This proposal addresses the agency's residual risk and technology review (RTR) of the National Emission Standards for Hazardous Air Pollutants (NESHAP) for Stationary Combustion Turbines. The Stationary Combustion Turbine NESHAP, subpart YYYY, was promulgated pursuant to section 112(d) of the Clean Air Act (CAA) on March 5, 2004. The NESHAP established emission limitations based on maximum achievable control technology (MACT) for controlling emissions of hazardous air pollutants (HAP) from stationary combustion turbines. The HAP emitted from stationary combustion turbines include formaldehyde, toluene, benzene, and acetaldehyde. This action implements the residual risk review requirements of CAA section 112(f)(2) and the technology review requirements of CAA section 112(d)(6). The statute directs the EPA to promulgate emission standards under CAA 112(f)(2) if such standards are required to provide an ample margin of safety to protect public health or to prevent, taking relevant factors into account, an adverse environmental effect. Any such standards are to be promulgated within eight years after promulgation of MACT standards under CAA section 112(d). CAA section 112(d)(6) requires the EPA to review and revise the MACT standards as necessary, taking into account developments in practices, processes and control technologies, no less often than every eight years. Pursuant to a court order, the EPA is obligated to complete the final action by March 13, 2020.