[Federal Register Volume 84, Number 196 (Wednesday, October 9, 2019)]
[Proposed Rules]
[Pages 54278-54352]
[FR Doc No: 2019-19875]

Vol. 84

Wednesday,

No. 196

October 9, 2019

Part II

Environmental Protection Agency

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40 CFR Part 63

National Emission Standards for Hazardous Air Pollutants: Generic
Maximum Achievable Control Technology Standards Residual Risk and
Technology Review for Ethylene Production; Proposed Rule

Federal Register / Vol. 84 , No. 196 / Wednesday, October 9, 2019 /
Proposed Rules

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ENVIRONMENTAL PROTECTION AGENCY

40 CFR Part 63

[EPA-HQ-OAR-2017-0357; FRL-9999-59-OAR]
RIN 2060-AT02

National Emission Standards for Hazardous Air Pollutants: Generic
Maximum Achievable Control Technology Standards Residual Risk and
Technology Review for Ethylene Production

AGENCY: Environmental Protection Agency (EPA).

ACTION: Proposed rule.

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SUMMARY: The U.S. Environmental Protection Agency (EPA) is proposing
amendments to the National Emission Standards for Hazardous Air
Pollutants (NESHAP): Generic Maximum Achievable Control Technology
Standards. The source category addressed in this action is Ethylene
Production. The EPA is proposing decisions concerning the residual risk
and technology review (RTR), including proposing amendments pursuant to
technology review for storage vessels and heat exchange systems. The
EPA is also proposing amendments to correct and clarify regulatory
provisions related to emissions during periods of startup, shutdown,
and malfunction (SSM), including removing general exemptions for
periods of SSM, adding work practice standards for periods of SSM where
appropriate, and clarifying regulatory provisions for certain vent
control bypasses. Lastly the EPA is proposing to add monitoring and
operational requirements for flares; and add provisions for electronic
reporting of performance test results and reports and Notification of
Compliance Status (NOCS) reports. We estimate that these proposed
amendments will reduce hazardous air pollutants (HAP) emissions from
this source category by 62 tons per year (tpy).

DATES: Comments. Comments must be received on or before November 25,
2019. 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 November 8, 2019.
    Public hearing. If anyone contacts us requesting a public hearing
on or before October 15, 2019, we will hold a hearing. Additional
information about the hearing, if requested, will be published in a
subsequent Federal Register document and posted at https://www.epa.gov/stationary-sources-air-pollution/acetal-resins-acrylic-modacrylic-fibers-carbon-black-hydrogen. See SUPPLEMENTARY INFORMATIONfor
information on requesting and registering for a public hearing.

ADDRESSES: You may send comments, identified by Docket ID No. EPA-HQ-
OAR-2017-0357 by any of the following methods:
     Federal eRulemaking Portal: https://www.regulations.gov/
(our preferred method). Follow the online instructions for submitting
comments.
     Email<EMAIL_ADDRESS>Include Docket ID No. EPA-
HQ-OAR-2017-0357 in the subject line of the message.
     Fax:<PHONE_NUMBER>. Attention Docket ID No. EPA-HQ-OAR-
2017-0357.
     Mail: U.S. Environmental Protection Agency, EPA Docket
Center, Docket ID No. EPA-HQ-OAR-2017-0357, 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 questions about this proposed
action, contact Andrew Bouchard, Sector Policies and Programs Division
(E-143-01), Office of Air Quality Planning and Standards, U.S.
Environmental Protection Agency, Research Triangle Park, North Carolina
27711; telephone number:<PHONE_NUMBER>; and email address:
<EMAIL_ADDRESS>For specific information regarding the risk
modeling methodology, contact Mark Morris, Health and Environmental
Impacts Division (C539-02), Office of Air Quality Planning and
Standards, U.S. Environmental Protection Agency, Research Triangle
Park, North Carolina 27711; telephone number:<PHONE_NUMBER>; and email
address<EMAIL_ADDRESS>For questions about monitoring and
testing requirements, contact Gerri Garwood, Sector Policies and
Programs Division (D-245-05), Office of Air Quality Planning and
Standards, U.S. Environmental Protection Agency, Research Triangle
Park, North Carolina 27711; telephone number:<PHONE_NUMBER>; and email
address<EMAIL_ADDRESS>For information about the applicability
of the NESHAP to a particular entity, contact Marcia Mia, Office of
Enforcement and Compliance Assurance (OECA), U.S. Environmental
Protection Agency, WJC South Building (Mail Code 2227A), 1200
Pennsylvania Avenue NW, Washington, DC 20460; telephone number: (202)
564-7042; and email address<EMAIL_ADDRESS>
SUPPLEMENTARY INFORMATION:
    Public hearing. Please contact Ms. Virginia Hunt at<PHONE_NUMBER>
or by email at<EMAIL_ADDRESS>to request a public hearing, to
register to speak at the public hearing, or to inquire as to whether a
public hearing will be held.
    Docket. The EPA has established a docket for this rulemaking under
Docket ID No. EPA-HQ-OAR-2017-0357. All documents in the docket are
listed in Regulations.gov. Although listed, 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 in hard copy. Publicly
available docket materials are available either electronically in
Regulations.gov or in hard copy at the EPA Docket Center, Room 3334,
WJC West Building, 1301 Constitution Avenue NW, Washington, DC. The
Public Reading Room is open from 8:30 a.m. to 4:30 p.m., Monday through
Friday, excluding legal holidays. The telephone number for the Public
Reading Room is<PHONE_NUMBER>, and the telephone number for the EPA
Docket Center is<PHONE_NUMBER>.
    Instructions. Direct your comments to Docket ID No. EPA-HQ-OAR-
2017-0357. The EPA's policy is that all comments received will be
included in the public docket without change and may be made available
online at https://www.regulations.gov/, including any personal
information provided, unless the comment includes information claimed
to be CBI or other information whose disclosure is restricted by
statute. Do not submit information that you consider to be CBI or
otherwise protected through https://www.regulations.gov/ or email. This

type of information should be submitted by mail as discussed below.
    The EPA may publish any comment received to its public docket.
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). For additional submission methods, the full EPA
public comment policy, information about CBI or multimedia submissions,
and general guidance on making effective comments, please visit https://www.epa.gov/dockets/commenting-epa-dockets.
    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 be
free of any defects or viruses. For additional information about the
EPA's public docket, visit the EPA Docket Center homepage at https://www.epa.gov/dockets.
    Submitting CBI. Do not submit information containing CBI to the EPA
through https://www.regulations.gov/ or email. 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, mark the outside of
the digital storage media as CBI and then 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 Instructions
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. 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. Send or deliver information identified as CBI only to the
following address: OAQPS Document Control Officer (C404-02), OAQPS,
U.S. Environmental Protection Agency, Research Triangle Park, North
Carolina 27711, Attention Docket ID No. EPA-HQ-OAR-2017-0357.
    Preamble Acronyms and Abbreviations. 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:

