[Federal Register Volume 79, Number 235 (Monday, December 8, 2014)]
[Proposed Rules]
[Pages 72873-72912]
[FR Doc No: 2014-27497]

Vol. 79

Monday,

No. 235

December 8, 2014

Part III

Environmental Protection Agency

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40 CFR Part 63

 National Emissions Standards for Hazardous Air Pollutants: Secondary
Aluminum Production; Proposed Rule

Federal Register / Vol. 79 , No. 235 / Monday, December 8, 2014 /
Proposed Rules

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ENVIRONMENTAL PROTECTION AGENCY

40 CFR Part 63

[EPA-HQ-OAR-2010-0544; FRL-9919-33-OAR]
RIN 2060-AQ40

National Emissions Standards for Hazardous Air Pollutants:
Secondary Aluminum Production

AGENCY: Environmental Protection Agency.

ACTION: Supplemental notice of proposed rulemaking.

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SUMMARY: This action supplements our notice of proposed rulemaking for
the national emissions standards for hazardous air pollutants (NESHAP)
for secondary aluminum production, which was published in the Federal
Register on February 14, 2012. In that action, the Environmental
Protection Agency (EPA) proposed decisions concerning the residual risk
and technology review for the Secondary Aluminum Production source
category and proposed amendments to correct and clarify rule
requirements. This supplemental proposal presents a revised risk review
(including a revised inhalation risk assessment, a refined multipathway
risk assessment, and an updated ample margin of safety analysis) and a
revised technology review for the Secondary Aluminum Production source
category. Similar to the 2012 proposal, we found risks due to emissions
of air toxics to be acceptable from this source category and we
identified no cost effective controls under the updated ample margin of
safety analysis or the technology review to achieve further emissions
reductions. Therefore, we are proposing no revisions to the numeric
emission standards based on these revised analyses. However, this
supplemental proposal supplements and modifies several of the proposed
technical corrections and rule clarifications that were originally
presented in the February 14, 2012 proposal; withdraws our previous
proposal to include affirmative defense provisions in the regulation;
proposes alternative compliance options for the operating and
monitoring requirements for sweat furnaces; and provides a revised cost
analysis for compliance testing. This action, if finalized, would
result in improved monitoring, compliance and implementation of the
rule.

DATES: Comments. Comments must be received on or before January 22,
2015. A copy of comments on the information collection provisions
should be submitted to the Office of Management and Budget (OMB) on or
before January 7, 2015.
    Public Hearing. If anyone contacts the EPA requesting a public
hearing by December 15, 2014, the EPA will hold a public hearing on
December 23, 2014 at the U.S. EPA building at 109 T.W. Alexander Drive,
Research Triangle Park, NC 27711. If you are interested in requesting a
public hearing or attending the public hearing, contact Ms. Virginia
Hunt at<PHONE_NUMBER> or at<EMAIL_ADDRESS>If the EPA holds a
public hearing, the EPA will keep the record of the hearing open for 30
days after completion of the hearing to provide an opportunity for
submission of rebuttal and supplementary information.

ADDRESSES: Comments. Submit your comments, identified by Docket ID No.
EPA-HQ-OAR-2010-0544, by one of the following methods:
     Federal eRulemaking Portal: http://www.regulations.gov.
Follow the online instructions for submitting comments.
     Email<EMAIL_ADDRESS>Include Attention Docket ID
No. EPA-HQ-OAR-2010-0544 in the subject line of the message.
     Fax:<PHONE_NUMBER>, Attention Docket ID No. EPA-HQ-OAR-
2010-0544.
     Mail: Environmental Protection Agency, EPA Docket Center
(EPA/DC), Mail Code 28221T, Attention Docket ID No. EPA-HQ-OAR-2010-
0544, 1200 Pennsylvania Avenue NW., Washington, DC 20460. Please mail a
copy of your comments on the information collection provisions to the
Office of Information and Regulatory Affairs, Office of Management and
Budget (OMB), Attn: Desk Officer for EPA, 725 17th Street NW.,
Washington, DC 20503.
     Hand/Courier Delivery: EPA Docket Center, Room 3334, EPA
WJC West Building, 1301 Constitution Avenue NW., Washington, DC 20004,
Attention Docket ID No. EPA-HQ-OAR-2010-0544. Such deliveries are only
accepted during the Docket's normal hours of operation, and special
arrangements should be made for deliveries of boxed information.
    Instructions: Direct your comments to Docket ID No. EPA-HQ-OAR-
2010-0544. 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 http://www.regulations.gov, including any personal
information provided, unless the comment includes information claimed
to be confidential business information (CBI) or other information
whose disclosure is restricted by statute. Do not submit information
that you consider to be CBI or otherwise protected through http://www.regulations.gov or email. The http://www.regulations.gov Web site
is an ``anonymous access'' system, 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 http://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 disk or CD-ROM 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 http://www.epa.gov/dockets.
    Docket: The EPA has established a docket for this rulemaking under
Docket ID No. EPA-HQ-OAR-2010-0544. All documents in the docket are
listed in the www.regulations.gov index. Although listed in the index,
some information is not publicly available, e.g., CBI or other
information whose disclosure is restricted by statute. Certain other
material, such as copyrighted material, is not placed on the Internet
and will be publicly available only in hard copy. Publicly available
docket materials are available either electronically in http://www.regulations.gov or in hard copy at the EPA Docket Center, Room
3334, EPA WJC West Building, 1301 Constitution Avenue NW., Washington,
DC. The Public Reading Room is open from 8:30 a.m. to 4:30 p.m., Monday
through Friday, excluding legal holidays. The telephone number for the
Public Reading Room is<PHONE_NUMBER>, and the telephone number for the
EPA Docket Center is<PHONE_NUMBER>.
    Public Hearing: If anyone contacts the EPA requesting a public
hearing by December 15, 2014, the public hearing will be held on
December 23, 2014 at the EPA's campus at 109 T.W. Alexander Drive,
Research Triangle Park, North Carolina. The hearing will begin at 1:00
p.m. (Eastern Standard Time) and conclude at 5:00 p.m. (Eastern
Standard Time). Please contact

Ms. Virginia Hunt at<PHONE_NUMBER> or at<EMAIL_ADDRESS>to
register to speak at the hearing or to inquire as to whether or not a
hearing will be held. The last day to pre-register in advance to speak
at the hearing will be December 22, 2014. Additionally, requests to
speak will be taken the day of the hearing at the hearing registration
desk, although preferences on speaking times may not be able to be
accommodated. If you require the service of a translator or special
accommodations such as audio description, please let us know at the
time of registration. If you require an accommodation, we ask that you
pre-register for the hearing, as we may not be able to arrange such
accommodations without advance notice.
    If no one contacts the EPA requesting a public hearing to be held
concerning this proposed rule by December 15, 2014, a public hearing
will not take place. If a hearing is held, it will provide interested
parties the opportunity to present data, views or arguments concerning
the supplemental notice of proposed rulemaking. The EPA will make every
effort to accommodate all speakers who arrive and register. Because the
hearing will be held at a U.S. government facility, individuals
planning to attend the hearing should be prepared to show valid picture
identification to the security staff in order to gain access to the
meeting room. Please note that the REAL ID Act, passed by Congress in
2005, established new requirements for entering federal facilities. If
your driver's license is issued by Alaska, American Samoa, Arizona,
Kentucky, Louisiana, Maine, Massachusetts, Minnesota, Montana, New
York, Oklahoma or the state of Washington, you must present an
additional form of identification to enter the federal building.
Acceptable alternative forms of identification include: Federal
employee badges, passports, enhanced driver's licenses and military
identification cards. In addition, you will need to obtain a property
pass for any personal belongings you bring with you. Upon leaving the
building, you will be required to return this property pass to the
security desk. No large signs will be allowed in the building, cameras
may only be used outside of the building and demonstrations will not be
allowed on federal property for security reasons.
    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 comments and
supporting information presented at the public hearing. Commenters
should notify Ms. Hunt if they will need specific equipment, or if
there are other special needs related to providing comments at the
hearings. Verbatim transcripts of the hearing and written statements
will be included in the docket for the rulemaking. 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 hearing to run either
ahead of schedule or behind schedule. Again, a hearing will not be held
unless requested. Please contact Ms. Virginia Hunt at<PHONE_NUMBER> or
at<EMAIL_ADDRESS>to request or register to speak at the hearing
or to inquire as to whether or not a hearing will be held.

FOR FURTHER INFORMATION CONTACT: For questions about this proposed
action, contact Ms. Rochelle Boyd, 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<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, NC 27711; telephone number:<PHONE_NUMBER>; fax number:
(919) 541-0840; and email address<EMAIL_ADDRESS>For information
about the applicability of the NESHAP to a particular entity, contact
Scott Throwe, Office of Enforcement and Compliance Assurance (OECA),
telephone number<PHONE_NUMBER>; and email address:
<EMAIL_ADDRESS>
SUPPLEMENTARY INFORMATION:
    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:
ACGIH American Conference of Government Industrial Hygienists
AEGL acute exposure guideline levels
AERMOD air dispersion model used by the HEM-3 model
AMOS ample margin of safety
ATSDR Agency for Toxic Substances and Disease Registry
BACT best available control technology
CAA Clean Air Act
CalEPA California Environmental Protection Agency
CBI confidential business information
CFR Code of Federal Regulations
D/F dioxins and furans
EJ environmental justice
EPA United States Environmental Protection Agency
ERPG Emergency Response Planning Guidelines
ERT Electronic Reporting Tool
HAP hazardous air pollutants
HCl hydrogen chloride
HEM-3 Human Exposure Model, Version 3
HF hydrogen fluoride
HI hazard index
HQ hazard quotient
ICR information collection request
IRIS Integrated Risk Information System
km kilometer
lb/yr pounds per year
LOAEL lowest-observed-adverse-effect level
MACT maximum achievable control technology
mg/m\3\ milligrams per cubic meter
MIR maximum individual risk
NAAQS National Ambient Air Quality Standard
NAICS North American Industry Classification System
NAS National Academy of Sciences
NATA National Air Toxics Assessment
NEI National Emissions Inventory
NESHAP National Emissions Standards for Hazardous Air Pollutants
NOAEL no observed adverse effects level
NRC National Research Council
NTTAA National Technology Transfer and Advancement Act
O&M operation and maintenance
OAQPS Office of Air Quality Planning and Standards
OECA Office of Enforcement and Compliance Assurance
OMB Office of Management and Budget
OM&M operation, maintenance and monitoring
PAH polycyclic aromatic hydrocarbons
PB-HAP hazardous air pollutants known to be persistent and bio-
accumulative in the environment
PEL probable effect levels
PM particulate matter
POM polycyclic organic matter
REL reference exposure level
RFA Regulatory Flexibility Act
RfC reference concentration
RfD reference dose
RTR residual risk and technology review
SAB Science Advisory Board
SAPU secondary aluminum processing unit
SBA Small Business Administration
SOP standard operating procedures
SSM startup, shutdown, and malfunction
TEQ toxic equivalents
THC total hydrocarbons
TOSHI target organ-specific hazard index
tpy tons per year
TRIM.FaTE Total Risk Integrated Methodology Fate, Transport and
Ecological Exposure model
TTN Technology Transfer Network
UBC used beverage containers
UF uncertainty factor
[micro]g/m\3\ microgram per cubic meter
UMRA Unfunded Mandates Reform Act
URE unit risk estimate
WHO World Health Organization
    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?
    C. What should I consider as I prepare my comments for the EPA?
II. Background Information
    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 is the history of the Secondary Aluminum Risk and
Technology Review?
    D. What data collection activities were conducted to support
this action?
III. Analytical Procedures
    A. How did we evaluate the post-MACT risks posed by the
Secondary Aluminum Production source category in the risk assessment
developed for this supplemental proposal?
    B. How did we consider the risk results in making decisions for
this supplemental proposal?
    C. How did we perform the technology review?
IV. Revised Analytical Results and Proposed Decisions for the
Secondary Aluminum Production Source Category
    A. What are the results of the risk assessment and analysis?
    B. What are our proposed decisions regarding risk acceptability,
ample margin of safety and adverse environmental effects based on
our revised analyses?
    C. What are the results and proposed decisions based on our
technology review?
    D. What other actions are we proposing?
    E. What compliance dates are we proposing?
V. Summary of the Revised 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. Paperwork Reduction Act
    C. Regulatory Flexibility Act
    D. Unfunded Mandates Reform Act
    E. Executive Order 13132: Federalism
    F. Executive Order 13175: Consultation and Coordination with
Indian Tribal Governments
    G. Executive Order 13045: Protection of Children from
Environmental Health Risks and Safety Risks
    H. Executive Order 13211: Actions Concerning Regulations That
Significantly Affect Energy Supply, Distribution, or Use
    I. National Technology Transfer and Advancement Act
    J. 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 regulated industrial source category that is the subject of
this supplemental proposal is listed in Table 1 of this preamble. Table
1 of this preamble is not intended to be exhaustive, but rather
provides a guide for readers regarding the entities likely to be
affected by this proposed action. These standards, once finalized, will
be directly applicable to affected sources. Federal, state, local and
tribal government entities are not affected by this proposed action. To
determine whether your facility would be affected, you should examine
the applicability criteria in the NESHAP. The Secondary Aluminum
Production source category includes any facility using clean charge,
aluminum scrap or dross from aluminum production, as the raw material
and performing one or more of the following processes: scrap shredding,
scrap drying/delacquering/decoating, thermal chip drying, furnace
operations (i.e., melting, holding, sweating, refining, fluxing or
alloying), recovery of aluminum from dross, in-line fluxing or dross
cooling.

