[Federal Register Volume 80, Number 31 (Tuesday, February 17, 2015)]
[Proposed Rules]
[Pages 8391-8439]
[FR Doc No: 2015-02055]

Vol. 80

Tuesday,

No. 31

February 17, 2015

Part II

Environmental Protection Agency

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40 CFR Part 63

National Emission Standards for Aerospace Manufacturing and Rework
Facilities Risk and Technology Review; Proposed Rule

Federal Register / Vol. 80 , No. 31 / Tuesday, February 17, 2015 /
Proposed Rules

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ENVIRONMENTAL PROTECTION AGENCY

40 CFR Part 63

[EPA-HQ-OAR-2014-0830; FRL-9922-10-OAR]
RIN 2060-AQ99

National Emission Standards for Aerospace Manufacturing and
Rework Facilities Risk and Technology Review

AGENCY: Environmental Protection Agency.

ACTION: Proposed rule.

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SUMMARY: The Environmental Protection Agency (EPA) is proposing
amendments to the national emissions standards for hazardous air
pollutants (NESHAP) for Aerospace Manufacturing and Rework Facilities
to address the results of the residual risk and technology review (RTR)
conducted as required under the Clean Air Act (CAA), and to correct
errors and deficiencies identified during the review of these
standards. The proposed amendments would add limitations to reduce
organic and inorganic emissions of hazardous air pollutants (HAP) from
specialty coating application operations; would remove the exemptions
from the emission limitations for periods of startup, shutdown and
malfunction (SSM) so that affected units would be subject to the
emission standards at all times; and would revise provisions to address
recordkeeping and reporting requirements applicable to periods of SSM.
This action also proposes other technical corrections. The EPA
estimates that implementation of this proposed rule will result in
reductions of 58 tons of HAP.

DATES: Comments. Comments must be received on or before April 3, 2015.
A copy of comments on the information collection provisions should be
submitted to the Office of Management and Budget (OMB) on or before
March 19, 2015.
    Public Hearing. If anyone contacts the EPA requesting a public
hearing by February 23, 2015, we will hold a public hearing on March 4,
2015. If you are interested in requesting a public hearing or attending
the public hearing, contact Ms. Pamela Garrett at<PHONE_NUMBER> or at
<EMAIL_ADDRESS>If the EPA holds a public hearing, the EPA will
keep the record of the hearing open for 30 days after completion of the
hearing to provide an opportunity for submission of rebuttal and
supplementary information.

ADDRESSES: Comments. Submit your comments, identified by Docket ID No.
EPA-HQ-OAR-2014-0830, by one of the following methods:
     Federal eRulemaking Portal: http://www.regulations.gov.
Follow the online instructions for submitting comments.
     Email<EMAIL_ADDRESS>Include Attention Docket ID
No. EPA-HQ-OAR-2014-0830 in the subject line of the message.
     Fax:<PHONE_NUMBER>, Attention Docket ID No. EPA-HQ-OAR-
2014-0830.
     Mail: Environmental Protection Agency, EPA Docket Center
(EPA/DC), Mail Code 28221T, Attention Docket ID No. EPA-HQ-OAR-2014-
0830, 1200 Pennsylvania Avenue NW., Washington, DC 20460. In addition,
please mail a copy of your comments on the information collection
provisions to the Office of Information and Regulatory Affairs, Office
of Management and Budget (OMB), Attn: Desk Officer for EPA, 725 17th
Street NW., Washington, DC 20503.
     Hand/Courier Delivery: EPA Docket Center, Room 3334, EPA
WJC West Building, 1301 Constitution Avenue NW., Washington, DC 20004,
Attention Docket ID No. EPA-HQ-OAR-2014-0830. 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-
2014-0830. The EPA's policy is that all comments received will be
included in the public docket without change and may be made available
online at http://www.regulations.gov, including any personal
information provided, unless the comment includes information claimed
to be confidential business information (CBI) or other information
whose disclosure is restricted by statute. Do not submit information
that you consider to be CBI or otherwise protected through http://www.regulations.gov or email. The http://www.regulations.gov Web site
is an ``anonymous access'' system, which means the EPA will not know
your identity or contact information unless you provide it in the body
of your comment. If you send an email comment directly to the EPA
without going through http://www.regulations.gov, your email address
will be automatically captured and included as part of the comment that
is placed in the public docket and made available on the Internet. If
you submit an electronic comment, the EPA recommends that you include
your name and other contact information in the body of your comment and
with any disk or CD-ROM you submit. If the EPA cannot read your comment
due to technical difficulties and cannot contact you for clarification,
the EPA may not be able to consider your comment. Electronic files
should not include special characters or any form of encryption and be
free of any defects or viruses. For additional information about the
EPA's public docket, visit the EPA Docket Center homepage at: http://www.epa.gov/dockets.
    Docket. The EPA has established a docket for this rulemaking under
Docket ID No. EPA-HQ-OAR-2014-0830. All documents in the docket are
listed in the http://www.regulations.gov index. Although listed in the
index, some information is not publicly available, e.g., CBI or other
information whose disclosure is restricted by statute. Certain other
material, such as copyrighted material, is not placed on the Internet
and will be publicly available only in hard copy. Publicly available
docket materials are available either electronically in regulations.gov
or in hard copy at the EPA Docket Center, Room 3334, EPA WJC West
Building, 1301 Constitution Avenue NW., Washington, DC. The Public
Reading Room is open from 8:30 a.m. to 4:30 p.m., Monday through
Friday, excluding legal holidays. The telephone number for the Public
Reading Room is<PHONE_NUMBER>, and the telephone number for the EPA
Docket Center is<PHONE_NUMBER>.
    Public Hearing. If a public hearing is requested by February 23,
2015, it will be held on March 4, 2015 at the EPA's Research Triangle
Park Campus, 109 T.W. Alexander Drive, Research Triangle Park, NC
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. Pamela Garrett at<PHONE_NUMBER> 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. If a hearing is requested, the last day to pre-register in
advance to speak at the hearing will be March 2, 2015.
    Additionally, requests to speak will be taken the day of the
hearing at the hearing registration desk, although preferences on
speaking times may not be able to be fulfilled. If you require the
service of a translator or special accommodations such as audio
description, please let us know at the time of registration. If you
require an accommodation, we ask that you preregister for the hearing,
as we may not be able to arrange such

accommodations without advance notice.
    If no one contacts the EPA requesting a public hearing to be held
concerning this proposed rule by February 23, 2015, a public hearing
will not take place. If a hearing is held, it will provide interested
parties the opportunity to present data, views or arguments concerning
the proposed action. The EPA will make every effort to accommodate all
speakers who arrive and register. Because the hearing will be held 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. Please note that the
REAL ID Act, passed by Congress in 2005, established new requirements
for entering federal facilities. If your driver's license is issued by
Alaska, American Samoa, Arizona, Kentucky, Louisiana, Maine,
Massachusetts, Minnesota, Montana, New York, Oklahoma or the state of
Washington, you must present an additional form of identification to
enter the federal building. Acceptable alternative forms of
identification include: federal employee badges, passports, enhanced
driver's licenses and military identification cards. In addition, you
will need to obtain a property pass for any personal belongings you
bring with you. Upon leaving the building, you will be required to
return this property pass to the security desk. No large signs will be
allowed in the building, cameras may only be used outside of the
building and demonstrations will not be allowed on federal property for
security reasons.
    The EPA may ask clarifying questions during the oral presentations,
but will not respond to the presentations at that time. Written
statements and supporting information submitted during the comment
period will be considered with the same weight as oral comments and
supporting information presented at the public hearing. Commenters
should notify Ms. Garrett if they will need specific equipment, or if
there are other special needs related to providing comments at the
hearing. 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. Again, a hearing will not be held
unless requested.

FOR FURTHER INFORMATION CONTACT: For questions about this proposed
action, contact Kim Teal, Sector Policies and Programs Division (D243-
02), Office of Air Quality Planning and Standards, U.S. Environmental
Protection Agency, Research Triangle Park, North Carolina 27711;
telephone number:<PHONE_NUMBER>; fax number:<PHONE_NUMBER>; and email
address<EMAIL_ADDRESS>For specific information regarding the risk
modeling methodology, contact Ted Palma, Health and Environmental
Impacts Division (C539-02), Office of Air Quality Planning and
Standards, U.S. Environmental Protection Agency, Research Triangle
Park, North Carolina 27711; telephone number:<PHONE_NUMBER>; fax
number:<PHONE_NUMBER>; and email address<EMAIL_ADDRESS>For
information about the applicability of the NESHAP to a particular
entity, contact Rafael Sanchez, Office of Enforcement and Compliance
Assurance (OECA),<PHONE_NUMBER><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:

AD Airworthiness Directive
AEGL acute exposure guideline level
AERMOD air dispersion model used by the HEM-3 model
ATSDR Agency for Toxic Substances and Disease Registry
BACT Best Achievable Control Technology
CAA Clean Air Act
CalEPA California EPA
CBI Confidential Business Information
CDX EPA's Central Data Exchange
CEDRI EPA's Compliance and Emissions Data Reporting Interface
CFR Code of Federal Regulations
CTG Control Technique Guideline document
EJ environmental justice
EPA Environmental Protection Agency
ERPG Emergency Response Planning Guidelines
ERT EPA's Electronic Reporting Tool
FAA Federal Aviation Administration
FR Federal Register
g/L grams/liter
HAP hazardous air pollutants
HCl hydrochloric acid
HEM-3 Human Exposure Model, Version 1.1.0
HF hydrogen fluoride
HI hazard index
HQ hazard quotient
HVLP high volume low pressure
IARC International Agency for Research on Cancer
ICR information collection request
IRIS Integrated Risk Information System
km kilometer
lb/gal pounds/gallon
LOAEL Lowest-observed-adverse-effect level
MACT maximum achievable control technology
mg/m\3\ milligrams per cubic meter
MIR maximum individual risk
mm Hg millimeters mercury
NAAQS National Ambient Air Quality Standards
NAICS North American Industry Classification System
NAS National Academy of Sciences
NATA National Air Toxics Assessment
NEI National Emission Inventory
NESHAP National Emissions Standards for Hazardous Air Pollutants
NOAA National Oceanic and Atmospheric Administration
NOAEL No-observed-adverse-effect levels
NRC National Research Council
NRDC Natural Resources Defense Council
NTP National Toxicology Program
NTTAA National Technology Transfer and Advancement Act
OAQPS Office of Air Quality Planning and Standards
OECA Office of Enforcement and Compliance Assurance
OEM original equipment manufacturer
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
PEL Probable effect level
POM polycyclic organic matter
ppm parts per million
PSD Prevention of Significant Deterioration
RACT Reasonably Available Control Technology
RBLC EPA's RACT/BACT/LAER Clearinghouse
RCRA Resource Conservation and Recovery Act of 1976
REL reference exposure level
RFA Regulatory Flexibility Act
RfC reference concentration
RfD reference dose
RoC Report of the Carcinogens
RTR residual risk and technology review
SAB Science Advisory Board
SCAQMD South Coast Air Quality Management District
SSM startup, shutdown and malfunction
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
VOC volatile organic compounds

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

I. General Information
    A. Does this action apply to me?
    B. Where can I get a copy of this document and other related
information?
    C. What should I consider as I prepare my comments for the EPA?
II. Background
    A. What is the statutory authority for this action?

