About the National Emission Inventory Database

National Emission Inventory (NEI) Database
for Criteria and Hazardous Air Pollutants

EPA's National Emission Inventory (NEI) database contains information about sources that emit criteria air pollutants and their precursors, and hazardous air pollutants. The database includes estimates of annual air pollutant emissions from point, nonpoint, and mobile sources in the 50 States, the District of Columbia, Puerto Rico, and the Virgin Islands. EPA collects information about sources and releases an updated version of the NEI database every three years.

EPA compiles the NEI database from these primary sources:

Emissions inventories compiled by state and local environmental agencies
Databases related to EPA's Maximum Achievable Control Technology (MACT) programs to reduce emissions of hazardous air pollutants
Toxic Release Inventory (TRI) data
For electric generating units, EPA's Emission Tracking System / Continuous Emissions Monitoring data (ETS/CEM) and Department of Energy fuel use data
For onroad sources, the Federal Highway Administration's estimate of vehicle miles traveled and emission factors from EPA's MOBILE computer model
For nonroad sources, EPA's NONROAD computer model
Previous emissions inventories, if states do not submit current data

More information about the NEI database and the compilation of emissions inventories is available in publications of EPA's Emission Factor and Inventory Group.

Pollutants

Criteria air pollutants are substances for which EPA has set health-based standards. Four of the six criteria air pollutants are included in the NEI database:

Carbon monoxide (CO)
Nitrogen oxides (NOx)
Sulfur dioxide (SO2)
Particulate matter (PM10 and PM2.5)

Ozone, the fifth criteria air pollutant, arises from photochemical reactions in the atmosphere rather than direct emissions from sources. Lead, the sixth criteria pollutant, is also a hazardous air pollutant ("lead compounds"). The NEI tracks lead emissions as a hazardous air pollutant.

The NEI also includes emissions of two substances that enable and facilitate the formation of criteria air pollutants:

Volatile organic compounds (VOCs)
Ammonia (NH3)

Volatile organic compounds react with nitrogen oxides in the atmosphere to form ozone. Ammonia reacts with nitrogen and sulfur compounds in the atmosphere, mainly nitric and sulfuric acids, to form particulate matter.

For AirData reports and maps, the NEI database provides annual emissions estimates of the criteria air pollutants and precursors identified above. NEI has county-level aggregate emissions for years 1990 through 2002, and emissions for individual point sources (facilities) for years 1996, 1999, and 2002. Because of changes in EPA emission inventory procedures, emissions for 1999 and later years may not be directly comparable with prior years, especially with regard to particulate matter emissions. Some facility IDs and names changed in the 1999 data, so it may not be possible to identify unambiguously the corresponding 1996 and 1999 data for some facilities.

Hazardous air pollutants (HAPs) are substances that are known or suspected to cause serious health problems such as cancer. The Clean Air Act defined an initial list of substances, and EPA currently identifies 188 HAPs. The list includes relatively common pollutants such as benzene, chlorine, methanol, and asbestos, as well as numerous less common substances. For details, see the complete list of hazardous air pollutants in the NEI database. EPA's Air Toxics Web site has further information about the sources of HAPs, and a list of the 188 pollutants. ("Air toxics" and "toxic air pollutants" are synonyms for hazardous air pollutants.) EPA has identified a subset of the 188 HAPs that pose the greatest potential for adverse health effects to the majority of the U.S. population living in urban areas. These 33 pollutants are called urban HAPs.

For AirData reports, the NEI database provides hazardous air pollutant emissions for years 1999 and 2002. County-level aggregate emissions are provided for all types of sources, and facility-level emissions are provided for each major point source.

Source Types

EPA categorizes pollution sources according to provisions of the Clean Air Act and Federal regulations based on that legislation. The characteristics of a pollution source determine which regulations govern its emissions.

The NEI database has these types of air pollution sources:

Stationary Sources

Sources that have a fixed location.

Point: Facilities that are identified individually by name and location.
Nonpoint: Facilities that are not identified individually, and diffuse sources, such as wildfires and agricultural tilling.

Mobile Sources

Sources that do not have a fixed location.

Onroad: Highway vehicles.
Nonroad: Vehicles or equipment not suitable for highway use, such as construction equipment, recreational boats, and aircraft.

Hazardous Air Pollutant Source Categories

For hazardous air pollutants, the Clean Air Act defines these categories of sources, which the NEI database incorporates:

Major: Point sources that emit or have the potential to emit at least 10 tons per year of any one HAP, or at least 25 tons per year of a combination of HAPs, are major sources. Examples of major sources are electric utility plants, chemical plants, steel mills, oil refineries, and hazardous waste incinerators. These sources may release air toxics from equipment leaks, when materials are transferred from one location to another, or during discharge through emissions stacks or vents. The NEI database has HAP emission estimates for each major source. As best as possible, point sources in the NEI have each been identified as either major or area, but this identification may not correspond to the official regulatory classification of some sources.
Area and Other (Nonpoint): The Clean Air Act used the term "area sources". In the NEI, these are called nonpoint sources.

Area sources are stationary sources that do not exceed the thresholds for major source designation. They emit less than 10 tons per year of a single HAP and less than 25 tons per year of all HAPs combined. Examples of area sources are neighborhood dry cleaners and gas stations. Though emissions from individual sources are often relatively small, collectively their emissions can be of concern, particularly where large numbers of sources are located in heavily populated areas. The NEI database has HAP emission estimates for some individual area sources and county aggregate emissions for others.

Other sources are wildfires and prescribed burning. The NEI database has county aggregate emissions for other sources.
Onroad Mobile: Licensed motor vehicles, including automobiles, trucks, buses, and motorcycles. The NEI database has county aggregate emissions for these sources.
Nonroad Mobile: 2- or 4-stroke and diesel engines, nonroad vehicles, aircraft, commercial marine vessels, and locomotives. The NEI database has county aggregate emissions for these sources.

Criteria Air Pollutant Source Categories

For criteria air pollutants and precursors, the NEI database has these categories of sources:

Point: Stationary sources that are identified individually by name and location, such as electric generating plants and factories. The NEI database has criteria pollutant emission estimates for each point source.
Nonpoint: Stationary sources that are not identified individually, such as offices and residences; and diffuse sources, such as wildfires, prescribed burning, and agricultural tilling. The NEI database has county aggregate emissions for nonpoint sources.
Onroad Mobile: Licensed motor vehicles, including automobiles, trucks, buses, and motorcycles. The NEI database has county aggregate emissions for these sources.
Nonroad Mobile: 2- or 4-stroke and diesel engines, nonroad vehicles, aircraft, commercial marine vessels, and locomotives. The NEI database has county aggregate emissions for these sources.