[Federal Register Volume 86, Number 5 (Friday, January 8, 2021)]
[Proposed Rules]
[Pages 1362-1390]
[FR Doc No: 2021-00174]

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ENVIRONMENTAL PROTECTION AGENCY

40 CFR Part 63

[EPA-HQ-OAR-2020-0560; FRL-10018-95-OAR]
RIN 2060-AU59

National Emission Standards for Hazardous Air Pollutants: Mercury
Cell Chlor-Alkali Plants Residual Risk and Technology Review

AGENCY: Environmental Protection Agency (EPA).

ACTION: Proposed rule.

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SUMMARY: The U.S. Environmental Protection Agency (EPA) is proposing
the results of the residual risk and technology review (RTR) of the
National Emission Standards for Hazardous Air Pollutants (NESHAP) for
mercury emissions from Mercury Cell Chlor-Alkali Plants, as required by
the Clean Air Act (CAA). The EPA is proposing to find risks due to
emissions of hazardous air pollutants (HAP) to be acceptable from the
Mercury Cell Chlor-Alkali Plants source category, and to determine that
the current NESHAP provides an ample margin of safety to protect public
health and that no more stringent standards are necessary to prevent,
taking into consideration costs, energy, safety, and other relevant
factors, an adverse environmental effect. The EPA is proposing to amend
the requirements for cell room fugitive mercury emissions to require
work practice standards for the cell rooms and to require instrumental
monitoring of cell room fugitive mercury emissions under the technology
review. Furthermore, under our technology review and maximum achievable
control technology (MACT) analysis, we are proposing to not require
conversion to non-mercury production technology and invite comments and
data and information regarding this proposed determination. In
addition, the EPA is proposing standards for fugitive chlorine
emissions from mercury cell chlor-alkali plants, which are not
currently regulated under the NESHAP. The EPA is proposing to address
applicability for thermal mercury recovery units when chlorine and
caustic are no longer produced in mercury cells. The EPA is also
proposing revisions related to emissions during periods of startup,
shutdown, and malfunction (SSM); provisions for electronic submission
of performance test results, performance evaluation reports, and
Notification of Compliance Status (NOCS) reports; and correction of
various compliance errors in the current rule.

DATES:
    Comments. Comments must be received on or before February 22, 2021.
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 February 8, 2021.
    Public hearing: If anyone contacts us requesting a public hearing
on or before January 13, 2021, we will hold a virtual public hearing.
See SUPPLEMENTARY INFORMATION for information on requesting and
registering for a public hearing.

ADDRESSES: You may send comments, identified by Docket ID No. EPA-HQ-
OAR-2020-0560, 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-2020-0560 in the subject line of the message.
     Fax:<PHONE_NUMBER>. Attention Docket ID No. EPA-HQ-OAR-
2020-0560.
     Mail: U.S. Environmental Protection Agency, EPA Docket
Center, Docket ID No. EPA-HQ-OAR-2020-0560, Mail Code 28221T, 1200
Pennsylvania Avenue NW, Washington, DC 20460.
     Hand Delivery or Courier (by scheduled appointment only):
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. Out of an abundance of

caution for members of the public and our staff, the EPA Docket Center
and Reading Room are closed to the public, with limited exceptions, to
reduce the risk of transmitting COVID-19. Our Docket Center staff will
continue to provide remote customer service via email, phone, and
webform. We encourage the public to submit comments via https://www.regulations.gov/ or email, as there may be a delay in processing
mail and faxes. Hand deliveries and couriers may be received by
scheduled appointment only. For further information on EPA Docket
Center services and the current status, please visit us online at
https://www.epa.gov/dockets.

FOR FURTHER INFORMATION CONTACT: For questions about this proposed
action, contact Phil Mulrine, Sector Policies and Programs Division
(D243-02), Office of Air Quality Planning and Standards, U.S.
Environmental Protection Agency, Research Triangle Park, North Carolina
27711; telephone number:<PHONE_NUMBER>; fax number:<PHONE_NUMBER>;
and email address<EMAIL_ADDRESS>For specific information
regarding the risk modeling methodology, contact James Hirtz, 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>;
fax number:<PHONE_NUMBER>; and email address<EMAIL_ADDRESS>
SUPPLEMENTARY INFORMATION:
    Participation in virtual public hearing. Please note that the EPA
is deviating from its typical approach because the President has
declared a national emergency. Due to the current Centers for Disease
Control and Prevention (CDC) recommendations, as well as state and
local orders for social distancing to limit the spread of COVID-19, the
EPA cannot hold in-person public meetings at this time.
    To request a virtual public hearing, contact<PHONE_NUMBER> or by
email at<EMAIL_ADDRESS>If requested, the virtual hearing
will be held on January 25, 2021. The hearing will convene at 9:00 a.m.
Eastern Time (ET) and will conclude at 3:00 p.m. ET. The EPA may close
a session 15 minutes after the last pre-registered speaker has
testified if there are no additional speakers. The EPA will announce
further details on the virtual public hearing at https://www.epa.gov//stationary-sources-air-pollution/mercury-cell-chloralkali-plants-national-emissions-standards.
    The EPA will begin pre-registering speakers for the hearing upon
publication of this document in the Federal Register, if a hearing is
requested. To register to speak at the virtual hearing, please use the
online registration form available at https://www.epa.gov/stationary-sources-air-pollution/mercury-cell-chloralkali-plants-national-emissions-standards or contact the public hearing team at (888) 372-
8699 or by email at<EMAIL_ADDRESS>The last day to pre-
register to speak at the hearing will be January 21, 2021. Prior to the
hearing, the EPA will post a general agenda that will list pre-
registered speakers in approximate order at: https://www.epa.gov/stationary-sources-air-pollution/mercury-cell-chloralkali-plants-national-emissions-standards.
    The EPA will make every effort to follow the schedule as closely as
possible on the day of the hearing; however, please plan for the
hearings to run either ahead of schedule or behind schedule.
    Each commenter will have 5 minutes to provide oral testimony. The
EPA encourages commenters to provide the EPA with a copy of their oral
testimony electronically (via email) by emailing it to Phil Mulrine at
<EMAIL_ADDRESS>The EPA also recommends submitting the text of
your oral testimony as written comments to the rulemaking docket.
    The EPA may ask clarifying questions during the oral presentations
but will not respond to the presentations at that time. Written
statements and supporting information submitted during the comment
period will be considered with the same weight as oral testimony and
supporting information presented at the public hearing.
    Please note that any updates made to any aspect of the hearing is
posted online at https://www.epa.gov/stationary-sources-air-pollution/mercury-cell-chloralkali-plants-national-emissions-standards. While the
EPA expects the hearing to go forward as set forth above, please
monitor our website or contact the public hearing team at (888) 372-
8699 or by email at<EMAIL_ADDRESS>to determine if there are
any updates. The EPA does not intend to publish a document in the
Federal Register announcing updates.
    If you require the services of a translator or a special
accommodation such as audio description, please pre-register for the
hearing with the public hearing team and describe your needs by January
15, 2021. The EPA may not be able to arrange accommodations without
advanced notice.
    Docket. The EPA has established a docket for this rulemaking under
Docket ID No. EPA-HQ-OAR-2020-0560. In addition to this docket
established for this rulemaking, relevant information can be found in
dockets for previous rulemakings; EPA-HQ-OAR-2002-0016 and EPA HQ-OAR-
2002-0017. All documents in the docket are listed in https://www.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. With the exception of
such material, publicly available docket materials are available
electronically in Regulations.gov.
    Instructions. Direct your comments to Docket ID No. EPA-HQ-OAR-
2020-0560. 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 electronically any information that you consider
to be CBI or other information whose disclosure is restricted by
statue. 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.
    The EPA is temporarily suspending its Docket Center and Reading
Room for public visitors, with limited exceptions, to reduce the risk
of transmitting COVID-19. Our Docket Center staff will continue to
provide remote customer service via email, phone, and webform. We
encourage the public to submit comments via https://www.regulations.gov/ as there may be a delay in processing mail and
faxes. Hand deliveries or couriers will be received by scheduled
appointment only. For further information and updates on EPA Docket
Center services, please visit us online at https://www.epa.gov/dockets.
    The EPA continues to carefully and continuously monitor information
from the CDC, local area health departments, and our Federal partners
so that we can respond rapidly as conditions change regarding COVID-19.
    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-2020-0560. Note that
written comments containing CBI and submitted by mail may be delayed
and no hand deliveries will be accepted.
    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:

