Welfare Risk and Exposure Assessment for 

Ozone 

Second External Review Draft 

Executive Summary 

 
EPA-452/P-14-003c 

February 2014 

Welfare Risk and Exposure Assessment for Ozone 

Second External Review Draft 

Executive Summary

U.S. Environmental Protection Agency 

Office of Air and Radiation 

Office of Air Quality Planning and Standards 

Health and Environmental Impacts Division 

Risk and Benefits Group 

Research Triangle Park, North Carolina 27711 
DISCLAIMER

This draft document has been prepared by staff from the Risk and Benefits Group, Health and 

Environmental Impacts Division, Office of Air Quality Planning and Standards, U.S. Environmental 

Protection Agency. Any findings and conclusions are those of the authors and do not necessarily reflect 

the views of the Agency. This draft document is being circulated to facilitate discussion with the Clean 

Air Scientific Advisory Committee to inform the EPA’s consideration of the ozone National Ambient 

Air Quality Standards. 

This information is distributed for the purposes of pre-dissemination peer review under 

applicable information quality guidelines. It has not been formally disseminated by EPA. It does not 

represent and should not be construed to represent any Agency determination or policy. 

Questions related to this preliminary draft document should be addressed to Travis Smith, U.S. 

Environmental Protection Agency, Office of Air Quality Planning and Standards, C539-07, Research 

Triangle Park, North Carolina 27711 (email: smith.jtravis@epa.gov). 

 
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SUMMARY CONCLUSIONS

he goals for this welfare risk and exposure 

assessment (REA) include (i) characterizing 

ambient ozone (O3) exposure and its relationship 

to ecological effects, and (ii) estimating the resulting 

impacts to several ecosystem services. We quantitatively 

characterize the impact of ambient O3 exposures on two 

important ecological effects – biomass loss and visible 

foliar injury – and quantitatively estimate impacts to the 

following ecosystem services: regulating services 

including carbon sequestration and pollution removal; 

provisioning services including timber production and 

agricultural harvesting; and cultural services such as 

recreation. We conduct both national-scale and case 

study analyses for these two ecological effects, and we 

also qualitatively assess impacts on additional ecosystem 

services, including hydrologic cycle, pollination 

regulation, and fire regulation (regulating services); 

commercial non-timber forest products and insect damage 

(provisioning services); and aesthetic and non-use values 

(cultural services). For each of these analyses, we use a 

biologically-relevant cumulative, seasonal form for O3

exposure, the W126 metric, which is measured as ppmhrs.

 For biomass loss, the Clean Air Scientific 

Advisory Committee (CASAC) recommended that EPA 

should consider options for W126 standard levels based 

on factors including a predicted one to two percent 

biomass loss for trees and a predicted five percent loss of 

crop yield. Small losses for trees on a yearly basis 

compound over time and can result in substantial biomass 

losses over the decades-long lifespan of a tree. For trees, 

the annual W126 index values leading to a one percent 

biomass loss range from approximately 4 to 10 ppm-hrs 

and leading to a two percent biomass loss range from 

approximately 7 to 14 ppm-hrs. For crops, the annual 

W126 index values leading to a five percent biomass loss 

range from approximately 12 to 17 ppm-hrs. The 

recommended biomass loss benchmark for crops occurs at 

higher W126 index values than for trees, suggesting that 

potential alternative standards that are protective of trees 

will also be protective of crops. 

Unlike for biomass, CASAC did not recommend 

a benchmark for foliar injury. In general, however, the 

results of several foliar injury analyses demonstrate a 

similar pattern – the proportion of biosites1 showing foliar

injury increases steeply with W126 index values up to

approximately 10 ppm-hrs and is relatively constant at 

W126 index levels above 10 ppm-hrs. While the

proportion of biosites with foliar injury differs, this

general pattern of response to W126 is seen in the foliar 

injury analyses stratified by soil moisture, by year, and by 

geographic region. 

In this REA, we analyzed the changes in risk 

across several potential alternative standard levels after 

adjusting air quality to just meet the existing standard2

and just meet alternative secondary standard levels. 

Overall, the largest reduction in O3-related welfare risk 

occurs when moving from recent ambient conditions to 

just meeting the existing standard. Some welfare 

exposures and risks remain after just meeting the existing 

standard and in many cases, just meeting potential 

alternative standard levels results in reductions in those 

remaining exposures and risks. 

INTRODUCTION

he U.S. Environmental Protection Agency (EPA) 

is conducting a review of the national ambient air 

quality standards (NAAQS) for O3 and related 

photochemical oxidants. This draft welfare REA focuses 

on assessments to inform consideration of the review of 

the secondary (welfare-based) NAAQS for O3. This draft 

REA, which is the second draft for this NAAQS review,

provides an assessment of exposure and risk associated 

with recent ambient concentrations of O3 and potential 

secondary standards. The REA builds on analyses done 

for the previous NAAQS review completed in 2008, 

expands the characterization of risk of ecological effects, 

and adds consideration of impacts to ecosystem services. 

