1.1 What data are included in the 2023 NEI release?

The NEI is a national compilation of air emission estimates of criteria air pollutants (CAPs), the precursors of CAPs, and hazardous air pollutants (HAPs). The hazardous air pollutants that are included in the NEI are based on Section 112(b) of the Clean Air Act. State, local and tribal (S/L/T) air agencies submit emission estimates to EPA and the Agency adds information from EPA emissions programs, such as the emission trading program, Toxics Release Inventory (TRI), and data collected during rule development or compliance testing. The NEI includes estimates of emissions from stationary sources (large and small industries, commercial, institutional and consumer), mobile sources, fires and biogenic emissions. EPA uses the NEI in rule development, non-attainment area designations, and as an input to various reports and assessments. This document discusses all components of the NEI and, where useful, highlights differences between the 2023 NEI and the most-recent publicly available full NEI release, the 2020 NEI. The NEI program develops emissions datasets, blends data from these multiple sources, and performs data processing steps that further enhance, quality assure and augment the compiled data.

EPA compiles the emissions data in the NEI at different levels of granularity, depending on the data category. For point sources (in general, large facilities), emissions inventories include emissions at a process level within a facility. EPA collects point source emissions data from S/L/T air agencies and adds data from the other EPA emissions programs including the Toxics Release Inventory, the Acid Rain Program, and the Greenhouse Gas Reporting Program. For nonpoint sources (typically smaller, yet pervasive sources) and mobile sources (both onroad and nonroad), the NEI includes emissions data as county totals. For commercial marine vessel sources, the NEI includes emissions data at the sub-county, polygon shape-level. For wildfires and prescribed burning, EPA estimates and provides emissions data as day-specific, coordinate-specific events for both smoldering and flaming components; however, the NEI includes these emissions as county totals.

The NEI includes the pollutants associated with the National Ambient Air Quality Standards (NAAQS), known as CAPs, HAPs associated with EPA’s Air Toxics Program, and, starting with the 2020 NEI, per- and polyfluoroalkyl (PFAS) compounds. The NEI includes greenhouse gases (GHGs) for fires, mobiles sources, and point sources, where voluntarily reported by SLTs. The CAPs have ambient concentration limits or are precursors for pollutants with such limits from the NAAQS program. The CAPs and precursors are lead (Pb), carbon monoxide (CO), nitrogen oxides (NOx), volatile organic compounds (VOCs), sulfur dioxide (SO2), particulate matter 10 microns or less (PM10), particulate matter 2.5 microns or less (PM2.5), and ammonia (NH3). The HAP pollutants include the 187 remaining HAP pollutants from the original 188 listed in Section 112(b) of the 1990 Clean Air Act Amendments. There are many different types of HAPs. For example, some are acid gases such as hydrochloric acid (HCl); others are heavy metals such as mercury (Hg), nickel and cadmium; and others are organic compounds such as benzene, formaldehyde, and acetaldehyde.