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Tuesday,

June 22, 2010

Part II

EnvironmentalProtection Agency40 CFR Parts 50, 53, and 58

Primary National Ambient Air QualityStandard for Sulfur Dioxide; Final Rule

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35520 Federal Register / Vol. 75, No. 119 / Tuesday, June 22, 2010/ Rules and Regulations

ENVIRONMENTAL PROTECTIONAGENCY

40 CFR Parts 50, 53, and 58

[EPAHQOAR20070352; 91604]

RIN 2060A048

Primary National Ambient Air Quality

Standard for Sulfur Dioxide

AGENCY: Environmental ProtectionAgency (EPA).

ACTION: Final rule.

SUMMARY: Based on its review of the airquality criteria for oxides of sulfur andthe primary national ambient air qualitystandard (NAAQS) for oxides of sulfuras measured by sulfur dioxide (SO2),EPA is revising the primary SO2NAAQS to provide requisite protectionof public health with an adequatemargin of safety. Specifically, EPA isestablishing a new 1-hour SO2 standardat a level of 75 parts per billion (ppb),

based on the 3-year average of theannual 99th percentile of 1-hour dailymaximum concentrations. The EPA isalso revoking both the existing 24-hourand annual primary SO2 standards.

DATES: This final rule is effective onAugust 23, 2010.

ADDRESSES: EPA has established adocket for this action under Docket IDNo. EPAHQOAR20070352. Alldocuments in the docket are listed onthe http://www.regulations.govWebsite. Although listed in the index, some

information is not publicly available,e.g., confidential business informationor other information whose disclosure isrestricted by statute. Certain othermaterial, such as copyrighted material,will be publicly available only in hardcopy form. Publicly available docketmaterials are available eitherelectronically through http://www.regulations.govor in hard copy atthe Air and Radiation Docket andInformation Center, EPA/DC, EPA West,Room 3334, 1301 Constitution Ave.,NW., Washington, DC. The PublicReading Room is open from 8:30 a.m. to4:30 p.m., Monday through Friday,excluding legal holidays. The telephonenumber for the Public Reading Room is(202) 5661744 and the telephonenumber for the Air and RadiationDocket and Information Center is (202)5661742.

FOR FURTHER INFORMATION CONTACT: Dr.Michael J. Stewart, Health andEnvironmental Impacts Division, Officeof Air Quality Planning and Standards,U.S. Environmental Protection Agency,Mail code C50406, Research TrianglePark, NC 27711; telephone: 919541

7524; fax: 9195410237; e-mail:[email protected].

SUPPLEMENTARY INFORMATION:

Table of Contents

The following topics are discussed inthis preamble:

I. Background

A. Summary of Revisions to the SO2Primary NAAQS

B. Statutory RequirementsC. Related SO2 Control ProgramsD. History of Reviews of the Primary

NAAQS for Sulfur OxidesE. Summary of Proposed Revisions to the

SO2 Primary NAAQSF. Organization and Approach to Final SO2

Primary NAAQS DecisionsII. Rationale for Decisions on the Primary

StandardsA. Characterization of SO2 Air Quality1. Anthropogenic Sources and Current

Patterns of SO2 Air Quality2. SO2 MonitoringB. Health Effects Information

1. Short-Term (5-Minute to 24-Hour) SO2Exposure and Respiratory Morbidity

Effectsa. Adversity of Short-Term Respiratory

Morbidity Effects2. Health Effects and Long-Term Exposures

to SO23. SO2-Related Impacts on Public HealthC. Human Exposure and Health Risk

CharacterizationD. Approach for Determining Whether To

Retain or Revise the Current StandardsE. Adequacy of the Current Standards1. Rationale for Proposed Decision2. Comments on the Adequacy of the

Current Standardsa. Comments on EPAs Interpretation of the

Epidemiologic Evidenceb. Comments on EPAs Interpretation of the

Controlled Human Exposure Evidencec. Comments on EPAs Characterization of

SO2-Associated Exposures and HealthRisks

3. Conclusions Regarding the Adequacy ofthe Current 24-Hour and AnnualStandards

F. Conclusions on the Elements of a NewShort-Term Standard

1. Indicatora. Rationale for Proposed Decision

b. Comments on Indicatorc. Conclusions on Indicator2. Averaging Timea. Rationale for Proposed Decision

b. Comments on Averaging Timec. Conclusions on Averaging Time3. Forma. Rationale for Proposed Decision

b. Comments on Formc. Conclusions on Form4. Levela. Rationale for Proposed Decision

b. Comments on Levelc. Conclusions on Level5. Retaining or Revoking the Current 24-

Hour and Annual Standardsa. Rationale for Proposed Decision

b. Comments on Retaining or Revoking theCurrent 24-Hour and Annual Standards

c. Conclusions on Retaining or Revokingthe Current 24-Hour and AnnualStandards

G. Summary of Decisions on PrimaryStandards

III. Overview of the Approach for Monitoringand Implementation

IV. Amendments to Ambient Monitoring andReporting Requirements

A. Monitoring Methods

1. Requirements for SO2 Federal ReferenceMethod (FRM)

a. Proposed Ultraviolet Fluorescence SO2FRM and Implementation

b. Public Commentsc. Conclusions on Ultraviolet Fluorescence

SO2 FRM and Implementation2. Requirements for Automated SO2

Methodsa. Proposed Performance Specifications for

Automated Methodsb. Public Commentsc. Conclusions for Performance

Specifications for SO2 AutomatedMethods

B. Network Design1. Approach for Network Design

a. Proposed Approach for Network Designb. Alternative Network Designc. Public Comments2. Modeling Ambient SO2 Concentrations3. Monitoring Objectivesa. Proposed Monitoring Objectives

b. Public Commentsc. Conclusions on Monitoring Objectives4. Final Monitoring Network Design5. Population Weighted Emissions Indexa. Proposed Use of the Population

Weighted Emissions Indexb. Public Commentsc. Conclusions on the Use of the

Population Weighted Emissions Index6. Regional Administrator Authoritya. Proposed Regional Administrator

Authorityb. Public Commentsc. Conclusions on Regional Administrator

Authority7. Monitoring Network Implementationa. Proposed Monitoring Network

Implementationb. Public Commentsc. Conclusions on Monitoring Network

ImplementationC. Data Reporting1. Proposed Data Reporting2. Public Comments3. Conclusions on Data Reporting

V. Initial Designation of Areas for the 1-HourSO2 NAAQS

A. Clean Air Act Requirements

1. Approach Described in Proposal2. Public CommentsB. Expected Designations Process

VI. Clean Air Act ImplementationRequirements

A. How This Rule Applies to TribesB. Nonattainment Area Attainment Dates1. Attaining the NAAQS2. Consequences of a Nonattainment Area

Failing To Attain by the StatutoryAttainment Date

C. Section 110(a)(1) and (2) NAAQSMaintenance/InfrastructureRequirements

1. Section 110(a)(1)(2) Submission

http://www.regulations.gov/http://www.regulations.gov/http://www.regulations.gov/mailto:[email protected]:[email protected]:[email protected]://www.regulations.gov/http://www.regulations.gov/http://www.regulations.gov/


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35521Federal Register / Vol. 75, No. 119 / Tuesday, June 22, 2010/ Rules and Regulations

1The legislative history of section 109 indicatesthat a primary standard is to be set at themaximum permissible ambient air level * * *

which will protect the health of any [sensitive]group of the population, and that for this purposereference should be made to a representativesample of persons comprising the sensitive grouprather than to a single person in such a group. S.Rep. No. 911196, 91st Cong., 2d Sess. 10 (1970).See also American Lung Assn v. EPA, 134 F. 3d388, 389 (DC Cir. 1998) (NAAQS must protect notonly average healthy individuals, but also sensitivecitizenschildren, for example, or people withasthma, emphysema, or other conditions renderingthem particularly vulnerable to air pollution. If apollutant adversely affects the health of thesesensitive individuals, EPA must strengthen theentire national standard.); Coalition of BatteryRecyclers Assn v. EPA, No. 091011 (DC Cir. May14, 2010) slip op. at 7 (same).

2EPA is currently conducting a separate reviewof the secondary SO2 NAAQS jointly with a reviewof the secondary NO2 NAAQS (see http://www.epa.gov/ttn/naaqs/standards/no2so2sec/index.htmlfor more information).

D. Attainment Planning Requirements1. SO2 Nonattainment Area SIP

Requirements2. New Source Review and Prevention of

Significant Deterioration Requirements3. General ConformityE. Transition From the Existing SO2

NAAQS to a Revised SO2 NAAQSVII. Appendix TInterpretation of the

Primary NAAQS for Oxides of Sulfur

and Revisions to the Exceptional EventsRule

A. Interpretation of the NAAQS for Oxidesof Sulfur

1. Proposed Interpretation of the Standard2. Comments on Interpretation of the

Standard3. Conclusions on Interpretation of the

StandardB. Exceptional Events Information

Submission ScheduleVIII. Communication of Public Health

InformationIX. Statutory and Executive Order Reviews

A. Executive Order 12866: RegulatoryPlanning and Review

B. Paperwork Reduction Act

C. Regulatory Flexibility ActD. Unfunded Mandates Reform ActE. Executive Order 13132: FederalismF. Executive Order 13175: Consultation

and Coordination With Indian TribalGovernments

G. Executive Order 13045: Protection ofChildren From Environmental Health &Safety Risks

H. Executive Order 13211: Actions ThatSignificantly Affect Energy Supply,Distribution or Use

I. National Technology Transfer andAdvancement Act

J. Executive Order 12898: Federal ActionsTo Address Environmental Justice inMinority Populations and Low-Income

PopulationsReferences

I. Background

A. Summary of Revisions to the SO2Primary NAAQS

Based on its review of the air qualitycriteria for oxides of sulfur and theprimary national ambient air qualitystandard (NAAQS) for oxides of sulfuras measured by sulfur dioxide (SO2),EPA is making revisions to the primarySO2 NAAQS so the standards arerequisite to protect public health withan adequate margin of safety, as

appropriate under section 109 of theClean Air Act (Act or CAA).Specifically, EPA is replacing thecurrent 24-hour and annual standardswith a new short-term standard basedon the 3-year average of the 99thpercentile of the yearly distribution of1-hour daily maximum SO2concentrations. EPA is setting the levelof this new standard at 75 ppb. EPA isadding data handling conventions forSO2by adding provisions for this new1-hour primary standard. EPA is alsoestablishing requirements for an SO2

monitoring network. These newprovisions require monitors in areaswhere there is an increased coincidenceof population and SO2 emissions. EPAis also making conforming changes tothe Air Quality Index (AQI).

