Law360, New York ( November 24, 2014, 11:27 AM EST) -- In Section 111 of the Clean Air Act, Congress expressly tasked the U.S. Environmental Protection Agency with regulating "sources" of pollution, by category, when the EPA administrator determines that the source category "causes, or contributes significantly to, air pollution which may reasonably be anticipated to endanger public health or welfare."[1] Congress directed that the 111(d) process was to be modeled after Section 110.[2] Having made the cause or contributes determination, the EPA may formulate, for a given source category, "the best system of emission reduction."[3] Once the EPA identifies the BSER, then the states may propose standards of performance that apply the BSER to the state's existing sources, in light of the state's circumstances and facilities.[4] One consideration in formulating standards of performance is "the remaining useful life of the existing source to which such standard applies."[5] In the event that a state declines to participate, the CAA generally allows the EPA "to prescribe a plan for a state in cases where the state fails to submit a satisfactory plan," and also permits federal enforcement "in cases where the state fails to enforce."[6] While rarely invoked, this has been the system since Section 111 became law....