    B. What is this source category and how does the current NESHAP
regulate its HAP emissions?
    C. What data collection activities were conducted to support
this action?
    D. What other relevant background information and data are
available?
    E. What litigation is related to this proposed action?
III. Analytical Procedures
    A. How did we estimate post-MACT risks posed by the source
category?
    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
    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?
    E. What other actions are we proposing?
    F. What compliance dates are we proposing?
V. Summary of Cost, Environmental and Economic Impacts
    A. What are the affected sources?
    B. What are the air quality impacts?
    C. What are the cost impacts?
    D. What are the economic impacts?
    E. What are the benefits?
VI. Request for Comments
VII. Submitting Data Corrections
VIII. Statutory and Executive Order Reviews
    A. Executive Order 12866: Regulatory Planning and Review and
Executive Order 13563: Improving Regulation and Regulatory Review
    B. Paperwork Reduction Act
    C. Regulatory Flexibility Act (RFA)
    D. Unfunded Mandates Reform Act (UMRA)
    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 (NTTAA)
    J. Executive Order 12898: Federal Actions to Address
Environmental Justice in Minority Populations and Low-Income
Populations

I. General Information

A. Does this action apply to me?

    Table 1 of this preamble lists the regulated industrial source
category that is the subject of this proposal. Table 1 is not intended
to be exhaustive, but rather to provide a guide for readers regarding
the entities that this proposed action is likely to affect. The
proposed standards, once promulgated, will be directly applicable to
the affected sources. Federal government entities may be affected by
this proposed action. Parties potentially affected by this action
include major and synthetic minor source installations that are owned
or operated by the Armed Forces of the United States (including the
Department of Defense and the Coast Guard) and the National Aeronautics
and Space Administration. As defined under the ``Surface Coating''
industry sector 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 Aerospace Manufacturing and Rework
Facilities source category is any facility engaged, either in part or
in whole, in the manufacture or rework of commercial, civil or military
aerospace vehicles or components and that are major sources as defined
in 40 CFR 63.2.

  Table 1--Industrial Source Category Affected by This Proposed Action
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       Source Category               NESHAP            NAICS code \a\
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Aerospace Manufacturing and   Aerospace             336411, 336412,
 Rework Facilities.            Manufacturing and     336413, 336414,
                               Rework Facilities.    336415, 336419,
                                                     481111, 481112,
                                                     481211, 481212,
                                                     481219.
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\a\ North American Industry Classification System.

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

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

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

    Submitting CBI. Do not submit information containing CBI to the EPA
through http://www.regulations.gov or email. Clearly mark the part or
all of the information that you claim to be CBI. For CBI information on
a disk or CD-ROM that you mail to the EPA, mark the outside of the disk
or CD-ROM as CBI and then identify electronically within the disk or
CD-ROM the specific information that is claimed as CBI. In addition to
one complete version of the comments that includes information claimed
as CBI, you must submit a copy of the comments that does not contain
the information claimed as CBI for inclusion in the public docket. If
you submit a CD-ROM or disk that does not contain CBI, mark the outside
of the disk or CD-ROM clearly that it does not contain CBI. Information
not marked as CBI will be included in the public docket and the EPA's
electronic public docket without prior notice. Information marked as
CBI will not be disclosed except in accordance with procedures set
forth in 40 Code of Federal Regulations (CFR) part 2. Send or deliver
information identified as CBI only to the following address: OAQPS
Document Control Officer (C404-02), OAQPS, U.S. Environmental
Protection Agency, Research Triangle Park, North Carolina 27711,
Attention Docket ID No. EPA-HQ-OAR-2014-0830.

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 HAP. For major sources, the technology-based
NESHAP must reflect the maximum degree of emission reductions of HAP
achievable (after considering cost, energy requirements and non-air
quality health and environmental impacts) and are commonly referred to
as maximum achievable control technology (MACT) standards.
    MACT standards must reflect the maximum degree of emissions
reduction achievable through the application of measures, processes,
methods, systems or techniques, including, but not limited to, measures
that: (1) Reduce the volume of or eliminate pollutants through process
changes, substitution of materials or other modifications; (2) enclose
systems or processes to eliminate emissions; (3) capture or treat
pollutants when released from a process, stack, storage or fugitive
emissions point; (4) are design, equipment, work practice or
operational standards (including requirements for operator training or
certification); or (5) are a combination of the above. CAA section
112(d)(2)(A) through (E). The MACT standards may take the form of
design, equipment, work practice or operational standards where the EPA
first determines either that: (1) A pollutant cannot be emitted through
a conveyance designed and constructed to emit or capture the pollutant,
or that any requirement for or use of, such a conveyance would be
inconsistent with law; or (2) the application of measurement
methodology to a particular class of sources is not practicable due to
technological and economic limitations. CAA section 112(h)(1) and (2).
    The MACT ``floor'' is the minimum control level allowed for MACT
standards promulgated under CAA section 112(d)(3) and may not be based
on cost considerations. For new sources, the MACT floor cannot be less
stringent than the emissions control that is achieved in practice by
the best-controlled similar source. The MACT floor for existing sources
can be less stringent than floors for new sources, but not less
stringent than the average emissions limitation achieved by the best-
performing 12 percent of existing sources in the category or
subcategory (or the best-performing five sources for categories or
subcategories with fewer than 30 sources). In developing MACT
standards, the EPA must also consider control options that are more
stringent than the floor. We may establish standards more stringent
than the floor based on considerations of the cost of achieving the
emission reductions, any non-air quality health and environmental
impacts and energy requirements.
    The EPA is then required to review these technology-based standards
and revise them ``as necessary (taking into account developments in
practices, processes and control technologies)'' no less frequently
than every 8 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).
CAA Section 112(f)(1) required that the EPA prepare a report to
Congress discussing (among other things) methods of calculating the
risks posed (or potentially posed) by sources after implementation of
the MACT standards, the public health significance of those risks and
the EPA's recommendations as to legislation regarding such remaining
risk. The EPA prepared and submitted the Residual Risk Report to
Congress, EPA-453/R-99-001 (Risk Report) in March 1999. CAA section
112(f)(2) then provides that if Congress does not act on any
recommendation in the Risk Report, the EPA must analyze and address
residual risk for each category or subcategory of sources 8 years after
promulgation of such standards pursuant to CAA section 112(d).
    Section 112(f)(2) of the CAA requires the EPA to determine for
source categories subject to MACT standards whether the emission
standards provide an ample margin of safety to protect public health.
Section 112(f)(2)(B) of the CAA expressly preserves the EPA's use of
the two-step process for developing standards to address any residual
risk and the agency's interpretation of ``ample margin of safety''
developed in the National Emissions Standards for Hazardous Air
Pollutants: Benzene Emissions from Maleic Anhydride Plants,
Ethylbenzene/Styrene Plants, Benzene Storage Vessels, Benzene Equipment
Leaks, and Coke By-Product Recovery Plants (Benzene NESHAP) (54 FR
38044, September 14, 1989). The EPA notified Congress in the Risk
Report that the agency intended to use the Benzene NESHAP approach in
making CAA section 112(f) residual risk determinations (EPA-453/R-99-
001, p. ES-11). The EPA subsequently adopted this approach in its
residual risk determinations and in a challenge to the risk review for
the Synthetic Organic Chemical Manufacturing source category, the
United States Court of Appeals for the District of Columbia Circuit
upheld as reasonable the EPA's interpretation that subsection 112(f)(2)
incorporates the approach established in the Benzene NESHAP. See NRDC
v. EPA, 529 F.3d 1077, 1083 (D.C. Cir. 2008) (``[S]ubsection
112(f)(2)(B) expressly incorporates the EPA's interpretation of the
Clean Air Act from the Benzene standard, complete with a citation to
the Federal Register.''); see also A Legislative History of the Clean
Air Act Amendments of 1990, vol. 1, p. 877 (Senate debate on Conference
Report).
    The first step in the process of evaluating residual risk is the
determination of acceptable risk. If risks are unacceptable, the EPA
cannot consider cost in identifying the emissions standards necessary
to bring risks to an acceptable level. The second step is the
determination of whether standards must be further revised in order to
provide an ample margin of safety to protect public health. The ample
margin of safety is the level at which the standards must be set,
unless an even more stringent standard is necessary to prevent, taking
into consideration costs, energy, safety and other relevant factors, an
adverse environmental effect.
1. Step 1--Determination of Acceptability
    The agency in the Benzene NESHAP concluded that ``the acceptability
of risk under section 112 is best judged on the basis of a broad set of
health risk measures and information'' and that the ``judgment on
acceptability cannot be reduced to any single factor.'' Benzene NESHAP
at 38046. The determination of what represents an ``acceptable'' risk
is based on a judgment of ``what risks are acceptable in the world in
which we live'' (Risk Report at 178, quoting NRDC v. EPA, 824 F. 2d
1146, 1165 (D.C. Cir. 1987) (en banc) (``Vinyl Chloride''), recognizing
that our world is not risk-free.
    In the Benzene NESHAP, we stated that ``EPA will generally presume
that if the risk to [the maximum exposed] individual is no higher than
approximately one in 10 thousand, that risk level is considered
acceptable.'' 54 FR 38045, September 14, 1989. We discussed the maximum
individual lifetime cancer risk (or maximum individual risk (MIR)) as
being ``the estimated risk that a person living near a plant would have
if he or she were

exposed to the maximum pollutant concentrations for 70 years.'' Id. We
explained that this measure of risk ``is an estimate of the upper bound
of risk based on conservative assumptions, such as continuous exposure
for 24 hours per day for 70 years.'' Id. We acknowledged that maximum
individual lifetime cancer risk ``does not necessarily reflect the true
risk, but displays a conservative risk level which is an upper-bound
that is unlikely to be exceeded.'' Id.
    Understanding that there are both benefits and limitations to using
the MIR as a metric for determining acceptability, we acknowledged in
the Benzene NESHAP that ``consideration of maximum individual risk * *
* must take into account the strengths and weaknesses of this measure
of risk.'' Id. Consequently, the presumptive risk level of 100-in-1
million (1-in-10 thousand) provides a benchmark for judging the
acceptability of maximum individual lifetime cancer risk, but does not
constitute a rigid line for making that determination. Further, in the
Benzene NESHAP, we noted that:

