Marco Vighi

Joint algal toxicity of 16 dissimilarly acting chemicals is predictable by the concept of independent action

For a predictive assessment of the aquatic toxicity of chemical mixtures, two competing concepts are available: concentration addition and independent action. Concentration addition is generally regarded as a reasonable expectation for the joint toxicity of similarly acting substances. In the opposite case of dissimilarly acting toxicants the choice of the most appropriate concept is a controversial issue. In tests with freshwater algae we therefore studied the extreme situation of multiple exposure to chemicals with strictly different specific mechanisms of action. Concentration response analyses were performed for 16 different biocides, and for mixtures containing all 16 substances in two different concentration ratios. Observed mixture toxicity was compared with predictions, calculated from the concentration response functions of individual toxicants by alternatively applying both concepts. The assumption of independent action yielded accurate predictions, irrespective of the mixture ratio or the effect level under consideration. Moreover, results even demonstrate that dissimilarly acting chemicals can show significant joint effects, predictable by independent action, when combined in concentrations below individual NOEC values, statistically estimated to elicit insignificant individual effects of only 1%. The alternative hypothesis of concentration addition resulted in overestimation of mixture toxicity, but differences between observed and predicted effect concentrations did not exceed a factor of 3.2. This finding complies with previous studies, which indicated near concentration-additive action of mixtures of dissimilarly acting substances. Nevertheless, with the scientific objective to predict multi-component mixture toxicity with the highest possible accuracy, concentration addition obviously is no universal solution. Independent action proves to be superior where components are well known to interact specifically with different molecular target sites, and provided that reliable statistical estimates of low toxic effects of individual mixture constituents can be given. With a regulatory perspective, however, fulfilment of both conditions may be regarded as an extraordinary situation, and hence concentration addition may be defendable as a pragmatic and precautionary default assumption.

Predicting the joint algal toxicity of multi-component s-triazine mixtures at low-effect concentrations of individual toxicants.

Herbicidal s-triazines are widespread contaminants of surface waters. They are highly toxic to algae and other primary producers in aquatic systems. This results from their specific interference with photosynthetic electron transport. Risk assessment for aquatic biota has to consider situations of simultaneous exposure to various of these toxicants. In tests with freshwater algae we predicted and determined the toxicity of multiple mixtures of 18 different s-triazines. The toxicity parameter was the inhibition of reproduction of Scenedesmus vacuolatus. Concentration-response analyses were performed for single toxicants and for mixtures containing all 18 s-triazines in two different concentration ratios. Experiments were designed to allow a valid statistical description of the entire concentration-response relationships, including the low concentration range down to EC1. Observed effects and effect concentrations of mixtures were compared to predictions of mixture toxicity. Predictions were calculated from the concentration-response functions of individual s-triazines by applying the concepts of concentration addition and independent action (response addition) alternatively. Predictions based on independent action tend to underestimate the overall toxicity of s-triazine mixtures. In contrast, the concept of concentration addition provides highly accurate predictions of s-triazine mixture toxicity, irrespective of the effect level under consideration and the concentration ratio of the mixture components. This also holds true when the mixture components are present in concentrations below their individual NOEC values. Concentrations statistically estimated to elicit non-significant effects of only 1% still contribute to the overall toxicity. When present in a multi-component mixture they can co-operate to give a severe joint effect. Applicability of the findings obtained with s-triazines to mixtures of other contaminants in aquatic systems and consequences for risk assessment procedures are discussed.

Ecological vulnerability in risk assessment — A review and perspectives

This paper reviews the application of ecological vulnerability analysis in risk assessment and describes new developments in methodology. For generic non-site-specific assessments (e.g. for the requirements of most European directives on dangerous chemicals) risk is characterised just on the basis of the ratio between an effect indicator and an exposure indicator. However, when the actual risk for a specific ecosystem is desired, the concept of ecological vulnerability may be more appropriate. This calls for a change in thinking, from sensitivity at the organism level to vulnerability at higher organization levels, and thus forms the link from laboratory toxicology to field effects at population, community or ecosystem level. To do so, biological and ecological characteristics of the ecosystems under concern are needed to estimate the ecological vulnerability. In this review we describe different vulnerability analysis methods developed for populations (of a single species), communities (consisting of different populations of species) and ecosystems (community and habitat combined). We also give some examples of methods developed for socio-ecological systems. Aspects that all methods share are the use of expert judgment, the input of stakeholders, ranking and mapping of the results, and the qualitative nature of the results. A new general framework is presented to guide future ecological vulnerability analysis. This framework can be used as part of ecological risk assessment, but also in risk management. We conclude that the further quantification of ecological vulnerability is a valuable contribution to vulnerability assessment.

