Executive Summary 

Overview 

This Regulatory Impact Analysis (RIA) provides EPA’s estimates of the range of the monetized 

human health benefits, control costs, and net benefits associated with meeting the revised suite of 

standards for fine particles (PM2.5) that were promulgated by EPA on September 21, 2006, as 

well as for meeting a one alternative. The final rule established a 24-hour standard of 35 μg/m3

and retained the annual standard of 15 μg/m3

. EPA also promulgated a final decision to retain 

the current 24-hour PM10 standards and to revoke the current annual PM10 standards, in order to 

maintain protection against the health and welfare effects of thoracic coarse particles (PM10-2.5). 

As was the case for the interim RIA accompanying the proposed rulemaking, due to data and 

modeling limitations preclude EPA from assessing the costs and benefits of retaining the existing 

PM10 standards. This summary outlines the basis for and approach used in the RIA, presents the 

key results and insights derived from the analyses, and highlights key uncertainties and 

limitations. 

In setting primary ambient air quality standards, EPA’s responsibility under the law is to 

establish standards that protect public health. The Clean Air Act (“Act”) requires EPA, for each 

criteria pollutant, to set a standard that protects public health with “an adequate margin of 

safety.” As interpreted by the Agency and the courts, the Act requires EPA to base this decision 

on health considerations; economic factors cannot be considered. 

This prohibition against the consideration of cost in the setting of the primary air quality 

standards, however, does not mean that costs, benefits or other economic considerations are 

unimportant or should be ignored. The Agency believes that consideration of costs and benefits 

is an essential decision making tool for the efficient implementation of these standards. The 

impacts of cost, benefits, and efficiency are considered by the states during this process, when 

states are making decisions regarding what timelines, strategies, and policies make the most 

sense. 

This PM2.5 NAAQS RIA is focused on development and analyses of illustrative control strategies 

to meet alternative suites of standards in 2020, the latest year by which the Clean Air Act 

generally requires full attainment of the new standards. Because the states are ultimately 

responsible for implementing strategies to meet the revised standards, the RIA provides insights 

and analysis of a limited number of illustrative control strategies that states might adopt to meet 

the revised standards. These strategies are subject to a number of important assumptions, 

uncertainties and limitations, which we document in the relevant portions of the analysis. 

EPA presents this analysis pursuant to Executive Order 12866 and the guidelines of OMB 

Circular A-4.1 These documents present guidelines for EPA to assess the incremental benefits 

and costs of the selected regulatory approach as well as one less stringent, and one more 

stringent, option. In this RIA, the 1997 standards represent the less stringent option, and the 

 

1

 For a copy of these requirements, see: http://www.whitehouse.gov/OMB/inforeg/eo12866.pdf and 

http://www.whitehouse.gov/omb/circulars/a004/a-4.html. 

ES-1 
alternative suite of standards including a tighter annual standard of 14 μg/m3

 together with the 

revised 24-hour standard of 35 μg/m3

 represents the more stringent option. 

ES.1 Approach to the Analysis 

The RIA consists of multiple analyses including an assessment of the nature and sources of 

ambient PM2.5; estimates of current and future emissions of relevant gases and particles that 

contribute to the problem; air quality analyses of baseline and alternative strategies; development 

of illustrative control strategies to attain the standards alternatives in future years; analyses of the 

incremental costs and benefits of attaining the alternative standards, together with an 

examination of key uncertainties and limitations; and a series of conclusions and insights gained 

from the analysis. 

Nature of PM2.5 

Particulate matter (PM) is a highly complex mixture of solid particles and liquid droplets that 

occur in the atmosphere together with numerous pollutant gases that interact with them. 