 1 A biosite is a plot of land on which data is collected 

regarding the incidence and severity of visible foliar 

injury on a variety of O3-sensitive plant species. 2 The existing secondary standard for O3 is set identical to 

the primary standard at a level of 0.075 ppm (75 ppb), 

based on the annual fourth-highest daily maximum 8-hour 

average concentration, averaged over three years.

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The W126 metric is a seasonal sum of 

hourly O3 concentrations, designed to 

measure the cumulative effects of O3

exposure on vulnerable plant and tree 

species. The W126 metric uses a 

sigmoidal weighting function to place less 

emphasis on exposure to low 

concentrations and more emphasis on 

exposure to high concentrations.

The REA also focuses on improving the characterization 

of the overall confidence in the risk estimates, including 

related uncertainties, by improving on the methods and 

data used in the previous analyses.

CONCEPTUAL FRAMEWORK

cosystem services are distinct from other 

ecosystem products and functions because there is 

human demand for these services. In the 

Millennium Ecosystem Assessment, ecosystem services 

are classified into four main categories:

• Provisioning -- products obtained from

ecosystems, such as the production of food and 

water.

• Regulating -- benefits obtained from the 

regulation of ecosystem processes, such as the 

control of climate and disease.

• Cultural -- the nonmaterial benefits that people 

obtain from ecosystems through spiritual 

enrichment, cognitive development, reflection, 

recreation, and aesthetic experiences.

• Supporting -- those services necessary for the 

production of all other ecosystem services, such 

as nutrient cycles and crop pollination.

In the previous review of the secondary NAAQS 

for O3, EPA focused the welfare risk assessment on 

estimating changes in biomass loss in forest tree species 

and yield loss in agricultural crops, quantifying foliar 

injury risk, and qualitatively considering effects on 

ecosystem services. In this review, EPA expanded the 

analysis to consider the broader array of impacts on 

ecosystem services resulting from known effects of O3

exposure on ecosystem functions. This expansion 

includes quantifying the risks not just to ecosystems, but 

also to the aspects of public welfare that depend on those 

ecosystems, i.e., ecosystem services. Figure ES-1

illustrates the relationships between the ecosystem 

services and aspects of public welfare that depend on 

those services.

Figure ES-1 Linkages Between Ecosystem Services 

Categories and Components of Human Well-Being (MEA, 

2005)

AIR QUALITY CONSIDERATIONS

he air quality information and analyses for this 

review build upon those in prior reviews and 

include: (1) summaries of recent ambient O3 data; 

(2) application of a methodology to extrapolate measured 

O3 concentrations to areas without monitors, including 

natural areas, such as national parks, that are important to 

a welfare effects assessment; and (3) adjustment of air 

quality to show the distributions of O3 when just meeting 

existing or potential alternative secondary standards. We 

use estimates of O3 exposure (measured as W126) (see 

the text box for a description of the W126 metric) to 

assess exposures and ecological risks associated with 

recent ambient conditions and just meeting the existing 

and alternative 

standards. While 

the existing O3

monitoring 

network has a 

largely urban 

focus, to address

ecosystem impacts 

of O3, it is equally 

important to focus 

E

T
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on monitoring in rural areas. The extent of monitoring 

coverage in non-urban areas has not significantly changed

since the previous review. Figure ES-2 shows the current 

O3 monitoring coverage in the U.S. – both urban and nonurban sites. To estimate O3 exposure in areas without 

monitors, particularly those gaps left by a sparse rural 

monitoring network in the western U.S., we use a spatial 

interpolation technique called Voronoi Neighbor 

Averaging (VNA) to create an air quality surface for the 

contiguous U.S. for each year from 2006 to 2010 and a 

surface for a 3-year average of 2006-2008 data.

 We also consider the changes in W126 index 

values after adjusting O3 concentrations to just meet the 

existing standard and potential alternative W126 standard 

levels of 15, 11, and 7 ppm-hrs. After adjusting the 

monitor values, we generate another 3-year average 

national-scale VNA surface for just meeting the existing 

standard. We then further adjust monitor data to just meet 

potential alternative W126 standard levels of 15, 11, and 

7 ppm-hrs. The adjusted surfaces are used as inputs to 

several assessments, including the geographic analysis to 

assess the effects of insect damage related to foliar injury, 

the national- and case study-scale biomass loss 

assessments, and the national park case studies for foliar 

injury. For the national-scale and screening-level foliar 

injury analyses, to better match the air quality data with 

short-term soil moisture data, we use the surfaces for the 

individual years from 2006 through 2010. 