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

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

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

    Id. at 38045. In some cases, these health measures and factors
taken together may provide a more realistic description of the
magnitude of risk in the exposed population than that provided by
maximum individual lifetime cancer risk alone.
    As noted earlier, in NRDC v. EPA, the court held that CAA section
112(f)(2) ``incorporates the EPA's interpretation of the Clean Air Act
from the Benzene Standard.'' The court further held that Congress'
incorporation of the Benzene standard applies equally to carcinogens
and non-carcinogens. 529 F.3d at 1081-82. Accordingly, we also consider
non-cancer risk metrics in our determination of risk acceptability and
ample margin of safety.
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, September 14, 1989.
    According to CAA section 112(f)(2)(A), if the MACT standards for
HAP ``classified as a known, probable, or possible human carcinogen do
not reduce lifetime excess cancer risks to the individual most exposed
to emissions from a source in the category or subcategory to less than
one in one million,'' the EPA must promulgate residual risk standards
for the source category (or subcategory), as necessary to provide an
ample margin of safety to protect public health. In doing so, the EPA
may adopt standards equal to existing MACT standards if the EPA
determines that the existing standards (i.e., the MACT standards) are
sufficiently protective. NRDC v. EPA, 529 F.3d 1077, 1083 (D.C. Cir.
2008) (``If EPA determines that the existing technology-based standards
provide an `ample margin of safety,' then the Agency is free to readopt
those standards during the residual risk rulemaking.'') The EPA must
also adopt more stringent standards, if necessary, to prevent an
adverse environmental effect,\1\ but must consider cost, energy, safety
and other relevant factors in doing so.
---------------------------------------------------------------------------

    \1\ ``Adverse environmental effect'' is defined as any
significant and widespread adverse effect, which may be reasonably
anticipated to wildlife, aquatic life or natural resources,
including adverse impacts on populations of endangered or threatened
species or significant degradation of environmental qualities over
broad areas. See 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, September 14, 1989, we stated as an
overall objective:

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

    The agency further stated that ``[t]he EPA also considers incidence
(the number of persons estimated to suffer cancer or other serious
health effects as a result of exposure to a pollutant) to be an
important measure of the health risk to the exposed population.
Incidence measures the extent of health risks to the exposed population
as a whole, by providing an estimate of the occurrence of cancer or
other serious health effects in the exposed population.'' Id. at 38045.
    In the ample margin of safety decision process, the agency again
considers all of the health risks and other health information
considered in the first step, including the incremental risk reduction
associated with standards more stringent than the MACT standard or a
more stringent standard that EPA has determined is necessary to ensure
risk is acceptable. In the ample margin of safety analysis, the agency
considers additional factors, including costs and economic impacts of
controls, technological feasibility, uncertainties and any other
relevant factors. Considering all of these factors, the agency will
establish the standard at a level that provides an ample margin of
safety to protect the public health, as required by CAA section 112(f).
54 FR 38046, September 14, 1989.

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

1. Description of the Aerospace Manufacturing and Rework Facilities
Source Category and Applicability.
    The NESHAP for the Aerospace Manufacturing and Rework Facilities
source category (henceforth referred to as the ``Aerospace NESHAP'')
was promulgated on September 1, 1995 (60 FR 45956) and codified at 40
CFR part 63, subpart GG. As promulgated in 1995, the Aerospace NESHAP
applies to the surface coating and related operations at each new and
existing affected source of HAP emissions at facilities that are major
sources and are engaged, either in part or in whole, in the manufacture
or rework of commercial, civil or military aerospace vehicles or
components. The requirements of the standards are nearly the same for
both new and existing sources. The Aerospace NESHAP (40 CFR 63.742)
defines ``aerospace vehicle or component'' as ``any fabricated part,
processed part, assembly of parts or completed unit, with the exception
of electronic components, of any aircraft, including, but not limited
to airplanes, helicopters, missiles, rockets, and space vehicles.''
Today, we estimate that 144 facilities are subject to the Aerospace
NESHAP. A complete list of facilities subject to the Aerospace NESHAP
is available in the Aerospace RTR database, which is available for
review in the docket for this proposed rulemaking. Section 63.741(c)
defines each affected source in the Aerospace Manufacturing and Rework
Facilities source category, and a facility could have a combination of
both new and existing affected sources. However, the emission standards
for new and existing affected sources are the same for nearly all
operations within subpart GG. The exceptions are the filter efficiency
requirements to control inorganic HAP emissions from primer and topcoat
spray application operations in 40 CFR 63.745 and for dry media
blasting operations in 40 CFR 63.746 and the requirements for controls
to reduce organic HAP emissions from chemical depainting operations in
40 CFR 63.746(c).
    The Aerospace NESHAP applies to organic HAP emissions from cleaning
operations, depainting operations, primer application operations,
topcoat application operations, chemical milling maskant application
operations and the handling and storage of waste. The rule also applies
to inorganic HAP emissions from primer and topcoat application
operations using spray equipment and depainting operations using dry
media blasting. The rule provides an exemption for primers, topcoats
and chemical milling maskants used in low-volumes which is defined as
189 liters (50 gallons) or less per formulation and for which the
combined annual total does not exceed 757 liters (200 gallons).
    The current Aerospace NESHAP explicitly excludes specialty coatings
from meeting any control requirements, as specified in 40 CFR 63.741(f)
and in 40 CFR 63.742 (i.e., the definitions for ``exterior primer,''
``primer,'' and ``topcoat''). Appendix A of the Aerospace NESHAP
defines 59 separate categories of specialty coatings.
    Although the EPA did not include emission limitations for specialty
coatings in the Aerospace NESHAP finalized in 1995 or in any subsequent
amendments, the EPA included VOC content limits for the 59 categories
of specialty coatings in the 1997 Aerospace Control Techniques
Guidelines (CTG) document.\2\ The CAA requires that state
implementation plans (SIPs) for certain ozone nonattainment areas be
revised to require the implementation of reasonably available control
technology (RACT) to control volatile organic compounds (VOC)
emissions. The EPA has defined RACT as the lowest emission limitation
that a particular source is capable of meeting by the application of
control technology that is reasonably available considering
technological and economic feasibility. The Aerospace CTG is intended
to provide state and local air pollution control authorities with an
information base, recommended emissions limitations and monitoring,
recordkeeping and reporting requirements for proceeding with their
analyses of reasonably available control technology (RACT) for their
own regulations to reduce VOC emissions from aerospace surface coating
operations.
---------------------------------------------------------------------------

    \2\ Guideline Series: Control of Volatile Organic Compound
Emissions from Coating Operations at Aerospace Manufacturing and
Rework Operations. Emission Standards Division, U. S. Environmental
Protection Agency, Office of Air and Radiation, Office of Air
Quality Planning and Standards, Research Triangle Park, NC 27711,
December 1997. Publication No. EPA-453/R-97-004.
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2. Organic and Inorganic HAP Emission Sources
    Organic HAP emissions from cleaning and depainting operations occur
from the evaporation of the volatile portion of the cleaning solvents
or chemical strippers. Cleaning emissions are typically fugitive in
nature and occur at most processing steps. Emissions from depainting
operations that occur within a booth or hangar are typically captured
and exhausted through a stack, although some emissions may be fugitive
in nature (e.g., open tanks).
    Organic HAP emissions from coating (primers, topcoats and chemical
milling maskants) application operations occur from the evaporation of
the solvent contained in the coatings. These emissions occur during the
application of the coatings on aerospace vehicles or parts, which may
take place in large open areas, such as hangars or in partially or
fully enclosed spaces, such as within spray booths.
    Organic HAP emissions from waste occur from evaporation of the
volatile portion of the waste while it is being handled or stored.
These emissions are fugitive in nature, occurring from each waste
container.
    Some coatings contain compounds that are inorganic HAP. Inorganic
HAP emissions from coatings occur during the application of the coating
if it is applied using spray guns. These inorganic HAP emissions are
paint particulates, commonly referred to as ``overspray,'' that do not
adhere to the surface being coated. Like the organic HAP emissions from
the operations, the emissions of the inorganic HAP may occur in large
open areas, such as hangars or in partially or fully enclosed spaces,
such as within spray booths. However, coatings that contain inorganic
HAP are typically applied in spray booths equipped with exhaust filters
to capture paint overspray. Inorganic HAP are not emitted from coatings
applied with non-spray methods, such as brushes, rollers or dip
coating, because the coating is not atomized with these methods.
    Inorganic HAP emissions from depainting operations may occur from
non-chemical methods, such as plastic and other types of dry media
blasting, used to strip an aerospace vehicle. (Chemical stripping
techniques do not release inorganic HAP.) These emissions occur as
particulates generated during the blasting process. The operation is
typically carried out within a large hangar equipped with a ventilation
system and particulate filtration device (e.g., a baghouse) or in
smaller enclosures, also equipped with filtration. The inorganic HAP
that are released from the depainting operations are primarily found in
the paint being stripped, although some stripping media may contain
trace amounts of inorganic HAP.

