&EPA
United State
EirviroiiwiU Protection
Agnncy
Health Risk and Exposure Assessment
for Ozone
Second External Review Draft
Executive Summary
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DISCLAIMER
This draft document has been prepared by staff from the Risk and Benefits Group, Health
and Environmental Impacts Division, Office of Air Quality Planning and Standards, U.S.
Environmental Protection Agency. Any findings and conclusions are those of the authors and do
not necessarily reflect the views of the Agency. This draft document is being circulated to
facilitate discussion with the Clean Air Scientific Advisory Committee to inform the EPA's
consideration of the ozone National Ambient Air Quality Standards.
This information is distributed for the purposes of pre-dissemination peer review under
applicable information quality guidelines. It has not been formally disseminated by EPA. It
does not represent and should not be construed to represent any Agency determination or policy.
Questions related to this preliminary draft document should be addressed to Dr. Bryan
Hubbell, U.S. Environmental Protection Agency, Office of Air Quality Planning and Standards,
C539-07, Research Triangle Park, North Carolina 27711 (email: hubbell.bryan@epa.gov).
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EPA-452/P-14-004f
February 2014
Health Risk and Exposure Assessment for Ozone
Second External Review Draft
Executive Summary
U.S. Environmental Protection Agency
Office of Air and Radiation
Office of Air Quality Planning and Standards
Health and Environmental Impacts Division
Risk and Benefits Group
Research Triangle Park, North Carolina 27711
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Health Risk and Exposure Assessment f<
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Executive Summary
Introduction
A
s part of the review of the ozone
National Ambient Air Quality Standards
(NAAQS), EPA has prepared this Risk
and Exposure Assessment (REA) to provide
estimates of exposures to O3 and resulting
mortality and morbidity health risks. The
health effects evaluated in this REA are
based on the findings of the O3 ISA (U.S.
EPA, 2012) that short term O3 exposures are
causally related to respiratory effects, and
likely causally related to cardiovascular
effects, and that long term O3 exposures
are likely causally related to respiratory
effects. The assessment evaluated total
exposures and risks associated with the full
range of observed O3 concentrations. In
addition, the REA estimated the incremental
changes in exposures and risks between just
meeting the existing standard of 75 ppb
and just meeting potential alternative
standard levels of 70, 65, and 60 ppb using
the form and averaging time of the existing
standard, which is the annual 4th highest
daily maximum 8-hour O3 concentration,
averaged over three consecutive years.
The results of the REA help to inform the O3
Policy Assessment (PA) in considering the
adequacy of the existing O3 standards, and
potential risk reductions associated with
potential alternative levels of the standard.
As described in the conceptual framework
and scope in Chapters 2 and 3,
respectively, the health REA discusses air
quality considerations (Chapter 4) and
evaluates exposures and lung function risk in
15 urban case study areas (Chapters 5 and
6, respectively) and risks based on
application of results of epidemiology
studies in a subset of 12 urban case study
areas (Chapter 7) . In addition, to place the
urban area analyses in a broader context,
the assessment estimated the national
burden of mortality associated with recent
O3 levels, and evaluated the
representativeness of the urban areas in
characterizing O3 exposures and risks across
the U.S. (Chapter 8). To further facilitate
interpretation of the results of the exposure
and risk assessment, Chapter? provides a
synthesis of the various results, focusing on
comparing and contrasting those results to
identify common patterns, or important
differences. It also includes an overall
integrated characterization of exposure and
risk in the context of key policy relevant
questions.
Conceptual Framework
and Scope
The REA provides information to answer
key policy-relevant risk questions with
regards to evaluation of the
adequacy of the existing standards
and evaluation of potential alternative
standards such as:
"To whaf extent do risk and/or exposure
analyses suggest that exposures of concern
for Os-re/afed health effects are likely to
occur with existing ambient levels of Os or
with levels that just meet the Os standard?
To what extent do alternative standards,
taking together levels, averaging times and
forms, reduce estimated exposures and risks
of concern attributable to Os and other
photochemical oxidants, and what are the
uncertainties associated with the estimated
exposure and risk reductions?"
In answering these questions, the REA
evaluates total exposures and risks
associated with the full range of observed
Os concentrations, as well as the
incremental changes in exposures and risks
for just meeting the existing standard and
just meeting several alternative standards.
