About the TCEQ Guidelines to Develop Toxicity Factors
This document is an update of and replacement for the previous TCEQ Regulatory Guidance-442 (RG-442), Guidelines to Develop Effects Screening Levels, Reference Values, and Unit Risk Factors. It is a technical guide written and used by the TCEQ to develop the following health- and welfare-based inhalation toxicity values, and health-based oral toxicity values:
- acute and chronic inhalation Reference Values (ReVs),
- chronic inhalation Unit Risk Factor (URF) values,
- acute and chronic inhalation Effects Screening Levels (ESLs),
- acute and chronic inhalation Air Monitoring Comparison Values (AMCVs),
- chronic oral reference dose (RfD) and slope factor (SFo) values.
Although this document is primarily written as guidance for the TCEQ staff, it also documents (largely by reference) the processes used to develop different toxicity values for any interested person with training in inhalation and oral toxicology and risk assessment.
ESLs and AMCVs
are chemical-specific air concentrations set to protect human health and welfare. ESLs are used in the air permitting program. Short-term ESLs are based on data concerning acute health effects, the potential for odors to be a nuisance, and effects on vegetation, while long-term ESLs are based on data concerning chronic health and vegetation effects. Welfare-based ESLs (odor and vegetation) are set based on effect threshold concentrations.
Health-based ESLs, however, are calculated from ReV and URF toxicity factors. ReVs and URFs are based on the most sensitive adverse health effect relevant to humans. Derivation of a ReV or URF begins with a toxicity assessment involving hazard identification and dose-response assessment based on the chemical’s mode of action. The resulting ReV and URF values are then used to calculate ESLs that correspond to no significant risk levels.
AMCV is a collective term used to describe chemical-specific air concentrations used to evaluate air monitoring data that are set to protect human health and welfare. Short-term AMCVs are based on data concerning acute health effects, odor potential, and acute vegetation effects, while long-term AMCVs are based on data concerning chronic health or vegetation effects. AMCVs may contain health-based ReVs and health- and welfare-based ESL values.
TCEQ Guidelines to Develop Toxicity Factors
The TCEQ Guidelines to Develop Toxicity Factors [hereafter referred to as Guidelines (RG-442)] has been finalized.
In June of 2011, the draft Guidelines underwent a letter peer review and public comment period which was organized by Toxicology Excellence for Risk Assessment (TERA). In April of 2012, the revised draft Guidelines underwent a second public comment period lasting 60 days.
The final version of the Guidelines (RG-442) incorporated comments from the Peer Review Report and the two rounds of public comments that increased its scientific and technical merit and clarity. The Texas Commission on Environmental Quality Responses to Peer Review Report and the Texas Commission on Environmental Quality Responses to Public Comments documents are available on the TCEQ Web page.
The Toxicology Division (TD) amended Section 3.9.1, Default Dosimetry Adjustments for Gases in the Guidelines (RG-442). This section was updated to include information on animal-to-human inhalation gas dosimetric adjustments based on recommendations in Advances in Inhalation Gas Dosimetry for Derivation of a Reference Concentration (RfC) and Use in Risk Assessment (USEPA 2012, EPA/600/R-12/044). Please refer to the White Paper for a discussion of the changes.
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