Overview of current models for assessing toxic impacts

6.3 Overview of current models for assessing toxic impacts

The potential use of modeling methods in relation to toxic substances and the WFD includes fugacity modeling and Quantitative Structure Activity Relationships (QSARs). The fugacity models can provide a link between pressures and concentrations of specific pollutants in the environment. Thus, they have a potential applicability in assessing the likelihood that surface water bodies will fail to meet the environmental quality standards and can be used to estimate critical target loads.

QSARs are theoretical models that can be used to predict the physicochemical, environmental and biological properties of molecules. The notion that there is a relation between chemical structure and biological activity is not new and QSARs have been developed for over a hundred years (Schultz et al. 2003). In the last 20 years there has been an enormous increase in the application of these models, due to the rapid increase in computing power and new methodology for molecular modeling and statistical analysis.

With respects of the WFD implementation requirements, the QSAR models can provide: • estimates of acute and/or chronic toxicity of substances to different trophic levels in cases where experimental data is missing, which can be used when setting EQS. • data on persistence and bioaccumulation in cases where experimental data is missing, which can be used when setting EQS. • QSAR models are frequently used to estimate physical parameters needed for fugacity modelling, i.e. partition coefficients, persistence and other parameters,

Moreover, the fugacity models can provide a link between pressures and concentrations of specific pollutants in the environment. Thus, they have a potential applicability in assessing the likelihood that surface water bodies will fail to meet the environmental quality standards and can be used to estimate critical target loads.

The development of reliable QSARs for toxicity requires a large amount of data from toxicity test that have been performed on different substances using a standardized protocol. This prerequisite cannot be fulfilled for a large number of species. Indeed, the primary reason for using toxicity QSARs is the lack of available toxicity data! In chemical risk assessment, one or a few species from each different trophic level are used to represent the toxicity to all species in that level. Most regulatory protocols include acute lethality for fish and planktonic crustacean, Daphnia spp., and chronic or reproduction data for algae, fish and Daphnia spp. Currently, QSARs are available for most substance groups for acute toxic effects to organisms used in chemical risk assessment.

Deteriorated ecological status may be caused also by other substances than the priority substances. QSARs models are potentially more useful in cases of other toxic substances since experimental data on properties and effects of the priority substances are, in general, much better known.

Endocrine disrupting chemicals (EDCs) are an area of growing concern in chemical risk assessment. Most of the end-points related to endocrine disruption used in QSAR modeling are in vitro tests related to receptor binding or biological effects of receptor binding, e.g. gene activation. QSARs are considered important and potentially useful due to the complexity and cost of in vitro and in vivo tests for endocrine disruption.

The current status of QSARs for endocrine disruption (Schmieder et al. 2003) are that most models are developed for specific substance groups. Some models have been developed based on diverse sets of chemicals with the purpose of screening new chemicals for EDCs. Focus is on the human estrogen receptor (hER) with few applications to other human receptors and ERs from rodents and calf. No QSARs for endocrine disruption in aquatic organisms have been found.

Van Leeuwen et al at University of Utrecht (Van Leeuwen and Vanderzandt 1992) have used QSAR estimates of toxicity of narcotic chemicals to predict no-effect levels (NELs) at the ecosystem level by means of recently developed extrapolation methods. Equilibrium partitioning theory was used to derive NELs for aquatic sediments and internal toxicant concentrations for aquatic organisms. Calculations were carried out for 102 narcotic substances.