1

Responses to Significant Comments on the 

2009 Proposed Rule on the 

Primary National Ambient Air Quality Standards 

for Sulfur Dioxide 

(December 8, 2009; 74 FR 64810) 

Docket Number OAR-2007-0352 

U.S. Environmental Protection Agency 

June 2010 
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Table of Contents 

I. Introduction............................................................................................................... 8 

II. Responses to Significant Comments on the Scientific Evidence and 

Exposure/Risk Information............................................................................................ 10 

A. Comments on the controlled human exposure evidence ................................. 10 

B. Comments on the epidemiology......................................................................... 22 

C. Comments on the air quality, exposure, and risk analyses............................. 34 

III. Responses to Significant Comments on the Adequacy of the Current 

Standards......................................................................................................................... 56 

IV. Comments on A New Short-Term SO2 Primary Standard............................. 59 

A. Indicator............................................................................................................... 59 

B. Averaging time .................................................................................................... 59 

C. Form..................................................................................................................... 61 

D. Level ..................................................................................................................... 65 

E. Retaining or revoking the current SO2 NAAQS .............................................. 73 

V. Technical Issues with Monitoring Requirements................................................. 78 

A: Comments on Proposed Monitoring Program.................................................... 79 

B: Comments on Monitoring Method....................................................................... 83 

C: Comments on deadlines for monitoring plans and deployment........................ 88 

D: Comments on Population Weighted Emissions Index (PWEI) monitors......... 93 

E: Comments on State-Level Emissions Monitors................................................. 103 

F: Comments on siting requirements and requesting waivers ............................. 107 

G: Comments on Proposed Alternative Network .................................................. 117 

H: Comments on data reporting.............................................................................. 121 

I: Comments on Cost of Monitoring Changes........................................................ 124 

J: Comments on the authority and responsibilities of Regional Administrators 

(RAs)........................................................................................................................... 126 

VI. Air Quality Index .............................................................................................. 128 

VII. Comments on the Process for Reviewing the SO2 Primary NAAQS ........... 131 

VIII. Interpretation of the Clean Air Act............................................................ 131 

IX. Comments on Implementation, Designations, and Exceptional Events ...... 136 

A. Comments on designations of non-attainment.................................................. 137 

B. Comments on date................................................................................................ 139 

C. Comments on data to be used ............................................................................ 141 

D. Future permitting................................................................................................ 147 

X. Responses to Significant Comments on Appendix T (Interpretation of Primary 

NAAQS and Exceptional Events Rules) ..................................................................... 153 

A. Comments on data completeness and data substitutions.......................... 153 

XI. Responses to Significant Comments on the Regulatory Impact Analysis ... 158 

XII. References.......................................................................................................... 163 
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Frequently Cited Documents 

The following documents are frequently cited throughout EPA=s response to comments, 

often by means of the short names listed below: 

Integrated Science Assessment (ISA): 

 EPA. (2008). ISA for Oxides of Sulfur - Health Criteria. National Center for 

Environmental Assessment, Research Triangle Park, NC. Available at: 

http://cfpub.epa.gov/ncea/cfm/recordisplay.cfm?deid=198843 

 

Preamble to the final rule (final notice): 

 Preamble to the Final Rule on the Review of the Primary National Ambient Air 

Quality Standards for Sulfur Dioxide; to be published in the Federal Register in 

June of 2010. 

Proposal notice: 

Primary National Ambient Air Quality Standard for Sulfur Dioxide: Proposed 

Rule. 74 FR 64810, December 8, 2009. 

Risk and Exposure Assessment (REA): 

EPA. (2009a). Risk and Exposure Assessment to Support the Review of the SO2 

Primary National Ambient Air Quality Standard. Office of Air Quality Planning 

and Standards, Research Triangle Park, NC. Available at: 

http://www.epa.gov/ttn/naaqs/standards/so2/s_so2_cr_rea.html
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Acronyms and Abbreviations 

A&B Alexander & Baldwin, Inc. 

AA The Aluminum Association 

ABR Association of Battery Recyclers, Inc. 

ACC American Chemistry Council 

AECT Association of Electric Companies of Texas 

AEPSC American Electric Power Service Corporation 

AERR Air Emissions Reporting Requirements 

AFPA American Forest & Paper Association 

AirQuality AirQuality Research and Logistics 

ALA American Lung Association 

ALA-SC American Lung Association State Chapters 

Alaska Alaska Department of Environmental Conservation 

Alexandria City of Alexandria, Virginia 

ACSBPP Annapolis Center for Science-Based Public Policy 

API American Petroleum Institute 

AQCD Air quality criteria document 

AQI Air quality index 

AQS Air quality system 

ASARCO American Smelting And Refining Company 

ASC The Amalgamated Sugar Company 

ATS American Thoracic Society 

BART Best available retrofit technology 

CAA Clean Air Act 

CAIR Clean Air Interstate Rule 

CASAC Clean Air Scientific Advisory Committee 

CBSA Core-based statistical area 

CE Consumers Energy 

CEM Continuous emission monitor 

CFR Code of Federal Regulations 

CIBO Council of Industrial Boiler Owners 

City of New York City of New York Law Department, Environmental Law Division 

CMAQ Community multi-scale air quality 

CO Carbon monoxide 

CRA Corn Refiners Association 

Delaware Delaware Department of Natural Resources and Environmental 

Control 

Dow The Dow Chemical Company 

DRSI Dominion Resources Services, Inc. 

Duke Energy Duke Energy Corporation 

ED visits Emergency department visits 

EDEC Empire District Electric Company 

EDF Environmental Defense Fund 

EEI Edison Electric Institute 

EGU Electric generating unit, or electricity generating utility 
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EPA Environmental Protection Agency 

EPRI Electric Power Research Institute 

ExxonMobil ExxonMobil Corporation 

FEM Federal equivalent method 

FEV1 Forced Expiratory Volume in the first second 

First Energy FirstEnergy Corporation 

Florida Florida Department of Environmental Protection 

FR Federal Register 

FRM Federal reference method 

GAM Generalized Additive Model 

GCLC Gulf Coast Lignite Coalition 

GDI Green Dubuque Inc. 

Georgia Georgia Department of Natural Resources 

Golder Golder Associates Inc. 

Hamilton Pepper Hamilton 

Harris County Harris County Public Health and Environmental Services 

H-GAC Houston-Galveston Area Council 

Houston City of Houston, TX 

Illinois Illinois Environmental Protection Agency 

Indiana Indiana Department of Environmental Management 

Iowa Iowa Department of Natural Resources/Iowa Department of Public 

Health 

ISA Integrated Science Assessment 

JRSC J.R. Simplot Company 

KUC Kennecott Utah Copper, LLC 

KYDEP Kentucky Department for Environmental Protection 

LAER Lowest Achievable Emission Rate 

LEC Lignite Energy Council 

Louisiana Chemical Louisiana Chemical Association, Louisana Mid-Continent Oil and 

Gas Association, and Louisana Pulp and Paper Association 

Marathon Petroleum Marathon Petroleum Company 

MDUC Montana-Dakota Utilities Co. 

MIDNRE Michigan Department of Natural Resources and Environment 

Missouri Missouri Department of Natural Resources, Air Pollution Control 

Program 

MSCC Montana Sulphur and Chemical Company Inc. 

MWV MeadWestvaco Corporation 

NAAQS National Ambient Air Quality Standards 

NACAA National Association of Clean Air Agencies 

NAM National Association of Manufacturers 

NEI National Emissions Index 

NESCAUM Northeast States for Coordinated Air Use Management 

New Mexico New Mexico Environment Department, Air Quality Bureau 

NMA National Mining Association 

NO2 Nitrogen dioxide 

North Carolina North Carolina Department of Environment and Natural Resources 
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NOx Nitrogen oxides 

NPRA National Petrochemical & Refiners Association 

NRDC Natural Resource Defense Council 

NRECA National Rural Electric Cooperative Association 

NSR New Source Review 

Nucor Nucor Corporation 

NYSDEC New York State Department of Environmental Conservation 

NYDOH New York State Department of Health 

O3 Ozone 

Ohio Ohio Environmental Protection Agency 

PCA Portland Cement Association 

PE Progress Energy 

Pennsylvania Pennsylvania Department of Environmental Protection 

Pepper Hamilton Pepper Hamilton 

PM Particulate matter 

PM10 Particulate matter with an aerodynamic diameter of less than 10 

micrometers 

PM2.5 Particulate matter with an aerodynamic diameter of less than 2.5 

micrometers 

ppb parts per billion 

PPLC PPL Corporation 

ppm parts per million 

PSD Prevention of significant deterioration 

PWEI Population weighted emissions index 

RA Regional administrator 

RACT Reasonable available control technology 

REA Risk and Exposure Assessment 

RIA Regulatory Impact Analysis 

Rio Tinto Rio Tinto Corporation 

RMA Rubber Manufacturers Association 

RRI RRI Energy, Inc. 

RTA Rio Tinto Alcan Sebree Works 

RTC Response to Comment 

SC Sierra Club 

SER Significant emission rate 

SIDS Sudden infant death syndrome 

SIL Significant impact level 

SIP State implementation plan 

SMC Significant monitoring concentration 

SO2 Sulfur dioxide 

South Carolina South Carolina Department of Health and Environmental Control 

South Dakota South Dakota Department of Environment and Natural Resources 

SOx Sulfur oxides 

Springfield City of Springfield, MO 

SAP Ad Hoc Committee of Sulfuric Acid Producers 

sRaw Specific Airway Resistance 
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TAB Texas Association of Business 

Texas Texas Commission on Environmental Quality 

TXOG Texas Oil and Gas Association 

TFI The Fertilizer Institute 

tpy tons per year 

UARG Utility Air Regulatory Group 

URL Upper range limit 

USDOI United States Department of Interior 

Vermont Vermont Air Pollution Control 

WEACT WE ACT for Environmental Justice (West Harlem Environmental 

Action, Inc.) 

WHO World Health Organization 

Wisconsin Wisconsin Department of Natural Resources 
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I. Introduction 

This document, together with the preamble to the final rule on the review of the primary 

national ambient air quality standards (NAAQS) for sulfur dioxide (SO2), presents the 

responses of the Environmental Protection Agency (EPA) to the public comments 

received on EPA’s proposal of December 8, 2009, proposing revisions to the primary 

NAAQS for SO2, as well as related proposed changes to the ambient air monitoring, 

reporting, and network design requirements for the primary SO2 NAAQS. 74 FR 64810. 

All significant issues raised in timely public comments have been addressed. Where 

comments were submitted after the close of the public comment period, EPA has 

responded to the extent feasible. 

Due to the number of comments that addressed similar issues, this response-to-comments 

document does not generally cross-reference each response to the commenter(s) which 

raised the particular issue involved, although commenters are identified in some cases 

where they provided particularly detailed comments that were used to frame the overall 

response on an issue. 

The responses presented in this document are intended to augment the responses to 

comments that appear in the preamble to the final rule or to address comments not 

discussed in the preamble to the final rule. Although portions of the preamble to the final 

rule are paraphrased in this document where useful to add clarity to responses, to the 

extent any (unintended) ambiguity is introduced by this paraphrasing, the preamble itself 

remains the definitive statement of the rationale for the positions adopted in the final rule. 

In many instances, particular responses presented in this document include cross 

references to responses on related issues that are located either in the preamble to the SO2

primary NAAQS final rule, or in this Response to Comments document. In other 

instances the comment is appropriately addressed by the Agency’s discussion in other 

parts of the record. All issues on which the Administrator is taking final action in the 

SO2 primary NAAQS final rule are addressed in the SO2 NAAQS rulemaking record. 

Accordingly, this Response to Comments document, together with the preamble to the 

SO2 primary NAAQS final rule and the information contained in the Integrated Science 

Assessment (ISA) (EPA, 2008), the Risk and Exposure Assessment (REA) (EPA, 2009a), 

and the Notice of Proposed Rulemaking should be considered collectively as EPA’s 

response to all of the significant comments submitted on EPA’s 2009 SO2 primary 

NAAQS proposed rule. This document incorporates directly or by reference the 

significant public comments addressed in the preamble to the SO2 NAAQS final rule as 

well as other significant public comments that were submitted on the proposed rule. 

Consistent with the final decisions presented in the notice of final rulemaking, comments 

on the following topics are addressed in this document: the scientific evidence and 

exposure/risk information (section II); the adequacy of the current SO2 standard to protect 

public health (section III); revisions to the current standard in terms of indicator (section 

IV.A), averaging time (section IV.B), form (section IV.C), level (section IV.D) and 
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retaining or revoking the current standards (section IV.E); revisions to the SO2

monitoring network (section V); the air quality index (VI); the process for reviewing the 

standard (section VII); interpretation of the Clean Air Act (section VIII); implementation, 

designations, and exceptional (section IX); and on data handling (section X). Finally, 

comments related to the Regulatory Impact Analysis are addressed (section XI). 

 
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II. Responses to Significant Comments on the Scientific 

Evidence and Exposure/Risk Information 

A. Comments on the controlled human exposure evidence 

(1) Comment: Several industry groups (e.g., API, UARG, ACC, ASC, CE, AEPSC, 

ASARCO, NMA, EEI, RTA) commented that EPA does not provide justification 

for changing the conclusion in the 1996 review that adverse effects were based on 

SO2 exposure concentrations that resulted in large decrements in lung function 

and moderate to severe respiratory symptoms in exercising asthmatics. That is, 

industry groups do not believe EPA has provided an adequate rationale for 

focusing the current review on moderate to severe decrements in lung function 

and/or respiratory symptoms ranging from mild (perceptible wheeze or chest 

tightness) to severe (breathing distress requiring the use of a bronchodilator). For 

example, UARG comments state: 

The genesis of this change is obscure. First, the Proposed Rule seems to 

postulate that the health risk to asthmatics who experience lung function 

effects without symptoms might be greater than the health risk to those 

who experience both symptoms and lung function decrements. See id. at 

64816/2. It is simply implausible that a failure to experience symptoms is 

worse than having them. Second, the Proposed Rule points to advice from 

CASAC as justification for the focus on moderate as well as large lung 

function decrements. Id. at 64817/3. But the Administrator culls this 

advice from comments of individual CASAC members in the transcript of 

a CASAC meeting; it is not reflected in the letter from the complete 

CASAC that followed that meeting. Finally, the Administrator simply 

provides no explanation of why she now considers mild symptoms to be 

adverse health effects. Clearly, this does not constitute a reasoned analysis 

in support of the change to the definition of adversity (see comments 

provided by UARG, p.10). 

Response: The commenters are correct that the ISA, and this review, focused on 

moderate or greater decrements in lung function in the presence or absence of 

respiratory symptoms in exercising asthmatics. With respect to the ISA, from 

individual level response data derived from controlled human exposure studies, 

the ISA characterized the number and percent of exercising asthmatics 

experiencing a ≥ 100% increase in specific airway resistance (sRaw ) or ≥ 15% 

decline in forced expiratory volume in the first second (FEV1). The results of this 

analysis indicated that exposure to SO2 concentrations as low as 200 -300 ppb for 

5 - 10 minutes resulted in approximately 5 - 30% of exercising asthmatics 

experiencing moderate or greater decrements in lung function (ISA, Table 3-1). 

In addition, at SO2 concentrations ≥ 400 ppb, 20 - 60% of exercising asthmatics 

experienced moderate or greater decrements in lung function. Notably, at SO2

concentrations ≥ 400 ppb the ISA noted that moderate or greater decrements in 
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lung function are frequently accompanied with reparatory symptoms, while at 200 

– 300 ppb there is limited evidence of respiratory symptoms. 

The decision in the ISA to focus on moderate or greater decrements in lung 

function with or without accompanying respiratory symptoms was not, as 

commenters contend, based solely on the advice of individual CASAC panel 

members. In part, as noted on page 3-4 of the ISA, this judgment reflects recent 

guidance provided by the American Thoracic Society (ATS, 2000). That is, the 

ISA states: “In their official statement, the ATS concluded that an air pollutioninduced shift in a population distribution of a given health-related endpoint (e.g., 

lung function in asthmatic children) should be considered adverse, even if this 

shift does not result in the immediate occurrence of illness in any one individual 

in the population (ISA, p.3-4).” Thus, the ISA concluded that these updated ATS 

guidelines could reasonably be interpreted to indicate that moderate or greater 

decrements in lung function, even without respiratory symptoms, could be of 

public health significance. (See also response #5 below, explaining that EPA 

would reach the same conclusion without reference to the ATS Guidelines.) 

Notably, this focus was also in agreement with CASAC comments. In their 

consensus letter to the Administrator dated August 8, 2008, CASAC advised EPA 

to place more of an emphasis on SO2 concentrations below 0.4 - 0.6 ppm (i.e. 

below 400 – 600 ppb), stating, “The clinical and epidemiological studies warrant 

a stronger conclusion about health effects at lower levels.” This counsel was 

based in part on the fraction of asthmatics shown to experience “...moderate or 

greater decrements in lung function at SO2 concentrations as low as 0.2 – 0.3 

ppm.” (Emphasis added.) Moreover, we note that this is in agreement with 

previous CASAC recommendations (Henderson 2006) and NAAQS review 

conclusions (EPA 2006, EPA 2007) indicating that moderate decrements in lung 

function can be clinically significant in some asthmatics. See Coalition of Battery 

Recyclers Association v. EPA, No. 09-1011 (D.C. Cir., May 14, 2010), slip 

opinion at 9 (reasonable for EPA to conclude that two IQ point loss is an adverse 

effect based in part on CASAC advice that such a decrement is significant). 

The commenter’s assertion that EPA’s interpretation of the controlled human 

exposure studies is grounded in the view that not having symptoms is worse than 

having them is misplaced. First, neither EPA nor the ISA made that statement. 

That language does not appear at 74 FR 64816 (the citation given by the 

commenter). Nonetheless, EPA believes that focusing on only more severe 

respiratory symptoms observed in controlled human exposure studies would be 

inappropriate and not protective of public health. As discussed in the final ISA (p. 

3-4 to 3-5), symptom perception is highly variable among asthmatics. This is 

particularly true in children, and may result in an increased risk of respiratory 

morbidity and mortality in those individuals who are poor perceivers of changes 

in pulmonary function, as they will be less likely to seek treatment. That is, 

individuals who experience moderate or greater decrements in lung function, but 

only mild or perhaps no respiratory symptoms at all are potentially at greater risk 

of developing hypoxia because they are less likely to seek medical treatment (as 
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noted by clinicians on the CASAC panel). Thus, it is possible that in some 

instances, a failure to have respiratory symptoms can lead to a more serious health 

outcomes. 

(2) Comment: Several industry groups (e.g., API, UARG, ACC, ACSBPP, Nucor, 

AEPSC, ABR, ASARCO, NMA, Dow, NRECA, NPRA, A&B) generally 

commented that the effects reported in exercising asthmatics following exposure 

to 200 - 300 ppb SO2 for 5-10 minutes should not be considered adverse by the 

Administrator. Commenters indicate that at 200 – 300 ppb, decrements in lung 

function reported in controlled human exposure studies in exercising asthmatics 

are not statistically significant, and they are not frequently accompanied by 

respiratory symptoms. Many of these groups indicate that EPA relied upon the 

advice of select CASAC members rather than consensus CASAC letters in 

determining adverse effects at 200 - 300 ppb SO2. 

Response: The commenters are correct that following 5-10 minute SO2

exposures at 200- 300 ppb, the moderate or greater decrements in lung function 

experienced by some exercising asthmatics are not statistically significant at the 

group mean level or frequently accompanied by respiratory symptoms. However, 

we strongly disagree that these effects are not adverse to the health of asthmatics. 

EPA’s rationale for considering health effects associated with 5-10 minute SO2

exposures from 200- 300 ppb as adverse is based on consensus CASAC advice 

and recommendations, EPA’s conclusions in previous reviews of other NAAQS, 

as well as ATS guidelines of what constitutes an adverse effect of air pollution. 

These considerations and similar comments are thoroughly described in sections 

II.B.1.a, II.E.2.b and II.F.4.b of the preamble to the final rule.

(3) Comment: A number of industry groups stated that it is inappropriate to analyze 

the effects of SO2 in terms of percentage of asthmatics affected, and that it is 

better to conduct analyses on group mean data. Industry groups (e.g., API, 

MSCC, ACSBPP) also stated that effects below 400 ppb should not be considered 

adverse because compared to the number of asthmatics experiencing decrements

in lung function, there were similar numbers of asthmatics experiencing increases

in lung function. For example API states: 

EPA’s conclusions about adverse effects in this range of SO2 exposure 

relies on selective consideration of negative responses in lung function in 

only a few individuals from one tail of the tail response distribution while 

ignoring similar positive and apparently beneficial responses in lung 

function, an approach that is not scientifically defensible (see comments 

provide by API; p. 14). 

Similarly, ACSBPP commented that: 

 

...day-today variation in pulmonary function test results, with a small 

portion of increases and decreases that are as large as the metric uses for 
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moderate decrements. EPA needs to consider the day-to-day variation in 

pulmonary function for asthmatics that may be affected by changes in 

meteorology, personal activities, exposure to indoor irritants and allergens, 

etc. in the analysis (see comments provide by ACSBPP; p. 6). 

Response: EPA strongly disagrees with the assertion that the analysis of 

percentage of asthmatics experiencing moderate or greater SO2-induced 

decrements in lung function reflects only normal variation in lung function 

unrelated to SO2 exposure. This analysis clearly shows an increase in the fraction 

of asthmatics affected with increasing SO2 concentrations, an effect which has 

been consistently demonstrated across studies. EPA further notes that both the 

ISA and REA have specifically considered and discussed statistical analyses of 

mean changes (group data) in lung function resulting from controlled exposures to 

SO2. See ISA at page 3-33, “Statistically significant decrements in lung function 

accompanied by respiratory symptoms including wheeze and chest tightness have 

been clearly demonstrated following exposure to 0.4-0.6 ppm SO2. Although 

studies have not reported statistically significant respiratory effects following 

exposure to 0.2-0.3 ppm SO2, some asthmatic subjects (5-30%) have been shown 

to experience moderate to large decrements in lung function at these exposure 

concentrations.” In addition, in comments prepared and submitted to the docket 

for API, William Linn’s analysis shows that, “...it is possible to pick out response 

data and statistical results from our studies that show significant (P < 0.05) 

pulmonary function changes at 0.2 ppm relative to control (filtered air, 0.0 ppm 

SO2),” although Dr. Linn did not consider these effects to be clinically significant. 

EPA’s conclusions from controlled human exposures to SO2 have been drawn 

from an evaluation of both individual and group mean response data. As stated on 

page 3-9 of the ISA, “Among asthmatics, both the magnitude of SO2-induced 

decrements in lung function and the percent of individuals affected have 

consistently been shown to increase with increasing exposure to SO2

concentrations between 0.2 and 1.0 ppm.” 

The commenters correctly point out that at the lowest concentration tested in freebreathing chamber studies (0.2 ppm), there are a similar number of asthmatics 

experiencing a moderate or greater decrease in lung function (i.e., ≥ 100 increase 

in sRaw or ≥ 15 decrease in FEV1) and experiencing what might be called a 

moderate improvement in lung function (i.e., ≥ 100 decrease in sRaw or ≥ 15 

increase in FEV1). This observation is consistent with data presented in Figures 

4-2 and 4-3 of the ISA showing essentially no SO2 -induced change in lung 

function at 0.2 ppm when averaged across asthmatics participating in the three 

cited studies conducted by William Linn. However, these figures also 

demonstrate that asthmatics who are sensitive to SO2 at a higher concentration 

(0.6 ppm) experience, on average, a greater decrement in lung function at lower 

concentrations, including 0.2 ppm, when compared with all subjects combined. 

Therefore, while some asthmatics are relatively insensitive to SO2-induced 

respiratory effects even at concentrations ≥ 0.6 ppm, there is clear empirical 

evidence that others experience significant bronchoconstriction following 
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exposures to both relatively high (0.6 ppm) and low (0.2 ppm) SO2

concentrations. Among these SO2-sensitive asthmatics, Figures 4-2 and 4-3 show 

a clear increase in bronchoconstriction with increasing SO2 concentration from 

0.2 - 0.4 ppm. Given this clear relationship of exposure and effect at all levels in 

the sensitive asthmatics (i.e. those who experienced significant decrements in lung 

function at the highest exposure concentration used (600 ppb)), EPA does not 

accept the commenter's premise that the results of the study do not demonstrate 

adverse effects. It should also be noted that significant effects have been 

observed at concentrations as low as 0.1 ppm (the lowest concentration tested) 

using mouthpiece exposure systems. Although these results cannot be compared 

directly with free-breathing chamber studies, they nonetheless provide valuable 

information regarding respiratory effects following exposures to low 

concentrations of SO2. These studies provide evidence of bronchoconstriction 

directly attributable to SO2 at the lowest concentrations tested among mild and 

moderate asthmatics. We further note that severe asthmatics could experience 

greater effects. See CASAC’s consensus letter to the Administrator dated August 

8, 2008, advising EPA to focus more on concentrations < 0.4 ppm, in part 

because, “...severe asthmatics were not part of these clinical studies, but it is not 

unreasonable to presume that they would have responded to even a greater 

degree.”

EPA also notes that a variety of factors such as variability in baseline conditions, 

time of day, exercise, and the laboratory exposure setting itself may affect lung 

function responses in subjects participating in controlled human exposure studies. 

Recognizing the potential of these factors to alter pulmonary function, 

investigators expose study participants to pollutant(s) of interest and a control 

atmosphere, typically filtered air. Exposures are conducted at the same time of 

day under the same laboratory conditions. By randomly assigning the order of 

these exposures, bias in response is distributed between the exposure days and 

improves the ability to discern the “true” effect of a pollutant exposure. Therefore, 

the other factors affecting lung function mentioned by the commenter are, to the 

extent possible, already controlled for by the design of these studies (see ISA, p.1-

4). 

(4) Comment: Several industry groups (e.g., UARG, API, ACC, Nucor) and 

organizations (e.g., EPRI) generally argue that EPA is not applying the 2000 ATS 

adversity guidelines (ATS 2000) with respect to population risk appropriately. 

These groups argue that since the effects of SO2 are transient and reversible 

within an hour, there is little chance of an asthmatic affected by these low levels 

of SO2 having diminished reserve lung function when confronted by another 

stimuli (e.g. viral infection). Groups also argue that variations in lung function is 

a hallmark of asthma and that there is no indication that the most sensitive 

exercising asthmatics represent a distinct subpopulation that may be affected in 

the manner described in the ATS Statement. 
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Response: ATS guidelines were not meant to provide strict rules related to the 

differentiation between adverse and non-adverse health effects of air pollution. As 

stated in the guidelines, “...the placement of dividing lines should be a societal 

judgment and consequently this committee does not propose specific boundaries 

for separating adverse from nonadverse effects.” Therefore, judgments regarding 

the adversity of effects following exposure to specific concentrations of SO2

represent EPA’s interpretation of the ATS guidelines. These judgments also 

reflect CASAC recommendations and conclusions drawn by EPA in previous 

NAAQS reviews (as described in final rule preamble sections II.B.1.a, II.E.2.b 

and II.F.4.b; see particularly final rule preamble section II.E.2.b and the response 

to Comment 5 in this section). 

Moreover, although it is correct that the decrements in lung function caused by 

exposure to SO2 in controlled human exposure studies have been shown to be 

transient, EPA disagrees with the assertion that there is little chance that an 

asthmatic would simultaneously be confronted by a secondary agent. Such 

exposure could very well occur, particularly considering the myriad of other 

stimuli known to affect lung function in asthmatics (e.g., allergens, cold air, 

coexposures to other pollutants—not just viral infections). In addition, those 

asthmatics with diminished reserve lung function caused by other factors such as 

an existing respiratory infection would be at higher risk of experiencing adverse 

respiratory effects if subsequently exposed to SO2 notwithstanding the transient 

nature of SO2-induced effects. 

(5) Comment: Multiple groups (e.g., ACSBPP) indicated that the ATS guidelines 

(ATS 2000) regarding adversity due to a population shift for a given health 

endpoint do not apply to SO2 effects because the ATS was referring to 

permanent decrements in lung function in a population. 

Response: EPA disagrees with the commenters’ interpretation of the ATS 

guidelines at the population level. The example given of an adverse effect at the 

population level given in the ATS Guidelines is as follows: 

 

A population of children with asthma could have a distribution of lung 

function such that no individual child has a level associated with 

significant impairment. Exposure to air pollution could shift the 

distribution toward lower levels without bringing any individual child to a 

level that is associated with clinically relevant consequences. Individuals 

within the population would, however, have diminished reserve function 

and are at potentially increased risk if affected by another agent, e.g., a 

viral infection. Assuming that the relationship between the risk factor and 

the disease is causal, the committee considered that such a shift in the risk 

factor distribution, and hence the risk profile of the exposed population, 

should be considered adverse, even in the absence of the immediate 

occurrence of frank illness (ATS, 2000; p. 668). 
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EPA does not believe that the above example was referring to a permanent loss in 

lung function. The Guidelines refer to permanent loss in lung function 

specifically when permanent effects are at issue, and further state that any 

permanent loss of lung function in an individual, or at the population level, is by 

definition an adverse effect: 

There is also epidemiologic evidence that air pollution may adversely 

affect lung growth or accelerate the age-related decline of lung function. 

Epidemiologic studies are limited in their power to detect such permanent 

effects and any evidence of association between air pollution exposure and 

permanent loss of function is indicative of an adverse effect at the 

population level. Some individuals may sustain clinically relevant, 

permanent losses of lung function. This committee considered that any 

detectable level of permanent lung function loss attributable to air 

pollution exposure should be considered as adverse. (ATS 2000; p. 671). 

The example quoted above (p. 668) does not mention permanent effects, 

“diminished reserve function” is not an inherently permanent effect, and, since 

any permanent effect is already deemed adverse under the Guidelines, EPA 

believes that the example would have no point if it was referring only to 

permanent effects. Thus, EPA is reasonably interpreting this part of the 

Guidelines to indicate that certain non-permanent effects at the population level 

may be considered to be adverse. 

Even without considering the 2000 ATS guidelines mentioned above, EPA would 

consider the asymptomatic decrements in lung function associated with 5- 10 

minute SO2 exposures as low as 200 ppb to be adverse. As described in the 

preamble to the final rule in sections II.B.1.a, II.E.2.b and II.F.4.b, these reasons 

include the conclusions drawn by EPA and CASAC in previous NAAQS reviews 

regarding the significance of decrements in lung function. That is EPA has stated 

that similar moderate or greater decrements in lung function (e.g., a ≥15% decline 

in FEV1 and/or ≥ 100% increase in sRaw) in people with pre-existing respiratory 

disease could result in clinical outcomes such as increased medication usage 

and/or disruption of normal activities (see REA, section 4.3, p. 35-36) which 

would also be considered adverse effects of air pollution under ATS guidelines 

(ATS 1985). In addition to these conclusions and CASAC advice from previous 

NAAQS reviews, EPA considered the consensus advice from CASAC in the 

current SO2 NAAQS review (Henderson 2008a; Henderson 2008b; Samet 2009; 

and final rule preamble sections II.B.1.a, II.E.2.b and II.F.4.b), as well as 

comments from individual clinicians on the CASAC panel during the current 

review (see final rule preamble sections II.B.1.a, II.E.2.b and II.F.4.b). See 

Coalition of Battery Recyclers Association v. EPA, No. 09-1011 (D.C. Cir., May 

14, 2010), slip opinion at 9 (reasonable for EPA to conclude that two IQ point 

loss is an adverse effect based in part on CASAC advice that such a decrement is 

significant). 
17

EPA finally notes that the current ISA also indicated that asymptomatic 

decrements in lung function can be clinically significant. The ISA states: 

 

...it is important to note that symptom perception is highly variable among 

asthmatics even during severe episodes of asthmatic bronchoconstriction. 

An asymptomatic decrease in lung function may pose a significant health 

risk to asthmatic individuals as it is less likely that these individuals will 

seek treatment (ISA, section 3.1.3; p. 3-4). 

Taken together, along with the consideration that severe asthmatics were not 

included in the controlled human exposure studies described in the ISA, the 

Administrator would have reasonably considered moderate or greater decrements 

in lung function in exercising asthmatics to be adverse, even without considering 

the 2000 ATS guidelines mentioned above. That being said however, she finds 

that the 2000 ATS guidelines provide strong support for considering these 

decrements in lung function, even without accompanying respiratory symptoms, 

as adverse. In addition, for the reasons outlined above, the Administrator further 

notes that she does not believe that only repeated exposures to SO2 should be 

considered adverse to the health of asthmatics. 

 (6) Comment: Several industry groups (e.g., API, UARG, ACC, CE) commented 

that EPA is incorrect in assuming that severe asthmatics would likely have a more 

pronounced response to SO2 exposures at a given level, or would respond to even 

lower levels of SO2. As support for their assertion multiple industry groups cite 

studies in the ISA stating that they included “severe asthmatics” as well as a study 

by Linn et al. (1987) concluding that among asthmatics, responses to SO2

exposure are not dependant on the clinical severity of asthma and that “the 

subjects with the highest risk [of temporary respiratory disturbances from ambient 

SO2] can be identified only be actually measuring their responses to SO2.” 

Response: We disagree with the assertion that severe asthmatics have been 

evaluated in 5 - 10 minute controlled human exposure studies. These comments 

and EPA responses are discussed in detail in section II.E.2.b of the preamble to 

the final rule. 

(7) Comment: EPRI generally commented that EPA is assuming that severe 

asthmatics were not included in controlled human exposure studies. However, 

EPRI contends that it is unlikely severe asthmatics would be exercising with the 

ventilation rates needed to be affected by SO2. 

Response: EPA strongly disagrees with the assertion that the most SO2-sensitive 

asthmatics have been included in controlled human exposure studies. EPA 

recognizes that it is possible that the most severe asthmatics would be less likely 

to experience SO2-induced bronchoconstriction due to exercise limitations. 

However, it is also possible that these asthmatics experience respiratory effects of 

SO2 exposure at lower ventilation than is required to produce a response in 
18

individuals with less severe disease. In addition, there is clearly a continuum of 

disease severity in the general population above the level of disease among those 

asthmatic subjects who participated in studies involving controlled exposures to 

SO2. It is therefore very possible that a significant fraction of these more severe 

asthmatics would be capable of experiencing increases in minute volume which 

would increase the fraction of SO2 reaching the tracheobronchial airways. 

This legitimate inference is supported by CASAC’s consensus advice that,

“...severe asthmatics were not part of these clinical studies, but it is not 

unreasonable to presume that they would have responded to even a greater 

degree.” Henderson (2008b; p. vi). 

(8) Comment: Multiple groups (e.g., ACSBPP, MWV, EPRI, Golder, AFPA, EEI, 

A&B) indicated that the respiratory effects of SO2 are not adverse because they 

are transient, reversible and/or can be alleviated through the use of medication. 

Moreover, they are similar to the effects experienced by asthmatics in response to 

common stimuli such as cold air, or psychological stress. 

Response: We agree that the respiratory effects of SO2 are transient and 

reversible. However, we strongly disagree that this indicates that such effects are 

not adverse to the health of asthmatics. EPA further disagrees that effects 

following SO2 exposure should not be considered adverse because they are 

similar to the effects experienced by asthmatics in response to common stimuli 

such as cold air. As noted above, judgments regarding the adversity of effects 

following SO2 exposure are described in detail in sections II.B.1.a, II.E.2.b, and 

II.F.4.b of the preamble to the final rule. However, additional information is 

provided below. 

Simply because the respiratory effects of SO2 in exercising asthmatics are 

transient and similar to those experienced by asthmatics in response to commonly 

encountered stimuli (e.g., cold air), it does not mean that such effects should not 

be considered adverse to the health of asthmatics. That is, following SO2

exposures ≥ 400 ppb, transient moderate or greater decrements in lung function 

are frequently accompanied by respiratory symptoms and this combination of 

effects would clearly be considered adverse under ATS guidelines (ATS 1985, 

ATS 2000). The observation that these respiratory effects are similar to those in 

response to commonly encountered stimuli does not change this conclusion. 

In addition, following SO2 exposures in the range of 200 - 300 ppb, controlled 

human exposure studies indicate that exposure to SO2 can result in an appreciable 

percentage of exercising asthmatics experiencing moderate or greater decrements 

in lung function. Notably, CASAC indicated that such decrements in lung 

function can be clinically significant in some asthmatics. As with exposures at 

higher levels, the observation that these respiratory effects are similar to those in 

response to commonly encountered stimuli does not change the following 

conclusions: 1) these respiratory effects are the direct result of exposure to SO2; 

and 2) these effects can be clinically significant in exercising asthmatics. We 
19

finally note that any respiratory effect experienced by exercising asthmatics in 

response to SO2 exposure would likely be in addition to those respiratory effects 

resulting from exposure to commonly encountered stimuli (that is, if co-exposure 

occurred). Thus, SO2 exposure could reasonably be judged to increase the risk of 

a more serious respiratory outcome (e.g., an asthma attack) because it could 

potentially reduce even further, an asthmatic’s lung function that has already been 

reduced by one or more commonly encountered stimuli. 

With respect to the respiratory effects of SO2 being alleviated through the use of 

medication, we note as discussed in the ISA (ISA, section 3.1.3.2), that some 

asthma medications have been shown to significantly reduce SO2-induced 

bronchoconstriction. However, the ISA further notes that even among severe 

asthmatics, the disease is often poorly controlled “...due to inadequate drug 

therapy or poor compliance among those who are on regular medication.”

Furthermore, EPA agrees with the statement from the 1985 ATS guidelines that 

medication usage in potentially susceptible populations should not be used as a 

justification to allow for increased pollutant concentrations. In addition, the ATS 

has recommended that detectable effects of air pollution on clinical measures 

(e.g., medication use) be considered adverse effects of air pollution (ATS, 1985) 

(9) Comment: Some health and environmental groups wanted more emphasis placed 

on mouthpiece studies at 100 ppb. For example ALA et al., stated: 

In its analysis of data from chamber studies in the ISA and in the REA, 

EPA focuses on studies of “free breathing” exposure. In doing so, EPA 

improperly and arbitrarily downplays important evidence that reported 

increased airway resistance, a measure of bronchoconstriction, in subjects 

with mild asthma at concentrations of 100 ppb. Regrettably, EPA does not 

rely on the mouthpiece studies in formulating its proposed standards (see 

comments provided by ALA et. al., p. 15).... In downplaying the 

mouthpiece studies, EPA ignores the large segment of people who rely on 

oral or oronasal breathing some or all of the time (see comments provided 

by ALA et. al., p. 16). 

Moreover, health and environmental groups note that effects at 100 ppb are 

backed up by studies in laboratory animals. 

Response: These comments and EPA’s responses were initially discussed in 

section II.F.4.b of the preamble to the final rule. However, EPA provides 

additional details below. 

EPA carefully considered the mouthpiece studies and continues to believe that 

these studies are not a reasonable proxy for actual exposure. In these studies, SO2

is delivered directly through the mouth, typically in conjunction with nasal 

occlusion. This allows a greater fraction of the inhaled SO2 to reach the 

tracheobronchial airways. Although we agree with commenters that some 
20

individuals do breathe oronasally both while at rest and during exercise, nasal 

ventilation still constitutes a significant percentage of total ventilation. The 

consequence is that individuals exposed to SO2 through a mouthpiece are likely to 

experience greater respiratory effects from a given SO2 exposure than they would 

in real life. Thus, as noted in the REA (REA, section 6.2) and in the proposal 

preamble (see section II.B.1.b), these mouthpiece studies only provide very 

limited evidence of decrements in lung function following exposure to 100 ppb 

SO2. Therefore, the Administrator reasonably did not place great weight on these 

mouthpiece studies when considering the appropriate level of a 1-hour SO2

standard. The commenters correctly point out that animal toxicology studies 

provide some evidence of an SO2-induced increase in airway hyperresponsiveness 

at concentrations of 100 ppb. However, these effects were observed in only one 

species, and only following repeat SO2 exposures. 

(10) Comment: MSCC generally commented that while annual and daily SO2

concentrations have fallen dramatically over the last 20 years, asthma prevalence 

has steadily increased. Thus, additional reductions in SO2 are unlikely to produce 

additional public health benefits. 

Response: EPA acknowledges that the etiology of asthma is complicated and 

likely involves multiple factors. However, EPA strongly disagrees with the 

assertion that additional reductions in SO2 will not benefit the health of 

asthmatics. Numerous controlled human exposure studies have consistently 

demonstrated that exposure to 5-minute peaks of SO2 causes decrements in lung 

function and/or respiratory symptoms in exercising asthmatics. Thus, limiting 

those 5-minute concentrations is very likely to produce public health benefits. 

In addition, as noted throughout this NAAQS review, numerous epidemiologic 

studies have reported associations between SO2 and markers of respiratory 

morbidity such as emergency department visits for asthma. Thus, it is similarly 

likely that limiting SO2 concentrations to below those in the locations these 

studies were conducted will produce public health benefits. 

(11) Comment: Rio Tinto generally commented that the ATS concept of enhancing 

the risk to a population to an unacceptable degree, without shifting the risks of 

any one individual to an unacceptable level is impractical. They note that taken to 

its logical conclusion, any concentration of SO2 would decrease reserve lung 

function and be unacceptable. 

Response: As noted in previous responses, ATS guidelines were not meant to 

provide strict rules related to the differentiation between adverse and non-adverse 

health effects of air pollution. As stated in the guidelines, “...the placement of 

dividing lines should be a societal judgment and consequently this committee does 

not propose specific boundaries for separating adverse from nonadverse effects.”

Therefore, EPA’s judgments regarding the adversity of effects following exposure 

to specific concentrations of SO2 represent our interpretation of the ATS 
21

guidelines along with CASAC recommendations and conclusions drawn by EPA 

in previous NAAQS reviews (as described in final rule sections II.B.1.a, II.E.2.b 

and II.F.4.b and responses to comments in this document). Moreover, the effects 

involved in the clinical studies are not minimal. Exposure at the 200 ppb 

benchmark can result in moderate or greater decrements in lung function, 

evidenced by a ≥15% decline in FEV1 and/or ≥ 100% increase in sRaw in an 

appreciable percentage of exercising asthmatics. As to the asserted inability to 

draw a line at any lung function decrement, the Administrator has rejected use of 

a 100 ppb benchmark and has placed minimal reliance on the (equivocal) 

evidence of lung function decrements associated with exposure at that benchmark 

level. The explanation for doing so (see section II.F.4.b of the preamble to the 

final rule) illustrates that there are reasonable means of differentiating between 

exposures which raise public health concern and those which do not.
22

B. Comments on the epidemiology 

(1) Comment: A number of commenters (e.g., API, UARG, ACSBPP, MSCC, 

Springfield, NPRA) indicated that EPA relies on only 10 epidemiologic studies as 

a basis for its proposed revisions to the SO2 NAAQS. Moreover, commenters 

indicated that the effects reported in these studies are small, sometimes null or 

negative and largely not statistically significant (ACSBPP specifically identifies 

studies by Peel et al., 2005, and Tolbert et al., 2007). 

Response: First, EPA disagrees with the commenters that the Agency relied on 

only ten epidemiologic studies as a basis for the proposed revisions to the primary 

SO2 NAAQS. Notably, the ISA’s conclusion that there was sufficient evidence 

to infer a causal relationship between respiratory morbidity and short-term (5-

minutes to 24-hours) exposure to SO2 (ISA, section 5.2) is based on the 

consistency, coherence, and plausibility of findings observed in controlled human 

exposure studies of 5 - 10 minutes, epidemiologic studies mostly using 1-hour 

daily maximum and 24-hour average SO2 concentrations, and animal 

toxicological studies using exposures of minutes to hours (id.). 

With respect to the epidemiologic evidence, EPA notes that since the last review 

of the SO2 NAAQS, there have been more than 50 peer-reviewed studies 

examining the relationship between SO2 concentrations and emergency 

department visits and hospital admissions for respiratory causes published 

worldwide. Taken together, the ISA concluded that these studies provide evidence 

to support an association between ambient SO2 concentrations and respiratory 

morbidity heath outcomes. (ISA, section 5-2, p. 5-5; see also ISA at p. 3-24) 

[“studies generally observed small, positive associations between ambient SO2

concentrations and ED visits and hospitalizations, particularly among children and 

older adults”]; ISA, p. 3-27 [“In summary, small, positive associations were 

observed between ambient SO2 concentrations and ED visits and asthma 

hospitalizations”]. The evidence from these studies is supported by additional 

panel studies of respiratory symptoms and medication use that demonstrate 

“consistent evidence of an association between ambient SO2 exposure and 

increased respiratory symptoms in children, particularly those with asthma or 

chronic respiratory symptoms” (ISA p. 5-11). The ISA concluded that these 

epidemiologic studies were consistent and coherent. This evidence was consistent 

in that associations were reported in studies conducted in numerous locations and 

with a variety of methodological approaches (ISA, section 5.2; p. 5-5). It was 

coherent in that respiratory symptom results from epidemiologic studies of shortterm (predominantly 1-hour daily maximum or 24-hour average) SO2

concentrations were generally in agreement with respiratory symptom results 

from controlled human exposure studies of 5 - 10 minutes. These results were 

also coherent in that the respiratory effects observed in controlled human 

exposure studies of 5 - 10 minutes provided a basis for a progression of 

respiratory morbidity that could lead to the emergency department visits and 

hospital admissions observed in epidemiologic studies (ISA, section 5.2; p. 5-5). 
23

In addition, the ISA found that when evaluated as a whole, SO2 effect estimates in 

multi-pollutant models generally remained positive and relatively unchanged 

when co-pollutants were included. Id. Therefore, although recognizing the 

uncertainties associated with separating the effects of SO2 from those of cooccurring pollutants (e.g., PM2.5), the ISA concluded that the limited available 

evidence indicates that the effect of SO2 on respiratory health outcomes appears to 

be generally robust and independent of the effects of gaseous co-pollutants, 

including NO2 and O3, as well as particulate co-pollutants, particularly PM2.5

(ISA, section 5.3; p. 5-9; see also id. at p. 5-5). 

Given the conclusions discussed above, the Administrator reasonably considered 

relevant air quality information available from locations where epidemiologic 

studies were conducted in determining the level of a revised standard. More 

specifically, in assessing the extent to which these studies and their associated air 

quality information could inform the level of a new 99th percentile 1-hour daily 

maximum standard for the U.S., air quality information from the U.S. and Canada 

was considered most relevant since these areas have similar monitor network 

designs and patterns of air quality. However, as described in proposal section 

II.F.4.a, SO2 concentrations reported for Canadian studies were not directly 

comparable to those reported for U.S. studies due to the use of different 

monitoring protocols in those studies. Thus, the Administrator focused on 99th

percentile air quality information available from locations where ten U.S. 

emergency department/hospitalization studies were conducted. However, we 

disagree with these commenters that the air quality information mentioned above 

was the sole basis for the range of levels proposed. The Administrator also 

considered the extent to which the level of a new 1-hour standard limited 5-

minute concentrations/exposures of concern identified from controlled human 

exposure studies. Thus, in considering a reasonable range of levels to propose 

and in adopting the ultimate 75 ppm 1-hour standard, the Administrator 

considered the epidemiologic and controlled human exposure evidence, as well as 

the air quality, exposure and risk information. 

We also generally disagree that epidemiologic effect estimates have to be large 

and/or statistically significant in order to be considered. When examining a large 

body of epidemiologic evidence, such as is available for the association of 

ambient SO2 concentrations and respiratory morbidity, it is appropriate to 

evaluate the trend of the direction and magnitude of effect estimates and evaluate 

the consistency of the directions and magnitudes of these effect estimates when 

determining evidence for causality, rather than focus on the statistical significance 

of each individual effect estimate. As noted above, the ISA concluded that these 

epidemiologic studies were consistent and coherent (including the results from 

Peel et al., 2005 and Tolbert et al., 2007). Additionally, it is not necessary for the 

effect estimates to be large in magnitude to be considered to have a relevant 

public health impact. For example, a 1% increase in daily ED visits or hospital 

admissions in a single urban area due to increased ambient SO2 concentrations 

translates into millions of annual excess ED visits and hospital admissions when 
24

applied to the entire US population. Such increases are clearly relevant to public 

health. See Coalition of Battery Recyclers Association v. EPA, No. 09-1011 

(D.C. Cir., May 14, 2010), slip opinion at 8 (“This assertion confuses the ‘critical 

distinction between population and individual risk’, wherein a small change ... at 

the level of an individual is a substantial change at the level of a population”). 

