Welfare Risk and Exposure Assessment for 

Ozone 

Final 

Executive Summary 

 
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EPA-452/R-14-005c 

August 2014 

Welfare Risk and Exposure Assessment for Ozone 

Final 

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 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. 

Questions on this document should be addressed to Dr. Bryan Hubbell at the U.S. 

Environmental Protection Agency, Office of Air Quality Planning and Standards, 109 TW Alexander 

Drive, C504-02, Research Triangle Park, North Carolina 27711 or email -- hubbell.bryan@epa.gov. 

 
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Summary

Welfare Risk and Exposure Assessment 

for Ozone, Final 

(August 2014)

SUMMARY CONCLUSIONS 

he goals for this welfare1 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 

                                                            

1

 A secondary, or welfare-based, standard provides public welfare 

protection, including protection against decreased visibility and against 

damage to animals, crops, vegetation, and buildings. 

2

 A biosite is a plot of land on which data was collected regarding the 

incidence and severity of visible foliar injury on a variety of O3-sensitive 

plant species. 

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 protect trees will also 

protect 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 biosites2

 showing foliar 

injury increases steeply with W126 index values up to 

approximately 10 ppm-hrs and is relatively constant 

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 and by year. 

In this REA, we analyzed the changes in O3

exposure and risk after adjusting air quality to just meet 

the existing standard3

 and to just meet potential 

alternative secondary standard levels. Overall, the largest 

reduction in O3-related ecological effects and associated 

ecosystem services occurs when moving from recent 

ambient conditions to just meeting the existing standard. 

Exposures and risks remaining after just meeting the 

existing standard, in many cases, can be reduced when 

just meeting potential alternative standard levels. 

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 welfare REA presents 

assessments to inform consideration of the review of the 

secondary (welfare-based) NAAQS for O3. This REA 

provides an assessment of exposure and risk associated 

with recent ambient concentrations of O3 and potential 

alternative secondary standards. The REA builds on 

3

 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 fourth-highest 

daily maximum 8-hour average concentration, averaged over three 

years. 

T 

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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. 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 public welfare. 

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 in this 

REA build upon those in prior reviews and 

include: (1) summaries of recent ambient O3

monitoring data; (2) an extrapolation of measured O3

concentrations to areas without monitors, including 

natural areas that are important to public welfare, such as 

national parks; and (3) adjustment of air quality to just 

meet the existing standard and potential alternative 

secondary 

standards. 

We use 

estimates of 

O3 exposure 

(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 on monitoring in rural areas. The 

extent of monitoring coverage in non-urban areas has not 

significantly changed since the previous review. Figure 

E 

T

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.
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(August 2014)

ES-2 shows the current O3 monitoring coverage in the 

U.S. – both urban and non-urban 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. When adjusting air quality to just meet the 

existing standard, only two of the nine U.S. regions 

remain 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) are between 9 and 12 ppm-hrs (Southeast – 11.9 

ppm-hrs and West North Central – 9.3 ppm-hrs). 

 The adjusted surfaces are used as inputs to 

several assessments, including the geographic analysis to 

assess the potential overlap between areas with elevated 

O3 and areas with elevated risks of fire and insect damage, 

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. We do not quantify fire or insect damage 

resulting from O3 exposure. In the Risk to Ecosystem 

Services section, we briefly discuss the ecosystem 

services associated with fire and insect damage on tree 

stands and timber production, and show the overlap of 

areas with higher W126 concentrations and higher risk of 

bark beetle infestation and fire risk. 

 The first step in assessing biomass loss for tree 

seedlings and crops is to identify the range of W126 index 

values corresponding to annual percent biomass loss 

benchmarks recommended by CASAC using exposureresponse (E-R) functions for 12 tree species and 10 crops, 

for which sufficient information is available. The 

estimated annual W126 index values are between 4 and 

10 ppm-hrs for a one percent biomass loss in tree 

seedlings, between 7 and 14 ppm-hrs for a two percent 

biomass loss in tree seedlings, and between 12 and 17 

ppm-hrs for a five percent biomass loss for crops. In 

general, estimates of annual percent biomass loss in tree 

seedlings are comparable to mature trees with a few 

exceptions. Next, we use these E-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 Forest 

Health Monitoring Network (FHM), which monitors the 

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

analyses identify the presence/absence of foliar injury. 

We also conduct analyses across years and different soil 

moisture categories. Over 81 percent of FHM biosites 

showed no visible foliar injury. Generally, the results 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 above 10 ppm-hrs. 

For biosites with greater than normal soil moisture, more 

biosites showed injury. Figure ES-3 shows the pattern 

seen in the foliar injury analyses stratified by soil 

moisture category. In addition, we see similar patterns 

when the foliar injury analysis is stratified by year. 

