[Federal Register Volume 79, Number 6 (Thursday, January 9, 2014)]
[Proposed Rules]
[Pages 1675-1731]
[FR Doc No: 2013-30132]

Vol. 79

Thursday,

No. 6

January 9, 2014

Part II

Environmental Protection Agency

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40 CFR Part 63

National Emission Standards for Hazardous Air Pollutants: Generic
Maximum Achievable Control Technology Standards; and Manufacture of
Amino/Phenolic Resins; Proposed Rule

Federal Register / Vol. 79 , No. 6 / Thursday, January 9, 2014 /
Proposed Rules

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ENVIRONMENTAL PROTECTION AGENCY

40 CFR Part 63

[EPA-HQ-OAR-2012-0133; FRL-9903-68-OAR]
RIN 2060-AR49

National Emission Standards for Hazardous Air Pollutants: Generic
Maximum Achievable Control Technology Standards; and Manufacture of
Amino/Phenolic Resins

AGENCY: Environmental Protection Agency (EPA).

ACTION: Proposed rule.

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SUMMARY: The EPA is proposing amendments, with regard to regulations
applicable to three industrial source categories, to two national
emission standards for hazardous air pollutants (NESHAP): NESHAP for
Source Categories: Generic Maximum Achievable Control Technology
Standards; and NESHAP: Manufacture of Amino/Phenolic Resins. The three
source categories addressed in this action are Acrylic and Modacrylic
Fibers Production, Polycarbonate Production and Amino/Phenolic Resins
Production. For all three of these source categories, the EPA is
proposing decisions concerning the residual risk and technology
reviews. The EPA is also proposing amendments to correct and clarify
regulatory provisions related to emissions during periods of startup,
shutdown and malfunction; add provisions for affirmative defense; add
requirements for electronic reporting of performance test results;
clarify provisions pertaining to open-ended valves and lines; add
monitoring requirements for pressure relief devices; and add standards
for previously unregulated hazardous air pollutant (HAP) emissions
sources for certain emission points. We estimate that these proposed
amendments will reduce HAP emissions from these three source categories
by a combined 22 tons per year.

DATES: Comments. Comments must be received on or before March 10, 2014.
A copy of comments on the information collection provisions should be
submitted to the Office of Management and Budget (OMB) on or before
February 10, 2014.
    Public Hearing. If anyone contacts the EPA requesting a public
hearing by January 24, 2014, we will hold a public hearing on February
10, 2014. If a hearing is requested, the last day to pre-register in
advance to speak at the hearing will be February 3, 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 no one contacts the EPA requesting a
public hearing to be held concerning this proposed rule by January 24,
2014, a public hearing will not take place. For further information on
the hearing, see section I.E of this preamble.

ADDRESSES: Comments. Submit your comments, identified by Docket ID No.
EPA-HQ-OAR-2012-0133, by one of the following methods:
     http://www.regulations.gov: Follow the online instructions
for submitting comments.
     Email<EMAIL_ADDRESS>Attention Docket ID No.
EPA-HQ-OAR-2012-0133.
     Fax:<PHONE_NUMBER>, Attention Docket ID No. EPA-HQ-OAR-
2012-0133.
     Mail: U.S. Postal Service, send comments to: EPA Docket
Center, EPA West (Air Docket), Attention Docket ID No. EPA-HQ-OAR-2012-
0133, U.S. Environmental Protection Agency, Mailcode: 2822T, 1200
Pennsylvania Ave. NW., Washington, DC 20460. Please include a total of
two copies. In addition, please mail a copy of your comments on the
information collection provisions to the Office of Information and
Regulatory Affairs, Office of Management and Budget (OMB), Attn: Desk
Officer for EPA, 725 17th Street NW., Washington, DC 20503.
     Hand Delivery: U.S. Environmental Protection Agency, EPA
West (Air Docket), Room 3334, 1301 Constitution Ave. NW., Washington,
DC 20004, Attention Docket ID No. EPA-HQ-OAR-2012-0133. Such deliveries
are only accepted during the Docket's normal hours of operation, and
special arrangements should be made for deliveries of boxed
information.
    Instructions. Direct your comments to Docket ID No. EPA-HQ-OAR-
2012-0133. 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 avoid the use of special characters or any form of encryption
and be free of any defects or viruses. For additional information about
the EPA's public docket, visit the EPA Docket Center homepage at:
http://www.epa.gov/dockets.
    Docket. The EPA has established a docket for this rulemaking under
Docket ID No. EPA-HQ-OAR-2012-0133. All documents in the docket are
listed in the 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 regulations.gov or in
hard copy at the EPA Docket Center, EPA West, Room 3334, 1301
Constitution Ave. NW., Washington, DC. The Public Reading Room is open
from 8:30 a.m. to 4:30 p.m., Monday through Friday, excluding legal
holidays. The telephone number for the Public Reading Room is (202)
566-1744, and the telephone number for the EPA Docket Center is (202)
566-1742.
    Public Hearing. If a public hearing is requested by January 24,
2014, it will be held on February 10, 2014, at the EPA's Research
Triangle Park Campus, 109 T.W. Alexander Drive, Research Triangle Park,
North Carolina 27711. The hearing will convene at 10:00 a.m. (Eastern
Standard Time) and end at 5:00 p.m. (Eastern Standard Time). A lunch
break will be held from 12:00 p.m. (Eastern Standard Time) until 1:00
p.m. (Eastern Standard Time). Please contact Ms. Virginia Hunt at (919)
541-0832 or at<EMAIL_ADDRESS>to request a

hearing, to determine if a hearing will be held and to register to
speak at the hearing, if one is held.

FOR FURTHER INFORMATION CONTACT: For questions about this proposed
action, contact Mr. Nick Parsons, Sector Policies and Programs Division
(E143-01), 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 Mr. Mark Morris,
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: (919)
541-5416; fax number:<PHONE_NUMBER>; email address:
<EMAIL_ADDRESS>For information about the applicability of these
three NESHAP to a particular entity, contact Ms. Tavara Culpepper,
Office of Enforcement and Compliance Assurance (OECA), telephone
number:<PHONE_NUMBER>; email address<EMAIL_ADDRESS>
SUPPLEMENTARY INFORMATION: Preamble Acronyms and Abbreviations. We use
multiple acronyms and terms in this preamble. While this list may not
be exhaustive, to ease the reading of this preamble and for reference
purposes, the EPA defines the following terms and acronyms here:

ACGIH American Conference of Governmental Industrial Hygienists
ADAF age-dependent adjustment factors
AEGL acute exposure guideline levels
AERMOD air dispersion model used by the HEM-3 model
AMF Acrylic and Modacrylic Fibers
APR Amino/Phenolic Resins
BACT best available control technology
CAA Clean Air Act
CalEPA California EPA
CBI Confidential Business Information
CDX Central Data Exchange
CEDRI Compliance and Emissions Data Reporting Interface
CFR Code of Federal Regulations
EJ environmental justice
EPA Environmental Protection Agency
ERPG Emergency Response Planning Guidelines
ERT Electronic Reporting Tool
FR Federal Register
GACT generally achievable control technology
HAP hazardous air pollutants
HCl hydrochloric acid
HEM-3 Human Exposure Model, Version 1.1.0
HI hazard index
HON National Emission Standards for Organic Hazardous Air Pollutants
From the Synthetic Organic Chemical Manufacturing Industry
HQ hazard quotient
ICR Information Collection Request
IRIS Integrated Risk Information System
km kilometer
LAER lowest achievable emission rate
LDAR leak detection and repair
MACT maximum achievable control technology
MACT Code Code within the NEI used to identify processes included in
a source category
mg/m\3\ milligrams per cubic meter
MIR maximum individual risk
NAAQS National Ambient Air Quality Standards
NAICS North American Industry Classification System
NAS National Academy of Sciences
NATA National Air Toxics Assessment
NEI National Emissions Inventory
NESHAP National Emissions Standards for Hazardous Air Pollutants
NIOSH National Institutes for Occupational Safety and Health
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
PAH polycyclic aromatic hydrocarbons
PB-HAP hazardous air pollutants known to be persistent and bio-
accumulative in the environment
PC Polycarbonate
POM polycyclic organic matter
ppm parts per million
PRD pressure relief device
RACT reasonably available control technology
RBLC RACT/BACT/LAER Clearinghouse
REL reference exposure level
RFA Regulatory Flexibility Act
RfC reference concentration
RfD reference dose
RTO regenerative thermal oxidizer
RTR residual risk and technology review
SAB Science Advisory Board
SBA Small Business Administration
SOCMI Synthetic Organic Chemical Manufacturing Industry
SOP standard operating procedures
SSM startup, shutdown and malfunction
TEQ toxic equivalency quotient
TLV threshold limit value
TOSHI target organ-specific hazard index
tpy tons per year
TRIM.FaTE Total Risk Integrated Methodology.Fate, Transport, and
Ecological Exposure model
TTN Technology Transfer Network
UF uncertainty factor
[mu]g/m\3\ microgram per cubic meter
UMRA Unfunded Mandates Reform Act
URE unit risk estimate
VCS voluntary consensus standards
VOC volatile organic compounds

