[Federal Register Volume 79, Number 127 (Wednesday, July 2, 2014)]
[Proposed Rules]
[Pages 37849-37895]
[FR Doc No: 2014-13490]

Vol. 79

Wednesday,

No. 127

July 2, 2014

Part II

Environmental Protection Agency

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40 CFR Part 63

National Emission Standards for Hazardous Air Pollutants: Off-Site
Waste and Recovery Operations; Proposed Rule

Federal Register / Vol. 79 , No. 127 / Wednesday, July 2, 2014 /
Proposed Rules

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ENVIRONMENTAL PROTECTION AGENCY

40 CFR Part 63

[EPA-HQ-OAR-2012-0360; FRL-9911-93-0A]
RIN 2060-AR47

National Emission Standards for Hazardous Air Pollutants: Off-
Site Waste and Recovery Operations

AGENCY: Environmental Protection Agency.

ACTION: Proposed rule.

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SUMMARY: The Environmental Protection Agency (EPA) is proposing
amendments to the national emission standards for hazardous air
pollutants (NESHAP) for off-site waste and recovery operations (OSWRO)
to address the results of the residual risk and technology review (RTR)
conducted under the Clean Air Act (CAA). In light of our residual risk
and technology review, we are proposing to amend the requirements for
leak detection and repair and the requirements for certain tanks. In
addition, the EPA is proposing amendments to revise regulatory
provisions pertaining to emissions during periods of startup, shutdown
and malfunction; add requirements for electronic reporting of
performance test results; revise the routine maintenance provisions;
clarify provisions pertaining to open-ended valves and lines; add
monitoring requirements for pressure relief devices; clarify provisions
for some performance test methods and procedures; and make several
minor clarifications and corrections.

DATES:
    Comments. Comments must be received on or before August 18, 2014. A
copy of comments on the information collection provisions should be
submitted to the Office of Management and Budget (OMB) on or before
August 1, 2014.
    Public Hearing. We do not plan to conduct a public hearing unless
requested. If requested, we will hold a public hearing on July 17,
2014. To request a hearing, please contact the person listed in the
following FOR FURTHER INFORMATION CONTACT section by July 14, 2014.

ADDRESSES:
    Comments. Submit your comments, identified by Docket ID No. EPA-HQ-
OAR-2012-0360, by one of the following methods:
     Federal eRulemaking Portal: http://www.regulations.gov.
Follow the online instructions for submitting comments.
     Email<EMAIL_ADDRESS>Include Docket ID No. EPA-
HQ-OAR-2012-0360 in the subject line of the message.
     Fax:<PHONE_NUMBER>, Attention Docket ID No. EPA-HQ-OAR-
2012-0360.
     Mail: Environmental Protection Agency, EPA Docket Center
(EPA/DC), Mail Code 28221T, Attention Docket ID No. EPA-HQ-OAR-2012-
0360, 1200 Pennsylvania Avenue NW., Washington, DC 20460. Please
include a total of two copies. In addition, please mail a copy of your
comments on the information collection provisions to the Office of
Information and Regulatory Affairs, Office of Management and Budget
(OMB), Attn: Desk Officer for EPA, 725 17th Street NW., Washington, DC
20503.
     Hand/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-2012-0360. Such deliveries are only
accepted during the Docket's normal hours of operation, and special
arrangements should be made for deliveries of boxed information.
    Instructions. Direct your comments to Docket ID No. EPA-HQ-OAR-
2012-0360. 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 proposed rule
under Docket ID No. EPA-HQ-OAR-2012-0360. All documents in the docket
are listed in the regulations.gov index. Although listed in the index,
some information is not publicly available, e.g., CBI or other
information whose disclosure is restricted by statute. Certain other
material, such as copyrighted material, is not placed on the Internet
and will be publicly available only in hard copy. Publicly available
docket materials are available either electronically in regulations.gov
or in hard copy at the EPA Docket Center, WJC West Building, Room 3334,
1301 Constitution Ave., NW., Washington, DC. The Public Reading Room is
open from 8:30 a.m. to 4:30 p.m., Monday through Friday, excluding
legal holidays. The telephone number for the Public Reading Room is
(202) 566-1744, and the telephone number for the EPA Docket Center is
(202) 566-1742.
    Public Hearing. If requested, we will hold a public hearing
concerning this proposed rule on July 17, 2014 in the Research Triangle
Park, North Carolina area. The EPA will provide further information
about the hearing at the following Web site, http://www.epa.gov/ttn/oarpg/t3main.html, if a hearing is requested. Persons interested in
presenting oral testimony at the hearing should contact Ms. Virginia
Hunt, Sector Policies and Programs Division (E143-01), Office of Air
Quality Planning and Standards, U.S. Environmental Protection Agency,
Research Triangle Park, NC 27711, telephone number<PHONE_NUMBER>, by
July 17, 2014. If no one requests to speak at the public hearing by
July 14, 2014, then a public hearing will not be held, and a
notification of such will be posted on http://www.epa.gov/ttn/oarpg/t3main.html.

FOR FURTHER INFORMATION CONTACT: For questions about this proposed
action, contact Ms. Paula Hirtz, Sector Policies and Programs Division
(E143-01), Office of Air Quality Planning and Standards, U.S.
Environmental Protection Agency, Research Triangle Park, NC 27711;
telephone number:<PHONE_NUMBER>; fax number:<PHONE_NUMBER>; and email
address<EMAIL_ADDRESS>For specific information regarding the
risk modeling methodology, contact Ms. Darcie Smith, Health and
Environmental Impacts Division (C504-06), Office of Air Quality
Planning and

Standards, U.S. Environmental Protection Agency, Research Triangle
Park, NC 27711; telephone number<PHONE_NUMBER>; fax number: (919) 541-
0840; and email address<EMAIL_ADDRESS>For information about
the applicability of the National Emission Standards for Hazardous Air
Pollutants (NESHAP) to a particular entity, contact Ms. Marcia Mia, EPA
Office of Enforcement and Compliance Assurance, telephone number (202)
564-7042; email address<EMAIL_ADDRESS>
SUPPLEMENTARY INFORMATION:

Preamble Acronyms and Abbreviations

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

AEGL--acute exposure guideline levels
AERMOD--air dispersion model used by the HEM-3 model
CAA--Clean Air Act
CalEPA--California EPA
CBI--Confidential Business Information
CDX--Central Data Exchange
CEDRI--Compliance and Emissions Data Reporting Interface
CFR--Code of Federal Regulations
EPA--Environmental Protection Agency
ERPG--Emergency Response Planning Guidelines
ERT--Electronic Reporting Tool
FR--Federal Register
HAP--hazardous air pollutants
HCl--hydrochloric acid
HEM-3--Human Exposure Mdel, Version 1.1.0
HF--hydrogen fluoride
HI--hazard index
HON--Hazardous Organic NESHAP
HQ--hazard quotient
ICR--Information Collection Request
IRIS--Integrated Risk Information System
km--kilometer
kPa--kilopascal
LDAR--leak detection and repair
LOAEL--lowest-observed-adverse-effect level
MACT--maximum achievable control technology
m\3\--cubic meter
mg/kg-day--milligrams per kilogram per day
mg/m\3\--milligrams per cubic meter
MIR--maximum individual risk
NAAQS--National Ambient Air Quality Standards
NAICS--North American Industry Classification System
NAS--National Academy of Sciences
NATA--National Air Toxics Assessment
NESHAP--National Emissions Standards for Hazardous Air Pollutants
NOAA--National Oceanic and Atmospheric Organization
NOAEL--no-observed-adverse-effect level
NRC--National Research Council
NTTAA--National Technology Transfer and Advancement Act
OAQPS--Office of Air Quality Planning and Standards
OMB--Office of Management and Budget
OSWRO--off-site waste and recovery operations
PB-HAP--hazardous air pollutants known to be persistent and bio-
accumulative in the environment
PEL--probable effect levels
POM--polycyclic organic matter
ppm--parts per million
PRD-- pressure relief device
PTE--permanent total enclosure
RCO--recuperative thermal oxidizer
RCRA--Resource Conservation and Recovery Act
REL--reference exposure level
RFA--Regulatory Flexibility Act
RfC--reference concentration
RfD--reference dose
RIA--Regulatory Impact Analysis
RTR--residual risk and technology review
SAB--Science Advisory Board
SBA--Small Business Administration
SCC--source classification code
S/L/Ts--State, local and tribal air pollution control agencies
SOP--standard operating procedures
SSM--startup, shutdown and malfunction
TEQ--toxicity equivalence factor
TOC--total organic compound
TOSHI--target organ-specific hazard index
tpy--tons per year
TRIM.FaTE--Total Risk Integrated Methodology.Fate, Transport and
Ecological Exposure model
TSDF--Solid Waste Treatment, Storage and Disposal Facility
TTN--Technology Transfer Network
UF--uncertainty factor
UMRA--Unfunded Mandates Reform Act
URE--unit risk estimate
VCS--voluntary consensus standards

    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?
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 are the results of the risk assessment and analyses?
    B. What are our proposed decisions regarding risk acceptability,
ample margin of safety and adverse environmental effects?
    C. What are the results of the technology review and our
proposed decisions?
    D. What other actions are we proposing?
    E. 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
    D. Unfunded Mandates Reform Act
    E. Executive Order 13132: Federalism
    F. Executive Order 13175: Consultation and Coordination With
Indian Tribal Governments
    G. Executive Order 13045: Protection of Children From
Environmental Health Risks and Safety Risks
    H. Executive Order 13211: Actions Concerning Regulations That
Significantly Affect Energy Supply, Distribution, or Use
    I. National Technology Transfer and Advancement Act
    J. Executive Order 12898: Federal Actions to Address
Environmental Justice in Minority Populations and Low-Income
Populations

A red-line version of the regulatory language that incorporates the
proposed changes in this action is available in the docket for this
action (Docket ID No. EPA-HQ-OAR-2012-0360).