    Table 1--NESHAP and Industrial Source Categories Affected by This
                             Proposed Action
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                                                                 NAICS
        Industrial source category                NESHAP        Code \a\
------------------------------------------------------------------------
Secondary Aluminum Production.............  Secondary........     331314
Primary Aluminum Production Facilities....  Aluminum.........     331312
Aluminum Sheet, Plate, and Foil             Production.......     331315
 Manufacturing Facilities.
Aluminum Extruded Product Manufacturing     .................     331316
 Facilities.
Other Aluminum Rolling and Drawing          .................     331319
 Facilities.
Aluminum Die Casting Facilities...........  .................     331521
Aluminum Foundry Facilities...............  .................     331524
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\a\ North American Industry Classification System.

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

    In addition to being available in the docket, an electronic copy of
this action is available on the Internet through EPA's Technology
Transfer Network (TTN) Web site, a forum for information and technology
exchange in various areas of air pollution control. Following signature
by the EPA Administrator, the EPA will post a copy of this supplemental
proposal at: http://www.epa.gov/ttn/atw/alum2nd/alum2pg.html. Following
publication in the Federal Register, the EPA will post the Federal
Register version of the proposal and key technical documents at this
same Web site. Information on the overall residual risk and technology
review program is available at the following Web site: http://www.epa.gov/ttn/atw/rrisk/rtrpg.html.

C. What should I consider as I prepare my comments for the EPA?

    Submitting CBI. Do not submit information containing CBI to the EPA
through http://www.regulations.gov or email. Clearly mark the part or
all of the information that you claim to be CBI. For CBI information on
a disk or CD-ROM that you mail to the EPA, mark the outside of the disk
or CD-ROM as CBI and then identify electronically within the disk or
CD-ROM 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 for inclusion in the public docket. If
you submit a CD-ROM or disk that does not contain CBI, mark the outside
of the disk or CD-ROM 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: Roberto
Morales, 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-2010-0544.

II. Background Information

A. What is the statutory authority for this action?

    Section 112 of the Clean Air Act (CAA) establishes a two-stage
regulatory process to address emissions of hazardous air pollutants
(HAP) from stationary sources. In the first stage, after the EPA has
identified categories of sources emitting one or more of the HAP listed
in CAA section 112(b), CAA section 112(d) requires us to promulgate
technology-based NESHAP for those sources. ``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. For major
sources, 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) and
are commonly referred to as maximum achievable control technology
(MACT) standards.
    MACT standards must reflect the maximum degree of emission
reduction achievable through the application of measures, processes,
methods, systems or techniques, including, but not limited to, measures
that (1) reduce the volume of or eliminate pollutants through process
changes, substitution of materials or other modifications; (2) enclose
systems or processes to eliminate emissions; (3) capture or treat
pollutants when released from a process, stack, storage or fugitive
emissions point; (4) are design, equipment, work practice or
operational standards (including requirements for operator training or
certification); or (5) are a combination of the above. CAA section
112(d)(2)(A) through (E). The MACT standards may take the form of
design, equipment, work practice or operational standards where the EPA
first determines either that (1) a pollutant cannot be emitted through
a conveyance designed and constructed to emit or capture the pollutant,
or that any requirement for, or use of, such a conveyance would be
inconsistent with law; or (2) the application of measurement
methodology to a particular class of sources is not practicable due to
technological and economic limitations. CAA section 112(h)(1) and (2).
    The MACT ``floor'' is the minimum control level allowed for MACT
standards promulgated under CAA section 112(d)(3) and may not be based
on cost considerations. For new sources, the MACT floor cannot be less
stringent than the emissions control that is achieved in practice by
the best-controlled similar source. The MACT floor for existing sources
can be less stringent than floors for new sources but not less
stringent than the average emissions limitation achieved by the best-
performing 12 percent of existing sources in the category or
subcategory (or the best-performing five sources for categories or
subcategories with fewer than 30 sources). In developing MACT
standards, the EPA must also consider control options that are more
stringent than the floor. We may establish standards more stringent
than the floor based on considerations of the cost of achieving the
emission reductions, any non-air quality health and environmental
impacts and energy requirements.
    The EPA is then required to review these technology-based standards
and revise them ``as necessary (taking into account developments in
practices, processes, and control technologies)'' no less frequently
than every eight years. CAA section 112(d)(6). In conducting this
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 second stage in standard-setting focuses on reducing any
remaining (i.e., ``residual'') risk according to CAA section 112(f).
Section 112(f)(1) required that the EPA prepare a report to Congress
discussing (among other things) methods of calculating the risks posed
(or potentially posed) by sources after implementation of the MACT
standards, the public health significance of those risks and the EPA's
recommendations as to legislation regarding such remaining risk. The
EPA prepared and submitted the Residual Risk Report to Congress, EPA-
453/R-99-001 (Risk Report) in March 1999. CAA section 112(f)(2) then
provides that if Congress does not act on any recommendation in the
Risk Report, the EPA must analyze and address residual risk for each
category or subcategory of sources 8 years after promulgation of such
standards pursuant to CAA section 112(d).
    Section 112(f)(2) of the CAA requires the EPA to determine for
source categories subject to MACT standards whether the emission
standards provide an ample margin of safety to protect public health.
Section 112(f)(2)(B) of the CAA expressly preserves the EPA's use of
the two-step process for developing standards to address any residual
risk and the agency's interpretation of ``ample margin of safety''
developed in the National Emissions Standards for Hazardous Air
Pollutants: Benzene Emissions from Maleic Anhydride Plants,
Ethylbenzene/Styrene Plants, Benzene Storage Vessels, Benzene Equipment
Leaks, and Coke By-Product Recovery Plants (Benzene NESHAP) (54 FR
38044, September 14, 1989). The EPA notified Congress in the Risk
Report that the agency intended to use the Benzene NESHAP approach in
making CAA section 112(f) residual risk determinations (EPA-453/R-99-
001, p. ES-11). The EPA subsequently adopted this approach in its
residual risk determinations and in a challenge to the risk review for
the Synthetic Organic Chemical Manufacturing source category, the
United States Court of Appeals for the District of Columbia Circuit
upheld as reasonable the EPA's interpretation that subsection 112(f)(2)
incorporates the approach established in the Benzene NESHAP. See NRDC
v. EPA, 529 F.3d 1077, 1083 (D.C. Cir. 2008)(``[S]ubsection
112(f)(2)(B) expressly incorporates the EPA's interpretation of the
Clean Air Act from the Benzene standard, complete with a citation to
the Federal Register.''); see also A Legislative History of the Clean
Air Act Amendments of 1990, vol. 1, p. 877 (Senate debate on Conference
Report).
    The first step in the process of evaluating residual risk is the
determination of acceptable risk. If risks are unacceptable, the EPA
cannot consider cost in identifying the emissions standards necessary
to bring risks to an acceptable level. The second step is the
determination of whether standards must be further revised in order to
provide an ample margin of safety to protect public health. The ample
margin of safety is the level at which the standards must be set,
unless an even more stringent standard is necessary to prevent, taking
into consideration costs, energy, safety and other relevant factors, an
adverse environmental effect.
1. Step 1--Determination of Acceptability
    The agency in the Benzene NESHAP concluded that ``the acceptability
of risk under section 112 is best judged on the basis of a broad set of
health risk measures and information'' and that the ``judgment on
acceptability cannot be reduced to any single factor.'' Benzene

NESHAP at 38046. The determination of what represents an ``acceptable''
risk is based on a judgment of ``what risks are acceptable in the world
in which we live'' (Risk Report at 178, quoting NRDC v. EPA, 824 F. 2d
1146, 1165 (D.C. Cir. 1987) (en banc) (``Vinyl Chloride''), recognizing
that our world is not risk-free.
    In the Benzene NESHAP, we stated that ``EPA will generally presume
that if the risk to [the maximum exposed] individual is no higher than
approximately one in 10 thousand, that risk level is considered
acceptable.'' 54 FR at 38045. We discussed the maximum individual
lifetime cancer risk (or maximum individual risk (MIR)) as being ``the
estimated risk that a person living near a plant would have if he or
she were exposed to the maximum pollutant concentrations for 70
years.'' Id. We explained that this measure of risk ``is an estimate of
the upper bound of risk based on conservative assumptions, such as
continuous exposure for 24 hours per day for 70 years.'' Id. We
explained that this measure of risk ``is an estimate of the upper bound
of risk based on conservative assumptions, such as continuous exposure
for 24 hours per day for 70 years.'' Id. We acknowledged that maximum
individual lifetime cancer risk ``does not necessarily reflect the true
risk, but displays a conservative risk level which is an upper-bound
that is unlikely to be exceeded.'' Id.
    Understanding that there are both benefits and limitations to using
the MIR as a metric for determining acceptability, we acknowledged in
the Benzene NESHAP that ``consideration of maximum individual risk . .
. must take into account the strengths and weaknesses of this measure
of risk.'' Id. Consequently, the presumptive risk level of 100-in-1
million (1-in-10 thousand) provides a benchmark for judging the
acceptability of maximum individual lifetime cancer risk, but does not
constitute a rigid line for making that determination. Further, in the
Benzene NESHAP, we noted that:

``[p]articular attention will also be accorded to the weight of
evidence presented in the risk assessment of potential
carcinogenicity or other health effects of a pollutant. While the
same numerical risk may be estimated for an exposure to a pollutant
judged to be a known human carcinogen, and to a pollutant considered
a possible human carcinogen based on limited animal test data, the
same weight cannot be accorded to both estimates. In considering the
potential public health effects of the two pollutants, the Agency's
judgment on acceptability, including the MIR, will be influenced by
the greater weight of evidence for the known human carcinogen.''

