Texas regulators have just released a new “Tier III” ESL Exceedances memo (July 10, 2025) that updates how the TCEQ’s toxicologists review air permit modeling. This guidance supersedes the draft 2001 and 2007 Toxicology memos and provides insight to how Toxicology has been evaluating chemicals. It provides thresholds for frequency and by concentration levels might exceed health-based or odor/vegetation Effects Screening Levels (ESLs) in permit reviews. Most importantly, this memo provides guidelines and are not legal standards: ESLs are screening tools, not enforceable standards like the NAAQS. Exceeding an ESL simply triggers closer review; it does not mean a permit will be denied. In fact, Tier III evaluations are done case by case by TCEQ’s Toxicology staff, who use their expert judgment on all factors (model conservatism, land use, monitoring data, etc.). In short, the new memo is a courtesy context for permit applicants – not a substitute for a formal toxicology review.

Background and Scope

TCEQ issues ESLs for hundreds of air toxics to protect health, avoid nuisance odor, or protect vegetation. Short-term (usually one-hour) and long-term (annual) ESLs are listed in the TCEQ’s Toxicity Factor Database, now part of the TAMIS (Texas Air Monitoring Information System) database which is always being updated. Applicants can download the latest ESLs from TAMIS to compare their model results to. When a modeled pollutant doesn’t screen out at Steps 0-6 of the Modeling and Effects Review Applicability (MERA) analysis and the modeled concentration exceeds the ESL at Step 7, Toxicology will perform a three tier evaluation. Tiers I and II are detailed in Appendix D of the MERA guidance. The July 2025 Tier III memo procedures further explains the general guidelines that Toxicology uses when evaluating a pollutant that makes it to Tier III. However, Toxicology stresses that every pollutant and situation is unique and only toxicology can decide how and if these guidelines apply.

Notably, the new memo does not apply in Air Pollutant Watch List (APWL) areas for the listed chemical. The APWL process focuses on sites already having elevated ambient levels. If your project is in an APWL county/city for the pollutant of interest, Tier III guidelines do not apply and stricter review applies. See TCEQ’s Air Pollutant Watch List page for details.

Guidelines vs. Standards: Key Points

• Screening Guidelines, Not Limits: ESLs and these Tier III metrics are for guidance only. They are screening levels used in permitting – not enforceable ambient standards. For example, if a model shows 1.5× an ESL (above the screening value), it does not automatically mean a violation; it means toxicologists will further evaluate. Likewise, Tier III’s suggested “allowable” exceedances serve to prompt discussion, not to set a legal cap. The memo explicitly cautions that nothing here substitutes for a toxicologist’s judgement and analysis.
• Case-by-Case Judgment: Every Tier III review is site- and pollutant-specific. The memo repeats that “only an assessment of risk by Toxicology can determine whether the guidelines described herein are applicable to any given permit”. Factors such as land use, nearby residents, model conservatism, existing pollutant levels, and data quality all play into a decision. These tables should be used as rough signposts and TCEQ toxicologists should be worked with when exceedances are flagged.
• Don’t Forget TAMIS: For any chemical, the official ESLs (interim or final) must be obtained from TCEQ’s TAMIS database. If the database does not include a chemical, a request must be sent to Toxicology Division staff for an ESL derivation. The Tier III memo organizes the ESL by type (interim vs. final, health-based vs. odor/vegetation). Be sure your permitting analysis uses the latest ESLs from TAMIS, as they are constantly being updated.

Allowable Exceedances under Tier III Guidelines

The memo provides tables that can be categorized into a few categories: interim ESL vs final ESL, short-term vs long-term, health-based vs odor- or vegetation-based, and carcinogenic and non-carcinogenic. Below is a summary.

• Interim ESLs allowances (short-term and long-term) are more conservative and allow higher exceedances and frequencies. The interim ESLs are based on limited data.
• Final Health-based ESLs allowances (short-term) are generally more stringent and allow lower exceedances and frequencies. The final ESLs have undergone a comprehensive development support document review.
• Final Health-based ESLs allowances (long-term) are broken up into carcinogenic and non-carcinogenic-based determination. Cancer causing pollutants will have more stringent allowances versus noncancerous pollutants. To identify whether a chemical’s ESL is based on cancer endpoints, review its Development Support Document (DSD) link when downloaded from TAMIS; not all chemicals with a DSD are cancer-based, and those without a DSD may be treated as noncancer.
• Final Odor/Vegetation ESLs allowances (short-term) are the most stringent allowances when compared to all other allowances.

Implications for Air Permitting

For Texas permit seekers, the new Tier III memo means staying proactive and should factor these guidelines into early project planning and modeling:

• Review Models Early: Compare your dispersion results to the ESL criteria above. If modeled peaks or annual averages exceed ESLs by large factors, identify how often those exceedances occur. Many times, a design tweak (lower emission rate or different stack height) can bring levels down under the guidelines.
• Coordinate with TCEQ: If you expect significant exceedances, engage TCEQ Toxicology early. Tier III reviews will consider context (e.g. wind data conservatism, existing air quality, etc.), so documenting these can help. Trinity Consultants can prepare brief risk summaries or compare to nearby ambient monitors if relevant.
• Use TAMIS Data: Always pull the latest ESL values from TAMIS to ensure you’re applying the correct thresholds. TAMIS also has toxicity factor info and the special notes memo that explains exceptions (e.g. how “PM” designations work).
• Remember the APWL Exception: If your site is in a watch-list area for a pollutant of concern, the Tier III caps above don’t apply. Instead, TCEQ will apply more conservative oversight. Permit seekers should check the APWL lists to see if their site is affected.

For any questions in regard to the July 2025 Tier III memo, contact Trinity’s Houston office at 713.552.1371.