6560-50-P

  E.O. 12866 Ferroalloys Production 2060-AQ11 Supplemental Proposal 20140313

                        ENVIRONMENTAL PROTECTION AGENCY
                                40 CFR Part 63
                      [EPA-HQ-OAR-2010-0895; FRL-XXXX-X]
                                RIN 2060-AQ-11
National Emissions Standards for Hazardous Air Pollutants: Ferroalloys Production

AGENCY: Environmental Protection Agency (EPA).
ACTION: Supplemental notice of proposed rulemaking.
SUMMARY: This action supplements our proposed amendments to the national emission standards for hazardous air pollutants for the ferroalloys production source category published in the Federal Register on November 23, 2011 (76 FR 72508). In that action, the Environmental Protection Agency (EPA) proposed amendments based on the initial technology and residual risk reviews for this source category. Today's action presents a new technology review and a new residual risk analysis for the ferroalloys production source category and proposes revisions to the standards based on those reviews. This action also proposes new compliance requirements to meet the revised standards. This action would result in significant environmental improvements through the reduction of fugitive manganese emissions and through more stringent emission limits for several processes.
DATES: Comments. Comments must be received on or before [INSERT DATE 45 DAYS AFTER DATE OF PUBLICATION IN THE FEDERAL REGISTER]. A copy of comments on the information collection provisions should be submitted to the Office of Management and Budget (OMB) on or before [INSERT THE DATE 30 DAYS AFTER DATE OF PUBLICATION IN THE FEDERAL REGISTER.]
      Public Hearing. If anyone contacts the EPA requesting a public hearing by June [insert specific date here], 2014, the EPA will hold a public hearing on July [insert specific date], 2014, from 10:00 a.m. to 4:00 p.m. at [insert specific location here] in Marietta, Ohio. If the EPA holds a public hearing, the EPA will keep the record of the hearing open for 30 days after completion of the hearing to provide an opportunity for submission of rebuttal and supplementary information.
ADDRESSES: Comments. Submit your comments, identified by Docket ID No. EPA-HQ-OAR-2010-0895, by one of the following methods:
      ::	http://www.regulations.gov: Follow the on-line instructions for submitting comments.
      oo       Email<EMAIL_ADDRESS>Attention Docket ID Number EPA-HQ-OAR-2010-0895.
       Fax:<PHONE_NUMBER>. Attention Docket ID Number EPA-HQ-OAR-2010-0895.
       Mail: U.S. Postal Service, send comments to: EPA Docket Center, EPA West Building (Air Docket), Attention Docket ID Number EPA-HQ-OAR-2010-0895, U.S. Environmental Protection Agency, Mailcode: 28221T, 1200 Pennsylvania Ave., NW, Washington, DC 20460. Please include a total of two copies.
       Hand Delivery: U.S. Environmental Protection Agency, EPA West Building (Air Docket), Room 3334, 1301 Constitution Ave., NW, Washington, DC 20004, Attention Docket ID Number EPA-HQ-OAR-2010-0895. Such deliveries are only accepted during the Docket's normal hours of operation, and special arrangements should be made for deliveries of boxed information. 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 17[th] Street, NW, Washington, DC 20503.
	Instructions. Direct your comments to Docket ID No. EPA-HQ-OAR-2010-0895. 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 www.regulations.gov or email. The http://www.regulations.gov website is an "anonymous access" system, which means the EPA will not know your identity or contact information unless you provide it in the body of your comment. If you send an email comment directly to the EPA without going through http://www.regulations.gov, your email address will be automatically captured and included as part of the comment that is placed in the public docket and made available on the Internet. If you submit an electronic comment, the EPA recommends that you include your name and other contact information in the body of your comment and with any disk or CD-ROM you submit. If the EPA cannot read your comment due to technical difficulties and cannot contact you for clarification, the EPA may not be able to consider your comment. Electronic files should not include special characters or any form of encryption and be free of any defects or viruses. For additional information about the EPA's public docket, visit the EPA Docket Center homepage at: http://www.epa.gov/dockets
	Docket. The EPA has established a docket for this rulemaking under Docket ID Number EPA-HQ-OAR-2010-0895. All documents in the docket are listed in the regulations.gov index. Although listed in the index, some information is not publicly available, e.g., CBI or other information whose disclosure is restricted by statute. Certain other material, such as copyrighted material, is not placed on the internet and will be publicly available only in hard copy. Publicly available docket materials are available either electronically in regulations.gov or in hard copy at the EPA Docket Center, EPA West 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<PHONE_NUMBER>, and the telephone number for the EPA Docket Center is<PHONE_NUMBER>.
      Public Hearing. If anyone contacts the EPA requesting a public hearing by [insert date], the public hearing will be held on [insert date], from 10:00 a.m. to 4:00 p.m. at [insert specific location] in Marietta Ohio. Persons interested in presenting oral testimony or inquiring as to whether a public hearing will be held should contact Ms. Virginia Hunt of the Sector Policies and Programs Division (E143-01), Office of Air Quality Planning and Standards, Environmental Protection Agency, Research Triangle Park, NC 27711; telephone number:<PHONE_NUMBER>; fax number:<PHONE_NUMBER>; email address<EMAIL_ADDRESS>
FOR FURTHER INFORMATION: For questions about this proposed action, contact Mr. Phil Mulrine, Sector Policies and Programs Division (D243-02), Office of Air Quality Planning and Standards, Environmental Protection Agency, Research Triangle Park, NC 27711; telephone<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 (C539 - 02), Office of Air Quality Planning and Standards, U.S. Environmental Protection Agency, Research Triangle Park, NC 27711; telephone number: (919) 541 - 2076; fax number: (919) 541 - 2076; and email address<EMAIL_ADDRESS>For information about the applicability of the National Emissions Standards for Hazardous Air Pollutants (NESHAP) to a particular entity, contact Cary Secrest, Office of Enforcement and Compliance Assurance (OECA), telephone number:<PHONE_NUMBER> and email address<EMAIL_ADDRESS>SUPPLEMENTARY INFORMATION:
Preamble Acronyms and Abbreviations
      We use multiple acronyms and terms in this preamble. While this list may not be exhaustive, to ease the reading of this preamble and for reference purposes, the EPA defines the following terms and acronyms here:
AEGL	acute exposure guideline levels
AERMOD	air dispersion model used by the HEM-3 model
ATSDR	Agency for Toxic Substances and Disease Registry
BLDS	bag leak detection system
BTF	Beyond the Floor
CAA	Clean Air Act
CalEPA	California EPA
CBI	Confidential Business Information
CFR	Code of Federal Regulations
EJ	environmental justice
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 Model, Version 1.1.0
HI	Hazard Index
HQ	Hazard Quotient
IRIS	Integrated Risk Information System
km	kilometer
LOAEL	lowest-observed-adverse-effect level
MACT	maximum achievable control technology
MACT Code	Code within the National Emissions Inventory used to identify processes included in a source category
mg/dscm	milligrams per dry standard cubic meter
mg/kg-day	milligrams per kilogram-day
mg/m[3]	milligrams per cubic meter
MIR	maximum individual risk
MRL	Minimal Risk Level
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
NOAEL	no-observed-adverse-effect level
NRC	National Research Council
NTTAA	National Technology Transfer and Advancement Act
OAQPS	Office of Air Quality Planning and Standards
OECA	Office of Enforcement and Compliance Assurance
OMB	Office of Management and Budget
PAH	polycyclic aromatic hydrocarbons
PB-HAP	hazardous air pollutants known to be persistent and bio-accumulative in the environment
PEL	probable effect level
PM	particulate matter
POM	polycyclic organic matter
ppm	parts per million
RDL 	representative method detection level
REL	reference exposure level
RFA	Regulatory Flexibility Act
RfC	reference concentration
RfD	reference dose
RTR	residual risk and technology review
SAB	Science Advisory Board
SBA	Small Business Administration
SSM	startup, shutdown, and malfunction
TOSHI	target organ-specific hazard index
TPY	tons per year
TRIM.FaTE	Total Risk Integrated Methodology.Fate, Transport, and Ecological Exposure model
TTN	Technology Transfer Network
UF	uncertainty factor
ug/dscm	micrograms per dry standard cubic meter
ug/m3	micrograms per cubic meter
UMRA	Unfunded Mandates Reform Act
UPL	Upper Prediction Limit
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 Information
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 is the history of the Ferroalloys Production Risk and Technology Review?
D. What data collection activities were conducted to support this action?
III. Analytical Procedures
A. For purposes of this supplemental proposal, how did we estimate the post-MACT risks posed by the Ferroalloys Production source category?
B. How did we consider the risk results in making decisions for this supplemental proposal?
C. How did we perform the technology review?
IV. Revised Analytical Results and Proposed Decisions for the Ferroalloys Production Source Category
A. What actions are we taking pursuant to CAA sections 112(d)(2) and 112(d)(3)?
B. What are the results of the risk assessment and analyses?
C. What are our proposed decisions regarding risk acceptability, ample margin of safety and adverse environmental effects based on our revised analyses?
D. What are the results and proposed decisions based on our technology review?
E. What other actions are we proposing?
F. What compliance dates are we proposing?

