The Clean Air Act (CAA) requires EPA to regulate substances listed as hazardous air pollutants (HAP) under CAA section 112(c). CAA section 112(c)(9) requires EPA to consider petitions to add or remove source categories. EPA reviews a petition to determine whether it provides adequate data and can be determined complete. If EPA decides that information is not adequate, the Administrator may use any authority available to them to acquire such information. Once the petition is determined to be complete, EPA must, within 12 months from the last receipt of information from the petitioners, either grant or deny the petition. On August 28, 2019, EPA received a petition to remove the Stationary Combustion Turbines source category from the list of categories of major sources. On November 19, 2019, December 2, 2020, and March 15, 2021, EPA received supplements to the petition. The EPA is currently evaluating the petition for completeness and will issue a notice to notify the petitioners and the public of its determination of whether the petition will be granted (a proposed rulemaking) or denied.