On September 16, 2026, the Washington Department of Ecology filed a CR-101 to begin rulemaking to update WAC 173-460, Washington’s air toxics permitting rule for new and modified stationary sources. While the filing represents only the first step in a multi-year rulemaking process, the scope of the revisions and the timeline suggest this could be one of the most consequential updates to the state’s air toxics program since major revisions were implemented in 2009.
WAC 173-460 (Controls for New Sources of Toxic Air Pollutants) establishes how toxic air pollutants are evaluated during Notice of Construction (NOC) permit reviews and determines whether proposed emissions present an acceptable risk to public health. Although the actual regulatory language has yet to be developed, Ecology has identified several areas under consideration that could significantly influence future permitting requirements and compliance demonstrations.
The Importance of WAC 173-460
Unlike many air quality regulations that focus on criteria pollutants such as particulate matter, sulfur dioxide, or nitrogen oxides, WAC 173-460 specifically addresses toxic air pollutants emitted from new and modified sources. Compliance with the rule often requires detailed air dispersion modeling and occasionally escalates to also need a detailed health risk assessment as part of the permitting process.
As Washington continues to expand its focus on environmental justice, emerging contaminants, and public health-based regulations, Ecology appears poised to revisit many of the foundational elements of the state’s air toxics program.
Key Areas Ecology Is Considering
While Ecology has not yet proposed specific regulatory language, the agency identified several topics that will be evaluated during the rulemaking process.
Acceptable Source Impact Levels and Toxic Air Pollutant Lists
One of the most significant potential changes involves updates to Acceptable Source Impact Levels (ASILs) and the list of regulated toxic air pollutants.
ASILs serve as the allowable model concentrations for a first-tier toxics analysis under WAC 173-460. Each pollutant that exceeds the respective Small Quantity Emission Rate (SQER) must be evaluated with air dispersion modeling to show compliance with the ASIL. The ASILs represents screening concentrations used during permitting reviews to determine whether a source may require additional evaluation through a Health Impact Assessment (HIA). Changes to these thresholds could affect how facilities demonstrate compliance and whether additional modeling or health risk assessment is required.
Ecology has also indicated it may evaluate adding per- and polyfluoroalkyl substances (PFAS) and other emerging chemicals of concern to the air toxics program. As regulatory attention on PFAS continues to expand nationwide, Washington could become one of the states taking a more formal approach to evaluating these compounds during air permitting reviews. Inclusion of PFAS could also lead to more engaged public interest.
Multiple Averaging Periods
Ecology is also considering revisions related to averaging periods for certain pollutants. Currently, permitting demonstrations rely on one specific averaging period to evaluate each individual toxic air pollutant. Ecology has indicated they will consider establishing multiple averaging periods for certain pollutants, which represents a departure from the current form of the rule. While the details remain unclear, this aspect of the rulemaking could significantly influence future permitting analyses. Changes to averaging periods can affect emission calculations, modeling assumptions, operational flexibility, and compliance demonstrations.
Treatment of Background Concentrations
Under the current rule, only the potential impacts of toxic air pollutants emitted from the proposed project are evaluated for a first-tier toxics assessment, without inclusion of background concentrations. Changes to how background concentrations are selected, calculated, or incorporated into permit reviews could affect the feasibility of projects in certain geographic areas, particularly those located in urban environments or industrial areas.
Emission Offsets and Toxic Equivalency Factors
The CR-101 also identifies emission offsets and toxic equivalency factors as areas for potential revision.
- Updates involving emission offsets could affect how facilities address increases and decreases in toxic air pollutant emissions associated with modification projects. It may provide clearer incentive for applicants to initiate and toxic reduction projects. Toxic equivalency factors can play an important role when evaluating groups of compounds with similar toxicological characteristics. Revisions to these factors or changes in how they are applied could influence permitting analyses and risk determinations.
Improving Rule Implementation
In addition to the technical changes listed above, Ecology indicated that it will consider updates intended to improve rule implementation.
While this category is broad, implementation-focused updates often include clarifications, procedural improvements, consistency measures, and administrative revisions that can affect how permitting requirements are applied in practice. These changes may ultimately prove just as important as the more technical revisions, particularly if they influence application requirements, permitting timelines, or regulatory certainty.
Potential Implications for New and Modified Sources
Although it is too early to predict the final language ibn the rule, facilities planning capital projects should recognize that changes to WAC 173-460 could affect future permitting strategies. Potential impacts could include additional evaluations for emerging contaminants such as PFAS, revised dispersion modeling requirements, changes to screening thresholds, updated approaches for addressing background concentrations, and increased scrutiny of air toxics inventories and emissions estimates.
Because the rulemaking timeline extends several years into the future, facilities currently in design or planning phases may want to consider how potential regulatory changes could affect long-term project schedules and permitting pathways.
Looking Ahead
Ecology’s current timeline suggests that stakeholder engagement meetings may begin in late 2026 or early 2027, with a formal proposed rule anticipated in early 2028 and final adoption expected in fall 2028. This extended schedule provides regulated facilities, industry groups, and community stakeholders substantial opportunity to participate in the process and provide input before specific regulatory changes are finalized. While the details remain uncertain, the breadth of topics identified in the CR-101 suggests this rulemaking could represent the most significant update to Washington’s air toxics permitting program since the major revisions adopted in 2009. Facilities with current or future permitting needs should monitor developments closely and begin considering how potential changes could influence project planning, permitting strategies, and long-term compliance obligations.
Trinity Consultants plans to actively participate in Ecology’s stakeholder outreach and rulemaking meetings throughout the development of the proposed revisions. As additional details, draft language, and technical analyses become available, we will continue evaluating the potential impacts to industrial facilities and their potential projects. Facilities interested in understanding how these changes may affect future projects should stay tuned for updates, insights, and regulatory alerts from Trinity as the rulemaking process progresses. For questions or assistance understanding how the proposed rulemaking may affect future projects, or for support with stakeholder engagement planning, please contact Angie Johnson at [email protected], or click Contact Us.