Id. at 38046. The agency also explained in the Benzene NESHAP that:

``[i]n establishing a presumption for MIR, rather than a rigid line
for acceptability, the Agency intends to weigh it with a series of
other health measures and factors. These include the overall
incidence of cancer or other serious health effects within the
exposed population, the numbers of persons exposed within each
individual lifetime risk range and associated incidence within,
typically, a 50 km exposure radius around facilities, the science
policy assumptions and estimation uncertainties associated with the
risk measures, weight of the scientific evidence for human health
effects, other quantified or unquantified health effects, effects
due to co-location of facilities, and co-emission of pollutants.''

Id. at 38045. In some cases, these health measures and factors taken
together may provide a more realistic description of the magnitude of
risk in the exposed population than that provided by maximum individual
lifetime cancer risk alone.

    As noted earlier, in NRDC v. EPA, the court held that section
112(f)(2) ``incorporates the EPA's interpretation of the Clean Air Act
from the Benzene Standard.'' The court further held that Congress'
incorporation of the Benzene standard applies equally to carcinogens
and non-carcinogens. 529 F.3d at 1081-82. Accordingly, we also consider
non-cancer risk metrics in our determination of risk acceptability and
ample margin of safety.
2. Step 2--Determination of Ample Margin of Safety
    CAA section 112(f)(2) requires the EPA to determine, for source
categories subject to MACT standards, whether those standards provide
an ample margin of safety to protect public health. As explained in the
Benzene NESHAP, ``the second step of the inquiry, determining an `ample
margin of safety,' again includes consideration of all of the health
factors, and whether to reduce the risks even further. . . . Beyond
that information, additional factors relating to the appropriate level
of control will also be considered, including costs and economic
impacts of controls, technological feasibility, uncertainties and any
other relevant factors. Considering all of these factors, the agency
will establish the standard at a level that provides an ample margin of
safety to protect the public health, as required by section 112.'' 54
FR at 38046, September 14, 1989.
    According to CAA section 112(f)(2)(A), if the MACT standards for
HAP ``classified as a known, probable, or possible human carcinogen do
not reduce lifetime excess cancer risks to the individual most exposed
to emissions from a source in the category or subcategory to less than
one in one million,'' the EPA must promulgate residual risk standards
for the source category (or subcategory), as necessary to provide an
ample margin of safety to protect public health. In doing so, the EPA
may adopt standards equal to existing MACT standards if the EPA
determines that the existing standards (i.e., the MACT standards) are
sufficiently protective. NRDC v. EPA, 529 F.3d 1077, 1083 (D.C. Cir.
2008) (``If EPA determines that the existing technology-based standards
provide an `ample margin of safety,' then the Agency is free to readopt
those standards during the residual risk rulemaking.'') The EPA must
also adopt more stringent standards, if necessary, to prevent an
adverse environmental effect,\1\ but must consider cost, energy, safety
and other relevant factors in doing so.
---------------------------------------------------------------------------

    \1\ ``Adverse environmental effect'' is defined as any
significant and widespread adverse effect, which may be reasonably
anticipated to wildlife, aquatic life or natural resources,
including adverse impacts on populations of endangered or threatened
species or significant degradation of environmental qualities over
broad areas. CAA section 112(a)(7).
---------------------------------------------------------------------------

    The CAA does not specifically define the terms ``individual most
exposed,'' ``acceptable level'' and ``ample margin of safety.'' In the
Benzene NESHAP, 54 FR at 38044-38045, September 14, 1989, we stated as
an overall objective:

In protecting public health with an ample margin of safety under
section 112, EPA strives to provide maximum feasible protection
against risks to health from hazardous air pollutants by (1)
protecting the greatest number of persons possible to an individual
lifetime risk level no higher than approximately 1-in-1 million and
(2) limiting to no higher than approximately 1-in-10 thousand [i.e.,
100-in-1 million] the estimated risk that a person living near a
plant would have if he or she were exposed to the maximum pollutant
concentrations for 70 years.

    The agency further stated that ``[t]he EPA also considers incidence
(the number of persons estimated to suffer cancer or other serious
health effects as a result of exposure to a pollutant) to be an
important measure of the health risk to the exposed population.
Incidence measures the extent of health risks to the exposed population
as a whole, by providing an estimate of the occurrence of cancer or
other serious health effects in the exposed population.'' Id. at 38045.
    In the ample margin of safety decision process, the agency again
considers all of the health risks and other health information
considered in the first step, including the incremental risk reduction

associated with standards more stringent than the MACT standard or a
more stringent standard that the EPA has determined is necessary to
ensure risk is acceptable. In the ample margin of safety analysis, the
agency considers additional factors, including costs and economic
impacts of controls, technological feasibility, uncertainties and any
other relevant factors. Considering all of these factors, the agency
will establish the standard at a level that provides an ample margin of
safety to protect the public health, as required by CAA section 112(f).
54 FR 38046, September 14, 1989.

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

    The Secondary Aluminum Production source category includes
facilities that produce aluminum from scrap aluminum material and
consists of the following operations: (1) Preprocessing of scrap
aluminum, including size reduction and removal of oils, coatings and
other contaminants; (2) furnace operations, including melting, in-
furnace refining, fluxing and tapping; (3) additional refining, by
means of in-line fluxing; and (4) cooling of dross. The following
sections include descriptions of the affected sources in the Secondary
Aluminum Production source category, the origin of HAP emissions from
these affected sources and factors affecting the emissions.
    Scrap aluminum is often preprocessed prior to melting.
Preprocessing steps may include shredding to reduce the size of
aluminum scrap; drying of oily scrap such as machine turnings and
borings; and/or heating in a scrap dryer, delacquering kiln or
decoating kiln to remove coatings or other contaminants that may be
present on the scrap. Heating of high iron content scrap in a sweat
furnace to reclaim the aluminum content is also a preprocessing
operation.
    Crushing, shredding and grinding operations are used to reduce the
size of scrap aluminum. Particulate matter (PM) and HAP metals
emissions are generated as dust from coatings and other contaminants
contained in the scrap aluminum.
    A chip dryer is used to evaporate oil and/or moisture from uncoated
aluminum chips and borings. Chip dryers typically operate at
temperatures ranging between 150 [deg]C to 400 [deg]C (300 [deg]F to
750 [deg]F). An uncontrolled chip dryer may emit dioxins and furans (D/
F) and total hydrocarbons (THC), of which some fraction is organic HAP.
    Painted and/or coated materials are processed in a scrap dryer/
delacquering kiln/decoating kiln to remove coatings and other
contaminants that may be present in the scrap prior to melting.
Coatings, oils, grease and lubricants represent up to 20 percent of the
total weight of these materials. Organic HAP, D/F and inorganic HAP
including particulate metal HAP are emitted during the drying/
delacquering/decoating process.
    Used beverage containers (UBC) comprise a major portion of the
recycled aluminum scrap used as feedstock by the industry. In scrap
drying/delacquering/decoating operations, UBC and other post-consumer
coated products (e.g., aluminum siding) are heated to an exit
temperature of up to 540 [deg]C (1,000 [deg]F) to volatilize and remove
various organic contaminants such as paints, oils, lacquers, rubber and
plastic laminates prior to melting. An uncontrolled scrap dryer/
delacquering kiln/decoating kiln emits PM (of which some fraction is
particulate metal HAP), hydrogen chloride (HCl), THC (of which some
fraction is organic HAP) and D/F.
    A sweat furnace is typically used to reclaim (or ``sweat'') the
aluminum from scrap with high levels of iron. These furnaces operate in
batch mode at a temperature that is high enough to melt the aluminum,
but not high enough to melt the iron. The aluminum melts and flows out
of the furnace while the iron remains in the furnace in solid form. The
molten aluminum can be cast into sows, ingots or T-bars that are used
as feedstock for aluminum melting and refining furnaces. Alternately,
molten aluminum can be fed directly to a melting or refining furnace.
An uncontrolled sweat furnace may emit D/F.
    Process (i.e., melting, holding or refining) furnaces are
refractory-lined metal vessels heated by an oil or gas burner to
achieve a metal temperature of about 760 [deg]C (1,400 [deg]F). The
melting process begins with the charging of scrap into the furnace. A
gaseous (typically, chlorine) or salt flux may be added to remove
impurities and reduce aluminum oxidation. Once molten, the chemistry of
the bath is adjusted by adding selected scrap or alloying agents, such
as silicon. Salt and other fluxes contain chloride and fluoride
compounds that may be released when introduced to the bath. HCl may
also be released when chlorine-containing contaminants (such as
polyvinyl chloride coatings) present in some types of scrap are
introduced to the bath. Argon and nitrogen fluxes are not reactive and
do not produce HAP. In a sidewell melting furnace, fluxing is performed
in the sidewell, and fluxing emissions from the sidewell are
controlled. In this type of furnace, fluxing is not typically done in
the hearth, and hearth emissions (which include products of combustion
from the oil and gas-fired furnaces) are typically uncontrolled.
    Process furnaces may process contaminated scrap which can result in
HAP emissions. In addition, fluxing agents may contain compounds
capable of producing HAP, some fraction of which is emitted from the
furnace. Process furnaces are significant sources of HAP emissions in
the secondary aluminum industry. An uncontrolled melting furnace which
processes contaminated scrap and uses reactive fluxes emits PM (of
which some fraction is particulate metal HAP), HCl and D/F.
    Process furnaces are divided into group 1 and group 2 furnaces.
Group 1 furnaces are unrestricted in the type of scrap they process and
the type of fluxes they can use. Group 2 furnaces process only clean
charge and conduct no reactive fluxing.
    Dross-only furnaces are furnaces dedicated to reclamation of
aluminum from drosses formed during the melting/holding/alloying
operations carried out in other furnaces. Exposure to the atmosphere
causes the molten aluminum to oxidize, and the flotation of the
impurities to the surface along with any salt flux creates ``dross.''
Prior to tapping, the dross is periodically skimmed from the surface of
the aluminum bath and cooled. Dross-only furnaces are typically rotary
barrel furnaces (also known as salt furnaces). A dross-only furnace
emits PM (of which some fraction is particulate metal HAP).
    Rotary dross coolers are devices used to cool dross in a rotating,
water-cooled drum. A rotary dross cooler emits PM (of which some
fraction is particulate metal HAP).
    In-line fluxers are devices used for aluminum refining, including
degassing, outside the furnace. The process involves the injection of
chlorine, argon, nitrogen or other gases to achieve the desired metal
purity. In-line fluxers are found primarily at facilities that
manufacture very high quality aluminum or in facilities with no other
means of degassing. An in-line fluxer operating without emission
controls emits HCl and PM.
    A summary description of requirements in the existing subpart RRR
NESHAP is provided below for the convenience of the reader. The
inclusion of this description, however, does not reopen the existing
rule requirements and we are neither reconsidering nor soliciting
public comment on the requirements

