Under the New Source Review (NSR) pre-construction permitting program, sources undergoing modifications need to determine whether their modification is considered a major modification and thus subject to NSR pre-construction permitting. A source owner determines if its source is undergoing a major modification under NSR, using a two-step applicability test. The first step is to determine if there is a "significant emission increase" of a regulated NSR pollutant from the proposed modification (Step 1), and the second step is to determine if there is a "significant net emission increase" of that pollutant (Step 2). In this action, we are proposing the consideration of emissions increases and decreases from a modification in Step 1 of the NSR major modification applicability test for all unit types (i.e., new, existing, and hybrid units).