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Emissions Factors & AP 42, Compilation of Air 

Pollutant Emission Factors 

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: 

E = A x EF x (1-ER/100)

where: 

E = emissions; 

A = activity rate; 

EF = emission factor, and 

ER =overall emission reduction efficiency, % 

AP-42, Compilation of Air Pollutant Emission Factors, has been published since 1972 as 

the primary compilation of EPA's emission factor information. It contains emission factors and 

process information for more than 200 air pollution source categories. A source category is a 

specific industry sector or group of similar emitting sources. The emission factors have been 

developed and compiled from source test data, material balance studies, and engineering 

estimates. The Fifth Edition of AP-42 was published in January 1995. Since then EPA has 

published supplements and updates to the fifteen chapters available in Volume I, Stationary 

Point and Area Sources. The latest emissions factors are available below on this website. 

Use the AP 42 Chapter webpage links below to access the document by chapter. 

Access to these emissions factors and other EPA reviewed stationary point and area source 

factors is also available from the WebFIRE application. WebFIRE provides fast and complete 

access to the Agency’s air emissions factors information. 

For information about emissions factors from highway vehicles and nonroad mobile sources, 

visit the Office of Transportation and Air Quality web site. 

AP 42, Fifth Edition 

Compilation of Air Pollutant Emission Factors, 

Technology Transfer Network 

Clearinghouse for Inventories & Emissions Factors

http://www.epa.gov/ttn/chief/ap42/index.html

Last updated on Wednesday, December 28, 2011

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Proposed and final rules related to emissions factors

October 14, 2009 - The Emissions Factors Program Improvements - Advanced notice of 

proposed rulemaking is available on the www.regulations.gov website , and the 

EPA's OAR and Policy Guidance Recent Additons webpage (PDF 290K). The comment period 

ended December 14, 2009. 

AP 42 FAQs

Answers to frequently asked questions about AP 42

Drafts

Draft Sections Under Review

AP 42 listing of supplements and updates.

Supplements A through F (1996 - 2000)

Updates 2001 to present

Older editions of AP-42, Volume 1 

This information is available for historical purposes only. For the most recent emissions 

factors, supported by the EPA, please see the table of contents below. 

Emissions Factors Procedures

Procedures for the Development of Emissions Factors from Stationary Sources. This page 

includes both the existing procedures and the review draft for the proposed new procedures. 

Volume 1: Stationary Point and Area Sources

Table of Contents, AP 42, Volume I, Fifth Edition

Cover page 

and Contents

Cover page, detailed Table of Contents, Publications in Series, Insertion 

Instructions, and Key Word Index (PDF 128K). This is current through the 

Fifth Edition, Supplement C of AP 42. For sections and chapters added 

after November 1997, see the chapter web pages below.

Introduction Introduction to AP 42, Volume I, Fifth Edition - January 1995 (PDF 

40K)

Chapter 1 External Combustion Sources

Chapter 2 Solid Waste Disposal

Chapter 3 Stationary Internal Combustion Sources

Chapter 4 Evaporation Loss Sources

Chapter 5 Petroleum Industry

Chapter 6 Organic Chemical Process Industry

Chapter 7 Liquid Storage Tanks

Chapter 8 Inorganic Chemical Industry

Chapter 9 Food and Agricultural Industries

Chapter 10 Wood Products Industry

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Chapter 11 Mineral Products Industry

Chapter 12 Metallurgical Industry

Chapter 13 Miscellaneous Sources

Chapter 14 Greenhouse Gas Biogenic Sources

Chapter 15 Ordnance Detonation

Appendix A Miscellaneous Data & Conversion Factors -- September 1985 (PDF 

103K)

Appendix B.1 Particle Size Distribution Data and Sized Emission Factors for 

Selected Sources -- October 1986 (PDF 2M)

Appendix B.2 Generalized Particle Size Distributions -- September 1996 (PDF 137K)

Appendix C.1 Procedures for Sampling Surface/Bulk Dust Loading -- July 1993 

(PDF 65K)

Appendix C.2 Procedures for Laboratory Analysis of Surface/Bulk Dust Loading 

Samples -- July 1993 (PDF 42K)

Draft Sections Under Review 

After the comment period, can I use a draft section? (TXT 1K)

Chapter & 

Section 

Description Comments Requested by: 

Chapter 15 Ordnance Detonation December 7, 2009

Section 4.12 Manufacture of Rubber 

Products

February 2, 2009

Section 2.4 Municipal Solid Waste 

Landfills

May 5, 2009

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