Atmospheric particles can be grouped according to various characteristics. For regulatory 

purposes, fine PM are measured as PM2.5. Particles are emitted directly from sources (referred to 

as primary PM) and are also formed through atmospheric chemical reactions (referred to as 

secondary PM). Primary PM2.5 consists of carbonaceous material (e.g. soot, and accompanying 

organics)—emitted from cars, trucks, heavy equipment, forest fires, and burning waste, as well 

as from coke ovens, metals from combustion and industrial processes, with some small 

contribution from crustal materials. Secondary PM2.5 forms in the atmosphere from precursor 

gases including sulfur and nitrogen oxides from power, industrial and other combustion and 

process sources, certain reactive organic gases from diesel and other mobile sources, solvents, 

fires, and biogenic sources such as trees, and ammonia from agricultural operations, natural, and 

other sources. Fine particles can be transported hundreds to thousands of miles from emissions 

sources. For this reason, fine particle concentrations in a particular area may have a substantial 

contribution from regional transport as well as local sources. As discussed more fully in 

Chapter 2, there are important regional differences in fine particle concentrations and 

composition that are important to recognize in developing control strategies. 

Overview of Air Quality Modeling Methodology/Baseline emissions forecasts

As a first step in the national assessment of alternatives, the analysis forecasts emissions and air 

quality in 2015 and 2020 under a regulatory base case that incorporates national, regional, state 

and local regulations that are already promulgated and/or adopted. This base case does not 

forecast actions states may take to implement the existing PM2.5 standards. The regulatory base 

case includes recent rules that will significantly reduce PM2.5 concentrations in future years by 

addressing emissions from the power generation sector - the Clean Air Interstate Rule (CAIR), 

the Clean Air Mercury Rule (CAMR), and the Clean Air Visibility Rule (CAVR, which also 

affects some industrial boiler emissions), and mobile sources through national rules for light and 

heavy-duty vehicles and non-road mobile sources. Current state programs that address these 

and other source categories that were on the books as of early 2005 are also modeled for future 

ES-2 
years. Based on the emissions forecasts, EPA developed annual and daily PM2.5 design value 

projections using the CMAQ model.2

 

Development and Application of Illustrative Control Strategies 

The air quality modeling results for the regulatory base case (Figure ES-1, ES-2) provided the 

starting point for developing illustrative control strategies to attain the 1997 as well as the 

revised and alternative suites of standards that are the focus of this RIA. The figures show that 

by 2020, while PM2.5 air quality would be significantly better than today under current 

requirements, several eastern and western States will need to develop and adopt additional 

controls to attain the revised standards. The modeling shown in Figure ES-2 suggests that under 

the revised suite of standards, greater reductions will be needed in some Western areas, 

particularly in California. 

We followed a three-step process to simulate attainment in each of the areas forecast to need 

additional controls to meet the revised and alternative standards: 1) We identified cost-effective 

controls to apply in each projected nonattainment area and then simulated the resulting air 

quality change in an air quality model; 2) For those areas that did not attain under 1) we 

developed and simulated the results of applying additional known emission controls that were 

not applied in the initial strategy, and then evaluated attainment status considering the 

uncertainty in the analyses; 3) For areas that we determined would still not attain under the more 

readily identifiable control strategies in 1) and 2), we used a combination of qualitative and 

quantitative analysis to estimate the costs and benefits of fully attaining the standards. This 

included identification of potential trends in pollution control measures (such as greater adoption 

of hydrogen fuel cell vehicles), extrapolation of costs based on existing technologies, and 

estimation of benefits by “rolling back” monitor values to just attain the standards. 

In developing strategies tailored to specific problem areas, we combined information from our 

air quality models and our emission control database. These combined data enabled us to 

selectively apply emission control measures on those industrial sources where it was most cost 

effective to do so—effectively generating the greatest estimated air quality improvement at the 

lowest cost. Because the national and regional programs summarized above (e.g. CAIR, mobile 

rules) will address a good portion of the regional transport contribution of PM2.5, the first set of 

controls to meet the 1997 and revised standards focus on reductions in local emissions. These 

local emissions are defined as those occurring in the projected nonattainment county and 

immediate surrounding counties in the MSA. In some cases, the local control strategy did not 

provide enough emission reductions to attain the standards. In that case, we explored emission 

controls among a broader set of counties within the state containing the projected nonattainment 

area. The exception to this approach is California, where, due to the extreme and widespread 

nature of the nonattainment problem, we considered controls throughout Southern California in 

the attainment strategies. 