ACC American Chemistry Council
AEGL acute exposure guideline level
AERMOD air dispersion model used by the HEM-3 model
AFPM American Fuel & Petrochemical Manufacturers
AMEL alternative means of emission limitation
APCD air pollution control device
ATSDR Agency for Toxic Substances and Disease Registry
BAAQMD Bay Area Air Quality Management District
BACT best available control technology
BDL elow detection levels
Btu British thermal units
BWON benzene waste operations NESHAP
CAA Clean Air Act
CalEPA California EPA
CBI Confidential Business Information
CDX Central Data Exchange
CEDRI Compliance and Emissions Data Reporting Interface
CEMS continuous emission monitoring system(s)
CFR Code of Federal Regulations
CMS continuous monitoring systems
CO carbon monoxide
CO2 carbon dioxide
CPMS continuous parametric monitoring system(s)
DLL detection level limited
EBU enhanced biological unit
ECHO enforcement and compliance history online
EFR external floating roof
EMACT ethylene production MACT
EPA Environmental Protection Agency
ERPG Emergency Response Planning Guideline
ERT Electronic Reporting Tool
FTIR Fourier transform infrared spectrometry
GACT generally available control technologies
HAP hazardous air pollutant(s)
HCl hydrochloric acid
HEM-3 Human Exposure Model, Version 1.1.0
HF hydrogen fluoride
HI hazard index
HQ hazard quotient
HRVOC highly reactive volatile organic compounds
IBR incorporation by reference
ICR Information Collection Request
IFR internal floating roof
IRIS Integrated Risk Information System
km kilometer
kPa kilopascals
LAER lowest achievable emission rate
LDAR leak detection and repair
LEL lower explosive limit
lpm liters per minute
MACT maximum achievable control technology
m\3\ cubic meter
mg/m\3\ milligrams per cubic meter
Mg/yr megagrams per year
MIR maximum individual risk
MMBtu million British thermal units
MON miscellaneous organic chemical manufacturing NESHAP
MPGF multi-point ground flare(s)
MTVP maximum true vapor pressure
NAAQS National Ambient Air Quality Standards
NAICS North American Industry Classification System
NATA National Air Toxics Assessment
NEI national emission inventory
NESHAP national emission standards for hazardous air pollutants
NHVcz net heating value in the combustion zone gas
NHVdil net heating value dilution parameter
NHVvg net heating value in the vent gas
NOCS notification of compliance status
NPDES National Pollutant Discharge Elimination System
NRDC Natural Resources Defense Council
NSPS new source performance standards
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
OSHA Occupational Safety and Health Administration
PAH polycyclic aromatic hydrocarbons
PB-HAP hazardous air pollutants known to be persistent and bio-
accumulative in the environment
PDF portable document format
PM particulate matter
PM2.5 particulate matter less than 2.5 microns in
diameter
POM polycyclic organic matter
ppm parts per million
ppmv parts per million by volume
ppmvd parts per million by volume, dry basis
ppmw parts per million by weight
PRA Paperwork Reduction Act
PRD pressure relief device(s)
psig pounds per square inch gauge
RACT reasonably available control technology
RATA relative accuracy test audit
REL reference exposure level

RFA Regulatory Flexibility Act
RfC reference concentration
RfD reference dose
RTR residual risk and technology review
SAB Science Advisory Board
SCAQMD South Coast Air Quality Management District
SCC source classification code
SOCMI synthetic organic chemical manufacturing industry
SSM startup, shutdown, and malfunction
TAB total annual benzene
TAC Texas Administrative Code
TCEQ Texas Commission on Environmental Quality
TOSHI target organ-specific hazard index
tpy tons per year
TRIM.FaTE Total Risk Integrated Methodology.Fate, Transport, and
Ecological Exposure model
TSM total selected metals
UF uncertainty factor
[micro]g/m\3\ microgram per cubic meter
UMRA Unfunded Mandates Reform Act
URE unit risk estimate
VCS voluntary consensus standards
VOC volatile organic compound(s)

    Organization of this document. The information in this preamble is
organized as follows below. In particular, section IV of this preamble
describes the majority of the agency's rationale for the proposed
actions in this preamble. Section IV.A specifies proposed monitoring
and operational requirements for flares in the ethylene production
source category to ensure that the level of control from the original
MACT standards is achieved by these air pollution control devices
(APCD). To ensure that CAA section 112 standards continuously apply
(Sierra Club v. EPA, 551 F.3d 1019 (D.C. Cir. 2008), section IV.A also
proposes work practice standards for periods of SSM for when flares are
used as an APCD, proposes work practice standards for periods of SSM
for certain vent streams (i.e. PRD releases and maintenance vents),
proposes clarifications for vent control bypasses for certain vent
streams (i.e., closed vent systems containing bypass lines, in situ
sampling systems, and flares connected to fuel gas systems), and
proposes work practice standards for decoking operations for ethylene
cracking furnaces (which is currently defined as a shutdown activity in
the Ethylene Production NESHAP).
    Section IV.B of this preamble summarizes the results of the risk
assessment while section IV.C of this preamble summarizes our proposed
decisions regarding the results of the risk assessment. Section IV.D of
this preamble summarizes the results of our technology review, and
proposes revisions for storage vessels and heat exchange systems.
Section IV.E of this preamble summarizes other changes we are
proposing, including general regulatory language changes related to the
removal of SSM exemptions, electronic reporting, and other minor
clarifications identified as part our review of the NESHAP and as part
of the other proposed revisions in this action. Lastly, section IV.F of
this preamble summarizes our rationale for the compliance dates we are
proposing.

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 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 taking in addition to those identified in
the risk and technology review?
    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?
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 13771: Reducing Regulations and Controlling
Regulatory Costs
    C. Paperwork Reduction Act (PRA)
    D. Regulatory Flexibility Act (RFA)
    E. Unfunded Mandates 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) and
1 CFR Part 51
    K. Executive Order 12898: Federal Actions To Address
Environmental Justice in Minority Populations and Low-Income
Populations

I. General Information

A. Does this action apply to me?

    Table 1 of this preamble lists the NESHAP and associated regulated
industrial source category that is 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, once promulgated, will be directly
applicable to the affected sources. Federal, state, local, and tribal
government entities would not be affected by this proposed action. As
defined in the National Emission Standards for Hazardous Air
Pollutants; Revision of Initial List of Categories of Sources and
Schedule for Standards Under Sections 112(c) and (e) of the Clean Air
Act Amendments of 1990 (61 FR 28197, June 4, 1996), the Ethylene
Production source category includes any chemical manufacturing process
unit in which ethylene and/or propylene are produced by separation from
petroleum refining process streams or by subjecting hydrocarbons to
high temperatures in the presence of steam.\1\ The ethylene production
unit includes the separation of ethylene and/or propylene from
associated streams such as a C4 product,\2\ pyrolysis gasoline, and
pyrolysis fuel oil. The ethylene production unit does not include the
manufacture of Synthetic Organic Chemical Manufacturing Industry
(SOCMI) chemicals such as the production of butadiene from the C4
stream and aromatics from pyrolysis gasoline.
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    \1\ In the June 4, 1996, document that revised the Initial List
of Source Categories, the EPA added seven categories of major
sources that included a source category listed as ``Ethylene
Processes,'' (61 FR 28197); however, subsequent regulatory actions
taken by the EPA, including the initial NESHAP development (e.g., 65
FR 76408, December 6, 2000) and current regulatory text at 40 CFR
part 63, subpart YY refer to the source category as ``Ethylene
Production.''
    \2\ The C4 product stream is a hydrocarbon product stream from
an ethylene production unit consisting of compounds with four carbon
atoms (e.g., butanes, butenes, butadienes).

