National Emission Standards for Hazardous Air Pollutants From Hazardous Waste Combustors: Residual Risk and Technology Review; Withdrawal of Proposed Revisions to Standards for Periods of Malfunction

Federal RegisterNov 10, 2025

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ENVIRONMENTAL PROTECTION AGENCY

40 CFR Part 63

[EPA-HQ-OAR-2004-0022; FRL-10654-01-OAR]

RIN 2060-AV96

National Emission Standards for Hazardous Air Pollutants From Hazardous Waste Combustors: Residual Risk and Technology Review; Withdrawal of Proposed Revisions to Standards for Periods of Malfunction

AGENCY:

Environmental Protection Agency (EPA).

ACTION:

Proposed rule and withdrawal of proposed rule.

SUMMARY:

This proposal presents the results of the U.S. Environmental Protection Agency's (EPA) residual risk and technology review for the National Emission Standards for Hazardous Air Pollutants (NESHAP) from Hazardous Waste Combustors (HWC) as required under the Clean Air Act (CAA). In this action, the EPA is proposing to establish emission limits and work practice standards for hydrogen fluoride and hydrogen cyanide emissions from HWC incinerators, cement kilns, solid fuel boilers, and liquid fuel boilers; eliminate the startup, shutdown, and malfunction (SSM) exemption; add a work practice standard for periods of SSM; add electronic reporting procedures and requirements; allow states to choose to exempt area sources from certain permitting requirements; and other clarifications and corrections. In response to comments received on certain aspects of the July 24, 2024, proposed revisions for periods of malfunction, the EPA is withdrawing that proposed rule and instead proposing different provisions to address periods of SSM.

DATES:

Comments.

Comments must be received on or before December 26, 2025. As of November 10, 2025, the proposed rule published on July 24, 2024, at 89 FR 59867, is withdrawn. Under the Paperwork Reduction Act (PRA), comments on the information collection provisions are best assured of consideration if the Office of Management and Budget (OMB) receives a copy of your comments on or before December 10, 2025.

Public hearing:

If anyone contacts us requesting a public hearing on or before November 15, 2025, we will hold a virtual public hearing. See

SUPPLEMENTARY INFORMATION

for information on requesting and registering for a public hearing.

ADDRESSES:

You may send comments, identified by Docket ID No. EPA-HQ-OAR-2004-0022, by any of the following methods:

•

Federal eRulemaking Portal:

https://www.regulations.gov/

(our preferred method). Follow the online instructions for submitting comments.

•

Email:

a-and-r-docket@epa.gov.

Include Docket ID No. EPA-HQ-OAR-2004-0022 in the subject line of the message.

•

Mail:

U.S. Environmental Protection Agency, EPA Docket Center, Docket ID No. EPA-HQ-OAR-2004-0022, Mail Code 28221T, 1200 Pennsylvania Avenue NW, Washington, DC 20460.

•

Hand/Courier Delivery:

EPA Docket Center, WJC West Building, Room 3334, 1301 Constitution Avenue NW, Washington, DC 20004. The Docket Center's hours of operation are 8:30 a.m.-4:30 p.m., Monday-Friday (except Federal holidays).

Instructions:

All submissions received must include the Docket ID No. for this rulemaking. Comments received may be posted without change to

https://www.regulations.gov/,

including any personal information provided. For detailed instructions on sending comments and additional information on the rulemaking process, see the

SUPPLEMENTARY INFORMATION

section of this document.

FOR FURTHER INFORMATION CONTACT:

For information about this proposed rule, contact U.S. EPA, Attn: Rachel Smoak, Mail Drop: E143-02, 109 T.W. Alexander Drive, P.O. Box 12055, RTP, North Carolina 27711; telephone number: (919) 541-0253; and email address:

smoak.rachel@epa.gov.

For specific information regarding the risk modeling methodology, contact U.S. EPA, Attn: Matt Woody, Ph.D., Mail Drop: C539-02, 109 T.W. Alexander Drive, P.O. Box 12055, RTP, North Carolina 27711; telephone number: (919) 541-1535; and email address:

woody.matt@epa.gov.

SUPPLEMENTARY INFORMATION:

Participation in virtual public hearing.

To request a virtual public hearing, contact the public hearing team at (888) 372-8699 or by email at

SPPDpublichearing@epa.gov.

If requested, the hearing will be held via virtual platform on November 25, 2025. The EPA may close a session 15 minutes after the last pre-registered speaker has testified if there are no additional speakers. The EPA will announce further details at

https://www.epa.gov/stationary-sources-air-pollution/hazardous-waste-combustors-national-emission-standards-hazardous.

The EPA will begin pre-registering speakers for the hearing no later than one business day after a request has been received. To register to speak at the virtual hearing, please use the online registration form available at

https://www.epa.gov/stationary-sources-air-pollution/hazardous-waste-combustors-national-emission-standards-hazardous

or contact the public hearing team at (888) 372-8699 or by email at

SPPDpublichearing@epa.gov.

The last day to pre-register to speak at the hearing will be November 22, 2025. Prior to the hearing, the EPA will post a general agenda that will list pre-registered speakers at:

https://www.epa.gov/stationary-sources-air-pollution/hazardous-waste-combustors-national-emission-standards-hazardous.

The EPA will make every effort to follow the schedule as closely as possible on the day of the hearing; however, please plan for the hearings to run either ahead of schedule or behind schedule. Each commenter will have four minutes to provide oral testimony. The EPA encourages commenters to submit the text of your oral testimony as written comments to the rulemaking docket. The EPA may ask clarifying questions during the oral presentations but will not respond to the presentations at that time. Written statements and supporting information submitted during the comment period will be considered with the same weight as oral testimony and supporting information presented at the public hearing.

Please note that any updates made to any aspect of the hearing will be posted online at

https://www.epa.gov/stationary-sources-air-pollution/hazardous-waste-combustors-national-emission-standards-hazardous.

While the EPA expects the hearing to go forward as set forth above, please monitor this website or contact the public hearing team at (888) 372-8699 or by email at

SPPDpublichearing@epa.gov

to determine if there are any updates. The EPA does not intend to publish a document in the

Federal Register

(FR) announcing updates.

If you require special accommodation such as audio description, please pre-register for the hearing with the public hearing team and describe your needs by November 17, 2025. The EPA may not be able to arrange accommodations without advanced notice.

Docket.

The EPA has established a docket for this proposed rule under Docket ID No. EPA-HQ-OAR-2004-0022. All documents in the docket are listed in the

https://www.regulations.gov/

index. Although listed in the index, some information is

not publicly available,

e.g.,

Confidential Business Information (CBI) or other information whose disclosure is restricted by statute. Certain other material, such as copyrighted material, is not placed on the internet and will be publicly available only as Portable Document Format (PDF) versions that can only be accessed on the EPA computers in the docket office reading room. Certain databases and physical items cannot be downloaded from the docket but may be requested by contacting the docket office at 202-566-1744. The docket office has up to 10 business days to respond to these requests. With the exception of such material, publicly available docket materials are available electronically at

https://www.regulations.gov.

Written Comments.

Submit your comments, identified by Docket ID No. EPA-HQ-OAR-2004-0022, at

https://www.regulations.gov/

(our preferred method), or the other methods identified in the

ADDRESSES

section. Once submitted, comments cannot be edited or removed from the docket. The EPA may publish any comment received to its public docket. Do not submit to the EPA's docket at

https://www.regulations.gov/

any information that you consider to be CBI or other information whose disclosure is restricted by statute. This type of information should be submitted as discussed in the

Submitting CBI

section of this document.

The EPA is soliciting comment on numerous aspects of the proposed rule. The EPA has indexed each comment solicitation with a unique identifier (

e.g.,

“C-1,” “C-2,” “C-3” . . .) to provide a consistent framework for effective and efficient provision of comments. Accordingly, we ask that commenters include the corresponding identifier when providing comments relevant to that comment solicitation. We ask that commenters include the identifier either in a heading or within the text of each comment (

e.g.,

“In response to C-1, . . .”) to make clear which comment solicitation is being addressed. We emphasize that we are not limiting comment to these identified areas and encourage provision of any other comments relevant to this proposed action.

Multimedia submissions (audio, video, etc.) must be accompanied by a written comment. The written comment is considered the official comment and should include discussion of all points you wish to make. The EPA will generally not consider comments or comment contents located outside of the primary submission (

i.e.,

on the Web, cloud, or other file sharing system). Please visit

https://www.epa.gov/dockets/commenting-epa-docket

for additional submission methods; the full EPA public comment policy; information about CBI or multimedia submissions; and general guidance on making effective comments.

The

https://www.regulations.gov/

website allows you to submit your comment anonymously, which means the EPA will not know your identity or contact information unless you provide it in the body of your comment. If you send an email comment directly to the EPA without going through

https://www.regulations.gov/,

your email address will be automatically captured and included as part of the comment that is placed in the public docket and made available on the internet. If you submit an electronic comment, the EPA recommends that you include your name and other contact information in the body of your comment and with any digital storage media you submit. If the EPA cannot read your comment due to technical difficulties and cannot contact you for clarification, the EPA may not be able to consider your comment. Electronic files should not include special characters or any form of encryption and should be free of any defects or viruses.

Submitting CBI.

Do not submit information containing CBI to the EPA through

https://www.regulations.gov/.

Clearly mark the part or all of the information that you claim to be CBI. For CBI information on any digital storage media that you mail to the EPA, note the docket ID, mark the outside of the digital storage media as CBI, and identify electronically within the digital storage media the specific information that is claimed as CBI. In addition to one complete version of the comments that includes information claimed as CBI, you must submit a copy of the comments that does not contain the information claimed as CBI directly to the public docket through the procedures outlined in

Written Comments

above. If you submit any digital storage media that does not contain CBI, mark the outside of the digital storage media clearly that it does not contain CBI and note the docket ID. Information not marked as CBI will be included in the public docket and the EPA's electronic public docket without prior notice. Information marked as CBI will not be disclosed except in accordance with procedures set forth in 40 Code of Federal Regulations (CFR) part 2.

Our preferred method to receive CBI is for it to be transmitted electronically using email attachments, File Transfer Protocol (FTP), or other online file sharing services (

e.g.,

Dropbox, OneDrive, Google Drive). Electronic submissions must be transmitted directly to the OAQPS CBI Office at the email address

oaqps_cbi@epa.gov,

and as described above, should include clear CBI markings and note the docket ID. If assistance is needed with submitting large electronic files that exceed the file size limit for email attachments, and if you do not have your own file sharing service, please email

oaqps_cbi@epa.gov

to request a file transfer link. If sending CBI information through the postal service, please send it to the following address: OAQPS Document Control Officer (C404-02), OAQPS, U.S. Environmental Protection Agency, 109 T.W. Alexander Drive P.O. Box 12055 RTP, North Carolina 27711, Attention Docket ID No. EPA-HQ-OAR-2004-0022. The mailed CBI material should be double wrapped and clearly marked. Any CBI markings should not show through the outer envelope.

Preamble acronyms and abbreviations. Throughout this preamble the use of “we,” “us,” or “our” is intended to refer to the EPA. We use multiple acronyms and terms in this preamble. While this list may not be exhaustive, to ease the reading of this preamble and for reference purposes, the EPA defines the following terms and acronyms here:

AEGL acute exposure guideline level

AERMOD air dispersion model used by the HEM model

APCD air pollution control device

AWFCO automatic waste feed cutoff

CAA Clean Air Act

CalEPA California EPA

CBI Confidential Business Information

CEDRI Compliance and Emissions Data Reporting Interface

CEMS continuous emission monitoring system

CFR Code of Federal Regulations

CfPT confirmatory performance test

CMS continuous monitoring system

CPT comprehensive performance test

DRE destruction and removal efficiency

EPA Environmental Protection Agency

ERPG emergency response planning guideline

ERT Electronic Reporting Tool

HAP hazardous air pollutant(s)

HBEL health-based emission limit

HCl hydrochloric acid

HCN hydrogen cyanide

HEM Human Exposure Model

HF hydrogen fluoride

HI hazard index

HQ hazard quotient

HWC hazardous waste combustor

ICR information collection request

IRIS Integrated Risk Information System

km kilometer

LOAEL lowest-observed-adverse-effect level

MACT maximum achievable control technology

mg/kg-day milligrams per kilogram per day

mg/m3 milligrams per cubic meter

MIR maximum individual risk

NAAQS National Ambient Air Quality Standards

NAICS North American Industry Classification System

NESHAP national emission standards for hazardous air pollutants

NOAEL no-observed-adverse-effect level

NRC National Research Council

NTTAA National Technology Transfer and Advancement Act

OAQPS Office of Air Quality Planning and Standards

OECA Office of Enforcement and Compliance Assurance

OMB Office of Management and Budget

PAH polycyclic aromatic hydrocarbons

PB-HAP hazardous air pollutants known to be persistent and bioaccumulative in the environment

PCDD/PCDF polychlorinated dibenzo-

-dioxins and polychlorinated dibenzofurans

PM particulate matter

POM polycyclic organic matter

ppm parts per million

REL reference exposure level

RFA Regulatory Flexibility Act

RfC reference concentration

RfD reference dose

RTR residual risk and technology review

SAB Science Advisory Board

SBA Small Business Administration

SSM startup, shutdown, and malfunction

TEQ toxic equivalency quotient

TOSHI target organ-specific hazard index

tpy tons per year

TRIM.FaTE Total Risk Integrated Methodology.Fate, Transport, and Ecological Exposure model

UF uncertainty factor

µg/m3 micrograms per cubic meter

UMRA Unfunded Mandates Reform Act

UPL upper prediction limit

URE unit risk estimate

VCS voluntary consensus standards

Table of Contents

I. General Information

A. Does this action apply to me?

B. Where can I get a copy of this document and other related information?

II. Background

A. What is the statutory authority for this proposed action?

B. What is this source category and how does the current NESHAP regulate its HAP emissions?

C. What data collection activities were conducted to support this action?

D. What other relevant background information and data are available?

III. Analytical Procedures and Decision-Making

A. How do we consider risk in our decision-making?

B. How do we perform the technology review?

C. How do we estimate post-MACT risk posed by the source category?

IV. Analytical Results and Proposed Decisions

A. What actions are we proposing pursuant to CAA sections 112(d)(2) and 112(d)(3)?

B. What are the results of the risk assessment and analyses?

C. What are our proposed decisions regarding risk acceptability, ample margin of safety, and adverse environmental effect?

