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).
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.
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 to conduct regulatory surveys, what the best control technologies to reduce emissions are, and the timelines associated.
This article, the final installment in our Subparts HH and HHH series, summarizes key regulatory updates for methanol emissions, metal Hazardous Air Pollutants (HAP) considerations, and additional MACT determinations affecting storage, loading, and process equipment the oil and gas sector, including:
- Methanol emissions
- Benzene, Toluene, Ethylbenzene, Xylene (BTEX) as a surrogate for dehydrator emissions
- Acid Gas Removal Units (AGRUs)
- Submerged filling Maximum Achievable Control Technology (MACT) determinations
- Transport loading submerged fill
- Performance reporting
- 2012 MACT floor analysis
Methanol Emissions at Natural Gas Transmission and Storage Facilities (Questions 3a, 3b, 6a, & 6b)
In 2023, EPA received Information Collection Request (ICR) questionnaire responses on methanol emissions from dehydrators at natural gas transmission storage facilities, and no methanol emissions were reported. Because data does not confirm methanol emissions, EPA is not proposing regulation for these facilities. EPA is requesting data on methanol emissions at these facilities, along with supporting documentation or rationale if emissions are not present and will evaluate new data during the comment period to determine whether methanol should be included in a future rulemaking.
BTEX as a Surrogate for Methanol Emissions (Questions 5a, 5b & 5c)
EPA is requesting comments on non-combustion compliance approaches for BTEX as a surrogate for methanol emissions from small glycol dehydrators. BTEX is generally a reliable surrogate for organic HAP but may not represent methanol emissions for dehydrators using non-combustion controls. Methanol is not naturally present in produced hydrocarbons and is often introduced as a hydrate inhibitor, resulting in variable presence in inlet streams. Methanol also has a higher affinity for water than BTEX, increasing its potential for release during regeneration. This suggests that methanol emissions could exceed levels predicted by BTEX-based equations. Available data show measurable methanol emissions across some dehydrators, with variability and limited representation in major source datasets.
Without robust facility-specific data showing a consistent relationship between BTEX and methanol under non-combustion controls, a separate standard may be warranted. EPA is requesting additional information, analyses, and data to evaluate whether BTEX can serve as a surrogate or if a methanol-specific standard may be warranted.
AGRUs (Questions 7a & 7b)
AGRUs may emit hazardous air pollutants, including BTEX, carbonyl sulfide (COS), and carbon disulfide (CS2), but direct emissions data are limited. EPA previously established MACT standards requiring 95 percent reduction for similar emission sources, such as glycol dehydrators and storage vessels with potential for flash emissions, based on the performance of the best-performing sources. Similar control technologies, particularly combustion devices, are frequently applied to AGRUs. In addition, EPA’s 1985 standards for acid gas removal required sulfur emission reductions ranging from 74 to 99 percent, with combustion identified as a control method that can also achieve at least 95 percent reduction of HAP. Based on these parallels, EPA proposes a 95 percent reduction as the MACT floor for AGRUs.
EPA is also evaluating whether AGRUs located upstream of natural gas processing plants could emit at major source levels and whether similar controls are warranted for these units. While control technologies are expected to perform similarly, data on emissions and configurations for these upstream AGRUs are limited. EPA is requesting comments on whether AGRUs at these locations should be subject to a 95 percent reduction standard, and on the existence, emissions, and control practices of AGRUs at natural gas transmission and storage facilities.
Submerged Fill MACT (Questions 8a & 8b)
EPA has historically identified submerged filling as a control method to reduce working losses from storage vessels at natural gas transmission and storage facilities. EPA applies the same rationale used for production and processing facilities to transmission and storage facilities. EPA concludes submerged filling represents the MACT floor achieved by the best-performing similar sources when the MACT floor was originally established in 1999; however, EPA notes that while submerged filling reduces working losses, it does not address flash emissions from storage vessels.
In the 2023 ICR responses, no storage vessels at natural gas transmission and storage facilities reported routing flash emissions, or any emissions, to control devices. Based on this absence of controls, EPA concludes that similar control devices were likely not in place in 1999. As a result, submerged filling is identified as the MACT floor for storage vessels without potential for flash emissions control. EPA is requesting comments on this determination. EPA specifically seeks information to support or refute whether submerged filling was widely used in 1999 to reduce emissions from storage vessels without flash emission control at production sites and major source natural gas processing plants, and whether this assumption remains supported.
Transport Loading Submerged Fill (Questions 9a & 9b)
As with storage vessels, submerged filling has historically been identified by EPA as a common and demonstrated method for reducing emissions from transport vessel loading at natural gas processing plants. Earlier rulemakings note that loading operations could occur using splash or submerged methods, with submerged filling recognized as the more controlled approach. Similar requirements were established for marine vessel loading and gasoline bulk plants, where submerged filling was required due to comparable equipment design, loading practices, and HAP emissions. EPA also indicates that submerged filling has long been used to reduce product loss. Based on these parallels, EPA concludes that submerged filling represents the level of control achieved by the best-performing sources in 1999 and supports its use as the MACT floor.
EPA also evaluated whether more stringent controls are warranted. In 2023 ICR responses, over 25 percent of major source natural gas processing plants reported using combustion devices to control loading emissions. Similar systems, including vapor recovery units, can achieve approximately 95 percent HAP reduction; however, EPA estimates the incremental cost effectiveness at $47,000 per ton of additional reduction, which exceeds previously determined reasonable levels. Based on this, EPA determined submerged filling remains an appropriate baseline. EPA is requesting comments on this MACT determination is supported by available data and whether available data support or refute the use of submerged filling in 1999 and today for both processing plants and transmission and storage facilities.
Performance Reports (Question 13)
The proposed reporting template incorporates facility-level information and emission source–specific data, including liquid loadout, dehydrator, produced water, and AGRU information. EPA will evaluate whether to revise these categories based on advancements since the last eight-year technology review. EPA is now requesting comments on the content, layout, and overall design for the performance report.
2012 MACT Floor Analysis (Question 16)
The 2012 MACT floor analysis for small glycol dehydrators revised the approach used to establish BTEX limits by incorporating statistical methods to address variability. Rather than relying solely on the average performance of the best-performing units, EPA used normalized BTEX emission rates to calculate both an average and a 99 percent upper prediction limit (UPL). This approach captures operational variability while reflecting top-performing sources.
EPA notes that this methodology was introduced in the final rule following revisions made in response to public comments. As a result, some stakeholders indicated that there was limited opportunity to evaluate and comment on the updated UPL-based approach. EPA is now requesting comment on whether the use of the 99 percent UPL appropriately represents the performance of the best-performing units and whether the data, assumptions, and statistical methods used to develop the MACT floor remain applicable for small glycol dehydrators.
Conclusion
This Delve series summarizes EPA’s proposed NESHAP Subparts HH and HHH updates, including source classification, Title V applicability, monitoring, and key technical issues. The proposal may affect how operators evaluate, control, and report emissions under the Clean Air Act. With EPA seeking input on 36 questions, operators should review the proposal, assess potential impacts, and identify where supporting data or comments are needed before the June 22, 2026, deadline.
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:
- Part 1: Overview of EPA’s Proposed Changes to Subparts HH & HHH
- Part 2: Source Classification, Permitting, and Emissions Calculations
- Part 3: LDAR Technologies, MACT Updates, and Compliance Timelines
- This article: Methanol, Metal HAPs, and Additional Stakeholder Considerations
Each article focuses on a key group of EPA’s requests for comment to help operators evaluate potential impacts and identify priorities ahead of the June 22, 2026, deadline.
