Under the Clean Air Act, one of the first questions a facility must answer is whether it qualifies as a “major source” or a “minor source.” That classification shapes nearly everything that follows — which permits apply, what control technology may be required, and how closely a facility’s emissions will be scrutinized going forward.

Major source status is generally determined by emissions thresholds of 100 or 250 tons per year, depending on the pollutant and the permitting program involved. Facilities that cross these thresholds may become subject to Title V permitting and, in some cases, Prevention of Significant Deterioration (PSD) review, which can include Best Available Control Technology (BACT) requirements. Minor sources face a lighter permitting burden, but they are not exempt from oversight — they remain subject to state permitting requirements and State Implementation Plan (SIP) obligations.

When the Emission Source Threshold Isn’t a Simple Calculation

Source classification is not always a straightforward tonnage exercise. In Utility Air Regulatory Group v. EPA (2014), the U.S. Supreme Court held that EPA could not require PSD or Title V permitting for a facility based solely on its greenhouse gas emissions, even though the statutory thresholds technically could have been read that way. The decision underscored that major source status depends on how emissions thresholds are applied pollutant by pollutant and program by program — not on a blanket interpretation that could sweep large numbers of additional facilities into major source status.

For facility operators, that case is a useful reminder that source classification is not always a simple threshold calculation. It can depend on how a given pollutant and permitting program interact, and misclassifying a facility — in either direction — can create compliance risk.

Why Classification Matters for Multi-Facility Air Compliance

For clients operating several facilities, source classification can vary from site to site, even within the same company. A facility that is a minor source in one state may be a major source in another, depending on production levels, control equipment, and local rules. Tracking each facility’s classification — and monitoring for changes that could shift a facility from minor to major — helps prevent gaps in permitting and reporting obligations before they become compliance issues.

Key Points

  • Major source status is generally determined by emissions thresholds of 100 or 250 tons per year, depending on the pollutant and program
  • Major sources may be subject to Title V permitting and PSD review, including BACT requirements
  • Minor sources face a lighter permitting burden but are still subject to state permitting and SIP requirements
  • Utility Air Regulatory Group v. EPA confirmed that major source status must be applied pollutant by pollutant and program by program
  • Multi-facility operators should track classification at each site, since it can vary by location and change over time

Supporting Air Quality Compliance Regardless of Classification

Compliance Assurance Associates, Inc. (CAA) helps clients manage visible emissions compliance across facility locations, including support for observer training, Method 9 certification, and accessible certification records — regardless of whether a facility is classified as a major or minor source.

Maintaining current observer certifications and organized compliance records can support a stronger compliance program as a facility’s operations, and its source classification, evolve over time.

To discuss facility classification or Method 9 compliance support, clients can reach CAA by email or phone.

Compliance Assurance Associates, Inc. (CAA) is a nationally recognized provider of smoke school training and opacity certification, helping facilities navigate visible emissions requirements embedded in Title V permits, State Implementation Plans, and other air compliance programs.