO2 exposure. In the
United States, approximately 10% of adults and 13% of children have
been diagnosed with asthma, and 6% of adults have been diagnosed with
COPD (ISA, section 4.4).
---------------------------------------------------------------------------

    \19\ The most current American Housing Survey (http://
www.census.gov/hhes/www/housing/ahs/ahs.html) is from 2007 and lists
a higher fraction of housing units within the 300 foot boundary.
According to Table 1A-6 from that report (http://www.census.gov/
hhes/www/housing/ahs/ahs07/tab1a-6.pdf), out of 128.2 million total
housing units in the United States, about 20 million were reported
by the surveyed occupant or landlord as being within 300 feet of a
4-or-more lane highway, railroad, or airport. That constitutes 15.6%
of the total housing units in the U.S. Assuming equal distributions,
with a current population of 306.3 million, that means that there
would be 47.8 million people meeting the 300 foot criteria.
---------------------------------------------------------------------------

    In considering the approach to setting the 1-hour standard, the
Administrator also notes that concerns with the proposed approach
expressed by the minority of CASAC Panel members included concern with
the uncertainty in the relationship between near-road and area-wide
NO2 concentrations, given that U.S. epidemiologic studies
have been based on concentrations measured at area-wide monitors.
However, as discussed by the majority of CASAC Panel members, a similar
uncertainty would be involved in setting a standard with the
alternative approach (Samet, 2009). The Administrator agrees with the
majority of CASAC Panel members and concludes that uncertainty in the
relationship between near-road and area-wide NO2
concentrations should be considered regardless of the approach selected
to set the standard. She recognizes that this uncertainty can and
should be taken into consideration when considering the level of the
standard.
    In drawing conclusions on the approach, the Administrator has
considered the extent to which each approach, in conjunction with the
ranges of standard levels discussed in the proposal, would be expected
to limit the distribution of NO2 concentrations across an
area and, therefore, would be expected to protect against risks
associated with NO2 exposures. Specifically, she has
considered the extent to which a standard set with each approach would
be expected to limit maximum NO2 concentrations and area-
wide NO2 concentrations.
    With regard to expected maximum concentrations, the Administrator
notes the following:
     A standard reflecting the maximum allowable NO2
concentration anywhere in an area would provide a relatively high
degree of confidence regarding the level of protection provided against
peak exposures, such as those that can occur on or near major roadways.
A standard level from anywhere within the proposed range (i.e., 80 to
100 ppb) would be expected to limit exposures to NO2
concentrations reported to increase airway responsiveness in
asthmatics.
     A standard reflecting the allowable area-wide
NO2 concentration would not provide a high degree of
confidence regarding the extent to which maximum NO2
concentrations would be limited. Maximum NO2 concentrations
would be expected to be controlled to varying degrees across locations
and over time depending on the NO2 concentration gradient
around roads. Given the expected variability in gradients across
locations and over time, most standard levels within the range
considered in the proposal with this option (i.e., 50 to 75 ppb) would
not be expected to consistently limit the occurrence of NO2
concentrations that have been reported to increase airway
responsiveness in asthmatics.
    With regard to expected area-wide concentrations, the Administrator
notes the following:
     The extent to which a standard reflecting the maximum
allowable NO2 concentration anywhere in an area would be
expected to limit area-wide NO2 concentrations would vary
across locations, e.g., depending on the NO2 concentration
gradient around roads. However, in conjunction with a standard level
from anywhere within the proposed range (i.e., 80-100 ppb), such an
approach would be expected to maintain area-wide NO2
concentrations below those measured in locations where key U.S.
epidemiologic studies have reported associations between ambient
NO2 and respiratory-related hospital admissions and
emergency department visits (based on available information regarding
the NO2 concentration gradient around roads as discussed
below).
     A standard reflecting the maximum allowable area-wide
NO2 concentration would provide a relatively high degree of
certainty regarding the extent to which area-wide NO2
concentrations are limited. In conjunction with a standard level from
anywhere within the range of