With regard to selecting alternative levels for
the 8-hour Os standards for evaluation in the
quantitative risk assessment, we base the
range of levels on the evaluations of the
evidence provided in the first draft PA,
which received support from the CASAC in
their advisory letter on the first draft PA. The
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first draft PA recommended evaluation of 8-
hour maximum concentrations in the range
of 60 to 70 ppb, with possible consideration
of levels somewhat below 60 ppb.
Os concentrations from 2006-2010 are used
in estimating exposures and risks for the 15
urban case study areas. Because of the
year-to-year variability in Os concentrations,
the assessment evaluates scenarios for
meeting the existing and potential
alternative standards based on multiple
years of Os data to better capture the high
degree of variability in meteorological
conditions, as well as reflecting years with
higher and lower emissions of Os precursors.
The 15 urban case study areas were
selected to be generally representative of
U.S. populations, geographic areas,
climates, and different Os and co-pollutant
levels. These urban case study areas include
Atlanta, GA; Baltimore, MD; Boston, MA;
Chicago, IL; Cleveland, OH; Dallas, TX;
Denver, CO; Detroit, Ml; Houston, TX; Los
Angeles, CA; New York, NY; Philadelphia,
PA; Sacramento, CA; St. Louis, MO; and
Washington, D.C.
We have identified the following goals for
the urban area exposure and risk
assessments: (1) to provide estimates of the
percent of people in the general population
and in at-risk populations and lifestages with
Os exposures above health-based
benchmark levels; (2) to provide estimates
of the percent of people in the general
population and in at-risk populations and
lifestages with impaired lung function
(defined based on decrements in FEV1)
resulting from exposures to Os; (3) to provide
estimates of the potential magnitude of
premature mortality associated with both
short-term and long-term Os exposures, and
selected morbidity health effects associated
with short-term Os exposures; (4) to evaluate
the influence of various inputs and
assumptions on risk estimates to the extent
possible given available methods and data;
(5) to gain insights into the spatial and
temporal distribution of risks associated with
Os concentrations just meeting existing and
alternative standards, patterns of risk
reduction associated with meeting
alternative standards relative to the existing
standard, and uncertainties in the estimates
of risk and risk reductions.
In working towards these goals, we follow a
conceptual framework, shown in the figure
below, comprised of air quality
characterization, review of relevant
scientific evidence on health effects,
modeling of exposure, modeling of risk, and
risk characterization. As shown in this
framework, modeling of personal exposure
and estimation of risks, which rely on
personal exposure estimates, are
implemented using the Air Pollution
Exposure model (APEX)1 (U.S. EPA, 2012a,b).
Modeling of population level risks for
endpoints based on application of results of
epidemiological studies is implemented
using the environmental Benefits Mapping
and Analysis Program (BenMAP)2, a peer
reviewed software tool for estimating risks
and impacts associated with changes in
ambient air quality (U.S. EPA, 2013). The
overall characterization of risk draws from
the results of the exposure assessment and
both types of risk assessment.
1 APEX is available for download at
http://www.epa.gov/ttn/fera/human_apex.html
2 BenMAP is available for download at
http://www.epa.gov/air/benmap/
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Policy Relevant Exposure
and Risk Questions
(Chapter2)
Exposure Assessment
APEX
Urban Scale
Assessment of
Individual Exposure
(Chapters)
Urban Scale Risk
Analyses Based on
Application of
Results from
Controlled Human
Exposure Studies
(Chapters)
I
Air Quality Characterization
(Chapter4)
Review of Health Evidence
(Chapter2)
RiskAssessment
BenMAP
Urban Scale Risk
Analyses Based on
Application of
Results from
Epidemic logical
Studies
(Chapter?)
National Scale Risk
Burden Basedon
Application of
Results from
Epidemiological
Studies
(Chapters)
Risk Cha racte rization
(Chapters)
Air Quality Considerations
In this analysis, we employed a
photochemical model-based adjustment
methodology (Simon et al, 2012) to
estimate the change in observed hourly O3
concentrations at a given set of monitoring
sites resulting from across-the-board
reductions in U.S. anthropogenic NOx
and/or VOC emissions. This information was
then used to adjust recent O3
concentrations (2006-2010) in the 15 case
study areas to reflect just meeting the
existing standard of 75 ppb and just meeting
potential alternative standard levels of 70,
65, and 60 ppb. Because the form of the
existing O3 standard is based on the 3-year
average of the 4th highest daily 8-hour
maximum, we simulate just meeting the
standard for two periods, 2006-2008 and
2008-2010.
