# Chapter 2. Integrative Health and Welfare Effects Overview

1 The subsequent chapters of this ISA will present the most policy

- relevant information related



### 2 to this review of t he NAAQS for O 3 . This chapter integrates the key findings from the disciplines



### 3 evaluated in this current assessment of the O 3 scientific literature, which includes the atmospheric



### 4 sciences, ambient air data analyses, exposure assessment, dosimetry, health studies



### 5 (e.g., toxicological, controlled human exposure, and epidemiologic), and welfare effects. The EPA



### 6 framework for causal determinations described in Chapter 1 has been applied to the body of



### 7 scientific evidence in order to collectively examine the he alth or welfare effects attributed to O 3 

### 8 exposure in a two

- step process.



### 9 As described in Chapter 1, EPA assesses the results of recent relevant publications, building



### 10 upon evidence available during the previous NAAQS review, to draw conclusions on the cau sal



### 11 relationships between relevant pollutant exposures and health or environmental effects. This ISA



### 12 uses a five

- level hierarchy that classifies the weight of evidence for causation:



### 13  Causal relationship



### 14  Likely to be a causal relationship



### 15  Suggestive of a causal relationship



### 16  Inadequate to infer a causal relationship



### 17  Not likely to be a causal relationship



### 18 Beyond judgments regarding causality are questions relevant to quantifying health or



### 19 environmental risks based on our understanding of the quantitative relationships between pollutant



### 20 exposures and health or welfare effects. Once a determination is made regarding the causal



### 21 relationship between the pollutant and outcome category, important questions regarding quantitative



### 22 relationships include:



### 23  What is the concentration

- response or dose

- response relationship?



### 24  Under what exposure conditions (amount deposited, dose or concentration, duration and



### 25 pattern) are effects observed?

Note: Hyperlinks to the reference citations throughout this document will take you to the NCEA HERO data base (Health and Environmental Research Online) at http://epa.gov/hero . HERO is a database of scientific literature used by U.S. EPA in the process of developing sc ience assessments suc h as the Integrated Science Assessments (ISA) and the Integrated Risk Information System (IRIS)

.

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1  What populations appear to be differentially affected i.e

., more susceptible to e ffects?



### 2  What elements of the ecosystem (e.g., types, regions, taxonomic groups, populations,



### 3 functions, etc.) appear to be affected or are more sensitive to effects?



### 4 To address these questions, in the second step of the EPA framework, the entirety of

quant itative evidence is evaluated to identify and characterize potential concentration

- response



### 6 relationships. This requires evaluation of levels of pollutant and exposure durations at which effects



### 7 were observed for exposed populations including potentially s usceptible populations.



### 8 This chapter summarizes and integrates the newly available scientific evidence that best



### 9 informs consideration of the policy

- relevant questions that frame this assessment, presented in
Chapter 1. Section 2.1 discusses the trends in ambient concentrations and sources of O 3 and provides



### 11 a brief summary of ambient air quality for short

- and long

- term exposure durations. Section 2.2



### 12 presents the evidence regarding personal exposure to ambient O 3 in outdoor and indoor



### 13 microenvironments, and it discusses the relationship between ambient O 3 concentrations and



### 14 exposure to O 3 from ambient sources. Section 2.3 provides a discussion of the dosimetry and mode

of action evidence for O 3 exposure. Section 2.4 integrates the evidence for studies tha t examine the



### 16 health effects associated with short

- and long

- term exposure to O 3 and discusses important



### 17 uncertainties identified in the interpretation of the scientific evidence. Section 2.5 provides a


### 18 discussion of policy

- relevant considerations, such as potentially susceptible populations, lag



### 19 structure, and the O 3 conce ntration

- response relationship. S ection 2.6 integrates the health evidence
from the different scientific disciplines and exposure durations. Finally, Section 2.7 summarizes the



### 21 evidence f or welfare effects related to O 3 exposure, and Section 2.8 reviews the literature on climate



### 22 and UV -B

.



# 2.1. Atmos pheric Chemis try and Ambient Concentrations

23 In the stratosphere, O 3 serves the beneficial role of blocking the Sun's harmful ultraviolet



### 24 radiation and preventing the majority of it from reaching the Earth's surface. In the troposphere,

however, O 3 and other photochemical oxidants are air pollutants with potentially harmful effects on



### 26 living organisms and materials. Chapter 3 of this review addresses the atmospheric chemistry



### 27 associated with tropospheric O 3 and other related photochemical oxidants and provides a detailed



### 28 analysis of recent surface

- level concentrations. This material builds on information reported in the



### 29 2006 O 3 AQCD (U.S. EPA, 2006, 088089 ) . Topics covered below and addressed in further detail in
Chapter 3 include: ( 1) physical and chemical processes of O 3 formation and removal ;


### 31 ( 2) atmospheric modeling ; ( 3) policy relevant background concentrations ; ( 4) monitoring techniques



### 32 and networks ; and ( 5) ambient concentrations.

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## 2.1.1. Phys ical and Chemical Proces s es

1 O zone in the troposphere is a secondary pollutant; it is formed by photochemical reactions of



### 2 precursor gasses and is not directly emitted from specific sources . Ozone and other oxidants, such as



### 3 peroxyacetyl nitrate (PAN) and hydrogen peroxide (H 2 O 2 ) form in p olluted areas by atmospheric



### 4 reactions involving two main classes of precursor pollutants: VOCs and NO X . Carbon monoxide
(CO) is also important for O 3 formation in polluted areas and in the remote troposphere. The



### 6 formation of O 3 , other oxidants and oxidat ion products from these precursors is a complex, nonlinear



### 7 function of many factors: ( 1) the intensity and spectral distribution of sunlight; ( 2) atmospheric



### 8 mixing; ( 3) concentrations of precursors in the ambient air and the rates of chemical reactions of



### 9 these precursors; and ( 4) processing on cloud and aerosol particles.

O zone is present not only in polluted urban atmospheres but throughout the troposphere, even



### 11 in remote areas of the globe. The same basic processes involving sunlight

- driven reactions of NO X , 

### 12 VOCs and CO contribute to O 3 formation throughout the troposphere. These processes also lead to



### 13 the formation of other photochemical products, such as PAN, nitric acid ( HN O 3 ), and sulfuric acid



### 14 (H 2 SO 4 ), and to other compounds, such as formaldehyde (H CHO) and other carbonyl compounds.



## 2.1.1.1. Gas Phas e Reac tio ns Lea din g to Ozone Form atio n and Los s

Photochemical processes involved in O 3 formation are relatively well understood and were



### 16 reviewed in detail in the 2006 O 3 AQCD (U.S. EPA, 2006, 088089 ) . The photochemical formation of



### 17 O 3 in the troposphere proceeds through the photolysis of nitrogen dioxide ( NO 2 ) to yield nitric oxide



### 18 (NO) and a ground

- state oxygen atom, O( 3 P), which then reacts with molecular oxygen ( O 2 ) to form



### 19 O 3 . Free radicals formed in the atmosphere through the oxidation of VOCs and CO proceed to

oxidize NO back to NO 2 , hence perpetuating the O 3 forming cycle. In urban areas, VOCs and CO are



### 21 both important for O 3 formation

. In nonurban vegetated areas, biogenic VOCs emitted from



### 22 vegetation tend to be the most important. In the remote troposphere, methane ( CH 4 ) – stru cturally the



### 23 simplest VOC

– and CO are the main carbon

- containing precursors to O 3 formation. A schematic



### 24 overview of the major photochemical cycles influencing O 3 in the troposphere and the stratosphere is
given in Figure 3 -1.



### 26 O zone is lost through a numb er of gas phase reactions and deposition to surfaces. The reaction



### 27 of O 3 with NO to produce NO 2 mainly results in the recycling of O 3 downwind via the



### 28 recombination of O( 3 P) with O 2 to reform O 3 . By itself, this reaction does not lead to a net change in



### 29 O 3 unless the NO 2 is converted to a stable end product such as HN O 3 or a temporary reservoir
product such as PAN . Ozone also reacts with unsaturated hydrocarbons and with hydrogen



### 31 containing free radicals (OH, HO 2 ). Recent field studies aimed at obtaining a better understanding of



### 32 atmospheric chemical processes involved in O 3 formation are discussed in Section 3.2.



### 33 Convective processes and small scale turbulence transport O 3 and other pollutants both upward



### 34 and downward throughout the planetary boundary layer and the free troposphere. In many areas of

the U.S., O 3 and its precursors can be transported over long distances, aided by vertical mixing. The

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1 transport of pollutants downwind of major urban centers is characterized by the development of



### 2 urban plumes. Meteorological conditions, small

- scale circulation patterns, localized chemistry, and



### 3 mountain barriers can influence mixing on a smaller scale, resulting in frequent heterogeneous O 3 

### 4 concentrations across an individual urban area. More details and observat ions of these processes are

included in Section 3.2 and Section 3.6.



## 2.1.1.2. Sourc e s of Pre c urs o rs Involve d in Ozone Formation

6 Emissions of O 3 precursor compounds ( NO X , VOCs, and CO) can be divided into



### 7 anthropogenic and natural source categories. Natural sources can be further divided into biogenic



### 8 from vegetation, microbes, and animals, and abiotic from biomass burning, lightning, and geogenic



### 9 sources. However, the distinction between natural sources and anthropogenic sources is often
difficult to make in practice, as human activities affect directly or indirectly emissions from what



### 11 would have been considered natural source s during the preindustrial era

. The magnitudes of O 3 

### 12 precursor sources are strongly location

- and time

- dependent and so average emission estim ates



### 13 should not be used to apportion sources of exposure

. More details on O 3 precursor emission



### 14 inventories are included in Section 3.2

.



## 2.1.2. Atmo s ph e ric Mo delin g

Chemistry

- transport models (CTMs) have been widely used to compute the interactions



### 16 among atmosphe ric pollutants and their transformation products, and the transport and deposition of



### 17 pollutants. They have also been widely used to improve our basic understanding of atmospheric



### 18 chemical processes and to develop control strategies

. The main components of a comprehensive



### 19 atmospheric chemistry modeling system are shown in Figure 3 -5 and are discussed in more detail in

Section 3.3.



### 21 The domains of CTMs extend from a few hundred kilometers on a side to the entire globe

. 

### 22 Most major regional (i.e., sub -continent al) scale air

- related modeling efforts at EPA rely on the



### 23 Community Multi

- scale Air Quality modeling system (CMAQ)

. CMAQ’s horizontal domain



### 24 typically extends over North America with efforts underway to extend it over the ent ire Northern
Hemisphere

. The up per boundary for CMAQ is typically set at 100 hPa, which is located on average



### 26 at about 16

- km altitude

. CMAQ is most often driven by the MM5 mesoscale meteorological model,



### 27 though it may be driven by other meteorological models including the Weather Resear ch Forecasting



### 28 ( WRF ) model and the Regional Atmospheric Modeling System (RAMS)

. Other major air quality



### 29 systems used for regional scale applications include The Comprehensive Air Quality Model with

extensions (CAMx) and the Weather Research and Forecast mo del with Chemistry (WRF/Chem).



### 31 Fine scale resolution is necessary to resolve features which can affect pollutant concentrations



### 32 such as urban heat island circulation; sea breezes; mountain and valley breezes; and the nocturnal



### 33 low

- level jet

. Horizontal domains are typically modeled by nesting a finer grid model within a larger



### 34 domain model of coarser resolution

. Caution must be exercised in using nested models because

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1 certain parameterizations like those for convection might be valid on a relatively coarse grid scale



### 2 but may not be valid on finer scales and because incompatibilities can occur at the model boundaries

. 

### 3 The use of finer resolution in CTMs will require advanced parameterizations of meteorological



### 4 processes such as boundary layer fluxes, deep con vection

, and clouds, and necessitate finer

- scale

inventories of land use

, source locations, and emission inventories

. 

### 6 Because of the large number of chemical species and reactions that are involved in the



### 7 oxidation of realistic mixtures of anthropogenic an d biogenic hydrocarbons, condensed mechanisms



### 8 must be used to simplify atmospheric models

. These mechanisms can be tested by comparison with



### 9 smog chamber data

. However, the existing chemical mechanisms often neglect many important
processes such as the for mation and subsequent reactions of long

- lived carbonyl compounds, the



### 11 incorporation of the most recent information about intermediate compounds, and heterogeneous



### 12 reactions involving cloud droplets and aerosol particles

. As a result, models such as CMAQ ha ve had



### 13 difficulties with capturing the regional nature of O 3 episodes, in part because of uncertainty in the



### 14 chemical pathways converting NO X to HN O 3 and recycling of NO X

. Each of the model components shown in Figure 3 -5 has associated uncertainties and th e


### 16 relative importance of these uncertainties varies with the modeling application. The largest errors in



### 17 photochemical modeling are still thought to arise from the meteorological and emissions inputs to



### 18 the model

. Algorithms must be used for simulating met eorological processes that occur on spatial



### 19 scales smaller than the model's grid spacing and for calculating the dependence of emissions on
meteorology and time

. Significant errors in emissions can occur if inappropriate assumptions are



### 21 used in these param eterizations

. 

### 22 The performance of CTMs must be evaluated by comparison with field data as part of a cycle



### 23 of model evaluations and subsequent improvements

. Discrepancies between model predictions and



### 24 observations can be used to point out gaps in current und erstanding of atmospheric chemistry and to
spur improvements in parameterizations of atmospheric chemical and physical processes

.



## 2.1.3. Policy Relevant Background Concentrations

26 The background concentrations of O 3 that are useful for risk and policy assessments informing



### 27 decisions about the NAAQS are referred to as policy

- relevant background (PRB) concentrations.



### 28 PRB concentrations have historically been defined by EPA as those concentrations that would occur



### 29 in the U.S. in the absence of anthropogenic emissions in continental North America (CNA) defined

here as the U.S., Canada, and Mexico. For this document, PRB concentrations include contributions



### 31 from natural sources everywhere in the world and from anthropogenic sources outside CN A.



### 32 Contributions to PRB O 3 in clude photochemical reactions involving natural emissions of



### 33 VOCs, NO X , and CO as well as the long

- range transport of O 3 and its precursors from outside CNA



### 34 and the stratospheric

- tropospheric exchange (STE) of O 3

. Natural sources of O 3 precursors include

b iogenic emissions, wildfires, and lightning

. Biogenic emissions from agricultural activities in CNA



### 36 are not considered in the formation of PRB O 3

. PRB O 3 sources and concentrations are summarized



### 37 here with further details in Section 3.4.

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## 2.1.3.1. Contributions from a n throp oge nic e mis s ions ou ts ide North Ame ric a

1 Because the mean tropospheric lifetime of O 3 is 3 0- 35 days

, O 3 can be transported from



### 2 continent to continent and around the globe in the Northern Hemisphere and O 3 produced by U .S.



### 3 emissions can be recirculat ed around northern mid

- latitudes back to the U .S

. High elevation sites are



### 4 most susceptible to the intercontinental transport of pollution , particularly during spring

. Surface

PRB O 3 contributions a re much smaller than those derived in the free troposphere because of



### 6 dilution and chemical destruction during downward transport to the surface

. There are no instances



### 7 where direct observation of PRB contributions from anthropogenic emissions outside North America



### 8 are directly observable ; careful screening of ob servations and application of photochemical models



### 9 must be used instead to estimate anthropogenic contributions to PRB from sources outside North

America.



## 2.1.3.2. Contributions from the s tra tos phere

11 O zone is produced naturally by photochemical reactions in the str atosphere and s ome of this



### 12 O 3 is transported downward into the troposphere throughout the year in a process known as



### 13 tropopause folding

. Maximum stratospheric contributions occur during late winter and early spring

, 

### 14 particularly behind cold front s that mix tropospheric and stratospheric air

. Stratospheric intrusions
that reach the surface are rare. Much more common are intrusions which penetrate only to the middle



### 16 and upper troposphere. However, O 3 transported to the upper and middle troposphere can still a ffect



### 17 surface concentrations through various exchange mechanisms that mix air from the free troposphere



### 18 with air in the planetary boundary layer

. T here is considerable uncertainty in the magnitude and



### 19 distribution of this potentially important source of tr opospheric O 3

.



## 2.1.3.3. Natural s o urces o f prec urs o rs to PRB Ozone forma tion

Biogenic sources of VOC and CO emissions contribute to precursors to PRB O 3 formation

. 

### 21 These sources were discussed above in Section 2.1.1.1 with further details in Section 3.2.



### 22 Biomass bu rning in the form of wildfires and prescribed fires contribute to NO X , CO and



### 23 VOCs, precursors to PRB O 3 formation

. Biomass burning exhibits strong seasonality and interannual



### 24 variability

, with most biomass burned during the local dry season

. T here is cons iderable uncertainty
in attributing the fraction of wildfire emissions to human activities because the emissions from



### 26 naturally occurring fires that would have been present in the absence of fire suppression practices are



### 27 not known.



### 28 Lightning is also a sou rce for NO X production

. Although total column estimates of lightning



### 29 produced NO X are substantial, this source does not contribute substantially to the NO X burden in the
continental boundary layer

. This is because only 2% of NO X production by lightning occ urs within



### 31 the boundary layer and most occurs in the free troposphere where much of the NO X produced is



### 32 converted to more oxidized nitrogen species during downward transport.

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## 2.1.3.4. Es tima ting PRB Concentra tions

There are two approaches to estimating PRB concentrations that have been considered thus

far

. The first involves using measurements and the second the use of chemistry

- transport models

. T he 2006 O 3 AQCD (Section 3.9) (U.S. EPA, 2006, 088089 ) noted that estimates of PRB
concentrations cannot be obtained solely by examining measurements of O 3 obtained at relatively

remote monitor ing sites in the U.S. because of the long

- range transport from anthropogenic source
regions within North America

. T he 2006 AQCD also noted that it is impossible to determine sources

of O 3 without ancillary data that could be used as tracers of sources or t o calculate photochemical
production and loss rates

. Furthermore, the use of monitoring data is limited to the edges of the

domain of interest because PRB O 3 entering from outside North America is destroyed over North
America either through chemical reacti ons or by deposition

. Within North America, PRB O 3 is only
produced by natural sources

. T herefore, t he current definition of PRB implies that only CTMs can be
used to estimate the range of PRB values

. A further advantage to using models is that the entire range

of O 3 concentrations in different environments can be used to evaluate model performance.
However, there may be specific instances such as stratospheric intrusions that occur on spatial scales
too fine to be resolved by the current generation of glob al CTMs.

E stimates of PRB concentrations for April

- May, 2001 ( Figure 3 -9 ) and June

- August, 2001

( Figure 3

- 10 ) from the GEOS

- Chem model used in the 2006 O 3 AQCD (U.S. EPA, 2 006, 088089 )

a re described in Section 3.4

. These e stimates indicate d that PRB O 3 concentrations in the U.S.

surface air were generally 15

- 35 ppb from June th rough August. Concentrations decline from spring
to summer and are generally <25 ppb under conditions conducive to high O 3 episodes

. PRB O 3 concentrations may be higher, especially at high altitude sites during the spring, due to enhanced

contributions fro m ( 1 ) pollution sources outside North America ; and ( 2 ) stratospheric O 3 exchange.

S imulated monthly mean concentrations in different quadrants of the U.S. are typically within

5 ppb v of observations at remote CASTNET sites, with no significant bias, except in the Southeast

in summer when the model is 8

- 12 ppb v too high . This bias might be d ue to excessive background

O 3 transported in from the Gulf of Mexico and the tropical Atlantic Ocean or to inaccuracies in

emissions inventories within the U.S

. The model reproduced the occurrences of relatively high O 3 at

r emote sites , and shows that these can generally be explained by North American pollution.
Although many of the features of the day

- to

- day variability of O 3 at relatively remote
monitoring sites in the U .S. are simulated reasonably well, uncertainties in the calculation of the
temporal variability of O 3 originating from different sources on shorter time scales must be
recognized. The uncertainties stem in part from an underestimate in the seasonal variabi lity in the

STE of O 3 , the geographical variability of this exchange, and the variability in the exchange between
the free troposphere and the planetary boundary layer in the model. In addition, the relatively coarse
spatial resolution in the model (2° × 2.5 °) limited the ability to provide separate estimates for cities
located close to each other, and so only regional estimates were provided for the 2006 O 3 AQCD

(U.S. EPA, 20 06, 088089 ).

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## 2.1.4. Mo nito rin g

2.1.4.1. Ro utine Mo nitorin g Te c hnique s

1 The Federal Reference Method (FRM) for O 3 measurement is called the Chemiluminescence



### 2 Method (CLM) and is based on the detection of chemiluminescence resulting from the reaction of O 3 

### 3 with ethylene gas

. The first ultraviolet (UV) absorption photometric analyzers were approved as



### 4 Federal Equivalent Methods (FEMs) in 1977 and gained rapid acceptance for NAAQS compliance
purposes due to ease of operation, relatively low cost, and reliability

. Almost all of the s tate or local



### 6 air monitoring stations (SLAMS) that reported data to EPA Air Quality System ( AQS ) from 2005 to



### 7 2009 used UV absorption photometer FEMs an d g reater than 96% of O 3 monitors met precision and



### 8 bias goal s during this period. The rationale, history, and calibration of O 3 measurements were



### 9 summarized in the 1996 O 3 AQCD (U.S. EPA, 1996, 017831 ) and the 2006 O 3 AQCD (U.S. EPA,

2006, 088089 ) and focused on the state of ambient O 3 measurements at that time as well as



### 11 evaluation of interferences and new developments

. Section 3.5 in this review includes the current



### 12 state of O 3 measu rements, interferences, and new developments for the period 2005

- 2009

. 

### 13 Satellite observations for O 3 are growing as a resource for many purposes, including model



### 14 evaluation, assessing emissions reductions, pollutant transport, and air quality management

. S atellite
remote sensing instruments do not directly measure the composition of the atmosphere

. Satellite



### 16 retrievals are conducted using the solar backscatter or thermal infrared emission spectra and a variety



### 17 of algorithms

. Most satellite measurement syste ms have been developed for stratospheric



### 18 measurement of the total O 3 column. Mathematical techniques have been developed and must be



### 19 applied to derive information from these systems about tropospheric O 3

.



## 2.1.4.2. Ambient Ozone Ne twork Des ign

To support the NAAQS, state and local monitoring agencies must operate O 3 monitors at



### 21 various locations depending on the area size and typical peak concentrations (expressed in



### 22 percentages below, or near the O 3 NAAQS)

. SLAMS make up the ambient air quality monitoring



### 23 sites that are primarily needed for NAAQS comparisons and include Photochemical Assessment



### 24 Monitoring Stations (PAMS), National Core (NCore), and all other State or locally

- operated stations
except for the monitors designated as special purpose monitors (SPMs)

. 

### 26 In 2 00 9 , there were 12 08 SLAMS O 3 monitors reporting values to the EPA AQS database



### 27 ( Figure 3 -16 ). Since O 3 levels decrease significantly in the colder parts of the year in many areas, O 3 

### 28 is required to be monitored at SLAMS monitoring sites only during the “O 3 season .” PAMS provides



### 29 more comprehensive data on O 3 in areas classified as serious, severe, or extreme nonattainment for

O 3

. There were a total of 119 PAMS reporting values to the EPA AQS data base in 200 9. NCore is a



### 31 new multi -pollutant monitoring netwo rk currently being implemented to meet multiple monitoring



### 32 objectives

. Ea ch state is required to operate at least one NCore site and the network will consist of



### 33 about 60 urban and 20 rural sites nationwide.

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1 The Clean Air Status and Trends Network (CASTNET) is a regional monitoring network



### 2 established to assess trends in acidic deposition and also provides concentration measurements of O 3

. 