AEGL acute exposure guideline level two
AERMOD air dispersion model used by the HEM-3 model
CAA Clean Air Act
CalEPA California EPA
CBI Confidential Business Information
CDX Central Data Exchange
CEDRI Compliance and Emissions Data Reporting Interface
CFR Code of Federal Regulations
ECHO EPA's Enforcement and Compliance History Online database
EPA Environmental Protection Agency
ERPG emergency response planning guidelines
ERT Electronic Reporting Tool
GACT generally available control technology
HAP hazardous air pollutant(s)
HCl hydrochloric acid
HEM-3 Human Exposure Model, Version 1.5.5
HF hydrogen fluoride
HI hazard index
HQ hazard quotient
ICR Information Collection Request
IRIS EPA's Integrated Risk Information System
km kilometer
MACT maximum achievable control technology
MIR maximum individual risk
NAAQS National Ambient Air Quality Standards
NAICS North American Industry Classification System
NEI National Emissions Inventory
NESHAP national emission standards for hazardous air pollutants
NOAEL No Observed Adverse Effect Level
NOCS Notification of Compliance Status report
NRDC Natural Resources Defense Council
NSPS new source performance standards
OMB Office of Management and Budget
OSHA Occupational Safety and Health Administration
PB-HAP hazardous air pollutants known to be persistent and bio-
accumulative in the environment
PDF portable document format
PM particulate matter
POM polycyclic organic matter
ppm parts per million
PRA Paperwork Reduction Act
REL reference exposure level
RfC reference concentration
RTR residual risk and technology review
SAB Science Advisory Board
SSM startup, shutdown, and malfunction
SV screening value
TOSHI target organ-specific hazard index
tpy tons per year
TRIM.FaTE Total Risk Integrated Methodology. Fate, Transport, and
Ecological Exposure model
UF uncertainty factor
URE unit risk estimate
USGS U.S. Geological Survey

    Organization of this document. The information in this preamble is
organized as follows:

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 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 pursuant to CAA sections 112(d)(2)
and 112(d)(3)?
    B. What are the results of the risk assessment and analyses?
    C. What are our proposed decisions regarding risk acceptability,
ample margin of safety, and adverse environmental effect?
    D. What are the results and proposed decisions based on our
technology review?
    E. What other actions are we proposing?
    F. What compliance dates are we proposing?
V. Summary of Cost, Environmental, and Economic Impacts
    A. What are the affected sources?
    B. What are the air quality impacts?
    C. What are the cost impacts?
    D. What are the economic impacts?
    E. What are the benefits?
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)
    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?

    The source category that is the subject of this proposal is Mercury
Cell Chlor-Alkali Plants regulated under 40 CFR part 63, subpart IIIII.
The North American Industry Classification System (NAICS) code for the
chlor-alkali industry is 325180. 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.
    In the Initial List of Categories of Sources Under Section
112(c)(1) of the Clean Air Act Amendments of 1990 (see 57 FR 31576,
July 16, 1992) and Documentation for Developing the Initial Source
Category List, Final Report (see EPA-450/3-91-030, July 1992), the EPA
listed the Chlorine Production source category. Subsequently, on
December 19, 2003, the EPA divided the Chlorine Production source
category into two subcategories because of the differences in the
production methods and the HAP emitted. These subcategories are: (1)
Mercury cell chlor-alkali plants; and (2) chlorine production plants
that do not rely upon mercury cells for chlorine production (e.g.,
diaphragm cell chlor-alkali plants, membrane cell chlor-alkali plants,
etc.). The EPA issued separate final actions in December 2003 to
address emissions of mercury from the mercury cell chlor-alkali plant
subcategory sources (68 FR 70904) and deleted the non-mercury cell
subcategory (68 FR 70948). This action addresses the Mercury Cell
Chlor-Alkali Plant source category, where a mercury cell chlor-alkali
plant is any facility where mercury cells are used to manufacture
product chlorine, product caustic, and by-product hydrogen and where
mercury may be recovered from wastes.

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/mercury-cell-chloralkali-plants-national-emissions-standards. 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.
    The proposed changes to the CFR that would be necessary to
incorporate the changes proposed in this action are set out in an
attachment to the memorandum titled Proposed Regulation Edits for 40
CFR part 63, subpart IIIII, available in the docket for this action
(Docket ID No. EPA-HQ-OAR-2020-0560). The document includes the
specific proposed amendatory language for revising the CFR and, for the
convenience of interested parties, a redline version of the regulation.
Following signature by the EPA Administrator, the EPA will also post a
copy of this memorandum and the attachments to https://www.epa.gov/stationary-sources-air-pollution/mercury-cell-chloralkali-plants-national-emissions-standards.

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 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 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 and revise the
standards as necessary taking into account any ``developments in
practices, processes, or control technologies.'' This review is
commonly referred to as the ``technology review.'' When the two reviews
are combined into a single rulemaking, it is commonly referred to as
the ``risk and technology review.'' The discussion that follows
identifies the most relevant statutory sections and briefly explains
the contours of the methodology used to implement these statutory
reqruirements. 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 defined in CAA section 112(a)(1) as those that
emit or have the potential to emit 10 tons per year (tpy) or more of a
single HAP or 25 tpy or more of any combination of HAP. All other
sources (not including motor vehicles or nonroad vehicles) are ``area
sources,'' as defined in CAA section 112(a)(2). 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.''
In certain instances, as provided in CAA section 112(h), the EPA may
set work practice standards in lieu of numerical emission standards.
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. 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 pursuant 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 Residual Risk Report that the Agency intended
to use the Benzene NESHAP approach in making CAA section 112(f)
residual risk determinations (EPA-453/R-99-001, p. ES-11). The EPA
subsequently adopted this approach in its residual risk determinations
and the United States Court of Appeals for the District of Columbia
Circuit upheld the EPA's interpretation that CAA section 112(f)(2)
incorporates the approach established in the Benzene NESHAP. See NRDC
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)
\1\ of approximately 1- in-10 thousand.'' (54 FR 38045). 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, we consider whether a more stringent standard is
necessary to prevent, taking into consideration costs, energy, safety,
and other relevant factors, an ``adverse environmental effect'' as
defined in CAA section 112(a)(7).
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    \1\ 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.
---------------------------------------------------------------------------

    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). The EPA is required to
address regulatory gaps, such as missing standards for listed HAP known
to be emitted from the source category. Louisiana Environmental Action
Network (LEAN) v. EPA, 955 F.3d 1088 (D.C. Cir. 2020).