I. General Information

A. Does this action apply to me?

    Table 1 of this preamble lists the NESHAP and associated 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. The Off-site Waste and Recovery
Operations source category was initially titled the ``Solid Waste
Treatment, Storage, and Disposal Facilities (TSDF)'' source category,
which included commercial facilities that treat, store or dispose of
any solid waste received from off-site, as well as commercial
facilities that recycle, recover and re-refine wastes received from
off-site.\1\ On October 13,

1994 (59 FR 51913), the EPA explained that the source category was
intended to represent those off-site waste and recovery operations that
are not specifically listed as a separate distinct NESHAP source
category such as hazardous waste incineration or municipal solid waste
landfills and changed the title of the Solid Waste TSDF source category
to ``Off-Site Waste and Recovery Operations'' to avoid confusion, to
better distinguish this source category from other source categories,
and to emphasize that this source category addresses only activities
that manage wastes received from off-site.
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    \1\ See Initial List of Categories of Sources Under Section
112(c)(1) of the Clean Air Act Amendments of 1990 (57 FR 31576, July
16, 1992); U.S. EPA. Documentation for Developing the Initial Source
Category List (EPA-450/3-91-030; July 1992).

                Table 1--NESHAP and Industrial Source Categories Affected By This Proposed Action
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            Source category                        NESHAP                   Examples of regulated entities
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Off-Site Waste and Recovery Operations.  Off-Site Waste and          Businesses or government agencies that
                                          Recovery Operations.        operate any of the following: Hazardous
                                                                      waste TSDF; Resource Conservation and
                                                                      Recovery Act (RCRA) exempt hazardous
                                                                      wastewater treatment facilities;
                                                                      nonhazardous wastewater treatment
                                                                      facilities other than publicly-owned
                                                                      treatment works; used solvent recovery
                                                                      plants; RCRA exempt hazardous waste
                                                                      recycling operations; used oil re-
                                                                      refineries.
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    This table is not intended to be exhaustive, but rather is meant to
provide a guide for readers regarding entities likely to be affected by
this action. If you have any questions regarding the applicability of
this action to a particular entity, consult either the air permitting
authority for the entity or your EPA regional representative, as listed
in 40 CFR 63.13 (General Provisions).

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 on the TTN's policy and guidance page for newly proposed or
promulgated rules at: http://www.epa.gov/ttn/oarpg/t3pfpr.html.
Following publication in the Federal Register, the EPA will post the
Federal Register version of the proposal and key technical documents on
the project Web site: http://www.epa.gov/ttn/atw/offwaste/oswropg.html.
Information on the overall RTR program is available at the following
Web site: http://www.epa.gov/ttn/atw/rrisk/rtrpg.html.

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

    Submitting CBI. Do not submit information containing CBI to the EPA
through http://www.regulations.gov or email. Clearly mark the part or
all of the information that you claim to be CBI. For CBI information on
a disk or CD-ROM that you mail to the EPA, mark the outside of the disk
or CD-ROM as CBI and then identify electronically within the disk or
CD-ROM the specific information that is claimed as CBI. In addition to
one complete version of the comments that includes information claimed
as CBI, you must submit a copy of the comments that does not contain
the information claimed as CBI for inclusion in the public docket. If
you submit a CD-ROM or disk that does not contain CBI, mark the outside
of the disk or CD-ROM clearly that it does not contain CBI. Information
not marked as CBI will be included in the public docket and the EPA's
electronic public docket without prior notice. Information marked as
CBI will not be disclosed except in accordance with procedures set
forth in 40 Code of Federal Regulations (CFR) part 2. Send or deliver
information identified as CBI only to the following address: Roberto
Morales, OAQPS Document Control Officer (C404-02), OAQPS, U.S.
Environmental Protection Agency, Research Triangle Park, North Carolina
27711, Attention Docket ID No. EPA-HQ-OAR-2012-0360.

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 hazardous air pollutants (HAPs) from stationary
sources. In the first stage, after the EPA has identified categories of
sources emitting one or more of the HAP listed in CAA section 112(b),
CAA section 112(d) requires us to promulgate technology-based NESHAP
for those sources. ``Major sources'' are those that emit or have the
potential to emit 10 tons per year (tpy) or more of a single HAP or 25
tpy or more of any combination of HAPs. For major sources, the
technology-based NESHAP must reflect the maximum degree of emission
reductions of HAPs achievable (after considering cost, energy
requirements and non-air quality health and environmental impacts) and
are commonly referred to as maximum achievable control technology
(MACT) standards.
    MACT standards must reflect the maximum degree of emissions
reduction achievable through the application of measures, processes,
methods, systems or techniques, including, but not limited to, measures
that (1) reduce the volume of or eliminate pollutants through process
changes, substitution of materials or other modifications; (2) enclose
systems or processes to eliminate emissions; (3) capture or treat
pollutants when released from a process, stack, storage or fugitive
emissions point; (4) are design, equipment, work practice or
operational standards (including requirements for operator training or
certification); or (5) are a combination of the above. CAA section
112(d)(2)(A)-(E). The MACT standards may take the form of design,
equipment, work practice or operational standards where the EPA first
determines either that (1) a pollutant cannot be emitted through a
conveyance designed and constructed to emit or capture the pollutant,
or that any requirement for, or use of, such a conveyance would be
inconsistent with law; or (2) the application of measurement
methodology to a particular class of sources is not practicable due to
technological and economic limitations. CAA section 112(h)(1)-(2).
    The MACT ``floor'' is the minimum control level allowed for MACT
standards promulgated under CAA section 112(d)(3) and may not be based
on cost considerations. For new sources, the MACT floor cannot be less
stringent than the emissions control that is achieved in practice by
the best-

controlled similar source. The MACT floor for existing sources can be
less stringent than floors for new sources but not less stringent than
the average emissions limitation achieved by the best-performing 12
percent of existing sources in the category or subcategory (or the
best-performing five sources for categories or subcategories with fewer
than 30 sources). In developing MACT standards, the EPA must also
consider control options that are more stringent than the floor. We may
establish standards more stringent than the floor based on
considerations of the cost of achieving the emission reductions, any
non-air quality health and environmental impacts and energy
requirements.
    The EPA is required to review these technology-based standards and
revise them ``as necessary (taking into account developments in
practices, processes, and control technologies)'' no less frequently
than every eight years. CAA section 112(d)(6). In conducting this
review, the EPA is not required to recalculate the MACT floor. Natural
Resources Defense Council (NRDC) v. EPA, 529 F.3d 1077, 1084 (D.C. Cir.
2008). Association of Battery Recyclers, Inc. v. EPA, 716 F.3d 667
(D.C. Cir. 2013).
    The second stage in standard-setting focuses on reducing any
remaining (i.e., ``residual'') risk according to CAA section 112(f).
Section 112(f)(1) required EPA to 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. Section 112(f)(2) then
provides that if Congress does not act on any recommendation in the
Report, EPA must analyze and address residual risk for each category or
subcategory of sources within 8 years after promulgation of such
standards pursuant to section 112(d).
    Section 112(f)(2) of the CAA requires the EPA to determine for
source categories subject to MACT standards whether the emission
standards provide an ample margin of safety to protect public health.
Section 112(f)(2)(B) of the CAA expressly preserves the EPA's use of
the two-step process for developing standards to address any residual
risk and the agency's interpretation of ``ample margin of safety''
developed in the National Emissions Standards for Hazardous Air
Pollutants: Benzene Emissions from Maleic Anhydride Plants,
Ethylbenzene/Styrene Plants, Benzene Storage Vessels, Benzene Equipment
Leaks, and Coke By-Product Recovery Plants (Benzene NESHAP) (54 FR
38044, September 14, 1989). The EPA notified Congress in the Risk
Report that the agency intended to use the Benzene NESHAP approach in
making CAA section 112(f) residual risk determinations (EPA-453/R-99-
001, p. ES-11). The EPA subsequently adopted this approach in its
residual risk determinations and in a challenge to the risk review for
the Synthetic Organic Chemical Manufacturing source category, the
United States Court of Appeals for the District of Columbia Circuit
upheld as reasonable the EPA's interpretation that subsection 112(f)(2)
incorporates the approach established in the Benzene NESHAP. See NRDC
v. EPA, 529 F.3d 1077, 1083 (D.C. Cir. 2008)(``[S]ubsection
112(f)(2)(B) expressly incorporates the EPA's interpretation of the
Clean Air Act from the Benzene standard, complete with a citation to
the Federal Register.''); see also A Legislative History of the Clean
Air Act Amendments of 1990, vol. 1, p. 877 (Senate debate on Conference
Report).
    The first step in the process of evaluating residual risk is the
determination of acceptable risk. If risks are unacceptable, the EPA
cannot consider cost in identifying the emissions standards necessary
to bring risks to an acceptable level. The second step is the
determination of whether standards must be further revised in order to
provide an ample margin of safety to protect public health. The ample
margin of safety is the level at which the standards must be set,
unless an even more stringent standard is necessary to prevent, taking
into consideration costs, energy, safety and other relevant factors, an
adverse environmental effect.
1. Step 1--Determination of Acceptability
    The agency in the Benzene NESHAP concluded that ``the acceptability
of risk under section 112 is best judged on the basis of a broad set of
health risk measures and information'' and that the ``judgment on
acceptability cannot be reduced to any single factor.'' Benzene NESHAP
at 38046. The determination of what represents an ``acceptable'' risk
is based on a judgment of ``what risks are acceptable in the world in
which we live'' (Risk Report at 178, quoting NRDC v. EPA, 824 F. 2d
1146, 1165 (D.C. Cir. 1987) (en banc) (``Vinyl Chloride''), recognizing
that our world is not risk-free.
    In the Benzene NESHAP, we stated that ``EPA will generally presume
that if the risk to [the maximum exposed] individual is no higher than
approximately one in 10 thousand, that risk level is considered
acceptable.'' 54 FR at 38045, 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 section
112(f)(2) ``incorporates the EPA's interpretation of the Clean Air Act
from the Benzene Standard.'' The court further held that Congress'
incorporation of the Benzene standard applies equally to carcinogens
and non-carcinogens. 529 F.3d at 1081-82. Accordingly, we also consider
non-cancer risk metrics in our determination of risk acceptability and
ample margin of safety.
2. Step 2--Determination of Ample Margin of Safety
    CAA section 112(f)(2) requires the EPA to determine, for source
categories subject to MACT standards, whether those standards provide
an ample margin of safety to protect public health. As explained in the
Benzene NESHAP, ``the second step of the inquiry, determining an `ample
margin of safety,' again includes consideration of all of the health
factors, and whether to reduce the risks even further . . . . Beyond
that information, additional factors relating to the appropriate level
of control will also be considered, including costs and economic
impacts of controls, technological feasibility, uncertainties and any
other relevant factors. Considering all of these factors, the agency
will establish the standard at a level that provides an ample margin of
safety to protect the public health, as required by section 112.'' 54
FR at 38046, September 14, 1989.
    According to CAA section 112(f)(2)(A), if the MACT standards for
HAP ``classified as a known, probable, or possible human carcinogen do
not reduce lifetime excess cancer risks to the individual most exposed
to emissions from a source in the category or subcategory to less than
one in one million,'' the EPA must promulgate residual risk standards
for the source category (or subcategory), as necessary to provide an
ample margin of safety to protect public health. In doing so, the EPA
may adopt standards equal to existing MACT standards if the EPA
determines that the existing standards (i.e., the MACT standards) are
sufficiently protective. NRDC v. EPA, 529 F.3d 1077, 1083 (D.C. Cir.
2008) (``If EPA determines that the existing technology-based standards
provide an `ample margin of safety,' then the Agency is free to readopt
those standards during the residual risk rulemaking.'') The EPA must
also adopt more stringent standards, if necessary, to prevent an
adverse environmental effect,\2\ but must consider cost, energy, safety
and other relevant factors in doing so.
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    \2\ ``Adverse environmental effect'' is defined as any
significant and widespread adverse effect, which may be reasonably
anticipated to wildlife, aquatic life or natural resources,
including adverse impacts on populations of endangered or threatened
species or significant degradation of environmental qualities over
broad areas. CAA section 112(a)(7).
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    The CAA does not specifically define the terms ``individual most
exposed,'' ``acceptable level'' and ``ample margin of safety.'' In the
Benzene NESHAP, 54 FR at 38044-38045, September 14, 1989, we stated as
an overall objective:

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

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

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

    The NESHAP for OSWRO was proposed on October 13, 1994 (59 FR
51913), promulgated on July 1, 1996 (61 FR 34140), and codified at 40
CFR part 63, subpart DD. The final rule was amended on July 20, 1999
(64 FR 38950). In general, the rule applies to waste management units
and recovery operations that are: (1) Located at major sources of HAP
emissions; and (2) used to manage, convey or handle used oil, used
solvent or waste received from other facilities and that contain at
least one of 97 organic HAP specified in the rule.\3\ The HAP emission
sources at facilities subject to the OSWRO NESHAP are tanks,
containers, surface impoundments, oil-water separators, organic-water
separators, process vents and transfer systems used to manage off-site
material and equipment leaks. The MACT standards regulate these
emissions sources through emission limits, equipment standards and work
practices.
---------------------------------------------------------------------------

    \3\ The OSWRO MACT rule defines ``waste,'' ``used oil'' and
``used solvent'' in 40 CFR 63.681 Definitions.
---------------------------------------------------------------------------

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

    Under the authority of CAA section 114, we sent questionnaires to
nine companies that own and operate OSWRO facilities. In the CAA
section 114 questionnaires, we asked for information about process
equipment, control devices, work practices, associated emission
reductions, point and fugitive emissions, and other aspects of facility
operations. We visited three facilities, and reviewed permit data from
18 state and local agencies. In addition, we reviewed several EPA
databases to identify facilities that may be part of the source
category. We also reviewed data in the EPA's National Emissions
Inventory (NEI) to identify emission sources and quantities of
emissions and the Toxics Release Inventory (TRI) to verify emissions
estimates.
    The data gathered through these activities are described further in
the memorandum Development of the RTR Emissions Dataset for the Off-
Site Waste and Recovery Operations Source Category, which is available
in the docket for this proposed rule.

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 for the
source category. The eight sections that follow this paragraph describe
how we estimated emissions and conducted the risk assessment. The
docket for this proposed rule contains the following document which
provides more information on the risk assessment inputs and models:
Draft Residual Risk Assessment for the Off-Site Waste and Recovery
Operations Source Category. The methods used to assess risks (as
described in the eight primary steps below) are consistent with those
peer-reviewed by a panel of the EPA's Science Advisory Board (SAB) in
2009 and described in their peer review report issued in 2010 \4\; they
are also consistent with the key recommendations contained in that
report.
---------------------------------------------------------------------------

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

1. How did we estimate actual emissions and identify the emissions
release characteristics?
    Data for 38 OSWRO facilities were used to create an RTR emissions
dataset (i.e., risk model input file). This RTR emissions dataset is
based on a combination of data gathered through the CAA section 114
questionnaire and the 2005 NEI. 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. Other databases, including the TRI and Envirofacts,
were consulted to verify emissions estimates and to identify facilities
that are part of the OSWRO source category. As part of our quality
assurance review, we reviewed the emissions data and release
characteristics data in the RTR emissions dataset to ensure the data
were accurate. We also checked the coordinates of each emission source
in the dataset using tools such as Google Earth and ArcView to ensure
the emission point locations were correct.
    While data for 38 OSWRO facilities were included in the RTR
emissions dataset, available data indicate there are 52 currently
operating major source facilities that are subject to the OSWRO MACT
standards. The remaining 14 facilities were not included in the
modeling file because the information available to the EPA, including
the NEI, did not attribute any amount of HAP emissions to off-site
waste and recovery operations at these facilities. It was also not
possible to discern from the emission point identifiers or
characteristics in the inventory which emissions could be attributed to
the OSWRO source category. We note that available permit information
indicates that five of these 14 facilities are only subject to off-site
waste HAP content determination requirements and are not subject to the
emissions standards and other requirements of the OSWRO NESHAP due to
the low amount of HAP in the off-site waste accepted by these
facilities. Also, available permit data indicates that two additional
facilities are not subject to the emissions standards and other
requirements of the OSWRO NESHAP because they comply instead with 40
CFR part 61, subpart FF, as allowed by the OSWRO NESHAP. For these
seven facilities, we would not expect any emission points to be labeled
as OSWRO emission points in the NEI because those emission points are
not subject to any OSWRO MACT emissions standards. We also did not
collect data from these facilities through our CAA section 114
questionnaire. As noted in section VI of this preamble, we are
requesting site-specific emissions data that would enable us to better
characterize the maximum risks from the OSWRO source category. A list
of the 52 facilities and additional information about the development
of the RTR emissions dataset is provided in the technical document:
Development of the RTR Emissions Dataset for the Off-Site Waste and
Recovery Operations Source Category, which is available in the docket
for this action.
2. How did we estimate MACT-allowable emissions?
    The available emissions data in the RTR emissions dataset include
estimates of the mass of HAP emitted during the specified annual time
period. In some cases, these ``actual'' emission levels are lower than
the emission levels required to comply with the MACT standards. The
emissions level allowed to be emitted by the MACT standards is referred
to as the ``MACT-allowable'' emissions level. We discussed the use of
both MACT-allowable and actual emissions in the final Coke Oven
Batteries residual risk rule (70 FR 19998-19999, April 15, 2005) and in
the proposed and final Hazardous Organic NESHAP residual risk rules (71
FR 34428, June 14, 2006, and 71 FR 76609, December 21, 2006,
respectively). In those previous actions, we noted that assessing the
risks at the MACT-allowable level is inherently reasonable since these
risks reflect the maximum level facilities could emit and still comply
with national emission standards. We also explained that it is
reasonable to consider actual emissions, where such data are available,
in both steps of the risk analysis, in accordance with the Benzene
NESHAP approach. (54 FR 38044, September 14, 1989.)
    We used the emissions data gathered from the 2005 NEI and responses
to the CAA section 114 questionnaire to estimate the MACT-allowable
emissions levels. We estimate that the actual emissions level is
representative of the MACT-allowable level for all emissions sources
except tanks and process vents. Based on responses to the CAA section
114 questionnaire, we estimate that MACT-allowable emissions from tanks
and process vents could be up to five times the actual emissions. For
some facilities, we cannot assign HAP emissions to a specific type of
emission source (e.g., a process vent) due to a lack of specificity in
the emission point identifiers in the NEI. For facilities where we
could identify specific emission source types, we applied a factor of 5
to the actual emissions attributable to tanks and process vents. A
factor of 1 was applied to the actual emissions for other emissions
sources (e.g., equipment leaks). For facilities where we could not
identify specific emission source types, we developed and applied a
factor of 2.5 to all the OSWRO emissions. The 2.5 factor is

based on the factor of 5 for tanks and process vents and information
from the responses to the CAA section 114 questionnaire indicating that
tank and process vent emissions comprise approximately half of the
total OSWRO emissions.
    For more detail about this estimate of the MACT-allowable
emissions, see the memorandum, MACT-Allowable Emissions for the Off-
Site Waste and Recovery Operations Source Category, which is available
in the docket for this action.
3. How did we conduct dispersion modeling, determine inhalation
exposures and estimate individual and population inhalation risks?
    Both long-term and short-term inhalation exposure concentrations
and health risks from the source category addressed in this proposal
were estimated using the Human Exposure Model (Community and Sector
HEM-3 version 1.1.0). The HEM-3 performs three primary risk assessment
activities: (1) Conducting dispersion modeling to estimate the
concentrations of HAP in ambient air, (2) estimating long-term and
short-term inhalation exposures to individuals residing within 50
kilometers (km) of the modeled sources \5\, and (3) estimating
individual and population-level inhalation risks using the exposure
estimates and quantitative dose-response information.
---------------------------------------------------------------------------

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

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

    \6\ U.S. EPA. Revision to the Guideline on Air Quality Models:
Adoption of a Preferred General Purpose (Flat and Complex Terrain)
Dispersion Model and Other Revisions (70 FR 68218, November 9,
2005).
    \7\ A census block is the smallest geographic area for which
census statistics are tabulated.
---------------------------------------------------------------------------

