SUPPLEMENTARY INFORMATION:

Throughout this document whenever “we,” “us,” or “our” is used, we mean the EPA.

I. Background Information

On August 5, 2013 (78 FR 47191), the EPA designated the Detroit area, a portion of Wayne County, Michigan, as nonattainment for the 2010 SO₂ NAAQS. On May 19, 2025 (90 FR 21228), the EPA approved Michigan's plan for attaining the 2010 SO₂ NAAQS for the Detroit area and for meeting other nonattainment area planning requirements of CAA sections 110, 172, 179 and 192. On August 14, 2025 (90 FR 39148), the EPA proposed to approve Michigan's redesignation request and maintenance plan for the Detroit nonattainment area for the 2010 SO₂ NAAQS.

II. Response to Comments

Upon publication of the August 14, 2025 (90 FR 39148), proposed rulemaking, the EPA opened a 30-day comment period, ending September 15, 2025. The EPA received one adverse comment, which is summarized below along with the EPA's response. The comment is included in the docket for this action.

Comment: The commenter suggests that there is insufficient evidence to demonstrate attainment in the Detroit area. Wayne County, in partnership with JustAir, has placed 100 air sensors in the county that have shown an average SO₂ level of 14.5 parts per billion (ppb) from May 1, 2024, when they began operation, to June 30, 2025. The commenter argues that the JustAir monitoring network demonstrates a ( printed page 16563) more granular measure of air quality than the five regulatory SO₂ monitors that Michigan EGLE operates in the Detroit area. While the commenter concedes that preliminary data from the JustAir sensors shows SO₂ levels below the primary SO₂ standard, the commenter suggests that the JustAir sensors have captured values that could indicate an exceedance of the recently revised secondary SO₂ standard. Additionally, the commenter states that the attainment demonstration relies on modeling using maximum allowable emissions but does not provide robust evidence that the applicable facilities are complying with the emission limits and associated requirements included in the plan.

Response: The reference to the secondary SO₂ standard is outside of the scope of this action, as this action is only addressing the primary standard for SO_2. For monitoring compliance with the NAAQS, the EPA utilizes an appendix of the CAA to address the requirements for monitoring quality assurance. The appendix, 40 CFR part 58 appendix A, specifies the minimum quality system requirements applicable to State and Local Monitoring Stations and other monitor types used to determine compliance with the NAAQS. The appendix addresses quality system requirements, measurement quality check requirements, calculations for data quality assessments, and reporting requirements. Therefore, the JustAir monitoring program, which does not follow this appendix, is not a reliable source of data for comparison to the NAAQS.

With regard to modeling using maximum allowable emissions, the EPA's April 23, 2014, “Guidance for 1-Hour SO₂ Nonattainment Area SIP Submissions” states that the EPA may make determinations of attainment based on the modeling, using allowable emissions, from the attainment demonstrations for the applicable SIP for the affected area. For a short-term ( i.e., 1-hour) standard, the EPA believes that dispersion modeling, using allowable emissions and addressing stationary sources in the affected area (and in some cases those sources located outside the nonattainment area which may affect attainment in the area) is technically appropriate, efficient and effective in demonstrating attainment in nonattainment areas because it takes into consideration combinations of meteorological and emission source operating conditions that may contribute to peak ground-level concentrations of SO_2. In reality, it is extremely unlikely that all sources would be operating at maximum emission rates simultaneously. The Detroit area met all compliance dates in Michigan's plan, and Michigan included compliance documentation as appendix D of its May 5, 2025, redesignation request, which is included in the docket for this action. Additionally, Michigan has shown that it maintains an active enforcement program to ensure ongoing compliance with the requirements included in the Detroit attainment plan.

Comment: The commenter raises concern that the primary 1-hour SO₂ NAAQS was set to protect against short-term SO₂ exposure and does not reflect long-term exposure risks. The commenter states that Wayne County residents have experienced health impacts from long-term, low-level exposure to SO_2, which the primary 1-hour standard does not address.

Response: Under section 109 of the CAA, the EPA sets primary, or health-based, NAAQS for all criteria pollutants to provide requisite protection of public health, including the health of at-risk populations, with an adequate margin of safety. In the EPA's June 22, 2010 (75 FR 35520), rulemaking strengthening the SO₂ NAAQS to the level of 75 ppb, the EPA provided a detailed rationale for the Administrator's determination that the 2010 SO₂ NAAQS would be protective of public health. This rationale included explicit consideration of protection for people, including children, with asthma. Specifically, the standard was based on direct evidence of SO₂ -related effects in controlled human exposure studies of exercising individuals with asthma, as well as epidemiologic evidence of associations between SO₂ concentrations in ambient air and respiratory-related emergency department visits and hospitalizations. This action does not address the primary SO₂ NAAQS itself but rather recognizes that the Detroit area has achieved the primary SO₂ NAAQS.

