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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:

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:

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 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 1985 through 1999, and emissions for individual point sources (facilities) for years 1996 and 1999. Because of changes in EPA emission inventory procedures, 1999 emissions may not be directly comparable with prior years, especially with regard to particulate matter emissions. Some facility IDs and names changed in the updated 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 year 1999. County-level aggregate emissions are provided for all types of sources, and facility-level emissions are provided for each major point source (see next section).

AirData previously reported 1996 HAP emissions, and we intended to add 1999 emissions when the inventory was complete. However, the methodology for compiling the 1999 emissions inventory differed significantly from the 1996 inventory. Comparing emissions from the two inventories might lead to invalid conclusions. Therefore AirData reports include only 1999 HAP emissions, which are more accurate and complete than the 1996 estimates.

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. 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
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.

AirData reports use alternative (former) names for some NEI source types for criteria air pollutants. The table below lists the corresponding current and alternative terminology.

Term Used in AirData Report Equivalent NEI Source Types
Area Source
(Tier Report)		 Nonpoint + Onroad + Nonroad

 

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