    In developing the risk assessment for chronic exposures, we used
the estimated annual average ambient air concentrations of each HAP
emitted by each source for which we have emissions data in the source
category. The air concentrations at each nearby census block centroid
were used as a surrogate for the chronic inhalation exposure
concentration for all the people who reside in that census block. We
calculated the MIR for each facility as the cancer risk associated with
a continuous lifetime (24 hours per day, 7 days per week, and 52 weeks
per year for a 70-year period) exposure to the maximum concentration at
the centroid of inhabited census blocks. Individual cancer risks were
calculated by multiplying the estimated lifetime exposure to the
ambient concentration of each of the HAP (in micrograms per cubic meter
([mu]g/m\3\)) by its unit risk estimate (URE). The URE is an upper
bound estimate of an individual's probability of contracting cancer
over a lifetime of exposure to a concentration of 1 microgram of the
pollutant per cubic meter of air. For residual risk assessments, we
generally use URE values from the EPA's Integrated Risk Information
System (IRIS). For carcinogenic pollutants without EPA IRIS values, we
look to other reputable sources of cancer dose-response values, often
using California EPA (CalEPA) URE values, where available. In cases
where new, scientifically credible dose response values have been
developed in a manner consistent with the EPA guidelines and have
undergone a peer review process similar to that used by the EPA, we may
use such dose-response values in place of, or in addition to, other
values, if appropriate.
    The EPA estimated incremental individual lifetime cancer risks
associated with emissions from the facilities in the source category as
the sum of the risks for each of the carcinogenic HAP (including those
classified as carcinogenic to humans, likely to be carcinogenic to
humans, and suggestive evidence of carcinogenic potential \8\) emitted
by the modeled sources. Cancer incidence and the distribution of
individual cancer risks for the population within 50 km of the sources
were also estimated for the source category as part of this assessment
by summing individual risks. A distance of 50 km is consistent with
both the analysis supporting the 1989 Benzene NESHAP (54 FR 38044,
September 14, 1989) and the limitations of Gaussian dispersion models,
including AERMOD.
---------------------------------------------------------------------------

    \8\ These classifications also coincide with the terms ``known
carcinogen, probable carcinogen, and possible carcinogen,''
respectively, which are the terms advocated in the EPA's previous
Guidelines for Carcinogen Risk Assessment, published in 1986 (51 FR
33992, September 24, 1986). Summing the risks of these individual
compounds to obtain the cumulative cancer risks is an approach that
was recommended by the EPA's Science Advisory Board (SAB) in their
2002 peer review of EPA's National Air Toxics Assessment (NATA)
entitled, NATA--Evaluating the National-scale Air Toxics Assessment
1996 Data--an SAB Advisory, available at: http://yosemite.epa.gov/
sab/sabproduct.nsf/214C6E915BB04E14852570CA007A682C/$File/
ecadv02001.pdf.
---------------------------------------------------------------------------

    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 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/m3) for inhalation exposure and in a dose expressed
in units of milligram per kilogram-day (mg/kg-day) for oral exposures),
at or below which no adverse health effects are anticipated for a
specified exposure duration''; or (3) as noted above, a scientifically
credible dose-response value that has been developed in a manner
consistent with the EPA guidelines and has undergone a peer review
process similar to that used by the EPA, in place of or in concert with
other values.
    The EPA also evaluated screening estimates of acute exposures and
risks for each of the HAP at the point of highest off-site exposure for
each facility (i.e., not just the census block centroids), assuming
that a person is located at this spot at a time when both the peak
(hourly) emissions rate and worst-case dispersion conditions occur. The
acute HQ is the estimated acute exposure divided by the acute dose-
response value. In each case, the EPA calculated acute HQ values using
best available, short-term dose-response values. These acute dose-
response values, which are described below, include the acute REL,
acute exposure guideline levels (AEGL) and emergency response planning
guidelines (ERPG) for 1-hour exposure durations. As discussed below, we
used conservative assumptions for emissions rates, meteorology and
exposure location for our acute analysis.
    As described in the CalEPA's Air Toxics Hot Spots Program Risk
Assessment Guidelines, Part I, The Determination of Acute Reference
Exposure Levels for Airborne Toxicants, an acute REL value (http://www.oehha.ca.gov/air/pdf/acuterel.pdf) is defined as, ``the
concentration level at or below which no adverse health effects are
anticipated for a specified exposure duration.'' Id. at page 2. Acute
REL values are based on the most sensitive, relevant, adverse health
effect reported in the peer-reviewed medical and toxicological
literature. Acute REL values are designed to protect the most sensitive
individuals in the population through the inclusion of margins of
safety. Because margins of safety are incorporated to address data gaps
and uncertainties, exceeding the REL does not automatically indicate an
adverse health impact.
    AEGL values were derived in response to recommendations from the
National Research Council (NRC). As described in Standing Operating
Procedures (SOP) of the National Advisory Committee on Acute Exposure
Guideline Levels for Hazardous Substances (http://www.epa.gov/oppt/aegl/pubs/sop.pdf),\9\ ``the NRC's previous name for acute exposure
levels--community emergency exposure levels--was replaced by the term
AEGL to reflect the broad application of these values to planning,
response, and prevention in the community, the workplace,
transportation, the military, and the remediation of Superfund sites.''
Id. at 2. 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.
---------------------------------------------------------------------------

    \9\ 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 non-regulatory 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/m3
(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 non-sensory 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 non-disabling odor,
taste, and sensory irritation or certain asymptomatic, non-sensory
effects.'' Id. Similarly, the document defines AEGL-2 values as ``the
airborne concentration (expressed as parts per million or milligrams
per cubic meter) of a substance above which it is predicted that the
general population, including susceptible individuals, could experience
irreversible or other serious, long-lasting adverse health effects or
an impaired ability to escape.'' Id.
    ERPG values are derived for use in emergency response, as described
in the American Industrial Hygiene Association's ERP Committee document
entitled, ERPGS Procedures and Responsibilities (http://sp4m.aiha.org/insideaiha/GuidelineDevelopment/ERPG/Documents/ERP-SOPs2006.pdf), which
states that, ``Emergency Response Planning Guidelines were developed
for emergency planning and are intended as health based guideline
concentrations for single exposures to chemicals.'' \10\ Id. at 1. The
ERPG-1 value is defined as ``the maximum airborne concentration below
which it is believed that nearly all individuals could be exposed for
up to 1 hour without experiencing other than mild transient adverse
health effects or without perceiving a clearly defined, objectionable
odor.'' Id. at 2. Similarly, the ERPG-2 value is defined as ``the
maximum airborne concentration below which it is believed that nearly
all individuals could be exposed for up to one hour without
experiencing or developing irreversible or other serious health effects
or symptoms which could impair an individual's ability to take
protective action.'' Id. at 1.
---------------------------------------------------------------------------

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

    As can be seen from the definitions above, the AEGL and ERPG values
include the similarly-defined severity levels 1 and 2. For many
chemicals, a severity level 1 value AEGL or ERPG has not been developed
because the types of effects for these chemicals are not consistent
with the AEGL-1/ERPG-1 definitions; in these instances, we compare
higher severity level AEGL-2 or ERPG-2 values to our modeled exposure
levels to screen for potential acute concerns. When AEGL-1/ERPG-1
values are available, they are used in our acute risk assessments.
    Acute REL values for 1-hour exposure durations are typically lower
than their corresponding AEGL-1 and ERPG-1 values. Even though their
definitions are slightly different, AEGL-1 values are often the same as
the corresponding ERPG-1 values, and AEGL-2 values are often equal to
ERPG-2 values. Maximum HQ values from our acute screening risk
assessments typically

result when basing them on the acute REL value for a particular
pollutant. In cases where our maximum acute HQ value exceeds 1, we also
report the HQ value based on the next highest acute dose-response value
(usually the AEGL-1 and/or the ERPG-1 value).
    To develop screening estimates of acute exposures in the absence of
hourly emissions data, generally we first develop estimates of maximum
hourly emissions rates by multiplying the average actual annual hourly
emissions rates by a default factor to cover routinely variable
emissions. We choose the factor to use partially based on process
knowledge and engineering judgment. The factor chosen also reflects a
Texas study of short-term emissions variability, which showed that most
peak emission events in a heavily-industrialized four-county area
(Harris, Galveston, Chambers and Brazoria Counties, Texas) were less
than twice the annual average hourly emissions rate. The highest peak
emissions event was 74 times the annual average hourly emissions rate,
and the 99th percentile ratio of peak hourly emissions rate to the
annual average hourly emissions rate was 9.\11\ Considering this
analysis, to account for more than 99 percent of the peak hourly
emissions, we apply a conservative screening multiplication factor of
10 to the average annual hourly emissions rate in our acute exposure
screening assessments as our default approach. However, we use a factor
other than 10 if we have information that indicates that a different
factor is appropriate for a particular source category. For this source
category, there was no such information available and the default
factor of 10 was used in the acute screening process.
---------------------------------------------------------------------------

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

    As part of our acute risk assessment process, for cases where acute
HQ values from the screening step were less than or equal to 1 (even
under the conservative assumptions of the screening analysis), acute
impacts were deemed negligible and no further analysis was performed.
In cases where an acute HQ from the screening step was greater than 1,
additional site-specific data were considered to develop a more refined
estimate of the potential for acute impacts of concern. For this source
category, there were no offsite acute values greater than 1, and no
refined estimates were developed. Ideally, we would prefer to have
continuous measurements over time to see how the emissions vary by each
hour over an entire year. Having a frequency distribution of hourly
emissions rates over a year would allow us to perform a probabilistic
analysis to estimate potential threshold exceedances and their
frequency of occurrence. Such an evaluation could include a more
complete statistical treatment of the key parameters and elements
adopted in this screening analysis. Recognizing that this level of data
is rarely available, we instead rely on the multiplier approach.
    To better characterize the potential health risks associated with
estimated acute exposures to HAP, and in response to a key
recommendation from the SAB's peer review of the EPA's RTR risk
assessment methodologies,\12\ we generally examine a wider range of
available acute health metrics (e.g., RELs, AEGLs) than we do for our
chronic risk assessments. This is in response to the SAB's
acknowledgement that there are generally more data gaps and
inconsistencies in acute reference values than there are in chronic
reference values. In some cases, when Reference Value Arrays \13\ for
HAP have been developed, we consider additional acute values (i.e.,
occupational and international values) to provide a more complete risk
characterization.
---------------------------------------------------------------------------