RISK TO VEGETATION AND ECOSYSTEMS

n this welfare REA, we quantified the impact of O3

exposure on two categories of ecological effects: (1) 

relative biomass loss for trees and crops, and (2) 

visible foliar injury. The results of these ecological 

assessments are inputs to ecosystem service assessments, 

which are described in more detail in the subsequent 

sections.3

 3 In this welfare REA, we do not quantify insect damage 

resulting from O3 exposure. In the next Section, Risk to 

Ecosystem Services, we briefly discuss the ecosystem 

services associated with insect damage on tree stands and 

timber production, including the overlap of areas with 

The first step in assessing biomass loss for tree 

seedlings and crops is to identify the range of W126 index 

values corresponding to benchmarks recommended by 

CASAC. To compare different levels of biomass loss to 

different W126 index values, we plot the concentrationresponse (C-R) functions for 12 tree species and 10 crops 

as a function of the percent biomass loss against varying 

index values of W126. For a one percent biomass loss in 

tree seedlings, the estimated annual W126 index values 

are between 4 and 10 ppm-hrs; for a two percent biomass 

loss in tree seedlings the estimated annual W126 index 

values are between 7 and 14 ppm-hrs; and for a five 

percent biomass loss for crops the estimated annual W126 

index values are between 12 and 17 ppm-hrs. In general, 

estimates of biomass loss in tree seedlings are comparable 

to mature trees with a few exceptions. Next, we use these 

C-R functions to determine the range of biomass loss 

associated with just meeting the existing and potential 

alternative W126 standards in analyses of individual 

species as well as combined analyses of individual 

species. 

To assess foliar injury at a national scale and 

identify potential benchmarks, we applied a national data 

set on foliar injury from the U.S. Forest Service’s (USFS) 

Forest Health Monitoring Network, which monitors the 

potential impacts of O3 on our nation’s forests. Our 

analyses provide results using both the presence/absence 

of foliar injury and elevated levels foliar injury. We also 

conduct analyses across years and different soil moisture 

categories. Over 81 percent of biosites in USFS’s data set 

showed no visible foliar injury. Generally, the results of 

all of these foliar injury analyses demonstrate a similar 

pattern – the proportion of biosites showing foliar injury

increases steeply with W126 index values up to

approximately 10 ppm-hrs and is relatively constant at 

W126 index levels above 10 ppm-hrs. We use these 

results to derive W126 benchmarks for visible foliar 

injury for six scenarios representing the full range of 

percentages of biosites showing visible foliar injury (i.e., 

any injury and elevated injury), including five scenarios 

considering soil moisture. For the “base scenario”, the 

benchmark represents the point above which there

 

higher W126 concentrations and risk of bark beetle 

infestation. 

I
ES-4

Figure ES-2 U.S. Ambient O3 Monitoring Sites in Operation During 2006-20104

 4 The map shows the location of all U.S. O3 monitors operating during the 2006-2010 period. The gray dots, which make up over 80 percent of the O3 monitoring network, are “State and Local 

Monitoring Stations” (SLAMS) monitors that are largely operated by state and local governments and largely focused on urban areas. The blue dots highlight two important subsets of the SLAMS 

network: “National Core” (NCore) multipollutant monitoring sites and the “Photochemical Assessment Monitoring Stations” (PAMS) network. The green dots represent the Clean Air Status and Trends 

Network (CASTNET) monitors, which are focused on rural areas. In 2010, there were about 80 CASTNET sites operating, with sites in the Eastern U.S. being operated by EPA and sites in the Western 

U.S. being operated by the National Park Service (NPS). The black dots represent “Special Purpose Monitoring Stations” (SPMS), which include about 20 rural monitors as part of the “Portable O3 

Monitoring System” (POMS) network operated by the NPS. 
ES-5

was a consistent percentage (17.7 percent) of biosites 

showing foliar injury, regardless of soil moisture. This 

analysis suggests that reductions in W126 index values at 

or above this benchmark (W126 > 10.46 ppm-hrs) are 

unlikely to substantially reduce the prevalence of foliar 

injury. Similarly, this analysis suggests that reductions in 

W126 index values below the base scenario benchmark 

are likely to relatively sharply reduce the prevalence of 

foliar injury. We also looked at alternative scenarios 

based on three different categories of soil moisture and 

the W126 index values associated with four different 

prevalences (e.g., 5%, 10%, 15% and 20% of biosites) of 

any foliar injury, and a final scenario based on a 5 percent

prevalence of foliar injury index greater than or equal to 

5. In total, we evaluated 13 different W126 benchmarks 

associated with the six foliar injury risk scenarios. The 

W126 benchmarks across the six scenarios range from 

3.05 ppm-hrs (five percent of biosites, normal moisture, 

any injury) up to 46.87 ppm-hrs (five percent of biosites, 

dry, elevated injury). These results suggest that soil 

moisture plays a role in foliar injury, potentially 

indicating that drought may provide some protection from 

foliar injury. In addition, we see similar patterns when 

the foliar injury analysis is stratified by year and 

geographic region.