D. What are the results and proposed decisions based on our technology review?

E. What other actions are we proposing?

F. What compliance dates are we proposing?

V. Summary of Cost, Environmental, and Economic Impacts

A. What are the affected sources?

B. What are the air quality impacts?

C. What are the cost impacts?

D. What are the economic impacts?

E. What are the benefits?

F. What analysis of children's environmental health did we conduct?

VI. Request for Comments

VII. Submitting Data Corrections

VIII. Statutory and Executive Order Reviews

A. Executive Order 12866: Regulatory Planning and Review and Executive Order 13563: Improving Regulation and Regulatory Review

B. Executive Order 14192: Unleashing Prosperity Through Deregulation

C. Paperwork Reduction Act (PRA)

D. Regulatory Flexibility Act (RFA)

E. Unfunded Mandate Reform Act (UMRA)

F. Executive Order 13132: Federalism

G. Executive Order 13175: Consultation and Coordination With Indian Tribal Governments

H. Executive Order 13045: Protection of Children From Environmental Health Risks and Safety Risks

I. Executive Order 13211: Actions Concerning Regulations That Significantly Affect Energy Supply, Distribution, or Use

J. National Technology Transfer and Advancement Act (NTTAA)

I. General Information

A. Does this action apply to me?

Table 1 of this preamble lists the NESHAP and associated regulated industrial source categories that are the subject of this proposal. Table 1 is not intended to be exhaustive but rather provides a guide for readers regarding the entities that this proposed action is likely to affect. The proposed standards, if finalized, would be directly applicable to the affected sources. State, local, and Tribal government entities do not own or operate sources that would be affected by this proposed action. The hazardous waste combustor (HWC) source category, which is the subject of this proposal, is regulated under 40 CFR part 63, subpart EEE, the National Emission Standards for Hazardous Air Pollutants from Hazardous Waste Combustors (HWC NESHAP). The HWC NESHAP includes hazardous waste combusting sources from five initial source categories: Hazardous Waste Incineration, Portland Cement Manufacturing, Clay Products Manufacturing (including lightweight aggregate kilns), Industrial Boilers, and Hydrochloric Acid (HCl) Production.

Hazardous waste combusting sources from five initial source categories are regulated as HWCs under 40 CFR part 63, subpart EEE, the HWC NESHAP. As defined in the

Initial List of Categories of Sources Under Section 112(c)(1) of the Clean Air Act Amendments of 1990

(57 FR 31576, July 16, 1992) and

Documentation for Developing the Initial Source Category List, Final Report

(EPA-450/3-91-030, July 1992), the “Hazardous Waste Incineration” source category includes any source that incinerates hazardous waste in “any furnace, or other device, used in the process of burning waste for the primary purpose of reducing the volume of the waste by removing combustible matter.” The “Portland Cement Manufacturing” source category includes “any facility engaged in manufacturing Portland cement by either the wet or dry process.” The “Clay Products Manufacturing” source category includes lightweight aggregate kilns and is defined as “any facility engaged in manufacturing of clay products such as brick, vitrified clay pipe, structural clay tile, and clay refractories.” The “Industrial Boilers” source category includes “boilers used in manufacturing, processing, mining, and refining or any other industry to provide steam, hot water, and/or electricity.” In 2004, the Industrial Boilers source category was combined with the Institutional/Commercial Boilers and the Process Heaters source categories into the Industrial/Commercial/Institutional Boilers and Process Heaters source category.

The “Hydrochloric Acid Production” source category includes “any facility engaged in the production of hydrochloric acid.”

70 FR 37819 (June 30, 2005).

Table 1—NESHAP and Source Categories Affected by This Proposed Action

Source category

NESHAP

NAICS code

Petroleum and coal products manufacturing

40 CFR part 63, subpart EEE

3241

Chemical manufacturing

40 CFR part 63, subpart EEE

325

Cement and concrete product manufacturing

40 CFR part 63, subpart EEE

3273

Other nonmetallic mineral product manufacturing

40 CFR part 63, subpart EEE

3279

Hazardous waste treatment and disposal

40 CFR part 63, subpart EEE

562211

Remediation and other waste management services

40 CFR part 63, subpart EEE

5629

North American Industry Classification System.

B. Where can I get a copy of this document and other related information?

In addition to being available in the docket, an electronic copy of this action is available on the internet. In accordance with 5 U.S. Code (U.S.C.) 553(b)(4), a brief summary of this rule may be found at

https://www.regulations.gov,

Docket ID No. EPA-HQ-OAR-2004-0022. Following signature by the Administrator, the EPA will post a copy of this proposed action at

https://www.epa.gov/stationary-sources-air-pollution/hazardous-waste-combustors-national-emission-standards-hazardous.

Following publication in the

Federal Register

, the EPA will post the

Federal Register

version of the proposal and key technical documents at this same web page. Information on the overall residual risk and technology review (RTR) program is available at

https://www.epa.gov/stationary-sources-air-pollution/risk-and-technology-review-national-emissions-standards-hazardous.

A memorandum showing the rule edits that would be necessary to incorporate the changes to 40 CFR part 63, subpart EEE, proposed in this action is available in the docket (Docket ID No. EPA-HQ-OAR-2004-0022). Following signature by the Administrator, the EPA also will post a copy of this document to

https://www.epa.gov/stationary-sources-air-pollution/hazardous-waste-combustors-national-emission-standards-hazardous.

II. Background

A. What is the statutory authority for this proposed action?

The statutory authority for this proposed action is provided by CAA sections 112, 301(a), and 502(a) (42 U.S.C. 7412, 7601(a), 7661a(a)). CAA section 112 establishes a two-stage regulatory process to develop standards for emissions of hazardous air pollutants (HAP) from stationary sources. Generally, the first stage involves establishing technology-based standards that reflect the maximum achievable control technology (MACT) or an appropriate alternative.

The second stage involves evaluating those standards within eight years to determine whether additional standards are needed to address any remaining risk associated with HAP emissions.

This second stage is commonly referred to as the “residual risk review.” In addition to the residual risk review, CAA section 112 also requires the EPA to review the standards every eight years and “revise as necessary” taking into account “developments in practices, processes, or control technologies.”

This review is commonly referred to as the “technology review.” When the two reviews are combined into a single rulemaking, it is commonly referred to as the “risk and technology review” (RTR). The discussion that follows identifies the most relevant statutory sections and briefly explains the contours of the methodology used to implement these statutory requirements.

42 U.S.C. 7412(d)(1)-(4).

Id.

7412(f)(2).

Id.

7412(d)(6).

In the first stage of the CAA section 112 standard-setting process, the EPA promulgates technology-based standards under CAA section 112(d) for categories of sources identified as emitting one or more of the HAP listed in CAA section 112(b). Sources of HAP emissions are either major sources or area sources, and CAA section 112 establishes different requirements for major and area source standards. “Major sources” are those that emit or have the potential to emit 10 tons per year (tpy) or more of a single HAP or 25 tpy or more of any combination of HAP.

All other sources are “area sources.”

For major sources, CAA section 112(d)(2) provides that the technology-based NESHAP must reflect the maximum degree of emission reductions of HAP achievable (after considering cost, energy requirements, and non-air quality health and environmental impacts). These standards are commonly referred to as MACT standards. CAA section 112(d)(3) also establishes a minimum control level for MACT standards, known as the MACT “floor,” based on emission controls achieved in practice by the best performing sources. In certain instances, as provided in CAA section 112(h), the EPA may set work practice standards in lieu of numerical emission standards. Under CAA section 112(h), the EPA may adopt a work practice standard in lieu of a numerical emission standard if it is “not feasible in the judgment of the Administrator to prescribe or enforce an emission standard for control of a hazardous air pollutant.”

CAA section 112(h)(2)(A) defines this phrase as applying in any situation where a HAP “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.”

This phrase is further defined in CAA section 112(h)(2)(B) as applying where “the Administrator determines that the application of measurement methodology to a particular class of sources is not practicable due to technological and economic limitations.”

The EPA has long considered situations where the majority of the measurements are below the detection limit as being a situation where measurement is not “technologically practicable” within the meaning of CAA section 112(h)(2)(B). Additionally, unreliable measurements raise issues of practicability, feasibility and enforceability. The application of measurement methodology in this situation would also not be “practicable due to . . . economic limitation” within the meaning of CAA section 112(h)(2)(B) because it would just result in cost expended to produce analytically

suspect measurements. The EPA also considers control options that are more stringent than the floor.

Standards more stringent than the floor are commonly referred to as “beyond-the-floor” standards. For area sources, CAA section 112(d)(5) allows the EPA to set standards based on generally available control technologies or management practices (GACT standards) in lieu of MACT standards.

42 U.S.C. 7412(a)(1).

Id.

7412(a)(2).

Id.

7412(h)(1).

Sierra Club

EPA,

479 F.3d 875, 883-84 (D.C. Cir. 2007); The EPA may “adopt[] a method to account for measurement imprecision that has a rational basis in the correlation between increased emission values and increased testing precision.”

Nat'l Ass'n of Clean Water Agencies

EPA,

734 F.3d 1115, 1154-55 (D.C. Cir. 2013).

42 U.S.C. 7412(h)(2)(A).

42 U.S.C. 7412(h)(2)(B).

Id.

7412(d)(2).

For categories of major sources and any area source categories subject to MACT standards, the second stage focuses on identifying and addressing any remaining (

i.e.,

“residual”) risk within eight years pursuant to CAA section 112(f). Specifically, CAA section 112(f)(2) requires the EPA to determine not later than eight years after establishment of the MACT standards whether promulgation of additional standards is needed to provide an ample margin of safety to protect public health or to prevent an adverse environmental effect. CAA section 112(d)(5) provides that this residual risk review is not required for categories of area sources subject to GACT standards. CAA section 112(f)(2)(B) expressly preserves the EPA's use of the two-step approach for developing standards to address any residual risk and the Agency's interpretation of “ample margin of safety” developed in the

National Emissions Standards for Hazardous Air Pollutants: Benzene Emissions from Maleic Anhydride Plants, Ethylbenzene/Styrene Plants, Benzene Storage Vessels, Benzene Equipment Leaks, and Coke By-Product Recovery Plants

(“Benzene NESHAP”).

The EPA notified Congress in the Residual Risk Report that the Agency intended to use the Benzene NESHAP approach in making CAA section 112(f) residual risk determinations (EPA-453/R-99-001, p. ES-11). The EPA subsequently adopted this approach in its residual risk determinations, and the United States Court of Appeals for the District of Columbia Circuit upheld the EPA's interpretation that CAA section 112(f)(2) incorporates the approach established in the Benzene NESHAP.

See NRDC

EPA,

529 F.3d 1077, 1083 (D.C. Cir. 2008).

54 FR 38044, Sept. 14, 1989.

The approach incorporated into the CAA and used by the EPA to evaluate residual risk and develop standards under CAA section 112(f)(2) is also a two-step approach. In the first step, the EPA determines whether risks are acceptable. This determination “considers all health information, including risk estimation uncertainty, and includes a presumptive limit on maximum individual lifetime [cancer] risk (MIR) of approximately 1 in 10 thousand”.

If risks are unacceptable, the EPA must determine the emission standards necessary to reduce risk to an acceptable level without considering costs. In the second step of the approach, the EPA considers whether the emission standards provide an ample margin of safety to protect public health “in consideration of all health information, including the number of persons at risk levels higher than approximately 1 in 1 million, as well as other relevant factors, including costs and economic impacts, technological feasibility, and other factors relevant to each particular decision”.

The EPA must promulgate emission standards necessary to provide an ample margin of safety to protect public health or determine that the standards being reviewed provide an ample margin of safety without any revisions. After conducting the ample margin of safety analysis, we consider whether a more stringent standard is necessary to prevent, taking into consideration costs, energy, safety, and other relevant factors, an adverse environmental effect.

54 FR 38045, Sept. 14, 1989. Although defined as “maximum individual risk,” MIR refers only to cancer risk. MIR, one metric for assessing cancer risk, is the estimated risk if an individual were exposed to the maximum level of a pollutant for a lifetime.

Id.

CAA section 112(d)(6) separately requires the EPA to review standards promulgated under CAA section 112 and revise them “as necessary (taking into account developments in practices, processes, and control technologies)” no less often than every eight years. In conducting this review, which we call the “technology review,” the EPA is not required to recalculate the MACT floors that were established during earlier rulemakings.

The EPA may consider cost in deciding whether to revise the standards pursuant to CAA section 112(d)(6).

Ass'n of Battery Recyclers, Inc.

EPA,

716 F.3d 667, 673-674 (D.C. Cir. 2013);

NRDC,

529 F.3d at 1084.

42 U.S.C. 7412(d)(2), (6);

Ass'n of Battery Recyclers,

617 F.3d at 673-74.

CAA sections 112(c)(3) and (k) require the EPA to identify and list the area source categories that represent 90 percent of the emissions of the 30 urban air toxics associated with area sources and subject them to standards under the CAA. CAA section 112(k)(3), which cross-references CAA section 112(c)(3), requires the EPA to identify a list of at least 30 air toxics that pose the greatest potential health threat in urban areas (the “urban” HAP). Taken together, these requirements are known as the Urban Air Toxics Strategy. These are the HAP that present the greatest threat to public health in the largest number of urban areas (CAA section 112(k)(3)(B)(i)). CAA sections 112(k)(3)(B)(ii) and 112(c)(3) also require the EPA to “assure that sources accounting for 90 percent or more of the 30 identified hazardous air pollutants are subject to standards.”

Louisiana Environmental Action Network (LEAN)

EPA,

the D.C. Circuit held that the EPA must address missing MACT standards for listed HAP known to be emitted from a major source category as part of its periodic review of MACT standards under CAA section 112(d)(6). 955 F.3d 1088 (D.C. Cir. 2020). In October 2022, Earthjustice filed an action in the U.S. District Court for the District of Columbia to compel the EPA to review and revise the HWC NESHAP under CAA sections 112(d)(6) and (f)(2) (

i.e.,

complete the RTR). In December 2024, the district court issued an order requiring that the EPA sign the final RTR rule for this source category by December 31, 2025, and establish standards for any previously unregulated HAP in the final RTR. Order,

Blue Ridge Envtl. Def. League

Regan,

22-cv-3134 (APM), at 4 (D.D.C. Dec. 12, 2024). The EPA is proposing this action in response to that court order.

Further, under CAA section 301(a) “[t]he Administrator is authorized to prescribe such regulations as are necessary to carry out his functions.” The EPA is also required to specify relevant test methods, best practices, procedures, or protocols and recordkeeping requirements for standards promulgated under CAA section 112.

Finally, CAA section 502(d)(l) requires each state to develop and submit to the EPA an operating permit program to meet the requirements of title V of the CAA and the EPA's implementing regulations at 40 CFR part 70 (“title V”). Major stationary sources of air pollution and certain other non-major sources are required to apply for and operate in accordance with title V operating permits that include emission limitations and other conditions as necessary to assure compliance with applicable requirements of the CAA, including the requirements of the applicable implementation plan.

B. What is this source category and how does the current NESHAP regulate its HAP emissions?

HWCs are incinerators, cement kilns, lightweight aggregate kilns, boilers, or HCl production furnaces that combust

hazardous waste for waste reduction, thermal energy recovery, and/or production of a product. Hazardous waste is defined under the Resource Conservation and Recovery Act (RCRA), which establishes a comprehensive regulatory structure overseeing the safe treatment, storage, and disposal of hazardous waste.

HWCs act as a disposal method for hazardous waste but can also provide other benefits to their owners and operators. The primary purpose of HWC incinerators is the destruction or volume reduction of hazardous waste. The primary purpose of HWC cement kilns is the production of cement using hazardous waste as a fuel to reduce the need for non-waste energy inputs. Similarly, HWC lightweight aggregate kilns use hazardous waste to provide energy for producing lightweight aggregate. HWC boilers produce thermal energy (often used in the form of steam), with hazardous waste often replacing the need for some non-waste fuel. An HWC HCl production furnace produces HCl, often using hazardous waste as a chlorine source.

