Use of exposure time and life expectancy in models for toxicity to aquatic organisms

Abstract

The exposure time is a variable which is usually not incorporated into models for toxicity. However, by the use of the internal concentrations as a measure of the toxicity of non-specific toxicants, rather than the ambient concentrations, this can be achieved. When the exposure time is relatively short, such as the typical exposure time of 96 hours for LC50 tests, the critical internal concentration, which is the internal concentration for lethality of aquatic organisms, is relatively constant for a particular non-specific toxicant and shows a much higher level of consistency than the LC50 measured in the ambient water. The bioconcentration process leading to the internal concentration with non-specific toxicants is a relatively well understood and predictable process. When the exposure time is relatively long, results on the measurement of the critical internal concentration with fish and crab species over different exposure times has demonstrated that the internal lethal concentration falls with increasing exposure time periods in a consistent and predictable manner. This is a reduction in the life expectancy of the test organism and so the overall biological effect is the result of two factors – internal concentration and the exposure time. A model can be developed to represent this process and used to estimate the critical internal concentration for any length of exposure periods. The model can be used to estimate chronic values of the internal concentration. It also provides information useful in assessing the risk to certain species due to the occurrence of residues of persistent non-specific toxicants in aquatic systems.