B. Statutory Requirements

Two sections of the Clean Air Act

(Act or CAA) govern the establishmentand revision of National Ambient AirQuality Standards NAAQS. Section 108of the Act directs the Administrator toidentify and list air pollutants that meetcertain criteria, including that the airpollutant in his judgment, cause[s] orcontribute[s] to air pollution which mayreasonably be anticipated to endangerpublic health and welfare and thepresence of which in the ambient airresults from numerous or diverse mobileor stationary sources. CAA section108(a)(1)(A) and (B). For those airpollutants listed, section 108 requires

the Administrator to issue air qualitycriteria that accurately reflect the latestscientific knowledge useful inindicating the kind and extent of allidentifiable effects on public health orwelfare which may be expected from thepresence of [a] pollutant in ambient air* * * Section 108(a)(2).

Section 109(a) of the Act directs theAdministrator to promulgate primaryand secondary NAAQS for pollutantsfor which air quality criteria have beenissued. Section 109(b)(1) defines aprimary standard as one the attainmentand maintenance of which in thejudgment of the Administrator, based on[the air quality] criteria and allowing anadequate margin of safety, are requisiteto protect the public health. 1 Section109(b)(1). A secondary standard, in turn,must specify a level of air quality theattainment and maintenance of which,in the judgment of the Administrator,

based on [the air quality] criteria, isrequisite to protect the public welfarefrom any known or anticipated adverseeffects associated with the presence of

such pollutant in the ambient air. 2Section 109(b)(2) This rule concernsexclusively the primary NAAQS foroxides of sulfur.

The requirement that primarystandards include an adequate margin ofsafety is intended to addressuncertainties associated withinconclusive scientific and technical

information available at the time ofstandard setting. It is also intended toprovide a reasonable degree ofprotection against hazards that researchhas not yet identified. Lead IndustriesAssociation v. EPA, 647 F.2d 1130, 1154(DC Cir 1980), cert. denied, 449 U.S.1042 (1980); American PetroleumInstitute v. Costle, 665 F.2d 1176, 1186(DC Cir. 1981), cert. denied, 455 U.S.1034 (1982). Both kinds of uncertaintiesare components of the risk associatedwith pollution at levels below those atwhich human health effects can be saidto occur with reasonable scientific

certainty. Thus, in selecting primarystandards that include an adequatemargin of safety, the Administrator isseeking not only to prevent pollutionlevels that have been demonstrated to beharmful but also to prevent lowerpollutant levels that may pose anunacceptable risk of harm, even if therisk is not precisely identified as tonature or degree. The CAA does notrequire the Administrator to establish aprimary NAAQS at a zero-risk level orat background concentration levels, seeLead Industries Association v. EPA, 647F.2d at 1156 n. 51, but rather at a levelthat reduces risk sufficiently so as to

protect public health with an adequatemargin of safety.

In addressing the requirement for amargin of safety, EPA considers suchfactors as the nature and severity of thehealth effects involved, the size of theat-risk population(s), and the kind anddegree of the uncertainties that must beaddressed. The selection of anyparticular approach to providing anadequate margin of safety is a policychoice left specifically to theAdministrators judgment. LeadIndustries Association v. EPA, 647 F.2dat 116162.

In setting standards that arerequisite to protect public health andwelfare, as provided in section 109(b),EPAs task is to establish standards thatare neither more nor less stringent thannecessary for these purposes. In sodoing, EPA may not consider the costsof implementing the standards.Whitman v. American Trucking

http://www.epa.gov/ttn/naaqs/standards/no2so2sec/index.htmlhttp://www.epa.gov/ttn/naaqs/standards/no2so2sec/index.htmlhttp://www.epa.gov/ttn/naaqs/standards/no2so2sec/index.htmlhttp://www.epa.gov/ttn/naaqs/standards/no2so2sec/index.htmlhttp://www.epa.gov/ttn/naaqs/standards/no2so2sec/index.htmlhttp://www.epa.gov/ttn/naaqs/standards/no2so2sec/index.htmlhttp://www.epa.gov/ttn/naaqs/standards/no2so2sec/index.html


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Associations, 531 U.S. 457, 471, 47576(2001).

Section 109(d)(1) of the Act requiresthe Administrator to periodicallyundertake a thorough review of the airquality criteria published under section108 and the NAAQS and to revise thecriteria and standards as may beappropriate. The Act also requires theAdministrator to appoint anindependent scientific reviewcommittee composed of seven members,including at least one member of theNational Academy of Sciences, onephysician, and one person representingState air pollution control agencies, toreview the air quality criteria andNAAQS and to recommend to theAdministrator any new * * * standardsand revisions of existing criteria andstandards as may be appropriate undersection 108 and subsection (b) of thissection. CAA section 109(d)(2). Thisindependent review function is

performed by the Clean Air ScientificAdvisory Committee (CASAC) of EPAsScience Advisory Board.

C. Related SO2 Control Programs

States are primarily responsible forensuring attainment and maintenance ofambient air quality standards once EPAhas established them. Under section 110of the Act, and related provisions, Statesare to submit, for EPA approval, Stateimplementation plans (SIPs) thatprovide for the attainment andmaintenance of such standards throughcontrol programs directed to sources of

the pollutants involved. The States, inconjunction with EPA, also administerthe prevention of significantdeterioration program that covers thesepollutants. See CAA sections 160169.In addition, Federal programs providefor nationwide reductions in emissionsof these and other air pollutants throughthe Federal motor vehicle and motorvehicle fuel control program under titleII of the Act (CAA sections 202250)which involves controls for emissionsfrom all moving sources and controls forthe fuels used by these sources; newsource performance standards under

section 111; and title IV of the Act (CAAsections 402416), which specificallyprovides for major reductions in SO2emissions. EPA has also promulgatedthe Clean Air Interstate Rule (CAIR) torequire additional SO2 emissionreductions needed in the eastern half ofthe United States to address emissionswhich contribute significantly tononattainment with, or interfere withmaintenance of, the PM NAAQS bydownwind States in the CAIR region.This rule was remanded by the DCCircuit, and although it remains in

effect, EPA is reevaluating it pursuant tothe court remand.

Currently, there are several areasdesignated as being in nonattainment ofthe primary SO2 NAAQS (see sectionVI). Moreover, as a result of this finalrule, additional areas could be classifiedas non-attainment. Certain States wouldthen be required to develop SIPs that

identify and implement specific airpollution control measures to reduceambient SO2 concentrations to attainand maintain the revised SO2 NAAQS,most likely by requiring air pollutioncontrols on sources that emit oxides ofsulfur (SOx).

D. History of Reviews of the PrimaryNAAQS for Sulfur Oxides

On April 30, 1971, the EPApromulgated primary SO2 NAAQS (36FR 8187). These primary standards,which were based on the findingsoutlined in the original 1969 Air QualityCriteria for Sulfur Oxides, were set at0.14 parts per million (ppm) averagedover a 24-hour period, not to beexceeded more than once per year, and0.030 ppm annual arithmetic mean. In1982, EPA published the Air QualityCriteria for Particulate Matter and SulfurOxides (EPA, 1982) along with anaddendum of newly publishedcontrolled human exposure studies,which updated the scientific criteriaupon which the initial standards were

based (EPA, 1982). In 1986, EPApublished a second addendumpresenting newly available evidencefrom epidemiologic and controlled

human exposure studies (EPA, 1986). In1988, EPA published a proposeddecision not to revise the existingstandards (53 FR 14926) (April 26,1988). However, EPA specificallyrequested public comment on thealternative of revising the currentstandards and adding a new 1-hourprimary standard of 0.4 ppm (400 ppb)to protect asthmatics against 510minute peak SO2 concentrations.

As a result of public comments on the1988 proposal and other post-proposaldevelopments, EPA published a secondproposal on November 15, 1994 (59 FR

58958). The 1994 re-proposal was basedin part on a supplement to the secondaddendum of the criteria document,which evaluated new findings on 510minute SO2 exposures in asthmatics(EPA, 1994a; EPA, 1994b). As in the1988 proposal, EPA proposed to retainthe existing 24-hour and annualstandards. EPA also solicited commenton three regulatory alternatives tofurther reduce the health risk posed byexposure to high 5-minute peaks of SO2if additional protection were judged to

be necessary. The three alternatives

were: (1) Revising the existing primarySO2 NAAQS by adding a new 5-minutestandard of 0.6 ppm (600 ppb) SO2; (2)establishing a new regulatory programunder section 303 of the Act tosupplement protection provided by theexisting NAAQS, with a trigger level of0.6 ppm (600 ppb) SO2, one expectedexceedance; and (3) augmenting

implementation of existing standards byfocusing on those sources or sourcetypes likely to produce high 5-minutepeak concentrations of SO2.

