Document ID: s3://data.kl3m.ai/documents/govinfo/USCOURTS/USCOURTS-caDC-13-01118/USCOURTS-caDC-13-01118-0/pdf.json

Parties Involved:
American Chemistry Council
Petitioner
Environmental Protection Agency
Respondent

Document Text:

United States Court of Appeals 

FOR THE DISTRICT OF COLUMBIA CIRCUIT

Argued December 3, 2015 Decided July 29, 2016 

No. 11-1108 

UNITED STATES SUGAR CORPORATION, 

PETITIONER

v. 

ENVIRONMENTAL PROTECTION AGENCY, 

RESPONDENT

AMERICAN CHEMISTRY COUNCIL, ET AL., 

INTERVENORS

Consolidated with 11-1124, 11-1134, 11-1142, 11-1145, 

11-1159, 11-1165, 11-1172, 11-1174, 11-1181, 13-1086, 

13-1087, 13-1091, 13-1092, 13-1096, 13-1097, 13-1098, 

13-1099, 13-1100, 13-1103 

On Petitions for Review of Final Action of the 

 United States Environmental Protection Agency 

William L. Wehrum Jr., David M. Friedland, and 

Douglas A. McWilliams argued the causes for Industry 

Petitioners. With them on the briefs were Allen A. Kacenjar, 

Katy M. Franz, Lisa Marie Jaeger, Sandra Y. Snyder, Peter 

USCA Case #13-1118 Document #1627694 Filed: 07/29/2016 Page 1 of 162
H. Wyckoff, Claudia M. O=Brien, Stacey L. VanBelleghem, Eli 

Hopson, Jane C. Luxton, Lauren E. Freeman, Elizabeth L. 

Horner, William F. Lane, Alan H. McConnell, Timothy S. 

Bishop, Kevin G. Desharnais, Chad M. Clamage, Ronald A. 

Shipley, Quentin Riegel, Linda E. Kelly, and Jeffrey A. 

Knight. Rachel Brand, Leslie A. Hulse, Harry M. Ng, Scott J. 

Stone, John P. Wagner, and Lee B. Zeugin entered 

appearances.

James S. Pew and Sanjay Narayan were on the briefs for 

Environmental Petitioners. Neil Gormley entered an 

appearance. 

Perry M. Rosen and Norman L. Rave, Jr., Attorneys, U.S. 

Department of Justice, argued the causes for respondent. 

With them on the brief was John C. Cruden, Assistant 

Attorney General, and Norman L. Rave, Jr., Attorney. Sam 

Hirsch and Madeline P. Fleisher, Attorneys, entered 

appearances. 

James S. Pew and Neil Gormley argued the causes for 

Environmental Respondent-Intervenors. With them on the 

briefs was Sanjay Narayan.

William L. Wehrum, Quentin Riegel, Linda E. Kelly, 

Patrick Forrest, Douglas A. McWilliams, Peter H. Wyckoff, 

Jeffrey A. Knight, Claudia M. O=Brien, Stacey L. 

VanBelleghem, Lisa Marie Jaeger, Sandra Y. Snyder, David 

M. Friedland, William F. Lane, Alan H. McConnell, Ronald 

A. Shipley, Carol F. McCabe, Suzanne Ilene Schiller, Michael 

Dillon, Charles Howland Knauss, Shannon S. Broome, 

Timothy S. Bishop, Kevin G. Desharnais, Chad M. Clamage, 

Lauren E. Freeman, Elizabeth L. Horner, Larry B. Alexander, 

and Leslie A. Hulse were on the brief for Industry IntervenorRespondents. Allen A. Kacenjar Jr., Rachel L. Brand, Harry 

M. Ng, Scott J. Stone, John P. Wagner, and Lee B. Zeugin

entered appearances. 

USCA Case #13-1118 Document #1627694 Filed: 07/29/2016 Page 2 of 162
No. 11-1125 

AMERICAN FOREST & PAPER ASSOCIATION, ET AL., 

PETITIONERS

v. 

ENVIRONMENTAL PROTECTION AGENCY, 

RESPONDENT

AMERICAN CHEMISTRY COUNCIL, ET AL., 

INTERVENORS

Consolidated with 11-1140, 11-1144, 11-1154, 11-1155, 

11-1161, 11-1171, 11-1173, 11-1180, 11-1183, 11-1188, 

13-1111, 13-1113, 13-1114, 13-1116, 13-1118, 13-1119, 

13-1120, 13-1121, 13-1123, 13-1124, 13-1127 

On Petitions for Review of a Final Action of the 

United States Environmental Protection Agency 

William L. Wehrum, Douglas A. McWilliams, and Jason 

T. Morgan argued the causes for Industry Petitioners. On the 

briefs were Richard G. Stoll, Leslie A. Hulse, Lisa Marie 

Jaeger, Sandra Y. Snyder, Peter H. Wyckoff, Jeffrey A. 

Knight, David M. Friedland, Jessalee Landfried, Michael B. 

Wigmore, Ronald A. Shipley, Chet M. Thompson, Linda E. 

Kelly, Quentin Riegel, William F. Lane, Alan H. McConnell, 

Carol F. McCabe, Suzanne Ilene Schiller, and Michael 

Dillon. David Y. Chung, Rachel L. Brand, Julia L. German,

Jeffrey W. Leppo, and Jane C. Luxton entered appearances.

USCA Case #13-1118 Document #1627694 Filed: 07/29/2016 Page 3 of 162
Neil Gormley and James S. Pew argued the causes and 

filed the briefs for Environmental Petitioners. 

Perry M. Rosen and Norman L. Rave, Jr., Attorneys, U.S. 

Department of Justice, argued the causes for respondent. 

With them on the brief was John C. Cruden, Assistant 

Attorney General. Madeline P. Fleisher, Attorney, entered an 

appearance. 

James S. Pew and Neil Gormley argued the causes and 

filed the briefs for Environmental Respondent-Intervenors. 

David M. Friedland and William L. Wehrum argued the 

causes for Industry Intervenor-Respondents. With them on 

the briefs were Jessalee Landfried, Leslie A. Hulse, Richard 

G. Stoll, Ronald A. Shipley, William F. Lane, Alan H. 

McConnell, James T. Morgan, Lisa Marie Jaeger, Sandra Y. 

Snyder, Jeffrey A. Knight, Shannon S. Broome, Carol 

McCabe, Suzanne Ilene Schiller, Michael Dillon, Linda E. 

Kelly, Quentin Riegel, and Charles H. Knauss. Scott J. Stone, 

Lori A. Rubin, and Jeffrey W. Leppo entered appearances. 

 

USCA Case #13-1118 Document #1627694 Filed: 07/29/2016 Page 4 of 162
No. 11-1141 

AMERICAN CHEMISTRY COUNCIL, 

PETITIONER

v. 

ENVIRONMENTAL PROTECTION AGENCY, 

RESPONDENT

AMERICAN FOREST & PAPER ASSOCIATION, ET AL., 

INTERVENORS

Consolidated with 11-1182, 11-1207, 11-1208, 13-1105, 

13-1107 

On Petitions for Review of a Final Action of the 

United States Environmental Protection Agency 

William L. Wehrum and David M. Friedland argued the 

causes for Industry Petitioners. On the briefs were Lisa Marie 

Jaeger, Sandra Y. Snyder, Jeffrey A. Knight, Quentin Riegel, 

and Leslie A. Hulse. Harry M. Ng, Scott J. Stone, and John P. 

Wagner entered appearances. 

Neil Gormley argued the cause for Environmental 

Petitioners. With him on the briefs was James S. Pew. 

Perry M. Rosen and Norman L. Rave, Jr., Attorneys, U.S. 

Department of Justice, argued the causes for respondent. 

With them on the brief was John C. Cruden, Assistant 

Attorney General. Madeline P. Fleisher, Attorney, entered an 

appearance. 

USCA Case #13-1118 Document #1627694 Filed: 07/29/2016 Page 5 of 162
David M. Friedland and William L. Wehrum argued the 

causes for Industry Intervenor-Respondents. With them on 

the briefs were Lisa Marie Jaeger, Sandra Y. Snyder, Jeffrey 

A. Knight, William F. Pedersen, Pamela A. Lacey, William F. 

Lane, Linda E. Kelly, Quentin Riegel, James W. Conrad, Jr., 

and Leslie A. Hulse. Harry M. Ng, Scott J. Stone, John P. 

Wagner, and Nidhi J. Thakar entered appearances. 

James S. Pew and Neil Gormley were on the brief for 

Environmental Respondent-Intervenors. 

USCA Case #13-1118 Document #1627694 Filed: 07/29/2016 Page 6 of 162
Before: HENDERSON, BROWN, and GRIFFITH, Circuit 

Judges. 

PER CURIAM: In these consolidated petitions for review, 

we address approximately thirty challenges to three 

regulations promulgated by the United States Environmental 

Protection Agency (EPA or Agency): (1) the “Major Boilers 

Rule,”1

 (2) the “Area Boilers Rule,”2 and (3) the 

“Commercial/Industrial Solid Waste Incinerators (CISWI) 

Rule.”3

 Collectively, these rules—all promulgated under the 

Clean Air Act (CAA or Act), 42 U.S.C. §§ 7401 et seq.—set 

emissions limits on certain combustion machinery known to 

release hazardous air pollutants (HAPs). Roughly one-half of 

 1

 National Emission Standards for Hazardous Air Pollutants 

for Major Sources: Industrial, Commercial, and Institutional Boilers 

and Process Heaters (2011 Major Boilers Rule), 76 Fed. Reg. 

15,608 (Mar. 21, 2011), as amended, National Emission Standards 

for Hazardous Air Pollutants for Major Sources: Industrial, 

Commercial, and Institutional Boilers and Process Heaters (2013 

Major Boilers Rule), 78 Fed. Reg. 7,138 (Jan. 31, 2013). 

2

 National Emission Standards for Hazardous Air Pollutants 

for Area Sources: Industrial, Commercial, and Institutional Boilers 

(2011 Area Boilers Rule), 76 Fed. Reg. 15,554 (Mar. 21, 2011), as 

amended, National Emission Standards for Hazardous Air 

Pollutants for Area Sources: Industrial, Commercial, and 

Institutional Boilers (2013 Area Boilers Rule), 78 Fed. Reg. 7,488 

(Feb. 1, 2013). 

3

 Standards of Performance for New Stationary Sources and 

Emission Guidelines for Existing Sources: Commercial and 

Industrial Solid Waste Incinerator Units (2011 CISWI Rule), 76 

Fed. Reg. 15,704 (Mar. 21, 2011), as amended, Commercial and 

Industrial Solid Waste Incineration Units: Reconsideration and 

Final Amendments; Non-Hazardous Secondary Materials that Are 

Solid Waste (2013 CISWI Rule), 78 Fed. Reg. 9,112 (Feb. 7, 

2013). 

USCA Case #13-1118 Document #1627694 Filed: 07/29/2016 Page 7 of 162
2 

the challenges are advanced by a group of municipal-electric 

organizations, industrial-trade associations, oil-and-gas 

industry representatives, and other entities that own and 

operate boilers, process heaters, and incinerators (Industry 

Petitioners). The other one-half are pressed by organizations 

interested in safeguarding the environment (Environmental 

Petitioners). 

I. BACKGROUND 

The three rules at issue address a common phenomenon: 

when combustion occurs, emissions result. The emissions 

include numerous materials, some of which pose risks to the 

environment in general and to human health in particular. 

Because combustion is an inevitable occurrence in the 

machinery that helps to power modern society, the Congress 

has authorized the EPA to provide for a regulatory framework 

that minimizes the deleterious effects of the incineration 

industry while simultaneously allowing it to operate. 

In 2013, the EPA finalized its efforts to do so for discrete 

types of combustion machinery: boilers, process heaters, and 

incinerators. Two of the three rules at issue—the Major 

Boilers Rule and the Area Boilers Rule—govern boilers and 

process heaters. The former are enclosed devices that use a 

controlled flame to heat water and convert it into steam or hot 

water. 40 C.F.R. § 63.11237. The latter are also enclosed 

devices that use a controlled flame but, instead of generating 

steam, they indirectly heat a “process material,” whether 

liquid, gas, or solid, or a “heat transfer material” like glycol or 

a mixture of glycol and water. Id. For simplicity, our use of 

“boilers” covers both machinery types. 

The two boiler-specific rules further divide the machinery 

into three categories: industrial, commercial, and 

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3 

institutional. See 2011 Area Boilers Rule, 76 Fed. Reg. at 

15,557. Industrial boilers are used for manufacturing, 

processing, mining, refining, and other similar operations. 

See id. Commercial boilers are used by shopping malls, 

laundromats, apartment complexes, restaurants, and hotels. 

See id. And institutional boilers include those used by, e.g., 

medical centers, schools, churches, prisons, and courthouses. 

See id. Collectively, over 200,000 boilers at over 100,000 

separate facilities must comply with the standards set out in 

the Major Boilers Rule or the Area Boilers Rule. 

The third rule that we address—the CISWI Rule—

governs combustion machinery known as “solid waste 

incineration unit[s].” 42 U.S.C. § 7429. The Act defines an 

incinerator as a “distinct operating unit of any facility” that 

burns solid waste from either commercial establishments, 

industrial establishments, or the general public. Id. 

§ 7429(g)(1). An incinerator subjects “waste material” to 

“high temperatures until it is reduced to ash.” Incinerator, 

NEW OXFORD AMERICAN DICTIONARY 853 (2d ed. 2005). 

Incinerators fall into different subcategories and, in the past, 

the EPA has issued rules governing many of them, including, 

e.g., municipal solid-waste incinerators, medical-waste 

incinerators, and sewage-sludge incinerators.4

 At issue in the 

CISWI Rule are incinerators located in commercial or 

industrial facilities that combust solid waste as defined in the 

Resource Conservation Recovery Act (RCRA), 42 U.S.C. 

§§ 6901 et seq. See 2011 CISWI Rule, 76 Fed. Reg. at 

15,706. 

 4

 See, e.g., 40 C.F.R. pt. 60, subpts. Cd, Ce, Eb, AAAA, 

BBBB, EEEE, FFFF, LLLL, MMMM. 

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4 

A. THE CLEAN AIR ACT, 42 U.S.C. §§ 7401 ET SEQ.

Enacted “to protect and enhance the quality of the 

Nation’s air resources so as to promote the public health and 

welfare and the productive capacity of its population,” 42 

U.S.C. § 7401(b)(1), the Act has been amended several times 

since the Congress first attempted to control air pollution via 

legislation in 1963. In 1970, the Congress required the EPA 

to identify and publish a list of HAPs, which the CAA defined 

as substances that increase “mortality,” “serious irreversible” 

illness, or “incapacitating reversible” illness. Clean Air 

Amendments of 1970, Pub. L. No. 91-604, § 4(a), 84 Stat. 

1676, 1685 (1970). The EPA had to set emission limits for 

every HAP based on the risk it posed to human health. See 

Sierra Club v. EPA (Sierra Club I), 353 F.3d 976, 979 (D.C. 

Cir. 2004). In other words, the EPA was to “consider[] levels 

of HAPs at which health effects are observed, factor[] in an 

ample margin of safety to protect the public health, and set 

emission restrictions accordingly.” Id. (quotation marks 

omitted). 

The risk-focused approach to capping HAP emissions left 

something to be desired. “In light of unrealistic time frames 

and scientific uncertain[t]y over which substances posed a 

threat to public health,” the EPA “only listed eight pollutants 

as hazardous between 1970 and 1990,” Nat. Res. Def. Council 

v. EPA (NRDC II), 529 F.3d 1077, 1079 (D.C. Cir. 2008), and 

set “emission standards for [only] seven of them,” Sierra Club 

I, 353 F.3d at 979; see also S. REP. NO. 101-228, at 3 (1989) 

(“Very little has been done since the passage of the 1970 Act 

to identify and control hazardous air pollutants.”). After 

twenty years of the risk-based approach, the Congress went 

back to the drawing board and, via the 1990 CAA 

Amendments, Pub. L. No. 101-549, 104 Stat. 2399 (1990), 

USCA Case #13-1118 Document #1627694 Filed: 07/29/2016 Page 10 of 162
5 

established the technology-based approach that governs 

today. See Sierra Club I, 353 F.3d at 979. 

1. 42 U.S.C. § 7412—“Hazardous Air Pollutants” 

The 1990 CAA Amendments overhauled the Act’s 

“Hazardous Air Pollutants” provision, codified at 42 U.S.C. 

§ 7412. Although earlier iterations of the Act had assigned 

HAPs-identification responsibility to the EPA, the slow pace 

at which the EPA discharged its duty prompted the Congress 

to create a list of pollutants itself.5

 See Sierra Club I, 353 

F.3d at 979-80 (citing 42 U.S.C. § 7412(b)). After identifying 

nearly two hundred HAPs that warranted emissions 

restrictions, see 42 U.S.C. § 7412(b)(1), the Congress directed 

the EPA, first, to identify the sources of each HAP, see id. 

§ 7412(c). The Agency then was to set emissions limits for 

each source that result in HAPs reduction to the greatest 

extent achievable by current technology. See generally Nat’l 

Ass’n for Surface Finishing v. EPA, 795 F.3d 1, 4 (D.C. Cir. 

2015) (citing 42 U.S.C. § 7412(b)(1), (c), (d)). 

a. Identifying and Categorizing HAP Sources 

The EPA’s first task is to create HAP-source categories 

and subcategories. See 42 U.S.C. § 7412(c). The Act 

distinguishes “major” from “area” sources, defining the 

former as “any stationary source or group of stationary 

sources” that neighbor each other, share common control, and 

emit (or have the potential to emit) either ten tons per year or 

more of any single HAP or twenty-five tons per year or more 

 5

 The EPA must keep the HAPs list current. See 42 U.S.C. 

§ 7412(b)(2), (3). 

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6 

of any HAP combination.6 Id. § 7412(a)(1). The latter are 

sources that do not emit enough HAPs to qualify as “major.” 

Id. § 7412(a)(2). Although the EPA must set stringent 

restrictions on major sources, it has discretion to set more 

lenient emissions caps on area sources. See id. § 7412(d)(5). 

Apart from the statutory distinction between major and 

area sources, the EPA has discretion to differentiate “among 

classes, types, and sizes of sources within a category or 

subcategory.” Id. § 7412(d)(1). Once the EPA finalizes 

HAPs-source categories and subcategories, the CAA 

mandates that it draw one final dividing line—between “new” 

sources and “existing” sources. See id. § 7412(d)(3). “New” 

sources are those “on which construction begins after EPA 

publishes emission standards,” Cement Kiln Recycling Coal.

v. EPA, 255 F.3d 855, 858 (D.C. Cir. 2001); most of the 

others are “existing” sources, see 42 U.S.C. § 7412(a)(10). 

But if an existing source experiences either a physical change 

or a change in operation method and the change increases 

HAP emissions by more than a de minimis amount, the Act 

mandates that the source meet the standards set for new 

sources. See id. § 7412(a)(5), (g). 

b. Setting Emission Standards for Major Sources—the 

“MACT” Standard 

After the EPA identifies HAP-source categories and 

subcategories, it then sets emissions limits for each. See id. 

§ 7412(d)(2). “[W]henever . . . feasible,” the caps must use 

numeric HAPs limits. Id. § 7412(h)(4). The size of the 

 6

 The CAA defines “stationary source” as “any building, 

structure, facility, or installation which emits or may emit any air 

pollutant.” 42 U.S.C. § 7411(a)(3). 

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7 

source—either “major” or “area”—dictates whether the EPA 

must set the numeric limit at the most stringent level that 

current technology allows or at the level set by “generally 

available control technologies.” Id. § 7412(d)(5). For major 

sources, the CAA directs the EPA to establish emissions caps 

that result in the “the maximum degree of reduction in 

emissions” that the EPA determines is “achievable.” Id.

§ 7412(d)(2). We refer to an emissions cap that reflects the 

current “maximum achievable control technology” as a 

“MACT” standard. See NRDC II, 529 F.3d at 1079. Setting a 

MACT standard is a two-step process. 

First, the EPA establishes a “MACT floor” for each 

category or subcategory. Sierra Club I, 353 F.3d at 980. The 

MACT floor ensures that all HAPs sources “at least clean up 

their emissions to the level that their best performing peers 

have shown can be achieved.” Id. For new sources—those 

built after promulgation of a HAPs limit, see 42 U.S.C. 

§ 7412(a)(4)—the MACT floor cannot be less stringent than 

the emissions levels achieved by the best performing similar 

source. Id. § 7412(d)(3). For existing sources in categories or 

subcategories that have thirty or more sources, the MACT 

floor cannot be less stringent than the average emissions 

limits achieved by the best performing 12 per cent of existing 

sources in that category or subcategory. Id. § 7412(d)(3)(A). 

And for existing sources in categories or subcategories with 

fewer than thirty sources, the MACT floor cannot be less 

stringent than the average emissions achieved by the best 

performing five sources. Id. § 7412(d)(3)(B). When setting 

the MACT floor, the EPA considers only the performance of 

the cleanest sources in a category or subcategory; it does not 

take into account other factors, including the cost of putting a 

source in line with its better-performing counterparts. See 

Cement Kiln, 255 F.3d at 857-58 (citing Nat’l Lime Ass’n v. 

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8 

EPA, 233 F.3d 625, 629 (D.C. Cir. 2000), as amended on 

denial of reh’g, No. 99-1325 (D.C. Cir. Feb. 14, 2001)). 

Second, the EPA must determine whether current 

technology makes it possible for a source to perform even 

better than the best performing similar source or sources. In 

other words, the CAA directs the EPA to consider whether it 

should set a “beyond-the-floor” MACT standard. Nat’l Lime 

Ass’n, 233 F.3d at 629. In determining whether a beyond-thefloor standard is “achievable,” the Agency must consider 

additional factors like “the cost of achieving such emission 

reduction,” “any non-air quality health and environmental 

impacts” and “energy requirements.” 42 U.S.C. § 7412(d)(2). 

It has broad discretion in its determination. See id.; cf. Nat’l 

Ass’n of Clean Water Agencies v. EPA (NACWA), 734 F.3d 

1115, 1157 (D.C. Cir. 2013) (noting, in section 7429 case, 

that “Congress gave EPA broad discretion in considering 

whether to go beyond-the-floor”). 

c. Setting Emission Standards for Area Sources—the 

“GACT” Standard 

Although the EPA must cap HAP emissions from major 

sources at the “maximum degree of reduction,” see 42 U.S.C. 

§ 7412(d)(2), it has discretion to set less stringent caps on 

emissions from area sources. Indeed, the EPA need not list 

categories of area sources at all unless: (A) it finds that the 

sources in that category or subcategory “present[] a threat of 

adverse effects” to the environment or human health, see id. 

§ 7412(c)(1), (3); or (B) control of a particular area source 

category or subcategory is necessary to ensure that sources 

accounting for at least 90 per cent of the aggregate emissions 

of the thirty HAPs the EPA believes “present the greatest 

threat to public health in the largest number of urban areas” 

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9 

are subject to CAA control, id. § 7412(c)(3), (k)(3)(B). If it 

finds that controlling emissions from a particular area source 

subcategory is necessary to achieve a 90 per cent reduction in 

the aggregate emissions of any of seven CAA-enumerated 

HAPs, section 7412(c)(6) requires the Agency to impose 

MACT caps on that subcategory. See id. § 7412(c)(6). 

With the exception of section 7412(c)(6)’s MACTstandard requirement, the EPA need not cap emissions from 

area sources at the MACT level. Instead, it may set more 

lenient emissions limits based on “generally available control 

technologies.” Id. § 7412(d)(5). We refer to these caps as 

GACT standards. The Act provides no guidance for setting 

GACT standards but the legislative history of the 1990 CAA 

Amendments describes GACT “as methods, practices and 

techniques [that] are commercially available and appropriate 

for application by the sources in the category considering 

economic impacts and the technical capabilities of the firms 

to operate and maintain the emissions control systems.” S.

REP. NO. 101-228, at 171 (1989). According to the EPA, it 

can and will consider the following in setting a GACT 

standard: 

 “costs and economic impacts . . . , which 

[are] particularly important when developing 

regulations for source categories that may 

have many small businesses . . . ”; 

 “the control technologies and management 

practices that are generally available to the 

area sources in the source category”; 

 “the standards applicable to major sources in 

the analogous source category to determine if 

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the control technologies and management 

practices are transferable and generally 

available to area sources”; and 

 “technologies and practices at area and major 

sources in similar categories to determine 

whether such technologies and practices 

could be considered generally available for 

the area source categories at issue.” 

2011 Area Boilers Rule, 76 Fed. Reg. at 15,556. And, unlike 

the EPA’s duty to consider a beyond-the-floor MACT 

standard, it need not consider a more stringent GACT 

standard. 

d. Work-Practice and Management-Practice Standards 

Although the CAA requires numeric emission standards 

where possible, the EPA can “promulgate a design, 

equipment, work practice, or operational standard, or 

combination thereof” if it determines that a numeric limit is 

“not feasible.” 42 U.S.C. § 7412(h)(1). In other words, the 

EPA can require that all sources in a given category or 

subcategory take a certain action (e.g., conduct a periodic 

tune-up) or install certain emissions-control technology (e.g., 

install a fabric filter). Although the EPA has discretion to 

impose a work-practice standard, the Act limits it by defining 

the operative phrase “not feasible” narrowly to mean: 

(A) a hazardous air pollutant or pollutants cannot 

be emitted through a conveyance designed 

and constructed to emit or capture such 

pollutant, or that any requirement for, or use 

of, such a conveyance would be inconsistent 

with any Federal, State or local law, or 

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11 

(B) the application of measurement methodology 

to a particular class of sources is not 

practicable due to technological and 

economic limitations. 

Id. § 7412(h)(2). 

Similarly, for area sources, the EPA can impose a 

“management-practice standard” in lieu of a numeric GACT 

standard. See id. § 7412(d)(5). A management-practice 

GACT standard is like a work-practice MACT standard in all 

ways but one—the EPA need not consider feasibility when 

setting management-practice standards. Compare id.

§ 7412(d)(2), with id. § 7412(d)(5). 

2. 42 U.S.C. § 7429—“Solid Waste Combustion” 

In addition to amending the Act’s “Hazardous Air 

Pollutants” provision, see id. § 7412, the 1990 CAA 

Amendments added to the U.S. Code section 7429, titled 

“Solid Waste Combustion.” Section 7429 regulates “solid 

waste incineration units” generally, see id. § 7429(a)(1)(A), 

and CISWI specifically, see id. § 7429(a)(1)(D). Although 

section 7412 requires the EPA to control emissions of nearly 

two hundred HAPs, see id. § 7412(d)(1), section 7429 

mandates that the EPA control emissions from only nine 

specific pollutants (as well as opacity, where appropriate), 

none of which the Congress included on its initial section 

7412 list, see id. § 7429(a)(4); see also Nat. Res. Def. Council 

v. EPA (NRDC I), 489 F.3d 1250, 1256 (D.C. Cir. 2007). We 

have held that this difference “makes 

promulgating . . . standards under [section 7412] and [section 

7429] mutually exclusive.” NACWA, 734 F.3d at 1119. In 

other words, if a source (or facility) is considered a CISWI 

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12 

and, therefore, regulated under section 7429, it cannot be 

regulated under section 7412. See id. 

Whether a source falls under section 7412 or section 

7429, “the statutory directive on setting MACT standards is 

virtually identical.” Id.; see also Nat’l Lime Ass’n, 233 F.3d 

at 631. That said, regulation under one section instead of the 

other “has practical consequences.” NACWA, 734 F.3d at 

1120. For example, section 7412 allows the EPA to impose a 

GACT standard for area sources only but section 7429 

requires the EPA to impose MACT standards for all covered 

units, regardless of their size. Compare 42 U.S.C. 

§ 7412(d)(1), (5), with id. § 7429(a)(1)(A); see also NRDC I, 

489 F.3d at 1256. Moreover, section 7412 mandates that the 

EPA control HAP emissions from “major source[s],” which 

the Act defines broadly to include “group[s] of stationary 

sources located within a contiguous area and under common 

control.” 42 U.S.C. § 7412(a)(1) (emphasis added). Section 

7429, in contrast, mandates that the EPA control emissions 

from “solid waste incineration unit[s],” which the Act defines 

more narrowly as “a distinct operating unit of any facility 

which combusts any solid waste material,” 42 U.S.C. 

§ 7429(g)(1) (emphases added). And finally, section 7429 

does not provide for work-practice standards. 

3. 42 U.S.C. §§ 7661 et seq.—“Title V Permits” 

Finally, the 1990 CAA Amendments added a provision to 

Title V of the Act that requires all owners and operators of 

HAP sources to obtain operating permits. See id. § 7661a. 

Title V does no more than consolidate “existing air pollution 

requirements into a single document, the Title V permit, to 

facilitate compliance monitoring” without imposing any new 

substantive requirements. Sierra Club v. Leavitt, 368 F.3d 

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13 

1300, 1302 (11th Cir. 2004). The legislative history of the 

1990 CAA Amendments indicates that the Congress required 

the “Title V permits” so that the public might “better 

determine the requirements to which the source is subject, and 

whether the source is meeting those requirements.” S. REP.

NO. 101-228, at 347. Although owners and operators of all 

major HAP sources must obtain Title V permits, see generally 

42 U.S.C. § 7661a(a), the EPA has discretion to exempt 

certain area source categories if it “finds that compliance with 

such requirements is impracticable, infeasible, or 

unnecessarily burdensome,” id. 

B. THE MAJOR BOILERS, AREA BOILERS,

AND CISWI RULES

On March 21, 2011, the EPA issued the first iteration of 

all three rules under review. That same day, however, the 

EPA announced that it intended to reconsider certain aspects 

of each rule. Not long after, multiple parties filed the 

petitions for review that we now address. Earlier, the EPA 

had concluded its reconsideration and issued the most recent 

iteration of the three rules. Because of this procedural quirk, 

each “rule” we address is in fact two separate rules—the 

EPA’s “final” 2011 version and its “final” 2013 version. The 

EPA’s analyses remained mostly consistent from 2011 to 

2013 and we indicate, where necessary, the instances in which 

the EPA changed course in a significant way. 

1. The Major Boilers Rule 

The Major Boilers Rule sets HAPs emission caps for all 

industrial, commercial, and institutional boilers that emit a 

large volume of HAPs. See 2011 Major Boilers Rule, 76 Fed. 

Reg. at 15,611. The EPA further divided the major boiler 

categories into subcategories based on the primary fuel 

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combusted by the boilers in the subcategory (e.g., coal, 

biomass, gas, etc.) and, for some subcategories, based on the 

method used to “feed” the fuel into the boiler. See 2013 

Major Boilers Rule, 78 Fed. Reg. at 7,144. For most of the 

subcategories, the EPA set a numeric MACT standard for four 

different HAPs: particulate matter (PM); hydrogen chloride 

(HCl); mercury (Hg); and carbon monoxide (CO). See id. at 

7,142 tbl.3; No. 11-1108 EPA Br. 9. The EPA used some of 

these HAPs—particularly CO—as a surrogate (or proxy) to 

set emissions limits for others on the section 7412(b) HAPs 

list. See 2013 Major Boilers Rule, 78 Fed. Reg. at 7,144-45. 

For the other major boiler subcategories, the EPA set a workpractice standard (specifically, a tune-up requirement) in lieu 

of numeric MACT standards. See 2011 Major Boilers Rule, 

76 Fed. Reg. at 15,613.7

 The EPA also established a tune-up 

work-practice standard to control for dioxin/furan emissions 

across all major boiler subcategories. 2013 Major Boilers 

Rule, 78 Fed. Reg. at 7,138. 

In addition to these emission standards, the Major Boilers 

Rule includes several other provisions relevant to the current 

petitions for review. 

a. The “Upper Prediction Limit” 

Several factors complicate the process of setting MACT 

floors. The first is the CAA itself, which mandates that all 

MACT floors (1) must be achievable, see 42 U.S.C. 

 7

 The four major boiler subcategories for which the EPA 

established work-practice standards include “[n]ew and existing 

units that have a designed heat input capacity of less than 10 

MMBtu/hr, and new and existing units in the Gas 1 (natural 

gas/refinery gas) subcategory and in the metal process furnaces 

subcategory.” 2011 Major Boilers Rule, 76 Fed. Reg. at 15,613. 

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§ 7412(d)(2); (2) must ensure continuous regulation of the 

covered sources, see id. § 7602(k); and (3) must be no less

stringent than the emissions levels being achieved by the bestcontrolled sources, see id. § 7412(d)(3). The second is that no 

source emits any HAP at a constant level; rather, HAP 

emissions fluctuate over time and for many reasons, 

including, e.g., “operation of control technologies, variation in 

combustion materials and combustion conditions, variation in 

operation of the unit itself, and variation associated with the 

emission measurement techniques.” Memorandum from 

Stephen D. Page, EPA Director of Air Quality Planning and 

Standards, EPA’s Response to Remand of the Record for 

Commercial and Industrial Solid Waste Incineration Units 

(Page Mem.) (July 14, 2014), at 3 (No. 11-1125 J.A. 1316). 

Finally, most sources do not measure their HAP emissions at 

all times and under all conditions.8

 Id. at 6. Instead, data are 

usually gathered when a source conducts a “three-run stack 

test.” Id. This test provides three “snapshots” of a source’s 

emissions in a limited set of conditions and, accordingly, it 

fails to demonstrate accurately a source’s emissions during all 

times and under all conditions. Id. 

