[Federal Register Volume 79, Number 216 (Friday, November 7, 2014)]
[Proposed Rules]
[Pages 66511-66590]
[FR Doc No: 2014-25872]

Vol. 79

Friday,

No. 216

November 7, 2014

Part III

Environmental Protection Agency

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40 CFR Parts 60 and 63

Phosphoric Acid Manufacturing and Phosphate Fertilizer Production RTR
and Standards of Performance for Phosphate Processing; Proposed Rule

Federal Register / Vol. 79 , No. 216 / Friday, November 7, 2014 /
Proposed Rules

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ENVIRONMENTAL PROTECTION AGENCY

40 CFR Parts 60 and 63

[EPA-HQ-OAR-2012-0522; FRL-9912-61-OAR]
RIN 2060-AQ20

Phosphoric Acid Manufacturing and Phosphate Fertilizer Production
RTR and Standards of Performance for Phosphate Processing

AGENCY: Environmental Protection Agency.

ACTION: Proposed rule.

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SUMMARY: The Environmental Protection Agency (EPA) is proposing
amendments to the National Emission Standards for Hazardous Air
Pollutants for the Phosphoric Acid Manufacturing and Phosphate
Fertilizer Production source categories and to new source performance
standards (NSPS) for several phosphate processing categories. The
proposed amendments address the results of the residual risk and
technology reviews (RTR) conducted as required under the Clean Air Act
(CAA), as well as other actions deemed appropriate during the review of
these standards. The proposed amendments include numeric emission
limits for mercury and work practice standards for hydrogen fluoride
(HF) from calciners; work practice standards for hazardous air
pollutant (HAP) emissions from gypsum dewatering stacks and cooling
ponds; emission standards requiring HF testing from various affected
sources; clarifications to the applicability and monitoring
requirements for both source categories to accommodate process
equipment and technology changes; changes to remove the exemptions for
startup, shutdown and malfunction; work practice standards for periods
of startup and shutdown; and revised provisions to address
recordkeeping and reporting requirements applicable to periods of
startup, shutdown and malfunction. The proposed amendments will reduce
mercury emissions, thereby reducing potential mercury exposure to
children, including the unborn. Further, the EPA has conducted an 8-
year review of the current NSPS for these source categories, and is
proposing that no revisions to the numeric emission limits for these
standards are appropriate.

DATES: Comments. Comments must be received on or before December 22,
2014. A copy of comments on the information collection provisions
should be submitted to the Office of Management and Budget (OMB) on or
before December 8, 2014.
    Public Hearing. If anyone contacts the EPA requesting to speak at a
public hearing by November 12, 2014, we will hold a public hearing on
November 24, 2014 on the EPA campus at 109 T.W. Alexander Drive,
Research Triangle Park, North Carolina.

ADDRESSES: Comments. Submit your comments, identified by Docket ID
Number EPA-HQ-OAR-2012-0522, by one of the following methods:
     Federal eRulemaking Portal: http://www.regulations.gov:
Follow the online instructions for submitting comments.
     Email<EMAIL_ADDRESS>Include Attention Docket ID
No. EPA-HQ-OAR-2012-0522 in the subject line of the message.
     Fax:<PHONE_NUMBER>, Attention Docket ID No. EPA-HQ-OAR-
2012-0522.
     Mail: Environmental Protection Agency, EPA Docket Center
(EPA/DC), Mail Code 28221T, Attention Docket ID No. EPA-HQ-OAR-2012-
0522, 1200 Pennsylvania Ave. NW., Washington, DC 20460. 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/Courier Delivery: EPA Docket Center, Room 3334, EPA
WJC Building, 1301 Constitution Ave. NW., Washington, DC 20004,
Attention Docket ID Number EPA-HQ-OAR-2012-0522. 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-
2012-0522. The EPA's policy is that all comments received will be
included in the public docket without change and may be made available
online at http://www.regulations.gov, including any personal
information provided, unless the comment includes information claimed
to be confidential business information (CBI) or other information
whose disclosure is restricted by statute. Do not submit information
that you consider to be CBI or otherwise protected through http://www.regulations.gov or email. The http://www.regulations.gov Web site
is an ``anonymous access'' system, which means the EPA will not know
your identity or contact information unless you provide it in the body
of your comment. If you send an email comment directly to the EPA
without going through http://www.regulations.gov, your email address
will be automatically captured and included as part of the comment that
is placed in the public docket and made available on the Internet. If
you submit an electronic comment, the EPA recommends that you include
your name and other contact information in the body of your comment and
with any disk or CD-ROM you submit. If the EPA cannot read your comment
due to technical difficulties and cannot contact you for clarification,
the EPA may not be able to consider your comment. Electronic files
should not include special characters or any form of encryption and be
free of any defects or viruses. For additional information about the
EPA's public docket, visit the EPA Docket Center homepage at: http://www.epa.gov/dockets.
    Docket. The EPA has established a docket for this rulemaking under
Docket ID Number EPA-HQ-OAR-2012-0522. 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, Room
3334, EPA WJC West Building, 1301 Constitution Avenue NW., Washington,
DC. The Public Reading Room is open from 8:30 a.m. to 4:30 p.m., Monday
through Friday, excluding legal holidays. The telephone number for the
Public Reading Room is<PHONE_NUMBER>, and the telephone number for the
EPA Docket Center is<PHONE_NUMBER>.
    Public Hearing. If anyone contacts the EPA requesting a public
hearing by November 12, 2014, the public hearing will be held on
November 24, 2014 at the EPA's campus at 109 T.W. Alexander Drive,
Research Triangle Park, North Carolina. The hearing will begin at 10:00
a.m. (Eastern Standard Time) and conclude at 5:00 p.m. (Eastern
Standard Time). There will be a lunch break from 12:00 p.m. to 1:00
p.m. Please contact Ms. Pamela Garrett at<PHONE_NUMBER> or
<EMAIL_ADDRESS>to register to speak at the hearing, or to
inquire about whether a hearing will be held. The last day to pre-
register in advance to speak at the hearings will be November 19, 2014.
Additionally, requests to speak will be taken the day of the hearing at
the hearing registration desk, although preferences on speaking times
may not

be able to be fulfilled. If you require the service of a translator or
special accommodations such as audio description, please let us know at
the time of registration. If you require an accommodation, we ask that
you pre-register for the hearing, as we may not be able to arrange such
accommodations without advance notice.
    The hearing will provide interested parties the opportunity to
present data, views or arguments concerning the proposed action. The
EPA will make every effort to accommodate all speakers who arrive and
register. Because this hearing is being held at U.S. government
facilities, individuals planning to attend the hearing should be
prepared to show valid picture identification to the security staff in
order to gain access to the meeting room. Please note that the REAL ID
Act, passed by Congress in 2005, established new requirements for
entering federal facilities. If your driver's license is issued by
Alaska, American Samoa, Arizona, Kentucky, Louisiana, Maine,
Massachusetts, Minnesota, Montana, New York, Oklahoma or the state of
Washington, you must present an additional form of identification to
enter the federal building. Acceptable alternative forms of
identification include: Federal employee badges, passports, enhanced
driver's licenses and military identification cards. In addition, you
will need to obtain a property pass for any personal belongings you
bring with you. Upon leaving the building, you will be required to
return this property pass to the security desk. No large signs will be
allowed in the building, cameras may only be used outside of the
building and demonstrations will not be allowed on federal property for
security reasons.
    The EPA may ask clarifying questions during the oral presentations,
but will not respond to the presentations at that time. Written
statements and supporting information submitted during the comment
period will be considered with the same weight as oral comments and
supporting information presented at the public hearing. Commenters
should notify Ms. Garrett if they will need specific equipment, or if
there are other special needs related to providing comments at the
hearings. Verbatim transcripts of the hearing and written statements
will be included in the docket for the rulemaking. The EPA will make
every effort to follow the schedule as closely as possible on the day
of the hearing; however, please plan for the hearing to run either
ahead of schedule or behind schedule.
    Again, a hearing will only be held if requested by November 12,
2014. Please contact Ms. Pamela Garrett at<PHONE_NUMBER> or at
<EMAIL_ADDRESS>or visit http://www.epa.gov/ttn/atw/phosph/phosphpg.html to determine if a hearing will be held. If the EPA holds
a public hearing, the EPA will keep the record of the hearing open for
30 days after completion of the hearing to provide an opportunity for
submission of rebuttal and supplementary information.

FOR FURTHER INFORMATION CONTACT: For questions about this proposed
action, contact Ms. Tina Ndoh, Sector Policies and Programs Division
(D243-02), Office of Air Quality Planning and Standards, U.S.
Environmental Protection Agency, Research Triangle Park, North Carolina
27711; telephone number:<PHONE_NUMBER>; fax number:<PHONE_NUMBER>;
and email address<EMAIL_ADDRESS>For specific information
regarding the risk modeling methodology, contact James Hirtz, Health
and Environmental Impacts Division (C539-02), Office of Air Quality
Planning and Standards, U.S. Environmental Protection Agency, Research
Triangle Park, North Carolina 27711; telephone number:<PHONE_NUMBER>;
fax number:<PHONE_NUMBER>; and email address<EMAIL_ADDRESS>For
information about the applicability of the national emissions standards
for hazardous air pollutants (NESHAP) or the NSPS to a particular
entity, contact Scott Throwe, Office of Enforcement and Compliance
Assurance, U.S. Environmental Protection Agency, William Jefferson
Clinton Building, Mail Code 2227A, 1200 Pennsylvania Avenue NW.,
Washington, DC 20460; telephone number:<PHONE_NUMBER>; and email
address<EMAIL_ADDRESS>
SUPPLEMENTARY INFORMATION:

Preamble Acronyms and Abbreviations

    We use multiple acronyms and terms in this preamble. While this
list may not be exhaustive, to ease the reading of this preamble and
for reference purposes, the EPA defines the following terms and
acronyms here:

ACI Activated Carbon Injection

AEGL Acute exposure guideline levels
AERMOD Air dispersion model used by the HEM-3 model
AFPC Association of Fertilizer and Phosphate Chemists
AOAC Association of Official Analytical Chemists
APF Ammonium phosphate fertilizer
BACT Best available control technology
BDL Below the method detection limit
BSER Best System of Emissions Reduction
CAA Clean Air Act
CalEPA California EPA
CA-REL California Reference Exposure Level
CBI Confidential Business Information
CDX Central Data Exchange
CEDRI Compliance and Emissions Data Reporting Interface
CEMS Continuous emissions monitoring system
CFR Code of Federal Regulations
CMS Continuous monitoring system
CPMS Continuous parameter monitoring system
DAP Diammonium phosphate
EPA Environmental Protection Agency
ERPG Emergency Response Planning Guidelines
ERT Electronic Reporting Tool
F Fluoride
FaTE Fate, Transport, and Ecological Exposure
FR Federal Register
FTIR Fourier transform infrared spectroscopy
gr/dscf Grams per dry standard cubic feet
GTSP Granular triple superphosphate
H Hydrogen
HAP Hazardous air pollutants
HCl Hydrogen chloride
HEM-3 Human Exposure Model, Version 1.1.0
HF Hydrogen fluoride
Hg Mercury
HI Hazard index
HQ Hazard quotient
ICR Information Collection Request
IRIS Integrated Risk Information System
km Kilometer
LAER Lowest achievable emissions rate
LOAEL Lowest-observed-adverse-effect level
MACT Maximum achievable control technology
MAP Monoammonium phosphate
mg/dscm Milligrams per dry standard cubic meter
mg/kg-day Milligrams per kilogram-day
mg/m\3\ Milligrams per cubic meter
MIBK Methyl isobutyl ketone
MIR Maximum individual risk
MRL Minimum risk level
NAAQS National Ambient Air Quality Standards
NAICS North American Industry Classification System
NATA National Air Toxics Assessment
NEI National Emissions Inventory
NESHAP National Emissions Standards for Hazardous Air Pollutants
NOAA National Oceanic and Atmospheric Administration
NOAEL No-observed-adverse-effect level
NRC National Research Council
NTTAA National Technology Transfer and Advancement Act
OAQPS Office of Air Quality Planning and Standards
OECA Office of Enforcement and Compliance Assurance
OMB Office of Management and Budget
P2O5 Phosphorus pentoxide
PB-HAP Hazardous air pollutants known to be persistent and bio-
accumulative in the environment
PEL Probable effect levels
PM Particulate matter
POM Polycyclic organic matter
PPA Purified phosphoric acid
ppm Parts per million

