2005), Bronx and Manhattan (NYS DOH, 2006), and 39 of the cities
evaluated by Zanobetti and Schwartz (2009) (U.S. EPA, 2009a, Figure
6-29).
---------------------------------------------------------------------------

    In addition to the above analyses, the Policy Assessment also
considered locations where health studies reported positive
associations with PM10-2.5 and where ambient PM10
concentrations were likely to have exceeded those allowed under the
current PM10 standard during the study period. Nine
locations met these criteria.\118\ Of these locations, all would also
likely have exceeded a 98th percentile PM10 standard with a
level at or below 87 [mu]g/m\3\ (U.S. EPA, 2011a, section 3.3.4).
---------------------------------------------------------------------------

    \118\ Pittsburgh (Chock et al., 2000), Coachella Valley (CA)
(Ostro et al., 2003), Phoenix (Mar et al., 2003; Wilson et al.,
2007), and 6 of the cities evaluated by Zanobetti and Schwartz
(2009) (U.S. EPA, 2009a, Figure 6-29).
---------------------------------------------------------------------------

    Therefore, among U.S. study locations where PM10-2.5-
associated health effects have been reported, some areas met the
current standard but would likely have violated a 98th percentile
PM10 standard with a level at or below 87 [mu]g/m\3\. In
contrast, the locations that violated the current standard would also
likely have violated a 98th percentile PM10 standard with a
level at or below 87 [mu]g/m\3\. Given this, the Policy Assessment
concludes that, compared to the current PM10 standard, a 24-
hour PM10 standard with a 98th percentile form could
potentially better target public health protection to locations where
we have more confidence that ambient PM10-2.5 concentrations
are associated with mortality and/or morbidity (U.S. EPA, 2011a, pp. 3-
45 to 3-46).
iii. Integration of Evidence-Based and Air Quality-Based Considerations
in the Policy Assessment
    In considering the integration of the evidence and air quality
information within the context of identifying potential alternative
standard levels for consideration, the Policy Assessment first notes
the following:

    (1) Analyses of air quality correlations suggest that a 98th
percentile 24-hour PM10 concentration as high as 87
[mu]g/m\3\ could be considered generally equivalent to the current
PM10 standard, over time and across the country.
    (2) A 98th percentile 24-hour PM10 standard with a
level at or below 87 [mu]g/m\3\ would be expected to maintain
PM10 and PM10-2.5 concentrations below those
present in U.S. locations where single-city studies have reported
PM10-2.5 effect estimates that are positive and
statistically significant (lowest concentration in such a location
was 91 [mu]g/m\3\). Although some single-city studies have reported
positive PM10-2.5 effect estimates in locations with 98th
percentile PM10 concentrations below 87 [mu]g/m\3\, these
effect estimates were not statistically significant.
    (3) Multi-city average 98th percentile PM10
concentrations were below 87 [mu]g/m\3\ for recent U.S. multi-city
studies, which have reported positive and statistically significant
PM10-2.5 effect estimates. However, the extent to which
effects reported in multi-city studies are associated with the
short-term air quality in any particular location is highly
uncertain.
    (4) Epidemiological studies have reported positive, and in a few
instances statistically significant, associations with
PM10-2.5 in some locations likely to have met the current
PM10 standard but not a PM10 standard with a
98th percentile form and a level at or below 87 [mu]g/m.\3\
    To the extent the above considerations are emphasized, the Policy
Assessment notes that a standard level as high as about 85 [mu]g/m\3\,
for a 24-hour PM10 standard with a 98th percentile form,
could be supported. Such a standard level would be expected to maintain
PM10 and PM10-2.5 concentrations below those
present in U.S. locations of single-city studies where
PM10-2.5 effect estimates have been reported to be positive
and statistically significant and below those present in some locations
where single-city studies reported PM10-2.5 effect estimates
that were positive, but not statistically significant. These include
some locations likely to have met the current PM10 standard
during the study periods. Given this, when compared to the current
standard, a 24-hour PM10 standard with a 98th percentile
form and a level at or below 85 [mu]g/m\3\ could have the effect of
focusing public health protection on locations where there is more
confidence that PM10-2.5 is associated with mortality and/or
morbidity.
    Given the above, the Policy Assessment concludes that a 98th
percentile standard with a level as high as 85 [mu]g/m\3\ could be
considered to the extent that more weight is placed on the
appropriateness of focusing public health protection in areas where
positive and statistically significant associations with
PM10-2.5 have been reported, and to the extent less weight
is placed on PM10-2.5 effect estimates that are not
statistically significant and/or that reflect estimates across multiple
cities. The Policy Assessment notes that it could be judged appropriate
to place less weight on PM10-2.5 effect estimates that are
not statistically significant given the relatively large amount of
uncertainty that is associated with the broader body of
PM10-2.5 health evidence, including uncertainty in the
extent to which health effects evaluated in epidemiological studies
result from exposures to PM10-2.5 itself, rather than one or
more co-occurring pollutants. This uncertainty, as well as other
uncertainties discussed above, are reflected in the Integrated Science
Assessment conclusions that the evidence is ``suggestive'' of a causal
relationship (i.e., rather than ``causal'' or ``likely causal'')
between short-term PM10-2.5 and mortality, respiratory
effects, and cardiovascular effects. In addition, the Policy Assessment
concludes that it could be appropriate to place less weight on 98th
percentile PM10 concentrations averaged across multiple
cities, given the uncertainty in

linking multi-city effect estimates with the air quality in any
particular location.
    However, the Policy Assessment also notes that, overall across the
U.S., based on recent air quality information (i.e., 2007-2009), fewer
people live in counties with 98th percentile 24-hour PM10
concentrations above 85 [mu]g/m\3\ than in counties likely to exceed
the current PM10 standard (U.S. EPA, 2011a, Table 3-2 and p.
3-48). These results could be interpreted to suggest that a 98th
percentile standard with a level of 85 [mu]g/m\3\ would decrease
overall public health protection compared to the current standard.
Based on this analysis of the number of people living in counties that
could violate the current and potential alternative PM10
standards, a 24-hour PM10 standard with a 98th percentile
form and a level between 75 and 80 [mu]g/m\3\ would provide a level of
public health protection that is generally equivalent, across the U.S.,
to that provided by the current standard. To the extent these
population counts are emphasized in comparing the public health
protection provided by the current and potential alternative standards,
and to the extent it is judged appropriate to set a revised standard
that provides at least the level of public health protection that is
provided by the current standard based on such population counts, the
Policy Assessment concludes that it would be appropriate to consider
standard levels in the range of approximately 75 to 80 [mu]g/m\3\
(Id.).
    The Policy Assessment concludes that alternative approaches to
considering the evidence could also lead to consideration of standard
levels below 75 [mu]g/m\3\. For example, a number of single-city
epidemiological studies have reported positive, though not
statistically significant, PM10-2.5 effect estimates in
locations with 98th percentile PM10 concentrations below 75
[mu]g/m\3\. Given that exposure error is particularly important for
PM10-2.5 epidemiological studies and can bias the results of
these studies toward the null hypothesis (see section IV.B.5 above), it
could be judged appropriate to place more weight on positive
associations reported in these epidemiological studies, even when those
associations are not statistically significant. In addition, the multi-
city averages of 98th percentile PM10 concentrations in the
locations evaluated by Zanobetti and Schwartz (2009) and Peng et al.
(2008) were 77 and 68 [mu]g/m\3\, respectively. Both of these multi-
city studies reported positive and statistically significant
PM10-2.5 effect estimates that remained positive in co-
pollutant models that included PM2.5, though only Zanobetti
and Schwartz (2009) reported PM10-2.5 effect estimates that
remained statistically significant in such co-pollutant models. Despite
uncertainties in the extent to which effects reported in these multi-
city studies are associated with the short-term air quality in any
particular location, emphasis could be placed on these multi-city
associations. The Policy Assessment concludes that, to the extent more
weight is placed on single-city studies reporting positive, but not
statistically significant, PM10-2.5 effect estimates and on
multi-city studies, it could be appropriate to consider standard levels
as low as 65 [mu]g/m\3\ (U.S. EPA, 2011a, p. 3-48). A standard level of
65 [mu]g/m\3\ would be expected to provide a substantial margin of
safety against health effects that have been associated with
PM10-2.5 and, as discussed above, could better focus
(compared to the current standard) public health protection on areas
where health studies have reported associations with
PM10-2.5.
    In considering potential alternative standard levels below 65
[mu]g/m\3\, the Policy Assessment notes that, as discussed above, the
overall body of PM10-2.5 health evidence is relatively
uncertain, with somewhat stronger support in U.S. studies for
associations with PM10-2.5 in locations with 98th percentile
PM10 concentrations above 85 [mu]g/m\3\ than in locations
with 98th percentile PM10 concentrations below 65 [mu]g/
m\3\. Specifically, the Policy Assessment notes the following (Id., p.
3-49):

    (1) Epidemiological studies, either single-city or multi-city,
have not reported positive and statistically significant
PM10-2.5 effect estimates in locations with 98th
percentile PM10 concentrations (multi-city average 98th
percentile concentrations in the case of multi-city studies) at or
below 65 [mu]g/m\3\.
    (2) Although some single-city morbidity studies have reported
positive, but not statistically significant, associations with
PM10-2.5 in locations with 98th percentile
PM10 concentrations below 65 [mu]g/m\3\, the results of
U.S. morbidity studies were generally less consistent than those of
mortality studies, with some PM10-2.5 effect estimates
being positive while others were negative (i.e., negative effect
estimates were reported in several studies conducted in Atlanta,
where the 98th percentile PM10 concentrations ranged from
67 [mu]g/m\3\ to 71 [mu]g/m\3\).
    (3) Although Bayes-adjusted single-city PM10-2.5
effect estimates were positive, but not statistically significant,
in some locations with PM10 concentrations below 65
[mu]g/m\3\, these effect estimates were based on the difference
between community-wide PM10 and PM2.5
concentrations. As discussed above, it is not clear how these
estimates of PM10-2.5 concentrations compare to those
more typically used in other studies to calculate
PM10-2.5 effect estimates. At present, few corroborating
studies are available that use other approaches (i.e., co-located
monitors, dichotomous samplers) to estimate/measure
PM10-2.5 in locations with 98th percentile
PM10 concentrations below 65 [mu]g/m\3\.

    In light of these limitations in the evidence for a relationship
between PM10-2.5 and adverse health effects in locations
with relatively low PM10 concentrations, along with the
overall uncertainties in the body of PM10-2.5 health
evidence as described above and in the Integrated Science Assessment,
the Policy Assessment concludes that while it could be judged
appropriate to consider standard levels as low as 65 [mu]g/m\3\, it is
not appropriate, based on the currently available body of evidence, to
consider standard levels below 65 [mu]g/m\3\.

D. CASAC Advice

    Following their review of the first and second draft Policy
Assessments, CASAC provided advice and recommendations regarding the
current and potential alternative standards for thoracic coarse
particles (Samet, 2010c,d). With regard to the existing PM10
standard, CASAC concluded that ``the current data, while limited, is
sufficient to call into question the level of protection afforded the
American people by the current standard'' (Samet, 2010d, p. 7).\119\ In
drawing this conclusion, CASAC noted the positive associations in
multi-city and single-city studies, including in locations with
PM10 concentrations below those allowed by the current
standard. In addition, CASAC gave ``significant weight to studies that
have generally reported that PM10-2.5 effect estimates
remain positive when evaluated in co-pollutant models'' and concluded
that ``controlled human exposure PM10-2.5 studies showing
decreases in heart rate variability and increases in markers of
pulmonary inflammation are deemed adequate to support the plausibility
of the associations reported in epidemiologic studies'' (Samet, 2010d,
p. 7). Given all of the above conclusions CASAC recommended that ``the
primary standard for PM10 should be revised'' (Samet, 2010d,
p. ii and p. 7). In discussing potential revisions, while CASAC noted
that the scientific evidence supports adoption of a standard at least
as stringent as current

standard, they recommended revising the current standard in order to
increase public health protection. In considering potential alternative
standards, CASAC drew conclusions and made recommendations in terms of
the major elements of a standard: Indicator, averaging time, form, and
level.
---------------------------------------------------------------------------

    \119\ With regard to limitations and uncertainties in the
evidence, CASAC endorsed the ISA weight of evidence conclusions for
PM10-2.5 (i.e., that the evidence is only ``suggestive''
of a causal relationship between short-term exposures and mortality,
respiratory effects, and cardiovascular effects) (Samet, 2009e;
Samet, 2009f).
---------------------------------------------------------------------------