V. Summary of the Revised Cost, Environmental and Economic Impacts
A. What are the affected sources?
B. What are the air quality impacts?
C. What are the cost impacts?
D. What are the economic impacts?
E. What are the benefits?

VI. Request for Comments
VII. Submitting Data Corrections
VIII. Statutory and Executive Order Reviews
A. Executive Order 12866: Regulatory Planning and Review and Executive Order 13563: Improving Regulation and Regulatory Review
B. Paperwork Reduction Act
C. Regulatory Flexibility Act
D. Unfunded Mandates Reform Act
E. Executive Order 13132: Federalism
F. Executive Order 13175: Consultation and Coordination with Indian Tribal Governments
G. Executive Order 13045: Protection of Children from Environmental Health Risks and Safety Risks
H. Executive Order 13211: Actions Concerning Regulations That Significantly Affect Energy Supply, Distribution, or Use
I. National Technology Transfer and Advancement Act
J. Executive Order 12898: Federal Actions to Address Environmental Justice in Minority Populations and Low-Income Populations

I. General Information
A. Does this action apply to me?
      Table 1 of this preamble lists the industrial source category that is the subject of this supplemental 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 finalized, will be directly applicable to the affected sources. Federal, state, local and tribal government agencies are not affected by this proposed action. As defined in the "Initial List of Categories of Sources Under Section 112(c)(1) of the Clean Air Act Amendments of 1990" (see 57 FR 31576, July 16, 1992), the "Ferroalloys Production" source category is any facility engaged in producing ferroalloys such as ferrosilicon, ferromanganese, and ferrochrome. The EPA redefined the Ferroalloys Production source category when it promulgated the 1999 Ferroalloys Production standard so that it now includes only major sources that produce products containing manganese (Mn). (64 FR 27450, May 20, 1999). The 1999 standard applies specifically to two ferroalloy product types: ferromanganese and silicomanganese.
Table 1. NESHAP AND INDUSTRIAL SOURCE CATEGORIES AFFECTED BY THIS PROPOSED ACTION

                                Source Category

                                    NESHAP

                                 NAICS code[a]
Ferroalloys Production
Ferroalloys Production
331110
[a] 2012 North American Industry Classification System

