United States
Environmental Protection
Agency
Enforcement Alert
Publication no. EPA 325-N-20-001
November 2020
EPA Reminder About Inappropriate Use of AP-42 Emission Factors
Purpose
This purpose of this Enforcement Alert is to remind permitting agencies, consultants, and regulated entities that
improperly using AP-42 emission factors can be costly to their businesses, inefficient, and in some circumstances, can
subject regulated entities to enforcement and penalties. The Environmental Protection Agency (EPA) is concerned that
some permitting agencies, consultants, and regulated entities may incorrectly be using AP-42 emission factors in place
of more representative source-specific emission values for Clean Air Act permitting and compliance demonstration
Consequences of Using AP-42 Factors
Permitting agencies, consultants, and regulated entities should be aware that even emission factors with more highly
rated AP-42 grades of "A" or "B" are only based on averages of data from multiple, albeit similar, sources (See the
Attachment for an overview of the history of AP-42 emission factors and the AP-42 emission factor rating system).
Accordingly, these factors are not likely to be accurate predictors of emissions from any one specific source, except in
very limited scenarios. While emission factors are helpful in making emission estimates for area-wide inventories for
specific source types, AP-42 provides the following warning:
"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 factor. As such, a permit limit using an AP-42 emission factor
would result in half of the sources being in noncompliance."1
With the advent of 1-hour and short-term National Ambient Air Quality Standards (NAAQS), permit limits must be able
to account for short term fluctuations. AP-42 emission factors also do not account for short term variation in emissions
as the emission factors are intended for use in developing area-wide annual or triannual inventories. In developing
emission factors, test data are typically taken from normal operating conditions and generally avoid conditions that can
cause short-term fluctuations in emissions. These short-term fluctuations in emissions can stem from variations in
process conditions, control device conditions, raw materials, ambient conditions, or other similar factors. This means
that if facilities use AP-42 emission factors as permit limits, facilities increase their chances of violating their short-term
permit limits. It also increases the likelihood of a geographic area's non-compliance with the NAAQS.
DISCLAIMER: This document aims to explain the application of certain EPA regulatory provisions using plain language. Nothing in
this Alert revises or replaces any regulatory provisions, any other part of the Code of Federal Regulations, the Federal Register, or
the Clean Air Act. Following the approaches for determining a single storage vessel's potential for VOC emissions and attempting
to comply with the closed vent system requirements as discussed in this Alert do not equate to or guarantee compliance with the
Clean Air Act, its implementing regulations, and associated state/local requirements. For more information, visit:
purposes.
1 AP-42, Fifth Edition Compilation of Air Pollutant Emissions Factors, Volume 1: Stationary Point and Area Sources. Introduction, p. 2
(emphasis added).
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It is also important to understand that there is a great deal of variability in the emissions data that are used to generate
the emission factors. This variability is not necessarily reflected in the emission factor. AP-42 describes this as follows:
"The extent of between-source variability that exists, even among similar individual sources, can be large
depending on process, control system, and pollutant. Although the causes of this variability are considered in
emission factor development, this type of information is seldom included in emission test reports used to
develop AP-42 factors. As a result, some emission factors are derived from tests that may vary by an order of
magnitude or more. Even when the major process variables are accounted for, the emission factors developed
may be the result of averaging source tests that differ by factors of five or more."2
In addition to potential permit noncompliance, or increased risk of area noncompliance with the NAAQS, using an
emission factor as an emission limit could have monetary implications for an individual source or permitting agency. For
example, many permitting agencies collect permitting fees based on the amount of pollution emitted. If a facility uses an
emission factor to estimate and report emissions, but the actual emission rate is lower than the emission factor, then
the facility will report more emissions and consequently pay more in fees. On the other hand, if a facility emits at a rate
above the emission factor, not only is the source violating its permit limit and the Clean Air Act, it is also not paying the
appropriate amount in fees.
