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Clearinghouse for Inventories & Emissions Factors

Basic Emissions Factors Information

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.

CHIEF Listserv

The CHIEF Listserv will notify you about new emissions factors, conferences, updates to software tools and any new additions to the Emissions Factors and Policy Applications Center.

Emissions Factors, AP 42

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. In this webpage you will find:
  • An Introduction to Emissions Factors Volume I
  • Chapters and Appendixes by Topic for Volume I
  • Notices of Chapter and/or Sections under Review
  • Procedures for Preparing Emissions Factors Documents
  • Emissions Factors Supplements
  • A section of Frequently Asked Questions (FAQs)
  • Older Editions of Emissions Factors Volume I


WebFIRE is EPA’s online emissions factor repository, retrieval, and development tool. The WebFIRE database contains EPA’s recommended emissions factors for criteria and hazardous air pollutants (HAP) for industrial and non-industrial processes. In addition, WebFIRE contains the individual data values used to develop the recommended factors and other data submitted to EPA by federal, state, tribal, and local agencies; consultants; and industries. For each recommended emissions factor and individual data value, WebFIRE contains descriptive information such as industry and source category type, control device information, the pollutants emitted, and supporting documentation.
WebFIRE will include data submitted to EPA from the Electronic Reporting Tool (ERT). Beginning January 1, 2012, industries are required to submit their source test data to EPA using the ERT application or the ERT approved spreadsheet. This information is shared with WebFIRE and is available to the public.

Emissions Factors and Emissions Estimation Tools

On this webpage you will find software and tools such as the:
  • TANKS is a desktop software program that estimates volatile organic compound (VOC) and hazardous air pollutant (HAP) emissions from fixed- and floating-roof storage tanks.
  • Other emissions tools, such as SPECIATE and WATER9.

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. AP 42, Compilation of Air Pollutant Emission Factors and the FIRE database should be checked for more current emissions factors.

Emissions Factors Related Resources

These resources contain information developed by the US 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. They should be used with discretion.

Emissions Factors Archive Guidance and Resource Materials

On this webpage you will find EPA documents and software related to 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.

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