 

2

 The methodologies for forecasting emissions and air quality and associated uncertainties are detailed in the 

Technical Support Document – “Air Quality Modeling Technique used for Multi-Pollutant Analysis?" 

(http://www.epa.gov/airmarkets/mp/aqsupport/airquality.pdf). The methodology used to derive the 98th percentile 

24-hour values is summarized in Chapter 4 of this RIA. 

ES-3 
Figure ES-1. Counties Projected to Violate the Revised PM2.5 NAAQS in 2020 

 Indicates county 

monitor exceeds 

daily standard of 35 

μg/m3

Indicates county 

monitor exceeds 

daily standard of 15 

μg/m3

Indicates county 

monitor exceeds 

both the annual 

standard of 15 μg/m3

and the daily 

standard of 35 μg/m3

With CAIR/CAMR/CAVR and Some Current Rules** Absent Additional Local Controls

* Projections as of September 2006. 

**Current rules include Title IV of CAA, NOx

SIP Call, and some existing State rules

Counties Projected to Exceed Revised Standards 

 

Annual 

and Daily Annual Only Daily Only 

 

2015 18 2 32 

2020 17 3 28 

Indicates county 

monitor exceeds 

daily standard of 35 

μg/m3

Indicates county 

monitor exceeds 

daily standard of 15 

μg/m3

Indicates county 

monitor exceeds 

both the annual 

standard of 15 μg/m3

and the daily 

standard of 35 μg/m3

With CAIR/CAMR/CAVR and Some Current Rules** Absent Additional Local Controls

* Projections as of September 2006. 

**Current rules include Title IV of CAA, NOx

SIP Call, and some existing State rules

Counties Projected to Exceed Revised Standards 

 

Annual 

and Daily Annual Only Daily Only 

 

2015 18 2 32 

2020 17 3 28 

Figure ES-2. Projected Reduction in Daily Design Value Needed to Attain the Revised 

Daily Standard of 35 μg/m3

 in 2020 

ES-4 

Less than 1 ug to attain

1 to 2 ug to attain

2 to 5 ug to attain

Greater than 5 ug to attain

standard in 2020 with 

measures needed to attain the 

1997 standards.

Incremental to baseline with CAIR/CAMR/CAVR and Mobile Source Rules without additional 

local controls for attainment of the current standards*

*Note that attainment with the 

1997 annual and daily 

standards by 2020 would 

potentially result in reductions 

in the number of 

nonattainment counties in 

2020, especially in the Eastern 

U.S. Our modeling suggests 

that Birmingham and Chicago 

would attain the revised daily 

Less than 1 ug to attain

1 to 2 ug to attain

2 to 5 ug to attain

Greater than 5 ug to attain

standard in 2020 with 

measures needed to attain the 

1997 standards.

Incremental to baseline with CAIR/CAMR/CAVR and Mobile Source Rules without additional 

local controls for attainment of the current standards*

*Note that attainment with the 

1997 annual and daily 

standards by 2020 would 

potentially result in reductions 

in the number of 

nonattainment counties in 

2020, especially in the Eastern 

U.S. Our modeling suggests 

that Birmingham and Chicago 

would attain the revised daily 
Given the baseline air quality forecast under the alternative standard (14, 35), we added a 

regional control program covering both utility and industrial sources of SO2 in portions of the 

Eastern US due to the number of projected nonattainment areas under the alternative standard, 

and prevalence of sulfate in the Eastern U.S. 

In general, we were able to model attainment with the alternative standard in most regions of the 

country with a mix of local or regional control strategies. The major exceptions are in California 

and Utah, where modeling of such strategies indicated that several counties would not attain the 

revised or alternative standards. 

ES-2. Results of Benefit-Cost Analysis 

Table ES-1 summarizes the net benefits of attaining a revised and alternative PM2.5 NAAQS. 

The table summarizes the full attainment benefits, economic costs and net benefits at 3 and 7 

percent discount rates. 