On May 22, 1996, EPA announced itsfinal decision not to revise the NAAQSfor SOx (61 FR 25566). EPA found thatasthmaticsa susceptible populationgroupcould be exposed to short-termSO2bursts resulting in repeatedexposure events such that tens orhundreds of thousands of asthmaticscould be exposed annually to lungfunction effects distinctly exceeding* * * [the] typical daily variation in

lung function

that asthmatics routinelyexperience, and found further thatrepeated occurrences should beregarded as significant from a publichealth standpoint. 61 FR at 25572,25573. Nonetheless, the agencyconcluded that the likelihood thatasthmatic individuals will be exposed* * * is very low when viewed from anational perspective, that 5-minutepeak SO[2] levels do not pose a broadpublic health problem when viewedfrom a national perspective, and thatshort-term peak concentrations of SO[2]do not constitute the type of ubiquitouspublic health problem for which

establishing a NAAQS would beappropriate. Id. at 25575. EPAconcluded, therefore, that it would notrevise the existing standards or add astandard to specifically address 5-minute exposures. EPA also announcedan intention to propose guidance, undersection 303 of the Act, to assist Statesin responding to short-term peaks ofSO2 and later initiated a rulemaking todo so (62 FR 210 (Jan. 2, 1997).

The American Lung Association andthe Environmental Defense Fundchallenged EPAs decision not toestablish a 5-minute standard. On

January 30, 1998, the Court of Appealsfor the District of Columbia Circuitfound that EPA had failed to adequatelyexplain its determination that norevision to the SO2 NAAQS wasappropriate and remanded thedetermination back to EPA for furtherexplanation. American Lung Assn v.EPA, 134 F. 3d 388 (DC Cir. 1998).Specifically, the court held that EPAhad failed to adequately explain the

basis for its conclusion that short-termSO2 exposures to asthmatics do notconstitute a public health problem,



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noting that the agency had failed toexplain the link between its finding thatrepeated short-term exposures weresignificant, and that there would be tensto hundreds of thousands of suchexposures annually to a susceptiblesubpopulation. 134 F. 3d at 392. Thecourt also rejected the explanation thatshort-term SO2bursts were localized,

infrequent, and site-specific as arational basis for the conclusion that nopublic health problem existed forpurposes of section 109: [N]othing inthe Final Decision explains whylocalized, site-specific, or eveninfrequent events might neverthelesscreate a public health problem,particularly since, in some sense, allpollution is local and site-specific* * *. Id. The court accordinglyremanded the case to EPA to adequatelyexplain its determination or otherwisetake action in accordance with theopinion. In response, EPA has collected

and analyzed additional air quality datafocused on 5-minute concentrations ofSO2. These air quality analysesconducted since the last review helpedinform the current review, which(among other things) address the issuesraised in the courts remand of theAgencys last decision.

EPA formally initiated the currentreview of the air quality criteria foroxides of sulfur and the SO2 primaryNAAQS on May 15, 2006 (71 FR 28023)with a general call for information.EPAs draft Integrated Review Plan forthe Primary National Ambient AirQuality Standards for Sulfur Dioxide

(EPA, 2007a) was made available inApril 2007 for public comment and wasdiscussed by the CASAC via a publiclyaccessible teleconference on May 11,2007. As noted in that plan, SOXincludes multiple gaseous (e.g., SO3)and particulate (e.g., sulfate) species.Because the health effects associatedwith particulate species of SOX have

been considered within the context ofthe health effects of ambient particles inthe Agencys review of the NAAQS forparticulate matter (PM), the currentreview of the primary SO2 NAAQS isfocused on the gaseous species of SOX

and does not consider health effectsdirectly associated with particulatespecies.

The first draft of the IntegratedScience Assessment for Oxides ofSulfur-Health Criteria (ISA) and theSulfur Dioxide Health Assessment Plan:Scope and Methods for Exposure andRisk Assessment (EPA, 2007b) werereviewed by CASAC at a public meetingheld on December 56, 2007. Based oncomments received from CASAC andfrom the public, EPA developed thesecond draft of the ISA and the first

draft of the Risk and ExposureAssessment to Support the Review ofthe SO2 Primary National Ambient AirQuality Standard (Risk and ExposureAssessment (REA)). These documentswere reviewed by CASAC at a publicmeeting held on July 3031, 2008. Basedon comments received from CASAC andthe public at this meeting, EPA released

the final ISA in September of 2008(EPA, 2008a; henceforth referred to asISA). In addition, comments receivedwere considered in developing thesecond draft of the REA. Importantly,the second draft of the REA containeda draft staff policy assessment thatconsidered the evidence presented inthe final ISA and the air quality,exposure, and risk characterizationresults presented in the second draftREA, as they related to the adequacy ofthe current SO2 NAAQS and potentialalternative primary SO2 standards. Thisdocument was reviewed by CASAC at a

public meeting held on April 1617,2009. In preparing the final REA report,which included the final staff policyassessment, EPA considered commentsreceived from CASAC and the public atand subsequent to that meeting. Thefinal REA containing the final staffpolicy assessment was completed inAugust 2009 (EPA 2009a; henceforthreferred to as REA)).

On December 8, 2009 EPA publishedits proposed revisions to the primarySO2 NAAQS. 74 FR 64810 presented anumber of conclusions, findings, anddeterminations proposed by theAdministrator. EPA invited general,specific, and/or technical comments onall issues involved with this proposal,including all such proposed judgments,conclusions, findings, anddeterminations. EPA invited specificcomment on the level, or range of levels,appropriate for such a standard, as wellas on the rationale that would supportthat level or range of levels. Thesecomments were carefully considered bythe Administrator as she made her finaldecisions, as described in this notice, onthe primary SO2 NAAQS

The schedule for completion of thisreview is governed by a judicial orderresolving a lawsuit filed in September2005, concerning the timing of thecurrent review. Center for BiologicDiversity v.Johnson (Civ. No. 051814)(D.D.C. 2007). The order that nowgoverns this review, entered by thecourt in August 2007 and amended inDecember 2008, provides that theAdministrator will sign, for publication,a final rulemaking concerning thereview of the primary SO2 NAAQS nolater than June 2, 2010.

E. Summary of Proposed Revisions tothe SO2 Primary NAAQS

For the reasons discussed in thepreamble of the proposal for the SO2primary NAAQS, EPA proposed to makerevisions to the primary SO2 NAAQS(and to add SO2 data handlingconventions) so the standards provide

requisite protection of public healthwith an adequate margin of safety.Specifically, EPA proposed to replacethe current 24-hour and annualstandards with a new short-term SO2standard. EPA proposed that this newshort-term standard would be based onthe 3-year average of the 99th percentile(or 4th highest) of the yearlydistribution of 1-hour daily maximumSO2 concentrations. EPA proposed to setthe level of this new 1-hour standardwithin the range of 50 to 100 ppb andsolicited comment on standard levels ashigh as 150 ppb. EPA also proposed toestablish requirements for an SO2

monitoring network at locations wheremaximum SO2 concentrations areexpected to occur and to add a newFederal Reference Method (FRM) formeasuring SO2 in the ambient air.Finally, EPA proposed to makecorresponding changes to the AirQuality Index for SO2.

F. Organization and Approach to FinalSO2 Primary NAAQS Decisions

This action presents theAdministrators final decisionsregarding the need to revise the currentSO2 primary NAAQS, and what those

revisions should be. Revisions to theprimary NAAQS for SO2, and therationale supporting those revisions, aredescribed below in section II.

An overview of the approach formonitoring and implementation ispresented in section III. Requirementsfor the SO2 ambient monitoring networkand for a new, additional FRM formeasuring SO2 in the ambient air aredescribed in section IV. EPAs currentplans for designations and forimplementing the revised SO2 primaryNAAQS are discussed in sections V andVI respectively. Related requirements

for data completeness, data handling,data reporting, rounding conventions,and exceptional events are described insection VII. Communication of publichealth information through the AQI isdiscussed in section VIII. A recitation ofstatutory authority and a discussion ofthose executive order reviews which arerelevant are provided in section IX.

Todays final decisions are based ona thorough review in the ISA ofscientific information on known andpotential human health effectsassociated with exposure to SO2 in the



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3A small number of sites, 98 total from 1997 to2007 of the approximately 500 SO2 monitors, andnot the same sites in all years, voluntarily reported5-minute block average data to AQS (ISA, section2.5.2). Of these, 16 reported all twelve 5-minuteaverages in each hour for at least part of the timebetween 1997 and 2007. The remainder reportedonly the maximum 5-minute average in each hour.

air. These final decisions also take intoaccount: (1) Assessments in the REA ofthe most policy-relevant information inthe ISA as well as quantitative exposureand risk analyses based on thatinformation; (2) CASAC Panel adviceand recommendations, as reflected in itsletters to the Administrator and itspublic discussions of the ISA and REA;

(3) public comments received during thedevelopment of the ISA and REA; and(4) public comments received on EPAsnotice of proposed rulemaking.

