[Federal Register Volume 76, Number 234 (Tuesday, December 6, 2011)]
[Proposed Rules]
[Pages 76260-76291]
[FR Doc No: 2011-29881]

Vol. 76

Tuesday,

No. 234

December 6, 2011

Part III

Environmental Protection Agency

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40 CFR Part 63

National Emissions Standards for Hazardous Air Pollutants: Primary
Aluminum Reduction Plants; Proposed Rule

Federal Register / Vol. 76 , No. 234 / Tuesday, December 6, 2011 /
Proposed Rules

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ENVIRONMENTAL PROTECTION

40 CFR Part 63

[EPA-HQ-OAR-2011-0797; FRL-9491-3]
RIN 2060-AQ92

National Emissions Standards for Hazardous Air Pollutants:
Primary Aluminum Reduction Plants

AGENCY: Environmental Protection Agency (EPA).

ACTION: Proposed rule.

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SUMMARY: The EPA is proposing amendments to the national emissions
standards for hazardous air pollutants for Primary Aluminum Reduction
Plants to address the results of the residual risk and technology
review that the EPA is required to conduct by the Clean Air Act. If
finalized, these proposed amendments would address previously
unregulated emissions (i.e., carbonyl sulfide (COS) emissions from new
and existing potlines and polycyclic organic matter (POM) emissions
from new and existing prebake potlines and existing pitch storage
tanks); remove the vertical stud Soderberg one (VSS1) potline
subcategory; reduce the MACT limits for POM emissions from horizontal
stud Soderberg (HSS) and VSS2 potlines; eliminate the startup, shutdown
and malfunction exemption in accordance with recent actions by the
United States Court of Appeals for the District of Columbia Circuit;
add provisions for facilities to avail themselves of an affirmative
defense in the event of a malfunction under certain conditions; and
make certain technical and editorial changes. The proposed emissions
limits for POM and COS are based on maximum achievable control
technology (MACT). While the proposed modifications would result in
some reduction in actual emissions of POM from existing pitch storage
tanks, reduce the potential emissions of POM from Soderberg potlines,
and prevent increases in emissions of COS and sulfur dioxide, the
health risks posed by actual emissions from this source category are
currently within the acceptable range and would not be reduced
appreciably by the proposed modifications.

DATES: Comments must be received on or before January 20, 2012. Under
the Paperwork Reduction Act, comments on the information collection
provisions are best assured of receiving full consideration if the
Office of Management and Budget (OMB) receives a copy of your comments
on or before January 5, 2012.
    Public Hearing. If anyone contacts the EPA requesting to speak at a
public hearing by December 16, 2011, a public hearing will be held on
December 21, 2011.

ADDRESSES: Submit your comments, identified by Docket ID Number EPA-HQ-
OAR-2011-0797, by one of the following methods:
     http://www.regulations.gov: Follow the on-line
instructions for submitting comments.
     Email<EMAIL_ADDRESS>Attention Docket ID Number
EPA-HQ-OAR-2011-0797.
     Fax:<PHONE_NUMBER>, Attention Docket ID Number EPA-HQ-
OAR-2011-0797.
     Mail: U.S. Postal Service, send comments to: EPA Docket
Center, EPA West (Air Docket), Attention Docket ID Number EPA-HQ-OAR-
2011-0797, U.S. Environmental Protection Agency, Mail Code: 2822T, 1200
Pennsylvania Ave. NW., Washington, DC 20460. Please include a total of
two copies. In addition, 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 Delivery: U.S. Environmental Protection Agency, EPA
West (Air Docket), Room 3334, 1301 Constitution Ave. NW., Washington,
DC 20004, Attention Docket ID Number EPA-HQ-OAR-2011-0797. 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 Number EPA-HQ-OAR-
2011-0797. The EPA's policy is that all comments received will be
included in the public docket without change and may be made available
on-line 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 avoid the use of special characters, 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/epahome/dockets.htm.
    Docket. The EPA has established a docket for this rulemaking under
Docket ID Number EPA-HQ-OAR-2011-0797. All documents in the docket are
listed in the http://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, EPA West,
Room 3334, 1301 Constitution Ave. NW., Washington, DC. The Public
Reading Room is open from 8:30 a.m. to 4:30 p.m., Monday through
Friday, excluding legal holidays. The telephone number for the Public
Reading Room is<PHONE_NUMBER>, and the telephone number for the EPA
Docket Center is<PHONE_NUMBER>.
    Public Hearing. If a public hearing is held, it will begin at 10
a.m. on December 21, 2011 and will be held at the EPA's campus in
Research Triangle Park, North Carolina, or at an alternate facility
nearby. Persons interested in presenting oral testimony or inquiring as
to whether a public hearing is to be held should contact Ms. Virginia
Hunt, Office of Air Quality Planning and Standards, Sector Policies and
Programs Division, (D243-02), U.S. Environmental Protection Agency,
Research Triangle Park, North Carolina 27711; telephone number: (919)
541-0832.

FOR FURTHER INFORMATION CONTACT: For questions about this proposed
action, contact Mr. David Putney, 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 Dr. Michael Stewart, Office of Air
Quality Planning and Standards, Health and Environmental Impacts
Division, Air Toxics Assessment Group (C504-06), U.S. Environmental
Protection Agency, Research Triangle Park, NC 27711; telephone number:
(919) 541-7524; fax number:<PHONE_NUMBER>; and email address:
<EMAIL_ADDRESS>For information about the applicability of the
proposed or current national emission standards for hazardous air
pollutants (NESHAP) for primary aluminum reduction plants to a
particular entity, contact the appropriate person listed in Table 1 of
this preamble.

 Table 1--List of EPA Contacts for the NESHAP Addressed in This Proposed
                                 Action
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         NESHAP for:            OECA Contact \1\      OAQPS Contact \2\
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Primary Aluminum Reduction    Patrick Yellin,       David Putney,
 Plants.<PHONE_NUMBER>,<PHONE_NUMBER>,
                               yellin.patrick@epa. <EMAIL_ADDRESS>                               gov.                  v
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\1\ EPA Office of Enforcement and Compliance Assurance.
\2\ EPA Office of Air Quality Planning and Standards.

SUPPLEMENTARY INFORMATION:

Preamble Acronyms and Abbreviations

    Several acronyms and terms used to describe industrial processes,
data inventories, and risk modeling are included in this preamble.
While this may not be an exhaustive list, the following terms and
acronyms are defined here for reference:

ADAF age-dependent adjustment factors
AEGL acute exposure guideline levels
AERMOD air dispersion model used by the HEM-3 model
AMOS ample margin of safety
ANPRM advance notice of proposed rulemaking
ATSDR Agency for Toxic Substances and Disease Registry
BACT best available control technology
BLDS bag leak detection system
CAA Clean Air Act
CBI Confidential Business Information
CEMS continuous emissions monitoring system
CFR Code of Federal Regulations
COS carbonyl sulfide
CTE central tendency exposure
EJ environmental justice
EPA Environmental Protection Agency
ERPG Emergency Response Planning Guidelines
ERT Electronic Reporting Tool
HAP hazardous air pollutants
HEM-3 Human Exposure Model, Version 3
HEPA high efficiency particulate air
HHRAP Human Health Risk Assessment Protocols
HI Hazard Index
HQ Hazard Quotient
ICR information collection request
IRIS Integrated Risk Information System
Km kilometer
LAER lowest achievable emissions rate
lb/yr pounds per year
MACT maximum achievable control technology
MACT Code Code within the NEI used to identify processes included in
a source category
MDL method detection level
mg/acm milligrams per actual cubic meter
mg/dscm milligrams per dry standard cubic meter
mg/m\3\ milligrams per cubic meter
MIR maximum individual risk
MRL minimum risk level
NAC/AEGL Committee National Advisory Committee for Acute Exposure
Guideline Levels for Hazardous Substances
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
ODW Office of Drinking Water
OECA Office of Enforcement and Compliance Assurance
OHEA Office of Health and Environmental Assessment
OMB Office of Management and Budget
PB-HAP hazardous air pollutants known to be persistent and bio-
accumulative in the environment
PM particulate matter
POM polycyclic organic matter
ppmv parts per million volume
RACT reasonably available control technology
RBLC RACT/BACT/LAER Clearinghouse
REL reference exposure level
RFA Regulatory Flexibility Act
RfC reference concentration
RfD reference dose
RIA Regulatory Impact Analysis
RTR residual risk and technology review
SAB Science Advisory Board
SBA Small Business Administration
SCC Source Classification Codes
SOP standard operating procedures
SSM startup, shutdown, and malfunction
TEQ toxic equivalency quotient
TOSHI target organ-specific hazard index
TPY tons per year
TRIM Total Risk Integrated Modeling System
TTN Technology Transfer Network
UF uncertainty factor
[micro]g/m \3\ microgram per cubic meter
UL upper limit
UMRA Unfunded Mandates Reform Act
UPL upper predictive limit
URE unit risk estimate
WHO World Health Organization
WWW worldwide web

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

I. General Information
    A. What is the statutory authority for this action?
    B. Does this action apply to me?
    C. Where can I get a copy of this document and other related
information?
    D. What should I consider as I prepare my comments for the EPA?
II. Background
    A. What is this source category and how did the MACT standard
regulate its HAP emissions?
    B. What data collection activities were conducted to support
this action?
III. Analyses Performed
    A. How did we address unregulated emission sources?
    B. How did we estimate risks posed by the source category?
    C. How did we consider the risk results in making decisions for
this proposal?
    D. How did we perform the technology review?
    E. What other issues are we addressing in this proposal?
IV. Analytical Results and Proposed Decisions
    A. What are the results of our analyses and proposed decisions
regarding unregulated emissions sources?
    B. What are the results of the risk assessments?
    C. What are our proposed decisions regarding risk acceptability
and ample margin of safety?
    D. What are the results and proposed decisions based on our
technology review?
    E. What other actions are we proposing?
    F. Compliance dates
V. Summary of Cost, Environmental, and Economic Impacts
    A. What are the affected sources?
    B. What are the air quality impacts?
    C. What are the cost impacts?
    D. What are the economic impacts?
    E. What are the benefits?
VI. Request for Comments

VII. Submitting Data Corrections
VIII. Statutory and Executive Order Reviews
    A. Executive Order 12866: Regulatory Planning and Review and
Executive Order 13563: Improving Regulation and Regulatory Review
    B. 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. What is the statutory authority for this action?