To compensate for the lack of adequate emissions data, 

the EPA uses a statistical tool known as the “upper prediction 

limit” (UPL) to account for the expected variability in 

emissions levels. See 2011 Major Boilers Rule, 76 Fed. Reg. 

at 15,630. The UPL, in turn, allows the Agency to set a 

MACT floor that is continuously achievable. Id. We discuss 

the UPL mechanics at greater length below, see infra § IV.C, 

but, in short, the EPA: (1) ranks all sources in a given 

 8

 As discussed below, however, the EPA does allow sources 

to demonstrate MACT compliance by use of “continuous 

monitors.” See infra § IV.I. 

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category based on their three-run stack-test data; 

(2) determines the HAP emissions level of the “best 

controlled similar source” to establish standards for new

sources, 42 U.S.C. § 7412(d)(3), and determines the average 

HAP emissions levels of the best performing 12 per cent of 

sources to establish standards for existing sources, id. 

§ 7412(d)(3)(A); and then (3) applies the UPL methodology 

to provide the cushion necessary to account for the expected 

peaks and valleys in HAP emissions not reflected in the threerun stack-test “snapshots.” See Page Mem. 4, 6. 

b. The “Pollutant-By-Pollutant” Approach 

In identifying the best performing sources in a given 

category, often the EPA could not identify a single source that 

controlled all HAPs better than all other sources. Instead, the 

EPA found that one source effectively controlled emissions 

from one HAP but was nonetheless one of the worstperforming sources at controlling emissions from a different 

HAP. For this reason, the EPA adopted a “pollutant-bypollutant” approach in setting MACT floors for major boiler 

subcategories. See 2011 Major Boilers Rule, 76 Fed. Reg. at 

15,622-23. That is, instead of identifying the one source that, 

on balance, best controlled all HAPs in the aggregate, the 

EPA used one source to set the MACT floor for, e.g., PM, and 

used a different source to set the MACT floor for, e.g., HCl. 

For at least two subcategories of major boilers—new heavy 

oil-fired units and existing stoker coal-fired units—the EPA’s 

pollutant-by-pollutant approach resulted in MACT floors that 

no source had achieved in toto. 

c. Startups, Shutdowns, and Malfunctions 

The EPA found it difficult to account for HAP emissions 

when sources start up, shut down, and malfunction. All three 

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occurrences alter HAP emissions and, historically, the EPA 

exempted sources from normal numeric MACT-standard 

compliance when these events occurred. See, e.g., Standards 

of Performance for New Stationary Sources, 42 Fed. Reg. 

57,125 (Nov. 1, 1977). Nevertheless, concluding that the Act 

“require[s] that there must be continuous section [7412]-

compliant standards” and observing that the exemption meant 

that “no section [7412] standard governs these events,” in 

2008 we vacated the exemption for startups, shutdowns, and 

malfunctions when the issue arose in a case challenging a 

different rule. Sierra Club v. EPA (Sierra Club III), 551 F.3d 

1019, 1027-28 (D.C. Cir. 2008) (emphasis added). 

In response to the Sierra Club III vacatur, the EPA 

established a work-practice standard in lieu of a numeric 

MACT standard during startup and shutdown periods (but not

during malfunctions) when it promulgated the Major Boilers 

Rule. See 2011 Major Boilers Rule, 76 Fed. Reg. at 15,613.9

 

It did so after determining that the “physical limitations and 

the short duration of startup and shutdown periods” made it 

technologically infeasible to conduct the requisite testing for 

numeric emissions limits. Id. A work-practice standard 

sufficed, in the EPA’s view, because “[p]eriods of startup, 

normal operations, and shutdown are all predictable and 

routine aspects of a source’s operations.” Id.

 9

 Specifically, the startup and shutdown work-practice 

standard requires a source to follow “the manufacturer’s 

recommended procedures for minimizing periods of startup and 

shutdown.” 2011 Major Boilers Rule, 76 Fed. Reg. at 15,613. “If 

manufacturer’s recommended procedures are not available,” the 

Major Boilers Rule provided that “sources must follow 

recommended procedures for a unit of similar design for which 

manufacturer’s recommended procedures are available.” Id. at 

15,642. 

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But because a malfunction is “sudden, infrequent, and not 

reasonably preventable,” id. (quoting 40 C.F.R. § 63.2), the 

EPA declined to treat a malfunction as a “distinct operating 

mode,” id. As a result, the EPA did not account for 

malfunctions when it set the MACT floors and it required 

sources to comply with all MACT floors even during periods 

of malfunction. Id. At the same time and recognizing that 

even the best equipment can fail and that such failure can 

spike emissions, the EPA added to the Major Boilers Rule “an 

affirmative defense to civil penalties for exceedances of 

numerical emission limits that are caused by malfunctions.” 

Id. In reviewing a challenge to a different EPA rule, however, 

we vacated a materially identical affirmative-defense 

provision and held that the EPA has no power under the CAA 

to create a defense to civil liability. See Natural Res. Def. 

Council v. EPA (NRDC III), 749 F.3d 1055, 1062-64 (D.C. 

Cir. 2014). Here, the EPA defends its decision not to address 

malfunctions by asserting that it will use its enforcement 

discretion regarding malfunctions on a case-by-case basis. 

d. The One-Time Energy Assessment 

The EPA also promulgated a “beyond-the-floor” 

requirement for all facilities with existing major boilers. See 

2011 Major Boilers Rule, 76 Fed. Reg. at 15,613. 

Specifically, the Major Boilers Rule mandates a “a one-time 

energy assessment . . . on the affected boilers and facility to 

identify any cost-effective energy conservation measures,” 

id., which assessment includes, inter alia, a review of fuel 

usage, energy management practices, and conservation 

measures, see 2013 Major Boilers Rule, 78 Fed. Reg. at 

7,198-99. In some respects, the energy assessment is limited: 

it (1) need occur only one time, see 40 C.F.R pt. 63, subpt. 

DDDDD tbl.3; (2) is “based on energy use by discrete 

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segments of a facility and not by a total aggregation of all 

individual energy using elements of a facility,” 2013 Major 

Boilers Rule, 78 Fed. Reg. at 7,146; and (3) does not require 

an owner or operator to implement any of the energy-saving 

findings the assessment makes. In one respect, however, it is 

expansive—it requires owners and operators to assess not 

only the boilers themselves but also other components 

“located on the site of the affected boiler that use energy 

provided by the boiler,” including “compressed air systems” 

as well as “facility heating, ventilation, and air conditioning 

systems.” 40 C.F.R. § 63.11237. 

e. The Health-Based Emissions Limits for HCl 

Although the EPA set numeric MACT standards to 

control HCl emissions, see 2013 Major Boilers Rule, 78 Fed. 

Reg. at 7,193-98 tbls.1 & 2, in an earlier iteration of the Major 

Boilers Rule, the EPA did not set MACT standards for HCl. 

See National Emission Standards for Hazardous Air 

Pollutants for Industrial, Commercial, and Institutional 

Boilers and Process Heaters (2004 Boilers Rule), 69 Fed. Reg. 

55,218, 55,227 (Sept. 13, 2004). Instead, the Agency opted 

for a less stringent health-based emissions limit under section 

7412(d)(4). See id. The EPA changed course after 

concluding that HCl emissions posed health concerns the 

Agency had not previously considered—in particular, the 

EPA feared the “potential cumulative public health and 

environmental effects” of HCl emissions, 2011 Major Boilers 

Rule, 76 Fed. Reg. at 15,643-44 (emphasis added)—and after 

recognizing that it did not have the requisite data to weigh 

adequately the newly identified health risks. 

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2. The Area Boilers Rule 

In the Area Boilers Rule, the EPA set emissions limits for 

the same three boiler categories it controlled in the Major 

Boilers Rule, see supra § I.B.1: industrial, commercial, and 

institutional boilers. See 2013 Area Boilers Rule, 78 Fed. Reg. 

at 7,488. It further split the categories into seven 

subcategories, see id., and set emissions limits for three of 

them, see id. at 7,517-18 tbls.1 & 2.10 These include: 

(1) coal-fired boilers (i.e., “any boiler that burns any solid 

fossil fuel and no more than 15 percent biomass,” 40 C.F.R. 

§ 63.11237); (2) oil-fired boilers (i.e., “any boiler that burns 

any liquid fuel and is not in either the biomass or coal 

subcategories,” id.); and (3) biomass-fired boilers (i.e., “any 

boiler that burns any” “biomass-based solid fuel that is not a 

solid waste” and “is not in the coal subcategory,” id.). See 

2013 Area Boilers Rule, 78 Fed. Reg. at 7,517-18 tbls.1 & 2. 

For these subcategories, the EPA set emissions limits for 

three HAPs: Hg, PM, and CO, with PM functioning as a 

surrogate for non-Hg urban metals and CO functioning as a 

surrogate for polycyclic organic matter (POM). See 2011 

Area Boilers Rule, 76 Fed. Reg. at 15,586. Because Hg and 

POM are both listed in section 7412(c)(6), the EPA had to set 

MACT standards for Hg and for CO (as surrogate for POM) 

for any area source category that, in the EPA’s view, required 

 10 As noted above, see supra § I.A.1.a, the EPA has some 

discretion in promulgating emissions limits for area HAP sources. 

Exercising its discretion, the EPA had previously determined that 

natural gas-fired area boilers did not emit HAPs at a level 

necessitating regulation. See National Emission Standards for 

Hazardous Air Pollutants for Area Sources: Industrial, Commercial, 

and Institutional Boilers (2010 Proposed Area Boilers Rule), 75 

Fed. Reg. 31,896, 31,900 (June 4, 2010). 

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MACT control to assure a 90 per cent reduction in the 

aggregate emissions of these two HAPs. See 42 U.S.C. 

§ 7412(c)(6). The Agency complied, setting numeric MACT 

standards for Hg and CO emissions from large coal-fired 

boilers and a MACT work-practice standard (specifically, a 

tune-up requirement) for emissions from small coal-fired 

boilers. See 2013 Area Boilers Rule, 78 Fed. Reg. at 7,488, 

7,517-18.11 It did not, however, set MACT standards for Hg 

and POM emissions from biomass or oil-fired boilers, finding 

it unnecessary to assure a 90 per cent reduction in aggregate 

emissions of those two HAPs. See 2011 Area Boilers Rule, 

76 Fed. Reg. at 15,566. 

Thus, with the exception of Hg and CO emissions from 

coal-fired boilers, the EPA had discretion to promulgate 

GACT standards for all other HAPs in all other source 

subcategories. See 42 U.S.C. § 7412(d)(5). Exercising this 

discretion resulted in the following standards: 

 

 11 As used in the Area Boilers Rule, the difference between 

“large” and “small” units depends on the heat-input capacity of the 

unit. See 2013 Area Boilers Rule, 78 Fed. Reg. at 7,488. It is not 

the same as the difference between “major” and “area” sources, 

which is based on the volume of HAPs a source emits. See 42 

U.S.C. § 7412(a).

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Boiler 

Subcategory Size Age Hg Limit 

Hg 

Type 

Coal 

Large New Numeric MACT 

Existing Numeric MACT 

Small

New TuneUp MACT 

Existing TuneUp MACT 

Biomass 

Large New --- --- 

Existing --- --- 

Small New --- --- 

Existing --- --- 

Oil 

Large New --- --- 

Existing --- --- 

Small New --- --- 

Existing --- --- 

Boiler 

Subcategory Size Age CO Limit 

CO 

Type 

Coal 

Large New Numeric MACT 

Existing Numeric MACT 

Small New Tune-up MACT 

Existing Tune-up MACT 

Biomass 

Large New Tune-up GACT 

Existing Tune-up GACT 

Small New Tune-up GACT 

Existing Tune-up GACT 

Oil 

Large New Tune-up GACT 

Existing Tune-up GACT 

Small New Tune-up GACT 

Existing Tune-up GACT 

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Boiler 

Subcategory Size Age PM Limit 

PM 

Type 

Coal 

Large New Numeric GACT 

Existing Tune-up GACT 

Small New Tune-up GACT 

Existing Tune-up GACT 

Biomass 

Large New Numeric GACT 

Existing Tune-up GACT 

Small New Tune-up GACT 

Existing Tune-up GACT 

Oil 

Large New Numeric GACT 

Existing Tune-up GACT 

Small New Tune-up GACT 

Existing Tune-up GACT 

2013 Area Boilers Rule, 78 Fed. Reg. at 7,488-89, 7,517-19. 

The Area Boilers Rule shares many of the same features 

as the Major Boilers Rule; for example, the Area Boilers Rule 

treats startups, shutdowns, and malfunctions in the same 

fashion as the Major Boilers Rule, see supra § I.B.1.c—i.e., 

the Area Boilers Rule creates work-practice (or managementpractice) standards for startup and shutdown periods but does 

not account for malfunctions at all, save for the Agency’s 

commitment to consider malfunctions on a case-by-case basis. 

See 2013 Area Boilers Rule, 78 Fed. Reg. at 7,496; 2011 Area 

Boilers Rule, 76 Fed. Reg. at 15,560-61. Additionally, the 

Area Boilers Rule imposes the same one-time energyassessment requirement for existing large area boilers that the 

Major Boilers Rule imposes for existing major boilers. See 

supra § I.B.1.d; see also 2013 Area Boilers Rule, 78 Fed. 

Reg. at 7,500; 2011 Area Boilers Rule, 76 Fed. Reg. at 

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15,560, 15,567-68. There are, however, two unique features 

of the Area Boilers Rule that warrant brief discussion. 

a. Exclusion of “Temporary Boilers” 

After the EPA promulgated the 2011 Area Boilers Rule 

but before it promulgated the 2013 version, it proposed an 

amendment to 40 C.F.R. § 63.11195 that added temporary 

boilers to the list of those boilers not regulated by 

section 7412. See National Emission Standards for 

Hazardous Air Pollutants for Area Sources: Industrial, 

Commercial, and Institutional Boilers (2011 Proposed Area 

Boilers Rule on Reconsideration), 76 Fed. Reg. 80,532, 

80,535 (Dec. 23, 2011). The EPA created the exclusion 

because, in its view, temporary boilers are “insignificant 

sources[] and were not included in the EPA’s analysis of the 

source category.” Id. The Agency eventually defined 

“temporary boiler” as “any gaseous or liquid fuel boiler that is 

designed to, and is capable of, being carried or moved from 

one location to another by means of, for example, wheels, 

skids, carrying handles, dollies, trailers, or platforms.” See 

2013 Area Boilers Rule, 78 Fed. Reg. at 7,491 (quoting 40 

C.F.R. § 63.11237). 

b. Title V Permit Exemption for Synthetic Area Sources 

As noted, see supra § I.A.3, Title V of the CAA imposes 

a permit requirement on all owners and operators of major 

and area HAP sources. See 42 U.S.C. § 7661a. The EPA, 

however, can exempt an area source subcategory if it finds 

“that compliance with such requirements is impracticable, 

infeasible, or unnecessarily burdensome on such categories.” 

Id. § 7661a(a). When it proposed the Area Boilers Rule in 

2010, the EPA considered exempting some area sources 

because, in its view, the existing restrictions on those sources 

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25 

made Title V duplicative. See 2010 Proposed Area Boilers 

Rule, 75 Fed. Reg. at 31,910-13. At the time, the EPA 

announced that it did not intend to exempt “synthetic” area 

sources (i.e., area sources that, but for existing air-pollution 

controls, would be considered major sources). Id. at 31,913. 

In so doing, the EPA reasoned that synthetic area sources: 

(1) more closely resemble major sources than area sources, 

(2) are often located in populous areas, and (3) have high 

HAP emissions potential when uncontrolled. Id. 

But in the 2011 Area Boilers Rule, the EPA changed 

course and exempted synthetic area sources from the Title V 

permitting requirement. See 76 Fed. Reg. at 15,578. It 

reasoned that the “observations and data . . . relied upon in 

other rulemakings for distinguishing between sources that 

became synthetic area sources due to controls and other 

synthetic and natural area sources did not necessarily apply to 

this source category.” Id. In its view, it no longer had 

“sufficient information” to distinguish synthetic area sources 

from the others it exempted and, accordingly, “the rationale 

for exempting most area sources subject to this rule . . . is also 

now relevant for” synthetic area sources. Id.; see also 2013 

Area Boilers Rule, 78 Fed. Reg. at 7,497. 

3. The CISWI Rule 

In the CISWI Rule, the EPA created four CISWI 

subcategories: (1) incinerators (i.e., “units designed to burn 

[solid] waste materials for the purpose of disposal”); 

(2) small, remote incinerators (“SRIs”) (i.e., units that burn 

small waste batches); (3) energy recovery units (“ERUs”) 

(i.e., units that would be classified as boilers but for the fact 

they combust solid waste); and (4) waste-burning kilns (i.e., 

units that would be classified as cement kilns if they did not 

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burn solid waste). 2013 CISWI Rule, 78 Fed. Reg. at 9,118. 

Initially, the EPA proposed a fifth subcategory—burn-off 

ovens—but eliminated burn-off ovens after comments 

revealed that it had greatly underestimated the number of 

units in that subcategory (36 versus 15,000) and that it lacked 

the requisite data to set limits for the units. See 2011 CISWI 

Rule, 76 Fed. Reg. at 15,734. Of the four CISWI 

subcategories, the EPA further divided the ERU subcategory 

(for CO emissions only) into coal-fired, biomass-fired and 

oil/gas-fired ERUs and it further divided the waste-burning 

kiln subcategory (again, for CO emissions only) into long and 

preheater/precalcinator kilns. See 2013 CISWI Rule, 78 Fed. 

Reg. at 9,118 tbl.2. 

The EPA then set numeric MACT limits for the section 

7429(a)(4) pollutants.12 See 2011 CISWI Rule, 76 Fed. Reg. 

at 15,709-10 tbl.1. Unlike the Major Boilers Rule and the 

Area Boilers Rule, the CISWI Rule contains no beyond-thefloor MACT standards. The EPA also declined to promulgate 

work-practice standards, concluding that it had no authority to 

do so because section 7429 includes no work-practice 

standard provision similar to that in section 7412. See id. at 

15,721. 

The CISWI Rule shares several features with the Major 

Boilers Rule. In the CISWI Rule, for instance, the EPA also 

used the UPL, see id. at 15,722-27, as well as the pollutantby-pollutant approach, see id. at 15,719-21, in setting MACT 

floors. Based in part on the differences between section 7412 

 12 These pollutants are (1) PM, (2) sulfur dioxide (SO2), 

(3) HCl, (4) nitrogen oxide (NOx), (5) CO, (6) lead (Pb), 

(7) cadmium (Cd), (8) Hg, (9) dioxins and dibenzofurans, and (10) 

opacity (where appropriate). 42 U.S.C. § 7429(a)(4). 

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and section 7429, the CISWI rule has four unique 

characteristics we briefly describe. 

a. Startups, Shutdowns, and Malfunctions 

As discussed, see supra § II.B.1.c, the EPA imposed a 

work-practice standard for major and area source boilers 

during periods of startup and shutdown but declined to make 

any regulatory modification for malfunctions. See 2011 

Major Boilers Rule, 76 Fed. Reg. at 15,613; 2011 Area 

Boilers Rule, 76 Fed. Reg. at 15,560-61. The CISWI Rule, 

however, makes no modification for any of these periods, 

mandating instead that the numeric MACT standards “apply 

at all times,” even when CISWI units are starting up or 

shutting down. 2011 CISWI Rule, 76 Fed. Reg. at 15,711, 

15,737-38. The Agency concluded that it had no legal 

authority under section 7429 to impose anything but a 

numeric MACT standard on CISWI units. See id. at 15,709 

tbl.1; see also id. at 15,737-38. 

b. The Record-Keeping Requirement 

Whether the EPA considers a combustion unit to be a 

boiler (and thus subject to section 7412) or a CISWI (and thus 

subject to section 7429) turns entirely on whether the unit 

combusts “solid waste.” See id. at 15,709. The term “solid 

waste” is defined in RCRA, 42 U.S.C. §§ 6901 et seq., and 

clarified by EPA regulation, see Identification of NonHazardous Secondary Materials that Are Solid Waste (NHSM 

Rule), 76 Fed. Reg. 15,456, 15,457 (Mar. 21, 2011). See also 

2011 CISWI Rule, 76 Fed. Reg. at 15,709. If the unit 

combusts solid waste, it is a CISWI. Id. 

The source owner or operator initially decides whether 

the material its combustion unit burns meets the definition of 

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28 

solid waste. See id. at 15,740. For this reason, the CISWI 

rule requires that the owner or operator of a combustion unit 

that burns materials “not clearly listed as traditional fuels” 

keep records explaining how the materials meet the regulatory 

definition of “non-solid waste.” Id.; see also 40 C.F.R. 

§ 60.2175(v). Failure to do so means, for the purposes of the 

EPA, that “the operating unit is a CISWI unit.” 40 C.F.R. 

§ 60.2265; see also 2013 CISWI Rule, 78 Fed. Reg. at 9,188. 

c. Emissions Averaging 

During the notice-and-comment period, certain industry 

entities urged the EPA to allow a facility containing more 

than one CISWI unit to demonstrate compliance with the 

CISWI MACT standards by averaging the HAP emissions of 

all units in the facility. See Commercial and Industrial Solid 

Waste Incineration Units: Reconsideration and Proposed 

Amendments; Non-Hazardous Secondary Materials that Are 

Solid Waste (2011 Proposed CISWI Rule on 

Reconsideration), 76 Fed. Reg. 80,452, 80,463 (Dec. 23, 

2011). Although it allowed facility-wide averaging in the 

Major Boilers Rule, the Agency declined to allow it for 

facilities with CISWI units. See id. The EPA explained, first, 

that “[t]he applicability of CISWI is such that each unit is an 

affected facility.” Id. In response to further comments, the 

EPA subsequently explained that it did “not believe [it had] 

the legal authority to allow emissions averaging in CISWI or 

under section [7429] generally because each individual unit is 

an affected facility.” Summary of Public Comments and 

Responses for Commercial and Industrial Solid Waste 

Incineration Units (CISWI Rule—Responses to Comments), 

EPA-HQ-OAR-2003-0119-2638-A2 (Dec. 2012), at 195. 

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29 

d. Treatment of Units that Begin Combusting Solid Waste 

Finally, in the preamble to the 2011 CISWI Rule, the 

EPA stated broadly that “[u]nits that begin combusting solid 

waste are considered existing sources under CISWI.” 76 Fed. 

Reg. at 15,714 (emphasis added). This categorical 

pronouncement drew objections from commentators who 

insisted that, if such units experienced an increase in HAP 

emissions, the units would meet the statutory definition of 

“modified solid waste incineration unit[s],” see 42 U.S.C. 

§ 7429(g)(3), and would, accordingly, be subject to the 

MACT standards for new units, see id. § 7429(g)(2). In the 

subsequent proposed CISWI Rule, the EPA clarified that 

“[a]n existing source will not be considered a new source 

solely due to a combustion material switch. Assuming new 

source applicability is not triggered, existing sources that 

change fuels or materials are considered existing 

sources . . . .” 2011 Proposed CISWI Rule on 

Reconsideration, 76 Fed. Reg. at 80,459. 

II. STANDARD OF REVIEW 

For each issue, the Petitioners argue that the EPA either 

misinterpreted the CAA, acted arbitrarily and capriciously, or 

both. We review the EPA’s construction of the statute under 

the two-part framework established in Chevron, U.S.A., Inc. v. 

Nat. Res. Def. Council, Inc., 467 U.S. 837 (1984). At 

Chevron step 1, we ask whether the Congress “has directly 

spoken to the precise question at issue”; if it has, we “must 

give effect to [its] unambiguously expressed intent.” Id. at 

842-43. In so doing, we examine the CAA’s text, structure, 

purpose, and legislative history to determine if the Congress 

has expressed its intent unambiguously. See Bell Atl. Tel. Co. 

v. FCC, 131 F.3d 1044, 1047 (D.C. Cir. 1997). If the statute 

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30 

is “silent or ambiguous with respect to the specific issue,” we 

proceed to Chevron step 2 and defer to the EPA’s 

interpretation so long as it is “based on a permissible 

construction of the statute.” Chevron, 467 U.S. at 842-43. 

The CAA authorizes the Court to “reverse any [EPA] 

action found to be . . . arbitrary, capricious, an abuse of 

discretion, or otherwise not in accordance with law.” 42 

U.S.C. § 7607(d)(9)(A). Our review is “narrow” and we will 

“not . . . substitute [our] judgment for that of the 

agency.” Motor Veh. Mfrs. Ass’n v. State Farm Mut. Auto. 

Ins. Co. (State Farm), 463 U.S. 29, 43 (1983). We “must 

uphold an agency’s action where [the agency] ‘has considered 

the relevant factors and articulated a rational connection 

between the facts found and the choice made,’ and has not 

‘relied on [improper] factors.’” Nat’l Ass’n of Clean Air 

Agencies v. EPA (NACAA), 489 F.3d 1221, 1228 (D.C. Cir. 

2007) (citations omitted) (quoting Allied Local & Reg’l Mfrs. 

Caucus v. EPA, 215 F.3d 61, 68 (D.C. Cir. 2000), and State 

Farm, 463 U.S. at 43). A rule is arbitrary and capricious if 

the agency: (1) “has relied on factors which Congress has not 

intended it to consider,” (2) “entirely failed to consider an 

important aspect of the problem,” (3) “offered an explanation 

for its decision that runs counter to the evidence before the 

agency,” or (4) “is so implausible that it could not be ascribed 

to a difference in view or the product of agency expertise.” 

State Farm, 463 U.S. at 43. 

We review the EPA’s factual determinations for 

substantial evidence. 5 U.S.C. § 706(2)(E). We also “owe[] 

particular deference to EPA when its rulemakings rest upon 

matters of scientific and statistical judgment within [its] 

sphere of special competence and statutory jurisdiction.” Am. 

Coke & Coal Chems. Inst. v. EPA, 452 F.3d 930, 941 (D.C. 

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31 

Cir. 2006). But “[w]e are hesitant to rubber-stamp EPA’s 

invocation of statistics without some explanation of the 

underlying principles or reasons why its formulas would 

produce an accurate result.” NACWA, 734 F.3d at 1145. 

III. INDUSTRY PETITIONERS’ CHALLENGES 

A. STARTUPS, SHUTDOWNS, AND MALFUNCTIONS

Industry Petitioners raise two sets of challenges to 

startup, shutdown, and malfunction periods: (1) a challenge to 

the EPA’s failure to take malfunctions into account in the 

Major Boilers and Area Boilers Rules and (2) a challenge to 

EPA’s failure to take into account periods of startup, 

shutdown, and malfunction in the CISWI Rule. For the 

reasons that follow, we reject all of the Industry Petitioners’ 

claims related to startups, shutdowns, and malfunctions. 

 

1. Periods of Malfunction in the Major Boilers and Area 

Boilers Rules 

First, Industry Petitioners challenge the Major Boilers 

and Area Boilers Rules’ failure to take malfunctions into 

account in setting MACT floors. See 2011 Major Boilers 

Rule, 76 Fed. Reg. at 15,613; 2011 Area Boilers Rule, 76 Fed. 

Reg. at 15,560-61. The EPA defends its refusal to account for 

malfunctions on the basis of (1) the impracticability of 

accounting for events that are necessarily unpredictable, and 

(2) the EPA’s assertion that it will use its prosecutorial 

discretion to determine on a case-by-case basis whether an 

exceedance of emission standards is attributable to an 

excusable malfunction or whether applicable regulatory 

penalties should be imposed instead. See No. 11-1108 EPA 

Br. 38; No. 11-1141 EPA Br. 29. 

 

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Both sides agree that malfunctions are inevitable in the 

operation of area and major boilers. According to the EPA, 

“even equipment that is properly designed and maintained can 

sometimes fail and . . . such failure can sometimes cause an 

exceedance of the relevant emission standard.” 2011 Major 

Boilers Rule, 76 Fed. Reg. at 15,613; 2011 Area Boilers Rule, 

76 Fed. Reg. at 15,561. Thus, the EPA defined a malfunction 

as a “sudden, infrequent, and not reasonably preventable 

failure of air pollution control and monitoring equipment, 

process equipment or a process to operate in a normal or usual 

manner.” 2011 Major Boilers Rule, 76 Fed. Reg. at 15,613 

(citing 40 C.F.R. § 63.2); 2011 Area Boilers Rule, 76 Fed. 

Reg. at 15,560 (same). In attempting to write rules to account 

for emissions, however, the EPA faced an intractable 

problem: how to account for a malfunction which is, by 

definition, unpredictable in terms of timing, duration, 

magnitude, and effect. While the existence of malfunctions is 

entirely predictable, the nature of those malfunctions is not, 

and it is the malfunction’s nature that affects emissions and 

thus is relevant to the application of emission limits. 

 

At first glance, the EPA’s chosen approach to 

malfunctions may seem counterintuitive, as the Agency 

appears to have several reasonable alternatives: it could 

exempt periods of malfunction entirely from the application 

of the emission standards; or it could apply the standards to 

malfunctions while giving boiler owners the opportunity to 

defend against a penalty by demonstrating they were not at 

fault for the malfunction. But the EPA has previously been 

stymied in its attempts to implement either of these solutions, 

as this court has concluded neither approach is consistent with 

the Agency’s enabling statutes. For instance, in Sierra Club 

III, the EPA attempted to exempt major sources from 

complying with emission standards during start up, shut 

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33 

down, and malfunction. See 551 F.3d at 1027-28. This court 

rejected that approach because the Congress “required that 

there must be continuous section 112-compliant standards” 

and so the EPA lacked discretion to exempt certain periods 

from compliance, regardless of their unpredictability. Id. at 

1027. In NRDC III, this court considered a challenge to the 

affirmative defense provision the EPA adopted for persons 

defending against civil suits under 42 U.S.C. § 7604(a), which 

allows “any person” to “commence a civil action on his own 

behalf” against any entity alleged to be in violation of an 

emission standard or limitation. The affirmative defense 

provision was meant to shield alleged violators from liability 

for certain emissions violations caused by “unavoidable” 

malfunctions; under the provision, therefore, “the district 

court [could] assess penalties only if violators fail[ed] to meet 

[their] burden of proving all of the requirements in the 

affirmative defense.” NRDC III, 749 F.3d at 1062 (internal 

quotation omitted). The court rejected this provision as an 

impermissible intrusion on the judiciary’s role. See id. at 

1063 (“[U]nder this statute, deciding whether penalties are 

‘appropriate’ in a given private civil suit is a job for the 

courts, not for EPA.”). 

Faced with an obvious dilemma, the EPA arrived at the 

approach it defends today. Malfunctions receive no special 

treatment and the EPA instead exercises “its enforcement 

discretion to address exceedances of emission limits that may 

be caused by such uncertain, unpredictable events, on a caseby-case basis.” No. 11-1108 EPA Br. 38; see also No. 11-

1141 EPA Br. 29. The EPA’s current treatment of 

malfunctions thus differs from its invalid affirmative defense 

provision because the Agency is exercising its own regulatory 

enforcement power on an ad hoc basis outside the context of 

citizen suits. When an exceedance occurs during a 

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34 

malfunction, the EPA determines what enforcement action—

if any—it should take by considering “the good faith efforts 

of the source to minimize emissions during malfunction 

periods, including preventative and corrective actions, as well 

as root cause analyses to ascertain and rectify excess 

emissions.” 2011 Major Boilers Rule, 76 Fed. Reg. at 15,613; 

see also 2011 Area Boilers Rule, 76 Fed. Reg. at 15,561 

(same). The EPA also considers whether the exceedance was 

in fact “not reasonably preventable” or whether it was 

“caused in part by poor maintenance or careless operation.” 

2011 Major Boilers Rule, 76 Fed. Reg. at 15,613 (citing 40 

C.F.R. § 63.2); see also 2011 Area Boilers Rule, 76 Fed. Reg. 

at 15,561 (same). 

For our purposes, we need not (indeed, must not) 

evaluate the policy implications of the EPA’s regulatory 

choice because our review is confined to determining whether 

the EPA’s regulation reflects a permissible reading of the 

applicable statute under Chevron. Here, we conclude that it 

does. The relevant statute requires only that the EPA set 

“achievable” standards, 42 U.S.C. § 7412(d)(2), and it defines 

achievability to be no less “than the emission control that is 

achieved in practice by the best controlled similar source,” 42 

U.S.C. § 7412(d)(3). The “best controlled similar source,” 

however, is unlikely to be a malfunctioning source, and the 

EPA is bound to enact a standard in keeping with emission 

limits achieved by that “best controlled similar source.” If 

anything, then, the statutory language on its face prevents the 

EPA from taking into account the effect of potential 

malfunctions when setting MACT emission standards. At the 

very least, the language permits the EPA to ignore 

malfunctions in its standard-setting and account for them 

instead through its regulatory discretion. Our Sierra Club III

decision confirms this. See 551 F.3d at 1027-28. Because the 

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35 

EPA had no option to exclude these unpredictable periods, its 

approach is reasonable. We therefore reject Industry 

Petitioners’ argument that the EPA either misinterpreted the 

CAA or acted arbitrarily and capriciously in failing to account 

for malfunctions when setting MACT floors in the Major and 

Area Boilers Rules. 