42 U.S.C. 6901-6992k.

HWCs may either burn only hazardous waste produced onsite or by the owner, which is referred to as a “captive” HWC, or burn hazardous waste produced offsite or by someone other than the owner, which is referred to as a “commercial” HWC. Facilities with captive HWCs typically use their HWC as a waste management strategy. The most common captive HWCs are solid fuel boilers, liquid fuel boilers, HCl production furnaces, and some incinerators. Facilities with commercial HWCs typically use their HWC for revenue generation. The most common commercial HWCs are cement kilns, lightweight aggregate kilns, and incinerators. The main line of business for some commercial HWC incinerators is waste management. There are approximately 160 HWCs located at approximately 90 facilities in the United States. In 2023, approximately 32.2 million tons of hazardous waste were generated in the United States, all of which must be treated or disposed of in ways that protect human health and the environment.

Hazardous waste incineration provided that disposal for approximately 1.1 million tons of that hazardous waste, and energy recovery in units like hazardous waste burning boilers accounted for an additional 1.4 million tons.

U.S. Environmental Protection Agency. (Last updated Dec. 30, 2024). Biennial Report Summary:

https://rcrapublic.epa.gov/rcra-hwip/trends-and-analysis/details/4.

U.S. Environmental Protection Agency. (Last updated Jul. 10, 2025). Biennial Report Management Methods:

https://rcrapublic.epa.gov/rcra-hwip/trends-and-analysis/details/3.

HWCs are regulated under both the CAA and RCRA. Under the CAA, all unit types are regulated under the HWC NESHAP. Prior to demonstrating compliance with the HWC NESHAP, incinerators were primarily regulated by 40 CFR part 264, subpart O, and cement kilns, lightweight aggregate kilns, boilers, and HCl production furnaces were primarily regulated by 40 CFR part 266, subpart H. For most sources, air emission standards and associated operating requirements are no longer contained in their RCRA permits. Sources continue to hold RCRA permits for activities related to hazardous waste management, including general facility standards, manifest requirements, closure, financial responsibility, and any risk-based emission limit and associated operating conditions deemed necessary to protect human health and the environment.

The HWC NESHAP, which was originally promulgated in 1999, regulated hazardous waste incinerators, cement kilns, and lightweight aggregate kilns.

These standards were vacated in 2001

and replaced with interim standards in 2002.

The EPA promulgated replacement standards for hazardous waste incinerators, cement kilns, and lightweight aggregate kilns and first-time standards for hazardous waste solid fuel boilers, liquid fuel boilers, and HCl production furnaces in 2005.

Subsequently, the EPA received four petitions for reconsideration of the final rule. In 2006, the EPA granted reconsideration for eight issues raised by the petitions

and in 2007 reopened the 2005 rule

(the “Solicitation of Comment on Legal Analysis”) to consider comments relating to an intervening decision by the D.C. Circuit.

The EPA took final action on the eight reconsideration issues, responded to comments on the Solicitation of Comment on Legal Analysis, and made technical corrections in 2008.

In response to a petition for reconsideration of the 2008 final rule, the EPA sought and received a full voluntary remand of the rule in 2009 to reexamine the HWC NESHAP in totality.

64 FR 52828 (September 30, 1999).

Cement Kiln Recycling Coal.

EPA,

255 F.3d 855, 872 (D.C. Cir. 2001).

67 FR 6792 (Feb. 13, 2002).

70 FR 59402 (Oct. 12, 2005).

71 FR 14665 (Mar. 23, 2006); 71 FR 52624 (Sept. 6, 2006).

72 FR 54875 (Sept. 27, 2007).

Sierra Club

EPA,

479 F.3d 875 (D.C. Cir. 2007).

73 FR 64068 (Oct. 28, 2008).

Sierra Club

EPA,

Docket No. 05-1441 (consolidated with Docket Nos. 05-1442, 05-1443, 05-1445, 05-1449) (D.C. Cir.).

In July 2024, the EPA issued a notice of proposed rulemaking for the HWC NESHAP regarding emission standards during periods of malfunction, electronic reporting provisions, emergency safety vent provisions, and other minor technical corrections.

The EPA is withdrawing certain aspects of that proposal in this document for the reasons explained in section IV.E. of this preamble. The EPA is instead proposing different requirements and soliciting comments on certain topics from the 2024 proposal that include emission standards during periods of malfunction and electronic reporting provisions in this notice of proposed rulemaking. The EPA will respond to other comments on aspects of the July 2024 proposal that are not withdrawn in the final action for this proposal.

89 FR 59867 (Jul. 24, 2024).

The key pollutants that the HWC NESHAP regulates include polychlorinated dibenzodioxins and furans (PCDD/PCDF); mercury (Hg); cadmium (Cd) and lead (Pb) as semi-volatile metals (SVM); arsenic (As), beryllium (Be), and chromium (Cr) as low-volatile metals (LVM); antimony (Sb), cobalt (Co), manganese (Mn), nickel (Ni), and selenium (Se) as non-enumerated metal HAP; HCl and chlorine gas; and other hydrocarbon HAP, including polychlorinated biphenyls (PCBs) and polycyclic aromatic hydrocarbons (PAHs). The HWC NESHAP also includes several other emission limits such as a carbon monoxide (CO) or total hydrocarbon (THC) limit associated with demonstrating good combustion practices, a destruction and removal efficiency (DRE) standard also for demonstrating good combustion practices, and a particulate matter (PM) emission limit in some subcategories.

The HWC NESHAP regulates HAP through a combination of numeric emission limits and surrogate standards, where compliance with one emission standard demonstrates compliance with the standard for another HAP. For example, emissions of non-PCDD/PCDF organic HAP, including PCBs and PAHs, are regulated by the combination of the DRE standard and either the CO or THC standard, as chosen by the source. Another example of a surrogate is the PM standard, which primarily regulates emissions of non-enumerated metal HAP. These metals are not regulated by

another metal HAP standard. Sources may also choose to regulate non-enumerated metal HAP directly as an alternative to the PM standard. An alternative health-based emission limit (HBEL) based on a site-specific risk assessment is also available for HCl and chlorine gas.

For more information on the alternative HBEL for HCl and chlorine gas,

see

70 FR 59413-25 (Oct. 12, 2005);

see also

69 FR 21298-305-06 (Apr. 20, 2004).

The HWC NESHAP regulates HAP emissions from HWCs at major and area sources, as defined by CAA sections 112(a)(1) and (2). The HWC NESHAP also requires both major and area sources to obtain a title V air permit. Major and area sources are subject to the same standards for HWC incinerators, cement kilns, and lightweight aggregate kilns. Area source HWC boilers and HCl production furnaces are only subject to the same emission standards as major sources for Hg, PCDD/PCDF, and non-PCDD/PCDF organic HAP. RCRA standards for Cd and Pb, Cr, HCl and chlorine gas, and PM under 40 CFR part 266, subpart H, apply to area source HWC boilers and HCl production furnaces unless an area source elects to comply with the HWC NESHAP major source standards in lieu of the RCRA standards. Area sources otherwise have the same requirements as major sources, including recordkeeping, reporting, operator training, the startup, shutdown, and malfunction (SSM) plan, and the automatic waste feed cutoff (AWFCO) system.

See

70 FR 59432 (Oct. 12, 2005) for further discussion on the similarities and differences between major and area source standards.

Periods of SSM are addressed under both the RCRA rules and the HWC NESHAP. The requirements of both rules apply simultaneously to HWCs. Under the RCRA rules, sources may choose to retain or revise certain RCRA permit conditions that are specific to periods of SSM, including a requirement not to feed most types of hazardous waste during periods of SSM. Alternatively, sources may choose to remove RCRA operating permit conditions specific to periods of SSM if an SSM plan has been developed under the HWC NESHAP and approved by the Administrator.

Most sources have chosen to remove conditions specific to periods of SSM from their RCRA permits because they have approved SSM plans under the HWC NESHAP.

See

40 CFR part 270, subpart I, for the integration of RCRA and CAA standards during periods of SSM.

The emission standards and operating requirements of the HWC NESHAP currently do not apply during periods of SSM.

However, there are two requirements relating to periods of SSM that apply to all HWCs. Specifically, all HWCs must develop an SSM plan and operate an AWFCO system. All HWCs are subject to the SSM plan provisions of the 40 CFR part 63, subpart A general provisions, including the requirements to develop an SSM plan and update it as necessary. Under the general provisions, if actions taken by the owner or operator cause the source to exceed any applicable emission limitation and are consistent with the procedures specified in the SSM plan, then the owner or operator must keep records and confirm in their reporting that their actions were consistent with the SSM plan. If actions taken by the owner or operator cause the source to exceed any applicable emission limitation and are not consistent with the procedures specified in the SSM plan, then the owner or operator must also record and report those actions.

In July 2024, the EPA proposed to remove the malfunction exemption from the HWC NESHAP, which, if finalized, would have required standards for periods of normal operation to apply at all times (89 FR 59870). After considering the comments received on that proposal, the EPA is withdrawing the proposed removal of the malfunction exemption and is instead proposing different requirements for periods of malfunction, as described in section IV.E of this preamble.

If an HWC uses their CAA SSM plan to comply with RCRA requirements, then the SSM plan must be submitted to the Administrator for review and approval and the SSM plan must include a description of potential causes of malfunctions and actions the source is taking to minimize the frequency and severity of those malfunctions. The Administrator must also approve any changes to the SSM plan if the changes may significantly increase emissions of HAP.

All HWCs are also required to operate an AWFCO system, which is a system that immediately (or within one minute in some circumstances) and automatically cuts off the hazardous waste feed to the HWC when an operating parameter limit (OPL) established per the HWC NESHAP is exceeded, an emission standard monitored by a continuous emission monitoring system (CEMS) is met or exceeded, the allowable combustion chamber pressure is exceeded, the span value of any continuous monitoring system (CMS) detector except a CEMS is met or exceeded, a CMS monitoring an emission level or an OPL established per the HWC NESHAP malfunctions, or any component of the AWFCO system fails. During an AWFCO, owners or operators must continue to send combustion gases to the air pollution control system while hazardous waste remains in the combustion chamber of the HWC. Hazardous waste feed to the HWC cannot restart until the OPLs and emission levels are within the specified limits, which typically takes no less than one hour. The AWFCO system must generally be tested at least weekly.

HWCs must comply with the described AWFCO system requirements during malfunctions, although an exceedance of an OPL or emission standard interlocked with the AWFCO system is not a violation of the HWC NESHAP if the corrective measures prescribed in the SSM plan are correctly followed.

Additionally, HWCs must comply with AWFCO requirements during periods of startup and shutdown if they burn hazardous waste during those periods. An exceedance of an OPL or emission standard interlocked with the AWFCO system is not a violation of the HWC NESHAP if the corrective measures prescribed in the SSM plan are correctly followed. If owners or operators of HWCs feed hazardous waste during periods of startup or shutdown, they must include waste feed restrictions and other appropriate operating conditions and limits in the SSM plan and interlock those OPLs with the AWFCO system. Under the RCRA incinerator (40 CFR part 264, subpart O) and boiler and industrial furnaces (BIF; 40 CFR part 266, subpart H) requirements, hazardous waste may be fed into an HWC during startup and shutdown if all OPLs are being met. This is typically the case shortly before startup ends and shortly after shutdown begins. In addition, certain types of hazardous waste may be fed during startup and shutdown under certain stipulations, regardless of whether OPLs for periods of normal operation are being met. One example is a waste that is only considered hazardous because it is ignitable and easily burned; an owner or operator might feed this waste into the combustor during startup and use the energy released by its combustion to raise the HWC's temperature to the allowable range for periods of normal operation. Most HWCs do not combust hazardous waste during startup and shutdown. In cases where an HWC does so, we expect that the HWC NESHAP's SSM plans closely mirror RCRA's restrictions on hazardous waste feed during periods of startup and shutdown.

C. What data collection activities were conducted to support this action?

The EPA conducted multiple data collection activities to support this action, including collecting HWC facility permits and emissions testing

information available through state and local authorities, a two-phased request for information under CAA section 114, and site visits to HWC facilities.

In its 1999 report to Congress on how the agency planned to address residual risks, the EPA stated that “source and emissions data can be derived from broad-scale emissions inventories,

specific data collection efforts with particular industries,

or information from regional, State, or local air toxics agencies.”

Residual Risk Report to Congress,

EPA-453/R-99-001 at 13 (March 1999) (emphasis added).

To develop the facility list, we began with a partial facility list provided by the EPA Office of Land and Emergency Management (OLEM), which administers the RCRA rules for HWCs. We also gathered a list of all facilities listed as subject to the HWC NESHAP in the EPA Office of Enforcement and Compliance Assurance's (OECA) Enforcement and Compliance History Online (ECHO) tool (

https://echo.epa.gov

). We reviewed and cross-referenced these lists and confirmed that facilities were subject to the HWC NESHAP by gathering title V air permits from the websites of state and local governments or agencies, where available. The resulting facility list is available in the docket for this proposed rule (Docket ID No. EPA-HQ-OAR-2004-0022).

The EPA also collected emissions test reports for the comprehensive performance test (CPT) and confirmatory performance test (CfPT) required by the HWC NESHAP. These test reports were gathered from the websites of state and local governments or agencies where available, from the EPA regional offices, and from some industry stakeholders who voluntarily provided courtesy copies. The CPT and CfPT reports provide unit- and site-specific emissions information for the HAP regulated by the HWC NESHAP and are the basis for emissions of the currently regulated HAP in the risk modeling for the HWC source category. The collected emissions test reports used to develop emissions for the HWC source category are available in the docket for this proposed rule (Docket ID No. EPA-HQ-OAR-2004-0022).

In August 2023 and January 2024, the EPA issued requests to collect information from HWC facilities owned and operated by nine entities (

i.e.,

corporations) pursuant to CAA section 114. These facilities were chosen to represent the six HWC subcategories and commercial and captive units. The August 2023 request was a questionnaire designed to collect comprehensive information about process equipment, control technologies, emissions, composition of the hazardous waste feed to the unit, periods of SSM, and other aspects of facility operations. Companies submitted responses (and follow-up responses) in November 2023. A copy of the questionnaire and the information not claimed as CBI by respondents is available in the docket for this proposed rule.

Docket ID No. EPA-HQ-OAR-2004-0022, Document ID No. EPA-HQ-OAR-2004-0022-0651.