 

(2) Comment: A number of commenters (e.g. API, UARG, ACC, ACSBPP, ABR, 

GCLC, CIBO, EEI) generally indicated that the results of epidemiologic studies 

are confounded by co-pollutants, and thus EPA cannot confidently identify SO2 as 

the causative agent of the resulting health outcome (e.g., ED visits). 

Response: EPA notes the response to this comment in section II.E.2.a of the 

preamble to the final rule. However, EPA provides additional information below. 

EPA consistently recognizes that other pollutants are also associated with health 

outcomes, as is reflected in the fact that EPA has established regulations to limit 

emissions of the particulate criteria pollutants as well as other gaseous criteria 

pollutants. In its assessment of the health evidence regarding SO2, EPA has 

carefully evaluated the potential for confounding, effect measure modification or 

other interactions between SO2 and other criteria pollutants, and concluded that 

the results attributable to SO2 are robust. See, e.g. ISA at pages 5-5 and 5-9. 

To evaluate the potential for confounding in the epidemiologic evidence, EPA 

focused especially on studies which used two-pollutant models (SO2 plus one 

other pollutant) as the inclusion of each additional pollutant in the model can 

decrease model stability and affect the precision of the effect estimate. This 

decrease in model stability is often reflected in wider confidence intervals, 

making it less likely for a statistically significant result to be observed. Thus, 

when a statistically significant effect estimate observed in a single pollutant 

model is no longer statistically significant in a copollutant model (even though the 

magnitude and direction of the effect estimate has not changed substantially), this 

may be an artifact of model instability. 

Although the presence of other pollutants in the ambient air mixture complicates 

efforts to quantify specific SO2-related health effects, a number of epidemiologic 

studies have evaluated associations with SO2 in models that also include cooccurring pollutants such as PM, O3, CO, and/or NO2. The evidence summarized 

in the ISA indicates that SO2 associations generally remain robust in these multipollutant models and supports a direct effect of short-term SO2 exposure on 

respiratory morbidity (see ISA Figure 3-8). This evidence supports the effects 

observed in experimental (i.e., controlled human exposure) studies that have 

evaluated respiratory symptoms and lung function (see ISA section 3.1.6), which 

provide plausibility and coherence of these effects. As noted, the ISA (section 

5.2) concluded that the robustness of epidemiologic findings to adjustment for copollutants, coupled with data from human experimental and animal studies, 
25

support a determination that the relationship between SO2 and respiratory 

morbidity is causal. 

(3) Comment: Industry commenters indicate that many epidemiologic studies relied 

upon by EPA did not employ multi-pollutant models to address potential 

confounding. Several groups indicated that lack of potential confounding by PM 

was especially problematic. Some commenters (e.g., ASARCO) also indicated 

out of the over 50 peer reviewed epidemiologic studies evaluated by EPA, only 

two found a statistically significant association between SO2 and emergency 

department visits when other pollutants were considered. 

Response: Results in ISA Figure 3-8 indicate that the association of SO2 with 

respiratory morbidity is robust to the addition of copollutants, including PM. 

While some individual studies may report specific findings that are more 

influenced by copollutants, Figure 3-8 clearly demonstrates the reasonableness of 

EPA’s conclusion of robustness. See Schwartz (1995), Burnett et al., (1997), NY 

DOH (2006), Lin et al., ((2003), Sunyer et al., (1997), Anderson et al., (1998), Ito 

et al., (2007), Hajat et al., (1999), Tsai et al., (2006), all of which are studies 

showed results which remained statistically significant for SO2 in two-pollutant 

models, many of which included PM.

(4) Comment: A number of industry commenters (e.g., API, UARG) indicate that 

EPA failed to discuss the only epidemiologic study looking at associations 

between hospital admissions for asthma, wheeze or shortness of breath and 5-

minute SO2 peaks (Donoghue and Thomas; 1999). Moreover, UARG indicates 

that EPA failed to account for a study by Erbas and Hyndman finding that “the 

effects of...sulfur dioxide were highly sensitive to model specification for both 

COPD and asthma [HAs].” UARG further states: 

The fact that EPA has failed to examine, or even acknowledge the 

existence of these studies after they were pointed out in public comments, 

is a striking indication that EPA has not fulfilled its duty under the CAA 

(see comments provided by UARG; p. 20). 

Response: EPA has reviewed both of the studies identified by commenters. The 

first, by Donoghue and Thomas (1999) is riddled with a number of limitations that 

in EPA’s view made it too unreliable to include in the ISA and REA. The 

limitations of this study include: 1) the statistical analysis included Poisson 

regression, and used GAM models and was conducted with S-Plus software. The 

study was published before Dominici et al. (2002) identified default GAM 

convergence issue and never reanalyzed, making it likely to be GAM impacted 

and hence unreliable (see EPA (2004), Air Quality Criteria for Particulate Matter, 

section 8.1.3.1 and 8.4.2 (EPA/600/P-99/002bF)); 2) The failure to present 

average, median, minimum or maximum SO2 concentrations. Instead only the 

percent of days when the maximum 5-minute SO2 concentration at any of the 10 

SO2 monitors exceeded 3 benchmark levels (800, 2145, and 5434 μg/m3

) is 
26

presented. 3) The failure to present the number of hospital visits or hospital 

admissions included in analyses. 4) The temporal scale of the exposure data is 5-

minute concentration, but temporal scale of outcome data is a 24 hour period. 

Thus, the precision of the 5-minute SO2 concentrations is lost on the less precise 

24-hour outcome data. For example, it isn’t clear if the hospital visit or hospital 

admission occurred before or after the 5-minute SO2 concentration was measured. 

5) A seasonal effect (by month) was observed, but the study authors did not adjust 

for influenza hospital visits to account for this potential confounder/effect 

modifier. 

 

Similarly, the study by Erbas and Hyndman (2005) was viewed by EPA to be too 

unreliable to include in the ISA and REA. The authors provide no information on 

the number of hospital admissions observed for COPD or asthma during the 3.5 

year study period. Additionally, there is no information that characterizes the 

concentration of SO2 during the study period.

(5) Comment: A number of commenters (e.g., API) indicate that EPA did not 

account for publication bias in the review of the SO2 NAAQS 

Response: EPA recognizes the possibility of publication bias, which can 

potentially occur in any field of study. In the discussion of the evaluation of 

epidemiologic studies, EPA observed that one of the advantages of multi-city 

studies is that “by their very nature can reduce uncertainty related to publication 

bias” (ISA, p. 3-1). 

EPA does not agree that reported associations between SO2 and health effects are 

an artifact of publication bias. EPA acknowledges that publication bias can result 

in potential overestimation of the estimated risk in a body of literature. However, 

for an individual study, factors such as exposure error or selection of results from 

an individual lag period from among several positive associations can result in 

underestimation of an effect estimate (i.e. a more pronounced bias in the other 

direction).

(6) Comment: A number of industry commenters indicated that EPA selectively 

presented results of epidemiologic studies. For example, API indicated that in the 

NYDOH (2006) analysis EPA selectively considered the positive associations in 

the Bronx NY, while not considering the negative associations in Manhattan (see 

comments by API; p. 17). 

Response: EPA disagrees that the results of the NYDOH (2006) from the Bronx 

and Manhattan were presented selectively. Effect estimates from both locations 

are presented in Figures 3-7 (p. 3-26) and 3-8 (p. 3-29) of the ISA. Additionally, 

on page 3-25, the ISA states, “Another study conducted in New York City 

(NYDOH, 2006) found a 10% (95% CI 5, 15) excess risk in asthma hospital 

admissions per 10 ppb increase in 24-hour average SO2 for Bronx residents, but a 

null association for the residents of Manhattan (-1% [95% CI: -11, 11]).” EPA 
27

fails to see how presenting effect estimates from both locations in the text and 

figures of the ISA constitutes a selective portrayal of the results. In addition, EPA 

notes that the REA and proposal included figures (see REA figures 5-2 and 5-3 

and proposal figures 2 and 3) with results from both the Bronx and Manhattan 

study areas. 

(7) Comment: A number of commenters (e.g., ACSBPP) argue that it is 

inappropriate for EPA to use the same epidemiologic analyses to suggest causal 

associations in reviews for multiple criteria pollutants. In general, these groups 

argue that the same epidemiologic analyses have been used to attribute health 

effects to O3, PM, NO2 and now, SO2. This is likely leading to double, triple, or 

quadruple counting of health effects. 

Response: EPA strongly disagrees that the agency is “double or triple counting” 

by attributing effects to SO2 in the current review that are attributed to different 

pollutants in other NAAQS reviews. EPA consistently recognizes that other 

pollutants are also associated with health outcomes, as is reflected in the fact that 

EPA has established regulations to limit emissions of the particulate criteria 

pollutants as well as other gaseous criteria pollutants. In its assessment of the 

health evidence regarding SO2, EPA has carefully evaluated the potential for 

confounding, effect measure modification or other interactions between SO2 and 

other criteria pollutants, and concluded that the results attributable to SO2 are 

robust (See Figure 3-8 on page 3-29 of ISA). The combination of this 

epidemiologic evidence with evidence from controlled human exposure and 

animal toxicological studies provides clear and convincing evidence of the 

specificity of these adverse respiratory effects to SO2 exposure. Together, these 

lines of evidence are consistent and coherent, explain the temporal and biologic 

gradients observed, and provide biological plausibility for the observed effects. 

See also Air Quality Criteria for Particulate Matter (EPA, 2004) at 8-254, 

indicating how EPA distinguishes effects attributable to PM from those 

attributable to SO2: “In many of these studies, PM with and without added 

components of gases thusly appears to be a key putative agent. However, care 

must be exercised in interpreting such results, taking into account what is known 

about the toxicology and clinical studies of the gases. It is often clear that these 

gases, at concentrations present of given the nature of the effects, do not carry 

sufficient biologic plausibility to substantially affect the results seen. For 

example, SO2 is mostly absorbed in upper airways under normal breathing 

conditions and, although it might affect airway neural reflexes to contribute to 

asthma exacerbation at typical U.S. ambient levels, it is not likely to exert 

sufficient effects on COPDS or CVD to contribute to excess morbidity and 

mortality.” 

(8) Comment: Some industry groups (e.g., API, UARG, ExxonMobil) commented 

that reliance on central monitors in epidemiologic studies leads to a high degree 

of exposure misclassification. For example, UARG states: “when all of the copollutants are measured with error, robustness of the association of one pollutant 
28

in multi-pollutant models is not sufficient to infer an absence of confounding (see 

comments provided by UARG; p. 18) 

Response: As noted in Section 2.6.4 of the ISA, although EPA agrees that SO2

measurements from central monitors is subject to potentially large measurement 

error when used to reflect population exposures in epidemiologic studies, the 

Agency disagrees with the suggestion that statistically significant associations 

between centrally monitored air quality concentrations and adverse health effects 

measured in these studies are invalid as a result. This section concluded that such 

measurement errors in community time-series and panel epidemiologic studies are 

more likely to underestimate the strength and the significance of any association 

between SO2 and any adverse health effects observed in the study, thereby 

decreasing the likelihood of an association reaching statistical significance and 

likewise decreasing the possibility of a false identification of an association 

(Section 2.6.4.4, p. 2-63). Thus, the measurement error makes it more difficult to 

detect a positive and statistically significant effect estimate, even when such an 

association exists. Moreover, the signal that drives statistical associations 

between ambient concentrations and health effects in time-series studies is the 

day-to-day changes in concentration, not the absolute daily values. Appropriately 

located central SO2 monitors can adequately characterize such day-to-day 

changes. Exposure studies have found statistically significant regression slopes 

between personal exposure to SO2 and ambient SO2 concentrations at fixed-site 

monitors (see ISA section 2.6.3.2, p. 2-56), indicating that fluctuations in ambient 

concentration are an important driver of fluctuations in SO2 exposure. 

Time-series epidemiologic studies evaluate associations between day-to-day 

changes in air pollution and health outcomes. Therefore, the measurement error 

argument cannot be used to nullify an effect that has been observed. If anything, 

as noted, it is likely that the real effects are likely to be larger than those that were 

estimated. Therefore, for the purposes of determining whether public health 

protection is warranted in light of the available evidence, EPA believes that it has 

interpreted the evidence from these epidemiologic studies reasonably. 

(9) Comment: Some industry groups (e.g., UARG) indicate that there is no evidence 

from controlled human exposure studies to suggest that brief exposures to low 

concentrations of SO2 could lead to hospital visits. For example, UARG states: 

Although no one would suggest that clinical studies could or should be 

conducted with the intent of sending people to the hospital, there is also no 

indication from the clinical studies of any population that would be 

susceptible to SO2 such that a brief exposure to a low concentration could 

lead to hospital visits (see comments provided by UARG; p. 16) 

Response: As described in Section 5.2 of the ISA (p. 5-2), the evidence from 

controlled human exposure, toxicological and epidemiologic studies for the 

respiratory health effects of SO2 are consistent with the mode of action of SO2 as 
29

it is currently understood. The immediate effect of SO2 on the respiratory system 

is bronchoconstriction. This response is mediated by chemosensitive receptors in 

the tracheobronchial tree. These receptors trigger reflexes at the central nervous 

system level resulting in bronchoconstriction, mucus secretion, mucosal 

vasodilation, cough and apnea followed by rapid shallow breathing. Asthmatics 

are more sensitive to the effects of SO2 likely resulting from preexisting 

inflammation associated with this disease. This inflammation may lead to 

enhanced release of mediators, alterations in the autonomic nervous system and/or 

sensitization of the chemosensitive receptors. These biological processes are 

likely to underlie decreased lung function and increased hyperresponsiveness 

observed in response to SO2 exposure. 

The asthmatics participating in studies involving controlled exposures to SO2

must be relatively healthy, and do not likely represent the most sensitive 

individuals in the population. Controlled human exposure studies include very 

small numbers of subjects, particularly when compared to the large populations 

considered in epidemiologic investigations. These populations do include the 

most sensitive individuals who are not eligible or willing to participate in 

controlled human exposure studies either due to study-specific inclusion or 

exclusion criteria, or self-selection. In addition, as presented in the ISA (p. 5-5) it 

is quite possible that effects observed in epidemiologic studies could be driven in 

large part by higher peak exposures within a 24-hour period. Thus, as noted in the 

ISA (id.), the effects of SO2 on respiratory symptoms, lung function, and airway 

inflammation observed in the controlled human exposure studies provide a basis 

for the progression of respiratory morbidity resulting in increased ED visits and 

hospital admissions, especially among susceptible populations (i.e., asthmatics). 

(10) Comment: Several industry commenters (e.g., UARG) indicated that there is a 

disconnect between effects seen at higher SO2 concentrations in controlled human 

exposure studies and those observed at lower concentrations in epidemiologic 

analyses. For example, UARG states: 

EPA recognizes that human clinical studies ‘provide directly applicable 

information for determining causality’” while for epidemiological studies 

“the degree of uncertainty introduced by confounding variables (e.g., other 

pollutants) affects the level of confidence that the health effects being 

investigated are attributable to SO2 exposures.” 74 Fed. Reg. at 64815/3 - 

64816/1. Thus, the discrepancy between the nature of effects attributed to 

SO2 in epidemiologic studies at ambient levels and those in clinical studies 

at much higher SO2 concentrations must call into question the reliability of 

the epidemiological studies as evidence of effects from SO2 exposure (see 

comments provided by UARG; p.17) 

Response: Please see the response to the previous comment that characterizes the 

progression from the respiratory effects observed in controlled human exposure 

studies to those observed in epidemiologic studies. In addition, although the 
30

concentrations investigated in controlled human exposure studies are higher than 

the mean concentrations reported in epidemiologic studies, it is important to note 

that epidemiologic studies conducted in the United States have reported 

associations between ambient SO2 concentrations measured at area-wide monitors 

in the current network and increased respiratory symptoms, emergency 

department visits, and hospital admissions. 

As noted in the ISA (Table 2-8, p. 2-33), 1-hour SO2 concentrations measured at 

area-wide monitors in the United States have been observed in the range of 50 to 

100 ppb, with maximum concentrations between 600 and 700 ppb. In addition, a 

very limited number of U.S. monitors report 5-minute SO2 concentrations, and 

upper-percentile 5 minute concentrations at these monitors can be 200-300 ppb or 

higher (ISA, Figure 2-31, p. 2-44). Based on these considerations, EPA believes 

that the SO2 concentrations that occurred in the locations of epidemiologic studies 

may have included concentrations that overlapped with those reported to cause 

respiratory effects in controlled human exposure studies. 

(11) Comment: Several industry commenters (e.g. ACSBPP) indicated that EPA 

downplayed major findings regarding uncertainty due to model selection. For 

example, ACSBPP states: 

Model selection uncertainty relates to confounding of air pollutant 

associations by temporal trends, weather and co-pollutants. During the last 

ozone review, EPA acknowledged that the uncertainties in the estimates of 

pollutant effects are understated by consideration of the statistical 

uncertainty of the fitted model alone. Much more uncertainty arises from 

the lack of information regarding the choice of appropriate models for 

adjusting confounding by other covariates, and the choice of appropriate 

lag structures (see comments provided by ACSBPP; p.8). 

Response: EPA has not ignored issues related to model specification for 

epidemiologic studies, such as selection of models and approaches to adjust for 

meteorological and temporal variables. As observed in the ISA (p. 3-1), extensive 

discussions of the issues surrounding model selection and model specification 

have been presented in the PM AQCD (EPA, 2004) and the O3 AQCD (EPA, 

2006) and are thus not reiterated at length in the ISA. The SOX ISA makes clear, 

however, that these issues were carefully considered in selecting studies for 

inclusion in the ISA and interpreting the results of the body of epidemiologic 

evidence. Indeed, EPA went to great lengths to do so, placing special emphasis 

on the epidemiologic studies which remained statistically significant in twopollutant models which included PM. See section II.F.4.e in the preamble to the 

proposed rule and section II.F.4.c in the preamble to the final rule. 

(12) Comment: Several commenters (e.g. ACSBPP) indicate that EPA chose the best 

lag times from studies with multiple lag times to support it’s assertion of a causal 

relationship between short-term SO2 exposure and adverse respiratory effects. 
31

Response: EPA reiterates that it believes in the importance of a comprehensive 

evaluation that considers and weighs a variety of evidence, including biological 

plausibility of associations between the various pollutants and health outcomes, 

and focuses on the stability of the size of the effect estimates in time-series studies 

considering both single- and multi-pollutant models, rather than just looking at 

statistical significance in a large number of alternative models as a basis to 

delineate between real and suspect associations. EPA finds that the approach of 

simply counting the number of statistically significant results across all models 

does not give adequate weight to important statistically significant results as a 

consequence. This has the effect of weighting all models equally, regardless of 

plausibility or statistical power, and it allows a lack of statistically significant 

results for one lag structure to essentially cancel out statistically significant results 

based on another lag structure. That is, EPA does not agree, for example, that a 

statistically significant association between emergency department visits for 

asthma and same day exposure to SO2 should be completely discounted by a 

finding in the same study that an association between emergency department 

visits for asthma and SO2 exposure several days prior to the ED visit is not 

statistically significant. Health effects associated with relatively more immediate 

exposures could well be the consequence of a biological mechanism that would 

not reasonably be expected to result in the same health effect several days after 

exposure. Thus, EPA does not believe that it is appropriate to simply average out 

statistically significant and nonsignificant results derived from models with 

different lags. 

EPA has an established hierarchy for choosing which effect estimates will be 

presented in the text and figures of the ISA when effect estimates from multiple 

lag days are presented: (1) effect estimates from distributed lags; (2) effect 

estimates for the average of multiple days (e.g., 0-2); (3) a priori lag days selected 

by the study authors; (4) if only individual lag days were presented, the estimate 

with the highest magnitude and level of statistical significance is selected from 

among the biologically plausible lag days available. 

(13) Comment: Commenters (e.g. ACSBPP) indicated that EPA did not discuss all of 

the results and/or all of the authors conclusions presented in the original 

publication (with regard to Sheppard 2003, Jaffe et al., 2003, Schwartz et al., 

1996, Wilson et al. 2005, Anderson et al. 1998)

Response: EPA may not discuss the author’s conclusions presented in the 

original publications because the interpretation of the results of a single study may 

differ greatly from the interpretation of a large body of evidence, which is what is 

evaluated and serves as the basis for causal determinations in the ISA. As 

mentioned in the response to epidemiology Comment 1, EPA evaluates the trend 

of the direction and magnitude of effect estimates and evaluates the consistency of 

the directions and magnitudes of these effect estimates when determining 
32

evidence for causality, rather than focusing on the statistical significance of each 

individual effect estimate. 

(14) Comment: ACSBPP indicated that there is a biologically implausible wide range 

of effect estimates in epidemiologic studies 

Response: In responding to similar comments made with regard to the 2004 PM 

AQCD, EPA has recognized that in the expansive body of literature, especially in 

the multi-city studies, there appears to be spatial heterogeneity in city-specific 

excess risk estimates for relationships between short-term ambient SO2

concentrations and acute health effects. The reasons for variation in effect 

estimates are not well understood. Factors likely contributing to the apparent 

heterogeneity include geographic differences in air pollution mixtures, and 

personal and sociodemographic factors potentially affecting SO2 exposure. 

The commenters focused on multi-city results (e.g., Sunyer et al., 2003; Barnett et 

al., 2005; Ballester et al., 2006), and raised a series of technical issues with the 

analytical approaches used, including the use of a standard analytical approach 

that did not allow for city-specific modeling of factors such as meteorology that 

likely vary from city to city, and the fact that many multi-city studies have 

combined results from communities with fewer available data that would likely 

not have been considered adequate for use in single-city analyses. Beyond the 

fact that these studies were conducted in Europe (Sunyer et al. 2003), Australia 

(Barnett et al. 2005), or focused on cardiac hospital admissions (Ballester et al. 

2006), and thus did not play a large role in the current regulatory process, the 

EPA does not agree that the issues raised diminish the value of these analyses. 

Another key factor that contributes to heterogeneity or variation between areas is 

a study’s statistical power. EPA observes the importance of statistical power for 

interpreting these results, and EPA agrees that it is reasonable to focus on results 

from studies with greater power. Despite there being some evidence for greater 

variation in magnitude and precision of SO2-health associations between 

geographic areas, EPA concluded in the ISA that the extensive body of 

epidemiology evidence demonstrated “an association between ambient SO2

concentrations and ED visits and hospitalizations for all respiratory causes, in 

particular among children and older adults (65+ years), and for asthma.” (EPA, 

2008, p. 3-28) 

(15) Comment: Some groups generally remarked that the term “robust” in the ISA is 

not used consistently or systematically across NAAQS reviews. For example, 

ACSBPP stated: 

In one instance, the ISA indicates that the term robust is used to indicate 

that there was little change in the magnitude of the central estimate, 

though statistical significance may have been lost. Since EPA is using 

similar methods to evaluate the epidemiology for other criteria pollutants, 
33

it is appropriate to consider the criterion for robustness used in other ISAs. 

In the draft CO ISA, EPA uses the fact that the association remains 

positive in copollutant models to define robustness. (see comments 

provided by ACSBPP; p. 19) 

Response: The term “robust” is used consistently throughout NAAQS reviews to 

indicate that results were essentially the same regardless of the manner in which 

the statistical models were specified. Statistical significance of the results is not 

considered in the application of the term “robust”. 

(16) Comment: EPRI generally commented that EPA should consider that all of the 

epidemiologic studies employed linear models that do not examine the possibility 

of a threshold at which health responses may occur. 

 

Response: EPA evaluated the concentration-response function and potential 

thresholds in Section 4.1 (p. 4-1 to 4-7) of the ISA. The ISA concluded that a 

clear increase in the magnitude of respiratory effects was observed with 

increasing exposure concentrations between 0.2 and 1.0 ppm during 5-10 minute 

SO2 exposures in controlled human exposure studies. The ISA also noted that the 

concentration-response function between short-term exposure to SO2 and 

respiratory morbidity could not be distinguished from linear across the entire 

concentration range. Population-level studies did not indicate a possible 

threshold, though due to limitations associated with observing a possible 

threshold in these studies the limited evidence was determined to be inconclusive 

regarding the presence of an effect threshold at current ambient levels. 

(17) Comment: With respect to the epidemiologic evidence, MSCC commented: 

There are other factors that may positively associate with increased levels 

of admissions and with increased levels of SO2 (if SO2 is present) – gusty, 

windy days, particulate loads, pollen, ambient temperature effects, patient 

activities, even days of the week, to name a few. Further, for such studies, 

the monitored levels, do not likely indicate the concentrations of actual 

SO2 exposure for the population, but at best a subset of any constrained 

very near the monitor(s). An association that is sometimes negative, 

sometimes positive, and rarely significant, is not compelling to use to 

conclude a direct cause/effect relationship (see comments provided by 

MSCC; p.5) 

Response: EPA evaluates each of the epidemiologic studies to ensure that 

potential confounding variables, including meteorological variables and day of 

week, are included in time-series studies of ED visits and hospital admissions to 

determine whether observed health effects are associated with air pollutant 

concentrations and not with these potential confounders. A time-series study that 

does not account for these potential confounders would have a difficult time 
34

getting accepted for publication in the peer-reviewed literature, and if it did, 

would not be included as evidence in the ISA. 

EPA disagrees that exposure measurement error is cause to ignore the 

observational evidence from epidemiologic evidence, and again reiterates (see 

response to epidemiology Comment 1) that the ISA’s conclusion that there was 

sufficient evidence to infer a causal relationship between respiratory morbidity 

and short-term (5-minutes to 24-hours) exposure to SO2 (ISA, section 5.2) is 

based on the consistency, coherence, and plausibility of findings observed in 

controlled human exposure studies of 5 - 10 minutes, epidemiologic studies 

mostly using 1-hour daily maximum and 24-hour average SO2 concentrations, and 

animal toxicological studies using exposures of minutes to hours (id.). 

(18) Comment: Some groups indicate that there is no evidence to suggest that 

children or older adults may be at increased risk of experiencing adverse 

respiratory effects following SO2 exposure. 

Response: The ISA concludes that there is “limited epidemiologic evidence to 

suggest that children and older adults (65+ years) are more susceptible to the 

adverse respiratory effects associated with ambient SO2 concentrations when 

compared to the general population” (ISA, pg. 4-14). Specifically, on page 4-12, 

the ISA states: 

 “A number of studies, investigating the association between ambient SO2

levels and ED visits or hospital admissions for all respiratory causes or 

asthma, stratified their analyses by age group. Figure 4-6 summarizes the 

evidence of age-specific associations between SO2 and acute respiratory 

ED visits and hospitalizations. Several studies demonstrated that excess 

risk of ED visits or hospitalizations for all respiratory causes was higher 

for children (e.g., Atkinson et al., 1999a; 1999b; Petroeschevsky et al. 

2001) and older adults (e.g., Petroeschevsky et al., 2001; Wilson et al., 

2005; Wong et al., 1999) when compared to the risk for adults or all ages 

together. This is more clearly depicted in the summary density curves in 

Figure 4-7 and Figure 4-8, created using the effect estimates presented in 

Figure 4-6. As shown in these two figures, the effect estimates for 

children and older adults are slightly larger than that for adults or all ages 

for both all respiratory diseases and asthma ED visits and 

hospitalizations.” 

C. Comments on the air quality, exposure, and risk analyses 

(1) Comment: Several commenters (e.g., API, UARG, ACSBPP, ACC, JSRC, ABR, 

ASARCO, GCLC) indicated that it was inappropriate for EPA to ‘degrade” 

current air quality to just meet the current 24-hour and/or annual standards. 

Industry groups argue that there is no plausible scenario that could lead to current 
35

air quality, which is well below the current standards, regressing back to “just 

meeting” the current standards. 

Response: EPA disagrees with the commenters. In determining whether the 

current primary standards are requisite to protect public health with an adequate 

margin of safety, EPA finds that it is appropriate to consider exposures and risk 

associated with air quality that has been adjusted to just meet the current 

standards. Similar comments and EPA’s responses are discussed in detail in 

section II.E.2.c of the final rule preamble. 

(2) Comment: Several commenters (e.g., API, UARG, ACSBPP, ACC, CE, TXOG, 

JRSC, ABR, ASARCO, NRECA) indicated that it was inappropriate for EPA to 

compare exposure and risks associated with just meeting alternative 1-hour 

standards to those just meeting the current SO2 standards. These groups argue the 

more relevant comparison to just meeting alternative 1-hour standards (or the 

current standards) is to the exposure and risk associated with “as is” air quality, 

which is well below the current SO2 standards. Moreover, these groups argue that 

if this comparison is done, there is little to no benefit to public health by adopting 

a 1-hour standard in the proposed range. 

Response: The response to this comment follows the same logic as that outlined 

in the preceding comment. Thus, EPA disagrees with the commenters that it is 

inappropriate to consider exposures and risk associated with air quality that has 

been adjusted to just meet the potential alternative standards in assessing what 

potential NAAQS could protect public health with an adequate margin of safety. 

Similar comments and EPA’s responses are discussed in detail in section II.E.2.c 

of the final rule preamble. 

(3) Comment: Several commenters (e.g., API, UARG, FE, MSCC, AECT, NAM) 

indicated that the proposed alternative standards are not necessary because SO2

emissions and/or ambient levels have fallen dramatically over the years and will 

continue to do so. Many of these groups also point out this will result in 

diminished risk estimates associated with ambient SO2 concentrations as well 

(which industry groups point out are already small). 

Response: As described in detail in final rule section II.E.2.c, EPA is required to 

review whether the present standards – not present air quality – are requisite to 

protect public health with an adequate margin of safety. Section 109 (b) (1). 

Similarly, EPA may not consider projected future air quality and its associated 

risks when reviewing the adequacy of the current standards to protect public 

health with an adequate margin of safety. 

(4) Comment: Several commenters (e.g., ACSBPP) generally indicated that the only 

relevant approach to assessing risks associated with SO2 exposure are those in the 

risk assessment chapter of the REA (Chapter 9). These groups argue that the air 

quality and exposure analyses presented in Chapters 8 and 9 of the REA 
36

substantially overestimate risks associated with SO2 exposure because they do not 

account for the fact that not all exposed asthmatics at elevated ventilation rates 

will experience a lung function response. Moreover, if only the results of the risk 

assessment are taken into account, one would see that there is very little risk of a 

lung function response in exposed asthmatic children at 150 ppb and above. 

Response: EPA disagrees with the commenters that primary emphasis should be 

placed on the results of the quantitative risk assessment when assessing the extent 

to which the current and potential alternative standards limit 5-minute benchmark 

exposures of potential concern and their associated health effects. As described in 

the REA (Chapters 7-9) and the proposal (section II.C), EPA used three different 

approaches (i.e., air quality analysis, exposure analysis, and quantitative risk 

assessment) for assessing exposures and risks associated with these 5-minute 

benchmarks identified from controlled human exposure studies. Rather than 

relying on just one of these approaches, EPA recognized that each of these 

analyses has strengths and limitations (see proposal preamble section II.C) and 

thus, reasonably considered the results of all of these analyses (albeit to different 

extents, see below) to evaluate the extent to which the current, and potential 

alternative 1-hour standards limit exposure and risks associated with these 5-

minute benchmark concentrations (see proposal preamble sections II.E.1.b and 

Tables 2 – 4). 

As noted in the final rule’s preamble (section II.E.2.c) however, the Administrator 

did place relatively more weight on the results of the St. Louis exposure analysis 

than she did on the results of the St. Louis quantitative risk assessment. EPA 

notes the results of the quantitative risk assessment likely have greater uncertainty 

than the results of the exposure analysis considered alone. That is, because the 

results of the exposure analysis are used as inputs into the quantitative risk 

assessment, the risk assessment will have all the uncertainties of the exposure 

analysis (see proposal preamble section II.C; 74 FR at 64823 and REA, section 

8.11), as well as the additional uncertainties inherent in the quantitative risk 

assessment (see proposal preamble section II.C; 74 FR at 64823 and REA, section 

9.4). EPA finally notes that the Administrator’s proposed and final decisions with 

regard to the adequacy of the current standards and the elements of a new shortterm standard appropriately placed substantial weight on the epidemiologic and 

controlled human exposure evidence presented in the ISA. 

As discussed above, EPA believes that it would be inappropriate to consider only 

the results of the quantitative risk assessment. Thus, we believe the commenter’s 

contention of relatively little risk of an adverse lung function response given a 1-

hour standard level of 150 ppb is not compelling in isolation. We believe that this 

is especially true given that the St. Louis exposure analysis estimates that a 1-hour 

standard at 150 ppb would only protect about 88 percent of asthmatic children at 

moderate or greater exertion in that city from experiencing at least one exposure 

per year greater than or equal to the 200 ppb 5-minute benchmark exposure of 

concern. 
37

 (5) Comment: Several commenters (e.g. ACSBPP) generally indicated that the risk 

estimates presented in Chapter 9 of the REA and in the proposal are substantially 

overestimated because they only consider decrements in lung function, 

independent of whether there were accompanying respiratory symptoms. 

Response: EPA disagrees that the quantitative risk estimates presented in 

Chapter 9 are substantially overestimated because they only consider decrements 

in lung function, independent of whether they were accompanied by respiratory 

symptoms. As noted above (see section II.A), and in detail in final rule preamble 

section II.E.2.b, EPA considered moderate or greater decrements in lung function, 

even without accompanying respiratory symptoms to be adverse effects of SO2

exposure. Thus, EPA believes that it was reasonable to consider the decrements 

in lung function health endpoint in its quantitative risk assessment. 

(6) Comment: Some commenters generally indicated (e.g., ACSBPP, TAB) that the 

scenarios under which an asthmatic at elevated ventilation (e.g., while exercising) 

is likely to come into contact with an SO2 benchmark concentration of concern is 

exceedingly rare and therefore not a public health issue appropriate for a NAAQS. 

In addition, some of these groups indicated that there were few monitored 5-

minute SO2 levels above 5-minute benchmark concentrations of concern. For 

example, ACSBPP indicates that Figure 7-19 of the REA shows that only a 

portion of the available monitors with 5-minute concentrations have exceedences 

of the various benchmarks, and at monitors with these exceedences, values above 

400 ppb are very rare (comments by ACSBPP; p. 27). 

Response: EPA disagrees with these comments. EPA first notes the results of 

the St. Louis exposure analysis indicating that numerous asthmatic children at 

moderate or greater exertion (e.g., while exercising) in St. Louis are estimated to 

experience at least one exposure per year above the 5-minute benchmark 

concentrations of concern identified from controlled human exposure studies (see 

REA Figure 8-19 and proposal Table 3 at 74 FR 64841). For example, if air 

quality is simulated to just meet the current standards in St. Louis, approximately 

24% and 73% of asthmatic children at moderate or greater exertion are estimated 

to experience at least one exposure per year above the 400 and 200 ppb 

benchmark exposures of concern, respectively. Id. 

In addition, the figure referenced by the commenter describes the number of 

days/year measured 5-minute SO2 concentrations from 98 monitors in 13 states 

from 1997 -2007 exceeded 5-minute benchmark concentrations identified from 

controlled human exposure studies. We first note that this subset of monitors that 

reported measured 5-minute SO2 concentrations was relatively small. That is, 

over the same time frame, there were 933 monitors in 49 states, the District of 

Columbia, Puerto Rico and the Virgin Islands measuring 1-hour SO2

concentrations. In addition, most of the monitors that did measure 5-minute 

concentrations only did so for a limited span of time, ranging from only several 
38

hundred, to at most, a few thousand hours in a year. REA Appendix A, Table 

A.5-1. Furthermore, it is important to consider that those monitors reporting 5-

minute concentrations do not represent data from a dedicated 5- minute 

monitoring network, but rather a voluntary submission of 5-minute values from 

existing monitors placed for the purpose of evaluating attainment of the 24-hour 

and annual average SO2 NAAQS, which does not require monitoring at locations 

of expected maximum concentrations (much less maximum 5-minute 

concentrations). Thus, there is considerable uncertainty that the relatively few 

monitors reporting 5-minute SO2 concentrations were sited in the proper locations 

to record 5-minute peaks of SO2 above benchmark values. 

Notwithstanding these decidedly non-optimized monitoring conditions, 

approximately 2% of the monitored days (approximately one week in a year) 

exceeded the 200 ppb benchmark. REA p. 120. EPA regards this amount of 

potential exposure to be significant. See also American Lung Ass’n v. EPA, 134 

F. 3d 388, 392 (D.C. Cir. 1998) (noting facts in previous review that “’repeated’ 

exposure is ‘significant’ and that thousands of asthmatics are exposed more than 

once a year”). Indeed, this comment appears to raise the same issue as that 

presented in the previous review: are repeated SO2 exposures resulting in adverse 

effects to a susceptible population a public health problem, even if the exposures 

are confined to relatively localized areas. American Lung Ass’n v. EPA, 134 F. 

3d at 392. The D.C. Circuit rejected the argument that such exposures were not a 

public health problem by virtue of being localized and infrequent, noting that 

hundreds of thousands of asthmatics were exposed to these events annually, and 

remanded the standard to EPA for want of a satisfactory explanation. The facts 

here, as set out in the REA, indicate that the current standard allows exposures 

exceeding health benchmarks (the benchmarks of 200 ppb and 400 ppb), in 

multiple areas and occurring on multiple days. See, e.g. REA Figure 7-19; see 

also CASAC letter of May 18, 2009 at p. 15 (“The panel agrees that the current 

24-hour and annual standards are not adequate to protect public health, especially 

in relation to short term exposures to SO2 (5-10 minutes) by exercising 

asthmatics.”) This risk-based information is one of the reasons EPA is 

determining that the present standards are not requisite to protect public health 

with an adequate margin of safety 

(7) Comment: UARG commented on the documentation and replicability of the data 

set used for the air quality data analyses. Concern was expressed regarding the 

number of monitors available and number of 5-minute and hourly measurements 

used in their analyses versus those used by EPA. 

Response: We have responded to this comment on the 1st draft REA by 

improving the documentation of the raw data sets used and the screening 

procedure to generate the final data set used for developing the PMR statistical 

model. See REA section 7.2.1, pages 73-766. In this section, there are 

descriptions regarding the types of data used for the air quality analyses including: 

1) monitors reporting 5-minute maximum SO2, 2) monitors reporting continuous 
39

5-minute SO2 concentrations and, 3) monitors reporting 1-hour SO2

concentrations. Briefly, all ambient monitoring data were downloaded from 

EPA’s Air Quality System (AQS). The processing of each data set is then 

adequately described given criteria for inclusion or exclusion from the final data 

set used for analysis. A final combined data set was constructed by EPA to 

maximize the number of 1-hour measurements that also had corresponding 5-

minute maximum concentrations. It was this final data set that was used to 

construct a statistical model used to predict 5-minute maximum concentrations 

from 1-hour concentrations. 

(8) Comment: A few commenters (ACC, UARG, API) requested improved 

documentation of and justification for the approach used to adjust ambient air 

quality data to simulate just meeting the current and alternative standards. 

Response: EPA has expanded the discussion regarding the proportional method 

used to adjust the air quality data (REA, section 7.2.4) and benchmark levels 

(REA, section 8.8.3), each justified by analyses conducted by Rizzo (2009). A 

comparison between historical, high concentration monitor data and recent, low 

concentration monitor data indicated that changes have occurred in a mostly 

proportional manner, giving support for the proportional method used to simulate 

alternative air quality scenarios. 

(9) Comment: ABR and ASARCO commented on the approach used to estimate 5-

minute maximum concentrations from 1-hour average SO2 concentrations. More 

specifically, concern was expressed regarding the peak-to-mean (PMR) ratios 

developed from a subset of ambient monitors and their application to the broader 

ambient monitoring network. These commenters claim that “if the characteristics 

and micro-scale conditions of the small subset of 5-minute concentration 

monitors, which significantly influence measured concentrations, differ from the 

broader monitoring network, then the estimated exposures are not accurately 

represented.” Differences in two identified ambient monitor characteristics (i.e., 

monitor objective and surrounding emission sources) are described by the 

commenters as either “increasing the uncertainty” in applying calculated PMRs to 

the broader monitoring network and leading “to an overestimation of exposures to 

SO2.” 

Response: EPA evaluated the attributes of both the ambient monitors that report 

5-minute SO2 concentrations and those monitors that report only 1-hour average 

concentrations (REA, section 7.2.2) and did so using several physical 

characteristics including monitor objective, scale, land-use, and setting; types of 

emission sources within 20 km of a monitor; and the population residing with 5, 

10, 15, or 20 km of a monitor. EPA acknowledged any differences where present, 

though in general, the composition of monitors from the two data sets was very 

similar regarding each of these physical attributes. EPA feels that the qualitative 

evaluation performed was a reasonable and informative comparison. 
40

EPA is well aware of the greater percentage of monitors identified as having a 

source-oriented objective among the monitors that reported 5-minute 

concentrations. EPA also recognizes that there can be high concentrations and 

significant concentration variability measured at these source-oriented monitors. 

This is the primary reason for developing a statistical model based on an ambient 

monitor’s concentration variability and the 1-hour concentration level (REA, 

section 7.2.3). The statistical model was designed to use information and 

relationships derived from the 5-minute measurements and appropriately assign 

PMRs to 1-hour concentration measurements. EPA calculated the coefficient of 

variation (COV) at each ambient monitors and used that to categorize every 

monitor into one of three COV categories (≤100%, <100-≤200%, and >200%). 

Then, distributions of PMRs were calculated from the available 5-minute 

measurement data to also reflect the differences in concentration levels as well as 

the concentration variability expected to occur at each of the monitors REA, 

section 7.2.3.2). Hence, our method is in agreement with the ABR/ASARCO 

comments made regarding the potential link between source characteristics, 

concentration variability, and 1-hour average concentration. 

(10) Comment: UARG commented that the coefficient of variation (COV) is an 

inappropriate summary statistic for the monitor data. They argue that the “COV 

measure is generally reserved for normally distributed data, which SO2

concentrations certainly are not.” 

Response: The COV statistic was not used by EPA as a descriptor to define a 

monitor’s ambient concentration distribution. It was used to categorize the 

monitor into one of three variability groups (low – ≤100% COV; medium <100-

≤200% COV, and high >200% COV). EPA acknowledges that measured ambient 

concentrations at a monitor may not necessarily reflect that of a normal 

distribution, and in fact considered this in the development of the PMR statistical 

model. An alternative approach was used that incorporated the geometric 

standard deviation (GSD) in categorizing the ambient monitors (i.e., a statistic 

more appropriate in describing the variability associated with a lognormal 

distribution). Five-minute maximum SO2 concentrations were estimating using a 

PMR statistical model that either used COV or GSD as the categorical variable. 

Results demonstrated that the PMR model using COV to define a monitor’s 

variability performed better than that using GSD (REA, section 7.2.3.4). 

(11) Comment: A few commenters (e.g., API, UARG, ASARCO) generally indicated 

that exposures and risk estimates presented in the REA and proposal are over 

estimates because they rely on emissions estimates from 2002 and that 

information on the stack emission characteristics (i.e., stack location, stack height, 

stack diameter, stack flow rate and stack temperature) are uncertain. For 

example, API points out that: “SO2 emissions fell from 111,057 thousand short 

tons nationally in 2002 to 77,685 thousand short tons nationally in 2008 (see 

comments provided by API; p.22) 
41

Response: We first note that values API cited are the 2002 and 2008 emission 

estimates for CO rather than SO2. Total SO2 emissions fell from 14,774 thousand 

short tons nationally in 2002 to 11,429 thousand short tons nationally in 2008, a 

national reduction of about 23%.1

 

EPA acknowledges there is uncertainty in using the 2002 NEI emissions data. 

EPA has appropriately characterized both the exposure and risk results associated 

with the “as is” modeling scenario as reflective of year 2002. No adjustment was 

made to reflect any apparent emission reductions (i.e., post-2002) in either study 

area. Therefore, the “as is” 2002 exposure and risk results should not necessarily 

be interpreted as those that might result from using current air quality conditions 

(e.g., year 2008). If there have been reductions to the specific SO2 emission 

sources modeled in these study areas relative to the 2002 emissions used, then the 

estimated exposure and risk results would likely be an overestimate of exposure 

and risk associated with recent year (e.g., 2008) emissions. 

However, as discussed above in the response to comments C(1) through C(3), 

EPA is interested in evaluating the risk of adverse health effects associated with a 

variety of air quality conditions including just meeting the current and alternative 

standards. Just as adjustments were made to 2001-2006 ambient monitor 

concentrations (REA, section 7.2.4), adjustments were made to the exposure 

benchmark level (REA, section 8.8.3) to reflect air quality just meeting the 

current or alternative standards. These hypothetical scenario-based investigations 

are independent of what current air quality conditions or emissions levels might 

actually be, assuming that the variability in measured ambient concentrations (and 

hence their emission sources) remain relatively stable. To justify this assumption 

as appropriate, EPA evaluated historical trends in ambient air quality (REA, 

section 7.4.2.5). 

EPA judged the 2002 NEI to be the most accurate, comprehensive SO2 emissions 

data base available for use in the exposure and risk analyses. As noted in the 

REA, the 2002 NEI emissions data were not used alone but were supplemented 

with Clean Air Markets Division (CAMD) Unit Level Emissions Database of 

continuous emissions monitoring (CEM) emissions data where available and 

Emissions Modeling System for Hazardous Pollutants model (EMS-HAP) data to 

specify temporal profiles. Because of the importance of accurate locations of 

emission sources relative to population receptors for exposure assessment, the 

locations of all emitting stacks were verified and corrected where necessary, 

based on GIS analysis, as described in the REA (section 8.4.3.1). In addition all 

port-related emission sources, which were characterized as nonpoint area 

emissions, had boundaries specified based on GIS analysis of aerial photographic 

images. 

 

1

 1970 - 2008 Average annual emissions, all criteria pollutants in MS Excel - June 2009. 

http://www.epa.gov/ttn/chief/trends/. 
42

(12) Comment: The REA included exposure analyses that were performed in the 

greater St. Louis metropolitan area and Green County, MO using U.S. EPA’s 

promulgated air quality dispersion model AERMOD (AERMIC MODEL) to 

estimate 1-hour ambient SO2 concentrations at the census block level. ABR and 

ASARCO commented that the model evaluation performed when AERMOD was 

originally developed showed that for short-term averages the ratio of the predicted 

concentration to the observed concentration ranged up to 1.35, and speculated that 

the discrepancies were due to the “conservativeness” of AERMOD in certain 

situations. 

Response: AERMOD has been evaluated against several data sets of observed 

concentrations. The results of the AERMOD development model evaluation 

showed, for short-term average concentrations, an overall average predicted-toobserved ratio of 1.03 with a range among sites from 0.76 to 1.35.2

 Thus, this 

evaluation does not provide evidence that AERMOD is inherently “conservative”, 

but rather suggests somewhat variable but unbiased model performance. 

A more relevant assessment of how AERMOD performed is provided by the 

comparison of model predicted to ambient monitor measured concentrations 

(REA, section 8.4.5). Two types of predicted-to-measured comparisons were 

made: (a) predicted and measured concentration distributions for a particular 

location were matched by percentile and compared, and (b) diurnal patterns of 

predicted and measured concentrations were compared by averaging 

concentrations for each hour of the day, e.g., the concentrations for the first hour 

of the day were averaged across all simulated days. Recognizing that 

uncertainties in model inputs can result in uncertainties in model predictions, 

particularly with respect to location, for both types of comparisons the predicted 

concentrations were processed for all modeling receptors within 4 km of the 

monitor site so that the observed values could be compared to both the predictions 

at the exact location of the monitor, as well as the range of predictions in the 

vicinity. The AERMOD 1-hour concentration predictions showed good 

agreement with measured concentrations, particularly for those at the upper end of 

the concentration distribution, concentrations most likely to be associated with 5-

minute benchmark level exceedances. 

(13) Comment: ASARCO commented that a substantial fraction of the SO2 emissions 

in Greene County were characterized as nonpoint sources, and generally 

questioned the approach and data used by EPA to model these nonpoint emission 

sources (e.g., the accuracy of the emissions estimates, assuming uniform emission 

densities across a census tract, and selection of emission release heights). 

Response: First of all, EPA has adopted the term “nonpoint” to refer to all 

stationary emission sources that are not inventoried at the facility-level, and hence 

these sources are not incorporated into the point source component of the NEI. 

 

2

 See AERMOD: Latest Features and Evaluation Results. EPA-454/R-03-003. 

http://www.epa.gov/ttn/scram/7thconf/aermod/aermod_mep.pdf
43

Rather these emissions are specified at the county-level. For the Greene County 

study area, to improve the spatial resolution of the nonpoint source emissions, the 

county level aggregate emissions data provided in the NEI were allocated to 

census tracts based on land use. For example, the county total of nonpoint 

industrial emissions was allocated to tracts in proportion the amount of industrial 

land use in each tract. Given the time, resources, and data available to conduct 

this exposure and risk assessment, it was not feasible to estimate the locations of 

these facilities any more precisely. Therefore, it was assumed for modeling 

purposes, that the emissions were released uniformly over the tract. 