    Table 1--NESHAP and Industrial Source Categories Affected By This
                             Proposed Action
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          Source category                  NESHAP         NAICS code \1\
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Ethylene Production...............  Generic Maximum               325110
                                     Achievable Control
                                     Technology
                                     Standards.
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\1\ 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. Following signature by the
EPA Administrator, the EPA will post a copy of this proposed action at
https://www.epa.gov/stationary-sources-air-pollution/acetal-resins-acrylic-modacrylic-fibers-carbon-black-hydrogen. Following publication
in the Federal Register, the EPA will post the Federal Register version
of the proposal and key technical documents at this same website.
Information on the overall RTR program is available at https://www3.epa.gov/ttn/atw/rrisk/rtrpg.html.
    A redline version of the regulatory language that incorporates the
proposed changes in this action is available in the docket for this
action (Docket ID No. EPA-HQ-OAR-2017-0357).

II. Background

A. What is the statutory authority for this action?

    The statutory authority for this action is provided by sections 112
and 301 of the Clean Air Act (CAA), as amended (42 U.S.C. 7401 et
seq.). Section 112 of the CAA establishes a two-stage regulatory
process to develop standards for emissions of HAP from stationary
sources. Generally, the first stage involves establishing technology-
based standards and the second stage involves evaluating those
standards that are based on maximum achievable control technology
(MACT) 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, the CAA also requires the EPA to review
standards set under CAA section 112 every 8 years to determine if there
are ``developments in practices, processes, or control technologies''
that may be appropriate to incorporate into the standards. 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.'' The discussion that follows
identifies the most relevant statutory sections and briefly explains
the contours of the methodology used to implement these statutory
requirements. A more comprehensive discussion appears in the document
titled CAA Section 112 Risk and Technology Reviews: Statutory Authority
and Methodology, in the docket for this rulemaking.
    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 source standards and area source standards.
``Major sources'' are those that emit or have the potential to emit 10
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.''
The EPA must also consider control options that are more stringent than
the floor. Standards more stringent than the floor are commonly
referred to as beyond-the-floor standards. In certain instances, as
provided in CAA section 112(h), the EPA may set work practice standards
where it is not feasible to prescribe or enforce a numerical emission
standard. For area sources, CAA section 112(d)(5) gives the EPA
discretion to set standards based on generally available control
technologies or management practices (GACT standards) in lieu of MACT
standards.
    The second stage in standard-setting focuses on identifying and
addressing any remaining (i.e., ``residual'') risk according to CAA
section 112(f). For source categories subject to MACT standards,
section 112(f)(2) of the CAA requires the EPA to determine 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. Section 112(d)(5) of the CAA provides that this
residual risk review is not required for categories of area sources
subject to GACT standards. Section 112(f)(2)(B) of the CAA further
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) (54 FR 38044, September 14, 1989). The EPA
notified Congress in the 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
(the Court) upheld the EPA's interpretation that CAA section 112(f)(2)
incorporates the approach established in the Benzene NESHAP. See NRDC
v. EPA, 529 F.3d 1077, 1083 (D.C. Cir. 2008).
    The approach incorporated into the CAA and used by the EPA to
evaluate residual risk and to develop standards under CAA section
112(f)(2) is 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)
\3\ of approximately 1 in 10 thousand.'' 54 FR 38045, September 14,
1989. If risks are unacceptable, the EPA must determine the emissions
standards necessary to reduce risk to an acceptable level without
considering costs. In the second step of the approach, the EPA
considers whether the emissions 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.''
Id. 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, the
EPA considers whether a more stringent standard is necessary to
prevent, taking into consideration costs, energy, safety, and other
relevant factors, an adverse environmental effect.
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    \3\ 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.
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    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 8 years. In
conducting this review, which we call the ``technology review,'' the
EPA is not required to recalculate the MACT floor. Natural Resources
Defense Council (NRDC) v. EPA, 529 F.3d 1077, 1084 (D.C. Cir. 2008).
Association of Battery Recyclers, Inc. v. EPA, 716 F.3d 667 (D.C. Cir.
2013). The EPA may consider cost in deciding whether to revise the
standards pursuant to CAA section 112(d)(6).

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

    The Ethylene Production MACT standards (herein called the EMACT
standards) for the Ethylene Production source category are contained in
the Generic Maximum Achievable Control Technology (GMACT) NESHAP which
also includes MACT standards for several other source categories. The
EMACT standards were promulgated on July 12, 2002 (67 FR 46258) and
codified at 40 CFR part 63, subparts XX and YY. As promulgated in 2002,
and further amended on April 13, 2005 (70 FR 19266), the EMACT
standards regulate HAP emissions from ethylene production units located
at major sources. An ethylene production unit is a chemical
manufacturing process unit in which ethylene and/or propylene are
produced by separation from petroleum refining process streams or by
subjecting hydrocarbons to high temperatures in the presence of steam.
The EMACT defines the affected source as all storage vessels, ethylene
process vents, transfer racks, equipment, waste streams, heat exchange
systems, and ethylene cracking furnaces and associated decoking
operations that are associated with each ethylene production unit
located at a major source as defined in CAA section 112(a).
    As of January 1, 2017, there were 26 ethylene production facilities
in operation and subject to the EMACT standards. This is based on our
search of the National Emission Inventory (NEI) and the EPA's
Enforcement and Compliance History Online (ECHO) database
(www.echo.epa.gov), and facility responses to our CAA section 114
request (see section II.C of this preamble for details about our CAA
section 114 request). We are also aware of the expansion and
construction of several facilities. Based upon this anticipated growth
for the Ethylene Production source category, we estimate that a total
of 31 ethylene production facilities will ultimately be subject to the
EMACT standards. A complete list of facilities that are currently
subject, or will be subject, to the EMACT standards is available in
Appendix A of the memorandum titled Review of the RACT/BACT/LAER
Clearinghouse Database for the Ethylene Production Source Category,
which is available in Docket ID No. EPA-HQ-OAR-2017-0357.

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

    In July 2014, the EPA issued a request, pursuant to CAA section
114, to collect information from ethylene production facilities owned
and operated by nine entities (i.e., corporations). This effort focused
on gathering comprehensive information about process equipment, control
technologies, point and fugitive emissions, and other aspects of
facility operations. Companies completed the survey and submitted
responses (and follow-up responses) to the EPA between October 2014 and
September 2015. Additionally, in April 2016, the EPA requested
historical monitoring and compliance data for heat exchange systems and
ethylene cracking furnaces, emissions source sampling for certain
pollutants for heat exchange systems, and stack testing for certain
pollutants for ethylene cracking furnaces under both normal operation
as well as during decoking operations. The results of these requests
were submitted to the EPA between the fall of 2016 and spring of 2017.
The EPA has used the collected information to fill data gaps, establish
the baseline emissions and control levels for purposes of the
regulatory reviews, to identify the most effective control measures,
and estimate the environmental and cost impacts associated with the
regulatory options considered and reflected in this proposed action.
The information not claimed as CBI by respondents is available in the
memorandum titled Data Received From Information Collection Request for
the Ethylene Production Source Category, in Docket ID No. EPA-HQ-OAR-
2017-0357.