    Section 112 of the 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 section 112(b) of the
CAA, section 112(d) of the CAA calls for us to promulgate national
emission standards for hazardous air pollutants (NESHAP) for those
sources. ``Major sources'' are those that emit or have the potential to
emit (PTE) 10 tons per year (tpy) or more of a single HAP or 25 tpy or
more of any combination of HAP. For major sources, these technology-
based standards must reflect the maximum degree of emission reductions
of HAP achievable (after considering cost, energy requirements and
nonair quality health and environmental impacts) and are commonly
referred to as maximum achievable control technology (MACT) standards.
    MACT standards are to reflect application of measures, processes,
methods, systems or techniques including, but not limited to, measures
which (1) reduce the volume of or eliminate emissions of 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)-(E). The MACT standard may take the form of a
design, equipment, work practice or operational standard where the EPA
first determines that either (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 sections 112(h)(1)-(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 emission control that is achieved in practice by the
best-controlled similar source. The MACT floors for existing sources
can be less stringent than floors for new sources, but they cannot be
less stringent than the average emission 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, we must also consider control options that are more
stringent than the floor. We may establish standards more stringent
than the floor (``beyond the floor'' standards) based on the
consideration of the cost of achieving the emissions reductions and any
nonair quality health and environmental impacts and energy
requirements. No beyond the floor standards are proposed in this
rulemaking action.
    The EPA is then required to review these technology-based standards
and to revise them ``as necessary (taking into account developments in
practices, processes, and control technologies)'' no less frequently
than every 8 years, under CAA section 112(d)(6). In conducting this
review, the EPA is not obliged to completely recalculate the prior MACT
determination. NRDC v. EPA, 529 F.3d 1077, 1084 (D.C. Cir. 2008).
    The second stage in standard-setting focuses on reducing any
remaining ``residual'' risk according to CAA section 112(f). This
provision requires, first, that the EPA prepare a Report to Congress
discussing (among other things) methods of calculating risk 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 this report (Residual Risk Report to
Congress, EPA-453/R-99-001) in March 1999. Congress did not act in
response to the report, thereby triggering the EPA's obligation under
CAA section 112(f)(2) to analyze and address residual risk.
    CAA section 112(f)(2) requires us to determine, for source
categories subject to MACT standards, whether the emissions standards
provide an ample margin of safety to protect public health. 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 1-in-1 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 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.
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    \1\ ``Adverse environmental effect'' is defined in CAA section
112(a)(7) 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.
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    Section 112(f)(2) of the CAA expressly preserves our use of a two-
step process for developing standards to address any residual risk and
our interpretation of ``ample margin of safety'' developed in the
National Emission 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
first step in this process is the determination of acceptable risk. The
second step provides for an ample margin of safety to protect public
health, which is the level at which the standards are set (unless a
more

stringent standard is required to prevent, taking into consideration
costs, energy, safety, and other relevant factors, an adverse
environmental effect).
    The terms ``individual most exposed,'' ``acceptable level,'' and
``ample margin of safety'' are not specifically defined in the CAA.
However, CAA section 112(f)(2)(B) preserves the interpretation set out
in the Benzene NESHAP, and the United States Court of Appeals for the
District of Columbia Circuit in NRDC v. EPA, 529 F.3d 1077, concluded
that the EPA's interpretation of subsection 112(f)(2) is a reasonable
one. See NRDC v. EPA, 529 F.3d at 1083 (``[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''). (D.C. Cir. 2008). See also, A Legislative History of the
Clean Air Act Amendments of 1990, volume 1, p. 877 (Senate debate on
Conference Report). We notified Congress in the Residual Risk Report to
Congress that we intended to use the Benzene NESHAP approach in making
CAA section 112(f) residual risk determinations (EPA-453/R-99-001, p.
ES-11).

    In the Benzene NESHAP, we stated as an overall objective:
    * * * in protecting public health with an ample margin of
safety, we strive 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 facility
would have if he or she were exposed to the maximum pollutant
concentrations for 70 years.

    The agency also stated that, ``The 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 risk 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.'' The agency
went on to conclude that ``estimated incidence would be weighed along
with other health risk information in judging acceptability.'' As
explained more fully in our Residual Risk Report to Congress, the EPA
does not define ``rigid line[s] of acceptability,'' but considers
rather broad objectives to be weighed with a series of other health
measures and factors (EPA-453/R-99-001, p. ES-11). 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'' (Residual Risk
Report to Congress, p. 178, quoting the Vinyl Chloride decision at 824
F.2d 1165) 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 1-in-10 thousand, that risk level is considered
acceptable.'' 54 FR 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 acknowledge 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
maximum individual lifetime cancer risk as a metric for determining
acceptability, we acknowledged in the 1989 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.
    The agency also explained in the 1989 Benzene NESHAP the following:
``In 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-kilometer (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.
    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 explained in the Benzene NESHAP, ``[e]ven though
the risks judged `acceptable' by the EPA in the first step of the Vinyl
Chloride inquiry are already low, 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.'' 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. 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 CAA section 112(f). 54 FR
38046.
    As discussed in the previous section of this preamble, we apply a
two-step process for developing standards to address residual risk. In
the first step, the EPA determines whether risks are acceptable. This
determination ``considers all health information, including risk
estimation uncertainty, and includes a presumptive limit on maximum
individual lifetime [cancer] risk (MIR) \2\ of approximately 1-in-10
thousand [i.e., 100-in-1 million].'' 54 FR 38045. In the second step of
the process, the EPA sets the standard at a level that provides an
ample margin of safety ``in consideration of all health information,
including the number of persons at risk levels higher than
approximately 1-in-1 million, as well as other relevant factors,
including costs and economic impacts, technological feasibility, and
other factors relevant to each particular decision.'' Id.
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    \2\ Although defined as ``maximum individual risk,'' MIR refers
only to cancer risk. MIR, one metric for assessing cancer risk, is
the estimated risk were an individual exposed to the maximum level
of a pollutant for a lifetime.
---------------------------------------------------------------------------

    In past residual risk determinations, the EPA presented a number of
human health risk metrics associated with emissions from the category
under review, including: The MIR; the numbers of persons in various
risk ranges; cancer incidence; the maximum noncancer hazard index (HI);
and the maximum acute noncancer hazard. In estimating risks, the EPA
considered

source categories under review that are located near each other and
that affect the same population. The EPA provided estimates of the
expected difference in actual emissions from the source category under
review and emissions allowed pursuant to the source category MACT
standard. The EPA also discussed and considered risk estimation
uncertainties. The EPA is providing this same type of information in
support of these actions.
    The agency acknowledges that the Benzene NESHAP provides
flexibility regarding what factors the EPA might consider in making our
determinations and how they might be weighed for each source category.
In responding to comment on our policy under the Benzene NESHAP, the
EPA explained that: ``The policy chosen by the Administrator permits
consideration of multiple measures of health risk. Not only can the MIR
figure be considered, but also incidence, the presence of noncancer
health effects, and the uncertainties of the risk estimates. In this
way, the effect on the most exposed individuals can be reviewed as well
as the impact on the general public. These factors can then be weighed
in each individual case. This approach complies with the Vinyl Chloride
mandate that the Administrator ascertain an acceptable level of risk to
the public by employing [her] expertise to assess available data. It
also complies with the Congressional intent behind the CAA, which did
not exclude the use of any particular measure of public health risk
from the EPA's consideration with respect to CAA section 112
regulations, and, thereby, implicitly permits consideration of any and
all measures of health risk which the Administrator, in [her] judgment,
believes are appropriate to determining what will `protect the public
health.' ''
    For example, the level of the MIR is only one factor to be weighed
in determining acceptability of risks. The Benzene NESHAP explains ``an
MIR of approximately 1-in-10 thousand should ordinarily be the upper
end of the range of acceptability. As risks increase above this
benchmark, they become presumptively less acceptable under CAA section
112, and would be weighed with the other health risk measures and
information in making an overall judgment on acceptability. Or, the
agency may find, in a particular case, that a risk that includes MIR
less than the presumptively acceptable level is unacceptable in the
light of other health risk factors.'' Similarly, with regard to the
ample margin of safety analysis, the Benzene NESHAP states that: ``EPA
believes the relative weight of the many factors that can be considered
in selecting an ample margin of safety can only be determined for each
specific source category. This occurs mainly because technological and
economic factors (along with the health-related factors) vary from
source category to source category.''

B. Does this action apply to me?

    The regulated industrial source category that is the subject of
this proposal is listed in Table 2 of this preamble. Table 2 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. As
defined in the source category listing report published by the EPA in
1992, the Primary Aluminum Reduction Plant source category is defined
as any facility which produced primary aluminum by the electrolytic
reduction process.