B. Where can I get a copy of this document and other related information?
      In addition to being available in the docket, an electronic copy of this action is available on the Internet through EPA's Technology Transfer Network (TTN) website, 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 website: http://www.epa.gov/ttn/atw/ferroa/ferropg.html. Information on the overall residual risk and technology review program is available at the following website: http://www.epa.gov/ttn/atw/rrisk/rtrpg.html.
C. What should I consider as I prepare my comments for the EPA?
      Submitting CBI. Do not submit information containing CBI to the EPA through http://www.regulations.gov or email. Clearly mark the part or all of the information that you claim to be CBI. For CBI information on a disk or CD-ROM that you mail to the EPA, mark the outside of the disk or CD-ROM as CBI and then identify electronically within the disk or CD-ROM the specific information that is claimed as CBI. In addition to one complete version of the comments that includes information claimed as CBI, you must submit a copy of the comments that does not contain the information claimed as CBI for inclusion in the public docket. If you submit a CD-ROM or disk that does not contain CBI, mark the outside of the disk or CD-ROM clearly that it does not contain CBI. Information not marked as CBI will be included in the public docket and the EPA's electronic public docket without prior notice. Information marked as CBI will not be disclosed except in accordance with procedures set forth in 40 Code of Federal Regulations (CFR) part 2. Send or deliver information identified as CBI only to the following address: Roberto Morales, OAQPS Document Control Officer (C404-02), OAQPS, U.S. Environmental Protection Agency, Research Triangle Park, North Carolina 27711, Attention Docket ID Number EPA-HQ-OAR-2010-0895.
II. Background Information
A. What is the statutory authority for this action?
Section 112 of the Clean Air Act (CAA) establishes a two-stage regulatory process to address emissions of hazardous air pollutants (HAPs) from stationary sources. In the first stage, after the EPA has identified categories of sources emitting one or more of the HAP listed in CAA section 112(b), CAA section 112(d) requires us to promulgate technology-based NESHAP for those sources. "Major sources" are those that emit or have the potential to emit 10 tons per year (tpy) or more of a single HAP or 25 tpy or more of any combination of HAPs. For major sources, the technology-based NESHAP must reflect the maximum degree of emission reductions of HAPs achievable (after considering cost, energy requirements and non-air quality health and environmental impacts) and are commonly referred to as maximum achievable control technology (MACT) standards.
MACT standards must reflect the maximum degree of emissions reduction achievable through the application of measures, processes, methods, systems or techniques, including, but not limited to, measures that (1) reduce the volume of or eliminate pollutants through process changes, substitution of materials or other modifications; (2) enclose systems or processes to eliminate emissions; (3) capture or treat pollutants when released from a process, stack, storage or fugitive emissions point; (4) are design, equipment, work practice or operational standards (including requirements for operator training or certification); or (5) are a combination of the above. CAA section 112(d)(2)(A)-(E). The MACT standards may take the form of design, equipment, work practice or operational standards where the EPA first determines either that (1) a pollutant cannot be emitted through a conveyance designed and constructed to emit or capture the pollutant, or that any requirement for, or use of, such a conveyance would be inconsistent with law; or (2) the application of measurement methodology to a particular class of sources is not practicable due to technological and economic limitations. CAA section 112(h)(1)-(2).
The MACT "floor" is the minimum control level allowed for MACT standards promulgated under CAA section 112(d)(3) and may not be based on cost considerations. For new sources, the MACT floor cannot be less stringent than the emissions control that is achieved in practice by the best-controlled similar source. The MACT floor for existing sources can be less stringent than floors for new sources but not less stringent than the average emissions limitation achieved by the best-performing 12 percent of existing sources in the category or subcategory (or the best-performing five sources for categories or subcategories with fewer than 30 sources). In developing MACT standards, the EPA must also consider control options that are more stringent than the floor. We may establish standards more stringent than the floor based on considerations of the cost of achieving the emission reductions, any non-air quality health and environmental impacts and energy requirements.
The EPA is then required to review these technology-based standards and revise them "as necessary (taking into account developments in practices, processes, and control technologies)" no less frequently than every eight years. CAA section 112(d)(6). In conducting this review, the EPA is not required to recalculate the MACT floor. Natural Resources Defense Council (NRDC) v. EPA, 529 F.3d 1077, 1084 (D.C. Cir. 2008). Association of Battery Recyclers, Inc. v. EPA, 716 F.3d 667 (D.C. Cir. 2013).
The second stage in standard-setting focuses on reducing any remaining (i.e., "residual") risk according to CAA section 112(f). This provision requires, first, that the EPA prepare a report to Congress discussing (among other things) methods of calculating the risks posed (or potentially posed) by sources after implementation of the MACT standards, the public health significance of those risks and the EPA's recommendations as to legislation regarding such remaining risk. The EPA prepared and submitted the Residual Risk Report to Congress, EPA - 453/R - 99 - 001 (Risk Report) in March 1999. Congress did not act in response, thereby triggering the EPA's obligation under CAA section 112(f)(2) to analyze and address residual risk.
Section 112(f)(2) of the CAA requires the EPA to determine for source categories subject to MACT standards whether the emission standards provide an ample margin of safety to protect public health. Section 112(f)(2)(B) of the CAA expressly preserves the EPA's use of the two-step process for developing standards to address any residual risk and the agency's interpretation of "ample margin of safety" developed in the National Emissions Standards for Hazardous Air Pollutants: Benzene Emissions from Maleic Anhydride Plants, Ethylbenzene/Styrene Plants, Benzene Storage Vessels, Benzene Equipment Leaks, and Coke By-Product Recovery Plants (Benzene NESHAP) (54 FR 38044, September 14, 1989). The EPA notified Congress in the Risk Report that the agency intended to use the Benzene NESHAP approach in making CAA section 112(f) residual risk determinations (EPA - 453/R - 99 - 001, p. ES - 11). The EPA subsequently adopted this approach in its residual risk determinations and in a challenge to the risk review for the Synthetic Organic Chemical Manufacturing source category, the United States Court of Appeals for the District of Columbia Circuit upheld as reasonable the EPA's interpretation that subsection 112(f)(2) incorporates the standards established in the Benzene NESHAP. See NRDC v. EPA, 529 F.3d 1077, 1083 (D.C. Cir. 2008)("[S]ubsection 112(f)(2)(B) expressly incorporates the EPA's interpretation of the Clean Air Act from the Benzene standard, complete with a citation to the Federal Register."); see also A Legislative History of the Clean Air Act Amendments of 1990, vol. 1, p. 877 (Senate debate on Conference Report).
The first step in the process of evaluating residual risk is the determination of acceptable risk. If risks are unacceptable, the EPA cannot consider cost in identifying the emissions standards necessary to bring risks to an acceptable level. The second step is the determination of whether standards must be further revised in order to provide an ample margin of safety to protect public health. The ample margin of safety is the level at which the standards must be set, unless an even more stringent standard is necessary to prevent, taking into consideration costs, energy, safety and other relevant factors, an adverse environmental effect.
1. Step 1-Determination of Acceptability
The agency in the Benzene NESHAP concluded that "the acceptability of risk under section 112 is best judged on the basis of a broad set of health risk measures and information" and that the "judgment on acceptability cannot be reduced to any single factor." Id. at 38046. The determination of what represents an "acceptable" risk is based on a judgment of "what risks are acceptable in the world in which we live" (Risk Report at 178, quoting NRDC v. EPA, 824 F. 2d 1146, 1165 (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. 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.
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, but must consider cost, energy, safety and other relevant factors in doing so.
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, we stated as an overall objective:
      In protecting public health with an ample margin of safety under section 112, EPA strives to provide maximum feasible protection against risks to health from hazardous air pollutants by (1) protecting the greatest number of persons possible to an individual lifetime risk level no higher than approximately 1-in-1 million and (2) limiting to no higher than approximately 1-in-10 thousand [i.e., 100-in-1 million] the estimated risk that a person living near a plant would have if he or she were exposed to the maximum pollutant concentrations for 70 years.

The agency further stated that "[t]he EPA also considers incidence (the number of persons estimated to suffer cancer or other serious health effects as a result of exposure to a pollutant) to be an important measure of the health risk to the exposed population. Incidence measures the extent of health risks to the exposed population as a whole, by providing an estimate of the occurrence of cancer or other serious health effects in the exposed population." Id. at 38045.
      In the ample margin of safety decision process, the agency again considers all of the health risks and other health information considered in the first step, including the incremental risk reduction associated with standards more stringent than the MACT standard or a more stringent standard that EPA has determined is necessary to ensure risk is acceptable. In the ample margin of safety analysis, the agency considers additional factors, including costs and economic impacts of controls, technological feasibility, uncertainties and any other relevant factors. Considering all of these factors, the agency will establish the standard at a level that provides an ample margin of safety to protect the public health, as required by CAA section 112(f). 54 FR 38046.
B. What is this source category and how does the current NESHAP regulate its HAP emissions?
      Ferroalloys are alloys of iron in which one or more chemical elements (such as chromium, manganese, and silicon) are added into molten metal. Ferroalloys are consumed primarily in iron and steel making and are used to produce steel and cast iron products with enhanced or special properties. The ferroalloys products that are the focus of the NESHAP are ferromanganese (FeMn) and silicomanganese (SiMn), which are produced by two facilities in the United States. One facility (Eramet) is located in Marrietta, Ohio and produces both FeMn and SiMn. The other plant (Felman) is located in Letart, West Virginia and produces only SiMn.
      Ferroalloys within the scope of this source category are produced using submerged electric arc furnaces, which are furnaces in which the electrodes are submerged into the charge. The submerged arc process is a reduction smelting operation. The reactants consist of metallic ores (ferrous oxides, silicon oxides, manganese oxides, etc.) and a carbon-source reducing agent, usually in the form of coke, charcoal, high- and low-volatility coal, or wood chips. Raw materials are crushed and sized, and then conveyed to a mix house for weighing and blending. Conveyors, buckets, skip hoists, or cars transport the processed material to hoppers above the furnace. The mix is gravity-fed through a feed chute either continuously or intermittently, as needed. At high temperatures in the reaction zone, the carbon source reacts with metal oxides to form carbon monoxide and to reduce the ores to base metal. The molten material (product and slag) is tapped from the furnace, sometimes subject to post-furnace refining, and poured into casting beds on the furnace room floor. Once the material hardens, it is transported to product crushing and sizing systems and packaged for transport to the customer.
      The NESHAP for Ferroalloys Production: Ferromanganese and Silicomanganese were promulgated on May 20, 1999 (64 FR 27450) and codified at 40 CFR part 63, subpart XXX. The 1999 NESHAP applies to all new and existing ferroalloys production facilities that manufacture ferromanganese or silicomanganese and are major sources or are co-located at major sources of HAP emissions.
      The existing Ferroalloys Production NESHAP rule applies to process emissions from the submerged arc furnaces, the metal oxygen refining process, and the product crushing equipment; process fugitive emissions from the furnace; and outdoor fugitive dust emissions sources such as roadways, yard areas, and outdoor material storage and transfer operations. For the electric (submerged) arc furnace process, the NESHAP specifies numerical emissions limits for particulate matter (as a surrogate for non-mercury (or particulate) metal HAP). The NESHAP also includes emissions limits for particulate matter (again as a surrogate for particulate metal HAP) for process emissions from the metal oxygen refining process and product crushing and screening equipment. Table 2 is a summary of the applicable limits in the existing Subpart XXX.
                    Table 2. Emission Limits in Subpart XXX