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

    We are relying on certain technical reports and memoranda that the
EPA developed for flares used as APCDs in the petroleum refinery sector
and new source performance standards (NSPS) (80 FR 75178, December 1,
2015). For completeness of the rulemaking record for this action and
for ease of reference in finding these items in the publicly available
Refinery rulemaking Docket, we are including in the docket for this
rulemaking (Docket ID No. EPA-HQ-OAR-2017-0357) a list of specific
technical support documents in Table 1 of the memorandum titled Control
Option Impacts for Flares Located in the Ethylene Production Source
Category. The Petroleum Refinery sector and NSPS rulemaking Docket is
located at Docket ID No. EPA-HQ-OAR-2010-0682.
    In addition, the EPA is incorporating into the docket for this
rulemaking, materials associated with a number of site-specific
alternative means of emission limitation (AMEL) requests for facilities
electing to use multi-point ground flares (MPGFs) as an APCD. These
site-specific AMEL requests for MPGFs have been approved by the EPA
because the MPGF can achieve at least equivalent reductions in
emissions as the underlying flare operational standards in various
NESHAP and/or NSPS. The EPA receives these AMEL requests because MPGF
are designed to operate above the current maximum permitted velocity
requirements for flares in the General Provisions at 40 CFR 63.11(b).
Given that the EPA has provided notice and sought comments on certain
specific AMEL requests, the underlying AMEL requests submitted by
industry, MPGF test data, technical memorandums, Federal Register
documents \4\ and other supporting and related material that formed the
basis of the AMEL requests and approved alternative operating
conditions have been placed in a publicly available docket at Docket ID
No. EPA-HQ-OAR-2014-0738. We consider all items in

Docket ID No. EPA-HQ-OAR-2014-0738 part of our rulemaking record as
well, given that this docket is specific to MPGF AMEL requests. We are,
therefore, incorporating this docket by reference in this rule.
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    \4\ 80 FR 8023, February 13, 2015; 80 FR 52426, August 31, 2015;
81 FR 23480, April 21, 2016; 82 FR 16392, April 4, 2017; 82 FR
27822, June 19, 2017; and 83 FR 18034, April 25, 2018.
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    Lastly, the EPA is incorporating into the docket for this
rulemaking, all materials associated with the development of the
current GMACT and EMACT standards from Docket ID No. A-97-17, Docket ID
No. A-98-22, and Docket ID No. OAR-2204-0411. Publicly available docket
materials are available either electronically at https://www.regulations.gov/,or in hard copy at the EPA Docket Center, EPA WJC
West Building, Room 3334, 1301 Constitution Ave. NW, Washington, DC.
The Public Reading Room is open from 8:30 a.m. to 4:30 p.m., Monday
through Friday, excluding legal holidays. The telephone number for the
Public Reading Room is<PHONE_NUMBER>, and the telephone number for the
EPA Docket Center is<PHONE_NUMBER>.

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.'' 54 FR 38046,
September 14, 1989. 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. 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.
    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 the HAP
emissions 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.\5\ 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 EPA's response to comments on our
policy under the Benzene NESHAP where the EPA explained that:
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    \5\ 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
exposure to the HAP to the level at or below which no adverse
chronic noncancer effects are expected; the HI is the sum of HQs for
HAP that affect the same target organ or organ system.

    ``[t]he 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 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'.''

See 54 FR 38057, September 14, 1989. Thus, the level of the MIR is only
one factor to be weighed in determining acceptability of risk. The
Benzene NESHAP explained that ``an 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.'' Id. at 38045. 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.'' Id. at
38061. We also consider the uncertainties associated with the various
risk analyses, as discussed earlier in this preamble, in our
determinations of acceptability and ample margin of safety.
    The EPA notes that it has not considered certain health information
to date in making residual risk determinations. At this time, 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.'' \6\
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    \6\ Recommendations of the SAB Risk and Technology Review (RTR)
Methods Panel are provided in their report, which is available at:
https://yosemite.epa.gov/sab/sabproduct.nsf/
4AB3966E263D943A8525771F00668381/$File/EPA-SAB-10-007-unsigned.pdf.

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    In response to the SAB recommendations, the EPA incorporates
cumulative risk analyses into its RTR risk assessments, including those
reflected in this proposal. 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 are concerned about the
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 significantly greater associated uncertainties than
the source category or facility-wide estimates. Such aggregate or
cumulative assessments would compound those uncertainties, making the
assessments too unreliable.

B. How do we perform the technology review?

    Our technology review 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
applying each development. This analysis informs our decision of
whether it is ``necessary'' to revise the emissions 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'':
     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
(or last updated) the NESHAP, we review a variety of data sources in
our investigation of potential practices, processes, or controls to
consider. 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, we provide a complete 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. The seven sections
that follow this paragraph describe how we estimated emissions and
conducted the risk assessment. The docket for this rulemaking contains
the following document, which provides more information on the risk
assessment inputs and models: Residual Risk Assessment for the Ethylene
Production Source Category in Support of the 2019 Risk and Technology
Review Proposed Rule. The methods used to assess risk (as described in
the seven 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;
\7\ and described in the SAB review report issued in 2010. They are
also consistent with the key recommendations contained in that report.
---------------------------------------------------------------------------

    \7\ U.S. EPA. Risk and Technology Review (RTR) Risk Assessment
Methodologies: For Review by the EPA's Science Advisory Board with
Case Studies--MACT I Petroleum Refining Sources and Portland Cement
Manufacturing, June 2009. EPA-452/R-09-006. https://www3.epa.gov/airtoxics/rrisk/rtrpg.html.
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1. How did we estimate actual emissions and identify the emissions
release characteristics?
    For each facility that we determined to be subject to the EMACT
standards (see section II.B of this preamble), we gathered emissions
data from Version 1 of the 2011 NEI. For each NEI record, we reviewed
the source classification code (SCC), emission unit, and process
descriptions, and then assigned the record to an emission source type
(i.e., each record was labeled storage vessel, process vent, transfer
rack, equipment leak, waste, heat exchange system, cracking furnace,
decoking pot, PRD, other ethylene source type, or non-ethylene source
type).
    In May 2014, the EPA provided member companies of the American
Chemistry Council (ACC) and the American Fuel & Petrochemical
Manufacturers (AFPM) an opportunity to voluntarily review their NEI
records for completeness and accuracy, given that these records would
form the underlying basis of our emissions modeling input files for the
residual risk review. The NEI records were sent in separate
Microsoft[supreg] Excel worksheet(s) via email to each company that
operates at least one facility in the Ethylene Production source
category. Each company was afforded an opportunity to review (and
revise, if necessary) emission values, emission release point
parameters, coordinates, and emission source type assignments. All
revisions and changes from these voluntary reviews were received
between June