                Table 2--NESHAP and Industrial Source Categories Affected by This Proposed Action
----------------------------------------------------------------------------------------------------------------
               Source category                            NESHAP               NAICS code \1\     MACT code \2\
----------------------------------------------------------------------------------------------------------------
Primary Aluminum Reduction Plants...........  Primary Aluminum Reduction                331312              0023
                                               Plants.
----------------------------------------------------------------------------------------------------------------
\1\ North American Industry Classification System.
\2\ Maximum Achievable Control Technology.

C. 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 proposal will also be available on the World Wide Web (WWW)
through the EPA's Technology Transfer Network (TTN). Following
signature by the EPA Administrator, a copy of this proposed action will
be posted on the TTN's policy and guidance page for newly proposed or
promulgated rules at the following address: http://www.epa.gov/ttn/atw/rrisk/rtrpg.html. The TTN provides information and technology exchange
in various areas of air pollution control.
    Additional information is available on the residual risk and
technology review (RTR) Web page at: http://www.epa.gov/ttn/atw/rrisk/rtrpg.html. This information includes source category descriptions and
detailed emissions estimates and other data that were used as inputs to
the risk assessments.

D. 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 comment that includes information claimed as
CBI, a copy of the comment that does not contain the information
claimed as CBI must be submitted 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 CFR part 2. Send or deliver information identified as CBI
only to the following address: Roberto Morales, OAQPS Document Control
Officer (C404-02), Office of Air Quality Planning and Standards, U.S.
Environmental Protection Agency, Research Triangle Park, North Carolina
27711, Attention Docket ID Number EPA-HQ-OAR-2011-0797.

II. Background

A. What is this source category and how did the MACT standard regulate
its HAP emissions?

    The NESHAP (or MACT rule) for the Primary Aluminum Reduction Plants
was promulgated on October 7, 1997 (62 FR 52407) and amended on
November 2, 2005 (70 FR 66285). The rule is applicable to facilities
with affected sources associated with the production of aluminum by
electrolytic reduction. Aluminum is produced from refined

bauxite ore (also known as alumina), using an electrolytic reduction
process in a series of cells called a ``potline.'' The raw materials
include alumina, coke, pitch and fluoride salts. According to
information available on the Web site of The Aluminum Association, Inc.
(http://www.aluminum.org) approximately 50 percent of the aluminum
produced in the U.S. comes from primary aluminum facilities. The two
main potline types are prebake (a newer, higher efficiency, lower-
emitting technology) and Soderberg (an older, lower efficiency, higher-
emitting technology). There are currently 15 facilities located in the
United States that are subject to the requirements of this NESHAP: 14
primary aluminum production plants and one carbon-only prebake anode
production facility. These 14 primary aluminum production plants have
approximately 53 potlines that produce aluminum. Each plant has a paste
production operation, and 12 of the 14 plants have anode bake furnaces.
Twelve of the 14 facilities utilize prebake potlines; the other 2
utilize Soderberg potlines. According to The Aluminum Association,
Inc., due to a decrease in demand for aluminum, four of the 14
facilities are currently idle including 1 Soderberg facility. The major
HAPs emitted by these facilities are carbonyl sulfide (COS), hydrogen
fluoride (HF), and polycyclic organic matter (POM), specifically
polycyclic aromatic hydrocarbons (PAH).
    The standards promulgated in 1997 and 2005 apply to emissions of
HF, measured using total fluorides (TF) as a surrogate, from all
potlines and anode bake furnaces and POM (as measured by methylene
chloride extractables) from Soderberg potlines, anode bake furnaces,
paste production plants and pitch storage tanks associated with primary
aluminum reduction. Affected sources under the rules are each potline,
each anode bake furnace (except for one that is located at a facility
that only produces anodes for use off-site), each paste production
plant, and each new pitch storage tank.
    The NESHAP designated seven subcategories of existing potlines
based primarily on differences in the process operation and
configuration. The control of primary emissions from the reduction
process is typically achieved by the installation of a dry alumina
scrubber (with a baghouse to collect the alumina and other particulate
matter). The MACT control technology typically used for anode bake
furnaces is a dry alumina scrubber, and a capture system vented to a
dry coke scrubber is used for control of paste production plants. See
Table 3 for the emission limits.

  Table 3--Summary of Current MACT Emission Limits for Existing Sources
             Under the 1997 NESHAP, and the 2005 Amendments
------------------------------------------------------------------------
            Source                  Pollutant          Emission limit
------------------------------------------------------------------------
Potlines: \1\
    CWPB1 potlines............  TF...............  0.95 kg/Mg (1.9 lb/
                                                    ton) of aluminum
                                                    produced.
    CWPB2 potlines............  TF...............  1.5 kg/Mg (3.0 lb/
                                                    ton) of aluminum
                                                    produced.
    CWPB3 potlines............  TF...............  1.25 kg/Mg (2.5 lb/
                                                    ton) of aluminum
                                                    produced.
    SWPB potlines.............  TF...............  0.8 kg/Mg (1.6 lb/
                                                    ton) of aluminum
                                                    produced.
    VSS1 potlines.............  TF...............  1.1 kg/Mg (2.2 lb/
                                                    ton) of aluminum
                                                    produced.
                                POM..............  1.2 kg/Mg (2.4 lb/
                                                    ton) of aluminum
                                                    produced.
    VSS2 potlines.............  TF...............  1.35 kg/Mg (2.7 lb/
                                                    ton) of aluminum
                                                    produced.
                                POM..............  2.85 kg/Mg (5.7 lb/
                                                    ton) of aluminum
                                                    produced.
    HSS potlines..............  TF...............  1.35 kg/Mg (2.7 lb/
                                                    ton) of aluminum
                                                    produced.
                                POM..............  2.35 kg/Mg (4.7 lb/
                                                    ton) of aluminum
                                                    produced.
Paste Production..............  POM..............  Install, operate, and
                                                    maintain equipment
                                                    for capture of
                                                    emissions and vent
                                                    to a dry coke
                                                    scrubber.
Anode Bake Furnace (collocated  TF...............  0.10 kg/Mg (0.20 lb/
 with a primary aluminum        POM..............   ton) of green anode.
 plant).                                           0.09 kg/Mg (0.18 lb/
                                                    ton) of green anode.
------------------------------------------------------------------------
\1\ CWPB1 = Center-worked prebake potline with the most modern reduction
  cells; includes all center-worked prebake potlines not specifically
  identified as CWPB2 or CWPB3.
CWPB2 = Center-worked prebake potlines located at Alcoa in Rockdale,
  Texas; Kaiser Aluminum in Mead, Washington; Ormet Corporation in
  Hannibal, Ohio; Ravenswood Aluminum in Ravenswood, West Virginia;
  Reynolds Metals in Troutdale, Oregon; and Vanalco Aluminum in
  Vancouver, Washington.
CWPB3 = Center-worked prebake potline that produces very high purity
  aluminum, has wet scrubbers as the primary control system, and is
  located at the primary aluminum plant operated by NSA in Hawesville,
  Kentucky.
HSS = Horizontal stud Soderberg potline.
SWPB = Side-worked prebake potline.
VSS1 = Vertical stud Soderberg potline at Northwest Aluminum in The
  Dalles, Oregon, or at Columbia Aluminum in Goldendale, Washington.
VSS2 = Vertical stud Soderberg potlines at Columbia Falls Aluminum in
  Columbia Falls, Montana.

 Table 4--Summary of Current MACT Emission Limits for New Sources Under
                   the 1997 NESHAP and 2005 Amendments
------------------------------------------------------------------------
            Source                  Pollutant          Emission limit
------------------------------------------------------------------------
All Potlines..................  TF...............  0.6 kg/Mg (1.2 lb/
                                                    ton) of aluminum
                                                    produced.
VSS1, VSS2, and HSS potlines..  POM..............  0.32 kg/Mg (0.63 lb/
                                                    ton) of aluminum
                                                    produced.
Paste Production..............  POM..............  Install, operate, and
                                                    maintain equipment
                                                    for capture of
                                                    emissions and vent
                                                    to a dry coke
                                                    scrubber.
Anode Bake Furnace (collocated  TF...............  0.01 kg/Mg (0.020 lb/
 with a primary aluminum        POM..............   ton) of green anode
 plant).                                           0.025 kg/Mg (0.05 lb/
                                                    ton) of green anode.
Pitch storage tanks...........  POM..............  Emission control
                                                    system designed and
                                                    operated to reduce
                                                    inlet emissions by
                                                    95 percent or
                                                    greater.
------------------------------------------------------------------------

    The 1997 NESHAP for primary aluminum reduction plants incorporates
new source performance standards for potroom groups; these emission
limits are listed in Table 4. The limits for new Soderberg facilities
apply to any Soderberg facility that adds a new potroom group to an
existing potline or is associated with a potroom group that meets the
definition of a modified or reconstructed potroom group. Since these
POM limits are very stringent, they effectively preclude the operation
of any new Soderberg potlines.
    Compliance with the emission limits in the current rule is
demonstrated by performance testing which can be addressed individually
for each affected source or according to emissions averaging
provisions. Monitoring requirements include monthly measurements of TF
secondary emissions, quarterly measurement of POM secondary emissions
and annual measurement of primary emissions, continuous parameter
monitoring for each emission control device, a monitoring device to
track daily weight of aluminum produced, daily inspection for visible
emissions, and daily inspection of wet roof scrubbers. Recordkeeping
for the rule is consistent with the General Provisions requirements
with the addition of recordkeeping for daily production of aluminum,
records supporting emissions averaging and records documenting the
portion of TF measured as particulate matter or gaseous form.