QA/QC Quality assurance/quality control
RACT Reasonably available control technology
RATA Relative accuracy test audit
RBLC RACT/BACT/LAER Clearinghouse
REL Reference exposure level
RFA Regulatory Flexibility Act
RfC Reference concentration
RfD Reference dose
RTR Residual risk and technology review
SAB Science Advisory Board
SBA Small Business Administration
SiF4 Silicon tetrafluoride
SPA Superphosphoric acid
SSM Startup, shutdown and malfunction
TOSHI Target organ-specific hazard index
tpy Tons per year
TRIM Total Risk Integrated Modeling System
TRIM.FaTE Total Risk Integrated Methodology.Fate, Transport, and
Ecological Exposure model
TTN Technology Transfer Network
UF Uncertainty factor
[mu]g/m\3\ Micrograms per cubic meter
UMRA Unfunded Mandates Reform Act
UPL Upper prediction limit
URE Unit risk estimate
VCS Voluntary consensus standards
WESP Wet electrostatic precipitator
WPPA Wet-process phosphoric acid
WWW World Wide Web

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

I. General Information
    A. Does this action apply to me?
    B. Where can I get a copy of this document and other related
information?
    C. What should I consider as I prepare my comments for the EPA?
II. Background
    A. What are the statutory authorities for this action?
    B. What are the source categories and how do the current NESHAP
and NSPS regulate emissions?
    C. What data collection activities were conducted to support
this action?
    D. What other relevant background information and data are
available?
III. Analytical Procedures
    A. How did we estimate post-MACT risks posed by the source
categories?
    B. How did we consider the risk results in making decisions for
this proposal?
    C. How did we perform the technology reviews for the NESHAP and
NSPS?
IV. Analytical Results and Proposed Decisions for the Phosphoric
Acid Manufacturing Source Category
    A. What actions are we taking pursuant to CAA sections 112(d)(2)
and 112(d)(3) for the Phosphoric Acid Manufacturing source category?
    B. What are the results of the risk assessment and analyses for
the Phosphoric Acid Manufacturing source category?
    C. What are our proposed decisions regarding risk acceptability,
ample margin of safety and adverse environmental effects for the
Phosphoric Acid Manufacturing source category?
    D. What are the results and proposed decisions based on our
technology review for the Phosphoric Acid Manufacturing source
category?
    E. What other actions are we proposing for the Phosphoric Acid
Manufacturing source category?
    F. What are the notification, recordkeeping and reporting
requirements for the Phosphoric Acid Manufacturing source category?
    G. What compliance dates are we proposing for the Phosphoric
Acid Manufacturing source category?
V. Analytical Results and Proposed Decisions for the Phosphate
Fertilizer Production Source Category
    A. What are the results of the risk assessment and analyses for
the Phosphate Fertilizer Production source category?
    B. What are our proposed decisions regarding risk acceptability,
ample margin of safety and adverse environmental effects for the
Phosphate Fertilizer Production source category?
    C. What are the results and proposed decisions based on our
technology review for the Phosphate Fertilizer Production source
category?
    D. What other actions are we proposing for the Phosphate
Fertilizer Production source category?
    E. What are the notification, recordkeeping and reporting
requirements for the Phosphate Fertilizer Production source
category?
    F. What compliance dates are we proposing for the Phosphate
Fertilizer Production source category?
VI. 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?
VII. Request for Comments
VIII. Submitting Data Corrections
IX. Statutory and Executive Order Reviews
    A. Executive Order 12866: Regulatory Planning and Review and
Executive Order 13563: Improving Regulation and Regulatory Review
    B. Paperwork Reduction Act
    C. Regulatory Flexibility Act
    D. Unfunded Mandates Reform Act
    E. Executive Order 13132: Federalism
    F. Executive Order 13175: Consultation and Coordination with
Indian Tribal Governments
    G. Executive Order 13045: Protection of Children from
Environmental Health Risks and Safety Risks
    H. Executive Order 13211: Actions Concerning Regulations That
Significantly Affect Energy Supply, Distribution, or Use
    I. National Technology Transfer and Advancement Act
    J. Executive Order 12898: Federal Actions to Address
Environmental Justice in Minority Populations and Low-Income
Populations

I. General Information

A. Does this action apply to me?

    Table 1 of this preamble lists the industrial source categories
that are the subject of this proposal. Table 1 is not intended to be
exhaustive but rather to provide a guide for readers regarding the
entities that this proposed action is likely to affect. The proposed
standards, once promulgated, will be directly applicable to the
affected sources. As defined in the ``Initial List of Categories of
Sources Under Section 112(c)(1) of the Clean Air Act Amendments of
1990'' (see 57 FR 31576, July 16, 1992), the ``Phosphoric Acid
Manufacturing'' source category is any facility engaged in the
production of phosphoric acid. The category includes, but is not
limited to, production of wet-process phosphoric acid (WPPA) and
superphosphoric acid (SPA). The ``Phosphate Fertilizer Production''
source category includes any facility engaged in the production of
phosphate-based fertilizers including, but not limited to, plants with
bulk-blend processes, fluid-mix processes or ammonia granulation
processes. Examples of phosphate fertilizers are: Monoammonium
phosphates (MAP) and diammonium phosphates (DAP) (or ammonium phosphate
fertilizer (APF)), and triple superphosphates (TSP).\1\
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    \1\ U.S. EPA. Documentation for Developing the Initial Source
Category List--Final Report, USEPA/OAQPS, EPA-450/3-91-030, July,
1992.

 Table 1--Industrial Source Categories Affected by This Proposed Action
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                                                   Examples of regulated
        Source category          NAICS Code \a\          entities
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Industrial....................            325312  Phosphoric Acid; and
                                                   Phosphate
                                                   Fertilizers.
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\a\ North American Industry Classification System.

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

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

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

    Submitting CBI. Do not submit information containing CBI to the EPA
through http://www.regulations.gov or email. Clearly mark the part or
all of the information that you claim to be CBI. For CBI information on
a disk or CD-ROM that you mail to the EPA, mark the outside of the disk
or CD-ROM as CBI and then identify electronically within the disk or
CD-ROM the specific information that is claimed as CBI. In addition to
one complete version of the comments that includes information claimed
as CBI, you must submit a copy of the comments that does not contain
the information claimed as CBI for inclusion in the public docket. If
you submit a CD-ROM or disk that does not contain CBI, mark the outside
of the disk or CD-ROM clearly that it does not contain CBI. Information
not marked as CBI will be included in the public docket and the EPA's
electronic public docket without prior notice. Information marked as
CBI will not be disclosed except in accordance with procedures set
forth in 40 Code of Federal Regulations (CFR) part 2. Send or deliver
information identified as CBI only to the following address: Roberto
Morales, OAQPS Document Control Officer (C404-02), OAQPS, U.S.
Environmental Protection Agency, Research Triangle Park, North Carolina
27711, Attention Docket ID Number EPA-HQ-OAR-2012-0522.

II. Background

A. What are the statutory authorities for this action?

1. NESHAP Authority
    Section 112 of the CAA establishes a two-stage regulatory process
to address emissions of hazardous air pollutants (HAPs) from stationary
sources. In the first stage, after the EPA has identified categories of
sources emitting one or more of the HAP listed in CAA section 112(b),
CAA section 112(d) requires us to promulgate technology-based NESHAP
for those sources. ``Major sources'' are those that emit or have the
potential to emit 10 tons per year (tpy) or more of a single HAP or 25
tpy or more of any combination of HAPs. For major sources, the
technology-based NESHAP must reflect the maximum degree of emission
reductions of HAPs achievable (after considering cost, energy
requirements and non-air quality health and environmental impacts) and
are commonly referred to as maximum achievable control technology
(MACT) standards.
    MACT standards must reflect the maximum degree of emissions
reduction achievable through the application of measures, processes,
methods, systems or techniques, including, but not limited to, measures
that (1) reduce the volume of or eliminate pollutants through process
changes, substitution of materials or other modifications; (2) enclose
systems or processes to eliminate emissions; (3) capture or treat
pollutants when released from a process, stack, storage or fugitive
emissions point; (4) are design, equipment, work practice or
operational standards (including requirements for operator training or
certification); or (5) are a combination of the above. CAA section
112(d)(2)(A)-(E). The MACT standards may take the form of design,
equipment, work practice or operational standards where the EPA first
determines either that (1) a pollutant cannot be emitted through a
conveyance designed and constructed to emit or capture the pollutant,
or that any requirement for, or use of, such a conveyance would be
inconsistent with law; or (2) the application of measurement
methodology to a particular class of sources is not practicable due to
technological and economic limitations. CAA section 112(h)(1)-(2).
    The MACT ``floor'' is the minimum control level allowed for MACT
standards promulgated under CAA section 112(d)(3) and may not be based
on cost considerations. For new sources, the MACT floor cannot be less
stringent than the emissions control that is achieved in practice by
the best-controlled similar source. The MACT floor for existing sources
can be less stringent than floors for new sources but not less
stringent than the average emissions limitation achieved by the best-
performing 12 percent of existing sources in the category or
subcategory (or the best-performing five sources for categories or
subcategories with fewer than 30 sources). In developing MACT
standards, the EPA must also consider control options that are more
stringent than the floor. We may establish standards more stringent
than the floor based on considerations of the cost of achieving the
emission reductions, any non-air quality health and environmental
impacts and energy requirements.
    The EPA is then required to review these technology-based standards
and revise them ``as necessary (taking into account developments in
practices, processes, and control technologies)'' no less frequently
than every eight years. CAA section 112(d)(6). In conducting this
review, the EPA is not required to recalculate the MACT floor. NRDC v.
EPA, 529 F.3d 1077, 1084 (D. C. Cir. 2008). Association of Battery
Recyclers, Inc. v. EPA, 716 F.3d 667 (D.C. Cir. 2013).
    The second stage in standard-setting focuses on reducing any
remaining (i.e., ``residual'') risk according to CAA section 112(f).
CAA section 112(f)(1) required that the EPA prepare a report to
Congress discussing (among other things) methods of calculating the
risks posed (or potentially posed) by sources after implementation of
the MACT standards, the public health significance of those risks and
the EPA's recommendations as to legislation regarding such remaining
risk. The EPA prepared and submitted the Residual Risk Report to
Congress, EPA-453/R-99-001 (Risk Report) in March 1999. CAA section
112(f)(2) then provides that if Congress does not act on any
recommendation in the Risk Report, the EPA must analyze and address
residual risk for each category or subcategory of sources 8 years after
promulgation of such standards pursuant to CAA section 112(d).
    CAA section 112(f)(2) of the CAA requires the EPA to determine for
source categories subject to MACT standards whether the emission
standards provide an ample margin of safety to protect public health.
CAA section 112(f)(2)(B) of the CAA expressly preserves the EPA's use
of the two-step process for developing standards to address any
residual risk and the agency's interpretation of ``ample margin of