2014 through October 2014, and then incorporated into the modeling
file.
    Also, as part of the mandatory July 2014 CAA section 114 request
(see section II.C of this preamble for details about our CAA section
114 request), the EPA asked companies to provide emission release point
parameters and coordinates, for all emission release points associated
with ethylene production if this information had not been previously
submitted as part of their voluntary review. In response to these
requests, companies also submitted process flow diagrams illustrating
the connectivity between each process and the emission release points.
We used all this information to reevaluate each NEI record in the
modeling file and to update emission release point parameter data. In
other words, we used the CAA section 114 response data wherever
possible in lieu of the 2011 NEI and/or voluntary review data.
    Finally, we reviewed each of the emission source types to
incorporate recent data and to ensure the data were complete and
representative. For instance, for the modeling file, we replaced the
2011 NEI ethylene cracking furnace and decoking operation emissions
data with the ethylene cracking furnace and decoking operation stack
test data that we received from the CAA section 114 responses because
we generally consider stack test data to be much more representative of
emissions from these operations than emission estimates made in the
absence of this data. For each of the other emission source types
associated with an ethylene production unit (i.e., storage vessels,
ethylene process vents, transfer racks, equipment leaks, waste streams,
and heat exchange systems), we compared emissions between all
facilities, and, based on this comparison, we observed some
inconsistencies with the reported emissions between the different
emission sources. For example, certain facilities did not report
emissions for an emission source type while others did so. Therefore,
we focused on the following two criteria to determine whether facility
emissions were both complete and representative: (1) A facility should
have emissions for all emission source types (provided that the
emission source type exists at the facility), and (2) a facility should
have emissions for all emission source types above source-specific
emission thresholds. If either of those criteria were not met for an
emission source type at a facility, then we applied a model emissions
profile to update the modeling file. These model emissions profiles, in
concert with the stack test data received from the CAA section 114
responses, were also used to develop model plants for the new ethylene
production facilities currently under construction and for recent major
expansions at existing facilities for which annual emissions data were
not available to the Agency. For further details on the assumptions and
methodologies used to estimate actual emissions, identify the emissions
release characteristics, develop model emissions profiles, and develop
model plants, see Appendix 1 of the document titled Residual Risk
Assessment for the Ethylene Production Source Category in Support of
the 2019 Risk and Technology Review Proposed Rule, which is available
in the docket for this rulemaking. We solicit comment on additional
information for the Ethylene Production source category that the EPA
could consider to estimate actual emissions.
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 19998-19999, April 15, 2005) and in the proposed
and final Hazardous Organic NESHAP RTR (71 FR 34428, June 14, 2006, and
71 FR 76609, December 21, 2006, respectively). 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 national emission standards. 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, September
14, 1989.)
    Apart from emissions from heat exchange systems and PRD releases,
we have determined that the actual emissions data are reasonable
estimates of the MACT-allowable emissions levels for the Ethylene
Production source category. For heat exchange systems, the MACT-
allowable emissions were assessed using a HAP speciation profile at the
annualized mass leak rate of 29.5 tpy allowed by the underlying MACT
standard at 40 CFR part 63, subpart XX. For atmospheric PRD releases,
the MACT-allowable emissions were assessed using a single atmospheric
PRD release identified from a review of excess emissions reported over
a 7.5-year period for approximately 30 percent of the facilities in the
source category.
    The ability to estimate MACT-allowable emissions from the actual
emissions dataset is largely dependent on the format of the standard
for a given emissions source as well as the types of controls employed
for the source. With respect to the various types of controls used
within the Ethylene Production source category, the most prevalent is
the use of a flare as a combustion control device. A flare can be used
to control emissions for a single emissions source, or, as is generally
the case, to control emissions from multiple emission sources/emission
source types.
    Flares are designed to handle a large range of variable flowrates
and compositions of combustible waste gases. Within the Ethylene
Production source category, flares generally control emissions from
multiple emission source types. Consideration of this, along with not
having a specific limit on how much gas can be combusted in a flare
(given that in many cases multiple emissions sources are being
controlled by this control device), means that it is extremely
difficult to determine an allowable emission rate for flares. For
purposes of this RTR, we have determined that flares in the Ethylene
Production source category are currently complying with certain design
and operational requirements that are generally expected to achieve 98-
percent destruction efficiencies or control. HAP emissions inventories
for flares in the Ethylene Production source category are developed
using engineering knowledge and, in many instances, presume this 98-
percent level of control. The Agency is unaware of any data that
suggest that flares used as controls in the Ethylene Production source
category are consistently over-controlling HAP emissions beyond 98-
percent control. And, while the Agency is proposing new operating
requirements for flares used as controls in this source category to
ensure at least 98-percent control given that more recent studies have
shown that some flares are operating less efficiently than 98-percent
control (see section IV.A.1 of this preamble), for purposes of the
MACT-allowable risk analysis, we are required to evaluate whether it is
necessary to tighten the existing MACT standard and subsequent level of
performance a flare is expected to

achieve. Thus, weighing all of these factors for flares, we believe
that the actual emission levels are a reasonable estimation of the
MACT-allowable emissions levels where the performance standards allow
the use of a flare as an APCD (e.g., storage vessels, ethylene process
vents, equipment leaks, transfer racks, and waste operations).
    For equipment leaks, which are currently subject to work practice
standards, there would be no difference between actual and MACT-
allowable emissions for facilities in the Ethylene Production source
category, provided the facilities are complying with the EMACT
standards as well as not conducting additional work practices proven to
reduce emissions beyond those required by the rule. We are aware of
only one rule in the State of Texas, which is the Texas Commission of
Environmental Quality (TCEQ) Highly Reactive Volatile Organic Compounds
(HRVOC) Rule (i.e., 30 TAC Chapter 115, Subchapter H, Division 3), that
may contain more stringent leak definitions and/or monitoring
frequencies for certain pieces of equipment for the eight facilities
located in Texas that might be subject to this rule. However, we note
based on our review of the Texas rule that specific facilities, which
are located in the Houston-Galveston-Brazoria area, still conduct a
leak detection and repair (LDAR) program using EPA Method 21; that the
vast majority of equipment (i.e., more than 95 percent of all equipment
surveyed in the CAA section 114 request), including almost all pieces
of equipment in gas and vapor service that would tend to highly
contribute to the overall equipment leak air emissions, are complying
with the same leak definition as in the EMACT standards; and that the
TCEQ HRVOC Rule generally requires quarterly monitoring while the EMACT
standards have varying degrees of monitoring frequencies depending on
the percentage of leaking equipment that could lead to more stringent,
the same, or less stringent frequencies that would require an EPA
Method 21 measurement and repair of a leaking component (if measured).
Therefore, weighing all of these factors for equipment leaks, we
determined that the actual emission levels for equipment leaks are a
reasonable estimation of the MACT-allowable emissions levels.
    For waste operations, the EMACT standards include various work
practice standards for the collection system of waste streams as well
as a performance standard for the treatment of these waste streams.
Assuming that the equipment in the collection system is maintained
properly and is in good working condition (as required), and that no
facilities are employing additional work practices proven to reduce
emissions beyond those required in the rule (we are unaware of any that
are doing additional work practices), there would be no difference in
the actual emissions level and the level allowed by the work practice
standards for the collection of waste streams. In general, for this
performance standard, it is possible that sources could over-control
emission sources resulting in the actual emissions being lower than the
MACT-allowable emissions. However, for waste operations, we are not
aware of any such over-control. Therefore, we believe that the actual
waste operations emission levels are a reasonable estimation of the
MACT-allowable emissions levels.
    For heat exchange systems, the EMACT standards include a LDAR work
practice where facilities are required to monitor for potential leaks
of HAP from process fluids into the cooling water of a heat exchange
system. Emissions of HAP from heat exchange systems result when leakage
of HAP from process fluids into the cooling water occurs and then that
cooling water is exposed to air (e.g., in a cooling tower for a closed-
loop system or from trenches/ponds in a once-through system). If a leak
is detected, it is only required to be repaired in a heat exchange
system if the exit mean concentration is at least 10 percent greater
than the entrance mean of the listed HAP (total or speciated) in Table
1 to subpart XX of 40 CFR part 63 (using a one-sided statistical
procedure at the 0.05 level of significance) and if it is at least 3.06
kilograms per hour (kg/hr). Therefore, for example, a leak of 3.05 kg/
hr or less of any HAP (total or speciated) that is listed in Table 1 to
subpart XX of 40 CFR part 63 need not be repaired. If we assume that
all the HAP at a 3.05 kg/hr leak rate would be emitted to the
atmosphere after the process fluids leak into cooling water and then
that cooling water is exposed to atmosphere, we would be left with an
annual MACT-allowable emissions level for heat exchange systems of 29.5
tpy (i.e., 3.05 kg/hr x 0.00110231 tons/kg x 8,760 hours per year (hr/
yr)) of HAP (total or speciated) listed in Table 1 to subpart XX of 40
CFR part 63. In order to determine a reasonable HAP speciation profile
to assess the MACT-allowable risk at the 29.5 tpy mass emission rate,
we reviewed historical heat exchange system compliance data gathered
under our CAA section 114 request. Given that 40 CFR part 63, subpart
XX requires a monitoring sensitivity that would enable detection of a
leak of 3.06 kg/hr or greater of the HAP listed in Table 1 to subpart
XX of 40 CFR part 63, we focused our analysis on determining a
reasonable HAP speciation profile based on historical leaks at or above
3.06 kg/hr. This was done for the purposes of removing records in the
dataset that have a higher level of uncertainty surrounding them (given
the monitoring sensitivity requirement in the rule), as well as to
remove the uncertainty in biased data where any reported historical
smaller leaks may have been predominately driven by data that were
reported at the detection level but were not actually measured. Thus,
upon reviewing the historical heat exchange system compliance data, we
found records of 10 speciated HAP leaks above 3.06 kg/hr that we
averaged for purposes of forming the basis of our HAP speciation
profile for the MACT-allowable emission level for heat exchange
systems. The HAP speciation profile analysis is available in Appendix 1
of the document titled Residual Risk Assessment for the Ethylene
Production Source Category in Support of the 2019 Risk and Technology
Review Proposed Rule, which is available in the docket for this
rulemaking.
    For ethylene cracking furnaces and associated decoking operations,
based on new information obtained through our CAA section 114 request,
we have determined that HAP are being emitted from these source types
and their actual emissions, which were measured from various ethylene
cracking furnaces and associated decoking operations during the stack
testing conducted pursuant to the CAA section 114 request, are allowed
by the rule. As such, we determined that the actual emissions are equal
to MACT-allowable emissions for these operations.
    Finally, in order to estimate the risk impacts of emissions from a
PRD release, we reviewed TCEQ's Air Emission Event Report Database
(http://www2.tceq.texas.gov/oce/eer/) over a 7.5-year period (i.e.,
January 1, 2010, to July 7, 2017) for roughly 30 percent of all
operating ethylene production facilities (i.e., seven of 26 ethylene
production facilities) in the source category that were chosen at
random and that have been in operation since January 1, 2010.
Accordingly, we believe these randomly selected facilities are a good
representation of all ethylene production facilities in the source
category. After reviewing TCEQ's database for reportable air emissions
events for these seven facilities over a 7.5-year period, we determined
that there were four reported emissions events that occurred from
atmospheric