Following the review of the August 2023 questionnaire information, the EPA issued an emissions testing request to the same nine entities in January 2024 to obtain emissions information about targeted pollutants and to characterize emissions of any HAP not currently regulated by the HWC NESHAP. Companies submitted responsive emissions testing results (and follow-up responses) between September 2024 and November 2024. The EPA did not receive emissions testing results from one company that temporarily ceased operation of their HWCs between January 2024 and September 2024. The January 2024 request generally required emissions testing of PCBs, PAHs, hydrogen fluoride (HF), hydrogen cyanide (HCN), hydrogen bromide (HBr), THC, and supporting measurements like oxygen and moisture, though the requests were tailored to the specific survey responses of each recipient. Notably, all HCl production furnaces indicated in their survey responses that they do not feed any fluorine to their units because it would contaminate their HCl product with HF, and so HCl production furnaces were not required to test for HF. The EPA has used the collected information to identify and quantify emissions of HAP not measured during a CPT or CfPT, fill data gaps, and estimate the public health, environmental, and cost impacts associated with the regulatory options considered in this proposed action. A copy of the emissions testing request and the information not claimed as CBI by respondents is available in the docket for this proposed rule (Docket ID No. EPA-HQ-OAR-2004-0022).

The EPA also conducted two site visits to HWC facilities in 2023. The primary goals of these site visits were to learn about the day-to-day operations of incinerators, solid fuel boilers, and cement kilns. Reports documenting these site visits are available in the docket for this proposed rule.

Docket ID No. EPA-HQ-OAR-2004-0022, Document ID Nos. EPA-HQ-OAR-2004-0022-0649 and EPA-HQ-OAR-2004-0022-0650.

D. What other relevant background information and data are available?

The EPA used emissions and supporting data from the National Emissions Inventory (NEI) based on emissions year 2022, supporting data from the database developed for the 2005 HWC NESHAP Final Rule, available RCRA trial or risk burn data, and CPT and CfPT stack test data from as many sources as possible to develop model file inputs for the residual risk assessment of sources subject to the HWC NESHAP.

The NEI is a database that contains information about sources that emit criteria air pollutants, their precursors, and HAP. The NEI contains data necessary for conducting risk modeling, including annual HAP emissions estimates from individual emissions sources at facilities and the related emissions release parameters. The database includes estimates of annual air pollutant emissions from point, nonpoint, and mobile sources in the 50 states, the District of Columbia, Puerto Rico, and the U.S. Virgin Islands. The EPA collects this information and releases a full, updated version of the NEI database every three years. The 2022 emissions data is not a full, triennial NEI; instead, the 2020 NEI was taken as a basis and more recent 2022 data was incorporated. In cases where we had emissions and release point parameters for the same HWC from multiple data sources (

e.g.,

the NEI, CPT, and August 2023 questionnaire), we prioritized information in the following order: August 2023 questionnaire, CPT or CfPT data, then NEI data. Additional information on the development of the modeling file can be found in the docket for this proposed rule (Docket ID No. EPA-HQ-OAR-2004-0022).

To identify control technologies in use and determine whether there have been developments in practices, processes, or control technologies to consider under the technology review, the EPA collected information from the August 2023 questionnaire, January 2024 emissions testing request, CPT and CfPT reports, consent decrees involving HWCs regulated by the HWC NESHAP, and the Reasonably Available Control Technology (RACT)/Best Available Control Technology (BACT)/Lowest Achievable Emission Rate (LAER) Clearinghouse (RBLC). The EPA established the RBLC to provide a central database of air pollution technology information (including technologies required in source-specific permits) to promote the sharing of information among permitting agencies

and to ais in identifying future control technology options that might apply to numerous sources within a category or only on a source-by-source basis.

The EPA also reviewed subsequent CAA regulatory actions for other source categories to determine whether there have been other developments in practices, processes, or control technologies that may also be applicable to the HWC NESHAP source category. Additional information about the technology review can be found in the docket for this proposed rule (Docket ID No. EPA-HQ-OAR-2004-0022).

U.S. Environmental Protection Agency. (Last updated Sept. 29, 2025). RACT/BACT/LAER Clearinghouse (RBLC) Basic Information:

https://www.epa.gov/catc/ractbactlaer-clearinghouse-rblc-basic-information.

III. Analytical Procedures and Decision-Making

In this section, we describe the analyses performed to support the proposed decisions for the RTR and other issues addressed in this proposal.

A. How do we consider risk in our decision-making?

As discussed in section II.A. of this preamble and in the Benzene NESHAP, in evaluating and developing standards under CAA section 112(f)(2), we apply a two-step approach to determine whether or not risks are acceptable and to determine if the standards provide an ample margin of safety to protect public health. As explained in the Benzene NESHAP, “the first step judgment on acceptability cannot be reduced to any single factor” and, thus, “[t]he Administrator believes that the acceptability of risk under section 112 is best judged on the basis of a broad set of health risk measures and information.”

Similarly, with regard to the ample margin of safety determination, “the Agency again considers all of the health risk and other health information considered in the first step” (

id.

). “Beyond that information, additional factors relating to the appropriate level of control will also be considered, including cost and economic impacts of controls, technological feasibility, uncertainties, and any other relevant factors” (

id.

54 FR 38046, Sept. 14, 1989.

The Benzene NESHAP approach provides flexibility regarding factors the EPA may consider in making determinations and how the EPA may weigh those factors for each source category.

The EPA conducts a risk assessment that provides estimates of the MIR posed by emissions of HAP that are carcinogens from each source in the source category, the hazard index (HI) for chronic exposures to HAP with the potential to cause noncancer health effects, and the hazard quotient (HQ) for acute exposures to HAP with the potential to cause noncancer health effects.

The assessment also provides estimates of the distribution of cancer risk within the exposed populations, cancer incidence, and an evaluation of the potential for an adverse environmental effect. The scope of the EPA's risk analysis is consistent with the explanation in the EPA's response to comments on our policy under the Benzene NESHAP:

See NRDC,

529 F.3d at 1082-1084.

The MIR is defined as the cancer risk associated with a lifetime of exposure at the highest concentration of HAP where people are likely to live. The HQ is the ratio of the potential HAP exposure concentration to the noncancer dose-response value; the HI is the sum of HQs for HAP that affect the same target organ or organ system.

The policy chosen by the Administrator permits consideration of multiple measures of health risk. Not only can the MIR figure be considered, but also incidence, the presence of non-cancer health effects, and the uncertainties of the risk estimates. In this way, the effect on the most exposed individuals can be reviewed as well as the impact on the general public. These factors can then be weighed in each individual case. This approach complies with the

Vinyl Chloride

mandate that the Administrator ascertain an acceptable level of risk to the public by employing his expertise to assess available data. It also complies with the Congressional intent behind the CAA, which did not exclude the use of any particular measure of public health risk from the EPA's consideration with respect to CAA section 112 regulations, and thereby implicitly permits consideration of any and all measures of health risk which the Administrator, in his judgment, believes are appropriate to determining what will “protect the public health.”

54 FR 38057, Sept. 14, 1989.

Thus, the level of the MIR is only one factor to be weighed in determining acceptability of risk. As the EPA explained in the Benzene NESHAP: “[A]n MIR of approximately one in 10 thousand should ordinarily be the upper end of the range of acceptability. As risks increase above this benchmark, they become presumptively less acceptable under [CAA] section 112 and would be weighed with the other health risk measures and information in making an overall judgment on acceptability. Or, the Agency may find, in a particular case, that a risk that includes an MIR less than the presumptively acceptable level is unacceptable in the light of other health risk factors.”

In other words, risks that include an MIR above 100-in-1 million (1-in-10 thousand) may be determined to be acceptable, and risks with an MIR below that level may be determined to be unacceptable, depending on the available health information. Similarly, with regard to the ample margin of safety analysis, the EPA stated in the Benzene NESHAP that: “EPA believes the relative weight of the many factors that can be considered in selecting an ample margin of safety can only be determined for each specific source category. This occurs mainly because technological and economic factors (along with the health-related factors) vary from source category to source category.”

We also consider the uncertainties associated with the various risk analyses, as discussed later in this preamble, in our determinations of acceptability and ample margin of safety.

Id.

at Sept. 14, 1989.

54 FR 38061, Sept. 14, 1989.

The EPA notes that, as a matter of longstanding practice, we do not attempt to quantify the HAP risk that may be associated with emissions from other facilities that do not include the source category under review, mobile source emissions, natural source emissions, persistent environmental pollution, or atmospheric transformation in the vicinity of the sources in the category. The EPA understands the potential importance of considering an individual's total exposure to HAP in addition to considering exposure to HAP emissions from the source category and facility. We recognize that such consideration may be particularly important when assessing noncancer risk, where pollutant-specific exposure health reference levels (

e.g.,

reference concentrations (RfCs)) are based on the assumption that thresholds exist for adverse health effects. For example, the EPA recognizes that, although exposures attributable to emissions from a source category or facility alone may not indicate the potential for increased risk of adverse noncancer health effects in a population, the exposures resulting from emissions from the facility in combination with emissions from all of the other sources (

e.g.,

other facilities) to which an individual is exposed may be sufficient to result in an increased risk of adverse noncancer health effects. In May 2010, the Science Advisory Board (SAB) advised the EPA “that RTR assessments will be most useful to decision makers and communities if results are presented in the broader context of aggregate and cumulative risks, including background

concentrations and contributions from other sources in the area.”

Recommendations of the SAB Risk and Technology Review Methods Panel are provided in their report, which is available at:

https://www.epa.gov/sites/default/files/2021-02/documents/epa-sab-10-007-unsigned.pdf.

In response to the SAB recommendations, the EPA incorporates cumulative risk analyses into its RTR risk assessments. The Agency (1) conducts facility-wide assessments, which include source category emission points, as well as other emission points within the facilities; (2) combines exposures from multiple sources in the same category that could affect the same individuals; and (3) for some persistent and bioaccumulative pollutants, analyzes the ingestion route of exposure. In addition, the RTR risk assessments consider aggregate cancer risk from all carcinogens and aggregated noncancer HQs for all noncarcinogens affecting the same target organ or target organ system.

Although we are interested in placing source category and facility-wide HAP risk in the context of total HAP risk from all sources combined in the vicinity of each source, we note there are uncertainties of doing so. Estimates of total HAP risk from emission sources other than those that we have studied in depth during this RTR review would have greater associated uncertainties than the source category or facility-wide estimates. We further note that CAA section 112(f)(2) does not require or authorize the EPA to promulgate standards based on cumulative assessments of a person's total exposure to HAP from all sources.

B. How do we perform the technology review?

The EPA's technology review primarily focuses on the identification and evaluation of developments in practices, processes, and control technologies that have occurred since the MACT standards were promulgated. Where we identify such developments, we analyze their technical feasibility, estimated costs, energy implications, and non-air environmental impacts. We also consider the emission reductions associated with the potential application of each development. This analysis informs our decision whether it is “necessary” to revise the emission standards. In addition, we consider the appropriateness of applying controls to new sources versus retrofitting existing sources. For this exercise, we consider any of the following to be a “development” :

Nat'l Ass'n for Surface Finishing

EPA,

795 F.3d 1, 11 (D.C. Cir. 2015) (upholding EPA's interpretation of what is considered “developments” under CAA section 112(d)(6) and deferring to EPA's methodology and balancing decisions for a technology review under the

Skidmore

standard of review).

• Any add-on control technology or other equipment that was not identified and considered during development of the original MACT standards;

• Any improvements in add-on control technology or other equipment (that were identified and considered during development of the original MACT standards) that could result in additional emissions reduction;

• Any work practice or operational procedure that was not identified or considered during development of the original MACT standards;

• Any process change or pollution prevention alternative that could be broadly applied to the industry and that was not identified or considered during development of the original MACT standards; and

• Any significant changes in the cost (including cost effectiveness) of applying controls (including controls the EPA considered during the development of the original MACT standards).

In addition to reviewing the practices, processes, and control technologies that were considered at the time we originally developed the NESHAP, we review a variety of data sources in our investigation of potential practices, processes, or controls. Pursuant to the D.C. Circuit's decision in

LEAN,

we also review available data to determine if there are any unregulated emissions of HAP within the source category and evaluate this data for use in developing new emission standards. The

LEAN

decision requires the EPA to address regulatory gaps when reviewing MACT standards, such as missing standards for listed air toxics known to be emitted from a major source category. See sections II.C. and II.D of this preamble for information on the specific data sources that were reviewed as part of the technology review.

C. How do we estimate post-MACT risk posed by the source category?

In this section, the EPA provides a description of the types of analyses that we generally perform during the risk assessment process. In some cases, we do not perform a specific analysis because it is not relevant. For example, in the absence of emissions of HAP known to be persistent and bioaccumulative in the environment (PB-HAP), we would not perform a multipathway exposure assessment. Where we do not perform an analysis, we state that we do not and provide the reason. While we present all of our risk assessment methods, we only present risk assessment results for the analyses actually conducted (see section IV.B. of this preamble).

The EPA conducts a risk assessment that provides estimates of the MIR for cancer posed by the HAP emissions from each source in the source category, the HI for chronic exposures to HAP with the potential to cause noncancer health effects, and the HQ for acute exposures to HAP with the potential to cause noncancer health effects. The assessment also provides estimates of the distribution of cancer risk within the exposed populations, cancer incidence, and an evaluation of the potential for an adverse environmental effect, taking into consideration the factors set out in CAA section 112(f). The following eight subsections describe how we estimated emissions and conducted the risk assessment. The docket for this proposed rule contains the following document which provides more information on the risk assessment inputs and models:

Residual Risk Assessment for the Hazardous Waste Combustor Source Category in Support of the 2025 Risk and Technology Review Proposed Rule.

The methods used to assess risk (as described in the eight primary steps below) are consistent with those described by the EPA in the document reviewed by a panel of the EPA's SAB in 2009;

and described in the SAB review report issued in 2010.

They are also consistent with the key recommendations contained in that report.

U.S. Environmental Protection Agency. (Last updated Mar. 20, 2012). Risk and Technology Review (RTR) Risk Assessment Methodologies: For Review by the EPA's Science Advisory Board—Case Studies—MACT I Petroleum Refining Sources and Portland Cement Manufacturing (EPA-452/R-09-006):

https://cfpub.epa.gov/si/si_public_record_report.cfm?LAB=OAQPS&dirEntryID=238928.

U.S. Environmental Protection Agency. (Last updated May 7, 2010). Review of EPA's draft entitled, “Risk and Technology Review (RTR) Risk Assessment Methodologies: For Review by the EPA's Science Advisory Board—Case Studies—MACT I Petroleum Refining Sources and Portland Cement Manufacturing”:

https://www.epa.gov/sites/default/files/2021-02/documents/epa-sab-10-007-unsigned.pdf.

1. How did we estimate actual emissions and identify the emissions release characteristics?

The EPA used the actual emissions and emissions release characteristics from the NEI based on emissions year 2022 for each HWC facility to create the initial risk modeling input file. For each NEI record, the EPA reviewed the standard classification code, emission unit description, and process description to classify each record as

either belonging to the source category (

i.e.,

the record represents emissions from an HWC) or not belonging to the source category. Source category emissions of HAP are stack emissions only because the source category is specific to the HWC unit. We included both stack and fugitive emissions in non-category records. We removed duplicate emission records for an HWC. For example, some facilities listed emissions from the HWC when the unit was and was not combusting hazardous waste separately. To consolidate these records, we removed the records identified as periods when hazardous waste was not being combusted. We then cross-referenced each source category record against the facility list and added units in the facility list that could not be identified in the NEI records. We identified emission release characteristics from emissions testing information, as available.