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

I. General Information
    A. Executive Summary
    B. Does this action apply to me?
    C. Where can I get a copy of this document and other related
information?
    D. What should I consider as I prepare my comments for the EPA?
    E. Public Hearing
II. Background
    A. What is the statutory authority for this action?
    B. What are the source categories and how did the MACT standards
regulate their HAP emissions?
    C. What data collection activities were conducted to support
this action?
    D. What other relevant background information and data are
available?
III. Analytical Procedures
    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 review?
IV. Analytical Results and Proposed Decisions for the AMF Source
Category
    A. What actions are we taking pursuant to CAA sections 112(d)(2)
and 112(d)(3)?
    B. What are the results of the risk assessment and analyses?
    C. What are our proposed decisions regarding risk acceptability,
ample margin of safety and adverse environmental effects?
    D. What are the results and proposed decisions based on our
technology review?
V. Analytical Results and Proposed Decisions for the APR Source
Category
    A. What actions are we taking pursuant to CAA sections 112(d)(2)
and 112(d)(3)?
    B. What are the results of the risk assessment and analyses?
    C. What are our proposed decisions regarding risk acceptability,
ample margin of safety and adverse environmental effects?
    D. What are the results and proposed decisions based on our
technology review?
VI. Analytical Results and Proposed Decisions for the PC Source
Category
    A. What are the results of the risk assessment and analyses?
    B. What are our proposed decisions regarding risk acceptability,
ample margin of safety and adverse environmental effects?
    C. What are the results and proposed decisions based on our
technology review?
VII. What other actions are we proposing?
    A. Startup, Shutdown and Malfunction
    B. Electronic Reporting
    C. Open-Ended Valves and Lines
    D. Flare Performance
VIII. What compliance dates are we proposing?
IX. 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?
X. Request for Comments

XI. Submitting Data Corrections
XII. 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
A red-line version of the regulatory language that incorporates the
proposed changes in this action is available in the docket for this
action (Docket ID No. EPA-HQ-OAR-2012-0133).

I. General Information

A. Executive Summary

1. Purpose of the Regulatory Action
    Section 112(d)(1) of the CAA requires the EPA to establish NESHAP
for source categories and subcategories of both major sources and area
sources of HAP that are listed for regulation under CAA section 112(c).
For major sources of HAP, under CAA sections 112(d)(2) and (3), the EPA
is required to set standards that reflect the emissions performance
achieved by the maximum achievable control technology (MACT) and by
other measures used at sources in the subject source category. For area
sources, under CAA section 112(d)(5) the EPA is allowed to instead
adopt standards reflecting generally achievable control technology
(GACT). Section 112(d)(6) of the CAA requires the EPA to review these
NESHAP regulations for each covered source category and to revise them
as necessary (taking into account developments in practices, processes
and control technologies) no less frequently than every 8 years.
Section 112(f)(2) of the CAA requires the EPA to assess, within 8 years
of promulgation of the original NESHAP for major sources and area
sources subject to MACT, the remaining risks due to emissions of HAP
from these source categories and determine whether the emissions
standards provide an ample margin of safety to protect public health.
Section 112(f)(5) provides that the EPA is not required to conduct this
latter review for area sources subject to GACT. We refer to these
reviews collectively as residual risk and technology reviews (RTRs).
    This action presents the results of, and proposed decisions based
on, the EPA's reviews of the following three source categories: Acrylic
and Modacrylic Fibers Production (AMF), Amino/Phenolic Resins
Production (APR) and Polycarbonate Production (PC). As detailed below,
the EPA is proposing amendments, based on the relevant RTR, to
regulations applicable to each of these three source categories. In
addition, we are also proposing amendments to the relevant regulations
to address the following: Emissions during periods of startup, shutdown
and malfunction; standards for previously unregulated HAP emissions
sources; revisions to require monitoring of pressure relief devices in
organic HAP service that release to the atmosphere; clarification of
provisions pertaining to open-ended valves and lines; and revisions to
require electronic reporting of performance test results.
2. Summary of the Major Provisions of the Regulatory Action in Question
    With regard to the AMF source category, the EPA has determined that
no amendments are needed for this source category based on the risk
review under CAA section 112(f). However, based on the technology
review under CAA section 112(d)(6), the EPA is proposing to eliminate
the less stringent of two currently available options for complying
with leak detection and repair program requirements--while retaining
the more stringent compliance requirement. In addition, under CAA
sections 112(d)(2) and (3), the EPA is proposing requirements to
address certain emission points that were not previously regulated.
    With regard to the APR source category, the EPA has determined that
no amendments are needed for this source category based on the risk and
technology reviews under CAA sections 112(d)(6) and 112(f). However,
under CAA sections 112(d)(2) and (3), the EPA is proposing requirements
to address certain emission points that were not previously regulated.
    With regard to the PC source category, the EPA has determined that
no amendments are needed for this source category based on the risk
review under CAA section 112(f). However, based on the technology
review under CAA section 112(d)(6), the EPA is proposing to eliminate
the less stringent of two currently available options for complying
with leak detection and repair program requirements--while retaining
the more stringent compliance requirement.
    The EPA is also proposing revisions to all three source categories
in four areas. First, the EPA is proposing to revise the standards so
that they apply at all times, including during periods of startup,
shutdown and malfunction (SSM). Second, the EPA is proposing to require
electronic reporting of performance test results. Third, the EPA is
clarifying the provisions regarding open-ended lines by adding a
definition for what constitutes a ``sealed'' open-ended line. Finally,
the EPA is proposing to require monitoring of pressure relief devices
(PRDs) in organic HAP service that release to the atmosphere, and that
a pressure release from such a PRD is a violation.
3. Costs and Emissions Reductions
    Table 1 below summarizes the estimated costs and potential
emissions reductions for this action. See section IX of this preamble
for further discussion of the costs and impacts.

      Table 1--Summary of the Costs and Emissions Reductions for the Proposed Acrylic and Modacrylic Fibers
           Production, Amino/Phenolic Resins Production and Polycarbonate Production NESHAP Amendments
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                                                      Number                                         Emissions
                 Source category                     affected      Capital costs    Annualized      reductions
                                                      plants            ($)        costs  ($/yr)       (tpy)
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Acrylic and Modacrylic Fibers Production........               1         $38,000          $6,000             0.2
Amino/Phenolic Resins Production................              18       1,500,000         400,000            20.1
Polycarbonate Production........................               4          67,000           9,400             2.1
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B. Does this action apply to me?

    Table 2 of this preamble lists the NESHAP and associated regulated
industrial source categories that are the subject of this proposal.
Table 2 is not intended to be exhaustive, but rather to provide a guide
for readers regarding entities that this proposed action is likely to
affect. The proposed standards, once finalized, will be directly
applicable to the affected sources. Federal, state, local and tribal
government entities would not be affected by this proposed action. As
defined in the ``Initial List of Categories of Sources Under Section
112(c)(1) of the Clean Air Act Amendments of 1990'' (see 57 FR 31576,
July 16, 1992), the ``Acrylic and Modacrylic Fibers Production'' source
category includes any facility engaged in manufacturing fibers in which
the fiber-forming substance is any long-chain, synthetic polymer
composed of at least 85 percent, by weight, acrylonitrile units. As
defined in the ``Initial List of Categories of Sources Under Section
112(c)(1) of the Clean Air Act Amendments of 1990'' (see 57 FR 31576,
July 16, 1992) and subsequently amended (see 65 FR 3276, January 20,
2000), the ``Amino/Phenolic Resins Production'' source category
includes any facility engaged in manufacturing amino resins or phenolic
resins. 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 ``Polycarbonate Production'' source category
includes any facility which manufactures a special class of polyester
formed from the dihydroxy compound and any carbonate diester or by
ester interchange.

    Table 2--NESHAP and Industrial Source Categories Affected by This
                             Proposed Action
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               NESHAP and source category                 NAICS Code \a\
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Generic Maximum Achievable         Acrylic and                    325220
 Control Technology Standards.      Modacrylic Fibers           (325222)
                                    Production.
                                   Polycarbonate                  325211
                                    Production.                 (325211)
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Amino/Phenolic Resins Production                                  325211
                                                                (325211)
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\a\ North American Industry Classification System 2012 (2007 in
  parenthesis).

