06- 096 C.M.R. ch. 100, § 19
"Best Available Retrofit Technology" means an emission limitation based on the degree of reduction achievable through the application of the best system of continuous emission reduction for each regulated pollutant which is emitted by an existing facility which emits or has the potential to emit any regulated pollutant at a rate equal to or greater than the emission rates for significant emissions as defined in this Chapter and which causes visibility impairment. The emission limitation must be established, on a case-by-case basis, taking into consideration the technology available, the costs of compliance, the energy and non-air quality environmental impacts of compliance, any pollution control equipment in use or in existence at the source, the remaining useful life of the source, and the degree of improvement in visibility which may reasonably be anticipated to result from the use of such technology. If the state determines that technological or economic limitations on the applicability of measurement methodology to a particular existing facility would make the imposition of an emission standard infeasible, it may instead prescribe a design, equipment, work practice, or other operational standard, or combination thereof, to require the application of BART. Such standard, to the degree possible, is to set forth the emission reduction to be achieved by implementation of such design, equipment, work practice or operation, and must provide for compliance by means which achieve equivalent results.
06- 096 C.M.R. ch. 100, § 19