Figure ES-3 shows the pattern seen in the foliar injury

analyses stratified by soil moisture category.

Figure ES-3 Cumulative Proportion of Sites with Any 

Visible Foliar Injury Present, by Soil Moisture 

Category

RISK TO ECOSYSTEM SERVICES

Figure ES-4 illustrates the overall relationships 

between some of the ecological effects of O3

exposure and the associated ecosystem services 

impacts. While we estimate the impact of ambient O3

exposures on biomass loss and foliar injury and the 

associated ecosystem services, because of a lack of 

sufficient data, methods, or resources we qualitatively or 

semi-quantitatively assess additional ecosystem services

potentially affected. 

Figure ES-4 Relationship between Ecological Effects 

of O3 Exposure and Ecosystem Services

Ecosystem Services Affected by Biomass Loss

cosystem services most directly affected by 

biomass loss include: (1) provision of food and 

fiber (provisioning), (2) carbon storage

(regulating), (3) pollution removal (regulating), and (4) 

habitat provision for wildlife, particularly habitat for 

threatened or endangered wildlife (supporting). We 

conduct national-scale and case-study scale analyses to 

estimate the ecological effect of biomass loss on several 

ecosystem services. Figure ES-5 provides a schematic of 

the relationships between the ecological effect of biomass 

loss and the analyses conducted to quantify the ecosystem

services affected.

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Welfare economics focuses on the optimal allocation of 

resources and goods and how those allocations affect 

total social welfare. Total welfare is also referred to as 

economic surplus, which is the overall benefit a society, 

composed of consumers and producers, receives when a 

good or service is bought or sold, given a quantity 

provided and a market price. Economic surplus is 

divided into two parts: consumer and producer surplus.

Figure ES-5 Conceptual Diagram of Relationship of 

Relative Biomass Loss to Ecosystem Services [The 

dashed box indicates those services for which direct 

quantification was not possible.]

 Using the Forest and Agricultural Optimization 

Model with Greenhouse Gases (FASOMGHG), we 

quantify the effects of biomass loss on 

timber production, agricultural 

harvesting, and carbon sequestration in 

a national-scale analysis.

5

 The analysis 

provided results for nine U.S. climate 

regions based on National Oceanic and 

Atmospheric Administration (NOAA) 

National Climate Data Center (NCDC) 

regions (see Figure ES-6 below). We 

consider these regions appropriate for our analyses 

because geographic patterns of both O3 and plant species 

are driven by climactic features such as temperature and 

 5 FASOMGHG is a national-scale model that provides a 

complete representation of the U.S. forest and agricultural 

sectors’ impacts of meeting alternative standards. 

FASOMGHG simulates the allocation of land over time 

to competing activities in both the forest and agricultural 

sectors. 

precipitation patterns. We use the O3 C-R functions for 

tree seedlings and crops to calculate relative yield loss, 

which is equivalent to relative biomass loss. Because the

forestry and agriculture sectors are related, and trade-offs 

occur between the sectors, we also calculate the resulting 

market-based welfare effects of O3 exposure in the 

forestry and agriculture sectors. 

Because most areas have W126 index values 

below 15 ppm-hrs after just meeting the existing O3 

standard, yield losses for commercial timber production 

are below one percent, with the exception of the 

Southwest, Southeast, Central, and South regions. For 

agricultural harvest, the largest yield changes occur 

between recent ambient conditions and just meeting the 

existing standard. Under recent ambient conditions, the 

West, Southwest, and Northeast regions generally have 

the highest yield losses at between 6.5 and 15 percent for 

winter wheat. Relative yield losses for winter wheat are 

less than one percent at potential alternative standard 

levels of 15, 11, and 7 ppm-hrs. For soybeans, yield 

losses above 1 percent remain after just meeting 15 ppmhrs in the Southwest and Central regions. Yield losses are 

below one percent after just meeting 11 and 7 ppm-hrs. 

Changes in yield are also associated with 

changes in consumer and producer/farmer surplus. The 

overall effect of changes in yield on forest ecosystem 

productivity depends on the 

composition of forest stands 

and the relative sensitivity 

of trees within those stands. 

Overall effect on 

agricultural yields and 

producer and consumer 

surplus depends on the (1) 

ability of producers/farmers 

to substitute other crops that are less O3 sensitive, and (2) 

responsiveness of demand and supply. Economic impacts 

from just meeting the existing and alternative standards 

were similar between the forestry and agricultural sectors 

-- consumer surplus generally increased in both sectors 

because higher productivity under lower W126 index 

values increased total yields and reduced market prices. 