PRDs (e.g., events where a PRD did not release emissions to an APCD
like a flare) on equipment in the Ethylene Production source category.
A closer inspection of these records, however, reveals that only one of
these events was actually an atmospheric PRD release on a properly
operating PRD. Therefore, for MACT-allowable emissions for PRD
releases, and in keeping with our conservative approach, we assumed
that each facility would have this reported release of HAP (i.e., 46.8
pounds (lbs) of 1,3-butadiene) occur once in a 7.5-year period (given
that this is the duration of the data we reviewed) and modeled an
annualized PRD release of HAP of 0.003 tpy of 1,3-butadiene from the
centroid of each ethylene production facility.
    For further details on the assumptions and methodologies used to
estimate MACT-allowable emissions, see Appendix 1 of the document
titled Residual Risk Assessment for the Ethylene Production Source
Category in Support of the 2019 Risk and Technology Review Proposed
Rule, which is available in the docket for this rulemaking.
3. How do we conduct dispersion modeling, determine inhalation
exposures, and estimate individual and population inhalation risks?
    Both long-term 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-3).\8\ The HEM-3
performs three primary risk assessment activities: (1) Conducting
dispersion modeling to estimate the concentrations of HAP in ambient
air, (2) estimating long-term 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.
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    \8\ For more information about HEM-3, go to https://www.epa.gov/fera/risk-assessment-and-modeling-human-exposure-model-hem.
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a. Dispersion Modeling
    The air dispersion model AERMOD, used by the HEM-3 model, is one of
the EPA's preferred models for assessing HAP concentrations from
industrial facilities.\9\ To perform the dispersion modeling and to
develop the preliminary risk estimates, HEM-3 draws on three data
libraries. The first is a library of meteorological data, which is used
for dispersion calculations. This library includes 1 year (2017) of
hourly surface and upper air observations from 824 meteorological
stations, selected to provide coverage of the United States and Puerto
Rico. A second library of United States Census Bureau census block \10\
internal point locations and populations provides the basis of human
exposure calculations (U.S. Census, 2010). 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.
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    \9\ 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, November 9,
2005).
    \10\ A census block is the smallest geographic area for which
census statistics are tabulated.
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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, September 14, 1989) and the limitations of Gaussian
dispersion models, including AERMOD.
    For each facility, we calculate the MIR as the cancer risk
associated with a continuous lifetime (24 hours per day, 7 days per
week, 52 weeks per year, 70 years) exposure to the maximum
concentration at the centroid of each inhabited census block. We
calculate individual cancer risk by multiplying the estimated lifetime
exposure to the ambient concentration of each HAP (in micrograms per
cubic meter ([mu]g/m\3\)) 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
microgram of the pollutant per cubic meter of air. For residual risk
assessments, we generally use UREs from the EPA's Integrated Risk
Information System (IRIS). 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 the 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 \11\ 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.
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    \11\ EPA's 2005 Guidelines for Carcinogen Risk Assessment
classifies carcinogens as: ``carcinogenic to humans,'' ``likely to
be carcinogenic to humans,'' and ``suggestive evidence of
carcinogenic potential.'' These classifications also coincide with
the terms ``known carcinogen, probable carcinogen, and possible
carcinogen,'' respectively, which are the terms advocated in EPA's
Guidelines for Carcinogen Risk Assessment, published in 1986 (51 FR
33992, September 24, 1986). In August 2000, the document,
Supplemental Guidance for Conducting Health Risk Assessment of
Chemical Mixtures (EPA/630/R-00/002), was published 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://
yosemite.epa.gov/sab/sabproduct.nsf/
214C6E915BB04E14852570CA007A682C/$File/ecadv02001.pdf.
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    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'' (https://iaspub.epa.gov/sor_internet/registry/termreg/searchandretrieve/glossariesandkeywordlists/search.do?details=&vocabName=IRIS%20Glossary). 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/mrls/index.asp); (2) the
CalEPA Chronic Reference Exposure Level (REL) (https://oehha.ca.gov/air/crnr/notice-adoption-air-toxics-hot-spots-program-guidance-manual-preparation-health-risk-0); or (3) as noted above, a scientifically
credible dose-response value that has been developed in a manner
consistent with the 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.
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
conservative assumptions about emission rates, meteorology, and
exposure location. In this proposed rulemaking, 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,\12\ we are revising 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 Ethylene Production Source Category in Support of the
2019 Risk and Technology Review Proposed Rule and in Appendix 5 of the
report: Technical Support Document for Acute Risk Screening Assessment.
We will be applying this revision in RTR rulemakings proposed on or
after June 3, 2019.
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    \12\ See, e.g., U.S. EPA. Screening Methodologies to Support
Risk and Technology Reviews (RTR): A Case Study Analysis (Draft
Report, May 2017. https://www3.epa.gov/ttn/atw/rrisk/rtrpg.html).
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    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, 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 (i.e., 99th percentile) co-occur and that a
person is present at the point of maximum exposure.
    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 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.'' \13\ 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 8 hours.\14\ They are
guideline levels for ``once-in-a-lifetime, short-term exposures to
airborne concentrations of acutely toxic, high-priority chemicals.''
Id. at 21. The AEGL-1 is specifically defined as ``the airborne
concentration (expressed as ppm (parts per million) or mg/m\3\
(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.''
Id. 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.'' Id.
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    \13\ 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.
    \14\ National Academy of Sciences, 2001. Standing Operating
Procedures for Developing Acute Exposure Levels for Hazardous
Chemicals, page 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).
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    ERPGs are ``developed for emergency planning and are intended as
health-based guideline concentrations for single exposures to
chemicals.'' \15\ Id. at 1. The ERPG-1 is defined as ``the maximum
airborne concentration below which it is believed that nearly all
individuals could be exposed for up to 1 hour without experiencing
other than mild transient adverse health effects or without perceiving
a clearly defined, objectionable odor.'' Id. at 2. Similarly, the ERPG-
2 is defined as ``the maximum airborne concentration below which it is
believed that nearly all individuals could be exposed for up to