### 3 CASTNET O 3 monitors operate year round and are primarily located in rural areas

. At the beginning



### 4 of 2010, there were 80 CASTNET sites located in, or near, rural areas

. The NPS also operates a
Portable Ozone Monitoring Systems (POMS) network

. The POMS couples the small, low

- power O 3 

### 6 monitor with a data logger, meteorological measurements, and solar power in a self contained system



### 7 for monitoring in remote locations . Twenty NPS POMS reported O 3 data to AQS in 20 10. A map of



### 8 the current and proposed rural NCore sites, along with the CASTNET, and the NPS POMS sites is



### 9 shown in Figure 3 -17.



## 2.1.5. Ambient Concentrations

Ozone is the on ly photochemical oxidant other than NO 2 that is routinely monitored and for



### 11 which a comprehensive database exists. Data for other photochemical oxidants typically have been



### 12 obtained only as part of special field studies. Most continuous O 3 monitors report hourly average



### 13 concentrations. This data can be used as reported 1

- h avg, or reported as a daily metric such as: (1)



### 14 the average of the hourly observations over a 24

- h period (24

- h avg); (2) the maximum hourly
observation occurring in a 24

- h period (1

- h da ily max); and (3) the maximum 8

- h running average of



### 16 the hourly observations occurring in a 24

- h period (8

- h daily max).



### 17 Section 3.6.1 includes an analysis of U.S. O 3 data reported to AQS between 2007 and 2009.



### 18 The median 1

- h daily max, 8

- h daily max, and 24

- h avg O 3 concentrations across all sites were 44,



### 19 40, and 29 ppb, respectively. The 98th percentiles of these same metrics across all sites were 86, 74,

and 55 ppb, respectively. The 8

- h daily max and 1

- h daily max metrics were highly correlated



### 21 (median r = 0.97, IQR = 0.96

- 0.98) while comparisons with the 24

- h avg metric were lower (e.g.,



### 22 median r = 0.83, IQR = 0.78

- 0.88 for comparison between the 24

- h avg and the 1

- h daily max). The



### 23 ratio and correlation between these metrics, however, can be very site

- specific.



## 2.1.5.1. Urba n

- Foc us ed S patial Va riability

24 AQS O 3 concentrations were used to investigate urban

- focused spatial variability in

Section 3.6.2. Figure 3 -22 contains the county

- scale 8 -h daily max O 3 concentrations from the



### 26 highest monitor within each U.S. county for 2007

- 2009 (top map) with seasonal stratification



### 27 (bottom 4 maps). This map is only meant to illustrate the general spatial and temporal distribution in



### 28 nationwide O 3 concentrations, and is limited by monitor availability, resulting in the majori ty of U.S.



### 29 counties not having available data. Furthermore, this map is not representative of O 3 concentrations
at all locations or times within the counties shown; considerable spatial variability can and does exist



### 31 within a county.



### 32 The highest 3 -yr avg ( 2007

- 2009) 8 -h daily max O 3 concentrations ( ≥ 50 ppb ) , shown in



### 33 Figure 3 -22, occur in counties in south ern California, Arizona, Colorado and Tennessee

. The lowest



### 34 monitored 3 -yr avg 8 -h daily max O 3 concentrations (<30 ppb ) occur in Pacific Coast counties in

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1 north ern California and Washington

, as well as in two n ortheastern counties in Pennsylvania and



### 2 Massachusetts. The seasonally

- stratified county

- scale maps in the lower half of Figure 3 -22 illustrate



### 3 the strong seasonality in 8 -h daily max O 3 concentrati ons.


### 4 To investigate urban

- scale variability, 20 focus cities were selected for closer analysis here and

in Section 3.6.2; these cities were selected based on their importance in O 3 epidemiology studies and



### 6 on their geographic distribution across the U.S. T he warm season (May

- September) distribution of



### 7 the 8 -h daily max O 3 concentrations from 2007

- 2009 for the 20 focus cities is included in Table 3

- 10

. 

### 8 Box plots of the distribution of 2007

- 2009 warm

- season 8 -h daily max O 3 data from each



### 9 individual monitor in the 20 focus cities were used in Section 3.6.2 to investigate individual city
variability in O 3 concentrations. Several cities had relatively little spatial variability in 8 -h daily max



### 11 O 3 concentrations (e.g., correlations ranging from 0.61 to 0.96 in A tlanta) while other cities exhibited



### 12 considerably more variability in O 3 concentrations (e.g., correlations ranging from

- 0.06 to 0.97 for



### 13 Los Angeles ). The negative and near

- zero correlations in Los Angeles were between monitors with a



### 14 relatively large se paration distance (>150 km), but even some of the closer monitor pairs were not
very highly correlated. Similar to the correlation, the coefficient of divergence (COD) was found to



### 16 be highly dependent on the urban area under investigation. As a result, cau tion should be observed in



### 17 using data from a sparse network of ambient O 3 monitors to approximate community

- scale



### 18 exposures.



## 2.1.5.2. Rural

- Focus e d Spa tial Variability

19 AQS O 3 data for monitors located within six rural monitoring sites were used in Section 3.6.2

to investigate rural -focused O 3 concentration variability. These rural monitoring sites tend to be less



### 21 directly affected by obvious anthropogenic pollution sources than urban sites. However, they can be



### 22 regularly affected by transport of O 3 or O 3 precursors from upwind urban areas, or by local



### 23 anthropogenic emissions within the rural areas such as emissions from motor vehicles, power



### 24 generation, biomass combustion, or oil and gas operations. As a result, monitoring data from these

rural locations are not unaf fected by anthropogenic emissions.



### 26 Box plots of 8 -h daily max O 3 concentrations measured at the 6 rural monitoring sites during



### 27 the warm season (May

- September) between 2007 and 2009 are shown in Figure 3 -37. The sites



### 28 include one in Adirondack State Park ( ADSP) on Whiteface Mountain in Upstate NY, one in Mount



### 29 Mitchell State Park (MMSP) in NC, five in Great Smoky Mountain National Park (SMNP) in NC
and TN, one in Rocky Mountain National Park (RMNP) in CO, one in San Bernardino National



### 31 Forest (SBNF), CA, an d two in Sequoia National Park (SENP), CA. Within SMNP, the median



### 32 warm

- season 8 -h daily max O 3 concentration ranged from 47 ppb at the lowest elevation site



### 33 (elevation = 564 m; site ID = 470090102) to 60 ppb at the highest elevation site (elevation = 2021 m;


### 34 site ID = 471550102), with correlations between the 5 sites ranging from 0.78 to 0.92 and CODs
ranging from 0.04 to 0.16. The correlation between the 2 sites in SENP was 0.86 and the COD was



### 36 0.09. A host of factors may contribute to variations observed at these rural sites, including proximity



### 37 to local O 3 precursor emissions, variations in boundary

- layer influences, meteorology and

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1 stratospheric intrusion as a function of elevation, and differences in wind patterns and transport



### 2 behavior due to local to pography. Expanded analyses of O 3 concentrations measured using the more



### 3 rural

- focused CASTNET monitoring network are included in Chapter 9

. 

### 4 Since O 3 produced from emissions in urban areas is transported to more rural downwind

locations, elevated O 3 concen trations can occur at considerable distances from urban centers. In



### 6 addition, major sources of O 3 precursors such as highways, power plants, biomass combustion, and



### 7 oil and gas operations are commonly found in rural areas, adding to the O 3 in these areas. Due to



### 8 lower chemical scavenging in nonurban areas, O 3 tends to persist longer in rural than in urban areas



### 9 which tends to lead to higher cumulative exposures in rural areas influenced by anthropogenic
precursor emissions. The persistently high O 3 concentr ations observed at many of these rural sites



### 11 investigated here indicate that cumulative exposures for humans and vegetation in rural areas can be



### 12 substantial and often higher than cumulative exposures in urban areas.



## 2.1.5.3. Na tiona l Tre n ds

13 Nationally, O 3 concentr ations have declined over the last decade, as shown in Figure 3 -41



### 14 from the 2010 National Air Quality Status and Trends report (U.S. EPA, 2010, 647278 ) . The
majority of this decline occurred before 2004 with national average concentrations remaining



### 16 relatively flat between 2004 and 2008. The large decreases in 2003 and 2004 coincides wi th NO X 

### 17 emissions reductions resulting from implementation of the NO X State Implementation Plan (SIP)



### 18 Call rule, which began in 2003 and was fully implemented in 2004. This rule was designed to reduce



### 19 NO X emissions from power plants and other large combusti on sour ces in the eastern U.S.

As noted in the 2006 O 3 AQCD (U.S. EPA, 2006, 088089 ) , trends in national parks and rural



### 21 areas are similar to nearby urban areas, reflecting the regional nature of O 3 pollution. However,



### 22 caution should be exercised in using trends calculated at national parks to infer contributions from



### 23 distant sources either inside or outside of North America because of the influence of regional



### 24 pollution.



## 2.1.5.4. Ho urly Va ria tions

O zone concentrations show a strong degree of diel variability resulting from daily patterns in



### 26 temperature, sunlight, and precursor emissions . Other factors, such as the relative importance of



### 27 transport versus local photochemical production and loss rates, the timing for entrainment of air from



### 28 the nocturnal residual boundary layer, and the diurnal variability in mixing layer height also play a



### 29 role in daily O 3 patterns. Urban diel variations investigated in Section 3.6.3.2 of this assessment
show no substantial change in patterns since the 2006 O 3 AQCD (U.S. EPA , 2006, 088089 ) . The 1 -h



### 31 max concentrations tend to occur in mid

- afternoon and 1 -h min concentrations tend to occur in early



### 32 morning, with more pronounced pe aks in the warm months relative to the cold months. Diel patterns



### 33 in O 3 have remained stable over the last 20 y ea rs , with times of occurrence of the daily maxima



### 34 varying by no more than an hour from year to year. There is city

- to

- city variability in these times,

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### 1 however, and caution is raised in extrapolating results from one city to another in determining the



### 2 time of day for O 3 maxima and minima.



## 2.1.5.5. As s ocia tions with Co

- pollutants

3 Since O 3 is a secondary pollutant formed in the atmosphere from precursor emiss ions, it is not



### 4 expected to be highly correlated with primary pollutants such as CO and NO X . Furthermore, O 3 

### 5 formation is strongly influenced by meteorology, entrainment, and transport of both O 3 and O 3 

### 6 precursors, resulting in a broad range in correlations with other pollutants which can vary



### 7 substantially with season. To investigate correlations with co -pollutant s, 8 -h daily max O 3 was


### 8 compared with co

- located 24 -h avg CO, SO 2 , NO 2 , PM 2.5 and PM 10 obtained from AQS for



### 9 2007

- 2009. Figure 3 -43 contains co -p ollutant box plots of the correlation between co -located



### 10 monitors for the year

- round data set and broken down by season.



### 11 The year

- round 8 -h daily max O 3 data exhibited a very wide range in correlations with all the



### 12 24 -h avg co

- pollutants. A clearer pattern emerged when the data are stratified by season with mostly



### 13 negative correlations in the winter and mostly positive correlations in the summer for all co



### 14 pollutants. The median seasonal correlations are modest at best with the highest positive correlation



### 15 at 0.52 for PM 2.5 in the summer and the highest negative correlation at

- 0.38 for PM 2.5 in the winter.



### 16 Spring and fall lie in between with spring having a slightly narrower distribution than fall for all co



### 17 pollutants. Expanded discussion of co

- pollutant c orrelation can be found in Section 3.6.4.



# 2.2. Human Expos ure

2.2.1. Expos ure Me as u re me n t

2.2.1.1. Me a s ure me n t o f Ozone Exposure

18 Passive badge samplers are the most widely used technique for measuring personal O 3 

### 19 exposure. The y operate on the nitrite

- nitrate conversion principle, and are convenient since they



### 20 require no pumps or wet chemistry in the field. They represent a cumulative (rather than continuous)



### 21 sample, and their detection limit makes them suitable for monitoring periods of 24 h ours or greater.



### 22 This limits their ap plicability in measuring short

- term daily fluctuations in personal exposure. Over a



### 23 24

- h period, the detection limit o f the badges is approximately 5

- 10 ppb , which may result in an



### 24 appreciable fraction of the samples being below the detection limit. An act ive sampler based on the



### 25 nitrite

- nitrate conversion reaction is also available, with a reported detection limit of 10 ppb -h,



### 26 enabling measurement of sub

- daily O 3 concentrations. A portable continuous O 3 monitor based on a



### 27 different principle, UV absorption , has recently become available. Its size and weight make it



### 28 suitable for use in a backpack configuration, although its use for personal exposure measurements



### 29 has been limited.

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39 Several studies described in the 2006 O 3 AQCD, along with a few new studies pub lished since,
describe the relationship between indoor O 3 concentration and the O 3 concentration immediately

outside the indoor microenvironment. These studies show that the indoor concentration is often
substantially lower than the outdoor concentration unless indoor sources are present. Low indoor O 3 concentrations can be explained by reaction of O 3 with surfaces and airborne constituents. However,

the indoor

- outdoor relationship is greatly affected by the air exchange rate; under conditions of high

air e xchange rate, such as open windows, the indoor O 3 concentration may approach the outdoor

concentration. In residential microenvironments, studies report indoor

- outdoor ratios ranging from
approximately 0.1

- 0.4, with the highest ratios observed in the summe r O 3 season and for homes with
increased window ventilation. A correlation of 0.58 was reported between indoor and outdoor O 3 concentrations, indicating that variations in outdoor concentration may be reflected indoors, though
the magnitude of the concentr ation is lower. Indoor

- outdoor ratios at schools were similar, with
higher ratios observed during the school day when opening doors and windows may lead to
increased air exchange rates. In vehicles, high air exchange rates that would normally lead to high

interior- exterior concentration ratios are offset by O 3 scavenging through vehicle

- emitted NO,
resulting in reported in

- vehicle concentrations that were approximately 50% of those measured at the

roadside.
The relationship between personal exposure and amb ient O 3 concentrations has been evaluated
in several research studies, many of which were conducted prior to 2005 and are discussed in the

2006 O 3 AQCD. The results of these studies indicate that personal exposures are moderately well

correlated with ambie nt concentrations, and that the ratio of personal exposure to ambient
concentration is higher in outdoor microenvironments and during the summer season. In situations
where a lack of correlation was observed, this may be due in part to a high proportion of personal

measurements below the detection limit. Correlations reported for daily or multi

- day measurements
range from approximately 0.3

- 0.8, with the upper end of the range reflecting longer

- duration (4 -day)
community average measurements that may limit t he influence of inter

- individual variability in

exposure. Hourly measurements in specific microenvironments show greater variability in
correlations between personal exposure and ambient concentration, with residential indoor

correlations <0.1 and outdoor correlations of 0.7 -0.9. Slopes from regression analyses of personal
exposure on ambient concentration generally ranged from approximately 0.1 -0.3. Higher slopes were
observed in studies that either adjusted for activity pattern and air exchange rate (0.54 ) or focused on
outdoor shoe cleaners (0.56), who may have increased exposure due to spending a substantial
fraction of the day outdoors. Ratios of personal exposure to ambient concentration showed similar
results, with a ratio of 0.3 reported for a year -r ound study in southern California, while ratios ranged

from 0.28

- 0.96 for outdoor workers, increasing with time spent outdoors.
Taken together, results from previous and recently published studies indicate that while the
relationship between personal exposures and ambient concentrations varies due to a number of
factors, such as activity patterns, housing characteristics, and season, O 3 concentrations measured at

central

- site monitors are representative of day

- to

- day changes in average personal O 3 exposure,

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1 which is the important parameter for time

- series epidemiologic studies. Another important finding is



### 2 that the magnitude of personal exposures is smaller than concentrations reported at fixed

- site



### 3 monitors due to time spent indoors and the low indoor penet ration of O 3

.



## 2.2.1.2. Co

- Exposure to Ozone a nd Othe r Polluta nts

4 Individuals may be exposed to other pollutants in conjunction with exposure to O 3

. Personal

exposure to O 3 shows variable association with personal exposure to other pollutants, with



### 6 differences in as sociation depending on factors such as season, city

- specific characteristics, and



### 7 spatial variability of the co -pollutant . For PM 2.5 , a rank correlation of 0.14 was reported between



### 8 daily O 3 and PM 2.5 exposures during spring and fall in Atlanta. Positive s lopes were reported during



### 9 summer in both Baltimore and Boston, although the slopes were somewhat different (0.21 and 0.72,
respectively). The summertime slope in Baltimore was higher for children (0.37) than for adults



### 11 (0.07), which may be the result of d ifferent activity patterns and time spent outdoors. Additional



### 12 evidence of variation by season and city is provided by the differing signs of the wintertime slopes,



### 13 with Baltimore showing a negative slope and Boston showing a positive slope. Interindividua l



### 14 variability likely played a role as well, since both cities showed a wide range (including both
negative and positive values) for individual

- specific personal O 3 - PM 2.5 slopes. For EC and NO 2 , 

### 16 near

- zero correlations were reported with O 3 during spring an d fall in Atlanta. These extremely low



### 17 correlations for the traffic

- related and spatially variable pollutants EC and NO 2 contrast with the



### 18 higher correlation observed for PM 2.5 , a regional pollutant.



### 19 In near

- road and on

- road microenvironments, correlations between O 3 and traffic

- related
pollutants are moderately to strongly negative, with the most strongly negative correlations observed



### 21 for NO 2 (- 0.8 to

- 0.9). This is consistent with the chemistry of NO oxidation, in which O 3 is



### 22 consumed to form NO 2 . The mo re moderate negative correlations observed for PM 2.5 , UFP, and VOC



### 23 may reflect reduced concentrations of O 3 in more polluted environments due to other scavenging



### 24 reactions. A similar process occurs indoors, where infiltrated O 3 reacts with airborne or surf ace

associated materials to form secondary compounds, such as formaldehyde. Although such reactions



### 26 decrease indoor O 3 exposure, they result in increasing exposure to other species which may



### 27 themselves have health effects.



## 2.2.2. Expos ure Modeling

28 Exposure s estim ates in urban areas may be improved by constructing a concentration surface



### 29 over a geographic domain using a model to compensate for missing data

. The calculated
concentration surface can then be used to estimate exposures outside residences, schools,



### 31 work places, roadways, or other locations of interest. This technique does not estimate exposure



### 32 directly because it does not account for activity patterns or concentrations in different



### 33 microenvironments. Most such modeling efforts have focused on the less

- rea ctive pollutants PM or



### 34 NO 2 . In a study that extended CALINE4 NO X modeling results to evaluate the impact on residential

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1 O 3 concentrations, O 3 concentrations were reduced by 0.51 ppb O 3 per 1 ppb NO X

. This intra -urban



### 2 traffic

- related variability in O 3 conce ntrations suggests that differences in traffic density between the



### 3 central site monitor and individual homes could result in either an overestimate or underestimate of



### 4 residential O 3

. A separate class of models, known as microenvironmental models, estimat e time -weighted



### 6 exposure for modeled individuals by summing exposure in each microenvironment visited during the



### 7 exposure period. Stochastic microenvironmental models , such as APEX and SHEDS, utilize



### 8 distribution s of pollut ant- related and individual

- level variables, such as ambient and local O 3 

### 9 concentration contributions and breathing rate respectively, to compute the distribution of individual

exposure s across the modeled population

. The models also have the capability to estimate received



### 11 dose through a dosimetry model. Using distributions of input parameters in the model framework



### 12 rather than point estimates allows the models to incorporate uncertainty and variability explicitly into



### 13 exposure estimates

. For the APEX model, an analysis has been conducted indicating that the



### 14 uncertainty in model exposure estimates for asthmatic children during moderate exercise is small to
moderate; however, APEX appears to substantially underestimate the frequency of multiple high



### 16 exposure events for a single individual. M icroenvironmental models, such as EMI, are also being



### 17 developed to use individual

- specific information derived from measurements or questionnaires,



### 18 rather than population distributions, to estimate exposures. This approach is particularly suitable for



### 19 pane l health studies where information is available for each participant, and may reduce uncertainty
in health effect estimates by improving exposure estimates.



## 2.2.3. Implic a tio ns fo r Epide miolo gic Stu die s

21 Exposure error can be an important contributor to variabilit y in epidemiologic study results

, 

### 22 although this may be less of an issue for O 3 because it is a secondary pollutant with relatively low



### 23 spatial variability across an urban area

. For example, an epidemiologic study in Atlanta observed



### 24 similar associations be tween heart rate variability (HRV) parameters and either ambient
concentrations or personal exposures of O 3 and PM 2.5 , another regional pollutant. The importance of



### 26 exposure error varies with study design and is dependent on the spatial and temporal aspect s of the



### 27 design. Several factors that could influence exposure estimates include nonambient exposure, spatial



### 28 and temporal variability, and the presence of O 3 in a mixture of pollutants. Nonambient exposure is



### 29 unlikely to influence health effect estimates because of the lack of indoor O 3 sources and because

indoor

- generated O 3 exposures are unlikely to be correlated with ambient O 3 exposure. Compared



### 31 with directly emitted pollutants such as CO and NO X , O 3 exhibits relatively low spatial variability



### 32 across u rban areas, as discussed in Chapter 3. Averaging data from a large number of samplers will



### 33 dampen intersampler variability , and use of multiple monitors over smaller land areas may allow for



### 34 more variability to be incorporated into an epidemiologic analysi s. Evidence from a study comparing
the effect of spatial variability on effect estimates for O 3 , PM 2.5 , NO 2 , and CO suggests that choice of



### 36 monitor for spatially homogenous pollutants such as O 3 may have little impact on the results of



### 37 epidemiologic studies. Season, however, may have a substantial effect due to much lower O 3

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1 concentrations during the winter, along with the higher correlations between ambient concentrations



### 2 and personal exposures observed during the summer. Studies conducted during the O 3 se ason or in



### 3 periods when communities are likely to have high air exchange rates (e.g., during mild weather) may



### 4 be less prone to exposure error than studies conducted only during winter. Year

- round studies that

include both the O 3 and non -O 3 seasons may hav e an intermediate level of exposure error. Exposure



### 6 to mixtures of pollutants containing O 3 also complicates interpretation of epidemiologic results.



### 7 Moderate to strong negative correlations between O 3 and traffic

- related pollutants, particularly NO 2 , 

### 8 make it difficult to determine to what extent O 3 - based effect estimates quantitatively reflect the



### 9 independent effect of O 3 itself, or the effect of another pollutant or pollutants in the mixture.
Interpretation of O 3 effects in the presence of PM is additiona lly complicated by the highly variable



### 11 correlations observed, which differ by city, season, and population characteristics (e.g., children



### 12 versus adults). Although these sources of exposure error should be considered in evaluating



### 13 epidemiologic results, previous and recently published exposure research indicate that O 3 

### 14 concentrations measured at central

- site monitors are indicative of day

- to

- day changes in average
personal O 3 exposure, making ambient concentrations a useful parameter for epidemiologic studi es.



# 2.3. Do s ime try a n d Mo d e o f Ac tio n

16 O zone is a highly reactive and poorly water soluble gas allowing it to penetrate into targets in



### 17 the lower respiratory tract. The fact that it is so chemically reactive suggests that the effective dose at



### 18 target sites exist s in the form of secondary oxidation products such as aldehydes and peroxides.



### 19 Reaction products are formed when O 3 interacts with components of the extracellular lining fluid
(ELF) such as lipids, proteins, and antioxidants . Ozone uptake relates directly to these ELF substrate



### 21 reactions and is termed 'reactive absorption'. The level and type of antioxidants varies between



### 22 species, regions of the respiratory tract itself, and can be altered by O 3 exposure. ELF constituents



### 23 appear in most cases to limit inte raction of O 3 with underlying tissues and to prevent penetration of



### 24 O 3 deeper into the lung. However, in some cases, the antioxidants and secondary oxidation products
formed in the aqueous phase might penetrate into the cells and cause injury . Ozone toxici ty is



### 26 observed to some extent in the nasal cavity, however further toxicity exists in the deep lung where



### 27 the ELF thickness narrows allowing O 3 to react directly with the epithelial cells and surface



### 28 macrophages.



## 2.3.1. Hu ma n a nd Animal Ozone Dos imetry

29 O 3 uptake efficiency is chemical

- reaction dependent, driven by the conversion of O 3 to
reaction products. The primary site of O 3 uptake and greatest O 3 dose in the lungs is the centriacinar



### 31 region (CAR), containing the respiratory bronchioles. Recent studies have pr ovided evidence for hot



### 32 spots of O 3 flux around bifurcations in the airways. Ozone uptake is 80

- 95% efficient in humans and



### 33 approximately 54% efficient in rats. The nasopharyngeal region provides defense against O 3 entering



### 34 the lungs and removes ~50% of th e absorbed O 3 in both species . Ozone uptake efficiency is sensitive

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1 to a number of factors. Fractional absorption will decrease with increased flow and increase



### 2 proportional to tidal volume (V T ). Increased breathing frequency (f B ) and oronasal breathing, a s


### 3 occurs during exercise, will shift the O 3 dose distribution deeper and lead to a greater dose to the



### 4 pulmonary region, increasing the potential of damage to bronchiolar and alveolar tissues. Individual
total airway O 3 uptake efficiency is also sensitive to large changes in O 3 concentration, exposure



### 6 time, and minute ventilation ( V E ) . Major sources of variability in absorption of O 3 include O 3 

### 7 concentration, exposure time, breathing frequency, minute volume, and tidal volume, but the



### 8 interindividual variat ion is the greatest source of variability uptake efficiency. However, to date, no



### 9 studies have shown that the large differences in biological response between subjects ( forced
expiratory volume in 1 sec [ FEV 1 ]

, bronchoalveolar lavage fluid [ BAL ], cell infl ammatory response,



### 11 etc.) are explainable by the differences in O 3 uptake

. 

