Why it comes first
Many NESHAP subparts apply only to major sources of HAP, some only to area sources, and some to both with different requirements. The boiler rules show the split: Subpart DDDDD covers boilers and process heaters at major sources, and Subpart JJJJJJ covers boilers at area sources.
“You are subject to this subpart if you own or operate an industrial, commercial, or institutional boiler or process heater as defined in § 63.7575 that is located at, or is part of, a major source of HAP, except as specified in § 63.7491.”
40 CFR 63.7485“You are subject to this subpart if you own or operate an industrial, commercial, or institutional boiler as defined in § 63.11237 that is located at, or is part of, an area source of hazardous air pollutants (HAP), as defined in § 63.2, except as specified in § 63.11195.”
40 CFR 63.11193
The definitions
The general definitions are in 40 CFR 63.2. Major source status is based on potential to emit HAP, considering controls, from all stationary sources in a contiguous area under common control. An area source is any source of HAP that isn't major. Some subparts define major source for their own category, so read the subpart's definitions as well.
“Major source means any stationary source or group of stationary sources located within a contiguous area and under common control that emits or has the potential to emit considering controls, in the aggregate, 10 tons per year or more of any hazardous air pollutant or 25 tons per year or more of any combination of hazardous air pollutants”
40 CFR 63.2“Area source means any stationary source of hazardous air pollutants that is not a major source as defined in this part.”
40 CFR 63.2
What goes into the number
Use potential to emit, not actual emissions. Include every HAP-emitting unit at the site under common control, including units no NESHAP covers, and compare both the largest single HAP and the total of all HAP with the definition.
Controls and operating limits lower potential to emit only if they are enforceable; see the definition of potential to emit in 40 CFR 63.2. Keep the HAP speciation behind each estimate, because the single-HAP comparison depends on it.
Oil and gas sites are counted differently
The oil and gas rules count major source status their own way. Subpart ZZZZ makes the determination for each surface site at production facilities. Subpart HH, for oil and natural gas production facilities, uses the definition in 40 CFR 63.761 and lets the owner estimate emissions from maximum throughput.
Subpart HH also requires a facility that is not major, but whose actual emissions are close, to update its determination every year. Read 40 CFR 63.760(c) before concluding a production facility is an area source.
“except that for oil and gas production facilities, a major source of HAP emissions is determined for each surface site.”
40 CFR 63.6585(b)“Facilities that are major or area sources of hazardous air pollutants (HAP) as defined in § 63.761.”
40 CFR 63.760(a)(1)“Emissions for major source determination purposes can be estimated using the maximum natural gas or hydrocarbon liquid throughput, as appropriate, calculated in paragraphs (a)(1)(i) through (iii) of this section.”
40 CFR 63.760(a)(1)“Any source that determines it is not a major source but has actual emissions of 5 tons per year or more of a single HAP, or 12.5 tons per year or more of a combination of HAP (i.e., 50 percent of the major source thresholds), shall update its major source determination within 1 year of the prior determination or October 15, 2012, whichever is later, and each year thereafter, using gas composition data measured during the preceding 12 months.”
40 CFR 63.760(c)
Area source doesn't mean no NESHAP
Area sources have standards too: Subpart HH covers TEG dehydration units at area sources, ZZZZ covers engines, and JJJJJJ covers boilers. Some area source rules also say whether being subject to them requires a Title V permit; ZZZZ is one.
“For area sources, the affected source includes each triethylene glycol (TEG) dehydration unit located at a facility that meets the criteria specified in paragraph (a) of this section.”
40 CFR 63.760(b)(2)“If you are an owner or operator of an area source subject to this subpart, your status as an entity subject to a standard or other requirements under this subpart does not subject you to the obligation to obtain a permit under 40 CFR part 70 or 71, provided you are not required to obtain a permit under 40 CFR 70.3(a) or 40 CFR 71.3(a) for a reason other than your status as an area source under this subpart.”
40 CFR 63.6585(d)
Staying an area source
A source near the line can take enforceable limits on HAP to stay an area source; the guide on writing a synthetic minor limit covers how. Subpart HH says it doesn't stand in the way. A source that later becomes major has to meet the major source requirements, so revisit the determination whenever equipment, throughput, or gas composition changes.
“Nothing in this paragraph is intended to preclude a source from limiting its potential to emit through other appropriate mechanisms that may be available through the permitting authority.”
40 CFR 63.760(a)(1)“A facility that is determined to be an area source, but subsequently increases its emissions or its potential to emit above the major source levels, and becomes a major source, must comply with all provisions of this subpart applicable to a major source starting on the applicable compliance date specified in paragraph (f) of this section.”
40 CFR 63.760(a)(1)
Document the determination
Keep a written determination with:
- The site boundary and the basis for contiguous area and common control
- Every HAP-emitting unit, with capacity and potential to emit for each HAP
- The factor, model, or analysis behind each estimate, with its source and date
- Each enforceable limit relied on, by permit and condition number
- The largest single HAP and the total, compared with the definition
- The date of the determination and the changes that would call for a new one
Sources
- 40 CFR 63.2: definitions for Part 63 (eCFR)
- 40 CFR 63.7485: Subpart DDDDD applicability (eCFR)
- 40 CFR 63.11193: Subpart JJJJJJ applicability (eCFR)
- 40 CFR 63.6585: Subpart ZZZZ applicability (eCFR)
- 40 CFR 63.760: Subpart HH applicability (eCFR)
- 40 CFR 63.761: Subpart HH definitions (eCFR)
This guide explains how programs fit together. Thresholds, limits, and dates appear only in the quoted text, linked to its source; read the full provision before relying on it.
AirPermitHub provides educational, engineering, research, and compliance-support information. Information provided by the platform does not constitute legal advice and should not replace review of applicable regulations, permits, agency guidance, or consultation with qualified professionals. Full disclaimer