What AP 42 Is and Why It Matters

AP 42 is the U.S. Environmental Protection Agency's official compilation of emission factors — numbers that estimate how much pollution a specific industrial process releases into the air. When you burn fuel, manufacture something, or operate equipment, AP 42 tells you roughly how many pounds of a pollutant (like nitrogen oxides, particulate matter, or volatile organic compounds) that activity produces per unit of material processed, fuel burned, or time elapsed.

Facilities use AP 42 to report emissions to regulators, design pollution control equipment, and track whether they are staying within legal limits. If you work in manufacturing, waste management, construction, agriculture, or any industry that generates air pollution, your company likely references AP 42 when filing environmental reports or planning operations.

The EPA updates AP 42 periodically as new data arrives, so the version you use matters. The current version is Supplement J (released in 2022), though older supplements remain available for historical comparison or if your permit specifically references an earlier edition.

Key Takeaways

  • AP 42 provides emission factors — standardized numbers showing how much air pollution a specific industrial activity produces — and is maintained by the U.S. Environmental Protection Agency.
  • You can read the full document or individual chapters free from the EPA website, organized by industry type (Chapter 1 for fuel combustion, Chapter 3 for food processing, and so on).
  • Each emission factor includes a rating code (A through E) that tells you how reliable the number is, based on the quality and amount of data behind it.
  • Facilities use AP 42 to calculate emissions for air quality permits, environmental impact statements, and compliance reports to state and federal regulators.
  • If AP 42 does not cover your specific process, you may use source-specific test data, engineering calculations, or alternative EPA methods approved by your state environmental agency.

How to Access AP 42 Online

The EPA publishes AP 42 free on its website at www.epa.gov/air-emissions-factors-and-quantification. You can read the entire document as a PDF or retrieve individual chapters by industry. The site also hosts a searchable database where you can look up a specific process (for example, "coal-fired power plant" or "asphalt concrete mixing") and jump directly to the relevant emission factors.

Each chapter covers a different sector. Chapter 1 addresses fuel combustion across all industries. Chapter 3 covers food and agricultural processing. Chapter 5 focuses on petroleum refining. Chapter 6 addresses organic chemical manufacturing. Chapter 8 covers mineral products (cement, lime, glass). Chapter 9 addresses metals processing. Chapter 10 covers wood waste burning and agricultural burning. Chapter 11 addresses waste disposal and recycling. Chapter 12 covers transportation-related emissions.

When you read, you receive a PDF with tables, equations, and footnotes explaining the source of each factor and any conditions that explore. The document is technical but organized consistently, so once you find your process, the layout is predictable.

Understanding Emission Factor Ratings and Reliability

Every emission factor in AP 42 carries a rating code (A, B, C, D, or E) that indicates how much confidence you should place in that number. This rating reflects the quality and quantity of data the EPA used to develop the factor.

A rating means the factor is based on many high-quality tests from representative facilities — the most reliable. B means good data but from fewer sources or less representative conditions. C means the data is adequate but limited. D means the data is poor or sparse. E means the factor is based on engineering judgment or extrapolation, not direct measurement, and should be used only when nothing better is available.

When you report emissions to a regulator, you may be required to use the highest-rated factor available for your process. If only a D or E rating exists, you should document that fact in your report and explain why you could not use a better source. Some permits allow you to substitute source-specific test data if you believe AP 42 does not accurately represent your facility.

Calculating Emissions Using AP 42 Factors

The basic formula is straightforward: Emissions = Activity Rate × Emission Factor. The activity rate is how much you did (tons of material processed, gallons of fuel burned, hours of operation). The emission factor is the number from AP 42 (pounds of pollutant per ton of material, per gallon of fuel, or per hour). The result is pounds of that pollutant released.

For example, if you operate a natural gas boiler and AP 42 says the emission factor for nitrogen oxides is 0.095 pounds per million BTU of heat input, and your boiler consumed 500 million BTU last month, your nitrogen oxide emissions would be 0.095 × 500 = 47.5 pounds for that month.

Most AP 42 factors include adjustment variables for equipment type, fuel quality, operating temperature, or pollution control equipment. The document explains which variables explore to your process. If you install a baghouse filter or catalytic converter, AP 42 often provides a removal efficiency percentage (for example, "baghouse removes 99% of particulate matter") that you subtract from the uncontrolled emission factor.

