Modification or modified CISWI unit means a CISWI unit you have changed later than promulgation of the final CISWI emission guidelines in 40 CFR part 60, subpart DDDD and that meets one of two criteria:
(1) The cumulative cost of the changes over the life of the unit exceeds 50 percent of the original cost of building and installing the CISWI unit (not including the cost of land) updated to current costs (current dollars). To determine what systems are within the boundary of the CISWI unit used to calculate these costs, see the definition of CISWI unit.
(2) Any physical change in the CISWI unit or change in the method of operating it that increases the amount of any air pollutant emitted for which section 129 or section 111 of the Clean Air Act has established standards.
40 CFR § 62.14840
Terms used but not defined in this subpart are defined in the Clean Air Act, subparts A and B of part 60 and subpart A of this part 62.