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

    The Chlorine Production source category was initially listed as a
category of major sources of HAP pursuant to section 112(c)(1) of the
CAA on July 16, 1992 (57 FR 31576). At the time of the initial listing,
the EPA defined the Chlorine Production source category as follows:

    The Chlorine Production Source Category includes any facility
engaged in the production of chlorine. The category includes, but is
not limited to, facilities producing chlorine by the following
production methods: Diaphragm cell, mercury cell, membrane cell,
hybrid fuel cell, Downs cell, potash manufacture, hydrochloric acid
decomposition, nitrosyl chloride process, nitric acid/salt process,
Kel-Chlor process, and sodium chloride/sulfuric acid process.\2\
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    \2\ Documentation for Developing the Initial Source Category
List. U.S. Environmental Protection Agency. EPA-450/3-91-030. July
1992. p. A-67. Available at: https://www3.epa.gov/ttn/atw/socatlst/socatpg.html.

    Based on the differences in the production methods and the HAP
emitted, the EPA decided to divide the Chlorine Production source
category into two subcategories: (1) Mercury cell chlor-alkali plants;
and (2) chlorine production plants that do not rely upon mercury cells
for chlorine production (diaphragm cell chlor-alkali plants, membrane
cell chlor-alkali plants, etc.). On July 3, 2002, the EPA issued
separate proposals to address emissions of mercury from the mercury
cell chlor-alkali plant subcategory sources (67 FR 44672) and emissions
of chlorine and hydrochloric acid (HCl) from both subcategories (67 FR
44713). Separate final actions were taken on both proposals on December
19, 2003. As part of these separate final actions, the EPA deleted the
non-mercury cell subcategory under the authority of CAA section
112(c)(9)(B)(ii) of the CAA (68 FR 70948).
    The final rule for the Mercury Cell Chlor-Alkali Plants subcategory
(68 FR 70904, December 19, 2003, codified at 40 CFR part 63 subpart
IIIII), which covers both major and area sources, included standards
for mercury emissions from two types of affected sources at plant sites
where chlorine and caustic are produced in mercury cells: Mercury cell
chlor-alkali production facilty affected sources and mercury recovery
facility affected sources. The rule prohibits mercury emissions from
new and reconstructed mercury cell chlor-alkali production facilities.
40 CFR 63.8190(a)(1). For existing mercury cell chlor-alkali production
facilities, the standards include emission limitations for mercury
emissions from process vents (including emissions from end-box
ventilation systems and hydrogen systems) and work practices for
fugitive mercury emissions from the cell room. 40 CFR 8190(a)(2),
8192(a) through (f). For new, reconstructed, and existing mercury
recovery facilities, the NESHAP includes emission limitations for
mercury emissions from oven type thermal recovery unit vents and non-
oven type thermal recovery unit vents. 40 CFR 63.8190(a)(3). The rule
did not include standards for chlorine or HCl, citing the authority of
section 112(d)(4) of the CAA (68 FR 70906). In its 2003 action (68 FR
70904), the EPA promulgated the initial Mercury Cell Chlor-Alkali
Plants NESHAP pursuant to CAA section 112(d)(2) and added the source
category to the EPA's Source Category List under CAA sections
112(c)(1), as well as under (c)(3) and (k)(3)(B) and (c)(6), in each
case because of the mercury emissions.

    Following promulgation of the 2003 Mercury Cell Chlor-Alkali Plants
NESHAP, the EPA received a petition to reconsider several aspects of
the rule from the Natural Resources Defense Council (NRDC). NRDC also
filed a petition for judicial review of the rule in the U.S. Court of
Appeals for the District of Columbia Circuit. In a letter dated April
8, 2004, the EPA granted NRDC's petition for reconsideration and on
July 20, 2004, the court placed the petition for judicial review in
abeyance pending the EPA's action on reconsideration. The EPA issued
proposals on June 11, 2008 (73 FR 33258), and on March 14, 2011 (76 FR
13852), to respond to the reconsideration petition. We discuss the
reconsideration and the 2008 and 2011 proposals further in section
IV.A.2 of this preamble.
    The use of mercury cell technology has been declining for decades
due to conversions to non-mercury processes and closures. For example,
in 1993, there were about 13 facilities in the U.S., and when we
initiated the development of this RTR proposed rule in early 2020,
there were two facilities operating. Since that time, one facility
(Ashta Chemicals in Ohio) ceased operating the mercury cell process.\3\
So, now only one mercury cell chlor-alkali plant remains in operation.
The one remaining mercury cell chlor-alkali facility is owned by
Westlake Chemical (operated by Eagle Natrium, LLC) and is located in
Marshall County, West Virginia. This is a large integrated chemical
production facility whose products include chlorine and caustic from
their chlor-alkali processes. In addition to the mercury cell process,
chlorine and caustic are also produced in diagraghm cells at the site.
---------------------------------------------------------------------------

    \3\ Ashta Chemicals in Ashtabula, Ohio, has stopped operating
the mercury cell process, and is on schedule to complete the
conversion to membrane cells by end of 2020. Source: Personal
communication, phone conversation: Between Brittany Johnson,
Environmental Manager, Ashta Chemicals and Phil Norwood, SC&A,
Contractor for U.S. EPA, December 4, 2020.
---------------------------------------------------------------------------

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

    Data sources used for this effort include the 2017 National
Emissions Inventory (NEI), title V permit information, conversations
with the West Virginia Department of Environmental Protection, and
conversations with facility representatives. The NEI data were
examined, and the processes and related emission sources associated
with the mercury cell chlor-alkali plant were identified. In addition,
information from data collection efforts from previous regulatory
efforts for the source category were consulted, including studies that
were conducted for the 2002 proposals, the 2003 final actions, and the
2008 and 2011 proposals cited above.

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

    There are other sources that are often used by the EPA in obtaining
information for RTRs. Examples include the EPA's Enforcement and
Compliance History Online (ECHO) database, the Reasonably Available
Control Technology/Best Available Control Technology/Lowest Achievable
Emission Rate Clearinghouse, and NESHAP for similar industries.
However, these sources were not utilized for the review for the Mercury
Cell Chlor-Alkali Plants NESHAP because (1) the mercury cell processes
are primarily sources of fugitive emissions and are unique such that
control measures and work practices from other industries would not be
applicable, and (2) since there is only one operating facility, it was
more practical to focus on the specifics of that single facility.

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 at
38046). 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 emissions of HAP
that are carcinogens from each source in the source category, the
hazard index (HI) for chronic exposures to HAP with the potential to
cause noncancer health effects, and the hazard quotient (HQ) for acute
exposures to HAP with the potential to cause noncancer health
effects.\4\ The assessment also provides estimates of the distribution
of cancer risk within the exposed populations, cancer incidence, and an
evaluation of the potential for an adverse environmental effect. The
scope of the EPA's risk analysis is consistent with the explanation in
the EPA's response to comments on our policy under the Benzene NESHAP:
---------------------------------------------------------------------------

    \4\ The MIR is defined as the cancer risk associated with a
lifetime of exposure at the highest concentration of HAP where
people are likely to live. The HQ is the ratio of the potential HAP
exposure concentration to the noncancer dose-response value; the HI
is the sum of HQs for HAP that affect the same target organ or organ
system.