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

    In addition to being available in the docket, an electronic copy of
this proposal 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 on the TTN's policy and guidance page for newly proposed or
promulgated rules at: http://www.epa.gov/ttn/oarpg/t3pfpr.html. The TTN
provides information and technology exchange in various areas of air
pollution control. Following publication in the Federal Register, the
EPA will post the Federal Register version of the proposal and key
technical documents on the project Web sites: http://www.epa.gov/ttn/atw/gmact/gmactpg.html and http://www.epa.gov/ttn/atw/amino/aminopg.html. 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.

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

    Submitting CBI. Do not submit information containing CBI to the EPA
through http://www.regulations.gov or email. Clearly mark the part or
all of the information that you claim to be CBI. For CBI information on
a disk or CD-ROM that you mail to the EPA, mark the outside of the disk
or CD-ROM as CBI and then identify electronically within the disk or
CD-ROM the specific information that is claimed as CBI. In addition to
one complete version of the 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: Nick
Parsons, c/o OAQPS Document Control Officer (C404-02), Office of Air
Quality Planning and Standards, U.S. Environmental Protection Agency,
Research Triangle Park, North Carolina 27711, Attn: Docket ID No. EPA-
HQ-OAR-2012-0133.

E. Public Hearing

    If a hearing is held, it 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, if held, will be at a U.S.
governmental facility, individuals planning to attend the hearing
should be prepared to show valid picture identification to the security
staff in order to gain access to the meeting room. 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. If a
hearing is held on February 10, 2014, written comments on the proposed
rule must be postmarked by March 10, 2014. Commenters should notify Ms.
Virginia Hunt if they will need specific equipment, or if there are
other special needs related to providing comments at the hearing. The
EPA will provide equipment for commenters to show overhead slides or
make computerized slide presentations if we receive special requests in
advance. Oral testimony will be limited to 5 minutes for each

commenter. The EPA encourages commenters to provide the EPA with a copy
of their oral testimony electronically (via email or CD) or in hard
copy form. Verbatim transcripts of the hearings 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. Information regarding the hearing
(including information as to whether or not one will be held) will be
available at: http://www.epa.gov/ttn/oarpg/t3main.html. Again, all
requests for a public hearing to be held must be received by January
24, 2014.

II. Background

A. What is the statutory authority for this action?

    Section 112 of the CAA establishes a two-stage regulatory process
to address emissions of HAP from stationary sources. In the first
stage, after the EPA has identified categories of sources emitting one
or more of the HAP listed in CAA section 112(b), CAA section 112(d)
requires us to promulgate technology-based NESHAP for those sources.
``Major sources'' are those that emit or have the potential to emit 10
tons per year (tpy) or more of a single HAP or 25 tpy or more of any
combination of HAPs. For major sources, the technology-based NESHAP
must reflect the maximum degree of emissions reductions of HAPs
achievable (after considering cost, energy requirements and non-air
quality health and environmental impacts) and are commonly referred to
as MACT standards.
    MACT standards must require 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
emission 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 that either: (1) a pollutant cannot be emitted through a
conveyance designed and constructed to emit or capture the pollutants
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
emissions reductions, any non-air quality health and environmental
impacts and energy requirements.
    The EPA is then required to review these technology-based standards
and revise them ``as necessary (taking into account developments in
practices, processes, and control technologies)'' no less frequently
than every eight years. CAA section 112(d)(6). In conducting this
review, the EPA is not required to recalculate the MACT floor. Natural
Resources Defense Council (NRDC) v. EPA, 529 F.3d 1077, 1084 (D.C. Cir.
2008). Association of Battery Recyclers, Inc. v. EPA, 716 F.3d 667
(D.C. Cir. 2013).
    The second stage in standard-setting focuses on reducing any
remaining (i.e., ``residual'') risk according to CAA section 112(f).
This provision requires, first, 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. Congress did
not act in response, thereby triggering the EPA's obligation under CAA
section 112(f)(2) to analyze and address residual risk.
    Section 112(f)(2) of the CAA requires the EPA to determine for
source categories subject to MACT standards whether the emission
standards provide an ample margin of safety to protect public health.
Section 112(f)(2)(B) of the CAA expressly preserves the EPA's use of
the two-step process for developing standards to address any residual
risk and the agency's interpretation of ``ample margin of safety''
developed in the National Emissions Standards for Hazardous Air
Pollutants: Benzene Emissions from Maleic Anhydride Plants,
Ethylbenzene/Styrene Plants, Benzene Storage Vessels, Benzene Equipment
Leaks, and Coke By-Product Recovery Plants (Benzene NESHAP) (54 FR
38044, September 14, 1989). The EPA notified Congress in the Risk
Report that the agency intended to use the Benzene NESHAP approach in
making CAA section 112(f) residual risk determinations (EPA-453/R-99-
001, p. ES-11). The EPA subsequently adopted this approach in its
residual risk determinations and in a challenge to the risk review for
the Synthetic Organic Chemical Manufacturing source category, the
United States Court of Appeals for the District of Columbia Circuit
upheld as reasonable the EPA's interpretation that subsection 112(f)(2)
incorporates the approach established in the Benzene NESHAP. See NRDC
v. EPA, 529 F.3d 1077, 1083 (D.C. Cir. 2008)(``[S]ubsection
112(f)(2)(B) expressly incorporates the EPA's interpretation of the
Clean Air Act from the Benzene standard, complete with a citation to
the Federal Register.''); see also A Legislative History of the Clean
Air Act Amendments of 1990, vol. 1, p. 877 (Senate debate on Conference
Report).
    The first step in the process of evaluating residual risk is the
determination of acceptable risk. If risks are unacceptable, the EPA
cannot consider cost in identifying the emissions standards necessary
to bring risks to an acceptable level. The second step is the
determination of whether standards must be further revised in order to
provide an ample margin of safety to protect public health. The ample
margin of safety is the level at which the standards must be set,
unless an even more stringent standard is necessary to prevent, taking
into consideration costs, energy, safety and other relevant factors, an
adverse environmental effect.

1. Step 1-Determination of Acceptability
    The agency in the Benzene NESHAP concluded that ``the acceptability
of risk under section 112 is best judged on the basis of a broad set of
health risk measures and information'' and that the ``judgment on
acceptability cannot be reduced to any single factor.'' Id. 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
(DC 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 38045. We discussed the maximum individual lifetime
cancer risk (or maximum individual risk (MIR)) as being ``the estimated
risk that a person living near a plant would have if he or she were
exposed to the maximum pollutant concentrations for 70 years.'' Id. We
explained that this measure of risk ``is an estimate of the upper bound
of risk based on conservative assumptions, such as continuous exposure
for 24 hours per day for 70 years.'' Id. We acknowledged that maximum
individual lifetime cancer risk ``does not necessarily reflect the true
risk, but displays a conservative risk level which is an upper-bound
that is unlikely to be exceeded.'' Id.
    Understanding that there are both benefits and limitations to using
the MIR as a metric for determining acceptability, we acknowledged in
the Benzene NESHAP that ``consideration of maximum individual risk * *
* must take into account the strengths and weaknesses of this measure
of risk.'' Id. Consequently, the presumptive risk level of 100-in-1
million (1-in-10 thousand) provides a benchmark for judging the
acceptability of maximum individual lifetime cancer risk, but does not
constitute a rigid line for making that determination. Further, in the
Benzene NESHAP, we noted that:

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

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

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

Id. at 38045. In some cases, these health measures and factors taken
together may provide a more realistic description of the magnitude of
risk in the exposed population than that provided by maximum individual
lifetime cancer risk alone.
    As noted earlier, in NRDC v. EPA, the court held that section
112(f)(2) ``incorporates the EPA's interpretation of the Clean Air Act
from the Benzene Standard.'' The court further held that Congress'
incorporation of the Benzene approach applies equally to carcinogens
and non-carcinogens. 529 F.3d at 1081-82. Accordingly, we also consider
non-cancer risk metrics in our determination of risk acceptability and
ample margin of safety.
2. Step 2--Determination of Ample Margin of Safety
    CAA section 112(f)(2) requires the EPA to determine, for source
categories subject to MACT standards, whether those standards provide
an ample margin of safety to protect public health. As explained in the
Benzene NESHAP, ``the second step of the inquiry, determining an `ample
margin of safety,' again includes consideration of all of the health
factors, and whether to reduce the risks even further . . . . Beyond
that information, additional factors relating to the appropriate level
of control will also be considered, including costs and economic
impacts of controls, technological feasibility, uncertainties and any
other relevant factors. Considering all of these factors, the agency
will establish the standard at a level that provides an ample margin of
safety to protect the public health, as required by section 112.'' 54
FR 38046.
    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 (DC Cir.
2008) (``If EPA determines that the existing technology-based standards
provide an 'ample margin of safety,' then the Agency is free to readopt
those standards during the residual risk rulemaking.'') The EPA must
also adopt more stringent standards, if necessary, to prevent an
adverse environmental effect,\1\ but must consider cost, energy, safety
and other relevant factors in doing so.
---------------------------------------------------------------------------

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

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

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

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

information considered in the first step, including the incremental
risk reduction associated with standards more stringent than the MACT
standard or a more stringent standard that 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.