Because the quantity demanded for most forestry and 

agricultural commodities is not highly responsive to 
ES-7

Consumer surplus is the difference between what a 

consumer would be willing to pay for a product and the price 

they have to pay for the product. For example, assume a 

consumer goes out to buy a CD player and he/she is willing to 

spend $250. When the shopper finds that the CD player is on 

sale for $150, economists would say that this shopper has a 

consumer surplus of $100, e.g., the difference between the 

$150 sale price and the $250 the consumer was willing to 

spend.

Producer surplus refers to the benefit, or profit, a producer 

receives from providing a good or service at a market price 

when they would have been willing to sell that good or service 

at a lower price. For example, if the amount the producer is 

willing to sell the CD player for is $75, and the producer sells 

the CD player for $150, the producer surplus is $75, e.g., the 

$150 sale price less the $75 price at which the producer was 

willing to sell.

changes in price, producer surplus often declines. In 

some cases, lower prices reduce producer profits more 

than can be offset by higher yields. For example, in 2040, 

the year with maximum changes in consumer and 

producer surplus, after just meeting the existing standard, 

the total producer surplus in the forestry 

sector is estimated to be $133 billion and 

total consumer surplus is estimated to be 

$935 billion. When adjusting to meeting 

alternative standards of 15, 11, and 7 

ppm-hrs, consumer surplus increases

$597 million, $712 million, and $779 

million (i.e., 0.06, 0.08, and 0.08 

percent), respectively, while producer 

surplus decreases $839 million, $858 

million, and $766 million, (i.e., about 0.6 

percent), respectively. (All estimates are 

in 2010$.)

Biomass loss due to O3 exposure 

can reduce carbon sequestration, and 

shifts between the forestry and 

agricultural sector can also affect carbon 

sequestration. Because most areas have 

W126 index values below 15 ppm-hrs after just meeting 

the existing O3 standard, a potential alternative standard 

of 15 ppm-hrs does not appreciably increase carbon 

sequestration. The majority of the enhanced carbon 

sequestration potential is in the forest biomass increases 

over time under potential alternative standards of 11 and 7 

ppm-hrs. On an annual basis, carbon sequestration at 11 

ppm-hrs is increased by about 20 million metric tons per 

year relative to just meeting the existing O3 standard, 

which is equivalent to taking about 4 million cars off the 

road.6

 Carbon sequestration at 7 ppm-hrs is increased by 

about 53 million metric tons per year relative to just 

meeting the existing standard, which is the equivalent of 

taking approximately 11 million cars off the road. 

In the case-study scale analyses, we use the iTree 

model to estimate the impact of biomass loss on tree 

growth and ecosystem services such as carbon 

 6 As calculated by the EPA Greenhouse Gas 

Equivalencies Calculator, available at 

http://www.epa.gov/cleanenergy/energyresources/calculator.html. 

sequestration and pollution removal provided by urban 

forests in five urban areas over a 25-year period.7

 

Relative to just meeting the existing standard, three of the 

urban areas (Atlanta, Chicago, and the urban areas of 

Tennessee) show gains in carbon sequestration at 

potential 

alternative 

standard levels of 

11 and 7 ppm-hrs. 

For example, 

relative to just 

meeting the 

existing standard, 

Chicago gains 

about 6,400 tons 

of carbon 

sequestration per 

year at 7 ppm-hrs, 

and the urban 

areas of 

Tennessee gain 

about 8,800 tons 

of carbon 

sequestration per year at 11 ppm-hrs and 20,000 tons of 

carbon sequestration per year at 7 ppm-hrs. These same 

three areas show gains in pollution removal (i.e., O3, 

carbon monoxide, nitrogen dioxide, and sulfur dioxide) at 

potential alternative standard levels of 11 and 7 ppm-hrs 

compared to meeting the existing standard. For example, 

relative to just meeting the existing standard, Chicago 

gains about 2,300 metric tons of pollution removal 

annually at 11 ppm-hrs and 6,500 metric tons of pollution 

removal annually at 7 ppm-hrs, and the urban areas of 

Tennessee gain about 5,300 metric tons of pollution 

removal annually at 11 ppm-hrs and 11,700 metric tons of 

pollution removal annually at 7 ppm-hrs. 

 7 The iTree model is a peer-reviewed suite of software 

tools provided by USFS.
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Figure ES-6 Map of the 9 NOAA Climate Regions used in the National-Scale Air Quality Adjustments
ES-9

Bags of ponderosa pine (Pinus ponderosa C. 

Lawson var. ponderosa) cones gathered in central 

Oregon for arts, crafts, and floral markets. 

Courtesy: U.S. Department of Agriculture

Syracuse and Baltimore do not realize gains in carbon 

sequestration or pollution removal, because W126 index 

values almost meet the alternative standards of 15, 11, 

and 7 ppm-hrs in those areas.

