MACT — 40 CFR Part 63

The 1990 Clean Air Act Amendments directed EPA to develop source-category-specific emission standards based on the best-performing technologies within each category. MACT represents the current mainstream of HAP regulation in the United States and supersedes or complements Part 61 for most source categories.

MACT structure

Part 63 is organized into subparts by source category. The most industrially relevant subparts for the process industries:

• Subpart H — Hazardous Organic NESHAP (HON) equipment leaks. Applies to equipment leaks in the synthetic organic chemical manufacturing industry (SOCMI). The HON is the equipment leak rule for SOCMI facilities subject to Part 63.
• Subpart CC — Petroleum Refineries (Refinery NESHAP / Refinery MACT). Applies to petroleum refineries subject to the Clean Air Act. Includes equipment leak requirements for components in VOC or HAP service.

Additional industrially relevant subparts:

• Subpart F — SOCMI source-category NESHAP (covers process vents and storage tanks; cross-references Subpart H for equipment leaks).
• Subpart G — SOCMI process vents, storage tanks, and wastewater.
• Subpart I — SOCMI for specific chemical categories (butadiene-, chlorine-, styrene-based processes).
• Subpart UUU — Catalytic Cracking Units, Catalytic Reforming Units, and Sulfur Recovery Units at petroleum refineries.
• Subpart Y — Marine Tank Vessel Loading Operations.
• Subpart EEEE — Organic Liquids Distribution.

Key Subpart H (HON) requirements

The HON is the equipment leak rule for SOCMI facilities under Part 63. It covers:

• Equipment in organic HAP service for 300 or more hours per year.
• Equipment containing or contacting fluid that is 5 percent or greater by weight of total organic HAP.
• Specific standards for equipment in gas/vapor, light liquid, and heavy liquid service.

Leak definition thresholds under the HON generally follow a 500 ppmv standard for many component categories, with variations by component type and service.

Key Subpart CC (Refinery NESHAP) requirements

The Refinery NESHAP covers petroleum refinery equipment leaks in organic HAP service. It also provides a higher maximum percent of leaking valves threshold for facilities using skip period provisions with connector monitoring. Many refineries subject to NSPS Subpart GGG or GGGa for VOC equipment leaks are simultaneously subject to Subpart CC for HAP equipment leaks, creating overlapping compliance requirements.

Relationship to NSPS

A facility can be subject to both an NSPS under 40 CFR Part 60 (for VOC emissions) and a MACT under 40 CFR Part 63 (for HAP emissions) for the same equipment. When requirements overlap, the more stringent standard generally applies. Some subparts explicitly address this overlap — for example, facilities subject to the Refinery NESHAP (Part 63 Subpart CC) but not subject to NSPS Subparts VV or GGG are not separately required to comply with NSPS Subpart VVa.

Enforcement

Part 63 is enforceable under Clean Air Act Section 113. Civil penalties are adjusted annually for inflation under 40 CFR 19.4. EPA's MACT enforcement priorities — particularly for the Refinery NESHAP and the HON — have produced significant enforcement settlements across the refining and petrochemical sectors.

The role of P&IDs

MACT equipment leak compliance depends entirely on a current component inventory. The source record is the governing P&ID. Every HON-subject valve, pump, and compressor seal must be traceable to a specific location on a specific drawing. P&ID drift is a structural vulnerability for any MACT compliance program.