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

    For the Primary Aluminum Reduction Plant source category, we
compiled a preliminary dataset using available information, reviewed
the data, and made changes where necessary. The preliminary dataset was
based on data in the 2002 National Emissions Inventory (NEI) Final
Inventory, Version 1 (made publicly available on February 26, 2006),
and the 2005 National Emissions Inventory (NEI), version 2.0 (made
publicly available in October 2008). The NEI is a database that
contains information about sources that emit criteria air pollutants,
their precursors, and HAP. The NEI database includes estimates of
annual air pollutant emissions from point and volume sources, emission
release characteristic data such as height, velocity, temperature and
location latitude/longitude coordinates.
    We reviewed the NEI datasets, corrected geographic coordinates and
stack parameters in consultation with the facilities, and made changes
based on available information. We also reviewed the emissions and
other data to identify data anomalies that could affect risk estimates.
The 2005 NEI was then updated to develop the 2005 National Air Toxics
Assessment (NATA) Inventory. Subsequently, in April 2011, we received
test data and other information through an Information Collection
Request (ICR) from 11 of the 15 facilities in the source category.
These ICR data were then used along with the 2005 NATA inventory data
to develop the emissions dataset for this source category, which
includes our best estimates of actual emissions of HAP for the
facilities. This dataset was then used in the risk modeling analyses to
estimate the risks due to actual emissions for the source category.
    POM emissions were allocated to specific POM compounds on the basis
of the fractional contributions of these compounds to the actual POM
emissions, as determined (as appropriate) from an average of test data
for two prebake potlines and an average of data from two Soderberg
facilities. Based on knowledge of the industry and previous testing, we
could reasonably expect emissions of approximately 23 POM specific POM
compounds from primary aluminum production facilities. The allocation
incorporated POM emissions at 50 percent of the detection limit for
those compounds ``reported as below detection limit.'' The use of 50
percent of the detection limit is more conservative than assuming that
these compounds were not present; an assumption that the compounds were
present at the detection limit would be an overestimation. The
assumption that these compounds were present at 50 percent of the
detection limit represented the midpoint of two extreme options. For
Soderberg potline stacks, six out of 38 measurements were below the
detection limit. For Soderberg potroom roof vents, 10 out of 38
measurements were below the detection limit. For prebake potline
stacks, 21 out of 38 measurements were below the detection limit. For
prebake potroom roof vents, 25 out of 38 measurements were below the
detection limit.
    To estimate allowable emissions, we analyzed the emissions data
gathered from the 2002 NEI, the 2005 NEI and responses to the ICR
described above. Based on that analysis, we estimated that allowable
emissions were generally about 1.5 times higher than actual emissions.
Therefore, to calculate allowable emissions we assumed that allowable
emissions were 1.5 times greater than actual emissions for all
facilities except for one idle Soderberg facility (Columbia Falls). For
Columbia Falls, which has the highest potential for emissions of all
the facilities, we evaluated site-specific data and estimated that
allowable emissions were about 1.9 times higher than actual emissions.
    Actual emissions of COS for the industry are estimated to be about
4,400 tons per year (tpy), with an average of about 330 tons per
facility. Actual emissions of HF are estimated to be about 1,900 tpy
with an average of about 160 tpy per facility. Estimated emissions of
speciated compounds of POM were much lower. Estimated actual emissions
of identified POM species totaled approximately 180 tpy for the
industry. Moreover, POM emissions are much higher from Soderberg
facilities compared to prebake facilities. The average POM emissions
from prebake facilities are about 4.5 tpy per facility, and the average
POM emissions for Soderberg facilities are about 60 tpy per facility.
We estimate that approximately one-third of the emissions of POM for
both types of potrooms come from the control device stack, and the
remainder are secondary emissions emitted from potroom vents. This
estimate is based on a summary of emissions derived from reports of
emission testing conducted at two prebake facilities and two Soderberg
facilities (``Industry Review of Draft POM Speciation and Emissions
Data,'' December 19, 2007).
    The emissions data, calculations and risk assessment inputs for the
Primary Aluminum Reduction Plant source category are described further
in Draft Development of the RTR Emissions Dataset for the Primary
Aluminum Production Source Category which is available in the docket
for this proposed rulemaking.

III. Analyses Performed

    In this section we describe the analyses performed to support the
proposed decisions for the RTR for this source category.

A. How did we address unregulated emissions sources?

    In the course of evaluating the Primary Aluminum Reduction Plant
source category, we identified certain HAP for which we failed to
establish emission standards in the original MACT. See National Lime v.
EPA, 233 F. 3d 625, 634 (DC Cir. 2000) (the EPA has ``clear statutory
obligation to set emissions standards for each listed HAP'').
    We evaluated establishing emissions limits for COS for the source
category and for POM for various emissions points that had not been
regulated in the 1997 MACT rule or in the 2005

amendments. Section 112(d)(3)(B) of the CAA requires that the MACT
standards for existing sources be at least as stringent as the average
emissions limitation achieved by the best performing five sources (for
which the Administrator has or could reasonably obtain emissions
information) in a category with fewer than 30 sources. The Primary
Aluminum source category consists of fewer than 30 sources.
    The EPA must exercise its judgment, based on an evaluation of the
relevant factors and available data, to determine the level of
emissions control that has been achieved by the best performing sources
under variable conditions. It is recognized in the case law that the
EPA may consider variability in estimating the degree of emissions
reduction achieved by best-performing sources and in setting MACT
floors. See Mossville Envt'l Action Now v. EPA, 370 F.3d 1232, 1241-42
(DC Cir 2004) (holding that the EPA may consider emissions variability
in estimating performance achieved by best-performing sources and may
set the floor at a level that a best-performing source can expect to
meet ``every day and under all operating conditions''). More details on
how we calculate MACT floors and how we account for variability are
described in the Draft MACT Floor Analysis for the Primary Aluminum
Source Category which is available in the docket for this proposed
action.
    Carbonyl sulfide (COS) was not regulated in the 1997 NESHAP or in
the 2005 amendments for Primary Aluminum Reduction Plants. In this
action we analyzed the available data and evaluated options for
developing MACT standards for this HAP. Based on all our analyses,
which are described in section IV.A of this preamble, we concluded that
establishing a standard based on a mass balance equation would be the
most appropriate approach. Therefore, we are proposing MACT standards
for COS in today's action based on use of a mass balance equation to
derive COS emissions based on data on anode coke sulfur content, anode
consumption and aluminum production.
    Polycyclic organic matter (POM) emissions from prebake potlines
were also not regulated in the 1997 NESHAP or in the 2005 amendments.
We are proposing MACT limits for new and existing prebake potlines in
today's action based on available data. Finally, the 1997 NESHAP
included MACT standards for new pitch storage tanks, which required a
95 percent reduction in emissions. However, the rule had no limits for
existing storage tanks. We are proposing that existing tanks will be
subject to the same standard (i.e., minimum of 95 percent reduction of
POM emissions). At least three facilities are currently achieving this
level of control on existing tanks.
    Further details about the analyses, the results and proposed
decisions regarding the proposed MACT limits pursuant to CAA section
112(d)(2) and 112(d)(3) are presented in section IV.A of this preamble.

B. How did we estimate risks posed by the source category?

    The EPA conducted risk assessments that provided estimates of the
MIR posed by the HAP emissions for each source in the category, the 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 assessments
also provided estimates of the distribution of cancer risks within the
exposed populations, cancer incidence and an evaluation of the
potential for adverse environmental effects for each source category.
The risk assessments consisted of seven primary steps, as discussed
below. The docket for this rulemaking contains the following document
which provides more information on the risk assessment inputs and
models: Draft Residual Risk Assessment for the Primary Aluminum
Reduction Plant Source Category. 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. Establishing the Nature and Magnitude of Actual Emissions and
Identifying the Emissions Release Characteristics
    As discussed in section II.B of this preamble, we used a dataset
consisting of the estimated actual and allowable emissions as the basis
for the risk assessment. In addition to the quality assurance (QA) of
the emissions and associated parameters contained in the dataset, we
also checked the coordinates of every facility in the dataset through
visual observations using tools such as Google Earth and ArcView. Where
coordinates were found to be incorrect, we identified and corrected
them to the extent possible. We also performed QA of the emissions data
and release characteristics to ensure there were no outliers.
2. Establishing the Relationship Between Actual Emissions and MACT-
Allowable Emissions Levels
    The available emissions data in the MACT dataset include estimates
of the mass of HAP actually emitted during the specified annual time
period. These ``actual'' emission levels are often lower than the
emission levels that a facility might be allowed to emit and still
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. This represents the highest emissions level that could
be emitted by the facility without violating the MACT standards.
    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 sources could
emit and still comply with national emission standards. But 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. (54 FR 38044, September 14, 1989.)
    Further explanation is provided in the document Draft Development
of the RTR Emissions Dataset for the Primary Aluminum Production Source
Category which is available in the docket for this proposed rulemaking.
3. Conducting Dispersion Modeling, Determining Inhalation Exposures and
Estimating Individual and Population Inhalation Risks
    Both long-term and short-term inhalation exposure concentrations
and health risks from each facility in the source category addressed in
this proposal were estimated using the Human Exposure Model (HEM)
(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
km of the modeled sources and (3) estimating individual and population-
level inhalation risks using the exposure estimates and quantitative
dose-response information.
    The dispersion model used by HEM-3 is AERMOD, which is one of the
EPA's preferred models for assessing pollutant concentrations from
industrial facilities.\4\ 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 (1991) of
hourly surface and upper air observations for more than 158
meteorological stations, selected to provide coverage of the United
States and Puerto Rico. A second library of United States Census Bureau
census block \5\ internal point locations and populations provides the
basis of human exposure calculations (Census, 2000). 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.
---------------------------------------------------------------------------