described. In addition, this summary description should not be relied
on to determine applicability of the regulatory provisions or
compliance obligations. The proposed decisions and rule amendments
addressed in section IV below are the only provisions on which we are
taking comment.
    The NESHAP for the Secondary Aluminum Production source category
were promulgated on March 23, 2000 (65 FR 15690) and codified at 40 CFR
part 63, subpart RRR (referred to from here on as subpart RRR in the
remainder of this document). The rule was amended at 67 FR 79808,
December 30, 2002; 69 FR 53980, September 3, 2004; 70 FR 57513, October
3, 2005 and 70 FR 75320, December 19, 2005. The existing subpart RRR
NESHAP regulates HAP emissions from secondary aluminum production
facilities that are major sources of HAP that operate aluminum scrap
shredders, thermal chip dryers, scrap dryers/delacquering kilns/
decoating kilns, group 1 furnaces, group 2 furnaces, sweat furnaces,
dross-only furnaces, rotary dross coolers and secondary aluminum
processing units (SAPUs). The SAPUs include group 1 furnaces and in-
line fluxers. The subpart RRR NESHAP regulates HAP emissions from
secondary aluminum production facilities that are area sources of HAP
only with respect to emissions of D/F from thermal chip dryers, scrap
dryers/delacquering kilns/decoating kilns, group 1 furnaces, sweat
furnaces and SAPUs.
    The secondary aluminum industry consists of approximately 161
secondary aluminum production facilities, of which the EPA estimates 53
to be major sources of HAP. The HAP emitted by these facilities are
metals, organic HAP, D/F, HCl and hydrogen fluoride (HF).
    Several of the secondary aluminum facilities are co-located with
primary aluminum, coil coating and possibly other source category
facilities. Natural gas boilers or process heaters may also be co-
located at a few secondary aluminum facilities.
    The standards promulgated in 2000 established emission limits for
PM as a surrogate for metal HAP, THC as a surrogate for organic HAP
other than D/F, D/F expressed as toxic equivalents and HCl as a
surrogate for acid gases including HF, chlorine and fluorine. HAP are
emitted from the following affected sources: Aluminum scrap shredders
(subject to PM standards), thermal chip dryers (subject to standards
for THC and D/F), scrap dryers/delacquering kilns/decoating kilns
(subject to standards for PM, D/F, HCl and THC), sweat furnaces
(subject to D/F standards), dross-only furnaces (subject to PM
standards), rotary dross coolers (subject to PM standards), group 1
furnaces (subject to standards for PM, HCl and D/F) and in-line fluxers
(subject to standards for PM and HCl). Group 2 furnaces and certain in-
line fluxers are subject to work practice standards. Table 2 provides a
summary of the current MACT emissions limits for existing and new
sources under the subpart RRR NESHAP.

[GRAPHIC] [TIFF OMITTED] TP08DE14.500

[GRAPHIC] [TIFF OMITTED] TP08DE14.501

[GRAPHIC] [TIFF OMITTED] TP08DE14.502

    Control devices currently in use to reduce emissions from affected
sources subject to the subpart RRR NESHAP include fabric filters for
control of PM from aluminum scrap shredders; afterburners for control
of THC and D/F from thermal chip dryers; afterburners plus lime-
injected fabric filters for control of PM, HCl, THC and D/F from scrap
dryers/delacquering kilns/decoating kilns; afterburners for control of
D/F from sweat furnaces; fabric filters for control of PM from dross-
only furnaces and rotary dross coolers; lime-injected fabric filters
for control of PM and HCl from in-line fluxers; and lime-injected
fabric filters for control of PM, HCl and D/F from group 1 furnaces.
All affected sources with add-on controls are also subject to design
requirements and operating limits to limit fugitive emissions.
    Compliance with the emission limits in the current rule is
demonstrated by an initial performance test for each affected source.
Repeat performance tests are required every 5 years. Area sources are
only subject to one-time performance tests for D/F. After the
compliance tests, facilities are required to monitor various control
parameters or conduct other types of monitoring to ensure continuous
compliance with the MACT standards. Owners or operators of sweat
furnaces that operate an afterburner that meets temperature and
residence time requirements are not required to conduct performance
tests.

C. What is the history of the Secondary Aluminum Risk and Technology
Review?

    On February 14, 2012 (77 FR 8576), we proposed that no amendments
to subpart RRR were necessary as a result of the residual risk and
technology review (RTR) conducted for the Secondary Aluminum Production
source category. In the same notice (77 FR 8576, which is referred to
as the 2012 proposal in the remainder of this Federal Register
document), we proposed amendments to correct and clarify existing
requirements in subpart RRR. In this supplemental proposal, we are
soliciting comment on modified proposed amendments to the subpart RRR
rule requirements and on alternative compliance options related to
sweat furnaces. The proposed revisions and alternative compliance
options, described in more detail later in this document, on which we
are soliciting comment are:
     Revised proposed limit on number of allowed furnace
operating mode changes per year (i.e., frequency) in proposed section
63.1514(e) of four times in any 6-month period, with the ability of
sources to apply to the appropriate authority for additional furnace
operating mode changes;
     Revised wording in proposed section 63.1511(b)(1) related
to testing under worst-case scenario clarifying under what conditions
the performance tests are to be conducted;
     Revised proposed requirements to account for fugitive
emissions during performance testing of uncontrolled furnaces,
including: (1) Installation of hooding according to American Conference
of Government Industrial Hygienists (ACGIH) guidelines; (2) application
of an assumption of 67 percent capture/control efficiency when
calculating emissions; or (3) in certain cases where installing ACGIH
hooding is impractical, allowing the facility to petition the
permitting authority for major sources or the Administrator for area
sources, for approval to use alternative testing procedures that will
minimize fugitive emissions;
     Revised proposed requirement that emission sources comply
with the emissions limits at all times including periods of startup and
shutdown. Definitions of startup and shutdown are

being proposed as well as an alternative method for demonstrating
compliance with emission limits;
     Revised proposed monitoring requirements in section
63.1510(d)(2) that require annual inspection of capture/collection
systems;
     Revised proposed compliance dates of 180 days for certain
requirements and 2 years for other requirements; and
     Revised operating and monitoring requirements for
demonstrating compliance for sweat furnaces.
    In addition, we are withdrawing our 2012 proposal to include
provisions establishing an affirmative defense in light of a recent
court decision vacating an affirmative defense in one of the EPA's
section 112(d) regulations. NRDC v. EPA, 749 F.3d 1055 (D.C. Cir. 2014)
(vacating affirmative defense provisions in Section 112(d) rule
establishing emission standards for Portland cement kilns).
    After reviewing the comments, data and other information received
after the 2012 proposal, we determined it is appropriate to present
certain revised analyses and revised proposed amendments in this
supplemental proposal to allow the public an opportunity to review and
comment on these revised analyses and revised proposed amendments.
    The 2012 proposal also contained other proposed requirements
(topics listed below) for which we have not made any changes to the
analyses, and, therefore, on which we are not seeking public comment in
this document. Other amendments or requirements that we proposed in
2012, which we are not re-opening for comment, are the following:
     Electronic reporting.
     ACGIH Guidelines.
     Lime injection rate.
     Flux monitoring.
     Cover flux.
     Bale breakers.
     Bag Leak Detection Systems (BLDS).
     Sidewell furnaces.
     Testing representative units.
     Initial performance tests.
     Scrap dryer/delacquering/decoating kiln definition.
     Group 2 furnace definition.
     HF emissions compliance.
     SAPU definition.
     Clean charge definition.
     Residence time definition.
     SAPU feed/charge rate.
     Dross-only versus dross/scrap furnaces.
     Applicability of rule to area sources.
     Altering parameters during testing with new scrap streams.
     Controlled furnaces that are temporarily idled for 24
hours or longer.
     Annual compliance certification for area sources.
    The comment period for the February 2012 proposal ended on April
13, 2012. We will address the comments we received during the public
comment period for the 2012 proposal, as well as comments received
during the comment period for this supplemental proposal, at the time
we take final action.
    Subpart RRR inadvertently uses several different terms for the
agency that has primary responsibility for implementation of certain
subpart RRR provisions. The terms used include ``responsible permitting
authority,'' ``permitting authority,'' ``applicable permitting
authority'' and ``delegated authority.'' Depending on the particular
state and whether the facility is a major or area source, the
permitting authority and the delegated authority for purposes of
subpart RRR may be the same or may differ. Therefore, the EPA deems it
appropriate to clarify for purposes of these specific subpart RRR
provisions that the ``permitting authority'' (defined in the General
Provisions as the Title V permitting authority) is the primary
implementing authority for major sources, and the Administrator is the
primary implementing authority for area sources. The General Provisions
define ``Administrator'' to mean the EPA Administrator or his or her
authorized representative (e.g., a state that has been delegated
authority to implement Subpart RRR).
    Where these terms for the implementing authority appear in this
supplemental proposal, we have made the necessary corrections. We plan
to correct the remainder of these references when we issue the final
rule.

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

    For the risk analysis performed for the 2012 proposal, we compiled
a dataset from two primary sources: (1) A nine-company testing
information collection request (ICR) sent in May 2010, and (2) an all-
company ICR sent to companies in February 2011. These data collection
efforts are described in the 2012 proposal, and a comprehensive
description of the emissions data, calculations and risk assessment
inputs are in the memorandum, Development of the RTR Risk Modeling
Dataset for the Secondary Aluminum Production Source Category (Docket
item EPA-HQ-OAR-2010-0544-0149).
    For the revised risk analysis conducted for this supplemental
proposal, changes were made in the methodology used to calculate
allowable emissions. Generally, allowable emissions were calculated for
the 2012 proposal as the product of the emissions limit for the
secondary aluminum emissions unit and the maximum production capacity
of the unit. For the revised emissions modeling for this supplemental
proposal, the amount of charge to the unit from the all-company ICR was
used in the allowable emissions calculation, rather than the maximum
production capacity of the unit. Uniformly assuming that every piece of
equipment is being used at maximum capacity results in an overestimate
of total aluminum throughput that is much larger than the actual
throughput for the facility as a whole. Moreover, if we assume maximum
production capacity coupled with the assumption that all HAP are being
emitted at the highest level allowed by the MACT rule (i.e., at the
level of the emissions limit), this results in an overly conservative
estimate of emissions. This overestimation is magnified for large
facilities, with multiple pieces of equipment. Therefore, for this
supplemental proposal, the amount of charge to the unit from the all-
company ICR was used in the allowable emissions calculation, rather
than the maximum production capacity of the unit. Furthermore, this
revised methodology is consistent with EPA's risk assessment
methodology performed in other RTR modeling projects. See National
Emission Standards for Hazardous Air Pollutants: Primary Lead Smelting;
proposed rule (76 FR 9410, February 17, 2011), National Emissions
Standards for Hazardous Air Pollutants: Secondary Lead Smelting;
proposed rule (76 FR 29032, May 19, 2011) and National Emissions
Standards for Hazardous Air Pollutants: Ferroalloys Production (76 FR
72508, November 23, 2011). For an in-depth description of the revised
risk modeling dataset, including changes in methodologies between the
emissions modeling for the 2012 proposal and the emissions modeling for
this supplemental proposal, see the memorandum, Development of the RTR
Supplemental Proposal Risk Modeling Dataset for the Secondary Aluminum
Production Source Category, available in this rulemaking docket.
    As part of the revised risk analysis, process equipment and unit
emissions data used in the emissions modeling for the 2012 proposal
were also reviewed. Since cancer risks were driven by D/F emissions in
the modeling done for the 2012 proposal, we focused our refined
assessment on the D/F emissions data. The other modeled pollutants had
considerably lower estimated risks (compared to D/F) and the estimated

risks for all these HAP were well below the presumptive acceptable risk
levels.
    For almost all facilities, the D/F emissions reported in the 2011
ICR responses were used for the revised modeling. However, for the
companies operating the 10 facilities that had the highest modeled risk
from actual emissions in the modeling for the 2012 proposal, we
requested and received results from additional compliance D/F testing
that was conducted since the 2011 ICR. The results for all test runs
associated with 2011 ICR responses and all test runs received as part
of the request for additional test data were averaged together for each
facility to provide more accurate estimates of the D/F emissions and
resulting risks for these facilities. A memorandum comparing the 2011
emissions data with the revised emissions data used for this
supplemental proposal and the reasons for differences is available in
the docket for this rulemaking. See Modeling Input Revisions for the
RTR Risk Modeling Dataset for the Secondary Aluminum Production Source
Category.
    We also revised emissions data for primary aluminum operations at
primary aluminum facilities that were co-located at secondary aluminum
facilities. The revised primary aluminum emissions data were based on
recent test data used in the supplemental proposed rulemaking for the
Primary Aluminum Production source category. These data included the
following:
     Additional emission test data for polycyclic organic
matter (POM) emissions from prebake potlines;
     Additional emission test data for PM emissions from
prebake and Soderberg potlines, anode bake furnaces and paste plants;
     Additional emission test data for speciated polycyclic
aromatic hydrocarbons (PAHs), speciated HAP metals, speciated
polychlorinated biphenyls (PCBs) and speciated D/Fs from potlines,
anode bake furnaces and paste plants.

