Target lipid model update and proposed refinement of HC5 calculation procedure

Area of Science:

• Environmental Chemistry
• Ecotoxicology
• Computational Toxicology

Background:

• The Target Lipid Model (TLM) has been a standard for assessing aquatic toxicity of organic chemicals for over 20 years.
• Existing TLM applications derive hazard concentrations (HC5) to protect 95% of aquatic species.
• The TLM database now includes extensive acute and chronic toxicity data for over 100 species and 400 chemicals.

Purpose of the Study:

• To re-evaluate and standardize lipid-water partitioning models within the TLM and PETROTOX models.
• To propose and validate an improved statistical approach for calculating chronic HC5 values.
• To ensure updated HC5 estimations are protective of 95% of aquatic species.

Main Methods:

• Evaluated various lipid-water partitioning models using statistical goodness-of-fit metrics.
• Selected the Abraham-type formulation of the Polyparameter Target Site Model (pp-TSM) as the preferred model.
• Developed a novel boot-strapped error estimation method for HC5 calculation, incorporating model parameter and toxicity distribution uncertainties.

Main Results:

• All evaluated lipid-water partitioning models showed similar predictive performance.
• The pp-TSM was identified as the preferred model due to its performance, accessibility, and ease of application.
• The new HC5 calculation method, using boot-strapped error estimation, produced protective values across acute, chronic, and sublethal toxicity data.

Conclusions:

• The updated TLM, incorporating the pp-TSM and the new HC5 calculation method, offers improved and reliable aquatic toxicity predictions.
• The proposed HC5 calculation method eliminates previous distributional constraints and accounts for additional error sources.
• These advancements support the continued application and refinement of the TLM for environmental risk assessment.