These are consolidated petitions for review from two final rules promulgated by D in 2004 and 2006 under the CAA to regulate hazardous air pollution from processing PCWPs. The outputs from PCWP processes include veneer, particleboard, oriented strandboard, hardboard, fiberboard, medium density fiberboard, as well as other products. At least six primary HAPs are released into the air. HAPs present, or may present a threat of adverse human health effects or adverse environmental effects. Ps contend in part that D exceeded its authority in creating a risk-based subcategory. Congress adopted the current version of Section 112 to require technology-based standards in place of the previous risk-based standards, which had not worked. D no longer had discretion to set emission standards for individual sources, nor to set whatever standards D deemed adequate. Section 112 thus mandates that D list and establish emission standards for each category and subcategory of 'major sources' that emit one or more of over 100 HAPs. The standards 'shall require the maximum degree of reduction in emissions' and reflect the 'maximum reduction in emissions which can be achieved by application of the best available control technology.' The 2004 Rule created a PCWP 'low risk' subcategory, which included sources that met the statutory criteria and additional requirements, such as annual emissions testing and reporting, set forth in the 2004 and 2006 Rules. D determined that the low-risk subcategory did not emit carcinogens in amounts resulting in a lifetime cancer risk exceeding one in a million to the most-exposed individual. The 2006 Rule, as relevant, reset the standard compliance date. Ps requested reconsideration of the low-risk subcategory and the delisting of sources under the 2004 Rule. D then proposed amendments to the 2004 Rule, which concerned such matters as the definition of 'affected source,' 'plywood and composite wood products manufacturing facility' (among others), procedures for the low-risk demonstration process, and other permitting and timing issues. Ps appealed.