The use of the model-based adjustment
methodology is an example of how we
have brought improvements into this review
that better represent current scientific
understanding. The model-based
adjustment methodology represents a
substantial improvement over the quadratic
rollback method used to adjust O3
concentrations in past reviews. For example,
while the quadratic rollback was a purely
mathematical technique which attempted
to reproduce the distribution of observed O3
concentrations just meeting various
standards, the new methodology uses
photochemical modeling to simulate the
response in O3 concentrations due to
changes in precursor emissions based on
current understanding of atmospheric
chemistry and transport. Second, quadratic
rollback used the same mathematical
formula to adjust concentrations at all
monitors within each urban case study area
for all hours, while model-based adjustment
methodology allows the adjustments to vary
both spatially across each case study area
and temporally across hours of the day and
across seasons. Finally, quadratic rollback
was designed to only allow decreases in O3
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concentrations, while the model-based
adjustment methodology allows both
increases and decreases in O3
concentrations, which more accurately
reflects the scientific understanding that
increases in O3 concentrations may occur in
response to reductions in NOx emissions in
some situations, such as in urban areas with
a large amount of NOx emissions.
Several general trends are evident in the
changes in O3 patterns across the case
study areas and across the different
standards under consideration. In all 15
case study areas, peak O3 concentrations
tended to decrease while the lowest O3
concentrations tended to increase as the
concentrations were adjusted to meet the
existing and potential alternative standards.
In addition, high and mid-range O3
concentrations generally decreased in rural
and suburban portions of the case study
areas, while O3 response to NOx reductions
was more varied within urban core areas. In
particular, while the annual 4th highest daily
maximum 8-hour concentrations generally
decreased in the urban core of the case
study areas in response to reductions in NOx
emissions, the seasonal mean of the daily
maximum 8-hour O3 concentrations did not
change significantly, though it did exhibit
some increases or decreases in the various
case study areas as the distribution of O3
was further adjusted to meet lower potential
alternative standards.
The adjustments to O3 to reflect just meeting
existing and potential alternative standards
are conducted by decreasing only
emissions of anthropogenic NOx and VOC
within the U.S. As such, the estimated
changes in exposure and risk, based on
these air quality changes, are solely
attributable to changes in U.S. emissions.
Human Exposure Modeling
T
he population exposure assessment
evaluated exposures to O3 using the
APEX exposure model which uses time-
activity diary and anthropometric data
coupled with local meteorology, population
demographics, and O3 concentrations to
estimate the percent of study groups above
exposure benchmarks. The analyses
examined exposure to O3 for the general
population, all school-aged children (ages
5-18), asthmatic school-aged children (ages
5-18), asthmatic adults (ages > 18), and
older persons (ages 65 and older), with a
focus on populations engaged in moderate
or greater exertion, for example, children
engaged in outdoor recreational activities.
Exposure is assessed in the 15 urban case
study areas for recent O3 (2006-2010) and
for O3 adjusted to just meet existing and
potential alternative standards for two
design value periods (2006-2008 and 2008-
2010). The analysis provided estimates of the
percent of several populations of interest
exposed to concentrations above three
health-relevant 8-hour average O3
exposure benchmarks: 60, 70, and 80 ppb.
These benchmarks were selected so as to
provide some perspective on the public
health impacts of OS-related health effects
that have been demonstrated in human
clinical and toxicological studies, but
cannot currently be evaluated in
quantitative risk assessments, such as lung
inflammation and increased airway
responsiveness. The ISA includes studies
showing significant effects at each of these
benchmark levels (U.S. EPA, 2012).
The analysis found that children are the
population of greatest concern for O3
exposures due to the greater amount of
time they spend outdoors engaged in
moderate or higher exertion activities and
the fact that children have the highest
percent of exposures of concern of any of
the at-risk populations. As a result, we focus
on the results for children in this discussion.
The two figures below show the average
across 2006-2010 of the percentage of
school-aged children experiencing 8-hour
exposure greater than 60 ppb for at least
one exposure (top) and for at least two
exposures (bottom) per year. Based on this
information, no more than 26 percent of any
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Atlanta
Baltimore
Boston
Chicago
Cleveland
Dallas
Denver
Detroit
Houston
Los Angeles
New York
Philadelphia
Sacramento
St. Louis
Washington
Atlanta
Baltimore
Boston
Chicago
Cleveland
Dallas
Denver
Detroit
Houston
Los Angeles
New York
Philadelphia
Sacramento
St. Louis
Washington
0% 1% 2% 3% 4% 5% 6% 7% 8%
Percent of All School-Age Children with at Least Two 8-hr Daily Max Exposure >= 60 ppb
standard level (ppb) I I 60 I I 65 E^H 70 ^CD 75
Average percent increases in percent of all school-age
children exposed at or above 60 ppb-8hr for each study
area over all years, for at least one exposure (left) and for at
least two exposures (right) per year.