I
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(August 2014)

Figure ES-2 U.S. Ambient O3 Monitoring Sites in Operation During 2006-201044 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

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 o

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

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 

network operated by the NPS. 
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(August 2014)

Figure ES-3 Cumulative Proportion of Sites with Any 

Visible Foliar Injury Present, by Soil Moisture 

Category

 Using the foliar injury benchmarks derived from 

the national scale analysis, we applied a screening-level 

approach using O3 exposure and soil moisture data for 214 

parks in the contiguous U.S. All scenarios assessed in the 

screening-level assessment reflect the special status of 

parks as areas designated for protection, and thus apply 

benchmarks corresponding to the presence of any visible 

foliar injury. We use these results to derive W126 

benchmarks for visible foliar injury for five scenarios 

representing the full range of percentages of biosites 

showing visible foliar injury, including four scenarios 

considering soil moisture. For the fifth scenario, or “base 

scenario”, the benchmark is the W126 index value where 

the slope of exposure-response relationship changes for 

all FHM biosites in all soil moisture categories. We 

looked at scenarios based on three different categories of 

soil moisture (i.e., wet, normal, dry) and the W126 index 

values associated with four different prevalences of any 

foliar injury (e.g., 5 percent, 10 percent, 15 percent and 20 

percent of biosites). In total, we evaluated ten different 

W126 benchmarks associated with the five foliar injury 

risk scenarios. The W126 benchmarks across the 

scenarios range from 3.05 ppm-hrs (five percent of 

biosites, normal moisture, any injury) up to 24.61 ppmhrs (15 percent of biosites, dry, any injury). These results 

suggest that soil moisture plays a role in foliar injury, 

potentially indicating that drought may provide some 

protection from foliar injury. 

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 

Figure ES-4 Relationship between Ecological Effects 

of O3 Exposure and Ecosystem Services 

associated ecosystem services, because of a lack of 

sufficient data, methods, or resources we qualitatively or 

semi-quantitatively assess additional ecosystem services 

potentially affected. 

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 

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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.

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 

                                                            

5

 FASOMGHG is a national-scale model that provides a complete 

representation of the U.S. forest and agricultural sectors’ impacts of just 

meeting alternative standards. FASOMGHG simulates the allocation of 

because geographic patterns of both O3 and plant species 

are driven by climatic features such as temperature and 

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

tree seedlings and crops to calculate relative yield loss, 

which equates to relative biomass loss. Because the 

forestry and agriculture sectors are linked, 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 are below 15 ppm-hrs after 

adjusting O3 distributions to just meet the existing 

standard (based on reducing nationwide emissions of 

NOx), 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 ppm-hrs 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 effects on 

land over time to competing activities in both the forest and agricultural 

sectors. 

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. 
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(August 2014)

Figure ES-6 Map of the 9 NOAA Climate Regions used in the Welfare Risk and Exposure Assessment
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(August 2014)

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. The overall 

economic effect of reduced O3 exposure on the forestry 

and agricultural sectors from just meeting the existing and 

alternative standards were 

similar between the 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 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 adjusting air quality to 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 air quality to just meet alternative W126 

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$ for the U.S. only.) 

Biomass loss due to O3 exposure can reduce 

carbon sequestration, and shifts between the forestry and 

agricultural sector can also affect carbon sequestration. 

                                                            

6

 As calculated by the EPA Greenhouse Gas Equivalencies Calculator, 

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

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 (meeting the 

existing 8-hour standard of 75 ppb increases carbon 

sequestration by 2,972 million metric tons per year). 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 

sequestration and pollution removal provided by urban 

forests in five urban study 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 

7

 The i-Tree model is a peer-reviewed suite of software tools provided 

by the U.S. Forest Service that provides urban forestry analysis. 

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 $70, and the 

producer sells the CD player for $150, the producer surplus is 

$80, e.g., the $150 sale price less the $70 price at which the 

producer was willing to sell. 
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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. Syracuse and 

Baltimore do not realize gains in carbon sequestration or 

pollution removal because the 2006-2008 W126 index 

values almost meet the alternative standard levels 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. Ozone 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 

                                                            

8

 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. The NSRE is conducted by the U.S. 

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. Visible foliar 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 with an annual payroll of $71 million (2006$). 

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,8

 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. 

Forest Service. Additional information can be located at 

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

T

Bags of ponderosa pine cones (Pinus ponderosa C. 

Lawson var. ponderosa) gathered in central Oregon for 

arts, crafts, and floral markets. 

Courtesy: U.S. Department of Agriculture
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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 areas designated for protection. In a screeninglevel assessment at 214 national parks, we apply the 

W126 benchmarks from the national analysis of foliar 

injury using FHM biosite data to five scenarios 

(representing the full range of percentages 

of biosites showing visible foliar injury) 

in order to provide an indication of the 

risk of visible foliar injury in each park. 

