[Federal Register Volume 86, Number 10 (Friday, January 15, 2021)]
[Proposed Rules]
[Pages 3906-3927]
[FR Doc No: 2021-00374]

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ENVIRONMENTAL PROTECTION AGENCY

40 CFR Part 63

[EPA-HQ-OAR-2020-0532; FRL-10018-49-OAR]
RIN 2060-AU64

National Emission Standards for Hazardous Air Pollutants: Cyanide
Chemicals Manufacturing 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) for the
National Emission Standards for Hazardous Air Pollutants (NESHAP) for
the Cyanide Chemicals Manufacturing source category as required under
the Clean Air Act (CAA). We are proposing to find that risk from
emissions of air toxics from this source category is acceptable, and
that the current standards provide an ample margin of safety to protect
public health. We are also proposing to find that there are no
developments in practices, processes, and control technologies, and, as
such, we are not proposing any development-based changes to the current
standards pursuant to the technology review. The EPA is, however,
proposing new emissions standards to address emissions from process
wastewater at existing sources. We are proposing to amend provisions
addressing startup, shutdown, and malfunction (SSM), to add electronic
reporting, and to update the reporting and recordkeeping requirements.
We do not expect these proposed amendments to result in changes in
emissions from the source category but anticipate improved monitoring,
compliance, and implementation of the existing standards.

DATES: Comments must be received on or before March 1, 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 16, 2021.
    Public hearing: If anyone contacts us requesting a public hearing
on or before January 21, 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-2020-0532, 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-2020-0532 in the subject line of the message.
     Fax:<PHONE_NUMBER>. Attention Docket ID No. EPA-HQ-OAR-
2020-0532.
     Mail: U.S. Environmental Protection Agency, EPA Docket
Center, Docket ID No. EPA-HQ-OAR-2020-0532, Mail Code 28221T, 1200
Pennsylvania Avenue NW, Washington, DC 20460.
     Hand/Courier Delivery: EPA Docket Center, WJC West
Building, Room 3334, 1301 Constitution Avenue NW, Washington, DC 20004.
The Docket Center's hours of operation are 8:30 a.m.-4:30 p.m., Monday-
Friday (except federal holidays).
    Instructions: All submissions received must include the Docket ID
No. for this rulemaking. Comments received may be posted without change
to https://www.regulations.gov/, including any personal information
provided. For detailed instructions on sending comments and additional
information on the rulemaking process, see the SUPPLEMENTARY
INFORMATION section of this document. 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 Nathan Topham, 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 for
public hearings 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 the public hearing
team at<PHONE_NUMBER> or by email at<EMAIL_ADDRESS>If
requested, the virtual hearing will be held on February 1, 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 at https://www.epa.gov/stationary-sources-air-pollution/acetal-resins-acrylic-modacrylic-fibers-carbon-black-hydrogen.
    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/acetal-resins-acrylic-modacrylic-fibers-carbon-black-hydrogen or contact the public hearing team at<PHONE_NUMBER> or
by email at<EMAIL_ADDRESS>The last

day to pre-register to speak at the hearing will be January 27, 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/acetal-resins-acrylic-modacrylic-fibers-carbon-black-hydrogen.
    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
<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 will
be posted online at https://www.epa.gov/stationary-sources-air-pollution/acetal-resins-acrylic-modacrylic-fibers-carbon-black-hydrogen. 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
22, 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-2020-0532. 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-
2020-0532. 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
statute. 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-2020-0532. 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
AERMOD air dispersion model used by the HEM-3 model
CAA Clean Air Act
CalEPA California EPA
CBI Confidential Business Information
CFR Code of Federal Regulations
EPA Environmental Protection Agency
ERPG emergency response planning guideline
ERT Electronic Reporting Tool
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
IRIS Integrated Risk Information System
km kilometer
MACT maximum achievable control technology
mg/kg-day milligrams per kilogram per day
mg/m3 milligrams per cubic meter
MIR maximum individual risk
NAAQS National Ambient Air Quality Standards
NAICS North American Industry Classification System
NESHAP national emission standards for hazardous air pollutants
NRC National Research Council
OAQPS Office of Air Quality Planning and Standards
OMB Office of Management and Budget
PAH polycyclic aromatic hydrocarbons
PB-HAP hazardous air pollutants known to be persistent and bio-
accumulative in the environment
PM particulate matter
POM polycyclic organic matter
ppm parts per million
REL reference exposure level
RFA Regulatory Flexibility Act
RfC reference concentration
RfD reference dose
RTR residual risk and technology review
SAB Science Advisory Board
SBA Small Business Administration
SSM startup, shutdown, and malfunction
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
[micro]g/m3 microgram per cubic meter
URE unit risk estimate
VCS voluntary consensus standards

    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 its HAP emissions?
    C. What data collection activities were conducted to support
this action?
    D. What other relevant background information and data are
available?
III. Analytical Procedures and Decision-Making
    A. How do we consider risk in our decision-making?
    B. How do we perform the technology review?
    C. How do we estimate post-MACT risk posed by the source
category?
IV. Analytical Results and Proposed Decisions
    A. What actions are we taking 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 cyanide
chemicals manufacturing major sources regulated under 40 CFR 63,
subpart YY. The North American Industry Classification System (NAICS)
codes for the cyanide chemicals manufacturing industry are 325188 and
325199. This list of categories and NAICS codes is not intended to be
exhaustive, but rather provides a guide for readers regarding the
entities that this proposed action is likely to affect. The proposed
standards, once promulgated, will be directly applicable to the
affected sources. Federal, state, local, and tribal government entities
would not be affected by this proposed action. As defined in the
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 Cyanide Chemicals
Manufacturing source category is any facility engaged in the production
of hydrogen cyanide or sodium cyanide. Hydrogen cyanide production
includes, but is not limited to, production of hydrogen cyanide using
any of the following methods: Reaction of methane and ammonia over a
platinum catalyst, reaction of methane and ammonia over a platinum-
rhodium catalyst, co-production with acrylonitrile (via Sohio process),
or pyrolysis of formaldehyde. Sodium cyanide production includes, but
is not limited to, production of sodium cyanide via the neutralization
process, or so-called wet process. In this process, hydrogen cyanide
reacts with sodium hydroxide solution usually in a reactor that
involves evaporation of water and crystallization of the product,
commonly called white cyanide.