1 hour without experiencing or developing irreversible or other serious
health effects or symptoms which could impair an individual's ability
to take protective action.'' Id. at 1.
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    \15\ ERPGS Procedures and Responsibilities. March 2014. American
Industrial Hygiene Association. Available at: https://www.aiha.org/get-involved/AIHAGuidelineFoundation/mergencyResponsePlanningGuidelines/Documents/ERPG%20Committee%20Standard%20Operating%20Procedures%20%20-%20March%202014%20Revision%20%28Updated%2010-2-2014%29.pdf.
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    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).
    For the acute inhalation risk assessment of the Ethylene Production
source category, we did not use the default acute emissions multiplier
of 10, but rather factors of 2, 4, 5, and 10, depending on the emission
process group. In general, hourly emissions estimates were based on
peak-to-mean ratios for 37 emission process groups ranging from a
factor of 2 to 10, with emissions from transfer racks and other
emission process groups where sufficient information did not exist to
adequately assess peak hourly emissions (e.g., flares controlling
various unknown emissions sources) having the highest hourly peak
emissions at a factor of 10 times the annual average. A further
discussion of why these factors were chosen can be found in Appendix 1
of the document titled Residual Risk Assessment for the Ethylene
Production Source Category in Support of the 2019 Risk and Technology
Review Proposed Rule, which is available in the docket for this
rulemaking.
    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 (even under the conservative assumptions of the screening
assessment), and no further analysis is performed for these HAP. In
cases where an acute HQ from the screening step is greater than 1, we
consider additional site-specific data to develop a more refined
estimate of the potential for acute exposures of concern. For this
source category, the data refinements employed consisted of determining
the highest HQ value that might occur outside facility boundaries.
These refinements are discussed more fully in the document titled
Residual Risk Assessment for the Ethylene Production Source Category in
Support of the 2019 Risk and Technology Review Proposed Rule, which is
available in the docket for this rulemaking.
4. How do we conduct the multipathway exposure and risk screening
assessment?
    The EPA conducted 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 determined
whether any sources in the source category emit any HAP known to be
persistent and bioaccumulative in the environment (PB-HAP), as
identified in the EPA's Air Toxics Risk Assessment Library (See Volume
1, Appendix D, at http://www.epa.gov/fera/risk-assessment-and-modeling-air-toxics-risk-assessment-reference-library).
    For the Ethylene Production source category, we identified PB-HAP
emissions of arsenic compounds, cadmium compounds, lead compounds,
mercury compounds, and polycyclic organic matter (POM) (of which
polycyclic aromatic hydrocarbons (PAH) is a subset), so we proceeded to
the next step of the evaluation. With the exception of lead, the human
health risk screening assessment for PB-HAP consists of three tiers. We
call this first evaluation the Tier 1 screening assessment. In a Tier 1
screening assessment, we determine whether the facility-specific
emission rates of PB-HAP are large enough to warrant further evaluation
of the human health risk through ingestion exposure under reasonable
worst-case conditions. To facilitate this step, we used 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 arsenic
compounds, cadmium compounds, chlorinated dibenzodioxins and furans,
mercury compounds, and POM. Based on the EPA estimates of toxicity and
bioaccumulation potential, the pollutants above represent a
conservative list for inclusion in multipathway risk assessments for
RTR rules. (See Volume 1, Appendix D at https://www.epa.gov/sites/production/files/201308/documents/volume_1_reflibrary.pdf.) 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.''
    We derive the Tier 1 screening threshold emission rates for these
PB-HAP (other than lead compounds) to correspond to a maximum excess
lifetime cancer risk of 1-in-1 million (i.e., for arsenic compounds,
polychlorinated dibenzodioxins and furans, and POM) or, for HAP that
cause noncancer health effects (i.e., cadmium compounds and mercury
compounds), a maximum HQ of 1. 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 (ingestion rates are decoupled into separate upper-bound
ingestion rates for the fisher, farmer, and gardener scenarios).
    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/farmer
scenario. 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. For
the Tier 2 farmer scenario, we assume the farmer consumes meat, eggs,
vegetables, and fruit grown near the facility. If further Tier 2
screening is necessary for the farmer scenario, we may apply the
gardener scenario. For the gardener scenario, we assume the gardener
only grows and consumes eggs, vegetables, and fruit products at the
same ingestion rate as the farmer. If PB-HAP emission rates do not
exceed a Tier 2 screening value of 1, we consider those PB-HAP
emissions to pose risks below a level of concern.
    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 USGS
waterbody data. If the PB-HAP emission rates for a facility exceed the
Tier 2 screening threshold emission rates and sufficient data are
available, we may conduct a Tier 3 screening assessment. If PB-HAP
emission rates do not exceed