Nor do we agree with the Industry Petitioners’ secondary 

argument that the EPA acted arbitrarily and capriciously by 

failing to set a work-practice or a GACT managementpractice standard for malfunction periods. First, the statute 

makes clear that these kinds of standards are to be set at the 

discretion of the EPA, so it would be difficult to interpret the 

statute consistently with its text while holding that the text’s 

permissive language in fact sets out a requirement that the 

Agency set work-practice or GACT management-practice 

standards. As to work-practice standards, “[t]he 

Administrator may, in lieu [of a numeric standard], 

promulgate a design, equipment, work practice, or operational 

standard, or combination thereof,” and any such standard set 

must “in the Administrator’s judgment [be] consistent with 

the provisions of subsection (d).” 42 U.S.C. § 7412(h)(1). As 

to GACT management-practice standards, “the Administrator 

may . . . elect to promulgate” such standards with respect to 

certain “categories and subcategories of area sources.” Id.

§ 7412(d)(5). It should go without saying that “may means 

may.” McCreary v. Offner, 172 F.3d 76, 83 (D.C. Cir. 1999) 

(internal quotations omitted). 

Second, the Petitioners have not demonstrated and the 

EPA does not concede that setting work-practice or GACT 

management-practice standards would even be feasible for 

periods of malfunction. As for work-practice standards, the 

EPA would have to conceive of a standard that could apply 

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36 

equally to the wide range of possible boiler malfunctions, 

ranging from an explosion to minor mechanical defects. Any 

possible standard is likely to be hopelessly generic to govern 

such a wide array of circumstances. Similar problems exist 

for setting GACT management practices. These management 

practices would also need to apply to the wide range of 

possible malfunctions, and the EPA would need to determine 

that the standard would “reduce emissions of hazardous air 

pollutants,” an evidence-based standard that is difficult 

(perhaps impossible) to apply to the unpredictable 

circumstances of malfunctions. 42 U.S.C. 

§ 7412(d)(5). Thus, we reject the Industry Petitioners’ 

argument that the EPA was required to set a work-practice or 

GACT management-practice standard for malfunction 

periods. 

In doing so, we are mindful that the EPA is not the only 

entity able to bring enforcement actions under the CAA, but 

that private citizens are also empowered to enforce emission 

standards by filing suit in district court. 42 U.S.C. 

§ 7604(a). Assurances that the EPA will use its prosecutorial 

discretion to account for malfunctions would mean little if 

private citizens could seek strict enforcement of those same 

standards. But as we stated in NRDC III, “the Judiciary, not 

any executive agency, determines ‘the scope’—including the 

available remedies—‘of judicial power vested by’ statutes 

establishing private rights of action.” 749 F.3d at 1063 

(quoting City of Arlington v. FCC, 133 S. Ct. 1863, 1871 

(2013)). Accordingly, in citizen suits under the CAA, “the 

courts determine, on a case-by-case basis, whether civil 

penalties are ‘appropriate.’” Id. Boiler operators can argue 

that penalties should not be assessed because of an 

unavoidable malfunction, and they can support that argument 

with other relevant facts, “such as the defendant’s ‘full 

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37 

compliance history and good faith efforts to comply.’” Id. 

(quoting 42 U.S.C. § 7413(e)(1)). The EPA can also provide 

supporting argumentation as intervenor or amicus. Id. Courts 

should not hesitate to exercise their judicial authority to craft 

appropriate civil remedies in the case of emissions 

exceedances caused by unavoidable malfunctions. 

2. Periods of Startup, Shutdown, and Malfunction in the 

CISWI Rule 

 In the CISWI Rule, the EPA made no modification for 

periods of startup, shutdown, or malfunction. The Industry 

Petitioners argue that failing to account for these periods 

violated the EPA’s statutory instruction to set “achievable” 

standards. Additionally, the Industry Petitioners claim it was 

arbitrary and capricious for the EPA to set work-practice 

standards for startup and shutdown periods under the Major 

Boilers Rule but not under the CISWI Rule. Both arguments 

are without merit. 

First, the EPA’s emission standards for small incinerators 

do take into account periods of shutdown and startup. The 

EPA based its standards for these machines on “short term 

stack tests for pollutants,” in which incinerators are monitored 

during the course of normal operation, which includes daily 

startup and shutdown periods. See 2011 CISWI Rule, 76 Fed. 

Reg. at 15,738. Thus, startup and shutdown times are already 

incorporated into the standards the EPA set, and what is more, 

nearly all pollutants are present in smaller numbers during 

startup and shutdown anyway, when incinerators are burning 

fuels alone rather than fuels and solid waste. See Standards of 

Performance for New Stationary Sources and Emission 

Guidelines for Existing Sources: Commercial and Industrial 

Solid Waste Incineration Units (2010 Proposed CISWI Rule), 

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38 

75 Fed. Reg. 31,938, 31,964 (June 4, 2010). Given this 

reality, the CISWI Rule satisfies the statutory standard of 

“achievability” and is not arbitrary and capricious. 

 

Second, as to periods of malfunctions, the same analysis 

applies to the CISWI Rule as applies to the Boilers 

Rules. The EPA adopted a reasonable interpretation of the 

CAA when it excluded periods of malfunction from its 

calculations of achievability given that malfunction periods 

are by their very nature unpredictable in terms of their effect 

on emissions. The EPA’s decision to account for 

malfunctions in its discretion is likewise a reasonable 

interpretation of 42 U.S.C. § 7412(d)(2) and (3). 

 

For these reasons, we reject the Industry Petitioners’ 

challenges to the EPA’s regulatory choices with regard to 

periods of startup, shutdown, and malfunction. 

B. THE POLLUTANT-BY-POLLUTANT APPROACH

The EPA must look to the performance of the best major 

boilers and CISWI incinerators when setting MACT floors for 

a pollutant. As described above, for new units, the EPA must 

set floors at the level achieved by the best similar unit in each 

subcategory. For existing units, the Agency must set floors at 

the level achieved by the best 12 per cent of similar units in 

each subcategory. 42 U.S.C. §§ 7412(d)(3)(A), 7429(a)(2). 

As a result, the EPA had to identify the best performing units 

in each subcategory when setting the MACT floors for the 

Major Boilers and CISWI Rules. But the EPA often could not 

identify a single unit or set of units that controlled all HAPs 

better than the other units in the subcategory. Instead, the 

EPA sometimes found that a unit might rank among the best 

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39 

in its subcategory at controlling emissions of one HAP, but 

among the worst at controlling emissions of a different HAP. 

To address this problem, the EPA adopted a “pollutantby-pollutant” approach in setting the MACT floors: instead of 

identifying the unit or units that best controlled all HAPs in 

the aggregate, the EPA used one unit or set of units to set the 

MACT floor for, e.g., PM, and used a different unit or set of 

units to set the MACT floor for, e.g., HCl. See 2011 Major 

Boilers Rule, 76 Fed. Reg. at 15,621-23; 2011 CISWI Rule, 

76 Fed. Reg. at 15,720-21. For at least two subcategories of 

major boilers—new heavy oil-fired units and existing stoker 

coal-fired units—the EPA’s pollutant-by-pollutant approach 

resulted in MACT floors that no unit in the subcategory had 

achieved in toto. Similarly, for small, remote incinerators 

(SRIs), the approach resulted in standards for existing units 

that only two of the 28 SRI units had met in toto, and 

standards for new units that no existing SRI had met in toto. 

The Industry Petitioners challenge the EPA’s use of the 

pollutant-by-pollutant approach. According to the Industry 

Petitioners, the CAA’s plain language requires the Agency to 

identify the best overall unit or set of units—not the best unit 

or set of units for a particular pollutant—in each subcategory 

when setting MACT floors. They further claim the EPA’s 

pollutant-by-pollutant approach was unreasonable with regard 

to SRIs because it resulted in a set of emission standards that 

no single unit in the subcategory had achieved in practice. 

We disagree, and conclude that the EPA’s pollutant-bypollutant approach is a reasonable interpretation and 

application of the statute. 

For the purposes of this challenge, the MACT floor 

provisions for major boilers and CISWI units are identical. 

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Under both provisions, the EPA must set emission standards 

for new units based on “the emissions control that is achieved 

in practice by the best controlled similar unit, as determined 

by the Administrator.” 42 U.S.C. § 7429(a)(2) (CISWI); see 

also id. § 7412(d)(3) (major boilers). For existing units, the 

MACT floor is based on “the average emissions limitation 

achieved by the best performing 12 percent of units in the 

category.” Id. § 7429(a)(2) (CISWI); see also id. 

§ 7412(d)(3)(A) (major boilers). 

The Industry Petitioners claim this language 

unambiguously forecloses the EPA’s pollutant-by-pollutant 

approach. For new units, they assert, the statute requires the 

EPA to find the single unit that performs best overall and use 

this unit—and only this unit—to set standards for all 

regulated pollutants. For example, if Incinerator 3 were 

deemed the best overall performer in a subcategory, then the 

EPA would use Incinerator 3’s emissions levels to set 

standards for PM, CO, and each of the other regulated 

pollutants. This would be true even if Incinerator 1 in the 

same subcategory had lower CO emissions and Incinerator 2 

had lower PM emissions. The Industry Petitioners also make 

this argument for existing sources. For these units, under 

their interpretation, the mandate to identify the “best 

performing 12 percent of units” required the EPA to use data 

from the 12 per cent of sources with the lowest overall 

emissions in the subcategory. In short, the Industry 

Petitioners argue that the best “unit” referred to by the 

provision cannot be a “hypothetical composite” of multiple 

units that result in standards for new units that no actual unit 

has met in practice with regard to every pollutant, or 

standards for existing units that 12 per cent of actual units 

have not met with regard to every pollutant. 

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The Industry Petitioners read too much into the statutory 

language. It is true that the statute requires the EPA to base 

MACT standards on what is “achieved” by the best “unit” or 

“12 percent of units.” But, as the EPA argues, the statute says 

nothing about how the Agency should determine which units 

are the best. Cf. Sierra Club v. EPA, 167 F.3d 658, 661 (D.C. 

Cir. 1999) (noting that section 7429(a) “on its own says 

nothing about how the performance of the best units is to be 

calculated”). Both the industry-favored method of choosing 

the best overall unit and the EPA’s method of choosing the 

best unit as to each particular pollutant facially comport with 

the statute’s mandate to determine which units are best. 

Because the statute is ambiguous as to how the EPA should 

identify those units, we must defer to the Agency’s choice so 

long as it is reasonable. See Sierra Club I, 353 F.3d at 990. 

Here, the EPA’s choice is reasonable. The statute 

provides that emission standards shall reflect “the maximum 

degree of reduction in emissions of [regulated pollutants] that 

the Administrator . . . determines is achievable for new or 

existing units in each category.” 42 U.S.C. § 7429(a)(2); see 

also id. § 7412(d)(2). It then provides that the “degree of 

reduction in emissions that is deemed achievable for new 

units in a category shall not be less stringent than the 

emissions control that is achieved in practice by the best 

controlled similar unit, as determined by the Administrator.” 

Id. § 7429(a)(2); see also id. § 7412(d)(3). Reading these 

provisions together, they support a pollutant-by-pollutant 

approach. The “best controlled similar unit” language does 

not exist in a vacuum; rather, it exists to measure the “degree 

of reduction in emissions that is deemed achievable.” Id.

§ 7429(a)(2); see also id. § 7412(d)(3). That “reduction in 

emissions” is the reduction in emissions of each pollutant 

listed in sections 7429(a)(4) and 7412(b)(1). The EPA’s 

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approach to setting standards on a pollutant-by-pollutant basis 

thus comfortably fits within this statutory scheme. 

Moreover, the Industry Petitioners have not explained 

how their preferred approach would better comport with the 

statute. Were the EPA required to determine which units 

perform best “overall,” we see at least two possibilities for 

how it could do so: First, the EPA could calculate a unit’s 

average emissions for each pollutant in consistent units of 

measurement, add these emissions together, and then choose 

the unit with the smallest overall sum in each subcategory. 

But this approach could produce arbitrary results, because the 

“best performing” overall unit might emit unusually low 

quantities of some pollutants and unusually high quantities of 

others. This would mean the emission standards for some 

pollutants would be lenient while others would be stringent, 

with no principled reason for the difference. Alternatively, 

the Agency could identify which source is best overall based 

on which emits the lowest level of the riskiest pollutants. But 

this approach would require the Agency to rank pollutants’ 

relative risks without any congressional guidance on how to 

do so. This approach would also contravene our previous 

understanding of the congressional intent behind the MACT 

floor provisions. As we have explained, the MACT floors 

“are to be based not on an assessment of the risks posed by 

[pollutants], but instead on the maximum achievable control 

technology (MACT) for sources in each category.” Sierra 

Club I, 353 F.3d at 980. 

The Industry Petitioners nevertheless argue that the 

CAA’s legislative history supports their preferred approach. 

In particular, they point to the floor comments of Senator 

Durenberger discussing the potential impact on MACT floors 

of mutually incompatible control technologies. 136 Cong. 

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Rec. S17,238 (daily ed. Oct. 26, 1990) (statement of Senator 

Durenberger). Mutually incompatible control technologies 

cannot be used at the same time and therefore present 

regulators with a dilemma. For example, say Technology 1 

and Technology 2 cannot be used together. If Technology 1 

is better at reducing PM than Technology 2, and Technology 

2 is better at reducing CO than Technology 1, the EPA would 

have to choose which of the two technologies to factor into 

emission standards. In such situations, Senator Durenberger 

anticipated that the “EPA should judge MACT to be the 

technology which best benefits human health and the 

environment on the whole.” Id. The Industry Petitioners 

argue this statement demonstrates that Congress intended the 

EPA to make an overall determination of which units are the 

best performing “on the whole.” 

Senator Durenberger’s statement does not support this 

broad principle. The statement merely explains that, where 

two technologies cannot be used together, the EPA should 

base MACT standards on the technology it considers best 

overall. Here, the Industry Petitioners do not identify any 

relevant control technologies that are mutually incompatible. 

Indeed, the EPA found in the CISWI Rule that “there is no 

technical reason why [the] air pollution control systems 

cannot be combined.” 2011 CISWI Rule, 76 Fed. Reg. at 

15,721; see also 2011 Major Boilers Rule, 76 Fed. Reg. at 

15,623 (“All available data for boilers and process heaters 

indicate that there is no technical problem achieving the floor 

levels contained in this final rule for each HAP 

simultaneously, using the MACT floor technology.”). There 

is thus no reason to believe that the EPA’s current MACT 

floor standards cannot be achieved. Instead, the Industry 

Petitioners merely insist that no units currently meet the 

EPA’s new unit standards with regard to every regulated 

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44 

pollutant in certain subcategories, and only a few sources 

meet all of the standards for existing units in the same 

subcategories. But, if the statute permits the EPA to 

determine which units are best on a pollutant-by-pollutant 

basis—and it does—then the EPA’s choice to adopt that 

approach does not become unlawful merely because few or no 

units have achieved those standards for all pollutants.

Finally, the Industry Petitioners argue that even if the 

pollutant-by-pollutant approach is reasonable in some 

circumstances, it is arbitrary and capricious as applied to 

certain SRIs because it exacerbates certain problems posed by 

the “batch” nature of SRIs. As explained at infra § III.E, 

SRIs burn waste in small batches. According to the 

Petitioners, this means that the SRIs that the EPA identified as 

best performing were, in reality, burning cleaner waste at the 

time emissions testing was done; they were not actually better 

than other units at removing or destroying waste. The 

pollutant-by-pollutant approach, the Industry Petitioners 

argue, “simply captures the results from units that happened 

to be burning wastes with low levels of that particular 

pollutant during testing,” and this reality makes it harder for 

SRI units to meet emission standards for all pollutants at the 

same time. No. 11-1125 Indus. Pet’rs’ Reply Br. 8 (emphasis 

omitted). 

This argument fails because the Industry Petitioners have 

not demonstrated that the Agency considered impermissible 

factors, failed “to consider important aspect[s] of the 

problem,” or offered an unreasonable explanation for its 

decision when setting the MACT floors for SRIs. See State 

Farm, 463 U.S. at 43. Rather, their argument is a back-door 

attempt to challenge the Agency’s alleged failure to consider 

waste inputs, which we reject below at infra § III.E. 

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Petitioners have also not shown that it is infeasible for the SRI 

units to meet the MACT floor standards or that any individual 

pollutant standard was not achieved in practice by an existing 

SRI unit. They merely assert, without evidence, that no 

existing unit burning high sulfur garbage can match the SO2

performance achieved by the unit the EPA used to set SO2

standards because that latter unit was burning low sulfur 

waste at the time of the emissions testing. But MACT floors 

are not unreasonable simply because they are difficult to 

achieve in practice. As such, we find the EPA’s pollutant-bypollutant approach to be a reasonable interpretation and 

application of the statute, and deny the Industry Petitioners’ 

challenge to the EPA’s use of this approach.

C. THE ENERGY-ASSESSMENT REQUIREMENT

The Major Boilers Rule and the Area Boilers Rule 

generally require sources with existing boilers to perform a 

one-time energy assessment. In the assessment, facilities 

must “identify energy conservation measures”—such as 

“process changes or other modifications to the facility”—

“that can be implemented to reduce the facility energy 

demand,” thereby “reduc[ing] fuel use.” 2011 Area Boilers 

Rule, 76 Fed. Reg. at 15,573; see also 2011 Major Boilers 

Rule, 76 Fed. Reg. at 15,632. While facilities must conduct 

the assessment, they need not implement its conclusions. See 

2011 Area Boilers Rule, 76 Fed. Reg. at 15,573; 2011 Major 

Boilers Rule, 76 Fed. Reg. at 15,632. 

The logic behind the assessment is straightforward. 

Boilers produce HAP emissions when fuel is combusted. 

Less combustion means fewer emissions. The EPA primarily 

justified the assessment as a beyond-the-floor MACT 

requirement under section 7412(d)(2). See 2011 Area Boilers 

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Rule, 76 Fed. Reg. at 15,573; 2011 Major Boilers Rule, 76 

Fed. Reg. at 15,632. With respect to certain biomass and oilfired boilers located at area sources, the assessment was 

justified as a GACT management practice under 

section 7412(d)(5). See 2011 Area Boilers Rule, 76 Fed. Reg. 

at 15,567. 

Industry Petitioners raise three principal challenges to the 

energy-assessment requirement, none of which have purchase. 

The first challenge claims that the energy assessment 

regulates aspects of facilities that are off limits to the EPA—

namely, the energy needs supplied by regulated boilers. 

Petitioners point to the language of the CAA, which requires 

the EPA to “list . . . categories and subcategories of major 

sources and area sources” of enumerated air pollutants. 42

U.S.C. § 7412(c)(1). “For the categories and subcategories 

the Administrator lists, the Administrator” must set 

“emissions standards under” section 7412(d). Id.

§ 7412(c)(2). As relevant here, the EPA defined the source 

categories to include “industrial boilers and commercial and 

institutional boilers.” 2011 Area Boilers Rule, 76 Fed. Reg. at 

15,557; 2011 Major Boilers Rule, 76 Fed. Reg. at 15,608. To 

the extent the assessment concerns parts of the facility other 

than the boiler itself, the Industry Petitioners claim it exceeds 

the EPA’s authority. 

The Industry Petitioners misapprehend both the scope of 

the assessment and the CAA. The assessment requires 

facilities to evaluate energy systems “located on the site of the 

affected boiler,” including “[p]rocess heating[,] compressed 

air systems[,] . . . facility heating, ventilation, and air 

conditioning systems,” and “[o]ther systems that use steam, 

hot water, process heat, or electricity, provided by the affected 

boiler.” 40 C.F.R. § 63.11237; see id. § 63.7575. Based on 

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that evaluation, facilities must compile a “comprehensive 

report detailing the ways to improve efficiency, the cost of 

specific improvements, [anticipated] benefits, and the time 

frame for recouping those investments.” 40 C.F.R. pt. 63, 

subpt. JJJJJJ tbl.2; id. pt. 63, subpt. DDDDD tbl.3. 

Contrary to the Industry Petitioners’ argument, the EPA 

has not “regulate[d] virtually every piece of equipment at all 

affected facilities.” No. 11-1141 Indus. Pet’rs’ Br. 19. Only 

“energy use systems” that “us[e] energy clearly produced by 

affected boilers” must be evaluated; facilities need not review 

the “total aggregation of all individual energy using segments 

of a facility.” 2013 Area Boilers Rule, 78 Fed. Reg. at 7,493 

(emphasis added); see also 2013 Major Boilers Rule, 78 Fed. 

Reg. at 7,188. The assessment focuses on “discrete segments 

of a facility,” such as “production area[s] or building[s]” 

associated with a particular boiler. 2013 Area Boilers Rule, 

78 Fed. Reg. at 7,493; see 2013 Major Boilers Rule, 78 Fed. 

Reg. at 7,188. Energy requirements satisfied by other 

sources—not by a HAP-emitting boiler—fall outside of that 

mandate. See 2011 Area Boilers Rule, 76 Fed. Reg. at 15,573 

(limiting the assessment to “specific portions of the source 

that directly affect emissions from the affected boiler”). And 

regulated facilities are under no obligation to implement the 

results they reach. In essence, rather than setting inflexible 

and generally applicable beyond-the-floor numeric limits, the 

EPA required facilities to take stock of the actual energy 

demands placed on their boilers. By reducing energy 

demands and associated fuel consumption, facilities could 

reduce HAP emissions. That requirement is more measured 

than the Industry Petitioners contend. 

And that measured requirement falls within the EPA’s 

statutory authority. The CAA authorizes the EPA to regulate 

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“major sources and area sources” of HAPs, and to subdivide 

those sources into categories and subcategories. 42 U.S.C. 

§ 7412(c)(1), (c)(2). To Industry Petitioners, the authority to 

subdivide sources means the EPA may only regulate the 

narrowest applicable categorization—in this instance, 

commercial and industrial boilers. But the statute does not 

require so rigid a reading. While the EPA is permitted to 

subdivide sources, each subdivision remains a component of 

either a major or area “source.” Dividing sources into 

categories and subcategories does not make them any less of a 

“source” subject to the EPA’s regulation. 

For that reason, the EPA explained that the Rules reach, 

respectively, “[a]ny area source facility using a boiler,” 2011 

Area Boilers Rule, 76 Fed. Reg. at 15,555 (emphasis added), 

and “major source facilities having affected boilers or process 

heaters,” 2011 Major Boilers Rule, 76 Fed. Reg. at 15,613 

(emphasis added). Likewise, the regulations implementing 

the energy assessment requirement apply to those who “own 

or operate an existing affected boiler,” not merely to the 

boiler itself. 40 C.F.R. § 63.11214(c); see id. § 63.7485. 

Going further, the relevant part of the CFR applies, by its own 

terms, to the “owner or operator of any stationary source.” Id. 

§ 63.1(b)(1). 

The Congress’s definition of the terms major and area 

source supports this reading. At bottom, both terms refer to a 

“stationary source.” See 42 U.S.C. § 7412(a)(1), (a)(2). 

Stationary source, in turn, means “any building, structure, 

facility, or installation which emits or may emit any air 

pollutant.” Id. § 7411(a)(3). Against that backdrop, the Rules 

apply to any “building, structure, facility, or installation” that 

contains a boiler emitting the specified HAPs. The EPA’s 

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regulatory authority reaches the relevant stationary source, of 

which the boiler is part. 

 That the EPA may regulate stationary sources does not 

mean it may regulate every nook and cranny of those sources. 

The CAA directs its authority to the establishment of 

emission standards; it does not provide some general power to 

superintend the business processes of plants and 

manufacturing facilities. In this case, however, we have no 

occasion to parse the precise parameters of the EPA’s 

authority to regulate aspects of area sources. It is enough to 

conclude that the challenged energy assessment—which 

applies only to systems that “us[e] energy clearly produced by 

affected boilers”—falls within the EPA’s authority under the 

CAA. 2013 Area Boilers Rule, 78 Fed. Reg. at 7,493; 2013 

Major Boilers Rule, 78 Fed. Reg. at 7,188. 

In the remaining two challenges, the Industry Petitioners 

take issue with the EPA’s justification of the energy 

assessment as a beyond-the-floor MACT standard and a 

GACT management-practice standard. We reject both 

challenges. 

The assessment represents a valid beyond-the-floor 

MACT standard.13 As discussed, after the Agency sets the 

MACT floor, it must determine “whether stricter standards 

are ‘achievable,’” Nat’l Lime Ass’n, 233 F.3d at 629 (quoting 

42 U.S.C. § 7412(d)(2)), considering costs, “any non-air 

 13 In addition to challenging the assessment as a beyond-thefloor measure, the Industry Petitioners claim the assessment 

represents an invalid work-practice standard. But “[t]he energy 

assessment is not . . . a work practice standard, and EPA makes no 

claim that it is.” No. 11-1141 EPA Br. 47 n.9. Therefore, we 

decline to address that contention. 

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quality health and environmental impacts and energy 

requirements,” 42 US.C. § 7412(d)(2). These “measures, 

processes, methods, systems or techniques includ[e], but [are] 

not limited to, measures which— 

(A) reduce the volume of, or eliminate emissions 

of, such pollutants through process changes, 

substitution of materials or other 

modifications, . . . 

(D) are design, equipment, work practice, or 

operational standards . . . or 

(E) are a combination of the above. 

Id. The EPA primarily justified the energy assessment as a 

beyond-the-floor measure designed to identify “process 

changes or other modifications to the facility” that would 

reduce fuel use and thereby reduce hazardous emissions. 

2011 Area Boilers Rule, 76 Fed. Reg. at 15,573; 2011 Major 

Boilers Rule, 76 Fed. Reg. at 15,632. 

The Industry Petitioners argue that the EPA skipped a 

step, imposing the energy assessment as a beyond-the-floor 

measure without first setting a relevant MACT floor. That is 

incorrect. The EPA first set a numeric MACT emissions limit 

for the categories and subcategories of sources subject to the 

energy assessment. See 40 C.F.R. pt. 63, subpt. JJJJJ tbl.1; id.

pt. 63, subpt. DDDDD tbl.2. The energy assessment 

represents a step beyond that—a measure designed to 

discover energy efficiencies that, once implemented, could 

decrease emissions below the floor level. 

Before setting a beyond-the-floor measure, the EPA must 

consider whether it is “achievable” based on a number of 

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factors, among them cost, “non-air quality health and 

environmental impacts and energy requirements.” 42 US.C. 

§ 7412(d)(2). The EPA did so here. To begin, the EPA 

adequately considered costs. In the EPA’s estimation, “[t]he 

one-time cost of an energy assessment ranges from $2500 to 

$55,000 depending on the size of the facility.” 2010 Proposed 

Area Boilers Rule, 75 Fed. Reg. at 31,907; National Emission 

Standards for Hazardous Air Pollutants for Major Sources: 

Industrial, Commercial, and Institutional Boilers and Process 

Heaters (2010 Proposed Major Boilers Rule), 75 Fed. Reg. 

32,006, 32,026 (June 4, 2010). Because saving fuel saves 

money, common sense suggested that sources would often 

find the energy assessment “cost-effective” to implement. 

2011 Area Boilers Rule, 76 Fed. Reg. at 15,568 (“By 

definition, any emission reduction [achieved as a result of the 

energy assessment] would be cost effective or else it would 

not be implemented.”); see also 2011 Major Boilers Rule, 76 

Fed. Reg. at 15,633. 

In addition to costs, the EPA considered non-air quality 

health and environmental impacts in general terms, 

concluding that “improving energy efficiency reduces 

negative impacts on the environment.” 2010 Proposed Area 

Boilers Rule, 75 Fed. Reg. at 31,907; 2010 Proposed Major 

Boilers Rule, 75 Fed. Reg. at 32,026. Given the nature of the 

assessment, the EPA’s somewhat terse analysis of health and 

environmental impacts suffices. Performing the assessment 

involves rudimentary tasks—examining the boiler and 

associated energy systems and drafting a report—that do not 

impose meaningful health or environmental impacts. The 

same holds for the EPA’s consideration of energy use 

requirements. Facilities would expend very little energy in 

conducting the one-time assessment, and could conserve 

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energy by implementing the results. The assessment therefore 

represents a lawful beyond-the-floor measure. 

We also find that the assessment is a valid GACT 

management practice. With respect to area sources, the EPA 

has discretion to require the use of “generally available 

control technologies or management practices . . . to reduce 

emissions of hazardous air pollutants.” 42 U.S.C. 

§ 7412(d)(5). The EPA justified the energy assessment as a 

GACT management practice for oil- and biomass-fired 

boilers. See 2011 Area Boilers Rule, 76 Fed. Reg. at 15,567. 

The Industry Petitioners challenge that justification, 

claiming the energy assessment—which does not require 

implementation—cannot “reduce emissions of hazardous air 

pollutants.” 42 U.S.C. § 7412(d)(5). We disagree. The EPA 

did not need to make implementation mandatory to make the 

assessment lawful. Under the CAA, the EPA may sometimes 

act with a soft touch, rather than a firm hand. Here, the EPA 

selected a soft touch, requiring an assessment but not 

implementation. It was not unreasonable for the EPA to 

conclude, “after considering the structure of the requirement, 

the incentives it presents, and the likely behavior of 

sources, . . . that sources will find it cost-effective to 

implement the conservation measures identified in the energy 

assessment.” 2011 Area Boilers Rule, 76 Fed. Reg. at 15,573. 

If the results were implemented, HAP emissions would be 

reduced. For present purposes, that is enough. 

For those reasons, we reject the Industry Petitioners’ 

challenges to the energy-assessment requirement. 

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D. RECORDKEEPING REQUIREMENT FOR CISWI UNITS

Section 7429 regulates combustion units that burn solid 

waste; units that do not burn solid waste will generally be 

regulated under section 7412. RCRA defines the term “solid 

waste” to mean (in part) “discarded material . . . resulting 

from industrial [or] commercial . . . operations.” 42 U.S.C. 

§ 6903(27); see id. § 7429(g)(6) (directing that “solid waste” 

carry “the meanings established by the Administrator 

pursuant to” RCRA). On the same day the EPA issued a rule 

setting emission standards for CISWI, it issued a separate rule 

fleshing out the meaning of solid waste in the context of 

combustion units. See NHSM Rule, 76 Fed. Reg. at 15,456. 

The NHSM Rule generally provides that “non-hazardous 

secondary materials that are combusted are solid wastes,”14

subject to several exceptions and exemptions. 40 C.F.R. 

§ 241.3(a). Among the exceptions, non-hazardous secondary 

materials that meet certain “legitimacy” criteria do not qualify 

as solid waste. See id. § 241.3(b), (d). Source owners and 

operators may also seek a finding from the EPA that 

particular materials do not constitute solid waste when 

combusted by a third party. Id. § 241.3(c). And the rule 

exempts altogether a variety of materials from the definition 

of solid waste, including “traditional fuels.” Id. § 241.2. 

The NHSM Rule is self-implementing: each source 

owner or operator must determine whether combusted 

materials meet the definition of solid waste. See 2011 CISWI 

Rule, 76 Fed. Reg. at 15,740. To ensure that owners and 

 14 The NHSM Rule defines non-hazardous secondary material 

to “mean[] a secondary material that, when discarded, would not be 

identified as a hazardous waste.” 40 C.F.R. § 241.2. 

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operators “review and apply” the NHSM Rule and its 

exceptions, the EPA issued strict recordkeeping requirements. 

Id. Owners and operators who determine the secondary 

materials they combust are not solid waste must “keep a 

record” justifying that decision. 40 C.F.R. § 60.2175(v). 

Failing to file records carries consequences. For units 

combusting discarded material other than traditional fuels, the 

failure to “keep and produce records” results in the 

determination that “the operating unit is a CISWI unit.” Id.

§§ 60.2265, 60.2875 (containing an identical provision). 

Industry Petitioners challenge this last provision of the 

CISWI Rule.15 They argue that the EPA cannot automatically 

treat units that fail to keep certain paperwork as CISWI units. 

Section 7429 permits regulation of “solid waste incineration 

units”—not units whose owners fail to file paperwork. As a 

result, the Industry Petitioners ask this court to invalidate the 

regulatory provision as exceeding the EPA’s statutory 

authority.16 

We decline the invitation. At Chevron’s first step, we 

find that “Congress did not speak directly, let alone clearly, to 

 15 In their reply brief, the Industry Petitioners clarify that they 

do not challenge the EPA’s authority to require sources to keep 

records. 

16 The Industry Petitioners also argue the EPA arbitrarily 

failed to provide sufficient notice of the recordkeeping 

presumption. We disagree. The Industry Petitioners had sufficient 

notice of the CISWI Rule, which was promulgated after notice and 

comment and “give[s] fair warning of the conduct it prohibits.” 

Gen. Elec. Co. v. EPA, 53 F.3d 1324, 1328 (D.C. Cir. 1995) 

(quoting Gates & Fox Co. v. OSHRC, 790 F.2d 154, 156 (D.C. Cir. 

1986)). 

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this issue.” Am. Chem. Council v. EPA, 337 F.3d 1060, 1064 

(D.C. Cir. 2003). Section 7429 regulates “solid waste 

incineration units,” a phrase that Congress defined “plainly 

and broadly to include ‘a distinct operating unit of any facility 

which combusts any solid waste material from commercial or 

industrial establishments or the general public.’” NRDC I, 

489 F.3d at 1257 (emphasis omitted) (quoting 42 U.S.C. 

§ 7429(g)(1)). In NRDC I, we vacated an earlier iteration of 

the CISWI Rule that narrowed the scope of that definition 

beyond what its language would bear. See id. at 1257-58. 

When the Congress commanded the EPA to regulate units 

that burn “any” solid waste, the Congress meant what it said. 