We qualitatively describe the potential effects of

O3 on other (non-timber) forest products that are 

harvested for commercial or subsistence activities, 

including edible fruits, nuts, berries, and sap; foliage, 

needles, boughs, and bark; grass, hay, alfalfa, and forage; 

herbs and medicinals; fuelwood, posts and poles; and 

Christmas trees. According to the ISA, O3 exposure 

causes biomass loss in sensitive woody and herbaceous 

species, which in turn could affect forest products used 

for arts, crafts, and florals. For example, Douglas Fir and 

Red Alder, among others, are used on the Pacific Coast 

for arts and crafts, particularly holiday crafts and 

decorations. Foliar injury impacts on O3-sensitive plants 

would potentially affect the harvest of leaves, needles, 

and flowers from these plants for decorative uses. The 

visible injury and early senescence caused by O3 in some 

evergreens may also reduce the value of a whole tree such 

as Christmas trees. Likewise, O3 can reduce the harvest of 

edible fruits, nuts, berries, and sap in O3-sensitive plants. 

According to the U.S. Census Bureau, the industry sector

for forest nurseries and gathering of forest products

employed 2,098 people, accounting for an annual payroll 

of $71 million (2006$) with an average annual income of 

$34,155). 

Ecosystem Services Affected by Visible Foliar 

Injury

he ecosystem services most likely to be affected 

by O3-induced visible foliar injury are aesthetic 

value and outdoor recreation (cultural services), 

which depend on the perceived scenic beauty of the 

environment. Studies of Americans’ perception of scenic 

beauty show that people tend to have reliable preferences 

for forests and vegetation with fewer damaged or dead 

trees and plants. Many outdoor recreation activities 

directly depend on the scenic value of the area, in 

particular scenic viewing, wildlife watching, hiking, and 

camping. These activities are enjoyed by millions of 

Americans every year and generate millions of dollars in 

economic value. According to the National Survey on 

Recreation and the 

Environment, some 

of the most popular 

outdoor activities 

are walking, 

including day 

hiking and 

backpacking; 

camping; bird 

watching; wildlife 

watching; and nature 

viewing. Total 

expenditures across 

wildlife watching 

activities, trail-based activities, and camp-based activities 

are approximately $200 billion dollars annually. Figure 

ES-7 shows the relationship between foliar injury and the 

analyses to assess affected ecosystem services.

Figure ES-7 Relationship between Visible Foliar 

Injury and Ecosystem Services

Enjoyment of recreation in national parks can be 

adversely affected by visible foliar injury, and national 

parks are designated as special areas in need of protection. 

In a screening-level assessment at 214 national parks, we 

apply the benchmarks from the national analysis of foliar 

T
ES-10

Mount Le Conte, Summer 

Great Smoky Mountains National Park 

Courtesy: NPS 

http://www.nps.gov/grsm/photosmultimedia/index.

htm

injury to six scenarios representing a range of percentages 

of biosites showing visible foliar injury. Based on lists 

from the National Park Service, 95 percent of the parks in 

this assessment contain at least one O3-sensitive species. 

Generally, benchmark scenarios corresponding to higher 

percentages of biosites showing foliar injury show fewer 

parks that exceed the benchmark criteria for those 

scenarios. During 2006 to 2010, 58 percent of parks 

exceeded the benchmark W126 corresponding to the base 

scenario (W126>10.46 ppm-hrs, 17.7 percent of biosites, 

without consideration of soil moisture, any injury) for at 

least three years. The analyses suggest that in order to 

substantially reduce the risk of foliar injury in these parks, 

the W126 index values would need to be reduced to be 

below 10.46 ppm-hrs. In addition, 98 percent, 80 percent, 

68 percent and 2 percent of parks would exceed the 

benchmark criteria corresponding to the 5 percent, 10

percent, 15 percent, and 20 percent prevalence scenarios

for at least 3 years within the 2006-2010 period. For the 

elevated injury scenario, 34 percent of parks would 

exceed the benchmark criteria (five percent of biosites, 

multiple moisture categories, elevated foliar injury) for at 

least three years. Because the screening-level assessment 

relies on annual estimates of W126 index values and soil 

moisture, we cannot fully evaluate just meeting the 

existing and alternative standards because they are based 

on the 3-year average air quality surfaces. However, we 

can observe that after adjusting the W126 surfaces to just 

meet the existing standard (3-year average), all of the 214 

parks are below 10.46 ppm-hrs, which corresponds to the 

annual W126 benchmark criteria for the base scenario.

We also assess foliar injury at a case-study scale. 