A new component of our benefits analysis is the expanded characterization of uncertainty about 

the impacts of PM on the risk of premature death. Since the publication of the RIA for the Clean 

Air Interstate Rule, we have completed a full-scale expert elicitation designed to more fully 

characterize the state of our understanding of the concentration-response function for PM-related 

premature mortality. The elicitation results form a major component of the current effort to use 

probabilistic assessment techniques to integrate uncertainty into the main benefits analysis. 

To reflect our expanded understanding of uncertainty, and to move us towards implementation of 

the recommendations of the National Research Council’s 2002 report “Estimating the Public 

Health Benefits of Proposed Air Pollution Regulations,” our summary benefits estimates are 

presented as ranges, and include additional information on the quantified uncertainty 

distributions surrounding the points on those ranges, derived from both the epidemiological 

studies and the expert elicitation. 

Tables ES-2 and ES-3 summarize the estimated benefits associated with attaining the revised and 

alternative PM2.5 standards, incremental to our modeled attainment strategy for the 1997 

standards. These tables include both the estimated reductions in the incidence of mortality and 

morbidity and the monetized value associated with these reductions in incidence. In addition to 

these health benefits, we estimate that, incremental to our modeled attainment strategy for the 

1997 standards, the monetary benefits associated with improvements in visibility in selected 

national parks and wilderness areas in 2020 will be $530 million for the revised standards, and 

$1,200 million for the alternative standards. 

Table ES-2 and ES-3 summarize the range of incidence and the range of total monetized benefits 

(health plus visibility) across several sources of mortality effect estimates that we used in our 

analysis. The ranges reflect two different sources of information about the impact of reductions 

in PM on reductions in the risk of premature death, including both the published epidemiology 

literature and an expert elicitation study conducted by EPA in 2006. Estimates based on the 

American Cancer Society study show benefits of meeting the revised 24-hour PM2.5 standard at 

$17 billion a year in 2020. In order to provide an indication of the sensitivity of the benefits 

estimates to alternative assumptions, in Chapter 5 we present a variety of benefits estimates 

based on both epidemiological studies (including the American Cancer Society Study and the Six 

ES-5
ES-6

Cities Study) and the expert elicitation. EPA intends to ask the Science Advisory Board to 

provide additional advice as to which scientific studies should be used in future RIAs to estimate 

the benefits of reductions in PM. 
Table ES-1: Comparison of Full Attainment Benefits with Social Costsf, Incremental to Attainment of 1997 St

Revised standard of 15/35 (μg/m3) Alternative standards of 14/3

Benefitsa Costsb Net benefitsc Benefitsa Costsb 

Benefits Based on Mortality Function from the American Cancer Society Study and Morbidity Functions from the Published Scientific Li

3% $17 $5.4 $12 $30 $7.9 7% $15 $5.4 $9 $26 $7.9 

Benefits Range Based on Expert Elicitation Derived Mortality Functions and Morbidity Functions from the Published Scientific Literature

Low Mean High Mean Low Mean High Mean Low Mean High Mean 

3% $9 $76 $5.4 $3.5 $70 $17 $140 $7.9 7% $8 $64 $5.4 $2.4 $59 $15 $120 $7.9 

eAlthough the overall range across experts is summarized in this table, the full uncertainty in the estimates is reflected by the results for the full set of 12 experts. The twelve e

mean effect estimate are not evenly distributed across the range illustrated by arraying the highest and lowest expert means. The distribution of benefits estimates associated w

responses can be found in Chapter 5. 

a Results reflect the use of two different discount rates: 3% and 7%, as recommended in EPA’s Guidelines for Preparing Economic Analyses (EPA, 2000b) and OMB Circula

rounded to two significant digits 

f For the purposes of comparison with the benefits, EPA uses the total social cost estimate which is slightly higher than the engineering costb Includes roughly $180 Million in supplemental engineering costs. c Estimates rounded to two significant digits after calculations. d based on Pope et al 2002, used as primary estimate in previous RIAs. 
Table ES-2. Estimated Reduction in Incidence of Adverse Health and Welfare Effects Associated with 

