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Toxicity Evaluation of Chemicals with Limited Toxicity Data. Roberta L. Grant, Ph.D. Toxicology Division - Chief Engineer’s Office Texas Commission on Environmental Quality. Outline. Toxicity assessments Chemicals with adequate toxicity data Chemicals with limited toxicity data.

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Toxicity Evaluation of Chemicals with Limited Toxicity Data


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    1. Toxicity Evaluation of Chemicals with Limited Toxicity Data Roberta L. Grant, Ph.D. Toxicology Division - Chief Engineer’s Office Texas Commission on Environmental Quality

    2. Outline • Toxicity assessments • Chemicals with adequate toxicity data • Chemicals with limited toxicity data

    3. Toxicity Assessment • The basic objective of a toxicity assessment is to identify what adverse health effects a chemical causes and how the appearance of these adverse effects depends on exposure level (dose)

    4. TCEQ Exposure Durations and Averaging Times • Acute – 1 hour intermittent exposure (short-term) • Chronic – lifetime exposure (long-term)

    5. Tiered Approach for Chemicals with Limited Toxicity Information

    6. Cumulative Normal Distribution

    7. Threshold of Concern Approach

    8. Threshold of Concern Approach • Obtain LC50 data and acute inhalation NOAELs from animal studies for 97 chemicals • Exposure duration: 1-hour intermittent exposure, but frequently included > 1 hr studies, and in some cases multiple day studies • Categorize chemicals into different acute inhalation toxicity potency classes using LC50 data and the Globally Harmonized System (GHS) of Classification and Labeling of Chemicals (United Nations 2005)

    9. Threshold of Concern (TOC)

    10. Calculate the 10th percentile of the cumulative percentage distribution of NOAELs in each category Cat 5 = 104 mg/m3 Cat 2 = 2 mg/m3 Cat 1 = 0.4 mg/m3 Cat 3&4 =12.6 mg/m3

    11. Threshold of Concern (TOC) • Divide the 10th percentile NOAEL values by 100 to account for human variability and uncertainty of animal to human extrapolation Cat 1: 0.4 mg/m3 / 100 = 4 ug/m3 Cat 2: 2 mg/m3 / 100 = 20 ug/m3 Cat 3&4: 12.6 mg/m3 / 100 = 125 ug/m3 Cat 5: 104 mg/m3 / 100 = 1000 ug/m3 • Use the LC50 data of a chemical to categorize it into a GHS category. Use the TOC for that category as a generic ESL

    12. Tier II Generic ESLsNOAEL to LC50 Ratio Approach

    13. NOAEL-to-LC50 Ratio Calculate the ratio between acute inhalation NOAELs and LC50 Calculate the 10th percentile ratio 10th percentile ratio = 0.0083

    14. NOAEL-to-LC50 Ratio • Divide the ratio of 0.0083 by 100 to account for human variability and uncertainty of animal to human extrapolation • Health-Protective Ratio = 0.000083 • LC50 data x 0.000083 = generic ESL

    15. TOC or NOAEL-to- LC50 Ratio Approach? • Both approaches use LC50 data, although the TOC approach is generally more conservative than the NOAEL-to- LC50 Ratio approach • Use information on the chemical (i.e., toxicity of chemical class or chemical structure) and a weight-of-evidence approach to decide which approach is most defensible • Choose the most conservative number if there is uncertainty in the quality of the LC50 data

    16. Summary • For chemicals with limited toxicity data, statistical or relative toxicity/potency approaches can be used to derive health-protective default or generic ESLs • Tier 1 – Threshold of Regulation • Tier II – Threshold of Concern toxicity potency classes or NOAEL-to-LC50 Ratio • Tier III – Relative Toxicity/Potency Approach

    17. ? ? ? QuestionsRoberta L. Grantrgrant@tceq.state.tx.us(512) 239-4115Toxicology Division Website:http://www.tceq.state.tx.us/implementation/tox/esl/guidelines/about.html ? ?