safety'' developed in the National Emissions Standards for Hazardous
Air Pollutants: Benzene Emissions from Maleic Anhydride Plants,
Ethylbenzene/Styrene Plants, Benzene Storage Vessels, Benzene Equipment
Leaks, and Coke By-Product Recovery Plants (Benzene NESHAP) (54 FR
38044, September 14, 1989). The EPA notified Congress in the Risk
Report that the agency intended to use the Benzene NESHAP approach in
making CAA section 112(f) residual risk determinations (EPA-453/R-99-
001, p. ES-11). The EPA subsequently adopted this approach in its
residual risk determinations and in a challenge to the risk review for
the Synthetic Organic Chemical Manufacturing source category, the
United States Court of Appeals for the District of Columbia Circuit
upheld as reasonable the EPA's interpretation that CAA section
112(f)(2) incorporates the approach established in the Benzene NESHAP.
See NRDC v. EPA, 529 F.3d 1077, 1083 (D.C. Cir. 2008)(``[S]ubsection
112(f)(2)(B) expressly incorporates the EPA's interpretation of the
Clean Air Act from the Benzene standard, complete with a citation to
the Federal Register.''); see also A Legislative History of the Clean
Air Act Amendments of 1990, vol. 1, p. 877 (Senate debate on Conference
Report).
    The first step in the process of evaluating residual risk is the
determination of acceptable risk. If risks are unacceptable, the EPA
cannot consider cost in identifying the emissions standards necessary
to bring risks to an acceptable level. The second step is the
determination of whether standards must be further revised in order to
provide an ample margin of safety to protect public health. The ample
margin of safety is the level at which the standards must be set,
unless an even more stringent standard is necessary to prevent, taking
into consideration costs, energy, safety and other relevant factors, an
adverse environmental effect.
a. Step 1-Determination of Acceptability
    The agency in the Benzene NESHAP concluded that ``the acceptability
of risk under section 112 is best judged on the basis of a broad set of
health risk measures and information'' and that the ``judgment on
acceptability cannot be reduced to any single factor.'' Benzene NESHAP
at 38046. The determination of what represents an ``acceptable'' risk
is based on a judgment of ``what risks are acceptable in the world in
which we live'' (Risk Report at 178, quoting NRDC v. EPA, 824 F. 2d
1146, 1165 (D.C. Cir. 1987) (en banc) (``Vinyl Chloride''), recognizing
that our world is not risk-free.
    In the Benzene NESHAP, we stated that ``EPA will generally presume
that if the risk to [the maximum exposed] individual is no higher than
approximately one in 10 thousand, that risk level is considered
acceptable.'' 54 FR at 38045, September 14, 1989. 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 acknowledged that maximum individual lifetime cancer
risk ``does not necessarily reflect the true risk, but displays a
conservative risk level which is an upper-bound that is unlikely to be
exceeded.'' Id.
    Understanding that there are both benefits and limitations to using
the MIR as a metric for determining acceptability, we acknowledged in
the Benzene NESHAP that ``consideration of maximum individual risk * *
* must take into account the strengths and weaknesses of this measure
of risk.'' Id. Consequently, the presumptive risk level of 100-in-1
million (1-in-10 thousand) provides a benchmark for judging the
acceptability of maximum individual lifetime cancer risk, but does not
constitute a rigid line for making that determination. Further, in the
Benzene NESHAP, we noted that:

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

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

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

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

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

(i.e., the MACT standards) are sufficiently protective. NRDC v. EPA,
529 F.3d 1077, 1083 (D.C. Cir. 2008) (``If EPA determines that the
existing technology-based standards provide an `ample margin of
safety,' then the Agency is free to readopt those standards during the
residual risk rulemaking.'') The EPA must also adopt more stringent
standards, if necessary, to prevent an adverse environmental effect,\2\
but must consider cost, energy, safety and other relevant factors in
doing so.
---------------------------------------------------------------------------

    \2\ ``Adverse environmental effect'' is defined as any
significant and widespread adverse effect, which may be reasonably
anticipated to wildlife, aquatic life or natural resources,
including adverse impacts on populations of endangered or threatened
species or significant degradation of environmental qualities over
broad areas. CAA section 112(a)(7).
---------------------------------------------------------------------------

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

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

    The agency further stated that ``[t]he EPA also considers incidence
(the number of persons estimated to suffer cancer or other serious
health effects as a result of exposure to a pollutant) to be an
important measure of the health risk to the exposed population.
Incidence measures the extent of health risks to the exposed population
as a whole, by providing an estimate of the occurrence of cancer or
other serious health effects in the exposed population.'' Id. at 38045,
September 14, 1989.
    In the ample margin of safety decision process, the agency again
considers all of the health risks and other health information
considered in the first step, including the incremental risk reduction
associated with standards more stringent than the MACT standard or a
more stringent standard that EPA has determined is necessary to ensure
risk is acceptable. In the ample margin of safety analysis, the agency
considers additional factors, including costs and economic impacts of
controls, technological feasibility, uncertainties and any other
relevant factors. Considering all of these factors, the agency will
establish the standard at a level that provides an ample margin of
safety to protect the public health, as required by CAA section 112(f).
54 FR 38046, September 14, 1989.
2. NSPS Authority
    New source performance standards implement CAA section 111, which
requires that each NSPS reflect the degree of emission limitation
achievable through the application of the best system of emission
reduction (BSER) which (taking into consideration the cost of achieving
such emission reductions, any nonair quality health and environmental
impact and energy requirements) the Administrator determines has been
adequately demonstrated.
    Existing affected facilities that are modified or reconstructed are
also be subject to NSPS. Under CAA section 111(a)(4), ``modification''
means any physical change in, or change in the method of operation of,
a stationary source which increases the amount of any air pollutant
emitted by such source or which results in the emission of any air
pollutant not previously emitted. Changes to an existing facility that
do not result in an increase in emissions are not considered
modifications.
    Rebuilt emission units would become subject to the NSPS under the
reconstruction provisions in 40 CFR 60.15, regardless of changes in
emission rate. Reconstruction means the replacement of components of an
existing facility such that: (1) The fixed capital cost of the new
components exceeds 50 percent of the fixed capital cost that would be
required to construct a comparable entirely new facility; and (2) it is
technologically and economically feasible to meet the applicable
standards (40 CFR 60.15).
    Section 111(b)(1)(B) of the CAA requires the EPA to periodically
review and, if appropriate, revise the standards of performance as
necessary to reflect improvements in methods for reducing emissions.
The EPA need not review an NSPS if the agency determines that such
review is not appropriate in light of readily available information on
the efficacy of the standard. When conducting the review under CAA
section 111(b)(1)(B), the EPA considers both (1) whether developments
in technology or other factors support the conclusion that a different
system of emissions reduction has become the ``best system of emissions
reduction'' and (2) whether emissions limitations and percent
reductions beyond those required by the current standards are achieved
in practice.

B. What are the source categories and how do the current NESHAP and
NSPS regulate emissions?

1. Description of Phosphoric Acid Manufacturing Source Category
    In 2014, 12 facilities in the United States manufacture phosphoric
acid. The basic step for producing phosphoric acid is acidulation of
phosphate rock. Typically, sulfuric acid, phosphate rock and water are
combined together and allowed to react to produce phosphoric acid and
gypsum. When phosphate rock is acidulated to manufacture WPPA, fluorine
contained in the rock is released. Fluoride (F) compounds,
predominately HF, are produced as particulates and gases that are
emitted to the atmosphere unless removed from the exhaust stream. Some
of these same F compounds also remain in the product acid and are
released as air pollutants during subsequent processing of the acid.
Gypsum is pumped as a slurry to ponds atop stacks of waste gypsum where
the liquids separate from the slurry and are decanted for return to the
process. The gypsum, which is discarded on the stack, is a solid waste
stream produced in this process. Five facilities concentrate WPPA to
make SPA, typically using the vacuum evaporation process. While one
manufacturer is permitted to use a submerged combustion process for the
production of SPA, that process was indefinitely shutdown on June 1,
2006. The majority of WPPA is used to produce phosphate fertilizers.
    Additional processes may also be used to further refine phosphoric
acid. At least two facilities have a defluorination process to remove F
from the phosphoric acid product, and one company uses a solvent
extraction process to remove metals and organics and to further refine
WPPA into purified phosphoric acid (PPA) for use in food manufacturing
or specialized chemical processes. In addition, four facilities have
processes to remove organics from the acid (i.e., the green acid
process).
    Sources of HF emissions from phosphoric acid plants include gypsum
dewatering stacks, cooling ponds, cooling towers, calciners, reactors,
filters, evaporators and other process equipment.
2. Federal Emission Standards Applicable to the Phosphoric Acid
Manufacturing Source Category
    The following federal emission standards are associated with the
Phosphoric Acid Manufacturing source category and are subject of this
proposed rulemaking:

     National Emission Standards for Hazardous Air Pollutants
from Phosphoric Acid Manufacturing Plants (40 CFR part 63, subpart AA);
     Standards of Performance for the Phosphate Fertilizer
Industry: Wet-Process Phosphoric Acid Plants (40 CFR part 60, subpart
T); and
     Standards of Performance for the Phosphate Fertilizer
Industry: Superphosphoric Acid Plants (40 CFR part 60, subpart U).
a. Phosphoric Acid Manufacturing NESHAP Emission Regulations
    The EPA promulgated 40 CFR part 63, subpart AA for the Phosphoric
Acid Manufacturing source category on June 10, 1999 (64 FR 31358). The
NESHAP established standards for major sources to control HAP emissions
from phosphoric acid facilities. Total F emission limits, as a
surrogate for the HAP HF, were set for WPPA process lines and SPA
process lines. For new sources, WPPA process lines are limited to
0.0135 pounds (lb) total F per ton (lb total F/ton) of equivalent
phosphorus pentoxide (P2O5), and SPA process
lines are limited to 0.00870 lb total F/ton of equivalent
P2O5. For existing sources, WPPA process lines
are limited to 0.020 lb total F/ton of equivalent
P2O5, SPA process lines using a vacuum
evaporation process are limited to 0.010 lb total F/ton of equivalent
P2O5, and SPA process lines using a submerged
combustion process are limited to 0.020 lb total F/ton of equivalent
P2O5.
    The NESHAP established emission limits for PM from phosphate rock
dryers and phosphate rock calciners as a surrogate for metal HAP. For
new sources, phosphate rock dryers are limited to 0.060 pounds PM per
ton (lb PM/ton) of phosphate rock feed, and phosphate rock calciners
are limited to 0.040 grains of PM per dry standard cubic feet (gr/
dscf). For existing sources, phosphate rock dryers are limited to
0.2150 lb PM/ton of phosphate rock feed, and phosphate rock calciners
are limited to 0.080 gr/dscf.
    Also, the NESHAP established an emission limit for methyl isobutyl
ketone (MIBK) for PPA process lines and work practices for cooling
towers. For new and existing sources, each product acid stream from PPA
process lines is limited to 20 parts per million (ppm) of MIBK, and
each raffinate stream from PPA process lines is limited to 30 ppm of
MIBK (compliance is based on a 30-day average of daily concentration
measurements).
b. Phosphoric Acid Manufacturing NSPS Emission Regulations
    The EPA promulgated 40 CFR part 60, subpart T for Wet-Process
Phosphoric Acid Plants on August 6, 1975 (40 FR 33154). The NSPS
established standards to control total F emissions from WPPA plants,
including reactors, filters, evaporators and hot wells. For new,
modified, and reconstructed sources WPPA plants are limited to 0.020 lb
total F/ton of equivalent P2O5.
    The EPA promulgated 40 CFR part 60, subpart U for Superphosphoric
Acid Plants on August 6, 1975 (40 FR 33155). The NSPS established
standards to control total F emissions from SPA plants, including
evaporators, hot wells, acid sumps and cooling tanks. For new, modified
and reconstructed sources, SPA plants are limited to 0.010 lb total F/
ton of equivalent P2O5.
3. Description of Phosphate Fertilizer Production Source Category
    In 2014, there are 11 operating facilities that produce phosphate
fertilizers, and most facilities can produce either MAP or DAP in the
same process train. However, approximately 80 percent of all ammonium
phosphates are produced as MAP. MAP and DAP plants are generally
collocated with WPPA plants since it is manufactured from phosphoric
acid and ammonia. The MAP and DAP manufacturing process consists of
three basic steps: Reaction, granulation and finishing operations such
as drying, cooling and screening. In addition, some of the fluorine is
liberated as HF and silicon tetrafluoride (SiF4), with the
majority being emitted as HF. Sources of F emissions from MAP and DAP
plants include the reactor, granulator, dryer, cooler, screens and
mills.
    TSP is made as run-of-the-pile-TSP (ROP-TSP) and granular TSP
(GTSP) by reacting WPPA with ground phosphate rock. The phosphoric acid
used in the GTSP process is appreciably lower in concentration (40-
percent P2O5) than that used to manufacture ROP-
TSP product (50- to 55- percent P2O5). The GTSP
process yields larger, more uniform particles with improved storage and
handling properties than the ROP-TSP process. Currently, no facilities
produce ROP-TSP or GTSP,\3\ although one facility retains an operating
permit to store GTSP.
---------------------------------------------------------------------------

    \3\ According to 2014 production and trade statistics issued by
International Fertilizer Industry Association (IFA).
---------------------------------------------------------------------------