                    New or Reconstructed or Existing Source

                                Affected Source

                       Applicable PM Emission Standards
                             Subpart XXX Reference
New or reconstructed
Submerged arc furnace
0.23 kilograms per hour per megawatt (kg/hr/MW) (0.51 pounds per hour per megawatt (lb/hr/MW) or 35 milligrams per dry standard cubic meter (mg/dscm) (0.015 grains per dry standard cubic foot (gr/dscf)
40 CFR 63.1652(a)(1) and (a)(2)
Existing
Open submerged arc furnace producing ferromanganese and operating at a furnace power input of 22 megawatts (MW) or less
9.8 kg/hr (21.7 lb/hr)
40 CFR 63.1652(b)(1)
Existing
Open submerged arc furnace producing ferromanganese and operating at a furnace power input greater than 22 MW
13.5 kg/hr (29.8 lb/hr)
40 CFR 63.1652(b)(2)
Existing
Open submerged arc furnace producing silicomanganese and operating at a furnace power input greater than 25 MW
16.3 kg/hr (35.9 lb/hr)
40 CFR 63.1652(b)(3)
Existing
Open submerged arc furnace producing silicomanganese and operating at a furnace power input of 25 MW or less
12.3 kg/hr (27.2 lb/hr)
40 CFR 63.1652(b)(4)
Existing
Semi-sealed submerged arc furnace (primary, tapping, and vent stacks) producing ferromanganese
11.2 kg/hr (24.7 lb/hr)
40 CFR 63.1652(c)
New, reconstructed, or existing
Metal oxygen refining process
69 mg/dscm (0.03 gr/dscf)
40 CFR 63.1652(d)
New or reconstructed
Individual equipment associated with the product crushing and screening operation
50 mg/dscm (0.022 gr/dscf)
40 CFR 63.1652(e)(1)
Existing
Individual equipment associated with the product crushing and screening operation
69 mg/dscm (0.03 gr/dscf)
40 CFR 63.1652(e)(2)

