On April 22, 2026, the U.S. Environmental Protection Agency (EPA) proposed updates to the National Emission Standards for Hazardous Air Pollutants (NESHAP) for the oil and gas sector through 40 CFR Part 63, Subparts HH (Oil and Natural Gas Production) and HHH (Natural Gas Transmission and Storage), as outlined in Part 1 of this series, “EPA Proposed Changes to NESHAP Subparts HH & HHH: Key Changes, Compliance, and Comment Deadlines”. This article builds on that foundation by focusing on how the proposal could affect source classification, air permitting, and compliance strategy for oil and gas operations.
The proposal includes potential changes to how certain affected emission points are regulated under the Clean Air Act (CAA) and includes 36 targeted questions for stakeholder input, which EPA has organized into a structured framework to gather feedback and technical data on regulatory approaches, emissions controls, and implementation considerations.
The EPA has requested comments, due June 22, 2026, from operators on how an update to Subpart HH could affect regulatory applicability (which facilities and emission points are covered), how EPA defines major source and area source status, and what the proposal could mean for Title V air permitting.
Major vs. Area Source Determination
The proposed rule redefines “associated equipment” by removing the phrase “except glycol dehydrators and storage vessels”. The removal of this phrase could affect how equipment is grouped for Subpart HH applicability. As a result, emissions from these sources would no longer require aggregation when determining major source status under Subparts HH and HHH. This change more closely aligns with the CAA which intended to regulate associated equipment as a major source, if such equipment individually has the potential to emit at a major source level of 10 tpy or more of a single hazardous air pollutant (HAP) or 25 tpy or more of combined HAPs.
EPA anticipates that this change could result in some facilities being reclassified from major sources to area sources, which may result in deregulation and cost savings. However, EPA is requesting information from operators to determine how many major sources will be affected by this definition change and the potential cost savings that could be realized if a major source changes to an area source classification.
As a result of reclassification, EPA is also concerned that facilities currently operating as a major source could remove emission control systems to an area source causing an increase in HAP emissions. If the rule is revised, it is recommended operators evaluate their source classification under Subpart HH while considering the rule change will not affect major source definitions under other sections of the CAA.
As stated above, this primarily affects upstream production facilities with glycol dehydrators, storage vessels with flashing emissions, and other associated equipment regulated under Subpart HH. Natural gas transmission and storage facilities regulated under Subpart HHH will likely be less affected by area source reclassification due to already being classified as a major sources without aggregation. However, upon reevaluation, a Subpart HHH affected facility could be reclassified as an area source and experience similar deregulation and cost savings.
Implications of Area Source Reclassification
One consequence of area source reclassification is that MACT emission control standards could be reduced or eliminated. Although there could be a reduction in required emission controls, regulation as a Subpart HH area source could include Benzene, Toluene, Ethylbenzene, and Xylene (BTEX) emission limits, rolling GLYCALC calculations, recordkeeping, and reporting.
- Compliance obligations that could relax with area source reclassification:
- Continuous compliance demonstrations tied to MACT limits
- §112(d)‑based performance tests
- MACT‑specific LDAR inspection frequencies
- MACT‑driven control device monitoring plans
- Semiannual MACT deviation reports
Facilities whose sole trigger for Title V major source applicability is HAP emissions at or above the ≥ 10 tpy of any single HAP or ≥ 25 tpy of combined HAP may no longer be a Title V major source. For operations that convert from a major source to an area source and subsequently transition from a Title V permit to a minor source air permit, could result in meaningful cost savings through reduced compliance reporting, simplified permit conditions, fewer monitoring requirements, and the elimination of MACT‑driven performance testing. Operators are unlikely to remove or downgrade existing HAP control systems, as continued compliance with area source thresholds is still required under Subpart HH. In practice, these controls also support worker safety and operational efficiency by limiting exposure to BTEX components and preventing product losses.
Additionally, operators would have to submit a state or federal permit modification approving this action. Removing controls would be unlikely with other air regulations such as best available control technology (BACT) and other state/federal operating permit requirements. Operators or industry groups should highlight redundant regulatory obligations when providing comments to EPA to illustrate that the removal of controls following a change in classification is unlikely.