    The CASAC agreed with staff's conclusions that the available
evidence supports consideration in the current review of retaining the
current PM10 indicator and the current 24-hour averaging
time (Samet, 2010c, Samet, 2010d). Specifically, with regard to
indicator, CASAC concluded that ``[w]hile it would be preferable to use
an indicator that reflects the coarse PM directly linked to health
risks (PM10-2.5), CASAC recognizes that there is not yet
sufficient data to permit a change in the indicator from
PM10 to one that directly measures thoracic coarse
particles'' (Samet, 2010d, p. ii). In addition, CASAC ``vigorously
recommends the implementation of plans for the deployment of a network
of PM10-2.5 sampling systems so that future epidemiological
studies will be able to more thoroughly explore the use of
PM10-2.5 as a more appropriate indicator for thoracic coarse
particles'' (Samet, 2010d, p. 7).
    The CASAC also agreed that the evidence supports consideration of a
potential alternative form. Specifically, CASAC ``felt strongly that it
is appropriate to change the statistical form of the PM10
standard to a 98th percentile'' (Samet, 2010d, p. 7). In reaching this
conclusion, CASAC noted that ``[p]ublished work has shown that the
percentile form has greater power to identify non-attainment and a
smaller probability of misclassification relative to the expected
exceedance form of the standard'' (Samet, 2010d. p. 7).
    With regard to standard level, in conjunction with a 98th
percentile form, CASAC concluded that ``alternative standard levels of
85 and 65 [mu]g/m\3\ (based on consideration of 98th percentile
PM10 concentration) could be justified'' (Samet, 2010d, p.
8). However, in considering the evidence and uncertainties, CASAC
recommended a standard level from the lower part of the range discussed
in the Policy Assessment, recommending a level ``somewhere in the range
of 75 to 65 [mu]g/m\3\'' (Samet, 2010d, p. ii).
    In making this recommendation, CASAC noted that the number of
people living in counties with air quality not meeting the current
standard is approximately equal to the number living in counties that
would not meet a 98th percentile standard with a level between 75 and
80 [mu]g/m\3\. CASAC used this information as the basis for their
conclusion that a 98th percentile standard between 75 and 80 [mu]g/m\3\
would be ``comparable to the degree of protection afforded to the
current PM10 standard'' (Samet, 2010d, p. ii). Given this
conclusion regarding the comparability of the current and potential
alternative standards, as well as their conclusion on the public health
protection provided by the current standard (i.e., that available
evidence is sufficient to call it into question), CASAC recommended a
level within a range of 75 to 65 [mu]g/m\3\ in order to increase public
health protection, relative to that provided by the current standard
(Samet 2010d, p. ii).

E. Administrator's Proposed Conclusions Concerning the Adequacy of the
Current Primary PM10 Standard

    In considering the evidence and information as they relate to the
adequacy of the current 24-hour PM10 standard, the
Administrator first notes that this standard is meant to protect the
public health against effects associated with short-term exposures to
PM10-2.5. In the last review, it was judged appropriate to
maintain such a standard given the ``growing body of evidence
suggesting causal associations between short-term exposure to thoracic
coarse particles and morbidity effects, such as respiratory symptoms
and hospital admissions for respiratory diseases, and possibly
mortality'' (71 FR 61185, October 17, 2006). Given the continued
expansion in the body of scientific evidence linking short-term
PM10-2.5 to health outcomes such as premature death and
hospital visits, discussed in detail in the Integrated Science
Assessment (U.S. EPA, 2009a, Chapter 6) and summarized above, the
Administrator provisionally concludes that the available evidence
continues to support the appropriateness of maintaining a standard to
protect the public health against effects associated with short-term
(e.g., 24-hour) exposures to PM10-2.5. In drawing
conclusions as to whether the current PM10 standard is
requisite (i.e., neither more nor less stringent than necessary) to
protect public health with an adequate margin of safety against such
exposures, the Administrator has considered:

    (1) The extent to which it is appropriate to maintain a standard
that provides some measure of protection against all
PM10-2.5, regardless of composition or source of origin;
    (2) The extent to which it is appropriate to retain a
PM10 indicator for a standard meant to protect against
exposures to ambient PM10-2.5; and
    (3) The extent to which the current PM10 standard
provides an appropriate degree of public health protection.

    With regard to the first point, in the last review the EPA
concluded that dosimetric, toxicological, occupational, and
epidemiological evidence supported retention of a primary standard to
provide some measure of protection against short-term exposures to all
thoracic coarse particles, regardless of their source of origin or
location, consistent with the Act's requirement that primary NAAQS
provide an adequate margin of safety (71 FR 61197, October 17, 2006).
In that review, the EPA concluded that a number of source types,
including motor vehicle emissions, coal combustion, oil burning, and
vegetative burning, are associated with health effects (U.S. EPA,
2004). In litigation of the decisions from the last review, the D.C.
Circuit affirmed the conclusion that it was appropriate to provide
``some protection from exposure to thoracic coarse particles * * * in
all areas'' (American Farm Bureau Federation v. EPA, 559 F. 3d at 532-
33).
    In considering this issue in the current review, the Administrator
judges that the expanded body of scientific evidence provides even more
support for a standard that protects against exposures to all thoracic
coarse particles, regardless of their location or source of origin.
Specifically, the Administrator notes that epidemiological studies have
reported positive associations between PM10-2.5 and
mortality or morbidity in a large number of cities across North
America, Europe, and Asia, encompassing a variety of environments where
PM10-2.5 sources and composition are expected to vary
widely. In considering this evidence, the Integrated Science Assessment
concludes that ``many constituents of PM can be linked with differing
health effects'' (U.S. EPA, 2009a, p. 2-26). While PM10-2.5
in most of these study areas is of largely urban origin, the
Administrator notes that some recent studies have also linked mortality
and morbidity with relatively high ambient concentrations of particles
of non-urban crustal origin. In considering these studies, she notes
the Integrated Science Assessment's conclusion that ``PM (both
PM2.5 and PM10-2.5) from crustal, soil or road
dust sources or PM tracers linked to these sources are associated with
cardiovascular effects'' (U.S. EPA, 2009a, p. 2-26).
    In light of this body of available evidence reporting
PM10-2.5-associated health effects across different
locations with a variety of sources, as well as the

Integrated Science Assessment's conclusions regarding the links between
adverse health effects and PM sources and composition, the
Administrator provisionally concludes in the current review that it is
appropriate to maintain a standard that provides some measure of
protection against exposures to all thoracic coarse particles,
regardless of their location, source of origin, or composition.
    With regard to the second point, in considering the appropriateness
of a PM10 indicator for a standard meant to provide such
public health protection, the Administrator notes that the rationale
used in the last review to support the unqualified PM10
indicator (see above) remains relevant in the current review.
Specifically, as an initial consideration, she notes that
PM10 mass includes both coarse PM (PM10-2.5) and
fine PM (PM2.5). As a result, the concentration of
PM10-2.5 allowed by a PM10 standard set at a
single level declines as the concentration of PM2.5
increases. At the same time, the Administrator notes that
PM2.5 concentrations tend to be higher in urban areas than
rural areas (U.S. EPA, 2005, p. 2-54, and Figures 2-23 and 2- 24) and,
therefore, a PM10 standard will generally allow lower
PM10-2.5 concentrations in urban areas than in rural areas.
    In considering the appropriateness of this variation in allowable
PM10-2.5 concentrations, the Administrator considers the
relative strength of the evidence for health effects associated with
PM10-2.5 of urban origin versus non-urban origin. She
specifically notes that, as described above and similar to the
scientific evidence available in the last review, the large majority of
the available evidence for thoracic coarse particle health effects
comes from studies conducted in locations with sources more typical of
urban and industrial areas than rural areas. While associations with
adverse health effects have been reported in some study locations where
PM10-2.5 is largely non-urban in origin (i.e., in dust storm
studies), particle concentrations in these study areas are typically
much higher than reported in study locations where the PM is of urban
origin. Therefore, the Administrator notes that the strongest evidence
for a link between PM10-2.5 and adverse health impacts,
particularly for such a link at relatively low particle concentrations,
comes from studies of urban or industrial PM10-2.5.
    The Administrator also notes that chemical constituents present at
higher levels in urban or industrial areas, including byproducts of
incomplete combustion (e.g. polycyclic aromatic hydrocarbons) emitted
as PM2.5 from motor vehicles as well as metals and other
contaminants emitted from anthropogenic sources, can contaminate
PM10-2.5 (U.S. EPA, 2004, p. 8-344; 71 FR 2665, January 17,
2006). While the Administrator acknowledges the uncertainty expressed
in the Integrated Science Assessment regarding the extent to which
particle composition can be linked to health outcomes based on
available evidence, she also considers the possibility that
PM10-2.5 contaminants typical of urban or industrial areas
could increase the toxicity of thoracic coarse particles in urban
locations.
    Given that the large majority of the evidence for
PM10-2.5 toxicity, particularly at relatively low particle
concentrations, comes from study locations where thoracic coarse
particles are of urban origin, and given the possibility that
PM10-2.5 contaminants in urban areas could increase particle
toxicity, the Administrator provisionally concludes that it remains
appropriate to maintain a standard that targets public health
protection to urban locations. Specifically, she concludes that it is
appropriate to maintain a standard that allows lower ambient
concentrations of PM10-2.5 in urban areas, where the
evidence is strongest that thoracic coarse particles are linked to
mortality and morbidity, and higher concentrations in non-urban areas,
where the public health concerns are less certain.
    Given all of the above considerations and conclusions, the
Administrator judges that the available evidence supports retaining a
PM10 indicator for a standard that is meant to protect
against exposures to thoracic coarse particles. In reaching this
judgment, she notes that, to the extent a PM10 indicator
results in lower allowable concentrations of thoracic coarse particles
in some areas compared to others, lower concentrations will be allowed
in those locations (i.e., urban or industrial areas) where the science
has shown the strongest evidence of adverse health effects associated
with exposure to thoracic coarse particles and where we have the most
concern regarding PM10-2.5 toxicity. Therefore, the
Administrator provisionally concludes that the varying amounts of
coarse particles that are allowed in urban vs. non-urban areas under
the 24-hour PM10 standard, based on the varying levels of
PM2.5 present, appropriately reflect the differences in the
strength of evidence regarding coarse particle effects in urban and
non-urban areas.120 121
---------------------------------------------------------------------------

    \120\ The Administrator recognizes that this relationship is
qualitative. That is, the varying coarse particle concentrations
allowed under the PM10 standard do not precisely
correspond to the variable toxicity of thoracic coarse particles in
different areas (insofar as that variability is understood).
Although currently available information does not allow any more
precise adjustment for relative toxicity, the Administrator believes
the standard will generally ensure that the coarse particle levels
allowed will be lower in urban areas and higher in non-urban areas.
Addressing this qualitative relationship, the D.C. Circuit held that
``[i]t is true that the EPA relies on a qualitative analysis to
describe the protection the coarse PM NAAQS will provide. But the
fact that the EPA's analysis is qualitative rather than quantitative
does not undermine its validity as an acceptable rationale for the
EPA's decision.'' 559 F. 3d at 535.
    \121\ The D.C. Circuit agreed with similar conclusions in the
last review and held that this rationale reasonably supported use of
an unqualified PM10 indicator for thoracic coarse
particles. American Farm Bureau Federation v. EPA, 559 F. 3d at 535-
36.
---------------------------------------------------------------------------

    In reaching this initial conclusion, the Administrator also notes
that, in their review of the second draft Policy Assessment, CASAC
concluded that ``[w]hile it would be preferable to use an indicator
that reflects the coarse PM directly linked to health risks
(PM10-2.5), CASAC recognizes that there is not yet
sufficient data to permit a change in the indicator from
PM10 to one that directly measures thoracic coarse
particles'' (Samet, 2010d, p. ii). In addition, CASAC ``vigorously
recommends the implementation of plans for the deployment of a network
of PM10-2.5 sampling systems so that future epidemiological
studies will be able to more thoroughly explore the use of
PM10-2.5 as a more appropriate indicator for thoracic coarse
particles'' (Samet, 2010d, p. 7). Given this recommendation, the
Administrator further judges that, although current evidence is not
sufficient to identify a standard based on an alternative indicator
that would be requisite to protect public health with an adequate
margin of safety across the United States, consideration of alternative
indicators (e.g., PM10-2.5) in future reviews is desirable
and could be informed by additional research, as described in the
Policy Assessment (U.S. EPA, 2011a, section 3.5).
    With regard to the third point, in evaluating the degree of public
health protection provided by the current PM10 standard, the
Administrator notes that the Policy Assessment discusses two different
approaches to considering the scientific evidence and air quality
information (U.S. EPA, 2011a, section 3.2.3). These different
approaches, which are described above in detail (section IV.C.1), lead
to different

conclusions regarding the appropriateness of the degree of public
health protection provided by the current PM10 standard. The
Administrator further notes that the primary difference between the two
approaches lies in the extent to which weight is placed on the
following (U.S. EPA, 2011a, section 3.2.3):

    (1) The PM10-2.5 weight-of-evidence classifications
presented in the Integrated Science Assessment concluding that the
existing evidence is suggestive of a causal relationship between
short-term PM10-2.5 exposures and mortality,
cardiovascular effects, and respiratory effects;
    (2) Individual PM10-2.5 epidemiological studies
reporting associations in locations that meet the current
PM10 standard, including associations that are not
statistically significant;
    (3) The limited number of PM10-2.5 epidemiological
studies that have evaluated co-pollutant models;
    (4) The limited number of PM10-2.5 controlled human
exposure studies;
    (5) Uncertainties in the PM10-2.5 air quality
concentrations used in epidemiological studies, given limitations in
PM10-2.5 monitoring data and the different approaches
used across studies to estimate ambient PM10-2.5
concentrations; and
    (6) Uncertainties and limitations in the evidence that tend to
call into question the presence of a causal relationship between
PM10-2.5 exposures and mortality/morbidity.