II. Rationale for Decisions on thePrimary Standards

This section presents the rationale forthe Administrators decision to revisethe existing SO2 primary standards byreplacing the current 24-hour andannual standards with a new 1-hour SO2standard at a level of 75 ppb, based onthe 3-year average of the annual 99thpercentile of 1-hour daily maximumconcentrations. As discussed more fully

below, this rationale takes into account:(1) Judgments and conclusionspresented in the ISA and the REA; (2)CASAC advice and recommendations asreflected in the CASAC panelsdiscussions of drafts of the ISA and REAat public meetings, in separate writtencomments, and in letters to theAdministrator (Henderson 2008a;Henderson 2008b; Samet, 2009); (3)public comments received at CASACmeetings during the development of theISA and the REA; and (4) publiccomments received on the notice ofproposed rulemaking.

In reaching this decision, EPA hasdrawn upon an integrative synthesis ofthe entire body of evidence on humanhealth effects associated with thepresence of SO2 in the ambient air, andupon the results of the quantitativeexposure and risk assessments reflectingthis evidence. As discussed below, this

body of evidence addresses a broadrange of health endpoints associatedwith exposure to SO2 in the ambient air.In considering this entire body ofevidence, EPA chose to focus most onthose health endpoints for which theISA found the strongest evidence of an

association with SO2 (see section II.Bbelow). Thus, the rationale for this finaldecision on the SO2 NAAQS focusedprimarily on respiratory morbidityfollowing short-term (5-minutes to 24-hours) exposure to SO2, for which theISA found a causal relationship.

As discussed below, a substantialamount of new research has beenconducted since EPAs last review of theSO2 NAAQS, with important newinformation coming from epidemiologicstudies in particular. In addition to thesubstantial amount of new

epidemiologic research, the ISAconsidered a limited number of newcontrolled human exposure studies andre-evaluated key older controlledhuman exposure studies. In evaluating

both the new and key older controlledhuman exposure studies, the ISAutilized updated guidelines published

by the American Thoracic Society (ATS)

on what constitutes an adverse effect ofair pollution (see ISA, section 3.1.3; p.34). Importantly, all controlled humanexposure and epidemiologic studiesevaluated in the ISA have undergoneintensive scrutiny through multiplelayers of peer review and opportunitiesfor public review and comment. Thus,the review of this information has beenextensive and deliberate.

After a background discussion of theprincipal emitting sources and currentpatterns of SO2 air quality and adescription of the current SO2monitoring network from which those

air quality patterns are obtained (sectionII.A), the remainder of this sectiondiscusses the Administrators rationalefor her final decisions on the primarystandards. Section II.B includes anoverview of the scientific evidencerelated to the respiratory effectsassociated with ambient SO2 exposure.This overview includes a discussion ofthe at-risk populations considered in theISA. Section II.C summarizes the keyapproaches taken by EPA to assessexposures and health risks associatedwith exposure to ambient SO2. SectionII.D summarizes the approach that was

used in the current review of the SO2

NAAQS with regard to consideration ofthe scientific evidence and the airquality, exposure, and risk-based resultsrelated to the adequacy of the currentstandards and potential alternativestandards. Sections II.E and II.F discuss,respectively, the Administratorsdecisions regarding the adequacy of thecurrent standards and the elements of anew short-term standard, taking intoconsideration public comments on theproposed decisions. Section II.Gsummarizes the Administratorsdecisions with regard to the SO2primary NAAQS.

A. Characterization of SO2 Air Quality

1. Anthropogenic Sources and CurrentPatterns of SO2 Air Quality

Anthropogenic SO2 emissionsoriginate chiefly from point sources,with fossil fuel combustion at electricutilities (66%) and other industrialfacilities (29%) accounting for themajority of total emissions (ISA, section2.1). Other anthropogenic sources ofSO2 include both the extraction of metalfrom ore as well as the burning of high

sulfur-containing fuels by locomotives,large ships, and equipment utilizingdiesel engines. SO2 emissions andambient concentrations follow a strongeast to west gradient due to the largenumbers of coal-fired electric generatingunits in the Ohio River Valley andupper Southeast regions. In the 12Consolidated Metropolitan StatisticalAreas (CMSAs) that had at least fourSO2 regulatory monitors from 20032005, 24-hour average concentrations inthe continental U.S. ranged from areported low of 1 ppb in Riverside, CAand San Francisco, CA to a high of 12ppb in Pittsburgh, PA and Steubenville,OH (ISA, section 2.5.1). In addition,outside or inside all CMSAs from 20032005, the annual average SO2concentration was 4 ppb (ISA, Table 28). However, spikes in hourlyconcentrations occurred. The mean 1-hour maximum concentration outside orinside CMSAs was 13 ppb, with amaximum value of greater than 600 ppboutside CMSAs and greater than 700ppb inside CMSAs (ISA, Table 28).

Temporal and spatial patterns of 5-minute peaks of SO2 are also importantgiven that controlled human exposurestudies have demonstrated thatexposure to these peaks can result inadverse respiratory effects in exercisingasthmatics (see section II.B below). Forthose monitors which voluntarilyreported 5-minute block average data,3when maximum 5-minuteconcentrations were reported, the

absolute highest concentration over theten-year period exceeded 4000 ppb, butfor all individual monitors, the 99thpercentile was below 200 ppb (ISA,section 2.5.2 Table 210). Medianconcentrations from these monitorsreporting 5-minute data ranged from1 ppb to 8 ppb, and the average for eachmaximum 5-minute level ranged from3 ppb to 17 ppb. Delaware,Pennsylvania, Louisiana, and WestVirginia had mean values for maximum5-minute data exceeding 10 ppb. Amongaggregated within-State data for the 16monitors from which all 5-minute

average intervals were reported, themedian values ranged from 1 ppb to 5ppb, and the means ranged from 3 ppbto 11 ppb (ISA, section 2.5.2 at 243).The highest reported concentration was921 ppb, but the 99th percentile values



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4A causal relationship is based on [e]vidence[that] is sufficient to conclude that there is a causalrelationship between relevant pollutant exposuresand the health outcome. That is, a positiveassociation has been observed between thepollutant and the outcome in studies in whichchance, bias, and confounding could be ruled outwith reasonable confidence. Evidence includes, forexample, controlled human exposure studies; orobservational studies that cannot be explain byplausible alternatives or are supported by otherlines of evidence (e.g. animal studies or mechanismof action information). Evidence includes replicated

and consistent high-quality studies by multipleinvestigators. ISA Table 12, at 111.

for aggregated within-State data were allbelow 90 ppb (id).

2. SO2 Monitoring

Although EPA established the SO2standards in 1971, uniform minimummonitoring network requirements forSO2 monitoring were only adopted inMay 1979. From the time of the

implementation of the 1979 monitoringrule through 2008, the SO2 monitoringnetwork has steadily decreased in sizefrom approximately 1496 sites in 1980to the approximately 488 sites operatingin 2008. At present, except for SO2monitoring required at National CoreMonitoring Stations (NCore stations),there are no minimum monitoringrequirements for SO2 in 40 CFR part 58Appendix D, other than a requirementfor EPA Regional Administratorapproval before removing any existingmonitors and a requirement that anyongoing SO2 monitoring must have at

least one monitor sited to measure themaximum concentration of SO2 in thatarea. EPA removed the specificminimum monitoring requirements forSO2 in the 2006 monitoring rulerevisions, except for monitoring atNCore stations, based on the fact thatthere were no SO2 nonattainment areasat that time, coupled with trendsshowing an increasing gap betweennational average SO2 concentrations andthe current 24-hour and annualstandards. The rule was also intended toprovide State, local, and Tribal airmonitoring agencies flexibility inmeeting perceived higher prioritymonitoring needs for other pollutants,or to implement the new multi-pollutantsites (NCore network) required by the2006 rule revisions (71 FR 61236,(October 6, 2006)). More information onSO2 monitoring can be found in sectionIV.

B. Health Effects Information

The ISA concluded that there wassufficient evidence to infer a causalrelationshipbetween respiratorymorbidity and short-term (5-minutes to24-hours) exposure to SO2 (ISA, section5.2). Importantly, we note that a causal

relationship is the strongest finding theISA can make.4 This conclusion was

based on the consistency, coherence,and plausibility of findings observed incontrolled human exposure studies of510 minutes, epidemiologic studiesmostly using 1-hour daily maximumand 24-hour average SO2concentrations, and animal toxicologicalstudies using exposures of minutes tohours (ISA, section 5.2). This evidence

is briefly summarized below anddiscussed in more detail in the proposal(see sections II.B.1 to II.B.5, see 74 FRat 64815821). We also note that the ISAjudged evidence of an association

between SO2 exposure and other healthcategories to be less convincing; otherassociations were judged to besuggestive but not sufficient to infer acausal relationship(i.e., short-term exposure to SO2 andmortality) or inadequate to infer thepresence or absence of a causalrelationship (i.e., short-term exposure toSO2 and cardiovascular morbidity, and

long-term exposure to SO2 andrespiratory morbidity, other morbidity,and mortality). Key conclusions fromthe ISA are described in greater detail inTable 53 of the ISA.