      The 1999 NESHAP established a building opacity limit of 20 percent that is measured during the required furnace control device performance test. The rule provides an excursion limit of 60 percent opacity for one 6-minute period during the performance test. The opacity observation is focused only on emissions exiting the shop due solely to operations of any affected submerged arc furnace. In addition, blowing taps, poling and oxygen lancing of the tap hole, burndowns associated with electrode measurements, and maintenance activities associated with submerged arc furnaces and casting operations are exempt from the opacity standards specified in §63.1653.
      For outdoor fugitive dust sources, as defined in §63.1652, the 1999 NESHAP requires that plants prepare and operate according to an outdoor fugitive dust control plan that describes in detail the measures that will be put in place to control outdoor fugitive dust emissions from the individual outdoor fugitive dust sources at the facility. The owner or operator must submit a copy of the outdoor fugitive dust control plan to the designated permitting authority on or before the applicable compliance date.
C. What is the history of the Ferroalloys Production Risk and Technology Review?
      Pursuant to section 112(f)(2) of the Clean Air Act, we first evaluated the residual risk associated with the Ferroalloys Production NESHAP in 2011. We also conducted a technology review, as required by section 112(d)(6) of the Clean Air Act. Finally, we also reviewed the 1999 MACT rule to determine if other amendments were appropriate. Based on the results of that previous residual risk and technology review, and the MACT rule review, we proposed amendments to subpart XXX on November 23, 2011 (76 FR 72508) (referred to from here on as the 2011 proposal in the remainder of this FR notice). The proposed amendments in the 2011 proposal which we are revisiting in today's supplemental proposal include the following:
      * proposed revisions to particulate matter (PM) standards for electric arc furnaces and local ventilation control devices;
      * emission limits for mercury, polycyclic aromatic hydrocarbons (PAHs), and hydrochloric acid (HCl);
      * proposed requirements to control process fugitive emissions based on full-building enclosure with negative pressure, or fenceline monitoring as an alternative;
      * a provision for emissions averaging; and
      * amendments to the monitoring, notification, recordkeeping and testing requirements.
      The comment period for the November 2011 proposal opened on November 23, 2011 and ended on January 31, 2012. We received significant comments from industry representatives, environmental organizations, and local community groups. We also met with stakeholders (from industry, community groups and environmental organizations) after proposal to further discuss their comments, concerns and related issues.
      After reviewing the comments, and after consideration of additional data and information received since the 2011 proposal, we determined it is appropriate to revise some of our analyses and publish a supplemental proposal. Therefore, in today's Notice of Supplemental Proposed Rulemaking we present revised analyses, and based on those analyses we are proposing revised amendments for the items listed above to allow the public an opportunity to review and comment on these revised analyses and revised proposed amendments.
      However, we also proposed other requirements in the 2011 proposal (listed below) for which we have made no revisions to the analyses, we are not proposing any changes, and are not reopening for public comment. The other requirements that we proposed in the 2011 proposal, for which we are not re-opening for comment, are the following:
         * PM standards for metal oxygen refining processes and crushing and screening operations;
         * emissions limits for formaldehyde;
         * elimination of SSM exemptions; and
         * electronic reporting.
      The comment period for the November 2011 proposal opened on November 23, 2011 and ended on January 31, 2012. We will address the comments we received during the public comment period for the 2011 proposal at the time we take final action.
	In the 2011 proposal, we also included information about several ATSDR health consultations and a study (Kim et al) that had been conducted in the Marietta area. We note that the Kim et al study was included in the 2012 ATSDR review of manganese.  Since the 2011 proposal, additional studies on the potential toxicity of manganese have been published. These studies add to the literature regarding potential health effects from exposure to manganese and will be included, along with the complete body of scientific evidence, in future reviews of manganese toxicity.
D. What data collection activities were conducted to support this action?
	Commenters on the 2011 proposal expressed concern that the data set used in the risk assessment did not adequately reflect current operations at the plants. In response to these comments, we worked with the facilities to address these concerns, and we obtained a significant amount of new data in order to establish a more robust data set than the data set we had for the 2011 proposal. Specifically, the plants provided data collected during their fall 2012 compliance tests, and, in response to an Information Collection Request (ICR) from the EPA in December 2012, they conducted more tests in the spring of 2013. This combined testing effort provided the following data:
   * Additional stack test data for arsenic, cadmium, chromium, lead, manganese, mercury, nickel, HCl, formaldehyde, PAH, polychlorinated biphenyls (PCB), and dioxins/furans;
   * Test data collected using updated, state-of-the-art test methods and procedures;
   * Hazardous air pollutant (HAP) test data for all operational furnaces;
   * Test data obtained during different seasonal conditions (i.e., spring and fall);
   * Test data for both products (ferromanganese and silicomanganese) for both furnaces at Eramet (Felman only produces silicomanganese).
      With the new data, we no longer have to extrapolate HAP emissions from a ratio of PM to HAP emissions from just one or two tested furnaces. We are also using test data collected using state-of-the-art test methods that provide better QA/QC of the test results. For mercury, test data were collected for the supplemental proposal using EPA Method 30B, which requires paired samples collected for each test run, in addition to a spiked sample during the 3-run test. Test data for PAH were collected using CARB 429, which provides greater sensitivity, precision, and identification of individual PAH compounds as compared to Method 0010 which was used for previous tests. We also received PCB and dioxin/furan test data that were collected using CARB 428, which uses high resolution instruments and provides a specific procedure for measuring PCBs in addition to dioxin/furans.
      Commenters also expressed concern that the estimated cost and operational impacts of the 2011 proposed process fugitive standards based on use of a total building enclosure requirement were significantly underestimated. In their comments both companies submitted substantial additional information and estimates regarding the elements, costs and impacts involved with constructing and operating a full building enclosure for their facilities. We also received comments saying that full-enclosure with negative pressure can lead to worker safety and health issues related to indoor air quality if the systems are  not designed and operated appropriately to provide sufficient air exchanges and air conditioning in the work space. Furthermore, in their comments, and in subsequent meetings and other communications, the companies also provided design and cost information for an alternative approach to substantially reduce fugitive emissions based on enhanced local capture and control of these emissions at each plant. In the summer of 2012 and fall of 2013, both plants submitted updated enhanced capture plans and cost estimates to implement those plans. We also consulted with outside ventilation experts and control equipment vendors to re-evaluate the costs of process fugitive capture as well as costs of other control measures such as activated carbon injection. We also gathered a substantial amount of opacity data from both facilities, and collected additional information regarding the processes, control technologies, and modeling input parameters (such as stack release heights and fugitive emissions release characteristics). We reviewed and evaluated all the data and information provided by the facilities, the ventilation experts and vendors, and revised our analyses accordingly.
III. Analytical Procedures
A. For purposes of this supplemental proposal, how did we estimate the post-MACT risks posed by the Ferroalloys Production source category?
      The EPA conducted a risk assessment that provided 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 noncancer health effects, and the hazard quotient (HQ) for acute exposures to HAP with the potential to cause noncancer health effects. The assessment also provided estimates of the distribution of cancer risks within the exposed populations, cancer incidence and an evaluation of the potential for adverse environmental effects for each source category. The risk assessment consisted of eight primary steps, as discussed in detail in the 2011 proposal. The docket for this rulemaking contains the following document which provides more information on the risk assessment inputs and models: Revised Draft Residual Risk Assessment for the Ferroalloys Production Source Category. The methods used to assess risks (as described in the eight primary steps below) are consistent with those peer-reviewed by a panel of the EPA's Science Advisory Board (SAB) in 2009 and described in their peer review report issued in 2010; they are also consistent with the key recommendations contained in that report.
1. How did we estimate actual emissions and identify the emissions release characteristics?
      As explained previously, the revised data set for the ferroalloys production source category, derived from the two existing ferromanganese and silicomanganese production facilities, constitutes the basis for the revised risk assessment. We estimated the magnitude of emissions using emissions test data collected through ICRs along with additional data submitted voluntarily by the companies. We also collected information regarding emissions release characteristics such as stack heights, stack gas exit velocities, stack temperatures, and source locations. In addition to the quality assurance (QA) of the source data for the facilities contained in the data set, we also checked the coordinates of every emission source in the data set through visual observations using tools such as GoogleEarth and ArcView. Where coordinates were found to be incorrect, we identified and corrected them to the extent possible. We also performed a QA assessment of the emissions data and release characteristics to ensure the data were reliable and that there were no outliers. The emissions data, and the methods used to estimate emissions from all the various emissions sources, are described in more detail in the technical document: Revised Draft Development of the RTR Emissions Dataset for the Ferroalloys Production 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 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.)
      For this supplemental proposal, we evaluated allowable stack emissions based on the level of control required by the 1999 MACT standards. We also evaluated the level of reported actual emissions and available information on the level of control achieved by the emissions controls in use. Further explanation is provided in the technical document: Revised Draft Development of the RTR Emissions Dataset for the Ferroalloys Production Source Category, which is available in the docket.