Emissions test data or values derived from emissions test data replaced or augmented NEI data for HWC emissions of PCDD/PCDF, HAP metals, HCl, chlorine gas, PCBs, PAHs, HF, and HCN. When we had CPT data or data responsive to the CAA section 114 emissions testing request for a unit, we used that data to estimate emissions. Most of the data were concentration data, and we calculated emissions on a tpy basis using the stack gas flow rate and by assuming that units operate 8,760 hours (hr) per year (

i.e.,

continuous operation). Some of the data were in a thermal concentration format (like pounds (lb) per million british thermal units (MMBTU)), and we used the thermal hazardous waste feedrate (MMBTU/hr) to calculate emissions. When we had units with data from multiple CPTs, test conditions, or runs, we used the mean to estimate annual emissions (tpy). For cement kilns with in-line raw mills, we calculated weighted averages to account for the typical time spent with the raw mill on (85 percent) and off (15 percent).

In accordance with the HWC NESHAP, owners and operators conduct CPTs at worst-case test conditions. Companies use multiple strategies to ensure that CPTs are conducted at worst-case conditions, including operating at worst-case operating parameter limits (

e.g.,

low combustion chamber temperature, high hazardous waste feed rate, high stack gas velocity) and intentionally adding extra HAP, HAP surrogates, or HAP precursors to the feed of the HWC to account for potential variability in the HWC feed. This means that emissions estimates based on CPT data are conservative, worst-case estimates. We expect that actual annual HAP emissions are lower than the estimates based on CPT data.

The EPA used CfPT data to help account for the conservative, worst-case estimates, where available. Unlike CPTs, CfPTs are conducted at normal operating conditions, and so we expect operating parameters to better reflect average operations of HWCs. CfPTs are only conducted for incinerators, cement kilns, lightweight aggregate kilns, and some liquid fuel boilers. Only PCDD/PCDF is measured when CfPTs are conducted. To scale the estimates produced by the CPT data to better resemble normal operating conditions, we developed ratios between the average stack gas flow rate during CPTs and CfPTs (“CPT adjustment factor”) and used them to adjust the CPT emissions estimates, with a maximum value of one.

The HWC NESHAP regulates metal HAP in groups: Hg; Cd and Pb are regulated as SVM; As, Be, and Cr are regulated as LVM (except for liquid fuel boilers, where the LVM standard regulates Cr only); and Sb, Co, Mn, Ni, and Se are typically regulated by PM surrogate. Because the HWC NESHAP regulates metals in groups, metals are often reported by group in CPT results; however, the residual risk review requires the emissions of each metal to be determined separately. Cr also required further speciation because Cr species vary widely in both physiochemical properties and toxicity. To separate these results, the EPA developed speciation factors for Cr, SVM, LVM, and the other metals (regulated using PM as a surrogate) by unit type using the database developed for the 2005 HWC NESHAP final rule.

Generally, we developed the speciation factors by calculating the ratio of the emission of the chemical species in question to the emission of the total group. We averaged speciation factors by unit type, and only the unit type averages were used to account for variability in chemical speciation. We also used standard Hg speciation factors to calculate emissions for Hg species with different physiochemical and toxicity properties. For liquid fuel boilers, the only regulated LVM is Cr, so speciation of Cr from LVM was not required. We did not speciate liquid fuel boiler emissions of As and Be from PM; instead, we collected separate emissions testing data (from trial or risk burns, or to demonstrate compliance with state or RCRA limits) for As and Be and used the data to estimate emissions.

Docket ID No. EPA-HQ-OAR-2004-0022, Document ID No. EPA-HQ-OAR-2004-0022-0433. Available at

https://www.epa.gov/stationary-sources-air-pollution/hazardous-waste-combustors-national-emission-standards-hazardous.

We did not have complete data to support unit-specific estimates for all units. The EPA made several informed assumptions to help fill the gaps. First, the EPA calculated average CPT adjustment factors for each unit type and applied those factors for units without a unit-specific CPT adjustment factor. Two HWC subcategories, solid fuel boilers and HCl production furnaces, are required to conduct CPTs but not CfPTs. We assumed that the unit type average CPT adjustment factor for these units was the average of the four other unit subcategory average CPT adjustment factors. Second, some companies have units that have been deemed identical units (“sister units”) for the purpose of demonstrating compliance with the HWC NESHAP. Typically, the companies perform emissions testing on one HWC of a set of sister units, and they attribute the results to the sister units. When an HWC did not have emissions data, but a sister unit did, we attributed those emissions to all sister unit HWCs without unit-specific data. Third, we calculated average emissions for each unit subcategory. We assumed that the emissions for an HWC without emissions data for a specific HAP were the average emissions of the unit subcategory. We applied this assumption widely for PCBs, PAHs, HF, and HCN because most of the emissions data represented a limited number of HWCs in the January 2024 emissions testing request. No emissions data were submitted in response to the January 2024 emissions testing request for lightweight aggregate kilns; instead, we combined the maximum concentration of each pollutant measured in the January 2024 emissions testing request and the average stack gas flow rate for lightweight aggregate kilns to estimate conservative emissions of these HAP from lightweight aggregate kilns. Fourth, for units with HAP emissions estimated by the HWC subcategory average emission, we substituted the estimated allowable emissions for the HWC subcategory average emission if the HWC subcategory average emission exceeded the estimated allowable emissions (calculated as described in section III.C.2. of this preamble). This affected HWCs with a known flow rate but no known concentration, and it allowed us to better estimate actual emissions from HWCs with smaller-than-average flow rates since the average emissions from the HWC subcategory are based on units with higher flow rates.

Additional information on the development of the modeling file for the HWC NESHAP source category, including the development of the actual emissions and emissions release characteristics, can be found in the docket for this proposed rule (Docket ID No. EPA-HQ-OAR-2004-0022).

2. How did we estimate MACT-allowable emissions?

The available emissions data in the RTR emissions dataset include estimates of the mass of HAP emitted during a specified annual time period. These “actual” emission levels are often lower than the emission levels allowed under the requirements of the current MACT standards. The emissions allowed under the MACT standards are referred to as the “MACT-allowable” emissions. We discussed the consideration of both MACT-allowable and actual emissions in the final Coke Oven Batteries RTR (70 FR 19992, 19998-99, Apr. 15, 2005) and in the proposed and final Hazardous Organic NESHAP RTR (71 FR 34421, 34428, June 14, 2006; 71 FR 76603, 76609, Dec. 21, 2006). In those actions, we noted that assessing the risk at the MACT-allowable level is inherently reasonable since that risk reflects the maximum level facilities could emit and still comply with NESHAP. We also explained that it is reasonable to consider actual emissions, where such data are available, in both steps of the risk analysis, in accordance with the Benzene NESHAP approach (54 FR 38044).

The current HWC NESHAP specifies numeric emission limits for PCDD/PCDF, HAP metals (directly, as groups, or through a surrogate), HCl, and chlorine gas for existing and new HWCs. These limits were used as the basis for calculating the MACT-allowable emissions. CPT stack gas flow rates and thermal hazardous waste feed rates were identified for each HWC, where possible. For HWCs without stack gas flow rate or thermal feed rate data, average rates were calculated for each type of HWC and used to fill gaps. These rates were combined with the emission limits and the assumption of 8,760 hours of operation per year (

i.e.,

24 hours per day, 365 days per year) to produce the upper bound of MACT-allowable emissions. In the case where a standard has two formats (

i.e.,

both a mass and thermal concentration basis), we took the greater of the two as the MACT-allowable emission. We then speciated the MACT-allowable emissions following the same procedures as the actual emissions, except that we speciated As and Be MACT-allowable emissions for liquid fuel boilers were speciated from PM. For PAHs, PCBs, HCN, and HF, we estimated the unit type allowable emissions from the average actual emissions for each unit type.

For the HWC source category, actual emissions tend to be lower than allowable emissions, in some cases much lower. This shows that HWCs are generally performing better than they are required to by the HWC NESHAP. We generally expect that actual emissions will be lower than allowable emissions because HWCs must demonstrate that their emissions are consistently below the emission limits, which practically means that they operate in such a manner as to be far enough below the emission limit that slight variations in the combustor operation would not cause them to exceed the emission limit. We use the full value of the emission limit to calculate allowable emissions. Another contributing factor in some cases could be that additional non-HWC NESHAP emission limits are established for HWCs under RCRA. Most HWCs have completed site-specific risk assessments using RCRA methodology and under specific provisions of RCRA. Some HWCs may have additional emission restrictions under RCRA based on those results. The methodologies in the RCRA site-specific risk assessments and this CAA residual risk review are not comparable, and one should not be used in lieu of the other. Additional information on the development of the modeling file for the HWC NESHAP source category, including the estimation of MACT-allowable emissions, can be found in the docket for this proposed rule (Docket ID No. EPA-HQ-OAR-2004-0022).

3. How do we conduct dispersion modeling, determine inhalation exposures, and estimate individual and population inhalation risk?

Both long- and short-term inhalation exposure concentrations and health risk from the source category addressed in this proposal were estimated using the Human Exposure Model (HEM).

The HEM performs three primary risk assessment activities: (1) conducting dispersion modeling to estimate the concentrations of HAP in ambient air; (2) estimating long- and short-term inhalation exposures to individuals residing within 50 kilometers (km) of the modeled sources; and (3) estimating individual and population-level inhalation risk using the exposure estimates and quantitative dose-response information.

For more information about HEM, go to

https://www.epa.gov/fera/risk-assessment-and-modeling-human-exposure-model-hem.

a. Dispersion Modeling

The air dispersion model AERMOD (American Meteorological Society/EPA Regulatory Model dispersion modeling system), used by the HEM model, is one of the EPA's preferred models for assessing air pollutant concentrations from industrial facilities.

To perform the dispersion modeling and to develop the preliminary risk estimates, HEM draws on three data libraries. The first is a library of meteorological data, which is used for dispersion calculations. This library includes one year (2019) of hourly surface and upper air observations from over 800 meteorological stations, selected to provide coverage of the United States and Puerto Rico. A second library of United States Census Bureau census block

internal point locations and populations provides the basis of human exposure calculations (U.S. Census, 2020). In addition, for each census block, the census library includes the elevation and controlling hill height, which are also used in dispersion calculations. A third library of pollutant-specific dose-response values is used to estimate health risk. These are discussed below.

U.S. EPA. Revision to the

Guideline on Air Quality Models: Adoption of a Preferred General Purpose (Flat and Complex Terrain) Dispersion Model and Other Revisions

(70 FR 68218, Nov. 9, 2005).

A census block is the smallest geographic area for which census statistics are tabulated.

b. Risk From Chronic Exposure to HAP

In developing the risk assessment for chronic exposures, we use the estimated annual average ambient air concentrations of each HAP emitted by each source in the source category. The HAP air concentrations at each nearby census block centroid located within 50 km of the facility are a surrogate for the chronic inhalation exposure concentration for all the people who reside in that census block. A distance of 50 km is consistent with both the analysis supporting the 1989 Benzene NESHAP (54 FR 38044) and the limitations of Gaussian dispersion models, including AERMOD.

For each facility, we calculate the MIR as the cancer risk associated with health protective assumptions, such as a continuous lifetime (24 hours per day, seven days per week, 52 weeks per year, 70 years) exposure to the maximum annual average concentration at the centroid of each inhabited census block. This is meant to provide an upper bound estimate of cancer risks as people

are unlikely to be in the same location for 70 years. We calculate individual cancer risk by multiplying the estimated lifetime exposure to the ambient concentration of each HAP (in micrograms per cubic meter (μg/m

)) by its unit risk estimate (URE). The URE is an upper-bound estimate of an individual's incremental risk of contracting cancer over a lifetime of exposure to a concentration of 1 μg/m

of air. For residual risk assessments, we currently use UREs from the EPA's Integrated Risk Information System (IRIS) when they are available. For carcinogenic pollutants without IRIS values, we look to other reputable sources of cancer dose-response values, often using California EPA (CalEPA) UREs, where available. In cases where new, scientifically credible dose-response values have been developed in a manner consistent with EPA guidelines and have undergone a peer review process similar to that used by the EPA, we may use such dose-response values in place of, or in addition to, other values, if appropriate. The pollutant-specific dose-response values used to estimate health risk are available at

https://www.epa.gov/fera/dose-response-assessment-assessing-health-risks-associated-exposure-hazardous-air-pollutants.

To estimate individual lifetime cancer risks associated with exposure to HAP emissions from each facility in the source category, we sum the risks for each of the carcinogenic HAP

emitted by the modeled facility. We estimate cancer risk at every census block within 50 km of every facility in the source category. The MIR is the highest individual lifetime cancer risk estimated for any of those census blocks. In addition to calculating the MIR, we estimate the distribution of individual cancer risks for the source category by summing the number of individuals within 50 km of the sources whose estimated risk falls within a specified risk range. We also estimate annual cancer incidence by multiplying the estimated lifetime cancer risk at each census block by the number of people residing in that block, summing results for all of the census blocks, and then dividing this result by a 70-year lifetime.

The EPA's 2005

Guidelines for Carcinogen Risk Assessment

identifies five recommended standard hazard descriptors: “Carcinogenic to Humans,” “Likely to Be Carcinogenic to Humans,” “Suggestive Evidence of Carcinogenic Potential,” “Inadequate Information to Assess Carcinogenic Potential,” and “Not Likely to Be Carcinogenic to Humans.” The first three are treated as carcinogenic and coincide with the terms “known carcinogen, probable carcinogen, and possible carcinogen,” respectively, which are the terms advocated in the EPA's

Guidelines for Carcinogen Risk Assessment,

published in 1986 (51 FR 33992, Sept. 24, 1986). In August 2000, the EPA published a document entitled

Supplemental Guidance for Conducting Health Risk Assessment of Chemical Mixtures

(EPA/630/R-00/002) as a supplement to the 1986 document. Copies of both documents can be obtained from

https://cfpub.epa.gov/ncea/risk/recordisplay.cfm?deid=20533&CFID=70315376&CFTOKEN=71597944.

Summing the risk of these individual compounds to obtain the cumulative cancer risk is an approach that was recommended by the EPA's SAB in their 2002 peer review of the EPA's National Air Toxics Assessment (NATA) titled

NATA—Evaluating the National-scale Air Toxics Assessment

1996 Data—an SAB Advisory, available at

https://archive.epa.gov/airtoxics/nata/web/html/sabrev.html.

To assess the risk of noncancer health effects from chronic exposure to HAP, we calculate either an HQ or a target organ-specific hazard index (TOSHI). We calculate an HQ when a single noncancer HAP is emitted. Where more than one noncancer HAP is emitted, we sum the HQ for each of the HAP that affects a common target organ or target organ system to obtain a TOSHI. The HQ is the estimated exposure divided by the chronic noncancer dose-response value, which is a value selected from one of several sources. The preferred chronic noncancer dose-response value is the EPA RfC, defined as “an estimate (with uncertainty spanning perhaps an order of magnitude) of a continuous inhalation exposure to the human population (including sensitive subgroups) that is likely to be without an appreciable risk of deleterious effects during a lifetime.”

