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Federal Register / Vol. 86, No. 5 / Friday, January 8, 2021 / Proposed Rules factors relevant to each particular decision. Id. The EPA must promulgate emission standards necessary to provide an ample margin of safety to protect public health or determine that the standards being reviewed provide an ample margin of safety without any revisions. After conducting the ample margin of safety analysis, we consider whether a more stringent standard is necessary to prevent, taking into consideration costs, energy, safety, and other relevant factors, an adverse environmental effect as defined in CAA section 112a7.
CAA section 112d6 separately requires the EPA to review standards promulgated under CAA section 112
and revise them as necessary taking into account developments in practices, processes, and control technologies no less often than every 8 years. In conducting this review, which we call the technology 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 EPA may consider cost in deciding whether to revise the standards pursuant to CAA section 112d6. The EPA is required to address regulatory gaps, such as missing standards for listed HAP known to be emitted from the source category.
Louisiana Environmental Action Network LEAN v. EPA, 955 F.3d 1088
D.C. Cir. 2020.

promulgation of additional standards is needed to provide an ample margin of safety to protect public health or to prevent an adverse environmental effect. Section 112d5 of the CAA
provides that this residual risk review is not required for categories of area sources subject to GACT standards.
Section 112f2B of the CAA further expressly preserves the EPAs use of the two-step approach for developing standards to address any residual risk and the Agencys 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 Residual Risk Report that the Agency intended to use the Benzene NESHAP approach in making CAA section 112f residual risk determinations EPA453/R99001, p. ES11. The EPA subsequently adopted this approach in its residual risk determinations and the United States Court of Appeals for the District of Columbia Circuit upheld the EPAs interpretation that CAA section 112f2
incorporates the approach established in the Benzene NESHAP. See NRDC v.
EPA, 529 F.3d 1077, 1083 D.C. Cir.
2008.
The approach incorporated into the CAA and used by the EPA to evaluate residual risk and to develop standards under CAA section 112f2 is a twostep approach. 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 1 of approximately 1in-10 thousand. 54 FR 38045. If risks are unacceptable, the EPA must determine the emissions standards necessary to reduce risk to an acceptable level without considering costs. In the second step of the approach, the EPA
considers whether the emissions standards provide an ample margin of safety to protect public health 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
The Chlorine Production Source Category includes any facility engaged in the production of chlorine. The category includes, but is not limited to, facilities producing chlorine by the following production methods: Diaphragm cell, mercury cell, membrane cell, hybrid fuel cell, Downs cell, potash manufacture, hydrochloric acid decomposition, nitrosyl chloride process, nitric acid/salt process, KelChlor process, and sodium chloride/sulfuric acid process.2

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

2 Documentation for Developing the Initial Source Category List. U.S. Environmental Protection Agency. EPA450/391030. July 1992. p. A67.
Available at: https www3.epa.gov/ttn/atw/
socatlst/socatpg.html.

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B. What is this source category and how does the current NESHAP regulate HAP
emissions?
The Chlorine Production source category was initially listed as a category of major sources of HAP
pursuant to section 112c1 of the CAA
on July 16, 1992 57 FR 31576. At the time of the initial listing, the EPA
defined the Chlorine Production source category as follows:

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Based on the differences in the production methods and the HAP
emitted, the EPA decided to divide the Chlorine Production source category into two subcategories: 1 Mercury cell chlor-alkali plants; and 2 chlorine production plants that do not rely upon mercury cells for chlorine production diaphragm cell chlor-alkali plants, membrane cell chlor-alkali plants, etc..
On July 3, 2002, the EPA issued separate proposals to address emissions of mercury from the mercury cell chloralkali plant subcategory sources 67 FR
44672 and emissions of chlorine and hydrochloric acid HCl from both subcategories 67 FR 44713. Separate final actions were taken on both proposals on December 19, 2003. As part of these separate final actions, the EPA deleted the non-mercury cell subcategory under the authority of CAA
section 112c9Bii of the CAA 68
FR 70948.
The final rule for the Mercury Cell Chlor-Alkali Plants subcategory 68 FR
70904, December 19, 2003, codified at 40 CFR part 63 subpart IIIII, which covers both major and area sources, included standards for mercury emissions from two types of affected sources at plant sites where chlorine and caustic are produced in mercury cells: Mercury cell chlor-alkali production facilty affected sources and mercury recovery facility affected sources. The rule prohibits mercury emissions from new and reconstructed mercury cell chlor-alkali production facilities. 40 CFR 63.8190a1. For existing mercury cell chlor-alkali production facilities, the standards include emission limitations for mercury emissions from process vents including emissions from end-box ventilation systems and hydrogen systems and work practices for fugitive mercury emissions from the cell room.
40 CFR 8190a2, 8192a through f.
For new, reconstructed, and existing mercury recovery facilities, the NESHAP includes emission limitations for mercury emissions from oven type thermal recovery unit vents and nonoven type thermal recovery unit vents.
40 CFR 63.8190a3. The rule did not include standards for chlorine or HCl, citing the authority of section 112d4
of the CAA 68 FR 70906. In its 2003
action 68 FR 70904, the EPA
promulgated the initial Mercury Cell Chlor-Alkali Plants NESHAP pursuant to CAA section 112d2 and added the source category to the EPAs Source Category List under CAA sections 112c1, as well as under c3 and k3B and c6, in each case because of the mercury emissions.

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