On September 30, 2021, EPA issued a start, shutdown, and malfunction (SSM) memo withdrawing its October 9, 2020, memo and returning to its 2015 policy. That 2015 policy states that exemptions from air emission exceedances limits during SSM events are inconsistent with the Clean Air Act. Air emissions can be briefly higher during SSM events because emitting units are not at steady-state operation, or some pollution control devices cannot be operated effectively or safely during these events.
Some State Implementation plans (SIPs) had discretionary exemptions for SSM emissions that exceed permit limits or provided affirmative defenses for violations. EPA reports that 12 states and local air pollution control agencies must now update their SIPs to conform with the 2015 policy.