When considering SO2 emissions, the majority of these nonpoint sources are 

comprised of small industrial and commercial/institutional facilities combusting 

fuels. For the fuel combustion categories, NEI estimates were derived from state 

level fuel consumption data, and allocated to counties based on employment data 

for appropriate sectors. In the case of industrial nonpoint sources, the fuel 

combustion data was first corrected for an estimate of fuel consumption by 

industrial point sources to avoid double-counting.3

As noted above, given the lack of location data for nonpoint sources, it was not 

feasible to specify the locations any more precisely than census tracts. If there are 

only a small number of nonpoint sources in a tract, this approach would tend to 

reduce the spatial variance of the concentration predictions compared to 

specifying precise locations for each small facility. The result could be either an 

over- or under-estimate of the number of exceedances at the population receptors. 

However, if there are numerous such facilities dispersed throughout the tract, this 

approach will provide a reasonable characterization of the spatial pattern of the 

emissions. 

EPA selected release heights for non-point area sources as 10.0 m for rural tracts 

and 20.0 m for urban tracts. Although no data were identified by EPA to estimate 

release heights for these nonpoint sources, the release heights were not selected 

“arbitrarily” as mentioned by the commenter. First, engineering judgment was 

used to estimate the release height for a small industrial or 

commercial/institutional facility of 2 to 3 stories. Then an estimate of plume rise 

was added, since AERMOD does not apply a plume rise algorithm to area 

sources. Finally a series of sensitivity simulations were conducted to characterize 

model performance at the ambient monitor locations in order to make final 

estimates for release parameters. 

Again, the model evaluation discussed in REA section 8.4.5 suggests that the 

approaches to modeling the nonpoint sources in both Greene County and the St. 

Louis metropolitan study areas resulted in acceptable model performance, 

 

3

 See Documentation for the Final 2002 Nonpoint Sector (Feb 06 Version) National Emission Inventory for 

Criteria and Hazardous Air Pollutants. 

ftp://ftp.epa.gov/EmisInventory/2002finalnei/documentation/nonpoint/2002nei_final_nonpoint_documentat

ion0206version.pdf . 
44

especially in the upper end of the concentration distribution, where exceedances 

of concentration thresholds of concern are most likely to occur. 

(14) Comment: UARG suggested that some of the exposure analysis could not be 

replicated with the data provided in the first draft REA. In particular they 

questioned the specification of block receptors within 20 km of major point 

sources, given a discrepancy between the number of block receptors stated in the 

draft report for the Southwest and James River Power Plants in Greene Co MO 

and the number they calculated based on location coordinates they derived from 

visual inspection of Google Earth. They suggest that the discrepancy may be due 

to a discrepancy in the location coordinates used for the REA and those they 

derived. 

Response: The UTM coordinates of each stack modeled for the final REA are 

listed in Appendix B, Table B.3-1. The discrepancy in the number of block 

receptors stated in the draft report and that found by the commenter is likely due 

to a discrepancy in the number of release points used to characterize the James 

River Power Plant. For this study we characterized the James River Power Plant 

with 10 emission release points, each with unique location parameters, while the 

commenter used only two. Hence, we found slightly more Census blocks within 

20 km of the plant than reported by the commenter. 

 

(15) Comment: Several commenters (e.g., API, UARG, Dominion) indicated EPA did 

not properly consider the exposure and risk results for Greene County rather, EPA 

focused on the larger risk estimates presented for St. Louis. 

Response: EPA acknowledges that more weight was placed on the exposure and 

risk results in St. Louis compared to Greene County. EPA believes this to be a 

reasonable approach: the St. Louis information provided more probative 

information as to the adequacy or inadequacy of the current standards. As stated 

in the REA: 

Exposures and risks have been estimated for two study areas in Missouri 

(i.e., Greene County and several counties representing the St. Louis urban 

area) which have significant emission sources of SO2. As noted in section 

8.10, there were differences in the number of exposures above benchmark 

values when the results of the Greene County and St. Louis exposure 

assessments were compared. Moreover, given that the results of the 

exposure assessment were used as inputs into the quantitative risk 

assessment, it was not surprising that there were also far fewer asthmatics 

at elevated ventilation rates estimated to have a moderate or greater lung 

function response in Greene county when compared to St. Louis. The 

difference in the St. Louis and Greene County exposure and quantitative 

risk results are likely indicative of the different types of locations they 

represent (see section 8.10). Greene County is a rural county with much 

lower population and emission densities, compared to the St. Louis study 

area which has population and emissions density similar to other urban 
45

areas in the U.S. It therefore follows that there would be greater 

exposures, and hence greater numbers and percentages of asthmatics at 

elevated ventilation rates experiencing moderate or greater lung function 

responses in the St. Louis study area. Thus, when considering the risk and 

exposure results as they relate to the adequacy of the current standards (as 

well as the need for considering potential alternative standards), the St. 

Louis results are more informative in that they suggest that the current 

standards may not adequately protect public health. Moreover, staff 

judges that the exposure and risk estimates for the St. Louis study area 

provide useful insights into exposures and risks for other urban areas in 

the U.S. with similar population and SO2 emissions densities (REA, p. 

364). 

 

(16) Comment: JRSC referenced prior comments by UARG stating that the individual 

data from controlled human exposure studies used in the quantitative risk 

assessment had significant errors associated with it, and that it was unclear if 

these errors had been corrected (comments by JRSC, p. 2). 

Response: EPA conducted a full quality assurance review of the controlled 

human exposure data, which included personal communication and technical 

guidance from the principal investigator of the original studies, William Linn. 

This review, presented in Johns and Simmons (2009), found a very limited 

number of minor errors in the original individual subject lung function data 

presented in the ISA. However, the results of the review did not change the 

conclusions presented in the ISA. In fact, the quality assurance review resulted in 

an increase in the percentage of responders reported in Table 3-1 of the ISA and 

9-3 of the REA. 

(17) Comment: UARG generally concluded that under all air quality scenarios EPA’s 

quantitative risk assessment substantially overestimated risk because EPA did not 

use proper methods to estimate the parameters of the exposure-response functions 

used in its analyses. UARG contends this is because many of the subjects in the 

controlled human exposure studies from which EPA’s exposure-response 

functions were derived (see REA, Table 9-3) were exposed to more than one SO2

concentration, yet EPA treated each exposure event as being independent (e.g., if 

the same subject was exposed to 200 and 300 ppb SO2, EPA considered these as 

representing two independent exposure events). UARG contends that 

observations from the same subject exposed to different SO2 concentrations are 

not independent observations and should not be treated as such. Notably, when 

UARG derived their own exposure-response functions taking into account that 

observations from the same subject exposed to different SO2 concentrations are 

not independent of each other, they estimated appreciably less risk than that 

estimated by EPA. 

Response: EPA notes that it is true that observations for the same subject across 

the different SO2 concentrations to which the subject was exposed are not 
46

completely independent observations – although for any given SO2 concentration, 

the observations are (mostly) independent.4

 The important question is what effect 

not considering the dependence of intra-subject observations across SO2

concentrations has on the estimated exposure-response relationship. 

To address this perceived problem of dependent observations, UARG estimates 

subject-specific exposure-response functions.5 Based on these subject-specific 

exposure-response functions, UARG estimates the two parameters in the 

(population-level) logistic and probit exposure-response functions necessary for 

use in the risk assessment. EPA plotted the logistic and probit exposure-response 

functions estimated by UARG along with the logistic and probit exposureresponse functions estimated by EPA, as well as the underlying data.6,7 These are 

shown in Figure 1. 

As Figure 1 shows, the logistic and probit exposure-response functions estimated 

by UARG (the blue curves) do not fit the underlying data (the proportions of 

subjects who responded at each exposure level) nearly as well as the functions 

estimated by EPA (the black curves). This suggests that, even though it deals 

with the “dependency problem,” the UARG approach has its own problems. 

 

4

 There were a few instances where a subject was exposed to the same SO2 concentration more than once. 

EPA noted this in the REA, stating the following: “Some subjects in the controlled human exposure studies 

participated in more than one study and were exposed to a given SO2 concentration more than once. 

However, because there were insufficient data to estimate subject-specific response probabilities, we 

assumed a single response probability (for a given definition of response) for all individuals and treated the 

repeated exposures for a single subject as independent exposures in the binomial distribution.” 

5

 UARG first claims that “the correct likelihood for such a case [in which there are several observations 

per subject] would be multinomial” and they set up a multinomial likelihood function. However, UARG 

appears not to use it. 

6

 The UARG curves are based on the estimated values given for the two parameters in each function 

presented in Table 4 of “Technical Comments on the Primary National Ambient Air Quality Standard for 

Sulfur Dioxide. Proposed Rule Docket no. EPA-HQ-OAR-2007-0352, by Edmund A.C. Crouch, Ph.D. and 

Laura C. Green, Ph.D., D.A.B.T. Cambridge Environmental Inc. Cambridge, Massachusetts. February 8, 

2010 (Docket No. EPA-HQ-OAR-2007-0352-1061.1) Corrected April 6, 2010. 

7

 The functional forms for the logistic and probit functions are given in (equation 9-3) and (equation 9-4), 

respectively, on p. 327 of the REA. The EPA-estimated curves and the underlying data are shown in 

Figure 9-2 on p. 330 of the REA. 
47

Figure 1. EPA- and UARG -Estimated Logistic and Probit Exposure-Response Functions 

(for Lung Function Response Defined as an Increase in sRaw of ≥100%) and Underlying 

Data

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

0 0.2 0.4 0.6 0.8 1

SO2 Concentration (ppm)

Response Rate

The original data

EPA probit

EPA 2-parameter logistic

UARG 2-parameter logistic

UARG probit

 

UARG is not specific about the nature of the problems that will result from using 

a likelihood function that does not consider intra-subject dependencies. They note 

only that “the consequence of using this incorrect likelihood for common subjects 

was to decrease (in the REA analysis) the estimated slope of the population E100

distribution function (the magnitude of the γ parameter in both [the logistic and 

probit exposure-response] models), and increase the estimated population fraction 

responding at low exposures (in the extrapolated region below 0.2 ppm SO2).”8

 

They note in a footnote that “this consequence was evaluated by performing the 

calculations both ways [i.e., the way EPA estimated the parameters and the way 

UARG estimated them].” That is, they concluded that EPA’s estimated curves 

are “wrong” because they differ markedly from their own curves. This presumes, 

however, that UARG’s curves are indeed “correct.” The comparison of how each 

of the curves reflects the underlying data (Figure 1) suggests otherwise. 

 

8

 From: Comments of the Utility Air Regulatory Group on the Risk and Exposure Assessment to Support 

the Review of the SO2 Primary National Ambient Air Quality Standards: Second Draft (March 2009). 

Docket ID No. EPA-HQ-OAR-2007-0352. June 11, 2009. Attachment 1 (Technical Comments by 

Edmund A. C. Crouch, Ph.D., Laura C. Green, Ph.D., D.A.B.T. and Joshua M. Galanter, M.D. on the SO2

REA (June 10, 2009), p. 12. 
48

While their approach of estimating subject-specific exposure-response functions 

addresses the issue of intra-subject dependencies, EPA notes that it creates other 

serious problems. In particular, EPA notes that there do not appear to be 

sufficient data for any of the subjects to estimate a subject-specific exposureresponse function.9

 The form of the individual-specific exposure-response 

function they chose is a quadratic function with three parameters. Thus for each 

subject they presumably had to estimate the subject-specific values of these three 

parameters. However, none of the subjects in the seven controlled human 

exposure studies on which EPA relied were exposed to more than three non-zero 

SO2 levels: over half were exposed to only one or two non-zero SO2 levels. 

Thus, EPA notes that it appears that the data for estimating these subject-specific 

exposure-response functions were severely limited, especially given that a large 

percentage of the total number of subjects had fewer exposures than the number 

of parameters UARG was attempting to estimate. UARG attempted to estimate 

three parameters in its exposure-response functions, but over fifty percent of 

subjects had only one or two exposures. It appears, moreover, that UARG’s 

population-level exposure-response function estimates depended on these subjectspecific exposure-response function estimates, and this could explain why 

UARG’s estimated population-level exposure-response functions do not fit the 

underlying controlled human exposure data nearly as well as the EPA-estimated 

functions. In attempting to address the problem of intra-subject dependencies, 

UARG seems to have created different problems. It is not surprising, then, that 

neither their estimated logistic function nor their estimated probit function fit the 

underlying data very well. 

There are other approaches to dealing with the intra-subject dependency issue that 

avoid the problem associated with the UARG approach. Not considering intrasubject dependencies is effectively omitting an individual-level effect from the 

model. This would be a problem if the omitted individual-level effect were 

correlated with the assignment of exposures (for example, if the hyper-responsive 

subjects were disproportionately exposed to the lowest levels), but there is no 

reason to suppose this is the case. Moreover, any possible bias in the estimated 

logit or probit model parameters is likely to be minor, since the majority of the 

subjects had only one or two exposures, so the degree of intra-subject 

dependencies is small; indeed, visual inspection of the EPA fitted curves in Figure 

1 does not suggest any substantial biases. 

EPA further notes that the approach we used to estimate the parameters of the 

exposure-response functions was not first introduced in this SO2 risk assessment; 

it was previously used in the O3 risk assessment completed in 2008, which used 

data from controlled human exposure studies much like the data used in the SO2

risk assessment. This approach was suggested to EPA by an applied statistician 

 

9

 This was noted in the Risk and Exposure Assessment, see 

http://www.epa.gov/ttn/naaqs/standards/so2/data/200908SO2REAFinalReport.pdf, p. 328. 
49

serving on the CASAC which at the time was overseeing the O3 NAAQS review. 

Notably, this approach allowed EPA to use all of the individual subject data. 

Consequently, as noted in section II.E.2.c of the preamble to the final rule, EPA 

does not accept UARG’s view that the methodology used in EPA’s quantitative 

risk assessment was inappropriate. 

 

(18) Comment: UARG generally concluded that the risk estimates presented in 

Chapter 9 of the REA and in the proposal are substantially overestimated because 

of the use of 50 ppb bins by EPA. 

More specifically, in their original comments submitted on June 11, 2009, UARG 

noted that EPA’s approach: 

 

...serves to overestimate the number of annual ‘events’ (defined as an 

increase in corrected relative sRaw of ≥100% among asthmatics in St. 

Louis). This is primarily because the REA analysis bins SO2 exposures 

into 50 pbb-wide bins. An accurate calculation must take into account the 

distribution of values, particularly in the lowest bin — wherein (i) ambient 

air concentrations range from zero to 50 ppb, and (ii) the REA predicts 

95% of the sRaw events.10 

That is, UARG concluded that the use of 50 ppb bins, combined with assigning all 

exposures within a bin the probability of an adverse lung function response at the 

midpoint of that bin (e.g., all exposures from 0 - 50 ppb were assigned the 

probability of an adverse lung function response at 25 ppb), resulted in a 

substantial overestimate of the total number of occurrences of lung function 

responses in asthmatics at moderate or greater exertion. UARG concludes that 

this is because the vast majority of exposures are occurring below the midpoint of 

the 0 - 50 ppb exposure bin (i.e., most exposures are occurring below 25 ppb), yet 

EPA is assigning these very low SO2 exposures the higher probability of a lung 

function response associated with the midpoint of the 0 -50 ppb exposure bin (i.e., 

the great majority of exposures were < 25 ppb but were assigned the probability 

of an adverse lung function response at 25 ppb). 

In follow-on comments submitted in February 2010,11 UARG takes advantage of 

a technique EPA used to streamline the estimation of exposures under the 

 

10 “Comments of the Utility Air Regulatory Group on the Risk and Exposure Assessment to Support the 

Review of the SO2 Primary National Ambient Air Quality Standards: Second Draft (March 2009).” 

Docket ID No. EPA-HQ-OAR-2007-0352. June 11, 2009. Attachment 1: “Technical Comments by 

Edmund A. C. Crouch, Ph.D., Laura C. Green, Ph.D., D.A.B.T. and Joshua M. Galanter, M.D. on the SO2

REA (June 10, 2009), p. 31. 

11 “Technical Comments on the Primary National Ambient Air Quality Standard for Sulfur Dioxide. 

Proposed Rule Docket No. EPA-HQ-OAR-2007-0352, by Edmund A.C. Crouch, Ph.D. and Laura C. 

Green, Ph.D., D.A.B.T. Cambridge Environmental Inc. Cambridge, Massachusetts. February 8, 2010 

(Docket No. EPA-HQ-OAR-2007-0352-1061.1) Corrected April 6, 2010. 
50

different air quality scenarios considered. As described in Section 8.8.3 of the 

July 2009 Final SO2 REA, 

 

... instead of proportionally adjusting the ambient concentrations, [EPA] 

proportionally adjusted the health effect benchmark levels used in each 

exposure modeling domain. The benchmark levels were adjusted rather 

than the air quality to reduce the processing time associated with the 

modeling of several thousands of receptors in each of the large exposure 

modeling domains. A proportional adjustment of the selected benchmark 

level (i.e., division by the adjustment factor) is mathematically equivalent 

to a proportional adjustment of the air quality concentration (i.e., 

multiplication by the adjustment factor). Therefore, the end effect of 

adjusting the exposure model input concentrations upward versus 

adjusting exposure model benchmark levels downward is identical. 

This “walkway” between SO2 concentrations under each alternative SO2 standard 

and SO2 concentrations under recent conditions allowed EPA to carry out the 

exposure analysis for all the different air quality scenarios in a single APEX run, 

resulting in an APEX output table with many SO2 exposure levels under year 

2002 “as is” air quality conditions. UARG used these many exposure levels 

under “as is” conditions and EPA’s factors for converting from recent conditions 

to each of the alternative air quality scenarios to create much smaller “bins.” 

Applying EPA’s approach of calculating the expected number of lung function 

responses at the midpoint of each “bin” to these much smaller “bins,” UARG 

showed that use of 50-ppb “bins” does indeed overstate the total number of 

occurrences of lung function response under each air quality scenario. UARG 

further notes that this methodological concern was raised in its comments on the 

second draft REA, but EPA failed to address this issue and relied heavily on this 

metric in the proposal with respect to the adequacy of the current and potential 

alternative standards. 

Response: EPA generally agrees with UARG’s technical comments that there is 

a substantial overestimation of the total occurrences of lung function responses 

because of the binning issues described above. However, we strongly disagree 

that: 1) this issue was not acknowledged in the final REA; and 2) the metric of 

total occurrences was relied on heavily in the policy assessment chapter of the 

REA (REA, chapter 10) and in the Administrator’s rationale with respect to the 

adequacy of the current and potential alternative standards. First, EPA did 

respond to this concern in the final REA. More specifically, page 344 of the final 

REA states: 

As noted in public comments on the 2nd draft SO2 REA, the assignment of 

response probability to the midpoint of the exposure bin combined with 

the lack of more finely divided intervals in this range can lead to 

significant overestimation of risks based on total occurrences of a defined 

lung function response. This is because the distribution of population 
51

exposures for occurrences is not evenly distributed across the bin, but 

rather is more heavily weighted toward the lower range of the bin. Thus, 

combining all exposures estimated to occur in the lowest bin with a 

response probability assigned to the midpoint of the bin results in a 

significant overestimate of the risk. Therefore, staff places less weight on 

the estimated number of occurrences of lung function responses. 

Thus, as noted in the final REA, less weight was placed on this metric in the 

quantitative risk assessment chapter (REA, chapter 9), and importantly, no weight 

was placed on this metric in either the policy assessment chapter of the REA 

(REA, chapter 10) or in the Administrator’s rationale sections of the proposal 

preamble. Rather, the policy assessment chapter of the REA and the 

Administrator’s rationale at the proposal considered the percent of exposed 

asthmatic children at moderate or greater exertion estimated to have at least one 

defined lung function response per year in St. Louis. Importantly, this metric is 

not appreciably affected by the binning issue raised in UARG’s comments. As 

stated on page 344- 345 of the final REA: 

This overestimation of total occurrences does not impact the risk metric 

expressed as incidence or percent incidence of a defined lung function 

response 1 or more times per year because the bulk of the exposures 

contributing to these risk metrics are not skewed toward the lower range of 

the reported exposure bins. 

This is confirmed by using the same technique used by UARG, described above, 

but applying it to this metric. The results, for asthmatic children in St. Louis, are 

shown in Table 1 below using the original 50-ppb bin approach and the more 

disaggregated bins identified by UARG. 
52

Table 1. Estimates of the Number of Asthmatic Children in St. Louis with at Least One 

Lung Function Response (Defined as an Increase in sRaw>=100%) Under Different Air 

Quality Scenarios Using 50 ppb Bins Compared with Estimates Using More Disaggregated 

Bins 

Using 50 

ppb Bins

Using More 

Disaggregated 

Bins

Using 50 

ppb Bins

Using More 

Disaggregated 

Bins

Recent conditions 588 591 100.5% 1.41% 1.42%

Current Standard 8021 8090 100.9% 19.23% 19.39%

Alternative Std: 99/50 396 406 102.5% 0.95% 0.97%

Alternative Std: 99/100 1221 1204 98.6% 2.93% 2.89%

Alternative Std: 99/150 2243 2214 98.7% 5.38% 5.31%

Alternative Std: 99/200 3369 3346 99.3% 8.08% 8.02%

Alternative Std: 99/250 4555 4541 99.7% 10.92% 10.89%

*There were 41714 asthmatic children in St. Louis in this analysis.

Number of Asthmatic 

Children Who Respond at 

Least Once

Percent of Asthmatic 

Children Who Respond at 

Least Once*

Air Quality Scenario

New Result 

as a Percent 

of Original 

Result

As Table 1 shows, the use of 50 ppb bins does not result in overestimates of the 

number or percent of asthmatic children in St. Louis with at least one lung 

function response across the air quality scenarios. While in some air quality 

scenarios the estimate based on the 50-ppb bin approach is slightly greater than 

the estimate based on more disaggregated bins, in others the estimate based on the 

50-ppb bin approach is slightly less than the estimate based on more 

disaggregated bins. The differences appear to be random and are judged not 

significant from a policy perspective. 

Finally, it is important to note that the Administrator’s rationale in the proposal 

regarding the adequacy of the current and potential alternative standards in 

general placed only limited reliance on the results of the quantitative risk 

assessment in St. Louis, with no reliance on the estimates of total occurrences. 

Rather, in addition to the substantial weight that she placed on the scientific 

evidence as described in the ISA, the Administrator placed relatively more weight 

on the results of the St. Louis exposure analysis (see section II.E.2.c of the 

preamble to the final rule for more information). 

(19) Comment: In their comments submitted on April 6, 2010, UARG notes that “the 

Final REA also omitted to take account of the additional information available on 

the experiments of Linn et al. (1983) that became available between the Draft 

REA and Final REA, another 69 samples with 14 positive results.”12 

 

12 “Technical Comments on the Primary National Ambient Air Quality Standard for Sulfur Dioxide. 

Proposed Rule Docket No. EPA-HQ-OAR-2007-0352, by Edmund A.C. Crouch, Ph.D. and Laura C. 

Green, Ph.D., D.A.B.T. Cambridge Environmental Inc. Cambridge, Massachusetts. February 8, 2010 

(Docket No. EPA-HQ-OAR-2007-0352-1061.1) Corrected April 6, 2010, p. 15. 
53

Response: While it was not possible to re-estimate the exposure-response 

functions incorporating this additional data mentioned by UARG, EPA did create 

a new graph showing the original and adjusted data as well as the EPA-estimated 

and the UARG-estimated logistic and probit exposure-response functions (based 

on the original set of data) for lung function response defined as an increase in 

sRaw of ≥100%. Shown below are all the original data points (black triangles) as 

well as the adjusted data points (red crosses) incorporating the additional data 

from Linn et al. (1983) for the three SO2 exposures for which that study provided 

data (0.2, 0.4, and 0.6 ppm). As shown in Figure 2 below, the three adjusted data 

points are all quite close to the original data points used to develop the exposureresponse functions; it is therefore highly unlikely that the incorporation of the 

Linn et al. (1983) data would alter the estimated exposure-response functions 

appreciably. 

Figure 2. EPA- and UARG (Crouch/Green)-Estimated Logistic and Probit ExposureResponse Functions (for Lung Function Response Defined as an Increase in sRaw of 

≥100%), Underlying Original Data, and Data Incorporating Linn et al. (1983) 

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

0 0.2 0.4 0.6 0.8 1

SO2 Concentration (ppm)

Response Rate

The original data

probit

2-parameter logistic

UARG 2-parameter logistic

UARG probit

Adjusted data - including Linn et al. 1983

(Computed Differences)

(20) Comment: Springfield pointed out that Table 10-5 of the REA contained an error. 

The 1-hour SO2 concentrations reported for Greene County were not from the 

monitor that recorded the highest SO2 concentrations in the county. Springfield 

was concerned that the exposure and risk estimates presented in the REA for 
54

Greene County were based on erroneous data, and thus did not accurately 

represent the impacts of SO2. 

Response: Springfield is correct that REA Table 10-5 identified the incorrect 

monitor as that recording the highest 1-hour daily maximum SO2 concentrations 

in Greene County. We note that Table 10-5 was only used to demonstrate that in 

most counties, the 99th percentile 1-hour daily maximum SO2 concentration 

corresponded to the 4th highest 1-hour daily maximum SO2 concentration and that 

the 98th percentile 1-hour daily maximum SO2 concentration corresponded to the 

7th -8th highest 1-hour daily maximum SO2 concentration. Thus, the results of the 

air quality, exposure and risk assessment for Greene County were not affected by 

this error. In addition, we note that in general, as noted above in response 8 of 

this section, the results of the Greene County exposure and risk assessments were 

not considered as informative as those in St. Louis. 

(21) Comment: Springfield commented: 

Springfield is the seat of Greene County, Missouri, a locale mentioned 

several times in the proposal and in the supporting Risk Exposure 

Analysis. Understandably, we noted with considerable interest the 

discussion of EPA’s exposure modeling for this area. We find it especially 

telling that the model returned exposure results that apparently were too 

inconsequential for further consideration (see, for example, p.64827, 

“(t)hus, when considering the risk and exposure results as they relate to 

the adequacy of the current standards, the REA concluded that the St. 

Louis results were more informative [than Greene County]...”). Despite 

this lack of any cogent exposure findings for our “rural” (population 

270,000) county, the proposed standard could very well place Greene 

County in nonattainment (see comments provided by Springfield, section 

II.d.ii) 

Response: Springfield is correct that EPA found the exposure and risk results for 

St. Louis more informative than those in Greene County for informing the 

adequacy of the current and potential alternative standards. As stated in the REA 

(and is quoted in the earlier response): 

Exposures and risks have been estimated for two study areas in Missouri 

(i.e., Greene County and several counties representing the St. Louis urban 

area) which have significant emission sources of SO2. As noted in section 

8.10, there were differences in the number of exposures above benchmark 

values when the results of the Greene County and St. Louis exposure 

assessments were compared. Moreover, given that the results of the 

exposure assessment were used as inputs into the quantitative risk 

assessment, it was not surprising that there were also far fewer asthmatics 

at elevated ventilation rates estimated to have a moderate or greater lung 

function response in Greene county when compared to St. Louis. The 
55

difference in the St. Louis and Greene County exposure and quantitative 

risk results are likely indicative of the different types of locations they 

represent (see section 8.10). Greene County is a rural county with much 

lower population and emission densities, compared to the St. Louis study 

area which has population and emissions density similar to other urban 

areas in the U.S. It therefore follows that there would be greater 

exposures, and hence greater numbers and percentages of asthmatics at 

elevated ventilation rates experiencing moderate or greater lung function 

responses in the St. Louis study area. Thus, when considering the risk and 

exposure results as they relate to the adequacy of the current standards (as 

well as the need for considering potential alternative standards), the St. 

Louis results are more informative in that they suggest that the current 

standards may not adequately protect public health. Moreover, staff 

judges that the exposure and risk estimates for the St. Louis study area 

provide useful insights into exposures and risks for other urban areas in 

the U.S. with similar population and SO2 emissions densities (REA, p. 

364). 

 

That being said, it is important to note several points. First, these analyses only 

estimated exposure and risks associated with modeled 5-minute benchmark 

concentrations of concern identified from the controlled human exposure 

literature: thus these considerations alone do not take into account 1-hour SO2

levels in U.S. locations epidemiologic studies reported positive effects between 

ambient SO2 and emergency department visits and/or hospitalizations (i.e., 

epidemiologic considerations taken into account by the Administrator in setting a 

new 1-hour standard at 75 ppb). In addition, Tables 7-12 to 7-14 of the REA 

indicate that when air quality is adjusted to just meet the current standards, 

Greene County had an appreciable number of days per year on average when 5-

minute SO2 concentrations exceeded 5-minute benchmark levels. This indicates 

5-minute concentrations of concern could be present in Greene County under the 

current standards, and it is thus possible with increased population growth and/or 

shifts in activity patterns, a greater number of SO2 exposures above 5-minute 

benchmark levels could result (if air quality just met the current standards). 

Finally, EPA notes that the NAAQS are national standards meant to provide 

protection in both rural and urban areas. 

(22) Comment: UARG generally commented that the risk and exposure assessment 

did not take into account the 3-hour secondary standard when estimating the 

extent to which the current standards limit 5-minute peaks above benchmark 

levels of concern. 

Response: Section 109(b)(1) defines a primary standard as one “the attainment 

and maintenance of which in the judgment of the Administrator, based on [the air 

quality] criteria and allowing an adequate margin of safety, are requisite to protect 

the public health.” Section 109(b)(1). In contrast, a secondary standard, must 

“specify a level of air quality the attainment and maintenance of which, in the 
56

judgment of the Administrator, based on [the air quality] criteria, is requisite to 

protect the public welfare from any known or anticipated adverse effects 

associated with the presence of such pollutant in the ambient air.”13 Thus, EPA 

found it reasonable when considering the adequacy of the current standards to 

protect public health, to only consider the primary SO2 standards in its air quality, 

exposure and risk analyses. 

III. Responses to Significant Comments on the Adequacy of 

the Current Standards 

(1) Comment: Many industry groups (e.g., API, UARG, ACC, DRSI, ExxonMobil, 

PE, CIBO, TFI, EEI, Dow, MWV, NMA, Duke Energy, GCLC) commented that, 

given the presence of numerous co-pollutants in the air, the epidemiologic studies 

do not support the contention that SO2 itself is causing health effects. For 

example, UARG stated: 

The epidemiological evidence cannot determine that SO2 is a cause of or a 

contributor to hospital admissions (“HA”), emergency department (“ED”) 

visits or respiratory symptoms, the effects cited in the Proposed Rule (see 

comments provided by UARG; p.3) 

Thus, these groups generally conclude that the epidemiologic evidence should not 

be considered when evaluating the adequacy of the current standards 

Response: These comments and EPA’s responses were presented in detail in 

section II.E.2.a of the preamble to the final rule. EPA also provided additional 

information on similar comments above in section II.B

(2) Comment: Many industry groups (e.g., API, ACC, PE, EEI, CIBO) commented 

that adverse health effects do not occur following 5 - 10 minute SO2 exposures < 

400 ppb. In addition, some groups (e.g., UARG, Duke Energy) commented that 

adverse respiratory effects do not occur in exercising asthmatics following SO2

exposures below 600 ppb. The disagreement is not whether effects occur in 

exercising asthmatics at these exposure levels and exposure durations. Rather, the 

issue is whether the effects experienced can properly be regarded as adverse. In 

general, these groups conclude that EPA’s judgment of adverse health effects at 

SO2 exposure levels below 600 or 400 ppb is inappropriately based on an unsound 

interpretation of ATS guidelines. More specifically, these groups generally 

contend that decrements in lung function without accompanying respiratory 

symptoms are not adverse effects of SO2 exposure, and that decrements in lung 

function in a percentage of exercising asthmatics does not represent a shift in lung 

function at the population level. Some of these groups also contend that EPA 

 

13 EPA is currently conducting a separate review of the secondary SO2 NAAQS jointly with a review of the 

secondary NO2 NAAQS (see http://www.epa.gov/ttn/naaqs/standards/no2so2sec/index.html for more 

information). 
57

followed the advice of individual CASAC members, rather than consensus 

CASAC written comments on the ISA and REA when concluding respiratory 

effects associated with SO2 exposures below 600 or 400 ppb are adverse. Thus, 

these groups generally conclude that 5-minute benchmark levels less than 600 or 

400 ppb should not be considered when evaluating the adequacy of the current 

standards. 

In addition to disagreeing with EPA’s proposed finding of adverse health effects 

following 5 - 10 minute SO2 exposures as low as 200 ppb, many industry groups 

(e.g., API, UARG, ACC, ExxonMobil) also disagreed with EPA that severe 

asthmatics were not included in controlled human exposure studies. That is, these 

groups contend that EPA is incorrect in assuming that severe asthmatics would 

likely have a more pronounced response to SO2 exposures at a given level, or 

would respond to even lower levels of SO2 and that this should be taken into 

account when judging the adequacy of the current standards. 

Response: EPA disagrees with these comments, and believes (as does CASAC) 

that the clinical evidence also supports the conclusion that the current standards 

are not requisite to protect public health with and adequate margin of safety. 

These comments and EPA’s responses were presented in detail in section II.E.2.b 

of the final rule. Similar comments and EPA responses were also discussed above 

in section II.A of this document. 

(3) Comment: Several commenters discussed the analyses of SO2-associated 

exposures and health risks presented in the REA. As in past reviews (e.g., EPA 

2005, EPA 2007), EPA has estimated risks associated with the current standards 

to inform judgments on the public health risks that could exist under different 

standard options. Some industry commenters (e.g., API, UARG, LEC, ASARCO, 

NRECA) concluded that when considering the adequacy of the current standards, 

the Administrator should consider exposures and risks associated with actual SO2

air quality rather than air quality allowed by the current NAAQS. These groups 

also note adjusting air quality to just meet the current and potential alternative 

standards is highly uncertain. They consequently challenged the relevance and 

appropriateness of EPA’s use of SO2 concentrations that have been simulated to 

just meet the current standards in assessing the adequacy of the current standards. 

Response: EPA disagrees with these comments. These comments and EPA’s 

responses were presented in detail in section II.E.2.c of the preamble to the final 

rule. Similar comments and EPA responses were also discussed above in section 

II.C. 

(4) Comment: UARG generally concluded that under all air quality scenarios, the 

results of EPA’s quantitative risk assessment (the third of the analyses conducted 

in the REA (chapter 9) are substantially overestimated because EPA did not use 

proper methods to estimate the parameters of the exposure-response functions 

used in its analyses. UARG contends this is because many of the subjects in the 
58

controlled human exposure studies from which EPA’s exposure-response 

functions were derived (see REA, Table 9-3) were exposed to more than one SO2

concentration, yet EPA treated each exposure event as being independent (e.g., if 

the same subject was exposed to 200 and 300 ppb SO2, EPA considered these as 

representing two independent exposure events). UARG contends that 

observations from the same subject exposed to different SO2 concentrations are 

not independent observations and should not be treated as such. Notably, when 

UARG derived their own exposure-response functions taking into account that 

observations from the same subject exposed to different SO2 concentrations are 

not independent of each other, they estimated appreciably less risk than that 

estimated by EPA. 

Response: EPA presented this comment and provided an initial qualitative 

response in section II.E.2.c of the preamble to the final rule. A more detailed 

technical response to this comment is discussed above in section II.C. 

(5) Comment: UARG concluded that EPA further overestimates the total number of 

occurrences of an adverse lung function response (i.e., total number of 

occurrences of increases in sRaw ≥100 or 200% and/or declines in FEV1 ≥ 15 or 

20%) in its quantitative risk assessment. More specifically, UARG concluded 

that the use of 50 ppb bins, combined with assigning all exposures within a bin 

the probability of an adverse lung function response at the midpoint of that bin 

(e.g., all exposures from 0 - 50 ppb were assigned the probability of an adverse 

lung function response occurring at 25 ppb), resulted in a substantial overestimate 

of the total number of occurrences of lung function responses in asthmatics at 

moderate or greater exertion. UARG generally concludes that this is because the 

vast majority of exposures of asthmatics at moderate or greater exertion are 

occurring below the midpoint of the 0 - 50 ppb exposure bin (i.e., most exposures 

are occurring below 25 ppb), yet EPA is assigning these very low SO2 exposures 

the higher probability of a lung function response associated with the midpoint of 

the 0 -50 ppb exposure bin. UARG contends that this results in a substantial 

overestimation of the total number of occurrences of lung function response in 

asthmatics and asthmatic children at moderate or greater exertion. UARG further 

notes that this methodological concern was raised in its comments on the second 

draft REA, but EPA failed to address this issue and relied heavily on this metric in 

the proposal with respect to the adequacy of the current and potential alternative 

standards. 

Response: EPA presented this comment and provided an initial qualitative 

response in section II.E.2.c of the preamble to the final rule. A more detailed 

technical response to the binning issue discussed in this comment can be found 

above in section II.C. 
59

IV. Comments on A New Short-Term SO2 Primary Standard 

This section discusses comments received on EPA’s proposed 1-hour standard. Some 

commenters provided comments on the cost or economic impact of monitoring, 

implementation, or compliance associated with the proposed SO2 NAAQS. As noted in 

section I.B of the preamble, the Clean Air Act bars consideration of costs in setting the 

NAAQS, and accordingly EPA has not considered costs, including the costs or economic 

impact of monitoring, implementation or compliance, in revising the primary SO2

NAAQS. 

A. Indicator 

Few public commenters directly addressed the issue of the indicator for the standard. 

These commenters generally endorsed the proposal to continue to use SO2 as the 

indicator for ambient SOx. 

 

B. Averaging time 

(1) Comment: As discussed above, industry commenters who disagreed with setting 

a new 1-hour standard generally based this conclusion on their interpretation of 

the scientific evidence and their conclusion that this evidence does not support the 

proposed revisions to the current SO2 NAAQS. 

Response: These comments and EPA’s responses were presented in detail in 

sections II.E.2.a and II.E.2.b of the preamble to the final rule.

(2) Comment: Some industry commenters (e.g. ASARCO, RTA, ABR, Nucor, 

KUC) and South Dakota expressed that EPA should have considered longer 

averaging times (e.g., 3-hour, 8-hour, 24-hour). In general, these groups 

concluded that a standard with a longer averaging time could potentially provide 

the same public health protection as a 1-hour standard, while also providing a 

more stable regulatory target. For example, in its comments, South Dakota states: 

“DENR recommends EPA evaluate a 3-hour or 8-hour standard to determine if 

these averaging periods are also protective of the public health. If they are, EPA 

should propose a 3-hour or 8-hour sulfur dioxide standard instead of a 1-hour 

standard. A longer averaging period would smooth out the variability of the 

upper range measurements and provide a more stabile standard” (see comments 

provided by South Dakota; p. 1) Similarly, RTA stated in its comments: “the 

short-term averaging period defined by EPA (i.e., 5-minutes to 24-hours) is not 

limited to only 5-minute, 1-hour and 24-hour averaging periods. EPA could 

explain in more detail why these three averaging periods were examined when 

considering appropriate averaging periods to limit short-term peaks of SO2...a 

longer term average could provide additional stability to the standard, while at the 
60

same time effectively protecting public health” (see comments provided by RTA; 

p. 3) 

Response: These comments and EPA’s responses were presented in detail in 

section II.F.2.b of the final rule

(3) Comment: Although health and environmental groups were supportive of setting 

a new 1-hour standard to protect against short-term exposures to SO2 (depending 

on the level of the 1-hour standard selected), these groups generally commented 

that a 5-minute standard to protect against health effects associated with 5-minute 

peaks would be optimal (e.g., ALA, Sierra Club, EDF). For example, in its 

comments ALA et al., stated: “We need a short-term SO2 standard, optimally a 5-

minute standard, to protect against bursts of pollution that can result from start-up, 

shutdown, upset, malfunction, downwash, complex terrain, atmospheric inversion 

conditions, and other situations” and that “EPA has over emphasized a concern 

about the stability of a 5-minute standard... The record does not show that any 

alleged instability of a 5-minute standard has any relevance to whether such a 

standard is requisite to protect public health (see comments provided by ALA et 

al., p. 11-12) 

Response: These comments and EPA’s responses were presented in detail in 

section II.F.2.b of the preamble to the final rule. We also again emphasize 

CASAC’s conclusion that “that a one-hour standard is the preferred averaging 

time”. Samet (2009) at p. 15.

(4) Comment: ALA generally commented that EPA was not setting a 5-minute 

standard because of concerns over the number of monitors needed. ALA notes 

that this concern is not lawful under the CAA. 

Response: As noted in final rule preamble sections II.F.2.b and II.F.2.c, EPA is 

not setting a 5-minute standard because of concerns over the stability of a 5-

minute standard, and the deleterious effects an unstable standard could have on 

public health protection. The number of monitors needed should a 5-minute 

standard be adopted was not a consideration in the Administrator’s rationale for 

not setting a 5-minute standard. 

 

In addition, as noted in final preamble section II.F.2.b, a 1-hour standard to 

protect against 5-minute exposures is in agreement with CASAC advice and 

recommendations. That is, CASAC stated that they were “in agreement with 

having a short-term standard and finds that the REA supports a 1-hour standard as 

protective of public health” (Samet 2009, p. 1). Similarly, in a CASAC statement 

addressing whether a 1-hour averaging time can adequately control 5 - 10 minute 

peak exposures and whether there should be a 5-minute averaging time, CASAC 

stated that the REA had presented a “convincing rationale” (Samet 2009, p. 16) 
61

for a 1-hour standard, and that “a one-hour standard is the preferred averaging 

time” (Samet 2009, p. 15). 

(5) Comment: ALA favored a rolling average rather than a block average for 

determining compliance with the NAAQS. That is, ALA stated: 

EPA has proposed a block average to determine compliance with the onehour daily maximum standard. That is, compliance would be measured 

against concentrations recorded in each hour of the day. However, this 

method of measuring compliance misses high concentrations that span the 

top of the hour. Our lungs do not differentiate between high concentrations 

that occur during hours beginning at the top of the hour, and all other 

hours in the day. To provide requisite health protection from short term 

exposures, EPA must base compliance on a rolling average for the onehour standard. Such a rolling average would be consistent with a not to be 

exceeded form of the standard (see comments provided by ALA et. al., p. 

14) 

 Response: EPA acknowledges that there are multiple ways of potentially 

determining compliance with the SO2 NAAQS. However, we disagree that a 

rolling average is needed to provide requisite protection against short-term SO2

exposures. The form of the standard being finalized is the 99th percentile of the 

distribution of 1-hour daily maximum SO2 concentrations averaged over 3 years. 

Thus, since EPA is already considering the 99th percentile of the distribution of 

the highest 1-hour SO2 concentrations in each day of the year, we believe that 

using a rolling average as opposed to a block average would result in negligible, if 

any, additional public health benefits. Moreover, we note that a rolling average 

for the 1-hour standard was not proposed 

C. Form 

(1) Comment: A number of industry groups (e.g., NAM, ASARCO) and South 

Dakota preferred a 98th percentile form. In general, their preference for a 98th

percentile form was based on their conclusion that a form based on the 98th

percentile would be more stable than a form based on the 99th percentile, and that 

a 98th percentile form is consistent with the forms selected in recent NAAQS 

reviews (i.e. PM2.5 and NO2). For example AirQuality stated: 

The Administrator should reconsider her proposal and choose instead the 

98th percentile (or equivalent nth highest value) form of the standard for 

the added reliability and stability it offers in determining compliance or 

progress towards attainment. This approach has been promulgated for 

recent revisions of the PM2.5 and NO2 standards and this consistency 

should be maintained with SO2 (see comments provided by AirQuality; p. 

1). 
62

Some groups also maintained that a 98th percentile form should be selected since a 

98th percentile form is consistent with the forms selected in recent NAAQS 

reviews (i.e. PM2.5 and NO2). 

Response: These comments and EPA’s responses were presented in detail in 

section II.F.3.b of the preamble to the final rule

(2) Comment: Dow commented that “using the 99th percentile could still cause an 

area to be considered a non-attainment area for an unusual event in one year, 

using the 98th percentile would, with the 3 year rolling average reduce this 

possibility and allow better identification of real issues where resources need to be 

deployed.” 

Response: EPA disagrees with this comment. The form of the standard is the 3-

year average of the 99th percentile of the annual distribution of 1-hour daily 

maximum SO2 concentrations. Moreover, analyses in the REA indicate that in 

most locations analyzed, the 99th percentile corresponds to the 4th highest SO2

concentration. Thus, an unusual event in one year would have to occur over at 

least four days, or at least on four different occasions (and each occasion would 

have to be on a different day). Moreover, as noted by the commenter, the use of 

the 3-year average will also mitigate this possibility. Finally, it is possible an 

unusual natural event could be considered an exceptional event and not result in 

an area being designated as being in nonattainment (see preamble to the final rule 

section VII.B). 

(3) Comment: MSCC commented that: 

In place of the 99th percentile, 3 year/1hour average proposed, 

consideration should reasonably be given instead to a standard that is 

reasonably protective of the 400 ppb threshold above, more directly 

related to it, and which recognizes that not every measured exceedence of 

a threshold concentration represents the failure of a feasible control 

strategy. This could take the form of a standard that, for example, 

identified 400 ppb as the excessive concentration for a 5-10 minute period 

in given hour, and required hourly compliance at the 97th, 98th or 99th 

percentile level for a given year, in conjunction with a requirement for 

reasonable data completeness in each calendar quarter. Replication should 

be required to eliminate flukes that could and do arise from measurement 

errors, random events such as fires, accidents, natural events, and other 

circumstances beyond reasonable human control, as well as predictable 

spikes arising from deliberate cultural events such as fireworks displays 

(see comments provided by MSCC; p. 6). 

Response: EPA disagrees with the commenter. As noted throughout the 

preamble to the final rule (e.g. see section II.E.2.b) and this RTC document (e.g., 
63

see section II.A), EPA considers the respiratory effects associated with 5- 10 

minute SO2 exposures as low as 200 ppb to be adverse to the health of asthmatics. 

Thus, we reject the idea that the form of the standard should be targeted to limit 5-

minute peaks above the 400 ppb threshold. Moreover, the form of the standard is 

the 3-year average of the 99th percentile of the annual distribution of 1-hour daily 

maximum SO2 concentrations. Thus, it is extremely unlikely that areas would be 

designated as nonattainment due to cultural events such as fireworks displays. 

(4) Comment: A number of health and environmental groups supported a 99th

percentile form, but expressed that they would prefer a more restrictive form, such 

as a no-exceedence based form. In addition, the Alexandria Department of 

Transportation and Environmental Services only recommended a no, or one 

exceedence based form. In general, these groups concluded that a more restrictive 

form would further limit the: 1) number of days an area could exceed the standard 

level and still attain the standard; and 2) the occurrence of 5-minute peaks of SO2

above benchmark levels.

Response: These comments and EPA’s responses were presented in detail in 

section II.F.3.b of the preamble to the final rule

(5) Comment: ALA generally commented that EPA favors a percentile form because 

it provides more stability than a more restrictive exceedence based form. 

Moreover, ALA indicates that EPA’s concerns over stability have no basis in fact 

and no relevance under the CAA. 

Response: ALA is incorrect in their suggestion that the primary reason EPA is 

setting a percentile based form is because of standard stability. The rationale for a 

percentile based form is discussed in detail in section II.F.3.b and II.F.3.c of the 

preamble to the final rule. In brief, this decision was based on multiple 

considerations including selecting a form of a new 1-hour standard that reflected 

that health evidence in the ISA indicating that the percentage of asthmatics 

affected and the severity of the response increases with increasing SO2

concentrations. Thus, a percentile form is appropriate in order to give due weight 

to years when 1-hour SO2 concentrations are well above the level of the standard, 

than to years when 1-hour SO2 concentrations are just above the level of the 

standard. Moreover, we note that a concentration based form is in agreement 

with CASAC advice that: “there is adequate information to justify the use of a 

concentration-based form averaged over 3 years” (Samet 2009, p. 16). 

(6) Comment: The CBD commented that: 

As a logical matter, it seems odd that EPA would reject an exceedancebased form due to its tendency to give less weight to years when 

concentrations are well above the standard, but would propose a three-year 

averaging period that could do the same thing, i.e., give diminished weight 
64

to years with high concentrations (see comments provided by the CBD p. 

6; note: footnotes have been omitted). 

