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The Clean Air Act (CAA) requires EPA to review maximum achievable control technology (MACT) standards and to revise them "as necessary (taking into account developments in practices, processes and control technologies)" no less frequently than every eight years. Under the "residual risk" provision of the CAA, EPA must evaluate the MACT standards within eight years after promulgation and promulgate standards, if required, to provide an ample margin of safety to protect public health or prevent an adverse environmental effect while considering the economic impacts of controls, technological feasibility, uncertainties and any other relevant factors. The EPA promulgated National Emission Standards for Hazardous Air Pollutants (NESHAP) for Primary Aluminum Reduction Plants (found in 40 CFR part 63, subpart LL) in 1997. This NESHAP applies to the owner or operator of each new pitch storage tank or new or existing potline, paste production plant, or anode bake furnace associated with primary aluminum reduction which is located at a major source, as defined in 40 CFR section 63.2. The EPA proposed standards addressing residual risk and technology developments on December 6, 2011. The EPA published a supplemental proposal on December 8, 2014. If finalized, the amendments will include new emissions standards for carbonyl sulfide emissions from new and existing potlines; new emissions standards for particulate matter (PM, as a surrogate for HAP metals) emissions from new and existing potlines, anode bake furnaces and paste production plants; new emissions standards for polycyclic organic matter (POM) emissions from new and existing prebake potlines and existing pitch storage tanks; new risk-based emissions standards for arsenic and nickel emissions from new and existing Soderberg potlines; reduce the emissions standards for POM from existing Soderberg potlines; eliminate the startup, shutdown and malfunction exemption; and make certain other technical and editorial changes.