4. Federal Emission Standards Applicable to the Phosphate Fertilizer
Production Source Category
    The following federal emission standards are associated with the
Phosphate Fertilizer Production source category and are subject of this
proposed rulemaking:
     National Emission Standards for Hazardous Air Pollutants
from Phosphate Fertilizers Production Plants (40 CFR part 63, subpart
BB);
     Standards of Performance for the Phosphate Fertilizer
Industry: Diammonium Phosphate Plants (40 CFR part 60, subpart V);
     Standards of Performance for the Phosphate Fertilizer
Industry: Triple Superphosphate Plants (40 CFR part 60, subpart W); and
     Standards of Performance for the Phosphate Fertilizer
Industry: Granular Triple Superphosphate Storage Facilities (40 CFR
part 60, subpart X).
a. Phosphate Fertilizer Production NESHAP Emission Regulations
    The EPA promulgated 40 CFR part 63, subpart BB for the Phosphate
Fertilizer Production source category on June 10, 1999 (64 FR 31358).
The NESHAP established standards for major sources to control HAP
emissions from phosphate fertilizer facilities. As a surrogate for HF,
the NESHAP set total F emission limits for DAP and/or MAP process lines
and GTSP process lines and storage buildings. The NESHAP also
established work practices for GTSP production. For new sources, DAP
and MAP process lines are limited to 0.058 lb total F/ton of equivalent
P2O5 feed. For existing sources, DAP and MAP
process lines are limited to 0.06 lb total F/ton of equivalent
P2O5 feed. For new sources, GTSP process lines
are limited to 0.1230 lb total F/ton of equivalent
P2O5 feed. For existing sources, GTSP process
lines are limited to 0.150 lb total F/ton of equivalent
P2O5 feed. For new and existing sources, GTSP
storage buildings are limited to 5.0x10-4 pounds of total F
per hour per ton of equivalent P2O5 stored.
b. Phosphate Fertilizer Production NSPS Emission Regulations
    The EPA promulgated 40 CFR part 60, subpart V for Diammonium
Phosphate Plants on July 25, 1977 (42 FR 37938). The NSPS established
standards to control total F emissions from granular DAP plants,
including reactors, granulators, dryers, coolers, screens and mills.
For new, modified and reconstructed sources, granular DAP plants are
limited to 0.06 lb total F/ton of equivalent P2O5
feed.

    The EPA promulgated 40 CFR part 60, subpart W for Triple
Superphosphate Plants on July 25, 1977 (42 FR 37938). The NSPS
established standards to control total F emissions from the production
of ROP-TSP and GTSP, and the storage of ROP-TSP. For new, modified and
reconstructed sources, production of ROP-TSP and GTSP and the storage
of ROP-TSP is limited to 0.20 lb total F/ton of equivalent
P2O5 feed.
    The EPA promulgated 40 CFR part 60, subpart X for Granular Triple
Superphosphate Storage Facilities on July 25, 1977 (42 FR 37938). The
NSPS established standards to control total F emissions from the
storage of GTSP, including storage or curing buildings (noted as
``piles'' in subpart X), conveyors, elevators, screens and mills. For
new, modified and reconstructed sources, the storage of GTSP is limited
to 5.0x10-4 pounds of total F per hour per ton of equivalent
P2O5 stored.
C. What data collection activities were conducted to support this
action?
    In April 2010, the EPA requested data, pursuant to CAA section 114,
from the seven companies that own and operate the 12 Phosphoric Acid
facilities and 11 Phosphate Fertilizer facilities. The EPA requested
available information regarding process equipment, control devices,
point and fugitive emissions, and other aspects of facility operations.
The seven companies completed the surveys for their facilities and
submitted the responses to the EPA in the fall of 2010. Additionally,
the EPA requested that the facilities conduct emissions tests in 2010
for certain HAP from specific processes. Pollutants tested included HF,
total F, PM and HAP metals. The facilities also conducted analyses of
the phosphate rock used in the manufacture of phosphoric acid. The
facilities submitted the results of these tests to the EPA in the fall
of 2010. The test results are available in the docket for this action.
    On January 24, 2014, the EPA issued another CAA section 114 survey
and testing request to certain facilities in order to gather additional
mercury (Hg) and HF emissions data from calciner operations, and
additional total F and HF emissions data from certain WPPA, SPA and APF
lines. The selection of WPPA, SPA and APF lines to be tested was based
on a review of the data received from the April 13, 2010 CAA section
114 survey request. In addition to the testing, the EPA requested
process production rate data concurrent with the duration of the
emissions testing (e.g., phosphoric acid production in tons per hour of
P2O5).
    For more information regarding the April 2010 CAA section 114 and
January 2014 CAA section 114 requests, refer to the memorandum,
``Information Collection and Additional Data Received for the
Phosphoric Acid and Phosphate Fertilizer Production Source
Categories,'' which is available in the docket for this action.
D. What other relevant background information and data are available?
    To support this proposed rulemaking, the EPA used information from
the EPA's National Emissions Inventory (NEI), and the RACT/BACT/LAER
Clearinghouse (RBLC) when performing the technology review and other
analyses. If emissions for a specific emission point were available in
the NEI, but test data were not available, we used the NEI data to
estimate emissions. This approach was primarily applicable to
combustion emissions. The EPA utilized the RBLC as a reference for
additional control technologies when performing the technology review.
See sections III.C, and IV.D, and V.C of this preamble for further
details on the use of these sources of information.
    Table 2 of this preamble summarizes the emissions data collected
for point sources and fugitive sources at phosphoric acid manufacturing
and phosphate fertilizer production facilities of HF, Total PM, Hg and
other HAP Metals. This includes emissions data from stack tests,
fugitive emission reports, and the NEI.

     Table 2--Summary of Emissions Data Collected for Point Sources and Fugitive Sources at Phosphoric Acid
                          Manufacturing and Phosphate Fertilizer Production Facilities
----------------------------------------------------------------------------------------------------------------
                                                                     Total PM                       HAP Metals
     Source category and emission point type         HF  (tpy)         (tpy)         Hg  (tpy)       (tpy) \a\
----------------------------------------------------------------------------------------------------------------
Phosphoric Acid Manufacturing:
    Point Sources...............................              38             162           0.019            1.07
    Fugitive Sources............................           2,155               0               0               0
    Total.......................................           2,193             162           0.019            1.07
Phosphate Fertilizer Production:
    Point Sources...............................            85.0             907            0.13            0.40
    Fugitive Sources............................          0.0051               0               0               0
    Total.......................................            85.0             907            0.13            0.40
----------------------------------------------------------------------------------------------------------------
\a\ HAP metals includes: antimony, arsenic, beryllium, cadmium, chromium (VI), chromium III, cobalt, lead,
  manganese, nickel, and selenium.

III. Analytical Procedures

    In this section, we describe the analyses performed to support the
proposed decisions for the RTR and other issues addressed in this
proposal.

A. How did we estimate post-MACT risks posed by the source categories?

    The EPA conducted a risk assessment that provides estimates of the
MIR posed by the HAP emissions from each source in the source category,
the hazard index (HI) for chronic exposures to HAP with the potential
to cause non-cancer health effects, and the hazard quotient (HQ) for
acute exposures to HAP with the potential to cause non-cancer health
effects. The assessment also provides estimates of the distribution of
cancer risks within the exposed populations, cancer incidence and an
evaluation of the potential for adverse environmental effects. The risk
assessment 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 Phosphate Fertilizer Production and
Phosphoric Acid Manufacturing. 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; \4\ they
are also consistent with the key

recommendations contained in that report.
---------------------------------------------------------------------------

    \4\ U.S. EPA SAB. Risk and Technology Review (RTR) Risk
Assessment Methodologies: For Review by the EPA's Science Advisory
Board with Case Studies--MACT I Petroleum Refining Sources and
Portland Cement Manufacturing, May 2010.
---------------------------------------------------------------------------

1. How did we estimate actual emissions and identify the emissions
release characteristics?
a. Estimation of Actual Emissions
    Data from our April 2010 CAA section 114 request were used for this
assessment. The EPA performed a review and thorough quality assurance/
quality control (QA/QC) of the data to identify any limitations and
issues. The EPA also contacted facility and industry representatives to
clarify details and resolve issues with their data submissions.
    The EPA updated the 2005 NEI data for the Phosphate Fertilizer
Production and Phosphoric Acid Manufacturing source categories with the
emissions data and corrections to facility and emission point locations
that we received from industry through the CAA section 114 request. The
data incorporation procedures are discussed in the memorandum,
``Emissions Data Used in Residual Risk Modeling: Phosphoric Acid and
Phosphate Fertilizer Production Source Categories,'' which is available
in the docket for this action. In a few limited instances, test data
were not available for an emission point available in the NEI, in which
case the existing emissions data in the 2005 NEI were used. The
following sections of this preamble describe each of the source
categories, including a discussion of the applicable information
sources used to estimate emissions.
b. Phosphoric Acid Manufacturing
    Phosphate rock is the starting material for the production of all
phosphate products. Once the rock reaches the phosphoric acid
production facility, phosphoric acid is typically produced using the
wet method, in which beneficiated ground phosphate rock (i.e.,
phosphate rock that has been processed to remove impurities) is reacted
with sulfuric acid and weak phosphoric acid to produce phosphoric acid
and phosphogypsum, a waste product. The phosphogypsum is disposed of on
site in waste piles known as gypsum dewatering stacks (which are also
referred to as ``gypsum stacks'' or ``gypstacks''). Phosphoric acid
facility emissions are both point sources and fugitive sources. Point
source emissions originate from equipment (e.g., reactors, filters,
evaporators and calciners) associated with phosphoric acid
manufacturing processes including WPPA process lines, SPA process lines
and PPA process lines. Fugitive emissions are released from cooling
ponds, cooling towers and gypsum dewatering stacks.
    In 2014, there are 12 phosphoric acid manufacturing facilities
operating in the United States. Based on the emissions dataset (see the
memorandum, ``Emissions Data Used in Residual Risk Modeling: Phosphoric
Acid and Phosphate Fertilizer Production Source Categories,'' which is
available in the docket for this action), all 12 of these facilities
are, or show the potential to be, major sources of HAP even though two
of these facilities identified themselves as area sources of HAP in
their response to our April 2010 CAA section 114 request. Ten of these
12 facilities are collocated with phosphate fertilizer production
facilities.
    Based on the emissions data provided with the CAA section 114
request or available in the NEI, the total HAP emissions for the
Phosphoric Acid Manufacturing source category are approximately 2,230
tpy. HF is the HAP emitted in the largest quantity across these 12
facilities, accounting for approximately 98 percent of the total HAP
emissions by mass. Persistent and bioaccumulative HAP (PB-HAP)
emissions reported from these facilities include Hg, Pb, dioxin,
polycyclic organic matter (POM) and cadmium compounds.
c. Phosphate Fertilizer Production
    Phosphate fertilizer operations are generally collocated with
phosphoric acid manufacturing facilities, which provide the feedstock
(phosphoric acid) for phosphate fertilizer production facilities.
Phosphate fertilizer is produced by reacting phosphoric acid and
ammonia, followed by granulation, drying, cooling and screening.
Emissions from each of these steps are included in the estimated point
source emissions for each facility. Phosphate fertilizer facilities
also send water to cooling ponds and, thus, contribute to the fugitive
emissions from these sources. However, the contribution from phosphate
fertilizer production sources to the fugitive emissions from the
cooling ponds is minimal. Therefore, we have assigned fugitive
emissions from cooling ponds to the Phosphoric Acid Manufacturing
source category.
    In 2014, there are 11 phosphate fertilizer production facilities
operating in the United States. Based on the emissions dataset (see the
memorandum, ``Emissions Data Used in Residual Risk Modeling: Phosphoric
Acid and Phosphate Fertilizer Production Source Categories,'' which is
available in the docket for this action), all 11 of these facilities
are, or show the potential to be, major sources of HAP even though one
of these facilities identified itself as an area source of HAP in their
response to our April 2010 CAA section 114 request. Ten of these 11
facilities are collocated with phosphoric acid manufacturing
facilities.
    Based on the emissions data provided with the CAA section 114
request or available in the NEI, the total HAP emissions for the
Phosphate Fertilizer Production source category are approximately 86
tpy. The HAP emitted in the largest quantity across these 11 facilities
is HF. HF accounts for 99 percent of the total emissions by mass. PB-
HAP emissions reported from these facilities include Hg, Pb, and
cadmium compounds.
2. How did we estimate MACT-allowable emissions?
    The available emissions data in the RTR emissions dataset include
estimates of the mass of HAP emitted during the specified annual time
period. In some cases, these ``actual'' emission levels are lower than
the emission levels required to comply with the MACT standards. The
emissions level allowed to be emitted by the MACT standards is referred
to as the ``MACT-allowable'' emissions level. We discussed the use of
both MACT-allowable and actual emissions in the final Coke Oven
Batteries residual risk rule (70 FR 19998-19999, April 15, 2005) and in
the proposed and final Hazardous Organic NESHAP residual risk rules (71
FR 34428, June 14, 2006, and 71 FR 76609, December 21, 2006,
respectively). In those previous actions, we noted that assessing the
risks at the MACT-allowable level is inherently reasonable since these
risks reflect the maximum level facilities could emit and still comply
with national emission standards. We also explained that it is
reasonable to consider actual emissions, where such data are available,
in both steps of the risk analysis, in accordance with the Benzene
NESHAP approach. (54 FR 38044, September 14, 1989.) Details on the
methodologies for calculating allowable emissions, as discussed below,
are provided in the memorandum, ``Emissions Data Used in Residual Risk
Modeling: Phosphoric Acid and Phosphate Fertilizer Production Source
Categories,'' which is available in the docket for this action.
a. Phosphoric Acid Manufacturing
    In the case of this particular source category, point sources
contribute only a small percentage of overall emissions. Therefore, as
a conservative approach, we used the emission limits and the

