Developing a National Emissions Inventory for 4, 4’-Methylene Diphenyl Diisocyanate

Over the last decade, the U.S. Environmental Protection Agency (EPA) has been developing, refining, and updating the National Toxics Inventory (NTI), a nationwide emissions inventory of 188 compounds and classes of compounds that are listed as hazardous air pollutants (HAPs) under the federal Clean Air Act. One of these compounds is 4, 4’-methylene diphenyl diisocyanate (MDI), which is widely used in the production of rigid polyurethane foams and also used to a lesser extent in coatings, adhesives, sealants, and elastomers. Both EPA and the manufacturers of MDI recognized that early NTI estimates of MDI emissions suffered from two major flaws: (1) because of the methodology used to collect NTI data, many MDI-emitting facilities were not included in the NTI; and (2) much of the available data on MDI emissions is based on the incorrect assumption that all MDI used by a facility is emitted when, in fact, MDI is normally reacted with a polyol and is therefore largely “consumed” in the manufacturing process. Thus, on one hand, early versions of the NTI omitted many facilities that should have been included. On the other hand, for facilities that were included in the NTI, MDI emissions tended to be overestimated – often substantially. In order to develop a more accurate inventory of nationwide MDI emissions, the Diisocyanates Panel of the American Chemistry Council (the Panel) has worked with EPA over the last several years to develop a more accurate inventory of MDI emissions. The Panel used a variety of sources, including EPA’s “Toxics Release Inventory” (TRI), in an effort to identify all sources of MDI emissions in the country. It then divided these sources into categories (eventually numbering 32) based on the way in which MDI is used in their manufacturing processes. Using a calculation method developed by what is now known as the Alliance for the Polyurethanes Industry (and approved by EPA in the early 1990s), the Panel then developed emissions estimates