April 21, 2022

AP-42 Emission Factors Not Always Appropriate

In November 2020, EPA released an enforcement alert about the inappropriate use of AP-42 emission factors. Emission factors were originally created in 1968, allow permitting agencies, consultants, and regulated entities to estimate a facility’s emissions without measuring each individual source of air pollution. The result is a cheaper but less accurate estimate of source-specific emissions for CAA permitting and compliance demonstrations. These estimates are critical for developing emission control strategies, determining the applicability of permitting and control programs, and ascertaining the effects of sources and appropriate mitigation schemes. EPA’s Fifth Edition of AP-42 was published in 1995 and currently contains more than 21,500 emission factors for over 200 air pollutants.

The agency is concerned both regulators and the regulated community are misusing the emission factors resulting in significant emission under- (and over-) estimates. AP-42 emission factors are graded on an “A – excellent” through “E – poor” scale, and even using A- and B-graded factors may inaccurately calculate emissions. This inaccuracy is because emission factors are developed from averaging emission test reports which may vary by more than an order of magnitude. Emission factors do not account for short-term variation in emissions, which is antithesis to permit limits based on the 1-hour and other short-term NAAQS. In some cases, permit violations due to inappropriate use of emission factors have resulted in civil penalties as high as $1.1 million and the installation of $70 million worth of air pollution control equipment. The agency warns:

“Use of these factors as source-specific permit limits and/or as emission regulation compliance determinations is not recommend 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 factor. As such, a permit limit using an AP-42 emission factor would result in half of the sources being in noncompliance.”

A facility should use the most representative emissions data when determining applicability, applying for a permit, or demonstrating compliance with permit limits. Numerous, more accurate alternatives to emission factors are available. In fact, EPA finds emission factors to be the least accurate (albeit cheapest) method to quantify source-specific emissions. These methods, in order of accuracy, are:

  1. Continuous emissions monitoring systems,
  2. Stack testing,
  3. Vendor guarantees and stack testing data from similar facilities,
  4. Material balance calculations,
  5. Optical remote sensing, and
  6. Emission factors.

Thus, when using emission factors to estimate emissions, evaluate the test conditions versus the current source and consider whether adjustments should be made.


©2022-2023 McCoy and Associates, Inc. All rights reserved.

McCoy and Associates has provided in-depth information to assist environmental professionals with complex compliance issues since 1982. Our seminars and publications are widely trusted by environmental professionals for their consistent quality, clarity, and comprehensiveness.



Considerable care has been exercised in preparing this document; however, McCoy and Associates, Inc. makes no representation, warranty, or guarantee in connection with the publication of this information. McCoy and Associates, Inc. expressly disclaims any liability or responsibility for loss or damage resulting from its use or for the violation of any federal, state, or municipal law or regulation with which this information may conflict. McCoy and Associates, Inc. does not undertake any duty to ensure the continued accuracy of this information.

This document addresses issues of a general nature related to the federal RCRA regulations. Persons evaluating specific circumstances dealing with the RCRA regulations should review state and local laws and regulations, which may be more stringent than federal requirements. In addition, the assistance of a qualified professional should be enlisted to address any site-specific circumstances.