levels discussed in the proposal (i.e., 50-75 ppb) with this
alternative approach, such a standard would be expected to maintain
area-wide NO2 concentrations below those measured in
locations where key U.S. epidemiologic studies have reported
associations between ambient NO2 and respiratory-related
hospital admissions and emergency department visits.
    Given the above considerations, the Administrator concludes that
both approaches, in conjunction with appropriate standard levels, would
be expected to maintain area-wide NO2 concentrations below
those measured in locations where key U.S. epidemiologic studies have
reported associations between ambient NO2 and respiratory-
related hospital admissions and emergency department visits. In
contrast, the Administrator concludes that only a standard reflecting
the maximum allowable NO2 concentration anywhere in an area,
in conjunction with an appropriate standard level, would be expected to
consistently limit exposures, across locations and over time, to
NO2 concentrations reported to increase airway
responsiveness in asthmatics. After considering the evidence and
uncertainties, and the advice of the CASAC Panel, the Administrator
judges that the most appropriate approach to setting a 1-hour standard
to protect against the distribution of short-term NO2
concentrations across an area, including the higher concentrations that
can occur around roads and result in elevated exposure concentrations,
is to set a standard that reflects the maximum allowable NO2
concentration anywhere in an area.
    In considering the level of a 1-hour NO2 standard that
reflects the maximum allowable NO2 concentration anywhere in
an area, the Administrator notes that there is no bright line clearly
directing the choice of level. Rather, the choice of what is
appropriate is a public health policy judgment entrusted to the
Administrator. This judgment must include consideration of the
strengths and limitations of the evidence and the appropriate
inferences to be drawn from the evidence and the exposure and risk
assessments. Specifically, the Administrator notes the following:
     Controlled human exposure studies have reported that
various NO2 exposure concentrations increased airway
responsiveness in mostly mild asthmatics (section II above and II.B.1.d
in proposal). These studies can inform an evaluation of the risks
associated with exposure to specific NO2 concentrations,
regardless of where those exposures occur in an area. Because
concentrations evaluated in controlled human exposure studies are at
the high end of the distribution of ambient NO2
concentrations (ISA, section <IP_ADDRESS>), these studies most directly
inform consideration of the risks associated with exposure to peak
short-term NO2 concentrations.
     Epidemiologic studies (section II.B.1.a and b) conducted
in the United States have reported associations between ambient
NO2 concentrations measured at area-wide monitors in the
current network and increased respiratory symptoms, emergency
department visits, and hospital admissions. Area-wide monitors in the
urban areas in which these epidemiologic studies were conducted are not
sited in locations where localized peak concentrations are likely to
occur. Thus, they do not measure the full range of ambient
NO2 concentrations across the area. Rather, the area-wide
NO2 concentrations measured by these monitors are used as
surrogates for the distribution of ambient NO2
concentrations across the area, a distribution that includes
NO2 concentrations both higher than (e.g., around major
roadways) and lower than the area-wide concentrations measured in study
locations. Epidemiologic studies evaluate whether area-wide
NO2 concentrations are associated with the risk of
respiratory morbidity. Available information on NO2
concentration gradients around roadways can inform estimates of the
relationship between the area-wide NO2 concentrations
measured in epidemiologic study locations and the higher NO2
concentrations likely to have occurred around roads in those locations,
which can then inform the decision on the level of a standard
reflecting the maximum allowable NO2 concentration anywhere
in an area.
     The risk and exposure analyses presented in the REA
provide information on the potential public health implications of
setting standards that limit area-wide NO2 concentrations to
specific levels. While the Administrator acknowledges the uncertainties
associated with these analyses which, as discussed in the REA, could
result in either over- or underestimates of NO2-associated
health risks, she judges that these analyses are informative for
considering the relative levels of public health protection that could
be provided by different standards.
    The Administrator's consideration of the controlled human exposure
evidence, epidemiologic evidence, and exposure/risk information are
discussed below specifically with regard to a decision on the level of
a standard that reflects the maximum allowable NO2
concentration anywhere in an area.
    In considering the potential for controlled human exposure studies
of NO2 and airway responsiveness to inform a decision on
standard level, the Administrator notes the following:
     NO2-induced increases in airway responsiveness,
as reported in controlled human exposure studies, are logically linked
to the adverse respiratory effects that have been reported in
NO2 epidemiologic studies.
     The meta-analysis of controlled human exposure data in the
ISA reported increased airway responsiveness in a large percentage of
asthmatics at rest following exposures at and above 100 ppb
NO2, the lowest NO2 concentration for which
airway responsiveness data are available in humans.
     This meta-analysis does not provide any evidence of a
threshold below which effects do not occur. The studies included in the
meta-analysis evaluated primarily mild asthmatics while more severely
affected individuals could respond to lower concentrations. Therefore,
it is possible that exposure to NO2 concentrations below 100
ppb could increase airway responsiveness in some asthmatics.
    In considering the evidence, the Administrator recognizes that the
NO2-induced increases in airway responsiveness reported for
exposures to NO2 concentrations at or above 100 ppb could be
adverse for some asthmatics. However, she also notes that important
uncertainties exist with regard to the extent to which NO2-
induced increases in airway responsiveness are adverse. Specifically,
she notes the following with regard to these uncertainties:
     The magnitude of the NO2-induced increase in
airway responsiveness, and the extent to which it is adverse, cannot be
quantified from the ISA meta-analysis (REA, section <IP_ADDRESS>).
     The NO2-induced increase in airway
responsiveness in resting asthmatics was typically not accompanied by
increased respiratory symptoms, even following exposures to
NO2 concentrations well above 100 ppb (ISA, section
<IP_ADDRESS>).
     The increase in airway responsiveness that was reported
for resting asthmatics was not present in exercising asthmatics (ISA,
Table 3.1-3).
    Taking into consideration all of the above, the Administrator
concludes that existing evidence supports the conclusion that the
NO2-induced increase in airway responsiveness at or above
100 ppb presents a risk of adverse

effects for some asthmatics, especially those with more serious (i.e.,
more than mild) asthma. The Administrator notes that the risks
associated with increased airway responsiveness cannot be fully
characterized by these studies, and thus she is not able to determine
whether the increased airway responsiveness experienced by asthmatics
in these studies is an adverse health effect. However, based on these
studies the Administrator concludes that asthmatics, particularly those
suffering from more severe asthma, warrant protection from the risk of
adverse effects associated with the NO2-induced increase in
airway responsiveness. Therefore, the Administrator concludes that the
controlled human exposure evidence supports setting a standard level no
higher than 100 ppb to reflect a cautious approach to the uncertainty
regarding the adversity of the effect. However, those uncertainties
lead her to also conclude that this evidence does not support setting a
standard level lower than 100 ppb.
    In considering the more serious health effects reported in
NO2 epidemiologic studies, as they relate to the level of a
standard that reflects the maximum allowable NO2
concentration anywhere in an area, the Administrator notes the
following:
     A cluster of 5 key U.S. epidemiologic studies (Ito et al.,
2007; Jaffe et al., 2003; Peel et al., 2005; Tolbert et al., 2007; and
a study by the New York State Department of Health, 2006) provide
evidence for associations between NO2 and respiratory-
related emergency department visits and hospital admissions in
locations where 98th percentile 1-hour daily maximum NO2
concentrations measured at area-wide monitors ranged from 85 to 94 ppb.
The Administrator judges it appropriate to place substantial weight on
this cluster of key U.S. epidemiologic studies in selecting a standard
level, as they are a group of studies that reported positive, and often
statistically significant, associations between NO2 and
respiratory morbidity in multiple cities across the United States.\20\
---------------------------------------------------------------------------

    \20\ Some of these studies also included susceptible and
vulnerable populations (e.g., children in Peel et al. (2005); poor
and minority populations in Ito et al., 2007).
---------------------------------------------------------------------------