See id.

In this case, the EPA included within the revised CISWI 

Rule a presumption designed to enforce the Congress’s 

command. Section 7429 nowhere addresses whether the EPA 

may establish presumptions to ensure its regulations reach all 

sources burning solid waste. At the same time, the Congress 

plainly intended the EPA to regulate sources burning “any” 

solid waste, a goal presumably advanced by the 

recordkeeping presumption. See id. Against that backdrop, 

we cannot conclude that the presumption offends the text or 

purpose of section 7429. 

Moving to Chevron’s second step, we conclude the 

recordkeeping presumption is reasonable. In American 

Chemistry Council, we upheld a regulation issued under 

RCRA defining hazardous waste to include any mixture or 

derivative of hazardous substances. See 337 F.3d at 1064-65. 

“[B]ecause many mixtures of and derivatives from hazardous 

wastes are themselves hazardous, it [was] reasonable for the 

EPA to assume that all such mixtures and derivatives are 

hazardous until shown otherwise.” Id. at 1065. In that 

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context, it made good sense for the EPA to “[p]lac[e] the 

burden upon the regulated entity” to show that a given 

substance lacked “hazardous characteristic[s].” Id. 

 Similar reasoning applies here. The EPA crafted the 

presumption to reach sources likely to be burning solid waste, 

namely, those burning discarded materials other than 

traditional fuels. See 42 U.S.C. § 6903(27) (defining “solid 

waste” to include, among other things, “discarded material”); 

40 C.F.R. § 241.2 (exempting traditional fuels, defined as 

“materials that are produced as fuels . . . that have not been 

discarded,” from the definition of solid waste). Such sources 

are subject to strict recordkeeping requirements. See 40 

C.F.R. § 60.2175(v). Within those confines, placing the 

burden on unit operators who have the mandatory obligation 

and the information to establish their non-regulable status is 

reasonable. Cf. Am. Chem. Council, 337 F.3d at 1065. 

 There is, however, a difference between the presumption 

in this case and the one we upheld in American Chemistry 

Council. The CISWI recordkeeping presumption appears to 

turn on the failure to file paperwork, rather than the presence 

of a regulated substance. However broadly the Congress 

defined “solid waste incineration unit” in section 7429, the 

Congress did not allow for the regulation of non-waste 

burning sources—even when those sources fail to file 

paperwork. Indeed, had the EPA attempted to regulate 

sources based purely on a failure to file paperwork, we may 

well have reached a different conclusion. 

But the CISWI presumption does not stretch so far. As 

explained, the presumption depends on factors beyond the 

mere failure to keep records. Sources subject to the 

presumption burn materials likely to qualify as solid waste, 

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and must satisfy demanding recordkeeping requirements. The 

EPA acted reasonably when it presumed such sources were 

burning solid waste. 

Despite the provision’s narrow reach, the Industry 

Petitioners fear it will sweep up sources not burning solid 

waste. To the extent that possibility exists, sources 

wrongfully regulated as CISWI have multiple forms of 

recourse. Most obviously, sources can prepare and file the 

records they were already required to make under 40 C.F.R. 

§ 60.2175(v). They can also avail themselves of procedures 

designed to identify non-waste materials in 40 C.F.R. § 241.3. 

The existence of these safety valves calms concerns that the 

presumption will regulate non-waste burning sources. 

 We therefore reject the Industry Petitioners’ challenges 

to the recordkeeping presumption.17 

E. WASTE-STREAM VARIANCE FOR SRI UNITS

The EPA regulated SRIs as a subcategory in the CISWI 

Rule. See Memorandum from Eastern Research Group, Inc., 

to Toni Jones, U.S. Environmental Protection Agency, CISWI 

Emission Limit Calculations for Existing and New Sources 

for the Reconsideration Final Rule (Jones Mem.) (Nov. 16, 

2012) (No. 11-1125 J.A. 1159, 1162). There are 28 SRI units, 

 17 The Industry Petitioners also contend that the CISWI Rule 

functions as a form of injunctive relief in violation of 42 U.S.C. 

§ 7413(a). That is incorrect. The provision is neither styled nor 

operated as a form of injunctive relief. Cf. 42 U.S.C. § 7413(a) 

(permitting the Administrator to issue, among other forms of relief, 

“an administrative penalty order” or “an order requiring [a person 

in violation of EPA regulations] to comply with such requirement 

or prohibition”). 

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all of which are located in Alaska, and the EPA had emissions 

data for nine of them. Id. As explained supra § I.B.3, the 

EPA used the pollutant-by-pollutant approach to establish 

MACT emission standards for these units. For new-unit 

standards, the EPA determined which of the nine units had the 

lowest emissions for a particular pollutant and set the MACT 

floor for that pollutant at the level achieved by the identified 

unit. See 42 U.S.C. § 7429(a)(2) (explaining that MACT 

floors for new units must be set at “the emissions control that 

is achieved in practice by the best controlled similar unit”). 

When setting MACT floors for existing units, the EPA had to 

calculate the average level of emissions achieved by the best 

performing 12 per cent of units. See id. It therefore 

determined which four sources had the lowest emissions for a 

given pollutant and set the emissions standard for that 

pollutant at the average level achieved by those four units. 

The Industry Petitioners argue that the EPA’s approach 

was unlawful because it failed to account for the unique role 

that waste inputs play in emissions from SRIs. Unlike larger 

incinerators, SRIs burn small batches of waste at a time. Some 

batches include cleaner waste, such as wood and cardboard, 

while others include waste, such as sewage, that generates 

large quantities of SO2 and other pollutants. Moreover, 

existing SRIs cannot use certain “end-of-stack” control 

technologies like wet scrubbers due to the Alaskan climate. 

The Industry Petitioners thus contend that emissions from 

SRIs are more closely tied to waste input than are emissions 

from other types of incinerators. This difference, they assert, 

required the EPA to take into account, when determining 

which SRI units were best performing for MACT floor 

purposes, the kind of waste an SRI unit was burning at the 

time of testing. Because the Agency did not do so, the 

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Industry Petitioners contend the MACT standards for SRIs are 

arbitrary and capricious. We disagree. 

To support their challenge, the Industry Petitioners 

advance two arguments, neither of which has merit. 

Petitioners first point to section 7429(a)(3), which directs the 

EPA to base emission standards on “methods and 

technologies for removal or destruction of pollutants before, 

during, or after combustion.” 42 U.S.C. § 7429(a)(3). 

According to the Industry Petitioners, this language requires 

the EPA to identify best performing units for MACT purposes 

by considering which units are best at removing or destroying 

pollutants. The Industry Petitioners assert that the Agency did 

not do this. Instead, they contend, the EPA set standards 

without regard to whether that unit happened to be burning 

cleaner waste. And, according to the Industry Petitioners, 

remote incinerators in Alaska cannot control their waste 

inputs because the core purpose of SRIs is to burn waste that 

is impracticably far from municipal landfills. The fact that 

emissions levels varied dramatically during test runs for the 

SRI units, they claim, is thus the result of random variance in 

the type of waste the unit was combusting, rather than any 

“method” or “technology” aimed at “removing” or 

“destroying” pollutants. 

The EPA responds that the approach it adopted for SRIs 

complies with section 7429(a)(3) because “waste 

segregation”—that is, diverting dirtier waste to landfills and 

burning only cleaner waste—is a “method . . . for removal . . . 

of pollutants before . . . combustion.” See 42 U.S.C. 

§ 7429(a)(3). In fact, during notice and comment, the EPA 

estimated that many SRIs would choose to comply with the 

MACT standards by segregating their waste instead of by 

installing expensive control technologies. See Jones Mem. 

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The Agency also determined that waste segregation was 

possible for SRIs because their waste often contained 

materials that could be recycled. Id. Finally, the Agency 

factored in any additional variance in emissions from these 

units by calculating the MACT floors according to the UPL 

formula described at supra §§ I.B.1.a, IV.C. For these 

reasons, the Agency contends, it did not need to consider 

further any variation in emissions that might be caused by 

differences in waste inputs for SRIs. 

The EPA has the better argument, based on both text and 

precedent. Textually, waste segregation plainly can be a 

“method[]” for “removal” of pollutants “before” combustion. 

See 42 U.S.C. § 7429(a)(3). Accordingly, the EPA, when 

setting MACT floors, could not have looked solely to 

technologies used to reduce emissions during combustion. 

Accord Sierra Club v. EPA (Sierra Club II), 479 F.3d 875, 

883 (D.C. Cir. 2007) (per curiam). Instead, the plain 

language of section 7429(a)(3) requires the Agency to 

consider whether emission reductions can be achieved by 

non-combustion-related controls such as using cleaner fuels 

or waste inputs. Accord id. The statute supports the approach 

that the Agency took here. 

Our holding in Sierra Club II confirms that our 

conclusion is correct. In that case, the EPA had 

acknowledged that kilns emitted lower levels of pollutants 

when burning cleaner clay but nevertheless based MACT 

standards only on the emission reductions achieved by control 

technology during the combustion process. Id. at 882. The 

Agency explained that clean clay existed only in certain areas 

and that transportation of the clay over long distances was 

impractical. Id. The EPA therefore considered only those 

emission reductions that were attributable to “deliberate steps 

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kiln operators [took] to reduce emissions rather than to the 

‘happenstance’ of being located near cleaner clay.” Id. at 

883. But we rejected that approach, finding that “the Clean 

Air Act requires neither an intentional action nor a deliberate 

strategy to reduce emissions.” Id. Instead, where “nontechnology factors” affect emission levels, we held the EPA 

must consider those effects when setting MACT floors. Id. 

Applying that same reasoning, the EPA acted reasonably 

when it decided to consider the emissions reduction that could 

be achieved by waste segregation in SRI units before 

combustion. This is true even if an element of 

“happenstance” plays into an SRI unit’s ability to segregate 

its waste. And, had the EPA instead determined that the best 

performers were those SRI units that most effectively reduced 

pollutants only during combustion, as the Industry Petitioners 

suggest, the resulting MACT standards may have run afoul of 

our holding in Sierra Club II. We cannot, as a result, find the 

Agency’s choice to avoid that outcome unreasonable. 

The Industry Petitioners’ second argument also comes up 

short. According to Petitioners, the EPA selected the best 

performers for SRIs merely because those units happened to 

be burning batches of cleaner waste at the time of the 

emissions test. They claim this happenstance resulted in test 

data that did not reasonably estimate the typical performance 

of the units, and thus misidentified the best performers. See 

Cement Kiln, 255 F.3d at 862 (finding that although the EPA 

has authority to estimate which units perform best, its 

methodology must “provide[] an accurate picture of the 

relevant sources’ actual performance”). Petitioners further 

argue that the Agency’s use of the UPL method to account for 

variability did not fix this problem because the EPA applies 

that method only after identifying the best performers.

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If the record supported this argument, it might well be 

persuasive; in NACWA, we accepted a similar contention that 

the EPA’s dataset for determining MACT floors must fairly 

represent a unit’s typical performance. See 734 F.3d at 1146. 

But the record here does not support the Industry Petitioners’ 

position. None of the evidence on which Petitioners rely can 

bear the weight they would have us place on it. 

First, Petitioners cite evidence indicating that XTO 

Energy, which operates the incinerator that the EPA deemed 

the best performer for SO2, was burning low-sulfur “waste 

wood, cardboard, and oily waste” during the relevant test 

runs. See ConocoPhillips Co., Comment on EPA’s Proposed 

National Emission Standards for Hazardous Air Pollutants, 

EPA-HQ-OAR-2003-0119 (Feb. 12, 2012) (No. 11-1125 J.A. 

1036). But the record does not show that the resulting test 

data were unrepresentative of XTO’s typical performance 

because the record says nothing about what XTO typically 

burns. Id. 

Second, Petitioners note that Drift River, the unit the 

EPA deemed the worst performer for SO2, had emissions 

results similar to XTO Energy’s when burning low-sulfur 

waste, but results over 1,000 times higher when burning highsulfur waste. See id. (No. 11-1125 J.A. 1032-33). But again, 

the record does not say anything about the type of waste Drift 

River typically burns or its sulfur content; it merely 

demonstrates that the unit’s test results varied greatly from 

one run to the next. See id. 

Third, Petitioners point to additional test data they 

provided for the Kuparuk unit, a source that met the EPA’s 

MACT standards for NOx. See id. (No. 11-1125 J.A. 1017, 

1027-28). They claim this data shows that the Kuparuk unit 

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“consistently” emits NOx levels exceeding that standard when 

burning sewage sludge. Id. This claim is both factually 

untrue—as the data reveals exceedances on only one day—

and says nothing about whether the test data that the EPA 

used was representative of Kuparuk’s typical performance. 

Id. 

Instead, the record supports the EPA’s assertion that it 

gave Petitioners “multiple opportunities” to present data on 

the variability of waste streams for SRIs, but Petitioners never 

provided a reasonable empirical basis upon which the Agency 

could adjust the MACT standards due to this variability. The 

Industry Petitioners have thus not met their burden to show 

that the EPA’s test data was unrepresentative of SRI units’ 

actual or typical performance. 

In sum, no record evidence suggests that the current SRI 

emission standards are not achievable. The Industry 

Petitioners instead offer only general statements about the 

“small batch” nature of SRIs and the difficulty of using waste 

segregation or other controls in remote locations. These 

factors alone do not call into question the EPA’s assertion that 

controls such as waste segregation and technology upgrades 

are a feasible means of achieving compliance with the MACT 

floors that it established. See 2011 CISWI Rule, 76 Fed. Reg. 

at 15,730 (explaining that the MACT floors will require SRIs 

to employ “the best demonstrated technologies that are 

technologically feasible at these facilities,” such as 

afterburners and waste segregation, and noting that such 

controls “are sufficient to meet the MACT floor limits”). As 

a result, the EPA’s action here was reasonable; the Agency 

did not need to account further for waste stream variance in 

setting MACT floor standards for these SRI units. 

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F. CARBON MONOXIDE AS A SURROGATE

In setting MACT standards for major boilers, the EPA 

used carbon monoxide (CO) as a surrogate for several of the 

HAPs that the Agency was required to regulate. A surrogate 

is another chemical that stands in as a proxy for the regulated 

HAP when the EPA sets numeric emission standards. The 

EPA regulates the surrogate in order to regulate the HAP, 

sometimes because the HAP itself is too difficult to measure. 

We have previously approved the use of surrogates where 

the EPA’s choice of a surrogate for the HAP is “reasonable.” 

See, e.g., Nat’l Lime Ass’n, 233 F.3d at 637. Here, the 

Industry Petitioners claim the EPA’s use of CO as a surrogate 

was not reasonable for a particular type of emissions—

organic HAP emissions from coal-fired boilers—for two 

reasons. First, the EPA based the MACT standards on 

datasets that contained numerous “non-detects” for these 

organic HAPs. Second, the Agency failed to explain why it 

used CO as a surrogate for major boilers, but used workpractice standards to regulate similar emissions from other 

types of boilers in another rule. We find no merit in either 

argument and, accordingly, deny this challenge. 

The Industry Petitioners base their first argument on a 

deficiency in the EPA’s dataset for coal-fired boilers’ 

emissions—i.e., the dataset contained numerous “non-detects” 

for organic HAP emissions. A test result is considered a 

“non-detect” when emissions testing returns a value below 

that which the test methods are capable of detecting. 

According to the Industry Petitioners, multiple non-detects in 

a dataset demonstrate that it is “not feasible” to set a numeric 

emission standard for the affected HAP. As a result, they 

argue, the EPA should have set work-practice standards for 

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these HAPs under section 7412(h)(2), which permits the EPA 

to set such standards when it is “not feasible” to set a numeric 

emission standard. See 42 U.S.C. § 7412(h)(2). 

This argument fails because Petitioners have not 

explained how the non-detects here made setting numeric 

emissions “not feasible,” as that term is defined in the CAA. 

The CAA expresses a clear preference for MACT emission 

standards and limits the EPA’s ability to fashion more flexible 

work-practice standards. Compare id. § 7412(d)(3) 

(providing that emission standards “shall not be less 

stringent” than the MACT floor), with id. § 7412(h)(1) 

(permitting work-practice standards only if MACT standards 

are “not feasible”). To set a work-practice standard for these 

emissions, in fact, the EPA would need to find that it is 

infeasible to set a numeric standard for a particular HAP. Id. 

§ 7412(h)(1). And, as relevant here, the statute defines setting 

a numeric standard as “not feasible” where “the application of 

measurement methodology to a particular class of sources is 

not practicable due to technological and economic 

limitations.” Id. § 7412(h)(2)(B). 

This is a high bar and Petitioners have not demonstrated 

that the non-detects they have identified meet it. During 

notice and comment, the Agency reasonably explained that 

non-detects are present in many of its datasets because they 

are inherent to the imprecision associated with measuring 

boiler emissions. See, e.g., 2011 Major Boilers Rule, 76 Fed. 

Reg. at 15,623. The EPA’s scientific conclusion that its data 

was nevertheless sufficient to set numeric standards receives 

an “extreme degree of deference.” Kennecott Greens Creek 

Mining Co. v. Mine Safety & Health Admin., 476 F.3d 946, 

954-55 (D.C. Cir. 2007) (quotations omitted). And the 

Industry Petitioners never explain here why the particular 

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level of non-detects found in this dataset nevertheless made a 

numeric standard infeasible. Although the Industry 

Petitioners point to several comments asserting that no coalfired boiler could meet the current numeric standards in all 

HAP categories, these general comments say nothing about 

the relevant question under the statute: whether it was feasible 

to establish numeric standards for organic HAP emissions in 

light of the non-detects in the coal-fired boiler datasets. 

We also reject the Industry Petitioners’ second argument 

that the EPA needed to explain why it established workpractice standards for other types of boilers in the unrelated 

“Utility MATS” rule. We take an “every tub on its own 

bottom” approach to the EPA’s setting of emission standards 

pursuant to the CAA. Sierra Club I, 353 F.3d at 986. The 

adequacy of the underlying justification offered by the 

Agency is what matters in an arbitrary-and-capricious 

review—not what the Agency did on a different record 

concerning a different industry. Id. As a result, we cannot 

find that it was unreasonable for the EPA to use CO as a 

surrogate in setting numeric standards for coal-fired boilers on 

this basis. Nor can we find that the EPA was required on 

reconsideration to explain the discrepancy between its 

approach to organic HAP emissions in these two rules, as 

Petitioners assert. See id. at 987 (“EPA could have noted 

where the bases for its decision in this case differed from 

those with respect to other decisions in other cases, as was 

done in the EPA’s brief to this court . . . but such explanations 

are not required given the different contexts of the various 

rulemakings.”). 

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G. HEALTH-BASED EMISSIONS LIMITATION FOR HCL

In the Major Boiler Rule, the EPA chose not to exercise 

its discretion to create more lenient emission standards for 

hydrogen chloride (HCl) based on health. The Industry 

Petitioners challenge this decision as arbitrary and capricious 

because, they claim, the Agency considered impermissible 

factors in reaching the decision and departed from its previous 

position without adequate justification. We disagree and hold 

the EPA reasonably chose not to establish a health-based 

emissions limitation for HCl. 

The EPA generally must establish emission standards for 

all listed pollutants emitted from a source category based on 

what the best performing similar sources have achieved, i.e., 

the MACT floor. The Agency, however, may consider 

adopting alternative health-based emission standards—which 

are more lenient—for pollutants with an established health 

threshold. See 42 U.S.C. § 7412(d)(4). The statutory 

language permitting these alternative standards is 

discretionary, providing that “[w]ith respect to pollutants for 

which a health threshold has been established, the 

Administrator may consider such threshold level, with an 

ample margin of safety, when establishing emission standards 

under this subsection.” Id. (emphasis added). But, even if the 

EPA considers, in its discretion, a health-based emission 

standard, the statutory text nowhere requires that the EPA 

adopt a more lenient standard than the MACT floor. This 

provision thus allows, but does not require, the EPA to adopt 

a standard more lenient than the MACT floor, subject to two 

critical restrictions: the Agency must determine (1) that there 

is an established health threshold, and (2) that the established 

threshold would provide “an ample margin of safety.” 

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Using this authority, the EPA considered and adopted 

health-based emission standards for HCl in an earlier 

rulemaking for major boilers. See 2004 Boilers Rule, 69 Fed. 

Reg. at 55,240-41. At the time, the Agency based its decision 

on three key findings: a health threshold was established for 

HCl, adverse health effects were unlikely at emissions below 

that level, and low HCl emissions from major source boilers 

made HCl a “particularly well-suited” candidate for more 

lenient standards. Id. at 55,241. The EPA also said, however, 

that it was not embracing a general policy for HCl, but would 

instead “undertake in each individual rule to determine 

whether it is appropriate to exercise [the Agency’s] 

discretion” to adopt such standards. Id. We later vacated that 

rule without considering the merits of the EPA’s HCl 

decision. See NRDC I, 489 F.3d 1250. 

The EPA again chose to consider a health-based standard 

for HCl in the current rulemaking, but this time declined to set 

such a standard. 2010 Major Boilers Rule, 75 Fed. Reg. at 

32,030. The EPA explained that it continued to interpret its 

authority under section 7412(d)(4) to require that it find a 

health threshold exists, with an ample margin of safety, before 

using its discretion to depart from an established MACT floor. 

Id. The Agency reasoned further that, even if it made a 

finding that a health threshold exists, the discretionary nature 

of the authority allowed it to weigh additional factors when 

choosing whether to adopt the more lenient health-based 

standard. Id. Those factors included: the potential for 

cumulative adverse health effects due to concurrent exposure 

to other HAPs or emissions from other nearby sources; 

potential impacts of increased emissions on ecosystems; and 

reductions in emissions of other pollutants, also known as 

“co-benefits,” achieved through enforcement of the HCl 

MACT floor. Id. at 32,030-31. 

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Applying this interpretation, the EPA suggested in its 

proposed rule that a health-based standard for HCl might not 

be appropriate because these additional health and 

environmental considerations cautioned against a more lenient 

emission standard. Id. at 32,031. The Agency acknowledged, 

in particular, that its decision in the 2004 rule was based on 

data that considered only the chronic respiratory effects of 

HCl exposure. Id. While affirming the validity of those 

findings, the EPA explained that those chronic impact studies 

did not consider the additional variables it had now identified, 

nor did it consider the potential acute or carcinogenic effects 

that might be caused by HCl exposure. Id. And, because of 

these potential (though unproven) risks, the Agency resolved 

that it currently lacked sufficient information to establish an 

HCl emission standard that would protect health with an 

ample margin of safety. Id. It thus requested additional data 

from stakeholders and the regulated community to help 

address its concerns, including information regarding the 

potential cumulative effects of HCl emissions from boilers 

and other nearby sources. Id. 

After receiving numerous comments on the issue, the 

EPA declined to set a health-based standard in the final rule 

for two primary reasons: (1) the comments had not provided 

sufficient data on potential cumulative health and 

environmental effects caused by HCl emissions from boilers 

and other nearby sources; and (2) the comments affirmed the 

potential co-benefits that limiting HCl emissions might have 

in lowering emissions of other HAP and non-HAP pollutants. 

2011 Major Boilers Rule, 76 Fed. Reg. at 15,643-44. 

According to the EPA, its consideration of these co-benefits 

was not a regulation of other pollutants; rather, it was simply 

choosing not to ignore the purpose of the CAA—to reduce the 

negative health and environmental effects of HAP 

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emissions—when exercising its discretionary authority under 

the Act. Id. at 15,644. 

The Industry Petitioners contend that the EPA’s 

consideration of the broad potential health and environmental 

impacts of HCl rendered the Agency’s decision arbitrary and 

capricious. In particular, they argue that the Agency based its 

decision on two impermissible factors that were not supported 

by the record: (1) the potential cumulative effects of 

emissions from boilers and other nearby sources, and (2) the 

co-benefits of setting a more stringent MACT floor standard 

for HCl. We disagree on both counts. 

The statutory text and purpose of section 7412(d)(4) 

amply support the Agency’s decision to consider potential 

cumulative risks associated with emissions from boilers and 

other nearby sources. Although other CAA provisions require 

the EPA to set emission standards based on the emissions 

from a particular source, section 7412(d)(4)’s plain language 

is not focused on emissions from any particular source. 

Compare 42 U.S.C. § 7412(d)(3) (instructing the EPA to set 

emission standards for sources at the level achieved in 

practice by the best controlled similar source), with id. 

§ 7412(d)(4) (containing no mention of emissions from a 

particular source). The EPA’s consideration of the 

cumulative impacts from these emissions is also relevant to 

the Agency’s statutory mandate to ensure that a health 

threshold would protect health with an “ample margin of 

safety.” As such, the Agency had discretion to consider the 

potential risks associated with the cumulative emissions of 

boilers and other nearby sources under this provision. 

The EPA was likewise free to consider potential cobenefits that might be achieved from enforcing the HCl 

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MACT floor. Section 7412(d)(4)’s text does not foreclose the 

Agency from considering co-benefits and doing so is 

consistent with the CAA’s purpose—to reduce the health and 

environmental impacts of hazardous air pollutants. The 

Agency was under no obligation to ignore the CAA’s purpose 

in making a final decision on whether to exercise a 

discretionary authority. 

The Industry Petitioners attempt to refute this 

straightforward conclusion by pointing to “restrictions” in 

another provision, section 7412(d)(2). No. 11-1108 Indus. 

Pet’rs’ Br. 55-56. This provision requires the EPA to 

consider costs, non-air quality health and environmental 

impacts, and energy requirements in setting maximum 

achievable emission standards. Petitioners contend that these 

same “restrictions” must be read into section 7412(d)(4). But, 

even if we assume Petitioners are correct that these factors 

restrict the Agency’s ability to consider other factors under 

section 7412(d)(2), that provision furthers the statute’s 

command to set the strictest possible emission standards 

above what has already been achieved (i.e., the MACT 

floors). Section 7412(d)(4), by contrast, is a permissive 

authority for the EPA to abandon already achieved emission 

standards. We do not read limits on the EPA’s authority to 

set more stringent standards into a provision laying out the 

EPA’s authority to set more lenient standards. If anything, 

the difference between the provisions cuts the other way. 

Section 7412(d)(4) does not specify the factors that 

Petitioners argue for, while section 7412(d)(2) does. This 

difference shows that Congress knew how to provide such 

limits where it found them necessary. We thus find no basis 

to conclude that the EPA could not consider potential 

cumulative effects or co-benefits in rejecting a more lenient 

health-based HCl standard. 

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Finally, the Industry Petitioners claim that the EPA’s 

decision was arbitrary because the Agency failed to support 

its reversal from the 2004 rule, in which it set health-based 

emission standards for HCl. Because the EPA changed its 

position, the Petitioners contend that the Agency had to 

present factual support for its decision to disregard the facts 

and circumstances that underlay its prior adoption of a healthbased HCl standard. See FCC v. Fox Television Stations, 

Inc., 556 U.S. 502, 516 (2009). The Agency failed to do this, 

Petitioners say, because it relied on a data gap regarding the 

potential cumulative effects of HCl exposure. But this 

argument fares no better than Petitioners’ first. 

At the outset, Petitioners misstate the EPA’s burden to 

justify its change in policy. Although an agency does not 

generally need to provide a more substantial explanation or 

reason for a policy change than for any other action, it must 

do so where “its new policy rests upon factual findings that 

contradict those which underlay its prior policy.” Id. at 515. 

In that circumstance, “it is not that further justification is 

demanded by the mere fact of policy change; but that a 

reasoned explanation is needed for disregarding facts and 

circumstances that underlay or were engendered by the prior 

policy.” Id. at 515-16. The EPA, therefore, was not required 

to refute the factual underpinnings of its prior policy with new 

factual data. The Agency only needed to provide a reasoned 

explanation for discounting the importance of the facts that it 

had previously relied upon. Id. 

The EPA did so here by explaining that its prior decision 

focused too narrowly on the chronic respiratory effects of HCl 

emissions without considering the broader implications of 

such emissions on health and environmental conditions. See 

2010 Proposed Major Boilers Rule, 75 Fed. Reg. at 32,030-

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31. In so doing, the EPA neither contradicted nor abandoned 

the factual findings it made in its earlier rulemaking. It 

instead acknowledged that those findings were more limited 

than what it now considered necessary to justify the exercise 

of its discretion to set a health-based standard. Id. For 

example, the Agency noted that: (1) little research had been 

done on HCl’s carcinogenicity or on the toxicity of mixtures 

of HCl and other respiratory irritants emitted from boilers; 

and (2) the Agency had no data about peak short-term 

emissions of HCl from major boilers that might create risks of 

acute exposure. Id.

These enumerated concerns were sufficient to support the 

Agency’s decision not to adopt a health-based standard. 

Section 7412(d)(4) does not require that the EPA present 

affirmative factual data to reject a health-based standard. The 

provision requires just the opposite: in order to impose a 

health-based standard, the Agency must find that a health 

threshold can be set that provides an ample margin of safety. 

The EPA here determined that it could not do so, in part 

because it lacked relevant data like that discussed above. 

2011 Major Boilers Rule, 76 Fed. Reg. at 15,643-44. In other 

words, the EPA could not determine that any health threshold 

would provide an ample margin of safety to protect health. 

Without such a finding, the EPA could not invoke its 

discretionary authority under the statute. Id. There was thus 

nothing impermissible in the EPA’s reliance on a lack of data 

in rejecting a more lenient health-based standard. The EPA’s 

decision not to adopt health-based emission standards for HCl 

was not arbitrary and capricious. 

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H. EMISSIONS AVERAGING OF MULTIPLE CISWI UNITS 

IN ONE FACILITY

Certain industry entities urged the EPA to allow facilities 

with more than one CISWI unit to demonstrate MACT 

compliance by showing that the average HAP emissions 

across all units at that location fell under the relevant cap. 

They pointed to the EPA’s allowance of emissions averaging 

in the Major Boilers Rule but the Agency defended its 

disparate treatment because, in its view, “[t]he applicability of 

CISWI is such that each unit is an affected facility.” See 2011 

Proposed CISWI Rule on Reconsideration, 76 Fed. Reg. at 

80,463. It subsequently elaborated that it did “not believe [it 

has] the legal authority to allow emissions averaging in 

CISWI or under section [7429] generally because each 

individual unit is an affected facility.” CISWI Rule—

Responses to Comments, at 195-96. The Industry Petitioners 

challenge the disallowance of facility-wide averaging for 

CISWIs, arguing that “unit” cannot mean “facility” because 

section 7429(g)(1) defines “solid waste incineration unit” as 

“a distinct operating unit of any facility” and therefore the 

EPA’s rule fails Chevron step 1. They also argue the EPA’s 

conflation of “unit” and “facility” is unreasonable, and thus 

violates Chevron step 2, because the EPA has allowed 

emissions averaging in a different section 7429 rule and in a 

number of section 7412 rules. 

Although the Industry Petitioners’ point is well taken—

the plain terms of the CAA foreclose the EPA’s conflation of 

a CISWI “unit” and “affected facility,” see 42 U.S.C. 

§ 7429(g)(1) (“facility” is comprised of “units”)—we agree 

that the EPA has no statutory authority to allow emissions 

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averaging under section 7429.18 Section 7429 requires the 

EPA to regulate emissions from all “solid waste incineration 

units,” 42 U.S.C. § 7429(a)(2); see also id. § 7429(a)(4), and 

the CAA defines a “solid waste incinerator unit” as “a distinct 

operating unit” of a “facility,” id. § 7429(g)(1) (emphasis 

added). In other words, because the CAA mandates that the 

EPA regulate each “distinct” CISWI unit in a “facility,” the 

EPA cannot allow emissions averaging of all CISWI units in 

a facility. See id. 

For this reason, the Industry Petitioners’ Chevron 

challenge fails, notwithstanding the EPA’s minimal 

explanation set forth in its proposed CISWI Rule. It is 

axiomatic that an agency must “articulate[] an adequate

explanation for its action,” Int’l Fabricare Inst. v. EPA, 972 

F.2d 384, 389 (D.C. Cir. 1992) (emphasis added); see also 

State Farm, 463 U.S. at 48, but the EPA’s failure to do so 

here cannot create statutory authority that does not exist. And 

because the EPA has no authority under section 7429 to allow 

emissions averaging of multiple CISWI units in one facility, 

the Petitioners’ Chevron argument does not carry the day.19 

 18 The EPA does have statutory authority under section 7412 

to allow facility-wide emissions averaging in the Major Boilers 

Rule. See 42 U.S.C. § 7412(a)(1) (“major source[s]” defined as 

“any stationary source or group of stationary sources located within 

a contiguous area and under common control” (emphasis added)); 

see also id. § 7411(a)(3) (“stationary source” defined as “any 

building, structure, facility, or installation which emits or may emit 

any air pollutant”). 

19 The EPA concedes that it once allowed, in a different rule, 

emissions averaging for units subject to section 7429 but has since 

concluded that it does not have the statutory authority to do so. 

Although the Industry Petitioners argue that the Agency arbitrarily 

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IV. ENVIRONMENTAL PETITIONERS’ 

CHALLENGES 

A. CARBON MONOXIDE AS A SURROGATE

As explained at supra §§ I.B.1 and III.F, the EPA used 

carbon monoxide (CO) as a surrogate for several nondioxin/furan organic HAPs when the Agency set the MACT 

floors for major boilers. In support of this approach, the EPA 

found that both CO and these HAPs were the products of 

“incomplete combustion.” 2010 Proposed Major Boilers 

Rule, 75 Fed. Reg. at 32,018. The Agency concluded as a 

result that CO was a reasonable surrogate because: 

(1) minimizing CO emissions would minimize these HAPs; 

(2) methods used for the control of these HAP emissions 

would be the same methods used to control CO emissions 

(i.e., good combustion or using an oxidation catalyst); 

(3) standards limiting CO emissions would result in decreases 

in these HAP emissions; and (4) establishing emission limits 

for individual organic HAPs would be impractical and costly. 

