U.S. EPA’s Proposed Amendments to the Ethylene Oxide Sterilizer Rule and Their Implications for Other Rules

Author: Lindsey Kroos

On March 17, 2026, the United States Environmental Protection Agency (U.S. EPA) proposed amendments to 40 CFR Part 63, Subpart O (Ethylene Oxide Emissions Standards for Sterilization Facilities). The amendments would reverse certain stringent requirements finalized just two years ago and are likely welcome changes for the ethylene oxide (EtO) sterilization industry, but the basis for the reversal will also likely have lasting impacts on other industries. Read on to learn why and what parts of the rule U.S. EPA is proposing to amend.

Rule History and Risk Considerations

The original National Emission Standard for Hazardous Air Pollutants (NESHAP) for EtO Sterilization Facilities was promulgated in 1994. U.S. EPA conducted the Risk and Technology Review (RTR) mandated by the Clean Air Act (CAA) in 2006 and determined that revisions to the rule were not necessary to protect public health and the environment. Of note, RTRs consist of two parts: the residual risk review under Section 112(f)(2) of the CAA, and the technology review under Section 112(d)(6) of the CAA. The risk review has historically been understood to be a one-time review, required within eight years of a CAA Section 112 standard’s promulgation. The technology review, on the other hand, is not a one-time review, and is required at least every eight years.

On April 13, 2023, U.S. EPA proposed amendments to Subpart O based on a second RTR. The technology-based portion of the review was standard, but the second risk review was unprecedented. Just a few weeks later, on April 23, 2023, U.S. EPA proposed amendments to the Hazardous Organic NESHAP (HON), some of which were also based on a second risk review. And on December 27, 2024, U.S. EPA proposed amendments to the NESHAP for the Polyether Polyols (PEPO) Production Industry in part based on a second risk review.

The common thread between these rules is the regulation of EtO. In 2016, U.S. EPA updated its Integrated Risk Information System (IRIS) inhalation unit risk estimate (URE) for EtO, resulting in EtO being deemed a higher risk than originally thought. U.S. EPA stated in the proposed rules that a second risk review was warranted due to this updated IRIS value.

Subpart O was finalized on April 4, 2024, the HON rule was finalized on May 16, 2024, and both included new or more stringent requirements for EtO based on the second risk review. On March 12, 2025, U.S. EPA announced that Subpart O would be reconsidered. On March 18, 2026, the PEPO rule was finalized without the second risk review-based requirements, and the amendments proposed to Subpart O would eliminate the second risk review-based requirements.

U.S. EPA explains in the proposed rule that the basis for rescinding the second risk review-based requirements is two-fold: because it misinterpreted its authority to conduct a second “discretionary” risk review and due to the uncertainty surrounding the 2016 IRIS value. Amendments to the HON rule have not been proposed as of the date of this blogpost, and future second risk reviews appear unlikely (unless litigation on these rules results in a court decision that contradicts U.S. EPA’s current interpretation).

Proposed Amendments

While recission of the second risk review-based requirements may get most of the attention, there are several proposed amendments to the rule, including the following:

• Rescinding the CAA Section 112(f)(2) percent reduction standards,
• Rescinding the CAA Section 112(f)(2) requirement to use an EtO continuous emissions monitoring system (CEMS),
• Rescinding the requirement for a Permanent Total Enclosure (PTE),
• Amending certain parametric monitoring requirements,
• Amending certain CAA Section 112(d)(6) percent reduction standards, and
• Various technical corrections and clarifications.

Each proposed amendment is discussed further below.

Rescinding the CAA Section 112(f)(2) standards

Based on the reasons described above, the second risk review-based percent reduction requirements for certain sterilization chamber vents (SCVs), aeration room vents (ARVs), chamber exhaust vents (CEVs), and Group 1 and Group 2 room air emissions with specified minimum annual EtO use are proposed to be removed. Most of the percent reduction requirements proposed for removal range from 99.99% to 98%. Instead, the affected sources would be subject to the applicable CAA Section 112(d)(2) and (3), 112(d)(5), or 112(d)(6) requirements. In some cases, the required percent reduction would be less stringent, but others would remain the same or similar.

Rescinding the CAA Section 112(f)(2) requirement to use an EtO CEMS

The requirement for facilities using 100 lb/yr or more of EtO to use an EtO CEMS to demonstrate compliance is proposed for removal because it “was based on the results of an unauthorized second residual risk assessment and risk-based standards-setting.” Instead, all facilities can choose to use EtO CEMS or an annual performance test and parametric monitoring to demonstrate compliance.