Attaining the Revised and Alternative Standards, Incremental to Attainment of the 1997 Standards (95 

Percent Confidence Intervals Provided in Parentheses) 

Estimate Revised Standards (15/35)

Alternative Revised Standards 

(14/35)

Mortality 

 Estimate based on American Cancer Society 

studya

2,500 

(1,000 – 4,100) 

4,400 

(1,700 – 7,100) 

 Range based on expert elicitation resultsb

 Low Mean 1,200 

(0 – 5,800) 

2,200 

(0 – 11,000) 

 High Mean 13,000 

(6,400 – 19,000) 

24,000 

(12,000 – 35,000) 

Morbidity 

Chronic bronchitis (age >25 and over) 2,600 

(490 – 4,800)

4,600 

(850—8,300) 

Nonfatal myocardial infarction (age >17) 5,000 

(2,700 – 7,200)

8,700 

(4,800 – 13,000) 

Hospital admissions—respiratory (all ages)b 530 

(260 – 800) 

980 

(490 – 1,500) 

Hospital admissions—cardiovascular (age >17)c 1,100 

(690 – 1,500) 

2,100 

(1,300 – 2,800) 

Emergency room visits for asthma (age <19) 1,200 

(730 – 1,700) 

3,200 

(1,900 – 4,500) 

Acute bronchitis (age 8–12) 7,300 

(–260 – 15,000) 

13,000 

(–440 – 25,000) 

Lower respiratory symptoms (age 7–14) 56,000 

(27,000 – 84,000) 

88,000 

(43,000 – 130,000) 

Upper respiratory symptoms (asthmatic children, age 

9–18) 

41,000 

(13,000 – 70,000) 

65,000 

(20,000 – 110,000) 

Asthma exacerbation (asthmatic children, age 6–18) 51,000 

(5,600 – 150,000) 

79,000 

(8,900 – 230,000) 

Work loss days (age 18–65) 350,000 

(300,000 – 390,000) 

550,000 

(480,000 – 620,000) 

Minor restricted-activity days (age 18–65) 2,000,000 

(1,700,000 – 2,300,000) 

3,300,000 

(2,700,000 – 3,800,000) 

a

 The estimate is based on the concentration-response (C-R) function developed from the study of the American Cancer Society cohort reported in 

Pope et al (2002), which has previously been reported as the primary estimate in recent RIAs. 

b

 The low mean estimate is based on the C-R function provided Expert K. The high mean estimate is based on the C-R function provided by 

Expert E. The expert elicitation project is described in greater detail in Chapter 5, and a complete report of the project is available on EPA’s 

website. . Although the overall range across experts is summarized in this table, the full uncertainty in the estimates is reflected by the results for 

the full set of 12 experts. The twelve experts’ judgments as to the likely mean effect estimate are not evenly distributed across the range 

illustrated by arraying the highest and lowest expert means. Likewise the 5th and 95th percentiles for these highest and lowest judgments of the 

effect estimate do not imply any particular distribution within those bounds. The distribution of mortality estimates associated with each of the 

twelve expert responses can be found in Chapter 5. 

ES-8
Table ES-3. Estimated Annual Monetized Benefits in 2020 of Illustrative Implementation 

Strategies for the Selected and Alternative PM2.5 NAAQS, Incremental to Attainment of 

the 1997 Standards 

Note: Unquantified benefits are not included in these estimates, thus total benefits are likely to be larger than 

indicated in this table. 

Total Full Attainment Benefitsa

 (billions 1999$) 

15/35 (μg/m3) 14/35 (μg/m3) 

 

Benefits Based on Mortality Function from the American Cancer Society Study and Morbidity Functions 

from the Published Scientific Literature b

Using a 3% discount rate $17 

($4.1 – $36) 

$30 

($7.3 - $63) 

Using a 7% discount rate $15 

($3.5 – $31) 

$26 

($6.4 - $54) 

Benefits Range Based on Expert Elicitation Derived Mortality Functions and Morbidity Functions from the 

Published Scientific Literaturec

Low Mean High Mean Low Mean High Mean 

Using a 3% discount rate $9 

($0.8 - $42) 

