Potential to emit, explained for engineers

The definition

Federal rules define potential to emit as the maximum capacity of a stationary source to emit a pollutant under its physical and operational design. Physical or operational limitations, such as control equipment or restrictions on hours of operation or on the type or amount of material combusted, stored, or processed, count as part of that design only if they are enforceable.

The PSD definition is at 40 CFR 52.21(b)(4), and the Title V definition is at 40 CFR 70.2. State rules usually adopt the same concept; read the definition in the program you are applying under.

Potential versus actual emissions

Actual emissions describe what a unit emitted in a period. PTE describes what it could emit at maximum design capacity with only enforceable limits applied. A facility that runs half the year still has a PTE based on year-round operation unless an enforceable limit says otherwise.

When limits count

A control device, an hours limit, or a throughput cap lowers PTE only when it is enforceable, typically as a permit condition or rule requirement with monitoring, recordkeeping, and reporting that show compliance. Taking such limits to stay below major source thresholds is often called becoming a synthetic minor source.

• Write the limit so it can be measured (hours, throughput, fuel use)
• Pair it with monitoring and records that prove compliance
• Make sure the control efficiency you claim matches what you can demonstrate

Fugitive emissions

Whether fugitive emissions count toward major source status depends on the source category and the program. The PSD and Title V rules each say when fugitives are included. Check the major source definition that applies before adding or excluding them.

Documenting the calculation

A defensible PTE calculation shows, for each unit and pollutant: the capacity basis, the emission factor with its source document, section, and revision date, operating hours, any control efficiency and the enforceable requirement behind it, and the arithmetic. AirPermitHub calculators record each of these and export the full calculation set.