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

    In addition to being available in the docket, an electronic copy of
this action is available on the internet. Following signature by the
EPA Administrator, the EPA will post a copy of this proposed action at
https://www.epa.gov/stationary-sources-air-pollution/acetal-resins-acrylic-modacrylic-fibers-carbon-black-hydrogen. Following publication
in the Federal Register, the EPA will post the Federal Register version
of the proposal and key technical documents at this same website.
Information on the overall RTR program is available at https://www3.epa.gov/ttn/atw/rrisk/rtrpg.html.
    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 YY, available in the docket for this action
(Docket ID No. EPA-HQ-

OAR-2020-0532). 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/acetal-resins-acrylic-modacrylic-fibers-carbon-black-hydrogen.

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 hazardous air pollutants (HAP) from stationary
sources. Generally, the first stage involves establishing technology-
based standards and the second stage involves evaluating those
standards that are based on maximum achievable control technology
(MACT) to determine whether additional standards are needed to address
any remaining risk associated with HAP emissions. This second stage is
commonly referred to as the ``residual risk review.'' In addition to
the residual risk review, the CAA also requires the EPA to review
standards set under CAA section 112 every 8 years 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
requirements. A more comprehensive discussion appears in the document
titled CAA Section 112 Risk and Technology Reviews: Statutory Authority
and Methodology, in the docket for this rulemaking.
    In the first stage of the CAA section 112 standard setting process,
the EPA promulgates technology-based standards under CAA section 112(d)
for categories of sources identified as emitting one or more of the HAP
listed in CAA section 112(b). Sources of HAP emissions are either major
sources or area sources, and CAA section 112 establishes different
requirements for major source standards and area source standards.
``Major sources'' are those that emit or have the potential to emit 10
tons per year (tpy) or more of a single HAP or 25 tpy or more of any
combination of HAP. All other sources are ``area sources.'' For major
sources, CAA section 112(d)(2) provides that the technology-based
NESHAP must reflect the maximum degree of emission reductions of HAP
achievable (after considering cost, energy requirements, and non-air
quality health and environmental impacts). These standards are commonly
referred to as MACT standards. CAA section 112(d)(3) also establishes a
minimum control level for MACT standards, known as the MACT ``floor.''
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 (the court) upheld the EPA's interpretation that CAA section
112(f)(2) incorporates the approach established in the Benzene NESHAP.
See NRDC v. EPA, 529 F.3d 1077, 1083 (D.C. Cir. 2008).
    The approach incorporated into the CAA and used by the EPA to
evaluate residual risk and to develop standards under CAA section
112(f)(2) is a two-step approach. In the first step, the EPA determines
whether risks are acceptable. This determination ``considers all health
information, including risk estimation uncertainty, and includes a
presumptive limit on maximum individual lifetime [cancer] risk (MIR)
\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.
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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.
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    CAA section 112(d)(6) separately requires the EPA to review
standards promulgated under CAA section 112 and revise them ``as
necessary (taking into account developments in practices, processes,
and control technologies)'' no less often than every 8 years. In
conducting this review, which we call the ``technology review,'' the
EPA is not required to recalculate the MACT floor. Natural Resources
Defense Council (NRDC) v. EPA, 529 F.3d 1077, 1084 (D.C. Cir. 2008).
Association of Battery Recyclers, Inc. v. EPA, 716 F.3d 667 (D.C. Cir.
2013). The EPA may consider cost in deciding whether to revise the

standards pursuant to CAA section 112(d)(6). The EPA is required to
address regulatory gaps, such as missing standards for listed air
toxics 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 its HAP emissions?