$76 

($19-$150) 

$17 

($1.7 - $77) 

$140 

($36 - $280) 

Using a 7% discount rate $8 

($0.8 - $36) 

$64 

($16 - $130) 

$15 

($1.6 - $66) 

$120 

($31 - $240) 

a

 Results reflect the use of two different discount rates: 3% and 7%, as recommended in EPA’s Guidelines for 

Preparing Economic Analyses (EPA, 2000b) and OMB Circular A-4 (OMB, 2003). Results are rounded to 

two significant digits. 

b

 The estimate is based on the concentration-response (C-R) function developed from the study of the American 

Cancer Society cohort reported in Pope et al (2002), which has previously been reported as the primary 

estimate in recent RIAs. 

c

 Although the overall range across experts is summarized in this table, the full uncertainty in the estimates is 

reflected by the results for the full set of 12 experts. The twelve experts’ judgments as to the likely mean 

effect estimate are not evenly distributed across the range illustrated by arraying the highest and lowest expert 

means. Likewise the 5th and 95th percentiles for these highest and lowest judgments of the effect estimate do 

not imply any particular distribution within those bounds. The distribution of benefits estimates associated 

with each of the twelve expert responses can be found in Chapter 5. 

ES-9
Table ES-4 summarizes the total annualized engineering and social costs of meeting the current 

standard and the alternative scenarios using 3 and 7 percent discount rates. Total annualized 

costs are estimated from a baseline inventory in 2020 that reflects controls for 

CAIR/CAMR/CAVR and other on-the-books rules. Based on engineering cost estimates, the 

incremental cost of the revised standards (15/35) is approximately $5.0 to $5.1 billion using 3 

and 7 percent discount rates, respectively. The incremental costs for the alternative standards are 

$6.8 to $6.9 billion using 3 and 7 percent discount rates, respectively. These cost numbers are 

highly uncertain because they include the extrapolated costs of full attainment in California and 

Salt Lake City. Approximately $4.5 billion of the incremental cost of achieving both 15/35 and 

14/35 is attributable to these extrapolated full attainment costs. An analysis of the costs and 

benefits of attaining the 1997 standards in 2015 is provided in Appendix A. 

For the purposes of comparison with the benefits, EPA uses the total social cost estimate which 

is slightly higher than the engineering cost. Total social costs (including the general equilibrium 

impacts on GDP) are estimated to be $5.4 billion in 2020 for the revised standards, and $7.9 

billion for the alternative standards. 

Table ES-4. Comparison of Total Annualized Engineering Costs Across PM NAAQS 

Scenarios (millions of 1999 dollars, 7% interest rate) a 

Scenario 

Source Category 

Revised Stds: 

15/35 

Alternative 

Revised Stds: 

14/35 

EGU’s $400 $1,100 

Mobile Sources $60 $60 

Non-EGU’s $380 $1,300 

Incremental Residual Cost of Full Attainmentb

East $3 $180 

West $300 $300 

California $4,000 $4,000

 Total of Residual Costs of Full Attainment $4,300 $4,500 

Total Annualized Costs (incremental to the current 

standard) – using a 7% interest rate $5,100 $7,000 

Total Annualized Costs (incremental to the current 

standard) – using a 3% interest rate $5,050 $6,800 

a Upon review of emissions and air quality results of the control strategies applied in this RIA, some areas had 

residual nonattainment problems (requiring additional emissions reductions to meet the standard) as a result of 

our initial selection of controls. The incremental costs of fully attaining in these areas (the residual cost of full 

attainment) reflect extrapolated costs of additional control measures that would be necessary to bring areas 

with residual nonattainment into compliance. Chapter 4 provides details of the assessment. 

b The incremental cost of residual nonattainment (beyond our modeled control strategy) for the West and 

California are extrapolated. The methodology used to derive these estimates is described in Chapter 6. These 

estimates are derived using a 7 percent discount rate. The incremental cost of residual non-attainment in the 

East are based on supplemental carbonaceous particle emission controls, which are detailed in Chapter 4.