CBD also commented that: 

EPA does not explain its three-year averaging proposal except to state that 

it “increases the stability of the standard.” Under the Clean Air Act, 

however, a NAAQS must be requisite to protect public health with an 

adequate margin of safety, regardless of cost and implementation issues. 

42 U.S.C. § 7409(b)(1); see also Whitman, 531 U.S. at 464-71. Therefore, 

if EPA in fact believes that a stable regulatory standard is requisite to 

protect public health, EPA should explain in its responses to comments 

why this is so, and further should explain why a three-year averaging 

period would be required to produce a stable regulatory standard (see 

comments provided by the CBD p. 7; note: footnotes have been omitted). 

Response: The commenter is correct that in part, EPA selected a concentration 

based form in order to give due weight to years when 1-hour SO2 concentrations 

are well above the level of the standard than to years when 1-hour SO2

concentrations are just above the level of the standard. However, EPA also 

considered that a concentration based form, averaged over three years, would 

likely be appreciably more stable than an exceedence based form. Selecting a 

form that is relatively stable is important to prevent locations from frequently 

shifting in and out of attainment thereby reducing public health protection by 

disrupting an area’s ongoing implementation plans and associated control 

programs. Thus, EPA notes that this decision is based on more stable standards 

providing increased public health protection and not on implementation or cost 

issues. See American Trucking Assn’s v. EPA, 283 F. 3d at 374-75.

(7) Comment: Some groups preferred the 4th highest form to the 99th percentile. For 

example, AirQuality stated that if a 99th percentile or 4th highest form is chosen 

over a 98th percentile or 7-8th highest form, than a 4th highest form is preferable. 

AirQuality states: 

The “4th” highest daily maximum concentration is preferable to the “99th” 

form in that it can provide an unambiguous determination of a “design 

value” in the face of missing data if concentrations are high. The 99th

percentile form does not provide that certainty if data are missing (see 

comments provided by AirQuality, p.2). 

Response: Appendix T Section 3.c describes two ways that a design value can 

still be considered valid even though it is incomplete. Section 3.c.i explains that 

even if the days are incomplete but the quarters are complete then the design 

value that is above the standard is valid. Also Section 3.c.ii provides for two 

diagnostic data substitution tests for use with data sets that do not meet the 75% 

requirement. These tests are complicated and may not be perceived as transparent 

to all the public, but EPA believes that this complexity is necessary and 
65

appropriate given the usefulness of the tests in allowing at least some areas to be 

clearly determined to be meeting or not meeting the NAAQS despite the data 

incompleteness. 

In addition, EPA notes that the 99th percentile was favored over the 4th highest 

form since it results in a sampling from the same part of the annual distribution of 

1-hour daily maximum SO2 concentrations regardless of the number of 1-hour 

daily maximum concentrations reported in a given year for a particular location. 

D. Level 

(1) Comment: Many industry groups (e.g., NAM, Golder, AFPA, AA, Marathon 

Petroleum) indicated that other environmental/health agencies (e.g., California 

Air Resource Board, World Health Organization (WHO)) have used the same 

health information from controlled human exposure and epidemiologic studies to 

set a 1-hour SO2 standard at a level higher than the proposed range of 50 - 100 

ppb. Thus, these groups generally contend that EPA is interpreting the health 

information too stringently. 

 

Response: EPA disagrees with these comments. The California Air Resource 

Board adopted its 1-hour standard in 1984 and retained this standard in 1995 (see 

http://www.arb.ca.gov/research/aaqs/caaqs/so2-1/so2-1.htm), while the WHO 

guidelines were last updated in 2005 (see 

http://www.euro.who.int/Document/E90038.pdf). Thus, especially with respect to 

the California 1-hour standard, it is highly unlikely that all the health information 

available in the current SO2 NAAQS review was available in these previous 

reviews. 

We also note that there are significant differences in the 1-hour standard being 

finalized by EPA and the existing WHO 10-minute recommendation. For 

example, the WHO recommendation is a ten minute recommendation equal to 

approximately 190 ppb (see http://www.euro.who.int/Document/E90038.pdf). 

Because of the difference in averaging times, in order for EPA to protect against 

similar 5-10 minute SO2 concentrations of concern with a 1-hour standard, that 

necessarily means that a 1-hour standard would have to be lower than the 190 ppb 

WHO guidelines. The WHO also has a 24-hour SO2 recommendation while EPA 

is revoking its 24-hour standard and using a 1-hour standard to protect against 

health effects associated with averaging times from 5-minutes to 24-hours. 

In addition, we note that the form of the standard EPA is finalizing differs from 

that of the California 1-hour standard. While the 1-hour standard EPA is 

finalizing is the 99th percentile of the distribution of 1-hour daily maximum SO2

concentrations averaged over 3 years, the California standard is more restrictive. 

That is, the form of the California standard indicates that the level is not to be 

exceeded (see http://www.arb.ca.gov/research/aaqs/aaqs2.pdf). Moreover, in 
66

addition to a 1-hour standard, California also has a 24-hour standard at a level of 

about 40 ppb. Thus, comparing only the levels of different short-term standards is 

often misleading. 

Finally, we note that the CAA entrusts the EPA Administrator to set standards for 

criteria pollutants that are in her judgment, adequate to protect public health with 

an adequate margin of safety, and that these standards are to be based on air 

quality criteria, not on standards established by other entities. 

(2) Comment: With regard to the controlled human exposure evidence, health and 

environmental groups generally concluded that a 1-hour SO2 standard no higher 

than 50 ppb is needed to protect against 5-minute SO2 benchmark exposures as 

low as 100 ppb identified from mouthpiece exposure studies, rather than the 200 

ppb 5-minute SO2 benchmark identified from “free breathing” controlled human 

exposure studies. More specifically, in their combined comments, ALA et. al., 

stated: 

In its analysis of data from chamber studies in the ISA and in the REA, 

EPA focuses on studies of free breathing exposure. In doing so, EPA 

improperly and arbitrarily downplays important evidence that reported 

increased airway resistance, a measure of bronchoconstriction, in subjects 

with mild asthma at concentrations of 100 ppb. Regrettably, EPA does not 

rely on the mouthpiece studies in formulating its proposed standards (see 

comments provided by ALA et. al., p. 15)...In downplaying the 

mouthpiece studies, EPA ignores the large segment of people who rely on 

oral or oronasal breathing some or all of the time (see comments provided 

by ALA et. al., p.16) 

Response: These comments and EPA’s responses were presented in detail in 

section II.F.4.b of the preamble to the final rule. Similar comments and EPA 

responses were also discussed above in section II.A.

(3) Comment: Health and environmental groups (e.g., ALA, ATS, EDF, NRDC, 

Sierra Club, CBD) and the Alexandria Department of Transportation and 

Environmental Services generally concluded that the epidemiologic evidence 

indicates that a standard no higher than 50 ppb is required to protect public health. 

For example, it its comments the CBD stated: 

Epidemiologic studies referenced in the Proposed Rule showed positive, 

and in many cases statistically significant, relationships between ambient 

SO2 concentrations and hospital admissions where 99th percentile 1-hour 

concentrations ranged from 50 - 460 ppb. Of these studies, two showed 

positive and sometimes statistically significant relationships in singlepollutant models at 50 ppb, and three studies showed statistically 

significant correlations at 78- 150 ppb in multi-pollutant models. These 

three multipollutant studies, moreover, “lend[] strong support . . . to the 

conclusion that SO2 effects are generally independent” of those of co-
67

pollutants like particulate matter. Giving these studies their proper weight, 

and allowing for an adequate margin of safety, EPA should set a one-hour 

NAAQS at a level no higher than the lowest concentration at which 

positive, adverse relationships have been demonstrated: 50 ppb (note that 

footnotes were omitted; see comments provided by CBD p.5).

Response: These comments and EPA’s responses were presented in detail in 

section II.F.4.b of the preamble to the final rule. We also again emphasize 

CASAC’s view that a 50 ppb 1-hour standard (99th percentile form) is by no 

means compelled, that no level lower than 50 should be considered, and that EPA 

could legitimately consider standards with higher levels. Samet (2009) at p. 16. 

EPA’s final determination as to level is consistent with this advice

(4) Comment: A number of industry groups did not support setting a new 1-hour 

SO2 standard. However, several of these groups (e.g., UARG, API, ASARCO) 

and the SC Chamber of Commerce concluded that, if EPA does choose to set a 

new 1-hour standard, the level of that standard should be ≥ 150 ppb. In addition, 

South Dakota and Ohio recommended standard levels at 150 ppb. As a basis for 

this recommendation, these groups generally emphasized uncertainties in the 

scientific evidence. As noted above, these commenters typically concluded that 

the available epidemiologic studies do not support the conclusion that SO2 causes 

the reported health effects. This was based on their assertion that the presence of 

co-pollutants in the ambient air precludes the identification of a specific SO2

contribution to reported effects. 

Response: These comments and EPA’s responses were presented in detail in 

section II.F.4.b of the preamble to the final rule.

(5) Comment: Industry groups (e.g., API, ABR, ASARCO) had several comments 

with respect to the epidemiologic study conducted by the NYDOH (NYDOH, 

2006). First, these groups generally concluded that the results of this study are 

mixed. That is, while SO2 effect estimates were positive and statistically 

significant even in multipollutant models with PM2.5 or NO2 in the Bronx, SO2

effect estimates were actually negative in Manhattan in both single and 

multipollutant models. These groups also contend that this report was not peerreviewed and that the authors of this study indicated that high correlations among 

pollutants in the Bronx made it difficult to confidently identify which pollutants 

are actually increasing risks. For these reasons, industry groups generally 

concluded that this study should not be relied upon by the Administrator in 

considering the appropriate level of anew 1-hour standard. 

Response: These comments and EPA’s responses were presented in detail in 

section II.F.4.b of the preamble to the final rule. Other comments and EPA 

responses with respect to this study are presented below. 
68

(6) Comment: Some groups (e.g., ASARCO, ABR) also commented that the 

NYDOH (NYDOH, 2006) results are uncertain because it was noted by the 

authors that the SO2 concentrations used to estimate relative risk associated with 

SO2 exposure were arbitrary (see ASARCO comments, p.6). Others commented 

(e.g., ACSBPP) that the lack of association in Manhattan is not consistent with a 

causal SO2 impact. Furthermore, others commented that the discrepant results in 

the Bronx and Manhattan could be explained by residual confounding by other 

factors such as correlation with other pollutants or smoking prevalence, which 

was not controlled for in the analysis. 

Response: These commenters misconstrued or misunderstood the text when they 

claimed the SO2 concentrations used to estimate the relative risk associated with 

SO2 exposure were arbitrary. On Page 21 of the NYDOH study (2006), the 

authors state, “It should be noted that the choice of concentration increments used 

to compute the RRs is an arbitrary one.” The commenters failed to include the 

word “increments” in their comments. What the study authors are referring to is 

that they present their results per some incremental change in exposure 

concentration. The ISA standardizes all of the studies so that effect estimates 

describing 24-hour average SO2 concentrations all have an incremental change of 

10 ppb and effect estimates describing 1-hour maximum SO2 concentrations all 

have an incremental change of 40 ppb. It is also common in the peer-reviewed 

literature to see investigators use the mean concentration, the interquartile range 

of concentrations, or the standard deviation as the incremental change. In this 

case, the authors use the mean SO2 concentration averaged over the two study 

sites (11 ppb; See Table 3, page 50 of NYDOH, 2006). What’s more, they 

present a comparison of relative risks computed using alternative concentration 

increments in Table 4b (page 52 of NYDOH, 2006). No where do the authors 

claim that the SO2 concentrations themselves were arbitrary. 

With respect to the lack of association in Manhattan, EPA notes that there were 

29,987 asthma visits in the Bronx during the study period, and only 5,014 asthma 

visits in Manhattan during this same period (See Table 1, page 38 of NYDOH, 

2006). The much smaller sample size in Manhattan makes it more difficult to 

detect a statistically significant association when compared to the sample size 

available in the Bronx. Additionally, the demographics of the population in the 

Bronx and Manhattan differ by age, race and socioeconomic status, all of which 

may be susceptibility factors that would explain why an association was observed 

in the Bronx and not Manhattan. 

In addition, we note that the authors acknowledge that "Variation in effects of 

unmeasured co-pollutants, such as indoor allergens, environmental tobacco smoke 

or local traffic and industrial emissions, might also influence the apparent 

differences in acute asthma ED responses to ambient air pollution observed in the 

two communities. Increased exposure to such local measured pollutants could 

directly increase baseline asthma morbidity and might also indirectly increase the 

response to changes in ambient air pollutants by increasing airway inflammation 
69

and hyper-responsiveness to acute airway irritants. Data were not available to 

address these possible effects in this report." In fact, by the nature of the study 

design, these potential confounders are not able to be measured on an individual 

level in any time-series studies of ED visits or hospital admissions. Thus this 

limitation applies to all time-series studies and is not specific to NYDOH, 2006. 

In light of this limitation, the authors still conclude that "The results suggest that 

the criteria pollutants PM2.5, SO2, O3 and NO2 had a statistically detectable impact 

on acute asthma ED visits in a community with a relatively high baseline rate of 

acute asthma exacerbations. In two-pollutant and three-pollutant regression 

models, O3 and SO2, and to a lesser extent maximum one-hour PM2.5, were the 

most robust pollutants. In other words, these pollutants exhibited less change in 

their effect estimates as additional pollutants were added to the models." 

Additional discussion with respect to the apparent discrepancy in results in the 

Bronx and Manhattan study areas can be found in section II.F.4.b of the preamble 

to the final rule.

(7) Comment: Some groups (e.g., ASARCO, ABR) also commented with respect to 

the NYDOH (NYDOH, 2006) analysis that the Administrator failed to note that 

the positive association in the Bronx was observed at biologically implausible lag 

times of 3 or 4 days but not at 0-2 days. 

Response: The commenters are incorrect in stating that the associations observed 

in the Bronx were observed at "biologically implausible lag times of 3 or 4 days, 

but not at 0-2 days". In Table 4a (page 51, NYDOH, 2006) the authors present 

the relative risk for the "5-day mean" which is the average of days 0, 1, 2, 3, and 

4. As noted in the response to Comment 12 in section II.B, EPA relies heavily on 

the effect estimates for the average of multiple days. Perhaps the commenters 

were referring to Figure 8 (page 71, NYDOH, 2006) which presents the relative 

risk for each of the single day lags from 0 to 4. Here, there are elevated relative 

risks on lag day 0, 2, 3, and 4 with the ones on days 2, 3, and 4 appearing to be 

statistically significant. Single lag day 2 is statistically significant and has the 

relative risk of greatest magnitude. This response is not biologically implausible. 

(8) Comment: With respect to Ito et al., (2007), industry groups generally 

commented that since the SO2 effect estimate did not remain statistically 

significant in multipollutant models with NO2, this study does not indicate an 

independent effect of SO2 on emergency department visits in the NYC study area. 

API specifically commented: 

 

The RR for an increase of 6 ppb SO2 was statistically significant (1.20; 

95% CI: 1.13, 1.28) and remained so when PM2.5, O3, or CO was included 

in the model, but became nonsignificant when NO2 was included in the 

model (RR not provided, 95% CI: 0.9, 1.1). Because associations with SO2

could be attributable to NO2, this study cannot be used to assess the effects 
70

of SO2 on health effects with small incremental increases in exposure (see 

comments provided by API; p. B-4). 

Response: These comments and EPA’s responses were presented in detail in 

section II.F.4.b of the preamble to the final rule

(9) Comment: With respect to Ito et al., (2007), ABR commented that the authors 

state: “ 

SO2 and CO showed lower monitor to monitor correlation and low withincity precision of the mean levels, indicating that these pollutants’ risk 

estimates could be biased in short term health effects models (see 

comments by ABR; p.5) 

ABR also notes that the authors concluded the study with a precaution against 

using multipollutant models in health analyses. 

Response: EPA agrees with Ito et al. (2007) that the lower monitor correlations 

and within-city precision could bias short-term health effects models. But as 

addressed in the response to other comments (see section II.B), EPA maintains 

that measurement errors in community time-series and panel epidemiologic 

studies are more likely to underestimate the strength and the significance of any 

association between SO2 and adverse health effects, thereby decreasing the 

likelihood of an association reaching statistical significance and likewise 

decreasing the possibility of a false identification of an association (Section 

2.6.4.4, p. 2-63). Thus, the measurement error makes it more difficult to detect a 

positive and statistically significant effect estimate, even when such an 

association exists. Moreover, the signal that drives statistical associations 

between ambient concentrations and health effects in time-series studies is the 

day-to-day changes in concentration, not the absolute daily values. Appropriately 

located central SO2 monitors can adequately characterize such day-to-day 

changes. Exposure studies have found statistically significant regression slopes 

between personal exposure to SO2 and ambient SO2 concentrations at fixed-site 

monitors (Section 2.6.3.2, p. 2-56), indicating that fluctuations in ambient 

concentration are an important driver of fluctuations in SO2 exposure. 

EPA agrees that multipollutant models should be used and interpreted cautiously 

in health analyses. Currently, these multipollutant models are the most common 

tools used by investigators to try to disentangle the effects of individual criteria 

pollutants. When examined across the body of evidence, EPA believes that the 

results of the multipollutant models can be informative about the robustness of 

associations observed in epidemiologic studies. 

(10) Comment: With respect to Schwartz (1995), industry groups generally 

commented that the results of this study are mixed, and therefore should not be 
71

considered by the Administrator. More specifically, these commenters noted that 

although the results in New Haven remained statistically significant in the 

presence of PM10, the SO2 effect estimate in Tacoma was reduced and no longer 

statistically significant in the presence of PM10. Commenters also noted that in 

both cities, the SO2 effect estimate was reduced and no longer statistically 

significant in the presence of O3. 

Response: These comments and EPA’s responses were presented in detail in 

section II.F.4.b of the preamble to the final rule

(11) Comment: Some commenters indicated that because EPA did not find the 

epidemiologic evidence adequate enough to support a quantitative risk assessment 

based on epidemiology, then these studies should not be relied upon for informing 

the level of a standard. 

Response: EPA disagrees with this comment. Using epidemiologic studies for 

the purpose of a quantitative risk assessment is a separate issue from using air 

quality information from locations in which epidemiologic studies were 

conducted to help inform the level of a new 1-hour standard. EPA judged that a 

quantitative risk assessment using concentration-response functions derived from 

any one epidemiologic study would not produce quantitative results that would be 

particularly useful in the decision making process. However, although no one 

epidemiologic study was appropriate for use in a quantitative risk assessment, 

EPA concluded that looking at SO2 air quality in the ten locations U.S. 

epidemiologic studies reported positive associations between ambient SO2 and 

emergency department visits or hospitalizations was informative with respect to 

setting the level of a new 1-hour standard. 

 

(12) Comment: Dow commented that “if EPA opts to set a 1-hour NAAQS for SO2, 

the expected difference between "monitor locations of expected maximum shortterm concentration" and area-wide concentrations should be considered in 

establishing the standard (see comments provided by Dow; p.2). 

Response: EPA set the level of a new 99th percentile 1-hour daily maximum SO2

standard at 75 ppb. In doing so, EPA considered that the monitors in three 

locations where epidemiologic studies (i.e., NYDOH (2006), Ito et al., (2007), 

Schwartz (1995)) reported statistically significant associations in multipollutant 

models with PM may not have recorded the highest 99th percentile 1-hour daily 

maximum SO2 concentration across a given study area. This consideration is 

described in section II.F.4.c of the preamble to the final rule. 

(13) Comment: In general, EPRI commented that using the epidemiology to set a 1-

hour standard is problematic because few of these studies considered a 1-hour 

averaging time 
72

Response: EPA acknowledges that many of the epidemiologic studies considered 

in the ISA and REA used longer averaging times than 1-hour (e.g., 24-hour, 3-

hour). However, we disagree with that using 1-hour air quality information from 

the areas these epidemiologic studies were conducted to inform a new 1-hour 

standard is problematic. EPA notes that just because a particular epidemiologic 

study used a longer averaging time, it does not rule out the possibility that the 

effects reported in those studies may be associated with shorter-term SO2

concentrations. In fact, when describing epidemiologic studies observing positive 

associations between ambient SO2 and respiratory symptoms, the ISA stated “that 

it is possible that these associations are determined in large part by peak 

exposures within a 24-hour period” (ISA, section 5.2 at p. 5-5). EPA notes that 

this assertion is based on both the large body of controlled human exposure 

studies reporting respiratory effects following 5 - 10 minute SO2 exposures, as 

well as a body of epidemiologic studies reporting positive associations between < 

24-hour ambient SO2 concentrations and respiratory morbidity endpoints. 

(14) Comment: In addition to generally concluding that the epidemiology is too 

uncertain to demonstrate that SO2 has an independent effect on the respiratory 

effects reported in those studies, many industry groups (e.g., API, ACC, PE, EEI, 

CIBO, CRA, Pepper Hamilton, RMA, Louisiana Chemical, A&B, TFI) also 

generally commented that adverse health effects do not occur following 5-10 

minute SO2 exposures < 400 ppb in controlled human exposure studies (an issue 

also discussed above in final rule section II.E.2). Thus, these groups generally 

maintained that the level of a 1-hour standard should not take into account 

limiting 5-minute peaks ≤ 200 ppb. From this argument, many of these groups 

further maintained that 1-hour standard levels ≥ 150 ppb are requisite to protect 

public health with an adequate margin of safety.

Response: These comments and EPA’s responses were presented in detail in 

section II.F.4.b of the preamble to the final rule.

(15) Comment: Both industry (e.g., Rio Tinto) and environmental/health groups (e.g., 

ALA et al.,) referenced the 5-minute peak to 1-hour mean ratios in REA Table 10-

1 as a rationale for selecting a specific level for a 1-hour SO2 standard to 

adequately protect against 5-minute SO2 concentrations of concern (notably, 

industry was generally opposed to a new 1-hour standard, but they still suggested 

standard levels in the event the Administrator chose to adopt such a standard). 

Response: EPA did not use the 5-minute peak to 1-hour mean ratios from REA 

Table 10-1 in its consideration for the level of a new 1-hour SO2 standard. 

Rather, EPA used these ratios as an initial consideration for an appropriate 

averaging time for a new short-term SO2 standard. As noted in the proposal, this 

information suggested that a 1-hour averaging time was likely to be more efficient 

at limiting 5-minute concentrations of concern than a 24-hour averaging time. As 

stated in footnote 19 of the preamble to the proposal: 
73

The analysis of peak to mean ratios was used as an initial screen to 

evaluate which averaging times could be suited to control 5-minute peaks 

of SO2. The more sophisticated analysis for ultimately determining that a 

one-hour averaging time set at an appropriate level could effectively limit 

these 5-minute peaks was the air quality, exposure, and risk analyses 

discussed in section II.F.4. (74 FR 64831) 

That is, EPA reasonably based its decision on the appropriate level for limiting 5-

minute concentrations of concern identified from controlled human exposure 

studies on the sophisticated air quality, exposure, and risk analyses presented in 

detail in chapters 7 – 9 of the REA and the summary of the estimates from those 

analyses presented in sections II.F.4.b and II.F.4.c of the preamble to the proposal 

(see especially Tables 2 - 4). However, as previously noted EPA did not place the 

same weight on all of these analyses. For example, EPA placed relatively more 

weight on the St. Louis exposure analysis than it did on the St. Louis quantitative 

risk assessment (see final rule preamble section II.E.2.c). 

We also note that the ratios presented in Table 10-1 are from a small number of 

monitors in areas voluntarily reporting 5-minute SO2 concentrations from the 

years 2004 -2006. Moreover, not all the monitors in Table 10-1 reported 5-

minute concentrations during the entire three year period (see REA, Table 10-1). 

Thus, while EPA finds this data was reasonable to use as an initial screen for an 

appropriate averaging time, we do not think it is a sufficient basis for selecting the 

level of a new 1-hour standard to protect against 5-minute concentrations of 

concern. Again, EPA relied on the more sophisticated air quality, exposure, and 

risk analyses as they indicate the extent to which a given level of a 1-hour 

standard limited 5-minute concentrations of concern. 

E. Retaining or revoking the current SO2 NAAQS 

(1) Comment: Public health (e.g., ALA, ATS) and environmental organizations 

(e.g., CBD, WEACT) were generally opposed to revoking the current 24-hour and 

annual standards. These groups generally concluded that the 24-hour standard 

should be revised while the annual standard should be retained. In support of this 

position, ALA et al., cited air quality information from the REA indicating that if 

air quality was simulated to just meet a 99th percentile 1-hour daily maximum 

standard in the proposed range of 50 – 100 ppb, then in some locations analyzed, 

99th percentile 24-hour average SO2 concentrations would be above 

concentrations (i.e., above 99th percentile 24-hour average concentrations) in 

cities where U.S. emergency department visit and hospital admission studies 

reported positive associations with SO2. In addition, many of these groups were 

opposed to revoking the current annual standard. In general, these groups 

concluded that given the uncertainties associated with SO2 exposure and longterm health effects, EPA should err on the side of being health protective and 

retain the existing annual standard. 
74

Response: These comments and EPA responses are discussed in detail in final 

rule section II.F.5.b. 

(2) Comment: NAACA, NESCAUM, and Vermont, while supportive of revoking 

the existing standards, also suggested EPA explore setting a new 24-hour standard 

to minimize the potential that multiple hours within a day would exceed a 1-hour 

standard 

Response: EPA generally agrees with the commenters that given the form of the 

standard selected, a 99th percentile daily maximum, there is potential for multiple 

hours within a day that would exceed the 1-hour level selected for that standard. 

However, given typical patterns in air quality data, the frequency of days having 

multiple hours above a given 1-hour level is likely rare. We also again emphasize 

CASAC’s conclusion that “that a one-hour standard is the preferred averaging 

time.” Samet (2009) at p. 15. 

(3) Comment: The CBD commented that: 

In proposing to revoke the 24-hour standard, EPA appears to assume that 

health effects found in epidemiologic studies based on 24-hour exposures 

are driven entirely or primarily by very short-term (five- to ten-minute) 

peak exposures. This assumption, however, is not borne out by the 

evidence (see comments provided by the CBD p. 7). 

Response: EPA provided a response to similar comments in final rule preamble 

section II.F.5.b. We further note that EPA’s rationale does not assume that all 24-

hour associations in epidemiologic studies are primarily being driven by 5- 10 

minute peak SO2 concentrations. Rather, it reasonably considers that the overall 

body of health evidence suggests that respiratory effects following SO2 exposure 

may be most related to averaging times ≤ 1-hour (see preamble section II.F.5.b). 

(4) Comment: The CBD commented that EPA should better explain why it proposed 

to revoke the current annual standard. The CBD also noted that: 

 

CASAC points out that EPA has not focused on causality in the same way 

in reviewing the health effects of other pollutants. EPA’s proposal also 

may run afoul of a long line of cases holding that a NAAQS must not only 

protect against known hazards, but also must protect against uncertain and 

incompletely understood hazards. See Am. Trucking Ass’ns, 283 F.3d at 

369; Am. Lung Ass’n, 134 F.3d at 389; Lead Indus. Ass’n, 647 F.2d at 

1152-55. (see comments provided by the CBD p. 8; note: footnotes have 

been omitted) 

Response: EPA provided its rationale for revoking the annual standard in final 

rule preamble section II.F.5.b. We note further, that although the commenter is 
75

correct that the adequate margin of safety standard affords authority to EPA to act 

to protect against uncertain or incompletely understood hazards, EPA is not 

compelled to act when there is evidence that no hazard exists. The ISA finds that 

there is inadequate evidence to infer a causal relationship between morbidity and 

mortality from long-term SO2 exposure. ISA p. 5-6; see also ISA p. 5-13 

explaining the lack of evidence of causality between long-term SO2 exposure and 

particular health end points. See American Farm Bureau Federation v. EPA, 559 

F. 3d 512, 538-39 (D.C. Cir. 2009) (“The EPA reasonably decided that an annual 

coarse particle standard is not necessary because, as the Criteria document and the 

Staff paper make clear, the latest scientific data do not indicate that long-term 

exposure to coarse particles poses a health risk”). 

With respect to CASAC, the CBD provided the following information in a 

footnote: 

See Samet 2009 at 9 (“Some discussion is needed that indicates why the 

risk assessment for this pollutant, in contrast to others, is limited only to 

health effects that are classified as sufficient to infer causality.”). 

EPA notes that we placed primary emphasis on those health endpoints for which 

the ISA concluded the strongest evidence. More specifically, the preamble to the 

final rule states (see introduction to section II): 

In reaching this decision, EPA has drawn upon an integrative synthesis of 

the entire body of evidence on human health effects associated with the 

presence of SO2 in the ambient air, and upon the results of quantitative 

exposure and risk assessments reflecting this evidence. As discussed 

below, this body of evidence addresses a broad range of health endpoints 

associated with exposure to SO2 in the ambient air. In considering this 

entire body of evidence, EPA chose to focus most on those health 

endpoints for which the ISA found the strongest evidence of an 

association with SO2 (see section II.B below). Thus, the rationale for this 

final decision on the SO2 NAAQS focused primarily on respiratory 

morbidity following short-term (5-minutes to 24-hours) exposure to SO2, 

for which the ISA found a causal relationship. 

(5) Comment: Amanda L. Matthews urged EPA to retain the annual standard in 

order to protect against long-term SO2 effects on prenatal/neonatal health 

outcomes. She contends that EPA ignored several key long-term SO2 exposure 

studies with regard to these health outcomes. In particular: Hoppenbrouwers 

(1981) with regard to Sudden Infant Death Syndrome (SIDS); Liu et al., (2003) 

with regard to increases in pre-term delivery and low birth weight; and Dales et 

al., (2004) with regard to SIDS

Response: As noted by the commenter, EPA found the association between 

ambient SO2 and long-term health effects to be inadequate to infer the presence or 
76

absence of a causal relationship. That is, the ISA found the long-term health 

evidence to be of insufficient quantity, quality, consistency, or statistical power to 

make a determination as to whether SO2 was truly associated with these health 

outcomes (ISA, Table 1-2). Notably, this included an evaluation of the possible 

associations between long-term (weeks to year) exposure to SO2 and adverse 

prenatal and neonatal outcomes. EPA further notes that the general scientific 

conclusions of the ISA were reviewed and accepted by CASAC. With regard 

specifically to the studies mentioned above, EPA did consider Liu et al., (2003) as 

well as Dales et al., (2004), but still found the overall breadth of the information 

relating long-term exposure to SO2 and adverse prenatal and neonatal outcomes 

inadequate to infer the presence or absence of a causal relationship. With respect 

to the study by Hoppenbrouwers et al. (1981), this study was not included in the 

final ISA because the focus of the document was on recent scientific studies, and 

re-evaluation of only some key older studies (primarily key older controlled 

human exposure studies). Additionally, the lag time used in the Hoppenbrouwers 

et al. (1981) study was 7 weeks, which is not consistent with any known 

mechanism by which SO2 may cause adverse health outcomes. That being said, 

EPA can confidently state that inclusion of this study in the ISA would not have 

changed the final conclusion with respect to prenatal and neonatal outcomes. 

(6) Comment: Amanda L. Matthews urged EPA to retain the annual standard 

generally commenting that a 1-hour standard in the proposed range will likely not 

prevent annual SO2 concentrations from exceeding the level of the current annual 

standard. 

Response: EPA disagrees with this comment. Table 10-4 of the REA gives a 

clear indication that 99th percentile 1-hour daily maximum standards in the range 

of 50 -100 ppb (and indeed up to 150 ppb) will likely prevent annual SO2

concentrations from exceeding the level of the current annual standard (i.e. 

approximately 30 ppb). In fact, Table 10-4 estimates that given a 100 ppb 

standard, the highest annual SO2 concentration in any county analyzed in this 

table would only be 15.5 ppb. Thus, a standard level of 75 ppb would be 

expected to keep annual SO2 concentrations even lower. EPA notes that REA 

Appendix Tables D-5 and D-6 show similar information. 

(7) Comment: Health and environmental groups urged EPA to maintain or lower the 

existing 24-hour standard. As evidence that both a standard designed to limit 5-

10 minute peaks of SO2 and a standard to limit 24-hour average concentrations of 

SO2 is needed, some of these groups cite SO2 WHO guidelines and standards in 

SO2 standards in California. For example, ALA states: 

...both World Health Organization and the state of California recommend 

or maintain both a short-term (hourly or 10-minutes) and a 24-hour 

standard. The World Health Organization has recommended both a 10 

minute standard and a 24-hour standard. Their 24- hour standard 

recommendation is for a standard of 7 ppb, far more stringent than the 
77

U.S. standard, and their recommendation for a 10-minute standard is 175 

ppb. The California Air Resources Board has established a 24-hour 

standard of 40 ppb, not to be exceeded. This standard is in addition to the 

state‘s one-hour standard for SO2 (note that footnotes have been omitted; 

see comments provided by ALA et. al., p. 9). 

Response: As noted above, the CAA entrusts the EPA Administrator to set 

standards for criteria pollutants that are in her judgment, adequate to protect 

public health with an adequate margin of safety, and that these standards are to be 

based on air quality criteria, not on standards established by other entities. 

That being said, EPA notes that a 99th percentile 1-hour daily maximum standard 

set at a level of 75 ppb is expected to limit 5-minute to 24-hour SO2

concentrations associated with respiratory morbidity effects in both epidemiologic 

and controlled human exposure studies. See sections II.F.2.b, II.F.2.c, II.F.4.b 

and II.F.4.c of the final rule preamble for more detail. 
78

 

V. Technical Issues with Monitoring Requirements 

 (1) Comment: LEC expressed concern that EPA’s proposed two-pronged monitoring 

network based on the Population Weighted Emissions Index (PWEI) metric and a 

state’s contribution to the national SO2 emissions inventory would likely result in 

monitors near major sources where SO2 “hot spots” are likely and fail to provide 

sufficient monitors in non-urban and rural areas to appropriately characterize 

background concentrations for permitting purposes. LEC suggests that EPA 

encourage states to site area-wide monitors to establish background concentration 

for use in permitting and to track air quality trends. When a state does not have 

an appropriate background monitor, LEC included suggestion on using sourceoriented monitoring data to estimate background. 

Response: As explained in sections III and IV of the preamble to the final rule, 

EPA intends to use a hybrid approach that combines monitoring and modeling, 

using each of these analytic tools where they are most appropriate and effective. 

As a result, EPA has revised the scope of the monitoring network is revised so 

that it is no longer required to be source-oriented in nature (i.e., “hot-spot” 

monitoring only). Instead the final monitoring network design now has the 

flexibility to address multiple monitoring objectives including source 

characterization, highest concentration, population exposure, general background, 

or regional transport. As a result of this flexibility, the commenter’s concern 

should be alleviated because, as part of the required monitoring network, states 

may site monitors to address the need for measuring background concentrations. 

(2) Comment: RRI recommended EPA change its monitoring guidance in order to 

“allow the use of emission distributions in a probabilistic implementation of 

compliance modeling for short-term probabilistic standards such as the 1-hour 

SO2 NAAQS.” 

Response: Under a hybrid analytical approach, EPA anticipates using both 

ambient monitoring and the modeling of SO2 sources in the implementation of the 

revised NAAQS. EPA anticipates that dispersion modeling will be used to assess 

impacts of large and medium sized sources. Therefore, as indicated in the final 

rule, EPA intends to develop guidance for the use of refined air quality dispersion 

modeling in implementing the new SO2 1-hour NAAQS and the issue of how to 

appropriately characterize emissions will be addressed in that guidance, as 

needed. 

(3) Comment: NACAA provided comments urging EPA to provide greater flexibility 

in the siting requirements for new SO2 monitors in order to avoid requiring the 

installation of redundant SO2 monitors and allow for states to use a broader range 

of criteria to determine the best locations for SO2 monitors. NACAA also included 

suggestions on when duplicative monitors could be removed. NYSDEC also 

noted that the two prong approach could result in duplicative monitoring. 
79

Response: Under a hybrid analytical approach, introduced in section III of the 

preamble to the final rule, the monitoring network is no longer required to be 

source-oriented in nature. As further explained in section IV.B of the preamble, 

the required monitoring network has the flexibility to have monitoring sites 

characterizing one or more of the following objectives: source characterization, 

highest concentration, population exposure, general background, or regional 

transport (discussed in section IV.B.3 of the preamble). EPA believes this 

flexibility alleviates much of the concern raised over redundant or unnecessary 

monitoring and, therefore, it is unnecessary to address the specific suggestions on 

reducing such duplicative monitoring. 

IV.A: Comments on Proposed Monitoring Program 

 (1) Comment: NYSDEC, noting the significant shift in SO2 emissions over the last 

three decades away from small and mobile emitters of SO2 to large, stationary 

sources, expressed concern in its comments that the “proposed SO2 monitoring 

network design does not adequately address this source transition and will not 

ensure that the largest point sources are adequately characterized.” To address 

this source transition effectively, NYSDEC recommended using a CBSA 

emissions threshold (50,000 tpy) and a point source emissions threshold (20, 000 

tpy) instead of a PWEI or state’s contribution to SO2 emissions approach for 

siting monitors. NESCAUM made similar threshold recommendations. 

Response: Under a hybrid analytical approach, introduced in section III of the 

preamble to the final rule, and further explained in section IV,V, and VI of the 

preamble, EPA anticipates using both ambient monitoring and the modeling of 

SO2 sources to assess compliance with the new 1-hour SO2 NAAQS. We believe 

that for a short-term 1-hour standard it is more technically appropriate, efficient, 

and effective to use modeling as the principal means of assessing compliance for 

medium to larger sources, and to rely more on monitoring for groups of smaller 

sources and sources not as conducive to modeling. As a result, EPA anticipates 

that SO2 sources will be adequately characterized by either, or both of, ambient 

monitoring and source modeling. 

(2) Comment: Indiana urged EPA to emphasize the placement of SO2 monitors in 

“areas that record higher values and away from those found to be below the level 

of the proposed NAAQS.” 

Response: Under EPA’s anticipated use of a hybrid analytical approach, as 

introduced in section III of the preamble to the final rule and the monitoring 

network design described in section IV, the final monitoring network design is 

more flexible than the proposed network design. The monitor objectives of the 

final network design are broadened in the final rule to include assessment of 

population exposure, general background concentrations, SO2 transport, long-term 

trends, and source oriented and/or highest concentration data. This increased 
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flexibility gives states the ability to select areas where they believe ambient 

monitoring is most warranted. As a result, we believe that the final network 

design allows states to emphasize placement of SO2 monitors in “areas that record 

higher values and away from those found to be below the level of the proposed 

NAAQS” as the commenter suggested. 

(3) Comment: The City of Alexandria expressed concern in its comments that, while 

an increased focus on monitoring SO2 concentrations in high impact areas is a 

positive change by EPA, Alexandria’s own experience in air quality monitoring 

“shows that local monitoring should only be used as a backstop, in order to assist 

in compliance verification, and only after air quality model-based limitations are 

developed and implemented by the large stationary sources that have the potential 

to contravene NAAQS protections.” Under the proposed network design, while 

new SO2 monitors could be placed to identify areas of localized nonattainment 

caused by discrete stationary sources, the commenter believes that sole reliance 

on these locally-sited monitors is misplaced. The commenter contends this is 

because monitoring methods cannot fully and comprehensively identify all areas 

of non-compliance caused by a single source’s maximum potential emissions, for 

all of the possible local meteorological conditions. Instead, the commenter 

suggests using modeling major sources to demonstrate compliance with the SO2

NAAQS. 

Response: Partly in response to these and other comments, and a re-examination 

of our historical approach to SO2 NAAQS implementation, we now anticipate 

using a hybrid analytic approach combining the use of monitoring and available 

modeling to assess compliance with the new 1-hour SO2 NAAQS. EPA discusses 

its revised intended approach in chapters III, IV, V, and VI of the preamble to the 

final rule, EPA believes that under a hybrid analytical approach, the commenter’s 

concerns should be addressed. 

(4) Comment: NACAA stated that “EPA and NACAA jointly developed a National 

Monitoring Strategy that provided a framework for operation and maintenance of 

the national air monitoring networks. The primary purpose of the National 

Monitoring Strategy was to gain a better understanding of the relationships and 

potential reaction in the atmosphere of various pollutants, develop a strategy to 

apply the lessons learned and use the information developed for a variety of 

purposes, not just for determining attainment/non-attainment.” NACAA 

concluded that it is “...concerned that EPA appears to be changing the focus of 

the monitoring program as currently described in the National Monitoring 

Strategy from recording ambient measurements to measuring emissions from 

sources, based on the more source-oriented monitoring lead, NO2 and SO2 

proposals.” 

Response: Under a hybrid analytical approach, introduced in section III of the 

preamble to the final rule, EPA anticipates using both ambient monitoring and 

source modeling in implementing the SO2 NAAQS (fully discussed in sections 
81

III, IV, V and VI of the preamble). As a result, the required monitoring network 

is no longer required to be source-oriented in nature, as explained in section IV.B 

of the preamble. The required monitoring network has the flexibility to have 

monitoring sites serve multiple monitoring objectives. EPA believes that under 

this approach, there is no longer cause for concern that SO2 monitoring sites can 

not be used in the multi-pollutant paradigm that the monitoring strategy promotes, 

and that the NCore network exemplifies. Further, NCore monitoring sites that 

measure SO2 are allowed to satisfy minimum monitoring requirements resulting 

from this rulemaking. 

 (5) Comment: Several commenters (ALA/EDF/NRDC/SC, CBD, and Missouri) 

urged EPA to revise its approach to determining how many SO2 monitors are 

required in each state to require more monitors. These commenters asked that 

more SO2 monitors be required in order to more directly monitor air quality 

around the nation’s many large SO2 emission sources. For example, 

ALA/EDF/NRDC/SC argues that “the proposed 348 monitors are grossly 

inadequate to detect peak concentrations from the 4,400 stationary sources, and an 

unknown number of area sources, which emit more than 50 tons of sulfur dioxide 

per year.” 

Response: To address these and other comments, EPA re-examined its proposed 

monitoring-focused approach and its historical approach to SO2 NAAQS 

implementation. As a result, EPA intends to use a hybrid analytical approach, as 

introduced in section III of the preamble to the final rule. Under this approach, 

EPA anticipates using both ambient monitoring and source modeling thus 

allowing more individual sources to be addressed, while maintaining a stable 

monitoring network, which can serve multiple monitoring objectives. EPA 

believes a hybrid analytical approach as described in sections III, IV, V, and VI 

addresses the concern raised by these commenters that the proposed monitoring 

network would be inadequate to detect peak concentrations from a large number 

of stationary sources. 

(6) Comment: NESCAUM commented that it does “not support EPA’s proposed 

requirement that any PWEI- or CBSA-triggered monitors ‘shall not count toward 

satisfying any required monitors resulting from the state emissions triggered 

requirements’ (74 FR 64880). Additional SO2 monitors should only be required if 

the CBSA-triggered monitor count is less than the state emissions triggered 

count.” NESCAUM also recommended exempting states from the emissionstriggered monitoring requirement if they had SO2 inventory of less than 0.1% of 

the national inventory. 

Response: In this comment, NESCAUM refers to the second prong of the 

proposed monitoring network which required minimum number of monitors 

based on the state’s contribution to the national SO2 emissions inventory. In the 

final monitoring network design, explained in section IV.B.4 of the preamble to 
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the final rule, EPA did not retain this second prong to the network design. As a 

result, the concerns raised by this commenter should no longer exist. 

(7) Comment: South Carolina submitted comments noting that the agency “believes 

the number of monitors, distributed as proposed, is greater than needed to provide 

a network sufficient to provide reporting to the general public and support 

compliance with the standards in areas where there is a reasonable potential to 

exceed the proposed standard...The number and allocation of monitors in the 

proposal using the PWEI appears to miss some high emission density areas and 

require monitoring in many locations unlikely to experience significant impact.” 

Response: The statement that “...the number of monitors, distributed as proposed, 

is greater than needed...” implies that the proposed network was viewed to be 

redundant and would have required unnecessary monitoring in certain areas. 

Under a hybrid analytical approach, introduced in section III and further 

explained in section IV.B of the preamble to the final rule, the number of required 

monitors is lessened, the monitoring network is no longer required to be sourceoriented in nature, and the network objectives of the final network design are now 

broadened to include assessment of population exposure, general background 

concentrations, SO2 transport, long-term trends, and source oriented and/or 

highest concentration data. As a result, the required monitoring network has the 

flexibility to have monitoring sites characterizing one or more of these objectives. 

EPA believes this flexibility alleviates the concern raised over redundant or 

unnecessary monitoring. 

The commenter also suggested that the proposed allocation of monitors based on 

the use of the PWEI may “...miss some high emission density areas and require 

monitoring in many locations unlikely to experience significant impact.” As 

explained in the preamble, EPA believes where high emissions density occurs due 

to large sources, the modeling component of our anticipated hybrid approach 

would be more technically appropriate and effective in assessing ambient SO2

concentrations and compliance with the SO2 NAAQS. In addition, states always 

have the prerogative to monitor where they believe monitoring is warranted above 

minimum monitoring requirements. Further, as discussed in section IV.B.6 of the 

preamble to the final rule, EPA Regional Administrators (working with states) 

may require additional monitoring above the minimum requirements if they 

believe monitoring objectives remain unfulfilled in an area even though a state 

may be satisfying minimum monitoring requirements. The commenter also 

suggested that the proposed monitoring approach could lead to “...monitoring in 

many locations unlikely to experience significant impact.” The final network 

design, discussed in section IV.B.4 in the preamble to the final rule, utilizes the 

PWEI, but with some changes in the breakpoints used to require monitors, 

ultimately requiring lesser number of monitors in the final network than by the 

proposed use of the PWEI. EPA believes that the anticipated use of modeling, the 

states and RA’s authority to require monitoring above the minimum requirements, 

and the changes to the PWEI breakpoints addresses the commenters concern. 
83

(8) Comment: City of New York commented that New York City presently has three 

SO2 monitors located in its metropolitan area and urges EPA to ensure that the 

final SO2 monitoring plan does not reduce the number of required monitors in the 

New York City area.

Response: Under the final network design, discussed in section IV.B.4 in the 

preamble to the final rule, and based on the 2005 NEI and 2008 Census data, EPA 

estimates that the New York City CBSA has a PWEI value that will require it to 

operate at least 3 monitors within the CBSA. 

IV.B: Comments on Monitoring Method 

(1) Comment: Two industry groups provided comments noting that, while EPA is 

proposing a significant change in the permitted total interferent limit for the new 

FRM, the interferent limits need to be further tightened in order to reduce the 

potential for false positive exceedances of the SO2 NAAQS. Given the stringent 

SO2 standards being proposed, AirQuality recommends “a more realistic 

specification would limit each interferent to no more than 3 ppb and total 

interference to no more than 12 ppb.” UARG recommends EPA “further limit the 

total of all interferents” without providing a specific number. NESCAUM also 

submitted a comment requesting EPA “tighten the NO interference metric for the 

new SO2 Federal Reference Method from 100 to 300 to 1. The proposed value of 

100 results in substantial NO interference at sites with low SO2 levels in urban 

areas, such as NCORE sites.” 

Response: As discussed in section IV.A.2.b of the preamble to the final rule, EPA 

reconsidered what is reasonably feasible with current technology to determine the 

final interferent equivalent limit requirements. In evaluating the limits, we must 

keep in mind that the test concentrations specified for most of the interferents (in 

Table B-3) are substantially higher than the concentrations normally observed in 

ambient air. A review of test data submitted for numerous SO2 FEM applications 

showed that it is not feasible to further lower the limit requirement of each 

interferent. However, in response to the NESCAUM comment, EPA has 

concluded that that the limit requirement for NO interferent for SO2 analyzers can 

be lowered to ±3 ppb (167:1) for the lower measurement range to reduce possible 

NO interference at sites in urban areas with low SO2 levels. In reconsidering the 

requirement for the total of all interferents, we also reassessed the high probability 

that many of the test interferents would not coexist in ambient air at these test 

concentrations. Therefore, we concluded that the limit requirement for total 

interference equivalent for SO2 analyzers can be eliminated entirely. 

(2) Comment: UARG requested that EPA provide additional information to prove 

that existing FEMs qualify as FRM and FEM under the proposed new definitions 

as EPA stated without providing supporting information. 
84

Response: As explained in section IV.A.1.b of the preamble to the final rule, 

there is no legal impediment to continued use of SO2 FEMs without redesignation 

pursuant to the new automated SO2 FRM. With respect to the technical issue 

raised by the commenter, these automated continuous monitoring methods have 

been tested against the test and performance requirements of 40 CFR Part 53, 

which are designed specifically to test such continuous methods. For decades, 

these FEMs have been gathering and reporting hourly averages (almost 

exclusively) for monitoring agencies and so are technically sound to provide 

monitoring data for determining compliance with the new one-hour NAAQS. 

