The pT-Value as an Indicator of Toxicity in the Aquatic Environment

Ecotoxicological information plays an important role in establishing effect-oriented standards for chemical substances. For various reasons, however, the approach of setting standards for individual compounds doesn’t guarantee an effective protection of ecosystems. Therefore the use of ecotoxicological indicators is advocated in addition to chemical standards. In this respect the pT-value is proposed as a biological sum parameter.

The pT-value is defined as ¹⁰log T/100, in which T stands for the number of times a sample (water or sediment) has to be diluted, (1/T) or concentrated (T), in order to produce an EC20 in the Microtox test. The scaling of the pT is chosen such that a pT-value of 0 and 1 corresponds with a maximum permissible risk level and a negligible risk, respectively.

The method and strategy for measuring the pT-value in the aquatic environment is presented. An example is given for the river Rhine, indicating that the river quality has improved since 1975, but still hasn’t reached the acceptable level.