### 12 Interspecies differences limit quantitative comparison between species ; however

, the acute and



### 13 chronic functional responses of laboratory animals to O 3 appear qualitatively homologo us to that of



### 14 the human making them a useful tool in determining mechanistic and cause

- effect relationships with

O 3 exposure. Recent studies have shown that varied O 3 response in different mouse strains was not



### 16 due to differences in delivered dose of O 3 to the lung but more likely genetic sensitivity. Dose



### 17 comparison between humans and rats shows that exercising humans accumulated 4

- 5 times higher



### 18 O 3 reactants in BAL compared to similarly exposed resting rats and it was necessary to expose



### 19 resting rats to 2 ppm O 3 to achieve increases in BAL protein and polymorphonuclear cells [ PMNs ]

similar to those of the 0.4 ppm exposed humans.



## 2.3.2. Po s s ible Pa thwa ys /Mo de s o f Ac tio n

21 Three distinct short

- term responses have been well

- characterized in humans challenged with



### 22 O 3 : decreased pulmonary function, airways inflammation, and increased bronchial reactivity. In



### 23 addition, evidence has been accumulating that O 3 exposure exacerbates, and possibly causes, asthma



### 24 and allergic airways disease in humans. Effects on the nasal airways and distal lung of humans,
including inflammation and injury, have also been described. Animal studies have demonstrated a



### 26 wide range of respiratory system effects. While the respiratory tract is the primary target tissue,



### 27 cardiovascular and other organ effects occur following short

- and long

- term exposures of animals to



### 28 O 3 . Mechanisms responsible for these effects are incompletely understood

. 

### 29 The initial key event in the O 3 toxicity pathway is the formation of secondary oxidation
products in the respiratory tract. Pathways for the removal of those products are also of great



### 31 importance. Due to the highly reactive nature of O 3 , direct reactions most likely involve components



### 32 of the ELF and/or plasma membranes of surface macrophages which extend beyond the ELF.



### 33 Reaction products likely mediate O 3 effects on respiratory tract epithelium.



### 34 A nother key event in the O 3 toxicity pathway is the activation of neural reflexes which leads to
decrements in pulmonary function. Evidence is accumulating that secondary ox idation products are



### 36 responsible for this effect. Eicosanoids have been implicated in humans while eicosanoids and



### 37 aldehydes are effective in animal models. Different receptors on bronchial C

- fibers have been shown

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39 to mediate separate effects of O 3 on pulm onary function. Nociceptor sensory nerves are involved in
the involuntary truncation of respiration which results in decreases in forced vi ta l capacity ( FVC )

, FEV 1 , tidal volume and an increase in respiratory frequency and pain upon deep inspiration. Opioi ds
block these responses while atropine does not. New evidence in an animal model suggests that
TRPA1 receptors on bronchial C

- fibers mediate this pathway. O zone exposure also results in
activation of vagal sensory nerves and a mild increase in airways obstruction measured as increased
specific airway resistance ( sRaw ) . Atropine and beta

- adrenergic agonists blocked this response in
one study indicating that the airway obstruction was due to bronchoconstriction. Other studies in
humans implicated SP release from bronchial C

- fibers resulting in airway narrowing due to either
neurogenic edema or bronchoconstriction. New evidence in an animal model suggests that the SP

NK receptor pathway caused bronchoconstriction following O 3 exposure . Considerable inter

indiv idual variability exists in O 3 responsiveness measured by decrements in pulmonary function.
Further, attenuation of these pulmonary function decrements occurs following O 3 exposure for
several consecutive days. Mechanisms responsible for these effects are not known but may be related
to inherent differences in neural sensitivity.
Injury and inflammation are additional key events in the O 3 toxicity pathway. Secondary
oxidation products have been implicated in a number of these processes. Although there may b e

inter

- species differences with respect to specific mediators, mechanisms involved in the acute

responses to O 3 include epithelial injury and airways neutrophilia. Longer

- term exposures may result
in mucus cell metaplasia of nasal epithelium or airways re modeling and fibrosis. Work from several
laboratories in humans and animal models suggest that O 3 triggers the release of tachykinins such as

SP from airway sensory nerves which could contribute to downstream effects including injury and

inflammation. New investigations show that O 3 exposure leads to the generation of hyaluronan
fragments which activate TLR4 and CD44

- dependent signaling pathways in macrophages and result
in a greater turnover of macrophage populations in the lung. Activation of these pathwa ys occurs later than the acute neutrophilic response suggesting that they may contribute to longer

- term effects

of O 3

. The mecha nisms involved in clearing O 3 - provoked inflammation remain to be clarified.
Similar to the pulmonary function responses, conside rable inter

- individual variability exists in O 3 responsiveness as measured by airways neutrophil i a. Further, attenuation of the inflammatory
response occurs following O 3 exposure for several consecutive days. However evidence suggests
that injury may conti nue despite the dampening of the inflammatory response during repeated
exposures. Mechanisms responsible for inter

- individual variability and response attenuation, or the
lack thereof, are not known. It should be noted that inflammation, as measured by air ways neutrophilia, is not correlated with decrements in pulmonary function as measured by spirometry.
Consequently, spirometric measures are not a good surrogate for the degree of inflammation in any
given individual following O 3 exposure. Furthermore, air ways neutrophilia may not be a good

indicator of O 3 - mediated lung injury.
Increased bronchial reactivity is a key event in the toxicity pathway of O 3 . It can be both a
rapidly occurring and persist e nt response, although adaptation can also occur during mul ti -day

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- mediated effects on smooth muscle
have been proposed to contribute to airway hyperresponsiveness ( AHR ) following O 3 exposure. Currently, more evidence has accumulated for the latter mechanism. In humans exposed to O 3 , atropine was found to block the early AHR response indicating the involvement of cholinergic
postganglionic pathways. Inhibition of arachidonic acid metabolism was ineffective in blocking this
response in humans while mixed results were found in animal models. Studies in O 3 -exposed
animals have demonstrated a role for SP release from bronchial C fibers in mediating neurally

mediated effects on smooth muscle. Later phases of increased bronchial reactivity may involve the

inducti on of interleukin ( IL )- 1beta which in turn upregulates SP production. In guinea pigs,
eosinophil- derived major basic protein contributed to the stimulation of cholinergic postganglionic
pathways. A novel role for hyaluronan in mediating the later phase eff ects of O 3 has recently been
demonstrated. High molecular weight polymers of hyaluronan normally found in t he ELF were
degraded following O 3 exposure in mice. The resulting hyaluronan fragments stimulated AHR in a

toll- like receptor ( TLR4 ) and CD44 recepto r- dependent manner. Previous work has shown that O 3 ­ mediated increases in lung permeability required a functioning TLR4 suggesting a possible
relationship between increased epithelial permeability and AHR in this model. Other cytokines and

chemokines have been implicated in the AHR response to O 3 in animal models.
Both older and more recent studies provide insight into the ability of O 3 to provoke asthma
exacerbations in humans. Greater airways inflammation and/or greater bronchial reactivity have

been dem onstrated in asthmatics compared to non

- asthmatics. This pre

- existing inflammation and
altered baseline bronchial reactivity may contribute to the enhanced bronchoconstriction seen in
asthmatics exposed to O 3 . Furthermore inflammation may contribute to O 3 - mediated AHR. Animal
studies have demonstrated a role for eosinophil

- derived proteins in mediating these effects. Since

air ways eosinophilia occurs in both allergic humans and allergic animal models, this pathway may

underlie the exacerbation of allergic a sthma by O 3 . In addition, differences have been noted in
epithelial cytokine expression in bronchial biopsy samples of healthy and asthmatic subjects. A Th2
phenotype, indicative of adaptive immune system activation and enhanced allergic responses, was

obs erved before O 3 exposure and was increased by O 3 exposure in asthmatics. Since eosinophilia is a
hallmark of a Th2 phenotype, these findings support links between allergic asthma, sensitivity to O 3 and adaptive immunity. Studies in humans and animal models also provide evidence for activation of
innate immunity by O 3 . In humans, O 3 exposure resulted in increased numbers of airways monocytes

and dendritic

- like cells. Altered expression of cell surface markers characteristic of innate immunity
and antigen pre sentation was observed on monocytes and macrophages. Recruitment of these
activated immune cells could lead to activation of allergen

- specific memory T cells in allergic
individuals and result in the exacerbation of existing asthma in response to an allergen trigger. In

animal studies, O 3 exposure primed the innate immune system and led to increased endotoxin

induced AHR by a mechanism involving hyaluronan and TLR4. The exaggerated immune response

to O 3 + endotoxin could lead to a more pronounced lung injur y response to a bacterial trigger.
Enhanced bronchial reactivity, airways eosinophilia, Th2 phenotype, recruitment of activated innate

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39 immune cells, and enhanced responsiveness to endotoxin all provide biological plausibility for
epidemiologic evidence of asthma exacerbations associated with exposure to O 3 . Thus, the influx of
immunomodulatory cells and the activation of innate and adaptive immunity leads to the
exacerbation of asthma and allergic responses which is emerging as a key event in the toxicity

p athway of O 3

. Recent studies in humans and animal models also provide evidence that O 3 exposure causes induction of AHR and allergic responses. Both activation of innate immunity and promotion of
adaptive immunity have been implicated. In humans, O 3 expos ure resulted in increased numbers of

dendritic

- like cells and levels of a cytokine associated with dendritic cell activation in the sputum,
suggesting the presence of a population of activated dendritic cells which could stimulate naïve

T- cells to promote the development of asthma. Evidence for activated dendritic cells was also found

in glutathione S transferase M1 ( GSTM1 ) null human subjects (S ection 5.2. 9 .1) and in allergen

sensitized animals exposed to O 3 . In the latter study, O 3 acted as an adjuvant fo r allergic sensitization
and the development of AHR by a mechanism involving TLR4. In a different animal model, O 3 ­ induced AHR required the presence of NKT cells and IL

- 17, both of which indicate innate immune
system activation . Ozone

- induced goblet cell m etaplasia has also been demonstrated. These findings
suggest that O 3 may be capable of causing new onset asthma and allergic responses in humans. Thus,
promotion of adaptive immunity and activation of innate immunity leads to the induction of AHR
and allergic responses which is emerging as a key event in the toxicity pathway of O 3

. Both older and more recent studies in animal models provide several mechanisms by which O 3 exposure could enhance susceptibility to lung infections. Both decreased mucociliary p article
clearance and decreased numbers and function of alveolar macrophage have been implicated. Recent
studies suggest that O 3 - mediated oxidation of SP

- A oxidation and priming of the innate immune
system may contribute to decreased pathogen clearance. Im mune dysfunction outside of the lung has

also been demonstrated. Thus

, immune system modulation is emerging as a key event in the O 3 toxicity pathway.

Studies in animals provide evidence for extrapulmonary effects of O 3 . Although it was
suggested that the se effects are directly mediated by secondary oxidation products formed in the lung

as a result of O 3 exposure, there is no evidence that these species enter the circulation. Alternatively,
extrapulmonary effects may be due to activation of neural reflexes or to release of diffusible
mediators which may initiate or propagate inflammatory responses in the vascular or systemic
compartments. Recent studies suggest that oxidative/nitrosative stress contributes to O 3 -induced

cardiovascular effects. Thus

, systemi c inflammation and vascular oxidative/nitrosative stress are
emerging as key events in the toxicity pathway of O 3

. Collectively

, older and more recent studies provide evidence for mechanisms which may

underlie the variability in responsiveness seen among individuals. Certain functional genetic
polymorphisms, pre

- existing conditions and diseases, lifestages and co

- exposures contribute to

enhanced susceptibility to O 3

. Adaptation may also be important, but it is incompletely understood

­
both in terms of the pathways involved and the resulting consequences.

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### 1 Overall, key events in the toxicity pathway of O 3 have been identified in humans and animal



### 2 models. They include the formation of secondary oxidation products in the lung, activation of neural



### 3 reflexes, pu lmonary injury and inflammation and increased bronchial reactivity. In addition,



### 4 evidence is accumulating that immune system modulation may lead to impaired host defense and the



### 5 exacerbation and/or induction of asthma and allergic responses

. Systemic infla mmation and vascular



### 6 oxidative/nitrosative stress may be critical to the extrapulmonary effects of O 3

.



# 2.4. Health Effects

7 This section evaluates the evidence from toxicological, controlled human exposure, and



### 8 epidemiologic studies that examined the health effe cts associated with short - and long

- term exposure



### 9 to O 3 . The results from the health studies evaluated in combination with the evidence from



### 10 atmospheric chemistry and exposure assessment studies contribute to the causal determinations made



### 11 for the health o utcomes discussed in this assessment ( Section 1.6 .4). In the following sections a



### 12 discussion of the causal determinations will be presented by exposure duration (i.e., short- or long



### 13 term exposure) for the health effects for which sufficient evidence was a vailable to conclude a



### 14 causal, likely to be causal or suggestive relationship . Although not presented in depth in this chapter,



### 15 a detailed discussion of the underlying evidence used to formulate each causal determination can be



### 16 found in Chapters 6 and 7.



## 2.4.1. Effe cts o f S ho rt

- Te rm Expo s ure to O zone

Table 2-1

. Summary of causal determinations for short

- term exposure to ozone

Outcome Causality Determination

Respiratory Effects Causal Relationship

Cardiovascular Effects Suggestive of a Causal Relationship

Cent ral Nervous System Effects Suggestive of a Causal Relationship Mortality Likely to be a Causal Relationship



## 2.4.1.1. Re s pira tory Effe c ts

17 The 2006 O 3 AQCD concluded that there was clear, consistent evidence of a causal



### 18 relationship between short

- term exposure t o O 3 and respiratory health effects (U.S. EPA, 2006,



### 19 088089 )

. This causal association was substantiated by the coherence of effects observed across



### 20 controlled human exposure, epidemiologic, and toxicological studies indicating associations of



### 21 short- term O 3 exposures with a range of respiratory health endpoints from respiratory tract



### 22 inflammation to respiratory hospital admissions (HA) and ED visits. Across disciplines, acute O 3 

### 23 exposures induced or were associated with statistically significant declines in lung function. An

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39 equally strong body of evidence from controlled human e xposure and toxicological studies

demonstrated O 3 - induced inflammatory responses, increased epithelial permeability, and airway
hyperresponsiveness (both specific and nonspecific). Toxicological studies provided additional

evidence for O 3 - induced impairmen t of host defenses. Coherent with inflammation and airway
hyperresponsiveness, epidemiologic studies consistently demonstrated positive associations of

increases in ambient O 3 concentrations with increases in respiratory symptoms and asthma

medication use in asthmatic children and with respiratory

- related hospital admissions and asthma

related emergency department ( ED ) visits. Although O 3 was consisten tly associated with

nonaccident al and cardiopulmonary mortality, the contribution of respiratory causes to these findings

was uncertain.
Building on the strong body of evidence presented in the 2006 AQCD, recent studies continue
to support associations between short

- term O 3 exposure and respiratory effects. In young healthy
adults exposed to O 3 for 6.6 h ours , s tudies demonstrate mean FEV 1 decrements of about 3% at
60 ppb

, 5% at 70 ppb

, and 6

- 8% at 80 ppb ( Section 6.2.1.2)

. These studies also show considerable
intersubject variability in responsiveness to O 3 , with the percentage of subjects with >10%

decrement in FEV 1 increasing with increasing concentration of O 3 exposure. The proportion

(uncorrected for filtered air [ FA ] responses) of individuals with >10% FEV 1 decrements ranges from

3 to 20% at an average O 3 exposure level of 60 ppb and from 17 to 29% at 80 ppb

. The collective body of epidemiologic evidence demonstrates associations between ambient O 3 and decrements in lung function, although recent studies contributed more mixed evidence. A
notable difference among newer studies is the limited investigation of populations engaged in
outdoor recreation, exercise, or work, which contributed to the strength of evidence in previous

AQCDs (U.S. EPA, 1996, 017831 ; U.S. EPA, 2006, 088089 ) . Some recent evidence suggests that
public attenti on to daily AQI may be reducing exposures of some groups. Recent epidemiologic
studies contributed insight into susceptibility factors for O 3 - associated respiratory morbidity. Among
subjects with atopy, asthmatics with concurrent respiratory infection, eld erly with AHR or obesity, or
groups with diminished antioxidant enzyme activity, lung function responses to ambient O 3 exposures generally were exacerbated. The susceptibility of these populations is supported by
extensive laboratory evidence (human and an imal) for O 3 - induced exacerbation of allergic
inflammation, increased susceptibility to bacterial and viral infections, exacerbation of O 3 -induced
AHR by obesity, and modulation of O 3 effects by the oxidative stress/antioxidant balance. In recent

controlle d human exposure studies, lung function responses to O 3 are enhanced in subjects with
higher body mass index ( BMI ).
As with lung function, recent controlled human exposure studies demonstrate increases in
respiratory symptoms in healthy, young adults following 5.6 - to 6.6- h exposure to O 3 at levels

<80 ppb. The collective body of epidemiologic studies strongly demonstrates positive associations of

ambient O 3 exposure with respiratory symptoms and asthma medication use among asthmatic
subjects, especially in populations with additional susceptibility factors such as asthmatics with
atopy, asthmatics with diminished antioxidant enzyme activity, or infants with asthmatic mothers.

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1 Recent studies in animals and in vitro models also continue to demonstrate O 3 -indu ced lung



### 2 injury and inflammatory responses. Building on the extensive experimental evidence, new



### 3 epidemiologic evidence emerged for ambient O 3 - associated increases in mediators of inflammation



### 4 measured in upper and lower airway samples, including eNO, cytokines such as IL

- 6 or IL

- 8, and
inflammatory cells such as eosinophils. Epidemiologic studies also report associations of increases in



### 6 ambient O 3 with decreased levels of glutathione and increased levels of malondialdehyde in airways.



### 7 At the time of the 20 06 O 3 AQCD, c ontrolled human studies of dietary antioxidant supplementation



### 8 had shown some protective effects of alpha- tocopher ol and ascorbate on lung function from O 3 

### 9 exposure, but not on the intensity of subjective symptoms and inflammatory response. Mo re recent
evidence indicates that diminished activity of oxidant metabolizing enzymes (e.g., GSTM1, GSTP1)



### 11 or intake of antioxidant vitamins influences inflammatory responses to O 3 exposure

. Across all three



### 12 disciplines, evidence suggests a role antioxidan t defenses in modulating responses to O 3

. 

### 13 Recent epidemiologic studies build upon the strong body of evidence that demonstrated



### 14 consistent positive associations between daily changes in O 3 exposure and respiratory

- related
hospital admissions and ED visits by demonstrating associations in diverse populations across the



### 16 U.S., Canada, and Europe

. In all

- year analyses, recent multicity studies and a multicontinent study



### 17 found an approximate 1.6

- 5.4% increase in all respiratory

- related hospital admissions and E D visits



### 18 for standardized increases in ambient O 3 concentrations 1 . Positive associations persisted in analyses



### 19 restricted to the summer season, but the magnitude varied depending on the study location.
Compared with studies reviewed in the 2006 O 3 AQCD, mo re recent studies examine associations



### 21 between short

- term O 3 exposure and specific respiratory outcomes . Although still limited in number,



### 22 both single

- and multicity studies found consistent, positive associations of daily changes in O 3 

### 23 concentrations with asthma and chronic obstructive pulmonary disease ( COPD ) hospital admissions



### 24 and ED visits. Evidence was more limited for pneumonia

. C onsistent with the conclusions of the

2006 O 3 AQCD (U.S. EPA, 2006, 088089 ) , i n studies that conducted seasonal analyses, larger effects



### 26 were estimated for the warm season or summer months than f or the cold season or for all seasons

, 

### 27 particularly for asthma and COPD . Although the current body of evidence did not include detailed



### 28 age

- stratified results, t he increased risk of asthma hospital admissions observed for children provide d



### 29 additional suppo rt for the conclusion from the 2006 O 3 AQCD that children are particularly
susceptible to O 3 - induced respiratory effects (U.S. EPA, 2006, 088089 )

. Among studies that



### 31 evaluated the potential confounding effects of co -pollutants

, O 3 effect estimates for respiratory



### 32 related hospital admissions and ED visits remained relatively rob ust upon the inclusion of PM and



### 33 gaseous pollutants in two

- pollutant models. Although the concentration

- response relationship



### 34 between short

- term O 3 exposure and respiratory

- related hospital admissions and ED visits has not
been extensively examined, prelim inary examination s found no evidence of a threshold between



### 36 short- term O 3 exposure and asthma hospital admissions and pediatric asthma ED visits

.

1 Effect estimates w ere standardized to a 20

- ppb increase for 24

- h avg O 3 , a 30

- ppb increase for 8

- h max O 3 , and a 40 ppb increase for 1

- h

max O 3

.

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38 New evidence extends the potential continuum of well

- established O 3 - associated respiratory
effects (e.g., air way inflammation; impaired host defense; lung function decrements; and respiratory
symptoms, ED visits, and hospital admissions) by demonstrating associations between ambient O 3 exposure and respiratory

- related mortality. The multicontinent APHENA study re ported primarily
positive associations with respiratory mortality in all

- year analyses, with stronger associations
observed in analyses restricted to the summer season. These findings were supported by U.S. and
European multicity studies, in which a majori ty of respiratory mortality effect estimates ranged from
a 2.3 to 6.8% increase per standardized increase in ambient O 3 concentrations. Although co -pollutant
confounding was not extensively examined, the O 3 - respiratory mortality relationship was moderately
to substantially sensitive (e.g., increased or attenuated) to inclusion of PM 10 in co

- pollutant models.

However, interpretation of these results requires caution due to the limited PM datasets used in these

studies

. In summary, new studies evaluated in th e current review support or expand upon the strong
body of evidence presented in the 2006 O 3 AQCD that short

- term O 3 exposure is causally associated
with respiratory health effects. Recent controlled human exposure studies demonstrate decreases in

FEV 1 in the range of 2.8 to 3.6% with prolonged O 3 exposures (6.6 h ours ) as low as 60 ppb in
concentration. By demonstrating O 3 - induced airway hyperresponsiveness, activation of neural
reflexes, allergic responses, lung injury, impaired host defense, and airway in flammation,
toxicological studies have characterized O 3 modes of action and provided biological plausibility for
epidemiologic observations of associations of ambient O 3 exposure with decreases in lung function
and increases in respiratory symptoms. The coherence of results across studies for O 3 - associated
changes in lung function, airway inflammation, and respiratory symptoms, in turn, provides the
biological plausibility for epidemiologic findings of consistently positive associations of ambient O 3

exposu re with respiratory hospital admissions and ED visits in diverse populations across the U.S.,
Europe, and Canada. Additionally, a multicontinent study and several multicity studies reported
positive associations between ambient O 3 exposures and respiratory mortality. New epidemiologic
studies provide evidence for associations of ambient O 3 exposure with biological markers of airway
inflammation and oxidative stress and indicated that groups with diminished antioxidant capacity or
comorbidities such as atopy , AHR, or obesity may have increased susceptibility to respiratory
morbidity associated with O 3 exposure. This new information is consistent with previously available
toxicological and clinical evidence as well as current information on modes of action. A common observation among epidemiologic studies of respiratory morbidity and mortality was stronger
associations in analyses restricted to warm seasons compared to cold seasons. Additionally, although

co

- pollutant confounding was evaluated infrequently, O 3 effect estimates generally remained
statistically significant in co

- pollutant models with PM 2.5 , PM 10 , or NO 2 . Collectively, the evidence
integrated across controlled human exposure, epidemiologic, and toxicological studies as well as
across the spectrum o f respiratory health endpoints continues to demonstrate that there is a causal
relationship between short

- term O 3 exposure and respiratory health effects.