    The MACT standards for the Cyanide Chemicals Manufacturing source
category are contained in the Generic Maximum Achievable Control
Technology (GMACT) NESHAP which also includes MACT standards for
several other source categories. The cyanide chemicals manufacturing
standards were promulgated on July 12, 2002, (67 FR 46258) and codified
at 40 CFR part 63, subpart YY. As promulgated in 2002, the cyanide
chemicals manufacturing standards regulate HAP emissions from cyanide
chemicals manufacturing units located at major sources. The HAP emitted
from the source category include cyanide compounds (hydrogen cyanide
and sodium cyanide), acetonitrile, and acrylonitrile.
    The NESHAP defines the affected source as each cyanide chemicals
manufacturing process unit (CCMPU). The rule states that the CCMPU is
the equipment assembled and connected by hard-piping or duct work to
process raw materials to manufacture, store, and transport a cyanide
chemicals product. A CCMPU shall be limited to any one of the
following: An Andrussow process unit, a Blausaure Methane Anlage
process unit, a sodium cyanide process unit, or a Sohio hydrogen
cyanide process unit. For the purpose of this subpart, a CCMPU includes
reactors and associated unit operations, associated recovery devices,
and any feed, intermediate and product storage vessels, product
transfer racks, and connected ducts and piping. A CCMPU also includes
pumps, compressors, agitators, pressure relief devices, sampling
connection systems, open-ended valves or lines, valves, connectors,
instrumentation systems, and control devices or systems.
    The NESHAP established emissions standards for process vents,
storage vessels, transfer racks, and equipment leaks. Cyanide process
vents are subject to a 98 weight-percent reduction of total HAP \2\
performance standard or 20 parts per million by volume (ppmv) total HAP
outlet exit concentration limit. For storage vessels in the Cyanide
Chemicals Manufacturing source category, sources may either choose to
comply with a 98 weight-percent reduction of hydrogen cyanide
performance standard, a 20 ppmv hydrogen cyanide exit outlet
concentration limit, or equipment standards (e.g., use a flare).
Transfer racks are subject to equipment standards or the same
performance standard or concentration limit \3\ as cyanide process
vents. Equipment leaks are subject to work practice standards required
by either 40 CFR part 63, subpart TT or subpart UU.
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    \2\ ``Dry end'' process vents at sodium cyanide units must meet
a 98 percent reduction performance standard for emissions of sodium
cyanide since this is the form of cyanide compounds emitted from
these emission points. The HAP emitted from other process vents that
make up the ``total HAP'' emitted from these sources are hydrogen
cyanide, acetonitrile, and acrylonitrile.
    \3\ Transfer racks emissions limits are expressed in terms of
hydrogen cyanide as this is the only HAP emitted from these sources.
---------------------------------------------------------------------------

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

    The EPA used a variety of resources to obtain data about facilities
and their emissions for use in our risk assessment. We used the EPA's
Enforcement and Compliance History Online (ECHO) database to develop a
list of potentially subject facilities. Using this list, we searched
state environmental agency websites and correspondence with industry to
obtain copies of title V permits to confirm whether facilities have
cyanide chemicals manufacturing subject to the NESHAP. Once the
facility list was finalized, the EPA used the 2017 National Emissions
Inventory (NEI) to get emissions data for each facility. We compared
the NEI data to title V permits to provide additional information
regarding the applicability of the Cyanide Chemicals Manufacturing
NESHAP. Further discussion of the methodology used to develop the
emissions dataset for the risk assessment can be found in the
memorandum titled Technical Support Document for the Cyanide Chemicals
Manufacturing NESHAP Residual Risk and Technology Review Proposal,
which is available in the docket for this action.

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

    We searched for information from the Reasonably Available Control
Technology, Best Available Control Technology, and Lowest Achievable
Emission Rate Clearinghouse (RBLC) database, reviewed title V permits
for each cyanide chemicals manufacturing facility, and reviewed
regulatory actions related to emissions controls at similar sources
that could be applicable to cyanide chemicals manufacturing. We
reviewed the RBLC to identify potential additional control
technologies. No additional control technologies applicable to cyanide
chemicals manufacturing were found using the RBLC. Additional
information related to the promulgation and subsequent amendments of
the NESHAP is available in docket ID: No. EPA-HQ-OAR-2004-0041.

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
EPA's response to comments on our policy under the Benzene NESHAP:
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    \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.
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    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
(or last updated) the NESHAP, we review a variety of data sources in
our investigation of potential practices, processes, or controls. 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 Cyanide
Chemicals Manufacturing 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 list of facilities subject to the NESHAP was created through
searching the EPA's ECHO database, the 2017 NEI, and state databases of
title V permits. The list of facilities is available in the memorandum
titled Technical Support Document for the Cyanide Chemicals
Manufacturing NESHAP Residual Risk and Technology Review Proposal. Once
the facility list was finalized, available emissions data were obtained
from the NEI. Title V permits were used to determine which emission
points at each facility are subject to the Cyanide Chemicals
Manufacturing NESHAP.
    We compared the NEI data to title V permits to confirm that the NEI
included all emission points listed as subject to the NESHAP according
to the permit. We evaluated latitudes and longitudes listed in the NEI
to ensure their accuracy using satellite imagery. All of the latitudes
and longitudes used in our dispersion modeling are in the modeling file
used for the proposed rule, which is available in Docket ID No. EPA-HQ-
OAR-2020-0532. Corrections were made to emission point characteristics
for one non-category emission point that appeared to have erroneous
stack velocity entered into the NEI. This emission point's stack
velocity was corrected to a default maximum value. All corrections made
to emission point parameters are documented in the modeling file,
available in Docket ID No. EPA-HQ-OAR-2020-0532.
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 and 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).
    We have determined that the actual emissions data are reasonable
estimates of the MACT-allowable emissions levels for the Cyanide
Chemicals Manufacturing source category. The ability to estimate MACT-
allowable emissions from the actual emissions dataset is largely
dependent on the format of the standard for a given emissions source as
well as the types of controls employed for the source. With respect to
the various types of controls used within the Cyanide Chemicals
Manufacturing source category, the most prevalent is the use of a flare
as a combustion control device. A flare can be used to control
emissions for a single emissions source, or, as is generally the case,
to control emissions from multiple emission sources/emission source
types. Flares are designed to handle a wide range of flowrates and
compositions of combustible waste gases. Within the Cyanide Chemicals
Manufacturing source category, flares generally control emissions from
multiple emission source types. Consideration of this, along with not
having a specific limit on how much gas can be combusted in a flare
(given that in many cases multiple emissions sources are being
controlled by this control device), means that it is extremely
difficult to determine an allowable emission rate for flares. We have
determined that flares in the Cyanide Chemicals Manufacturing