1. Short-Term (5-minute to 24-hour) SO2Exposure and Respiratory MorbidityEffects

The ISA examined numerouscontrolled human exposure studies andfound that moderate or greaterdecrements in lung function (i.e., 15%decline in Forced Expiratory Volume(FEV1) and/or 100% increase inspecific airway resistance (sRaw)) occur

in some exercising asthmatics exposedto SO2 concentrations as low as200300 ppb for 510 minutes. The ISAalso found that among asthmatics, boththe percentage of individuals affected,and the severity of the responseincreased with increasing SO2concentrations. That is, at 510 minuteconcentrations ranging from 200300ppb, the lowest levels tested in free

breathing chamber studies,approximately 530% percent ofexercising asthmatics experiencedmoderate or greater decrements in lungfunction (ISA, Table 31). At

concentrations of 400600 ppb,moderate or greater decrements in lungfunction occurred in approximately 2060% of exercising asthmatics, andcompared to exposures at 200300 ppb,a larger percentage of asthmaticsexperienced severe decrements in lungfunction (i.e., 20% decrease in FEV1and/or 200% increase in sRaw; ISA,Table 31). Moreover, at SO2concentrations 400 ppb (510 minute

exposures), moderate or greaterdecrements in lung function were oftenstatistically significant at the groupmean level and frequently accompanied

by respiratory symptoms. Id.The ISA also found that in locations

meeting the current SO2 NAAQS,numerous epidemiologic studiesreported positive associations between

ambient SO2 concentrations andrespiratory symptoms in children, aswell as emergency department visitsand hospitalizations for all respiratorycauses and asthma across multiple agegroups. Moreover, the ISA concludedthat these epidemiologic studies wereconsistent and coherent. This evidencewas consistent in that associations werereported in studies conducted innumerous locations and with a varietyof methodological approaches (ISA,section 5.2; p. 55). It was coherent inthat respiratory symptom results fromepidemiologic studies of short-term

(predominantly 1-hour daily maximumor 24-hour average) SO2 concentrationswere generally in agreement withrespiratory symptom results fromcontrolled human exposure studies of510 minutes. These results were alsocoherent in that the respiratory effectsobserved in controlled human exposurestudies of 510 minutes furtherprovided a basis for a progression ofrespiratory morbidity that could lead tothe increased emergency departmentvisits and hospital admissions observedin epidemiologic studies (ISA, section5.2; p. 55). In addition, the ISA foundthat when evaluated as a whole, SO2

effect estimates in multi-pollutantmodels generally remained positive andrelatively unchanged when co-pollutants were included. Therefore,although recognizing the uncertaintiesassociated with separating the effects ofSO2 from those of co-occurringpollutants, the ISA concluded that thelimited available evidence indicates thatthe effect of SO2 on respiratory healthoutcomes appears to be generally robustand independent of the effects ofgaseous co-pollutants, including NO2and O3, as well as particulate co-pollutants, particularly PM2.5

(ISA, section 5.3; p. 59).The ISA also found that therespiratory effects of SO2 wereconsistent with the mode of action as itis currently understood from animaltoxicological and controlled humanexposure studies (ISA, section 5.2; p. 52). The immediate effect of SO2 on therespiratory system is

bronchoconstriction. This response ismediated by chemosensitive receptorsin the tracheobronchial tree. Activationof these receptors triggers centralnervous system reflexes that result in



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6This aspect of susceptibility is referred to asvulnerability in the proposal and in the ISA.

7The ISA cites one chamber study withintermittent exercise where healthy and asthmatic

Continued

likelihood of an adverse outcome givena specific exposure in comparison withthe general population (American LungAssociation, 2001). The susceptibility ofan individual to SO2 can encompass amultitude of factors which representnormal developmental phases or lifestages (e.g., age) or biologic attributes(e.g., gender); however, other factors

(e.g., socioeconomic status (SES)) mayinfluence the manifestation of diseaseand also increase an individualssusceptibility (American LungAssociation, 2001). In addition,populations may be at increased risk toSO2 due to an increase in their exposureduring certain life stages (e.g.,childhood or old age) or as a result ofexternal factors (e.g., SES) thatcontribute to an individual beingdisproportionately exposed to higherconcentrations than the generalpopulation.6 It should be noted that insome cases specific populations may be

affected by multiple susceptibilityfactors. For example, a population thatis characterized as having low SES mayhave less access to healthcare resultingin the manifestation of a disease, whichincreases their susceptibility to SO2,while they may also reside in a locationthat results in disproportionately highexposure to SO2.

To examine whether SO2differentially affects certainpopulations, stratified analyses are oftenconducted in epidemiologicinvestigations to identify the presenceor absence of effect modification. Athorough evaluation of potential effect

modifiers may help identify susceptiblepopulations that are at increased risk toSO2 exposure. These analyses are basedon the proper identification ofconfounders and subsequent adjustmentfor them in statistical models, whichhelps separate a spurious from a truecausal association. Although the designof toxicological and human clinicalstudies does not allow for an extensiveexamination of effect modifiers, the useof animal models of disease and thestudy of individuals with underlyingdisease or genetic polymorphisms doallow for comparisons between

subgroups. Therefore, the results fromthese studies, combined with thoseresults obtained through stratifiedanalyses in epidemiologic studies,contribute to the overall weight ofevidence for the increased susceptibilityof specific populations to SO2. Thosepopulations identified in the ISA to bepotentially at greater risk ofexperiencing an adverse health effectfrom SO2 were described in detail in the

proposal (section II.B.5) and include: (1)Those with pre-existing respiratorydisease; (2) children and older adults;(3) persons who spend increased timeoutdoors or at elevated ventilation rates;(4) persons with lower SES; and (5)persons with certain genetic factors.

As discussed in the proposal (sectionII.B.5.g, 74 FR at 64821), large

proportions of the U.S. population arelikely to be at increased risk ofexperiencing SO2-related health effects.In the United States, approximately 7%of adults and 9% of children have beendiagnosed with asthma. Notably, theprevalence and severity of asthma ishigher among certain ethnic or racialgroups such as Puerto Ricans, AmericanIndians, Alaskan Natives, and AfricanAmericans (EPA 2008b). Furthermore, ahigher prevalence of asthma amongpersons of lower SES and an excess

burden of asthma hospitalizations andmortality in minority and inner-city

communities have been observed (EPA,2008b). In addition, population groupsbased on age comprise substantialsegments of individuals that may bepotentially at risk for SO2-related healthimpacts. Based on U.S. census data from2000, about 72.3 million (26%) of theU.S. population are under 18 years ofage, 18.3 million (7.4%) are under 5years of age, and 35 million (12%) are65 years of age or older. There is alsoconcern for the large segment of thepopulation that is potentially at risk toSO2-related health effects because ofincreased time spent outdoors atelevated ventilation rates (those who

work or play outdoors). Overall, theconsiderable size of the populationgroups at risk indicates that exposure toambient SO2 could have a significantimpact on public health in the UnitedStates.

C. Human Exposure and Health RiskCharacterization

To put judgments about SO2-associated health effects into a broaderpublic health context, EPA has drawnupon the results of the quantitativeexposure and risk assessments.

Judgments reflecting the nature of the

evidence and the overall weight of theevidence are taken into consideration inthese quantitative exposure and riskassessments. These assessments includeestimates of the likelihood thatasthmatic children at moderate orgreater exertion (e.g. while exercising)in St. Louis or Greene County, Missouriwould experience SO2 exposures ofpotential concern. In addition, theseanalyses include an estimate of thenumber and percent of exposedasthmatic children in these locationslikely to experience SO2-induced lung

function responses (i.e., moderate orgreater decrements in lung functiondefined in terms of sRaw or FEV1) undervarying air quality scenarios (i.e.,current air quality and air qualitysimulated to just meet the current orpotential alternative standards). Theseassessments also characterize the kindand degree of uncertainties inherent in

such estimates.As previously mentioned, the ISA

concluded that the evidence for anassociation between respiratorymorbidity and short-term SO2 exposurewas sufficient to infer a causalrelationship (ISA, section 5.2) and thatthe definitive evidence for thisconclusion was from the results of 510minute controlled human exposurestudies demonstrating decrements inlung function and/or respiratorysymptoms in exercising asthmatics (ISA,section 5.2). Accordingly, the air qualityand exposure analyses and their

associated risk characterizations focusedon 5-minute concentrations of SO2 inexcess of potential health effect

benchmark values derived from thecontrolled human exposure literature(see proposal section II.C.1, 74 FR at64821, and REA, section 6.2). These

benchmark levels are not potentialstandards, but rather are SO2 exposureconcentrations which representexposures of potential concern whichare used in these analyses to estimatepotential exposures and risks associatedwith 5-minute concentrations of SO2.The REA considered 5-minute

benchmark levels of 100, 200, 300, and400 ppb in these analyses, butespecially noted exceedances orexposures with respect to the 200 and400 ppb 5-minute benchmark levels.These benchmark levels werehighlighted because (1) 400 ppbrepresents the lowest concentration infree-breathing controlled humanexposure studies where moderate orgreater lung function decrementsoccurred which were often statisticallysignificant at the group mean level andwere frequently accompanied byrespiratory symptoms; and (2) 200 ppbis the lowest level at which moderate or

greater decrements in lung function infree-breathing controlled humanexposure studies were found in someindividuals, although these lungfunction changes were not statisticallysignificant at the group mean level.Notably, 200 ppb is also the lowest levelthat has been tested in free-breathingcontrolled human exposure studies(REA, section 4.2.2).7



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children were exposed to 100 ppb SO2 in a mixturewith ozone and sulfuric acid. The ISA notes thatcompared to exposure to filtered air, exposure tothe pollutant mix did not result in statisticallysignificant changes in lung function or respiratorysymptoms (ISA, section 3.1.3.4).

8Benchmark values derived from the controlledhuman exposure literature were associated with a5-minute averaging time. However, as noted in

footnote 3 above, only 98 ambient monitors locatedin 13 States from 19972007 reported measured 5-minute SO2 concentrations since such monitoring isnot required (see section II.A.2 and section IV). Incontrast, 809 monitors in 48 States, DC, Puerto Rico,and the Virgin Islands reported 1-hour SO2concentrations over a similar time period.Therefore, to broaden analyses to areas wheremeasured 5-minute SO2 concentrations were notavailable, the REA utilized a statistical relationshipto estimate the highest 5-minute level in an hour,given a reported 1-hour average SO2 concentration(REA, section 6.4). Then, similar to measured 5-minute SO2 concentrations, statistically estimated5-minute SO2 concentrations were compared to 5-minute potential health effect benchmark values(REA, chapters 7 and 8, respectively).