EPA has clarified the proposed regulatory text so that the rules state 

unambiguously that both FRMs apply to the new one-hour standard (as well as to 

the 24-hour and annual standard so long as they are retained), as do all presently 

designated FEMs. 

(3) Comment: AirQuality requested EPA to reevaluate its decision to not change the 

span drift requirement at the 80% URL which EPA left at +5.0%. “Ambient air 

monitors in the 21st century should be able to hold span drift to no more than 

+2.5% under the conditions specified in EPA testing and EPA should adjust the 

span drift specification (at 80% URL) accordingly.” 

Response: In response to this comment, EPA has reevaluated the span drift limits. 

Based on information from FEM testing and manufacturers’ data (EPA, 2009c), 

EPA has concluded that the span drift requirement at 80% URL can be lowered to 

±3%, as stated in section IV.A.2.b of the preamble to the final rule. 

(4) Comment: ExxonMobil expressed support for EPA “allowing monitoring 

agencies to choose mobile monitoring that meets monitoring quality requirements. 

This would, over time, expand geographic coverage and be responsive to changes 

in roadway emissions and community co-location relationships, while facilitating 

optimal deployment of monitoring resources.” 

Response: EPA appreciates the corroboration expressed in this comment and 

agrees with ExxonMobil that the revised performance specifications of automated 

SO2 methods included in Table B-1 of 40 CFR Part 53 for noise, lower detection 

limit, interference equivalent, zero drift, span drift, lag time, rise time, fall time, 

and precision (discussed in section IV.A.2.a and A.2.c of the preamble of the final 

rule) will allow for monitoring that meets various quality requirements and will be 

responsive to changing needs.

(5) Comment: Missouri provides recommendations for several technical changes to 

EPA’s proposed monitoring method. These changes are: 

o Page 64870 Appendix A-1, section 2.4, Sampling considerations – “While 

the use of a particulate filter can be a good practice it can also cause 

problems. Whether or not a sample line filter is required has normally 

been spelled out in the reference and equivalency descriptions; some 

require the filter and some don't. The wording here seems to make it a 
85

universal requirement. We suggest changing the wording or removing this 

section and referencing the List of Designated Reference and Equivalent 

from EPA's National Exposure Research Laboratory.” 

o Page 64871 Appendix A-1, sections 4.1 .1 and 4.1 .2. – “These two 

sections cause confusion. Since the goal is to control the flow outputs of 

the dilution system to within 2% of a NIST traceable standard, we propose 

combining the two sections and making this concept clearer. For example: 

‘The air and standard gas combined dilution system shall measure and 

control both the air and standard gas flows. Each of these flows will be 

calibrated to a NIST traceable standard to deliver flows accurate to within 

+/- 2% of the NIST traceable standard.’” 

o Page 64871 Appendix A-1, section 4.1 .6.1 – “We have not seen problems 

with using cylinders with concentrations below 50 parts per million (ppm). 

A problem with using cylinders with concentrations of 50 ppm and greater 

is that, in order to dilute to a concentration approximately equal to the 

standard, e.g., 0.055 ppm, requires using the normal 10 liters per minute 

(lpm) air flow controller and 100 cubic centimeters per minute (cc/min) 

gas flow controller at less accurate limits of their design range, 9 lpm and 

10 cc/m. Unless this section is based on recent study, we recommend 

consulting with manufacturers.” 

Response: 1) The use of a particulate filter on the sample inlet line may or may 

not be required for existing specific FRM or FEM analyzers, depending on 

manufacturer requirements, which are reflected in EPA/ORD’s List of Designated 

Reference and Equivalent Methods. Missouri’s suggestion is a reasonable one. 

However, for new SO2 FRM analyzers, EPA believes that the benefits of an inlet 

line particulate filter and the uniformity provided by a mandatory filter 

requirement outweigh possible disadvantages of using such a filter. Therefore, 

the particulate filter requirement of section 2.4 of the FRM has not been changed. 

2) Sections 4.1.1 and 4.1.2 of the FRM described the separate required functions 

of (1) flow regulation and (2) flow measurement. These separate functions 

correspond to the separate schematic boxes for each of these two functions shown 

in Figure 2 of the FRM. However, both functions are often combined in a single 

device, such as a mass flow controller. In response to this comment to reduce 

possible confusion regarding flow control and measurement, the language of these 

sections has been revised to describe both the flow rate regulation and flow rate 

measurement functions together for the SO2 standard gas (4.1.1) and for the 

dilution air (4.1.2). This arrangement is appropriate because the flow rate 

measurement devices may need to be calibrated differently and specifically for 

nitrogen (standard gas) and for air. 

3) In response to this comment, which we believe is very appropriate, the 

minimum concentration requirement for the SO2 calibration standard, specified in 

section 4.1.6.1 of the FRM, was re-evaluated. EPA has concluded that this 

requirement can be changed to 10 ppm. The language of section 4.1.6.1 has been 
86

revised to incorporate this change. These changes to the language of the FRM are 

discussed in Section IV.A.1.b of the preamble.

(6) Comment: A number of organizations expressed support for EPA’s proposed 

FRM using UVF as a means of both improving the accuracy of monitoring data 

and reducing the operational cost of FRMs. (AirQuality, CE, H-GAC, Houston, 

KYDEP, MSCC, North Carolina, Pennsylvania, South Carolina, South Dakota, 

UARG, and Wisconsin) 

Response: EPA appreciates the corroboration expressed in these comments. As 

discussed in section IV.A.1.c of the preamble to the final rule, we are finalizing 

the new automated SO2 FRM based on UVF technology as Appendix A-1 to 40 

CFR Part 50, titled “Reference Measurement Principle and Calibration Procedure 

for the Measurement of Sulfur Dioxide in the Atmosphere (Ultraviolet 

Fluorescence Method).”

 (7) Comment: Both North Carolina and South Carolina urge EPA to retain existing 

FRM and FEM standards so that monitoring agencies can avoid additional 

monitoring expenditures and continue to utilize existing equipment as long as the 

data produced from the instruments can meet the monitoring and data quality 

objectives. Similarly, NYSDEC recommended EPA permit “existing FEMs to be 

retained while superseding the existing FRM” rather than keep the existing FRM 

for a number of years. NYSDEC also commented that the existing FEMs should 

be retained because “the existing standard range FEMs typically used by State and 

Local monitoring agencies are acceptable for SO2 monitoring for comparison to 

the range of the proposed standard.” In addition, AirQuality also expressed 

support for utilizing “existing FEM monitors for initial attainment designation 

(before they are phased out)” even if they “may create some difficulties” because 

“there is no real alternative.” 

Response: EPA concurs with these comments. Section IV.A.1.b of the preamble 

to the final rule explains that there are sound reasons for not withdrawing the 

existing FRM at this time, as doing so would result inappropriately in cancellation 

of existing FEM designations as well. The consequent costs and disruptions to 

State and Local monitoring networks are clearly unwarranted given that existing 

FEMs (and the existing FRM as well) accurately measure ambient one-hour SO2

concentrations. Because supersession of an FRM, as defined in §53.16, requires 

cancellation of FEMs based on the superseded FRM, EPA is exercising its 

discretion not to supersede the existing FRM but rather to simply add a second 

FRM for SO2. As described in section IV.A.1.c of the preamble, EPA plans to 

rescind the original manual FRM at a future time when the new FRM analyzers 

have permeated the network. 

(8) Comment: UARG’s comments included the claim that EPA is violating the CAA 

by proposing to have multiple FRM standards effective at the same time. 
87

According to UARG’s interpretation of the CAA, EPA is only allowed to have 

one operational FRM standard at a time for a NAAQS. 

Response: As discussed in section IV.A.1.b of the preamble to the final rule, there 

is nothing in the Act which mandates a single FRM for each NAAQS. There are 

sound policy reasons for not withdrawing the existing FRM at this time, as this 

would result in cancellation of all existing SO2 FEM designations as well. See § 

53.16 (b). Cancellation of the FEM designations of all these SO2 analyzers now 

would be potentially very disruptive to State, Local, and other monitoring 

networks, and is technically unwarranted given that existing FEMs and the 

existing FRM accurately measure ambient one-hour SO2 concentrations. 

Therefore, EPA is exercising its discretion not to supersede the existing SO2 FRM 

and add a second FRM. 

(9) Comment: Center for Biological Diversity recommends in its comments that 

“EPA should move more expeditiously in phasing out the existing FRM and 

implementing new performance standards.” 

Response: In section IV.A.1.b of the preamble to the final rule, we describe how 

withdrawing the existing FRM at this time would result in cancellation of existing 

FEM designations as well and so would be potentially very disruptive to State, 

local, and other monitoring networks. We have identified no negative impact from 

retaining the existing FRM for some period of time to support the approval of 

existing FEMs. However, as stated in section IV.A.1.c of the preamble, EPA 

plans to rescind this manual method at a future time when the new FRM analyzers 

have permeated the network. 

(10) Comment: AirQuality submitted comments supportive of EPA for “seeking to 

upgrade the performance specifications for both FRM and FEM SO2 monitors. 

Response: EPA appreciates the corroboration expressed in these comments. As 

discussed in section IV.A.2.a and A.2.c of the preamble of the final rule, EPA has 

included revised performance specifications for automated SO2 methods in Table 

B-1 of 40 CFR Part 53 for noise, lower detection limit, interference equivalent, 

zero drift, span drift, lag time, rise time, fall time, and precision. In addition, to 

address the need for more sensitive lower measurement ranges for SO2 analyzers, 

EPA has added a separate set of performance requirements that would apply 

specifically to narrower measurement ranges, i.e. ranges extending from zero to 

concentrations less than 0.5 ppm. 

(11) Comment: Wisconsin recommends EPA revise its monitoring requirements “to 

specifically recognize digital recorders rather than maintaining the tie to analog 

technology” in Section 53.21 in order to increase the time resolution of SO2 

monitoring data. 
88

Response: In section IV.A.2.b of the preamble of the final rule, EPA 

acknowledges that industry has moved away from strip chart recording 

technology to digital data recording. The language of §53.21 calls for a graphic 

representation of analyzer responses to test concentrations to facilitate visual 

examination of test results. It allows any “alternative measurement data recording 

device” as long as it can provide such a graphic representation. Describing the 

analog strip chart recorder in this section provides a clear and appropriate model 

to help define the type of graphic representation needed for the Part 53 tests. EPA 

believes that the language of §53.21 is adequately broad to permit digital or other 

types of data recording devices. With regard to acquisition of field monitoring 

data, there is no requirement for use of chart recorders in lieu of digital or other 

types of data recorders as long as hourly (or shorter time) averages can be 

accurately recorded. 

IV.C: Comments on deadlines for monitoring plans and deployment 

(1) Comment: Louisiana Chemical requested EPA provide 6 additional months “for 

states to submit a monitoring plan (i.e. until January 1, 2012).” 

Response: The final network design, discussed in section IV.B.4 of the preamble 

to the final rule, requires fewer monitors than proposed. Further, states have the 

flexibility to use required monitors to serve multiple monitoring objectives, as 

discussed in section IV.B.3 of the preamble. As a result, EPA believes that much 

of the existing monitoring network will be able to satisfy minimum monitoring 

requirement promulgated in this rule. EPA roughly estimates that 41 CBSAs will 

have to establish a new site, or insert an SO2 analyzer into existing sites within 

those CBSAs that currently do not have an SO2 analyzer operating. As a result, 

and as discussed in section IV.B.7 of the preamble, EPA does not believe extra 

time for network plan development or deployment is necessary. 

(2) Comment: South Carolina requested revisions to the expectations for the 

proposed monitoring plan such that “the Annual Monitoring Plan due July 1, 

2011, should only be expected to contain a general description of the expected 

SO2 monitoring network unless the monitoring organization plans to implement 

some SO2 monitoring as part of the calendar year 2012 Annual Monitoring Plan. 

The 2013 Annual Monitoring Plan (due to EPA by July 1, 2012) is the appropriate 

document for review and comment of any additions or modifications required for 

the January 1, 2013 implementation of the SO2 required monitoring.” 

Response: The final network design, discussed in section IV.B.4 of the preamble 

to the final rule, requires fewer monitors than proposed. Further, states have the 

flexibility to use required monitors to serve multiple monitoring objectives, as 

discussed in section IV.B.3 of the preamble. As a result, EPA believes that much 

of the existing monitoring network will be able to satisfy minimum monitoring 

requirements promulgated in this rule. EPA roughly estimates that only 41 
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CBSAs will have to establish a new site, or insert an SO2 analyzer at existing 

sites, within those CBSAs that currently do not have an SO2 analyzer operating. 

Therefore, as discussed in section IV.B.7 of the preamble, EPA believes that 

states will have sufficient time to develop annual monitoring plans that include 

the requirements of this rulemaking by July 1, 2011. 

(3) Comment: Ohio provided comments on the proposed monitoring plan that 

expressed significant skepticism that large SO2 emitter states would be able to 

complete all the necessary work before the July 1, 2011 deadline for state 

monitoring plans. Ohio points out that a state-wide analysis that distributes 18 

PWEI monitors in 10 counties and 9 emissions triggered monitors (under the 

proposed network design) within the state is a large undertaking requiring 

significant analysis and resources. 

Response: EPA believes that states will have sufficient time to develop annual 

monitoring plans that include the requirements of this rulemaking by July 1, 2011 

because the final network design, discussed in section IV.B.4 of the preamble to 

the final rule, requires fewer monitors than proposed. Further, states have the 

flexibility to use required monitors to serve multiple monitoring objectives, as 

discussed in section IV.B.3 of the preamble. As a result, EPA believes that much 

of the existing monitoring network will be able to satisfy minimum monitoring 

requirements promulgated in this rule.

(4) Comment: ALA/EDF/NRDC/SC recommended EPA accelerate the deployment 

schedule for SO2 monitors because “the sooner monitors are in place, the sooner 

the public will experience the health benefits of the new standard.” 

Response: Rather than a monitoring-focused approach to assess ambient SO2

concentrations and compliance with the SO2 NAAQS, EPA anticipates using a 

hybrid analytic approach that combines monitoring and modeling, using each of 

these analytic tools where they are most appropriate and effective. We anticipate 

placing greater emphasis on modeling than did the proposed rule as the most 

technically appropriate, efficient, and readily available method for assessing 

short-term ambient SO2 concentrations in areas with point sources. This projected 

change in approach would necessarily result in a lesser emphasis on the less 

appropriate, more expensive, and slower to establish monitoring sites than did the 

proposed rule. Therefore, the minimum requirements for the SO2 monitoring 

network is of a smaller scale than proposed, and we do not expect monitoring to 

become the primary method by which ambient concentrations are compared to the 

new 1-hour SO2 NAAQS. States have the flexibility to use required monitors to 

serve multiple monitoring objectives, as discussed in section IV.B.3 of the 

preamble. As a result, EPA believes that much of the existing monitoring 

network will be able to satisfy minimum monitoring requirement promulgated in 

this rule. However, EPA received comment from some states urging us to 

consider their burden (both financially and with regard to personnel) of 

implementing multiple rulemakings with overlapping schedules. With this 
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consideration in mind and the anticipated use of modeling described above, EPA 

does not believe there is sufficient reason to accelerate the deployment of SO2

monitors sooner than the proposed schedule. As discussed in section IV.B.7 of 

the preamble, the schedule requires submission of annual monitoring plans that 

will reflect the requirements of this rulemaking by July 1, 2011, and is further 

requiring that the network deployment be complete by January 1, 2013.

(5) Comment: Louisiana Chemical requested EPA provide a one-year extension to 

the proposed SO2 monitor deployment deadline so that state agencies would have 

until January 1, 2014 to install the required SO2 monitors. 

Response: The final network design, discussed in section IV.B.4 of the preamble 

to the final rule, requires fewer monitors than proposed. Further, states have the 

flexibility to use required monitors to serve multiple monitoring objectives, as 

discussed in section IV.B.3 of the preamble. As a result, EPA believes that much 

of the existing monitoring network will be able to satisfy minimum monitoring 

requirement promulgated in this rule. EPA roughly estimates that 41 CBSAs will 

have to establish a new site, or insert an SO2 monitors into existing sites within 

those CBSAs that currently do not have an SO2 analyzer operating. As a result, 

and as discussed in section IV.B.7 of the preamble, EPA does not believe extra 

time for network plan development or deployment is necessary. 

 (6) Comment: Three state agencies requested EPA allow state agencies to deploy 

SO2 monitors on a phased schedule. Wisconsin’s comments requested that state 

agencies be allowed to install half of the requisite SO2 monitors by the proposed 

deadline of January 1, 2013 with the second half of the required SO2 monitors 

being installed by January 1, 2014. According to Wisconsin, the phased 

installation schedule is necessary because “state and local agencies will need 

more time to establish the network because establishing new sites near point 

sources is time-consuming, and expensive.” South Carolina provided comments 

requesting that, if EPA was going to move forward with the proposed SO2 

monitoring plan, it allow state agencies phase in the implementation of SO2 

monitors “with the largest source/highest probability population exposure areas 

designated for implementation in 2013 (some proportion of the highest PWEI 

monitors) and establishment of the remaining PWEI and the state level emissions 

triggered monitoring required by the following year.” Finally, Iowa also requests 

a phased implementation schedule for all states required to install more than four 

SO2 monitors that would extend the final deadline for all SO2 monitors to be 

deployed by two years (January 1, 2015). 

Response: The final network design, discussed in section IV.B.4 of the preamble 

to the final rule, requires fewer monitors than proposed. Further, states have the 

flexibility to use required monitors to serve multiple monitoring objectives, as 

discussed in section IV.B.3 of the preamble. As a result, EPA believes that much 

of the existing monitoring network will be able to satisfy the minimum 
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monitoring requirement promulgated in this rule. EPA roughly estimates that 41 

CBSAs will have to establish a new site, or insert an SO2 monitor into existing 

sites within those CBSAs that currently do not have an SO2 monitor operating. 

As a result, and as discussed in section IV.B.7 of the preamble, EPA does not 

believe extra time for network plan development or deployment is necessary. 

 (7) Comment: Both NYSDEC and MIDNRE provided comments arguing that EPA’s 

implementation deadline for SO2 monitors was not realistic given the scope of the 

work, limited state funding, and the implementation of other monitoring programs 

during the same time period. 

Response: The final network design, discussed in section IV.B.4 of the preamble 

to the final rule, requires fewer monitors than proposed. Further, states have the 

flexibility to use required monitors to serve multiple monitoring objectives, as 

discussed in section IV.B.3 of the preamble. As a result, EPA believes that much 

of the existing monitoring network will be able to satisfy minimum monitoring 

requirement promulgated in this rule. EPA roughly estimates that 41 CBSAs will 

have to establish a new site, or insert an SO2 analyzer into existing sites within 

those CBSAs that currently do not have an SO2 analyzer operating. Given the 

fewer number of required monitors and the flexibility of monitoring objectives 

discussed in section IV.B.7 of the preamble, EPA does not believe extra time for 

network plan development or deployment is necessary. 

(8) Comment: Four state agencies expressed support for the proposed 5-year network 

review process being linked with SO2 network adjustments (Indiana, South 

Carolina, South Dakota, and Wisconsin). South Carolina and South Dakota also 

recommended that network adjustments also be allowed on an annual basis as part 

of the annual monitoring plan. 

Response: In section IV.B.7 of the preamble to the final rule, EPA has clarified 

that existing requirements for both annual and 5-year reviews are appropriate 

tools to ensure that the SO2 network is sited and operated appropriately. In 

response to the comments, adjustments to the network may be made on annual 

basis as part of annual network plans required in 40 CFR §58.10. However EPA 

does not intend for required monitors to move from spot to spot on a frequent (i.e. 

annual) basis. In particular, annual monitoring plans would be expected to 

identify, for example, if an area has minimum monitoring requirement where it 

previously did not as a result of updated PWEI calculations. 

(9) Comment: North Carolina recommended that “if the EPA is going to base the 

monitoring requirements on emission inventories, then it would make sense to 

make adjustments to the monitoring network on the same cycle that the statewide 

emissions inventories are updated, i.e., either every 3 years or 6 years to coincide 

with AERR inventory requirements.” 
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Response: In section IV.B.7 of the preamble to the final rule, EPA has clarified 

that existing requirements for both annual and 5-year reviews are appropriate 

tools to ensure that the SO2 network is sited and operated appropriately. 

Regarding network adjustments, annual adjustments to the network may be made, 

and documented in annual monitoring plans. However EPA does not intend for 

required monitors to move from spot to spot on a frequent (i.e. annual) basis. 

Annual monitoring plans would be expected to identify, for example, if an area 

now has minimum monitoring requirement where it previously did not as a result 

of updated PWEI calculations. EPA notes that the Air Emissions Reporting 

Requirement (AERR) requires full inventory updates every three years. However, 

as discussed in section IV.B.5 of the preamble, EPA believes that the NEI is an 

appropriate tool by which to calculate PWEI values that are used to determine 

minimum monitoring requirements for SO2 monitoring. The process by which the 

NEI will be updated is being adjusted (by use of the Emissions Inventory System 

[EIS]) in a manner that will allow for more frequent insertion of state-supplied 

emissions data, resulting in a more up-to-date inventory. Any introduction of 

actual reductions that may be occurring (such as federally enforceable reductions, 

like MACT rules) will therefore be reflected more quickly in the inventory. EPA 

believes that the NEI is limited only if states choose not to submit data for 

inclusion, or in a timely manner, and thus, strongly encourages states to submit all 

available emissions data as frequent as feasible to ensure the NEI is as accurate as 

possible. 

(10) Comment: Five organizations opposed EPA’s proposed five-year update schedule 

for SO2 monitors. NYSDEC saw no need for an every five-year update if state 

agencies were “permitted to use the best available data to determine where and 

how many monitors are needed.” Alaska opposed the proposal on the grounds that 

the update process was not necessary if EPA followed Alaska’s other 

recommendation to allow state agencies to remove SO2 monitors that no longer 

demonstrate concentrations of concern.” Ohio opposed the proposal, because as 

the proposal is currently being interpreted, states will be required to duplicate “the 

same analysis every five years under the network assessment. Such a requirement 

would be duplicative, unnecessary, and overly burdensome.” NACAA and Illinois 

both considered the five-year siting update process to be duplicative given the 

annual and 5-year network reviews that are already in place. 

Response: As discussed in section IV.B.7 of the preamble to the final rule, EPA 

has clarified that existing requirements for both annual and 5-year reviews are 

appropriate tools to ensure that the SO2 monitoring network will be sited and 

operated appropriately, per 40 CFR §58.10. EPA did not intend for states to 

execute a parallel, and duplicate review, every 5-years solely for the SO2 network. 

Annual adjustments to the network may be made, and documented in annual 

monitoring plans, however EPA does not intend for required monitors to move 

from spot to spot on a frequent (i.e. annual) basis. Annual monitoring plans 

would be expected to identify, for example, if an area now has minimum 
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monitoring requirement where it previously did not as a result of updated PWEI 

calculations. 

Regarding Alaska’s recommendation to allow state agencies to remove SO2

monitors that no longer demonstrate concentrations of concern, EPA notes that 

existing rules (40 CFR § 58.14 (c)) allow states to remove any monitors above 

minimum monitoring requirements if certain conditions are met, and provided 

Regional Administrator approval.

IV.D: Comments on Population Weighted Emissions Index (PWEI) 

monitors 

(1) Comment: Four state agencies provided comments criticizing EPA’s PWEI 

metric as too simplistic and/or flawed to be the only tool for determining where 

prong 1 SO2 monitors are to be located (Iowa, NESCAUM, NYSDEC, and 

Wisconsin). Wisconsin “believes that modeling, population distribution, and local 

conditions must be considered in determining whether monitoring is required.” 

Response: Under a hybrid analytical approach, introduced in section III of the 

preamble to the final rule, EPA anticipates using both ambient monitoring and 

source modeling in implementing the SO2 NAAQS (discussed in sections III, IV, 

V and VI of the preamble). As a result, the required monitoring network is no 

longer required to be source-oriented in nature, as explained in section IV.B of the 

preamble. The required monitoring network has the flexibility to have monitoring 

sites characterizing one or more of the following objectives: source 

characterization, highest concentration, population exposure, general background, 

or regional transport. EPA believes that the flexibility of how monitors may serve 

varied monitor objectives in the final network design is balanced by the continued 

use of the PWEI to require them. The use of the PWEI provides a mechanism to 

focus required monitoring in areas where there is an increased coincidence of 

population and SO2 emissions. 

(2) Comment: Four commenters expressed concern with the use of CBSAs for 

determining the number and location of SO2 monitors. Delaware does not favor 

EPA’s proposed CBSA/PWEI metric for SO2 monitors because it does not 

properly take into account interstate CBSAs that require special consideration “for 

determining the need for additional monitors beyond the current PWEI calculation 

method.” 

Response: There is precedent for using CBSAs (which encompasses both 

Metropolitan Statistical Areas [MSAs] and Micropolitan Statistical Areas) as a 

means to identify urban areas (e.g. the requirement of NO2 monitors in CBSAs 

based on population [40 CFR Part 58 Appendix D, section 4.3]). EPA believes 

that in situations where a CBSA covers more than one state, or monitoring 

agency, those entities are in a position to negotiate with each other (and with EPA 

Regional assistance) how best to satisfy minimum monitoring requirements for 
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that CBSA. In all cases, states always have the prerogative to conduct additional 

monitoring above the minimum requirements for an area as they see fit. 

(3) Comment: NESCAUM provided comments seeking EPA clarification on how 

monitoring issues will be resolved for CBSAs that span across EPA regions and 

consequently have separate Regional Administrators. 

Response: EPA believes that in situations where a CBSA covers more than one 

state, or monitoring agency, those entities are in a position to negotiate with each 

other (and with EPA Regional assistance) how best to satisfy minimum 

monitoring requirements for that CBSA. Similarly, EPA expects that the 

Regional Administrators, and possibly EPA Headquarters, would also work 

together as necessary in dealing with any cross boundary issues that may arise. 

(4) Comment: South Carolina and Missouri requested in their comments that EPA 

relax the requirement that SO2 monitors be located within the boundaries of the 

assigned CBSA in order “to allow monitoring organizations the ability to best 

address sources and potential maximum impacts associated with an area without 

unnecessary restrictions.” 

Response: EPA intends to use a hybrid analytical approach, introduced in section 

III of the preamble to the final rule, that combines monitoring and modeling, 

using each of these analytic tools where they are most appropriate and effective. 

In addition, the scope of the monitoring network is revised so that it is no longer 

required to be source-oriented in nature (i.e., “hot-spot” monitoring only). Instead 

the final monitoring network design now has the flexibility to address multiple 

monitoring objectives including source characterization, highest concentration, 

population exposure, general background, or regional transport. Thus the final 

monitoring network design is more flexible than the proposed network design as 

discussed in section IV.B of the preamble. This increased flexibility gives states 

the ability to select areas where they believe ambient monitoring is most 

warranted within a CBSA with required monitoring. However, EPA believes that 

requiring monitors in CBSAs by using the PWEI metric appropriately focuses 

monitors into areas with increased population and SO2 emissions. EPA has also 

included a provision to allow a required monitor to be sited outside of its parent 

CBSA. As explained in section IV.B.3 of the preamble, a source-oriented/high 

concentration monitor may be placed outside of its parent CBSA if it is sited to 

characterize a source inside that parent CBSA whose PWEI value triggered 

minimum monitoring requirements. Further, as discussed in section IV.B.6, EPA 

Regional Administrators (working with states) may require additional monitoring 

above the minimum requirements if they believe monitoring objectives remain 

unfulfilled in an area (including areas outside of CBSAs) even though a state may 

be satisfying minimum monitoring requirements. Finally, states always have the 

prerogative to conduct additional monitoring above the minimum requirements 

for an area as they see fit. 
95

(5) Comment: South Carolina commented that EPA “needs to develop a better 

definition for Core-Based Statistical Areas (CBSA) which is not explicitly defined 

by the U.S. Census Bureau.” In addition, South Carolina “strongly encourages 

EPA to specifically seek public comment on the definition and use of CBSA, to 

include an annual list of counties comprising each CBSA in the context of the 

standard implementation and monitoring requirements.” 

Response: EPA believes that the term CBSA is adequately defined by the U.S. 

Office of Management and Budget (OMB). OMB defines metropolitan and 

micropolitan statistical areas according to published standards that are applied to 

Census Bureau data. The general concept of a metropolitan or micropolitan 

statistical area is that of a core area containing a substantial population nucleus, 

together with adjacent communities having a high degree of economic and social 

integration with that core. Currently defined metropolitan and micropolitan 

statistical areas are based on application of 2000 standards (which appeared in the 

Federal Register on December 27, 2000) to 2000 decennial census data. Current 

metropolitan and micropolitan statistical area definitions were announced by 

OMB effective June 6, 2003 

(http://www.census.gov/population/www/metroareas/aboutmetro.html). 

(6) Comment: Both South Carolina and LEC provided comments noting that the use 

of PWEI as the metric for locating prong 1 SO2 monitors will emphasize urban 

areas at the expense of non-urban and rural areas which will be left with few if 

any SO2 monitors. Without an area-wide monitoring network, state agencies will 

find it difficult conduct SO2 permitting in these areas (LEC) and will “lose the 

broader context needed to evaluate” SO2 concentration data (South Carolina). 

Response: : Under a hybrid analytical approach, as introduced in section III of the 

preamble to the final rule, the final monitoring network design is more flexible 

than the proposed network design, as discussed in section IV.B of the preamble. 

The scope of the monitoring network is revised so that it is no longer required to 

be source-oriented in nature (i.e., “hot-spot” monitoring only). Instead, the final 

monitoring network design now has the flexibility to address multiple monitoring 

objectives including source characterization, highest concentration, population 

exposure, general background, or regional transport. This increased flexibility 

gives states the ability to select areas where they believe ambient monitoring is 

most warranted within a CBSA with required monitoring. As a result, states may 

site monitors to address the need to measure background concentrations or to 

provide data for a broader context to evaluate ambient SO2 concentrations. 

EPA notes that it currently has no minimum monitoring requirements (except for 

NCore) for SO2, but as of the 3rd quarter of 2009, EPA estimated that 

approximately 433 SO2 sites were operating nationwide, based on data states have 

submitted to AQS. Under the final network design, and including monitors 

required at NCore sites, EPA estimates that 191 monitors would be required to be 
96

operational by January 1, 2013. That number is well short of the 433 analyzers 

operating in late 2009. EPA believes that states have and likely will continue to 

place SO2 monitors in various locations (including non-urban areas) to produce 

information upon which PSD permitting determinations may be based. 

(7) Comment: Wisconsin commented that “if EPA is intent on placing monitors in 

areas of higher population, it may be useful to establish a population threshold for 

evaluating exposure potential within a CBSA to ensure that monitoring is directed 

toward the higher priority locations.” Wisconsin’s recommendation is that EPA 

should “consider allowing monitoring agencies the flexibility, under the first 

prong, to determine whether SO2 monitors would be suitable in metropolitan 

statistical areas with populations between 50,000–350,000” or “in rural areas, if 

necessary, and that such monitors would fulfill prong 1 requirements.” 

 

Response: Under a hybrid analytical approach, as introduced in section III of the 

preamble to the final rule, the final monitoring network design is more flexible 

than the proposed network design, as discussed in section IV.B of the preamble. 

This increased flexibility gives states the ability to select areas where they believe 

ambient monitoring is most warranted within a CBSA with required monitoring. 

EPA’s contemplated use of the hybrid analytical approach, results in a lesser 

emphasis on the monitoring network design to assess compliance with the SO2

NAAQS than did the proposed rule. Therefore, the minimum requirements for 

the SO2 monitoring network are smaller than those proposed. EPA does not 

believe that the minimum monitoring requirement should be further relaxed than 

the final network design, in order to have a monitoring network coupled with 

source modeling under a hybrid analytical approach, which is focused to provide 

information to support protecting public health, in this case by focusing 

monitoring in areas where there is an increased coincidence of population and 

SO2 emissions. In regard to monitors being necessary in more rural areas, states 

always have the prerogative to conduct additional monitoring above the minimum 

requirements for an area as they see fit. Further, as discussed in section IV.B.6 of 

the preamble, EPA Regional Administrators (working with states) may require 

additional monitoring above the minimum requirements if they believe 

monitoring objectives remain unfulfilled in an area (including areas outside of 

CBSAs) even though a state may be satisfying minimum monitoring 

requirements. 

(8) Comment: Iowa requested that EPA consider supplementing the “2005 NEI with 

the most recent and representative point source emissions inventory data 

available, using both CEMs data and emissions reports from Type A major 

emitting facilities. Additionally, recent federally enforceable reductions not 

reflected in current emissions inventories, such as may be expected from Best 

Available Retrofit Technology (BART) requirements, should be considered.” 

Response: EPA believes that the NEI is an appropriate tool by which to calculate 

PWEI values that are used to determine minimum monitoring requirements for 
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SO2 monitoring. As discussed in section IV.B.5 in the preamble to the final rule, 

the process by which the NEI will be updated is being adjusted in a manner that 

will allow for more frequent insertion of state supplied emissions data, resulting 

in a more up-to-date inventory. Any introduction of actual reductions that may be 

occurring will therefore be reflected more quickly in the inventory, a result that 

the commenter appears to desire. EPA believes that the NEI is limited only if 

states chose not to submit available information timely for inclusion, and thus, we 

strongly encourage states to submit all available emissions data as frequent as 

feasible to ensure the NEI is as accurate as possible. 

 (9) Comment: Pennsylvania provided comments recommending that thresholds be 

included within the PWEI calculation method to focus monitoring in high 

population areas that could potentially violate the proposed new one-hour SO2 

standards. Pennsylvania recommended that prong one monitors be limited to 

“urbanized areas with a population of at least 50,000 people or greater where 

there is a sulfur dioxide source or combination of sources within 50 miles 

emitting a total of at least 20,000 tons of SO2 per year.” 

NYSDEC comments included remarks that “the PWEI calculation method should 

be changed to remove or reduce the influence of population in the calculation,” 

and “because the PWEI approach does not appear to be more predictive, the 

Department does not recommend a PWEI breakpoint. Instead, the Department 

recommends a CBSA emissions value of 50,000 tpy as a threshold where 

monitoring would be required. In addition, if the site has a 3 year design value 

that is less than 1⁄2 of the standard, that monitor should no longer be required.” 

(.0145) Similarly, NESCAUM recommended EPA “adopt an emissions-only 

approach, resulting in fewer CBSA monitors” including the use of a “threshold of 

50,000 tpy CBSA SO2 emissions to trigger the first CBSA monitor, and a second 

CBSA monitor required when emissions exceed 200,000 tpy. CBSAs with 

emissions under these thresholds could be monitored under the state emissions 

triggered prong.” 

Response: EPA discusses this issue and the examples for alternative thresholds in 

section IV.B.5 of the preamble to the final rule. As explained there, EPA believes 

that these thresholds neither provide an adequate number of monitors nor do they 

include an adequate number of CBSAs that should have monitors. Using the 

commenters’ thresholds would result in too sparse a network to meet the 

appropriate monitoring objectives. EPA believes that the broader scope of the 

monitoring objectives of the network is best served by using the PWEI metric to 

focus ambient monitoring into areas where there is an increased coincidence of 

population and SO2 emissions. 

(10) Comment: Center for Biological Diversity complained that EPA’s proposed 

PWEI calculation method is too weighted towards monitoring high population 

areas and “could effectively deny the health benefits of a short-term SO2 standard 

to residents of areas with some of the nation’s highest total SO2 emissions” 

because they live in rural areas. 
98

Response: Partly in response to these and other comments and an examination of 

our historical approach to SO2 NAAQS implementation, we now anticipate using 

a hybrid analytic approach combining the use of monitoring and available 

modeling to assess compliance with the new 1-hour SO2 NAAQS. Rather than a 

monitoring-focused approach EPA anticipates using each of these analytic tools 

where they are most appropriate and effective. We anticipate placing greater 

emphasis on modeling than did the proposed rule as the most technically 

appropriate, efficient, and readily available method for assessing short-term 

ambient SO2 concentrations in areas with point sources. This projected change in 

approach would necessarily result in a lesser emphasis on the monitoring network 

than did the proposed rule. Therefore, the minimum requirements for the SO2

monitoring network is of a smaller scale than proposed, and we do not expect 

monitoring to become the primary method by which ambient concentrations are 

compared to the new 1-hour SO2 NAAQS. EPA discusses its intended approach 

in sections III, IV, V, and VI of the preamble to the final rule. EPA believes that 

under a hybrid analytical approach, the commenter’s point about the lack of 

monitoring in CBSAs with low populations but with high emissions, (e.g., greater 

than 100, 000 tpy) is no longer a concern. EPA notes that states always have the 

prerogative to conduct additional monitoring above the minimum requirements in 

any area they see fit. 

(11) Comment: Missouri’s comments included the recommendation that EPA adopt 

the alternative PWEI equation PWEI = [Population + Emissions (TPY) * 100] / 

100 because this equation “allows for highly populated areas with lower 

emissions to have necessary monitoring while emphasizing areas with lower 

populations that have higher emissions. The thresholds for this proposal would be 

PWEI over 225,000 - three monitors, PWEI over 75,000 - two monitors, and 

PWEI over 15,000 - one monitor. The new approach would require any area with 

over 15,000 tons per year of S02 to have at least one monitor.” According to 

Missouri’s calculations, “the total number of required monitors using the revised 

PWEI would be 250 including 187 CBSAs as compared to 231 monitors in 131 

CBSAs under EPA's proposal.” 

Response: EPA anticipates using a hybrid analytic approach, as introduced in 

section III of the preamble to the final rule, combining the use of monitoring and 

available modeling to assess compliance with the new 1-hour SO2 NAAQS. 

Rather than a monitoring-focused approach, EPA anticipates using each of these 

analytic tools where they are most appropriate and effective. This projected 

change in approach would necessarily result in a lesser emphasis on monitoring 

than did the proposed rule. EPA has adjusted the final network design, discussed 

in section IV.B.4 in the preamble to the final rule, which uses the PWEI metric, 

but with some changes in the breakpoints used to require monitors, which 

ultimately results in requiring fewer monitors than the number proposed 

(discussed in section IV.B.5 of the preamble). EPA believes that fewer monitors 

are needed to serve monitoring objectives under a hybrid analytical approach, as 
99

compared to either the proposed number or in the example provided by the 

commenter 

 (12) Comment: Three organizations recommended EPA revise the PWEI calculation 

method in order to reduce the number of SO2 monitors that would be placed in 

areas unlikely to have SO2 concentrations violating EPA’s proposed new SO2 

standards (Illinois, NACAA, and North Carolina). According to NACAA, “EPA 

should limit the total number of monitors required in CBSAs based on additional 

metrics, such as total number of monitors, historical data, area, trends analysis 

and/or modeling, and allow for removal of monitors” if SO2 concentrations prove 

to not be close to violating the SO2 NAAQS. Similarly, South Carolina finds fault 

with the proposed PWEI equation because it doesn’t take into consideration “the 

area of the CBSA, relative distribution of population or emissions within or 

outside the boundaries” which can “lead to inconsistency and possibly inefficient 

use of resources across the CBSAs.” South Carolina’s recommended solution to 

this problem is for EPA to “develop a refined Index incorporating the proposed 

PWEI population and emissions elements, but truly normalizing the result to 

better account for the differences in CBSAs. CBSA area, population distribution 

and source categories in and near the CBSAs are among the possible factors that 

should be considered” 

Response: Under a hybrid analytical approach, as introduced in section III of the 

preamble to the final rule, the final monitoring network design is more flexible 

than the proposed network, as discussed in section IV.B of the preamble. The 

final network design, discussed in section IV.B.4 in the preamble to the final rule, 

utilizes the PWEI metric, but with some changes in the breakpoints used to 

require monitors, resulting in fewer required monitors than proposed. Under our 

current approach, states are free to use such metrics suggested above to aid in 

siting their required monitors within a given CBSA. One commenter states that 

the use of PWEI can “...lead to inconsistency and possibly inefficient use of 

resources across the CBSAs.” On a national scale, EPA believes that no 

nationally applied network will always perfectly fit every area of the country. 

EPA has finalized a minimally required network which we believe is necessary to 

ensure that a network of adequate size and focus will be operated. The number of 

minimally required monitors in this rule is much less than the number estimated 

to be in operation at the time of this rulemaking. EPA does not believe that the 

application of the PWEI will lead to an inefficient use of resources. Further, the 

final network design allows states more flexibility in monitor siting than that 

proposed (as discussed in section IV.B.3 of the preamble), which should further 

prevent states from operating a minimally required monitor in a fashion that they 

would consider to be inefficient. 

 (13) Comment: MIDNRE’s comments included a recommendation that EPA modify 

the PWEI calculation method to take advantage of natural break points between 

the PWEI values of CBSAs when determining how many SO2 monitors are 

required in an area so that CBSAs with almost identical PWEI scores are not 
100

arbitrarily required to have different numbers of SO2 monitors deployed. 

Similarly, South Carolina commented that the “break points in the PWEI 

approach for the number of monitors (1,000,000, 10,000 and 5,000) are arbitrary. 

Any index developed to determine potential for population exposure and the 

number of required monitors should have a logical and documented basis for the 

breakpoints used to determine the minimum monitoring requirements.” South 

Carolina also recommended “the PWEI be truly normalized to better address the 

differences in area, population and source type and distribution between CBSAs.” 

 

Response: Under a hybrid analytical approach, introduced in section III of the 

preamble to the final rule, the final network design (discussed in section IV.B.4) 

still uses the PWEI as a means by which to require monitoring. However, the 

application of the PWEI has been adjusted, as discussed in section IV.B.5 of the 

preamble, where EPA did consider the MIDNRE suggestion in the final rule. 

 (14) Comment: Both Florida and Louisiana Chemical recommended that EPA revise 

its PWEI calculation method to take into account the impact of stack heights on 

SO2 concentrations when calculating the number of SO2 monitors required for a 

CBSA and their location. 

Response: Under a hybrid analytical approach, as introduced in section III of the 

preamble to the final rule, the final monitoring network design is more flexible 

than the proposed network design, as discussed in section IV.B of the preamble. 

The required monitoring network no longer must be wholly source-oriented in 

nature, but rather can serve multiple monitoring objectives, as discussed in section 

IV.B.3. As a result EPA does not believe it necessary for states to consider 

individual source characteristics when requiring a given number of monitors for 

an area. Further, states now have increased flexibility in deciding the location of 

required monitors, which should address the commenter’s concerns.

 (15) Comment: Iowa commented that EPA should “abandon use of the PWEI in 

designing the monitoring network. The PWEI is scientifically unsound as it fails 

to address: 1) the characteristics of the dominant SO2 source type, 2) population 

distributions, 3) recent and representative emissions data, 4) the number of SO2 

sources in a state, and 5) the role of atmospheric dispersal in relating emissions to 

ambient impacts.” 

Response: Under a hybrid analytical approach, as introduced in section III of the 

preamble to the final rule, the final monitoring network design is more flexible 

than the proposed network design, as discussed in section IV.B of the preamble. 

EPA expects the hybrid analytical approach to combine modeling and monitoring, 

using each where they are most appropriate and effective. The required 

monitoring network is no longer wholly source-oriented in nature, and can now 

serve multiple monitoring objectives, as discussed in section IV.B.3. However, 

EPA is retaining the use of the PWEI metric to appropriately focus monitoring 

resources into areas with increased coincidence of population and SO2 emissions. 
101

As states decided where to place their required monitoring, under the final 

network design requirements, they can consider the characteristics of the 

dominant SO2 source type in that area, population distributions, and the role of 

atmospheric dispersal in relating emissions to ambient impacts. For some of 

these, modeling may be a more appropriate and effective tool. If a state feels that 

more monitors are required, above the minimum requirements, to address any one 

of a myriad of issues, EPA notes that states always have the prerogative to 

conduct additional monitoring in any locations they see fit consistent with the 

overall monitoring objectives. 

With regard to ‘recent’ emissions data being utilized in the PWEI, as discussed in 

section IV.B.5 of the preamble, EPA believes that the NEI is an appropriate tool 

by which to calculate PWEI values. The process by which the NEI will be 

updated is being adjusted in a manner that will allow for more frequent insertion 

of state-supplied emissions data, resulting in a more up-to-date inventory. Any 

introduction of actual reductions that may be occurring will therefore be reflected 

more quickly in the inventory. EPA believes that the NEI is limited only if states 

choose not to submit data for inclusion, or in a timely manner, and thus, strongly 

encourages states to submit all available emissions data as frequent as feasible to 

ensure the NEI is as accurate as possible. 

 

(16) Comment: Wisconsin remarked in its comments that, “in states such as 

Wisconsin, with large CBSA containing relatively small population centers and 

large, dispersed sources, placing monitors at the maximum downwind location 

does not necessarily result in effective protection of public health. Some CBSAs 

may have excess monitors and others may have inadequate coverage based on 

local conditions. The maximum hourly downwind locations may not correlate 

well with population centers.” 

Response: Under a hybrid analytical approach, as introduced in section III of the 

preamble to the final rule, EPA anticipates using both modeling and monitoring 

where each of these is most appropriate and effective. In the example that 

Wisconsin gives of small population centers with large sources, Wisconsin’s 

concern could be addressed by using modeling rather than monitoring as it may 

be more conducive in assessing ambient SO2 concentrations and compliance with 

the SO2 NAAQS. In addition, the final monitoring network design is more 

flexible than the proposed network design, as discussed in section IV.B of the 

preamble. The required monitoring network is no longer wholly source-oriented 

in nature, and can now serve multiple monitoring objectives, as discussed in 

section IV.B.3, including monitors sited to specifically characterize population 

exposure. 

(17) Comment: Many commenters expressed concerns about inaccuracies in the 2005 

NEI data and recommended that states be permitted to utilize the “best available 

emissions data when selecting the sources where monitors will be established” 

rather than being limited to the NEI inventory. (DSRI, MIDNRE, NESCAUM, 
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NYSDEC, North Carolina, South Carolina, South Dakota, UARG, and 

Wisconsin) 

Response: In response to the comments, EPA recognizes that states do receive 

and submit emissions information for some sources on an annual basis, but PWEI 

calculations must be made using the most recent NEI data. As discussed in 

section IV.B.5 of the preamble, EPA believes that the NEI is an appropriate tool 

to calculate PWEI values that are used to determine minimum monitoring 

requirements for SO2 monitoring. The process by which the NEI will be updated 

is being adjusted in a manner that will allow for more frequent insertion of state 

supplied emissions data, resulting in a more up-to-date inventory. Any 

introduction of actual reductions that may be occurring (such as federally 

enforceable reductions, like MACT rules) will therefore be reflected more quickly 

in the inventory. EPA believes that the NEI is limited only if states choose not to 

submit data for inclusion, or in a timely manner, and thus, strongly encourages 

states to submit all available emissions data as frequent as feasible to ensure the 

NEI is as accurate as possible. 

(18) Comment: Delaware expressed concern that basing monitor allocation and siting 

on emissions inventories “may not be the most relevant method for sourceoriented monitoring network design. Facilities with large emissions at high 

elevations often have a much smaller impact on 1-hour concentrations than 

smaller facilities with low level emissions. Unless the site allocation method can 

incorporate critical information such as stack height, this method is unlikely to 

produce the most meaningful results.” 

Response: Rather than the proposed monitoring-focused approach to assess 

compliance with the SO2 NAAQS, EPA anticipates using a hybrid analytical 

approach, introduced in section III of the preamble to the final rule, which 

combines monitoring and modeling using each of these where they are most 

appropriate and effective which should address the commenters concerns. As 

further explained in sections IV, V, and VI of the preamble, EPA anticipates that 

both ambient monitoring and the modeling of SO2 sources would play a role in 

the implementation of the revised SO2 primary NAAQS. Since it is anticipated 

that dispersion modeling will be used to assess impacts of individual SO2 sources, 

the required monitoring network is no longer required to be wholly sourceoriented in nature, and now can serve multiple monitoring objectives as discussed 

in section IV.B.3 of the preamble. 

(19) Comment: Pennsylvania, concerned about EPA’s over-reliance on the NEI for 

determining SO2 monitor locations, proposed “that if the emission inventory is to 

be used, that a source threshold be developed, similar to lead, where modeling 

would be used to determine if the expected maximum one-hour SO2 concentration 

is at least 50 percent of the NAAQS for those facilities emitting 50,000 tons per 

year based on the most recent inventory available at the time of final rule.... The 

determination of facilities should also take into account the installation of control 
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devices (scrubbers) that would reduce SO2 emissions to less than 50,000 tons per 

year if the installation of the devices would occur prior to January 1, 2013.... This 

approach would allow states and regions to analyze the effect of stack height on 

local ambient concentrations.” 

