The Clean Air Act (CAA or the Act) requires each State Implementation Plan (SIP) to contain adequate provisions prohibiting emissions that will have certain adverse air quality effects in other states. On September 26, 2018, the State of Idaho (Idaho or the State) made a submission to the Environmental Protection Agency (EPA) to address these requirements for the 2015 8-hour ozone National Ambient Air Quality Standards (NAAQS). The EPA is approving the submission as meeting the requirement that each SIP contain adequate provisions to prohibit emissions that will significantly contribute to nonattainment or interfere with maintenance of the 2015 8-hour ozone NAAQS in any other state.