     A single study (Delfino et al., 2002) provides mixed
evidence for NO2 effects (i.e., respiratory symptoms) in a
location with a 98th percentile 1-hour daily maximum NO2
concentration, as measured by an area-wide monitor, of 50 ppb. In that
study, most of the reported NO2 effect estimates were
positive, but not statistically significant. Given the variability in
the NO2 effect estimates in this study, as well as the lack
of studies in other locations with similarly low NO2
concentrations, the Administrator judges it appropriate to place
limited weight on this study, compared to the cluster of 5 studies as
noted above.
    Given these considerations, the Administrator concludes that the
epidemiologic evidence provides strong support for setting a standard
that limits the 98th percentile of the distribution of 1-hour daily
maximum area-wide NO2 concentrations to below 85 ppb. This
judgment takes into account the determinations in the ISA, based on a
much broader body of evidence, that there is a likely causal
association between exposure to NO2 and the types of
respiratory morbidity effects reported in these studies. Given the
considerations discussed above, the Administrator judges that it is not
necessary, based on existing evidence, to set a standard that maintains
peak area-wide NO2 concentrations to below 50 ppb.
    In considering specific standard levels supported by the
epidemiologic evidence, the Administrator notes that a level of 100
ppb, for a standard reflecting the maximum allowable NO2
concentration anywhere in the area, would be expected to maintain area-
wide NO2 concentrations well below 85 ppb, which is the
lowest 98th percentile concentration in the cluster of 5 studies. With
regard to this, she specifically notes the following:
     If NO2 concentrations near roads are 100%
higher than concentrations away from roads, a standard level of 100 ppb
would limit area-wide concentrations to approximately 50 ppb.
     If NO2 concentrations near roads are 30% higher
than concentrations away from roads, a standard level of 100 ppb would
limit area-wide concentrations to approximately 75 ppb.
    The Administrator has also considered the NO2 exposure
and risk information within the context of the above conclusions on
standard level. Specifically, she notes that the results of exposure
and risk analyses were interpreted as providing support for limiting
area-wide NO2 concentrations to no higher than 100 ppb.
Specifically, these analyses estimated that a standard that limits
area-wide NO2 concentrations to approximately 100 ppb or
below would be expected to result in important reductions in
respiratory risks, relative to the level of risk permitted by the
current annual standard alone. As discussed above, a standard
reflecting the maximum allowable NO2 concentration with a
level of 100 ppb would be expected to maintain area-wide NO2
concentrations to within a range of approximately 50 to 75 ppb. Given
this, the Administrator concludes that a standard level of 100 ppb is
consistent with conclusions based on the NO2 exposure and
risk information.
    Finally, the Administrator notes that a standard level of 100 ppb
is consistent with the consensus recommendation of CASAC.
    Given the above considerations and the comments received on the
proposal, the Administrator determines that the appropriate judgment,
based on the entire body of evidence and information available in this
review, and the related uncertainties, is a standard level of 100 ppb
(for a standard that reflects the maximum allowable NO2
concentration anywhere in an area). She concludes that such a standard,
with the averaging time and form discussed above, will provide a
significant increase in public health protection compared to that
provided by the current annual standard alone and would be expected to
protect against the respiratory effects that have been linked with
NO2 exposures in both controlled human exposure and
epidemiologic studies. Specifically, she concludes that such a standard
will limit exposures at and above 100 ppb for the vast majority of
people, including those in at-risk groups, and will maintain maximum
area-wide NO2 concentrations well below those in locations
where key U.S. epidemiologic studies have reported that ambient
NO2 is associated with clearly adverse respiratory health
effects, as indicated by increased hospital admissions and emergency
department visits.
    In setting the standard level at 100 ppb rather than a lower level,
the Administrator notes that a 1-hour standard with a level lower than
100 ppb would only result in significant further public health
protection if, in fact, there is a continuum of serious, adverse health
risks caused by exposure to NO2 concentrations below 100 ppb
and/or associated with area-wide NO2 concentrations well-
below those in locations where key U.S. epidemiologic studies have
reported associations with respiratory-related emergency department
visits and hospital admissions. Based on the available evidence, the
Administrator does not believe that such assumptions are warranted.
Taking into account the uncertainties that remain in interpreting the
evidence from available controlled human exposure and epidemiologic
studies, the Administrator notes that the likelihood of obtaining
benefits to public health with a standard set below

100 ppb decreases, while the likelihood of requiring reductions in
ambient concentrations that go beyond those that are needed to protect
public health increases.
    Therefore, the Administrator judges that a standard reflecting the
maximum allowable NO2 concentration anywhere in an area set
at 100 ppb is sufficient to protect public health with an adequate
margin of safety, including the health of at-risk populations, from
adverse respiratory effects that have been linked to short-term
exposures to NO2 and for which the evidence supports a
likely causal relationship with NO2 exposures. The
Administrator does not believe that a lower standard level is needed to
provide this degree of protection. These conclusions by the
Administrator appropriately consider the requirement for a standard
that is neither more nor less stringent than necessary for this purpose
and recognizes that the CAA does not require that primary standards be
set at a zero-risk level or to protect the most sensitive individual,
but rather at a level that reduces risk sufficiently so as to protect
the public health with an adequate margin of safety.