Another potential monetary implication for facilities is an enforcement action assessing penalties for violating the Clean
Air Act. As described in a 2006 report issued by the EPA Inspector General:
"...according to EPA enforcement records, three industries - petroleum refineries, wood products, and ethanol
production - operated with insufficient control equipment primarily because emission limits were significantly
underestimated due to the emission factors used. EPA, through separate enforcement actions, required
companies in these industries to install additional emission controls, resulting in the combined reduction of over
1,000,000 tons of pollutants."3
For example, the EPA Inspector General's 2006 report documented an EPA investigation in the Wood Products industry
that found a nationwide pattern of Clean Air Act violations by one company. EPA found that the company had used an
AP-42 emission factor designated as "poor" for volatile organic compound (VOC) emissions that resulted in the company
underestimating such emissions and claiming that its facilities were not subject to permitting requirements. To resolve
the violations, the company entered into a consent decree with the United States, which required the company to pay a
civil penalty of $1.1 million and to install air pollution control equipment at a cost of $70 million.4
One example of a present-day concern is the use of a default vapor pressure value for estimating VOC emissions from
heated tanks that store heavy refinery liquids such as No. 6 fuel oil. The true vapor pressure (TVP) of a stored liquid is
important when calculating the emissions from tanks using the equations in AP-42, Chapter 7, Liquid Storage Tanks. The
default vapor pressure is only an estimate and may not be correct for every blend of No. 6 fuel oil. Direct emissions
testing of No. 6 fuel oil tanks and TVP testing in 2012 and 2013, suggested that in those cases the use of the default
vapor pressure in AP-42 had resulted in emissions estimates that were understated by a factor of 100 for permitting and
reporting purposes. Reliance on the default vapor pressure in AP-42 and the resulting emission factors, instead of
directly measuring VOC emissions and vapor pressure, can be very costly for businesses as shown by two recently
concluded cases, summarized in the following two boxes.
2	AP-42, Fifth Edition Compilation of Air Pollutant Emissions Factors, Volume 1: Stationary Point and Area Sources. Introduction, p. 3
(emphasis added).
3	U.S. EPA Office of Inspector General, EPA Can Improve Emissions Factors Development and Management, Report No. 2006-P-
00017, March 22, 2006.
4	Id.
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Sprague Resources LP operates heated asphalt and No. 6
fuel oil storage tanks at seven facilities across New
England. Applying VOC testing results rather than AP-42
estimates, EPA found that Sprague had unpermitted
facilities that required permits, and also had facilities
with permits that failed to fully account for VOC
emissions. Sprague entered into a settlement with the
United States and the Commonwealth of Massachusetts
that required the company to pay $350,000 civil
penalties, obtain revised state air pollution control
permits, limit the amount of asphalt and No. 6 fuel oil
stored in and passed through the tanks at six facilities,
and provide odor controls on tanks at two facilities.
Global Partners LP operates heated asphalt and No. 6
fuel oil storage tanks at a facility in South Portland,
Maine. Applying VOC testing results rather than AP-42
estimates, EPA found that Global's permit failed to fully
account for VOC emissions. Global entered into a
settlement with the United States that required the
company to obtain a revised state air pollution control
permit, limit the amount of asphalt and No. 6 fuel oil
stored in and passed through the tanks at the facility,
install odor controls on tanks, pay a $40,000 penalty,
and invest $150,000 in a local wood-stove replacement
project.
Regulated entities of any size who voluntarily discover, promptly disclose, expeditiously correct, and take steps to
prevent recurrence of potential violations may be eligible for a reduction or elimination of any civil penalties that
otherwise might apply. Most violations can be disclosed and processed via EPA's automated online "eDisclosure" system
(seehttps://www.epa.gov/compliance/epas-edisclosure). To learn more about the EPA's violation disclosure policies,
including conditions for eligibility, please review EPA's Audit Policy website at https://www.epa.gov/compliance/epas-
audit-policy. Many states also offer incentives for self-policing; please check with the appropriate state agency for more
information.
What Can Be Done?
Consultants and facility owners/operators should obtain and use the most representative emissions data, which in many cases
may be source-specific emissions data, when determining applicability, applying for a permit, or demonstrating compliance with
permit limits.
Various EPA publications (e.g., https://www.epa.gov/emc) describe the benefits and limitations of different ways to quantify
source-specific emissions. These techniques in order of accuracy are:
•	Continuous Emissions Monitoring System (CEMS) - CEMs offers a highly accurate source-specific method that continuously
monitors the emissions coming out of a particular stack; however, although the accuracy of this method is high, the cost is
also the highest at $20,000-$50,000 per year.
•	Stack Testing - Like a CEMS, source-specific data are generated at a particular stack but emissions are only measured for a
specific time, typically for a few hours during normal operations. Costs for stack testing typically run $20,000, but testing may
only be necessary every 2 to 5 years.
•	Vendor Guarantees and Stack Test Data from Similar Facilities - If representative source-specific data cannot be obtained,
emissions information from equipment vendors, particularly emission performance guarantees or actual test data from
similar equipment, is a better source of information for permitting decisions than an AP-42 emission factor.