B. What are the source categories and how did the MACT standards
regulate their HAP emissions?

1. Acrylic and Modacrylic Fibers Production Source Category
    The NESHAP for Acrylic and Modacrylic Fibers Production (``AMF MACT
standards''), with the exception of wastewater processes, were
promulgated on June 29, 1999 (64 FR 34854), and codified at 40 CFR part
63, subpart YY. The provisions for wastewater were promulgated
separately on November 22, 1999 (64 FR 63695), and also codified at 40
CFR part 63, subpart YY. The AMF MACT standards were established in a
consolidated rulemaking for certain small source categories consisting
of five or fewer major sources. The standards for these source
categories were developed under the EPA's Generic MACT program.
    Acrylic and modacrylic fibers are manufactured fibers in which the
fiber-forming substance is a long-chain synthetic polymer containing
acrylonitrile units. The fiber-forming substance in acrylic fibers is
composed of at least 85 percent acrylonitrile units by weight, whereas
modacrylic fibers are less than 85 but at least 35 percent
acrylonitrile units by weight. These acrylic and modacrylic fibers have
been used in textiles (including apparel, carpet, awnings, tents,
sandbags and auto upholstery) and in industrial applications like
concrete reinforcements and industrial filters. These fibers are also
used as carbon fiber precursors. Carbon fibers developed from acrylic
fibers have high tensile strength and are used in aerospace
applications, such as aircraft airframes and engine structures, as well
as other applications where light weight and high strength are needed,
including racing car bodies, golf club shafts, bicycle frames, fishing
rods, automobile springs, sailboat masts and many other items.
    The production of AMF involves a polymerization reaction process
using either a solution or suspension process in either a batch or
continuous mode. The resulting polymer (called ``spin dope'') is spun
into fibers using either wet or dry spinning techniques. The spun
fibers are then treated to remove excess solvent and to improve fiber
characteristics through processes such as washing, stretching, crimping
and drying.
    Sources of HAP emissions from the production of AMF include: (1)
Storage vessels used to store acrylonitrile monomer and co-monomers;
(2) process vents on reactors, vessels and storage vessels used for
acrylic polymerization, monomer recovery, fiber spinning and solvent
recovery operations; (3) fugitive emissions from AMF spinning lines;
(4) wastewater treatment systems; and (5) equipment leaks. In the
production of AMF, HAP are used primarily as raw materials or reaction
inhibitors in the polymerization reaction process. The AMF MACT
standards include emission limits for existing and new fiber spinning
lines using spin dope from a suspension polymerization process, new
sources using a solution polymerization process and for process vents
at all facilities. The AMF MACT standards include a combination of
equipment standards and work practices for equipment leaks and
wastewater, and a combination of equipment standards and emission
limits for storage vessels.
    To meet the requirements of the AMF MACT standards, the emissions
from storage vessels are typically controlled either by floating roofs
or fixed roofs that route emissions through a closed vent system to a
combustion or recovery device. Emissions from wastewater are generally
controlled by equipment modifications (e.g., covers on surface
impoundments, containers and drain systems) and pretreatment to remove
HAP and biodegradation or pretreatment and discharge to a publicly
owned treatment works for biodegradation. Emissions from equipment
leaks are typically reduced by leak detection and repair (LDAR) work
practice programs. Controls for process vents include combustion or
recovery devices, and controls for fiber spinning lines include
enclosure of the spinning and washing areas with venting to a
combustion or recovery device.
    We identified one major source currently operating that is subject
to the AMF MACT standards. Acrylonitrile accounts for the majority of
the HAP emissions from the AMF processes at this facility
(approximately 32 tpy and over 99 percent of the total HAP emissions by
mass). The only other HAP reported by this facility is hydroquinone
(approximately 3 lbs/yr). As we have stated previously, other organic
HAP, where present, would only be associated with those pollutant
streams containing acrylonitrile, and where sources control
acrylonitrile emissions, comparable levels of control will be achieved
for other organic HAP emitted from AMF facilities. See NESHAP: Generic
Maximum Achievable Control Technology (Generic MACT); Final Rule,
Process Wastewater Provisions; Proposed Rule, 64 FR 34854, 34858 (June
29, 1999). The same is true here--hydroquinone is emitted only from
equipment leaks, and equipment leaks are already subject to control
through the LDAR program in the rule.
    We estimate that the actual emissions levels for all emission
sources are representative of the MACT-allowable levels (i.e., the
maximum emission levels allowed if in compliance with the MACT
standards), as we are not aware of any situations in which the facility
is conducting additional work practices or operating a control device
such that it achieves a greater emission reduction than required. For
more detail about this estimate of the ratio of actual-to-MACT-
allowable emissions and the estimation of the MACT-allowable emission
levels (and associated risks and impacts), see the memorandum, MACT
Allowable Emissions and Risks for the Acrylic and Modacrylic Fibers,
Amino/Phenolic Resins, and Polycarbonate Production Source Categories,
available in the docket for this action (EPA-HQ-OAR-2012-0133).
2. Amino/Phenolic Resins Production
    The NESHAP for the Manufacture of Amino/Phenolic Resins (``APR MACT
standards''; also referred to as Group III Polymers and Resins) were
promulgated on January 20, 2000 (65 FR 3275), and codified at 40 CFR
part 63, subpart OOO. The APR MACT standards apply to major sources and
regulate HAP emissions resulting from the manufacture of amino resins
or phenolic resins. These two products can broadly be classified as
formaldehyde-based thermosetting resins. An amino resin is a resin
produced through the reaction of formaldehyde, or a formaldehyde-
containing solution, with one or more compounds that contain an amino
group; these compounds include melamine, urea and urea derivatives. A
phenolic resin is a resin that is a condensation product of
formaldehyde and phenol, or a formaldehyde substitute and/or a phenol
substitute.