G. Annual Standard

    In the proposal, the Administrator noted that some evidence
supports a link between long-term exposures to NO2 and
adverse respiratory effects and that CASAC recommended in their
comments prior to the proposal that, in addition to setting a new 1-
hour standard to increase public health protection, the current annual
standard be retained. CASAC's recommendation was based on the
scientific evidence and on their conclusion that a 1-hour standard
might not provide adequate protection against exposure to long-term
NO2 concentrations (Samet, 2008b).
    With regard to an annual standard, CASAC and a large number of
public commenters (e.g., NACAA, NESCAUM; agencies from States including
CA, IN, MO, NC, NY, SC, TX, VA; Tribal organizations including Fon du
Lac and the National Tribal Air Organization; environmental/medical/
public health groups including ACCP, ALA, AMA, ATS, CAC, EDF, EJ, GASP,
NACPR, NAMDRC, NRDC) agreed with the proposed decision to maintain an
annual standard, though their recommendations with regard to the level
of that annual standard differed (see below).
    As noted above, CASAC recommended ``retaining the current standard
based on the annual average'' based on the ``limited evidence related
to potential long-term effects of NO2 exposure and the lack
of strong evidence of no effect'' and that ``the findings of the REA do
not provide assurance that a short-term standard based on the one-hour
maximum will necessarily protect the population from long-term
exposures at levels potentially leading to adverse health effects''
(Samet, 2008b). A number of State agencies and organizations also
recommended maintaining the current level of the annual standard (i.e.,
53 ppb). This recommendation was based on the conclusion that, while
some evidence supports a link between long-term NO2
exposures and adverse respiratory effects, that evidence is not
sufficient to support a standard level either higher or lower than the
current level. In addition, a number of industry groups (e.g., AAM,
API, Dow, INGAA, UARG) recommended retaining the level of the current
annual standard but, as described above, did so within the context of a
recommendation that EPA should not set a new 1-hour standard.
    In contrast, some environmental organizations and medical/public
health organizations as well as a small number of States (e.g., ALA,
EDF, EJ, NRDC, and organizations in CA) recommended setting a lower
level for the annual standard. These commenters generally supported
their recommendation by pointing to the State of California's annual
standard of 30 ppb and to studies where long-term ambient
NO2 concentrations have been associated with adverse
respiratory effects such as impairments in lung function growth.
    As discussed above (II.B.3), the evidence relating long-term
NO2 exposures 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 the case of respiratory morbidity, the
ISA (section <IP_ADDRESS>) concluded that ``The high correlation among
traffic-related pollutants made it difficult to accurately estimate the
independent effects in these long-term exposure studies.'' Given these
uncertainties associated with the role of long-term NO2
exposures in causing the reported effects, the Administrator concluded
in the proposal that, consistent with the CASAC recommendation,
existing evidence is not sufficient to justify setting an annual
standard with either a higher or lower level than the current standard.
Commenters have not submitted any new analyses or information that
would change this conclusion. Therefore, the Administrator does not
agree with the commenters who recommended a lower level for the annual
standard.
    The Administrator judges that her conclusions in the proposal
regarding the annual standard remain appropriate. Specifically, she
continues to agree with the conclusion that, though some evidence does
support the need to limit long-term exposures to NO2, the
existing evidence for adverse health effects following long-term
NO2 exposures does not support either increasing or
decreasing the level of the annual standard. In light of this and
considering the recommendation from CASAC to retain the current level
of the annual standard, the Administrator judges it appropriate to
maintain the level of the annual standard at 53 ppb.

H. Summary of Final Decisions on the Primary NO2 Standard

    For the reasons discussed above, and taking into account
information and assessments presented in the ISA and REA, the advice
and recommendations of the CASAC, and public comments, the
Administrator has decided to revise the existing primary NO2
standard. Specifically, the Administrator has determined that the
current annual standard by itself is not requisite to protect public
health with an adequate margin of safety. In order to provide
protection for asthmatics and other at-risk populations against an
array of adverse respiratory health effects related to short-term
NO2 exposure, the Administrator is establishing a short-term
NO2 standard defined by the 3-year average of the 98th
percentile of the yearly distribution of 1-hour daily maximum
NO2 concentrations. She is setting the level of this
standard at 100 ppb, which is to reflect the maximum allowable
NO2 concentration anywhere in an area. In addition to
setting a new 1-hour standard, the Administrator retains the current
annual standard with a level of 53 ppb. The new 1-hour standard, in
combination with the annual standard, will provide protection for
susceptible groups against adverse respiratory health effects
associated with short-term exposures to NO2 and effects
potentially associated with long-term exposures to NO2.

III. Amendments to Ambient Monitoring and Reporting Requirements

    The EPA is finalizing several changes to the ambient air
monitoring, reporting, and network design requirements for the
NO2 NAAQS. This section discusses the changes we are
finalizing which are intended to support the proposed 1-

hour NAAQS and retention of the current annual NAAQS as discussed in
Section II. Ambient NO2 monitoring data are used to
determine whether an area is in violation of the NO2 NAAQS.
Ambient NO2 monitoring data are collected by State, local,
and Tribal monitoring agencies (``monitoring agencies'') in accordance
with the monitoring requirements contained in 40 CFR parts 50, 53, and
58.

