Ecological Thresholds of Toxicological Concern: A Review

Abstract

The ecological threshold of toxicological concern (ecoTTC) is analogous to traditional human health-based TTCs but with derivation and application to ecological species. An ecoTTC is computed from the probability distribution of predicted no effect concentrations (PNECs) derived from either chronic or extrapolated acute toxicity data for toxicologically or chemically similar groups of chemicals. There has been increasing interest in using ecoTTCs in screening level environmental risk assessments and a computational platform has been developed for derivation with aquatic species toxicity data (https://envirotoxdatabase.org/). Current research and development areas include assessing mode of action-based chemical groupings, conservatism in estimated PNECs and ecoTTCs compared to existing regulatory values, and the influence of taxa (e.g., algae, invertebrates, and fish) composition in the distribution of PNEC values. The ecoTTC continues to develop as a valuable alternative strategy within the toolbox of traditional and new approach methods for ecological chemical assessment. This brief review article describes the ecoTTC concept and potential applications in ecological risk assessment, provides an overview of the ecoTTC workflow and how the values can be derived, and highlights recent developments and ongoing research. Future applications of ecoTTC concept in different disciplines are discussed along with opportunities for its use.

Keywords: aquatic; ecological; mode of action; predicted no effect concentration; threshold of toxicological concern.

Figure 1

Schematic outlining the process for deriving a CTD (chemical toxicity distribution) or ecological threshold of toxicological concern (ecoTTC) using the EnviroTox platform (Connors et al., 2019). Targeted distributions can be created by using specific filtering criteria (e.g., MOA, chemical category, phys/chem property). The Predicted No Effect Concentration (PNEC) calculator is used to apply the correct, region-specific assessment factor based on the relative amount of data available for each test compound.

Figure 2

Example ecological threshold of toxicological concern (ecoTTC) probability distribution. Each data point in the distribution represents a chemical-specific predicted no effect concentration (PNEC) value. PNECs are calculated based on the relative amount of data (acute and chronic, for each trophic level). A shaded cell in the matrix tables indicates the trophic level with the lowest effect concentration; this value is divided by the appropriate assessment factor to derive the PNEC for each chemical in the distribution. In this example the ecoTTC is the 5th percentile of the distribution and is bounded by the lower (LCL) and upper (UCL) confidence limits.