Substitutes for formaldehyde include acetaldehyde or furfuraldehyde.
Substitutes for phenol include other phenolic-starting compounds such
as cresols, xylenols, p-tert-butylphenol, p-phenylphenol and
nonylphenol. Formaldehyde, phenol, acetaldehyde and cresols are HAP,
but the other reactants are not. Amino/phenolic resins are used in the
manufacture of plywood, particle board, adhesives, wood furniture and
plastic parts.
    Generally, the production of APR entails four processes: (1) Raw
material (i.e., solvent and catalyst) storage and refining; (2) polymer
formation in a reactor; (3) material recovery; and (4) finishing (e.g.,
cooling, filtering, drying or pulverizing).
    Sources of HAP emissions from the production of APR include reactor
batch process vents, non-reactor batch process vents, continuous
process vents, equipment leaks, wastewater, storage vessels and heat
exchangers. In the production of APR, HAP are used primarily as
reactants or extraction solvents. The APR MACT standards include a
combination of equipment standards and emission limits for the various
emission sources.
    To meet the requirements of the APR MACT standards, the typical
control techniques used to reduce emissions include LDAR programs for
heat exchangers and other equipment. Boilers, combustion and recovery
devices may be used to control emissions from batch process vents.
    We identified 18 currently-operating facilities subject to the APR
MACT standards. Methanol, formaldehyde and phenol account for the
majority of the HAP emissions from the APR production processes at
these facilities (approximately 357 tpy and 96 percent of the total HAP
emissions by mass). A variety of other chemicals are used in the
production of APR, and these facilities also reported emissions of 23
other HAP. Emissions of three persistent bioaccumulative HAP (PB-HAP)
are reported in the data set for this source category, including lead
compounds, cadmium compounds, and polycyclic organic matter (POM)
(which includes polyaromatic hydrocarbons (PAH)).
    We estimate that the actual emissions levels for all sources are
representative of the MACT-allowable levels (i.e., the maximum emission
levels allowed if in compliance with the MACT standards), as we are not
aware of any situations in which facilities are conducting additional
work practices or operating a control device such that it achieves a
greater emission reduction than required, except batch process vents.
As it is possible that the capture systems and control devices used at
some facilities achieve greater emission reductions than what is
required by the NESHAP for batch process vents, the MACT-allowable
level for organic HAP emissions from reactor batch process vents could
be up to 3.4 times the actual emissions and the MACT-allowable level
for organic HAP emissions from non-reactor batch process vents could be
up to 1.6 times the actual emissions for some facilities in this source
category. For more detail about this estimate of the ratio of actual-
to-MACT-allowable emissions and the estimation of MACT-allowable
emission levels (and associated risks and impacts), see the memorandum,
MACT Allowable Emissions and Risks for the Acrylic and Modacrylic
Fibers, Amino/Phenolic Resins, and Polycarbonate Production Source
Categories, available in the docket for this action (EPA-HQ-OAR-2012-
0133).
3. Polycarbonate Production Source Category
    The NESHAP for Polycarbonate Production (``PC MACT standards''),
with the exception of wastewater processes, were promulgated on June
29, 1999 (64 FR 34854), and codified at 40 CFR part 63, subpart YY. The
provisions for wastewater were promulgated separately on November 22,
1999 (64 FR 63695), and also codified at 40 CFR part 63, subpart YY.
Along with the AMF and other source categories, the PC source category
standards were established in a consolidated rulemaking for certain
small source categories consisting of five or fewer major sources. The
standards for these source categories were developed under the EPA's
Generic MACT program.
    Polycarbonates are thermoplastic polymers that can be either
transparent or opaque, are heat resistant and are scratch and impact
resistant. These properties make PC useful in a variety of
applications, including as a dielectric in capacitors, car headlights,
water bottles, sports helmets, compact discs and DVDs, eyewear lenses,
medical devices, toys and other products.
    The production of PC involves a polymerization reaction process
using either a solution or suspension process in either a batch or
continuous mode. All production of PC in the United States is currently
based on the polymerization reaction of bisphenols with phosgene in the
presence of catalysts, solvents (mainly methylene chloride) and other
additives. After the reaction, the resulting polymer is purified and
sent to a recovery process to remove remaining methylene chloride. The
resin is dried and stored in silos.
    All phosgene used as a feedstock for the production of PC is
produced onsite to reduce potential hazards associated with
transporting and storing this material. The phosgene is fed directly
from dedicated phosgene production equipment to PC polymerization
process equipment. Consequently, phosgene production is integrated with
the production of PC; the production of PC cannot occur without the
other process operating. Since dedicated phosgene production units are
integral to the PC production process, the EPA considers such phosgene
production units to be part of the PC source category (63 FR 55178,
October 18, 1998).
    Sources of HAP emissions from the production of PC include storage
vessels used to store methylene chloride and other organic solvents;
process vents on polymerization, polymer solution purification and
solvent recovery equipment; wastewater treatment systems; and equipment
leaks. In the production of PC, HAP are used as monomers, co-monomers
and solvents in the polymerization reaction. The PC MACT standards
include emission limits for continuous process vents. The PC MACT
standards include a combination of equipment standards and work
practices for equipment leaks and wastewater and a combination of
equipment standards and emission limits for storage vessels.
    To meet the requirements of the PC MACT standards, the typical
control devices used to reduce emissions from storage vessels are fixed
roofs with emissions routed through a closed vent system to a
combustion or recovery device. Emissions from wastewater are generally
controlled by equipment modifications (e.g., covers on surface
impoundments, containers and drain systems) and treatment to remove the
HAP, including steam stripping followed by recovery or combustion of
the stripped HAP. Emissions from equipment leaks are typically reduced
by leak detection and repair work practice programs. Controls for
continuous and batch process vents include combustion or recovery
devices.
    We identified four currently-operating facilities subject to the PC
MACT standards. Methylene chloride, ethyl chloride and triethylamine
account for the majority of the HAP emissions from the PC production
processes at these facilities (approximately 330 tpy and over 99
percent of the total HAP emissions by mass). Phosgene and chlorobenzene
emissions were also reported from the PC production processes at these
facilities.

    We estimate that the actual emissions levels for all sources are
representative of the MACT-allowable levels (i.e., the maximum emission
levels allowed if in compliance with the MACT standards), as we are not
aware of any situations in which facilities are conducting additional
work practices or operating a control device such that it achieves a
greater emission reduction than required, except storage vessels. As it
is possible that the capture systems and control devices used at some
facilities achieve greater HAP emission reductions than what is
required by the NESHAP for some storage vessels, depending on the
vessel capacity and vapor pressure of the stored material, the MACT-
allowable level of HAP emissions could be up to 2.5 times the actual
emissions for storage vessels in this source category. For more detail
about this estimate of the ratio of actual to MACT-allowable emissions
and the estimation of the MACT-allowable emission levels (and
associated risks and impacts), see the memorandum, MACT Allowable
Emissions and Risks for the Acrylic and Modacrylic Fibers, Amino/
Phenolic Resins, and Polycarbonate Production Source Categories,
available in the docket for this action (EPA-HQ-OAR-2012-0133).

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

    To perform the risk assessments for these source categories, we
developed data sets for the APR and PC source categories based on
information in the 2005 National Emissions Inventory (NEI) (available
at http://www.epa.gov/ttnchie1/net/2005inventory.html). The NEI is a
database that contains information about sources that emit criteria air
pollutants, their precursors and HAP. The database includes estimates
of annual air pollutant emissions from point, nonpoint and mobile
sources in the 50 states, the District of Columbia, Puerto Rico and the
Virgin Islands. The EPA collects this information and releases an
updated version of the NEI database every 3 years. We reviewed the NEI
data and made adjustments where necessary to ensure the proper
facilities were included and to ensure the proper processes were
allocated to each source category. We also reviewed the emissions and
other data to identify data anomalies that could affect risk estimates,
such as whether a pollutant was expected to be emitted from facilities
in a source category or whether an emission point was located within a
facility's fenceline. The NEI data were also reviewed by industry trade
groups, including the American Chemistry Council and the Society of
Chemical Manufacturers and Affiliates, as well as several state air
agencies. Where the EPA received new information from the industry and
air agency review, including updated emissions data and process
information, facility closure information and information that some
facilities were not subject to the APR or PC MACT standards, we revised
the NEI data where we concluded the comments supported such adjustment.
We used this reviewed and revised data set to conduct the risk
assessment and other analyses for each source category. Due to the
conservative nature of our emissions estimates, as described in the
emissions data memo cited below, we believe that the data set provides
a conservative estimate for use in assessing the risk from these source
categories. Further details on the changes made to the 2005 NEI data
can be found in the memorandum, Emissions Data and Acute Risk Factor
Used in Residual Risk Modeling: Acrylic and Modacrylic Fibers, Amino/
Phenolic Resins, and Polycarbonate Production, available in the docket
for this action (EPA-HQ-OAR-2012-0133).
    To perform the risk assessment for the AMF source category, we
developed a data set based on information submitted to the EPA for this
purpose by the one operating facility in the source category. On
February 23, 2012, the EPA visited this facility, Cytec Carbon Fibers,
LLC, located in Piedmont, South Carolina. The purpose of this visit was
to better understand the acrylic fiber production processes, the
controls in place to reduce HAP emissions and the characteristics of
the emission points at this facility. As part of this visit, the EPA
requested that facility personnel examine the 2008 NEI HAP inventory
data that the EPA had for the facility. The EPA provided this data to
the facility prior to the site visit to give the facility the
opportunity to correct or update the data. After review of the data,
the facility submitted updated information, and the updated data formed
the basis for the data set used for modeling.

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

    To conduct the technology review, we reviewed information developed
since these rules were originally promulgated in 1999 and 2000. Since
those rules have been promulgated, the EPA has developed other air
toxics regulations for a number of other source categories that emit
organic HAP from the same type of emission sources that are present in
the three source categories included in this technology review. In
these other air toxic regulatory actions, we consistently evaluated any
new practices, processes and control techniques. For this technology
review, we took into account the regulatory requirements and/or
technical analyses associated with these other regulatory actions to
identify any practices, processes and control techniques considered in
these efforts that could possibly be applied to the source categories
addressed in this action.
    We also downloaded from the reasonably available control technology
(RACT)/best available control technology (BACT)/lowest achievable
emission rate (LAER) Clearinghouse for processes in the AMF, APR and PC
source categories with permits dating back to the promulgation dates of
each MACT regulation. Finally, we conducted an online search of all
relevant publications, journals, permits and other documents to
identify any new practices, processes or control technologies for HAP
emissions sources since the dates of promulgation of the standards.
    To evaluate unregulated emission points at facilities regulated by
the APR MACT standards, we relied on existing data submitted to the EPA
during development of the existing APR MACT standards. To evaluate
unregulated emission points for the AMF MACT standards, we relied
primarily on data submitted to the EPA by the one operating facility in
the source category, along with information gathered during the EPA's
visit to the facility.