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, and (3) estimating individual and population-level inhalation risks using the exposure estimates and quantitative dose-response information.
      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. 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 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.
      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 (μg/m3)) 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.
      In the case of nickel compounds, to provide a health-protective estimate of potential cancer risks, we used the IRIS URE value for nickel subsulfide in the assessment for the 2011 proposed rule for ferroalloys production. Based on past scientific and technical considerations, the determination of the percent of nickel subsulfide was considered a major factor for estimating the extent and magnitude of the risks of cancer due to nickel-containing emissions. Nickel speciation information for some of the largest nickel-emitting sources (including oil combustion, coal combustion, and others) suggested that at least 35 percent of total nickel emissions may be soluble compounds and that the URE for the mixture of inhaled nickel compounds (based on nickel subsulfide, and representative of pure insoluble crystalline nickel) could be derived to reflect the assumption that 65 percent of the total mass of nickel may be carcinogenic.
      Based on consistent views of major scientific bodies (i.e., National Toxicology Program (NTP) in their 12[th] Report of the Carcinogens (ROC), International Agency for Research on Cancer (IARC), and other international agencies) that consider all nickel compounds to be carcinogenic, we currently consider all nickel compounds to have the potential of being carcinogenic to humans. The 12th Report of the Carcinogens states that the "combined results of epidemiological studies, mechanistic studies, and carcinogenic studies in rodents support the concept that nickel compounds generate nickel ions in target cells at sites critical for carcinogenesis, thus allowing consideration and evaluation of these compounds as a single group." Although the precise nickel compound (or compounds) responsible for the carcinogenic effects in humans is not always clear, studies indicate that nickel sulfate and the combinations of nickel sulfides and oxides encountered in the nickel refining industries cause cancer in humans (these studies are summarized in a review by Grimsrud et al., 2010). The major scientific bodies mentioned above have also recognized that there are differences in toxicity and/or carcinogenic potential across the different nickel compounds.
      In the inhalation risk assessment for proposal, to take a conservative approach, we considered all nickel compounds to be as carcinogenic as nickel subsulfide and applied the IRIS URE for nickel subsulfide without a factor to reflect the assumption that 100 percent of the total mass of nickel may be as carcinogenic as pure nickel subsulfide. However, given that there are two additional URE values derived for exposure to mixtures of nickel compounds, as a group, that are 2-3 fold lower than the IRIS URE for nickel subsulfide, the EPA also considers it reasonable to use a value that is 50 percent of the IRIS URE for nickel subsulfide for providing an estimate of the lower end of the plausible range of cancer potency values for different mixtures of nickel compounds. In the public comments provided in response to the proposal and available in the docket, one facility provided additional data in the form of a laboratory test report that indicated it would be unlikely that 100 percent of the nickel from the furnace would be in the form of nickel subsulfide. Given this new information, and the above assumptions about the cancer potency for different mixtures of nickel compounds, we believe it is reasonable to apply the URE for nickel refinery dust to all nickel compounds in the revised data set for the supplemental proposal.
      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) 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) and the limitations of Gaussian dispersion models, including AERMOD.
      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 (MRL) (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 (μg/m[3]) for inhalation exposure and in a dose expressed in units of milligram per kilogram-day (mg/kg-day) for oral exposures), at or below which no adverse health effects are anticipated for a specified exposure duration"; or (3), as noted above, a scientifically credible dose-response value that has been developed in a manner consistent with the EPA guidelines and has undergone a peer review process similar to that used by the EPA, in place of or in concert with other values.
      As mentioned above, in order to characterize non-cancer chronic effects, and in response to key recommendations from the SAB, the EPA selects dose-response values that reflect the best available science for all HAP included in RTR risk assessments. More specifically, for a given HAP, EPA examines the availability of inhalation reference values from the sources included in our tiered approach (e.g., IRIS first, ATSDR second, CalEPA third) and determines which inhalation reference value represents the best available science. Thus, as new inhalation reference values become available, EPA will typically evaluate them and determine whether they should be given preference over those currently being used in RTR risk assessments.
      There is an existing IRIS RfC for manganese (Mn) published in 1993. This value was used in the RTR risk assessment supporting the Ferroalloys Notice of Proposed Rulemaking. However, since the 2011 proposal, ATSDR has published an assessment of Mn toxicity (2012) which includes a chronic inhalation value (i.e., an ATSDR Minimal Risk Level or MRL). Both the 1993 IRIS RfC and the 2012 ATSDR MRL were based on the same study (Roels et al., 1993).  In developing their assessment, ATSDR used updated dose-response modeling methodology (benchmark dose approach) and considered recent pharmacokinetic findings to support their MRL derivation.  In the interest of using the best available science, the EPA has chosen in this instance to rely on the ATSDR MRL for Mn in the current ferroalloys supplemental proposal.
      The EPA also evaluated screening estimates of acute exposures and risks for each of the HAP at the point of highest potential off-site exposure for each facility. To do this EPA estimated the risks when both the peak hourly emissions rate and worst-case dispersion conditions occur. We also assume that a person is located at the point of highest impact during that same time. In accordance with our mandate in Section 112 of the Clean Air Act, we use the point of highest off-site exposure to assess the potential risk to the maximally exposed individual.  The acute HQ is the estimated acute exposure divided by the acute dose-response value. In each case, the EPA calculated acute HQ values using best available, short-term dose-response values. These acute dose-response values, which are described below, include the acute REL, acute exposure guideline levels (AEGL) and emergency response planning guidelines (ERPG) for 1-hour exposure durations. As discussed below, we used conservative assumptions for emissions rates, meteorology and exposure location for our acute analysis.
      As described in the CalEPA's Air Toxics Hot Spots Program Risk Assessment Guidelines, Part I, The Determination of Acute Reference Exposure Levels for Airborne Toxicants, an acute REL value (http://www.oehha.ca.gov/air/pdf/acuterel.pdf) is defined as "the concentration level at or below which no adverse health effects are anticipated for a specified exposure duration." Id. at page 2. Acute REL values are based on the most sensitive, relevant, adverse health effect reported in the peer-reviewed medical and toxicological literature. Acute REL values are designed to protect the most sensitive individuals in the population through the inclusion of margins of safety. Because margins of safety are incorporated to address data gaps and uncertainties, exceeding the REL does not automatically indicate an adverse health impact.
      AEGL values were derived in response to recommendations from the National Research Council (NRC). As described in Standing Operating Procedures (SOP) of the National Advisory Committee on Acute Exposure Guideline Levels for Hazardous Substances (http://www.epa.gov/oppt/aegl/pubs/sop.pdf), "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.
      The document lays out the purpose and objectives of AEGL by stating that "the primary purpose of the AEGL program and the National Advisory Committee for Acute Exposure Guideline Levels for Hazardous Substances is to develop guideline levels for once-in-a-lifetime, short-term exposures to airborne concentrations of acutely toxic, high-priority chemicals." Id. at 21. In detailing the intended application of AEGL values, the document states that "[i]t is anticipated that the AEGL values will be used for regulatory and nonregulatory purposes by U.S. Federal and state agencies and possibly the international community in conjunction with chemical emergency response, planning, and prevention programs. More specifically, the AEGL values will be used for conducting various risk assessments to aid in the development of emergency preparedness and prevention plans, as well as real-time emergency response actions, for accidental chemical releases at fixed facilities and from transport carriers." Id. at 31.
      The AEGL - 1 value is then specifically defined as "the airborne concentration (expressed as ppm (parts per million) or mg/m[3] (milligrams per cubic meter)) of a substance above which it is predicted that the general population, including susceptible individuals, could experience notable discomfort, irritation, or certain asymptomatic nonsensory effects. However, the effects are not disabling and are transient and reversible upon cessation of exposure." Id. at 3. The document also notes that, "Airborne concentrations below AEGL - 1 represent exposure levels that can produce mild and progressively increasing but transient and nondisabling odor, taste, and sensory irritation or certain asymptomatic, nonsensory effects." Id. Similarly, the document defines AEGL - 2 values as "the airborne concentration (expressed as parts per million or milligrams per cubic meter) of a substance above which it is predicted that the general population, including susceptible individuals, could experience irreversible or other serious, long-lasting adverse health effects or an impaired ability to escape." Id.
      ERPG values are derived for use in emergency response, as described in the American Industrial Hygiene Association's ERP Committee document entitled, ERPGS Procedures and Responsibilities (http://sp4m.aiha.org/insideaiha/GuidelineDevelopment/ERPG/Documents/ERP-SOPs2006.pdf), which states that, "Emergency Response Planning Guidelines were developed for emergency planning and are intended as health based guideline concentrations for single exposures to chemicals." 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.
      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 99[th] percentile ratio of peak hourly emissions rate to the annual average hourly emissions rate was 9. 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, data were available to determine process-specific factors. Some processes, for example the electric arc furnaces, operate continuously so there are no peak emissions. These processes received a factor of 1 in the acute assessment. Other processes, for example tapping and casting, have specific cycles, with peak emissions occurring for a part of that cycle (e.g., 30 minutes during a 2-hour period). For these processes, we used a factor of 4 in the acute assessment.  Even with data available to develop process-specific factors, our acute assessment is still conservative in that it assumes that every process releases its peak emissions at the same hour, and that this is the same hour as the worst-case dispersion conditions. This results in a highly conservative exposure scenario. A further discussion of why this factor of 4 was chosen can be found in the memorandum, Revised Draft Development of the RTR Emissions Dataset for the Ferroalloys Production Source Category, available in the docket for this rulemaking.