    \4\ 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).
    \5\ A census block is generally 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 concentration of each of the
HAP emitted by each 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 an inhabited census block. 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) by its unit risk estimate (URE), which 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 the 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.
    Polycyclic organic matter (POM), a carcinogenic HAP with a
mutagenic mode of action, is emitted by the facilities in this source
category.\6\ For this compound group,\7\ the age-dependent adjustment
factors (ADAF) described in the EPA's Supplemental Guidance for
Assessing Susceptibility from Early-Life Exposure to Carcinogens \8\
were applied. This adjustment has the effect of increasing the
estimated lifetime risks for POM by a factor of 1.6. In addition,
although only a small fraction of the total POM emissions were not
reported as individual compounds, the EPA expresses carcinogenic
potency for compounds in this group in terms of benzo[a]pyrene
equivalence, based on evidence that carcinogenic POM has the same
mutagenic mechanism of action as benzo[a]pyrene. For this reason, the
EPA's Science Policy Council \9\ recommends applying the Supplemental
Guidance to all carcinogenic polycyclic aromatic hydrocarbons for which
risk estimates are based on relative potency. Accordingly, we have
applied the ADAF to the benzo[a]pyrene equivalent portion of all POM
mixtures.
---------------------------------------------------------------------------

    \6\ U.S. EPA. Performing risk assessments that include
carcinogens described in the Supplemental Guidance as having a
mutagenic mode of action. Science Policy Council Cancer Guidelines
Implementation Work Group Communication II: Memo from W.H. Farland,
dated October 4, 2005.
    \7\ See the Risk Assessment for Source Categories document
available in the docket for a list of HAP with a mutagenic mode of
action.
    \8\ U.S. EPA. Supplemental Guidance for Assessing Early-Life
Exposure to Carcinogens. EPA/630/R-03/003F, 2005. http://www.epa.gov/ttn/atw/childrens_supplement_final.pdf.
    \9\ U.S. EPA. Science Policy Council Cancer Guidelines
Implementation Workgroup Communication II: Memo from W.H. Farland,
dated June 14, 2006.
---------------------------------------------------------------------------

    Incremental individual lifetime cancer risks associated with
emissions from the source category were estimated 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 \10\) emitted by the
modeled source. Cancer incidence and the distribution of individual
cancer risks for the population within 50 km of any source were also
estimated for the source category as part of these assessments by
summing individual risks. A distance of 50 km is consistent with both
the analysis supporting the 1989 Benzene NESHAP (54 FR 38044) and the
limitations of Gaussian dispersion models, including AERMOD.
---------------------------------------------------------------------------

    \10\ 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 EPA's
NATA entitled, 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.
---------------------------------------------------------------------------

    To assess risk of noncancer 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 for chronic exposures is the estimated
chronic exposure divided by the chronic reference level, which is
either the EPA reference concentration (RfC), defined as ``an estimate
(with uncertainty spanning perhaps an order of magnitude) of a
continuous inhalation exposure to the human population (including
sensitive subgroups) that is likely to be without an appreciable risk
of deleterious effects during a lifetime,'' or, in cases where an RfC
from the EPA's IRIS database is not available, a value from the
following prioritized sources: (1) The agency for Toxic Substances and
Disease Registry Minimum Risk Level, which is defined as ``an estimate
of daily human exposure to a substance that is likely to be without an
appreciable risk of adverse effects (other than cancer) over a
specified duration of exposure''; (2) the CalEPA Chronic Reference
Exposure Level (REL), which is defined as ``the concentration level 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.
    Screening estimates of acute exposures and risks were also
evaluated for each of the HAP at the point of highest off-site exposure
for each facility (i.e., not just the census block centroids), assuming
that a person is located at this spot at a time when both the peak
(hourly) emission rates from each emission point at the facility and
worst-case dispersion conditions occur. The acute HQ is the estimated
acute exposure divided by the acute dose-response value. In each case,
acute HQ values were calculated 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
emission 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.'' Acute REL values are
based on the most sensitive, relevant, adverse health effect reported
in the medical and toxicological literature. Acute REL values are
designed to protect the most sensitive sub-populations (e.g.,
asthmatics) by the inclusion of margins of safety. Since margins of
safety are incorporated to address data gaps and uncertainties,
exceeding the acute REL does not automatically indicate an adverse
health impact.
    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/opptintr/aegl/pubs/sop.pdf),\11\ ``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.''
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.'' The document lays
out the purpose and objectives of AEGL by stating (page 21) 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.''
In detailing the intended application of AEGL values, the document
states (page 31) 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.''
---------------------------------------------------------------------------

    \11\ NAS, 2001. Standing Operating Procedures for Developing
Acute Exposure Levels for Hazardous Chemicals, page 2.
---------------------------------------------------------------------------

    The AEGL-1 value is then specifically defined as ``the airborne
concentration of a substance above which it is predicted that the
general population, including susceptible individuals, could experience
notable discomfort, irritation, or certain asymptomatic nonsensory
effects. However, the effects are not disabling and are transient and
reversible upon cessation of exposure.'' The document also notes (page
3) 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.'' Similarly, the document defines
AEGL-2 values as ``the airborne concentration (expressed as ppm or mg/
m\3\) 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.''
    ERPG values are derived for use in emergency response, as described
in the American Industrial Hygiene Association's document entitled,
Emergency Response Planning Guidelines (ERPG) Procedures and
Responsibilities (http://www.aiha.org/1documents/committees/ERPSOPs2006.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.'' \12\ 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.'' 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 1 hour without
experiencing or developing irreversible or other serious health effects
or symptoms which could impair an individual's ability to take
protective action.''
---------------------------------------------------------------------------

    \12\ 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; in these instances, higher severity level AEGL-2 or ERPG-2
values are compared to our modeled exposure levels to assess potential
for acute concerns.
    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 similar to
the corresponding ERPG-1 values, and AEGL-2 values are often similar 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, we developed
estimates of maximum hourly emission rates by multiplying the average
actual annual hourly emission rates by a factor to cover routinely
variable emissions. Acute risk modeling is conducted under the
assumption that peak emissions are ten times greater than long term
average

emissions, in the absence of information regarding the variability of
the emissions.
    With respect to routine variable emissions, primary aluminum
potlines have a more consistent emissions profile than many other
sources because these emissions actually reflect the average of the
emissions from approximately 100 individual pots which operate in
cycles that are not in phase with each other. Thus any variability
associated with aluminum levels or electrode replacement for a
particular pot may be damped out by the other pots at different stages.
Alcoa provided to EPA a series of hourly hydrogen fluoride
concentration data for two potlines at their Wenatchee facility.
Approximately 2,075 consecutive hourly readings were provided based on
Fourier Transform Infrared measurements at the roof vents. Alcoa found
that the ratio of the maximum HAP emission rate to the average HAP
emission rate for these two potlines were 2.7 and 5.6. Only one value
out 2,075 consecutive hour samples (0.05 percent) was more than 5 times
the average (i.e., 99.95 percent of values were less than 5 times the
average).
    This dataset was then combined and subjected to two statistical
analysis techniques: The upper prediction limit (UPL) calculated
assuming a log-normal distribution after adjusting for temporal
correction and extreme value theory. The average of the concentration
values is 514 [micro]g/m\3\. The 99 percent UPL was calculated at 2,215
[micro]g/m, which corresponds to 4.3 times the mean.
    Using the extreme value theory, the 99.9 percentile estimate of the
generalized extreme value distribution (corresponding to 1 observation
in 1000) was 2,306 [micro]g/m\3\, which corresponds to 4.5 times the
mean. Based on these data, a source category factor of 5 times the
average hourly emissions rate, rather than the default factor of 10,
was used in the acute screening assessment.
    When worst-case HQ values from the initial acute screen step were
less than 1, acute impacts were deemed negligible and no further
analysis was performed. In cases where an acute HQ value from the
screening step indicated the potential for acute risk, we further
analyzed these values by considering additional site-specific data to
develop a relatively more refined estimate of the potential for acute
impacts of concern. This site-specific data includes the facility
layout that was used to distinguish facility property from an area
where the public could be exposed. These refinements are discussed in
the Draft Residual Risk Assessment for the Primary Aluminum Production
Source Category document, which is available in the docket for this
proposed rulemaking.
    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 emission 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.
However, we recognize that having this level of data is rare, hence our
use of the multiplier approach.
    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,\13\ we generally examine a wider range of
available acute health metrics 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.
Comparisons of the estimated maximum off-site 1-hour exposure levels
are not typically made to occupational levels for the purpose of
characterizing public health risks in RTR assessments. This is because
they are developed for working-age adults and are not generally
considered protective for the general public. We note that occupational
ceiling values are, for most chemicals, set at levels higher than a 1-
hour AEGL-1.
---------------------------------------------------------------------------

    \13\ 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.
---------------------------------------------------------------------------