Specifically, we assess O3-exposure risk at three national 

parks – Great Smoky Mountains National Park, Rocky 

Mountain National Park, and Sequoia/Kings National 

Parks. For each park, we assess the potential impact of 

O3-related foliar injury on recreation (cultural services) by 

considering information on visitation patterns, 

recreational activities and visitor expenditures. We 

include percent cover of species sensitive to foliar injury 

and focus on the overlap between recreation areas within 

the park and alternative W126 standard levels. 

In the Great Smoky Mountains National Park, 

there are 37 sensitive species across vegetative strata, and 

2011 visitor spending exceeded $800 million. Seasonal 

O3 concentrations in the park have been among the

highest in eastern U.S. -- under recent ambient conditions, 

44 percent of the park has W126 index values above 15 

ppm-hrs. After adjustments to just meeting the existing 

standard of 75 ppb, no area in the park exceeds an 

alternative W126 standard level of 7 ppm-hrs. Rocky 

Mountain National Park has seven sensitive species, 

including Quaking Aspen. In 2011, visitor spending at 

the park was over $170 million. Under recent ambient 

conditions, all of the park has W126 index values above 

15 ppm-hrs. When adjusted to just meeting the existing 

standard, 59 percent of the park would have W126 index 

values between 7 and 11 ppm-hrs. When adjusted to just 

meeting an alternative W126 standard level of 15 ppmhrs, no area in the park exceeds an alternative W126 

standard level of 7 ppm-hrs. In Sequoia/Kings National 

Parks, there are 12 sensitive species across vegetative 

strata, and 2011 visitor spending was over $97 million. 

When adjusted to just meet the existing standard, no area 

in the park exceeds an alternative W126 standard level of 

7 ppm-hrs.
ES-11

Between 93 and 96 percent of NSRE survey 

respondents stated that provision of habitat 

for wild plants and animals is moderately, 

very, or extremely important to them.

Between 90 and 97 percent of NSRE survey 

respondents stated that maintaining existence, 

option, or bequest values is moderately, very, or 

extremely important to them.

Additional Ecosystem Services Affected

ecause of a lack of data, methods, or resources

we qualitatively or semi-quantitatively assess

additional ecosystem services potentially 

affected, including cultural, supporting, 

regulating, and provisioning services. Cultural services

include non-use values that can be directly or indirectly 

impacted by O3 exposure. When people value a resource 

even though they may never visit the resource or derive 

any tangible benefit from it, they perceive an existence 

value. When the resource is valued as a legacy to future 

generations, bequest value exists. Additionally, there 

exists an option value to knowing that you may visit a 

resource at some point in the future. The National Survey 

on Recreation and the Environment (NSRE) is an ongoing 

survey of a random sample of adults over the age of 16 on 

their interactions with the environment.8

 NSRE data 

indicate that Americans have very strong preferences for 

existence, option, and bequest values related to forests –

90 to 97 percent of survey respondents indicated it is 

moderately, 

very, or 

extremely 

important to 

them to 

maintain existence values, to maintain option values, and 

to maintain bequest values.

The supporting service of community 

composition, or structure, is affected by O3 exposure 

because some species are more resistant to the negative 

effects of O3 and are able to out-compete more susceptible 

species. Changes in community composition underlie 

possible changes in associated services such as herbivore 

grazing, production of preferred species of timber, and 

preservation of 

habitat for 

unique or 

endangered 

communities 

or species. 

 8 The NSRE is conducted by the U.S. Forest Service. 

Additional information can be located at 

http://www.srs.fs.usda.gov/trends/nsre-directory/.

The NSRE provides data on the values that survey 

respondents place on the provision of habitat for wild 

plants and animals – between 93 and 96 percent of survey 

respondents indicated it is important to them to preserve 

wildlife habitat, to preserve unique wild plants and 

animals, and to protect rare or endangered species. 

Regulating services include air quality, water 

quantity and quality, climate, erosion, fire regulation, and 

pollination regulation. Regulation of the water cycle can 

be adversely affected by the effects of O3 on plants. 

Studies of O3-impacted forests in eastern Tennessee in or 

near the Great Smoky Mountains has shown that ambient 

O3 exposures resulted in increased water use in O3-

sensitive species, which led to decreased modeled lateseason stream flow in those watersheds. Ecosystem 

services potentially affected by such a loss in stream flow 

could include habitat for species (e.g., trout) that depend 

on an optimum stream flow or temperature. Additional 

downstream effects could potentially include a reduction 

in the quantity and/or quality of water available for 

irrigation or drinking and for recreational use. Ninety-one 

percent of NSRE respondents ranked water quality 

protection as either an extremely or very important benefit 

of wilderness. 