•	Material Balance Calculations - While the material balance calculations are not generally considered as accurate as direct
measurements, they may provide more reliable average emission estimates for certain sources where a high percentage of
material is lost to the atmosphere (e.g., solvent VOC emissions). The costs for recordkeeping are approximately $2,000-
$10,000 per year. This method works well for materials and processes that are well understood.
•	Optical Remote Sensing - Measurement techniques involving differential absorption light detection and ranging (known as
DIAL) and solar occultation flux or SOF can be used to measure emissions from sources such as coke ovens, storage tanks,
wastewater treatment plants, and process units that are otherwise difficult to measure by other means. Measurement bias
on the order of ±30 percent is expected but the data can be more accurate than engineering estimates or emission factors.
•	Emission Factors - When source-specific emissions or other more reliable approaches are unavailable, AP-42 emission
factors may be the only way to estimate emissions. Again, the limitations of the factor in accurately representing the facility's
emissions and the environmental/financial risk of using the emission factor for a particular situation should be carefully
considered. Remember, AP-42 emission factors should only be used as a last resort. Even then the facility assumes all risk
associated with their use!
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Attachment - History of AP-42
Before the EPA existed, the U.S. Public Health Service (PHS)
published "A Compilation of Air Pollutant Emission Factors"
in 1968.* The purpose of the report was to assist the
various agencies responsible for compiling air pollution
emission inventories for communities across the nation by
providing them with relevant data. PHS recognized that
measuring each individual source of air pollution in a particular airshed was impractical, and so, to simplify the airshed
emission inventory process, while still maintaining a reasonably accurate inventory, PHS developed emission factors
based on the technical literature and a limited number of source-specific tests. The resulting emission factors were
simple averages of the rate at which pollutants were emitted from the burning or processing of a given quantity of
material. In some cases, emission factors were based on only one or two data points.
With the creation of the EPA, publication of the emission factors was continued with "Compilation of Air Pollutant
Emission Factors, Second Edition," by the EPA Office of Air Quality Planning and Standards in 1973.
The 3rd and 4th editions of AP-42 were released in 1977 and 1985. EPA published the most recent AP-42, the 5th edition in
1995s, and has published multiple supplements and updates since. Currently, AP-42 contains more than 21,500
emission factors for over 200 air pollutants. Within AP-42, each emission factor is given a rating between "A" (excellent)
and "E" (poor) (see Table 1 below). It is important to note that half of the emission factors are rated "D" or "E" and one-
fifth are unrated. This means that less than one-third of the emission factors are rated between "Excellent" and
"Average."
As we work to improve our ability to estimate emissions nationally, the grading in AP-42 helps us better understand the
quality of the data. But even factors that are rated "A" or "B" are not designed to be used by a single source where
other, more reliable, site-specific, data are available.
Table 1: Explanation of AP-42 Emission Factor Quality Ratings
Rating
Explanation
"A" - Excellent
Emission factor is developed from tests conducted with sound, or generally sound, methodology. Test
data are from many randomly chosen facilities and the source category population is sufficiently
specific to minimize variability. Data may, or may not, be reported in enough detail for adequate
validation.
"B" - Above Average
Same as "A," but test data are from a "reasonable number" of facilities. Although no specific bias is
evident, it's not clear if the facilities represent a random sample of the industry. The source category
population is sufficiently specific to minimize variability.
"C" - Average
Same as "B," but the factor can be developed from an unproven or new methodology. Test data may
be lacking a significant amount of background information. Although no specific bias is evident, it's
not clear if the facilities tested represent a random sample of the industry. The source category
population is specific enough to minimize variability.
"D" - Below Average
Same as "C," but test data are from a small number of facilities, and there may be reason to suspect
the facilities do not represent a random sample of the industry. There may also be evidence of
variability within the source population.
"E" - Poor
Factor is developed from: (1) tests based on an unproven or new methodology, or tests that may be
lacking a significant amount of background information, or (2) tests based on a generally unacceptable
method, but the method may provide an "order of magnitude" value for the source. Facilities tested
may not represent a random sample of the industry and there is evidence of variability within the
source category population.
* The PHS assigned the number 999-AP-42 to this
publication. 999 was the series number, AP was an
abbreviation for air pollution, and 42 was the
document number. Thus, the origin of today's AP-42!
5 AP-42, Fifth Edition Compilation of Air Pollutant Emissions Factors, Volume 1: Stationary Point and Area Sources. Introduction, pp.
9-10.
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