3. Regulation of Organic and Inorganic HAP Emissions in the Aerospace
NESHAP
    The Aerospace NESHAP specifies numerical emission limits for
organic HAP emissions from primer, topcoat, chemical milling maskant
application operations and chemical depainting operations; equipment
and filter efficiency requirements for dry media blasting depainting
operations and spray applied coating operations; composition
requirements and equipment standards for cleaning operations; and work
practice standards for waste handling and storage operations.
    The organic HAP emission rate for primers is 540 grams/liter (g/L)
(4.5 pounds/gallon (lb/gal)) (less water) for general aviation rework
facilities; 650 g/L (5.4 lb/gal) (less water) for large commercial
aircraft; or 350 g/L (2.9 lb/gal) for other primers (40 CFR
63.745(c)(1) and (2)). For topcoats and self-priming topcoats the
emission rate is 420 g/L (3.5 lb/gal) (less water); and 540 g/L (4.5
lb/gal) (less water) for primers and self-priming topcoats at general
aviation rework facilities (40 CFR 63.745(c)(3) and (4)).
Alternatively, a control system can be used to capture and control
emissions from the primer or topcoat application operation (40 CFR
63.745(d)). The system must achieve an overall control efficiency of 81
percent. Further, the Aerospace NESHAP specifies which types of coating
application techniques may be used (40 CFR 63.745(f)). The Aerospace
NESHAP also provides operating requirements for the application of
primers or topcoats that contain inorganic HAP, including control of
spray booth exhaust streams with either particulate filters or
waterwash systems (40 CFR 63.745(g)). The primer and topcoat limits and
control requirements do not apply to specialty coatings defined in
Appendix A to subpart GG.
    The organic HAP emission content limits for chemical milling
maskants for use with Type I chemical milling solutions is 622 g/L (5.2
lb/gal) (less water) and 160 g/L (1.3 lb/gal) (less water) for use with
Type II chemical milling solutions (40 CFR 63.747(c)). Alternatively, a
control system that achieves an overall control efficiency of 81
percent can be used to capture and control emissions from the maskant
application operation (40 CFR 63.747(d)). These requirements do not
apply to touch-up of scratched surfaces or damaged maskant and touch-up
of trimmed edges.
    For cleaning operations (including hand-wipe cleaning), the
Aerospace NESHAP specifies that cleaning solvents meet certain
composition requirements or that the cleaning solvents have a composite
vapor pressure of no more than 45 millimeters mercury (mm Hg) (24.1 in.
water) (40 CFR 63.744(b)). Work practice measures are also required (40
CFR 63.744(a)). Four work practice alternative techniques are specified
for spray gun cleaning, and work practice standards are specified for
flush cleaning operations (40 CFR 63.744(c) and (d)).
    The Aerospace NESHAP also specifies requirements for depainting
operations. Where there are no controls for organic HAP emissions from
chemical depainting operations, the rule prohibits organic HAP
emissions from chemical depainting operations, with the exception that
26 gallons of HAP-containing chemical stripper (or alternatively 190
pounds of organic HAP) may be used for each commercial aircraft
stripped, or 50 gallons (or 365 pounds of organic HAP) for each
military aircraft for spot stripping and decal removal (40 CFR
63.746(b)(1) through (3)). Where there are controls for organic HAP
emissions from chemical depainting, emissions must be reduced by 81
percent for controls installed before the effective date, and by 95
percent for controls installed on or after the effective date (40 CFR
63.746(c)). For non-chemical depainting operations that generate
inorganic HAP emissions from dry media blasting, the operation must be
performed in an enclosed area or in a closed cycle depainting system
and the air stream from the operation must pass through a dry filter
system meeting a minimum efficiency specified in the rule, through a
baghouse or through a waterwash system before being released to the
atmosphere (40 CFR 63.746(b)(4)).
    The handling and storage of waste that contains HAP must be
conducted in a manner that minimizes spills (40 CFR 63.748).

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

    In February 2011, the EPA issued an information collection request
(ICR), pursuant to CAA section 114, to approximately 1,300 facilities
that were thought to potentially own and operate Aerospace
Manufacturing and Rework Facilities. Information was requested on
operations subject to the Aerospace NESHAP (coatings, blast depainting
operations, solvent depainting operations and solvent cleaning
operations) as well as specialty coatings, chemical milling and metal
finishing operations, composite processing, storage tanks and
wastewater treatment. Information was also requested on booth
characteristics and control devices and location coordinates (latitude
and longitude) of emission stacks and operations. The ICR requested
available information regarding coating and solvent usage, process
equipment, control devices used, point and fugitive HAP emissions,
practices used to control HAP emissions and other aspects of facility
operations. A total of 87 major source facilities and 57 synthetic
minor facilities responded to the survey and were included in the risk
modeling analysis. The remaining facilities were either area source
facilities, not aerospace manufacturing or rework facilities or closed
facilities, or the ICR was returned undeliverable. We received data on
coating and solvent usage, chemical milling, metal finishing,
depainting operations, composite processing operations, storage tanks,
wastewater treatment operations and use of add-on control devices. From
these data, we were able to calculate HAP emissions for each of the
major source and synthetic minor facilities that responded to the
survey.
    In October 2012, the EPA issued a request for stack test data under
the authority of section 114 of the CAA. This request was sent to 9
parent companies for 18 facilities, requesting stack emissions testing
data for selected coating operations and spray booths and blast
depainting, composite processing and metal finishing operations
believed to represent the various processes and capture and control
configurations used by the industry. All facilities either responded to
the survey or provided information indicating the operations for which
we requested stack testing had been shutdown.
    In September 2013, the EPA issued an additional request to the same
companies requesting supplemental testing to confirm the content of the
coatings used in the October 2012 stack testing. These data were used
to speciate emissions for individual coatings and to develop the
default chromium speciation profile for processes included in the 2011
ICR.
    In May 2014, the EPA solicited industry review of the EPA's draft
modeling file records (e.g., estimated emissions and emission
estimation methods) that were developed based on the results of the
data collection efforts described above and the 2011 National Emission
Inventory (NEI) and 2005 National Air Toxics Assessment (NATA)
discussed in section II.D of this preamble. Of the 171 facilities
contacted, 84 facilities responded. Of the 171 facilities contacted,
the EPA determined that 144 are in operation

and subject to the NESHAP and 27 facilities are closed or not subject
to the Aerospace NESHAP (e.g., are area sources). The 144 facilities
that were determined to be in operation and subject to the NESHAP are
included in the model input file for the risk assessment.

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

    The 2011 NEI provided supplemental information for this RTR. 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. The
NEI includes information necessary for conducting risk modeling,
including annual HAP emissions estimates from individual emission
points at facilities and the related emissions release parameters. For
each emission record that was needed for the model input file for the
risk assessment (hereafter referred to as the ``RTR emissions
dataset'') that was not available from the 2011 ICR responses, the EPA
used available data in the 2011 NEI as the first alternative. The NEI
emission records used included annual HAP emissions estimates for
boilers, engines, chemical manufacturing processes, secondary metal
production processes, heaters, soil remediation, transportation
equipment, waste disposal, welding and other miscellaneous
manufacturing processes that were not included in the 2011 ICR.
Individual chromium emissions estimates were excluded from the modeling
file if they were found to overlap with a regulated process.
    The 2005 NATA also provided supplemental data for the RTR emissions
dataset for this RTR. The 2005 NATA includes annual HAP emissions
estimates for three Aerospace Manufacturing and Rework Facilities that
are not in the 2011 NEI. These data were incorporated into the RTR
emissions dataset, and include emission data for space heaters, boilers
and underground fuel tanks at the facilities. Although the 2005 NATA
data is outdated, we thought it important to ensure we had accounted
for all the major sources in the source category and given that we did
not have data on three of the facilities, EPA augmented our RTR
emissions dataset with this data for three of the 144 facilities. We
expect to have updated NATA soon and will consider the impact on the
three sources, as appropriate. NATA is the EPA's ongoing evaluation of
air toxics in the United States. The EPA developed NATA as a screening
tool for state/local/tribal agencies to prioritize pollutants, emission
sources and locations of interest for further study in order to gain a
better understanding of population risks. NATA assessments do not
incorporate refined information about emission sources, but rather use
general information about sources to develop estimates of risks which
are more likely to overestimate impacts than underestimate them. NATA
provides estimates of the risk of cancer and other serious health
effects from breathing (inhaling) air toxics in order to inform both
national and more localized efforts to identify and prioritize air
toxics, emission source types and locations which are of greatest
potential concern in terms of contributing to population risk.

E. What litigation is related to this proposed action?

    In 2007, the United States Court of Appeals for the District of
Columbia Circuit found that the EPA had erred in establishing emissions
standards for sources of HAP in the NESHAP for Brick and Structural
Clay Products Manufacturing and Clay Ceramics Manufacturing, 67 FR
26690 (May 16, 2003), and consequently vacated the rules.\3\ Among
other things, the court found EPA erred by failing to regulate
processes that emitted HAP, in some instances by establishing a MACT
floor of ``no control.'' In this action we are proposing to correct the
same error in the Aerospace NESHAP by proposing to remove the exemption
for specialty coatings found at 40 CFR 63.741(f) and instead add limits
for specialty coatings (including adhesives, adhesive bonding primers
and sealants).
---------------------------------------------------------------------------

    \3\ Sierra Club v. EPA, 479 F. 3d 875 (D.C. Cir. March 13,
2007).
---------------------------------------------------------------------------

    In a separate case, the court vacated portions of two provisions in
the EPA's CAA section 112 regulations that govern emissions of HAP
during periods of SSM.\4\ Specifically, the court vacated the SSM
exemption contained in 40 CFR 63.6(f)(1) and 40 CFR 63.6(h)(1), holding
that under section 302(k) of the CAA, emissions standards or
limitations must be continuous in nature and that the SSM exemption
violates the CAA's requirement that some section 112 standards apply
continuously. In this action, we are also proposing to revise these
provisions for Aerospace Manufacturing and Rework Facilities
operations, as discussed in section IV.E.2 of this preamble.
---------------------------------------------------------------------------

    \4\ Sierra Club v. EPA, 551 F. 3d 1019 (D.C. Cir. 2008), cert.
denied, 130 S. Ct 1735 (2010).
---------------------------------------------------------------------------

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 category?

    The EPA conducted a risk assessment that provides estimates of the
MIR posed by the HAP emissions from each source in the source category,
the hazard index (HI) for chronic exposures to HAP with the potential
to cause non-cancer health effects and the hazard quotient (HQ) for
acute exposures to HAP with the potential to cause non-cancer health
effects. The assessment also provides estimates of the distribution of
cancer risks within the exposed populations, cancer incidence and an
evaluation of the potential for adverse environmental effects. The
seven sections that follow this paragraph describe how we estimated
emissions and conducted the risk assessment. The docket for this
rulemaking contains the following document that provides more
information on the risk assessment inputs and models: Residual Risk
Assessment for the Aerospace Manufacturing and Rework Facilities Source
Category in Support of the January, 2015 Risk and Technology Review
Proposal, January 2015. The methods used to assess risks (as described
in the 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;\5\ they are also consistent
with the key recommendations contained in that report.
---------------------------------------------------------------------------