Note: New York level 60 was not modeled. We do not know what the
percent risk would be for NY under the 60 ppb alternative standard, but it
would not necessarily be zero.
study group in any study area was exposed
at least once at or above the 60 ppb-8hr
benchmark, when meeting the existing
standard. When meeting a standard level of
70 ppb, less than 20 percent of any study
group in any study area was exposed
at least once at or above the 60 ppb-
8hr exposure benchmark. Meeting a
standard level of 65 ppb is estimated to
reduce the percent of persons at or
above an exposure benchmark of 60
ppb-8hr to 10 percent or less of any
study group and study area.
For the exposure benchmark of 70 ppb-
8hr, less than 10 percent of any study
group, including all school-age
children, in any study area, was
exposed at least once at or above the
exposure benchmark when meeting
the existing standard. For the highest
exposure benchmark of 80 ppb-8hr, less
than 1 percent of any study group in
any study area was exposed at least
once at or above the exposure
benchmark when meeting the existing
standard. These percentages are even
smaller when meeting the lower
alterative standard levels.
For two or more exceedances at the 60
ppb-8hr benchmark, less than 15
percent of any study group in any study
area experience 8-hour exposure
greater than 60 ppb-8hrwhen meeting
the existing standard. There were no
persons estimated to experience any
multi-day exposures at or above 80
ppb-8hr for any study group in any
study area, while 2.2 percent or less of
persons were estimated to experience
two or more exposures at or above 70
ppb-8hr, when meeting the existing
standard or any of the alternative
standard levels.
In addition, the exposure assessment
also identified the specific
microenvironments and activities that
contribute most to exposure and
evaluated at what times and how long
individuals were in key microenvironments
and were engaged in key activities, with a
focus on persons experiencing the highest
daily maximum 8-hour exposure within each
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study area. That analysis found that: (1)
Children are an important exposure
population subgroup, largely as a result of
the combination of high levels of outdoor
time and engagement in moderate or high
exertion level activities. (2) Persons spending
a large portion of their time outdoors during
afternoon hours experienced the highest 8-
hour O3 exposure concentrations given that
O3 concentrations in other
microenvironments were simulated to be
lower than ambient concentrations. (3)
Highly exposed children on average spend
half of their outdoor time engaged in
moderate or greater exertion levels, such as
in sporting activities. Highly exposed adults
also spent their outdoor time engaged in
moderate or greater exertion levels though
on average, not as frequently as children.
Health Risks Based on
Controlled Human Exposure
Studies
This analysis uses the estimates of
exposure from APEX, combined with
results from controlled human exposure
studies, to estimate the number and
percent of at-risk populations (all children,
children with asthma, adults aged 18-35,
adults aged 36-55, and outdoor workers)
experiencing selected decrements in lung
function. The analysis focuses on estimates
of the percent of each at-risk population
experiencing a reduction in lung function for
three different levels of impact: 10, 15, and
20 percent decrements in FEV1. These levels
of impact were selected based on the
literature discussing the adversity associated
with increasing lung function decrements
(US EPA, 2012, Section 6.2.1.1; Henderson,
2006). Lung function decrements of 10
percent and 15 percent in FEV1 are
considered moderate decrements; 10
percent is considered potentially adverse for
people with lung disease, while a 15
percent is potentially adverse for active
healthy people. A 20 percent decrement in
FEV1 is considered a large decrement that is
potentially adverse for healthy people and
can potentially cause more serious effects in
people with lung disease.
Two models were used to estimate lung
function risks. One model was based on
application of a population level exposure-
response (E-R) function consistent with the
approach used in the previous O3 review,
and the other model was based on
application of an individual level risk
function (the McDonnell-Stewart-Smith (MSS)
model), which is being introduced in this
review. The main differences between the
two models are that the MSS model includes
responses for a wider range of exposure
protocols (under different levels of exertion,
lengths of exposures, and patterns of
exposure concentrations) than the
exposure-response model of previous
reviews. Both models have a logistic form
and are less sensitive to changes at very low
concentrations of O3 than to higher O3
concentrations. As a result, the models
show very few FEV1 responses > 10% when
ambient concentrations are below 20 ppb
and very few FEV1 responses > 15% when
ambient concentrations are below 40 ppb.