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## 2.4.1.2. Ca rdiova s c ula r Effe c ts

1 In past O 3 AQCDs

, the effects of O 3 to the cardiovascular system could not be thoroughly



### 2 evaluated due to the paucity of informa tion available. However, in recent years, investigation of O 3 ­ 

### 3 induced cardiovascular events has advanced

. In general, compared with the epidemiologic evidence,



### 4 the toxicological evidence is more supportive of an O 3 - induced cardiovascular effect

. Epidemiologic
evidence does not consistently demonstrate a positive relationship between short

- term O 3 exposure



### 6 and cardiovascular morbidity. However, most epidemiologic studies have not extensively



### 7 investigated the cardiovascular effects of O 3 exposure in susceptible populations, which may further



### 8 support the toxicological findings. Although the epidemiologic evidence is limited, single -city



### 9 studies reviewed in the 2006 O 3 AQCD (U.S. EPA, 2006, 088089 ) , rece nt multicity studies, and the
multicontinent APHENA study provide evidence of consistent positive associations between short



### 11 term O 3 exposure and cardiovascular mortality. However, in contrast with respiratory effects, there is



### 12 weak coherence between the associations for cardiovascular morbidity and mortality. Further, there



### 13 is no apparent biological mechanism to explain the association observed between short

- term O 3 

### 14 exposure and cardiovascular mortality and not for cardiovascular morbidity.
Animal toxicological studies (O 3 concentration 0.5

- 0.8 ppm ) provide evidence for O 3 -induced



### 16 card iovascular effects, specifically enhanced ischemia/reperfusion injury, disrupted NO -induced



### 17 vascular reactivity , decreased cardiac function, and increased HRV. The observed increase in HRV is



### 18 supported by a recent controlled human exposure study that also finds increased high frequency



### 19 HRV, but not altered blood pressure, following O 3 exposure. Toxicological studies investigating the

role of O 3 in heart rate regulation are mixed with both bradycardic and tachycardic responses



### 21 observed. These changes in card iac function provide evidence for O 3 - induced alterations in the



### 22 autonomic nervous system leading to cardiovascular complications. Epidemiologic studies showing



### 23 positive associations between O 3 and arrhythmias confirm the development of autonomic



### 24 dysfunctio n following O 3 exposure. It is still uncertain how O 3 inhalation may cause systemic
toxicity, however the cardiovascular effects of O 3 found in animals correspond to the development



### 26 and maintenance of a extrapulmonary oxidative, proinflammatory environment

. 

### 27 Overall, animal toxicological studies provide stronger evidence for O 3 exposure leading to



### 28 cardiovascular morbidity than epidemiologic evidence which observed a lack of coherent evidence.



### 29 Based on the relatively strong body of toxicological evidence, an d the consistent evidence of an

association between O 3 and cardiovascular mortality, but weak coherence and biological plausibility



### 31 for O 3 - induced cardiovascular morbidity, the generally limited body of evidence is suggestive of a



### 32 causal relationship betwe en relevant short

- term exposures to O 3 and cardiovascular effects.



## 2.4.1.3. Ce n tra l Ne rvo us Sys te m Effe c ts

33 In rodents, O 3 exposure has been shown to cause physicochemical changes in the brain



### 34 indicative of oxidative stress and inflammation. Recent toxicological stu dies add to earlier evidence
that acute exposures to O 3 can produce a range of effects on the central nervous system (CNS) and

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1 behavior. Previously observed effects, including neurodegeneration, alterations in neurotransmitters,



### 2 short- and long

- term memory , and sleep patterns, have been further supported by recent studies. In



### 3 instances where pathology and behavior are both examined, animals exhibit decrements in behaviors



### 4 tied to the brain regions or chemicals found to be affected or damaged. For example, d amage in the
hippocampus, which is important for memory acquisition, was correlated with impaired performance



### 6 in tests designed to assess memory. Thus the brain is functionally affected by O 3 exposure. The



### 7 single epidemiolog ic study conducted showed that O 3 affects memory in humans as well, albeit on a



### 8 long

- term exposure basis. Notably, exposure to O 3 levels as low as 0.25 ppm has resulted in



### 9 progressive neurodegeneration and deficits in both short

- and long

- term memory in rodents.
Additionally, changes in the CNS, including biochemical, cellular, and behavioral effects, have been



### 11 observed in animals whose sole exposure occurred in utero , at levels as a low as 0.3 ppm . Although



### 12 evidence from epidemiologic and controlled human exposure studies is lacking, the toxicological



### 13 evidence for the impact of O 3 on the brain and behavior is strong, and is suggestive of a causal



### 14 relationship between O 3 exposure and adverse CNS effects.



## 2.4.1.4. Mo rtality

The 2006 O 3 AQCD concluded that the overall body of evidence was highly sugg estive that O 3 

### 16 directly or indirectly contributes to non

- accidental and cardiopulmonary

- related mortality, but



### 17 additional research was needed to more fully establish underlying mechanisms by which such effects



### 18 occur

. The evaluation of new multicity studies that examined the association between short

- term O 3 

### 19 exposure and mortality found evidence which supports the conclusions of the 2006 O 3 AQCD

. These
new studies reported consistent positive associations between short

- term O 3 exposure and total



### 21 (nonaccident al) mortality, with associations being stronger during the warm season, as well as



### 22 additional support for associations between O 3 exposure and cardiovascular mortality being similar



### 23 or larger in magnitude compared to respiratory mortality. Additionally, th ese new studies examined



### 24 previously identified areas of uncertainty in the O 3 - mo rtality relationship.

Recent studies further examined potential confounders (i.e., co

- pollutants and seasonality) of



### 26 the O 3 - mo rtality relationship (Section 6.6.2.1). Unlike previous studies that were limited to primarily



### 27 examining the confounding effects of PM 10 , the se studies expanded their analyses to include multiple



### 28 PM indices (i.e., PM 10 , PM 2.5 , and PM components). Co

- pollutant models found evidence that



### 29 associations betwe en O 3 and total mortality were robust to the inclusion of PM 10 or PM 2.5 , while

other studies found evidence for a modest reduction (~20

- 30%) when examining PM 10 and by age



### 31 group or cause

- specific mortality (i.e., cardiovascular). Additional reductions in O 3 - mo rtality risk



### 32 estimates were also observed when examining PM components, specifically sulfate, in co

- pollutant



### 33 models. Overall, the impact of PM indices on O 3 - mortality risk estimates was found to be much



### 34 smaller than the variation in O 3 - mortality risk estimates across cities. Although some studies suggest

that O 3 - mortality risk estimates may be confounded by PM or its chemical components, the



### 36 interpretation of these results requires caution due to the limited datasets used in these studies. When



### 37 examini ng the potential for seasonal confounding of the O 3 - mo rtality relationship it was observed

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29 that the extent of smoothing or the methods used for adjustment can influence O 3 risk estimates
because of the opposing seasonal trends of O 3 and mortality when not instituting enough degrees of

freedom (df) to control for temporal/seasonal trends

. The multicity studies evaluated in this review also examined the potential regional
heterogeneity in O 3 - mo rtality risk estimates ( Section 6.6.2.2). These studies provide e vidence which
suggests generally higher O 3 - mo rtality risk estimates in northeastern U.S. cities with some regions
showing no associations between O 3 exposure and mortality (e.g., S outhwest, u rban M idwest).
Multicity studies that examined individual

- and co mmunity

- level characteristics identified factors
that may explain the observed regional heterogeneity in O 3 - mo rtality risk estimates as well as
characteristics of populations potentially susceptible to O 3 - related health effects. An examination of
community

- level characteristics found an increase in the O 3 - mo rtality risk estimates in cities with
higher unemployment, percentage of the population Black/African

- American, percentage of the
working population that uses public transportation, lower temperatures, a nd lower prevalence of
central air conditioning.

Additional studies were evaluated that examined factors, such as multi

- day effects, mortality
displacement, adaptation, and whether a threshold exists in the O 3 - mo rtality relationship, which may

influence t he shape of the O 3 - mortality concentration

- response ( C-R) curve. An examination of
multiday effects in a U.S. and European multicity study found conflicting evidence for mortality
displacement, but the evidence suggest s that the positive associations betwe en O 3 and mortality are
observed mainly in the first few days after exposure. Additionally, a U.S. multicity study found
evidence of an adaptive response to O 3 exposure, with the highest risk estimates earlier in the O 3 season (i.e., July) and diminished e ffects later (i.e., August). However, the evidence of adaptive
effects has an implication for the interpretation of multi

- day effects, and requires further analysis.
Analyses that specifically focused on the O 3 - mo rtality C

- R relationship found no evidence of a
threshold, but did observe evidence for potential differences in the C

- R relationship across cities.

Overall, this evidence support s the 2006 O 3 AQCD which concluded that “if a population threshold

level exists in O 3 health effects, it is likely near the lower limit of ambient O 3 concentrations in the

U.S.” (U.S. EPA, 2006, 088089 ). Taken together, the body of evidence indicates that there is likely

to be a causal relationship between short

- term exposures to O 3 and all

- cause mortality

.

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## 2.4.2. Effe cts o f Lon g

- Te rm Expo s ure to O zone

Table 2-2. Summary of causal determinations for l ong
- term exposure to ozone

Outcome Causality Determination Respiratory Effects Likely to be a Causal Relationship

Cardiovascular Effects Suggestive of a Causal Relationship Mortality Suggestive of a Causal Relationship Reproductive and Developmental Su ggestive of a Causal Relationship Central Nervous System Effects Suggestive of a Causal Relationship Cancer, Mutagenicity, and Genotoxicity Inadequate to Infer a Causal Relationship



## 2.4.2.1. Re s pira tory Effe c ts

1 The epidemiologic studies reviewed in the 2006 O 3 AQCD detected no associations between



### 2 long

- term O 3 exposures and asthma

- related symptoms, asthma prevalence or allergy to common



### 3 aeroallergens among children after controlling for covariates. Little evidence was available to relat e


### 4 long

- term exposure to current ambient O 3 concentrations to deficits in the growth of lung function in



### 5 children. Additionally, limited evidence was available evaluating the relationship between long

- term



### 6 O 3 levels and pulmonary inflammation and other endpoints.



### 7 Recent studies ex amine the relationship between long

- term O 3 exposure and new onset asthma



### 8 in children ( Section 7.2.1). Studies have provided evidence for a relationship between different



### 9 genetic variants ( e.g., heme oxygenase [ HMOX ] , GST s , arginase [ ARG ] ) that , in combina tion with



### 10 O 3 exposure , are related to new onset asthma

. Studies using a cross

- sectional design provide support



### 11 for a relationship between long

- term O 3 exposure and health effects in asthmatics , including



### 12 bronchi tic symptoms and respiratory

- related school a bsences

. Additionally, c hronic O 3 exposure was


### 13 related to childhood asthma hospita l admissions

. 

### 14 Studies of long

- term exposure to O 3 and pulmonary function effects are inconclusive , with



### 15 some new epidemiologic studies relat ing effects at higher exposure lev els

. Information from



### 16 toxicological studies in adult and infant non

- human primates indicates that long

- term exposure to O 3 

### 17 during development can result in irreversible morphological changes in the lung along with changes



### 18 in pulmonary function.



### 19 The strong est evidence for a relationship between long

- term O 3 exposure and respiratory



### 20 morbidity in recent studies demonstrates associations between long

- term measures of O 3 exposure


### 21 and new

- onset asthma in children and increased respiratory symptom effects in asth matics. While the



### 22 evidence may be limited , these U.S. multi -community prospective cohort studies demonstrate that



### 23 asthma risk is associated with the important relationships between genetic variability, environmental



### 24 O 3 exposure, and behavior. These relatio nships are complex. The genes evaluated in these studies are



### 25 both key candidates in the oxidative stress pathway and have been shown to play an important role in

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1 asthma development. Reduced risk for asthma development is reported in some studies in childre n


### 2 living in low -O 3 communities. Other recent studies provide coherent evidence for long

- term O 3 

### 3 exposure and respiratory morbidity effects such as first asthma hospitalization and respiratory



### 4 symptoms in asthmatics. Studies considering other pollutants pro vide data supporting the notion that

the effects related to O 3 are independent from potential effects of the other pollutants. Some studies



### 6 provide evidence for a positive concentration

- response relationship. Generally, the epidemiologic



### 7 and toxicological evidence provides a compelling case that support s the hypothesis that a


### 8 relationship exists between long

- term exposure to ambient O 3 and me asures of respiratory morbidity.



### 9 The 2006 O 3 AQCD concluded the evidence was suggestive but inconclusive at that time . Building
upon that evidence, the more recent epidemiologic evidence, combined with toxicological studies in



### 11 rodents and non

- human primates, provides biologically plausible evidence that there is likely to be



### 12 a causal relationship between long

- term exposu re to O 3 and respiratory morbidity

.



## 2.4.2.2. Cardiovas cular Effec ts

13 Previous AQCDs did not address the cardiovascular effects of long

- term O 3 exposure

. The



### 14 evidence remains limited; however the emerging data is supportive of a role for O 3 in chronic

cardiovascular diseases

. Two epidemiologic studies have investigated cardiovascular morbidity after



### 16 long

- term O 3 exposure and both assessed cardiovascular disease related biomarkers

. A study of the



### 17 relationship between O 3 and cardiovascular mortality reported no association after adjustment for



### 18 PM 2.5 levels. Additional epidemiologic studies on cardiovascular morbidity and mortality after long



### 19 term exposure have not been published.
Toxicological evidence of long

- term O 3 exposure is also limited but three strong toxicologi cal



### 21 studies have been published since the previous AQCD. These studies provide evidence for O 3 

### 22 enhanced atherosclerosis and ischemia/reperfusion injury, corresponding with development of a



### 23 systemic oxidative, proinflammatory environment (Section 7.3.1.2). Although questions exist for



### 24 how O 3 inhalation causes systemic effects, a recent study proposes a mechanism for development of
vascular pathology that involves activation of LOX

- 1 by O 3 oxidized lipids. This activation may also



### 26 be responsible for O 3 induce d changes in genes involved in proteolysis, thrombosis, and



### 27 vasoconstriction. Taking into consideration the positive toxicological studies reported, the generally



### 28 limited body of evidence is suggestive of a causal relationship between relevant long

- term



### 29 ex posures to O 3 and cardiovascular effects

.



## 2.4.2.3. Re prod uc tive a nd De ve lo pme nta l Effe c ts

The 2006 O 3 AQCD concluded that the limited number of studies that investigated O 3 

### 31 demonstrated no associations between O 3 and birth outcomes, with the possible exception of b irth



### 32 defects

. The current review included an expanded body of evidence of the associations between O 3 

### 33 and reproductive and developmental effects

. Recent toxicological studies provide evidence for an



### 34 effect of prenatal exposure to O 3 on ultrastructural chan ges in bronchiole development, alterations in
placental and offspring cytokines, and increased offspring airway hyper -reactivity (Section 7.4.7).

March 2011 2- 29 DRAFT – DO NOT CITE OR QUOTE

### 5

10

15

20

25

30

1 Also, there is limited toxicological evidence for an effect of prenatal and early life exposure on



### 2 central ner vous system effects, including laterality, brain morphology, neurobehavioral



### 3 abnormalities, and sleep aberration (Section 7.4.8)

. Recent epidemiologic studies have begun to



### 4 explore the effects of O 3 on sperm quality, and provide limited evidence for decrements in sperm
concentration, while there is limited toxicological evidence for testicular degeneration caused by O 3 

### 6 (Section 7.4.2)

. There is no evidence that prenatal or early life O 3 concentrations are associated with



### 7 infant mortality

. 

### 8 Collectively, the re is limited though positive toxicological evidence for O 3 -induced



### 9 developmental effects, including effects on pulmonary structure and function and central nervous
system effects

. Limited epidemiologic evidence exists for an association with O 3 concentrat ion and



### 11 decreased sperm concentration

. A recent toxicological study provides limited evidence for a possible



### 12 biological mechanism (histopathology showing impaired spermatogenesis) for such an association

. 

### 13 Additionally, though the evidence for an associatio n between O 3 concentrations and adverse birth



### 14 outcomes is generally inconsistent, there are several influential studies that indicate an association
with reduced birth weight and restricted fetal growth

. Taking into consideration the positive evidence



### 16 for developmental and reproductive outcomes from toxicological and epidemiological studies, and



### 17 the few influential birth outcome studies, the evidence is suggestive of a causal relationship



### 18 between long

- term exposures to O 3 and reproductive and developmental effects

.



## 2.4.2.4. Ce n tra l Ne rvo us Sys te m Effe c ts

19 R ecent r esearch in the area of O 3 - induced neurotoxicity has included several long

- term
exposure studies. Notably, the first epidemiologic study to examine the relationship between O 3 

### 21 exposure and neurobehavioral effe cts observed an association between annual O 3 levels and an



### 22 aging- related cognitive performance decline in tests measuring coding ability and attention/short



### 23 term memory. This observation is supported by studies in rodents which demonstrate oxidative stres s


### 24 in the brain and associated decrements in behavioral tests, including those measuring memory, after
subchronic exposure to 0.25 ppm O 3 . Additionally, neurobehavioral changes are evident in animals



### 26 whose only exposure to O 3 occurred in utero

. Collectively , the limited epidemiologic and



### 27 toxicological evidence is coherent and is suggestive of a causal relationship between O 3 exposure


### 28 and adverse CNS effects.



## 2.4.2.5. Ca nc e r, Muta ge nic ity a nd Ge n otoxic ity

29 The 2006 O 3 AQCD (U.S. EPA, 2006, 088089 ) reported that evidence did not support ambient

O 3 as a pulmonary carcinogen

. Since the 2006 AQ CD, very few epidemiologic and toxicological



### 31 studies have been published that examine O 3 as a carcinogen, but collectively, study results indicate



### 32 that O 3 may contribute to DNA damage

. Overall, the evidence is inadequate to determine if a



### 33 causal relationsh ip exists between ambient O 3 exposures and cancer

.

March 2011 2- 30 DRAFT – DO NOT CITE OR QUOTE

## 2.4.2.6. Mo rtality

1 T he 2006 O 3 AQCD concluded that an insufficient amount of evidence existed “to suggest a



### 2 causal relationship between chronic O 3 exposure and increased risk for mortality in humans”



### 3 (U.S. EPA, 2006, 088089 )

. Two additional studies have been conducted since the last re view, an



### 4 ecologic study that finds no association between mortality and O 3 , and a reanalysis of the ACS



### 5 cohort that specifically points to a relationship between long

- term O 3 exposure and an increased risk



### 6 of respiratory mortality (Section 7.7.1)

. The find ings from the reanalysis of the ACS cohort are



### 7 consistent and coherent with the evidence from epidemiologic, controlled human exposure, and



### 8 animal toxicological studies for the effects of short

- and long

- term exposure to O 3 on respiratory



### 9 effects

. Addition ally, the evidence for short

- and long

- term respiratory morbidity provides biological



### 10 plausibility for mortality due to respiratory disease

. Collectively, the evidence is suggestive of a



### 11 causal relationship between long

- term O 3 expos ures and all

- cause mort ality

.



# 2.5. P olicy Relevant Cons iderations

2.5.1. Potentially Sus ceptible Populations

12 Upon evaluating the association between short

- and long

- term exposure to O 3 and various



### 13 health outcomes, studies also attempted to identify populations that are more susceptible 1 to O 3

. 

### 14 These studies did so by conducting stratified epidemiologic analyses; by examining individuals with



### 15 an underlying health condition in controlled human exposure studies; or by developing animal



### 16 models that mimic the pathophysiologic al conditions associa ted with an adverse health effect. These



### 17 studies identified a multitude of factors that could potentially contribute to whether an individual is



### 18 susceptible to O 3 ( Table 8-1 ). The examination of susceptible populations to O 3 exposure allows for



### 19 the NAAQS t o provide an adequate margin of safety for both the general population and for sensitive



### 20 populations.



### 21 The populations identified in Chapter 8 that are most susceptible to O 3 - related health effects



### 22 are individuals with influenza/infection, individuals with asthma, and older age groups. There were a



### 23 small number of studies on influenza/infection but both reported influenza/infection to modify the



### 24 association between O 3 exposure and respiratory effects, with individuals having influenza or an



### 25 infection being at increased susceptibility. Asthma as a susceptibility factor was supported by



### 26 controlled human exposure and toxicological studies, as well as some evidence from epidemiologic



### 27 studies. Most studies comparing age groups reported greater effects of short

- te rm O 3 exposure on


### 28 mortality among older adults. Diet and obesity are also both likely susceptibility factors. Multiple



### 29 epidemiologic, controlled human exposure, and toxicological studies reported that diets deficient in

1 P opulations that have a greater likelihood of experiencing health effects related to exposure to an air pollutant (e.g., O 3 ) due to a variety

of factors including, but not limited to: genetic background, birth outcomes (e.g., low birth weight, birth defects), race, sex, lifestage, lifestyle (e.g., smoking status, nutrition), preexisting disease, SES (e.g., educational attainment, reduced access to health care), and characteristics that may modify exposure to O 3 (e.g., time spent outdoors)

March 2011 2- 31 DRAFT – DO NOT CITE OR QUOTE

### 5

10

15

20

25

30

35

1 Vitamins E and C are associated with susceptibility to O 3 - related health effects. Similarly, studies of



### 2 effect measure modification by BMI observed greater O 3 - related respiratory decrements for



### 3 individuals who were obese.



### 4 Other potential factors [pre

- existing conditions (such as COPD and CVD) young age, sex, and
multiple genes (such as GSTM1, GSTP1, HMOX

- 1, NQO1

, and TNF

- α ) ] provided some evidence



### 6 of susceptibility, but further evidence is needed. In addition, examination of modification of the



### 7 associations between O 3 exposure and health effects by SES and race were available in a limited



### 8 number of studies, and demonstrat ed possible increased odds of health effects related to O 3 exposure


### 9 among those with low SES and Blacks.
Individuals involved in outdoor activities were examined in a recent study but no effect



### 11 modification was observed. However, previous evidence along w ith biological plausibility from



### 12 toxicological and controlled human studies has shown this population to be susceptible to O 3 - related



### 13 health effects. The only studies examining effect measure modification by diabetes examined O 3 

### 14 exposure and cardiovascular outcomes, but none of the studies reported any change in the
association by diabetes.



### 16 Studies of air conditioning use, physical conditioning, and smoking were conducted but not



### 17 much evidence was available to determine whether susceptibility to O 3 - related health effects is



### 18 present for these factors. Toxicological studies also identified hyperthyroidism and the lifestage of



### 19 gestation to be factors warranting further examination. Future research on these will provide
additional insight into whether these fact ors affect susceptibility to O 3 - related health effects.



## 2.5.2. La g Stru c tu re o f O zone

- Mo rbidity and O zone

- Mo rtality As s o cia tion s

21 Epidemiologic studies have attempted to identify the time

- frame in which exposure to O 3 can


### 22 impart a health effect. Although O 3 exposu re

- response relationships have traditionally been



### 23 examined using air quality data for a defined lag period ( e.g. , 1 day or average of 0 -1 day s), the



### 24 relationship can potentially be influenced by a multitude of factors, such as the underlying
susceptibility of an individual (e.g., age, pre

- existing diseases), which could increase or decrease the



### 26 lag times observed.



### 27 An attempt has been made to identify whether certain lag periods are more strongly associated



### 28 with specific health outcomes. The epidemiologic evidence evaluated in the 200 6 O 3 AQCD



### 29 indicated that one of the remaining uncertainties in characterizing the O 3 - mo rtality relationship was
identifying the appropriate lag structure (e.g., single

- day lags versus distributed lag model).