    \5\ 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?
    Data for 144 Aerospace Manufacturing and Rework Facilities were
used to create the RTR emissions dataset, as described in section II.C
of this preamble. The emissions sources included in the RTR emissions
dataset includes the following types of sources currently regulated by
the Aerospace NESHAP: Primer/topcoat application operations, waste
handling operations, chemical milling maskant application

operations, cleaning operations and chemical and blast depainting
operations. The RTR emissions dataset also includes the following types
of sources not currently regulated by the Aerospace NESHAP: Specialty
coatings, composite processing, chemical milling and metal finishing,
wastewater, storage tanks, boilers, engines, chemical manufacturing
processes, secondary metal production processes, heaters, soil
remediation, transportation equipment, waste disposal, welding and
other miscellaneous manufacturing processes. These emission sources
include both fugitive emissions and stack emissions. This RTR emissions
dataset is based primarily on data gathered through the CAA section 114
questionnaire, as described in section II.C of this preamble. This
dataset was supplemented with data received from the 2012 ICR for stack
testing data and the 2013 request for information on coatings analyses
(as described in section II.C of this preamble), the 2011 NEI (as
described in section II.D of this preamble) and the 2005 NATA (as
described in section II.D of this preamble). The sources noted above
provided all of the emissions data in the RTR emissions dataset and
nearly all of the facility specific data needed to conduct the risk
modeling analysis. However, there were limited instances where default
values were used to fill gaps in the facility-specific data used in the
risk modeling analysis. Examples of default values used to fill these
data gaps were default values used for stack height and other release
point parameters, and percentages used to segregate mercury and
chromium compounds into separate species. Use of defaults is discussed
in detail in the memorandum, Aerospace Manufacturing and Rework
Facilities RTR Modeling File Preparation, December 2014, available in
the docket for this action (Modeling File Preparation Memo).
    The RTR emissions dataset was refined following an extensive
quality assurance check of source locations, emission release
characteristics and annual emission estimates. We checked the
coordinates of each emission source in the dataset using ArcGIS to
ensure the emission point locations were correct. Also, as discussed in
section II.C of this preamble, in May 2014, the EPA solicited industry
review of the dataset and made corrections, as needed. For further
information on the EPA's quality assurance review, see the Modeling
File Preparation Memo available in the docket for this action.
    A list of the 144 facilities and additional information used to
develop the RTR emissions dataset is available in the Aerospace RTR
database, and documentation on the development of this database is
provided in the Modeling File Preparation Memo, both of which are
available in the docket for this action.
2. How did we estimate MACT-allowable emissions?
    The available emissions data in the RTR emissions dataset include
estimates of the actual 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.)
    We used the RTR emissions dataset discussed in section III.A.1 of
this preamble to estimate MACT-allowable emissions levels. Facilities
were asked to provide a multiplier in the 2011 ICR survey to scale up
average hourly emissions to maximum hourly emissions for air dispersion
modeling, given that each facility typically has a large number of
emission points and it would be difficult to determine the maximum
hourly emissions from each emission point. Many of the facilities
reported multipliers that were based on, for example, scaling
production from 2,000 hours to 8,760 hours per year or from one shift
per day to three shifts. However, using these values would have led to
unrealistically high ``allowable'' emission values because of
limitations in the market for new aerospace vehicles and for rework
services, and because many facilities have permit restrictions on their
total annual emissions. Therefore, the EPA did not use maximum hourly
emissions and instead chose to use a multiplier based on current and
historical industry capacity utilization factors. The EPA chose to use
a single multiplier of 1.02 to scale average annual emissions to
allowable annual emissions. The allowable emissions multiplier is based
on the difference between 2008 production utilization rate of 83.1
percent and the 20-year historical maximum production utilization rate
from 1990 of 85.0 percent (85 / 83.1 = 1.02). The docket for this
rulemaking contains information on the development of estimated MACT-
allowable emissions in the Modeling File Preparation Memo.
3. How did we conduct dispersion modeling, determine inhalation
exposures and estimate individual and population inhalation risks?
    Both long-term and short-term inhalation exposure concentrations
and health risks from the source category addressed in this proposal
were estimated using the Human Exposure Model (Community and Sector
HEM-3 version 1.1.0). The HEM-3 performs three primary risk assessment
activities: (1) Conducting dispersion modeling to estimate the
concentrations of HAP in ambient air, (2) estimating long-term and
short-term inhalation exposures to individuals residing within 50
kilometers (km) of the modeled sources \6\ and (3) estimating
individual and population-level inhalation risks using the exposure
estimates and quantitative dose-response information.
---------------------------------------------------------------------------

    \6\ This metric comes from the Benzene NESHAP. See 54 FR 38046.
---------------------------------------------------------------------------

    The air dispersion model used in the analysis, the AERMOD model, is
one of the EPA's preferred models for assessing pollutant
concentrations from industrial facilities.\7\ To perform the dispersion
modeling and to develop the preliminary risk estimates, HEM-3 draws on
three data libraries. The first is a library of meteorological data,
which is used for dispersion calculations. This library includes 1 year
(2011) of hourly surface and upper air observations for more than 800
meteorological stations, selected to provide coverage of the United
States and Puerto Rico. A second library of United States Census Bureau
census block \8\ 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.
---------------------------------------------------------------------------

    \7\ 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).
    \8\ A census block is the smallest geographic area for which
census statistics are tabulated.
---------------------------------------------------------------------------

    In developing the risk assessment for chronic exposures, we used
the estimated annual average ambient air concentrations of each HAP
emitted by each source for which we have emissions data in the source
category. The air concentrations at each nearby census block centroid
were used as a surrogate for the chronic inhalation exposure
concentration for all the people who reside in that census block. We
calculated the MIR for each facility as the cancer risk associated with
a continuous lifetime (24 hours per day, 7 days per week and 52 weeks
per year for a 70-year period) exposure to the maximum concentration at
the centroid of inhabited census blocks. Individual cancer risks were
calculated by multiplying the estimated lifetime exposure to the
ambient concentration of each of the HAP (in micrograms per cubic meter
([mu]g/m\3\)) by its unit risk estimate (URE). The URE is an upper
bound estimate of an individual's probability of contracting cancer
over a lifetime of exposure to a concentration of 1 microgram of the
pollutant per cubic meter of air. For residual risk assessments, we
generally use URE values from the EPA's Integrated Risk Information
System (IRIS). For carcinogenic pollutants without IRIS values, we look
to other reputable sources of cancer dose-response values, often using
California EPA (CalEPA) URE values, where available. In cases where
new, scientifically credible dose response values have been developed
in a manner consistent with the EPA guidelines and have undergone a
peer review process similar to that used by the EPA, we may use such
dose-response values in place of, or in addition to, other values, if
appropriate.
    The EPA estimated incremental individual lifetime cancer risks
associated with emissions from the facilities in the source category as
the sum of the risks for each of the carcinogenic HAP (including those
classified as carcinogenic to humans, likely to be carcinogenic to
humans and suggestive evidence of carcinogenic potential \9\) emitted
by the modeled sources. Cancer incidence and the distribution of
individual cancer risks for the population within 50 km of the sources
were also estimated for the source category as part of this assessment
by summing individual risks. A distance of 50 km is consistent with
both the analysis supporting the 1989 Benzene NESHAP (54 FR 38044,
September 14, 1989) and the limitations of Gaussian dispersion models,
including AERMOD.
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    \9\ These classifications also coincide with the terms ``known
carcinogen, probable carcinogen, and possible carcinogen,''
respectively, which are the terms advocated in the EPA's previous
Guidelines for Carcinogen Risk Assessment, published in 1986 (51 FR
33992, September 24, 1986). Summing the risks of these individual
compounds to obtain the cumulative cancer risks is an approach that
was recommended by the EPA's SAB in their 2002 peer review of EPA's
NATA entitled, NATA--Evaluating the National-scale Air Toxics
Assessment 1996 Data--an SAB Advisory, available at: http://
yosemite.epa.gov/sab/sabproduct.nsf/
214C6E915BB04E14852570CA007A682C/$File/ecadv02001.pdf.
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    To assess the risk of non-cancer health effects from chronic
exposures, we summed the HQ for each of the HAP that affects a common
target organ system to obtain the HI for that target organ system (or
target organ-specific HI, TOSHI). The HQ is the estimated exposure
divided by the chronic reference value, which is a value selected from
one of several sources. First, the chronic reference level can be the
EPA reference concentration (RfC) (http://www.epa.gov/riskassessment/glossary.htm), defined as ``an estimate (with uncertainty spanning
perhaps an order of magnitude) of a continuous inhalation exposure to
the human population (including sensitive subgroups) that is likely to
be without an appreciable risk of deleterious effects during a
lifetime.'' Alternatively, in cases where an RfC from the EPA's IRIS
database is not available or where the EPA determines that using a
value other than the RfC is appropriate, the chronic reference level
can be a value from the following prioritized sources: (1) The Agency
for Toxic Substances and Disease Registry (ATSDR) Minimum Risk Level
(http://www.atsdr.cdc.gov/mrls/index.asp), which is defined as ``an
estimate of daily human exposure to a hazardous substance that is
likely to be without an appreciable risk of adverse non-cancer health
effects (other than cancer) over a specified duration of exposure'';
(2) the CalEPA Chronic Reference Exposure Level (REL) (http://www.oehha.ca.gov/air/hot_spots/pdf/HRAguidefinal.pdf), which is defined
as ``the concentration level (that is expressed in units of micrograms
per cubic meter ([mu]g/m\3\) for inhalation exposure and in a dose
expressed in units of milligram per kilogram-day 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.
    As mentioned above, in order to characterize non-cancer chronic
effects, and in response to key recommendations from the SAB, the EPA
selects dose-response values that reflect the best available science
for all HAP included in RTR risk assessments.\10\ More specifically,
for a given HAP, the EPA examines the availability of inhalation
reference values from the sources included in our tiered approach
(e.g., IRIS first, ATSDR second, CalEPA third) and determines which
inhalation reference value represents the best available science. Thus,
as new inhalation reference values become available, the EPA will
typically evaluate them and determine whether they should be given
preference over those currently being used in RTR risk assessments.
---------------------------------------------------------------------------

    \10\ 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.
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    The EPA also evaluated screening estimates of acute exposures and
risks for each of the HAP at the point of highest potential off-site
exposure for each facility. To do this, the EPA estimated the risks
when both the peak hourly emissions rate and worst-case dispersion
conditions occur. We also assume that a person is located at the point
of highest impact during that same time. In accordance with our mandate
in section 112 of the CAA, we use the point of highest off-site
exposure to assess the potential risk to the maximally exposed
individual. The acute HQ is the estimated acute exposure divided by the
acute dose-response value. In each case, the EPA calculated acute HQ
values using best available, short-term dose-response values. These
acute dose-response values, which are described below, include the
acute REL, acute exposure guideline levels (AEGL) and emergency
response planning guidelines (ERPG) for 1-hour exposure durations. As
discussed below, we used conservative assumptions for emissions rates,
meteorology and exposure location for our acute analysis.