    The policy chosen by the Administrator permits consideration of
multiple measures of health risk. Not only can the MIR figure be
considered, but also incidence, the presence of non-cancer health
effects, and the uncertainties of the risk estimates. In this way,
the effect on the most exposed individuals can be reviewed as well
as the impact on the general public. These factors can then be
weighed in each individual case. This approach complies with the
Vinyl Chloride mandate that the Administrator ascertain an
acceptable level of risk to the public by employing his expertise to
assess available data. It also complies with the Congressional
intent behind the CAA, which did not exclude the use of any
particular measure of public health risk from the EPA's
consideration with respect to CAA section 112 regulations, and
thereby implicitly permits consideration of any and all measures of
health risk which the Administrator, in his judgment, believes are
---------------------------------------------------------------------------
appropriate to determining what will ``protect the public health''.

(54 FR at 38057). 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. In other words, risks that include
an MIR above 100-in-1 million may be determined to be acceptable, and
risks with an MIR below that level may be determined to be
unacceptable, depending on all of the available health information.
Similarly, with regard to the ample margin of safety analysis, the EPA
stated in the Benzene NESHAP that: ``EPA believes the relative weight
of the many factors that can be considered in selecting an ample margin
of safety can only be determined for each specific source category.
This occurs mainly because technological and economic factors (along
with the health-related factors) vary from source category to source
category.'' 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.'' \5\
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    \5\ Recommendations of the SAB Risk and Technology Review
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.
---------------------------------------------------------------------------

    In response to the SAB recommendations, the EPA incorporates
cumulative risk analyses into its RTR risk assessments. The Agency (1)
conducts facility-wide assessments, which include source category
emission points, as well as other emission points within the
facilities; (2) combines exposures from multiple sources in the same
category that could affect the same individuals; and (3) for some
persistent and bioaccumulative pollutants, analyzes the ingestion route
of exposure. In addition, the RTR risk assessments consider aggregate
cancer risk from all carcinogens and aggregated noncancer HQs for all
noncarcinogens affecting the same target organ or target organ system.
    Although we are interested in placing source category and facility-
wide HAP risk in the context of total HAP risk from all sources
combined in the vicinity of each source, we 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 primarily focuses on the identification and
evaluation of developments in practices, processes, and control
technologies that have occurred since the MACT standards were
promulgated. Where we identify such developments, we analyze their
technical feasibility, estimated costs, energy implications, and non-
air environmental impacts. We also consider the emission reductions
associated with 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
the NESHAP, we review a variety of data sources in our investigation of
potential practices, processes, or controls to consider. We also review
the NESHAP and the available data to determine if there are any
unregulated emissions of HAP within the source category and evaluate
this data for use in developing new emission standards. 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 Mercury Cell Chlor-Alkali Plant Source Category in
Support of the 2020 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; \6\ and described in the
SAB review report issued in 2010. They are also consistent with the key
recommendations contained in that report.
---------------------------------------------------------------------------

    \6\ 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.
---------------------------------------------------------------------------

1. How did we estimate actual emissions and identify the emissions
release characteristics?
    The HAP emissions from the single mercury cell chlor-alkali plant
includes mercury and chlorine. Hydrochloric acid historically had been
associated with these facilities, but based on recent reviews of
available information and discussions with Westlake Chemical, we
conclude that any HCl emissions from the remaining operating facility
in West Virginia are due to non-source category emissions sources such
as HCl production operations (i.e., they are not emitted by an affected
source subject to the standards applicable to mercury cell chlor-alkali
plants). The mercury emissions are emitted from several emission
sources within the mercury cell chlor-alkali facility affected source
at the one operating mercury cell chlor-alkali plant, which, for the
purposes of the source category risk assessment, have been categorized
into two general emission process groups: (1) Process vents and (2)
fugitives from the mercury cell room building. Based on available data,
we conclude the chlorine emissions are mostly or entirely emitted as
fugitive emissions associated with the cell room or from pipes or other
equipment used to pump the product chlorine to the chlorine storage
units or other associated equipment in the mercury cell chlor-alkali
facility affected source. The main source of emissions data used in our
analyses was the NEI data submitted for calendar year 2017. Data on the
numbers, types, dimensions, and locations of the emission points and
non-point sources for each facility were obtained from the NEI and
Google Earth\TM\. A description of the data, approach, and rationale
used to develop actual HAP emissions estimates is discussed in more
detail in the document, Development of the Residual Risk Review
Emissions Dataset for the Mercury Cell Chlor-Alkali Plants Source
Category, which is available in the docket (Docket ID No. EPA-HQ-OAR-
2020-0560).
2. How did we estimate MACT-allowable emissions?
    The available emissions data in the RTR emissions dataset include
estimates of the mass of HAP emitted during a specified annual time
period. These ``actual'' emission levels are often lower than the
emission levels allowed under the requirements of the current MACT
standards. The emissions allowed under the MACT standards are referred
to as the ``MACT-allowable'' emissions. We discussed the consideration
of both MACT-allowable and actual emissions in the final Coke Oven
Batteries RTR (70 FR 19992, 19998 through 19999, April 15, 2005) and in
the proposed and final Hazardous Organic NESHAP RTR (71 FR 34421,
34428, June 14, 2006, and 71 FR 76603, 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)
    For the Mercury Cell Chlor-Alkali Plants source category, the EPA
assumed actual emissions are equal to allowable emissions. Allowable
emissions are the estimated emissions that would occur under normal
full-capacity operating conditions and as allowed under the applicable
MACT standards. There is no available data that suggests the facility
is operating at less than full capacity. There is also no evidence that
the facility is controlling point source emissions to a degree greater
than the emission limitations or that they are performing practices in
excess of the required work practices for the control of fugitive
emissions. This means that they are not reducing emissions beyond the
levels required by the MACT standards which would result in actual
emissions being less than allowable emissions. In addition, a review of
ECHO indicates no enforcement actions for violations of the title V
operating limits over the last 5 years, which would result in actual
emissions being greater than allowable. Therefore, we are comfortable
with the assumption that actual emissions are equal to the allowable
emissions.
3. How do we conduct dispersion modeling, determine inhalation
exposures, and estimate individual and population inhalation risk?
    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).\7\ 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.
---------------------------------------------------------------------------

    \7\ For more information about HEM-3, go to https://www.epa.gov/fera/risk-assessment-and-modeling-human-exposure-model-hem.
---------------------------------------------------------------------------

a. Dispersion Modeling
    The air dispersion model AERMOD, used by the HEM-3 model, is one of
the EPA's preferred models for assessing air pollutant concentrations
from industrial facilities.\8\ 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 (2016) 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 \9\
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.
---------------------------------------------------------------------------

    \8\ 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).
    \9\ A census block is the smallest geographic area for which
census statistics are tabulated.
---------------------------------------------------------------------------

b. Risk From Chronic Exposure to HAP
    In developing the risk assessment for chronic exposures, we use the
estimated annual average ambient air concentrations of each HAP emitted
by each source in the source category. The HAP air concentrations at
each nearby census block centroid located within 50 km of the facility
are a surrogate for the chronic inhalation exposure concentration for
all the people who reside in that census block. A distance of 50 km is
consistent with both the analysis supporting the 1989 Benzene NESHAP
(54 FR 38044) and the limitations of Gaussian dispersion models,
including AERMOD.
    For each facility, we calculate the MIR as the cancer risk
associated with 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 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 \10\ 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.
---------------------------------------------------------------------------