Id. Although several commenters challenged aspects of this 

reasoning, the EPA ultimately stuck with its decision to use 

CO as a surrogate for non-dioxin/furan organic HAP 

emissions, without further explanation, in the final Major 

Boilers Rule. See 2013 Major Boilers Rule, 78 Fed. Reg. at 

7,145 (explaining the EPA was denying Sierra Club’s petition 

to reconsider the suitability of CO as a surrogate for nonorganic HAPs based on the reasoning provided by the Agency 

in the 2010 proposed rule). 

 

changed its position, the fact that the EPA may have acted outside 

its authority in a rule is not at issue here. “[P]revious statutory 

violations,” of course, “cannot excuse” new ones. New Jersey v. 

EPA, 517 F.3d 574, 583 (D.C. Cir. 2008). 

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The Environmental Petitioners challenge this decision, 

arguing that the EPA has not adequately explained how 

setting emission standards for CO will accomplish what the 

statute plainly requires: that the EPA set emission standards 

for organic HAPs at the average level achieved by the best 

performers with regard to those HAPs. We agree and remand 

to the EPA to adequately explain how CO acts as a reasonable 

surrogate for non-dioxin/furan organic HAPs. We do not, 

however, vacate the current emission standards because we 

conclude that the Agency will likely be able to adequately 

explain its decision on remand and that vacatur would prove 

substantially disruptive. 

 The EPA may use a surrogate to regulate HAPs under 

section 7412 where “reasonable.” See, e.g., Nat’l Lime Ass’n, 

233 F.3d at 637. To be reasonable, the emission standard set 

for the surrogate must reflect what the best source or best 12 

per cent of sources in the relevant subcategory achieved with 

regard to the HAP. See Sierra Club I, 353 F.3d at 984. This 

requires the surrogate’s emissions to share a close relationship 

with the emissions of the HAP. Id. One crucial factor we 

have identified for determining whether that close relationship 

exists is the availability of alternative control technologies. 

See id. at 985. These technologies regulate the HAP without 

impacting a surrogate’s emissions, or regulate the surrogate 

without impacting the HAP. Id. As we have explained, the 

importance of this factor to our reasonableness analysis “is 

clear: if EPA looks only to [the surrogate], but HAPs are 

reduced [in another] way that does not reduce [the surrogate], 

the best achieving sources, and what they can achieve with 

respect to HAPs, might not be properly identified.” Id. 

In the Major Boilers Rule, the EPA proposed using CO as 

a surrogate because, as relevant here: (1) the lowest possible 

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CO emissions resulted in the lowest possible HAP emissions, 

and (2) the same combustion and oxidation control methods 

reduce both types of emissions. See 2010 Proposed Major 

Boilers Rule, 75 Fed. Reg. at 32,018. But, during notice and 

comment, the EPA failed to directly consider and respond to 

several comments that introduced evidence suggesting that 

other control technologies and methods could be effectively 

used to reduce HAP emissions without also impacting CO 

emissions, or vice versa. See, e.g., Inst. of Clean Air Cos., 

Comments on National Emission Standards for Hazardous Air 

Pollutants for Major Sources, EPA-HQ-OAR-2002-0058 

(Aug. 23, 2010), at 20-21 (No. 11-1108 J.A. 822-23); 

Responses to Public Comments on EPA’s National Emission 

Standards for Hazardous Air Pollutants for Major Sources, 

vol. 2, EPA-HQ-OAR-2002-0058 (Feb. 2011) (No. 11-1108 

J.A. 1033, 1035-36, 1049-52). The EPA ultimately decided to 

use CO as a surrogate for all non-dioxin/furan organic HAPs 

in its final rule without ever addressing whether such 

alternative control technologies and methods might be used to 

lower organic HAP emissions further. See 2011 Major 

Boilers Rule, 76 Fed. Reg. at 15,654; 2013 Major Boilers 

Rule, 78 Fed. Reg. at 7,138. Instead, the Agency responded 

by doubling down on its assertion that both CO and organic 

HAP emissions were the product of poor combustion and, as a 

result, optimal combustion would minimize the emissions of 

both CO and non-dioxin/furan organic HAPs. 2013 Major 

Boilers Rule, 78 Fed. Reg. at 7,145. But this response was no 

response at all to the substantial concerns raised in the 

comments that other variables might also affect emissions. 

Although we afford an agency’s scientific decision “an 

extreme degree of deference,” see Kennecott Greens, 476 

F.3d at 954-55 (quoting Hüls Am., Inc. v. Browner, 83 F.3d 

445, 452 (D.C. Cir. 1996)), we cannot uphold an agency 

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decision that does not consider all relevant factors or fails to 

establish a reasonable connection to the facts in the record. 

Cf. Ethyl Corp. v. EPA, 51 F.3d 1053, 1064 (D.C. Cir. 1995). 

The EPA could not conclude that CO acts as a reasonable 

surrogate in this statutory context without at least considering 

a key factor: whether the best performing boilers might be 

using alternative control technologies and methods that 

reduce organic HAP emissions beyond what they achieve by 

regulating CO alone. See Sierra Club I, 353 F.3d at 985. We 

therefore reject the EPA’s contention that its reason for using 

CO as a surrogate—that good combustion would minimize 

both CO and non-dioxin/furan organic HAP emissions—was 

alone sufficient to support its decision. 

We recognize that there might be a context where a 

surrogate’s use is reasonable despite the presence of 

alternative control methods or technologies, but the Agency 

does not explain why it did not need to even consider whether 

such methods might further reduce HAPs here. For example, 

if the EPA used a surrogate that was closely correlated to the 

HAP and set surrogacy emission standards at a level that 

would eliminate HAP emissions altogether, the Agency might 

not need to account for alternative control technologies in its 

final rule. In that case, the use of the surrogate would not call 

into question whether the Agency had regulated the HAP as 

required by the statute because, after all, nothing is better than 

eliminating HAP emissions entirely. But the Agency offers 

us no analogous explanation or supportive data here. 

Although it is possible that all of the challenged CO emission 

standards are in fact set at such a level, the Agency has not 

defended the rule on such reasoning. Indeed, the Agency 

failed to consider or even comment directly on this issue, 

including whether certain post-combustion processes might 

increase organic HAP emissions without a corresponding 

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increase in CO emissions. We cannot ignore such an 

oversight in this context. 

We reject, however, the Environmental Petitioners’ other 

argument that combustion-related issues preclude the EPA 

from using CO as a surrogate for non-dioxin/furan organic 

HAPs. The Petitioners contend that the EPA’s decision to use 

CO was arbitrary because record evidence demonstrated a 

breakdown in the correlation between CO and organic HAP 

emissions at CO emission levels below 130 parts per million 

(ppm). But the EPA explained that this apparent breakdown 

was most likely caused by the difficulty of measuring the 

regulated HAP at such extremely low emission levels, rather 

than by a flaw in the correlation between CO and organic 

HAPs. 2013 Major Boilers Rule, 78 Fed. Reg. at 7,144-45; 

Memorandum from Eastern Research Group, Inc. to Jim 

Eddinger, EPA, Revised MACT Floor Analysis for the 

Industrial, Commerical, and Institutional Boilers and Process 

Heaters National Emission Standards for Hazardous Air 

Pollutants—Major Source (2012 MACT Floor Memorandum) 

(Aug. 2012), at 11-12 (No. 11-1108 J.A. 1462-63). This is 

precisely the sort of scientific judgment to which we must 

defer and accordingly, we do so on this point. See Kennecott 

Greens, 476 F.3d at 954-55. The Environmental Petitioners 

fail to provide any reason to believe that organic HAP 

emissions can, in fact, be accurately measured at such low 

levels. And the Agency’s explanation also addresses why the 

EPA discounted record evidence regarding extremely high 

burn temperatures that demonstrated a potential breakdown in 

the CO and organic HAP relationship as HAP emissions 

approached zero. 

Still, the EPA’s failure to address substantial record 

evidence on the potential availability of alternative control 

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technologies or methods rendered the Agency’s use of CO as 

a surrogate for certain organic HAPs arbitrary and capricious. 

We thus remand the portion of the Major Boilers Rule 

providing for CO’s use as a surrogate for non-dioxin/furan 

organic HAPs to the Agency for further consideration. We do 

not, however, vacate the current emission standards based on 

CO’s use as a surrogate. We may remand without vacatur 

where there is a likelihood of (1) cure on remand, and (2) a 

substantial disruptive effect that would result from vacatur. 

See Heartland Reg’l Med. Ctr. v. Sebelius, 566 F.3d 193, 197-

98 (D.C. Cir. 2009). Here, vacatur would cause substantial 

disruptive effects by removing emission limits for the 

regulated HAPs. And it is likely that the EPA will be able to 

adequately explain its use of CO on remand after properly 

considering the matter. As a result, we decline to vacate the 

current standards in the interim. 

B. EXCLUSION OF CERTAIN UNITS FROM MACT ANALYSIS

In the Major Boilers Rule, the EPA created subcategories 

based primarily on the fuel combusted. See 2013 Major 

Boilers Rule, 78 Fed. Reg. at 7,142. To qualify for certain 

subcategories, the EPA required that a source burn a fuel 

mixture comprised of only 10 per cent of the subcategorydefining fuel. See, e.g., id. at 7,193 (“Unit designed to burn 

solid fuel subcategory means any boiler . . . that burns . . . at 

least 10 percent solid fuel . . . in combination with liquid fuels 

or gaseous fuels.” (emphasis added)). Notwithstanding the 

low bar for inclusion, we conclude, and discuss at greater 

length below, see infra § IV.J, that the EPA reasonably 

exercised its discretion when it subcategorized boilers this 

way.

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We cannot say the same about the EPA’s exclusion of 

certain high-performing units from its MACT-floor 

calculation. Although the EPA allowed sources that combust 

only 10 per cent of a subcategory-defining fuel to join that 

subcategory, it declined to consider emissions from any 

source that burned less than 90 per cent of the subcategorydefining fuel when determining the average emissions level of 

the best performing sources in setting MACT floors for 

existing sources. And when it set a subcategory’s MACT 

floors for new sources, the Agency declined to consider the 

emissions levels from any source that did not burn 100 

per cent of the fuel. This disparate treatment makes a 

difference; several sources excluded from the MACT-floor 

determination were among the best performing sources (or, in 

some cases, the single best performing source) in that fuelbased subcategory. 

The CAA, however, demands that source subcategories 

take the bitter with the sweet. Section 7412 mandates, 

without ambiguity, that the EPA set the MACT floor at the 

level achieved by the best performing source, or the average 

of the best performing sources, in a subcategory. See 42 

U.S.C. § 7412(d)(3)(A), (B). It thus follows that if the EPA 

includes a source in a subcategory, it must take into account 

that source’s emissions levels in setting the MACT floor. 

The Agency, however, claims discretion to omit from 

MACT-floor computation sources it considers dissimilar. In 

support, it cites section 7412(d)(3), which provides that 

MACT standards must be no less stringent than “the best 

controlled similar source, as determined by the [EPA].” Id. 

§ 7412(d)(3) (emphases added). Our decision in Sierra Club 

II, 479 F.3d 875, however, forecloses this argument. In 

Sierra Club II, the EPA set MACT standards for brick and 

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ceramic kilns. Id. at 879. For some subcategories, the EPA 

based its MACT-floor determination on “the pollution control 

devices used by the second-best performers,” not the best 

performers. Id. (emphasis added). Although the EPA argued 

that it “reasonably construe[d] the term ‘best performing’ . . . 

to allow it to consider whether retrofitting kilns with a 

particular pollution control technology is technically 

feasible,” id. at 880 (alterations in original), we held that the 

EPA could not circumvent the requirement that it base the 

MACT floor “on the emission level actually achieved by the 

best performers (those with the lowest emission levels).” Id. 

at 880-81 (citing Cement Kiln, 255 F.3d at 861) (emphasis in 

original). We reach the same conclusion here. 

The EPA tries to distinguish Sierra Club II, arguing that 

the issue in that case “was whether [the] EPA could exclude 

all units using the most-effective emission control technique 

because it might not be applicable to all existing units”; 

however, “[h]ere, [the] EPA is excluding a test result that is 

unrepresentative of typical operations of units in the 

subcategory, and thus is inappropriate to use in establishing 

the MACT floor.” No. 11-1108 EPA Br. 81. But it makes no 

difference whether the EPA exempts from consideration units 

with certain highly effective technology or units with 

impressive test results driven by the fuel combination it 

combusts. Either approach contravenes our holding in Sierra 

Club II that the EPA cannot ignore “the emission level 

actually achieved by the best performers (those with the 

lowest emission levels)” in the subcategory. 479 F.3d at 880 

(emphasis omitted). In any event, the EPA has not simply 

excluded aberrant test results; it has excluded an entire class 

of units—those burning less than 90 per cent of the 

subcategory’s fuel—even though every one of those units fits 

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the subcategory’s parameters. This is no different from what 

we rejected in Sierra Club II. 

The EPA insists that if a source is “unrepresentative of 

typical operations of units in the subcategory,” it is 

“inappropriate to use [it] in establishing the MACT floor.” 

No. 11-1108 EPA Br. 81. Not so. “The idea is to set limits 

that, as an initial matter, require all sources in a category to at 

least clean up their emissions to the level that their best 

performing peers have shown can be achieved.” Sierra Club 

I, 353 F.3d at 980 (citing 42 U.S.C. § 7412(d)(3)). For this 

reason, an unusually high-performing source should be 

considered; indeed its performance suggests that a more 

stringent MACT standard is appropriate. Accordingly, we 

vacate the MACT standards for all major boiler subcategories 

that would have been affected had the EPA considered all 

sources included in the subcategories.20

C. UPPER PREDICTION LIMIT

Sections 7412 and 7429 create MACT-floor criteria that, 

for our purpose, are materially the same. Compare 42 U.S.C. 

 20 In its brief, the EPA argued that the Environmental 

Petitioners’ challenge was moot either because the challenged 

MACT standards had been remanded for other reasons or because 

inclusion of the allegedly dissimilar sources would not have 

affected the MACT standard. During oral argument, however, it 

conceded that it misunderstood the scope of the Petitioners’ 

argument, which argument challenges unremanded MACT 

standards that have in fact been affected by the EPA’s decision to 

omit certain high-performing sources from its MACT-floor 

analysis. See Oral Arg. Recording pt. B at 48:28-49:22. We 

believe that the Environmental Petitioners’ challenge is not moot 

and has not been waived. 

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§ 7412(d), with id. § 7429(a)(2). In both provisions, the CAA 

mandates that MACT floors have maximum stringency but 

also be continuously achievable. See id. § 7412(d)(2), (k); id. 

§ 7429(a)(2); id. § 7602(k). Satisfying the statutory criteria is 

no easy task, especially because no source emits any HAPs at 

a constant level. See Page Mem. 6. Rather, emissions levels 

fluctuate over time and for many reasons. See id. at 3.21 We 

have held, see Mossville Envtl. Action Now v. EPA, 370 F.3d 

1232, 1242 (D.C. Cir. 2004), and recently reaffirmed, see 

NACWA, 734 F.3d at 1133-34, that the EPA can consider this 

variability when setting MACT floors. 

Further complicating the task is the way in which sources 

typically measure emissions. Virtually all of the data the EPA 

collects to set MACT floors come from the three-run stack 

test. Page Mem. 6. The three-run stack test, as the name 

suggests, involves three measurements of the source’s 

emissions taken over a short time period (i.e., no more than a 

few days) with each of the three test “runs” lasting from one 

hour to four hours. Id. at 3. Because the tests provide three 

“snapshots” of a source’s emissions performance, they cannot 

accurately represent the source’s full range of emissions over 

all times and under all conditions. Id. at 3-4. Because stack 

testing typically involves “three separate runs,” however, it 

“will in most cases show some of a particular source’s 

 21 See also Page Mem. 2-3 (“This variability occurs due to a 

number of factors, including measurement variability (both 

sampling and analysis) and short term fluctuations in the emission 

levels that result from short-term changes in fuels, processes, 

combustion conditions, and controls.”). 

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variability over the short period of time during which testing 

was conducted.” Id. at 6 (emphasis added).22 

1. NACWA, 734 F.3d 1115 (D.C. Cir. 2013) 

Based on the limitations inherent in stack testing, the 

EPA concluded that it could not set MACT floors based on 

that testing alone. It began using the UPL to account for the 

HAPs-emissions variety that stack-testing data do not reflect. 

See NACWA, 734 F.3d at 1122. The Agency did so in several 

rules promulgated in 2011, including not only the Major 

Boilers Rule and the CISWI Rule but also the Sewage Sludge 

Incinerator Rule addressed in NACWA. See id. In that case, 

the petitioners challenged the EPA’s UPL use, arguing that 

the Agency failed to establish that the UPL fairly represented 

the “average emissions limitation achieved” by the best 

performing sources to set the Sewage Sludge Incinerator 

MACT floors and, accordingly, was “unlawful and arbitrary.” 

Id. at 1130. We agreed in part. See id. at 1119. 

Specifically, we struggled to pin down the EPA’s precise 

interpretation of the phrase “average emissions limitation 

achieved by the best performing 12 percent of units.” Id. at 

1142-43 (quoting 42 U.S.C. § 7429(a)(2)).23 As best we 

could tell, the EPA defended its use of the UPL as follows: 

“[b]ecause the [UPL] represents the value which [the EPA] 

 22 See also Page Mem. 5 (“[E]ven single three run tests, which 

are performed over a short period of time, typically show different 

emissions levels during each individual test run.”). 

23 See also NACWA, 734 F.3d at 1142 (“[I]t seems EPA has 

adopted yet another interpretation of the phrase ‘average emissions 

limitation achieved by the best performing 12 percent of units.’” 

(emphasis added)). 

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can expect the mean (i.e., average) of three future 

observations (3-run average) to fall below, based upon the 

results of the independent sample size from the same 

population, the [UPL] reflects average emissions.” Id. at 

1142 (quoting Standards of Performance for New Stationary 

Sources and Emission Guidelines for Existing Sources: 

Sewage Sludge Incineration Units, 76 Fed. Reg. 15,372, 

15,389 (Mar. 21, 2011)) (emphasis added) (some alteration in 

original). In our view, however, “the word ‘average’ . . . 

seems to mean the average emissions limitation that the 

existing population of the best-performing 12 percent of 

incinerators has achieved.” Id. (emphases added). 

Despite these doubts, we reasoned that the EPA could 

have “plausibl[y]” concluded that the UPL represents the 

“average emissions limitation achieved” by the best 

performing sources. Id. at 1143. That said, we were not 

willing to assume the EPA’s responsibility of “supply[ing] a 

reasoned basis” for its UPL use. Id. (quoting Bowman 

Transp., Inc. v. Ark.-Best Freight Sys., Inc., 419 U.S. 281 

(1974)). For that reason, we remanded—but did not vacate, 

see id. at 1161—the UPL portion of the Sewage Sludge 

Incinerator Rule and ordered the EPA to “clarify how the 

[UPL] represents the average emissions limitation achieved 

by the best performing 12 percent.” Id. at 1143 (internal 

quotation marks omitted).24 

 24 See also NACWA, 734 F.3d at 1151 (“[W]hile we determine 

that [the] EPA’s use of the [UPL] may be lawful, we are remanding 

this portion of its rulemaking for further explanation on the issue[] 

of how the upper prediction limit represents the average emissions 

limitation achieved . . . .” (internal quotation marks omitted)). 

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Because the EPA also used the UPL in the Major Boilers 

Rule and the CISWI Rule, the Agency moved for a limited 

remand of the current petitions so that it could include its 

revised UPL explanation in the administrative records of these 

two regulations.25 See Page Mem. 2. On July 14, 2014, the 

EPA published a fifteen-page memorandum authored by 

Stephen D. Page, the EPA Director of Air Quality Planning 

and Standards (Page Memorandum), in response to NACWA. 

See id. at 1. The EPA’s current explication of the UPL is now 

before us.26

 25 In NACWA, we had other problems with the EPA’s use of 

the UPL. Specifically, the EPA had explained that “a smaller 

dataset may have greater variability, and thus a higher [UPL].” 

NACWA, 734 F.3d at 1144. We instructed the EPA not only to 

explain its use of the UPL in general but also to “explain why the 

[UPL] could still be considered accurate given a small dataset” in 

particular. Id. at 1144-45 (emphasis added). In its remand motion, 

the EPA represented that it could “adequately explain why [its] use 

of the UPL in general is consistent with Clean Air Act requirements 

through a remand of the record for a limited time” but that “the 

question of whether the UPL is an appropriate statistical method for 

small data sets requires more analysis . . . [along with] additional 

notice and comment rulemaking.” No. 11-1108 Mot. for Remand 

9, 13 (Feb. 28, 2014). We agreed and, for this reason, the only 

issue we decide today is whether the EPA carried its burden of 

establishing, as a general matter, that the UPL reasonably estimates 

the average emissions level achieved by the best performing source 

or sources to set MACT floors. 

26 The Environmental Petitioners urge us to ignore the Page 

Memorandum, insisting that it “provide[s] a series of new 

interpretations and assertions that, rather than ‘explaining’ the prior 

record, instead contradict and revise the agency’s earlier position,” 

in contravention of NACWA and the scope of the remand the 

Agency requested regarding the Major Boilers Rule and the CISWI 

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2. The Page Memorandum 

The Page Memorandum recognized our “concern about 

the interpretation [we] believed [the] EPA was taking” of the 

word “average.” Page Mem. 3. It clarified that the Agency 

“does not interpret the term ‘average’” to mean “the average 

of a future 3-run compliance test.” Id. (emphasis added) 

(quoting NACWA, 734 F.3d at 1143). Rather, it explained that 

the “EPA interprets the average to mean the average 

emissions over time,” based not only on the “average of all 

emissions test data from the best performing source or 

sources” but also on “information regarding the variability of 

emissions.” Id. (emphasis added). 

In the EPA’s judgment, “variability is a key factor in 

establishing” MACT standards because “[e]ach MACT 

standard is based on limited data from sources whose 

emissions are expected to vary over their long term 

 

Rule. No. 11-1108 Envtl. Pet’rs’ Br. 41. But our NACWA decision 

did not, as the Petitioners would have it, require the EPA to adopt 

our belief that the Agency construed “average” to mean “the 

average of a future 3-run compliance test.” See NACWA, 734 F.3d 

at 1143. Rather, we asked the EPA to clarify how, in its view, the 

UPL “represents the ‘average emissions limitation achieved by the 

best performing 12 percent.’” Id. (emphasis added). Nor do we 

think that the EPA altered its initial basis for using the UPL, which 

the EPA has consistently held out as “a statistical formula designed 

to estimate a MACT floor level that is equivalent to the average of 

the best performing sources based on future compliance tests.” 

2011 Major Boilers Rule, 76 Fed. Reg. at 15,630 (emphasis added). 

What the EPA failed to do before NACWA was to explain how the 

UPL functions and why it is a reasonable way to calculate 

“average” emissions levels. The Page Memorandum does precisely 

that. 

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performance.” Id. Specifically, “[t]he available emissions 

data are generally in the form of short term, three-run stack 

tests, with each test run lasting for between 1 and 4 hours.” 

Id. For this reason, the EPA concluded that it did not have 

information “encompass[ing] the emissions performance of a 

source over time.” Id. (emphasis added). And because the 

“EPA interprets ‘emissions performance’ . . . to mean the 

emissions of a source over the long term, rather than just 

during a short-term stack test,” the EPA found it necessary to 

“appl[y] a methodology that predicts the actual emissions 

levels the source is achieving at times other than when stack 

testing was conducted.” Id. at 3-4 (emphases added). 

The UPL is the methodology the EPA selected to account 

for these limitations. Id. at 4. “[A] value derived from widely 

accepted and commonly used statistical principles,” the UPL 

“represents the upper end of a prediction interval.” Id. In 

layman’s terms, the UPL uses an equation that considers 

(1) the average of the best performing source or sources’ 

stack-test results (i.e., the mean); (2) the pattern the stack-test 

results create (i.e., the distribution); (3) the variability in the 

best performing source or sources’ stack-test results (i.e., the 

variance); and (4) the total number of stack tests conducted 

for the best performing source or sources (i.e., the sample 

size). Id. at 4-5.

The UPL, however, cannot demonstrate with absolute 

certainty the average emissions levels achieved by the best 

performing sources at all times (indeed, certainty is 

impossible without continuous monitoring). See id. Instead, 

the UPL equation produces a range of values that is expected, 

given the variance in the relevant stack-test data, to 

encompass the average emissions levels achieved by the best 

performing sources a specified percentage of the time. Id. at 

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4. To establish the MACT floor, the EPA calibrated the UPL 

equation to produce a range in which the average emissions 

levels of the best performing source or sources would be 

expected to fall 99 per cent of the time, which is referred to as 

a 99 per cent confidence interval. Id. Once the EPA had this 

range, it set the MACT floor at the top level of that range—

hence, the “upper” in “upper prediction limit”—to arrive at a 

figure that, 99 out of 100 times, it expected the average 

emissions levels of the best performing sources to “achieve.” 

Id. Or, in the EPA’s words, “the 99 percent UPL is the level 

of emissions that” the EPA is “99 percent confident is 

achieved by the average source represented in a dataset over a 

long-term period based on its previous, measured 

performance history as reflected in short term stack-test data.” 

Id. 

One of the equations the EPA used to calculate the UPL 

is as follows:27

 27 The EPA used “one of several equations” to calculate the 

UPL depending on “certain characteristics of [the] dataset,” 

including the distribution of data within the dataset. Page Mem. 4. 

Here, we set out the equation the EPA used for a dataset with a 

“normal distribution.” Id. at 10. For our review, we need not 

recount the other, somewhat more complicated equations the EPA 

used in determining the UPL for datasets with, e.g., a “lognormal 

distribution.” See id. (“Even though they differ due to separate 

mathematical properties associated with each distribution, the UPL 

equations share a common format . . . .”); see generally id. at 11 

(describing lognormal distribution equation). 

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NACWA, 734 F.3d at 1139. In this equation: 

 “x̄” is the mean; 

 “t(0.99, n-1)” is a value called the “t-statistic,” the 

statistical tool used to set the confidence interval 

(here, 99 per cent); 

 “n” is the sample size; 

 “m” is the number of stack tests that were run to 

calculate the mean (“x̄”); because most stack tests 

involve 3 “runs,” m usually equals 3; 

 “s” represents the “standard deviation.” 

See id.; see also Page Mem. 10-11. 

3. Instant Challenges to UPL 

After the EPA issued the Page Memorandum, the 

Environmental Petitioners renewed their argument that the 

UPL represents neither (1) the “average” emissions limit of 

the best performing source or sources in a subcategory, nor 

(2) the emissions levels “achieved” by the best performing 

sources in a subcategory. We believe that the EPA has 

carried its burden of demonstrating that the UPL “reflect[s] a 

reasonable estimate of the emissions achieved in practice by 

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the best performing sources.” Cement Kiln, 255 F.3d at 871-

72 (internal quotation marks omitted); see also NACWA, 734 

F.3d at 1148 (“[H]aving decided to account for variability, 

and having decided to estimate that variability, EPA bears the 

burden of demonstrating with substantial evidence that its 

estimate is reasonable.”). 

Our conclusion is driven, in large part, by the deference 

we owe the EPA when it determines how best to meet the 

technical challenges in its area of expertise. Indeed, the EPA 

“typically has wide latitude in determining the extent of datagathering necessary to solve a problem” and, for that reason, 

we have “accorded Chevron deference to [its] interpretation 

of [the CAA] as allowing it to estimate MACT floors.” 

NACWA, 734 F.3d at 1131. Moreover, “the requirement that 

the existing unit floors not be less stringent than the average 

emissions limitation achieved by the best performing 12 

percent of units does not, on its own, dictate how the 

performance of the best units is to be calculated,” id. (internal 

quotation marks omitted)—“[f]loors need not be perfect 

mirrors of the best-performers’ emissions,” Cement Kiln, 255 

F.3d at 871. So long as the EPA “demonstrate[s] with 

substantial evidence—not mere assertions”—that the UPL 

“allows a reasonable inference as to the performance of the 

top 12 percent of units,” NACWA, 734 F.3d at 1131

(quotations omitted) (emphasis added), the EPA has 

conducted reasoned decision making. 

The Agency has done so here. The Page Memorandum 

explains the limitations of stack-test data—i.e., the 

“snapshots” cannot reflect the best performing source’s or 

sources’ average emissions levels at all times and under all 

operating conditions. Page Mem. 6. The Page Memorandum 

also explains that the Agency chose the UPL as a tool 

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“derived from widely accepted and commonly used statistical 

principles,” id. at 4, that “reasonably account[s] for variability 

in the emissions of . . . sources,” id. at 2. Finally, the Page 

Memorandum plugs the analytical gap we identified in 

NACWA—it thoroughly explains how and why the UPL 

accounts for the variance and therefore how and why it 

reasonably represents the emissions level “achieved by the 

average source” or sources. Id. at 3-5. In so doing, the EPA 

has “clarif[ied],” to our satisfaction, “how the upper 

prediction limit represents the average emissions limitation 

achieved.” NACWA, 734 F.3d at 1143 (internal quotation 

marks omitted). 

The Environmental Petitioners’ arguments to the contrary 

are unavailing. Their primary objection is that the UPL 

cannot reasonably estimate the “average” emissions level 

achieved by the best performing source or sources because the 

UPL represents “a level [the] EPA expects any future 

compliance test by any [source] in the top 12 percent to fall 

below.” No. 11-1108 Envtl. Pet’rs’ Br. 35 (emphases added) 

(internal quotation marks omitted).28 But the Page 

Memorandum counters the Environmental Petitioners’ 

mistaken understanding of what the UPL represents.29 

According to the EPA, “the UPL does not represent the worst 

emissions performance of the best performing units at any 

 28 See also No. 11-1108 Envtl. Pet’rs’ Reply Br. 15 (“It is, as 

its name indicates, an upper limit—the emissions limitation that 

every member of the best-performing 12 percent will fall 

below . . . .” (emphasis in original) (quotation marks omitted)). 

29 The Environmental Petitioners’ argument rests, at least in 

part, on their contention that we should not consider the Page 

Memorandum at all. We decline their invitation to ignore the 

explanation we ordered the EPA to provide. 

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time.” Page Mem. 4 (emphasis in original).30 It is instead 

“the average level expected to have been achieved over time” 

by the best performing source or sources. Id. (emphasis in 

original). “In other words, the 99 percent UPL is the level of 

emissions that [the EPA is] 99 percent confident is achieved 

by the average source . . . over a long-term period based on 

its previous, measured performance history as reflected in 

short term stack test data.” Id. (emphasis added). 

Next, the Environmental Petitioners criticize the Page 

Memorandum’s explanation that the UPL represents the longterm average emissions levels achieved because “the first 

element of the UPL equation is the average of the short-term 

emissions test data from the best-performing sources.” Id. In 

their view, the UPL is no different from “saying that, over 

time, the average of 1, 2, and 3 = 2 + 500 because the first 

element in the equation (2) is the average of 1, 2, and 3.” No. 

11-1108 Envtl. Pet’rs’ Br. 48. But the UPL does not simply 

tack an arbitrary increase on top of the stack-test average of 

the best performing sources. Rather, the UPL “allows [the] 

EPA to use emissions test data and the data characteristics,” 

which include “the distribution and sample size, along with 

the intrinsic variability associated with those data,” to 

estimate “an emissions limit based on a specified level of 

confidence such that an average best performing existing 

 30 See also Page Mem. 5 (It is “generally . . . reasonable to 

establish a [MACT floor] standard that all the best performing 12 

percent of existing sources can meet without any modification 

because the statute requires the Agency to establish the standard at 

the average level of performance of the best 12 percent of sources.” 

(emphasis in original)); id. at 14 (“[T]he MACT floor represents the 

average emission level achieved by the best performing sources, 

not the worst emission level achieved by those sources.” (emphases 

in original)). 

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source would not be expected to exceed the limit a specified 

number of times.” Page Mem. 6 (emphases added). In other 

words, the UPL does not simply add an arbitrarily chosen 

value but instead turns entirely on the features inherent in the 

stack-test data and how those features reflect the natural 

variance in emissions experienced by the best performing 

sources over time. See id. at 4 (“[T]he MACT floor 

calculation takes into account the inherent variability in 

emissions performance to more accurately reflect the range of 

the best performing sources’ emissions over time.” (emphasis 

added)).31 Thus, as the Page Memorandum amply 

demonstrates, see id., the EPA’s use of the UPL is not 

arbitrary. 

The Environmental Petitioners also attack the results 

produced by the UPL. They provide a series of charts that, in 

their view, demonstrate that the UPL sets MACT floors far 

too high to comport with the CAA’s mandate that floors 

represent “the maximum degree of reduction in emissions.” 

See 42 U.S.C. § 7412(d)(2). True, some of the charts show 

that the EPA has set a MACT floor above the highest 

emissions level recorded by the best performing sources’ 

stack testing. See No. 11-1108 Envtl. Pet’rs’ Br. 14-15; No. 