Joint algal toxicity of phenylurea herbicides is equally predictable by concentration addition and independent action

Photosynthesis-inhibiting phenylurea derivatives, such as diuron, are widely used as herbicides. Diuron concentrations clearly exceeding the predicted-no-effect concentration have been regularly measured in European freshwater systems. The frequently observed exposure to mixtures of phenylureas additionally increases the hazard to aquatic primary producers. Fluctuating numbers and concentrations of individual toxicants make experimental testing of every potential mixture unfeasible. Thus, predictive approaches to the mixture hazard assessment are needed. For this purpose, two concepts are at hand, both of which make use of known toxicities of the individual components but are based on opposite mechanistic suppositions: Concentration addition is based on the idea of similar mechanisms of action, whereas independent action assumes dissimilarly acting mixture components. On the basis of pharmacological reasoning, it was therefore anticipated that the joint algal toxicity of phenylurea mixtures would be predictable by concentration addition. Indeed, we could demonstrate a high predictive power of concentration addition for these combinations. Surprisingly, however, the opposite concept of independent action proved to be equally valid, because both concepts predicted virtually identical mixture toxicities. This exceptional case has previously been derived from theoretical considerations. Now, the tested phenylurea mixtures serve as an example for the practical relevance of this situation for multicomponent mixtures.

Determination of n-octanol/water partition coefficient (Kow) of pesticide critical review and comparison of methods

Abstract Octanol/water partition coefficients (log Kow) for 87 chemicals representing the main classes of pesticides have been determined by means of three different estimation methods (RP-HPLC, ClogP, calculation from water solubility), and the results have been compared with experimental values (measured mainly with Slow Stirring or Shake Flask methods), collected through a survey of the literature. On the basis of a critical evalution of all available data, a selected value for each pesticide has been proposed. Values and limitations of the three estimation methods has been discussed.

BIOCONCENTRATION OF ORGANIC CHEMICAL VAPOURS IN PLANT LEAVES : THE AZALEA MODEL

Abstract Experimental data on the accumulation and release kinetics of azalea leaves exposed to constant vapour levels of alachlor, dieldrin and 3,4,3′,4′-tetrachlorobiphenyl are reported. Calculated leaf/air bioconcentration factors for these and other 11 organic chemical vapours are used to improve a correlation with the 1- octanol water and air/water equilibrium partition coefficients.

Chlorinated hydrocarbons in pine needles in Europe : fingerprint for the past and recent use

Pine needles have been demonstrated as a useful monitoring matrix for the evaluation of the tropospheric contamination levels of persistent chlorinated hydrocarbons, such as DDTs, HCHs, and HCB. Global chlorinated hydrocarbon distribution has been investigated with major attention to remote areas, while the factors affecting the distribution trends in regions of major use are less known. Six countries in Europe were analyzed by the transect sampling mode. Homogeneous contamination intensities were present within each transect, and correspondence factor analysis was used for the characterization of the typical distribution patterns

QSARs for organotin compounds on Daphnia magna

Abstract A QSAR study has been carried out on several organotin compounds using physical and topological parameters (log P, pKa, 1 x and 1 x v ) and acute toxicity data on Daphnia magna . Equations with significant correlation and high predictive capacity have been found.

Water quality objectives for mixtures of toxic chemicals: problems and perspectives

The need to develop water quality objectives not only for single substances but also for mixtures of chemicals seems evident. For that purpose, the conceptual basis could be the use of the two existing biometric models: concentration addition (CA) and independent action (IA), which is also called response addition. Both may allow calculation of the toxicity of mixtures of chemicals with similar modes of action (CA) or dissimilar modes of action (IA), respectively. The joint research project Prediction and Assessment of the Aquatic Toxicity of Mixtures of Chemicals (PREDICT) within the framework of the IVth Environment and Climate Programme of the European Commission, provided the opportunity to address (a) chemometric and QSAR criteria to classify substances as supposedly similarly or dissimilarly acting; (b) the predictive values of both models for the toxicity of mixtures at low, statistically nonsignificant effect concentrations of the individual components; and (c) the predictability of mixture toxicity at higher levels of biological complexity. In this article, the general outline, methodological approach, and some preliminary findings of PREDICT are presented. A procedure for classifying chemicals in relation to their structural and toxicological similarities has been developed. The predictive capabilities of CA and IA models have been demonstrated for single species and, to some extent, for multispecies testing. The role of very low effect concentrations in multiple mixtures has been evaluated. Problems and perspectives concerning the development of water quality objectives for mixtures are discussed.

Chlorinated dioxins: volatilization from soils and bioconcentration in plant leaves.

Polychlorinated dibenzodioxins (PCDDs) are a group of xenobiotics of extreme environmental interest, by virtue of their high toxic potential, coupled with high bioaffinity and resistance to degradation. One in particular (2,3,7,8tetrachloro dibenzo-p-dioxin, 2,3,7,8-TCDD), is probably the most poisonous substance ever introduced into the environment. PCDDs are not intentionally produced, but mainly arise from combustion processes and certain industrial activities; all the sources of these substances are not yet completely known (Hutzinger and Fiedler 1989). However, loads to the environment are certainly significant as PCDD residues may be found in many different environmental matrices (Jones and Bennet 1989).