9EPA recently conducted a complete qualityassurance review of all individual subject data. Theresults of this review did not substantively changeany of the entries in ISA, Table 31, and did notin any way affect the conclusions of the ISA (seeJohns and Simmons, 2009).

The REA utilized three approaches tocharacterize health risks. In the firstapproach, for each air quality scenario,statistically estimated 5-minute SO2concentrations 8 and measured ambient5-minute SO2 concentrations werecompared to the 5-minute potentialhealth effect benchmark levelsdiscussed above (REA, chapter 7). This

air quality analysis included allavailable ambient monitoring data aswell as a more detailed analysis in 40counties. The air quality analysis wasconsidered a broad characterization ofnational air quality and humanexposures that might be associated withthese 5-minute SO2 concentrations. Anadvantage of the air quality analysis isits relative simplicity; however, there isuncertainty associated with theassumption that SO2 air quality canserve as an adequate surrogate for totalexposure to ambient SO2. Actualexposures might be influenced by

factors not considered by this approach,including small-scale spatial variabilityin ambient SO2 concentrations (whichmight not be represented by the currentfixed-site ambient monitoring network)and spatial/temporal variability inhuman activity patterns. A moredetailed overview of the air qualityanalysis and its associated limitationsand uncertainties is provided in theproposal (see sections II.C.2, 74 FR at64822 and II.C.3, 74 FR at 64823,respectively) and the air quality analysisis thoroughly described in the REA(chapter 7).

In the second approach, an inhalation

exposure model was used to generatemore realistic estimates of personalexposures in asthmatics (REA, chapter8). This analysis estimated temporallyand spatially variablemicroenvironmental 5-minute SO2concentrations and simulated

asthmatics contact with these pollutantconcentrations while at moderate orgreater exertion (i.e., while at elevatedventilation rates). The approach wasdesigned to estimate exposures that arenot necessarily represented by theexisting ambient monitoring data and to

better represent the physiologicalconditions corresponding with the

respiratory effects reported in controlledhuman exposure studies. AERMOD, anEPA dispersion model, was used toestimate 1-hour ambient SO2concentrations using emissionsestimates from stationary, non-point,and where applicable, port sources. TheAir Pollutants Exposure (APEX) model,an EPA human exposure model, wasthen used to estimate populationexposures using the estimated hourlycensus block level SO2 concentrations.From the 1-hour census blockconcentrations, 5-minute maximum SO2concentrations within each hour were

estimated by APEX (REA, section 8.7.1)using the statistical relationshipmentioned above in footnote 8.Estimated exposures to 5-minute SO2levels were then compared to the 5-minute potential health effect

benchmark levels discussed above. Thisapproach to assessing exposures wasmore resource intensive than usingambient levels as an indicator ofexposure; therefore, the final REAincluded the analysis of two locations:St. Louis and Greene County, MO.Although the geographic scope of thisanalysis was limited, the approachprovided estimates of SO2 exposures in

asthmatics and asthmatic children in St.Louis and Greene Counties, and thusserved to complement the broader airquality characterization. A moredetailed overview of this exposureanalysis and its associated limitationsand uncertainties is provided in theproposal (see sections II.C.2, 74 FR at64822 and II.C.3, 74 FR at 64823,respectively) and the exposure analysisis thoroughly described in the REA(chapter 8).

The third approach was a quantitativerisk assessment. This approachcombined results from the exposure

analysis (i.e., the number of exposedtotal asthmatics or asthmatic childrenwhile at moderate or greater exertion)with exposure-response functionsderived from individual level data fromcontrolled human exposure studies (seeISA, Table 31 and Johns (2009) 9) toestimate the percentage and number of

exposed asthmatics and asthmaticchildren in St. Louis and Greene Countylikely to experience a moderate orgreater lung function response (i.e.,decrements in lung function defined interms of FEV1 and sRaw) under the airquality scenarios mentioned above(REA, chapter 9). A more detailedoverview of this analysis and its

associated limitations and uncertaintiesis provided in the proposal (see sectionsII.C.2, 74 FR at 64822 and II.C.3, 74 FRat 64823, respectively) and thequantitative risk analysis is thoroughlydescribed in the REA (chapter 9).

Notably, for the reasons described inthe REA (REA, section 10.3.3) and theproposal (see section II.E.1.b, 74 FR at64827), when considering the St. Louisand Greene County exposure and riskresults as they relate to the adequacy ofthe current standards, the REAconcluded that the St. Louis resultswere more informative in terms of

ascertaining the extent to which thecurrent standards protect against healtheffects linked to the various benchmarks(linked in turn to 5-minute SO2exposures). The results in fact suggestedthat the current standards may notadequately protect public health (REA,section 10.3.3, p. 364). Moreover, theREA judged that the exposure and riskestimates for the St. Louis study areaprovided useful insights into exposuresand risks for other urban areas in theU.S. with similar population and SO2emissions densities (id.). For similarreasons, the St. Louis results were moreinformative for ascertaining the

adequacy of the potential alternativestandards under consideration.

Key results of the air quality,exposure, and risk analyses werepresented in the policy assessmentchapter of the REA (chapter 10) andsummarized in the proposal (see Tables24 in the preamble to the proposedrule). In considering these results, theproposal noted that these analysessupport that 5-minute SO2 exposures,reasonably judged important from apublic health perspective, wereassociated with air quality adjustedupward to simulate just meeting the

current standards (see proposal, sectionII.E.1.c, 74 FR at 64828). Moreover,these results indicated that 99thpercentile 1-hour daily maximumstandard levels in the range of 50100ppb could substantially limit exposuresof asthmatic children at moderate orgreater exertion from 5-minute SO2concentrations 400 ppb, andappreciably limit exposures of thesechildren from 5-minute SO2concentrations 200 ppb (REA, p. 392393). Results of these analyses alsoindicated that a 1-hour standard at 150



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ppb could still substantially limitexposures of asthmatic children atmoderate or greater exertion from 5-minute SO2 concentrations 400 ppb,

but would provide these childrenappreciably less protection fromexposure to 5-minute SO2concentrations 200 ppb (REA, p. 395396).

D. Approach for Determining WhetherTo Retain or Revise the CurrentStandards

EPA notes that the final decision onretaining or revising the current primarySO2 standards is a public health policyjudgment to be made by theAdministrator. This judgment has beeninformed by a recognition that theavailable health effects evidence reflectsa continuum consisting of ambientlevels of SO2 at which scientistsgenerally agree that health effects arelikely to occur, through lower levels atwhich the likelihood and magnitude ofthe response become increasinglyuncertain. The Administrators finaldecisions draw upon scientificinformation and analyses related tohealth effects, population exposures andrisks; judgments about the appropriateresponse to the range of uncertaintiesthat are inherent in the scientificevidence and analyses; and commentsreceived from CASAC and the public.

To evaluate whether the currentprimary SO2 standards are adequate orwhether revisions are appropriate, EPAhas used an approach in this reviewdescribed in chapter 10 of the REA

which builds upon the approaches usedin reviews of other criteria pollutants,including the most recent reviews of theNO2, Pb, O3, and PM NAAQS (EPA,2008c; EPA, 2007c; EPA, 2007d; EPA,2005), and reflects the latest body ofevidence and information that iscurrently available, as reflected by theISA. As in other recent reviews, EPAconsidered the implications of placingmore or less weight or emphasis ondifferent aspects of the scientificevidence and the exposure-/risk-basedinformation, recognizing that the weightto be given to various elements of the

evidence and exposure/risk informationis part of the public health policyjudgments that the Administrator willmake in reaching decisions on thestandard.

A series of general questions framedthis approach to considering thescientific evidence and exposure-/risk-

based information. First, EPAsconsideration of the scientific evidenceand exposure/risk information withregard to the adequacy of the currentstandards has been framed by thefollowing questions:

To what extent does evidence that hasbecome available since the last reviewreinforce or call into question evidence forSO2-associated effects that were identified inthe last review? To what extent has evidence for different

health effects and/or susceptible populationsbecome available since the last review?

To what extent have uncertaintiesidentified in the last review been reduced

and/or have new uncertainties emerged? To what extent does evidence and

exposure-/risk-based information that hasbecome available since the last reviewreinforce or call into question any of the

basic elements (indicator, averaging time,form, and level) of the current standard?

To the extent that the availableevidence and exposure-/risk-basedinformation suggests it may beappropriate to consider revision of thecurrent standards, EPA considers thatevidence and information with regard toits support for consideration of astandard that is either more or lessstringent than the current standards.

This evaluation is framed by thefollowing questions:

Is there evidence that associations,especially causal or likely causalassociations, extend to ambient SO2concentrations as low as, or lower than, theconcentrations that have previously beenassociated with health effects? If so, what arethe important uncertainties associated withthat evidence? Are exposures above benchmark levels

and/or health risks estimated to occur inareas that meet the current standard? If so,are the estimated exposures and health risksimportant from a public health perspective?What are the important uncertainties

associated with the estimated risks?To the extent that there is support for

consideration of a revised standard, EPAthen considers the specific elements ofthe standard (indicator, averaging time,form, and level) within the context ofthe currently available information. Inso doing, the Agency addresses thefollowing questions regarding theelements of the standard:

Does the evidence provide support forconsidering a different indicator for gaseousSOX? Does the evidence provide support for

considering different, or additional averagingtimes? What ranges of levels and forms of

alternative standards are supported by theevidence, and what are the associateduncertainties and limitations? To what extent do specific averaging

times, levels, and forms of alternativestandards reduce the estimated exposuresabove benchmark levels and risks attributableto exposure to ambient SO2, and what are theuncertainties associated with the estimatedexposure and risk reductions?