A. Monitoring Methods

    We are finalizing the proposed changes regarding the NO2
Federal Reference Method (FRM) or Federal Equivalent Method (FEM)
analyzers. Specifically, we are continuing to use the NO2
chemiluminescence FRM and are finalizing the requirement that any
NO2 FRM or FEM used for making primary NAAQS decisions must
be capable of providing hourly averaged concentration data. The
following paragraphs provide background and rationale for the continued
use of the chemiluminescence FRM and the decision to finalize the
proposed changes.
1. Chemiluminescence FRM and Alternative Methods
    The current monitoring method in use by most State and local
monitoring agencies is the gas-phase chemiluminescence FRM (40 CFR Part
50, Appendix F), which was implemented into the NO2
monitoring network in the early 1980s. EPA did not propose to
discontinue using the chemiluminescence FRM, although we received some
comments from industry (Alliance of Automobile Manufacturers, Edison
Electric, and the National Petrochemical and Refiners Association)
raising concerns about using a method that is subject to known
interferences from certain species of oxides of nitrogen known as
NOZ. Important components of ambient NOZ include
nitrous acid (HNO2), nitric acid (HNO3), and the
peroxyacetyl nitrates (PANs).
    The issue of concern in public comments is that the reduction of
NO2 to NO on the MoOX converter substrate used in
chemiluminescence FRMs is not specific to NO2; hence,
chemiluminescence method analyzers are subject to varying interferences
produced by the presence in the air sample of the NOZ
species listed above and others occurring in trace amounts in ambient
air. This interference is often termed a ``positive artifact'' in the
reported NO2 concentration since the presence of
NOZ results in an over-estimate in the reported measurement
of the actual ambient NO2 concentration. This interference
by NOZ compounds has long been known and evaluated
(Fehsenfeld et al., 1987; Nunnermacker et al., 1998; Parrish and
Fehsenfeld, 2000; McClenny et al., 2002; U.S. Environmental Protection
Agency, 1993, 2006a). Further, as noted in the ISA (ISA Section 2.3),
it appears that interference by NOZ on chemiluminescence
FRMs is not more than 10 percent of the reported NO2
concentration during most or all of the day during winter (cold
temperatures), but larger interference ranging up to 70 percent can be
found during summer (warm temperatures) in the afternoon at sites away
and downwind from strong emission sources.
    The EPA acknowledges that the NOZ interference in the
reported NO2 concentrations collected well downwind of
NOX source areas and in relatively remote areas away from
concentrated point, area, or mobile sources is significantly larger
than the NOZ interference in NO2 measurements
taken in urban cores or other areas with fresh NOX
emissions. To meet the primary objective of monitoring maximum
NO2 concentrations in an area, the EPA is requiring
NO2 monitors to be placed in locations of the expected
highest concentrations, not in relatively remote areas away from
NOX sources. The required monitors resulting from the
network design discussed below in Section III.B will require monitors
to be placed near fresh NOX sources or in areas of dense
NOX emissions, where NO2 concentrations are
expected to be at a maximum, and interference from NOZ
species is at a minimum. Therefore, EPA believes that the positive
artifact issue, although present, is small, relative to the actual
NO2 being measured. As a result EPA believes the
chemiluminescence FRM is suitable for continued use in the ambient
NO2 monitoring network, as the potential positive bias from
NOZ species is not significant enough to discontinue using
the chemiluminescence FRM.
    EPA also received support from some industry groups (e.g. Savannah
River Nuclear Solutions, Teledyne API, and the Utility Air Regulatory
Groups) and States (e.g., MODEQ and NCDENR) to further the development
of alternative methods in determining NO2 concentrations.
Such alternative methods include the photolytic- chemiluminescence
method and cavity ring-down spectroscopy. As a result, EPA will
continue working with commercial and industrial vendors, to identify
and evaluate such new technologies. These efforts may include field
testing instruments and further characterizing methods in a laboratory
setting to assess their potential as future reference or equivalent
methods, and their role in more directly measuring NO2.
2. Allowable FRM and FEMs for Comparison to the NAAQS
    The current CFR language does not prohibit the use of any
particular NO2 FRM or FEM to be used in comparison to the
standard.\21\ There are designated wet chemical methods that are only
able to report ambient concentration values averaged across multiple
hours. With the establishment of a 1-hour NAAQS, any FRM or FEM which
is a wet chemical based method would not be appropriate for use in
determining compliance of the 1-hour NAAQS because they are unable to
report hourly data. EPA addressed this issue by proposing and
finalizing that only those methods capable of providing 1-hour
measurements will be comparable to the NAAQS.
---------------------------------------------------------------------------

    \21\ A list of approved FRM and FEMs is maintained by EPA's
Office of Research and Development, and can be found at: http://
www.epa.gov/ttn/amtic/files/ambient/criteria/reference-equivalent-
methods-list.pdf.
---------------------------------------------------------------------------

a. Proposed Changes to FRM and FEMs That May Be Compared to the NAAQS
    EPA proposed that only those FRMs or FEMs that are capable of
providing hourly averaged concentration data may be used for comparison
to the NAAQS.
b. Comments
    EPA received comments from some State and industry groups (e.g.
Missouri, North Carolina, and Air Quality Research and Logistics)
supporting the proposed approach to only allowing those FRMs or FEMs
that are capable of providing hourly averaged concentration data may be
used for comparison to both the annual and 1-hour NAAQS, and did not
receive any public comments that objected to the proposed approach.
c. Decisions on Allowable FRM and FEMs for Comparison to the NAAQS
    Accordingly, EPA is finalizing the proposed changes to 40 CFR Part
58 Appendix C to allow only data from FRM or FEMs that are capable of
providing hourly data to be used for comparison to both the annual and
1-hour NAAQS.

B. Network Design

    With the establishment of a 1-hour NO2 NAAQS intended to
limit exposure to maximum concentrations that may occur anywhere in an
area, EPA recognizes that the data from the current NO2
network is inadequate to fully assess compliance with the revised