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 risk assessments that provided estimates of the
MIR posed by the HAP emissions from each source in each 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 assessments also provided estimates of the
distribution of cancer risks within the exposed populations, cancer
incidence and an evaluation of the potential for adverse environmental
effects for each source category. The risk assessment consisted of
eight primary steps, as discussed below. The docket for this rulemaking
contains the following documents which provide more

information on the risk assessment inputs and models: Draft Residual
Risk Assessment for the Acrylic and Modacrylic Fibers Production Source
Category, Draft Residual Risk Assessment for the Amino/Phenolic Resins
Production Source Category, and Draft Residual Risk Assessment for the
Polycarbonate Production Source Category. The methods used to assess
risks (as described in the eight 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
\2\; they are also consistent with the key recommendations contained in
that report.
---------------------------------------------------------------------------

    \2\ U.S. EPA SAB. Risk and Technology Review (RTR) Risk
Assessment Methodologies: For Review by the EPA's Science Advisory
Board with Case Studies--MACT I Petroleum Refining Sources and
Portland Cement Manufacturing, May 2010.
---------------------------------------------------------------------------

1. How did we estimate actual emissions and identify the emissions
release characteristics?
    As discussed in section II.C of this preamble, we created the
preliminary data sets for the APR and PC source categories using data
in the 2005 NEI, supplemented by data collected from industry, industry
trade associations and state air agencies (when available). For the AMF
source category, we used data collected from the one facility subject
to the AMF MACT standards.
2. How did we estimate MACT-allowable emissions?
    The available emissions data in the MACT 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.)
    As described above, the actual emissions data were compiled based
on the NEI and information gathered from facilities through industrial
trade associations and state air agencies for the APR and PC source
categories and through the one facility subject to the AMF MACT
standards. To estimate emissions at the MACT-allowable level, we
developed a ratio of MACT-allowable to actual emissions for each
emissions source type in each source category, based on the level of
control required by the MACT standards compared to the level of
reported actual emissions and available information on the level of
control achieved by the emissions controls in use. For example, if
there was information to suggest several facilities in a source
category were controlling storage tank emissions by 98 percent while
the MACT standards required only 92-percent control, we would estimate
that MACT-allowable emissions from these emission points could be as
much as four times higher (8-percent allowable emissions compared with
2 percent actually emitted), and the ratio of MACT-allowable to actual
would be 4:1 for this emission point type at the facilities in this
source category. After developing these ratios for each emission point
type in each source category, we next applied these ratios on a
facility-by-facility basis to the maximum chronic risk values from the
inhalation risk assessment to obtain facility-specific maximum risk
values based on MACT-allowable emissions. Further explanation of this
evaluation is provided in the technical document, MACT Allowable
Emissions and Risks for the Acrylic and Modacrylic Fibers, Amino/
Phenolic Resins, and Polycarbonate Production Source Categories,
available in the docket for this action (EPA-HQ-OAR-2012-0133).
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 categories 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 \3\; and (3) estimating
individual and population-level inhalation risks using the exposure
estimates and quantitative dose-response information.
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    \3\ 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.\4\ To perform the dispersion modeling and to
develop the preliminary risk estimates, HEM-3 draws on three data
libraries. The first is a library of meteorological data, which is used
for dispersion calculations. This library includes 1 year of hourly
surface and upper air observations for 189 meteorological stations,
selected to provide coverage of the United States and Puerto Rico. A
second library of United States Census Bureau census block \5\ internal
point locations and populations provides the basis of human exposure
calculations (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.
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    \4\ U.S. EPA. Revision to the Guideline on Air Quality Models:
Adoption of a Preferred General Purpose (Flat and Complex Terrain)
Dispersion Model and Other Revisions (70 FR 68218, November 9,
2005).
    \5\ A census block is generally the smallest geographic area for
which census statistics are tabulated.
---------------------------------------------------------------------------

    In developing the risk assessment for chronic exposures, we used
the estimated annual average ambient air concentration of each 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.
    With regard to formaldehyde (one of the primary HAP emitted by
facilities in the APR source category), the EPA determined in 2004 that
the Chemical Industry Institute of Toxicology (CIIT) cancer dose-
response value for formaldehyde (5.5 x 10-9 per mg/m\3\) was
based on better science than the IRIS cancer dose-response value (1.3 x
10-5 per mg/m\3\). Thus, we switched at that time from using
the IRIS value to the CIIT value in risk assessments supporting
regulatory actions. Based on subsequent published research, however,
the EPA changed its determination regarding the CIIT model and, in
2010, the EPA returned to using the 1991 IRIS value. The EPA has been
working on revising the formaldehyde IRIS assessment, and the National
Academy of Sciences (NAS) completed its review of the EPA's draft in
April of 2011.\6\ The EPA is reviewing the public comments and the NAS
independent scientific peer review. The EPA will follow the NAS Report
recommendations and will present results obtained by implementing the
biologically-based dose-response (BBDR) model for formaldehyde. The EPA
will compare these estimates with those currently presented in the
External Review draft of the assessment and will discuss their
strengths and weaknesses. As recommended by the NAS committee,
appropriate sensitivity and uncertainty analyses will be an integral
component of implementing the BBDR model. The draft IRIS assessment
will be revised in response to the NAS peer review, and public comments
and the final assessment will be posted on the IRIS database. In the
interim, we will present findings using the 1991 IRIS value as a
primary estimate, and may also consider other information as the
science evolves. As noted above and described in the risk assessment,
the IRIS URE for formaldehyde is 1.3 x 10-5 mg/m\3\,
whereas, the CIIT URE for formaldehyde is 5.5 x 10-9 mg/
m\3\.
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    \6\ http://www.nap.edu/catalog.php?record_id=13142.
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    We note here that several carcinogens have a mutagenic mode of
action.\7\ Of these compounds, POM is emitted by facilities in the APR
source category. For these compounds, the age-dependent adjustment
factors (ADAF) described in the EPA's Supplemental Guidance for
Assessing Susceptibility from Early-Life Exposure to Carcinogens \8\
were applied. This adjustment has the effect of increasing the
estimated lifetime risks for these pollutants by a factor of 1.6.\9\ In
addition, the EPA expresses carcinogenic potency for compounds in the
POM group in terms of benzo[a]pyrene equivalence, based on evidence
that carcinogenic POM have the same mutagenic mechanism of action as
does benzo[a]pyrene. For this reason, the EPA's Science Policy Council
\10\ recommends applying the Supplemental Guidance to all carcinogenic
polycyclic aromatic hydrocarbons for which risk estimates are based on
relative potency. Accordingly, we have applied the ADAF to
benzo[a]pyrene equivalent portion of all POM mixtures.
---------------------------------------------------------------------------

    \7\ U.S. EPA, 2006. Performing risk assessments that include
carcinogens described in the Supplemental Guidance as having a
mutagenic mode of action. Science Policy Council Cancer Guidelines
Implementation Workgroup Communication II: Memorandum from W.H.
Farland dated June 14, 2006. http://epa.gov/osa/spc/pdfs/CGIWGCommunication_II.pdf.
    \8\ U.S. EPA, 2005. Supplemental Guidance for Assessing Early-
Life Exposure to Carcinogens. EPA/630/R-03/003F. http://www.epa.gov/ttn/atw/childrens_supplement_final.pdf.
    \9\ Only one of these mutagenic compounds, benzo[a]pyrene, is
emitted by any of the sources covered by this proposal.
    \10\ U.S. EPA, 2005. Science Policy Council Cancer Guidelines
Implementation Workgroup Communication I: Memorandum from W.H.
Farland dated October 4, 2005, to Science Policy Council. http://www.epa.gov/osa/spc/pdfs/canguid1.pdf.
---------------------------------------------------------------------------

    The EPA estimated incremental individual lifetime cancer risks
associated with emissions from the facilities in the source categories
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 \11\)
emitted by the modeled sources. Cancer incidence and the distribution
of individual cancer risks for the population within 50 km of any
source were also estimated for the source categories as part of these
assessments by summing individual risks. A distance of 50 km is
consistent with both the analysis supporting the 1989 Benzene NESHAP
(54 FR 38044) and the limitations of Gaussian dispersion models,
including AERMOD.
---------------------------------------------------------------------------

