To evaluate EPA’s progress in reducing air toxic emissions through the Maximum Achievable Control Technology (MACT) standards and to identify sources that may be modeled as part of residual risk assessments, operations within facilities that are subject to MACT standards are identified in the NTI by 4-digit MACT codes. [note that the term NTI (National Toxics Inventory) has since been replaced with NEI and that the codes are now 6 digits]

A MACT category is one for which emissions limitations have been or are being developed under section 112(d) of the Clean Air Act (National Emissions Standards for Hazardous Air Pollutants). EPA sets source category, technology based standards through its MACT program that sharply reduce emissions of HAPs. EPA’s ATW web site includes information on the MACT source categories and the MACT program (www.epa.gov/ttn/atw/eparules.html). The tagging of data with MACT codes allows EPA to determine reductions attributable to the MACT program. The NTI associates MACT codes corresponding to MACT source categories with stationary major and [section-112] area source data. MACT codes are assigned at the process level or at the site level in the point source data, e.g., the MACT code for municipal waste combustors (MWCs) is assigned at the site level whereas the MACT code for petroleum refinery catalytic cracking is assigned at the process level. MACT codes are also assigned to source categories in the nonpoint source file.