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Under the New Source Review (NSR) preconstruction permitting program, stationary sources undergoing modifications need to determine whether their physical or operational changes are a "major modification" based on the emissions increase that would result from the change. The term "project aggregation" refers to the grouping of multiple, related physical or operational changes at a facility into a single project, such that their emission increases or decreases would be combined for purposes of determining NSR applicability. In January 2009, the EPA issued an interpretive rule that required changes to be aggregated into a single project if they are "substantially related" and created a presumption against aggregating projects that occur 3 or more years apart. This 2009 rule is currently stayed and under reconsideration by EPA. This action will finalize the reconsideration proceeding of the rule.