    \11\ These classifications also coincide with the terms ``known
carcinogen, probable carcinogen, and possible carcinogen,''
respectively, which are the terms advocated in the EPA's previous
Guidelines for Carcinogen Risk Assessment, published in 1986 (51 FR
33992, September 24, 1986). Summing the risks of these individual
compounds to obtain the cumulative cancer risks is an approach that
was recommended by the EPA's SAB in their 2002 peer review of the
EPA's National Air Toxics Assessment (NATA) entitled, NATA--
Evaluating the National-scale Air Toxics Assessment 1996 Data--an
SAB Advisory, available at: http://yosemite.epa.gov/sab/
sabproduct.nsf/214C6E915BB04E14852570CA007A682C/$File/
ecadv02001.pdf.
---------------------------------------------------------------------------

    To assess 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 level, which is a value selected from
one of several sources. First, the chronic reference level can be the
EPA reference concentration (RfC) (http://www.epa.gov/riskassessment/glossary.htm), defined as ``an estimate (with uncertainty spanning
perhaps an order of magnitude) of a continuous inhalation exposure to
the human population (including sensitive subgroups) that is likely to
be without an appreciable risk of deleterious effects during a
lifetime.'' Alternatively, in cases where an RfC from the EPA's IRIS
database is not available or where the EPA determines that using a
value other than the RfC is appropriate, the chronic reference level
can be a value from the following prioritized sources: (1) The Agency
for Toxic Substances and Disease Registry 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 off-site exposure for
each facility (i.e., not just the census block centroids), assuming
that a person is located at this spot at a time when both the peak
(hourly) emission rates and worst-case dispersion conditions occur. 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 emission rates, meteorology and
exposure location for our acute analysis.
    As described in the CalEPA's Air Toxics Hot Spots Program Risk
Assessment Guidelines, Part I, The Determination of Acute Reference
Exposure Levels for Airborne Toxicants, an acute REL value (http://www.oehha.ca.gov/air/pdf/acuterel.pdf) is defined as ``the
concentration level at or below which no adverse health effects are
anticipated for a specified exposure duration.'' 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 value does not automatically
indicate an adverse health impact.
    AEGL values were derived in response to recommendations from the
National Research Council (NRC). As described in Standing Operating
Procedures (SOP) of the National Advisory Committee on Acute Exposure
Guideline Levels for Hazardous Substances (http://www.epa.gov/opptintr/aegl/pubs/sop.pdf),\12\ ``the NRC's previous name for acute exposure
levels--community emergency exposure levels--was replaced by the term
AEGL to reflect the broad application of these values to planning,
response and prevention in the community, the workplace,
transportation, the military and the remediation of Superfund sites.''
Id. at 2. This document also states that AEGL values ``represent
threshold exposure limits for the general public and are applicable to
emergency exposures ranging from 10 minutes to eight hours.'' Id. at 2.
---------------------------------------------------------------------------

    \12\ NAS, 2001. Standing Operating Procedures for Developing
Acute Exposure Levels for Hazardous Chemicals, page 2.
---------------------------------------------------------------------------

    The document lays out the purpose and objectives of AEGL by stating
that ``the primary purpose of the AEGL program and the National
Advisory Committee for Acute Exposure Guideline Levels for Hazardous
Substances is to develop guideline levels for once-in-a-lifetime,
short-term exposures to airborne concentrations of acutely toxic, high-
priority chemicals.'' Id. at 21. In detailing the intended application
of AEGL values, the document states that ``[i]t is anticipated that the
AEGL values will be used for regulatory and nonregulatory purposes by
U.S. Federal and state agencies, and possibly the international
community in conjunction with chemical emergency response, planning and
prevention programs. More specifically, the AEGL values will be used
for conducting various risk assessments to aid in the development of
emergency preparedness and prevention plans, as well as real-time
emergency response actions, for accidental chemical releases at fixed
facilities and from transport carriers.'' Id. at 31.
    The AEGL-1 value is then specifically defined as ``the airborne
concentration (expressed as ppm (parts per million) or mg/m\3\
(milligrams per cubic meter)) of a substance above which it is
predicted that the general population, including susceptible
individuals, could experience notable discomfort, irritation, or
certain asymptomatic nonsensory effects. However, the effects are not
disabling and are transient and reversible upon cessation of
exposure.'' Id. at 3. The document also notes that, ``Airborne
concentrations below AEGL-1 represent exposure levels that can produce
mild and progressively increasing but transient and nondisabling odor,
taste, and sensory irritation or certain asymptomatic, nonsensory
effects.'' Id. Similarly, the document defines AEGL-2 values as ``the
airborne concentration (expressed as parts per million or milligrams
per cubic meter) of a substance above which it is predicted that the
general population, including susceptible individuals, could experience
irreversible or other serious, long-lasting adverse health effects or
an impaired ability to escape.'' Id.
    ERPG values are derived for use in emergency response, as described
in the American Industrial Hygiene Association's ERP Committee document
entitled, 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.'' \13\ 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.
---------------------------------------------------------------------------

    \13\ 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.\14\ 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 these
source categories, a factor of 10 was applied to all emissions, with
one exception. A factor of two was applied for emissions from equipment
leaks for all three source categories. A further discussion of why
these factors were chosen can be found in the memorandum, Emissions
Data and Acute Risk Factor Used in Residual Risk Modeling: Acrylic and
Modacrylic Fibers, Amino/Phenolic Resins, and Polycarbonate Production,
available in the docket for this action (EPA-HQ-OAR-2012-0133).
---------------------------------------------------------------------------

    \14\ 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 using a
peak-to-mean hourly emissions ratio based on source category-specific
knowledge or data (rather than the default factor of 10) and using the
site-specific facility layout to distinguish facility property from an
area where the public could be exposed. These refinements are discussed
more fully in the Draft Residual Risk Assessment for the Acrylic and
Modacrylic Fibers Production Source Category, Draft Residual Risk
Assessment for the Amino/Phenolic Resins Production Source Category,
and Draft Residual Risk Assessment for the Polycarbonate Production
Source Category, which are available in the docket for this action.
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,\15\ 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 \16\ for
HAP have been developed, we consider additional acute values (i.e.,
occupational and international values) to provide a more complete risk
characterization. As a result, for most chemicals, the 15-minute
occupational ceiling values are set at levels higher than a one-hour
AEGL-1, making comparisons to them irrelevant unless the AEGL-1 or
ERPG-1 levels are exceeded (U.S. EPA 2009). Such is not the case when
comparing the available acute inhalation health effect reference values
for formaldehyde (U.S. EPA 2009). See section V.B.2 of this preamble
for additional information on the acute dose-response values for
formaldehyde.
---------------------------------------------------------------------------

    \15\ 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.
    \16\ 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.
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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://www.epa.gov/ttn/fera/risk_atra_vol1.html).
    For the AMF and PC source categories, we did not identify emissions
of any PB-HAP. Because we did not identify PB-HAP emissions, no further
evaluation of multipathway risk was conducted for these source
categories.
    For the APR source category, we identified emissions of lead
compounds (1 facility), cadmium compounds (2 facilities) and POM
(analyzed as benzo(a)pyrene toxic equivalency quotient (TEQ)) (2
facilities). Because one or more of these PB-HAP are emitted by at
least one facility in the APR source category, we proceeded to the
second step of the evaluation. In this step, we determined whether the
facility-specific emissions rates of each 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 thresholds for each
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.
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 the PB-HAP were compared to the emission rate threshold values
for each of the PB-HAP identified 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.
    For the purpose of developing emissions rates for our Tier I TRIM-
Screen, we derived emission levels for each PB-HAP (other than lead) at
which the maximum excess lifetime cancer risk would be 1-in-1 million
or, for HAP that cause non-cancer health effects, the maximum hazard
quotient would be 1. If the emissions rate of any PB-HAP exceeds the
Tier I screening emissions rate for any facility, we conduct a Tier II
multipathway screen. In the Tier II screen, the location of each
facility that exceeds the Tier I emission rate is used to refine the
assumptions associated with the environmental scenario while
maintaining the exposure scenario assumptions. We then adjust 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
lead compounds, rather than developing a screening emissions rate for
them, we compared maximum estimated chronic inhalation exposures with
the level of the current National Ambient Air Quality Standard (NAAQS)
for lead. Values below the level of the primary (health-based) lead
NAAQS were considered to have a low potential for multi-pathway risk.
    For further information on the multipathway analysis approach, see
the Draft Residual Risk Assessment for the Acrylic and Modacrylic
Fibers Production Source Category, Draft Residual Risk Assessment for
the Amino/Phenolic Resins Production Source Category, and Draft
Residual Risk Assessment for the Polycarbonate Production Source
Category, which are available in the docket for this action.
5. How did we assess risks considering emissions control options?
    In addition to assessing baseline inhalation risks and screening
for potential multipathway risks, we also estimated risks considering
the potential emissions reductions that would be achieved by the
control options under consideration. In these cases, the expected
emissions reductions were applied to the specific HAP and emissions
points in the source category dataset to develop corresponding
estimates of risk and incremental risk reductions.
6. 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 persistent bioaccumulative HAP
(PB-HAP) and two acid gases. The five PB-HAP are cadmium, dioxins/
furans, polycyclic organic matter (POM), mercury (both inorganic
mercury and methyl mercury) and lead. The two acid gases are hydrogen
chloride (HCl) and hydrogen fluoride (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 (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 cadmium
compounds, dioxins/furans, POM and mercury in soil, sediment and water.
For lead, we currently do not have the ability to calculate these
concentrations using the TRIM.Fate model. Therefore, to evaluate the
potential for adverse environmental effects from lead, we compare the
HEM modeled inhalation exposures from the source category emissions of
lead with the level of the secondary National Ambient Air Quality
Standard (NAAQS) for lead.\17\ We consider values below the level of
the secondary lead NAAQS to be unlikely to cause adverse environmental
effects.
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    \17\ The secondary lead NAAQS is a reasonable measure of
determining whether there is an adverse environmental effect since
it was established considering ``effects on soils, water, crops,
vegetation, man-made materials, animals, wildlife, weather,
visibility and climate, damage to and deterioration of property, and
hazards to transportation, as well as effects on economic values and
on personal comfort and well-being.''
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    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 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 to estimate the
potential for an adverse environmental effect.
    The EPA acknowledges that other HAP beyond the seven HAP discussed
above may have the potential to cause adverse environmental effects.
Therefore, the EPA may include other relevant HAP in its environmental
risk screening in the future, as modeling science and resources allow.
The EPA invites comment on the extent to which other HAP emitted by the
source category may cause adverse environmental effects. Such
information should include references to peer-reviewed ecological
effects benchmarks that are of sufficient quality for making regulatory
decisions, as well as information on the presence of