III. Analytical Procedures

A. How did we evaluate the post-MACT risks posed by the Secondary
Aluminum Production source category in the risk assessment developed
for this supplemental proposal?

    The EPA conducted a risk assessment that provides estimates of the
MIR posed by the HAP emissions from each source in the source category,
the hazard index (HI) for chronic exposures to HAP with the potential
to cause noncancer health effects and the hazard quotient (HQ) for
acute exposures to HAP with the potential to cause noncancer health
effects. The assessment also provides estimates of the distribution of
cancer risks within the exposed populations, cancer incidence and an
evaluation of the potential for adverse environmental effects. 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 used for this
revised assessment: Residual Risk Assessment for the Secondary Aluminum
Production Source Category in Support of the 2014 Supplemental
Proposal. The methods used to assess risks (as described in the seven
primary steps below) are consistent with those peer-reviewed by a panel
of the EPA's Science Advisory Board (SAB) in 2009 and described in
their peer review report issued in 2010; \3\ they are also consistent
with the key recommendations contained in that report.
---------------------------------------------------------------------------

    \3\ U.S. EPA SAB. 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, May 2010.
---------------------------------------------------------------------------

1. How did we estimate actual emissions and identify the emissions
release characteristics?
    As explained in section II.D above, the revised RTR emissions
dataset for the Secondary Aluminum Production source category
constitutes the basis for the revised risk assessment. This includes
recent test data received from the primary aluminum facilities that
were co-located at secondary aluminum production facilities. We
estimated the magnitude of emissions using emissions test data
collected through ICRs along with more recent data submitted by
companies with facilities identified as the highest risk facilities for
D/F emissions in the 2012 risk analysis. We also reviewed the
information regarding emissions release characteristics such as stack
heights, stack gas exit velocities, stack temperatures and source
locations. In addition to the data quality checks performed on the
source data for the facilities contained in the dataset, we also
verified the coordinates of every emission source in the dataset
through visual observations using Google Earth. We also performed data
quality checks on the emissions data and release characteristics. The
revised emissions data, the data quality checks and the methods used to
estimate emissions from all the various emissions sources, are
described in more detail in the technical documents: Development of the
RTR Supplemental Proposal Risk Modeling Dataset for the Secondary
Aluminum Production Source Category and Modeling Input Revisions for
the RTR Risk Modeling Dataset for the Secondary Aluminum Production
Source Category, which are available in the docket for this action.
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 the specified annual time
period. In some cases, these ``actual'' emission levels are lower than
the emission levels required to comply with the MACT standards. The
emissions level allowed to be emitted by the MACT standards is referred
to as the ``MACT-allowable'' emissions level. We discussed the use of
both MACT-allowable and actual emissions in the final Coke Oven
Batteries residual risk rule (70 FR 19998-19999, April 15, 2005) and in
the proposed and final Hazardous Organic NESHAP residual risk rules (71
FR 34428, June 14, 2006 and 71 FR 76609, December 21, 2006,
respectively). In those previous actions, we noted that assessing the
risks at the MACT-allowable level is inherently reasonable since these
risks reflect the maximum level facilities could emit and still comply
with national emission standards. We also explained that it is
reasonable to consider actual emissions, where such data are available,
in both steps of the risk analysis, in accordance with the Benzene
NESHAP approach (54 FR 38044, September 14, 1989).
    For this supplemental proposal, we evaluated allowable stack
emissions based on the level of control required by the subpart RRR
MACT standards. As described in section II.D above, changes were made
in the methodology used to calculate the allowable emissions for the
revised risk analysis conducted for this supplemental proposal. In the
2012 proposal, allowable emissions were calculated using the emissions
limits for the 67 secondary aluminum emissions units and the maximum
production capacity of each unit. For the revised emissions modeling,
the actual amount of charge to the unit from the all-company ICR was
used in the allowable emissions calculation, rather than the maximum
production capacity of the unit. The methodology used to calculate
allowable emissions is explained in more detail in the technical
documents: Development of the RTR Supplemental Proposal Risk Modeling
Dataset for the Secondary Aluminum Production

Source Category and Modeling Input Revisions for the RTR Risk Modeling
Dataset for the Secondary Aluminum Production Source Category, which
are available in the docket for this action.
3. How did we conduct dispersion modeling, determine inhalation
exposures and estimate individual and population inhalation risks?
    Both long-term and short-term inhalation exposure concentrations
and health risks from the source category addressed in this proposal
were estimated using the Human Exposure Model (Community and Sector
HEM-3 version 1.1.0). 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 \4\, and (3) estimating
individual and population-level inhalation risks using the exposure
estimates and quantitative dose-response information.
---------------------------------------------------------------------------

    \4\ This metric comes from the Benzene NESHAP. See 54 FR 38046.
---------------------------------------------------------------------------

    The air dispersion model used by the HEM-3 model (AERMOD) is one of
the EPA's preferred models for assessing pollutant concentrations from
industrial facilities.\5\ 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 (2011) of
hourly surface and upper air observations for more than 800
meteorological stations, selected to provide coverage of the United
States and Puerto Rico. A second library of United States Census Bureau
census block \6\ 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 unit risk factors and other
health benchmarks is used to estimate health risks. These risk factors
and health benchmarks are the latest values recommended by the EPA for
HAP and other toxic air pollutants. These values are available at
http://www.epa.gov/ttn/atw/toxsource/summary.html and are discussed in
more detail later in this section.
---------------------------------------------------------------------------

    \5\ 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).
    \6\ A census block is the smallest geographic area for which
census statistics are tabulated.
---------------------------------------------------------------------------

    In developing the risk assessment for chronic exposures, we used
the estimated annual average ambient air concentrations of each HAP
emitted by each major source and D/F emissions from each area source
for which we have emissions data in the source category. The air
concentrations at each nearby census block centroid were used as a
surrogate for the chronic inhalation exposure concentration for all the
people who reside in that census block. We calculated the MIR for each
facility as the cancer risk associated with a continuous lifetime (24
hours per day, 7 days per week and 52 weeks per year for a 70-year
period) exposure to the maximum concentration at the centroid of
inhabited census blocks. Individual cancer risks were calculated by
multiplying the estimated lifetime exposure to the ambient
concentration of each of the 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 probability 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
URE values from the EPA's Integrated Risk Information System (IRIS).
For carcinogenic pollutants without EPA IRIS values, we look to other
reputable sources of cancer dose-response values, often using
California EPA (CalEPA) URE values, where available. In cases where
new, scientifically credible dose response values have been developed
in a manner consistent with the EPA guidelines and have undergone a
peer review process similar to that used by the EPA, we may use such
dose-response values in place of, or in addition to, other values, if
appropriate.
    The EPA estimated incremental individual lifetime cancer risks
associated with emissions from the facilities in the source category as
the sum of the risks for each of the carcinogenic HAP (including those
classified as carcinogenic to humans, likely to be carcinogenic to
humans and suggestive evidence of carcinogenic potential \7\) emitted
by the modeled sources. Cancer incidence and the distribution of
individual cancer risks for the population within 50 km of the sources
were also estimated for the source category as part of this assessment
by summing individual risks. A distance of 50 km is consistent with
both the analysis supporting the 1989 Benzene NESHAP (54 FR 38044,
September 14, 1989) and the limitations of Gaussian dispersion models,
including AERMOD.
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    \7\ These classifications also coincide with the terms ``known
carcinogen, probable carcinogen, and possible carcinogen,''
respectively, which are the terms advocated in the EPA's previous
Guidelines for Carcinogen Risk Assessment, published in 1986 (51 FR
33992, September 24, 1986). Summing the risks of these individual
compounds to obtain the cumulative cancer risks 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: http://yosemite.epa.gov/sab/sabproduct.nsf/
214C6E915BB04E14852570CA007A682C/$File/ecadv02001.pdf.
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    To assess the risk of non-cancer health effects from chronic
exposures, we summed the HQ for each of the HAP that affects a common
target organ system to obtain the HI for that target organ system (or
target organ-specific HI, TOSHI). The HQ is the estimated exposure
divided by the chronic reference value, which is a value selected from
one of several sources. First, the chronic reference level can be the
EPA reference concentration (RfC) (http://www.epa.gov/riskassessment/glossary.htm), 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.'' Alternatively, in cases where an RfC from the EPA's IRIS
database is not available or where the EPA determines that using a
value other than the RfC is appropriate, the chronic reference level
can be a value from the following prioritized sources: (1) The Agency
for Toxic Substances and Disease Registry (ATSDR) Minimum Risk Level
(http://www.atsdr.cdc.gov/mrls/index.asp), which is defined as ``an
estimate of daily human exposure to a hazardous substance that is
likely to be without an appreciable risk of adverse non-cancer health
effects (other than cancer) over a specified duration of exposure'';
(2) the CalEPA Chronic Reference Exposure Level (REL) (http://www.oehha.ca.gov/air/hot_spots/pdf/HRAguidefinal.pdf), which is defined
as ``the concentration level (that is expressed in units of micrograms
per cubic meter ([mu]g/m\3\) for inhalation exposure and in a dose
expressed in units of milligram per kilogram-day (mg/kg-day) for oral
exposures), at or below which no adverse health effects are anticipated
for a specified exposure duration''; 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, in place of or in
concert with other values.
    The EPA also evaluated screening estimates of acute exposures and
risks for each of the HAP at the point of highest potential off-site
exposure for each facility. To do this, the EPA estimated the risks
when both the peak hourly emissions rate and worst-case dispersion
conditions occur. We also assume that a person is located at the point
of highest impact during that same time. In accordance with our mandate
in section 112 of the CAA, we use the point of highest off-site
exposure to assess the potential risk to the maximally exposed
individual. In some cases, the agency may choose to refine the acute
screen by also assessing the exposure that may occur at a centroid of a
census block. The acute HQ is the estimated acute exposure divided by
the acute dose-response value. In each case, the EPA calculated acute
HQ values using best available, short-term dose-response values. These
acute dose-response values, which are described below, include the
acute REL, acute exposure guideline levels (AEGL) and emergency
response planning guidelines (ERPG) for 1-hour exposure durations. As
discussed below, we used conservative assumptions for emissions rates,
meteorology and exposure location for our acute analysis.
    As described in the CalEPA's Air Toxics Hot Spots Program Risk
Assessment Guidelines, Part I, The Determination of Acute Reference
Exposure Levels for Airborne Toxicants, an acute REL value (http://www.oehha.ca.gov/air/pdf/acuterel.pdf) is defined as ``the
concentration level at or below which no adverse health effects are
anticipated for a specified exposure duration.'' Id. at page 2. Acute
REL values are based on the most sensitive, relevant, adverse health
effect reported in the peer-reviewed medical and toxicological
literature. Acute REL values 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.
    As we state above, in assessing the potential risks associated with
acute exposures to HAP, we do not follow a prioritization scheme and,
therefore, we consider available dose-response values from multiple
authoritative sources. In the RTR program, the EPA assesses acute risk
using toxicity values derived from one hour exposures.
    AEGL values were derived in response to recommendations from the
National Research Council (NRC). As described in Standing Operating
Procedures (SOP) of the National Advisory Committee on Acute Exposure
Guideline Levels for Hazardous Substances (http://www.epa.gov/oppt/aegl/pubs/sop.pdf),\8\ ``the NRC's previous name for acute exposure
levels--community emergency exposure levels--was replaced by the term
AEGL to reflect the broad application of these values to planning,
response, and prevention in the community, the workplace,
transportation, the military, and the remediation of Superfund sites.''
Id. at 2. This document also states that AEGL values ``represent
threshold exposure limits for the general public and are applicable to
emergency exposures ranging from 10 minutes to eight hours.'' Id. at 2.
---------------------------------------------------------------------------