      As part of our acute risk assessment process, for cases where acute HQ values from the screening step were less than or equal to 1 (even under the conservative assumptions of the screening analysis), acute impacts were deemed negligible and no further analysis was performed. In cases where an acute HQ from the screening step was greater than 1, additional site-specific data were considered to develop a more refined estimate of the potential for acute impacts of concern. For this source category, the data refinements employed consisted of determining that the receptor with the maximum concentration was off of plant property. These refinements are discussed more fully in the Draft Residual Risk Assessment for the Ferroalloys Production Source Category, which is available in the docket for this source category. Ideally, we would prefer to have continuous measurements over time to see how the emissions vary by each hour over an entire year. Having a frequency distribution of hourly emissions rates over a year would allow us to perform a probabilistic analysis to estimate potential threshold exceedances and their frequency of occurrence. Such an evaluation could include a more complete statistical treatment of the key parameters and elements adopted in this screening analysis. Recognizing that this level of data is rarely available, we instead rely on the multiplier approach.
      To better characterize the potential health risks associated with estimated acute exposures to HAP, and in response to a key recommendation from the SAB's peer review of the EPA's RTR risk assessment methodologies, 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 for HAP have been developed, we consider additional acute values (i.e., occupational and international values) to provide a more complete risk characterization.
4. How did we conduct the multipathway exposure and risk screening?
      The EPA conducted a screening analysis examining the potential for significant human health risks due to exposures via routes other than inhalation (i.e., ingestion). We first determined whether any sources in the source 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 Ferroalloys Production source category, we identified emissions of cadmium compounds, chlorinated dibenzodioxins and furans, lead compounds, mercury compounds, and polycyclic organic matter. Because one or more of these PB-HAP are emitted by at least one facility in the Ferroalloys Production source category, we proceeded to the second step of the evaluation. In this step, we determined whether the facility-specific emissions rates of each of the emitted PB - HAP were large enough to create the potential for significant non-inhalation human health risks under reasonable worst-case conditions. To facilitate this step, we developed emissions rate thresholds for each PB - HAP using a hypothetical upper-end screening exposure scenario developed for use in conjunction with the EPA's Total Risk Integrated Methodology.Fate, Transport, and Ecological Exposure (TRIM.FaTE) model. We conducted a sensitivity analysis on the screening scenario to ensure that its key design parameters would represent the upper end of the range of possible values, such that it would represent a conservative but not impossible scenario. The facility-specific emissions rates of each of the PB - HAP were compared to the emission rate threshold values for each of the PB - HAP identified to assess the potential for significant human health risks via non-inhalation pathways. We call this application of the TRIM.FaTE model the Tier I TRIM-screen or Tier I screen.
      For the purpose of developing emissions rates for our Tier I TRIM-screen, we derived emission levels for each PB-HAP (other than lead) at which the maximum excess lifetime cancer risk would be 1-in-1 million or, for HAP that cause non-cancer health effects, the maximum hazard quotient would be 1. If the emissions rate of any PB-HAP exceeds the Tier I screening emissions rate for any facility, we conduct a 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.
      If the PB-HAP emissions for a facility exceed the Tier II screening emissions rate, and data are available, we may decide to conduct a more refined multipathway assessment. A refined assessment replaces some of the assumptions made in the Tier II screen, with site-specific data. The refined assessment also uses the TRIM.FaTE model and facility-specific emission rate thresholds that are created for each PB-HAP. For the ferroalloys production source category, we did conduct a refined multipathway assessment for one facility in the category. A detailed discussion of the approach for this assessment can be found in Appendix 10 (Technical Support Document:  Human Health Multipathway Residual Risk Assessment for the Ferroalloys Production Source Category) of the risk assessment document.
      In evaluating the potential multi-pathway risk from emissions of lead compounds, rather than developing a screening emissions rate for them, we compared maximum estimated chronic inhalation exposures with the level of the current National Ambient Air Quality Standard (NAAQS) for lead. Values below the level of the primary (health-based) lead NAAQS were considered to have a low potential for multi-pathway risk.
      For further information on the multipathway analysis approach, see the Draft Residual Risk Assessment for the Ferroalloys Production Source Category, which is available in the docket for this action.
5. How did we assess risks considering the revised emissions control options?
      In addition to assessing baseline inhalation risks and potential multipathway risks, we also estimated risks considering the emissions reductions that would be achieved by the control options under consideration in this supplemental proposal. In these cases, the expected emissions reductions were applied to the specific HAP and emissions points in the source category data set to develop corresponding estimates of risk that would exist after implementation of the proposed amendments in today's action.
6. How did we conduct the environmental risk screening assessment?
a. Adverse Environmental Effect
      The EPA has developed a screening approach to examine the potential for adverse environmental effects as required under section 112(f)(2)(A) of the CAA. Section 112(a)(7) of the CAA defines "adverse environmental effect" as "any significant and widespread adverse effect, which may reasonably be anticipated, to wildlife, aquatic life, or other natural resources, including adverse impacts on populations of endangered or threatened species or significant degradation of environmental quality over broad areas."
b. Environmental HAP
      The EPA focuses on seven HAP, which we refer to as "environmental HAP," in its screening analysis: five persistent bioaccumulative HAP (PB-HAP) and two acid gases. The five PB-HAP are cadmium, dioxins/furans, polycyclic organic matter (POM), mercury (both inorganic mercury and methyl mercury) and lead. The two acid gases are hydrogen chloride (HCl) and hydrogen fluoride (HF). The rationale for including these seven HAP in the environmental risk screening analysis is presented below.
      The 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 increase as does the potential for adverse effects. The five PB-HAP we evaluate as part of our screening analysis account for 99.8 percent of all PB-HAP emissions (on a mass basis from the 2005 NEI).
      In addition to accounting for almost all of the mass of PB-HAP emitted, we note that the TRIM.Fate model that we use to evaluate multipathway risk allows us to estimate concentrations of cadmium compounds, dioxins/furans, POM and mercury in soil, sediment and water. For lead, we currently do not have the ability to calculate these concentrations using the TRIM.Fate model. Therefore, to evaluate the potential for adverse environmental effects from lead, we compare the 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. We consider values below the level of the secondary lead NAAQS as unlikely to cause adverse environmental effects.
      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 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 Administration (NOAA)) or state agencies.
      Benchmarks for all effect levels are not available for all PB-HAP and assessment endpoints. In cases where multiple effect levels were available for a particular PB-HAP and assessment endpoint, we use all of the available effect levels to help us to determine whether ecological risks exist and, if so, whether the risks could be considered significant and widespread.
d. Ecological assessment endpoints and benchmarks for acid gases
      The environmental screening analysis also evaluated potential damage and reduced productivity of plants due to direct exposure to acid gases in the air. For acid gases, we evaluated the following ecological assessment endpoint:
      * Local terrestrial plant communities with foliage exposed to acidic gaseous HAP in the air.
      The selection of ecological benchmarks for the effects of acid gases on plants followed the same approach as for PB-HAP (i.e., we examine all of the available 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 ferroalloys production source category sources emitted any of the seven environmental HAP. For the ferroalloys production source category, we identified emissions of five of the PB HAP (cadmium, mercury, lead, dioxins, and polycyclic organic matter) and one acid gas (HCl).
      Because one or more of the seven environmental HAP evaluated are emitted by the facilities 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 is analyzed differently as discussed earlier. In the first tier, we determined whether the maximum facility-specific emission rates of each of the emitted environmental HAP were large enough to create the potential for adverse environmental effects under reasonable worst-case environmental conditions. These are the same environmental conditions used in the human multipathway exposure and risk screening analysis.
      To facilitate this step, TRIM.FaTE was run for each PB-HAP under hypothetical environmental conditions designed to provide conservatively high HAP concentrations. The model was set to maximize runoff from terrestrial parcels into the modeled lake, which in turn, maximized the chemical concentrations in the water, the sediments, and the fish. The resulting media concentrations were then used to back-calculate a screening threshold emission rate that corresponded to the relevant exposure benchmark concentration value for each assessment endpoint. To assess emissions from a facility, the reported emission rate for each PB-HAP was compared to the screening threshold emission rate for that PB-HAP for each assessment endpoint. If emissions from a facility do not exceed the Tier I threshold, the facility "passes" the screen, and therefore, is not evaluated further under the screening approach. If emissions from a facility exceed the Tier I threshold, we evaluate the facility further in Tier II.
      In Tier II of the environmental screening analysis, the screening emission thresholds are adjusted to account for local meteorology and the actual location of lakes in the vicinity of facilities that did not pass the Tier I screen. The modeling domain for each facility in the tier II analysis consists of eight octants. Each octant contains 5 modeled soil concentrations at various distances from the facility (5 soil concentrations x 8 octants = total of 40 soil concentrations per facility) and 1 lake with modeled concentrations for water, sediment and fish tissue. In the tier II environmental risk screening analysis, the 40 soil concentration points are averaged to obtain an average soil concentration for each facility for each PB-HAP. For the water, sediment and fish tissue concentrations, the highest value for each facility for each pollutant is used. If emission concentrations from a facility do not exceed the Tier II threshold, the facility passes the screen, and is typically not evaluated further. If emissions from a facility exceed the Tier II threshold, the facility does not pass the screen and, therefore, may have the potential to cause adverse environmental effects. Such facilities are evaluated further to investigate factors such as the magnitude and characteristics of the area of exceedance.
