EPA Method 21 — Determination of Volatile Organic Compound Leaks

Method 21 is not a regulation in itself — it is the measurement procedure that LDAR regulations incorporate by reference. When 40 CFR Subpart VVa specifies that a valve in gas service is leaking at 500 ppmv above background, the 500 ppmv measurement is made using Method 21.

What Method 21 specifies

Method 21 defines:

• Instrumentation requirements — the portable VOC detector must have specific sensitivity, response time, and calibration characteristics. Flame ionization detectors (FIDs) are the most common Method 21 instrument; some applications use photoionization detectors (PIDs) or catalytic oxidation detectors.
• Calibration requirements — daily calibration with specified reference gases, drift checks, and response factor determination.
• Sampling procedure — how the probe is positioned at the potential leak interface (valve stem, pump seal, flange, connector), sampling duration, and measurement recording.
• Background measurement — how ambient VOC concentration is established so that leak measurements can be reported as “above background.”
• Data quality objectives — performance criteria the measurement must meet to be considered valid.

Leak definition thresholds

Method 21 provides the measurement procedure; the applicable LDAR regulation specifies the leak definition threshold. Common thresholds:

• 500 ppmv above background — applied to valves under NSPS Subpart VVa, many NESHAPs, and the Hazardous Organic NESHAP (HON).
• 10,000 ppmv above background — applied under older NSPS provisions (Subpart VV) and for certain component types.
• 2,000 ppmv — applied to pumps under some rules.
• Other thresholds — applied to specific component types and service classifications under specific rules.

Method 21 versus Optical Gas Imaging (OGI)

Method 21 is instrument-based: a technician physically walks every regulated component with a probe, takes a direct measurement at the leak interface, and records the result. This produces a high-quality, defensible measurement but is labor-intensive.

Optical Gas Imaging (OGI), codified as 40 CFR Part 60 Appendix K, uses a specialized infrared camera to visualize VOC plumes. OGI surveys components from standoff distance rather than probe contact, which allows faster coverage of large populations.

Recent EPA rulemakings — including the 2024 NSPS OOOOb and OOOOc for oil and gas facilities — allow Appendix K (OGI) as an alternative to Method 21 for compliance. Other rules still require Method 21 as the primary method or as a verification method when OGI is used.

The role of P&IDs

Method 21 measurements are taken at specific components. Each component must be identified, located, and attributable to a specific location in the facility. The source record for component identification and location is the governing P&ID. A Method 21 survey that cannot trace each measurement back to a specific component on a specific drawing is not defensible under audit.