permitted production capacity specified in the title V permit for each
facility to calculate allowable emissions for point sources. Because
emission limits are in terms of total F (pounds of total F per ton of
P2O5 production), and not the HAP HF, emissions
for total F were used as a surrogate for HF when calculating allowable
emissions. If emissions limits were not available in the title V
permit, we used the emission limits for existing sources in the current
NESHAP subpart AA. Because emissions limits for metals and MIBK are not
listed in the permits, we calculated allowable emissions using the
emissions as measured in the stack tests for the CAA section 114
request, and scaled these emissions up using the permitted capacity.
Allowable point source emissions are as much as 59 times higher than
actual total F emissions, about 8 times higher than actual metal
emissions, and about 2 times higher than actual MIBK emissions at
phosphoric acid manufacturing processes.
    For fugitive emissions of HF from gypsum dewatering stacks, cooling
ponds and cooling towers, the EPA estimated that actual emissions were
equivalent to allowable emissions. We do not expect fugitive emissions
to increase from these sources with an increase in production rate, or
increase significantly during a process upset, as emissions from these
large fugitive sources are the cumulative result of many decades of
stacking gypsum waste product and re-circulating cooling water. Because
of their general homeostatic nature, we expect only minor changes in
cooling pond emissions over time. We also anticipate that emissions are
higher during daylight hours and warmer months due to the increased
evaporation rate associated with higher ambient temperatures. Test data
for these sources were obtained during the spring and summer seasons
and during daylight hours. Therefore, emissions would not be expected
to increase significantly beyond the levels measured during the tests.
We expect that the emission factors and range of estimates (high,
medium and low) that we developed, based on the test data for the
spring and summer seasons obtained from industry, account sufficiently
for any changes to emissions as ambient conditions change. For more
information on the development of emission factors, see the memorandum,
``Emissions Data Used in Residual Risk Modeling: Phosphoric Acid and
Phosphate Fertilizer Production Source Categories,'' which is available
in the docket for this action.
b. Phosphate Fertilizer Production
    Similar to phosphoric acid manufacturing, point sources contribute
only a small percentage of overall emissions from this particular
source category. Therefore, as a conservative approach, we used the
emission limits (expressed in pounds of total F per ton of
P2O5 production) and the permitted production
capacity specified in the title V permit for each facility to calculate
point source allowable emissions for total F, as a surrogate for HF. If
emissions limits were not available in the title V permit, we used the
limits for existing sources in the current NESHAP subpart BB. Because
emissions limits for metals are not listed in the permits, we
calculated allowable emissions using the emissions test data collected
by the CAA section 114 request, and scaled these emissions up using the
permitted capacity. Allowable point source emissions are as much as 11
times higher than actual total F emissions and about 2 times higher
than actual metal at phosphate fertilizer production processes.
3. How did we conduct dispersion modeling, determine inhalation
exposures and estimate individual and population inhalation risks?
    Both long-term and short-term inhalation exposure concentrations
and health risks from the source category addressed in this proposal
were estimated using the Human Exposure Model (Community and Sector
HEM-3 version 1.1.0). The HEM-3 performs three primary risk assessment
activities: (1) Conducting dispersion modeling to estimate the
concentrations of HAP in ambient air, (2) estimating long-term and
short-term inhalation exposures to individuals residing within 50
kilometers (km) of the modeled sources,\5\ and (3) estimating
individual and population-level inhalation risks using the exposure
estimates and quantitative dose-response information.
---------------------------------------------------------------------------

    \5\ This metric comes from the Benzene NESHAP. See 54 FR 38046.
---------------------------------------------------------------------------

    The air dispersion model used by the HEM-3 model (AERMOD) is one of
the EPA's preferred models for assessing pollutant concentrations from
industrial facilities.\6\ To perform the dispersion modeling and to
develop the preliminary risk estimates, HEM-3 draws on three data
libraries. The first is a library of meteorological data, which is used
for dispersion calculations. This library includes 1 year (2011) of
hourly surface and upper air observations for more than 800
meteorological stations, selected to provide coverage of the United
States and Puerto Rico. A second library of United States Census Bureau
census block \7\ internal point locations and populations provides the
basis of human exposure calculations (U.S. Census, 2010). In addition,
for each census block, the census library includes the elevation and
controlling hill height, which are also used in dispersion
calculations. A third library of pollutant unit risk factors and other
health benchmarks is used to estimate health risks. These risk factors
and health benchmarks are the latest values recommended by the EPA for
HAP and other toxic air pollutants. These values are available at:
http://www.epa.gov/ttn/atw/toxsource/summary.html and are discussed in
more detail later in this section.
---------------------------------------------------------------------------

    \6\ 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).
    \7\ A census block is the smallest geographic area for which
census statistics are tabulated.
---------------------------------------------------------------------------

    In developing the risk assessment for chronic exposures, we used
the estimated annual average ambient air concentrations of each HAP
emitted by each source for which we have emissions data in the source
category. The air concentrations at each nearby census block centroid
were used as a surrogate for the chronic inhalation exposure
concentration for all the people who reside in that census block. We
calculated the MIR for each facility as the cancer risk associated with
a continuous lifetime (24 hours per day, 7 days per week and 52 weeks
per year for a 70-year period) exposure to the maximum concentration at
the centroid of inhabited census blocks. Individual cancer risks were
calculated by multiplying the estimated lifetime exposure to the
ambient concentration of each of the HAP (in micrograms per cubic meter
([mu]g/m\3\)) by its unit risk estimate (URE). The URE is an upper
bound estimate of an individual's probability of contracting cancer
over a lifetime of exposure to a concentration of 1 microgram of the
pollutant per cubic meter of air. For residual risk assessments, we
generally use URE values from the EPA's Integrated Risk Information
System (IRIS). For carcinogenic pollutants without EPA IRIS values, we
look to other reputable sources of cancer dose-response values, often
using California EPA (CalEPA) URE values, where available. In cases
where new, scientifically credible dose response values have been
developed in a manner consistent with the EPA guidelines and have
undergone a peer review process similar to that used by

the EPA, we may use such dose-response values in place of, or in
addition to, other values, if appropriate.
    The EPA estimated incremental individual lifetime cancer risks
associated with emissions from the facilities in the source category as
the sum of the risks for each of the carcinogenic HAP (including those
classified as carcinogenic to humans, likely to be carcinogenic to
humans, and suggestive evidence of carcinogenic potential \8\) emitted
by the modeled sources. Cancer incidence and the distribution of
individual cancer risks for the population within 50 km of the sources
were also estimated for the source category as part of this assessment
by summing individual risks. A distance of 50 km is consistent with
both the analysis supporting the 1989 Benzene NESHAP (54 FR 38044,
September 14, 1989) and the limitations of Gaussian dispersion models,
including AERMOD.
---------------------------------------------------------------------------

    \8\ 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
National Air Toxics Assessment (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.
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    To assess the risk of non-cancer health effects from chronic
exposures, we summed the HQ for each of the HAP that affects a common
target organ system to obtain the HI for that target organ system (or
target organ-specific HI, TOSHI). The HQ is the estimated exposure
divided by the chronic reference value, which is either the EPA
reference concentration (RfC) (http://www.epa.gov/riskassessment/glossary.htm), defined as ``an estimate (with uncertainty spanning
perhaps an order of magnitude) of a continuous inhalation exposure to
the human population (including sensitive subgroups) that is likely to
be without an appreciable risk of deleterious effects during a
lifetime,'' 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
(http://www.atsdr.cdc.gov/mrls/index.asp), which is defined as ``an
estimate of daily human exposure to a hazardous substance that is
likely to be without an appreciable risk of adverse non-cancer health
effects (other than cancer) over a specified duration of exposure'';
(2) the CalEPA Chronic Reference Exposure Level (REL) (http://www.oehha.ca.gov/air/hot_spots/pdf/HRAguidefinal.pdf), which is defined
as ``the concentration level (that is expressed in units of micrograms
per cubic meter ([mu]g/m\3\) for inhalation exposure and in a dose
expressed in units of milligram per kilogram-day (mg/kg-day) for oral
exposures), at or below which no adverse health effects are anticipated
for a specified exposure duration''; or (3), as noted above, a
scientifically credible dose-response value that has been developed in
a manner consistent with the EPA guidelines and has undergone a peer
review process similar to that used by the EPA, in place of or in
concert with other values.
    The EPA also evaluated screening estimates of acute exposures and
risks for each of the HAP at the point of highest potential off-site
exposure for each facility. To do this, the EPA estimated the risks
when both the peak hourly emissions rate and worst-case dispersion
conditions occur. We also assume that a person is located at the point
of highest impact during that same time. In accordance with our mandate
in section 112 of the CAA, we use the point of highest off-site
exposure to assess the potential risk to the maximally exposed
individual. The acute HQ is the estimated acute exposure divided by the
acute dose-response value. In each case, the EPA calculated acute HQ
values using best available, short-term dose-response values. These
acute dose-response values, which are described below, include the
acute REL, acute exposure guideline levels (AEGL) and emergency
response planning guidelines (ERPG) for 1-hour exposure durations. As
discussed below, we used conservative assumptions for emissions rates,
meteorology and exposure location for our acute analysis.
    As described in the CalEPA's Air Toxics Hot Spots Program Risk
Assessment Guidelines, Part I, The Determination of Acute Reference
Exposure Levels for Airborne Toxicants, an acute REL value (http://www.oehha.ca.gov/air/pdf/acuterel.pdf) is defined as ``the
concentration level at or below which no adverse health effects are
anticipated for a specified exposure duration.'' Id. at page 2. Acute
REL values are based on the most sensitive, relevant, adverse health
effect reported in the peer-reviewed medical and toxicological
literature. Acute REL values are designed to protect the most sensitive
individuals in the population through the inclusion of margins of
safety. Because margins of safety are incorporated to address data gaps
and uncertainties, exceeding the REL does not automatically indicate an
adverse health impact.
    AEGL values were derived in response to recommendations from the
National Research Council (NRC). As described in Standing Operating
Procedures (SOP) of the National Advisory Committee on Acute Exposure
Guideline Levels for Hazardous Substances (http://www.epa.gov/oppt/aegl/pubs/sop.pdf),\9\ ``the NRC's previous name for acute exposure
levels--community emergency exposure levels--was replaced by the term
AEGL to reflect the broad application of these values to planning,
response, and prevention in the community, the workplace,
transportation, the military, and the remediation of Superfund sites.''
Id. at 2.
---------------------------------------------------------------------------