g. Acid Gas Methodology
      The environmental screening analysis evaluates the potential phytotoxicity and reduced productivity of plants due to chronic exposure to acid gases. The environmental risk screening methodology for acid gases is a single-tier screen that compares the average off-site ambient air concentration over the modeling domain to ecological benchmarks for each of the acid gases. Because air concentrations are compared directly to the ecological benchmarks, emission-based thresholds are not calculated for acid gases as they are in the ecological risk screening methodology for PB-HAPs.
      For purposes of ecological risk screening, EPA identifies a potential for adverse environmental effects to plant communities from exposure to acid gases when the average concentration of the HAP around a facility exceeds the LOAEL ecological benchmark. In such cases, we further investigate factors such as the magnitude and characteristics of the area of exceedance (e.g., land use of exceedance area, size of exceedance area) to determine if there is an adverse environmental effect.
For further information on the environmental screening analysis approach, see the Revised Draft Residual Risk Assessment for the Ferroalloys Production 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 of interest, but also emissions of HAP from all other emissions sources at the facility for which we have data. However, for the Ferroalloys Production source category, we did not identify other HAP emissions sources located at these facilities. Thus, we did not perform a separate facility wide risk assessment.
8. How did we consider uncertainties in risk assessment
      In the Benzene NESHAP, we concluded that risk estimation uncertainty should be considered in our decision-making under the ample margin of safety framework. Uncertainty and the potential for bias are inherent in all risk assessments, including those performed for this proposal. Although uncertainty exists, we believe that our approach, which used conservative tools and assumptions, ensures that our decisions are health protective and environmentally protective. A brief discussion of the uncertainties in the emissions dataset, dispersion modeling, inhalation exposure estimates and dose-response relationships follows below. A more thorough discussion of these uncertainties is included in the Revised Draft Development of the RTR Emissions Dataset for the Ferroalloys Production Source Category and the other uncertainties are described in more detail in the Revised Draft Residual Risk Assessment for the Ferroalloys Production Source Category, which is available in the docket for this action.
a. Uncertainties in the Emissions Dataset
      Although the development of the RTR 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.
      As described above and in the emissions technical document, we gathered a substantial amount of emissions test data for the stack emissions from both facilities. Therefore, the level of uncertainty in the estimates of HAP emissions from the stacks is relatively low. Regarding fugitive emissions, we lack direct quantitative measurements of these emissions, therefore, we had to rely on available emissions factors and other technical information to derive the best estimates of emissions for these emissions. To estimate these fugitive emissions, we relied on information and observations gathered through several site visits by the EPA technical experts, reviewed and evaluated all available emissions factors, and analyzed other relevant information such as the measured ratios of HAP metals to particulate matter, estimated capture efficiencies of the various ventilation hoods currently used to capture and control some of the fugitive emissions, and the production rates for various products. Based on this information, we have derived the best estimates of fugitive emissions from these sources. Details are described in the Revised Draft Development of the RTR Emissions Dataset for the Ferroalloys Production Source Category, which is available in the docket for this action. Nevertheless, there are still some uncertainties regarding the precise quantities of fugitive HAP being emitted from these plants.
b. Uncertainties in Dispersion Modeling
      We recognize there is uncertainty in ambient concentration estimates associated with any model, including the EPA's recommended regulatory dispersion model, AERMOD. In using a model to estimate ambient pollutant concentrations, the user chooses certain options to apply. For RTR assessments, we select some model options that have the potential to overestimate ambient air concentrations (e.g., not including plume depletion or pollutant transformation). We select other model options that have the potential to underestimate ambient impacts (e.g., not including building downwash). Other options that we select have the potential to either under- or overestimate ambient levels (e.g., meteorology and receptor locations). On balance, considering the directional nature of the uncertainties commonly present in ambient concentrations estimated by dispersion models, the approach we apply in the RTR assessments should yield unbiased estimates of ambient HAP concentrations.
c. Uncertainties in Inhalation Exposure
      The EPA did not include the effects of human mobility on exposures in the assessment. Specifically, short-term mobility and long-term mobility between census blocks in the modeling domain were not considered. 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).
      In addition, the assessment predicted the chronic exposures at the centroid of each populated census block as surrogates for the exposure concentrations for all people living in that block. Using the census block centroid to predict chronic exposures tends to over-predict exposures for people in the census block who live farther from the facility and under-predict exposures for people in the census block who live closer to the facility. Thus, using the census block centroid to predict chronic exposures may lead to a potential understatement or overstatement of the true maximum impact, but is an unbiased estimate of average risk and incidence. We reduce this uncertainty by analyzing large census blocks near facilities using aerial imagery and adjusting the location of the block centroid to better represent the population in the block, as well as adding additional receptors where the block population is not well represented by a single location.
      The assessment evaluates the cancer inhalation risks associated with pollutant exposures over a 70-year period, which is the assumed lifetime of an individual. In reality, both the length of time that modeled 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 overstatement of 25 to 30 percent of exposures.
      In addition to the uncertainties highlighted above, there are several factors specific to the acute exposure assessment that EPA conducts as part of the risk review under section 112 of the CAA that should be highlighted. The accuracy of an acute inhalation exposure assessment depends on the simultaneous occurrence of independent factors that may vary greatly, such as hourly emissions rates, meteorology and the presence of humans at the location of the maximum concentration. In the acute screening assessment that we conduct under the RTR program, we assume that peak emissions from the source category and worst-case meteorological conditions co-occur, thus resulting in maximum ambient concentrations. These two events are unlikely to occur at the same time, making these assumptions conservative.  We then include the additional assumption that a person is located at this point during this same time period.  For this source category, these assumptions would tend to be worst-case actual exposures as it is unlikely that a person would be located at the point of maximum exposure during the time of worst-case impact.
d. Uncertainties in Dose-Response Relationships
      There are uncertainties inherent in the development of the dose-response values used in our risk assessments for cancer effects from chronic exposures and non-cancer effects from both chronic and acute exposures. Some uncertainties may be considered quantitatively, and others generally are expressed in qualitative terms. We note as a 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 Revised Draft Risk Assessment for the Ferroalloys Production 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). In some circumstances, the true risk could be as low as zero; however, in other circumstances the risk could be greater. 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).
      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, e.g., factors of 10 or 3, used in the absence of compound-specific data; where data are available, UF may also be developed using compound-specific information. When data are limited, more assumptions are needed and more UF are used. Thus, there may be a greater tendency to overestimate risk in the sense that further study might support development of reference values that are higher (i.e., less potent) because fewer default assumptions are needed. However, for some pollutants, it is possible that risks may be underestimated.
      While collectively termed "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 unspeciated (e.g., glycol ethers), we conservatively use the most protective reference value of an individual compound in that group to estimate risk. Similarly, for 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.
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.
      We also completed a refined multi-pathway assessment for this supplemental proposal. The refined assessment contains considerably less uncertainty compared to the Tier I and Tier II screens. Nevertheless, some uncertainties also exist with the refined assessments. The refined multi-pathway assessment and related uncertainties are described in detail in the risk document Appendix 10, Revised Draft Residual Risk Assessment for the Ferroalloys Production Source Category, which is available in the docket for this action.
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.
	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, which was 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, which was 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 evaluates 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, where applicable. 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 screening methods is provided in Appendix 4 of the document "Technical Support Document for TRIM-Based Multipathway Tiered Screening Methodology for RTR: Summary of Approach and Evaluation." Also, see the Revised Draft Residual Risk Assessment for Ferroalloys Production Source Category, available in the docket for this action.
B. How did we consider the risk results in making decisions for this supplemental 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) of approximately [1-in-10 thousand] [i.e., 100-in-1 million]." 54 FR 38045. 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.
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 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. 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'."

54 FR at 38057. Thus, the level of the MIR is only one factor to be weighed in determining acceptability of risks. The Benzene NESHAP explained that "an MIR of approximately 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.