Introduction to Species Sensitivity Distributions (SSD) and HC5

Little Pro on 2019-06-06

Species Sensitivity Distributions (SSD) are a very important technique used in ecological risk assessment. It is primarily used to derive predicted no effect concentrations (PNECs) for environment risk assessment.

Principles of SSD

Different species show different sensitivities to the same chemical substance and the variation between those species can be described by a statistical distribution. If you have conducted many eco-toxicity tests on the same substance using multiple species (fish, invertebrates and plants) and obtained many eco-toxicity endpoints, you can use SSD, the statistical approach, to draw a curve such as the following one and derive HC5 (hazardous concentration for 5% of species).


Derived HC5 can then be used to calculate PNEC by dividing it with an additional safety factor (1-5). 

  • PNEC = HC5/AF

If you only have a small dataset (i.e. algae, daphnid and fish), the SSD approach will not be applicable. In that case, you usually need to divide the lowest NOEC value with larger safety factor (typically 10).

How to Derive PNEC

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 Tags: Topics - CRAEnvironmental Risk Assessment