NAAQS. As a result, EPA is promulgating new NO2 network
design requirements. The following sections provide background,
rationale, and details for the final changes to the NO2
network design requirements.
1. Two-Tiered Network Design
    A two-tiered monitoring network is appropriate for the
NO2 NAAQS because one tier (the near-road network) reflects
the much higher NO2 concentrations that occur near-road and
the second-tier (area-wide) characterizes the NO2
concentrations that occur in a larger area such as neighborhood or
urban areas. The ISA (Section 2.5.4 and 4.3.6) stated that
NO2 concentrations in heavy traffic or on freeways ``can be
twice the residential outdoor or residential/arterial road level,''
that ``exposure in traffic can dominate personal exposure to
NO2,'' and that ``NO2 levels are strongly
associated with distance from major roads (i.e., the closer to a major
road, the higher the NO2 concentration).'' The exposure
assessment presented in the REA estimated that roadway-associated
exposures account for the majority of exposures to peak NO2
concentrations (REA, Figures 8-17, 8-18). Monitoring studies suggest
that NO2 concentrations near roads can be considerably
higher than those in the same area but away from the road (e.g., by 30-
100%, see section II.A.2), where pollutants typically display peak
concentrations on or immediately adjacent to roads, producing a
gradient in pollutant concentrations where concentrations decrease with
increasing distance from roads. Since the intent of the revised NAAQS
is to limit exposure to peak NO2 concentrations that occur
anywhere in an area, monitors intended to measure the maximum allowable
NO2 concentration in an area should include measurements of
the peak concentrations that occur on and near roads due to on-road
mobile sources. The first tier of the network design, which focuses
monitoring near highly trafficked roads in urban areas where peak
NO2 concentrations are likely to occur, is intended to
measure maximum concentrations anywhere in an area, particularly those
due to on-road mobile sources since roadway-associated exposures
account for the majority of exposures to peak NO2
concentrations. The basis for the second tier of the network design is
to measure the highest area-wide concentrations to characterize the
wider area impact of a variety of NO2 sources on urban
populations. Area-wide monitoring of NO2 also serves to
maintain continuity in collecting data to inform long-term pollutant
concentration trends analysis and support ongoing health and scientific
research.
    This section discusses the two-tier network design approach
compared to the alternative network design which was also presented for
comment in conjunction with a solicitation for comment on an
alternative NAAQS. The alternative network design concept was based
entirely on requiring only monitors that would be considered area-wide,
while not requiring any near-road monitoring sites. The details of the
two-tier network design, including how many monitors are required,
where they are to be located, and the related siting criteria are
discussed in subsequent sections.
a. Proposed Two-Tier Network Design
    EPA proposed a two-tier network design composed of (1) near-road
monitors which would be placed in locations of expected maximum 1-hour
NO2 concentrations near heavily trafficked roads in urban
areas and (2) monitors located to characterize areas with the highest
expected NO2 concentrations at the neighborhood and larger
spatial scales (also referred to as ``area-wide'' monitors). As an
alternative, and in conjunction with a solicitation for comment on an
alternative NAAQS, EPA solicited comment on a network comprised of only
area-wide monitors.
b. Comments
    EPA received many comments on the overall two-tier network design,
with those who made statements with a relatively clear position on the
issue generally falling into four categories: (1) Those who support the
adoption of the proposed two-tier design approach, (2) those who
support the adoption of the two-tier concept, but with modifications,
(3) those who only supported the adoption of the alternative network
design, and (4) those who encourage EPA to commit to further research
of the near-road environment by monitoring near-roads, but not to use
near-road data for regulatory purposes, and therefore support the
alternative network design in which EPA solicited comment on a network
design composed only of area-wide monitors.
    Those commenters who generally supported the proposed two-tier
network, included CASAC (while there was not a consensus, a majority
were in support of the proposed network design), public health
organizations (e.g., AACPR, ACCP, AMA, ATA, and NAMDRC), several State
groups (e.g., the New York City Law Department and the Metropolitan
Washington Air Quality Committee), and some industry commenters (e.g.,
American Chemistry Council, The Clean Energy Group, and Dow Chemical).
    Those commenters who supported the adoption of the two-tier network
design concept, but suggested modifications to the actual design
included some health and environmental organizations (e.g., ALA, EDF,
EJ, and the NRDC), some States (e.g., California, the Central
Pennsylvania Clean Air Board, Harris County (Texas), Iowa, New York,
San Joaquin Air Pollution Control District, Spokane Regional Clean Air
Agency (SRCAA), the Texas Commission on Environmental Quality, and
Wisconsin), and some industry commenters, including the American
Petroleum Institute and the Utility Air Regulatory Group, who are cited
by other industry commenters. We believe that although these commenters
made suggestions to modify the proposed two-tier network design, they
are indicating that it is an acceptable approach. Their comments and
suggestions are discussed in greater detail in the following sections.
    Those commenters who only supported the adoption of the alternative
network design included State and industry groups (e.g., Indiana
Department of Environmental Management, the New York Department of
Transportation (NYSDOT), Alliance of Automobile Manufacturers, and the
Engine Manufacturers Association). These commenters typically made
comments on the two-tier network design, but did not do so in a way
that clearly supported near-road research.
    EPA received comments from some States or State organizations
(e.g., National Association of Clean Air Agencies (NACAA), the
Northeast States for Coordinated Air Use Management (NESCAUM), and 10
other individual States or State groups) and industry commenters (e.g.,
Consumers Energy, Edison Electric, and the National Association of
Manufacturers) that encouraged EPA to further research the near-road
environment, opposing use of near-road monitoring data for regulatory
purposes, and supported the adoption of the alternative network design
for regulatory purposes. For example, with regard to implementing the
two-tier network design that includes near-road regulatory monitoring,
NACAA stated that ``* * *a major new network--particularly one that is
inherently complicated and untried--should not be rolled out without
the benefit of an effective near-road monitoring research program that
can address many of the relevant data questions, and inform the
specific siting requirements of the rule.'' The NAM stated that
``conducting such

