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PREPUBLICATION COPY â Uncorrected Proofs 123 Appendix D Ethylene Dibromide Ethylene dibromide (EDB [(C2H4Br2]) is added to leaded aviation gasoline as a scavenger to help remove lead, which is volatilized from tetraethyl lead (TEL [(Pb(C2H5)4)]) during aircraft engine operation. EDB, is transformed to lead bromide (PbBr2) when the vaporized lead from TEL reacts with the bromine during combustion. The ethylene is burned as fuel. The removal of this lead is critical to the performance, reliability, and durability of the engine. The Toxic Substances Control Act (TSCA) requires the U.S. Environmental Protection Agency (EPA) to prioritize chemical substances for risk evaluation. In accordance with TSCA section 6(b) and 40 CFR Â§ 702.7, on March 21, 2019, EPA initiated the prioritization process for 20 chemical substances identified as candidates for High-Priority Substance designation.1 EDB was among the 20 chemical substances identified for risk evaluation. Subsequently, on August 23, 2019, EPA proposed to designate the same 20 chemical substances as High-Priority Substances for risk evaluation.2 Under TSCA section 6(b)(1)(B) and implementing regulations (40 CFR Â§ 702.3), a High- Priority Substance is defined as âa chemical substance that [EPA] concludes, without consideration of costs or other non-risk factors, may present an unreasonable risk of injury to health or the environment because of a potential hazard and a potential route of exposure under the conditions of use, including an unreasonable risk to a potentially exposed or susceptible subpopulation identified as relevant by [EPA].â A designation as a High-Priority Substance is not a finding of unreasonable risk. Rather, when prioritization is complete, for those chemicals designated as High-Priority Substances, EPA will have evidence on hazards and exposures that may support a finding that the substance may present an unreasonable risk of injury to health or the environment under the conditions of use. Final designation of a High-Priority Substance initiates the risk evaluation process (40 CFR Â§ 702.17), which culminates in a finding of whether or not the chemical substance presents an unreasonable risk of injury to health or the environment under the conditions of use. On April 9, 2020, EPA published a notice of the availability of two reports related to the draft risk assessment for EDB and sought comment on these reports.3 The reports, âDraft Scope of the Risk Evaluation for Ethylene Dibromide CASRN 106-93-4â and âDraft Scope of the Risk Evaluation for Ethylene Dibromide Supplemental File: Data Extraction and Data Evaluation Tables for Physical-Chemical Property Studies CASRN: 106-93-4â and the related public comments can be found in EPA public docket EPA-HQ-OPPT-2018-0488. The timeline for EPA completing its assessment of whether EDB should be designated as a High-Priority Substance is not clear and any discussion of follow-on regulatory action is premature. However, this is of great significance to the future composition of additive packages for leaded aviation gasoline. 1 EPA Initiation of Prioritization Under the Toxic Substances Control Act (TSCA), 84 Federal Register 10491 (March 21, 2019). 2 EPA. Proposed High-Priority Substance Designations Under the Toxic Substances Control Act (TSCA), 84 Federal Register 44300 (August 23, 2019). 3 85 Federal Register 19941, April 9, 2020.