Based on lists from the National Park 

Service, 95 percent of the parks in this 

assessment contain at least one O3-

sensitive species. Generally, scenarios for 

higher percentages of FHM biosites 

showing foliar injury have fewer parks 

that exceed the benchmarks for those 

scenarios across multiple years. During 

2006 to 2010, 58 percent of parks 

exceeded the W126 benchmark 

corresponding to the base scenario 

                                                            

9

 The prevalence scenarios are also discussion on page ES-5. 

(W126>10.46 ppm-hrs, all biosites in all soil moisture 

categories) for at least three years. In addition, 98 

percent, 80 percent, 68 percent and 2 percent of parks 

would exceed the benchmark criteria corresponding to the 

prevalence scenarios (i.e., 5 percent, 10 percent, 15 

percent, and 20 percent) for at least 3 years within the 

2006-2010 period.9

 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 

air quality surfaces to just meet the existing standard (3-

year average), all of the 214 parks are below 10.46 ppmhrs, which corresponds to the annual W126 benchmark 

for the base scenario. 

 We also assess foliar injury at three national 

parks – Great Smoky Mountains National Park, Rocky 

Mountain National Park, and Sequoia/Kings Canyon 

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 was 

above 15 ppm-hrs. 

After adjustments to 

just meet the existing 

Mount Le Conte, Summer 

Great Smoky Mountains National Park 

Courtesy: NPS 

http://www.nps.gov/grsm/photosmultimedia/index.htm
ES-11

Executive 

Summary

Welfare Risk and Exposure Assessment 

for Ozone, Final 

(August 2014)

standard of 75 ppb, no area in the park exceeds 7 ppmhrs. Rocky Mountain National Park has seven sensitive 

species, including Quaking Aspen. In 2011, visitor 

spending was over $170 million. Under recent ambient 

conditions, all of the park is above 15 ppm-hrs. When 

adjusted to just meet the existing standard, 59 percent of 

the park would be between 7 and 11 ppm-hrs. When 

adjusted to just meet an alternative standard level of 15 

ppm-hrs, no area in the park exceeds 7 ppm-hrs. In 

Sequoia/Kings Canyon 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 7 

ppm-hrs. 

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. Surveys indicate that 

Americans have very strong preferences for existence, 

bequest, and option 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. 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 decreased late-season stream 

flow modeled 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 potentially 

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 areas of moderate or high fire risk and (i) recent

W126 index values and (ii) air quality adjusted to just 

meet existing and alternative standard levels. The areas 

with 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 with high fire risk 

had W126 values over 7 ppm-hrs, and 74 percent had 

W126 values over 15 ppm-hrs. After just meeting the 

existing standard, almost all of the mixed conifer forest 

area with high fire risk is below 7 ppm-hrs. At the 

alternative standard level of 15 ppm-hrs, less than one 

percent of the high fire risk area is above 7 ppm-hrs. At 

B
ES-12

Executive 

Summary

Welfare Risk and Exposure Assessment 

for Ozone, Final 

(August 2014)

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

high fire risk area is below 7 ppm-hrs. 

 O3 exposure may increase 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 U.S. Forest Service 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 high pine beetle risk area is below 7 

ppm-hrs. At the alternative standard level of 15 ppm-hrs, 

all of the high pine beetle risk area is less than 7 ppm-hrs. 

CONCLUSIONS 

his welfare REA provides analyses that further 

inform the following policy-relevant questions10: 

(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 

                                                            

10 The policy-relevant questions were identified in the Integrated Review 

Plan for the Ozone National Ambient Air Quality Standards. 

the scientific information evaluated in the Integrated 

Science Assessment, air quality analyses, and the welfare 

REA; and (3) what are the important uncertainties and 

limitations in the evidence 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 nontimber forest products, recreation, and aesthetic 

and non-use values, we qualitatively assess 

potential impacts to services. 

 The analyses in this REA found 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 standard. This finding should be 

considered in the context of potential uncertainties in the 

actual responsiveness of W126 values in all areas to the 

emissions reductions used in the adjustments to just meet 

the existing standard. Keeping these potential 

uncertainties in mind, at an alternative standard level 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. 

 Despite uncertainties inherent in any complex, 

quantitative analysis, the overall body of scientific 

evidence underlying the ecological effects and associated 

ecosystem services evaluated in this welfare REA is 

strong, and the methods used to quantify associated risks 

are scientifically sound. 

T 

Southern Pine Beetle Damage 

Courtesy: Ronald F. Billings, Texas Forest Service. 

Bugwood.org 
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United States 

Environmental Protection Agency 

Office of Air Quality Planning and Standards 

Health and Environmental Impacts Division 

Research Triangle Park, NC 

Publication No.