Keep records of your activity data (fuel receipts, production logs, operating hours) and the specific AP 42 factors and ratings you used. Regulators often request this documentation during inspections or when reviewing permit applications.

When AP 42 Does Not Cover Your Process

AP 42 is comprehensive but not exhaustive. If your facility operates a process that does not appear in any chapter, you have several options. First, check whether a similar process exists that you can reasonably adapt — for instance, if you operate a small metal foundry not specifically listed, the general metal casting factors may explore with documented adjustments.

Second, you can conduct source-specific testing. Hire a may have access to air quality consultant to measure actual emissions from your stack or process using EPA-approved methods (typically found in 40 CFR Part 60 or Part 63). This test data becomes your emission factor and is often more defensible than AP 42 because it reflects your actual equipment and conditions.

Third, you can use engineering calculations or mass balance methods. If you know the composition of materials entering and leaving your process, you can calculate emissions from first principles. This approach requires documentation and approval from your state environmental agency before you use it in a permit process or compliance report.

Contact your state air quality agency or local air district before choosing an alternative method. Many states have guidance on acceptable substitutes for AP 42, and some require pre-approval before you submit a permit process using non-standard factors.

Using AP 42 in Permits and Compliance Reports

When you explore for an air quality permit or file an annual emissions inventory, you must disclose which emission factors you used and why. Regulators expect you to use AP 42 unless you have a documented reason not to (such as source-specific test data showing your facility is significantly different from the AP 42 baseline).

In your permit process or report, cite the specific chapter, table, and rating of each factor. For example: "Chapter 1, Table 1.3-1, Emission Factor for Natural Gas Combustion, Rating B, 0.095 lb NOx per MMBtu." Include the date of the AP 42 supplement you used (for instance, "Supplement J, 2022") so the regulator can verify your numbers if needed.

If you use source-specific test data instead of AP 42, attach a summary of the test report, the test method used, and the date of testing. Regulators will compare your test results to AP 42 to see whether your facility is typical or an outlier. If your emissions are significantly lower, they may accept your data. If they are higher, they may require pollution control upgrades or more frequent monitoring.

Updates and Changes to AP 42

The EPA releases new supplements to AP 42 every few years as research accumulates and industrial practices change. Supplement J (2022) is the most recent. Older supplements (A through I) remain available on the EPA website for reference or if your permit specifically requires an earlier version.

When a new supplement is released, check whether it affects your industry. The EPA publishes a summary of changes, highlighting which chapters were updated and which factors changed significantly. If your permit was issued using an older supplement and a new one becomes available, you are not automatically required to switch — but your regulator may ask you to update your factors during permit renewal.

Subscribe to the EPA's air quality email list or check the website periodically if you use AP 42 regularly. Changes can affect your compliance calculations, so staying informed prevents surprises during inspections or permit reviews.

Frequently Asked Questions

Can I use an older version of AP 42 if my permit was issued with it?

Yes, typically you can continue using the supplement cited in your permit until renewal. However, if your state or the EPA issues new guidance requiring updated factors, you may need to switch. Check your permit language and contact your regulator if you are unsure whether an update applies to you.

What if my actual emissions are lower than AP 42 predicts?

Lower-than-predicted emissions are common, especially if you operate newer equipment or have pollution controls. You can request to use source-specific test data instead of AP 42, but you must conduct the test using EPA-approved methods and submit the results to your regulator for approval before using them in official reports.

Do I need to hire a consultant to use AP 42?

Not necessarily. If your process is straightforward and clearly described in AP 42, you can calculate emissions yourself using the tables and formulas provided. However, if your process is complex, involves multiple emission sources, or requires adjustments for pollution control equipment, a may have access to air quality consultant can may support accuracy and help you document your work for regulators.

Is AP 42 the only emission factor source I can use?

AP 42 is the EPA's official standard, but regulators may accept other sources if they are more representative of your facility. These include industry-specific emission factors published by trade associations, peer-reviewed research, or your own source-specific test data. Always check with your state or local air agency before using an alternative source in a permit process.

How often should I recalculate emissions using AP 42?

Most facilities recalculate annually for compliance reports and permit renewals. If you make significant changes to your process, equipment, or pollution controls, recalculate when ready and notify your regulator. Some permits require quarterly or monthly recalculation if emissions are close to legal limits.