The Clean Air Act (CAA), directs D to establish emission standards for hazardous air pollutants based on the 'maximum achievable control technology' known as MACT. D must list categories and subcategories of hazardous air pollutant emissions sources, then set MACT standards for each category at a level requiring the maximum degree of reduction in emissions of the hazardous air pollutants subject to this section (including a prohibition on such emissions, where achievable) that the Administrator, taking into consideration the cost of achieving such emission reduction, and any non-air quality health and environmental impacts and energy requirements, determines is achievable for new or existing sources in the category or subcategory to which such emission standard applies. D promulgated revised emission standards for hazardous waste combustors. D proposed its usual three-year compliance period. For certain types of combustors, D proposed an early cessation program under which kilns and on-site incinerators that decide against making the improvements necessary to continue burning hazardous waste under the new standards would be required to 'immediately stop burning hazardous waste when the owner or operator first determines that [compliance will not be achieved] by the applicable date.' D adopted a final rule requiring owners and operators of hazardous waste combustion facilities to submit a Notification of Intent to Comply, Kilns and on-site incinerators that indicate an 'intent not to comply'--the focus of this case--must cease burning hazardous waste within two years of the effective date. Never before had D required emission sources to choose between complying or ceasing the regulated activity. Ps argued that D lacks statutory authority to impose such an unprecedented requirement. D stated that its new type of agenda is intended to expeditiously limit the burning of hazardous waste by those sources who do not intend to come into compliance with the requirements of the emission standards. Ps challenged the early cessation program in that it will produce no environmental benefits because hazardous waste currently burned by kilns and on-site incinerators will simply be shifted to commercial incinerators operating under the same emission standards. Ps petitioned for review.