    As described in the CalEPA's Air Toxics Hot Spots Program Risk
Assessment Guidelines, Part I, The Determination of Acute Reference
Exposure Levels for Airborne Toxicants, an acute REL value (http://www.oehha.ca.gov/air/pdf/acuterel.pdf) is defined as ``the
concentration level at or below which no adverse health effects are
anticipated for a specified exposure duration.'' Id. at page 2. Acute
REL values are based on the most sensitive, relevant, adverse health
effect reported in the peer-reviewed medical and toxicological
literature. Acute REL values are designed to protect the most sensitive
individuals in the population through the inclusion of margins of
safety. Because margins of safety are incorporated to address data gaps
and uncertainties, exceeding the REL does not automatically indicate an
adverse health impact.
    AEGL values were derived in response to recommendations from the
National Research Council (NRC). As described in Standing Operating
Procedures (SOP) of the National Advisory Committee on Acute Exposure
Guideline Levels for Hazardous Substances (http://www.epa.gov/oppt/aegl/pubs/sop.pdf),\11\ ``the NRC's previous name for acute exposure
levels--community emergency exposure levels--was replaced by the term
AEGL to reflect the broad application of these values to planning,
response and prevention in the community, the workplace,
transportation, the military and the remediation of Superfund sites.''
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.
---------------------------------------------------------------------------

    \11\ National Academy of Sciences (NAS), 2001. Standing
Operating Procedures for Developing Acute Exposure Levels for
Hazardous Chemicals, page 2.
---------------------------------------------------------------------------

    The document lays out the purpose and objectives of AEGL by stating
that ``the primary purpose of the AEGL program and the National
Advisory Committee for Acute Exposure Guideline Levels for Hazardous
Substances is to develop guideline levels for once-in-a-lifetime,
short-term exposures to airborne concentrations of acutely toxic, high-
priority chemicals.'' Id. at 21. In detailing the intended application
of AEGL values, the document states that ``[i]t is anticipated that the
AEGL values will be used for regulatory and nonregulatory purposes by
U.S. federal and state agencies and possibly the international
community in conjunction with chemical emergency response, planning and
prevention programs. More specifically, the AEGL values will be used
for conducting various risk assessments to aid in the development of
emergency preparedness and prevention plans, as well as real-time
emergency response actions, for accidental chemical releases at fixed
facilities and from transport carriers.'' Id. at 31.
    The AEGL-1 value is then specifically defined as ``the airborne
concentration (expressed as ppm (parts per million) or mg/m\3\
(milligrams per cubic meter)) of a substance above which it is
predicted that the general population, including susceptible
individuals, could experience notable discomfort, irritation, or
certain asymptomatic nonsensory effects. However, the effects are not
disabling and are transient and reversible upon cessation of
exposure.'' Id. at 3. The document also notes that, ``Airborne
concentrations below AEGL-1 represent exposure levels that can produce
mild and progressively increasing but transient and nondisabling odor,
taste, and sensory irritation or certain asymptomatic, nonsensory
effects.'' Id. Similarly, the document defines AEGL-2 values as ``the
airborne concentration (expressed as parts per million or milligrams
per cubic meter) of a substance above which it is predicted that the
general population, including susceptible individuals, could experience
irreversible or other serious, long-lasting adverse health effects or
an impaired ability to escape.'' Id.
    ERPG values are derived for use in emergency response, as described
in the American Industrial Hygiene Association's Emergency Response
Planning Committee document titled, ERPGS Procedures and
Responsibilities (http://sp4m.aiha.org/insideaiha/GuidelineDevelopment/ERPG/Documents/ERP-SOPs2006.pdf), which states that, ``Emergency
Response Planning Guidelines were developed for emergency planning and
are intended as health based guideline concentrations for single
exposures to chemicals.'' \12\ 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.
---------------------------------------------------------------------------

    \12\ ERP Committee Procedures and Responsibilities. November 1,
2006. American Industrial Hygiene Association.
---------------------------------------------------------------------------

    As can be seen from the definitions above, the AEGL and ERPG values
include the similarly defined severity levels 1 and 2. For many
chemicals, a severity level 1 value AEGL or ERPG has not been developed
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.\13\ 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.
---------------------------------------------------------------------------

    \13\ See http://www.tceq.state.tx.us/compliance/field_ops/eer/index.html or the docket to access the source of these data.
---------------------------------------------------------------------------

    For this source category, the default value was not utilized. A
peak 1-hour emission multiplier of 1.2 times the annual emissions was
utilized for the entire source category. This value was developed from
current and historical industry capacity utilization factors. The
emissions from this category are generally dependent on the amount of
HAP in the coatings and the amount of coating applied, and would only
vary in a significant manner if production increased. Therefore, the
EPA based the acute emissions multiplier on potential changes in
production. The acute emissions multiplier is based on the difference
between 2008 production utilization rate of 83.1 percent and the
maximum production utilization rate of 100 percent, which has not been
realized in 20 years of historical data (100 / 83.1 = 1.2). The docket
for this rulemaking contains information on the development of
estimated MACT-acute emissions in the Modeling File Preparation Memo. A
further discussion of why this factor was chosen can be found in
Appendix 1 of the Modeling File Preparation Memo, available in the
docket for this rulemaking.
    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 this source
category, the data refinements employed consisted of evaluating the
off-site extent of any exceedances of the acute health benchmarks.
These refinements are discussed more fully in the Modeling File
Preparation Memo, which is available in the docket for this source
category. 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,\14\ 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 \15\ for
HAP have been developed, we consider additional acute values (i.e.,
occupational and international values) to provide a more complete risk
characterization.
---------------------------------------------------------------------------

    \14\ 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.
    \15\ U.S. EPA. (2009) Chapter 2.9, Chemical Specific Reference
Values for Formaldehyde, in Graphical Arrays of Chemical-Specific
Health Effect Reference Values for Inhalation Exposures (Final
Report). U.S. Environmental Protection Agency, Washington, DC, EPA/
600/R-09/061, and available online at http://cfpub.epa.gov/ncea/cfm/recordisplay.cfm?deid=211003.
---------------------------------------------------------------------------

4. How did we conduct the multipathway exposure and risk screening?
    The EPA conducted a screening analysis examining the potential for
significant human health risks due to exposures via routes other than
inhalation (i.e., ingestion). We first determined whether any sources
in the Aerospace Manufacturing and Rework Facilities source category
emitted any HAP known to be persistent and bioaccumulative in the
environment (PB-HAP). The PB-HAP compounds or compound classes are
identified for the screening from the EPA's Air Toxics Risk Assessment
Library (available at http://www2.epa.gov/fera/risk-assessment-and-modeling-air-toxics-risk-assessment-reference-library).
    For the Aerospace Manufacturing and Rework Facilities source
category, we identified emissions of cadmium, dioxins/furans, POM,
mercury (both inorganic mercury and methyl mercury) and lead compounds.
Because one or more of these PB-HAP are emitted by at least one
facility in the Aerospace Manufacturing and Rework Facilities source
category, we proceeded to the next step of the evaluation. In this
step, we determined whether the facility-specific emissions rates of
the emitted PB-HAP were large enough to create the potential for
significant non-inhalation human health risks under reasonable worst-
case conditions. To facilitate this step, we developed emissions rate
screening levels for several PB-HAP using a hypothetical upper-end
screening exposure scenario developed for use in conjunction with the
EPA's Total Risk Integrated Methodology.Fate, Transport, and Ecological
Exposure (TRIM.FaTE) model. The PB-HAP with emissions rate screening
levels are: Lead, cadmium, chlorinated dibenzodioxins and furans,
mercury compounds and POM. We conducted a sensitivity analysis on the
screening scenario to ensure that its key design parameters would
represent the upper end of the range of possible values, such that it
would represent a conservative, but not impossible scenario. The
facility-specific emissions rates of these PB-HAP were compared to the
emission rate screening levels for these PB-HAP to assess the potential
for significant human health risks via non-inhalation pathways. We call
this application of the TRIM.FaTE model the Tier 1 TRIM-screen or Tier
1 screen.
    For the purpose of developing emissions rates for our Tier 1 TRIM-
screen, we derived emission levels for these PB-HAP (other than lead
compounds) at which the maximum excess lifetime cancer risk would be 1-
in-1 million (i.e., for polychlorinated dibenzodioxins and furans and
POM) or, for HAP that cause non-cancer health effects (i.e., cadmium
compounds and mercury compounds), the maximum HQ would be 1. If the
emissions rate of any PB-HAP included in the Tier 1 screen exceeds the
Tier 1 screening emissions rate for any facility, we conduct a second
screen, which we call the Tier 2 TRIM-screen or Tier 2 screen.
    In the Tier 2 screen, the location of each facility that exceeded
the Tier 1 emission rate is used to refine the assumptions associated
with the environmental scenario while maintaining the exposure scenario
assumptions. A key assumption that is part of the Tier 1 screen is that
a lake is located near the facility; we confirm the existence of lakes
near the facility as part of the Tier 2 screen. We then adjust the
risk-based Tier 1 screening level for each PB-HAP for each facility
based on an understanding of how exposure concentrations estimated for
the screening scenario change with meteorology and environmental
assumptions. PB-HAP emissions that do

not exceed these new Tier 2 screening levels are considered to pose no
unacceptable risks. If the PB-HAP emissions for a facility exceed the
Tier 2 screening emissions rate and data are available, we may decide
to conduct a more refined Tier 3 multipathway assessment. There are
several analyses that can be included in a Tier 3 screen depending upon
the extent of refinement warranted, including validating that the lake
is fishable and considering plume-rise to estimate emissions lost above
the mixing layer. If the Tier 3 screen is exceeded, the EPA may further
refine the assessment.
    In evaluating the potential multipathway risk from emissions of
lead compounds, rather than developing a screening emissions rate for
them, we compared maximum estimated chronic inhalation exposures with
the level of the current National Ambient Air Quality Standard (NAAQS)
for lead.\16\ Values below the level of the primary (health-based) lead
NAAQS were considered to have a low potential for multipathway risk.
---------------------------------------------------------------------------

    \16\ In doing so, EPA notes that the legal standard for a
primary NAAQS--that a standard is requisite to protect public health
and provide an adequate margin of safety (CAA section 109(b))--
differs from the CAA section 112(f) standard (requiring among other
things that the standard provide an ``ample margin of safety'').
However, the lead NAAQS is a reasonable measure of determining risk
acceptability (i.e., the first step of the Benzene NESHAP analysis)
since it is designed to protect the most susceptible group in the
human population--children, including children living near major
lead emitting sources (73 FR 67002/3; 73 FR 67000/3; 73 FR 67005/1).
In addition, applying the level of the primary lead NAAQS at the
risk acceptability step is conservative, since that primary lead
NAAQS reflects an adequate margin of safety.
---------------------------------------------------------------------------

    For further information on the multipathway analysis approach, see
the Residual Risk Assessment for the Aerospace Manufacturing and Rework
Facilities Source Category in Support of the January, 2015 Risk and
Technology Review Proposal, January 2015, which is available in the
docket for this action.
5. How did we conduct the environmental risk screening assessment?
a. Adverse Environmental Effect
    The EPA conducts a screening assessment to examine the potential
for adverse environmental effects as required under section
112(f)(2)(A) of the CAA. Section 112(a)(7) of the CAA defines ``adverse
environmental effect'' as ``any significant and widespread adverse
effect, which may reasonably be anticipated, to wildlife, aquatic life
or other natural resources, including adverse impacts on populations of
endangered or threatened species or significant degradation of
environmental quality over broad areas.''
b. Environmental HAP
    The EPA focuses on seven HAP, which we refer to as ``environmental
HAP,'' in its screening analysis: Five PB-HAP and two acid gases. The
five PB-HAP are cadmium, dioxins/furans, POM, mercury (both inorganic
mercury and methyl mercury) and lead compounds. 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 nationally from stationary sources (on a mass basis from
the 2005 NEI).
    In addition to accounting for almost all of the mass of PB-HAP
emitted, we note that the TRIM.FaTE model that we use to evaluate
multipathway risk allows us to estimate concentrations of cadmium
compounds, dioxins/furans, POM and mercury in soil, sediment and water.
For lead compounds, we currently do not have the ability to calculate
these concentrations using the TRIM.FaTE model. Therefore, to evaluate
the potential for adverse environmental effects from lead compounds, we
compare the estimated HEM-modeled exposures from the source category
emissions of lead with the level of the secondary NAAQS for lead.\17\
We consider values below the level of the secondary NAAQS for lead to
be unlikely to cause adverse environmental effects.
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    \17\ The secondary NAAQS for lead is a reasonable measure of
determining whether there is an adverse environmental effect since
it was established considering ``effects on soils, water, crops,
vegetation, man-made materials, animals, wildlife, weather,
visibility and climate, damage to and deterioration of property, and
hazards to transportation, as well as effects on economic values and
on personal comfort and well-being.''
---------------------------------------------------------------------------