a near road [research] monitoring program would allow EPA to collect
necessary data that can be used to better understand the health impacts
associated with short term NO2 exposures.''
    The EPA notes that the existing scientific research referenced in
the proposal and throughout this final rule show that there are on- and
near-road peaks of NO2 concentrations, relative to upwind or
background levels, which exist due to on-road mobile source emissions.
This research, as a body of evidence, also identifies the multiple
local factors that affect how, where, and when peak NO2
concentrations occur on or near a particular road segment. These
factors include traffic volume, fleet mix, roadway design, congestion
patterns, terrain, and meteorology. The EPA and States have access to
such data typically through Federal, State, and/or local departments of
transportation or other government organizations, and, as a result, are
in a position to implement a near-roar monitoring network that is
intended to measure maximum expected NO2 concentrations
resulting from on-road mobile source emissions. Further, EPA notes that
near-road monitoring is not a new objective for the ambient air
monitoring community as near-road carbon monoxide monitoring has been a
part of ongoing, long-term, routine networks for nearly three decades.
As a result, there is experience within EPA (both OAR and ORD) and
State and local agencies on conducting ambient monitoring near-roads.
In addition, EPA intends to develop guidance with input from all
stakeholders to assist with implementation of the monitoring
requirements, which is discussed in section III.B.5. EPA believes that
the existing science and research provide a sufficient base of
information to require a near-road monitoring network and that the
collective experience that exists in the ambient monitoring community
will allow for successful implementation of that network. EPA also
believes that through adherence of requirements for near-road site
selection and siting criteria discussed in sections III.B.6 and
III.B.7, respectively, that the two-tier network design will provide a
network that has a reasonable degree of similarity across the country
where the required near-road monitors are targeting the maximum
NO2 concentrations in an area attributable to on-road mobile
sources.
    Some industry commenters (e.g., Engine Manufacturers Association,
the South Carolina Chamber of Commerce, and the South Carolina
Manufacturers Alliance) who supported the adoption of the alternative
network design suggested that monitoring in the near-road environment
would not be indicative of exposure for general populations, and that
EPA should not focus on the near-road environment when requiring
monitoring. For example, the South Carolina Chamber of Commerce and the
South Carolina Manufacturers Alliance both state that ``it appears the
proposed monitoring network will result in a collection of microscale
data, which is not at all representative of air quality relevant to
population exposure.''
    The EPA notes that the intent of a near-road monitoring is to
support the revised NAAQS by assessing peak NO2
concentrations that may occur anywhere in an area. EPA recognizes that
there is variability in the properties (such as traffic counts, fleet
mix, and localized features) among the road segments that may exist in
an area, but on the whole, roads are ubiquitous, particularly in urban
environments. Consequently, a substantial fraction of the population is
potentially exposed to relatively higher concentrations of
NO2 that can occur in the near-road environment. The 2007
American Housing Survey (http://www.census.gov/hhes/www/housing/ahs/
ahs07/ahs07.html) estimates that over 20 million housing units are
within 300 feet (91 meters) of a 4-lane highway, airport, or railroad.
Using the same survey, and considering that the average number of
residential occupants in a housing unit is approximately 2.25, it is
estimated that at least 45 million American citizens live near 4-lane
highways, airports, or railroads. Although that survey includes
airports and railroads, roads are the most pervasive of the three,
indicating that a significant amount of the general population live
near roads. Furthermore, the 2008 American Time Use Survey (http://
www.bls.gov/tus/) reported that the average U.S. civilian spent over 70
minutes traveling per day. Accordingly, EPA concludes that monitors
near major roads will address a component of exposure for a significant
portion of the general population that would otherwise not be
addressed.
    The majority of State commenters, regardless of their position on
the proposed network design, along with some industry commenters,
observed that there was a need for funding the monitoring network.
These comments urged EPA to provide the resources needed to implement
and operate the required monitoring network. EPA notes that it has
historically funded part of the cost of the installation and operation
of monitors used to satisfy Federal monitoring requirements. EPA
understands these concerns, although the CAA requirements from which
this final rule derives (CAA sections 110, 310(a) and 319) are not
contingent on EPA providing funding to States to assist in meeting
monitoring requirements. However, EPA intends to work with NACAA and
the State and local air agencies in identifying available State and
Tribal Air Grant (STAG) funds and consider the increased resource needs
that may be needed to plan, implement, and operate this revised set of
minimum requirements.
c. Conclusions Regarding the Two-Tier Network Design
    The EPA believes that requiring near-road monitors in urban areas
as part of the network design are necessary to protect against risks
associated with exposures to peak concentrations of NO2
anywhere in an area. The combination of increased mobile source
emissions and increased urban population densities can lead to
increased exposures and associated risks, therefore urban areas are the
appropriate areas to concentrate required near-road monitoring efforts.
The EPA also recognizes the need to have monitors in neighborhood and
larger spatial scale locations away from roads that represent area-wide
concentrations. These types of monitors serve multiple important
monitoring objectives including comparison to the NAAQS, photochemical
pollutant assessment, ozone forecasting, characterization of point and
area source impacts, and by providing historical trends data for
current and future epidemiological health research. In some situations,
when coupled with data from near-road monitors, area-wide monitors may
also assist in the determination of spatial variation of NO2
concentrations across a given area and provide insight to the gradients
that exist between near-road or stationary source oriented
concentrations and area-wide concentration levels.
    After considering the scientific data and the public comments
regarding the proposed network design, the Administrator concludes that
a two-tier network design composed of (1) near-road monitors which
would be placed in locations of expected maximum 1-hour NO2
concentrations near heavily trafficked roads in urban areas and (2)
monitors located to characterize areas with the maximum expected
NO2 concentrations at the neighborhood and larger spatial
scales (also referred to as ``area-wide'' monitors) are needed to
implement the 1-hour NO2 NAAQS and

support the annual NAAQS. The details of this two-tier network design
are discussed in the following eight sections.
2. First Tier (Near-Road Monitoring Component) of the NO2
Network Design
    This section provides background, rationale, and details for the
final changes to the first tier of the two-tier NO2 network
design. In particular, this section will focus on the thresholds that
trigger monitoring requirements. Near-road site selection and siting
criteria details will be discussed in subsequent sections.
a. Proposed First Tier (Near-Road Monitoring Component) of the Network
Design
    EPA proposed that the first tier of the two-tier NO2
monitoring network design focus monitors in locations of expected
maximum 1-hour concentrations near major roads in urban areas. As noted
in the previous section, the exposure assessment presented in the REA
estimated that roadway-associated exposures account for the majority of
exposures to peak NO2 concentrations (REA, Figures 8-17, 8-
18). Since the combination of increased mobile source emissions and
increased urban population densities leads to increased exposures and
associated risks, the Administrator judges that urban areas are the
appropriate areas in which to concentrate required near-road monitoring
efforts. Therefore, we proposed that a minimum of one near-road
NO2 monitor be required in Core Based Statistical Areas
(CBSAs) with a population greater than or equal to 350,000 persons.
Based on 2008 Census Bureau statistics, EPA estimated this would result
in approximately 143 monitoring sites in as many CBSAs.
    We also proposed that a second near-road monitor be required in
CBSAs with a population greater than or equal to 2,500,000 persons, or
in any CBSAs with one or more road segments with an Annual Average
Daily Traffic (AADT) count greater than or equal to 250,000. Based on
2008 Census Bureau statistics and data from the 2007 Highway
Performance Monitoring System (HPMS) maintained by the U.S. Department
of Transportation (DOT) Federal Highway Administration (FHWA), this
particular element of the minimum monitoring requirements would have
added approximately 24 \22\ sites to the approximate 143 near-road
sites in CBSAs that already would have had one near-road monitor
required due to the 350,000 population threshold. Overall, the first
tier of the proposed network design was estimated to require 167 near-
road sites in 143 CBSAs.
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    \22\ Of the 24 additional sites, 22 are estimated to be
triggered due to a population of 2,500,000 while 2 (Las Vegas, NV
and Sacramento, CA) are estimated to be triggered by the presence of
one or more road segments with 250,000 AADT since they do not have a
population of 2,500,000 people.
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b. Comments
    The EPA received comments from some industry and public health
organizations (e.g. Dow Chemical, ATS, and the AMA) supporting the
proposed approach to use population thresholds for triggering minimum
near-road monitoring requirements. For example, Dow Chemical Company
stated that ``Dow comments that the proposed population thresholds are
reasonable for implementation of the new network design and that we
don't see a need to establish a threshold lower than 350,000 people for
the lower bound.''
    The EPA received comments from some States and State groups
suggesting that a combination of population and AADT counts or just
AADT counts should be used to trigger minimum near-road monitoring
requirements. For example, the San Joaquin Air Pollution Control
District in California suggested that we modify minimum monitoring
requirements so that one near-road NO2 monitor is required
for any CBSA with a population of 350,000 people which also had one or
more road segments with AADT counts of 125,000 or more. In another
example, Harris County Public Health and Environmental Services
(HCPHES) suggested that ``* * * rather than specifying population
limits for the monitoring, HCPHES supports a metric like the Annual
Average Daily Traffic (AADT) as a threshold for requiring a near-road
monitor. An initial focus on an AADT in excess of 250,000 is acceptable
as a starting point but EPA should revisit that level and consider
lowering it to 100,000 in five years.'' AASHTO \23\ and NYDOT \23\
suggested that EPA could set a threshold at 140,000 AADT for requiring
near-road monitors rather than using population thresholds.
---------------------------------------------------------------------------