    \10\ The 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 the
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.
---------------------------------------------------------------------------

    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. 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,\11\ we
revised our treatment of meteorological data to use reasonable worst-
case air dispersion conditions in our acute risk screening assessments
instead of worst-case air dispersion conditions. This revised treatment
of meteorological data and the supporting rationale are described in
more detail in Residual Risk Assessment for the Mercury Cell Chlor-
Alkali Plant Source Category in Support of the 2020 Risk and Technology
Review Proposed Rule, and in Appendix 5 of the report:

Technical Support Document for Acute Risk Screening Assessment. This
revised approach has been used in this proposed rule and in all other
RTR rulemakings proposed on or after June 3, 2019.
---------------------------------------------------------------------------

    \11\ 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).
---------------------------------------------------------------------------

    To assess the potential acute risk to the maximally exposed
individual, we use the peak hourly emission rate for each emission
point,\12\ reasonable worst-case air dispersion conditions (i.e., 99th
percentile), and the point of highest off-site exposure. Specifically,
we assume that peak emissions from the source category and reasonable
worst-case air dispersion conditions co-occur and that a person is
present at the point of maximum exposure.
---------------------------------------------------------------------------

    \12\ In the absence of hourly emission data, we develop
estimates of maximum hourly emission rates by multiplying the
average actual annual emissions rates by a factor (either a
category-specific factor or a default factor of 10) to account for
variability. This is documented in Residual Risk Assessment for
Mercury Cell Chlor-alkali Plants Source Category in Support of the
2020 Risk and Technology Review Proposed Rule, and in Appendix 5 of
the report: Technical Support Document for Acute Risk Screening
Assessment. Both are available in the docket for this rulemaking.
---------------------------------------------------------------------------

    To characterize the potential health risks associated with
estimated acute inhalation exposures to a HAP, we generally use
multiple acute dose-response values, including acute RELs, acute
exposure guideline levels (AEGLs), and emergency response planning
guidelines (ERPG) for 1-hour exposure durations, if available, to
calculate acute HQs. The acute HQ is calculated by dividing the
estimated acute exposure concentration by the acute dose-response
value. For each HAP for which acute dose-response values are available,
the EPA calculates acute HQs.
    An acute REL is defined as ``the concentration level at or below
which no adverse health effects are anticipated for a specified
exposure duration.'' \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.
---------------------------------------------------------------------------

    \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).
---------------------------------------------------------------------------

    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 one
hour without experiencing or developing irreversible or other serious
health effects or symptoms which could impair an individual's ability
to take protective action.'' Id. at 1.
---------------------------------------------------------------------------

    \15\ ERPGS Procedures and Responsibilities. March 2014. American
Industrial Hygiene Association. Available at: https://www.aiha.org/get-involved/AIHAGuidelineFoundation/EmergencyResponsePlanningGuidelines/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 this source category, we used a default acute emissions
multiplier of 10 as we have no information to suggest another factor to
account for variability in hourly emissions data is more appropriate.
    In our acute inhalation screening risk assessment, acute impacts
are deemed negligible for HAP for which acute HQs are less than or
equal to 1, and no further analysis is performed for these HAP. In
cases where an acute HQ from the screening step is greater than 1, we
assess the site-specific data to ensure that the acute HQ is at an off-
site location. For this source category, the data refinements employed
consisted of estimating the highest HQ that might occur outside
facility boundaries with the use of satellite imagery of the facility
with receptor locations. These refinements are discussed more fully in
the Residual Risk Assessment for the Mercury Cell Chlor-Alkali Plant
Source Category in Support of the 2020 Risk and Technology Review
Proposed Rule, which is available in the docket for this source
category.
4. How do we conduct the multipathway exposure and risk screening
assessment?
    The EPA conducts a tiered screening assessment examining the
potential for significant human health risks due to exposures via
routes other than inhalation (i.e., ingestion). We first determine
whether any sources in the source category emit any HAP known to be
persistent and bioaccumulative in the environment, as identified in the
EPA's Air Toxics Risk Assessment Library (see Volume 1, Appendix D, at
https://www.epa.gov/fera/risk-assessment-and-modeling-air-toxics-risk-assessment-reference-library).
    For the Mercury Cell Chlor-Alkali Plant source category, mercury
emissions were the only PB-HAP emitted by the source category, so we

proceeded to the next step of the evaluation. Except for lead, the
human health risk screening assessment for PB-HAP consists of three
progressive tiers. In a Tier 1 screening assessment, we determine
whether the magnitude of the facility-specific emissions of PB-HAP
warrants further evaluation to characterize human health risk through
ingestion exposure. To facilitate this step, we evaluate emissions
against previously developed screening threshold emission rates for
several PB-HAP that are based on a hypothetical upper-end screening
exposure scenario developed for use in conjunction with the EPA's Total
Risk Integrated Methodology. Fate, Transport, and Ecological Exposure
(TRIM.FaTE) model. The PB-HAP with screening threshold emission rates
are arsenic compounds, cadmium compounds, chlorinated dibenzodioxins
and furans, mercury compounds, and polycyclic organic matter (POM).
Based on the EPA estimates of toxicity and bioaccumulation potential,
these pollutants represent a conservative list for inclusion in
multipathway risk assessments for RTR rules. (See Volume 1, Appendix D
at https://www.epa.gov/sites/production/files/2013-08/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 (SV).''
    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 SV is greater than 1), we conduct a second
screening assessment, which we call the Tier 2 screening assessment.
The Tier 2 screening assessment separates the Tier 1 combined fisher
and farmer exposure scenario into fisher, farmer, and gardener
scenarios that retain upper-bound ingestion rates.
    In the Tier 2 screening assessment, the location of each facility
that exceeds a Tier 1 screening threshold emission rate is used to
refine the assumptions associated with the Tier 1 fisher and farmer
exposure scenarios at that facility. A key assumption in the Tier 1
screening assessment is that a lake and/or farm is located near the
facility. As part of the Tier 2 screening assessment, we use a U.S.
Geological Survey (USGS) database to identify actual waterbodies within
50 km of each facility and assume the fisher only consumes fish from
lakes within that 50 km zone. We also examine the differences between
local meteorology near the facility and the meteorology used in the
Tier 1 screening assessment. We then adjust the previously-developed
Tier 1 screening threshold emission rates for each PB-HAP for each
facility based on an understanding of how exposure concentrations
estimated for the screening scenario change with the use of local
meteorology and the USGS lakes database.
    In the Tier 2 farmer scenario, we maintain an assumption that the
farm is located within 0.5 km of the facility and that the farmer
consumes meat, eggs, dairy, vegetables, and fruit produced near the
facility. We may further refine the Tier 2 screening analysis by
assessing a gardener scenario to characterize a range of exposures,
with the gardener scenario being more plausible in RTR evaluations.
Under the gardener scenario, we assume the gardener consumes home-
produced eggs, vegetables, and fruit products at the same ingestion
rate as the farmer. The Tier 2 screen continues to rely on the high-end
food intake assumptions that were applied in Tier 1 for local fish
(adult female angler at 99th percentile fish consumption \16\) and
locally grown or raised foods (90th percentile consumption of locally
grown or raised foods for the farmer and gardener scenarios \17\). If
PB-HAP emission rates do not result in a Tier 2 SV greater than 1, we
consider those PB-HAP emissions to pose risks below a level of concern.
If the PB-HAP emission rates for a facility exceed the Tier 2 screening
threshold emission rates, we may conduct a Tier 3 screening assessment.
---------------------------------------------------------------------------

