Evaluation of PNEC values: extrapolation from Microtox, algae, daphnid, and fish data to HC5

In order to evaluate the risk to the environment from long term exposure of any discharged substance, toxicity thresholds are estimated, and particularly the Predicted No Effect Concentration (PNEC). This concentration can be estimated by the classic assessment factor approach or by statistical methods. These are more scientifically sound but they require several (at least 5-6) chronic ecotoxicity data, implying greater cost and time. New extrapolation methods derived from the statistical concept but requiring less data have been studied. Results show that methods based on chronic data are more reliable than methods based on acute data but the improvement is quite small. Considering the costs of chronic tests compared to acute tests, approaches based on acute data are an attractive alternative. A simple regression on the mean of the acute data gives the best results.