4. Conducting Multi-Pathway Exposure and Risk Screening
    The potential for significant human health risks due to exposures
via routes other than inhalation (i.e., multi-pathway exposures) and
the potential for adverse environmental impacts were evaluated in a
three-step process. In the first step, we determined whether any
facilities emitted any PB-HAP (HAP known to be persistent and bio-
accumulative in the environment). There are 14 PB-HAP compounds or
compound classes identified for this screening in the EPA's Air Toxics
Risk Assessment Library (available at http://www.epa.gov/ttn/fera/risk_atra_vol1.html). They are cadmium compounds, chlordane,
chlorinated dibenzodioxins and furans,
dichlorodiphenyldichloroethylene, heptachlor, hexachlorobenzene,
hexachlorocyclohexane, lead compounds, mercury compounds, methoxychlor,
polychlorinated biphenyls, POM, toxaphene and trifluralin.
    Since POM is a PB-HAP and is emitted by all facilities in this
source category, we proceeded to the second step of the evaluation to
screen for potentially significant multi-pathway risks due to POM
emissions. In this step, we determined whether the facility-specific
emission rates of POM were large enough to create the potential for
significant non-inhalation human or environmental risks under
reasonable worst-case conditions. To facilitate this step, we have
developed emission rate thresholds for each PB-HAP using a hypothetical
worst-case screening exposure scenario developed for use in conjunction
with the EPA's TRIM.FaTE model. The hypothetical screening scenario was
subjected to a sensitivity analysis to ensure that its key design
parameters were established such that environmental media
concentrations were not underestimated (i.e., to minimize the
occurrence of false negatives or results that suggest that risks might
be acceptable when, in fact, actual risks are high) and to also
minimize the occurrence of false positives for human health endpoints.
We call this application of the TRIM.FaTE model TRIM-Screen. The
facility-specific emission rates of POM were compared to the TRIM-
Screen emission threshold values for POM to assess the potential for
significant human health risks or environmental risks via non-
inhalation pathways.
5. Assessing Risks Considering Emissions Control Options
    In addition to assessing baseline inhalation risks and screening
for potential multi-pathway risks, where appropriate, we also estimated
risks considering the potential emission reductions that would be
achieved by the particular control options under consideration. In
these cases, the expected emissions reductions were applied to the
specific HAP and emissions sources in the source category dataset to
develop corresponding estimates of risk reductions.
6. Conducting Other Risk-Related Analyses: Facility Wide Assessments
    To put the source category risks in context, for our residual risk
reviews, we also 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 these facility wide assessments 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.
Eleven of the primary aluminum reduction plants are collocated with
secondary aluminum production operations. Based on a general knowledge
of these facilities, we believe that the Primary Aluminum sources are
the largest sources of HAP emissions at each of them. Moreover, we plan
to do a facility wide assessment for each of these eleven facilities in
an upcoming RTR rulemaking for the Secondary Aluminum source category.
Therefore, we did not perform a facility wide risk assessment for these
eleven facilities as part of today's action. For the four primary
aluminum facilities that are not collocated with secondary aluminum
production operations, the risk assessment performed as part of today's
action is a facility wide risk assessment.
7. Considering Uncertainties in Risk Assessment
    Uncertainty and the potential for bias are inherent in all risk
assessments, including those performed for the Primary Aluminum source
category addressed in this proposal. Although uncertainty exists, we
believe that our approach, which used conservative tools and
assumptions, ensures that our decisions are health-protective. A brief
discussion of the uncertainties in the emissions datasets, dispersion
modeling, inhalation exposure estimates and dose-response relationships
follows below. A more thorough discussion of these uncertainties is
included in the risk assessment documentation (referenced earlier)
available in the docket for this action.
a. Uncertainties in the Emissions Datasets
    Although the development of the MACT dataset involved QA/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 inaccurate, errors in estimating emissions values and other
factors. The emission estimates considered in this analysis generally
are annual totals for certain years that 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 a multiplication factor of 5 applied
to the average annual hourly emission rate, which is intended to
account for emission fluctuations due to normal facility operations.
b. Uncertainties in Dispersion Modeling
    While the analysis employed the EPA's recommended regulatory
dispersion model, AERMOD, we recognize that there is uncertainty in
ambient concentration estimates associated with any model, including
AERMOD. In circumstances where we had to choose between various model
options, where possible, model options (e.g., rural/urban, plume
depletion, chemistry) were selected to provide an overestimate of
ambient air concentrations of the HAP rather than underestimates.
However, because of practicality and data limitation reasons, some
factors (e.g., meteorology, building downwash) have the potential in
some situations to overestimate or underestimate ambient impacts. For
example, meteorological data were taken from a single year (1991), and
facility locations can be a significant distance from the sites where
these data were taken. Despite these uncertainties, we believe that at
off-site locations and census block centroids, the approach considered
in the dispersion modeling analysis should generally yield
overestimates of ambient HAP concentrations.
c. Uncertainties in Inhalation Exposure
    The effects of human mobility on exposures were not included in the
assessment. Specifically, short-term mobility and long-term mobility
between census blocks in the modeling domain were not considered.\14\
The assumption of not considering short or long-term population
mobility does not bias the estimate of the theoretical MIR, nor does it
affect the estimate of cancer incidence since 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 risk
levels.
---------------------------------------------------------------------------

    \14\ 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.
---------------------------------------------------------------------------

    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 further 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 it is an unbiased
estimate of average risk and incidence.
    The assessments evaluate the cancer inhalation risks associated
with continuous pollutant exposures over a 70-year period, which is the
assumed lifetime of an individual. In reality, both the length of time
that modeled emissions 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 United States facilities) will influence the risks posed by a given
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 rare cases, where a facility maintains or increases
its emission levels beyond 70 years, residents live beyond 70 years at
the same location, and the residents spend most of their days at that
location, then the risks could potentially be underestimated. Annual
cancer incidence estimates from exposures to emissions from these
sources would not be affected by uncertainty in the length of time
emissions sources operate.
    The exposure estimates used in these analyses assume chronic
exposures to ambient 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, these
levels are typically lower. This factor has the potential to result in
an overstatement of 25 to 30 percent of exposures.\15\
---------------------------------------------------------------------------

    \15\ U.S. EPA. National-Scale Air Toxics Assessment for 1996.
(EPA 453/R-01-003; January 2001; page 85.)
---------------------------------------------------------------------------

    In addition to the uncertainties highlighted above, there are
several other factors specific to the acute exposure assessment. 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 human activity patterns. In this
assessment, we assume that individuals remain for 1 hour at the point
of maximum ambient concentration as determined by the co-occurrence of
peak emissions and worst-case meteorological conditions. These
assumptions would tend to overestimate actual exposures since it is
unlikely that a person would be located at the point of maximum
exposure during the time of worst-case impact.
d. Uncertainties in Dose-Response Relationships
    There are uncertainties inherent in the development of the dose-
response values used in our risk assessments for cancer effects from
chronic exposures and noncancer effects from both chronic and acute
exposures. Some uncertainties 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 2005 Cancer Guidelines; namely, that ``the
primary goal of the 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 documentation, 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).\16\ In some circumstances, the true risk could be as
low as zero; however, in other circumstances, the risk could also be
greater.\17\ 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).
---------------------------------------------------------------------------

    \16\ IRIS glossary (http://www.epa.gov/NCEA/iris/help_gloss.htm).
    \17\ 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.
---------------------------------------------------------------------------

    Chronic noncancer reference (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 daily oral
exposure (RfD) or of a continuous inhalation exposure (RfC) 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 includes consideration of both uncertainty and
variability. When there are gaps in the available information, UF are
applied to derive reference values that are intended to protect against
appreciable risk of deleterious effects. The UF are commonly default
values,\18\ 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. While
collectively termed ``uncertainty factor,'' 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. 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).
---------------------------------------------------------------------------

    \18\ 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 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.
---------------------------------------------------------------------------

    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 peer-reviewed reference
values for cancer and noncancer effects for all pollutants emitted by
the sources included in this assessment, some HAP

continue to have no reference values for cancer or chronic noncancer or
acute effects. Since exposures to these pollutants cannot be included
in a quantitative risk estimate, an understatement of risk for these
pollutants at environmental exposure levels is possible. For a group of
compounds that are either unspeciated or do not have reference values
for every individual compound (e.g., glycol ethers), we conservatively
use the most protective reference value to estimate risk from
individual compounds in the group of compounds.
    Additionally, chronic reference values for several of the compounds
included in this assessment are currently under the EPA IRIS review,
and revised assessments may determine that these pollutants are more or
less potent than the current value. We may re-evaluate residual risks
for the final rulemaking if these reviews are completed prior to our
taking final action for this source category and a dose-response metric
changes enough to indicate that the risk assessment supporting this
notice may significantly understate human health risk.
e. Uncertainties in the Multi-Pathway and Environmental Effects
Screening Assessment
    We generally assume that when exposure levels are not anticipated
to adversely affect human health, they also are not anticipated to
adversely affect the environment. For each source category, we
generally rely on the site-specific levels of PB-HAP emissions to
determine whether a full assessment of the multi-pathway and
environmental effects is necessary. For this source category, we only
performed a multi-pathway screening assessment for PB-HAP. Thus, it is
important to note that potential PB-HAP multi-pathway risks are biased
high.