11-1108 Envtl. Pet’rs’ Reply Br. 23. But this does not mean 

that the UPL is an arbitrary “average” proxy—for at least two 

reasons. 

 31 See also Page Mem. 6-7 (“[T]he UPL equation that is used 

to account for variability and [to] calculate the MACT floor 

standard depends on the distribution of the data.”); id. at 11 (“The 

UPL . . . is directly related to the confidence level and to the 

variance, meaning that as either of these values go up or down, so 

does the UPL value.”). 

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First, the charts selectively included are generated from 

data sets with considerable variance between the highest 

recorded stack test and the lowest. Unsurprisingly, if a 

handful of “snapshots” in a data set demonstrate that 

emissions levels experience high spikes and low plummets at 

discrete times, it is more likely that the average emissions 

level achieved by the best performing sources at all times 

might be high. This is because a data set with high variability 

will produce a higher UPL than a data set with low variability, 

even if the two sets share the same average. In other words, 

the UPL takes large variance into account and therefore 

naturally goes higher to arrive at the 99 per cent certainty the 

EPA thinks is appropriate.32 Second, where the UPL 

suggested a MACT floor higher than the results of the stack 

tests, it often did so by insubstantial amounts. Indeed, for at 

least one chart, “the limit is a mere 4 millionths of a pound per 

million Btu above the emissions test results of best 

performers, an unalarming amount given that the 

methodology is supposed to account for variable results.” 

No. 11-1108 Indus. Intervenors’ Br. 10 (emphases in 

original). For these reasons, the Environmental Petitioners 

have not convinced us that the EPA failed to satisfy the 

 32 The EPA “selected the 99 percent level in order to provide 

reasonable assurance that the limit can be met at all times by a 

source with emissions at the average level achieved by the best 

performing source or sources.” Page Mem. 10. The Environmental 

Petitioners have not challenged the EPA’s choice of a 99 per cent 

confidence level, as opposed to a lower level of certainty, and we 

express no opinion on that choice. And we reiterate that the more 

specific concerns we had with the UPL when we decided 

NACWA—in particular, the UPL’s accuracy “given a small 

dataset”—are not before us. 734 F.3d at 1144-45. 

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“minimal standard[] of rationality” that we require. Ethyl 

Corp. v. EPA, 541 F.2d 1, 36 (D.C. Cir. 1976) (en banc). 

Finally, the Environmental Petitioners insist that “[t]he 

UPL predicts a level that hypothetical future tests will fall 

below, rather than estimating what boilers actually achieved,” 

in contravention of the requirement that MACT floors “reflect 

what the best-performing sources achieved.” No. 11-1108 

Envtl. Pet’rs’ Reply Br. 24 (internal quotation marks omitted). 

But the Environmental Petitioners ignore the Page 

Memorandum’s explanation that, because the UPL is not 

time-dependent, it “not only is a prediction of the emissions 

performance of those sources in tests conducted in the future, 

but is also an indication of the range of current average 

emissions performance of those units.” Page Mem. 3;33

see 

also No. 11-1108 Indus. Intervenors’ Br. 9 (“Because this 

statistical method is not time-dependent, it is equally valid for 

predicting past performance (i.e., the range of emissions 

levels expected to have been experienced in the past by the 

best performers during periods when actual emissions testing 

was not underway) and future performance.”). 

We believe that the UPL “reflect[s] a reasonable estimate 

of the emissions achieved in practice by the best-performing 

sources,” Cement Kiln, 255 F.3d at 871-72 (internal quotation 

marks omitted), and, accordingly, we reject the 

Environmental Petitioners’ challenge to it. 

 33 See also Page Mem. 4 (“[T]he 99 percent UPL is the 

emissions level that the source would be predicted to be below 99 

out of 100 performance tests, including emissions tests conducted 

in the past, present, and future.”); id. at 10 (“The confidence level, 

in this case 99 percent, is the percentage of measurements (past, 

present, and future) that are predicted to fall at or below the UPL 

value.”). 

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D. BEYOND-THE-FLOOR STANDARDS FOR CISWI UNITS

The EPA declined to set beyond-the-floor standards for 

CISWI units. The Environmental Petitioners challenge that 

determination in three primary respects, each of which we 

reject.34

 

Section 7429 of the CAA directs the EPA to set MACT 

standards in two steps. It first sets a floor level based on the 

best performing sources. See 42 U.S.C. § 7429(a)(2). Next, it 

determines “whether a more restrictive standard is 

‘achievable,’” NRDC III, 749 F.3d at 1057, “taking into 

consideration the cost of achieving such emission reduction, 

and any non-air quality health and environmental impacts and 

 34 Although the EPA does not argue that the court lacks 

jurisdiction to consider this argument, Environmental Petitioners 

raise the issue defensively, contending that they satisfied the CAA’s 

administrative exhaustion provision. We agree. During the 

rulemaking process, the Petitioners comprehensively critiqued the 

EPA’s proposed rationale for rejecting beyond-the-floor standards. 

See, e.g., Natural Resources Defense Council, Comments on 

Standards of Performance for New Stationary Sources and 

Emission Guidelines for Existing Sources, EPA-HQ-OAR-2003-

0119 (Aug. 23, 2010), at 11-16 (No. 11-1125 J.A. 668-73). Many 

of those comments challenged the EPA’s consideration of costs and 

other factors—the same types of issues Petitioners now ask the 

Panel to resolve. Because the Environmental Petitioners raised the 

relevant issues “with reasonable specificity” during the period for 

public comment, our jurisdiction is not in question. 42 U.S.C. 

§ 7607(d)(7)(B); see Portland Cement Ass’n v. EPA, 665 F.3d 177, 

186 (D.C. Cir. 2011) (“While we certainly require some degree of 

foresight on the part of commenters, we do not require telepathy. 

We should be especially reluctant to require advocates for affected 

industries and groups to anticipate every contingency.”). 

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energy requirements,”35 42 U.S.C. § 7429(a)(2). The “EPA 

calls these stricter requirements ‘beyond-the-floor’ 

standards.” NRDC III, 749 F.3d at 1057. 

In section 7429, the “Congress gave EPA broad 

discretion in considering whether to go beyond-the-floor.” 

NACWA, 734 F.3d at 1157. The Congress required the EPA 

to consider a variety of factors without telling the EPA how to 

weigh them. That calculus belongs to the EPA’s discretion. 

See 42 U.S.C. § 7429(a)(2) (delegating to the EPA 

Administrator the responsibility to “tak[e] into consideration” 

the statutory factors). Against that backdrop, challenges to 

the EPA’s beyond-the-floor determinations “must clear a high 

bar, as we are at our most deferential when an agency is 

‘making predictions, within its area of special expertise, at the 

frontiers of science.’” NACWA, 734 F.3d at 1156 (quoting 

Husqvarna, 254 F.3d at 199). 

When establishing MACT standards for CISWI, the EPA 

declined to establish beyond-the-floor standards in the 

proposed rule, see 2010 Proposed CISWI Rule, 75 Fed. Reg. 

at 31,956-59, and the final rule, see 2011 CISWI Rule, 76 

Fed. Reg. at 15,729-32. The EPA also declined requests to 

reconsider that decision. See Memorandum from Eastern 

Research Group, Inc., to Amy Hambrick, U.S. Environmental 

Protection Agency, Revised Draft CISWI Reconsideration 

Issues (Dec. 20, 2012), at 22-23 (No. 11-1125 J.A. 1219-20). 

 35 EPA interprets the statutory factor of “cost” to permit 

consideration of cost-effectiveness, NRDC III, 749 F.3d at 1060-61, 

which is often calculated “on [a] per ton of emissions removed 

basis,” Husqvarna AB v. EPA, 254 F.3d 195, 200 (D.C. Cir. 2001). 

We have previously upheld that interpretation. See, e.g., NRDC III, 

749 F.3d at 1060-61. 

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The first challenge targets several instances in which the 

EPA refused to require sources to adopt, as a beyond-the-floor 

measure, controls that most sources would employ to meet the 

MACT floor standard. In each instance, the EPA determined 

that the relative costs outweighed the expected emissions 

gains. In the first such case, the EPA decided not to require 

liquid-fired energy recovery units to install dry sorbent 

injection and fabric filters as a beyond-the-floor measure, 

despite the fact that “four of the six” units would need to 

install those systems to meet the floor standard. 2011 CISWI 

Rule, 76 Fed. Reg. at 15,731. That decision satisfied the 

statute. Had the EPA mandated the control measures, the 

remaining two units would have needed to expend “$1.1 

million per year” to achieve only a small emissions reduction, 

“which translates into an incremental cost-effectiveness of 

about $230,000 per ton” of emission. Id. Nothing in section 

7429(a)(2) requires the Agency to impose a cost so 

disproportionate to the expected emissions gains. 

The Environmental Petitioners take issue with two other 

decisions along these lines. In the first, the EPA declined to 

set beyond-the-floor mercury control measures for kilns, 

citing a cost-effectiveness of roughly $351 million per ton. 

See Memorandum from Eastern Research Group, Inc., to Toni 

Jones, U.S. Environmental Protection Agency, Final 

Reconsideration Beyond-the-Floor Analysis for CISWI Units 

(Reconsideration Mem.) (Dec. 20, 2012), at ¶ 3.4.2 (No. 11-

1125 J.A. 1232). In the second, a $26,000 per-ton 

implementation cost led the Agency not to establish stricter 

carbon monoxide control measures for calciner kilns. See id.

¶ 3.4.3. Energy use—a factor mandated in section 

7429(a)(2)—also entered the equation. With respect to 

calciner kilns, the technology used to reduce carbon 

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monoxide would also increase energy requirements, and 

therefore increase energy costs. See id. In each of these 

decisions, the EPA reasonably applied the statutory factors. 

That Petitioners would have weighed the costs differently 

provides no grounds to displace the EPA’s otherwise 

reasonable determination. 

In the second challenge to the decision not to set beyondthe-floor standards, the Environmental Petitioners contend the 

Agency arbitrarily failed to set emission levels lower than the 

MACT floor for categories likely to adopt technology capable 

of meeting those lower levels. Specifically, according to the 

Environmental Petitioners, the EPA knew waste-burning kilns 

and energy recovery units would adopt fabric filters that 

“achieve particulate matter emissions levels dramatically 

lower than the floor, but refused to set the standard at that 

lower level.” See No. 11-1125 Envtl. Pet’rs’ Br. 50. 

That is incorrect. The Environmental Petitioners spin this 

yarn based on a line in the proposed rulemaking. There, the 

Agency speculated that kilns and energy recovery units would 

adopt fabric filters to comply with the MACT floor limit, and 

would “likely achieve a level of performance” below the 

floor. 2010 Proposed CISWI Rule, 75 Fed. Reg. at 31,958. 

That statement represented a preliminary prediction, which 

was subject to change during the notice-and-comment 

process. And change it did. In the final rulemaking, the EPA 

further subcategorized the energy recovery unit subcategory 

and revised the MACT floor for waste-burning kilns. See

2013 CISWI Rule, 78 Fed. Reg. at 9,122 (explaining the 

changes). New information received during the rulemaking 

inspired those changes, which the EPA made after 

considering the statutory factors. See id.; Reconsideration 

Mem., ¶ 2.3-3.4.5. The evidence does not suggest that the 

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EPA refused to set beyond-the-floor emission levels it knew 

were reasonably achievable.36 

In the third challenge, the Environmental Petitioners take 

issue with three determinations that rested on factors other 

than cost. First, the EPA declined to require coal-fired energy 

recovery units to adopt linkageless boiler management 

systems as a beyond-the-floor measure for carbon monoxide. 

See Reconsideration Mem., ¶ 2.3.1.1. While acknowledging 

that linkageless systems were available at “fairly low-cost,” 

the EPA concluded it had insufficient data to determine the 

“actual reductions this control option would achieve” relative 

to an alternative control system. Id. 

The EPA acted reasonably. The record suggests the EPA 

had scant evidence on the efficacy of linkageless control 

measures applied to coal-fired energy recovery units. See id. 

Had the Agency imposed a stricter standard based on controls 

for which it had precious little (if any) evidence, a reviewing 

court may well have concluded the decision lacked “a rational 

connection between the facts found and the choice made.” 

State Farm, 463 U.S. at 43 (internal quotation marks omitted). 

Second, the EPA rejected regenerative thermal oxidizers 

as a beyond-the-floor control for carbon monoxide in solid 

waste energy recovery units. See 2011 CISWI Rule, 76 Fed. 

Reg. at 15,732. Thermal oxidizers could do the job “but 

likely at a far greater energy requirement (specifically natural 

 36 This argument suffers from an additional flaw: the 

Environmental Petitioners appear to treat as interchangeable 

proposed emissions rules for new units with the final rules 

applicable to existing ones. That apples-and-oranges comparison 

underscores the weakness of the argument. 

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gas) [and] with comparable control efficiency” as carbon 

monoxide catalysts, which “some units will need to install to 

meet the MACT floor . . . limits.” Id. In other words, even 

though oxidizers work as well as carbon catalysts, oxidizers 

would be unsuitable because they use more energy. See id.

(concluding that beyond-the-floor controls “would be 

unreasonable for this subcategory due to additional cost and 

energy impacts”). 

The Environmental Petitioners contend that the EPA 

failed to “suggest that these natural gas requirements are high 

in an absolute sense or relevant to achievability.” No. 11-

1125 Envtl. Pet’rs’ Br. 51. We agree that the EPA’s analysis 

is less than fully satisfying. Among other reasons, nowhere 

did the Agency estimate the per-ton cost of mandating 

thermal oxidizers, or compare the energy costs relative to 

other control measures. 

Despite these imperfections, we reject the challenge. See 

Dist. Hosp. Partners, L.P. v. Burwell, 786 F.3d 46, 61 (D.C. 

Cir. 2015) (“[I]mperfection alone does not amount to arbitrary 

decision-making.”). The EPA’s somewhat sparse analysis on 

this issue reflects a somewhat sparse record. At bottom, the 

Agency rejected thermal oxidizers because it lacked sufficient 

evidence to support their utility, at least compared with 

control measures whose efficacy and costs were better known. 

The Agency’s determination should be read in context. 

Elsewhere in the final rule, the EPA expanded on the energy 

and environmental impacts of thermal oxidizers, concluding 

that “[t]he combustion of fuel needed to generate additional 

electricity and to operate [thermal oxidizer] controls would 

yield slight increases in emissions, including NOX, CO, PM, 

and SO2 and an increase in CO2 emissions.” 2011 CISWI 

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Rule, 76 Fed. Reg. at 15,744. The EPA addressed another 

statutory factor—cost—by reasonable implication. Energy—

natural gas, in this case—is not free. A technology that 

demands “far greater energy requirement[s]” naturally comes 

at a cost. See id. at 15,732. 

Though courts are powerless to “supply a reasoned basis 

for the agency’s action that the agency itself has not given,” 

“[w]e will . . . uphold a decision of less than ideal clarity if 

the agency’s path may reasonably be discerned.” State Farm, 

463 U.S. at 43 (internal quotation marks omitted). Here, the 

Agency’s path may reasonably be discerned: mandating 

thermal oxidizers was not achievable due to increased energy 

demands and a corollary increase in cost, see 2011 CISWI 

Rule, 76 Fed. Reg. at 15,732 (declining to set a beyond-thefloor limit “due to additional cost and energy impacts”). For 

those reasons, EPA did not act unreasonably.

Third, and finally, the Environmental Petitioners 

challenge the rejection of dry sorbent injection and wet 

scrubbers as beyond-the-floor measures for waste-burning 

kilns. The EPA determined those measures would be costeffective (at only $5,000 per ton) but declined to require them 

due to “uncertainty” surrounding “the appropriate control 

system that some existing kilns would need to employ to 

meet” a stricter standard, “especially kilns that use ingredients 

with a high sulfur content.” See Reconsideration Mem., 

¶ 3.4.5. Adding to that uncertainty, the EPA could not 

“account for potential costs at existing sources for additional 

scrubber water and spent sorbent.” Id. As before, the EPA 

reached a reasonable conclusion in the face of imperfect 

information. Had the EPA set a beyond-the-floor standard 

based on sorbent injection and wet scrubbers, the Agency 

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would have been flying blind. In avoiding that course, we 

conclude the EPA acted reasonably. 

E. REGULATION OF CERTAIN CISWI UNITS

The final CISWI Rule did not contain emission standards 

for burn-off ovens, cyclonic burn barrels, foundry sand 

reclamation units, soil treatment units, and space heaters. The 

Environmental Petitioners claim that the EPA unlawfully 

exempted these units from regulation by creating 

subcategories that capture only a subset of the units that the 

Agency is required to regulate as CISWI. The EPA, however, 

protests that it did not exempt these five types of units from 

regulation. Rather, the Agency determined that it lacked 

sufficient data to regulate the units at this time, and, with 

respect to some, it received comments suggesting the units 

were not CISWI.37

 37 The EPA asserts that it has not made a final decision with 

regard to the regulation of the five units at issue here—a claim that 

calls into question our jurisdiction, which under the CAA is limited 

to “final” actions. See Portland Cement, 665 F.3d at 193 (citing 42 

U.S.C. § 7607(b)). We disagree with the Agency. Because the 

statutory deadline for the EPA to establish emission standards for 

all CISWI has passed, see 42 U.S.C. § 7429(a)(1)(D), “the 

promulgated regulations must be deemed the [A]gency’s complete 

response in compliance with the statutory requirement[].” Hercules 

Inc. v. EPA, 938 F.2d 276, 282 (D.C. Cir. 1991) (internal quotation 

marks omitted). Accordingly, “even if [the Agency] promulgates 

additional . . . rules sometime in the future, petitioners’ claim that 

the existing final regulations are unlawful remains reviewable by 

this court.” Id. (internal quotation marks omitted). Moreover, the 

EPA did not signal in the administrative record that it was 

“continu[ing] the rulemaking process” as to these five units. 

Portland Cement, 665 F.3d at 194 (holding that the EPA’s action 

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We agree with the Environmental Petitioners that the 

Agency has violated its nondiscretionary statutory duty (1) to 

promulgate standards with respect to cyclonic burn barrels, 

and (2) to determine whether the remaining four types of units 

fall within the statutory definition of CISWI. The CAA 

requires the EPA to “establish performance standards . . . for 

each category of solid waste incineration units” no later than 

November 15, 1994. See 42 U.S.C. § 7429(a)(1)(A), (D). 

The statute then defines “solid waste incineration unit” as a 

“distinct operating unit of any facility which combusts any 

solid waste material from commercial or industrial 

establishments or the general public.” Id. § 7429(g)(1) 

(emphasis added). That provision unambiguously requires the 

EPA to set emission standards for “any facility that combusts 

any commercial or industrial solid waste material at all,” 

subject only to the listed statutory exceptions. NRDC I, 489 

F.3d at 1257-58. Because the statutory deadline to regulate 

these units has long passed, the EPA has “breached a nondiscretionary duty” if it has failed to promulgate standards for 

any facilities combusting solid waste from commercial or 

industrial establishments that do not fit into the listed 

exceptions. Sierra Club v. EPA, 992 F.2d 337, 346 (D.C. Cir. 

1993); cf. id. (explaining that the “plain language” of a similar 

provision in RCRA “obligates the Agency to issue, by the 

deadline, revisions for all facilities” covered by the statute 

and therefore “does not contemplate partial compliance”). 

The Agency makes no effort to claim that cyclonic burn 

barrels fall outside the statutory definition for CISWI units. 

 

was not “final” under 42 U.S.C. § 7607(b) because the Agency 

expressly stated in its final rule that the rulemaking process 

remained underway). We therefore need not consider whether our 

conclusion regarding finality would change had it done so. 

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Nor could it—both the administrative record and the EPA’s 

brief make clear that cyclonic burn barrels “combust” solid 

waste. See 2011 Proposed CISWI Rule on Reconsideration, 

76 Fed. Reg. at 80,460 (describing a cyclonic burn barrel as 

“a combustion device for waste materials”); No. 11-1125 

EPA Br. 68 (same); see also 42 U.S.C. § 7429(g)(1) (defining 

“solid waste incineration unit” as a “distinct operating unit of 

any facility which combusts any solid waste material from 

commercial or industrial establishments or the general 

public”). Because they combust solid waste, cyclonic burn 

barrels clearly fall within the statutory definition of “solid 

waste incineration unit” and, as established above, the EPA 

had a nondiscretionary statutory duty to establish emission 

standards for all these units by 1994. We therefore conclude 

that the Agency violated that duty by failing to promulgate 

emission standards for cyclonic burn barrels. 

The EPA protests that it reasonably chose not to regulate 

cyclonic burn barrels at this time, given how little information 

it had on them. According to the EPA, comments revealed 

there were many more cyclonic burn barrels in use than 

originally thought, the Agency lacked data on these units, and 

it was “difficult, if not impossible, to test such units for the 

section 7429 pollutants.” No. 11-1125 EPA Br. 69. But this 

argument misses the point: in light of the unambiguous 

statutory command to promulgate numeric standards for all 

solid waste incineration units, the EPA had no discretion to 

avoid regulating any such units—even if its choice to avoid 

regulating these units would have been otherwise reasonable. 

The Agency was obligated to collect the data it needed, and 

Congress gave it the authority to do so. See 42 U.S.C. 

§ 7414(a) (explaining that for the purpose of regulating solid 

waste combustion under section 7429, the EPA may, for 

example, require owners and operators of those units to 

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sample emissions, keep records, and offer other information 

that the Agency needs). Moreover, the Agency provides no 

evidence that it would be infeasible to set emission standards 

for these units. Instead, the EPA merely states that it 

“received information” that measuring emissions is difficult, 

“if not impossible,” but points to no comments or evidence 

supporting this assertion. 2011 Proposed CISWI Rule on 

Reconsideration, 76 Fed. Reg. at 80,460. 

The EPA also had a duty to determine whether the other 

challenged sources—burn-off ovens (including foundry sand 

reclamation units), soil treatment units, and space heaters—

were units that “combust” solid waste. Several commenters 

told the Agency that these units fell within the statutory 

definition of CISWI, and the EPA itself initially viewed some 

of these units as combusting waste. See, e.g., CISWI Rule—

Responses to Comments, at 74-76; 2010 Proposed CISWI 

Rule, 75 Fed. Reg. at 31,941. Under these circumstances, the 

Agency was obligated to determine whether the units in fact 

combust solid waste. Yet the EPA concedes it never made 

that determination. As we have explained, the EPA had a 

nondiscretionary duty to promulgate standards for all solid 

waste combustion units. This obligation includes the 

subsidiary duty to determine whether the units identified by 

the commenters in fact combust solid waste. Any other 

conclusion would allow the Agency to ignore its statutory 

mandate altogether by not taking the initial step of identifying 

such units. 

The CAA unambiguously requires that the Agency 

establish standards for all CISWI units. As a result, we grant 

the Environmental Petitioners’ petition for review on this 

issue and remand to the Agency to set emission standards for 

cyclonic burn barrels. The EPA must also determine whether 

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the remaining four types of units are CISWI units and, if it 

finds that they are, it must set standards for them as well. 

F. DELISTING UNDER 42 U.S.C. § 7412(C) 

In contrast to major source subcategories (all of which 

the EPA must control), the CAA does not require the EPA to 

control emissions in every area source subcategory. See 42 

U.S.C. § 7412(c)(1), (3). The Act does, however, mandate 

that the EPA control area source emissions if the area source 

subcategory meets certain criteria. Section 7412(c)(1), for 

instance, requires the EPA to control any area source 

subcategory upon the Agency’s finding that emissions from 

the sources in the subcategory jeopardize either the 

environment or human health. See id. § 7412(c)(3). If so, the 

EPA can establish either a MACT or a GACT standard. See 

id. § 7412(d)(5). Similarly, if the EPA finds that capping 

emissions from an area source subcategory is necessary to 

achieve a 90 per cent reduction in the aggregate emissions of 

one of seven CAA-enumerated HAPs, section 7412(c)(6) 

requires the Agency to impose caps in that subcategory as 

well. See id. § 7412(c)(6). Upon that finding, however, the 

EPA must impose a MACT standard. Id. 

In addition to prescribing requirements for inclusion of 

area source subcategories, the CAA provides a mechanism for 

removal of area source subcategories that, in the EPA’s view, 

no longer need to be controlled. Specifically, the EPA can 

“delete” any subcategory if it finds that no source or group of 

sources in it (1) emits cancer-causing HAPs at a volume 

sufficient to increase the lifetime risk of cancer in the 

population by more than one in one million and (2) emits noncancer-causing HAPs at a level in excess of that which is 

adequate “to protect public health with an ample margin of 

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safety” and to prevent against environmental harm. Id. 

§ 7412(c)(9)(B). The section 7412(c)(9) process is known as 

“delisting.” 

In 1998, the EPA identified several area source boiler 

subcategories—including oil-fired, industrial wood, 

commercial oil-fired and commercial wood-combustion 

boilers—as contributors to the “90 per centum of the 

aggregate emissions” of Hg and POM under section 

7412(c)(6). See Source Category Listing for Section 

112(d)(2) Rulemaking Pursuant to Section 112(c)(6) 

Requirements, 63 Fed. Reg. 17,838, 17,839 (Apr. 10, 1998). 

When it decided to “list” these sources, however, the EPA 

included a caveat. It explained that it used the best emissions 

information it had at the time to conclude that these boiler 

subcategories produced enough Hg and POM emissions to 

justify section 7412(c)(6) control but it also admitted that it 

could not “assure that this calculation of the 90 percent will 

remain constant.” Id. at 17,840. 

The caveat proved prescient. When the EPA issued the 

2010 Proposed Area Boilers Rule, it decided it needed to 

regulate only coal-fired boilers at the MACT level to control 

90 per cent of Hg emissions. See 75 Fed. Reg. at 31,898. 

And when it finalized the 2011 Area Boilers Rule, the Agency 

similarly decided that it needed to regulate only coal-fired 

boilers at the MACT level to control 90 per cent of POM 

emissions. See 76 Fed. Reg. at 15,566. 

For this reason, the EPA established GACT, rather than 

MACT, standards for the oil-fired and biomass-fired area 

source subcategories regarding these two pollutants. See id. 

It did not, however, make any of the “delisting” findings 

required by section 7412(c)(9) when it removed these area 

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source subcategories from section 7412(c)(6)’s purview. See 

2011 Area Boilers Rule, 76 Fed. Reg. at 15,566 (“[W]e have 

not removed or ‘delisted’ oil-fired and biomass-fired area 

source boilers by this action. We are not promulgating 

MACT-based regulations at this time because they are 

unnecessary to meet the requirements of CAA section 

112(c)(6).”). The Environmental Petitioners challenge the 

EPA’s imposition of GACT standards, arguing that, because 

once the EPA “listed” these sources under section 

7412(c)(6)’s MACT requirement, the CAA mandates that the 

EPA “delist” them under section 7412(c)(9) before putting 

them under the more lenient GACT standards. In their view, 

the EPA’s contrary approach fails at Chevron step 1. The 

EPA responds that section 7412(c)(9) applies only if it 

decides to “delist” a subcategory entirely from section 7412 

regulation, resulting in neither MACT nor GACT restrictions. 

Because section 7412(c)(9) does not unambiguously 

apply to section 7412(c)(6) and because the EPA’s 

interpretation of section 7412(c)(9)’s delisting requirement is 

reasonable, we uphold the EPA’s decision as permissible 

under Chevron step 2. Section 7412(c)(9) provides that the 

EPA “may delete any source category from the list under this 

subsection” on its finding that the source category is not a 

threat to human health or the environment. 42 U.S.C. 

§ 7412(c)(9)(B). The inclusion of a singular “list” to govern 

“this subsection” seems, most naturally, to refer to the list 

contemplated by section 7412(c)(1), which states that the 

EPA “shall publish, and shall from time to time . . . revise . . . 

a list of all categories and subcategories of . . . area sources 

(listed under paragraph (3)).” Id. § 7412(c)(1) (emphasis 

added). In other words, it appears that section 7412(c)(1) 

directs the EPA to create one “list” of source categories and 

subcategories to subject to emission controls and section 

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7412(c)(9) instructs how to remove source categories from 

that list. This conclusion finds support in section 7412(c)(1)’s 

cross reference to “paragraph (3)” of section 7412(c), which 

lays out the circumstances under which the EPA “shall list” 

area source categories for emissions control. Id. § 7412(c)(3). 

In the Environmental Petitioners’ view, section 

7412(c)(9) also applies to a second, subsidiary list—that 

contemplated by section 7412(c)(6), requiring imposition of 

the MACT standard. Granted, section 7412(c)(6) mandates 

that the EPA “shall . . . list” source categories and 

subcategories if doing so is necessary to control 90 per cent of 

the aggregate emissions from seven enumerated pollutants. 

Id. § 7412(c)(6) (emphasis added). But the use of the verb 

“list” in section 7412(c)(6) does not unambiguously establish 

that 7412(c)(9), titled “[d]eletions from the list,” applies. 

Because section 7412(c)(9) is ambiguous, we defer to the 

EPA so long as its interpretation is “based on a permissible 

construction.” Chevron, 467 U.S. at 842-43. And the EPA’s 

reading of section 7412(c)(9)—that it applies only if the EPA 

wants to remove a category from all section 7412 

regulation—is reasonable. 

First, the EPA’s approach harmonizes sections 

7412(c)(1), 7412(c)(3), and 7412(c)(9). Because the EPA 

must find that an area source “presents a threat of adverse 

effects to human health or the environment” before it 

regulates the source category at all, id. § 7412(c)(3), it makes 

sense to require the EPA to find that “no source in the 

category or subcategory . . . exceed[s] a level which is 

adequate to protect public health . . . and no adverse 

environmental effect will result from emissions from any 

source” before it completely deregulates that category, id. 

§ 7412(c)(9). It makes less sense to require the EPA to make 

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the same findings before it opts for GACT instead of MACT 

standards, which occurs when the EPA removes a source from 

section 7412(c)(6)’s purview but continues to regulate it 

under section 7412(c)(1). 

Second, the EPA’s approach is consistent with our 

decision in New Jersey, 517 F.3d 574. There, we held that 

“the only way EPA could remove [a source category] from 

the section [7412(c)(1)] list was by satisfying section 

[7412(c)(9)’s] requirements.” Id. at 582. In other words, New 

Jersey held that the EPA cannot remove a source category 

from all section 7412 regulation without delisting it; it said 

nothing about the process by which the EPA moves source 

categories from section 7412(c)(6). 

Finally, the Petitioners’ argument would freeze the 

EPA’s decision as to which sources need to be controlled to 

reach the requisite 90 per cent emissions reduction for the 

section 7412(c)(6) pollutants until it determines that “no 

source in the category or subcategory . . . exceed[s] a level 

which is adequate to protect public health . . . and no adverse 

environmental effect will result from emissions from any 

source.” 42 U.S.C. § 7412(c)(9). This, in turn, would hamper 

the EPA’s ability to respond to updated data, thereby 

substantially complicating its attempts to control the 

pollutants. Nothing in the CAA suggests that the Congress 

intended to so hamstring the Agency. 

G. TITLE V PERMIT EXEMPTION FOR SYNTHETIC BOILERS

The EPA has discretion to exempt one or more area 

source categories from Title V permitting requirements upon 

a finding “that compliance with such requirements is 

impracticable, infeasible, or unnecessarily burdensome on 

such categories.” 42 U.S.C. § 7661a(a). The EPA originally 

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proposed exempting some area source categories because 

existing “testing, monitoring, notification, and recordkeeping 

requirements” rendered Title V permitting cumulative. 2010 

Proposed Area Boilers Rule, 75 Fed. Reg. at 31,910. At the 

time, however, the EPA elected not to exempt synthetic area 

sources as one of those categories. Id. at 31,913. Synthetic 

area sources are boilers that “naturally” emit pollutants at a 

major source level but which qualify as area sources due to 

the voluntary adoption of air pollution control technologies. 

Id. Despite its initial stance, the EPA ultimately decided to 

exempt all area sources—including synthetic area sources—

from Title V’s permitting requirements. See 2011 Area 

Boilers Rule, 76 Fed. Reg. at 15,578. 

Environmental Petitioners argue the EPA’s decision to 

exclude synthetic boilers from Title V licensing requirements 

is arbitrary and capricious for two reasons. First, they say, the 

EPA arbitrarily concluded synthetic area sources would bear 

the same level of burden as other area sources in complying 

with Title V permitting requirements, rather than a lesser one. 

See No. 11-1141 Envtl. Pet’rs’ Br. 39-43. And second, they 

contend the EPA arbitrarily dismissed the additional 

compliance benefits of Title V licensing for these synthetic 

sources. See id. at 43-47. Under State Farm, “an agency rule 

[is] arbitrary and capricious if the agency . . . offered an 

explanation of its decision that runs counter to the evidence 

before the agency.” 463 U.S. at 43. A court may not accept 

an agency’s “post hoc rationalizations” for its 

decisionmaking. Id. at 49. 