source category are currently complying with design and operational
requirements that are generally expected to achieve 98 percent
destruction efficiencies or control, which is the level of control
required by the NESHAP. HAP emissions inventories for flares in the
Cyanide Chemicals Manufacturing source category are developed using
engineering knowledge and, in many instances, presume this 98 percent
level of control. The Agency is unaware of any data that suggest that
flares used as controls in the Cyanide Chemicals Manufacturing source
category are consistently overcontrolling HAP emissions beyond 98
percent control. Thus, weighing all of these factors for flares, we
determined that the actual emission levels are a reasonable estimation
of the MACT-allowable emissions levels where the performance standards
allow the use of a flare as an air pollution control device (e.g.,
storage vessels, process vents, and transfer racks).
    For equipment leaks, which are currently subject to work practice
standards, there would be no difference between actual and MACT-
allowable emissions for facilities in the Cyanide Chemicals
Manufacturing source category, provided the facilities are complying
with the MACT standards as well as not conducting additional work
practices that would reduce emissions beyond those required by the
rule. We are aware of only one rule in the state of Texas, the Texas
Commission of Environmental Quality (TCEQ) Highly Reactive Volatile
Organic Compounds (HRVOC) Rule (i.e., 30 TAC Chapter 115, Subchapter H,
Division 3), that may contain more stringent leak definitions and/or
monitoring frequencies for certain pieces of equipment for the three
facilities located in Texas that might be subject to this rule.
However, based on our review of the Texas rule, we note the following:
(1) Specific facilities located in the Houston-Galveston-Brazoria area
still conduct a leak detection and repair (LDAR) program using EPA
Method 21; (2) the vast majority of equipment, including almost all
pieces of equipment in gas and vapor service that would tend to
contribute considerably to the overall equipment leak air emissions,
are complying with the same leak definition as in the MACT standards;
and (3) the TCEQ HRVOC Rule generally requires quarterly monitoring
while the MACT standards have varying monitoring frequencies depending
on the percentage of leaking equipment that could lead to more
stringent, the same, or less stringent frequencies that would require
an EPA Method 21 measurement and repair of a leaking component (if
measured). Therefore, considering these factors for equipment leaks, we
determined that the actual emission levels for equipment leaks are a
reasonable estimation of the MACT-allowable emissions levels.
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 826 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.
    No data are available on the carcinogenic effects of cyanide
compounds in humans via inhalation. Under the U.S. EPA (2005a)
Guidelines for Carcinogen Risk Assessment, there is ``inadequate
information to assess the carcinogenic potential'' of cyanide
compounds.
    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.
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    \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.
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    To assess the risk of noncancer health effects from chronic
exposure to HAP, we calculate either an HQ or a target organ-specific
hazard index (TOSHI). We calculate an HQ when a single noncancer HAP is
emitted. Where more than one noncancer HAP is emitted, we sum the HQ
for each of the HAP that affects a common target organ or target organ
system to obtain a TOSHI. The HQ is the estimated exposure divided by
the chronic noncancer dose-response value, which is a value selected
from one of several sources. The preferred chronic noncancer dose-
response value is the EPA RfC, defined as ``an estimate (with
uncertainty spanning perhaps an order of magnitude) of a continuous
inhalation exposure to the human population (including sensitive
subgroups) that is likely to be without an appreciable risk of
deleterious effects during a lifetime'' (https://iaspub.epa.gov/sor_internet/registry/termreg/searchandretrieve/glossariesandkeywordlists/search.do?details=&vocabName=IRIS%20Glossary). In cases where an RfC
from the EPA's IRIS is not available or where the EPA determines that
using a value other than the RfC is appropriate, the chronic noncancer
dose-response value can be a value from the following prioritized
sources, which define their dose-response values similarly to the EPA:
(1) The Agency for Toxic Substances and Disease Registry (ATSDR)
Minimum Risk Level (https://www.atsdr.cdc.gov/mrls/index.asp); (2) the
CalEPA Chronic Reference Exposure Level (REL) (https://oehha.ca.gov/air/crnr/notice-adoption-air-toxics-hot-spots-program-guidance-manual-preparation-health-risk-0); or (3) as noted above, a scientifically
credible dose-response value that has been developed in a manner
consistent with the EPA guidelines and has undergone a peer review
process similar to that used by the EPA. The pollutant-specific dose-
response values used to estimate health risks are available at https://www.epa.gov/fera/dose-response-assessment-assessing-health-risks-associated-exposure-hazardous-air-pollutants.
    Cyanide is extremely toxic to humans. Acute (10-minute) inhalation
exposure to 579 milligrams per cubic meter (mg/m\3\) of hydrogen
cyanide will cause death in 50 percent of exposed humans. Nonlethal
exposures to hydrogen cyanide gas will cause a variety of effects in
humans, such as headache, dizziness, upper respiratory irritation,
cough, altered sense of smell, nasal congestion, nosebleed, and
difficulty breathing. Chronic (long-term) inhalation exposure of humans
to cyanide results primarily in effects on the central nervous system.
Other effects in humans include cardiovascular and respiratory effects,
effects to the endocrine system (e.g., thyroid enlargement, altered
iodine uptake), and irritation to the eyes and skin. However, short
term exposure levels below the acute REL and chronic exposures below
the RfC are not likely to cause adverse effects.
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 Cyanide Chemicals
Manufacturing 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.
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    \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).
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    To assess the potential acute risk to the maximally exposed
individual, we use the peak hourly emission rate for each emission
point,\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.
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    \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 the
Cyanide Chemicals Manufacturing 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.
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    \13\ CalEPA issues acute RELs as part of its Air Toxics Hot
Spots Program, and the 1-hour and 8-hour values are documented in
Air Toxics Hot Spots Program Risk Assessment Guidelines, Part I, The
Determination of Acute Reference Exposure Levels for Airborne
Toxicants, which is available at https://oehha.ca.gov/air/general-info/oehha-acute-8-hour-and-chronic-reference-exposure-level-rel-summary.
    \14\ National Academy of Sciences, 2001. Standing Operating
Procedures for Developing Acute Exposure Levels for Hazardous
Chemicals, page 2. Available at https://www.epa.gov/sites/production/files/2015-09/documents/sop_final_standing_operating_procedures_2001.pdf. Note that the
National Advisory Committee for Acute Exposure Guideline Levels for
Hazardous Substances ended in October 2011, but the AEGL program
continues to operate at the EPA and works with the National
Academies to publish final AEGLs (https://www.epa.gov/aegl).
---------------------------------------------------------------------------