C. How did we consider the risk results in making decisions for this
proposal?

    In evaluating and developing standards under section 112(f)(2), as
discussed in section I.A of this preamble, we apply a two-step process
to address residual risk. In the first step, the EPA determines whether
risks are acceptable. This determination ``considers all health
information, including risk estimation uncertainty, and includes a
presumptive limit on maximum individual lifetime [cancer] risk (MIR)
\19\ of approximately 1-in-10 thousand [i.e., 100-in-1 million]'' (54
FR 38045). In the second step of the process, the EPA sets the standard
at a level that provides an ample margin of safety ``in consideration
of all health information, including the number of persons at risk
levels higher than approximately 1-in-1 million, as well as other
relevant factors, including costs and economic impacts, technological
feasibility, and other factors relevant to each particular decision''
(Id.)
---------------------------------------------------------------------------

    \19\ Although defined as ``maximum individual risk,'' MIR refers
only to cancer risk. MIR, one metric for assessing cancer risk, is
the estimated risk were an individual exposed to the maximum level
of a pollutant for a lifetime.
---------------------------------------------------------------------------

    In past residual risk actions, the EPA has presented and considered
a number of human health risk metrics associated with emissions from
the category under review, including: The MIR; the numbers of persons
in various risk ranges; cancer incidence; the maximum non-cancer hazard
index (HI); and the maximum acute non-cancer hazard (72 FR 25138, May
3, 2007; 71 FR 42724, July 27, 2006). In more recent proposals (75 FR
65068, October 21, 2010, and 75 FR 80220, December 21, 2010), the EPA
also presented and considered additional measures of health
information, such as estimates of the risks associated with the maximum
level of emissions which might be allowed by the current MACT standards
(see, e.g., 75 FR 65068, October 21, 2010, and 75 FR 80220, December
21, 2010). The EPA also discussed and considered risk estimation
uncertainties. The EPA is providing this same type of information in
support of the proposed actions described in this Federal Register
notice.
    The agency is considering all available health information to
inform our determinations of risk acceptability and ample margin of
safety under CAA section 112(f). Specifically, as explained in the
Benzene NESHAP, ``the first step judgment on acceptability cannot be
reduced to any single factor'' and thus ``[t]he Administrator believes
that the acceptability of risk under [previous] section 112 is best
judged on the basis of a broad set of health risk measures and
information'' (54 FR 38046). Similarly, with regard to making the ample
margin of safety determination, as stated in the Benzene NESHAP ``[in
the ample margin decision, the agency again considers all of the health
risk and other health information considered in the first step. Beyond
that information, additional factors relating to the appropriate level
of control will also be considered, including cost and economic impacts
of controls, technological feasibility, uncertainties, and any other
relevant factors.'' Id.
    The agency acknowledges that the Benzene NESHAP provides
flexibility regarding what factors the EPA might consider in making
determinations and how these factors might be weighed for each source
category. In responding to comment on our policy under the Benzene
NESHAP, the EPA explained that: ``The policy chosen by the
Administrator permits consideration of multiple measures of health
risk. Not only can the MIR figure be considered, but also incidence,
the presence of non-cancer health effects, and the uncertainties of the
risk estimates. In this way, the effect on the most exposed individuals
can be reviewed as well as the impact on the general public. These
factors can then be weighed in each individual case. This approach
complies with the Vinyl Chloride mandate that the Administrator
ascertain an acceptable level of risk to the public by employing [her]
expertise to assess available data. It also complies with the
Congressional intent behind the CAA, which did not exclude the use of
any particular measure of public health risk from the EPA's
consideration with respect to CAA section 112 regulations, and,
thereby, implicitly permits consideration of any and all measures of
health risk which the Administrator, in [her] judgment, believes are
appropriate to determining what will `protect the public health' '' (54
FR 38057).
    Thus, the level of the MIR is only one factor to be weighed in
determining acceptability of risks. The Benzene NESHAP explained that
``an MIR of approximately 1-in-10 thousand should ordinarily be the
upper end of the range of acceptability. As risks increase above this
benchmark, they become presumptively less acceptable under CAA section
112, and would be weighed with the other health risk measures and
information in making an overall judgment on acceptability. Or, the
agency may find, in a particular case, that a risk that includes MIR
less than the presumptively acceptable level is unacceptable in the
light of other health risk factors'' (Id. at 38045). Similarly, with
regard to the ample margin of safety analysis, the EPA stated in the
Benzene NESHAP that: ``* * * the EPA believes the relative weight of
the many factors that can be considered in selecting an ample margin of
safety can only be determined for each specific source category. This
occurs mainly because technological and economic factors (along with
the health-related factors) vary from source category to source
category'' (Id. at 38061).

D. How did we perform the technology review?

    Our technology review focused on the identification and evaluation
of developments in practices, processes, and control technologies that
have

occurred since the Primary Aluminum Reduction Plant NESHAP was
promulgated.
    Based on our analyses of the data and information collected from
industry and the trade organization representing all facilities subject
to the NESHAP, our general understanding of the industry, and other
available information in the literature on potential controls for this
industry, we identified no new developments in practices, processes,
and control technologies. For the purpose of this exercise, we
considered any of the following to be a ``development'':
     Any add-on control technology or other equipment that was
not identified and considered during development of the 1997 Primary
Aluminum Reduction Plant NESHAP.
     Any improvements in add-on control technology or other
equipment (that were identified and considered during development of
the 1997 Primary Aluminum Reduction Plant NESHAP) that could result in
significant additional emissions reduction.
     Any work practice or operational procedure that was not
identified or considered during development of the 1997 Primary
Aluminum Reduction Plant NESHAP.
     Any process change or pollution prevention alternative
that could be broadly applied to the industry and that was not
identified or considered during development of the 1997 Primary
Aluminum Reduction Plant NESHAP.
    We also consulted the EPA's RACT/BACT/LAER Clearinghouse (RBLC) to
identify potential technology advances. Control technologies classified
as RACT (Reasonably Available Control Technology), BACT (Best Available
Control Technology), or LAER (Lowest Achievable Emissions Rate) apply
to stationary sources depending on whether the sources exist or new and
on the size, age, and location of the facility. BACT and LAER (and
sometimes RACT) are determined on a case-by-case basis, usually by
State or local permitting agencies. The EPA established the RBLC to
provide a central database of air pollution technology information
(including technologies required in source-specific permits) to promote
the sharing of information among permitting agencies and to aid in
identifying future possible control technology options that might apply
broadly to numerous sources within a category or apply only on a
source-by-source basis. The RBLC contains over 5,000 air pollution
control permit determinations that can help identify appropriate
technologies to mitigate many air pollutant emissions streams. We
searched this database to determine whether it contained any practices,
processes, or control technologies for the types of processes covered
by the Primary Aluminum Reduction Plant NESHAP. No such practices,
processes, or control technologies were identified in this database.

E. What other issues are we addressing in this proposal?

    In addition to the analyses described above, we also reviewed other
aspects of the MACT standards for possible revision as appropriate and
necessary. Based on this review we have identified aspects of the MACT
standards that we believe need revision.
    This includes proposing revisions to the startup, shutdown and
malfunction (SSM) provisions of the MACT rule in order to ensure that
they are consistent with a recent court decision in Sierra Club v. EPA,
551 F. 3d 1019 (DC Cir. 2008). In addition, we are proposing other
changes to the rule which are not based on residual risk. These include
establishing MACT floor-based standards for POM emissions from prebake
potlines, COS emissions from all potlines, and design standards for
control of POM emissions from existing pitch storage tanks. We are also
proposing changes to the rule related to affirmative defense for
exceedance of an emission limit during a malfunction. The analyses and
proposed decisions for these actions are presented in section IV of
this preamble.

IV. Analytical Results and Proposed Decisions

    This section of the preamble provides the results of our RTR for
the Primary Aluminum Reduction Plant source category and our proposed
decisions concerning changes to the Primary Aluminum Reduction Plant
NESHAP.

A. What are the results of our analyses and proposed decisions
regarding unregulated emissions sources?

    The current MACT rule has no standards for POM from prebake
potlines. Prebake facilities have significantly lower POM emissions
compared to Soderberg facilities. Nevertheless, these emissions are not
negligible. We are proposing to establish MACT emission limits for POM
from prebake potlines in this action. The typical controls used on
these prebake potlines to limit the primary (i.e., stack) emissions,
and which reflect the MACT floor level of control, are dry alumina
scrubbers (with a baghouse). We calculated MACT floor limits for these
potlines based on the limited available data. We also considered
possible controls beyond the MACT floor, such as wet roof scrubbers,
but we estimated that these beyond-the-floor controls would only
achieve approximately an additional 30 percent reduction in secondary
(i.e., roof vent) emissions and that the costs of these additional
controls would be quite high (e.g., well over $100 million in capital
costs for the industry). We estimate that the cost of controlling POM
from prebake potroom secondary emissions would be approximately
$800,000 per ton. Therefore, we are proposing emission limits for POM
from prebake potlines, after considering variability in emissions using
a 99% upper prediction level approach, based on the MACT floor. We are
proposing a POM emission limit for new prebake potlines equal to the
lowest limit for existing prebake potlines (developed from data
obtained from the best performing sources (center-worked prebake one)
facilities). More details about the data and analyses used to derive
the MACT limits, and explanation of the beyond-the-floor analyses, are
provided in the technical document Draft MACT Floor Analysis for the
Primary Aluminum Production Source Category which is available in the
docket for this proposed action. The proposed limits for prebake
potlines are shown in Table 5.