Addition of Methanol to List of HAPs
EPA is also proposing to explicitly include methanol as a regulated hazardous air pollutant under Subparts HH and HHH. This change introduces new considerations for facilities that currently rely on BTEX as a surrogate for organic HAP emissions.
Instead of establishing separate emission limits for each individual compound, EPA has historically relied on BTEX as a surrogate to represent emission of organic HAPs for small dehydrators. Because BTEX compounds are typically absorbed into glycol and more likely to be emitted than other HAPs, controls that reduce BTEX are generally effective at reducing other non‑methanol HAPs.
Under this proposal, EPA would add methanol to the list of applicable HAPs in Subparts HH and HHH. There is some concern that the current method of using BTEX as a surrogate for organic HAP emissions is not well suited for methanol due to its water solubility being higher than BTEX compounds. EPA is requesting information to help develop an alternative compliance approach or updated emission limit equations for methanol, in addition to the BTEX emission limit equations already included in the regulation for small dehydrators. EPA is requesting operators provide inlet gas analyses that include methanol either directly or through industry groups to evaluate applicable methanol control.
Updates to Emission Calculation Methods
EPA is proposing updates to the compliance equations for small dehydrators to streamline compliance demonstration. The proposed updated equations, which differ for existing and new units, use throughput, BTEX concentrations, and constants based on an Upper Prediction Limit (UPL) to account for normal operating variability. Facilities could also demonstrate compliance using either measured BTEX concentrations or alternative reportable detection limits (RDL).
The proposed RDL equation addresses cases where very low inlet BTEX concentrations produce impractically low emission limits that are difficult to measure or enforce. EPA is also requesting updated data on BTEX detection limits, as prior assumptions (0.116 ppmv) may not reflect current conditions. The EPA is not proposing any changes to the large dehydrator provisions within Subparts HH and HHH.
Air Quality Modeling and Software Updates
EPA is also requesting comment on adding PromaxTM v.6 and other modeling programs as alternatives to GlycalcTM and on whether to revise the standard to allow the use of generic software rather than naming specific tools within NESHAP. With GlycalcTM now classified as Legacy Software that will no longer be updated or supported, EPA is considering removing references to GlycalcTM from Subparts HH and HHH.
Allowing PromaxTM as an explicitly approved modeling tool reflects current industry practice as PromaxTM is widely used, actively maintained, and better suited to model modern gas compositions and operating conditions. This change would not alter emission limits or regulatory expectations but would simply align the rule with current engineering practice. For operators, this reduces risk associated with relying on unsupported software and provides greater confidence that modeling results will be accepted by regulators. Continuing to cite an outdated tool (GlycalcTM), introduces unnecessary uncertainty for both operators and regulators, particularly when questions arise regarding model accuracy, compatibility, or the availability of technical support.
Conclusion
EPA’s proposed updates to Subparts HH and HHH introduce changes to how facilities may be classified, permitted, and evaluated for compliance. Revisions to the definition of associated equipment, potential reclassification of major sources to area sources, and new considerations for methanol emissions all have implications for regulatory applicability and compliance strategy.
At the same time, the proposal reflects EPA’s broader effort to modernize the rule by aligning definitions with the Clean Air Act, updating emissions calculation approaches, and incorporating current modeling practices. While some changes may reduce regulatory burden, they also introduce new considerations that operators should evaluate carefully within the context of existing permitting and compliance frameworks.
Delve Deeper with our in-Depth Guide to EPA’s Proposed Changes to Subparts HH and HHH
This article is part of Trihydro’s four-part series on EPA’s proposed updates to NESHAP Subparts HH and HHH:
- Start here: Overview of EPA’s Proposed Changes to Subparts HH & HHH
- This article: Source classification, permitting, and emissions calculations
- Next: LDAR and MACT compliance updates, including OGI and Appendix K
- Final: Methanol, metal HAPs, and additional stakeholder considerations
Each article focuses on a core set of EPA’s requests for comment to help operators evaluate potential impacts and identify priorities ahead of the June 22, 2026 deadline.