    In evaluating the different possible approaches to considering the
public health protection provided by the current PM10
standard, the Administrator first notes that when the available
PM10-2.5 scientific evidence and its associated
uncertainties are considered, the Integrated Science Assessment
concludes that the evidence is suggestive of a causal relationship
between short-term PM10-2.5 exposures and mortality,
cardiovascular effects, and respiratory effects. As discussed in
section IV.B.1 above and in more detail in the Integrated Science
Assessment (U.S. EPA, 2009a, section 1.5), a suggestive determination
is made when the ``[e]vidence is suggestive of a causal relationship
with relevant pollutant exposures, but is limited because chance, bias
and confounding cannot be ruled out.'' In contrast, the Administrator
notes that she is proposing to strengthen the annual fine particle
standard based on a body of scientific evidence judged sufficient to
conclude that a causal relationship exists (i.e., mortality,
cardiovascular effects) or is likely to exist (i.e., respiratory
effects) (section III.B). The suggestive judgment for
PM10-2.5 reflects the greater degree of uncertainty
associated with this body of evidence, as discussed above in detail
(sections IV.B.5 and IV.C.1) and as summarized below.
    The Administrator notes that the important uncertainties and
limitations associated with the scientific evidence and air quality
information raise questions as to whether public health benefits would
be achieved by revising the existing PM10 standard. Such
uncertainties and limitations include the following:

    (1) While PM10-2.5 effect estimates reported for
mortality and morbidity were generally positive, most were not
statistically significant, even in single-pollutant models. This
includes effect estimates reported in some study locations with
PM10 concentrations above those allowed by the current
24-hour PM10 standard.
    (2) The number of epidemiological studies that have employed co-
pollutant models to address the potential for confounding,
particularly by PM2.5, remains limited. Therefore, the
extent to which PM10-2.5 itself, rather than one or more
co-pollutants, contributes to reported health effects remains
uncertain.
    (3) Only a limited number of experimental studies provide
support for the associations reported in epidemiological studies,
resulting in further uncertainty regarding the plausibility of the
associations between PM10-2.5 and mortality and morbidity
reported in epidemiological studies.
    (4) Limitations in PM10-2.5 monitoring data and the
different approaches used to estimate PM10-2.5
concentrations across epidemiological studies result in uncertainty
in the ambient PM10-2.5 concentrations at which the
reported effects occur, increasing uncertainty in estimates of the
extent to which changes in ambient PM10-2.5
concentrations would likely impact public health.
    (5) The lack of a quantitative PM10-2.5 risk
assessment further contributes to uncertainty regarding the extent
to which any revisions to the current PM10 standard would
be expected to improve the protection of public health, beyond the
protection provided by the current standard (see section III.B.5
above).
    (6) The chemical and biological composition of
PM10-2.5, and the effects associated with the various
components, remains uncertain. Without more information on the
chemical speciation of PM10-2.5, the apparent variability
in associations across locations is difficult to characterize.

    In considering these uncertainties and limitations, the
Administrator notes in particular the considerable degree of
uncertainty in the extent to which health effects reported in
epidemiological studies are due to PM10-2.5 itself, as
opposed to one or more co-occurring pollutants. As discussed above,
this uncertainty reflects the fact that there are a relatively small
number of PM10-2.5 studies that have evaluated co-pollutant
models, particularly co-pollutant models that have included
PM2.5, and a very limited body of controlled human exposure
evidence supporting the plausibility of a causal relationship between
PM10-2.5 and mortality and morbidity at ambient
concentrations. The Administrator notes that these important
limitations in the overall body of health evidence introduce
uncertainty into the interpretation of individual epidemiological
studies, particularly those studies reporting associations with
PM10-2.5 that are not statistically significant. Given this,
the Administrator reaches the provisional conclusion that it is
appropriate to place relatively little weight on epidemiological
studies reporting associations with PM10-2.5 that are not
statistically significant in single-pollutant and/or co-pollutant
models.
    With regard to this provisional conclusion, the Administrator notes
that, for single-city mortality studies conducted in the United States
where ambient PM10 concentration data were available for
comparison to the current standard, positive and statistically
significant PM10-2.5 effect estimates were only reported in
study locations that would likely have violated the current
PM10 standard during the study period (U.S. EPA, 2011a,
Figure 3-2). In U.S. study locations that would likely have met the
current standard, PM10-2.5 effect estimates for mortality
were positive, but not statistically significant (U.S. EPA, 2011a,
Figure 3-2). In considering U.S. study locations where single-city
morbidity studies were conducted, and which would likely have met the
current PM10 standard during the study period, the
Administrator notes that PM10-2.5 effect estimates were both
positive and negative, with most not statistically significant (U.S.
EPA, 2011a, Figure 3-3).
    In addition, in considering the single-city analyses for the
locations evaluated in the multi-city study by Zanobetti and Schwartz
(2009), the Administrator notes that associations in most of these
locations were not statistically significant and that this was the only
study to estimate ambient PM10-2.5 concentrations as the
difference between county-wide PM10 and PM2.5
mass. As discussed above, it is not clear how computed
PM10-2.5 measurements, such as those used by Zanobetti and
Schwartz (2009), compare with the PM10-2.5 concentrations
obtained in other studies either by direct measurement with a
dichotomous sampler or by calculating the difference using co-located
samplers (U.S. EPA,

2009a, section <IP_ADDRESS>).\122\ For these reasons, the Administrator
notes that there is considerable uncertainty in interpreting the
associations in these single-city analyses.
---------------------------------------------------------------------------

    \122\ As noted in section IV.B.5 above and in the Policy
Assessment (U.S. EPA, 2011a, p. 3-16), there are also important
uncertainties in estimates of ambient PM10-2.5
concentrations based on the difference between PM10 mass
and PM2.5 mass, as measured at co-located monitors.
---------------------------------------------------------------------------

    The Administrator acknowledges that an approach to considering the
available scientific evidence and air quality information that
emphasizes the above considerations differs from the approach taken by
CASAC. Specifically, CASAC placed a substantial amount of weight on
individual studies, particularly those reporting positive health
effects associations in locations that met the current PM10
standard during the study period. In emphasizing these studies, as well
as the limited number of supporting studies that have evaluated co-
pollutant models and the small number of supporting experimental
studies, CASAC concluded that ``the current data, while limited, is
sufficient to call into question the level of protection afforded the
American people by the current standard'' (Samet, 2010d, p. 7) and
recommended revising the current PM10 standard (Samet,
2010d).
    The Administrator has carefully considered CASAC's advice and
recommendations. She notes that in making its recommendation on the
current PM10 standard, CASAC did not discuss its approach to
considering the important uncertainties and limitations in the health
evidence, and did not discuss how these uncertainties and limitations
are reflected in its recommendation. As discussed above, such
uncertainties and limitations contributed to the conclusions in the
Integrated Science Assessment that the PM10-2.5 evidence is
only suggestive of a causal relationship, a conclusion that CASAC
endorsed (Samet, 2009e,f). Given the importance of these uncertainties
and limitations to the interpretation of the evidence, as reflected in
the weight of evidence conclusions in the Integrated Science Assessment
and as discussed above, the Administrator judges that it is appropriate
to consider and account for them when drawing conclusions about the
potential implications of individual PM10-2.5 health studies
for the current standard.
    In light of the above approach to considering the scientific
evidence, air quality information, and associated uncertainties, the
Administrator reaches the following provisional conclusions:

    (1) Given the important uncertainties and limitations associated
with the overall body of health evidence and air quality information
for PM10-2.5, as discussed above and as reflected in the
Integrated Science Assessment weight-of-evidence conclusions; given
that PM10-2.5 effect estimates for the most serious
health effect, mortality, were not statistically significant in U.S.
locations that met the current PM10 standard and where
coarse particle concentrations were either directly measured or
estimated based on co-located samplers; and given that
PM10-2.5 effect estimates for morbidity endpoints were
both positive and negative in locations that met the current
standard, with most not statistically significant; when viewed as a
whole the available evidence and information suggests that the
degree of public health protection provided against short-term
exposures to PM10-2.5 does not need to be increased
beyond that provided by the current PM10 standard.\123\
---------------------------------------------------------------------------

    \123\ This is not to say that the EPA could not adopt or revise
a standard for a pollutant for which the evidence is suggestive of a
causal relationship. Indeed, with respect to thoracic coarse
particles itself, the D.C. Circuit noted that ``[a]lthough the
evidence of danger from coarse PM is, as the EPA recognizes,
`inconclusive', the agency need not wait for conclusive findings
before regulating a pollutant it reasonably believes may pose a
significant risk to public health.'' American Farm Bureau Federation
v. EPA 559 F. 3d at 533. As explained in the text above, it is the
Administrator's provisional judgment that significant uncertainties
presented by the evidence and information before her in this review,
both as to causality and as to concentrations at which effects may
be occurring, best support a decision to retain rather than revise
the current primary 24-hour PM10 standard.
---------------------------------------------------------------------------

    (2) Given that positive and statistically significant
associations with mortality were reported in single-city U.S. study
locations likely to have violated the current PM10
standard, the degree of public health protection provided by the
current standard is not greater than warranted.\124\
---------------------------------------------------------------------------

    \124\ There are similarities with the conclusions drawn by the
Administrator in the last review. There, the Administrator concluded
that there was no basis for concluding that the degree of protection
afforded by the current PM10 standards in urban areas is
greater than warranted, since potential mortality effects have been
associated with air quality levels not allowed by the current 24-
hour standard, but have not been associated with air quality levels
that would generally meet that standard, and morbidity effects have
been associated with air quality levels that exceeded the current
24-hour standard only a few times. 71 FR at 61202. In addition, the
Administrator concluded that there was a high degree of uncertainty
in the relevant population exposures implied by the morbidity
studies suggesting that there is little basis for concluding that a
greater degree of protection is warranted. Id. The D.C. Circuit in
American Farm Bureau Federation v. EPA explicitly endorsed this
reasoning. 559 F. 3d at 534.

    In reaching these provisional conclusions, the Administrator notes
that the Policy Assessment also discusses the potential for a revised
PM10 standard (i.e., with a revised form and level) to be
``generally equivalent'' to the current standard, but to better target
public health protection to locations where there is greater concern
regarding PM10-2.5-associated health effects (U.S. EPA,
2011a, sections 3.3.3 and 3.3.4).\125\ In considering such a potential
revised standard, the Policy Assessment discusses the large amount of
variability in PM10 air quality correlations across
monitoring locations and over time (U.S. EPA, 2011a, Figure 3-7) and
the regional variability in the relative degree of public health
protection that could be provided by the current and potential
alternative standards (U.S. EPA, 2011a, Table 3-2). In light of this
variability, the Administrator notes the Policy Assessment conclusion
that no single revised PM10 standard (i.e., with a revised
form and level) would provide public health protection equivalent to
that provided by the current standard, consistently over time and
across locations (U.S. EPA, 2011a, section 3.3.4). That is, a revised
standard, even one that is meant to be ``generally equivalent'' to the
current PM10 standard, could increase protection in some
locations while decreasing protection in other locations.
---------------------------------------------------------------------------

    \125\ As discussed in detail above (section IV.C.2.d) and in the
Policy Assessment (U.S. EPA, 2011a, sections 3.3.3 and 3.3.4), a
revised standard that is generally equivalent to the current
PM10 standard could provide a degree of public health
protection that is similar to the degree of protection provided by
the current standard, across the United States as a whole. However,
compared to the current PM10 standard, such a generally
equivalent standard would change the degree of public health
protection provided in some specific areas, providing increased
protection in some locations and decreased protection in other
locations.
---------------------------------------------------------------------------

    In considering the appropriateness of revising the current
PM10 standard in this way, the Administrator notes the
following:

    (1) As discussed above, positive PM10-2.5 effect
estimates for mortality were not statistically significant in U.S.
locations that met the current PM10 standard and where
coarse particle concentrations were either directly measured or
estimated based on co-located samplers, while positive and
statistically significant associations with mortality were reported
in locations likely to have violated the current PM10
standard.
    (2) Also as discussed above, effect estimates for morbidity
endpoints in locations that met the current standard were both
positive and negative, with most not statistically significant.
    (3) Important uncertainties and limitations associated with the
overall body of health evidence and air quality information for
PM10-2.5, as discussed above and as reflected in the
Integrated Science Assessment weight-of-evidence conclusions, call
into question the extent to which the type of quantified and refined
targeting of public health protection envisioned under a revised
standard could be reliably accomplished.