    \16\ Burger, J. 2002. Daily consumption of wild fish and game:
Exposures of high end recreationists. International Journal of
Environmental Health Research, 12:343-354.
    \17\ U.S. EPA. Exposure Factors Handbook 2011 Edition (Final).
U.S. Environmental Protection
    Agency, Washington, DC, EPA/600/R-09/052F, 2011.
---------------------------------------------------------------------------

    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.
    For further information on the multipathway assessment approach,
see the Residual Risk Assessment for the Mercury Cell Chlor-Alkali
Plant Source Category in Support of the Risk and Technology Review 2020
Proposed Rule, which is available in the docket for this action.
5. How do we conduct the environmental risk screening assessment?
a. Adverse Environmental Effect, Environmental HAP, and Ecological
Benchmarks
    The EPA conducts a screening assessment to examine the potential
for an adverse environmental effect as required under 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 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 Residual Risk
Assessment for the Mercury Cell Chlor-Alkali Plant Source Category in
Support of the Risk and Technology Review 2020 Proposed Rule, which is
available in the docket for this action.
b. Environmental Risk Screening Methodology
    For the environmental risk screening assessment, the EPA first
determined whether any facilities in the Mercury Cell Chlor-Alkali
Plant source category emitted any of the environmental HAP. For the
Mercury Cell Chlor-Alkali Plant source category, we identified
emissions of mercury and HCl. 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 National Ambient Air Quality Standards (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 square kilometers;
the percentage of the modeled area around each facility that exceeds
the ecological benchmark for each acid gas; and the area-weighted
average SV around each facility (calculated by dividing the area-
weighted average concentration over the 50-km modeling domain by the
ecological benchmark for each acid gas). For further information on the
environmental screening assessment approach, see Appendix 9 of the
Residual Risk Assessment for the Mercury Cell Chlor-Alkali Plant Source
Category in Support of the Risk and Technology Review 2020 Proposed
Rule, which is available in the docket for this action.
6. How do we conduct facility-wide assessments?
    To put the source category risks in context, we typically examine
the risks

from the entire ``facility,'' where the facility includes all HAP-
emitting operations within a contiguous area and under common control.
In other words, we examine 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 2014 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. The facility-wide file was then
used to analyze risks due to the inhalation of HAP that are emitted
``facility-wide'' for the populations residing within 50 km of each
facility, consistent with the methods used for the source category
analysis described above. For these facility-wide risk analyses, the
modeled source category risks were compared to the facility-wide risks
to determine the portion of the facility-wide risks that could be
attributed to the source category addressed in this proposal. We also
specifically examined the facility that was associated with the highest
estimate of risk and determined the percentage of that risk
attributable to the source category of interest. The Residual Risk
Assessment for the Mercury Cell Chlor-Alkali Plant Source Category in
Support of the Risk and Technology Review 2020 Proposed Rule, available
through the docket for this action, 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.
    For this source category, we conducted the facility-wide assessment
using a dataset that the EPA compiled from the 2017 NEI. We used the
NEI data for the facility and did not adjust any category or ``non-
category'' data. Therefore, there could be differences in the dataset
from that used for the source category assessments described in this
preamble. We analyzed 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, we
made a reasonable attempt to identify the source category risks, and
these risks were compared to the facility-wide risks to determine the
portion of facility-wide risks that could be attributed to the source
category addressed in this proposal. We also specifically examined the
facility that was associated with the highest estimate of risk and
determined the percentage of that risk attributable to the source
category of interest. The Residual Risk Assessment for the Mercury Cell
Chlor-Alkali Plant Source Category in Support of the Risk and
Technology Review 2020 Proposed Rule, available through the docket for
this action, 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 Residual Risk Assessment for the
Mercury Cell Chlor-Alkali Plant Source Category in Support of the Risk
and Technology Review 2020 Proposed Rule, which is available in the
docket for this action. 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 the EPA's 2005 Guidelines for Carcinogen Risk Assessment;
namely, that ``the primary goal of EPA actions is protection of human
health; accordingly, as an Agency policy, risk assessment procedures,
including default options that are used in the absence of scientific
data to the contrary, should be health protective'' (the EPA's 2005
Guidelines for Carcinogen Risk Assessment, pages 1 through 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.\18\
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.\19\
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,\20\ 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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    \18\ IRIS glossary (https://ofmpub.epa.gov/sor_internet/registry/termreg/searchandretrieve/glossariesandkeywordlists/search.do?details=&glossaryName=IRIS%20Glossary).
    \19\ 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.
    \20\ 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.
    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 actual exposure scenario. In most
cases, it is unlikely that a person would be located at the point of
maximum exposure during the time when peak emissions and reasonable
worst-case air dispersion conditions occur simultaneously.
f. Uncertainties in the Multipathway and Environmental Risk Screening
Assessments
    For each source category, we generally rely on site-specific levels
of PB-HAP or environmental HAP emissions to determine whether a refined
assessment of the impacts from multipathway exposures is necessary or
whether it is necessary to perform an environmental screening
assessment. This determination is based on the results of a three-
tiered screening assessment that relies on the outputs from models--
TRIM.FaTE and AERMOD--that estimate environmental pollutant
concentrations and human exposures for five PB-HAP (dioxins, POM,
mercury, cadmium, and arsenic) and two acid gases (HF and HCl). 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.\21\
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    \21\ 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 RTRs.
    Input uncertainty is concerned with how accurately the models have
been configured and parameterized for the assessment at hand. For Tier
1 of the multipathway and environmental screening assessments, we
configured the models to avoid underestimating exposure and risk. This
was accomplished by selecting upper-end values from nationally
representative datasets for the more influential parameters in the
environmental model, including selection and spatial configuration of
the area of interest, lake location and size, meteorology, surface
water, soil characteristics, and structure of the aquatic food web. We
also assume an ingestion exposure scenario and values for human
exposure factors that represent reasonable maximum exposures.
    In Tier 2 of the multipathway and environmental screening
assessments, we refine the model inputs to account for meteorological
patterns in the vicinity of the facility versus using upper-end
national values, and we identify the actual location of lakes near the
facility rather than the default lake location that we apply in Tier 1.
By refining the screening approach in Tier 2 to account for local
geographical and meteorological data, we decrease the likelihood that
concentrations in environmental media are overestimated, thereby
increasing the usefulness of the screening assessment. In Tier 3 of the
screening assessments, we refine the model inputs again to account for
hour-by-hour plume-rise and the height of the mixing layer. We can also
use those hour-by-hour meteorological data in a TRIM.FaTE run using the
screening configuration corresponding to the lake location. These
refinements produce a more accurate estimate of chemical concentrations
in the media of interest, thereby reducing the uncertainty with those
estimates. The assumptions and the associated uncertainties regarding
the selected ingestion exposure scenario are the same for all three
tiers.
    For the environmental screening assessment for acid gases, we
employ a single-tiered approach. We use the modeled air concentrations
and compare those with ecological benchmarks.
    For all tiers of the multipathway and environmental screening
assessments, our approach to addressing model input uncertainty is
generally cautious. We choose model inputs from the upper end of the
range of possible values for the influential parameters used in the
models, and we assume that the exposed individual exhibits ingestion
behavior that would lead to a high total exposure. This approach
reduces the likelihood of not identifying high risks for adverse
impacts.
    Despite the uncertainties, when individual pollutants or facilities
do not exceed screening threshold emission rates (i.e., 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, the EPA may
evaluate other relevant HAP in the future, as modeling science and
resources allow.