Because the individual level E-R function
approach allows for a more complete
estimate of risk, we focus on the results of
the MSS model for this discussion.
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.Atlanta
Baltimore
Boston
Chicago
Cleveland
Dallas
Denver
Detroit
Houston
New York
Philadelphia
Sacramento
St Lotlis
0°o 2°o 4°o 6°o S°o 10°o 12°o 14°o 16°o 18°o
percent of school-aaeclclukkeu \\itliFEVl decrement > 10°o
standard level (ppb) I riO I 165 I I "0 I I "5
Average percent increases in percent of all school-age
children with FEV1 decrement > 10 percent in each study
area over all years
Note: New York level 60 was not modeled. We do not know what the
percent risk would be for NY under the 60 ppb alternative standard, but it
would not necessarily be zero.
the existing standard level of 75 ppb
and aboutl 3.3 percent for the
alternative standard level of 70 ppb.
The length of the brown bar is the
incremental risk reduction (3 percent)
in going from the existing standard of
75 ppb to the 70 ppb alternative
standard. The pattern of reductions
for lung function decrements larger
than 15 and 20 percent are similar.
Health Risks Based on
Application of Results
of Epidemiological
Studies
T.
Lung function risks were estimated for each
of the 15 urban case study areas for recent
air quality (2006-2010) and for air quality
adjusted to just meet existing and
alternative standards for two design value
periods (2006-2008 and 2008-2010). As with
the exposure assessment, we focus on lung
function decrements in children as they are
the populations likely to have the greatest
percentage at risk due to higher levels of
exposure and greater levels of exertion. The
figure above shows the risks just meeting
the existing and potential alternative
standard levels, where risk is taken to be the
average value for each study area (over all
years) of the percent of school-aged
children with FEV1 decrement of 10 percent
or greater. This figure shows that there are
significant decreases in incremental risk for
all 15 cities in the progression from the level
of the existing standard, 75 ppb to the
alternative standard levels of 70, 65, and 60
ppb. The risks in this figure for Washington,
DC, for example, are about 16.3 percent for
he epidemiology-based risk
assessment evaluated mortality
and morbidity risks from short-
term exposures, as well as mortality
risks from long-term exposures to O3,
by applying concentration-response
(C-R) functions derived from epidemiology
studies. Most of the endpoints evaluated in
epidemiology studies are for the entire study
population. Because most mortality and
hospitalizations occur in older persons, the
risk estimates for this portion of the analysis
are thus more focused in adults rather than
children, and thus differ in focus compared
to the human exposure and lung function
risk assessments. The analysis included both
a set of urban area case studies and a
national-scale assessment.
The urban case study analyses evaluated
mortality and morbidity risks, including
emergency department (ED) visits,
hospitalizations, and respiratory symptoms
associated with recent O3 concentrations
(2006-2010) and with O3 concentrations
adjusted to just meet the existing and
alternative O3 standards. Mortality and
hospital admissions (HA) were evaluated in
12 urban areas (a subset of the 15 urban
areas evaluated in the exposure and lung
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function risk assessments), while ED visits and
respiratory symptoms were evaluated in a
subset of areas with supporting
epidemiology studies. The 12 urban areas
were: Atlanta, GA; Baltimore, MD; Boston,
MA; Cleveland, OH; Denver, CO; Detroit, Ml;
Houston, TX; Los Angeles, CA; New York, NY;
Philadelphia, PA; Sacramento, CA; and St.
Louis, MO. The urban case study analyses
focus on risk estimates for the middle year of
each three-year attainment
simulation period (2006-2008 and
2008-2010) in order to provide
estimates of risk for a year with
generally higher O3 levels (2007) and
a year with generally lower O3 levels
(2009).
In previous reviews, O3 risks were
estimated for the portion of total O3
attributable to North American
anthropogenic sources (referred to in
previous O3 reviews as "policy
relevant background"). In contrast,
this assessment provides risk estimates
for the urban areas for O3
concentrations down to zero,
reflecting the lack of evidence for a
detectable threshold in the C-R
functions (ISA, 2012), and the
understanding that U.S. populations
may experience health risks
associated with O3 resulting from
emissions from all sources, both
natural and anthropogenic, and
within and outside the U.S.