    Given all of the above considerations, the Administrator notes that
there is a

large amount of uncertainty in the extent to which public health would
be improved by changing the locations to which the PM10
standard targets protection. Therefore, she reaches the provisional
conclusion that the current PM10 standard should not be
revised in order to change that targeting of protection.
    In considering all of the above, including the scientific evidence,
the air quality information, the associated uncertainties, and CASAC's
advice, the Administrator reaches the provisional conclusion that the
current 24-hour PM10 standard is requisite (i.e., neither
more protective nor less protective than necessary) to protect public
health with an adequate margin of safety against effects that have been
associated with PM10-2.5. In light of this provisional
conclusion, the Administrator proposes to retain the current
PM10 standard in order to protect against health effects
associated with short-term exposures to PM10-2.5.
    The Administrator recognizes that her proposed conclusions and
decision to retain the current PM10 standard differ from
CASAC's recommendations, stemming from the differences in how the
Administrator and CASAC considered and accounted for the evidence and
its limitations and uncertainties. In light of CASAC's views and
recommendation to revise the current PM10 standard, the
Administrator welcomes the public's views on these different approaches
to considering and accounting for the evidence and its limitations and
uncertainties, as well as on the appropriateness of revising the
primary PM10 standard, including revising the form and level
of the standard.

F. Administrator's Proposed Decision on the Primary PM10 Standard

    For the reasons discussed above, and taking into account the
information and assessments presented in the Integrated Science
Assessment and the Policy Assessment and the advice and recommendations
of CASAC, the Administrator proposes to retain the current primary
PM10 standard. The Administrator solicits comment on all
aspects of this proposed decision, including her rationale for reaching
the provisional conclusion that the current PM10 standard is
requisite to protect public health with an adequate margin of safety
and the provisional conclusion that it is not appropriate to revise the
current PM10 standard by setting a ``generally equivalent''
standard with the goal of better targeting public health protection.

V. Communication of Public Health Information

    Sections 319(a)(1) and (3) of the CAA require the EPA to establish
a uniform air quality index for reporting of air quality. These
sections specifically direct the Administrator to ``promulgate
regulations establishing an air quality monitoring system throughout
the United States which utilizes uniform air quality monitoring
criteria and methodology and measures such air quality according to a
uniform air quality index'' and ``provides for daily analysis and
reporting of air quality based upon such uniform air quality index * *
*'' In 1979, the EPA established requirements for index reporting (44
FR 27598, May 10, 1979). The requirement for State and local agencies
to report the AQI appears in 40 CFR 58.50 and the specific requirements
(e.g., what to report, how to report, reporting frequency,
calculations) are in appendix G to 40 CFR part 58.
    Information on the public health implications of ambient
concentrations of criteria pollutants is currently made available
primarily by AQI reporting through EPA's AIRNow Web site.\126\ The
current AQI has been in use since its inception in 1999.\127\ It
provides accurate, timely, and easily understandable information about
daily levels of pollution (40 CFR 58.50). The AQI establishes a
nationally uniform system of indexing pollution levels for ozone,
carbon monoxide, nitrogen dioxide, PM and sulfur dioxide. The AQI is
also recognized internationally as a proven tool to effectively
communicate air quality information to the public. In fact, many
countries have created similar indices based on the AQI.
---------------------------------------------------------------------------

    \126\ See http://www.airnow.gov/.
    \127\ In 1976, the EPA established a nationally uniform air
quality index, then called the Pollutant Standard Index (PSI), for
use by State and local agencies on a voluntary basis (41 FR 37660,
September 7, 1976). In August 1999, the EPA adopted revisions to
this air quality index (64 FR 42530, August 4, 1999) and renamed the
index the AQI.
---------------------------------------------------------------------------

    The AQI converts pollutant concentrations in a community's air to a
number on a scale from 0 to 500. Reported AQI values enable the public
to know whether air pollution levels in a particular location are
characterized as good (0-50), moderate (51-100), unhealthy for
sensitive groups (101- 150), unhealthy (151-200), very unhealthy (201-
300), or hazardous (301-500). The AQI index value of 100 typically
corresponds to the level of the short-term (e.g., daily or hourly
standard) NAAQS for each pollutant. Below an index value of 100, an
intermediate value of 50 was defined either as the level of the annual
standard if an annual standard has been established (e.g.,
PM2.5, nitrogen dioxide), or as a concentration equal to
one-half the value of the short-term standard used to define an index
value of 100 (e.g., carbon monoxide). An AQI value greater than 100
means that a pollutant is in one of the unhealthy categories (i.e.,
unhealthy for sensitive groups, unhealthy, very unhealthy, or
hazardous) on a given day. An AQI value at or below 100 means that a
pollutant concentration is in one of the satisfactory categories (i.e.,
moderate or good). Decisions about the pollutant concentrations at
which to set the various AQI breakpoints that delineate the various AQI
categories for each pollutant specific sub-index within the AQI draw
directly from the underlying health information that supports the NAAQS
review.
    Historically, state and local agencies have primarily used the AQI
to provide general information to the public about air quality and its
relationship to public health. For more than a decade, many states and
local agencies, as well as the EPA and other Federal agencies, have
been developing new and innovative programs and initiatives to provide
more information to the public, in a more timely way. These
initiatives, including air quality forecasting, real-time data
reporting through the AIRNow Web site, and air quality action day
programs, can serve to provide useful, up-to-date, and timely
information to the public about air pollution and its effects. Such
information will help individuals take actions to avoid or to reduce
exposures to ambient pollution at levels of concern to them and can
encourage the public to take actions that will reduce air pollution on
days when levels are projected to be at levels of concern to local
communities. Thus, these programs have significantly broadened the ways
in which state and local agencies can meet the nationally uniform AQI
reporting requirements, and are contributing to state and local efforts
to provide community health protection and to attain or maintain
compliance with the NAAQS. The EPA and state and local agencies
recognize that these programs are interrelated with AQI reporting and
with the information on the effects of air pollution on public health
that is generated through the periodic review, and revision when
appropriate, of the NAAQS.
    In recognition of the proposed change to the primary annual
PM2.5 standard summarized in section III.F above, the EPA
proposes a conforming change to the PM2.5 sub-index of the
AQI to be

consistent with the proposed change to the annual standard. The health
effects information that supports the proposed decisions on the
PM2.5 standards, as discussed in section III.B above, is
also the basis for the proposed decisions on the AQI discussed below in
this section. The EPA intends to finalize conforming changes to the AQI
in conjunction with the Agency's final decisions on the primary annual
and 24-hour PM2.5 standards, if revisions to such standards
are promulgated.
    With respect to an AQI value of 50, as discussed above, the
historical approach is to set it at the same level of the annual
standard, if there is one. This is consistent with the current AQI sub-
index for PM2.5, in which the current AQI value of 50 is set
at 15 [mu]g/m\3\, consistent with the level of the current primary
annual PM2.5 standard. The EPA sees no basis for deviating
from this approach in this review. Thus, the EPA proposes to set an AQI
value of 50 within a range of 12 to 13 [mu]g/m\3\, 24-hour average,
consistent with the proposed annual PM2.5 standard level
(section III.F). The final AQI value of 50 will be set at the level of
the annual PM2.5 standard that is promulgated.
    With respect to an AQI value of 100, which is the basis for
advisories to individuals in sensitive groups, there are two general
approaches that could be used to select the associated PM2.5
level. By far the most common approach, which has been used with the
other sub-indices as noted above, is to set an AQI value of 100 at the
same level as the short-term standard. The EPA recognizes that some
state and local air quality agencies have expressed a strong preference
that the Agency set an AQI value of 100 equal to any short-term
standard. These agencies typically express the view that this linkage
is useful for the purpose of communicating with the public about the
standard, as well as providing consistent messages about the health
impacts associated with daily air quality. The EPA proposes to use this
approach to set the AQI value of 100 at 35 [mu]g/m\3\, 24-hour average,
consistent with the proposal to retain the current 24-hour
PM2.5 standard (section III.F). If the 24-hour standard is
set at a different level, the EPA proposes to set an AQI value of 100
at the level of the 24-hour PM2.5 standard that is
promulgated.
    An alternative approach is to directly evaluate the health effects
evidence to select the level for an AQI value of 100. This was the
approach used in the 1999 rulemaking to set the AQI value of 100 at a
level of 40 [mu]g/m\3\, 24-hour average,\128\ when the 24-hour standard
level was 65 [mu]g/m\3\. This alternative approach was used in the case
of the PM2.5 sub-index because the annual and 24-hour
PM2.5 standards set in 1997 were designed to work together,
and the intended degree of health protection against short-term risks
was not defined by the 24-hour standard alone, but by the combination
of the two standards working in concert. Indeed, at that time, the 24-
hour standard was set to provide supplemental protection relative to
the principal protection provided by the annual standard. The EPA is
soliciting comment on this alternative approach in recognition that, as
proposed, the 24-hour PM2.5 standard is intended to continue
to provide supplemental protection against effects associated with
short-term exposures of PM2.5 by working in conjunction with
the annual standard to reduce 24-hour exposures to PM2.5.
The EPA recognizes that some state and local air quality agencies have
expressed support for this alternative approach. Using this alternative
approach could result in consideration of a lower level for an AQI
value of 100, based on the discussion of the health information
pertaining to the level of the 24-hour standard in section III.E.4
above. The EPA encourages state and local air quality agencies that use
the AQI to comment on both the approach and the level at which to set
an AQI value of 100 together with any supporting rationale.
---------------------------------------------------------------------------

    \128\ Currently, we are cautioning members of sensitive groups
at the AQI value of 100 at 35 [mu]g/m\3\, 24-hour average,
consistent with more recent guidance from EPA with regard to the
development of State emergency episode contingency plans (Harnett,
2009, Attachment B).
---------------------------------------------------------------------------

    With respect to an AQI value of 150, this level is based upon the
same health effects information that informs the selection of the level
of the 24-hour standard and the AQI value of 100. The AQI value of 150
was set in the 1999 rulemaking at a level of 65 [mu]g/m\3\, 24-hour
average. In considering what level to propose for an AQI value of 150,
we believe that the health effects evidence indicates that the level of
55 [mu]g/m\3\, 24-hour average, is appropriate to use \129\ in
conjunction with an AQI value of 100 set at the proposed level of 35
[mu]g/m\3\. Thus, if the EPA sets an AQI value of 100 at the
PM2.5 level of 35 [mu]g/m\3\, 24-hour average, the Agency
proposes to set an AQI value of 150 at the PM2.5 level of 55
[mu]g/m\3\, 24-hour average. If, however, the EPA decides to set an AQI
value of 100 at a lower level, then the EPA would adjust an AQI value
of 150 proportionally. The Agency's approach to selecting the levels at
which to set the AQI values of 100 and 150 inherently recognizes that
the epidemiological evidence upon which these decisions are based
provides no evidence of discernible thresholds, below which effects do
not occur in either sensitive groups or in the general population, at
which to set these two breakpoints. Therefore, EPA concludes the use of
a proportional adjustment would be appropriate.
---------------------------------------------------------------------------

    \129\ We note that this level is consistent with the level
recommended in the more recent EPA guidance (Harnett, 2009,
Attachment B), which is in use by many State and local agencies.
---------------------------------------------------------------------------