Comment: The commenter raises concerns over the potential for increased pollution from facilities within the Detroit area, citing that redesignating the area to attainment risks signaling that efforts to reduce emissions are no longer a priority. The commenter states that the decreased SO₂ levels in the Detroit area suggest that the current nonattainment status of the area has successfully led to lower pollution in the area and removing the enhanced scrutiny and enforcement measures that come with nonattainment status may lead to increased emissions. Additionally, the commenter contends that the measures put forth by Michigan within the maintenance plan for the area are reactive rather than preventative, as contingency measures would be triggered only after exceedances occur. Therefore, the commenter suggests that the maintenance plan should contain proactive measures that would be triggered at the first signs of SO₂ increases.

Response: A redesignation to attainment does not remove any emission control measures for existing sources that are already adopted into the EPA approved SIP for Michigan. As discussed in the proposal for this action, Michigan's redesignation request includes a demonstration that attainment of the SO₂ NAAQS was attributable to permanent and enforceable emissions reductions. Further, Michigan's redesignation request includes a plan to maintain the SO₂ NAAQS, which includes an attainment emission inventory, a maintenance demonstration, a commitment for continued air quality monitoring, a process for verification of continued attainment, and a contingency plan, which all will support measures to ensure the Detroit area remains in attainment and does not violate the SO₂ NAAQS in the future.

In the “General Preamble for the Implementation of Title I of the Clean Air Act Amendments of 1990,” published on April 16, 1992 (57 FR 13498), the EPA provides discussion of contingency measures specifically for SO₂ . This guidance states that in many cases, attainment revolves around compliance of a single source or a small set of sources with emission limits shown to provide for attainment. Although this guidance applies to contingency measures under section 172(c)(9), the EPA applies a similar policy with respect to contingency measures for SO₂ required in maintenance plans under section 175A(d). The requirement to submit contingency measures in accordance with section 175A of the CAA can be adequately addressed for SO₂ by the operation of a comprehensive enforcement program, which can quickly identify and address sources that might be causing exceedances of the NAAQS.

As discussed in the proposal, Michigan has demonstrated that it maintains a comprehensive and active enforcement program capable of prompt action to remedy compliance issues. Beyond this, as part of its contingency plan, Michigan commits to adopt and expeditiously implement necessary corrective actions in the event of a violation of the standard, or if an annual 99th percentile daily maximum 1-hour SO₂ concentration of 79 ppb or above ( printed page 16564) occurs in a single calendar year in the Detroit area.

Comment: The commenter states that the attainment framework does not adequately capture the cumulative impacts of long-term exposure to pollution. The commenter states that residents living in the Detroit area are disproportionately exposed to environmental hazards, including SO₂ and other pollutants. The commenter also notes that SO₂ can react in the atmosphere to form particulate matter and argues that elevated levels of SO₂ could lead to health effects from both exposure to SO₂ and particulate matter.

Response: The EPA is committed to protecting human health. For all criteria pollutants, the EPA ensures that the NAAQS are set to provide an adequate margin of safety for both the general population and susceptible populations that are potentially at increased risk for health effects in response to exposure to ambient air pollution.

When considering the primary SO₂ NAAQS, the EPA took into account the Integrated Science Assessment (ISA) of scientific information on known and potential human health effects associated with exposure to SO₂ in the air via the (1) Assessments in the Risk and Exposure Assessment (REA) of the most policy-relevant information in the ISA as well as quantitative exposure and risk analyses based on that information; (2) Clean Air Scientific Advisory Committee (CASAC) Panel advice and recommendations, as reflected in its letters to the Administrator and its public discussions of the ISA and REA; (3) public comments received during the development of the ISA and REA; and (4) public comments received on the EPA's notice of proposed rulemaking. Additionally, the EPA referred to the ISA regarding health effects and long-term exposures to SO₂ which addressed and concluded the long-term health evidence to be of insufficient quantity, quality, consistency, or statistical power to make a determination as to whether SO₂ was truly associated with these health outcomes. Therefore, when strengthening the SO₂ standard, the EPA drew upon an entire body of evidence on human health effects associated with the presence of SO₂ in the ambient air, and upon the results of the quantitative exposure and risk assessments reflecting this evidence.

In this action, the EPA is recognizing that the Detroit area has achieved the primary SO₂ NAAQS and is approving Michigan's maintenance plan for the area. While the EPA appreciates the commenter's concerns regarding other pollutants in the Detroit area, the EPA is not evaluating the air quality in the Detroit area with respect to other NAAQS in this action.

