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Texas Toxics Effects Screening Level (ESL) for Air Quality Impacts Analysis

Posted: October 1st, 2020

Authors: Tanner H. 

Effects Screening Levels (ESLs) are used by the Texas Commission on Environmental Quality (TCEQ) to determine when a more in-depth review of air toxics modeling is necessary.  The human health and welfare effects evaluation is an integral part of the air permitting process in Texas.  For case-by-case permits including minor and major sources, prevention of significant deterioration (PSD), and nonattainment new source review (NNSR), TCEQ requires an air toxics impacts analysis for air contaminants based on the ESLs for the respective air contaminants.  ESLs are comprised of short-term (1-hour average) and long-term (annual average) concentrations for various toxics.  The primary focus of this blog is to provide guidance for air toxics modeling in case-by-case and amendment applications where offsite concentrations are over the ESL, and when they are under the ESL.

An air toxics impact analysis must be performed for any site that has the potential to emit a substance that is included on, but not limited to, TCEQ’s ESL list.  TCEQ’s ESL list changes often, so it is important to obtain the most recent ESL list for air contaminants which may be involved in an air permitting project.  To do this, follow TCEQ’s instructions on how to download the ESL list from the Texas Air Monitoring Information System (TAMIS) database.  If a chemical is not found from TAMIS, you can request ESL values by sending a form to TCEQ’s toxicology group.

TCEQ uses a Modeling and Effects Review Applicability (MERA) analysis to determine if an in-depth review is required.  The MERA is a nine-step (steps 0 to 8) analysis that determines the scope of air dispersion modeling and effects review and is documented in TCEQ’s MERA guidance document titled “Modeling and Effects Review Applicability (MERA)”.  The key to correctly completing the MERA analysis is to determine at which step a chemical “falls out” of the MERA, which is done by evaluating the criteria of the certain step.  The MERA begins with analysis of the net potential emissions increase and increases in complexity from project potential emissions to site-wide modeling.  Modeling can be performed at any point after step 3 and is a requirement starting on step 7.  For chemicals which do not “fall out” before step 7, a site-wide air toxics modeling analysis must be performed following TCEQ’s Air Dispersion Modeling Team (ADMT) guidance.

When a project air toxics modeling analysis is performed and reveals off-site concentrations below the levels listed in the table within the MERA reference guide step 4 for Maintenance, Startup, and Shutdown (MSS) and production emissions, no further analysis is required.  When a site-wide air toxics modeling analysis is performed and reveals off-site concentrations below the ESL, there is no further analysis required.  The MERA analysis, along with any site-wide air toxics modeling files, are then ready to be submitted to TCEQ with the rest of the air permit application or amendment.  For site-wide modeling only, TCEQ’s Toxicology Division will then evaluate the modeling based on a three-tiered approach, which is outlined in Appendix D of TCEQ’s MERA guidance document.

When the results of an air toxics modeling analysis are greater than the short-term ESL for a substance due to MSS emissions, there may still be a way to avoid an in-depth review.  TCEQ allows some exceedances to the short-term ESL.  Planned MSS emissions for certain compounds may exceed the ESL a certain amount of times, depending on the magnitude of the ESL exceedance.  The number of allowable exceedances is determined using the following table from MERA step 5B:

For modeling scenarios that exceed the ESL and do not meet the criteria for these exceptions, an in-depth review will be required.  An in-depth review is the third tier of the Toxicology Division’s evaluation procedure (Appendix D of TCEQ’s MERA guidance document) in which they consider the health and welfare impacts of the compound to determine if the impact is acceptable, allowable, or unacceptable.

For more information or if you have any questions regarding this blog, please reach out to ALL4 Houston Office Project Engineer Tanner Henson at thenson@all4inc.com or 281-937-7553 x308.

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