IV. Analytical Results and Proposed Decisions

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

1. MACT standards for Chlorine Emissions
    In addition to mercury, based on the NEI, the Westlake, West
Virginia, mercury cell chlor-alkali facility emits an estimated 0.24
tpy fugitive emissions of chlorine from the mercury cell chlor-alkali
production facility affected source. Chlorine is not emitted from
mercury thermal recovery units and furthermore, the facility does not
have a mercury thermal recovery unit at the site. In the 2003 final
rule, the EPA made the decision not to regulate chlorine and HCl in the
Mercury Cell Chlor-Alkali Plant NESHAP based on the authority under
section 112(d)(4) of the CAA. Specifically, the EPA based this decision
on the ``determination that no further control is necessary because
chlorine and HCl are ``health threshold pollutants,'' and chlorine and
HCl levels emitted from chlorine production processes are below their
threshold values within an ample margin of safety.'' (68 FR 70906,
December 19, 2003).
    However, the EPA has determined that it must now propose standards
for all HAP emissions from the source category, including emissions of
chlorine, pursuant to CAA section 112(d)(2) and (3).\22\ As discussed
in section III.C.1 above, while there are HCl emissions from the direct
synthesis HCl production units at the Westlake, West Virginia,
facility, they are not from processes that are part of the mercury cell
chlor-alkali plant. Therefore, no emission limitations or work
practices are being proposed for HCl since the emissions are not from
parts of the site that are within the mercury cell chlor-alkali plant.
As a result, we are only required to propose standards for chlorine
emissions pursuant to CAA section 112(d)(2) and (3).
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    \22\ The EPA not only has authority under CAA section 112(d)(2)
and (3) to set MACT standards for previously unregulated HAP
emissions at any time, but is required to address any previously
unregulated HAP emissions as part of its periodic review of MACT
standards under CAA section 112(d)(6). LEAN v. EPA, 955 F.3d at
1091-1099.
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    Fugitive chlorine emissions occur from equipment leaks in the cell
room and throughout the other parts of the mercury cell chlor-alkali
production facility affected source that handle and process the
chlorine gas produced. As stated previously, mercury recovery units are
not sources of chlorine emissions.

    Section 112 of the CAA generally directs that standards be
specified as numerical emission standards, if possible. However, if it
is determined that it is not feasible to prescribe or enforce a
numerical emission standard, CAA section 112(h) indicates that a
design, equipment, work practice, or operational standard may be
specified, provided the criteria of CAA section 112(h)(2) are met.
Those criteria define ``not feasible to prescribe or enforce an
emission standard'' to mean any situation in which the EPA determines
that: (1) A HAP or pollutants cannot be emitted through a conveyance
designed and constructed to emit or capture such pollutant, or that any
requirement for, or use of, such a conveyance would be inconsistent
with any federal, state, or local law, or (2) the application of
measurement methodology to a particular class of sources is not
practicable due to technological and economic reasons. Most fugitive
chlorine emission sources at mercury cell chlor-alkali plants are
associated with cell rooms. Potential fugitive chlorine emissions are
also located in the chlorine processing area. For both the cell room
and the chlorine processing area, the fugitive chlorine emissions are
primarily due to equipment leaks. Due to the nature of equipment leaks
(i.e., low flow rate, occurring from individual pieces of equipment,
high variability in time, and location of occurrence) it is
technologically and economically impractical to collect the emissions
and route them to a control device. As such, we believe that it is not
feasible to either prescribe or enforce numerical emission limit(s) for
fugitive chlorine emissions from cell rooms or any other location at
the facility, under both of the criteria set forth in CAA section
112(h)(2)(A) and (B). Consequently, these proposed standards address
fugitive chlorine emission sources at existing mercury cell chlor-
alkali production facility affected sources through the establishment
of work practice standards. As the NESHAP already effectively prohibits
the construction or reconstruction of a mercury cell chlor-alkali
production facility, there is no need to establish a new source MACT
floor for fugitive chlorine emissions.
    There are many incentives for the identification and correction of
chlorine leaks and to reduce fugitive chlorine emissions throughout the
mecury cell chlor-alkali plant. First, chlorine is a primary product of
the process, so lost chlorine equals lost product and lost profit.
Second, chlorine, particularly ``wet'' chlorine, is very corrosive to
process equipment. Therefore, prompt repair of chlorine leaks reduces
damange to process equipment. These corrosive properties also mean that
small leaks can quickly become large leaks, which could result in
chlorine releases that are dangerous to plant workers and the
surrounding community. For these reasons, the Westlake, West Virginia,
facility has a program in place to identify and repair fugitive
chlorine leaks across the plant. Specifically, Westlake operators
perform inspections during each shift to identify leaks of chlorine.
Therefore, leaks are detected and corrective actions implemented to
minimize and reduce any fugitive chlorine emissions. Based on available
information, we understand that the method Westlake uses to identify
leaks of chlorine from each piece of equipment is olfactory
observations of chlorine gas. If leaks are detected using the olfactory
method, the facility takes immediate actions to fix the identified
leaks. Furthermore, Westlake has chlorine sensors installed and
operated throughout the relevant process units. If one of these sensors
measures a chlorine concentration of 2 parts per million by volume
(ppmv) or greater, the facility takes action to identify and fix leaks.
Since there is only one currently operating mercury cell chlor-alkali
plant in the country, the MACT floor for existing sources is
represented by the practices in place at the Westlake facility to
reduce chlorine fugitive emissions.
    As noted above, it is technologically and economically impractical
to collect the emissions from every potential leak source at a facility
and route them to a control device. The cell room building is generally
under negative pressure and the air is routed through the roof vents.
As a beyond-the-floor option for fugitive chlorine emissions, we
considered requiring the air from the roof vents to be routed to a
scrubber or other control device. However, the volume of the air flow
from the Westlake cell room is over 700 million cubic feet per day, or
almost 500,000 cubic feet per minute. It would be technically
infeasible for any control device to handle this volume of gas
throughput. Therefore, we rejected this beyond-the-floor option.
    Therefore, we are proposing the MACT floor level of control which
represents the procedures in place at the Westlake, West Virginia,
site. We developed the work practices in the proposed amendments to
reflect these procedures, along with associated recordkeeping and
reporting requirements to demonstrate compliance. Specifically, we are
proposing that facilities must identify and inspect each piece of
equipment that contains chlorine gas with a concentration of at least 5
percent chlorine by volume throughout the mercury cell chlor-alkali
production facility affected source for leaks at least once each 12
hours. We are requesting comment on whether the 5 percent by volume
threshold for defining equipment that must be inspected for chlorine
leaks is the appropriate threshold for identifying equipment with the
potential to generate fugitive emissions of chlorine gas. Equipment
that is under negative pressure would be excluded from this
requirement. The method that we are proposing to identify leaks of
chlorine from each piece of equipment is olfactory observations of
chlorine gas. However, we solicit comments regarding other methods
(e.g., auditory or visual) that should also be allowed as a method to
identify leaks.
    When a leak is detected, we are proposing that a first attempt at
repair be conducted within 1 hour of detection and that the leak be
repaired within 1 day of detection. We are proposing that a leak is
repaired when the evidence of the olfactory observation is eliminated.
    Additionally, we are proposing that chlorine sensors be installed
and operated continuously (at least one measure every 15 minutes)
throughout the affected source. Each time one of these sensors measures
a chlorine concentration of 2 ppmv or greater, the proposed rule would
require a complete inspection for leaks of all equipment containing 5
percent chlorine by volume within 1 hour of detection. The chlorine
sensors that the facility uses must have a detection limit of 2 ppm or
less. Furthermore, we propose the sensor must be calibrated and
maintained following the manufacturer's recommendations.
    We are requesting comment on several aspects of the proposed
requirements related to the use of chlorine sensors to identify leaks
that may occur between the 12-hour regular inspections. First, we are
requesting comment on where these ambient sensors should be located to
ensure that chlorine emissions are detected by the ambient monitors.
The proposed rule requires that they be placed throughout the mercury
cell chlor-alkali manufacturing facility affected source, which
includes ``all cell rooms and ancillary operations used in the
manufacture of product chlorine, product caustic, and by-product
hydrogen.'' We are requesting comment whether the rule should specify
areas of the facility where sensors should be located and whether it
should specify a