Rescinding the requirement for a PTE

While not based on the second risk review, the requirement to use a PTE for room air emissions is proposed to be removed for two possible reasons. First, U.S. EPA had finalized this requirement based on their understanding that many EtO sterilization facilities already employed this technology; however, recognizing that implementing PTEs would require substantial time, they extended the compliance deadline from the proposal from two years to three years. They further explained that additional mechanisms for extending the compliance deadline beyond three years were available, including a 2-year Presidential Exemption, which was granted to 22 facilities via a July 17, 2025 proclamation. Now in this proposed reconsideration, U.S. EPA explains that they have determined EtO sterilization facilities do not consistently employ PTEs, and that the need for a PTE depends on a facility’s design and configuration, such that the requirement for a PTE can be established on a case-by-case basis during the state permitting process.

Second, U.S. EPA explains that they have “discretion whether to remove the PTE requirement as a compliance mechanism” because “the PTE requirement is not compelled by the D.C. Circuit’s decision in Louisiana Environmental Action Network v. Environmental Protection Agency (LEAN).” U.S. EPA goes on to explain that other rulemakings have implied that the LEAN decision required them to regulate previously unregulated pollutants as well as regulated pollutants emitted in a different matter (e.g., fugitive), but that they are clarifying “that when conducting a CAA section 112(d)(6) review,” they are “not obligated under the interpretation adopted in LEAN to prescribe particular standards for emission points with respect to pollutants already regulated under the NESHAP.” Therefore, because EtO was already regulated, U.S. EPA is not required to regulate EtO that is emitted in a different manner (e.g., fugitive). This change in interpretation has implications for many rules that U.S. EPA is currently reviewing or reconsidering and will likely be litigated if finalized.

Amending certain parametric monitoring requirements

The monitoring parameters and the procedure for setting operating parameter limits during performance tests is proposed to be updated for acid-water scrubbers, thermal oxidizers, catalytic oxidizers, gas-solid reactors, and other control devices. The purpose of the updates is to establish continuous monitoring based on a 1-hour block average basis rather than the current 3-hour block average basis and to better align the parameters with performance of the control devices. In particular, acid-water scrubbers would require monitoring of the scrubber liquid-to-gas ratio, scrubber liquid temperature, and scrubber liquid pH rather than the scrubber liquid level, ethylene glycol concentration, and scrubber liquid pH. Thermal oxidizers would require monitoring of the combustion chamber temperature rather than “the temperature in or immediately downstream of the firebox,” as well as flue gas flow rate. Catalytic oxidizers would continue to monitor the inlet and outlet temperatures of the catalyst bed, but would also require monitoring of the flue gas flow rate. Gas-solid reactors would require monitoring of the pressure drop across the reactor and establishing an upper gas-solid reactor outlet EtO concentration operating limit. During annual performance tests, the average parameter values for each control device measured during the test would become the unit’s ongoing operating limits.

Amending certain CAA Section 112(d)(6) standards

The percent reduction requirement for new ARVs at facilities that use at least 10 tpy of EtO is proposed to be reduced from 99.9% to 99.6%. The 99.9% reduction scenario was finalized in the 2024 rule because the cost-per-ton of EtO removed was lower than the 99.6% reduction scenario. However, U.S. EPA explains that the 99.6% reduction scenario would be equal to the percent reduction for existing ARVs, and therefore the cost analysis did not account for the use of existing “ductwork, control devices, and other existing infrastructure” that could be shared among existing and new ARVs, resulting in a lower cost-per-ton of EtO removed than the 99.9% reduction.

Various technical corrections and clarifications

U.S. EPA has proposed to make the following technical corrections and clarifications:

• Correcting cross-reference and typographical errors,
• Making clarifying technical corrections,
• Updating the definitions of operating day, single-item sterilization, sterilization operation, and indoor EtO storage,
• Updating the requirement for EtO CEMS inlet time-sharing,
• Correcting requirements for flow rate monitors,
• Adding timing requirements for Relative Accuracy Test Audits (RATAs), and
• Making technical corrections to Performance Specification (PS) 19 and Procedure 7 of 40 CFR Part 60 Appendix F.

What’s Next?

U.S. EPA has requested comments on 21 specific questions identified throughout the preamble to the proposed rule. The compliance deadline for CAA Section 112(d) standards of April 6, 2027 for existing sources, and upon startup for new sources, is proposed to be retained. Compliance with the CAA Section 112(f)(2) standards would have been April 6, 2026, but U.S. EPA states “existing sources that currently are subject to both section 112(d) standards and section 112(f)(2) standards promulgated in the 2024 Final Rule would only be subject to the applicable section 112(d) standards.” However, comments on the proposed reconsideration are due by May 1, 2026, after the April 6, 2026 compliance date for CAA Section 112(f)(2) standards (except for those facilities granted an extension), and a final rule would likely not be in place for several months after that.

This is just one of several recent rulemaking actions where U.S. EPA has put forth new interpretations that conflict with the previous administration’s actions and interpretations. We are keeping track of updates to this rule and other rules that were made more stringent during the previous administration. Please reach out to me or your ALL4 Project Manager with questions or for more information.