    \12\ The SAB peer review of RTR Risk Assessment Methodologies is
available at: http://yosemite.epa.gov/sab/sabproduct.nsf/
4AB3966E263D943A8525771F00668381/$File/EPA-SAB-10-007-unsigned.pdf.
    \13\ U.S. EPA. (2009) Chapter 2.9 Chemical Specific Reference
Values for Formaldehyde in Graphical Arrays of Chemical-Specific
Health Effect Reference Values for Inhalation Exposures (Final
Report). U.S. Environmental Protection Agency, Washington DC, EPA/
600/R-09/061, and available online at http://cfpub.epa.gov/ncea/cfm/recordisplay.cfm?deid=211003.
---------------------------------------------------------------------------

4. How did we conduct the multipathway exposure and risk screening?
    The EPA conducted a screening analysis examining the potential for
significant human health risks due to exposures via routes other than
inhalation (i.e., ingestion). Initially, we determined whether any
sources in the source category emitted any hazardous air pollutants
known to be persistent and bioaccumulative in the environment (PB-HAP).
The PB-HAP compounds or compound classes are identified for the
screening from the EPA's Air Toxics Risk Assessment Library (available
at http://www.epa.gov/ttn/fera/risk_atra_vol1.html).
    For the OSWRO source category, we identified emissions of
polycyclic organic matter (POM) (analyzed as benzo(a)pyrene toxicity
equivalence factor (TEQ)), polychlorinated biphenyls,
hexachlorobenzene, chlordane, lindane (gamma hch), methoxyclor,
toxaphene, heptachlor, and trifluralin. Because one or more of these
PB-HAP are emitted by at least one facility in the OSWRO 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
thresholds 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 thresholds are: Lead,
cadmium, chlorinated dibenzodioxins and furans, mercury compounds, and
polycyclic organic matter (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 threshold values for these PB-HAP to assess the potential
for significant human health risks via non-inhalation pathways. We call
this application of the TRIM.FaTE model the Tier I TRIM-screen or Tier
I screen.
    For the purpose of developing emissions rates for our Tier I TRIM-
screen, we derived emission levels for these PB-HAP (other than 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 hazard quotient would be
1. If the emissions rate of any PB-HAP included in the Tier I screen
exceeds the Tier I screening emissions rate for any facility, we
conduct a second screen, which we call the Tier II TRIM-screen or Tier
II screen. In the Tier II screen, the location of each facility that
exceeded the Tier I emission rate is used to refine the assumptions
associated with the environmental scenario while maintaining the
exposure scenario assumptions. We then adjust the risk-based Tier I
screening level for each PB-HAP for each facility based on an
understanding of how exposure concentrations estimated for the

screening scenario change with meteorology and environmental
assumptions. PB-HAP emissions that do not exceed these new Tier II
screening levels are considered to pose no unacceptable risks. When
facilities exceed the Tier II screening levels, it does not mean that
multipathway impacts are significant, only that we cannot rule out that
possibility based on the results of the screen. These facilities may be
further evaluated for multipathway risks using the TRIM.FaTE model.
    For further information on the multipathway analysis approach, see
the Draft Residual Risk Assessment for the Off-Site Waste and Recovery
Operations Source Category, which is available in the docket for this
action.
5. How did we assess risks considering emissions control options?
    In addition to assessing baseline inhalation risks and screening
for potential multipathway risks, we also estimated risks considering
the potential emission reductions that would be achieved by the control
options under consideration. In these cases, the expected emission
reductions were applied to the specific HAP and emission points in the
RTR emissions dataset to develop corresponding estimates of risk and
incremental risk reductions.
6. How did we conduct the environmental risk screening assessment?
a. Adverse Environmental Effect
    The EPA has developed a screening approach to examine the potential
for adverse environmental effects as required under section
112(f)(2)(A) of the CAA. Section 112(a)(7) of the CAA defines ``adverse
environmental effect'' as ``any significant and widespread adverse
effect, which may reasonably be anticipated, to wildlife, aquatic life,
or other natural resources, including adverse impacts on populations of
endangered or threatened species or significant degradation of
environmental quality over broad areas.''
b. Environmental HAP
    The EPA focuses on seven HAP, which we refer to as ``environmental
HAP,'' in its screening analysis: Five persistent bioaccumulative HAP
(PB-HAP) and two acid gases. The five PB-HAP are cadmium, dioxins/
furans, polycyclic organic matter (POM), mercury (both inorganic
mercury and methyl mercury) and lead 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 National Ambient Air
Quality Standard (NAAQS) for lead.\14\ We consider values below the
level of the secondary lead NAAQS to be unlikely to cause adverse
environmental effects.
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    \14\ The secondary lead NAAQS is a reasonable measure of
determining whether there is an adverse environmental effect since
it was established considering ``effects on soils, water, crops,
vegetation, man-made materials, animals, wildlife, weather,
visibility and climate, damage to and deterioration of property, and
hazards to transportation, as well as effects on economic values and
on personal comfort and well-being.'' 73 FR 66964, November 12,
2008.
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    Due to their well-documented potential to cause direct damage to
terrestrial plants, we include two acid gases, HCl and HF, in the
environmental screening analysis. According to the 2005 NEI, HCl and HF
account for about 99 percent (on a mass basis) of the total acid gas
HAP emitted by stationary sources in 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
following population-level ecological assessment endpoint to screen for
indirect HAP exposures of top consumers via the bioaccumulation of HAP
in food chains:
     Piscivorous (i.e., fish-eating) wildlife consuming PB-HAP-

contaminated fish from nearby water bodies.
    For cadmium compounds, dioxins/furans, POM and mercury, we
identified the available ecological benchmarks for each assessment
endpoint. An ecological benchmark represents a concentration of HAP
(e.g., 0.77 ug of HAP per liter of water) that has been linked to a
particular environmental effect level (e.g., a no-observed-adverse-
effect level (NOAEL)) through scientific study. For PB-HAP, we
identified, where possible, ecological benchmarks at the following
effect levels:
    Probable effect levels (PEL): Level above which adverse effects are
expected to occur frequently.
    Lowest-observed-adverse-effect level (LOAEL): The lowest exposure
level tested at which there are biologically significant increases in
frequency or severity of adverse effects.
    No-observed-adverse-effect levels (NOAEL): The highest exposure
level tested at which there are no biologically significant increases
in the frequency or severity of adverse effect.
    We established a hierarchy of preferred benchmark sources to allow
selection of benchmarks for each environmental HAP at each ecological
assessment endpoint. In general, the EPA sources that are used at a
programmatic level (e.g., Office of Water, Superfund Program) were
used, if available. If not, the EPA benchmarks used in regional
programs (e.g., Superfund) were used. If benchmarks were not available
at a programmatic or regional level, we used benchmarks developed by
other federal agencies (e.g., National Oceanic and Atmospheric
Organization (NOAA)) or state agencies.
    Benchmarks for all effect levels are not available for all PB-HAP
and assessment endpoints. In cases where multiple effect levels were
available for a particular PB-HAP and assessment endpoint, we use all
of the available effect levels to help us to determine whether
ecological risks exist and, if so, whether the risks could be
considered significant and widespread.
d. Ecological Assessment Endpoints and Benchmarks for Acid Gases
    The environmental screening analysis also evaluated potential
damage and reduced productivity of plants due to direct exposure to
acid gases in the air. For acid gases, we evaluated the following
ecological assessment endpoint:
     Local terrestrial plant communities with foliage exposed
to acidic gaseous HAP in the air.
    The selection of ecological benchmarks for the effects of acid
gases on plants followed the same approach as for PB-HAP (i.e., we
examine all of the available chronic benchmarks). For HCl, the EPA
identified chronic benchmark concentrations. We note that the benchmark
for chronic HCl exposure to plants is greater than the reference
concentration for chronic inhalation exposure for human health. This
means that where the EPA includes regulatory requirements to prevent an
exceedance of the reference concentration for human health, additional
analyses for adverse environmental effects of HCL would not be
necessary.
    For HF, the EPA identified chronic benchmark concentrations for
plants and evaluated chronic exposures to plants in the screening
analysis. High concentrations of HF in the air have also been linked to
fluorosis in livestock. However, the HF concentrations at which
fluorosis in livestock occur are higher than those at which plant
damage begins. Therefore, the benchmarks for plants are protective of
both plants and livestock.
e. Screening Methodology
    For the environmental risk screening analysis, the EPA first
determined whether any facilities in the OSWRO source category emitted
any of the seven environmental HAP. For the OSWRO source category, we
identified emissions of POM, 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 threshold emission rate that corresponded to the
relevant exposure benchmark concentration value for each assessment
endpoint. To assess emissions from a facility, the reported emission
rate for each PB-HAP was compared to the screening threshold emission
rate for that PB-HAP for each assessment endpoint. If emissions from a
facility do not exceed the Tier I threshold, the facility ``passes''
the screen, and therefore, is not evaluated further under the screening
approach. If emissions from a facility exceed the Tier I threshold, we
evaluate the facility further in Tier II.
    In Tier II of the environmental screening analysis, the screening
emission thresholds are adjusted to account for local meteorology and
the actual location of lakes in the vicinity of facilities that did not
pass the Tier I screen. The modeling domain for each facility in the
Tier II analysis consists of eight octants. Each octant contains 5
modeled soil concentrations at various distances from the facility (5
soil concentrations x 8 octants = total of 40 soil concentrations per
facility) and 1 lake with modeled concentrations for water, sediment
and fish tissue. In the Tier II environmental risk screening analysis,
the 40 soil concentration points are averaged to obtain an average soil
concentration for each facility for each PB-HAP. For the water,
sediment and fish tissue concentrations, the highest value for each
facility for each pollutant is used. If emission concentrations from a
facility do not exceed the Tier II threshold, the facility passes the
screen, and typically is not evaluated further. If emissions from a
facility exceed the Tier II threshold, the facility does not pass the
screen and, therefore, may have the potential to cause adverse
environmental effects. Such facilities are evaluated further to
investigate factors such as the magnitude and characteristics of the
area of exceedance.
g. Acid Gas Methodology
    The environmental screening analysis evaluates the potential
phytotoxicity and reduced productivity of plants due to chronic
exposure to acid gases. The environmental risk screening methodology
for acid gases is a single-tier screen that compares the average off-
site ambient air concentration over the modeling domain to ecological
benchmarks for each of the acid gases. Because air concentrations are
compared directly to the ecological benchmarks, emission-based
thresholds are not calculated for acid gases as they