ES-10
ES-3. Uncertainties and Limitations 

Air Quality Modeling and Emissions 

• Overall, the air quality model performs well in predicting monthly to seasonal 

concentrations, similar to other state-of-the-science air quality model applications for 

PM2.5. However, there is less certainty in analyses involving 24-hour model predictions 

than those involving longer-term averages concentrations and performance is better for 

the Eastern U.S. than for the West. In both the East and West, secondary carbonaceous 

aerosols are the most challenging species for the modeling system to predict in terms of 

evaluation against ambient data. 

• Underestimation biases in the mobile source emission inventories lead to uncertainty as 

to the relative contribution of mobile source emissions to overall PM levels. 

• Additional uncertainty is introduced as a result of our limited understanding concerning 

the collective impact on future-year emission estimates from economic growth estimates, 

increases in technological efficiencies, and limited information on the effectiveness of 

future control programs. 

• The regional scale used for air quality modeling can understate the effectiveness of 

controls on local sources in urban areas as compared to area-wide or regional controls. 

This serves to obscure local-scale air quality improvements that result from urban-area 

controls. 

Controls and Cost

• The technologies applied and the emission reductions achieved in these analyses may not 

reflect emerging control devices that could be available in future years to meet any 

requirements in SIPs or upgrades to some current devices that may serve to increase 

control levels. 

• The effects from “learning by doing” are not accounted for in the emission reduction 

estimates for point and area sources. It is possible that an emissions control technology 

may have better performance in reducing emissions due to greater understanding of how 

best to operate and maintain the technology. As a result, we may understate the emission 

reductions estimated by these analyses. The mobile source control measures do account 

for these learning by doing effects. 

• The effectiveness of the control measures in these analyses is based on an assumption 

that these controls are well maintained throughout their equipment life (the amount of 

time they are assumed to operate). To the extent that a control measure is not well 

maintained, the control efficiency may be less than estimated in these analyses. Since 

these control measures must operate according to specified permit conditions, however, it 

is expected that the maintenance of controls should yield control efficiencies at or very 

close to those used in these analyses. As a result, we may overstate the emission 

reductions estimated by these analyses. 

• The application of area source control technologies in these analyses assume that a 

ES-11
constant estimate for emission reduction is reasonable despite variation in the extent or 

scale of application (e.g. dust control plans at construction sites ). To the extent that there 

are economies of scale in area source control applications, we may overstate the emission 

reductions estimated by these analyses. 

• The cost extrapolation method used to develop full attainment costs is highly uncertaint 

and may significantly under or overstate future costs of full attainment. 

Benefits

• This analysis assumes that inhalation of fine particles is causally associated with 

premature death at concentrations near those experienced by most Americans on a daily 

basis. Although biological mechanisms for this effect have not yet been specifically 

identified, the weight of the available epidemiological, toxicological, and experimental 

evidence supports an assumption of causality. The impacts of including a probabilistic 

representation of causality are explored using the results of the expert elicitation. 

• This analysis assumes that all fine particles, regardless of their chemical composition, are 

equally potent in causing premature mortality. This is an important assumption, because 

the composition of PM produced via transported precursors emitted from EGUs may 

differ significantly from direct PM released from automotive engines and other industrial 

sources. In accordance with advice from the CASAC, EPA has determined that no clear 

scientific grounds exist for supporting differential effects estimates by particle type, 

based on information in the most recent Criteria Document. In chapter 5, we provide a 

decomposition of benefits by PM component species to provide additional insights into 

the makeup of the benefits associated with reductions in overall PM2.5 mass (See Tables 

5-32 and 5-33). 

• This analysis assumes that the concentration-response (CR) function for fine particles is 

approximately linear within the range of ambient concentrations under consideration 

(above the assumed threshold of 10 μg/m3). Thus, we assume that the CR functions are 

applicable to estimates of health benefits associated with reducing fine particles in areas 

with varied concentrations of PM, including both regions that are in attainment with 

PM2.5 standards and those that do not meet the standards. However, we examine the 

impact of this assumption by looking at alternative thresholds in a sensitivity analysis. 