    \9\ National Academy of Sciences (NAS), 2001. Standing Operating
Procedures for Developing Acute Exposure Levels for Hazardous
Chemicals, page 2.
---------------------------------------------------------------------------

    This document also states that AEGL values ``represent threshold
exposure limits for the general public and are applicable to emergency
exposures ranging from 10 minutes to eight hours.'' Id. at 2. The
document lays out the purpose and objectives of AEGL by stating that
``the primary purpose of the AEGL program and the National Advisory
Committee for Acute Exposure Guideline Levels for Hazardous Substances
is to develop guideline levels for once-in-a-lifetime, short-term
exposures to airborne concentrations of acutely toxic, high-priority
chemicals.'' Id. at 21. In detailing the intended application of AEGL
values, the document states that ``[i]t is anticipated that the AEGL
values will be used for regulatory and nonregulatory purposes by U.S.
federal and state agencies and possibly the international community in
conjunction with chemical emergency response, planning, and prevention
programs. More specifically, the AEGL values will be used for
conducting various risk assessments to aid in the development of
emergency preparedness and prevention plans, as well as real-time
emergency response actions, for accidental chemical releases at fixed
facilities and from transport carriers.'' Id. at 31.
    The AEGL-1 value is then specifically defined as ``the airborne
concentration (expressed as ppm (parts per million) or mg/m\3\
(milligrams per cubic meter)) of a substance above which it is
predicted that the general population, including susceptible
individuals, could experience notable discomfort, irritation, or
certain asymptomatic

nonsensory effects. However, the effects are not disabling and are
transient and reversible upon cessation of exposure.'' Id. at 3. The
document also notes that, ``Airborne concentrations below AEGL-1
represent exposure levels that can produce mild and progressively
increasing but transient and nondisabling odor, taste, and sensory
irritation or certain asymptomatic, nonsensory effects.'' Id.
Similarly, the document defines AEGL-2 values as ``the airborne
concentration (expressed as parts per million or milligrams per cubic
meter) of a substance above which it is predicted that the general
population, including susceptible individuals, could experience
irreversible or other serious, long-lasting adverse health effects or
an impaired ability to escape.'' Id.
    ERPG values are derived for use in emergency response, as described
in the American Industrial Hygiene Association's ERP Committee document
titled, ERPGS Procedures and Responsibilities (http://sp4m.aiha.org/insideaiha/GuidelineDevelopment/ERPG/Documents/ERP-SOPs2006.pdf), which
states that, ``Emergency Response Planning Guidelines were developed
for emergency planning and are intended as health based guideline
concentrations for single exposures to chemicals.'' \10\ Id. at 1. The
ERPG-1 value is defined as ``the maximum airborne concentration below
which it is believed that nearly all individuals could be exposed for
up to 1 hour without experiencing other than mild transient adverse
health effects or without perceiving a clearly defined, objectionable
odor.'' Id. at 2. Similarly, the ERPG-2 value is defined as ``the
maximum airborne concentration below which it is believed that nearly
all individuals could be exposed for up to one hour without
experiencing or developing irreversible or other serious health effects
or symptoms which could impair an individual's ability to take
protective action.'' Id. at 1.
---------------------------------------------------------------------------

    \10\ ERP Committee Procedures and Responsibilities. November 1,
2006. American Industrial Hygiene Association.
---------------------------------------------------------------------------

    As can be seen from the definitions above, the AEGL and ERPG values
include the similarly-defined severity levels 1 and 2. For many
chemicals, a severity level 1 value AEGL or ERPG has not been developed
because the types of effects for these chemicals are not consistent
with the AEGL-1/ERPG-1 definitions; in these instances, we compare
higher severity level AEGL-2 or ERPG-2 values to our modeled exposure
levels to screen for potential acute concerns. When AEGL-1/ERPG-1
values are available, they are used in our acute risk assessments.
    Acute REL values for 1-hour exposure durations are typically lower
than their corresponding AEGL-1 and ERPG-1 values. Even though their
definitions are slightly different, AEGL-1 values are often the same as
the corresponding ERPG-1 values, and AEGL-2 values are often equal to
ERPG-2 values. Maximum HQ values from our acute screening risk
assessments typically result when basing them on the acute REL value
for a particular pollutant. In cases where our maximum acute HQ value
exceeds 1, we also report the HQ value based on the next highest acute
dose-response value (usually the AEGL-1 and/or the ERPG-1 value).
    To develop screening estimates of acute exposures in the absence of
hourly emissions data, generally we first develop estimates of maximum
hourly emissions rates by multiplying the average actual annual hourly
emissions rates by a default factor to cover routinely variable
emissions. We choose the factor to use partially based on process
knowledge and engineering judgment. The factor chosen also reflects a
Texas study of short-term emissions variability, which showed that most
peak emission events in a heavily-industrialized four-county area
(Harris, Galveston, Chambers and Brazoria Counties, Texas) were less
than twice the annual average hourly emissions rate. The highest peak
emissions event was 74 times the annual average hourly emissions rate,
and the 99th percentile ratio of peak hourly emissions rate to the
annual average hourly emissions rate was 9.\11\ Considering this
analysis, to account for more than 99 percent of the peak hourly
emissions, we apply a conservative screening multiplication factor of
10 to the average annual hourly emissions rate in our acute exposure
screening assessments as our default approach. However, we use a factor
other than 10 if we have information that indicates that a different
factor is appropriate for a particular source category. For this source
category, we applied a multiplication factor of 10 to all emission
sources except for HF emissions from the gypsum dewatering stacks and
cooling ponds. The EPA used a multiplication factor of 1 for gypsum
dewatering stacks and cooling ponds based upon the stability of HF
releases from this emission source. Section III.A.2.a of this preamble
as well as the memorandum, ``Emissions Data Used in Residual Risk
Modeling: Phosphoric Acid Manufacturing and Phosphate Fertilizer
Production,'' which is available in the docket for this rulemaking,
discusses our rationale for choosing this factor.
---------------------------------------------------------------------------

    \11\ See http://www.tceq.state.tx.us/compliance/field_ops/eer/index.html or docket to access the source of these data.
---------------------------------------------------------------------------

    As part of our acute risk assessment process, for cases where acute
HQ values from the screening step were less than or equal to 1 (even
under the conservative assumptions of the screening analysis), acute
impacts were deemed negligible and no further analysis was performed.
In cases where an acute HQ from the screening step was greater than 1,
additional site-specific data were considered to develop a more refined
estimate of the potential for acute impacts of concern. For these
source categories, the data refinements employed consisted of, in some
cases, the use of a refined emissions multiplier for individual
emission process groups to estimate the peak hourly emission rates in
lieu of using the default emission multiplier of 10(x) the annual
average 1-hour emission rate.
    For the two source categories, we conducted a review of the layout
of emission points at the facilities to ensure they were located within
the facility boundaries as well as to identify the maximum off-site
acute impact receptor for the facilities that did not screen out during
the initial base model run.
    Ideally, we would prefer to have continuous measurements over time
to see how the emissions vary by each hour over an entire year. Having
a frequency distribution of hourly emissions rates over a year would
allow us to perform a probabilistic analysis to estimate potential
threshold exceedances and their frequency of occurrence. Such an
evaluation could include a more complete statistical treatment of the
key parameters and elements adopted in this screening analysis.
Recognizing that this level of data is rarely available, we instead
rely on the multiplier approach.
    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,\12\ we generally examine a wider range of
available acute health metrics (e.g., RELs, AEGLs) than we do for our
chronic risk assessments. This is in response to the SAB's
acknowledgement

that there are generally more data gaps and inconsistencies in acute
reference values than there are in chronic reference values. In some
cases, when Reference Value Arrays \13\ for HAP have been developed, we
consider additional acute values (i.e., occupational and international
values) to provide a more complete risk characterization.
---------------------------------------------------------------------------

    \12\ 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.
    \13\ U.S. EPA. (2009) Chapter 2.9 Chemical Specific Reference
Values for Formaldehyde in Graphical Arrays of Chemical-Specific
Health Effect Reference Values for Inhalation Exposures (Final
Report). U.S. Environmental Protection Agency, Washington, DC, EPA/
600/R-09/061, and available online at http://cfpub.epa.gov/ncea/cfm/recordisplay.cfm?deid=211003.
---------------------------------------------------------------------------

4. How did we conduct the multipathway exposure and risk screening?
    The EPA conducted a screening analysis examining the potential for
significant human health risks due to exposures via routes other than
inhalation (i.e., ingestion). We first determined whether any sources
in the source categories emitted any hazardous air pollutants known to
be persistent and bioaccumulative in the environment (PB-HAP). The PB-
HAP compounds or compound classes are identified for the screening from
the EPA's Air Toxics Risk Assessment Library (available at http://www2.epa.gov/fera/risk-assessment-and-modeling-air-toxics-risk-assessment-reference-library).
    For the Phosphoric Acid Manufacturing source category, we
identified PB-HAP emissions of cadmium compounds, Pb compounds, Hg
compounds, POM and dioxin. For the Phosphate Fertilizer Production
Source Category, we identified PB-HAP emissions of cadmium compounds,
Pb compounds, and Hg compounds.
    Because one or more of these PB-HAP are emitted by at least one
facility in the two source categories, we proceeded to the next step of
the evaluation. In this step, we determined whether the facility-
specific emissions rates of the emitted PB-HAP were large enough to
create the potential for significant non-inhalation human health risks
under reasonable worst-case conditions. To facilitate this step, we
developed emissions rate screening levels for several PB-HAP using a
hypothetical upper-end screening exposure scenario developed for use in
conjunction with the EPA's Total Risk Integrated Methodology. Fate,
Transport and Ecological Exposure (TRIM.FaTE) model. The PB-HAP with
emissions rate screening levels are: Pb, cadmium, chlorinated
dibenzodioxins and furans, Hg compounds and POM. We conducted a
sensitivity analysis on the screening scenario to ensure that its key
design parameters would represent the upper end of the range of
possible values, such that it would represent a conservative but not
impossible scenario. The facility-specific emissions rates of each of
these PB-HAP were compared to the emission rate screening levels for
these PB-HAP to assess the potential for significant human health risks
via non-inhalation pathways. We call this application of the TRIM.FaTE
model the Tier I TRIM-screen or Tier I screen.
    For the purpose of developing emissions rates for our Tier I TRIM-
screen, we derived emission levels for these PB-HAP (other than Pb
compounds) at which the maximum excess lifetime cancer risk would be 1-
in-1 million (i.e., for polychlorinated dibenzodioxins and furans and
POM) or, for HAP that cause non-cancer health effects (i.e., cadmium
compounds and Hg compounds), the maximum HQ would be 1. If the
emissions rate of any PB-HAP included in the Tier I screen exceeds the
Tier I screening emissions rate for any facility, we conduct a second
screen, which we call the Tier II TRIM-screen or Tier II screen. In the
Tier II screen, the location of each facility that exceeded the Tier I
emission rate is used to refine the assumptions associated with the
environmental scenario while maintaining the exposure scenario
assumptions. We then adjusted the risk-based Tier I screening level for
each PB-HAP for each facility based on an understanding of how exposure
concentrations estimated for the screening scenario change with
meteorology and environmental assumptions. PB-HAP emissions that do not
exceed these new Tier II screening levels are considered to pose no
unacceptable risks. When facilities exceed the Tier II screening
levels, it does not mean that multipathway impacts are significant,
only that we cannot rule out that possibility based on the results of
the screen. These facilities may be further evaluated for multipathway
risks using the TRIM.FaTE model.
    In evaluating the potential multi-pathway risk from emissions of Pb
compounds, rather than developing a screening emissions rate for them,
we compared maximum estimated chronic inhalation exposures with the
level of the current NAAQS for Pb.\14\ Values below the level of the
primary (health based) Pb NAAQS were considered to have a low potential
for multi-pathway risk.
---------------------------------------------------------------------------

    \14\ In doing so, the EPA notes that the legal standard for a
primary NAAQS--that a standard is requisite to protect public health
and provide an adequate margin of safety (CAA section 109(b))--
differs from the CAA section 112(f) standard (requiring among other
things that the standard provide an ``ample margin of safety'').
However, the Pb NAAQS is a reasonable measure of determining risk
acceptability (i.e., the first step of the Benzene NESHAP analysis)
since it is designed to protect the most susceptible group in the
human population--children, including children living near major
lead emitting sources (73 FR 67002/3; 73 FR 67000/3; 73 FR 67005/1).
In addition, applying the level of the primary Pb NAAQS at the risk
acceptability step is conservative, since that primary Pb NAAQS
reflects an adequate margin of safety.
---------------------------------------------------------------------------