Fire regime regulation is also negatively affected 

by O3 exposure. For example, O3 exposure may 

contribute to forest susceptibility to wildfires in southern 

California by increasing leaf turnover rates and litter, 

increasing fuel loads on the forest floor. In a case-study 

scale analysis, we develop maps that overlay the mixed 

conifer forest area of California with (i) areas of moderate 

or high fire risk and recent W126 index values and (ii) air 

quality adjusted to just meet existing and alternative 

standard levels. The highest fire risk and highest W126 

levels overlap with each other, as well as with significant 

portions of mixed conifer forest. Under recent conditions, 

over 97 percent of mixed conifer forest area has W126 

index values over 7 ppm-hrs and fire risk, and 74 percent 

has W126 index values over 15 ppm-hrs and fire risk. 

After just meeting the existing standard, almost all of the 

mixed conifer forest area with fire risk is below 7 ppmhrs. At the alternative standard level of 15 ppm-hrs, less 

than one percent of the fire risk area is above 7 ppm-hrs. 

At alternative standard levels of 11 and 7 ppm-hrs, all of 

the fire risk area is below 7 ppm-hrs. 

B
ES-12

Southern Pine Beetle Damage 

Courtesy: Ronald F. Billings, Texas Forest 

Service. Bugwood.org

O3 exposure results in increased susceptibility to 

infestation by some chewing insects, including the 

southern pine beetle and western bark beetle. These 

infestations can cause economically significant damage to 

tree stands and the 

associated timber 

production 

(provisioning 

service). In the 

short-term, the 

immediate 

increase in timber 

supply that results 

from the 

additional 

harvesting of 

damaged timber depresses prices for timber and benefits 

consumers. In the longer-term, the decrease in timber 

available for harvest raises timber prices, potentially 

benefitting producers. The USFS reports timber 

producers have incurred losses of about $1.4 billion 

(2010$), and wood-using firms have gained about $966 

million, due to beetle outbreaks between 1977 and 2004. 

We develop maps that overlay the forest areas at risk of 

basal area loss from pine beetle infestation with W126 

index values. After just meeting the existing standard, 

most of the pine beetle risk area is below 7 ppm-hrs. At 

the alternative standard level of 15 ppm-hrs, all of the 

pine beetle risk area is less than 7 ppm-hrs. 

CONCLUSIONS

his welfare REA provides analyses that further inform 

the following policy-relevant questions9

: (1) in 

considering alternative standards, to what extent do 

alternative levels reduce estimated exposures and welfare 

risks attributable to O3; (2) what range of alternative 

standard levels should be considered based on the 

scientific information evaluated in the ISA, air quality 

analyses, and the welfare REA; and (3) what are the 

important uncertainties and limitations in the evidence 

 9 The policy-relevant questions were identified in the 

Integrated Review Plan for the Ozone National Ambient

Air Quality Standards.

and assessments and how might those uncertainties and 

limitations be taken into consideration in identifying 

alternative secondary standards for consideration. To 

develop information to help inform these questions, we 

quantify ecological effects based on the relationship with 

the W126 metric and assess the associated impacts on 

ecosystem services. For some ecosystem services, such 

as commercial non-timber forest products, recreation, and 

aesthetic and non-use values, we qualitatively assess 

potential impacts to services. 

For biomass loss, a one to two percent loss for 

trees corresponds to an annual W126 index value of 4 to 

14 ppm-hrs, and a five percent loss for crops corresponds 

to an annual W126 index value of 12 to 17 ppm-hrs. For 

visible foliar injury, across all biosites, the proportion of 

biosites showing injury generally levels off around a 

W126 index value of 10 ppm-hrs; however, for biosites 

with greater than normal soil moisture, the proportion of 

sites with damage can be greater.

We estimate that some exposures and risks 

remain after just meeting the existing standard and that in 

many cases, just meeting potential alternative standard 

levels results in reductions in those exposures and risks. 

Overall, the largest reduction in O3 exposure-related 

welfare risk occurs when moving from recent ambient 

conditions to just meet the existing secondary standard of

75 ppb. When adjusting for meeting the existing 

standard, only two of the nine U.S. regions have W126 

index values remaining above 15 ppm-hrs (West -- 18.9 

ppm-hrs and Southwest – 17.7 ppm-hrs). Four regions 

(East North Central, Northeast, Northwest, and South) 

would meet 7 ppm-hrs, and two regions (Southeast and 

West North Central) have index values between 9 and 12 

ppm-hrs (Southeast – 11.9 ppm-hrs and West North 

Central – 9.3 ppm-hrs). At an alternative standard of 15 

ppm-hrs, ambient O3 exposure and related risk are not 

appreciably different than they are after just meeting the 

existing standard. Meeting alternative standard levels of 

11 ppm-hrs and 7 ppm-hrs results in smaller risk 

reductions compared to the decreases in risk from meeting 

the existing standard relative to recent conditions. 

T
United States 

Environmental Protection Agency 

Office of Air Quality Planning and Standards 

Health and Environmental Impacts Division 

Research Triangle Park, NC 

Publication No.