Response: Pennsylvania’s suggestion is similar in some regards to the alternative 

network design, described which was provided in the proposed rule for comment. 

In the proposed rule, EPA took comment on alternative network design; however 

it is not adopting that alternative network design which should alleviate the 

concerns expressed by the commenter. Instead, EPA anticipates using a hybrid 

analytical approach, as introduced in section III of the preamble to the final rule. 

The final monitoring network design is more flexible than the proposed network 

design, as discussed in section IV.B of the preamble. The required monitoring 

network is no longer wholly source-oriented in nature, as modeling of source may 

in effect provide source-oriented concentration data. We now anticipate using a 

hybrid analytic approach combining the use of monitoring and available modeling 

to assess compliance with the new 1-hour SO2 NAAQS. We anticipate placing 

greater emphasis on modeling than did the proposed rule as it is technically 

appropriate, efficient, and readily available method for assessing short-term 

ambient SO2 concentrations in areas with large point sources. Thus the alternative 

network design or the approach suggested by Pennsylvania, where modeling 

informs where monitoring is to occur is not necessary and the projected change in 

our approach should address the commenters’ concerns. . 

IV.E: Comments on State-Level Emissions Monitors 

(1) Comment: H-GAC commented that it supports EPA’s proposed state-level 

emissions monitoring plan of locating monitors in areas where maximum SO2 

concentrations are expected. NESCAUM also supports source-oriented SO2 

monitoring but with the caveat that state agencies ought to have the authority to 

remove SO2 monitors after three-years of deployment if the design value is less 

than 50% of the standard. NESCAUM also supports “a minimum number of sites 

within a state, based on the state emissions triggered monitor count as proposed 

by EPA.” 

Response: Under a hybrid analytical approach, as introduced in section III of the 

preamble to the final rule, the final monitoring network design is more flexible 

than the proposed network design, as discussed in section IV.B of the preamble. 

The final network design does not retain the proposed requirement for monitors 

based solely on state level emissions (the second prong of the proposed network 

design), as discussed in section IV.B.4 of the preamble. The required monitoring 

network is no longer wholly source-oriented in nature, and can now serve 

multiple monitoring objectives, as discussed in section IV.B.3. EPA believes 

state and local air agencies should consider monitoring, as appropriate, those 

sources which are not as conducive to dispersion modeling. Such sources include 

(1) sources classified as non-point sources.(“area-sources”) such as shipping 
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ports, (2) a source situated in area of complex terrain and/or situated in a complex 

meteorological regime, (3) locations that have multiple, relatively small sources 

with overlapping plumes. EPA also notes that existing rules (40 CFR § 58.14 (c 

)) allow states to remove any monitors above minimum monitoring requirements 

if certain conditions are met, and provided Regional Administrator approval.

(2) Comment: Pennsylvania commented that it opposes the proposed design of the 

state-level emissions monitoring program and requests that EPA remove the 

program in favor of allowing Regional Administrators and state and local air 

pollution control agencies “to determine the number and location of any 

additional SO2 monitors.” Similarly, South Carolina commented that state-level 

emissions monitors are unnecessary and redundant if EPA allows state agencies 

more flexibility in siting PWEI monitors such as by allowing the monitors to be 

located outside the boundaries of the CBSA. 

Response: Under a hybrid analytical approach, as introduced in section III of the 

preamble to the final rule, the final monitoring network design is more flexible 

than the proposed network design, as discussed in section IV.B of the preamble. 

The final monitoring network design does not retain the proposed requirement for 

monitors based solely on state level emissions (the second prong of the proposed 

network design), as discussed in section IV.B.4 of the preamble. 

 (3) Comment: Several organizations provided comments in opposition to EPA’s 

proposed requirement that every state have at least one state-level emissions 

monitor. South Carolina recommended that “EPA should not require one monitor 

in every state unless the NCore site can serve as the sole monitor.” AirQuality 

recommended EPA not require state-level emissions monitors for any states that 

contribute less than 0.5% to the nation’s total SO2 emissions. In addition, Vermont 

recommended that state’s that emit less than 0.1% of the nation’s total SO2 

emissions should be permitted to site their one required state-level emissions 

monitor at the state’s discretion rather than at an in-state SO2 emissions source 

since no emissions sources in such a state would be likely to generate an SO2 

concentration of concern. 

Response: Under a hybrid analytical approach, as introduced in section III of the 

preamble to the final rule, the final monitoring network design no longer requires 

monitors based solely on state level emissions (the second prong of the proposed 

network design), as discussed in section IV.B.4 of the preamble. The removal of 

the second prong of the monitoring network in the final network design should 

address the commenters’ concerns. 

(4) Comment: Both Delaware and South Dakota provided comments in support of 

EPA’s “requirement that each state have at least one monitor” in the state-level 

emission monitor prong of the network design. (Delaware and South Dakota) 
105

Response: Under a hybrid analytical approach, as introduced in section III of the 

preamble to the final rule, the final monitoring network design is more flexible 

than the proposed network design, as discussed in section IV.B of the preamble, 

and no longer will require monitors based solely on state level emissions (the 

second prong of the proposed network design), as discussed in section IV.B.4 of 

the preamble. However, if a state feels that more monitors are required, above the 

minimum requirements, to address any one of a myriad of issues, EPA notes that 

states always have the prerogative to conduct additional monitoring in any 

locations they see fit consistent with overall monitoring objectives.

(5) Comment: Iowa expressed concern that “it will be difficult to obtain an accurate 

picture of the attainment status of the ambient air in any state based on the 

number of monitors allocated in the proposed rule.” 

Response: Rather than the proposed monitoring-focused approach to assess 

compliance with the SO2 NAAQS, EPA anticipates using a hybrid analytical 

approach, introduced in section III of the preamble to the final rule, which 

combines monitoring and modeling using each of these where they are most 

appropriate and effective. As further explained in sections IV, V, and VI of the 

preamble, EPA anticipates that both ambient monitoring and the modeling of SO2

sources would play a role in the implementation of the revised SO2 primary 

NAAQS and provide more accurate information on what areas may or may not be 

in attainment than the proposed monitoring-focused approach. 

(6) Comment: NYSDEC recommended that state-level emissions monitors be used 

exclusively for source-oriented monitoring but also be permitted “to replace 

PWEI monitors if the source is within a CBSA.” 

Response: Under a hybrid analytical approach, as introduced in section III of the 

preamble to the final rule, the final monitoring network design is more flexible 

than the proposed network design, as discussed in section IV.B of the preamble, 

and no longer requires monitors based solely on state level emissions (the second 

prong of the proposed network design), as discussed in section IV.B.4 of the 

preamble.

(7) Comment: NYSDEC recommended “that monitoring be considered for any point 

source with greater than 20,000 tpy SO2 emissions.” 

Response: Under a hybrid analytical approach, introduced in section III of the 

preamble to the final rule, and further explained in sections IV, V, and VI of the 

preamble, EPA anticipates using both ambient monitoring and the modeling of 

SO2 sources to play a role in the implementation of the revised NAAQS. If a state 

believes that monitoring is necessary to evaluate source impacts (whether sources 

have been modeled or not), the state may focus its required monitors for that 

purpose (for sources inside CBSAs which have minimum monitoring 

requirements). If a state believes that a source outside of a CBSA with minimum 
106

monitoring requirements warrants ambient monitoring resources to characterize 

source impacts, EPA notes that states always have the prerogative to conduct 

monitoring above the minimum requirements as they see fit consistent with the 

over all monitoring objectives. Further, Regional Administrators also have the 

authority (as discussed in section IV.B.6 of the preamble) to require additional 

monitors above the minimum requirements, if monitoring objectives are not being 

met in an area although the state may be fulfilling minimum monitoring 

objectives. EPA expects Regional Administrators to work with states to address 

in such circumstances.

(8) Comment: NACAA and North Carolina commented that for siting state-level 

emissions monitors, “EPA must allow agencies to determine the most 

scientifically defensible location, while taking into account potential exposures 

and access to locations with adequate siting.” (NACAA and North Carolina) 

Similarly, Dow commented “that EPA's final rule should provide additional 

flexibility with respect to locating monitors within state boundaries solely based 

on their contribution to annual SO2 emissions.” 

Response: Under a hybrid analytical approach, as introduced in section III of the 

preamble to the final rule, the final monitoring network design is more flexible 

than the proposed network design, as discussed in section IV.B of the preamble. 

The required monitoring network is no longer wholly source-oriented in nature, 

and can serve multiple monitoring objectives, as discussed in section IV.B.3. The 

greater flexibility to site monitors to meet different and varied monitoring 

objectives should address the concerns expressed by these commenters. If a state 

believes that monitoring resources outside of a CBSA with minimum monitoring 

requirements are warranted, EPA notes that states always have the prerogative to 

conduct monitoring above the minimum requirements as they see fit consistent 

with the overall monitoring objectives.

(9) Comment: RRI and API cautioned that “if a state’s SO2 monitoring network does 

not include suitably sited background monitors, but instead only includes sourceoriented monitors, specific EPA guideline methods for using this source oriented 

monitoring data to estimate background for compliance modeling need to be 

developed.” (API and RRI) 

Response: Under a hybrid analytical approach, as introduced in section III of the 

preamble to the final rule, the final monitoring network design is more flexible 

than the proposed network design, as discussed in section IV.B of the preamble. 

The required monitoring network is no longer wholly source-oriented in nature, 

and now can serve multiple monitoring objectives, discussed in section IV.B.3, 

including non-source oriented monitors such as those characterizing general 

background. However, EPA does not intend for the required network to be 

shifted to only provide ‘background’ values for modeling purposes, as we believe 

that such an action would be at the expense of other monitoring objectives. EPA 

encourages states to consider monitoring near sources which are not conducive to 
107

modeling, such as (1) sources classified as non-point sources (. “area-sources”) 

such as shipping ports, (2) a source situated in area of complex terrain and/or 

situated in a complex meteorological regime, (3) locations that have multiple, 

relatively small sources with overlapping plumes, and also consider monitoring 

for population exposures. Because monitoring objectives can be broader now and 

include monitoring for background, EPA believes the commenters concern is 

addressed and specific guideline methods to estimate background from sourceoriented monitoring data are not needed at present. 

(10) Comment: Ohio criticized EPA’s proposed monitoring plan for being too stateoriented in its monitor allocation process. Ohio recommended that “allocation of 

monitors based on emissions should not be based upon state borders but rather 

areas of highest concentration regardless of boundaries.... An analysis, based 

upon the final level of the standard, should be conducted to determine what level 

of emissions have an impact on achieving the standard followed by an analysis 

nationally to determine which areas of emissions necessitate monitors (again, 

regardless of boundaries). States are capable of working with our neighbors to 

determine which state would be in the best position to site and operate a monitor.” 

Dow expressed a similar view in noting that “assigning monitors based on which 

state has the most emissions may be inconsistent with the ‘area of highest 

concentration’ criteria” given that the emissions from major SO2 sources “are 

more likely to impact distant areas than local areas and the resultant ground level 

concentrations are often minimal.” 

Response: Under a hybrid analytical approach, as introduced in section III of the 

preamble to the final rule, the final monitoring network design is more flexible 

than the proposed network design, as discussed in section IV.B of the preamble, 

and no longer requires monitors based solely on state level emissions (the second 

prong of the proposed network design), as discussed in section IV.B.4 of the 

preamble. The final network design is intended to focus the minimally required 

monitors into areas that have an increased coincidence of population and SO2

emissions through the use of the PWEI, as discussed in sections IV.B.4 and 

IV.B.5 of the preamble. As states decide where to place their required 

monitoring, under the final network design requirements, they can consider a 

number of monitoring objectives, including identifying the highest concentrations. 

States may find modeling more useful and appropriate to meet certain objectives 

such as identifying peak concentrations near large sources. EPA also notes that 

states always have the prerogative to conduct additional monitoring in any 

locations they see fit consistent with the overall monitoring objectives. 

IV.F: Comments on siting requirements and requesting waivers 

(1) Comment: City of New York supports EPA’s position that SO2 monitors should 

be focused on measuring “maximum ground-level concentrations in areas of both 

higher population and higher emissions.” 
108

Response: Under a hybrid analytical approach, as introduced in section III of the 

preamble to the final rule, the final monitoring network design is more flexible 

than the proposed network design, as discussed in section IV.B of the preamble. 

The required monitoring network is no longer wholly source-oriented in nature, as 

modeling of source may be more appropriate, efficient, and effective in providing 

information on source-oriented concentration data. However, EPA notes that as 

explained in section IV.B.3, states should consider monitoring near sources which 

are not as conducive to modeling, such as (1) sources classified as non-point 

sources (“area-sources”) such as shipping ports, (2) a source situated in area of 

complex terrain and/or situated in a complex meteorological regime, (3) locations 

that have multiple, relatively small sources with overlapping plumes, and also to 

consider monitoring for population exposures.

(2) Comment: Multiple organizations provided comments in support of EPA 

focusing the SO2 monitoring program on measuring maximum concentrations of 

SO2 in the ambient air near major sources of SO2 emissions. (ATS, Center for 

Biological Diversity, Harris County, Houston, and NYSDEC). As 

ALA/EDF/NRDC/SC notes, “People in rural areas have long been neglected by a 

monitoring network strategy that places most monitors in large cities. Large SO2 

sources such as smelters and power plants have created pollution hotspots in less 

populated areas. The people who live, work, and breathe in the vicinity of these 

sources are those most in need of protection from SO2 air pollution.” In addition, 

NYSDEC argued that “by adequately monitoring significant sources of SO2 we 

expect that we will be protecting more populated areas.” 

Response: In response to these and other comments and an examination of our 

historical approach to SO2 NAAQS implementation, we anticipate using a hybrid 

analytical approach, as introduced in section III of the preamble to the final rule 

which combines the use of monitoring and available modeling to assess 

compliance with the new 1-hour SO2 NAAQS. The final monitoring network 

design is focused, through use of the PWEI, into areas where there is increased 

coincidence of population and SO2 emissions (section IV.B.4 and IV.B.5 of the 

preamble). Under this approach, EPA expects many SO2 sources in more rural 

areas to be modeled to characterize their ground-level impacts which should 

address the concerns expressed by the commenters. In addition, there are 

provisions that allow monitoring outside of urban areas where minimum 

monitoring requirements are focused. First, states always have the prerogative to 

conduct monitoring above the minimum requirements as they see fit consistent 

with the overall monitoring objectives (see 40 CFR Part 58, Appendix D, section 

1.1). Second, as discussed in section IV.B.6, EPA Regional Administrators 

(working with states) may require additional monitoring above the minimum 

requirements if they believe monitoring objectives remain unfulfilled in an area 

(urban or rural) even though a state may be satisfying minimum monitoring 

requirements. 
109

(3) Comment: A number of organizations expressed a preference for EPA to use 

population exposure to high ambient air SO2 concentrations as the primary 

criterion for determining where SO2 monitors would be located. Many 

organizations provided comments similar to NPRA’s that for the “purposes of 

designing a monitoring system to support implementation of a NAAQS, 

monitoring plans should not include locations where there is little or no prospect 

of public exposure, but must be broadly related to ambient air conditions faced by 

the public at large.” (ABR, Exxon Mobil, Golder, NPRA, PCA, Rio Tinto, RTA, 

UARG, Delaware, Indiana, North Carolina, South Carolina, South Dakota, and, 

Wisconsin) 

Response: Under a hybrid analytical approach, as introduced in section III of the 

preamble to the final rule, the final monitoring network design is focused, through 

use of the PWEI, into areas where there is increased coincidence of population 

and SO2 emissions (section IV.B.4 and IV.B.5 of the preamble). We now 

anticipate using an approach combining the use of monitoring and available 

modeling to assess compliance with the new 1-hour SO2 NAAQS. Therefore, the 

required monitoring network is no longer wholly source-oriented in nature, and 

can now serve multiple monitoring objectives, as discussed in section IV.B.3, 

including population exposure. Further, EPA encourages states to consider 

monitoring near sources which are not as conducive to modeling, such as (1) 

sources classified as non-point sources (i.e., “area-sources”) such as shipping 

ports, (2) a source situated in area of complex terrain and/or situated in a complex 

meteorological regime, (3) locations that have multiple, relatively small sources 

with overlapping plumes, and also to consider monitoring for population 

exposures. 

 .

(4) Comment: Five organizations requested that state and local agencies be provided 

the authority to remove SO2 monitors: “based on an assessment of local 

conditions and risks (Wisconsin), ”if the ambient data is not likely to violate the 

NAAQS” (MIDNRE), “if measured design values at the site are less than 75% of 

the selected standard level” (AirQuality), or if “the data show that the 

concentrations in the area are <75% of the NAAQS” in a CBSA with only one 

monitor or “concentrations are <80% of the NAAQS” for a CBSA with more than 

one monitor (Alaska and NACAA). AirQuality also remarked that the decision to 

remove SO2 monitors should “include relevant stakeholder input that includes 

consideration of continuing monitoring of a ‘clean air’ area because the monitor 

provides the community confidence the operations of nearby sources are well 

within health protective limits.” 

Response: Under existing regulation (40 CFR 58.14(c)), there is a process states 

may utilize to receive EPA Regional Administrator approval to shut down an 

existing SO2 monitoring site. However, EPA notes that this applies to monitors in 

excess of the established minimum monitoring requirements in 40 CFR Part 58 

Appendix D. 
110

 (5) Comment: Several organizations requested EPA provide greater flexibility for 

state agencies in the siting and operation of monitoring stations and conducting 

network reviews in order to protect the public health as well as possible (Iowa, 

MIDNRE, and South Carolina). 

Response: Under a hybrid analytical approach, as introduced in section III of the 

preamble to the final rule, the final monitoring network design has greater 

flexibility than the proposed network design, as discussed in section IV.B of the 

preamble. The final monitoring network design now has the flexibility to address 

multiple monitoring objectives including source characterization, highest 

concentration, population exposure, general background, or regional transport 

(discussed in section IV.B.3 of the preamble).

(6) Comment: South Carolina commented that EPA’s “monitoring requirement must 

include the ability for states to address the needs for area and regional background 

concentration measurements” by allowing for at least some area-wide monitoring 

stations to stay in there current locations. 

Response: Under a hybrid analytical approach, as introduced in section III of the 

preamble to the final rule, the final monitoring network design has greater 

flexibility than the proposed network design, as discussed in section IV.B of the 

preamble. The required monitoring network is no longer wholly source-oriented 

in nature, and now can serve multiple monitoring objectives, as discussed in 

section IV.B.3, including siting of non-source oriented monitors such as those 

characterizing background concentrations (within a CBSA where monitoring is 

required). If a state believes that monitoring resources outside of a CBSA with 

minimum monitoring requirements are warranted, EPA notes that states always 

have the prerogative to conduct monitoring above the minimum requirements as 

they see fit consistent with the overall monitoring objectives.

 

(7) Comment: MIDNRE requested that state agencies “have the flexibility to 

substitute a nearby pre-existing monitoring station that may be located at a 

population-oriented location if its location is within a prescribed distance (e.g., 

three kilometers).” 

Response: Under a hybrid analytical approach, as introduced in section III of the 

preamble to the final rule, the final monitoring network design has greater 

flexibility than the proposed network design, as discussed in section IV.B of the 

preamble. The required monitoring network is no longer wholly source-oriented 

in nature, and now can serve multiple monitoring objectives, as discussed in 

section IV.B.3. Regarding the commenter’s specific suggestion of allowing 

existing monitors to be used, this comment was directed at the proposed network 

design’s requirement that all minimally required monitors be source-oriented. 

Under the more flexible final network design, EPA believes the commenter’s 

concern should be alleviated.
111

(8) Comment: RRI commented that EPA “should encourage states to provide a 

stakeholder process so that affected emission sources can be consulted in the 

state’s planning process for the monitor placement, which may include a 

dispersion modeling analysis conducted by the state.” 

Response: Under a hybrid analytical approach, a state’s monitoring network plan, 

including any source-oriented monitoring that might be conducted under the plan 

will go through a public notice process. As required in 40 CFR 58.10, states 

develop an annual monitoring network plan, which would describe its monitoring 

network including any proposed or forthcoming changes to the monitoring 

network, which is available for public inspection prior to submission to EPA. 

RRI’s comment regarding the use of a dispersion modeling analysis to determine 

monitor placement is directed at the proposed and alternative network design on 

which EPA took comment but is not adopting in the final rule. 

(9) Comment: City of New York requested EPA “require that states provide an 

opportunity for affected municipalities to provide relevant information and 

suggestions about monitor locations, and that states shall consider such 

information in their proposals to the EPA” because oftentimes local agencies have 

information useful to the siting of air quality monitors such as the “location of 

existing sensitive populations, future demographic shifts, and the scope of 

planned developments, and other governmental initiatives.” 

Response: Monitoring network plans that states develop for the SO2 monitoring 

network will go through a public notice process. As required in 40 CFR §58.10, 

states develop an annual monitoring network plan, which would describe its 

monitoring network including any proposed or forthcoming changes to the 

monitoring network, which is available for public inspection prior to submission 

to EPA.

 (10) Comment: MIDNRE recommended EPA conduct a feasibility study to justify 

“the need for multiple SO2 monitors within a single CBSA and the need for new 

monitoring stations.” In the feasibility study, “the maximum modeled 

concentrations should be compared with the design values generated by urban 

areas that already operate a rich density of SO2 monitors. By comparing the 

ambient data and isopleths created from it with dispersion modeling outputs, 

relationships between monitoring and modeling data can be estimated for CBSAs. 

This could provide insights into how ambient stations can be substituted for 

maximum concentration sites, leveraging the infrastructure. The level of the 

NAAQS may need to be adjusted downward to retain the same level of protection 

as if a maximum concentration site were used.” 

Response: Under a hybrid analytical approach, as introduced in section III of the 

preamble to the final rule, the final monitoring network design is focused on, 

through use of the PWEI, into areas where there is increased coincidence of 

population and SO2 emissions (section IV.B.4 and IV.B.5 of the preamble). 
112

Under this approach we anticipate combining the use of monitoring and available 

modeling to assess compliance with the new 1-hour SO2 NAAQS. Therefore, the 

required monitoring network no longer must be wholly source-oriented in nature, 

and now can serve multiple monitoring objectives, as discussed in section IV.B.3, 

while the modeling of sources in effect provide source-oriented concentration 

data. EPA believes the more flexible final network design addresses the concerns 

raised by the commenter, significantly relieving what some considered a 

redundant or unnecessary monitoring burden on states, reducing the need for new 

monitoring sites that would have been required under the proposal, and only 

requiring multiple monitors in urban areas where the relatively highest 

coincidences of population and SO2 emissions are occurring. 

(11) Comment: API and RRI commented that “EPA should explicitly consider the 

needs of compliance modeling in siting monitors,” making sure to “avoid doublecounting regional background concentrations to be obtained from appropriate 

monitors, which are added to modeled impacts from a specific facility” (API and 

RRI). 

Response: We recognize the need for and distinct role that representative 

background monitored concentrations play in support of modeling demonstrations 

for compliance with the standards. However, we feel the monitoring language in 

this final rule as written appropriately addresses the needs of compliance 

modeling by including the measurement of background concentrations as one of 

the monitoring objectives. Further, we believe the monitoring rule is not an 

appropriate regulatory forum to address the needs identified in the comment 

further. As indicated in the final rule, EPA intends to develop guidance for the 

use of refined air quality dispersion modeling in implementing the new SO2 1-

hour NAAQS and the issue of how to appropriately define background 

concentrations will be addressed in that guidance, as needed. 

(12) Comment: RRI and LEC suggested EPA develop a more robust method for 

calculating background concentrations of SO2 by “assigning background on an 

hour-by-hour basis from one or more monitors, accounting for wind direction and 

excluding hours when a monitor is directly downwind of the its existing source. 

Another approach could be the development of SO2 background climatology 

according to wind direction and season, based upwind monitors that are not 

influenced by local sources.” RRI continued by stating that “refined modeling 

approaches that consider realistic combinations of source impacts and regional 

background concentrations are critical to avoid false identifications of nonexistent non-attainment areas.” 

Response: EPA recognizes the technical challenges associated with conducting a 

cumulative ambient air quality impact analysis given the 1-hour averaging time 

and the statistical form of the new standard. Existing guidance for demonstrating 

compliance with NAAQS through dispersion modeling is expected to be generally 

applicable and adequate for the new hourly SO2 standard, and that guidance 
113

provides an appropriate amount of flexibility to address specific issues that may 

arise on a case-by-case basis. However, EPA expects to develop additional 

modeling guidance to support the proper implementation of the new standard. 

Further, we believe the monitoring rule is not an appropriate regulatory forum to 

address the needs identified in the comment further. 

(13) Comment: RRI urged EPA not to allow “states to use ‘hot spot’ monitors to 

characterize regional background concentrations that are needed for 

comprehensive modeling analyses.” In addition, RRI requested EPA allow states 

to permit “affected facilities to conduct refined modeling to determine whether 

there is a need for monitoring near these facilities, and where any monitor should 

be placed.” 

Response: Under a hybrid analytical approach, as introduced in section III of the 

preamble to the final rule, the final monitoring network design is focused, through 

use of the PWEI, into areas where there is increased coincidence of population 

and SO2 emissions (section IV.B.4 and IV.B.5 of the preamble). Under this 

approach we anticipate combining the use of monitoring and available modeling 

to assess compliance with the new 1-hour SO2 NAAQS. Therefore, the required 

monitoring network no longer must be wholly source-oriented in nature, and now 

can serve multiple monitoring objectives, as discussed in section IV.B.3, while 

the modeling of sources in effect provide source-oriented concentration data. 

Because of the greater flexibility of the final monitoring network to address 

multiple monitoring objectives including general background (discussed in 

section IV.B.3 of the preamble), EPA expects that non-source oriented monitoring 

data will be available for use as background concentration input for modeling 

analysis. Regarding the “need for monitoring” near facilities, states should 

consider monitoring near sources which are not as conducive to modeling, such as 

(1) sources classified as non-point sources (i.e., “area-sources”) such as shipping 

ports, (2) a source situated in area of complex terrain and/or situated in a complex 

meteorological regime, (3) locations that have multiple, relatively small sources 

with overlapping plumes.

(14) Comment: Louisiana Chemical recommends that the placement of SO2 monitors 

“be based on projected emissions rather than historical data” since “significant 

reductions in levels of sulfur dioxide emissions have occurred since 2005 and 

these will continue to decrease. Decisions on monitor locations should be based 

on enforceable agreements, permits and regulations in the process of 

implementation.” 

Response: Rather than the proposed monitoring-focused approach to assess 

compliance with the SO2 NAAQS, EPA anticipates using a hybrid analytical 

approach, introduced in section III of the preamble to the final rule, which 

combines monitoring and modeling using each of these where they are most 

appropriate and effective which should address the commenters concerns. Since 

it is anticipated that dispersion modeling will be used to assess impacts of 
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individual SO2 sources, the required monitoring network is no longer wholly 

source-oriented in nature, and now can serve multiple monitoring objectives, as 

discussed in section IV.B.3, while the modeling of sources in effect provide 

source-oriented concentration data. This approach should address the 

commenter’s concerns regarding the appropriate location of monitors. 

(15) Comment: MSCC requests “greater consideration...be given so that the limited 

monitoring resources are placed in the areas judged most likely to benefit from 

the measurements–most likely to a) have populations at risk, b) have 

concentrations above the threshold of the standard, and c) most likely to have a 

feasible means of correcting any problem found.” 

Response: Under a hybrid analytical approach, as introduced in section III of the 

preamble to the final rule, the final monitoring network design is focused, through 

use of the PWEI, into areas where there is increased coincidence of population 

and SO2 emissions (section IV.B.4 and IV.B.5 of the preamble). The required 

monitoring network no longer must be wholly source-oriented in nature, and now 

can serve multiple monitoring objectives, as discussed in section IV.B.3, while 

the modeling of sources in effect provide source-oriented concentration data. 

EPA believes the final network design, in combination with the projected use of 

modeling, alleviates the concerns raised by the commenter.

(16) Comment: Dow recommended EPA relocate “only a portion of the current 

monitors during the first few years of short term data collection” in order that data 

from the new monitoring stations can be compared with the pre-existing 

monitoring stations and community monitors to gain a better understanding of the 

strengths and weaknesses of the new monitoring system. Dow also expressed 

concern that “the new monitoring network could result in some monitoring results 

that are variable and responsive to small changes in placement.” 

Response: Under a hybrid analytical approach, as introduced in section III of the 

preamble to the final rule, the final monitoring network design is focused, through 

use of the PWEI, into areas where there is increased coincidence of population 

and SO2 emissions (section IV.B.4 and IV.B.5 of the preamble). Under this 

approach we anticipate combining the use of monitoring and available modeling 

to assess compliance with the new 1-hour SO2 NAAQS. Thus, the required 

monitoring network is no longer wholly source-oriented in nature, and now can 

serve multiple monitoring objectives, discussed in section IV.B.3, while the 

modeling of sources may be more efficient and effective in providing sourceoriented concentration data. EPA believes the final network design, in 

combination with the projected use of modeling, should alleviate the concerns 

raised by the commenter.

(17) Comment: City of Alexandria commented that, based on its own monitoring 

experiences, “local monitoring should only be used as a backstop, in order to 

assist in compliance verification, and only after air quality model-based 
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limitations are developed and implemented by the large stationary sources that 

have the potential to contravene NAAQS protections.” 

Response: Partly in response to these and other comments and an examination of 

our historical approach to SO2 NAAQS implementation, we now anticipate using 

a hybrid analytic approach, as introduced in section III of the preamble to the final 

rule, combining the use of monitoring and available modeling to assess 

compliance with the new 1-hour SO2 NAAQS. Rather than a monitoring-focused 

approach EPA anticipates using each of these analytic tools where they are most 

appropriate and effective. We anticipate placing greater emphasis on modeling 

than did the proposed rule as the most technically appropriate, efficient, and 

readily available method for assessing short-term ambient SO2 concentrations in 

areas with large point sources. Under this approach, the required monitoring 

network no longer must be wholly source-oriented in nature, and now can serve 

multiple monitoring objectives, as discussed in section IV.B.3, while the 

modeling of sources in effect provide source-oriented concentration data. EPA 

believes states should consider monitoring in areas or near sources that are not 

conducive to modeling, such as (1) sources classified as non-point sources (“areasources”) such as shipping ports, (2) a source situated in area of complex terrain 

and/or situated in a complex meteorological regime, (3) locations that have 

multiple, relatively small sources with overlapping plumes. This approach should 

address the commenter’s concern. 

(18) Comment: AirQuality recommended EPA “consider adding simple particle 

detectors to SO2 sites to further characterize the nature of the plume. If other such 

parameters are to be required at SO2 sites EPA should be prepared to fund the 

capital cost of the equipment involved.” 

Response: While EPA does not disagree with the reasoning of the commenter’s 

suggestion, EPA is only addressing the SO2 monitoring network design here. 

Particulate matter is a separate NAAQS pollutant which is on another time-line 

for review. However, EPA encourages and applauds state efforts to make 

monitoring sites multi-pollutant. 

(19) Comment: Wisconsin requested “a clarification on the minimum monitoring 

requirements presented on page 64850. When EPA discusses minimum SO2 

monitoring requirements, the third requirement is ‘any ongoing SO2 monitoring 

must have a least one monitor sited to measure the maximum concentration of 

SO2 in that area.’ Further clarification is requested for the term ‘area’. What is the 

special extent that one existing monitor must measure?” 

Response: Due to the change of approach from proposal, this comment is now 

moot. 

(20) Comment: Several organizations provided comments in support of granting SO2 

monitor waivers when violation of the SO2 NAAQS has been proven to be 
116

unlikely. Iowa recommended that “existing monitors that record low values” 

continue to be considered “subject to removal in accordance with existing 

regulations contained in 40 CFR 58 Subpart B paragraph 58.14” as well as being 

considered for a waiver when “there are no monitoring locations in the prescribed 

area where site specific ambient impact analyses predict a NAAQS violation, 

there are no monitoring locations available in the modeled “hot spot” that meet 

EPA siting requirements, or the modeled “hot spot” occurs in an area that is 

inaccessible or unpopulated.” MIDNRE recommended EPA permit a monitoring 

requirement to be waived “if modeling shows maximum concentrations are less 

than 50 percent of the NAAQS” so that state agencies can efficiently use their 

limited budgets. (Iowa, MIDNRE, and South Carolina) 

Response: Under a hybrid analytical approach, as introduced in section III of the 

preamble to the final rule, the required monitoring network no longer must be 

wholly source-oriented in nature, and now can serve multiple monitoring 

objectives, as discussed in section IV.B.3, while the modeling of sources we 

believe would be more efficient and effective in providing source-oriented 

concentration data. Further, fewer monitors are required in the final network 

design versus that number which was proposed. Due to the flexibility in the 

monitoring network design and the adjustments made to the PWEI thresholds by 

which monitoring is required (discussed in section IV.B.4 and IV.B.5), EPA does 

not believe that a waiver provision is necessary. Regarding site shutdown, 40 

CFR 58.14(c) explains how a state can seek EPA Regional Administrator 

approval to shut down an existing SO2 monitoring site. However, EPA notes that 

this applies to monitors in excess of the established minimum monitoring 

requirements in 40 CFR Part 58 Appendix D. 

(21) Comment: Three commenters expressed concern that the “two pronged approach 

in the proposed regulation will lead to duplicative monitoring in some areas and 

require monitors in areas where monitors are not needed” and recommend EPA 

allow monitors to be removed “(1) where there is only one monitor for a given 

source or Core Based Statistical Area (CBSA) and data show that the 

concentrations in the area are less than 75 percent of the National Ambient Air 

Quality Standards (NAAQS); or (2) where there is more than one monitor in a 

given CBSA or for a given source and data show that concentrations are less than 

80 percent of the NAAQS.” (North Carolina, NACAA, and Ohio) 

Response: Under a hybrid analytical approach, as introduced in section III of the 

preamble to the final rule, the required monitoring network no longer must be 

wholly source-oriented in nature, and now can serve multiple monitoring 

objectives, discussed in section IV.B.3, while the modeling of sources would be 

more efficient and effective in providing source-oriented concentration data. EPA 

believes an approach that combines use of monitoring and modeling and not 

requiring monitors under a second prong as proposed alleviates the commenters’ 

concerns regarding duplicative monitoring. In addition, due to the flexibility in 

the monitoring network design and the adjustments made to the PWEI thresholds 
117

by which monitoring is required (discussed in section IV.B.4 and IV.B.5), EPA 

does not believe that a waiver provision is necessary. 

(22) Comment: Wisconsin recommends that the “proposed rule should provide 

flexibility for states to eliminate monitors from CBSAs based on an assessment of 

local conditions and risks and EPA Regional Directors should have the ability to 

approve waivers or exceptions to the monitoring requirements in the rule.” 

Wisconsin also recommends “EPA allow flexibility for monitor siting in rural 

areas, if necessary, and that such monitors would fulfill prong 1 requirements.” 

Response: Under a hybrid analytical approach, as introduced in section III of the 

preamble to the final rule, the required monitoring network no longer must be 

wholly source-oriented in nature, and now can serve multiple monitoring 

objectives including siting monitors in rural areas consistent with the monitoring 

objectives, as discussed in section IV.B.3, while the modeling of sources in effect 

provide source-oriented concentration data. Due to the flexibility in the 

monitoring network design and the adjustments made to the PWEI thresholds by 

which monitoring is required (discussed in section IV.B.4 and IV.B.5), EPA does 

not believe that a waiver provision is necessary. Regarding the request for 

“...EPA Regional Directors should have the ability to approve waivers or 

exceptions to the monitoring requirements in the rule”, EPA disagrees with such a 

concept. The minimum monitoring requirements are a ‘minimum’ to ensure that 

a network of sufficient size and focus is maintained to support the NAAQS.

IV.G: Comments on Proposed Alternative Network 

(1) Comment: Five commenters expressed support for the proposed alternative 

monitoring network and the use of dispersion modeling to determine where SO2 

monitors should be located. For instance Iowa recommended “that the final rule 

contains provisions that require monitors to be sited only at locations where 

dispersion modeling indicates that the NAAQS is violated” (Alexandria, 

Delaware, South Dakota, and Center for Biological Diversity). Delaware’s 

support for the alternative monitoring network did come with the recommendation 

that EPA revise its approach to “include the impact of modeled concentrations on 

populated areas, and not rely on modeled ambient concentrations alone to 

determine the number or location of monitoring sites.” 

Response: Under a hybrid analytical approach, as introduced in section III of the 

preamble to the final rule, the final monitoring network design is focused, through 

use of the PWEI, into areas where there is increased coincidence of population 

and SO2 emissions (section IV.B.4 and IV.B.5 of the preamble). Under this 

approach, we anticipate combining the use of monitoring and available modeling 

to assess compliance with the new 1-hour SO2 NAAQS. Thus, the required 

monitoring network no longer must be wholly source-oriented in nature, and now 

can serve multiple monitoring objectives, discussed in section IV.B.3, while the 

modeling of sources we believe would be more efficient and effective in 
118

providing source-oriented concentration data. EPA believes that at hybrid 

analytical approach is superior to both the proposed network design and the 

alternative network design because redundant and/or unnecessary monitoring is 

reduced, required monitoring can serve multiple monitoring objectives, and 

modeling can provide data that would have otherwise only been collected by a 

relative fewer number of source-oriented monitors compared to the number of 

sources that can be modeled. EPA’s reasons for believing that modeling SO2

sources is more effective than just source-oriented SO2 monitoring is discussed in 

detail in section IV.B.2 of the preamble. 

(2) Comment: Five commenters expressed opposition to EPA’s proposed alternative 

monitoring network with three of the organizations opposing the proposed 

alternative network design because it “would not distinctly use population as a 

factor in deciding where monitors should be placed.” (AirQuality, South 

Carolina, and UARG). In addition, NPRA opposed EPA’s proposed alternative 

network because it would “effectively make the SO2 NAAQS a “source oriented” 

NAAQS” which is “not supported by the clear language of the CAA.” Finally, 

Indiana opposed the alternative network because the state agency preferred “to 

determine placement of the monitors” rather than have the monitor locations 

determined strictly from dispersion modeling. 

Response: In the final rule, EPA is not adopting the alternative monitoring 

network. Instead, under a hybrid analytical approach, as introduced in section III 

of the preamble to the final rule, the final monitoring network design is focused, 

through use of the PWEI, into areas where there is increased coincidence of 

population and SO2 emissions (section IV.B.4 and IV.B.5 of the preamble). Under 

this approach, we anticipate combining the use of monitoring and available 

modeling to assess compliance with the new 1-hour SO2 NAAQS. Thus, the 

required monitoring network no longer must be wholly source-oriented in nature, 

and now can serve multiple monitoring objectives, as discussed in section IV.B.3, 

while the modeling of sources we believe could be more appropriate, efficient, 

and effective in providing source-oriented concentration data. EPA believes that 

a hybrid analytical approach is superior to both the proposed network design and 

the alternative network design because redundant and/or unnecessary monitoring 

is reduced, required monitoring can serve multiple monitoring objectives, and 

modeling can provide data that would have otherwise only been collected by a 

relative fewer number of source-oriented monitors compared to the number of 

sources that can be modeled. EPA’s reasons for believing that modeling SO2

sources is more effective than just source-oriented SO2 monitoring is discussed in 

detail in section IV.B.2 of the preamble.

(3) Comment: API provided comments recommending that if dispersion modeling 

was used in the monitor location process that “EPA should encourage states to 

provide a stakeholder process so that affected emission sources can be consulted 

in the state’s planning process for the monitor placement...If an affected source 

does not agree with the state’s analysis on the placement of one or more SO2 
119

monitors, they should have the right to provide their own study for state review 

and consideration if they choose to do so.” 

Response: Under a hybrid analytical approach, as introduced in section III of the 

preamble to the final rule, the required monitoring network no longer must be 

wholly source-oriented in nature, and now can serve multiple monitoring 

objectives, as discussed in section IV.B.3, while the modeling of sources we 

believe would be more efficient and effective in providing source-oriented 

concentration data. EPA is encouraging states to consider monitoring in areas or 

near sources that are not conducive to modeling, such as (1) sources classified as 

non-point sources (i.e., “area-sources”) such as shipping ports, (2) a source 

situated in area of complex terrain and/or situated in a complex meteorological 

regime, (3) locations that have multiple, relatively small sources with overlapping 

plumes. The decision on the appropriate locations for the required monitors falls 

on the states, who will detail such decisions in their annual monitoring network 

plan. States must develop annual monitoring network plans under 40 CFR 

§58.10, and the plan must be available to the public prior to submission to EPA. 

(4) Comment: UARG opposes the proposed use of AERMOD for any requisite 

dispersion modeling because it would “simply not be appropriate for siting a 

monitor at the likely site of the expected maximum 1-hour concentration–the 

model has been validated by comparing it with observed high ambient 

concentrations, but its performance for matching concentrations at a specific time 

or location has not been evaluated.” 

Response: The revised approach for monitoring shifts the focus of monitoring 

toward characterizing ambient concentrations that are best achieved through 

ambient monitoring, such as areas with a large number of smaller SO2 sources or 

areas with complex meteorological influences, such as coastlines, that may be 

more difficult to characterize through modeling. Under a hybrid analytical 

approach (as introduced in section III of the preamble to the final rule), modeling 

will typically not be relied upon to site a single source-oriented ambient monitor. 

The limitation of a single monitor to adequately account for peak hourly SO2

ambient concentrations in the vicinity of SO2 emission sources, even if dispersion 

modeling was conducted to site the monitor, is part of the rationale for revising 

the focus and objectives for the ambient monitoring network. 

(5) Comment: NYSDEC expressed concern in its comments that “using 

AERSCREEN or SCREEN3 may not be the most practical approach for 

determining the number and location of required monitors” because “the data 

required to run the models are only readily available for permitted facilities,” 

which would result in incomplete and inaccurate dispersion models. 

Response: Under a hybrid analytical approach, as introduced in section III of the 

preamble to the final rule, the required monitoring network no longer must be 

wholly source-oriented in nature, and now can serve multiple monitoring 
120

objectives, as discussed in section IV.B.3, while the reliance on appropriate 

refined dispersion modeling would be used to provide source-oriented 

concentration data. 

(6) Comment: RRI and LEC suggested EPA develop a more robust method for 

calculating background concentrations of SO2 by “assigning background on an 

hour-by-hour basis from one or more monitors, accounting for wind direction and 

excluding hours when a monitor is directly downwind of the its existing source. 

Another approach could be the development of SO2 background climatology 

according to wind direction and season, based upwind monitors that are not 

influenced by local sources.” (RRI (.0388), LEC (.0257)) RRI continued by 

stating that “refined modeling approaches that consider realistic combinations of 

source impacts and regional background concentrations are critical to avoid false 

identifications of non-existent non-attainment areas.” (RRI (.0389)) 

Response: EPA recognizes the technical challenges associated with conducting a 

cumulative ambient air quality impact analysis given the 1-hour averaging time 

and the statistical form of the new standard. Existing guidance for demonstrating 

compliance with NAAQS through dispersion modeling is expected to be generally 

applicable and adequate for the new hourly SO2 standard, and that guidance 

provides an appropriate amount of flexibility to address specific issues that may 

arise on a case-by-case basis. However, EPA expects to develop additional 

modeling guidance to support the proper implementation of the new standard. 

(7) Comment: NYSDEC recommended that if the alternative monitoring network 

approach is pursued, potential monitoring should be limited “to the source types 

of highest concern (e.g. smelters, coal burning plants, >20,000 tpy emission 

sources). In addition, modeling can be used in areas of uncertainty, as an 

auxiliary or complementary tool to other methods for assessing the likelihood of 

NAAQS exceedance.” 

Response: EPA has not pursued the alternative network design. The final 

network design is discussed in section IV.B of the preamble to the final rule. 

(8) Comment: North Carolina made several recommendations for how the alternative 

monitoring network proposal could be improved. First, North Carolina suggested 

that any “facility whose smokestack SO2 emissions are above a determined 

emission rate would be required to perform modeling to demonstrate that the SO2 

concentration at the fence-line does not exceed whatever ambient standard EPA 

establishes. If modeling does not demonstrate compliance, the facility could then 

be required to reduce emissions from the stack, install continuous emissions 

monitoring (CEM) in the stack itself, or require a fence-line monitor at the target 

facility. This will allow flexibility to work with facilities to determine if 

monitoring is required and will reduce the cost and consequences of 

nonattainment.” Second, EPA could create a monitoring approach similar “to the 

lead network where modeling can be used to demonstrate whether the facility 
121

impacts the NAAQS. Facilities where the model indicates the ambient air is a 

certain amount below the NAAQS” would not have to monitor. “This approach 

would also reward states that have taken actions to reduce SO2 emissions from 

their facilities and place the monitoring burden on states that have more 

significant SO2 emissions.” 

Response: EPA has not pursued the alternative network design, but instead has 

identified a hybrid analytical approach, as introduced in section III of the 

preamble to the final rule, where the required monitoring network no longer must 

be wholly source-oriented in nature, and now can serve multiple monitoring 

objectives, as discussed in section IV.B.3, while the modeling of sources we 

believe would be more efficient and effective in providing source-oriented 

concentration data. This combination of monitoring and modeling is anticipated 

to provide the data for implementation of and designations under the revised 

primary SO2 NAAQS. EPA believes that a hybrid analytical approach embodies 

much of the suggestions made by North Carolina, thus addressing their concerns. 

(9) Comment: Missouri recommended “EPA should not require states to conduct 

refined modeling in every case that the screening tools show a violation. Many 

times, the screening tools are overly conservative and might lead to analysis in 

areas that is not warranted. Many states will have a better indication of areas that 

might violate the new NAAQS without this type of screening approach due to 

previous permit modeling exercises or additional ambient monitoring data 

collected by third parties.” 

Response: This comment is no longer relevant under a hybrid analytical 

approach, as discussed in sections III, IV, V, and VI in the preamble to the final 

rule. 

IV.H: Comments on data reporting 

(1) Comment: Iowa commented that EPA will need to emphasize that “long term 

monitoring commitments are required, as computation of design values at a 

monitoring site requires at least three years of data.” 

Response: EPA concurs with Iowa that monitoring sites are long term 

commitments. EPA expects that many of the existing sites in urban area with 

required minimum monitors will be used to meet minimum monitoring 

requirements of this final rule, thus continuing operation of some sites that could 

be already considered ‘long-term’ sites. 

(2) Comment: NESCAUM recommended if a “source-oriented site has or is expected 

to have SO2 levels at 75% or more of the NAAQS... that five-minute wind data 

also be collected at the site.” 
122

Response: EPA did not propose, and is not finalizing a requirement for wind data 

at source oriented monitoring sites. EPA encourages states, as appropriate, or 

possible, to collect meteorological data at such sites in order to better characterize 

source impacts on the monitoring site. 

(3) Comment: Both Alaska and NACAA oppose requiring states to measure and 

report data at increments below one hour. In Alaska’s comments, the Department 

notes that upgrading its “current data collection infrastructure to capture, store 

and review data on a smaller time scale than one hour would impose substantial 

costs” because Alaska does not currently possess monitors capable of reporting 

five-minute SO2 concentrations. Similarly, NACAA commented that while report 

five-minute SO2 concentrations “may be easily achievable for some agencies with 

the appropriate data collection infrastructure, requiring all agencies to perform 

these duties impose significant costs.” 

Response: EPA notes that all continuous UVF analyzers produce readings that 

can be aggregated into time intervals as short as 1-minute. Such units should 

easily be capable of reporting 5-minute averages. EPA believes that the 

prevalence of data acquisition systems that can acquire monitor output signals and 

formulate averages as short as 1-minute is nearly ubiquitous among state, local, 

and tribal agencies. Such data systems can typically be configured to store and 

report 1-minute averages to a central database which subsequently can be 

configured to aggregate data into the desired averaging period (e.g., 5-minute, 15-

minute, and 60-minute). Although agencies may be most familiar with reporting 

averages over 60-minute periods, the capability to create alternative average 

periods and report such averages in AQS compatible formats likely resides within 

most current data systems. EPA acknowledges that some investment may be 

necessary for those few agencies whose data systems cannot currently meet the 

required reporting requirements for SO2; however, EPA also believes that the 

investment in such systems will provide efficiencies in data handling across all 

continuous measurements that will outweigh the initial cost.

(4) Comment: Missouri recommends EPA require SO2 concentrations to be reported 

in ppm rather than ppb in order to be consistent with other the reporting of other 

pollutants and the Air Quality Index. 