minimum number of sensors. We are requesting comment on the types
(i.e., detection methodology) of devices that should be used, the
appropriate detection limit for these devices, and whether the devices
should be subject to the continuous parameter monitoring requirements
in 40 CFR 63.8 of the General Provisions of part 63. We are requesting
comment on the appropriate sampling time and whether the proposed
requirement that a measurement be taken every 15 minutes is
appropriate, as well as the proposed 2 ppmv concentration level that
triggers action (i.e., additional inspections). In conjunction, we are
requesting comment on whether action should be required based on a
single measurement above the 2 ppmv action level, or whether it should
be required when measurements averaged over a specified time period
exceed 2 ppmv (e.g., if the one-hour average concentration is greater
than 2 ppmv). Finally, the proposed rule generically requires that
records of all chlorine concentration measurements be maintained. We
are requesting comments on whether the rule should include data
acquisition system and data format requirements, and if so, what
associated requirements might be appropriate.
    The proposed rule would require that initial attempts at corrective
actions of leaks be taken within 1 hour of detection, and the leak be
repaired within 1 day of the date of detection. Records would be
required to document the equipment containing more than 5 percent by
volume of chlorine and the dates and times the inspections occurred.
For each leak identified, records would also be required identifying
the piece of equipment with the leak, the date and time it was
identified, the date and time a first attempt to repair the leak was
performed, the date and time the leak was stopped and repaired, and a
description of the repair made to stop the leak. Records would also be
required of any deviation from these work practices. Also, the number
of leaks found and repaired during the reporting timeframe and any
deviations from the work practices would be included in the periodic
report.
2. Reconsideration Petition and Beyond-the-Floor Analysis for Mercury
    In early 2004, the EPA received a petition for reconsideration
pursuant to CAA section 307(d)(7)(B) and a petition for judicial review
under CAA section 307(b)(1) from the NRDC regarding the 2003 Mercury
Cell Chlor-Alkali MACT standards. In the petition for reconsideration,
NRDC claimed that the EPA failed to conduct the required beyond-the-
floor analysis under CAA section 112(d)(2) regarding whether to
prohibit mercury emissions from existing sources, as the rule did for
new and reconstructed sources. In a letter dated April 8, 2004, the EPA
informed NRDC that it had granted the petition for reconsideration and
would respond to NRDC's petition in a subsequent notice of proposed
rulemaking. On July 20, 2004, the court put the litigation into
abeyance and directed the EPA to file periodic status reports.
    In 2006 and 2007, the EPA conducted a testing program to measure
fugitive mercury emissions at two selected facilities to inform the
reconsideration. The EPA provided final reports regarding the results
of the study to NRDC as required by a joint stipulation filed in the
litigation. Both of the studied facilities are no longer operational.
On June 11, 2008 (73 FR 33258), the EPA published a proposed rule that
provided the EPA's proposed response to the petition for
reconsideration, which would require facilities to install and operate
a continuous mercury monitoring system in the ``upper portions of the
cell room'' and continue to perform the work practice standards (with
reduced recordkeeping and reporting requirements and no floor-level
monitoring). The EPA received comments from Oceana, PPG Industries, the
Chlorine Institute, Olin Chlor-alkali Products, and an anonymous
submittal.
    Subsequently, in 2011, the EPA published a new proposed rulemaking
in response to the petition for reconsideration (76 FR 13852, March 14,
2011). The new proposed rule contained two options that the EPA was
considering. The first option was to require remaining existing
facilities to convert to a non-mercury technology to produce chlorine
as a beyond-the-floor measure under CAA section 112(d)(2). The second
option included the combination of the continuous cell room monitoring
program and work practice program originally proposed in 2008 as a
beyond-the-floor measure. Like for the 2008 proposed rule, the EPA
received a number of comments from various stakeholders both for and
against the 2011 proposed rulemaking. All of the EPA's technical
analyses for the proposed rulemakings, public comments, and other
supporting information regarding the 2008 and 2011 proposals are
available in the docket for the proposals (Docket ID No. EPA-HQ-OAR-
2002-0017). No final action has been taken on the 2008 or 2011
proposals, or to respond to the petition for reconsideration, and the
litigation concerning the 2003 NESHAP remains in abeyance with the EPA
still subject to the court's order to file periodic status reports.
    In conjunction with this proposed RTR action under CAA sections
112(d)(6) and 112(f)(2), the EPA, pursuant to CAA sections 112(d)(2)
and (3), re-evaluated whether a beyond-the-floor requirement that
facilities must convert to a non-mercury technology within 3 years
would still be appropriate based on updated analyses compared to those
supporting the 2011 proposal. In 2011 there were four such facilities
still in operation. Two of these facilities were the subject of the
EPA's studies of fugitive mercury emissions over 2006 and 2007, and
they have since shut down. As described above, only one operating
facility remains in the U.S. that uses the mercury cell process to
produce chlorine. Based on our updated analysis, contained in the
docket for this proposed rule, we estimate the capital costs would be
about $69 million for the one remaining facility to convert to a non-
mercury process. However, there would be savings over time due to the
elimination of compliance costs associated with mercury and the higher
efficiency and energy savings of switching to the membrane technology.
The estimated annual costs, after accounting for the expected savings,
are $2.8 million per year for the one remaining mercury cell facility.
Based on reported mercury emissions, the cost effectiveness of the
conversion is estimated to be $22,000 per pound of mercury emissions
eliminated. However, we also note that the cost-effectiveness estimate
is uncertain because, first, mercury emissions are based on
calculations and assumptions regarding the facility's emissions (no
test data are available for this facility), and second, because there
are uncertainties with the cost estimates from the 2011 proposal as
being transferable to the remaining facility. In the 2011 proposal, the
estimated cost effectiveness was $20,000 per pound for the industry
(see 76 FR 13852, March 14, 2011), but this was substantially based on
the studies conducted for the two no longer operating sources.
    Based on consideration of the updated costs and cost effectiveness
and uncertainties, and given the passage of time, and the fact that the
cost-effectiveness data and analysis done in 2011 were based on two
facilities that are no longer operating, we question whether those 2011
analyses would still be transferable to the one remaining operating
facility. Consequently, we are not proposing in this action to require
the elimination of mercury as a beyond-