### 31 Currently , many inves tigators have chosen to examine the lag structure of associations between O 3 

### 32 concentration and health outcome instead of focusing on a priori lag times. This approach is



### 33 informative because if effects are cumulative, higher overall risks may exist than wou ld be observed



### 34 for any given single day lag. An examination of lag times used in the epidemiologic studies
evaluated in this assessment can provide further insight on the relationship between O 3 exposure and



### 36 morbidity and mortality outcomes.

March 2011 2- 32 DRAFT – DO NOT CITE OR QUOTE

### 5

10

15

20

25

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35 

## 2.5.2.1. O zone

- Res pira tory Effe c t As s oc ia tio ns

1 Collectively , recent epidemiologic studies of lung function, respiratory symptoms, and



### 2 biological markers of airway inflammation and oxidative stress examined associations with single



### 3 day ambient O 3 exposures lagged from 0 to 7 day s as well as concentrations averaged over 2 to 19



### 4 days. Lag 0 and 1 ambient O 3 exposures were associated with decreases in lung function and
increases in respiratory symptoms, airway inflammation, and oxidative stress. In several studies,



### 6 multiday averages of O 3 exposure were associated with these endpoints, indicating that exposures



### 7 accumulated over several days may be important or may be subject to less measurement error.



### 8 Studies have suggested that O 3 - related respiratory morbidity may occur via multiple



### 9 mechanisms with varying time courses of action

. Many epidemiologic studies simultaneously

examined associations of short

- term ambient O 3 exposure with lung function, respiratory symptoms,



### 11 and biological markers of airway inflammation and oxidative stress and found inconsistent



### 12 associations among endpoints whether evaluated at the same or different lags of O 3 exposure. In



### 13 most cases, investigators examined a limited number of O 3 exposure lags and did not assign different



### 14 O 3 exposure lags for each endpoint based on hypothesized mechanisms of action. These limitations
may explain some of the inconsistencies in associations of O 3 with different respiratory health



### 16 endpoints and may contribute to uncertainty over the important lags of ambient O 3 exposure for



### 17 parti cular respiratory endpoints.



### 18 In studies of respiratory hospital admissions and ED visits, results were somewhat sensitive to



### 19 the lag day selected (i.e., reduced when using a single

- day lag and increased when using a distributed
lag , up to 0 -3 days) , thoug h when analyses were restricted to summer months only, the lag period



### 21 tended to influence the results much less. Overall, a mong studies that examined a range of single -day



### 22 lags and multiday averages , evidence did not overwhelmingly point to stronger immedi ate, delayed,



### 23 or cumulative effects of O 3 exposure on respiratory effects

.



## 2.5.2.2. O zone

- Morta lity As s oc ia tions

24 Epidemiologic studies that focused on the association between short

- term O 3 exposure and
mortality (i.e., all

- cause, respiratory and cardiovascular) mos tly examined a priori lag structures of



### 26 either 1 or 0

- 1 days. Although mortality studies do not often examine alternative lag structures,



### 27 several recent studies have conducted more extensive analysis of lag structure to investigate



### 28 “mortality displacement” (i.e., deaths are occurring in frail individuals and exposure is only moving



### 29 the day of death to a day slightly earlier) , with varying results (Section 6.6.2.4)

. An examination of

multi

- day effects in a U.S. and European multicity study found conflicting evidence for mortality



### 31 displacement, but the evidence suggest s that the positive associations between O 3 and mortality are



### 32 observed mainly in the first few days after exposure. A study conducted in 48 U.S. cities during the



### 33 warm season (i.e., June

- August) for the years 1989

- 2000 found that larger risk estimates were



### 34 observed for distributed lag model s compared to the lag 0 day estimates ; however, larger risk
estimates at lag 0

- 3 compared to 0

- 20 provide additional support for O 3 - induced mortality effects

March 2011 2- 33 DRAFT – DO NOT CITE OR QUOTE

### 1 oc curring within the first few days after exposure. Although not a traditional mortality displacement



### 2 study, the APHENA study found no indication that a distributed lag model with up to a 2

- day lag



### 3 yielded meaningfully larger O 3 mortality risk estimates tha n the lag 0

- 1 and lag 1 results

. Finally , a


### 4 study conducted in 21 European cities reports that using single

- day exposures may overestimate the



### 5 effects on all

- cause and cardiovascular mortality, but underestimated the ef fects on respiratory



### 6 mortality. Thus, the results in part suggest evidence of mortality displacement for all

- cause and



### 7 cardiovascular mortality. However, it should be noted that the difference in results observed across



### 8 studies may be due to the different model specifications used. Collectivel y, these studies suggest that



### 9 the positive associations between O 3 and mortality are observed mainly in the first few days after



### 10 exposure

. Overall, the evidence suggest s that estimating the mortality risk using a single day of O 3 

### 11 exposure may underestimate the public health impact, but the extent of multi

- day effects appear to be



### 12 limited to a few days.



## 2.5.3. O zone Concentration

- Res pons e Relations hip

13 An important consideration in characterizing the O 3 - mo rbidity and mortality association is



### 14 whether the C

- R relation ship is linear across the full concentration range that is encountered or if



### 15 there are concentration ranges where there are departures from linearity (i.e., nonlinearity). In this



### 16 ISA studies have been identified that attempt to characterize the shape of t he O 3 C- R curve along



### 17 with possible O 3 “thresholds” (i.e., O 3 levels which must be exceeded in order to elicit a health



### 18 response). The controlled human exposure and epidemiologic studies that examined the shape of the



### 19 C- R curve and the potential presence o f a threshold have indicated a generally linear C

- R function



### 20 with no indication of a threshold for O 3 concen trations greater than 30 or 40 ppb , which corresponds



### 21 with PRB and the lower bound of O 3 concentrations included in the C

- R functions

.



## 2.5.3.1. Concentration

- Re s p ons e Re la tions hip Cha ra c te rize d by Co ntrolle d Hu ma n Expos ure Studies

22 Controlled human exposure studies have provided strong and quantifiable C-R data on the



### 23 human health effects of O 3

. The magnitude of respiratory effects in these studies is generally a


### 24 function of O 3 exposure, i.e., the product of concentration (C), minute ventilation (V E ), and exposure



### 25 duration. Recent studies provide evidence for a smooth C-R curve without indication of a threshold



### 26 in young healthy adults, exposed during moderate ex ercise for 6.6 h ours to O 3 concentrations of



### 27 between 40 and 120 ppb (Figure 6

- 1).



## 2.5.3.2. Concentration

- Res pons e Relations hip Charac terized by Epidemiologic Studies

28 A study examin in g the C-R relationship found no evidence of a threshold between short

- term



### 29 O 3 expo sure and pediatric asthma ED visits. One study reports that both quintile and loess dose



### 30 response analyses (Figure 6

- 11) suggest that there are elevated associations with O 3 at relatively low

March 2011 2- 34 DRAFT – DO NOT CITE OR QUOTE

### 5

10

15

20

25

30

35

1 concentrations

, between 30 and 40 ppb, with stronger evidence at concentrations of 40 ppb and



### 2 above

. In an additional analysis, using a smooth function the authors examined whether the shape of



### 3 the C

- R curve for short- term exposure to O 3 and asthma hospital admissions (i.e., both general and



### 4 ICU for all ages) is linear . When comparing the curve to a linear fit line the authors found that the
linear fit is a reasonable approximation of the C-R relationship between O 3 and asthma hospital



### 6 admissions around and below the current NAAQS (Figure 6-9)

. Although the C-R relation ship



### 7 between short

- term O 3 exposure and respiratory

- related hospital admissions and ED visits has not



### 8 been extensively examined, preliminary examination s found no evidence of a threshold between



### 9 short- term O 3 exposure and pediatric asthma ED visits

. E viden ce associating long

- term O 3 exposure to first asthma hospital admission in a C-R



### 11 relationship is provided i n a retrospective cohort study (Figure 7.3)

. 

### 12 Evaluation of the short- term exposure to O 3 - mortality C-R relationship is difficult due to the



### 13 highly h eterogeneous O 3 - mortality associations among regions in multicity studies (using log

- linear



### 14 models). In addition, there are numerous issues that may influence the shape of the O 3 - mortality C-R
relationship that warrant examination including: multi

- day effe cts (distributed lags), potential



### 16 adaptation and mortality displacement (i.e., hastening of death by a short period). Several recent



### 17 studies applied a variety of statistical approaches to examine the shape of the O 3 - mortality C-R



### 18 relationship and whether a threshold exists. The se studies did not find any evidence that supports a



### 19 threshold for the association between short

- term exposure to O 3 and mortality. It was also
demonstrated that the heterogeneity in the O 3 - mo rtality relationship across cities (or reg ions)



### 21 complicates the interpretation of a combined C-R curve and threshold analysis. Additionally, given



### 22 the effect modifiers identified in the mortality analyses that are also expected to vary regionally (e.g.,



### 23 temperature, air conditioning prevalence), a national or combined analysis may not be appropriate to



### 24 identify whether a threshold exists in the O 3 - mo rtality C

- R relationship.
Additionally, several studies of long

- term exposure to O 3 and birth outcomes have



### 26 characterized the C-R relationship. Eviden ce from the southern California Children's Health Study



### 27 identified a C-R relationship of birth weight with 24

- h avg O 3 concentrations averaged over the



### 28 entire pregnancy that was clearest above the 30 ppb level (Figure 7

- 4). Relative to the lowest decile



### 29 of 24

- h avg O 3 , estimates for the next 5 lowest deciles were approximately

- 40 g to

- 50 g, with no
clear trend and with 95% confidence bounds that included zero. The highest four deciles of O 3 

### 31 exposure showed an approximately linear decrease in birth weight, and all four 95% CIs excluded



### 32 zero, and r anged from mean decreases of 74 grams to decreases of 148 grams. Another study



### 33 conducted in southern California reported increased risks for cardiac birth defects in a dose

- response


### 34 manner with second

- mo nth O 3 exposure.



# 2.6. In te g ra tio n o f Ozone Health Effects

This section summarizes the main conclusions of this assessment regarding the health effects



### 36 of O 3 and the concentrations at which those effects are observed

. The conclusions from the previous

March 2011 2- 35 DRAFT – DO NOT CITE OR QUOTE

### 1 NAAQS review and the causality determinations from this review are summarized in Table 2 -3. This



### 2 section also integrates the evidence from short

- and long

- term expos u re studies across scientific



### 3 disciplines (i.e., controlled human exposure studies, toxicology, and epidemiology ) in interpreting



### 4 the health effects evidence that spans from prenatal development to death. The clearest evidence for



### 5 health effects associated with short

- term exposure to O 3 is provided by studies of respiratory effects

. 

### 6 The combined health effects evide nce supports a causal relationship for this outcome

. The evidence



### 7 is also sufficient to infer a relationship that is likely to be causal for short

- term exposure to O 3 and



### 8 mortality and long

- term exposure to O 3 and respiratory effects

.



### Table 2 -3. Summary of evidence from epidemiologic, controlled human exposure, and animal toxicological studies on the health effects associated with short

- and long

- term exposure to

ozone

**Health Outcome Conclusion**  **s from**  **2006 O 3**  **AQCD Conclusions from 2011**  **1 st**  **Draft ISA** 

Respiratory effects Short

- T erm E xposure to O 3 The overall evidence supports a causal relationship between acute ambient O 3 exposures and increased respiratory morbidity
outcomes. Causal r elationship

Lung function Results from controlled human exposure studies and animal toxicological studies provide clear evidence of causality for the associations observed between acute ( ≤ 24 h) O 3 exposure and relatively small, but statistically significant declines in lung function observed in numerous recent epidemiologic studies. Declines in lung function are particularly noted in children, asthmatics, and adults who work or exercise outdoors. Recent controlled human exposure studies demonstrated decreases in FEV 1 in the range of 2.8 to 3.6% with O 3 exposures 6.6 h in duration and as low as 60 ppb in concentration. The collective body of epidemiologic evidence demonstrates associations between acute ambient O 3 exposure and decrements in lung function, part i cularly in asthmatics, children, and adults who work or exercise outdoors.

Airway hyperrespon siveness Evidence from human clinical and animal toxicological studies clearly indicate that acute exposure to O 3 can induce airway hyperreactivity, thus likely placing atopic asthmatics at greater risk for more prolonged bouts of breathing difficulties du e to airway constriction in response to various airborne allergens or other triggering stimuli. A limited number of studies have observed airway hyperresponsiveness in rodents and guinea pigs after exposure to less than 300 pp b O 3

. As previously reported in the 2006 O 3 AQCD, 50 pp b O 3 induced airway hyperresponsiveness in certain strains of rats, suggesting a genetic component.

Pulmonary inflammation, injury and oxidative stress The extensive human clinical and animal toxicological evidence, together with the limited available epidemiologic evidence, is clearly indicative of a causal role for O 3 in inflammatory responses in the airways. E pidemiologic studies provided new evidence for associations of ambient O 3 with mediators of airway inflammation and oxid ative stress and indicated that groups with diminished antioxidant capacity or comorbidities such as atopy, AHR, or obesity may have increased susceptibility to respiratory morbidity associated with O 3 exposure. Generally, these studies were conducted in locations where the 8

- h max O 3 concentration ranged from 31 to 66 ppb

.

Respiratory symptoms and medication use Young healthy adult subjects exposed in clinical studies to O 3 concentrations ≥ 80 pp b for 6 to 8 h during moderate exercise exhibit symptoms of cough and pain on deep inspiration. The epidemiologic evidence shows significant associations between acute exposure to ambient O 3 and increases in a wide variety of respiratory symptoms (e.g., cough, wheeze, production of phlegm, and shortness of breath) and medication use in asthmatic children. The collective body of epidemiologic evidence demonstrates positive associations between acute exposure to ambient O 3 and respiratory symptom s (e.g., cough, wheeze, production of phlegm, and shortness of breath) in asthmatic children. Generally, these studies were conducted in locations where the 8

- h max O 3 concentration ranged from 17 to 66 ppb

.

March 2011 2- 36 DRAFT – DO NOT CITE OR QUOTE**Health Outcome Conclusion**  **s from**  **2006 O 3**  **AQCD Conclusions from 2011**  **1 st**  **Draft ISA** 

Lung host defenses Toxicological studies provid ed extensive evidence that acute O 3 exposures as low as 80 to 5 00 pp b can cause increases in susceptibility to infectious diseases due to modulation of lung host defenses. A single controlled human exposure study found decrements in the ability of alveolar macrophages to phagocytose microorganisms upon exposure to 8 0 to 1 00 pp b O 3

. Recent studies build on prior evidence that O 3 can affect multiple aspects of innate and acquired immunity, including macrophage function, immune cell proliferation, and natural killer cell function with acute O 3 exposures as low as 80 pp b

.

Previous toxicological evidence indicated that Further evidence for O 3 - induced allergic skewing O 3 exposure skews immune responses toward Allergic and asthma is provided by a few recent studies in rodents an allergic phenotype, and enhances the related responses using exposure concentrations as low as development and severity of asthma

- related 2 00 pp b

. responses such as AHR.

Aggregate popul ation time

- series studies Hospital admissions, observed that ambient O 3 concentrations are ED visits, and positively and robustly associated with physician visits respiratory

- related hospitalizations and asthma ED visits during the warm season. Strong evidence demonstrated associations of ambient O 3 with res piratory hospital admissions and ED visits in diverse populations across the U.S., Europe, and Canada. Generally, these studies were conducted in locations where the 8- h max O 3 concentration ranged from 18 to 60 ppb

.

Respiratory Mortality Aggregate popula tion time

- series studies specifically examining mortality from respiratory causes were limited in number and showed inconsistent associations between acute exposure to ambient O 3 exposure and respiratory mortality. Recent multicity time

- series studies and a multicontinent study consistently demonstrated associations between ambient O 3 and respiratory

- related mortality visits in diverse populations across the U.S., Europe, and Canada. Generally, these studies were conducted in locations where the 8

- h max O 3 concentration ranged from 20 to 63 ppb

. The limited evidence is highly suggestive that O 3 directly and/or indirectly contributes to Cardiovascular effects cardiovascular

- related morbidity, but much Suggestive of a Causal Relationship remains to be done to more fully substantiate the associa tion.

Toxicological studies report that acute exposures to O 3 are associated with alterations Central nervous system in neurotransmitters, motor activity, short

- and Suggestive of a Causal Relationship effects long

- term memory, sleep patterns, and histolo gical signs of neurodegeneration.

The evidence is highly suggestive that O 3 directly or indirectly contributes to non

- All

- cause mortality Likely to be a Causal Relations hip accidental and cardiopulmonary

- related mortality.

Long

- term Exposure to O 3 The current evidence is suggestive but Respiratory effects inconclusive for respiratory health effects from Likely to be a Causal Relationship long

- term O 3 exposure.

New onset asthma No Studies Evidence for a relationship be tween different genetic variants (HMOX, GST, ARG) that, in combination with O 3 exposure, are related to

n ew onset asthma. These results were observed when subjects living in areas where the mean annual 8

- h max O 3 concentration was 55.2 ppb

, compared to tho se who lived where it was 38.4 ppb

. Chronic O 3 exposure was related to first childhood asthma hospital admissions in a Asthma hospital positive concentration

- response relationship

. No Studies admissions Generally, these studies were conducted in locations where the 8

- h max O 3 concentration ranged from 30 to 41 ppb

.

March 2011 2- 37 DRAFT – DO NOT CITE OR QUOTE**Health Outcome Conclusion**  **s from**  **2006 O 3**  **AQCD Conclusions from 2011**  **1 st**  **Draft ISA** 

Pulmonary structure and function Epidemiologic studies observed that reduced lung function growth in children was associated with seasonal exposure to O 3 ; however, cohort studies of annual or m ultiyear O 3 exposure observed little clear evidence for impacts of longer

- term, relatively low

- level O 3 exposure on lung function development in children. Animal toxicological studies reported chronic O 3 ­ induced structural alterations in several regions of the respiratory tract including the
centr i acinar region. Morphologic evidence from studies using exposure regimens that mimic seasonal exposure patterns report increased lung injury compared to conventional chronic stable exposures. Evidence for pulmonary function effects is inconclusive, with some new epidemiologic studies relating effects at exposure levels from 30 to 65 ppb

. Information from toxicological studies indicates that long

- term exposure ( 5 00 pp b ) during gestation or development can result in i rreversible morphological changes in the lung, which in turn can influence pulmonary function.

Pulmonary inflammation, injury and oxidative stress Extensive human clinical and animal toxicological evidence, together with limited epidemiologic evidence ava ilable, suggests a causal role for O 3 in inflammatory responses in the airways Several epidemiologic and toxicology studies (as low as 5 00 pp b ) add to observations of O 3 - induced inflammation and injury.

Allergic responses Limited epidemiologic evidence supported an association between ambient O 3 and allergic symptoms. Little if any information was available from toxicological studies. Evidence relates positive outcomes of allergic response and O 3 exposure but with variable strength for the effect estimates ; exposure to O 3 may increase total IgE in adult asthmatics. Allergic antibody levels in rodents were increased by exposure to O 3 concentrations as low as 2 00 pp b

Respiratory mortality Studies of cardio

- pulmonary mortality were insufficient to suggest a causal relationship between chronic O 3 exposure and increased risk for mortality in humans A single study demonstrated that exposure to O 3 (1- h max 45 to 60 ppb ) elevated the risk of death from respiratory causes and this effect was robust to the inclusion of PM 2.5
Cardiovascular Effects No studies from previous review Suggestive of a Causal Relationship

Limited evidence for a relationship between air pollution and birth

- related health outcomes, Reproductive and including mortality, premature births, low birth Suggestive of a Causal Relationship developmental effects weights, and birth defects, with little evidence being found for O 3 effects.

Toxicological studies report that acute exposures to O 3 are associated with alterati ons Central nervous system in neurotransmitters, motor activity, short and Suggestive of a Causal Relationship effects long term memory, sleep patterns, and histological signs of neurodegeneration.

Little evidence for a relationship between Cancer chronic O 3 exposure and increased risk of lung Inadequate to infer a Causal Relationship

cancer.
There is little evidence to suggest a causal All

- cause m ortality relationship between chronic O 3 exposure and Suggestive of a Causal Relationship increased risk for mortality in humans.



## 2.6.1. Respi rato ry He alth Effe ct s

1 Collectively, there is a vast amount of evidence spanning several decades that supports a



### 2 causal association between exposure to O 3 and a spectrum of respiratory effects, including



### 3 development of the respiratory system; pulmonary structure, inflammation, injury and function;



### 4 changes in host defense; new onset asthma and asthma exacerbations; respiratory symptoms; ED



### 5 visits and hospital admissions for respiratory diseases; and even death attributable to respiratory



### 6 disease (Figure 2

- 1)

. The majority of this evidence is derived from studies investigating short

- term



### 7 exposure (i.e., days to weeks) to O 3 , although more recent evidence demonstrates that long

- term



### 8 exposure (i.e., months to years) may also be detrimental to the respiratory syst em.

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### Figure 2 -1. Snapshot of evidence for the spectrum of respiratory health effects associated with short

- and long

- term exposure to ozone

. Green=Animal Toxicological Studies; Blue=Controlled Human Exposure Studies;
﷒ Purple=Epidemiologic Studies; AM=Al veolar Macrophage

.

March 2011 2- 39 DRAFT – DO NOT CITE OR QUOTE 

### 1

### 1 Mechanistic evidence for the effect of O 3 on the respiratory system was initially characterized



### 2 in the 1996 O 3 AQCD, and identified a variety of lung lipid changes which may be further



### 3 metabolized to produce numerous biologically acti ve mediators that can affect host defenses, lung



### 4 function, the immune system and other functions. As summarized in Section 2.3 and fully



### 5 characterized in Chapter 5 key events in the toxicity pathway of O 3 have been identified in humans



### 6 and animal models. T hey include the formation of secondary oxidation products in the lung,



### 7 activation of neural reflexes, pulmonary injury and inflammation and increased bronchial reactivity.



### 8 In addition, evidence is accumulating that influx of immunomodulatory cells, activat ion of innate



### 9 and adaptive immunity, induction of AHR and allergic responses, impairment of host defense,



### 10 systemic inflammation and vascular oxidative/nitrosative stress may also be critical to the O 3 

### 11 toxicity pathway (Figure 2

- 2).



### Solid arrows denote ﷒ Schemati c depicting key events in the ozone toxicity pathway

. Figure 2

- 2. pathways for which there is greater certainty. Broken arrows represent pathways of emerging interest.