organisms located near facilities within the source category that such
benchmarks indicate could be adversely affected.
c. Ecological Assessment Endpoints and Benchmarks for PB-HAP
    An important consideration in the development of the EPA's
screening methodology is the selection of ecological assessment
endpoints and benchmarks. Ecological assessment endpoints are defined
by the ecological entity (e.g., aquatic communities including fish and
plankton) and its attributes (e.g., frequency of mortality). Ecological
assessment endpoints can be established for organisms, populations,
communities or assemblages, and ecosystems.
    For PB-HAP except for lead, 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, 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 mercury, 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.
    No-observed-adverse-effect levels (NOAEL): The highest exposure
level tested at which there are no biologically significant increases
in the frequency or severity of adverse effect.
    We established a hierarchy of preferred benchmark sources to allow
selection of benchmarks for each environmental HAP at each ecological
assessment endpoint. In general, the EPA sources that are used at a
programmatic level (e.g., Office of Water, Superfund Program) were
used, 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., NOAA) or state agencies.
    Benchmarks for all effect levels are not available for all PB-HAP
and assessment endpoints. In cases where multiple effect levels were
available for a particular PB-HAP and assessment endpoint, we use all
of the available effect levels to help us to determine whether
ecological risks exist and, if so, whether the risks could be
considered significant and widespread.
d. Ecological Assessment Endpoints and Benchmarks for Acid Gases
    The environmental screening analysis also evaluated potential
damage and reduced productivity of plants due to direct exposure to
acid gases in the air. For acid gases, we evaluated the following
ecological assessment endpoint:
     Local terrestrial plant communities with foliage exposed
to acidic gaseous HAP in the air.
    The selection of ecological benchmarks for the effects of acid
gases on plants followed the same approach as for PB-HAP (i.e., we
examine all of the available benchmarks). For HCl, the EPA identified
chronic benchmark concentrations. We note that the benchmark for
chronic HCl exposure to plants is greater than the reference
concentration for chronic inhalation exposure for human health. This
means that where 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, 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, EPA first determined
whether any facilities in the AMF, APR and PC source categories emitted
any of the seven environmental HAP. For the AMF and PC source
categories, we did not identify emissions of any of the seven
environmental HAP included in the screen. Because we did not identify
environmental HAP emissions, no further evaluation of environmental
risk was conducted for those source categories. For the APR source
category, we identified emissions of lead compounds (1 facility),
cadmium compounds (2 facilities) and POM (analyzed as benzo(a)pyrene
TEQ) (2 facilities).
    Because one or more of the seven environmental HAP evaluated are
emitted by at least one facility in the APR source category, we
proceeded to the second step of the evaluation.
f. PB-HAP Methodology
    For cadmium, mercury, POM and dioxins/furans, the environmental
screening analysis consists of two tiers, and lead is 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 sediments, and
the fish. The resulting media concentrations were then used to back-
calculate a screening threshold 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 threshold emission
rate for that PB-HAP for each assessment endpoint. If emissions from a
facility do not exceed the Tier I threshold, the

facility ``passes'' the screen, and therefore, is not evaluated further
under the screening approach. If emissions from a facility exceed the
Tier I threshold, we evaluate the facility further in Tier II.
    In Tier II of the environmental screening analysis, the screening
emission thresholds 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 threshold, the facility passes the
screen, and typically is not evaluated further. If emissions from a
facility exceed the Tier II threshold, 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
thresholds are not calculated for acid gases as they are in the
ecological risk screening methodology for PB-HAPs.
    For purposes of ecological risk screening, 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 the Acrylic and
Modacrylic Fibers Production Source Category, Draft Residual Risk
Assessment for the Amino/Phenolic Resins Production Source Category,
and Draft Residual Risk Assessment for the Polycarbonate Production
Source Category, which are available in the docket for this action.
7. 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 emissions sources at the facility for which we have
data. The emissions data for generating these ``facility-wide'' risks
were obtained from the 2005 NEI for the APR and PC source categories,
and from the 2008 NEI for the AMF source category. 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 three
source categories addressed in this proposal. The Draft Residual Risk
Assessment for the Acrylic and Modacrylic Fibers Production Source
Category, Draft Residual Risk Assessment for the Amino/Phenolic Resins
Production Source Category, and Draft Residual Risk Assessment for the
Polycarbonate Production Source Category, available through the docket
for this action, provide 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.
8. 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 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 the Acrylic and
Modacrylic Fibers Production Source Category, Draft Residual Risk
Assessment for the Amino/Phenolic Resins Production Source Category,
and Draft Residual Risk Assessment for the Polycarbonate Production
Source Category, which are available in the docket for this action
(EPA-HQ-OAR-2012-0133).
a. Uncertainties in the Emissions Datasets
    Although the development of the RTR 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 emissions 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 emissions rates for the
acute effects screening assessment were based on an emission adjustment
factor applied to the average annual hourly emissions rates, which are
intended to account for emission fluctuations due to normal facility
operations.
b. Uncertainties in Dispersion Modeling
    We recognize there is uncertainty in ambient concentration
estimates associated with any model, including the EPA's recommended
regulatory dispersion model, AERMOD. In using a model to estimate
ambient pollutant concentrations, the user chooses certain options to
apply. For RTR assessments, we select some model options that have the
potential to overestimate ambient air concentrations (e.g., not
including plume depletion or pollutant transformation). We select other
model options that have the potential to underestimate ambient impacts
(e.g., not including building downwash). Other options that we select
have the potential to either under- or overestimate ambient levels
(e.g., meteorology and receptor

locations). On balance, considering the directional nature of the
uncertainties commonly present in ambient concentrations estimated by
dispersion models, the approach we apply in the RTR assessments should
yield unbiased estimates of ambient HAP concentrations.
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.\18\ 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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    \18\ Short-term mobility is movement from one microenvironment
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 receptors where the block
population is not well represented by a single location.
    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 receptors 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 emissions sources at facilities actually operate (i.e., more or
less than 70 years) and the domestic growth or decline of the modeled
industry (i.e., the increase or decrease in the number or size of
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 overstatement of 25 to 30 percent of exposures.\19\
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    \19\ 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 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 human
activity patterns. In this assessment, we assume that individuals
remain for 1 hour at the point of maximum ambient concentration as
determined by the co-occurrence of peak emissions and worst-case
meteorological conditions. These assumptions would tend to be worst-
case actual exposures as it is unlikely that a person would be located
at the point of maximum exposure during the time of worst-case impact.
d. Uncertainties in Dose-Response Relationships
    There are uncertainties inherent in the development of the dose-
response values used in our risk assessments for cancer effects from
chronic exposures and 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