06- 096 C.M.R. ch. 100, § 80
"MACT emission limitation for new HAP sources" means the emission limitation pursuant to Section 112 of the CAA which is not less stringent than the emission limitation achieved in practice by the best controlled similar source, and which reflects the maximum degree of reduction in emissions of hazardous air pollutants (including a prohibition on such emissions, where achievable) that the EPA, or the Department, taking into consideration the cost of achieving such emission reduction, and any nonair quality health and environmental impacts and energy requirements, determines is achievable by sources in the category or subcategory to which the standard applies. If the EPA has either proposed a relevant emission standard pursuant to section 112(d) or section 112(h) of the CAA or adopted a presumptive MACT determination for the source category which includes the constructed or reconstructed major source, then the MACT requirements applied to the constructed or reconstructed major source shall consider those MACT emission limitations and requirements of the proposed standard or presumptive MACT determination.
06- 096 C.M.R. ch. 100, § 80