are in the ecological risk screening methodology for PB-HAPs.
    For purposes of ecological risk screening, the EPA identifies a
potential for adverse environmental effects to plant communities from
exposure to acid gases when the average concentration of the HAP around
a facility exceeds the LOAEL ecological benchmark. In such cases, we
further investigate factors such as the magnitude and characteristics
of the area of exceedance (e.g., land use of exceedance area, size of
exceedance area) to determine if there is an adverse environmental
effect.
    For further information on the environmental screening analysis
approach, see the Draft Residual Risk Assessment for the Off-Site Waste
and Recovery Operations Source Category, which is available in the
docket for this action.
7. How did we conduct facility-wide assessments?
    To put the source category risks in context, we typically examine
the risks from the entire ``facility,'' where the facility includes all
HAP-emitting operations within a contiguous area and under common
control. In other words, we examine the HAP emissions not only from the
source category emission points of interest, but also emissions of HAP
from all other emission sources at the facility for which we have data.
The emissions data for estimating these ``facility-wide'' risks were
obtained from the 2005 NEI (available at http://www.epa.gov/ttn/atw/nata2005). 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 Draft Residual
Risk Assessment for the Off-Site Waste and Recovery Operations Source
Category 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.
8. How did we consider uncertainties in risk assessment?
    In the Benzene NESHAP, we concluded that risk estimation
uncertainty should be considered in our decision-making under the ample
margin of safety framework. Uncertainty and the potential for bias are
inherent in all risk assessments, including those performed for this
proposal. Although uncertainty exists, we believe that our approach,
which used conservative tools and assumptions, ensures that our
decisions are health protective and environmentally protective. A brief
discussion of the uncertainties in the 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 Draft Residual Risk Assessment for the
Off-Site Waste and Recovery Operations Source Category, which is
available in the docket for this action.
a. Uncertainties in the RTR Emissions Dataset
    Although the development of the RTR emissions dataset involved
quality assurance/quality control processes, the accuracy of emissions
values will vary depending on the source of the data, the degree to
which data are incomplete or missing, the degree to which assumptions
made to complete the datasets are accurate, errors in emission
estimates and other factors. The emission estimates considered in this
analysis generally are annual totals for certain years and they do not
reflect short-term fluctuations during the course of a year or
variations from year to year. The estimates of peak hourly emission
rates for the acute effects screening assessment were based on an
emission adjustment factor applied to the average annual hourly
emission rates, which are intended to account for emission fluctuations
due to normal facility operations.
b. Uncertainties in Dispersion Modeling
    We recognize there is uncertainty in ambient concentration
estimates associated with any model, including the EPA's recommended
regulatory dispersion model, AERMOD. In using a model to estimated
ambient pollutant concentrations, the user chooses certain options to
apply. For RTR assessments, we select some model options that have the
potential to overestimate ambient air concentrations (e.g., not
including plume depletion or pollutant transformation). We select other
model options that have the potential to underestimate ambient impacts
(e.g., not including building downwash). Other options that we select
have the potential to either under- or overestimate ambient levels
(e.g., meteorology and receptor locations). On balance, considering the
directional nature of the uncertainties commonly present in ambient
concentrations estimated by dispersion models, the approach we apply in
the RTR assessments should yield unbiased estimates of ambient HAP
concentrations.
c. Uncertainties in Inhalation Exposure
    The EPA did not include the effects of human mobility on exposures
in the assessment. Specifically, short-term mobility and long-term
mobility between census blocks in the modeling domain were not
considered.\15\ The approach of not considering short or long-term
population mobility does not bias the estimate of the theoretical MIR
(by definition), nor does it affect the estimate of cancer incidence
because the total population number remains the same. It does, however,
affect the shape of the distribution of individual risks across the
affected population, shifting it toward higher estimated individual
risks at the upper end and reducing the number of people estimated to
be at lower risks, thereby increasing the estimated number of people at
specific high risk levels (e.g., 1-in-10 thousand or 1-in-1 million).
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    \15\ Short-term mobility is movement from one micro-environment
to another over the course of hours or days. Long-term mobility is
movement from one residence to another over the course of a
lifetime.
---------------------------------------------------------------------------

    In addition, the assessment predicted the chronic exposures at the
centroid of each populated census block as surrogates for the exposure
concentrations for all people living in that block. Using the census
block centroid to predict chronic exposures tends to over-predict
exposures for people in the census block who live farther from the
facility and under-predict exposures for people in the census block who
live closer to the facility. Thus, using the census block centroid to
predict chronic exposures may lead to a potential understatement or
overstatement of the true maximum impact, but is an unbiased estimate
of average risk and incidence. We reduce this uncertainty by analyzing
large census blocks near facilities using aerial imagery and adjusting
the location of the block centroid to better represent the population
in the block, as well as adding additional receptor locations where the
block population is not well represented by a single location.
    The assessment evaluates the cancer inhalation risks associated
with

pollutant exposures over a 70-year period, which is the assumed
lifetime of an individual. In reality, both the length of time that
modeled emission sources at facilities actually operate (i.e., more or
less than 70 years) and the domestic growth or decline of the modeled
industry (i.e., the increase or decrease in the number or size of
domestic facilities) will influence the future risks posed by a given
source or source category. Depending on the characteristics of the
industry, these factors will, in most cases, result in an overestimate
both in individual risk levels and in the total estimated number of
cancer cases. However, in the unlikely scenario where a facility
maintains, or even increases, its emissions levels over a period of
more than 70 years, residents live beyond 70 years at the same
location, and the residents spend most of their days at that location,
then the cancer inhalation risks could potentially be underestimated.
However, annual cancer incidence estimates from exposures to emissions
from these sources would not be affected by the length of time an
emissions source operates.
    The exposure estimates used in these analyses assume chronic
exposures to ambient (outdoor) levels of pollutants. Because most
people spend the majority of their time indoors, actual exposures may
not be as high, depending on the characteristics of the pollutants
modeled. For many of the HAP, indoor levels are roughly equivalent to
ambient levels, but for very reactive pollutants or larger particles,
indoor levels are typically lower. This factor has the potential to
result in an overestimate of 25 to 30 percent of exposures.\16\
---------------------------------------------------------------------------

    \16\ U.S. EPA. National-Scale Air Toxics Assessment for 1996.
(EPA 453/R-01-003; January 2001; page 85.)
---------------------------------------------------------------------------

    In addition to the uncertainties highlighted above, there are
several factors specific to the acute exposure assessment that should
be highlighted. The accuracy of an acute inhalation exposure assessment
depends on the simultaneous occurrence of independent factors that may
vary greatly, such as hourly emissions rates, meteorology and human
activity patterns. In this assessment, we assume that individuals
remain for 1 hour at the point of maximum ambient concentration as
determined by the co-occurrence of peak emissions and worst-case
meteorological conditions. These assumptions would tend to be worst-
case actual exposures as it is unlikely that a person would be located
at the point of maximum exposure 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 Draft Residual Risk Assessment for the Off-Site Waste and
Recovery Operations Source Category, which is available in the docket
for this action.
    Cancer URE values used in our risk assessments are those that have
been developed to generally provide an upper bound estimate of risk.
That is, they represent a ``plausible upper limit to the true value of
a quantity'' (although this is usually not a true statistical
confidence limit).\17\ In some circumstances, the true risk could be as
low as zero; however, in other circumstances the risk could be
greater.\18\ When developing an upper bound estimate of risk and to
provide risk values that do not underestimate risk, health-protective
default approaches are generally used. To err on the side of ensuring
adequate health protection, the EPA typically uses the upper bound
estimates rather than lower bound or central tendency estimates in our
risk assessments, an approach that may have limitations for other uses
(e.g., priority-setting or expected benefits analysis).
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    \17\ IRIS glossary (http://www.epa.gov/NCEA/iris/help_gloss.htm).
    \18\ An exception to this is the URE for benzene, which is
considered to cover a range of values, each end of which is
considered to be equally plausible, and which is based on maximum
likelihood estimates.
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    Chronic non-cancer RfC and reference dose (RfD) values represent
chronic exposure levels that are intended to be health-protective
levels. Specifically, these values provide an estimate (with
uncertainty spanning perhaps an order of magnitude) of a continuous
inhalation exposure (RfC) or a daily oral exposure (RfD) to the human
population (including sensitive subgroups) that is likely to be without
an appreciable risk of deleterious effects during a lifetime. To derive
values that are intended to be ``without appreciable risk,'' the
methodology relies upon an uncertainty factor (UF) approach (U.S. EPA,
1993, 1994) which considers uncertainty, variability and gaps in the
available data. The UF are applied to derive reference values that are
intended to protect against appreciable risk of deleterious effects.
The UF are commonly default values,\19\ e.g., factors of 10 or 3, used
in the absence of compound-specific data; where data are available, UF
may also be developed using compound-specific information. When data
are limited, more assumptions are needed and more UF are used. Thus,
there may be a greater tendency to overestimate risk in the sense that
further study might support development of reference values that are
higher (i.e., less potent) because fewer default assumptions are
needed. However, for some pollutants, it is possible that risks may be
underestimated.
---------------------------------------------------------------------------