    With respect to an AQI value of 500, a review of the history of the
AQI value of 500 for PM10 and of the AQI value of 500 for
PM2.5 is useful background. The current AQI value of 500 for
PM10 was set in 1987 at the level of 600 [mu]g/m\3\, 24-hour
average, on the basis of increased mortality associated with historical
wintertime pollution episodes in London (52 FR 24687 to 24688, July 1,
1987). Particle concentrations during these episodes, measured by the
British Smoke method, were in the range of 500 to 1000 [mu]g/m\3\. In
the 1987 rulemaking that established the upper bound index value for
PM10, the EPA cited a generally held opinion that the
British Smoke method measures PM with a cutpoint of approximately 4.5
microns (52 FR 24688, July 1, 1987). In establishing this value for
PM10, the EPA assumed that concentrations of
PM10, which includes both coarse and fine particles, during
episodes of concern, would be about 100 [mu]g/m\3\ higher than the PM
concentration measured in terms of British Smoke (52 FR 24688, July 1,
1987). The upper bound index value of 600 [mu]g/m\3\ was developed by
selecting the lower end of the range of harmful concentrations during
the historical wintertime pollution episodes in London (500 [mu]g/m\3\)
and adding a margin of 100 [mu]g/m\3\ to account for this measurement
difference. The current PM2.5 concentration corresponding to
an AQI value of 500 set in the 1999 rulemaking is 500 [mu]g/m\3\, 24-
hour average.\130\ Because there were few PM2.5 monitoring
data available at that time, the decision was based on the stated
assumption that PM concentrations measured by the British Smoke method
were approximately equivalent to PM2.5 concentrations. In
considering whether it is appropriate to retain or revise the AQI value
of 500 for PM2.5, the EPA notes that the 1999 rulemaking was
based on an assumption of approximate equivalence between the British
Smoke

method and the current PM2.5 method. This assumption is not
entirely consistent with the view cited in 1987 that the British Smoke
method has a size cutpoint of 4.5 microns (52 FR 24688, July 1, 1987),
such that it would be reasonable to expect based on considering size
cutpoint alone that a level of 500 [mu]g/m\3\ based on the British
Smoke method would generally be equivalent to a somewhat lower level
based on the current PM2.5 method. Nonetheless, more recent
comparisons between British Smoke and PM2.5 measurement
methods (Heal, et al., 2005; Chaloulakou, et al., 2005) suggest that on
average British Smoke can be less than or more than PM2.5,
but generally represents a larger fraction in the seasons and locations
when PM2.5 predominantly results from directly emitted
carbonaceous particles such as from combustion sources. More generally,
the EPA recognizes that extremely high PM concentrations that would
most likely be associated with combustion sources (e.g., coal burning
in historic the London event, wildfires in contemporary U.S.
environments) are typically dominated by fine particles, such that
there may be very little difference between these measurement methods
at such high levels.
---------------------------------------------------------------------------

    \130\ We note that a level of 350 [mu]g/m\3\ is recommended for
an AQI value of 500 in the more recent EPA guidance (Harnett, 2009,
Attachment B).
---------------------------------------------------------------------------

    Further, in considering the body of more recent health effects
evidence available in this review, the EPA concludes that there is
little information about more recent air pollution episodes similar to
the wintertime pollution episodes in London and associated impacts on
community health upon which to base a decision. Thus, the EPA concludes
that it remains appropriate to use the historical wintertime pollution
episodes in London as the basis for setting an AQI value of 500 for
PM2.5 as described above because it is still the best
available directly relevant information. Nonetheless, the EPA takes
note of a limited number of more recent studies cited in the Integrated
Science Assessment that evaluated wood smoke health impacts which found
effects such as cardiovascular morbidity and mortality as well as
respiratory effects, albeit at much lower levels (U.S. EPA, 2009a,
sections 6.2 and 6.6). These more recent health studies may provide
some support for considering a lower PM2.5 level for an AQI
value of 500.
    Based on the above considerations, the EPA concludes that it is
appropriate to propose to retain the current level of 500 [mu]g/m\3\,
24-hour average, for the AQI value of 500. The EPA solicits comment on
alternative approaches to setting a level for the AQI value of 500 and
on alternative levels that commenters believe may be appropriate as
well as supporting information and rationales for such alternative
levels. The EPA also solicits any additional information, data,
research or analyses that may be useful to inform a final decision on
the appropriate level to set the AQI value of 500.
    For the intermediate breakpoints in the AQI between the values of
150 and 500, the EPA proposes PM2.5 concentrations that
generally reflect a linear relationship between increasing index values
and increasing PM2.5 values. The available scientific
evidence of health effects related to population exposures to
PM2.5 concentrations between the level of the 24-hour
standard and an AQI value of 500 suggest a continuum of effects in this
range, with increasing PM2.5 concentrations being associated
with increasingly larger numbers of people likely to experience such
effects. The generally linear relationship between AQI values and
PM2.5 concentrations in this range is consistent with the
health evidence. This also is consistent with the Agency's practice of
setting breakpoints in symmetrical fashion where health effects
information does not suggest particular levels.
    Table 2 below summarizes the proposed breakpoints for the
PM2.5 sub-index.\131\ Table 2 shows the intermediate
breakpoints for AQI values of 200, 300 and 400 based on a linear
interpolation between the proposed levels for AQI values of 150 and
500. If a different level were to be set for an AQI value of 150 or
500, intermediate levels would be calculated based on a linear
relationship between the selected levels for AQI values of 150 and 500.
---------------------------------------------------------------------------

    \131\ As discussed in section VII.C below, the EPA is also
proposing to update the data handling procedures for reporting the
AQI and corresponding updates for other AQI-sub-indices presented in
Table 2 of appendix G of 40 CFR part 58.

            Table 2--Proposed Breakpoints for PM2.5 Sub-Index
------------------------------------------------------------------------
                                                   Proposed breakpoints
          AQI category            Index values     ([mu]g/m\3\, 24-hour
                                                         average)
------------------------------------------------------------------------
Good...........................            0-50          0.0-(12.0-13.0)
Moderate.......................          51-100         (12.1-13.1)-35.4
Unhealthy for Sensitive Groups.         101-150                35.5-55.4
Unhealthy......................         151-200               55.5-150.4
Very Unhealthy.................         201-300              150.5-250.4
Hazardous......................         301-400              250.5-350.4
                                        401-500              350.5-500.4
------------------------------------------------------------------------

    In proposing to retain the 500 level for the AQI as described
above, we note that the EPA is not proposing to establish a Significant
Harm Level (SHL) for PM2.5. The SHL is an important part of
air pollution Emergency Episode Plans, which are required for certain
areas by CAA section 110(a)(2)(G) and associated regulations at 40 CFR
51.150, under the Prevention of Air Pollution Emergency Episodes
program. The Agency believes that air quality responses established
through an Emergency Episode Plan should be developed through a
collaborative process working with State and Tribal air quality,
forestry and agricultural agencies, Federal land management agencies,
private land managers and the public. Therefore, if in future
rulemaking EPA proposes revisions to the Prevention of Air Pollution
Emergency Episodes program, the proposal will include a SHL for
PM2.5 that is developed in collaboration with these
organizations. As discussed in the 1999 Air Quality Index Reporting
Rule (64 FR 42530), if a future rulemaking results in a SHL that is
different from the 500 value of the AQI for PM2.5, the AQI
will be revised accordingly.

VI. Rationale for Proposed Decisions on the Secondary PM Standards

    This section presents the rationale for the Administrator's
proposed decisions to revise the current suite of secondary PM
standards by adding a distinct standard for PM2.5 to address
PM-related

visibility impairment while retaining the current secondary
PM2.5 and PM10 standards to address the other
welfare effects considered in this review. In particular, this section
presents background information on EPA's previous and current reviews
of the secondary PM standards (section VI.A), information on visibility
impairment (section VI.B), conclusions on the adequacy of the current
secondary PM2.5 standards to protect against PM-related
visibility impairment (section VI.C), conclusions on alternative
standards to protect against PM-related visibility impairment (section
VI.D), conclusions on secondary PM standards to address other PM-
related welfare effects (section VI.E), and a summary of the
Administrator's proposed decisions on the secondary PM standards
(section VI.F).

A. Background

    The current suite of secondary PM standards is identical to the
current suite of primary PM standards, including 24-hour and annual
PM2.5 standards and a 24-hour PM10 standard. The
current secondary PM2.5 standards are intended to provide
protection from PM-related visibility impairment, whereas the entire
suite of secondary PM standards is intended to provide protection from
other PM-related effects on public welfare, including effects on
sensitive ecosystems, materials damage and soiling, and climatic and
radiative processes.
    The approach used for reviewing the current suite of secondary PM
standards builds upon and broadens the approaches used in previous PM
NAAQS reviews. The following discussion focuses particularly on the
current PM2.5 standards related to visibility impairment and
provides a summary of the approaches used to review and establish
secondary PM2.5 standards in the last two reviews (section
VI.A.1); judicial review of the 2006 standards that resulted in the
remand of the secondary annual and 24-hour PM2.5 NAAQS to
the EPA (section VI.A.2); and the current approach for evaluating the
secondary PM2.5 standards (section VI.A.3).
1. Approaches Used in Previous Reviews
    The original secondary PM2.5 standards were established
in 1997 and a revision to the 24-hour standard was made in 2006. The
approaches used in making final decisions on secondary standards in
those reviews, as well as the current review, utilize different ways to
consider the underlying body of scientific evidence. They also reflect
an evolution in EPA's understanding of the nature of the effect on
public welfare from visibility impairment, from an approach focusing
only on Federal Class I area visibility impacts to a more multifaceted
approach that also considers PM-related impacts on non-Federal Class I
area visibility, such as in urban areas. This evolution has occurred in
conjunction with the expansion of available PM data and information
from associated studies of public perception, valuation, and personal
comfort and well-being.
    In 1997, the EPA revised the identical primary and secondary PM
NAAQS in part by establishing new identical primary and secondary
PM2.5 standards. In revising the secondary standards, the
EPA recognized that PM produces adverse effects on visibility and that
impairment of visibility was being experienced throughout the U.S., in
multi-state regions, urban areas, and remote mandatory Federal Class I
areas alike. However, in considering an appropriate level for a
secondary standard to address adverse effects of PM2.5 on
visibility, the EPA concluded that the determination of a single
national level was complicated by regional differences. These
differences included several factors that influence visibility such as
background and current levels of PM2.5, composition of
PM2.5, and average relative humidity. Variations in these
factors across regions could thus result in situations where attaining
an appropriately protective concentration of fine particles in one
region might or might not provide adequate protection in a different
region. The EPA also determined that there was insufficient information
at that time to establish a level for a national secondary standard
that would represent a threshold above which visibility conditions
would always be adverse and below which visibility conditions would
always be acceptable.
    Based on these considerations, the EPA assessed potential
visibility improvements in urban areas and on a regional scale that
would result from attainment of the new primary standards for
PM2.5. The agency concluded that the spatially averaged form
of the annual PM2.5 standard was well suited to the
protection of visibility, which involves effects of PM2.5
throughout an extended viewing distance across an urban area. Based on
air quality data available at that time, many urban areas in the
Northeast, Midwest, and Southeast, as well as Los Angeles, were
expected to see perceptible improvement in visibility if the annual
PM2.5 primary standard were attained. The EPA also concluded
that attainment of the 24-hour PM2.5 standard in some areas
would be expected to reduce, to some degree, the number and intensity
of ``bad visibility'' days, resulting in improvement in the 20 percent
of days having the greatest impairment over the course of a year.
    Having concluded that attainment of the annual and 24-hour
PM2.5 primary standards would lead to visibility
improvements in many eastern and some western urban areas, the EPA also
considered whether these standards could provide potential improvements
to visibility on a regional scale. Based on information available at
the time, the EPA concluded that attainment of secondary
PM2.5 standards set identical to the primary
PM2.5 standards would be expected to result in visibility
improvements in the eastern U.S. at both urban and regional scales, but
little or no change in the western U.S., except in and near certain
urban areas.
    The EPA then considered the potential effectiveness of a regional
haze program, required by sections 169A and 169B of the CAA \132\ to
address those effects of PM on visibility that would not be addressed
through attainment of the primary PM2.5 standards. The
regional haze program would be designed to address the widespread,
regionally uniform type of haze caused by a multitude of sources. The
structure and requirements of sections 169A and 169B of the CAA provide
for visibility protection programs that can be more responsive to the
factors contributing to regional differences in visibility than can
programs addressing a nationally applicable secondary NAAQS. The
regional haze visibility goal is more protective than a secondary NAAQS
since the goal addresses any anthropogenic impairment rather than just
impairment at levels determined to be adverse to public welfare. Thus,
an important factor considered in the 1997 review was whether a
regional haze program, in conjunction with secondary standards set
identical to the suite of PM2.5 primary standards, would
provide appropriate protection for visibility in non-Federal Class I
areas. The EPA concluded that the two programs and associated control
strategies should provide such protection due to the regional
approaches needed to manage

emissions of pollutants that impair visibility in many of these areas.
---------------------------------------------------------------------------

    \132\ In 1977, Congress established as a national goal ``the
prevention of any future, and the remedying of any existing,
impairment of visibility in mandatory Federal Class I areas which
impairment results from manmade air pollution'', section 169A(a)(1)
of the CAA. The EPA is required by section 169A(a)(4) of the CAA to
promulgate regulations to ensure that ``reasonable progress'' is
achieved toward meeting the national goal.
---------------------------------------------------------------------------