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39 Recent toxicological studies of long

- term exposure to O 3 occurring throughout various
lifestages, beginning with prenatal and early life exposures, provide novel evidence for effects on
development of the respiratory system, including ultrastructural changes in bronchiole development,
alterations in placental and offspring cytokine s, and increased offspring airway hyper

- reactivity

( Section 7.4.7)

. The strongest evidence for O 3 - induced effects on the developing lung comes from a
series of experiments using infant rhesus monkeys repeatedly exposed to 0.5 ppm O 3 starting at

one mon th o f age

. Functional changes in the conducting airways of infant rhesus monkeys exposed

to either O 3 alone or O 3 + antigen were accompanied by a number of cellular and morphological
changes

. In addition to these functional and cellular changes, significant structural changes in the
respiratory tract were observed

. Importantly, the O 3 - induced structural pathway changes persisted
after recovery in filtered air for six mon ths after cessation of the O 3 exposures

. Exposure to O 3 has

also been associated with simila r types of alterations in pulmonary structure in all adult laboratory
animal species studied, from rats to monkeys (U.S. EPA, 1996, 017831 )

. In addition to effects on the development and structure of the respiratory tract, there is

extensive evidence for the effect of short

- term exposure to O 3 on pulmonary inflammation

. Previous
evidence from controlled human exposure studies indicated that O 3 causes an inflammatory response
in the lungs (U.S. EPA, 1996, 017831 )

. This infl ammatory response to O 3 was detected after a single

1-h exposure with exercise to O 3 concentrations of 0.3 ppm ; the increased levels of some
inflammatory cells and mediators persisted for at least 18 h ours

. Toxicological studies provided

additional evidenc e for increases in permeability and inflammation in rabbits at levels as low as

0.1 ppm O 3

. Evidence summarized in the 2006 O 3 AQCD demonstrated that inflammatory responses
were observed subsequent to 6.6 h O 3 exposure to the lowest tested level of 0.08 pp m in healthy
human adults, while animal toxicological studies provided extensive evidence that short

- term
(1 -3 hours ) O 3 exposure as low as 0.1

- 0.5 ppm could cause lung inflammatory responses. The
limited epidemiologic evidence demonstrated an association between short

- term ambient O 3 exposure and airway inflammation in children (1 -h max O 3 of approximately 0.1 ppm )

. The most
recent epidemiologic studies provide additional supporting evidence by demonstrating associations

of ambient O 3 with mediators of air way inflammation and indicating that groups with diminished
antioxidant capacity or comorbidities such as atopy, AHR, or obesity may have increased
susceptibility to respiratory morbidity associated with O 3 exposure ( Section s 6.2.4 and 8 .1 ).
The normal inflammatory response in lung tissue is part of host defense that aids in removing
microorganisms or particles that have reached the distal airways and alveolar surface

. The 1996 O 3 AQCD concluded that short

- term exposure to elevated concentrations of O 3 resu lted in alterations in
these host defense mechanisms in the respiratory system

. Specifically, toxicological studies of short

term exposures as low as 0.1 ppm O 3 were shown to decrease the ability of alveolar macrophages to
ingest particles, and short

- term exposures as low as 0.08 ppm for 3 h ours prevented mice from
resisting infection with streptococcal bacteria, resulting in mortality

. Similarly, alveolar macrophages
removed from the lungs of human subjects after 6.6 h ours of exposure to 0.08 and 0.10 ppm O 3 resulted in a decreased ability to ingest microorganisms, indicating some impairment of host defense

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. These altered host defense mechanisms can lead to susceptibility to respiratory infections,

which are associated with increased risk of asth ma when occurring in early life

. In addition to pulmonary inflammation and host defenses, recent epidemiologic evidence has
revealed an association between long

- term exposure to O 3 and new onset asthma (Section 7.2.1)

. Studies have provided evidence for a relationship between different genetic variants ( e.g., HMOX,

GST, ARG ) that , in combination with O 3 exposure , are related to new onset asthma

. This is the first
time that evidence has extended beyond the association of exposure to O 3 and asthma exacerbati ons to suggest that long

- term exposure to O 3 may play a role in the development of the disease and

contribute to incident cases of asthma.
The most commonly observed and strongest evidence for respiratory effects associated with

short- term exposure to O 3 a re increased frequency of breathing and decreased tidal volume (i.e.,
rapid, shallow breathing)

. Previous c ontrolled human exposure studies demonstrated O 3 -induced
decrements in pulmonary function, characterized by alterations in lung volumes and flow and airway
resistance and responsiveness for multihour exposures (up to 7 h ours ) to O 3 concentrations as low as

0.08 ppm (U.S. EPA, 1996, 017831 )

. A series of mobile laboratory studies of lung function and
respiratory symptoms reported pulmonary function decrements at mean ambient O 3 concentrations

of 0.14 ppm in exercising healthy adolescents and increased respiratory symptoms and pulmonary

function decrements at 0.15 ppm in heavily exercising athletes and at 0.17 ppm in lightly exercising
healthy and asthmatic subjects

. Epidemiologic and animal toxicological evidence is coherent with
the results of the controlled human exposure studies, both indicating decrements in lung function

upon O 3 exposure

. Combined statistical analysis of six epidemiologic studies in children at summer
camp demonstrated decrements in FEV 1 of 0.50 mL /ppb with previous hour O 3 concentration. For
preadolescent children exposed to 120 ppb (0. 12 ppm ) ambient O 3 , this amounted to an average

decrement of 2.4

- 3.0% in FEV 1 . Two key studies of lung function measurements before and after

well

- defined outdoor exercise events in adults yielded exposure

- respo nse slopes of 0.40 and

1.35 mL /ppb

. Animal toxicological studies reported similar respiratory effects in rats at exposures as

low as 0.2 ppm O 3 for 3 h ours

. The 2006 O 3 AQCD characterized the controlled human exposure
and animal toxicological studies as pr oviding clear evidence of causality for the associations

observed between acute ( ≤ 24 h ours ) O 3 exposure and relatively small, but statistically significant
declines in lung function observed in numerous recent epidemiologic studies. Declines in lung

funct ion were particularly noted in children, asthmatics, and adults who work or exercise outdoors

. Recent studies in animals and in vitro models described inflammatory and injury responses mediated
by toll

- like receptors (e.g., TLR4, TL2), receptors for TNF or IL

- 1, multiple signaling pathways
(e.g., p38, JNK, NFκB, MAPK/AP

- 1), and oxidative stress ( Section 6.2.3.3 )

. Recent controlled
human exposure studies examined lower concentration O 3 exposures (40

- 80 ppb ) and demonstrated

that FEV 1 , respiratory symptoms, and inflammatory responses were affected b y O 3 exposures of

6.6 hours and in the range of 60 to 80 ppb ( Section 6.2.1.2). These studies demonstrated decreases in

FEV 1 in the range of 2.8 to 3.6% with O 3 exposures 6.6 h in duration and as low as 60 ppb in

concentration . Recent e pidemiologic studies provide greater insight into subject factors that may

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39 increase susceptibility for O 3 - associated respiratory morbidity. It was in these poten tially susceptible
populations (e.g., asthmatics with atopy or concurrent respiratory infection, infants with asthm atic
mothers, elderly with AHR or obesity, or groups with diminished antioxidant capacity ) that O 3 ­ associated decreases in lung function tended to be observed

. O zone exposure has been shown to result in airway hyperresponsiveness (both specific and

non

- spe cific), epithelial permeability, and respiratory tract inflammation. Increased airway
responsiveness is an important consequence of exposure to O 3 because its presence means a change
in airway smooth muscle reactivity and implies that the airways are predisposed to narrowing on
inhalation of a variety of stimuli (e.g., specific allergens, SO 2 , cold air)

. Specifically, acute (2 or

3 hours ) exposure to 0.25 or 0.4 ppm O 3 was found to cause increases in airway responsiveness in
response to allergen challenges among allergic asthmatic subjects who characteristically already had
somewhat increased airway responsiveness at baseline.
In addition to alterations in lung volumes and flow, changes in pulmonary function due to

exposure to O 3 may be elicited as respirato ry symptoms (e.g., coughing, wheezing, shortness of

breath)

. The 1996 O 3 AQCD identified an association between respiratory symptoms and increasing

ambient O 3 , particularly among asthmatic children

. In the 2006 O 3 AQCD, the evidence was
extended to include young healthy adult subjects that exhibited symptoms of cough and pain on deep
inspiration after exposure to 0.08 ppm O 3 for 6

- 8 h ours during moderate exercise

. The increase in the
incidence of cough was found in controlled human exposure studies as low a s 0.12 ppm in healthy
adults during 1

- 3 h ours with very heavy exercise and other respiratory symptoms, such as pain on
deep inspiration and shortness of breath, were observed at 0.16 -0.18 ppm with heavy and very heavy

exercise

. Previous epidemiologic evidence showed significant associations between acute exposure

to ambient O 3 and increases in a wide variety of respiratory symptoms (e.g., cough, wheeze,
production of phlegm, and shortness of breath) in asthmatic children (U.S. EPA, 2006, 088089 )

. Epidemiologic studies also indicated that acute O 3 exposure is likely associated with increased

asthma medication use in asthmatic children

. Similar to what was observed for pulmonary function,

r ecent epidemiologic studies provided greater insight into subject factors that may increase
susceptibility for O 3 - associated respiratory symptom s . It was in these potentially susceptible
populations (e.g., asthmatics with atopy or concurrent respiratory infection, infants with asthmatic
mothers, elderly with AHR or obesity, or groups with diminished antioxidant capacity ) that O 3 ­
associated i ncreas es in respiratory symptoms tended to be observed

. Additionally, recent evidence
provides evidence for an association between long

- term exposure to O 3 and respiratory symptoms

( Section 7.2.2).
When respiratory symptoms, asthma exacerbations, or other respiratory diseases become too
serious to be cared for at home, they can result in visits to hospital emergency departments (ED) or
hospital admissions

. The frequency of these types of ED visits and hospital admissions is associated

with ambient O 3 concentratio ns

. Summertime daily hospital admissions for respiratory causes in
various locations of eastern North America consistently reported a relationship with ambient levels

of O 3 in studies reviewed in the 1996 O 3 AQCD. This association remained even when consid ering

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38 only concentrations below 0.12 ppm O 3

. The 2006 O 3 AQCD concluded that aggregate population

time

- series studies demonstrate a positive and robust association between ambient O 3 concentrations
and respiratory

- related hospitalizations and asthma ED vis its during the warm season

. Recent
epidemiologic time

- series studies included additional multicity and multicontinent studies in which

short- term increases in ambient O 3 concentrations were consistently associated with increases in
respiratory hospital adm issions and ED visits across diverse populations, geographic locations, and
range of O 3 concentrations (Section 6.2.7)

. There is also recent evidence for an association between
respiratory hospital admissions and long

- term exposure to O 3 ( Section 7.2.2).

F inally, in very serious cases, O 3 exposure may contribute to death from respiratory causes.

Recent evidence from s everal multicity and multicontinent studies demonstrated associations

between increases in short- term exposure to ambient O 3 concentrations an d increases in respiratory
mortality ( Section 6.6.2.5)

. Similarly, a study of long

- term exposure to ambient O 3 concentrations

also demonstrated an association between O 3 and increases in respiratory mortality (Section 7.7.1)

. Evidence from these new mortality studies is consistent and coherent with the evidence from
epidemiologic, controlled human exposure, and animal toxicological studies for the effects of short

and long

- term exposure to O 3 on respiratory effects

. Additionally, the evidence for short

- an d long

term respiratory morbidity provides biological plausibility for mortality due to respiratory disease

. In summary, recent studies support or build upon the strong body of evidence presented in the

1996 and 2006 O 3 AQCDs that short

- term O 3 exposure i s causally associated with adverse
respiratory health effects. Recent controlled human exposure studies demonstrated decreases in

FEV 1 in the range of 2.8 to 3.6% with O 3 exposures 6.6 h ours in duration and as low as 60 ppb in
concentration. Equally strong evidence demonstrated associations of ambient O 3 with respiratory
hospital admissions and ED visits in diverse populations across the U.S., Europe, and Canada. Most
effect estimates ranged from a 1.4 to 2.9% increase in daily ED visits or hospital admissi ons and

were observed in locations with mean 1 -h max O 3 concentrations between 11 and 60 ppb . Several
multicity and multicontinent studies reported associations between increases in ambient O 3 concentrations and increases in respiratory mortality. Individu al

- level epidemiologic studies
provided new evidence for associations of ambient O 3 with mediators of airway inflammation and
oxidative stress and indicated that groups with diminished antioxidant capacity or comorbidities such
as atopy, AHR, or obesity may have increased susceptibility to respiratory morbidity associated with

O 3 exposure. These recent epidemiologic findings provided support for ambient O 3 exposure having
similar effects and modes of action as those observed in studies with experimental exp osures. By
demonstrating O 3 - induced airway hyperresponsiveness, activation of neural reflexes (indicative of
decreased pulmonary function) , allergic responses, lung injury, impaired host defense, and airway
inflammation, toxicological studies have characte rized O 3 modes of action and have provided
biological plausibility for epidemiologic associations of ambient O 3 exposure with lung function and
respiratory symptoms, hospital admissions, ED visits, and mortality. Together, the evidence
integrated across co ntrolled human exposure, epidemiologic, and toxicological studies and across the

March 2011 2- 44 DRAFT – DO NOT CITE OR QUOTE

### 5

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35

1 spectrum of respiratory health endpoints continues to demonstrate that there is a causal



### 2 relationship between short

- term O 3 exposure and respiratory health effects.



### 3 The strong est evidence for a relationship between long

- term O 3 exposure and respiratory



### 4 morbidity in recent studies demonstrates associations between long

- term measures of O 3 exposure

and new

- onset asthma in children and increased respiratory symptom effects in asth matics. While the



### 6 evidence may be limited , these U.S. multi -community prospective cohort studies demonstrate that



### 7 asthma risk is associated with the important relationships between genetic variability, environmental



### 8 O 3 exposure, and behavior. Other recent studies provide coherent evidence for long

- term O 3 

### 9 exposure and respiratory morbidity effects such as first asthma hospitalization and respiratory
symptoms in asthmatics. Generally, the epidemiologic and toxicological evidence provides a



### 11 compelling case th at support s the hypothesis that a relationship exists between long

- term exposure to



### 12 ambient O 3 and me asures of respiratory morbidity

. Building upon that evidence , the more recent



### 13 epidemiologic evidence, combined with toxicological studies in rodents and no n- human primates,



### 14 provides biologically plausible evidence that there is likely to be a causal relationship between

long

- term exposure to O 3 and respiratory morbidity

.



## 2.6.2. Mortality Effects

16 The 2006 O 3 AQCD concluded that the overall body of evidence was highl y suggestive that



### 17 short- term exposure to O 3 directly or indirectly contributes to non

- accidental and cardiopulmonary



### 18 related mortality, but additional research was needed to more fully establish underlying mechanisms



### 19 by which such effects occur

. The evalua tion of new multicity studies that examined the association

between short

- term O 3 exposure and mortality found evidence which supports the conclusions of the



### 21 2006 O 3 AQCD

. These new studies reported consistent positive associations between short

- term O 3 

### 22 ex posure and total (nonaccidental) mortality, with associations being stronger during the warm



### 23 season, as well as additional support for associations between O 3 exposure and cardiovascular



### 24 mortality being similar or larger in magnitude compared to respirator y mortality. Additionally, these
new studies examined previously identified areas of uncertainty in the O 3 - mo rtality relationship

. 

### 26 Taken together, the body of evidence indicates that there is likely to be a causal relationship



### 27 between short

- term exposures to O 3 and all

- cause mortality

. 

### 28 T he 2006 O 3 AQCD concluded that an insufficient amount of evidence existed “to suggest a



### 29 causal relationship between chronic O 3 exposure and increased risk for mortality in humans”

(U.S. EPA, 2006, 088089 )

. Two additional studies have been conducted since the last review, an



### 31 ecologic study that fi nds no association between mortality and O 3 , and a reanalysis of the ACS



### 32 cohort that specifically points to a relationship between long

- term O 3 exposure and an increased risk



### 33 of respiratory mortality

. The findings from the reanalysis of the ACS study are c onsistent and



### 34 coherent with the evidence from epidemiologic, controlled human exposure, and animal
toxicological studies for the effects of short

- and long

- term exposure to O 3 on respiratory effects

. 

### 36 Additionally, the evidence for short

- and long

- term resp iratory morbidity provides biological

March 2011 2- 45 DRAFT – DO NOT CITE OR QUOTE

### 5

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1 plausibility for mortality due to respiratory disease

. Collectively, the evidence is suggestive of a



### 2 causal relationship between long

- term O 3 exposures and mortality

.



## 2.6.3. Ca rdio va s cular He alth Effec ts

3 In past O 3 AQCDs the effects of short- and long

- term exposure to O 3 on the cardiovascular



### 4 system could not be thoroughly evaluated due to the paucity of information available. However,
studies investigating O 3 - induced cardiovascular events have advanced in the last two decades

. 

### 6 Overall, there is limited, mixed evidence for cardiovascular effects in epidemiologic studies. Animal



### 7 toxicological studies provide more evidence for O 3 exposure leading to cardiovascular morbidity

. 

### 8 The toxicological studies demonstrate O 3 - induced cardio vascular effects, specifically enhanced



### 9 ischemia/reperfusion injury with or without the corresponding development of a systemic oxidative,
proinflammatory environment, disrupted NO

- induced vascular reactivity, decreased cardiac function,



### 11 and increased HRV

. Taking into consideration the positive toxicological studies reported, the



### 12 generally limited body of evidence is suggestive of a causal relationship for both relevant short



### 13 and long

- term exposures to O 3 and cardiovascular effects.



## 2.6.4. Central Nervous Sys tem Effects

14 In rodents, O 3 exposure has been shown to cause physicochemical changes in the brain

indicative of oxidative stress and inflammation. Recent toxicological studies add to earlier evidence



### 16 that short- and long

- term exposures to O 3 can produce a range of effects on the central nervous



### 17 system and behavior. Previously observed effects, including neurodegeneration, alterations in



### 18 neurotransmitters, short

- and long

- term memory, and sleep patterns, have been further supported by



### 19 recent studies. In instances where pathology and behavior are both examined, animals exhibit
decrements in behaviors tied to the brain regions or chemicals found to be affected or damaged. The



### 21 single epidemiolog ic study conducted showed that long

- term exposure to O 3 affects memory in



### 22 humans as well. Notably, exposure to O 3 levels as low as 0.25 ppm has resulted in progressive



### 23 neurodegeneration and deficits in both short

- and long

- term memory in rodents. Additionally,



### 24 changes in the CNS, including biochemical, cellular, and behavioral effects, have been observed in
animals whose sole exposure occurred in utero , at levels as a low as 0.3 ppm

. Together the evidence



### 26 from studies of short

- and long

- term exposure to O 3 is suggestive of a causal relationship between



### 27 O 3 exposure and adverse CN S effects.



## 2.6.5. Re p ro du c tive a n d Develo p me ntal Effe cts

28 There is limited though positive toxicological evidence for O 3 - induced developmental effects,



### 29 including effects on pulmonary structure and function and central nervous system effects

. Limited
epidemiologic evidence exists for an association with O 3 concentration and decreased sperm



### 31 concentration

. A recent toxicological study provides limited evidence for a possible biological



### 32 mechanism (histopathology showing impaired spermatogenesis) for such an association

.

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### 5

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1 Additionally, though the evidence for an association between O 3 concentrations and adverse birth



### 2 outcomes is generally inconsistent, there are several influential studies that indicate an association



### 3 with reduced birth weight and restricted fetal growth

. Overall, the evidence is suggestive of a causal



### 4 relationship between long

- term exposures to O 3 and reproductive and developmental effects

.



## 2.6.6. Cancer and Muta genicity and Genotoxicity

The 2006 O 3 AQCD (U.S. EPA, 2006, 088089 ) reported that evidence did not support ambient



### 6 O 3 as a pulmonary carcinogen

. Since the 2006 AQCD, very few epidemiologic and toxicological



### 7 studies have been published that examine O 3 as a carcinogen, but collectively, study results indicate



### 8 that O 3 may contribute to DNA damage

. Overall, the evidence is inadequate to determine if a



### 9 causal relationship exists be tween ambient O 3 exposures and cancer

.



# 2.7. Effe c ts o n Ve g e ta tio n a n d Eco s ys te m s

Chapter 9 presents the most policy

- relevant information related to this review of the NAAQS



### 11 for the effects of O 3 on vegetation and ecosystems . This section integrates the key find ings from the



### 12 disciplines evaluated in this current assessment of the O 3 scientific literature, which includes plant



### 13 physiology, biochemistry, whole plant biology, ecosystems and exposure

- response

. 

### 14 Ozone effects at small scales, such as the leaf of an ind ividual plant, can result in effects at a
continuum of larger scales. Figure 2 -3 is a simplified diagram of the major pathway through which



### 16 O 3 enters plants and the major endpoints O 3 may affect from small to large scales. The sections of



### 17 C hapter 9 are organized around this paradigm of effects at the cellular and subcellu l ar level followed



### 18 by consideration of the whole plant and finally, O 3 impacts on ecosystem

- level processes . Ozone



### 19 enters leaves through stomata, and can alter stomatal conductance and disr upt CO 2 fixation ( S ection

9.4). These effects can change rates of leaf gas exchange, growth and reproduction at the individual



### 21 plant level (S ection 9.5 ). Those O 3 - induced effects can translate from the individual plant level to the



### 22 ecosystem level, and cau se changes in ecosystem services, such as C storage, water production,



### 23 nutrient cycling, and community composition (Section 9.6). The EPA framework for causal



### 24 determinations described in Chapter 1 has been applied to the body of scientific evidence to

coll ectively examine effects attributed to O 3 exposure (Table 2

- 4).

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### Figure 2

- 3. The effects of ozone at leaf, plant and ecosystem scales

.

March 2011 2- 48 DRAFT – DO NOT CITE OR QUOTE

### Table 2- 4. Summary of ozone causal determinations for vegetation and ecosystem effects

Vegetation and Ecosystem Effect s Causality Determination
Reduced Vegetation Growth Causal Alteration of Vegetation Reproduction Causal Visible Foliar Injury Effects on Vegetation Causal Alteration of Leaf Gas Exchange in Vegetation Causal Reduced Yield and Quality of Agricultural Crops Causal Reduced Productivity in Terrestrial Ecosystems Causal Reduced Carbon (C) Sequestration in Terrestrial Ecosystems Likely Causal Alteration of Terrestrial Ecosystem Water Cycling Likely Causal

Alteration of Below

- ground Biogeochemical Cycles Causal Alteration of Terrestrial Community Composition Likely Causal



## 2.7.1. Me ch a nis ms Go ve rnin g Res p on s e

1 Section 9.4 focuses on the effects of O 3 stress on plants and their responses to that stress on the



### 2 molecular, biochemical and physiological levels. Many of the studies focus on the molecular



### 3 mechanisms that underlie the observed biochemical and physiological changes observed in many



### 4 plant species in response to O 3 exposure. The results support and strengthen those reported in the



### 5 2006 O 3 AQCD. The most s ignificant change in this section from the 2006 O 3 AQCD is the emphasis



### 6 on molecular mechanisms as new techniques, such as those used in evaluating transcriptomes (total



### 7 set of RNA transcripts in a particular cell at a particular time) and proteomes (total set of proteins



### 8 expressed in a particular cell at a particular time), have been utilized to perform very comprehensive



### 9 analyses of changes in gene transcription and protein expression in plants exposed to O 3 . These



### 10 newer molecular studies not only provide very important and wide

- ranging information regarding



### 11 the many mechanisms of plant responses to O 3 , they also allow for the analysis of interactions



### 12 between various biochemical pathways which are induced in response to O 3 . However, many of



### 13 these studies a re conducted in artificial conditions with model plants which are typically exposed to



### 14 very high, short doses of O 3 . Therefore, additional work remains to elucidate whether these plant



### 15 responses are transferable to other plant species exposed to more reali stic ambient conditions.



### 16 Ozone is taken up into leaves through open stomata. Once inside the substomatal cavity, O 3 is



### 17 thought to rapidly react with the aqueous layer surrounding the cell (apoplast) to form breakdown

). These • 2 ) and peroxy radicals (HO • ), hydroxyl radicals (HO ־ 

### 18 products such as H 2 O 2 , superoxide (O 2 

### 19 radicals may play a role in signaling processes and may also interact with sensitive molecules both



### 20 outside and inside the cell to cause damage. This process was very comprehensively described i n the



### 21 2006 O 3 AQCD and is only summarized in this document in Section 9.4.2.



### 22 As plants have been shown to respond to O 3 exposure very rapidly, this response must result



### 23 from a plant's ability to sense the presence of O 3 and/or its breakdown products and th en


### 24 communicate that information to the nucleus to initiate changes in gene expression. While it remains

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multiple simultaneous mechanisms by which O 3 can be sensed, and how much variation exists in O 3 sensing between species and exposure conditions, some progress has been made in the understanding
of this process since the 2006 O 3 AQCD. Experimental evidence described in Section 9.4.3.1
suggests that O 3 and/or its breakdown products may be directly sensed by apoplastic receptor
proteins (although they have not yet been identified). Additionally, a change in cellular redox state
due to plant exposure to O 3 could be the manner in which plants sense the presence o f the pollutant.
Once the plant has sensed the presence of the pollutant, there is much evidence to suggest that
mitogen

- activated protein kinases (MAPK) play an important role in communicating signals to the
nucleus that result in gene expression changes in response to O 3 . Calcium has also been implicated to
play a role in the signal transduction processes. To summarize, the evidence to date suggests there
may be several mechanisms by which plants sense the presence of O 3 and then communicate this
signal t o the nucleus to induce changes in gene expression.
New technologies have allowed for the evaluation of changes in the entire transcriptome and
proteome, rather than analyzing the modification of the expression of individual genes and proteins;

the result s of these studies are presented in Section 9.4.3.2. While transcriptome and proteome
analyses per se were not previously addressed, the 2006 O 3 AQCD did provide much information
regarding changes in gene expression and protein quantity of individual genes and proteins in

O 3 - treated plants. In the transcriptome and proteome studies described here, O 3 exposure conditions
(concentration, duration of exposure), plant species and sampling times vary significantly; however,
functional classification of the genes and proteins that are either up

- or down

- regulated by plant
exposure to O 3 exhibit common trends. In summary, genes involved in plant defense, signaling, and
those associated with the synthesis of plant hormones and secondary metabolism are generally up

r egulated in plants exposed to O 3 , while those related to photosynthesis and general metabolism are
typically down

- regulated. Proteome studies support these results by demonstrating concomitant
increases or decreases in the proteins encoded by these genes. The transcriptome and proteome
results support and enhance the findings of the 2006 O 3 AQCD.

