The Clean Air Act (CAA) calls for the Administrator to add a substance to the CAA section 112(b)(1) list, otherwise known as the HAP list, “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 Agency 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 late 2023 [88 FR 62711 (September 13, 2023)] and will be promulgated in Summer 2026.