In cases where an RfC from the EPA's IRIS is not available or where the EPA determines that using a value other than the RfC is appropriate, the chronic noncancer dose-response value can be a value from the following prioritized sources, which define their dose-response values similarly to the EPA: (1) the Agency for Toxic Substances and Disease Registry (ATSDR) Minimum Risk Level (

https://www.atsdr.cdc.gov/minimal-risk-levels/about/index.html

); (2) the CalEPA Chronic Reference Exposure Level (REL) (

https://oehha.ca.gov/air/general-info/oehha-acute-8-hour-and-chronic-reference-exposure-level-rel-summary

); or (3) as noted above, a scientifically credible dose-response value that has been developed in a manner consistent with EPA guidelines and has undergone a peer review process similar to that used by the EPA. The pollutant-specific dose-response values used to estimate health risks are available at

https://www.epa.gov/fera/dose-response-assessment-assessing-health-risks-associated-exposure-hazardous-air-pollutants.

U.S. Environmental Protection Agency. (Last updated May 2, 2025). IRIS Glossary:

https://www.epa.gov/iris/iris-glossary.

c. Risk From Acute Exposure to HAP That May Cause Health Effects Other Than Cancer

For each HAP for which appropriate acute inhalation dose-response values are available, the EPA also assesses the potential health risks due to acute exposure. For these assessments, the EPA makes health protective assumptions about emission rates, meteorology, and exposure location. As part of our efforts to continually improve our methodologies to evaluate the risks that HAP emitted from categories of industrial sources pose to human health and the environment,

we revised our treatment of meteorological data to use reasonable worst-case air dispersion conditions in our acute risk screening assessments instead of worst-case air dispersion conditions. This revised treatment of meteorological data and the supporting rationale are described in more detail in

Residual Risk Assessment for the Hazardous Waste Combustor Source Category in Support of the 2025 Risk and Technology Review Proposed Rule

and in appendix 5 of the report:

Technical Support Document for Acute Risk Screening Assessment,

which are available in the docket for this proposed rule. This revised approach has been used in this proposed rule and in all other RTR rulemakings proposed on or after June 3, 2019.

See, e.g.,

U.S. EPA.

Screening Methodologies to Support Risk and Technology Reviews (RTR): A Case Study Analysis

(Report, Sept. 2018).

https://cfpub.epa.gov/si/si_public_record_report.cfm?Lab=OAQPS&dirEntryID=307074.

See for example, 85 FR 40740, (July 7, 2020) (Organic Liquids Distribution RTR); 85 FR 40386, (July 6, 2020) (Ethylene Production RTR); 85 FR 15608, (March 18, 2020) (Solvent Extraction for Vegetable Oil Production RTR).

To assess the potential acute risk to the maximally exposed individual, we use the peak hourly emission rate for each emission point,

reasonable worst-case air dispersion conditions (

i.e.,

99th percentile), and the point of highest off-site exposure. Specifically, we assume that peak emissions from the source category and reasonable worst-

case air dispersion conditions co-occur and that a person is present at the point of maximum exposure.

In the absence of hourly emission data, we develop estimates of maximum hourly emission rates by multiplying the average actual annual emission rates by a factor (either a category-specific factor or a default factor of 10) to account for variability. We used the default factor of 10 for this risk assessment because we did not have hourly emissions data. This is documented in

Residual Risk Assessment for the Hazardous Waste Combustor Source Category in Support of the 2025 Risk and Technology Review Proposed Rule

and in appendix 5 of the report:

Technical Support Document for Acute Risk Screening Assessment.

Both are available in the docket for this rulemaking.

To characterize the potential health risks associated with estimated acute inhalation exposures to a HAP, we generally use multiple acute dose-response values, including acute RELs, acute exposure guideline levels (AEGLs), and emergency response planning guidelines (ERPG) for 1-hour exposure durations, if available, to calculate acute HQs. The acute HQ is calculated by dividing the estimated acute exposure concentration by the acute dose-response value. For each HAP for which acute dose-response values are available, the EPA calculates acute HQs.

An acute REL is defined as “the concentration level at or below which no adverse health effects are anticipated for a specified exposure duration.”

Acute RELs are based on the most sensitive, relevant, adverse health effect reported in the peer-reviewed medical and toxicological literature. They are designed to protect the most sensitive individuals in the population through the inclusion of margins of safety. Because margins of safety are incorporated to address data gaps and uncertainties, exceeding the REL does not automatically indicate an adverse health impact. AEGLs represent threshold exposure limits for the general public and are applicable to emergency exposures ranging from 10 minutes to eight hours.

They are guideline levels for “once-in-a-lifetime, short-term exposures to airborne concentrations of acutely toxic, high-priority chemicals.”

The AEGL-1 is specifically defined as “the airborne concentration (expressed as ppm (parts per million) or mg/m

(milligrams per cubic meter)) of a substance above which it is predicted that the general population, including susceptible individuals, could experience notable discomfort, irritation, or certain asymptomatic nonsensory effects. However, the effects are not disabling and are transient and reversible upon cessation of exposure.” The document also notes that “Airborne concentrations below AEGL-1 represent exposure levels that can produce mild and progressively increasing but transient and nondisabling odor, taste, and sensory irritation or certain asymptomatic, nonsensory effects.”

AEGL-2 are defined as “the airborne concentration (expressed as parts per million or milligrams per cubic meter) of a substance above which it is predicted that the general population, including susceptible individuals, could experience irreversible or other serious, long-lasting adverse health effects or an impaired ability to escape.”

CalEPA issues acute RELs as part of its

Air Toxics Hot Spots Program,

and the 1-hour and 8-hour values are documented in

Air Toxics Hot Spots Program Risk Assessment Guidelines, Part I, The Determination of Acute Reference Exposure Levels for Airborne Toxicants,

which is available at

https://oehha.ca.gov/air/general-info/oehha-acute-8-hour-and-chronic-reference-exposure-level-rel-summary.

National Academy of Sciences, 2001. Standing Operating Procedures for Developing Acute Exposure Levels for Hazardous Chemicals, at 2. Available at

https://www.epa.gov/sites/production/files/2015-09/documents/sop_final_standing_operating_procedures_2001.pdf.

Note that the National Advisory Committee for Acute Exposure Guideline Levels for Hazardous Substances ended in October 2011, but the AEGL program continues to operate at the EPA and works with the National Academies to publish final AEGLs

(https://www.epa.gov/aegl

Id.

at 21.

Id.

ERPGs are developed by the American Industrial Hygiene Association (AIHA) for emergency planning and are intended to be health-based guideline concentrations for single exposures to chemicals. The ERPG-1 is the maximum airborne concentration established by AIHA below which it is believed that nearly all individuals could be exposed for up to one hour without experiencing other than mild transient adverse health effects or without perceiving a clearly defined, objectionable odor. Similarly, the ERPG-2 is the maximum airborne concentration established by AIHA below which it is believed that nearly all individuals could be exposed for up to one hour without experiencing or developing irreversible or other serious health effects or symptoms which could impair an individual's ability to take protective action.

An acute REL for 1-hour exposure durations is typically lower than its corresponding AEGL-1 and ERPG-1. Even though their definitions are slightly different, AEGL-1s are often the same as the corresponding ERPG-1s, and AEGL-2s are often equal to ERPG-2s. The maximum HQs from our acute inhalation screening risk assessment typically result when we use the acute REL for a HAP. In cases where the maximum acute HQ exceeds 1, we also report the HQ based on the next highest acute dose-response value (usually the AEGL-1 and/or the ERPG-1). In our acute inhalation screening risk assessment, acute impacts are deemed negligible for HAP for which acute HQs are less than or equal to 1, and no further analysis is performed for these HAP. In cases where an acute HQ from the screening step is greater than 1, we assess the site-specific data to ensure that the acute HQ is at an off-site location. For this source category, the data refinements employed consisted of reviewing satellite imagery of the locations of the maximum acute HQ values to determine if the maximum was off facility property. For any maximum value that was determined to be on facility property, the next highest value that was off facility property was used. These refinements are discussed more fully in the

Residual Risk Assessment for the Hazardous Waste Combustor Source Category in Support of the 2025 Risk and Technology Review Proposed Rule,

which is available in the docket for this proposed rule.

4. How do we conduct the multipathway exposure and risk screening assessment?

The EPA conducts a tiered screening assessment examining the potential for significant human health risks due to exposures via routes other than inhalation (

i.e.,

ingestion). We first determine whether any sources in the source category emit any HAP known to be persistent and bioaccumulative in the environment, as identified in the EPA's Air Toxics Risk Assessment Library.

See

volume 1, appendix D, at

https://www.epa.gov/fera/risk-assessment-and-modeling-air-toxics-risk-assessment-reference-library.

For the HWC source category, we identified PB-HAP emissions of As compounds, Cd compounds, PCDD/PCDF, Pb, polycyclic organic matter (POM), and Hg, so we proceeded to the next step of the evaluation. Except for Pb, the human health risk screening assessment for PB-HAP consists of three progressive tiers. In a Tier 1 screening assessment, we determine whether the magnitude of the facility-specific emissions of PB-HAP warrants further evaluation to characterize human health risk through ingestion exposure. To facilitate this step, we evaluate emissions against previously developed screening threshold emission rates for several PB-HAP that are based on a hypothetical upper-end screening exposure scenario developed for use in conjunction with the EPA's Total Risk Integrated Methodology.Fate, Transport, and Ecological Exposure (TRIM.FaTE) model. The PB-HAP with screening threshold emission rates are As compounds, Cd compounds, PCDD/PCDF, Hg compounds, and POM. Based on the EPA estimates of toxicity and bioaccumulation potential, these pollutants represent a conservative list for inclusion in multipathway risk assessments for RTR rules.

In this assessment, we compare the facility-

specific emission rates of these PB-HAP to the screening threshold emission rates for each PB-HAP to assess the potential for significant human health risks via the ingestion pathway. We call this application of the TRIM.FaTE model the Tier 1 screening assessment. The ratio of a facility's actual emission rate to the Tier 1 screening threshold emission rate is a “screening value.”

Id.

We derive the Tier 1 screening threshold emission rates for these PB-HAP (other than Pb compounds) to correspond to a maximum excess lifetime cancer risk of 1-in-1 million (

i.e.,

for As compounds, PCDD/PCDF, and POM) or, for HAP that cause noncancer health effects (

i.e.,

Cd compounds and Hg compounds), a maximum HQ of one. If the emission rate of any one PB-HAP or combination of carcinogenic PB-HAP in the Tier 1 screening assessment exceeds the Tier 1 screening threshold emission rate for any facility (

i.e.,

the screening value is greater than 1), we conduct a second screening assessment, which we call the Tier 2 screening assessment. The Tier 2 screening assessment separates the Tier 1 combined fisher and farmer exposure scenario into fisher, farmer, and gardener scenarios that retain upper-bound ingestion rates.

In the Tier 2 screening assessment, the location of each facility that exceeds a Tier 1 screening threshold emission rate is used to refine the assumptions associated with the Tier 1 fisher and farmer exposure scenarios at that facility. A key assumption in the Tier 1 screening assessment is that a lake and/or farm is located near the facility. As part of the Tier 2 screening assessment, we use a U.S. Geological Survey (USGS) database to identify actual waterbodies within 50 km of each facility and assume the fisher only consumes fish from lakes within that 50 km zone. We also examine the differences between local meteorology near the facility and the meteorology used in the Tier 1 screening assessment. We then adjust the previously developed Tier 1 screening threshold emission rates for each PB-HAP for each facility based on an understanding of how exposure concentrations estimated for the screening scenario change with the use of local meteorology and the USGS lakes database.

In the Tier 2 farmer scenario, we maintain an assumption that the farm is located within 0.5 km of the facility and that the farmer consumes meat, eggs, dairy, vegetables, and fruit produced near the facility. We may further refine the Tier 2 screening analysis by assessing a gardener scenario to characterize a range of exposures, with the gardener scenario being more plausible in RTR evaluations. Under the gardener scenario, we assume the gardener consumes home-produced eggs, vegetables, and fruit products at the same ingestion rate as the farmer. The Tier 2 screen continues to rely on the high-end food intake assumptions that were applied in Tier 1 for local fish (adult female angler at 99th percentile fish consumption)

and locally grown or raised foods (90th percentile consumption of locally grown or raised foods for the farmer and gardener scenarios).

If PB-HAP emission rates do not result in a Tier 2 screening value greater than 1, we consider those PB-HAP emissions to pose risks below a level of concern. If the PB-HAP emission rates for a facility exceed the Tier 2 screening threshold emission rates, we may conduct a Tier 3 screening assessment.

Burger, J. (2002). Daily consumption of wild fish and game: Exposures of high end recreationists.

International Journal of Environmental Health Research,

12, 343-354:

https://doi.org/10.1080/0960312021000056393

U.S. Environmental Protection Agency. (Last updated Mar. 21, 2022). Exposure Factors Handbook 2011 Edition (Final Report):

https://iris.epa.gov/document/&deid=236252.

There are several analyses that can be included in a Tier 3 screening assessment, depending upon the extent of refinement warranted, including validating that the lakes are fishable, locating residential/garden locations for urban and/or rural settings, considering plume-rise to estimate emissions lost above the mixing layer, and considering hourly effects of meteorology and plume-rise on chemical fate and transport (a time-series analysis). If necessary, the EPA may further refine the screening assessment through a site-specific assessment.

In evaluating the potential multipathway risk from emissions of Pb compounds, rather than developing a screening threshold emission rate, it is our longstanding practice to compare maximum estimated chronic inhalation exposure concentrations to the level of the current National Ambient Air Quality Standard (NAAQS) for Pb.

Values below the level of the primary (health-based) Pb NAAQS are considered to have a low potential for multipathway risk.

In doing so, the EPA notes that the legal standard for a primary NAAQS—that a standard is requisite to protect public health and provide an adequate margin of safety (CAA section 109(b))—differs from the CAA section 112(f) standard (requiring, among other things, that the standard provide an “ample margin of safety to protect public health”). However, the primary lead NAAQS is a reasonable measure of determining risk acceptability (

i.e.,

the first step of the Benzene NESHAP analysis) since it is designed to protect the most susceptible group in the human population—children, including children living near major lead emitting sources. 73 FR 67002, (Oct. 18, 2006); 73 FR 67000; 73 FR 67005. In addition, applying the level of the primary lead NAAQS at the risk acceptability step is conservative, since that primary lead NAAQS reflects an adequate margin of safety.

For further information on the multipathway assessment approach, see the

Residual Risk Assessment for the Hazardous Waste Combustor Source Category in Support of the Risk and Technology Review 2025 Proposed Rule,

which is available in the docket for this proposed rule.

5. How do we assess risks considering emissions control options?

In addition to assessing baseline inhalation risks and screening for potential multipathway risks, we also estimate risks considering the potential emission reductions that would be achieved by the control options under consideration. In these cases, the expected emission reductions are applied to the specific HAP and emission points in the RTR emissions dataset to develop corresponding estimates of risk and incremental risk reductions.