    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 acute factors between 2 and 10,
depending on the type of source, to estimate peak hourly emissions from
annual emissions estimates for input into the risk assessment modeling
analysis. Specifically, we used a factor of 2 for process vents and
equipment leaks, a factor of 4 for storage vessels, and a factor of 10
for transfer racks. A further discussion of why these factors were
chosen can be found in the memorandum, Technical Support Document for
the Cyanide Chemicals Manufacturing NESHAP Residual Risk and Technology
Review Proposal, available in the docket for this rulemaking.
    In our acute inhalation screening risk assessment, acute impacts
are deemed negligible for HAP for which acute HQs are less than or
equal to 1, 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, no data were conducted.
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 Cyanide Chemicals Manufacturing source category, we
identified potential PB-HAP emissions of arsenic, cadmium, lead,
mercury, and polycyclic organic matter (POM) based on entries in the
NEI. We note that for the Cyanide Chemicals Manufacturing source
category, we modeled these pollutants to provide a conservative
assessment of risks because these pollutants are included in the NEI.
However, we do not believe these HAP are emitted from the cyanide
chemicals manufacturing process. Very small amounts of these HAP are
included in the NEI as byproducts of fuel combustion and are unrelated
to cyanide chemicals manufacturing.
    After identifying potential PB-HAP emissions, the next step of the
evaluation is a tiered screening assessment. 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 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.
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    There are several analyses that can be included in a Tier 3
screening assessment, depending upon the extent of refinement
warranted, including validating that the lakes are fishable, locating
residential/garden locations for urban and/or rural settings,
considering plume-rise to estimate emissions lost above the mixing
layer, and considering hourly effects of meteorology and plume-rise on
chemical fate and transport (a time-series analysis). If necessary, the
EPA may further refine the screening assessment through a site-specific
assessment.
    In evaluating the potential multipathway risk from emissions of
lead compounds, rather than developing a screening threshold emission
rate, we compare maximum estimated chronic inhalation exposure
concentrations to the level of the current National Ambient Air Quality
Standard (NAAQS) for lead.\18\ Values below the level of the primary
(health-based) lead NAAQS are considered to have a low potential for
multipathway risk.
---------------------------------------------------------------------------

    \18\ In doing so, the EPA notes that the legal standard for a
primary NAAQS--that a standard is requisite to protect public health
and provide an adequate margin of safety (CAA section 109(b))--
differs from the CAA section 112(f) standard (requiring, among other
things, that the standard provide an ``ample margin of safety to
protect public health''). However, the primary lead NAAQS is a
reasonable measure of determining risk acceptability (i.e., the
first step of the Benzene NESHAP analysis) since it is designed to
protect the most susceptible group in the human population--
children, including children living near major lead emitting
sources. 73 FR 67002/3; 73 FR 67000/3; 73 FR 67005/1. In addition,
applying the level of the primary lead NAAQS at the risk
acceptability step is conservative, since that primary lead NAAQS
reflects an adequate margin of safety.
---------------------------------------------------------------------------

    For further information on the multipathway assessment approach,
see the Residual Risk Assessment for the Cyanide Chemicals
Manufacturing 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 hydrochloric acid (HCl)
and hydrogen fluoride (HF).
    HAP that persist and bioaccumulate are of particular environmental
concern because they accumulate in the soil, sediment, and water. The
acid gases, HCl and HF, are included due to their well-documented
potential to cause direct damage to terrestrial plants. In the
environmental risk screening assessment, we evaluate the following four
exposure media: terrestrial soils, surface water bodies (includes
water-column and benthic sediments), fish consumed by wildlife, and
air. Within these four exposure media, we evaluate nine ecological
assessment endpoints, which are defined by the ecological

