Ecological Structure Activity Relationships (ECOSAR) is a tool for predicting the aquatic toxicity of industrial chemicals, developed by the U.S. Environmental Protection Agency (EPA) and the Syracuse Research Corporation (SRC).

Aquatic toxicity of untested chemicals is predicted based on their structural similarity to chemicals for which aquatic studies are available.

The toxicity data used to build the SARs are collected from publicly available experimental studies and confidential submissions provided to the U.S. EPA New Chemicals Program.

ECOSAR contains a library of class-based QSARs for predicting aquatic toxicity, overlaid with an expert decision tree for selecting the appropriate chemical class. ECOSAR derives toxicity values for three general types of chemicals:

  • Neutral Organics, i.e. nonionizable and nonreactive chemicals acting via simple nonpolar narcosis (baseline toxicity)
  • Organic Chemicals with Excess Toxicity, i.e. organic chemicals presenting a more specific mode of toxicity based on the presence of reactive functional groups (Hermens 1990). These chemicals can be more toxic than predicted by baseline toxicity equations to one or more aquatic organisms. Separate QSARs have been developed for several chemical classes identified as presenting excess toxicity to at least one or more species
  • Surfactant (Surface-Active) Organic Chemicals, i.e. chemicals that can greatly reduce the surface tension of water when used in very low concentrations.

The QSARs in ECOSAR for both neutral organics and classes with excess toxicity are based on a linear mathematical relationship between the predicted log Kow values (EPI Suite KOWWIN prediction) and the corresponding log of the measured toxicity values (mmol/L) for a suite of training set chemicals within each class of interest.

The studies collected for the training set chemicals in ECOSAR undergo an extensive data validation step to ensure appropriateness for inclusion in the model.