III. What action is the EPA taking?

The EPA is redesignating the Detroit area from nonattainment to attainment for the 2010 SO₂ NAAQS in accordance with Michigan's May 5, 2025, request. The EPA has determined that the area is attaining the 2010 SO₂ NAAQS and that the improvement in air quality is due to permanent and enforceable SO₂ emission reductions in the area. The EPA is also approving Michigan's maintenance plan, which is designed to ensure that the area will continue to maintain the 2010 SO₂ NAAQS.

In accordance with 5 U.S.C. 553(d) of the Administrative Procedure Act (APA), this action shall become effective immediately upon publication. The immediate effective date for this action is authorized under 5 U.S.C. 553(d)(1).

Section 553(d)(1) of the APA provides that final rules shall not become effective until 30 days after publication in the Federal Register “except . . . a substantive rule which grants or recognizes an exemption or relieves a restriction.” The purpose of this provision is to “give affected parties a reasonable time to adjust their behavior before the final rule takes effect.” Omnipoint Corp. v. Fed. Commc'n Comm'n, 78 F.3d 620, 630 (D.C. Cir. 1996); see also United States v. Gavrilovic, 551 F.2d 1099, 1104 (8th Cir. 1977) (quoting legislative history). However, when the agency grants or recognizes an exemption or relieves a restriction, affected parties do not need a reasonable time to adjust because the effect is not adverse. The EPA has determined that this rule relieves a restriction because this rule relieves sources in the area of Nonattainment New Source Review permitting requirements; instead, upon the effective date of this action, sources will be subject to less restrictive Prevention of Significant Deterioration permitting requirements. For this reason, the EPA finds that under 5 U.S.C. 553(d)(1) it is appropriate for this action to become effective on the date of publication of this action.

IV. Statutory and Executive Order Reviews

Under the CAA, redesignation of an area to attainment and the accompanying approval of a maintenance plan under section 107(d)(3)(E) are actions that affect the status of a geographical area and do not impose any additional regulatory requirements on sources beyond those imposed by state law. A redesignation to attainment does not in and of itself create any new requirements, but rather results in the applicability of requirements contained in the CAA for areas that have been redesignated to attainment. Moreover, the Administrator is required to approve a SIP submission that complies with the provisions of the CAA and applicable Federal regulations. 42 U.S.C. 7410(k); 40 CFR 52.02(a). Thus, in reviewing SIP submissions, the EPA's role is to approve State choices, provided that they meet the criteria of the CAA. Accordingly, this action merely approves State law as meeting Federal requirements and does not impose additional requirements beyond those imposed by State law. For that reason, this action:

• Is not a significant regulatory action subject to review by the Office of Management and Budget underExecutive Order 12866 (58 FR 51735, October 4, 1993);
• Is not subject toExecutive Order 14192 (90 FR 9065, February 6, 2025) because SIP actions are exempt from review under Executive Order 12866;
• Does not impose an information collection burden under the provisions of the Paperwork Reduction Act (44 U.S.C. 3501 et seq.);
• Is certified as not having a significant economic impact on a substantial number of small entities under the Regulatory Flexibility Act (5 U.S.C. 601 et seq.);
• Does not contain any unfunded mandate or significantly or uniquely affect small governments, as described in the Unfunded Mandates Reform Act of 1995 (Pub. L. 104-4);
• Does not have federalism implications as specified inExecutive Order 13132 (64 FR 43255, August 10, 1999);
• Is not subject toExecutive Order 13045 (62 FR 19885, April 23, 1997) because it approves a State program;
• Is not a significant regulatory action subject toExecutive Order 13211 (66 FR 28355, May 22, 2001); and
• Is not subject to requirements of section 12(d) of the National Technology Transfer and Advancement Act of 1995 (15 U.S.C. 272 note) because application of those requirements would be inconsistent with the CAA.

In addition, the SIP is not approved to apply on any Indian reservation land or in any other area where the EPA or an Indian Tribe has demonstrated that a Tribe has jurisdiction. In those areas of Indian country, the rule does not have Tribal implications and will not impose substantial direct costs on Tribal governments or preempt Tribal law as specified by Executive Order 13175 (65 FR 67249, November 9, 2000). ( printed page 16565)

This action is subject to the Congressional Review Act, and the EPA will submit a rule report to each House of the Congress and to the Comptroller General of the United States. This action is not a “major rule” as defined by 5 U.S.C. 804(2).

Under section 307(b)(1) of the CAA, petitions for judicial review of this action must be filed in the United States Court of Appeals for the appropriate circuit by June 1, 2026. Filing a petition for reconsideration by the Administrator of this final rule does not affect the finality of this action for the purposes of judicial review nor does it extend the time within which a petition for judicial review may be filed, and shall not postpone the effectiveness of such rule or action. This action may not be challenged later in proceedings to enforce its requirements. (See section 307(b)(2).)

For the reasons stated in the preamble, title 40 CFR parts 52 and 81 are amended as follows:

91 FR 16562