entity and its attributes. For PB-HAP (other than lead), both
community-level and population-level endpoints are included. For acid
gases, the ecological assessment evaluated is terrestrial plant
communities.
    An ecological benchmark represents a concentration of HAP that has
been linked to a particular environmental effect level. For each
environmental HAP, we identified the available ecological benchmarks
for each assessment endpoint. We identified, where possible, ecological
benchmarks at the following effect levels: probable effect levels,
lowest-observed-adverse-effect level, and no-observed-adverse-effect
level. In cases where multiple effect levels were available for a
particular PB-HAP and assessment endpoint, we use all of the available
effect levels to help us to determine whether ecological risks exist
and, if so, whether the risks could be considered significant and
widespread.
    For further information on how the environmental risk screening
assessment was conducted, including a discussion of the risk metrics
used, how the environmental HAP were identified, and how the ecological
benchmarks were selected, see Appendix 9 of the Residual Risk
Assessment for the Cyanide Chemicals Manufacturing 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 Cyanide Chemicals
Manufacturing source category emitted any of the environmental HAP. For
the Cyanide Chemicals Manufacturing source category, we identified
potential emissions of arsenic, cadmium, lead, mercury, POM, and one
acid gas, HCl, based on entries in the NEI. Because one or more of the
environmental HAP evaluated may be emitted by at least one facility in
the source category, we proceeded to the second step of the evaluation.
As noted above, we modeled these emissions to err on the side of an
overly conservative analysis because they are included in the NEI;
however, we do not believe these HAP are emitted from the Cyanide
Chemicals Manufacturing source category. The NEI entries for these HAP
from these sources are likely the result of emissions factors that are
used for fuel combustion and are unrelated to cyanide chemicals
manufacturing.
c. PB-HAP Methodology
    The environmental screening assessment includes six PB-HAP, arsenic
compounds, cadmium compounds, dioxins/furans, POM, mercury (both
inorganic mercury and methyl mercury), and lead compounds. With the
exception of lead, the environmental risk screening assessment for PB-
HAP consists of three tiers. The first tier of the environmental risk
screening assessment uses the same health-protective conceptual model
that is used for the Tier 1 human health screening assessment.
TRIM.FaTE model simulations were used to back-calculate Tier 1
screening threshold emission rates. The screening threshold emission
rates represent the emission rate in tons of pollutant per year that
results in media concentrations at the facility that equal the relevant
ecological benchmark. To assess emissions from each facility in the
category, the reported emission rate for each PB-HAP was compared to
the Tier 1 screening threshold emission rate for that PB-HAP for each
assessment endpoint and effect level. If emissions from a facility do
not exceed the Tier 1 screening threshold emission rate, the facility
``passes'' the screening assessment, and, therefore, is not evaluated
further under the screening approach. If emissions from a facility
exceed the Tier 1 screening threshold emission rate, we evaluate the
facility further in Tier 2.
    In Tier 2 of the environmental screening assessment, the screening
threshold emission rates are adjusted to account for local meteorology
and the actual location of lakes in the vicinity of facilities that did
not pass the Tier 1 screening assessment. For soils, we evaluate the
average soil concentration for all soil parcels within a 7.5-km radius
for each facility and PB-HAP. For the water, sediment, and fish tissue
concentrations, the highest value for each facility for each pollutant
is used. If emission concentrations from a facility do not exceed the
Tier 2 screening threshold emission rate, the facility ``passes'' the
screening assessment and typically is not evaluated further. If
emissions from a facility exceed the Tier 2 screening threshold
emission rate, we evaluate the facility further in Tier 3.
    As in the multipathway human health risk assessment, in Tier 3 of
the environmental screening assessment, we examine the suitability of
the lakes around the facilities to support life and remove those that
are not suitable (e.g., lakes that have been filled in or are
industrial ponds), adjust emissions for plume-rise, and conduct hour-
by-hour time-series assessments. If these Tier 3 adjustments to the
screening threshold emission rates still indicate the potential for an
adverse environmental effect (i.e., facility emission rate exceeds the
screening threshold emission rate), we may elect to conduct a more
refined assessment using more site-specific information. If, after
additional refinement, the facility emission rate still exceeds the
screening threshold emission rate, the facility may have the potential
to cause an adverse environmental effect.
    To evaluate the potential for an adverse environmental effect from
lead, we compared the average modeled air concentrations (from HEM-3)
of lead around each facility in the source category to the level of the
secondary NAAQS for lead. The secondary lead NAAQS is a reasonable
means of evaluating environmental risk because it is set to provide
substantial protection against adverse welfare effects which can
include ``effects on soils, water, crops, vegetation, man-made
materials, animals, wildlife, weather, visibility and climate, damage
to and deterioration of property, and hazards to transportation, as
well as effects on economic values and on personal comfort and well-
being.''
d. Acid Gas Environmental Risk Methodology
    The environmental screening assessment for acid gases evaluates the
potential phytotoxicity and reduced productivity of plants due to
chronic exposure to HF and HCl. The environmental risk screening
methodology for acid gases is a single-tier screening assessment that
compares modeled ambient air concentrations (from AERMOD) to the
ecological benchmarks for each acid gas. To identify a potential
adverse environmental effect (as defined in section 112(a)(7) of the
CAA) from emissions of HF and HCl, we evaluate the following metrics:
the size of the modeled area around each facility that exceeds the
ecological benchmark for each acid gas, in acres and 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 Cyanide Chemicals Manufacturing

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 2017 NEI. The source category records
of that NEI dataset were removed, evaluated, and updated as described
in section II.C of this preamble: What data collection activities were
conducted to support this action? 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 Cyanide Chemicals Manufacturing 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 Cyanide
Chemicals Manufacturing 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
Cyanide Chemicals Manufacturing 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.\19\
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.\20\
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,\21\ 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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    \19\ IRIS glossary (https://ofmpub.epa.gov/sor_internet/registry/termreg/searchandretrieve/glossariesandkeywordlists/search.do?details=&glossaryName=IRIS%20Glossary).
    \20\ 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.
    \21\ 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.\22\
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    \22\ In the context of this discussion, the term ``uncertainty''
as it pertains to exposure and risk encompasses both variability in
the range of expected inputs and screening results due to existing
spatial, temporal, and other factors, as well as uncertainty in
being able to accurately estimate the true result.
---------------------------------------------------------------------------

    Model uncertainty concerns whether the model adequately represents
the actual processes (e.g., movement and accumulation) that might occur
in the environment. For example, 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)?