The EPA has authority under the CAA to exempt sources 

from Title V permitting requirements if those requirements 

would be “impracticable, infeasible, or unnecessarily 

burdensome” on the area source. 42 U.S.C. § 7661a(a). The 

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EPA previously developed a four-factor balancing test to 

determine whether Title V’s requirements are “unnecessarily 

burdensome.” See Exemption of Certain Area Sources from 

Title V Operating Permit Programs, 70 Fed. Reg. 75,320, 

75,323 (Dec. 19, 2005). Under this test, the EPA considers 

whether: (1) Title V permitting would result in significant 

improvements in compliance with emission standards; 

(2) whether Title V permitting would impose significant 

burdens on the area source category; (3) whether the costs are 

justified, taking into account potential gains; and (4) whether 

there are existing enforcement programs in place sufficient to 

ensure compliance. See id. at 75,323-26. The EPA also must 

consider, consistent with the legislative history of the CAA, 

whether exemption would “adversely affect public health, 

welfare, or the environment.” Id. at 75,333-34. These factors 

are considered in combination and not every factor must point 

in favor of exemption for the EPA to choose that course. See 

id. at 75,323. 

In its 2010 Proposed Area Boilers Rule, the EPA applied 

this balancing test and excluded almost all area source boilers 

except synthetic boilers that achieved “area” status via 

installation of a control technology (although it exempted 

those that achieved “area” status through operational 

changes). The EPA provided an extensive rationale for its 

decision to exclude these “natural” area sources from Title 

V’s permitting requirements. See 2010 Proposed Area 

Boilers Rule, 75 Fed. Reg. at 31,910-13. With respect to 

factor one, the EPA found its proposed rule already required 

“direct monitoring of emissions,” both continuously and 

periodically, recordkeeping that would allow for additional 

monitoring, and “semi-annual reporting to assure 

compliance.” Id. at 31,911. Moreover, under the proposed 

rule, “records are required to be maintained in a form suitable 

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and readily available for expeditious review” for up to five 

years. Id. The EPA acknowledged Title V permitting could 

provide some additional compliance benefits; specifically, 

that Title V has an every-six-month monitoring and reporting 

requirement. See id. But the EPA ultimately concluded the 

monitoring, recordkeeping, and reporting requirements of its 

proposed rule were sufficient to assure compliance: “Given 

the nature of the operations at most area sources and the types 

of requirements in this rule, Title V would not significantly 

improve those compliance requirements.” Id. 

As to the second factor, the EPA noted that subjecting 

most area sources to Title V would “impose[] certain burdens 

and costs that do not exist outside of the [t]itle V program.” 

Id. at 31,912. One of the EPA’s major concerns was that 

“requiring permits for the large number of area sources could, 

at least in the first few years of implementation, potentially 

adversely affect public health, welfare, or the environment by 

shifting [s]tate agencies[’] resources away from assuring 

compliance for major sources with existing permits to issuing 

new permits for these area sources, potentially reducing 

overall air program effectiveness.” Id. at 31,913. For the 

third factor, the EPA concluded the costs of compliance 

would “impose a significant burden on many of the 

approximately 137,000 facilities affected by this proposed 

rule” with only “low” potential gains in compliance. Id. at 

31,912. Finally, for the fourth factor, the EPA determined 

that “[s]tate delegated programs are sufficient to assure 

compliance with this [rule],” and noted that the Agency 

retains authority to enforce this rule “anytime.” Id. The EPA 

therefore proposed exempting these area sources from the 

permitting requirements. See id. Environmental Petitioners 

are not currently challenging the exemption for non-synthetic 

area boilers.

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However, in this 2010 rulemaking, the EPA also 

explained precisely why it declined to exempt synthetic area 

sources that installed air pollution controls from Title V 

requirements. First, the EPA noted these synthetic area 

sources “represent less than one percent of the total number of 

sources that will be subject to the final rule.” Id. at 31,913. 

The EPA also characterized these sources as “much more like 

the major sources” that are not exempt from Title V 

permitting requirements. Id. Further distinctions included 

that “many of these sources are located in cities, and often in 

close proximity to residential and commercial centers where 

large numbers of people live and work,” that they “have 

significantly higher emissions potential when uncontrolled” 

(even compared to synthetic boilers that adopted operational 

limits to attain area source status), and that many of these 

sources “are large facilities with comprehensive compliance 

programs in place” as opposed to small facilities, like schools 

or hospitals. Id. Given these distinctions, the EPA concluded 

additional public involvement and compliance oversight 

through Title V was “important to ensure that these sources 

are maintaining their emissions at the area source level.” Id. 

But the EPA shifted its position in the 2011 Area Boilers 

Rule by deciding to exempt all area sources, including 

synthetic sources. See 76 Fed. Reg. at 15,578. The EPA 

provided only a cursory explanation for this shift, noting how 

a further review of the record led it to conclude “observations 

and data we have relied upon in other rulemakings for 

distinguishing between sources that became synthetic area 

sources due to controls and other synthetic and natural area 

sources [do] not necessarily apply to this source category.” 

Id. (emphasis added). Because the EPA asserted it no longer 

had “sufficient information” to identify control-technologydependent synthetic sources, it decided to apply the same 

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rationale used to exempt “natural” sources to these synthetic 

sources. Id. (“[T]he rationale for exempting most area 

sources subject to this rule . . . is also now relevant for sources 

which we proposed to permit [under Title V].”). But—even if 

the EPA truly cannot distinguish between synthetic sources 

relying on control technologies and other sources—it does not 

invariably follow that the justifications the Agency relied on 

for exempting “natural” sources under the four-factor 

balancing test can be transposed onto these synthetic sources. 

Cf. Sierra Club II, 479 F.3d at 884 (“We agree with the Sierra 

Club that EPA’s use of work practice standards instead of 

emission floors violates section 7412(h). That provision 

allows EPA to substitute work practice standards for emission 

floors only if measuring emissions levels is technologically or 

economically impracticable. Here, EPA never determined 

that measuring emissions from ceramic kilns was 

impracticable; it determined only that it lacked emissions data 

from ceramics kilns. EPA thus had no basis under section 

7412(h) for using work practice standards.”). 

In its next iteration of the rule, the EPA endeavored to 

further explain its exemption of synthetic sources. The EPA 

again stated it “lacked sufficient information” to distinguish 

these synthetic sources from other area sources. See 2011 

Proposed Area Boilers Rule on Reconsideration, 76 Fed. Reg. 

at 80,538. The Sierra Club challenged this exemption in a 

comment, and the EPA responded with “additional analysis” 

of the synthetic exemption. Id. In this analysis, the EPA first 

reiterated the difference in number between the two types: 

estimating there to be at least 48 control-technologydependent synthetic sources versus 137,000 other area 

sources, most of which are located at small facilities like 

schools, hospitals, and churches. See id. The EPA then 

provided a new rationale for the exemption: that these 

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synthetic facilities “may already have a Title V permit for 

other reasons.” Id. The EPA also found that “synthetic area 

sources would likely be subject to more stringent permit and 

monitoring requirements than natural area sources” because 

they have a “legal duty to use the control equipment” to keep 

them at an “area” level. Id. (emphasis added). Finally, the 

EPA made several assertions about the similarities between 

synthetic and natural sources. Specifically, that synthetic 

sources are “similar in size and sophistication to those that are 

natural area sources,” that their “uncontrolled emissions are 

generally on the same order of magnitude as the emissions of 

natural sources,” and that “the facilities and owners are 

comparable in size.” Id. The EPA provided no data or 

examples in support of these assertions, which appear to 

directly contradict the distinctions the EPA listed in its earlier 

version of the rule. Compare id., with 2010 Proposed Area 

Boilers Rule, 75 Fed. Reg. at 31,913. In its final rule, the 

EPA declined to make any changes to its Title V 

exemptions—exempting all area sources including synthetic 

sources using a control technology. See 2013 Area Boilers 

Rule, 78 Fed. Reg. at 7,497. 

Based on this record, the EPA’s reasoning has several 

fatal flaws that render its exemption decision arbitrary. The 

EPA put forward two primary justifications for exempting 

synthetic sources: (1) that it could not necessarily rely on 

existing data for distinguishing the different type of sources, 

and (2) that these facilities are “similar in size and 

sophistication” to natural area sources. See 2011 Proposed 

Area Boilers Rule on Reconsideration, 76 Fed. Reg. at 

80,538. The second justification flatly contradicts the EPA’s 

earlier, extensive discussion about how these synthetic 

sources have higher emissions potential and are often located 

on large sites with existing compliance programs, in addition 

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to being uniquely few in number and generally found near 

cities. 2010 Proposed Area Boilers Rule, 75 Fed. Reg. at 

31,912-13. These factors all undercut the EPA’s assertion 

that synthetic sources are “similar”—in size, sophistication, or 

otherwise—to natural sources. With respect to the lack of 

data for distinguishing, the EPA was able to estimate in its 

proposed rule that 48 synthetic sources would have been 

affected by this rule—which suggests the EPA possesses 

some mechanism for distinguishing the types. See 2011 

Proposed Area Boilers Rule on Reconsideration, 76 Fed. Reg. 

at 80,538. Moreover, the EPA does not explain why the data 

it used in prior rulemakings to distinguish these source types 

is not accurate in this context. Environmental Petitioners also 

point out that, “to qualify for area-source status, synthetic area 

sources must notify the EPA or the state permitting authority 

of the limits on their emissions,” such that the EPA “need 

only ask these authorities to identify the sources operating in 

their states.” No. 11-1141, Envtl. Pet’rs’ Br. 39-40. The EPA 

never endeavors to explain why that mechanism (or any other 

existing mechanism) is insufficient for identifying synthetic 

area sources. 

Because its justifications for the final rule contradict 

earlier findings, the EPA must provide some reasoning to 

explain why its final decision “runs counter to the evidence 

before the agency.” State Farm, 463 U.S. at 43. The EPA’s 

proffered explanation fails. This court has “often declined to 

affirm an agency decision if there are unexplained 

inconsistencies in the final rule.” See Dist. Hosp. Partners, 

786 F.3d at 59; see also Gulf Power Co. v. FERC, 983 F.2d 

1095, 1101 (D.C. Cir. 1993) (“[W]hen an agency takes 

inconsistent positions . . . it must explain its reasoning.”); 

Gen. Chem. Corp. v. United States, 817 F.2d 844, 846 (D.C. 

Cir. 1987) (holding agency action to be arbitrary because its 

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analysis was “internally inconsistent and inadequately 

explained”). The EPA had a duty here to examine and justify 

the “key assumptions” underlying its decision, and it failed to 

do so. See Appalachian Power Co. v. EPA, 135 F.3d 791, 818 

(D.C. Cir. 1998) (“EPA retains a duty to examine key 

assumptions as part of its affirmative burden of promulgating 

and explaining a nonarbitrary, non-capricious rule.” (internal 

quotation marks omitted)). 

The EPA’s major oversight was its failure to explain why 

the rationale it used to exempt natural area sources from Title 

V could be identically applied to synthetic area sources. One 

of the Agency’s main justifications for exempting natural area 

sources was that their prolific numbers might overwhelm state 

and local regulatory agencies, diverting resources from other 

important environmental programs, thereby harming public 

health and welfare. The EPA never explained why requiring 

48 synthetic area sources to comply with Title V would strain 

government resources to a comparable degree as would 

requiring the 137,000 natural area sources to comply. As 

discussed above, the EPA also did not explain how it 

suddenly determined these synthetic area sources were 

“similar in size and sophistication” to natural sources, when it 

had previously articulated several key differences. It is 

particularly unclear how these synthetic sources could have 

“uncontrolled emissions . . . generally on the same order of 

magnitude as the emissions of natural area sources.” 2011 

Proposed Area Boilers Rule on Reconsideration, 76 Fed. Reg. 

at 80,538. Given that synthetic sources are defined as 

“major” sources that have artificially reduced their emissions 

to an “area” level, it is difficult to understand how the 

uncontrolled emissions of these sources would be similar to 

natural area sources. Additionally, the EPA asserted that 

synthetic source “facilities and owners are comparable in 

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size” to natural sources. Id. This contradicts earlier findings 

that synthetic sources tend to be large, located on sites with 

existing compliance plans, and near population-dense areas. 

The EPA provides no data or explanation to support this shift. 

The EPA relies on another problematic premise when it 

claims the potential benefits of subjecting synthetic area 

sources to Title V requirements are low. Both the EPA and 

Industry Intervenors argue that the added benefits of Title V 

would be minimal for these synthetic sources, relying solely 

on the rationale given for natural sources. But the EPA 

originally asserted “additional public involvement and 

compliance assurance requirements through title V [are] 

important to ensure that these sources are maintaining their 

emissions at the area source level.” 2010 Proposed Area 

Boilers Rule, 75 Fed. Reg. at 31,913 (emphasis added). The 

EPA never explains why these additional benefits were 

considered “important” before but are now “not important” 

simply because it allegedly determined that synthetic sources 

may be hard to distinguish from natural sources. The 

difficulty in identifying synthetic sources says nothing about 

the benefits that may be gained by requiring Title V permits, 

assuming the sources can be identified. Synthetic sources 

retain the attributes which first motivated the EPA to subject 

them to Title V permitting: they tend to be near cities, 

specifically near large residential populations, and they have 

greater emission potential if their control technology is 

removed, turned off, or not kept up to standards. The EPA 

arguably finds Title V’s additional compliance benefits 

unnecessary because synthetic sources have “a legal duty to 

use the control equipment” and that use is “not optional.” 

2011 Proposed Area Boilers Rule on Reconsideration, 76 Fed. 

Reg. at 80,538. But that observation does not speak to the 

need for public oversight; just because facilities are obligated 

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to use the control technology does not mean they will always 

do so. Title V’s process requires facilities to submit 

compliance documentation every six months—far more 

frequently than under the EPA’s current rule—which expands 

the opportunity for public oversight and compliance. Perhaps 

this “legal duty” provides a stronger incentive for compliance 

than public oversight but, if so, the EPA still fails to explain 

how. 

Similarly, for factor three’s balancing of costs and 

benefits, the EPA never justifies applying to natural sources—

which tend to be small sites like schools, hospitals, and 

churches—the same rationale it applies to these larger 

synthetic sources, which tend to be located at refineries, 

chemical plants, and factories. Given these distinctions, it is 

at least possible this balancing would lead to a different 

outcome for synthetic sources. Taken as a whole, the EPA’s 

analysis fails to explain why several of the facts and 

characteristics it relied on for its initial assessment are no 

longer relevant—creating several glaring inconsistencies in 

the rulemaking record. The EPA offers no plausible reason 

for applying the results of the four-factor test for natural 

sources wholesale to these control-technology-dependent 

synthetic sources. We do not hold, however, that the EPA can 

never remove synthetic area sources from the ambit of Title V 

compliance. The outcome the EPA ultimately reached may 

be reasonable; however, “[n]ot only must an agency’s decreed 

result be within the scope of its lawful authority, but the 

process by which it reaches that result must be logical and 

rational.” Allentown Mack Sales & Serv., Inc. v. NLRB, 522 

U.S. 359, 374 (1998). The EPA should have applied its fourfactor balancing test directly to synthetic sources or, at a 

minimum, provided an explanation for adopting the natural 

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source balancing test that is not premised on inconsistencies 

in the record. 

With respect to remedy, there is a strong possibility that 

the Agency can properly explain its decision to exclude 

synthetic boilers from the Title V permitting requirement; 

moreover, vacating the decision would be unnecessarily 

disruptive for synthetic boiler operators who, in the interim, 

would not know whether they needed to begin the expensive, 

time-consuming process of obtaining a Title V permit. See

Allied-Signal, Inc. v. U.S. Nuclear Regulatory Comm’n, 988 

F.2d 146, 150-51 (D.C. Cir. 1993). We therefore remand this 

issue (without vacating) for further explanation by the EPA. 

H. GACT STANDARD DETERMINATIONS

With few exceptions, the EPA has broad discretion to 

choose how to control area source emissions. For instance, 

the EPA has discretion to choose between GACT and MACT 

standards in the majority of cases. See 42 U.S.C. 

§ 7412(d)(5). Even if the EPA chooses a MACT standard, it 

has discretion—although somewhat circumscribed—to set a 

work-practice standard instead of a numeric standard. Id. 

§ 7412(h)(1). And the EPA has discretion when choosing 

among different GACT-standard options. See id.

§ 7412(d)(5). 

Accordingly, we must uphold the EPA’s GACT-standard 

determinations so long as it “has considered the relevant 

factors and articulated a rational connection between the facts 

found and the choice made, and has not relied on [improper] 

factors.” Nat’l Ass’n of Clean Air Agencies, 489 F.3d at 1228 

(citations and internal quotation marks omitted). But for all 

of the discretion the EPA enjoys, it must nonetheless 

demonstrate that it exercised its judgment in a reasoned way. 

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The cases establishing this principle are legion. See, e.g., 

Transactive Corp. v. United States, 91 F.3d 232, 236 (D.C. 

Cir. 1996) (agency must “identif[y] and explain[] the 

reasoned basis for its decision”); Int’l Fabricare Inst., 972 

F.2d at 389 (agency must “examine[] the relevant data and 

. . . articulate[] an adequate explanation for its action”). The 

EPA need not go to great lengths to meet its burden; indeed, 

we “uphold a decision of less than ideal clarity” so long as 

“the agency’s path may reasonably be discerned.” State 

Farm, 463 U.S. at 43 (quoting Ark.-Best Freight Sys., Inc., 

419 U.S. at 286). 

With these principles in mind, we address the 

Environmental Petitioners’ two challenges to the EPA’s 

discretionary decisions regarding the Area Boilers Rule. 

1. EPA’s Selection of GACT Standards 

for Non-Hg Metals 

The Environmental Petitioners argue that the EPA failed 

to support its decision-making when it established MACT 

standards for Hg and POM emissions from some coal-fired 

boilers but declined to regulate non-Hg emissions under the 

MACT standard from the same boilers. We agree. Although 

the EPA thoroughly explained why it chose to impose one 

GACT standard instead of another, nothing in the record 

explains why the EPA decided to impose GACT standards 

instead of MACT standards in the first place. Despite the 

Agency’s broad discretion, we cannot sustain its action in the 

absence of some explanation for why GACT standards are 

more appropriate than MACT standards for these sources and 

types of pollutants. See Transactive Corp., 91 F.3d at 236. 

For this reason, we remand (but do not vacate) the EPA’s 

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fired boilers. See Sierra Club, 167 F.3d at 664; Nat’l Lime 

Ass’n, 233 F.3d at 634-35. 

2. EPA’s Selection of Certain GACT Standards 

The Environmental Petitioners also challenge several of 

the EPA’s choices among different GACT standards. As 

noted, see supra § I.A.1.c, the CAA provides virtually no 

instruction regarding GACT standards but the standards 

generally take the form of “methods, practices and techniques 

which are commercially available and appropriate for 

application by the sources in the category considering 

economic impacts and the technical capabilities of the firms 

to operate and maintain the emissions control systems.” S.

REP. NO. 101-228, at 171 (1989). Because the EPA has 

ample discretion to choose the appropriate GACT standard, 

we will affirm its choices so long as we can discern reasoned 

decision-making from the record. State Farm, 463 U.S. at 43. 

For the reasons set forth below, we can do so here and, 

accordingly, we reject the Environmental Petitioners’ GACTfocused challenges. 

First, the Environmental Petitioners challenge the data set 

the EPA used to arrive at the numeric GACT standards for 

non-Hg-metal emissions from coal-fired boilers. Specifically, 

they contend that the EPA set the GACT limit based on 

boilers with no control technology, which resulted in a 

numeric standard of 0.42 lb/mmBtu. They insist that the EPA 

should instead have examined boilers outfitted with fabric 

filters, which would have resulted in a numeric standard of 

0.03 lb/mmBtu. The EPA, however, thoroughly explained 

why it considered the uncontrolled boiler data set. 

Specifically, the controlled data set derives from the EPA’s 

“New Source Performance Standards” (NSPS) data, which, in 

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the Agency’s view, could be used to set the non-Hg-metal 

GACT standard for boilers with a heat input capacity of 30 

mmBtu/hr or greater but did not suffice for boilers with a 

lower heat input capacity. For this reason, the EPA examined 

its original data set, found that none of the coal-fired boilers 

in that set used control technology and, accordingly, set the 

GACT numeric standard at the emissions level achieved by 

the best performing uncontrolled source in that data set (i.e., 

0.42 lb/mmBtu). We are satisfied that the EPA exercised its 

discretion in a reasoned manner and, accordingly, we do not 

disturb it. See Transactive Corp., 91 F.3d at 236. 

Next, the Environmental Petitioners challenge the EPA’s 

decision to establish a tune-up requirement as a GACT 

management-practice standard for Hg and POM emissions 

from large biomass-fired and oil-fired boilers. In their view, 

other, more restrictive control technologies, including 

multiclones,38 are “generally available” and their availability 

mandates that the EPA set numeric standards based on boilers 

that use those controls. But the EPA explained its approach: 

A boiler tune-up requirement would potentially 

result in the same non-mercury metallic HAP 

reduction as a PM emission limit based on 

performance of multiclones but would also 

reduce emissions of organic HAP. In addition 

the cost of a boiler tune-up appears minimal 

compared to the cost for testing and 

 38 A multiclone is a PM “mechanical separator[].” See 2010 

Proposed Area Boilers Rule, 75 Fed. Reg. at 31,908. It diverts 

particles from the exhaust stream by creating a circular air flow. 

See id.

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monitoring to demonstrate compliance with an 

emission limit. 

See 2010 Proposed Area Boilers Rule, 75 Fed. Reg. at 31,908. 

The EPA also explained that multiclones were “minimally 

effective” for controlling non-Hg metals, ineffective for POM 

and Hg, and expensive. Id. Because the EPA’s decision to 

impose a tune-up requirement fits within its “technical 

expertise,” we owe the Agency an “extreme degree of 

deference” so long as its explanation is rational. Catawba 

Cty., N.C. v. EPA, 571 F.3d 20, 41 (D.C. Cir. 2009) (quoting 

City of Waukesha v. EPA, 320 F.3d 228, 247 (D.C. Cir. 

2003)). And because its explanation was rational, we reject 

the Petitioners’ challenge thereto. 

Finally, the Petitioners challenge the EPA’s decision to 

set a tune-up requirement as a management-practice standard 

for small biomass-fired and oil-fired area boilers. The EPA 

adopted this approach because measuring PM emissions for 

smaller boilers is “not feasible.” 2010 Proposed Area Boilers 

Rule, 75 Fed. Reg. at 31,906. When the EPA explained its 

decision regarding small biomass-fired and oil-fired area 

boilers, it provided the same reasons it gave for its use of a 

tune-up requirement for small coal-fired area boilers, which 

we address (and uphold), infra, § IV.M. For those reasons, 

we reject the challenge to the EPA’s tune-up requirement for 

small biomass-fired and oil-fired area boilers. 

I. 30-DAY ROLLING AVERAGE

As discussed, see supra § I.B.1.a, when the EPA sets a 

MACT floor, it begins by examining data generated by stack 

testing. Once the MACT standard is established, however, a 

source may (and in some cases, must) demonstrate 

compliance by implementing “continuous monitoring” instead 

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of conducting additional stack tests.39 See 2011 Proposed 

CISWI Rule on Reconsideration, 76 Fed. Reg. at 80,464-65. 

For a source using a continuous monitor, the EPA determines 

MACT-standard compliance based on the source’s thirty-day 

“rolling average.” Id. at 80,465. 

The calculation of a thirty-day rolling average is 

straightforward: the average of a source’s daily emissions for 

the immediately preceding thirty days. Each day produces a 

new rolling average and each “average is a separate 

compliance determination.” No. 11-1125 EPA Br. 88 n.17. 

In the EPA’s view, this “allow[s] operators sufficient 

flexibility for operational and control device adjustments 

should they be needed for short term fuel or waste 

characteristics variability.” 2011 Proposed CISWI Rule on 

Reconsideration, 76 Fed. Reg. at 80,465. The EPA also 

concluded that thirty-day rolling average violations will occur 

almost as frequently as violations of shorter rolling-average 

periods. Id. 

The CAA vests the EPA with authority to “prescribe 

procedures and methods for determining compliance and for 

monitoring and analysis of pollutants.” 42 U.S.C. § 7661c(b). 

We have emphasized that the EPA has “broad discretion in 

selecting a monitoring regime that ensures compliance, and as 

long as it reasonably articulate[s] the basis for its decision, 

[we] will defer to the informed discretion of the Agency, 

 39 As the name suggests, a continuous monitoring system 

measures the source’s emissions at all times and generally takes one 

of two forms: (1) a continuous parameter monitor, which measures, 

e.g., a source’s temperature, pressure or oxygen content; or (2) a 

continuous emissions monitor, which measures the pollutant 

concentration in the source’s emissions. 

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recognizing that analysis of this issue requires a high level of 

expertise.” White Stallion Energy Ctr., LLC v. EPA, 748 F.3d 

1222, 1255 (D.C. Cir. 2014) (per curiam) (citation and 

internal quotation marks omitted), rev’d on other grounds by 

Michigan v. EPA, 135 S. Ct. 2699 (2015). Notwithstanding 

this deference, the Environmental Petitioners argue that 

allowing a source to demonstrate compliance by way of a 

thirty-day rolling average not only fails Chevron review but is 

also arbitrary. We disagree. 

First, they argue that the thirty-day rolling average fails at 

Chevron step 1 because it allows sources to emit HAPs 

continuously at the UPL-established MACT floor. Because 

they do not believe that the UPL represents the average 

emissions level achieved by the best performing sources, they 

argue that, ipso facto, allowing sources to continuously emit 

HAPs at the UPL level means that sources are permitted to 

emit at levels higher than the average levels achieved by the 

best performing sources. Because we have already concluded 

that the UPL is in fact a reasonable proxy for the average 

emissions level achieved by the best performing sources, see 

supra § IV.C, the Environmental Petitioners’ premise is 

inaccurate. And because the “total emissions from a unit 

complying with a rolling average must still be below the total 

emissions from a unit emitting continuously at the level of the 

standard,” No. 11-1125 EPA Br. 90, the Environmental 

Petitioners’ Chevron step 1 argument fails. 

The Environmental Petitioners’ Chevron step 2 argument 

fares no better. The EPA explained that (1) it expects to catch 

violations using a thirty-day rolling average “almost as much 

as for a shorter term average” and (2) it believes the longer 

average to be more effective in addressing “[c]oncerns of 

variability outside the operators[’] control such as fuel 

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content, seasonal factors, load cycling, and infrequent hours 

of needed operation.” 2011 Proposed CISWI Rule on 

Reconsideration, 76 Fed. Reg. at 80,465. Because the EPA 

“reasonably articulate[d] the basis for its decision,” we uphold 

it. White Stallion, 748 F.3d at 1255 (internal quotation marks 

omitted). 

Finally, we conclude that the EPA’s allowance of thirtyday rolling averaging does not reflect an arbitrary change in 

position. Although the Petitioners cite other rules that, in 

their view, manifest that the EPA once believed that longer 

averaging periods resulted in less stringent enforcement, most 

of the rules they cite have nothing to do with MACT-setting 

or MACT compliance40 and none evidences an unexplained or 

unjustified deviation. Similarly, the Petitioners point to the 

EPA’s explanation in the 2011 CISWI Rule that “24-hour 

block averages . . . would be inconsistent with the sampling 

time for the stack test data” to indicate an arbitrary change in 

position. See 76 Fed. Reg. at 15,728. But the EPA made this 

statement while discussing why stack-test data and 

continuous-monitoring data could not be used in tandem to set 

a MACT level, which says nothing about allowing emissions 

 40 The only exception is the EPA’s 1996 Medical-Waste 

Incinerators Rule, which provides that “[t]he period of time over 

which emissions are measured and then averaged to determine 

compliance with the regulation . . . must correspond to the period of 

time over which emission levels were measured and averaged in 

determining the emission limits included in the regulation.” 

Standards of Performance for New Stationary Sources and 

Emission Guidelines for Existing Sources: Medical Waste 

Incinerators, 61 Fed. Reg. 31,736, 31,748 (June 20, 1996). This 

twenty-year-old statement, however, does not detract from the 

EPA’s well-reasoned defense of the thirty-day rolling average in 

the CISWI Rule. 

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averaging—long-term or otherwise—to gauge MACT-floor 

compliance. See id. And even if the Environmental 

Petitioners had directed us to a real about-face, the EPA’s 

justification for allowing the thirty-day rolling average 

convinces us that any change was not arbitrary. 

J. FUEL-COMBUSTION-BASED SUBCATEGORIES

Section 7412 provides that the EPA may distinguish 

among “classes, types, and sizes” of sources when 

establishing emission standards. 42 U.S.C. § 7412(c), (d)(1). 

Under this authority, the EPA created subcategories of major 

source boilers based on the fuel the boiler was designed to 

burn. 2013 Major Boilers Rule, 78 Fed. Reg. at 7,144. The 

Environmental Petitioners challenge this decision on three 

grounds: First, they claim that the text of the statute 

forecloses the EPA from creating such subcategories. Next, 

they argue that the EPA’s subcategories are arbitrary because 

they permit a boiler to switch subcategories from year to year. 

Finally, they contend that the EPA’s action was arbitrary 

because the Agency failed to demonstrate with substantial 

evidence that burning a different fuel alters the boiler’s class, 

type, or size. 

These arguments fail. Section 7412(d) gives the EPA 

discretion to create subcategories based on boiler type, and 

nothing in the statute forecloses the Agency from doing so 

based on the type of fuel a boiler was designed to burn. Nor 

was the EPA’s decision to create such subcategories arbitrary 

and capricious. The Agency considered the relevant factors in 

coming to a reasoned decision that the type of fuel a boiler is 

designed to burn impacts the feasibility of emission standards. 

And, finally, the EPA based its technical judgment on 

sufficient record evidence. As a result, we deny the 

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Environmental Petitioners’ challenge to the EPA’s 

subcategorization of major source boilers based on the type of 

fuel the boiler is designed to burn. 

The Environmental Petitioners first claim that the text of 

the CAA forecloses the EPA from creating subcategories of 

“types” of boilers based on the fuel a boiler burns because a 

single boiler may use different fuels over the course of its 

lifetime. This may be true, but the Petitioners never explain 

what it is about the word “type” that bars the EPA from 

regulating a boiler that burns “x” differently from a boiler that 

burns “y.” According to its ordinary meaning, “type” is easily 

broad enough to accommodate changes in boiler 

characteristics from year to year. See OXFORD ENGLISH

DICTIONARY (2013) (defining “type” as a “general form, 

structure, or character distinguishing a particular kind, group, 

or class of beings or objects”). There is no textual reason then 

to assume that a boiler’s type must be written in stone. 

Nor does our understanding of “type” write it out of the 

statute, as the Petitioners contend. The EPA has done what 

the term plainly encompasses: it has distinguished among 

boilers based on the kind of fuel the boiler burned over the 

last year. It is thus not surprising that we have interpreted a 

similar provision to permit distinctions based on fuel inputs. 

See Sierra Club v. Costle, 657 F.2d 298, 318-19 (D.C. Cir. 

1981) (holding that the text of 42 U.S.C. § 7411, which 

allows the EPA to “distinguish among classes, types and 

sizes,” permits distinctions based on variations in the sulfur 

content of coal used by utility plants). Likewise, we conclude 

that section 7412’s undefined and unrestricted use of class, 

type, or size does not foreclose the EPA’s interpretation. 

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This court will, as a result, defer to the EPA’s 

interpretation so long as it is reasonable. See, e.g., Sierra 

Club I, 353 F.3d at 990. And here, it is. The Agency 

explained that boilers vary in their designs depending on the 

type of fuel they burn. 2010 Proposed Major Boilers Rule, 75 

Fed. Reg. at 32,016-17. These differences, according to the 

Agency, affect boiler emissions and the feasibility of emission 

controls. Id. And, because design constraints also restrict a 

boiler’s ability to switch fuels, the Agency concluded that it 

could determine a boiler’s type by looking at the fuel it had 

burned over the previous 12-month period. Id. at 32,014. 

The Environmental Petitioners point to nothing in the record 

that calls into question either of these technical judgments, 

which receive great deference. See NRDC I, 489 F.3d at 

1375. Nor do the Petitioners offer any additional reasons in 

support of their argument that the Agency has ventured 

beyond its authority under the statute. In fact, the EPA’s 

reasoning from the emissions data is consistent with the very 

existence of a subcategorization authority because the grant of 

this authority implicitly acknowledges that the EPA may need 

to set different emission standards within a category of major 

sources based on what is achievable for a subset of those 

sources. Because the statutory text readily encompasses the 

EPA’s interpretation for the reasons explained above, and 

because the Environmental Petitioners offer no additional 

argument as to why the EPA’s interpretation was 

unreasonable, we reject the Petitioners’ Chevron challenge to 

the EPA’s interpretation of its subcategorization authority. 

The Environmental Petitioners nevertheless claim that the 

EPA’s subcategories are arbitrary because a boiler is not of a 

different type when it can be a boiler “designed to burn coal” 

one year, and a boiler “designed to burn biomass” the next. 

But this argument fails for the same reasons as the Chevron

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argument we just rejected. The fact that boilers may switch 

from one type to another over time does not, alone, render a 

subcategorization arbitrary. With no discernable basis to find 

the EPA’s choice here questionable, much less arbitrary, we 

reject this argument too. 