The two figures to the right show the
results of the mortality (top) and
respiratory hospital admissions
(bottom) risk assessments for all 12
urban areas associated with short-
term exposure to O3, showing the
effect on the incidence per 100,000
population just meeting the existing
75 ppb standard and potential
alternative O3 standards of 70, 65,
and 60 ppb in 2007. The overall trend
across urban areas is small decreases
in mortality and morbidity risk as air
quality is adjusted to just meet
incrementally lower standard levels. In New
York, there are somewhat greater
decreases, reflecting the relatively large
emission reductions used to adjust air quality
to just meet the 65 ppb alternative
standard, and the substantial change in the
distribution of O3 concentrations that
resulted. Risks vary substantially across
urban areas; however, the general pattern
of reductions across the alternative
Trend in ozone-related mortality across standard
levels (deaths per 100,000)
-Atlanta, GA
-Baltimore, MD
Boston, MA
-Cleveland, OH
-Denver.CO
Detroit, Ml
-Houston, TX
-LosAngeles, CA
New York, NY
-Philadelphia, PA
-Sacramento, CA
St. Louis, MO
75ppb
70ppb
65ppb
60ppb
Trend in ozone-related HA across standard levels
(HA per 100,000)
-Atlanta, GA
-Baltimore, MD
Boston, MA
-Cleveland, OH
-Denver.CO
Detroit, Ml
•Houston, TX
-LosAngeles, CA
New York, NY
-Philadelphia, PA
-Sacramento, CA
St. Louis, MO
75ppb
70ppb
65ppb
60ppb
Impacts of just meeting existing and alternative standard
levels on short-term mortality risk per 100,000 population (top)
and on respiratory hospital admissions risk per 100,000
population for 2007 (bottom)
Nofe: New York level 60 was not modeled. We do not know what the
percent risk would be for NY under the 60 ppb alternative standard, but it
would not necessarily be zero.
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standards is similar between urban areas.
Risks are generally slightly lower in 2009
relative to 2007; though the patterns of
reductions are very similar between the two
years. On average, compared with
meeting the existing standard, mortality and
respiratory hospitalization risks decrease by
5% or less for a level of 70 ppb, 10% or less for
a level of 65 ppb, and 15% or less for a level
of 60 ppb. Larger risk reductions are
estimated on days with higher O3.
We also evaluated mortality risks in the 12
urban areas associated with long-term O3
exposures (based on the April to September
average of the peak daily one-hour
maximum concentrations). The figure below
shows the results of long-term mortality risk
assessments for all 12 urban areas, showing
the effect on the incidence per 100,000
population just meeting the existing
standard and potential alternative O3
standard levels of 70, 65, and 60 ppb in
2007. Risks from long-term exposures after
just meeting the existing standard are
substantially greater than risks from short-
term exposures, ranging from 16 to 20
percent of respiratory mortality across urban
areas. However, the percent reductions in
Trend in ozone-related mortality across standard
levels (deaths per 100,000)
o 25
E 10
75ppb
70ppb
65ppb
60ppb
Impacts of just meeting existing and alternative standard levels on
long-term mortality risk per 100,000 population for 2007
risks are similar to those for mortality from
short-term exposures, e.g., less than 10
percent reduction in risk relative to just
meeting the existing standard in most areas
when just meeting the 70 ppb and 65 ppb
alternative standards, and less than 20
percent reductions when just meeting the
60 ppb alternative standard level.
Mortality and morbidity risks generally do not
show large responses to meeting existing or
alternative levels of the standard for several
reasons. First, these risks are based on C-R
functions that are approximately linear
along the full range of concentrations, and
therefore reflect the impact of changes in
O3 along the complete range of 8-hour
average O3 concentrations. This includes
days with low baseline O3 concentrations
that are predicted to have increases in O3
concentrations, as well as days with higher
starting O3 concentrations that are
predicted to have decreases in O3
concentrations as a result of just meeting
existing and potential alternative standards.