    For further information on the multipathway analysis approach, see
the memorandum, ``Draft Residual Risk Assessment for Phosphate
Fertilizer Production and Phosphoric Acid Manufacturing,'' which is
available in the docket for this action.
5. How did we conduct the environmental risk screening assessment?
a. Adverse Environmental Effect
    The EPA has developed a screening approach to examine the potential
for adverse environmental effects as required under section
112(f)(2)(A) of the CAA. Section 112(a)(7) of the CAA defines ``adverse
environmental effect'' as ``any significant and widespread adverse
effect, which may reasonably be anticipated, to wildlife, aquatic life,
or other natural resources, including adverse impacts on populations of
endangered or threatened species or significant degradation of
environmental quality over broad areas.''
b. Environmental HAP
    The EPA focuses on seven HAP, which we refer to as ``environmental
HAP,'' in its screening analysis: Five PB-HAP and two acid gases. The
five PB-HAP are cadmium, dioxins/furans, POM, Hg (both inorganic
mercury and methyl mercury) and Pb compounds. The two acid gases are
HCl and HF. The rationale for including these seven HAP in the
environmental risk screening analysis is presented below.
    HAP that persist and bioaccumulate are of particular environmental
concern because they accumulate in the soil, sediment and water. The
PB-HAP are taken up, through sediment, soil, water, and/or ingestion of
other organisms, by plants or animals (e.g., small fish) at the bottom
of the food chain. As larger and larger predators consume these
organisms, concentrations of the PB-HAP in the animal tissues increases
as does the potential for adverse effects. The five PB-HAP we evaluate
as part of

our screening analysis account for 99.8 percent of all PB-HAP emissions
nationally from stationary sources (on a mass basis from the 2005 NEI).
    In addition to accounting for almost all of the mass of PB-HAP
emitted, we note that the TRIM.FaTE model that we use to evaluate
multipathway risk allows us to estimate concentrations of for cadmium
compounds, dioxins/furans, POM and Hg in soil, sediment and water. For
Pb compounds, we currently do not have the ability to calculate these
concentrations using the TRIM.FaTE model. Therefore, to evaluate the
potential for adverse environmental effects from Pb compounds, we
compare the estimated HEM-modeled exposures from the source category
emissions of Pb with the level of the secondary NAAQS for Pb.\15\ We
consider values below the level of the secondary Pb NAAQS to be
unlikely to cause adverse environmental effects.
---------------------------------------------------------------------------

    \15\ The secondary Pb NAAQS is a reasonable measure of
determining whether there is an adverse environmental effect since
it was established considering ``effects on soils, water, crops,
vegetation, man-made materials, animals, wildlife, weather,
visibility and climate, damage to and deterioration of property, and
hazards to transportation, as well as effects on economic values and
on personal comfort and well-being.''
---------------------------------------------------------------------------

    Due to their well-documented potential to cause direct damage to
terrestrial plants, we include two acid gases, HCl and HF, in the
environmental screening analysis. According to the 2005 NEI, HCl and HF
account for about 99 percent (on a mass basis) of the total acid gas
HAP emitted by stationary sources in the U.S. In addition to the
potential to cause direct damage to plants, high concentrations of HF
in the air have been linked to fluorosis in livestock. Air
concentrations of these HAP are already calculated as part of the human
multipathway exposure and risk screening analysis using the HEM3-AERMOD
air dispersion model, and we are able to use the air dispersion
modeling results to estimate the potential for an adverse environmental
effect.
    The EPA acknowledges that other HAP beyond the seven HAP discussed
above may have the potential to cause adverse environmental effects.
Therefore, the EPA may include other relevant HAP in its environmental
risk screening in the future, as modeling science and resources allow.
The EPA invites comment on the extent to which other HAP emitted by the
source categories may cause adverse environmental effects. Such
information should include references to peer-reviewed ecological
effects benchmarks that are of sufficient quality for making regulatory
decisions, as well as information on the presence of organisms located
near facilities within the source category that such benchmarks
indicate could be adversely affected.
c. Ecological Assessment Endpoints and Benchmarks for PB-HAP
    An important consideration in the development of the EPA's
screening methodology is the selection of ecological assessment
endpoints and benchmarks. Ecological assessment endpoints are defined
by the ecological entity (e.g., aquatic communities including fish and
plankton) and its attributes (e.g., frequency of mortality). Ecological
assessment endpoints can be established for organisms, populations,
communities or assemblages, and ecosystems.
    For PB-HAP (other than Pb compounds), we evaluated the following
community-level ecological assessment endpoints to screen for organisms
directly exposed to HAP in soils, sediment and water:
     Local terrestrial communities (i.e., soil invertebrates,
plants) and populations of small birds and mammals that consume soil
invertebrates exposed to PB-HAP in the surface soil.
     Local benthic (i.e., bottom sediment dwelling insects,
amphipods, isopods and crayfish) communities exposed to PB-HAP in
sediment in nearby water bodies.
     Local aquatic (water-column) communities (including fish
and plankton) exposed to PB-HAP in nearby surface waters.
    For PB-HAP (other than Pb compounds), we also evaluated the
following population-level ecological assessment endpoint to screen for
indirect HAP exposures of top consumers via the bioaccumulation of HAP
in food chains:
     Piscivorous (i.e., fish-eating) wildlife consuming PB-HAP-
contaminated fish from nearby water bodies.
    For cadmium compounds, dioxins/furans, POM and Hg, we identified
the available ecological benchmarks for each assessment endpoint. An
ecological benchmark represents a concentration of HAP (e.g., 0.77 ug
of HAP per liter of water) that has been linked to a particular
environmental effect level (e.g., a no-observed-adverse-effect level
(NOAEL)) through scientific study. For PB-HAP we identified, where
possible, ecological benchmarks at the following effect levels:
     Probable effect levels (PEL): Level above which adverse
effects are expected to occur frequently.
     Lowest-observed-adverse-effect level (LOAEL): The lowest
exposure level tested at which there are biologically significant
increases in frequency or severity of adverse effects.
     NOAEL: The highest exposure level tested at which there
are no biologically significant increases in the frequency or severity
of adverse effect.
    We established a hierarchy of preferred benchmark sources to allow
selection of benchmarks for each environmental HAP at each ecological
assessment endpoint. In general, the EPA sources that are used at a
programmatic level (e.g., Office of Water, Superfund Program) were
used, if available. If not, the EPA benchmarks used in regional
programs (e.g., Superfund) were used. If benchmarks were not available
at a programmatic or regional level, we used benchmarks developed by
other federal agencies (e.g., National Oceanic and Atmospheric
Administration (NOAA) or state agencies.
    Benchmarks for all effect levels are not available for all PB-HAP
and assessment endpoints. In cases where multiple effect levels were
available for a particular PB-HAP and assessment endpoint, we use all
of the available effect levels to help us to determine whether
ecological risks exist and, if so, whether the risks could be
considered significant and widespread.
d. Ecological Assessment Endpoints and Benchmarks for Acid Gases
    The environmental screening analysis also evaluated potential
damage and reduced productivity of plants due to direct exposure to
acid gases in the air. For acid gases, we evaluated the following
ecological assessment endpoint:
     Local terrestrial plant communities with foliage exposed
to acidic gaseous HAP in the air.
    The selection of ecological benchmarks for the effects of acid
gases on plants followed the same approach as for PB-HAP (i.e., we
examine all of the available chronic benchmarks). For HCl, the EPA
identified chronic benchmark concentrations. We note that the benchmark
for chronic HCl exposure to plants is greater than the reference
concentration for chronic inhalation exposure for human health. This
means that where the EPA includes regulatory requirements to prevent an
exceedance of the reference concentration for human health, additional
analyses for adverse environmental effects of HCl would not be
necessary.

    For HF, the EPA identified chronic benchmark concentrations for
plants and evaluated chronic exposures to plants in the screening
analysis. High concentrations of HF in the air have also been linked to
fluorosis in livestock. However, the HF concentrations at which
fluorosis in livestock occur are higher than those at which plant
damage begins. Therefore, the benchmarks for plants are protective of
both plants and livestock.
e. Screening Methodology
    For the environmental risk screening analysis, the EPA first
determined whether any facilities in the Phosphoric Acid Manufacturing
source category and Phosphate Fertilizer Production source category
emitted any of the seven environmental HAP. For the Phosphoric Acid
Manufacturing source category, we identified emissions of cadmium,
dioxin, Hg, Pb, POM, HCl and HF. For the Phosphate Fertilizer
Production source category, we identified emissions of cadmium, Hg, Pb
and HF.
    Because one or more of the seven environmental HAP evaluated are
emitted by at least one facility in the source categories, we proceeded
to the second step of the evaluation.
f. PB-HAP Methodology
    For cadmium, Hg, POM and dioxins/furans, the environmental
screening analysis consists of two tiers, while Pb compounds are
analyzed differently as discussed earlier. In the first tier, we
determined whether the maximum facility-specific emission rates of each
of the emitted environmental HAP were large enough to create the
potential for adverse environmental effects under reasonable worst-case
environmental conditions. These are the same environmental conditions
used in the human multipathway exposure and risk screening analysis.
    To facilitate this step, TRIM.FaTE was run for each PB-HAP under
hypothetical environmental conditions designed to provide
conservatively high HAP concentrations. The model was set to maximize
runoff from terrestrial parcels into the modeled lake, which in turn,
maximized the chemical concentrations in the water, the sediment and
the fish. The resulting media concentrations were then used to back-
calculate a screening level emission rate that corresponded to the
relevant exposure benchmark concentration value for each assessment
endpoint. To assess emissions from a facility, the reported emission
rate for each PB-HAP was compared to the screening level emission rate
for that PB-HAP for each assessment endpoint. If emissions from a
facility do not exceed the Tier I screening level, the facility
``passes'' the screen, and, therefore, is not evaluated further under
the screening approach. If emissions from a facility exceed the Tier I
screening level, we evaluate the facility further in Tier II.
    In Tier II of the environmental screening analysis, the emission
rate screening levels are adjusted to account for local meteorology and
the actual location of lakes in the vicinity of facilities that did not
pass the Tier I screen. The modeling domain for each facility in the
Tier II analysis consists of eight octants. Each octant contains 5
modeled soil concentrations at various distances from the facility (5
soil concentrations x 8 octants = total of 40 soil concentrations per
facility) and 1 lake with modeled concentrations for water, sediment
and fish tissue. In the Tier II environmental risk screening analysis,
the 40 soil concentration points are averaged to obtain an average soil
concentration for each facility for each PB-HAP. For the water,
sediment and fish tissue concentrations, the highest value for each
facility for each pollutant is used. If emission concentrations from a
facility do not exceed the Tier II screening level, the facility passes
the screen, and is typically not evaluated further. If emissions from a
facility exceed the Tier II screening level, the facility does not pass
the screen and, therefore, may have the potential to cause adverse
environmental effects. Such facilities are evaluated further to
investigate factors such as the magnitude and characteristics of the
area of exceedance.
g. Acid Gas Methodology
    The environmental screening analysis evaluates the potential
phytotoxicity and reduced productivity of plants due to chronic
exposure to acid gases. The environmental risk screening methodology
for acid gases is a single-tier screen that compares the average off-
site ambient air concentration over the modeling domain to ecological
benchmarks for each of the acid gases. Because air concentrations are
compared directly to the ecological benchmarks, emission-based
screening levels are not calculated for acid gases as they are in the
ecological risk screening methodology for PB-HAPs.
    For purposes of ecological risk screening, the EPA identifies a
potential for adverse environmental effects to plant communities from
exposure to acid gases when the average concentration of the HAP around
a facility exceeds the LOAEL ecological benchmark. In such cases, we
further investigate factors such as the magnitude and characteristics
of the area of exceedance (e.g., land use of exceedance area, size of
exceedance area) to determine if there is an adverse environmental
effect.
    For further information on the environmental screening analysis
approach, see the ``Draft Residual Risk Assessment for Phosphate
Fertilizer Production and Phosphoric Acid Manufacturing'', which is
available in the docket for this action.
6. How did we conduct facility-wide assessments?
    To put the source category risks in context, we typically examine
the risks from the entire ``facility,'' where the facility includes all
HAP-emitting operations within a contiguous area and under common
control. In other words, we examine the HAP emissions not only from the
source category emission points of interest, but also emissions of HAP
from all other emission sources at the facility for which we have data.
We examined ``facility-wide'' risks using 2005 NEI data and modeling as
described in sections IV.B.5 and V.A.5 of this preamble.
    We analyzed risks due to the inhalation of HAP that are emitted
``facility-wide'' for the populations residing within 50 km of each
facility, consistent with the methods used for the source category
analysis described above. For these facility-wide risk analyses, the
modeled source category risks were compared to the facility-wide risks
to determine the portion of facility-wide risks that could be
attributed to each of the source categories addressed in this proposal.
For the facilities in these source categories, we estimated the maximum
inhalation cancer and chronic non-cancer risks associated with all HAP
emissions sources at the facility, including emissions sources that are
not part of the source categories but are located within a contiguous
area and are under common control. We specifically examined the
facility that was associated with the highest estimate of risk and
determined the percentage of that risk attributable to the source
category of interest. The results of these facility-wide assessments
are summarized in sections IV and V of this preamble. The ``Draft
Residual Risk Assessment for Phosphate Fertilizer Production and
Phosphoric Acid Manufacturing'' available through the docket for this
action provides the methodology and results of the facility-wide
analyses, including all facility-wide risks and the percentage of
source category contribution to facility-wide risks.