Finally, the Environmental Petitioners contend that the 

Agency failed to demonstrate with sufficient evidence, rather 

than mere assertions, that burning a different fuel makes the 

boiler a different class, type, or size. The Petitioners largely 

fail to develop this argument and, regardless, the EPA easily 

met its burden. The EPA based its decision on documented 

emissions data, several reports provided by the National 

Energy Technology Laboratory on boiler operations, and 

operating manuals provided by boiler manufacturers. See, 

e.g., Summary of Public Comments and Responses for 

National Emission Standards for Hazardous Air Pollutants for 

Major Sources: Industrial, Commercial, and Institutional 

Boilers and Process Heaters, EPA-HQ-OAR-2002-0058-

3511-A1 (Dec. 2012), at 558-63. These sources support the 

EPA’s decision to distinguish boilers based on the type of fuel 

they are designed to burn and the Agency’s conclusion that 

boilers designed for one fuel type are unlikely to use another 

fuel type. Id. The Petitioners present no contrary evidence, 

nor do they attack the validity or accuracy of the data that the 

EPA relied upon. We thus find no merit in the Petitioners’ 

various challenges to the EPA’s decision to subcategorize 

major boilers based on the fuel the boiler is designed to burn. 

K. “UNITS THAT BEGIN COMBUSTING SOLID WASTE”

AS “EXISTING” SOURCES

Section 7429(a)(2) distinguishes between “existing” and 

“new” CISWI units. The former must comply with floors set 

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at the “average emissions limitation achieved by the best 

performing 12 percent of units” and the latter must comply 

with stricter floors set at the level achieved by “the best 

controlled similar unit.” 42 U.S.C. § 7429(a)(2). “Modified” 

units, defined as units “at which modifications have occurred” 

that either experience changes that cost more than 50 per cent 

of the original construction price or result in increased 

emissions, see id. § 7429(g)(3), must be treated as “new,” see 

id. § 7429(g)(2). 

The preamble to the 2011 CISWI Rule states, “[u]nits 

that begin combusting solid waste are considered existing

sources.” 76 Fed. Reg. at 15,714 (emphasis added). 

Commentators objected that this blanket statement 

contravened the Act’s plain terms, which mandate that the 

EPA treat such sources as “new,” not “existing,” if they meet 

the section 7429(g)(3) requirements. In its subsequent 2011 

Proposed CISWI Rule on Reconsideration, the EPA refined 

its position: “An existing source will not be considered a new 

source solely due to a combustion material switch. Assuming 

new source applicability is not triggered, existing sources that 

change fuels or materials are considered existing 

sources . . . .” 76 Fed. Reg. at 80,459. 

The Environmental Petitioners argue that the EPA’s 

broad statement in the 2011 CISWI Rule indicates that it 

impermissibly changed its treatment of “modified sources” in 

contravention of the CAA. The EPA, however, agrees that 

any CISWI unit fitting the statutory criteria for a modified 

source must comply with new-unit MACT levels, not 

existing-unit MACT levels. See 42 U.S.C § 7429(g)(2). It 

also recognizes that its categorical statement in the 2011 

CISWI Rule “may have been imprecise” and, in any event, it 

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argues that the Environmental Petitioners have taken its 

statement out of context. See No. 11-1125 EPA Br. 73. 

We agree with the Agency. The EPA’s later statement 

made clear that it intended to treat “sources that change fuels 

or materials” as “existing sources” unless “new source 

applicability,” as mandated by the Act, is “triggered.” See 

2011 Proposed CISWI Rule on Reconsideration, 76 Fed. Reg. 

at 80,459. Moreover, the Agency provided its more precise 

statement while discussing specifically what constitutes a 

“modification” for the CISWI Rule. See id. (“An existing 

source will not be considered a new source solely due to a 

combustion material switch.”). It made its earlier, 

“imprecise” comment, in contrast, while describing when a 

fuel change could mean the difference between regulation 

under section 7412 or section 7429. See 2011 CISWI Rule, 

76 Fed. Reg. at 15,714. Convinced that the EPA has not 

impermissibly changed the statutory definition of “modified” 

CISWI, we reject the Petitioners’ challenge. 

L. EXCLUSION OF “TEMPORARY” BOILERS FROM AREA 

BOILERS RULE

In the final 2013 Area Boilers Rule, the EPA excluded 

“temporary boilers” from regulation under section 7412. See

78 Fed. Reg. at 7,491. The Rule defined “temporary boilers” 

as “any gaseous or liquid fuel boiler that is designed to, and is 

capable of, being carried or moved from one location to 

another by means of, for example, wheels, skids, carrying 

handles, dollies, trailers, or platforms.” Id. Moreover, a 

boiler is not a temporary boiler if any of the following apply: 

 

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(1) The equipment is attached to a foundation. 

(2) The boiler or a replacement remains at a 

location within the facility and performs the 

same or similar function for more than 12 

consecutive months, unless the regulatory 

agency approves an extension. An extension 

may be granted by the regulatory agency 

upon petition by the owner or operator of a 

unit specifying the basis for such a request. 

Any temporary boiler that replaces a 

temporary boiler at a location within the 

facility and performs the same or similar 

function will be included in calculating the 

consecutive time period unless there is a gap 

in operation of 12 months or more. 

(3) The equipment is located at a seasonal 

facility and operates during the full annual 

operating period of the seasonal facility, 

remains at the facility for at least 2 years, and 

operates at that facility for at least 3 months 

of each year. 

(4) The equipment is moved from one location 

to another within the facility but continues to 

perform the same or similar function and 

serve the same electricity, steam, and/or hot 

water system in an attempt to circumvent the 

residence time requirements of this 

definition. 

2013 Area Boilers Rule, 78 Fed. Reg. at 7,491-92. 

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 Environmental Petitioners challenge this exclusion as a 

violation of the EPA’s obligations under the CAA to regulate 

all boilers listed under section 7412. By its own terms, the 

2011 Area Boilers Rule “applies to all existing and new 

industrial boilers, institutional boilers, and commercial boilers 

located at area sources. Boiler means an enclosed combustion 

device having the primary purpose of recovering thermal 

energy in the form of steam or hot water.” 76 Fed. Reg. at 

15,557; see also 76 Fed. Reg. at 15,554 (“EPA is 

promulgating national emission standards for control of 

hazardous air pollutants from two area source categories: 

Industrial boilers and commercial and institutional boilers.”). 

Environmental Petitioners claim the general term “boiler” 

necessarily encompasses temporary boilers: “[T]he category 

of ‘boilers’ plainly includes temporary boilers, just as the 

category of ‘courts’ includes federal courts, or the category of 

‘dogs’ includes brown dogs.” No. 11-1141, Envtl. Pet’rs’ 

Reply Br. 7. According to Petitioners, then, sections 7412(c) 

and 7412(d) of the CAA require the EPA to issue emission 

standards for temporary boilers as well as “permanent” 

boilers. See 42 U.S.C. § 7412(c)(2) (“For the categories and 

subcategories the Administrator lists, 

the Administrator shall establish emission standards . . . .”); 

id. § 7412(d)(1) (“The Administrator shall promulgate 

regulations establishing emission standards for each category 

or subcategory of major sources and area sources of 

hazardous air pollutants listed for regulation . . . .”). 

 To the extent Environmental Petitioners challenge as 

unreasonable the EPA’s justifications for declining to set 

emission standards for temporary boilers, they cannot prevail. 

“Under arbitrary-and-capricious review, EPA’s 

determinations are presumptively valid provided [they] meet[] 

a minimum rationality standard.” Nat’l Ass’n for Surface 

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Finishing, 795 F.3d at 7 (internal quotation marks omitted). 

So the question is whether the EPA offered a sufficiently 

rational explanation for its exclusion of temporary boilers. 

The EPA has done so here. First, contrary to Petitioners’ 

claims, temporary boilers were never considered an 

inexorable part of the “industrial boiler” category section 

7412 requires the EPA to regulate. While the EPA only 

listed generic area source categories—“industrial boilers” and 

“institutional/commercial boilers”—in its 1999 rulemaking, it 

has since refined these broad categories pursuant to its 

statutory authority. See National Air Toxics Program: The 

Integrated Urban Strategy, 64 Fed. Reg. 38,706, 38,721 tbl.2 

(July 19, 1999). In doing so, the EPA excluded several other 

subgroups of boilers that might otherwise be read as falling 

under one of the general boiler categories. See, e.g., 2013 

Area Boilers Rule, 78 Fed. Reg. at 7,492 (excluding boilers 

already regulated by other MACT standards); 2011 Proposed 

Area Boilers Rule on Reconsideration, 76 Fed. Reg. at 80,539 

(excluding electric and residential boilers as not part of either 

source category). The EPA’s clarification that temporary 

boilers were never considered part of the “industrial boilers” 

category was simply another refinement, as contemplated by 

the statute. See 42 U.S.C § 7412(e)(4) (precluding judicial 

review until the EPA has issued its final emission standards 

for a category or subcategory). 

 Second, as both the EPA and Industry Intervenors note, 

the parallel rule for major source boilers has always explicitly 

excluded temporary boilers from its “industrial boiler” 

categorization. See 40 C.F.R. § 63.7491(j). The EPA thus 

considered commenters’ requests to add a similar clarification 

to the 2013 Area Boilers Rule and reasonably decided to do 

so. See, e.g., American Forest & Paper Association, 

Comments on Proposed Area Source Rule (AF&PA 

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Comments), EPA-HQ-OAR-2006-0790-1939 (Aug. 23, 

2010), at 58 (No. 11-1141 J.A. 389). EPA explained this 

choice in its proposed rule: 

Owners and operators of regulated sources 

have pointed out that temporary boilers are 

small (less than 10 MMBtu/hr heat input) and 

are generally owned and operated by 

contractors, rather than the facility. As a 

result, they are not included in the facility’s 

operating permits because state and federal 

CAA operating permit programs have 

historically classified such units as 

insignificant sources. The owners and 

operators also noted that compliance with the 

work practice requirements applicable to these 

small boilers would be complicated because 

they are typically located on site for less than a 

year, but would be subject to biennial 

management practice requirements. We agree 

that the source category identified in subpart 

JJJJJJ should specifically exclude these 

temporary boilers because they have been 

considered insignificant sources, and were not 

included in the EPA’s analysis of the source 

category. 

2011 Proposed Area Boilers Rule on Reconsideration, 76 Fed. 

Reg. at 80,535. 

 The unique nature of temporary boilers favors their 

exclusion. These boilers tend to be rented for use on a 

temporary basis and come in “shop-fabricated package 

designs.” AF&PA Comments, at 58 (No. 11-1141 J.A. 389). 

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Temporary boilers also “typically only fire gas or liquid fossil 

fuels (natural gas or distillate oil) which may be cleaner than 

the boiler(s) they are temporarily replacing. In addition, these 

units often do not have exhaust stacks that meet EPA Method 

1 requirements for application of test methods.” Id. 

Regardless, during the rulemaking, Environmental Petitioners 

argued the EPA had “not explained why this is a distinction 

that justifies differential treatment, let alone an exemption.” 

See Area Boilers Rule—Responses to Comments, at 65. The 

EPA responded by explaining that rather than having “created 

a category or subcategory of ‘temporary boilers’ and then 

exempted them from the standards,” the Agency never 

“intend[ed] to regulate temporary boilers under the area 

source standards” in the first place. See id. The EPA further 

noted that, “[b]y their nature of being temporary, these boilers 

operate in place of another non-temporary boiler while that 

boiler is being constructed, replaced or repaired, in which 

case we counted the non-temporary boiler as the one being 

regulated.” Id. Finally, the Agency concluded regulation of 

temporary boilers was not necessary to meet its statutory 

emission requirements under sections 7412(c)(6) and 

7412(c)(3) of the CAA. Id. In its final rule, the EPA 

reiterated this explanation: “Similar to residential boilers, we 

did not intend to regulate temporary boilers under the area 

source standards because they are not part of either the 

industrial boiler source category or the 

commercial/institutional boiler source category.” 2013 Area 

Boilers Rule, 78 Fed. Reg. at 7,491. The final regulation also 

included a detailed explanation of how EPA decided on its 

limited definition of “temporary boilers.” See id. at 7,499. 

 The evidence before the Agency supported its decision to 

exclude temporary boilers. Indeed, the EPA “cogently 

explain[ed]” why it exercised its discretion in this manner, 

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such that this court concludes the Agency’s choice “was the 

product of reasoned decision making.” U.S. Telecom Ass’n v. 

FCC, 227 F.3d 450, 460 (D.C. Cir. 2000). We therefore 

uphold the EPA’s exclusion of temporary boilers from 

regulation of area source boilers. 

M. WORK-PRACTICE STANDARDS FOR COAL-FIRED 

BOILERS

When setting emission limits for area sources, the EPA 

enjoys greater discretion than when setting limits for major 

sources. With respect to major sources, the EPA has to 

promulgate MACT standards, see 42 U.S.C. § 7412(d)(2), 

whereas for area sources the EPA can generally promulgate 

more lenient GACT standards, see id. § 7412(d)(5). The 

CAA, however, singles out seven particularly hazardous 

pollutants that require stricter regulatory standards, even for 

area sources.

41 Under section 7412(c)(6), the EPA must “list 

categories and subcategories of sources assuring that sources 

accounting for not less than 90 per centum of the aggregate 

emissions of each such pollutant” are regulated. The EPA 

listed a variety of area sources under section 7412(c)(6) in its 

1998 rulemaking based on their Hg and POM emissions. See

Proposed 2010 Area Boilers Rule, 75 Fed. Reg. at 31,898; 

Source Category Listing for Section 112(d)(2) Rulemaking 

Pursuant to Section 112(c)(6) Requirements, 63 Fed. Reg. 

17,838, 17,849-50 (Apr. 10, 1998). But the Agency 

subsequently refined that list and ultimately concluded only 

 41 These seven pollutants are: (i) alkylated lead compounds, 

(ii) polycyclic organic matter (POM), (iii) hexachlorobenzene, (iv) 

mercury (Hg), (v) polychlorinated biphenyls, (vi) 2,3,7,8-

tetrachlorodibenzofurans, and (vii) 2,3,7,8-tetrachlorodibenzo-pdioxin. See 42 U.S.C. § 7412(c)(6). 

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coal-fired area boilers needed to be listed to meet the statute’s 

90 per cent emissions threshold. See 2010 Proposed Area 

Boilers Rule, 75 Fed. Reg. at 31,898; see also No. 11-1141 

EPA Br. 14 (“[T]he coal-fired subcategory is responsible for 

over 82 percent of the mercury emissions from the [area 

source] category in the inventory, even though it represents 

only 2 percent of the boilers in the category.”). 

 Under section 7412(c)(6), the EPA was therefore 

required to set either a MACT limit under section 7412(d)(2), 

a health threshold under section 7412(d)(4), or a workpractice standard under section 7412(h) for all coal-fired 

boilers. The Agency chose to set MACT numerical emission 

limits for Hg and CO42 at new and existing large coal-fired 

boilers. 2013 Area Boilers Rule, 78 Fed. Reg. at 7,488. 

However, the EPA found it “technologically and 

economically impracticable to apply [its] measurement 

methodology to . . . small sources,” and so it chose to institute 

a work-practice standard43 for all new and existing small coalfired boilers. Id. at 7,488-89. This work-practice standard 

requires small coal-fired units to be periodically tuned up but 

does not impose any numeric emission limit. See id. The 

EPA similarly decided that, for large coal-fired boilers 

undergoing a startup or a shutdown, a work-practice 

standard—rather than a numeric emission standard—was 

 42 Because the EPA chose to regulate POM emissions 

indirectly—by using CO emissions as a surrogate—the standards it 

set under section 7412(c)(6) are for CO rather than POM. See 2013 

Area Boilers Rule, 78 Fed. Reg. at 7,488, 7,503. 

43 In their brief, Environmental Petitioners alternate between 

the terms “operational standards” and “work-practice standards,” 

both of which fall under section 7412(d)(2)(D). This opinion will 

use “work-practice standards” for simplicity. 

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most practicable. See id. at 7,518 tbl.2 (requiring owners of 

large “[e]xisting or new coal-fired” boilers to “[m]inimize the 

boiler’s startup and shutdown periods and conduct startups 

and shutdowns according to the manufacturer’s recommended 

procedures”). 

 Environmental Petitioners challenge the EPA’s decision 

to employ work-practice standards as a violation of 

7412(d)(2)’s mandate to achieve the “maximum degree of 

reduction in emissions.” We examine Petitioner’s statutory 

argument step-by-step, as it hinges on the interplay between 

several statutory provisions. First, section 7412(c)(6)—which 

governs regulation of Hg and POM emissions—requires the 

Administrator to regulate sources of these pollutants under 

either section 7412(d)(2) or (d)(4). Section 7412(d)(4) allows 

the Administrator to establish health-based emission 

standards; it is not implicated here. Instead, the EPA decided 

to regulate coal-fired boilers under section 7412(d)(2). 

Section 7412(d)(2) instructs the Administrator to achieve “the 

maximum degree of reduction in emissions of the hazardous 

air pollutants . . . that the Administrator, taking into 

consideration the cost of achieving such emissions reduction, 

and any non-air quality health and environmental impacts and 

energy requirements, determines is achievable for new or 

existing sources.” The Administrator is authorized to use 

several means to achieve this reduction including 

implementing “design, equipment, work practice, or 

operational standards . . . as provided in [section 7412(h)].” 

42 U.S.C. § 7412(d)(2)(D). Section 7412(h)(1) states: “[I]f it 

is not feasible in the judgment of the Administrator to 

prescribe or enforce an emission standard for control of a 

[pollutant], the Administrator may, in lieu thereof, promulgate 

a . . . work-practice standard . . . , which in the 

Administrator’s judgment is consistent with the provisions of 

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subsection (d) or (f) of this section.” Petitioners do not 

dispute the EPA’s ability to set work-practice standards here; 

they instead focus on section 7412(h)’s requirement that any 

such standards be “consistent with” subsection (d)—which 

requires the “maximum degree of reduction in emissions.” 

According to Petitioners, the EPA’s decision to set these 

particular work-practice standards fails at both Chevron steps. 

 With respect to Chevron step 1, Petitioners argue the 

“EPA does not claim the operational standards [for coal-fired 

boilers] are ‘consistent with the provisions of subsection (d) 

or (f)’ of § 7412.” No. 11-1141 Envtl. Pet’rs’ Br. 33. In other 

words, because these work-practice standards “do not even 

purport” to be consistent with section 7412(d)’s mandate to 

maximize reduction of emissions, “they are unlawful under 

Chevron step one.” Id. Petitioners point to the EPA’s 

specific findings to support this claim: for large coal-fired 

boilers, the EPA found that mercury emissions could be 

reduced by 75 to 82 per cent through the use of a fabric filter. 

Id. But, according to Petitioners, the “EPA admits the tuneup program [for small coal-fired boilers] will reduce 

emissions by only one percent.” Id. And, with respect to 

large coal-fired boilers undergoing startup or shutdown, 

Petitioners argue the “EPA does not claim that ‘following the 

manufacturer’s recommended procedures’ during startup and 

shutdown will reduce emissions at all.” Id.

 At the familiar Chevron step 1, the court must “first 

examine the statute de novo, employing traditional tools of 

statutory construction.” Nat’l Ass’n of Clean Air Agencies, 

489 F.3d at 1228 (internal quotation marks omitted). If the 

Congress’s intent is clear, then the Agency’s interpretation is 

afforded no deference, and the court “must give effect to the 

unambiguously expressed intent of Congress.” Id.

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In this case, Environmental Petitioners place too much 

emphasis on certain snippets of the statute without examining 

the larger context. For one, Petitioners seem to argue that the 

EPA must adopt work-practice standards that result in the 

maximum possible reduction of emissions, without taking into 

account any other considerations. But section 7412(d)(2) 

itself belies this claim: it says the EPA must promulgate 

standards that require “the maximum degree of reduction in 

emissions . . . that the Administrator, taking into 

consideration the cost of achieving such emission reduction, 

and any non-air quality health and environmental impacts 

and energy requirements, determines is achievable.” 42 

U.S.C. § 7412(d)(2) (emphasis added). This portion of the 

statute explicitly defers to the Administrator’s judgment 

regarding a standard’s “achievability,” even though it directs 

him to consider particular factors in making that assessment. 

Section 7412(h) similarly requires the Administrator to adopt 

a work-practice standard that in his judgment is consistent 

with section 7412(d)(2)’s mandate. We therefore cannot 

accept Petitioners’ suggestion that Congress unambiguously 

required the EPA to adopt standards that result in the 

maximum reduction of emissions that is technologically 

feasible. 

Environmental Petitioners’ challenge to these workpractice standards as unreasonable under Chevron step 2 and 

arbitrary under State Farm presents a closer call. With 

respect to Chevron step 2, the court must “uphold an agency’s 

interpretation if it is reasonable.” Ariz. Pub. Serv. Co. v. EPA, 

211 F.3d 1280, 1287 (D.C. Cir. 2000). And, “even where 

EPA’s construction satisfies Chevron, [the court] still must 

ensure that its action is not otherwise arbitrary and capricious. 

The arbitrary and capricious standard is ‘[h]ighly deferential,’ 

and it ‘presumes the validity of agency action.’” Nat’l Ass’n 

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of Clean Air Agencies, 489 F.3d at 1228 (citations omitted). 

As long as an agency has “considered the relevant factors and 

articulated ‘a rational connection between the facts found and 

the choice made,’” then its decision must be upheld. Allied 

Local & Reg’l Mfrs. Caucus, 215 F.3d at 68. 

Petitioners mount both a “facial” and a substantive 

challenge to the EPA’s rationale for adopting work-practice 

standards. First, Petitioners claim the EPA’s decision is 

arbitrary because it fails “to reconcile its approach with the 

statutory requirement [of section 7412(d)(2)].” No. 11-1141 

Envtl. Pet’rs’ Br. 34. Specifically, Petitioners insist the EPA 

must explicitly state somewhere that these particular workpractice standards are “consistent” with section 7412(d)(2). 

See id. at 33-34. Otherwise, Petitioners contend, the Court 

must simply “assume that the Agency heeded § 7412(h)’s 

‘consistent with’ requirement, notwithstanding the EPA’s 

failure to acknowledge and apply that requirement in the 

record.” No. 11-1141 Envtl. Pet’rs’ Reply Br. 10. 

The Agency responds that, “by identifying the tune-up 

and startup/shutdown requirements as ‘work practices,’ [it] is 

stating that those standards are issued under section 7412(h) 

and are consistent with the requirements of section 7412(d) 

(i.e., MACT).” No. 11-1141, EPA Br. 71. The EPA did 

acknowledge it has an obligation to maximize emission 

reductions under section 7412(d)(2) when promulgating 

work-practice standards. See 2011 Area Boilers Rule, 76 Fed. 

Reg. at 15,568 (“CAA section 112(h) authorizes the 

Administrator to promulgate [a work-practice standard] 

consistent with the provisions of CAA sections 112(d) or 

(f) . . . .”). However, Petitioners are correct that the Agency 

did not make a finding on the record that these work-practice 

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standards would achieve the highest emissions reduction 

possible. 

But the lack of an explicit statement does not 

automatically condemn this portion of the rule. See Bowman, 

419 U.S. at 286 (“[W]e will uphold a decision of less than 

ideal clarity if the agency’s path may reasonably be 

discerned.”). The Petitioners offer no support for their 

contention that the EPA must make an express finding that its 

standards are “consistent” with section 7412(d)(2). Nor does 

our conclusion requires us to merely “assume” that the 

Agency’s actions comport with section 7412(d)(2). Instead, 

as we usually do when presented with such arguments, we 

review the rulemaking record to determine whether the 

justifications the EPA offered for adopting these work 

practices standards were permissible. 

1. Small Coal-Fired Boilers

First, with respect to small coal-fired boilers, the EPA 

determined that a biennial tune-up requirement would best 

comply with section 7412(h)’s requirements. As a starting 

point, the EPA surveyed a sample of state regulations 

mandating various work-practice standards for small coalfired boilers; the Agency found that ten states required tuneups, two required periodic inspections, one required operator 

training, and one required operation in accordance with 

manufacturer specifications. 2011 Area Boilers Rule, 76 Fed. 

Reg. at 15,573-74. The EPA thus concluded that tune-ups 

were the most typical work-practice standard employed for 

this type of boiler. Id. The Agency also found that regular 

tune-ups could lower HAP emissions by increasing the 

efficiency of small coal-fired boilers. See id. at 15,575 (“A 

tune-up performed to the manufacturer’s specifications would 

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ensure the highest energy efficiency and reduce fuel usage, 

which will ultimately reduce HAP emissions.”); see also 2010 

Proposed Area Boilers Rule, 75 Fed. Reg. at 31,908 (“A 

boiler tune-up provides potential savings from energy 

efficiency improvements and pollution prevention. . . . In 

addition, the cost of a boiler tune-up appears minimal 

compared to the cost for testing and monitoring to 

demonstrate compliance with an emission limit.”).44

The EPA elected to implement a work-practice standard 

because the typical method used to measure emissions of Hg 

and CO could not be used to sample emissions from stacks 

with small diameters (less than 12 inches). See 2011 Area 

Boilers Rule, 76 Fed. Reg. at 15,568. Because many small 

coal-fired boilers have stacks with diameters below 12 

inches—and because many of these boilers “do not currently 

have sampling ports or a platform for accessing the exhaust 

stack”—the Agency concluded the cost of testing and 

monitoring these small boilers would “present an excessive 

burden for smaller sources.” Id. The Agency’s consideration 

of cost effectiveness is particularly appropriate in this context 

because the “vast majority” of area source boilers are 

“generally owned and operated by small entities,” which 

would be disproportionately burdened by a numeric emissions 

limit. See Fact Sheet: Final Adjustments to the Air Toxics 

Standards for Industrial, Commercial, and Institutional 

Boilers at Area Source Facilities, 1, 2 (No. 11-1141 J.A. 684-

 44 The EPA’s discussion of boiler tune-up advantages occurs 

mainly in the context of its decision to select a GACT standard 

rather than any numeric emission standards for certain boilers. 

Environmental Petitioners challenge this decision on similar 

grounds, see supra § IV.H. But the benefits of periodic tune-ups 

also apply to the coal-fired boilers at issue here. 

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85); see also 2010 Proposed Area Boilers Rule, 75 Fed. Reg. 

at 31,906 (“The results of the analysis indicate that total 

compliance costs exceed 3 percent (and can reach as high as 

19 percent) of the average firm revenues for 79 percent of the 

facilities.”). 

Environmental Petitioners counter that while tune-ups 

may minimally reduce HAP emissions, they do not maximize 

this reduction per section 7412(d)(2)’s mandate.45 But 

Petitioners view section 7412(d) too myopically; under that 

section, the Administrator is empowered to adopt standards 

that “tak[e] into consideration the cost of achieving such 

emission reduction.” 42 U.S.C. § 7412(d)(2). The EPA here 

examined the costs of imposing a numeric emission standard 

on small coal-fired boilers and found that option “not 

feasible” due to high costs and monitoring difficulties, 

considerations equally permissible under section 7412(d)(2). 

Petitioners argue that requiring the use of a fabric filter would 

have resulted in greater reductions, but they are unable to 

point to any small coal-fired boiler that currently uses such a 

filter. See 2010 Proposed Area Boilers Rule, 75 Fed. Reg. at 

31,906 (“For existing [small] area source boilers, the only 

work practice being used that potentially controls mercury 

 45 Petitioners also point to a comment they made in the record 

arguing that a tune-up standard “would not achieve emission 

reductions that are consistent with the definition of MACT,” and 

urge that the EPA never addressed these concerns. See National 

Association of Clean Air Agencies, Comments on EPA Proposals 

for Regulation of Hazardous Air Pollutants (HAPS), EPA-HQOAR-2006-0790, EPA-HQ-OAR-2002-0058, EPA-HQ-OAR2003-0119 (Aug. 23, 2010), at 21-22 (No. 11-1141 J.A. 417-18). 

But this comment was specifically addressed to gas-fired boilers, 

and it is inapposite to the EPA’s consideration of standards for 

coal-fired boilers. 

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and POM emissions is a boiler tune-up.”). Evidence before 

the agency in fact indicated that the best performing small 

coal boilers for POM emission use no add-on controls. See

Memorandum from Amanda Singleton & Brandon Long, 

Eastern Research Group, to Jim Eddinger, EPA 

(MACT/GACT Mem.), App. E-2a (No. 11-1141 J.A. 540). It 

was therefore reasonable for the EPA, when considering 

costs, to conclude that biennial tune-ups would allow for the 

maximum “achievable” reduction in emissions. 

Petitioners’ most compelling argument involves the 

EPA’s lack of data on small coal-fired boilers. As they point 

out, the EPA’s summary of its 2008 combustion survey makes 

no mention of any small coal-fired boilers. See

MACT/GACT Mem., App. D-3, tbl.1 (No. 11-1141 J.A. 523). 

And the EPA never directly addressed whether control 

technologies, such as fabric filters, were useable by small 

boilers; “[t]he only claim EPA made in the record is that tuneups are the most effective option that [small] coal-fired 

boilers . . . are currently using, not that tune-ups yield the 

maximum reduction ‘achievable.’” No. 11-1141 Envtl. 

Pet’rs’ Reply Br. 11. In Sierra Club II, this court agreed with 

Sierra Club’s challenge to the EPA’s use of a work-practice 

standard instead of an emission floor because the “EPA never 

determined that measuring emissions from ceramics kilns was 

impracticable; it determined only that it lacked emissions data 

from ceramics kilns. EPA thus had no basis under section 

7412(h) for using work practice standards.” 479 F.3d at 884. 

That context is somewhat distinguishable, given that the 

statute there explicitly required the EPA to make a 

“feasibility” finding (as discussed above), but it could be 

argued that the EPA here lacked the data to determine 

whether tune-ups were “consistent with” section 7412(d)(2). 

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Ultimately, though, the high level of deference afforded 

the EPA counsels in favor of upholding this work-practice 

standard. Although the EPA did not explicitly state that tuneups were the best option to reduce emissions while still 

“considering costs,” this finding can be inferred from the 

record as a whole. For instance, the EPA found that “[n]one 

of the States for which we have an inventory have an 

applicable emissions limit” for small coal-fired boilers, except 

New Jersey, which actually has tune-ups as its work-practice 

standard. 2010 Proposed Area Boilers Rule, 75 Fed. Reg. at 

31,909. Based on these findings, it can reasonably be inferred 

that—given the prevalence of these small boilers—at least a 

few would be using a control technology if that were 

technologically or economically feasible. Because numeric 

emissions cannot easily be measured from these smaller 

sources and the costs of outfitting them with such technology 

would be cost prohibitive, the EPA’s choice of tune-ups as the 

work-practice standard is sufficiently reasonable to uphold 

under both Chevron step 2 and State Farm. The Agency’s 

choice is consistent with section 7412(h)’s “feasibility” 

requirement and with section 7412(d)(2)’s instruction to 

maximize emission reductions while also considering costs. 

2. Large Coal-Fired Boilers Undergoing Startup or 

Shutdown 

 The record for large coal-fired boilers undergoing startup 

or shutdown is less extensive but again the EPA’s 

determination is reasonable. While large coal-fired boilers 

are required to meet numeric emission standards during 

“normal” operations, the EPA adopted a work-practice 

standard for the temporary periods of startup and shutdown. 

See 40 C.F.R. § 63.11214 (“[M]inimize the boiler’s startup 

and shutdown periods following the manufacturer’s 

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recommended procedures, if available. If manufacturer’s 

recommended procedures are not available, you must follow 

recommended procedures for a unit of similar design for 

which manufacturer’s recommended procedures are 

available.”). Environmental Petitioners contend that the EPA 

never stated this practice would reduce emissions at all, and 

therefore it has not met its burden under section 7412(d)(2). 

But we have already explained that no express finding of 

consistency with section 7412(d)(2) need be made. Here, the 

record suggests that the work-practice standard the Agency 

chose would reduce emissions, and we therefore can 

“reasonably [] discern[]” the Agency’s path. Bowman, 419 

U.S. at 286. Specifically, the EPA explained that requiring 

boilers to operate in startup and shutdown mode for 

“sufficient time to conduct the required test runs [to impose 

numeric standards] could result in higher emissions than 

would otherwise occur.” 2011 Major Boilers Rule, 76 Fed. 

Reg. at 15,642. Industry stakeholders also pointed out that “it 

is very common . . . for certain control devices to be out of 

operation during periods of start-up due to the nature of the 

equipment.” See American Chemistry Council, Comments 

on EPA’s Proposed Rule for National Emission Standards for 

Hazardous Air Pollutants for Area Sources, EPA-HQ-OAR2006-0790 (Aug. 23, 2010), at 31 (No. 11-1141 J.A. 386). 

Because the control technology is temporarily offline, “it is 

likely that emissions will exceed the standards proposed 

[during that time period].” Id. A work-practice standard that 

requires facilities to minimize the time their boilers spend in 

startup or shutdown thus seems calculated to maximally 

reduce emissions during those periods—and Petitioners fail to 

provide any viable alternative. We therefore conclude the 

EPA’s decision to adopt these work-practice standards for 

large coal-fired boilers during startup and shutdown was 

reasonable. 

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V. CONCLUSION 

 For the foregoing reasons, we grant the petitions in part 

and deny them in part. Specifically, we vacate the MACT 

standards for all major boiler subcategories that would have 

been affected had the EPA considered all sources included in 

the subcategories, as explained at supra § IV.B. We 

also remand, without vacatur, to the EPA to: (1) adequately 

explain how CO acts as a reasonable surrogate for nondioxin/furan organic HAPs; (2) set emission standards for 

cyclonic burn barrels; (3) determine whether burn-off ovens, 

soil treatment units, and space heaters are CISWI units and, if 

so, to set standards for those types of units; (4) adequately 

explain the exclusion of synthetic boilers from Title V’s 

permitting requirements; and (5) adequately explain the 

choice of GACT standards over MACT standards for non-Hg 

metals. 

So ordered. 

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