Second, these risks reflect changes in the
urban-area wide monitor average, which
will not be as responsive to air quality
adjustments as the design value monitor,
and which includes monitors with
both decreases and increases in 8-
hour concentrations. Third, the days
and locations with predicted
increases in O3 concentrations
(generally those with low to
midrange starting O3
concentrations) resulting from just
meeting the existing or alternative
standard levels generally are
frequent enough to offset days and
locations with predicted decreases
in O3. The focus of the
epidemiological studies on urban
area-wide average O3
concentrations, and the lack of
thresholds coupled with the linear
nature of the C-R functions mean
that in this analysis, the impact of a
peak-based standard (which seeks
to reduce peak concentrations
•Atlanta, GA
•Baltimore, MD
Boston, MA
leveland,OH
Denver, CO
Detroit, Ml
•Houston, TX
•Los Angeles, CA
New York, NY
•Philadelphia, PA
Sacramento, CA
St. Louis, MO
Note: New York level 60 was not modeled. We do not know what the percent risk would
be for NY under the 60 ppb alternative standard, but it would not necessarily be zero.
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regardless of effects on low or mean
concentrations) on estimates of mortality
and morbidity risks based on results of those
studies is relatively small. However, we are
not able to draw strong conclusions about
the results across urban areas, because of
the limited number of urban areas
represented for most of the endpoints.
The national-scale epidemiology-based risk
assessment evaluated only mortality
associated with recent O3 concentrations
across the entire U.S for 2006-2008. The
national-scale assessment is a complement
to the urban scale analysis, providing both a
broader assessment of OS-related health
risks across the U.S. It demonstrates that
there are O3 risks across the U.S, not just in
urban areas, even though the O3 levels in
many areas were lower than the existing
standard level. We estimated 15,000
premature OS-related non-accidental
deaths (all ages) annually associated with
short-term exposure to recent O3 levels
across the continental U.S. for 2007, May-
September. For long-term mortality, we
estimated 45,000 premature OS-related
adult (age 30 and older) respiratory deaths
annually for 2007, April-September. While we
did not assess the changes in risk at a
national level associated with just meeting
existing and potential alternative standards,
just meeting existing and potential
alternative standards would likely reduce O3
concentrations both in areas that are not
meeting those standards and in locations
surrounding those areas, leading to risk
reductions that are not captured by the
urban scale analysis.
Representativeness of
Exposure and Risk Results
As part of this assessment, we conducted
several analyses to determine the
extent to which our selected urban
areas represent: (1) the highest mortality
and morbidity risk areas in the U.S.; and (2)
the types of patterns of O3 air quality
changes that we estimate would be
experienced by the overall U.S. population
in response to emissions reductions that
would decrease peak O3 concentrations to
meet the existing standard or lower
alternative O3 standard levels.
We selected urban areas for the exposure
and risk analyses based on criteria that
included O3 levels, at-risk populations, and
related factors that were designed to
ensure we captured areas and populations
likely to experience high O3 exposures and
risks. Based on the comparisons of
distributions of risk characteristics, the
selected urban case study areas represent
urban areas that are among the most
populated in the U.S., have relatively high
peak O3 levels, and capture well the range
of city-specific mortality risk effect estimates.
The analyses found that the O3 mortality risk
for short-term O3 exposures in the 12 urban
study areas are representative of the full
distribution of U.S. OS-related mortality,
representing both high end and low end risk
counties. For the long-term exposure
related mortality risk metric, the 12 urban
study areas are representative of the central
portion of the distribution of risks across all
U.S. counties, however, the selected 12
urban areas do not capture the very highest
(greater than 98th percentile) or lowest (less
than 25th percentile) ends of the national
distribution of long-term exposure-related
OS-related risk.
While we selected urban areas to represent
those populations likely to experience
elevated risks from OS exposure, we did not
include amongst the selection criteria the
responsiveness of OS in the urban area to
decreases in OS precursor emissions that
would be needed to just meet existing or
potential alternative standards. The
additional analyses we conducted suggest
that many of the urban case study areas
may show OS responses that are typical of
other large urban areas in the U.S., but may
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not represent the response of O3 in other
populated areas of the U.S. These other
areas, including suburban areas, smaller
urban areas, and rural areas, would be
more likely than our urban case study areas
to experience area-wide average
decreases in mean O3 concentrations and,
therefore, decreases in mortality and
morbidity risks, as O3 standards are met.
Even though large urban areas have high
population density, the majority of the U.S.
population lives outside of these types of
urban core areas, and thus, a large
proportion of the population is likely to
experience greater mortality and morbidity
risk reductions in response to reductions in 8-
hour O3 concentrations than are predicted
by our modeling in the 12 selected urban
case study areas.
Because our selection strategy for risk
modeling was focused on identifying areas
with high risk, we tended to select large
urban population centers. This strategy was
largely successful in including urban areas in
the upper end of the O3 risk distribution.