    \8\ National Academy of Sciences (NAS), 2001. Standing Operating
Procedures for Developing Acute Exposure Levels for Hazardous
Chemicals, page 2.
---------------------------------------------------------------------------

    The document lays out the purpose and objectives of AEGL by stating
that ``the primary purpose of the AEGL program and the National
Advisory Committee for Acute Exposure Guideline Levels for Hazardous
Substances is to develop guideline levels for once-in-a-lifetime,
short-term exposures to airborne concentrations of acutely toxic, high-
priority chemicals.'' Id. at 21. In detailing the intended application
of AEGL values, the document states that ``[i]t is anticipated that the
AEGL values will be used for regulatory and nonregulatory purposes by
U.S. Federal and state agencies and possibly the international
community in conjunction with chemical emergency response, planning,
and prevention programs. More specifically, the AEGL values will be
used for conducting various risk assessments to aid in the development
of emergency preparedness and prevention plans, as well as real-time
emergency response actions, for accidental chemical releases at fixed
facilities and from transport carriers.'' Id. at 31.
    The AEGL-1 value is then 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.'' Id. at 3. 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. Similarly, the document defines AEGL-2 values 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.
    ERPG values are derived for use in emergency response, as described
in the American Industrial Hygiene Association's Emergency Response
Planning (ERP) Committee document titled, ERPGS Procedures and
Responsibilities (https://www.aiha.org/get-involved/AIHAGuidelineFoundation/EmergencyResponsePlanningGuidelines/Documents/ERP-SOPs2006.pdf), which states that, ``Emergency Response Planning
Guidelines were developed for emergency planning and are intended as
health based guideline concentrations for single exposures to
chemicals.'' \9\ Id. at 1. The ERPG-1 value 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 value 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.
---------------------------------------------------------------------------

    \9\ ERP Committee Procedures and Responsibilities. November 1,
2006. American Industrial Hygiene Association.
---------------------------------------------------------------------------

    As can be seen from the definitions above, the AEGL and ERPG values
include the similarly-defined severity levels 1 and 2. For many
chemicals, a severity level 1 value AEGL or ERPG has not been developed
because the types of effects for these chemicals are not consistent
with the AEGL-1/ERPG-1 definitions; in these instances, we compare
higher severity level AEGL-2 or ERPG-2 values to our modeled exposure
levels to screen for potential

acute concerns. When AEGL-1/ERPG-1 values are available, they are used
in our acute risk assessments.
    Acute REL values for 1-hour exposure durations are typically lower
than their corresponding AEGL-1 and ERPG-1 values. Even though their
definitions are slightly different, AEGL-1 values are often the same as
the corresponding ERPG-1 values and AEGL-2 values are often equal to
ERPG-2 values. Maximum HQ values from our acute screening risk
assessments typically result when basing them on the acute REL value
for a particular pollutant. In cases where our maximum acute HQ value
exceeds 1, we also report the HQ value based on the next highest acute
dose-response value (usually the AEGL-1 and/or the ERPG-1 value).
    To develop screening estimates of acute exposures in the absence of
hourly emissions data, generally we first develop estimates of maximum
hourly emissions rates by multiplying the average actual annual hourly
emissions rates by a default factor to cover routinely variable
emissions. We choose the factor to use partially based on process
knowledge and engineering judgment. The factor chosen also reflects a
Texas study of short-term emissions variability, which showed that most
peak emission events in a heavily-industrialized four-county area
(Harris, Galveston, Chambers and Brazoria Counties, Texas) were less
than twice the annual average hourly emissions rate. The highest peak
emissions event was 74 times the annual average hourly emissions rate
and the 99th percentile ratio of peak hourly emissions rate to the
annual average hourly emissions rate was 9.\10\ Considering this
analysis, to account for more than 99 percent of the peak hourly
emissions, we apply a conservative screening multiplication factor of
10 to the average annual hourly emissions rate in our acute exposure
screening assessments as our default approach. However, we use a factor
other than 10 if we have information that indicates that a different
factor is appropriate for a particular source category. For this source
category, there was no such information available and the default
factor of 10 was used in the acute screening process.
---------------------------------------------------------------------------

    \10\ See http://www.tceq.state.tx.us/compliance/field_ops/eer/index.html or docket to access the source of these data.
---------------------------------------------------------------------------

    Ideally, we would prefer to have continuous measurements over time
to see how the emissions vary by each hour over an entire year. Having
a frequency distribution of hourly emissions rates over a year would
allow us to perform a probabilistic analysis to estimate potential
threshold exceedances and their frequency of occurrence. Such an
evaluation could include a more complete statistical treatment of the
key parameters and elements adopted in this screening analysis.
Recognizing that this level of data is rarely available, we instead
rely on the multiplier approach.
    As part of our acute risk assessment process, for cases where acute
HQ values from the screening step are less than or equal to 1 (even
under the conservative assumptions of the screening analysis), acute
impacts are deemed negligible and no further analysis is performed. In
cases where an acute HQ from the screening step are greater than 1,
additional site-specific data would be considered to develop a more
refined estimate of the potential for acute impacts of concern.
However, for this source category, no acute values were greater than 1.
Therefore, further refinement was not performed.
    To better characterize the potential health risks associated with
estimated acute exposures to HAP, and in response to a key
recommendation from the SAB's peer review of the EPA's RTR risk
assessment methodologies,\11\ we generally examine a wider range of
available acute health metrics (e.g., RELs, AEGLs) than we do for our
chronic risk assessments. This is in response to the SAB's
acknowledgement that there are generally more data gaps and
inconsistencies in acute reference values than there are in chronic
reference values. In some cases, when Reference Value Arrays \12\ for
HAP have been developed, we consider additional acute values (i.e.,
occupational and international values) to provide a more complete risk
characterization.
---------------------------------------------------------------------------

    \11\ The SAB peer review of RTR Risk Assessment Methodologies is
available at: http://yosemite.epa.gov/sab/sabproduct.nsf/
4AB3966E263D943A8525771F00668381/$File/EPA-SAB-10-007-unsigned.pdf.
    \12\ U.S. EPA. (2009) Chapter 2.9 Chemical Specific Reference
Values for Formaldehyde in Graphical Arrays of Chemical-Specific
Health Effect Reference Values for Inhalation Exposures (Final
Report). U.S. Environmental Protection Agency, Washington, DC, EPA/
600/R-09/061 and available online at http://cfpub.epa.gov/ncea/cfm/recordisplay.cfm?deid=211003.
---------------------------------------------------------------------------

4. How did we conduct the multipathway exposure and risk screening?
    The EPA conducted a screening analysis examining the potential for
significant human health risks due to exposures via routes other than
inhalation (i.e., ingestion). We first determined whether any major
sources in the source category emitted any HAP known to be persistent
and bioaccumulative in the environment (PB-HAP). The PB-HAP compounds
or compound classes are identified for the screening from the EPA's Air
Toxics Risk Assessment Library (available at: http://www2.epa.gov/fera/risk-assessment-and-modeling-air-toxics-risk-assessment-reference-library). Since D/F is the only pollutant for which subpart RRR area
sources are regulated under CAA section 112(d), this was the only PB-
HAP evaluated in this screening analysis for area sources.
    For major sources in the Secondary Aluminum Production source
category, we identified emissions of cadmium compounds, D/F, lead
compounds, mercury compounds and POM. Because one or more of these PB-
HAP are emitted by at least one facility in the Secondary Aluminum
Production source category, we proceeded to the next step of the
evaluation. In this step, we determined whether the facility-specific
emissions rates of the emitted PB-HAP were large enough to create the
potential for significant non-inhalation human health risks under
reasonable worst-case conditions. To facilitate this step, we developed
emissions rate screening levels for several PB-HAP using 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 emissions rate
screening levels are: lead, cadmium, D/F, mercury compounds and POM. We
conducted a sensitivity analysis on the screening scenario to ensure
that its key design parameters would represent the upper end of the
range of possible values, such that it would represent a conservative
but not impossible scenario. The facility-specific emissions rates of
these PB-HAP were compared to the emission rate screening levels for
these PB-HAP to assess the potential for significant human health risks
via non-inhalation pathways. We call this application of the TRIM.FaTE
model the Tier 1 TRIM-screen or Tier 1 screen.
    For the purpose of developing emissions rates for our Tier 1 TRIM-
screen, we derived emission levels for these PB-HAP (other than lead
compounds) at which the maximum excess lifetime cancer risk would be 1-
in-1 million (i.e., for D/F and POM) or, for HAP that cause non-cancer
health effects (i.e., cadmium compounds and mercury compounds), the
maximum HQ would be 1. If the emissions rate of any PB-HAP included in
the Tier 1 screen exceeds the Tier 1 screening emissions rate for any
facility, we conduct a

second screen, which we call the Tier 2 TRIM-screen or Tier 2 screen.
    In the Tier 2 screen, the location of each facility that exceeded
the Tier 1 emission rate is used to refine the assumptions associated
with the environmental scenario while maintaining the exposure scenario
assumptions. A key assumption that is part of the Tier 1 screen is that
a lake is located near the facility; we confirm the existence of lakes
near the facility as part of the Tier 2 screen. We then adjust the
risk-based Tier 1 screening level for each PB-HAP for each facility
based on an understanding of how exposure concentrations estimated for
the screening scenarios for the subsistence fisher and the subsistence
farmer change with meteorology and environmental assumptions. PB-HAP
emissions that do not exceed these new Tier 2 screening levels are
considered to pose no unacceptable risks. If the PB-HAP emissions for a
facility exceed the Tier 2 screening emissions rate and data are
available, we may decide to conduct a more refined Tier 3 multipathway
screening analysis. There are several analyses that can be included in
a Tier 3 screen depending upon the extent of refinement warranted,
including validating that the lake is fishable and considering plume-
rise to estimate emissions lost above the mixing layer. If the Tier 3
screen is exceeded, the EPA may further refine the assessment.
    For this source category, we conducted a Tier 3 screening analysis
for six major sources with Tier 2 cancer screen values greater than or
equal to 50 times the Tier 2 threshold for the subsistence fisher
scenario. The major sources represented the highest screened cancer
risk for multipathway impacts. Therefore, further screening analyses
were not performed on the area sources. A detailed discussion of the
approach for this risk assessment can be found in Appendix 8 of the
Residual Risk Assessment for the Secondary Aluminum Production Source
Category in Support of the 2014 Supplemental Proposal.
    In evaluating the potential multipathway risk from emissions of
lead compounds, rather than developing a screening emissions rate for
them, we compared maximum estimated chronic inhalation exposures with
the level of the current National Ambient Air Quality Standard (NAAQS)
for lead.\13\ Values below the level of the primary (health-based) lead
NAAQS were considered to have a low potential for multipathway risk.
---------------------------------------------------------------------------