    For these reasons, the EPA concluded that a national regional haze
program, combined with a nationally applicable level of protection
achieved through secondary PM2.5 standards set identical to
the primary PM2.5 standards, would be more effective for
addressing regional variations in the adverse effects of
PM2.5 on visibility than would be national secondary
standards for PM with levels lower than the primary PM2.5
standards. The EPA further recognized that people living in certain
urban areas may place a high value on unique scenic resources in or
near these areas, and as a result might experience visibility problems
attributable to sources that would not necessarily be addressed by the
combined effects of a regional haze program and PM2.5
secondary standards. The EPA concluded that in such cases, state or
local regulatory approaches, such as past action in Colorado to
establish a local visibility standard for the City of Denver, would be
more appropriate and effective in addressing these special situations
because of the localized and unique characteristics of the problems
involved. Visibility in an urban area located near a mandatory Federal
Class I area could also be improved through state implementation of the
then-current visibility regulations, by which emission limitations can
be imposed on a source or group of sources found to be contributing to
``reasonably attributable'' impairment in the mandatory Federal Class I
area.
    Based on these considerations, in 1997 the EPA set secondary
PM2.5 standards identical to the primary PM2.5
standards, in conjunction with a regional haze program under sections
169A and 169B of the CAA, as the most appropriate and effective means
of addressing the public welfare effects associated with visibility
impairment. Together, the two programs and associated control
strategies were expected to provide appropriate protection against PM-
related visibility impairment and enable all regions of the country to
make reasonable progress toward the national visibility goal.
    In 2006, EPA revised the suite of secondary PM2.5
standards to address visibility impairment by making the suite of
secondary standards identical to the revised suite of primary
PM2.5 standards. The EPA's decision regarding the need to
revise the suite of secondary PM2.5 standards reflected a
number of new developments that had occurred and sources of information
that had become available following the 1997 review. First, the EPA
promulgated a Regional Haze Program in 1999 (65 FR 35713, July 1, 1999)
which required states to establish goals for improving visibility in
Federal Class I areas and to adopt control strategies to achieve these
goals. Second, extensive new information from visibility and fine
particle monitoring networks had become available, allowing for updated
characterizations of visibility trends and PM concentrations in urban
areas, as well as Federal Class I areas. These new data allowed the EPA
to better characterize visibility impairment in urban areas and the
relationship between visibility and PM2.5 concentrations.
Finally, additional studies in the U.S. and abroad provided the basis
for the establishment of standards and programs to address specific
visibility concerns in a number of local areas. These studies (Denver,
Phoenix, and British Columbia) utilized photographic representations of
visibility impairment and produced reasonably consistent results in
terms of the visual ranges found to be generally acceptable by study
participants. The EPA considered the information generated by these
studies useful in characterizing the nature of particle-induced haze
and for informing judgments about the acceptability of various levels
of visual air quality in urban areas across the U.S. Based largely on
this information, the Administrator concluded that it was appropriate
to revise the secondary PM2.5 standards to provide increased
protection from visibility impairment principally in urban areas, in
conjunction with the regional haze program for protection of visual air
quality in Federal Class I areas.
    In so doing, the Administrator recognized that PM-related
visibility impairment is principally related to fine particle
concentrations and that perception of visibility impairment is most
directly related to short-term, nearly instantaneous levels of visual
air quality. Thus, in considering whether the then-current suite of
secondary standards would provide the appropriate degree of protection,
he concluded that it was appropriate to focus on just the 24-hour
secondary PM2.5 standard to provide requisite protection.
    The Administrator then considered whether PM2.5 mass
remained the appropriate indicator for a secondary standard to protect
visibility, primarily in urban areas. The Administrator noted that PM-
related visibility impairment is principally related to fine particle
levels. Hygroscopic components of fine particles, in particular
sulfates and nitrates, contribute disproportionately to visibility
impairment under high humidity conditions. Particles in the coarse mode
generally contribute only marginally to visibility impairment in urban
areas. With the substantial addition to the air quality and visibility
data made possible by the national urban PM2.5 monitoring
networks, an analysis conducted for the 2006 review found that, in
urban areas, visibility levels showed far less difference between
eastern and western regions on a 24-hour or shorter time basis than
implied by the largely non-urban data available in the 1997 review. In
analyzing how well PM2.5 concentrations correlated with
visibility in urban locations across the U.S., the 2005 Staff Paper
concluded that clear correlations existed between 24-hour average
PM2.5 concentrations and calculated (i.e., reconstructed)
light extinction, which is directly related to visual range (U.S. EPA,
2005, p. 7-6). These correlations were similar in the eastern and
western regions of the U.S. These correlations were less influenced by
relative humidity and more consistent across regions when
PM2.5 concentrations were averaged over shorter, daylight
time periods (e.g., 4 to 8 hours) when relative humidity in eastern
urban areas was generally lower and thus more similar to relative
humidity in western urban areas. The 2005 Staff Paper noted that a
standard set at any specific PM2.5 concentration would
necessarily result in visual ranges that vary somewhat in urban areas
across the country, reflecting the variability in the correlations
between PM2.5 concentrations and light extinction. The 2005
Staff Paper concluded that it was appropriate to use PM2.5
as an indicator for standards to address visibility impairment in urban
areas, especially when the indicator is defined for a relatively short
period (e.g., 4 to 8 hours) of daylight hours (U.S. EPA, 2005, p. 7-6).
Based on their review of the Staff Paper, most CASAC Panel members also
endorsed such a PM2.5 indicator for a secondary standard to
address visibility impairment (Henderson, 2005a. p. 9). Based on the
above considerations, the Administrator concluded that PM2.5
should be retained as the indicator for fine particles as part of a
secondary standard to address visibility protection, in conjunction
with averaging times from 4 to 24 hours.
    In considering what level of protection against PM-related
visibility impairment would be appropriate, the Administrator took into
account the results of the public perception and attitude surveys
regarding the acceptability of various degrees of visibility impairment
in the U.S. and

Canada, state and local visibility standards within the U.S., and
visual inspection of photographic representations of several urban
areas across the U.S. In the Administrator's judgment, these sources
provided useful but still quite limited information on the range of
levels appropriate for consideration in setting a national visibility
standard primarily for urban areas, given the generally subjective
nature of the public welfare effect involved. Based on photographic
representations of varying levels of visual air quality, public
perception studies, and local and state visibility standards, the 2005
Staff Paper had concluded that 30 to 20 [mu]g/m\3\ PM2.5
represented a reasonable range for a national visibility standard
primarily for urban areas, based on a sub-daily averaging time (U.S.
EPA, 2005, p. 7-13). The upper end of this range was below the levels
at which illustrative scenic views are significantly obscured, and the
lower end was around the level at which visual air quality generally
appeared to be good based on observation of the illustrative views.
This concentration range generally corresponded to median visual ranges
in urban areas within regions across the U.S. of approximately 25 to 35
km, a range that was bounded above by the visual range targets selected
in specific areas where state or local agencies placed particular
emphasis on protecting visual air quality. In considering a reasonable
range of forms for a PM2.5 standard within this range of
levels, the 2005 Staff Paper had concluded that a concentration-based
percentile form was appropriate, and that the upper end of the range of
concentration percentiles for consideration should be consistent with
the 98th percentile used for the primary standard and that the lower
end of the range should be the 92nd percentile, which represented the
mean of the distribution of the 20 percent most impaired days, as
targeted in the regional haze program (U.S. EPA, 2005 pp. 7-11 to 7-
13). While recognizing that it was difficult to select any specific
level and form based on then-currently available information
(Henderson, 2005a, p. 9), the CASAC Panel was generally in agreement
with the ranges of levels and forms presented in the 2005 Staff Paper.
    The Administrator also considered the level of protection that
would be afforded by the proposed suite of primary PM2.5
standards (71 FR 2681, January 17, 2006), on the basis that although
significantly more information was available than in the 1997 review
concerning the relationship between fine PM levels and visibility
across the country, there was still little available information for
use in making the relatively subjective value judgment needed in
selecting the appropriate degree of protection to be afforded by such a
standard. In so doing, the Administrator compared the extent to which
the proposed suite of primary standards would require areas across the
country to improve visual air quality with the extent of increased
protection likely to be afforded by a standard based on a sub-daily
averaging time. Based on such an analysis, the Administrator observed
that the predicted percent of counties with monitors not likely to meet
the proposed suite of primary PM2.5 standards was actually
somewhat greater than the predicted percent of counties with monitors
not likely to meet a sub-daily secondary standard with an averaging
time of 4 daylight hours, a level toward the upper end of the range
recommended in the 2005 Staff Paper, and a form within the recommended
range. Based on this comparison, the Administrator tentatively
concluded that revising the secondary 24-hour PM2.5 standard
to be identical to the proposed revised primary PM2.5
standard (and retaining the then-current annual secondary
PM2.5 standard) was a reasonable policy approach to
addressing visibility protection primarily in urban areas. In proposing
this approach, the Administrator also solicited comment on a sub-daily
(4- to 8-hour averaging time) secondary PM2.5 standard (71
FR 2675 to 2781, January 17, 2006).
    In commenting on the proposed decision, the CASAC requested that a
sub-daily standard to protect visibility ``be favorably reconsidered''
(Henderson, 2006a, p.6). The CASAC noted three cautions regarding the
proposed reliance on a secondary PM2.5 standard identical to
the proposed 24-hour primary PM2.5 standard: (1)
PM2.5 mass measurement is a better indicator of visibility
impairment during daylight hours, when relative humidity is generally
low; the sub-daily standard more clearly matches the nature of
visibility impairment, whose adverse effects are most evident during
the daylight hours; using a 24- hour PM2.5 standard as a
proxy introduces error and uncertainty in protecting visibility; and
sub-daily standards are used for other NAAQS and should be the focus
for visibility; (2) CASAC and its monitoring subcommittees had
repeatedly commended EPA's initiatives promoting the introduction of
continuous and near-continuous PM monitoring, and recognized that an
expanded deployment of continuous PM2.5 monitors would be
consistent with setting a sub-daily standard to protect visibility; and
(3) the analysis showing a similarity between percentages of counties
not likely to meet what the CASAC Panel considered to be a lenient 4-
to 8-hour secondary standard and a secondary standard identical to the
proposed 24-hour primary standard was a numerical coincidence that was
not indicative of any fundamental relationship between visibility and
health. The CASAC Panel further stated that ``visual air quality is
substantially impaired at PM2.5 concentrations of 35 [mu]g/
m\3\'' and that ``[i]t is not reasonable to have the visibility
standard tied to the health standard, which may change in ways that
make it even less appropriate for visibility concerns'' (Henderson,
2006a, pp. 5 to 6).
    In reaching a final decision, the Administrator focused on the
relative protection provided by the proposed primary standards based on
the above-mentioned similarities in percentages of counties meeting
alternative standards, and on the limitations in the information
available concerning studies of public perception and attitudes
regarding the acceptability of various degrees of visibility impairment
in urban areas, as well as on the subjective nature of the judgment
required. In so doing, the Administrator concluded that caution was
warranted in establishing a distinct secondary standard for visibility
impairment and that the available information did not warrant adopting
a secondary standard that would provide either more or less protection
against visibility impairment in urban areas than would be provided by
secondary standards set equal to the proposed primary PM2.5
standards.
2. Remand of 2006 Secondary PM2.5 Standards
    As noted above in section II.B.2 above, several parties filed
petitions for review challenging EPA's decision to set the secondary
NAAQS for fine PM identical to the primary NAAQS. On judicial review,
the D.C. Circuit remanded to EPA for reconsideration the secondary
NAAQS for fine PM because the Agency's decision was unreasonable and
contrary to the requirements of section 109(b)(2). American Farm Bureau
Federation v. EPA, 559 F. 3d 512 (D.C. Cir., 2009).
    The petitioners argued that EPA's decision lacked a reasoned basis.
First, they asserted that EPA never determined what level of visibility
was ``requisite to protect the public welfare.'' They argued that EPA
unreasonably