    We are proposing standards pursuant to CAA section 112(d)(2) for
process wastewater from existing cyanide chemical manufacturing process
units, which was previously unregulated.\23\ During development of the
initial MACT standards, we identified process wastewater at existing
sources as a potential source of emissions of hydrogen cyanide,
acetonitrile, and acrylonitrile. See 65 FR 76408, 76411, and 76413,
December 6, 2000, for a discussion of the HAP emitted from cyanide
chemicals manufacturing. At that time, we identified measures
undertaken at cyanide chemicals manufacturing facilities to comply with
other NESHAP as the ``MACT floor,'' but we did not include these
measures in 40 CFR part 63, subpart YY for existing cyanide chemical
manufacturing process units. Based on our review, we are proposing to
find that these measures reflect the best performing sources in the
source category. The results and proposed decisions based on the
analyses performed pursuant to CAA section 112(d)(2) and (3) are
presented below.
---------------------------------------------------------------------------

    \23\ 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 F3d at 1091-
1099.
---------------------------------------------------------------------------

    For this proposal, we reviewed title V permits for facilities
subject to the Cyanide Chemicals Manufacturing NESHAP and determined
that all cyanide chemicals manufacturing facilities are co-located with
processes subject to the Hazardous Organic NESHAP (HON) or
substantively similar requirements. In the 2000 NESHAP proposal, we
stated that wastewater treatment units at cyanide chemicals
manufacturing facilities are typical of synthetic organic chemicals
manufacturing facilities subject to the HON. The wastewater
requirements of the HON are already an approved means of compliance for
wastewater emission sources subject to 40 CFR part 63, subpart YY as
stated in 40 CFR 63.1100(g)(5). We are proposing to require compliance
with HON wastewater requirements for process

wastewater at existing sources, which will ensure all affected sources
at cyanide chemicals manufacturing facilities are subject to MACT
standards. We are proposing these requirements for cyanide chemicals
manufacturing existing sources because such requirements represent: (1)
The measures employed by the best performing sources in the category;
and (2) an already acceptable means of compliance for wastewater
emissions at sources subject to subpart YY. We believe that these
requirements will not require additional controls or emissions
reductions since existing sources we have identified as subject to the
Cyanide Chemicals Manufacturing NESHAP are already subject to the HON
or substantively identical wastewater requirements in another NESHAP.
    We are also adding the HON requirements for waste management units
upstream of an open or closed biological treatment process to the new
source standard to ensure demonstrable compliance measures are in place
for these sources; however, we believe these measures would already be
employed by any new sources to achieve the combined 93 percent capture
and control of HAP emissions from wastewater required for process
wastewater emissions at new sources subject to the Cyanide Chemicals
Manufacturing NESHAP.
    We have identified three HAP that may be present in process
wastewater streams at cyanide chemicals manufacturing facilities:
Hydrogen cyanide, acetonitrile, and acrylonitrile. We are proposing to
include hydrogen cyanide in the calculations required to determine
compliance with the wastewater standard for the Cyanide Chemicals
Manufacturing source category to ensure all HAP potentially present in
process wastewater are subject to MACT standards. The other two HAP
that may be present in cyanide chemicals manufacturing wastewater
(acetonitrile and acrylonitrile) are already included in the list of
compounds subject to the HON wastewater provisions. We do not expect
significant amounts of hydrogen cyanide to be present in these process
wastewater streams. When developing the 2002 NESHAP, facilities that
were surveyed reported very low levels of hydrogen cyanide in their
wastewaters with one exception. The only facility that had high levels
of hydrogen cyanide in its wastewater used add-on controls to remove
the hydrogen cyanide prior to discharge. That facility was the basis
for the ``new source'' MACT floor. We expect any facilities with high
levels of hydrogen cyanide in their wastewater would already possess
add-on controls similar to those present at the single existing source
with high levels of hydrogen cyanide in order to meet effluent
discharge limits and protect the biological wastewater treatment
systems used at these facilities. We are including hydrogen cyanide in
these calculations to ensure that all HAP emitted by the source
category are subject to MACT standards.
    Nevertheless, we are seeking comment on whether facilities would
need to install additional controls, achieve additional emissions
reductions, or incur significant costs as a result of the proposed
standards for process wastewater. For this proposed rule, we did not
identify any new control technologies or developments in existing
technologies to evaluate as ``beyond-the-floor'' controls other than
the controls evaluated during the initial MACT standards. We did not
find any data to support changing the conclusion that application of
the new source MACT limit for process wastewater emissions to existing
sources is unreasonable (See 65 FR 76419 and Docket Item No. EPA-HQ-
OAR-2004-0041-0003).

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

1. Chronic Inhalation Risk Assessment Results
    The EPA estimated inhalation risk based on actual and allowable
emissions, which we determined are the same for this category. The
estimated baseline inhalation MIR posed by the source category is 5-in-
1 million based on actual emissions and MACT-allowable emissions. The
total estimated cancer incidence based on actual or allowable emission
levels is 0.004 excess cancer cases per year, or one case every 250
years. Emissions of acrylonitrile from process vents account for 95
percent of the cancer incidence. Approximately 61,653 people are
exposed to cancer risk greater than or equal to 1-in-1 million based
upon actual and allowable emissions (see Table 1 of this preamble).
    The maximum chronic noncancer TOSHI values for the source category
were estimated to be 1 for neurological effects based on actual and
allowable emissions. For both actual and allowable emissions, risk was
driven by hydrogen cyanide emissions from process vents, wastewater,
and equipment leaks.