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

    While collectively termed ``UF,'' these factors account for a
number of different quantitative considerations when using observed
animal (usually rodent) or human toxicity data in the development of
the RfC. The UF are intended to account for: (1) Variation in
susceptibility among the members of the human population (i.e., inter-
individual variability); (2) uncertainty in extrapolating from
experimental animal data to humans (i.e., interspecies

differences); (3) uncertainty in extrapolating from data obtained in a
study with less-than-lifetime exposure (i.e., extrapolating from sub-
chronic to chronic exposure); (4) uncertainty in extrapolating the
observed data to obtain an estimate of the exposure associated with no
adverse effects; and (5) uncertainty when the database is incomplete or
there are problems with the applicability of available studies.
    Many of the UF used to account for variability and uncertainty in
the development of acute reference values are quite similar to those
developed for chronic durations, but they more often use individual UF
values that may be less than 10. The UF are applied based on chemical-
specific or health effect-specific information (e.g., simple irritation
effects do not vary appreciably between human individuals, hence a
value of 3 is typically used), or based on the purpose for the
reference value (see the following paragraph). The UF applied in acute
reference value derivation include: (1) Heterogeneity among humans; (2)
uncertainty in extrapolating from animals to humans; (3) uncertainty in
lowest observed adverse effect (exposure) level to no observed adverse
effect (exposure) level adjustments; and (4) uncertainty in accounting
for an incomplete database on toxic effects of potential concern.
Additional adjustments are often applied to account for uncertainty in
extrapolation from observations at one exposure duration (e.g., 4
hours) to derive an acute reference value at another exposure duration
(e.g., 1 hour).
    Not all acute reference values are developed for the same purpose
and care must be taken when interpreting the results of an acute
assessment of human health effects relative to the reference value or
values being exceeded. Where relevant to the estimated exposures, the
lack of short-term dose-response values at different levels of severity
should be factored into the risk characterization as potential
uncertainties.
    Although every effort is made to identify appropriate human health
effect dose-response assessment values for all pollutants emitted by
the sources in this risk assessment, some HAP emitted by this source
category are lacking dose-response assessments. Accordingly, these
pollutants cannot be included in the quantitative risk assessment,
which could result in quantitative estimates understating HAP risk. To
help to alleviate this potential underestimate, where we conclude
similarity with a HAP for which a dose-response assessment value is
available, we use that value as a surrogate for the assessment of the
HAP for which no value is available. To the extent use of surrogates
indicates appreciable risk, we may identify a need to increase priority
for new IRIS assessment of that substance. We additionally note that,
generally speaking, HAP of greatest concern due to environmental
exposures and hazard are those for which dose-response assessments have
been performed, reducing the likelihood of understating risk. Further,
HAP not included in the quantitative assessment are assessed
qualitatively and considered in the risk characterization that informs
the risk management decisions, including with regard to consideration
of HAP reductions achieved by various control options.
    For a group of compounds that are not speciated (e.g., glycol
ethers), we conservatively use the most protective reference value of
an individual compound in that group to estimate risk. Similarly, for
an individual compound in a group (e.g., ethylene glycol diethyl ether)
that does not have a specified reference value, we also apply the most
protective reference value from the other compounds in the group to
estimate risk.
e. Uncertainties in the Multipathway Assessment
    For each source category, we generally rely on site-specific levels
of PB-HAP emissions to determine whether a refined assessment of the
impacts from multipathway exposures is necessary. This determination is
based on the results of a two-tiered screening analysis that relies on
the outputs from models that estimate environmental pollutant
concentrations and human exposures for four PB-HAP. Two important types
of uncertainty associated with the use of these models in RTR risk
assessments and inherent to any assessment that relies on environmental
modeling are model uncertainty and input uncertainty.\20\
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    \20\ In the context of this discussion, the term ``uncertainty''
as it pertains to exposure and risk encompasses both variability in
the range of expected inputs and screening results due to existing
spatial, temporal, and other factors, as well as uncertainty in
being able to accurately estimate the true result.
---------------------------------------------------------------------------

    Model uncertainty concerns whether the selected models are
appropriate for the assessment being conducted and whether they
adequately represent the actual processes that might occur for that
situation. An example of model uncertainty is the question of whether
the model adequately describes the movement of a pollutant through the
soil. This type of uncertainty is difficult to quantify. However, based
on feedback received from previous EPA Science Advisory Board reviews
and other reviews, we are confident that the models used in the screen
are appropriate and state-of-the-art for the multipathway risk
assessments conducted in support of RTR.
    Input uncertainty is concerned with how accurately the models have
been configured and parameterized for the assessment at hand. For Tier
I of the multipathway screen, we configured the models to avoid
underestimating exposure and risk. This was accomplished by selecting
upper-end values from nationally-representative data sets for the more
influential parameters in the environmental model, including selection
and spatial configuration of the area of interest, lake location and
size, meteorology, surface water and soil characteristics and structure
of the aquatic food web. We also assume an ingestion exposure scenario
and values for human exposure factors that represent reasonable maximum
exposures.
    In Tier II of the multipathway assessment, we refine the model
inputs to account for meteorological patterns in the vicinity of the
facility versus using upper-end national values and we identify the
actual location of lakes near the facility rather than the default lake
location that we apply in Tier I. By refining the screening approach in
Tier II to account for local geographical and meteorological data, we
decrease the likelihood that concentrations in environmental media are
overestimated, thereby increasing the usefulness of the screen. The
assumptions and the associated uncertainties regarding the selected
ingestion exposure scenario are the same for Tier I and Tier II.
    For both Tiers I and II of the multipathway assessment, our
approach to addressing model input uncertainty is generally cautious.
We choose model inputs from the upper end of the range of possible
values for the influential parameters used in the models, and we assume
that the exposed individual exhibits ingestion behavior that would lead
to a high total exposure. This approach reduces the likelihood of not
identifying high risks for adverse impacts.
    Despite the uncertainties, when individual pollutants or facilities
do screen out, we are confident that the potential for adverse
multipathway impacts on human health is very low. On the other hand,
when individual pollutants or facilities do not screen out, it does not
mean that multipathway impacts are significant, only that we cannot
rule out that possibility and that a refined multipathway analysis for
the

site might be necessary to obtain a more accurate risk characterization
for the source category.
    For further information on uncertainties and the Tier I and II
screening methods, refer to the risk document Appendix 4, ``Technical
Support Document for TRIM-Based Multipathway Tiered Screening
Methodology for RTR.''
f. Uncertainties in the Environmental Risk Screening Assessment
    For each source category, we generally rely on site-specific levels
of environmental HAP emissions to perform an environmental screening
assessment. The environmental screening assessment is based on the
outputs from models that estimate environmental HAP concentrations. The
same models, specifically the TRIM.FaTE multipathway model and the
AERMOD air dispersion model, are used to estimate environmental HAP
concentrations for both the human multipathway screening analysis and
for the environmental screening analysis. Therefore, both screening
assessments have similar modeling uncertainties.
    Two important types of uncertainty associated with the use of these
models in RTR environmental screening assessments--and inherent to any
assessment that relies on environmental modeling--are model uncertainty
and input uncertainty.\21\
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    \21\ In the context of this discussion, the term
``uncertainty,'' as it pertains to exposure and risk assessment,
encompasses both variability in the range of expected inputs and
screening results due to existing spatial, temporal, and other
factors, as well as uncertainty in being able to accurately estimate
the true result.
---------------------------------------------------------------------------

    Model uncertainty concerns whether the selected models are
appropriate for the assessment being conducted and whether they
adequately represent the movement and accumulation of environmental HAP
emissions in the environment. For example, does the model adequately
describe the movement of a pollutant through the soil? This type of
uncertainty is difficult to quantify. However, based on feedback
received from previous EPA Science Advisory Board reviews and other
reviews, we are confident that the models used in the screen are
appropriate and state-of-the-art for the environmental risk assessments
conducted in support of our RTR analyses.
    Input uncertainty is concerned with how accurately the models have
been configured and parameterized for the assessment at hand. For Tier
I of the environmental screen for PB-HAP, we configured the models to
avoid underestimating exposure and risk to reduce the likelihood that
the results indicate the risks are lower than they actually are. This
was accomplished by selecting upper-end values from nationally-
representative data sets for the more influential parameters in the
environmental model, including selection and spatial configuration of
the area of interest, the location and size of any bodies of water,
meteorology, surface water and soil characteristics and structure of
the aquatic food web. In Tier I, we used the maximum facility-specific
emissions for the PB-HAP (other than lead compounds, which were
evaluated by comparison to the secondary lead NAAQS) that were included
in the environmental screening assessment and each of the media when
comparing to ecological benchmarks. This is consistent with the
conservative design of Tier I of the screen. In Tier II of the
environmental screening analysis for PB-HAP, we refine the model inputs
to account for meteorological patterns in the vicinity of the facility
versus using upper-end national values, and we identify the locations
of water bodies near the facility location. By refining the screening
approach in Tier II to account for local geographical and
meteorological data, we decrease the likelihood that concentrations in
environmental media are overestimated, thereby increasing the
usefulness of the screen. To better represent widespread impacts, the
modeled soil concentrations are averaged in Tier II to obtain one
average soil concentration value for each facility and for each PB-HAP.
For PB-HAP concentrations in water, sediment and fish tissue, the
highest value for each facility for each pollutant is used.
    For the environmental screening assessment for acid gases, we
employ a single-tiered approach. We use the modeled air concentrations
and compare those with ecological benchmarks.
    For both Tiers I and II of the environmental screening assessment,
our approach to addressing model input uncertainty is generally
cautious. We choose model inputs from the upper end of the range of
possible values for the influential parameters used in the models, and
we assume that the exposed individual exhibits ingestion behavior that
would lead to a high total exposure. This approach reduces the
likelihood of not identifying potential risks for adverse environmental
impacts.
    Uncertainty also exists in the ecological benchmarks for the
environmental risk screening analysis. We established a hierarchy of
preferred benchmark sources to allow selection of benchmarks for each
environmental HAP at each ecological assessment endpoint. In general,
EPA benchmarks used at a programmatic level (e.g., Office of Water,
Superfund Program) were used if available. If not, we used EPA
benchmarks used in regional programs (e.g., Superfund Program). If
benchmarks were not available at a programmatic or regional level, we
used benchmarks developed by other agencies (e.g., NOAA) or by state
agencies.
    In all cases (except for lead compounds, which were evaluated
through a comparison to the NAAQS), we searched for benchmarks at the
following three effect levels, as described in section III.A.6 of this
preamble:
    1. A no-effect level (i.e., NOAEL).
    2. Threshold-effect level (i.e., LOAEL).
    3. Probable effect level (i.e., PEL).
    For some ecological assessment endpoint/environmental HAP
combinations, we could identify benchmarks for all three effect levels,
but for most, we could not. In one case, where different agencies
derived significantly different numbers to represent a threshold for
effect, we included both. In several cases, only a single benchmark was
available. In cases where multiple effect levels were available for a
particular PB-HAP and assessment endpoint, we used all of the available
effect levels to help us to determine whether risk exists and if the
risks could be considered significant and widespread.
    The EPA evaluated the following seven HAP in the environmental risk
screening assessment: cadmium, dioxins/furans, POM, mercury (both
inorganic mercury and methyl mercury), lead compounds, HCl and HF.
These seven HAP represent pollutants that can cause adverse impacts for
plants and animals either through direct exposure to HAP in the air or
through exposure to HAP that is deposited from the air onto soils and
surface waters. These seven HAP also represent those HAP for which we
can conduct a meaningful environmental risk screening assessment. For
other HAP not included in our screening assessment, the model has not
been parameterized such that it can be used for that purpose. In some
cases, depending on the HAP, we may not have appropriate multipathway
models that allow us to predict the concentration of that pollutant.
The EPA acknowledges that other HAP beyond the seven HAP that we are
evaluating may have the potential to cause adverse environmental
effects and, therefore, the