 Table 5--Proposed Emission Limits for New and Existing Prebake Potlines
------------------------------------------------------------------------
            Source                  Pollutant          Emission limit
------------------------------------------------------------------------
Existing Prebake:
    CWPB1 potlines............  POM..............  0.31 kg/Mg (0.62 lb/
                                                    ton) of aluminum
                                                    produced.
    CWPB2 potlines............  POM..............  0.65 kg/Mg (1.3 lb/
                                                    ton) of aluminum
                                                    produced.
    CWPB3 potlines............  POM..............  0.63 kg/Mg (1.26 lb/
                                                    ton) of aluminum
                                                    produced.
    SWPB potlines:............  POM..............  0.33 kg/Mg (0.65 lb/
                                                    ton) of aluminum
                                                    produced.
New Prebake:

    All prebake potline types.  POM..............  0.31 kg/Mg (0.62 lb/
                                                    ton) of aluminum
                                                    produced.
------------------------------------------------------------------------

    As mentioned above, the current MACT rule has no standards for COS.
It is very difficult and quite expensive to measure total COS emissions
because the concentrations of secondary emissions are below the
detection limit of the EPA reference method. However, stack tests are
feasible and have been completed. Moreover, emissions studies have been
completed using an experimental test method to estimate COS emissions
from these secondary emissions sources (Determination of COS to
SO2 Ratio in Smelting Process Emissions at the Alcoa Warrick
Operations, 4 August 1995). We have been able to use the experimental
test results along with stack test data and data on sulfur content of
input materials to estimate total COS emissions. We have determined
that there is a direct relationship between the COS emissions and the
sulfur content of raw materials. The results of these studies indicate
that an estimated 8 percent of the sulfur present in the coke (used to
make anodes) is converted to COS emissions.
    Given the technical difficulties of measuring secondary COS
emissions directly, and given that there is a direct relationship
between sulfur content of input materials and COS emissions, we
developed a mass balance equation for calculating COS emissions. Using
this approach, we developed a proposed MACT standard for COS using the
mass balance equation. The equation derives monthly COS emission rates
based on anode coke sulfur content, anode consumption and aluminum
production, as follows:
[GRAPHIC] [TIFF OMITTED] TP06DE11.005

Where:

ECOS = the facility wide emission rate of COS during the
calendar month in pounds per ton of aluminum produced;
K = factor accounting for molecular weights and conversion of sulfur
to carbonyl sulfide = 234;
Y = the tons of anode used at the facility during the calendar
month;
Z = the tons of aluminum produced at the facility during the
calendar month; and
%S = the weighted average sulfur content of the anode coke utilized
in the production of aluminum during the calendar month (e.g., if
the weighted average sulfur content of the anode coke utilized
during the calendar month was 2.5%, then %S = 0.025).

    Using this method, we are proposing a MACT floor limit for COS for
existing facilities at 3.9 pounds of COS per ton of aluminum produced
(lb/ton Al), based on data obtained from the five facilities with the
lowest calculated COS emissions and adjustment to account for
variability using a 99% upper prediction limit approach. With regard to
costs for this standard, we estimate that all facilities will be able
to meet this limit with minimal additional costs (e.g., calculating COS
emissions and the associated monitoring, recordkeeping and reporting).
With regard to new sources, the MACT floor limit for COS for new
facilities is proposed at 3.1 lb COS/ton Al, based on data obtained
from the facility with the lowest calculated COS emissions and
adjustment to account for variability.
    We also considered beyond-the-floor options for COS. For example,
we assessed the feasibility and costs of proposing that all existing
facilities meet a limit of 3.1 lb COS/ton Al. We estimate that a limit
at this level would impact 5 facilities, result in 220 tpy reductions
of COS emissions, at a total cost of $13,000,000 (or $2.6 million per
facility) per year. However, there are significant uncertainties
regarding the future availability and costs of the associated lower-
sulfur anode coke. The Primary Aluminum industry obtains most of their
coke as a by-product from the gas and oil refinery industry. It is our
understanding that currently available coke with low sulfur contents
could be very hard to obtain in the future and will likely be much more
expensive. This situation is expected due to the following: (1) The
type of crude oil input at refineries in the future is generally
expected to be heavier and, therefore, less likely to result in ``anode
grade coke'' that has the structure necessary for use in anode
production; (2) the type of crude oil input at refineries in the future
is generally expected to have higher sulfur content because the per
barrel cost of heavy sour (i.e., high-sulfur) crude oil is so much
lower than light sweet (i.e., low-sulfur) crude oil; (3) refineries
initially designed to process light sweet crude oil are being converted
to process heavy sour crude oil at a rapid pace worldwide due to
refinery economics; (4) refineries are designed to desulfurize the
product streams (gasoline, diesel, etc.), not the crude oil input, and
the sulfur in the crude oil tends to concentrate in the petroleum coke
(i.e., the ``bottoms''); (5) unwillingness of refineries to
preferentially process light sweet crude oil in place of heavy sour
crude oil due to unfavorable economics (i.e., refineries will not
modify their operations to change the quality of a by-product such as
petroleum coke); and (6) the lack of leverage that primary aluminum
companies have over the quality of this by-product, as coke is a very
low profit item for refineries and anode grade coke represents less
than 20% of all the petroleum coke produced worldwide. Thus, based on
future availability of low-sulfur coke, the true long term costs could
exceed the present estimated cost of $13,000,000 per year.
    We also evaluated the feasibility and costs of another beyond-the-
floor option of requiring that all existing facilities meet a limit of
3.5 lb COS/ton Al. We estimate that a limit at this level would impact
2 facilities, result in 52 tpy reductions of COS emissions, at a total
cost of $2,000,000 (or $1 million per facility) per year. Once again,
these estimated costs could be significant underestimates of the true
long-term costs. The uncertainties and concerns about the future
availability and costs of the required low-sulfur content coke that are
described above for the 3.1 lb COS/ton Al option are also a concern for
this 3.5 lb COS/ton Al option.
    We also considered control options including incineration and
scrubbing of COS. The cost of incineration would be quite high due to
the volume (typically millions of cubic feet per minute) and the
relatively low temperature of the exhaust stream (typically less than
200 [deg]F). Incineration also involves the disadvantage of the
generation of sulfur dioxide and other pollutants. Similarly, the cost
of scrubbers would be quite high and involve the disadvantage of
generating a waste sludge stream.
    Given the analyses and conclusions described above, we are
proposing a MACT standard for COS for existing facilities based on the
MACT floor analysis, which is a limit of 3.9 lb COS/ton Al. With regard
to new sources, we are proposing a MACT standard for COS based on the
MACT floor analysis, which is a limit of 3.1 lb COS/ton Al.
    With regard to POM emissions from pitch storage tanks, the 1997
NESHAP included MACT standards for new pitch

storage tanks, which required a 95 percent reduction in POM emissions.
However, the 1997 NESHAP had no limits for existing storage tanks. We
are proposing in today's action that existing tanks will be subject to
the same standard (i.e., minimum of 95 percent reduction of POM
emissions). At least three facilities are currently achieving this
level of control. We estimate that eight facilities would be affected
by this standard and would need to add controls, at a total annualized
cost of about $21,000 per facility. We also estimate that this would
achieve 1.6 tons reductions in POM emissions per year.
    A non-contact single stage, refrigerated, water cooled condenser
system was considered as a beyond the floor option for POM from pitch
storage tanks. However, we believe the associated cost (estimated at
$184,000 per year, per facility) is not justified by the incremental
control of HAP (estimated at 0.081 tons per year for the industry).

B. What are the results of the risk assessments?

    For the Primary Aluminum source category, we conducted an
inhalation risk assessment for all HAP emitted. We also conducted
multi-pathway screening analyses for PB-HAP emitted (i.e., POM).
Results of the risk assessment are presented briefly below and in more
detail in the residual risk documentation referenced in section III of
this preamble, which is available in the docket for this action.
    Table 6 of this preamble provides an overall summary of the results
of the inhalation risk assessment.

                                      Table 6--Primary Aluminum Reduction Plant Inhalation Risk Assessment Results
--------------------------------------------------------------------------------------------------------------------------------------------------------
    Maximum individual cancer risk (in 1 million) \1\        Estimated                 Maximum chronic non-cancer
----------------------------------------------------------  population     Estimated            TOSHI \2\
                                                           at increased     annual    ----------------------------
                                               Based on       risk of       cancer       Based on      Based on         Worst-case  maximum refined
      Based on actual emissions level          allowable   cancer  >=1-    incidence      actual       allowable     screening acute non-cancer HQ \3\
                                               emissions       in-1       (cases per     emissions     emissions
                                               level 4 5      million        year)         level         level
--------------------------------------------------------------------------------------------------------------------------------------------------------
30.........................................          100        41,000         0.005           0.4           0.6   HQREL 10 (HF)
                                                                                HQAEGL-1
                                                                                4 (HF)
--------------------------------------------------------------------------------------------------------------------------------------------------------
\1\ Estimated maximum individual excess lifetime cancer risk due to HAP emissions from the source category.
\2\ Maximum TOSHI. The target organ with the highest TOSHI for the primary aluminum source category is the skeletal system.
\3\ See section III.B of this preamble for explanations of acute dose-response values.
\4\ The facility with the highest MIR based on allowable emissions is the Columbia Falls facility. Notably, this facility has not operated in
  approximately 2 years and therefore, the EPA did not generate risk estimates (i.e., MIR, TOSHI, and acute screening values) based on actual emissions
  for this facility.
\5\ The highest MIR based on allowable emissions from an operating facility is estimated to be up to 50 in one million, for the operating Soderberg
  facility.

    The results of the chronic inhalation cancer risk assessment
indicate that, based on estimates of current actual emissions, the
maximum individual lifetime cancer risk (MIR) could be up to 30 in one
million, with emissions of POM \20\ primarily from potline roof vents
(secondary emissions) and anode bake furnaces driving these risks. The
highest MIR of up to 30 in one million based on actual emissions is due