rejected the target level of protection recommended by its staff, while
failing to provide a target level of its own. The court agreed, stating
that ``the EPA's failure to identify such a level when deciding where
to set the level of air quality required by the revised secondary fine
PM NAAQS is contrary to the statute and therefore unlawful.
Furthermore, the failure to set any target level of visibility
protection deprived the EPA's decision-making of a reasoned basis.''
559 F. 3d at 530.
    Second, the petitioners challenged EPA's method of comparing the
protection expected from potential standards. They contended that EPA
relied on a meaningless numerical comparison, ignored the effect of
humidity on the usefulness of a standard using a daily averaging time,
and unreasonably concluded that the primary standards would achieve a
level of visibility roughly equivalent to the level the EPA staff and
CASAC deemed ``requisite to protect the public welfare.'' The court
found that EPA's equivalency analysis based on the percentages of
counties exceeding alternative standards ``failed on its own terms.''
The same table showing the percentages of counties exceeding
alternative secondary standards, used for comparison to the percentages
of counties exceeding alternative primary standards to show
equivalency, also included six other alternative secondary standards
within the recommended CASAC range that would be more ``protective''
under EPA's definition than the adopted primary standards. Two-thirds
of the potential secondary standards within the CASAC's recommended
range would be substantially more protective than the adopted primary
standards. The court found that EPA failed to explain why it looked
only at one of the few potential secondary standards that would be less
protective, and only slightly less so, than the primary standards. More
fundamentally, however, the court found that EPA's equivalency analysis
based on percentages of counties demonstrated nothing about the
relative protection offered by the different standards, and that the
tables offered no valid information about the relative visibility
protection provided by the standards. 559 F. 3d at 530-31.
    Finally, the Staff Paper had made clear that a visibility standard
using PM2.5 mass as the indicator in conjunction with a
daily averaging time would be confounded by regional differences in
humidity. The court noted that EPA acknowledged this problem, yet did
not address this issue in concluding that the primary standards would
be sufficiently protective of visibility. 559 F. 3d at 530. Therefore,
the court granted the petition for review and remanded for
reconsideration the secondary PM2.5 NAAQS.
3. General Approach Used in the Policy Assessment for the Current
Review
    The approach used in this review broadens the general approaches
used in the last two PM NAAQS reviews by utilizing, to the extent
available, enhanced tools, methods, and data to more comprehensively
characterize visibility impacts. As such, the EPA is taking into
account considerations based on both the scientific evidence
(``evidence-based'') and a quantitative analysis of PM-related impacts
on visibility (``impact-based'') to inform conclusions related to the
adequacy of the current secondary PM2.5 standards and
alternative standards that are appropriate for consideration in this
review. As in past reviews, the EPA is also considering that the
secondary NAAQS should address PM-related visibility impairment in
conjunction with the Regional Haze Program, such that the secondary
NAAQS would focus on protection from visibility impairment principally
in urban areas in conjunction with the Regional Haze Program that is
focused on improving visibility in Federal Class I areas. The EPA again
recognizes that such an approach is the most appropriate and effective
means of addressing the public welfare effects associated with
visibility impairment in areas across the country.
    The Policy Assessment draws from the qualitative evaluation of all
studies discussed in the Integrated Science Assessment (U.S. EPA,
2009a). Specifically, the Policy Assessment considers the extensive new
air quality and source apportionment information available from the
regional planning organizations, long-standing evidence of PM effects
on visibility, and public preference studies from four urban areas
(U.S. EPA, 2009a, chapter 9), as well as the integration of evidence
across disciplines (U.S. EPA, 2009a, chapter 2). In addition, limited
information that has become available regarding the characterization of
public preferences in urban areas has provided some new perspectives on
the usefulness of this information in informing the selection of target
levels of urban visibility protection. On these bases, the Policy
Assessment again focuses assessments on visibility conditions in urban
areas.
    The conclusions in the Policy Assessment reflect EPA staff's
understanding of both evidence-based and impact-based considerations to
inform two overarching questions related to: (1) The adequacy of the
current suite of PM2.5 standards and (2) what potential
alternative standards, if any, should be considered in this review to
provide appropriate protection from PM-related visibility impairment.
In addressing these broad questions, the discussions in the Policy
Assessment were organized around a series of more specific questions
reflecting different aspects of each overarching question (U.S. EPA,
2011a, Figure 4-1). When evaluating the visibility protection afforded
by the current or any alternative standards considered, the Policy
Assessment takes into account the four basic elements of the NAAQS:
indicator, averaging time, level, and form.

B. PM-Related Visibility Impairment

    As discussed below, the rationale for the Administrator's proposed
decision regarding secondary PM standards to protect against visibility
impairment focuses on those considerations most influential in the
Administrator's proposed decisions, including consideration of: (1) The
latest scientific information on visibility effects associated with PM
as described in the Integrated Science Assessment (U.S. EPA, 2009a);
(2) insights gained from assessments of correlations between ambient
PM2.5 and visibility impairment prepared by EPA staff in the
Visibility Assessment (U.S. EPA, 2010b); and (3) specific conclusions
regarding the need for revisions to the current standards (i.e.,
indicator, averaging time, form, and level) that, taken together, would
be requisite to protect the public welfare from adverse effects on
visual air quality.
    This section outlines key information contained in the Integrated
Science Assessment, the Visibility Assessment and the Policy Assessment
on: (1) The nature of visibility impairment, including the relationship
between ambient PM and visibility, temporal variations in light
extinction, periods during the day of interest for assessing visibility
conditions, and exposure durations of interest and (2) public
perceptions and attitudes about visibility impairment and the impacts
of visibility impairment on public welfare.
1. Nature of PM-Related Visibility Impairment
    New research conducted by regional planning organizations in
support of the Regional Haze Rule, as discussed in chapter 9 of the
Integrated Science Assessment, continues to support and refine EPA's
understanding of the effect of PM on visibility and the source

contributions to that effect in rural and remote locations. Additional
by-products of this research include new insights regarding the
regional source contributions to urban visibility impairment and better
characterization of the increment in PM concentrations and visibility
impairment that occur in many cities (i.e., the urban excess) relative
to conditions in the surrounding rural areas (i.e., regional
background). Ongoing urban PM2.5 speciated and aggregated
mass monitoring has produced new information that has allowed for
updated characterization of current visibility levels in urban areas.
Information from both of these sources of PM data, while useful, has
not however changed the fundamental and long understood science
characterizing the contribution of PM, especially fine particles, to
visibility impairment. This science, briefly summarized below, provides
the basis for the Integrated Science Assessment designation of the
relationship between PM and visibility impairment as causal.
a. Relationship Between Ambient PM and Visibility
    Visibility impairment is caused by the scattering and absorption of
light by suspended particles and gases in the atmosphere. The combined
effect of light scattering and absorption by both particles and gases
is characterized as light extinction, i.e., the fraction of light that
is scattered or absorbed in the atmosphere. Light extinction is
quantified by a light extinction coefficient with units of 1/distance,
which is often expressed in the technical literature as 1/(1 million
meters) or inverse megameters (abbreviated Mm-1). When PM is
present in the air, its contribution to light extinction typically
greatly exceeds that of gases.
    The amount of light extinction contributed by PM depends on the
particle size distribution and composition, as well as its particle
concentration. If details of the ambient particle size distribution and
composition (including the mixing of components) are known, Mie theory
can be used to accurately calculate PM light extinction (U.S. EPA,
2009a, chapter 9). However, routine monitoring rarely includes
measurements of particle size and composition information with
sufficient detail for such calculations. To make estimation of light
extinction more practical, visibility scientists have developed a much
simpler algorithm, known as the IMPROVE algorithm,\133\ to estimate
light extinction using routinely monitored fine particle
(PM2.5) speciation and coarse particle mass
(PM10-2.5) data. In addition, relative humidity information
is needed to estimate the contribution by liquid water that is in
solution with hygroscopic PM components (U.S. EPA, 2009a, section
<IP_ADDRESS>; U.S. EPA, 2010b, chapter 3). There is both an original and a
revised version of the IMPROVE algorithm (Pitchford et al., 2007). The
revised version was developed to address observed biases in the
predictions using the original algorithm under very low and very high
light extinction conditions.\134\ These IMPROVE algorithms are
routinely used to calculate light extinction levels on a 24-hour basis
in Federal Class I areas under the Regional Haze Program.
---------------------------------------------------------------------------

    \133\ The algorithm is referred to as the IMPROVE algorithm
because it was developed specifically to use the aerosol monitoring
data generated at network sites and with equipment specifically
designed to support the IMPROVE program and was evaluated using
IMPROVE optical measurements at the subset of sites that make those
measurements (Malm et al., 1994).
    \134\ These biases were detected by comparing light extinction
estimates generated from the IMPROVE algorithm to direct optical
measurements in a number of rural Federal Class I areas.
---------------------------------------------------------------------------

    In either version of the IMPROVE algorithm, the concentration of
each of the major aerosol components is multiplied by a dry extinction
efficiency value and, for the hygroscopic components (i.e., ammoniated
sulfate and ammonium nitrate), also multiplied by an additional factor
to account for the water growth to estimate these components'
contribution to light extinction. Both the dry extinction efficiency
and water growth terms have been developed by a combination of
empirical assessment and theoretical calculation using typical particle
size distributions associated with each of the major aerosol
components. They have been evaluated by comparing the algorithm
estimates of light extinction with coincident optical measurements.
Summing the contribution of each component gives the estimate of total
light extinction per unit distance denoted as the light extinction
coefficient (bext), as shown below for the original IMPROVE algorithm.

bext [ap] 3 x f(RH) x [Sulfate]
    + 3 x f(RH) x [Nitrate]
    + 4 x [Organic Mass]
    + 10 x [Elemental Carbon]
    + 1 x [Fine Soil]
    + 0.6 x [Coarse Mass]
    + 10

    Light extinction (bext) is in units of Mm-1, the mass
concentrations of the components indicated in brackets are in units of
[mu]g/m\3\, and f(RH) is the unitless water growth term that depends on
relative humidity. The final term of 10 Mm-1 is known as the
Rayleigh scattering term and accounts for light scattering by the
natural gases in unpolluted air. The dry extinction efficiency for
particulate organic mass is larger than those for particulate sulfate
and nitrate principally because the density of the dry inorganic
compounds is higher than that assumed for the PM organic mass
components.
    For the first two terms, ``sulfate'' is defined in terms of
ammonium sulfate and ``nitrate'' is defined in terms of ammonium
nitrate. Since IMPROVE does not include ammonium ion monitoring, the
assumption is made that all sulfate is fully neutralized ammonium
sulfate and all nitrate is assumed to be ammonium nitrate.\135\ Though
often reasonable, neither assumption is always true (see U.S. EPA,
2009a, section <IP_ADDRESS>). In the eastern U.S. during the summer there is
insufficient ammonia in the atmosphere to neutralize the sulfate fully.
Fine particle nitrates can include sodium or calcium nitrate, which are
the fine particle fraction of generally much coarser particles due to
nitric acid interactions with sea salt at near-coastal areas (sodium
nitrate) or nitric acid interactions with calcium carbonate in crustal
aerosol (calcium nitrate). Despite the simplicity of the algorithm, it
performs reasonably well and permits the contributions to light
extinction from each of the major components (including the water
associated with the sulfate and nitrate compounds) to be separately
approximated.
---------------------------------------------------------------------------

    \135\ To calculate ammonium sulfate, multiply the CSN
measurement of the sulfate ion by 1.375. To calculate ammonium
nitrate, multiply the CSN measurement of the nitrate ion by 1.29
(Lowenthal and Kumar, 2006).
---------------------------------------------------------------------------

    The f(RH) term reflects the increase in light scattering caused by
particulate sulfate and nitrate under conditions of high relative
humidity. Particles with hygroscopic components (e.g., particulate
sulfate and nitrate) contribute more light extinction at higher
relative humidity than at lower relative humidity because they change
size in the atmosphere in response to ambient relative humidity
conditions. For relative humidity below 40 percent the f(RH) value is
1, but it increases to 2 at approximately 66 percent, 3 at
approximately 83 percent, 4 at approximately 90 percent, 5 at
approximately 93 percent, and 6 at approximately 95 percent relative
humidity. The result is that both particulate sulfate and nitrate are
more efficient per unit mass in light extinction than any other aerosol
component for relative humidity above

approximately 85 percent where their total light extinction efficiency
exceeds the 10 m\2\/g associated with elemental carbon (EC). Based on
this algorithm, particulate sulfate and nitrate are estimated to have
comparable light extinction efficiencies (i.e., the same dry extinction
efficiency and f(RH) water growth terms), so on a per unit mass
concentration basis at any specific relative humidity they are treated
as equally effective contributors to visibility effects.
    As noted above, particles with hygroscopic components (e.g.,
particulate sulfate and nitrate) contribute more light extinction at
higher relative humidity than at lower relative humidity because they
change size in the atmosphere in response to ambient relative humidity
conditions. PM containing elemental or black carbon (BC) absorbs light
as well as scattering it, making it the component with the greatest
light extinction contributions per unit of mass concentration, except
for the hygroscopic components under high relative humidity
conditions.\136\
---------------------------------------------------------------------------

    \136\ The IMPROVE algorithm does not explicitly separate the
light-scattering and light-absorbing effects of elemental carbon.
---------------------------------------------------------------------------