The 2006 O 3 AQCD included a discussion on the role of phytohormones, including salicylic
acid, ethylene and jasmonic acid, in plant response to O 3 . Many additional studies using microarray
technology (used to determine changes in the transcriptome) and a variety of Arabidopsis mutants
are described in Section 9.4.3.3 and support the conclusions from the 2006 O 3 AQCD. Transcriptome
analysis has also illuminated the complex interactions that exist between these hormones to better
define plant response to O 3 . To summarize, the results indicate that while ethylene and salicylic acid
are needed to develop O 3 - induced leaf lesions, jasmonic acid acts antagonistically to et hylene and
salicylic acid to limit the spread of the lesions. Abscisic acid, in addition to its role in regulating
stomatal aperture, may also act antagonistically to the jasmonic acid signaling pathway. Nitric oxide
has also been proposed to play a role i n regulating O 3 - induced changes in gene expression; however,
its role is not yet well defined. Changes in phytohormones and the interactions between them reveal
some of the complexity of plant responses to an oxidative stressor such as O 3

.

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39 Antioxidant met abolites, such as ascorbate and glutathione, and the enzymes that regenerate
them are a critical part of plant defense responses to oxidative stress. The role of ascorbate, which is
located in several cellular compartments and also in the apoplast, was com prehensively evaluated in

the 2006 O 3 AQCD as a first line of defense (due to its location in the apoplast) against oxidative
stress. Ascorbate has also been the focus of studies investigating differences in O 3 tolerance between
plant species or cultivars/ genotypes within one species. While the studies evaluated for the current
document support the important role of ascorbate, several studies suggest that ascorbate quantity,
especially in the apoplast, is not the primary factor in determining plant toleranc e to O 3 . In summary,
antioxidant metabolites and enzymes increase in quantity in plants exposed to O 3 . In most cases,
there is a correlation between the degree to which these defensive systems are induced and the
ability of the plant to tolerate exposure to O 3 . This up

- regulation of antioxidant defenses and the
need to keep antioxidant metabolites in a reduced state requires a significant shift in C metabolism
away from growth and reproduction to sustain the energy needs of the plant for defense.

While decl ines in C fixation as a result of plant exposure to O 3 were extensively described in

the 2006 O 3 AQCD, some recent studies (described in Section 9.4.5.1 ) of O 3 - induced declines in
photosynthesis have focused also on O 3 effects on the light reactions. Decli nes in the Fv/Fm ratio (a
measure of the maximum efficiency of the light reactions of photosynthesis) were observed in
several studies using a variety of plant species and exposure conditions. Additionally, O 3 increased

the coefficient of non

- photochemical quenching in several species, an indication that defense and
repair mechanisms of a non

- photochemical nature are activated in these plants while less absorbed
light is being used to drive photosynthesis. This indicates a shift away from photosynthesis to
defense, resulting in negative impacts on growth and reproduction.

Section 9.4.5.2 evaluates the effects of O 3 on respiration. While C assimilation declines in O 3 exposed plants, respiration is generally up

- regulated. These increases in respiration are th ought to
result from a plant's greater energy needs for defense (maintaining its antioxidant metabolites in a
reduced state) and repair. The increased energy needs will negatively impact plant growth and
reproduction.
Secondary metabolism is most often up

- regulated in a variety of species exposed to either

acute or chronic O 3 exposures as a part of a generalized plant defense mechanism. Changes in gene
expression, quantity and activity of enzymes associated with secondary metabolism and alterations

in secon dary metabolite quantity have been documented in plants exposed to O 3 . Some secondary
metabolites, such as flavonoids and polyamines, are of particular interest as they are known to have
antioxidant properties. Investigations on the importance of isoprenes in plant response to O 3 have
revealed conflicting results; however, there is some evidence to suggest that they may play a
protective role. In summary, secondary metabolites increase in quantity in O 3 - treated plants as part of
a generalized plant defense response. Some secondary metabolites are of particular importance in

O 3 - treated plants as they may have antioxidant functions. Increased synthesis of secondary
metabolites represents a large energy investment of the plant into defense responses and away fr om growth and reproduction.

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1 Section 9.4.6 focuses on O 3 - induced changes in stomatal function. Stomata play a critical role



### 2 in limiting O 3 uptake into the plant by reducing stomatal aperture. Declines in stomatal conductance



### 3 in response to O 3 have been documented for many plant species, and much evidence suggests that



### 4 this results from increases in intercellular CO 2 concentration due to reductions in C fixation.
Additionally, sensitivity of some plants to O 3 has been related to a sluggish stomatal response, in



### 6 which plants are unable to close their stomata rapidly in response to O 3 . To summarize, stomatal



### 7 response to O 3 can help to determine plant sensitivity to the pollutant, and the decreases in stomatal



### 8 conductance are thought to be related to declines in C fixation rates. Reduced stomatal conductance



### 9 will decrease rates of C assimilation and lead to diminished growth and reproduction in plants.



## 2.7.2. Na ture o f Effec ts on Veg eta tio n

Ambient O 3 concentrations have long been known to cause visible foliar injury, d ecreases in



### 11 photosynthetic rate, decreases in growth, and decreases in the quality and yield of some plant species



### 12 (U.S. EPA, 1978, 040586 ; U.S. EPA, 1984, 029711 ; U.S. EPA, 1996, 080827 ; U.S. EPA, 2006,



### 13 088089 ) . Numerous studies have related O 3 exposure to plant responses, with mo st research effort



### 14 focused on the growth of tree seedlings and the yield of crops as endpoints. The response of a plant
species to O 3 exposure depends upon many factors, including genetic characteristics, biochemical



### 16 and physiological status, and previous and current exposure to other stressors. The associated



### 17 sections in Section 9.5 focus mainly on studies published since the release of the 2006 O 3 AQCD



### 18 (U.S. EPA, 2006, 088089 ) . However, because much O 3 research was conducted prior to the 2006 O 3 

### 19 AQCD, the conclusions presented below are collectively based on this ISA as well a s the 1978,

1986, 1996, and 2006 AQCDs (U.S. EPA, 1978, 040586 ; U .S. EPA, 1984, 029711 ; U.S. EPA, 1996,



### 21 080827 ; U.S. EPA, 200 6, 088089 ).



## 2.7.2.1. Effe c ts o n Wo ody a nd He rba c e ous Ve ge ta tio n

Growth and Biomass Allocation

22 The previous O 3 AQCDs concluded that there is strong and consistent evid ence that ambient



### 23 concentrations of O 3 decrease growth in numerous plant species across the U.S. Studies published



### 24 since the last review continue to support that conclusion ( Section 9.5.2.1 ).

A recently published meta

- analysis of 263 studies reported that current ambient O 3 

### 26 concentrations (~40 ppb) significantly decreased annual total biomass growth of forest species by an



### 27 average of 7%, with potentially greater decreases (11 to 17%) in areas that have higher O 3 

### 28 concentrations and as background O 3 increases in the future. This meta

- analysis demonstrates the



### 29 coherence of O 3 effects across numerous studies and species using a variety of experimental
techniques. A study conducted on mature forest trees reported that the cumulative effects of ambient



### 31 levels of O 3 decreased seasonal stem growth by 30

- 50% for most of the species in a high O 3 - year in



### 32 comparison to a low O 3 - year

.

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1 Since the 2006 O 3 AQCD

, several studies were published based on the Aspen free

- air carbon



### 2 dioxide /O 3 enrichment (FACE) experiment using “fr ee air

, ” O 3 , and CO 2 exposures in a forest in



### 3 Wisconsin. It was found that O 3 caused reductions in total biomass relative to the control in aspen,



### 4 paper birch, and sugar maple communities during the first seven years of stand development.

Overall, the stud ies at the Aspen FACE experiment were consistent with many of the open -top



### 6 chamber (OTC) studies that were the foundation of previous O 3 NAAQS reviews. These results



### 7 strengthen our understanding of O 3 effects on forests and demonstrate the relevance of the



### 8 knowledge gained from trees grown in open

- top chamber studies.



### 9 In recent studies, O 3 was shown to have either negative, non

- significant, or positive effects on
root biomass and root:shoot ratio. While the findings of individual studies were mixed, recent meta



### 11 analyses have generally indicated that O 3 reduced C allocated to roots.



### 12 For some annual species, particularly crops, the endpoint for an assessment of the risk of O 3 

### 13 exposure can be defined as yield or growth, e.g., production of grain. For plants grown in mixtures



### 14 such as hayfields, and natural or semi

- natural grasslands (including native nonagricultural species),
endpoints other than production of biomass may be important. Such endpoints include biodiversity



### 16 or species composition, and measures of pl ant quality. Effects may also result from competitive



### 17 interactions among plants in mixed

- species communities. Most of the available data on non -crop


### 18 herbaceous species are for grasslands with many of the recent studies conducted in Europe.



### 19 Evidence is suff icient to conclude that there is a causal relationship between O 3 exposure and reduced growth of woody and herbaceous vegetation.



### Reproduction

21 Studies during recent decades have demonstrated O 3 effects on different stages of plant



### 22 reproduction ( Section 9.5 .2.2 ) . Several recent studies published since the 2006 O 3 AQCD further



### 23 demonstrate the effects of O 3 on reproductive processes in herbaceous and woody plant species.



### 24 The impacts of O 3 on reproductive development can occur by influencing (1) age at time of
initial flowering, particularly in long

- lived trees that often have long juvenile periods of early growth



### 26 without flower and seed production; (2) flower bud initiation and development; (3) pollen



### 27 germination and pollen tube growth; and (4) seed, fruit, or cone yields and seed quality.



### 28 Evidence is sufficient to conclude that there is a causal relationship between O 3 exposure


### 29 and changes in reproduction of vegetation.



### Visible Foliar Injury

Visible foliar injury resulting from exposure to O 3 has been well cha racterized and



### 31 documented over several decades of research on many tree, shrub, herbaceous, and crop species



### 32 (U.S. EPA, 1978, 040586 ; U.S. EPA, 1984, 029711 ; U.S. EPA, 1996, 080827 ; U.S. EPA, 2006,



### 33 088089 ) ( Section 9.5.2.3 ). Ozone

- induced visible foliar injury symptoms on certain b ioindicator



### 34 plant species are considered diagnostic as they have been verified experimentally in exposure

response studies, using exposure methodologies such as continuous stirred tank reactors (CSTRs),

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1 OTCs, and free

- air fumigation. Experimental evidence has clearly established a consistent



### 2 association of visible injury with O 3 exposure, with greater exposure often resulting in greater and



### 3 more prevalent injury. Since the 2006 O 3 AQCD, several multiple

- year field surveys of O 3 -induced



### 4 visible foliar injury have been conducted at National Wildlife Refuges in Maine, Michigan, New
Jersey, and South Carolina. New sensitive species showing visible foliar injury continue to be



### 6 identified from field surveys and verified in controlled exposure studies.



### 7 The use of b iological indicators in field surveys to detect phytotoxic levels of O 3 is a



### 8 longstanding and effective methodology. The USDA Forest Service through the Forest Health



### 9 Monitoring (FHM) Program (1990

- 2001) and currently the Forest Inventory and Analysis (FIA )
Program has been collecting data regarding the incidence and severity of visible foliar injury on a



### 11 variety of O 3 sensitive plant species throughout the U.S. The network has provided evidence that O 3 

### 12 concentrations were high enough to induce visible symp toms on sensitive vegetation. From repeated



### 13 observations and measurements made over a number of years, specific patterns of areas experiencing



### 14 visible O 3 injury symptoms can be identified.
In addition, a study assessed the risk of O 3 - induced visible folia r injury on bioindicator plants



### 16 in 244 national parks in support of the National Park Service's Vital Signs Monitoring Network. The



### 17 results of the study demonstrated that the risk of visible foliar injury was high in 65 parks (27%),



### 18 moderate in 46 parks (1 9%), and low in 131 parks (54%). Some of the well

- known parks with a high



### 19 risk of O 3 - induced visible foliar injury include : Gettysburg, Valley Forge, Delaware Water Gap, Cape
Cod, Fire Island, Antietam, Harpers Ferry, Manassas, Wolf Trap Farm Park, Mammoth Cave, Shiloh,



### 21 Sleeping Bear Dunes, Great Smoky Mountains, Joshua Tree, Sequoia and Kings Canyon, and



### 22 Yosemite.



### 23 Evidence is sufficient to conclude that there is a causal relationship between ambient O 3


### 24 exposure and the occurrence of O 3 - induced visible foli ar injury on sensitive vegetation across

the U.S.



### Gas Exchange

26 There is strong experimental evidence over several decades of research that exposure to O 3 

### 27 reduces photosynthesis and alters stomatal conductance in a wide variety of plant species. The mode



### 28 of action, as characterized in Section 9.4 and in previous reviews, provides biological plausibility for



### 29 O 3 effects on leaf gas exchange.
In compiling more than 55 studies, a meta -analysis reported that current O 3 concentrations in



### 31 the northern hemisphere are decreasing photosynthesis (11%) and stomatal conductance (13%)



### 32 across tree species. It was also found that younger trees less than four years old ) were affected less



### 33 by O 3 than older trees. Further, the authors also found that decreases in photosynthesis are consistent



### 34 with the cumulative uptake of O 3 into the leaf. In contrast, several studies reported that O 3 exposure may result in loss of stomatal control, incomplete stomatal closure at night and a decoupling of



### 36 photosynthesis and stomatal conductance, which may have implications for whole

- plant water use



### 37 ( Section 9.6.3 ).

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1 Evidence is sufficient to conclude that there is a causal relationship between O 3 exposure


### 2 and the alteration of leaf gas exchange in vegetation.



## 2.7.2.2. Agric ultura l Cro ps

Yield and Crop Qua lity

3 The detrimental effect of O 3 on crop production has been recognized since the 1960s and a



### 4 large body of research has subsequently stemmed from those initial findings. Previous O 3 AQCDs
have extensively reviewed this body of literature (U.S. EPA, 2006, 088089 ) . Recent experimental



### 6 studies of O 3 effects on crops are discusse d in Section 9.5.3 and summarized in Table s 9- 3 and 9

- 16

. 

### 7 Current O 3 concentrations across the U.S. are high enough to cause yield loss for a variety of



### 8 agricultural crops including, but not limited to, soybean, wheat, cotton, potato, watermelon, beans,



### 9 tu rnip, onion, lettuce, and tomato. Continued increases in O 3 concentration may further decrease
yield in these sensitive crops while also initiating yield losses in less sensitive crops. Despite the



### 11 well

- documented yield losses due to increasing O 3 concentr ation, there is still a knowledge gap



### 12 pertaining to the exact mechanism of O 3 - induced yield loss. Research has linked increasing O 3 

### 13 concentration to decreased photosynthetic rates and accelerated senescence, which are related to



### 14 yield.
Recent modeling rese arch has correlated satellite air

- column observations with direct air



### 16 sampling O 3 data and modeled the yield

- loss due to O 3 over the continuous tri

- state area of Illinois,



### 17 Iowa and Wisconsin. This modeling data correlates well with the previous results fro m FACE

- type



### 18 experiments and OTC experiments.



### 19 New research is beginning to consider the mechanism of damage caused by long, lower O 3 concentration (so

- called chronic exposure) compared to short, very high O 3 concentration (so

- called



### 21 acute exposure). Both ty pes of O 3 exposure cause damage to agricultural crops, but through very



### 22 different mechanisms. Until recently, most research on the mechanism of O 3 damage has used acute



### 23 exposure studies. It has become clear that the same cellular and biochemical processes involved in



### 24 the response to acute O 3 exposure are not involved in response to chronic O 3 exposure, yet both
cause yield

- loss in agriculturally important crops.



### 26 In addition, new research has highlighted the effects of O 3 on crop quality. Increasing O 3 

### 27 conce ntration decreases nutritive quality of grasses, decreases macro

- and micro -nutrient



### 28 concentrations in fruits and vegetable crops, and decreases cotton fiber quality. These areas of



### 29 research require further investigation to determine the mechanism and dose

- responses.

Evidence is sufficient to conclude that there is a causal relationship between O 3 exposure


### 31 and reduced yield and quality of agricultural crops.



## 2.7.2.3. Fa c tors Tha t Mo dify Fu nc tio na l a nd Gro wth Re s pons e

32 Many biotic and abiotic factors, including insect s, pathogens, root microbes and fungi,



### 33 temperature, water and nutrient availability, and other air pollutants, as well as elevated CO 2 ,

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1 influence or alter plant response to O 3 . These modifying factors were comprehensively reviewed in



### 2 the 2006 O 3 AQCD. A li mited number of studies published since 2006 provide further support for



### 3 our understanding of the role of these interactions in modifying O 3 - induced plant responses and are



### 4 discussed in Section 9.5.4

.



## 2.7.3. Ecos ys tems and Services

Ozone has been found to alter plant physiological processes such as growth, biomass



### 6 allocation, reproduction and gas exchange (Section 9.5 ). Those O 3 - induced effects at the individual



### 7 plant scale have the potential to translate to effects at the ecosystem level, and cause changes in



### 8 bio geochemical cycling and community composition. Information presented in the associated section



### 9 ( Section 9.6 ) was collected at multiple scales, ranging from responses at the population level to the
ecosystem level. The effects of O 3 on ecosystem productivit y, C sequestration, water cycling,



### 11 nutrient cycling, and community composition are reviewed.



## 2.7.3.1. Productivity and Carbon Se ques tration

12 During the previous NAAQS reviews, there were very few studies that investigated the effect



### 13 of O 3 exposure on ecosystem prod uctivity and C sequestration. Recent studies from long

- term FACE



### 14 experiments provided evidence of the association of O 3 exposure and reduced productivity at the
ecosystem level. Elevated O 3 reduced stand

- level biomass by 13

- 23% at Aspen FACE after 7 years



### 16 of O 3 exposure, and annual volume growth by 9.5 m 3 /ha at the Kranzberg Forest (Germany) FACE.



### 17 Studies at the leaf and plant scales showed that O 3 reduced photosynthesis and plant growth, which



### 18 provided coherence and biological plausibility for the decrease in ecosystem productivity. Results



### 19 across different ecosystem models were consistent with the FACE experimental evidence, which

showed that O 3 reduced ecosystem productivity.



### 21 Although O 3 generally causes negative effects on plant growth, the magnitude of the response



### 22 varies among plant communities. For example, O 3 had little impact on white fir, but greatly reduced



### 23 growth of ponderosa pine in southern California. Ozone decreased net primary production (NPP) of



### 24 most forest types by 7

- 8% in Mid

- Atlantic reg ion, but had small impacts on spruce

- fir forest, which
was decreased by only 1%. Among crop species, the estimated yield loss for wheat (7

- 12%) and



### 26 soybean (6

- 16%) were higher than rice (3

- 4%) and maize (3

- 5%).



### 27 In addition to plant growth, other indicator s that are typically estimated by model studies



### 28 include net ecosystem CO 2 exchange (NEE), C sequestration, and crop yield. Model simulations



### 29 consistently found that O 3 exposure caused negative impacts on those indicators ( Section 9.6.2,

Table 9 -5) , but the severity of these impacts was influenced by multiple interactions of biologi cal



### 31 and environmental factors. For example, the largest O 3 - induced crop yield losses occurred in high



### 32 production areas exposed to high O 3 concentrations, such the Midwest and the Mississippi Valley



### 33 regions of the U.S.

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1 The suppression of ecosystem C sinks results in more CO 2 accumulation in the atmosphere.



### 2 Globally, the indirect radiative forcing, reported in Watts/square meter (W/m 2 ), caused by O 3 

### 3 exposure through lowering ecosyst em C sink (0.62

- 1.09 W/m 2 ) could have an even greater impact



### 4 on global warming than the direct radiative forcing of O 3 (0.89 W/m 2 ). Ozone could also affect
regional C budgets through interacting with multiple factors, such as N deposition, elevated CO 2 an d



### 6 land use history. Model simulations suggested that O 3 partially offset the growth stimulation caused



### 7 by elevated CO 2 and N deposition in both Northeast

- and Mid Atlantic

- region forest ecosystems of



### 8 the U.S.



### 9 The evidence is sufficient to infer that there is a causal relationship between O 3 exposure and reduced productivity, and there is likely to be causal relationship between O 3 exposure and



### 11 reduced carbon sequestration in terrestrial ecosystems.



## 2.7.3.2. Wa te r Cyc ling

12 Although the evidence was from a limited numb er of field and modeling studies, these



### 13 findings showed an association of O 3 exposure and the alteration of water cycle at the ecosystem



### 14 level. Field studies suggested that peak hourly O 3 exposure increased the rate of water loss from
several tree species, and led to a reduction in the late

- season modeled stream flow in three forested



### 16 watersheds in eastern Tennessee. Evidence of sluggish stomatal responses during O 3 exposure was


### 17 found in their study and several other studies ( Section 9.6.3 ), which provided biological plausibility



### 18 for the observed higher water loss at the ecosystem level. However, many experiments, mostly based



### 19 on short

- term O 3 exposure, found that O 3 generally reduced stomatal conductance. The O 3 -induced
reduction in stomatal aperture is the biological assumption for most process

- based models.



### 21 Therefore, results of those models normally found that O 3 reduced water loss. For example, one


### 22 study found that O 3 damage and N limitation together reduced evapotranspiration and increase



### 23 runoff.



### 24 Althou gh the direction of the response differed among studies, the evidence is sufficient to

conclude that there is likely to be a causal relationship between O 3 exposure and the alteration



### 26 of ecosystem water production.



## 2.7.3.3. Below

- Ground Processes

27 Since the 2006 O 3 AQCD, more evidence has shown that although the responses are often



### 28 species specific, O 3 altered the quality and quantity of C input to soil, microbial community



### 29 composition, and C and nutrient cycling. Biogeochemical cycling of below

- ground processes is

d riven by C input from plants. Studies at the leaf and plant level have provided biologically plausible



### 31 mechanisms, such as reduced photosynthetic rates, increased metabolic cost, and reduced root C



### 32 allocation ( Section 9.6.4 ) for the association of O 3 expos ure and the alteration of below -ground



### 33 processes.

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1 Results from Aspen FACE and other experimental studies consistently found that O 3 reduced



### 2 litter production and altered C chemistry, such as soluble sugars, soluble phenolics, condensed



### 3 tannins, lignin, and macro/micro nutrient concentration in litter. The changes in substrate quality and



### 4 quantity could alter microbial metabolism under elevated O 3 , and therefore soil C and nutrient
cycling. Several studies indicated that O 3 generally suppressed soil enzyme activities. However, the



### 6 impact of O 3 on litter decomposition was inconsistent and varied among species, sites and exposure



### 7 length. Ozone had small impact on dynamics of micro and macro nutrients, except for N. Ozone was



### 8 found to reduce N release from leaf litter and decrease gross N mineralization, which could



### 9 potentially decrease N availability to plants.
Studies from the Aspen FACE experiment suggested that the response of below -ground



### 11 C cycle to O 3 exposure, such as litter decomposition, soil respirati on and soil C content, changed



### 12 over time. For example, in the early part of the experiment (1998

- 2003), O 3 had no impact on soil



### 13 respiration but reduced the formation rates of total soil C under elevated CO 2 . However, after 10

­ 

### 14 11 y ea rs of exposure, O 3 was found to increase soil respiration but have no significant impact on soil

C formation under elevated CO 2 ( Section 9.6.4.1 ).