    \13\ 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'').
However, the 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 analysis approach, see
the Residual Risk Assessment for the Secondary Aluminum Production
Source Category in Support of the 2014 Supplemental Proposal, which is
available in the docket for this action.
5. How did we conduct the environmental risk screening assessment?
a. Adverse Environmental Effect
    The EPA conducts a screening assessment to examine the potential
for adverse environmental effects 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.''
b. Environmental HAP
    The EPA focuses on seven HAP, which we refer to as ``environmental
HAP,'' in its screening analysis: Five PB-HAP and two acid gases. The
five PB-HAP are cadmium, D/F, POM, mercury (both inorganic mercury and
methyl mercury) and lead compounds. The two acid gases are HCl and HF.
The rationale for including these seven HAP in the environmental risk
screening analysis is presented below.
    The HAP that persist and bioaccumulate are of particular
environmental concern because they accumulate in the soil, sediment and
water. The PB-HAP are taken up, through sediment, soil, water and/or
ingestion of other organisms, by plants or animals (e.g., small fish)
at the bottom of the food chain. As larger and larger predators consume
these organisms, concentrations of the PB-HAP in the animal tissues
increase as does the potential for adverse effects. The five PB-HAP we
evaluate as part of our screening analysis account for 99.8 percent of
all PB-HAP emissions nationally from stationary sources (on a mass
basis from the 2005 National Emissions Inventory (NEI)).
    In addition to accounting for almost all of the mass of PB-HAP
emitted, we note that the TRIM.FaTE model that we use to evaluate
multipathway risk allows us to estimate concentrations of cadmium
compounds, D/F, POM and mercury in soil, sediment and water. For lead
compounds, we currently do not have the ability to calculate these
concentrations using the TRIM.FaTE model. Therefore, to evaluate the
potential for adverse environmental effects from lead compounds, we
compare the estimated HEM-modeled exposures from the source category
emissions of lead with the level of the secondary NAAQS for lead.\14\
We consider values below the level of the secondary lead NAAQS as
unlikely to cause adverse environmental effects.
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    \14\ The secondary lead NAAQS is a reasonable measure of
determining whether there is an adverse environmental effect since
it was established considering ``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.''
---------------------------------------------------------------------------

    Due to their well-documented potential to cause direct damage to
terrestrial plants, we include two acid gases, HCl and HF, in the
environmental screening analysis. According to the 2005 NEI, HCl and HF
account for about 99 percent (on a mass basis) of the total acid gas
HAP emitted by stationary sources in the U.S. In addition to the
potential to cause direct damage to plants, high concentrations of HF
in the air have been linked to fluorosis in livestock. Air
concentrations of these HAP are already calculated as part of the human
multipathway exposure and risk screening analysis using the HEM3-AERMOD
air dispersion model, and we are able to use the air dispersion
modeling results to estimate the potential for an adverse environmental
effect.
    The EPA acknowledges that other HAP beyond the seven HAP discussed
above may have the potential to cause adverse environmental effects.
Therefore, the EPA may include other relevant HAP in its environmental
risk screening in the future, as modeling science and resources allow.
The EPA invites comment on the extent to which other HAP emitted by the
source category may cause adverse environmental effects. Such
information should include references to peer-reviewed ecological
effects benchmarks that are of sufficient quality for making

regulatory decisions, as well as information on the presence of
organisms located near facilities within the source category that such
benchmarks indicate could be adversely affected.
c. Ecological Assessment Endpoints and Benchmarks for PB-HAP
    An important consideration in the development of the EPA's
screening methodology is the selection of ecological assessment
endpoints and benchmarks. Ecological assessment endpoints are defined
by the ecological entity (e.g., aquatic communities including fish and
plankton) and its attributes (e.g., frequency of mortality). Ecological
assessment endpoints can be established for organisms, populations,
communities or assemblages and ecosystems.
    For PB-HAP (other than lead compounds), we evaluated the following
community-level ecological assessment endpoints to screen for organisms
directly exposed to HAP in soils, sediment and water:
     Local terrestrial communities (i.e., soil invertebrates,
plants) and populations of small birds and mammals that consume soil
invertebrates exposed to PB-HAP in the surface soil;
     Local benthic (i.e., bottom sediment dwelling insects,
amphipods, isopods and crayfish) communities exposed to PB-HAP in
sediment in nearby water bodies; and
     Local aquatic (water-column) communities (including fish
and plankton) exposed to PB-HAP in nearby surface waters.
    For PB-HAP (other than lead compounds), we also evaluated the
following population-level ecological assessment endpoint to screen for
indirect HAP exposures of top consumers via the bioaccumulation of HAP
in food chains;
     Piscivorous (i.e., fish-eating) wildlife consuming PB-HAP-
contaminated fish from nearby water bodies.
    For cadmium compounds, D/F, POM and mercury, we identified the
available ecological benchmarks for each assessment endpoint. An
ecological benchmark represents a concentration of HAP (e.g., 0.77
[mu]g of HAP per liter of water) that has been linked to a particular
environmental effect level through scientific study. For PB-HAP we
identified, where possible, ecological benchmarks at the following
effect levels:
     Probable effect levels (PEL): Level above which adverse
effects are expected to occur frequently;
     Lowest-observed-adverse-effect level (LOAEL): The lowest
exposure level tested at which there are biologically significant
increases in frequency or severity of adverse effects; and
     No-observed-adverse-effect levels (NOAEL): The highest
exposure level tested at which there are no biologically significant
increases in the frequency or severity of adverse effect.
    We established a hierarchy of preferred benchmark sources to allow
selection of benchmarks for each environmental HAP at each ecological
assessment endpoint. In general, the EPA sources that are used at a
programmatic level (e.g., Office of Water, Superfund Program) were used
in the analysis, if available. If not, the EPA benchmarks used in
regional programs (e.g., Superfund) were used. If benchmarks were not
available at a programmatic or regional level, we used benchmarks
developed by other federal agencies (e.g., National Oceanic and
Atmospheric Administration (NOAA)) or state agencies.
    Benchmarks for all effect levels are not available for all PB-HAP
and assessment endpoints. 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.
d. Ecological Assessment Endpoints and Benchmarks for Acid Gases
    The environmental screening analysis also evaluated potential
damage and reduced productivity of plants due to direct exposure to
acid gases in the air. For acid gases, we evaluated the following
ecological assessment endpoint:
     Local terrestrial plant communities with foliage exposed
to acidic gaseous HAP in the air.
    The selection of ecological benchmarks for the effects of acid
gases on plants followed the same approach as for PB-HAP (i.e., we
examine all of the available benchmarks). For HCl, the EPA identified
chronic benchmark concentrations. We note that the benchmark for
chronic HCl exposure to plants is greater than the reference
concentration for chronic inhalation exposure for human health. This
means that where the EPA includes regulatory requirements to prevent an
exceedance of the reference concentration for human health, additional
analyses for adverse environmental effects of HCl would not be
necessary.
    For HF, the EPA identified chronic benchmark concentrations for
plants and evaluated chronic exposures to plants in the screening
analysis. High concentrations of HF in the air have also been linked to
fluorosis in livestock. However, the HF concentrations at which
fluorosis in livestock occur are higher than those at which plant
damage begins. Therefore, the benchmarks for plants are protective of
both plants and livestock.
e. Screening Methodology
    For the environmental risk screening analysis, the EPA first
determined whether any of the major source facilities in the Secondary
Aluminum Production source category emitted any of the seven
environmental HAP. We identified emissions of five of the PB-HAP
(cadmium, mercury, lead, D/F, PAHs) and two acid gases (HCl and HF).
Because one or more of the seven environmental HAP evaluated were
emitted by facilities in the source category, we proceeded to the
second step of the evaluation. Since D/F is the only pollutant for
which subpart RRR area sources are regulated under CAA section 112(d),
this was the only PB-HAP evaluated in this screening analysis.
f. PB-HAP Methodology
    For cadmium, mercury, POM and D/F, the environmental screening
analysis consists of two tiers, while lead compounds are analyzed
differently as discussed earlier. In the first tier, we determined
whether the maximum facility-specific emission rates of each of the
emitted environmental HAP for the major sources were large enough to
create the potential for adverse environmental effects under reasonable
worst-case environmental conditions. This same assessment was done for
area sources for D/F because this is the only pollutant for which
subpart RRR area sources are regulated under CAA section 112(d). These
are the same environmental conditions used in the human multipathway
exposure and risk screening analysis.
    To facilitate this step, TRIM.FaTE was run for each PB-HAP under
hypothetical environmental conditions designed to provide
conservatively high HAP concentrations. The model was set to maximize
runoff from terrestrial parcels into the modeled lake, which in turn,
maximized the chemical concentrations in the water, the sediments and
the fish. The resulting media concentrations were then used to back-
calculate a screening level emission rate that corresponded to the
relevant exposure benchmark

