EPA proposed to find that it is not “appropriate and necessary” to regulate hazardous air pollutant (HAP) emissions from coal- and oil-fired electric generating units (EGUs), thereby reversing its prior cost conclusions underlying the Mercury and Air Toxics Standards (MATS), but otherwise proposed to leave the regulation in effect; the Agency also seeks comment on whether it has the authority or obligation to delist EGUs from CAA §112(c) and rescind the MATS, and whether it should establish a subcategory for emissions of acid gas HAP from existing EGUs firing eastern bituminous coal refuse.