7. How did we consider uncertainties in risk assessment?
    In the Benzene NESHAP, we concluded that risk estimation
uncertainty should be considered in our decision-making under the ample
margin of safety framework. Uncertainty and the potential for bias are
inherent in all risk assessments, including those performed for this
proposal. Although uncertainty exists, we believe that our approach,
which used conservative tools and assumptions, ensures that our
decisions are health protective and environmentally protective. A brief
discussion of the uncertainties in the RTR emissions datasets,
dispersion modeling, inhalation exposure estimates and dose-response
relationships follows below. A more thorough discussion of these
uncertainties is included in the Draft Residual Risk Assessment for
Phosphate Fertilizer Production and Phosphoric Acid Manufacturing,
which is available in the docket for this action.
a. Uncertainties in the RTR Emissions Datasets
    Although the development of the RTR emissions datasets involved
quality assurance/quality control processes, the accuracy of emissions
values will vary depending on the source of the data, the degree to
which data are incomplete or missing, the degree to which assumptions
made to complete the datasets are accurate, errors in emission
estimates and other factors. The emission estimates considered in this
analysis generally are annual totals for certain years, and they do not
reflect short-term fluctuations during the course of a year or
variations from year to year. The estimates of peak hourly emission
rates for the acute effects screening assessment were based on an
emission adjustment factor applied to the average annual hourly
emission rates, which are intended to account for emission fluctuations
due to normal facility operations.
b. Uncertainties in Dispersion Modeling
    We recognize there is uncertainty in ambient concentration
estimates associated with any model, including the EPA's recommended
regulatory dispersion model, AERMOD. In using a model to estimated
ambient pollutant concentrations, the user chooses certain options to
apply. For RTR assessments, we select some model options that have the
potential to overestimate ambient air concentrations (e.g., not
including plume depletion or pollutant transformation). We select other
model options that have the potential to underestimate ambient impacts
(e.g., not including building downwash). Other options that we select
have the potential to either under- or overestimate ambient levels
(e.g., meteorology and receptor locations). On balance, considering the
directional nature of the uncertainties commonly present in ambient
concentrations estimated by dispersion models, the approach we apply in
the RTR assessments should yield unbiased estimates of ambient HAP
concentrations.
c. Uncertainties in Inhalation Exposure
    The EPA did not include the effects of human mobility on exposures
in the assessment. Specifically, short-term mobility and long-term
mobility between census blocks in the modeling domain were not
considered.\16\ The approach of not considering short or long-term
population mobility does not bias the estimate of the theoretical MIR
(by definition), nor does it affect the estimate of cancer incidence
because the total population number remains the same. It does, however,
affect the shape of the distribution of individual risks across the
affected population, shifting it toward higher estimated individual
risks at the upper end and reducing the number of people estimated to
be at lower risks, thereby increasing the estimated number of people at
specific high risk levels (e.g., 1-in-10 thousand or 1-in-1 million).
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    \16\ Short-term mobility is movement from one micro-environment
to another over the course of hours or days. Long-term mobility is
movement from one residence to another over the course of a
lifetime.
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    In addition, the assessment predicted the chronic exposures at the
centroid of each populated census block as surrogates for the exposure
concentrations for all people living in that block. Using the census
block centroid to predict chronic exposures tends to over-predict
exposures for people in the census block who live farther from the
facility and under-predict exposures for people in the census block who
live closer to the facility. Thus, using the census block centroid to
predict chronic exposures may lead to a potential understatement or
overstatement of the true maximum impact, but is an unbiased estimate
of average risk and incidence. We reduce this uncertainty by analyzing
large census blocks near facilities using aerial imagery and adjusting
the location of the block centroid to better represent the population
in the block, as well as adding additional receptor locations where the
block population is not well represented by a single location.
    The assessment evaluates the cancer inhalation risks associated
with pollutant exposures over a 70-year period, which is the assumed
lifetime of an individual. In reality, both the length of time that
modeled emission sources at facilities actually operate (i.e., more or
less than 70 years) and the domestic growth or decline of the modeled
industry (i.e., the increase or decrease in the number or size of
domestic facilities) will influence the future risks posed by a given
source or source category. Depending on the characteristics of the
industry, these factors will, in most cases, result in an overestimate
both in individual risk levels and in the total estimated number of
cancer cases. However, in the unlikely scenario where a facility
maintains, or even increases, its emissions levels over a period of
more than 70 years, residents live beyond 70 years at the same
location, and the residents spend most of their days at that location,
then the cancer inhalation risks could potentially be underestimated.
However, annual cancer incidence estimates from exposures to emissions
from these sources would not be affected by the length of time an
emissions source operates.
    The exposure estimates used in these analyses assume chronic
exposures to ambient (outdoor) levels of pollutants. Because most
people spend the majority of their time indoors, actual exposures may
not be as high, depending on the characteristics of the pollutants
modeled. For many of the HAP, indoor levels are roughly equivalent to
ambient levels, but for very reactive pollutants or larger particles,
indoor levels are typically lower. This factor has the potential to
result in an overestimate of 25 to 30 percent of exposures.\17\
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    \17\ U.S. EPA. National-Scale Air Toxics Assessment for 1996.
(EPA 453/R-01-003; January 2001; page 85.)
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    In addition to the uncertainties highlighted above, there are
several factors specific to the acute exposure assessment that the EPA
conducts as part of the risk review under section 112 of the CAA that
should be highlighted. The accuracy of an acute inhalation exposure
assessment depends on the simultaneous occurrence of independent
factors that may vary greatly, such as hourly emissions rates,
meteorology and the presence of humans at the location of the maximum
concentration. In the acute screening assessment that we conduct under
the RTR program, we assume that peak emissions from the source category
and worst-case meteorological conditions co-occur, thus resulting in
maximum ambient concentrations. These two

events are unlikely to occur at the same time, making these assumptions
conservative. We then include the additional assumption that a person
is located at this point during this same time period. For this source
category, these assumptions would tend to be worst-case actual
exposures as it is unlikely that a person would be located at the point
of maximum exposure during the time when peak emissions and worst-case
meteorological conditions occur simultaneously.
d. Uncertainties in Dose-Response Relationships
    There are uncertainties inherent in the development of the dose-
response values used in our risk assessments for cancer effects from
chronic exposures and non-cancer effects from both chronic and acute
exposures. Some uncertainties may be considered quantitatively, and
others generally are expressed in qualitative terms. We note as a
preface to this discussion a point on dose-response uncertainty that is
brought out in the EPA's 2005 Cancer Guidelines; \18\ namely, that
``the primary goal of EPA actions is protection of human health;
accordingly, as an Agency policy, risk assessment procedures, including
default options that are used in the absence of scientific data to the
contrary, should be health protective'' (EPA 2005 Cancer Guidelines,
pages 1-7). This is the approach followed here as summarized in the
next several paragraphs. A complete detailed discussion of
uncertainties and variability in dose-response relationships is given
in the Draft Residual Risk Assessment for Phosphate Fertilizer
Production and Phosphoric Acid Manufacturing, which is available in the
docket for this action.
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    \18\ Guidelines for Carcinogen Risk Assessment, EPA/630/P-03/
001F, March 2005, Risk Assessment Forum, U.S. Environmental
Protection Agency, Washington, DC.
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    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).\19\ In some circumstances, the true risk could be as
low as zero; however, in other circumstances the risk could be
greater.\20\ When developing an upper bound estimate of risk and to
provide risk values that do not underestimate risk, health-protective
default approaches are generally used. To err on the side of ensuring
adequate health protection, the EPA typically uses the upper bound
estimates rather than lower bound or central tendency estimates in our
risk assessments, an approach that may have limitations for other uses
(e.g., priority-setting or expected benefits analysis).
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    \19\ Upper bound, IRIS glossary (http://www.epa.gov/NCEA/iris/help_gloss.htm).
    \20\ An exception to this is the URE for benzene, which is
considered to cover a range of values, each end of which is
considered to be equally plausible, and which is based on maximum
likelihood estimates.
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    Chronic non-cancer RfC and reference dose (RfD) values represent
chronic exposure levels that are intended to be health-protective
levels. Specifically, these values provide an estimate (with
uncertainty spanning perhaps an order of magnitude) of a continuous
inhalation exposure (RfC) or a daily oral exposure (RfD) to the human
population (including sensitive subgroups) that is likely to be without
an appreciable risk of deleterious effects during a lifetime. To derive
values that are intended to be ``without appreciable risk,'' the
methodology relies upon an uncertainty factor (UF) approach (U.S. EPA,
1993, 1994) 21 22 which considers uncertainty, variability
and gaps in the available data. The UF are applied to derive reference
values that are intended to protect against appreciable risk of
deleterious effects. The UF are commonly default values,\23\ 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.
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    \21\ U.S. EPA. Reference Dose (RfD): Description and Use in
Health Risk Assessments. Dated March 1993.
    \22\ U.S. EPA. Methods for Derivation of Inhalation Reference
Concentrations and Application of Inhalation Dosimetry. EPA/600/8-
90/066F. Dated October 1994.
    \23\ According to the NRC report, Science and Judgment in Risk
Assessment (NRC, 1994) ``[Default] options are generic approaches,
based on general scientific knowledge and policy judgment, that are
applied to various elements of the risk assessment process when the
correct scientific model is unknown or uncertain.'' The 1983 NRC
report, Risk Assessment in the Federal Government: Managing the
Process, defined default option as ``the option chosen on the basis
of risk assessment policy that appears to be the best choice in the
absence of data to the contrary'' (NRC, 1983a, p. 63). Therefore,
default options are not rules that bind the agency; rather, the
agency may depart from them in evaluating the risks posed by a
specific substance when it believes this to be appropriate. In
keeping with the EPA's goal of protecting public health and the
environment, default assumptions are used to ensure that risk to
chemicals is not underestimated (although defaults are not intended
to overtly overestimate risk). See EPA, 2004, An Examination of EPA
Risk Assessment Principles and Practices, EPA/100/B-04/001 available
at: http://www.epa.gov/osa/pdfs/ratf-final.pdf.
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    While collectively termed ``UF,'' these factors account for a
number of different quantitative considerations when using observed
animal (usually rodent) or human toxicity data in the development of
the RfC. The UF are intended to account for: (1) Variation in
susceptibility among the members of the human population (i.e., inter-
individual variability); (2) uncertainty in extrapolating from
experimental animal data to humans (i.e., interspecies differences);
(3) uncertainty in extrapolating from data obtained in a study with
less-than-lifetime exposure (i.e., extrapolating from sub-chronic to
chronic exposure); (4) uncertainty in extrapolating the observed data
to obtain an estimate of the exposure associated with no adverse