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The Clean Air Act (CAA) requires the Administrator to add a substance to the hazardous air pollutant (HAP) list, CAA section 112(b)(1) upon a showing by the petitioner or on the Administrator's own determination that the substance is an air pollutant and that emissions, ambient concentrations, bioaccumulation, or deposition of the substance are known to cause or may reasonably be anticipated to cause adverse effects to human health or adverse environmental effects. The EPA has determined that action is needed to develop regulatory requirements directly addressing the impacts on regulated entities from the addition of a HAP to ensure the effective and efficient implementation of requirements triggered by the addition of a new HAP. This rule was proposed in 2023 (88 FR 62711; Sept. 13, 2023).