The questions outlined above havebeen addressed in the REA. Thefollowing sections present

considerations regarding the adequacyof the current standards andconclusions on the elements of a newshort-term standard in terms ofindicator, averaging time, form, andlevel.

E. Adequacy of the Current Standards

This section discusses considerations

related to the decision as to whether thecurrent 24-hour and annual SO2 primaryNAAQS are requisite to protect publichealth with an adequate margin ofsafety. Specifically, section II.E.1provides an overview of the rationalesupporting the Administrators proposalthat the current standards do notprovide adequate public healthprotection; section II.E.2 discussespublic comments received on theadequacy of the current standards; andsection II.E.3 discusses theAdministrators final decision onwhether the current SO2 primary

NAAQS is requisite to protect publichealth with an adequate margin ofsafety, as required by sections 109(d)and (b) of the Act.

1. Rationale for Proposed Decision

In the proposal, the Administratorinitially concluded that the current 24-hour and annual SO2 NAAQS were notadequate to protect public health withan adequate margin of safety (seesection II.E.4, 74 FR at 64829). Inreaching this conclusion, sheconsidered the: (1) Scientific evidenceand conclusions in the ISA; (2) exposureand risk information presented in the

REA; (3) conclusions of the policyassessment chapter of the REA; and (4)views expressed by CASAC. Theseconsiderations are discussed in detail inthe proposal (see section II.E., 74 FR at64826) and are summarized in thissection.

In the proposal the Administratornoted the following in considering theadequacy of the current 24-hour andannual primary SO2 standards: The conclusion of the ISA that the

results of controlled human exposureand epidemiologic studies form aplausible and coherent data set that

supports a causal relationship betweenshort-term (5-minutes to 24-hours) SO2exposures and adverse respiratoryeffects, and that the epidemiologicevidence (buttressed by the clinicalevidence) indicates that the effects seenin the epidemiologic studies areattributable to exposure to SO2 (ISA,section 5.2). The conclusion of the ISA that [i]n

the epidemiologic studies, respiratoryeffects were observed in areas where themaximum ambient 24-h avg SO2concentration was below the current 24-



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h avg NAAQS level * * *. (ISA,section 5.2, p. 52.) and so would occurat ambient SO2 concentrations that arepresent in locations meeting the current24-hour NAAQS. These respiratory effects also

occurred in areas with annual airquality levels considerably lower thanthose allowed by the current annual

standard, indicating that the currentannual standard is also not providingprotection against short-term healtheffects reported in epidemiologicstudies (ISA, section 5.2). Analyses in the REA supporting

that 5-minute exposures, reasonablyjudged important from a public healthperspective (i.e., respiratory effectsjudged to be adverse to the health ofasthmatics, see sections II.B.1.c above,and II.E.2.b below), were associatedwith air quality adjusted upward tosimulate just meeting the current 24-hour and annual standards. CASAC advice that the current

24-hour and annual standards are notadequate to protect public health,especially in relation to short termexposures to SO2 (510 minutes) byexercising asthmatics (Samet, 2009,p. 15).

Based on these considerations(discussed in more detail in theproposal, see sections II.E.1 and II.E.2),the Administrator proposed that thecurrent 24-hour and annual SO2standards are not requisite to protectpublic health with an adequate marginof safety against adverse respiratoryeffects associated with short-term

(5-minute to 24-hour) SO2 exposures. Inconsidering approaches to revising thecurrent standards, the Administratorinitially concluded it appropriate toconsider setting a new 1-hour standard.The Administrator noted that a 1-hourstandard would likely provide increasedpublic health protection, especially formembers of at-risk groups, from therespiratory effects described in bothepidemiologic and controlled humanexposure studies.

2. Comments on the Adequacy of theCurrent Standards

This section discusses publiccomments on the proposal that eithersupported or opposed theAdministrators proposed decision torevise the current SO2 primary NAAQS.Comments on the adequacy of thecurrent standards that focused on thescientific and/or the exposure/risk basisfor the Administrators proposedconclusions are discussed in sectionsII.E.2.aII.E.2.c. Comments on theepidemiologic evidence are consideredin section II.E.2.a. Comments on thecontrolled human exposure evidence

are considered in section II.E.2.b.Comments on human exposure andhealth risk assessments are consideredin section II.E.2.c. To the extent thesecomments on the evidence andinformation are also used to justifycommenters conclusions on decisionsrelated to indicator, averaging time,form, or level, they are noted as well in

the appropriate sections below (II.F.1II.F.4, respectively). The summaries ofcomments, and responses thereto,presented below are not exclusive: othercomments and responses are beingincluded in the Response to Comment(RTC) Document which is part of therecord for this rulemaking (EPA, 2010).

Many public commenters agreed withthe proposal that based on the availableinformation, the current SO2 standardsare not requisite to protect public healthwith an adequate margin of safety andthat revisions to the standards aretherefore appropriate. Among those

calling for revisions to the standardswere environmental groups (e.g., SierraClub, WEACT for Environmental

Justice, Center for Biological Diversity,(CBD) Environmental Defense Fund(EDF), Natural Resources DefenseCouncil (NRDC)); medical/public healthorganizations (e.g., American LungAssociation (ALA), American ThoracicSociety (ATS)); State environmentalorganizations (e.g., National Associationof Clean Air Agencies (NACAA),Northeast States for Coordinated AirUse Management (NESCAUM); Stateenvironmental agencies (e.g., suchagencies in DE, IA, IL, MI, NY, NM, OH,

PA, TX, VT); the Fond du Lac Band ofLake Superior Chippewa (Fond du Lac)Tribe, local groups (e.g., Houston-Galveston Area Council, AlexandriaDepartment of Transportation andEnvironmental Services) and mostindividual commenters (13,000). Thesecommenters generally concluded thatthe current SO2 standards need to berevised and that a more stringentstandard is needed to protect the healthof susceptible population groups. Insupporting the need to adopt a morestringent NAAQS for SO2, thesecommenters often referenced the

conclusions of CASAC, as well asevidence and information presented inthe proposal. As such, the rationaleoffered by these commenters wasconsistent with that presented in theproposal to support the Administratorsproposed decision to revise the currentSO2 NAAQS.

Most industry commenters (e.g.,Utility Air Regulatory Group (UARG),American Petroleum Institute (API),Arizona Public Service, NationalPetrochemical & Refiners Association(NPRA), Montana-Dakota Utilities Co.,

Dominion Resources, Council ofIndustrial Boiler Owners (CIBO), EdisonElectric Institute (EEI), Duke Energy,National Mining Association (NMA));and some organizations (e.g., TexasAssociation of Business, The AnnapolisCenter for Science-Based Public Policy(ACSBPP), South Carolina Chamber ofCommerce) opposed the proposed

revisions to the SO2 primary NAAQS. Insupporting their views, industrycommenters generally concluded thatEPA did not appropriately consideruncertainties associated with theepidemiologic and controlled humanexposure evidence.

More specifically, with respect to theepidemiologic studies, many of thesecommenters concluded that results ofthese studies are confounded by co-pollutants and thus too uncertain todetermine whether SO2 is trulyassociated with the health outcomes

being measured (e.g., hospital

admissions; Federal Register seebelow). With respect to the controlledhuman exposure studies, manycommenters were critical of the 5-minute benchmark levels that werederived from these studies andsubsequently used by EPA in the airquality, exposure, and risk analyses.These groups were particularlyconcerned about the Administratorsreliance on the 200 ppb 5-minute

benchmark level in assessing theadequacy of the current and potentialalternative standards. In general, manyindustry groups maintained that adverserespiratory effects did not occur

following 510 minute SO2 exposures< 400 ppb (e.g., API, EEI, CIBO) andsome groups stated that even at SO2concentrations 400 ppb, reportedeffects may not be of clinical concern,and thus are likely not adverse (e.g.,UARG). Many industry groups (e.g.,API, UARG) also disagreed with EPAs(and CASACs) conclusions that severeasthmatics were not included in thesecontrolled human exposure studies, andthat severe asthmatics would likely havea more pronounced response to SO2exposures at a given level, or wouldrespond to even lower levels of SO2.

In responding to these specificcomments, we note that theAdministrator relied in the proposal onthe evidence, information, andjudgments contained in the ISA and theREA (including the policy assessmentchapter), as well as on the advice ofCASAC. In considering the evidence,information, and judgments of the ISAand the REA, the Agency notes thatthese documents have been reviewedand discussed extensively by CASAC atmultiple public meetings (see above,section I.D) and in their letters to the



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10As noted in the proposal (see sections II.D.1, 74FR at 6482464825 and II.F.4.a, 74 FR at 64835),there is special sensitivity in this review indisentangling SO2-related effects from PM-relatedeffects (especially sulfate PM).

EPA Administrator. Thus, it isimportant to note that CASAC generallyaccepted the key findings andconclusions presented in both the ISAand REA (see Henderson 2008a,Henderson 2008b, and Samet, 2009).

a. Comments on EPAs Interpretation ofthe Epidemiologic Evidence

Many industry groups (e.g., API,UARG, American Chemistry Council(ACC), Dominion Resources,ExxonMobil, Progress Energy, CIBO,The Fertilizer Institute, EEI, DowChemical Company (Dow),MeadWestvaco Corporation (MWV),(NMA) and some organizations (e.g.,ACSBPP) commented that, given thepresence of numerous co-pollutants inthe air, the epidemiologic studies do notsupport the contention that SO2 itself iscausing health effects. For example,UARG stated: The epidemiologicalevidence cannot determine that SO2 isa cause of or a contributor to hospitaladmissions (HA), emergencydepartment (ED) visits or respiratorysymptoms, the effects cited in theProposed Rule.