a Tier 2 screening value of 1, we consider those PB-HAP emissions to
pose risks below a level of concern. If, based on additional analysis
and review, it is determined that no subsistence farming operations are
in the area, then the farmer scenario is not used in Tier 3 and only
gardener screening values are reported. If information obtained
suggests that subsistence farming operations do not exist, the EPA
considers the gardener scenario to be the most possible in all RTR
evaluations.
    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 if the Tier 3 screening assessment indicates that risks
above levels of concern cannot be ruled out.
    In evaluating the potential multipathway risk from emissions of
lead compounds, rather than developing a screening threshold emission
rate, we compare maximum estimated chronic inhalation exposure
concentrations to the level of the current National Ambient Air Quality
Standard (NAAQS) for lead.\16\ Values below the level of the primary
(health-based) lead NAAQS are considered to have a low potential for
multipathway risk.
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    \16\ 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 67002/3; 73 FR 67000/3; 73 FR 67005/1. 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.
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    For further information on the multipathway assessment approach,
see the document titled Residual Risk Assessment for the Ethylene
Production Source Category in Support of the 2019 Risk and Technology
Review Proposed Rule, which is available in the docket for this
rulemaking.
5. How did 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 section
112(f)(2)(A) of the CAA. Section 112(a)(7) of the CAA 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.''
    The EPA focuses on eight HAP, which are referred to as
``environmental HAP,'' in its screening assessment: Six PB-HAP and two
acid gases. The PB-HAP included in the screening assessment are arsenic
compounds, cadmium compounds, dioxins/furans, POM, mercury (both
inorganic mercury and methyl mercury), and lead compounds. The acid
gases included in the screening assessment are hydrochloric acid (HCl)
and hydrogen fluoride (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 lead), 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, and no-observed-adverse-effect
level. 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 to 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 document titled
Residual Risk Assessment for the Ethylene Production Source Category in
Support of the 2019 Risk and Technology Review Proposed Rule, which is
available in the docket for this rulemaking.
b. Environmental Risk Screening Methodology
    For the environmental risk screening assessment, the EPA first
determined whether any facilities in the Ethylene Production source
category emitted any of the environmental HAP. For the Ethylene
Production source category, we identified emissions of arsenic
compounds, cadmium compounds, HCl, hydrofluoric acid, lead, mercury,
and POM. Because one or more of the environmental HAP evaluated 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, arsenic
compounds, cadmium compounds, dioxins/furans, POM, mercury (both
inorganic mercury and methyl mercury), and lead compounds. With the
exception of lead, 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 back-calculate Tier 1
screening threshold emission rates. The screening threshold emission
rates represent the emission rate in tons of pollutant per year 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 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., 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
lead, we compared the average modeled air concentrations (from HEM-3)
of lead around each facility in the source category to the level of the
secondary NAAQS for lead. The secondary lead 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.''
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 section 112(a)(7) of the
CAA) 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 km\2\; 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
document titled Residual Risk Assessment for the Ethylene Production
Source Category in Support of the 2019 Risk and Technology Review
Proposed Rule, which is available in the docket for this action.
6. How did 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 the HAP emissions not only 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 this source category, we conducted the facility-wide assessment
using a dataset compiled from the 2011 NEI. The source category records
of that NEI dataset were removed, evaluated, and updated as described
in section II.C of this preamble. Once a quality assured source
category dataset was available, it was placed back with the remaining
records from the NEI for that facility. Also, because a preliminary
screening of facility-wide risks based on the 2011 NEI indicated the
potential for ethylene oxide to be a whole facility risk driver, we
updated the facility-wide modeling file for ethylene oxide emissions
using the 2014 NEI data set given that this was the best available data
for this pollutant. 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 document titled: Residual Risk Assessment for
the Ethylene Production Source Category in Support of the 2019 Risk and
Technology Review Proposed Rule, which is available in the docket for
this rulemaking, 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.
7. 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
conservative 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
below. Also included are those uncertainties specific to our acute
screening assessments, multipathway screening assessments, and our
environmental risk screening assessments. A more thorough discussion of
these uncertainties is included in the document titled Residual Risk
Assessment for the Ethylene Production Source Category in Support of
the 2019 Risk and Technology Review Proposed Rule, which is available
in the docket for this rulemaking. 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 annual totals for certain years, and they do not
reflect short-term fluctuations during the course of a year or
variations from year to year. 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 there is uncertainty in ambient concentration
estimates associated with any model, including the EPA's recommended
regulatory dispersion model, 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 under- or overestimate ambient levels
(e.g., meteorology and receptor locations). On balance, 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
location 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 every effort is made 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 likely dominate 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 under nor
overestimate when looking at the maximum individual risk 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 in qualitative terms. We note, as a
preface to this discussion, a point on dose-response uncertainty that
is stated in 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'' (EPA's 2005
Guidelines for Carcinogen Risk Assessment, page 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.\17\
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.\18\
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,\19\ 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.
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    \17\ IRIS glossary (https://ofmpub.epa.gov/sor_internet/registry/termreg/searchandretrieve/glossariesandkeywordlists/search.do?details=&glossaryName=IRIS%20Glossary).
    \18\ 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.
    \19\ 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.
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    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., 4 hours) to derive an acute dose-
response value at another exposure duration (e.g., 1 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 are
lacking 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 above, there are
several factors specific to the acute exposure assessment that the EPA
conducts as part of the risk review under section 112 of the CAA. The
accuracy of an acute inhalation exposure assessment depends on the
simultaneous occurrence of independent factors that may vary greatly,
such as hourly emissions 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 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 (dioxins, POM,
mercury, cadmium, and arsenic) and two acid gases (HF and hydrogen
chloride). For lead, we use AERMOD to determine ambient air
concentrations, which are then compared to the secondary NAAQS standard
for lead. 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.\20\
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    \20\ 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.
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    Model uncertainty concerns whether the model adequately represents
the actual processes (e.g., movement and accumulation) that might occur
in the environment. For example, does the model adequately describe 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 RTR.
    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., screen out), 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: Arsenic,
cadmium, dioxins/furans, lead, mercury (both inorganic and methyl
mercury), 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 HAP 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, 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 taking in addition to those identified in the
risk and technology review?

    In addition to the proposed actions on the risk review and
technology review discussed further in this section, we are proposing
the following: (1) Adding monitoring and operational requirements for
flares used as APCDs; (2) consistent with Sierra Club v. EPA, 551 F.3d
1019 (D.C. Cir. 2008), ensuring that CAA section 112 standards apply
continuously by proposing to add provisions and clarifications for
periods of SSM and bypasses, including for PRD releases, bypass lines
on closed vent systems, in situ sampling systems, maintenance
activities, and certain gaseous streams routed to a fuel gas system;
and (3) consistent with Sierra Club v. EPA, 551 F.3d 1019 (D.C. Cir.
2008), proposing to remove the shutdown exemption for decoking
operations (i.e., the decoking of ethylene cracking furnace radiant
tubes) and add work practice standards for this emission source. The
results and proposed decisions based on the analyses performed pursuant
to CAA section 112(d)(2) and (3) are presented below.
1. Flares
    The EPA is proposing under CAA section 112(d)(2) and (3) to amend
the operating and monitoring requirements for flares used as APCDs in
the Ethylene Production source category. We have determined that the
current requirements for flares are not adequate to ensure the level of
destruction efficiency needed to conform with the EMACT standards. As
previously explained, with respect to the various types of controls
used within the Ethylene Production source category, a flare is the
most prevalent APCD. A flare can be used to control emissions from
either a single emissions source (e.g., ethylene process vent), or
multiple emission sources (e.g., storage vessels, process vents, and
transfer racks). In the development of the EMACT standards, the EPA
stated that ``It is generally accepted that combustion devices achieve
a 98 weight-percent reduction in HAP emissions. . .'' (65 FR 76428,
December 6, 2000). The requirements applicable to flares, which are
used to control emissions from various emission sources in this source
category, are set forth in the General Provisions to 40 CFR part 63 and
cross-referenced in 40 CFR part 63, subpart SS for storage vessels,
ethylene process vents, transfer racks, and equipment leaks; and set
forth in the General Provisions to 40 CFR part 60 and cross-referenced
in 40 CFR part 61, subpart FF for waste operations. In general, flares
used as APCDs are expected to achieve 98-percent HAP destruction
efficiencies when designed and operated according to the requirements
in the General Provisions. Studies on flare performance,\21\ however,
indicate that these General Provisions requirements are inadequate to
ensure proper performance of flares at refineries and other
petrochemical facilities (including ethylene production units),
particularly when either assist steam or assist air is used. In
addition, over the last decade, flare minimization efforts at these