EPA may evaluate other relevant HAP in the future, as modeling science
and resources allow.
    Further information on uncertainties and the Tier I and II
environmental screening methods is provided in Appendix 5 of the
document ``Technical Support Document for TRIM-Based Multipathway
Tiered Screening Methodology for RTR: Summary of Approach and
Evaluation.'' Also, see the Draft Residual Risk Assessment for the Off-
Site Waste and Recovery Operations Source Category, available in the
docket for this action.

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

    As discussed in section II.A of this preamble, in evaluating and
developing standards under section 112(f)(2), we apply a two-step
process to address residual risk. In the first step, the EPA determines
whether risks are acceptable. This determination ``considers all health
information, including risk estimation uncertainty, and includes a
presumptive limit on maximum individual lifetime [cancer] risk (MIR)
\22\ of approximately [1-in-10 thousand] [i.e., 100-in-1 million].'' 54
FR 38045, September 14, 1989. If risks are unacceptable, the EPA must
determine the emissions standards necessary to bring risks to an
acceptable level without considering costs. In the second step of the
process, the EPA considers whether the emissions standards provide an
ample margin of safety ``in consideration of all health information,
including the number of persons at risk levels higher than
approximately 1-in-1 million, as well as other relevant factors,
including costs and economic impacts, technological feasibility, and
other factors relevant to each particular decision.'' Id. The EPA must
promulgate tighter emission standards if necessary to provide an ample
margin of safety.
---------------------------------------------------------------------------

    \22\ Although defined as ``maximum individual risk,'' MIR refers
only to cancer risk. MIR, one metric for assessing cancer risk, is
the estimated risk were an individual exposed to the maximum level
of a pollutant for a lifetime.
---------------------------------------------------------------------------

    In past residual risk actions, the EPA considered a number of human
health risk metrics associated with emissions from the categories under
review, including the MIR, the number of persons in various risk
ranges, cancer incidence, the maximum non-cancer HI and the maximum
acute non-cancer hazard. See, e.g., 72 FR 25138, May 3, 2007; 71 FR
42724, July 27, 2006. The EPA considered this health information for
both actual and MACT-allowable emissions. See, e.g., 75 FR 65068,
October 21, 2010; 75 FR 80220, December 21, 2010; 76 FR 29032, May 19,
2011. The EPA also discussed risk estimation uncertainties and
considered the uncertainties in the determination of acceptable risk
and ample margin of safety in these past actions. The EPA considered
this same type of information in support of this action.
    The agency is considering these various measures of health
information to inform our determinations of risk acceptability and
ample margin of safety under CAA section 112(f). As explained in the
Benzene NESHAP, ``the first step judgment on acceptability cannot be
reduced to any single factor'' and thus ``[t]he Administrator believes
that the acceptability of risk under [previous] section 112 is best
judged on the basis of a broad set of health risk measures and
information.'' 54 FR 38046, September 14, 1989. Similarly, with regard
to the ample margin of safety determination, ``the Agency again
considers all of the health risk and other health information
considered in the first step. Beyond that information, additional
factors relating to the appropriate level of control will also be
considered, including cost and economic impacts of controls,
technological feasibility, uncertainties, and any other relevant
factors.'' Id.
    The Benzene NESHAP approach provides flexibility regarding factors
the EPA may consider in making determinations and how the EPA may weigh
those factors for each source category. In responding to comment on our
policy under the Benzene NESHAP, the EPA explained that:

    ``[t]he policy chosen by the Administrator permits consideration
of multiple measures of health risk. Not only can the MIR figure be
considered, but also incidence, the presence of non-cancer health
effects, and the uncertainties of the risk estimates. In this way,
the effect on the most exposed individuals can be reviewed as well
as the impact on the general public. These factors can then be
weighed in each individual case. This approach complies with the
Vinyl Chloride mandate that the Administrator ascertain an
acceptable level of risk to the public by employing [her] expertise
to assess available data. It also complies with the Congressional
intent behind the CAA, which did not exclude the use of any
particular measure of public health risk from the EPA's
consideration with respect to CAA section 112 regulations, and
thereby implicitly permits consideration of any and all measures of
health risk which the Administrator, in [her] judgment, believes are
appropriate to determining what will `protect the public health'.''

See 54 FR at 38057, September 14, 1989. Thus, the level of the MIR is
only one factor to be weighed in determining acceptability of risks.
The Benzene NESHAP explained that ``an MIR of approximately one in 10
thousand should ordinarily be the upper end of the range of
acceptability. As risks increase above this benchmark, they become
presumptively less acceptable under CAA section 112, and would be
weighed with the other health risk measures and information in making
an overall judgment on acceptability. Or, the Agency may find, in a
particular case, that a risk that includes MIR less than the
presumptively acceptable level is unacceptable in the light of other
health risk factors.'' Id. at 38045. Similarly, with regard to the
ample margin of safety analysis, the EPA stated in the Benzene NESHAP
that: ``EPA believes the relative weight of the many factors that can
be considered in selecting an ample margin of safety can only be
determined for each specific source category. This occurs mainly
because technological and economic factors (along with the health-
related factors) vary from source category to source category.'' Id. at
38061. We also consider the uncertainties associated with the various
risk analyses, as discussed earlier in this preamble, in our
determinations of acceptability and ample margin of safety.
    The EPA notes that it has not considered certain health information
to date in making residual risk determinations. At this time, we do not
attempt to quantify those HAP risks that may be associated with
emissions from other facilities that do not include the source
categories in question, mobile source emissions, natural source
emissions, persistent environmental pollution or atmospheric
transformation in the vicinity of the sources in these categories.
    The agency understands the potential importance of considering an
individual's total exposure to HAP in addition to considering exposure
to HAP emissions from the source category and facility. We recognize
that such consideration may be particularly important when assessing
non-cancer risks, where pollutant-specific exposure health reference
levels (e.g., RfCs) are based on the assumption that thresholds exist
for adverse health effects. For example, the agency recognizes that,
although exposures attributable to emissions from a source category or
facility alone may not indicate the potential for increased risk of
adverse non-cancer health effects in a population, the exposures
resulting from emissions from the facility in combination with
emissions from all of the other sources (e.g., other facilities) to
which an individual is exposed may be sufficient to result in increased
risk of adverse non-cancer health effects. In

May 2010, the SAB advised the EPA ``that RTR assessments will be most
useful to decision makers and communities if results are presented in
the broader context of aggregate and cumulative risks, including
background concentrations and contributions from other sources in the
area.'' \23\
---------------------------------------------------------------------------

    \23\ EPA's responses to this and all other key recommendations
of the SAB's advisory on RTR risk assessment methodologies (which is
available at: http://yosemite.epa.gov/sab/sabproduct.nsf/
4AB3966E263D943A8525771F00668381/$File/EPA-SAB-10-007-unsigned.pdf)
are outlined in a memo in this proposed rule docket from David
Guinnup entitled, EPA's Actions in Response to the Key
Recommendations of the SAB Review of RTR Risk Assessment
Methodologies.
---------------------------------------------------------------------------

    In response to the SAB recommendations, the EPA is incorporating
cumulative risk analyses into its RTR risk assessments, including those
reflected in this proposal. The agency is: (1) Conducting facility-wide
assessments, which include source category emission points as well as
other emission points within the facilities; (2) considering sources in
the same category whose emissions result in exposures to the same
individuals; and (3) for some persistent and bioaccumlative pollutants,
analyzing the ingestion route of exposure. In addition, the RTR risk
assessments have always considered aggregate cancer risk from all
carcinogens and aggregate non-cancer hazard indices from all non-
carcinogens affecting the same target organ system.
    Although we are interested in placing source category and facility-
wide HAP risks in the context of total HAP risks from all sources
combined in the vicinity of each source, we are concerned about the
uncertainties of doing so. Because of the contribution to total HAP
risk from emission sources other than those that we have studied in
depth during this RTR review (i.e., those sources located at facilities
within the source category), such estimates of total HAP risks would
have significantly greater associated uncertainties than the source
category or facility-wide estimates. Such aggregate or cumulative
assessments would compound those uncertainties, making the assessments
too unreliable.

C. How did we perform the technology review?

    Our technology review focused on the identification and evaluation
of developments in practices, processes and control technologies that
have occurred since the MACT standards were promulgated. Where we
identified such developments, in order to inform our decision of
whether it is ``necessary'' to revise the emissions standards, we
analyzed the technical feasibility of applying these developments, and
the estimated costs, energy implications, non-air environmental
impacts, as well as considering the emission reductions. We also
considered the appropriateness of applying controls to new sources
versus retrofitting existing sources.
    Based on our analyses of the available data and information, we
identified potential developments in practices, processes and control
technologies. For this exercise, we considered any of the following to
be a ``development'':

     Any add-on control technology or other equipment that
was not identified and considered during development of the original
MACT standards.
     Any improvements in add-on control technology or other
equipment (that were identified and considered during development of
the original MACT standards) that could result in additional
emissions reduction.
     Any work practice or operational procedure that was not
identified or considered during development of the original MACT
standards.
     Any process change or pollution prevention alternative