            TABLE 1--Inhalation Risk Assessment Summary For Cyanide Chemicals Manufacturing \1\ Source Category (40 CFR Part 63, Subpart YY)
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                           Maximum        Estimated       Estimated
                                                         individual     population at   annual cancer
           Risk assessment                Number of    cancer risk (1- increased risk     incidence         Maximum chronic      Maximum screening acute
                                       facilities \2\   in-1 million)  of cancer >= 1-   (cases per       noncancer TOSHI \4\        noncancer HQ \5\
                                                             \3\        in-1 million        year)
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                                Baseline Actual Emissions
--------------------------------------------------------------------------------------------------------------------------------------------------------
Source Category......................              13               5          61,653           0.004  1 (neurological)........  1 (REL)
Facility-Wide........................              13             200         266,532            0.04  1 (neurological)........  .......................
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                              Baseline Allowable Emissions
--------------------------------------------------------------------------------------------------------------------------------------------------------
Source Category......................              13               5          61,653           0.004  1 (neurological)........  .......................
--------------------------------------------------------------------------------------------------------------------------------------------------------
\1\ Based on actual and allowable emissions.
\2\ Number of facilities evaluated in the risk assessment. Includes 13 operating facilities subject to 40 CFR part 63, subpart YY.
\3\ Maximum individual excess lifetime cancer risk due to HAP emissions from the source category.
\4\ Maximum TOSHI. The target organ with the highest TOSHI for the Cyanide Chemicals Manufacturing source category is the neurological system.
\5\ The maximum estimated acute exposure concentration was divided by available short-term threshold values to develop an array of HQ values. The acute
  HQ shown was based upon the lowest acute 1-hour dose-response value, the REL for hydrogen cyanide. When an HQ exceeds 1, we also show the HQ using the
  next lowest available acute dose-response value.

2. Screening Level Acute Risk Assessment Results
    Based on our screening analysis of reasonable worst-case acute
exposure to actual emissions from the category, no HAP exposures result
in an HQ greater than 1 based upon the 1-hour REL. As discussed in
section III.C.3.c of this preamble, for this source category, we used
acute factors between 2 and 10, depending on the type of source.
Specifically, we used a factor of 2 for process vents and equipment
leaks, a factor of 4 for storage vessels, and a factor of 10 for
transfer racks. A further discussion of why these factors were chosen
can be found in the memorandum, Technical Support Document for the
Cyanide Chemicals Manufacturing NESHAP Residual Risk and Technology
Review Proposal, available in the docket for this rulemaking.
3. Multipathway Risk Screening Results
    Three of the 13 facilities in this source category reported
emissions of PB-HAP in the NEI which include POM (of which polycyclic
aromatic hydrocarbons is a subset), lead compounds, arsenic compounds,
cadmium compounds, and mercury compounds. We note that for the Cyanide
Chemicals Manufacturing source category, while we modeled these
emissions, none of these HAP are expected to be emitted from the source
category and they were only modeled to provide a conservative estimate
of risk because they were included in the NEI. To identify potential
multipathway health risks from PB-HAP other than lead, we first
performed a tiered screening assessment (Tiers 1, 2, and 3) based on
emissions of PB-HAP emitted from each facility in the source category.
Arsenic emissions from a single facility exceeded the Tier 1 cancer
screening threshold emission rate with a maximum SV of 2. No facilities
had POM emissions exceeding the Tier 1 cancer screening threshold
emission rate. Mercury emissions from a single facility exceeded the
Tier 1 noncancer screening threshold emission rate with a maximum SV of
2. No facilities had cadmium emissions exceeding the Tier 1 noncancer
screening threshold emission rate. For the facilities and HAP for which
the Tier 1 threshold emissions rates were exceeded (i.e., SV greater
than 1), we conducted a Tier 2 screening analysis. In the Tier 2
screening analysis, no facilities had an SV greater than 1.
Specifically, the maximum Tier 2 cancer SV was less than 1 for both the
farmer scenario for arsenic (0.4) and the fisher scenario for mercury
(0.3).
    Further facility details on the multipathway screening analysis can
be found in Appendix 10 of the Residual Risk Assessment for the Cyanide
Chemical Manufacturing Source Category in Support of the Risk and
Technology Review 2020 Proposed Rule.
    An SV in any of the tiers is not an estimate of the cancer risk or
a noncancer HQ. Rather, an SV represents a high-end estimate of what
the risk or HQ may be. For example, facility emissions resulting in an
SV of 2 for a non-carcinogen can be interpreted to mean that we are
confident that the HQ would be lower than 2. Similarly, facility
emissions resulting in a cancer SV of 20 for a carcinogen means that we
are confident that the cancer risk is lower than 20-in-1 million. Our
confidence comes from the health-protective assumptions that are
incorporated into the screens: We choose inputs from the upper end of
the range of possible values for the influential parameters used in the
screens and we assume food consumption behaviors that would lead to
high total exposure. This risk assessment estimates the maximum hazard
for mercury and cadmium through fish consumption based on upper bound
screens and the maximum excess cancer risks from POM and arsenic
through ingestion of fish and farm produce.
    In evaluating the potential for adverse health effects from
emissions of lead, the EPA compared modeled annual lead concentrations
to the secondary NAAQS level for lead (0.15 [mu]g/m\3\, arithmetic mean
concentration over a 3-month period). The highest annual average lead
concentration, 0.00000065 [micro]g/m\3\, is orders of magnitude below
the NAAQS level for lead, indicating a low potential for adverse health
impacts.
4. Environmental Risk Screening Results
    As described in section III.A of this preamble, we conducted an
environmental risk screening assessment for the Cyanide Chemical
Manufacturing source category for the following pollutants: arsenic,
cadmium, HCl, lead, mercury (methyl mercury and mercuric chloride), and
POM. As noted in our discussion of the multipathway risk assessment
results, these HAP are not associated with cyanide chemicals
manufacturing and are not emitted from the source category. There were
NEI entries for small amounts of these pollutants and we chose to model
these emissions to err on the side of an overly conservative
assessment.
    In the Tier 1 screening analysis for the above PB-HAP (other than