• A key assumption underlying the entire analysis is that the forecasts for future emissions 

and associated air quality modeling are valid. Because we are projecting emissions and 

air quality out to 2020, there are inherent uncertainties in all of the factors that underlie 

the future state of emissions and air quality levels. 

ES-12
ES-4. Conclusions and Insights 

EPA’s analysis has estimated the health and welfare benefits of reductions in ambient 

concentrations of particulate matter resulting from a set of illustrative control strategies to reduce 

emissions of PM2.5 precursors. The results suggest there will be significant additional health and 

welfare benefits arising from reducing emissions from a variety of sources in and around 

projected nonattaining counties in 2020. While 2020 is the latest date by which states would 

generally need to demonstrate attainment with the revised standards, it is expected that benefits 

(and costs) will begin occurring earlier, as states begin implementing control measures to show 

progress towards attainment. 

There are several important factors to consider when evaluating the relative benefits and costs of 

the attainment strategies for the revised 15/35 and alternative 14/35 standards: 

 California accounts for a large share of the total benefits and costs for both of the 

evaluated standards (80 percent of the benefits and 78 percent of the costs of attaining the 

revised standards, and 50 percent of the benefits and 58 percent of the costs of attaining 

the alternative standards). Because we were only able to model a small fraction of the 

emissions controls that might be needed to reach attainment in California, the proportion 

of California benefits in the “residual attainment” category are large relative to other 

areas of the U.S. Both the benefits and the costs associated with the assumed reductions 

in California are particularly uncertain. 

 The comparative magnitudes and distributions of benefits estimates for the revised and 

alternative standards are significantly affected by differences in assumed attainment 

strategies. As noted above, attainment with the revised standards was simulated using 

mainly local reductions, while a supplemental eastern regional SO2 reduction program 

was used for the alternative. Under the assumptions in the analyses, the regional strategy 

used in meeting the alternative standard resulted in significant additional benefits in 

attainment areas than the local area strategy used for the revised standard. This makes 

the difference in benefits between the revised and alternative standards larger than can be 

accounted for by only the 1 μg/m3 lower annual level for the alternative standards. 

 Given current scientific uncertainties regarding the contribution of different components 

to the effects associated with PM2.5 mass, this analysis continues to assume the 

contribution is directly proportional to their mass. In the face of uncertainties regarding 

this assumption, we believe that strategies which reduce a wide array of types of PM and 

precursor emissions will have more certain health benefits than strategies that are more 

narrowly focused. For this reason, the analysis provides a rough basis for comparing the 

assumed benefits associated with different components for different strategies. The 

illustrative attainment strategy for the revised standards results in a more balanced mix of 

reductions in different PM2.5 components than does the regional strategy for the alterative 

standards. Until a more robust scientific basis exists for making reliable judgments about 

the relative toxicity of PM, it will not be possible to determine whether the strategy of 

reducing a wide array of PM types is the optimal approach. 

 Because of the limitations and uncertainties in the emissions and air quality components 

of our assessment, the specific control strategies that might be the most effective in 

helping areas to reach attainment are still very uncertain. For example, the high 

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likelihood of mobile sources emissions being significantly understated biases the analyses 

by requiring additional controls from other sources in both the base case and the analyses 

of the 1997, revised, and alternative standards. 

 Previous analyses have focused on measuring cost-effectiveness by comparing control 

measures in terms of cost per ton of emissions reduced. In those analyses, direct PM 

controls usually appear to be less cost-effective because the cost per ton is in the tens of 

thousands of dollars per ton, while SO2 and NOx controls are on the order of thousands 

of dollars per ton. The current analysis demonstrates that when considered on a cost per 

microgram reduced basis, controls on directly emitted PM are often the most costeffective, because of the significant local contribution of direct PM emissions to 

nonattaining monitors in urban areas. This finding suggests that states should consider 

ranking controls on a cost per microgram basis rather than a cost per ton basis to increase 

the overall cost-effectiveness of attainment strategies. 

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