Use of Reduced Life Expectancy Model for Evaluation of Effects of Exposure Time on Aquatic Toxicity
The effect of exposure time is an important concept in toxicity that is long recognised but less well understood. This effect is particularly important when the exposure time is long such as in the assessment of health risks of environmental contaminants. The effect of exposure time may also be considered to include the kinetic effect of toxicant uptake within the target organism. This makes it even more difficult to understand the processes involved. Traditionally, the effect of exposure time has been empirically correlated by the use of the Haber's rule or its various modified forms. The basic premise of the Haber's rule is that the product of exposure concentration and the corresponding exposure time when the toxic effect is observed is a constant. A lower exposure concentration leads to a longer exposure time, which is generally consistent with experimental observations. However, this relationship cannot be expected to hold true when the exposure concentration is low and/or the exposure time is long. This is because of the fact that, when the concentration approaches zero, the exposure time must approach infinite to keep the concentration time product constant. As living organisms have finite life expectancies, this requirement cannot physically be met. In our earlier study on the effects of long exposure time and internal lethal concentrations of nonspecific toxicants in fish, a simple mathematical model based on the concept of life expectancy reduction was developed. This Reduced Life Expectancy Model can be used to describe the relationship between internal lethal concentration (ILC50) and the exposure period (LT50). However, a major problem with the use of ILC50 is the lack of ILC50 data in the scientific literature. In this paper, another form of the Reduced Life Expectancy Model, in which the ILC50 is replaced with the more commonly used LC50, was used for the evaluation of the effects of exposure time on aquatic toxicity. A number of sets of toxicity data with different aquatic organisms and toxicants were evaluated with the Reduced Life Expectancy Model. Results indicated that while many of the experimental data sets can be satisfactorily correlated with the simple Reduced Expectancy Model, significant deviations occurred for some data sets. For these cases, a Two Stage Reduced Life Expectancy Model was further developed to satisfactorily correlate the data sets. The two stage model was based on the assumption that the time course of toxicants in the organism can have two phases in the peripheral system and the central system.
15th International Symposium on Toxicity Assessment, Hong Kong, Jul.3-8, 2011
Environmental Sciences not elsewhere classified