Basic Information of Air Emissions Factors and Quantification

On This Page:

• About Emission Factors
  • What is an Emissions Factor?
  • Emissions Factors Procedures
  • Emission Factors Uncertainty
  • Locating and Estimating Reports
  • Emission Factors Related Resources
  • Emission Factors Archive Guidance and Resource Materials

About Emissions Factors 

Emissions factors have long been the fundamental tool in developing national, regional, state, and local emissions inventories for air quality management decisions and in developing emissions control strategies. Emission factors in AP-42 are neither EPA-recommended emission limits (e.g., best available control technology or BACT, or lowest achievable emission rate or LAER) nor standards (e.g., National Emission Standard for Hazardous Air Pollutants or NESHAP, or New Source Performance Standards or NSPS). Use of these factors as source-specific permit limits and/or as emission regulation compliance determinations is NOT recommended by EPA. Because emission factors essentially represent an average of a range of emission rates, approximately half of the subject sources will have emission rates greater than the emission factor and the other half will have emission rates less than the emission factor. As such, a permit limit using an AP-42 emission factor could result in approximately half of the sources being in noncompliance.

An emissions factor is a representative value that attempts to relate the quantity of a pollutant released to the atmosphere with an activity associated with the release of that pollutant. These factors are usually expressed as the weight of pollutant divided by a unit weight, volume, distance, or duration of the activity emitting the pollutant (e.g., kilograms of particulate emitted per megagram of coal burned). Such factors facilitate estimation of emissions from various sources of air pollution. In most cases, these factors are simply averages of all available data of acceptable quality, and are generally assumed to be representative of long-term averages for all facilities in the source category (i.e., a population average).

The general equation for emissions estimation is:

Procedures for the Development of Emissions Factors from Stationary Sources. This page contains the finalized procedures document for emissions factor development. 

Emissions Factors Uncertainty

Uncertainty is dependent on the kind of emissions released, the number of tests used to determine the emissions factor, the appropriate decision level (or percentile) within the distribution range, and the number of similar emissions units within a specific area. EPA released a draft review for public comment on April 24, 2007.

Since the posting of the Draft Emission Factor Uncertainty Assessment for comment on April 24, 2007, we have received numerous inquiries from interested stakeholders questioning whether the Agency intends to include the effect of uncertainty on a source's applicable emission-related requirements, or whether states or local authorities need to apply the effect of uncertainty to determine area compliance with NAAQS standards. As originally stated in the Draft Assessment, we do not attempt to evaluate or provide guidance on the application of emissions factor uncertainty in making environmental decisions. Moreover, any such decisions would be made only after a formal notice and comment rulemaking process. However, EPA did not intend any change in current practices that would warrant rulemaking. Rather, our intent is to educate sources and regulators about the accuracy of emission factors and to improve such accuracy through the incorporation of the results of direct emissions testing into the estimation of future emission factors.

For more information, visit the Emissions Factor Uncertainty Assessment report webpage.

Locating and Estimating Reports

This report series, titled Locating and Estimating Air Toxic Emissions from Sources of (source category or substance) characterizes the source categories for which emissions of a toxic substance have been identified. The reports are available on the Locating and Estimating (L&E) Documents webpage. AP 42, Compilation of Air Pollutant Emission Factors and the WebFIRE database should be checked for more current emissions factors.

Emissions Factors Related Resources

These resources contain information developed by the U.S. EPA or by outside parties which have undergone limited EPA and external review and are felt to be potentially useful, but which are not contained in AP 42.  The reports are available on the Related Emissions Factors Documents webpage. They should be used with discretion.

Emissions Factors Archive Guidance and Resource Materials

The Emissions Factors Archive Guidance and Resource Materials webpage includes EPA documents emissions factors and the CHIEF (Clearinghouse for Inventories & Emissions Factors) Archive Database. The archive is a collection of older reports and software manuals, as well as references and background reports cited in the Compilation of Air Pollutant Emission Factors, AP-42, Volume I: Stationary Point & Area Sources. Most of the material is in a scanned PDF format.