However, this also led to an
overrepresentation of the populations living
in locations where we estimate increasing
mean seasonal O3 would occur in response
to decreases in O3 precursor emissions that
would be needed to just meet existing or
alternative standards. The implication of this
is that our estimates of mortality and
morbidity risk reductions for the selected
urban areas should not be seen as
representative of potential risk reductions for
most of the U.S. population, and are likely to
understate the average risk reduction that
would be experienced across the
population.
Synthesis
To facilitate interpretation of the results
of the exposure and risk assessment, this
assessment provides a synthesis of the
various results, focusing on comparing and
contrasting those results to identify common
patterns, or important differences.
Consistent with the available evidence, we
estimated exposures relative to several
health-based exposure benchmarks, lung
function risks based on a threshold
exposure-response model of lung function
decrements, and mortality and morbidity
risks based on non-threshold C-R functions.
These three different analyses result in
differing sensitivities of results to changes in
O3. Because the three metrics are affected
differently by changes in O3 at low
concentration levels, it is important to
understand these changes in O3 at low
concentrations in interpreting differences in
the results across metrics.
The exposure benchmark analysis is the least
sensitive to changes in O3 in the lower part
of the distribution of starting O3
concentrations, because the lowest of the
exposure benchmarks is at 60 ppb, above
the portion of the distribution of starting O3
concentrations where we saw increases.
Since the modeled exposures will always be
less than or equal to the monitor
concentrations, a benchmark of exposure
at 60 ppb is above the range of O3
concentrations where the model-based
adjustment approach estimates increases in
concentrations. Thus, this metric is most
reflective of the decreases in O3 at high
concentrations that are expected to result
from just meeting the existing and potential
alternative standards.
The lung function risk analysis is less sensitive
than the mortality and morbidity risk
assessments to changes at very low
concentrations of O3, because the risk
function is logistic and shows little response
at lower O3 dose rates that tend to occur
when ambient concentrations are lower
(generally less than 20 ppb for the 10
percent FEV1 decrement and generally less
than 40 ppb for the 15 percent FEV1
decrement). However, because there are
still some increases that occur in the 50 to 60
ppb range where the estimated risk is more
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responsive, there may be some reduction in
the net risk decrease.
The mortality and morbidity risk assessment is
the analysis that is most sensitive to the
increases in O3 in the lower part of the
distribution of starting O3 concentrations
that we estimated would occur as the
existing and alternative standards are met in
some urban areas. Mean O3 concentrations
for the urban areas change little between
air quality scenarios for meeting the existing
and alternative standards, because mean
concentrations reflect both the increases in
O3 at lower concentrations and the
decreases in O3 occurring on days with high
O3 concentrations. This leads to small net
changes in mortality and morbidity risk
estimates for many of the urban case study
areas. However, both the net change in risk
and the distribution of risk across the range
of O3 concentrations may be relevant in
considering the degree of additional
protection provided by just meeting existing
and alternative standards.
In conclusion, we have estimated that
exposures and risks remain after just meeting
the existing standards and that that in many
cases, just meeting potential alternative
standard levels results in reductions in those
exposures and risks. Meeting potential
alternative standards has larger impacts on
metrics that are not sensitive to changes in
lower O3 concentrations. When meeting the
70, 65, and 60 ppb alternative standards,
the percent of children experiencing
exposures above the 60 ppb health
benchmark falls to less than 20 percent, less
than 10 percent, and less than 3 percent in
the worst O3 year for all 15 case study urban
areas, respectively. Lung function risk also
drops considerably as lower standards are
met. When meeting the 70, 65, and 60 ppb
alternative standards, the percent of
children with lung function decrements
greater than or equal to 10 percent in the
worst year falls to less than 21 percent, less
than 18 percent, and less than 14 percent in
the worst O3 year for all 15 case study urban
areas, respectively. Mortality and respiratory
hospitalization risks decrease by 5% or less
for a level of 70 ppb, 10% or less for a level of
65 ppb, and 15% or less for a level of 60 ppb.
These smaller changes in the mortality and
morbidity risks, relative to the exposures and
lung function risk reductions, reflect the
impact of increasing O3 on low
concentration days, and the non-threshold
nature of the C-R function. Larger mortality
and morbidity risk reductions are estimated
on days with higher baseline O3
concentrations.
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United States Office of Air Quality Planning and Standards Publication No. EPA-452/P-14-004f
Environmental Protection Air Quality Strategies and Standards Division February 2014
Agency Research Triangle Park, NC
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