concentration value for each assessment endpoint. To assess emissions
from a facility, the reported emission rate for each PB-HAP was
compared to the screening level emission rate for that PB-HAP for each
assessment endpoint. If emissions from a facility do not exceed the
Tier 1 screening level, the facility ``passes'' the screen, and,
therefore, is not evaluated further under the screening approach. If
emissions from a facility exceed the Tier 1 screening level, we
evaluate the facility further in Tier 2.
    In Tier 2 of the environmental screening analysis, the emission
rate screening levels 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 screen. The modeling domain for each facility in the
Tier 2 analysis consists of eight octants. Each octant contains 5
modeled soil concentrations at various distances from the facility (5
soil concentrations x 8 octants = total of 40 soil concentrations per
facility) and one lake with modeled concentrations for water, sediment
and fish tissue. In the Tier 2 environmental risk screening analysis,
the 40 soil concentration points are averaged to obtain an average soil
concentration for each facility for each PB-HAP. For the water,
sediment and fish tissue concentrations, the highest value for each
facility for each pollutant is used. If emission concentrations from a
facility do not exceed the Tier 2 screening level, the facility passes
the screen, and is typically not evaluated further. If emissions from a
facility exceed the Tier 2 screening level, the facility does not pass
the screen and, therefore, may have the potential to cause adverse
environmental effects. Such facilities are evaluated further to
investigate factors such as the magnitude and characteristics of the
area of exceedance.
g. Acid Gas Methodology
    The environmental screening analysis evaluates the potential
phytotoxicity and reduced productivity of plants due to chronic
exposure to acid gases. The environmental risk screening methodology
for acid gases is a single-tier screen that compares the average off-
site ambient air concentration over the modeling domain to ecological
benchmarks for each of the acid gases. Because air concentrations are
compared directly to the ecological benchmarks, emission-based
screening levels are not calculated for acid gases as they are in the
ecological risk screening methodology for PB-HAP.
    For purposes of ecological risk screening, the EPA identifies a
potential for adverse environmental effects to plant communities from
exposure to acid gases when the average concentration of the HAP around
a facility exceeds the LOAEL ecological benchmark. In such cases, we
further investigate factors such as the magnitude and characteristics
of the area of exceedance (e.g., land use of exceedance area, size of
exceedance area) to determine if there is an adverse environmental
effect. For further information on the environmental screening analysis
approach, see the Residual Risk Assessment for the Secondary Aluminum
Production Source Category in Support of the 2014 Supplemental
Proposal, which is available in the docket for this action.
6. How did we conduct facility-wide assessments?
    To put the source category risks in context, we typically examine
the risks from the entire ``facility,'' where the facility includes all
HAP-emitting operations within a contiguous area and under common
control. In other words, we examine the HAP emissions not only from the
source category of interest, but also emissions of HAP from all other
emissions sources at the facility for which we have data. For the
Secondary Aluminum Production source category, we had nine facilities
that were co-located with primary aluminum reduction plants.
7. How did we consider uncertainties in risk assessment?
    In the Benzene NESHAP, we concluded that risk estimation
uncertainty should be considered in our decision-making under the ample
margin of safety framework. 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 protective 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. A more thorough discussion of these
uncertainties is included in the Development of the RTR Supplemental
Proposal Risk Modeling Dataset for the Secondary Aluminum Production
Source Category and Modeling Input Revisions for the RTR Risk Modeling
Dataset for the Secondary Aluminum Production Source Category, which
are available in the docket for this action. The other uncertainties
are described in more detail in the Residual Risk Assessment for the
Secondary Aluminum Production Source Category in Support of the 2014
Supplemental Proposal, which is available in the docket for this
action.
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 of 10 applied to the average annual hourly
emission rates for all emission process groups, which are intended to
account for emission fluctuations due to normal facility operations. A
description of the development of the emissions dataset is in section
II.D of this preamble and in the documents, Development of the RTR
Supplemental Proposal Risk Modeling Dataset for the Secondary Aluminum
Production Source Category and Modeling Input Revisions for the RTR
Risk Modeling Dataset for the Secondary Aluminum Production Source
Category, which are in the docket for this rulemaking.
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.
c. Uncertainties in Inhalation Exposure
    The EPA did not include the effects of human mobility on exposures
in the assessment. Specifically, short-term mobility and long-term
mobility between census blocks in the modeling domain were not
considered.\15\ The approach of not considering short or long-term
population mobility does not bias the estimate of the theoretical MIR
(by definition), nor does it affect the estimate of cancer incidence
because the total population number remains the same. It does, however,
affect the shape of the distribution of individual risks across the
affected population, shifting it toward higher estimated individual
risks at the upper end and reducing the number of people estimated to
be at lower risks, thereby increasing the estimated number of people at
specific high risk levels (e.g., 1-in-10 thousand or 1-in-1 million).
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    \15\ Short-term mobility is movement from one micro-environment
to another over the course of hours or days. Long-term mobility is
movement from one residence to another over the course of a
lifetime.
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    In addition, the assessment predicted the chronic exposures at the
centroid of each populated census block as surrogates for the exposure
concentrations for all people living in that block. Using the census
block centroid to predict chronic exposures tends to over-predict
exposures for people in the census block who live farther from the
facility and under-predict exposures for people in the census block who
live closer to the facility. Thus, using the census block centroid to
predict chronic exposures may lead to a potential understatement or
overstatement of the true maximum impact, but is an unbiased estimate
of average risk and incidence. We reduce this uncertainty by analyzing
large census blocks near facilities using aerial imagery and adjusting
the location of the block centroid to better represent the population
in the block, as well as adding additional receptor locations where the
block population is not well represented by a single location.
    The assessment evaluates the cancer inhalation risks associated
with pollutant exposures over a 70-year period, which is the assumed
lifetime of an individual. In reality, both the length of time that
modeled emission sources at facilities actually operate (i.e., more or
less than 70 years) and the domestic growth or decline of the modeled
industry (i.e., the increase or decrease in the number or size of
domestic facilities) will influence the future risks posed by a given
source or source category. Depending on the characteristics of the
industry, these factors will, in most cases, result in an overestimate
both in individual risk levels and in the total estimated number of
cancer cases. However, in the unlikely scenario where a facility
maintains, or even increases, its emissions levels over a period of
more than 70 years, residents live beyond 70 years at the same
location, and the residents spend most of their days at that location,
then the cancer inhalation risks could potentially be underestimated.
However, annual cancer incidence estimates from exposures to emissions
from these sources would not be affected by the length of time an
emissions source operates.
    The exposure estimates used in these analyses assume chronic
exposures to ambient (outdoor) levels of pollutants. Because most
people spend the majority of their time indoors, actual exposures may
not be as high, depending on the characteristics of the pollutants
modeled. For many of the HAP, indoor levels are roughly equivalent to
ambient levels, but for very reactive pollutants or larger particles,
indoor levels are typically lower. This factor has the potential to
result in an overestimate of 25 to 30 percent of exposures.\16\
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    \16\ U.S. EPA. National-Scale Air Toxics Assessment for 1996.
(EPA 453/R-01-003; January 2001; page 85.)
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    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 that
should be highlighted. 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 humans at the location of the maximum
concentration. In the acute screening assessment that we conduct under
the RTR program, we assume that peak emissions from the source category
and worst-case meteorological conditions co-occur, thus, resulting in
maximum ambient concentrations. These two events are unlikely to occur
at the same time, making these assumptions conservative. We then
include the additional assumption that a person is located at this
point during this same time period. For this source category, these
assumptions would tend to be worst-case actual exposures as it is
unlikely that a person would be located at the point of maximum
exposure during the time when peak emissions and worst-case
meteorological conditions occur simultaneously.
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 non-cancer effects from both chronic and acute
exposures. Some uncertainties may be considered quantitatively, and
others generally are expressed in qualitative terms. We note as a
preface to this discussion a point on dose-response uncertainty that is
brought out in the EPA's 2005 Cancer Guidelines; namely, that ``the
primary goal of EPA actions is protection of human health; accordingly,
as an Agency policy, risk assessment procedures, including default
options that are used in the absence of scientific data to the
contrary, should be health protective'' (EPA 2005 Cancer Guidelines,
pages 1-7). This is the approach followed here as summarized in the
next several paragraphs. A complete detailed discussion of
uncertainties and variability in dose-response relationships is given
in the Residual Risk Assessment for the Secondary Aluminum Production
Source Category in Support of the 2014 Supplemental Proposal, which is
available in the docket for this action.
    Cancer URE values used in our risk assessments are those that have
been developed to generally provide an upper bound estimate of risk.
That is, they represent a ``plausible upper limit to the true value of
a quantity'' (although this is usually not a true statistical
confidence limit).\17\ In some circumstances, the true risk could be as
low as zero; however, in other circumstances the risk could be
greater.\18\ When developing an upper bound estimate of risk and to
provide risk values that do not underestimate risk, health-protective
default approaches are generally used. To err on the side of ensuring
adequate health protection, the EPA typically uses the upper bound
estimates rather than lower bound or central tendency estimates in our
risk assessments, an approach that may have limitations for

other uses (e.g., priority-setting or expected benefits analysis).
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    \17\ IRIS glossary (http://www.epa.gov/NCEA/iris/help_gloss.htm).
    \18\ An exception to this is the URE for benzene, which is
considered to cover a range of values, each end of which is
considered to be equally plausible, and which is based on maximum
likelihood estimates.
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    Chronic non-cancer RfC and reference dose (RfD) values represent
chronic exposure levels that are intended to be health-protective
levels. Specifically, these values provide an estimate (with
uncertainty spanning perhaps an order of magnitude) of a continuous
inhalation exposure (RfC) or a daily oral exposure (RfD) to the human
population (including sensitive subgroups) that is likely to be without
an appreciable risk of deleterious effects during a lifetime. To derive
values that are intended to be ``without appreciable risk,'' the
methodology relies upon an uncertainty factor (UF) approach (U.S. EPA,
1993, 1994) which considers uncertainty, variability and gaps in the
available data. The UF are applied to derive reference values that are
intended to protect against appreciable risk of deleterious effects.
The UF are commonly default values,\19\ e.g., factors of 10 or 3, used
in the absence of compound-specific data; where data are available, UF
may also be developed using compound-specific information. When data
are limited, more assumptions are needed and more UF are used. Thus,
there may be a greater tendency to overestimate risk in the sense that
further study might support development of reference values that are
higher (i.e., less potent) because fewer default assumptions are
needed. However, for some pollutants, it is possible that risks may be
underestimated.
---------------------------------------------------------------------------

    \19\ According to the NRC report, Science and Judgment in Risk
Assessment (NRC, 1994) ``[Default] options are generic approaches,
based on general scientific knowledge and policy judgment, that are
applied to various elements of the risk assessment process when the
correct scientific model is unknown or uncertain.'' The 1983 NRC
report, Risk Assessment in the Federal Government: Managing the
Process, defined default option as ``the option chosen on the basis
of risk assessment policy that appears to be the best choice in the
absence of data to the contrary'' (NRC, 1983a, p. 63). Therefore,
default options are not rules that bind the agency; rather, the
agency may depart from them in evaluating the risks posed by a
specific substance when it believes this to be appropriate. In
keeping with the EPA's goal of protecting public health and the
environment, default assumptions are used to ensure that risk to
chemicals is not underestimated (although defaults are not intended
to overtly overestimate risk). See EPA, 2004, An Examination of EPA
Risk Assessment Principles and Practices, EPA/100/B-04/001 available
at: http://www.epa.gov/osa/pdfs/ratf-final.pdf.
---------------------------------------------------------------------------

    While collectively termed ``UF,'' these factors account for a
number of different quantitative considerations when using observed
animal (usually rodent) or human toxicity data in the development of
the RfC. The UF are intended to account for: (1) Variation in
susceptibility among the members of the human population (i.e., inter-
individual variability); (2) uncertainty in extrapolating from
experimental animal data to humans (i.e., interspecies differences);
(3) uncertainty in extrapolating from data obtained in a study with
less-than-lifetime exposure (i.e., extrapolating from sub-chronic to
chronic exposure); (4) uncertainty in extrapolating the observed data
to obtain an estimate of the exposure associated with no adverse
effects; and (5) uncertainty when the database is incomplete or there
are problems with the applicability of available studies.
    Many of the UF used to account for variability and uncertainty in
the development of acute reference values are quite similar to those
developed for chronic durations, but they more often use individual UF
values that may be less than 10. The UF are applied based on chemical-
specific or health effect-specific information (e.g., simple irritation
effects do not vary appreciably between human individuals, hence a
value of 3 is typically used), or based on the purpose for the
reference value (see the following paragraph). The UF applied in acute
reference value derivation include: (1) Heterogeneity among humans; (2)
uncertainty in extrapolating from animals to humans; (3) uncertainty in
lowest observed adverse effect (exposure) level to no observed adverse
effect (exposure) level adjustments; and (4) uncertainty in accounting
for an incomplete database on toxic effects of potential concern.
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 reference value at another exposure duration
(e.g., 1 hour).
    Not all acute reference 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 reference value or
values being exceeded. Where relevant to the estimated exposures, the
lack of short-term dose-response values at different levels of severity
should be factored into the risk characterization as potential
uncertainties.
    Although every effort is made to identify appropriate human health
effect dose-response assessment values for all pollutants emitted by
the sources in this risk assessment, some HAP emitted by this source
category are lacking dose-response assessments. Accordingly, these
pollutants cannot be included in the quantitative risk assessment,
which could result in quantitative estimates understating HAP risk. To
help to alleviate this potential underestimate, where we conclude
similarity with a HAP for which a dose-response assessment value is
available, we use that value as a surrogate for the assessment of the
HAP for which no value is available. To the extent use of surrogates
indicates appreciable risk, we may identify a need to increase priority
for new IRIS assessment of that substance. We additionally note that,
generally speaking, HAP of greatest concern due to environmental
exposures and hazard are those for which dose-response assessments have
been performed, reducing the likelihood of understating risk.
    For a group of compounds that are unspeciated (e.g., glycol
ethers), we conservatively use the most protective reference 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 reference value, we also apply the most
protective reference value from the other compounds in the group to
estimate risk.
e. Uncertainties in the Multipathway Assessment
    For each source category, we generally rely on site-specific levels