6. How do we conduct the environmental risk screening assessment?

a. Adverse Environmental Effect, Environmental HAP, and Ecological Benchmarks

The EPA conducts a screening assessment to examine the potential for an adverse environmental effect as required under CAA section 112(f)(2)(A). This section authorizes the Agency to adopt more stringent standards than MACT standards, if necessary, “to prevent, taking into consideration costs, energy, safety, and other relevant factors, an adverse environmental effect.” CAA section 112(a)(7) defines “adverse environmental effect” as “any significant and widespread adverse effect, which may reasonably be anticipated, to wildlife, aquatic life, or other natural resources, including adverse impacts on populations of endangered or threatened species or significant degradation of environmental quality over broad areas.”

In conducting the screening assessment during the risk review, under CAA section 112(f)(2)(A), it is the EPA's long-standing practice to focus on eight HAP, which are referred to as “environmental HAP”: six PB-HAP and two acid gases. The PB-HAP included in the screening assessment are As

compounds, Cd compounds, PCDD/PCDF, POM, Hg (both inorganic and methylmercury), and Pb compounds. The acid gases included in the screening assessment are HCl and HF.

HAP that persist and bioaccumulate are of particular environmental concern because they accumulate in the soil, sediment, and water. The acid gases, HCl and HF, are included due to their well-documented potential to cause direct damage to terrestrial plants. In the environmental risk screening assessment, we evaluate the following four exposure media: terrestrial soils, surface water bodies (includes water-column and benthic sediments), fish consumed by wildlife, and air. Within these four exposure media, we evaluate nine ecological assessment endpoints, which are defined by the ecological entity and its attributes. For PB-HAP other than Pb, both community-level and population-level endpoints are included. For acid gases, the ecological assessment evaluated is terrestrial plant communities.

An ecological benchmark represents a concentration of HAP that has been linked to a particular environmental effect level. For each environmental HAP, we identified the available ecological benchmarks for each assessment endpoint. We identified, where possible, ecological benchmarks at the following effect levels: probable effect levels, lowest-observed-adverse-effect level (LOAEL), and no-observed-adverse-effect level (NOAEL). In cases where multiple effect levels were available for a particular PB-HAP and assessment endpoint, we use all of the available effect levels to help us determine whether ecological risks exist and, if so, whether the risks could be considered significant and widespread.

For further information on how the environmental risk screening assessment was conducted, including a discussion of the risk metrics used, how the environmental HAP were identified, and how the ecological benchmarks were selected, see appendix 9 of the

Residual Risk Assessment for the Hazardous Waste Combustor Source Category in Support of the Risk and Technology Review 2025 Proposed Rule,

which is available in the docket for this proposed rule.

b. Environmental Risk Screening Methodology

For the environmental risk screening assessment, the EPA first determined whether any facilities in the HWC source category emitted any of the environmental HAP. For the HWC source category, we identified emissions of As compounds, Cd compounds, PCDD/PCDF, Pb, POM, Hg, HCl, and HF. Because one or more of the environmental HAP evaluated—As compounds, Cd compounds, PCDD/PCDF, Pb, POM, Hg, HCl, and HF—are emitted by at least one facility in the source category, we proceeded to the second step of the evaluation.

c. PB-HAP Methodology

The environmental screening assessment includes six PB-HAP—As compounds, Cd compounds, PCDD/PCDF, POM, Hg (both inorganic and methylmercury), and Pb compounds. With the exception of Pb, the environmental risk screening assessment for PB-HAP consists of three tiers. The first tier of the environmental risk screening assessment uses the same health-protective conceptual model that is used for the Tier 1 human health screening assessment. TRIM.FaTE model simulations were used to calculate Tier 1 screening threshold emission rates. The screening threshold emission rates represent the emission rate in tpy that results in media concentrations at the facility that equal the relevant ecological benchmark. To assess emissions from each facility in the category, the reported emission rate for each PB-HAP was compared to the Tier 1 screening threshold emission rate for that PB-HAP for each assessment endpoint and effect level. If emissions from a facility do not exceed the Tier 1 screening threshold emission rate, the facility “passes” the screening assessment and therefore is not evaluated further under the screening approach. If emissions from a facility exceed the Tier 1 screening threshold emission rate, we evaluate the facility further in Tier 2.

In Tier 2 of the environmental screening assessment, the screening threshold emission rates are adjusted to account for local meteorology and the actual location of lakes in the vicinity of facilities that did not pass the Tier 1 screening assessment. For soils, we evaluate the average soil concentration for all soil parcels within a 7.5-km radius for each facility and for each PB-HAP. For the water, sediment, and fish tissue concentrations, the highest value for each facility for each pollutant is used. If emission concentrations from a facility do not exceed the Tier 2 screening threshold emission rate, the facility “passes” the screening assessment and typically is not evaluated further. If emissions from a facility exceed the Tier 2 screening threshold emission rate, we evaluate the facility further in Tier 3.

As in the multipathway human health risk assessment, in Tier 3 of the environmental screening assessment, we examine the suitability of the lakes around the facilities to support life and remove those that are not suitable (

e.g.,

lakes that have been filled in or are industrial ponds), adjust emissions for plume-rise, and conduct hour-by-hour time-series assessments. If these Tier 3 adjustments to the screening threshold emission rates still indicate the potential for an adverse environmental effect (

i.e.,

the facility emission rate exceeds the screening threshold emission rate), we may elect to conduct a more refined assessment using more site-specific information. If, after additional refinement, the facility emission rate still exceeds the screening threshold emission rate, the facility may have the potential to cause an adverse environmental effect.

To evaluate the potential for an adverse environmental effect from Pb, we compared the average modeled air concentrations (from HEM) of Pb around each facility in the source category to the level of the secondary Pb NAAQS. The secondary Pb NAAQS is a reasonable means of evaluating environmental risk because it is set to provide substantial protection against adverse welfare effects, which can include “effects on soils, water, crops, vegetation, man-made materials, animals, wildlife, weather, visibility and climate, damage to and deterioration of property, and hazards to transportation, as well as effects on economic values and on personal comfort and well-being.”

CAA section 302(h) describes effects on welfare. 42 U.S.C. 7602(h).

d. Acid Gas Environmental Risk Methodology

The environmental screening assessment for acid gases evaluates the potential phytotoxicity and reduced productivity of plants due to chronic exposure to HF and HCl. The environmental risk screening methodology for acid gases is a single-tier screening assessment that compares modeled ambient air concentrations (from AERMOD) to the ecological benchmarks for each acid gas. To identify a potential adverse environmental effect (as defined in CAA section 112(a)(7)) from emissions of HF and HCl, we evaluate the following metrics: the size of the modeled area around each facility that exceeds the ecological benchmark for each acid gas, in acres and square kilometers; the percentage of the modeled area around each facility that exceeds the ecological benchmark for each acid gas; and the area-weighted average screening value

around each facility (calculated by dividing the area-weighted average concentration over the 50-km modeling domain by the ecological benchmark for each acid gas). For further information on the environmental screening assessment approach, see appendix 9 of the

Residual Risk Assessment for the Hazardous Waste Combustor Source Category in Support of the Risk and Technology Review 2025 Proposed Rule,

which is available in the docket for this proposed rule.

7. How do we conduct facility-wide assessments?

To put the source category risks in context, we typically examine the risks from the entire facility, where the facility includes all HAP-emitting operations within a contiguous area and under common control. In other words, we examine not only the HAP emissions from the source category emission points of interest, but also emissions of HAP from all other emission sources at the facility for which we have data. For the HWC source category, we conducted the facility-wide assessment using a dataset compiled from the NEI based on emissions year 2022. The source category records of that NEI dataset were removed, evaluated, and updated as described in section II.C. of this preamble. Once we completed the quality assurance review, the dataset was placed back with the remaining records from the NEI for that facility. The facility-wide file was then used to analyze risks due to the inhalation of HAP that are emitted facility-wide for the populations residing within 50 km of each facility, consistent with the methods used for the source category analysis described above. For these facility-wide risk analyses, the modeled source category risks were compared to the facility-wide risks to determine the portion of the facility-wide risks that could be attributed to the source category addressed in this proposal. We also specifically examined the facility that was associated with the highest estimate of risk and determined the percentage of that risk attributable to the source category of interest. The

Residual Risk Assessment for the Hazardous Waste Combustor Source Category in Support of the Risk and Technology Review 2025 Proposed Rule,

available through the docket for this proposed rule, provides the methodology and results of the facility-wide analyses, including all facility-wide risks and the percentage of source category contribution to facility-wide risks.

8. How do we consider uncertainties in risk assessment?

Uncertainty and the potential for bias are inherent in all risk assessments, including those performed for this proposal. Although uncertainty exists, we believe that our approach, which used health protective tools and assumptions, ensures that our decisions are health and environmentally protective. A brief discussion of the uncertainties in the RTR emissions dataset, dispersion modeling, inhalation exposure estimates, and dose-response relationships follows. Also included are those uncertainties specific to our acute screening assessments, multipathway screening assessments, and environmental risk screening assessments. A more thorough discussion of these uncertainties is included in the

Residual Risk Assessment for the Hazardous Waste Combustor Source Category in Support of the Risk and Technology Review 2025 Proposed Rule,

which is available in the docket for this proposed rule. If a multipathway site-specific assessment was performed for this source category, a full discussion of the uncertainties associated with that assessment can be found in appendix 11 of that document,

Site-Specific Human Health Multipathway Residual Risk Assessment Report.

a. Uncertainties in the RTR Emissions Dataset

Although the development of the RTR emissions dataset involved quality assurance/quality control processes, the accuracy of emissions values will vary depending on the source of the data, the degree to which data are incomplete or missing, the degree to which assumptions made to complete the datasets are accurate, errors in emission estimates, and other factors. The emission estimates considered in this analysis generally are emissions during worst-case scenario performance tests corrected based on a stack gas flow rate or hazardous waste thermal concentration feed rate more typical of normal operations. Results were averaged across multiple years, where available, and emissions averages across HWC unit subcategories were used when specific emissions data was not available. The estimates of peak hourly emission rates for the acute effects screening assessment were based on an emission adjustment factor applied to the average annual hourly emission rates, which are intended to account for emission fluctuations due to normal facility operations.

b. Uncertainties in Dispersion Modeling

We recognize that there is uncertainty in ambient concentration estimates associated with any model, including AERMOD. In using a model to estimate ambient pollutant concentrations, the user chooses certain options to apply. For RTR assessments, we select some model options that have the potential to overestimate ambient air concentrations (

e.g.,

not including plume depletion or pollutant transformation). We select other model options that have the potential to underestimate ambient impacts (

e.g.,

not including building downwash). Other options that we select have the potential to either underestimate or overestimate ambient levels (

e.g.,

meteorology and receptor locations). On average, considering the directional nature of the uncertainties commonly present in ambient concentrations estimated by dispersion models, the approach we apply in the RTR assessments should yield unbiased estimates of ambient HAP concentrations. We also note that the selection of meteorology dataset locations could have an impact on the risk estimates. As we continue to update and expand our library of meteorological station data used in our risk assessments, we expect to reduce this variability.

c. Uncertainties in Inhalation Exposure Assessment

Although we make every effort to identify all of the relevant facilities and emission points, as well as to develop accurate estimates of the annual emission rates for all relevant HAP, the uncertainties in our emission inventory are likely the highest-contributing factors of the uncertainties in the exposure assessment. Some uncertainties in our exposure assessment include human mobility, using the centroid of each census block, assuming lifetime exposure, and assuming only outdoor exposures. For most of these factors, there is neither an underestimate nor overestimate when looking at the MIR or the incidence, but the shape of the distribution of risks may be affected. With respect to outdoor exposures, actual exposures may not be as high if people spend time indoors, especially for very reactive pollutants or larger particles. For all factors, we reduce uncertainty when possible. For example, with respect to census block centroids, we analyze large blocks using aerial imagery and adjust locations of the block centroids to better represent the population in the blocks. We also add additional receptor locations where

the population of a block is not well-represented by a single location.

d. Uncertainties in Dose-Response Relationships

There are uncertainties inherent in the development of the dose-response values used in our risk assessments for cancer effects from chronic exposures and noncancer effects from both chronic and acute exposures. Some uncertainties are generally expressed quantitatively, and others are generally expressed qualitatively. We note, as a preface to this discussion, a point on dose-response uncertainty that is stated in the EPA's

2005 Guidelines for Carcinogen Risk Assessment;

namely that “the primary goal of EPA actions is protection of human health; accordingly, as an Agency policy, risk assessment procedures, including default options that are used in the absence of scientific data to the contrary, should be health protective” (the EPA's

2005 Guidelines for Carcinogen Risk Assessment,

at 1-7). This is the approach followed here as summarized in the next paragraphs.

Cancer UREs used in our risk assessments are those that have been developed to generally provide an upper-bound estimate of risk.

That is, they represent a “plausible upper limit to the true value of a quantity” (although this is usually not a true statistical confidence limit). In some circumstances, the true risk could be as low as zero; however, in other circumstances the risk could be greater.

Chronic noncancer RfC and reference dose (RfD) values represent chronic exposure levels that are intended to be health-protective levels. To derive dose-response values that are intended to be “without appreciable risk,” the methodology relies upon an uncertainty factor (UF) approach,

which considers uncertainty, variability, and gaps in the available data. The UFs are applied to derive dose-response values that are intended to protect against appreciable risk of deleterious effects.

U.S. Environmental Protection Agency. (Last updated Jun. 22, 2022). Integrated Risk Information System (IRIS) Glossary:

https://sor.epa.gov/sor_internet/registry/termreg/searchandretrieve/glossariesandkeywordlists/search.do?details=&vocabName=IRIS%20Glossary&filterTerm=unit%20risk&checkedAcronym=false&checkedTerm=false&hasDefinitions=false&filterTerm=unit%20risk&filterMatchCriteria=Contains.

An exception to this is the URE for benzene, which is considered to cover a range of values, each end of which is considered to be equally plausible, and which is based on maximum likelihood estimates.

See

A Review of the Reference Dose and Reference Concentration Processes,

U.S. EPA, December 2002, and

Methods for Derivation of Inhalation Reference Concentrations and Application of Inhalation Dosimetry,

U.S. EPA, 1994.

Many of the UFs used to account for variability and uncertainty in the development of acute dose-response values are quite similar to those developed for chronic durations. Additional adjustments are often applied to account for uncertainty in extrapolation from observations at one exposure duration (

e.g.,

four hours) to derive an acute dose-response value at another exposure duration (

e.g.,

one hour). Not all acute dose-response values are developed for the same purpose, and care must be taken when interpreting the results of an acute assessment of human health effects relative to the dose-response value or values being exceeded. Where relevant to the estimated exposures, the lack of acute dose-response values at different levels of severity should be factored into the risk characterization as potential uncertainties.

Uncertainty also exists in the selection of ecological benchmarks for the environmental risk screening assessment. We established a hierarchy of preferred benchmark sources to allow selection of benchmarks for each environmental HAP at each ecological assessment endpoint. We searched for benchmarks for three effect levels (

i.e.,

no-effects level, threshold-effect level, and probable-effect level), but not all combinations of ecological assessment/environmental HAP had benchmarks for all three effect levels. Where multiple effect levels were available for a particular HAP and assessment endpoint, we used all of the available effect levels to help us determine whether risk exists and whether the risk could be considered significant and widespread.

