(a) Except as provided in Subsection (b), in determining whether a proposed change at an existing facility is a modification, the commission may not consider the effect on emissions of:
(1) any air pollution control method applied to a source; or
(2) any decreases in emissions from other sources.
(b) In determining whether a proposed change at an existing facility that meets the criteria of Section 382.003(9)(E) results in a net increase in allowable emissions, the commission shall consider the effect on emissions of:
(1) any air pollution control method applied to the facility;
(2) any decreases in allowable emissions from other facilities that have received a preconstruction permit or permit amendment no earlier than 120 months before the change will occur; and
(3) any decreases in actual emissions from other facilities that meet the criteria of Section 382.003(9)(E)(i) or (ii).
(c) Nothing in this section shall be construed to limit the application of otherwise applicable state or federal requirements, nor shall this section be construed to limit the commission's powers of enforcement under this chapter.