    Due to their well-documented potential to cause direct damage to
terrestrial plants, we include two acid gases, HCl and HF, in the
environmental screening analysis. According to the 2005 NEI, HCl and HF
account for about 99 percent (on a mass basis) of the total acid gas
HAP emitted by stationary sources in the U.S. In addition to the
potential to cause direct damage to plants, high concentrations of HF
in the air have been linked to fluorosis in livestock. Air
concentrations of these HAP are already calculated as part of the human
multipathway exposure and risk screening analysis using the HEM3-AERMOD
air dispersion model, and we are able to use the air dispersion
modeling results to estimate the potential for an adverse environmental
effect.
    The EPA acknowledges that other HAP beyond the seven HAP discussed
above may have the potential to cause adverse environmental effects.
Therefore, the EPA may include other relevant HAP in its environmental
risk screening in the future, as modeling science and resources allow.
The EPA invites comment on the extent to which other HAP emitted by the
source category may cause adverse environmental effects. Such
information should include references to peer-reviewed ecological
effects benchmarks that are of sufficient quality for making regulatory
decisions, as well as information on the presence of organisms located
near facilities within the source category that such benchmarks
indicate could be adversely affected.
c. Ecological Assessment Endpoints and Benchmarks for PB-HAP
    An important consideration in the development of the EPA's
screening methodology is the selection of ecological assessment
endpoints and benchmarks. Ecological assessment endpoints are defined
by the ecological entity (e.g., aquatic communities including fish and
plankton) and its attributes (e.g., frequency of mortality). Ecological
assessment endpoints can be established for organisms, populations,
communities or assemblages and ecosystems.
    For PB-HAP (other than lead compounds), we evaluated the following
community-level ecological assessment endpoints to screen for organisms
directly exposed to HAP in soils, sediment and water:
     Local terrestrial communities (i.e., soil invertebrates,
plants) and populations of small birds and mammals that consume soil
invertebrates exposed to PB-HAP in the surface soil.
     Local benthic (i.e., bottom sediment dwelling insects,
amphipods, isopods and crayfish) communities exposed to

PB-HAP in sediment in nearby water bodies.
     Local aquatic (water-column) communities (including fish
and plankton) exposed to PB-HAP in nearby surface waters.
    For PB-HAP (other than lead compounds), we also evaluated the
population-level ecological assessment endpoint to screen for indirect
HAP exposures of top consumers via the bioaccumulation of HAP in food
chains. The endpoint evaluated was 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 [micro]g 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., National Oceanic and Atmospheric
Administration (NOAA)) or state agencies.
    Benchmarks for all effect levels are not available for all PB-HAP
and assessment endpoints. In cases where multiple effect levels were
available for a particular PB-HAP and assessment endpoint, we use all
of the available effect levels to help us to determine whether
ecological risks exist and, if so, whether the risks could be
considered significant and widespread.
d. Ecological Assessment Endpoints and Benchmarks for Acid Gases
    The environmental screening analysis also evaluated potential
damage and reduced productivity of plants due to direct exposure to
acid gases in the air. For acid gases, we evaluated the ecological
assessment endpoint of local terrestrial plant communities with foliage
exposed to acidic gaseous HAP in the air.
    The selection of ecological benchmarks for the effects of acid
gases on plants followed the same approach as for PB-HAP (i.e., we
examine all of the available chronic benchmarks). For HCl, the EPA
identified chronic benchmark concentrations. We note that the benchmark
for chronic HCl exposure to plants is greater than the reference
concentration for chronic inhalation exposure for human health. This
means that where the EPA includes regulatory requirements to prevent an
exceedance of the reference concentration for human health, additional
analyses for adverse environmental effects of HCl would not be
necessary.
    For HF, the EPA identified chronic benchmark concentrations for
plants and evaluated chronic exposures to plants in the screening
analysis. High concentrations of HF in the air have also been linked to
fluorosis in livestock. However, the HF concentrations at which
fluorosis in livestock occur are higher than those at which plant
damage begins. Therefore, the benchmarks for plants are protective of
both plants and livestock.
e. Screening Methodology
    For the environmental risk screening analysis, the EPA first
determined whether any facilities in the Aerospace Manufacturing and
Rework Facilities source category emitted any of the seven
environmental HAP. For the Aerospace Manufacturing and Rework
Facilities source category, we identified emissions of five PB-HAP and
two acid gases as the environmental HAP. The five PB-HAP are cadmium,
dioxins/furans, POM, mercury (both inorganic mercury and methyl
mercury) and lead compounds. The two acid gases are HCl and HF.
    Because one or more of the seven environmental HAP evaluated are
emitted by at least one facility in the 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, while lead compounds are
analyzed differently as discussed earlier. In the first tier, we
determined whether the maximum facility-specific emission rates of each
of the emitted environmental HAP 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 level emission rate that corresponded to the
relevant exposure benchmark concentration value for each assessment
endpoint. To assess emissions from a facility, the reported emission
rate for each PB-HAP was compared to the screening level emission rate
for that PB-HAP for each assessment endpoint. If emissions from a
facility do not exceed the Tier 1 screening level, the facility
``passes'' the screen, and, therefore, is not evaluated further under
the screening approach. If emissions from a facility exceed the Tier 1
screening level, we evaluate the facility further in Tier 2.
    In Tier 2 of the environmental screening analysis, the emission
rate screening levels are adjusted to account for local meteorology and
the actual location of lakes in the vicinity of facilities that did not
pass the Tier 1 screen. The modeling domain for each facility in the
Tier 2 analysis consists of eight octants. Each octant contains five
modeled soil concentrations at various distances from the facility (5
soil concentrations x 8 octants = total of 40 soil concentrations per
facility) and one lake with modeled concentrations for water, sediment
and fish tissue. In the Tier 2 environmental risk screening analysis,
the 40 soil concentration points are averaged to obtain an average soil
concentration for each facility for each PB-HAP. For the water,
sediment and fish tissue concentrations, the highest value for each
facility for each pollutant is used. If emission concentrations from a
facility do not exceed the Tier 2 screening level, the facility passes
the screen, and typically is not evaluated further. If emissions from a
facility exceed the Tier 2 screening level, the facility does not

pass the screen and, therefore, may have the potential to cause adverse
environmental effects. Such facilities are evaluated further to
investigate factors such as the magnitude and characteristics of the
area of exceedance.
g. Acid Gas Methodology
    The environmental screening analysis evaluates the potential
phytotoxicity and reduced productivity of plants due to chronic
exposure to acid gases. The environmental risk screening methodology
for acid gases is a single-tier screen that compares the average off-
site ambient air concentration over the modeling domain to ecological
benchmarks for each of the acid gases. Because air concentrations are
compared directly to the ecological benchmarks, emission-based
screening levels are not calculated for acid gases as they are in the
ecological risk screening methodology for PB-HAP.
    For purposes of ecological risk screening, the EPA identifies a
potential for adverse environmental effects to plant communities from
exposure to acid gases when the average concentration of the HAP around
a facility exceeds the LOAEL ecological benchmark. In such cases, we
further investigate factors such as the magnitude and characteristics
of the area of exceedance (e.g., land use of exceedance area, size of
exceedance area) to determine if there is an adverse environmental
effect.
    For further information on the environmental screening analysis
approach, see the Residual Risk Assessment for the Aerospace
Manufacturing and Rework Facilities Source Category in Support of the
January, 2015 Risk and Technology Review Proposal, January 2015, which
is available in the docket for this action.
6. How did we conduct facility-wide assessments?
    To put the source category risks in context, we typically examine
the risks from the entire ``facility,'' where the facility includes all
HAP-emitting operations within a contiguous area and under common
control. In other words, we examine the HAP emissions not only from the
source category emission points of interest, but also emissions of HAP
from all other emission sources at the facility for which we have data.
    The emissions inventories developed from the 2011 and 2012 ICRs,
2011 NEI and 2005 NATA include emissions information for all emissions
sources at the facilities that are part of the Aerospace Manufacturing
and Rework Facilities source category. These include sources currently
regulated by the Aerospace NESHAP: Primer/topcoat application
operations, waste handling operations, chemical milling maskant
application operations, cleaning operations and chemical and blast
depainting operations. These also include emission sources not
currently regulated by the Aerospace NESHAP: Specialty coatings,
composite processing, chemical milling and metal finishing, wastewater,
storage tanks, boilers, engines, chemical manufacturing processes,
secondary metal production processes, heaters, soil remediation,
transportation equipment, waste disposal, welding and other
miscellaneous manufacturing processes.
    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 the source category addressed in this proposal. We
specifically examined the facility that was associated with the highest
estimate of risk and determined the percentage of that risk
attributable to the source category of interest. The Residual Risk
Assessment for the Aerospace Manufacturing and Rework Facilities Source
Category in Support of the January, 2015 Risk and Technology Review
Proposal, January 2015, available through the docket for this action,
provides the methodology and results of the facility-wide analyses,
including all facility-wide risks and the percentage of source category
contribution to facility-wide risks.
7. How did we consider uncertainties in risk assessment?
    In the Benzene NESHAP, we concluded that risk estimation
uncertainty should be considered in our decision-making under the ample
margin of safety framework. Uncertainty and the potential for bias are
inherent in all risk assessments, including those performed for this
proposal. Although uncertainty exists, we believe that our approach,
which used conservative tools and assumptions, ensures that our
decisions are health protective and environmentally protective. A brief
discussion of the uncertainties in the RTR emissions dataset,
dispersion modeling, inhalation exposure estimates and dose-response
relationships follows below. A more thorough discussion of these
uncertainties is included in the Residual Risk Assessment for the
Aerospace Manufacturing and Rework Facilities Source Category in
Support of the January, 2015 Risk and Technology Review Proposal,
January 2015, which is available in the docket for this action.
a. Uncertainties in the RTR Emissions Dataset
    Although the development of the RTR emissions dataset involved
quality assurance/quality control processes, the accuracy of emissions
values will vary depending on the source of the data, the degree to
which data are incomplete or missing, the degree to which assumptions
made to complete the datasets are accurate, and errors in emission
estimates and other factors. The emission estimates considered in this
analysis are annual totals for certain years, and they do not reflect
short-term fluctuations during the course of a year or variations from
year to year. The estimates of peak hourly emission rates for the acute
effects screening assessment were based on an emission adjustment
factor applied to the average annual hourly emission rates, which are
intended to account for emission fluctuations due to normal facility
operations.
b. Uncertainties in Dispersion Modeling
    We recognize there is uncertainty in ambient concentration
estimates associated with any model, including the EPA's recommended
regulatory dispersion model, AERMOD. In using a model to estimate
ambient pollutant concentrations, the user chooses certain options to
apply. For RTR assessments, we select some model options that have the
potential to overestimate ambient air concentrations (e.g., not
including plume depletion or pollutant transformation). We select other
model options that have the potential to underestimate ambient impacts
(e.g., not including building downwash). Other options that we select
have the potential to either under- or overestimate ambient levels
(e.g., meteorology and receptor locations). On balance, considering the
directional nature of the uncertainties commonly present in ambient
concentrations estimated by dispersion models, the approach we apply in
the RTR assessments should yield unbiased estimates of ambient HAP
concentrations.
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 micro-environment
to another over the course of hours or days. Long-term mobility is
movement from one residence to another over the course of a
lifetime.
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    In addition, the assessment predicted the chronic exposures at the
centroid of each populated census block as surrogates for the exposure
concentrations for all people living in that block. Using the census
block centroid to predict chronic exposures tends to over-predict
exposures for people in the census block who live farther from the
facility and under-predict exposures for people in the census block who
live closer to the facility. Thus, using the census block centroid to
predict chronic exposures may lead to a potential understatement or
overstatement of the true maximum impact, but is an unbiased estimate
of average risk and incidence. We reduce this uncertainty by analyzing
large census blocks near facilities using aerial imagery and adjusting
the location of the block centroid to better represent the population
in the block, as well as adding additional receptor locations where the
block population is not well represented by a single location.
    The assessment evaluates the cancer inhalation risks associated
with pollutant exposures over a 70-year period, which is the assumed
lifetime of an individual. In reality, both the length of time that
modeled emission sources at facilities actually operate (i.e., more or
less than 70 years) and the domestic growth or decline of the modeled
industry (i.e., the increase or decrease in the number or size of
domestic facilities) will influence the future risks posed by a given
source or source category. Depending on the characteristics of the
industry, these factors will, in most cases, result in an overestimate
both in individual risk levels and in the total estimated number of
cancer cases. However, in the unlikely scenario where a facility
maintains, or even increases, its emissions levels over a period of
more than 70 years, residents live beyond 70 years at the same
location, and the residents spend most of their days at that location,
then the cancer inhalation risks could potentially be underestimated.
However, annual cancer incidence estimates from exposures to emissions
from these sources would not be affected by the length of time an
emissions source operates.
    The exposure estimates used in these analyses assume chronic
exposures to ambient (outdoor) levels of pollutants. Because most
people spend the majority of their time indoors, actual exposures may
not be as high, depending on the characteristics of the pollutants
modeled. For many of the HAP, indoor levels are roughly equivalent to
ambient levels, but for very reactive pollutants or larger particles,
indoor levels are typically lower. This factor has the potential to
result in an overestimate of 25 to 30 percent of exposures.\19\
---------------------------------------------------------------------------