Response: Since the standard itself is expressed in parts per billion (rather than in 

fractional parts per million), EPA believes it appropriate for SO2 concentrations to 

be reported the same way.

(5) Comment: Seven organizations submitted comments in opposition to EPA 

proposal to require state and local agencies to report the maximum 5-minute block 

average because the reporting requirement would represent too great a burden on 

state finances and labor and because the additional reporting requirements would 

not be necessary in order to demonstrate compliance with the proposed new SO2 

standard. (Iowa, NYSDEC, LEC, RRI, South Carolina, and KYDEP) 
123

Response: EPA has finalized the requirement to report the maximum 5-minute 

data block occurring in each hour. EPA’s reasons for this requirement are 

discussed in detail in section IV.C of the preamble to the final rule. 

(6) Comment: AirQuality recommended “hourly met data should be reported to EPA 

while the maximum 5-minute block average met data should be reported at the 

State’s option” so that EPA will be able “to accurately determine the source of the 

measured SO2.” 

Response: EPA did not propose, and is not finalizing a requirement for wind data 

at source oriented monitoring sites. EPA encourages states, as appropriate, or 

possible, to collect meteorological data at such sites in order to better characterize 

source impacts on the monitoring site.

(7) Comment: Six organizations provided comments supporting the requirement that 

state agencies submit all 12 5-minute block SO2 concentrations to EPA along with 

the 1-hour average either because the organization considered reporting all 12 5-

minute concentrations “less resource intensive than selecting the 5-minute max 

for each hour” (NYSDEC) or because it “will provide higher quality data for 

future NAAQS reviews” (AirQuality). Center for Biological Diversity, 

Wisconsin, ATS, and ALA/EDF/NRDC/SC) 

Response: EPA has finalized the requirement to report the maximum 5-minute 

data block occurring in each hour. EPA’s reasons for this requirement are 

discussed in detail in section IV.C of the preamble to the final rule.

(8) Comment: Three organizations submitted comments opposing the requirement 

that state agencies provide all 12 5-minute block SO2 concentrations because the 

requirement would create too great a reporting burden for financially strapped 

state agencies. (Missouri, South Carolina, and South Dakota) 

Response: EPA has finalized the requirement to report the maximum 5-minute 

data block occurring in each hour, as discussed in detail in section IV.C of the 

preamble to the final rule.

(9) Comment: Both ALA/EDF/NRDC/SC and South Dakota provided comments 

supporting the requirement that state agencies report the highest 5-minute rolling 

average SO2 concentration from each hour. In its justification for supporting a 

highest 5-minute rolling average reporting requirement, ALA/EDF/NRDC/SC 

noted “the rolling average will yield data more appropriate to the evaluation of 

health concerns than block averaging which is arbitrary and does not actually 

report the highest 5-minute concentration.” (South Dakota and 

ALA/EDF/NRDC/SC) In addition, Center for Biological Diversity expressed 

support for EPA requiring states to report the highest 5-minute rolling average 

SO2 concentration for each hour at SO2 monitors where possible. 
124

Response: EPA has finalized the requirement to report the maximum 5-minute 

data block occurring in each hour. See discussion in section IV.C of the preamble 

to the final rule.

(10) Comment: Three organizations provided comments opposing the requirement that 

state agencies report highest 5-minute rolling average SO2 concentrations because 

the use of rolling averages would be burdensome to financially strapped state 

agencies in charge of the SO2 monitoring network. (North Carolina, Indiana, and 

South Carolina) 

Response: EPA has finalized the requirement to report the maximum 5-minute 

data block occurring in each hour, as discussed in detail in section IV.C of the 

preamble to the final rule.

IV.I: Comments on Cost of Monitoring Changes 

(1) Comment: Numerous commenters expressed serious concern about large the cost 

of the ambitious monitoring network EPA is proposing and how it will be funded. 

These commenters said that state governments are already struggling with budget 

cuts and the addition of a new SO2 monitoring program on top of other monitoring 

program expansions is likely to result in cuts to state air quality monitoring 

programs, regardless of federal requirements, without federal funding assistance. 

Many comments called on the federal government to significantly increase federal 

funding for the development and operation of the proposed SO2 monitoring 

network in general terms. (NYSDEC, Delaware, Indiana, AEPSC, NESCAUM, 

Illinois, South Carolina, Texas, Wisconsin, CE, Ohio). Others explicitly requested 

that EPA fund the new SO2 monitoring program with Section 103 funds rather 

than leave the monitoring network unfunded or funded with Section 105 funds 

(which require matching state and local funds). (Florida, Iowa, Pennsylvania , 

MIDNRE , North Carolina , Alaska, NACAA, KYDEP, and Vermont) 

Response: EPA notes that $15 million in new STAG funding has been proposed 

in the President's budget for fiscal year 2011 for the purpose of supporting new 

monitoring equipment purchases related to the NAAQS. EPA also notes that 

additional STAG funding was proposed to support "core" programs which can 

include activities such as ambient monitoring. EPA will work with the Regional 

Offices, NACAA, and monitoring agencies to develop a plan for allocating these 

funds to the new monitoring needs related to recent NAAQS revisions. 

(2) Comment: Several commenters expressed concern that EPA significantly 

underestimated the deployment and operation costs for new SO2 monitors. For 

example Vermont estimated that the associated costs for a source-oriented 

monitor would “be at least 30 to 40 percent higher than EPA's estimates.” 

(NYSDEC, Delaware, Alaska, NACAA, and Vermont). 
125

Response: EPA uses its best available estimation in determining costs for network 

deployment and operations, as required for inclusion the Information Collection 

Review (ICR) that accompanies each rulemaking. 

(3) Comment: NACAA and Vermont suggested in their comments that EPA consider 

“promulgating regulations that make the sources pay directly for the monitoring 

efforts carried out by state and local agencies” as a means of addressing the 

funding crisis for SO2 monitoring and other NAAQS monitoring. (NACAA and 

Vermont). 

Response: EPA is not in a position to address or execute such an action as part of 

this rulemaking. EPA notes that $15 million in new STAG funding has been 

proposed in the President's budget for fiscal year 2011 for the purpose of 

supporting new monitoring equipment purchases related to the NAAQS. EPA 

also notes that additional STAG funding was proposed to support "core" 

programs which can include activities such as ambient monitoring. EPA will 

work with the Regional Offices, NACAA, and monitoring agencies to develop a 

plan for allocating these funds to the new monitoring needs related to recent 

NAAQS revisions. 

(4) Comment: Georgia requested in its comments that EPA withdraw its proposed 

monitoring program proposal “until a holistic analysis of the total costs is 

conducted and adequate new funding is identified and assured.”

Response: EPA understands that each individual NAAQS review is not occurring 

in a vacuum. However, EPA will not suspend the promulgation of monitoring 

rules that directly support revised NAAQS. EPA notes that $15 million in new 

STAG funding has been proposed in the President's budget for fiscal year 2011 

for the purpose of supporting new monitoring equipment purchases related to the 

NAAQS. EPA also notes that additional STAG funding was proposed to support 

"core" programs which can include activities such as ambient monitoring. EPA 

will work with the Regional Offices, NACAA, and monitoring agencies to 

develop a plan for allocating these funds to the new monitoring needs related to 

recent NAAQS revisions. 

(5) Comment: Several commenters suggested that EPA should not implement any 

new FRM or FEM changes until sufficient new funding is procured to fund the 

deployment of the entire new SO2 monitoring network because the funding 

challenge for the SO2 monitoring network will only be made worse by the forced 

retirement of functional SO2 monitors that no longer meet the new FRM and FEM 

standards. (MIDNRE, AEPSC). 

Response: Under this rulemaking, there will not be an entirely new monitoring 

network. The promulgation of a new FRM, which coexists with the existing FRM 

and FEMs, will not force the retirement of functional and currently operating 

FEMs. 
126

(6) Comment: Several commenters recommended that, because of the limited funds 

for deploying a new SO2 monitoring network, dispersion modeling should be 

utilized along with other criteria (i.e. population exposure) to determine which 

SO2 monitors are the highest priorities to deploy so that a smaller, but still health 

protective, SO2 monitoring network can be deployed. (North Carolina, South 

Carolina, Vermont) 

Response: As discussed in section IV.B.3 and IV.B.4 of the preamble to the final 

rule, the final monitoring network design calls for less monitoring sites and is 

more flexible than the proposed network design. As a result, EPA anticipates that 

only 41 new monitors will need to be deployed to satisfy minimum monitoring 

requirements. Therefore, we believe that the final network design alleviates the 

commenters concerns. 

IV.J: Comments on the authority and responsibilities of Regional 

Administrators (RAs) 

(1) Comment: Several commenters expressed support for requiring RAs be in “close 

consultation with the [appropriate] state agency” when determining when to 

require additional SO2 monitors (North Carolina, Louisiana Chemical, South 

Carolina, and AirQuality). Similarly, AirQuality recommended EPA include 

provisions requiring RAs to provide a public comment period for proposed 

additional SO2 monitors before a final decision is made. Finally, Louisiana 

Chemical suggested EPA require RAs to “consider certain objective factors when 

determining whether to require any additional ambient SO2 monitors to the 

network.” 

Response: EPA agrees with the commenters’ suggestions, and expects that 

Regional Administrators would work with state and local air agencies when 

evaluating and possibly requiring additional monitors above the minimum 

requirements. Regarding public input, any additional monitoring that a Regional 

Administrator might require of a state would be included in that state’s annual 

monitoring plan. States must develop annual monitoring network plans as 

required by 40 CFR 58.10, those plans must be available to the public prior to 

submission to EPA. 

(2) Comment: Several organizations recommended that an RA’s authority to require 

additional SO2 monitors be contingent upon the RA/EPA providing all the 

additional funding necessary to deploy and operate the additional monitoring 

stations (NYSDEC, Delaware, North Carolina, South Carolina, and AirQuality). 

recommended that additional SO2 monitors required by the RA but dispute by 

state, local, or tribal monitoring authorities should be funded by the federal 

government “for three years or until enough data is collected to reach an 

attainment designation and address the concern.” (.0843) 
127

Response: In the event that an RA does require additional monitors above the 

minimum required, EPA will work with the monitoring agencies to develop a plan 

on how funds might be allocated for any new monitoring. 

(3) Comment: ALA/EDF/NRDC/SC recommended EPA consider the likelihood of 

RAs requiring additional SO2 monitors when creating the federal budget for the 

monitoring program. 

Response: EPA always tries to create as accurate a budget as possible, and if any 

information regarding RA required monitoring is available as budgets are being 

developed, such information would be considered in the budget creation process. 

(4) Comment: Florida recommended EPA grant RAs the authority to approve 

reductions in the number of required SO2 monitors if it can be shown that 

marginal SO2 monitors will not significantly benefit the public health. 

Response: EPA disagrees with the concept that allows “RAs...to approve 

reductions in the number of required SO2 monitors if it can be shown that 

marginal SO2 monitors will not significantly benefit the public health.” EPA 

notes that this comment was directed at the proposed monitoring network design 

which required a greater number of monitors compared to the final network 

design which is part of a hybrid analytical approach (introduced in section III of 

the preamble of the final rule). The minimum monitoring requirements are a 

‘minimum’ to ensure that a network of sufficient size and focus is maintained to 

support the NAAQS.

(5) Comment: Florida recommended EPA “allow for the regional administrator to 

approve adjustment of the monitoring requirements to provide for flexibility for 

monitors to address multiple purposes.” 

Response: Under a hybrid analytical approach, as introduced in section III of the 

preamble to the final rule, the required monitoring network no longer must be 

wholly source-oriented in nature, and now can serve multiple monitoring 

objectives, as discussed in section IV.B.3, while the modeling of sources would 

be more efficient and effective in providing source-oriented SO2 concentration 

data. Due to the flexibility in the monitoring network design EPA believes the 

commenter’s concerns are alleviated.

(6) Comment: AirQuality recommended “EPA should consider adding guidance to 

its monitoring network proposal that focuses on school-oriented sites in section d. 

‘Monitoring required by the regional administrator’ [64854] to clarify if schools 

may be an example of ‘a case where a source having modest emissions still has 

high potential to cause a violation of the NAAQS in a community or 

neighborhood.’” 
128

Response: Under the final rule, Regional Administrators may consider requiring 

monitoring wherever monitoring objectives are not being met. EPA anticipates 

that in situations such as the example given by the commenter, the Regional 

Administrators would consider on a case-by-case basis the need for additional 

monitors in such situations. 

VI. Air Quality Index 

(1) Comment: Pennsylvania commented that EPA must provide timely 

implementation rules and guidance for issues such as SHLs, emergency action 

levels, and the updated AQI in order to adequately protect public health and the 

environment. 

Response: EPA agrees with this comment which is why we have revised the Air 

Quality Index (AQI). EPA did not propose to change breakpoints at the higher 

end of the AQI scale (from 200 to 500), which would apply to state contingency 

plans or the Significant Harm Level (40 CFR 51.16), because the information 

from this review does not inform decisions about breakpoints at those higher 

levels. 

(2) Comment: NESCAUM commended EPA’s proposal to revise the AQI 

concurrently with the NAAQS. In a February 8, 2007, letter to EPA, NESCAUM 

indicated that the current AQI is not well designed for its current uses (e.g., 

addressing real-time exposures with additional messaging at lower levels 

approaching the standard). They stated that EPA should revisit and overhaul the 

AQI to address the multiple purposes it now serves and better serve public health 

protection (e.g., adjust the AQI to reflect shorter averaging times, account for 

multi-pollutant cumulative impacts, and consider additional contaminants). 

Response: Structural changes to the AQI, called an overhaul by this commenter, 

would have to be made through a separate rulemaking. 

(3) Comment: Letters from a mass comment campaign sponsored by the Sierra Club, 

and comments from NACAA, South Carolina, and Vermont support revisions to 

the AQI to conform with the new SO2 primary standard, if it is promulgated. 

Response: EPA agrees with these commenters. 

(4) Comment: North Carolina strongly recommended that EPA establish the AQI 

breakpoints for SO2 when they propose the final SO2 NAAQS rule. North 

Carolina recommended that a 100 AQI correspond with the short-term SO2

NAAQS, consistent with other NAAQS pollutants. The 50 and 150 AQI 

breakpoints should also be consistent with the other NAAQS pollutants. North 
129

Carolina is concerned that failing to adequately define the AQI breakpoints might 

lead to problems such as those with the PM2.5 AQI. 

Response: EPA has identified all of the changed breakpoints in the AQI, and has 

made revisions to the 50 and 150 AQI breakpoints that are consistent with recent 

decisions made for other NAAQS. 

(5) Comment: Air quality commented that the range for the AQI 50 breakpoint is 

acceptable because the approach EPA used to arrive at it is consistent with that of 

previous standards. 

Response: EPA agrees with this commenter. 

(6) Comment: NYSDEC recommended that the requirement for hourly AQI 

forecasts be delayed at least one year so that the data being collected and 

forecasted is shown to be reliable. In the meantime, EPA may want to make the 

SO2 AQI based on a longer interval (e.g., 24-hour) that can be more accurately 

forecasted (NYSDEC) EPA should provide guidance for forecasting 1-hour SO2

data and for communicating these forecasts to the public in a timely manner. 

Response: EPA recommends and encourages air quality forecasting but it is not 

required (64 FR 42548, August 4, 1999). We agree that there will be new 

challenges associated with creating and communicating an SO2 forecast. We will 

work with State agencies that want to develop an SO2 forecasting program on 

issues including, but not limited to, forecasting for short term periods. 

(7) Comment: While Wisconsin supports changing the AQI concurrent with the new 

NAAQS, they believe it is impractical to incorporate SO2 into its forecasting and 

public health notification program because SO2 does not behave like a regional 

pollutant. Exceedances may occur with little or no warning and for two hours or 

less. They requested EPA consider the resources necessary for public 

communications at the state and local levels, particularly in areas where other air 

quality exceedances are relatively rare. 

Response: EPA recommends and encourages air quality forecasting but it is not 

required (64 FR 42548, August 4, 1999). We agree that there will be new 

challenges associated with creating and communicating an SO2 forecast. We will 

work with State agencies that want to develop an SO2 forecasting program on 

issues including, but not limited to, forecasting for short term periods. 

(8) Comment: NESCAUM commends EPA on its proposal to revise the Air Quality 

Index (AQI) concurrently with the SO2 NAAQS. NESCAUM has some questions 

regarding how EPA envisions reporting and forecasting SO2 concentrations for 

the AQI using source-oriented monitors. For example, how does EPA expect 

geographic regions to be delineated, given the more localized nature of a sourceoriented network? 
130

Response: This final rule departs from the proposed rule in that it allows for a 

combined monitoring and modeling approach. Because of this, the monitoring 

network is not required to be wholly source-oriented in nature. States have 

flexibility to allow required monitoring sites to serve multiple monitoring 

objectives including characterizing source impacts, highest concentrations, 

population exposure, background, and regional transport. Further, EPA expects 

that much of the existing network will be retained by states to satisfy the 

minimum monitoring requirements. This means that it is unlikely that AQI 

reporting and forecasting will be heavily driven by source-oriented monitors. 

Rather, many of the existing monitors (a majority of which are community-wide 

monitors) will remain in place, which prevents the need for new geographic 

regions to be delineated. 

(9) Comment: Additionally, NACAA requests that EPA work with state and local 

agencies to develop a forecast model and appropriate guidance for incorporating 

SO2 into their daily air quality forecasts. 

Response: We agree that there will be new challenges associated with creating 

and communicating an SO2 forecast. We will work with State agencies that want 

to develop an SO2 forecasting program on issues including, but not limited to, 

forecasting for short term periods. 

(10) Comment: The State of Vermont APCD supports the EPA proposal to revise the 

SO2 AQI concurrently with the primary SO2 NAAQS. 

The ability to provide timely and geographically relevant air quality 

reporting, alerts and forecasts are important components of public health 

protection. However, we have some questions regarding how the EPA 

envisions reporting and forecasting SO2 concentrations for the AQI using 

source oriented monitors. For example, how does EPA expect geographic 

regions to be delineated, given the more localized nature of a sourceoriented network? Additionally, we also urge the EPA to work with the 

states to revisit and overhaul the AQI in light of the multiple purposes it 

now serves, including: adjusting the AQI to reflect shorter averaging 

times; accounting for multi-pollutant (i.e., cumulative) impacts; and 

considering additional contaminants. By so doing, public health protection 

would be better served. 

Response: This final rule departs from the proposed rule in that it allows for a 

combined monitoring and modeling approach. Because of this, the monitoring 

network is not required to be wholly source-oriented in nature. States have 

flexibility to allow required monitoring sites to serve multiple monitoring 

objectives including characterizing source impacts, highest concentrations, 

population exposure, background, and regional transport. Further, EPA expects 
131

that much of the existing network will be retained by states to satisfy the 

minimum monitoring requirements. This means that it is unlikely that AQI 

reporting and forecasting will be heavily driven by source-oriented monitors. 

Rather, many of the existing monitors (a majority of which are community-wide 

monitors) will remain in place, which prevents the need for new geographic 

regions to be delineated. 

VII. Comments on the Process for Reviewing the SO2 Primary 

NAAQS 

(1) Comment: SAP generally commented that there was insufficient monitoring data 

for establishing a new 1-hour SO2 standard. This commenter contends that EPA 

should propose to place additional monitors by 2013 and then review whether a 

new 1-hour standard is required. 

Response: EPA disagrees with the commenter. Using the existing monitoring 

network numerous epidemiologic studies have reported positive associations 

between mostly 24-hour average and 1-hour daily maximum SO2 concentrations 

and serious health outcomes such as hospital admissions. We further note that 

CASAC has approved: 1) the science underlying the determination in the ISA that 

there is a causal relationship between respiratory morbidity and short-term (5-

minutes to 24-hour) exposure to SO2; and 2) analyses in the REA estimating the 

extent to which a 1-hour standard will protect asthmatics from 5-minute SO2

concentrations of concern identified from controlled human exposure studies. 

Finally, we note EPA is legally obligated to make a decision by June 2, 2010. 

VIII. Interpretation of the Clean Air Act 

(1) Comment: EPA cannot take into account costs of compliance on industries and 

state governments in establishing NAAQS; however it can and should take into 

account the effects of its policies on the economic health of the people affected, 

and particularly on economically February 8, 2010 Page 9 disadvantaged groups 

who appear to experience asthma disproportionately. It should not fail to consider 

the effects of that economic health and of economic stress (and distress) on the 

people’s health in the affected areas. Put another way, the transfer of production 

jobs available to less educated persons from the US to China, from nonattainment areas to non-classified areas, and from less educated US laborers and 

craftsmen to well-paid engineers may be good for world productivity or good for 

a working man in China or in another less regulated area – outside the nonattainment area, but the economic stress is not without stress or health impact on 

the persons displaced or left behind or their children. 
132

Response: However deeply felt the commenter’s argument is, it has no legal 

merit and in fact has been rejected repeatedly. In Whitman v.American Trucking 

Associations. 531 U.S. 457, 466 (2001), the United States Supreme Court 

considered the argument that "the economic cost of implementing a very stringent 

standard might produce health losses sufficient to offset the health gains achieved 

in cleaning the air -- for example, by closing down whole industries and thereby 

impoverishing the workers and consumers dependent upon those industries." The 

Court continued, "That is unquestionably true, and Congress was unquestionably 

aware of it." The Court went on the reject the argument based on the statutory 

text: "requisite to protect the public health with an adequate margin of safety" 

makes no mention of cost, unlike the plethora of other CAA provisions which do. 

The Circuit Court of Appeals for the District of Columbia Circuit has likewise 

rejected the argument. "AISI next contends that EPA erred in refusing to consider 

the health consequences of unemployment in determining the primary standards 

for particulate matter. This claim is entirely without merit. In three previous 

cases this court has emphatically stated that section 109 does not permit EPA to 

consider such costs in promulgating national ambient air quality standards. It is 

only health effects relating to pollutants in the air that EPA may consider. See 

[section 108 (a) (2) ("Air quality criteria for an air pollutant shall accurately 

reflect the latest scientific knowledge useful in indicating the kind and extent of 

all identifiable effects on public health or welfare which may be expected from 

the presence of such pollutant in the ambient air, in varying quantities.") 

Consideration of costs associated with alleged health risks from unemployment 

would be flatly inconsistent with the statute, legislative history and case law on 

this point." NRDC v. EPA, 902 F. 2d 962, 972-73 (D.C. Cir. 1990). 

(2) Comment: The Clean Air Act requires that health effects be projected with a 

reasonable degree of scientific certainty. The commenter believes that setting a 1-

hour standard in the range of 50-100 ppb is not supported by the scientific 

evidence. The Agency’s review and utilization of the information contained in 

the ISA and the REA do not meet the requirements of the CAA, and EPA’s 

promulgation of a final standard within the range proposed would be more 

stringent than necessary. (Progress Energy) 

Response: EPA firmly believes that the standards adopted in the final rule are 

well supported by the air quality criteria as set out in the ISA and REA, and has 

explained why in the preambles to the proposed and final rules, as well as this 

Response to Comment Document. Nor are the standards more stringent than 

necessary, given that they are established at a level slightly below the 99th

percentile 1-hour daily maximum SO2 concentrations in the epidemiologic studies 

EPA reasonably considered to be especially probative. 

The commenter’s statement that “health effects must be projected with a 

reasonable degree of scientific certainty” under the CAA (unsupported by legal 

citation), is also misleading. For example, the requirement in the Act that primary 
133

NAAQS provide an “adequate margin of safety” specifically directs EPA to act 

even if a risk is uncertain in nature or degree, or not even identified. See S. Rep. 

No. 91-1196, at 10 (1970); see also American Farm Bureau et al. v. EPA, 559 F. 

3d 512, 532-33 (D.C. Cir. 2009) (“Although the evidence of danger from coarse 

PM is, as the EPA recognizes, ‘inconclusive’, the agency need not wait for 

conclusive findings before regulating a pollutant it reasonably believes may pose 

a significant risk to public health....’The Clean Air Act requires EPA to 

promulgate protective primary NAAQS even where, as here, the pollutant’s risks 

cannot be quantified or precisely identified as to nature or degree’” (internal 

citations omitted)). In the situation here, where the ISA finds a causal 

relationship between respiratory morbidity and short-term exposure to SO2, and 

support in the epidemiologic evidence and risk analysis for all of EPA’s decisions 

as to the elements of a revised standard, there is certainly sufficient certainty for 

EPA to revise the standard. 

(3) Comment: UARG commented that when reviewing an existing NAAQS (which 

previously was determined to be at the level requisite to protect human health 

according to the CAA), the Administrator must provide a reasoned explanation 

for any conclusion that the existing NAAQS is no longer at the level requisite to 

protect public health with an adequate margin of safety. The commenter cites in 

support Motor Vehicle Mfrs. Ass’n v. State Farm Mut. Auto. Ins. Co., 463 U.S. 

29,42 (1983) (“an agency changing course by rescinding a rule is obligated to 

supply a reasoned analysis for the change beyond that which may be required 

when an agency does not act in the first instance”); Atchison, Topeka and Santa 

Fe Ry. Co. v. Wichita Bd. of Trade, 412 U.S. 800, 808 (1973) (an agency must 

“explain its departure from prior norms”); and AT&T Corp. v. FCC, 263 F.3d 729, 

736 (D.C. Cir. 2001) (reasoned decision making standard requires explanation of 

departure from prior decision). The commenter continues, “Rather, the changed 

conclusions are the result of changes in EPA’s interpretation of the evidence. 

Only if she can supply a “reasoned analysis” for these changes can the 

Administrator rely on them for a change to the 1996 conclusion that effects of 

public health significance begin at 600 ppb, or for a reversal of the 1996 decision 

that no new short-term NAAQS was needed to protect public health with an 

adequate margin of safety. She has failed to present such an analysis.” 

Response: EPA must fully explain the basis for its decision in this NAAQS 

review (including explanation of policies and judgments involved in those 

decisions, and explanations of how any changes in the science since the last 

review were evaluated in making a decision. CAA section 307 (d) (3), (d) (6) (A) 

and (B). EPA has done so here. EPA, however, is not bound by previous NAAQS 

determinations, and carries no special burden of explanation (beyond providing a 

reasoned explanation) if it deviates from prior determinations. The cases the 

commenter cites are inapposite. The plurality opinion in Atchison, Topeka & 

Santa Fe Ry. Co. involved a situation where the Interstate Commerce 

Commission, in an adjudicative proceeding, issued a rate order which deviated 

from prior settled adjudicative precedent of the Commission (412 U.S. at 805-06). 
134

The Commission had a settle principle (referred to in the opinion as “settled rule”) 

addressing the general issue involved in the case, and attempted to distinguish the 

pending rate determination from that principle. The adequacy of the 

Commission’s explanation for a departure from the settled principle was the basic 

issue on judicial review. 

The Court addressed this issue with an overall focus on the need for an agency to 

explain its decision, as the fundamental precondition for adequate judicial review. 

An agency needs to explain its reasoning in order for a court to determine if the 

decision was with Congress’ delegation of statutory authority. In that context, an 

adjudicatory agency could, through a course of agency action, establish a settled 

rule or principle that embodies the agency’s informed judgment on the 

appropriate way to carry out the policies committed to it by Congress. In those 

circumstances a court reviewing an agency decision should properly presume that 

the agency’s settled principle embodies the agency’s view on what will best carry 

out the policies of Congress, calling for an adequate explanation by the agency if 

it departs from its own norm. The basic purpose of both the presumption and the 

need to explain a deviation from the settle norm derives from the obligation that 

an agency explain how its decision conforms to the policies committed to it by 

Congress. An agency may establish such a settled norm, and if it does then that 

norm provides an explanation for agency action in future cases. Once such a 

norm has been established, however, an agency must explain a deviation from it 

so the reviewing court can determine whether the agency’s decision is still 

consistent with the policies committed to it by Congress. 

EPA does not see that this situation has any relevance to the periodic NAAQS 

review process, or that EPA established any settled rule for interpretation of SO2

clinical studies in the last review (much less establish one through a trial-type 

adjudicatory process) as in Atchison, Topeka & Santa Fe Ry. Co.

To the contrary, section 109 mandates a periodic review based on an updated 

review of the science. Prior NAAQS determinations, based on the then current 

science, do not have status as adjudicative precedent, “settled rule”, or settled 

interpretation. The required review of the air quality criteria is to ensure that 

NAAQS decision are based on “the latest scientific knowledge” regarding effects 

of the pollutant in question, and this on-going dynamic process extends to 

interpretations of scientific evidence. 

Nor does the statement from State Farm that agencies changing an existing rule 

are “obligated to supply a reasoned analysis beyond that which may be required 

when an agency does not act in the first instance” (463 U.S. at 42) support the 

proposition that there is some special burden over and beyond the requirements of 

section 109 (b) and (d) necessary to justify a change. The action reviewed in that 

case was a rescission of an existing rule adopted pursuant to a statutory scheme 

which required that standards be practicable. That is unlike the statutory scheme 

of the NAAQS, which not only requires periodic review, but requires that those 
135

reviews be based on air quality criteria reflecting “the latest scientific knowledge” 

regarding the effects of the pollutant in question. 

(4) Comment: API commented that although the Proposed Rule acknowledges that 

the SO2 REA estimated “short-term exposures and potential human health risks” 

associated with recent ambient SO2 levels, 74 Fed. Reg. at 64,821, the 

Administrator completely ignores those estimates in considering whether a new 1-

hour NAAQS is necessary for public health protection. Those analyses, however, 

are very relevant for addressing the risks in a real world context. See Whitman, 

531 U.S. at 494-95 (Breyer, J., concurring in part and concurring judgment). They 

indicate that exposures and risks associated with just attaining the 1-hour NAAQS 

that the Administrator has proposed (and therefore apparently deems “tolerable”) 

22 are comparable to those associated with recent air quality. 

Response: The issue here is whether, in determining for purposes of section 109 

(d) (1) whether it is appropriate to revise the current primary NAAQS for SO2, 

EPA should consider the air quality allowed by that current standard or existing 

air quality. First, it is apparent that the provision requires review of the current 

standard, not current air quality. Second, EPA is to consider the test in section 

109 (b) (1) in determining if it is “appropriate” for EPA to revise the standard (see 

section 109 (d)(1)). Section 109 (b) (1) requires that a primary NAAQS – not air 

quality at the time of a NAAQS – be requisite to protect the public health with an 

adequate margin of safety. Again, it is the standard itself which is at issue. Third, 

section 109 (d) (1) requires EPA to consider any revised air quality criteria in 

determining if it is appropriate to revise the NAAQS. Those criteria require 

consideration of “the latest scientific knowledge useful in indicating the kind and 

extent of all identifiable effects on public health ... which may be expected from 

the presence of such pollutant in the ambient air, in varying quantities” and thus 

certainly authorize consideration of the effects the pollutant has at quantities 

allowed by the current standard. Consequently, EPA is at the least allowed to 

consider conditions allowed by the current standard in determining if it is 

appropriate to revise it. 

The commenters cite part of Justice Breyer’s concurring opinion in American 

Trucking Assn’s, but that opinion is not applicable here. It does not discuss the 

question of whether EPA may consider actual or allowable air quality in deciding 

whether or not to revise a primary NAAQS. 

(5) Comment: NPRA, which supports comments submitted by API, said that the 

CAA neither requires nor supports the proposed 1-hour SO2 NAAQS because the 

proposed standard is four times lower than the lowest tested concentration in 

human exposure clinical trials and is eight times lower than the level at which 

statistically significant human exposure results occur. Without further substantial 

evidence from clinical human exposure trials, the proposed and alternative SO2

NAAQS are “arbitrary and capricious.” 
136

Response: This comment ignores the difference in averaging times, so that the 

commenter’s comparison is apples to oranges. Moreover, it is entirely 

permissible for EPA to base determinations as to a standard’s level on 

epidemiologic studies, and indeed is required to consider such studies in 

developing standards since they “reflect the latest scientific knowledge useful in 

indicating the kind and extent of all identifiable effects on public health ... which 

may be expected from the presence of [SO2} in the ambient air”, i.e. part of the air 

quality criteria. CAA section 108 (a) (2). 

(6) Comment: LEC, API, and RREIE commented that the definition of ambient air 

quality is interpreted too broadly to include areas that are hypothetically but not 

practically (or legally or safely) accessible to the public and that dispersion 

modeling is often applied to these areas. (LEC, API, RRI, RRI) 

Response: EPA is now contemplating a hybrid approach to implementation 

which includes increased utilization of dispersion modeling. See sections III-VI 

of the preamble to the final rule. 

(7) Comment: PE, supporting the comments of UARG and EEI, commented that 

EPA review and utilization of the information in the ISA and the REA do not 

meet the requirements of the CAA. EPA’s promulgation of a final standard 

within the proposed range would be more stringent than necessary. 

Response: EPA disagrees. The primary standard adopted in this rule is requisite 

to protect public health with an adequate margin of safety for the reasons given in 

section II.F of the preamble to the final rule. 

IX. Comments on Implementation, Designations, and 

Exceptional Events 

(1) Comment: Missouri believes that the Reasonable Future Progress requirement 

under CAA Section 171 should not be interpreted to mean a generally linear 

progression towards attainment. Missouri’s previous analyses showed that many 

of the SO2 sources are coal combustion and other large single-point emitters, and 

EPA acknowledges that these types of sources will likely be responsible for most 

of the impact in non-attainment areas. Step-down functions are not possible with 

these types of sources, Missouri claims. Missouri suggested EPA consider an allin-one approach that would allow for a single control process (e.g., scrubber) on a 

large source to bring an area into attainment and meet the requirements of 

Reasonable Future Progress, reasonable available control technology, reasonable 

available control measures, and contingency measures for this type of nonattainment area. Such an approach would only be allowed with EPA Regional 

Office approval upon verification that it meets statutory requirements. 

Response: This final rule is not taking final action regarding any implementation 

issues that may become the subject of future SIP rulemaking actions. Missouri’s 
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comments would be more appropriately raised in the context of a specific SIP 

action, and are beyond the scope of today’s final rule. Therefore, EPA is not 

taking a position here regarding whether it agrees with the approach stated by the 

commenter above. 

(2) Comment: A commenter stated that the annual SO2 standard should not be 

revoked until EPA approves SIPs in attainment areas under the future SO2 

secondary standard, which may also be based on an annual averaging time. 

Response: EPA disagrees with the comment. This rulemaking concerns only the 

primary standards for SO2. 74 Fed. Reg. at 64812 n. 2. The annual SO2 standard 

is a primary standard, not a secondary standard. See 40 CFR section 50.4 (a). 

 The exclusive secondary standard for SO2 is the 3-hour standard codified in 40 

CFR section 50.5. EPA is not determining the adequacy of this secondary 

standard in this review or this rulemaking, as just noted. The commenter’s 

request to retain the annual primary standard until SIPs reflecting a new 

secondary standard are approved is effectively a request to amend the present 

secondary standard, and is therefore inappropriate given the scope of this review. 

 In any case, in the event that any substantive responsive to this comment is 

required, air quality information indicates that a 1-hour standard of 75 ppb is 

estimated to generally keep annual SO2 concentrations well below the level of the 

current annual standard. 74 FR at 64845. Thus, there would be no loss of 

protection to public welfare due to revocation of the annual primary standard. 

A. Comments on designations of non-attainment 

 General comments on designations of non-attainment 

(1) Comment: NESCAUM, NACAA, and ALA/EDF/NRDC/SC support EPA’s 

proposal to keep non-attainment designations and subsequent requirements under 

the current SO2 NAAQS in effect until the non-attainment area submits, and EPA 

approves, a SIP for the new SO2 NAAQS. The commenters believe this is needed 

to maintain the needed public health protection and regulatory coverage until a 

new and workable SO2 reduction plan is in place. ALA/EDF/NRDC/SC 

suggested keeping the current standards in place for at least a year during the 

transition to the new standards in order to comply with CAA provisions and to 

avoid arbitrarily and unjustifiably relaxing the health protections. (NESCAUM 

Response: EPA appreciates the support of the commenters on the transition 

strategy for SO2. 

(2) Comment: Pennsylvania commented that EPA must provide timely 

implementation rules and guidance for issues such as SILs, emergency action 

levels, and the updated AQI in order to adequately protect public health and the 

environment. 
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Response: See preamble section V.D.2. related to NSR and PSD requirements 

and section VII related to the AQI which addresses this comment.

(3) Comment: Sulfuric Acid Producers commented that EPA should examine if the 

proper regulatory infrastructure at the state and local levels is ready to address an 

increase in SO2 non-attainment. EPA should also provide guidance for LAER 

and RACT for SO2. EPA should also address if these programs will have a 

material effect on reducing the negative health impacts that are claimed in the 

REA. 

Response: As stated in the implementation section of the preamble to the final 

NAAQS review notice, EPA believes that generally there are appropriate 

guidance and regulations currently in place to address the revised SO2 NAAQS. 

However, EPA agrees that providing additional guidance on modeling for the new 

1-hour standard is needed, and will be providing that guidance in time to help 

states and sources implement the new NAAQS. EPA also believes that the 

anticipated approach discussed in the preamble for designating areas for SO2 also 

would provide states with sufficient time to address any control plans necessary to 

address areas that may be violating the revised NAAQS and to provide public 

health protection by helping areas that are violating the NAAQS attain the 

standard as expeditiously as practicable. 

(4) Comment: Though EPA does not specifically indicate an approach for setting 

non-attainment area boundaries for the new SO2 NAAQS, MIDNRE is concerned 

that EPA is considering using CBSAs, since EPA repeatedly mentions using 

CBSAs in determining PWEI and since EPA has often designated large, multicounty areas as non-attainment for other pollutants. MIDNRE believes CBSAs 

may be too large for SO2 designations, since EPA is treating SO2 as sourcespecific and heterogeneous in a given area. MIDNRE strongly recommends 

flexibility in setting non-attainment area boundaries, many of which should be 

smaller than CBSAs and maybe even smaller than counties. 

Response: As stated in the designations section of the final SO2 NAAQS 

preamble, the EPA intends to initially consider the county line as the presumptive 

nonattainment boundary for SO2 unless the State provides justification in its 

recommendation as to why a different boundary would be more appropriate. 

Today’s final rule does not bind states or EPA to treating the county line as the 

nonattainment boundary, but only explains the starting point for area-specific 

analyses. This approach would provide States with a clear starting point yet 

would allow the flexibility to show that a smaller or larger boundary is more 

appropriate. It is also consistent with the approach that EPA has taken for other 

NAAQS. 

 (5) Comment: ALA/EDF/NRDC/SC commented that EPA has historically and 

incorrectly made designations only when new standards are adopted. The new 

monitoring network is likely to identify many areas with high SO2 levels. As 
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such, in the final rule, ALA/EDF/NRDC/SC urged EPA to include a timetable for 

the post-2012 evaluation of the attainment status of all monitored counties. 

Response: The commenters essentially claim that SO2 areas initially designated as 

“unclassifiable” and which subsequently violate the revised SO2 NAAQS need to 

be redesignated to “nonattainment” to ensure that such areas take action to bring 

them into attainment. Section 107(d)(3) of the CAA provides EPA with 

discretion to redesignate areas at any time following initial designations. As 

discussed in the preamble to the final rule, we believe our expected approach to 

implementation of the new 1-hour SO2 NAAQS would fully address the 

commenters’ concern. Under CAA section 110(a)(1) all areas designated as 

“unclassifiable” would need to submit a SIP revision that demonstrates attainment 

with both monitoring and modeling data, as well as include measures to attain the 

SO2 revised NAAQS if necessary. Under the CAA, that SIP revision will be due 

three years after promulgation of the NAAQS or, June 2013. By utilizing section 

110(a)(1) of the CAA, the EPA’s expected approach would result in States with 

unclassifiable areas implementing the revised SO2 NAAQS sooner than if EPA 

were to rely exclusively upon redesignation under section 107(d)(3) to trigger a 

SIP demonstrating attainment. In any event, if States are unable to submit SIPs 

showing attainment by 2013, EPA has the authority to redesignate the area as 

“nonattainment,” in addition to disapproving the SIP and promulgating a FIP. 

 

We anticipate that under the expected approach discussed in the final rule, the 

delay in implementation reflected in the commenters’ concerns should not occur, 

and that many areas in the country would be able to demonstrate attainment in 

their section 110(a)(1) SIPs--the effect of which would be more areas attaining the 

revised NAAQS as expeditiously as practicable. 

B. Comments on date 

(1) Comment: UARG is concerned that the following proposal is an error, with 

regards to how 2011 data is to be included for SO2 designations: “2011 data must 

be flagged and detailed event documentation submitted 60 days after the end of a 

calendar quarter in which the event occurred or by March 31, 2011.” According 

to UARG, this March, 2011 deadline seems odd for a number of reasons: (a) it 

seems arbitrary, and (b) it would not allow for consideration of a full calendar 

year of 2011 data (74 FR 64873/1 defines a year as a calendar year, so 

considering any 2011 data requires that all 2011 data be considered). UARG 

recommended the deadline for detailed 2011 documentation submittal be March 

31, 2012, or as an alternative, 60 days after the end of each calendar quarter; the 

deadline should be March 31, 2013, if necessary to qualify FRMs and FEMs 

under new specifications. 

Response: The commenter notes that the incorrect March 31, 2011 date as shown 

in the proposal for flagging and documenting 2011 data should be corrected to 

March 31, 2012, and will be corrected in the Final Rule Preamble, Regulatory 
140

Text and Tables. Noting this date correction, the Agency stated in the proposal, 

“If a state intends 2011 data to be considered in SO2 designations, 2011 data must 

be flagged and detailed event documentation submitted 60 days after the end of 

the calendar quarter in which the event occurred or by March 31, 2012, whichever 

date occurs first. Again, EPA believes these deadlines will be feasible because 

experience suggest that exceptional events affecting SO2 data are few in number 

and easily assessed, so no state is likely to have a large workload.” A deadline of 

March 31, 2012 is the midpoint of the first 6 months of 2012, and provides 

equally 3 months for states and 3 months for EPA to process exceptional events 

claims associated with the last quarter of calendar year 2011. The Agency 

continues to believe that three months is adequate time for a state to review and 

document any SO2-related exceptional event that may occur in the last three 

months of 2011. Since the initial area designations must be finalized by EPA by 

July 2012, the Agency does not believe it is reasonable to establish a deadline 

later than March 31, 2012. EPA needs sufficient time to review any statesubmitted documentation during April through June of 2012 and make final 

decisions regarding exceptional events claims from 2011 prior to finalizing the 

designations in July 2012. 

(2) Comment: ALA/EDF/NRDC/SC commented that this proposal suggests that SO2

Part D SIPs be due 18 months from the effective date of an area’s non-attainment 

designation, and that the attainment date will be determined from the nonattainment effective date. CAA Section 191 requires submission of the Part D 

SIP within 18 months “of the designation,” and Section 192 requires attainment as 

quickly as possible but no later than 5 years from the non-attainment designation 

date. ALA/EDF/NRDC/SC stated that in neither of these cases does the statue 

allow measurement of the relevant time from the effective date of designation. 

Section 107 requires promulgation of designations within specified timeframes, 

and it does not provide for delayed effective dates. 

Response: The statute uses different language to establish deadlines for issuing 

designations under section 107 and submitting SIPs under section 192. Under 

section 107(d)(1)(A), states must submit designations recommendations no later 

than 1 year “after promulgation of a new or revised” NAAQS, and under section 

107(d)(1)(B)(i) EPA must promulgate designations no later than 2 years “from the 

date of promulgation of the new or revised” NAAQS. The courts have interpreted 

“promulgation” as meaning the date of signature of a final rule and its broad 

dissemination, so the time period under which states are to recommend 

designation and EPA is to issue designations does not need to await the effective 

date of the rule, which is later than its promulgation. In contrast, under sections 

191 and 192, the dates for starting the requirements for nonattainment area SIP 

submittal and attainment are the dates “of the designation” and “of the 

nonattainment designation,” respectively. These provisions do not refer to the 

date of “promulgation,” as under section 107, but rather to the action taken, which 

EPA believes is reasonable to interpret as meaning the date that the state or area 
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actually becomes a nonattainment area (i.e., the effective date of EPA’s 

designation action). 

C. Comments on data to be used 

(1) Comment: NPRA, supporting API comments, noted that EPA is proposing to 

conclusively designate an area with a violating monitor as being in nonattainment, while exercising undefined discretion when the opposite occurs. 

NPRA thinks it is unjustified to treat data differently when the information is 

valid and comes from the same type of monitors that are sited according to 

existing EPA regulations and guidance. 

Response: First, in this rulemaking, EPA is not taking conclusive action on any 

area’s designation, nor did EPA propose to take such conclusive action. Second, 

as we explain in sections III, IV, V and VI of the final rule preamble, for SO2

EPA has long taken the position that the use of monitoring alone cannot always be 

relied upon to identify sources that cause or contribute to NAAQS violations. 

Sulfur dioxide is emitted from sources which vary in size and stack height, 

causing a “patchwork” of violations--not all of which can be measured by a 

single, or even several, monitors. Therefore, in order to get a true picture of 

ambient air conditions in an area, EPA has traditionally depended upon both 

monitoring and modeling data, as appropriate, to accurately confirm that an area 

is in compliance with the NAAQS. This approach to SO2 designations is 

consistent with that taken in previous actions, including those referred to in the 

final rule preamble. In addition, the CAA itself does not prescribe the type of 

data by which EPA’s designation decisions may be informed, or what relative 

weight to give different types of data when they lead to different conclusions. For 

example, CAA section 107(d)(3)(A) refers to “air quality data, planning and 

control considerations, or any other air quality-related considerations the 

Administrator deems appropriate” as forming the basis for a redesignation 

decision. This certainly will allow EPA to take reasonable action in response to 

situations where, for example, modeling data indicate an area is not attaining the 

standard, while monitoring data might not reveal a violation. 

(2) Comment: In order to obviate the need for states to install monitors at great cost 

where concentrations of SO2 are a small fraction of the NAAQS, and to ensure 

that areas lacking robust monitoring data can be designated as non-attainment, 

NESCAUM and VTPAC recommended that EPA allow modeling be used in 

conjunction with monitoring data to better determine non-attainment areas. 

Response: See sections III and IV of the preamble for the discussion on EPA’s 

anticipated approach to the use of monitoring and modeling for the revised SO2

standard. 
142

(3) Comment: In modeling compliance with a statistically-based standard, API 

recommended that EPA consider applying realistic emissions profiles that are 

based on the expected frequency distribution of emissions. 

Response: EPA currently plans to develop further guidance regarding modeling 

demonstrations of compliance with the NAAQS. EPA is taking API’s 

recommendation under advisement, but today’s final rule is not taking final action 

in response to API’s request. 

(4) Comment: RRI and API recommend that EPA develop refined modeling tools 

and guidance that will not penalize sources by being overly conservative, 

especially when considering combined impacts of the primary sources and 

background sources. It would be burdensome to states if highly conservative 

modeling results created false new non-attainment designations. 

Response: EPA understands the importance of appropriately characterizing 

contributions from background sources, whether through representative 

monitoring data or through explicit modeling of background source emissions, as 

part of dispersion modeling analyses for compliance demonstrations recognizing 

the new form of the NAAQS. EPA expects to provide guidance on procedures for 

determining background concentrations, combining monitored background 

concentrations and modeled impacts to estimate the total air quality concentration 

to support the implementation phase of the NAAQS.

(5) Comment: By the 2012 deadline for determining attainment/non-attainment for 

the new SO2 NAAQS, PCA is concerned that less than three years of short-term 

data will be available from the existing monitoring network, and even less data 

will be available from the proposed new monitoring network. PCA stated that it 

seems premature to determine attainment/non-attainment based on the existing 

monitoring network (which EPA admits is not representative of areas of 

maximum impact) and to then require states to prepare and submit SIPs to address 

non-attainment designations. It seems imprudent to go through this burdensome 

process while knowing that different, deeper, and more widespread emissions cuts 

will be needed to address non-attainment identified by the proposed new 

monitoring network. 

Response: As we explain in sections III, IV, V and VI of the preamble to the final 

rule, we expect to follow an approach that would designate most areas in the 

country as “unclassifiable” in June 2012, pending receipt of additional data from 

SO2 monitors and modeling conducted following our development of further 

modeling guidance under the 1-hour SO2 standard. Areas that present both 

modeling and monitoring data that support a designation of “attainment” by June 

2012 and that EPA finds demonstrate attainment would be designated as such. 

Areas for which monitoring or refined modeling data show sources cause or 

contribute to NAAQS violations would be designated as “nonattainment.” We 

believe following such an approach would avoid the problem identified by the 
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commenter, as it would be far less dependent upon receipt of monitoring data 

under the new network. 