### 16 The evidence is sufficient to infer that there is a causal relationship between O 3 exposure


### 17 and the alteration of below

- ground biog eochemical cycles.



## 2.7.3.4. Community Compos ition

18 In the 2006 O 3 AQCD, the impact of O 3 exposure on species competition and community



### 19 composition was assessed. Ozone was found to cause a significant decline in ponderosa and Jeffrey
pine in the San Bernardino Mountains in southern California . Ozone

- exposure also tended to shift



### 21 the grass

- legume mixtures in favor of grass species (U.S. EPA, 2006, 088089 ) . Since the 2006 O 3 

### 22 AQCD, more evidence has shown that O 3 exposure changed the competitive interactions and led to



### 23 loss of O 3 sensitive species or genotypes. Studies at plant level found th at the severity of O 3 damage



### 24 on growth, reproduction and foliar injury varied among species (Section 9.6.5), which provided the
biological plausibility for the alteration of community composition. Additionally, research since the



### 26 last review has shown that O 3 can alter community composition and diversity of soil microbial



### 27 communities.



### 28 The decline of conifer forests under O 3 exposure was continually observed in several regions.



### 29 Ozone damage was believed to be an important causal factor in the dramatic decline of sacred fir in
the valley of Mexico, as well as c embran pine in southern France and Carpathian Mountains. Results



### 31 from the Aspen FACE site indicated that O 3 could alter community composition of broadleaf forests



### 32 as well. At the Aspen FACE site, O 3 redu ced growth and increased mortality of a sensitive aspen



### 33 clone, while the O 3 tolerant clone emerged as the dominant clone in the pure aspen community. In



### 34 the mixed aspen

- birch and aspen

- maple communities, O 3 reduced the competitive capacity of aspen

compare d to birch and maple.



### 36 The tendency for O 3 - exposure to shift the biomass of grass

- legume mixtures in favor of grass



### 37 species, was reported in the 2006 O 3 AQCD (U.S. EPA, 2006

, 088089 ) and has been generally

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1 confirmed by recent studies. However, in a high elevation mature/species

- rich grass

- legume pasture,



### 2 O 3 fumigation showed no significant impact on community composition.



### 3 Ozone exposure not only altered community composition of plant species, but also



### 4 microorganisms. The shift in community composition of bacteria and fungi has been observed in
both natural and agricultural ecosys tems, although no general patterns could be identified.



### 6 The evidence is sufficient to conclude that there is likely to be a causal relationship between



### 7 O 3 exposure and the alteration of community composition.



## 2.7.4. Air Qu ality Indic es

8 Exposure indices are metri cs that quantify exposure as it relates to measured plant damage



### 9 (i.e., reduced growth). They are summary measures of monitored ambient O 3 concentrations over
time intended to provide a consistent metric for reviewing and comparing exposure

- response effect s


### 11 obtained from various studies. No new information is available since 2006 that alters the basic



### 12 conclusions put forth in the 2006 and 1996 O 3 AQCDs (U.S. EPA, 1996, 080827 ; U.S. EPA, 2006,



### 13 088089 ) . These AQCDs focused on the rese arch used to develop various exposure indices to help



### 14 quantify effects on growth and yield in crops, perennials, and trees (primarily seedlings). The
performance of indices was compared through regression analyses of earlier studies designed to



### 16 support the estimation of predictive O 3 exposure

- response models for growth and/or yield of crops



### 17 and tree (seedling) species.



### 18 The main conclusions from the 1996 and 2006 O 3 AQCDs (U. S. EPA, 1996, 080827 ;


### 19 U.S. EPA, 2006, 088089 ) regarding an index based on ambient exposure are still valid. These key

conclusions can be restated as follows:



### 21  O 3 effects in plants are cumulative;



### 22  higher O 3 concentrations appear to be more important than lower concentrations in



### 23 eliciting a response;



### 24  plant sensitivit y to O 3 varies with time of day and plant development stage; and

 exposure indices that cumulate hourly O 3 concentrations and preferentially weight the



### 26 higher concentrations have better statistical fits to growth/yield response data than do the



### 27 mean and pea k indices.



### 28 Various weighting functions have been used, including threshold

- weighted (e.g., SUM06) and



### 29 continuous sigmoid

- weighted (e.g., W126) functions. Based on statistical goodness -of- fit tests, these

cumulative, concentration

- weighted indices could not be differentiated from one another using data



### 31 from previous exposure studies. Additional statistical forms for O 3 exposure indices are summarized



### 32 in Section 9.7 of this ISA. The majority of studies published since the 2006 O 3 AQCD (2006,

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1 088089 ) do not change earlier conclusions, including the importance of peak concentrations, and the



### 2 duration and occurrence of O 3 exposures in altering plant growth and yield.



### 3 Given the current state of knowledge and the best available data, exposure indices that



### 4 cumulate and differentially weight the higher hourly average concentrations and also include the

mid

- level values continue to offer the most defensible approach for use in developing response



### 6 functions and comparing studies, as well as for defining future indices for vegetation protection.



## 2.7.4.1. Mo de le d Ozone De p os ition or "Flux"

7 Another approa ch for improving risk assessment of vegetation response to ambient O 3 is based



### 8 on determining the O 3 concentration from the atmosphere that enters the leaf (i.e., flux or



### 9 deposition). Interest has been increasing in recent years, particularly in Europe, in using
mathematically tractable flux models for O 3 assessments at the regional, national, and European



### 11 scale. While some efforts have been made in the U.S. to calculate O 3 flux into leaves and canopies,



### 12 little information has been published relating these fluxes to effects on vegetation. There is also



### 13 concern that not all O 3 stomatal uptake results in a yield reduction, which depends to some degree on



### 14 the amount of internal detoxification occurring with each particular species. Those species having
high amounts of detoxification potential may, in fact, show little relationship between O 3 stomatal



### 16 uptake and plant response. The lack of data in the U.S. and the lack of understanding of



### 17 detoxification processes have made this technique less viable for vulnerabi lity and risk assessments



### 18 in the U.S.



## 2.7.4.2. Night

- Time Exp os ures

19 A 12

- h (8 :00 a.m. to 8 :00 p.m. ) daylight period for a W126 cumulating exposure was proposed
in 2007 and 2009 following the release of the 2006 O 3 AQCD and was based primarily on evidence



### 21 that the conditions for uptake of O 3 into the plant occur mainly during the daytime hours. Plants have



### 22 the highest stomatal conductance during the daytime and atmospheric turbulent mixing is greatest



### 23 then as well (U.S. EPA, 2006, 088089 ) . Recent reviews of the literature reported that a large number



### 24 of species had varying degrees of noc turnal stomatal conductance. In general, stomatal conductance
at night is at a much lower rate compared to daytime conductance. For significant nocturnal stomatal



### 26 flux and O 3 effects to occur, specific conditions must exist. A susceptible plant with noctur nal



### 27 stomatal conductance and low defense must be growing in an area with relatively high night

- time O 3 

### 28 and appreciable nocturnal turbulence. It is unclear how many areas there are in the U.S. where these



### 29 conditions occur. More information is needed in thes e locations in order to assess the local O 3 patterns, micrometeorology and responses of potentially vulnerable plant species.



## 2.7.5. Expos ure

- Response

31 None of the information on effects of O 3 on vegetation published since the 2006 O 3 AQCD has



### 32 modified the assess ment of quantitative exposure

- response relationships that was presented in that

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- response models
by computing them using the W126 metric, cumulated over 90 days. Almost all of the experimental

research on the effects of O 3 on growth or yield of plants p ublished since 2006 used only two levels
of exposure. In addition, hourly O 3 concentration data that would allow calculations of exposure
using the W126 scale are generally unavailable. However, two long

- term experiments, one with a
crop species (soybean), one with a tree species (aspen), have produced data that can be used to
validate the exposure

- response models presented in the 2006 O 3 AQCD, and methodology used to

derive them.
Quantitative characterization of exposure

- response in the 2006 O 3 AQCD was b ased on
experimental data generated for that purpose by the National Crop Loss Assessment Network
(NCLAN) and EPA National Health and Environmental Effects Research Laboratory, Western
Ecology Division (NHEERL

- WED) projects, using OTCs to expose crops and trees seedling to O 3

. In recent years, yield and growth results for two of the species that had provided extensive exposure

response information in those projects have become available from studies that used FACE
technology, which is intended to provide co nditions much closer to natural environments. The robust
methods that were used previously with exposure measured as SUM06 were applied to the NCLAN

and NHEERL

- WED data with exposure measured as W126, in order to derive single

- species median
models for soy bean and aspen from studies involving different genotypes, years, and locations. The
resulting models were used to predict the change in yield of soybean and biomass of aspen between
the two levels of exposure reported in current FACE experiments. Results from these new
experiments were exceptionally close to predictions from the models. The accuracy of model
predictions for two widely different plant species provides support for the validity of the
corresponding multiple

- species models for crops and trees in the NCLAN and NHEERL

- WED
projects. However, variability among species in those projects indicates that the range of sensitivity
is likely quite wide. This was confirmed by a recent experiment with cottonwood in a naturally
occurring gradient of exposure , which established the occurrence of species with responses
substantially more severe under currently existing conditions than are predicted by the median model
for multiple species.

Results from several meta

- analyses have provided approximate values for responses of yield
of soybean, wheat, rice and other crops under broad categories of exposure, relative to charcoal

filtered air. Additional reports have summarized yield data for six crop species under various broad
comparative exposure categories, and re viewed 263 studies that reported effects on tree biomass.
However, these analyses have proved difficult to compare with exposure

- response models, especially
given that exposure was not exp ressed on the same W126 scale.

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# 2.8. Th e Role o f Tro p o s ph e ric Ozone in Cl imate Change and UV

- B Effects

1 Atmospheric O 3 plays an important role in the Earth's energy budget by interacting with



### 2 incoming solar radiation and outgoing infrared radiation. Tropospheric O 3 makes up only a small



### 3 portion of the total column of O 3 , but it has important incremental effects on the overall radiation



### 4 budget. Chapter 10 assesses the specific role of tropospheric O 3 in the earth's radiation budget and
how perturbations in tropospheric O 3 might affect climate through its role as a greenhouse gas



### 6 ( Section 10.2), and health, ecology and welfare through its role in shielding the earth’s surface from



### 7 solar ultraviolet radiation ( Section 10.3 ).



## 2.8.1. Tro po s p h eric Ozo n e as a Greenhouse Gas

8 Tropospheric O 3 is a major greenhouse gas , and increases in its abundan ce may contribute to



### 9 climate change according to the 2007 climate assessment by the Intergovernmental Panel on Climate
Change (IPCC). Models calculate that the global burden of tropospheric O 3 has doubled since the



### 11 preindustrial era, while observations ind icate that in some regions O 3 may have increased by factors



### 12 as great as 4 or 5

. The se increase s are tied to the rise in emissions of O 3 precursors from human



### 13 activity, mainly fossil fuel consumption and agricultural processes

. The impact on climate of the O 3 

### 14 change since preindustrial times has been estimated to be about 25

- 40% of anthropogenic CO 2 impact and about 75% of anthropogenic CH 4 impact according to the IPCC, ranking it third in



### 16 importance among the greenhouse gases.



### 17 The metric frequently used to estimate the potential climate impact of O 3 is called radiative



### 18 forcing (RF). RF is a change in the radiative balance at the tropopause or at the top of the



### 19 atmosphere when a perturbation is introduced in the earth

- atmosphere

- ocean system. The units of RF
are energy flux per unit area, or W/ m 2 , and p ositive values indicate warming while negative values



### 21 indicate cooling

. The IPCC estimates a radiative forcing of 0.35 W/m 2 for the change in tropospheric



### 22 O 3 since the preindustrial era, compared to 1.66 W/m 2 fo r CO 2 and 0.48 W/m 2 for CH 4 . The error



### 23 bars encompassing the tropospheric O 3 radiative forcing estimate range from 0.25 to 0.65 W/m 2

, 

### 24 making it relatively more uncertain than the long

- lived greenhouse gases. Despite these

uncertainties, there is a causal r elationship between tropospheric O 3 and radiative forcing

. 

### 26 RF does not take into account the climate feedbacks that could amplify or dampen the actual



### 27 surface temperature response

. Quantifying the change in surface temperature requires a complex



### 28 climate si mulation in which all important feedbacks are accounted for. As these processes are not



### 29 well understood or easily modeled , the surface temperature response to a given RF is highly
uncertain and can vary greatly among models and from region to region within the same model

. 

### 31 Despite these uncertainties, there is likely to be a causal relationship between tropospheric O 3 

### 32 and climate change

.

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## 2.8.2. Tro po s p h eric Ozo n e a nd UV

- B rela ted e ffe cts

Ultraviolet (UV) radiation emitted from the Sun contains sufficient energy whe n it reaches the
Earth to break (photolyze) chemical bonds in molecules, thereby leading to damaging effects on
living organisms and materials. Atmospheric O 3 plays a crucial role in reducing exposure to solar

UV radiation at the Earth's surface. Ozone in the stratosphere is responsible for the majority of this
shielding effect, as approximately 90% of total atmospheric O 3 is located there over mid

- latitudes.
Ozone in the troposphere provides supplemental shielding of radiation in the wavelength band from

2 80

- 315 nm , UV

- B radiation. UV

- B radiation has important effects on human health and ecosystems,
and is associated with materials damage.

Adverse human health effects associated with solar UV

- B radiation exposure include
erythema, skin cancer, ocular damage , and immune system suppression. A potential human health

benefit of increased UV

- B exposure involves the UV

- induced production of vitamin D which may
help reduce the risk of metabolic bone disease

, type I diabetes

, mellitus

, and rheumatoid arthritis

, and may provide beneficial immunomodulatory effects on multiple sclerosis, insulin

- dependent

diabetes mellitus, and rheumatoid arthritis.
Adverse ecosystem and materials damage effects are also associated with solar UV

- B radiation

exposure. T errestrial ecosyst em effects from increased UV

- B radiation include reduced plant
productivity and plant cover, changes in biodiversity, susceptibility to infection, and increases in
natural UV protective responses. In general, however, these effects are small for moderate U V- B

increases at mid

- latitudes. Aquatic ecosystem effects from increased UV

- B radiation include
sensitivity in growth, immune response, and behavioral patterns of aquatic organisms.

B iogeochemical cycles

, particularly the carbon cycle, can also be influenced by increased UV

- B

radiation with effects ranging from UV

- induced increases in CO 2 uptake through soil respiration to

UV

- induced release of CO 2 through photodegradation of above

- ground plant litter. Changes in solar
UV radiation may also have effects on carbon cycling and CO 2 uptake in the oceans as well as
release of dissolved organic matter from sediment and algae. Finally, m aterials damage from

increased UV

- B radiation include UV

- induced photodegradation of wood and plastics.
There is a lack of publish ed studies that critically examine the incremental health or welfare
effects (adverse or beneficial) attributable specifically to changes in UV

- B exposure resulting from
perturbations in tropospheric O 3 concentrations. While the effects are expected to be small, they
cannot yet be critically assessed within reasonable uncertainty . Overall, the evidence is inadequate

to determine if a causal relationship exists between tropospheric O 3 and UV

- B related health

and welfare effects.

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# 2.9. Summary of Caus al Determinati ons for Health Effects an d Welfare Effects

1 This chapter has provided an overview of the underlying evidence used in making the causal



### 2 determinations for the health and welfare effects of O 3

. This review builds upon the conc l usions of



### 3 the previous AQCDs fo r O 3 (U.S. EPA, 1978, 040586 ; U.S. EPA, 1984, 029711 ; U.S. EPA, 1996,



### 4 017831 ; U.S. EPA, 1996, 080827 ; U.S. EPA, 2006, 088089 ).



### 5 The evaluation of the epidemiologic, toxicological, and controlled human exposure studies



### 6 published since the completion of the 2006 O 3 AQCD have provided additional evidence for O 3 ­ 

### 7 related health outcomes . Table 2

- 5 provides an overview of the causal determinations for all of the



### 8 health outcomes evaluated . Causal determinations for O 3 and welfare effects are included in



### 9 Table 2-6 , while causal determinations for climate change and UV

- B effects are in Table 2 -7.



### 10 Detailed discussions of the scientific evidence and rationale for these causal determinations ar e


### 11 provided in subsequent chapters of this ISA.



### Table 2 -5 . Summary of ozone causal determinations by exposure duration and health outcome

Health Outcome Conclusion s from 2006 O 3 AQCD Conclusions from 2011 1st Draft ISA

Short

- Term Exposure to O 3

Respirat ory effects The overall evidence supports a causal relationship between acute ambient O 3 exposures and increased respiratory morbidity outcomes. Causal R elationship

Cardiovascular effects The limited evidence is highly suggestive that O 3 directly and/or i ndirectly contributes to cardiovascular

- related morbidity, but much remains to be done to more fully substantiate the association. Suggestive of a Causal Relationship

Central nervous system effects Toxicological studies report that acute exposures to O 3 a re associated with alterations in neurotransmitters, motor activity, short and long term memory, sleep patterns, and histological signs of neurodegeneration. Suggestive of a Causal Relationship

All

- cause mortal ity The evidence is highly suggestive that O 3 directly or indirectly contributes to non

- accidental and cardiopulmonary

- related mortality. Likely to be a Causal Relationship
Long

- term Exposure to O 3
Respiratory effects The current evidence is suggestive but inconclusive for respiratory health effect s from long

- term O 3 exposure. Likely to be a Causal Relationship

Cardiovascular Effects No studies from previous review Suggestive of a Causal Relationship

Reproductive and developmental effects Limited evidence for a relationship between air pollution a nd birth

- related health outcomes, including mortality, premature births, low birth weights, and birth defects, with little evidence being found for O 3 effects. Suggestive of a Causal Relationship

Central nervous system effects Toxicological studies report that acute exposures to O 3 are associated with alterations in neurotransmitters, motor activity, short and long term memory, sleep patterns, and histological signs of neurodegeneration. Suggestive of a Causal Relationship

Cancer Little evidence for a rel ationship between chronic O 3 exposure and increased risk of lung cancer. Inadequate to infer a Causal Relationship

Mortality There is little evidence to suggest a causal relationship between chronic O 3 exposure and increased risk for mortality in humans. Suggestive of a Causal Relationship

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### Table 2 -6 . Summary of ozone causal determination for welfare effects

Vegetation and Ecosystem
Effects Conclusions from 2006 O 3 AQCD Conclusions from 2011 1st
Draft ISA

Reduced Vegetation Growth Data published since the 1996 AQCD strengthen previous conclusions that there is strong evidence that current ambient O 3 concentrations cause decreased growth and biomass accumulation in annual, perennial and woody plants, including agronomic crops, annuals, shrubs, grasses, and trees. Causal Relationship

Alteration of Vegetation Reproduction For several decades, studies have demonstrated O 3 effects on different stages of reproduction. Causal Relationship

Visible Foliar Injury Effects on Vegetation Data published since the 1996 AQCD strengthen previous conclusions that there is strong evidence that current ambient O 3 concentrations cause impaired aesthetic quality of many native plants and trees by increasing foliar injury. Causal Relationship

Alteration of Leaf Gas Exchan ge in Vegetation Ozone exposure reduces photosynthesis , and the mechanisms of this reduction are better understood as a result of the research since the 1996 AQCD. Causal Relationship

Reduced Yield and Quality of Agricultural Crops Data published since th e 1996 AQCD strengthen previous conclusions that there is strong evidence that current ambient O 3 concentrations cause decreased yield and/or nutritive quality in a large number of agronomic and forage crops. Causal Relationship

Reduced Productivity in Te rrestria l Ecosystems There is evidence that O 3 is an important stressor of ecosystems and that the
effects of O 3 on individual plants and processes are scaled up through the ecosystem, affecting net primary productivity. Causal Relationship

Reduced Carbon (C) Sequestration in Terrestrial Ecosystems Limited studies from previous review Likely to be a Causal Relationship

Alteration of Terrestrial Ecosystem Water Cycling Ecosystem water quantity may be affected by O 3 exposure at the landscape level. Likely t o be a Causal Relationship

Alteration of Below

- ground Biogeochemical Cycles Ozone

- sensitive species have well known responses to O 3 exposure , including altered C allocation to below

- ground tissues, and altered rates of leaf and root production, turnover, and decomposition. These shifts can affect overall C and N loss from the ecosystem in terms of respired C, and leached aqueous dissolved organic Causal Relationship

and inorganic C and N.

Alteration of Terrestrial Community Composition Ozone may be affe cting above

- and below

- ground community composition through impacts on both growth and reproduction. Significant changes in plant community composition resulting directly from O 3 exposure have been
demonstrated. Likely to be a Causal Relationship



### T

able 2

- 7. Summary of ozone causal determination for climate change and UV

- B effects

Effects Conclusions from 2006 O 3 AQCD Conclusions from 2011 1st Draft ISA

Radiative Forcing Climate forcing by O 3 at the regional scale may be its most important impact o n climate

. Causal Relationship

Climate Change While more certain estimates of the overall importance of global

- scale forcing due to tropospheric O 3 await further advances in monitoring and chemical transport modeling, the overall body of scientific eviden ce suggests that high concentrations of O 3 on the regional
scale could have a discernable influence on climate , leading to surface temperature and hydrological cycle changes. Likely to be a Causal Relationship

UV

- B Related Health and
Welfare Effects UV -B has not been studied in sufficient detail to allow for a credible health benefits assessment. In conclusion, the effect of changes in surface

- level O 3 concentrations on UV

induced health outcomes cannot yet be critically assessed within reasonable uncerta inty. Inadequate to Determine if a Causal Relationship Exists

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# References

U.S. EPA (1978). Air quality criteria for ozone and other photochemical oxidants (Report No. EPA/600/8 -78/004). Washington, DC: U.S. Environmental Protection Agency, Office of R esearch and Development. 040586

U.S. EPA (1984). Air quality criteria for ozone and other photochemical oxidants, v 3 (Report No. EPA/600/8 -84/020A).

Research Triangle Park, NC: U.S. Environmental Protection Agency, Office of Research and Development. http://nepis.epa.gov/Exe/ZyPURL.cgi?Dockey=2000AVEV.txt. 029711

U.S. EPA (1996). Air quality criteria for ozone and related photochemical oxidants (Report No. EPA/600/P

- 93/004AF). Research Triangle Park, NC: U.S. Environmental Protection Agency, Office of Research and Development. (NTIS No. PB94173127) 017831

U.S. EPA (1996). Air quality criteria for ozone and related photochemical oxidants Volume II of III (Report No.

EPA/600/P

- 93/004BF). Research Triangle Park, NC: U.S. Environmental Protection Agency, Office of Research and Development. 080827

U.S. EPA (2006). Air quality criteria for ozone and related photochemical oxidants (Report No. EPA/600/R -05/004AF). Research Triangle Park, NC: U.S. Environmental Protection Agency, Office of Research and Development. http://cfpub.epa.gov/ncea/cfm/recordisplay.cfm?deid=149923

. 088089

U.S. EPA (2010). Our nation's air: Status and trends through 2008 (Report No. EPA/454/R

- 09/002). Research Triangle Park, NC: U.S. Environmental Protection Agency, Office of Air Quality Planning and Standards.

htt p://www.epa.gov/airtrends/2010/report/fullreport.pdf

. 647278

Note: Hyperlinks to the reference citations throughout this document will take you to the NCEA HERO database (Health and
Environmental Research O nline) at http://epa.gov/hero . HERO is a database of scientific literature used by U.S. EPA in the process of developing science assessments such as the Integrated Science Assessments (ISAs) and the Integrated Risk Informa tion System (IRIS).

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