    With regard to the fifth and sixth terms, the fine soil component
is based on measurement of five elements: Aluminum (Al), silicon (Si),
calcium (Ca), iron (Fe), and titanium (Ti).\137\ Inspection of the PM
component-specific terms in the simple original IMPROVE algorithm shows
that most of the PM2.5 components contribute 5 times or more
light extinction than a similar concentration of PM10-2.5.
---------------------------------------------------------------------------

    \137\ Consistent with calculations used in the IMPROVE network
and the Regional Haze Program, the fine soil component is calculated
using the following formula:
    Fine Soil = 2.20 x [Al] + 2.49 x [Si] + 1.63 x [Ca] + 2.42 x
[Fe] + 1.94 x [Ti].
---------------------------------------------------------------------------

    Subsequent to the development of the original IMPROVE algorithm, an
alternative algorithm (variously referred to as the ``revised
algorithm'' or the ``new algorithm'' in the literature) has been
developed. It employs a more complex split-component mass extinction
efficiency to correct biases believed to be related to particle size
distributions, a sea salt term that can be important for remote coastal
areas, a different multiplier for organic carbon for purposes of
estimating organic carbonaceous material,\138\ and site-specific
Rayleigh light scattering terms in place of a universal Rayleigh light
scattering value. These features of the revised IMPROVE algorithm are
described in section <IP_ADDRESS> of the Integrated Science Assessment,
which also presents a comparison of the estimates produced by the two
algorithms for rural areas. Compared to the original algorithm, the
revised IMPROVE algorithm can yield higher estimates of current light
extinction levels in urban areas on days with relatively poor
visibility (Pitchford, 2010). This difference is primarily attributable
to the split-component mass extinction efficiency treatment in the
revised algorithm rather than to the inclusion of a sea salt term or
the use of site-specific Rayleigh scattering values.
---------------------------------------------------------------------------

    \138\ The revised IMPROVE algorithm uses a multiplier of 1.8
instead of 1.4 as used in the original algorithm for the mean ratio
of organic mass to organic carbon.
---------------------------------------------------------------------------

    As mentioned above, particles are not the only contributor to
ambient visibility conditions. Light scattering by gases also occurs in
ambient air. Under pristine atmospheric conditions, naturally occurring
gases such as elemental nitrogen and oxygen cause what is known as
Rayleigh scattering. Rayleigh scattering depends on the density of air,
which is a function primarily of the elevation above sea level, and can
be treated as a site-dependent constant. The Rayleigh scattering
contribution to light extinction is only significant under pristine
conditions. The only other commonly occurring atmospheric gas to
appreciably absorb light in the visible spectrum is nitrogen dioxide.
Nitrogen dioxide forms in the atmosphere from nitrogen oxide emissions
associated with combustion processes. These combustion processes also
emit PM at levels that generally contribute much higher light
extinction than the nitrogen dioxide (i.e., nitrogen dioxide absorption
is generally less than approximately 5 percent of the light extinction,
except where emission controls remove most of the PM prior to releasing
the remaining gases to the atmosphere). The final term in the IMPROVE
algorithm of 10 Mm-1 is known as the Rayleigh scattering
term and accounts for light scattering by the natural gases in
unpolluted air. The remainder of this section focuses on the
contribution of PM, which is typically much greater than that of gases,
to ambient light extinction, unless otherwise specified.
    In the following discussions, visual air quality is characterized
in terms of both light extinction, as discussed above, and an
alternative scale for characterizing visibility--the deciview scale--
that is defined directly in terms of light extinction (expressed in
units of Mm-1) by the following equation: \139\

    \139\ As used in the Regional Haze Program, the term
bext refers to light extinction due to PM2.5,
PM10-2.5, and ``clean'' atmospheric gases. In the Policy
Assessment, in focusing on light extinction due to PM2.5,
the deciview values include only the effects of PM2.5 and
the gases. The ``Rayleigh'' term associated with clean atmospheric
gases is represented by the constant value of 10 Mm-1.
Omission of the Rayleigh term would create the possibility of a
negative deciview values when the PM2.5 concentration is
very low.
---------------------------------------------------------------------------

Deciview (dv) = 10 ln (bext/10 Mm-1).

    The deciview scale is frequently used in the scientific and
regulatory literature on visibility, as well as in the Regional Haze
Program. In particular, the deciview scale is used in the public
perception studies that were considered in the past and current reviews
to inform judgments about an appropriate degree of protection to be
provided by a secondary NAAQS.
b. Temporal Variations of Light Extinction
    Particulate matter concentrations and light extinction in urban
environments vary from hour-to-hour throughout the 24-hour day due to a
combination of diurnal changes in meteorological conditions and
systematic changes in emissions activity (e.g., rush hour traffic).
Generally, low mixing heights at night and during the early morning
hours tend to trap locally produced emissions, which are diluted as the
mixing height increases due to heating during the day. Low temperatures
and high relative humidity at night are conducive to the presence of
ammonium nitrate particles and water growth by hygroscopic particles
compared with the generally higher temperatures and lower relative
humidity later in the day. These combine to make early morning the most
likely time for peak urban light extinction. Superimposed on such
systematic time-of-day variations are the effects of synoptic
meteorology (i.e., those associated with changing weather) and
regional-scale air quality that can generate peak light extinction
impacts any time of day. The net effects of the systematic urban- and
larger-scale variations are that peak daytime PM light extinction
levels can occur any time of day, although in many areas they most
often occur in early morning hours (U.S. EPA, 2010b, sections 3.4.2 and
3.4.3; Figures 3-9, 3-10, and 3-12).
    This temporal pattern in urban areas contrasts with the general
lack of a strong diurnal pattern in PM concentrations and light
extinction in most Federal Class I areas, reflective of a relative lack
of local sources as compared to urban areas. The use in the

Regional Haze Program of 24-hour average concentrations in the IMPROVE
algorithm is consistent with this general lack of a strong diurnal
pattern in Federal Class I areas.
c. Periods During the Day of Interest for Assessment of Visibility
    Visibility is typically associated with daytime periods because
people are outside more during the day than at night and there are more
viewable scenes at a distance during the day than at night. The Policy
Assessment recognizes, however, that physically PM light extinction
behaves the same at night as during the day, enhancing the scattering
of anthropogenic light, contributing to the ``skyglow'' within and over
populated areas, adding to the total sky brightness, and contributing
to the reduction in contrast of stars against the background. These
effects produce the visual result of a reduction in the number of
visible stars and the disappearance of diffuse or subtle phenomena such
as the Milky Way. The extinction of starlight is a secondary and minor
effect also caused by increased PM scattering and absorption.
    However, there are significant and important differences between
daytime and nighttime visual environments with regard to how light
extinction per se relates to visual air quality (or visibility) and
public welfare. First, daytime visibility has dominated the attention
of those who have studied the visibility effects of air pollution,
particularly in urban areas. As a result, little research has been
conducted on nighttime visibility and the state of the science is not
comparable to that associated with daytime visibility impairment. As
noted in the Policy Assessment, no urban-focused preference or
valuation studies providing information on public preferences for
nighttime visual air quality have been identified (U.S. EPA, 2011a, p.
4-17). Second, in addition to air pollution, nighttime visibility is
affected by the addition of light into the sight path from numerous
sources, including anthropogenic light sources in urban environments
such as artificial outdoor lighting, which varies dramatically across
space, and natural sources including the moon, planets, and stars.
Light sources and ambient light conditions are typically five to seven
orders of magnitude dimmer at night than in sunlight. Moonlight, like
sunlight, introduces light throughout an observer's sight path at a
constant angle. On the other hand, dim starlight emanates from all over
the celestial hemisphere while artificial lights are concentrated in
cities and illuminate the atmosphere from below. These different light
sources will yield variable changes in visibility as compared to what
has been established for the daytime scenario, in which a single
source, the sun, is by far the brightest source of light. Third, the
human psychophysical response (e.g., how the human eye sees and
processes visual stimuli) at night is expected to differ (U.S. EPA,
2009a, section 9.2.2).
    Given the above, the Policy Assessment notes that the science is
not available at this time to support adequate characterization
specifically of nighttime PM light extinction conditions and the
related effects on public welfare (U.S. EPA, 2011a, p. 4-18). Thus, the
Policy Assessment focuses its assessments of PM visibility impacts in
urban areas on daylight hours. For simplicity, and because perceptions
and welfare effects from light extinction-related visual effects during
the minutes of actual sunrise and sunset have not been explored,
daylight hours are defined as those hours entirely after the local
sunrise time and before the local sunset time.
    In so doing, the Policy Assessment notes that the 24-hour averaging
time used in the Regional Haze Program includes nighttime conditions
(U.S. EPA, 2011a, p. 4-18). It also notes, however, that the goal of
the Regional Haze Program is to address any manmade impairment of
visibility without regard to distinctions between daylight and
nighttime conditions. Moreover, because of the lack of strong diurnal
patterns in most Federal Class I areas, both nighttime and daylight
visibility are strongly correlated with 24-hour average visibility
conditions, so a 24-hour averaging period is suitable for driving both
daylight and nighttime visibility towards their natural conditions.
Also, the focus on 24-hour average visibility allows the Regional Haze
Program to make use of more practically obtained ambient speciated PM
measurements of adequate accuracy than if a shorter averaging period
were used, which is an important consideration especially given the
remoteness of many Federal Class I area monitoring sites and given the
low PM concentrations that must be measured accurately in such areas.
    In addition, when natural conditions such as fog and rain cause
poor visibility, it can be reasonably assumed that the light extinction
properties of the air that are attributable to air pollution are not
important from a public welfare perspective. Thus, it is appropriate to
give special treatment to such periods when considering whether current
PM2.5 standards adequately protect public welfare from PM-
related visibility impairment. In evaluating alternative sub-daily
standards, the Policy Assessment addresses this issue by screening out
hours with particularly high relative humidity. As discussed further
below, the Policy Assessment uses a relative humidity screen of 90
percent on the basis that it serves as a reasonable surrogate for
excluding hours affected by fog and rain (U.S. EPA, 2011a, p. 4-18).
d. Exposure Durations of Interest
    The roles that exposure duration and variations in visual air
quality within any given exposure period play in determining the
acceptability or unacceptability of a given level of visual air quality
has not been investigated via preference studies. In the preference
studies available for this review, subjects were simply asked to rate
the acceptability or unacceptability of each image of a haze-obscured
scene, without being provided any suggestion of assumed duration or of
assumed conditions before or after the occurrence of the scene
presented. Preference and/or valuation studies show that atmospheric
visibility conditions can be quickly assessed and preferences
determined. A momentary glance at an image of a scene (i.e., less than
a minute) is enough for study participants to judge the acceptability
or unacceptability of the viewed visual air quality conditions.
Moreover, individual participants in general consistently judge the
acceptability of same-scene images that differed only with respect to
light extinction levels when these images were presented repeatedly for
such short periods. That is, individuals generally did not say that a
higher-light extinction image was acceptable while saying a lower-light
extinction, same-scene image was unacceptable, even though they could
not compare images side-to-side. However, the Policy Assessment does
not have information about what assumptions, if any, the participants
may have made about the duration of exposure in determining the
acceptability of the images and EPA staff is unaware of any studies
that characterize the extent to which different frequencies and
durations of exposure to visibility conditions contribute to the degree
of public welfare impact that occurs.
    In the absence of such studies, the Policy Assessment considers a
variety of circumstances that are commonly expected to occur in
evaluating the potential impact of visibility impairment on the public
welfare based on available information (U.S. EPA, 2011a, pp. 4-19 to 4-
20). In some

circumstances, such as infrequent visits to scenic vistas in natural or
urban environments, people are motivated specifically to take the
opportunity to view a valued scene and are likely to do so for many
minutes to hours to appreciate various aspects of the vista they choose
to view. In such circumstances, the viewer may consciously evaluate how
the visual air quality at that time either enhances or diminishes the
experience or view. However, the public also has many more
opportunities to notice visibility conditions on a daily basis in
settings associated with performing daily routines (e.g., during
commutes and while working, exercising, or recreating outdoors). These
scenes, whether iconic or generic, may not be consciously viewed for
their scenic value and may not even be noticed for periods comparable
to what would be the case during purposeful visits to scenic visits,
but their visual air quality may still affect a person's sense of
wellbeing. Research has demonstrated that people are emotionally
affected by low visual air quality, that perception of pollution is
correlated with stress, annoyance, and symptoms of depression, and that
visual air quality is deeply intertwined with a ``sense of place,''
affecting people's sense of the desirability of a neighborhood (U.S.
EPA, 2009a, section 9.2.4). Though it is not known to what extent these
emotional effects are linked to different periods of exposure to poor
visual air quality, providing additional protection against short-term
exposures to levels of visual air quality considered unacceptable by
subjects in the context of the preference studies would be expected to
provide some degree of protection against the risk of loss in the
public's ``sense of wellbeing.''