Although EPA has recognized thatmultiple factors can contribute to theetiology of respiratory disease and thatmore than one air pollutant couldindependently impact respiratoryhealth, we continue to judge, asdiscussed in the ISA, that the availableevidence supports the conclusion thatthere is an independent effect of SO2 onrespiratory morbidity. In reaching thisjudgment, we recognize that a major

methodological issue affecting SO2epidemiologic studies concerns the

evaluation of the extent to which otherair pollutants, particular PM2.5,10 mayconfound or modify SO2-related effectestimates. The use of multi-pollutantregression models is a commonapproach for evaluating potentialconfounding by co-pollutants inepidemiologic studies. It is thereforeimportant to note that when the ISAevaluated U.S. and internationalepidemiologic studies employing multi-pollutant models, SO2 effect estimatesgenerally remained positive and

relatively unchanged when co-pollutants, including PM, were included(see ISA, p. 55). Therefore, althoughrecognizing the uncertainties associatedwith separating the effects of SO2 fromthose of co-occurring pollutants, the ISAconcluded that the limited availableevidence indicates that the effect of SO2on respiratory health outcomes appears

to be generally robust and independentof the effects of gaseous co-pollutants,including NO2 and O3, as well asparticulate co-pollutants, particularlyPM2.5 (ISA, section 5.2; p. 59).

In considering questions ofconfounding and causation, theepidemiologic studies should not beconsidered in a vacuum. As emphasized

by the ISA, and endorsed by CASAC,controlled human exposure studiesprovide support for the plausibility ofthe associations reported inepidemiologic studies (ISA, section 55;Henderson 2008a; Henderson 2008b).These controlled human exposurestudies exposed exercising asthmatics to510 minute peaks of SO2 and reporteddecrements in lung function and/orrespiratory symptoms in up to 60% ofthese individuals (depending onexposure concentration; see ISA, Table53; p. 511). Thus, these experimentalstudy results provide strong support for

an independent contribution of SO2 tothe respiratory health effects reported inepidemiologic studies: The effects ofSO2 on respiratory symptoms, lungfunction, and airway inflammationobserved in the human clinical studiesusing peak exposures further provides a

basis for a progression of respiratorymorbidity resulting in increasedemergency department visits andhospital admissions. Collectively, thesefindings provide biological plausibilityfor the observed association betweenambient SO2 levels and emergencydepartment visits and hospitalizationsfor all respiratory diseases and asthma,

notably in children and older adults.* * * (ISA, section 5.2 at p. 55).Thus, EPA is not relying solely on theepidemiologic studies to evaluatewhether associations reported in thesestudies (e.g., associations withemergency department visits) are likelythe result of ambient SO2 exposure.

b. Comments on EPAs Interpretation ofthe Controlled Human ExposureEvidence

Many industry groups (e.g., API, ACC,Progress Energy, EEI, CIBO) commentedthat adverse health effects do not occur

following 510 minute SO2 exposures< 400 ppb. In addition, some groups(e.g., UARG) commented that adverserespiratory effects do not occur inexercising asthmatics following SO2exposures below 600 ppb. Thedisagreement is not whether effectsoccur in exercising asthmatics at theseexposure levels and exposure durations.Rather, the issue is whether the effectsexperienced can properly be regarded asadverse. In general, these groupsconclude that EPAs judgment ofadverse health effects at SO2 exposure

levels below 600 or 400 ppb isinappropriately based on an unsoundinterpretation of ATS guidelines. Morespecifically, these groups generallycontend that decrements in lungfunction without accompanyingrespiratory symptoms are not adverseeffects of SO2 exposure, and thatdecrements in lung function in a

percentage of exercising asthmatics doesnot represent a shift in lung function atthe population level. Some of thesegroups also contend that EPA followedthe advice of individual CASACmembers, rather than consensus CASACwritten comments on the ISA and REAwhen concluding respiratory effectsassociated with SO2 exposures below600 or 400 ppb are adverse.Furthermore, some groups contend thateffects below 400 ppb should not beconsidered adverse because comparedto the number of asthmaticsexperiencing decrements in lung

function, there were similar numbers ofasthmatics experiencing increases inlung function. EPA disagrees with thesecomments, and believes that the clinicalevidence also supports the conclusionthat the current standards are notrequisite to protect public health withand adequate margin of safety.

The Agency disagrees that adverserespiratory effects do not occur inexercising asthmatics following 510minute SO2 exposures ranging from400600 ppb. As previously mentioned,at SO2 concentrations ranging from 400600 ppb, moderate or greaterdecrements in lung function occur in

approximately 2060% of exercisingasthmatics (again, defined in terms of a 15% decline in FEV1 or 100% increasein sRaw; ISA, Table 31). Moreover, atconcentrations 400 ppb, decrements inlung function are often statisticallysignificant at the group mean level, andare frequently accompanied byrespiratory symptoms (ISA, Table 51).ATS guidelines on what constitutes anadverse health effect of air pollutionclearly state that reversible loss of lungfunction in combination with thepresence of symptoms should beconsidered adverse (ATS 1985, 2000).

Moderate or greater decrements in lungfunction accompanied by respiratorysymptoms fit this description. Thus, theAgencys conclusion of adverse healtheffects associated with SO2concentrations 400 ppb is consistentwith ATS guidelines.

The Agency also disagrees withindustry commenters regarding theadversity of the respiratory effects seenin exercising asthmatics following 510minute SO2 exposures ranging from200300 ppb. As mentioned above(section II.B.1), and discussed more



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11See hearing transcripts from EPA Clean AirScientific Advisory Committee (CASAC), July 3031 2008, Sulfur Oxides-Health Criteria (part 3 of 4)pages 211213). These transcripts can be found inDocket ID No. EPAHQORD20060260. Availableat http://www.regulations.gov.

fully in the proposal (see section II.B.3,74 FR at 64819), the ISA reported thatexposure to SO2 concentrations as lowas 200300 ppb for 510 minutes resultsin approximately 530% of exercisingasthmatics experiencing moderate orgreater decrements in lung function. In2000, the ATS updated its guidelines onwhat constitutes an adverse health

effect of air pollution. These guidelinesindicated that exposure to air pollutionthat increases the risk of an adverseeffect to the entire population isadverse, even though it may notincrease the risk of any individual to anunacceptable level (ATS 2000). Forexample, ATS notes that a population ofasthmatics could have a distribution oflung function such that no individualhas a level associated with significantimpairment. Exposure to air pollutioncould shift the distribution to lowerlevels that still do not bring anyindividual to a level that is associated

with clinically relevant effects.However, this would be consideredadverse because individuals within thepopulation would have diminishedreserve function, and therefore would beat increased risk if affected by anotheragent (ATS 2000).

Considering the 2000 ATS guidelines,the results of the clinical studiesconducted at 200300 ppb werereasonably interpreted by EPA toindicate an SO2-induced shift in theselung function measurements for a subsetof this population. That is, anappreciable percentage of this

population of exercising asthmaticswould be expected to experiencemoderate or greater decrements in lungfunction in response to SO2concentrations as low as 200 ppb, andthus would be expected to havediminished reserve lung function. As aresult, this sub-population would be atgreater risk of a more severe response ifaffected by another respiratory agent(e.g., viral infection, or O3).

EPA is also mindful of CASACcomments on this issue following thesecond draft ISA. The second draft ISAplaced relatively little weight on healtheffects associated with SO2 exposures at

200300 ppb. CASAC strongly disagreedwith this characterization of the healthevidence. Their consensus letterfollowing the second draft ISA states:

Our major concern is the conclusions inthe ISA regarding the weight of the evidencefor health effects for short-term exposure tolow levels of SO2. Although the ISA presentsevidence from both clinical andepidemiological studies that indicate healtheffects occur at 0.2 ppm or lower, the finalchapter emphasizes health effects at 0.4 ppmand above * * * CASAC believes the clinicaland epidemiological evidence warrants

stronger conclusions in the ISA regarding theavailable evidence of health effects at 0.2ppm or lower concentrations of SO2. Theselection of a lower bound concentration forhealth effects is very important because theISA sets the stage for EPAs risk assessmentdecisions. In its draft Risk and ExposureAssessment (REA) to Support the Review ofthe SO2 Primary National Ambient AirQuality Standards (July 2008), EPA chose a

range of 0.4 ppm0.6 ppm SO2concentrations for its benchmark analysis. As

CASAC will emphasize in a forthcomingletter on the REA, we recommend that alower bound be set at least as low as 0.2 ppm.(Henderson 2008a)

EPA also notes the similar CASACcomments on the first draft of the REA.The consensus CASAC letter followingthe 1st draft REA states:

The CASAC believes strongly that theweight of clinical and epidemiology evidenceindicates there are detectable clinicallyrelevant health effects in sensitivesubpopulations down to a level at least aslow as 0.2 ppm SO2. These sensitive

subpopulations represent a substantialsegment of the at-risk population.(Henderson 2008b; p. 1)

See Coalition of Battery RecyclersAssociation v. EPA, No. 091011 (DCCir., May 14, 2010), slip opinion at 9,holding that it was reasonable for EPAto con

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