 (6) Comment: UARG commented that non-attainment decisions should be based on 

monitors that qualify as federal reference monitors under EPA’s new 

specifications. 

Response: EPA is not taking final action on any designation in this rulemaking. 

We expect, based on our discussed anticipated approach, to initially designate 

most areas in the country as “unclassifiable” in June 2012, pending receipt of 

additional data. Under section 110(a)(1) SIPs we expect states to rely upon 

appropriate modeling and monitoring data to support demonstrations of 

attainment and maintenance of the revised SO2 NAAQS--demonstrations that will 

be assisted greatly by new rules that will reduce SO2 levels nationwide.

(7) Comment: UARG, CE (supporting comments by UARG and EE), and NRECA 

commented that EPA has two options for making initial attainment/nonattainment designations with regard to whatever 1-hour NAAQS it adopts. These 

commenters believe that first, EPA could designate all areas as unclassifiable until 

adequate data have been collected with the new FRM. Second, if EPA believes 

that many existing monitors might qualify as new FRMs or FEMs, EPA could 

extend the deadline for attainment/non-attainment designations by a year. These 

commenters argued that EPA recently used CAA § 107(d)(1)(B) to defer 

designations for the 2008 ozone NAAQS by a year while it reconsiders that rule 

(75 FR 2936, Jan. 19, 2010), and UARG, CE, and NRECA believe it would be 

just as appropriate to use CAA § 107(d)(1)(B) to extend the SO2 designation 

owing to an absence of FRM or FEM data. EPA could use that year to determine 

which monitors would qualify under the new FRM and FEM definitions. For 

those monitors that qualify, EPA could then appropriately rely on existing data to 

make designations. 

Response: As the commenters suggest, under the anticipated approach discussed 

in sections III, IV, V and VI of the preamble the EPA expects to designate most 

areas as “unclassifiable” pending receipt of additional air quality data and refined 

SO2 modeling from the states confirming their status as either meeting or not 

meeting the new SO2 NAAQS. The commenters are correct that under section 

107(d)(1)(B)(i), the CAA provides the EPA an additional third year to complete 

initial designations in the event that there is insufficient data on which to base 

designations. EPA considered this option, but expects that it would not be 

necessary to resolve the timing problem the commenters raised. Instead, we 

believe that an approach like that discussed in the preamble would allow States to 

develop the necessary data to support section 110(a)(1) SIP demonstrations of 

attainment and maintenance of the new 1-hour SO2 NAAQS that rely upon 

expected SO2 reductions to be achieved by the attainment date. 
144

Furthermore, by designating most areas as “unclassifiable” pending receipt of 

additional data under the new SO2 monitoring rules and refined modeling, we 

would be enabling states to account for new national and regional rules that we 

anticipate will have significant SO2 benefits. 

Regarding monitoring methods, as discussed in section IV.A of the preamble to 

the final rule, there may be more than one FRM and associated FEMs that may 

provide data for comparison to the NAAQS. As a result, the current FEMs in use 

in the SO2 network are appropriate for continued use to provide SO2 

concentrations for comparison to the NAAQS. 

(8) Comment: ACC recommended that EPA make attainment/non-attainment 

designations in 2012 based on current monitoring data, not as unclassified in 

certain cases as proposed. 

Response: For most areas, EPA does not expect to receive either monitoring or 

refined modeling data in order to issue attainment or nonattainment designations 

by 2012. In such situations where areas cannot be classified on the basis of 

available information as meeting or not meeting the NAAQS, CAA section 

107(d)(1)(A)(iii) requires that EPA issue an “unclassifiable” designation. We 

expect initial designations in June 2012 would be based on monitoring data plus 

refined modeling data that States choose to include to demonstrate that an area is 

or is not meeting the new SO2 NAAQS. Absent definitive data from necessary 

monitoring and refined modeling, the EPA expects to designate most areas as 

“unclassifiable.” This approach would still allow States to request an initial 

designation of “attainment,” provided that in their recommendations to EPA due 

June 2011, they included necessary clean monitoring data, as well as refined 

modeling data to support a designation of “attainment.” 

(9) Comment: MSCC commented that if one three-year period at one monitor is 

good enough for non-attainment designation, it should also be good enough for 

attainment designation, rebuttable by further failure in the future. 

Response: This comment essentially makes the same claim as NPRA’s comment 

below. As we explained above, due to the unique characteristics of SO2, EPA has 

not traditionally relied exclusively upon monitoring to support designations and 

attainment determinations for the SO2 NAAQS. While this final rulemaking is 

not taking final action on any specific area’s designation, and while all future 

designation actions under the new 1-hour SO2 NAAQS will be based on their own 

facts, as we explained in sections III, IV, V and VI of the preamble, we anticipate 

using a hybrid approach in issuing designations that would continue to rely upon 

appropriate modeling and monitoring data to ensure that our conclusions 

regarding specific areas are accurate. 

 

Furthermore, by designating most areas as “unclassifiable” pending receipt of 

additional data under the new SO2 monitoring rules and refined modeling, we 
145

would be enabling States to account for new and regional national rules that we 

anticipate will have significant SO2 benefits. 

(10) Comment: NPRA, which supports comments by API, said that EPA’s proposal to 

discriminate against monitors demonstrating attainment is contrary to the CAA 

and cannot be finalized. NPRA states that the CAA requires EPA to utilize the 

monitoring information that is available within the statutory period for 

designations, and that, following the promulgation of a new or revised NAAQS, 

the governor of each state must submit to EPA a list of all areas of attainment, 

non-attainment, and unclassifiable in the state, and EPA can modify the 

submission so long as the state is notified and has an opportunity to demonstrate 

that the modification is inappropriate. 

Response: First, in this final rulemaking promulgating the new 1-hour SO2 

NAAQS and associated monitoring requirements, EPA is not taking final action 

on any approach toward issuing designations. EPA will issue future designations 

under CAA section 107 based on the information that is before the agency at that 

time. We therefore disagree with the commenter's assertion that the proposed 

NAAQS represented a "proposal to discriminate against monitors demonstrating 

attainment," as the NAAQS proposal did not in itself propose to either accept or 

reject any such monitoring data for any specific area. While we explain in today's 

notice that we expect to follow EPA's longstanding approach of relying upon both 

monitoring and modeling to inform future designations actions, only in such 

future actions will the agency's approach to this issue become final. 

Second, we note that the Clean Air Act does not specify the kind of data upon 

which a future designation action must be based. Regarding initial designations, 

section 107(d)(1) makes no reference to the type of information that may inform a 

designation. Rather, section 107(d)(1) repeatedly states that EPA may issue a 

designation that the agency "deems appropriate," indicating that EPA has 

substantial discretion in issuing designations. See, e.g., NACAA v. EPA, 489 F.3d 

1221, 1229 (D.C. Cir. 2007) (characterizing EPA's authority to promulgate 

aircraft engine emission standards under CAA section 231(a)(3) "as [it] deems 

appropriate" as "both explicit and extraordinarily broad" and as leaving "a gap for 

the agency to fill" (emphasis in original)). In fact, in its only reference to the type 

of data that may inform initial designations, section 107(d)(1)(A)(iii) uses the 

phrase “on the basis of available information,” without restriction. Moreover, 

CAA section 107(d)(3)(A) provides that EPA may issue re-designations "on the 

basis of air quality data, planning and control considerations, or any other air 

quality -related considerations the Administrator deems appropriate," without 

narrowing the type of information EPA may consider or specifying in what way 

EPA must treat any information. Thus, we do not agree that the Act supports the 

commenter's objection. 

 (11) Comment: ASC requested a detailed analysis of how predictive air dispersion 

modeling results will be used for attainment/non-attainment designations under 

the new proposed NAAQS. For example, “how will modeling be used to 
146

determine impacts to at-risk people? The proposed rule needs to include a 

discussion of the accuracy and precision of the predicted modeling results 

compared to actual ambient monitored concentrations.” 

Response: AERMOD, EPA’s preferred model for NSR/PSD permitting under 

Appendix W, 40 CFR Part 51, has been extensively evaluated based on 17 field 

study databases, several of which included hourly SO2 monitored concentrations 

from operating facilities. Although AERMOD results were not meant to be 

compared against monitors on a "paired in time and space" basis, past AERMOD 

simulations, including those used for SO2 Risk and Exposure Assessment, have 

shown good agreement with monitoring data. EPA feels that with proper 

emissions data, source characterization, and meteorological data, AERMOD 

results will generally compare well against ambient measurements.

(12) Comment: One commenter stated that the proposed NAAQS presents a host of 

complications related to nonattainment area attainment demonstrations that will 

need to be fully addressed in the “Guideline on Air Quality Modeling” (Appendix 

W, 40 CFR part 51) before implementation. 

Response: EPA expects to provide technical guidance on modeling and analyses 

as part of SIP demonstrations. We expect that the models currently required for 

NSR/PSD permitting under our Guideline on Air Quality Modeling (40 CFR Part 

51, Appendix W) will be suitable for demonstrating compliance with the revised 

SO2 NAAQS. 

(13) Comment: The commenter states that the proposed rule states that AERMOD, 

CALPUFF, and all other guideline models, such as Offshore and Coastal 

Dispersion Model (OCD) will need to be modified to provide for the 99th 

percentile (4th highest), or possibly the 98th percentile (8th highest) daily 1-hour 

concentration per year at each model receptor. 

Response: At this time, EPA plans to issue further guidance on use of the 

AERMOD dispersion model to accommodate the new 1-hour standard. EPA is 

also developing a generic AERMOD post-processor to aid in calculating the 

design values for criteria pollutants for new and proposed NAAQS, including 

SO2. As with the recently published NO2 NAAQS, EPA will publish a notice on 

the SCRAM website regarding modeling for the new 1-hour SO2 NAAQS. 

 (14) Comment: The commenter further states that the proposed SO2 standard will 

present much greater compliance hurdles in comparison to the present standard. 

The commenter states that the manner that background concentrations are 

incorporated into the modeling results will be critical for realistic and not overly 

conservative compliance demonstrations. (RRI Energy) 

Response: EPA understands the importance of background concentrations and 

combining them with modeling results for compliance demonstrations 
147

recognizing the new form of the NAAQS. EPA expects to provide guidance on 

procedures for determining background concentrations, and combining 

background concentrations and modeled impacts to estimate the total air quality 

concentration to support the implementation phase of the NAAQS. 

(15) Comment: One commenter states that the existing EPA models which are used 

for the implementation of the permitting program are not reliable for a short term 

1-hr standard for SO2. The commenter further states that the current models are 

not capable of predicting 1-hr SO2 impacts. (South Dakota) 

Response: EPA’s regulatory model for NSR/PSD permitting under Appendix W, 

40 CFR Part 51 has been extensively evaluated based on 17 field study databases, 

several of which included hourly SO2 monitored concentrations from operating 

facilities. Since all applications of the model are based on hourly predictions, 

there is no information to support the commenter’s claims and no basis to expect 

that model performance of AERMOD will not be appropriate for this 1-hour 

standard. 

D. Future permitting 

(1) Comment: CIBO commented on the nexus between the proposed rule and the 

NSR and PSD permitting programs. “A new SO2 1-hour standard of 50-100 ppb 

would make it nearly impossible for industries to obtain permits for any changes 

that involve increased use of sulfur-bearing fuels or raw materials. Industries 

require workable permitting systems in order to expand, modernize equipment, 

and meet future energy demands.” CIBO is disappointed that the regulatory 

impact analysis does not realistically assess this economic impact on the nation’s 

manufacturing base and jobs, and believes non-EGU point sources will bear the 

brunt of the proposed rule. “Overall, the new hourly SO2 standard will be 

impractical when applied to the major New Source Review program. 

Response: The commenter states that a new 1-hour SO2 standard of 50-100 ppb 

would make it nearly impossible for industries to obtain permits for increased use 

of sulfur bearing fuels and therefore non-EGUs will bear the brunt of the revised 

standard. The commenter did not provide any specific examples or analysis to 

support these claims. Furthermore, EPA does not construe the CAA to allow the 

Agency to take such implementation considerations into account when 

establishing NAAQS to protect public health with an adequate margin of safety. 

The CAA and EPA’s implementing rules generally require compliance with or 

minimal impact on the NAAQS as a condition of PSD permitting. For 

nonattainment NSR permitting the CAA provides that, as a prerequisite to being 

permitted to construct and operate a new or modified stationary source, increased 

emissions from such source be offset by reductions in existing emissions so as to 

ensure reasonable progress toward attainment of the NAAQS. 
148

(2) Comment: ASARCO, supporting comments by API, UARG, and NMA, 

commented that three years of on-site meteorological data would be required to 

demonstrate compliance with the three-year average NAAQS, which, in their 

opinion, is unreasonable for typical industrial projects that are time sensitive. 

Response: The definition of the form of the 1-hour SO2 NAAQS, based on a 

three-year average, is in relation to ambient monitoring data, and has no 

implication with regard to the length of meteorological data period required for 

dispersion modeling. The recommendations in Section 8.3.1.2 of EPA's 

Guideline on Air Quality Models (40 CFR Part 51, Appendix W) regarding the 

length of meteorological data record for dispersion modeling would apply for the 

new 1-hour SO2 NAAQS. That provision states that “[t]he use of 5 years of 

NWS meteorological data or at least l year of site specific data is required."

(3) Comment: ASARCO, supporting comments by API, UARG, and NMA, 

commented that in order to implement the PSD program, EPA should adopt SILs 

such that facilities can determine if a project is “significant” from an air quality 

perspective. “The development of SILs and other appropriate thresholds has often 

lagged far behind the implementation of the new NAAQS, resulting in significant 

implementation difficulties in obtaining PSD permits.” 

Response: EPA agrees that the SIL is an useful screening tool for implementing 

the PSD requirements for SO2 and all other regulated NSR pollutants. EPA plans 

to undertake rulemaking to establish a 1-hour SO2 SIL as a screening tool to 

facilitate implementing the 1-hour SO2 NAAQS in the PSD permitting program. 

In addition, we are considering providing guidance which could include an 

interim SIL value for SO2 that could be used by state permitting authorities while 

the final rule for a SIL is being completed, provided that the states establish an 

appropriate record for individual permitting actions based on the supporting 

technical information provided by EPA showing that a particular screening value 

reflects a de minimis impact level. 

 

(4) Comment: ASARCO, supporting comments by API, UARG, and NMA, 

commented that if the 1-hour NAAQS is less than the 3-hour or 24-hour NAAQS, 

these increment values will potentially exceed the 1-hour NAAQS. This 

contradicts CAA Section 163(b)(2) as it relates to PSD increments – that section 

lists allowable increases in SO2 concentrations above the baseline concentration. 

Response: EPA acknowledges that, because of the stringency of the new onehour SO2 NAAQS, the 3-hour and 24-hour SO2 increments allow greater 

increases in ambient concentrations of SO2 than does the new 1-hour SO2

NAAQS level. The CAA clearly provides that the amount of deterioration 

allowed by any increment cannot cause the NAAQS to be exceeded. Therefore, 

the new 1-hour SO2 NAAQS would govern the amount of air quality deterioration 

that would be allowed in an area over the 3-hour and 24-hour averaging periods. 
149

It should also be pointed out that because the existing 3-hour and 24-hour 

increments for SO2 are defined by the CAA at section 163(b), EPA cannot alter 

those increments without prior congressional action authorizing EPA to make any 

necessary changes. EPA does not interpret section 163(b) as establishing a right 

to degrade air quality to the level of the increments that would preclude EPA from 

establishing the 1-hour SO2 NAAQS level established in this action. See, 

National Corn Growers Ass’n v. EPA, 291 F.3d 1, 12 (D.C. Cir. 2002). 

(5) Comment: CRA commented that a stringent short-term SO2 standard greatly 

increases the difficulty of modeling to demonstrate compliance, since the standard 

that these plants are held to may not necessarily reflect the annual or daily average 

emission rates of affected facilities. ) CRA said that the current SO2 increment 

standards are appropriate for dispersion modeling requirements under the new 

source review program, and that it would be unnecessary to promulgate a 1-hour 

PSD increment for SO2. 

Response: Section 166(a) of the CAA authorizes EPA to promulgate “regulations 

for the prevention of significant deterioration” for pollutants for which NAAQS 

are promulgated after 1977. Thus, we believe that it would be appropriate to 

consider the need for a short-term increment for SO2 consistent with the 1-hour 

averaging period for the new SO2 NAAQS. Historically, EPA has developed 

increments for each applicable averaging period for which a NAAQS has been 

promulgated. However, the court determined in earlier NO2 increment litigation 

that increments for a particular pollutant do not necessarily need to match the 

averaging periods that have been established for NAAQS for the same pollutant. 

Environmental Defense Fund, Inc. v. EPA, 898 F.2d 183, 189-190 (D.C. Cir. 

1990)(“... the ‘goals and purposes’ of the PSD program, set forth in § 160, are not 

identical to the criteria on which the ambient standards are based.”) We intend to 

evaluate the need for a 1-hour SO2 increment in a subsequent rulemaking. 

(6) Comment: DRSI, supporting the comments by UARG, commented that if EPA 

intends to specify new PSDs, SILs, SERs, and SMC requirements specific to a 

new 1-hour standard, then it should conduct a separate notice-and-comment 

rulemaking to allow adequate opportunity for public review and input. 

Response: EPA does intend to use a separate rulemaking to propose screening 

tools for the new 1-hour SO2 NAAQS that would be established in regulations. 

As described in our response to an earlier comment, we are also considering the 

development of guidance that suggests an interim SIL for SO2 that state 

permitting authorities may use implement the new 1-hour SO2 NAAQS if they 

establish an appropriate record. We recognize the importance of public 

participation in the ultimate development of any SIL that is used in permitting. 

An interim SIL reflected in any EPA guidance should be subject to public 

comment whenever it is applied by a permitting authority in permitting action. 

The public would also have an opportunity to comment on any permanent SIL 

EPA proposed to establish by rulemaking. 
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(7) Comment: Alexandria commented that it is false to say that the SIP and NSR 

programs are effective means of addressing the public health impacts of existing 

large stationary sources that violate the NAAQS. First, many of the “oldest, 

dirtiest pollution sources” are essentially perpetually grandfathered from the 

requirement to operate with the controls required to ensure compliance with the 

NAAQS (while new sources must adhere to strict procedures and controls). 

Second, recent changes in the federal NSR program have allowed large stationary 

sources significant flexibility in determining what time periods constitute their 

baseline. The result of the above two points is that the reliance on the SIP-based 

monitoring method of NAAQS implementation practically guarantees that many 

thousands to millions of people throughout the country will continue for many 

years to suffer the health effects of frequent and chronic exposures in excess of 

the short-term NAAQS. This may follow many years, possibly decades of 

exposure from the emissions of proximate single sources at levels exceeding those 

corresponding to adverse health effects. Third, when emissions from existing 

major sources are shown by themselves to violate health standards, vast resources 

must be committed. For example, the NSR program (both major and minor, and 

its requisite ambient air quality standards compliance demonstration) is triggered 

only when a physical modification at a source leads to annual emission increases. 

However, many large electricity-generating units and other stationary sources use 

physical modifications to modify their operating conditions, often to improve 

financial performance. These physical modifications can increase plant output, 

though annual emissions decrease or do not change from historical baseline. 

Response: EPA generally agrees with the comment that the NSR program applies 

to existing sources when they modify the existing source (e.g, modify existing 

units or add additional units) and that the NSR program does not specifically 

address existing violations of the NAAQS by un-modified existing sources. 

However, another provision of the CAA, section 110(a)(1) and (a)(2)(A) requires 

that the state implementation plans demonstrate compliance (including schedules 

and timetables) with the NAAQS for existing sources. Such provisions are not 

limited to the regulation of only new sources or sources that propose to modify 

their existing facilities. In addition, areas that are found to be in violation of the 

NAAQS may be required to control those sources that are determined to be 

contributing to violations of the NAAQS through designation of the area as 

nonattainment, or through the implementation of a SIP call for the affected area. 

In these cases, states are required to adopt appropriate controls, or emissions 

limits, on the affected sources in order to bring about attainment in the area. 

(8) Comment: Alexandria commented that EPA should not rely solely on any 

number of locally-cited monitors. This is because (a) the SIP process, and its 

categorical source control measures, is designed for assuring that regional 

pollutant levels comply with NAAQS, and (b) monitoring methods cannot fully 

and comprehensively identify all areas of non-compliance caused by a single 

source’s maximum potential emissions for all possible local meteorological 
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conditions. A SIP remedy to a NAAQS contravention by a single source may 

take at least eight years, given the following timeline for an area for which 

conditions indicate a potential violation of the new NAAQS by a particular 

source: (1) regulators might assign a monitor(s) to the area, (2) public money 

would need to be expended for an air quality monitoring analysis, ensuring that 

those monitors be correctly located at the discrete points where emissions and 

meteorology together produce impacts exceeding the NAAQS, (3) after three 

years of modeling, a non-attainment area is defined, and (4) after another year, 

control measures commensurate with the NAAQS would be stipulated through the 

SIP (Alexandria) Instead, Alexandria requests that EPA propose and implement a 

streamlined procedure by which local air quality agencies can petition a single 

major source to demonstrate NAAQS compliance using standardized modeling 

techniques prescribed by the “Guideline.” These results would then be used to 

determine an emissions control regime for a source that is NAAQS compliant. 

Alexandria stated that EPA has invested significant resources in developing 

standard modeling tools, preprocessors, and guidelines, such that an existing 

major source should now readily be able to calculate how their maximum 

potential emissions affect nearby ambient pollutant levels. 

Response: In sections III, IV V and VI of the preamble to the final rule, we 

discuss our anticipated approach to implementing the new 1-hour NAAQS. We 

expect that many, if not all of Alexandria’s concerns, would be satisfactorily 

addressed by such an approach. 

(9) Comment: ALA/EDF/NRDC/SC commented that it is unlawful to use SILs 

and/or SMCs to waive or limit the CAA requirements for air quality impact 

analyses. For SILs: The CAA unequivocally mandates that a proposed PSD 

source or modification must demonstrate that its emissions “will not cause or 

contribute” to a violation of increments or of any NAAQS. “No major emitting 

facility” may be constructed unless it meets this requirement, and the emissions 

from a facility may not cause or contribute to an exceedance of “any” increment 

“for any pollutant in any area” or to an exceedance of “any” NAAQS. The statute 

requires that each permit applicant must conduct an air quality analysis, including 

obtaining continuous air quality monitoring data, which is “gathered for the 

purposes of determining whether emissions from such facility will exceed the 

maximum allowable increases or the maximum allowable concentration permitted 

under this part.” For SMCs: Section 165(e)(2) mandates a full year of continuous 

air quality monitoring for each major source subject to the PSD program, without 

exception, other than a limited provision that allows for less than a year’s worth 

of monitoring based on a determination “that a complete analysis for such 

purposes may be accomplished in a shorter period.” ALA stated that exception 

“hardly amounts to the authority to waive monitoring entirely,” which, in their 

opinion, is what SMCs do. 

Response: The EPA disagrees with this commenter’s claim that it is unlawful to 

use SILs and SMC as screening tools to provide a reasonable implementation 
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approach for PSD/NSR purposes. We believe that the concept of a SIL, or SMC, 

is grounded on the de minimis principles described by the court in Alabama 

Power v. Costle, 636 F.2d 323, 362 (D.C. Cir. 1979) (Alabama Power). In that 

case reviewing EPA’s 1978 PSD regulations, the court recognized that “there is 

likely a basis for an implication of de minimis authority to provide exemption 

when the burdens of regulation yield a gain of trivial or no value.” Alabama 

Power at 360. Additional discussion of EPA’s legal authority to promulgate SILs 

and SMCs are provided in 72 Fed. Reg. 54112, 54139-41 (Sept. 21, 2007). 

(10) Comment: Missouri commented that the revised 3-hour, 24-hour, and annual SO2

increment standards could be addressed in a way similar to how Congress 

resolved the PM10 issue, though the issue is not quite the same since the PM10 

issue involved a new size distinction. Based on the PM10 issue, though, Congress’ 

intent is clearly only to require increment standards for pollutants and averaging 

times where the public or environment needs protection. It does not make sense 

to continue to include an increment evaluation for an averaging time that does not 

exist. It is very difficult to believe that a standard that is proposed to be tightened 

to this extent could be allowed to determine the issuing of permits to sources 

using the same principles that were adopted under the current standard. The more 

difficult question is, according to Missouri, "What is the correct course of action 

for areas that have already triggered a minor source baseline and have violated 

one of the existing increment standards or are near an increment violation?" 

Another critical issue, in their opinion, is the implementation of new significant 

impact levels, significant monitoring concentrations, and significant emission 

rates for the new standard. 

Response: As described in an earlier response, the CAA explicitly defines 

increments for SO2 that EPA cannot alter without prior authorization from 

Congress. EPA recognizes that there may be little need for annual and 24-hour 

SO2 increments following the revocation of the annual and 24-hour NAAQS. 

Nevertheless, these increments must remain intact until and unless Congress 

decides to amend the CAA to itself make, or authorize EPA to make, any 

necessary changes. 

(11) Comment: Dow, supporting ACC and API comments, recommended that EPA 

clarify that compliance with the new standard should be determined only by the 

ambient monitoring network. Dow clarified that state permitting agencies do not 

have to require regulated entities to conduct air dispersion modeling unless 

required by a review of the PSD air permit application. 

Response: Under the anticipated implementation approach discussed in sections 

III, IV, V and VI of the preamble to the final rule, EPA expects to rely upon both 

monitoring and refined dispersion modeling to inform designations and 

determinations of whether implementation plans show attainment of the new 1-

hour NAAQS. We note that section 110(a)(2)(K) of the CAA provides that the 
153

Administrator may prescribe the use of ambient modeling for the purpose of 

predicting the effect on ambient air quality of any (emphasis added) emissions for 

which there is a NAAQS. Therefore, the use of ambient modeling need not be 

limited to only sources that must meet the PSD requirements. A prerequisite to 

the issuance of any construction permit for a stationary source is that it 

demonstrates that its emissions do not cause or contribute to a violation of any 

NAAQS. See, e.g., 40 CFR 51.160(a), (b), (f). Meeting this requirement may not 

in all cases require a modeling demonstration; however, we have in the past 

concluded that modeling is sometimes necessary to demonstrate NAAQS 

compliance as part of the issuance of even minor source permits, and we expect 

this to continue to be true. EPA’s implementing rules for applying ambient 

modeling [see 40 CFR part 51, Appendix W, Guideline on Air Quality Models] 

contain the specific modeling requirements for existing and new sources. States 

are authorized under the SIP to use ambient modeling for these purposes. 

Nothing in the CAA suggests that only ambient monitoring, instead of modeling, 

must be used to determine source impacts and compliance with the NAAQS. The 

rules at 40 CFR 51.160 (f) requiring state plans to contain legally enforceable 

procedures also specify that the modeling meet the requirements of Appendix W. 

See 51.160 (f). 

X. Responses to Significant Comments on Appendix T 

(Interpretation of Primary NAAQS and Exceptional Events 

Rules) 

A. Comments on data completeness and data substitutions 

(1) Comment: Iowa commented that they would prefer to couple a lower NAAQS 

level with simple design value calculations that are transparent to the public, 

rather than couple a higher NAAQS level with a complicated design value. As 

EPA moves towards one-hour standards for many gaseous pollutants, it is 

desirable to standardize the design value calculations across the pollutants. Iowa 

recommended using a quarterly data capture of at least 75% to produce a valid 

annual 98th or 99th percentile for design value calculations. The final form of the 

standard must be robust so as to more effectively control air pollution. According 

to Iowa, regulating the tails of the SO2 distribution will cause great year-to-year 

variability in attainment, and the modeling hot spots that dictate monitor 

placement may also be diffuse and poorly defined when regulating the SO2 

distribution tails. 

Response: EPA agrees with the comment on data completeness. The final rule 

incorporates this completeness requirement, but also provides for two diagnostic 

data substitution tests for use with data sets that do not meet the 75% requirement. 

These tests are complicated and may not be perceived as transparent to all the 

public, but EPA believes that this complexity is necessary and appropriate given 
154

the usefulness of the tests in allowing at least some areas to be clearly determined 

to be meeting or not meeting the NAAQS despite the data incompleteness.

(2) Comment: NYSDEC does not believe that data substitution should be employed 

to make attainment/non-attainment designations. 

Response: This issue is discussed in the NFR Preamble text, Section VII, 

Appendix T--Interpretation of the Primary NAAQS for Oxides of Sulfur and 

Revisions to the Exceptional Events Rule, Interpretation of the NAAQS for Oxides 

of Sulfur (Section VII.A). See also the response to the above comment.

(3) Comment: UARG and DSRI agreed that there should be completeness criteria for 

compliance data, but disagree with EPA’s suggested approach. UARG and DSRI 

argued that the data completeness rule should include a system for determining 

when incomplete data should be used, rather than giving discretion to the 

Administrator (such discretion is in conflict with the current data completeness 

rule and opens the possibility of seemingly arbitrary decisions). Since only 75% 

of the expected data are required for completeness, when hypothetical data are 

substituted for missing data, enough substituted data should be used to bring the 

expected data to 75% rather than including enough substituted data to bring the 

expected data to 100%. The goal of the monitoring program should be a high 

degree of accuracy in ambient SO2 measurements, and this accuracy will increase 

not just with an increasing percentage of expected data but also with an increasing 

percentage of actual monitoring data. 

Response: The issue of substituting data up to 75% versus 100% completeness as 

part of the diagnostic data substitution tests is discussed in the NFR Preamble 

text, Section VII, Appendix T--Interpretation of the Primary NAAQS for Oxides 

of Sulfur and Revisions to the Exceptional Events Rule. The commenter also 

disagrees with a proposed provision allowing the Administrator to use incomplete 

data even in a situation in which the data substitution test does not produce a 

validated design value. The final rule retains this provision, which also is a 

feature of the data interpretation appendices for the PM2.5 and the 1-hour NO2

NAAQS. EPA believes that situations may arise in which using incomplete data 

is appropriate but which cannot be anticipated at this time. EPA notes that the 

public will have opportunity to comment on any such use of incomplete data that 

has regulatory consequences for a state, because EPA provides public notice prior 

to any final designation, clean data determination, or SIP approval/disapproval 

action.

(4) Comment: Wisconsin commented that EPA should consider allowing data to be 

qualified rather than invalidated for certain quality control failures like baseline 

drift and some calibration verification failures. Wisconsin agreed that some 

situations may call for using data substitution, but there may be many situations 

where the data gatherer/analyst notices a data bias that can be adjusted for, 

leading to a better estimate of actual concentrations versus using a data 
155

substitution procedure. For example, a verification standard may exceed limits by 

a couple percent when the sample concentrations are near baseline–that is, the 

magnitude seems reliable but the absolute concentration is questionable. In this 

case, under the current data handling procedures, this important qualifying 

information would be lost. Wisconsin proposed that the data analyst be allowed 

to build in any necessary margin of error to the data. 

Response: Section 2.a of Appendix T says that all data required to be submitted to 

AQS meeting requirements of part 58 shall be used in design value calculations. 

The situations and alternative courses of action described in the comment arise 

prior to submission of data to AQS, and are not addressed in Appendix T. Such 

issues are addressed by EPA monitoring guidance, which is outside the scope of 

this rulemaking. 

(5) Comment: Several commenters expressed support for EPA’s proposed 75% 

completeness requirement for daily and quarterly data. 

Response: EPA agrees with the comment on data completeness. The NFR does 

what the comment suggests.

(6) Comment: North Carolina and AirQuality expressed support for EPA’s proposed 

substitution criteria, although AirQuality only explicitly approves of the criteria if 

the form of the standard is the 4th highest concentration. 

Response: This issue is discussed in the NFR Preamble text, Section VII, 

Appendix T--Interpretation of the Primary NAAQS for Oxides of Sulfur and 

Revisions to the Exceptional Events Rule. 

(7) Comment: Iowa “do not favor the provisions in the proposed rule that disallow 

substitution of secondary monitor data for primary monitor data except in cases 

when the primary monitor is permanently removed from the site” because it will 

decrease data quality and consequently recommended that the “provision be 

removed from the final rule.” 

Response: This issue is discussed in the NFR Preamble text, Section VII, 

Appendix T--Interpretation of the Primary NAAQS for Oxides of Sulfur and 

Revisions to the Exceptional Events Rule. 

(8) Comment: Iowa recommended EPA allow “states to upload hourly data from 

multiple monitors to the AQS database” in order to “construct a composite data 

set in AQS for which each hourly value represents the average of the valid hourly 

data collected for each instrument at the monitoring site. The data from the 

composite data set could then be used for computation of design values.” 
156

Response: This issue is discussed in the NFR Preamble text, Section VII, 

Appendix T--Interpretation of the Primary NAAQS for Oxides of Sulfur and 

Revisions to the Exceptional Events Rule. 

(9) Comment: MIDNRE, concerned about the inherent instability of a short-term SO2 

standard (1, 3, or 8-hour), provided comments recommending EPA modify the 

methodology for siting SO2 monitors so that state agencies can “average monitors 

within a specified area and compare the averaged level to the standard for 

attainment purposes or to classify a subset of data as not applicable for 

comparison of the NAAQS, in the same way that the original fine particulate 

network was set up.” MIDNRE also expressed concern that “both the PWEI and 

contribution to national SO2 emissions are emission based, and thus, redundant. 

The redundancy creates an added, unnecessary financial burden by requiring 

additional monitors. For states with a high contribution to the national inventory 

as well as many large urban areas, the number of monitoring sites and the short 

implementation time line would create an excessive burden.” 

Response: The commenter makes two suggestions said to be tied to a concern that 

the form and level of the proposed standard may cause some sites to flip in and 

out of attainment. The commenter provided a graph showing the variation of the 

three-year averages of various 1-hour SO2 metrics from 1993 to 2007 for a 

monitoring site in Detroit, MI. It is not clear whether the "1st high" and other 

metrics shown in the graph are based on all hours of the year, or on daily 

maximum 1-hour values. 

EPA's first response is that the graph provided by the commenter does not show a 

great deal of instability of the 4th high metric, which (assuming the graph is 

presenting the 4th high daily maximum value) is essentially the same as the 

proposed 99th percentile form of the standard. The value of this metric ranged 

between 40 and 80 ppb during this time period, and was appreciably outside the 

40 to 60 ppb range only in the three 3-year periods that included data from 1999. 

EPA recognizes that while using a 3-year average reduces the instances of areas 

flipping in and out of attainment, there likely will always be some such cases in 

areas whose long term average air quality is close to the standard such that a 

single particularly bad year causes the design value to exceed the standard for 

three successive years. 

The commenter's first suggestion is that states be allowed to "average monitors" 

within a specified area and compare the averaged levels to the standard for 

attainment purposes. It is not clear whether the suggestion envisions averaging 

same-hour concentrations across sites before identifying the 99th percentile (for 

example) value of the daily maximum 1-hour concentration, or averaging the 

three-year average of annual 99th percentile values across sites. The first 

approach has the potential to greatly reduce the level of protection afforded by a 

NAAQS set at a given level, because in any single hour the extent of the area of 

high SO2 concentration may be quite limited such that averaging across sites 
157

(even sites not widely separated in monitoring network terms) could result in an 

average value substantially below the actual concentration to which some 

individuals may be exposed. The second approach could have a lesser tendency 

to reduce the level of protection, if the sites being averaged have similar 

transport/dilution relationships to the same dominant SO2 emission source(s) so 

that they experience a similar distribution of 1-hour concentrations during the 

year although not matched across sites hour-by-hour. However, the second 

approach could still tend to result in a reported concentration below the highest 

concentration to which some people may be exposed. EPA sees no advantage to 

such an approach. EPA notes that if spatial averaging were applied to monitoring 

data, the Administrator would have to consider this feature when selecting the 

level of the NAAQS. EPA also notes that in any case a spatial averaging 

approach could only be appropriate and relevant if multiple monitors are used 

near single sources, which EPA in general believes would often be a poor use of 

monitoring resources given the number of sources with potential to cause 

violations of the NAAQS. 

The second suggestion is that a subset of monitors be classified as not applicable 

for comparison to the NAAQS, and the commenter makes reference to the fine 

PM network. The 1997 final rule for the PM2.5 NAAQS established two new 

standards: an annual standard based on PM2.5 concentrations from single or 

multiple community-oriented monitors; and a 24-hour standard based on 24-hour 

PM2.5 concentrations at population-oriented monitors within an area. 71 FR 

61144, 61146, 61164-65 (October 17, 2006). Also see 40 CFR 58.30. This form 

of the annual standard was intended to characterize area-wide PM2.5 

concentrations, in conjunction with a 24-hour standard designed to provide 

adequate protection against localized peak or seasonal PM2.5 levels. The 24-hr 

standard was deigned to provide protection for people residing in or near localized 

areas of elevated concentrations. Id. These provisions were based largely on the 

specific evidence before EPA in that NAAQS review, which was focused largely 

on epidemiologic evidence with only limited evidence from clinical studies. EPA 

made various public health policy judgments concerning the appropriate level, 

averaging time, and form of the PM2.5 standards in light of that evidence.

The body of evidence in this SO2 review is quite different, including important 

clinical evidence as well as epidemiologic evidence. This body of evidence did 

not lead EPA to propose provisions concerning community-oriented monitors or 

population oriented monitors as a way to provide appropriate public health 

protection from short-term exposure to SO2. The commenter raises concerns over 

the stability of a 1-hour SO2 standard; however the monitoring provisions for the 

PM2.5 NAAQS described above were not based on concerns over the stability of 

the standard. The issue of the stability of the standard is important, and it is 

addressed in this review through the use of a percentile form and a 3 year average. 

Commenter presents no evidence or argument supporting use of a different 

approach to address the issue of stability of the standard. 
158

XI. Responses to Significant Comments on the Regulatory 

Impact Analysis 

(1) Comment: Benefits from direct exposure to SO2 are small; most of the benefits 

comes from emissions of particulate matter. (CIBO, AECT, South Carolina 

Chamber of Commerce, PE, EEI, UARG). 

Response: EPA concurs that the monetized benefits of reduced SO2 exposure 

appear small when compared to the monetized benefits of reduced PM2.5

exposure. This result is consistent with other recent RIAs, where the PM2.5 cobenefits represent a large proportion of total monetized benefits. This result is 

amplified in this RIA by the decision not to quantify SO2-related premature 

mortality and other morbidity endpoints due to the uncertainties associated with 

estimating those endpoints. Studies have shown that there is a relationship 

between SO2 exposure and premature mortality, but that relationship is limited by 

potential confounding. Because premature mortality generally comprises over 

90% of the total monetized benefits, this decision may substantially underestimate 

the monetized health benefits of reduced SO2 exposure. 

EPA does not concur with the characterization of the benefits being restricted to 

only the monetized SO2-related health benefits. As there are several other 

categories of benefits associated with the SO2 reductions, EPA believes it is 

appropriate to compare the total monetized benefits with the costs. It is 

inappropriate to consider only one category of monetized benefits in isolation of 

the other benefits anticipated from the rule. 

(2) Comment: At the low levels of exposure being studied, co-pollutant impact may 

be improperly attributed to SO2. (AEPSC, UARG) 

Response: EPA concurs that co-pollutants present in the ambient air may 

contribute to the health effects attributed to SO2 in single pollutant models, which 

could lead to overestimating the SO2 risks if those co-pollutants are highly 

correlated with SO2. Where available, we have selected multipollutant effect 

estimates to control for the potential confounding effects of co-pollutants; these 

include NYDOH (2006), Schwartz et al. (1994) and O’Conner et al. (2007). 

Because the majority of the monetized benefits are from PM2.5-related premature 

mortality and we did not estimate SO2-related mortality, it is unlikely that 

potential confounding would have a substantial effect on the total monetized 

benefits. 

(3) Comment: Very few benefits come from reducing exposure to SO2; virtually all 

of the benefits come from reduced exposure to particulate matter. PM co-benefits 
159

should not be addressed in this RIA. (CIBO, PE, South Carolina Chamber of 

Commerce, EEI. UARG). 

Response: EPA does not concur that the PM2.5 co-benefits should excluded from 

the total monetized benefits. Because SO2 is a precursor to PM2.5, reducing SO2

emissions in the projected non-attainment areas will also reduce PM2.5 formation, 

human exposure and the incidence of PM2.5-related health effects. The PM cobenefits are directly related to the reduction of SO2 emissions, not ancillary 

controls installed to reduce PM. 

Reducing SO2 emissions is associated with a variety of benefit categories, only 

some of which we were able to quantify and monetize in this analysis. In this 

analysis, we monetized the benefits associated with some SO2-related morbidity 

endpoints and PM-related mortality and morbidity endpoints, but we did not 

monetize the benefits associated with SO2-related mortality, acid deposition, 

mercury methylation from sulfate, or visibility. 

The goal of an RIA is to provide a comprehensive estimate of all the anticipated 

costs and benefits associated with a regulatory action, regardless of the underlying 

justification for the action. Co-benefits that occur as a result of a regulatory 

action are appropriate to include in the RIA, and it is appropriate to compare the 

all of the monetized benefits with the costs. It is inappropriate to consider only 

one category of monetized benefits in isolation of the other benefits anticipated 

from the rule. 

(4) Comment: Health benefits from PM controls are “double-counted” with the PM 

NAAQS and other rules. (UARG, Pepper Hamilton, AA).

 

Response: EPA does not concur that the PM co-benefits estimated for this rule are 

“double-counted” with the PM NAAQS. The emission reductions estimated for 

NAAQS analyses are incremental to those estimated for previous NAAQS 

analyses. Reducing SO2 also reduces PM2.5 formation, human exposure and the 

incidence of PM2.5-related health effects, regardless of the rule that requires the 

reduction. Because the control strategies to attain the NAAQS are illustrative, 

there may be unavoidable degree of overlap with rules that will be promulgated 

subsequent to this rulemaking. However, both the costs and benefits would be 

double-counted in that scenario. 

Note also that EPA models PM2.5-related premature mortality using a nothreshold, log-linear model, which is consistent with the conclusion in the 

Integrated Science Assessment for Particulate Matter (EPA, 2009c). The 

emission reductions from this rule occur in areas with varied concentrations of 

PM2.5, including both regions that are in attainment with fine particle standard and 

would not be required to develop control strategies for the fine particulates, and 

those that would not be in attainment. 
160

(5) Comment: There is a NAAQS underway for fine particulates; if virtually all the 

benefits are from PM, then control PM and not SO2. Claiming PM mortality as a 

benefit in the SO2 NAAQS is misleading and misplaced. (RMA, PCA).

 

Response: The level of the SO2 NAAQS is determined on the basis of the health 

effects associated with SO2 exposure. Emission reductions from other rules such 

as the PM NAAQS may help an area achieve attainment, but those emission 

reductions do not affect the health basis for the SO2 NAAQS. 

(6) Comment: Consideration of PM co-benefits may be appropriate for longer-term 

standards with longer averaging periods; it is inappropriate to consider those 

benefits in a short-term standard based on short-term effects. (CRA)

Response:  EPA does not concur that it is inappropriate to consider PM cobenefits for a short-term SO2 standard. Although the averaging period of the 

standard could affect the emission reductions needed to meet the standard, the 

averaging period of the standard is irrelevant to PM2.5 formation from those SO2 

emissions. 

(7) Comment: EPA’s assertion that there would be high PM mortality is among the 

least justified of all EPA’s impact assumptions. (PCA).

Response: EPA does not concur with the characterization of PM-related 

premature mortality as the “least justified” assumption. After evaluating the body 

of scientific literature, the Integrated Science Assessment for Particulate Matter 

concluded that the relationship between both short-term and long-term exposure 

to PM2.5 is causally associated with premature morality (EPA, 2009c). 

(8) Comment: The total monetized costs exceed the benefits from reducing SO2

emissions by a significant amount. (Pepper Hamilton, NPRA, AECT, TAB, EEI, 

UARG)

Response:  Total monetized benefits of the SO2 standard exceed the costs by a 

substantial margin. EPA believes it is inappropriate to consider only one category 

of monetized benefits (direct SO2 health impacts) in isolation of the other benefits 

(particulate matter health impacts) anticipated from compliance with the standard. 

(9) Comment: The RIA may substantially underestimate the health benefits of 

reducing exposure to SO2 in cities like New York by relying on national data to 

estimate base incidence rates. (City of New York)

Response:  EPA concurs that the PM2.5 co-benefits are based on national averages, 

and thus do not reflect local variability in population density, meteorology, 

exposure, baseline health incidence rates, or other local factors. This 

methodology might lead to underestimates in some locations such as New York 

City and overestimates in other locations, but EPA believes that the overall 
161

national estimate is not biased. (Based on projections, EPA estimates that New 

York City would attain the alternate standards without additional controls.) 

(10) Comment: Any revision to the SO2 NAAQS that makes those standards more 

stringent will result in increased cost of electricity to cooperative consumerowners. Because cooperatives have a disproportionate amount of fossil-fuel fired 

generation when compared to the electric utility industry as a whole, these 

negative impacts could be disproportionately higher for electric co-ops and their 

consumer owners – particularly in rural areas. (NRECA). 

Response: In the RIA for the proposal, EPA included an economic impact 

analysis of the impact of the illustrative control strategies for areas to meet the 50 

ppb alternative standard. That economic impact analysis shows that the electric 

power industry will only experience annualized costs of less than 0.4 percent of 

total industry revenues in 2020. While we do not provide impacts for individual 

power plants and their owners, the impact estimates do suggest that, at a broad 

scale, the electric power industry, as affected by illustrative control strategies such 

as those included in this RIA, will not experience substantial impacts from 

compliance with this NAAQS. 

(11) Comment: This RIA does not realistically assess the economic impact on the 

nation’s manufacturing base and jobs. Non-EGU point sources will bear the brunt 

of the proposed rule and the economic impact on existing industrial facilities, 

including potential job losses, has not been accounted for in the RIA. (CIBO). 

Response: In the RIA for the proposal, EPA included an economic impact 

analysis of the impact of the illustrative control strategies for areas to meet the 50 

ppb alternative standard. That economic impact analysis shows that industries 

with non-EGU point sources affected by the illustrative control strategies will 

only experience annualized costs of less than 0.01 percent of total industry 

revenues in 2020. While we do not provide impacts for individual facilities and 

firms, the impact estimates do suggest that, at a broad scale, industries potentially 

affected by illustrative control strategies such as those included in this RIA will 

not experience substantial impacts from compliance with this NAAQS. 

(12) Comment: Neither the Proposed Rule nor its accompanying Regulatory Impact 

Analysis (RIA) discuss the Proposed Rule’s potentially significant economic 

impacts on sulfuric acid plants. Neither document attempts to compare those 

economic impacts to the relatively small contribution of the sulfuric acid 

manufacturing industry to ambient SO2. (Ad Hoc Committee of Sulfuric Acid 

Producers). 

Response: In the RIA for the proposal, EPA included an economic impact 

analysis of the impact of the illustrative control strategies for areas to meet the 50 

ppb alternative standard. That economic impact analysis shows that industries 

with non-EGU point sources such as the chemical manufacturing industry (which 
162

includes sulfuric acid plants) that may be affected by this NAAQS will only 

experience annualized costs of less than 0.01 percent of total industry revenues in 

2020. While we do not provide impacts for individual facilities and firms, the 

impact estimates do suggest that, at a broad scale, sulfuric acid plants that may be 

potentially affected by illustrative control strategies such as those included in this 

RIA will not experience substantial impacts from compliance with this NAAQS. 

(13) Comment: The RIA states that EPA was unable to identify SO2 emissions control 

measures that would be capable of bringing some counties into attainment of such 

a SO2 standard. (Ad Hoc Committee of Sulfuric Acid Producers). 

Response: In this analysis we were not able to find controls that would bring all 

areas into attainment with the alternative standards in all areas. It is uncertain 

what controls States would put in place to attain a tighter standard. We should 

also note that because of data and resource limitations for this broad national 

analysis, we are not able to adequately represent in this analysis the impacts of 

some local emission control programs. 

(14) Comment: The RIA also does not make clear how or if retrofits and replacements 

of boilers burning residual oil and switching to natural gas or distillate heating oil 

is accounted for in the estimated costs of reducing SO2 emissions. (City of New 

York)

Response: The RIA does not include any retrofits or replacements of boilers 

burning residual oil and switching to natural gas. The control measure considered 

for these sources was flue gas desulfurization. We should also note that because 

of data and resource limitations for this broad national analysis, we are not able to 

adequately represent in this analysis the impacts of some local emission control 

programs, or specific strategies that may be available to some sources in some 

areas. 
163

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