    \23\ AASHTO, NESCAUM, and NYDOT did not support the two-tier
network design; however they provided suggestions on how the network
design might be modified if the EPA were to finalize requirements
for near-road monitors. In the case of AASHTO and NYDOT, their
suggestions were made with the suggestion that EPA use a separate
rulemaking process to require monitors.
---------------------------------------------------------------------------

    EPA is finalizing the population-only threshold approach to trigger
near-road monitoring, as the first step in the process of establishing
the first-tier of near-road monitors, and for identifying the
appropriate number and locations for siting these monitors. EPA
believes that the uncertainty in defining specific national AADT counts
is too great to support use in this first step of the alternative
approaches suggested by the commenters. EPA notes that, in general,
roads with higher AADT counts have relatively higher amounts of mobile
source emissions, leading to an increased potential for relatively
higher on-road and roadside NO2 concentrations. This concept
is supported, for example, by Gilbert et al., 2007, who state that the
NO2 concentrations analyzed in their study are significantly
associated with traffic counts. In part, these suggestions by
commenters to include AADT counts as part of, or independently as, a
threshold for requiring monitors appears to be aimed at increasing the
focus of the near-road network to locations where NO2
concentrations are expected to be highest. However these suggestions
would also, in effect, reduce the size of the required network compared
to the network that EPA had proposed. The differences in fleet mix,
roadway design, congestion patterns, terrain, and local meteorology
amongst road segments that may have identical AADTs are quite variable
and affect the NO2 concentrations on and near those
segments. The available data and related technical and scientific
quantification of what particular AADT count might be expected to
contribute to some specific NO2 concentration is
insufficient to establish a specific, nationally applicable AADT count
threshold that could be used as part of a population-AADT combination,
or a distinct AADT count, to require all near-road monitors. Therefore,
EPA chose not to utilize a population-AADT or an AADT-only threshold to
trigger all minimally required near-road monitoring because of the lack
of a quantitative, nationally applicable relationship between a certain
AADT threshold and an expected NO2 concentration. Instead,
EPA is finalizing the proposed population-only threshold approach to
trigger a minimum of one monitor in a CBSA. In larger CBSAs, EPA does
require, at a minimum, a second monitor based on either an AADT count
of 250,000 or a population threshold of 2,500,00 or more persons in a
CBSA as described more fully below. EPA believes this approach for
siting near-road monitoring provides a greater degree of certainty in
covering a large segment of the total population (66%, which is
explained below) and will provide data on exposure from geographically
and spatially diverse areas where a larger number of people

are likely to be exposed to peak NO2 concentrations.
    Some commenters (e.g., AASHTO,\23\ NESCAUM,\23\ NYDEC, NYDOT \23\)
suggested focusing multiple near-road monitors only in relatively
larger CBSAs than those which were proposed. For example, NYDEC
suggested that EPA require, at minimum, two near-road monitors in any
CBSA of 2,500,000 people or more, but not in CBSAs below that
population threshold. In their comments, they point out the variety of
near-road environments that exist in the larger CBSAs such as New York
City.
    EPA notes that the larger CBSAs, such as those with a population of
2,500,000 or more persons, are more likely to have a greater number of
major roads across a potentially larger geographic area, and a
corresponding increase in potential for exposure in different settings
(evidenced in the U.S. Department of Transportation (U.S. DOT) Federal
Highway Administration (FHWA) ``Status of the Nation's Highways,
Bridges, and Transit: 2006 Conditions and Performance'' document which
is discussed below). This is the primary reasoning behind the
requirement for two monitors in CBSAs with more than 2,500,000 people.
EPA also believes that having multiple monitors in the largest CBSAs
will allow better understanding of the differences that may exist
between roads in the same CBSA due to fleet mix, congestion patterns,
terrain, or geographic locations. However, EPA believes that a network
with substantially fewer monitors in correspondingly fewer CBSAs, as
the commenters suggested, would lead to an insufficient monitoring
network lacking a balanced approach needed for a regulatory network
intended to support the revised NAAQS on a national basis.
    On a related note to those comments that suggested focusing more
near-road monitors only in the larger CBSAs, EPA proposed that any
CBSAs with one or more road segments with an Annual Average Daily
Traffic (AADT) count greater than or equal to 250,000 must have a
second monitor if they do not already have two near-road monitors
because of the population threshold. Such an AADT-triggered monitor
would account for situations where a relatively less populated area has
a very highly trafficked road. In this case, EPA notes that because
those road segments with 250,000 AADT have been identified by U.S. DOT
FHWA (http://www.fhwa.dot.gov/policyinformation/tables/02.cfm) as being
the top 0.03 percent of the most traveled public road segments, that