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Air Emissions Factors and Quantification

AP-42 Frequent Questions

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Can I Download AP-42 Sections From The Internet?

Yes! The Compilation of Air Pollutant Emission Factors, Volume I: Stationary Point and Area Sources is available on the AP-42: Compilation of Air Emission Factors webpage.


Can I Download Emissions Factors For Mobile Sources From The Internet?

AP-42 Volume II: Mobile Sources is no longer maintained. More current mobile source emissions factors are available using the Office of Transportation and Air Quality (OTAQ) mobile source models.

However, for reference purposes, OTAQ continues to post parts of the most recent mobile sources AP-42, Volume II on their web site. In particular, Appendix H is still useful in documenting the emissions factors produced by MOBILE5, and in some cases carried over into MOBILE6 without additional documentation.


I Have Been Using Sections Of AP-42 To Obtain Emissions Factors. However, I Have Questions About How These Emissions Factors Are Obtained. Is There a Way To Get The Test Methodologies That Were Used To Determine The Emissions Factors?

The AP-42 Background Documents provide background information on how the AP-42 emissions factors were obtained. These documents include a literature review, emissions factor methodologies and reference materials. Most background documents are available on this site under the corresponding AP-42 chapter page.


What Do The Emissions Factors Ratings Mean?

The following information is from the Introduction of the Compilation of Air Pollutant Emission Factors, Volume I: Stationary Point and Area Sources.

Emissions factor ratings are best characterized as follows:

  • A = Excellent. Emission factor is developed primarily from A and B rated source test data taken from many randomly chosen facilities in the industry population. The source category population is sufficiently specific to minimize variability.
  • B = Above average. Emission factor is developed primarily from A or B rated test data from a moderate number of facilities. Although no specific bias is evident, is not clear if the facilities tested represent a random sample of the industry. As with the A rating, the source category population is sufficiently specific to minimize variability.
  • C = Average. Emission factor is developed primarily from A, B, and C rated test data from a reasonable number of facilities. Although no specific bias is evident, it is not clear if the facilities tested represent a random sample of the industry. As with the A rating, the source category population is sufficiently specific to minimize variability.
  • D = Below average. Emission factor is developed primarily from A, B and C rated test data from a small number of facilities, and there may be reason to suspect that these facilities do not represent a random sample of the industry. There also may be evidence of variability within the source population.
  • E = Poor. Factor is developed from C and D rated test data from a very few number of facilities, and there may be reason to suspect that the facilities tested do not represent a random sample of the industry. There also may be evidence of variability within the source category population.
  • U = Unrated (Only used in the L&E documents). Emission factor is developed from source tests which have not been thoroughly evaluated, research papers, modeling data, or other sources that may lack supporting documentation. The data are not necessarily "poor," but there is not enough information to rate the factors according to the rating protocol. "U" ratings are commonly found in L&E documents and FIRE rather than in AP-42.

Can I Use Draft Sections Of AP-42?

AP-42 sections designated as 'final' have completed the public comment process and all issues have been resolved. Sections designated as 'draft' reflect the fact that the comment period on these sections has passed, but not all issues have been resolved. EPA might receive additional data or comments that would cause a re-evaluation of the available data and possibly open another comment period. Users are encouraged to use factors from finalized sections, if available, but may decide that the draft emissions factors provide better estimates after reviewing the supporting documentation.


There Are Many Sections Of AP-42 That Have Not Been Completed. The Description Of Their Status Was Recently Changed From "WORK IN PROGRESS," or "WORK SUSPENDED - AT THIS TIME, THERE ARE NO PLANS TO UPDATE THIS SECTION" to "THIS IS A PLACEHOLDER HEADING SHOULD EPA DETERMINE AT SOME FUTURE DATE THAT DEVELOPMENT OF A SECTION IS WARRANTED." Why Was The Description Changed?

The new wording is a better indication of the actual status of EPA's efforts in these areas. Years ago, work was initiated in these areas, however, in most cases, no emissions data were made available for review and the sections were not developed. EPA has no plans to develop new sections for the placeholder headings at this time.