Although we make every effort to identify appropriate human health effect dose-response values for all pollutants emitted by the sources in this risk assessment, some HAP emitted by this source category lack dose-response assessments. Accordingly, these pollutants cannot be included in the quantitative risk assessment, which could result in quantitative estimates understating HAP risk. To help to alleviate this potential underestimate, where we conclude similarity with a HAP for which a dose-response value is available, we use that value as a surrogate for the assessment of the HAP for which no value is available. To the extent use of surrogates indicates appreciable risk, we may identify a need to increase priority for an IRIS assessment for that substance. We additionally note that, generally speaking, HAP of greatest concern due to environmental exposures and hazard are those for which dose-response assessments have been performed, reducing the likelihood of understating risk. Further, HAP not included in the quantitative assessment are assessed qualitatively and considered in the risk characterization that informs the risk management decisions, including consideration of HAP reductions achieved by various control options.

For a group of compounds that are unspeciated (

e.g.,

glycol ethers), we conservatively use the most protective dose-response value of an individual compound in that group to estimate risk. Similarly, for an individual compound in a group (

e.g.,

ethylene glycol diethyl ether) that does not have a specified dose-response value, we also apply the most protective dose-response value from the other compounds in the group to estimate risk.

e. Uncertainties in Acute Inhalation Screening Assessments

In addition to the uncertainties highlighted in section III.C.8. of this preamble, there are several factors specific to the acute exposure assessment that the EPA conducts as part of the risk review under CAA section 112(f). The accuracy of an acute inhalation exposure assessment depends on the simultaneous occurrence of independent factors that may vary greatly, such as hourly emission rates, meteorology, and the presence of a person. In the acute screening assessment that we conduct under the RTR program, we assume that peak emissions from the source category and reasonable worst-case air dispersion conditions (

i.e.,

99th percentile) co-occur. We then include the additional assumption that a person is located at this point at the same time. Together, these assumptions represent a reasonable worst-case actual exposure scenario. In most cases, it is unlikely that a person would be located at the point of maximum exposure during the time when peak emissions and reasonable worst-case air dispersion conditions occur simultaneously.

f. Uncertainties in the Multipathway and Environmental Risk Screening Assessments

For each source category, we generally rely on site-specific levels of PB-HAP or environmental HAP emissions to determine whether a refined assessment of the impacts from multipathway exposures is necessary or whether it is necessary to perform an environmental screening assessment. This determination is based on the

results of a three-tiered screening assessment that relies on the outputs from models—TRIM.FaTE and AERMOD—that estimate environmental pollutant concentrations and human exposures for five PB-HAP (PCDD/PCDF, POM, Hg, Cd compounds, and As compounds) and two acid gases (HF and HCl). For Pb, we use AERMOD to determine ambient air concentrations, which are then compared to the secondary Pb NAAQS. Two important types of uncertainty associated with the use of these models in RTR risk assessments and inherent to any assessment that relies on environmental modeling are model uncertainty and input uncertainty.

In the context of this discussion, the term “uncertainty” as it pertains to exposure and risk encompasses both

variability

in the range of expected inputs and screening results due to existing spatial, temporal, and other factors, as well as

uncertainty

in being able to accurately estimate the true result.

Model uncertainty concerns whether the model adequately represents the actual processes (

e.g.,

movement and accumulation) that might occur in the environment. For example, if the model adequately describes the movement of a pollutant through the soil. This type of uncertainty is difficult to quantify. However, based on feedback received from previous EPA SAB reviews and other reviews, we are confident that the models used in the screening assessments are appropriate and state-of-the-art for the multipathway and environmental screening risk assessments conducted in support of RTRs.

Input uncertainty is concerned with how accurately the models have been configured and parameterized for the assessment at hand. For Tier 1 of the multipathway and environmental screening assessments, we configured the models to avoid underestimating exposure and risk. This was accomplished by selecting upper-end values from nationally representative datasets for the more influential parameters in the environmental model, including selection and spatial configuration of the area of interest, lake location and size, meteorology, surface water, soil characteristics, and structure of the aquatic food web. We also assume an ingestion exposure scenario and values for human exposure factors that represent reasonable maximum exposures.

In Tier 2 of the multipathway and environmental screening assessments, we refine the model inputs to account for meteorological patterns in the vicinity of the facility versus using upper-end national values, and we identify the actual location of lakes near the facility rather than the default lake location that we apply in Tier 1. By refining the screening approach in Tier 2 to account for local geographical and meteorological data, we decrease the likelihood that concentrations in environmental media are overestimated, thereby increasing the usefulness of the screening assessment. In Tier 3 of the screening assessments, we refine the model inputs again to account for hour-by-hour plume-rise and the height of the mixing layer. We can also use those hour-by-hour meteorological data in a TRIM.FaTE run using the screening configuration corresponding to the lake location. These refinements produce a more accurate estimate of chemical concentrations in the media of interest, thereby reducing the uncertainty with those estimates. The assumptions and the associated uncertainties regarding the selected ingestion exposure scenario are the same for all three tiers.

For the environmental screening assessment for acid gases, we employ a single-tiered approach. We use the modeled air concentrations and compare those with ecological benchmarks.

For all tiers of the multipathway and environmental screening assessments, our approach to addressing model input uncertainty is generally cautious. We choose model inputs from the upper-end of the range of possible values for the influential parameters used in the models, and we assume that the exposed individual exhibits ingestion behavior that would lead to a high total exposure. This approach reduces the likelihood of not identifying high risks for adverse impacts.

Despite the uncertainties, when individual pollutants or facilities do not exceed screening threshold emission rates (

i.e.,

“passes”), we are confident that the potential for adverse multipathway impacts on human health is very low. On the other hand, when individual pollutants or facilities do exceed screening threshold emission rates, it does not mean that impacts are significant, only that we cannot rule out that possibility and that a refined assessment for the site might be necessary to obtain a more accurate risk characterization for the source category.

The EPA evaluates the following HAP in the multipathway and/or environmental risk screening assessments, where applicable: As compounds, Cd compounds, PCDD/PCDF, Pb, Hg (both inorganic and methylmercury), POM, HCl, and HF. These HAP represent pollutants that can cause adverse impacts either through direct exposure to HAP in the air or through exposure to HAP that are deposited from the air onto soils and surface waters and then through the environment into the food web. These HAP represent those for which we can conduct a meaningful multipathway or environmental screening risk assessment. For other HAP not included in our screening assessments, the model has not been parameterized such that it can be used for that purpose. In some cases, depending on the HAP, we may not have appropriate multipathway models that allow us to predict the concentration of that pollutant. The EPA acknowledges that other HAP beyond these that we are evaluating may have the potential to cause adverse effects and, therefore, the EPA may evaluate other relevant HAP in the future, as modeling science and resources allow.

IV. Analytical Results and Proposed Decisions

A. What actions are we proposing pursuant to CAA sections 112(d)(2) and 112(d)(3)?

In this proposal, we are proposing actions to address unregulated HAP pursuant to the D.C. Circuit's decision in

LEAN.

The D.C. Circuit has held that the EPA is required to address any previously unregulated HAP emissions as part of its periodic review of MACT standards under CAA section 112(d)(6). Based on a review of available information pursuant to the

LEAN

decision, we are proposing the following pursuant to CAA sections 112(d)(2), (d)(3), and (h)(1):

See LEAN,

955 F.3d at 1091-99.

• Numeric emission limits for HF and HCN for major source HWC solid fuel boilers.

• Work practice standard for HF for major source HWC incinerators.

• Work practice standard for HF and numeric emission limit for HCN for major source HWC cement kilns.

• Work practice standard for HF and numeric emission limits for HCN for major source liquid fuel boilers.

The results and proposed decisions based on the analyses performed pursuant to CAA sections 112(d)(2) and (3) are presented below, with separate discussion for each subcategory and HAP.

Consistent with the EPA's longstanding position, we do not believe that we are required to regulate emissions of HF or HCN from area sources in the HWC NESHAP because the EPA did not identify either HF or HCN as urban HAP pursuant to CAA sections 112(k)(3)(B) and 112(c)(3)

and because CAA section 112(c)(6) does not identify either HF or HCN as a pollutant of specific concern. Neither HF nor HCN is an urban HAP or a pollutant of specific concern, therefore, we are not proposing any emission limits for HF or HCN for area sources. All emission standards discussed here are only for HWCs at facilities that are major sources of HAP.

64 FR 38706, 38715, (July 19, 1999);

see, e.g.,

Proposed Gas-Fired Melting Furnaces Located at

Wool Fiberglass Manufacturing Area Sources NESHAP, 78 FR 22370, 22375-76, (Apr. 15, 2013); 80 FR 45280, 45319-20, (July 29, 2015) (finalized as proposed).

As previously noted, the D.C. Circuit has held that the EPA must address any previously unregulated HAP known to be emitted from major sources as part of its periodic review of MACT standards under CAA section 112(d)(6).

LEAN,

955 F.3d at 1091-99. The order issued by the D.C. District Court addressing our obligations to review and revise the HWC NESHAP also requires the EPA to establish standards for any previously unregulated HAP in this rulemaking. Order,

Blue Ridge Envtl. Def. League

Regan,

22-cv-3134 (APM) (D.D.C. Dec. 12, 2024). During the technology review, the EPA identified HF and HCN as unregulated HAP through permit review and emissions testing. We also collected information on PCBs and PAHs in the emission testing request. Although we mistakenly identified PCBs as an unregulated HAP to the D.C. District Court, we more recently conducted a careful analysis of the HWC NESHAP's rule record, which revealed that the EPA already promulgated MACT standards for PCBs and PAHs through the combination of the DRE and CO or THC standards as a surrogate for non-PCDD/PCDF organic HAP.

Because PCBs are already regulated through surrogacy, no additional emission standards are required. Therefore, we are not proposing additional standards regulating PCB emissions.

70 FR 59433 (Oct. 12, 2005); 80 FR 31473 (June 3, 2015).

To address the missing HF and HCN standards, we are proposing emission limits for HF and HCN under CAA sections 112(d)(2), (d)(3), and (h)(2) as described in this section. While the proposed emission limits for these HAP were calculated under CAA sections 112(d)(3) and (h)(2), we are soliciting comment on setting the HF and HCN standards pursuant to CAA section 112(d)(6) rather than setting the HF and HCN standards exclusively pursuant to CAA section 112(d)(2), (d)(3), and (h)(2) (C-1). Although the D.C. Circuit held in

LEAN

that the EPA is required to address previously unregulated HAP from major sources during a CAA section 112(d)(6) technology review, it is not entirely clear how that process functions under the statutory text. The difference in the approach would be that we would not be constrained to any minimum stringency level and would, therefore, not conduct a beyond-the-floor analysis. We would not anticipate any cost or impact differences associated with setting the HF and HCN limits pursuant to CAA section 112(d)(6) as compared to CAA sections 112(d)(2) and (3). The estimated costs would be for testing, recordkeeping, and reporting. It bears noting that the standards under review were first promulgated in 2005 and our review found an overall reduction of emissions from this source category that could likely be attributed to concerted efforts of sources since promulgation. We are also soliciting comments, data, and other information regarding the analyses for our proposed MACT floor standards, the beyond-the-floor options, and our determinations (C-2).

1. Solid Fuel Boilers

a. Hydrogen Fluoride

The EPA is proposing MACT standards for HF emissions from solid fuel boilers. As further explained below, the EPA is also soliciting comment on establishing an HBEL under CAA section 112(d)(4) for HF emissions from solid fuel boilers.

Under the D.C. Circuit's decision in

LEAN,

the EPA must set emission limits for major sources with known unregulated HAP emissions as part of its periodic review of MACT standards under CAA section 112(d)(6). These standards can take at least three forms: technology-based standards that reflect the maximum reductions of HAP achievable (after considering cost, energy requirements, and non-air health and environmental impacts) and are commonly referred to as MACT standards; an HBEL for HAP with an established health threshold; or a work practice standard when another standard is not feasible to prescribe or enforce. Because the EPA did not have previous HF emissions data, the EPA collected HF emissions data from one HWC solid fuel boiler in the January 2024 emissions testing request, and this boiler had detected emissions of HF in all emissions test runs. Based on that emissions test data, the EPA considers that a numerical emission standard is feasible to prescribe and enforce for emissions of HF from solid fuel boilers.

We are proposing MACT emission limits for HF emissions from solid fuel boilers. CAA section 112(d)(3)(B) provides that MACT shall not be less stringent than “the average emission limitation achieved by the best performing 5 sources (for which the Administrator has or could reasonably obtain emissions information) in the category or subcategory for categories or subcategories with fewer than 30 sources.” Because we have HF emissions data for only one of the seven solid fuel boilers, the proposed MACT floor is based on the HF data for this unit. In determining the level of the MACT floor, we used the Upper Prediction Limit (UPL) method to account for variability in solid fuel boiler performance and calculated the MACT floor at 6.2 parts per million by volume (ppmv) HF, dry basis and corrected to seven percent oxygen.

Based on available data, the EPA estimates that all solid fuel boilers would be able to meet the MACT floor limit with no additional controls.

MACT Floor and Beyond-the-Floor Analysis for Hazardous Waste Combustors, which is available in the docket for this proposed rulemaking (Docket ID No. EPA-HQ-OAR-2004-0022); The UPL “reflect[s] a reasonable estimate of the emissions achieved in practice by the best-performing sources.”

U.S. Sugar Corp.

EPA,

830 F.3d 579, 639 (D.C. Cir. 2016) (alteration in original).

For new sources, CAA section 112(d)(3) provides that the MACT shall not be less stringent than “the emission control that is achieved in practice by the best controlled similar source, as determined by the Administrator.” Because we only have HF emissions data from one solid fuel boiler, the proposed MACT floor limit for new sources is the same as the MACT floor limit for existing sources: 6.2 ppmv HF, dry basis and corrected to seven percent oxygen.

When establishing an emission standard pursuant to CAA section 112, the EPA also determines whether to control emissions “beyond-the-floor” (BTF) after considering the costs, non-air quality health and environmental impacts, and energy requirements of such more stringent control.

Further, CAA section 112 does not prescribe a methodology for the Agency's costs analysis. Therefore, where cost is a consideration for standard setting under CAA section 112(d)(2), we have historically used cost-effectiveness (cost/ton-reduced) in supporting analyses.

The EPA solicits comment

on whether strategies other than cost per ton of pollutant reduced for considering cost when evaluating beyond-the-floor standards would be more appropriate (C-3).

Nat'l Lime Ass'n

EPA,

233 F.3d 625, 634 (D.C. Cir. 2000) (“Once the Agency sets statutory floors, it then determines, considering cost and the other factors listed in section 7412(d)(2), whether stricter standards are `achievable.' The Agency calls such stricter requirements `beyond-the-floor' standards.”).

See NRDC

EPA,

749 F.3d 1055, 1060 (D.C. Cir. 2014) (upholding the EPA's consideration of cost-effectiveness as a component of the CAA section 112(d)(2) cost analysis);