    \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 the EPA
conducts as part of the risk review under section 112 of the CAA that
should be highlighted. The accuracy of an acute inhalation exposure
assessment depends on the simultaneous occurrence of independent
factors that may vary greatly, such as hourly emissions rates,
meteorology and the presence of humans at the location of the maximum
concentration. In the acute screening assessment that we conduct under
the RTR program, we assume that peak emissions from the source category
and worst-case meteorological conditions co-occur, thus, resulting in
maximum ambient concentrations. These two events are unlikely to occur
at the same time, making these assumptions conservative. We then
include the additional assumption that a person is located at this
point during this same time period. For this source category, these
assumptions would tend to be worst-case actual exposures as it is
unlikely that a person would be located at the point of maximum
exposure during the time when peak emissions and worst-case
meteorological conditions occur simultaneously.
d. Uncertainties in Dose-Response Relationships
    There are uncertainties inherent in the development of the dose-
response values used in our risk assessments for cancer effects from
chronic exposures and non-cancer effects from both chronic and acute
exposures. Some uncertainties may be considered quantitatively, and
others generally are expressed in qualitative terms. We note as a
preface to this discussion a point on dose-response uncertainty that is
brought out in the EPA's 2005 Cancer Guidelines; namely, that ``the
primary goal of EPA actions is protection of human health; accordingly,
as an Agency policy, risk assessment procedures, including default
options that are used in the absence of scientific data to the
contrary, should be health protective'' (EPA 2005 Cancer Guidelines,
pages 1-7). This is the approach followed here as summarized in the
next several paragraphs. A complete detailed discussion of
uncertainties and variability in dose-response relationships is given
in the Residual Risk Assessment for the Aerospace Manufacturing and
Rework Facilities Source Category in Support of the January, 2015 Risk
and Technology Review Proposal, January 2015, which is available in the
docket for this action.
    Cancer URE values used in our risk assessments are those that have
been developed to generally provide an upper bound estimate of risk.
That is, they represent a ``plausible upper limit to the true value of
a quantity'' (although this is usually not a true statistical
confidence limit).\20\ In some circumstances, the true risk could be as
low as zero; however, in other circumstances the risk could be
greater.\21\ When developing an upper bound estimate of risk and to
provide risk values that do not underestimate risk, health-protective
default approaches are generally used. To err on the side of ensuring
adequate health protection, the EPA typically uses the upper bound
estimates rather than lower bound or central tendency estimates in our
risk assessments, an approach that may have limitations for other uses
(e.g., priority-setting or expected benefits analysis).
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    \20\ IRIS glossary (http://www.epa.gov/NCEA/iris/help_gloss.htm).
    \21\ An exception to this is the URE for benzene, which is
considered to cover a range of values, each end of which is
considered to be equally plausible, and which is based on maximum
likelihood estimates.
---------------------------------------------------------------------------

    Chronic non-cancer RfC and reference dose (RfD) values represent
chronic exposure levels that are intended to be health-protective
levels. Specifically, these values provide an estimate (with
uncertainty spanning perhaps an order of magnitude) of a continuous
inhalation exposure (RfC) or a daily oral exposure (RfD) to the human
population

(including sensitive subgroups) that is likely to be without an
appreciable risk of deleterious effects during a lifetime. To derive
values that are intended to be ``without appreciable risk,'' the
methodology relies upon an uncertainty factor (UF) approach (U.S. EPA,
1993, 1994) 22 23 which considers uncertainty, variability
and gaps in the available data. The UF are applied to derive reference
values that are intended to protect against appreciable risk of
deleterious effects. The UF are commonly default values \24\ (e.g.,
factors of 10 or 3) used in the absence of compound-specific data;
where data are available, UF may also be developed using compound-
specific information. When data are limited, more assumptions are
needed and more UF are used. Thus, there may be a greater tendency to
overestimate risk in the sense that further study might support
development of reference values that are higher (i.e., less potent)
because fewer default assumptions are needed. However, for some
pollutants, it is possible that risks may be underestimated.
---------------------------------------------------------------------------

    \22\ U.S. EPA. Reference Dose (RfD): Description and Use in
Health Risk Assessments. Dated March 1993.
    \23\ U.S. EPA. Methods for Derivation of Inhalation Reference
Concentrations and Application of Inhalation Dosimetry. EPA/600/8-
90/066F. Dated October 1994.
    \24\ According to the NRC report, Science and Judgment in Risk
Assessment (NRC, 1994) ``[Default] options are generic approaches,
based on general scientific knowledge and policy judgment, that are
applied to various elements of the risk assessment process when the
correct scientific model is unknown or uncertain.'' The 1983 NRC
report, Risk Assessment in the Federal Government: Managing the
Process, defined default option as ``the option chosen on the basis
of risk assessment policy that appears to be the best choice in the
absence of data to the contrary'' (NRC, 1983a, p. 63). Therefore,
default options are not rules that bind the agency; rather, the
agency may depart from them in evaluating the risks posed by a
specific substance when it believes this to be appropriate. In
keeping with EPA's goal of protecting public health and the
environment, default assumptions are used to ensure that risk to
chemicals is not underestimated (although defaults are not intended
to overtly overestimate risk). See EPA, 2004, An Examination of EPA
Risk Assessment Principles and Practices, EPA/100/B-04/001 available
at: http://www.epa.gov/osa/pdfs/ratf-final.pdf.
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    While collectively termed ``UF,'' these factors account for a
number of different quantitative considerations when using observed
animal (usually rodent) or human toxicity data in the development of
the RfC. The UF are intended to account for: (1) Variation in
susceptibility among the members of the human population (i.e., inter-
individual variability); (2) uncertainty in extrapolating from
experimental animal data to humans (i.e., interspecies differences);
(3) uncertainty in extrapolating from data obtained in a study with
less-than-lifetime exposure (i.e., extrapolating from sub-chronic to
chronic exposure); (4) uncertainty in extrapolating the observed data
to obtain an estimate of the exposure associated with no adverse
effects; and (5) uncertainty when the database is incomplete or there
are problems with the applicability of available studies.
    Many of the UF used to account for variability and uncertainty in
the development of acute reference values are quite similar to those
developed for chronic durations, but they more often use individual UF
values that may be less than 10. The UF are applied based on chemical-
specific or health effect-specific information (e.g., simple irritation
effects do not vary appreciably between human individuals, hence a
value of 3 is typically used), or based on the purpose for the
reference value (see the following paragraph). The UF applied in acute
reference value derivation include: (1) Heterogeneity among humans; (2)
uncertainty in extrapolating from animals to humans; (3) uncertainty in
lowest observed adverse effect (exposure) level to no observed adverse
effect (exposure) level adjustments; and (4) uncertainty in accounting
for an incomplete database on toxic effects of potential concern.
Additional adjustments are often applied to account for uncertainty in
extrapolation from observations at one exposure duration (e.g., 4
hours) to derive an acute reference value at another exposure duration
(e.g., 1 hour).
    Not all acute reference values are developed for the same purpose
and care must be taken when interpreting the results of an acute
assessment of human health effects relative to the reference value or
values being exceeded. Where relevant to the estimated exposures, the
lack of short-term dose-response values at different levels of severity
should be factored into the risk characterization as potential
uncertainties.
    For a group of compounds that are unspeciated (e.g., glycol
ethers), we conservatively use the most protective reference value of
an individual compound in that group to estimate risk. Similarly, for